Enhanced naloxone potency and the development of narcotic tolerance

When various biogenic amine modifying drugs were administered to mice which were pretreated with morphine, none of the drugs except atropine and haloperidol altered the usual doubling of the naloxone efficacy which is observed in morphine-pretreated mice. Atropine appeared to decrease the potency of naloxone. Haloperidol, which did not alter naloxone potency in normal mice, further increased the naloxone potency in morphine-pretreated mice. When the biogenic amine modifiers were injected chronically to mice during the development of morphine tolerance, again none of the drugs, except haloperidol and atropine altered the development of tolerance. Haloperidol appeared to enhance and atropine to inhibit the degree of tolerance. These results indicate that it is the alteration in the narcotic-induced enhancement of naloxone potency and not the mere alteration of noloxone potency by various drugs, which can be used as an indicator of the alterations in the development of tolerance.