Three binding sites of 125I-iodocyanopindolol, i.e. beta 1, beta 2-adrenergic and 5HT1B-serotonergic receptors in rat brain determined by the displacement and Scatchard analysis

Abstract

The selectivity of binding of 125I-iodocyanopindolol (125I-ICYP) to beta 1-, beta 2-adrenergic and 5HT1B-serotonergic receptors were analysed using both the Scatchard and displacement methods. When the binding of 125I-ICYP to the rat brain membrane was examined by means of the displacement analysis with l-metoprolol, triphasic displacement curves were observed. Analyses carried out on the basis of the displacement of 125I-ICYP by beta-selective antagonists, alpha-adrenergic agonists and antagonists, and serotonergic agonists and antagonists indicated the correspondence of the super high, high- and low-affinity sites of 125I-ICYP binding to beta 2-, beta 1-adrenergic and 5HT1B-receptors, respectively. By contrast, the Scatchard analysis revealed the biphasic character of the 125I-ICYP binding and the complete inhibition of the binding to high- and low-affinity sites by l-metoprolol (30 microM) and 5-hydroxytryptamine (5HT, 10 microM), respectively, suggesting the correspondence of the high- and low-affinity sites to beta-adrenergic and 5HT1B-receptors. Thus, regarding the determination of the selectivity of binding of a radioligand, the displacement analysis is more informative than the Scatchard analysis. In the case of 125I-ICYP the separation of beta 1- and beta 2-adrenergic receptors becomes feasible only with the displacement method.