Influence of oxytocin on nociception and morphine antinociception

In the hot plate test the intracerebroventricular (i.c.v.) injection of oxytocin produced a significant decrease in nociception, starting from the dose of 1 microgram/rat. A comparable effect was obtained with 10-200 times higher intraperitoneal (i.p.) doses. The i.c.v. injection of the oxytocin antagonist d(CH2)5-Tyr(Me)-[Orn8]-vasotocin, while having no influence per se, completely prevented the antinociceptive effect of an equal i.c.v. dose of oxytocin. The antinociceptive effect of oxytocin was also prevented by naltrexone, and oxytocin caused a small but significant increase of the antinociceptive effect of morphine and of its duration. These data indicate that pharmacological amounts of oxytocin produce antinociception, that occurs through the activation of oxytocin receptors; endogenous opioid systems seem to be involved altogether.