Labelling of Rat Brain ß-Adrenoceptors: (3H)CGP-12177 or (125I)Iodocyanopindolol?

Abstract

Binding of (¹²⁵I) iodocyanopindolol (ICYP) and (³H) CGP-12177 to rat brain homogenates was characterized and compared. ICYP was shown to bind to both ß-adrenergic and serotonin1B (5HT_1B) receptors whereas (³H)CGP-12177 only labelled the first ones. The addition of 10 μM serotonin (5HT) prevented ICYP binding to 5HT receptors and under these experimental conditions both ligands labelled a similar total number of ß-adrenoceptors in the different rat brain regions. ICYP displayed a higher affinity for cerebellar (mainly ß₂-subtype) than for cerebral cortex ß-adrenoceptors (mainly ß-subtype) suggesting a subtype selectivity. A multiple displacement binding approach using CGP-20712A, a ß₁-subtype ligand, as competitor revealed a 2.6 fold selectivity of ICYP for the ß₂-adrenoceptor subtype. On the other hand, (³H)CGP-12177 binds only to ß-adrenoceptors and is not subtype selective in the rat brain homogenate. Considering both its high specificity and its lack of subtype selectivity (³H)CGP-12177 seems to be a more suitable ligand than ICYP to non-selectively label ß-adrenoceptors in rat brain.