A comparative study of alpha2- and beta-adrenoceptor distribution in pigeon and chick brain

The pharmacological properties and anatomical distribution of alpha2-, beta1- and beta2-adrenoceptors in pigeon and chick brains were studied by both homogenate binding and tissue section autoradiography. [3H]Bromoxidine (alpha2-adrenoceptor-), [3H]CGP 12177 (beta-adrenoceptor) and [125I]cyanopindolol (beta-adrenoceptor) were used as radioligands. In both species, [3H]bromoxidine binding to avian brain tissue showed a pharmacological profile similar to that previously reported for alpha2-adrenoceptors in mammals. Regarding the anatomical distribution, the areas with the highest densities of alpha2-adrenoceptors in the pigeon brain included the hyperstriatum, nuclei septalis, tectum opticum and some brainstem nuclei. Most beta-adrenoceptors found in tissue membranes and sections from chick and pigeon brain were of the beta2 subtype, in contrast to what has been reported in the mammalian brain, where the beta1 subtype is predominant. A striking difference was found between the two species regarding the densities of these receptors: while pigeon brain was extremely rich in [125I]cyanopindolol binding throughout the brain (mainly cerebellum) in the pigeon, the levels of labelling in the chick brain were much lower; the exception was the cerebellum, which displayed a higher density than other parts of the brain in both species. Overall, our results support the proposed anatomical equivalences between a number of structures in the avian and mammalian encephalon.