Up-regulation of the neuronal serotoninergic phenotype in relation to astrocytic population was studied in primary cultures of rat embryonic rostral raphe. Short treatments (18 hr at day in vitro 4) with brain-derived neurotrophic factor (BDNF) or dibutyryl-cAMP (dBcAMP) increased the number of serotoninergic neurons by approximately 80% and approximately 40%, respectively, and markedly enhanced the branching (by 11-fold and 5-fold, respectively) and total length (by 4-fold and 2.5-fold, respectively) of their neurites. Concomitantly, under BDNF treatment, the astrocyte population was decreased by half and became mostly protoplasmic-like. In contrast, dBcAMP treatment also reduced the astrocytic cell density (by one-third) but induced a stellate morphology. Similar short treatment with the astrocyte-derived S100beta factor induced no modification of the serotonin (5-HT) neuronal phenotype nor of astrocytes morphology. Both BDNF- and cAMP-induced effects were abolished by simultaneous treatment with the specific tyrosine kinase inhibitor genistein, suggesting a role for the high-affinity BDNF receptor tyrosine kinase (TrkB). These data suggest that BDNF and cAMP, but not S100beta, rapidly induce both an up-regulation of the 5-HT neuronal phenotype and modifications of the neighboring astrocytes in a TrkB-dependent manner.
