Developmental regulation of the serotonergic transmitter phenotype in rostral and caudal raphe neurons by transforming growth factor-βs

Abstract

Serotonergic (5-HT) neurons of the CNS develop as two separate clusters, a rostral and a caudal group, within the brain stem raphe. We show here that the transforming growth factors -β2 and -β3 (TGF-β) and the TGF-β type II receptor are expressed in the embryonic rat raphe, when 5-HT neurons develop and differentiate. To investigate putative roles of TGF-βs in the regulation of 5-HT neuron development we have generated serum-free cultures isolated either from the rostral or the caudal embryonic rat raphe, respectively. In cultures from the caudal E14 raphe saturating concentrations (5 ng/ml) of TGF-β2 and -β3 augmented numbers of tryptophan hydroxylase (TpOH) - immunoreactive neurons and cells specifically taking up 5,7- dihydroxytryptamine (5,7-DHT) by about 1.7-fold over a period of 4 days. Treatment with TGF-βs also increased uptake of 3H-5HT uptake about 1.7- fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of delayed short-term treatments. Comparing rostral and caudal raphe cultures from different embryonic ages suggests that distinct effects of TGF-βs reflect the responsiveness of 5-HT neurons at different ages rather than of different origins.