Alterations in brain and plasma concentrations of alpha-melanocyte-stimulating hormone during habituation of prey-catching behavior in toads.

Alpha-melanocyte-stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) promote habituation of prey-catching behavior in toads. We tested the hypothesis that habituation is associated with alterations in the activity of alpha-MSH neurons in a toad, Bufo cognatus. We used immunocytochemistry and RIA to determine the organization and distribution of alpha-MSH neurons in the brain. In addition, we measured brain alpha-MSH and plasma concentrations in toads at different times before and during stimulus specific habituation of the turning response to a rotating prey dummy. Cell groups immunoreactive for alpha-MSH-/and ACTH were detected only in the infundibular hypothalamus. Projections from these cells innervated the olfactory nucleus, nucleus accumbens, septum, habenular nucleus, the preoptic area, the hypothalamus, the caudal thalamus, optic tectum and brain stem. The rank order of alpha-MSH concentrations in the brain of Bufo cognatus was caudal thalamus/hypothalamus (T/HT) > preoptic area (PO) > or = rostral telencephalon (Tel) > brain stem (BS) > optic tectum (OT). The only difference observed in tissue alpha-MSH levels between habituated and non-habituated toads was an elevation in T/HT alpha-MSH levels in control toads after 20 min in the testing apparatus. In contrast, many differences were observed in both groups as a function of time spent in the test apparatus. Concentrations of alpha-MSH in the T/HT and PO increased while concentrations in the Tel decreased over time for both control and habituated toads. Plasma alpha-MSH titers rose over time in control toads but not in habituated toads. Our findings suggest that habituation per se is not associated with dramatic alterations in brain or plasma alpha-MSH levels. On the other hand, prolonged confinement causes pronounced alterations in the activity of brain and pituitary proopiomelanocortin cells.