Basic fibroblast growth factor (bFGF) acts on both neurons and glia to mediate the neurotrophic effects of astrocytes on LHRH neurons in culture

Luteinizing hormone‐releasing hormone (LHRH) neurons play a pivotal role in the neuroendocrine control of mammalian reproduction. Astrocytes were shown to be involved in the regulation of LHRH neuronal function, but little is known about the contribution of astroglial‐derived factors in the regulation of LHRH neuron development. In order to gain insight into the mechanisms regulating the development of these cells, at morphological and biochemical levels we characterized the neurotrophic effects exerted by young astrocytes (maintained in culture for 8 days in vitro) and old astrocytes (maintained 26 days) on the differentiation, proliferation, and phenotypic expression of immortalized hypothalamic LHRH (GT1‐1) neurons in vitro. Culturing GT1‐1 cells in the presence of young glia for different time intervals caused a marked acceleration in the acquisition of their neuronal phenotype. At all times examined, GT1‐1 cells cocultured with young glia exhibited a significantly greater extension of processes/cell, larger number of processes/cell and greater surface area of growth cones than GT1‐1 cells grown over nonglial adhesive substrates (polylysine). By contrast, when GT1‐1 neurons were cocultured with old glia, the length of neuronal processes and the growth cone surface area were significantly lower than in control GT1‐1 neurons cultured in the absence of glia. At 3 days in vitro (DIV), GT1‐1 neurons cocultured with young glia exhibited a 50% lower incorporation of [3H]thymidine than GT1‐1 neurons cultured without glia. By contrast, in the presence of old glia [3H]thymidine incorporation was significantly higher in cells cocultured with glia than in GT1‐1 neurons cultured alone. Localization of the proliferating cells by dual immunohistochemical staining revealed that the incorporation of bromodeoxiuridine (BrdU) was restricted to nuclei of GT1‐1 neurons when these were cocultured with young glia, but associated with both neurons and astrocytes in the presence of old glia. At the functional level, coculture of GT1‐1 neurons with young glia increased the spontaneous release of LHRH as compared to GT1‐1 neurons grown in the absence of glia. By contrast, in the presence of old glia LHRH release in the medium was significantly lower than in controls. Conditioned medium of young glia (ACM‐Y) induced significant neurotrophic and functional effects on GT1‐1 cells, but these effects were 50% less potent than the coculture itself. Heat denaturation of ACM‐Y totally a