Clonidine-induced potentiation of reflex vagal bradycardia in anaesthetized cats

The interactions between clonidine and the selective alpha 1- and alpha 2-adrenoceptor antagonists, prazosin and yohimbine, on the angiotensin-induced reflex vagal bradycardia have been investigated in propranolol-treated chloralose-anaesthetized cats. Clonidine (1-10 microgram intracisternally) had little effect on the pressor responses to angiotensin, only slightly increased the peak but markedly increased the duration of the reflex bradycardia. Neither prazosin nor yohimbine (200 microgram intracisternally of either antagonist) had any effect on the reflex bradycardia to angiotensin. Yohimbine greatly diminished the clonidine-induced potentiation of the response, but prazosin was without effect. Clonidine appears to potentiate the vagal reflex bradycardia by stimulating central alpha 2-adrenoceptors.