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Molecular diversity of neocortical GABAergic interneurones
Abstract
In addition to being the main source of inhibition in the adult brain, GABAergic interneurones are instrumental in pacing the activity of large ensembles of principal cells. GABAergic interneurones have unique features that enable them to contribute to the generation of synchronized network activity thereby shaping principal cell behaviour. Whereas the anatomical and physiological characteristics of certain interneuronal types have been studied extensively over the last decades, the molecular diversity of interneurones is a more recent focus of investigation in this field. Molecular cloning and expression analysis of many receptor families often revealed differential expression in GABAergic interneurones and pyramidal cells. Here we review recent findings regarding the molecular diversity of GABAergic interneurones in the neocortex. Better knowledge about differential gene expression in GABAergic interneurones is the basis for further investigations aimed at understanding the contribution of specific proteins in interneurones to network function.
... Most of the variety in cortical neuron types is seen among interneurons, although they are numerically a minority (DeFelipe & Jones, 1988). The range of morphologies, physiological properties and innervation targets of these interneurons powerfully shapes the response properties of the cortex, allowing incoming activity to evoke diverse responses for use in cortical processing (White, Benshalom, & Hersch, 1984;Gupta, Wang, & Markram, 2000;Blatow, Caputi, & Monyer, 2005;Huang, Di Cristo, & Ango, 2007). ...
... On the postsynaptic side, there are systematic differences in interneuron receptor properties. For example, interneuron AMPA receptors have a characteristic subunit makeup (in rat: Kondo et al., 2000) and are typically less calcium-permeant than those of pyramidal neurons (reviewed by Blatow, Caputi, & Monyer, 2005). ...
... The only fundamental distinction is between the major lineages: Cajal-Retzius neurons from the pallial margins, interneurons from the subpallial ganglionic eminences, and principal neurons from pallial radial glia. Within these key populations, a range of gene expression repertoires and connectional preferences is possible (Blatow, Caputi, & Monyer, 2005;Cobos et al. 2006;Fishell, 2007), and certain features tend to cluster together, perhaps under the influence of shared promoters (Cobos et al. 2006;e.g. in rat: Toledo-Rodriguez et al., 2004). To this end, promoter-based labelling (e.g. ...
NOTE: This chapter gets few tracked citations, most people just cite the book. If you find it useful, please cite it as:
Kirkcaldie MTK. Neocortex. In: Watson CR, Paxinos G, Puelles L, editors. The Mouse Nervous System. Academic Press; 2012. pp. 52–111.
Thank you!
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A comprehensive review of the cells, structure and organisation of the mouse neocortex. The major classifications of principal neurons and interneurons are reviewed, including morphology, histochemical markers and developmental origins. Laminar, connectional and regional organisation and the relationship with thalamic nuclei are also described. The 750 references are chosen for mouse-specific data, rather than descriptions of other species or generic "mammal cortex".
... Another school of thought believes that the simple dichotomy of excitation and inhibition from the deterministic view is misleading because it can originate from simple experimental designs that have limited application to the natural physiology of the cortex (Fritschy, 2008). Findings that inhibition is important for both triggering and regulating plasticity (Blatow et al., 2005;Floyer-Lea et al., 2006;Frangou et al., 2019) argue against the deterministic view, suggesting that the direction of the shift is not important. Instead, the view advocates that if the change in E/I balance is significantly different from the normal range of transient fluctuation of the balance across time, it can be physiologically purposeful (Dehghani et al., 2016;Steel et al., 2020). ...
... Our understanding of how a shift of E/I balance in a particular direction can affect neural plasticity remains to be elucidated because animal studies show that both excitation and inhibition are important for both promoting and regulating plasticity. The sheer diversity of GABAergic interneurons in the cortex and their sophisticated functional organizations argue against the deterministic view (Blatow et al., 2005), which advocates that an increase in excitation unilaterally potentiates plasticity while its reduction diminishes it. Instead, E/I balance is a complex, dynamic status where the roles of excitation and inhibition in the cortex can be multifarious. ...
An interlude of dark exposure for about 1 week is known to shift excitatory/inhibitory (E/I) balance of the mammalian visual cortex, promoting plasticity and accelerating visual recovery in animals that have experienced cortical lesions during development. However, the translational impact of our understanding of dark exposure from animal studies to humans remains elusive. Here, we used magnetic resonance spectroscopy as a probe for E/I balance in the primary visual cortex (V1) to determine the effect of 60 min of dark exposure, and measured binocular combination as a behavioural assay to assess visual plasticity in 14 normally sighted human adults. To induce neuroplastic changes in the observers, we introduced 60 min of monocular deprivation, which is known to temporarily shift sensory eye balance in favour of the previously deprived eye. We report that prior dark exposure for 60 min strengthens local excitability in V1 and boosts visual plasticity in normal adults. However, we show that it does not promote plasticity in amblyopic adults. Nevertheless, our findings are surprising, given the fact that the interlude is very brief. Interestingly, we find that the increased concentration of the excitatory neurotransmitter is not strongly correlated with the enhanced functional plasticity. Instead, the absolute degree of change in its concentration is related to the boost, suggesting that the dichotomy of cortical excitation and inhibition might not explain the physiological basis of plasticity in humans. We present the first evidence that an environmental manipulation that shifts cortical E/I balance can also act as a metaplastic facilitator for visual plasticity in humans. image
Key points
A brief interlude (60 min) of dark exposure increased the local concentration of glutamine/glutamate but not that of GABA in the visual cortex of adult humans.
After dark exposure, the degree of the shift in sensory eye dominance in favour of the previously deprived eye from short‐term monocular deprivation was larger than that from only monocular deprivation.
The neurochemical and behavioural measures were associated: the magnitude of the shift in the concentration of glutamine/glutamate was correlated with the boost in perceptual plasticity after dark exposure.
Surprisingly, the increase in the concentration of glutamine/glutamate was not correlated with the perceptual boost after dark exposure, suggesting that the physiological mechanism of how E/I balance regulates plasticity is not deterministic. In other words, an increased excitation did not unilaterally promote plasticity.
... Data about expression of metabotropic glutamate receptors in cortical GABAergic interneurons are sparse. Only part of SOM+, CB+ and CR+ neurons express mGluR1a while this subunit was not demonstrated in PV+ interneurons [79]. GABA A receptor subunits alpha1 and beta 2-3 are expressed in FS cells [79] One of the neuromodulatory sources of activity onto the GABAergic interneuron is through nicotinic acetylcholine signalling. ...
... Only part of SOM+, CB+ and CR+ neurons express mGluR1a while this subunit was not demonstrated in PV+ interneurons [79]. GABA A receptor subunits alpha1 and beta 2-3 are expressed in FS cells [79] One of the neuromodulatory sources of activity onto the GABAergic interneuron is through nicotinic acetylcholine signalling. Nicotinic signalling plays important role in cognitive and memory processes. ...
The calcium binding protein parvalbumin (PV) in the mammalian neocortex is expressed in a subpopulation of cortical GABAergic inhibitory interneurons. PV – producing interneurons represent the largest subpopulation of neocortical inhibitory cells, exhibit mutual chemical and electrical synaptic contacts and are well known to generate gamma oscillation. This review summarizes basic data of the distribution, afferent and efferent connections and physiological properties of parvalbumin expressing neurons in the neocortex. Basic data about participation of PV-positive neurons in cortical microcircuits are presented. Autaptic connections, metabolism and perineuronal nets (PNN) of PV positive neurons are also discussed.
... In the neocortex, the exact location at the synapse (or extrasynaptically) of the different types of receptors as well as their subunit composition and their level of expression are relatively unknown, though they are likely to be area and cell-type specific (Blatow et al., 2005). Of interesting note, it has been shown that in contrast to glutamatergic synapses located on excitatory neurons, glutamatergic synapses located on inhibitory neurons express a type of AMPA receptor that does not contain the GluR2 subunit, therefore rendering it permeable to Ca 2+ in addition to Na + and K + (Hestrin, 1993;Jonas et al., 1994). ...
... Of interesting note, it has been shown that in contrast to glutamatergic synapses located on excitatory neurons, glutamatergic synapses located on inhibitory neurons express a type of AMPA receptor that does not contain the GluR2 subunit, therefore rendering it permeable to Ca 2+ in addition to Na + and K + (Hestrin, 1993;Jonas et al., 1994). Furthermore, these excitatory synapses show little contribution of the NMDA current to the postsynaptic membrane potential depolarization (Thomson et al., 1996), though NMDA receptor mRNA is found in the cytoplasm of inhibitory neurons (Blatow et al., 2005). ...
The neocortex is the most distinctive feature of the mammalian brain and it is considered to be the substrate of high-order cognitive functions. The nature and the arrangement of the diverse neuronal elements constituting its multiple areas have received longstanding attention. Progressively such anatomical and functional investigations are undertaken in the context of an intact living being. It offers the possibility of examining the role of particular areas, networks or even individual neurons during various behavioral states. Synaptic connectivity and synaptic transmission have been traditionally investigated in reduced preparations. Typically, electrophysiological and optical techniques have been used to control and record the propagation of electrical activity between two or more neurons in acute brain slices in vitro. The purpose of this thesis is the investigation of synaptic connectivity and synaptic transmission within the intact neocortex of the living mouse. Here I took advantage of the recent development of optogenetics, in combination with electrophysiology and two-photon microscopy to systematically and directly record synaptic transmission between a single excitatory neuron and two main types of GABAergic neurons in layer 2/3 of the mouse barrel cortex in vivo. Overall, I discovered stronger excitatory connections onto GABAergic neurons than onto excitatory neurons, irrespective of the absolute or relative locations of the pre- and postsynaptic neurons somas. I further revealed that parvalbumin-expressing (PV) and somatostatin-expressing (Sst) GABAergic neurons received excitatory inputs that were similar in magnitude, but were more reliable and faster in PV neurons than in Sst neurons. Exploring postsynaptic responses to multiple presynaptic action potentials elicited at high frequency, I found a strong short-term facilitation accompanied by significant input summation in Sst neurons, but little short-term dynamics with no summation in PV neurons. Lastly, I compared the amplitude of single action potential-evoked postsynaptic responses as a function of neocortical activity level and found that it was unchanged in both neuron types. Overall, the results of this thesis provide new insights into the functioning of microcircuits in vivo while confirming many findings from reduced preparations. In the future, it will be interesting to extend these initial in vivo measurements to other neuron and synapse types, particularly in awake animals engaged in different behavioral states.
... This hypothesis is intriguing, but direct evidence of how NMDAR antagonists selectively act on specific populations of PFC interneurons remains unclear. In fact, little is known regarding the properties of NMDARs in cortical interneurons (Blatow et al, 2005). Some evidence suggests that NMDAR-mediated excitatory responses exist in cortical interneurons, but these findings are controversial. ...
... Although numerous studies have reported a developmental change in NMDARs from different cortical regions, most of the studies have focused on NMDAR functions in pyramidal cells (Monyer et al, 1994; Kumar and Huguenard, 2003; Liu et al, 2004). Little is known regarding the NMDARs in cortical interneurons (Blatow et al, 2005). To record the AMPA and NMDA currents, we applied whole-cell patch clamp to individual interneurons. ...
In the prefrontal cortex, N-methyl-D-aspartic acid (NMDA) receptors (NMDARs) are critical not only for normal prefrontal functions but also for the pathological processes of schizophrenia. Little is known, however, about the developmental properties of NMDARs in the functionally diverse sub-populations of interneurons. We investigated the developmental changes of NMDARs in rat prefrontal interneurons using patch clamp recording in cortical slices. We found that fast-spiking (FS) interneurons exhibited properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA currents distinct from those in regular spiking (RS) and low-threshold spiking (LTS) interneurons, particularly during the adolescent period. In juvenile animals, most (73%) of the FS cells demonstrated both AMPA and NMDA currents. The NMDA currents, however, gradually became undetectable during cortical development, with most (74%) of the FS cells exhibiting no NMDA current in adults. In contrast, AMPA and NMDA currents in RS and LTS interneurons were relatively stable, without significant changes from juveniles to adults. Moreover, even in FS cells with NMDA currents, the NMDA/AMPA ratio dramatically decreased during the adolescent period but returned to juvenile level in adults, compared with the relatively stable ratios in RS and LTS interneurons. These data suggest that FS interneurons in the prefrontal cortex undergo dramatic changes in glutamatergic receptors during the adolescent period. These properties may make FS cells particularly sensitive and vulnerable to epigenetic stimulation, thus contributing to the onset of many psychiatric disorders, including schizophrenia.
... At the cortical level, inhibition is largely mediated via activation of gamma-aminobutyric acid (GABA) receptors. GABA is a principal inhibitory neurotransmitter in the brain tissue of mammalians with a rich structural diversity of receptors and a dense representation of GABA-ergic interneurons in the neocortex (e.g., [9]). For example, GABA-ergic activity in the primary motor cortex (M1) has been reported to play a prominent role in the fine tuning of corticospinal excitability during movement initiation and movement withdrawal [2] or during movement preparation [5,10]. ...
This review summarizes behavioral and neurophysiological aspects of inhibitory control affected by a single bout of cardiovascular exercise. The review also examines the effect of a single bout of cardiovascular exercise on these processes in young adults with a focus on the functioning of prefrontal pathways (including the left dorsolateral prefrontal cortex (DLPFC) and elements of the prefrontal-basal ganglia pathways). Finally, the review offers an overview on the potential effects of cardiovascular exercise on GABA-ergic and glutamatergic neurotransmission in the adult brain and propose mechanisms or processes that may mediate these effects. The main findings show that a single bout of cardiovascular exercise can enhance inhibitory control. In addition, acute exercise appears to facilitate activation of prefrontal brain regions that regulate excitatory and inhibitory pathways (specifically but not exclusively the prefrontal-basal-ganglia pathways) which appear to be impaired in older age. Based on the reviewed studies, we suggest that future work examine the beneficial effects of exercise on the inhibitory networks in the aging brain.
... Those that express PV frequently display a non-adapting firing pattern, while those that express CCK often exhibit an adapting firing pattern (Kawaguchi, 1995;Kawaguchi and Kondo, 2002;Povysheva et al., 2007). CCK expressing cells may also posses receptors such as 5 -HT3, nicotinic (a7), mGluRl and the cannabinoid type 1 (CBl) receptor, which allows them to be modulated by additional transmitters (Blatow et al., 2005;Bodor et al., 2005). ...
Interneurons are a highly diverse population of cell that mediate inhibition within the neocortex, while a relatively uniform population - pyramidal cells - provide excitation. The modulation of neurotransmission between subtypes of interneuron and pyramidal cells by cannabinoids and NMDA receptors was investigated. Dual whole-cell recordings from interneurons and pyramidal cells were obtained from layers II-V of rat (postnatal days 17-22) sensorimotor cortex. During recording, neurons were filled with biocytin for subsequent histological processing to recover the neuroanatomy of recorded cells, allowing for reconstruction and analysis. At excitatory connections, the response in amplitude of EPSPs received by interneurons as a reaction to changes in the holding potential of the postsynaptic cell seemed to coincide with the type of interneuron. NMDA receptors appeared to contribute to EPSPs received by all cell types. Connections from interneurons to pyramidal cells either underwent a DSI protocol during control conditions and in the presence of the CB1 receptor inverse agonist/antagonist AM 251 (8 μM), or were recorded in the presence of the non-selective cannabinoid receptor agonist anandamide (9 μM), followed by AM 251. The reduction in average IPSP amplitude at connections sensitive to DSI was blocked by AM 251, indicating that the effects of DSI were mediated by the CB1 receptor. AM 251 significantly increased the average IPSP amplitude from control in half of the connections tested, suggesting that there is either a tonic activation of CB1 receptors or that they are constitutively active at these pairs. AM 251 did not increase the average IPSP amplitude in two thirds of anandamide sensitive connections, implying that the reduction in average IPSP amplitude was due to a CB1 receptor independent mechanism. To investigate as to whether CB2 receptors were involved in modulating GABAergic inhibition within the neocortex, spontaneous IPSPs were recorded from layer II/III pyramidal cells and challenged with cannabinoid receptor ligands. The results suggest that CB2 receptors modulate GABAA receptor mediated inhibition within the neocortex.
... In rodent brains GABAergic neurons represent only 10%-15% of the entire cortical neuronal population and are the primary source of inhibition (Meyer et al., 2011). This class of neurons is composed of molecularly and morphologically heterogeneous subgroups (for review see, Blatow et al., 2005;Ascoli et al., 2008;Xu et al., 2010;Rudy et al., 2011;Caputi et al., 2013;DeFelipe et al., 2013;Tremblay et al., 2016). The seminal studies by Ramon y Cajal established the presence of a large number of cells with ''short-axons'' in the brain (Ramon Y Cajal et al., 1988). ...
Previous studies have shown that parvalbumin-expressing neurons (CC-Parv neurons) connect the two hemispheres of motor and sensory areas via the corpus callosum, and are a functional part of the cortical circuit. Here we test the hypothesis that layer 5 CC-Parv neurons possess anatomical and molecular mechanisms which dampen excitability and modulate the gating of interhemispheric inhibition. In order to investigate this hypothesis we use viral tracing to determine the anatomical and electrophysiological properties of layer 5 CC-Parv and parvalbumin-expressing (Parv) neurons of the mouse auditory cortex (AC). Here we show that layer 5 CC-Parv neurons had larger dendritic fields characterized by longer dendrites that branched farther from the soma, whereas layer 5 Parv neurons had smaller dendritic fields characterized by shorter dendrites that branched nearer to the soma. The layer 5 CC-Parv neurons are characterized by delayed action potential (AP) responses to threshold currents, lower firing rates, and lower instantaneous frequencies compared to the layer 5 Parv neurons. Kv1.1 containing K+ channels are the main source of the AP repolarization of the layer 5 CC-Parv and have a major role in determining both the spike delayed response, firing rate and instantaneous frequency of these neurons.
... Unlike acute drug effects, which occur in an intact system, epileptogenesis and recurrent seizures in chronic epilepsy likely refl ect pathological disturbances of neuronal circuits that may have multiple origins. Furthermore, the simple view that GABAergic transmission acts like a break preventing overexcitation of neuronal circuits has been challenged by the highly sophisticated anatomical and functional organization of GABAergic interneurons in cerebral cortex (Blatow et al., 2005;Markram et al., 2004). Rather, GABAergic function is required for fi netuning of neuronal circuits and its infl uence on cell fi ring and network oscillations is constrained spatially and temporally (Mann and Paulsen, 2007;Tukker et al., 2007). ...
GABA(A) receptors mediate most of the fast inhibitory transmission in the CNS. They form heteromeric complexes assembled from a large family of subunit genes. The existence of multiple GABA(A) receptor subtypes differing in subunit composition, localization and functional properties underlies their role for fine-tuning of neuronal circuits and genesis of network oscillations. The differential regulation of GABA(A) receptor subtypes represents a major facet of homeostatic synaptic plasticity and contributes to the excitation/inhibition (E/I) balance under physiological conditions and upon pathological challenges. The purpose of this review is to discuss recent findings highlighting the significance of GABA(A) receptor heterogeneity for the concept of E/I balance and its relevance for epilepsy. Specifically, we address the following issues: (1) role for tonic inhibition, mediated by extrasynaptic GABA(A) receptors, for controlling neuronal excitability; (2) significance of chloride ion transport for maintenance of the E/I balance in adult brain; and (3) molecular mechanisms underlying GABA(A) receptor regulation (trafficking, posttranslational modification, gene transcription) that are important for homoeostatic plasticity. Finally, the relevance of these findings is discussed in light of the involvement of GABA(A) receptors in epileptic disorders, based on recent experimental studies of temporal lobe epilepsy (TLE) and absence seizures and on the identification of mutations in GABA(A) receptor subunit genes underlying familial forms of epilepsy.
... At the cortical level, motor inhibition is largely mediated via activation of gammaaminobutyric acid (GABA) receptors. GABA is a principal inhibitory neurotransmitter in the brain tissue of mammalians with a rich structural diversity of receptors and a dense representation of GABA-ergic interneurons in the neocortex (e.g., Blatow et al., 2005). Deficiencies or abnormalities in GABA-ergic activity have been documented in many cognitive and/or M a n u s c r i p t 5 suggest that age differences in interhemispheric connectivity affect functional brain activity (Langan et al., 2010). ...
The ability to inhibit actions, one of the hallmarks of human motor control, appears to decline with advancing age. Evidence for a link between changes in inhibitory functions and poor motor performance in healthy older adults has recently become available with transcranial magnetic stimulation (TMS). Overall, these studies indicate that the capacity to modulate intracortical (ICI) and interhemispheric (IHI) inhibition is preserved in high-performing older individuals. In contrast, older individuals exhibiting motor slowing and a declined ability to coordinate movement appear to show a reduced capability to modulate GABA-mediated inhibitory processes. As a decline in the integrity of the GABA-ergic inhibitory processes may emerge due to age-related loss of white and gray matter, a promising direction for future research would be to correlate individual differences in structural and/or functional integrity of principal brain networks with observed changes in inhibitory processes within cortico-cortical, interhemispheric, and/or corticospinal pathways. Finally, we underscore the possible links between reduced inhibitory functions and age-related changes in brain activation patterns.
... As in the cerebral cortex (Freund and Buzsaki, 1996;Markram et al., 2004;Blatow et al., 2005;Somogyi and Klausberger, 2005), different subtypes of BLA interneurons regulate the activity of principal cells in distinct ways because they target different postsynaptic domains (Muller et al., 2003(Muller et al., , 2006(Muller et al., , 2007Bienvenu et al., 2012) and receive contrasting sets of inputs (Smith et al., 2000). For instance, parvalbumin-immunoreactive (PV + ) interneurons appear to be mainly involved in feedback inhibition: they receive strong excitatory inputs from BLA projection cells, but very few from the cerebral cortex (Smith et al., 2000) and they form numerous inhibitory synapses on the somatic, axonal, and proximal dendritic domains of projection neurons (Pitkänen and Amaral, 1993;Sorvari et al., 1995;Smith et al., 1998;Kemppainen and Pitkänen, 2000;McDonald and Betette, 2001; reviewed in Spampanato et al., 2011). ...
The present study was undertaken to shed light on the synaptic organization of the rat basolateral amygdala (BLA). The BLA contains multiple types of GABAergic interneurons that are differentially connected with extrinsic afferents and other BLA cells. Previously, it was reported that parvalbumin immunoreactive (PV(+) ) interneurons receive strong excitatory inputs from principal BLA cells but very few cortical inputs, implying a prevalent role in feedback inhibition. However, because prior physiological studies indicate that cortical afferents do trigger feedforward inhibition in principal cells, the present study aimed to determine whether a numerically important subtype of interneurons, expressing calbindin (CB(+) ), receives cortical inputs. Rats received injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHAL) in the perirhinal cortex or adjacent temporal neocortex. Light and electron microscopic observations of the relations between cortical inputs and BLA neurons were performed in the lateral (LA) and basolateral (BL) nuclei. Irrespective of the injection site (perirhinal or temporal neocortex) and target nucleus (LA or BL), approximately 90% of cortical axon terminals formed asymmetric synapses with dendritic spines of principal BLA neurons, while 10% contacted the dendritic shafts of presumed interneurons, half of which were CB(+) . Given the previously reported pattern of CB co-expression among GABAergic interneurons of the BLA, these results suggest that a subset of PV-immunonegative cells that express CB, most likely the somatostatin-positive interneurons, are important mediators of cortically-evoked feedforward inhibition in the BLA. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.
... The vast majority of cortical GABAergic cells are interneurons that only make local connections. GABAergic interneurons are extremely diverse, differing in shape, electrophysiological properties, and the combination of neuropeptides and calcium-binding proteins that they express (Peters and Kara, 1985b;Cavanagh and Parnavelas, 1988;DeFelipe, 1993;Kawaguchi and Kondo, 2002;Blatow et al., 2005;Tomson and Lamy, 2007;PING et al., 2008;Karagiannis et al., 2009;Xu et al., 2010;Vitalis and Rossier, 2011;Tricoire and Vitalis, 2012;DeFelipe et al., 2013). Using these differentiating characteristics, one can at a first approximation distinguish four main classes of interneurons populating the somatosensory cortex (PING et al., 2008;DeFelipe et al., 2013). ...
Cortical circuits control higher-order cognitive processes and their function is highly dependent on their structure that emerges during development. The construction of cortical circuits involves the coordinated interplay between different types of cellular processes such as proliferation, migration, and differentiation of neural and glial cell subtypes. Among the multiple factors that regulate the assembly of cortical circuits, 5-HT is an important developmental signal that impacts on a broad diversity of cellular processes. 5-HT is detected at the onset of embryonic telencephalic formation and a variety of serotonergic receptors are dynamically expressed in the embryonic developing cortex in a region and cell-type specific manner. Among these receptors, the ionotropic 5-HT3A receptor and the metabotropic 5-HT6 receptor have recently been identified as novel serotonergic targets regulating different aspects of cortical construction including neuronal migration and dendritic differentiation. In this review, we focus on the developmental impact of serotonergic systems on the construction of cortical circuits and discuss their potential role in programming risk for human psychiatric disorders.
... GluR2-lacking CP-AMPARs have recently received considerable attention because of their postulated role in synaptic plasticity (Liu & Cull-Candy, 2000;Clem & Barth, 2006;Plant et al. 2006;Adesnik J Physiol 588.15 & Nicoll, 2007) and neurological disorders (Tanaka et al. 2000;Cull-Candy et al. 2006;Liu et al. 2006;Isaac et al. 2007;Liu & Zukin, 2007). Although a previous study reported that fast-spiking (FS) interneurons in the rat motor cortex had a relatively small NMDA contribution (Angulo et al. 1999a), the AMPA and NMDA subtypes expressed by functionally identified neocortical interneurons remained limited (Blatow et al. 2005), particulary in the prefrontal cortex (PFC). Recent studies indicated that Ca 2+ influx in the dendrites of neocortical interneurons is mainly through CP-AMPARs (Goldberg et al. 2003) and that pyramidal neurons in the somatosensory cortex lost CP-AMPARs before postnatal day (PD)16 (Kumar et al. 2002). ...
Abnormal influx of Ca(2+) is thought to contribute to the neuronal injury associated with a number of brain disorders, and Ca(2+)-permeable AMPA receptors (CP-AMPARs) play a critical role in the pathological process. Despite the apparent vulnerability of fast-spiking (FS) interneurons in neurological disorders, little is known about the CP-AMPARs expressed by functionally identified FS interneurons in the developing prefrontal cortex (PFC). We investigated the development of inwardly rectifying AMPA receptor-mediated currents and their correlation with NMDA receptor-mediated currents in FS interneurons in the rat PFC. We found that 78% of the FS interneurons expressed a low rectification index, presumably Ca(2+)-permeable AMPARs, with only 22% exhibiting AMPARs with a high rectification index, probably Ca(2+) impermeable (CI). FS interneurons with CP-AMPARs exhibited properties distinct from those expressing CI-AMPARs, although both displayed similar morphologies, passive membrane properties and AMPA currents at resting membrane potentials. The AMPA receptors also exhibited dramatic changes during cortical development with significantly more FS interneurons with CP-AMPARs and a clearly decreased rectification index during adolescence. In addition, FS interneurons with CP-AMPARs exhibited few or no NMDA currents, distinct frequency-dependent synaptic facilitation, and protracted maturation in short-term plasticity. These data suggest that CP-AMPARs in FS interneurons may play a critical role in neuronal integration and that their characteristic properties may make these cells particularly vulnerable to disruptive influences in the PFC, thus contributing to the onset of many psychiatric disorders.
... Expression patterns of neurotransmitter receptors and voltage-gated channels determine key biophysical properties, and molecular markers such as calcium-binding proteins and neuropeptides also identify specific neuronal types (Blatow et al., 2005;Toledo-Rodriguez et al., 2005;Sugino et al., 2006). Morphological studies provided essential insights into how interneurons function within the modular architecture of cerebral cortex (Douglas and Martin, 2004) and subfields of hippocampus (Bernard and Wheal, 1994). ...
The morphological and electrophysiological diversity of inhibitory cells in hippocampal area CA3 may underlie specific computational roles and is not yet fully elucidated. In particular, interneurons with somata in strata radiatum (R) and lacunosum-moleculare (L-M) receive converging stimulation from the dentate gyrus and entorhinal cortex as well as within CA3. Although these cells express different forms of synaptic plasticity, their axonal trees and connectivity are still largely unknown. We investigated the branching and spatial patterns, plus the membrane and synaptic properties, of rat CA3b R and L-M interneurons digitally reconstructed after intracellular labeling. We found considerable variability within but no difference between the two layers, and no correlation between morphological and biophysical properties. Nevertheless, two cell types were identified based on the number of dendritic bifurcations, with significantly different anatomical and electrophysiological features. Axons generally branched an order of magnitude more than dendrites. However, interneurons on both sides of the R/L-M boundary revealed surprisingly modular axodendritic arborizations with consistently uniform local branch geometry. Both axons and dendrites followed a lamellar organization, and axons displayed a spatial preference toward the fissure. Moreover, only a small fraction of the axonal arbor extended to the outer portion of the invaded volume, and tended to return toward the proximal region. In contrast, dendritic trees demonstrated more limited but isotropic volume occupancy. These results suggest a role of predominantly local feedforward and lateral inhibitory control for both R and L-M interneurons. Such a role may be essential to balance the extensive recurrent excitation of area CA3 underlying hippocampal autoassociative memory function.
... With genetically marked subtypes of GABAergic neurons (Tamamaki et al., 2003;Blatow et al., 2005), it is possible to explore the subtype specific regulation of migration in experiments in vivo. Additionally, it may be possible to test for the possible consequences of the link between cell migration and network activity. ...
Available evidence converges to suggest that during the early development of the cerebral cortex, the emergence of the spontaneous network activity chronologically overlap with the end of the cell migration period in the developing cortex. We approached the functional regulation of neuronal migration in a culture model of neocortical networks, using time lapses to detect migratory movements, calcium-imaging to assess the activity of migratory neurons, and immunocytochemical methods to identify the migratory cells retrospectively. In cell cultures, early physiological development and cell migration are reproduced at a local network level, thus allowing the study of the interrelationships between cell migration and network development independent of the topographical complexity. Neurons migrate at least until 12 days in vitro and GABAergic neurons migrate faster compared with non-GABAergic neurons. A decline of migratory activity was coincident with the development of spontaneous synchronous network activity. Migrating interneurons did not participate in synchronous network activity, but interneurons that ended cell migration during observation time frequently engaged in synchronous activity within less than an hour. Application of GABA(A) and ionotropic glutamate receptor antagonists significantly increased the number of migrating GABAergic neurons without changing the dynamics of the migratory movements. Thus, neurotransmitters released by early network activity might favor the termination of neuronal migration. These results reinforce the idea that network activity plays an important role in the development of late-born GABAergic cells.
... They receive slower, depressing EPSPs from and deliver slower IPSPs to neighboring pyramids. CCKcontaining cells are quite distinct from PV-cells, being a primary recipient of 5-hydroxytryptamine (5-HT) synapses, unique in expressing 5-HT3 and nicotinic (7) receptors (Blatow et al., 2005 ) and with calbindin (Cb)interneurones , unique in bearing presynaptic CB1 (cannabinoid type 1) Functional maps of neocortical local circuitry ...
This review aims to summarize data obtained with different techniques to provide a functional map of the local circuit connections made by neocortical neurones, a reference for those interested in cortical circuitry and the numerical information required by those wishing to model the circuit. A brief description of the main techniques used to study circuitry is followed by outline descriptions of the major classes of neocortical excitatory and inhibitory neurones and the connections that each layer makes with other cortical and subcortical regions. Maps summarizing the projection patterns of each class of neurone within the local circuit and tables of the properties of these local circuit connections are provided.
This review relies primarily on anatomical studies that have identified the classes of neurones and their local and long distance connections and on paired intracellular and whole-cell recordings which have documented the properties of the connections between them. A large number of different types of synaptic connections have been described, but for some there are only a few published examples and for others the details that can only be obtained with paired recordings and dye-filling are lacking. A further complication is provided by the range of species, technical approaches and age groups used in these studies. Wherever possible the range of available data are summarised and compared. To fill some of the more obvious gaps for the less well-documented cases, data obtained with other methods are also summarized.
... Unlike acute drug effects, which occur in an intact system, epileptogenesis and recurrent seizures in chronic epilepsy likely refl ect pathological disturbances of neuronal circuits that may have multiple origins. Furthermore, the simple view that GABAergic transmission acts like a break preventing overexcitation of neuronal circuits has been challenged by the highly sophisticated anatomical and functional organization of GABAergic interneurons in cerebral cortex (Blatow et al., 2005; Markram et al., 2004 ). Rather, GABAergic function is required for fi netuning of neuronal circuits and its infl uence on cell fi ring and network oscillations is constrained spatially and temporally (Mann and Paulsen, 2007; Tukker et al., 2007). ...
GABA(A) receptors mediate most of the fast inhibitory transmission in the CNS. They form heteromeric complexes assembled from a large family of subunit genes. The existence of multiple GABA(A) receptor subtypes differing in subunit composition, localization and functional properties underlies their role for fine-tuning of neuronal circuits and genesis of network oscillations. The differential regulation of GABA(A) receptor subtypes represents a major facet of homeostatic synaptic plasticity and contributes to the excitation/inhibition (E/I) balance under physiological conditions and upon pathological challenges. The purpose of this review is to discuss recent findings highlighting the significance of GABA(A) receptor heterogeneity for the concept of E/I balance and its relevance for epilepsy. Specifically, we address the following issues: (1) role for tonic inhibition, mediated by extrasynaptic GABA(A) receptors, for controlling neuronal excitability; (2) significance of chloride ion transport for maintenance of the E/I balance in adult brain; and (3) molecular mechanisms underlying GABA(A) receptor regulation (trafficking, posttranslational modification, gene transcription) that are important for homoeostatic plasticity. Finally, the relevance of these findings is discussed in light of the involvement of GABA(A) receptors in epileptic disorders, based on recent experimental studies of temporal lobe epilepsy (TLE) and absence seizures and on the identification of mutations in GABA(A) receptor subunit genes underlying familial forms of epilepsy.
... It has been speculated that differential NMDA receptor subunits distributed on interneurons, particularly PV-ir neurons, may be responsible for NMDA receptor dysfunction (Homayoun and Moghaddam, 2007;Javitt, 2004;Lindsley et al., 2006;Olney and Farber, 1995), but direct evidence is still missing. The distributions of NMDA receptor subtypes in cortical interneurons are also unclear (Blatow et al., 2005). Some studies suggested that interneurons lack NMDA receptor-mediated responses (Goldberg et al., 2003;Ling and Benardo, 1995). ...
The selective involvement of a subset of neurons in many psychiatric disorders, such as gamma-aminobutyric acid (GABA)-ergic interneurons in schizophrenia, creates a significant need for in-depth analysis of these cells. Here we introduce a combination of techniques to examine the relative gene expression of N-methyl-d-aspartic acid (NMDA) receptor subtypes in GABAergic interneurons from the rat prefrontal cortex. Neurons were identified by immunostaining, isolated by laser microdissection and RNA was prepared for reverse transcription polymerase chain reaction (RT-PCR) and real-time PCR. These experimental procedures have been described individually; however, we found that this combination of techniques is powerful for the analysis of gene expression in individual identified neurons. This approach provides the means to analyze relevant molecular mechanisms that are involved in the neuropathological process of a devastating brain disorder.
... Based on evidence obtained in the hippocampus (Freund and Buzsáki, 1996;Somogyi and Klausberger, 2005) and neocortex (Markram et al., 2004;Blatow et al., 2005), it is likely that these various types of interneurons structure the activity of principal cells in distinct ways because they target different postsynaptic domains and receive contrasting sets of inputs. For instance, PV interneurons of the BLA appear to be mainly involved in feedback inhibition: they receive strong excitatory inputs from BLA projection cells but very few from the cortex (Smith et al., 2000) and they form numerous inhibitory synapses on the somatic, axonal, and proximal dendritic domains of projection neurons (Pitkänen and Amaral, 1993;Sorvari et al., 1995;Smith et al., 1998;Kemppainen and Pitkänen, 2000;McDonald and Betette, 2001). ...
The lateral nucleus of the amygdala is the entry point of most sensory inputs into the amygdala. However, the way information is processed and distributed within the lateral nucleus still eludes us. To gain some insight into this issue, we have examined the spatial organization of excitatory and inhibitory connections in the lateral nucleus. To this end, we performed whole-cell recordings of principal lateral amygdala neurons and studied their responses to local pressure applications of glutamate in coronal and horizontal slices of the guinea-pig amygdala. In coronal sections, glutamate puffs performed at a distance from the recorded cells usually evoked inhibitory responses, except when the recorded neuron was adjacent to the external capsule, in which case excitatory responses could be evoked from ejection sites along the external capsule. In contrast, glutamate puffs evoked a mixture of excitatory and inhibitory responses in horizontal slices. Excitatory responses were generally evoked from stimulation sites located lateral to the recorded cell whereas inhibitory responses were commonly elicited from medial stimulation sites, irrespective of their rostrocaudal position. These findings confirm and extend previous tract-tracing studies where it was found that intrinsic connections within the lateral amygdala prevalently run in the dorsoventral and lateromedial directions. However, our results also reveal a hitherto unsuspected level of spatial heterogeneity in the intrinsic circuit of the lateral amygdala. The prevalence of excitatory responses in horizontal slices coupled to the ubiquity of inhibitory responses in coronal slices suggest that the lateral amygdala network is designed to allow associative interactions within the rostrocaudal plane while preventing runaway excitation locally.
... Several classes of interneurons with distinct functional properties regulate the inhibitory activity in the neocortex [33]. The activity of interneurons appears to be mainly regulated by the major inhibitory neurotransmitter gamma-aminobutyric acid (GABA) [34] and the major excitatory neurotransmitter, glutamate [35]. The concentration of GABA could not be measured by the MRS technique used here. ...
A combination of magnetoencephalography and proton magnetic resonance spectroscopy was used to correlate the electrophysiology of rapid auditory processing and the neurochemistry of the auditory cortex in 15 healthy adults. To assess rapid auditory processing in the left auditory cortex, the amplitude and decrement of the N1m peak, the major component of the late auditory evoked response, were measured during rapidly successive presentation of acoustic stimuli. We tested the hypothesis that: (i) the amplitude of the N1m response and (ii) its decrement during rapid stimulation are associated with the cortical neurochemistry as determined by proton magnetic resonance spectroscopy.
Our results demonstrated a significant association between the concentrations of N-acetylaspartate, a marker of neuronal integrity, and the amplitudes of individual N1m responses. In addition, the concentrations of choline-containing compounds, representing the functional integrity of membranes, were significantly associated with N1m amplitudes. No significant association was found between the concentrations of the glutamate/glutamine pool and the amplitudes of the first N1m. No significant associations were seen between the decrement of the N1m (the relative amplitude of the second N1m peak) and the concentrations of N-acetylaspartate, choline-containing compounds, or the glutamate/glutamine pool. However, there was a trend for higher glutamate/glutamine concentrations in individuals with higher relative N1m amplitude.
These results suggest that neuronal and membrane functions are important for rapid auditory processing. This investigation provides a first link between the electrophysiology, as recorded by magnetoencephalography, and the neurochemistry, as assessed by proton magnetic resonance spectroscopy, of the auditory cortex.
... The major targets of monoaminergic afferents in the prefrontal cortex, amygdala, and hippocampal formation are GABAergic interneurons (Freund et al., 1990;Brinley-Reed and McDonald, 1999;Rosenkranz and Grace, 1999;Grace and Rosenkranz, 2002;Grace, 2002). There are multiple populations of local circuit neurons, with different specific functions, characteristic interconnections, and cellular properties (Schwartzkroin et al., 1990), which can be neurochemically, physiologically, and morphologically characterized (Sloviter, 1989;Kisvarday et al., 1990;Nusser et al., 1995;Kemppainen and Pitkanen, 2000;Markram et al., 2004;Monyer and Markram, 2004;Blatow et al., 2005;Rainnie et al., 2006;Woodruff and Sah, 2007). The different GABAergic subpopulations exert their inhibitory function either at the level of the soma and axon hillock (¼ Parvalbumin (PARV)-positive) or at the dendrites (¼ Calbindin-D28k (CaBP-D28k)-positive) of spine-bearing neocortical, hippocampal, and amygdalar pyramidal neurons (Ribak et al., 1993;Seress et al., 1993;Sorvari et al., 1996;Gulyas et al., 1999;McDonald and Betette, 2001;Freund, 2003;Freund and Katona, 2007). ...
Emotional experience during early life has been shown to interfere with the development of excitatory synaptic networks in the prefrontal cortex, hippocampus, and the amygdala of rodents and primates. The aim of the present study was to investigate a developmental "homoeostatic synaptic plasticity" hypothesis and to test whether stress-induced changes of excitatory synaptic composition are counterbalanced by parallel changes of inhibitory synaptic networks. The impact of repeated early separation stress on the development of two GABAergic neuronal subpopulations was quantitatively analyzed in the brain of the semiprecocial rodent Octodon degus. Assuming that PARV- and CaBP-D28k-expression are negatively correlated to the level of inhibitory activity, the previously described reduced density of excitatory spine synapses in the dentate gyrus of stressed animals appears to be "amplified" by elevated GABAergic inhibition, reflected by reduced PARV- (down to 85%) and CaBP-D28k-immunoreactivity (down to 74%). In opposite direction, the previously observed elevated excitatory spine density in the CA1 region of stressed animals appears to be amplified by reduced inhibition, reflected by elevated CaPB-D28k-immunoreactivity (up to 149%). In the (baso)lateral amygdala, the previously described reduction of excitatory spine synapses appears to be "compensated" by reduced inhibitory activity, reflected by dramatically elevated PARV- (up to 395%) and CaPB-D28k-immunoreactivity (up to 327%). No significant differences were found in the central nucleus of the amygdala, the piriform, and somatosensory cortices and in the hypothalamic paraventricular nucleus. Thus during stress-evoked neuronal and synaptic reorganization, a homeostatic balance between excitation and inhibition is not maintained in all regions of the juvenile brain.
This chapter summarizes many of the cellular alterations that give rise to an imbalance between excitation and inhibition, and to a hypersynchronous neuronal discharge. Contributing intrinsic neuronal properties include aberrant channel function and/or location that may cause excessive discharge and/or inadequate hyperpolarization of excitatory (projection) cells or loss of inhibition resulting from changes (damage to) inhibitory interneurons. Altered/abnormal synaptic function may also be critical, such as activity-dependent changes in receptor properties and/or plasticity of synaptic connectivity; such changes may affect both excitatory (e.g., mediated by glutamatergic synapses) and inhibitory (e.g., local GABA circuits) function. Finally, dysfunction of extracellular influences, such as those mediated by glial activities, changes in extracellular spaces, effects of hormonal modulators, and dependent on an intact vascular system, can result in hyperexcitability and seizures discharge. These cellular changes can result from genetic disorders, from aberrant developmental programs, or from trauma-induced brain injury. The net effect of these changes - often multiple and complex - can lead to a focal epilepsy such as temporal lobe epilepsy (if the abnormalities are restricted to temporal lobe structures) or to a more generalized phenotype (e.g., generalized epilepsy with febrile seizures plus, GEFS+). Insight into underlying cellular and molecular abnormalities of a given epilepsy phenotype will, we hope, lead to better and earlier treatments.
The Journal of Physiology supported a symposium to celebrate the past achievements and continuing influence of the work of the late Professor Eberhard H. Buhl, held at the University of Leeds in september 2004. The purpose of the symposium was to bring together former colleagues and collaborators of the late Professor Buhl to provide a forum for the discussion of the state of the art in his principal research field: the correlation between the anatomical structure and emergent function of neuronalnetworks.
Inhibition modulates receptive field properties and integrative responses of neurons in cortical circuits. The contribution of specific interneuron classes to cortical circuits and emergent responses is unknown. Here, we examined neuronal responses in primary visual cortex (V1) of adult Dlx1(-/-) mice, which have a selective reduction in cortical dendrite-targeting interneurons (DTIs) that express calretinin, neuropeptide Y, and somatostatin. The V1 neurons examined in Dlx1(-/-) mice have reduced orientation selectivity and altered firing rates, with elevated late responses, suggesting that local inhibition at dendrites has a specific role in modulating neuronal computations. We did not detect overt changes in the physiological properties of thalamic relay neurons and features of thalamocortical projections, such as retinotopic maps and eye-specific inputs, in the mutant mice, suggesting that the defects are cortical in origin. These experimental results are well explained by a computational model that integrates broad tuning from dendrite-targeting and narrower tuning from soma-targeting interneuron subclasses. Our findings suggest a key role for DTIs in the fine-tuning of stimulus-specific cortical responses.
The cytoarchitectonic similarities of different neocortical regions have given rise to the idea of 'canonical' connectivity between excitatory neurons of different layers within a column. It is unclear whether similarly general organizational principles also exist for inhibitory neocortical circuits. Here we delineate and compare local inhibitory-to-excitatory wiring patterns in all principal layers of primary motor (M1), somatosensory (S1) and visual (V1) cortex, using genetically targeted photostimulation in a mouse knock-in line that conditionally expresses channelrhodopsin-2 in GABAergic neurons. Inhibitory inputs to excitatory neurons derived largely from the same cortical layer within a three-column diameter. However, subsets of pyramidal cells in layers 2/3 and 5B received extensive translaminar inhibition. These neurons were prominent in V1, where they might correspond to complex cells, less numerous in barrel cortex and absent in M1. Although inhibitory connection patterns were stereotypical, the abundance of individual motifs varied between regions and cells, potentially reflecting functional specializations.
Interneurone bilden eine extrem heterogene Population, die eine differenzierte inhibitorische Kontrolle von Erregungsbildung und Erregungsfluss in cortikalen Schaltkreisen ermöglicht. In dieser Arbeit wurde eine Charakterisierung der Interneurone des Gyrus dentatus durchgeführt, der als Eingangsstruktur in den hippocampalen Schaltkreis eine strategisch wichtige Position einnimmt. Zur unvoreingenommenen intraexperimentellen Zellselektion wurde die GAD67-eGFP-knock-in Mauslinie, bei der abhängig von der Expression der GABA-synthetisierenden Enzymisoform GAD67 das Fluoreszenzprotein eGFP gebildet wird, zuerst immunhistochemisch validiert und dann als Modellsystem verwendet.
Bei 105 der eGFP-markierten Interneurone wurden mithilfe der Patch-Clamp-Technik die Morphologie sowie passive und aktive elektrophysiologische Membraneigenschaften bestimmt. Bei 19 Interneuronen wurde zudem eine dreidimensionale Neurolucida-Rekonstruktion durchgeführt. Die lichtmikroskopische Analyse sowie eine auf den quantitativen Daten der rekonstruierten Neurone basierende hierarchische Clusteranalyse
ergab eine morphologische Einteilung in fünf Klassen: BC-Zellen (Axon in der Körnerzellschicht), HICAP-Zellen (Axon in der inneren Molekularschicht), HIPP-Zellen (Axon in der mittleren und äusseren Molekularschicht), TML-Zellen (Axon in der gesamten Molekularschicht) und ML-Zellen (Soma und Axon in der Molekularschicht). Diese Einteilung ist vor allem durch die Schichtspezifität der Axonprojektion gekennzeichnet und weist auf eine differenzierte Regulation der unterschiedlichen Prinzipalzellkompartimente (Soma vs. Dendriten) sowie der Eingangstrakte (Tractus perforans und associativus) des Gyrus dentatus hin.
Bei der Analyse der elektrophysiologischen Eigenschaften der identifizierten Interneuronklassen fielen charakteristische Aktionspotentialmuster sowie zahlreiche statistisch signifikante Unterschiede in den passiven und aktiven Membraneigenschaften zwischen den Klassen auf. Eine auf den physiologischen Charakteristika von 65 identifizierten Zellen basierende hierarchische lusteranalyse ergab eine Unterteilung, die der morphologischen Klassifizierung im Wesentlichen entsprach. Daraus folgt, dass Interneurone des Gyrus dentatus in fünf distinkte, durch ihre Morphologie und ihre Physiologie gekennzeichnete Typen unterteilt werden können. Diese könnten unterschiedliche funktionelle Aufgaben im Schaltkreis des Gyrus dentatus wie z.B. die Synchronisation von Prinzipalzellensembles oder die Kontrolle synaptischer Eingänge an den Prinzipalzelldendriten übernehmen.
The study examined the neurophysiological correlates of age-related changes in the coordination of hand and foot movements. Young and older adults (N=30) performed cyclical isodirectional and non-isodirectional hand-foot movements with contralateral and ipsilateral limb combinations. Motor evoked potentials (MEPs) and silent period durations following transcranial magnetic stimulation (TMS) were measured from the right extensor carpi radialis (ECR) muscle during the interlimb coordination tasks. Older adults demonstrated lower coordination stability than younger adults, particularly when performing non-isodirectional movements with ipsilateral limbs. For all coordination tasks, MEP amplitude was lower in older compared with young participants. Young adults showed significantly longer silent period durations when the coordination pattern involved ipsilateral limbs than during contralateral limb coordination. In contrast, silent period durations did not differ between contralateral and ipsilateral limb coordination in older adults. These results suggest that deterioration in motor performance with advancing age may be associated with a decreased ability to modulate inhibitory function.
Excitatory cortical neurons form fine-scale networks of precisely interconnected neurons. Here we tested whether inhibitory cortical neurons in rat visual cortex might also be connected with fine-scale specificity. Using paired intracellular recordings and cross-correlation analyses of photostimulation-evoked synaptic currents, we found that fast-spiking interneurons preferentially connected to neighboring pyramids that provided them with reciprocal excitation. Furthermore, they shared common fine-scale excitatory input with neighboring pyramidal neurons only when the two cells were reciprocally connected, and not when there was no connection or a one-way, inhibitory-to-excitatory connection. Adapting inhibitory neurons shared little or no common input with neighboring pyramids, regardless of their direct connectivity. We conclude that inhibitory connections and also excitatory connections to inhibitory neurons can both be precise on a fine scale. Furthermore, fine-scale specificity depends on the type of inhibitory neuron and on direct connectivity between neighboring pyramidal-inhibitory neuron pairs.
Projection neurons in the developing cerebral cortex of rodents are basically born near the ventricle and migrate radially to beneath the marginal zone, whereas their cortical interneurons are generated in the ventral telencephalon and migrate tangentially to the cortex. The origins and migratory profiles of each interneuron subtype have been studied extensively in the last decade, and an enormous effort has been made to clarify the cellular and molecular mechanisms that regulate interneuron migration. More recently, the interaction between projection neurons and migrating interneurons, including how they are incorporated into their proper layers, has begun to be analyzed. In this review, I outline the most recent findings in regard to these issues and discuss the mechanisms underlying the development of cortical cytoarchitecture.
Rostral ventral medulla (RVM) contains significant numbers of local GABAergic neurons which may subserve respiratory chemosensory and baroreceptor reflexes. Nicotinic mechanisms stimulate release of GABA in certain brainstem neurons. Whether the GABAergic neurons at RVM express nicotinic cholinergic receptors (nAChRs) is not known. We used glutamic acid decarboxylase 67-kDa isoform (GAD67) and parvalbumin (PV) as anatomical markers to identify the GABAergic neurons of the RVM and caudal pons and performed double labeling to evaluate the expression of alpha-7 and alpha-4 nAChRs by GAD67 and PV-imnunoreactive (ir) cells at these sites. GAD67-ir cells were found at the ventrolateral pontomedullary border in areas adjacent to the A5 noradrenergic cell group and overlapping the facial nucleus lateral subnuclei and para-facial zones. Of 205 GAD67-ir cells labeled at these sites, 74% exhibited immunoreactivity for alpha-7 nAChRs. Alpha-4 immunoreactivity was also present in 35% of GAD67-ir cells at these sites. The PV-ir cells of RVM and caudal pons were found medial to the facial nucleus and lateral to the pyramid in a column distinct from the GAD67-ir cells. Virtually all the PV-ir cells demonstrated immunoreactivity for alpha-4 nAChR (95%) and alpha-7 (93%) subunits of nAChRs. Differential expression of GAD67 and PV by neurons at the pontomedullary border implies that PV may not be a valid marker for GABAergic neurons. The expression of alpha-4 and alpha-7 nAChRs by GAD67-ir cells suggests nicotinic cholinergic modulation of GABAergic signaling at these ventrolateral pontomedullary sites.
The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined:
When gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in vertebrate brain, binds to its receptor it activates a chloride channel. Neurotransmitter action at the GABAA receptor is potentiated by both benzodiazepines and barbiturates which are therapeutically useful drugs (reviewed in ref. 1). There is strong evidence that this receptor is heterogeneous. We have previously isolated complementary DNAs encoding an alpha- and a beta-subunit and shown that both are needed for expression of a functional GABAA receptor. We have now isolated cDNAs encoding two additional GABAA receptor alpha-subunits, confirming the heterogeneous nature of the receptor/chloride channel complex and demonstrating a molecular basis for it. These alpha-subunits are differentially expressed within the CNS and produce, when expressed with the beta-subunit in Xenopus oocytes, receptor subtypes which can be distinguished by their apparent sensitivity to GABA. Highly homologous receptor subtypes which differ functionally seem to be a common feature of brain receptors.
AMPA receptor (AMPAR) subunits expression was studied in nitric oxide synthase (NOS)-positive neurons of the adult rat cortex, striatum, and hippocampus, by a double-labeling approach, combining nonradioactive in situ hybridization and immunocytochemistry. The majority of cortical and hippocampal NOS-immunopositive neurons were characterized by a predominant expression of GluR-A and -D mRNA and low or undetectable expression of GluR-B and -C mRNA. In the striatum, the expression profile of AMPAR subunits in NOS-positive neurons differed from that in the other two regions. This is reflected in the overall low expression of all AMPA receptor subunits and the paucity of GluR-D subunit expression that contrasts with the high expression of this subunit in NOS-positive cells in the hippocampus. Double-labeling experiments revealed a substantial correspondence between mRNA and protein levels of AMPAR subunits. Further evidence for the regional diversity of NOS-positive neurons is derived from the expression analysis of glutamate decarboxylase (GAD)-65 and -67 mRNAs. NOS-positive neurons expressed high levels of GAD-65, but not -67 in the cortex, high levels of both forms in the hippocampus, and low or undetectable levels of both mRNAs in the striatum. Despite of these differences, NOS-positive neurons share the common feature of low GluR-B subunit expression, suggesting the presence of AMPAR channels with high Ca2+ permeability, regardless of the regional location. The relative resistance of NOS-positive interneurons in neurodegenerative diseases suggests that glutamate receptor-mediated Ca2+ influx alone does not suffice to explain neuronal vulnerability, and additional factors have thus to be considered.
Partially synchronous 40-Hz oscillations of cortical neurons have been implicated in cognitive function. Specifically, coherence of these oscillations between different parts of the cortex may provide conjunctive properties to solve the 'binding problem': associating features detected by the cortex into unified perceived objects. Here we report an emergent 40-Hz oscillation in networks of inhibitory neurons connected by synapses using GABAA (gamma-aminobutyric acid) receptors in slices of rat hippocampus and neocortex. These network inhibitory postsynaptic potential oscillations occur in response to the activation of metabotropic glutamate receptors. The oscillations can entrain pyramidal cell discharges. The oscillation frequency is determined both by the net excitation of interneurons and by the kinetics of the inhibitory postsynaptic potentials between them. We propose that interneuron network oscillations, in conjunction with intrinsic membrane resonances and long-loop (such as thalamocortical) interactions, contribute to 40-Hz rhythms in vivo.
The family of mammalian genes related to the Drosophila Shaker gene, consisting of four subfamilies, is thought to encode subunits of tetrameric voltage-gated K+ channels. There is compelling evidence that subunits of the same subfamily, but not of different subfamilies, form heteromultimeric channels in vitro, and thus, each gene subfamily is postulated to encode components of an independent channel system. In order to identify cells with native channels containing subunits of one of these subfamilies (Shaw-related or ShIII), the cellular distribution of ShIII transcripts was examined by Northern blot analysis and in situ hybridization. Three of four ShIII genes (KV3.1, KV3.2, and KV3.3) are expressed mainly in the CNS. KV3.4 transcripts are also present in the CNS but are more abundant in skeletal muscle. In situ hybridization studies in the CNS reveal discrete and specific neuronal populations that prominently express ShIII mRNAs, both in projecting and in local circuit neurons. In the cerebral cortex, hippocampus, and caudate-putamen, subsets of neurons can be distinguished by the expression of specific ShIII mRNAs. Each ShIII gene exhibits a unique pattern of expression; however, many neuronal populations expressing KV3.1 transcripts also express KV3.3 mRNAs. Furthermore, KV3.4 transcripts are present, albeit at lower levels, in several of the neuronal populations that also express KV3.1 and/or KV3.3 mRNAs, revealing a high potential for heteromultimer formation between the products of three of the four genes. Expression of ShIII cRNAs in Xenopus oocytes was used to explore the functional consequences of heteromultimer formation between ShIII subunits. Small amounts of KV3.4 cRNA, which expresses small, fast-inactivating currents when injected alone, produced fast-inactivating currents that are severalfold larger when coinjected with an excess of KV3.1 or KV3.3 cRNA. This amplification is due to both an increase in single-channel conductance in the heteromultimeric channels and the observation that less than four, perhaps even a single KV3.4 subunit is sufficient to impart fast-inactivating properties to the channel. The oocyte experiments indicate that the apparently limited, low-level expression of KV3.4 in the CNS is potentially significant. The anatomical studies suggest that heteromultimer formation between ShIII proteins might be a common feature in the CNS. Moreover, the possibility that the subunit composition of heteromultimers varies in different neurons should be considered, since the ratios of overlapping signals change from one neuronal population to another. In order to proceed with functional analysis of native ShIII channels, it is important to known which subunit compositions might occur in vivo. The studies presented here provide important clues for the identification of native homo- and heteromultimeric ShIII channels in neurons.
In the cortex fast excitatory synaptic currents onto excitatory pyramidal neurons and inhibitory nonpyramidal neurons are mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors exhibiting cell-type-specific differences in their kinetic properties. AMPA receptors consist of four subunits (GluR1-4), each existing as two splice variants, flip and flop, which critically affect the desensitization properties of receptors expressed in heterologous systems. Using single cell reverse transcription PCR to analyze the mRNA of AMPA receptor subunits expressed in layers I-III neocortical neurons, we find that 90% of the GluR1-4 in nonpyramidal neurons are flop variants, whereas 92% of the GluR1-4 in pyramidal neurons are flip variants. We also find that nonpyramidal neurons predominantly express GluR1 mRNA (GluR1/GluR1-4 = 59%), whereas pyramidal neurons contain mainly GluR2 mRNA (GluR2/GluR1-4 = 59%). However, the neuron-type-specific splicing is exhibited by all four AMPA receptor subunits. We suggest that the predominance of the flop variants contributes to the faster and more extensive desensitization in nonpyramidal neurons, compared to pyramidal cells where flip variants are dominant. Alternative splicing of AMPA receptors may play an important role in regulating synaptic function in a cell-type-specific manner, without changing permeation properties.
The type 3 serotonin receptor (5-HT 3 R) is a ligand-gated ion channel whose presence in the CNS has been established by radioligand binding, in situ hybridization, and immunohistochemical analysis. To analyze further the role of the 5-HT 3 R in the CNS, we used in situ hybridization and immunocytochemistry to determine that 5-HT 3 R-expressing neurons are mainly GABA-containing cells in the rat telencephalon. We determined that 5-HT 3 R/GABA-containing neurons do not exhibit somatostatin immunoreactivity but often contain cholecystokinin (CCK) immunoreactivity. 5-HT 3 R-expressing cells with CCK immunoreactivity were observed in the neocortex, olfactory cortex, hippocampus, and amygdala. The 5-HT 3 R/CCK interneurons represent between 35 and 66% of the total population of CCK-containing cells in the neocortex.
Further characterization of the 5-HT 3 R/GABAergic neurons was based on their calcium-binding protein immunoreactivity and showed that these neurons lack parvalbumin (PV) and represent a subpopulation of calbindin (CB)-containing interneurons that were preferentially present in the CA1–CA3 subfield of the hippocampus. Although some 5-HT 3 R/GABAergic neurons with calretinin (CR) were found in the neocortex, olfactory cortex, hippocampus, and amygdala, these neurons were more often present in the agranular insular and piriform cortices.
We conclude that the neuronal expression of the 5-HT 3 R is selective within the GABA neuron population in the rat telencephalon. These 5-HT 3 R-expressing interneurons might contain CCK, CB, and CR. We suggest that serotonin through the 5-HT 3 R may regulate GABA and CCK neurotransmission in the telencephalon.
The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n = 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n = 30) and/or calbindin (n = 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.
Native AMPA receptors (AMPARs) were investigated in neocortical fast-spiking (FS) and regular-spiking nonpyramidal (RSNP) cells. The onset of and recovery from desensitization as well as current rectification and single-channel conductance were studied by using fast glutamate application to outside-out patches. The GluR1-4 subunit, flip/flop splicing, and R/G editing expression patterns of functionally characterized cells were determined by single-cell reverse transcription-PCR to correlate the subunit composition of native AMPARs with their functional properties. Our sample, mostly constituted by RSNP neurons, predominantly expressed GluR3 flip and GluR2 flop. In individual cells, flip/flop splicing of each subunit appeared to be regulated independently, whereas for R/G editing all subunits were either almost fully edited or unedited. We confirmed that the relative GluR2 expression controls the permeation properties of native AMPARs, whereas none of the single molecular parameters considered appeared to be a key determinant of the kinetics. FS neurons displayed AMPARs with relatively homogeneous functional properties characterized by fast desensitization, slow recovery from desensitization, marked inward rectification, and large single-channel conductance. In contrast, these parameters varied over a wide range in RSNP neurons, and their combination resulted in various AMPAR functional patterns. Indeed, in different cells, fast or slow desensitization was found to be associated with either slow or fast recovery from desensitization. Similarly, fast or slow kinetics was associated with either strong or weak rectification. Our results suggest that kinetic and permeation properties of native AMPARs can be regulated independently in cortical neurons and probably do not have the same molecular determinants.
Physiological, morphological and immunohistochemical characteristics of non-pyramidal cells in frontal cortex of young rats were studied in vitro by whole-cell recording and biocytin injection. Several groups of GABAergic non-pyramidal cells were identified: (i) parvalbumin fast-spiking (FS) cells with low input resistances and spikes of short duration, including extended plexus (basket) cells and chandelier cells. These cells showed abrupt episodes of non-adapting repetitive discharges; (ii) late-spiking (LS) cells exhibiting slowly developing ramp depolarizations, including neurogliaform cells; (iii) the remaining groups contained both burst-spiking (BS) or regular-spiking (RS) non-pyramidal (NP) cells. BSNP cells exhibited bursting activity (two or more spikes on slow depolarizing humps) from hyperpolarized potentials. Both these physiological types corresponded to a range of morphologies: (i) somatostatin-containing Martinotti cells with ascending axonal arbors to layer I (some were also positive for calbindin D28k); (ii) VIP-containing double bouquet cells with descending axonal arbors as well as arcade cells (these included small cells immunoreactive for CCK or calretinin). Each subtype of cells made GABAergic synapses onto relatively specific portions of cortical cells, but similar domains were innervated by multiple classes of GABA cells.
Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.
The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.
The cellular mechanisms by which neuronal nicotinic cholinergic receptors influence many aspects of physiology and pathology in the neocortex remain primarily unknown. Whole-cell recordings and single-cell reverse transcription (RT)-PCR were combined to analyze the effect of nicotinic receptor agonists on different types of neurons in acute slices of rat neocortex. Nicotinic receptor agonists had no effect on pyramidal neurons and on most types of interneurons, including parvalbumin-expressing fast spiking interneurons and somatostatin-expressing interneurons, but selectively excited a subpopulation of interneurons coexpressing the neuropeptides vasoactive intestinal peptide (VIP) and cholecystokinin. This excitation persisted in the presence of glutamate, GABA, and muscarinic receptor antagonists and in the presence of tetrodotoxin and low extracellular calcium, suggesting that the depolarization was mediated through the direct activation of postsynaptic nicotinic receptors. The responses were blocked by the nicotinic receptor antagonists dihydro-beta-erythroidine and mecamylamine and persisted in the presence of the alpha7 selective nicotinic receptor antagonist methyllycaconitine, suggesting that the involved nicotinic receptors lacked the alpha7 subunit. Single-cell RT-PCR analysis indicated that the majority of the interneurons that responded to nicotinic stimulation coexpressed the alpha4, alpha5, and beta2 nicotinic receptor subunits. Therefore, these results provide a role for non-alpha7 nicotinic receptors in the selective excitation of a subpopulation of neocortical interneurons. Because the neocortical interneurons expressing VIP have been proposed previously to regulate regional cortical blood flow and metabolism, these results also provide a cellular basis for the neuronal regulation of cortical blood flow mediated by acetylcholine.
Kv3.1 is a voltage-gated, fast activating/deactivating potassium (K(+)) channel with a high-threshold of activation and a large unit conductance. Kv3.1 K(+) channels are expressed in fast-spiking, parvalbumin-containing interneurons in cortex, hippocampus, striatum, the thalamic reticular nucleus (TRN), and in several nuclei of the brain stem. A high density of Kv3.1 channels contributes to short-duration action potentials, fast afterhyperpolarizations, and brief refractory periods enhancing the capability in these neurons for high-frequency firing. Kv3.1 K(+) channel expression in the TRN and cortex also suggests a role in thalamocortical and cortical function. Here we show that fast gamma and slow delta oscillations recorded from the somatomotor cortex are altered in the freely behaving Kv3.1 mutant mouse. Electroencephalographic (EEG) recordings from homozygous Kv3.1(-/-) mice show a three- to fourfold increase in both absolute and relative spectral power in the gamma frequency range (20-60 Hz). In contrast, Kv3.1-deficient mice have a 20-50% reduction of power in the slow delta range (2-3 Hz). The increase in gamma power is most prominent during waking in the 40- to 55-Hz range, whereas the decrease in delta power occurs equally across all states of arousal. Our findings suggest that Kv3. 1-expressing neurons are involved in the generation and maintenance of cortical fast gamma and slow delta oscillations. Hence the Kv3. 1-mutant mouse could serve as a model to study the generation and maintenance of fast gamma and slow delta rhythms and their involvement in behavior and cognition.
The regional and cellular localisation of gamma-aminobutyric acid(A) (GABA(A)) receptors was investigated in the human basal ganglia using receptor autoradiography and immunohistochemical staining for five GABA(A) receptor subunits (alpha(1), alpha(2), alpha(3), beta(2, 3), and gamma(2)) and other neurochemical markers. The results demonstrated that GABA(A) receptors in the striatum showed considerable subunit heterogeneity in their regional distribution and cellular localisation. High densities of GABA(A) receptors in the striosome compartment contained the alpha(2), alpha(3), beta(2, 3), and gamma(2) subunits, and lower densities of receptors in the matrix compartment contained the alpha(1), alpha(2), alpha(3), beta(2,3), and gamma(2) subunits. Also, six different types of neurons were identified in the striatum on the basis of GABA(A) receptor subunit configuration, cellular and dendritic morphology, and chemical neuroanatomy. Three types of alpha(1) subunit immunoreactive neurons were identified: type 1, the most numerous (60%), were medium-sized aspiny neurons that were immunoreactive for parvalbumin and alpha(1), beta(2,3), and gamma(2) subunits; type 2 (38%) were medium-sized to large aspiny neurons immunoreactive for calretinin and alpha(1), alpha(3), beta(2,3), and gamma(2) subunits; and type 3 (2%) were large sparsely spiny neurons immunoreactive for alpha(1), alpha(3), beta(2,3), and gamma(2) subunits. Type 4 neurons were calbindin-positive and immunoreactive for alpha(2), alpha(3), beta(2,3), and gamma(2) subunits. The remaining neurons were immunoreactive for choline acetyltransferase (ChAT) and alpha(3) subunit (type 5) or were neuropeptide Y-positive with no GABA(A) receptor subunit immunoreactivity (type 6). The globus pallidus contained three types of neurons: types 1 and 2 were large neurons and were immunoreactive for alpha(1), alpha(3), beta(2,3), and gamma(2) subunits and for parvalbumin alone (type 1) or for both parvalbumin and calretinin (type 2); type 3 neurons were medium-sized and immunoreactive for calretinin and alpha(1), beta(2, 3), and gamma(2) subunits. These results show that the subunit composition of GABA(A) receptors displays considerable regional and cellular variation in the human striatum but are more homogeneous in the globus pallidus.
Inhibitory interneurons are critical to sensory transformations, plasticity and synchronous activity in the neocortex. There are many types of inhibitory neurons, but their synaptic organization is poorly understood. Here we describe two functionally distinct inhibitory networks comprising either fast-spiking (FS) or low-threshold spiking (LTS) neurons. Paired-cell recordings showed that inhibitory neurons of the same type were strongly interconnected by electrical synapses, but electrical synapses between different inhibitory cell types were rare. The electrical synapses were strong enough to synchronize spikes in coupled interneurons. Inhibitory chemical synapses were also common between FS cells, and between FS and LTS cells, but LTS cells rarely inhibited one another. Thalamocortical synapses, which convey sensory information to the cortex, specifically and strongly excited only the FS cell network. The electrical and chemical synaptic connections of different types of inhibitory neurons are specific, and may allow each inhibitory network to function independently.
A puzzling feature of the neocortex is the rich array of inhibitory interneurons. Multiple neuron recordings revealed numerous
electrophysiological-anatomical subclasses of neocortical γ-aminobutyric acid–ergic (GABAergic) interneurons and three types
of GABAergic synapses. The type of synapse used by each interneuron to influence its neighbors follows three functional organizing
principles. These principles suggest that inhibitory synapses could shape the impact of different interneurons according to
their specific spatiotemporal patterns of activity and that GABAergic interneuron and synapse diversity may enable combinatorial
inhibitory effects in the neocortex.
Networks of GABAergic interneurons are implicated in synchronizing cortical activity at gamma frequencies (30-70 Hz). Here we demonstrate that the combined electrical and GABAergic synaptic coupling of basket cells instantaneously entrained gamma-frequency postsynaptic firing in layers 2/3 of rat somatosensory cortex. This entrainment was mediated by rapid curtailment of gap junctional coupling potentials by GABAA receptor-mediated IPSPs. Electron microscopy revealed spatial proximity of gap junctions and GABAergic synapses on somata and dendrites. Electrical coupling alone entrained postsynaptic firing with a phase lag, whereas unitary GABAergic connections were ineffective in gamma-frequency phasing. These observations demonstrate precise spatiotemporal mechanisms underlying action potential timing in oscillating interneuronal networks.
Electrical coupling by gap junctions is an important form of cell-to-cell communication in early brain development. Whereas glial cells remain electrically coupled at postnatal stages, adult vertebrate neurons were thought to communicate mainly via chemical synapses. There is now accumulating evidence that in certain neuronal cell populations the capacity for electrical signaling by gap junction channels is still present in the adult. Here we identified electrically coupled pairs of neurons between postnatal days 12 and 18 in rat visual cortex, somatosensory cortex, and hippocampus. Notably, coupling was found both between pairs of inhibitory neurons and between inhibitory and excitatory neurons. Molecular analysis by single-cell reverse transcription-PCR revealed a differential expression pattern of connexins in these identified neurons.
Gamma oscillations synchronized between distant neuronal populations may be critical for binding together brain regions devoted to common processing tasks. Network modeling predicts that such synchrony depends in part on the fast time course of excitatory postsynaptic potentials (EPSPs) in interneurons, and that even moderate slowing of this time course will disrupt synchrony. We generated mice with slowed interneuron EPSPs by gene targeting, in which the gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was altered to drive expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit GluR-B. GluR-B is a determinant of the relatively slow EPSPs in excitatory neurons and is normally expressed at low levels in gamma-aminobutyric acid (GABA)ergic interneurons, but at high levels in the GAD-GluR-B mice. In both wild-type and GAD-GluR-B mice, tetanic stimuli evoked gamma oscillations that were indistinguishable in local field potential recordings. Remarkably, however, oscillation synchrony between spatially separated sites was severely disrupted in the mutant, in association with changes in interneuron firing patterns. The congruence between mouse and model suggests that the rapid time course of AMPA receptor-mediated EPSPs in interneurons might serve to allow gamma oscillations to synchronize over distance.
A novel, local inhibitory circuit in layer 2/3 of rat somatosensory cortex is described that connects pyramidal cells reciprocally with GABAergic vasoactive intestinal polypeptide-immunoreactive bipolar interneurons. In paired whole-cell recordings, the glutamatergic unitary responses (EPSPs or EPSCs) in bipolar cells evoked by repetitive (10 Hz) stimulation of a pyramidal cell show strong frequency-dependent depression. Unitary IPSPs evoked in pyramidal cells by repetitive stimulation of bipolar cells, on average, maintained their amplitude. This suggests that the excitatory synapses on bipolar cells act as a low-pass filter in the reciprocal pyramid-to-bipolar circuit. The EPSCs in bipolar cells are mediated predominantly by AMPA receptor (AMPAR) channels. AMPARs desensitize rapidly and recover slowly from desensitization evoked by a brief pulse of glutamate. In slices, reduction of AMPAR desensitization by cyclothiazide (50-100 microm) or conditioning steady-state desensitization induced by application of extracellular AMPA (50 nm) or glutamate (50 microm) strongly reduced synaptic depression. It is concluded that in the local circuits between pyramidal and bipolar cells the desensitization of AMPARs in bipolar cells contributes to low-pass feedback inhibition of layer 2/3 pyramidal neurons by bipolar cells.
The chapter discusses the aspiny nonpyramidal neurons with short-axon or aspiny interneurons in the neocortex, unless otherwise specified. The chapter provides some historical views whose roots lie in the observations of Cajal, and discusses certain recent general aspects of neocortical aspiny interneurons. Interneurons show a great variety of morphological, biochemical, and physiological types. They constitute approximately 15-30% of the total population of neurons. All interneurons have a more or less dense axonal arborization distributed near the cell body, mainly within the area occupied by their dendritic field. Most interneurons are GABAergic, and express a number of different neurotransmitters (or their synthesizing enzymes), neuropeptides and calcium binding proteins. Thus, interneurons are biochemically, and widely heterogeneous. Some interneurons are common to all species, and therefore, are considered as basic elements of cortical circuits. Others may represent evolutionary specializations, which are characteristic of particular mammalian subgroups, and thus, cannot be taken as essential, or general, features of cortical organization.
In native brain membranes the principal excitatory neurotransmitter l-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.
The regional and cellular localisation of gamma-aminobutyric acidA (GABAA) receptors was investigated in the human basal ganglia using receptor autoradiography and immunohistochemical staining for five GABAA receptor subunits (α1, α2, α3, β2,3, and γ2) and other neurochemical markers. The results demonstrated that GABAA receptors in the striatum showed considerable subunit heterogeneity in their regional distribution and cellular localisation. High densities of GABAA receptors in the striosome compartment contained the α2, α3, β2,3, and γ2 subunits, and lower densities of receptors in the matrix compartment contained the α1, α2, α3, β2,3, and γ2 subunits. Also, six different types of neurons were identified in the striatum on the basis of GABAA receptor subunit configuration, cellular and dendritic morphology, and chemical neuroanatomy. Three types of α1 subunit immunoreactive neurons were identified: type 1, the most numerous (60%), were medium-sized aspiny neurons that were immunoreactive for parvalbumin and α1, β2,3, and γ2 subunits; type 2 (38%) were medium-sized to large aspiny neurons immunoreactive for calretinin and α1, α3, β2,3, and γ2 subunits; and type 3 (2%) were large sparsely spiny neurons immunoreactive for α1, α3, β2,3, and γ2 subunits. Type 4 neurons were calbindin-positive and immunoreactive for α2, α3, β2,3, and γ2 subunits. The remaining neurons were immunoreactive for choline acetyltransferase (ChAT) and α3 subunit (type 5) or were neuropeptide Y-positive with no GABAA receptor subunit immunoreactivity (type 6). The globus pallidus contained three types of neurons: types 1 and 2 were large neurons and were immunoreactive for α1, α3, β2,3, and γ2 subunits and for parvalbumin alone (type 1) or for both parvalbumin and calretinin (type 2); type 3 neurons were medium-sized and immunoreactive for calretinin and α1, β2,3, and γ2 subunits. These results show that the subunit composition of GABAA receptors displays considerable regional and cellular variation in the human striatum but are more homogeneous in the globus pallidus. J. Comp. Neurol. 415:313–340, 1999. © 1999 Wiley-Liss, Inc.
Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restricted distribution in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like immunoreactive neurons appear to be GABAergic. Therefore, cannabinoids and cannabinoid receptors may play a role in modulating GABAergic neurons.
The distribution of connexin36 (Cx36) in the adult rat brain and retina has been analysed at the protein (immunofluorescence) and mRNA (in situ hybridization) level. Cx36 immunoreactivity, consisting primarily of round or elongated puncta, is highly enriched in specific brain regions (inferior olive and the olfactory bulb), in the retina, in the anterior pituitary and in the pineal gland, in agreement with the high levels of Cx36 mRNA in the same regions. A lower density of immunoreactive puncta can be observed in several brain regions, where only scattered subpopulations of cells express Cx36 mRNA. By combining in situ hybridization for Cx36 mRNA with immunohistochemistry for a general neuronal marker (NeuN), we found that neuronal cells are responsible for the expression of Cx36 mRNA in inferior olive, cerebellum, striatum, hippocampus and cerebral cortex. Cx36 mRNA was also demonstrated in parvalbumin-containing GABAergic interneurons of cerebral cortex, striatum, hippocampus and cerebellar cortex. Analysis of developing brain further revealed that Cx36 reaches a peak of expression in the first two weeks of postnatal life, and decreases sharply during the third week. Moreover, in these early stages of postnatal development Cx36 is detectable in neuronal populations that are devoid of Cx36 mRNA at the adult stage. The developmental changes of Cx36 expression suggest a participation of this connexin in the extensive interneuronal coupling which takes place in several regions of the early postnatal brain.
NMDA receptors are composed of proteins from two families: NMDAR1, which are required for channel activity, and NMDAR2, which modulate properties of the channels. The mRNA encoding the NMDAR2D subunits has a highly restricted pattern of expression: in the forebrain, it is found in only a small subset of cortical, neostriatal and hippocampal neurons. We have used a quantitative double-label in situ hybridization method to examine the expression of NMDAR2D mRNA in neurochemically defined populations of neurons. In the neostriatum. NMDAR2D was expressed by the interneuron populations marked by preprosomatostatin (SOM), the 67-kDa form of glutamic acid decarboxylase (GAD67), parvalbumin (PARV), and choline acetyltransferase (ChAT) mRNAs but not by the projection neurons expressing β-preprotachykinin (SP) or preproenkephalin (ENK) mRNAs. In the neocortex, NMDAR2D expression was observed in only a small number of neurons, but these included almost all of the SOM-, GAD67- and PARV-expressing interneurons. In the hippocampus. NMDAR2D was not present in pyramidal or granule cells, but was abundant in SOM-, GAD67-, and PARV-positive interneurons. NMDAR2D expression appears to be a property shared by interneurons in several regions of the brain. The unique electrophysiological characteristics conveyed by this subunit, which include resistance to blockade by magnesium ion and long channel offset latencies, may be important for the integrative functions of these neurons. NMDAR2D-containing receptor complexes may prove to be important therapeutic targets in human disorders of movement. In addition the presence of NMDAR2D subunits may contribute to the differential vulnerability of interneurons to excitotoxic injury.
Recording of glutamate-activated currents in membrane patches was combined with RT-PCR-mediated AMPA receptor (AMPAR) subunit mRNA analysis in single identified cells of rat brain slices. Analysis of AMPARs in principal neurons and interneurons of hippocampus and neocortex and in auditory relay neurons and Bergmann glial cells indicates that the GluR-B subunit in its flip version determines formation of receptors with relatively slow gating, whereas the GluR-D subunit promotes assembly of more rapidly gated receptors. The relation between Ca2+ permeability of AMPAR channels and the relative GluR-B mRNA abundance is consistent with the dominance of this subunit in determining the Ca2+ permeability of native receptors. The results suggest that differential expression of GluR-B and GluR-D subunit genes, as well as splicing and editing of their mRNAs, account for the differences in gating and Ca2+ permeability of native AMPAR channels.
The expression of two alternative splice variants, Flip and Flop, in mRNAs encoding the four AMPA-selective glutamate receptors (GluR-A, -B, -C, and -D) was studied in the developing brain by in situ hybridization. These receptors are expressed prominently before birth, and patterns of distribution for Flip versions remain largely invariant during postnatal brain development. In contrast, the Flop versions are expressed at low levels prior to postnatal day 8. Around this time, the expression of Flop mRNAs increases throughout the brain, reaching adult levels by postnatal day 14. Thus, receptors carrying the Flop module appear to participate in mature receptor forms.
The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.
In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.
Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.
An in situ study of mRNAs encoding NMDA receptor subunits in the developing rat CNS revealed that, at all stages, the NR1 gene is expressed in virtually all neurons, whereas the four NR2 transcripts display distinct expression patterns. NR2B and NR2D mRNAs occur prenatally, whereas NR2A and NR2C mRNAs are first detected near birth. All transcripts except NR2D peak around P20. NR2D mRNA, present mainly in midbrain structures, peaks around P7 and thereafter decreases to adult levels. Postnatally, NR2B and NR2C transcript levels change in opposite directions in the cerebellar internal granule cell layer. In the adult hippocampus, NR2A and NR2B mRNAs are prominent in CA1 and CA3 pyramidal cells, but NR2C and NR2D mRNAs occur in different subsets of interneurons. Recombinant binary NR1-NR2 channels show comparable Ca2+ permeabilities, but marked differences in voltage-dependent Mg2+ block and in offset decay time constants. Thus, the distinct expression profiles and functional properties of NR2 subunits provide a basis for NMDA channel heterogeneity in the brain.
Tau is abundant in the axon, whereas MAP2 is found in the cell body and dendrites. To understand their differential localization, we performed transfection studies on primary cultured neurons using tagged tau, MAP2, MAP2C, and their chimeric/deletion mutants. We found that MAP2 was prevented from entering the axon by its N-terminal projection domain and that microtubule binding of tau was stronger in the axon than in the cell body and dendrites, whereas that of MAP2/MAP2C was tighter in the cell body and dendrites than in the axon. These binding properties were determined by their microtubule-binding domains and were suggested to be regulated by phosphorylation, at least in the case of tau. Thus, the suppressed axonal transit of MAP2 and locally regulated microtubule binding may play important roles for their sorting in neurons.
Fast excitatory synaptic transmission in the CNS is mediated by AMPA-type glutamate receptor (GluR) channels. Heterologous expression suggested that the Ca2+ permeability of these receptors critically depends on the subunit composition. Using patch-clamp techniques in brain slices, we found that the Ca2+ permeability of native AMPA-type GluRs was markedly higher in nonpyramidal (PCa/PK approximately 0.63) than in pyramidal (PCa/PK approximately 0.05) neurons of rat neocortex. Analysis of mRNA in single cells indicated that the relative abundance of GluR-B-specific mRNA was significantly lower in nonpyramidal (GluR-B/GluR-non-B approximately 0.3) than in pyramidal (GluR-B/GluR-non-B approximately 3) cells. This suggests that differences in relative abundance of GluR-B-specific mRNA generate functional diversity of AMPA-type GluRs in neurons with respect to Ca2+ permeability.
The use of immunocytochemical and histochemical methods to study colocalization of two or more chemical compounds in the same neuron has revealed that there is a great diversity of neocortical neurons. Neurons double stained for two or more substances are found in all layers, except in layer I, in all cortical areas and species studied so far, but there seem to be differences in the proportions and numbers of neurons double stained for a particular combination of substances between different species, and between cortical layers and cortical areas of the same species.
With few (though important) exceptions, the majority of classic neurotransmitters, neuropeptides, calciumbinding proteins, and cell surface molecules currently known to be present in the neocortex are localized mainly, if not exclusively, in smooth nonpyramidal cells. Therefore, chemical diversity of neurons in the neocortax is mainly a feature of the population of nonpyramidal cells. It seems clear that no combinations of double staining are found exclusively in a single morphological type of nonpyramidal neuron. However, in some species, certain combinations are found selectively in particular groups of smooth nonpyramidial neurons. Conversely, the same morphological type of neuron can display a variety of chemical characteristics depending on the species and cortical area and layer in which it is located. These studies have so far proved useful to characterize and identify cortical nouronal subpopulations and cortical circuits anatomically, but they probably have functional significance as well.
Cannabinoid receptor mRNA was localized in adult rat brain by 35S-tailed oligonucleotide probes and in situ hybridization histochemistry. Labelling is described as uniform or non-uniform depending on the relative intensities of individual cells expressing cannabinoid receptor mRNA within a given region or nucleus. Uniform labelling was found in the hypothalamus, thalamus, basal ganglia, cerebellum and brainstem. Non-uniform labelling that resulted from the presence of cells displaying two easily distinguishable intensities of hybridization signals was observed in several regions and nuclei in the forebrain (cerebral cortex, hippocampus, amygdala, certain olfactory structures). Olfactory-associated structures, basal ganglia, hippocampus, and cerebellar cortex displayed the heaviest amounts of labelling. Many regions that displayed cannabinoid receptor mRNA could reasonably be identified as sources for cannabinoid receptors on the basis of well documented hodologic data. Other sites that were also clearly labelled could not be assigned as logical sources of cannabinoid receptors. The localization of cannabinoid receptor mRNA indicates that sensory, motor, cognitive, limbic, and autonomic systems should all be influenced by the activation of this receptor by either exogenous cannabimimetics, including marijuana, or the yet unknown endogenous "cannabinoid" ligand.
We used in situ hybridization and immunocytochemistry to investigate the presence of GABA in neurons that express the type 3 serotonin receptor (5-HT3R). Quantitative analysis indicated that more than 90% of 5-HT3R expressing cells are GABAergic in the neocortex and hippocampus. The co-existence of 5-HT3R and GABA in cortical and hippocampal neurons indicates that serotonin, via 5-HT3R, can affect GABA release and suggests the participation of 5-HT3R in the inhibitory regulation of forebrain neurons.
Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors are assembled from the four subunits GluR1, 2, 3, 4 (or GluRA, B, C, D). AMPA channels that do not contain the GluR2 subunit are permeable to calcium. Recent studies indicate that excitotoxic as well as epileptic and ischemic cell damage may be mediated not only by N-methyl-Daspartate receptors, but also by AMPA receptors. The majority of interneurons in the hippocampus are resistant, but subsets of interneurons are consistently damaged in different disease states. Single immunolabeling using antibodies against AMPA receptor subunits, together with double immunolabeling for calcium-binding proteins (parvalbumin, calbindin and calretinin) and the neuropeptide somatostatin, were performed to study GluR1-4 immunoreactivity in interneuronal populations and principal cells. The ultrastructure of GluR1-4 labeled neurons was also examined using electron microscopy. With the exception of calbindin-positive interneurons, GluR2/3 was absent from hippocampal interneurons in both rat and monkey. In the rat, interneurons were more strongly immunoreactive against GluR1 than principal cells. In the monkey, immunoreactivity for GluR4 in interneurons was stronger than for GluR1. All GluR subunits were confined to spines, dendritic membrane and cytoplasm surrounding the nucleus but absent from axons and presynaptic terminals. Our findings suggest that hippocampal principal cells and interneurons express different complements of AMPA receptor subunits. Furthermore, the absence of GluR2 and/or GluR3 in both vulnerable and resistant interneurons subtypes indicates that knowledge of receptor subunit composition is not sufficient to predict neuronal vulnerability.
Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate–putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer.
D1 and D2 receptors have been described in different populations of efferent pyramidal neurons of the rat frontal cortex. Combined in situ hybridization and immunocytochemistry show here that these two subtypes are expressed in cortical GABAergic interneurons, with D1 and D2 mainly found in a subpopulation containing parvalbumin, whereas only 10% of the calbindin neurons express D1 receptors. These data indicate that various DA agonists may influence inhibitory circuits by distinct dopamine receptor subtypes.
Encoding of information in the cortex is thought to depend on synchronous firing of cortical neurons. Inhibitory neurons are known to be critical in the coordination of cortical activity, but how interaction among inhibitory cells promotes synchrony is not well understood. To address this issue directly, we have recorded simultaneously from pairs of fast-spiking (FS) cells, a type of gamma-aminobutyric acid (GABA)-containing neocortical interneuron. Here we report a high occurrence of electrical coupling among FS cells. Electrical synapses were not found among pyramidal neurons or between FS cells and other cortical cells. Some FS cells were interconnected by both electrical and GABAergic synapses. We show that communication through electrical synapses allows excitatory signalling among inhibitory cells and promotes their synchronous spiking. These results indicate that electrical synapses establish a network of fast-spiking cells in the neocortex which may play a key role in coordinating cortical activity.
Cannabinoids can modulate motor behaviour, learning and memory, cognition and pain perception. These effects correlate with the expression of the cannabinoid receptor 1 (CB1) and with the presence of endogenous cannabinoids in the brain. In trying to obtain further insights into the mechanisms underlying the modulatory effects of cannabinoids, CB1-positive neurons were determined in the murine forebrain at a single cell resolution. We performed a double in situ hybridization study to detect mRNA of CB1 in combination with mRNA of glutamic acid decarboxylase 65k, neuropeptide cholecystokinin (CCK), parvalbumin, calretinin and calbindin D28k, respectively. Our results revealed that CB1-expressing cells can be divided into distinct neuronal subpopulations. There is a clear distinction between neurons containing CB1 mRNA either at high levels or low levels. The majority of high CB1-expressing cells are GABAergic (gamma-aminobutyric acid) neurons belonging mainly to the cholecystokinin-positive and parvalbumin-negative type of interneurons (basket cells) and, to a lower extent, to the calbindin D28k-positive mid-proximal dendritic inhibitory interneurons. Only a fraction of low CB1-expressing cells is GABAergic. In the hippocampus, amygdala and entorhinal cortex area, CB1 mRNA is present at low but significant levels in many non-GABAergic cells that can be considered as projecting principal neurons. Thus, a complex mechanism appears to underlie the modulatory effects of cannabinoids. They might act on principal glutamatergic circuits as well as modulate local GABAergic inhibitory circuits. CB1 is very highly coexpressed with CCK. It is known that cannabinoids and CCK often have opposite effects on behaviour and physiology. Therefore, we suggest that a putative cross-talk between cannabinoids and CCK might exist and will be relevant to better understanding of physiology and pharmacology of the cannabinoid system.
By coupling glutamate to the IP(3) signaling pathway, group I metabotropic receptors can increase intracellular Ca(2+) concentration, and might thus contribute to excitotoxicity. To identify neurons that might be vulnerable to such injury, we performed immunofluorescence histochemistry for metabotropic glutamate receptor 1alpha (mGluR1alpha) in the cerebral cortex of adult rat. mGluR1alpha was in somata and dendrites of a subset of non-pyramidal neurons scattered throughout the cerebral cortex. To further characterize mGluR1alpha-positive neurons, we investigated its colocalization with several neurochemical markers. Nearly all mGluR1alpha-positive cells were interneurons immunopositive for gamma-aminobutyric acid. The majority (70-80%) of mGluR1alpha-immunopositive neurons were double-labeled for somatostatin. Approximately half of calretinin-positive neurons and 30% of calbindin-positive neurons expressed mGluR1alpha. In contrast, parvalbumin-expressing neurons were rarely positive for mGluR1alpha. Neurons staining strongly for mGluR1alpha were also positive for GluR1. These results indicated that mGluR1alpha is expressed by specific classes of GABAergic neurons in the neocortex, and suggests a mechanism by which these neurons may be especially vulnerable to excitotoxic injury.
μ-Opioid receptor-expressing neurons in the rat cerebral neocortex were characterized by an immunolabeling method with an antibody to a carboxyl terminal portion of the receptor. They were small, bipolar, vertically elongated, non-pyramidal neurons, and scattered mainly in layers II–IV. We examined chemical characteristics of μ-opioid receptor-expressing neocortical neurons by the double immunofluorescence method. Almost all neuronal cell bodies expressing μ-opioid receptor-like immunoreactivity showed immunoreactivity for GABA, suggesting that they were cortical inhibitory interneurons. μ-Opioid receptor-immunoreactive neurons were further studied by the double staining method with markers for the subgroups of cortical GABAergic neurons. Immunoreactivities for vasoactive intestinal polypeptide, corticotropin releasing factor, choline acetyltransferase, calretinin and cholecystokinin were found in 92, 79, 67, 35 and 35% of μ-opioid receptor-immunoreactive cortical neurons, respectively. In contrast, less than 10% of μ-opioid receptor-immunoreactive neurons showed immunoreactivity for parvalbumin, calbindin, somatostatin, neuropeptide Y or nitric oxide synthase. Moreover, μ-opioid receptor-immunoreactive neurons very frequently exhibited preproenkephalin immunoreactivity, but not preprodynorphin immunoreactivity.
Local-circuit, gamma-aminobutyric acid-releasing inhibitory interneurons of the hippocampus and cortex have traditionally been considered as the regulators of principal neuron activity--the yin to the excitatory yang. Recent evidence indicates that, in addition to that role, their network connectivity and the properties of their intrinsic voltage-gated currents are finely tuned to permit inhibitory interneurons to generate and control the rhythmic output of large populations of both principal cells and other populations of inhibitory interneurons. This review brings together recently described properties and emerging principles of interneuron function that indicate a much more complex role for these cells than just providers of inhibition.
Neural processing occurs in parallel in distant cortical areas even for simple perceptual tasks. Associated cognitive binding is believed to occur through the interareal synchronization of rhythmic activity in the gamma (30-80 Hz) range. Such oscillations arise as an emergent property of the neuronal network and require conventional chemical neurotransmission. To test the potential role of gap junction-mediated electrical signaling in this network property, we generated mice lacking connexin 36, the major neuronal connexin. Here we show that the loss of this protein disrupts gamma frequency network oscillations in vitro but leaves high frequency (150 Hz) rhythms, which may involve gap junctions between principal cells (Schmitz et al., 2001), unaffected. Thus, specific connexins differentially deployed throughout cortical networks are likely to regulate different functional aspects of neuronal information processing in the mature brain.
Inhibitory interneurons often generate synchronous activity as an emergent property of their interconnections. To determine the role of electrical synapses in such activity, we constructed mice expressing histochemical reporters in place of the gap junction protein Cx36. Localization of the reporter with somatostatin and parvalbumin suggested that Cx36 was expressed largely by interneurons. Electrical synapses were common among cortical interneurons in controls but were nearly absent in knockouts. A metabotropic glutamate receptor agonist excited LTS interneurons, generating rhythmic inhibitory potentials in surrounding neurons of both wild-type and knockout animals. However, the synchrony of these rhythms was weaker and more spatially restricted in the knockout. We conclude that electrical synapses containing Cx36 are critical for the generation of widespread, synchronous inhibitory activity.
A central debate regarding neocortical function concerns the degree to which the underlying microcircuitry is stereotypically organized. Stereotypy reflects invariance in structure and function, as a result of common genetic templates and environmental conditions, whereas uniqueness can be caused by genetic variations, differences in environmental conditions as well as random processes. Stereotypy is an appealing concept because it provides strong support for determinism in the formation of neuronal microcircuits and in the relationship between their specific structure and function.