Interference with glutamate antiporter system xc− enables post‐hypoxic long‐term potentiation in hippocampus

• Published in: Experimental physiology Vol. 109; no. 9; pp. 1572 - 1592
• Main Authors: Heit, Bradley S., Chu, Alex, McRay, Alyssa, Richmond, Janet E., Heckman, Charles J., Larson, John
• Format: Journal Article
• Language: English
• Published: Oxford John Wiley & Sons, Inc 01.09.2024; Wiley
• Subjects: Adenosine A1 receptors; Anoxia; Calcium influx; cystine/glutamate transporter; Depolarization; Excitotoxicity; Genotypes; Glutamate receptors; Glutamic acid receptors (ionotropic); Hippocampus; Hypoxia; Ischemia; Latency; Long-term potentiation; N-Methyl-D-aspartic acid receptors; Neural plasticity; Neurotransmission; Paired-pulse facilitation; Receptor mechanisms; stroke; Synaptic plasticity; Synaptic transmission
• ISSN: 0958-0670; 1469-445X; 1469-445X
• DOI: 10.1113/EP092045

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. What is the central question of this study? Does glutamate antiporter system xc− influence recovery in the ischemic penumbra? What is the main finding and its importance? Our data reveal that mild hypoxia elicits a persistent potentiation of synaptic transmission in hippocampus when system xc− is genetically deleted or pharmacologically inhibited. This ‘post‐hypoxic long‐term potentiation (LTP)’ engages similar mechanims as classical NMDAR‐dependent LTP induced by theta‐burst stimulation (TBS). Because this neuroplasticity potentially fosters recovery in penumbral tissue, these findings portend system xc− antagonism as a post‐stroke therapeutic intervention. (a) When a cerebral blood vessel becomes occluded during stroke, two distinct regions form: the ischaemic core and the ischaemic penumbra. The core suffers severe oxygen deprivation (anoxia), glutamate excitotoxicity, anoxic depolarization and rapid cell death. The penumbra, which is the outer rim of the core, experiences mild ischaemia (hypoxia) and altered synaptic signalling but retains the potential to be spared via adaptations in neuronal excitability. (b–e) Employing hippocampal slice electrophysiology, we modelled the ischaemic penumbra by subjecting wild‐type (WT) mice and system xc− knockout (xCT−/−) mice to hypoxia. When compared to pre‐hypoxia baseline responses (PRE), adenosine‐mediated suppression of synaptic responses during hypoxia (HYP) was equivalent between genotypes. xCT−/− responses, however, displayed potentiation in the post‐hypoxia phase (POST), which resembled LTP. This post‐hypoxic LTP was abolished by the NMDA receptor antagonist DAP5, but was unaffected by the A1R antagonist CPX and system xc− inhibitor CPG. WT responses phenocopied post‐hypoxic LTP after treatment with CPG or when incubated in elevated Ca2+ concentrations. Collectively, these data suggest that post‐hypoxic LTP with system xc− interference is driven by enhanced postsynaptic depolarization and (1) removal of Mg2+ from NMDARs which (2) allows Ca2+ influx, thus (3) increasing AMPA receptor insertion. Moreover, these findings reveal a potential mechanism for improved recovery in penumbral tissue when system xc− is absent or inhibited.