Regulation of the transmitter phenotype of rostral and caudal groups of cultured serotonergic raphe neurons.

We have studied the regulation of survival and serotonergic markers by neurotrophins and several trophically active cytokines in neurons cultured from the embryonic rat raphe region under defined conditions. At embryonic day 14, saturating concentrations of brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4 and basic fibroblast growth factor elicited a two- to 2.5-fold increase in numbers of tryptophan hydroxylase- and serotonin-immunoreactive neurons over a four-day culture period. Transforming growth factor beta-1 and glial cell line-derived neurotrophic factor were less potent, while fibroblast growth factor-5 was only marginally effective. Distinct responses to different factors were noted depending on embryonic age and regional origin of serotonergic neurons. Thus, brain-derived neurotrophic factor augmented numbers of tryptophan hydroxylase-positive neurons at embryonic day 16 by a factor of seven, but only 1.5- to two-fold when cultures were established from day 13 or 14 embryos. In cultures of rostral serotonergic groups (B4-B9), numbers of tryptophan hydroxylase-positive neurons decreased in the absence of factors, whereas numbers of tryptophan hydroxylase-immunoreactive neurons in cultures from caudal serotonergic groups (B1-B3) increased during a 12-day culture period. There was no evidence that serotonergic neurons undergo apoptosis (as visualized by terminal deoxynucleotidyl transferase dUTP nick end labeling) or proliferate (as visualized by 5-bromodeoxyuridine incorporation) in culture. Numbers of serotonergic neurons also increased when cultures were treated with a brief 24-h pulse of brain-derived neurotrophic factor, supporting the notion that changes in numbers of serotonergic neurons reflected alterations of phenotype rather than cell death or proliferation. The ability of cells to specifically take up the serotonin analog 5,7-dihydroxytryptamine was also up-regulated by brain-derived neurotrophic factor in both rostral and caudal raphe cultures. Lability of the serotonergic phenotype was further suggested by the observation that ciliary neurotrophic factor fully prevented the brain-derived neurotrophic factor-mediated increase in tryptophan hydroxylase-positive neurons. The effect of ciliary neurotrophic factor was dependent on the presence of astrocytes. We conclude that serotonergic neurons show spatially and temporally distinct responses to neurotrophic factors, which seem to have a profound influence of the transmitter phenotype rather than on survival.