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The pharmacological properties of [3H]-WB4101, [3H]-clonidine and [3H]-dihydroalprenolol binding in chick brain membranes display the characteristics known for alpha 1-, alpha 2- and beta-adrenergic binding sites, respectively. Kinetic studies performed at different embryonic and post-hatching ages have shown one binding component for each one of the above radioactive ligands. The ontogeny of alpha 1, alpha 2 and beta binding sites was studied in cerebral hemispheres, optic lobes, brain stem and cerebellum. In all brain regions studied, the development of alpha 2 binding sites precedes that of alpha 1 and beta, and a very significant decrease of alpha 2 number was observed in the cerebellum, brain stem and optic lobes afterwards. The autoradiographic localization of adrenergic receptors was studied in the optic lobes and cerebellum. In the optic lobes the superficial layers of stratum griseum and fibrosum showed a strong selective labelling of alpha 1, alpha 2 and beta binding sites and the strong selective labelling of alpha 2 binding sites extended to the layer of stratum opticum. Among the nuclei located in the optic lobe only the nucleus mesencephalis lateralis pars dorsalis (MLD) exhibited a strong selective labelling for alpha 1 binding sites while, for beta binding sites, not only the MLD, but also the nucleus isthmic pars parvocellularis (Ipc) and the nucleus isthmic pars magnocellularisa (Imc) exhibited strong labelling. In the cerebellum strong selective labelling for alpha 1 and beta receptors was seen in the molecular layer. Labelling of the granule cell layer was almost equally strong for alpha 1 but significantly less for beta binding sites. No significant labelling could be detected for alpha 2 binding sites.

In the present work [3H]-WB4101, [3H]-DHA, and [3H]-clonidine were used for the study of the localization of alpha 1, alpha 2, and beta adrenergic receptors in the chick brain. The highest concentration of [3H]-WB4101 was observed in the nucleus pretectalis, followed by the nucleus brachium conjunctivum descendens. The superficial layers of stratum griseum fibrosum superficiale, the nucleus mesencephalis lateralis pars dorsalis, and the locus coeruleus showed concentrations of [3H]-WB4101 binding higher than 300 fmoles/mg protein. Concentrations of [3H]-DHA binding higher than 300 fmoles/mg protein were observed in the paleostriatum, the external part of nucleus pretectalis, the nucleus isthmi parvocellularis, the nucleus mesencephalis lateralis pars dorsalis, the dorsal nucleus of oculomotor center, and the molecular layer of cerebellum. Locus coeruleus was the only area of chick brain which showed concentration of [3H]-clonidine binding higher than 300 fmoles/mg protein. With few exceptions, [3H]-clonidine binding was very low and in the telencephalon it was undetectable.

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