Effects of Sino-Aortic Denervation on the Acute Circulatory Responses to Centrally Administered 6-Hydroxydopamine and of Clonidine in Conscious Rabbits

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Abstract
In intact animals the circulatory responses evoked by transmitter released from specific CNS neurons, or by centrally administered drugs, depend on the direct effects initiated in the CNS and secondary reflex changes in autonomic activity. The contribution of the arterial baroreceptor reflexes to the mean arterial pressure (MAP) and heart rate responses produced by intracisternal (i.c.) clonidine and during the first few hours after i.c. 6-hydroxydopamine (6-OHDA), which releases transmitter from noradrenergic neurons, was studied. The direct and reflex components were assessed from the differences in responses between intact and sino-aortic denervated (SAD) rabbits, with the latter''s responses taken to approximate the direct CNS-induced effects. The direct effects of transmitter release were an early fall in MAP followed by a late pressor effect, and an early bradycardia followed by a late tachycardia. The early responses were similar to the direct effects of i.c. clonidine and to the findings observed in pontine rabbits with intact baroreceptors. In intact rabbits, feedback through the arterial baroreceptors reduced the fall in MAP with clonidine, but had no effect on the heart rate changes. It also eliminated the early depressor response to 6-OHDA but had little effect on the early bradycardia; however, the late pressor response was little affected, and the late tachycardia was masked by a reflex bradycardia. Arterial baroreceptor reflexes evidently can modify some effects initiated in the CNS, but some of the pathways are not involved in this interaction.