Complement factor C5a and C5a receptor contribute to morphine tolerance and withdrawal-induced hyperalgesia in rats

Morphine is a potent opioid analgesic. However, the repeated use of morphine causes tolerance and hyperalgesia. Neuroinflammation has been reported to be involved in morphine tolerance and withdrawal-induced hyperalgesia. The complement system is a crucial effector mechanism of immune responses. The present study investigated the roles of complement factor C5a and C5a receptor (C5aR) in the development of morphine tolerance and withdrawal-induced hyperalgesia. In the present study, the levels of C5a and C5aR were increased in the L5 lumbar spinal cords of morphine-tolerant rats. The administration of C5a promoted the development of hyperalgesia and the expression of spinal antinociceptive tolerance to intrathecal morphine in both mechanical and thermal test. However, these phenomena caused by morphine were significantly attenuated by the C5aR antagonist PMX53. These results suggest that complement activation within the spinal cord is involved in morphine tolerance and withdrawal-induced hyperalgesia. C5a and C5aR may serve as novel targets for the control of morphine tolerance and withdrawal-induced hyperalgesia.