Centrally induced reduction in sympathetic tone-a postsynaptic alpha-adrenoceptor-stimulating action of imidazolines

Naphazoline or oxymetazoline (both 30 mug/kg) were injected into the cisterna magna of anaesthetized cats and reduced blood pressure, heart rate and the electrical discharge rate of small fibre bundles of the preganglionic sympathetic splanchnic nerve. Cats were depleted of endogenous noradrenaline by pretreatment with reserpine (5 mg/kg, 18 h) and alpha-methyl-p-tyrosine (twice 300 mg/kg, 18 and 2 h). In these animals, intracisternal injection of 30 mug/kg oxymetazoline exerted a decrease of sympathetic discharges similar to that described for non-pretreated animals. In noradrenaline-depleted cats intracisternal injection of 1 mug/kg clonidine also decreased the sympathetic discharges. It is concluded that these imidazolines exert their sympathoinhibitory and cardiovascular effects by stimulation of postsynaptic alpha-adrenoceptors in the CNS.