PHARMACOLOGICAL AND MOLECULAR DISCRIMINATION OF BRAIN I-2-IMIDAZOLINERECEPTOR SUBTYPES

I-2-imidazoline receptors labelled with [H-3]-idazoxan in the rabbit a nd rat brains displayed high and low affinity, respectively, for the g uanidide amiloride; reinforcing the previous definition of I-2A-imidaz oline receptors expressed in the rabbit brain and I-2B-imidazoline rec eptors expressed in the rat brain. Other drugs tested displayed biphas ic curves in competition experiments, indicating the existence of high and low affinity sites for both subtypes of I-2-imidazoline receptors . Among the drugs studied, bromoxidine, moxonidine, (+)- and (-)-medet omidine and clorgyline were more potent on the high and/or low affinit y sites of I-2B- than on their corresponding of I-2A-imidazoline recep tors (K-iH ratios 20 to 65). No correlation was found for the potencie s of the drugs tested at the low affinity sites of both I-2-imidazolin e receptor subtypes. Preincubation (30 min at 25 degrees C) with 10(-6 ) M clorgyline reduced by 60% the B-max of [H-3]-idazoxan binding to I -2B-imidazoline receptors in the rat brain, but it did not affect the binding parameters of the radioligand saturation curves to I-2A-imidaz oline receptors in the rabbit brain. These results indicated that I-2A - and I-2B-imidazoline receptor subtypes differ in the pharmacological profiles of their high and low affinity sites and in the ability to i rreversibly bind clorgyline. In rat cortical membranes western blot de tection of immunoreactive imidazoline receptor proteins revealed a dou ble band of similar to 29/30 kDa and two less intense bands of similar to 45 and similar to 66 kDa. In rabbit cortical membranes the antibod y used detected proteins of similar to 30, similar to 57 and similar t o 66 kDa. It is suggested that different imidazoline receptor proteins (similar to 45 vs similar to 57 kDa) may account for the different ph armacological profiles of I-2-imidazoline receptor subtypes.