Influence of central pretreatment with 6-hydroxydopamine on the hypotensive effect of clonidine.

Intracisternal (i.ci.) pretreatment of rabbits with 6-hydroxydopamine (6-OH-DA) (3 times 0.5 mg/kg) reduced the norepinephrine content of the brainstem significantly by 52% but did not diminish the hypotensive and bradycardiac action of 2-(2,6-dichloroanilino)-2-imidazoline (clonidine; Catapresan; Catapres) (doses 10, 30 and 100 microgram/kg i.v. and 1 microgram/kg i.ci.) in anaesthetized animals. Pretreatment with 6-OH-DA did not abolish the enhancement by clonidine (100 microgram/kg i.v.) of reflex bradycardia elicited by angiotensin 0.2 microgram/kg i.v. The conclusion is drawn that clonidine acts directly on central alpha-adrenergic receptors independent of storage and synthesis of endogenous catacholamines in the central adrenergic neurons.