Selectivity of neuronal [3H]GABA accumulation in the visual cortex as revealed by Golgi staining of the labeled neurons

Advances in thin tissue Golgi-Cox impregnation: fast, reliable methods for multi-assay analyses in rodent and non-human primate brain

In 1873 Camillo Golgi discovered a staining technique that allowed for the visualization of whole neurons within the brain, initially termed 'the black reaction' and is now known as Golgi impregnation. Despite the capricious nature of this method, Golgi impregnation remains a widely used method for whole neuron visualization and analysis of dendritic arborization and spine quantification. We describe a series of reliable, modified 'Golgi-Cox' impregnation methods that complement some existing methods and have several advantages over traditional whole brain 'Golgi' impregnation. First, these methods utilize 60-100μm thick brain sections, which allows for fast, reliable impregnation of neurons in rats (7-14 days) and non-human primates (NHP) (30 days) while avoiding the pitfalls of other 'rapid Golgi' techniques traditionally employed with thin sections. Second, these methods employ several common tissue fixatives, resulting in high quality neuron impregnation in brain sections from acrolein, glutaraldehyde, and paraformaldehyde perfused rats, and in glutaraldehyde perfused NHP brain tissue. Third, because thin sections are obtained on a vibratome prior to processing, alternate sections of brain tissue can be used for additional analyses such as immunohistochemistry or electron microscopy. This later advantage allows for comparison of, for example, dendrite morphology in sections adjacent to pertinent histochemical markers or ultrastructural components. Finally, we describe a method for simultaneous light microscopic visualization of both tyrosine hydroxylase immunohistochemistry and Golgi impregnation in the same tissue section. Thus, the methods described here allow for fast, high quality Golgi impregnation and conserve experimental subjects by allowing multiple analyses within an individual animal.

Combined physiological-anatomical approaches to study lateral inhibition

In the visual cortex, large basket cells form the cellular basis of long-range lateral inhibition. The present paper focuses on combinations of methods with which large basket cells can be studied in the context of extensive neuronal representations. In the first approach, the topographic relationship between large basket axons and known functional representations such as orientation, direction, and ocular dominance is analysed. Functional mapping is carried out using extracellular electrode recordings or optical imaging of intrinsic signals followed by 3-dimensional anatomical reconstruction of biocytin stained large basket cells in the same regions. In the second approach, the contribution of lateral inhibition to orientation and direction selectivity is assessed using the GABA inactivation paradigm and direct inhibitory projections from the inactivation to recording sites are demonstrated with biocytin staining and injections of [3H]nipecotic acid, a radioactive marker for GABAergic cells. The limitation of these approaches is that they can only be used in cortical regions which lie on the surface of the brain.

Cellular heterogeneity in cerebral cortex: a study of the morphology of pyramidal neurones in visual areas of the marmoset monkey

The morphological characteristics of the basal dendritic fields of layer III pyramidal neurones were determined in visual areas in the occipital, parietal, and temporal lobes of adult marmoset monkeys by means of intracellular iontophoretic injection of Lucifer yellow. Neurones in the primary visual area (V1) had the least extensive and least complex (as determined by Sholl analysis) dendritic trees, followed by those in the second visual area (V2). There was a progressive increase in size and complexity of dendritic trees with rostral progression from V1 and V2, through the "ventral stream," including the dorsolateral area (DL) and the caudal and rostral subdivisions of inferotemporal cortex (ITc and ITr, respectively). Neurones in areas of the dorsal stream, including the dorsomedial (DM), dorsoanterior (DA), middle temporal (MT), and posterior parietal (PP) areas, were similar in size and complexity but were larger and more complex than those in V1 and V2. Neurones in V1 had the lowest spine density, whereas neurones in V2, DM, DA, and PP had similar spine densities. Neurones in MT and inferotemporal cortex had relatively high spine densities, with those in ITr having the highest spine density of all neurones studied. Calculations based on the size, number of branches, and spine densities revealed that layer III pyramidal neurones in ITr have 7.4 times more spines on their basal dendritic fields than those in V1. The differences in the extent of, and the number of spines in, the basal dendritic fields of layer III pyramidal neurones in the different visual areas suggest differences in the ability of neurones to integrate excitatory and inhibitory inputs. The differences in neuronal morphology between visual areas, and the consistency in these differences across New World and Old World monkey species, suggest that they reflect fundamental organisational principles in primate visual cortical structure.

The vesicular GABA transporter, VGAT, localizes to synaptic vesicles in sets of glycinergic as well as GABAergic neurons

A transporter thought to mediate accumulation of GABA into synaptic vesicles has recently been cloned (McIntire et al., 1997). This vesicular GABA transporter (VGAT), the first vesicular amino acid transporter to be molecularly identified, differs in structure from previously cloned vesicular neurotransmitter transporters and defines a novel gene family. Here we use antibodies specific for N- and C-terminal epitopes of VGAT to localize the protein in the rat CNS. VGAT is highly concentrated in the nerve endings of GABAergic neurons in the brain and spinal cord but also in glycinergic nerve endings. In contrast, hippocampal mossy fiber boutons, which although glutamatergic are known to contain GABA, lack VGAT immunoreactivity. Post-embedding immunogold quantification shows that the protein specifically associates with synaptic vesicles. Triple labeling for VGAT, GABA, and glycine in the lateral oliva superior revealed a higher expression of VGAT in nerve endings rich in GABA, with or without glycine, than in others rich in glycine only. Although the great majority of nerve terminals containing GABA or glycine are immunopositive for VGAT, subpopulations of nerve endings rich in GABA or glycine appear to lack the protein. Additional vesicular transporters or alternative modes of release may therefore contribute to the inhibitory neurotransmission mediated by these two amino acids.

Neurons in the ventral subiculum, amygdala and entorhinal cortex which project to the nucleus accumbens: their input from somatostatin-immunoreactive boutons

Neurons in the hippocampus, amygdala and entorhinal cortex which project to the nucleus accumbens were labelled retrogradely following injection of horseradish peroxidase. The injections were targetted on the medial part of the nucleus accumbens, but some injection sites included the whole nucleus. Projection neurons in all three areas were found to be spiny, and from the entorhinal cortex and ventral subiculum of the hippocampus they were pyramidal neurons. Somatostatin (S28(1-12)-immunoreactive neurons were found in all parts of the three limbic areas examined. They were found to have various morphologies, but in the electron microscope all had the ultrastructural characteristics of interneurons. In the hippocampus the stratum lacunosum was found to contain the most immunoreactive fibres while most cells lay in the stratum oriens. In the amygdala the densest staining for both cells and fibres was in the central nucleus. In the entorhinal cortex somatostatin-immunoreactive fibres and cells seemed to have no preferential distribution. Examination of somatostatin-immunoreactive profiles in the electron microscope revealed that the majority of synaptic contacts were made with dendrites, many of which were spine-bearing. In the light microscope somatostatin-immunoreactive fibres could be seen to lie near the somata and proximal dendrites of neurons that projected to the nucleus accumbens. In the electron microscope it was found that somatostatin-immunoreactive boutons were in symmetrical synaptic contact with the somata and proximal dendrites of neurons in the ventral subiculum, entorhinal cortex and amygdala which project to the nucleus accumbens.

Electron microscopy of in vivo recorded and intracellularly injected inferior olivary neurons and their GABAergic innervation in the cat

This paper reports on the detailed morphology of inferior olivary neurons in the cat following electrophysiological examination, intracellular injection with horseradish peroxidase, and gamma aminobutyric acid (GABA) immunocytochemistry. The activity of olivary cells was recorded intracellularly in vivo and their response to mesodiencephalic stimulation was tested. In a number of cases their response to stimulation of the contralateral superior cerebellar peduncle was also tested. Mesodiencephalic stimulation resulted in monosynaptic, and superior peduncle stimulation in disynaptic activation of cells in the medial accessory and principal olivary subdivisions. Rebound olivary activity was usually only found after mesodiencephalic stimulation. Light microscopic investigation of osmicated and Araldite embedded Vibratome sections was facilitated considerably when performing the osmication in a glucose solution. Peroxidase labeled olivary cells, like that earlier described for Golgi-impregnated material, possess a complex globular dendritic geometry. Especially, and unlike Golgi material, the abundance of exceptionally long and complex spiny appendages could be appreciated. The axons usually stemmed from first order dendrites and did not give rise to recurrent axon collaterals. The ultrastructural analysis of this material, mainly from serial sections, was combined with postembedding GABA immunohistochemistry. In this way, GABAergic as well as non-GABAergic profiles were studied in conjunction with HRP labeled cellular elements. The GABAergic terminals usually contained pleomorphic vesicles and made symmetrical synapses whereas non-GABAergic terminals nearly always formed asymmetrical synapses and contained round or oval vesicles. Most, if not all, HRP labeled spiny appendages were incorporated in glomeruli. A particular spiny appendage may contribute more than one spine head to a glomerular core, which, on average, consisted of spiny elements of six different neurons. A glomerular core is surrounded by approximately the same amounts of GABAergic and non-GABAergic boutons. Also, all spiny appendages, and most of their individual spine heads, are contacted by GABAergic as well as non-GABAergic boutons. Spiny appendages on the axon hillock may be incorporated in dendritic glomeruli, however, most synapses with the hillock were made by GABAergic boutons. The combined physiological and morphological observations imply that 1) the cerebellar nuclei can exert an excitatory influence on inferior olivary neurons through a mesodiencephalic relay, 2) the GABAergic nucleo-olivary input seems to be capable of diminishing the oscillatory tendencies of olivary neurons, and 3) the mesodiencephalic (non-GABAergic) and cerebellar (GABAergic) input may subserve a timing function since these inputs systematically impinge upon the same olivary spines.

Intracellular labeling of neurons in the medial accessory olive of the cat: II. Ultrastructure of dendritic spines and their GABAergic innervation

In order to describe the morphology of dendritic spines of identified neurons in the cat inferior olive together with their gamma-aminobutyric acid (GABA) synaptic input, a technique was used combining intracellular labeling of horseradish peroxidase with postembedding gold-immunocytochemistry. With this technique physiologically identified olivary cells were reconstructed with the light microscope, and the horseradish peroxidase reaction product and immunogold labeling were subsequently examined in serial sections at the ultrastructural level. In addition, a degenerating neuron was observed, resulting in a triple labeling in single ultrathin sections. Quantitative and three-dimensional analysis showed that the dendritic spines were composed of long, thin stalks ending in one or more spine heads. The spines of cells located in the caudal half of the medial accessory olive (type I cells, characterized by dendrites which run away from the soma) were found to be less complex than those of cells located rostrally in this olivary subnucleus (type II cells, characterized by dendrites which tend to turn back towards the soma). Most, if not all, of the spines of both cell types were located within glomeruli. On average, the spines within individual glomeruli originated from 6 different dendrites (with a maximum of 8). Different spines within the same glomerulus were never derived from different dendrites of the same olivary neuron, but single spines frequently gave rise to several spine heads, which could be located either within different glomeruli or inside a single glomerulus. The glomerular spine heads originating from the same spine were rarely located near one another. All spines and most of the spine heads were contacted by both GABAergic and non-GABAergic terminals. Most of the GABAergic terminals contained pleomorphical vesicles and displayed symmetric synapses whereas the non-GABAergic terminals showed usually round to oval vesicles and asymmetric synapses.

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.

Interplexiform cells of the goldfish retina

Dopaminergic and glycinergic interplexiform cells (IPCs) in the goldfish retina were impregnated by using two new Golgi protocols. The two cell types have markedly different morphological characteristics: Dopaminergic IPCs have primary dendrites that descend into and stratify in the inner plexiform layer, where they give rise to processes that project to the outer plexiform layer. Conversely, glycinergic IPCs have primary dendrites that ascend to the outer plexiform layer and from this dendritic arbor, many processes then project into the inner plexiform layer. The apparent coverage of dopaminergic IPCs is almost four times that of glycinergic IPCs. Even so, the coverage of each glycinergic IPC in the outer plexiform layer allows it to provide an accurate copy of the S-space to the inner plexiform layer. Considering the known GABAergic and glycinergic synaptologies in the inner plexiform layer, the glycinergic IPC must form a major element in the retinal circuitry of the goldfish.

Axo-axonic chandelier cells in the rat fascia dentata: Golgi-electron microscopy and immunocytochemical studies

Synaptic transmission can be blocked very efficiently by inhibitory synapses on axon initial segments. Inhibitory chandelier cells forming synapses on the axon initial segment of pyramidal neurons have been found in the neocortex and hippocampus proper. Here we describe an axo-axonic local circuit neuron in the rat fascia dentata that establishes synaptic contacts with axon initial segments of numerous dentate granule cells. Examination of a large number of Golgi-impregnated nongranule cells in the fascia dentata of rats revealed a group of neurons with characteristics of chandelier cells. Thus these cells exhibited an extensive axonal plexus within the granular layer that characteristically formed vertical aggregations of axonal varicosities. The cell bodies of these neurons were located in the inner molecular layer or in the outer part of the granular layer. Their dendrites invaded the molecular layer, suggesting an afferent innervation similar to that of the granule cells. Well impregnated putative axo-axonic cells were gold-toned for an electron microscopic analysis. The cell bodies and dendrites of these neurons exhibited characteristic ultrastructural features of nongranule cells, i.e., large amounts of perinuclear cytoplasm, infoldings of the nuclear membrane, and a large number of synaptic contacts on the perikaryon and on the smooth dendritic shafts. The axon originating from the cell body or from a proximal dendrite gave rise to numerous vesicle-filled varicosities that almost exclusively formed symmetric synaptic contacts with axon initial segments. A semiquantitative study of five axonal complexes demonstrated that 92.3% of identified postsynaptic elements were initial segments of granule cell axons. Immunostaining with antibodies against glutamate decarboxylase (GAD) and parvalbumin (PARV) revealed a subpopulation of neurons that very much resembled the Golgi-impregnated axo-axonic cells with regard to cell body location, dendritic arborization, and fine structural characteristics of perikarya and dendrites. GAD and PARV were found to be coexistent in these cells. Moreover, we found GAD- and PARV-immunoreactive terminals in symmetric synaptic contact with axon initial segments of granule cells. The present study has shown a hitherto unknown axo-axonic cell in the rat fascia dentata. On the basis of our immunocytochemical findings, we hypothesize that this cell exerts a strong inhibitory effect on dentate granule cells. This way, signal transmission from the fascia dentata to the hippocampus proper within the "trisynaptic pathway" can efficiently be controlled by a group of highly specialized neurons.

Pharmacological analysis of cortical circuitry

The cortical circuitry of the visual cortex has been worked out in great detail. Anatomical investigations reveal stereotyped connections within cortical columns and specific long-range connections between distant columns. Pharmacological techniques for blocking the activity in individual cortical layers or columns allow the microdissection of the cortical circuit. These studies could relate specific functional roles to particular cortical connections.

Postnatal development of lamina III/IV nonpyramidal neurons in rabbit auditory cortex: quantitative and spatial analyses of Golgi-impregnated material

We have studied the postnatal development of lamina III/IV spine-free nonpyramidal neurons in the auditory cortex of the New Zealand white rabbit. The morphology and dendritic branching pattern of single cells impregnated with a Golgi-Cox variant were analyzed with the aid of camera lucida drawings and three-dimensional reconstructions obtained with a computer microscope. Sample sizes of 20 neurons were obtained at birth (day 0), postnatal day (PD) 3, 6, 9, 12, 15, 21, and 30 days of age. Normative data were also available from PD-60 and young adult rabbits studied previously. At birth, lamina II-IV have not yet emerged from the cortical plate; immature nonpyramidal neurons at the bottom of the cortical plate (presumptive layer IV) are characterized by short, vertically oriented dendrites. Growth-cone-like structures are present along the shafts and at the tips of the dendrites. At birth, soma area and total dendritic length are, respectively, 34 and 10% of adult values. The cortical plate acquires a trilaminar appearance at PD-3. The six-layered cortex is present by PD-6. During the first postnatal week dendritic length increases fourfold and is accompanied by a significant increase in both terminal and preterminal dendritic growth cones. At the onset of hearing at PD-6, there is a significant proliferation of dendrites and branches to 144 and 200% of adult levels, respectively. These supernumerary dendrites are rapidly lost during the second postnatal week, at which time the somata and dendrites become covered with spines. The loss of higher-order dendrites occurs more gradually; the number of dendritic branches is still 116% of adult values at PD-30. Spine density peaks between days PD-12 and PD-15, and then gradually diminishes until the cells are sparsely spined or spine free by PD-30. Total dendritic length increases in a linear fashion up to PD-15, at which time it is 80% of adult values. An analysis of terminal and intermediate branches demonstrated that the increase in total dendritic length after PD-6 is due entirely to the growth of terminal dendrites. Total dendritic length attains adult levels by PD-30. Spatial analyses revealed that a vertical orientation of dendrites is present at birth. Associated with the onset of hearing at PD-6, there is an explosive elaboration of dendrites toward the pial surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotoninergic innervation of the cat cerebral cortex

Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.

Serotonergic axons form basket-like terminals in cerebral cortex

Serotonergic axons in the posterior cerebral cortex of the cat were demonstrated immunohistochemically using a monoclonal antibody to serotonin (5-HT). This technique reveals the presence of a dense serotonergic innervation of single cortical neurons at the light microscopic level. 5-HT axons with large varicosities (1-6 microns in diameter) form distinct, basket-like arrays around counterstained somata principally in layer I. In each basket one or more axons encircle and make repeated contact with the soma. Some axons extend from the soma and apparently climb along the dendrites of the target neuron. The climbing 5-HT axons form a stellate or horizontal pattern suggesting that the target cells are non-pyramidal neurons of the supragranular layers.

Non-pyramidal hippocampal projection neurons: a light and electron microscopic study

Following injections of horseradish peroxidase conjugated with wheat germ agglutinin into the medial nucleus accumbens of the rat, a large number of projecting pyramidal neurons in the hippocampus were retrogradely labelled. In addition to this major projection, a few retrogradely labelled cells were tentatively identified at the light microscopic level as non-pyramidal neurons. These presumptive non-pyramidal neurons were found in all hippocampal layers, although they were mainly outside the stratum pyramidale, in the stratum oriens. Ultrastructurally, in serial sections, the non-pyramidal nature of 20 of these neurons was confirmed by their characteristic features such as deeply indented nuclei, occasional intranuclear inclusions, and symmetric and asymmetric synaptic contacts with their somata. Possible transmitters used by these neurons are discussed.

Neuronal uptake and laminar distribution of tritiated aspartate, glutamate, gamma-aminobutyrate and glycine in the prestriate cortex of squirrel monkeys: correlation with levels of cytochrome oxidase activity and their uptake in area 17

The neuronal uptake and laminar distribution of cortically injected tritium-labeled gamma-aminobutyrate (GABA), aspartic acid, glutamate and glycine was examined in the prestriate cortex of squirrel monkeys. The intent of this investigation was not to examine the role of these amino acids as neurotransmitters, but to correlate the distribution of tritium-labeled neurons with their levels of cytochrome oxidase activity. A comparison of the number of these labeled neurons was made between the metabolically active "puff" and the less active "nonpuff" regions. In addition, these results were contrasted with the findings in area 17. With each tritiated amino acid tested, labeled neurons that had either high or low levels of cytochrome oxidase activity were present in all laminae. However, the density of labeled neurons varied between lamina for a given amino acid as well as between different amino acids. While many neurons that were cytochrome oxidase-reactive were also tritium-labeled, cytochrome oxidase activity was not a prerequisite for the sequestering of tritium label. In fact, many of the labeled neurons exhibited relatively low levels of cytochrome oxidase activity. Similar to area 17, few aspartate- or glutamate-labeled neurons were present in laminae II-III. The number of labeled neurons for both amino acids increased in laminae IV-VI, with the greatest increase observed in laminae V-VI. Gamma-aminobutyrate-labeled neurons were more prevalent in laminae I and upper II than in the other laminae, whereas in area 17, a greater proportion of the labeled neurons were found in laminae V-VI. With the exception of the uppermost laminae, where GABA-labeled neurons were more abundant, the number of glycine-labeled neurons was significantly greater throughout most laminae than with the other amino acids examined. The density of glycine-labeled neurons in lamina IV, however, was significantly less than the number observed in lamina III even though lamina III was farther away from the injection site which was at the boundary between laminae V-VI. Glycine-labeled neurons were, on average, larger than those labeled with any other amino acid. Similar to area 17, more GABA- and glycine-labeled neurons were observed within the puff regions than in nonpuff regions. No puff/nonpuff differences were observed in the distribution of leucine-injected controls. Labeled neurons for each amino acid included stellate-, fusiform- and pyramidal-shaped cells, each of varying sizes. However, outside the intensely labeled injection sites, no GABA-labeled pyramidal cells were observed.(ABSTRACT TRUNCATED AT 400 WORDS)

GABA immunoreactive neurons in rat visual cortex

An antiserum to gamma-aminobutyric acid (GABA) was used in a light and electron microscopic immunocytochemical study to determine the morphology and distribution of GABA-containing neurons in the rat visual cortex and to ascertain whether all classes of nonpyramidal neurons in this cortex are GABAergic. The visual cortex used for light microscopy was prepared in such a way that the antibody penetrated completely through tissue sections, and in these sections large numbers of GABA immunoreactive neurons were apparent. The labeled neurons could be identified as being either multipolar, bitufted, bipolar, or horizontal neurons. In layers II through VIa, GABA immunostained cells were distributed uniformly and accounted for approximately 15% of all neurons, but in layer I all neurons appeared to be immunostained. Electron microscopy of GABA immunostained visual cortex prepared to ensure good fine structural preservation confirmed the presence in layers II through VIa of numerous immunoreactive bipolar neurons, both small and large varieties, as well as multipolar and bitufted neurons. Additionally, electron microscopy reveals that astrocytes are frequently GABA immunoreactive. From a correlated light and electron microscopic evaluation of neurons in GABA immunostained visual cortex, it was possible to confirm which kinds of neurons are GABAergic and what proportion of the neuronal population they represent. Thus, from an analysis of some 950 neurons, it was found that pyramidal neurons were never immunoreactive and that except for 20% of the bipolar cell population, all examples of other types of nonpyramidal neurons encountered in this material were GABA immunoreactive.

Synaptic connections of axo-axonic (chandelier) cells in human epileptic temporal cortex

The human temporal cortex contains a type of interneuron, identified by Golgi impregnation which, like the axo-axonic or chandelier cells found in animals, establishes Gray's type II synaptic contacts exclusively with the axon initial segments of pyramidal cells. Each terminal segment is composed of 3-12 boutons to form a "chandelier"-like appearance. For the two human axo-axonic cells analysed in this study we could identify 269 and 86 bouton rows respectively, which represents an equivalent number of postsynaptic pyramidal cells. A terminal bouton row from one of these Golgi-impregnated cells was shown to be in synaptic contact with the axon initial segment of a Golgi-impregnated pyramidal cell. The very specific nature of the target of axo-axonic cells, together with their highly divergent axonal arborization, means that they are ideally placed to control the output of a large population of pyramidal cells. Since previous studies in animals have shown the GABAergic nature of axo-axonic cells it is possible that human axo-axonic cells could be involved in the generation of epileptic activity or in the control of its propagation.

Structure of the olfactory bulb of the hedgehog (Erinaceus europaeus): description of cell types in the granular layer

The cytoarchitecture of the olfactory bulb and the cell types in the granular layer of adult hedgehogs have been studied with the Golgi method. The mitral cell layer does not stand out as a monolayer as in most mammals; it is arranged as a diffuse stratum with mitral cells displaced into the external plexiform layer. The external plexiform layer is exceedingly thick and contains the branches of peripheral processes of granule cells and displaced mitral and tufted cells. The granular layer contains granule cells and varieties of short-axon cells. Among granule cells a type of cell with an elaborate system of protrusions close to the cell body has been found. Four main varieties of short-axon cells are described. These include cells with local or extended axons, according to the branching pattern of their axons inside the granular layer or extending into the external plexiform layer as well. Short-axon cells were also classified as cells with smooth and spinous dendrites. A variety of cell with smooth dendrites and elaborate axonal system reaching the periglomerular zone is described. This type of cell has been found frequently in the olfactory bulb of the hedgehog. In comparison to several other mammals, short-axon cells in the olfactory bulb of the hedgehog have been found to be particularly abundant and to have more complex axonal systems. It is suggested that some of them may represent inhibitory interneurons acting upon granule and periglomerular cells, playing an important role in the centrifugal pathway controlling the olfactory input.

Neurons accumulating [3H]gamma-aminobutyric acid (GABA) in supragranular layers of cat primary auditory cortex (AI)

The classes of neurons accumulating exogenously injected, tritiated gamma-aminobutyric acid [( 3H]GABA) were studied in the supragranular layers in the primary auditory field of the adult cat. The size, laminar locus, and somatodendritic profiles of labeled neurons were studied light microscopically in frozen- or Vibratome-sectioned, 30 micron thick material, and in semithin, 1-2 micron thick, plastic-embedded high-resolution autoradiographic preparations. The chief goals of the study were to determine which types of cells could be identified as accumulating [3H]GABA in layers I, II and III, and to establish possible relationships between these cells and neurons described in Golgi studies of these layers, and the neurons found, in parallel investigations of the connections of the primary auditory field, to participate as ipsilateral corticocortical and commissural cells of origin. The principal findings are: that neurons in every layer in the primary auditory field take up tritiated gamma-aminobutyric acid; that their Nissl-counterstained somata have a smaller average area, and a smaller range of areas, than do the unlabeled cells; that more than one type of labeled neuron-as defined by somatic size and shape, height:width ratios, and nuclear membrane morphology-could be identified in each layer; that none of the labeled neurons had a soma with a pyramidal configuration; that the labeled cells are comparable in size, shape, and laminar distribution to some populations of non-pyramidal ipsilateral corticocortical cells of origin in layers II and III, and perhaps to certain classes of commissurally projecting, layer III non-pyramidal neurons; and finally, that only a rather small proportion-perhaps 10% or less, except in layer I-of the supragranular cells appear to accumulate labeled material. With regard to the identity of particular classes of neurons accumulating silver grains above background in the individual layers, in layer I, 2 of the 4 types of neurons characterized in Golgi preparations take up gamma-aminobutyric acid and the remaining 2 types may also, and the relative number of labeled cells appears to be higher than in the other layers; in layer II, 2 of the 9 varieties are labeled, and 4 other types may also be; and in layer III, 2 of the 11 types take up gamma-aminobutyric acid, and 5 other varieties may as well. Three types of non-pyramidal layer II cells that project ipsilaterally from AI to the second auditory cortical field, AII, possibly accumulate gamma-aminobutyric acid; 3 types of commissural non-pyramidal cells of origin linking AI to AI appear to be labeled by gamma-aminobutyric acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholecystokinin-immunoreactive boutons in synaptic contact with hippocampal pyramidal neurons that project to the nucleus accumbens

Neurons in the hippocampal formation of the rat that project to the medial nucleus accumbens were identified following the retrograde transport of a conjugate of horseradish peroxidase with wheat germ agglutinin. The great majority of such projecting neurons were located in the ventral subiculum and were pyramidal in shape; the pyramidal nature of 25 such retrogradely labelled neurons was established by Golgi impregnation. In material processed to reveal both retrogradely labelled cells and cholecystokinin-immunoreactivity, no immunoreactive projecting neurons were found. However, 48 identified projecting neurons, probably pyramidal, were found to receive input from cholecystokinin-immunoreactive boutons that formed symmetrical synaptic contacts with the soma or proximal dendrites. It is suggested that one function of cholecystokinin-immunoreactive neurons in the hippocampal formation might be to influence the output of the pyramidal neurons that project to the nucleus accumbens. Since this pathway is one of the main links between the limbic system and the basal ganglia, it is conceivable that changes in the cholecystokinin levels in the hippocampus, as found in schizophrenia, might influence behaviour through the pathway connecting the hippocampus with the nucleus accumbens.

gamma-Aminobutyric acid immunoreactivity in mouse barrel field: a light microscopical study

The barrel field of the mouse somatosensory cortex (SmI) was investigated immunocytochemically using an antiserum against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). GABA-immunopositive cells and processes are grouped largely in the barrel side, whereas the barrel hollow is only weakly immunostained. The GABA-immunopositive cells have an ellipsoidal appearance similar to that of non-pyramidal class II barrel neurones described previously in Golgi impregnation studies of the mouse and rat barrel field.

Laminar and cellular localization of cytochrome oxidase in the cat striate cortex

Cytochrome oxidase (C.O.) was histochemically localized in the cat striate cortex at the light and electron microscopic levels. The results indicate that the oxidative metabolic activity within the cat striate cortex may vary between (1) different laminae, (2) neurons and glia, (3) different neuron types, (4) dendrite and soma of the same cell, (5) different types of dendrites, (6) different segments of the same dendrite, and (7) different classes of symmetric and asymmetric axon terminals. Maximal laminar C.O. staining was localized within geniculoreceptive layer IV. Darkly reactive neurons include the large (presumed corticotectal) pyramids of layer V, and various classes of large and medium-sized presumed GABAergic nonpyramidal cells sparsely distributed throughout layers II-VI. The small and medium-sized pyramids of layers II, III, V, and VI, as well as many of the smaller presumed GABAergic neurons, were only lightly or moderately reactive. The darkly reactive neurons tended to be those that received convergent or proximally localized asymmetric axosomatic synapses, implying that they are strongly driven by excitatory synaptic input. The darkly reactive nonpyramids resembled those that form GAD+, symmetric axosomatic synapses with pyramidal cells. The dark reactivity of the symmetric synaptic terminals indicates that they mediate strong inhibition of neuronal discharge. The dark reactivity of a class of large asymmetric terminals in layer IV is likely to represent highly active geniculocortical terminals. The predominant distribution of elevated C.O. reactivity in dendrites is correlated with reported sites of (1) convergent excitatory synaptic input, (2) maximal field potentials, (3) highly active ion transport, and (4) Na+, K+-ATPase.

Glutamic acid decarboxylase and somatostatin immunoreactivities in rat visual cortex

Antibodies to glutamic acid decarboxylase (GAD) and somatostatin (SS) were used to determine the laminar distribution and morphology of GAD- and SS-immunoreactive neurons and terminals in rat visual cortex. The present study demonstrates that GAD-immunoreactive neurons constitute several morphologically distinct subclasses of neurons in rat visual cortex. These subclasses of neurons can be distinguished by differences in soma size, soma shape, dendritic branching patterns, axonal arborizations, and location in the neuropil. GAD-immunoreactive neurons are found throughout all layers of visual cortex. They have nonpyramidal morphology and constitute roughly 15% of the total neuronal population. The laminar pattern of GAD-immunoreactive puncta is uneven, with a prominent band of terminals in layer IV. Numerous large GAD-positive puncta surround the somata and proximal dendrites of pyramidal cells in layers II, III, and V. SS-immunoreactive neurons constitute a less numerous and more restricted population of nonpyramidal neurons. Their somata are located mainly in layers II, III, V, and VI. Very few, if any, SS-immunoreactive neurons are found in layers I and IV. SS-immunoreactive terminals are arranged along vertical and diagonal collateral branches that have a beaded appearance. Finally, many neurons in the supra- and infragranular layers and in the white matter are immunoreactive to both glutamic acid decarboxylase and somatostatin. This coexistence of immunoreactivity to both GAD and SS may characterize a broad subclass of cortical nonpyramidal neurons.

Projection of olfactory bulb efferents to layer I GABAergic neurons in the entorhinal area. Combination of anterograde degeneration and immunoelectron microscopy in rat

Glutamic acid decarboxylase (GAD) immunoelectron microscopy in combination with anterograde degeneration was applied in rats to study the synaptic targets of olfactory bulb afferents to the lateral subdivision (LEA) of the entorhinal area (EA). Immunoreactive neurons and terminals are scattered throughout all layers of LEA. After olfactory bulb resection, terminal degeneration occurs in layer Ia of EA. Using the electron microscope we examined serial thin sections of 12 and 14 immunoreactive neurons sampled from layer Ia of the dorsal (DLEA) and ventral (VLEA) subdivisions of LEA, respectively. The morphology of all these neurons is similar: they are small (short axis 5-9 micron, long axis 7-12 micron) and possess eccentrically located, indented nuclei provided with filamentous nuclear rodlets. The immunoreactive neurons have thin, smooth dendrites which usually emerge abruptly from the somata. We observed a single cilium on 5 of the immunoreactive neurons. In layer Ia of both DLEA and VLEA, the somata of the immunoreactive neurons are contacted by degenerating, non-immunoreactive boutons showing asymmetric synaptic junctions. In addition to these boutons, 4 other categories of axo-somatic terminals can be distinguished: normal, non-immunoreactive boutons forming asymmetric synapses and containing spherical synaptic vesicles; normal, non-immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles; normal, non-immunoreactive boutons with asymmetric synapses, containing dense-cored vesicles in addition to spherical synaptic vesicles; and normal, immunoreactive boutons with symmetric synapses and pleomorphic synaptic vesicles. It is suggested that the GAD-immunoreactive neurons which receive olfactory bulb input correspond to local circuit neurons with intralaminar axons which innervate each other as well as the distal segments of the apical dendrites of projection neurons with cell bodies in layers II and III. Thus, the olfactory input in EA seems to be wired not only for excitation of layers II and III pyramidal neurons but also for feed-forward inhibition using GABAergic intermediary neurons, strategically located in the area of termination of olfactory bulb fibers.

The role of the gamma-aminobutyrate system in the interactions of cortical evoked potentials

The interactions between acoustic and somatosensory evoked potentials were examined in the anterior suprasylvian gyrus of the cat. Under conditions of barbiturate anaesthesia, occlusion was the dominant form of interaction. gamma-Aminobutyrate in local and intravenous application, and baclofen and diazepam in intravenous application significantly deepened the occlusion. gamma-Aminobutyrate antagonists, picrotoxin and bicuculline, in subconvulsive doses decreased occlusion or turned it into facilitation. gamma-Aminobutyrate agonists and gamma-aminobutyrate depressed and gamma-aminobutyrate antagonists enhanced the amplitude of the evoked potentials but the interactions by themselves proved independent from the absolute amplitudes. The interactions between evoked potentials of different modalities in the association cortex of the cat can be regarded as an expression of the actual equilibrium between excitatory and inhibitory interneuronal systems.

Correlation between cytochrome oxidase staining and the uptake and laminar distribution of tritiated aspartate, glutamate, gamma-aminobutyrate and glycine in the striate cortex of the squirrel monkey

The cellular uptake and laminar distribution of tritium-labeled gamma-aminobutyrate, aspartate, glutamate and glycine were examined in the primary visual cortex of squirrel monkeys. The purpose was to correlate the distribution of these labeled neurons with their level of cytochrome oxidase activity, particularly in laminae II-III (puffs) and adjacent non-puff regions. In general, tritium-labeled neurons that had either high or low levels of cytochrome oxidase activity were present in all laminae with each amino acid tested; however, their density varied between laminae and with the amino acid injected. Specifically, in laminae II-III, very few neurons were labelled with either of the putative excitatory amino acids (aspartate and glutamate). An increased uptake for both was observed in lamina IVC, with the greatest increase for each occurring in laminae V and VI. Significantly more neurons in each lamina were labeled with the putative inhibitory transmitters (gamma-aminobutyrate and glycine) than with either aspartate or glutamate. gamma-Aminobutyrate-labeled neurons were more prevalent in lamina II than III, and an increase in labeling was observed in laminae IV-VI, with the most prominent increase found in laminae V and VI. Glycine-labeled neurons were larger, more uniformly distributed and more abundant throughout all cortical laminae than those labeled with the other amino acids. Significantly more gamma-aminobutyrate- and glycine-labeled neurons were found in the puff regions than in the non-puff areas. No difference was found between puff and non-puff regions for the tritium-labeled leucine controls. Labeled neurons included stellate, fusiform and pyramidal-shaped cells of varying sizes; however, gamma-aminobutyrate-labeled pyramidal cells were not observed outside of the intense injection site. Large glycine-labeled cytochrome-oxidase-reactive pyramidal cells (24-32 micron in diameter) were present at the boundary between laminae V and VI. In addition, a row of large glycine-labeled, fusiform neurons were present in lamina IVB. With each amino acid injected, the tritium-labeled neurons that were darkly reactive for cytochrome oxidase were, on average, larger than the tritium-labeled neurons that were only lightly reactive for cytochrome oxidase. Thus, each of the four amino acids tested had its unique pattern of distribution in the primate striate cortex. Whether one or all of them served as neurotransmitter(s) for distinct neuronal groups is beyond the scope of this study. Glycine, in particular, might be used in part or in whole for metabolic purposes.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine hydroxylase-immunoreactive boutons in synaptic contact with identified striatonigral neurons, with particular reference to dendritic spines

Tyrosine hydroxylase-immunoreactive fibres in the rat neostriatum were studied in the electron microscope in order to determine the nature of the contacts they make with other neural elements. The larger varicose parts of such fibres contained relatively few vesicles and rarely displayed synaptic membrane specializations; however, thinner parts of axons (0.1-0.4 micron) contained many vesicles and had symmetrical membrane specializations, indicative of en passant type synapses. By far the most common postsynaptic targets of tyrosine hydroxylase-immunoreactive boutons were dendritic spines and shafts, although neuronal cell bodies and axon initial segments also received such input. Six striatonigral neurons in the ventral striatum were identified by retrograde labelling with horseradish peroxidase and their dendritic processes were revealed by Golgi impregnation using the section-Golgi procedure. The same sections were also developed to reveal tyrosine hydroxylase immunoreactivity and so we were able to study immunoreactive boutons in contact with the Golgi-impregnated striatonigral neurons. Each of the 280 immunoreactive boutons examined in the electron microscope displayed symmetrical synaptic membrane specializations: 59% of the boutons were in synaptic contact with the dendritic spines, 35% with the dendritic shafts and 6% with the cell bodies of striatonigral neurons. The dendritic spines of striatonigral neurons that received input from immunoreactive boutons invariably also received input, usually more distally, from unstained boutons that formed asymmetrical synaptic specializations. A study of 87 spines along the dendrites of an identified striatonigral neuron showed that the most common type of synaptic input was from an individual unstained bouton making asymmetrical synaptic contact (53%), while 39% of the spines received one asymmetrical synapse and one symmetrical immunoreactive synapse. It is proposed that the spatial distribution of presumed dopaminergic terminals in synaptic contact with different parts of striatonigral neurons has important functional implications. Those synapses on the cell body and proximal dendritic shafts might mediate a relatively non-selective inhibition. In contrast, the major dopaminergic input that occurs on the necks of dendritic spines is likely to be highly selective since it could prevent the excitatory input to the same spines from reaching the dendritic shaft. One of the main functions of dopamine released from nigrostriatal fibres might thus be to alter the pattern of firing of striatal output neurons by regulating their input.

Morphology of Golgi-Cox-impregnated barrel neurons in rat SmI cortex

Golgi-Cox-impregnated neurons in the barrel cortex of the rat were studied qualitatively and quantitatively. Adult rat brains were sectioned perpendicular to or parallel to the cortical representation of the large facial vibrissae at 125 micron. Cortical laminar and barrel boundaries were identified from the Nissl counterstain. Over 200 well-impregnated neurons in cortical layers I-IV were selected for classification and further detailed study. Three broad classes of neurons were recognized: (1) pyramidal cells with conical somata, a stout apical dendrite, and spines; (2) class I nonpyramidal cells having small spherical somata and spiny dendrites; and (3) class II nonpyramidal cells having larger ellipsoid somata and smooth or beaded dendrites. The class I cells were further subdivided into "star pyramids" (cells with an apical dendrite) and spiny stellate cells (cells in which all dendrites were of similar length). The class II cells also were subdivided into multiform cells (with multiple dendrites radiating from the soma) and bipolar cells (with two principal dendritic trunks arising from the superficial and deep aspects of the soma). The position of these various cell types in the superficial cortical laminae was mapped in sections normal to the pia. Numerous examples of the class I and class II neurons were drawn with respect to the barrels in layer IV and the extent of their processes noted. Finally, approximately 250 barrel-related class I and II neurons were studied quantitatively using a computer-microscope and digitizing tablet. The density of the Golgi-impregnated neurons corresponds to the pattern of cell density seen with the Nissl counterstain. The various cell types are not uniformly distributed as a function of cortical depth. Cells with apical dendrites were found principally in the supragranular layers and star pyramids in the superficial one-half of layer IV. Spiny stellate cells are concentrated in layer IV and the smooth cells are present in greatest number in deep layer III and deeper layer IV. On the basis of these distributions we suggest that layer IV be subdivided into two sublaminae. The class I and class II neurons can be distinguished according to quantitative criteria which apply in either plane of section used. Class I neurons have smaller projected somal areas, more proximal dendritic branching, and shorter dendrites when class I and II neurons are measured in three dimensions.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics and distribution of muscimol binding sites in cat visual cortex

In vitro receptor binding techniques were used to study the characteristics and distribution of [3H]muscimol binding sites in cat visual cortex. [3H]muscimol, a specific GABA agonist, labeled a single population of binding sites with a Kd of 18 nM. Specific binding was saturable, reversible, and was blocked by the addition of GABA or (+)-bicuculline. Autoradiograms revealed that the highest density of [3H]muscimol binding sites occurred in cortical layer IV. Little variation between the various visual cortical areas was noted in contrast to marked regional heterogeneity within subcortical structures.

Cholinergic synapses in the rat brain: a correlated light and electron microscopic immunohistochemical study employing a monoclonal antibody against choline acetyltransferase

Using a monoclonal antibody to choline acetyltransferase, immunoreactive synaptic boutons were identified in the neostriatum, cingulate cortex, basolateral nucleus of the amygdala, hippocampus and interpeduncular nucleus of the rat. The synapses were generally symmetrical although some asymmetrical membrane specializations were observed. Postsynaptic targets included perikarya, dendritic shafts and dendritic spines.

Characterization of pallidonigral neurons in the rat by a combination of Golgi impregnation and retrograde transport of horseradish peroxidase: their monosynaptic input from the neostriatum

After injection of horseradish peroxidase, or its conjugate with wheatgerm agglutinin, into the substantia nigra of rats, retrogradely labelled cells were found in the globus pallidus. Forty-six of these neurons were also impregnated by the Golgi procedure and then gold-toned: their somata ranged from 15 to 30 microns in diameter and these pallidonigral neurons had from two to five primary dendrites that were long and smooth, that branched infrequently and that bore occasional spines on their distal regions. Most of the neurons studied came from the lateral part of the globus pallidus. At the ultrastructural level, the identified pallidonigral neurons were found to have deeply infolded nuclei and an abundant cytoplasm; their perikarya were richly innervated by two distinct types of bouton, both of which formed symmetrical synaptic contacts. The dendrites of pallidonigral neurons were ensheathed in boutons, the majority forming symmetrical synaptic contacts. After placement of electrolytic lesions in the rostro-dorsal neostriatum, degenerating boutons were found in symmetrical synaptic contact with the cell bodies and dendrites of six identified pallidonigral neurons. It is concluded that pallidonigral neurons belong to the Golgi category of large pallidal neurons with smooth dendrites and that they receive monosynaptic input from the neostriatum. Thus, in addition to the direct striatonigral pathway, the neostriatum can influence the substantia nigra by a monosynaptic relay through the globus pallidus, which might allow other pallidal afferents to influence the transfer of information from neostriatum to substantia nigra.

Response properties and morphological identification of neurons in the cat motor cortex

Intracellular recordings and morphological identification of neurons using intracellular HRP staining were performed in the cat motor cortex. By thalamic ventrolateral (VL) or cerebellar nucleus stimulation, pyramidal cells in layer III, fast pyramidal tract neurons (PTNs) and stellate cells in layers II and III were activated with short latency and fast rising EPSPs, while pyramidal cells in layer II and slow PTNs showed longer latency and slow rising EPSPs. This difference may be related to activation through the deep and superficial thalamocortical projections. Although pyramidal cells in layer VI did not respond orthodromically to VL or cerebellar stimulation, some of them proved to receive the recurrent action of PTNs because of the response to stimulation of the cerebral peduncle (CP). One aspinous stellate cell in layer III was activated by CP as well as VL stimulation. This cell was supposed to be an inhibitory interneuron responsible for both recurrent and VL-evoked inhibition.

Aspiny neurons and their local axons in the neostriatum of the rat: a correlated light and electron microscopic study of Golgi-impregnated material

Three types of neuron with smooth (aspiny) dendrites could be distinguished in the Golgi-impregnated rat neostriatum. Examples of each type of aspiny neuron were found with local axon collaterals within the neostriatum and these were selected for gold- toning and examination in the electron microscope. One type of aspiny neuron had an elongated, usually spindle-shaped, medium-size soma with two, or rarely three, primary dendrites originating from opposite poles of the cell; one example of this type of neuron had two separate axons. The second type of aspiny neuron had a nearly round, medium-size soma with four primary dendrites that branched profusely quite close to the cell body. A third type of aspiny neuron had a very large polygonal-shaped cell body. Afferent axon terminals were found in synaptic contact with the dendrites and cell bodies of all three types of aspiny neuron. Axon collaterals of each type of neuron displayed varicosities which, when examined in the electron microscope, were frequently found to be boutons making synaptic contact. All such synaptic contacts had symmetrical membrane specializations and the most common postsynaptic targets were dendritic shafts, sometimes spine-bearing. Dendritic spines themselves also received synapses from each type of neuron. No axosomatic synapses involving boutons of identified axons were found. One example of a synapse between an axon collateral of an aspiny neuron and one of the same neuron's dendrites (an ' autapse ') was demonstrated by electron microscopy. It is concluded that the synaptic terminals of at least four types of neuron, the three aspiny types described here and the medium-size densely spiny neuron, participate in local circuit interactions in the neostriatum.

Morphometry of spine-free nonpyramidal neurons in rabbit auditory cortex

A study of the morphometry and laminar distribution of spine-free nonpyramidal neurons in electrophysiologically verified primary auditory cortex was carried out in adult rabbits. By using image-combining computer microscopy, the locations of all impregnated neurons in 300-micrometers Golgi-Cox Nissl sections through the auditory cortex were determined. Spine-free non-pyramidal neurons constitute nearly 72% of the nonpyramidal neurons present. They are distributed in a band extending from 450 to 750 micrometers beneath the pial surface corresponding to laminae III and IV. A combination of dendritic stick, Fourier, and statistical analyses revealed a highly significant spatial orientation of their dendrite systems along a vertical axis parallel to the apical dendrites of pyramidal neurons. A significant tangential orientation of dendrites along a dorsal-ventral axis was also found. A radial analysis of the dendrite systems revealed that the pronounced vertical orientation of spine-free nonpyramidal neurons is due to (1) directed dendritic growth along the vertical axis, (2) decreased branching and rapid termination of tangentially oriented dendrites, and (3) increased branching of vertically growing dendrites. The radial analysis also revealed that the longest branches are those directed toward the white matter.

Glutamate decarboxylase-immunoreactive terminals of Golgi-impregnated axoaxonic cells and of presumed basket cells in synaptic contact with pyramidal neurons of the cat's visual cortex

Glutamate decarboxylase (GAD)-immunoreactive varicosities were found around cell bodies of nonimmunoreactive and immunoreactive neurons in the cat's visual cortex; they also occurred along apical dendrites and axon initial segments of pyramidal neurons. By examination in the electron microscope of structures first identified in the light microscope, it was established that the GAD-immunoreactive varicosities were boutons in symmetrical synaptic contact with pyramidal cells in layers II-IV. More than 90% of 142 boutons surrounding the cell bodies of 20 pyramidal neurons were immunoreactive for GAD. Since such a high proportion of the axosomatic boutons are GAD-immunoreactive, it is likely that the terminals of basket cells are included in this population and so the basket cell probably uses gamma-aminobutyrate as a transmitter, as suggested by previous authors. Almost all the 68 boutons in symmetrical contact with the axon initial segments of six pyramidal neurons could be shown to be GAD-immunoreactive, which makes it very likely that the boutons of axoaxonic cells contain GAD-immunoreactivity. This was established unequivocally for an individual Golgi-impregnated axoaxonic cell by combining Golgi impregnation and immunocytochemistry in the same sections: A Golgi-impregnated axoaxonic cell whose cell body was in layer II gave rise to numerous terminal segments, some of which were examined in the electron microscope after gold-toning. These boutons were in synaptic contact with axon initial segments and not only contained the Golgi precipitate but were also immunoreactive for GAD. It is concluded that the axoaxonic cell in the visual cortex uses gamma-aminobutyrate as a transmitter. An individual axoaxonic cell in layer II/III was filled with horseradish peroxidase by intracellular iontophoresis. The very extensive local axonal field was composed of 330 terminal bouton rows in layer II/III and a sparse descending collateral projection to infragranular layers. A computer-assisted reconstruction of the axonal field in three dimensions revealed the following: The main output of the cell is to pyramidal neurons that lie deeper than the soma; the axonal arborization occupies an area of 400 micron in the anteroposterior axis and extends 200 micron along the mediolateral axis; the terminal bouton rows in layer II/III form clusters about 50 micron wide running approximately at right angles to the border between areas 17 and 18, with an intercluster interval of about 100 micron. These findings suggest that the terminals of an individual axoaxonic cell could be contained within one ocular dominance column but that there may be inhomogeneities in the weighting of the axoaxonic input to pyramidal cells in the supragranular layers.

Laminal distribution of gamma-aminobutyric acid (GABA) in the occipital cortex of rats: evidence as a neurotransmitter

The laminal distribution of gamma-aminobutyric acid (GABA) in the rat occipital cortex was determined by an enzymatic procedure following decapitation or microwave irradiation (5 kW for 1.1 s). No significant differences were observed in the concentrations of GABA among 5 layers after microwave irradiation. However, when the animals were sacrificed by decapitation, the concentrations varied among the layers with the highest being detected in the internal granular layer (lamina IV) followed by the external granular (II), external pyramidal (III) and internal pyramidal layers (V). In these layers, GABA was significantly higher than in the other two layers, the molecular (I) and multiform layers (VI). The results are discussed in terms of the neurotransmitter function of GABA in the occipital cortex.

Synaptic connections of morphologically identified and physiologically characterized large basket cells in the striate cortex of cat

Neurons were studied in the striate cortex of the cat following intracellular recording and iontophoresis of horseradish peroxidase. The three selected neurons were identified as large basket cells on the basis that (i) the horizontal extent of their axonal arborization was three times or more than the extent of the dendritic arborization; (ii) some of their varicose terminal segments surrounded the perikarya of other neurons. The large elongated perikarya of the first two basket cells were located around the border of layers III and IV. The radially-elongated dendritic field, composed of beaded dendrites without spines, had a long axis of 300-350 microns, extending into layers III and IV, and a short axis of 200 microns. Only the axon, however, was recovered from the third basket cell. The lateral spread of the axons of the first two basket cells was 900 microns or more in layer III and, for the third cell, was over 1500 microns in the antero-posterior dimension, a value indicating that the latter neuron probably fulfills the first criterion above. The axon collaterals of all three cells often branched at approximately 90 degrees to the parent axon. The first two cells also had axon collaterals which descended to layers IV and V and had less extensive lateral spreads. The axons of all three cells formed clusters of boutons which could extend up a radial column of their target cells. Electron microscopic examination of the second basket cell showed a large lobulated nucleus and a high density of mitochondria in both the perikarya and dendrites. The soma and dendrites were densely covered by synaptic terminals. The axons of the second and third cells were myelinated up to the terminal segments. A total of 177 postsynaptic elements was analysed, involving 66 boutons of the second cell and 89 boutons of the third cell. The terminals contained pleomorphic vesicles and established symmetrical synapses with their postsynaptic targets. The basket cell axons formed synapses principally on pyramidal cell perikarya (approximately 33% of synapses), spines (20% of synapses) and the apical and basal dendrites of pyramidal cells (24% of synapses). Also contacted were the perikarya and dendrites of non-pyramidal cells, an axon, and an axon initial segment. A single pyramidal cell may receive input on its soma, apical and basal dendrites and spines from the same large basket cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal subpopulations in cat retina which accumulate the GABA agonist, (3H)muscimol: a combined Golgi and autoradiographic study

Golgi impregnation techniques were combined with electron microscopic autoradiography of (3H-muscimol in order to provide morphological identification of labeled neurons in the cat retina. This gamma-aminobutyric acid (GABA) agonist has been shown to label the same neurons which accumulate (3H)GABA. Selected cells were photographed and drawn by the aid of a camera lucida drawing tube prior to being thin sectioned for autoradiography. The (3H)muscimol-accumulating neurons which were identified include an interplexiform cell, five classes of conventional amacrine cell, and another cell with its soma located in the ganglion cell layer which is either a ganglion cell or a displaced amacrine. The conventional amacrine cells were compared with the recent morphological classification of Kolb et al. (Vision Res. 21: 1081-1114, '81) and identified as A2, A10, A13, A17, and A19 amacrine cells. These cells exhibit a widespread distribution providing input to all five strata of the inner plexiform layer.

Four types of neuron in layer IVab of cat cortical area 17 accumulate 3H-GABA

Roughly 10% of the neurons in layer IVab of cat area 17 accumulate exogenous 3H-gamma-aminobutyric acid (GABA) but how many types of neuron comprise this population was unknown. We characterized these neurons by partial reconstruction of their somas from serial electron microscope autoradiograms and distinguished four types. GABA 1 was large (greater than 16.5 micron) and dark with a dense distribution of synaptic terminals, substantial geniculate input to the soma, and a moderate accumulation of GABA. GABA 2 was small (less than 13 micron) and pale, also with a dense distribution of terminals but without evidence of somatic geniculate input, and a moderate accumulation of GABA. GABA 3 was radially fusiform (20 micron X 8 micron) with varicose dendrites, a sparse distribution of synaptic terminals, and a heavy accumulation of GABA. GABA 4 was medium in size (15 micron) with a moderate distribution of synaptic terminals and a heavy accumulation of GABA. Reasons are presented for believing that each of these four categories of GABA-accumulating neuron represents a fundamental cell type.

The section-Golgi impregnation procedure. 1. Description of the method and its combination with histochemistry after intracellular iontophoresis or retrograde transport of horseradish peroxidase

A method is described for impregnating neurons by the Golgi procedure in sections of brain tissue that are of a thickness 80-100 microns, so allowing a variety of histochemical procedures to be carried out prior to Golgi-impregnation. The method has been applied to the retina of the adult primate and to brain sections incubated to reveal horseradish peroxidase activity with substrates that give electron-dense reaction products. Sections containing neurons that have been filled with horseradish peroxidase by intracellular iontophoresis following electrophysiological characterization, or containing neurons retrogradely labelled by horseradish peroxidase, can be impregnated by this Golgi procedure. Subsequent gold-toning of the Golgi-impregnated neurons makes it possible to study, in the electron-microscope, both Golgi-impregnated and peroxidase-containing neurons and their afferent and efferent synaptic contacts within the same section. The procedure also makes it possible to repeat Golgi impregnation on sections already Golgi-impregnated and gold-toned, or to repeat the Golgi impregnation if the first impregnation is not satisfactory. The procedure is illustrated by examples from the monkey retina, cat visual cortex and rat neostriatum.

The section-Golgi impregnation procedure. 2. Immunocytochemical demonstration of glutamate decarboxylase in Golgi-impregnated neurons and in their afferent synaptic boutons in the visual cortex of the cat

Sections of the cat's visual cortex were stained by an antiserum to glutamate decarboxylase using the peroxidase-antiperoxidase method; they were then impregnated by the section Golgi procedure and finally the Golgi deposit was replaced by gold. Neurons containing glutamate decarboxylase immunoreactivity were found in all layers of the visual cortex, without any obvious pattern of distribution. Fifteen immunoreactive neurons were also Golgi-impregnated and gold-toned, which enabled us to study the morphology and synaptic input of identified GABAergic neurons. These neurons were found to be heterogeneous both with respect to the sizes and shapes of their perikarya and the branching patterns of their dendrites. All the immunoreactive, Golgi-impregnated neurons had smooth dendrites, with only occasional protrusions. The synaptic input of glutamate decarboxylase-immunoreactive neurons was studied in the electron microscope. Immunoreactive neurons received immunoreactive boutons forming symmetrical synapses on their cell bodies. The Golgi-impregnation made it possible to study the input along the dendrites of immunoreactive neurons. One of the large neurons in layer III whose soma was immunoreactive was also Golgi-impregnated: it received numerous non-immunoreactive asymmetrical synaptic contacts along its dendrites and occasional ones on its soma. The same neuron also received a few boutons forming symmetrical synaptic contacts along its Golgi-impregnated dendrites; most of these boutons were immunoreactive for glutamate decarboxylase. Glutamate decarboxylase-immunoreactive boutons were also found in symmetrical synaptic contact with non-immunoreactive neurons that were Golgi-impregnated. A small pyramidal cell in layer III was shown to receive several such boutons along its somatic membrane. It is concluded that the combination of immunoperoxidase staining and Golgi impregnation is technically feasible and that it can provide new information. The present study has shown that there are many morphologically distinct kinds of aspiny GABAergic neurons in the visual cortex; that the predominant type of synaptic input to the dendrites of such neurons is from boutons forming asymmetrical synapses, but that some of the GABAergic neurons also receive a dense symmetrical synaptic input on their cell bodies, and occasional synapses along their dendrites, from the boutons of other GABAergic neurons. These findings provide a morphological basis, firstly, for a presumed powerful excitatory input to GABAergic interneurons and, secondly, for the disinhibition which has been postulated from electrophysiological studies to occur in the cat's visual cortex.