The central action of clonidine and its antagonism

1. We have examined the central actions of clonidine (2-(2-6-dichlorphenylamine)-2-imidazoline hydrochloride). It has been confirmed that when infused into the vertebral artery at 2 mug/min, it caused a decrease in blood pressure and a slight increase in heart rate. The same dose given intravenously or into the carotid artery had no effect.2. Intravertebral clonidine also greatly reduced the reflex response to carotid occlusion and the effects of an intravertebral infusion of angiotensin (1 ng/kg)/min.3. This central action of clonidine was antagonized by the adrenergic neurone blocking drug bethanidine (4-5 mg/kg intravenously) even after the cervical cord had been transected at C(4)-C(6) suggesting that bethanidine also has central actions.4. Other drugs which also antagonized the central effects of clonidine were guanethidine (4-5 mg/kg intravenously), bretylium (10 mg/kg intravenously) and phentolamine (0.2 mg/kg intravenously).5. It is suggested that there are central adrenergic neurones which inhibit cardiovascular autonomic reflexes and that the central autonomic effects of clonidine are due to stimulation of inhibitory adrenoceptors. The antagonism by adrenergic neurone blocking drugs of the effect of clonidine could therefore be due to blockade of these inhibitory pathways.6. The central action of clonidine could only be demonstrated when a high concentration was infused into the vertebral artery and could not be shown with oral doses of (20 mug/kg)/day for seven days. It is concluded that the hypotensive action of therapeutic doses is unlikely to be due to the central action of clonidine.