The endogenous tripeptide Tyr-Gly-Gly as a possible metabolite of opioid peptides in rat brain: identification, regional distribution, effects of lesions and formation in depolarized slices

Using a sensitive radioimmunoassay, the tripeptide Tyr-Gly-Gly (YGG) which corresponds to the N-terminal sequence of opioid peptides was detected in rat brain and identified by HPLC. Its regional distribution paralleled that of (Met5)enkephalin (YGGFM), a marker of enkephalin neurons. Ablation of these neurons in the striato-pallidal pathway by intrastriatal kainate, induced a significant decrease in YGG levels in caudateputamen and globus pallidus (-49%), consistent with the hypothesis that YGG originates from enkephalin neurons. When pallidal slices were incubated under various conditions, YGG was mainly found in the incubation medium indicating a predominantly extracellular localization. Depolarization of these slices by a K+-stimulus elicited a release of YGGFM accompanied by a marked increase in YGG levels. Bestatin and amastatin further enhanced YGG levels, reflecting the participation of aminopeptidases in the metabolism of the tripeptide and its precursor. Captopril, an inhibitor of the angiotensin-converting enzyme (ACE) showed no effect on the recovery of YGGFM and YGG. In contrast, the formation of YGG was completely prevented by Thiorphan (IC50 value = 9 nM) and phosphoramidon, two inhibitors of "enkephalinase" (EC 3.4.24.11; membrane metallo-endopeptidase), thus identifying the latter as the YGG-forming enzyme. The K+-induced increase in YGG + YGGFM levels in medium containing bestatin exceeded by about 60% the amount of YGGFM released from tissues, suggesting that YGG was mainly formed by extracellular hydrolysis of the various opioid fragments of the proenkephalin molecule. In vivo, YGG levels of cerebral regions were also markedly reduced in rats treated with acetorphan, a parenterally active "enkephalinase" inhibitor. All data suggest that YGG levels constitute an index of opioid peptide release.