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Melatonin inhibits reproductive activity through changes of serotonergic activity in the brain of freshwater catfish (Mystus cavasius)
... Among all environmental cues, photoperiod is the most important, providing an exact year-on-year index of time. Many fishes utilize it for regulating the reproductive BPG endocrine axis (Bromage et al., 2001;Shao et al., 2013;Lee et al., 2017;Badruzzaman et al., 2020). To switch on and off gonadal development, these fishes rely on increasing photoperiod after the winter solstice and declining photoperiod after the summer solstice (Chaube and Joy, 2002;Badruzzaman et al., 2013). ...
... The rhythmic profile of melatonin in most fishes (except salmonids) is tightly controlled by photoperiod and circadian clocks that reach their zenith at midnight and their nadir during the day. Moreover, the duration of nocturnal melatonin secretion is directly proportional to the length of darkness for each day; therefore, fish synthesize more melatonin during longer nights in winter than during shorter nights in summer (Reiter and Tan, 2010;Maitra et al., 2013;Badruzzaman et al., 2020). Thus, environmental regulation of daily and seasonal melatonin variations affects daily/seasonal physiological and behavioral functions such as sexual maturation, growth, immune response, and migration in fish Falcón et al., 2010;Moniruzzaman et al., 2016;Sánchez-Vázquez et al., 2019;Ciani et al., 2021). ...
... These conditions induce ovarian development more effectively than other light pulses or Nanda-Hamner cycles. In these cases, with a 1 h nocturnal light pulse, melatonin likely influences the BPG axis because it is high during scotophase and low during photophase and has an inhibitory effect on ovarian maturation in many teleosts (Khan and Joy, 1990;Senthilkumaran and Joy, 1995;Ghosh and Nath, 2005;Maitra et al., 2013;Moniruzzaman et al., 2016;Badruzzaman et al., 2013Badruzzaman et al., , 2020. In addition, due to longer night with an incomplete photoperiod (night illumination), the duration of nocturnal melatonin secretion may be higher than during a complete photoperiod, which negatively impacts ovarian development in this species. ...
In the freshwater catfish, Mystus cavasius, locally known as gulsha, ovarian maturation is triggered by long-day conditions. Using dopaminergic neuronal activity in the brain, the purpose of this study was to identify the brain’s detection of a nocturnal light pulse that induced ovarian development. Since direct inhibition of pituitary gonadotropin release is exerted by dopamine (DA), it may serve as a neuromodulator of photoperiodic stimulation in teleosts. We studied functional effects of photoperiodicity on dopaminergic rhythmicity in gulsha brain. Nocturnal illumination and Nanda-Hamner photocycles revealed that ovarian development is induced by a 1 h light pulse between zeitgeber time (ZT) 12 and 13. Daily fluctuations in DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and DOPAC/DA were observed under a 12L:12D photoperiod. Fish exhibited increased levels during the daytime and decreased levels at night. Rhythmic patterns of dopaminergic activity also showed clear circadian oscillations under constant light, but not constant dark conditions. After 7 days of exposure to long photoperiod (14L:10D), DA, DOPAC and DOPAC/DA in the brain at ZT12 and ZT16 were significantly higher than during a short photoperiod (10L:14D). Melatonin-containing water inhibited the release of DA and DOPAC 6 h and 24 h after treatment, respectively, and DOPAC/DA 6 h after treatment. This inhibition was blocked by the melatonin receptor antagonist, luzindole. These results suggest that a 1 h nocturnal light pulse induces ovarian development through alteration of dopaminergic neuronal excitability in the brain, via oscillation in melatonin triggered by photic stimuli, which may interfere with the reproductive endocrine axis in gulsha.
... Moreover, treatment with exogenous melatonin by oral route with melatonin-rich pellets may cause a decrease in spawning frequency, number of spawned eggs, Gonado-Somatic Index (GSI) in females, and decreased sperm count, spermatocrit, and spermatozoa activity index in male Nile tilapia (Oreochromis niloticus) 85 . Similarly, another recent study in freshwater catfish (Mystus cavasius) revealed that administration of melatonin by the same oral route could reduce the level of GSI, the number of vitellogenic oocytes in the ovaries, and can repress serotonergic activity during the spawning season 87 . This indicates that, like exogenous melatonin treatment by injection, the application of melatonin with fish feed also has an inhibitory effect on gonadal maturity during the spawning phase. ...
... Hence, melatonin has never been implemented in large-scale aquaculture despite being a potent candidate for regulating fish reproduction. Recently, at least two reports were documented, indicating that the oral supplementation of melatonin also has a significant effect on reproductive functions in fish species 86,87 . This invites us to think about the extra-pineal or extra-retinal sources of melatonin, especially those not controlled by photoperiodic regulation, like gut melatonin. ...
Pinealocytes of the pineal gland in vertebrates mainly synthesize melatonin (5-methoxy-N-acetyl-tryptamine). Moreover, melatonin is synthesized in several extra-pineal cells, including the photoreceptor cells of the retina, the cells of the gut, and the hepatocytes of the liver in different vertebrates, including fish species. One of the remarkable features of pineal and retinal melatonin is that it is produced rhythmically in synchronization with the environmental Light-Dark (LD) cycle, with a daily nighttime peak. However, the melatonin synthesis in tissue/cells from the extra-pineal and extra-retinal origin(s) may not always undergo photoperiod-regulated daily variations but is also dependent on the environmental food entrainment factors (in the gut), acting as the most reliable synchronizer(s) in its daily rhythm features. Moreover, the regulation of the liver and ovary (important for fish reproduction) is unclear. In this review, we attempt a comparative account of the nature and regulation of endogenous melatonin synthesis between a source like the pineal gland and many other non-pineal origins, which have gained serious attention in the last ten years. We also review the functions of melatonin in regulating fish ovarian growth and maturation. The physiological melatonin levels, manipulated either endogenously (by photoperiodic modulations) or exogenously (by injections or by feeds), have tremendous effects on reproductive events in fish at the age of its first maturity, as revealed in recent findings. Characterization and identification of the importance of pineal gland melatonin in the growth of the oocytes via the hypothalamic-pituitary-gonadal axis have been explored several years back. The identification of melatonin receptors about fourteen years back on the wall of developing oocyte spurt the breakthrough, which introduced the concept of direct control of melatonin on developing oocytes. Thus, this review gains uniqueness by addressing the latest developments recorded in the field of melatonin and fish reproduction, particularly in improving oocyte maturation. Nonetheless, an attempt has been made to underline approaches that need to be developed to apply the molecule in large-scale aquaculture.
... This experiment was conducted during active breeding season. Gonadal development was suppressed by restricted feeding (Badruzzaman et al., 2020a;2020b). As a result, at the onset of the experiment, the GSI was low and ovaries of all fish were occupied by immature oocytes at the perinucleolus stage (PNS). ...
... However, the intensity of dopaminergic inhibition on the BPG axis is relatively variable among fish species (Dufur et al., 2010). Therefore, it is important to note that melatonin-containing pellets suppress ovarian development in this species, suggesting that melatonin-DA may influence reproductive activity of the BPG axis (Badruzzaman et al., 2020a). Melatonin stimulates DA release in vitro from brain slices in culture medium in sapphire devils (Badruzzaman et al., 2013). ...
... This experiment was conducted during active breeding season. Gonadal development was suppressed by restricted feeding (Badruzzaman et al., 2020a;2020b). As a result, at the onset of the experiment, the GSI was low and ovaries of all fish were occupied by immature oocytes at the perinucleolus stage (PNS). ...
... However, the intensity of dopaminergic inhibition on the BPG axis is relatively variable among fish species (Dufur et al., 2010). Therefore, it is important to note that melatonin-containing pellets suppress ovarian development in this species, suggesting that melatonin-DA may influence reproductive activity of the BPG axis (Badruzzaman et al., 2020a). Melatonin stimulates DA release in vitro from brain slices in culture medium in sapphire devils (Badruzzaman et al., 2013). ...
... This experiment was conducted during active breeding season. Gonadal development was suppressed by restricted feeding (Badruzzaman et al., 2020a;2020b). As a result, at the onset of the experiment, the GSI was low and ovaries of all fish were occupied by immature oocytes at the perinucleolus stage (PNS). ...
... However, the intensity of dopaminergic inhibition on the BPG axis is relatively variable among fish species (Dufur et al., 2010). Therefore, it is important to note that melatonin-containing pellets suppress ovarian development in this species, suggesting that melatonin-DA may influence reproductive activity of the BPG axis (Badruzzaman et al., 2020a). Melatonin stimulates DA release in vitro from brain slices in culture medium in sapphire devils (Badruzzaman et al., 2013). ...
... The synchronization of sexual maturation and reproduction with the environmental light-dark cycle is mediated through melatonin, synthesized at night by the pineal gland and retina, which acts as an internal chemical messenger of environmental signals to optimize reproductive success (Falcon et al., 2007(Falcon et al., , 2010. Experiments evident that melatonin plays a role in the regulation of many reproductive processes in the regulation of the HPG axis depending on the species, time, mode and duration of administration (Renuka et al., 2010;Hasan et al., 2014;Maitra et al., 2016;Moniruzzaman et al., 2016;Badruzzaman et al., 2020). Exogenous melatonin treatment significantly decreased the expression of GnRH and GTH genes in several fish species. ...
... Melatonin is known as the biological clock molecule in vertebrates, as it regulates the sleep-wake rhythm and seasonal rhythm of an individual (Pandi-Perumal et al. 2008). Melatonin also performs various biological activities such as regulating reproductive development, inhibiting cancer progression, participating in the immune response, and controlling antioxidant stress (Acuna-Castroviejo et al. 2014;Badruzzaman et al. 2020; Communicated by B. Pelster. Tan et al. 2003). ...
Melatonin is a multifunctional bioactive molecule present in almost all organisms and has been gradually used in the aquaculture industry in recent years. Energy metabolism is an essential process for individuals to maintain their life activities; however, the process through which melatonin regulates energy metabolism in aquatic animals remains unclear. The present study aimed to conduct a comprehensive analysis of the regulatory mechanism of melatonin for energy metabolism in Cherax destructor by combining metabolomics analysis with the detection of the key substance content, enzymatic activity, and gene expression levels in the energy metabolism process after culturing with dietary melatonin supplementation for 8 weeks. Our results showed that dietary melatonin increased the content of glycogen, triglycerides, and free fatty acids; decreased lactate levels; and promoted the enzymatic activity of pyruvate kinase (PK), malate dehydrogenase (MDH), and acetyl-CoA carboxylase. The results of gene expression analysis showed that dietary melatonin also increased the expression levels of hexokinase, PK, MDH, lactate dehydrogenase, lipase, and fatty acid synthase genes. The results of metabolomics analysis showed that differentially expressed metabolites were significantly enriched in lysine degradation and glycerophospholipid metabolism. In conclusion, our study demonstrates that dietary melatonin increased oxidative phosphorylation, improved glucose utilization, and promoted storage of glycogen and lipids in C. destructor. These lipids are used not only for energy storage but also to maintain the structure and function of cell membranes. Our results further add to the understanding of the mechanisms of energy regulation by melatonin in crustaceans.
... These results further confirm our previous conclusion that per2 participates in the regulation of the circadian rhythm of organisms exposed to a short light illumination period and may play an important role in the pituitary gland. Furthermore, the expression of Amper2 was the highest under the 8L: 16D condition in the pituitary gland in the long-term experiments, which may be explained by the fact that fish synthesize more melatonin during winter nights compared to summer nights (Reiter et al. 2010;Badruzzaman et al. 2020). Taken together, our findings indicated that the expression of Amper2 was only significantly upregulated under long-term short light treatment, which may be related to the participation of per2 in regulating the physiological changes induced by 8L: 16D during long winters at high latitudes. ...
Clock genes are essential for the daily functions of vertebrates and invertebrates and participate in a wide variety of biological functions including biochemical, physiological, and behavioral processes. Circadian clock disorders are detrimental to both humans and fish, and therefore research on the regulation of genes involved in the circadian clock has attracted increasing attention. Period 2 (per2) is an important gene whose expression is closely related to melatonin secretion, in addition to being affected by the photoperiod length. To gain more insights into the potential biological function of per2, we have cloned and expressed per2 in Amur minnow (Phoxinus lagowskii). We also analyzed melatonin content and AANAT2 gene expression under different short or long-term photoperiod conditions. In the present study, the secretion of melatonin in P. lagowskii was highest in the dark phase under short-term experiments. In the pituitary, the expression of the per2 gene was highest during the dark phase in both the 16L: 8D and 8L: 16D groups. The expression of the per2 gene in the brain was also highest during the dark phase in the 12L: 12D group. Though the expression of the per2 gene was not the highest in the dark phase for the other groups, there were no significant differences compared to the highest value. In addition, in the brain and pituitary, AANAT2 expression was highest during the light phase, except in the 8L: 16D group. Our findings suggested that Amper2 may participate in the feedback regulation of melatonin by regulating the expression of AANAT2. Fluorescence in situ hybridization analyses confirmed that per2 mRNA was expressed during the dark phase, especially in 16L: 8D group. Immunohistochemistry analyses confirmed that PER2 protein was expressed higher in 8L: 16D group. The dual luciferase experiment first validated the promotion of Wnt/β-Catenin expression by the per2 gene. Our results provide novel insights into the expression and location of per2, which contributes to a better understanding of the role of clock genes in the circadian rhythm and seasonal responses of high-latitude fish and might provide useful information of practical interest for high-latitude aquaculture.
... In fish, both the eye and the pineal gland contain photoreceptor cells (Davies and Bromage, 2002;Taranger et al., 2003). Artificial photoperiod manipulation is a commonly used technique in aquaculture to optimize fish production and to regulate the timing of reproductive performance (Badruzzaman et al., 2020;Bromage et al., 2001;Guerrero-Tortolero et al., 2010). For fish, due to high blood sensitivity, a range of blood-based biomarker studies have been conducted to understand the potential effects of light and changes in seasonal photoperiod (Burgos-Aceves et al., 2019;Seibel et al., 2021). ...
Global climate change due to anthropogenic activities affects the dynamics of aquatic communities by altering the adaptive capacities of their inhabitants. Analysis of blood provides valuable insights in the form of a comprehensive representation of the physiological and functional status of fish under various environmental and treatment conditions. This review synthesizes currently available information about blood biomarkers used in climate change induced stress responses in fish. Alterations in informative blood-based indicators are used to monitor the physiological fitness of individual fishes or entire populations. Specific characteristics of fish blood, such as serum and plasma metabolites, cell composition, cellular abnormalities, cellular and antioxidant enzymes necessitate adapted protocols, as well as careful attention to experimental designs and meticulous interpretation of patterns of data. Moreover, the sampling technique, transportation, type of culture system, acclimation procedure, and water quality must all be considered for valid interpretation of hemato-biochemical parameters. Besides, blood collection, handling, and storage time of blood samples can all have significant impacts on the results of a hematological analysis, so it is optimal to perform hemato-biochemical evaluations immediately after blood collection because long-term storage can alter the results of the analyses, at least in part as a result of storage-related degenerative changes that may occur. However, the scarcity of high-throughput sophisticated approaches makes fish blood examination studies promising for climate-driven stress responses in fish.
... In addition, specific subtypes of melatonin receptors are expressed in the pituitary of mudskipper Boleophthalmus pectinirostris (Hong et al. 2014), orange-spotted grouper E. coioides (Chai et al. 2013), Atlantic salmon Salmo salar (Ciani et al. 2019), and medaka Oryzias latipes (Maugars et al. 2020). These findings indicate that the pituitary gland is a target of melatonin, and partially support the potential involvement of melatonin in reproduction including vitellogenesis (Badruzzaman et al. 2020;Imamura et al. 2021) . In this regard, it was reported that melatonin suppresses vitellogenesis in the liver through downregulation of gonadotropin genes in the pituitary (Sébert et al. 2008;Alvarado et al. 2015;Fukunaga et al. 2019). ...
In the reproductive season, lunar-synchronized fish spawn during species-specific moon phases. It remains unknown how they perceive cues from the moon for initiating gonadal development and subsequent spawning. We hypothesized that fish use moonlight to synchronize reproductive events. Using the honeycomb grouper Epinephelus merra, a tropical Serranidae, spawns around the time of the full moon, we report the successful induction of spawning in this species by controlling light condition at night. Following 3 months of acclimatization under long days (14:10) at 28 ± 1.0°C, mature fish were reared under an artificial full (experimental group) or new (controls) moon during scotophase. Rearing fish under these conditions for 1 month resulted in successful spawning in the experimental group between 21h00 and 01h30. Similar results were obtained when control fish were subdivided into artificial full and new moon groups and reared for an additional month. Fish in the experimental group had a high gonadosomatic index and abundant yolk-laden oocytes in the ovary. Therefore, moonlight triggers gonadal development and subsequent spawning in lunar-synchronized fish with a full-moon preference.
... However, the co-injection experiment with cGnIH-III and melatonin showed that the surge induced by cGnIH-III peptide is diminsed by the melatonin. LHRHa is a well know positive regulator of HPG axis in fishes and other animals while melatonin has negative impacts on reproductive axis in fishes during breeding phase (Bhattacharya et al., 2007;Servili et al., 2013;Badruzzaman et al., 2020). So here the synergistic experiments of cGnIH-III peptide with these positive and negative regulators confirmed the positive effect of GnIH in L. catla. ...
After the discovery of Gonadotropin-inhibitory hormone (GnIH) in birds in 2000, it showed different roles in different vertebrate classes and even in different species of same classes. In birds and mammals, GnIH inhibits the expression of gonadotropins during reproduction, while in fishes it exerts both inhibitory and stimulatory effect on reproduction. The current study evaluates the role of GnIH during reproduction in Labeo catla. The partial cDNA sequence of GnIHR1 and GnIHR3 receptor genes was identified by degenerate PCR. The mRNA expression analysis of GnIHRs during different reproductive phases showed that the expression of all three GnIH receptor genes is highest during spawning phase. The expression of GnIH receptors is detected in both brain and gonads except for GnIHR3 which only expressed in gonads. The in vivo experiments with GnIH antagonist, RF313 drastically reduced the expression level of reproduction related genes like LH, FSH, and GnRH at 1h post-injection. In another experiment the surge induced by cGnIH-III peptide on gonadotropins gene expression is further increased when co-injected with LHRHa. However, co-injection of melatonin along with cGnIH-III peptide had opposite effects. These results showed that the GnIH/GnIHRs system has positive effect on reproduction in L. catla.
... It has been shown that the effect of melatonin could be indirect by modifying neurotransmitters like serotonin. Melatonin inhibited hypothalamic serotonin (5-HT) content and turnover (activity) in H. fossilis (Senthilkumaran and Joy 1995a) and brain 5-HT, 5-HIAA, and 5-HIAA/5-HT in another catfish Mystus cavasius (Badruzzaman et al. 2020). Summing up, the present data and previous studies in the catfish support predominantly an inhibitory role of melatonin on the BPG axis during gonadal recrudescence. ...
... It has been shown that the effect of melatonin could be indirect by modifying neurotransmitters like serotonin. Melatonin inhibited hypothalamic serotonin (5-HT) content and turnover (activity) in H. fossilis (Senthilkumaran and Joy 1995a) and brain 5-HT, 5-HIAA, and 5-HIAA/5-HT in another catfish Mystus cavasius (Badruzzaman et al. 2020). Summing up, the present data and previous studies in the catfish support predominantly an inhibitory role of melatonin on the BPG axis during gonadal recrudescence. ...
Abstract In seasonally breeding vertebrates, extrinsic factors like photoperiod and temperature are major determinants, controlling the annual reproductive cycle. In teleosts, kisspeptin, which occurs in two molecular forms: kisspeptin1 (Kiss1) and kisspetin2 (Kiss2), has been reported to alter gonadotropin (Lh and Fsh) secretion but its effect on gonadotropin-releasing hormone (Gnrh) secretion is not unequivocally proved. In the
catfish Heteropneustes fossilis, we isolated and characterized kiss2 and gnrh2 cDNAs and the present work reports effects of altered photo-thermal conditions and melatonin (MT, a pineal hormone) on their expressions in the brain. The exposure of the catfish to long photoperiod (LP, 16 h light) at normal temperature (NT) or high temperature (HT, 28 °C) at normal photoperiod (NP) for 14 or 28 days stimulated both kiss2 and gnrh2 expression in both gonad resting and preparatory phases with the combination of LP + HT eliciting maximal effects. Short photoperiod (SP, 8 h light) under NT or HT altered the gene expression according to the reproductive phase and temperature. MT that mediates photothermal signals to the brain inhibited brain kiss2 and gnrh2 gene expression in the NP + HT, LP + NT, and SP + NT groups. The altered photo-thermal conditions elicited changes in steroidogenic pathway as evident from changes in plasma E2, progesterone, and testosterone levels. The results show that brain kiss2-gnrh2 signaling is involved in photo-thermal-mediated mechanisms controlling reproduction.
Keywords kiss2 . gnrh2 . Expression . Catfish . Brain
Imidacloprid (IMI), one of the most frequently used neonicotinoid insecticides in agriculture, is resided in surface water worldwide and poses a threat to aquatic organisms. Melatonin (MT) provides effective protection against insecticide-induced toxicity, nevertheless, the toxic effects and whether MT attenuates intestinal injury caused by IMI exposure in the common carps remains poorly explored. Previous studies have reported adverse effects of IMI exposure on intestinal health status. Therefore, we first demonstrated that IMI altered the composition and function of the intestinal microbiota, destroying the integrity of intestinal ultrastructure, increasing intestinal permeability. Meanwhile, metagenomic sequencing and ELISA kits results hypothesized that peptidoglycan (PGN) is an IMI-triggered intestinal microbial metabolite. Subsequently, we thus further elucidated that IMI induced an increase in intestinal tight junction permeability by inducing PGN secretion in vitro model. MT addition dramatically attenuated IMI-induced intestinal toxicity by remitting PGN synthesis and thus resecuring tight junction permeability, thereby reducing intestinal injury. SB203580 was supplied as a P38MAPK inhibitor to alleviate the increased permeability of tight junctions induced by IMI/PGN. Therefore, these findings confirmed that MT protects against IMI-induced intestinal injury by negatively regulating PGN/P38MAPK pathway to antagonize the increased tight junction permeability.
As an important hormone, melatonin participates in endocrine regulation of diverse functions in vertebrates. Its biosynthesis is catalyzed by four cascaded enzymes, among them, arylalkylamine N-acetyltransferase (AANAT) is the most critical one. Although only single aanat gene has been identified in most groups of vertebrates, researchers including us have determined that fish have the most diverse of aanat genes (aanat1a, aanat1b, and aanat2), playing various potential roles such as seasonal migration, amphibious aerial vision, and cave or deep-sea adaptation. With the rapid development of genome and transcriptome sequencing, more and more putative sequences of fish aanat genes are going to be available. Related phylogeny and functional investigations will enrich our understanding of AANAT functions in various fish species.
Circadian rhythm is an intrinsic mechanism developed by organisms to adapt to external environmental signals. Nowadays, owing to the job and after-work entertainment, staying up late – Circadian rhythm disorders (CRD) are common. CRD is linked to the development of fatty liver, type 2 diabetes, and chronic gastroenteritis, which affecting the body’s metabolic and inflammatory responses via multi-organ crosstalk (gut-liver-brain axis, etc.). However, studies on the mechanisms of multi-organ interactions by CRD are still weak. Current studies on therapeutic agents for CRD remain inadequate, and phytochemicals have been shown to alleviate CRD-induced syndromes that may be used for CRD-therapy in the future. Tea, a popular phytochemical-rich beverage, reduces glucolipid metabolism and inflammation. But it is immature and unclear in the mechanisms of alleviation of CRD-mediated syndrome. Here, we have analyzed the threat of CRD to hosts and their offspring’ health from the perspective of the “gut-liver-brain” axis. The potential mechanisms of tea in alleviating CRD were further explored. It might be by interfering with bile acid metabolism, tryptophan metabolism, and G protein-coupled receptors, with FXR, AHR, and GPCR as potential targets. We hope to provide new perspectives on the role of tea in the prevention and mitigation of CRD.
• Highlights
• The review highlights the health challenges of CRD via the gut-liver-brain axis.
• CRD research should focus on the health effects on healthy models and its offspring.
• Tea may prevent CRD by regulating bile acid, tryptophan, and GPCR.
• Potential targets for tea prevention and mitigation of CRD include FXR, AHR and GPCR.
• A comprehensive assessment mechanism for tea in improving CRD should be established.
Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.
Zebrafish represent the one alternative vertebrate, genetic model system to mice that can be easily manipulated in a laboratory setting. With the teleost Medaka (Oryzias latipes), which now has a significant following, and over 30,000 other fish species worldwide, there is great potential to study the biology of environmental adaptation using teleosts. Zebrafish are primarily used for research on developmental biology, for obvious reasons. However, fish in general have also contributed to our understanding of circadian clock biology in the broadest sense. In this review, we will discuss selected areas where this contribution seems most unique. This will include a discussion of the issue of central versus peripheral clocks, in which zebrafish played an early role; the global nature of light sensitivity; and the critical role played by light in regulating cell biology. In addition, we also discuss the importance of the clock in controlling the timing of fundamental aspects of cell biology, such as the temporal control of the cell cycle. Many of these findings are applicable to the majority of vertebrate species. However, some reflect the unique manner in which “fish” can solve biological problems, in an evolutionary context. Genome duplication events simply mean that many fish species have more gene copies to “throw at a problem”, and evolution seems to have taken advantage of this “gene abundance”. How this relates to their poor cousins, the mammals, remains to be seen.
Differential effects of photoperiod and temperature on the temporal modulation of gonadotrophin subunit genes (glycoprotein α, gpα), follicle-stimulating hormone β (fshβ) and luteinizing hormone β (lhβ) expression were investigated in the stinging catfish Heteropneustes fossilis. Female H. fossilis were exposed to varying photoperiod and temperature conditions for 14 and 28 days in the early preparatory phase of the annual reproductive cycle. Gonadotrophin subunit gene expression, gonado-somatic index (IG), ovarian histology and plasma steroid hormone levels were evaluated. The exposure of H. fossilis to long photoperiod (LP) of 16 h light or high temperature (HT) at 28 ± 2° C (mean ± s.e.), alone or in combination, resulted in significant increases in gpα, fshβ and lhβ messenger (m)RNA levels, IG, plasma oestradiol-17β (E2), testosterone (T) and progesterone (P4) levels. The ovaries were filled with advanced yolky oocytes. On the other hand, the short photoperiod (SP) of 8 h light exposure decreased the transcript levels with higher inhibition in the normal temperature (NT) group at 18 ± 2° C (mean ± s.e.) than the HT group at 28 ± 2° C. Furthermore, the inhibition reached the highest level in total darkness (TD) of 24 h light deprivation under NT conditions at 18 ± 2° C. Consequently, the SP and TD treatments inhibited the IG, plasma E2 and T levels and ovarian development. The exposure to high temperature at 28 ± 2° C also modified the short photoperiod effect by elevating plasma E2 level. The plasma T level changed only mildly while the plasma P4 level showed the greatest fluctuations; the level reached the nadir in the SP + HT group but increased in the SP + NT group on day 28. A two-way ANOVA of the data showed differential effects of photoperiod and temperature; photoperiod produced a highly significant effect on fshβ expression while temperature had a highly significant effect both on lhβ and gpα levels. Thus, the differential expression of the gpα by the environmental variables ensures temporal synchronization of ovarian development and spawning.
Reproduction in most fish is seasonal or periodic, and the spawning occurs in an appropriate season to ensure maximum survival of the offspring. The sequence of reproductive events in an annual cycle is largely under the control of a species-specific endogenous timing system, which essentially relies on a well-equipped physiological response mechanism to changing environmental cues. The duration of solar light or photoperiod is one of the most predictable environmental signals used by a large number of animals including fish to coordinate their seasonal breeding. In vertebrates, the pineal gland is the major photoneuroendocrine part of the brain that rhythmically synthesizes and releases melatonin (N-acetyl-5-methoxytryptamine) into the circulation in synchronization with the environmental light–dark cycle. Past few decades witnessed an enormous progress in understanding the mechanisms by which melatonin regulates seasonal reproduction in fish and in other vertebrates. Most studies emphasized hormonal actions of melatonin through its high-affinity, pertussis toxin-sensitive G-protein (guanine nucleotide-binding protein)-coupled receptors on the hypothalamus–pituitary–gonad (HPG) axis of fish. However, the discovery that melatonin due to its lipophilic nature can easily cross the plasma membrane of all cells and may act as a potent scavenger of free radicals and stimulant of different antioxidants added a new dimension to the idea explaining mechanisms of melatonin actions in the regulation of ovarian functions. The basic concept on the actions of melatonin as an antioxidant emerged from mammalian studies. Recently, however, some new studies clearly suggested that melatonin, apart from playing the role of a hormone, may also be associated with the reduction in oxidative stress to augment ovarian functions during spawning. This review thus aims to bring together the current knowledge on the role of melatonin as a hormone as well as an antioxidant in the control of fish reproduction and shape the current working hypotheses supported by recent findings obtained in carp or based on knowledge gathered in mammalian and avian species. In essence, this review highlights potential actions of melatonin as a hormone in determining temporal pattern of spawning and as an antioxidant in regulating oocyte maturation at the downstream of HPG axis in fish.
Melatonin is synthesized in peripheral locations of vertebrates, including the gastrointestinal tract (GIT). In teleost, information regarding this topic is scarce. Here we studied the presence and synthesis of melatonin at the rainbow trout GIT. Different sections of trout GIT (from esophagus to hindgut) were dissected out and assayed for contents of melatonin, serotonin (5-HT) and its metabolite, 5-hydroxyindole acetic acid, as well as for aanat1, aanat2 and hiomt mRNA abundance. A trout group was pinealectomized to evaluate changes in plasma and gut melatonin content. Finally, the daily profile of melatonin and 5-HT content, and aanat1, aanat2 and hiomt mRNA abundance were analyzed in gut of trout kept under normal lighting, and then under constant darkness. Melatonin was detected in all GIT regions with higher concentrations in the muscular wall than in the mucosa, a similar trend to that of 5-HT. In contrast, transcripts of melatonin synthesis enzymes were more abundant in the mucosa. Pinealectomy did not affect melatonin levels in midgut and hindgut either at day or at night. Additionally, no daily rhythms could be defined for melatonin content in gut tissues but increases during late light phase and at midnight occurred. However, aanat1, aanat2 and hiomt mRNA abundance showed clear daily rhythms with peaks at night. These rhythms remained with a 3-h phase advanced peak in fish exposed to constant darkness. Our results provide clear evidence for a local synthesis of melatonin in trout GIT that might be influenced by the content of 5-HT in the tissue. The process is affected by environmental light cycle and is likely to be under circadian regulation.
The neuroendocrine mechanism regulates reproduction through the hypothalamo-pituitary-gonadal (HPG) axis which is evolutionarily conserved in vertebrates. The HPG axis is regulated by a variety of internal as well as external factors. Serotonin, a monoamine neurotransmitter, is involved in a wide range of reproductive functions. In mammals, serotonin regulates sexual behaviors, gonadotropin release and gonadotropin-release hormone (GnRH) secretion. However, the serotonin system in teleost may also play unique role in the control of reproduction as the mechanism of reproductive control in teleosts is not always the same as in the mammalian models. In fish, the serotonin system is also regulated by natural environmental factors as well as chemical substances. In particular, selective serotonin reuptake inhibitors (SSRIs) are commonly detected as pharmaceutical contaminants in the natural environment. Those factors may influence fish reproductive functions via the serotonin system. This review summarizes the functional significance of serotonin in the teleosts reproduction.
Melatonin receptor gene expression as well as melatonin synthesis and secretion activities were examined in the pineal gland of the grass puffer, which exhibits unique lunar/tidal cycle-synchronized mass spawing: spawning occurs before high tide on the day of spring tide during spawing season. Melatonin synthesizing activity was assessed by the abundance of arylalkylamine N-acetyltransferase 2 (AANAT2) mRNA. The amount of aanat2 mRNA was low during light phase and initiated to increase after the light was turned off. The secretion of melatonin from primary pineal organ culture was stimulated after the light was turned off and ceased immediately after the light was turned on. The expression levels of four melatonin receptor subtype genes (mel1a1.4, mel1a1.7, mel1b, and mel1c) showed synchronous variations, and the levels tended to be high during the dark phase under light/dark conditions. These results suggest that the action of melatonin on the pineal gland is highly dependent on light and photoperiod, possibly with stronger action during night time. Under constant darkness, the expression of four melatonin receptor subtype genes showed unique ultradian oscillations with the period of 14.0–15.4 h, suggesting the presence of a circatidal oscillator in the pineal gland. The present results indicate that melatonin may serve local chronobiological functions in the pineal gland. These cyclic expressions of melatonin receptor genes in the pineal gland may be important in the control of the lunar/tidal cycle-synchronized mass spawning in the grass puffer.
All mammal behaviors and functions exhibit synchronization with environmental rhythms. This is accomplished through an internal mechanism that generates and modulates biological rhythms. The circadian timing system, responsible for this process, is formed by connected neural structures. Pathways receive and transmit environmental cues to the central oscillator, the hypothalamic suprachiasmatic nucleus, which mediates physiological and behavioral alterations. The suprachiasmatic nucleus has three major inputs: The retinohypothalamic tract (a direct projection from the retina), the geniculohypothalamic tract (an indirect photic projection originating in the intergeniculate leaflet), and a dense serotonergic plexus from the raphe nuclei. The serotonergic pathway, a source of non-photic cues to the suprachiasmatic nucleus, modulates its activity. The importance of raphe nuclei in circadian rhythms, especially in photic responses, has been demonstrated in many studies. Serotonin is the raphe neurotransmitter that triggers phase shifts, inhibits light-induced phase-shifts, and plays a role in controlling the sleep-wake cycle. All data to date have demonstrated the importance of the raphe, through serotonergic afferents, in adjusting circadian rhythms and must therefore be considered a component of the circadian timing system. The aim of this paper is to review the literature addressing the involvement of serotonin in the modulation of circadian rhythm.
In the neuroendocrine system controlling fish reproduction, dopamine (DA) acts as a gonadotropin inhibitory factor and plays a role in regulating gonadal development of certain species. The present study examined the effects of chemical destruction of dopaminergic neurons in the brain on DA production and ovarian development in the sapphire devil Chrysiptera cyanea, a reef-associated damselfish. The avidin-biotin-peroxidase complex method using an antibody against tyrosine hydroxylase (TH), a critical enzyme in the DA synthesis pathway, identified a population of dopaminergic neurons with somata in the anteroventral preoptic nucleus of the diencephalon and fibers terminating in the proximal pars distalis of the pituitary. Maintaining fish in seawater containing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at 0.02 and 0.2 µg/mL for 2 days resulted in decreases in DA, 3,4-dihydroxyphenylacetic acid (DOPAC; DA metabolite), and DA metabolic rate in the whole brain. The number of TH-positive neurons in the diencephalon decreased after 0.02 µg/mL MPTP treatment for 2 days. These results suggest that MPTP treatment destroys TH-positive neurons in the diencephalon, thereby decreasing the synthesis and release of DA from the brain. This treatment rescued ovarian development in fish with artificially retracted ovaries during the spawning season. The gonadosomatic index of MPTP-treated fish 5 and 7 days after treatment was significantly higher than that of control fish. Oocytes in the vitellogenic stages were observed in the ovaries of MPTP-treated fish, but not in control fish. These results suggest that DA in the brain drives ovarian development in the sapphire devil.
A temporal relationship between the serum profiles of pineal-hormone melatonin (N-acetyl-5-methoxytryptamine) and two major ovarian steroids, i.e., 17-β estradiol (E2) and 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β DHP) or maturation inducing hormone (MIH), has been sought for the first time in any free-living teleost. The study was carried under natural conditions in a carp Catla catla following collection of blood samples at different time-points in a diurnal cycle during each of the four distinct reproductive phases in an annual cycle. The levels of melatonin and the steroids were measured following specific RIA and ELISA techniques, respectively. A time-bound change in the serum melatonin was noted in a 24 h cycle, though the pattern of fluctuations was not identical throughout the reproductive cycle. Melatonin reached its peak during the mid-night in each reproductive phase, other than the preparatory phase (January–March) when the diurnal peak value was recorded in the late dark phase. In an annual cycle, melatonin titer was found to be maximum during the post-spawning phase (September–December). However, in each reproductive phase, a minimum daily value of melatonin was recorded in the mid-day. Serum E2, irrespective of reproductive phase, exhibited a daily peak in the mid-day and nadir in the early morning. However, in a seasonal cycle, E2 was found to be maximum in the spawning phase (July–August). The serum MIH, however, did not show any significant daily variations, and underwent significant changes in an annual cycle with a very low value in the preparatory phase followed by a gradual increase in the pre-spawning (April–June), a peak in the spawning phase, and was undetectable during the post-spawning phase. The simple correlation-coefficient analysis of the seasonal peak values of melatonin revealed a significant negative correlation with photoperiods, water temperature and serum E2, while a positive correlation was found between the seasonal values of serum E2, photoperiod and water temperature. In a seasonal cycle, serum MIH values exhibited a positive correlation with both water temperature and serum E2, and a negative correlation with melatonin. Collectively, the present study underlines the importance of environmental photo-thermal conditions, especially water temperature, as the functional correlate of seasonal changes in serum melatonin and ovarian steroids in carp.
Daily variation in melatonin receptor (MT1 and MT2) density in three specific tissues-brain, retina, and ovary-and its temporal relationship with serum melatonin were evaluated for the first time in a freshwater teleost, the carp Catla catla, under natural as well as altered photoperiods in different reproductive phases of the annual cycle. Cosinor analysis was used to determine rhythmic features of the serum melatonin and receptors (MT1 and MT2) in different tissues. In each photoperiodic group, irrespective of season, the daily minimum serum melatonin level was noted at midday. However, the daily peak value of melatonin varied in relation to both photo-schedules and reproductive phases. Under natural photoperiods (NPs; duration varied with seasons) and short photoperiods (SPs; light [L]:dark [D] 8:16), it occurred in the late dark phase during the preparatory phase, and at midnight in the remaining parts of the annual cycle. On the other hand, in each reproductive phase, compared to corresponding NP carp, the daily melatonin peak under long photoperiods (LPs; L:D 16:8) exhibited a phase delay of ∼2-3 h (occurring during the late dark phase). The melatonin levels at each sampling point were highest during the postspawning phase and lowest during the spawning phase, irrespective of the photoperiodic history of the fish. In each tissue, Western blot analysis revealed a band at ∼37 kDa and a band at ∼36 kDa corresponding to the molecular weights of native MT1 and MT2 receptor proteins, respectively, with the band intensity of MT1 always being higher than that of a 36-kDa protein. The content of both melatonin receptor proteins varied significantly according to the studied tissue (being highest in the retina, intermediate in the brain, and lowest in the ovary), time in the daily cycle (peak at midnight and fall at midday), and reproductive phase in the annual cycle (highest in the spawning phase and lowest in the postspawning phase). Remarkably, no significant effects of altered photoperiod were detected on any rhythm parameters of either MT1 or MT2 in any of the studied tissues. Collectively, the results of the present study suggest a role of photoperiod in determining daily and seasonal profiles of serum melatonin, but not its receptor proteins, on the ovary or on any nongonad tissues in carp.
In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.
While gonadotropin-releasing hormone (GnRH) is considered as the major hypothalamic factor controlling pituitary gonadotrophins in mammals and most other vertebrates, its stimulatory actions may be opposed by the potent inhibitory actions of dopamine (DA) in teleosts. This dual neuroendocrine control of reproduction by GnRH and DA has been demonstrated in various, but not all, adult teleosts, where DA participates in an inhibitory role in the neuroendocrine regulation of the last steps of gametogenesis (final oocyte maturation and ovulation in females and spermiation in males). This has major implications for inducing spawning in aquaculture. In addition, DA may also play an inhibitory role during the early steps of gametogenesis in some teleost species, and thus interact with GnRH in the control of puberty. Various neuroanatomical investigations have shown that DA neurones responsible for the inhibitory control of reproduction originate in a specific nucleus of the preoptic area (NPOav) and project directly to the region of the pituitary where gonadotrophic cells are located. Pharmacological studies showed that the inhibitory effects of DA on pituitary gonadotrophin production are mediated by DA-D2 type receptors. DA-D2 receptors have now been sequenced in several teleosts, and the coexistence of several DA-D2 subtypes has been demonstrated in a few species. Hypophysiotropic DA activity varies with development and reproductive cycle and probably is controlled by environmental cues as well as endogenous signals. Sex steroids have been shown to regulate dopaminergic systems in several teleost species, affecting both DA synthesis and DA-D2 receptor expression. This demonstrates that sex steroid feedbacks target DA hypophysiotropic system, as well as the other components of the brain-pituitary gonadotrophic axis, GnRH and gonadotrophins. Recent studies have revealed that melatonin modulates the activity of DA systems in some teleosts, making the melatonin-DA pathway a prominent relay between environmental cues and control of reproduction. The recruitment of DA neurons for the neuroendocrine control of reproduction provides an additional brain pathway for the integration of various internal and environmental cues. The plasticity of the DA neuroendocrine role observed in teleosts may have contributed to their large diversity of reproductive cycles.
Importance of melatonin (N-acetyl-5-methoxytryptamine) in the regulation of oocyte maturation has been studied in a carp Catla catla. Melatonin secretory cells were immunocytochemically localized only in the end vesicle. Diurnal and seasonal studies indicated that the serum levels of melatonin exhibit a minimum diurnal value in the mid-day of all seasons, but the peak value is attained either at mid-night or just before the onset of light. Moreover, highest seasonal value of melatonin was noted in the post-spawning phase. Administration of melatonin at different doses (25, 50, or 100 mug/100 g body weight) for 1, 15, or 30 days resulted in either pro- or anti-gonadal effects depending on the reproductive seasons. In vitro study revealed that incubation of oocytes with melatonin 4h prior to addition of MIH in the medium led to an accelerated rate of oocyte maturation through the formation of a complex of two proteins (MPF), cyclin B and cyclin dependant kinase, Cdc2. Moreover, melatonin pre-incubation considerably increased MIH stimulation of histone H1 phosphorylation as compared to MIH alone. Taken together, gathered information promotes the idea of a physiological role of melatonin in the maturation of oocytes in Catla catla.
In Channa punctatus, a significant daily variation in hypothalamic 5-HT level and monoamine oxidase (MAO) activity was noticed in preparatory phase (February), but not in prespawning (May) or postspawning (November) phases. Hypothalamic dopamine (DA) and noradrenaline (NA), on the other hand, showed marked daily variation in their levels during all the three seasons with peak values in the photophase. The overall activity of MAO (mean +/- SEM on 24-hr period) increased from November to May through February, whereas the 5-HT content which was high in November decreased during February and May. The NA and DA levels were low in November and February and high in May. The catecholamine (CA) content and MAO activity increased with increasing photoperiod and temperature which is indicative of an enhanced CA metabolism.
Effects of daily administration of melatonin for 15 days were evaluated with respect to ovarian activities and plasma gonadotropin (GtH II) and vitellogenin (Vg) levels in intact (INT) and pinealectomized (Px) female catfish, C. batrachus, during preparatory (April), prespawning (May and June), spawning (July) and post-spawning (September) periods. Px (saline control groups) caused a stimulatory effect during preparatory (with respect to Vg synthesis and incorporation) and prespawning (with respect to Vg synthesis) periods whereas no effect was observed during spawning and post-spawning periods with respect to the reproductive parameters studied. During April, melatonin-treatment significantly decreased plasma GtH II levels and percentage of vitellogenic oocytes without any significant changes in plasma Vg levels and gonadosomatic index (GSI). During early prespawning period, in May, 50microg melatonin brought about a significant reduction in plasma GtH II levels in INT group, whereas 100microg caused a decrease in all parameters; on the other hand, in Px groups both dose levels proved to be inhibitory. In June (late prespawning period) melatonin-treatment could not bring about any change in GSI and plasma Vg levels compared to the control groups regardless of Px but plasma GtH II and mean number of yolky oocytes were significantly reduced in melatonin-treated INT group. During spawning period (July) melatonin inhibited the GSI, mean number of yolky oocytes and plasma GtH II levels without affecting plasma Vg levels. In September (post-spawning period), melatonin did inhibit both GSI and plasma GtH II levels. The results, thus, indicate that melatonin showed variable effects (inhibitory and/or no effect) to GSI, mean number of yolky oocytes and plasma Vg levels but a consistent inhibiton of plasma GtH II levels indicating that melatonin may control the reproduction by blocking the GtH II release from the pituitary via affecting the hypothalamo-hypophysial axis.
The seasonal, daily and lunar control of reproduction involves photoperiodic, circadian and lunar changes in the activity of kisspeptin, gonadotropin-inhibitory hormone (GnIH) and gonadotropin-releasing hormone (GnRH) neurons. These changes are brought through complex networks of light-, time- and non-photic signal-dependent control mechanisms, which are mostly unknown at present. The grass puffer, Takifugu alboplumbeus, a semilunar spawner, provides a unique and excellent animal model to assess this question because its spawning is synchronized with seasonal, daily and lunar cycles. In the diencephalon, the genes for kisspeptin, GnIH and their receptors showed similar expression patterns with clear seasonal and daily oscillations, suggesting that they are regulated by common mechanisms involving melatonin, circadian clock and water temperature. For implications in semilunar-synchronized spawning rhythm, melatonin receptor genes showed ultradian oscillations in expression with the period of 14.0-15.4 h in the pineal gland. This unique ultradian rhythm might be driven by circatidal clock. The possible circatidal clock and circadian clock in the pineal gland may cooperate to drive circasemilunar rhythm to regulate the expression of the kisspeptin, GnIH and their receptor genes. On the other hand, high temperature (over 28°C) conditions, under which the expression of the kisspeptin and its receptor genes is markedly suppressed, may provide an environmental signal that terminates reproduction at the end of breeding period. Taken together, the periodic regulation of the kisspeptin, GnIH and their receptor genes by melatonin, circadian clock and water temperature may be important in the precisely-timed spawning of the grass puffer.
The present study aimed to evaluate antioxidant role of melatonin in determining seasonality of ovarian growth in adult carp Catla catla. Accordingly, an identical regimen of exogenous melatonin administration (100 μg/100 g body wt./day for 15 days) was followed during the preparatory, pre-spawning and spawning phases of an annual reproductive cycle. The study did not include post-spawning phase, when the ovaries were completely regressed and devoid of any healthy growing follicles. The ovarian response was evaluated by determining relative number of developing oocytes as well as measuring the levels of melatonin, oxidative stress [using malondialdehyde (MDA) as the marker], both enzymatic [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST)] and non-enzymatic [reduced glutathione (GSH)] antioxidants in the ovarian homogenates. Due to melatonin treatment, oocyte growth was accelerated in the preparatory phase, but retarded in the pre-spawning and spawning phases of annual cycle. Conversely, melatonin administration in each reproductive phase led to a significant reduction of MDA and elevations of SOD, CAT, GPx, GST, GSH as well as melatonin levels in the ovary. As a result, melatonin titers in the ovary always showed a negative correlation with MDA and a positive correlation with SOD, CAT, GST, GPx, as well as GSH levels. However, melatonin content of ovary and the values of gono-somatic index in melatonin treated carp displayed a positive correlation in the preparatory phase and a negative correlation in the remaining parts of reproductive cycle. Thus it seems likely that melatonin by acting as an antioxidant reduces intra-ovarian oxidative stress throughout the seasons of follicular growth, while exogenous melatonin administration exerts pro-gonadal influences during the preparatory phase, but anti-gonadal effects during the pre-spawning and spawning phases of reproductive cycle.
The major objective of present study was to demonstrate the actions of exogenous melatonin on ovaprim (synthetic GnRH and domperidone)-induced final oocyte maturation focusing oxidative status of pre-ovulatory follicles in carp Catla catla. Accordingly, gravid carp during early spawning phase of reproductive cycle were injected with melatonin and/or ovaprim at different time intervals, or luzindole (a pharmacological blocker of melatonin receptors) before them. The effects were studied on the latency period, the rate of germinal vesicle breakdown (GVBD; a visual marker of final oocyte maturation) in the oocytes, and the levels of maturation promoting factor (MPF), oxidative stress, different antioxidants, melatonin and MT1 melatonin receptor protein in the extracts of pre-ovulatory follicles as well. Notably, melatonin treatment two hours before ovaprim injection resulted in shortest latency period as well as highest rate of GVBD and MPF formation. Exogenous melatonin, irrespective of injection schedule, caused significant reduction in intra-follicular oxidative stress and an increase in the levels of both enzymatic and non-enzymatic antioxidants, melatonin and its receptor protein. Concentrations of ovarian melatonin in each fish exhibited a significant negative correlation with the level of oxidative stress, but a positive correlation with the rate of GVBD and the activity/level of different antioxidants. However, no significant effects of melatonin and/or ovaprim were detected in luzindole pre-treated carp. Collectively, current study provides the first evidence that melatonin pre-treatment in carp ameliorates ovaprim actions on the process of final oocyte maturation by MPF formation and alleviates oxidative stress in pre-ovulatory follicles by stimulating different antioxidants.
Life evolves in a cyclic environment, and to be successful, organisms must adapt not only to their spatial habitat, but also to their temporal habitat. How do plants and animals determine the time of year so they can anticipate seasonal changes in their habitats? In most cases, day length or photoperiod acts as the principal external cue for determining seasonal activity. For organisms not living at the bottom of the ocean or deep in a cave, day follows night, and the length of the day changes predictably throughout the year. These changes in photoperiod provide the most accurate signal for predicting upcoming seasonal conditions. Measuring day length allows plants and animals to anticipate and adapt to seasonal changes in their environments in order to optimally time key developmental events including seasonal growth and flowering of plants, annual bouts of reproduction, dormancy and migration in insects, and the collapse and re-growth of the reproductive system that drives breeding seasons in mammals and birds. Although research on photoperiodic time measurement originally integrated work on plants and animals, recent work has focused more narrowly and separately on plants, invertebrates, or vertebrates. As the fields have become more specialized there has been less interaction across the broader field of photoperiodism. As a result, researchers in each area often needlessly repeat both theoretical and experimental work. However, over the past decade, intense work on daily and seasonal rhythms in fruit flies, mustard plants, and hamsters and mice, has led to remarkable progress in understanding the phenomenology, as well as the molecular and genetic mechanisms, underlying circadian rhythms and clocks. This book was developed to further this type of cooperation among scientists from all related disciplines.
In fish like in mammals, dopamine (DA) is a major catecholaminergic neurotransmitter that contributes to many functions of the nervous system like sensory perception, tuning of sensori-motor cues, and hypothalamic and pituitary functions. In the eel, DA inhibits gonadal development, and juvenile silver eels remain blocked at a prepubertal stage if their reproductive migration does not occur. From data in other teleosts and vertebrates, it is suggested that DA would be involved also in the last steps of eel reproduction (oocyte maturation, ovulation, and spermiation) as well as in eel reproductive migration (locomotion and olfaction). Investigating dopaminergic systems in the eel may help in understanding the mechanisms of its complex life cycle and provide new data for its conservation and reproduction. In this article we review the biosynthesis and catabolism of catecholamines and discuss available methods to investigate brain dopaminergic systems in vertebrates and their application to the eel. Immunocytochemistry, in situ hybridization, and different tracing methods are used to map dopaminergic neurons and projections in the brain and pituitary and infer their potential functions. Moreover, variations in dopaminergic activity may be approached by means of quantitative methods like quantitative real-time RT-PCR and HPLC. These tools are currently used to study dopaminergic systems in the eel brain, their anatomy, regulation, and potential roles with special emphasis on the regulation of reproduction and reproductive migration.
To assess a possible antistress role of melatonin in fish, we orally administered melatonin to rainbow trout for 10 d and then kept the fish under normal or high stocking density conditions during the last 4 d. Food intake; biochemical parameters in plasma (cortisol, glucose, and lactate concentrations); liver (glucose and glycogen concentrations, and glycogen synthase activity); enzyme activities of amylase, lipase, and protease in foregut and midgut; and content of the hypothalamic neurotransmitters dopamine and serotonin, as well as their oxidized metabolites, 3,4-dihydroxyphenylacetic acid and 5-hydroxy-3-indoleacetic acid, were evaluated under those conditions. High stocking density conditions alone induced changes indicative of stress conditions in plasma cortisol concentrations, liver glycogenolytic potential, the activities of some digestive enzymes, and the 3,4-dihydroxyphenylacetic acid-to-dopamine and 5-hydroxy-3-indoleacetic acid-to-serotonin ratios in the hypothalamus. Melatonin treatment in nonstressed fish induced an increase in liver glycogenolytic potential, increased the activity of some digestive enzymes, and enhanced serotoninergic and dopaminergic metabolism in hypothalamus. The presence of melatonin in stressed fish resulted in a significant interaction with cortisol concentrations in plasma, glycogen content, and glycogen synthase activity in liver and dopaminergic and serotoninergic metabolism in the hypothalamus. In general, the presence of melatonin mitigated several of the effects induced by stress, supporting an antistress role for melatonin in rainbow trout.
1.1. Day-night variations of hypothalamic serotonergic activity were noted in goldfish maintained on a 16L/8D, 20°C regime but were not found in fish kept on an 8L/16D, 20°C regime.2.2. Pinealectomy had a significant effect on hypothalamic serotonin content and serotonergic activity in fish exposed to the long, but not short, photoperiod.3.3. Melatonin administration to control animals (100 μg per day for 10 days) modified the day-night variations of hypothalamic serotonin content and serotonergic activity regardless of photoperiod.4.4. The effects of melatonin on hypothalamic serotonin content in pinealectomized animals were dependent upon the photoperiod regime, whereas effects on serotonergic activity were not.5.5. It is concluded that the goldfish pineal organ and melatonin may play a role in hypothalamic adjustments to changing photoperiods.
Intact females of Oryzias latipes exposed to 14 hours of light per day for 40 days continued to lay eggs for as long as 35 days. Under a similar photoperiod, pinealectomy shortened the period of oviposition to four days and enucleation to seven days. Moreover, the mean gonosomatic indices (GSI: the gonadal weight expressed as a percentage of the body weight) of intact, pineal-ectomized, and blinded female fish under the same photoperiod were 10.56 ± 0.53, 3.77 ± 0.31, and 6.31 ± 0.48, respectively. Fish receiving eight hours of light per day laid eggs for only two to seven days, whether they were intact or operated. The GSI's were slightly larger in both pinealectomized and enucleated females than in intact ones (P < 0.1).
The significant difference in GSI between blinded females subjected to the eight-hour photoperiod (3.45 ± 0.48) suggests the presence of an extraocular photoreceptive organ mediating the effects of light on the gonad.
The shortening of the period of oviposition in pinealectomized fish seems to show the involvement of the pineal in gonadal development and oviposition; probably the pineal functions as the photoreceptor.
The effects of different photoperiod regimes, and total darkness on day-night variations in hypothalamic serotonin (5-HT)
content and turnover index (TI), and monoamine oxidase (MAO) activity that occur exclusively during the gonadal preparatory
phase (February–March) were investigated in female Heteropneustes fossilis. Exposure of the fish to long photoperiods (16L:8D; light between 06.00–22.00h, for 40 days elevated both the midphotophase
and midscotophase values of 5-HT content and MAO, and abolished their day-night differences. The daily variations of 5-HT-TI
was further intensified by the treatment. Under short photoperiods (4L:20D; light between 18.00–22.00h for 40 days), there
was a significant decrease in both 5-HT content and TI, a reversal of the day-night variations of 5-HT content and MAO activity,
and loss of the 5-HT-TI pattern. In fish maintained in total darkness the day-night variations were not found and there was
a significant inhibition of 5-HT. Exposure of the fish to continuous light abolished the day-night variations of these correlates
and elevated their values at both intervals. When the LD cycle was reversed, there was a corresponding shift in the day-night
patterns. The gonadosomatic index (GSI) was significantly elevated in the long photoperiod and continuous illumination groups,
significantly decreased in the short photoperiod and total darkness groups, and unchanged in the reversed LD cycle regime.
The results indicate that the day-night variations of 5-HT and MAO are photoperiod-dependent and are controlled by the prevailing
LD cycle during the gonadal preparatory phase of the annual reproductive cycle. The photoperiod effects on the gonadal activity
may be mediated through the hypothalamic serotonergic system.
Photoperiod, or length of day, has a predictable annual cycle, making it an important cue for the timing of seasonal behavior and development in many organisms. Photoperiod is widely used among temperate and polar animals to regulate the timing of sexual maturation. The proper sensing and interpretation of photoperiod can be tightly tied to an organism's overall fitness. In photoperiodic mammals and birds the thyroid hormone pathway initiates sexual maturation, but the degree to which this pathway is conserved across other vertebrates is not well known. We use the threespine stickleback Gasterosteus aculeatus, as a representative teleost to quantify the photoperiodic response of key genes in the thyroid hormone pathway under controlled laboratory conditions. We find that the photoperiodic responses of the hormones are largely consistent amongst multiple populations, although differences suggest physiological adaptation to various climates. We conclude that the thyroid hormone pathway initiates sexual maturation in response to photoperiod in G. aculeatus, and our results show that more components of this pathway are conserved among mammals, birds, and teleost fish than was previously known. However, additional endocrinology, cell biology and molecular research will be required to define precisely which aspects of the pathway are conserved across vertebrates.
The circadian system of mammals is composed of a hierarchy of oscillators that function at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator throughout the body, yet this clock system is adapted to different functional contexts. In the central suprachiasmatic nucleus (SCN) of the hypothalamus, a coupled population of neuronal circadian oscillators acts as a master pacemaker for the organism to drive rhythms in activity and rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which in turn provides stability to the overall temporal architecture of the organism. Throughout the majority of the cells in the body, cell-autonomous circadian clocks are intimately enmeshed within metabolic pathways. Thus, an emerging view for the adaptive significance of circadian clocks is their fundamental role in orchestrating metabolism.
Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.
Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.
Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes. Request by email to get a copy of the article
Many tropical wrasses show a daily pattern of spawning with gamete release typically near daytime high tide. The environmental cues the fish obtains from day-night and tidal cycles to ensure spawning synchrony and how those cues are transduced, however, are not fully understood. To gain insight into these issues, the involvement of monoamines in mediating endogenous day-night and tidal rhythms in the threespot wrasse, Halichoeres trimaculatus, were examined. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC, a metabolite of DA), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) in the brain of the fish were measured with high-performance liquid chromatography and electrochemical detection. DOPAC and the metabolic rate of DA activity (DOPAC/DA) were found to increase during the day and decrease during the night for fish held under a natural photoperiod. Fish acclimated to a 12:12 light-dark cycle and to constant dark conditions exhibited similar changes, whereas fish acclimated to constant light conditions exhibited little or no change. Intraperitoneal injection of melatonin resulted in a significant reduction in DOPAC/DA. Furthermore, DOPAC/DA was significantly lower in fish held at 3m compared to 0m depth, suggesting that hydrostatic pressure influences DA metabolic rate. These results indicate that light and hydrostatic pressure control dopaminergic turnover in the brain of threespot wrasse. Day-night and tidal changes in these two factors therefore may be the main environmental cues the fish uses to synchronize its spawning activity.
It is possible to manipulate fish gonadal function through exogenous melatonin. However gonadal responses to melatonin vary and depend on time, mode and duration of the administration of the hormone. The present study describes the effects of different photoperiods and melatonin treatments on the ovarian kinetics of the fish Channa punctatus. Fish held in long days (LD 14:10) were injected melatonin daily (10 microg im) either at 08.00 or 17.00 h, respectively or immersed in melatonin water (100 microg/l) for 24 or 15 h daily. In another experiment fish held in long days or continuous light (LL; LD 24.00) were immersed in melatonin water (100 microg/l) either for 24 or 15 h daily. Both experiments had appropriate controls. The GSI (Gonadosomatic index=gonadal wt./100g body weight) increased (P<0.01) in fish immersed in melatonin water daily for 24h when compared either with the GSI of control fish or with fish held in melatonin water daily for 15 h (17.00-08.00 h). The GSI decreased (P<0.01) in fish that received melatonin daily by injection mode. Data from follicular kinetics largely corroborate the GSI data. Fish exposed to melatonin water daily for 24h had more vitellogenic follicles (VF) and fewer atretic follicles (AF). There was a general decrease (P<0.01) in previtellogenic follicles (PVF) in all treated groups. Melatonin by injection mode did not affect the number of VF but it significantly increased (P<0.01) the AF. In the other experiment, the GSI increased (P<0.01) in fish held in long days and immersed in melatonin water 24h daily. However, the GSI decreased in fish held in long days and immersed in melatonin water for a restricted period (between 17.00 and 08.00 h). The GSI of fish held in LL and immersed in melatonin water daily for 24h increased (P<0.01), whereas it decreased (P<0.01) in fish that were immersed in melatonin water daily for 15 h. The data from follicular kinetics revealed a decrease in PVF of fish held in LL and in all the melatonin-exposed groups irrespective of the duration of exposure to melatonin. However the VF increased markedly in fish held in LL and immersed in melatonin water daily for 24h. VF of fish held in LL but immersed in melatonin water daily for restricted period decreased. In fish held in long days and immersed in melatonin water daily either for 24 or 15 h the VF number increased (P<0.01). Marked increase in AF was seen fish held in LL and exposed to melatonin-containing water daily for restricted period. The effect of melatonin on reproduction seems to depend on the photoperiod and duration of exposure to melatonin. In fish held either in long days or LL, daily restricted exposure to melatonin tends to inhibit gonads, whereas continuous exposure to melatonin either stimulate or has no effect. The results apparently support the view that duration of exposure to melatonin signals darkness.
1. The 5-hydroxytryptamine (5-HT, serotonin) concentration in the brain of male specimens of the teleost, Fundulus grandis, was determined for 24-hr periods.2. The mean 5-HT concentrations in whole brains was higher betweeen 8:30 a.m. and 4:30 p.m. than between 8:30 p.m. and 4:30 a.m.3. Male Fundulus grandis exhibited a diurnal swimming activity pattern; maximum activity occurred at 12:30 p.m. and minimum activity was at 12:30 a.m.4. Males injected with 5-HT were less active than the saline-injected controls.5. Male fishes kept in constant darkness for one week had significantly less brain 5-HT than males kept in constant light.6. It is suggested that 5-HT has a normal role in regulating the swimming activity of this fish.
The effects of daily administration of melatonin or ovine prolactin for 20 days were evaluated with respect to the response of the hypophysial-ovarian system in the catfish. Heteropneustes fossilis, during the prespawning and spawning periods. In the prespawning period, daily treatment with melatonin (20 or 100 μg/fish) or prolactin (100 or 250 μg/fish) significantly inhibited vitellogenesis and induced follicular atresia in the ovary, whereas at the level of the pituitary, only the higher dose of melatonin (100 μg/fish) brought about a significant reduction in the number of gonadotrophs. In the spawning period, melatonin (20 or 100 μg/fish) and prolactin (100 or 250 μg/fish) treatment induced significant ovarian regression, and the higher doses of melatonin (100 μg/fish) and prolactin (250 μg/fish) alone significantly reduced the number of pituitary gonadotrophs. Cessation of treatment with melatonin or prolactin resulted in restoration of activity in the pituitary and ovaries. Evidence is presented to show that the inhibition exerted by melatonin or prolactin is tentative and that this inhibitory influence is abolished following withdrawal of treatment.
Immunoreactive serotonin (ir-5HT) containing cells were localized in the brain and pituitary gland of the platyfish by use of immunoperoxidase procedures. In the brain, ir-neurons were found lining the wall of the third ventricle and in its lateral and posterior recesses. More caudally, ir-perikarya were found in the valvular portion of the cerebellum and in the raphe region. Ir-5HT was also localized within the pineal gland in fish that had been sacrificed before 1:00 p.m. Within the pituitary gland, ir-5HT was localized in periodic acid Schiff-positive cells of the pars intermedia of all fish while, in only a few animals, less intense immunoreactivity was also present in gonadotrophs of the caudal pars distalis.
Melatonin-treated Fundulus similis collected in January or May and maintained on a long photoperiod had significantly smaller gonads than control animals. The effect of melatonin was independent of the time of day that it was administered. Melatonin treatment of F. similis maintained on a short photoperiod in May retarded testicular enlargement; melatonin treatment of animals on a short photoperiod in January was without effect. The gonadal enlargement seen in control Hyla cinerea maintained on a long photoperiod in December was inhibited by melatonin.
In Heteropneustes fossilis, contents and turnovers of hypothalamic catecholamines (CA) and activities of dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT) showed significant seasonal variations with significantly high day values. The seasonal pattern of dopamine (DA) on one hand and that of noradrenaline (NA) and adrenaline (A) on the other hand showed an inverse relationship, the former decreasing and the latter increasing during the progress of gonadal recrudescence. The DBH and PNMT levels were low in the resting phase and increased to the peak in the prespawning (DBH) and spawning (PNMT) phases. Maintenance of the fish under long photoperiods (16L:8D) and high temperature (28 +/- 2 degrees) stimulated the NA and A, and DBH and PNMT activities, and suppressed the DA mechanism, the changes being maximal in the raised temperature groups. In the resting phase (December), ovariectomy (OVX) or estradiol-17 beta (E2) replacement in 4-week ovariectomized fish did not produce any significant effects on the CA and enzyme activities. On the contrary, in the prespawning phase (May), OVX produced differential and biphasic responses on CA and the enzymes. The contents and turnovers of both NA and A increased significantly at 2-5 weeks and decreased in the sixth week. However, the reverse was true for DA. The DBH and PNMT activities (assayed only 3, 4, and 6 weeks after OVX) were elevated significantly in the third and fourth weeks but decreased in the sixth week. Plasma levels of gonadotropin (GTH) increased significantly at all durations of OVX in a bimodal pattern while the E2 levels decreased consistently. Supplementation with a low dose (0.1 microgram/g BW) of E2 restored the NA and A and enzyme activities while the higher doses (0.5, 1.0, and 5.0 micrograms/g BW) depleted them. The reverse was true for DA. The low dose of E2 restored the GTH level while the higher ones inhibited it significantly. These results indicate that both environmental photoperiod and temperature and E2-negative feedback act on the CA to modulate GTH secretion.
The effects of serotonin (5-hydroxytryptamine; 5-HT) alone, and in combination with luteinizing hormone-releasing hormone analog (LHRHa), on plasma levels of maturational gonadotropin (GtH II) in the post-vitellogenic female and spermiating male Atlantic croaker were investigated during mid-light, late-light, mid-dark, and late-dark phases of the day-night (12 hr light: 12 hr dark) cycle. Although LHRHa stimulated GtH II secretion throughout the day-night cycle, maximal stimulation was observed during the mid-dark phase. Serotonin significantly potentiated LHRHa-induced GtH II secretion at all time points tested except during the late-dark phase. The plasma GtH II response to LHRHa was markedly reduced, and the response to 5-HT was abolished in regressed Atlantic croaker. Pretreatment of the fish with a 5-HT2 antagonist, LY 53,857 maleate, almost completely abolished the stimulatory effect of 5-HT on LHRHa-induced GtH II secretion, whereas a 5-HT1 antagonist, methiothepin mesylate, failed to influence the stimulatory effect of 5-HT on GtH II secretion. In addition, a 5-HT2 agonist, (+/-) DOI hydrochloride, potentiated the effect of 5-HT on basal and LHRHa-induced GtH II secretion, while a 5-HT1 agonist, m-chlorophenylpiperazine dihydrochloride, did not. The results clearly demonstrate time- and season-dependent stimulatory effects of LHRHa and 5-HT on plasma GtH II levels in Atlantic croaker. Moreover, the stimulatory effect of 5-HT appears to be mediated by 5-HT2 receptors in this species.
In Heteropneustes fossilis, significant annual variations in serotonin (5-HT) and monoamine oxidase (MAO) were found in the hypothalamus both at 12 and 24 hr with high values of content and turnover index (TI) during gonadal recrudescence, and low values during gonadal quiescence. The 5-HT content reached high levels during April-July (late preparatory, prespawning, and spawning phases). The TI of 5-HT showed two peaks in its midday value; a major peak occurred in March (midpreparatory phase) and a minor one in June (prespawning phase). High MAO activity was seen in June (prespawning phase) and low values during February, March, and April (preparatory phase). Significant day-night variations in 5-HT content and TI, and MAO activity were observed in the hypothalamus and telencephalon in February and March (early and midpreparatory phase), but not at other times of the year. In the whole brain and separate regions such as the thalamus, midbrain, and hindbrain no day-night patterns were seen. A comparison of the data shows that the increase in 5-HT content and TI from February to March was significantly higher in the hypothalamus than in telencephalon and the opposite is true for MAO. The results suggest that the high hypothalamic activities of 5-HT and MAO during recrudescence is related to breeding activity; the day-night variations during the early and midpreparatory phase in these variables may be related to initiation of breeding in this species.
Breeding in the three-spined stickleback is stimulated by long but not by short photoperiods in many seasons. The aim of the present study was to test the hypothesis that melatonin plays a role in the inhibitory effect of short photoperiod in this species. Adult nonbreeding males were kept either under constant light (Experiment 1) or under a stimulatory long photoperiod (16L 8D, Experiment 2), in water containing 0, 20, or 80 microg/liter melatonin for 16 hr/day for 28 days during the spring. These melatonin treatments were intended to simulate the daily melatonin pattern of a nonstimulatory short photoperiod. In the second experiment, fish were also kept under a nonstimulatory short photoperiod (8L 16D). In the natural breeding season the only germ cells found in the stickleback testes are spermatozoa and spermatogonia, a condition found in many fish under all treatments. In the first experiment, spermatogenesis was not influenced by melatonin. However, testes also containing spermatocytes and spermatids were more common in fish kept under 8L 16D and fish treated with 80 microg/liter melatonin than in 16L 8D controls in Experiment 2. Kidney hypertrophy, an androgen-dependent male secondary sexual characteristic in the stickleback, appeared in most males kept under constant light or 16L 8D and was not influenced by melatonin treatment. In contrast, control males kept under 8L 16D in Experiment 2 did not display kidney hypertrophy. Therefore, the presence of an extended period of elevated melatonin did not prevent the stimulatory effects of long photoperiod on development of this secondary sexual characteristic in the stickleback.
We report the distribution of serotonin immunoreactive (5-HT-ir) structures in the brain of the adult Senegalese sole, Solea senegalensis, using the streptavidin-biotin-peroxidase complex immunohistochemical method. We have found a wide distribution of immunoreactive fibers throughout the entire brain. 5-HT-ir cell bodies appeared restricted to some periventricular nuclei associated with the diencephalic recesses, and in the rhombencephalic reticular formation and inferior olivary region. Specifically, cerebrospinal fluid-contacting serotoninergic cells were found within the pars dorsalis and pars ventralis of the nucleus recessus lateralis, in the paraventricular organ and in the nucleus recessus posterioris. In the brainstem, 5-HT-ir perikarya appear within the superior and inferior raphe, the nucleus reticularis superioris, the nucleus interpeduncularis and the inferior olive. Although positive fibers were not found in the neurohypophysis, a few 5-HT-ir cells were identified in the adenohypophysis. This distribution is compared with those found in other fishes and discussed in the context of putative roles of 5-HT as a neuroendocrine factor and neurotransmitter in the Senegalese sole.
Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute a small family of monooxygenases that utilize tetrahydropterins as substrates. When from eukaryotic sources, these enzymes are composed of a homologous catalytic domain to which are attached discrete N-terminal regulatory domains and short C-terminal tetramerization domains, whereas the bacterial enzymes lack the N-terminal and C-terminal domains. Each enzyme contains a single ferrous iron atom bound to two histidines and a glutamate. Recent mechanistic studies have begun to provide insights into the mechanisms of oxygen activation and hydroxylation. Although the hydroxylating intermediate in these enzymes has not been identified, the iron is likely to be involved. Reversible phosphorylation of serine residues in the regulatory domains affects the activities of all three enzymes. In addition, phenylalanine hydroxylase is allosterically regulated by its substrates, phenylalanine and tetrahydrobiopterin.
The pineal organ and its secretory product melatonin are regarded as synchronizers of daily rhythms to the external light/dark (LD) cycle. In fish, the pineal organ acts as a direct photoreceptor, transducing light information into neural and humoral (melatonin) signals. In the present study, we investigate a possible role for the pineal organ and melatonin in the regulation of feeding rhythms of rainbow trout, Oncorhynchus mykiss. We used individual rainbow trout placed in an insulated room at constant temperature (14 degrees C). Fish were self-fed ad lib by means of self-feeders coupled to a computer that continuously recorded demand-feeding activity. Before and after pinealectomy, the fish were exposed to a LD cycle of 16:8 h and then constant light (LL) to test the effect of pinealectomy on demand-feeding rhythms. Feeding records revealed that trout fed exclusively during daytime (96% of feeding confined to the light phase), and that removal of the pineal organ did not disrupt this daily feeding profile, with synchronization to the LD cycle persisting. Moreover, the appearance of circadian feeding rhythms was not affected by pinealectomy: most of the operated fish free-ran with an average tau longer than 24 h. Plasma melatonin rhythms persisted in the pinealectomized trout, but with small amplitude. These results suggest that the pineal may not be the site of the pacemaker that controls feeding rhythms in trout, although further research is required to study the involvement of other photoperiod-transducing systems and melatonin (of nonpineal origin) in the regulation and expression of circadian rhythms in this species.
Exposure of female catfish, Heteropneustes fossilis to 30-day regimes of long photoperiod (16L), elevated temperature (28 +/- 2 degrees C), or a combination of both stimulated brain tyrosine hydroxylase (TH) activity significantly over that of control fish held in natural conditions in gonad resting (10.5L:13.5D, 10 +/- 2 degrees C) and preparatory (12.5L:11.5D,18 +/- 2 degrees C) phases. The response was high in the combination group in both phases. The increase in TH activity was higher in forebrain regions (telencephalon and hypothalamus) than medulla oblongata. Exposure of the fish to short photoperiod (8L:16D) and total darkness decreased the enzyme activity significantly in both resting and preparatory phases regardless of the temperature. The inhibition was high in fish held under total darkness. Gonadosomatic index (GSI) was significantly elevated in long photoperiod and high temperature groups, alone or in combination, and decreased significantly in short photoperiod (only in preparatory phase) and total darkness groups. Administration of the serotonin precursor 5-hydroxytryptophan (5-HTP; 5mg/100g body weight [BW], three daily intraperitoneal [i.p.] injections prior to sacrificing) stimulated TH activity in fish held under long and normal photoperiods in both phases. Three daily injections of the serotonin blocker parachlorophenylalanine (p-CPA; 10mg/100g BW) and melatonin (75 microg/100g BW) prior to sacrificing inhibited brain TH activity significantly in both phases. GSI was significantly stimulated by 5-HTP, and inhibited by both p-CPA and melatonin injections. Changes in TH activity and GSI can be correlated and explained on the basis of previous reports on changes in catecholamine activity that modulates gonadotropin secretion in the catfish. Further, the photoperiod and temperature-induced changes in TH activity may be modulated by alterations in serotonergic activity.
Fish plays an important role in the Bangladeshi diet, constituting the main animal food in rural households. Fish consumption is dominated by wild small (length<25 cm) indigenous fish species (SIS). A study was conducted to evaluate the dietary contribution of vitamin A and calcium from fish in rural Bangladesh. Seasonal fish consumption was surveyed in 84 rural households in Kishoreganj district using a 5 day recall method. Fifty-nine of the households practised aquaculture, producing carps and SIS in small ponds. The intra-household distribution of a standardised fish dish was surveyed in 20 households by weighing. The contribution of vitamin A and calcium from fish was calculated as a nutrient contribution ratio, expressing the nutrient intake from fish relative to the recommended intake at household level. The mean amount of fish consumed in all households was 37 g raw edible parts/person/day (median=27) in July 1997. 82 g raw edible parts/person/day (median=64) in October 1997, and 55 g raw edible parts/person/day (median=42) in February 1998. A total of 44 fish species was recorded in the diet. SIS contributed 84% of the total fish intake. Fish bought in the local markets was the most important source of fish in both fish-producing and non-fish-producing households. The intra-household fish distribution showed that the heads of the households (all males) were favoured over females and other males. In October, the; mean nutrient contribution ratio for vitamin A was 40% (median=23%) and for calcium was 32% (median=26%). SIS contributed more than 99% of vitamin A and calcium intakes from the total fish intake. Mola, a vitamin A-rich SIS, was produced in 34 of the fish-producing households. Mola harvested from the ponds and consumed in the households contributed, on average, 21% (median=18%) of the household recommended intake of vitamin A in a 7 month period. SIS is an important and perhaps irreplaceable dietary source of vitamin A and calcium, and promotion of the availability and accessibility of SIS for the population in rural Bangladesh should be given priority.
Pineal organ and its hormone melatonin (N-acetyl-5-methoxytryptamine) is likely involved in timing and synchronisation of many internal processes, such as reproduction, with annual changes in environmental cues, i.e., photoperiod and water temperature. The seasonal changes in melatonin profile in stickleback brains related to the following reproductive phases were examined, and the link between melatonin concentrations and the stages of spawning cycle was analysed. Two wild populations of sticklebacks were exposed to annual environmental changes in their natural habitats. Brains, gonads, kidneys and livers were collected over 2 years. Melatonin was measured using RIA and the indices, gonadosomatic (GSI), nephrosomatic (NSI) and hepatosomatic (HSI), were calculated. The role of melatonin, as a component of internal calendar engaged in the control of seasonal breeding in this species, is discussed. The extremely high melatonin levels observed in early spring (March) and autumn (October) seem to mark out a time frame for spawning in sticklebacks. The seasonal pattern of melatonin production and identified development stages of gonads suggests the potential inhibitory effect of the hormone on stickleback reproduction in shortening photoperiod and stimulatory effect in lengthening photoperiod.