Immortalized hypothalamic luteinizing hormone-releasing hormone (LHRH) neurons induce a functional switch in the growth factor responsiveness of astroglia: involvement of basic fibroblast growth factor

Recent evidence indicates that astroglial-derived growth factors (GFs) part icipate in the development of luteinizing hormone-releasing hormone (LHRH) neurons, but it is still unknown whether LHRH neurons may exert a reciproca l modulation of glial cell function. Using immortalized hypothalamic LHRH ( GT(1-1)) neurons in co-culture with glial cells, we have recently shown tha t basic fibroblast growth factor (bFGF) plays a prominent role in the glial -induced acquisition of the mature LHRH phenotype by GT(1-1) cells. We have resorted to this model and combined biochemical and morphological approach es to study whether the response of glial cells to a number of GFs (includi ng bFGF;, insulin-like growth factor I, IGF-I, epidermal growth factor, ECF and insulin) expressed during LHRH neuron differentiation, is modulated by co-culture with pure LHRH neurons. Pre-treatment of hypothalamic astrocyte s with an inactive ('priming') dose of bFGF for 12 h powerfully increased a stroglia proliferative response to IGF-I (10 ng/ml), EGF (10 g/ml) and insu lin (10 mug/ml), inducing a 65-100% increase in the [H-3]thymidine incorpor ation compared to untreated cultures. When astroglial cells and developing GT(1-1) neurons were co-cultured for 5 days in vitro (DIV), the [H-3]thymid ine incorporation was significantly higher than in astroglial cells culture d without neurons. Application of the different GFs to the co-culture for e ither 12 or 24 h further stimulated DNA synthesis to various extent accordi ng to the GF applied and the time of application. Localization of the proli ferating cells by dual immunohistochemical staining, followed by cell count ing and bromodeoxiuridine (BrdU) labeling index calculation, revealed that the incorporation of BrdU was restricted to the nuclei of LHRH-immunopositi ve neurons. Such changes were accompanied by extensive morphological altera tions of astroglial and LHRH fiber networks, whereas neutralization of bFGF activity in GT(1-1) neuron-glial co-cultures by a bFGF-antibody, dramatica lly counteracted the observed effects. The functional switch of astroglia p roliferative response to GFs coupled to the potent morphological and functi onal modifications of developing glia and pure LHRH neurons observed in vit ro, support a bidirectional interaction between immortalized LHRH neurons a nd astroglial cells and identify bFGF as a key player in this crosstalk. (C ) 2000 ISDN. Published by Elsevier Science Ltd. All rights reserved.