Hyperbaric oxygen depolarizes solitary complex neurons in tissue slices of rat medulla oblongata

Hyperbaric oxygen (HBO2) at approximately 3 atmospheres absolute (ATA) pressure is toxic to the mammalian CNS due to excessive O2 free radical production. No study has ever determined the effects of < or = 3 ATA of O2 on the membrane potential and firing rate of neurons in the mammalian brainstem. Likewise, no study has ever determined the effects of < or = 3 ATA pressure per se on brainstem neurons. Accordingly, we initiated intracellular recordings at 1 ATA in solitary complex neurons in slices (300 microns) of rat caudal medulla oblongata that were maintained inside a 72 liter hyperbaric chamber. Helium, which is inert and without narcotic effect at moderate levels of hyperbaria, was used to hydrostatically compress the submerged brain slice to determine the effects of pressure per se. Tissue oxygen tension and extracellular pH were also measured during exposure to hyperbaric gases. Six of 19 neurons were affected by hyperbaric helium; 5 cells were depolarized and 1 cell was hyperpolarized. Input resistance (Rin) either increased (n = 1) or decreased (n = 3). When control perfusate (0.95 ATA O2) was switched to perfusate saturated with 98% O2 (balance CO2, pH = 7.3-7.4, pO2 = 2.5-3.4 ATA; 2-18 minutes of exposure) in a separate pressure vessel, 8 of 13 neurons were depolarized and 5 neurons were insensitive. In the 8 O2-responsive neurons, Rin either increased (n = 5), decreased (n = 2) or was unchanged (n = 1). Three of 8 neurons depolarized by HBO2 were also depolarized by hyperbaric helium, usually with an additional change in Rin. We conclude that hydrostatic (helium) pressure and HBO2 independently increase excitability in certain solitary complex neurons. We hypothesize that these responses contribute, in part, to neural events that either precede or occur during CNS O2 toxicity.