Free Radical Production Is Increased During Hyperoxia And Exacerbated By Hypercapnic Acidosis (HA) In CO 2 ‐Chemosensitive Caudal Solitary Complex (cSC) Neurons In Rat Medullary Slices

Abstract

CNS O2 toxicity (CNS-OT) seizures are the limiting factor when breathing hyperbaric O2 in diving & submarine medicine. O2-induced seizures are the result of oxidative and nitrosative stress that lead to CNS hyperexcitability. The risk for CNS-OT increases during CO2 retention. The cSC is an important cardiorespiratory integrative center that is stimulated during both HA & hyperoxia. The stimulatory effects of O2 & CO2 on cellular excitability suggests these molecules modulate one another in the CNS-OT mechanism. We have tested the following hypotheses in rat brain slices: 1) hyperoxia (95%O2) increases the production of reactive O2 & N2 species (RO/NS) in cSC cells (control O2, 40%), and 2) addition of HA exacerbates the rate of RO/NS production. We also investigated the effects of CO2-anesthesia on RO/NS production during subsequent administration of HA & hyperoxia. Fluorescence microscopy was used to measure the production rates of superoxide (˙O2-), nitric oxide (˙NO), and an aggregate pool of RO/NS (ONOO-, ˙NO2,˙CO3-) as the slope of fluorescence intensity (ΔFI/min). Results show the ΔFI/min of ⋅O2- and ⋅NO increased during hyperoxia, but were only seen in the presence of NOS and SOD inhibitors or SOD mimetics, respectively. Addition of HA increased the ΔFI/min of the aggregate pool, but only during hyperoxia. The effects of HA during hyperoxia are greater when CO2 anesthesia is not used prior to tissue harvesting. We conclude that 1) hyperoxia produces an enhanced redox and nitrosative environment for cSC neurons, 2) HA exacerbates downstream RO/NS production in an O2-dependent manner, and 3) use of CO2 anesthesia before euthanasia may blunt the effects of HA. ONR