Investigations into the mechanism of morphine and ethanol inhibition in the guinea pig ileum longitudinal muscle strip

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Abstract
It has been noted that the analgesic property of ethanol bears a marked resemblance to that of morphine. The purpose of this study was to determine if the mechanism of action of morphine and ethanol was similar using the guinea pig ileal longitudinal muscle strip (GPI-LMS). Ethanol (35–260 mM) depressed the twitch response and the acetylcholine- (ACh-), KCl-, and BaCl2-induced contractions to the same extent while having no significant effect on the binding of [3H]quinuclidinyl benzilate ([3H]QNB) to muscarinic receptors. Morphine (53–530 nM) inhibited the twitch response and to a lesser extent BaCl2- and KCl-induced contractions while having no significant effect on either ACh-induced contractions or the binding of [3H]QNB to muscarinic receptors. Naloxone and increased [Ca2+] reversed the inhibitory effects of morphine but not ethanol. Ethanol appears to inhibit a site after interaction of ACh with the receptor. Mechanism of inhibition of BaCl2 response is also different as naloxone and increased [Ca2+] reverse morphine but not ethanol inhibition. Ethanol inhibition in GPI-LMS does not involve the opiate receptor.

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