RILMENIDINE LOWERS ARTERIAL-PRESSURE VIA IMIDAZOLE RECEPTORS IN BRAIN-STEM C1 AREA

We sought to determine the site of action and receptor type responsible for the antihypertensive actions of rilmenidine, an oxazoline analogue of clonidine. In anesthetized paralyzed rats decerebration did not alter the dose dependent reductions in arterial pressure and heart rate elicited by i.v. drug. Rilmenidine microinjected bilaterally into the C1 area of the rostral ventrolateral medulla (RVL), but not nucleus tractus solitarii (NTS) nor caudal ventrolateral medulla (CVL), elicited dose-dependent falls in arterial pressure and heart rate at doses an order of magnitude less than required systemically. Prior microinjection into the C1 area of the selective alpha-2-adrenoceptor antagonist SKF-86466, even at high doses, failed to modify the hypotension to i.v. rilmenidine. However, microinjection of 3- to 10-fold lower doses of idazoxan, a ligand for imidazole as well as alpha-2-adrenoceptors, blocked the effects. Rilmenidine also competed with the clonidine analogue [H-3]p-aminoclonidine ([H-3]PAC) at specific binding sites in membranes of bovine ventrolateral medulla and frontal cortex. In RVL rilmenidine competed with binding to imidazole and alpha-2-adrenergic binding sites with a 30-fold selectivity for the imidazole binding sites. In frontal cortex binding was of lower affinity and restricted to alpha-2-adrenergic sites. We conclude that rilmenidine, like clonidine, acts to lower arterial pressure by an action on imidazole receptors in the C1 area of RVL. The higher selectivity of rilmenidine for imidazole to alpha-2-adrenoceptors as compared to clonidine may explain the lower sedative effects of rilmenidine.