Anoxic insults cause hyperexcitability and cell death in mammalian neurons. Conversely, in anoxia-tolerant turtle brain, spontaneous electrical activity is suppressed by anoxia (i.e., spike arrest; SA) and cell death does not occur. The mechanism(s) of SA is unknown but likely involves GABAergic synaptic transmission, because GABA concentration increases dramatically in anoxic turtle brain. We investigated this possibility in turtle cortical neurons exposed to anoxia and/or GABA A/B receptor (GABAR) modulators. Anoxia increased endogenous slow phasic GABAergic activity, and both anoxia and GABA reversibly induced SA by increasing GABA A Rmediated postsynaptic activity and Cl − conductance, which eliminated the Cl − driving force by depolarizing membrane potential (∼8 mV) to GABA receptor reversal potential (∼−81 mV), and dampened excitatory potentials via shunting inhibition. In addition, both anoxia and GABA decreased excitatory postsynaptic activity, likely via GABA B R-mediated inhibition of presynaptic glutamate release. In combination, these mechanisms increased the stimulation required to elicit an action potential >20-fold, and excitatory activity decreased >70% despite membrane potential depolarization. In contrast, anoxic neurons cotreated with GABA A+B R antagonists underwent seizure-like events, deleterious Ca 2+ influx, and cell death, a phenotype consistent with excitotoxic cell death in anoxic mammalian brain. We conclude that increased endogenous GABA release during anoxia mediates SA by activating an inhibitory postsynaptic shunt and inhibiting presynaptic glutamate release. This represents a natural adaptive mechanism in which to explore strategies to protect mammalian brain from low-oxygen insults.western painted turtle | cerebral cortex | channel arrest | pyramidal neurons | natural anesthetic mechanism W hen deprived of oxygen, mammalian neurons are unable to produce sufficient ATP to meet cellular demands (1, 2). As a result, the Na + /K + ATPase (Na + pump) fails and neuronal membrane potential (V m ) becomes unsustainable and anoxic depolarization (AD) follows, causing electrical hyperexcitability, deleterious Ca 2+ influx, and spreading depression in the penumbral region (2, 3). Numerous studies have focused on the role of glutamatergic N-methyl-D-aspartate receptors (NMDARs) in this mechanism, and although NMDAR blockade prevents glutamatergic excitotoxicity (4), it does not prevent AD-mediated injury or postinsult apoptotic cell death (5). Thus, it is not surprising that clinical interventions targeting glutamate receptors alone have been largely ineffective against anoxic or ischemic damage (6), and therefore examination of alternative mechanisms to limit excitability during such insults is necessary.A potential therapeutic alternative to directly antagonizing excitatory pathways is to up-regulate inhibitory mechanisms such as those mediated by γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the mature mammalian CNS (7). GABAergic mechanisms are not strongly recr...