[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2, a proposed antagonist of the nociceptin receptor, is a potent and stable agonist in the rat spinal cord

[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is an nociceptin analogue which has been shown to be a selective antagonist of the nociceptin receptor in peripheral tissues. We now report that intrathecal [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 produced a dose-dependent depression of the nociceptive flexor reflex in rats, an effect that is similar to nociceptin. The duration of depression produced by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 was significantly more prolonged than by nociceptin. The reflex depressive effect of nociceptin was not blocked by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2. The results indicated that the proposed nociceptin receptor antagonist [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is a potent agonist in rat spinal cord and more resistant to enzymatic degradation compared to nociceptin.