Abstract
Melanocyte stimulating hormone (MSH), which produces pigmentary changes in amphibians, affects learning and extinction of aversive and appetitive habits in mammals. This study examined the effects of synthetic α-MSH on learning and extinction of two shock motivated T-maze tasks. The performance of pigmented rats with large or small lesions of the posterior neocortex was compared to the performance of intact rats. Results of the first problem showed a significant effect of the brain lesion but no differences between rats treated with MSH or saline. When switched to the second problem, rats injected with MSH took longer to learn the task than controls receiving saline. A relearning test 5 weeks later without injections showed that rats previously receiving MSH retained less than controls. The results suggest that MSH produced increased motivational arousal which led to delayed extinction of the first task, and therefore, slower learning of the second task.
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