Serotonin receptor activation leads to neurite outgrowth and neuronal survival

Serotonin 5-HT1 receptors are implicated in anxiety and depression. These receptors belong to the family A of G-protein-coupled receptors and couple to inhibitory G-proteins. Recent studies show that chronic activation of 5-HT1A receptors leads to proliferation of hippocampal neurons suggesting that neurogenesis contributes to the effects of antidepressants. However, the molecular mechanisms and pathways involved are not understood. We used Neuro 2A cells transfected with 5-HT1A receptors and SK-N-SH cells endogenously expressing the receptor to examine the effect of receptor activation on neuronal survival and neurite outgrowth. We find that receptor activation leads to increased neurite outgrowth that can be blocked by the receptor selective antagonist and by treatment with pertussis toxin or lactacystin implicating inhibitory G-proteins and proteasomal degradation in this process. Interestingly, the small G-protein Rap and the transcription factor STAT-3 are also involved since reducing the levels of Rap1 (using small interfering RNA) or STAT-3 (using dominant negative STAT3) significantly blocks 5-HT1A-receptor-mediated neurite outgrowth. The observed increase in the phosphorylation of Src and STAT-3, at sites leading to their activation, further supports a crucial role for these proteins in neurite outgrowth. We also find that prolonged activation of endogenous 5-HT1A receptors leads to increased cell survival even under starving conditions; this is completely blocked by co-treatment with the antagonist. Taken together, these findings indicate that activation of the 5-HT1A receptor leads to a number of neurotropic events by activating a series of signal transduction molecules leading to long-term changes required for neurogenesis.