Morphine and β-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 ileum^3,4. 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.