. Hyperbaric oxygen and chemical oxidants stimulate CO2/H ϩ -sensitive neurons in rat brain stem slices. J Appl Physiol 95: 910-921, 2003. First published April 18, 2003 10.1152/japplphysiol.00864. 2002-Hyperoxia, a model of oxidative stress, can disrupt brain stem function, presumably by an increase in O2 free radicals. Breathing hyperbaric oxygen (HBO 2) initially causes hyperoxic hyperventilation, whereas extended exposure to HBO 2 disrupts cardiorespiratory control. Presently, it is unknown how hyperoxia affects brain stem neurons. We have tested the hypothesis that hyperoxia increases excitability of neurons of the solitary complex neurons, which is an important region for cardiorespiratory control and central CO2/H ϩ chemoreception. Intracellular recordings were made in rat medullary slices during exposure to 2-3 atm of HBO 2, HBO 2 plus antioxidant (Trolox C), and chemical oxidants (N-chlorosuccinimide, chloramine-T). HBO 2 increased input resistance and stimulated firing rate in 38% of neurons; both effects of HBO 2 were blocked by antioxidant and mimicked by chemical oxidants. Hypercapnia stimulated 32 of 60 (53%) neurons. Remarkably, these CO 2/H ϩ -chemosensitive neurons were preferentially sensitive to HBO2; 90% of neurons sensitive to HBO2 and/or chemical oxidants were also CO2/H ϩ chemosensitive. Conversely, only 19% of HBO2-insensitive neurons were CO2/H ϩ chemosensitive. We conclude that hyperoxia decreases membrane conductance and stimulates firing of putative central CO2/H ϩ -chemoreceptor neurons by an O2 free radical mechanism. These findings may explain why hyperoxia, paradoxically, stimulates ventilation. central chemoreception; reactive oxygen species; cardiorespiratory control; intracellular recording; hyperoxia THE CENTRAL NERVOUS SYSTEM (CNS) is especially sensitive to oxidative stress. For example, hyperoxia, which is a popular model of oxidative stress, induced by breathing high levels of oxygen at hyperbaric pressure [i.e., hyperbaric oxygen (HBO 2 )], can rapidly disrupt neural function and result in CNS O 2 toxicity (16). Neurological responses to hyperoxia vary, depending on the oxygen tension in the brain and the duration of exposure. For example, the CNS response to hyperoxia can range from moderate, but reversible, changes in neural activity (7,49), to violent and reversible seizures at higher levels of oxygen (16), to irreversible motor deficits and ultimately death at the highest dosages of hyperoxia (16). In each of these instances, the effects of hyperoxia on the CNS are thought to result from increased production and accumulation of O 2 free radicals and subsequent oxidation of cellular components vital to maintaining normal mechanisms of neuronal excitability (16).The cardiorespiratory centers of the brain stem, similarly, are sensitive to a broad range of inspired oxygen. Hypoxia increases alveolar ventilation, primarily by stimulation of the peripheral chemoreceptors (18), although a central stimulatory effect on ventilation has also been reported (47, 64). Paradoxica...