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The nervous systems of Cnidarians
Abstract
Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved. The basic plan of the cnidarian nervous system is that of a nerve net which, at some locations, has condensed to form nerve plexuses, or circular or longitudinal nerve tracts which may be syncytia. At the ultrastructural level, many cnidarian neurons have the combined characteristics of sensory, motor, inter- and neurosecretory neurons and thus appear to be multifunctional. We propose that these multifunctional neurons resemble the ancestors of the more specialized neurons that we find in higher animals today. The primitive nervous system of cnidarians is strongly peptidergic: from a single sea anemone species Anthopleura elegantissima, we have now isolated 16 different novel neuropeptides. These peptides are biologically active and cause inhibitions or contractions in muscle preparations or isolated muscle cells from sea anemones. The various peptides are located in at least six distinct sets of neurons showing that sea anemone neurons have already specialized with respect to their peptide content. Using immuno-electronmicroscopy, we have found that the peptides are located in neuronal dense-cored vesicles associated with both synaptic and non-synaptic release sites. All these data indicate that evolutionarily "old" nervous systems use peptides as transmitters. We have also investigated the biosynthesis of the cnidarian neuropeptides. These neuropeptides are made as large precursor proteins which contain multiple (up to 36) copies of immature neuropeptides. Thus, the biosynthesis of neuropeptides in cnidarians is very efficient and comparable to that of higher invertebrates, such as molluscs and insects, and vertebrates.
... Natural models for systems of this type exist among the high biodiversity and biological complexity present in aquatic life forms. For instance, marine life forms (e.g., sea jellies, sea stars, sea anemones, and coral) [4] exemplify organic organisms that rely on natural soft-hydrogel structures embedding complex hierarchical chemo mechanical systems to productively sustain life and respond to simple environmental stimuli that include chemical concentration (chemo taxis), [5] physical sheer/ stress (mechano taxis), [6] and light (photo taxis). [7] While the mechanisms of action have important differences in each case, all rely on specific exploitations of complex multiscale/gradient attributes of materials mechanics that have been the subjects of recent efforts to better model and predict shape change outcomes for such biomimetic 4D systems. ...
... Hydrogel and other polymer-based soft materials have attracted considerable attention for use in actuators in consequence of their capacities to provide autonomous functional responses to a variety of stimuli. [2a-d] Many of these systems are architecturally or chemically inspired by bio-origin materials or organisms [4,9] or are biomimetic in the sense that they utilize ambient environmental or energetic inputs to induce physical changes or perform elementary forms of work. [2h,10] Ionotropic hydrogels that rapidly cure upon exposure to cationic salt solutions have been the subject of numerous studies and applications in this regard [11] and notable means for fabricating complex structures from them at millimeter (and larger) scales have been reported in the literature. ...
... As seen in the rheology of the two compositions prior to curing, the Gel 4 (AlgThk) blend is significantly more viscoelastic (higher phase angle; Figure 1d) and a softer gel (lower magnitude of complex shear modulus; Figure 1e) in comparison to the Gel 3 (AlgLap). (1)(2)(3)(4)7) correspond to Table 1 ink compositions. Sea star bio-inspiration for S-SAA (right, scale 5-13 cm) reproduced with permission. ...
Direct‐ink writing (DIW), a rapidly growing and advancing form of additive manufacturing, provides capacities for on‐demand tailoring of materials to meet specific requirements for final designs. The penultimate challenge faced with the increasing demand of customization is to extend beyond modification of shape to create 4D structures, dynamic 3D structures that can respond to stimuli in the local environment. Patterning material gradients is foundational for assembly of 4D structures, however, there remains a general need for useful materials chemistries to generate gray scale gradients via DIW. Here, presented is a simple materials assembly paradigm using DIW to pattern ionotropic gradients in hydrogels. Using structures that architecturally mimic sea‐jelly organisms, the capabilities of spatial patterning are highlighted as exemplified by selectively programming the valency of the ion‐binding agents. Spatial gradients, when combined with geometry, allow for programming the flexibility and movement of iron oxide nanoparticle–loaded ionotropic hydrogels to generate 4D‐printed structures that actuate in the presence of local magnetic fields. This work highlights approaches to 4D design complexity that exploits 3D‐printed gray‐scale/gradient mechanics. Patterning gradients is foundational for the development of 4D‐printed structures. A novel, simple materials assembly paradigm for direct‐ink writing is presented to translate design complexity to net form fabrication via embedding gray scale gradients of complex form.
... Cnidaria and Ctenophora are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle [9][10][11][12][13][14][15]. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net [11,13,[15][16][17] that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters [15][16][17][18][19][20]. It was reported that cnidarian soft corals [21] and box jellyfish [22,23] exhibit periods of quiescence, a pre-requisite for sleep-like states, prompting us to ask whether sleep is present in Cnidaria. ...
... Cnidaria and Ctenophora are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle [9][10][11][12][13][14][15]. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net [11,13,[15][16][17] that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters [15][16][17][18][19][20]. It was reported that cnidarian soft corals [21] and box jellyfish [22,23] exhibit periods of quiescence, a pre-requisite for sleep-like states, prompting us to ask whether sleep is present in Cnidaria. ...
... To our knowledge, our finding is the first example of a sleep-like state in an organism with a diffuse nerve net [7,8], suggesting that this behavioral state arose prior to the evolution of a centralized nervous system. Though at least 600 million years of evolution separate cnidarians from bilaterians [9][10][11][12][13][14][15][16], many aspects of the nervous system are conserved, including neuropeptides and neurotransmitters [15][16][17][18][19][20]. One such conserved molecule, melatonin [32], promotes sleep in diurnal vertebrates, including zebrafish [33] and humans [34], and induces quiescence in invertebrates [35]. ...
Do all animals sleep? Sleep has been observed in many vertebrates, and there is a growing body of evidence for sleep-like states in arthropods and nematodes. Here we show that sleep is also present in Cnidaria, an earlier-branching metazoan lineage. Cnidaria and Ctenophora are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters . It was reported that cnidarian soft corals and box jellyfish exhibit periods of quiescence, a pre-requisite for sleep-like states, prompting us to ask whether sleep is present in Cnidaria. Within Cnidaria, the upside-down jellyfish Cassiopea spp. displays a quantifiable pulsing behavior, allowing us to perform long-term behavioral tracking. Monitoring of Cassiopea pulsing activity for consecutive days and nights revealed behavioral quiescence at night that is rapidly reversible, as well as a delayed response to stimulation in the quiescent state. When deprived of nighttime quiescence, Cassiopea exhibited decreased activity and reduced responsiveness to a sensory stimulus during the subsequent day, consistent with homeostatic regulation of the quiescent state. Together, these results indicate that Cassiopea has a sleep-like state, supporting the hypothesis that sleep arose early in the metazoan lineage, prior to the emergence of a centralized nervous system.
... R. Soc. B 370: 20150286 [61,67,128,132,133,135,136,[138][139][140][141][142]. Therefore, the evolutionary origin of the nerve net appears to be closely linked to that of the musculature [44]. ...
... This aggregation process might have occurred around adherens junctions, as suggested by the shared presence of both N-and E-cadherin in adherens and synaptic junctions [154]. In line with a division of labour scenario, cnidarian sensory neurons and ganglion cells are related to the extent that they cannot be clearly separated into two distinct cell types [139]. To test these hypotheses further, the molecular profiling of cell types constituting the nerve net in cnidarians and ctenophores will be especially rewardingand also solve the yet unsettled question of nervous system homology between ctenophores and other neuralians [155]. ...
... B 370: 20150286 musculature. Adding to this, numerous studies report paracrine release of structurally related, amidated neuropeptides of the RFamide and other families from cnidarian nerve net neurons [136,139,157,160]. ...
Prerequisite for tracing nervous system evolution is understanding of the body plan, feeding behaviour and locomotion of the first animals in which neurons evolved. Here, a comprehensive scenario is presented for the diversification of cell types in early metazoans, which enhanced feeding efficiency and led to the emergence of larger animals that were able to move. Starting from cupshaped, gastraea-like animals with outer and inner choanoflagellate-like cells, two major innovations are discussed that set the stage for nervous system evolution. First, the invention of a mucociliary sole entailed a switch from intra- to extracellular digestion and increased the concentration of nutrients flowing into the gastric cavity. In these animals, an initial nerve net may have evolved via division of labour from mechanosensory-contractile cells in the lateral body wall, enabling coordinated movement of the growing body that involved both mucociliary creeping and changes of body shape. Second, the inner surface of the animals folded into metameric series of gastric pouches, which optimized nutrient resorption and allowed larger body sizes. The concomitant acquisition of bilateral symmetry may have allowed more directed locomotion and, with more demanding coordinative tasks, triggered the evolution of specialized nervous subsystems. Animals of this organizational state would have resembled Ediacarian fossils such as Dickinsonia andmay have been close to the cnidarian–bilaterian ancestor. In the bilaterian lineage, the mucociliary sole was used mostly for creeping, or frequently lost. One possible remnant is the enigmatic Reissner’s fibre in the ventral neural tube of cephalochordates and vertebrates. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
... Many studies confirm that cnidarian neurons label with antibodies to a range of neuropeptides but not to monoamines or acetylcholine (Grimmelikhuijzen and Westfall, 1995;Satterlie, 2019). Studies on Hydra have illustrated that peptide-gated ion channels (HyNaC) are almost exclusively used for fast neuromuscular transmission, and their presence in other cnidarians and in ctenophores and placozoans is taken to suggest this might be the case also in those groups (Assmann et al., 2014). ...
... Neurotransmitters have long been used to identify neurons in a range of tissues, from vertebrates to jellyfish (Torrealba and Carrasco, 2004). They are useful markers, but not all neurons label even in a single animal, and although developing custom antibodies has been a typical approach, these still do not identify all neurons (Grimmelikhuijzen and Westfall, 1995), and in some cases, the antibodies can also label non-neuronal cells (e.g. Faltine-Gonzalez and Layden, 2019). ...
... Many studies confirm that cnidarian neurons label with antibodies to a range of neuropeptides but not to monoamines or acetylcholine (Grimmelikhuijzen and Westfall, 1995;Satterlie, 2019). Studies on Hydra have illustrated that peptide-gated ion channels (HyNaC) are almost exclusively used for fast neuromuscular transmission, and their presence in other cnidarians and in ctenophores and placozoans is taken to suggest this might be the case also in those groups (Assmann et al., 2014). ...
... Neurotransmitters have long been used to identify neurons in a range of tissues, from vertebrates to jellyfish (Torrealba and Carrasco, 2004). They are useful markers, but not all neurons label even in a single animal, and although developing custom antibodies has been a typical approach, these still do not identify all neurons (Grimmelikhuijzen and Westfall, 1995), and in some cases, the antibodies can also label non-neuronal cells (e.g. Faltine-Gonzalez and Layden, 2019). ...
The evolution of animal diversity is strongly affected by the origin of novel cell and tissue types and their interactions with each other. Understanding the evolution of cell types will shed light on the evolution of novel structures, and in turn highlight how animals diversified. Several cell types may also have been lost as animals simplified – for example did sponges have nerves and lose them? This book reveals the interplay between gains and losses and provides readers with a better grasp of the evolutionary history of cell types. In addition, the book illustrates how new cell types allow a better understanding permitting the discrimination between convergence and homology.
... Hym-370 and Hym-357 had been identified in a screen for myoactive peptides [22][23][24][25] . Rfamide III is a neuropeptide belonging to a family of FMRFamide-like peptides conserved in invertebrates and vertebrates [26][27][28][29] . Previous publications [26][27][28][29][30] have demonstrated that the expression patterns of these three genes are unique in that each gene is expressed in different subpopulations of neurons, forming distinct localized expression patterns along the body axis with sharp boundaries (Fig. 4a). ...
... Rfamide III is a neuropeptide belonging to a family of FMRFamide-like peptides conserved in invertebrates and vertebrates [26][27][28][29] . Previous publications [26][27][28][29][30] have demonstrated that the expression patterns of these three genes are unique in that each gene is expressed in different subpopulations of neurons, forming distinct localized expression patterns along the body axis with sharp boundaries (Fig. 4a). To assess their antimicrobial activity, the neuropeptides were chemically synthesized and tested in microdilution susceptibility assays against eight bacterial species. ...
Colonization of body epithelial surfaces with a highly specific microbial community is a fundamental feature of all animals, yet the underlying mechanisms by which these communities are selected and maintained are not well understood. Here, we show that sensory and ganglion neurons in the ectodermal epithelium of the model organism hydra (a member of the animal phylum Cnidaria) secrete neuropeptides with antibacterial activity that may shape the microbiome on the body surface. In particular, a specific neuropeptide, which we call NDA-1, contributes to the reduction of Gram-positive bacteria during early development and thus to a spatial distribution of the main colonizer, the Gram-negative Curvibacter sp., along the body axis. Our findings warrant further research to test whether neuropeptides secreted by nerve cells contribute to the spatial structure of microbial communities in other organisms.
... The genes associated with neuroactive ligand-receptor interactions, the dendritic tree, and postsynaptic neurotransmitter receptor activity expanded, increasing neuromodulation. Cnidarians were among the first species to develop a neural system capable of sensing and relaying information about their environment [65,66]. To date, the evolutionary origins of the nervous system have been fairly restricted, as studies have focused only on a few model species, such as Hydra magnipapillata (Hydrozoa) and N. vectensis, and have included stony corals very infrequently [65]. ...
The Late Devonian period was known for disturbances such as lower temperatures and abnormal ocean carbon and nitrogen levels, leading to the extinction of approximately 21% of genus-level and 16% of family-level marine organisms. However, evolutionary responses of marine organisms to hardships have not yet been fully explored, even though these organisms may soon face another extinction event. Stony corals, one of the few marine organisms that survived the Late Devonian Period, may provide some insight into the adaptive evolution mechanism underlying survival in unstable marine environments. The current study revealed that the gene families related to signal transduction and immunity, such as G protein-coupled receptors and Toll-like receptors, expanded in stony coral ancestors (SCAs), possibly improving the efficiency of stress and immune responses and maintaining internal environmental homeostasis. Interestingly, the first horizontal gene transfer event of MSHA from actinomycetes to corals and the subsequent expansion in SCAs were discovered. MSHA encodes D-inositol 3-phosphate glycosyltransferase, which is naturally found in actinomycetes and is responsible for the synthesis of mycothiol with antibacterial properties. The MSHA family members diverged throughout the development of stony corals, but their essential function in glycosyl transfer remained unchanged. Therefore, the evolutionary history of ancient coral shows that efficient signal transduction and increased immunity may have driven the survival of SCAs throughout the Late Devonian period, which may provide new insights into how current corals avoid extinction.
... Hydroids belong to phylum Cnidaria, and Bilateria and Cnidaria clades are phylogenetically close which makes the first ones an interesting group of organisms to study neuro-processes. Unitary hydroids have a well-structured and documented CNS (Grimmelikhuijzen and Westfall, 1995;Koizumi, 2016), but evidence of a nervous system in colonial cnidarians is scarce. In his research article, Kosevich reports the presence of a colonial nervous system in the coenosarc of the athecate Clava multicornis, the thecate Dynamena pumila, and the thecate Obelia longissima, all of which are species of colonial hydroids with interconnected zooids. ...
... Previously examined Brachyury ortholog of P. carnea clusters with Brachyury3 group according to our analysis (Figure 1), and its expression was detected not in the oral, but in the aboral ectoderm (Spring et al., 2002). In D. pumila, Brachyury3 expression also was detected in discrete triangular and bottle-like ectodermal cells (Figure 5g-j), morphologically similar to sensory cells of cnidarian nerve system (Grimmelikhuijzen & Westfall, 1995). This is of interest considering that Brachyury gene acts as neural repressor in the anthozoan Nematostella (Schwaiger et al., 2021). ...
Brachyury, a member of T-box gene family, is widely known for its major role in mesoderm specification in bilaterians. It is also present in non-bilaterian metazoans, such as cnidarians, where it acts as a component of an axial patterning system. In this study, we present a phylogenetic analysis of Brachyury genes within phylum Cnidaria, investigate differential expression and address a functional framework of Brachyury paralogs in hydrozoan Dynamena pumila. Our analysis indicates two duplication events of Brachyury in the cnidarian lineage: in the common ancestor of the Medusozoa clade and at the base of the class Hydrozoa. We designate result of the first step as Brachyury2 and of the second as Brachyury3.
Brachyury1 and 2 display a conservative expression pattern marking the oral pole of the body axis in D. pumila. On the contrary, Brachyury3 expression was detected in scattered presumably nerve cells of the D. pumila larva. Pharmacological modulations indicated that Brachyury3 is not under regulation of cWnt signalling in contrast to the other two Brachyury genes. Divergence in expression patterns and regulation suggest neofunctionalization of Brachyury3 in hydrozoans.
... The most important feature of cnidarians is the presence of stinging cells, or cnidocytes, with specialized organelles, which comprise capsules capable of firing hollow threads that can be used for hunting, defense, or attachment [2]. Cnidocytes, sensory cells that have sensory cilia at the apical side, and a network of ganglion cells form the nerv ous system in Cnidarians, which can have different struc tures depending on the organism [8,9]. Free swimming jellyfish or medusae have nerve rings consisting of neu rites (neuron axons) at the edge of the umbrella. ...
A unique set of features and characteristics of species of the Cnidaria phylum is the one reason that makes them a model for a various studies. The plasticity of a life cycle and the processes of cell differentiation and development of an integral multicellular organism associated with it are of a specific scientific interest. A new stage of development of molecular genetic methods, including methods for high-throughput genome, transcriptome, and epigenome sequencing, both at the level of the whole organism and at the level of individual cells, makes it possible to obtain a detailed picture of the development of these animals. This review examines some modern approaches and advances in the reconstruction of the processes of ontogenesis of cnidarians by studying the regulatory signal transduction pathways and their interactions.
... Самым важ ным отличительным признаком Стрекающих является наличие стрекательных клеток или книдоцитов со специализированными органел лами, которые представляют собой капсулы с выстреливающими нитями, использующимися для охоты, защиты и прикрепления [2]. Книдо циты вместе с сенсорными клетками, которые имеют чувствительные реснички на апикальных частях, выходящих на поверхность эпителиев, и сетью ганглиозных клеток формируют нервную систему Стрекающих, которая различается по строению в зависимости от жизненной формы организма [8,9]. У свободноплавающих медуз по краю колокола имеются нервные кольца, состоящие из нейритов (отростков нейронов), кроме того, у многих медуз есть специализиро ванные органы чувств -светочувствительные глазки и органы равновесия [10,11]. ...
Исключительный набор особенностей и характеристик представителей типа Cnidaria (Стрекающие) делает
их модельным объектом для широкого круга исследований. Особый научный интерес представляют плас#
тичность жизненного цикла и связанные с ним процессы клеточной дифференцировки и развития целост#
ного многоклеточного организма. Новый уровень развития молекулярно#генетических методов, в том чис#
ле использование методов широкомасштабного секвенирования геномов, транскриптомов и эпигеномов,
как на уровне целого организма, так и на уровне отдельных клеток, делает возможным получение детальной
картины развития этих животных. В представленном обзоре рассматриваются современные подходы и дос#
тижения с использованием методов широкомасштабного секвенирования в реконструкции процессов он#
тогенеза Cnidaria путём изучения регуляторных путей клеточной трансдукции и их взаимодействий.
Abstract—A unique set of features and characteristics of species of the Cnidaria phylum is the one reason that makes them
a model for a various studies. The plasticity of a life cycle and the processes of cell differentiation and development of an
integral multicellular organism associated with it are of a specific scientific interest. A new stage of development of molec�
ular genetic methods, including methods for high�throughput genome, transcriptome, and epigenome sequencing, both at
the level of the whole organism and at the level of individual cells, makes it possible to obtain a detailed picture of the devel�
opment of these animals. This review examines some modern approaches and advances in the reconstruction of the process�
es of ontogenesis of cnidarians by studying the regulatory signal transduction pathways and their interactions.
... Sea anemones have a simple nervous system and lack the lowest brain base or central information processing mechanism. Without true muscle tissue and visual capacity, they rely on nematocysts on their tentacles to release venom for predation, defense, and intraspecific competition [8,9]. Nematocytes present in all cnidarians produce highly complex venom-filled organelles known as nematocysts [10]. ...
As primitive metazoa, sea anemones are rich in various bioactive peptide neurotoxins. These peptides have been applied to neuroscience research tools or directly developed as marine drugs. To date, more than 1100 species of sea anemones have been reported, but only 5% of the species have been used to isolate and identify sea anemone peptide neurotoxins. There is an urgent need for more systematic discovery and study of peptide neurotoxins in sea anemones. In this review, we have gathered the currently available methods from crude venom purification and gene cloning to venom multiomics, employing these techniques for discovering novel sea anemone peptide neurotoxins. In addition, the three-dimensional structures and targets of sea anemone peptide neurotoxins are summarized. Therefore, the purpose of this review is to provide a reference for the discovery, development, and utilization of sea anemone peptide neurotoxins.
... Mechanosensory and chemosensory cells are found scattered throughout the epithelium of cnidarians (Cormier & Hessinger, 1980;Donaldson et al., 1980;Grimmelikhuijzen & Westfall, 1995;Mackie et al., 1988;Satterlie, 2002;Westfall, 1973;Westfall & Kinnamon, 1978). Though being primarily visually guided, the cubozoans also receive input from other senses (Golz & Thurm, 1993;Satterlie, 2011). ...
Box jellyfish have an elaborate visual system and perform advanced visually guided behaviours. However, the rhopalial nervous system (RNS), believed to be the main visual processing centre, only has 1000 neurons in each of the four eye carrying rhopalia. We have examined the detailed structure of the RNS of the box jellyfish Tripedalia cystophora, using immuno‐labelling with antibodies raised against four putative neuropeptides (T. cys RFamide, VWamide, RAamide and FRamide). In the RNS T. cys RF‐, VW‐ and RAamide antibodies stain sensory neurons, the pit eyes, the neuropil, and in peptide specific subpopulations of stalk associated neurons and giant neurons. Furthermore, RFamide ir+ neurites are seen in the epidermal stalk nerve, whereas VWamide antibodies stain the gastrodermal stalk nerve. RFamide has the most widespread expression including in the ring and radial nerves, the pedalium nerve plexus and in the tentacular nerve net. RAamide is the putative neurotransmitter in the motor neurons of the subumbrellar nerve net and VWamide is a potential marker for neuronal differentiation as it is found in subpopulations of undifferentiated cells both in the rhopalia and in the bell. The results from the FRamide antibodies were not included as only few cells were stained and in an unreproducible way. Our studies show hitherto unseen details of the nervous system of T. cystophora, and allowed us to identify specific functional groups of neurons. This identification is important for understanding visual processing in the RNS and enables experimental work, directly addressing the role of the different neuropeptides in vision.
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... Les coraux sont des animaux majoritairement coloniaux, c'est-à-dire qu'ils vivent en colonies d'individus appelés polypes ( Figure 5AB) reliés les uns aux autres par une cavité commune (appelée cavité gastro-vasculaire ou coelentéron) et par un réseau nerveux commun (Grimmelikhuijzen and Westfall 1995). Chaque polype est composé d'un orifice unique, appelé bouche, qui assure à la fois les fonctions de nutrition et d'excrétion. ...
Les coraux Scléractiniaires se développent généralement dans la zone photique peu profonde, exposée au rayonnement ultraviolet (UVs), la composante la plus dangereuse du rayonnement solaire. Le rayonnement UVs augmente avec le réchauffement climatique et s’ajoute à l’ensemble des pressions auxquelles sont soumis les coraux. Les enjeux de cette thèse ont été 1) de mieux comprendre les effets des UVs sur la réponse physiologique des coraux, les flux de matière organique et les bactéries associées au mucus et au corail; et 2) de caractériser l’effet combiné des UVs et d’une augmentation de température, et/ou d’un changement de disponibilité en sels nutritifs. Les résultats obtenus montrent tout d’abord que l’exposition des coraux aux UVs amplifie l’effet négatif de la température sur leur physiologie. Il en est de même pour l’absence en sels nutritifs, essentiels pour la physiologie corallienne. Nos résultats indiquent également que la sensibilité des coraux à un stress UV dépend de l’espèce étudiée et de la densité de symbiontes présents dans les tissus. L’effet négatif des UVs augmente avec la densité de symbiontes, vraisemblablement dû à la formation d’espèces réactives de l’oxygène (ROS) qui provoquent des dommages à l’organisme. Dans cette thèse, nous avons montré que la voie de signalisation JNK (c-Jun N-terminal kinase), hautement conservée au sein des êtres vivants, est impliquée dans la gestion de ces espèces réactives et que son inhibition entraine un blanchissement très rapide des coraux sous UVs et forte température. Finalement, l’excrétion de matière organique ainsi que les bactéries associées sont également impactés par les UVs ce qui pourrait contribuer à d’importants changements biochimiques dans l’eau des récifs coralliens. Les travaux de cette thèse apportent de nouvelles connaissances sur les effets des UVs sur les coraux et soulignent l’importance de les prendre en considération lors de nos prédictions sur le devenir des récifs coralliens face au réchauffement climatique.
... At the base of the apex, ganglia neurons can form a nerve ring that allows coordinated behaviors [11]. Although cnidarian nervous systems are highly peptidergic with peptide-gated ion channels as receptors [12,13], they share with bilaterians all the basic properties of synaptic conduction and chemical neurotransmission [14,15], even though the neuromuscular junction might have evolved independently in cnidarians and bilaterians [16]. Still, the transcriptional factors that control neurogenesis are largely evolutionarily conserved [2,5,9,[17][18][19][20][21] and cnidarian nervous systems might represent the first evolutionary attempt of centralized nervous system in eumetazoans [11]. ...
The nervous system is produced and maintained in adult Hydra through the continuous production of nerve cells and mechanosensory cells (nematocytes or cnidocytes). De novo neurogenesis occurs slowly in intact animals that replace their dying nerve cells, at a faster rate in animals regenerating their head as a complete apical nervous system is built in few days. To dissect the molecular mechanisms that underlie these properties, a precise monitoring of the markers of neurogenesis and nematogenesis is required. Here we describe the conditions for an efficient BrdU-labeling coupled to an immunodetection of neuronal markers, either regulators of neurogenesis, here the homeoprotein prdl-a, or neuropeptides such as RFamide or Hym-355. This method can be performed on whole-mount animals as well as on macerated tissues when cells retain their morphology. Moreover, when antibodies are not available, BrdU-labeling can be combined with the analysis of gene expression by whole-mount in situ hybridization. This co-immunodetection procedure is well adapted to visualize and quantify the dynamics of de novo neurogenesis. Upon continuous BrdU labeling, the repeated measurements of BrdU-labeling indexes in specific cellular populations provide a precise monitoring of nematogenesis as well as neurogenesis, in homeostatic or developmental conditions.KeywordsHydra nervous systemInterstitial stem cellsNeurogenesisNematogenesisIn situ hybridizationImmunofluorescenceHydroxyureaBrdUprdl-a
Hym-355
RFamide
... By contrast, Cnidaria (e.g. jellyfish and sea anemones), possess diffuse nerve nets [11] and mechanical stimulation of Calliactis parasitica produces nervous impulses, strong stimulation (i.e. potentially nociceptive) evoking a closure reflex [12]. ...
Nociceptors, i.e. sensory neurons tuned to detect noxious stimuli, are found in numerous phyla of the Animalia kingdom and are often polymodal, responding to a variety of stimuli, e.g. heat, cold, pressure and chemicals, such as acid. Owing to the ability of protons to have a profound effect on ionic homeostasis and damage macromolecular structures, it is no wonder that the ability to detect acid is conserved across many species. To detect changes in pH, nociceptors are equipped with an assortment of different acid sensors, some of which can detect mild changes in pH, such as the acid-sensing ion channels, proton-sensing G protein-coupled receptors and several two-pore potassium channels, whereas others, such as the transient receptor potential vanilloid 1 ion channel, require larger shifts in pH. This review will discuss the evolution of acid sensation and the different mechanisms by which nociceptors can detect acid.
This article is part of the Theo Murphy meeting issue ‘Evolution of mechanisms and behaviour important for pain’.
... (Tardent and Weber, 1976;Epp and Tardent, 1978). Several distinct neuronal subtypes, with different functional properties and with consistent and specific location in the animal, can be recognized morphologically and immunochemically (Grimmelikhuijzen and Westfall, 1995;Koizumi, 2002). A thin nerve ring (consisting of four different subsets of neurons) connected to the nervous net has been observed between the hypostome and the tentacle zone (Koizumi, 2002). ...
... Like GnRH receptors, many of the classes of receptors resolved in our global phylogeny (Fig. 3) are restricted to bilaterian taxa, but a single receptor class, RFamide receptors, has representation in both bilaterian and non-bilaterian lineages. This finding is consistent with a wealth of previous data showing the prevalence of peptidergic transmitters in cnidarian nervous systems (Grimmelikhuijzen et al., 1996;Grimmelikhuijzen and Westfall, 1995) and is also intriguing in light of the fact that the placozoan Trichoplax adhaerens lacks neurons altogether. Interestingly, the two earliest branching taxa in our genomic pipeline, the sponge A. queenslandica, which lacks neurons, and the comb jelly M. leidyi, which possess neurons Moroz et al. (2014), both lack RFamide receptors in our analyses. ...
The discovery of genes related to gonadotropin-releasing hormones (GnRH) and their receptors from diverse species has driven important advances in comparative endocrinology. However, our view of the evolutionary histories and nomenclature of these gene families has become inconsistent as several different iterations of GnRH and receptor relationships have been proposed. Whole genome sequence data are now available for most of the major lineages of animals, and an exhaustive view of the phylogenies of GnRH and their receptors is now possible. In this paper, we leverage data from publically available whole genome sequences to present anew phylogenomic analysis of GnRH and GnRH receptors and the distant relatives of each across metazoan phylogeny. Our approach utilizes a phylogenomics pipeline that searches datafrom 36 whole genome sequences and conducts phylogenetic analyses of gene trees. We provide a comprehensive analysis of the major groupings of GnRH peptides related hormones and their receptors and provide some suggestions for a new nomenclature. Our study provides a framework for understanding the functional diversification of this family of neuromodulatory peptides and their receptors.
... Originally identified in mollusc ganglia (Price & Greenberg, 1977), the FaRP family of peptides has been found in every major metazoan phylum, from coelenterates to chordates (e.g. Schneider & Taghert, 1988;Aarnisalo & Panula, 1995;Grimmelikhuijzen & Westfall, 1995). FMRFamide was demonstrated to be cardioexcitatory in molluscs and mammals and strongly myoactive in helminths (Geary et al., 1999). ...
We present the first results of immunocytochemical (ICC) observations on serotonin (5-HT) and FMRFamide (Phe-Met-Arg-Phe-NH2) immunoreactivity patterns in the rotifer nervous system investigated using a confocal laser scanning microscope (CLSM). Three species of rotifers are studied: Platyias patulus (Plationus patulus, Segers et al., 1993; Hydrobiologia 268: 1–8), Euchlanis dilatata; and Asplanchna herrickii. Independently from their systematic position, these species possess similar nerve structures. However, some differences were observed in the innervation of the corona, mastax, foot, and mostly in the pattern of the cerebral neurons. The general numbers of 5-HT-immunoreactive (IR) and FMRFamide-IR neurons are low (10–34), but constant for each species. The sizes of the neurons vary from 2, 5 to 10 µm. From 4 to 14 cerebral neurons lie at different levels and are arranged into an X- or a ring- or a curved arch shape. One or two pairs of neurons are localized along longitudinal nerve cords. Double staining of 5-HT and FMRFamide-IR elements shows no co-localization.
... The protein FMRFamide (Phe Met Arg Phe NH 2 ) is a tetrapeptide neurotransmitter, a member of the family of RFamide peptides (FaRP), all sharing the same C terminal RFamide sequence. Originally identified in mollusc ganglia (Price and Greenberg 1977), the FaRP family of peptides have been found in every major metazoan phylum, from coelenterates to chordates (e.g., Schneider and Taghert 1988;Grimmelikhuijzen and Westfall 1995;Aarnisalo and Panula 1995;Halton et al. 1999). FMRFamide was demonstrated to be cardioexcitatory in molluscs and mammals and strongly myoactive in helminths (Geary et al. 1999). ...
... (Stedman 1995) Neuron -multifunctional (combined characteristics of sensory, motor, inter-and neurosecretory neurons), morphological and functional unit of the nerve net consisting of the nerve cell body and processes, axon and dendrite(s) (sometimes these processes are referred to as neurites), communicating through electrical conduction or secretion of neuropeptides. (Fautin and Mariscal 1991;Grimmelikhuijzen and Westfall 1995) Nodule -a small node, a circumscribed mass of tissue, or knob-like or knotty swelling of tissue. (Stedman 1995) Nonseptate -not divided internally by partitions. ...
... A chemo-and/or mechanoreceptive function has been claimed for Hydra sensory neurons (Kass-Simon & Hufnagel, 1991). However, sensory neurons of Cnidaria, as well as their ganglion neurons, may be multifunctional sensory-inter-moto-neurosecretory neurons presenting from one to four such functions (Westfall, 1973;Westfall & Kinnamon, 1978;Grimmelikhuijzen & Westfall, 1995). A photoreceptive function of a ciliated sensory neuron conforms with the ciliary line of photoreceptors (Eakin, 1972), although recent studies on the identification of the gene Pax6 (a morphogenetic master gene discovered in the visual pigment of all animal phyla) in jellyfish and Hydra (Sun et al., 1997) could support the theory of the monophyletic origin of the various types of visual structures and suggest strong analogies between the elementary molecular mechanisms of different phototransductive pathways (Gehring, 2002). ...
Hydra's response to a light pulse is a phase shift of the state of bioelectric activity correlated with the periodic shortening-elongation behaviour. The direction and absolute value of a phase shift depend on intensity, direction, application phase (along the periodic activity state). and wavelength of the light pulse. Repetitive pulses entrain the behavioural cycle. The period of the behavioural cycle depends on intensity and wavelength of steady background illumination; however. the light effect is not exerted isotropically along all the phases of the behavioural cycle. Inferences are drawn on light influence on the behaviour pacemaking mechanism. By using polyclonal antibodies against squid rhodopsin, an opsin-like protein has, presumably in sensory cells.
... It appears that Hydra, an animal with one of the most primitive nervous systems, extensively uses peptides as information-transmitting substances. The Hydra nervous system has been visualized by a number of antibodies to biologically active peptides (Grimmelikhuijzen and Westafall, 1995). Takahashi et al. (Takahashi et al., 1997) reported abundant peptides from Hydra tissues that were, potentially, biologically active. ...
Tentacle ball formation (TBF) in Hydra elicited by S-methylglutathione (GSM) was modulated by a number of biologically active peptides. Hydra fed on Artemia, which had been hatched in a common salt solution supplemented with LiCl and ZnCl2, easily induced TBF in response to GSM after pretreatment with trypsin. After Hydra were treated with 100 pg/ml trypsin for 10 min, the response to GSM (TBF) was sensitively suppressed by acidic fibroblast growth factor and other biologically active peptides for >10 h. Various peptides, but not transforming growth factor beta (TGF-β), suppressed GSM-induced TBF in a specific pattern for each peptide. However, TGF-β was unique in that it did not suppress the response to GSM, but nullified the suppressive effect of other peptides. Only active TGF-β nullified the suppressive effect of the peptides, and the latent form of TGF-β neither suppressed GSM-induced TBF nor nullified the suppressive effect of other peptides. Members of the TGF-β family suppressed GSM-induced TBF. These results indicate that all peptides examined, except for TGF-β, suppressed the response to GSM in a manner specific to each peptide. This assay system would be useful in identification of biologically active peptides.
... Of relevance to SCZ and ASD, as well as other neurological disorders, is evidence that the core components of the nervous system and immune system are conserved in vertebrates. Indeed, the core components of the synapse are found in cnidarians, which form primitive nerve networks, and evolved around 680 million years ago (Galliot et al., 2009;Grimmelikhuijzen & Westfall, 1995). Furthermore, many synaptic genes are found in sponges, the oldest-surviving metazoan phyletic lineage, which actually lack synapses (Kosik et al., 2008;Srivastava et al., 2010). ...
... Hydra has few visible tissue boundaries, including one between the tentacle zone and the tentacles, one between the peduncle and the basal disc and one that separates the bud from the parent animal at the end of the budding process, which is Hydras way of asexual reproduction (Böttger and Hassel, 2012). The diffuse cnidarian nervous system with multifunctional neurons is arranged in a net like structure with some condensed areas and probably represents an ancestral form in nervous system evolution (Grimmelikhuijzen and Westfall, 1995). Many major developmental signalling pathways, including Notch (Käsbauer et al., 2007) and BMP (Reinhardt et al., 2004) have been identified in Hydra indicating an early origin of metazoan signalling modules. ...
Eph receptors and ephrins are important players in axon guidance, cell sorting and boundary formation. Both the receptors and the ligands are integrated transmembrane proteins and signalling is bidirectional. The prevalent outcome of signal transduction is repulsion of adjacent cells or cell populations. Eph/ephrins have been identified in all multicellular animals from human to sponge, their functions however appear to have been altered during evolution. Here we have identified four Eph receptors and three class B ligands in the cnidarian Hydra vulgaris, indicating that those are the evolutionary older ones. In situ hybridisation experiments revealed a striking complementarity of expression of receptors and ligands in tentacles and in developing buds. This suggests that the original function of ephrin signalling may have been in epithelial cell adhesion and the formation of tissue boundaries.
... At the functional level, synaptic transmission in cnidarians relies on fast neurotransmitters (glutamate, GABA, glycine) and slow neurotransmitters (catecholamine, serotonin, neuropeptides) for sensory-signal conduction [133]. At the ultrastructural level, many cnidarian neurons have multifunctional traits of sensory, neurosecretory and stimulatory attributes [134]. Significantly, the genome of A. digitifera encodes the expression of a ciliary neurotrophic factor, which is a polypeptide hormone and nerve growth factor that promotes neurotransmitter synthesis, neurite outgrowth and regeneration [135]. ...
Background
Contemporary coral reef research has firmly established that a genomic approach is urgently needed to better understand the effects of anthropogenic environmental stress and global climate change on coral holobiont interactions. Here we present KEGG orthology-based annotation of the complete genome sequence of the scleractinian coral Acropora digitifera and provide the first comprehensive view of the genome of a reef-building coral by applying advanced bioinformatics.
Description
Sequences from the KEGG database of protein function were used to construct hidden Markov models. These models were used to search the predicted proteome of A. digitifera to establish complete genomic annotation. The annotated dataset is published in ZoophyteBase, an open access format with different options for searching the data. A particularly useful feature is the ability to use a Google-like search engine that links query words to protein attributes. We present features of the annotation that underpin the molecular structure of key processes of coral physiology that include (1) regulatory proteins of symbiosis, (2) planula and early developmental proteins, (3) neural messengers, receptors and sensory proteins, (4) calcification and Ca ²⁺ -signalling proteins, (5) plant-derived proteins, (6) proteins of nitrogen metabolism, (7) DNA repair proteins, (8) stress response proteins, (9) antioxidant and redox-protective proteins, (10) proteins of cellular apoptosis, (11) microbial symbioses and pathogenicity proteins, (12) proteins of viral pathogenicity, (13) toxins and venom, (14) proteins of the chemical defensome and (15) coral epigenetics.
Conclusions
We advocate that providing annotation in an open-access searchable database available to the public domain will give an unprecedented foundation to interrogate the fundamental molecular structure and interactions of coral symbiosis and allow critical questions to be addressed at the genomic level based on combined aspects of evolutionary, developmental, metabolic, and environmental perspectives.
... 3 Most of the neurophysiological features of more "advanced" metazoan nervous systems are actually present at the cnidarian grade, including multifunctional neurons, action potentials, synapses, and chemical neurotransmission. For Grimmelikhuijzen and Westfall (1995) the existence of such features shows cnidarians to be "near the main line" of evolution, and suggests that the study of their nervous systems will illuminate some of the properties of nervous systems ancestral to the higher metazoans. ...
This review considers some important landmarks in animal evolution, asking to what extent specialized action-selection mechanisms play a role in the functional architecture of different nervous system plans, and looking for “forced moves” or “good tricks” (see Dennett, D., 1995, Darwin’s Dangerous Idea, Penguin Books, London) that could possibly transfer to the design of robot control systems. A key conclusion is that while cnidarians (e.g. jellyfish) appear to have discovered some good tricks for the design of behavior-based control systems—largely lacking specialized selection mechanisms—the emergence of bilaterians may have forced the evolution of a central ganglion, or “archaic brain”, whose main function is to resolve conflicts between peripheral systems. Whilst vertebrates have many interesting selection substrates it is likely that here too the evolution of centralized structures such as the medial reticular formation and the basal ganglia may have been a forced move because of the need to limit connection costs as brains increased in size.
... A vital staining procedure with reduced Methylene Blue by Unna's method (Pantin, 1948) can be used alternatively to silver staining experiments on whole mounts and reveals both the nerve net and sensory system (Batham et al., 1960). The use of antisera against RFamides has demonstrated that these neuropeptides are widely found in the phylum Cnidaria (Grimmelikhuijzen and Westfall, 1995;Grimmelikhuijzen et al., 1996). RFamides are produced by a plexus of neurons and thus, antisera against RFamide can be used in addition to silver or vital stainings. ...
The phylum Cnidaria is the most primitive phylum with a well-developed nervous system. Planula larvae and polyps display a diffuse nerve net (plexus), which is densest in the polyp hypostome. In contrast, the nervous system of the medusa is more complexly structured and reflects the anatomical needs of a well differentiated non-sessile animal. We analyzed the nervous system of two life stages of the hydrozoan Podocoryne carnea. Nerve nets of both polyps and developing medusae were examined in whole mounts and gelatin sections by using antibodies and vital staining with reduced Methylene Blue. In the polyp, both RFamide positive nerve cells and tyrosine-tubulin containing nerve cells form an ectodermal plexus. However, apical neuronal concentration is stressed by a particular nerve ring formed by tyrosine-tubulin positive nerve cells in the hypostome above the tentacle zone. This apical nerve ring is not detected with antisera against RFamide. In developing medusa buds, the earliest detected RFamide positive nerve cells occur at stage 4 at the location of the prospective ring canal. The nerve net of the developing medusa is fully differentiated at bud stage 8. Similar results were obtained with the anti tyrosine-tubulin antibody. Strikingly, two different nerve nets were discovered which connect the medusa bud with the plexus of the gonozoid, suggesting neuronal control by the polyp during medusa bud development. Vital staining with reduced Methylene Blue (Unna's) identified not only nerve cells at the ring canal but also bipolar cells within the radial canal. These cells may fulfill sensory functions.
... The basic plan of the cnidarian nervous system is a diffuse network of nerve cells, but in some members of this group, such as medusae, nerve cells also can aggregate in nerve plexuses, nerve rings, or sense organs. Sea anemones have complex neuronal nets and nerve plexuses in both the inner and outer epithelial layers (Grimmelikhuijzen and Westfall, 1995). From sea anemones, a variety of neuropeptides, including the closely related Antho-RWamide I (<Glu-Ser-Leu-Arg-Trp-NH2) and Antho-RWamide II (<Glu-Gly-Leu-Arg-Trp-NH2) have been isolated (Graff and Grimmelikhuijzen, 1988a, b;Grimmelikhuijzen et al., 1992). ...
Light microscopic studies have shown that the sea anemone neuropeptides Antho-RWamides I $( and II $( are located in neurons associated with the oral sphincter muscle of the sea anemone Calliactis parasitica. In the present ultrastructural study, using the immunogold technique, we found Antho-RWamide-like material in the granular vesicles of neurons that make synaptic contacts with the myonemes of both gastrodermal and oral sphincter muscle cells of Calliactis. Gastrodermal nerve cells contained immunoreactive granular vesicles averaging 149.3 ± 4.1 nm in diameter; smaller granular vesicles (47.5 ± 2.5 nm) were present at a labelled synapse. Neurites associated with the sphincter muscle had immunoreactive granular vesicles averaging 78.8 ± 3.3 nm in diameter with smaller granular vesicles (63 ± 4.4 nm) at three labelled neuromuscular synapses. All Antho-RWamide-immunoreactive vesicles were irregularly granular, unlike the typical dense-cored vesicles observed at some other synapses in sea anemones. No evidence was found of storage or release at nonsynaptic sites (paracrine secretion). The Antho-RWamide immunoreactive neurites innervate the sphincter muscle fibers directly rather than through intermediate neuronal pathways. This is the first ultrastructural evidence of a neuropeptide at a coelenterate neuromuscular synapse.
Background
As the sister group to all Bilateria, representatives of the phylum Cnidaria (sea anemones, corals, jellyfishes, and hydroids) possess a recognizable and well-developed nervous system and have attracted considerable attention over the years from neurobiologists and evo-devo researchers. Despite a long history of nervous system investigation in Cnidaria, most studies have been performed on unitary organisms. However, the majority of cnidarians are colonial (modular) organisms with unique and specific features of development and function. Nevertheless, data on the nervous system in colonial cnidarians are scarce. Within hydrozoans (Hydrozoa and Cnidaria), a structurally "simple" nervous system has been described for Hydra and zooids of several colonial species. A more complex organization of the nervous system, closely related to the animals' motile mode of life, has been shown for the medusa stage and a few siphonophores. Direct evidence of a colonial nervous system interconnecting zooids of a hydrozoan colony has been obtained only for two species, while it has been stated that in other studied species, the coenosarc lacks nerves.
Methods
In the present study, the presence of a nervous system in the coenosarc of three species of colonial hydroids - the athecate Clava multicornis , and thecate Dynamena pumila and Obelia longissima - was studied based on immunocytochemical and ultrastructural investigations.
Results
Confocal scanning laser microscopy revealed a loose system composed of delicate, mostly bipolar, neurons visualized using a combination of anti-tyrosinated and anti-acetylated a-tubulin antibodies, as well as anti-RF-amide antibodies. Only ganglion nerve cells were observed. The neurites were found in the growing stolon tips close to the tip apex. Ultrastructural data confirmed the presence of neurons in the coenosarc epidermis of all the studied species. In the coenosarc, the neurons and their processes were found to settle on the mesoglea, and the muscle processes were found to overlay the nerve cells. Some of the neurites were found to run within the mesoglea.
Discussion
Based on the findings, the possible role of the colonial nervous system in sessile hydroids is discussed.
Invertebrates have proven to be extremely useful models for gaining insights into the neural and molecular mechanisms of sensory processing, motor control, and higher functions, such as feeding behavior, learning and memory, navigation, and social behavior. Their enormous contribution to neuroscience is due, in part, to the relative simplicity of invertebrate nervous systems and, in part, to the large cells found in some invertebrates, like mollusks. Because of the organizms’ cell size, individual neurons can be surgically removed and assayed for expression of membrane channels, levels of second messengers, protein phosphorylation, and RNA and protein synthesis. Moreover, peptides and nucleotides can be injected into individual neurons. Other invertebrate systems such as Drosophila and Caenorhabditis elegans are ideal models for genetic approaches to the exploration of neuronal function and the neuronal bases of behavior. The Oxford Handbook of Invertebrate Neurobiology reviews neurobiological phenomena, including motor pattern generation, mechanisms of synaptic transmission, and learning and memory, as well as circadian rhythms, development, regeneration, and reproduction. Species-specific behaviors are covered in chapters on the control of swimming in annelids, crustacea, and mollusks; locomotion in hexapods; and camouflage in cephalopods. A unique feature of the handbook is the coverage of social behavior and intentionality in invertebrates. These developments are contextualized in a chapter summarizing past contributions of invertebrate research as well as areas for future studies that will continue to advance the field.
Brachyury, a member of T-box gene family, is widely known for its major role in mesoderm specification in bilaterians. It is also present in non-bilaterian metazoans, such as cnidarians, where it acts as a component of an axial patterning system. In this study, we present a phylogenetic analysis of Brachyury genes within phylum Cnidaria, investigate differential expression and address a functional framework of Brachyury paralogs in hydrozoan Dynamena pumila. Our analysis indicates two duplication events of Brachyury within the cnidarian lineage. The first duplication likely appeared in the medusozoan ancestor, resulting in two copies in medusozoans, while the second duplication arose in the hydrozoan ancestor, resulting in three copies in hydrozoans. Brachyury1 and 2 display a conservative expression pattern marking the oral pole of the body axis in D. pumila. On the contrary, Brachyury3 expression was detected in scattered presumably nerve cells of the D. pumila larva. Pharmacological modulations indicated that Brachyury3 is not under regulation of cWnt signaling in contrast to the other two Brachyury genes. Divergence in expression patterns and regulation suggest neofunctionalization of Brachyury3 in hydrozoans.
How are animals equipped with only diffusely distributed nerve nets able to generate complex behaviors? A new study in Hydra vulgaris proposes that a peptide-based 'wireless connectome' works in concert with two familiar network-coordinating mechanisms to generate the animal's remarkable somersault behavior.
The development of aquaculture is closely related to human life. Crustaceans play essential roles in aquaculture. However, the cultivation of crustaceans has been plagued by various diseases, causing substantial economic losses. At present, the occurrence of shrimp diseases is directly related to environmental stress and physiological dysfunction, and especially environmental stress has become an important factor inducing diseases. Therefore, how to regulate the immune defence of crustaceans under environmental stress has become the primary problem in the prevention and control of shrimp diseases. The neuroendocrine‐immune system plays a vital role in maintaining homeostasis and enhancing environmental adaptability. Besides, in the evolutionary sense, with the theory that the nervous system and the immune system have a common origin put forward, the research on the neuroendocrine‐immune system of invertebrates has gradually become a hot spot. This review describes the evolution and communication mechanism of the neuroendocrine system and immune system in vertebrates and invertebrates, summarizes the characteristics of the neuroendocrine system and immune system in crustaceans, and focuses on how environmental stress affects the regulation of the neuroendocrine system on the immune system in crustaceans, as well as the influence of these regulations on host immune defence. This review also introduced the immune regulation of crustacean intestines under environmental stress and the effect of pathogen stress on crustacean immunity, hoping to lay a theoretical foundation for the follow‐up study of crustacean stress resistance and provide theoretical guidance for the healthy aquaculture of crustaceans.
Feelings are conscious mental events that represent body states as they undergo homeostatic regulation. Feelings depend on the interoceptive nervous system (INS), a collection of peripheral and central pathways, nuclei and cortical regions which continuously sense chemical and anatomical changes in the organism. How such humoral and neural signals come to generate conscious mental states has been a major scientific question. The answer proposed here invokes (1) several distinctive and poorly known physiological features of the INS; and (2) a unique interaction between the body (the ‘object’ of interoception) and the central nervous system (which generates the 'subject' of interoception). The atypical traits of the INS and the direct interactions between neural and non‐neural physiological compartments of the organism, neither of which is present in exteroceptive systems, plausibly explain the qualitative and subjective aspects of feelings, thus accounting for their conscious nature.
The phylum Cnidaria contains an estimated 11 000+ living species and is almost exclusively an aquatic group, primarily marine but with some freshwater species and a few terrestrial parasitic species. Cnidarians reproduce both sexually and asexually, most species incorporating both methods. They produce gametes (eggs and sperm), can be monoecious, producing both eggs and sperm, or dioecious, with individuals of separate sexes (gonochoric). Cnidarians are soft‐bodied, diploblastic (two cell layers) metazoans (animal kingdom), with primary radial symmetry (although with some variations). Threats to cnidarians include climate change, infectious diseases, development of shorelines, increase in surface runoff, land‐based sources of pollution, ship groundings, and damaging fishing techniques (dynamite, nets, etc.). As a possible indicator of some of these detrimental changes, there has been an increase in the frequency of jelly blooms.
Sleep is almost universally present throughout the animal kingdom, yet how sleep has evolved is not known. From an evolutionary aspect, a majority of animals that have been studied so far exhibit sleep or sleep-like states. Studies on sleep in different species belonging to vertebrate and invertebrate categories have provided novel insights into the evolution of sleep. The presence of a sleep-like state in jellyfish, a simple diploblastic animal having a noncephalized brain and no centralized nervous system, suggests that sleep/sleep-like state might have evolved even before brain cephalization occurred. The sleep-like state has been reported through observational studies in several species such as coelenterates, nematodes (roundworm), annelids, arthropods and mollusks, fishes, amphibians, reptiles, birds, and mammals. Electrophysiological correlates have also been recorded in a few species. The changes in low-frequency waves and spike-like activity in the EEG during the sleep-like state have been reported in crayfish, Drosophila, octopuses, some frogs, green iguanas, and box turtles. The activity of some EEG electrical waves changes from active to sleep-like state in all these organisms. This electrical activity could be the electrophysiological signature of the sleep-like state in nonmammalian and nonavian species. It is possible that such electrophysiological correlates evolved phylogenetically in mammalian and avian sleep. In this chapter, we have attempted to provide a comprehensive overview of the current knowledge about the evolution and presence of a standard signature of sleep across different species.
In Hydra the nervous system is composed of neurons and mechano‐sensory cells that differentiate from interstitial stem cells, which also provide gland cells and germ cells. The adult nervous system is actively maintained through continuous de novo neurogenesis that occurs at two distinct paces, slow in intact animals and fast in regenerating ones. Surprisingly Hydra vulgaris survive the elimination of cycling interstitial cells and the subsequent loss of neurogenesis if force‐fed. By contrast, H. oligactis animals exposed to cold temperature undergo gametogenesis and a concomitant progressive loss of neurogenesis. In the cold‐sensitive strain Ho_CS, this loss irreversibly leads to aging and animal death. Within four weeks, Ho_CS animals lose their contractility, feeding response and reaction to light. Meanwhile, two positive regulators of neurogenesis, the homeoprotein prdl‐a and the neuropeptide Hym‐355, are no longer expressed, while the “old” RFamide‐expressing neurons persist. A comparative transcriptomic analysis performed in cold‐sensitive and cold‐resistant strains confirms the down‐regulation of classical neuronal markers during aging but also shows the up‐regulation of putative regulators of neurotransmission and neurogenesis such as AHR, FGFR, FoxJ3, Fral2, Jagged, Meis1, Notch, Otx1, TCF15. The switch of Fral2 expression from neurons to germ cells suggests that in aging animals, the neurogenic program active in interstitial stem cells is re‐routed to germ cells, preventing de novo neurogenesis and impacting animal survival.
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Neuropeptides play critical roles in cnidarian development. However, although they are known to play key roles in settlement and metamorphosis, their temporal and spatial developmental expression has not previously been characterized in any coral. We here describe Acropora millepora LWamide and RFamide and their developmental expression from the time of their first appearance, using in situ hybridization and FMRFamide immunohistochemistry. AmRFamide transcripts first appear in the ectoderm toward the oral end of the planula larva following blastopore closure. This oral bias becomes less apparent as the planula develops. The cell bodies of AmRFamide-expressing cells are centrally located in the ectoderm, with narrow projections to the mesoglea and to the cell surface. As the planula approaches settlement, AmRFamide expression disappears and is undetectable in the newly settled polyp. Expressing cells then gradually reappear as the polyp develops, becoming particularly abundant on the tentacles. AmLWamide transcripts first appear in ectodermal cells of the developing planula, with minimal expression at its two ends. The cell bodies of expressing cells lie just above the mesoglea, in a position distinct from those of AmRFamide-expressing cells, and have a narrow projection extending across the ectoderm to its surface. AmLWamide-expressing cells persist for most of the planula stage, disappearing shortly before settlement, but later than AmRFamide-expressing cells. As is the case with AmRFamide, expressing cells are absent from the polyp immediately after settlement, reappearing later on its oral side. AmLWamide expression lags that of AmRFamide in both its disappearance and reappearance. Antibodies to FMRFamide stain cells in a pattern similar to that of the transcripts, but also cells in areas where there is no expression revealed by in situ hybridization, most notably at the aboral end of the planula and in the adult polyp. Adult polyps have numerous staining cells on the tentacles and oral discs, as well as an immunoreactive nerve ring around the mouth. There are scattered staining cells in the coenosarc between polyps and staining cells are abundant in the mesenterial filaments. The above results are discussed in the context of our knowledge of the behavior of coral planulae at the time of their settlement and metamorphosis. Corals are facing multiple environmental threats, and these results both highlight the need for, and bring us a step closer to, a mechanistic understanding of a process that is critical to their survival.
In all bilaterian taxa, the nervous system is formed of two major components, the central nervous system (CNS) and the peripheral nervous system (PNS). Axons of sensory neurons projecting towards the CNS and axons of motor neurons going in the opposite direction form peripheral nerves. The basic architecture of the central nervous system falls into three main types: subepithelial, basiepithelial, and invaginated. The nervous system of invertebrate deuterostomes-echinoderms, hemichordates, cephalochordates, and urochordates-consists of a basiepithelial nerve plexus with local condensations that, in most cases, derive from invaginations of the neuroepithelium. Synapses are the points of contact between neurons, or between neurons and other target cells, such as muscle, gland, or sensory receptor. This chapter discusses some well-studied circuits in a number of different invertebrate systems. It presents some of the important concepts that structure our understanding of how groups of interconnected neurons control behavior.
Anatomy is the study of the structure of an organism. This chapter provides information on the basic gross and microscopic anatomy of the corals from the subclasses Hexacorallia (order Scleractinia) and Octocorallia (order Alcyonacea). Black corals (subclass Hexacorallia, order Antipatharia) are mentioned briefly. The chapter first presents the histology of the hexacorals, then of the octocorals, about which less is known. It also talks about Cnidarian anatomy, and Anthozoan corals. The phylum Cnidaria is distinguished from all others by a key epithelial cell type, the cnidocyte that produces a specialized organelle, the cnidocyst (or cnida) that contains a toxin and tubule that everts when triggered. Scleractinian corals can be imperforate with solid aragonite skeletons or perforate with tubes lined with tissue piercing the skeleton. Understanding is still lacking of the biochemistry and metabolic function of some anatomical structures, such as the red-pigmented portions of faviid mesenterial filaments or the calicoblasts.
The freshwater polyp, Hydra has long been used as a model for studying mechanisms controlling pattern formation and the axial polarity of the body column. It has been suggested that body patterning is dependent on the presence of internal morphogens. A variety of different candidate molecules, in Hydra and other cnidarians, have been proposed as putative morphogens. They are derived from different cellular sources and the evidence for their ability to act as morphogens is critically assessed. The actions of any morphogens will be mediated by signal transduction pathways and such pathways have been demonstrated in cnidarians. The phosphatidylinositol-protein kinase C system appears to be the major pathway involved in pattern formation. Cnidarians are able to respond to external cues which trigger key events in their life cycle, such as metamorphosis. Internal neurohormones, particularly members of the GLWamide family of neuropeptides, transmit the signals to all parts of the body and the signalling cascades involved in these events have been explored. Colonial forms possess the ability to recognise each other as self or non-self. Interactions between neighbouring colonies lead either to fusion or to the destruction of one of the colonies. The phenomenon, which may be linked to the competition for living space, is reminiscent of the allorecognition/allorejection responses observed in other invertebrates and vertebrates. The presence of these mechanisms in such simple organisms as cnidarians suggests that they arose early in evolution. To date, the signalling systems involved in the behaviours in cnidarians have not been studied in any depth.
The puzzle of how complex nervous systems emerged remains unsolved. Comparative studies of neurodevelopment in cnidarians and bilaterians suggest that this process began with distinct integration centres that evolved on opposite ends of an initial nerve net. The 'apical nervous system' controlled general body physiology, and the 'blastoporal nervous system' coordinated feeding movements and locomotion. We propose that expansion, integration and fusion of these centres gave rise to the bilaterian nerve cord and brain.
Gibst du bei Google ‚plant consciousness’ ein, findest du auf der ersten Trefferseite neben Wikipedia ungefähr drei Typen von Seiten: Esoterik-Seiten, wo dir erklärt wird, dass alles mit allem verbunden ist und Menschen nichts Besonderes, sondern nur ein Teil der großen und ganzen Oneness der Natur sind; Vegetarier-Seiten, wo sich energisch gegen die Idee der fühlenden Pflanze gewehrt wird; und wissenschaftsjournalistische Seiten, in denen auf nüchterne Art die faszinierenden neuen Erkenntnisse der zeitgenössischen Pflanzenforschung präsentiert werden. Die ersten philosophischen Beiträge findet man dagegen erst auf der zweiten und dritten Trefferseite. Sie behandeln die mit diesem Thema aufkommenden ethischen Probleme.
Einen dieser philosophischen Beiträge werde ich als Ausgangspunkt nehmen, dabei aber die ethische Fragestellung vollkommen ignorieren. Was mich stattdessen interessiert, ist, wie für und gegen Pflanzenbewusstsein argumentiert wird. Dafür werde ich die Kommentare der Leser dieses Artikels als roten Faden benutzen und mit eigenen Argumenten das Sprachspiel mitspielen, ohne dabei zu beanspruchen, mich der Wahrheit zu nähern. Im Gegenteil, ich werde versuchen deutlich zu machen, dass sprachliche Argumente an sich ein schlechtes Mittel sind, um die Frage nach dem Pflanzenbewusstsein zu entscheiden.
Den lebensfernen Sprachspielereien setze ich dann das lebensnahe Video entgegen. Das auf Film festgehaltene, mit eigenen Augen gesehene Verhalten der Pflanzen macht das sichtbar, was uns normalerweise verborgen bleibt und was sprachliche Argumente nicht übersetzen können. Durch visuelle Konfrontation erhaltene Evidenz ist viel wichtiger und überzeugender als sprachliche Argumente, wenn die Frage nach dem Pflanzenbewusstsein beantwortet werden soll. Um nicht nur gut zu argumentieren und sich gegenseitig auszusmarten, sondern um wirklich herauszufinden, wie sich Sachen verhalten, muss die Philosophie ihren Sprachfetisch überwinden und auf anderen Ebenen das Thema bearbeiten, im Fall des Bewusstseins auf der visuellen Ebene.
Genauso wie man sehen kann, dass Tiere Schmerzen empfinden und intentional handeln, kann man sehen, dass Pflanzen intentional handeln und ein Bewusstsein haben. Zeitgenössische Philosophie muss bei diesem genuin philosophischen Thema die neuen Möglichkeiten nutzen, die ihnen seit der Omnipräsenz des Internets zur Verfügung stehen. Jenes hat nicht nur das Spektrum an Möglichkeiten die Welt zu erklären erweitert, sondern auch verändert, wie Menschen die Welt erklärt haben wollen.
Bevor wir uns aber all diesen Themen widmen, müssen zuallererst zwei wichtige Fragen beantwortet werden: Die Frage, was mit Bewusstsein gemeint ist, denn dass jeder darunter etwas anderes versteht, führt bei dieser Geschichte immer zu ziemlich vielen Missverständnissen; und die Frage, wieso man überhaupt auf die Idee kommt, Pflanzen Bewusstsein zuzuschreiben.
This chapter discusses metamorphosis from the larval to the polyp state and pattern formation in larvae, polyps, and colonies. Metamorphosis and pattern formation in larvae and colonies have been studied by experiments with Hydractinia, and pattern formation in polyps, by experiments with Hydra. In Hydractinia, the larval state is stable until external cues trigger the onset of metamorphosis. Hydractinia, a rather typical hydrozoan with a simple structure, is the best studied. There exist female and male animals, which release sperm and oocytes, respectively, into the surrounding water. Following fertilization, cleavage takes place, leading to different cell types and an elongated body shape. Hydrozoa provides an access to some elementary processes, including regulation of cell proliferation and differentiation as well as interaction of and communication among cells. Hydractinia has several technical advantages. It is possible to get thousands of larvae several times a week throughout the year. Metamorphosis can be triggered deliberately. Substances applied to the surrounding seawater are easily taken up by the larvae and influence the development. Various approaches are used to study pattern formation in hydrozoa, including regeneration and transplantation techniques, methods of molecular biology, and biochemical methods. Models play the role of a link between the different fields and the different species.
Most animals that display a bilateral symmetry (bilaterians) share homologous regulatory genes involved in head development. Recently, homologues of several of these genes have been cloned from animals that are radially organized, such as coral, sea anemones, jellyfish or hydra (cnidarians). Surprisingly, some of these are expressed apically and/or during apical patterning in hydrozoans, suggesting that head patterning is much older than previously thought.
Hydra belongs to the class Hydrozoa in the phylum Cnidaria. Hydra is a model animal whose cellular and developmental data are the most abundant among cnidarians. Hence, I discuss the developmental neurobiology of hydra. The hydra nerve net is a mosaic of neural subsets expressing a specific neural phenotype. The developmental dynamics of the nerve cells are unique. Neurons are produced continuously by differentiation from interstitial multipotent stem cells. These neurons are continuously displaced outwards along with epithelial cells and are sloughed off at the extremities. However, the spatial distribution of each neural subset is maintained. Mechanisms related to these phenomena, i.e., the position-dependent changes in neural phenotypes, are proposed. Nerve-net formation in hydra can be examined in various experimental systems. The conditions of nerve-net formation vary among the systems, so we can clarify the control factors at the cellular level by comparing nerve-net formation in different systems. By large-scale screening of peptide signal molecules, peptide molecules related to nerve-cell differentiation have been identified. The LPW family, composed of four members sharing common N-terminal L(or I)PW, inhibits nerve-cell differentiation in hydra. In contrast, Hym355 (FPQSFLPRG-NH3) activates nerve differentiation in hydra. LPWs are epitheliopeptides, whereas Hym355 is a neuropeptide. In the hypostome of hydra, a unique neuronal structure, the nerve ring, is observed. This structure shows the nerve association of neurites. Exceptionally, the tissue containing the nerve ring shows no tissue displacement during the tissue flow that involves the whole body. The neurons in the nerve ring show little turnover, although nerve cells in all other regions turn over continuously. These associations and quiet dynamics lead me to think that the nerve ring has features similar to those of the central nervous system in higher animals.
Behavioural coordination in sea anemones involves both excitation and inhibition. Examples include reciprocal inhibition between longitudinal and circular muscles (Batham and Pantin, 1954), inhibitory actions during peristalsis (Ewer, 1960; McFarlane, 1974a), joint excitatory and inhibitory innervation of sphincter muscle (Lawn, 1976) and inhibition of neural pacemakers (McFarlane, 1974b).
Coelenterates are generally assumed to be primitive animals with simple nervous systems. We believe, however, that in at least one group, the sea anemones, the nervous system is in reality rather complex, a view supported by the growing number of neuropeptides recently extracted from these animals (Grim-melikhuijzen et al. 1990a,b). Three of these peptides (Antho-RFamide and the Antho-RWamides I and II) have demonstrable physiological actions on sea-anemone muscle preparations (McFarlane et al. 1987, 1990; McFarlane and Grimmelikhuijzen, 1991) and, in the case of the Antho-RWamides, on isolated muscle cells (McFarlane et al. 1991). Here we consider a fourth neuropeptide from sea anemones, Antho-RNamide (L-3-phenyllactyl-Leu-Arg-Asn-NH2) (Grim-melikhuijzen et al. 1990), and show that it has opposite actions on adjacent antagonistic muscles in sea anemones.
Antho-RFamide is a neuropeptide isolated from the sea anemone Anthopleura elegantissima. Antho- RFamide immunoreactivity was localized in four different populations of neuronal vesicles in the tentacle nerve plexus of Anthopleura. Small, opaque, neuronal vesicles, averaging 49 nm in diameter, were gold-labeled at two- way synapses. Heterogranular vesicles, averaging 184 nm in diameter, were gold-labeled in a neuronal swelling ad- jacent to a muscle cell process. These vesicles were similar in size to a third class of gold-labeled dense-cored vesicles. A fourth class of immunogold-labeled vesicles observed in neuronal swellings had light cores and averaged 129 nm in diameter. Using 5-nm gold particles, we observed a heavy labeling of the granular cores of the dense-cored vesicles, suggesting that the immunoreactivity is specific to the vesicle core. The ultrastructural demonstration of Antho-RFamide immunoreactivity in interneuronal syn- aptic vesicles, together with the immunofluorescence and electrophysiological studies of other investigators, suggest that Antho-RFamide plays a role in neurotransmission in sea anemones.
Neuropeptides containing the carboxyl-terminal sequence Arg-Phe-NH2 are found throughout the animal kingdom and are important substances mediating neuronal communication. Here, we have cloned the cDNA coding for the precursor protein of the sea anemone neuropeptide (Antho-RFamide) < Glu-Gly-Arg-Phe-NH2. This precursor is 334 amino acids in length and contains 19 copies of unprocessed Antho-RFamide (Gln-Gly-Arg-Phe-Gly), which are tandemly arranged in the C-terminal part of the protein. Paired basic residues (Lys-Arg) or single basic residues (Arg) occur at the C-terminal side of each Antho-RFamide sequence. These are likely signals for posttranslational cleavage. The processing signals at the N-terminal side of each Antho-RFamide sequence, however, include acidic residues. Processing at these amino acids must involve either an amino- or an endopeptidase that cleaves C-terminally of aspartic acid or glutamic acid residues. Such processing is, to our knowledge, hitherto unknown for peptidergic neurons. The Antho-RFamide precursor also contains two copies of the putative Antho-RFamide-related peptide Phe-Gln-Gly-Arg-Phe-NH2 and one copy of Tyr-Val-Pro-Gly-Arg-Tyr-NH2. In addition, the precursor protein harbors four other putative neuropeptides that are much less related to Antho-RFamide. This report shows that the biosynthetic machinery for neuropeptides in coelenterates, the lowest animal group having a nervous system, is already very efficient and similar to that of higher invertebrates, such as mollusks and insects, and vertebrates.
Neuropeptides are synthesized as large precursor proteins that undergo posttranslational cleavages and modifications to produce bioactive peptides. Here, we have cloned two closely related precursor proteins for the sea anemone neuropeptide Antho-RFamide (<Glu-Gly-Arg-Phe-NH2) from Anthopleura elegantissima. The first precursor (435 amino acids long) contains 13 copies of immature Antho-RFamide (Gln-Gly-Arg-Phe-Gly) and nine other, Antho-RFamide-related neuropeptide sequences that are in the C-terminal part of the protein. The second precursor (429 amino acid residues) harbors 14 copies of immature Antho-RFamide and eight other related peptide sequences. Each copy of Antho-RFamide or Antho-RFamide-related peptide is followed, at its C-terminal side, by a single Arg residue, which is an established signal for posttranslational cleavage. At the N terminus of each Antho-RFamide sequence, however, basic residues are lacking, and instead one or more acidic residues occur. These acidic residues are the cleavage sites for a new type of processing enzyme occurring in neurons. This enzyme could either be an amino- or endopeptidase hydrolyzing at the C-terminal side of Asp or Glu residues. The N-terminal regions of the two precursor proteins harbor eight copies of the putative neuropeptide sequence Pro-Gln-Phe-Trp-Lys-Gly-Arg-Phe-Ser and three additional, closely related sequences. The total number of all established and putative neuropeptides that may be cleaved from the precursors is 33. Thus, the Antho-RFamide precursors beong to the most complex peptide precursor proteins known so far.
This volume provides a particularly timely survey of invertebrate peptide hormones. Interest has been growing in invertebrate peptide hormones. This interest has focused upon two important and related aspects, both of which are fully covered in this volume. First, many of these peptides are neurohormones with chemical characteristics resembling, sometimes closely, established vertebrate neurohormonal peptides. In this way these findings have had considerable impact on our standing of the origin and evolution of peptide regulators. Second, with the availability of techniques such as HPLC and cDNA probes, which have allowed detailed study of vertebrate peptides, significant advances have been made in our understanding of the physiology and biochemistry of native invertebrate peptides. The volume aims to provide a synthesis between these two aspects of investigative activity. As such, it should have a broad appeal to scientists from a number of disciplines.
Invertebrate chemical synapses are characterized by a diversity of presynaptic vesicles and membrane-associated structures. They have in common with classical chemical synapses of vertebrates a pair of parallel densified membranes with a uniformly wide intercellular cleft containing intracleft material, a presynaptic aggregation of clear or dense-cored vesicles, and usually one or more mitochondria with nearby microtubules in the synaptic terminal or axonal varicosity. At these conventional synaptic foci some vesicles have thin filamentous connections to the presynaptic membrane. Invertebrate neuromuscular junctions often appear morphologically similar to interneuronal synapses because they lack the postsynaptic infoldings of vertebrate muscles. Electrical synapses presumably appear in all metazoa as morphologically identifiable gap junctions in which there is cytoplasmic continuity between two cells separated by a 2–3-nm-wide intercellular gap. In addition to these conventional synapses there are dyads, spine synapses, neuro-secretory-motor junctions, neuromuscular junctions with presynaptic dense bars, and gap junctions with vesicles in various invertebrate groups.
This chapter is divided into three parts. The first part is a general survey, describing briefly the discovery of some neuropeptides and what can be learned about their occurrence, biosynthesis, action etc. The first part will mainly deal with vertebrate neuropeptides. The second part gives an up-to-date list and a discussion of established invertebrate neuropeptides. The last part deals with the structure, localisation and action of neuropeptides in cnidarians.
Although giant axons were described in a cnidarian a hundred years ago (Korotneff, 1884), this discovery was lost through an historical accident and it is only in recent years that escape responses mediated by outsize or giant axons comparable to those found in polychaetes, squid, and others have again come to light. The best examples are from the Class Hydrozoa, specifically one particular trachyline jellyfish, Aglantha digitale (Figure 1A), and a number of siphonophores of which the best known is Nanomia cara. The greater part of this chapter will deal with these examples. Table I presents the taxonomic relationships of these animals. Giant axons are known in the Class Scyphozoa, but they coordinate normal locomotion, not escape per se, and the same is true in a number of hydromedusae (see reviews by Passano, 1982; Spencer and Schwab, 1982). These cases will not be covered here.
In the sea anemone Calliactis parasitica endodermal application of the anthozoan neuropeptide Antho-RFamide (<Glu-Gly-Arg-Phe-amide), at a concentration of 10−6 or 10−7moll−1, caused a long-lasting increase in tone, contraction frequency and contraction amplitude in several slow muscle groups but had no effect on contractions in fast muscles. The effects were investigated further in isolated muscle preparations. Ectodermal application to whole animals had no effect on muscle contractions. Both ectodermal and endodermal application, at 10−7moll−1, raised electrical activity in an ectodermal conduction system, the SSI, but had no effect on an endodermal conduction system, the SS2. Electrical activity in the SS2 was increased by application at 10−6moll−1 to the endoderm but not to the ectoderm. The peptide had no effect on the through-conducting nerve net. It is concluded that contractions evoked by Antho-RFamide may be partly due to neuronal activity, but probably also involve direct excitation of the muscles. The diverse excitatory actions of Antho-RFamide suggest that it may be a neurotransmitter or neuromodulator in sea anemones.
The rapid co-ordination of the beat had been identified with a specialized system of neurones which spreads over the muscles from the marginal ganglia. By analogy with other invertebrates this has been called the giant fibre system.
The feeding response and the spasm are co-ordinated by a separate net, called ‘the diffuse nerve-net’, which is both physiologically and histologically distinct from the giant fibre system.
The histological structures of the two nerve-nets and the marginal ganglia are described.
Although the diffuse nerve-net contains sensory cells, it also acts upon the muscles, and a double motor innervation is indicated.
The two nerve-nets meet and interact at the marginal ganglia. An attempt has been made to interpret the neurone structure of the ganglia.
Parker's theory of the origin of the nervous system is discussed along with later interpretations. Attention today has shifted from the cellular to the molecular level, and it has become clear that many of the molecules and mechanisms thought of as typically neuronal have homologs or counterparts in non-nervous cells and unicellular organisms. This applies to signalling chemicals, receptors, second messenger systems and ion channels, and also to the production of electrical events. Parker's view of sponges as a group lacking nerves but possessing independent effectors is still acceptable, but some sponges (and also higher animals) employ non-nervous signalling pathways to coordinate their effectors. Thus, nerves are not always necessary for coordinated behavior. Cnidarians like hydra have seemingly simple, two-dimensional nervous systems with little or no centralization, but even such systems can be surprisingly complex, and the more advanced cnidarians show neurophysiological specializations as sophisticated as those of many higher invertebrates. Examples of ingenious cnidarian solutions to behavioral problems are given. No existing animals have 'elementary' nervous systems if that term implies the existence of crude or inefficient functional adaptations.
An antiserum to the sequence Arg-Phe-amide (RFamide) was used to stain the nervous systems of various physonectid siphonophores. In the stem of Nanomia bijuga, this antiserum stained an ectodermal nerve net, which was interrupted, at regular intervals, by transverse collars of neurons. Injection of Lucifer yellow into the “giant axon” of the stem showed that this axon was dye-coupled to an ectodermal nerve net that resembled the RFamide-positive network. Ectodermal nets of neurons were also found in the pneumatophore, gastrozooids, tentacles and tentilla. At the junctions of the pneumatophore, the gastrozooids, the dactylozooids and the gonozooids with the stem, and at the junctions of tentacles and tentilla, collars or rings of neurons occurred. The stem was connected to the phyllozooids and nectophores by muscular lamellae, which were bordered by chains of neurons. At the margin of the nectophores, an immunoreactive nerve ring was found. Connected to this ring and located in the“seitliche Zapfen” (“sidely-located patche”), were two agglomerations of nerve cells. On the upper side of the bell margin, positioned at 90° relative to the “seitliche Zapfen”, a delta-shaped neuronal structure was found. This structure was connected to the nerve ring and was associated with a muscle, which ran a short distance along the exumbrellar surface.
The nervous systems of Agalma elegans, Forskalia edwardsi, Forskalia leuckarti and Halistemma rubrum resembled that of Nanomia bijuga in all major respects.
Coelenterates have the simplest nervous system in the animal kingdom, and it was probably within this group of animals that nervous systems first evolved. Present day coelenterates are diverse and comprise two phyla. The classes Hydrozoa (for example Hydra), Cubozoa (“box jellyfishes”), Scyphozoa (“true jellyfishes”) and Anthozoa (for example sea anemones and corals) constitute the phylum Cnidaria. A companion phylum is the Ctenophora (“combjellies”) or Acnidaria. Most Hydrozoa, Cubozoa and Scyphozoa have a life cycle including a polyp (sessile) and medusa (mobile) form. In Anthozoa, the medusa is lacking, whereas in Ctenophora no polyps occur. Coelenterates can either live individually or in colonies. Many Hydrozoa and Anthozoa form colonies of polyps (e.g. corals), but also mixed colonies of polyps and medusae exist. The physonectid siphonophores, for example, are swimming hydrozoans consisting of a long stem to which numerous medusae and various forms of polyps are attached.
The numbers, types, and distributions of neurons in a hypostome of Hydra littoralis were determined from electron micrographs of serial (0.25 μm thick) sections. In 1,080 serial sections examined we found 75 sensory cells and 949 centrally located ganglion cells. More than 96% of the 1,024 neurons identified had a single cilium.
Sensory cells were most numerous near the apex of the hypostome. Proceeding away from the apex, they steadily decreased in numbers; at 120 μm they were no longer observed. Ganglion cells were bimodally distributed; some were associated with sensory cells at the apex, but most were found at the sites of tentacle origin.
We observed, throughout the hypostome, a total of 64 neuronal clusters (three or more contiguous neurons), with an average of five and a maximum of 11 neurons in a cluster. Clusters were distributed similarly to ganglion cells: an initial concentration of clusters near the apex; the majority at the hypostometentacle junctions.
Each neuron identified was traced through succeeding sections in which it was observed. We used a three coordinate system to create a three-dimensional reconstruction of the neuronal locations in the hypostome. Although the functional significance of the neuronal distributions we observed is unknown, we suggest that neurons at the apex of the hypostome transduce sensory information involved in feeding behavior. The neuronal concentrations at sites of tentacle origin may be responsible for initiating Contraction Burst Pulses associated with rhythmic behavioral patterns of Hydra or coordinating tentacle movements involved in prey capture, ingestion or locomotion.
Speculations about evolution have an irresistable fascination, partly because they are hard to prove wrong, partly because they make us think about origins and look for clues in development. Cnidarians may have been the first metazoans to evolve, although this seems rather unlikely in view of the fact that they are all carnivores, preying on other metazoans. Given, however, that of the surviving phyla they alone retain the presumably ancestral (diploblastic) body plan, it becomes especially interesting to look at their nervous systems for clues as to how nerves evolved. This is not just a selling point for use in grant applications. There really is no better group in which to look for clues. At the same time, given the 700 million years which have elapsed since cnidarians appeared in the fossil record, any clues about nervous origins can only be of the most general nature. Cnidarian nerves are not obviously primitive in functional terms, only in the way they are laid out as center-less nets. Of course the layout can be far more complex than this. Some of them do have centres. At the last count, the jellyfish Aglantha had six, possibly seven, physiologically distinct neuronal subsets running in the marginal nerve rings. My efforts to understand their interactions have aged me prematurely. Other speakers at this meeting have looked at ionic channels, morphogenetic factors and neurochemicals for clues. Molecular biology may (probably will) eventually help us reconstruct phylogeny, and place the cnidarians in their proper place on the many-branched tree of neural evolution, but we are still far from this point. As of now, we do not know in what precursor cell line neurons first arose, through what stages they passed to assume their now familiar form, or what functions they originally served.
1.Swimming in the cubomedusaCarybdea rastonii is controlled by a subumbrellar nerve net. Neurons that make up this net, including “giant” neurons, make random synaptic contacts with each other and with the circular subumbrellar swimming muscles (Figs. 1–3).2.Extracellularly recorded swimming impulses originate in the rhopalia and spread throughout the subumbrellar nerve net, initiating contractions of the subumbrellar musculature (Fig. 4).3.Intracellular recordings from the subumbrellar giant neurons indicate that all-or-none overshooting action potentials precede each swimming contraction (Fig. 6). Synaptic depolarizations were occasionally recorded alone, and triggering an action potential (Fig. 6C, E).4.Extracellularly recorded muscle potentials exhibit frequency-dependent facilitation, with normal swimming at about 80% of the maximal contraction of the muscle sheet (Figs. 4 and 5).5.Intracellular recordings from subumbrellar muscle cells reveal graded depolarizations with each contraction of the muscle sheet. The jagged potentials are initially small and increase in amplitude with the first few contractions in a series (Fig. 7). The increases in muscle cell depolarizations may be related to the facilitation in the size of extracellularly recorded muscle potentials.6.Pacemakers of the four rhopalia interact by a dominance hierarchy; the rhopalium with the highest firing frequency controls swimming. Dominance shifts have been observed in two-rhopalia preparations (Fig. 8 A), and facilitation of the musculature often occurs independently of rhopalial sequence (Fig. 8B).7.The swimming system ofCarybdea is comparable to the “giant fiber nerve net (GFNN)” of other scyphomedusae.
Examination of sections of the marginal ganglion of the jellyfish Cyanea and the hydromedusan Phialidium by the electron microscope, in a region where nervous tissue is readily identified on account of its abundance, reveals the following features.
Nerve-cell bodies and axons are crowded together without special glial cells. The axons form a layer between the cell-bodies and the mesogloea and the spaces between them are continuous with other intercellular spaces and with the mesogloea.
Features typical of nerve-cells in other animals are mitochondria, Golgi region (= γ-cytomembranes), neurotubules (= canaliculi) about 16 mµ wide, and several types of vesicle ranging in size from 50 to 200 mµ, including synaptic vesicles of 50 to 100 mµ.
Features not typical of nerve-cells are the modified (possibly sensory) cilia on the dendrites of bipolar cells and the absence of clumps of Nissl substance and neurofilaments.
Synapses between axons (or with a perikaryon) have a synaptic cleft of 18 to 22 mµ and a crowded row of synaptic vesicles within the neurones on each side of the synapse.
With two plates (figs. 3 and 4) SUMMARY The nerve fibres of Aurellia aurita were originally described by E. A. Shafer from gold preparations. This work has been repeated with Holmes's method of silver staining on the slide. Results have beert obtained substantially in agreement with those of Shafer. The same nerves can be seen in the living animal when a phase-contrast micro-scope or oblique illumination is used. By the use of these techniques, a study has been made of the plan of the nerve net over the subumbrellar surface of the bell. This work is intended as an anatomical foundation for physiological studies to be described elsewhere.
SUMMARY 1. Two discrete networks of neurones in the outer nerve-ring of Poly- orchis penicillatus can be identified by their physiological and morphologi- cal characteristics. 2. The 'B' system is characterized by the regular, spontaneous firing pattern that can be recorded intracellularly. Bursts of up to six spikes are produced in response to a rapid reduction in the light intensity. 3. Neurones of the 'B' system are electrically coupled to one another. 4. Action potentials in the 'B' system produce unitary EPSPs in swim- ming motor neurones and in epithelial cells overlying the outer nerve-ring. 5. Lucifer Yellow injected into a 'B' neurone diffuses rapidly through neighbouring neurones to reveal a condensed network of neurones in the centre of the nerve-ring and a more diffuse network passing up and around each tentacle. 6. The 'O' system is characterized by very regular (approx. 1 Hz), spontaneous membrane potential oscillations. Action potentials are never recorded. 7. Neurones of the 'O' system are electrically coupled to one another. 8. There is evidence of interaction between the 'O' system and swimming motor neurones. 9. Lucifer Yellow injected into an 'O' neurone diffuses through member neurones to show an anastomosing network of neurones extending across the width of the outer nerve-ring and tracts of neurones extending up the sides of each tentacle towards the ocelli. 10. The restriction of injected Lucifer Yellow to each of the networks and the blockade of interaction between systems by Mg2"1" anaesthesia are evidence that signalling between different central networks is by chemical means. 11. The adaptive advantages of this type of functional organization of central neurones in radially symmetrical animals are discussed. Such an organization is compared with that found in bilateral animals.
Suction electrodes attached to tentacles of the sea anemone Calliactis parasitica record regular bursts of activity associated with the through‐conducting nerve net. Most bursts consist of 10–15 pulses at a frequency of 1 every 4 sec to 1 every 10 sec. The interval between bursts is usually 10–20 min. Regularity in pulse number and frequency in successive bursts suggests that the activity originates from a pacemaker. Bursts are always followed by slow contraction of endodermal longitudinal (parietal) muscles after a short delay, and endo‐dermal circular muscles after a long delay. A simple model for nervous pacemaker control of rhythmic contractions cannot be proposed as slow contractions can also occur in the absence of recorded nerve net activity.
POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Ångstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.
Two types of nerve cells, sensory and ganglion cells, were identified in the epidermis of the hypostome of Pelmatohydra robusta by light and electron microscopy. In the study of distribution of these cells, the presence of a circumhypostomal nerve ring in the epidermis was revealed, although hydras have been considered to possess only a diffuse nervous system or socalled nerve net. The nerve ring, which encircled the hypostome, was constituted by several clusters of ganglion cells, thick bundles of many neurites connecting these clusters, and a small number of individual ganglion cells located along the bundles. In the nerve ring, some of the lamellae protruding from the ganglion cells were frequently myelinated and wrapped the cell bodies of neighboring ganglion cells, and other lamellae were arranged in concentric circles.
Silver impregnations, immunofluorescence microscopy, and electron microscopy of the nervous system of Velella confirm previous reports that there are two nerve nets, one composed of small and the other of “giant” neurites. Only one of these systems, the small-fibered open one, shows FMRFamide-like immunoreactivity. It appears to be primarily a sensory network. Despite presence of a neuropeptide in these neurons, they did not contain dense-cored vesicles. The “giant” nerve net (closed system) shows many connections that appear syncytial in the silver preparations. While it is confirmed that gap junctions are present between some neurites in the closed system, it is likely that fusion of neurites also occurs and that the system is a partial syncytium. Membrane complexes with gap junctions are abundant in the cytoplasm. It is suggested that fusion occurs by the engulfment of small neurons by large, resulting in an excess of cell membrane, which is internalized with gap junctions still intact. These internalized membranes appear to break up into vesicles eventually. A similar process may occur in the “giant” swimming motor neuron net of the medusa Polyorchis.
Nerve cells in hydra have been studied with the light and electron microscopes. Techniques for observing bi-polar, tri-polar, multi-polar and sensory cells are described. Evidence is presented that these cells form a major part of the nervous system in hydra.
The ultrastructural studies indicate the following: (1) There are three types of nerve cells: ganglionic, sensory and neurosecretory. (2) Nerve cells are concentrated at the base of tentacles, hypostome and basal regions. (3) Nerve cells are situated adjacent to the muscle fibers of the epithelio-muscular cells. Criteria for identification of nerve cells at the ultrastructural level are presented. The nerve cells in hydra appear to be structurally different from similar cells in higher invertebrates and vertebrates. An important distinction is the lack of specialized synaptic vesicles in hydra.
The appearance, dimensions, and location of secretory droplets in cells identified as neurosecretory cells are discussed. The droplets resemble closely neurosecretory droplets in nerve cells of higher invertebrates and vertebrates. The mechanism for the release of neurosecretory material in hydra appears to be similar to that suggested for higher organisms.
It is suggested that one type of secretory droplet in hydra nerve cells may represent a growth stimulating principle that is believed to control growth and differentiation (Burnett, '66). The relationship of nerve cells to muscle processes is discussed.
By using a radioimmunoassay for the sequence Arg-Phe-amide, the peptide pGlu-Gly-Arg-Phe-amide was isolated from the pennatulid Renilla köllikeri and sequenced. This peptide is a neuropeptide and is produced in high amounts in Renilla tissue (10 wet wt).
1.
Postsynaptic potentials can be recorded intracellularly from epitheliomuscular cells overlying the inner nerve-ring in the medusaPolyorchis
penicillatus (Cnidaria, Hydrozoa) (Fig. 1). These postsynaptic potentials lead to the generation of muscle action potentials which propagate through the swimming-muscle sheets. It is the swimming motor neuron network that innervates this epithelium. An alternative motor pathway is present that involves a network of small, multipolar neurons that is interpolated between swimming motor neurons and overlying epithelial cells (Fig. 2).
2.
That the postsynaptic potentials recorded are due to release of a chemical transmitter is supported by the following evidence: (a) PSPs have a constant delay
([`(x)] = 3.2\textms)(\bar x = 3.2{\text{ms)}}
following the presynaptic spike (Figs. 3, 4); (b) high Mg++ concentrations reduce the amplitude of PSPs and eventually block transmission (Fig. 10); (c) there is no electrical coupling between presynaptic neurons and postsynaptic epithelial cells.
3.
There is an inverse relationship between the duration of presynaptic action potentials and the amplitude of PSPs (Figs. 5, 6). The duration of presynaptic action potentials is a reflection of the degree of synchrony of spiking in the motor network so that short duration motor spikes are associated with synchronous firing. In such cases, the simultaneous release of transmitter substance at a number of neighbouring synapses will cause rapid temporal summation of PSPs in postsynaptic cells. Similarly, long duration presynaptic spikes are associated with asynchronous transmitter release and consequently with small PSPs (Figs. 8, 9).
4.
Changes in PSP amplitude are seen in all postsynaptic cells of a localised region (Fig. 7).
5.
The muscle action potential can be separated into two components, the velar and subumbrellar action potentials (Fig. 11). This biphasic nature of muscle action potentials recorded in the synaptic region results from all-or-none action potentials that are generated at the velar and subumbrellar borders of this region conducting back electrotonically. These action potentials made to conduct antidromically towards the synapses by electrical stimulation of the muscle sheets decrement as they travel through the synaptic region (Fig. 12). The nature of electrical coupling between epithelial cells in the synaptic non-muscular region and the muscle sheets proper must be different.
6.
Larger amplitude PSPs are associated with muscle action potentials that follow with a shorter latency, and that have the two components (velar and subumbrellar) following each other more rapidly (Figs. 5, 9).
7.
Action potentials in the motor network are brought into phase as they conduct around the margin. This leads to more synchronous activation of synapses and hence larger PSPs at regions distant from the initiation site of the motor spike. The resulting decrease in the latency of muscle APs at these distant sites will automatically compensate for the conduction delay of motor spikes.
Antisera to the sequence Arg-Phe-amide (RF-amide) have a high affinity to the nervous system of fixed hydroid polyps. Whole-mount incubations of several Hydra species with RFamide antisera visualize the three-dimensional structure of an ectodermal nervous system in the hypostome, tentacles, gastric region and peduncle. In the hypostome of Hydra attenuata a ganglion-like structure occurs, consisting of numerous sensory cells located in a region around the mouth opening and a dense plexus of processes which project mostly radially towards the bases of the tentacles. In Hydra oligactis an ectodermal nerve ring was observed lying at the border of hypostome and tentacle bases. This nerve ring consists of a few large ganglion cells with thick processes forming a circle around the hypostome. This is the first direct demonstration of a nerve ring in a hydroid polyp.
Incubation of Hydractinia echinata gastrozooids with RFamide antisera visualizes an extremly dense plexus of neuronal processes in body and head regions. A ring of sensory cells around the mouth opening is the first group of neurons to show RFamide immunoreactivity during the development of a primary polyp. In gonozooids the oocytes and spermatophores are covered with strongly immunoreactive neurons.
All examples of whole-mount incubations with RF-amide antisera clearly show that hydroid polyps have by no means a “diffuse nerve net”, as is often believed, and that neuronal centralization and plexus formation are common in these animals. The examples also show that treatment of intact fixed animals with RFamide antisera is a useful technique to study the anatomy or development of a principal portion of the hydroid nervous system.
Patterning processes during embryonic development of Hydractinia echinata were analysed for alterations in morphology and physiology as well as for changes at the cellular level by means of treatment with proportioning altering factor (PAF). PAF is an endogenous factor known to change body proportions and to stimulate nerve cell differentiation in hydroids (Plickert 1987, 1989). Applied during early embryogenesis, this factor interferes with the proper establishment of polarity in the embryo. Instead of normal shaped planulae with one single anterior and one single posterior end, larvae with multiple termini develop. Preferentially, supernumerary posterior ends, which give rise to polyp head structures during metamorphosis, form while anterior ends are reduced. The formation of such polycaudal larvae coincide with an increase in the number of interstitial cells and their derivatives at the expense of epithelial cells. Treatment of further advanced embryonic stages causes an increase in length, presumably due to the general stimulation of cell proliferation observed in such embryos. Also, the spatial arrangement of cells (i.e. cells in proliferation and RFamide (Arg-Phe-amide immunopositive nerve cells) is altered by PAF. Larvae that develop from treated embryos display altered physiological properties and are remarkably different from normal planulae with respect to their morphogenetic potential: (1) Larvae lose their capacity to regenerate missing anterior parts; isolated posterior larva fragments form regenerates of a bicaudal phenotype. (2) In accordance with the frequently observed reduction of anterior structures, the capacity to respond to metamorphosis-inducing stimuli decreases. (3) The morphogenetic potential to form basal polyp parts is found to be reduced. In contrast, the potential to form head structures during metamorphosis increases, since primary polyps with supernumerary hypostomes and tentacles metamorphose from treated animals.
A mouse monoclonal antibody was prepared by using homogenized fragments of crude umbrella material of the hydromedusa Podocoryne carnea as an antigen. The selected clone produced an IgG (mAb sm-1) which decorated smooth muscle cells of hydrozoans. Immunohistochemical testing of mAb sm-1 on whole-mount preparations revealed reactivity with a cytoplasmic, formaldehyde-resistant antigen present in the smooth muscle cells, but absent in all other cell-types. The antibody can therefore be used as a selective and highly sensitive marker to trace the pattern of the smooth muscle system in hydrozoans. The tight association between smooth muscle cells and nerve cells which show FMRFamide-like immunoreactivity can be demonstrated in whole-mount preparations of the hydromedusa Podocoryne carnea with a polyclonal anti-FMRFamide antiserum and in double-labelling experiments.
Thesis (doctoral)--Universität München, 1890.
Evolutionary “old” nervous systems such as those of coelenterates are peptidergic: Using various radioimmunoassays we have now isolated 13 novel neuropeptides from sea anemones and several others from hydrozoan polyps and medusae. These peptides are all structurally related and contain the C-terminal sequence Arg-X-NH 2 or Lys-X-NH 2 , where X is Ala, Asn, Ile, Phe, Pro or Trp. Three neuropeptides have a novel N-terminal L-3-phenyllactyl residue, which protects against degradation by nonspecific aminopeptidases. The neuropeptides from sea anemones are produced by different sets of neurones and have excitatory or inhibitory actions on isolated muscle preparations, suggesting that they are neurotransmitters or neuromodulators. We have also cloned the precursor protein for the sea-anemone neuropeptide Antho-RFamide (<Glu-Gly-Arg-Phe-NH 2 ). In Calliactis parasitica this precursor harbours 19 copies of immature Antho-RFamide (Gln-Gly-Arg-Phe-Gly) together with 7 other, putative neuropeptide sequences. The precursor of Anthopleura elegantissima contains 14 copies of Antho-RFamide and 19 other, putative neuropeptides. This shows that the biosynthetic machinery for neuropeptides in coelenterates, the lowest animal group having a nervous system, is already very efficient and similar to that of higher invertebrates, such as molluscs and insects, and vertebrates.
Cellular interrelationships and synaptic connections in tentacles of several species of coelenterates were examined by means of electron microscopy to determine if neuromuscular pathways were present. The presence of sensory cells, ganglion cells, epitheliomuscular cells, interneuronal synapses, and neuromuscular junctions suggests that neuromuscular pathways are present in coelenterates. Naked axons without sheath cells form several synapses en passant with the same and with different epitheliomuscular cells as well as with nematocytes and other neurons. Interneuronal synapses and neuromuscular and neuronematocyte junctions have clear or dense-cored vesicles (700–1500 Å in diameter) associated with a dense cytoplasmic coat on the presynaptic membrane, a cleft (100–300 Å in width) with intracleft filaments, and a subsynaptic membrane with a dense cytoplasmic coat. At scyphozoan neuromuscular junctions there is a subsurface cisterna of endoplasmic reticulum, which is separated from the epitheliomuscular cell membrane by a narrow cytoplasmic gap (100–300 Å in width) . Neuromuscular junctions in coelenterates resemble en passant axonal junctions with smooth muscle in higher animals.
Morphological evidence is presented for a simple reflex involving a two-cell (sensory or ganglion-epitheliomuscular cell) or three-cell (sensory-ganglion-epitheliomuscular cell) pathway that may result in the coordinated contraction of the longitudinal muscle in tentacles of coelenterates.
Central neurons in Polyorchis (Hydromedusae) were impaled with microelectrodes, and conventional resting potentials were obtained.
The waveform of action potentials recorded concurrently with swimming events shows evidence of electrotonic coupling between
these neurons, which are also directly photosensitive and receive excitatory synaptic input from other conduction systems.
The electrical correlates of activity in the effector systems responsible for swimming, crumpling and postural changes have been recorded in the anthomedusan Polyorchis penicillatus.
Motor spikes (pre‐swim pulses), that initiate swimming contractions, appear without delay at distant sites on the inner nerve‐ring in unstimulated preparations. Levels of Mg ⁺⁺ anaesthesia which block the neuromuscular junctions between PSP giant neurons and swimming muscle do not affect PSP activity.
Swimming muscle potentials can be recorded from subumbrella and velar muscle sheets using extra‐ and intracellular electrodes. These action potentials have a distinct plateau and are propagated in a myoid fashion. Resting potentials average −70 mV with spikes overshooting zero by some 62 mV. The effects of repetitive stimulation are described.
Extracellular recordings indicate that neuronal pathways may play a major role in mediating crumpling, unlike many other species where epithelial pathways are more important. Endodermal spikes recorded intracellularly from the radial and ring canals have amplitudes of some 92 mV arising from resting potentials that average −55 mV. Repetitive stimulation causes a decrease in amplitude and increase in duration of epithelial action potentials.
Tentacle length is controlled by a pacemaker system located in both nerve rings. The frequency of spikes (PTPs) generated by this system determines the length and tonus of tentacles. The neuromuscular junctions between the motor neurons and tentacle muscle are Mg ⁺⁺ sensitive and show facilitating properties.
The gross and fine morphology of the major effector systems in the anthomedusan, Polyorchis penicillatus, is described and discussed in relation to the known physiological and behavioral properties of these systems. Swimming is controlled by an anastomosing network of giant neurons within the inner nerve ring and radial nerves. Although these neurons may be coupled by gap junctions it is likely that they form a syncytium. The photosensitivity of the "giants" is attributed to reflexive membranes within the cytoplasm. Giant neurons act as both the pre- and postsynaptic cell when forming synapses with other neurons of the inner nerve ring. Neuromuscular synapses between "giants" and the striated swimming muscle are found around the margin and along the radii. Swimming muscle cells are connected laterally by gap junctions and end-to-end by desmosomes which are sometimes elaborated with extra-thick filaments. Unstriated sphincter and radial muscles, the major muscles associated with crumpling, are both greatly folded over mesogloeal ridges and have processes that cross the mesogloea to contact the ring and radial canals, respectively. Synapses or other sites that might be responsible for exciting these muscles during crumpling have not been found. The ability of the endodermal lamella and canals to propagate action potentials can be accounted for by the numerous gap junctions that are seen in these tissues. The precise location where excitation is transferred to the nervous system to initiate crumpling is not known but epithelial bridges crossing the mesogloea are likely routes. Synapses between neurons originating in the outer nerve ring and tentacle longitudinal muscle can account for the control of tentacle length. Neurons of the outer nerve ring also synapse onto velar, radial fibers and the sphincter muscle. The inner and outer nerve rings have nervous connections. The organisation of the outer nerve ring and the arrangement of nerves within the endodermal plexus is described. A diagram showing the major connections and interactions of components of the effector systems is presented.
Using serial-sectioning techniques for conventional transmission and high-voltage electron microscopy, we characterized the ultrastructural features and synaptic contacts of the sensory cell in tentacles ofHydra. The sensory cell has an apical specialization characterized by a recessed cilium surrounded by three rodlike stereocilia. This ciliary—stereociliary complex constitutes the receptive or dendritic pole of the sensory cell. The dense filamentous cores of the stereocilia project proximally into a narrow circumciliary cytoplasmic region connected by septate junctions to marginal processes of an enveloping epitheliomuscular cell. The central cilium has a characteristic marginal flare midway along its length and a dense filamentous substructure at its base. Pairs of branched, striated rootlets extend from the axial centriole into a mitochondria-rich region of the cell. Pigment-like granules are present in the cytoplasm around the circumciliary space. The perikaryon is characterized by an elongate nucleus surrounded by a narrow rim of cytoplasm containing prominent Golgi complexes, assorted vacuoles and dense-cored vesicles, free ribosomes, short segments of rough endoplasmic reticulum, microtubules, glycogen particles, and lipid droplets.
Generally, one or two thin, naked axons extend laterally from the perikaryon into the nerve net region above the myonemes of the large epitheliomuscular cells. Within the axons are found occasional aggregates of dense-cored vesicles anden passant synapses characterized by the presence of clear or dense-cored vesicles in contact with paramembranous densities and associated intracleft cross filaments. Using these ultrastructural criteria, we demonstrated for the first time that the granule-containing sensory cells have synaptic contacts with other neurons, nematocytes, and epitheliomuscular cells; hence, we considered these cells to be sensory–motor–interneurons with neurosecretory granules. We hypothesize that this unique, apparently multifunctional neuron may be a modern representative of a primitive stem cell that gave rise evolutionarily to the sensory cells, motor neurons, interneurons, and neurosecretory cells of higher animals.
This chapter discusses some recent evidence on cell differentiation, cell movement, cell interaction, and morphogenesis in hydra with particular emphasis on spatial patterns and prepatterns. It discusses some general concepts of the biological pattern formation, with emphasis on physicochemical mechanisms capable of forming reproducible spatial patterns. Spatial patterns of differentiated cells may result from the regulation of the time sequences and quantities, in which various cell types are produced. Hydra is a few millimeters long and has an asymmetrical polar organization with tentacles, hypostome, and gastric column, including the budding zone, peduncle, and basal disk. The most interesting feature of hydra is its high power of regeneration. The evidence on regeneration and transplantation shows that prepatterns are involved in directing the positions of new heads and feet. Prepatterns in hydra exhibit a set of features typical of many other biological systems. This lateral inhibition type can be accounted for by short-range activation, long-range inhibition, and certain kinetic conditions that lead, starting from near-uniform initial conditions, to the firing of stable self-regulating patterns that show a set of properties of many biological patterns.
A cardioexcitatory substance from ganglia of the clam Macrocallista nimbosa, formerly designated peak C, is the tetrapeptide
amide Phe-Met-Arg-Phe-NH2. Its structure was determined by the combined use of Edman dansyl degradation and tryptic digestion.
The structure was confirmed by synthesis. This neuropeptide is active at about 10(-8)M when assayed on molluscan muscle.
The complexity of the hydrozoan planular nervous system was examined. Using a whole-mount technique with indirect immunofluorescence, the spatial pattern of ganglionic cells showing RFamide-like immunoreactivity was visualized. RFamide antiserum bound a subset of ganglionic cells in the anterior and upper middle regions of the planula and a few ganglionic cells in the upper tail region. Labeled cells consisted of bipolar and multipolar neurons. Stained processes from these cells formed a three-dimensional nerve net that followed the contour of the mesoglea; such fibers were striking in terms of their large numbers, long lengths, and organization into distinct bundles. Labeled fibers were seen to contact other ganglionic cells, sensory cells, epithelio-muscle cells, the mesoglea, and the outside free surface. All stained cell bodies and fibers were found in the ectoderm. Using the same technique the reappearance of RFamide-positive ganglionic cells in epithelial tissue of chimeric grafts of planulae was observed. Interstitial cells capable of forming RFamide-positive ganglionic cells underwent extensive anterior-posterior migrations in the grafts, moved into the epithelial tissue, and differentiated into RFamide-positive ganglionic cells. Stained repopulated ganglionic cells always formed in the same position in the epithelial tissue as was observed in control planulae suggesting that the expression of RFamide-like substances may be position dependent in the planula.
The anatomy and developmental dynamics of the nerve ring in the hypostome of Hydra oligactis were examined immunocytochemically with an antiserum against a neuropeptide and with neuron-specific monoclonal antibodies. The nerve ring is unique in the mesh-like nerve net of hydra. It is a distinct neuronal complex consisting of a thick nerve bundle running circumferentially at the border between the hypostome and tentacle zone. Immunostaining showed that the nerve ring was heterogeneous and contained at least four different subsets of neurons. During head regeneration and budding, the nerve ring appeared only after the nerve net of ganglion and sensory cells had formed. Every epithelial cell is continuously displaced with neurons toward either head or foot in an adult hydra. However, the ectoderm in the immediate vicinity of, and including, the nerve ring constitutes a stationary zone that is not displaced. Tissue immediately above this zone is displaced toward the tip of the hypostome, while tissue below is displaced along the tentacles. Correspondingly, the production of new neurons in the ring as measured by their differentiation kinetics is much slower than in surrounding areas. Thus, the nerve ring is static and stable in contrast to the dynamic features of the nerve net of hydra.
Using a radioimmunoassay against the C-terminal sequence Arg-Pro-NH2 (RPamide), we have isolated the peptide Leu-Pro-Pro-Gly-Pro-Leu-Pro-Arg-Pro-NH2 (Antho-RPamide) from an extract of the sea anemone Anthopleura elegantissima. Antho-RPamide is located in neurons of sea anemones. Application of low concentrations of Antho-RPamide to tentacle preparations of sea anemones strongly increased the frequency and duration of spontaneous contractions, suggesting that this peptide is involved in neurotransmission. Antho-RPamide has a free N-terminus, yet its X-Pro-Pro sequence makes it relatively resistant to degradation by nonspecific aminopeptidases. Thus, we have discovered another strategy by which sea anemones protect the N-termini of their bioactive neuropeptides.
Using a radioimmunoassay for the sequence Arg-Phe-NH2 (RFamide), we have isolated the peptide less than Glu-Trp-Leu-Lys-Gly-Arg-Phe-NH2 (Pol-RFamide II) from acetic acid extracts of the hydromedusa Polyorchis penicillatus. This peptide is a neuropeptide and constitutes a peptide family together with less than Glu-Leu-Leu-Gly-Gly-Arg-Phe-NH2 (Pol-RFamide I), the first neuropeptide isolated from Polyorchis, and less than Glu-Gly-Arg-Phe-NH2 (Antho-RFamide), a neuropeptide isolated from sea anemones and sea pansies.
Antho-RWamide I (less than Glu-Ser-Leu-Arg-Trp-NH2) and Antho-RWamide II (less than Glu-Gly-Leu-Arg-Trp-NH2), the second and third anthozoan neuropeptides to be identified, both induced slow contractions of several endodermal muscles in four species of sea anemone. In a fifth species, Protanthea simplex, Antho-RWamide II, but not Antho-RWamide I, evoked contractions of body wall muscles. Isolated, trimmed sphincter muscle preparations of Calliactis parasitica contracted at a threshold concentration of 10(-9) mol l-1 Antho-RWamide II. Antho-RWamide II was more potent than Antho-RWamide I. Unlike the responses to Antho-RFamide (the first anthozoan neuropeptide described), these were simple contractions with no change in spontaneous activity. The Antho-RWamides did not excite electrical activity in any of the three known conducting systems (the through-conducting nerve net and the slow systems 1 and 2), indicating that they may be acting directly on endodermal muscles. Application of peptides to smooth muscle cells, isolated from the sphincter of C. parasitica, confirmed that Antho-RWamide I and II act directly on the muscle. We conclude that the Antho-RWamides may be neurotransmitters at some neuromuscular synapses in sea anemones.
We have isolated and sequenced the neuropeptide L-3-phenyllactyl-Phe-Lys-Ala-NH2 from the sea anemone Anthopleura elegantissima. This neuropeptide (named Antho-KAamide) has the unusual N-terminal L-3-phenyllactyl blocking group which has recently also been discovered in 2 other neuropeptides from sea anemones. We propose that the L-3-phenyllactyl residue renders Antho-KAamide resistant to nonspecific aminopeptidases, thereby increasing the stability of the neuropeptide after neuronal release. The existence of the L-3-phenyllactyl residue in 3 neuropeptides isolated so far suggests that this blocking group is more generally occurring.
The nervous systems of the scyphomedusae Chrysaora hysoscella, Cyanea capillata and Cyanea lamarckii (Phylum Cnidaria) were stained using an anti-serum against the anthozoan neuropeptide Antho-RFamide. Staining was widespread in all three species. In Chrysaora, the antiserum revealed nerve nets in the subumbrella and exumbrella ectoderm, in both faces of the oral lobes, and in the endoderm lining the subumbrella and exumbrella surfaces of the gastric cavity. The most prominent staining occurred in a dense plexus of neurons in the ectoderm at the base of the tentacles. This nerve net sent projections into the subumbrella ectoderm. For the most part, staining in the two species of Cyanea was similar to that in Chrysaora, with a few exceptions. These include the presence, in Cyanea, of an obvious tentacular nerve tract and nerve nets associated with clusters of cnidocytes in the tentacles. Radioimmunoassays of extracts from Chrysaora and Cyanea lamarkii revealed that both species contain large amounts of Antho-RFamide-like material (up to 55 nmol/animal). The results indicate that Antho-RFamide-like neuropeptides are widespread in scyphomedusae.