!a-MSH- and novelty-induced emotional responses in rats: Associated changes in plasma corticosterone and brain catecholamines.

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Abstract
Defecation responses induced in rats by exposure to novel environments did not habituate if the animals were treated with .alpha.-melanocyte stimulating hormone (.alpha.-MSH). The MSH-treated rats sustained high defecation responses on 6 successive days of exposure, whereas control rats habituated. The role of plasma 11-hydroxycorticosterone (11-OHCS) levels and brain catecholamines in the novelty-induced defecation following .alpha.-MSH administration was investigated. MSH treatment alone increased plasma 11-OHCS levels in the resting condition and lowered brain dopamine (DA) but not norepinephrine (NE) concentrations. Plasma 11-OHCS levels in MSH-treated rats were not significantly increased on successive days of combined MSH and novelty treatments but whole brain DA and NE concentrations were both lowered. A strong negative correlation was observed between defecation responses and brain DA concentrations on successive days of the joint MSH and novelty treatment. .alpha.-MSH treatment had no effect on step-down activity.