Inhibition of dynorphin-converting enzymes prolongs the antinociceptive effect of intrathecally administered dynorphin in the mouse formalin test

The effects of peptidase inhibitors on the antinociception induced by intrathecally (i.t.) administered dynorphin A and dynorphin B in the mouse formalin test were examined. When administered i.t. 5 min before the injection of 0.5% formalin solution into the dorsal surface of a hindpaw, dynorphin A (0.5-2 nmol) and dynorphin B (2-8 nmol) produced a dose-dependent and significant reduction of the paw-licking response. Dynorphin A (2 nmol) and dynorphin B (8 nmol)-induced antinociception disappeared completely within 90 min and 60 min, respectively. p-Hydroxymercuribenzoate, a cysteine proteinase inhibitor, and phosphoramidon, an endopeptidase 24.11 inhibitor simultaneously administered with dynorphin A or dynorphin B, significantly prolonged antinociception induced by both dynorphins. However, captopril, an angiotensin-converting enzyme inhibitor, bestatin (a general aminopeptidase inhibitor) and a serine proteinase inhibitor phenylmethanesulfonyl fluoride, were inactive. Dynorphin-converting enzyme(s) transform dynorphin-related peptides to [Leu⁵]enkephalin and [Leu⁵]enkephalin-Arg⁶. Neither [Leu⁵]enkephalin nor [Leu⁵]enkephalin-Arg⁶, even at high dose (10 nmol), produced any antinociceptive effect. However, [Leu⁵]enkephalin-Arg⁶, but not [Leu⁵]enkephalin, produced a significant antinociceptive effect when co-administered with phosphoramidon. Therefore, the prolongation of the antinociception induced by both dynorphins in the presence of phosphoramidon, may be due to the inhibition of [Leu⁵]enkephalin-Arg⁶ degradation. The present results indicate that dynorphin-converting enzyme(s) may be important enzyme(s) responsible for terminating dynorphin-A- and dynorphin-B-induced antinociception at the spinal cord level in mice.