RS-45041-190 - A SELECTIVE, HIGH-AFFINITY LIGAND FOR I-2 IMIDAZOLINE RECEPTORS

1 RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline showed high affinity for I-2 imidazoline receptors labelled by [H-3]-idazoxan in rat (pK(i) = 8.66 +/- 0.09), rabbit (pK(i) = 9.37 +/- 0.07), dog (pK(i) = 9.32 +/- 0.18) and baboon kidney (pK(i) = 8.85 +/- 0.12), but had very low affinity for alpha(2)-adrenoceptors in rat cerebral cortex (pK(i) = 5.7 +/- 0.09). 2 RS-45041-190 showed low affinity for other adrenoceptors, dopamine, 5-hydroxytryptamine, and muscarinic receptors and dihydropyridine binding sites (selectivity ratio > 1000). 3 RS-45041-190 showed moderate potency for the inhibition of monoamine oxidase A in vitro (pIC(50) = 6.12), but had much lower potency for monoamine oxidase B (pIC(50) = 4.47), neither of which equated with its affinity for I-2 receptors. 4 RS-45041-190 (0.001 to 3 mg kg(-1), i.v. and 1 ng-50 mu g i.c.v.) had only small, transient effects on blood pressure and heart rate in anaesthetized rats. In conscious rats, RS-45041-190 had no effect on body core temperature or tail skin temperature (1 mg kg(-1), s.c.) or on activity or rotarod performance (10 mg kg(-1), i.p.). There were also no effects on barbiturate sleeping time in mice after doses of 1-10 mg kg(-1), i.p. 5 RS-45041-190 (10 and 25 mg kg(-1), i.p.) significantly increased food consumption in rats for up to 4 h after dosing, but unlike idazoxan (10 mg kg(-1), i.p.) did not increase water consumption. 6 RS-45041-190 is therefore a selective, high-affinity ligand at I-2 imidazoline receptors and its hyperphagic effect may suggest a role for I-2 imidazoline receptors in the modulation of appetite. However, in the absence of a selective agonist it is unclear whether this ligand is an agonist or an antagonist at I-2 receptors.