Augmentation of vagal reflex bradycardia by central ?1adrenoceptors in the cat

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Abstract
Vagal reflex bradycardia was induced in anaesthetized cats with high level spinal axotomy by electrical stimulation of either the carotid sinus nerves or a depressor nerve. In both preparations reflex bradycardia increased with the rate of stimulation. Injection of 1 μg/kg clonidine into a lateral cerebral ventricle augmented reflex bradycardia in response to carotid sinus nerve stimulation while the same dose of clonidine was ineffective when given intravenously. The antagonistic effect of intracerebroventricular yohimbine (50 μg/kg) indicated that the effect of clonidine was due to its α1agonistic action. In contrast to carotid nerve stimulation the reflex bradycardia in response to depressor nerve stimulation was affected neither by intracerebroventricular injection of clonidine (2 μg/kg) nor by yohimbine (100 μg/kg). It is concluded that in the cat, the function of the central parts of the baroreceptor reflex which originate from the carotid sinus area is augmented by stimulation of α1adrenoceptors while the function of those parts originating from the aortic area is not.