Post-hypoxic changes in rat cortical neuron GABA A receptor function require L-type voltage-gated calcium channel activation

Hypoxia modifies GABA_A receptor (GABA_AR) function and can cause seizures, encephalopathy or myoclonus. To characterize the effects of hypoxia on neuronal GABA_ARs, we subjected rat cortical neurons to 1% O₂ for 2, 4 or 8 h, followed by recovery times of 0-96 h, and used whole-cell and perforated patch-clamp recording to assess GABA_AR currents and pharmacology. Hypoxic exposure for 4 h caused downregulation of maximal GABA current immediately following hypoxia and after 48 h recovery without changing the EC₅₀ for GABA. Two- and eight-hour hypoxic exposures had inconsistent effects on GABA_AR currents. Maximal diazepam potentiation was increased immediately following 4 h hypoxia, while potentiation by zolpidem was increased after 48 h recovery. Pentobarbital enhancement and zinc inhibition of GABA currents were unchanged. Hypoxia also caused a depolarizing shift in the reversal potential of GABA-induced Cl⁻ currents after 24 h recovery. The L-type voltage-gated calcium channel (L-VGCC) blocker, nitrendipine, during hypoxia or control treatment prevented the reduction in GABA_AR currents, and increased control currents over baseline. Nitrendipine also prevented the increase in zolpidem potentiation 48 h after hypoxia, and blocked the depolarizing shift in Cl⁻ reversal potential 24 h after hypoxia. The effects of hypoxia on maximal GABA_AR currents, zolpidem pharmacology and Cl⁻ reversal potential thus require depolarization-induced calcium entry via L-VGCCs, and constitutive L-VGCC activity appears to reduce maximal GABA_AR currents in control neurons via a calcium-dependent process. Calcium-dependent modulation of GABA_AR currents via L-VGCCs may be a fundamental regulatory mechanism for GABA receptor function.