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 striatum. We found that the 5-HT biosynthetic enzyme tryptophan hydroxylase (TPH) was expressed in nearly 10% of neurons following treatment with an extract derived from adult raphe tissue. This effect was mimicked by co-treatment with a growth factor (aFGF, bFGF or BDNF; but not GDNF, IGF-1, EGF or TGF) and the neurotransmitter 5-HT (but not GABA, dopamine, glutamate) and/or a protein kinase activator (IBMX, forskolin, TPA). Treatment with combined 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 metabolite 5-HIAA (4.24+/-0.11 ng) in maximally stimulated cultures. These findings demonstrate that it is possible to promote the differentiation of serotonergic phenotypic traits in developing brain neurons in culture.