alpha-melanocyte-stimulating hormone and habituation of prey-catching behavior in the Texas toad, Bufo speciosus

We investigated dose-dependent effects of alpha-melanocyte-stimulating horm one (alpha-MSH) on habituation in the Texas toad, Bufo speciosus. Additiona lly, we determined changes in plasma and brain levels of alpha-MSH followin g peripheral administration of the peptide or following exposure to an ethe r stressor. The ability of alpha-MSH to facilitate acquisition of habituati on was dose dependent. Plasma alpha-MSH concentrations were elevated within 5 min of dorsal lymph sac injection and remained elevated up to 600% over controls after 30 min. Administration of 50 mu g alpha-MSH had no effect on plasma corticosterone levels. Radiolabeled alpha-MSH was detected in cereb rospinal fluid microdialysates within minutes of peripheral injection. Conc entrations of alpha-MSH in the telencephalon and preoptic area were signifi cantly lowered after ether exposure, whereas levels in the optic tectum, th alamus/hypothalamus, brainstem, and plasma were unchanged. We conclude that alpha-MSH administered peripherally facilitates habituation in a dose-depe ndent fashion. Our results confirm that the effects of alpha-MSH are indepe ndent of corticosterone secretion. The peptide is cleared rapidly into the bloodstream and enters the cerebrospinal fluid after dorsal lymph sac injec tion. Neuronal alpha-MSH may help toads gather information about their envi ronment when exposed to certain stressors. (C) 1999 Academic Press.