Effects of melanocyte-stimulating hormone (MSH) and melatonin on passive avoidance and on an emotional response

The present experiment investigated the opposite effects of synthetic alpha-MSH and Melatonin on acquisition and extinction of a passive avoidance response (PAR) and on emotionality, as indexed by defecation, in the PA box. It was found that intraperitoneal (IP) administration of alpha-MSH delayed extinction and increased defecation responses whereas IP administration of Melatonin facilitated extinction of the PAR and decreased defecation. The present experiment confirmed MSH-Melatonin opposition on memory and on the defecation response.