Reinforcer interactions under concurrent schedules of food, water, and intravenous morphine

Responding by six rats was maintained under a concurrent chained fixed-ratio 1, fixed-ratio 9 schedule (conc chain FR1 FR9 ) of food, water, and morphine presentations. The subjects had continuous access to the schedule contingencies on a reversed 12-h light-dark cycle. Local rates and temporal patterns were very similar for responding maintained by the three reinforcers with food and water intake occurring predominantly during the dark cycle, while morphine infusions were evenly distributed. Food and water extinction (24-h duration) decreased the number of ratios completed on both the food and water levers. Moreover, food extinction resulted in a large increase in I.V. morphine self-administration. Morphine extinction increased responding on the morphine lever while almost eliminating responding on the water lever. Changes in the dose of morphine (2.5-40 mg/kg/injection) did not significantly affect food and water intake, but were inversely related to responding on the morphine lever. Saline substitutions resulted in effects similar to those observed during morphine extinction. The schedule used in this study provides a method for examining the specificity of a number of pharmacological and neurochemical manipulations.