Prolonged exposure to hypobaric hypoxia transiently reduces GABAA receptor number in mice cerebral cortex

The central nervous system is severely affected by hypoxic conditions, which produce alterations in neural cytoarchitecture and neurotransmission, resulting in a variety of neuropathological conditions such as convulsive states, neurobehavioral impairment and motor CNS alterations. Some of the neuropathologies observed in hypobaric hypoxia, corresponding to high altitude conditions, have been correlated with a loss of balance between excitatory and inhibitory neurotransmission, produced by alterations in glutamatergic and GABAergic receptors. In the present work, we have studied the effect of chronic hypobaric hypoxia (506 hPa, 18 h/day×21 days) applied to adult male mice on GABA_A receptors from cerebral cortex, to determine whether hypoxic exposure may irreversibly affect central inhibitory neurotransmission. Saturation curves for [³H]GABA specifically bound to GABA_A receptors in isolated synaptic membranes showed a 30% decrease in maximal binding capacity after hypoxic exposure (B_max control, 4.70±0.19, hypoxic, 3.33±0.10 pmol/mg protein), with no effect on GABA binding sites affinity (K_d control: 159.3±13.3 nM, hypoxic: 164.2±15.1 nM). Decreased B_max values were observed up to the 10th post-hypoxic day, returning to control values by the 15th post-hypoxic day. Pharmacological properties of GABA_A receptor were also affected by hypoxic exposure, with a 45 to 51% increase in the maximal effect by positive allosteric modulators (pentobarbital and 5α-pregnan-3α-ol-20-one). We conclude that long-term hypoxia produces a significant but reversible reduction on GABA binding to GABA_A receptor sites in cerebral cortex, which may reflect an adaptive response to this sustained pathophysiological state.