Behavioral and physiological effects of alpha-melanocyte-stimulating hormone in the Texas Toad (Bufo speciosus)

Alpha-melanocyte-stimulating hormone (a-MSH) is a tridecapeptide derived from the pro-opiomelanocortin (POMC) gene product. This and other POMC peptides have been shown to influence learning and memory process in a variety of testing paradigms. a-MSH secretion is also highly influenced by stress, and as a result, the neuroactive properties of a-MSH were investigated in the Texas toad, Bufo speciosus, using the well-documented prey-catching behavior as a model for assessing learning as measured by habituation to a false prey stimulus. a-MSH administered peripherally facilitated habituation of prey-oriented turning responses in a dose-related manner. Further experiments showed that dorsal lymph sac injections of a-MSH elevated plasma a-MSH levels in a rapid and dose-dependent manner without influencing plasma corticosterone levels. Although plasma a-MSH levels were significantly increased following dorsal lymph sac injection of a-MSH, a-MSH levels in the brain regions investigated were not significantly raised above control animals treated with vehicle. Ether stress was found to decrease a-MSH levels in the telencephalon and preoptic area by 90 min following stress, with a-MSH levels in these regions returning to baseline by 120 min. The stressor was not found to have any effect on other brain regions investigated. In vivo microdialysis revealed low levels of a-MSH delivery from peripheral circulation into cerebral ventricular circulation within 30 min following peripheral administration. The data reported herein suggest that a-MSH facilitates habituation of the prey-catching behavior independent of glucocorticoid influence and that peripheral a-MSH has a high degree of clearance into general circulation, and a small clearance into cerebral circulation.