Interference with glutamate antiporter system xc - enables post-hypoxic long-term potentiation in hippocampus.

Our group previously showed that genetic or pharmacological inhibition of the cystine/glutamate antiporter, system xc -, mitigates excitotoxicity after anoxia by increasing latency to anoxic depolarization, thus attenuating the ischaemic core. Hypoxia, however, which prevails in the ischaemic penumbra, is a condition where neurotransmission is altered, but excitotoxicity is not triggered. The present study employed mild hypoxia to further probe ischaemia-induced changes in neuronal responsiveness from wild-type and xCT KO (xCT-/-) mice. Synaptic transmission was monitored in hippocampal slices from both genotypes before, during and after a hypoxic episode. Although wild-type and xCT-/- slices showed equal suppression of synaptic transmission during hypoxia, mutant slices exhibited a persistent potentiation upon re-oxygenation, an effect we termed 'post-hypoxic long-term potentiation (LTP)'. Blocking synaptic suppression during hypoxia by antagonizing adenosine A1 receptors did not preclude post-hypoxic LTP. Further examination of the induction and expression mechanisms of this plasticity revealed that post-hypoxic LTP was driven by NMDA receptor activation, as well as increased calcium influx, with no change in paired-pulse facilitation. Hence, the observed phenomenon engaged similar mechanisms as classical LTP. This was a remarkable finding as theta-burst stimulation-induced LTP was equivalent between genotypes. Importantly, post-hypoxic LTP was generated in wild-type slices pretreated with system xc - inhibitor, S-4-carboxyphenylglycine, thereby confirming the antiporter's role in this phenomenon. Collectively, these data indicate that system xc - interference enables neuroplasticity in response to mild hypoxia, and, together with its regulation of cellular damage in the ischaemic core, suggest a role for the antiporter in post-ischaemic recovery of the penumbra.

Tonic extracellular glutamate and ischaemia: glutamate antiporter system xc - regulates anoxic depolarization in hippocampus.

In stroke, the sudden deprivation of oxygen to neurons triggers a profuse release of glutamate that induces anoxic depolarization (AD) and leads to rapid cell death. Importantly, the latency of the glutamate-driven AD event largely dictates subsequent tissue damage. Although the contribution of synaptic glutamate during ischaemia is well-studied, the role of tonic (ambient) glutamate has received far less scrutiny. The majority of tonic, non-synaptic glutamate in the brain is governed by the cystine/glutamate antiporter, system xc - . Employing hippocampal slice electrophysiology, we showed that transgenic mice lacking a functional system xc - display longer latencies to AD and altered depolarizing waves compared to wild-type mice after total oxygen deprivation. Experiments which pharmacologically inhibited system xc - , as well as those manipulating tonic glutamate levels and those antagonizing glutamate receptors, revealed that the antiporter's putative effect on ambient glutamate precipitates the ischaemic cascade. As such, the current study yields novel insight into the pathogenesis of acute stroke and may direct future therapeutic interventions. KEY POINTS: Ischaemic stroke remains the leading cause of adult disability in the world, but efforts to reduce stroke severity have been plagued by failed translational attempts to mitigate glutamate excitotoxicity. Elucidating the ischaemic cascade, which within minutes leads to irreversible tissue damage induced by anoxic depolarization, must be a principal focus. Data presented here show that tonic, extrasynaptic glutamate supplied by system xc - synergizes with ischaemia-induced synaptic glutamate release to propagate AD and exacerbate depolarizing waves. Exploiting the role of system xc - and its obligate release of ambient glutamate could, therefore, be a novel therapeutic direction to attenuate the deleterious effects of acute stroke.

Synaptic and Network Contributions to Anoxic Depolarization in Mouse Hippocampal Slices.

Ischemic stroke remains the third leading cause of death and leading cause of adult disability worldwide. A key event in the pathophysiology of stroke is the anoxic depolarization (AD) of neurons in the ischemic core. Previous studies have established that both the latency to AD and the time spent in AD prior to re-oxygenation are predictors of neuronal death. The present studies used hippocampal slices from male and female mice to investigate the electrophysiological events that affect latency to AD after oxygen deprivation. The results confirm that the epoch between AD and re-oxygenation largely determines the magnitude of synaptic recovery after anoxic challenge. Using a selective antagonist of adenosine A1 receptors, we also confirmed that adenosine released during anoxia (ANOX) suppresses synaptic glutamate release; however, this action has no effect on AD latency or the potential for post-anoxic recovery of synaptic transmission. In contrast, antagonism of AMPA- and NMDA-type glutamate receptors significantly prolongs the latency to AD and alters the speed and synchrony of associated depolarizing waves. Experiments using slices with fields Cornu ammonis 3 (CA3) and Cornu ammonis 1 (CA1) disconnected showed that AD latency is longer in CA1 than in CA3; however, the early AD in CA3 is propagated to CA1 in intact slices. Finally, AD latency in CA1 was found to be longer in slices from female mice than in those from age-matched male mice. The results have implications for stroke prevention and for understanding brain adaptations in hypoxia-tolerant animals.