Effects of intraventricularly injected 6-hydroxydopamine on carotid sinus baroreceptor reflex in rabbits

Effects of 6-hydroxydopamine (6-OH-DA) injected into the lateral brain ventricle on the carotid sinus baroreceptor reflex were studied in rabbits anesthetized with alpha-chloralose and urethane. As short term effects, injection of 500 mug/kg of 6-OH-DA caused a fall in blood pressure and heart rate, enhanced the depressor and bradycardia responses to electrical stimulation of the carotid sinus nerve (CSN), and inhibited the pressor response to carotid occlusion. These effects reached the maximum with 2 hrs and disappeared by the 4th hr. Intraventricular injection of noradrenaline (NA) could mimic most of these effects. At 4.5 hrs after injection of 6-OH-DA, NA content of the brain was definitely reduced: 21% of control in the hypothalamus and 14% in the pons-medulla. Rabbits treated with 6-OH-DA under penthobarbital anesthesia 24 hrs before showed a slight fall in resting blood pressure and almost normal baroreceptor function. Intraventricular application of phentolamine abolished the responses to CSN stimulation in 6-OH-DA pretreated as well as normal animals. These results suggest that the acute effects of 6-OH-DA are based on the increased release of NA from the affected nerve terminals and that noradrenergic neurons are involved in the central pathway of baroreceptor reflex. Moreover, the relfex may be functionally maintained by a small portion of brain NA content, even when noradrenergic neurons are greatly affected by 6-OH-DA.