Spinal ERK2 activation through δ2-opioid receptors contributes to nociceptive behavior induced by intrathecal injection of leucine-enkephalin.

Intrathecal (i.t.) injection of leucine-enkephalin (Leu-ENK), co-administered with peptidase inhibitors, phosphoramidon (an endopeptidase 24.11 inhibitor), and bestatin (a general aminopeptidase inhibitor), produced behaviors consisting of the biting and/or licking of the hindpaw and the tail along with hindlimb scratching directed toward the flank, which peaked at 10-15 min after an injection. This characteristic behavior was not observed in mice treated with i.t. Leu-ENK alone. We also investigated the effect of the extracellular signal-regulated kinase (ERK) in spinal processing of nociception induced by i.t. co-administration of Leu-ENK with phospharamidon and bestatin. Western blot analysis of phospho-ERK (pERK) showed a significant increase of pERK2 in the lumbar spinal cord in response to i.t. Leu-ENK co-injected with peptidase inhibitors. The MAP kinase-ERK inhibitor, U0126 dose-dependently attenuated the nociceptive behavior and spinal ERK activation to i.t. Leu-ENK co-injected with peptidase inhibitors. Furthermore, the nociceptive behavior and spinal ERK activation evoked by i.t. Leu-ENK in combination with peptidase inhibitors were inhibited by co-administration of the non-selective δ-opioid receptor antagonist, naltrindole, the selective δ2-opioid receptor antagonist, naltriben, the non-competitive N-methyl-D-aspartate (NMDA) antagonist, MK-801 or the non-selective nitric oxide synthase inhibitor, L-NAME, the selective nNOS inhibitor, N(ω)-propyl-L-arginine or the selective iNOS inhibitor, W1400, but not by the selective δ1-receptor antagonist, BNTX (7-benzylidenenaltrexone). These results suggest that spontaneous nociceptive behaviors produced by i.t. co-administration of Leu-ENK with peptidase inhibitors may be induced by an activation of the glutamate-NO-ERK pathway through the δ2-opioid receptor in the dorsal spinal cord.