Hyperbaric oxygen and chemical oxidants stimulate CO₂/H⁺-sensitive neurons in rat brain stem slices

Abstract

. 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...

Characterization of the chemosensitive response of individual solitary complex neurons from adult rats

“…The current study examined SC neurons exclusively taken from the caudal region of the SC from adult rats. We found very similar results to the results from the neonatal studies (5) and the results from Mulkey et al (22) in that 57% were activated with a CI of 177 Ϯ 8% and 9% were inhibited with a CI of 63 Ϯ 10%. The similar percentages of activated and inhibited SC neurons using a variety of techniques suggests that we are randomly sampling SC neurons, or else all techniques (perforated patch, whole cell patch, sharp tipped electrodes) have a similar bias, which seems unlikely.…”

“…Studies done on SC neurons from adult rats have only looked at the percentage of neurons that were activated by HA (7,10,21,22). In a mix of caudal and more rostral slices, 35% of SC neurons from adult rats were found to be activated in response to HA (7), while in caudal slices only, 53% of SC neurons from adult rats were activated in response to HA (22). The current study examined SC neurons exclusively taken from the caudal region of the SC from adult rats.…”

“…The brain stem was then removed and submerged in aCSF equilibrated with 5% CO2/95% O2 gas mixture. Transverse slices (300 m) were prepared on a vibratome (Pelco 101, series 1000) beginning at the obex and extending rostrally for ϳ1 mm, as previously described (22). The first slice that we used was the slice just caudal to area postrema, and we also used the next two slices going rostrally (the middle slice contained the area postrema).…”

“…Sharp-tip recordings were used to assure that our whole cell recordings did not cause washout of the electrical response to HA as previously described (11,22,40). Sharp-tip electrodes were also fabricated from borosilicate glass, filled with 3 M potassium acetate solution, and were pulled to a tip resistance of ϳ100 -150 M⍀ using a one-stage Flaming/Brown micropipette puller (P87, Sutter Instruments, Novato, CA).…”

“…PUTATIVE CENTRAL CHEMOSENSITIVE neurons have mostly been studied in neonatal rats in various localized regions within the brain stem, including the solitary complex [SC, including neurons from both the nucleus tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMN)] (5,7,10,22), ventrolateral medulla (54), retrotrapezoid nucleus (RTN) (24,44), medullary raphé (40,53), pre-Bötzinger region (47), and the locus coeruleus (LC) (14,32,35). All of these regions increase ventilation when focally acidified (4,26).…”