Three Binding Sites of ¹²⁵I-Iodocyanopindolol, i.e. β₁, β₂-Adrenergic and 5HT_1B-Serotonergic Receptors in Rat Brain Determined by the Displacement and Scatchard Analysis

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The selectivity of binding of ¹²⁵I-iodocyanopindolol (¹²⁵I-ICYP) to β₁-, β₂-adrenergic and 5HT_1B-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 β-selective antagonists, α-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 β₂-, β₁-adrenergic and 5HT_1B-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 (30μM) and 5-hydroxytryptamine (5HT, 10 μM), respectively, suggesting the correspondence of the high- and low-affinity sites to β-adrenergic and 5HT_1B-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 β₁- and β₂-adrenergic receptors becomes feasible only with the displacement method.