[H-3]2-(2-benzofuranyl)-2-imidazoline: A new selective high affinity radioligand for the study of rabbit brain imidazoline I-2 receptors

This is the first study characterising the binding of the new imidazoline I-2 receptor selective radioligand [H-3]2-(2-benzofuranyl)-2-imidazoline (2-BFI) to rabbit brain membranes. [H-3]2-BFI binding was found to be saturable and of high affinity identifying two binding sites with K-D1 = 0.27 nM, B-max = 111.2 fmol mg(-1) protein and K-D2 = 8.97 nM, B-max = 268 fmol mg(-1) protein. Specific binding represented greater than 90% of total binding. Kinetic studies revealed that the binding was rapid and reversible and also showed [H-3]2-BFI interacted with these two sites or two affinity states. In competition binding studies against [H-3]2-BFI (0.3-lnM) idazoxan, 2-BFI, cirazoline, guanabenz, naphazoline, amiloride and BU224 (2-(4,5-dihydroimidaz-2-yl-quinoline) displaced with high affinity. In contrast the alpha(2)-adrenoceptor antagonists efaroxan and rauwolscine, the I-1 site selective drug moxonidine, the monoamine oxidase-A inhibitor clorgyline and the proposed endogenous imidazoline receptor ligand, agmatine, were weak at displacing [H-3]2-BFI binding. These findings are consistent with [H-3]2-BFI recognising imidazoline receptors of the I-2 subtype in rabbit brain.