Distribution of the AMPA2 glutamate receptor subunit in adult cat visual cortex

In this study, we revealed the distribution of the AMPA2 glutamate receptor subunit (AMPA2) in the visual cortical areas 17 and 18 of the adult cat by means of different techniques. In situ hybridization, with a cat-specific radioactively labeled oligonucleotide probe, showed that AMPA2 mRNA was expressed mainly in cortical layers II/III and V/VI with a lower expression in layer IV and practically no signal in layer I. Immunocytochemistry, using a polyclonal AMPA2 subunit-specific antibody, showed immunoreactivity almost exclusively in the somata and dendrites of pyramidal neurons in cortical layers II/III and V/VI. Only a very faint signal was detected in layer IV. Neurons with little or no AMPA2 have AMPA receptors that are highly permeable to calcium. By determining the location of AMPA2, this study therefore provides a clear examination of the distribution of Ca²⁺-impermeable AMPA receptors over the supra- and infragranular layers of cat visual cortex. The functional implication of the absence of AMPA2 in cortical layer IV and thus the presence of Ca²⁺-permeable AMPA receptors in this layer, is still speculative and has yet to be elucidated.     

Electron-lucent degenerating geniculate terminals in cat striate cortex

Degenerating geniculate axon terminals in cat striate cortex have been previously described as electron-dense. After electrolytic lesion of the lateral geniculate nucleus, we observed degenerating terminals in layer 4 of striate cortex which were electron-lucent. The lucent terminals --which co-exist with the dense terminals--are characterized by a pale matrix, large size, distorted mitochondria, and a paucity of synaptic vesicles. They preferentially (82.5%) contact dendritic spines. Lucent terminals were common in layer 4, rare in layer 6, and absent from layers 1 through 3 and layer 5. This distribution is consistent with the projection of the lateral geniculate nucleus to the striate cortex. Thus, geniculate terminals undergo both the electron-lucent and electron-dense degeneration reactions in cat striate cortex, and the lucent terminals make a significant contribution to the amount of degeneration present. The relationship of lucent degeneration to other forms of degeneration is discussed.     

Stimulus-analyzing mechanisms in the cat visual cortex

Units in the cat's visual cortex were observed while the orientation and intensity of illumination of a “preferred” visual stimulus was varied. In addition to showing the typical sensitivity to stimulus orientation the units were also sensitive to intensity. Almost all (52 of 56) increased firing rate with increased intensity. Thus, even at higher levels of the visual pathway where neurons are noted for their ability to abstract elements of form from the total visual input, intensity continues to exert a major influence. For a given neuron various combinations of intensity and orientation produced the same response. The results do not support a “place” theory of pattern recognition.     

Plasticity of cat visual cortex

Monocular eyelid closure in kittens mimics certain visual deficits in humans that result in amblyopia ex anopsia. I have now studied the effects of monocular eyelid closure in cat upon the slow-wave response recorded from visul cortex. The pattern of changes in the response closely paralleled the changes in visual function of the amblyopic eye, in particular the suppression during binocular vision.     

Striate cortex and visual acuity functions in the cat.

Using a two-choice visual discrimination paradigm, thresholds for size (gratings), parallelness (parallel vs. non-parallel lines), contour alignment (vernier offset), and angularity (polygon figures) were behaviorally determined in cats before and after ablations of portions of the geniculo-cortical system. Animals with a total loss of cortical area 17, and with a loss, in some cases, of up to 90% of areas 18 (with and without infringement into area 19), showed about a 30% reduction in grating acuity, a three-fold increase in parallelness and angularity thresholds, and a total loss of contour alignment ability. Control animals with ablations sparing area 17 showed no significant threshold changes. All animals were able to learn classic form discriminations postoperatively, but those with area 17-18 lesions at a somewhat slower than normal rate. Control procedures indicated that all tested discrimination capabilities did not depend on luminance differences between targets, local flux cues within the targets, or on the animals' use of residual portions of area 17 representing the peripheral visual field. Since the cat has multiple thalamo-cortical visual pathways, the results of the present study are consistent with the hypothesis that pathways parallel to the geniculo-striate system are capable of processing spatial information of considerable detail. The results also suggest, however, that the geniculo-striate system is uniquely necessary for the processing of the finest attributes of spatial contours.     

Enrichment of cholinergic synaptic terminals on GABAergic neurons and coexistence of immunoreactive GABA and choline acetyltransferase in the same synaptic terminals in the striate cortex of the cat

The synaptic circuits underlying cholinergic activation of the cortex were studied by establishing the quantitative distribution of cholinergic terminals on GABAergic inhibitory interneurons and on non-GABAergic neurons in the striate cortex of the cat. Antibodies to choline acetyltransferase and GABA were used in combined electron microscopic immunocytochemical experiments. Most of the cholinergic boutons formed synapses with dendritic shafts (87.3%), much fewer with dendritic spines (11.5%), and only occasional synapses were made on neuronal somata (1.2%). Overall, 27.5% of the postsynaptic elements, all of them dendritic shafts, were immunoreactive for GABA, thus demonstrating that they originate from inhibitory neurons. This is the highest value for the proportion of GABAergic postsynaptic targets obtained so far for any intra- or subcortical afferents in cortex. There were marked variations in the laminar distribution of targets. Spines received synapses most frequently in layer IV (23%) and least frequently in layers V-VI (3%); most of these spines also received an additional synapse from a choline acetyltransferase-negative bouton. The proportion of GABA-positive postsynaptic elements was highest in layer IV (49%, two-thirds of all postsynaptic dendritic shafts), and lowest in layers V-VI (14%). The supragranular layers showed a distribution similar to that of the average of all layers. The quantitative distribution of targets postsynaptic to choline acetyltransferase-positive terminals is very different from the postsynaptic targets of GABAergic boutons, or from the targets of all boutons in layer IV reported previously. In both cases the proportion of GABA-positive dendrites was only 8-9% of the postsynaptic elements. At least 8% of the total population of choline acetyltransferase-positive boutons, presumably originating from the basal forebrain, were also immunoreactive for GABA. This raises the possibility of cotransmission at a significant proportion of cholinergic synapses in the cortex. The present results demonstrate that cortical GABAergic neurons receive a richer cholinergic synaptic input than non-GABAergic cells. The activation of GABAergic neurons by cholinergic afferents may increase the response specificity of cortical cells during cortical arousal thought to be mediated by the basal forebrain. The laminar differences indicate that in layer IV, at the first stage of the processing of thalamic input, the cholinergic afferents exert substantial inhibitory influence in order to raise the threshold and specificity of cortical neuronal responses. Once the correct level of activity has been set at the level of layer IV, the influence can be mainly facilitatory in the other layers.     

Temporal interactions in the cat visual system. I. Orientation-selective suppression in the visual cortex

The perception of a visual contour depends on the spatial and temporal context in which it is viewed. Interactions between visual contours are believed to underlie a wide range of perceptual phenomena, including geometric illusions and aftereffects, contrast adaptation, and visual masking. The physiological mechanisms that might underlie such interactions were studied in the visual cortex of the cat by recording responses of single neurons to pairs of brief stationary stimuli that were separated in time. The results revealed a long-lasting, orientation-selective suppression, termed "paired-pulse suppression," which was strongest at the cell's preferred orientation, but which was more broadly tuned for orientation than the excitatory response of the cell. Although the strength and duration of the suppression varied widely, some degree of response reduction was present in most cells studied. The function of this suppression may be to regulate the gain with which visual inputs are transmitted to cortical neurons, thus preventing response saturation and positive feedback.     

A study of inhibitory antagonism in cat visual cortex.

Since there seems to be good evidence that GABA may act as an inhibitory neurotransmitter in the mammalian cortex, we tested the effects of an antagonist of GABA, namely the alkaloid bicuculine, on the response properties of visual cortex neurons, using a computer-controlled stimulus presentation system to assess quantitatively the changes in receptive field organization after the drug. Complex cells were most affected, increasing both evoked and spontaneous activity and losing some of their specificities for stimulus parameters such as orientation and direction. Hyper-complex cells lost their inhibitory flanks, responding equally well to long and short bars after the drug. Simple cells were the least affected, usually becoming somewhat depressed after the drug. Preliminary tests with another inhibitory amino acid antagonist, strychnine, showed that it excited simple cells, indicating that possibly more than one inhibitory transmitter is at work in the cortex. The results are discussed with relation to the synaptic anatomy of the cortex, and it is concluded that a class of stellate cells, using GABA, is a likely candidate for the transmitter of some intracortical inhibition.     

Clustered intrinsic connections in cat visual cortex

The intrinsic connections of the cortex have long been known to run vertically, across the cortical layers. In the present study we have found that individual neurons in the cat primary visual cortex can communicate over suprisingly long distances horizontally (up to 4 mm), in directions parallel to the cortical surface. For all of the cells having widespread projections, the collaterals within their axonal fields were distributed in repeating clusters, with an average periodicity of 1 mm. This pattern of extensive clustered projections has been revealed by combining the techniques of intracellular recording and injection of horseradish peroxidase with three-dimensional computer graphic reconstructions. The clustering pattern was most apparent when the cells were rotated to present a view parallel to the cortical surface. The pattern was observed in more than half of the pyramidal and spiny stellate cells in the cortex and was seen in all cortical layers. In our sample, cells made distant connections within their own layer and/or within another layer. The axon of one cell had clusters covering the same area in two layers, and the clusters in the deeper layer were located under those in the upper layer, suggesting a relationship between the clustering phenomenon and columnar cortical architecture. Some pyramidal cells did not project into the white matter, forming intrinsic connections exclusively. Finally, the axonal fields of all our injected cells were asymmetric, extending for greater distances along one cortical axis than along the orthogonal axis. The axons appeared to cover areas of cortex representing a larger part of the visual field than that covered by the excitatory portion of the cell's own receptive field. These connections may be used to generate larger receptive fields or to produce the inhibitory flanks in other cells' receptive fields.     

A study of tachykinin-immunoreactivity in the cat visual cortex

The localization of tachykinin-immunoreactivity in the cat visual cortex (area 17) was investigated using immunohistochemical methods. Strong laminar specificity was observed, with immunoreactivity highest in layer V, followed by layers I, VI, II and III, and the lowest density in layer IV. Most of the immunoreactive product was localized in neuronal processes. A few immunopositive cell bodies were also present. The immunopositive neurons were non-pyramidal, multipolar, or bipolar in shape, and mostly found in layer V. There were particularly dense immunopositive fibers and varicosities around somata in layer V. These may represent tachykinin-containing presynaptic terminals (boutons). The results provide anatomical evidence that tachykinin may primarily affect layer V neurons in the cat visual cortex.     

Heterogeneity of GABAergic cells in cat visual cortex

Antibodies against neuropeptides and against a vitamin D-dependent calcium-binding protein (CaBP) label small cells with nonpyramidal-like morphology in the cat visual cortex (areas 17, 18, and 19). Since GABAergic cells are interneurons, a double-staining procedure was used to test for the coexistence of cholecystokinin (CCK), somatostatin (SRIF), neuropeptide Y (NPY), corticotropin-releasing factor (CRF), vasoactive intestinal polypeptide (VIP), and CaBP with glutamic acid decarboxylase (GAD). Our results show that CRF and VIP do not coexist with GAD, while the 3 other peptides and CaBP do. Hence GAD-positive cells can be subdivided into 4 broad groups: (1) cells that are only GAD-positive, (2) cells that are GAD- and CaBP-positive, (3) GAD-positive neurons also containing CCK, and (4) GAD-positive cells that also contain SRIF. A small subset of class 2 also contains SRIF and most cells of class 4 also contain NPY. The 4 classes of GAD-positive cells differ in laminar position: class 1 predominates in layers IV and V, classes 2 and 3 in the upper laminae (II and III), and class 4 in the deepest layer (VI).     

Organization of direction preferences in cat visual cortex

Single unit recordings were made from the visual cortex of 5 adult cats. Visual stimuli were used to determine the stimulus orientation and direction of movement preferred by cortical cells. Analysis of the sequence of neurons recorded along each electrode penetration and their direction preferences indicates that neurons preferring similar directions of movement are clustered together in the cortex.     

Postsynaptic potentials in cat visual cortex: dependence on polarization.

During the investigation of visually evoked postsynaptic potentials (PSPs) of visual cortical neurons, we recorded cell activity under different levels of membrane potential. In some cases, however, dependence of these PSPs on the level of membrane polarization appears to be inconsistent with the conventional scheme. One disagreement was the reduction, instead of an increase, of excitatory potentials during hyperpolarization of the cell. The other point was that depolarization of the cell often leads to increase of the amplitude of both excitatory and inhibitory postsynaptic potentials. This inconsistency may suggest the involvement of voltage-dependent ion channels in generating PSPs to visual stimuli. A possible way of separating the excitatory and inhibitory components of the response by polarization of the cell in spite of the presence of voltage-dependent channels and possible implications of this mechanism in the visual cortex are discussed.     

Topography of orientation centre connections in the primary visual cortex of the cat.

The functional topography of lateral connections to orientation-centre zones was studied by optical imaging of intrinsic signals in combination with tracer injections (fluorescent beads and biocytin) and electrophysiological recordings. Three-dimensional reconstruction of anterogradely labelled axon terminals and retrogradely labelled somata revealed a uniform distribution across all orientations in a non-patchy manner. The overall lateral extent of the labelling was 3-4 mm in layer 3, that is about half of the extent observed for orientation domain connections in the same layer. These bulk injection data are in contrast with the reportedly sharp orientation tuning of neurons of centre zones and suggest that orientation specificity here does not require highly specific connections. Nonetheless, another plausible scenario is that orientation centre connections are orientation specific but their specificity present at the single cell level cannot be revealed by bulk labelling due to their large spatial overlap.     

Immunocytochemical study of GABAA receptors in the cat visual cortex

The laminar distribution and morphological structures associated with GABA_A receptor immunoreactivity in the cat visual cortex were studied by using two different polyclonal antibodies directed either against the purified GABA_A receptor protein (antibody “967”) or against a specific domain of the β₁-subunit of the GABA_A receptor (antibody “Q”). Immunoblots of cat visual cortex tissue with these antibodies revealted that antibody “Q” recognizes only one subunit, namely the β₁-subunit of the GABA_A receptor, and that antibody “967” recognizes three subunits. Both antibodies produced very similar staining patterns, indicating that the β₁-subunit may be an essential component of the GABA_A receptor in the cat visual cortex. The typical staining pattern showed a clear membrane structure around neuronal somata. Using cell body shape criteria, immunopositive neurons included both pyramidal cells in cortical layers II, III, and V, and nonpyramidal cells in all cortical layers. Immunopositive neurons were uniformly distributed in layers II to VI, whereas the density of immunopositive cells in layer I was lower. Some immunopositive neurons were also found in the white matter underlying the visual cotex. In gray matter, immunopositive structures also included dendrites, especially the proximal dendrites, and axon initial segments of pyramidal neurons. The immunopositive processes usually ran vertically toward the pial surface. Some astrocytes were also immunostained. They were localized in layer I and in the white matter. The overall pattern of immunostaining was similar in areas 17, 18, and 19. © 1993 Wiley-Liss, Inc.     

Interrelation among properties of visual cortex neurons in the cat

In acute experiments on immobilized cats 13 functional characteristics of 96 visual cortex neurons were investigated. Regressional and cluster analyses were used to divide these neurons into two subgroups with different density and degree of connections between characteristics. The receptive fields of cells of the first subgroup were localized relatively centrally in the visual field, those of the second subgroup were localized more often on the periphery. A valuable correlation was found in the half of the studied characteristics. In each subgroup the more centrally localized cells with small receptive fields had relatively shorter latencies, lower thresholds, shorter temporal summation, wider intensity range and greater differential sensitivity; their responses were phasic, with high-frequency discharges. The density of valuable correlation of the characteristics varied from 0.21 to 0.99. The amount of these correlations in the first subgroup was two times higher than in the second one. The possible mechanisms of the correlation between the properties of the visual cortex neurons are discussed, as well as their differences in two subgroups and in the cortex and LGB.