Basic and Regulatory Mechanisms of In Vitro Release of Met‐Enkephalin from the Dorsal Zone of the Rat Spinal Cord

Abstract

Under control conditions, superfused slices of the dorsal half of the lumbar enlargement from adult rats released Met-enkephalin-like material (MELM) that behaved as authentic Met-enkephalin under two different chromatographic procedures (Bio-gel filtration, HPLC). MELM release increased markedly on exposure of slices to batrachotoxin (0.5 μM) or to an excess of K⁺ (28 and 56 mM instead of 5.6 mM). The K ⁺ -evoked release was totally dependent on the presence of Ca²⁺ in the super-fusing fluid whereas the spontaneous efflux of MELM was only partially Ca²⁺-dependent. Further experiments performed with tissues of polyarthritic rats indicated that the increase in their MELM levels was associated with a lower fractional rate constant of MELM release, therefore suggesting that spinal Met-enkephalin turnover might be reduced in chronically suffering animals. Examination of the possible modulation of MELM release by various neuroactive compounds present within the dorsal horn revealed that cholecystokinin (10 μM), but not its desulphated derivative, substance P-sulphoxide (10 μM), and to a lesser extent substance P, enhanced the K⁺-evoked MELM release. In contrast, γ-aminobutyric acid (10 μM) and (–)-baclofen (1 μM) partially prevented the stimulatory effect of K⁺ on MELM release. Other compounds such as serotonin, somatostatin, and neurotensin altered neither the spontaneous nor the K⁺-evoked release of MELM.