'Aortic baroreceptor' neurons in the nucleus tractus solitarius in rats: convergence of cardiovascular inputs as revealed by heartbeat-locked activity.

Rat aortic depressor nerve (ADN) contains only baroreceptor afferents. We identified 'aortic baroreceptor' neurons in the nucleus tractus solitarius (NTS) as those responding to electrical stimulation of the ADN and attempted to demonstrate convergence of cardiovascular mechanoreceptor inputs in these 'baroreceptor' neurons. In chloralose-urethane-anesthetized rats, ADN stimulation evoked either short or long latency responses (SLR, LLR) in 193 neurons of the NTS. 28 (SLR, 15; LLR, 13) demonstrated ongoing activities with cardiac rhythm despite the fact that the ADN had been cut peripherally. In 12 (SLR, 5; LLR, 7) of the 28 neurons, heartbeat-locked activity was abolished by carotid occlusion (CO), and augmented by methoxamine-induced blood pressure elevation, indicating that the heartbeat-locked activity originated from carotid sinus baroreceptors (CSB). In 11 neurons (SLR, 6; LLR, 5), the heartbeat-locked activity was not affected by CO but was abolished by topical application of lidocain on the ipsilateral cervical vagus, suggesting that the heartbeat-locked activity originated mostly from cardiac mechanoreceptors. The origin of the heartbeat-locked activity of the remaining 5 neurons could not be determined. The onsets, as well as peaks of the heartbeat-locked activity of vagal origin appeared significantly earlier than those of CSB origin. In conclusion, NTS neurons receive converging projection not only from the two major arterial baroreceptors but also from the arterial baroreceptors and cardiac mechanoreceptors, thereby integrating sensory information of vascular and cardiac origins.