Regulation of spinal nociceptin expression by neuropathic pain

Abstract

Nociceptin is the endogenous ligand of a new opioid receptor, the opioid receptor-like-1 (ORL1) receptor. Chronic inflammatory pain causes an increase in the expression of nociceptin and the ORL1 receptor in the dorsal horn of rat spinal cord, thus indicating an involvement of the endogenous nociceptin/ORL1 system in mechanisms of pathological pain. This study investigates the influence of neuropathic pain on the expression of nociceptin using immunohistochemistry. To induce neuropathic pain, a ligation of the sciatic nerve was performed in 12 rats under general anesthesia. A sham operation was performed in 12 rats of the control group. Nerve ligation caused a significant ipsilateral thermal hyperalgesia, a typical sign of neuropathic pain. The paw withdrawal latency was decreased by 45.7+/-4.9% ( p<0.05) at day 5 and by 37.3+/-1.8% ( p<0.05) at day 10. Although hyperalgesia was fully present after 5 days, no changes in nociceptin immunoreactivity in the lumbar spinal cord were detected at this time point. Ten days after nerve ligation, there was a 2.46+/-0.38 fold ( p<0.05) bilateral increase in nociceptin immunoreactivity in the lamina superficiales (I and II), with a notable increase in the inner lamina II at the level of L4. Further investigations are necessary to elucidate the relationship between neuropathic pain, the nociceptin-ORL1 receptor system and potential therapeutic options.