PHARMACOLOGICAL CHARACTERIZATION OF [H-3] IDAZOXAN, [H-3] RX821002 AND P-[I-125]IODOCLONIDINE BINDING TO ALPHA(2)-ADRENOCEPTORS IN RAT CEREBRAL CORTICAL MEMBRANES

Binding characteristics of alpha(2)-adrenoceptors in rat cerebral cort ical membranes were compared using the antagonist radioligands [H-3]id azoxan, 3]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([H-3]RX821 002), and the partial agonist radioligand 25]2-[2,6-(dichloro-4-iodoph enyl)imino]imidazoline ([I-125]iodoclonidine). With [H-3]RX821002 and alpha(2)-adrenoceptor subtype-selective competitors, both alpha(2A/D)- and alpha(2C)-adrenoceptor subtypes were detected, suggesting rat cor tical membranes contain approximately 90% alpha(2A/D)-adrenaceptor sub type and 10% alpha(2C)-adrenoceptor subtype. Only alpha(2A/D)-adrenoce ptors were detected with [H-3]idazoxan and [I-125]iodoclonidine. All t hree radioligands bound to a single high affinity site (K-d = 0.3-1.6 nM). However, the densities of sites labeled by [H-3]idazoxan and [I-1 25]iodoclonidine were 50% greater than the density labeled by [H-3]RX8 21002, likely representing non-adrenoceptor binding sires. The density of [I-125]iodoclonidine binding sites in glycylglycine buffer was sim ilar to that labeled by [H-3]RX821002. These results suggest that: (1) alpha(2A/D)-adrenoceptors are the predominant subtype in rat cerebral cortex, (2) demonstrate that the small number of alpha(2C)-adrenocept ors in this tissue can be detected using prazosin to displace [H-3]RX8 21002 binding, and (3) non-adrenoceptor binding with [I-125]iodoclonid ine can be minimized with the use of glycylglycine buffer.