Neuronal membrane potential is mildly depolarized in the anoxic turtle cortex

Neuronal membrane potential (E_m) regulates the activity of excitatory voltage-sensitive channels. Anoxic insults lead to a severe loss of E_m and excitotoxic cell death (ECD) in mammalian neurons. Conversely, anoxia-tolerant freshwater turtle neurons depress energy usage during anoxia by altering ionic conductance to reduce neuronal excitability and ECD is avoided. This wholesale alteration of ion channel and pump activity likely has a significant effect on E_m. Using the whole-cell patch clamp technique we recorded changes in E_m from turtle cortical neurons during a normoxic to anoxic transition in the presence of various ion channel/pump modulators. E_m did not change with normoxic perfusion but underwent a reversible, mild depolarization of 8.1 ± 0.2 mV following anoxic perfusion. This mild anoxic depolarization (MAD) was not prevented by the manipulation of any single ionic conductance, but was partially reduced by pre-treatment with antagonists of GABA_A receptors (5.7 ± 0.5 mV), cellular bicarbonate production (5.3 ± 0.2 mV) or K⁺ channels (6.0 ± 0.2 mV), or by perfusion of reactive oxygen species scavengers (5.2 ± 0.3 mV). Furthermore, all of these treatments induced depolarization in normoxic neurons. Together these data suggest that the MAD may be due to the summation of numerous altered ion conductance states during anoxia.