Morphine and beta-endorphin inhibit release of noradrenaline from cerebral cortex but not of dopamine from rat striatum.

Abstract

β-ENDORPHIN is a large polypeptide containing 31 amino acid residues, the first five of which are identical with methionine-enkephalin1 which has potent naloxone-sensitive analgesic properties2 and opiate activity in receptor binding assays in guinea pig ileum3,4. It has been shown that morphine depresses the potassium-induced release of 3H-noradrenaline in the rat cerebral cortex5 and of 3H-dopamine in the rat striatum6. These effects are blocked by the opiate antagonist naloxone and are probably due to the activation of presynaptic inhibitory opiate receptors located in noradrenergic and dopaminergic nerve endings of the rat brain. While met-enkephalin reproduced the effects of morphine in rat cortex slices7, it failed to reduce the release of dopamine from the rat striatum6. In view of these results, it was considered of interest to examine under similar experimental conditions, the effects of morphine and of β-endorphin on the potassium-stimulated release of 3H-dopamine from striatal slices and of 3H-noradrenaline from cerebral cortex slices of the rat8. In contrast to Loh et al.6, who more recently have not been able to reproduce their data (E. L. Way, personal communication), we find no evidence for the inhibition of the potassium-stimulated overflow of 3H-dopamine by β-endorphin and morphine in the striatal slices of the rat.