Antinociceptive effects of morphine and U-50,488H on vaginal distension in the anaesthetized rat

The antinociceptive activity of the kappa- and mu-opioid receptor agonists, (+/-)-U-50,488H and morphine, was examined in a vaginal distension model in anaesthetized female rats. Vaginal distension induced a reproducible cardiovascular response (CVR) which was inhibited in a dose related manner by morphine (0.03 1.0 mg/kg i.v., ED50 = 0.16 mg/kg) and (+/-)-U-50,488H (0.08 - 1.6 mg/kg i.v., ED50 = 0.49 mg/kg). Morphine (0.3 mu g/rat) administered i.c.v. inhibited the CVR by 81.6 +/- 7.9 % whereas (+/-)-U-50,488H (30 - 300 mu g/rat) was inactive by this route. A low dose of naloxone (30 mu g/kg i.v.) blocked the effect of morphine but not that of (+/-)-U-50,488H. The kappa-opioid antagonist, nor-binaltorphimine (10 mg/kg s.c.) abolished the response to (+/-)-U-50,488H but not that of morphine. This demonstrates that both central and peripheral mu-opioid receptors may be involved in morphine-induced antinociception whereas the kappa-opioid agonist, (+/-)-U-50,488H, blocks vaginal nociception by acting on peripheral kappa-opioid receptors.