Role of mu-opioid receptor in nociceptive modulation in anterior cingulate cortex of rats

Lots of studies have demonstrated that anterior cingulate cortex plays important roles in the pain perception and pain modulation. The present study explored the role of mu-opioid receptor in nociceptive modulation in anterior cingulate cortex of rats with neuropathic pain. Neuropathic pain model was set up by chronic constriction injury of the left sciatic nerve of rats. The hindpaw withdrawal latency to thermal and mechanical stimulation, by hot plate and Randall Selitto Test respectively, was used to evaluate the rat's responses to noxious stimulation. Results showed that intra-anterior cingulate cortex injection of morphine could induce the antinociception dose-dependently. By intra-anterior cingulate cortex injection of opioid receptor antagonist, the morphine-induced antinociception could be attenuated by naloxone, as well as much significantly by the selective mu-opioid receptor antagonist β-funaltrexamine, indicating that mu-opioid receptor is involved in the morphine-induced antinociception in anterior cingulate cortex of rats with neuropathic pain. The morphine-induced antinociception was much more decreased in rats with neuropathic pain than that in normal rats, and there was a significant decrease in mu-opioid receptor messenger RNA levels in anterior cingulate cortex of rats with neuropathic pain, indicating that there may be a down-regulation in mu-opioid receptor expression in anterior cingulate cortex of rats with neuropathic pain. To further confirm the role of mu-opioid receptor in morphine-induced antinociception in anterior cingulate cortex, normal rats were received intra-anterior cingulate cortex administration of small interfering RNA targeting mu-opioid receptor and it was found that there was a down-regulation in mu-opioid receptor messenger RNA levels, as well as a down-regulation in mu-opioid receptor expression in anterior cingulate cortex tested by real-time polymerase chain reaction and western blotting. Furthermore, the morphine-induced antinociceptive effect decreased significantly in rats with small interfering RNA targeting mu-opioid receptor, which indicated that knockdown mu-opioid receptor in anterior cingulate cortex could also attenuate morphine-induced antinociceptive effect. These results strongly suggest that mu-opioid receptor plays a significant role in nociceptive modulation in anterior cingulate cortex of rats.