Pharmacological characterization of [³H]idazoxan, [³H]RX821002 and p-[¹²⁵I]iodoclonidine binding to α₂-adrenoceptors in rat cerebral cortical membranes

Binding characteristics of α₂-adrenoceptors in rat cerebral cortical membranes were compared using the antagonist radioligands [³H]idazoxan, [³H]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([³H]RX821002), and the partial agonist radioligand [¹²⁵I]2-[2,6-(dichloro-4-iodophenyl)imino]imidazoline ([¹²⁵I]iodoclonidine). With [³H]RX821002 and α₂-adrenoceptor subtype-selective competitors, both α₂A/D- and α₂C-adrenoceptor subtypes were detected, suggesting rat cortical membranes contain approximately 90% α₂A/D-adronoceptor subtype and 10% α₂C-adrenoceptor subtype. Only α₂A/D-adrenoceptors were detected with [³H]idazoxan and [¹²⁵I]iodoclonidine. All three radioligands bound to a single high affinity site (K_d = 0.3-1.6 nM). However, the densities of sites labeled by [³H]idazoxan and [¹²⁵I]iodoclonidine were 50% greater than the density labeled by [³H]RX821002, likely representing non-adrenoceptor binding sites. The density of [¹²⁵I]iodoclonidine binding sites in glycylglycine buffer was similar to that labeled by [³H]RX821002. These results suggest that: (1) α_2A/D-adrenoceptors are the predominant subtype in rat cerebral cortex, (2) demonstrate that the small number of α_2C-adrenoceptors in this tissue can be detected using prazosin to displace [³H]RX821002 binding, and (3) non-adrenoceptor binding with [¹²⁵I]iodoclonidine can be minimized with the use of glycylglycine buffer.