REAL-TIME IMAGING OF INTRINSIC OPTICAL SIGNALS DURING EARLY EXCITOTOXICITY EVOKED BY DOMOIC ACID IN THE RAT HIPPOCAMPAL SLICE

Excitotoxicity involves neuronal depolarization and eventual cell deat h primarily through excess activation of glutamate receptors. Neuronal cell swelling is considered an early excitotoxic event mediated by io nic influx (mainly Na+ and Cl-) followed by water. Changes in the intr insic optical signals of nerve tissue correlate with neuronal activity such that light transmittance (LT) increases across the brain slice a s cells swell. The present study examined the effects of domoic acid, a potent excitotoxic food contaminant and glutamate analogue, on intri nsic optical signals in the rat hippocampal slice. A brief 1-min expos ure to 10 mu M domoate at 22 degrees C elevated LT by 58% in the apica l dendritic region of CA1 and to a lesser extent in the molecular laye r of the upper dentate gyrus. The responses peaked by 5 min and slowly reversed during a 30-min wash. The same responses were evoked by a 1- min application of 10 mu M ha-amino-3-hydroxy-5-methyl-isoxazole-4-pro pionate (AMPA) at 22 degrees C. Minor changes were observed in the CA3 region and the lower blade of the dentate gyrus. At 37 degrees C, exp osure to 10 mu M domoate for 10 min resulted in apparent irreversible neuronal damage in the CA1 and upper dentate regions. The Na+ channel blocker tetrodotoxin (1 mu M) eliminated the evoked CA1 population spi ke but not the LT increase, indicating that the domoate signal is not associated with action potential discharge pre- or post-synaptically. However, the response to domoate at 22 degrees C was reversibly blocke d by the nonspecific glutamate receptor antagonist kynurenate and the non-N-methyl-D-aspartate (non-NMDA) receptor antagonists 6-cyano-7-nit roquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (D NQX). The response was not blocked by the NMDA receptor antagonist 2-a mino-5-phosphonovaleate (AP-5) nor the kainate receptor blocker gamma- D-glutamylaminomethyl sulfonate (GAMS). Relative tissue resistance (R( REL)) measured across the CA1 dendritic region increased rapidly in re sponse to domoate and fell slowly over 30 min, which paralleled the LT response described above. The increase in R(REL) was blocked by kynur enate. We propose that domoate binding to AMPA receptors opens channel s mediating ionic influx, presumably Na+ followed passively by Cl-. Wa ter follows, producing prolonged postsynaptic swelling in the CA1 and dentate regions where AMPA receptors are most abundant. At higher temp erature this swelling can progress to permanent neuronal injury. Imagi ng intrinsic optical signals allows a real-time view of early excitoto xic events and may prove useful in assessing potentially therapeutic a gents that reduce damage induced by excitotoxic agents or ischemia.