We investigated dose-dependent effects of α-melanocyte-stimulating hormone (α-MSH) on habituation in the Texas toad, Bufo speciosus. Additionally, we determined changes in plasma and brain levels of α-MSH following peripheral administration of the peptide or following exposure to an ether stressor. The ability of α-MSH to facilitate acquisition of habituation was dose dependent. Plasma α-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 μg α-MSH had no effect on plasma corticosterone levels. Radiolabeled α-MSH was detected in cerebrospinal fluid microdialysates within minutes of peripheral injection. Concentrations of α-MSH in the telencephalon and preoptic area were significantly lowered after ether exposure, whereas levels in the optic tectum, thalamus/hypothalamus, brainstem, and plasma were unchanged. We conclude that α-MSH administered peripherally facilitates habituation in a dose-dependent fashion. Our results confirm that the effects of α-MSH are independent of corticosterone secretion. The peptide is cleared rapidly into the bloodstream and enters the cerebrospinal fluid after dorsal lymph sac injection. Neuronal α-MSH may help toads gather information about their environment when exposed to certain stressors.
- α- melanocyte-stimulating hormone