Intra-amygdalar injection of DAMGO: effects on c-Fos levels in brain sites associated with feeding behavior

It is well known that the mu opioid agonist, Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), increases food intake in rats when injected into a variety of brain sites including the central nucleus of the amygdala (CeA). Immunohistochemical studies measuring c-Fos immunoreactivity (IR) suggest that the CeA contributes to opioid-related feeding. In the current study, we injected 2 nmol of DAMGO and measured food intake, c-Fos IR levels in various brain sites involved in feeding behavior, and mu opioid receptor internalization. We also studied the effect of CeA-injected DAMGO on LiCl-induced increases in c-Fos IR in the amygdala. As was expected, intra-CeA injection of DAMGO increased food intake of rats over a 4-h period. DAMGO injection into the CeA also resulted in mu opioid receptor internalization in the CeA, indicating activation of mu opioid receptor expressing neurons in this site. Administration of DAMGO into the CeA increased c-Fos IR levels in the shell of the nucleus accumbens (NAcc), but not in 17 other brain sites that were studied. We also found that intra-CeA injection of DAMGO, prior to LiCl injection, decreased c-Fos IR levels in the CeA compared to vehicle-injected rats. Thus, intra-CeA administration of DAMGO may increase feeding, in part, by activating neurons in the shell of the nucleus accumbens and by inhibiting activity of selected neurons in the CeA.