Alterations in the BDNF–mTOR Signaling Pathway in the Spinal Cord Contribute to Hyperalgesia in a Rodent Model of Chronic Restraint Stress

Stress is a non-specific, systemic, physiological response of the body to strong internal and external environmental stimuli. Accumulating evidence has suggested that stress, particularly chronic restraint stress (CRS), can reduce pain threshold and increase pain sensitivity. However, pathogenic and therapeutic mechanisms underlying CRS remain unclear. Here, we aimed to investigate roles of the brain-derived neurotrophic factor (BDNF)-mammalian target of rapamycin (mTOR) signaling pathway in CRS-induced abnormal pain sensitivity. CRS was successfully mimicked 7 days after model development, and paw withdrawal mechanical threshold (PWMT) and tail-flick latency (TFL) were evaluated. CRS significantly altered BDNF and mTOR phosphorylation in the anterior cingulate cortex and spinal cord but not in the hippocampus. On day 7, a single dose of 7,8-dihydroxyflavone, an activator of BDNF-tropomyosin receptor kinase B, was administered via intraperitoneal or intrathecal injection. Notably, only the intrathecal injection improved PWMT and TFL. Additionally, an intraperitoneal injection of rapamycin, an mTOR inhibitor, failed to induce any behavioral changes, whereas a single intrathecal injection of rapamycin improved abnormal CRS-induced PWMT and TFL. In conclusion, CRS can induce abnormal pain sensitivity, probably by altering the BDNF-mTOR signaling pathway in the spinal cord.