Sex differences in baroreflex (BRx) function are well documented. Hormones likely contribute to this dimorphism, but many functional aspects remain unresolved. Our lab has been investigating a subset of vagal sensory neurons that constitute nearly 50% of the total population of myelinated aortic baroreceptors (BR) in female rats but less than 2% in male rats. Termed "Ah," this unique phenotype has many of the nonoverlapping electrophysiological properties and chemical sensitivities of both myelinated A-type and unmyelinated C-type BR afferents. In this study, we utilize three distinct experimental protocols to determine if Ah-type barosensory afferents underlie, at least in part, the sex-related differences in BRx function. Electron microscopy of the aortic depressor nerve (ADN) revealed that female rats have less myelin (P < 0.03) and a smaller fiber cross-sectional area (P < 0.05) per BR fiber than male rats. Electrical stimulation of the ADN evoked compound action potentials and nerve conduction profiles that were markedly different (P < 0.01, n = 7 females and n = 9 males). Selective activation of ADN myelinated fibers evoked a BRx-mediated depressor response that was 3-7 times greater in female (n = 16) than in male (n = 17) rats. Interestingly, the most striking hemodynamic difference was functionally dependent upon the rate of myelinated barosensory fiber activation. Only 5-10 Hz of stimulation evoked a rapid, 20- to 30-mmHg reduction in arterial pressure of female rats, whereas rates of 50 Hz or higher were required to elicit a comparable depressor response from male rats. Collectively, our experimental results are suggestive of an alternative myelinated baroreceptor afferent pathway in females that may account for, at least in part, the noted sex-related differences in autonomic control of cardiovascular function.

Concomitant with menopause there is a significant reduction in the capacity of the baroreflex (BRx). Loss of so‐called sex hormones is but one factor. Our previous in situ BRx studies have validated and quantified the contribution of a sex specific subpopulation of low‐threshold myelinated aortic baroreceptor afferents (Ah‐type) in eliciting depressor dynamics not observed in male rats. Ovariectomy greatly reduces the excitability of these Ah‐type afferents while the excitability of low threshold myelinated A‐type and high threshold unmyelinated C‐type afferents is unchanged.OBJECTIVEDetermine if the ovariectomy‐mediated loss in excitability of Ah‐type afferents alters the depressor response evoked by this sex specific baroreceptor afferent pathway. Age‐matched, normal (NF) and ovariectomized (OVX) female rats were prepared for selective, bipolar electrical stimulation of the left aortic depressor nerve. The OVX (n = 9) presented with a significantly diminished parasympathetic mediated reduction in mean arterial pressure in comparison to the NF (n = 16). Interestingly, the most significant changes in depressor dynamics occurs at electrical thresholds and frequencies most closely aligned with Ah‐type baroreceptor fibers. We conclude that these sex specific afferents may provide a neuromodulatory pathway uniquely associated with the hormonal regulation of BRx function. HL072012, HL081819