Mu-opioid receptor is involved in beta-endorphin-induced feeding in goldfish

The present study evaluated the central effects of selective opioid receptor subtype agonists and antagonists on food intake in satiated goldfish. Significant increases in feeding behavior occurred in goldfish injected with beta-endorphin, the kappa agonist, U-50488, the delta agonist, [D-Pen(2),D-Pen(5)] enkephalin (DPEN), and the mu agonist, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol] enkephalin (DAMGO). On the other hand, the different receptor antagonists used: nor-binaltorphamine (nor-BNI) for kappa, 7-benzidilidenenaltrexone (BNTX) for delta(1), naltriben for delta(2), beta-funaltrexamine (beta-FNA) for mu, and naloxonazine for mu(1), by themselves, did not modify ingestion or slightly reduced it. The feeding stimulation by beta-endorphin was antagonized by beta-FNA and naloxonazine, but not by nor-BNI, BNTX, or naltriben. These data indicate that the mu-opioid receptor is involved in the modulation of the feeding behavior in goldfish.