Affective consequences and subsequent effects on morphine self-administration of d-ala²-methionine enkephalin

Albino rats implanted with intracerebroventricular (ICV) cannulae were used to investigate the effects of ICV administration of d-ala2-methionine enkephalin. Potentially positive affective consequences were assessed by observing rats’ movements in an alley, one compartment of which had previously been paired with drug administration. Like morphine (10 mg/kg, intra-peritoneally—IP), this enkephalin analogue (10 µg, ICV) produced a tendency for rats to move toward the place where they had previously experienced the drug’s effects. In another experiment, the same dose of d-ala2-methionine enkephalin was not sufficient to produce a conditioned taste aversion, as did the 10-mg/kg IP dose of morphine. Rats with a prior history of administration of either d-ala2-methionine enkephalin or systemic morphine subsequently consumed significantly more sweetened morphine solution than control animals in a voluntary oral consumption situation with tap water also available. Collectively, these results suggest that enkephalin administration may produce a positive affective state without aversive components and potentiate voluntary consumption of morphine.