Action of peptidase inhibitors on methionine5-enkephalin-arginine6-phenylalanine7 (YGGFMRF) and methionine5-enkephalin (YGGFM) metabolism and on electroacupuncture antinociception.

In the spinal cord Met5-enkephalin-Arg6-Phe7 (YGGFMRF) is located in small interneurons of the dorsal and ventral horns. From these storage sites, YGGFMRF can be released by perfusing the subarachnoidal spaces of the spinal cord with artificial spinal fluid containing substance P. In vitro YGGFMRF can be hydrolyzed readily by a dipeptidyl carboxypeptidase. In order to ascertain whether this reaction is physiologically relevant, we measured the content of YGGFMRF and Met5-enkephalin (YGGFM) in subarachnoidal space perfusate in presence and in absence of captopril, bestatin and thiorphan using substance P to activate the release of opioid peptides. Without peptidase inhibitors, the efflux of YGGFMRF and YGGFM was hardly detectable. The addition of captopril to the perfusion medium increased the substance P (10(-7) M)-induced release of YGGFMRF markedly but it increased the efflux of YGGFM to a much smaller extent. When captopril and bestatin were added together the amount of YGGFMRF present in the perfusate was further increased slightly. In contrast, the YGGFM content in the same perfusate was increased greatly by bestatin and only slightly by thiorphan. To characterize the pharmacological profile of these peptidase inhibitors, we compared electroacupuncture antinociception with and without intrathecal injections of captopril and bestatin. This antinociception, as measured by tail-flick latency, was potentiated by the intrathecal injection of captopril and bestatin. These results taken together suggest that YGGFMRF released in the perfusate of the arachnoidal space by substance P is metabolized by both dipeptidyl carboxypeptidase and aminopeptidase.