View Program - 63rd Annual Drosophila Research Conference

Serotonergic neurons produce extensively branched axons that fill most of the central nervous system, where they modulate behaviors such as mood, sleep, appetite, locomotion, and cognition. Proper behavioral output therefore depends on the precise outgrowth and targeting of serotonergic axons during development. To assist in this process, serotonergic neurons utilize serotonin as a trophic signaling molecule prior to it assuming its canonical role as a neurotransmitter. This process, termed autoregulation, plays a negative role in Drosophila by limiting axon outgrowth, branching, and varicosity development. Yet the underlying mechanism of serotonin autoregulation remains unknown. In non-serotonergic neurons, activation of serotonin receptors causes downstream signaling events that are linked with F-actin depolymerization, growth cone collapse, and reduced neurite outgrowth. We therefore hypothesized that serotonin autoreceptors initiate autoregulation in serotonergic neurons. To test this, we adapted a primary neuron culture system in which Drosophila serotonergic neurons could be grown and unambiguously identified using Gal4-dependent expression of the fluorescent protein tdTomato. Next, we initiated autoregulation by applying exogenous serotonin to the culture. We found reduced axon outgrowth and branching, suggesting serotonergic neurons in culture respond similarly as previously reported in vivo. Lastly, we used pharmacological activation of serotonin autoreceptors to test their role in autoregulation. Axons treated with receptor agonists showed reduced outgrowth and branching, thereby mimicking the effects of serotonin application. This suggests that serotonin initiates autoregulation through activation of autoreceptors.

Serotonin autoreceptors regulate Drosophila serotonergic axon morphology in vitro