Platelet-mediated changes to neuronal glutamate receptor expression at sites of microthrombosis following experimental subarachnoid hemorrhage

Object. Glutamate is important in the pathogenesis of brain damage after cerebral ischemia and traumatic brain injury. Notably, brain extracellular and cerebrospinal fluid as well as blood glutamate concentrations increase after experimental and clinical trauma. While neurons are one potential source of glutamate, platelets also release glutamate as part of their recruitment and might mediate neuronal damage. This study investigates the hypothesis that platelet microthrombi release glutamate that mediates excitotoxic brain injury and neuron dysfunction after subarachnoid hemorrhage (SAH). Methods. The authors used two models, primary neuronal cultures exposed to activated platelets, as well as a whole-animal SAH preparation. Propidium iodide was used to evaluate neuronal viability, and surface glutamate receptor staining was used to evaluate the phenotype of platelet-exposed neurons. Results. The authors demonstrate that thrombin-activated platelet-rich plasma releases glutamate, at concentrations that can exceed 300 mu M. When applied to neuronal cultures, this activated plasma is neurotoxic, and the toxicity is attenuated in part by glutamate receptor antagonists. The authors also demonstrate that exposure to thrombin-activated platelets induces marked downregulation of the surface glutamate receptor glutamate receptor 2, a marker of excitotoxicity exposure and a possible mechanism of neuronal dysfunction. Linear regression demonstrated that 7 days after SAH in rats there was a strong correlation between proximity to microthrombi and reduction of surface glutamate receptors. Conclusions. The authors conclude that platelet-mediated microthrombosis contributes to neuronal glutamate receptor dysfunction and might mediate brain injury after SAH.