New pool of cortical interneuron precursors in the early postnatal dorsal white matter.

The migration of cortical γ-aminobutyric acidergic interneurons has been extensively studied in rodent embryos, whereas few studies have documented their postnatal migration. Combining in vivo analysis together with time-lapse imaging on cortical slices, we explored the origin and migration of cortical interneurons during the first weeks of postnatal life. Strikingly, we observed that a large pool of GAD65-GFP-positive cells accumulate in the dorsal white matter region during the first postnatal week. Part of these cells divides and expresses the transcription factor paired box 6 indicating the presence of local transient amplifying precursors. The vast majority of these cells are immature interneurons expressing the neuronal marker doublecortin and partly the calcium-binding protein calretinin. Time-lapse imaging reveals that GAD65-GFP-positive neurons migrate from the white matter pool into the overlying anterior cingulate cortex (aCC). Some interneurons in the postnatal aCC express the same immature neuronal markers suggesting ongoing migration of calretinin-positive interneurons. Finally, bromodeoxyuridine incorporation experiments confirm that a small fraction of interneurons located in the aCC are generated during the early postnatal period. These results altogether reveal that at postnatal ages, the dorsal white matter contains a pool of interneuron precursors that divide and migrate into the aCC.

Excess of serotonin affects embryonic interneuron migration through activation of the serotonin receptor 6.

The discovery that a common polymorphism (5-HTTLPR, short variant) in the human serotonin transporter gene (SLC6A4) can influence personality traits and increase the risk for depression in adulthood has led to the hypothesis that a relative increase in the extracellular levels of serotonin (5-HT) during development could be critical for the establishment of brain circuits. Consistent with this idea, a large body of data demonstrate that 5-HT is a strong neurodevelopmental signal that can modulate a wide variety of cellular processes. In humans, serotonergic fibers appear in the developing cortex as early as the 10th gestational week, a period of intense neuronal migration. In this study we hypothesized that an excess of 5-HT could affect embryonic cortical interneuron migration. Using time-lapse videometry to monitor the migration of interneurons in embryonic mouse cortical slices, we discovered that the application of 5-HT decreased interneuron migration in a reversible and dose-dependent manner. We next found that 5-HT6 receptors were expressed in cortical interneurons and that 5-HT6 receptor activation decreased interneuron migration, whereas 5-HT6 receptor blockade prevented the migratory effects induced by 5-HT. Finally, we observed that interneurons were abnormally distributed in the cerebral cortex of serotonin transporter gene (Slc6a4) knockout mice that have high levels of extracellular 5-HT. These results shed new light on the neurodevelopmental alterations caused by an excess of 5-HT during the embryonic period and contribute to a better understanding of the cellular processes that could be modulated by genetically controlled differences in human 5-HT homeostasis.

Functional N-methyl-D-aspartate receptors in O-2A glial precursor cells: a critical role in regulating polysialic acid-neural cell adhesion molecule expression and cell migration.

The capacity for long-distance migration of the oligodendrocyte precursor cell, oligodendrocyte-type 2 astrocyte (O-2A), is essential for myelin formation. To study the molecular mechanisms that control this process, we used an in vitro migration assay that uses neurohypophysial explants. We provide evidence that O-2A cells in these preparations express functional N-methyl-D-aspartate (NMDA) receptors, most likely as homomeric complexes of the NR1 subunit. We show that NMDA evokes an increase in cytosolic Ca2+ that can be blocked by the NMDA receptor antagonist AP-5 and by Mg2+. Blocking the activity of these receptors dramatically diminished O-2A cell migration from explants. We also show that NMDA receptor activity is necessary for the expression by O-2A cells of the highly sialylated polysialic acid-neural cell adhesion molecule (PSA-NCAM) that is required for their migration. Thus, glutamate or glutamate receptor ligands may regulate O-2A cell migration by modulating expression of PSA-NCAM. These studies demonstrate how interactions between ionotropic receptors, intracellular signaling, and cell adhesion molecule expression influence cell surface properties, which in turn are critical determinants of cell migration.

Murine clusterin: molecular cloning and mRNA localization of a gene associated with epithelial differentiation processes during embryogenesis.

Clusterin is a broadly distributed glycoprotein constitutively expressed by various tissues and cell types, that has been shown to be involved in cell-cell adhesion and expressed during cellular differentiation in vitro. To assess the suggested participation of clusterin in these processes in vivo, we have cloned the cDNA encoding murine clusterin and studied the cellular distribution of clusterin mRNA during murine embryogenesis. Sequence analysis of the cDNA encoding murine clusterin revealed 92 and 75% sequence identity with the rat and human cDNAs, respectively, and conservation of the predicted structural features which include alpha-helical regions and heparin-binding domains. From 12.5 d of development onwards, the clusterin gene is widely expressed in developing epithelia, and selectively localized within the differentiating cell layers of tissues such as the developing skin, tooth, and duodenum where proliferating and differentiating compartments are readily distinguished. In addition, transient and localized clusterin gene expression was detected in certain morphogenetically active epithelia. In the lung, abundant gene transcripts were detected in cuboidal epithelial cells of the terminal lung buds during branching morphogenesis, and in the kidney, clusterin gene expression in the epithelial cells of comma and S-shaped bodies coincided with the process of polarization. Our results demonstrate the in vivo expression of the clusterin gene by differentiating epithelial cells during murine embryogenesis, and provide novel evidence suggesting that clusterin may be involved in the differentiation and morphogenesis of certain epithelia.

PSA-NCAM is required for activity-induced synaptic plasticity.

Hippocampal organotypic slice cultures maintained 10-20 days in vitro express a high level of the polysialylated embryonic form of neural cell adhesion molecule (NCAM) (PSA-NCAM). Treatment of the cultures with endoneuraminidase-N selectively removed polysialic acid (PSA) from NCAM and completely prevented induction of long-term potentiation (LTP) and long-term depression (LTD) without affecting cellular or synaptic parameters. Similarly, slices prepared from transgenic mice lacking the NCAM gene exhibited a decaying LTP. No inhibition of N-methyl-D-aspartic acid receptor-dependent synaptic responses was detected. Washout of the enzyme resulted in reexpression of PSA immunoreactivity which correlated with a complete recovery of LTP and LTD. This reexpression was blocked by TTX and low calcium and enhanced by bicuculline. Taken together, these results indicate that neuronal activity regulates the expression of PSA-NCAM at the synapse and that this expression is required for the induction of synaptic plasticity.

Recruiting new neurons from the subventricular zone to the rat postnatal cortex: an organotypic slice culture model.

The neurogenic subventricular zone (SVZ) of the lateral ventricle is a potential source for neuronal replacement in the postnatal or adult neocortex after injury. Here we present a novel model system to directly explore the cellular mechanisms of this process. In order to visualize directed migration from the SVZ towards the cortex, we transplanted green fluorescent protein-labeled progenitor/stem cells into the SVZ of newborn rats. At 2 days after transplantation, we generated organotypic slice cultures and applied fluorescent time-lapse imaging to explore directly the migration and integration of donor cells into the host tissue for up to 2 weeks. Our studies revealed that subventricular grafts provide a significant number of immature neurons to neocortical regions. In the cortex, immature neurons first migrate radially towards the pial surface and then differentiate into GABAergic interneurons. We conclude that our model system presents a novel and effective experimental paradigm to evaluate the recruitment of SVZ-derived neurons into the postnatal cortex, a phenomenon that may represent a potential route for cortical repair.

Extracellular proteolysis in the adult murine brain.

Plasminogen activators are important mediators of extracellular metabolism. In the nervous system, plasminogen activators are thought to be involved in the remodeling events required for cell migration during development and regeneration. We have now explored the expression of the plasminogen activator/plasmin system in the adult murine central nervous system. Tissue-type plasminogen activator is synthesized by neurons of most brain regions, while prominent tissue-type plasminogen activator-catalyzed proteolysis is restricted to discrete areas, in particular within the hippocampus and hypothalamus. Our observations indicate that tissue-type plasminogen activator-catalyzed proteolysis in neural tissues is not limited to ontogeny, but may also contribute to adult central nervous system physiology, for instance by influencing neuronal plasticity and synaptic reorganization. The identification of an extracellular proteolytic system active in the adult central nervous system may also help gain insights into the pathogeny of neurodegenerative disorders associated with extracellular protein deposition.

Cell biology of polysialic acid.

The unusual carbohydrate polysialic acid (PSA), attached uniquely to neural cell adhesion molecule (NCAM) through a developmentally regulated process, modulates neural cell interactions. Major advances in the past two years have increased our understanding of PSA biosynthesis and regulation. Of particular interest is the cloning of the genes encoding polysialyltransferases (PSTs) and the finding that a single enzyme is able to confer polysialylation to NCAM. The electrical activity of neurons and transmembrane signalling are probably major players in controlling both PSA biosynthesis and its expression at the cell surface. A direct causal relationship between PSA expression and activity-induced synaptic plasticity has been reported.

Progressive neuronal and motor dysfunction in mice overexpressing the serine protease inhibitor protease nexin-1 in postmitotic neurons.

Perturbation of the homeostasis between proteases and their inhibitors has been associated with lesion-induced or degenerative neuronal changes. Protease nexin-1 (PN-1), a secreted serine protease inhibitor, is constitutively expressed in distinct neuronal cell populations of the adult CNS. In an earlier study we showed that transgenic mice with ectopic or increased expression of PN-1 in postnatal neurons have altered synaptic transmission. Here these mice are used to examine the impact of an extracellular proteolytic imbalance on long-term neuronal function. These mice develop disturbances in motor behavior from 12 weeks on, with some of the histopathological changes described in early stages of human motor neuron disease, and neurogenic muscle atrophy in old age. In addition, sensorimotor integration, measured by epicranial multichannel recording of sensory evoked potentials, is impaired. Our results suggest that axonal dysfunction rather than cell death underlies these phenotypes. In particular, long projecting neurons, namely cortical layer V pyramidal and spinal motor neurons, show an age-dependent vulnerability to PN-1 overexpression. These mice can serve to study early stages of in vivo neuronal dysfunction not yet associated with cell loss.

Coexpression of vasopressin and oxytocin in hypothalamic supraoptic neurons of lactating rats.

Magnocellular hypothalamic neurons in the rat supraoptic nucleus (SON) normally produce either vasopressin (VP) or oxytocin (OT). Here we demonstrate that many magnocellular neurons in the SON of lactating rats synthesize both hormones at the same time. We show the colocalization of the messenger (m) RNA that encodes the VP precursor with OT-neurophysin; OT mRNA with VP-neurophysin, the C-terminal glycopeptide of the VP precursor, and VP itself, and the presence of both mRNAs in the same cell. At the light microscopic level quantitative studies show that on the second day of lactation, 17% of the SON neurons produce both hormones, on the fifth day 13%, and on the ninth day 9%. Two days after lactation the number of cells that are positive for both hormones returns to the control level (2-3%). We also show by means of electron microscopic immunohistochemistry that both peptides (or their precursors) are present in the same neurosecretory vesicles in nerve endings in the posterior lobe of lactating rats. At the electron microscopic level quantitative studies show that on the second day of lactation 21% of the terminals contain mixed vesicles; this number increases to 24% by the fourth day and is down to 5% by the 15th day, a level similar to that found in control rats. Since the double-labeled cells seemed to be producing additional VP as opposed to OT, we hypothesized that the former should affect urinary osmolality. Urine samples of lactating rats show a significant (5-fold) increase in urine osmolality during lactation (highest on the second day). The increase in osmolality correlated with the increase in the number of VP positive cells during lactation. We suggest that magnocellular neurons that ordinarily synthesize little or no VP can produce this antidiuretic hormone to help the animal compensate for the loss of water associated with lactation.

The hypothalamic paraventricular nucleus has a pivotal role in regulation of prolactin release in lactating rats.

The affect of paraventricular nucleus (PVN) lesions on PRL secretory response to suckling was studied in adult female rats. Basal levels of PRL were similar in the control and lesioned groups. Substantial decreases in PRL levels occurred after separation of pups from their mothers in the control as well as lesioned animals. When mothers and pups were reunited, the circulating PRL concentrations of the control groups rose immediately from basal values of 50-100 micrograms/liter to reach peaks of 450-550 micrograms/liter. PVN lesions significantly decreased the suckling-induced rise of PRL levels. Furthermore, PVN lesions abolished the high amplitude, episodic pattern of PRL release in continuously lactating rats. These findings are consistent with the view that PVN neurons produce PRL releasing factor(s), which is (are) required for normal secretory patterns of PRL in lactating rats.

Pituitary binding of vasopressin is altered by experimental manipulations of the hypothalamo-pituitary-adrenocortical axis in normal as well as homozygous (di/di) Brattleboro rats.

In the present study we report the properties of vasopressin (VP) receptors in the anterior pituitary gland and show that the number of these receptors is markedly affected by adrenalectomy and hypothalamic lesions. VP-binding activity was assayed in particulate fractions of rat anterior pituitary glands using tritium-labeled arginine VP ([3H] AVP) as tracer. In the presence of Mg2+ the radioligand interacted with a single class of high affinity, low capacity binding sites. Magnesium ions modulated the affinity of the receptors but had no effect on binding capacity. Guanine nucleotides decreased the amount of tracer bound in a dose-dependent manner by increasing the dissociation constant (Kd) of the binding reaction by approximately 2-fold. Increasing the concentration of Mg2+ did not prevent this effect. Bilateral adrenalectomy (ADX) decreased pituitary AVP-binding activity: binding fell by 30% 4 h after surgery and declined further to 10% or less of control at 4 days. The decrease in binding was primarily due to a reduction in the number of receptors. Daily administration of corticosterone inhibited the reduction of binding activity at 4 days in a dose-dependent manner. Destruction of hypophyseotropic VP neurons by means of surgical lesioning of the hypothalamic paraventricular nucleus or the medial basal hypothalamus abolished the effect of ADX on pituitary AVP binding at 24 h but only attenuated the degree of receptor loss at 4 days. Furthermore, the lesions themselves caused a significant (approximately 30%) reduction in receptor number 4-7 days after hypothalamic surgery. Adrenalectomy reduced pituitary AVP-binding activity in homozygous (di/di) Brattleboro rats. The extent as well as the time course of the loss of receptor activity resembled that in normal rats. Rat anterior pituitary segments were exposed to synthetic CRF, AVP, or oxytocin (all 10(-7) M) for 4 h in vitro, and [3H] AVP-binding activity was subsequently determined. Both AVP and oxytocin reduced the amount of radioligand bound, while CRF had no effect. These observations allow the following conclusions: Magnesium ions and guanine nucleotides modulate the affinity of pituitary AVP receptors by different mechanisms and have no effect on binding capacity; Pituitary receptors for AVP are regulated by the amount of AVP released by paraventricular nucleus neurons as well as through a mechanism that requires the presence of corticosterone; Homozygous Brattleboro rats may respond to ADX by increased hypothalamic release of an endogenous ligand for pituitary AVP receptors.

Stress-induced increase in vasopressin and corticotropin-releasing factor expression in hypophysiotrophic paraventricular neurons.

The concerted action of CRF and vasopressin (VP) plays a critical role in regulating ACTH release from anterior pituitary cells. In this study, we have explored the expression of these neurohormones in hypophysiotropic paraventricular neurons after repeated exposure of rats to immobilization stress. Cell by cell quantitative in situ hybridization was used to evaluate the steady state level of mRNAs coding for VP and CRF. We found that 16 daily stress exposures resulted in a significant increase in the average cellular level of CRF and VP mRNAs (150% and 200% of control levels, respectively). Moreover, in the repeatedly stressed group, the number of VP-expressing parvicellular neurons was approximately doubled relative to the control value. Using quantitative immunoelectron microscopy, VP- and CRF-immunoreactive sites were assessed in the dense core vesicle compartment of CRF axon terminals in the external zone of the median eminence. We found that after repeated stress, the immunolabeling of VP was augmented, while that of CRF was slightly decreased. Concurrently, we observed a significant increase in the proportion of CRF nerve terminals that were VP positive (from 50% in controls to 90% in stressed animals). We conclude that the observed changes in CRF neurons may represent a physiological response to increased functional demand and may lead to alterations in the composition of the ACTH-releasing signal.

Glucocorticoid receptor in magnocellular neurosecretory cells.

Quantitative in vitro autoradiography, cytosol receptor assay in punched brain tissue, and immunocytochemistry have revealed that the glucocorticoid receptor is present in the rat supraoptic nucleus (SON). Based on its binding characteristics the receptor appears to be the type II glucocorticoid receptor. With the use of a monoclonal antibody against purified liver glucocorticoid receptor, immunostaining was found in magnocellular neurosecretory neurons in the SON, but not in magnocellular neurons in the paraventricular nucleus. Immunoreactive cells seem to be concentrated in ventral parts of the SON where vasopressin cells were previously shown to be located. One to 2 weeks after bilateral adrenalectomy, there was a substantial decrease in glucocorticoid receptor immunostaining in magnocellular as well as other types of neurons in various brain regions. Administration of synthetic glucocorticoids (RU 28362 or dexamethasone) induced a robust increase in the intensity of immunostaining in cell nuclei of neurosecretory cells. The presence of glucocorticoid receptors in the SON suggests that glucocorticoids may affect vasopressin synthesis or/and secretion through a direct action on magnocellular neurons.

Distribution of tyrosine-hydroxylase (TH)-immunoreactive neurons in the diencephalon of the pigeon (Columba livia domestica).

The distribution of tyrosine-hydroxylase (TH)-immunoreactive cell bodies and fibers in the diencephalon has been investigated with immunohistological techniques in the pigeon. The results suggest that TH is present in a number of morphologically distinct neuronal systems. Preoptic and hypothalamic TH neurons were subdivided into a medial periventricular and a lateral group. The medial group starts with a rostral collection of small cells in the preoptic region. A significantly larger collection of TH neurons occupies the paraventricular nucleus (PVN) (stratum cellulare internum) and mainly consists of large multipolar cells. Further caudally, the main concentration of cells is in the hypothalamic posteromedial and the periventricular regions of the tuberoinfundibular (arcuate) nucleus. No TH neuron was found in the ventral and lateral parts of the tuberoinfundibular region, suggesting that the prominent tuberoinfundibular dopaminergic system described in mammals is absent in the pigeon. This further substantiated by the relative scarcity of TH immunoreactive fibers and varicosities in the neurohemal zone of the median eminence (ME). The caudalmost components of the medial group appear to be continuous with the large population of TH neurons distributed in the midline of the mesencephalon. Tyrosine-hydroxylase-immunopositive cells have not been found in the paraventricular organ. The lateral group consists of TH neurons loosely arranged in the lateral hypothalamus, including regions of the supraoptic nucleus and hypothalamic posterolateral nucleus. Tyrosine-hydroxylase containing neurons vary widely in size, shape, and dendritic arborization in each diencephalic region. However, it is possible to distinguish two main cell types. Small bipolar neurons with two simple arborizing dendrites were concentrated in the medial periventricular system. The second type of cell is large, multipolar with four to five branching dendrites. This latter cell type occurs mainly in the lateral system and in the PVN. Major fiber bundles containing TH immunoreactivity were identified in the lateral and periventricular hypothalamus. The paraventricular organ and the organum vasculosum laminae terminalis contained the densest arborization of fibers and varicosities. In the ME, dense innervation was found in the subependymal layer. Dense arborizations of TH positive fibers and varicosities were located in the septal nuclei and the paleostriatum augmentatum.

Distribution and projections of cholecystokinin-immunoreactive neurons in the hypothalamic paraventricular nucleus of rat.

Analysis of coronal sections from colchicine-treated rat brains reveals that CCK-immunoreactivity (CCK-ir) is present in two distinguishable neuronal systems in the paraventricular nucleus (PVN). More than 60% of these cells were found to be typical parvicellular neurons; the remainder were magnocellular neurons. The magnocellular CCK-ir neurons were concentrated in the medial magnocellular subdivision, while more caudally they formed a ring around a zone of unstained magnocellular neurons. Immunostained parvicellular neurons predominate in medial and periventricular parvicellular subdivisions. The efferent projections of CCK-ir neurons were investigated by looking for retrograde accumulation of CCK-ir in cell bodies after selective knife cuts. A parasagittal cut of the lateral retrochiasmatic area as well as transection of the rostral median eminence resulted in an accumulation of CCK-ir material in a large number of both parvi- and magnocellular neurons. After pituitary stalk lesions, however, increased staining was only seen in magnocellular neurons. It is inferred that the magnocellular (presumed oxytocin-CCK) cells send their axons to the pituitary, whereas axons of CCK-ir parvicellular neurons appear to terminate in the median eminence. After transection of the medial forebrain bundle (MFB), immunostaining increased in a small number of scattered transected fibers proximal to the knife cut and in a few perikarya in the PVN, indicating that very few CCK cells may send descending fibers to the lower brainstem.

Hypothalamic paraventricular nucleus: a quantitative analysis of cytoarchitectonic subdivisions in the rat.

The boundaries of subnuclei of the hypothalamic paraventricular nucleus (PVN) were outlined on the basis of a step by step computer-assisted image analysis of coronal, serial sections of the hypothalamus in rats. The cell size-frequency histogram, cell packing density, and the cell number were determined for each PVN subdivision. Three principal cell types were distinguished: small, short diameter (d) = 6-10 microns; medium-sized, d = 10-13 microns; and large, d = 13-19 microns. None of these cell types occur exclusively in any PVN subdivision. According to the predominant cell type, the PVN can be divided into magnocellular, mediocellular and parvicellular subnuclei. Large cells appear to form one subdivision in which the cell packing density is relatively stable. Although the cell sizes showed a relatively large variation, no definitive subgroups (parts) could be distinguished. On the basis of cell packing density, the parvicellular and mediocellular subnuclei can be further divided. The mediocellular cell group can be divided into a dorsal and a posterior subdivision. The parvicellular zone is subdivided into a periventricular and a large medial subdivision. Based on inhomogeneities in cell packing density, the medial subdivision can be further divided into anterior, dorso-lateral, ventromedial and caudal parts. The estimated total cell number in the PVN (in both sides of the brain) is about 21,500. On average, 19% of this population, i.e., 4,200 neurons, was calculated in the magnocellular (neurosecretory) division. Out of this, only 66%, about 2,700 cells, falls into the category of large cells. There are approximately 5,700 neurons in the posterior and dorsal mediocellular, and about 11,600 in the periventricular and medial parvicellular subdivisions.

Neuronal architecture in the rat central nucleus of the amygdala: a cytological, hodological, and immunocytochemical study.

The organization of neurons in the rat central nucleus of the amygdala (CNA) has been examined by using Nissl stain and immunocytochemical and retrograde tracing techniques. Four main subdivisions were identified on the basis of quantitative analyses of Nissl-stained material: medial (CM), lateral (CL), lateral capsular (CLC), and ventral (CV). An intermediate subdivision (CI), previously described by McDonald ('82), was apparent only in animals that had HRP-WGA injected into the bed nucleus of the stria terminalis. Large populations of neurotensin-, corticotropin-releasing factor (CRF)-, and enkephalin-immunoreactive neurons were present within the lateral divisions (mainly CL), although they were also seen within CM. Somatostatin-immunoreactive neurons were distributed mainly within CL and CM. Within CL, neurotensin- and enkephalin-immunoreactive neurons were more numerous laterally whereas CRF- and somatostatin-immunoreactive neurons were more numerous medially. Substance P-immunoreactive neurons were almost exclusively confined to CM. Only a few cholecystokinin- and vasoactive-polypeptide-immunoreactive neurons were seen in the CNA, and they were observed within CL, CV, and CM. The majority of neurons projecting to the dorsal medulla, hypothalamus, and ventral tegmental area were located within CM, although a significant number of cells were also seen within CL. Efferent projections to the bed nucleus of the stria terminalis were found to arise from neurons located within all subdivisions of the CNA. Thus, the distributional patterns of peptidergic and efferent neurons were not confined to individual cytoarchitectonically- defined subdivisions of the CNA. Rather, the results suggest overlapping medial to the lateral trends. Comparisons with the results of previous studies indicate that peptidergic and afferent terminal distribution patterns are more restricted to individual cytoarchitectonically defined subregions of the CNA. These observations suggest that the detailed cytoarchitecture of the CNA more likely reflects the functional integration of afferents rather than the organization of the CNA output neurons.

Organization of vasotocin-immunoreactive cells and fibers in the canary brain.

The distribution of vasotocin (VT)-immunoreactive neuronal perikarya and fibers in the canary (Serinus canaria) was investigated with immunohistological techniques. The results suggest that most VT-stained cell bodies are located in three diencephalic regions. First, a large number of densely packed neurons are found in the paraventricular nucleus (PVN) and the anterior preoptic nucleus. Neurons here vary widely in size and shape. Small-size rounded neurons and large-size multipolar neurons appear to concentrate in separate subdivisions. Second, a series of loosely organized cell groups of medium- to large-size cells occurs in the lateral parts of the hypothalamus. These aggregates of neurons apparently correspond to subdivisions of the supraoptic nucleus (SON). Third, diffusely distributed, lightly stained cells are found dorsal to the paraventricular nucleus in the dorsal diencephalon. A number of cells of this group seem to be located in the basal septal area and bed nucleus of the stria terminalis. Immunoreactive fibers and varicosities concentrate in brain regions that are associated with neuroendocrine, autonomic, and limbic functions. Axons from the PVN and SON form compact bundles of the hypothalamohypophysial tract in the lateral hypothalamus and then funnel into the internal zone of the medium eminence (ME). Furthermore, a heavy innervation seems to be present in the palisadal, external zone of the ME. A substantial number of fibers appear to leave the PVN toward extrahypothalamic areas. Most extrahypothalamic VT fibers innervate telencephalic and brainstem regions that are thought to be involved in mediation of limbic and autonomic functions. These areas include the lateral and medial septum, the lateral habenula, the substantia grisea centralis, the area ventralis (Tsai), the locus coeruleus, raphe nuclei, the nucleus tractus solitarii, and lateral medulla. In addition, fibers with immunoreactivity for VT innervate structures such as the optic tectum and the nucleus ovoidalis that have been implicated in sensory processing of visual and auditory information. Finally, VT fibers and varicosities occur in centers including the nucleus robustus archistriatalis and nucleus intercollicularis that have been implicated in vocal control.

Contribution of the neural cell adhesion molecule to neuronal and synaptic plasticity.

The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM contribute to many aspects of the development and plasticity of the central nervous system. This includes mechanisms of cell differentiation and migration, neurite outgrowth, establishment of specific patterns of synaptic connections, synaptic plasticity and long-term potentiation. How NCAM and PSA-NCAM contribute to regulate all these different mechanisms remains essentially unknown. Adhesive properties appear to be important, but recent studies also point to possible interactions between NCAM and PSA-NCAM with intracellular signalling cascades that are essential to biological functions. Some of these mechanisms are discussed and a hypothesis is proposed based on the existence of cross-talk between these molecules and signalling pathways mediated by growth factors.