Long-term orexigenic effects of AgRP-(83-132) involve mechanisms other than melanocortin receptor blockade

Overexpression of agouti-related peptide (AgRP), an endogenous melanocortin (MC) 3 and 4 receptor antagonist (MC3/4-R), causes obesity. Exogenous AgRP (83-132) increases food intake, but its duration and mode of action are unk nown. We report herein that doses as low as 10 pmol can have a potent effec t on food intake of rats over a 24-h period after intracerebroventricular i njection. Additionally, a single third ventricular dose as low as 100 pmol in rats produces a robust increase in food intake that persists for an enti re week. AgRP-(83-132) completely blocks the anorectic effect of MTII (MC3/ 4-R agonist), given simultaneously, consistent with a competitive antagonis t action. However, when given 24 h prior to MTII, AgRP-(83-132) is ineffect ive at reversing the anorectic effects of the agonist. These results suppor t a critical role of MC tone in limiting food intake and indicate that the orexigenic effects of AgRP-(83-132) are initially mediated by competitive a ntagonism at MC receptors but are sustained by alternate mechanisms.