Synchronous oscillatory activity in immature cortical network is driven by GABAergic preplate neurons.

Neurons dissociated from embryonic cerebral rat cortex form a differentiated network of synaptic connections and develop synchronous oscillatory network activity with the beginning of the second week in culture. During an initial phase lasting 3-4 d, synchronous calcium transients can be blocked completely by either CNQX or bicuculline, showing that both glutamatergic and GABAergic neurons are required for the generation of this form of activity. By manipulating dissociation and growth conditions, cultures containing different populations of GABAergic neurons were obtained. These cultures revealed that a distinct population of large GABAergic neurons is a key element in the generation of synchronous oscillatory network activity. A minimal number of two large GABAergic neurons per square millimeter are required for the occurrence of synchronous activity. Changes in the density of all other types of GABAergic or non-GABAergic neurons has no influence on the synchronous activity. Electron microscopic analysis shows that the large GABAergic neurons form an interconnected network. Exceptionally high somatodendritic innervation and extended axonal arborization enable these neurons to collect electric network activity and to distribute it effectively throughout the neuronal network. Additional experiments indicated that most neurons developing in culture to large GABAergic neurons are derived from the primordial plexiform layer and reside in the subplate at the time of birth. We suggest that they function as an integrating element that synchronizes neuronal activity during early cortical development by collecting incoming extrinsic and intrinsic signals and distributing them effectively throughout the developing cortical plate.

Serotoninergic innervation of the ferret cerebral cortex. I. Adult pattern.

We have investigated the serotoninergic innervation of the adult ferret cerebral cortex with immunohistochemical techniques. Distribution pattern of serotoninergic fibers in the ferret neocortex is characterized by a decrease in the density of fibers as one moves from the pial surface towards the white matter. Throughout the entire cerebral cortex, the serotoninergic fibers are very dense within the supragranular layers, especially within layer 1. In contrast, granular and infragranular layers exhibit only a sparse innervation. Although this general pattern of innervation is roughly the same in all cortical areas, significant variations in the fiber density are apparent in different regions. Areas 17, 1, 6, and 8 (primary visual cortex, presumptive somatosensory cortex, presumptive motor cortex, and prefrontal cortex, respectively) are described in more detail to illustrate the diversity of the serotoninergic innervation patterns. The density of innervation is highest in areas 1 and 6, intermediate in area 8, and lowest in area 17. It is noteworthy that while areas 1, 6, and 8 show a marked decrease in fiber density at the boundary between layer 3 and 4, the less strongly innervated area 17 shows a change in density in the transition from layer 2 to layer 3. The types of fibers found within the ferret cortex are similar to those described in other mammalian species. The bulk of the innervation is made by very fine fusiform axons with small ovoid varicosities. In addition to this fiber type, axons with thick round varicosities and some smooth nonvaricose axons were found. The latter types occur in very small numbers within the supragranular layers and mostly in more anterior cortical regions. While the general innervation pattern and the fiber types are similar to those described in the cat cerebral cortex, the pericellular baskets found in the cat cortex (Mulligan and Törk, J Comp Neurol 270:86-110, 1988) are not seen in the ferret.

Serotoninergic innervation of the ferret cerebral cortex. II. Postnatal development.

We have investigated the serotoninergic innervation of the ferret cortex from the day of birth to adulthood with immunohistochemical techniques. Due to the premature birth of ferrets, this period spans the entire generation of cells located within the upper cortical layers and their subsequent migration to their final positions. Already at birth, serotoninergic fibers innervate the developing cortex. This innervation is most dense within the marginal zone, the subplate region, and the lower portion of the cortical plate. As long as cell migration continues, serotoninergic fibers enter the expanding portions of the cortex. Only the region just below the marginal zone where newly arriving cells are added to the cortical plate is not innervated by the ingrowing fibers. When the bulk of cell migration ceases, during the third postnatal week, this gap disappears and the fibers gradually form a continuous innervation from the pia to the ventricle. As the cortex matures, the serotoninergic fibers become successively confined to the upper layers, to generate the adult pattern. In the adult ferret cortex, the highest innervation density is found within layers 1, 2, and 3, with a much sparser innervation within the lower layers (Voigt and de Lima, J. Comp. Neurol. 314:403-414, 1991). The dense innervation in the deep cortical layers is only transient, virtually disappearing toward adulthood. These results suggest that serotoninergic axons innervate cortical layers as soon as newly arriving cells reach their final positions within the cortex. This early innervation lends support to the idea that serotonin may play a role during development of the cerebral cortex.

Cholinergic innervation of the cat striate cortex: a choline acetyltransferase immunocytochemical analysis.

In area 17 of adult cats the morphology, distribution, and synaptology of cholinergic elements were examined by immunocytochemical methods with a monoclonal antibody against choline acetyltransferase (ChAT). ChAT(+) fibers are present throughout the entire depth of the cortex but are particularly dense in layer I. Typically these fibers are very thin and possess numerous irregularly spaced varicosities. Except in layer I and deep layer VI, where the fibers tend to run parallel to the pial surface, they appear to be randomly oriented. At the electron microscope level, immunolabeling was present in unmyelinated fibers of irregular contour and diameter. Most of the ChAT(+) varicose profiles contained mitochondria and round vesicles. Synaptic complexes were relatively infrequent and tended to be of the symmetrical type. They were located mostly on dendritic shafts and only rarely on cell bodies and dendritic spines. Both pyramidal and nonpyramidal cells were found to be innervated by cholinergic afferents. These anatomical data are consistent with the known physiology of acetylcholine in the visual cortex, which indicates that it acts as a modulator of cortical excitability.

The brainstem projection to the lateral geniculate nucleus in the cat: identification of cholinergic and monoaminergic elements.

The pontomesencephalic projection to the dorsal lateral geniculate nucleus (dLGN) of the cat was analyzed by combining retrograde transport of rhodamine-labeled latex spheres and immunohistochemistry. After injections of latex beads into the dLGN, sections of the brainstem were treated immunohistochemically for choline acetyltransferase (ChAT), serotonin (Ser), tyrosine hydroxylase (TH), and dopamine-beta-hydroxylase (DBH). Essentially, six regions in the brainstem contained retrogradely labeled cells: the superior colliculus, the parabigeminal nucleus, the dorsal raphe nuclei, the parabrachial area of the central tegmental field, the marginal nucleus of the brachium conjunctivum, and the nucleus coeruleus. Furthermore, isolated retrogradely labeled cells were present in the central nucleus of the raphe, in the cuneiform nucleus, and in the periaqueductal gray. Most serotoninergic double-labeled cells were found in the medial and lateral divisions of the dorsal raphe nuclei, but a few were also present in the central nucleus of the raphe. In the sections immunostained for ChAT, double-labeled cells were located in the central tegmental field, in the marginal nucleus of the brachium conjunctivum, and in the nucleus coeruleus. In the sections treated for TH and DBH, double-labeled cells showed a similar distribution, and like the ChAT(+) cells, they were located mainly in the central tegmental field, in the marginal nucleus of the brachium conjunctivum, and in the nucleus coeruleus. In these regions the cholinergic and noradrenergic cells that projected to the lateral geniculate nucleus were intermingled, the former predominating rostrally and the latter caudally. The majority of retrogradely labeled cells were located in the region of the central tegmental field in the vicinity of the brachium conjunctivum, and most of these cells were also ChAT-immunoreactive. We, therefore, conclude that the cholinergic projection is the most important of the central core projections ascending to the dLGN.

The serotoninergic fibers in the dorsal lateral geniculate nucleus of the cat: distribution and synaptic connections demonstrated with immunocytochemistry.

The distribution, morphology, and synaptic contacts of serotoninergic fibers were studied with immunocytochemical methods in the lateral geniculate complex of the cat. The serotonin-immunoreactive fibers are diffusely distributed throughout the main laminae of the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN) and reach a particular density in the ventral lateral geniculate nucleus (vLGN). The labeled fibers are in most cases very thin and sometimes varicose. There is no obvious order in their distribution pattern except that they sometimes partially encircle the unlabeled cell bodies of the dLGN. The synaptic connections of the serotoninergic fibers were investigated mainly in the A laminae of the dLGN. Few synaptic complexes were found, most of them with asymmetric morphology. The postsynaptic elements were small dendritic profiles. Perisomatic serotoninergic fibers were seen, but no convincing synaptic contacts were found between labeled fibers and cell somata. In the dLGN, serotoninergic profiles were almost exclusively confined to the extraglomerular neuropile. In the PGN serotoninergic fibers also contacted dendritic profiles and formed asymmetrical synapses, but as in the geniculate, synaptic specializations were very rare.

Identification of two distinct populations of gamma-aminobutyric acidergic neurons in cultures of the rat cerebral cortex.

Two types of neurons containing gamma-aminobutyric acid (GABA) were identified in cultures of embryonic rat neocortex. Large GABAergic neurons were already present 4 hours after plating, whereas small ones appeared later. Both types were shown to be neurons by double labeling with GABA and microtubule-associated protein 2 (MAP2) immunocytochemistry. The large GABAergic neurons represented less than 5% of the adherent cells, developed neurites rapidly, and progressed synchronously through the polarization and differentiation steps characteristic of the whole neuronal population. During the second week in culture, these GABA-immunoreactive cells developed into large, stellate neurons with fairly homogeneous morphology and poorly ramified, straight dendrites. At the same time, the GABAergic neuropil increased greatly, and neurites of GABAergic neurons showed advancing maturity and smoothness. The axon of each cell covered extensive areas of the culture, frequently encircling the somata of unlabeled neurons in a basket-like fashion. Significant numbers of small GABAergic cells developed only in the absence of the mitotic inhibition routinely used to control glial proliferation. These late-born GABAergic neurons went through neuritogenesis when most of the other neurons were already forming synapses on their somatodendritic surfaces. In mature cultures, they had a multipolar or fusiform morphology with spine-bearing dendrites. They had small somata and were often present inside clusters of neurons. Their short axons showed no obvious basket-like pattern of arborization. Thus, the two types of GABAergic neurons identified in cortical cultures differed in their morphology, distribution, and developmental history. We propose that intercellular interactions during early synaptogenesis may play a role in the development of different morphological types of GABAergic neurons in vitro.

Synaptophysin immunohistochemistry reveals inside-out pattern of early synaptogenesis in ferret cerebral cortex.

Synaptogenesis in the ferret cerebral cortex was examined from the day of birth to adulthood with an antibody against synaptophysin at the light and electron microscopic levels. Due to the premature birth of ferrets, the generation of cells destined to the upper cortical layers and their subsequent migration to their final positions in the cortical plate are largely postnatal events. Throughout the newborn ferret cerebral cortex, a high amount of synaptophysin immunoreactivity was present within the marginal zone and subplate region. Staining was also conspicuous within the forming cortical plate. The typical layering pattern of synaptophysin immunoreactivity in the developing cortical plate correlated with the migration pattern of cortical neurons. The synaptic density was lowest directly below the marginal zone, where the youngest neurons just stopped their migration. Below this zone, the density of the synaptic staining increased gradually toward lower (and older) cortical plate layers. As the cortex expanded, the synaptophysin immunoreactivity pattern closely followed the expansion, suggesting that synapses were formed in a given layer shortly after the cells migrating to this layer reached their final position. As soon as cell migration had finished, the entire cortical plate contained dense synaptophysin immunoreactivity, in a pattern similar to that observed in the adult animal. During cortical development, a rostrocaudal and a laterodorsal gradient of synaptogenesis was observed. At any given time, rostral and lateral regions of the cerebral cortex were more advanced in their development than caudal and dorsal regions. Electron microscopic examination of synaptophysin immunoreactivity in the developing cerebral cortex of ferrets confirmed that labeling was solely associated with synaptic vesicles. These vesicles were typically, but not exclusively, confined to synaptic boutons. Especially around the end of the first postnatal week, long fiber profiles loaded with synaptic vesicles were occasionally detected. As some of these fibers also showed en passant synapses along their course, we concluded that synaptic vesicle labeling may be reliably used to study synaptogenesis at the light microscopic level. A systematic analysis of samples from postnatal days 0 and 7 corroborated this conclusion, showing that synaptic profile distribution completely matched the distribution of synaptophysin immunoreactivity seen in the light microscope. In conclusion, synaptogenesis begins as soon as migratory cells reach their final position in the cortical plate. As long as cell migration continues, synaptogenesis is under the constraints of neurogenesis, following its gradients.

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.

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.

Neuritic differentiation and synaptogenesis in serum-free neuronal cultures of the rat cerebral cortex.

To better understand the dynamics of the cellular processes involved in early neocortical development, we studied the neuritic differentiation and synaptogenesis of dispersed neurons grown in serum-free cultures under a wide variety of culture conditions. Microtubule-associated protein (MAP2), phosphorylated neurofilament (SMI 31) and synaptophysin immunocytochemistry was complemented with time-lapse studies. During the first week in vitro dissociated cortical neurons developed from roundish cells without processes to neurons with axons and differentiated dendrites, going through five distinct phases. The sequence of these phases was unaltered in a wide range of culturing methods, but the timing of the steps varied among cultures started with different cell densities. Synaptic terminals were first observed after 3-4 days in vitro, coincident with the beginning of dendritic differentiation. Synaptogenesis progressed at least until the end of the third week in vitro, despite a decline in cell density during the second week in vitro. The process of cellular differentiation of cerebral cortical neurons in vitro resembled the development of these cells in the intact tissue, suggesting that organized cell migration is not a prerequisite for the differentiation of single cortical neurons.

Direct synaptic connections of axons from superior colliculus with identified thalamo-amygdaloid projection neurons in the rat: possible substrates of a subcortical visual pathway to the amygdala.

The aim of the present study was to identify synaptic contacts from axons originating in the superior colliculus with thalamic neurons projecting to the lateral nucleus of the amygdala. Axons from the superior colliculus were traced with the anterograde tracers Phaseolus vulgaris leucoagglutinin or the biotinylated and fluorescent dextran amine "Miniruby." Thalamo-amygdaloid projection neurons were identified with the retrograde tracer Fluoro-Gold. Injections of Fluoro-Gold into the lateral nucleus of the amygdala labeled neurons in nuclei of the posterior thalamus which surround the medial geniculate body, viz. the suprageniculate nucleus, the medial division of the medial geniculate body, the posterior intralaminar nucleus, and the peripeduncular nucleus. Anterogradely labeled axons from the superior colliculus terminated in the same regions of the thalamus. Tecto-thalamic axons originating from superficial collicular layers were found predominantly in the suprageniculate nucleus, whereas axons from deep collicular layers were detected in equal density in all thalamic nuclei surrounding the medial geniculate body. Double-labeling experiments revealed an overlap of projection areas in the above-mentioned thalamic nuclei. Electron microscopy of areas of overlap confirmed synaptic contacts of anterogradely labeled presynaptic profiles originating in the superficial layers of the superior colliculus with retrogradely labeled postsynaptic profiles of thalamo-amygdaloid projection neurons. These connections may represent a subcortical pathway for visual information transfer to the amygdala.

Morphology of the cells within the inferior temporal gyrus that project to the prefrontal cortex in the macaque monkey.

The primate neocortex possesses an extraordinary degree of regional specialization. Virtually all cortical functions are dependent upon a complex system of reciprocal connections between related cortical regions that allow for distributed information processing. Although some aspects of the organization of these corticocortical projections are understood, little is known about the morphology and afferents to the cells of origin of long corticocortical projections in primates. We combined intracellular injection of Lucifer Yellow (LY) in fixed tissue with in vivo retrograde transport of fast blue to study the dendritic morphology of neurons within the inferior temporal gyrus (ITG) and the superior temporal sulcus (STS) that furnish corticocortical projections to the prefrontal cortex. The fast blue retrogradely labeled cells formed two clearly defined bands within the inferior temporal cortex: a supragranular band that corresponded to layer III, and an infragranular band that corresponded to layers V and VI. After Lucifer Yellow intracellular filling, these retrogradely labeled cells projecting to the prefrontal cortex were found to be morphologically very heterogeneous. Although all filled cells had spiny dendrites, they presented a wide range of cell body sizes and dendritic tree morphologies. In layer III, the majority of cells were typical pyramids of various sizes. In layers V-VI, numerous typical pyramidal cells were present. In addition, significant numbers of modified pyramidal forms were found, including vertical and horizontal fusiform cells, asymmetrical pyramids and multipolar cells. The entire dendritic arbor of individual subtypes in layers III, V, and VI was restricted to a few cortical layers, but as a group these cells had dendrites spanning the whole cortical depth. We suggest that corticocortically projecting cells are distinct from subcortically projecting cells and consist of a defined set of morphological and functional subgroups, each of which is driven by a distinct set of afferents and likely possesses different response properties.

Horseradish peroxidase labeling of cat tectotectal cells.

By means of unilateral intracollicular injections of horseradish peroxidase (HRP), the cells of origin of the tectotectal projection in cats were detected in the opposite colliculus. The 9 injected animals were divided into two groups in accordance to the location of their injections along the collicular rostrocaudal axis. The majority of the labeled cells were obtained from two injections performed at the level of the commissure of the superior colliculus (231 cells) whereas 5 injections performed at lower levels led to labeling of 49 cells. In two animals, the injections did not cause labeling of cells, a fact that was attributed to the superficiality of the injections. Concerning the distribution of the labeled cells over the collicular layers and along the collicular rostrocaudal axis, there was no difference between the two groups of animals. Among the 280 labeled cells, 1 cell (0.35%) was found in stratum griseum superficiale, 83 cells (29.64%) in stratum opticum, 182 cells (65.00%) in stratum griseum intermediale and 14 cells (5.00%) in stratum griseum profundum. Ninety-two per cent of the cells were located in the rostral portion of the colliculus, in the region between the collicular rostral tip and the collicular rostral plane that corresponds to the vertical meridian representation. Eight per cent of the cells were concentrated in a zone comprising the lower half of stratum opticum and the upper half of stratum griseum intermediale, a location that corresponds to the transition between the visual-sensory and the visual-motor colliculus. On the basis of laminar segregation of collicular functions, the possibility that the tectotectal projection could be related to visual orienting, following and attentive collicular functions through the suppression of activity (a non-enhancement effect?) of homologous cells in the opposite colliculus is discussed.

Axonal connections of thalamic posterior paralaminar nuclei with amygdaloid projection neurons to the cholinergic basal forebrain in the rat.

Stimulation of the amygdala elicits cortical activation mediated by the corticopetal basal forebrain. An unresolved question is whether the involved amygdala neurons that project to the basal forebrain receive direct thalamic input. Using combined retrograde and anterograde tracing techniques, we demonstrate a monosynaptic contact between thalamic posterior paralaminar nuclei axons and neurons in the central amygdala that project to corticopetal cholinergic basal forebrain regions. These contacts may constitute the morphological substrate for the induction of fast cortical arousal and attention triggered by emotional events.

Regulation of early spontaneous network activity and GABAergic neurons development by thyroid hormone.

Early in development spontaneous activity modulates survival and connectivity of neurons and thus plays a crucial role in the formation of neural networks. The emergence of synchronous activity in cultured neocortical networks initially is driven by large GABAergic interneurons. Here we studied the impact of thyroid hormone on early network development and especially on the development of large GABAergic neurons. Triiodothyronine enhances the frequency of early spontaneous synchronous network activity and an overall increase in network connectivity is indicated by the increased density of glutamatergic and GABAergic synapses. The hormone-induced increase of activity parallels cell type-specific changes in neuronal soma size and cell density, with strong effects on somatic and axonal growth of large GABAergic interneurons. Interestingly, large GABAergic neuron growth is both activity- and hormone-regulated. Blocking neuronal activity by tetrodotoxin or the glutamate receptor blockers D-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione disodium reveals a direct contribution of triiodothyronine to somatic growth, which also precedes the formation of synchronous network activity. The hormone-mediated effects on spontaneous activity and on large GABAergic neurons growth can be blocked by the nuclear thyroid hormone receptors antagonist 1-850. Thus, our data suggest that triiodothyronine actions result in functional maturation of early cortical networks and cell type-specific structural alterations. The increase in spontaneous activity might initially follow the growth of the large GABAergic neurons, which show an exquisite sensitivity to the presence of thyroid hormones. For the most part, however, the hormone-mediated growth of the GABAergic neurons relies strongly on the maturation of glutamatergic synaptic activity.

Astroglia inhibit the proliferation of neocortical cells and prevent the generation of small GABAergic neurons in vitro.

We quantitatively studied the dynamics of rat neocortical precursor proliferation in vitro, and additionally examined the effects of neuron-glia interactions on the proliferation and differentiation of neurons, and particularly of gamma-aminobutyric acid (GABA)-containing cells. In cultures grown on glia-free substrate, cellular proliferation was detected at least until the end of the second week in vitro, but most neurons which expressed detectable amounts of microtubule-associated protein at 12 days in vitro were generated early during the first week. Further double-labelling experiments, combining 5'-bromo-2'-deoxyuridine with GABA or beta-tubulin III immunohistochemistry, provided direct evidence that neuronal proliferation continued through the second week in vitro, and that a population of small GABAergic neurons was generated between 3 and 12 days in vitro. Culturing cells on a glial substrate significantly reduced the generation of small GABAergic cells and strongly inhibited the total cell proliferation. Inhibition also occurred if astrocytes were added to the culture after 6 days in vitro, but was significantly decreased if cells were grown on a fixed glial substrate, suggesting that the effect might be at least partially mediated by active interactions between neurons and glia. In conclusion, our results show that the sustained proliferation of precursor cells in neocortical cultures is necessary for the differentiation of small GABAergic neurons, and that mature astroglia effectively inhibit the proliferation of neocortical precursors thereby affecting the appearance of a population of GABAergic cells.

The cholinergic innervation of the visual thalamus: an EM immunocytochemical study.

In this study we demonstrate at the ultrastructural level that both the dorsal lateral geniculate nucleus (dLGN), the visual relay of the thalamus, and the perigeniculate nucleus (PGN), the visual segment of the thalamic reticular nucleus (TRN), are densely innervated by fibres with Choline-Acetyl-Transferase (ChAT) like immunoreactivity. These axons make synaptic contacts with interneurones considered to be inhibitory, both in the PGN and within the synaptic glomeruli of the dLGN. In addition, Chat positive terminals form intra- and extraglomerular synapses with dendrites thought to arise from relay cells. We interpret these results as evidence for direct cholinergic modulation of both relay cells and inhibitory interneurones.