Morphine lethality in rats: effects of various central receptor blocking agents.

A previous report of a lethal potentiation between sublethal doses of morphine and the beta-adrenergic blocker propranolol was confirmed for Sprague-Dawley rats. An alpha-adrenergic blocker, phentolamine, also showed a lesser but significant potentiation, as did a moderate dose of atropine. No synergism was noted between morphine and methylatropine, haloperidol or methysergide. Phentolamine and a lower dose of atropine, which did not synergize with morphine, both added significantly to the lethality of the combination of morphine and propranolol, whereas methylatropine, haloperidol and methysergide had no significant effect to synergize or antagonize mortality.