Papers of the Week

Neural plasticity, especially central sensitization, is essential for developing and maintaining neuropathic pain. Unfortunately, the analgesic potency of morphine is greatly reduced in animal models and patients with neuropathic pain. We hypothesized that pre-activation of spinal N-methyl-D-aspartate receptors (NMDARs) by agonist or neuropathic pain facilitated the development of morphine-induced analgesic tolerance. We therefore investigated the effects of spinal NMDAR activation, induced by neuropathic pain, on the development of morphine-induced analgesic tolerance in male Sprague-Dawley rats. Four days of chronic constriction injury (CCI) induced upregulation of spinal NR1. Once established, spinal central sensitization accelerated the development of morphine-induced analgesic tolerance. Continuous ropivacaine infusion prevented CCI-induced increases in spinal Substance P (SP), NR1, and TRPV1. Blockade of peripheral nociceptive inputs prevented chronic morphine-induced increases in spinal SP, NR1, and TRPV1 and a rightward shift of the morphine dose-response curve in the CCI model. These findings suggest that pre-activation of spinal NMDARs contributes to central sensitization and potentiates the development of morphine-induced analgesic tolerance. Interruption of the peripheral nociceptive inputs during the induction phase could prevent spinal central sensitization and retain morphine efficacy, thereby delaying the development of morphine-induced tolerance in patients with neuropathic conditions.