Interpretation of intrinsic optical signals and calcein fluorescence during acute excitotoxic insult in the hippocampal slice

Immediate (acute) neuronal damage in response to overstimulation of glutama te receptors results from toxic exposure to food poisons acting as glutamat e analogues. Glutamate agonist application evokes dramatic intrinsic optica l signals (IOSs) in the rat hippocampal slice preparation, particularly in the CA1 region. Theoretically IOSs are generated by alterations to neuronal and glial structure that change light transmittance (LT) in live brain tis sue. To better understand such signals, IOSs evoked by the glutamate agonis t N-methyl-D-aspartate were imaged in the rat hippocampal slice. We correla ted these excitotoxic signals with: (1) biophysical principles governing li ght transport, (2) tissue volume changes as measured using a free intracell ular fluorophore (calcein), (3) dendritic morphology visualized by dye inje ction, and (4) standard histopathology. In theory LT elevation evoked durin g acute excitotoxic swelling is generated lay change to subcellular structu re that reduces light scattering during cell swelling. However, in responsi ve dendritic regions, initial LT elevation caused by cell swelling was over ridden by the formation of dendritic beads, a conformation that increased l ight scattering (thereby reducing LT) even as the calcein signal demonstrat ed that the tissue continued to swell. Thus IOS imaging reveals acute somat ic and dendritic damage during excitotoxic stress that can be monitored acr oss slices of brain tissue in real time. (C) 1999 Academic Press.