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KEY POINTS: The hippocampal CA1 region is highly vulnerable to ischaemic stroke. Two forms of AMPA receptor (AMPAR) plasticity an anoxic form of long term potentiation and a delayed increase in Ca(2+) permeable (CP) AMPARs contribute to this susceptibility by increasing excitotoxicity. In CA1 the acid sensing ion channel 1a (ASIC1a) is known to facilitate LTP and contribute to ischaemic acidotoxicity. We have examined the role of ASIC1a in AMPAR ischaemic plasticity in organotypic hippocampal slice cultures exposed to oxygen glucose deprivation (a model of ischaemic stroke) and in hippocampal pyramidal neuron cultures exposed to acidosis. We find that ASIC1a activation promotes both forms of AMPAR plasticity and that neuroprotection by inhibiting ASIC1a circumvents any further benefit of blocking CP AMPARs. Our observations establish a new interaction between acidotoxicity and excitotoxicity and provide insight into the role of ASIC1a and CP AMPARs in neurodegeneration. Specifically we propose that ASIC1a activation drives certain post ischaemic forms of CP AMPAR plasticity. ABSTRACT: The CA1 region of the hippocampus is particularly vulnerable to ischaemic damage. While NMDA receptors play a major role in excitotoxicity it is thought to be exacerbated in this region by two forms of post ischaemic AMPA receptor (AMPAR) plasticity namely anoxic long term potentiation (a LTP) and a delayed increase in the prevalence of Ca(2+) permeable GluA2 lacking AMPARs (CP AMPARs). The acid sensing ion channel 1a (ASIC1a) which is expressed in CA1 pyramidal neurons is also known to contribute to post ischaemic neuronal death and to physiologically induced LTP. This raises the question does ASIC1a activation drive the post ischaemic forms of AMPAR plasticity in CA1 pyramidal neurons? We have tested this by examining organotypic hippocampal slice cultures (OHSCs) exposed to oxygen glucose deprivation (OGD) and dissociated cultures of hippocampal pyramidal neurons (HPNs) exposed to low pH (acidosis). We find that both a LTP and the delayed increase in the prevalence of CP AMPARs are dependent on ASIC1a activation during ischaemia. Indeed acidosis alone is sufficient to induce the increase in CP AMPARs. We also find that inhibition of ASIC1a channels circumvents any potential neuroprotective benefit arising from block of CP AMPARs. By demonstrating that ASIC1a activation contributes to post ischaemic AMPAR plasticity our results identify a functional interaction between acidotoxicity and excitotoxicity in hippocampal CA1 cells and provide insight into the role of ASIC1a and CP AMPARs as potential drug targets for neuroprotection. We thus propose that ASIC1a activation can drive certain forms of CP AMPAR plasticity and that inhibiting ASIC1a affords neuroprotection.

Transient anoxia/hypoglycaemia in organotypic hippocampal slice cultures, a model of transient brain ischaemia, ultimately results in delayed cell death. Although the mechanisms underlying this delayed death remain unknown, an increase in excitatory drive has been postulated. We report here that transient anoxia/hypoglycaemia in rat hippocampal slice cultures resulted in a 70-80% enhancement of evoked, alpha-amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid (AMPA) receptor-mediated, excitatory responses lasting over 60 min. This effect was prevented by blockade of N-methyl-d-aspartate (NMDA) receptors, did not involve changes of paired-pulse facilitation ratio, but was associated with a 50% increase in amplitude, but not frequency, of spontaneous miniature excitatory postsynaptic currents (mEPSCs). Consistent with this, paired recordings revealed the appearance of AMPA receptor-mediated EPSCs at previously silent synapses and occlusion by prior induction of long-term potentiation (LTP). Transient anoxia/hypoglycaemia further resulted in a 63% potentiation of evoked NMDA receptor-dependent synaptic responses, accounting for the 20% increase in ratio of AMPA to NMDA responses. No change in rectification properties of AMPA receptor-mediated currents could be detected within the first hour following anoxia/hypoglycaemia-induced potentiation. Western blot analyses of slice cultures exposed to either control conditions or a short anoxia/hypoglycaemia revealed a marked, 50-70% increase of GluR1, GluR2/3 and NR1 subunits 1 h, but not 15 min, after the anoxic/hypoglycaemic episode. This increase was blocked by an inhibitor of protein synthesis. Together these results indicate that a transient anoxia/hypoglycaemia is associated with a marked enhancement of excitatory transmission sharing similarities with the mechanisms underlying LTP, and is correlated with an increased synthesis of excitatory receptor subunits.