Impaired and enhanced shuttle box avoidance behavior following ventromedial hypothalamic lesions in rats

Abstract

VMH-lesioned female Long-Evans hooded rats held to preoperative body weight acquired a shuttle box avoidance response only as rapidly as control animals at both a moderate and high shock level when intertrial interval crossings were not punished, and at a moderate shock intensity with punished ITI crossings. Both groups displayed 90–95% asymptotic avoidance behavior under all three conditions. Obese rats with VMH lesions displayed impaired avoidance behavior under these conditions, displaying only 55–60% asymptotic avoidance behavior after 110 trials. The impaired avoidance behavior by obese rats was not due to immobility, for they emitted as many unpunished ITI crossings as control animals. Both lean and obese rats with VMH lesions avoided a significantly greater number of shocks than control animals at a high shock intensity when ITI crossings were punished, with control animals averaging only 20% avoidance responding after 110 trials. Lean VMH rats again performed better than obese rats, displaying 85% and 60% asymptotic avoidance behavior, respectively. Lean VMH rats made more punished ITI crossings than control animals at the high shock intensity, but there was no difference between the unoperated and obese VMH-lesioned animals. Previous discrepant results with shuttle box avoidance experiments are attributed to different testing conditions, although strain differences are also possible.