IMAGING NMDA-INDUCED AND KAINATE-INDUCED INTRINSIC OPTICAL SIGNALS FROM THE HIPPOCAMPAL SLICE

1. Brain ischemia causes excess release and accumulation of glutamate that binds to postsynaptic receptors. This opens ionotropic channels t hat mediate neuronal depolarization and ionic fluxes that can lead to neuronal death. 2. The CA1 pyramidal cell region of the hippocampus is particularly susceptible to this neurotoxic process. Brain cell swell ing is considered an early excitotoxic event, but remains poorly under stood and documented. As cells swell, light transmittance (LT) increas es through brain tissue, so we hypothesized that brief exposure to glu tamate agonists would elicit cell swelling that could be imaged in rea l time in the hippocampal slice. 3. A 1-min bath application of 100 mu M N-methyl-D-aspartate (NMDA) or 100 mu M kainate at 22 degrees C gre atly increased LT, particularly in the dendritic regions of CA1. The r esponse peaked by 2-3 min and slowly reversed over the subsequent 20 m in following exposure. Peak LT increases were >50% in CA1 stratum radi atum and >20% in both CA1 stratum oriens and the dendritic region of t he dentate gyrus, all areas with a high concentration of NMDA and alph a-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors. Th e CA3 stratum radiatum, which contains fewer of these receptors, showe d a comparatively small LT increase. 4. The NMDA receptor antagonist 2 -amino-5-phosphonovalerate (AP-5) [but not 6-cyano-7-nitroquinoxaline- 2,3-dione (CNQX)] blocked the CA1 response to NMDA, whereas the non-NM DA receptor antagonist CNQX (but not AP-5) blocked the response to kai nate. The relative tissue resistance measured across CA1 stratum radia tum increased after NMDA or kainate exposure with a time course simila r to the LT change described above. The increase in relative tissue re sistance was blocked by kynurenate, a nonspecific glutamate antagonist Increases in both LT and tissue resistance provide two independent li nes of evidence that cell swelling rapidly developed in CA1 dendritic areas after activation of NMDA or AMPA receptors. 5. This swelling at 22 degrees C was accompanied by a temporary loss of the evoked CA1 fie ld potential. However, at 37 degrees C the dendritic swelling rapidly progressed to an irreversible LT increase (swelling) of the CA1 cell b odies accompanied by a permanent loss of the evoked field. 6. We propo se that dendritic swelling mediated by NMDA and AMPA receptors is an e arly excitotoxic event that can herald permanent damage to CA1 neurons , those cells most vulnerable to ischemic insult.