[H-3] RS-45041-190 - A SELECTIVE HIGH-AFFINITY RADIOLIGAND FOR I-2 IMIDAZOLINE RECEPTORS

1 RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) is an I-2 imi dazoline receptor ligand with the highest affinity and selectivity so far described; [H-3]-RS-45041-190 has a tritium atom attached to the 7 -position on the isoindoline ring. 2 [H-3]-RS-45041-190 binding to rat kidney membranes was saturable (B-max = 223.1 +/- 18.4 fmol mg(-1) pr otein) and of high affinity (K-d= 2.71 +/- 0.59 nM). Kinetic studies r evealed that the binding was rapid and reversible, with [H-3]-RS-45041 -190 interacting with two sites or two affinity states. 3 Competition studies showed that 60-70% of [H-3]-RS-45041-190 binding (1 nM) was sp ecifically to imidazoline binding sites of the I-2 subtype, characteri zed by high affinity for idazoxan (pIC(50) 7.85 +/- 0.03) and cirazoli ne (pIC(50) 8.16 +/- 0.05). The remaining 30-40% was displaced specifi cally by the monoamine oxidase A inhibitors, clorgyline and pargyline. 4 alpha(1)- and alpha(2)-adrenoceptor, I-1 imidazoline, histamine, 5- hydroxytryptamine or dopamine receptor ligands had low affinity sugges ting that [H-3]-RS-45041-190 did not label receptors of these classes. 5 In autoradiography studies, [H-3]-RS-45041-190 labelled discrete re gions of rat brain corresponding to the distribution of I-2 subtypes, notably the subfornical organ, arcuate nucleus, interpeduncular nucleu s, medial habenular nucleus and lateral mammillary nucleus, and additi onal sites in the locus coeruleus, dorsal raphe and dorsomedial hypoth alamic nucleus. 6 [H-3]-RS-45041-190 therefore labels I-2 receptors wi th high affinity, and an additional site which has high affinity for s ome monoamine oxidase inhibitors.