TRPM2 and CaMKII signaling drives excessive GABAergic synaptic inhibition following ischemia

Excitotoxicity and the concurrent loss of inhibition are well-defined mechanisms driving acute elevation in excitatory/ inhibitory (E/I) balance and neuronal cell death following an ischemic insult to the brain. Despite the high prevalence of long-term disability in survivors of global cerebral ischemia (GCI) as a consequence of cardiac arrest, it remains unclear whether E/I imbalance persists beyond the acute phase and negatively affects functional recovery. We previously demonstrated sustained impairment of long-term potentiation (LTP) in hippocampal CA1 neurons correlating with deficits in learning and memory tasks in a murine model of cardiac arrest/ cardiopulmonary resuscitation (CA/CPR). Here, we use CA/CPR and an in vitro ischemia model to elucidate mechanisms by which E/I imbalance contributes to ongoing hippocampal dysfunction in male mice. We reveal increased postsynaptic GABA A receptor (GABA A R) clustering and function in the CA1 region of the hippocampus that reduces E/I ratio. Importantly, reduced GABA A R clustering observed in the first 24 hours rebounds to an elevation of GABAergic clustering by 3 days post-ischemia. This increase in GABAergic inhibition required activation of the Ca 2+ -permeable ion channel transient receptor potential melastatin-2 (TRPM2), previously implicated in persistent LTP and memory deficits following CA/CPR. Furthermore, we find Ca 2+ -signaling, likely downstream of TRPM2 activation, upregulates Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) activity, thereby driving the elevation of postsynaptic inhibitory function. Thus, we propose a novel mechanism by which inhibitory synaptic strength is upregulated in the context of ischemia and identify TRPM2 and CaMKII as potential pharmacological targets to restore perturbed synaptic plasticity and ameliorate cognitive function. Significance Statement Excitatory/ inhibitory (E/I) imbalance drives long-term disability in numerous central nervous system disorders, including cerebral ischemia. Previous studies indicated ischemia-induced hippocampal synaptic plasticity deficits contribute to long-term cognitive impairment, yet the mechanisms underlying hippocampal dysfunction are poorly defined. Here, we combine in vivo and in vitro approaches to demonstrate elevated GABA A receptor clustering and function contribute to a reduction in hippocampal E/I balance and deficits in long-term potentiation at delayed timepoints following ischemia. We further identify ongoing activation of the TRPM2 ion channel and Ca 2+ -dependent kinase, CaMKII, are required for the ischemia-induced enhancement of GABAergic synaptic inhibition, highlighting promising new targets to improve post-ischemic long-term functional recovery.