Influence of naloxone on the effects of LSD in monkeys.

Three stump-tailed monkeys were trained in an appetitive behavioral task. Administration of 0.1 mg/kg of lysergic acid diethylamide (LSD) moderately disrupted performance acutely, and produced abnormal motor activity. With repeated administration of LSD, motor activity and task responding returned to normal. When 1.0 mg/kg of naloxone was administered prior to administration of LSD in a crossover design, appetitive responding was abolished acutely in all animals and behavior was substantially more disrupted than with LSD alone. Tolerance did not occur with repeated administration of this combination of drugs. Endorphins and/or opiate receptors may act to attenuate the effects of hallucinogenic agents, and may subserve the development of tolerance to these effects.