Regional Metabolism of Met‐Enkephalin and Cholecystokinin on Intact Rat Brain Slices: Characterization of Specific Peptidases

Abstract: The metabolism of Met‐enkephalin and cholecystokinin (CCK) 8‐(sulfated) by intact microslices was studied in rat brain regions. Incubation of brain slices with Met‐enkephalin (400 µM) resulted in a linear rate of disappearance of parent peptide and appearance of metabolic fragments whose rate of accumulation was specific to brain region. The degradative rate (pmol/min/mg of protein) of Met‐enkephalin was high in caudate‐putamen (5,160 ± 120) and lower in nucleus accumbens (3,630 ± 110) and frontal cortex (3,180 ± 120). Inhibition of aminopeptidases decreased Met‐enkephalin degradation (50–97% vs. control) in frontal cortex but was less effective in caudate‐putamen (20–34%). Tyr‐Gly‐Gly and Phe‐Met were recovered in caudate‐putamen and nucleus accumbens, whereas negligible quantities of these fragments were recovered from frontal cortex. Phosphoramidon, an inhibitor of neutral endopeptidase 24.11, decreased Met‐enkephalin degradation in caudate‐putamen (14%) but had no effect on that in frontal cortex. A cocktail of bestatin or leuhistin (inhibitors of aminopeptidases), phosphoramidon, and captopril (an inhibitor of angiotensin converting enzyme) protected Met‐enkephalin from degradation (recovery >95%) in caudate‐putamen. CCK 8‐(sulfated) degradation on slices from caudate‐putamen, nucleus accumbens, and frontal cortex was not altered by inhibitors of neutral endopeptidase 24.11, metalloendopeptidase 24.15, angiotensin converting enzyme, or thiol proteases. Inhibitors of either aminopeptidases or serine proteases produced small reductions (13–30%) in CCK degradation in each region. These data provide evidence for regional and structural specificity in terminating the actions of neuropeptides.