Sex Differences in Responses of Nucleus Tractus Solitarii Neurons to Acute Tissue Hypoxia

Obstructive sleep apnea (OSA) is a risk factor associated with cardiovascular diseases, such as hypertension. Females are protected from the hypertensive and tachycardia effects of chronic intermittent hypoxia model of OSA. The present study examined the change in nucleus tractus solitarius (nTS) intracellular calcium level response to exposure to acute hypoxia in male rats and in female rats and the role of voltage gated calcium channel (VGCC) in mediating the response. Adult rat brainstem slices (250 μm thick) containing commissural and caudal nTS were incubated for 45 min with 10 μM Fura-2AM and 30 μL of F127 at room temperature and then washed for 20 min in artificial cerebrospinal fluid (aCSF) bubbled with 95% O2/5%CO2. A single slice was transferred to the recording chamber on an upright epifluorescent microscope and superfused with normal aCSF bubbled with 20% O2/5%CO2 balanced with N2 at a rate of 2.5 ml/min. Acute hypoxia was established by exposing hindbrain slices to aCSF bubbled with 95% N2/5% CO2. In slices where the role of VGCC was being studied, the slice was preincubated with 20 µM nifedipine in aCSF before the recording and was exposed to hypoxia in the presence of the same drug during the recording period. Fluorescence of Fura-2AM was excited by epi-illumination with light filtered alternatively at 340 or 380 nm while the emitted light passed through a barrier filter (510 nm). Pairs of 340 and 380 nm images were acquired at intervals of 5 s and analyzed off-line with NIS-Elements AR 3.2 software to yield 340/380 ratio. A total of 22 sections were examined from 3 male and 3 female rats and an average of 6 cells/slice were analyzed in each section. Five minutes of hypoxic aCSF triggered a greater increase from baseline in slices from male rats compared to female rats (males: 3±0.16%, n=49 vs females: 2.2+0.19%, n=40; P2+]i as a response to acute tissue hypoxia in caudal NTS is differentially regulated between male and female rats and is also dependent, at least in part, upon Ca2+ influx through L-type voltage gated calcium channel in the males. As hypoxia-induced elevations in intracellular Ca2+ are likely to alter caudal NTS neuronal function under hypoxic conditions, its differential regulation in males and females might mediate some of the protection seen at organismal level.