PROJECT SUMMARY Antidepressants have been front-line treatments for chronic pain for years. However, clinical efficacy of selective serotonin reuptake inhibitors (SSRI’s) has been moderate and highly variable across different conditions, which calls for a deeper understanding of 5-HT’s role in modulating pain, in an anatomical and receptor-specific manner. One area of recent interest is the amygdala. After neuropathic injury, neural activity in the amygdala changes during the chronification process, while changes in 5-HT have also been detected. Inhibition of the 5-HT2C receptor in the basolateral nucleus of the amygdala (BLA) “permits” SSRI-induced reduction of pain in rodents. In addition, genetic disruption of 5-HT2C prevents neuropathic pain development. Taken together these data support the hypothesis that increases in 5-HT signaling through the 5-HT2C contribute to the development and maintenance of chronic pain and ultimately influence the ability of SSRI’s to be used to treat pain. To test this hypothesis, it is necessary to monitor 5-HT dynamics and neural activity over time in the BLA after neuropathic injury from acute through chronic phases. Currently no implantable sensor is capable of multi- channel detection of both phasic and tonic 5-HT release while recording electrophysiology over days. In this proposal, we introduce novel flexible multi-electrode arrays “GC-MEAs” that made of glassy carbon microelectrode sites and interconnects on flexible and ultrathin polymer substrate. The GC-MEAs will be capable of phasic 5-HT (with fast scan cyclic voltammetry) and tonic 5-HT (with square wave voltammetry) detections, as well as neural activity recordings in vivo over 4 weeks. Our GC-MEAs represent a substantive technological breakthrough in the understanding of 5-HT dynamics. First (Aim 1), we will optimize the fabrication process and materials to obtain the most robust device with stable, sensitive and selective detection. Next (Aim 2) we will validate this technology in the BLA acutely and chronically for four weeks. Finally (Aim 3), we will monitor tonic and phasic 5-HT as well as neural activities in the BLA of mice during acute and chronic phases of a Spared Nerve Injury (SNI), with or without SSRI treatment and 5-HT2C inhibition in BLA.