Characterization of serotonergic neurons in the central nervous system of Drosophila melanogaster from a functional, biochemical and temporal perspective

Serotonin is a classical small-molecule neurotransmitter with known effects on developmental processes. Errant serotonergic function is thought to be the basis of numerous human diseases. Previous studies have shown a role for serotonin in the fly peripheral nervous system. In this study, we show that serotonin modulates the development of serotonergic varicosities within the intact fly central nervous system. We have developed an assay to quantify the number of varicosities within the larval ventral nerve cord. Decreasing endogenous serotonin results in an increase in varicosity number in the fly nerve cord, while addition of excess exogenous serotonin decreases native varicosity number. The acute effects of excess serotonin on varicosity density are reversible and selectively modulate serotonergic varicosities, sparing dopaminergic structure. Cocaine decreases serotonergic varicosity levels, presumably by acutely increasing synaptic serotonin levels. Pharmacological and genetic studies suggest that these effects are mediated through serotonin G-protein-coupled receptors. Secondly, a serotonin affinity resin was constructed to identify serotonin binding proteins from the adult fruit fly. Sufficient purification was achieved following affinity purification and electrophoresis to directly sequence protein bands of interest. Actin, known to interact with serotonin, bound tightly to the affinity resin and may have been stabilized by additional interactions with tropomyosin. Elongation factor Tu was also recovered via affinity purification. This technique offers a method to selectively identify proteins with potential interactions with serotonin. Finally, loss of serotonergic and dopaminergic neurons may have serious implications for normal brain function. Drosophila models of neurodegenerative diseases utilize the short life-span and simple anatomy of the fly to characterize the molecular and genetic processes characteristic to each dysfunctional state. In this study, we examine the normal variability in cell loss for the serotonergic and dopaminergic neurons within the adult fly ventral nerve cord. The loss of these neurons following metamorphosis is highly variable. Aging has no net effect on cell loss for these neurons for the first three weeks of adulthood, though the variation between age-matched individual adults was significant. Apoptosis may function during metamorphosis to regulate cell number and contribute to the observed variability.