Antinociceptive properties of oxymorphazole in the mouse

Oxymorphazole (17-methyl-6,7-dehydro-3,14-dihydroxy-4,5 α-epoxy-6,7:3′,4′-pyrazolomorphinan), a hydrophilic opioid, given intracerebroventricularly (2.5–50 nmol) or intrathecally (0.3–5 nmol) dose-dependently produced tail-flick inhibition in male CD-1 mice. However, oxymorphazole given subcutaneously even at high doses (10–80 mg/kg) produced weak tail-flick inhibition. Oxymorphazole given intraperitoneally (0.1 to 10 mg/kg) dose-dependently inhibited abdominal constriction response induced by intraperitoneally injection of 0.6% acetic acid. Oxymorphazole given intracerebroventricularly (25 nmol) or intrathecally (5 nmol) induced tail-flick inhibition was blocked by pretreatment with the μ-opioid receptor antagonist d-Phe-Cys-Tyr-d-Orn-Thr-Pen-Thr-NH2, but not κ-opioid receptor antagonist nor-binaltrophimine. The δ-opioid receptor antagonist, naltrindole, blocked the tail-flick inhibition induced by oxymorphazole given intrathecally but not intracerebroventricularly. The inhibition of the abdominal constriction response by oxymorphazole given intraperitoneally was blocked by intraperitoneally pretreatment with naloxone, but not naltrindole or nor-binaltrophimine. Thus, oxymorphazole given systemically produces antinociception only with the abdominal constriction test, but not the tail-flick test, suggesting that it produces the antinociception at the peripheral sites when administered systemically. The oxymorphazole-induced antinociception is mainly mediated by the stimulation of μ-opioid receptors when given either centrally or systemically and also the δ-opioid receptors when given intrathecally. The lack of central antinociceptive effect of oxymorphazole given systemically may have interesting clinical implications.