Stress facilitates pain perception and sensitizes pain pathways, but the underlying mechanism is still unclear. The purpose of this study was to investigate whether the activation of 5-hydroxytryptamine (5-HT) subtype-3 receptor in the spinal cord contributes to somatic hyperalgesia induced by repeated three-day forced swim in the estradiol replacement rats after ovariectomy. Somatic sensitivity was assessed by thermal withdrawal latency to radiant heat and mechanical withdrawal threshold to von Frey filaments. The expression of 5-HT3A receptor in the L4–L5 dorsal spinal cord was examined by Western blot. Repeated forced swim stress reduced the thermal withdrawal latency and mechanical withdrawal threshold, and the presence of estradiol exaggerated this hyperalgesia. The expression of 5-HT3A receptor in the L4–L5 dorsal spinal cord increased significantly following repeated forced swim in estradiol replacement rats. Intrathecal injection of 5-HT3 receptor antagonist Y-25130 blocked the somatic hyperalgesia induced by forced swim stress. These data indicate that 5-HT3 receptor activation through the descending facilitation system contributes to the somatic hyperalgesia evoked by forced swim stress. The results may provide a new therapeutic avenue for alleviating pain induced by stress.
Keywords: 5-HT receptor; Pain; descending facilitation; forced swim; spinal cord; stress-induced hyperalgesia.