Ethanol‐Induced Increase of Agouti‐Related Protein (AgRP) Immunoreactivity in the Arcuate Nucleus of the Hypothalamus of C57BL/6J,but not 129/SvJ,Inbred Mice

Background: The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor, pro‐opiomelanocortin (POMC). Previous research has shown that MC receptor (MCR) agonists reduce, and MCR antagonists increase, ethanol consumption in rats and mice. Consistently, genetic deletion of the endogenous MCR antagonist, agouti‐related protein (AgRP), causes reductions of ethanol‐reinforced lever pressing and binge‐like ethanol drinking in C57BL/6J mice. Ethanol also has direct effects on the central MC system, as chronic exposure to an ethanol‐containing diet causes significant reductions of α‐melanocyte stimulating hormone (α‐MSH) immunoreactivity in specific brain regions of Sprague‐Dawley rats. Together, these observations suggest that the central MC system modulates neurobiological responses to ethanol. To further characterize the role of the MC system in responses to ethanol, here we compared AgRP and α‐MSH immunoreactivity in response to an acute injection of saline or ethanol between high ethanol drinking C57BL/6J mice and moderate ethanol drinking 129/SvJ mice. Methods: Mice received an intraperitoneal (i.p.) injection of ethanol (1.5 g/kg or 3.5 g/kg; mixed in 0.9% saline) or an equivolume of 0.9% saline. Two hours after injection, animals were sacrificed and their brains were processed for AgRP and α‐MSH immunoreactivity. Results: Results indicated that acute ethanol administration triggered a dose‐dependent increase in AgRP immunoreactivity in the arcuate (ARC) of C57BL/6J mice, an effect that was not evident in the 129/SvJ strain. Although acute administration of ethanol did not influence α‐MSH immunoreactivity, C57BL/6J mice had significantly greater overall α‐MSH immunoreactivity in the ARC, dorsomedial, and lateral regions of the hypothalamus relative to the 129/SvJ strain. In contrast, C57BL/6J mice displayed significantly lower α‐MSH immunoreactivity in the medial amygdala. Conclusions: The results show that acute ethanol exposure has direct effects on endogenous AgRP activity in ethanol preferring C57BL/6J mice. It is suggested that ethanol‐induced increases in AgRP may be part of a positive feedback system that stimulates excessive binge‐like ethanol drinking in C57BL/6J mice. Inherent differences in α‐MSH immunoreactivity may contribute to differences in neurobiological responses to ethanol that are characteristically observed between the C57BL/6J and 129/SvJ inbred strains of mice.