EVIDENCE FOR MORPHINE AND MORPHINE-LIKE ALKALOID RESPONSES RESISTANT TO NALOXONE BLOCKADE IN THE RAT VAS-DEFERENS
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1979
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Abstract
The application of morphine or surrogates to the isolated rat was deferens maintained at 37.degree. C in Tyrode solution produced an increase in the electrically induced muscular twitch. Leucine enkephal in or D-2-D-Ala-Met enkephalinamide produced a dose-dependent inhibition of the muscular twitch. The effect of morphine and derivatives was not antagonized by naloxone, but the depression caused by the opiate pentapeptides or .beta.-endorphin was readily antagonized and reversed by naloxone. Tolerance developed to the in vitro effect of morphine; vasa deferentia obtained from tolerant-dependent rats were about 6 .times. less sensitive to the effect of morphine and about 5 .times. less sensitive to the depression caused by leucine enkephalin as compared to their respective paired, placebo implanted control rats.