5-HT1A autoregulates serotonergic axon outgrowth by initiating actin depolymerization

Serotonergic neurons extend long, highly branched axons throughout the brain during development and are responsible for the modulation of many behaviors. Various behavioral and neurological disorders are associated with aberrant serotonergic axon structure. Because proper behavioral output is dependent on precise outgrowth and targeting of serotonergic axons, it is important to understand how serotonergic axon outgrowth is regulated during development. Our previous pharmacological experiments suggest that autoreceptor 5-HT1A regulates axon outgrowth and branching of Drosophila serotonergic neurons in vitro. To further investigate this, I used a genetic approach to decrease expression of 5-HT1A, which rescued axon outgrowth during 5-HT treatment, supporting our earlier pharmacological Findings. To confirm that 5-HT1A is an autoreceptor in our cultured neurons, I used a genetic reporter line and immunofluorescence to label serotonergic neurons expressing 5-HT1A. I found that 5-HT1A expression was required for serotonergic neurons to respond to 5-HT treatment. Together, these results suggest that autoreceptor 5-HT1A regulates axon outgrowth of Drosophila serotonergic neurons during development, however, the underlying mechanism remains unknown. To address this, I investigated the downstream cytoskeletal mechanisms of 5-HT1A mediated regulation. Using live cell imaging, I measured axon length before and after acute 5-HT treatment and found that acute addition of 5-HT may reduce axon length. Kymograph analysis of docked mitochondria revealed that this reduction was not due to axon retraction. Finally, I used a fixed cell approach to compare G/F actin ratios in treated vs untreated neurons and found that 5-HT1A activation led to actin depolymerization in growth cones of serotonergic neurons. Together, these results suggest that 5-HT1A autoreceptors limit axon outgrowth by initiating intracellular signaling cascades that lead to growth cone collapse.