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

Abstract

1 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. 2 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. 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 μg/kg) lowered mean arterial blood pressure by 19·3 mmHg, while after 6-OHD it fell only 94 mmHg. 4 These studies suggest that the central hypotensive effect of clonidine is dependent on the integrity of central monoaminergic neurones.