Open-loop characteristics of the arterial baroreflex after blockade of unmyelinated baroreceptors with resiniferatoxin

- We investigated the open-loop characteristics of the baroreflex.
- Resiniferatoxin blocks C-fiber baroreceptor activity.
- The neural arc and total reflex arc had lower response ranges after resiniferatoxin.
- The peak sympathetic nerve activity response was largely unchanged.
- A-fibers maintain gain at normal pressures and C-fibers at high pressures.

Arterial baroreceptors can be divided into two categories dependent on whether their axons are myelinated (A-fiber) or unmyelinated (C-fiber). We investigated the effect of periaxonal resiniferatoxin (RTX), a blocker of C-fiber baroreceptor activity, on the open-loop static characteristics of the arterial baroreflex. The baroreceptor region of the right aortic depressor nerve was isolated, and intra-baroreceptor region pressure (BRP) was changed from 60 to 180 mm Hg in 10 anesthetized Sprague-Dawley rats. Open-loop static characteristics of the neural arc from BRP to efferent sympathetic nerve activity (SNA), peripheral arc from SNA to arterial pressure (AP), and total reflex arc from BRP to AP were estimated. Although blocking C-fiber activity with RTX resulted in a lower response range (33.7 ± 4.6% and 49.4 ± 4.8%, P < 0.01) and higher minimum SNA (78.0 ± 4.7% and 53.6 ± 5.0%, P < 0.001) of the steady-state neural arc, the peak SNA response to BRP was greater at a BRP of 160 mm Hg (− 37.87 ± 5.83% and − 26.28 ± 4.90%, P = 0.01). RTX also resulted in a lower response range (27.8 ± 5.0 mm Hg and 40.9 ± 5.2 mm Hg, P < 0.01) and higher minimum AP (92.4 ± 4.7 mm Hg and 79.1 ± 4.9 mm Hg, P < 0.001) of the total reflex arc. Despite these changes, the maximum slope of the neural arc and the maximum gain of the total reflex arc did not differ significantly after RTX. These data suggest that A-fiber baroreceptors can regulate AP and maintain the maximum gain when systemic AP is around the normal operating range. In contrast, C-fiber baroreceptors are critically important for reductions in SNA and AP when systemic AP is raised above the normal operating range.