Effects of various μ- and δ-opioid ligands on food intake in the meat-type chick

The present study was designed to examine the effects of μ- and δ-opioid receptor ligands on feeding behavior in meat-type chicks. Intracerebroventricular (ICV) injection of naltrexone (μ- and δ-antagonist), β-funaltrexamine (β-FNA; μ-antagonist), ICI-174,864 (ICI; δ-antagonist), or naloxonazine (NAL; μ1-antagonist) significantly decreased deprivation-induced feeding at 30 min postinjection. Co-injection of β-FNA, but not NAL, significantly blocked the depressive effect of [d-Ala2, N-MePhe4, Gly5-ol]-enkephalin (μ-opioid agonist) under ad libitum conditions. Central injection of ICI attenuated significant effects of [d-Pen2,5]-enkephalin (δ-opioid agonist) on feeding behavior in ad libitum fed chicks. Co-injection of β-FNA, but not ICI, significantly attenuated the orexigenic effect of [d-Ala2, d-Leu3]-enkephalin (μ- and δ-opioid agonist). These results suggest that the endogenous opioid peptides, which act on the μ- and/or δ-opioid receptor, have an important role in feeding behavior in the central nervous system of meat-type chicks.