Expression of highly polysialylated NCAM in the neocortex and piriform cortex of the developing and the adult rat

Expression of polysialylated N-CAM in the central nervous system of adult canaries and its possible relation to function

Polysialylated neuronal cell adhesion molecule (PSA-N-CAM) is a cell surface molecule associated with neurons that undergo changes in configuration or spatial translocation. In both cases, this molecule is thought to reduce the adhesivity of these cells or of their processes, which can thereby insinuate themselves into the existing parenchyma. We used a monoclonal antibody specific to PSA to offer what we believe is the first account of the distribution of PSA-N-CAM in the adult songbird brain. This antibody stained a diversity of cell classes and processes, as follows: 1) a subset of ventricular zone cells; 2) migrating cells thought to be neuroblasts; 3) a subset of differentiated neurons; 4) some brain surface astrocytes; 5) some tanycytes; 6) the neuropil of some regions; 7) some axonal fibers; and 8) possibly some synapses. Our results demonstrate also, for the first time, the wide distribution of a very numerous population of migrating cells in the telencephalon and the seasonal regulation of PSA-N-CAM expression in a part of the adult brain known to undergo seasonal changes in cell recruitment and function. However, we did not find PSA-N-CAM associated with young migrating cells in the high vocal center (HVC), nor was there PSA-N-CAM in the robust nucleus of the archistriatum (RA), which is known to receive new axonal endings from HVC. In these instances spatial translocation may occur with the assistance of other surface molecules.

Polysialylation as a regulator of neural plasticity in rodent learning and aging

Although generally accepted to play an important role in development, the precise functional significance of NCAM remains to be elucidated. Correlative and interventive studies suggest a role for polysialylated NCAM in neurite elaboration. In the adult NCAM polysialylation continues to be expressed in regions of the central nervous system which retain neuroplastic potential. During memory formation modulation of polysialylation on the synapse-enriched isoform of NCAM occurs in the hippocampus. The polysialylated neurons of this structure have been located at the border of the granule cell layer and hilar region of the dentate and their number increases dramatically during memory consolidation. The converse is also true for a profound decline in the basal number of polysialylated neurons occurs with ageing when neural plasticity becomes attenuated. In conclusion, it is suggested that NCAM polysialylation regulates ultrastructural plasticity associated with synaptic elaboration.

Expression of neural cell adhesion molecules, NCAMs, and their polysialylated forms, PSA-NCAMs, in the developing rat pituitary gland

Neural cell adhesion molecules (NCAMs) can undergo post-translational modifications, such as the addition of polysialic acid chains, thus generating PSA-NCAMs, which are expressed mainly during development. Since polysialylation considerably modifies NCAM adhesivity, expression of NCAMs and PSA-NCAMs has been investigated in the developing hypophysis by immunohistochemistry. At embryonic day 13 (E13), an antibody against NCAM outlined all cellular profiles in the entire Rathke's pouch; this labelling persisted until adulthood. NCAM expression increased in all lobes during development and concerned all pituitary cell types. In contrast, at E13, PSA-NCAMs were only detected in the neural lobe, solely constituted of pituicytes at this stage, and the tuberal lobe, the only lobe expressing hormonal mRNA at the same stage. PSA-NCAMs expression increased in the neural lobe at E17 with the arrival of the neurosecretory fibres and persisted into adulthood. In the anterior lobe, PSA-NCAMs appeared at E15 where their distribution was similar to that of the differentiating corticotrophic cells; at subsequent stages, their expression extended to the whole anterior lobe. Only two cell types, corticotrophic and somatotrophic cells, remained labelled in the adult gland. In the intermediate lobe, melanotrophic cells never expressed PSA-NCAMs but these were expressed on folliculo-stellate cells at birth, preceding the onset of innervation. These results suggest that NCAMs and PSA-NCAMs play a role in pituitary histogenesis, cell differentiation and neurointermediate lobe innervation.

Straying phenomenon of migrating LHRH neurons and highly polysialylated NCAM in the chick embryo

The present study on unilaterally incompletely placodectomized chick embryos revealed that arrival of migrating LHRH neurons into the forebrain area is dependent on the presence of a central projection of the olfactory nerve. When a fragment of the lateral and medial olfactory epithelium was spared the damage, a small number of LHRH-ir cells were found to migrate into the forebrain along a thin NCAM-H-expressing fiber bundle of the olfactory nerve. In embryos with large lesions which destroyed the lateral olfactory epithelium, the poorly developed NCAM-H-positive olfactory nerve fibers were arrested where they meet the NCAM-positive medial nasal branch of the ophthalmic nerve and frequently fused with this branch fiber bundle. In these embryos, no LHRH-ir cells were detected in the forebrain area. They were deviated from their regular migratory course to the NCAM-H-positive medial nasal branch fiber bundle. This straying phenomenon indicates that some structural support is apparently needed for the migration of LHRH-ir cells, but their migratory route is not completely programmed in their precursor cells in the olfactory placode. In cultures of placodal tissues, the coexistence of migrating LHRH-ir cells with NCAM-H-expressing neural elements was always confirmed. It is suggested therefore that not only the structural support for the migration but also the interaction between LHRH-ir cells and NCAM-H-expressing neural elements is a prerequisite for successful LHRH neuronal migration.

Expression of the neural cell adhesion molecule (NCAM) and polysialic acid during taste bud degeneration and regeneration

Taste receptor cells are replaced throughout life, accompanied by continuing synaptogenesis between newly formed taste cells and first-order gustatory fibers. The neural cell adhesion molecule (NCAM) is expressed by a subset of taste cells in adult rodents and appears on gustatory nerve fibers during development prior to differentiation of the taste buds. We employed antibodies against the extracellular domain of the NCAM polypeptide (mAb 3F4) and against polysialic acid (PSA) residues found on embryonic forms of NCAM (mAb 5A5) to investigate the relationship between the expression of these molecules and the innervation of taste buds in adult rats. In unoperated rats, anti-NCAM recognized a subset of cells within the vallate taste buds and also the fibers of the glossopharyngeal (IXth) nerve, including those innervating the gustatory epithelium. Taste bud cells did not express PSA but mAb 5A5 immunoreactivity was observed on some fibers of the IXth nerve, including a few that entered the taste buds. Bilateral crush of the IXth nerve resulted in the loss of NCAM expression from the gustatory epithelium within 8 days. As IXth nerve fibers reinnervated the epithelium, NCAM expression was seen first in the nerve, followed by increased expression in the epithelium as the taste cells differentiated from their precursors. PSA expression by fibers of the IXth nerve did not return to normal until well after the regeneration of the vallate taste buds. The present results demonstrate that taste cell expression of NCAM is dependent upon innervation by the IXth nerve and that NCAM expression appears in the nerve prior to its expression in the differentiating epithelium during regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of polysialylated neural cell adhesion molecule by proliferating cells in the subependymal layer of the adult rat, in its rostral extension and in the olfactory bulb

The highly sialylated isoform of the neural cell adhesion molecule is thought to be expressed predominantly in the developing nervous system, where it is implicated in a variety of dynamic events linked to neural morphogenesis. It has become increasingly evident, however, that this "embryonic" neural cell adhesion molecule isoform continues to be expressed in certain adult neuronal systems, and in particular, in those that can undergo structural plasticity. In the present study, we performed light microscopic immunocytochemistry with an antibody specific for polysialylated neural cell adhesion molecule and confirmed our earlier observations [Bonfanti L. et al. (1992) Neuroscience 49, 419-436] showing polysialylated neural cell adhesion molecule-immunoreactive cells in the subependymal layer of the lateral ventricle of the adult rat, a region where cell proliferation continues into the postnatal period. In addition, we used an antibody raised against the proliferating cell nuclear antigen and found that proliferating cells continue to be visible in this area, even in the adult. Double immunolabeling showed that many of these newly generated cells displayed high polysialylated neural cell adhesion molecule immunoreactivity. Cells from a portion of the subependymal layer migrate to the olfactory bulb and contribute to the continual replacement of its granule neurons [Luskin M. B. (1993) Neuron 11, 173-189]. We found polysialylated neural cell adhesion molecule-immunoreactive cells all along the pathway purported to be followed by the newly generated cells to their final destination and in neurons corresponding to granular and periglomerular cells in the olfactory bulb. Our present observations thus support the contention that polysialylation is a feature of neurons capable of dynamic change and may contribute to the molecular mechanisms permitting cell proliferation and migration not only during development but also in the adult.

Cell adhesion molecules in the early developing mouse retina: retinal neurons show preferential outgrowth in vitro on L1 but not N-CAM

Both L1 and N-CAM are present on optic axons early in the developing mouse retina and optic nerve. In in vitro assays on substrates of purified cell adhesion molecules cells derived from E13 mouse retinae showed vigorous neurite extension on L1 but not on N-CAM. Although retinal neurons on N-CAM showed only limited attachment to the substrate, they were able to form lamellipodia immediately around the cell perimeter. In contrast, similarly derived cortical cells showed extensive neurite outgrowth on both substrates. Under these culture conditions, nearly all of the L1 and N-CAM present in the cell membrane appeared to be sequestered on the lower surface of the growth cones and neurites, indicating that most of these cell adhesion molecules were involved in homophilic interactions. Our results suggest differential roles for L1 and N-CAM in initiation and establishment of the optic pathway.

Pattern of expression of highly polysialylated neural cell adhesion molecule in the developing and adult rat striatum

In rats, morphological and synaptic maturation of the striatum, a brain area involved in the control of movement and in cognitive behaviour, proceeds for several weeks postnatally. Little is known, however, about the molecular events associated with the final maturation of the striatum. In particular, there is little information on molecules playing a role in cell adhesion, a phenomenon of particular importance for neuronal development. We have examined the time course and topography of expression of the highly polysialylated form of the neural cell adhesion molecule in the rat striatum during postnatal development and in the adult, and compared it to growth-associated protein-43, a marker of axonal growth. As earlier during development [Aaron L. I. and Chesselet M.-F. (1989) Neuroscience 28, 701-710], immunolabelling for polysialylated neural cell adhesion molecule was very intense in the entire striatum at postnatal days 17-19. At postnatal days 21 and 22, loss of polysialylated neural cell adhesion molecule immunoreactivity in the caudal part of the striatum contrasted with the persistence of immunoreactivity at more rostral levels. Most of the striatum was devoid of polysialylated neural cell adhesion molecule immunoreactivity by postnatal day 25. At this age, as well as in the striatum of adult rats, immunolabelling was only observed along the ventricular edge of the striatum. In contrast to polysialylated neural cell adhesion molecule immunoreactivity, immunolabelling for growth-associated protein-43 had reached its adult pattern by postnatal day 17, indicating that polysialylated neural cell adhesion molecule persists beyond the period of major axonal growth. In the adult, an area of stronger growth associated protein-43 immunoreactivity overlapped with the region which retained immunoreactivity to polysialylated neural cell adhesion molecule. The results indicate that, in the developing rat striatum, the neural cell adhesion molecule remains highly sialylated not only during the ingrowth of cortical and nigral inputs but also during the formation of dendritic spine and synaptogenesis. Loss of polysialyated neural cell adhesion molecule occurs at the time of emerging spontaneous activity in cerebral cortex, and precedes the development of mature responses to cortical stimulation and adult membrane properties in a majority of striatal neurons.

Localization of c-fos, c-jun, and hsp70 mRNA expression in brain after neonatal hypoxia-ischemia

The sites of expression of early response mRNAs were determined in the brains of 7-day-old rat pups exposed to unilateral carotid artery ligation followed by 3 h of hypoxia. Pups were sacrificed after recovery periods ranging from 10 min to 24 h. In agreement with our previous northern blot analysis, in situ hybridization of coronal brain sections to probes for c-fos, c-jun, and heat-inducible hsp70 revealed a marked induction and subsequent disappearance of all three mRNAs during this time period. We observed co-localization of the 2 immediate early gene (IEG) mRNAs, c-fos and c-jun, which encode proteins that act in combination to regulate subsequent gene expression. These mRNAs were expressed in all regions known to be vulnerable to permanent injury in this model, such as the cortex, hippocampus, and striatum, as well as in other regions that are spared from permanent damage, such as contralateral cortex and lateral ventricular neuroepithelium. The temporal and regional co-localization of c-fos and c-jun suggests that the transcriptional regulatory activity of their protein products could play a role in plasticity associated with death or recovery from injury in the immature brain. Hsp70 mRNA expression was induced in nearly all of the animals that were positive for IEG mRNAs. Although the most frequent site of expression for all three mRNAs was the ipsilateral cerebral cortex, hsp70 expression was restricted to the ipsilateral hemisphere and absent from a number of structures that were positive for c-fos and c-jun. In addition, the patterns of expression of hsp70 within specific structures frequently differed from those of the IEGs, implying that although both cellular early response systems are activated in this model, their specific functions are carried out within different microenvironments.

NCAM immunoreactivity on mossy fibers and reactive astrocytes in the hippocampus of epileptic rats

Sprouting and synaptogenesis of mossy fibers develop in adult hippocampus after epilepsy. In control conditions, mossy fibers constitute the main afferent pathway to the Ammon's horn, where they mainly innervate CA3 pyramidal cells, but after treatment with the convulsant agent, kainate, mossy fibers also innervate granule cell dendrites generating recurrent excitatory circuits which may contribute to the maintenance of the epileptic condition. In the present study we show an enhanced immunoreactivity to neural cell adhesion molecules (NCAMs), a family of membrane glycoproteins involved in axonal growth. NCAM immunoreactivity is enriched on cytoplasmic membranes of axon shafts that are likely to be mossy fiber collaterals. NCAM immunoreactivity was also observed on the cytoplasmic membranes of reactive astrocytes, at the axon-glial contacts. Our results therefore suggest that there is an interaction of newly developed mossy fibers with other fibers and glial cells. This interaction may be mediated by NCAMs. Taking into account the trophic properties of NCAMs we suggest that they regulate the sprouting, growing and synaptogenesis of mossy fibers in epileptic conditions.

Expression of highly polysialylated neural cell adhesion molecule in calcitonin-producing cells

Calcitonin-producing cells are endocrine derivatives of the neural crest and have several neuron-like properties. Expression of the neural cell adhesion molecule in calcitonin-producing cells was examined using two types of antibodies to neural cell adhesion molecule: monoclonal antibody 12E3 recognizes the polysialic acid portion of highly polysialylated neural cell adhesion molecule, and monoclonal antibody AF11 and polyclonal antiserum react with the polypeptide portion common to three major isoforms of neural cell adhesion molecule. An immunohistochemical study revealed that highly polysialylated neural cell adhesion molecule was expressed both in fetal rat thyroidal calcitonin-producing cells and in a calcitonin-producing cell line, rMTC 6-23, established from explantable neoplasm of rat calcitonin-producing cells. The neural cell adhesion molecule in the rMTC 6-23 cells was further characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis. Two anti-neural cell adhesion molecule monoclonal antibodies, 12E3 and AF11, revealed a broad positive band around 200,000-250,000 mol. wt in solubilized proteins. When the polysialic acids were eliminated by neuraminidase treatment, the immunoreactivity to monoclonal antibody 12E3 was completely abolished, and core polypeptide corresponding to neural cell adhesion molecule with a molecular weight of 120,000 was detected by monoclonal antibody AF11. These results suggest that cells of the calcitonin-producing cell line express on their surfaces highly polysialylated 120,000 mol. wt form of neural cell adhesion molecule polypeptide.

Transient outgrowth from retinae implanted in the neonatal rat cerebral cortex

Retinae placed in the cortex of neonatal rodents show substantial outgrowth, both along the subpial margin and within the cortical white matter, which is not sustained beyond 6 days post-transplantation. These results, in combination with previous studies, suggest that optic axon outgrowth is sustained for a finite period, after which it requires target-derived influences for its long-term maintenance.

Distribution and possible roles of the highly polysialylated neural cell adhesion molecule (NCAM-H) in the developing and adult central nervous system

The neural cell adhesion molecule (NCAM) is a cell surface glycoprotein which is thought to mediate cell adhesion and recognition. During developmental stages, NCAM is highly polysialylated (NCAM-H) by a unique alpha-2,8-linked polysialic acid chain (PSA), and this PSA portion of NCAM-H has been found to be closely associated with various developmental processes of the nervous system. Further, recent immunohistochemical investigations have revealed that even in the adult nervous system, a persistent PSA expression has been found confined to several regions: the olfactory bulb, the piriform cortex, the hippocampal dentate gyrus, the hypothalamus, some nuclei of the medulla and the dorsal horn of the spinal cord, which are related directly or indirectly to sensory systems. Moreover, in the dentate gyrus and olfactory bulb the expression is connected with adult neurogenesis that may add new neuronal circuits to the adult neural tissue. Therefore, the possible role of NCAM-H in the central nervous system may be associated not only with neural development, but also with adult functions, such as the processing system of sensory information and neuronal plasticity.

Highly polysialylated NCAM expression in the developing and adult rat spinal cord

The expression of a highly polysialylated neural cell adhesion molecule (NCAM-H) appeared in motor neurons, presumptive commissural neurons and floor plate at embryonic day 12, and then spread throughout the spinal cord during late embryonic and early postnatal stages. In the adult stage, the expression almost disappeared, but remained in the superficial laminae of the dorsal horn, the lateral spinal nucleus and the area around the central canal. These results suggest that the NCAM-H expression of the spinal cord is involved in the developmental events and possibly in the processing system of somatic and/or visceral information during the adult stage.

Distribution of NCAM-180 and polysialic acid in the developing tectum mesencephali of the frog Discoglossus pictus and the salamander Pleurodeles waltl

The 180 kDa component of the neural cell adhesion molecule (NCAM-180), total NCAM (NCAM-total) and the polysialic acid modification of NCAM (PSA) show similar temporal and spatial regulation in the developing tecta of Pleurodeles waltl (salamander) and Discoglossus pictus (frog). Whereas NCAM-total is found throughout the tectal tissue on neurons and glia, NCAM-180 is only found on non-proliferating neurons and in fiber layers. PSA is expressed by a subset of NCAM-180-positive cells. Western blots show that there is little polysialylated NCAM-140 in the developing amphibian tectum. Regions unstained for PSA and NCAM-180 correspond precisely to the growth zones of the tectum. NCAM-180 and PSA are not present in tecta of early larvae. Staining intensity is strongest at midlarval stages for both antigens. At metamorphosis, PSA is strongly downregulated, whereas NCAM-180 is downregulated in juvenile animals. Both antigens are still present in fiber layers of adult animals. In dissociated tissue culture of the frog tectum, NCAM-180 is not present on astrocytes, but on neuronal cells. Expression is enhanced at cell contact sites, suggesting that NCAM-180 is involved in cell contact stabilization. This study shows that general features of temporal and spatial regulation of NCAM isoforms and PSA are highly conserved in frog and salamander tecta, despite large differences in the rate of cell migration and the degree of lamination in these homologous brain regions.

Origin of luteinizing hormone-releasing hormone (LHRH) neurons in the chick embryo: effect of the olfactory placode ablation

A unilateral olfactory placodectomy resulted in the absence of luteinizing hormone-releasing hormone-immunoreactive (LHRH-ir) cells in the olfactory-forebrain axis of the operated side, whereas the development of the LHRH neuronal system was not disturbed on the unoperated side. In the embryos in which a fragment of the medial olfactory epithelium was spared the damage, a small number of LHRH-ir cells were detected in the nasal region of the operated side, where the lack of the central projection of the olfactory nerve caused stagnation of LHRH-ir cells, no ir-cells being found in the brain area. These results suggest that LHRH neurons originate in the olfactory placode and then migrate into the forebrain along the olfactory nerve.

Mapping of the distribution of polysialylated neural cell adhesion molecule throughout the central nervous system of the adult rat: an immunohistochemical study

In the nervous system, the neural cell adhesion molecule changes at the cell surface during development, from a form highly enriched in polysialic residues to several isoforms containing much less sialic acid, and is thought to participate in the structuring of neuronal groups and in the establishment of neuronal connections. Recent observations have indicated, however, that it may not be restricted to developing tissues since it is still present in certain adult neuronal centres which can undergo morphological reorganization. In this study, therefore, we examined systematically the distribution of polysialylated neural cell adhesion molecule immunoreactivity throughout the central nervous system of adult male and female rats, using light microscopic immunocytochemistry and immunoblot analysis with an antibody that specifically recognizes the polysialic residues of the molecule. Concomitantly, we compared this immunoreactivity to that due to all isoforms of the neural cell adhesion molecule, detected with a polyclonal serum raised against the NH2-terminal of the protein. Immunoreactivity due to the polysialylated isoform was consistently visualized in several discrete areas of the adult brain and spinal cord. An intercellular punctate immunolabelling characterized the staining in certain hypothalamic and thalamic nuclei, superficial laminae of the dorsal horn of the spinal cord, ventral portion of the dentate gyrus of the hippocampus, lateral geniculate, parabrachial and habenular nuclei, bed nucleus of the stria terminalis, mesencephalic central gray and olfactory bulb. In other areas, such as the piriform cortex, dorsal aspect of the dentate gyrus and fimbria and lamina X of the spinal cord, isolated neuronal-like cells were either completely filled with immunolabel or showed a surface reaction on their cell bodies and processes. Highly immunoreactive isolated glial-like cells were also noted within the ependymal layer of the central canal and lateral ventricles and at times in the peripheral white matter of the spinal cord. In contrast to this discrete localization, staining due to all isoforms of the neural cell adhesion molecule was widespread and diffuse throughout the brain and spinal cord. The expression of the polysialylated isoform in the supraoptic nucleus and hippocampus was confirmed by immunoblot analysis; it occurred together with weakly sialylated isoforms. No obvious differences were detected in the amount or distribution of immunoreactivity due to the polysialylated isoform in relation to the sex or age of the animals (between three and 12 months of age). Our study thus demonstrates that well-defined areas of the central nervous system of the adult rat continue to express the polysialylated isoform of the neural cell adhesion molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

The persistent expression of a highly polysialylated NCAM in the dentate gyrus of the adult rat

The expression of a highly polysialylated form of the neural cell adhesion molecule (NCAM-H), often termed 'embryonic NCAM', has been investigated in the hippocampal formation of developing and adult rats. To determine the immunohistochemical localization of NCAM-H, a monoclonal antibody that reacts with the polysialic acid portion of NCAM-H was used. In the late embryonic and early postnatal periods, immunoreactivity for NCAM-H was found throughout the hippocampal formation, except for the ventricular layer. Thereafter, the immunoreactivity gradually decreased and almost vanished in most parts in the adult. However, a strong immunoreactivity remained on a number of cells in the dentate gyrus of adult rats, particularly in the deepest part of the granular layer. The immunoreactive arborized dendrites, mostly arising from the primary apical pole of the granule cells, were found to enter the molecular layer. The mossy fibers also were positive. Electron-microscopic examination of the hilus portion showed that the immunoreactivity was detected on the plasma membrane of some axons in the mossy fiber bundles. Since postnatal neurogenesis is known to continue into adulthood in the deepest part of the granule cell layer of the dentate gyrus, these results suggest that, in the adult dentate gyrus, NCAM-H is expressed by newly generated granule cells, and that the NCAM-H-expressing new cells may participate in the formation of new neural circuits.

The ontogeny of luteinizing hormone-releasing hormone (LHRH) producing neurons in the chick embryo: possible evidence for migrating LHRH neurons from the olfactory epithelium expressing a highly polysialylated neural cell adhesion molecule

The development of neurons expressing luteinizing hormone-releasing hormone (LHRH) has been studied immunohistochemically in the chick embryo from the 3.5 embryonic day (ED) to the day of hatching. At ED-3.5, LHRH-immunoreactive neurons were first detected in the medial epithelium of the olfactory pit, but their appearance in the brain was delayed to ED-4.5. On EDs-6-7, cords of the LHRH-immunoreactive cells extended across the nasal septum towards the ventromedial forebrain with the olfactory nerve. By double staining for LHRH and, a highly polysialylated form of neural cell adhesion molecule (NCAM-H), the LHRH-positive neurons in the olfactory-forebrain system were found strongly NCAM-H-positive. At ED-8, a marked decrease in the number of LHRH-positive cells in the olfactory epithelium and a concomitant increase in the LHRH-positive cells in the forebrain area were noted. From ED-11 to the day of hatching, the majority of LHRH-positive neurons tended to move into their usual adult position, whereas the LHRH-positive cells had almost disappeared in the olfactory epithelium. No LHRH-immunoreactive neurons were found strongly positive to NCAM-H. These results suggest that LHRH neurons originate from the olfactory placode, then as they develop they migrate across the nasal septum and enter the forebrain with the olfactory nerve. The close association of NCAM-H with the developing LHRH neurons raises the possibility that NCAM-H plays some role in guiding the migrating LHRH neurons from the olfactory epithelium to the forebrain.