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The role of NMDA receptors in excitotoxicity Bakshi, Deeksha


NMDA receptors are glutamate-gated cation channels named after their prototypical selective agonist NMDA. The channels occur as multiple subtypes, which are formed from interactions between different receptor subunits. NMDA receptor subunits are classified into three families: NR1, NR2A-D, and NR3A, B. NMDA receptors are implicated in HD pathology. During HD, a subset of medium-sized aspiny interneurons in the striatum that co-localize SST, NPY, and the enzyme NOS are selectively spared. In contrast, medium-sized spiny cells that constitute 80 % of all striatal neurons undergo selective neurodegeneration. While it was suggested that the interneurons survive because they lack NMDA receptors, studies including from our lab have shown the presence of NR1 in SST-positive striatal neurons. The finding of NR1 expression and co-localization with SST-positive neurons indicates that NMDA receptor-induced toxicity may be regulated in a receptor-specific manner. Therefore, the present study was conducted to investigate whether NMDA application leads to toxicity that is receptor-specific in HEK293 cells stably transfected with NR1, NR2A, or NR2B. The main findings of this study indicate that NMDA application causes cell death, which varies in intensity and nature, depending upon the NMDA concentration applied, and the receptor-type expressed by the cells. Cells expressing NR1 were found to undergo apoptosis but not necrosis, while cells expressing NR2A/NR2B underwent both apoptosis and necrosis in a receptor-specific manner. In cells expressing NR2A/NR2B, exposure to low concentrations of NMDA resulted in cell death that was predominantly apoptotic. In contrast, exposure to high concentrations of NMDA produced mostly necrosis. In cells expressing NR1, NMDA application caused apoptosis, which exhibited a gradual increase in response to greater concentrations of NMDA. In addition, cell death through apoptosis and/or necrosis was determined to be the greatest at all NMDA concentrations in cells expressing NR2B, followed by those expressing NR2A, and then NR1. Taken together, these results indicate that the activation of receptors formed by NR1, NR2A, or NR2B have different toxic consequences. Thus, the selective neurodegeneration observed during HD may be due to the variation in expression levels of NR1, NR2A, and NR2B between medium-sized aspiny interneurons and medium-sized spiny projection neurons.

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