Synaptic relationships of a type of GABA-immunoreactive neuron (clutch cell), spiny stellate cells and lateral geniculate nucleus afferents in layer IVC of the monkey striate cortex

A Quantitative Comparison of Inhibitory Interneuron Size and Distribution between Mouse and Macaque V1, Using Calcium-Binding Proteins

The mouse is a useful and popular model for studying of visual cortical function. To facilitate the translation of results from mice to primates, it is important to establish the extent of cortical organization equivalence between species and to identify possible differences. We focused on the different types of interneurons as defined by calcium-binding protein (CBP) expression in the layers of primary visual cortex (V1) in mouse and rhesus macaque. CBPs parvalbumin (PV), calbindin (CB), and calretinin (CR) provide a standard, largely nonoverlapping, labeling scheme in macaque, with preserved corresponding morphologies in mouse, despite a slightly higher overlap. Other protein markers, which are relevant in mouse, are not preserved in macaque. We fluorescently tagged CBPs in V1 of both species, using antibodies raised against preserved aminoacid sequences. Our data demonstrate important similarities between the expression patterns of interneuron classes in the different layers between rodents and primates. However, in macaque, expression of PV and CB is more abundant, CR expression is lower, and the laminar distribution of interneuron populations is more differentiated. Our results reveal an integrated view of interneuron types that provides a basis for translating results from rodents to primates, and suggest a reconciliation of previous results.

A Neuronal Network Model of the Primate Visual System: Color Mechanisms in the Retina, LGN and V1

Color plays a key role in human vision but the neural machinery that underlies the transformation from stimulus to perception is not well understood. Here, we implemented a two-dimensional network model of the first stages in the primate parvocellular pathway (retina, lateral geniculate nucleus and layer 4C[Formula: see text] in V1) consisting of conductance-based point neurons. Model parameters were tuned based on physiological and anatomical data from the primate foveal and parafoveal vision, the most relevant visual field areas for color vision. We exhaustively benchmarked the model against well-established chromatic and achromatic visual stimuli, showing spatial and temporal responses of the model to disk- and ring-shaped light flashes, spatially uniform squares and sine-wave gratings of varying spatial frequency. The spatiotemporal patterns of parvocellular cells and cortical cells are consistent with their classification into chromatically single-opponent and double-opponent groups, and nonopponent cells selective for luminance stimuli. The model was implemented in the widely used neural simulation tool NEST and released as open source software. The aim of our modeling is to provide a biologically realistic framework within which a broad range of neuronal interactions can be examined at several different levels, with a focus on understanding how color information is processed.

Generation of diverse cortical inhibitory interneurons

First described by Ramon y Cajal as 'short-axon' cells over a century ago, inhibitory interneurons in the cerebral cortex make up ~20-30% of the neuronal milieu. A key feature of these interneurons is the striking structural and functional diversity, which allows them to modulate neural activity in diverse ways and ultimately endow neural circuits with remarkable computational power. Here, we review our current understanding of the generation of cortical interneurons, with a focus on recent efforts to bridge the gap between progenitor behavior and interneuron production, and how these aspects influence interneuron diversity and organization. WIREs Dev Biol 2018, 7:e306. doi: 10.1002/wdev.306 This article is categorized under: Nervous System Development > Vertebrates: General Principles.

The Diversity of Cortical Inhibitory Synapses

The most typical and well known inhibitory action in the cortical microcircuit is a strong inhibition on the target neuron by axo-somatic synapses. However, it has become clear that synaptic inhibition in the cortex is much more diverse and complicated. Firstly, at least ten or more inhibitory non-pyramidal cell subtypes engage in diverse inhibitory functions to produce the elaborate activity characteristic of the different cortical states. Each distinct non-pyramidal cell subtype has its own independent inhibitory function. Secondly, the inhibitory synapses innervate different neuronal domains, such as axons, spines, dendrites and soma, and their inhibitory postsynaptic potential (IPSP) size is not uniform. Thus, cortical inhibition is highly complex, with a wide variety of anatomical and physiological modes. Moreover, the functional significance of the various inhibitory synapse innervation styles and their unique structural dynamic behaviors differ from those of excitatory synapses. In this review, we summarize our current understanding of the inhibitory mechanisms of the cortical microcircuit.

The neocortex of cetartiodactyls. II. Neuronal morphology of the visual and motor cortices in the giraffe (Giraffa camelopardalis)

The present quantitative study extends our investigation of cetartiodactyls by exploring the neuronal morphology in the giraffe (Giraffa camelopardalis) neocortex. Here, we investigate giraffe primary visual and motor cortices from perfusion-fixed brains of three subadults stained with a modified rapid Golgi technique. Neurons (n = 244) were quantified on a computer-assisted microscopy system. Qualitatively, the giraffe neocortex contained an array of complex spiny neurons that included both "typical" pyramidal neuron morphology and "atypical" spiny neurons in terms of morphology and/or orientation. In general, the neocortex exhibited a vertical columnar organization of apical dendrites. Although there was no significant quantitative difference in dendritic complexity for pyramidal neurons between primary visual (n = 78) and motor cortices (n = 65), there was a significant difference in dendritic spine density (motor cortex > visual cortex). The morphology of aspiny neurons in giraffes appeared to be similar to that of other eutherian mammals. For cross-species comparison of neuron morphology, giraffe pyramidal neurons were compared to those quantified with the same methodology in African elephants and some cetaceans (e.g., bottlenose dolphin, minke whale, humpback whale). Across species, the giraffe (and cetaceans) exhibited less widely bifurcating apical dendrites compared to elephants. Quantitative dendritic measures revealed that the elephant and humpback whale had more extensive dendrites than giraffes, whereas the minke whale and bottlenose dolphin had less extensive dendritic arbors. Spine measures were highest in the giraffe, perhaps due to the high quality, perfusion fixation. The neuronal morphology in giraffe neocortex is thus generally consistent with what is known about other cetartiodactyls.

Inhibitory interneuron classes express complementary AMPA-receptor patterns in macaque primary visual cortex

Glutamate receptors mediate excitatory neurotransmission. A very prevalent type of glutamate receptor in the neocortex is the AMPA receptor (AMPAR). AMPARs mediate fast synaptic transmission and their functionality depends on the subunit composition. In primary visual cortex (area V1), the density and subunit composition of AMPARs differ among cortical layers and among cell types. The AMPARs expressed by the different types of inhibitory interneurons, which are crucial for network function, have not yet been characterized systematically. We investigated the distribution of AMPAR subunits in macaque V1 for three distinct subpopulations of inhibitory interneurons: parvalbumin-immunoreactive (PV-IR) interneurons, calbindin-immunoreactive (CB-IR) interneurons, and calretinin-immunoreactive (CR-IR) interneurons. We found that PV-IR cells, which have previously been identified as fast spiking, show high expression of the GluA2 and GluA3 subunits. In contrast, CB-IR and CR-IR cells, which tend to be intermediate spiking, show high expression of the GluA1 and GluA4 subunits. Thus, our data demonstrate that the expression of AMPARs divides inhibitory interneurons in macaque V1 into two categories that are compatible with existing classification methods based on calcium-binding proteins and firing behavior. Moreover, our findings suggest new approaches to target the different inhibitory interneuron classes pharmacologically in vivo.

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.

RC3/neurogranin is expressed in pyramidal neurons of motor and somatosensory cortex in normal and denervated monkeys

RC3/neurogranin is a neuron-specific calpacitin located in the cytoplasm and, especially, in dendrites and dendritic spines of cortical neurons, involved in many aspects of excitatory transmission and long-term potentiation. We investigated RC3 expression in pyramidal cortical neurons and interneurons of the motor and somatosensory cortex of normal Macaca fascicularis by means of double immunofluorescence and with techniques that combine immunohistochemistry and radioactive in situ hybridization. We show that RC3 is expressed in virtually all pyramidal neurons and spiny stellate neurons of neocortical areas 4, 3b, 1, 2, 5, 7, and SII, but not in the majority of cortical interneurons. RC3 protein and mRNA are tightly colocalized with the alpha subunit of CaM kinase II and the 200-kD, nonphosphorylated neurofilament, whereas they are absent from cells expressing the 27-kD, vitamin D-dependent calbindin and parvalbumin. In order to investigate possible activity-dependent regulation of the expression of RC3, we compared these results with those obtained from monkeys subjected to chronic peripheral cutaneous denervation of the first finger. We found that the pattern of distribution of RC3 in motor and somatosensory cortices after nerve cut did not differ from normal.

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and gamma-aminobutyric acidergic cortical cells in the prelimbic cortex of the rat

Although the reciprocal interconnections between the prefrontal cortex and the mediodorsal nucleus of the thalamus (MD) are well known, the involvement of inhibitory cortical interneurons in the neural circuit has not been fully defined. To address this issue, we conducted three combined neuroanatomical studies on the rat brain. First, the frequency and the spatial distribution of synapses made by reconstructed dendrites of nonpyramidal neurons were identified by impregnation of cortical cells with the Golgi method and identification of thalamocortical terminals by degeneration following thalamic lesions. Terminals from MD were found to make synaptic contacts with small dendritic shafts or spines of Golgi-impregnated nonpyramidal cells with very sparse dendritic spines. Second, a combined study that used anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry indicated that PHA-L-labeled terminals from MD made synaptic junctions with GABA-immunoreactive dendritic shafts and spines. Nonlabeled dendritic spines were found to receive both axonal inputs from MD with PHA-L labelings and from GABAergic cells. In addition, synapses were found between dendritic shafts and axon terminals that were both immunoreactive for GABA. Third, synaptic connections between corticothalamic neurons that project to MD and GABAergic terminals were investigated by using wheat germ agglutinin conjugated to horseradish peroxidase and postembedding GABA immunocytochemistry. GABAergic terminals in the prelimbic cortex made symmetrical synaptic contacts with retrogradely labeled corticothalamic neurons to MD. All of the synapses were found on cell somata and thick dendritic trunks. These results provide the first demonstration of synaptic contacts in the prelimbic cortex not only between thalamocortical terminals from MD and GABAergic interneurons but also between GABAergic terminals and corticothalamic neurons that project to MD. The anatomical findings indicate that GABAergic interneurons have a modulatory influence on excitatory reverberation between MD and the prefrontal cortex.

Cortical interneurons: from Cajal to 2001

After the collective work of many investigators, beginning with the early studies of Cajal, the following main [figure: see text] conclusions may be drawn regarding the morphology, biochemical characteristics and synaptic connections of interneurons: 1. Interneurons show a great variety of morphological, biochemical and physiological types. They constitute approximately 15-30% of the total population of neurons. 2. Because of the heterogeneity of interneurons and the lack of consensus as to which characteristics are essential for an individual neuron to be considered a member of a given cell type, there is no definitive classification of interneurons. Nevertheless, certain interneurons can be readily recognized by their unique morphological characteristics, or they can be more generally divided into subgroups on the basis of their biochemical characteristics, patterns of axonal arborization, or synaptic connections with pyramidal cells. 3. All interneurons have a more or less dense axonal arborization distributed near the cell body, mainly within the area occupied by their dendritic field. However, some interneurons may display, in addition, prominent long, horizontal or vertical axonal collaterals. [figure: see text] 4. Most interneurons form symmetrical synapses with both pyramidal cells and other interneurons, with the exception of chandelier cells, which only form synapses with the axon initial segment of pyramidal cells. Furthermore, interneurons are not only connected by chemical synapses (unidirectional connections), but they may also form electrical synapses through gap junctions (bidirectional) in a specific manner. 5. With the exception of chandelier cells, other types of interneurons include among their synaptic targets more than one type of postsynaptic element. But the degree of preference for these postsynaptic elements varies markedly between different types of interneurons. 6. The number of synapses made by a single axon originating from a given interneuron on another neuron is on the order of ten or less. Since, in general, cortical neurons receive many more interneuronal (symmetrical) synapses (on the order of a few hundred or thousand), a considerable convergence of various types of interneurons to pyramidal cells and interneurons appears to occur. 7. Most interneurons are GABAergic and also express a number of different neurotransmitters (or their synthesizing enzymes), neuropeptides and calcium-binding proteins. Thus, interneurons are, biochemically, widely heterogeneous. 8. Some of the morphologically identifiable neurons can be characterized by their particular biochemical characteristics, and some biochemically definable subgroups of interneurons display a particular morphology. However, different morphological types of GABAergic neurons may share one or several neurotransmitters, neuroactive substances and/or other molecular markers. Therefore, a great variety of subgroups of morphologically and biochemically identifiable neurons exist. 9. Some interneurons appear to be common to all species and, therefore, may be considered as basic elements of cortical circuits, whereas 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. 10. Given the complexity of cortical circuits and the areal and species differences, it is impossible to draw a "sufficiently" complete basic diagram of cortical microcircuitry that is valid for all cortical areas and species.

Immunoreactivity for Taurine Characterizes Subsets of Glia, GABAergic and non-GABAergic Neurons in the Neo- and Archicortex of the Rat, Cat and Rhesus Monkey: Comparison with Immunoreactivity for Homocysteic Acid

The cerebral cortex is an area rich in taurine (2-aminoethanesulphonic acid), but only limited information exists regarding its cellular distribution. We therefore examined taurine-like immunoreactivity in the cerebral cortex of the rat, cat and macaque monkey using antiserum directed against glutaraldehyde-conjugated taurine. Immunostaining was assessed at the light and electron microscopic level, and patterns obtained in light microscopic studies were compared to those produced with antiserum to gamma-aminobutyric acid (GABA) and homocysteic acid (HCA). In all three species, strong taurine-like immunoreactive perivascular endothelial cells, pericytes and oligodendrocytes were found. These cells were located throughout the neuropil, which itself showed a low level of immunoreactivity. In rats and cats, a small number of weakly taurine-enriched neurons were observed, particularly in superficial layers. In all cortical areas of the macaque, however, glial staining was matched by strong, selective staining of subpopulations of cortical neurons which were distributed in a bilaminar pattern involving layers II/III and VI. In addition, in primary visual cortex, area 17, immunopositive neurons were also present in sublayer IVCbeta, while in the hippocampus strongly taurine-positive neurons were most conspicuous in the granule cell layer of the dentate gyrus. In all regions, strongly taurine-positive neurons constituted only a subpopulation of the neurons occupying a given layer. Examination of adjacent sections for GABA immunoreactivity showed that the most strongly taurine-positive neurons in layers II/III were immunoreactive for GABA. The cells located in layers IVCbeta and VI, and the granule cells of the dentate gyrus, however, were GABA-negative. The morphological features of these latter groups suggested that the antiserum to taurine identifies subsets of spiny stellate, small pyramidal and dentate granule cells. None of these neurons showed immunoreactivity with antiserum to HCA in the primate; HCA-positive glia were found along the pial and white matter boundaries of the cortex, and showed no overlap with strongly taurine-positive glial elements. Although a transmitter role for taurine may be unlikely, particularly in view of its enrichment in subpopulations of both inhibitory and excitatory cells, the capacity of taurine to influence membrane-associated functions in excitable tissues, and its selective distribution demonstrated here, provides the potential for a contribution to communication between cortical cells.

Formation of dendritic spines with GABAergic synapses induced by whisker stimulation in adult mice

During development, alterations in sensory experience modify the structure of cortical neurons, particularly at the level of the dendritic spine. Are similar adaptations involved in plasticity of the adult cortex? Here we show that a 24 hr period of single whisker stimulation in freely moving adult mice increases, by 36%, the total synaptic density in the corresponding cortical barrel. This is due to an increase in both excitatory and inhibitory synapses found on spines. Four days after stimulation, the inhibitory inputs to the spines remain despite total synaptic density returning to pre-stimulation levels. Functional analysis of layer IV cells demonstrated altered response properties, immediately after stimulation, as well as four days later. These results indicate activity-dependent alterations in synaptic circuitry in adulthood, modifying the flow of sensory information into the cerebral cortex.

Axonal topography of cortical basket cells in relation to orientation, direction, and ocular dominance maps

The axonal (bouton) distributions of a layer 4 clutch cell (CC), two layer 3 medium-sized basket cells (MBC), and a layer 3 large basket cell (LBC) to orientation, direction, and ocular dominance maps were studied quantitatively. 1) The CC provided exclusively local projections (<380 microm from the soma) and contacted a narrow "niche" of functional representations. 2) The two MBCs emitted local projections (75% and 79% of all boutons), which were engaged with isoorientations (61% and 48%) and isodirections, and long-range projections (25% and 21%, >313 microm and >418 microm), which encountered cross-orientation sites (14% and 12%) and isoorientation sites (7% and 5%). Their direction preferences were mainly perpendicular to or opposite those of local projections. 3) The LBC provided the majority (60%) of its boutons to long-range distances (>437 microm). Locally, LBC boutons showed a rather balanced contribution to isoorientations (19%) and cross-orientations (12%) and preferred isodirections. Remotely, however, cross-orientation sites were preferred (31% vs. 23%) and the directional output was balanced. 4) Monte Carlo simulations revealed that the differences between the orientation specificity of local and long-range projections cannot be explained by a homogeneous lateral distribution of the boutons. 5) There was a similar eye preference in the local and long-range projection fields of the MBCs. The LBC contacted both contra- and ipsilateral eye domains. 6) The basket axons showed little laminar difference in orientation and direction topography. The results suggest that an individual basket cell can mediate a wide range of effects depending on the size and termination pattern of the axonal field.

Calretinin in the entorhinal cortex of the rat: distribution, morphology, ultrastructure of neurons, and co-localization with gamma-aminobutyric acid and parvalbumin

Calretinin is a marker that differentially labels neurons in the central nervous system. We used this marker to distinguish subtypes of neurons within the general population of neurons in the entorhinal cortex of the rat. The distribution, morphology, and ultrastructure of calretinin-immunopositive neurons in this cortical area were documented. We further analyzed the co-localization of the marker with gamma-aminobutyric acid (GABA) and studied whether calretinin-positive neurons project to the hippocampal formation. Methods used included single-label immunocytochemistry at the light and electron microscopic level, retrograde tracing combined with immunocytochemistry, and double-label confocal laser scanning microscopy (CLSM). The entorhinal cortex contained calretinin-positive cells in a scattered fashion, in all layers except layer IV (lamina dissecans). Bipolar and multipolar dendritic configurations were present, displaying smooth dendrites. Bipolar cells had a uniform morphology whereas the multipolar calretinin cell population consisted of large neurons, cells with long ascending dendrites, horizontally oriented neurons, and small spherical cells. Retrograde tracing combined with immunocytochemistry showed that calretinin is not present in cells projecting to the hippocampus. Few synapic contacts between calretinin-positive axon terminals and immunopositive cell bodies and dendrites were seen. Most axon terminals of calretinin fibers formed asymmetrical synapses, and immunopositive axons were always unmyelinated. Results obtained in the CLSM indicate that calretinin co-exists in only 18-20% of the GABAergic cell population (mostly small spherical and bipolar cells). Thus, the entorhinal cortex contains two classes of calretinin interneurons: GABA positive and GABA negative. The first class is presumably a classical, GABAergic inhibitory interneuron. The finding of calretinin-immunoreactive axon terminals with asymmetrical synapses suggests that the second class of calretinin neuron is a novel type of a (presumably excitatory) interneuron.

Termination of the geniculocortical projection in the striate cortex of macaque monkey: a quantitative immunoelectron microscopic study

The goal of this present study was to derive a new estimate of the synaptic contribution of the dorsal lateral geniculate nucleus (dLGN) to the subdivisions of its main recipient layer, layer 4C, of striate cortex of macaque monkey. The projection from the dLGN and its terminal boutons within layer 4C were visualized by immunodetection of the calcium binding protein, parvalbumin (PV), which is expressed in relay cells of the dLGN. The proportion of asymmetric synapses formed by PV-positive boutons within the alpha and beta sublayers of 4C was estimated by using a nonbiased stereological counting method. The proportion of asymmetric synapses contributed by the PV-positive boutons to layer 4Calpha is 8.7%; to 4Cbeta is 6.9%. Assuming all the PV-positive asymmetric synapses derive from the dLGN relay cells, this gives a ratio of dLGN synapses per neuron of 192 in layer 4Calpha and 128 in layer 4Cbeta. Thus, the recurrent excitatory input from neighboring cortical neurons must play an important part in responses of the neurons lying at the input stage of the cortical circuit.

Salient features of synaptic organisation in the cerebral cortex

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.

5-HT2A receptor antagonists inhibit potassium-stimulated gamma-aminobutyric acid release in rat frontal cortex

Several drugs selective for the serotonin 5-HT2A receptor were tested for their effects on spontaneous and K(+)-evoked [3H] gamma-aminobutyric acid (GABA) release from slices of rat frontal cortex. Under K+ stimulation, the antagonists ketanserin, spiperone, R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenethyl)]- 4-piperidinemethanol (MDL 100,907) and ritanserin inhibited GABA release by 12-31%. Rats were treated with the serotonin-depleting agent para-chlorophenylalanine and with the serotonergic neurotoxin para-chloroamphetamine. In para-chlorophenylalanine-treated animals, stimulated GABA release in the presence of ketanserin remained depressed. In animals treated with both para-chlorophenylalanine and para-chloroamphetamine, ketanserin or the hallucinogenic agonist (2,5-dimethoxy-4-iodophenyl)-2-aminoethane (2C-I) each appeared to decrease stimulated GABA release but this was not significant. However, when ketanserin and 2C-I were both present in the superfusion buffer an additive inhibitory effect was observed, and GABA release was decreased 30%. These results suggest that serotonin facilitates GABA release in cortex via 5-HT2A receptors and that the functional response of this system is resistant to serotonin depletion.

Synaptic output of physiologically identified spiny stellate neurons in cat visual cortex

Spiny stellate neurons of area 17 of the cat's visual cortex were physiologically characterised and injected intracellularly with horseradish peroxidase. Six neurons from sublamina 4A were selected. Five had the S-type of simple receptive fields; one had a complex receptive field. Their axons formed boutons mainly in layers 3 and 4. An electron microscopic examination of 45 boutons showed that each bouton formed one asymmetric synapse on average. Spines were the most frequent synaptic target (74%); dendritic shafts formed the remainder (26%). On the basis of ultrastructural characteristics, 8% of the target dendrites were characterised as originating from smooth gamma-aminobutyrate-ergic (GABAergic) neurons. Thus the major output of spiny stellate neurons is to other spiny neurons, probably pyramidal neurons in layer 3 and spiny stellates in layer 4.

Polyneuronal innervation of spiny stellate neurons in cat visual cortex

Our hypothesis was that spiny stellate neurons in layer 4 of cat visual cortex receive polyneuronal innervation. We characterised the synapses of four likely sources of innervation by three simple criteria: the type of synapse, the target (spine, dendritic shaft), and the area of the presynaptic bouton. The layer 6 pyramids had the smallest boutons and formed asymmetric synapses mainly with the dendritic shaft. The thalamic afferents had the largest boutons and formed asymmetric synapses mainly with spines. The spiny stellates had medium-sized boutons and formed asymmetric synapses mainly with spines. We used these to make a "template" to match against the boutons forming synapses with the spiny stellate dendrite. Of the asymmetric synapses, 45% could have come from layer 6 pyramidal neurons, 28% from spiny stellate neurons, and 6% from thalamic afferents. The remaining 21% of asymmetric synapses could not be accounted for without assuming some additional selectivity of the presynaptic axons. Additional asymmetric synapses may come from a variety of sources, including other cortical neurons and subcortical nuclei such as the claustrum. Of the symmetric synapses, 84% could have been provided by clutch cells, which form large boutons. The remainder, formed by small boutons, probably come from other smooth neurons in layer 4, e.g., neurogliaform and bitufted neurons. Our analysis supports the hypothesis that the spiny stellate receives polyneuronal innervation, perhaps from all the sources of boutons in layer 4. Although layer 4 is the major recipient of thalamic afferents, our results show that they form only a few percent of the synapses of layer 4 spiny stellate neurons.

Precision and variability in postsynaptic target selection of inhibitory cells in the hippocampal CA3 region

Non-pyramidal cells were filled intracellularly with biocytin in the CA3 region of the guinea-pig hippocampus in vitro, within or close to stratum pyramidale. On the basis of camera lucida reconstructions and electron microscopy, six different cell types with distinct laminar distribution of axon terminals could be distinguished. The axon of three axo-axonic cells, three typical basket cells, and atypical basket cells of two types arborized in the perisomatic and proximal dendritic region of CA3 pyramidal cells. Two cells with axons innervating the distal dendritic segments of pyramidal cells were also found; one terminated in stratum radiatum and the other in stratum lacunosum-moleculare. Electron microscopy demonstrated that symmetrical synapses were formed by the labelled boutons on axon initial segments, somata, and proximal or distal dendrites of mostly pyramidal neurons. Axo-axonic cells showed absolute target selectivity for axon initial segments, whereas for the other cells the distribution of contacted elements was determined by the laminar distribution of axon terminals. In two cases, where additional cells were labelled with biocytin, multiple (up to nine) light microscopically identified contacts (presumed synaptic contacts) were established by the interneurons on several pyramidal cells and on an axo-axonic cell. Our results show that a restricted set of inhibitory cells, with somata within or close to CA3 stratum pyramidale, possess variable patterns of axonal arborization. Various types of postsynaptic elements are contacted, but precision in selecting certain targets and ignoring others is maintained within a particular cell type and layer. In contrast to the diversity of axonal arbors the structure of the dendritic trees shows no consistent differences, suggesting that the cells may be activated by a similar set of afferents. It seems probable that the innervation of precise regions of postsynaptic pyramidal cells by different types of interneurons--often in conjunction with particular excitatory afferents (Han et al., Eur. J. Neurosci., 5, 395-410, 1993)--underlies functional differences in inhibitory synaptic actions.

Electron microscopy of Golgi-impregnated interneurons: notes on the intrinsic connectivity of the cerebral cortex

The Golgi-electron microscope technique has opened new avenues to explore the synaptic organization of the brain. In this article, we shall discuss basic methodological principles necessary to analyze axonal arborizations with this combined technique. To illustrate the applications of the method, we shall review the forms and distribution of the synapses in which the axonal arborizations of local cortical interneurons engage.

Expression of GABA and GABAA receptors by neurons of the subplate zone in developing primate occipital cortex: evidence for transient local circuits

Several lines of evidence suggest that the transient subplate zone of the embryonic mammalian telencephalon could influence cortical development through synaptic or trophic interactions with growing cortical afferents and migrating neurons. Since such interactions may involve neurotransmitters and their receptor molecules, we have examined the expression of GABA and subunits of the GABAA/benzodiazepine receptor complex in the occipital lobe of embryonic rhesus monkeys by immunochemistry and in situ hybridization. We found that during the second half of gestation, when the subplate zone reaches peak maturity in this species, many neurons can be immunolabeled with both GABA antisera and monoclonal antibodies against GABAA receptor subunits. The most robust labeling occurs at approximately embryonic day (E)125 (birth is at E165). Electron microscopic observations of receptor subunit-immunolabeled material confirmed that subunits of the GABAA receptor are localized in the subplate neurons and their dendritic processes. In many instances the reaction product is associated with the plasma membranes of labeled processes, some of which form symmetrical synapses with small unlabeled axon terminals. The results of in situ hybridization are in accord with the results of receptor subunit immunochemistry. From E80 to E141, hybridization signal for GABAA receptor subunit mRNA occurs in the subplate zone and increases steadily to peak levels between E125 and E141. The present results reveal that all the elements necessary for the formation of functional GABAergic synaptic circuitry are present in the subplate zone. Further, the ages showing the most pronounced receptor and transmitter expression in this primate coincide with the ingrowth of major cortical afferent systems. Taken together, these findings suggest that GABAergic local neuronal circuits in the subplate may be involved in the development of long tract connections stationed in this zone prior to their transfer to the overlying cortical plate.

Organization and plasticity of GABA neurons and receptors in monkey visual cortex

The GABA neurons of monkey area 17 are a morphologically and chemically heterogeneous population of interneurons that are normally distributed most densely within the geniculocortical recipient zones of the visual cortex. In adult monkeys deprived of visual input from one eye, the levels of immunoreactivity for GABA and GAD within neurons of these geniculocortical zones is reduced. Similar changes are seen in the levels of proteins that make up the GABAA receptor sub-type. The effects of monocular deprivation on other substances suggest that specific types of GABA neurons, such as those in which the tachykinin neuropeptide family and parvalbumin coexist with GABA, are greatly influenced by changes in visual input. That some proteins remain normal within deprived-eye neurons and that other proteins are increased indicates the changes in the GABA cells of the cortex are not the result of a general reduction in protein synthesis. Comparisons of what is known about the morphological and synaptic features of GABA cells in area 17 and the characteristics of cells affected by monocular deprivation suggests that certain classes, such as the clutch cell, may be preferential targets of deprivation. Such a selective loss of certain GABA neurons would have broad implications for the possible physiological plasticity of cortical cells, for if ongoing studies determine that specific receptive field properties are affected by monocular deprivation in adults, the correlation of functional properties and classes of GABA cells would be possible.

Postnatal development of thalamic recipient neurons in the monkey striate cortex: I. Comparison of spine acquisition and dendritic growth of layer 4C alpha and beta spiny stellate neurons

A quantitative study has been made from Golgi impregnations of the maturation of dendrites and their spines on spiny stellate neurons in the macaque monkey primary visual cortex. The neurons studied lay within either the alpha or the beta division of lamina 4C; previous workers have shown the alpha division neurons to be contacted by thalamic axon terminals arising from the magnocellular division of the lateral geniculate nucleus (LGN) of the thalamus and the beta division neurons to be contacted by parvocellular LGN inputs. Most thalamic terminals and perhaps the majority of other type 1 (Colonnier, '81), presumed excitatory, inputs to these cells make synaptic contacts on the tips of their dendritic spines. Measurement was made of relative changes in the total number of spines on these alpha and beta spiny neurons over age by measuring both spine density along the dendrites and dendritic arbor size in single 90-microns sections from Golgi rapid preparations. Our previous work (Lund et al., '77; Boothe et al., '79) showed a marked proliferation and attrition of spines and dendritic branches to occur in the early postnatal weeks; Rakic et al. ('86) have since proposed that there is a cortexwide synchrony of synapse acquisition and loss during this same period. However, different visual capacities channelled via the magnocellular and parvicellular geniculate relays show different maturational rates (Harwerth et al., '86). This study indicates that the anatomical maturation of spines on the alpha and beta neurons is not temporally coincident from birth to 30 weeks. During this period, phases of spine acquisition and loss on alpha neurons precedes similar phases on beta neurons. The alpha neurons carry a peak spine population at 5-8 weeks postnatal, whereas the beta neurons carry their peak spine populations between 8 and 24 weeks postnatal. At all ages prior to 30 weeks, the two sets of neurons carry quite different total spine populations. Close to 30 weeks of age, the total spine coverage has fallen on both sets of neurons and becomes identical between the alpha and beta neurons. In animals aged 30 weeks to adult, spine coverage per neuron is maintained at a common figure for the alpha and beta neurons despite further growth and disparate dendritic arbor sizes and different local spine densities in the two groups; this suggests that some common sampling paradigm between pre- and postsynaptic elements is adopted by the alpha and beta neurons and also suggests the development of a close functional correlation between the two sets of neurons.

Organization of pyramidal neurons in area 17 of monkey visual cortex

In sections of area 17 of monkey visual cortex treated with an antibody to MAP2 the disposition of the cell bodies and dendrites of the neurons is readily visible. In such preparations it is evident that the apical dendrites of the pyramidal cells of layer VI form fascicles that pass into layer IV, where most of them gradually taper and form their terminal tufts. In contrast, the apical dendrites of the smaller layer V pyramidal cells come together in a more regular fashion. They form clusters that pass through layer IV and into layer II/III where the apical dendrites of many of the pyramidal cells in that layer add to the clusters. In horizontal sections taken through the middle of layer IV, these clusters of apical dendrites are found to have an average center-to-center spacing of about 30 microns, and it is proposed that each cluster of apical dendrites represents the axis of a module of pyramidal cells that has a diameter of about 30 microns and contains about 142 neurons. The MAP2 antibody reaction also reveals that some pyramidal cells in layers IVA and IVB have their cell bodies arranged into cones. There are about 118 such cones beneath 1 mm2 of cortical surface and the apical dendrites of the pyramidal cells within them bundle together at the apex of each cone to pass into layer III. Surrounding the cones of neurons there are horizontally aligned, thin dendrites. The location of these dendrites coincides with the dark walls of the honeycomb pattern seen in layer IVA after cytochrome oxidase reactions, or after the parvocellular input from the lateral geniculate nucleus has been labeled. Thus the cones of pyramidal cells within upper layer IV fit into the pockets of the honeycomb pattern. Below the cones of pyramidal cells are the outer Meynert cells within layer IVB, and the cell bodies of these large neurons are disposed so that they preferentially lie beneath the neuropil between the cones of pyramids. It is suggested that pyramidal cell modules are a basic feature of the cerebral cortex, and that these are combined together by afferent inputs to the cortex to generate the systems of functional columns.

The relationship between simple and double opponent cells

Little is known about the formation of double opponent cells (DOCs) from geniculate afferents. Three LGN cell types have been considered as DOC precursors. No simple wiring scheme based on these cell types is consistent with the available evidence. The color and luminance multiplexed signal of P beta ganglion cells (Type I receptive fields) contains the information necessary to construct DOCs, provided that filtering operations can separate the two signals. Electrophysiological and anatomical evidence is consistent with Type I cells being filtered prior to the formation of DOCs. Cortical Type II and Type III cells can also be created by filtering.

Distribution of GABA immunoreactivity in the optic tectum of the frog: a light and electron microscopic study

GABA immunoreactivity was studied in the optic tectum of the frog, Rana esculenta, by postembedding immunohistochemical methods at the light and electron microscopic levels. Nearly one-third of the total population of tectal cells appeared to be GABA-immunoreactive. The proportion of stained neurons was highest in layer 9 (61%), and they occurred less frequently in layers 7 (21%) and 6 (27%). Stained perikarya represented a population of small neurons with a diameter of 8-10 microns. Large cell bodies in layer 7 or at the top of layer 6, and cells of origin of the mesencephalic trigeminal tract in layer 2, were devoid of labelling. Axon terminals and dendrites displaying immunoreactivity for GABA were observed in all of the plexiform layers. On the basis of ultrastructural characteristics two types of GABA-positive axon terminals and two variations of GABA-immunoreactive dendrites were distinguished. Synaptic relations of GABA-immunoreactive and GABA-negative axons as well as dendrites were also studied. Besides a wide variety of axodendritic synapses, dendrodendritic synaptic appositions were also revealed. The results suggest that various inhibitory mechanisms are involved in tectal circuits, which have to be incorporated into future neuronal models concerning visual information processing in the optic tectum of the frog.

Synaptic connections of neurones identified by Golgi impregnation: characterization by immunocytochemical, enzyme histochemical, and degeneration methods

For more than a century the Golgi method has been providing structural information about the organization of neuronal networks. Recent developments allow the extension of the method to the electron microscopic analysis of the afferent and efferent synaptic connections of identified, Golgi-impregnated neurones. The introduction of degeneration, autoradiographic, enzyme histochemical, and immunocytochemical methods for the characterization of Golgi-impregnated neurones and their pre- and postsynaptic partners makes it possible to establish the origin and also the chemical composition of pre- and postsynaptic elements. Furthermore, for a direct correlation of structure and function the synaptic interconnections between physiologically characterized, intracellularly HRP-filled neurones and Golgi-impregnated cells can be studied. It is thought that most of the neuronal communication takes place at the synaptic junction. In the enterprise of unravelling the circuits underlying the synaptic interactions, the Golgi technique continues to be a powerful tool of analysis.

GABAergic synapses in the brain identified with antisera to GABA and its synthesizing enzyme, glutamate decarboxylase

GABA is a known inhibitory neurotransmitter in the mammalian brain. The site of GABAergic synapses can be determined with immunocytochemical methods that localize either GABA or its synthesizing enzyme, glutamate decarboxylase (GAD). In general, GABAergic axon terminals contain pleomorphic synaptic vesicles and form symmetric synapses. However, a small number of GABAergic axon terminals in selected brain regions (spinal cord, cerebellum, superior colliculus, striatum, globus pallidus, inferior olive, and substantia nigra) form asymmetric synapses. GAD- and GABA-immunoreactive processes that contain synaptic vesicles participate in every known morphological type of chemical synapse. These include axosomatic, axodendritic, axospinous, initial segment, axoaxonic, dendrodendritic, serial, reciprocal, and ribbon synapses. Although GABAergic synapses form a heterogeneous group, they most commonly form axosomatic, axodendritic, and initial segment synapses in the brain and spinal cord. These findings provide helpful guidelines for the identification of GABAergic synapses in future studies.

Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. I. Morphometric analysis

An examination of material prepared for conventional electron microscopy has indicated that there are at least four different types of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells in the rat visual cortex. One type of terminal synapses with the initial axon segment and it is derived from the chandelier cell. Because the location and features of these terminals allow them to be readily recognized, chandelier cell terminals were used to determine the extent of morphometric variability that can exist among terminals originating from one cell type. It was found that there is a wide range of mean synaptic vesicle size among chandelier terminals, so that calculated mean vesicle profile diameters for individual terminals can be between 32 and 39 nm. Similar ranges of mean synaptic vesicle sizes also exist among populations of the other three axon terminal types. These terminal types are referred to as 'large', 'medium-sized', and 'dense' terminals. The large terminals synapse with the cell bodies of layer II/III pyramids and their profiles often measure 1.5 X 0.8 microns. The large terminals contain rather loosely packed pleomorphic vesicles and they frequently synapse with a second neuronal element. The medium-sized terminals are smaller, being 1.0 X 0.6-0.8 microns in size, and their synaptic vesicles are usually more closely packed than those within the large terminals. The medium-sized terminals are the ones encountered most frequently on the cell bodies of pyramidal cells and they can also occur on the axon hillock and initial axon segment. The dense terminals are usually flattened against the cell body, and they contain rather rounded and closely packed synaptic vesicles, which often seem to be enmeshed in a rather dark cytoplasmic matrix. This matrix and the close packing of the vesicles makes these terminals appear to be more dense than the others. It is now necessary to determine the origins of the large, medium and dense terminals, and to ascertain if they all use GABA as their neurotransmitter.

Arborisation pattern and postsynaptic targets of physiologically identified thalamocortical afferents in striate cortex of the macaque monkey

The monosynaptic targets of different functional types of geniculocortical axons were compared in the primary visual cortex of monkeys. Single thalamocortical axons were recorded extracellularly in the white matter by using horseradish-peroxidase-filled pipettes. Their receptive fields were mapped and classified as corresponding to those of parvi- or magnocellular neurons in the lateral geniculate nucleus. The axons were then impaled and injected intraaxonally with horseradish peroxidase. Two magnocellular (MA) and two parvicellular (PA) axons were successfully recovered and reconstructed in three dimensions. The two MA axons arborised mainly in layer 4C alpha, as did the two PA axons in layer 4C beta. Few collaterals formed varicosities in layer 6. Both MA axons had two large, elongated clumps of bouton (approx. 300-500 x 600-1,200 microns each) and a small clump. One PA axon had two clumps (each with a core appr. 200 microns in diameter); the other had only one (appr. 150-200 microns in axon had 1,380; one MA axon had 3,200 boutons; and those of the more extensive MA axon were not counted. The distribution of postsynaptic targets as well as the number of synapses per bouton has been established for a sample of 150 PA boutons and 173 MA boutons from serial ultrathin sections. The MA axons made on average 2.1 synapses per bouton compared to 1.79 for one PA axon and 2.6 for the other. The sample of boutons taken from the two physiological types of axons contacted similar proportions of dendritic spines (52-68%), shafts (33-47%), and somata (0-3%). The postsynaptic elements were further characterized by immunostaining for GABA. All postsynaptic perikarya and some of the dendrites (4.5-9.5% of all targets) were positive for the amino acid. Near the thalamic synapse GABA-negative dendritic shafts frequently contained lamellar bodies, an organelle identical in structure to spine apparatus. Dendritic shafts and spines postsynaptic to the thalamocortical boutons frequently received an adjacent synapse from GABA-immunoreactive boutons. The similarity between the magno-and parvicellular axons in their targeting of postsynaptic elements, including the GABAergic neurons, suggests that the structural basis of the physiological differences between 4C alpha and 4C beta neurons should be sought in other aspects of the circuitry of layer 4C, such as local cortical circuits, or in the far greater horizontal extent of the thalamocortical and GABAergic axons in layer 4C alpha compared to those in the beta subdivision.

Ultrastructural analysis of somatostatin-immunoreactive neurons and synapses in the temporal and occipital cortex of the macaque monkey

Somatostatin-containing neurons and terminals have been analyzed in monkey temporal and occipital cortex by using light and electron microscopic immunohistochemistry. An antibody against Somatostatin-28, that was shown previously preferentially to label fibers (Morrison et al.: Brain Research 262:344-351, 1983), was utilized. As expected, few cell bodies were labeled. At the electron microscopic level, labeled cells presented a characteristic asymmetric position of the nucleus and very few symmetric or asymmetric synapses on the somatic surface. In all areas examined, somatostatin fibers formed a dense plexus in the most superficial layers (I-upper III). The density of labeled fibers in intermediate (deep III-IV) and deep layers (V-VI) varied considerably among areas. The synaptic relationships of the immunoreactive fibers were analyzed and postsynaptic targets quantified in V1, V2, and the superior and inferior temporal gyrus (STG and ITG, respectively). The synapses formed by somatostatin-labeled boutons were of the symmetric type (type II) and the primary postsynaptic targets were dendritic shafts. No regional differences were found in the distribution of the postsynaptic targets in layers I-upper III. The pattern of synapses in the deep layers was examined in STG. The frequency and distribution of postsynaptic targets was similar to the superficial layers of STG and the other temporal and occipital regions. In intermediate layers of the temporal cortex areas there was an increase in the proportion of synapses on dendritic spines. In a correlated light and electron microscopic analysis we examined synapses made by radial fibers in these regions and found that although the main targets are distal dendritic shafts, almost 40% of synapses were on dendritic spines. We suggest that the radial fibers may originate from a specialized interneuron, previously described as the double bouquet cell, and that this particular subset of somatostatin-containing double bouquet cells is likely to exhibit a very high degree of regional heterogeneity with a preference for association cortices with extensive corticocortical convergence.

Synaptic organization of serotonin-immunoreactive fibers in primary visual cortex of the macaque monkey

The macaque neocortex is very densely innervated by serotonin-containing fibers. The highest density of these fibers is in primary sensory regions such as the primary visual cortex. By using an antibody against serotonin, we analyzed the distribution and morphology of serotonin-immunoreactive fibers and synapses in the primary visual cortex of the adult cynomolgus monkey. In addition, we quantified the laminar distribution of labeled varicosities and the distances between varicosities in single fibers. While serotonin-immunoreactive fibers are found in all cortical layers, at least three bands of heightened density of innervation were readily recognized that were coincident with 1) layer IIIB to IVC alpha, 2) layer VA, and 3) layer VIB. Layer IVC alpha of area 17 contained more varicosities per unit area than any other sublayer. There was a high degree of variability in the intervaricosity distances along single fibers; more than half were longer than 10 microns. At the electron microscopic level, synaptic contacts were also observed throughout the entire thickness of area 17, with the highest frequency in layer IV. The labeled varicosities were packed with electron-lucent synaptic vesicles and formed synaptic complexes with small, but conspicuous, post-synaptic densities. Dendritic shafts were the most common postsynaptic target of the labeled synapses. Among these characteristically slender post-synaptic shafts, profiles with structural features of both spiny and smooth dendrites were observed. The small diameter of most of the postsynaptic dendrites indicated that distal dendrites were preferentially contacted by serotonin-immunoreactive varicosities. Although direct identification of the postsynaptic neurons will be required for complete characterization of this circuitry, the distribution of serotonin-immunoreactive varicosities suggests that serotoninergic interactions in the primary visual cortex of the cynomolgus monkey are directed predominantly at the distal dendrites of granular and infragranular neurons rather than at targets in the supragranular layers.

Distribution of GABAergic neurons and axon terminals in the macaque striate cortex

Antisera to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) have been used to characterize the morphology and distribution of presumed GABAergic neurons and axon terminals within the macaque striate cortex. Despite some differences in the relative sensitivity of these antisera for detecting cell bodies and terminals, the overall patterns of labeling appear quite similar. GABAergic axon terminals are particularly prominent in zones known to receive the bulk of the projections from the lateral geniculate nucleus; laminae 4C, 4A, and the cytochrome-rich patches of lamina 3. In lamina 4A, GABAergic terminals are distributed in a honeycomb pattern which appears to match closely the spatial pattern of geniculate terminations in this region. Quantitative analysis of axon terminals that contain flat vesicles and form symmetric synaptic contacts (FS terminals) in lamina 4C beta and in lamina 5 suggest that the prominence of GAD and GABA axon terminal labeling in the geniculate recipient zones is due, at least in part, to the presence of larger GABAergic axon terminals in these regions. GABAergic cell bodies and their initial dendritic segments display morphological features characteristic of nonpyramidal neurons and are found in all layers of striate cortex. The density of GAD and GABA immunoreactive neurons is greatest in laminae 2-3A, 4A, and 4C beta. The distribution of GABAergic neurons within lamina 3 does not appear to be correlated with the patchy distribution of cytochrome oxidase in this region; i.e., there is no significant difference in the density of GAD and GABA immunoreactive neurons in cytochrome-rich and cytochrome-poor regions of lamina 3. Counts of labeled and unlabeled neurons indicate that GABA immunoreactive neurons make up at least 15% of the neurons in striate cortex. Layer 1 is distinct from the other cortical layers by virtue of its high percentage (77-81%) of GABAergic neurons. Among the other layers, the proportion of GABAergic neurons varies from roughly 20% in laminae 2-3A to 12% in laminae 5 and 6. Finally, there are conspicuous laminar differences in the size and dendritic arrangement of GAD and GABA immunoreactive neurons. Lamina 4C alpha and lamina 6 are distinguished from the other layers by the presence of populations of large GABAergic neurons, some of which have horizontally spreading dendritic processes. GABAergic neurons within the superficial layers are significantly smaller and the majority appear to have vertically oriented dendritic processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunogold demonstration of GABA in synaptic terminals of intracellularly recorded, horseradish peroxidase-filled basket cells and clutch cells in the cat's visual cortex

To identify the putative transmitter of large basket and clutch cells in the cat's visual cortex, an antiserum raised against GABA coupled to bovine serum albumen by glutaraldehyde and a postembedding, electron microscopic immunogold procedure were used. Two basket and four clutch cells were revealed by intracellular injection of horseradish peroxidase. They were identified on the basis of the distribution of their processes and their synaptic connections. Large basket cells terminate mainly in layer III, while clutch cells which have a more restricted axon, terminate mainly in layer IV. Both types of neuron have a small radial projection. They establish type II synaptic contacts and about 20-30% of their synapses are made with the somata of other neurons, the rest with dendrites and dendritic spines. Altogether 112 identified, HRP-filled boutons, the dendrites of three clutch cells and myelinated axons of both basket and clutch cells were tested for the presence of GABA. They were all immunopositive. The postsynaptic neurons received synapses from numerous other GABA-positive boutons in addition to the horseradish peroxidase-filled ones. Dendritic spines that received a synapse from a GABA-positive basket or clutch cell bouton also received a type I synaptic contact from a GABA-negative bouton. A few of the postsynaptic dendrites, but none of the postsynaptic somata, were immunoreactive for GABA. The fine structural characteristics of the majority of postsynaptic targets suggested that they were pyramidal and spiny stellate cells. These results provide direct evidence for the presence of immunoreactive GABA in identified basket and clutch cells and strongly suggest that GABA is a neurotransmitter at their synapses. The laminar distribution of the synaptic terminals of basket and clutch cells demonstrates that some GABAergic neurons with similar target specificity segregate into different laminae, and that the same GABAergic cells can take part in both horizontal and radial interactions.