Hyperoxia and Hypercapnic Acidosis Increase Free Radical Production and Cellular Excitability in Rat Caudal Solitary Complex Brain Slices

Abstract

The caudal solitary complex (cSC) is a cardiorespiratory integrative center in the dorsal medulla oblongata that plays a vital role in the central CO2‐chemoreceptive network. Neurons in this area respond to hypercapnic acidosis (HA) by a depolarization of the membrane potential and increase in firing rate, however a definitive mechanism for this response remains unknown. Likewise, CO2‐chemoreceptive neurons in the cSC respond to hyperoxia in a similar fashion, but via a free radical mediated mechanism. It still remains unknown if the response to increased pO2 is merely an increase in redox signaling, or if it's the result of a pathological state of redox stress. Importantly, free radical production is known to be stimulated by increasing pO2, and can be exacerbated downstream by the addition of CO2 and its subsequent acidosis. Conditions of hyperoxia in combination with HA can therefore become detrimental in several scenarios, including O2 toxicity seizures in divers and stranded submariners, as well as in cases of ischemia‐reperfusion injury and sleep apneas. As such, we sought to not only determine how O2 and CO2 interact to affect cellular excitability in the cSC, but also if these cells exhibited increases in redox signaling and/or stress. We employed sharp‐electrode intracellular electrophysiology to study whole‐cell electrical responses to varied combinations of hyperoxia (0.4–0.95/1.95 ATA O2) and HA (0.05–0.1 ATA CO2). Additionally, we used fluorescence microscopy under similar conditions to study changes in the production rates of various free radicals, including superoxide (·O2−), nitric oxide (·NO), and a downstream aggregate pool of CO2/H+‐dependent reactive oxygen and nitrogen species (RONS). Finally, we used several colorimetric assays to measure markers of oxidative and nitrosative stress, including malondialdehyde, 3‐nitrotyrosine, and protein carbonyls. Our hypothesis for these experiments was that hyperoxia and HA alone could produce effects, but would be more pronounced when used together. As such, we saw that ~89% of cells tested that were stimulated by both hyperoxia and HA showed larger firing rate responses to HA with an increased background of O2 (0.9–1.9 ATA) after showing a smaller response or no response to HA in control levels of O2 (0.4 ATA). Additionally, we noted that while the rate of ·O2− production did not increase in response to hyperoxia, it was actively consumed by ·NO and SOD. The rate of ·NO production did increase during hyperoxia compared to control. Downstream, our aggregate pool of RONS showed no increased production rates during HA in control O2, while significantly increasing during HA with background hyperoxia. Finally, no significant effects were seen when probing for markers of redox stress in response to hyperoxia and hypercapnic hyperoxia. Overall, these results suggest that that increased excitability seen in cSC neurons during hypercapnic hyperoxia is the result of physiological redox signaling rather than pathological redox stress. Further research needs to be done to determine how this redox mechanism is specifically resulting in increased cellular excitability.Support or Funding InformationONR Undersea Medicine