Factors affecting voluntary morphine intake in self-maintained addicted rats
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- Weeks, J.R. & Collins, R.J. Psychopharmacologia (1964) 6: 267. doi:10.1007/BF00413156
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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.