The pituitary hormone MSH has been tested in several different behavioral systems. It was found to increase resistance to extinction of a conditioned active avoidance response, a passive avoidance response, and an appetitive response. Perseverance, general activity levels, and memory could not be shown to fully explain the actions of MSH in the tasks studied, but subtle interactions may have been involved. Some studies in rats and human beings indicated an effect of MSH upon the attentive process, particularly visual attention. It is possible that MSH increases emotional arousal which leads to increased attention to some environmental cues. Electrical correlates of MSH activity have been found in animal and man, and endogenous MSH appears to respond to several types of stress. It is speculated that during the course of evolution, MSH has maintained its 'adaptive' value for the organism, but that this may apply only to simple tasks. The conclusion is reached that in mammals MSH exhibits definite extra pigmentary actions upon behavior.
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