The effects of melanocortin peptides and corticosterone on habituation in the great plains toad, Bufo cognatus.

We investigated the ability of various melanocortin peptides and corticosterone to influence habituation of prey-catching behavior in the toad, Bufo cognatus. Male toads were injected with various melanocortin peptide fragments or corticosterone 30 min prior to acquisition. Adrenocorticotropin (ACTH[1-39]), ACTH[4-10], and N-acetyl ACTH[1-13] amide (alpha-MSH) significantly decreased the number of turning reactions during acquisition in relation to controls. The effects of the noncorticotropic ACTH fragments were rapid and transient, occurring within the first 20 min of acquisition. Corticosterone caused a slight but significant decrease in the number of turning reactions. Neither des-acetyl alpha-MSH nor (Nle4, D-Phe7) alpha-MSH had any effect on acquisition. ACTH[1-39] was the only peptide that delayed extinction. The ability of alpha-MSH to facilitate acquisition was not observed in the presence of the alpha-MSH antagonist U-76188E. These data suggest that the effects of ACTH on habituation are, in part, independent of effects on glucocorticoid secretion. Apparently, similar structural requirements are necessary for the behavioral effects of melanocortins in amphibians and mammals.