Quantitative distribution of GABA-immunoreactive neurons in the visual cortex (area 17) of the cat

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons. J. Neurophysiol. 78: 2287-2295, 1997. Using neocortical layer I neurons as a model for GABAergic interneurons, we have studied gamma-aminobutyric acid-A (GABAA) receptor-mediated spontaneous inhibitory postsynaptic currents (IPSCs) and modulation by metabotropic glutamate receptors (mGluRs). In the presence of 0.5 mu M tetrodotoxin (TTX) and ionotropic glutamate receptor antagonists and under symmetrical Cl- conditions, the mean amplitude of miniature IPSCs (mIPSCs) was approximately 50 pA at a holding potential of -70 mV with individual events ranging from 10 to 400 pA. Averaged mIPSCs had a 10-90% rise time of approximately 0.6 ms. The decay was double exponential. The fast component had a time constant of approximately 4 ms and comprised approximately 40% of the total amplitude. The slow component had a time constant of approximately 22 ms. The frequency of spontaneous IPSCs (sIPSCs), recorded in the absence of TTX, was increased by bath application of the mGluR agonist 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD; 10-100 mu M) or the group I mGluR selective agonist quisqualic acid (Quis; 0.5-1 mu M). Under identical conditions, mIPSCs were not affected. The kinetics of sIPSCs and mIPSCs were not altered by ACPD or Quis. Quis (1 mu M) induced an inward current of approximately 70 pA at a holding potential of -70 mV, whereas ACPD (40-200 mu M) induced a smaller inward current. This current was linear over the voltage range -70 to +30 mV and reversed polarity near 0 mV. In current-clamp recordings, both Quis and ACPD induced a depolarization and action potential firing in layer I and deeper layer interneurons. We conclude that neocortical layer I neurons receive GABAA receptor-mediated inhibitory synaptic inputs. Activation of mGluRs, possibly mGluR1 and/or mGluR5, causes an enhancement of inhibitory synaptic transmission by directly depolarizing corticalGABAergic interneurons through the opening of nonselective cation channels.

Brain-derived neurotrophic factor mediates the activity-dependent regulation of inhibition in neocortical cultures

The excitability of cortical circuits is modulated by interneurons that release the inhibitory neurotransmitter GABA. In primate and rodent visual cortex, activity deprivation leads to a decrease in the expression of GABA. This suggests that activity is able to adjust the strength of cortical inhibition, but this has not been demonstrated directly. In addition, the nature of the signal linking activity to GABA expression has not been determined. Activity is known to regulate the expression of the neurotrophin brain-derived neurotrophic factor (BDNF), and BDNF has been shown to influence the phenotype of GABAergic interneurons. We use a culture system from postnatal rat visual cortex to test the hypothesis that activity is regulating the strength of cortical inhibition through the regulation of BDNF. Cultures were double-labeled against GABA and the neuronal marker MAP2, and the percentage of neurons that were GABA-positive was determined. Blocking spontaneous activity in these cultures reversibly decreased the number of GABA-positive neurons without affecting neuronal survival. Voltage-clamp analysis of inhibitory currents demonstrated that activity blockade also decreased GABA-mediated inhibition onto pyramidal neurons and raised pyramidal neuron firing rates. All of these effects were prevented by incubation with BDNF during activity blockade, but not by neurotrophin 3 or nerve growth factor. Additionally, blockade of neurotrophin signaling mimicked the effects of activity blockade on GABA expression. These data suggest that activity regulates cortical inhibition through a BDNF-dependent mechanism and that this neurotrophin plays an important role in the control of cortical excitability.

Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex

With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (-50 to -70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to -30 mV or by the use of a Mg2+-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of approximately 0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of approximately 1.2 ms and comprised approximately 80% of the total amplitude. A small slow component had a decay time constant of approximately 4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at -60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca2+ permeable.

Morphology and physiology of cortical neurons in layer I

The electrophysiological and morphological properties of layer I neurons were studied in visual cortex slices from 7- to 19-d-old rats using whole-cell recording and biocytin labeling. A heterogeneous population of small, nonpyramidal neurons was found. Approximately one third of the cells we recorded were neurogliaform cells; another third were multipolar neurons with axons descending out of layer I. The remaining cells were heterogeneous and were not classified. In slices from 7- to 10-d-old animals only, we identified Cajal-Retzius cells. Neurogliaform neurons had a very dense local axonal field, which was largely contained within layer I. Cells with descending axons had a relatively sparse local axonal arbor and projected at least to layer II and sometimes deeper. Spiking in neurogliaform neurons was followed by an afterdepolarizing potential, whereas spiking in cells with descending axons was followed by a slow after-hyperpolarizing potential (AHP). In addition, neurogliaform cells exhibited less spike broadening and a larger fast AHP after single spikes than did cells with descending axons. Generally, cells in layer I received synaptic inputs characterized as either GABA- or glutamate-mediated, suggesting the presence of excitatory and inhibitory inputs. With their output largely limited to layer I, neurogliaform cells could synapse with other layer I neurons, the most distal dendritic branches of pyramidal cells, or the dendrites of layer II/III interneurons, which invade layer I. Cells with descending axons could contact a wide variety of cortical cells throughout their vertical projection.

Correlation between kinetics and RNA splicing of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in neocortical neurons

In the cortex fast excitatory synaptic currents onto excitatory pyramidal neurons and inhibitory nonpyramidal neurons are mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors exhibiting cell-type-specific differences in their kinetic properties. AMPA receptors consist of four subunits (GluR1-4), each existing as two splice variants, flip and flop, which critically affect the desensitization properties of receptors expressed in heterologous systems. Using single cell reverse transcription PCR to analyze the mRNA of AMPA receptor subunits expressed in layers I-III neocortical neurons, we find that 90% of the GluR1-4 in nonpyramidal neurons are flop variants, whereas 92% of the GluR1-4 in pyramidal neurons are flip variants. We also find that nonpyramidal neurons predominantly express GluR1 mRNA (GluR1/GluR1-4 = 59%), whereas pyramidal neurons contain mainly GluR2 mRNA (GluR2/GluR1-4 = 59%). However, the neuron-type-specific splicing is exhibited by all four AMPA receptor subunits. We suggest that the predominance of the flop variants contributes to the faster and more extensive desensitization in nonpyramidal neurons, compared to pyramidal cells where flip variants are dominant. Alternative splicing of AMPA receptors may play an important role in regulating synaptic function in a cell-type-specific manner, without changing permeation properties.

Noise, neural codes and cortical organization

Cortical circuitry must facilitate information transfer in accordance with a neural code. In this article we examine two candidate neural codes: information is represented in the spike rate of neurons, or information is represented in the precise timing of individual spikes. These codes can be distinguished by examining the physiological basis of the highly irregular interspike intervals typically observed in cerebral cortex. Recent advances in our understanding of cortical microcircuitry suggest that the timing of neuronal spikes conveys little, if any, information. The cortex is likely to propagate a noisy rate code through redundant, patchy interconnections.

Synchronization of inhibitory neurones in the guinea-pig hippocampus in vitro

1. Intracellular recordings were obtained from pyramidal, granule and hilar cells in transverse slices of guinea-pig hippocampus to examine synaptic interactions between GABAergic neurones. 2. In the presence of the convulsant compound 4-aminopyridine (4-AP), after fast excitatory amino acid (EAA) neurotransmission was blocked pharmacologically, large amplitude inhibitory postsynaptic potentials (IPSPs) occurred rhythmically (every 4-8 s) and synchronously in all principal cell populations (triphasic synchronized IPSPs). In the presence of the GABAA receptor blocker picrotoxin (PTX), a large amplitude IPSP continued to occur spontaneously in all principal cells simultaneously (monophasic synchronized IPSP). 3. Burst firing occurred simultaneously in a group of hilar neurones (synchronized bursting neurones) coincident with triphasic synchronized IPSPs in principal cells. After PTX was added, the bursts and the underlying depolarizing synaptic potentials were completely suppressed in some of the synchronized bursting neurones (type I hilar neurones), while others (type II hilar neurones) continued to fire in bursts coincident with monophasic synchronized IPSPs in principal cells. Intense hyperpolarization blocked burst firing and revealed underlying attenuated spikes of less than 10 mV, but did not uncover any underlying depolarizing synaptic potentials. 4. In type II hilar neurones, during sufficient hyperpolarization, spontaneous activity consisted of attenuated spikes. With depolarization, the small spikes began to trigger full size action potentials. These data suggest the presence of electrotonically remote spike initiation sites. 5. The morphology of synchronized bursting neurones was revealed by intracellular injection of the fluorescent dye Lucifer Yellow. Attempts to inject dye into one type II hilar neurone often resulted in the labelling of two to four cells (dye coupling). Dye coupling was not observed in type I hilar neurones. 6. These findings indicate that excitatory interactions synchronizing the firing of GABAergic neurones can occur in the absence of fast EAA neurotransmission. GABAergic neurones can become synchronized via their recurrent collaterals through the depolarizing action of synaptically activated GABAA receptors. In addition, a subpopulation of GABAergic neurones can become synchronized by a mechanism probably involving electrotonic coupling.

The topography of tangential inhibitory connections in the postnatally developing and mature striate cortex of the cat

Clustered intrinsic connections in the striate cortex of kittens originate from an unclustered, diffusely organized pattern prevailing during the first postnatal week. In order to study the participation of inhibitory neurons in this reorganization of the connections, we determined the topography of the inhibitory tangenital connections in the striate cortex of cats ranging in age between 7 and 330 days by combining retrograde transport of fluorescent microspheres with GABA immunohistochemistry. After small intracortical injections of tracer, neurons containing either microspheres only (non-GABAergic neurons) or GABA-like immunoreactivity in addition to microspheres (GABAergic neurons) are labelled at various horizontal distances from the injection. At the end of the first postnatal week, both GABAergic and non-GABAergic neurons are distributed in the horizontal plane in an unclustered fashion. During the second postnatal week, the tangential connections rearrange rapidly to form clusters. The tendency of the cells to form clusters is much weaker, however, in GABAergic than in non-GABAergic neurons. In regions > 500 microns distant from the centre of injection approximately 90% of the non-GABAergic neurons (range 87.5-92.6%) but only 63% (range 57.1-72.3%) of the GABAergic neurons reside within the clusters formed by the non-GABAergic neurons. These proportions do not change systematically with age. In the regions outside the non-GABAergic clusters, GABAergic neurons appear to be evenly distributed and not to aggregate in clusters. From postnatal day 7 forward GABAergic neurons largely retain their overall distribution and density in the horizontal plane. When considering all cortical layers (including the superficial white matter) the lateral spread of the GABAergic neurons is more restricted than that of the non-GABAergic neurons. Systematic changes in the lateral spread of inhibitory connections according to postnatal age were not observed. We conclude that, like the non-GABAergic neurons, the GABAergic neurons have attained an adult-like topography in the horizontal plane by about the end of the second postnatal week. From that time until adulthood they display much weaker clustering, a higher relative occurrence of short axon collaterals and a more restricted lateral distribution than do the excitatory neurons.

Distribution of GABA-containing neurons in human frontal cortex: a quantitative immunocytochemical study

Fresh biopsy specimens of human cerebral cortex were collected from patients suffering from deep-seated tumors requiring resection. GABAergic neurons were revealed in 50-microns-thick sections, for pre-embedding, and 1-micron-thick sections, for post-embedding GABA immunocytochemistry. In both thick and thin sections, the reaction product was found in neuronal cell bodies and in small profiles in the neuropil. In both preparations, GABA-containing somata were distributed evenly throughout the depth of the cortex. As best appreciated in the thicker sections, GABA-immunoreactive neurons belonged to a variety of morphological cell types with multipolar, bitufted or bipolar, and horizontal dendritic arbors. In the semi-thin sections sampled in the frontal cortex, the proportion of these neurons could be accurately evaluated and was found to be 21.2% +/- 4.8% of all cortical neurons. The average size of GABA-immunoreactive neurons was, in each layer, smaller than that of immunonegative neurons. The average soma size of both neuronal populations, immunoreactive and immunonegative for GABA, increased with depth. The comparison between the rat, cat, macaque monkey, and human GABAergic interneurons revealed similarities among primate brains, contrasting with the parameters (morphology, size, density) measured in rodents. These data are pertinent to the involvement of the GABAergic neurons in the shaping of receptive-field properties of cortical neurons in healthy brains and in pathologies involving the impairment of inhibitory neurotransmission.

Developmental changes in layer I of the human neocortex during prenatal life: a DiI-tracing and AChE and NADPH-d histochemistry study

The development of fetal layer I (marginal zone, MZ) was studied in the human neocortex by using DiI tracing and AChE and NADPH-d histochemistry, and examining the Nissl-stained material of the Yakovlev Collection. We describe the sequential maturation of the Cajal-Retzius (CR) cells and the granule cells of the subpial granular layer (SGL), and the close relationship between both. The first CR cells appear in the primordial plexiform layer, at 6 gestational weeks (GW). After the formation of the cortical plate, they settle under the pial surface. At 13 GW, the SGL begins to form around the CR cells. The horizontal members of a polymorphic population of CR cells begin to mature at 13 GW, and the intermediate and vertical forms differentiate at 16 and 18 GW, respectively. All CR cells project into an axonal plexus in the lower third of the MZ. From 18 GW, CR cells and SGL become segregated, and the polymorphic CR cells lie now below the SGL, with which they remain connected by ascending processes. Granule cells invade the lower MZ contacting somata and processes of CR cells. The somata of vertical CR cells elongate until 23/24 GW when they show degenerative signs. After 24 GW, all polymorphic CR cells die. Granule cells degenerate after 24 GW; the SGL disappears at 28/30 GW. A population of persisting CR cells, morphologically different from the transient polymorphic forms, appears in a subpial position and survives in small numbers throughout life. Small non-CR neurons differentiate first in the lower half of layer I, thereafter also in the upper half. Histochemically, all CR cells are AChE-positive; they contain NADPH-d only transiently at 20 GW. We propose that CR cells and SGL provide a transient innervation network for the developing cortical plate at a time when the definitive fiber systems of the molecular layer are not yet established.

Numerical data on neocortical neurons in adult rat, with special reference to the GABA population

The disector method was used to estimate the numerical density of neurons (number per unit volume) and their actual number per column (number under a given area of pial surface), in the occipital (monocular segment of the primary visual area, Oc1M), the parietal (somatosensory barrelfield area, Par1) and the frontal cortex (primary motor area, Fr1) of adult rat. Values were first obtained for all neurons in each layer, and then for GABA neurons as identified with postembedding immunocytochemistry on semithin sections. The numerical density of neurons in the frontal cortex (34,000/mm3) was significantly lower than in the two other neocortical areas (occipital: 52,000; parietal: 48,000/mm3). The GABA population showed a similar difference and consequently represented an equivalent proportion of total (15%) in the three cortical areas. Across layers, there was an alternate distribution of low and high density of neurons from layers II-III to VI in the three cortical areas, with the highest density in layer IV of the two sensory areas. The laminar changes in density of the GABA neurons were not as pronounced as those of the overall population. Consequently, the layers with the highest overall neuronal densities tended to have a lower proportion of GABA neurons and vice versa. There were more neurons under 1 mm2 of surface in the parietal (90,000) than the occipital or the frontal cortex (71,000), which was also true of the GABA neurons. The greater number of neurons per column in the parietal cortex was mostly imputable to layer IV, the main recipient of thalamic axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology and distribution of neuropeptide-containing neurons in human cerebral cortex

Biopsies of human cerebral cortex were fixed by immersion and immunostained for the detection of neuropeptides in neuronal cell bodies and axons. Four neuropeptides (neuropeptide Y, somatostatin, , substance P and cholecystokinin) were visualized in a series of adjacent sections. All populations of immunoreactive neurons had a morphology characteristic of interneurons, with variations in dendritic arborizations and laminar distribution. The cholecystokinin-immunoreactive neurons were most numerous in the supragranular layers, whereas neurons containing the other three peptides occurred mainly in infragranular layers, or even in neurons populating the subcortical white matter. Quantitatively, each population of neuropeptide-containing neurons accounted for 1.4-2.5% of the total neuronal population. The distribution of these neurons varied slightly between cytoarchitectonic divisions, with substance P- and somatostatin-immunoreactive neurons dominating in the temporal lobe and cholecystokinin-immunoreactive neurons in the frontal lobe. Neuropeptide Y-immunoreactive neurons dominated in the gray matter of the frontal half of the hemisphere and in the subcortical white matter of the caudal half of the hemisphere. Furthermore, co-existence of neuropeptide Y or substance P immunoreactivity within somatostatin-immunoreactive neurons could be demonstrated using double labeling immunofluorescence techniques. The axonal plexuses immunoreactive for neuropeptide Y, somatostatin, or substance P were distributed in all layers, with a strong predominance of horizontally oriented fibers in layer I, a moderate plexus of randomly oriented fibers in the supra- and infragranular layers, and a slightly weaker innervation of layer IV. Immunoreactive axons formed, in addition, complex terminal arbors, mostly in older subjects, suggesting that they resulted from an as yet undefined aging process. The present study underlines several aspects of the organization of the neuropeptide-containing neurons of the human cerebral cortex, which are of particular interest in the light of the involvement of these neurons in several neurodegenerative diseases.

Presence of GABA-immunoreactive neurons within intracortical patches in area 18 of the cat

In cat visual cortex, horizontal, intracortical connections spread laterally to link together specific columnar sites. When visualized by retrograde tracers, these intracortical connections appear as periodic, columnar patches of dense cellular labeling interspersed with areas of much less dense labeling. We looked for anatomical evidence for direct inhibition among the patchy, horizontal connections in area 18, by combining retrograde labeling using wheat germ agglutinin (WGA) conjugated to horseradish peroxidase (HRP) with immunohistochemistry using an antiserum against the inhibitory neurotransmitter gamma-amino butyric acid (GABA). We found numerous double-labeled cells associated with some, but not all, of the local patches nearest to the injection site. In the superficial layers, the GABA-immunoreactive cells also labeled with WGA-HRP were confined to a zone approximately 1.0 mm from the center of the injection, while the double-labeled cells in the deeper layers spanned greater distances, up to 3.0 mm from the injection center. These more distant, double-labeled cells in the deeper layers were located on the edges or outside of the patches of dense labeling. Thus, all of the more distant intracortical patches, as well as some of the more proximal patches were devoid of double-labeled cells--a finding which suggests that direct inhibition may occur among only a selected group of the 'short range' intracortical patches and among none of the long-range patches.

Cellular organization of reciprocal patchy networks in layer III of cat visual cortex (area 17)

There is no direct information available concerning the exact spatial characteristics of long-range axons and their relationship with the patchy phenomena observed after extracellular injection of retrograde tracers. In the present study, using the recently introduced neuronal tracer biocytin, we demonstrate by detailed three-dimensional reconstruction of 10 pyramidal cells in layer III, that their clustered axonal terminals form a specific patchy network in layers II and III. The reconstructed network occupied an area of 6.5 x 3.5 mm parallel to the cortical surface elongated in an anteroposterior direction. The average centre-to-centre distance between patches within the network was 1.1 mm. On average, the axonal field of each of the 10 pyramidal cells contained a total of 417 boutons at four to eight distinct sites (patches), and in each patch, an average of 79 boutons was provided by the same cell. The identified connections between the patches were predominantly reciprocal. Detailed analyses have shown that many pyramidal cells of the network are directly interconnected so that each of them can receive one to four, chiefly axospinous, contacts onto the distal segment of its apical and basal dendrites from the axon of another pyramidal cell belonging to a different patch labelled from the same injection site. We hypothesize that the possible functional role of the network is to link remote sites with similar physiological characteristics, such as orientation preference, supporting the model of Mitchison and Crick [(1982) Proc. natn. Acad. Sci. U.S.A. 79, 3661-3665].

GABA neuronal subpopulations in cat primary auditory cortex: co-localization with calcium binding proteins

GABA immunoreactive neurons are present in all layers of cat AI and in the subjacent white matter; they are most numerous in layer II, the superficial half of layer III and layer IV. Double labeling immunofluorescence reveals that subpopulations of the GABA neurons are immunoreactive for the calcium binding proteins (CaBP), calbindin (28 kDa vitamin D-dependent calcium binding protein) and parvalbumin. Both proteins are present exclusively with GABA neurons but in subpopulations that are entirely separate. The two proteins together are present in approximately 70-75% of the GABA neurons; the largest group of GABA neurons displaying no CaBP immunoreactivity is in layers I-IIIA and VI. Calbindin immunoreactive neurons are present in two bands within cat AI: a superficial band, made up of numerous stained somata and processes, that includes layers II and IIIA and a deeper band, containing fewer neurons, that is coextensive with layer VI. Isolated calbindin somata are scattered between the two bands and very rarely in the subcortical white matter. Parvalbumin immunoreactive neurons are very densely packed in layers IIIB and layer IV, and include the majority of GABA neurons in layer IV; they are also numerous in layer VI. Parvalbumin immunoreactive neurons are much less numerous in layers II, IIIA and V and are absent from layer I. Light microscopic analyses suggest that the two subpopulations of GABA/CaBP neurons include several morphological types. In addition to the intrinsic somata and processes, numerous axons in white matter subjacent to AI are immunoreactive for either or both of the two proteins. These data demonstrate that cat AI is similar to other cortical areas in other species in possessing subpopulations of GABA neurons that express the CaBPs, calbindin and parvalbumin.

A functional microcircuit for cat visual cortex

1. We have studied in vivo the intracellular responses of neurones in cat visual cortex to electrical pulse stimulation of the cortical afferents and have developed a microcircuit that simulates much of the experimental data. 2. Inhibition and excitation are not separable events, because individual neurones are embedded in microcircuits that contribute strong population effects. Synchronous electrical activation of the cortex inevitably set in motion a sequence of excitation and inhibition in every neurone we recorded. The temporal form of this response depends on the cortical layer in which the neurone is located. Superficial layer (layers 2+3) pyramidal neurones show a more marked polysynaptic excitatory phase than the pyramids of the deep layers (layers 5+6). 3. Excitatory effects on pyramidal neurones, particularly the superficial layer pyramids, are in general not due to monosynaptic input from thalamus, but polysynaptic input from cortical pyramids. Since the thalamic input is transient it does not provide the major, sustained excitation arriving at any cortical neurone. Instead the intracortical excitatory connections provide the major component of the excitation. 4. The polysynaptic excitatory response would be sustained well after the stimulus, were it not for the suppressive effect of intracortical inhibition induced by the pulse stimulation. 5. Intracellular recording combined with ionophoresis of gamma-aminobutyric acid (GABA) agonists and antagonists showed that intracortical inhibition is mediated by GABAA and GABAB receptors. The GABAA component occurs in the early phase of the impulse response. It is reflected in the strong hyperpolarization that follows the excitatory response and lasts about 50 ms. The GABAB component occurs in the late phase of the response, and is reflected in a sustained hyperpolarization that lasts some 200-300 ms. Both components are seen in all cortical pyramidal neurones. However, the GABAA component appears more powerful in deep layer pyramids than superficial layer pyramids. 6. The microcircuit simulates with good fidelity the above data from experiments in vivo and provides a novel explantation for the apparent lack of significant inhibition during visual stimulation. The basic circuit may be common to all cortical areas studied and thus the microcircuit may be a 'canonical' microcircuit for neocortex.

Expression of glutamic acid decarboxylase mRNA in normal and monocularly deprived cat visual cortex

Neurons expressing glutamic acid decarboxylase (GAD) mRNA were localized by in situ hybridization in normal and monocularly deprived cat visual cortex by using single-stranded RNA probes transcribed from cDNAs cloned in vectors with the T3 and T7 RNA polymerase promoters. In Northern blot analyses, these RNA probes identified 2 forms of GAD mRNA, one of which is approximately 200 bases longer than the other which has previously been identified. The distribution of neurons containing GAD mRNA was compared with the distribution of immunocytochemically identified GABA neurons and in both cases the highest density of labeled neurons was found in layers II, III, and upper VI. All other cellular layers contained a homogeneous, but lower density of labeled cells. Cells expressing GAD mRNA outnumbered GABA immunostained neurons by approximately 10%, but colocalization of GAD mRNA with GABA immunocytochemistry revealed that the two methodologies were detecting the same neuronal population. To determine whether decreased retinal activity affected the levels of GAD mRNA in adult cats, neurons containing GAD mRNA were counted in normal and monocularly deprived visual cortex. However, the number of cells expressing GAD mRNA did not change following monocular deprivation.

Quantitative distribution of GABA-immunoreactive neurons in cetacean visual cortex is similar to that in land mammals

Sections of the anterior portion of the visual cortex in the lateral gyrus of the Black Sea porpoise were studied to determine the neuronal architecture and numerical density, and the distribution of neurons immunoreactive to gamma-aminobutyric acid (GABA). Cytoarchitecture and neuronal density are similar to those described in another cetacean, the bottlenose dolphin. GABA-positive neurons are distributed through all layers of the visual cortex but are especially dense in layers II and III, and comprise some 20% of the total neuronal population in this part of the cortex. The distribution of GABA-positive neurons is similar to that found in land mammals.

Laminar distribution of GABA (gamma-aminobutyric acid) in the dorsal lateral geniculate nucleus, Area 17 and Area 18 of the visual cortex, and the superior colliculus of the cat

The laminar distribution of gamma-aminobutyric acid (GABA) was studied in certain structures of the visual system of the adult cat. A microassay method to measure GABA (10(-12) mol) was established using enzymatic cycling of NADP-NADPH. In the dorsal lateral geniculate nucleus, GABA concentration was highest in lamina A (average concentration 23 mmol/kg dry weight) and lowest in lamina C. In the visual cortex (Areas 17 and 18), the concentration of GABA was 10-12 mmol/kg dry weight in layers I-IV and 5-8 mmol/kg dry weight in layers V and VI. No significant difference was found in the GABA distribution in Areas 17 and 18. In the superior colliculus, the highest level of GABA was found in the upper part of the superficial gray layer (40 mmol/kg dry weight), whereas the deep layers contained GABA at a concentration of 23-28 mmol/kg dry weight. The results of the GABA distribution measurements revealed an orderly, layer-specific disposition of the neurotransmitter in the cat visual system. GABA may play an important role in the function of the visual system.

GABA immuno-electron microscopic study of the nucleus of the optic tract in the rabbit

The pretectal nucleus of the optic tract (NOT) was investigated immunocytochemically with an antiserum against gamma aminobutyric acid (GABA) employing the pre-embedding peroxidase antiperoxidase technique at the light microscopic level and the postembedding colloidal gold technique at the electron microscopic level. GABA immunoreactivity was observed in cell bodies of different sizes and as punctate structures in the neuropil. In the electron microscope, besides immunoreactive dendrites, four different types of terminals were found to be GABA-immunopositive; three types of terminals with clustered and flattened vesicles (F-profile) and a fourth type with pleomorphic vesicles, presumably of dendritic origin (P-profile). Both P- and F-profiles formed symmetrical synapses with dendritic profiles arranged in clusters ensheathed by glial elements. GABA-immunopositive terminals were observed in synaptic contact with somata and retinal terminals (R-profiles) that were always GABA-immunonegative. Some GABA-immunopositive somata showed presynaptic contacts with dendrites. The presence of GABA in numerous distinct elements in the NOT and the diversity in labeled somata and terminals demonstrate the importance of the inhibitor neurotransmitter in the NOT and suggest that its function is not limited to interneurons.

Lateral interactions at direction-selective striate neurones in the cat demonstrated by local cortical inactivation

1. Single neurones were recorded with glass-coated tungsten electrodes from area 17 of the cat's visual cortex. The cats were anaesthetized and artificially respirated with a mixture of halothane, nitrous oxide and oxygen. 2. For local cortical inactivation a multibarrel pipette was placed 0.5-2.5 mm posterior (or anterior) to the recording site, at a depth of 400-600 micron. Four separate barrels of the pipette were filled with gamma-aminobutyric acid (GABA); the fifth was filled with Pontamine Sky Blue for labelling of the centre of the inactivation site. 3. Direction-selective cells, of differing optimal orientations and preferred directions of motion, were classified as simple or complex and tested with computer-controlled stimuli presented on an oscilloscope. 4. During continuous recording GABA was microionophoretically applied for different durations and with different ejection currents. The effectiveness of GABA microionophoresis was evident from the direct GABAergic effects (strong overall inhibition of the recorded cells) observed with high ejection currents and prolonged application. 5. Two discrete effects could be observed during local inactivation distant from the cortical cell under study: an increase of the response in either the non-preferred or the preferred direction; or a decrease of the response in the preferred direction. All GABA-induced changes were reversible. 6. The depressant action of GABA was independent of the relative topography between recording and inactivation site and affected mainly the response to the preferred direction of stimulus motion. 7. Disinhibition was only observed when the stimulus-evoked response moved on the cortical map in a direction from the GABA pipette towards the recording electrode. It is concluded that GABA reversibly silences inhibitory interneurones that are situated in the vicinity of the micropipette tip and are involved in generation of direction selectivity. 8. No fundamental differences between cells from different cortical layers were observed. The disinhibitory effects of GABA inactivation were more pronounced and more frequently seen in simple cells (61%) than in complex cells (38%), while the opposite was true for reduced excitation during lateral GABA inactivation (observed in 62% of the complex vs. 39% of the simple cells). Accordingly, lateral inhibition statistically prevails in simple cells and lateral excitation in complex cells. 9. Among the inhibitory and excitatory mechanisms affected by lateral GABA inactivation, inhibition is organized with a higher topographic specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

gamma-Aminobutyric acid (GABA) immunoreactivity in mouse and rat first somatosensory (SI) cortex: description and comparison

The location and morphological characteristics of gamma-aminobutyric acid (GABA)-immunopositive cells and their processes were studied in rat and mouse first somatosensory (SI) cortex (including 'barrels') in layer IV, and layers above (I-III), and below (V and VI). In coronal sections of SI cortex of both species GABA-immunopositive cells and punctate profiles were found in each of layers I-VI. The cells were of various sizes; the largest, located in layers III and V of each species, resemble the large basket cells seen in Golgi-impregnated material. Most of the immunopositive cells were multipolar and circular or ellipsoidal in shape, but occasionally bipolar cells with fusiform perikarya were also seen. In coronal sections, immunopositive cells did not form a characteristic pattern. GABA-immunopositive cells were observed to be most numerous in the supragranular layers whereas GABA-positive punctate profiles were more numerous in layer IV. In tangential sections from layer IV of SI cortex of both species, GABA-immunopositive cells, processes and punctate profiles were visible throughout the entire barrel field. The pattern of distribution of immunopositive cells was similar (a) in two different morphological groups--i.e. the posteromedial barrel subfield (PMBSF) and the anterolateral barrel subfield (ALBSF) in rat barrel field, and (b) in PMBSF barrels of both rat and mouse (excluding differences due to structural dissimilarities between rat and mouse barrels). GABA-immunopositive neurons were grouped mainly in the barrel side and septum and were visible frequently in small clusters. In barrels of both species GABA-immunopositive cells were of a variety of sizes and ranged in shape from ellipsoidal to circular.

Distribution of neurons and glia in the visual cortex (area 17) of the adult albino rat: a quantitative description

The neuronal and glial cell composition of the rat visual cortex (area 17) has been determined quantitatively using stereological techniques. The volume numerical densities (number of cells per mm3 of cortex) of neurons and of the principal glial cell types (astroglia, oligodendroglia, and microglia) were calculated from tangential semithin resin sections spaced at regular intervals 50 micron apart throughout the entire depth of the visual cortex. From measurements of cortical and laminar thickness the separate volume numerical densities of neurons and glial cells were derived for each lamina in the cortex. In addition, the absolute numbers of cells in each lamina under 1 mm2 of cortical surface were calculated. The mean cortical volume numerical density of neurons was 60,020 +/- 3840/mm3 (mean +/- SEM; n = 8), and 49,040 +/- 2610/mm3 for the combined glial cell types. Astroglia, oligodendroglia, and microglia were present in a ratio of 6:3:1 respectively. It was determined from neuronal and glial somatic volume estimates that the somata of these cells occupied approximately 13.5% of unit cortical volume, with 81.3% of the unit volume being occupied by cortical neuropil. Using previously published reports that described the laminar composition of neurons in terms of the relative proportions of pyramidal and non-pyramidal cells, the laminar volume numerical densities for these neuronal categories have been derived. In addition, it has been estimated that under 1 mm2 of cortical surface there are 79,500 pyramidal and 7790 non-pyramidal neurons distributed throughout layers 1-6 of the rat visual cortex.

Laminar distribution of GABA-immunoreactive neurons and processes in area 18 of the cat

Intracortical inhibition mediated by the neurotransmitter gamma-amino butyric acid (GABA) plays a crucial role in the formation of the physiological response properties of neurons in mammalian visual cortex. Using a potent antibody developed against the amino acid neurotransmitter. GABA (Immunoclear Co., USA), we have identified GABA-immunoreactive neuronal somata and processes in area 18 of the cat. The GABA-positive neurons included multipolar, bipolar and bitufted but not pyramidal cell types. The density of GABA-immunoreactive neurons was higher in Layers I-IV (range 182-205 cells/mm2) than in Layers V-VI (range 78-92 cells/mm2). The mean areal measurement of GABA-immunoreactive somatic profiles was 135 micron2 (s.d. = 66). We observed numerous GABA-immunoreactive fiber fragments predominantly in Layers I, III, IV and V. Most of the fibers were oriented with their long axis tangential to the cortical surface, but, vertically-oriented fibers were observed as well. Many of the fibers were axonal-like and appeared beaded in Layer I and occasionally in II, while most of the fibers in the other layers appeared myelinated. A dense GABA-immunoreactive neuropil was present in Layer V. Results from our studies provide immunohistochemical evidence for a system of GABA-immunoreactive neurons and axonal collaterals which presumably mediate inhibition in the visual cortex. Since many of the GABA-immunoreactive fibers were oriented tangential to the cortical surface, the structural elements required for inhibition between functional columns are present. GABA-mediated inhibition both within and between functional columns likely assists in the formation of receptive field properties in the visual cortex.

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.

A quantitative investigation of the neuronal composition of the rat dorsal lateral geniculate nucleus using GABA-immunocytochemistry

The proportion of neurons immunoreactive for gamma-aminobutyric acid (GABA), and their rostrocaudal distribution in the dorsal lateral geniculate nucleus of the rat, were determined quantitatively using post-embedding GABA-immunochemistry on semithin resin embedded coronal sections followed by stereological analysis. The mean total volume numerical density of neurons (total number of neurons per mm3) in the dLGN was 67,077 +/- 4412 mm-3 (mean +/- SEM; n = 5), comprising a mean volume numerical density for GABA-immunopositive neurons of 14,584 +/- 1324 mm-3, and a mean volume numerical density of GABA-immunonegative neurons of 52,493 +/- 3419 mm-3, GABA-immunopositive neurons constituted 21.7 +/- 0.5% of the total neuronal composition of the rat dorsal lateral geniculate nucleus. Although no rostrocaudal variation was detected in the total volume numerical density of neurons, the relative proportion of GABA-immunopositive neurons was significantly lower in the caudal segment (18.1 +/- 0.6%) compared with the middle (24.9 +/- 0.9%) and the rostral segments (22.1%). Furthermore, on the basis of somatic size distributions, GABA-immunonegative neurons were seen to be significantly smaller in the caudal segment than in the more anterior two segments. The somatic size of GABA-immunopositive neurons showed no rostrocaudal variation through the dorsal lateral geniculate nucleus. These data provide a morphological correlate for the structural and functional subdivision of the dorsal lateral geniculate nucleus described previously in electrophysiological and morphological studies.