Characteristics of ^^125 I-Iodocyanopindolol Binding to β-Adrenergic and Serotonin-1B Receptors of Rat Brain

The present study was designed to examine the specificity of β-adrenergic antagonists for β_1 -, β_2 -adrenergic and 5HT_1B -serotonergic receptors by the competitive interaction with ^^125 I-iodocyanopindolol (^^125 I-ICYP) as a radioligand. The β_1 -adrenoceptors were preferred by acebutolol, atenolol, betaxolol, practolol, and I-, dl- and d-metoprolol, while butoxamine and ICI-118,551 preferred β_2 -adrenoceptors. The selectivities of these β_1 - and β_2 -antagonists are well-known, but alprenolol which is known as a non-selective antagonist was 7.2-fold more selective for the β_2 -adrenoceptors in the present study. All β-antagonists used were more selective towards β-adrenoceptors as compared with 5HT_1B -receptors. Good correlations were observed between the potencies of β-adrenoceptor antagonists for inhibition of ^^125 I-ICYP binding to β_1 - and β_2 -adrenoceptor sites and their potencies for inhibiting the binding of the same radioligand to 5HT_1B -serotonergic receptor sites. These results suggest that β-adrenoceptor antagonists can bind to β-adrenoceptors and 5HT_1B -receptors.