Binding of an imidazolidine (clonidine), an oxazoloazepin (B-HT 933) and a thiazoloazepin (B-HT 920) to rat brain α-adrenoceptors and relation to cardiovascular effects

Abstract

The specific binding to α-adrenoceptors in crude plasma membrane preparations of the rat brain was studied by means of 3H-clonidine (specific radioactivity 26.7 Ci/mmole). Equilibrium binding of 3H-clonidine could be described adequately according to a two-site model with a minor population of high affinity sites (K D1 = 0.4 nM) and a major population of low affinity sites (K D2 = 6.1 nM). The heterogeneity of 3H-clonidine binding was also indicated by a biphasic association rate in kinetic binding studies. In competition experiments with 3H-clonidine concentrations of 0.5 or 4.0 nM respectively, concentration-dependent displacement was observed with the non-radioactive compounds: clonidine, B-HT 920 (2-amino-6-ally--5,6,7,8-tetrahydro-4H-thiazolo-[5,4-d]-azepin-dihydrochloride) and B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxozolo-[5,4-d]-azepinddihydrochloride). IC 50 values of 3, 21 and 160 nM or 10, 63 and 380 nM respectively were thereby evaluated. Cardiovascular effects were estimated in rats. The blood pressure increase in spinal animals was taken as parameter for α-adrenoceptor stimulation at peripheral vascular sites. The bradycardic effect in vagotomized animals was taken as parameter for central nervous sympathoinhibition. The ranking order of the potency of the three drugs was the same in both in vivo tests and parallels the in vitro binding affinities at both binding sites: clonidine > B-HT 920 > B-HT 933. These results indicate the similarity of the α-adrenoceptor structures in brain membrane preparations, at peripheral vascular sites and at central sympathoinhibitory sites.