IDENTIFICATION, PARTIAL CHARACTERIZATION, AND HYPOTHALAMIC DISTRIBUTION OF KAPPA-OPIOID, MU-OPIOID, AND DELTA-OPIOID RECEPTORS IN A PASSERINE SONGBIRD (JUNCO-HYEMALIS)

Brain tissues obtained from a passerine songbird (dark-eyed junco, Junco hyemalis) were used to identify and partially characterize central opioid receptors. We found that [H-3]EKC (putative kappa ligand), [H-3]DAMGO (putative mu ligand), and [H-3]DPDPE and [H-3]pCl-DPDPE (two putative delta ligands) bind to brain tissue preparations specifically, in a time-dependent fashion, and with a high affinity (K(d)'S less-than-or-equal-to 5 nM). Binding sites are present at low concentrations (B(max) < 120 fmol/mg protein), and they are pharmacologically selective. In vitro autoradiography studies revealed a high density of delta receptors in hypothalamic regions (ventromedial and lateral hypothalamus) that regulate feeding behavior. Together with previous studies, these observations suggest that the central influence of opioids on avian food consumption depends on mechanisms located in hypothalamic regions.