INTERACTIONS BETWEEN MORPHINE AND THE OPIOID‐LIKE PEPTIDES IN THE RAT VAS DEFERENS

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Abstract
Morphine, methadone, levorphanol, pethidine, etonitazene and related morphine‐like alkaloids produced an increase in the electrically‐evoked muscular contraction of the rat vas deferens. In contrast, the enkephalins and β‐endorphin caused inhibition of the twitching. The concentration of β‐endorphin required to inhibit by 50% the muscular twitch was about 50 to 100 times less than that of the enkephalins. Pretreatment of the vasa with morphine antagonized the inhibition of the neuromuscular transmission caused by either β‐endorphin or enkephalin. Conversely, pretreatment with β‐endorphin sensitized the vasa to the increase in twitch tension caused by morphine. Morphine did not alter the sensitivity to exogenously administered noradrenaline, dopamine or potassium.