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 m ouse formalin test were examined. When administered i.t. 5 min before the injection of 0.5% formalin solution into the dorsal surface of a h indpaw, 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-Hydr oxymercuribenzoate, a cysteine proteinase inhibitor, and phosphoramido n, an endopeptidase 24.11 inhibitor simultaneously administered with d ynorphin A or dynorphin B, significantly prolonged antinociception ind uced by both dynorphins. However, captopril, an angiotensin-converting enzyme inhibitor, bestatin (a general aminopeptidase inhibitor) and a serine proteinase inhibitor phenylmethanesulfonyl fluoride, were inac tive. Dynorphin-converting enzyme(s) transform dynorphin-related pepti des to [Leu(5)]enkephalin and [Leu(5)]enkephalin-Arg(6). Neither [Leu( 5)]enkephalin nor [Leu(5)]enkephalin-Arg(6), even at high dose (10 nmo l), produced any antinociceptive effect. However, [Leu(5)]enkephalin-A rg(6), but not [Leu(5)]enkephalin, produced a significant antinocicept ive effect when co-administered with phosphoramidon. Therefore, the pr olongation of the antinociception induced by both dynorphins in the pr esence of phosphoramidon, may be due to the inhibition of [Leu(5)]enke phalin-Arg(6) degradation. The present results indicate that dynorphin -converting enzyme(s) may be important enzyme(s) responsible for termi nating dynorphin-A- and dynorphin-B-induced antinociception at the spi nal cord level in mice.