Spinal ERK activation via NO-cGMP pathway contributes to nociceptive behavior induced by morphine-3-glucuronide.

Intrathecal (i.t.) injection of morphine-3-glucuronide (M3G), a major metabolite of morphine without analgesic actions, produces a severe hindlimb scratching followed by biting and licking in mice. The pain-related behavior evoked by M3G was inhibited dose-dependently by i.t. co-administration of tachykinin NK(1) receptor antagonists, sendide, [D-Phe(7), D-His(9)] substance P(6-11), CP-99994 or RP-67580 and i.t. pretreatment with antiserum against substance P. The competitive NMDA receptor antagonists, D-APV and CPP, the NMDA ion-channel blocker, MK-801 or the competitive antagonist of the polyamine recognition site of NMDA receptor ion-channel complex, ifenprodil, produced inhibitory effects on i.t. M3G-evoked nociceptive response. The NO-cGMP-PKG pathway, which involves the extracellular signal-regulated kinase (ERK), has been implicated as mediators of plasticity in several pain models. Here, we investigated whether M3G could influence the ERK activation in the NO-cGMP-PKG pathway. The i.t. injection of M3G evoked a definite activation of ERK in the lumbar dorsal spinal cord, which was prevented dose-dependently by U0126, a MAP kinase-ERK inhibitor. The selective nNOS inhibitor N(omega)-propyl-l-arginine, the selective iNOS inhibitor W1400, the soluble guanylate cyclase inhibitor ODQ and the PKG inhibitor KT-5823 inhibited dose-dependently the nociceptive response to i.t. M3G. In western blotting analysis, inhibiting M3G-induced nociceptive response using these inhibitors resulted in a significant blockade of ERK activation induced by M3G in the spinal cord. Taken together, these results suggest that activation of the spinal ERK signaling in the NO-cGMP-PKG pathway contributes to i.t. M3G-evoked nociceptive response.