Hyperoxia increases free radical production in rat brain slices in the dorsal CO2‐chemosensitive area of the caudal medulla oblongata (1092.5)

Abstract

Hyperoxia stimulates breathing and activates putative CO2 chemoreceptor neurons in the caudal solitary complex (cSC). We have previously shown that 95%O2‐the typical level of control O2 used for brain slice studies‐establishes an abnormally high tissue pO2 throughout the slice and is a powerful stimulant of neuronal activity in the cSC when using a lower level of control O2 (40%). Likewise, hyperbaric O2 has similar stimulatory effects on firing rate of cSC neurons, particularly CO2‐excited neurons, which are mimicked by chemical oxidants and abolished by antioxidants. Now we test the hypothesis that 95%O2 increases the rate of production of reactive O2 and N2 species in the cSC in slices maintained in 40%O2 (control). Fluorescence microscopy was used to measure the rate of superoxide (⋅O2‐, DHE) and nitric oxide (⋅NO, DAF‐FM DA) production. Our findings show that the rates of ⋅O2 and ⋅NO production increased, respectively, in 95%O2 vs. 40%O2, but only in the presence of NOS and SOD inhibitors (to prevent consumption of ⋅O2‐ by its reactions with ⋅NO and SOD) or SOD mimetics (to prevent consumption of ⋅NO by its reaction with ⋅O2‐). We conclude that 1) using 95%O2 in slices produces an enhanced redox and nitrosative environment that modulates excitability of cSC neurons, including CO2‐chemosensitive neurons; and 2) measurement of ⋅O2‐ using DHE or ⋅NO using DAF‐FM DA is affected (blunted) by downstream reactions that consume the reactive species of interest.Grant Funding Source: ONR