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

1. Clonidine (1 mug/kg), given by intracisternal injection to anaesthetized rabbits, lowered mean arterial blood pressure by 33 mmHg and heart rate by 32 beats/min.2. In animals pre-treated with 6-hydroxydopamine (6-OHD 500 mug/kg intracisternally) 7-10 days before, intracisternal clonidine (1 mug/kg) reduced mean arterial blood pressure by only 2.5 mmHg and heart rate by 4 beats/minute.3. 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 mug/kg) lowered mean arterial blood pressure by 19.3 mmHg, while after 6-OHD it fell only 9.4 mmHg.4. These studies suggest that the central hypotensive effect of clonidine is dependent on the integrity of central monoaminergic neurones.