SELECTIVE REDUCTION OF GLUR2 PROTEIN IN ADULT HIPPOCAMPAL CA3 NEURONSFOLLOWING STATUS EPILEPTICUS BUT PRIOR TO CELL LOSS

Kainic acid (KA) induces status epilepticus and delayed neurodegenerat ion of CA3 hippocampal neurons. Downregulation of glutamate receptor 2 (GluR2) subunit mRNA [the pha-amino-3-hydroxy-5-methyl-4-isoxazole-pr opionic acid (AMPA) subunit that limits Ca2+ permeability] is thought to a play role in this neurodegeneration, possibly by increased format ion of Ca2+ permeable AMPA receptors. The present study examined early hippocampal decreases in GluR2 mRNA and protein following kainate-ind uced status epilepticus and correlated expression changes with the app earance of dead or dying cells by several histological procedures. At 12 h, in situ hybridization followed by emulsion dipping showed nonuni form decreases in GluR2 mRNA hybridization grains overlying morphologi cally healthy-appearing CA3 neurons. GluR1 and N-methyl-D-aspartate re ceptor mRNAs were unchanged. At 12-16 h, when little argyrophilia or c ells with some features of apoptosis were detected by silver impregnat ion or electron microscopy, single immunohistochemistry with GluR2 and GluR2/3 subunit-specific antibodies demonstrated a pattern of decreas ed GluR2 receptor protein within CA3 neurons that appeared to predict a pattern of damage, similar to the mRNA observations. Double immunola beling showed that GluR2 immunofluorescence was depleted and that GluR 1 immunofluorescence was sustained in clusters of the same CA3 neurons . Quantitation of Western blots showed increased GluR1:GluR2 ratios in CA3 but not in CA1 or dentate gyrus subfields. Findings indicate that the GluR1:GluR2 protein ratio is increased in a population of CA3 neu rons prior to significant cell loss. Data are consistent with the ''Gl uR2 hypothesis'' that reduced expression of GluR2 subunits will increa se formation of AMPA receptors permeable to Ca2+ and predict vulnerabi lity to a particular subset of pyramidal neurons following status epil epticus. (C) 1998 Wiley-Liss, Inc.