Plasma concentrations of alpha-melanotropin in the rat during the acquisition and extinction of conditioned avoidance behaviour and during the acquisition of maze learning behaviour.
Concentrations of immunoassayable alpha-melanotropin in plasma were increased above control levels during the acquisition of conditioned avoidance behaviour but not during the extinction of this behaviour. There were no significant variations in levels of alpha-melanotropin in plasma during the acquisition of reward motivated maze learning behaviour. Throughout the latter test hormone concentrations were increased above normal levels. Non-specific stress was thought to be an important factor in the production of increases in concentrations of alpha-melanotropin in plasma. Acute physical stress, such as that caused by the electric foot shock of the avoidance studies, produced the greatest increases, while the smaller increase noted during maze learning was attributed to the stress of the chronic food deprivation schedule used in this test. The data did not support a role for systemic melanotropin in the processes of learning and memory as there were no consistent correlations between levels of alpha-melanotropin in plasma and the parameters of learning in the tests. Although a role for stress-induced alpha-melanotropin release from the pituitary gland in the enhancement of attention was thought possible, actions of exogenous melanotropins on behaviour could be due to effects on melanotropin-containing systems present within the central nervous system.