Neuropeptide processing in regional brain slices: Effect of conformation and sequence

The central enzymatic stability of des-enkephalin-γ-endorphin and its synthetic analogs [cycloN(α6), C(δ11)]β-endorphin-[6-17] and [Pro⁷,Lys(Ac)⁹]-β-endorphin[6-17] was studied in vitro using a newly developed, regionally dissected rat brain slice, time course incubation procedure. Tissue slice viability was estimated as the ability of the brain slice to take up or release γ-[³H]aminobutyric acid after high K⁺ stimulation. Results demonstrated stability of uptake/release up to 5 hr of incubation, suggesting tissue viability over this period. The estimated half-life of peptides based on the results obtained in our incubation protocol suggest that the peptides studied are metabolized at different rates in the individual brain regions tested. A good correlation exists between the high enzyme activity of neutral endopeptidase (EC 3.4.24.11) and the rapid degradation of des-enkephalin-γ-endorphin and [cycloN(α6),C(δ11)]β-endorphin-[6-17] in caudate putamen. Proline substitution combined with lysine acetylation appears to improve resistance to enzymatic metabolism in caudate putamen and hypothalamus. However, cyclization of des-enkephalin-γ-endorphin forming an amide bond between the α-NH₂ of the N-terminal threonine and the γ-COOH of glutaminc acid did not improve peptide stability in any brain region tested. The present study has shown that the brain slice technique is a valid and unique approach to study neuropeptide metabolism in small, discrete regions of rat brain where peptides, peptidases and receptors are colocalized and that specific structural modifications can improve peptide stability.