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

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

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

Synaptic relationships involving local axon collaterals of pyramidal neurons in the cat motor cortex

The intracortical synaptic relationships of pyramidal neurons in the cat motor cortex were studied by intracellular recording and labeling techniques. Neurons that responded with monosynaptic excitatory postsynaptic potentials (EPSPs) to microstimulation in the somatosensory cortex were identified by intracellular recordings. Long-term potentiation (LTP) was evoked in all of these neurons (n = 15), following tetanic stimulation (50 Hz, 5 s) of their afferents from the somatosensory cortex. Three of these cells (cells A-C) were identified as pyramidal neurons, following intracellular injections of Neurobiotin. The intracortical axon collaterals of these labeled cells arborized extensively, forming terminal clusters both in close proximity to the parent soma and along their long, horizontal branches. Terminal clusters in both the proximal and in the distal termination zones of each of the cells were studied by electron microscopy. In their proximal arborization zones, the axon collaterals of the labeled pyramidal neurons synapsed preferentially with dendritic spines belonging to other pyramidal cells. In contrast, in their distal terminal clusters, the axon collaterals of each of the cells formed synapses in different proportions with different postsynaptic targets. The distal axon collaterals of cell A formed 86% of their synapses with pyramidal neurons; those of cell B formed 64% of their synapses with pyramidal cells, the remaining synapses with the dendritic shafts and somata of nonpyramidal neurons, and those of cell C provided most of their output (68%) to nonpyramidal, presumably inhibitory neurons. These findings suggest a high selectivity of intrinsic axon collaterals to form specific patterns of synapses. The patterns of synaptic interactions formed by these intrinsic axon collaterals may be a substrate for shaping and modulating representation maps in the motor cortex.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

Local circuit neurons of developing and mature macaque prefrontal cortex: Golgi and immunocytochemical characteristics

A study has been made of the nonpyramidal, local circuit neurons in developing and mature macaque monkey prefrontal cortex with Golgi and immunocytochemical techniques. The area chosen for study is located between the cingulate gyrus and the ventral bank of the principal sulcus, and contains areas 9 and 46 as described by Walker (J. Comp. Neurol. 73:59-86, '40). In Golgi studies, the unique axonal features of impregnated neurons made possible the identification of thirteen separate classes of local circuit neurons. Five of these cell types, in their general characteristics, resembled classes identified in human prefrontal cortex, as well as in other cortical areas of macaque monkeys and other species. Measurements of the scale of axon arbors and dendritic fields of the Golgi-stained local circuit neurons also suggested particular spatial relationships of certain classes to the scale of intrinsic lattice connections made by the axons of pyramidal neurons in the same region. Similarities in morphology between cells described in human prefrontal cortex and neuron varieties described in this study indicate that this region of monkey prefrontal cortex may serve as a useful model for neuron populations in human prefrontal cortex. Sufficient morphological detail was present in immunocytochemical studies to suggest one or more identifying biochemical characteristics for seven of the thirteen classes of local circuit neurons. The calcium binding proteins, parvalbumin, calbindin D-28K, and calretinin, were found in chandelier and wide arbor neurons, neurogliaform cells, and double bouquet neurons, respectively. In addition, cholecystokinin immunoreactivity was present in medium arbor neurons and in narrow arbor cells connecting layers 2 and 4. Somatostatin 28(1-12) immunoreactivity was detected in beaded axon neurons in layers 5 and 6. This biochemical characterization of local circuit neurons, although incomplete, confirms the separate identity of at least some of the varieties distinguished by Golgi morphology, and allows a start to be made on studies examining changes in their functional state. The general inhibitory nature of these interneurons suggests that they are likely to play a crucial role in determining patterns of neural activation in the prefrontal cortex.

Network of GABAergic large basket cells in cat visual cortex (area 18): implication for lateral disinhibition

Anatomical and immunohistochemical data indicate that, in addition to pyramidal neurons, nonpyramidal cells are exposed to perisomatic inhibition mediated by gamma-aminobutyric acid (GABA)-containing terminals. However, no direct information is available as yet for the origin of GABAergic inputs to morphologically identified GABAergic neurons. In the present paper, we studied the topographical and synaptic relationship between identified GABAergic large basket cells and their immunohistochemically characterized target neurons revealed by parvalbumin-(PV) and GABA immunostaining in the same material. Extracellularly applied biocytin labelled a total of 36 and 9 large basket cells in layers III and V, respectively. Of these, the axonal arborizations of two basket cells, BC1 and BC2, were reconstructed. The axon of BC1 occupied an area of about 2.3 x 2.2 mm2 in layer III, providing a total of 2,755 terminals. The axon of BC2 showed an overall extent of 3.8 x 1.7 mm2 in layer V elongated in the anteroposterior direction, and gave off 1,599 terminals. Immunostaining for PV was carried out to reveal putative nonpyramidal targets for BC1 and BC2. It was found that in addition to immunonegative cells, they established an average of 4-6 perisomatic contacts onto each of 58 (BC1) and 33 (BC2) PV-immunopositive neurons. For electron microscopic verification, 23 terminals apposing the somata of 12 PV-immunopositive neurons were selected. Each terminal was found to establish symmetrical (type II) contacts with its targeted cell. Furthermore, the distribution of soma area of the targeted PV-immunopositive cells and of identified large basket cells showed remarkable similarity, implying that the two populations were actually the same. In addition, the average horizontal distance between neighbouring PV-immunopositive target cells was found to be about 100 microns both in layers III and V. The results suggest that in area 18 the same large basket cell provides direct inhibition to certain pyramidal cells and facilitation to other pyramidal neurons, by inhibiting their presynaptic large basket cells at regular intervals.

Pyramidal cells in piriform cortex receive a convergence of inputs from monoamine activated GABAergic interneurons

Previously, serotonin (5-HT) was shown to increase inhibitory post-synaptic potentials (IPSPs) in layer II pyramidal cells, and excite a subpopulation of interneurons located on the layer II/III border of piriform cortex in rat in vitro brain slices. In the present study, the effects of norepinephrine (NE) and dopamine (DA) on these two populations of neurons were examined in brain slices using intracellular and extracellular recordings. All three monoamines increased GABAergic IPSPs in many pyramidal cells; overall, 5-HT was most effective in eliciting IPSPs (58% of cells), followed by NE (45%), then DA (24%). Commonly, pyramidal cells responded with an increase in IPSPs to more than one of the monoamines. The increase in IPSPs was found to include an increase in the frequency of IPSPs present at baseline, as well as recruitment of additional IPSPs of different amplitudes. In interneurons the effects of the monoamines paralleled that which was found for the pyramidal cells. Thus, all three monoamines increased the firing rate of many interneurons; again 5-HT was most effective (56%), followed by NE (51%), then DA (42%). In about 10% of the interneurons the monoamines inhibited cell firing. Interneurons frequently had responses to more than one of the monoamines. The excitatory amino acid (EAA) antagonist, kynurenic acid (200-400 microM), spared most 5-HT and NE responses on interneurons, suggesting that these effects were directly mediated. We conclude that IPSPs elicited by monoamines in pyramidal cells result from a convergence of inputs from populations of layer II/III interneurons that are activated by one, two or all three of the monoamines.

Visual responsiveness and direction selectivity of cells in area 18 during local reversible inactivation of area 17 in cats

We have investigated the effects of inactivation of localized sites in area 17 on the visual responses of cells in visuotopically corresponding regions of area 18. Experiments were performed on adult normal cats. The striate cortex was inactivated by the injection of nanoliters of lidocaine hydrochloride or of gamma-aminobutyric acid (GABA) dissolved in a staining solution. Responses of the simple and complex cells of area 18 to optimally oriented light and dark bars moving in the two directions of motion were recorded before, during, and after the drug injection. Two main effects are described. First, for a substantial number of cells, the drug injection provoked an overall reduction of the cell's visual responses. This nonspecific effect largely predominated in the complex cell family (76% of the units affected). This effect is consistent with the presence of long-range excitatory connections in the visual cortex. Second, the inactivation of area 17 could affect specific receptive-field properties of cells in area 18. The main specific effect was a loss of direction selectivity of a number of cells in area 18, mainly in the simple family (more than 53% of the units affected). The change in direction selectivity comes either from a disinhibitory effect in the nonpreferred direction or from a reduction of response in the preferred direction. It is proposed that the disinhibitory effects were mediated by inhibitory interneurones within area 18. In a very few cases, the change of directional preference was associated with a modification of the cell's response profile. These results showed that the signals from area 17 are necessary to drive a number of units in area 18, and that area 17 can contribute to, or at least modulate, the receptive-field properties of a large number of cells in the parastriate area.

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

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

A comparison of the electrophysiological properties of morphologically identified cells in layers 5B and 6 of the rat neocortex

In vitro studies performed in mammalian brain slices have shown that cortical neurons differ in their intrinsic membrane properties. In the rodent cortex these properties are related to a specific cell morphology and synaptic connectivity in some cells but not in others. Due to their small size, little is known about the intrinsic membrane properties of layer 6 cells, however, and it is not clear whether cell morphology is related to electrophysiological properties in this layer. We used a combination of intracellular recording and dye-filling to study the electrophysiological and morphological characteristics of layer 6 cells of the rat sensorimotor cortex in vitro and compared their properties to those of large layer 5B pyramidal cells. Our sample of 24 filled and anatomically reconstructed cells in layer 6 confirms previous Golgi studies that showed them to be a morphologically diverse group consisting of regularly and irregularly oriented pyramidal cells and spiny nonpyramidal cells. Regular layer 6 pyramidal cells differed with respect to the length of their apical dendrites and extent of their axonal arborizations, while irregularly oriented pyramidal cells consisted of sideways or inverted pyramidal cells of variable size and morphology. Spiny nonpyramidal cells included bi-tufted and multi-polar cell types that differed in size and extent of dendritic trees. Many layer 6 cells showed long horizontal axon collaterals in layer 6, and an oblique or vertical projection to layer 4. Stimulation with intracellular constant current pulses revealed that the morphological diversity was mirrored by a similar electrophysiological diversity. Most layer 6 cells were capable of firing trains of action potentials characterized by an initial doublet or triplet followed by a train of single spikes (phasic-tonic mode). The majority of layer 6 cells could fire in either a tonic (single spikes only) mode with low strength current input and a phasic-tonic pattern with higher current strengths. A minority fired either always phasic-tonic or tonic-only spike trains. The size and sequence of spike afterpotentials during low-rate repetitive firing was highly variable in layer 6 cells suggesting that the relative importance of ionic currents responsible for spike repolarization and afterpotentials varied from cell to cell. Subthreshold responses showed prominent inward rectification, while hyperpolarizing "sag" was present in most cells tested. In comparison, large layer 5B pyramidal cells fired either phasic-tonic only or both phasic-tonic and tonic patterns. A minority of cells were capable of firing repetitive bursts, while the remainder fired repetitive single spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

gamma-Aminobutyric acid-containing basal forebrain neurons innervate inhibitory interneurons in the neocortex

The basal forebrain-neocortex pathway--involved in higher cognitive processing, selective attention, and arousal--is considered one of the functionally most important ascending subcortical projections. The mechanism by which this relatively sparse subcortical pathway can control neuronal activity patterns in the entire cortical mantle is still unknown. The present study in the cat provides evidence that gamma-aminobutyric acid-containing basal forebrain neurons participate in the neocortical projection and establish multiple synaptic connections with gamma-aminobutyric acid-releasing interneurons containing somatostatin or parvalbumin. We propose that a mechanism by which the numerically small ascending pathways can exert a powerful global effect in the neocortex is by the selective innervation of gamma-aminobutyric acid-releasing interneurons, which, in turn, control the activity of large populations of pyramidal cells through their extensive axon arborizations. Finally, these results demonstrate a direct anatomical link between two cell populations implicated in Alzheimer disease pathology: basal forebrain neurons and cortical somatostatin cells.

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

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

Receptor subtypes involved in callosally-induced postsynaptic potentials in rat frontal agranular cortex in vitro

A slice preparation of rat frontal agranular cortex preserving commissural inputs has been used for intracellular recording from layer V pyramidal cells, in order to characterize the synaptic potentials induced by stimulation of the corpus callosum and to reveal the subtypes of amino acid receptors involved. Stimulation of the corpus callosum induced EPSPs followed by early IPSPs with a peak latency of 30 +/- 2 ms and late IPSPs with a peak latency of 185 +/- 18 ms. Reversal potentials for early and late IPSPs were -75 +/- 5 mV (early) and -96 +/- 5 mV (late). Late IPSPs were more dependent on extracellular K+ concentration. The early IPSPs were blocked by GABAA antagonists, bicuculline and picrotoxin, whereas the late IPSPs were reduced by the GABAB antagonist, phaclofen. CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), an antagonist of non-NMDA (N-methyl-D-aspartate) receptors, suppressed both EPSPs and late IPSPs at 5 microM. Early IPSPs remained at this concentration but were suppressed by 20 microM CNQX. In Mg(2+)-free solution, EPSPs were larger and more prolonged than in control solution. These enhanced EPSPs persisted after 5 to 20 microM CNQX, but were reduced in amplitude, and their onset was delayed by 3.6 +/- 0.8 ms. The remaining EPSPs were suppressed by 50 microM APV (DL-2-amino-5-phosphono-valeric acid), an antagonist of NMDA receptors. In Mg(2+)-free solution containing 5 to 20 microM CNQX, the late IPSPs were not diminished. The remaining late IPSPs were suppressed by APV or by phaclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAergic interneurons containing calbindin D28K or somatostatin are major targets of GABAergic basal forebrain afferents in the rat neocortex

The arborization pattern and postsynaptic targets of the GABAergic component of the basal forebrain projection to neo- and mesocortical areas have been studied by the combination of anterograde tracing and pre- and postembedding immunocytochemistry. Phaseolus vulgaris leucoagglutinin (PHAL) was iontophoretically delivered into the region of the diagonal band of Broca, with some spread of the tracer into the substantia innominata and ventral pallidum. A large number of anterogradely labelled varicose fibres were visualized in the cingulate and retrosplenial cortices, and a relatively sparse innervation was observed in frontal and occipital cortical areas. Most of the labelled axons were studded with large en passant varicosities (Type 1), whereas the others (Type 2) had smaller boutons often of the drumstick type. Type 1 axons were distributed in all layers of the mesocortex with slightly lower frequency in layers 1 and 4. In the neocortex, layer 4, and to a smaller extent upper layer 5 and layer 6 contained the largest number of labelled fibres, whereas only a few fibres were seen in the supragranular layers. Characteristic type 2 axons were very sparse but could be found in all layers. Most if not all boutons of PHAL-labelled type 1 axons were shown to be GABA-immunoreactive by immunogold staining for GABA. Altogether 73 boutons were serially sectioned and found to make symmetrical synaptic contacts mostly with dendritic shafts (66, 90% of total targets), cell bodies (6, 8.2% of total), and with one spine. All postsynaptic cell bodies, and the majority of the dendritic shafts (44, 60.3% of total targets) were immunoreactive for GABA. Thus at least 68.5% of the total targets were GABA-positive, but the majority of the dendrites not characterized immunocytochemically for technical reasons (15.1%) also showed the fine structural characteristics of nonpyramidal neurons. The target interneurons included some of the somatostatin- and calbindin-containing subpopulations, and a small number of parvalbumin-containing neurons, as shown by double immunostaining for PHAL and calcium-binding proteins or neuropeptides. We suggest that the innervation of inhibitory interneurons having extensive local axon arborizations may be a mechanism by which basal forebrain neurons-most notably those containing GABA--have a powerful global effect on the majority of principal cells in the entire cortical mantle.

Neuronal dysfunction at the border of focal lesions in cat visual cortex

Traditional concepts assume that traumatic or ischemic brain lesions are surrounded by regions with depressed neuronal function. More recently hyperactivity gained increasing attention as excitotoxic mechanisms become effective at certain stages of neuronal injury. Single cell recordings in the surrounding of small focal lesions in the cat visual cortex revealed both types of functional pathology 1-30 days after lesioning. A rim of suppressed neurons surrounded a completely silent core. Cells further away from the lesion showed bursts and long lasting hyperactivity with extremely high discharge rates. Consequently, the volume of disturbed tissue was considerably larger than the region of initial cell death. This halo of dysfunction may be important for neurological symptoms evoked by cortical lesions.

Intrinsic connections in cat visual cortex: a combined anterograde and retrograde tracing study

Area 18 of cat visual cortex was examined for intrinsic axons following small, columnar injections of an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Locally projecting axons radiated from the injection site and branched to form 10-15 discrete, approximately circular patches 500-750 microns in diameter consisting of many bouton-studded terminal arborizations. Labeled fibers and boutons ramified densely in layers I, II/II, V, and VI, and were noticeably less dense in layer IV. Afferent and efferent pathways originating from the same cortical columns were studied by injecting a mixture of PHA-L and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Between 10 and 15 patches of cells retrogradely labeled by WGA-HRP surrounded each injection site. Within a patch, labeled cells were found in all layers and included both pyramidal and non-pyramidal cells. The distribution of PHA-L labeling was similar to that obtained when PHA-L was injected alone. Most often, the labeled patches resulting from injections of such mixtures contained both anterograde and retrograde labeling. However, patches consisting of retrograde labeling alone and of anterograde labeling alone were also observed, indicating that the local connections linking neighboring cortical columns were not always reciprocal.

Differential distribution of GABA- and serotonin-containing afferents on an identified central neuron

The distributions of gamma-aminobutyric acid (GABA)- and serotonin (5-HT)-containing terminals impinging on the surface of the Mauthner (M-) cell were studied at the light microscopic level using double immunofluorescent labeling and were compared with that of the glycine receptor. The latter was visualized indirectly, using a monoclonal mouse antibody which recognizes its 93-kDa associated protein. This neuron has two large principal dendrites: one extending ventrorostrally (ventral dendrite) and the other dorsolaterally (lateral dendrite). There are also two other classes of smaller processes: one that projects ventrally (small ventral dendrites) and one penetrating in the axon cap (cap dendrites), a peculiar neuropil surrounding the initial segment of the M-cell axon. A cellular regionalization of these afferent systems was found: GABA boutons, labeled for glutamic acid decarboxylase (GAD), were localized preferentially on the lateral dendrite while 5-HT-filled endings predominated on the ventral one. The density of these two classes of inputs was comparable in the other areas of the M-cell: less of their terminals were in contact with the soma outside the axon cap, and more numerous boutons, which presented either GABA or 5-HT immunoreactivities, were apposed to the small ventral dendrites. This preferential pattern of innervation differed with the ubiquitous presence of glycine receptor clusters on the M-cell membrane. Finally no evidence of a colocalization of GABA and 5-HT in afferent endings was detected at any portion of the M-cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Horizontal interactions between visual cortical neurones studied by cross-correlation analysis in the cat

1. To explore the functional significance of horizontal neural connections in the extent of a 'hypercolumn' of the cat visual cortex, we carried out cross-correlation analysis of spike trains recorded simultaneously from a pair of neurones separated horizontally by less than 1 mm. 2. Significantly correlated firings, which were found in sixty-eight pairs of cells among 327 pairs analysed, were classified into three types on the basis of their functional implications: (1) excitatory interactions, (2) inhibitory interactions and (3) common inputs to both neurones of a pair from other sources. 3. Of these three types, common inputs were encountered most frequently. Excitatory interactions were always accompanied by common inputs. Inhibitory interactions were observed least frequently. 4. The proportion of cell pairs with correlated firings was high in pairs with a horizontal separation of less than 200 microns and decreased markedly with a horizontal separation of more than 400 microns. 5. Regarding laminar locations of cells, common inputs and excitatory interactions were often observed in layers II + III and V, whereas laminar bias was not seen in inhibitory interactions. 6. With respect to difference in orientation preference between two cells, all the three types of correlations were observed, mostly in cell pairs with a difference of less than 45 deg. In particular, common inputs and excitatory interactions were often seen in cell pairs with matched orientation preferences, but inhibitory interactions were found mostly in those with slightly different orientation preferences. In addition, common inputs and excitatory interactions tended to be found between cells with the same eye preference. 7. These results suggest that horizontal functional interactions exist mainly in a range of up to 400 microns as far as the extent of a hypercolumn of the visual cortex is concerned, and these interactions operate effectively between cortical cells with similar receptive field properties except for inhibitory interactions.

Synaptic organization of cortico-cortical connections from the primary visual cortex to the posteromedial lateral suprasylvian visual area in the cat

The synaptic organization of the projection from the cat striate visual cortex to the posteromedial lateral suprasylvian cortical area (PMLS) was examined. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophorectically delivered into area 17, and anterogradely labeled fibers were revealed in PMLS by means of an immunocytochemical detection method. Most axons and presumptive terminal swellings were found in layers III and IV. The neuronal elements (n = 190) that were postsynaptic to anterogradely labeled boutons were quantitatively analyzed. All anterogradely labeled cortico-cortical boutons (n = 182) established type 1 synapses. The results show that 83% of the postsynaptic targets were dendritic spines, probably belonging to pyramidal cells. Dendritic shafts constituted 17% of the targets. The dendritic shafts postsynaptic to cortico-cortical boutons were studied for the presence of gamma-aminobutyric acid (GABA) with a postembedding immunogold method. Most dendritic shafts (85%) that were tested were found to be GABA-positive, demonstrating that they originate from local inhibitory neurons. Taking into account that most postsynaptic targets were spines and extending the results of the immunocytochemical testing to the total population of postsynaptic dendrites, it was calculated that at least 14% of targets originated from GABA-positive cells. Thus cortico-cortical axons establish direct monosynpatic connections mainly with pyramidal and to a lesser extent with GABAergic nonpyramidal neurons in area PMLS, providing both feedforward excitation and feedforward inhibition to a visual associational area known to be involved in the processing of motion information. The results are consistent with previously demonstrated deficits in physiological properties of neurons in PMLS following removal of cortico-cortical afferents.

Subtraction inhibition combined with a spiking threshold accounts for cortical direction selectivity

We have modeled simple-cell direction selectivity by a nonlinearity consisting of a subtraction inhibition followed by half-wave rectification and compared the performance of this model to that of different versions of the elaborated Reichardt detector for similar inputs and parameter settings. Not only does the subtraction model fit the experimental data more closely than the elaborated Reichardt detector, but the subtraction model also is more plausible from a physiological and anatomical point of view. Moreover, the subtraction model operates optimally at plausible spatiotemporal parameter settings. Therefore, we conclude that there is no need to invoke specific synaptic interactions, such as implied in the Reichardt detector, to account for simple-cell direction selectivity.

Targets of horizontal connections in macaque primary visual cortex

Pyramidal neurons within the cerebral cortex are known to make long-range horizontal connections via an extensive axonal collateral system. The synaptic characteristics and specificities of these connections were studied at the ultrastructural level. Two superficial layer pyramidal cells in the primate striate cortex were labeled by intracellular injections with horseradish peroxidase (HRP) and their axon terminals were subsequently examined with the technique of electron microscopic (EM) serial reconstruction. At the light microscopic level both cells showed the characteristic pattern of widespread, clustered axon collaterals. We examined collateral clusters located near the dendritic field (proximal) and approximately 0.5 mm away (distal). The synapses were of the asymmetric/round vesicle variety (type I), and were therefore presumably excitatory. Three-quarters of the postsynaptic targets were the dendritic spines of other pyramidal cells. A few of the axodendritic synapses were with the shafts of pyramidal cells, bringing the proportion of pyramidal cell targets to 80%. The remaining labeled endings were made with the dendritic shafts of smooth stellate cells, which are presumed to be (GABA)ergic inhibitory cells. On the basis of serial reconstruction of a few of these cells and their dendrites, a likely candidate for one target inhibitory cell is the small-medium basket cell. Taken together, this pattern of outputs suggests a mixture of postsynaptic effects mediated by consequence the horizontal connections may well be the substrate for the variety of influences observed between the receptive field center and its surround.

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.

Patterns of synaptic input on corticocortical and corticothalamic cells in the cat visual cortex. I. The cell body

Immunocytochemical and electron microscopic methods were used to examine the ultrastructure and synaptology of callosal and corticothalamic pyramidal cell somata in the cat visual cortex (area 17). Callosal and corticothalamic cells were labeled after injection of horseradish peroxidase (HRP) in the contralateral visual cortex or in the ipsilateral lateral geniculate nucleus. The synaptic relationship between each of the two populations of pyramidal cells and cells containing the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) was examined at the light and electron microscope level using the combined techniques of retrograde transport of HRP and GABA immunocytochemistry. We found that callosal and corticothalamic cell somata have an ultrastructure and synaptology that distinguishes them from each other. Reconstructions from electron micrographs of serial sections revealed that the vast majority of synapses (89-96%) on the cell body of pyramidal cells were formed by GABAergic axon terminals, and that within each population of pyramidal cells there was variability in the number and density of axosomatic synapses. Callosal pyramidal cells received a greater number and higher density of axosomatic synapses than corticothalamic cells. These data suggest that callosal cells receive more inhibition than corticothalamic cells at the level of their somata.

Patterns of synaptic input on corticocortical and corticothalamic cells in the cat visual cortex. II. The axon initial segment

In the present study we examined the synaptology of the axon initial segments (AISs) of three populations of retrogradely labeled pyramidal cells: callosal, corticothalamic, and ipsilateral corticocortical projecting cells. The cells were labeled by horseradish peroxidase injected into the contralateral visual cortex, the ipsilateral lateral geniculate nucleus, or area 19 of the ipsilateral hemisphere. The AISs of these cells were completely reconstructed from tracings of serial electron micrographs and the number and type of synapses ending on them determined. These data were compared with that described in the companion paper (Fariñas and DeFelipe, J Comp Neurol 1991; 304:53-69), in which the ultrastructure and synaptology of the somata of callosal and corticothalamic cells was investigated. Together, these data permit comparisons to be made between the somatic and axonic innervation of the same cells. Our results show that each population of efferent cells examined received a characteristic and rather homogeneous number (in comparison with the number of axosomatic synapses) of axoaxonic synapses: 16 to 23 for AISs of callosal cells, 22 to 28 for ipsilateral corticocortical cells, and 1 to 5 for corticothalamic cells. Since most axoaxonic synapses are likely to be GABAergic, it is concluded that each population of pyramidal cells receives a distinct GABAergic inhibitory synaptic profile.

Mechanisms of inhibition in cat visual cortex

1. Neurones from layers 2-6 of the cat primary visual cortex were studied using extracellular and intracellular recordings made in vivo. The aim was to identify inhibitory events and determine whether they were associated with small or large (shunting) changes in the input conductance of the neurones. 2. Visual stimulation of subfields of simple receptive fields produced depolarizing or hyperpolarizing potentials that were associated with increased or decreased firing rates respectively. Hyperpolarizing potentials were small, 5 mV or less. In the same neurones, brief electrical stimulation of cortical afferents produced a characteristic sequence of a brief depolarization followed by a long-lasting (200-400 ms) hyperpolarization. 3. During the response to a stationary flashed bar, the synaptic activation increased the input conductance of the neurone by about 5-20%. Conductance changes of similar magnitude were obtained by electrically stimulating the neurone. Neurones stimulated with non-optimal orientations or directions of motion showed little change in input conductance. 4. These data indicate that while visually or electrically induced inhibition can be readily demonstrated in visual cortex, the inhibition is not associated with large sustained conductance changes. Thus a shunting or multiplicative inhibitory mechanism is not the principal mechanism of inhibition.

Influence of GABA-induced remote inactivation on the orientation tuning of cells in area 18 of feline visual cortex: a comparison with area 17

We have investigated the effect of iontophoretically applying the inhibitory transmitter gamma-aminobutyric acid (GABA) through four pipettes, each located at a horizontal distance of some 500-600 microns from the recording site, on the orientation tuning of cells in areas 17 and 18 of the cat visual cortex for moving the stationary flash-presented bar stimuli. Forty-five of 74 cells tested in area 18 (61%) showed a significant (greater than 25%) increase in orientation tuning width (at half the maximum response) during GABA application, which reflected an increase in response to non-optimal orientations. The mean orientation tuning width of these cells increased by 79%, and the ratio of responses to the orientation orthogonal to the optimum and to the optimum increased from 0.16 to 0.46. The results were similar to those from area 17, in which 36 of 54 cells (66%) showed significant broadening of orientation tuning during GABA application, with a 90% increase in mean tuning width and an increase in the relative response to the orientation orthogonal to the optimum from 0.17 to 0.42. The distributions of cells in areas 17 and 18 with respect to the magnitude of GABA-induced effects on orientation tuning width were not significantly different (mean increase in tuning width: area 17, 102%; area 18, 87%). Although most cells were tested only with moving bars, comparable effects of remote GABA application on orientation tuning were observed when stationary flash-presented bars were used. Of 11 cells thus tested in area 18, seven showed significantly broader tuning during GABA application, with a 132% increase in mean tuning width. In some 25% of cells in each area which showed a significant effect of GABA application on orientation tuning the response to at least one non-optimal orientation exceeded, during GABA application, the response to the previous optimum. There was essentially no correlation between the changes in orientation tuning and changes in the level of spontaneous activity or in the response to the optimum orientation during GABA application. Thus, an increase in the general excitability of recorded cells or the loss of an unspecific inhibitory input cannot account for the effects of GABA application on orientation tuning. Remote GABA application presumably inactivated cells with different preferred orientations from that of the recorded cell. It is thus argued that the observed broadening of orientation tuning during GABA application reflected the loss of an inhibitory input tuned to non-optimal orientations.(ABSTRACT TRUNCATED AT 400 WORDS)

A functional microcircuit for cat visual cortex

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

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

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

Inhibitory processes in the flash evoked potential of the monkey

Flash visual evoked potentials (VEP) and concurrent multiple unit activity (MUA) were recorded from closely spaced intracortical sites in unanesthetized monkeys before and after intracortical injection of the GABAA antagonist bicuculline. Laminar VEP profiles were subjected to current source density (CSD) analysis to localize the transmembrane current flows contributing to the generation of the field potentials. Before bicuculline, the first large VEP component, N40, was generated principally within the parvocellular thalamorecipient sublamina 4Cb. After bicuculline injection, the current sinks associated with N40 spread throughout lamina 4, consistent with a release of intracortical inhibition mediated by GABA. A subsequent component, P65, believed to represent recurrent inhibitory activity within 4Cb, was greatly diminished in size after bicuculline injection. The laminar pattern of current sources and sinks coincident with this component was more complicated after bicuculline, reflecting the summation of current flows associated with disinhibited lamina 4 activity. Bicuculline also altered the responses of neuronal elements in laminae 3 and 5, evidenced by large increases in MUA in these laminae that began approximately 50 msec after stimulation. Finally, bicuculline diminished the degree of intracortical ocular dominance, implicating GABAergic mechanisms in the maintenance and refinement of ocular input segregation within cortical columns.

Characteristics of miniature inhibitory postsynaptic currents in CA1 pyramidal neurones of rat hippocampus

1. Recordings were made in vitro from chloride-loaded CA1 rat hippocampal pyramidal neurones in the presence of tetrodotoxin (TTX) to examine miniature inhibitory postsynaptic currents (IPSCs). 2. Most spontaneous synaptic events recorded before TTX was applied, and all events that were resolved in the presence of TTX, were blocked by the GABAA receptor antagonist bicuculline. 3. At 25 degrees C, averaged miniature IPSCs had time to peak of about 3 ms and in most cases decayed with a single time constant close to 25 ms. 4. With a driving force for chloride ions between 70 and 80 mV, the mean miniature IPSC amplitude was 19.6-27.9 pA, yielding a conductance of 258-326 pS. The mean amplitude of unitary IPSCs recorded before TTX was applied was in the range of 31-73 pA. 5. When intervals between miniature IPSCs were compared with an exponential distribution, there was an excess of events at intervals shorter than 5 ms. Some individual events appeared to represent the nearly simultaneous release of two inhibitory quanta. 6. Miniature IPSC amplitude distributions were better fitted with the sum of two Gaussians than with one Gaussian. The variance in amplitude of a single quantal event exceeded that of the baseline noise. 7. Comparison of the conductance changes corresponding to the first Gaussian distribution with single GABA channel data suggests that one inhibitory quantum opens twelve to twenty chloride channels and that GABA molecules bind once to a postsynaptic receptor.

Control of Neuronal Output by Inhibition at the Axon Initial Segment

We examine the effect of inhibition on the axon initial segment (AIS) by the chandelier (“axoaxonic”) cells, using a simplified compartmental model of actual pyramidal neurons from cat visual cortex. We show that within generally accepted ranges, inhibition at the AIS cannot completely prevent action potential discharge: only small amounts of excitatory synaptic current can be inhibited. Moderate amounts of excitatory current always result in action potential discharge, despite AIS inhibition. Inhibition of the somadendrite by basket cells enhances the effect of AIS inhibition and vice versa. Thus the axoaxonic cells may act synergistically with basket cells: the AIS inhibition increases the threshold for action potential discharge, the basket cells then control the suprathreshold discharge.

Combining retrograde tracing, intracellular injection, anterograde degeneration and electron microscopy to reveal synaptic links

Synaptic circuitry was investigated by combining retrograde tracing, intracellular staining, anterograde degeneration and electron microscopy in the same piece of tissue. This methodological procedure was successfully applied to disentangle a disynaptic neuronal chain, which originated in the olfactory bulb, was synaptically relayed in the entorhinal cortex and terminated in the ipsilateral hippocampus. Presumed entorhinal relay cells were retrogradely labelled from their hippocampal termination site by means of a fluorescent tracer. Subsequently, the marked projection neurones were intracellularly injected with Lucifer yellow in fixed slice preparations. Following a simple photo-conversion procedure, dye filled cells were processed for electron microscopy. The origin of presynaptic afferents to identified relay cells was revealed ultrastructurally after lesion induced anterograde degeneration of olfactory mitral cell axons. Due to its reliability, technical simplicity and a high degree of selectivity the new approach is considered an appropriate tool for unravelling neuronal networks.

Thalamic and extrathalamic connections of the dysgranular unresponsive zone in the grey squirrel (Sciurus carolinensis)

The connections of the cortical dysgranular "unresponsive zone" (UZ) (Sur et al.: J. Comp. Neurol. 179:425-450, '78) in the grey squirrel were studied with horseradish peroxidase and autoradiographic techniques. The results of these experiments show that the major subcortical connections of the unresponsive zone are in large part reciprocal. Connections are distributed within the thalamus in a poorly defined region including restricted portions of several nuclei that lie along the rostral, dorsal, and caudal borders of the ventral posterior nucleus. Additional thalamic connections of the UZ terminate in the reticular nucleus and are reciprocally related to the paralaminar and central median nuclei. Extrathalamic terminations were observed in the zona incerta, the intermediate and deep layers of the superior colliculus, the red nucleus, and several subdivisions of the pontine nuclei. The similarity between the pattern of subcortical connections of the UZ in the grey squirrel and patterns reported for the parietal septal region in rats (Chapin and Lin: J. Comp. Neurol. 229:199-213, '84) and for area 3a in primates (Friedman and Jones: J. Neurophysiol. 45:59-85, '81), suggests that the UZ in the grey squirrel may represent a counterpart of at least part of area 3a as described in primates. The results are further discussed with respect to a possible role of the thalamus in control or modulation of interhemispheric circuits and of the UZ in the modulation of nociceptive and kinesthetic pathways through the thalamus. Finally, the term parietal dysgranular cortex (PDC) is proposed as an alternative to denote the region currently called the unresponsive zone.

The callosal projection in cat visual cortex as revealed by a combination of retrograde tracing and intracellular injection

The neuronal composition of callosally projecting cells in cat visual cortex was determined with a combination of retrograde labelling and intracellular injection. Fluorescent tracers were stereotaxically injected into the proximity of the area 17/18 border, corresponding to the representation of the visual vertical meridian. In fixed slice preparations of homotopic regions of the contralateral hemisphere retrogradely labelled cells were filled with Lucifer Yellow. Of more than a hundred injected cells a morphological variety of pyramidal cells, located in cortical layers II-IV and VI, constituted the prevalent cell class in the contralateral projection. A minor proportion of spiny stellate cells was encountered in layer IV. Despite the presence of a contralaterally projecting smooth stellate cell, presumed to be a basket cell, it is concluded that the efferents to contralateral visual cortex predominantly arise from pyramidal and spiny stellate cells. Thus, in agreement with findings from anterograde degeneration studies, the interhemispheric pathway most likely conveys a direct excitatory input to postsynaptic target cells.

Intracellular lucifer yellow injection in fixed brain slices combined with retrograde tracing, light and electron microscopy

The present paper contains a full methodological description of iontophoretic Lucifer Yellow injections in fixed brain slices in mammals. In brief, cortical tissue was either perfused or immersion-fixed in paraformaldehyde. After Vibratome sectioning, tissue slices were transferred to epifluorescence microscopes equipped with long distance objectives. Under visual guidance, neurons were selectively impaled with Lucifer Yellow-filled electrodes and intracellularly injected until all dendrites appeared brightly fluorescent. Excellent dendritic staining was obtained in both perfusion-fixed cat visual cortex and immersion-fixed human brain biopsies. Dendritic spines, varicosities and growth cones could be readily discerned. Filling of axonal collaterals was, however, incomplete. Callosally projecting neurons in cat visual cortex were retrogradely traced with a mixture of the fluorescent tracers Fast Blue and DiI. Subsequently the morphology of labelled cells was determined by intracellular Lucifer Yellow injection. Although the Fast Blue fluorescence had become undetectable in filled neurons the granular red appearance of DiI was still discernible. Hence the neuronal composition of even relatively sparse projections can be demonstrated. To obtain permanent preparations, dye-filled neurons were immersed in a diaminobenzidine solution and irradiated with epifluorescent illumination until all visible fluorescence had faded. Photo-oxidation resulted in the intracellular formation of a homogeneously distributed brown reaction product visible with the light microscope. Brief osmication enhanced the staining contrast, thus providing a Golgi-like image. Subsequent electron microscopy of photo-converted cells showed the fine granular nature of the electron opaque reaction product, thus revealing numerous cytological features. The precipitate was homogeneously distributed throughout the entire cytoplasm and nucleus, extending into dendrites and axon. Any apparent leakage of the label into the extracellular space was not observed. Intracellular staining in fixed tissue yields a high number of neurons with extensive filling of dendritic arbors. Photo-oxidation provides stable, non-fading preparations with the option of subsequent electron microscopy. In addition, the technique can be combined with immunocytochemistry and a variety of fluorescent tracer substances. These features, combined with its high selectivity and relative methodological simplicity, render the method to be a promising alternative to classical neuroanatomical approaches.

Molecular determinants of GABAergic local-circuit neurons in the visual cortex

Golgi impregnation, intracellular marking techniques and immunocytochemistry have led to the identification of several distinct GABAergic cell types. Co-localization of neuropeptides or calcium-binding proteins has provided additional markers for GABAergic cells. Recently, immunological or lectin probes have helped to identify additional subsets of GABAergic neurons. In combination with other immunocytochemical and anatomical approaches, these probes are now being used to link molecular composition to cellular architecture in the visual cortex.

Dendritic transformations on random synaptic inputs as measured from a neuron's spike train--modeling and simulation

Extracellular spike trains recorded from central nervous system neurons reflect the random activations from a multitude of presynaptic cells making contacts mainly on the extensive dendritic trees. The dendritic potential variations are propagated towards the trigger zone where action potentials are generated. In this paper, two dendritic propagation modes are modeled: passive and quasi-active. Synaptic bombardments are modeled as being applied apically, somatically, or distributed over the dendritic tree. The resulting simulated neuronal spike trains are analyzed by point process techniques. Dendritic inputs resulted in a tendency for random bursting, interspike interval histograms with a long tail and coefficients of variation larger than one. The autocorrelation histograms reflected dynamics of the dendritic tree and they were able to discriminate between a passive or a quasi-active propagation mode and between dendritic and somatic synaptic inputs.

The cholinergic nucleus basalis: a key structure in neocortical arousal

Single unit studies indicate that increased activity in the cholinergic nucleus basalis (NB) correlates with behavioral activation and neocortical desynchronization. Lesions of the NB result in neocortical slow delta waves, similar to the action of antimuscarinic drugs, and the lesion releases the oscillation of GABAergic neurons in the reticular nucleus of the thalamus, resulting in high voltage neocortical spindles. Extensive damage of the thalamus does not produce slowing of neocortical activity but it abolishes neocortical spindles. We suggest that the NB plays a key role in neocortical activation by a) blocking the afterhyperpolarizations and accommodation in neocortical pyramidal neurons and b) suppressing the rhythm generation in the reticular nucleus-thalamocortical circuitry. We further suggest that the NB system may serve as a structural basis for the concept of the generalized activation described by Moruzzi and Magoun (1949).

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

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

Morphology of neurons in area 4 gamma of the cat's cortex studied with intracellular injection of HRP [corrected and issued with original paging in J Comp Neurol 1988 Nov 8;277(2)]

Neurons in laminae II, III, V, and VI of area 4 gamma of the cat motor cortex were studied following intracellular penetration with an HRP-filled micro-electrode. Antidromic and synaptic responses produced by stimulation of the cerebral peduncles and/or of the ventrolateral nucleus of the thalamus were investigated. Horseradish peroxidase was then iontophoresed into the same neurons to allow examination of their detailed morphology. The morphology of pyramidal neurons whose somata were located in a particular lamina was similar but differed from that of pyramidal neurons in other laminae. The modified pyramidal neurons of lamina II had a truncated apical dendrite or did not possess an obvious apical dendrite, even though the ascending dendritic branches were longer and more extensive than the "basal" branches. As was the case for the pyramidal cells in other laminae, the axons of these lamina II modified pyramidal cells descended toward the white matter; their somata were generally pyramidal in shape, and their dendrites were spiny. All pyramidal neurons except some of lamina VI had ascending dendrites which terminated in a tuft in lamina I, subpially. No intracortical collaterals were seen originating from the axons of lamina II or of lamina VI pyramidal neurons. Lamina III pyramidal neurons had extensive short and long axon collaterals which contributed synaptic boutons to all laminae of the cortex. Pyramidal neurons of lamina V had fewer axon collaterals whose synaptic boutons were restricted to laminae V and VI. All somata of pyramidal tract neurons (PRNs), identified by antidromic responses from peduncular stimulation, were located in lamina V, except for one which was located in lamina VI. Recurrent collaterals of pyramidal neurons were activated by peduncular stimulation. Recurrent excitatory postsynaptic potentials (epsps) could be evoked in fast PTNs, slow PTNs, other pyramidal neurons of lamina V, and pyramidal neurons of lamina VI at latencies between 1.3 and 6.25 msec. In some slow PTNs, a recurrent inhibitory postsynaptic potential of long duration was the predominant response. Stimulation of the ventrolateral nucleus of the thalamus resulted in epsps in pyramidal neurons of lamina III, V, and VI at latencies between 1.0 and 5.0 msec.

Alterations in excitatory and GABAergic inhibitory connections in hippocampal transplants

Solid pieces of embryonic hippocampal tissue were implanted in a cavity formed by aspiration of the fimbria-fornix and the overlying cingulate cortex in adult rats. Six to 8 months after the transplantation, chronic recording electrodes were implanted into the graft and the host hippocampi for the recording of electroencephalogram and unit activity in the freely moving animal. Irregularly occurring sharp waves or electroencephalogram spikes and concurrent synchronous discharge of large groups of neurons dominated the electrical activity of the grafts, in contrast to the situation in normal animals. Light microscopy and GABA immunocytochemistry in the grafts revealed that the three major cell types of the hippocampal formation, i.e. pyramidal neurons, dentate granule cells and GABA-immunoreactive interneurons were present in the hippocampal grafts. At the ultrastructural level, however, significant alterations in connectivity were observed. The most striking finding was the absence or sparse occurrence of synapses on the axon initial segments of pyramidal neurons. The axon initial segments are normally densely covered by GABAergic synapses derived from a specialized type of interneuron, the chandelier or axo-axonic cell. On the other hand, numerous GABA-immunoreactive terminals were found in synaptic contact with somata of pyramidal neurons, suggesting that other types of GABAergic interneurons and their efferent connections may have developed in a normal manner. The cell bodies of pyramidal neurons received, in addition, several asymmetric synapses from GABA-negative terminals. These presumably excitatory synapses are not present on the somata of pyramidal cells in the normally developing hippocampus. We hypothesize that the somatic excitatory synapses originate, at least in part, from the axon collaterals of the neighbouring pyramidal cells in the graft. We suggest that the hyperexcitability of the neuronal circuitry within the graft is due to reduced inhibition (lack of axo-axonic synapses) coupled with increased collateral excitation of the pyramidal neurons.

Inhibition contributes to orientation selectivity in visual cortex of cat

Neurons in the visual cortex are selectively responsive to light or dark bars presented at particular orientations. On the basis of physiological data, this orientation selectivity is hypothesized as being due at least partially to intracortical inhibitory mechanisms. But this hypothesis has been challenged by intracellular recordings indicating that excitatory inputs themselves are orientation-selective, so inhibition may not contribute to the observed selectivity. Also, there is controversy about the presence of intracortical horizontal connections mediating inhibition for selectivity and about the theoretical validity of such inhibitory connections. Using cross-correlation analysis of the activities of two neurons recorded simultaneously, we find that inhibitory interactions exist between cells with somewhat different, but not orthogonal, orientation preferences. This suggests that intracortical horizontal inhibition operates between 'orientation columns' to sharpen the orientation tuning of cortical neurons.