Transcriptional landscape of the dorsal raphe serotonin neurons rendering stress resiliency

Major depressive disorder (MDD) is a serious and large social problem, yet the pathophysiology of MDD and the action mechanism of antidepressants are still poorly understood. A number of studies have reported that activation and inactivation of serotonin neurons in the dorsal raphe nucleus (DRN) cause antidepressant-like effects and depressive-like behaviors, respectively. Also, their physiological neural activities are increased when mice were chronically administered an SSRI and decreased in mice exposed to chronic social defeat stress (CSDS), a mouse model of depression. However, the molecular mechanism underlying these neural activity changes in DRN serotonin neurons remains unclear. In this study, we performed a DRN serotonin neuron-specific comprehensive gene expression analysis by using Translating Ribosome Affinity Purification (TRAP) technology in both chronic SSRI-treated mice as a model of antidepressant treatment and CSDS mice as a model of depression. It revealed that many gene expression changes were the opposite between SSRI-treated mice and CSDS-susceptible mice. Among these, we identified S100a10 as a prodepressive gene in DRN serotonin neurons, and we found that Interleukin-4 (IL-4) - Signal Transducer and Activator of Transcription 6 (STAT6) pathway and 5-HT1B receptor were the upstream and downstream molecules of S100a10, respectively. Our findings provide insights into molecular mechanisms underlying the action of antidepressants and stress resiliency.