, Volume 6, Issue 4, pp 267-279

Factors affecting voluntary morphine intake in self-maintained addicted rats

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Self-maintained morphine addicted rats were prepared by implanting chronic venous cannulas and fitting the rats with a device permitting relatively free movement and also enabling them to obtain morphine injections at will by pressing a lever. Three factors modifying voluntary morphine intake were studied. 1. Using a continuous reinforcement schedule, progressively decreasing the size of the morphine dose led to a greater number of doses daily. Compensation was incomplete in that the total daily morphine intake decreased. 2. Progressively increasing a fixed ratio reinforcement schedule up to about FR-75, caused rats to continue responding on the lever until the dose was obtained. Above FR-75 responding became intermittent and daily morphine decreased as the time interval between doses increased. 3. Continuous intravenous infusion of a second drug, leaving voluntary access to morphine at FR-10, led to decreased morphine intake following infusion of morphine itself, codeine and meperidine. Nalorphine infusion increased morphine intake. Effectiveness of infusions varied with the infusion rate.

A preliminary account of this work was presented at the 25th Meeting of the Committee on Drug Addiction and Narcotics, Natl. Res. Council, Natl. Acad. Sci., Ann Arbor, Michigan, 15–17 February 1963.