A neurochemically distinct third channel in the macaque dorsal lateral geniculate nucleus.

The primate visual system is often divided into two channels, designated M and P, whose signals are relayed to the cerebral cortex by neurons in the magnocellular and parvicellular layers of the dorsal lateral geniculate nucleus. We have identified a third population of geniculocortical neurons in the dorsal lateral geniculate nucleus of macaques, which is immunoreactive for the alpha subunit of type II calmodulin-dependent protein kinase. This large third population occupies interlaminar regions (intercalated layers) ventral to each principal layer. Retrograde labeling of kinase-immunoreactive cells from the primary visual cortex shows that they provide the geniculocortical input to cytochrome oxidase-rich puffs in layers II and III.

Activity-dependent regulation of GABA expression in the visual cortex of adult monkeys.

Levels of the inhibitory transmitter, GABA, and its synthesizing enzyme, GAD, appear to be regulated in the visual cortex of young adult monkeys in an activity-dependent manner. In monkeys subjected to monocular deprivation by eye removal, tetrodotoxin injection, or eyelide suture, the number of GABA and GAD immunoreactive neurons in deprived-eye columns of the cortex is reduced by up to 50%. This effect is unaccompanied by cell death and is reversible. After cessation of TTX injection or reopening of the eyes, the number of immunostained cells returns to normal. The effect appears after 4-5 days of eye removal or tetrodotoxin injection, but only after 7-16 weeks of eyelid suture. In the latter case, it is more severe in the younger monkeys. The reversible reduction in GABA and GAD immunostaining extends out of layer IVC into lay IVA and to neurons around but not in cytochrome oxidase periodicities of layer III. This may indicate selective vulnerability of GABA cells sensitive to high spatial frequency.

Neuronal chemistry and functional organization in the primate visual system.

Beginning with the first step of visual processing and proceeding outward from that point, the neurons involved in different aspects of vision are distinct. Stated simply, neurons doing different things look different. They often display distinct morphological features and they usually express different molecules. In addition, neurons that perform a common function usually aggregate together to form recognizable layers or compartments that can be studied in isolation because they are neurochemically distinct. Here is found, then, a junction of two major domains in neuroscience research, as discovery of molecular diversity among neurons is exploited to study organization and function of the primate visual system.

Differential Calcium Binding Protein Immunoreactivity Distinguishes Classes of Relay Neurons in Monkey Thalamic Nuclei.

The distributions of neurons displaying immunoreactivity for two calcium binding proteins, parvalbumin and 28Kd calbindin, were studied in the thalamus of M. fascicularis. Colocalization experiments were carried out to determine the extent to which parvalbumin- and calbindin-like immunoreactivity was found in the same cells and the extent to which either was localized in GABAergic interneurons. Anterograde and retrograde tracing experiments involving the fluorescent tracer, fast blue, were also used to determine that cells expressing the calcium binding proteins projected upon the cerebral cortex. In the dorsal thalamus, nuclei are distinguished by different patterns of parvalbumin-like and calbindin-like immunoreactivity. In certain nuclei, for example the lateral dorsal and anterior pulvinar, neurons express immunoreactivity for only one of the calcium binding proteins. In others, neurons in different layers, for example the dorsal lateral geniculate nucleus, or in different compartments, for example the intralaminar nuclei, express immunoreactivity for either parvalbumin or calbindin; in other nuclei, for example the ventral group, neurons are mixed and immunoreactivity for parvalbumin and calbindin is commonly colocalized. In the ventral thalamus and epithalamus, similar patterns are observed. Colocalization of parvalbumin- and GABA-immunoreactivity is found in all cells of the reticular nucleus but only in certain cells in selected nuclei of the dorsal thalamus, namely the dorsal lateral geniculate and magnocellular medial geniculate. No calbindin-positive cells are also GABA-positive. Most parvalbumin and/or calbindin positive cells in the dorsal thalamus project to the cerebral cortex, as indicated by the retrograde tracing studies, and many parvalbumin positive fibres entering the cerebral cortex could also be shown to contain fast blue anterogradely transported from a thalamic injection. Most of the major sensory and motor pathways entering the dorsal thalamus express parvalbumin immunoreactivity. The optic tract also expresses calbindin immunoreactivity but most other calbindin positive fibres entering the thalamus ascend in the midbrain tegmentum. The differential distributions of parvalbumin and calbindin implied by these results suggest that thalamic cells belonging to different functional systems and projecting differentially upon the cerebral cortex can be distinguished by differential expression of these or closely related calcium binding proteins. This may yield clues to their differential responsivity to afferent driving.

Compartmental organization of layer IVA in human primary visual cortex.

Immunostaining for three neuronal proteins, nonphosphorylated neurofilament protein (with antibody SMI-32), calbindin, and parvalbumin, was used to examine the organization of layer IV in human primary visual cortex (area 17 or V1) specifically to determine whether, similar to the case in macaque V1, layer IVA is present and is divided into neurochemically distinct compartments. All three proteins are expressed by neurons that are unevenly distributed in layer IV of human V1; immunostaining for each protein includes a thin band corresponding to layer IVA of classic cytoarchitectonic studies. In this band, nonphosphorylated neurofilament protein immunoreactivity is present in relatively broad clusters of pyramidal cell somata and dendrites that appear as upwardly protruding parts of intense immunostaining in layer IVB, whereas immunoreactivity for calbindin and parvalbumin exists in somata of nonpyramidal neurons and in thin, dense clusters of punctate profiles. In tangential sections through layer IVA, the three proteins are seen in distinct compartments. Calbindin- and parvalbumin-immunostained neurons make up a thinly walled honeycomb or lattice, whereas neurons immunostained for nonphosphorylated neurofilament protein occupy the central lacunae. Direct comparison shows that neurons immunostained for calbindin occupy regions in layer IVA complementary to those immunostained for nonphosphorylated neurofilament protein. These data demonstrate a basic similarity in the organization of layer IV in macaques and humans. Layer IVA specifically is organized into complementary and neurochemically distinct compartments, including what appears to be a geniculocortically innervated and parvicellular-driven lattice and the interstitial lacunae formed by the periodic, upward protrusion of magnocellular-dominated layer IVB neurons.

Thalamic relay nuclei for cerebellar and certain related fiber systems in the cat.

Anterograde labeling techniques were used to define the terminal distributions in the thalamus of afferents arising in the deep cerebellar nuclei, entopeduncular nucleus and substantia nigra. Anterograde and retrograde labeling methods were then used to determine the extent of the cortical projections of the cerebellar relay nuclei. The cerebellar projection to the contralateral ventral nuclei of the thalamus terminates in a zone which is separate from that receiving pallido- and nigrothalamic fibers. None of the zones of termination of these fiber systems corresponds to commonly recognized cytoarchitectonic divisions. Instead, they include parts of the ventroanterior (VA), ventrolateral (VL) and principal ventromedial (VMp) nuclei. Some cells within the zone of termination of cerebellar afferents project to parietal cortex (areas 5 and 7). A further, distinct group of cells in this zone projects to motor cortex (area 4). But projections to area 4 also arise from small groups of cells: (a) in the zone receiving nigro- and pallidothalamic fibers; (b) in the part of VL, distinct from the cerebellar terminal zone, in which spinothalamic fibers terminate. Cerebellar, nigral, and entopeduncular fibers also terminate in the intralaminar nuclei. These projections are far greater in extent than those arising in the spinal cord. Some parts of the intralaminar nuclei are dominated by a particular afferent system, while others show substantial overlap of the terminal zone of several afferent systems.

Intracortical connectivity of architectonic fields in the somatic sensory, motor and parietal cortex of monkeys.

Anterograde and retrograde transport methods were used to study the corticocortical connectivity of areas 3a, 3b, 1, 2, 5, 4 and 6 of the monkey cerebral cortex. Fields were identified by cytoarchitectonic features and by thalamic connectivity in the same brains. Area 3a was identified by first recording a short latency group I afferent evoked potential. Attempts were made to analyze the data in terms of: (1) routes whereby somatic sensory input might influence the performance of motor cortex neurons; (2) possible multiple representations of the body surface in the component fields of the first somatic sensory area (SI). Apart from vertical interlaminar connections, two types of intracortical connectivity are recognized. The first, regarded as "non-specific," consists of axons spreading out in layers I, III and V-VI from all sides of an injection of isotope; these cross architectonic borders indiscrimininately. They are not unique to the regions studied. The second is formed by axons entering the white matter and re-entering other fields. In these, they terminate in layers I-IV in one or more mediolaterally oriented strips of fairly constant width (0.5--1 mm) and separated by gaps of comparable size. Though there is a broadly systematic topography in these projections, the strips are probably best regarded as representing some feature other than receptive field position. Separate representations are nevertheless implied in area 3b, in areas 1 and 2 (together), in areas 3a and 4 (together) and in area 5; with, in each case, the representations of the digits pointed at the central sulcus. Area 3b is not connected with areas 3a or 4, but projects to a combined areas 1 and 2. Area 1 is reciprocally connected with area 3a and area 2 reciprocally with area 4. The connectivity of area 3a, as conventionally identified, is such that it is probably best regarded not as an entity, but as a part of area 4. Areas identified by others as area 3a should probably be regraded as parts of area 3b. Parts of area 5 that should be more properly considered as area 2, and other parts that receive thalamic input not from the ventrobasal complex but from the lateral nuclear complex and anterior pulvinar, are also interconnected with area 4. More posterior parts of area 5 are connected with laterally placed parts of area 6. A more medial part of area 6, the supplementary motor area, occupies a pivotal position in the sensory-motor cortex, for it receives fibers from areas 3a, 4, 1, 2 and 5 (all parts), and projects back to areas 3a, 4 and 5.

Variability in the terminations of GABAergic chandelier cell axons on initial segments of pyramidal cell axons in the monkey sensory-motor cortex.

Chandelier cell axons were studied in the sensory-motor cortex of adult monkeys. The axonal fields of Golgi-impregnated chandelier cells in layer II in motor cortex are flattened sagittally. The vertical terminal portions of the axons varied both in length and in the numbers converging to form terminations of greater or lesser complexity. Golgi-impregnated plexuses were embedded in plastic and resectioned serially at 2.5-3.0 micrograms. A single axonal field could have as many as 400 terminal rows. All lie 3-13 micrograms beneath pyramidal cell somata. These terminations are not randomly distributed but instead, form clusters. Further resectioning the plastic sections for electron microscopy revealed that all the terminations are on the initial axon segments of pyramidal cells and all form symmetric synaptic contacts. In immunocytochemical material stained for glutamic acid decarboxylase (GAD), the enzyme involved in the synthesis of GABA, GAD-positive boutons were found to form symmetric synaptic contacts with a variety of postsynaptic elements including the axon hillocks and axon initial segments of pyramidal cells. Serial reconstructions from electron micrographs revealed GAD-positive terminals synapsing with the axon initial segment of pyramidal cells joined by cytoplasmic bridges and forming vertically oriented rows identical to those of chandelier cell terminals identified positively in the resectioned Golgi material. The GAD-positive terminals forming initial segment synapses were never continuous with GAD-positive terminals forming axo hillock synapses. The latter probably arise from basket cell axons. Initial segments of pyramidal cell axons in layers II and III were contacted by more GAD-positive terminals than the initial segments of pyramidal cell axons in layer V. The largest pyramidal cells in layer III received the most synapses. Many larger pyramidal cells, identified as callosally projecting cells by the retrograde transport of horseradish peroxidase (HRP), were shown in serial electron micrographs to possess large numbers of initial segment synapses, comparable to those seen in the immunocytochemical material. Serial reconstructions of pyramidal cell axons from axon hillock to the first myelin internode in resectioned Golgi, immunocytochemical and HRP material showed that the number of synapses varied from 2 to 52 for layers II and III and from 2 to 26 for layer V. The number of synapses on the axon hillocks varied from zero to 12. The variability in these terminations may be an important factor in the shaping of the functional properties of the pyramidal cells.

Evidence for coexistence of GABA and dopamine in neurons of the rat olfactory bulb.

Immunoreactivities for gamma-aminobutyric acid (GABA) and the dopamine-synthesizing enzyme tyrosine hydroxylase (TH) were localized ultrastructurally and colocalized at the light microscopic level in neurons of the rat main olfactory bulb. By means of a simultaneous indirect immunofluorescence technique, GABA and TH immunoreactivities were found to coexist in a large number of neurons in the glomerular and external plexiform layers. Virtually all the TH-immunoreactive periglomerular neurons also contained GABA immunoreactivity (GABA-I) while there was an additional number of GABA-immunoreactive periglomerular cells (27%) which did not contain TH immunoreactivity (TH-I). In contrast, the numerous tufted-type neurons in the glomerular and superficial external plexiform layers which contained TH-I did not contain GABA-I. In the external plexiform layer (EPL), 41% of the immunoreactive neurons contained GABA-I alone, 24% contained TH-I alone, and 35% contained both. EPL neurons containing GABA-I only or both GABA-I and TH-I never exhibited tufted cell morphological characteristics and were generally of the short-axon type. Electron microscopic examination of GABA-I and TH-I elements in the glomerular layer detected morphologically similar periglomerular perikarya and intraglomerular processes immunoreactive for each substance and other neurons and processes of the same type containing neither GABA-I or TH-I. These data indicate that the classical neurotransmitters GABA and dopamine coexist in large numbers of neurons in the rat main olfactory bulb including characteristic periglomerular cells and certain other local-circuit neuronal types.

Morphology of synapses formed by cholecystokinin-immunoreactive axon terminals in regio superior of rat hippocampus.

Immunocytochemical and electron microscopic methods were used to examine neurons in regio superior of rat hippocampus displaying cholecystokinin octapeptide-like immunoreactivity. Cholecystokinin-immunoreactive synaptic terminals and somata are found in all layers of regio superior but are most numerous in stratum pyramidale. The vast majority of terminals form symmetric synaptic contacts onto the somata and proximal dendrites of hippocampal pyramidal cells and onto smaller dendrites which may also arise from pyramidal cells. A very small number of cholecystokinin-immunoreactive terminals form synapses that appear asymmetric and contact dendritic shafts or spines. The somata of some pyramidal cells receive symmetric synapses from cholecystokinin-immunoreactive terminals that are joined by cytoplasmic bridges to form parts of pericellular baskets. These and adjacent pyramidal cell somata are also contacted by terminals that are not immunoreactive for cholecystokinin. No cholecystokinin-positive terminals contacted the initial segments of pyramidal cell axons. Cholecystokinin-immunoreactive cells are found in all layers of regio superior. Their somata receive a few symmetric synapses, most of which are formed by terminals not immunoreactive for cholecystokinin. Their dendrites receive a greater number of both symmetric and asymmetric contacts, some of which are immunoreactive for cholecystokinin. We conclude the following: The localization of cholecystokinin immunoreactivity in synaptic terminals contacting the somata and dendrites of hippocampal pyramidal cells is consistent with the suggestion that cholecystokinin acts as a neurotransmitter at these sites and at sites in other parts of the cerebral cortex. Results from the present and previous studies suggest that cholecystokinin-like immunoreactivity may co-exist with gamma-aminobutyrate in some non-pyramidal neurons of regio superior. Cholecystokinin-immunoreactive terminals arise mainly from non-pyramidal cells intrinsic to the hippocampus, one class of which appears to be a type of basket cell.

A correlative electron microscopic study of basket cells and large GABAergic neurons in the monkey sensory-motor cortex.

Large basket cells were identified in Golgi and horseradish peroxidase labeled material from the sensory-motor cortex of adult monkeys. Their morphology was correlated at the light and electron microscopic level with large comparable cells stained immunocytochemically for glutamate decarboxylase. In Golgi-impregnated material these cells have a very large cell body and dendrites that extend through several layers of the cortex with a predominant vertical orientation. The axon is only stained for a few micrometers. The same cells studied electron microscopically in serial sections after gold-toning show very distinctive ultrastructural characteristics: the cell bodies contain a large number of organelles, the nuclei are rounded with homogeneously dispersed chromatin and synapsing onto the somata are many axon terminals, both symmetrical and asymmetrical but the symmetrical type forms 70-80% of the total; dendrites also receive a large number of both symmetrical and asymmetrical synaptic contacts. All the axons of basket cells become myelinated and the Golgi labeling of the initial segments is interrupted at the commencement of the first myelin internode. The axon initial segments receive several symmetrical synaptic contacts in the proximal one-third of their length. The axonal arborization of a basket cell retrogradely labeled in the somatosensory cortex after intracortical injection of horseradish peroxidase was analyzed in detail. The mainly horizontal axonal collaterals of this cell are myelinated for most of their trajectory and have a preferred orientation in the anteroposterior dimension. These axonal collaterals, although very long (more than 1.8 mm), at intervals give rise to a small number of short unmyelinated terminal branches that bear a series of boutons terminaux forming a multi-terminal ending. The multi-terminal endings surround somata and proximal dendrites of pyramidal and non-pyramidal cells. Dense pericellular terminations (baskets or nests) like those drawn by Ramón y Cajal and Marin-Padilla are not formed by the axon of a single basket cell. Thus, basket formations are presumably formed by converging axons from several basket cells. Immunocytochemical material was stained for glutamate decarboxylase, the enzyme involved in the synthesis of gamma-aminobutyrate (GABA). This shows that large glutamate decarboxylase-positive neurons of the same size as those positively identified as basket cells in the Golgi and horseradish peroxidase material have virtually the same morphological characteristics, at both the light and electron microscope levels, as the basket cells.(ABSTRACT TRUNCATED AT 400 WORDS)

A microcolumnar structure of monkey cerebral cortex revealed by immunocytochemical studies of double bouquet cell axons.

Immunocytochemical methods were used to study 28,000 mol. wt calbindin and tachykinin immunoreactivity in the monkey cerebral cortex. Calbindin and tachykinin immunoreactivity give rise to a generally different pattern of staining of cell bodies and terminal-like puncta. However, the staining of long, vertically-oriented bundles of processes--identical to classical double bouquet cell axonal arborizations--is the most prominent feature of the pattern of both calbindin- and tachykinin-immunoreactive staining. These bundles form a widespread and regular columnar system descending from layer II to layers III-V. The bundles are most evident in layer III where, in tangential sections, they have a density of 7-15 bundles/10,000 microns 2 with a center-to-center spacing of 15-30 microns. The distribution of immunoreactive bundles through the cortex is not homogeneous; somatic sensory, auditory, and visual areas display a large number of calbindin-immunoreactive bundles while tachykinin-immunoreactive bundles are only numerous in the auditory areas and in area 18 of the visual cortex. In the motor cortex (area 4) few or no immunoreactive bundles are visualized with either antibody. Correlative light and electron microscope analysis of tachykinin immunoreactive bundles in the primary auditory cortex shows that the tachykinin-positive axons of the bundles form symmetrical synaptic contacts with dendritic shafts (57%) and spines (43%). Frequently, several immunoreactive boutons that arise from the same fiber are seen climbing along the surfaces of vertically-oriented, non-immunoreactive processes which include myelinated and unmyelinated axons and probably glial processes. The same ultrastructural features and a similar synaptic distribution were found in a previous study [DeFelipe et al. (1989) Brain Res. 503, 49-54] of calbindin-positive bundles in the somatic sensory cortex (areas 3a and 1). Despite the virtually identical morphological features of tachykinin- and calbindin-immunoreactive bundles, colocalization studies demonstrate little coexistence of the two antigens in somata and none in the axonal bundles of double bouquet cells. These data suggest that the double bouquet cell is a chemically heterogeneous, but ubiquitous morphological type of cortical interneuron, whose uniquely bundled axonal system, which is probably GABAergic, imposes a fundamental microcolumnar organization upon the cerebral cortex.

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

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

A method for fixation of previously fresh-frozen human adult and fetal brains that preserves histological quality and immunoreactivity.

A method is described that enables fixation of previously fresh-frozen and stored adult and fetal human or animal brains. The method involves fixing during thawing under controlled, cryoprotected conditions and is compatible with good histological quality and the preservation of enzymatic activity and immunoreactivity of many neural antigens. It offers considerable advantages for the storage of large amounts of tissue from which multiple samples can be taken and processed under fixation and other conditions that can be optimized for a variety of methods, many of which may be incompatible if the whole brain is fixed in a single fixative prior to storage.

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

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

Distributions of certain neuropeptides in the primate thalamus.

The distributions of fibers and terminals immunoreactive for somatostatin (SRIF), neuropeptide Y (NPY), substance P (SP) and cholecystokinin octapeptide (CCK), were studied in the diencephalon of cynomolgus monkeys. Immunoreactivity for all 4 peptides is found in extrinsic afferent fibers innervating the dorsal thalamus, ventral thalamus and epithalamus. The distributions of such fibers are more extensive than previously described and include many relay nuclei in their zones of terminations. SP fibers are particularly concentrated in the ventral posteromedial nucleus. All peptides are especially concentrated in fibers in the intralaminar and reticular nuclei. Afferent fibers immunoreactive for each of the 4 peptides approach the thalamus by two pathways. An anterior route is formed by the classical periventricular system ascending from the hypothalamus to the epithalamus. A posterior pathway ascends in the lateral midbrain tegmentum and provides innervation to posterior, intralaminar, and many relay nuclei, plus the ventral thalamus. A basal forebrain pathway, containing SRIF and NPY immunoreactive fibers, enters the thalamus in association with the ansa lenticularis and SP fibers also ascend from the substantia nigra.

Synapses of double bouquet cells in monkey cerebral cortex visualized by calbindin immunoreactivity.

In the monkey neocortex, immunoreactivity for the 28-kDa vitamin D-dependent calcium binding protein (Calbindin) is contained in a set of GABAergic intrinsic neurons whose small size and laminar locations render them very distinct from a second set of GABAergic intrinsic neurons that show immunoreactivity for another calcium-binding protein, parvalbumin. A conspicuous feature of many calbindin-immunoreactive cells is their possession of long, vertically oriented bundles of immunoreactive processes that descend or ascend vertically through several cortical layers. These are components of the radial fasciculi of the cortex and are here shown by correlative electron microscopic immunocytochemistry to consist of both immunoreactive dendrites and unmyelinated axons. The morphology of the bundles and the parent cells indicates that the cells are classical double bouquet cells. The calbindin-positive axons in the radial fasciculi in the present study formed symmetric synapses on unlabeled dendritic shafts (62%) and spines (38%). Despite the close-packed nature of the immunoreactive axons, relatively few terminals of the same axon converged on a single postsynaptic profile. The postsynaptic profiles were identified in certain cases as side branches of pyramidal cell apical and basal dendrites. Mainstem apical dendrites generally did not receive synapses derived from the calbindin-positive axons. These results indicate that double bouquet cells can be distinguished both by their GABAergic character and by their possession of calbindin immunoreactivity. They are probably major contributors to the vertical flow of inhibitory influences across laminae of the cerebral cortex.

Temporal sequence of neurotransmitter expression by developing neurons of fetal monkey visual cortex.

The developing fetal monkey visual cortex was studied immunocytochemically from 110-155 days post-conception in order to localize cell populations immunoreactive (ir) for gamma-aminobutyric acid, Substance P, cholecystokinin-octapeptide, somatostatin, neuropeptide Y, and proenkephalin A peptide (BAM-18). The area 17/18 border and all cortical laminae identified in the adult visual cortex were discernible from the youngest age examined. All ir-cell populations studied were present at each fetal age. However, despite a relatively adult-like cytoarchitecture, all ir-cell populations studied displayed patterns of immunostaining which were unlike those described in adult visual cortex, and showed significant changes in laminar distribution, morphology, and numbers over the time course of gestation examined. Despite the differences in the patterns of immunostaining between the fetal and adult visual cortex, ir-cell populations intrinsic to the developing visual cortex exhibited adult-like combinations of co-localized transmitters and peptides. The developing monkey cortex also contains ir-cell populations, particularly BAM-18-ir cells, which have not been detected immunocytochemically in the adult monkey cortex. Differences between the fetal and the adult ir-cell populations might be accounted for by cell death, morphological transformation, secondary migration or changes in gene expression for neurotransmitters and neuropeptides.

Choline acetyltransferase-immunoreactive neurons in fetal monkey cerebral cortex.

Two monoclonal antibodies to choline acetyltransferase (ChAT) were used to stain the cerebral cortex of fetal monkeys at 110-150 days post-conception. In addition to a small number of immunostained fibers, cells resembling typical non-pyramidal neurons were immunostained in developing layers V and VI and in the subjacent white matter of each area examined (sensory-motor and visual areas). ChAT-immunoreactive neurons have not been described in the cerebral cortex of adult primates, but the present observations indicate such neurons exist in the developing primate cortex.

Distribution of callosal fibers around the hand representations in monkey somatic sensory cortex.

Single or multiunit recording of responses to natural peripheral stimuli was used to map the representations of the upper limb and adjacent body parts in the postcentral gyrus of monkeys in which callosally projecting cells and/or axons had been prepared for labeling by previous section of the corpus callosum or contralateral injection of horseradish peroxidase. Subsequent demonstration of electrode tracks in relation to labeled cells and axons showed that no neurons with receptive fields distal to the elbow lay in callosally connected zones, irrespective of the cytoarchitectonic field or body representation in which they lay.