BINDING OF [H-3] PARA-AMINOCLONIDINE TO 2 SITES, ALPHA-2-ADRENOCEPTORS AND IMIDAZOLINE BINDING-SITES - DISTRIBUTION OF IMIDAZOLINE BINDING-SITES IN RAT-BRAIN

Binding sites labeled by [3H]p-Aminoclonidine ([3H]PAC were investigated by the competitive analysis with imidazoline and non-imidazoline derivatives. Phenylethylamine derivatives displaced only the part of specific sites for [3H]PAC, which was considered as α2-adrenoceptor, whereas imidazoline derivatives, such as clonidine and tolazoline, competed for a further specific binding of [3H]PAC to the non-adrenergic sites, in addition to the α2-adrenoceptor. Because the non-adrenergic sites were specific for the imidazoline structure, they were termed imidazoline sites. The imidazoline sites were not distributed uniformly among rat brain regions. In striatum, hippocampus and medulla oblongata, they occupied 39.6, 33.0 and 36.5% of the specific binding of [3H]PAC, respectively. Saturation isotherms revealed that Kd and Bmax of imidazoline sites for [3H]PAC were 3.09 ± 0.59 nM, 27.4 ± 1.7 fmol/mg protein and 2.23 ± 0.29 nM, 21.0 ± 1.5 fmol/mg protein in striatum and hippocampus, respectively. Because imidazoline binding sites also displayed weak affinities for imidazole compounds, such as histamine and cimetidine, the imidazoline site may be a subtype of histamine H2-receptor. © 1990.