Hypothalamic neurons control a variety of important hormonal and behavioral functions. Little is known, however, about the neurotrophic factors that these neurons may require for survival and/or maintenance of their differentiated functions. We conducted experiments to examine this issue, utilizing a combination of immunohistochemical, in situ hybridization and cell culture approaches. We found that the low affinity receptor for nerve growth factor (p75 NGFR) is present in small subsets of hypothalamic peptidergic neurons identified as such by their content of galanin, luteinizing hormone-releasing hormone (LHRH) and vasointestinal peptide (VIP). More prominently, however, examination of hypothalamic dopaminergic (DA) neurons for the presence of p75 NGFR-like immunoreactivity revealed that the receptor was present on tyrosine hydroxylase (TH)-positive neurons of the zona incerta and periventricular region, but not on neuroendocrine DA neurons of the tuberoinfundibular region. In situ hybridization experiments using a p75 NGFR cRNA confirmed this distribution. Regardless of the presence or absence of p75 NGFR, neither DA group expresses trkA mRNA, indicating that these two major hypothalamic subsets of DNA neurons are NGF-insensitive. A substantial fraction of TH mRNA-positive cells in the zona incerta expresses trkB mRNA, which encodes the receptor for brain derived neurotrophic factor (BDNF); in turn BDNF supports the in vitro survival of hypothalamic TH neurons bearing p75-NGFR, suggesting that BDNF is trophic for DNA neurons of the zona incerta. In contrast, tuberoinfundibular DA neurons do not express trkB mRNA, but some have trkC mRNA, which encodes the receptor for neurotrophin-3 (NT-3). The in vitro survival of TH neurons devoid of p75-NGFR is supported by NT-3, implying that NT-3 may be trophic for a subset of tuberoinfundibular DA neurons. These results suggest that, in spite of expressing an identical neurotransmitter phenotype, anatomically and functionally segregated DA neurons of the neurodendocrine brain are sustained by different neurotrophic factors.

Injury of the nervous system triggers a complex series of repair mechanisms that include production of neurotrophic and mitogenic factors by cells neighboring the injured area. While trauma of most parts of the brain results in loss of function, lesions of certain regions of the female hypothalamus enhance the secretory activity of a group of specialized neurons that produce luteinizing hormone-releasing hormone (LHRH), the neuropeptide that controls sexual development. The increased output of LHRH causes sexual precocity by prematurely activating the neuroendocrine reproductive axis. Recent studies have implicated transforming growth factor alpha (TGF alpha) produced by reactive astrocytes in the process by which lesions hasten sexual maturation, and have suggested that the stimulatory actions of TGF alpha on LHRH neurons require the intermediacy of epidermal growth factor receptors (EGFRs). In the present study, we examined the changes in EGFR gene expression following lesions of the preoptic-anterior hypothalamic area (POA-AHA) of immature female rats, identified the cell types where EGFR synthesis increases, and assessed the biochemical activity of the newly formed EGFR protein. RNase protection assays demonstrated that the lesion significantly increased the levels of a predominant mRNA transcript encoding the full-length, membrane-spanning EGFR, but did not affect those of a much less abundant, alternatively spliced mRNA that encodes a truncated, presumably secreted form of EGFR. Following lesions, antibody-induced EGFR kinase activity increased twofold. Antibodies directed against a peptide sequence contained within the carboxy terminus of EGFR showed intense EGFR immunoreactivity in cells surrounding the lesion site; double immunohistochemistry identified these cells as astrocytes since EGFR immunoreactivity was colocalized with that of glial fibrillary acidic protein, an astrocytic marker. That these changes result from an increase in EGFR gene expression was indicated by the elevated levels of EGFR mRNA detected by in situ hybridization in cells of the same area. Although POA-AHA lesions did not result in appearance of EGFR in LHRH neurons themselves, EGFR-positive cells and processes were seen in close proximity to LHRH neurons and their nerve terminals, particularly in the area surrounding the lesion. Since TGF alpha gene expression is also increased in reactive astrocytes of POA-AHA lesions and blockade of EGFR prevented the advancing effect of the lesion on puberty (Junier et al., 1991b), the present results support the concept that, in lesioned animals, TGF alpha stimulates LHRH secretion indirectly via a paracrine mechanism that involves its interaction with EGFRs located on astroglial cells.

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