Effects of altering motivation for food in rats trained with food reinforcement to discriminate between d-amphetamine and saline injections

Abstract

Previous studies have shown that altering motivation typically affects stimulus generalization in animals trained to discriminate exteroceptive stimuli, but few studies have evaluated the effects of manipulating motivation on drug stimuli. In the few published studies, motivation levels were manipulated by arranging different feeding conditions prior to stimulus generalization tests with rats trained to discriminate morphine from vehicle and in pigeons trained to discriminate phencyclidine or pentobarbital from vehicle. In the present study, rats maintained at 80% of free-feeding weights were trained to discriminate between injections of 1.0mg/kg d-amphetamine and saline in a two-lever food-reinforced operant procedure. Generalization tests were then conducted with a range of d-amphetamine doses (0, 0.03, 0.1, and 0.3, 1.0mg/kg) when the rats were not fed before experimental sessions (high motivation) and when they were pre-fed 1g of food (moderate motivation) or their daily ration of food (low motivation) 1h before test sessions. Changing the motivation level significantly affected response rate and latency to the first response in generalizations tests, but did not significantly affect mean percentage of drug-appropriate responding (a continuous measure) or percentage of animals that selected the drug-appropriate lever (a quantal measure). The present findings indicate that manipulating motivation for food minimally impacts d-amphetamine discrimination, however, the range of conditions used to examine the effects of motivating operations on stimulus control by d-amphetamine drugs and other drugs is limited and the topic may warrant further investigation.