Mechanisms governing the differentiation of a serotonergic phenotype in culture

In this study, we explored whether a serotonergic (5-HT) phenotype could be novelly induced in the phenotypically plastic neurons of the developing st riatum. We found that the 5-HT biosynthetic enzyme tryptophan hydroxylase ( TPH) was expressed in nearly 10% of neurons following treatment with an ext ract derived from adult raphe tissue. This effect was mimicked by co-treatm ent with a growth factor (aFGF, bFGF or BDNF; but not GDNF, IGF-1, EGF or T GF) and the neurotransmitter 5-HT (but not GABA, dopamine, glutamate) and/o r a protein kinase activator (IBMX, forskolin, TPA). Treatment with combine d factors (aFGF+5-HT+IBMX+forskolin+TPA) yielded the greatest level of TPH induction (15.6%). Moreover, TPH was enzymatically active (112.8+/-36 pmol/ mg per h) and produced detectable levels of 5-HT (2.12+/-0.30 ng) and its m etabolite 5-HIAA (4.24+/-0.11 ng) in maximally stimulated cultures. These f indings demonstrate that it is possible to promote the differentiation of s erotonergic phenotypic traits in developing brain neurons in culture. (C) 2 000 Elsevier Science B.V. All rights reserved.