Regulation of serotonergic neuronal development in Drosophila. H.I. Rieff , B.G. Condron. Dept.Biology, U.Virginia, Charlottesville, VA.

   Serotonergic function is regulated in part by the serotonin transporter (SERT). We have investigated the molecular signals that regulate the embryonic onset of SERT expression and how these signals relate to the axon guidance of Drosophila serotonergic neurons. SERT activity is detected in embryonic fly serotonergic neurons using an in situ enzymatic assay to histochemically measure serotonin uptake. We found that SERT activity, as well as SERT mRNA, is first seen in the serotonergic neurons after their axons cross the midline (beginning around stage 15). This suggests that molecular cues at the midline are crucial for the induction of SERT. SERT activity is lost when the serotonergic cell bodies are separated from the midline. In contrast, a number of genetic manipulations indicate a lesser role for the midline. Combined cellular and genetic analyses will further elucidate the role of midline signals.
   We have used the SERT enzymatic assay in two screens to determine the signaling pathways important for serotonergic neuronal development. First, we have initiated a directed screen to examine SERT regulation in known Drosophila mutants and in flies with specific genes mis-expressed in the serotonergic neurons (accomplished using an eagle-GAL4 driver). We found that two ETS-domain transcription factors, pointed and anterior open, dramatically affect SERT regulation. This suggests that a number of signaling pathways may regulate SERT activity during neuronal development. Second, we have begun an undirected deficiency screen to detect loci important for the proper axon guidance and differentiation of the fly serotonergic neurons. Initial results indicate that a number of loci are important for serotonergic neuronal development. Further results will likely reveal the relationship between axon guidance and regulation of cell fate in the development of Drosophila serotonergic neurons.