Ac. Mackinnon et al., [H-3] RS-45041-190 - A SELECTIVE HIGH-AFFINITY RADIOLIGAND FOR I-2 IMIDAZOLINE RECEPTORS, British Journal of Pharmacology, 116(2), 1995, pp. 1729-1736
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.