Mechanisms for activation of aortic baroreceptor C‐fibres in rabbits and rats

Abstract

In an earlier study, we examined the pressure-response characteristics of rat aortic baroreceptors with C-fibre (non-medullated) afferents. Compared with aortic baroreceptor fibres with A-fibre (medullated) afferents, the C-fibres were activated at higher pressures and discharged more irregularly when stimulated with a steady level of pressure. Here we examine the relationship between discharge and the aortic diameter in these two types of afferents in rats and rabbits. An in vitro aortic arch/aortic nerve preparation was used to record single-fibre activity simultaneously with aortic arch pressure and diameter. Diameter was measured using a highly sensitive non-contact photoelectric device. Baroreceptor discharge was characterized by stimulating the nerve endings with either slow pressure ramps from subthreshold to 200-250 mmHg, at a rate of rise of 2 mmHg s-1, or pressure steps from subthreshold to suprathreshold levels, at amplitudes of 110-180 mmHg. In response to these inputs, C-fibres in rabbits (conduction velocities= 0.8-2.2 m s-1) behaved much like those in rats. The C-fibres had significantly higher pressure thresholds (95 +/- 3 mmHg vs. 53 +/- 2 mmHg; mean +/- SEM), lower threshold frequencies (2.4 +/- 0.5 vs. 27.7 +/- 1.8 spikes s-1), lower maximum discharge frequencies (22.7 +/- 2.3 vs. 65 +/- 5.8 spikes s-1) and more irregular discharge in response to a pressure step when compared with A-fibres (conduction velocities of 8-16 m s-1). When plotted against diameter, C-fibre ramp-evoked discharge increased gradually at first, and then rose steeply at increasingly higher ramp pressures where aortic diameter became relatively constant. In contrast, A-fibre discharge was linearly related to diameter over a wide range of pressure. These results suggest two interpretations: (1) The relation between stretch and C-fibre discharge is highly non-linear, with a marked increase in sensitivity at large diameters. (2) C-fibres are stimulated by changes in intramural stress rather than stretch.

Sensing of Blood Pressure Increase by Transient Receptor Potential Vanilloid 1 Receptors on Baroreceptors

“…Thus, TRPV1-expressing baroreceptive afferents are important for the feedback control of blood pressure stability. In general, A␦-fiber baroreceptors have lower pressure thresholds, higher mean discharge rates, and higher sensitivities than C-fiber baroreceptors (Coleridge et al, 1973;Thorén et al, 1977;Seagard et al, 1990;Kunze and Andresen, 1991;Thoren et al, 1999). These characteristic differences between A␦-and C-fibers support the notion that A␦-fibers are primarily important for baroreflex responses at normal and low blood pressure ranges, whereas C-fibers are mainly involved in baroreceptive regulation in the high blood pressure range.…”

“…The baroreceptive afferents are divided into myelinated (A␦-fiber) and unmyelinated (C-fiber) nerve fibers based on the presence of myelination and conduction velocity of the nerve axon (Krauhs, 1979). In general, A␦-fibers have lower pressure thresholds, higher firing rates, and more regular discharges than C-fibers (Thoren et al, 1999). Anodal blocking of A␦-fibers results in a significant decrease in baroreflex sensitivity without changing the baseline levels of arterial blood pressure (ABP) (Seagard et al, 1993).…”

Differences in the dynamic baroreflex characteristics of unmyelinated and myelinated central pathways are less evident in spontaneously hypertensive rats

“…In contrast, the same intervention only reduces SNA to ϳ40% in SD rats (12,13). Because C-fiber baroreceptors typically function at highpressure levels (24,25), the difference in the maximum SNA suppression suggests larger contribution of C-fiber baroreceptors to the SNA regulation in WKY compared with SD rats. Again, the dynamic properties of C-fiber axons could be different between WKY and SD rats, or otherwise, the mass of C-fiber axons may be different between WKY and SD rats and affected the degree of nonderivative characteristics.…”

“…One concern about both studies is that there was no attempt to selectively stimulate one of the two types of baroreceptor afferents: myelinated (A-fiber) and unmyelinated (C-fiber) afferents. In addition to the differences in baroreceptor transduction properties such as threshold pressure and firing frequency (4,11,20,24,25), there are significant differences in the dynamic characteristics between A-fiber and C-fiber central pathways (27). The A-fiber central pathway has strong derivative characteristics, whereas the C-fiber central pathway exhibits relatively weak derivative characteristics.…”

Nonlinear identification of the total baroreflex arc: chronic hypertension model

“…Therefore, downstream dynamic functioning pertaining to the control of vascular properties and/or cardiac contractility must have specifically been enhanced. 3) It may be possible that the linear component (faster dynamic system) and nonlinear component (squaring and slower dynamic systems) of the Uryson model correspond to myelinated and unmyelinated fiber pathways of the total arc (17,19) and that these pathways are differentially impacted in SHR relative to WKY.…”