[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 imidazoline 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) protein) and of high affinity (K-d= 2.71 +/- 0.59 nM). Kinetic studies revealed 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 specifically to imidazoline binding sites of the I-2 subtype, characterized by high affinity for idazoxan (pIC(50) 7.85 +/- 0.03) and cirazoline (pIC(50) 8.16 +/- 0.05). The remaining 30-40% was displaced specifically 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 suggesting that [H-3]-RS-45041-190 did not label receptors of these classes. 5 In autoradiography studies, [H-3]-RS-45041-190 labelled discrete regions of rat brain corresponding to the distribution of I-2 subtypes, notably the subfornical organ, arcuate nucleus, interpeduncular nucleus, medial habenular nucleus and lateral mammillary nucleus, and additional sites in the locus coeruleus, dorsal raphe and dorsomedial hypothalamic nucleus. 6 [H-3]-RS-45041-190 therefore labels I-2 receptors with high affinity, and an additional site which has high affinity for some monoamine oxidase inhibitors.