In vitro release of [5-methionine]enkephalin and [5-leucine]-enkephalin from the rat globus pallidus

Abstract

Endogenous [5-methionine]enkephalin (Met-enkephalin) and [5-leucine]enkephalin (Leu-enkephalin) were released from perfused slices of rat globus pallidus by increased K+ in a Ca2+-dependent manner. Tissue perfused for 40 min contained only 26% of the Met-enkephalin and 44% of the Leu-enkephalin found in the freshly dissected tissue. After perfusion, the mean (.+-. [standard error of the mean]) ratio (wt/wt) of Met-enkephalin to Leu-enkephalin was 3.4 .+-. 0.2 compared with 5.8 .+-. 0.2 in the fresh tissue. The degradation of trace amounts of synthetic [3H]enkephalins in the perfusing medium during stimulated release seems to reflect the accelerated degradation of enkephalin released from the tissue: 63% of the Met-enkephalin and 23% of the Leu-enkephalin were degraded in a medium containing bacitracin (30 .mu.g/ml). The mean ratio (wt/wt) of the Met-enkephalin to the Leu-enkephalin recovered after release by exposure of slices to 50 mM K+ was 2.7 .+-. 0.3. When perfusates were corrected for degradation, this ratio increased to about 5.5 which is higher than that found in the perfused tissue. The differences in release, tissue loss and catabolism of the 2 enkephalins might be reflecting differences in the metabolic systems operating on the pentapeptides, but this interpretation must be validated by in vivo release experiments. Both enkephalins could be considered candidate neurotransmitters in the rat globus pallidus.