Involvement of opioid receptors in the CGRP-induced antinociception in the nucleus accumbens of rats.

It has been demonstrated that calcitonin gene-related peptide (CGRP) plays important roles in the modulation of nociception in the nucleus accumbens (NAc) of rats. The present study is performed to explore the possible involvement of opioid receptors in the CGRP-induced antinociception in the NAc of rats. Intra-NAc administration of CGRP induces significant increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. Interestingly, the CGRP-induced antinociceptive effects are inhibited by following intra-NAc injection of the opioid receptor antagonist naloxone, suggesting that the opioid receptors are involved in the CGRP-induced antinociception in the NAc of rats. Furthermore, the CGRP-induced antinociception is attenuated by intra-NAc injection of mu-opioid receptor (MOR) antagonist beta-funaltrexamine (beta-FNA) and kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI), but not by delta-receptor (DOR) antagonist naltrindole. In the present study, we also demonstrated that there was no significant difference between the CGRP-induced antinociception and the morphine-induced antinociception in the NAc in rats. The results of the present study demonstrate that both mu- and kappa-opioid receptors are involved in the CGRP-induced antinociception in the NAc of rats.