Distribution of neuronal melanocortins in the spadefoot toad Spea multiplicata and effects of stress

We examined the effects of an acute stressor on regional alpha-melanocyte-stimulating hormone (αMSH) content in the New Mexican spadefoot toad, Spea multiplicata. We first used immunocytochemistry along with radio-immunoassay (RIA) to examine the distribution of pro-opiomelanocortin (POMC) neurons in the brain of Spea. Neurons immunoreactive for αMSH, β-endorphin, and corticotropin were observed in the preoptic nucleus and ventral infundibulum of the hypothalamus. Ascending immunoreactive fibers projected to the olfactory nucleus, nucleus accumbens, and striatum. Numerous immunoreactive fibers were also observed in the hypothalamus. The thalamus/hypothalamus (T/HT) contained the greatest concentrations of αMSH as determined by RIA followed by the preoptic area (PO). Levels in the telencephalon, brain stem, and optic tectum (OT) were 14-23 times lower than in the T/HT. Exposure to a brief stressor elevated αMSH levels in the PO and OT. We conclude that Spea possesses two distinct POMC neuronal cell groups, one located in the anterior preoptic area and one located in the ventral infundibulum. Ascending projections to the basal ganglia might play an indirect role in tectal regulation and the control of prey-catching behavior. Exposure to an acute stressor alters brain αMSH content in Spea, although there are regional and temporal differences in the response pattern compared to Bufo. These findings are consistent with the notion that neuronal melanocortins influence how frogs and toads gather information about their environment during stress.