A COMPARATIVE-STUDY OF ALPHA(2)-ADRENOCEPTOR AND BETA-ADRENOCEPTOR DISTRIBUTION IN PIGEON AND CHICK BRAIN

The pharmacological properties and anatomical distribution of alpha(2) -, beta(1)- and beta(2)-adrenoceptors in pigeon and chick brains were studied by both homogenate binding and tissue section autoradiography. [H-3]Bromoxidine (alpha(2)-adrenoceptor-), [H-3]CGP 12177 (beta-adren oceptor) and [(1)25l]cyanopindolol (beta-adrenoceptor) were used as ra dioligands. In both species, [H-3]bromoxidine binding to avian brain t issue showed a pharmacological profile similar to that previously repo rted for alpha(2)-adrenoceptors in mammals. Regarding the anatomical d istribution, the areas with the highest densities of alpha(2)-adrenoce ptors in the pigeon brain included the hyperstriatum, nuclei septalis, tectum opticum and some brainstem nuclei. Most beta-adrenoceptors fou nd in tissue membranes and sections from chick and pigeon brain were o f the beta(2) subtype, in contrast to what has been reported in the ma mmalian brain, where the beta(1) subtype is predominant. A striking di fference was found between the two species regarding the densities of these receptors: while pigeon brain was extremely rich in [(125)l]cyan opindolol binding throughout the brain (mainly cerebellum) in the pige on, the levels of labelling in the chick brain were much lower; the ex ception was the cerebellum, which displayed a higher density than othe r parts of the brain in both species. Overall, our results support the proposed anatomical equivalences between a number of structures in th e avian and mammalian encephalon.