Synaptic connections, axonal and dendritic patterns of neurons immunoreactive for cholecystokinin in the visual cortex of the cat

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

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

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

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

Examination of the effects of cholecystokinin 26-33 and neuropeptide Y on responses of visual cortical neurons of the cat

Extracellular recordings were made from 160 neurons in area 17 (n = 120) and area 18 (n = 40) of the visual cortex of anesthetized cats. Cells were classified according to their receptive field properties and their intracortical positions were evaluated histologically. Cholecystokinin 26-33, antagonists, (cholecystokinin 27-32, cholecystokinin 27-33 and proglumide), amino acids, neuropeptide Y and solvent vehicle (control), were administered to cells by microiontophoresis (cholecystokinin and neuropeptide Y) or by pressure (neuropeptide Y). The results of the tests with cholecystokinin 26-33 fell into four categories: enhancement (31%), suppression (24%), mixed, i.e. either biphasic responses or dose-related alterations in the direction of effect (20%), and no effect (25%). Enhancements of the visually elicited response were more prevalent in simple (43%) and unimodal/movement-sensitive (34%) cells than in complex (7%) cells. The converse was true for suppressions: 19% of simple cells, 24% of unimodal/movement-sensitive cells, and 31% of complex cells were suppressed. Thirty per cent of the unaffected cells were complex or unimodal/movement-sensitive; only 14% were simple. Cells in layers II-IV were more likely to have firing enhanced than suppressed by cholecystokinin 26-33. The converse was true for cells in layers V and VI, where 50% of responses were suppressed and only 22% were enhanced. Unaffected cells were found predominantly in layer III of areas 17, and the lower part of layer III and layer IV of area 18. Cholecystokinin 26-33 sometimes exerted delayed, response-suppressant effects; it also occasionally elevated responsiveness preferentially within the upper ranges (10-20 degrees/s) of velocity tuning curves. Cholecystokinin 26-33 altered the response-suppressant action of GABA in 11 of 19 visually sensitive cells. The peptide potentiated the visual responsiveness in half of the cells where cholecystokinin 26-33 diminished the GABA-induced suppressions (n = 8). The presumed antagonists either exerted no effect on firing or on cholecystokinin 26-33-induced effects, or had cholecystokinin 26-33-like actions themselves. There was a reversible partial antagonism of the effects of cholecystokinin 26-33 on only two of 11 cells tested. Neuropeptide Y injected by pressure or administered iontophoretically had variable and inconsistent effects on the visually evoked responses of 29 additional neurons from those described above. These effects were indistinguishable from those of the vehicle whether spontaneous activity, magnitude of the visually elicited response, spatial integrity of the RF substructure, orientation or velocity tuning was assessed.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

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.

Subpopulations of GABAergic neurons containing parvalbumin, calbindin D28k, and cholecystokinin in the rat hippocampus

The possible coexistence of calbindin D28k with parvalbumin and of calbindin D28k with cholecystokinin was studied in nonpyramidal cells of the rat dorsal hippocampal formation. Neighbouring Vibratome sections were immunostained either for calbindin D28k and parvalbumin or for calbindin D28k and cholecystokinin. The cells, halved during sectioning, were identified in both sections immunostained for different antigens. The coexistence of calbindin D28k and parvalbumin in the same neuron was rare throughout the hippocampal formation with the exception of stratum oriens of the CA1 region, where 9.6% of the parvalbumin-immunoreactive cells also contained calbindin D28k. In stratum radiatum of the CA3 region, calbindin D28k and cholecystokinin coexisted in 12.5% and 21.2% of the calbindin D28k and cholecystokinin-immunoreactive cells, respectively. In other regions of the hippocampal formation, the two markers coexisted in less than 5% of the cells of either type. The present results demonstrate that calbindin D28k-, parvalbumin- and cholecystokinin-containing nonpyramidal cells represent largely nonoverlapping cell populations and may thus be involved in different inhibitory circuits.

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.

Cholecystokinin innervation of monkey prefrontal cortex: an immunohistochemical study

Knowledge of the circuitry of chemically identified systems in primate prefrontal cortex is limited. Although cholecystokinin is very abundant in prefrontal cortex (Geola et al.: Journal of Clinical Endocrinology and Metabolism 53(2):270-275, 1981; Taquet et al.: Neuroscience 27(3):871-883, 1988), the organization of cholecystokinin-containing structures in primate prefrontal cortex has not been investigated. Using immunohistochemical and retrograde transport techniques, we characterized the cholecystokinin innervation of prefrontal cortex in macaque monkeys. The use of two antibodies directed against different portions of the cholecystokinin molecule revealed that distinct forms of the molecule were differentially localized in the same cortical neurons. These small, nonpyramidal cholecystokinin-positive neurons had a variety of somal morphologies and the density of labeled cells did not differ among cytoarchitectonic regions. Labeled neurons had a distinctive laminar distribution with the greatest density of cells present in layers II-superficial III. Labeled fibers also had a distinctive laminar pattern of distribution that differed from that of the immunoreactive neurons. In granular prefrontal cortex, terminal fields were evident in layers II, IV, and VI, with the greatest density in layer VI. Agranular area 24 exhibited a bilaminar pattern of immunoreactivity with a band in layer II and a very dense terminal field in layers V-VI. A high density of cholecystokinin-binding sites has been found in layers III-IV of prefrontal cortex and other association areas in the monkey; this finding has been attributed to possible cholecystokinin-containing afferents from the thalamus (Kritzer et al.: Journal of Comparative Neurology 263:418-435, 1987). The mediodorsal nucleus of the thalamus is known to be a source of afferents which terminate in layer IV of prefrontal cortex. However, combined retrograde transport and immunohistochemical techniques failed to reveal the presence of cholecystokinin-positive neurons in the mediodorsal nucleus of the thalamus that project to prefrontal cortex. These findings, and other observations, suggest that the terminal field in layer IV is formed by descending axons that arise from cholecystokinin-containing neurons in layers II and superficial III. This study demonstrates that the cholecystokinin innervation of prefrontal cortex has a laminar specific organization that is preserved across cytoarchitectonic regions. This distribution of immunoreactive structures suggests a distinctive role of cholecystokinin in cortical circuitry that is common to every region of prefrontal cortex.

Cortical neurons expressing the cholecystokinin gene in the rat: distribution in the adult brain, ontogeny, and some of their projections

Recent studies of neuronal cholecystokinin (CCK) expression performed with more sensitive techniques have demonstrated that the distribution of the expression of this peptide is more widespread than previously thought. In the present study, hybridization histochemistry was used to map cortical neurons expressing the CCK gene in adult and developing rats. Retrograde tracing with Fluorogold in combination with hybridization histochemistry was used to demonstrate some of the projections of these neurons. Neurons expressing the CCK gene were found in all areas of the neo- and allocortices. They were of several morphological types, including pyramidal neurons, and were found in almost all layers, albeit at different relative numbers and with different levels of expression. Generally, layers II and III, deep layer V, and layer VI had many neurons expressing CCK mRNA. Cortical CCK expression was first detected on the 15th day of gestation in the primordial plexiform layer. Expression developed thereafter in a regular and continuous fashion until an adult-like pattern was present on the 21st day after birth. Cortical neurons containing CCK mRNA were found in almost all the projections studied. Many neurons in both neo- and allocortical areas with cortico-cortical, associational, and commissural pathways contained CCK mRNA. Similarly, numerous corticostriatal neurons contained CCK mRNA; however, only a few corticothalamic neurons expressed CCK mRNA. These results demonstrate that in the rat cortex the distribution of projection neurons expressing CCK is much more widespread than had been previously shown and will stimulate further investigations into the role of CCK in these neurons.

Synaptic connections of neurones identified by Golgi impregnation: characterization by immunocytochemical, enzyme histochemical, and degeneration methods

For more than a century the Golgi method has been providing structural information about the organization of neuronal networks. Recent developments allow the extension of the method to the electron microscopic analysis of the afferent and efferent synaptic connections of identified, Golgi-impregnated neurones. The introduction of degeneration, autoradiographic, enzyme histochemical, and immunocytochemical methods for the characterization of Golgi-impregnated neurones and their pre- and postsynaptic partners makes it possible to establish the origin and also the chemical composition of pre- and postsynaptic elements. Furthermore, for a direct correlation of structure and function the synaptic interconnections between physiologically characterized, intracellularly HRP-filled neurones and Golgi-impregnated cells can be studied. It is thought that most of the neuronal communication takes place at the synaptic junction. In the enterprise of unravelling the circuits underlying the synaptic interactions, the Golgi technique continues to be a powerful tool of analysis.

VIP- and PHI-immunoreactivity in olfactory centers of the adult cat

The purpose of the study was to determine the morphology and distribution of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive (VIP- and PHI-ir) neurons and innervation patterns in the main and accessory olfactory bulb, anterior olfactory nucleus, and piriform cortex of the adult cat. In these centers, VIP- and PHI-immunoreactive material are present in the same neuronal types, respectively, therefore summarized as VIP/PHI-ir neurons. In the main olfactory bulb, the majority of VIP/PHI-ir neurons are localized in the external plexiform layer. These neurons give rise to two or more locally branching axons. They form boutons on mitral and external tufted cell bodies. According to the morphology and location, we have classified these neurons as Van Gehuchten cells. Some VIP/PHI-ir neurons are present in the glomerular layer. They have small somata and give rise to dendrites branching exclusively into glomeruli. We have classified these neurons as periglomerular cells. In the granule cell layer, neurons with long apical dendrites and one locally projecting axon are present. In the accessory olfactory bulb, VIP/PHI-ir neurons are localized in the mixed external/mitral/internal plexiform layer. They represent Van Gehuchten cells. In the anterior olfactory nucleus and piriform cortex, VIP/PHI-ir bipolar basket neurons are present. They are localized mainly in layers II/III. These neurons are characterized by a bipolar dendritic pattern and by locally projecting axons forming basket terminals on large immunonegative cell somata. Because of their common morphological features, we summarize them as the retrobulbar VIP/PHI-ir interneuron population. The PHI-ir neurons display the same morphology as the VIP-ir cells. However, they are significantly lower in number with a ratio of VIP-ir to PHI-ir cells about 2:1 in the main and accessory olfactory bulb and in the anterior olfactory nucleus. By contrast, in the piriform cortex the ratio is about 1:1.

Ultrastructural analysis of somatostatin-immunoreactive neurons and synapses in the temporal and occipital cortex of the macaque monkey

Somatostatin-containing neurons and terminals have been analyzed in monkey temporal and occipital cortex by using light and electron microscopic immunohistochemistry. An antibody against Somatostatin-28, that was shown previously preferentially to label fibers (Morrison et al.: Brain Research 262:344-351, 1983), was utilized. As expected, few cell bodies were labeled. At the electron microscopic level, labeled cells presented a characteristic asymmetric position of the nucleus and very few symmetric or asymmetric synapses on the somatic surface. In all areas examined, somatostatin fibers formed a dense plexus in the most superficial layers (I-upper III). The density of labeled fibers in intermediate (deep III-IV) and deep layers (V-VI) varied considerably among areas. The synaptic relationships of the immunoreactive fibers were analyzed and postsynaptic targets quantified in V1, V2, and the superior and inferior temporal gyrus (STG and ITG, respectively). The synapses formed by somatostatin-labeled boutons were of the symmetric type (type II) and the primary postsynaptic targets were dendritic shafts. No regional differences were found in the distribution of the postsynaptic targets in layers I-upper III. The pattern of synapses in the deep layers was examined in STG. The frequency and distribution of postsynaptic targets was similar to the superficial layers of STG and the other temporal and occipital regions. In intermediate layers of the temporal cortex areas there was an increase in the proportion of synapses on dendritic spines. In a correlated light and electron microscopic analysis we examined synapses made by radial fibers in these regions and found that although the main targets are distal dendritic shafts, almost 40% of synapses were on dendritic spines. We suggest that the radial fibers may originate from a specialized interneuron, previously described as the double bouquet cell, and that this particular subset of somatostatin-containing double bouquet cells is likely to exhibit a very high degree of regional heterogeneity with a preference for association cortices with extensive corticocortical convergence.

Early postnatal development of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive structures in the cat visual cortex

The early postnatal development of neurons containing vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) has been analyzed in visual areas 17 and 18 of cats aged from postnatal day (P) 0 to adulthood. Neuronal types are established mainly by axonal criteria. Both peptides occur in the same neuronal types and display the same postnatal chronology of appearance. Several cell types are transient, which means that they are present in the cortex only for a limited period of development. According to their chronology of appearance the VIP/PHI-immunoreactive (ir) cell types are grouped into three neuronal populations. The first population comprises six cell types which appear early in postnatal life. The pseudohorsetail cells of layer I possess a vertically descending axon which initially gives rise to recurrent collaterals, then forms a bundle passing layers III to V, and finally, horizontal terminal fibers in layer VI. The neurons differentiate at P 4 and disappear by degeneration around P 30. The neurons with columnar dendritic fields of layers IV/V are characterized by a vertical arrangement of long dendrites ascending or descending parallel to each other, thus forming an up to 600 microns long dendritic column. Their axons always descend and terminate in broad fields in layer VI. The neurons appear at P 7 and are present until P 20. The multipolar neurons of layer VI occur in isolated positions and have broad axonal territories. The neurons differentiate at P 7 and persist into adulthood. Bitufted to multipolar neurons of layers II/III have axons descending as a single fiber to layer VI, where they terminate. The neurons appear at P 12 and persist into adulthood. The four cell types described above issue a vertically oriented fiber architecture in layers II-V and a horizontal terminal plexus in layer VI which is dense during the second, third and fourth week. Concurrent with the disappearance of the two transient types the number of descending axonal bundles and the density of the layer VI plexus is reduced, but the latter is maintained during adulthood by the two persisting cell types. Two further cell types belong to the first population: The transient bipolar cells of layers IV, V, and VI have long dendrites which extend through the entire cortical width. Their axons always descend, leave the gray matter, and apparently terminate in the upper white matter. The neurons differentiate concurrently with the pseudohorsetail cells at P 4, are very frequent during the following weeks, and eventually disappear at P 30.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Early postnatal development of cholecystokinin-immunoreactive structures in the visual cortex of the cat

The early postnatal development of cholecystokinin-immunoreactive (CCK-ir) neurons was analyzed in visual areas 17 and 18 of cats aged from postnatal day 0 to adulthood. Neurons were classified mainly by axonal criteria. According to their chronology of appearance neurons are grouped into three neuronal populations. The first population consists of five cell types which appear perinatally in areas 17 and 18. Four of them have axons terminating in layer VI. Neurons with columnar dendritic fields of layers IV and V display a conspicuous dendritic arborization with the long dendrites always arranged parallel to each other. This way they form a vertically oriented dendritic column. The neurons differentiate at around P 2 and are present until the end of the second postnatal week. They disappear possibly by degeneration and cell death. Multipolar neurons of layer VI have long dendrites and axonal domains of up to 800 micron in diameter. Three percent of these neurons send out two axons instead of only one. Neurons differentiate at P 0 and the cell type persists into adulthood. Bitufted to multipolar neurons of layer V constitute a frequent type; 10% of these cells issue two axons. They differentiate at P 2 and the type survives into adulthood. Bitufted to multipolar neurons of layers II/III appear at P 2 and send their axons into layer VI. So, early postnatally an axonal connection from superficial cortical layers to layer VI is established. The cell type persists into adulthood. The fifth cell type of the first population is constituted by the neurons of layer I with intralaminar axons which differentiate at P 2. Although they derive from the early marginal zone, the cell type survives into adulthood. The second population consists of two cell types which appear around the end of the second and during the third postnatal week in areas 17 and 18. Multipolar neurons of layer II have horizontally or obliquely arranged basket axons which, during the second postnatal month, form patches of high fiber and terminal density along the layer I/II border. Neurons with descending main axons issuing horizontal and oblique collaterals of layers II-IV form broad axonal fields. The third population in area 17 is constituted by three cell types: Bitufted neurons with axons descending in form of loose bundles of layers II/III differentiate during the fifth postnatal week. Small basket cells of layers II/III with locally restricted axonal plexuses and somewhat larger basket cells of layer IV appear during the sixth and seventh week.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Synaptic organization of serotonin-immunoreactive fibers in primary visual cortex of the macaque monkey

The macaque neocortex is very densely innervated by serotonin-containing fibers. The highest density of these fibers is in primary sensory regions such as the primary visual cortex. By using an antibody against serotonin, we analyzed the distribution and morphology of serotonin-immunoreactive fibers and synapses in the primary visual cortex of the adult cynomolgus monkey. In addition, we quantified the laminar distribution of labeled varicosities and the distances between varicosities in single fibers. While serotonin-immunoreactive fibers are found in all cortical layers, at least three bands of heightened density of innervation were readily recognized that were coincident with 1) layer IIIB to IVC alpha, 2) layer VA, and 3) layer VIB. Layer IVC alpha of area 17 contained more varicosities per unit area than any other sublayer. There was a high degree of variability in the intervaricosity distances along single fibers; more than half were longer than 10 microns. At the electron microscopic level, synaptic contacts were also observed throughout the entire thickness of area 17, with the highest frequency in layer IV. The labeled varicosities were packed with electron-lucent synaptic vesicles and formed synaptic complexes with small, but conspicuous, post-synaptic densities. Dendritic shafts were the most common postsynaptic target of the labeled synapses. Among these characteristically slender post-synaptic shafts, profiles with structural features of both spiny and smooth dendrites were observed. The small diameter of most of the postsynaptic dendrites indicated that distal dendrites were preferentially contacted by serotonin-immunoreactive varicosities. Although direct identification of the postsynaptic neurons will be required for complete characterization of this circuitry, the distribution of serotonin-immunoreactive varicosities suggests that serotoninergic interactions in the primary visual cortex of the cynomolgus monkey are directed predominantly at the distal dendrites of granular and infragranular neurons rather than at targets in the supragranular layers.

Synaptic connections of an interneuron with axonal arcades in the cat visual cortex

A single, isolated interneuron with axonal arcades in the cat visual cortex was analysed in detail by both light and electron microscopy. The neuron was impregnated by the Golgi-Kopsch method, gold-toned, and processed for electron microscopy using the ethanolic phosphotungstic acid (PTA) staining method of Bloom & Aghajanian (1968). These methods, in combination, resulted in the successful identification of a large number of synaptic boutons arising from the axon of the cell under study. We examined serially at the electron microscope level 210 boutons of the axonal arborization of the cell. Of these, 152 formed identifiable symmetrical synaptic contacts with a variety of postsynaptic elements. The vast majority of the postsynaptic targets were dendritic profiles, which represented 95.7% of all the synaptic contacts identified. Only one example was observed of two labelled boutons making contacts with the same postsynaptic element; the rest were apparently on different elements. This distribution of synapses, characterized by the lack of convergence, is very similar to that reported by other authors for a certain kind of double bouquet cell which, in turn, shares some morphological features with the neurons with axonal arcades. It is suggested that fine details of the geometry of the axonal arborization of a given cell are an important reflection of the distribution of its synapses.

CCK-immunoreactive terminals form different types of synapses in the rat and monkey hippocampus

Electronmicroscopic immunocytochemical analysis of the types and patterns of synaptic contacts formed by cholecystokinin (CCK)-containing terminals in the CA1 and CA3 region of the rat and monkey hippocampus reveals numerous symmetric synaptic contacts on cell bodies and dendritic shafts of pyramidal cells in both species. In the monkey, however, CCK-immunoreactive terminals also form asymmetric synaptic contacts with dendritic spines, such contacts are absent or very rare in the rat hippocampus. The present finding in primate hippocampus provides evidence that the same neuropeptides can be found in both symmetric and asymmetric contacts and may be added to other evidence challenging the traditional concept that symmetric synapses mediate exclusively inhibitory and asymmetric exclusively excitatory transmission. Furthermore, although our comparative analysis confirms considerable similarities in the distribution of CCK-containing elements in primate and rodent hippocampus it also revealed a potentially important difference in synaptoarchitecture that should be taken into account in extrapolations from one species to the other.

Evidence for interlaminar inhibitory circuits in the striate cortex of the cat

An interlaminar, ascending, and GABAergic projection is demonstrated in the striate cortex of the cat. We have examined a basket cell, with soma and smooth dendrites in layers V and VI, that was injected intracellularly with HRP in the kitten. Three-dimensional reconstruction of its axon revealed a horizontal plexus in layer V and upper VI, extending about 1.8 mm anteroposteriorly and 0.8 mm mediolaterally; a dense termination in the vicinity of the soma in layers V and VI; and an ascending tuft terminating in layers II and III in register above the soma and about 250 microns in diameter. Many boutons of this cell contacted neuronal somata and apical dendrites of pyramidal cells and subsequent electron microscopy showed that these boutons formed type II synaptic contacts with these structures. A random sample of postsynaptic targets (n = 199) in layers III, V, and VI showed that somata (20.1%), dendritic shafts (38.2%), and dendritic spines (41.2%) were contacted. The fine structural characteristics of postsynaptic elements indicated that the majority originated from pyramidal cells. Direct identification of postsynaptic neurons was achieved by Golgi impregnation of four large pyramidal cells in layer V, which were contacted on their somata and apical dendrites by between three and 34 boutons of the HRP-filled basket cell. Layer IV neurons were not contacted. Golgi-impregnated neurons similar to the HRP-filled basket cell were also found in the deep layers. The axonal boutons of one of them were studied; it also formed type II synapses with somata and apical dendrites of pyramidal cells. Boutons of the HRP-filled neuron were shown to be GABA-immunoreactive by the immunogold method. This is direct evidence in favour of the GABAergic nature of deep layer basket cells with ascending projections. The existence of an ascending GABAergic pathway was also demonstrated by injecting [3H]GABA into layers II and III. The labelled amino acid was transported retrogradely by a subpopulation of GABA-immunoreactive cells in layers V and VI, in addition to cells around the injection site. The axonal pattern and mode of termination of deep basket cells make them a candidate for producing or enhancing directional selectivity, a characteristic of layer V cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Light and electron microscopical immunocytochemistry of 5'-nucleotidase in rat cerebellum

5'-Nucleotidase in nervous tissue has so far not been localised at the ultrastructural level using immunocytochemical techniques. We have now applied monoclonal antibodies and a polyclonal antiserum raised against this ecto-enzyme and describe the distribution of 5'-nucleotidase antigenicity in rat cerebellum both at the light and electron microscopic levels. Within all cerebellar layers, 5'-nucleotidase immunoreactivity was found on plasma membranes of glial elements, i.e. Bergmann glial cell processes crossing the molecular layer, astrocytic end-feet around blood vessels and glial cell extensions surrounding single Purkinje cells. In the granular layer, 5'-nucleotidase immunoreactivity was present on glial membranes interposed between granule cells. Neuronal cells or processes were devoid of immunoreactivity. The immunocytochemical results were compared with conventional 5'-nucleotidase histochemistry. Both techniques showed the same ecto-localisation of the enzyme and favour the view of 5'-nucleotidase being predominantly situated at glial plasma membranes.

Immunogold demonstration of GABA in synaptic terminals of intracellularly recorded, horseradish peroxidase-filled basket cells and clutch cells in the cat's visual cortex

To identify the putative transmitter of large basket and clutch cells in the cat's visual cortex, an antiserum raised against GABA coupled to bovine serum albumen by glutaraldehyde and a postembedding, electron microscopic immunogold procedure were used. Two basket and four clutch cells were revealed by intracellular injection of horseradish peroxidase. They were identified on the basis of the distribution of their processes and their synaptic connections. Large basket cells terminate mainly in layer III, while clutch cells which have a more restricted axon, terminate mainly in layer IV. Both types of neuron have a small radial projection. They establish type II synaptic contacts and about 20-30% of their synapses are made with the somata of other neurons, the rest with dendrites and dendritic spines. Altogether 112 identified, HRP-filled boutons, the dendrites of three clutch cells and myelinated axons of both basket and clutch cells were tested for the presence of GABA. They were all immunopositive. The postsynaptic neurons received synapses from numerous other GABA-positive boutons in addition to the horseradish peroxidase-filled ones. Dendritic spines that received a synapse from a GABA-positive basket or clutch cell bouton also received a type I synaptic contact from a GABA-negative bouton. A few of the postsynaptic dendrites, but none of the postsynaptic somata, were immunoreactive for GABA. The fine structural characteristics of the majority of postsynaptic targets suggested that they were pyramidal and spiny stellate cells. These results provide direct evidence for the presence of immunoreactive GABA in identified basket and clutch cells and strongly suggest that GABA is a neurotransmitter at their synapses. The laminar distribution of the synaptic terminals of basket and clutch cells demonstrates that some GABAergic neurons with similar target specificity segregate into different laminae, and that the same GABAergic cells can take part in both horizontal and radial interactions.