Huidobro F, Huidobro-Toro J P, Miranda H
Br J Pharmacol. 1980 Dec;70(4):519-25. doi: 10.1111/j.1476-5381.1980.tb09770.x.
1 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 beta-endorphin caused inhibition of the twitching. 2 The concentration of beta-endorphin required to inhibit by 50% the muscular twitch was about 50 to 100 times less than that of the enkephalins. 3 Pretreatment of the vasa with morphine antagonized the inhibition of the neuromuscular transmission caused by either beta-endorphin or enkephalin. 4 Conversely, pretreatment with beta-endorphin sensitized the vasa to the increase in twitch tension caused by morphine. 5 Morphine did not alter the sensitivity to exogenously administered noradrenaline, dopamine or potassium.
