Handling facilitates the acquisition of lever-pressing for brain self-stimulation in the posterior hypothalamus of rats

Abstract

Adult male rats implanted with stimulating electrodes in the posterior hypothalamus-ventral tegmental area (PH-VTA) were allowed to learn to lever press for intracranial self-stimulation (ICSS) without any priming by the experimenter or use of external cues. Animals that had been handled for seven days prior to testing acquired the lever-pressing task more rapidly than non-handled animals. Prior handling had no effect on the amount of locomotor activity in a novel environment nor on the number of non-reinforced lever presses. This suggested that the effect of handling on ICSS acquisition was not due to a change in locomotor activity or exploratory behavior. Implanted rats that were injected before each session with d-amphetamine (0.25 mg/kg) or saline did not differ in their rates of acquisition of ICSS in PH-VTA. However, injected animals acquired ICSS more slowly than the non-injected (handled and non-handled) animals. These results suggest that mild stressors applied before acquisition testing can alter the rate of learning of an operant task for ICSS in the posterior hypothalamic region and that small routine manipulations of animals can affect learning in operant training situations.