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

Neurochemical features and synaptic connections of large physiologically-identified GABAergic cells in the rat frontal cortex

Physiological and morphological properties of large non-pyramidal cells immunoreactive for cholecystokinin, parvalbumin or somatostatin were investigated in vitro in the frontal cortex of 18-22-day-old rats. These three peptides were expressed in separate populations including large cells. Cholecystokinin cells and parvalbumin cells made boutons apposed to other cell bodies, but differed in their firing patterns in response to depolarizing current pulses. Parvalbumin cells belonged to fast-spiking cells. Parvalbumin fast-spiking cells also included chandelier cells. In contrast, cholecystokinin cells were found to be regular-spiking non-pyramidal cells or burst-spiking non-pyramidal cells with bursting activity from hyperpolarized potentials (two or more spikes on slow depolarizing humps). Large somatostatin cells belonged to the regular-spiking non-pyramidal category and featured wide or ascending axonal arbors (wide arbor cells and Martinotti cells) which did not seem to be apposed to the somata so frequently as large cholecystokinin and parvalbumin cells. For electron microscopic observations, another population of eight immunohistochemically-uncharacterized non-pyramidal cells were selected: (i) five fast spiking cells including one chandelier cell which are supposed to contain parvalbumin, and (ii) three large regular-spiking non-pyramidal cells with terminals apposed to somata, which are not considered to include somatostatin cells, but some of which may belong to cholecystokinin cells. The fast-spiking cells other than a chandelier cell and the large regular-spiking non-pyramidal cells made GABA-positive synapses on somata (4% and 12% of the synapses in two small to medium fast-spiking cells, 22% and 35% of the synapses in two large fast-spiking cells, and 10%, 18% and 37% of the synapses in three large regular-spiking non-pyramidal cells). A few terminals of the fast-spiking and regular-spiking non-pyramidal cells innervated GABAergic cells. About 30% of the fast-spiking cell terminals innervated spines, but few of the regular-spiking non-pyramidal cell terminals did. A fast-spiking chandelier cell made GABA-positive synapses on GABA-negative axon initial segments. These results suggest that large GABAergic cells are heterogeneous in neuroactive substances, firing patterns and synaptic connections, and that cortical cells receive heterogeneous GABAergic somatic inputs.

Differentially interconnected networks of GABAergic interneurons in the visual cortex of the cat

Networks of GABAergic neurons have been implicated in neuronal population synchronization. To define the extent of cellular interconnections, we determined the effect, number, and subcellular distribution of synapses between putative GABAergic neurons in layers II-IV of the cat visual cortex using paired intracellular recordings in vitro followed by correlated light and electron microscopy. All neurons having interneuronal electrophysiological properties were classified by their postsynaptic target profile and were identified as basket (BC; n = 6), dendrite-targeting (DTC; n = 1), and double bouquet (DBC; n = 2) cells. In four out of five anatomically fully recovered and reconstructed cell pairs, synaptic connections were found to be reciprocal. Generally BCs established synaptic junctions closer (21 +/- 20 micron) to postsynaptic somata than did DBCs (43 +/- 19 micron; p < 0.01). The unitary number of synapses (n values, 10, 7, and 20) in each of three BC-to-BC pairs was higher than that in three BC-to-DBC (n values, 1, 2, and 2) and three DBC-to-BC (n values, 1, 4, and 4) connections (p < 0.05). A BC innervated a DTC through two synaptic junctions. Unitary postsynaptic effects mediated by five BCs could be recorded in two BCs, two DBCs, and a DTC. The BCs elicited short-duration fast IPSPs, similar to those mediated by GABAA receptors. At a membrane potential of -55.0 +/- 6.4 mV, unitary IPSPs (n = 5) had a mean amplitude of 919 +/- 863 microV. Postsynaptic response failures were absent when an IPSP was mediated by several release sites. Thus, distinct GABAergic interneurons form reciprocally interconnected networks. The strength of innervation and the proximal placement of synapses suggest a prominent role for BCs in governing the activity of intracortical GABAergic networks in layers II-IV.

The variable discharge of cortical neurons: implications for connectivity, computation, and information coding

Cortical neurons exhibit tremendous variability in the number and temporal distribution of spikes in their discharge patterns. Furthermore, this variability appears to be conserved over large regions of the cerebral cortex, suggesting that it is neither reduced nor expanded from stage to stage within a processing pathway. To investigate the principles underlying such statistical homogeneity, we have analyzed a model of synaptic integration incorporating a highly simplified integrate and fire mechanism with decay. We analyzed a "high-input regime" in which neurons receive hundreds of excitatory synaptic inputs during each interspike interval. To produce a graded response in this regime, the neuron must balance excitation with inhibition. We find that a simple integrate and fire mechanism with balanced excitation and inhibition produces a highly variable interspike interval, consistent with experimental data. Detailed information about the temporal pattern of synaptic inputs cannot be recovered from the pattern of output spikes, and we infer that cortical neurons are unlikely to transmit information in the temporal pattern of spike discharge. Rather, we suggest that quantities are represented as rate codes in ensembles of 50-100 neurons. These column-like ensembles tolerate large fractions of common synaptic input and yet covary only weakly in their spike discharge. We find that an ensemble of 100 neurons provides a reliable estimate of rate in just one interspike interval (10-50 msec). Finally, we derived an expression for the variance of the neural spike count that leads to a stable propagation of signal and noise in networks of neurons-that is, conditions that do not impose an accumulation or diminution of noise. The solution implies that single neurons perform simple algebra resembling averaging, and that more sophisticated computations arise by virtue of the anatomical convergence of novel combinations of inputs to the cortical column from external sources.

Evidence for a contribution of lateral inhibition to orientation tuning and direction selectivity in cat visual cortex: reversible inactivation of functionally characterized sites combined with neuroanatomical tracing techniques

We have previously reported that cells in cat areas 17 and 18 can show increases in response to non-optimal orientations or directions, commensurate with a loss of inhibition, during inactivation of laterally remote, visuotopically corresponding sites by iontophoresis of gamma-aminobutyric acid (GABA). We now present anatomical evidence for inhibitory projections from inactivation sites to recording sites where 'disinhibitory' effects were elicited. We made microinjections of [3H]-nipecotic acid, which selectively exploits the GABA re-uptake mechanism, < 100 microm from recording sites where cells had shown either an increase in response to non-optimal orientations during inactivation of a cross-orientation site (n = 2) or an increase in response to the non-preferred direction during inactivation of an iso-orientation site with opposite direction preference (n = 5). Retrogradely labelled GABAergic neurons were detected autoradiographically and their distribution was reconstructed from series of horizontal sections. In every case, radiolabelled cells were found in the vicinity of the inactivation site (three to six within 150 microm). The injection and inactivation sites were located in layers II/III-IV and their horizontal separation ranged from 400 to 560 microm. In another experiment, iontophoresis of biocytin at an inactivation site in layer III labelled two large basket cells with terminals in close proximity to cross-orientation recording sites in layers II/III where disinhibitory effects on orientation tuning had been elicited. We argue that the inactivation of inhibitory projections from inactivation to recording sites made a major contribution to the observed effects by reducing the strength of inhibition during non-optimal stimulation in recurrently connected excitatory neurons presynaptic to a recorded cell. The results provide further evidence that cortical orientation tuning and direction selectivity are sharpened, respectively, by cross-orientation inhibition and iso-orientation inhibition between cells with opposite direction preferences.

Salient features of synaptic organisation in the cerebral cortex

The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.

The absence of a major Ca2+ signaling pathway in GABAergic neurons of the hippocampus

The Ca2+/calmodulin-dependent protein phosphatase 2B or calcineurin (CN) participates in several Ca2+-dependent signal transduction cascades and, thus, contributes to the short and long term regulation of neuronal excitability. By using a specific antibody to CN, we demonstrate its absence from hippocampal interneurons and illustrate a physiological consequence of such CN deficiency. Consistent with the lack of CN in interneurons as detected by immunocytochemistry, the CN inhibitors FK-506 or okadaic acid significantly prolonged N-methyl-D-aspartate channel openings recorded in the cell-attached mode in hippocampal principal cells but not those recorded in interneurons. Interneurons were also devoid of Ca2+/calmodulin-dependent protein kinase IIalpha, yet many of their nuclei contained the cyclic AMP-responsive element binding protein. On the basis of the CN and Ca2+/calmodulin-dependent protein kinase IIalpha deficiency of interneurons, entirely different biochemical mechanisms are expected to govern Ca2+-dependent neuronal plasticity in interneurons versus principal cells.

Binocular cross-orientation suppression in the cat's striate cortex

When a cortical cell is activated by an optimal sinusoidal grating, its response can be attenuated by a superimposed second grating oriented orthogonally to the optimal stimulus. This effect is known as cross-orientation suppression (COS). In previous work, monocular characteristics have been explored and interocular tests have been conducted in an attempt to locate the origin of the suppression. In this study, we have recorded extracellularly from cortical cells to investigate the binocular characteristics of COS. Our hypothesis is that binocular disparity influences the strength of the effect. Our results do not support this supposition. We find that binocular COS is as strong as monocular COS, but disparity changes are of no consequence. We also conducted interocular tests in which the optimal grating and the orthogonal mask were seen by separate eyes. Although most interocular effects were weak, they were present in almost every cell and spanned a wide range of suppression strengths. We also tested the effect of asynchronous presentation of optimal and orthogonal gratings. These temporal offsets did not affect the strength of COS. We conclude that the suppressive mechanism underlying COS is primarily monocular and acts prior to the convergence of the two monocular streams.

Absence of a prevalent laminar distribution of IPSPs in association cortical neurons of cat

Absence of a prevalent laminar distribution of IPSPs in association cortical neurons of cat. J. Neurophysiol. 78: 2742-2753, 1997. The depth distribution of inhibitory postsynaptic potentials (IPSPs) was studied in cat suprasylvian (association) cortex in vivo. Single and dual simultaneous intracellular recordings from cortical neurons were performed in the anterior part of suprasylvian gyrus (area 5). Synaptic responses were obtained by stimulating the suprasylvian cortex, 2-3 mm anterior to the recording site, as well as the thalamic lateral posterior (LP) nucleus. Neurons were recorded from layers 2 to 6 and were classified as regular spiking (RS, n = 132), intrinsically bursting (IB, n = 24), and fast spiking (FS, n = 4). Most IB cells were located in deep layers (below 0.7 mm, n = 19), but we also found some IB cells more superficially (between 0.2 and 0.5 mm, n = 5). Deeply lying corticothalamic neurons were identified by their antidromic invasion on thalamic stimulation. Neurons responded with a combination of excitatory postsynaptic potentials (EPSPs) and IPSPs to both cortical and thalamic stimulation. No consistent relation was found between cell type or cell depth and the amplitude or duration of the IPSPs. In response to thalamic stimulation, RS cells had IPSPs of 7.9 +/- 0.9 (SE) mV amplitude and 88.9 +/- 6.4 ms duration. In IB cells, IPSPs elicited by thalamic stimulation had 7.4 +/- 1.3 mV amplitude and 84.7 +/- 14.3 ms duration. The differences between the two (RS and IB) groups were not statistically significant. Compared with thalamically elicited inhibitory responses, cortical stimulation evoked IPSPs with higher amplitude (12.3 +/- 1.7 mV) and longer duration (117 +/- 17.3 ms) at all depths. Both cortically and thalamically evoked IPSPs were predominantly monophasic. Injections of Cl- fully reversed thalamically as well as cortically evoked IPSPs and revealed additional late synaptic components in response to cortical stimulation. These data show that the amount of feed forward and feedback inhibition to cat's cortical association cells is not orderly distributed to distinct layers. Thus local cortical microcircuitry goes beyond the simplified structure determined by cortical layers.

Interneurons immunoreactive for vasoactive intestinal polypeptide (VIP) are extensively innervated by parvalbumin-containing boutons in rat primary somatosensory cortex

Sensory perception results from the synchronized action of large ensembles of cortical neurons. Receptive field properties of such neurons in sensory areas strongly depend on circuits utilizing the inhibitory amino acid transmitter gamma-aminobutyric acid (GABA). GABAergic neurons often co-localize neuropeptides and/or calcium-binding proteins in a cell-type specific manner. We have taken advantage of this fact to study the synaptic circuitry involving presynaptic parvalbumin-containing boutons (originating from horizontally extensive basket cells) and postsynaptic VIP-immunoreactive GABAergic targets which mostly have a vertically oriented axonal field. Abundant appositions between parvalbumin-immunoreactive boutons and all VIP-stained neurons were observed at the light microscopic level. The numbers of contacts ranged between three and well over 20 for single VIP cells. The higher numbers were especially frequent in the supragranular and granular layers contacting the numerous bipolar, as well as multipolar VIP cells located there; but the VIP-immunoreactive neurons in the infragranular layers were also targeted by parvalbumin-immunoreactive boutons without exception, albeit in more variable, mostly lower numbers. Correlated electron microscopic investigations revealed that virtually all of these light microscopically observed appositions resembled symmetric synaptic specializations. The vast majority were located on the soma or proximal dendrites of the VIP-positive neurons. Since pyramidal cells, in turn, represent a major target for the parvalbumin and VIP synapses, the activation of these synapses may lead to coherent oscillations providing the necessary clock function to synchronize pyramidal cell discharges, both across and within cortical columns.

Residual binocular interactions in the striate cortex of monkeys reared with abnormal binocular vision

We investigated the nature of residual binocular interactions in the striate cortex (V1) of monkey models for the two most common causes of visual dysfunction in young children, specifically anisometropia and strabismus. Infant rhesus monkeys were raised wearing either anisometropic spectacle lenses that optically defocused one eye or ophthalmic prisms that optically produced diplopia and binocular confusion. Earlier psychophysical investigations had demonstrated that all subjects exhibited permanent binocular vision deficits and, in some cases, amblyopia. When the monkeys were adults, the responses of individual V1 neurons were studied with the use of microelectrode recording techniques while the animals were anesthetized and paralyzed. The manner in which the signals from the two eyes were combined in individual cells was investigated by dichoptically stimulating both eyes simultaneously with drifting sine wave gratings. In both lens- and prism-reared monkeys, fewer neurons had balanced ocular dominances and greater numbers of neurons were excited by only one eye. However, many neurons that appeared to be monocular exhibited clear binocular interactions during dichoptic stimulation. For the surviving binocular neurons, the maximum binocular response amplitudes were lower than normal; fewer neurons, particularly complex cells, were sensitive to relative interocular spatial phase disparities; and the remaining disparity-sensitive neurons exhibited lower degrees of binocular interaction. In prism-reared monkeys, an unusually high proportion of complex cells exhibited binocular suppression during dichoptic stimulation. Binocular contrast summation experiments showed that for both cooperative and antagonistic binocular interactions, contrast signals from the two eyes were combined by individual neurons in a normal linear fashion in both lens- and prism-reared monkeys. The observed binocular deficits appear to reflect a reduction in functional inputs from one eye and/or spatial imprecision in the monocular receptive fields rather than an aberrant form of binocular interaction. In the prism-reared monkeys, the predominance of suppression suggests that inhibitory connections were, however, less susceptible to diplopia and confusion than excitatory connections. Overall, there were many parallels between V1 physiology in our monkey models and the residual vision of humans with anisometropia or strabismus.

Nucleus reticularis neurons mediate diverse inhibitory effects in thalamus

Detailed information regarding the contribution of individual gamma-aminobutyric acid (GABA)-containing inhibitory neurons to the overall synaptic activity of single postsynaptic cells is essential to our understanding of fundamental elements of synaptic integration and operation of neuronal circuits. For example, GABA-containing cells in the thalamic reticular nucleus (nRt) provide major inhibitory innervation of thalamic relay nuclei that is critical to thalamocortical rhythm generation. To investigate the contribution of individual nRt neurons to the strength of this internuclear inhibition, we obtained whole-cell recordings of unitary inhibitory postsynaptic currents (IPSCs) evoked in ventrobasal thalamocortical (VB) neurons by stimulation of single nRt cells in rat thalamic slices, in conjunction with intracellular biocytin labeling. Two types of monosynaptic IPSCs could be distinguished. "Weak" inhibitory connections were characterized by a significant number of postsynaptic failures in response to presynaptic nRt action potentials and relatively small IPSCs. In contrast, "strong" inhibition was characterized by the absence of postsynaptic failures and significantly larger unitary IPSCs. By using miniature IPSC amplitudes to infer quantal size, we estimated that unitary IPSCs associated with weak inhibition resulted from activation of 1-3 release sites, whereas stronger inhibition would require simultaneous activation of 5-70 release sites. The inhibitory strengths were positively correlated with the density of axonal swellings of the presynaptic nRt neurons, an indicator that characterizes different nRt axonal arborization patterns. These results demonstrate that there is a heterogeneity of inhibitory interactions between nRt and VB neurons, and that variations in gross morphological features of axonal arbors in the central nervous system can be associated with significant differences in postsynaptic response characteristics.

Fast IPSPs elicited via multiple synaptic release sites by different types of GABAergic neurone in the cat visual cortex

1. The effects of synapses established by smooth dendritic neurones on pyramidal and spiny stellate cells were studied in areas 17 and 18 of the cat visual cortex in vitro. Paired intracellular recordings with biocytin-filled electrodes and subsequent light and electron microscopic analysis were used to determine the sites of synaptic interaction. 2. All smooth dendritic cells established type II synapses previously shown to be made by terminals containing GABA, therefore the studied cells are probably GABAergic. Three classes of presynaptic cell could be defined, based on their efferent synaptic target preference determined from random samples of unlabelled postsynaptic cells. (a) Basket cells (n = 6) innervated mainly somata (49.9 +/- 13.8%) and dendritic shafts (45.2 +/- 10.7%) and, to a lesser extent, dendritic spines (4.9 +/- 4.6%). (b) Dendrite-targeting cells (n = 5) established synapses predominantly on dendritic shafts (84.3 +/- 9.4%) and less frequently on dendritic spines (11.2 +/- 6.7%) or somata (4.5 +/- 4.7%). (c) Double bouquet cells (n = 4) preferred dendritic spines (69.2 +/- 4.2%) to dendritic shafts (30.8 +/- 4.2%) as postsynaptic targets and avoided somata. 3. Interneurones formed 5240 +/- 1600 (range, 2830-9690) synaptic junctions in the slices. Based on the density of synapses made by single interneurones and the volume density of GABAergic synapses, it was calculated that an average interneurone provides 0.66 +/- 0.20% of the GABAergic synapses in its axonal field. 4. The location of synaptic junctions on individual, identified postsynaptic cells reflected the overall postsynaptic target distribution of the same GABAergic neurone. The number of synaptic junctions between pairs of neurones could not be predicted from light microscopic examination. The number of electron microscopically verified synaptic sites was generally smaller for the dendritic domain and larger for the somatic domain than expected from light microscopy. All presynaptic cells established multiple synaptic junctions on their postsynaptic target cells. A basket cell innervated a pyramidal cell via fifteen release sites; the numbers of synapses formed by three dendrite-targeting cells on pyramidal cells were seventeen and eight respectively, and three on a spiny stellate cell; the interaction between a double bouquet cell and a postsynaptic pyramidal cell was mediated by ten synaptic junctions. 5. All three types of interneurone (n = 6; 2 for each type of cell) elicited short-latency IPSPs with fast rise time (10-90%; 2.59 +/- 1.02 ms) and short duration (at half-amplitude, 15.82 +/- 5.24 ms), similar to those mediated by GABAA receptors. 6. Average amplitudes of unitary IPSPs (n = 6) were 845 +/- 796 microV (range, 134-2265 microV). Variability of IPSP amplitude was moderate, the average ratio of IPSP and baseline noise variance was 1.54 +/- 0.96. High frequency activation of single presynaptic dendrite-targeting cells led to an initial summation followed by use-dependent depression of the averaged postsynaptic response. Double bouquet cell-evoked IPSPs, recorded in the soma, had a smaller amplitude than those evoked by the other two cell types. In all connections, transmission failures were rare or absent, particularly when mediated by a high number of release sites. 7. The results demonstrate that different types of neocortical GABAergic neurones innervate distinct domains on the surface of their postsynaptic target cells. Nevertheless, all three types of cell studied here elicit fast IPSPs and provide GABAergic input through multiple synaptic release sites with few, if any, failures of transmission.

Loss of inhibitory synapses on the soma and axon initial segment of pyramidal cells in human epileptic peritumoural neocortex: implications for epilepsy

The peritumoural neocortex removed from epileptic patients represents an important region for research because of its possible relationship to the generation, maintenance, and propagation of seizures. The peritumoural neocortex removed from an epileptic patient showing a regrowth of an anaplastic astrocytoma was examined in detail using immunocytochemistry for gamma-aminobutyric acid, glutamic acid decarboxylase, parvalbumin, nonphosphorylated neurofilament protein, glial fibrillary acidic protein, and histocompatibility antigen HLA-DR. The patterns of immunostaining were compared with the cytoarchitecture and myeloarchitecture in adjacent sections, and with the patterns of immunostaining observed in normal control neocortex. Furthermore, quantitative electron microscopy was used to compare the synaptic densities of presumptive excitatory and inhibitory synapses between regions showing different grades of cytoarchitectural and neurochemical alterations in the peritumoural neocortex, and to compare these regions with normal neocortex. A variety of changes in synaptic circuits in the peritumoural neocortex was found, but it appears that neurons within the less abnormal-looking regions were involved in altered synaptic circuits that might contribute to epileptic activity. In these regions, the most prominent change was the loss of inhibitory synapses on the soma and axon initial segment of pyramidal cells, but numerous excitatory synapses were present on their dendrites that would make these neurons hyperexcitable. However, the most abnormal regions histologically were likely a primary zone for progression of the tumour, with many surviving neurones, but which received and formed very few synapses; thus, they were probably unrelated to the initiation, maintenance, or propagation of seizures.

GABA-induced inactivation of functionally characterized sites in cat striate cortex: effects on orientation tuning and direction selectivity

Microiontophoresis of gamma-aminobutyric acid (GABA) was used to reversibly inactivate small sites of defined orientation/direction specificity in layers II-IV of cat area 17 while single cells were recorded in the same area at a horizontal distance of approximately 350-700 microns. We compared the effect of inactivating iso-orientation sites (where orientation preference was within 22.5 deg) and cross-orientation sites (where it differed by 45-90 deg) on orientation tuning and directionality. The influence of iso-orientation inactivation was tested in 33 cells, seven of which were subjected to alternate inactivation of two iso-orientation sites with opposite direction preference. Of the resulting 40 inactivations, only two (5%) caused significant changes in orientation tuning, whereas 26 (65%) elicited effects on directionality: namely, an increase or a decrease in response to a cell's preferred direction when its direction preference was the same as that at an inactivation site, and an increase in response to a cell's nonpreferred direction when its direction preference was opposite that at an inactivation site. It is argued that the decreases in response to the preferred direction reflected a reduction in the strength of intracortical iso-orientation excitatory connections, while the increases in response were due to the loss of iso-orientation inhibition. Of 35 cells subjected to cross-orientation inactivation, only six (17%) showed an effect on directionality, whereas 21 (60%) showed significant broadening of orientation tuning, with an increase in mean tuning width at half-height of 126%. The effects on orientation tuning were due to increases in response to nonoptimal orientations. Changes in directionality also resulted from increased responses (to preferred or nonpreferred directions) and were always accompanied by broadening of tuning. Thus, the effects of cross-orientation inactivation were presumably due to the loss of a cross-orientation inhibitory input that contributes mainly to orientation tuning by suppressing responses to nonoptimal orientations. Differential effects of iso-orientation and cross-orientation inactivation could be elicited in the same cell or in different cells from the same inactivation site. The results suggest the involvement of three different intracortical processes in the generation of orientation tuning and direction selectivity in area 17: (1) suppression of responses to nonoptimal orientations and directions as a result of cross-orientation inhibition and iso-orientation inhibition between cells with opposite direction preferences; (2) amplification of responses to optimal stimuli via iso-orientation excitatory connections; and (3) regulation of cortical amplification via iso-orientation inhibition.

Morphological properties of intracellularly labeled layer I neurons in rat neocortex

The morphology of neurons in layer I of rat neocortex, including Cajal-Retzius (CR) cells, was studied by using intracellular biocytin staining in brain slices obtained from rats during the first 22 postnatal days. Within the first postnatal week, horizontal bipolar neurons or CR cells were prominent in layer I. Typically, CR cells had one main dendrite and one axon originating from opposite poles of the somata. Even though the main dendrites and axons could be quite long, complex dendritic or axonal arbors were not observed. Starting around postnatal day 6 (PN 6), CR cells were less frequently observed. From PN 10 to PN 21, nonpyramidal neurons with diverse morphologies became the main neuronal component in layer I. The somata of layer I nonpyramidal neurons were quite variable in size and shape. Dendrites were smooth or sparsely spiny, and the dendritic trees were mainly restricted to layer I, covering an area with a diameter of about 200 microns. Axon collaterals of these cells formed elaborate arbors with diameters of around 700 microns in layer I and extending, in many cases, to layer II/III and even layer IV. This extensive axonal plexus provides a rich anatomical base on which layer I neurons, functioning as local circuit elements, may interact with each other and with neurons in other layers.

Regional and cellular fractionation of working memory

This chapter recounts efforts to dissect the cellular and circuit basis of a memory system in the primate cortex with the goal of extending the insights gained from the study of normal brain organization in animal models to an understanding of human cognition and related memory disorders. Primates and humans have developed an extraordinary capacity to process information "on line," a capacity that is widely considered to underlay comprehension, thinking, and so-called executive functions. Understanding the interactions between the major cellular constituents of cortical circuits-pyramidal and nonpyramidal cells-is considered a necessary step in unraveling the cellular mechanisms subserving working memory mechanisms and, ultimately, cognitive processes. Evidence from a variety of sources is accumulating to indicate that dopamine has a major role in regulating the excitability of the cortical circuitry upon which the working memory function of prefrontal cortex depends. Here, I describe several direct and indirect intercellular mechanisms for modulating working memory function in prefrontal cortex based on the localization of dopamine receptors on the distal dendrites and spines of pyramidal cells and on interneurons in the prefrontal cortex. Interactions between monoamines and a compromised cortical circuitry may hold the key to understanding the variety of memory disorders associated with aging and disease.

Laminar origins of inhibitory synaptic inputs to pyramidal neurons of the rat neocortex

1. Inhibitory neuron-pyramidal cell interactions were investigated in slices of rat somatosensory cortex in which excitatory synaptic transmission was blocked with bath-applied glutamate receptor antagonists. Local inhibitory neurons were excited by focal pressure ejections of small (approximately 40 pl) volumes of 1-10 mM acetylcholine. 2. The frequency of inhibitory postsynaptic potentials (IPSPs) ("responses per trial' or R/T) declined as the stimulation distance was increased. Inhibitory inputs were most prevalent in layer II/III regular spiking (RS) pyramidal neurons (30 cells) where median R/T was 0.020. In layer V, the median R/T was 0.024 for RS neurons (25 cells), but significantly lower for burst-firing (IB) neurons (17 cells), where median R/T was 0.007 (P = 0.039). 3. IPSPs in individual layer V pyramidal cells were recorded with CsCl electrodes. In eight neurons, spontaneous picrotoxin-sensitive IPSPs were recorded and found to display a wide range of 10-90% rise times (1-34 ms), not correlated with amplitude (0.2-18 mV). For a further ten pyramidal neurons, extracellular stimulating electrodes were placed simultaneously in layers II/III and V/VI in order to evoke pairs of IPSPs whose waveforms were averaged and compared. In seven cells, IPSPs evoked from layer II/III (distal location) had longer 10-90% rise times than IPSPs evoked from layer V/VI stimulating electrodes (proximal location). In addition, "proximal' IPSPs could always be reversed by membrane depolarization whereas "distal' ones could not (n = 4/4). 4. This study showed that pyramid cell-inhibitory neuron interconnections are extensive but their spatial organization varies with cell class and with cortical layer. In addition, pyramidal neurons can receive inhibitory inputs from locations on their apical dendrites.

Imaging of radiocarbon-labelled tracer molecules in neural tissue using accelerator mass spectrometry

Autoradiography is widely and successfully used to image the distribution of radiolabelled tracer molecules in biological samples. The method is, however, limited in resolution and sensitivity, especially for 14C. Here we describe a new method for imaging 14C-labelled tracers in sections of biological tissue. A highly focused beam of gallium ions bombards the tissue, which is eroded (sputtered) into constituent atoms, molecules and secondary ions. The 14C ions are detected in the secondary beam by the most sensitive method available, namely accelerator mass spectrometry. The specimen is scanned pixel by pixel (1 x 2 microm), generating an image in a manner analogous to scanning electron microscopy. The method can thus be regarded as a specialized form of scanning secondary ion mass spectrometry (SIMS), referred to here as SIAMS (ref. 2). We have used SIAMS to localize the neurotransmitter gamma-aminobutyric acid (GABA) in thin sections of cerebral cortex, and show that it can generate 14C images that are much improved on 14C autoradiography. A scan takes 10-20 min and reveals individual axons, neurons and glial cells at high sensitivity. In principle, the resolution could be increased by up to tenfold, and the method could be extended to some other nuclides.

Neuronal hyperexcitability and reduction of GABAA-receptor expression in the surround of cerebral photothrombosis

Changes of neuronal excitability and gamma-aminobutyric acid (GABAA)-receptor expression were studied in the surround of photothrombotic infarcts, which were produced in the sensorimotor cortex of the rat by using the rose bengal technique. In a first series of experiments, multiunit recordings were performed on anesthetized animals 2-3 mm lateral from the lesion. Mean discharge frequency was considerably higher in recordings from lesioned animals (> 100 Hz in the first postlesional week) compared with control animals (mean, 15 Hz). These alterations were already present after 1 day but were most pronounced 3 to 7 days after lesion induction. Thereafter the hyperexcitability declined again, although it remained visible up to 4 months. In a second series of experiments, the GABAA-receptor expression was studied autoradiographically. This revealed a reduction of GABAA receptors in widespread brain areas ipsilateral to the lesion. The reduction was most pronounced in the first days after lesion induction and declined with longer intervals. It is concluded that cortical infarction due to photothrombosis leads to a long-lasting and widespread reduction of GABAA-receptor expression in the surround of the lesion, which is associated with an increased neuronal excitability. Such alterations may be responsible for epileptic seizures that can be observed in some patients after stroke and may contribute to neurologic deficits after stroke.

Inhibition synchronizes sparsely connected cortical neurons within and between columns in realistic network models

Networks of compartmental model neurons were used to investigate the biophysical basis of the synchronization observed between sparsely-connected neurons in neocortex. A model of a single column in layer 5 consisted of 100 model neurons: 80 pyramidal and 20 inhibitory. The pyramidal cells had conductances that caused intrinsic repetitive bursting at different frequencies when driven with the same input. When connected randomly with a connection density of 10%, a single model column displayed synchronous oscillatory action potentials in response to stationary, uncorrelated Poisson spike-train inputs. Synchrony required a high ratio of inhibitory to excitatory synaptic strength; the optimal ratio was 4 : 1, within the range observed in cortex. The synchrony was insensitive to variation in amplitudes of postsynaptic potentials and synaptic delay times, even when the mean synaptic delay times were varied over the range 1 to 7 ms. Synchrony was found to be sensitive to the strength of reciprocal inhibition between the inhibitory neurons in one column: Too weak or too strong reciprocal inhibition degraded intra-columnar synchrony. The only parameter that affected the oscillation frequency of the network was the strength of the external driving input which could shift the frequency between 35 to 60 Hz. The same results were obtained using a model column of 1000 neurons with a connection density of 5%, except that the oscillation became more regular. Synchronization between cortical columns was studied in a model consisting of two columns with 100 model neurons each. When connections were made with a density of 3% between the pyramidal cells of each column there was no inter-columnar synchrony and in some cases the columns oscillated 180 degrees out of phase with each other. Only when connections from the pyramidal cells in each column to the inhibitory cells in the other column were added was synchrony between the columns observed. This synchrony was established within one or two cycles of the oscillation and there was on average less than 1 ms phase difference between the two columns. Unlike the intra-columnar synchronization, the inter-columnar synchronization was found to be sensitive to the synaptic delay: A mean delay of greater than 5 ms virtually abolished synchronization between columns.

Microcircuitry of forward and feedback connections within rat visual cortex

Visual cortex in mammals is composed of many distinct areas that are linked by reciprocal connections to form a multilevel hierarchy. Ascending information is sent via forward connections from lower to higher areas and is thought to contribute to the emergence of increasingly complex receptive field properties at higher levels. Descending signals are transmitted via feedback connections from higher to lower areas and are believed to provide information about the context in which a stimulus appears, to contribute to modulation of visual responses by attention, and to play a role in memory processes. To determine whether forward and feedback pathways in rat visual cortex constitute distinct intracortical circuits, we have studied the distribution of reciprocal corticocortical inputs to pyramidal cells and gamma-aminobutyric acid (GABA)ergic interneurons. For this purpose, we chose forward and feedback connections between primary visual cortex and the secondary extrastriate lateromedial (LM) area as a model system. Pathways were traced with the axonal marker phaseolus vulgaris-leucoagglutinin. Labeled terminals were identified in the electron microscope, and GABA immunocytochemistry was used to identify the postsynaptic dendritic shafts of GABAergic interneurons. In both pathways, inputs to pyramidal cells were directed preferentially to dendritic spines and not to shafts. In the forward pathway, 90% of labeled inputs were distributed to pyramidal cells and 10% to interneurons. This proportion was similar to that of nearby unlabeled connections in the neuropil, indicating that forward connections are not selective for pyramidal cells or interneurons. In sharp contrast, feedback connections were significantly different from the unlabeled connections and supplied almost exclusively pyramidal cells (98%). Feedback inputs to GABAergic neurons were five times weaker (2%) relative to the forward direction. These structural differences suggest that disynaptic GABAergic inhibition is much stronger in forward than in feedback pathways. Recent physiological experiments have confirmed this prediction (Shao et al. [1995] Soc. Neurosci. Abstr., 21:1274) and we, therefore, conclude that relatively small anatomical differences in the microcircuitry can have important functional consequences. It remains an open question whether generally reciprocal interareal circuits at all levels of the cortical hierarchy are organized in similar fashion.

Double labeling of GABA and cytochrome oxidase in the macaque visual cortex: quantitative EM analysis

In the primate striate cortex, cytochrome oxidase (CO)-rich puffs differ from CO-poor interpuffs in their metabolic levels and physiological properties. The neurochemical basis for their metabolic and physiological differences is not well understood. The goal of the present study was to examine the relationship between the distribution of gamma aminobutyric acid (GABA)/non-GABA synapses and CO levels in postsynaptic neuronal profiles and to determine whether or not a difference existed between puffs and interpuffs. By combining CO histochemistry and postembedding GABA immunocytochemistry on the same ultrathin sections, the simultaneous distribution of the two markers in individual neuronal profiles was quantitatively analyzed. In both puffs and interpuffs, GABA-immunoreactive (GABA-IR) neurons were the only cell type that received both non-GABA-IR (presumed excitatory) and GABA-IR (presumed inhibitory) axosomatic synapses, and they had three times as many mitochondria darkly reactive for CO than non-GABA-IR neurons, which received only GABA-IR axosomatic synapses. GABA-IR neurons and terminals in puffs had a larger mean size, about twice as many darkly reactive mitochondria, and a higher ratio of non-GABA-IR to GABA-IR axosomatic synapses than those in interpuffs (2.3:1 vs. 1.6:1; P < 0.01). There were significantly more synapses of both non-GABA-IR and GABA-IR types in the neuropil of puffs than of interpuffs; however, the ratio of non-GABA-IR to GABA-IR synapses was significantly higher in puffs (2.86:1) than in interpuffs (2.08:1; P < 0.01). Our results are consistent with the hypothesis that the level of oxidative metabolism in postsynaptic neurons and neuronal processes is tightly governed by the strength and proportion of excitatory over inhibitory synapses. Thus, the present results suggest that (1) GABA-IR neurons in the macaque striate cortex have a higher level of oxidative metabolism than non-GABA ones because their somata receive direct excitatory synapses and their terminals are more tonically active; (2) the higher proportion of presumed excitatory synapses in puffs imposes a greater energy demand there than in interpuffs; and (3) excitatory synaptic activity may be more prominent in puffs than in interpuffs because puffs receive a greater proportion of excitatory synapses from multiple sources including the lateral geniculate nucleus, which is not known to project to the interpuffs.

Simple codes versus efficient codes

Transmission of information is an important function of cortical neurons, so it is conceivable that they have evolved to transmit information efficiently, with low noise and high temporal precision. Such precision is consistent with the output generated by various working models that mimick neuronal activity, from simple integrate-and-fire models to elaborate numerical simulations of realistic-looking neurons. But our current inability to match this data with neurons' detailed spike-generating mechanisms in vivo allows us a wide latitude in interpreting the significance of the various components of their spike code. One extreme hypothesis, the 'simple' model, is that each neuron is noisy and slow, performing a simple computation and transmitting a small amount of information. A competing hypothesis, the 'efficient' model, postulates that a neuron transmits large amounts of information through precise, complex, single-spike computations. Both hypotheses are broadly consistent with the available data. The conflict may only be resolved with the development of new measurement techniques that will allow one to investigate directly the properties that make a neuron efficient--that is, to be able to measure highly transient, localized events inside the thinnest dendrites, which are currently experimentally inaccessible.

Excitatory synaptic connections onto rat hippocampal inhibitory cells may involve a single transmitter release site

1. Whole-cell tight-seal records of excitatory postsynaptic currents (EPSCs) were made from inhibitory cells in the CA3 region of thin hippocampal slices. We tested the hypothesis that excitatory synaptic connections made on inhibitory cells involve few transmitter release sites. 2. EPSCs impinging on inhibitory cells had a time to peak of 0.4-3.8 ms and an amplitude of 8-90 pA at a holding potential of -60 mV. They were suppressed by the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonovaleric acid (APV). 3. Addition of tetrodotoxin (TTX) and Co2+ to the external solution reduced the frequency of EPSCs from 0.90 to 0.25 s-1 (n = 24 cells). In the majority of cells EPSC amplitude distributions were not significantly changed. 4. Increasing Ca2+ and reducing Mg2+ in the external solution, in order to enhance the probability of transmitter release, did not change EPSC amplitude distributions. In contrast, amplitude histograms for IPSCs recorded from pyramidal cells were shifted to higher mean values in this solution. 5. EPSCs were elicited in inhibitory cells by electrical stimulation via a glass pipette placed near to pyramidal cells in stratum pyramidale. EPSCs elicited by weak stimuli had similar amplitude distributions to excitatory synaptic events recorded in the presence of TTX and Co2+. 6. These findings suggest excitatory synaptic connections made with CA3 inhibitory cells involve few or possibly just one transmitter release site.

Scanning laser photostimulation: a new approach for analyzing brain circuits

A new technique for understanding the organization of complex circuits in the vertebrate brain, scanning laser photostimulation, is described. This approach is based on the photolysis of a caged form of the excitatory neurotransmitter glutamate. Computer-controlled photostimulation and whole cell recording in brain slices allow the construction of detailed maps of the position, strength, sign and number of inputs converging on a single postsynaptic neuron. Scanning laser photostimulation offers many advantages over current techniques: spatial resolution is superb, fibers of passage are not activated, and thousands of presynaptic locations can be stimulated. This review describes the technique of photostimulation, outlines the instrumentation, necessary to implement it, and discusses the interpretation of photostimulation-derived data. Several examples of applications, ranging from mapping circuits in the mammalian visual cortex to determining receptor distributions on single neurons are considered. Although still in its early stages, scanning laser photostimulation offers neuroscientists a powerful tool for determining the organization and function of local brain circuits.

The neocortex. An overview of its evolutionary development, structural organization and synaptology

By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this "new" structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neocortical population. The evolutionary development of the pyramidal neurons can be traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as "psychic cells". Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1-6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of parvalbumin immunoreactivity in the cerebral cortex of the cat

Parvalbumin immunoreactivity in the developing neocortex of the cat progresses following specific laminar, areal, and, in a particular area, roughly anteroposterior gradients. Parvalbumin immunoreactivity first occurs in basket cells and later in chandelier neurons. Pyramid-like immunoreactive neurons are also transitorily observed from the second to the third week in layer V of the auditory association-related areas. Parvalbumin-immunoreactive neurons first appear in the primary somatosensory cortex and primary auditory and visual areas, followed by the primary motor and polysensory association areas and, finally, the auditory association areas and cortical areas related to the limbic system. In addition to cortical neurons, three fiber systems are immunolabeled with antiparvalbumin antibodies: thalamocortical, callosal, and ipsilateral corticocortical. Parvalbumin-immunoreactive thalamocortical fibers appear during the first month of postnatal life. Parvalbumin-immunoreactive callosal and ipsilateral corticocortical fibers are seen from the fourth postnatal week onward. Because all parvalbumin-immunoreactive cortical neurons in adulthood are nonpyramidal inhibitory cells, the present findings suggest that a number of ipsilateral corticocortical and callosal connections may be inhibitory.

Inactivation of the infragranular striate cortex broadens orientation tuning of supragranular visual neurons in the cat

Intracortical inhibition is believed to enhance the orientation tuning of striate cortical neurons, but the origin of this inhibition is unclear. To examine the possible influence of ascending inhibitory projections from the infragranular layers of striate cortex on the orientation selectivity of neurons in the supragranular layers, we measured the spatiotemporal response properties of 32 supragranular neurons in the cat before, during, and after neural activity in the infragranular layers beneath the recorded cells was inactivated by iontophoretic administration of GABA. During GABA iontophoresis, the orientation tuning bandwidth of 15 (46.9%) supragranular neurons broadened as a result of increases in response amplitude to stimuli oriented about +/- 20 degrees away from the preferred stimulus angle. The mean (+/- SD) baseline orientation tuning bandwidth (half width at half height) of these neurons was 13.08 +/- 2.3 degrees. Their mean tuning bandwidth during inactivation of the infragranular layers increased to 19.59 +/- 2.54 degrees, an increase of 49.7%. The mean percentage increase in orientation tuning bandwidth of the individual neurons was 47.4%. Four neurons exhibited symmetrical changes in their orientation tuning functions, while 11 neurons displayed asymmetrical changes. The change in form of the orientation tuning functions appeared to depend on the relative vertical alignment of the recorded neuron and the infragranular region of inactivation. Neurons located in close vertical register with the inactivated infragranular tissue exhibited symmetric changes in their orientation tuning functions. The neurons exhibiting asymmetric changes in their orientation tuning functions were located just outside the vertical register. Eight of these 11 neurons also demonstrated a mean shift of 6.67 +/- 5.77 degrees in their preferred stimulus orientation. The magnitude of change in the orientation tuning functions increased as the delivery of GABA was prolonged. Responses returned to normal approximately 30 min after the delivery of GABA was discontinued. We conclude that inhibitory projections from neurons within the infragranular layers of striate cortex in cats can enhance the orientation selectivity of supragranular striate cortical neurons.

Relationship between lateral inhibitory connections and the topography of the orientation map in cat visual cortex

The functional and structural topography of lateral inhibitory connections was investigated in visual cortical area 18 using a combination of optical imaging and anatomical tracing techniques in the same tissue. Orientation maps were obtained by recording intrinsic signals in regions of 8.4-19 mm2. To reveal the inhibitory connections provided by large basket cells, biocytin was iontophoretically injected at identified orientation sites guided by the pattern of surface blood vessels. The axonal and dendritic fields of two retrogradely labelled large basket cells were reconstructed in layer III. Their axonal fields extended up to 1360 microns from the parent somata. In addition to single basket cells, the population of labelled basket cell axons was also studied. For this analysis anterogradely labelled basket axons running horizontally over 460-1280 microns from the core of an injection site in layer III were taken into account. The distribution of large basket cell terminals according to orientation preferences of their target regions was quantitatively assessed. Using the same spatial resolution as the orientation map, a frequency distribution of basket cell terminals dependent on orientation specificity could be derived. For individual basket cells, the results showed that, on average, 43% of the terminals provided input to sites showing similar orientation preferences (+/- 30 degrees) to those of the parent somata. About 35% of the terminals were directed to sites representing oblique-orientation [+/- (30-60) degrees], and 22% of them terminated at cross-orientation sites [+/- (60-90) degrees]. Furthermore, the possible impact of large basket cells on target cells at different distances and orientation preferences was estimated by comparing the occurrence of orientation preferences with the occurrence of basket terminals on the distance scale. It was found that a basket cell could elicit iso-orientation inhibition with a high impact between 100-400 and 800-1200 microns, strong cross-orientation inhibition at approximately 400-800 microns, and oblique-orientation inhibition between 300-500 and 700-900 microns from the parent soma. The non-isotropic topography of large basket axons suggests a complex function for this cell class, possibly including inhibition related to orientation and direction selectivity depending on the location of the target cells and possible target selectivity.

Extensive integration field beyond the classical receptive field of cat's striate cortical neurons--classification and tuning properties

Length- and width-summation curves of striate cortex cells revealed that there exist facilitatory, inhibitory or disinhibitory integration fields (IF) beyond the sides and ends of the classical receptive field (RF). The extent of the IFs is most frequently 2-5 times the size of the RFs. The tuning properties of IFs were studied using an annular surround grating patch while an optimal centre patch was placed at the excitatory RF to continuously activate the cell. The results show that, for most cells, the orientation, spatial frequency and speed tuning of the IFs were similar to, but broader than, the tuning of the RF, whereas the direction selectivity of the IF was not as pronounced as that of the RF. The possible functional significance of the IF is discussed.

Influences of horizontal connections on visual responses in rabbit striate cortex

The goal of this study was to examine the role of horizontal connections in rabbit striate neurons. Anaesthetized rabbits were prepared in the usual fashion for single-cell recordings in area 17 of the visual cortex. We compared responses evoked by moving and stationary stimuli prior to, during and after recovery from lateral microinjection of either lidocaine (n = 61), gamma-aminobutyric acid (GABA, n = 18) or bicuculline (n = 8) 2 mm from the recording site. This procedure allows evaluation of the contribution of neighbouring neurons in visual responses. Results showed that striate neurons are dependent on the adjacent cells' excitability. Modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli. Lateral inactivation preferentially influenced non-directional over direction-selective units. This influence usually resulted in the non-directional neuron becoming directional by attenuation of the visually driven response in one direction. Simple and complex cells tended to be influenced differently by lateral inactivation. Simple cells became less responsive, whereas complex cells became more responsive. This dichotomy among cellular types suggests that simple cells receive mainly excitatory horizontal influences, while complex cells are contacted mostly by lateral inhibitory inputs.

Interaction between 5-HT1A and nicotinic cholinergic receptors in the regulation of water maze navigation behavior

The interaction between serotonin (5-HT)1A and nicotinic cholinergic receptors in the regulation of spatial navigation behavior in the Morris water maze (WM) test was studied. Pretraining intraperitoneal (i.p.) injections of a combination of subthreshold doses of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (a 5-HT1A receptor agonist) at 30 micrograms/kg and mecamylamine (a nicotinic cholinergic receptor antagonist) a 2500 micrograms/kg greatly impaired WM navigation to a hidden platform and slightly, but not statistically significantly, impaired WM navigation to a visible platform. Post-training i.p. injections of this combination had no effect on WM navigation performance. Serotonin depletion induced by p-chlorophenylalanine (PCPA) increased the performance impairing action of pretraining injected combination of 8-OH-DPAT 30 micrograms/kg and mecamylamine 2500 micrograms/kg. In trained rats combined injections of 8-OH-DPAT 30 micrograms/kg and mecamylamine 2500 micrograms/kg given pretraining had no effect on the navigation to a hidden platform located in a familiar or in a novel position. Pretraining trial injected combination of hexamethonium 2000 micrograms/kg (a peripherally acting nicotinic antagonist) and 8-OH-DPAT 30 micrograms/kg had no effect on navigation. These data suggest that a combined treatment with a 5-HT1a receptor agonist and a nicotinic cholinergic receptor antagonist more severely impair non-mnemonic acquisition performance processes than consolidation and retrieval processes.

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.

The effects of chronic haloperidol administration on GABA-immunoreactive axon terminals in rat medial prefrontal cortex

Several reports have suggested that chronic haloperidol (HAL) treatment induces ultrastructural changes in synapses of substantia nigra, corpus striatum, and medial prefrontal cortex (mPFC) of rat brain. The effects of HAL on specific cortical transmitter systems, however, are not well characterized. Recent studies have indicated that there may be a loss of gamma-aminobutyric acid (GABA)ergic cells in anterior cingulate cortex of schizophrenic subjects and this hypothesis has prompted interest in the question of whether dopamine receptor antagonists, such as HAL, may influence the activity of this transmitter system. This current report describes a quantitative light microscopic analysis of GABA-immunolabeled axosomatic terminals in mPFC of rats treated with HAL decanoate (0.5 mg/kg/day, i.m.) for a period of 4 months. GABA-containing terminals were visualized with an avidin-biotin immunoperoxidase method for localizing anti-GABA antibodies. Computer-assisted image processing was employed to determine the total number of pixels representing GABA-immunoreaction product in axon terminals that were in direct apposition to pyramidal cell bodies. Drug-treated animals showed a significant increase in the number of pixels representing GABA-immunoreaction product in axosomatic terminals of layers II, III, VI, and VI (93%, 63%, 31%, and 43%, respectively). These data are consistent with the idea that chronic HAL administration may be associated with a significant increase in the amount of GABA present in terminals surrounding pyramidal neurons of rat mPFC. The fact that GABA-containing terminals showed the greatest increase in layer II is not consistent with the known distribution of dopamine afferents to this region which is lowest in superficial laminae. Based on the laminar distribution of non-dopaminergic receptor types that have a high affinity for HAL, the effect of this drug on GABAergic transmission could potentially involve changes that are mediated through mechanisms in which 5-HT2 or sigma opiate receptors play a role.

Interocular control of neuronal responsiveness in cat visual cortex

Neurons in the cat primary visual cortex are selective for particular contour orientations but their responsiveness can vary under certain conditions. After prolonged stimulation (adaptation), the contrast sensitivity of cortical cells is reduced and the 'gain' (the strength of response as a function of contrast) falls. The response to an optimal contour is also reduced when a different stimulus is superimposed on the receptive field in the same eye. Here we report that the sudden appearance of an inappropriate stimulus in one eye can interocularly suppress the activity of cortical neurons if they are already responding to an optimally oriented stimulus in the other eye. In strabismic cats, whose cortical neurons lack binocular facilitation, even contours of similar orientation shown to the two eyes trigger such suppression. This interocular control of cortical responsiveness could serve to veto signals from one eye under conditions that would otherwise cause double vision and perceptual confusion.

A comparison of synapses onto the somata of intrinsically bursting and regular spiking neurons in layer V of rat SmI cortex

Regular spiking (RS) and intrinsically bursting (IB) neurons show distinct differences in their inhibitory responses. Under various conditions, the synaptic responses of RS cells display marked inhibitory postsynaptic potentials (IPSPs), whereas the responses of most IB cells do not (Silva et al: Soc Neurosci Abstr 14:883, 1988; Chagnac-Amitai and Connors: J Neurophysiol 61:747, 62:1149, 1989; Connors and Gutnick: TINS 13:99, 1990). This investigation is designed to determine if differences in the inhibitory responses of RS versus IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified by using intracellular recording and labeling, examined with the light microscope, and then serial thin-sectioned prior to examination with the electron microscope. Axonal terminals presynaptic to their somata and proximal dendrites were identified and classified according to criteria described by Peters and coworkers (Peters et al: J Neurocytol 19:584, 1990; Peters and Harriman: J Neurocytol 19:154, 1990; 21:679, 1992). The locations of these boutons were displayed on the surfaces of 3-D reconstructions of the somata and proximal dendrites. The reconstructions were produced directly from the serial thin sections by using a novel, electron microscopic, image-processing computer resource. Our analysis showed no significant difference in the types and concentration of boutons presynaptic to the cell bodies and proximal dendrites of intrinsically bursting versus regular spiking neurons. We conclude that the differences observed in the inhibitory responses of intrinsically bursting versus regular spiking neurons cannot be explained by differences in the concentrations of synapses onto their somata.

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

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

Map of the synapses formed with the dendrites of spiny stellate neurons of cat visual cortex

The synaptic input of six spiny stellate neurons in sublamina 4A of cat area 17 was assessed by electron microscopy. The neurons were physiologically characterized and filled with horseradish peroxidase in vivo. After processing the neurons were reconstructed at the light microscopic level using computer-assisted methods and analyzed quantitatively. The extensive branching of the dendritic tree about 50 microns from the soma meant that the distal branches constituted five times the length of proximal dendrite. Proximal and distal portions of a single dendrite from each neuron were examined in series of ultrathin sections (1,456 sections) in the electron microscope. The majority (79%) of the 263 synapses examined were asymmetric; the remainder (21%) were symmetric. Symmetric synapses formed 35% of synapses sampled on proximal dendrites and were usually located on the shaft. They formed only 4% of synapses sampled on distal dendrites. Spines accounted for less than half of the total asymmetric synapses (45%); the remainder were on shafts. Symmetric synapses formed with four of 92 spines. Nine spines formed no synapses. Spiny stellate neurons in cat visual cortex appear to differ considerably from pyramidal neurons in having a significant asymmetric (excitatory) synaptic input to the dendritic shaft.

Local circuit neurons immunoreactive for calretinin, calbindin D-28k or parvalbumin in monkey prefrontal cortex: distribution and morphology

In the cerebral cortex, local circuit neurons provide critical inhibitory control over the activity of pyramidal neurons, the major class of excitatory efferent cortical cells. The calcium-binding proteins, calretinin, calbindin, and parvalbumin, are expressed in a variety of cortical local circuit neurons. However, in the primate prefrontal cortex, relatively little is known, especially with regard to calretinin, about the specific classes or distribution of local circuit neurons that contain these calcium-binding proteins. In this study, we used immunohistochemical techniques to characterize and compare the morphological features and distribution in macaque monkey prefrontal cortex of local circuit neurons that contain each of these calcium-binding proteins. On the basis of the axonal features of the labeled neurons, and correlations with previous Golgi studies, calretinin appeared to be present in double-bouquet neurons, calbindin in neurogliaform neurons and Martinotti cells, and parvalbumin in chandelier and wide arbor (basket) neurons. Calretinin was also found in other cell populations, such as a distinctive group of large neurons in the infragranular layers, but it was not possible to assign these neurons to a known cell class. In addition, although the animals studied were adults, immunoreactivity for both calretinin and calbindin was found in Cajal-Retzius neurons of layer I. Dual labeling studies confirmed that with the exception of the Cajal-Retzius neurons, each calcium-binding protein was expressed in separate populations of prefrontal cortical neurons. Comparisons of the laminar distributions of the labeled neurons also indicated that these calcium-binding proteins were segregated into discrete neuronal populations. Calretinin-positive neurons were present in greatest density in deep layer I and layer II, calbindin-immunoreactive cells were most dense in layers II-superficial III, and parvalbumin-containing neurons were present in greatest density in the middle cortical layers. In addition, the relative density of calretinin-labeled neurons was approximately twice that of the calbindin- and parvalbumin-positive neurons. However, within each group of labeled neurons, their laminar distribution and relative density did not differ substantially across regions of the prefrontal cortex. These findings demonstrate that calretinin, calbindin, and parvalbumin are markers of separate populations of local circuit neurons in monkey prefrontal cortex, and that they may be useful tools in unraveling the intrinsic inhibitory circuitry of the primate prefrontal cortex in but normal and disease states.

Synapses on axon collaterals of pyramidal cells are spaced at random intervals: a Golgi study in the mouse cerebral cortex

In this study we investigated the arrangement of synapses on local axon collaterals of Golgi-stained pyramidal neurons in the mouse cerebral cortex. As synaptic markers we considered axonal swellings visible at high magnification under the light microscope. Such axonal swellings coincide with synaptic boutons, as has been demonstrated in a number of combined light and electron microscopic studies. These studies also indicated that, in most cases, one bouton corresponds precisely to one synapse. Golgi-impregnated axonal trees of 20 neocortical pyramidal neurons were drawn with a camera lucida. Axonal swellings were marked on the drawings. Most swellings were 'en passant'; occasionally, they were situated at the tip of short, spine-like processes. On axon collaterals, the average interval between swellings was 4.5 microns. On the axonal main stem, the swellings were always less densely packed than on the collaterals. Statistical analysis of the spatial distribution of the swellings did not reveal any special patterns. Instead, the arrangement of swellings on individual collaterals follows a Poisson distribution. Moreover, the same holds to a large extent for the entire collection of pyramidal cell collaterals. This suggests that a single Poisson process, characterized by only one rate parameter (number of synapses per unit length), describes most of the spatial distribution of synapses along pyramidal cell collaterals. These findings do not speak in favour of a pronounced target specificity of pyramidal neurons at the synaptic level. Instead, our results support a probabilistic model of cortical connectivity.

Hippocampal pyramidal cells excite inhibitory neurons through a single release site

Morphologically a synapse consists of a presynaptic release site containing vesicles, a postsynaptic element with membrane specialization, and a synaptic cleft between them. The number of release sites shapes the properties of synaptic transmission between neurons. Although excitatory interactions between cortical neurons have been examined, the number of release sites remains unknown. We have now recorded excitatory postsynaptic potentials evoked by single pyramidal cells in hippocampal interneurons and visualized both cells using biocytin injections. Light and electron microscopy showed that excitatory postsynaptic potentials were mediated by a single synapse. We also reconstructed the entire axon arborization of single pyramidal cells, filled in vivo, in sections counterstained for parvalbumin, which selectively marks basket and axo-axonic cells. Single synaptic contacts between pyramidal cells and parvalbumin-containing neurons were dominant (> 80%), providing evidence for high convergence and divergence in hippocampal networks.

Functional and structural topography of horizontal inhibitory connections in cat visual cortex

The functional organization of long-horizontal inhibitory connections was studied in cat visual cortical area 17, using a combination of electrophysiological recording and anatomical tracing in the same tissue. Orientation maps were obtained by recording multiunit activity from layer III at regular intervals (100-300 microns) in a region of approximately 1.3 mm2 of cortex at a depth corresponding to the location of the basket cell axons reconstructed later. Before the physiological mapping, the neuronal tracer biocytin had been iontophoretically injected at one functionally characterized site. On the basis of light microscopic features a total of five biocytin-labelled large basket axons, BC1-BC5, were reconstructed from series of horizontal sections of two cats. The parent somata and dendritic fields of three axons (BC1, BC4 and BC5) could also be reconstructed. The axonal field of basket cell BC1 had an overall lateral spread of 1.8 mm. The axons of basket cells BC4 and BC5 spanned a distance of 3.05 and 2.85 mm, respectively. The distribution pattern of histologically reconstructed recording sites and of five labelled basket cell axons were directly compared in the same sections. The results show that a single large basket cell provides input to regions representing the whole range of orientations, i.e. iso-orientation (+/- 30 degrees), oblique orientation (+/- [30-60] degrees) and cross-orientation (+/- [60-90] degrees) to that at the basket cell's soma. Furthermore, the differential effect mediated by the same large basket cell at sites of different orientation preference was numerically estimated for two basket cells (BC4 and BC5) whose preferred orientations could be determined on the basis of recording sites adjacent to their parent somata. We counted the number of axonal terminals of these basket cells at iso-, oblique- and cross-orientation sites and found no significant difference in the average density of terminals at sites of either orientation preference. The functional topography of large basket cell axons indicates that the same basket cell can mediate iso-, oblique- and cross-orientation inhibition at different sites. Hence, we assume that large basket cells serve a complex physiological role depending on the location of target cells in the orientation map.

Complete axon arborization of a single CA3 pyramidal cell in the rat hippocampus, and its relationship with postsynaptic parvalbumin-containing interneurons

The complete axon arborization of a single CA3 pyramidal cell has been reconstructed from 32 (60 microns thick) sections from the rat hippocampus following in vivo intracellular injection of neurobiotin. The same sections were double-immunostained for parvalbumin--a calcium-binding protein selectively present in two types of GABAergic interneurons, the basket and chandelier cells--in order to map boutons of the pyramidal cell in contact with dendrites and somata of these specific subsets of interneurons visualized in a Golgi-like manner. The axon of the pyramidal cell formed 15,295 boutons, 63.8% of which were in stratum oriens, 15.4% in stratum pyramidale and 20.8% in stratum radiatum. Only 2.1% of the axon terminals contacted parvalbumin-positive neurons. Most of these were single contacts (84.7%), but double or triple contacts (15.3%) were also found. The majority of the boutons terminated on dendrites (84.1%) of parvalbumin-positive cells, less frequently on cell bodies (15.9%). In order to estimate the proportion of contacts representing synapses, 16 light microscopically identified contacts between boutons of the filled pyramidal cell axon and the parvalbumin-positive targets were examined by correlated electron microscopy. Thirteen of them were found to be asymmetrical synapses, and in the remaining three cases synapses between the labelled profiles could not be confirmed. We conclude that the physiologically effective excitatory connections between single pyramidal cells and postsynaptic inhibitory neurons are mediated by a small number of contacts, mostly by a single synapse. This results in a high degree of convergence and divergence in hippocampal networks.