Modification of morphine analgesia by drugs affecting adrenergic and tryptaminergic mechanisms

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Abstract
The effects of drugs that modify adrenergic or tryptaminergic mechanisms were tested on the analgesic action of morphine in mice. Analgesia was assessed by the hot plate method and phenylquinonewrithing method. Reserpine antagonized the analgesic action of morphine in both tests, the maximal effects occurring 6–8 h after the administration of reserpine. p-Chlorophenylalanine antagonized the analgesic action of morphine as assessed by the writhing method but not by the hot plate method. The analgesic action of morphine was not modified in either test by pretreatment with α-methyl-p-tyrosine, propranolol, phentolamine or methysergide. These results suggest that the analgesic action of morphine, as measured in the writhing test, may be mediated by 5-hydroxytryptamine but that other mechanisms may be involved in the hot plate test.