Sites of actions of alpha 2 agonists in the brain and periphery

Alpha 2 agonists including rilmenidine, clonidine and alpha methylnoradrenaline all seem to have a predominantly central action in lowering blood pressure. Studies using the peripheral decarboxylase inhibitor alpha-methyldopa hydrazine in both experimental animals and humans support the predominant central action of the active metabolite of methyldopa, alpha methylnoradrenaline. Comparative studies on the antihypertensive potency of selective alpha 2 agonists like rilmenidine, guanfacine, clonidine and lofexidine suggest that the relative ability to bind to alpha 1 and alpha 2 adrenoceptors is not critical but the ability to enter the central nervous system is essential for antihypertensive efficacy.