NR1/NR2B receptor Activation induces Cytoskeletal Proteolysis in Partial Ischemic Rat Hippocampal Formation

Jung, Yong Wook; Moon, Il Soo; Ko, Bok Hyun

Abstract

In the rat brain, partial ischemia causes a delayed neuronal degeneration that occurs hours to days after reoxygenation. It is generally thought that the ischemic damage is initiated by neurotoxicity mediated through glutamate receptors, particulaly NMDA subtypes. Calcium entry through the NMDA receptor is responsible for the synaptic plasiticity and neuronal pathology. Degradation of MAP-2 and NF200, a major components of neuronal cytoskeleton, by Ca2+-dependent protease after NMDA receptor activation has been postulated in delayed neuronal damage. Calcium-activated protease calpain, excessive degradation of MAP-2, together with the calpain-sensitive microtubule and neurofilaments, would be expected to disrupt intracellular transport- and membrane-related functions that is vital to neurons. Changed of NR subunit 2A, 2B, MAP2 and NF200 in rat hippncampal postsynaptic density[PSD] after partial ischemic injury were investigated though immunoblot analyses. To understand the effect of Ca2+, influx through NMDA receptors on neuronal damage which is manifested by cytoskeletal disruption, morphological change was examined through immunohistochemistry and routine staining method. We found that immunoreactivity to NR2B receptor subuit in the hippocampal formation PSD was upregulated while MAP2 and NF200 was down-regulted at 18 hours after initial partial ischemic insult. On the other hand, morphological changes of neuronal cell in partial ischemic conditions were manifested as eosinophilic inclusion bodies in the cytoplasm which is progression of neuronal damage after 6 days. Calcium influx through NR1/NR2B receptor channel may activate intracellular proteases which would degrade cytoskeleton. Proteolysis of cytoskeleton leads to its reorganization and eventually damages normal function of cell membrane which cause neuronal cell death.