Aortic Depressor Response is Blunted Following Ovariectomy: Evidence for Involvement of Central but not Peripheral Afferent Pathways in Rat

Abstract

Sex differences in aortic baroreflex (BRx) function are well documented. In female rats, selective electrical stimulation of myelinated aortic baroreceptor fibers evokes a much greater reduction in blood pressure and heart rate than in age‐matched males. Our lab has a longstanding interest in the electrophysiological properties of vagal neurons that give rise to these sexually dimorphic barosensory fibers. We have previously shown that myelinated “Ah‐type” afferent neurons lose their capacity for sustained repetitive discharge after ovariectomy (OVX). Furthermore, 4 or more weeks after OVX the enhanced BRx response of female rats is blunted. This longitudinal study aims to quantify the transition in BRx dynamics over the first 4 weeks after OVX. Age‐matched cohorts of normal (OVI, n = 11) and OVX rats (n = 26) were prepared for bipolar electrical stimulation of aortic depressor nerve. Rats at 1 week post OVX were not significantly different than OVI controls. However, rats that were 2 to 4 weeks post OVX exhibited blunted depressor responses that were 3 – 4 times smaller than OVI. To determine whether Ah‐type afferent fiber conduction was being compromised, we measured electrically evoked compound action potentials (CAP) from additional cohorts of OVI (n = 10) and OVX rats (n = 15) at 2, 3 and 4 weeks after OVX. These studies revealed no significant changes in the recruitment profile of Ah myelinated fibers nor the integrated electrical energy of the CAP. Interestingly, at 4 or more weeks following OVX the CAP showed evidence of temporal dispersion. Collectively, our results suggest that the onset of early changes in BRx function after OVX are likely driven by central mechanisms as the evoked conduction profile of Ah barosensory fibers appears unaltered.