Inhibitory action of intracerebroventricular injections of norepinephrine and clonidine on ventricular extrasystoles induced by stimulating hypothalamus in rabbits

The effects of bilateral intracerebroventricular (icv) injections of norepinephrine (NE) and clonidine on hypothalamic stimulation-induced ventricular extrasystoles (VE) were investigated in alpha-chloralose and urethane anaesthetized rabbits. NE and clonidine reduced the number of HVE. The effects of NE were antagonized by pretreatment with phentolamine and yohimbine, but unaffected by pretreatment with prazosin. Pretreatment with yohimbine could also abolish the effects of clonidine. It is concluded that the effects of reduction in VE induced by NE and clonidine may be mediated by activating central alpha 2-adrenoceptors rather than alpha 1-adrenoceptors.