Evidence for chemosensitive fibers in the aortic depressor nerve in mice but not in rats

Abstract

A “J‐shaped” response of the aortic depressor nerve (ADN) to changes in aortic pressure has been described in rabbits. Specifically, at low pressure a paradoxical increase in ADN activity was noted during the diastolic phase of the cardiac cycle. We studied the physiological significance of this phenomenon. Lowering blood pressure (BP, Na‐nitroprusside) in anesthetized rats (n=4) abolished any ADN activity, suggesting that the “J‐shaped” response does not exist in rats. In anesthetized mice (spontaneously breathing 100% O2, n=11) total and systolic ADN activities mirrored BP changes elicited by Na‐nitroprusside and phenylephrine. In contrast, diastolic discharge activity showed an inverse relationship with BP, i.e., a decrease in BP caused an increase in diastolic ADN activity and vice versa. Electrical stimulation of the ADN (n=11 mice) during different phases of the cardiac cycle decreased BP and heart rate independently of the phase of the cardiac cycle, suggesting that central nervous system processing of ADN activity is independent of the cardiac cycle. Switching the inspiratory gas from 100% O2 to room air caused an immediate increase in diastolic ADN activity by 27.7±4.9% (n=4 mice) even before the onset of hypoxia‐induced hypotension. These results suggest that the ADN in mice (and possibly rabbits) but not in rats contains chemosensitive nerve fibers originating from glomus cells in the aortic bodies.