Identification, partial characterization, and hypothalamic distribution of κ, μ, and δ 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 [³H]EKC (putative κ ligand), [³H]DAMGO (putative μ ligand), and [³H]DPDPE and [³H]pCl-DPDPE (two putative δ ligands) bind to brain tissue preparations specifically, in a time-dependent fashion, and with a high affinity (K_d's ≦ 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 δ 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.