Role of alpha-MSH in the regulation of consummatory behavior: immunohistochemical evidence

Central injection of alpha -melanocyte-stimulating hormone (alpha -MSH) dec reases food intake, suggesting a role for this peptide in the mediation of satiety. Inasmuch as alpha -MSH also supports the development of taste aver sions under certain conditions, the nature of its influence on ingestive be havior, i.e., whether it is related to satiety or aversion, remains unclear . In the present studies, we used immunostaining, including that for c-Fos as a marker of neuronal activation, to further substantiate the physiologic al role for alpha -MSH in the regulation of consummatory behavior. We found that an increase in activation of alpha -MSH neurons in the arcuate nucleu s coincided with meal termination. Administration of powerful aversive agen ts, LiCl and CuSO4, did not stimulate alpha -MSH cells but did induce prono unced activation of oxytocin (OT) and vasopressin (VP) neurons, the final c omponents of circuitry mediating aversion. We observed fewer Fos-positive O T/VP neurons after alpha -MSH injection into the lateral ventricle or into the hypothalamic paraventricular nucleus, treatments that cause mild or no aversion, respectively. The degree of activation of OT/VP neurons parallele d the magnitude of aversive response to a given treatment. Our data support the hypothesis that, in the arcuate nucleus, alpha -MSH acts as a satiety mediator independent from aversion-related mechanisms.