Antinociceptive effect produced by intracerebroventricularly administered dynorphin A is potentiated by p-hydroxymercuribenzoate or phosphoramidon inthe mouse formalin test

The antinociceptive effects of intracerebroventricularly (i.c.v.) administe red dynorphin A, an endogenous agonist for K-opioid receptors, in combinati on with various protease inhibitors were examined using the mouse formalin test in order to clarify the nature of the proteases involved in the degrad ation of dynorphin A in the mouse brain. When administered i.c.v. 15 min be fore the injection of 2% formalin solution into the dorsal surface of a hin dpaw, 1-4 nmol dynorphin A produced a dose-dependent reduction of the nocic eptive behavioral response consisting of licking and biting of the injected paw during both the first (0-5 min) and second (10-30 min) phases. When co -administered with p-hydroxymercuribenzoate (PHMB), a cysteine protease inh ibitor, dynorphin A at the subthreshold dose of 0.5 nmol significantly prod uced an antinociceptive effect during the second phase. This effect was sig nificantly antagonized by nor-binaltorphimine, a selective K-opioid recepto r antagonist, but not by naltrindole, a selective delta -opioid receptor an tagonist. At the same dose of 0.5 nmol, dynorphin A in combination with pho sphoramidon, an endopeptidase 24.11 inhibitor, produced a significant antin ociceptive effect during both phases. The antinociceptive effect was signif icantly antagonized by naltrindole, but not by nor-binaltorphimine. Phenylm ethanesulfonyl fluoride (PMSF), a serine protease inhibitor, bestatin, a ge neral aminopeptidase inhibitor, and captopril, an angiotensin-converting en zyme inhibitor, were all inactive. The degradation of dynorphin A by mouse brain extracts in vitro was significantly inhibited only by the cysteine pr otease inhibitors PHMB and N-ethylmaleimide, but not by PMSF, phosphoramido n, bestatin or captopril. The present results indicate that cysteine protea ses as well as endopeptidase 24.11 are involved in two steps in the degrada tion of dynorphin A in the mouse brain, and that phosphoramidon inhibits th e degradation of intermediary delta -opioid receptor active fragments enkep halins which are formed from dynorphin A. (C) 2001 Elsevier Science B.V. Al l rights reserved.