Effects of a Lower Level of Control O₂, Hyperoxia and Hypercapnia on Solitary Complex Neurons in Medullary Tissue Slices

Studies of neuronal O ₂ -/CO ₂ -chemosensitivity in the brainstem often employ tissue slices maintained in medium equilibrated with 95%O ₂ , which we now know produces hyperoxia, increased production of reactive species, and oxidative stress. Accordingly, the objectives of this study were to determine if solitary complex (SC) neurons in medullary slices, incubated in 40%O ₂ , exhibit spontaneous, stable activity and are responsive to hyperoxia and hypercapnia. Medullary slices (300-400µm, P3-P21) were harvested in 95%O ₂ + 5%CO ₂ (2-4°C) and immediately transferred to 40%O ₂ + 5%CO ₂ (22-24°C). Whole-cell recordings (~37°C) of 69 neurons show a mean resting membrane potential of -42mV with spontaneous and evoked action potentials that overshoot 0mV. Whole cell recordings were made within 2-7.5hr post-slicing. To date, 25 neurons were tested using hyperoxia (95% O ₂ + 5% CO ₂ ), hypercapnia (40% O ₂ + 10%CO ₂ ) and hyperoxic hypercapnia (90%O ₂ + 10%CO ₂ ). The majority of neurons (13) were insensitive to hyperoxia/hypercapnia. Eight neurons were stimulated by hyperoxia and five by hypercapnia. Of these, four were stimulated by hyperoxic hypercapnia. Two neurons were inhibited by hypercapnia and one was inhibited by hyperoxia. We conclude that medullary slices can be maintained in 40%O ₂ for many hours for studies of CO ₂ /O ₂ -chemosensitivity at normobaric pressure. (ONR N000140710890, NIH R01 HL 56683-09).