Distribution of neuronal alpha MSH in Spea multiplicata: effects of stress
It has been previously demonstrated in Bufo sp. that stress induces alterations in neuronal and pituitary melanocortins. Here I examined the effects of an acute stressor on regional brain alpha-melanocyte-stimulating hormone (a-MSH) content in the New Mexican spadefoot toad, Spea multiplicata. First the distribution of a-MSH neurons in Spea was seen using immunocytochemistry combined with radioimmunoassay (RIA). a-MSH-ir cells were observed in the preoptic nucleus and ventral infundibulum of the hypothalamus. Ascending immunoreactive fibers projected to the nucleus accumbens and olfactory nucleus of the telencephalon. Some fibers reach the amygdala and the optic tectum. High concentrations of a-MSH were detected in the hypothalamus, the location of the largest population of a-MSH-ir cell bodies with lower amounts in the preoptic area, telencephalon, brainstem, and optic tectum as determined by RIA. Exposure to a brief stressor elevated a-MSH levels in the preoptic area and optic tectum 60-min after exposure.
I conclude that the distribution of neuronal melanocortins in Spea resembles the pattern observed in Bufo, with an exception of the preoptic cell group, which has not been observed in latter species. As in Bufo, the activity of melanocortin neurons is altered during stress. Melanocortins have been implicated in the control of visual learning in toads. This work suggests that the stress-induced alteration in neuronal melanocortin activity may influence the behavior of the toad and how they gather information about their environment.