Developmental regulation of early serotonergic neuronal differentiation: the role of brain-derived neurotrophic factor and membrane depolarization.

The RN46A cell line was derived from Embryonic Day 13 rat medullary raphe cells by infection with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen. This cell line is neuronally restricted and constitutively differentiates following a shift to nonpermissive temperature. Undifferentiated RN46A cells express low levels of tryptophan hydroxylase (TPH), low-affinity neurotrophin receptor (p75NTR), and trkB immunoreactivities, but no detectable levels of serotonin (5HT) immunoreactivity. TrkB, p75NTR, and TPH, but not 5HT, expressions increase with differentiation and treatment with brain-derived neurotrophic factor (BDNF). 5HT synthesis in RN46A cells requires initial treatment with BDNF, followed by growth under partial membrane depolarizing conditions. Embryonic raphe cultures treated similarly with BDNF and partial depolarizing conditions also demonstrate increased 5HT synthesis. The sodium-dependent transporter for 5HT reuptake is present in undifferentiated RN46A cells, and the apparent Km and Bmax are unchanged by differentiation or BDNF treatment and membrane depolarization. The high-affinity 5HT1A receptor is present in both undifferentiated and differentiated RN46A cells, and while the Kd is unaffected by differentiation or BDNF/membrane depolarization, the Bmax increases 20-fold after differentiation and 3.5-fold further with BDNF under depolarizing conditions. The expression of the synaptic vesicular monoamine transporter, as determined by the binding of [125I]iodovinyltetrabenazine, also increases in RN46A cells with differentiation. However, 5HT release is constitutive and is independent of acute membrane depolarization. Collectively these data indicate that distinct aspects of serotonin metabolism are differentially regulated during development and suggest that 5HT may function as a developmental signal in an autocrine loop during early serotonergic differentiation.