Inhibition of trigemino-hypoglossal reflex in rats by oxytocin is mediated by μ and κ opioid receptors

Recent studies showed that oxytocin plays an important role in the modulation of pain at different levels of the central nervous system. The present study was undertaken to investigate the effect of oxytocin on trigemino-hypoglossal reflex in rats. With the experimental settings used in this study, we have demonstrated that oxytocin showed significant analgesic effect after intracerebroventricular administration in rats, as assayed by the amplitude of the retractory movements of the tongue after tooth pulp stimulation. Antinociceptive effect of oxytocin was inhibited by subsequent perfusion of cerebral ventricles with oxytocin antagonist, [deamino-Cys1-D-Tyr(OEt)2-Thr4-Orn8]-oxytocin, atosiban. An involvement of opioid system in the oxytocin-induced analgesia was studied after intracerebroventricular administration of different opioid antagonists: non-selective naloxone, μ-selective β-funaltrexamine, δ-selective naltrindole, and κ-selective nor-binaltorphimine. It was shown that inhibition of antinociceptive effects was mediated through μ and κ opioid receptors, indicating that there is a synergy between oxytocin and opioid systems in transmitting and modulating pain stimuli. Co-administration of oxytocin and a μ-selective endogenous opioid ligand endomorphin-2 did not significantly increase the antinociceptive activity of endomorphin-2.