PLG and FMRFamide--endogenous peptides with marked inhibitory effects on opioid-induced feeding.

Social conflict and defeat in mice leads to an activation of endogenous opiate systems and the display of marked feeding behavior. Intraperitoneal administrations of prolyl-leucyl-glycinamide (PLG 0.01-10 mg/kg) leads to a dose-dependent inhibition of defeat-induced feeding that is analogous to that obtained after treatment with either the endogenous peptide FMRFamide (Phe-Met-Arg-Phe-NH2), or the prototypic opiate antagonist, naloxone. These results suggest that PLG, and FMRFamide, or related small peptides may function as endogenous antagonists of opioid-induced feeding.