Central noradrenergic neurones and the cardiovascular actions of clonidine in the rabbit

Abstract

Clonidine (1 μg/kg), given by intracisternal injection to anaesthetized rabbits, lowered mean arterial blood pressure by 33 mmHg and heart rate by 32 beats/min. In animals pre‐treated with 6‐hydroxydopamine (6‐OHD 500 μg/kg intracisternally) 7–10 days before, intracisternal clonidine (1 μg/kg) reduced mean arterial blood pressure by only 2·5 mmHg and heart rate by 4 beats/minute. The hypotensive action of intravenous clonidine was reduced to 49% of control by pre‐treatment with intracisternal 6‐OHD. In unanaesthetized normal animals intravenous clonidine (30 μg/kg) lowered mean arterial blood pressure by 19·3 mmHg, while after 6‐OHD it fell only 94 mmHg. These studies suggest that the central hypotensive effect of clonidine is dependent on the integrity of central monoaminergic neurones. 1973 British Pharmacological Society