Brain-derived neurotrophic factor and trkB are essential for cAMP-mediatedinduction of the serotonergic neuronal phenotype

Serotonergic neurons in the central nervous system are crucial in the contr ol of autonomic functions and behavior. Mechanisms by which development and maintenance of the serotonergic transmitter phenotype is regulated include activation of protein kinase A (PKA). Using cultures established from the E14 rat raphe we show here that forskolin (10 mu M) increases numbers of ne urons expressing tryptophan hydroxylase (TpOH), the key enzyme of serotonin synthesis, and uptake of the false serotonergic transmitter 5,7-dihydroxyt ryptamine (5,7-DHT). As shown by shortterm treatments the effect is due to phenotype induction rather than survival. To begin to understand downstream or parallel signaling pathways required for the PKA-mediated induction of serotonergic markers, we have studied the putative implication of brain-der ived neurotrophic factor (BDNF) and its receptor trkB. Treatment of raphe n eurons with forskolin induced BDNF mRNA assayed by competitive RT-PCR. More over, trkB-IgG receptor bodies fully prevented the forskolin-induced numeri cal increase in TpOH- and 5,7-DHT-positive cells suggesting an implication of a TrkB-activated pathway. TrkC-IgG had no effect. K252b, a specific inhi bitor of trk kinase activity likewise abolished the induction of serotonerg ic markers by forskolin. In turn, the inductive effect of BDNF on serotoner gic markers was blocked by KT5720, a specific inhibitor of PKA. Taken toget her, these data suggest that cc-activation of cAMP- and trkB-dependent sign aling pathways plays a crucial role in the regulation of the serotonergic n euronal phenotype. (C) 2000 Wiley-Liss, Inc.