Distribution of mu, delta, and kappa opioid receptor binding sites in the brain of the one‐day‐old domestic chick (Gallus domesticus): An in vitro quantitative autoradiographic study

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Abstract
Three highly specific opioid ligands—[D‐Ala2, Gly‐ol.]‐enkephalin (DAGO) for mu (m̈) receptor sites, [D‐Pen2, D‐Pen5]‐enkephalin (DPDPE) for delta (δ) sites, and U‐69593 for kappa (κ) sites—were used to determine the regional distribution of the three major subtypes of opioid receptor binding sites in the brains of 1‐day‐old domestic chicks by the technique of quantitative receptor autoradiography. Whilst there was a degree of heterogeneity in the binding levels of each of the ligands, some notable similarities existed in the binding of the μ and κp ligands in several forebrain regions, and in the optic tectum of the midbrain where μ and δ binding was very high. In the forebrain there was a high level of binding of μ and κ ligands in the hyperstriatum, and for the μ ligand there was a very distinct lamination of binding sites in hyperstriatum accessorium, intercalatum supremum, dorsale and ventrale. Levels of binding of the μ and κ ligands were also high in nucleus basalis, and (for μ only) in the neostriatum. The distribution of binding of the δ specific ligand in the forebrain showed marked differences to that of μ and κ, being particularly low in the hyperstriatum and neostriatum. Very high levels of labelling of δ binding sites were, however, found in the nucleus rotundus. Binding of the three ligands was generally low or absent in the cerebellum and medulla, apart from a distinct labelling of the granule cell layer by the μ‐ligand. A kinetic analysis was made of the binding of the three ligands to whole forebrain sections using scintillation counting methods. The μ‐ligand DAGO had the highest affinity, KD (1.0 n mol L−1), followed by the κ ligand U69593 (4.6 n mol L−1), and then the δ ligand DPDPE (6.0 n mol L−1). The Bmax varied from 14.6 f mol/mg tissue for the μ ligand to 6.3 fmol/mg tissue (δ), and 4.3 fmol/mg tissue (κ). However, an analysis of the kinetics of binding of the μ, δ, and κ ligands to individual forebrain regions by quantitative receptor autoradiography revealed a considerable variation in the affinity for binding sites and in maximal binding levels. A previous study was made of the distribution of the three opioid binding sites in adult pigeon brain by Reiner et al. (Journal of Comparative Neurology 280:359–382, 1989). The present data show some similarities to the earlier study, but there are also marked dissimilarities which may be due either to species specific, or age related, differences. In the domestic chick the greatest densities of opiate receptor binding sites appear to coincide with regions involved in higher order sensory processing and memory storage.