IMAGING CELL-VOLUME CHANGES AND NEURONAL EXCITATION IN THE HIPPOCAMPAL SLICE

Brain cell swelling is a consequence of seizure, ischemia or excitotox icity. Changes in light reflectance from cortical surface are now used to monitor brain activity but these intrinsic signals are poorly unde rstood. The objectives of this study were first, to show that changes in light transmittance were correlated with cell volume and second, to image increases in light transmittance as they related to neuronal ac tivation. Transverse hippocampal slices from the rat were used for the study. Brief exposure (4-6 min) to hypo-osmotic artificial cerebrospi nal fluid (-40 mOsm) elevated light transmittance consistently and rev ersibly in most regions of the slice and particularly in CA1 dendritic regions. Neither zero-Ca2+ artificial cerebrospinal fluid nor tetrodo toxin altered the transmittance increase and its subsequent reversal, suggesting that it was dependent on osmolality but independent of syna ptic transmission and neuronal firing. The amplitude of the CA1 popula tion spike evoked from Schaffer collaterals increased concomitantly wi th the hypo-osmotic increase in light transmittance, providing evidenc e that the extracellular tissue resistance increased. Hyper-osmotic ar tificial cerebrospinal fluid (+40 mOsm) containing impermeant mannitol consistently lowered light transmittance and the amplitude of the pop ulation spike. Glycerol (+40 mOsm), which is cell permeant, did not ha ve an affect. Taken together these observations indicate that osmotic challenge alters light transmittance by inducing changes in cell volum e. Transmittance increases induced by hypo-osmotic artificial cerebros pinal fluid or 10 mu M kainate were small in the CA1 cell body region compared to dendritic regions. Similarly, orthodromic stimulation of a xons terminating in stratum oriens or in stratum radiatum evoked trans mittance increases only in their respective postsynaptic areas. In con trast, the cell body region and its adjacent proximal-apical dendrites (both sites of action potential initiation) could display dramatic in creases in light transmittance upon brief exposure to 20 mM K+, The re sponse, which may represent neuronal damage, was blocked in tetrodotox in. Antidromic stimulation evoked a weak response in these same proxim al areas. We conclude that activity-dependent increases in light trans mittance across brain slices primarily reveal glial and neuronal swell ing associated with excitatory synaptic input and action potential dis charge. The signal can be imaged in real time to reveal neuronal activ ation, not only among hippocampal areas, but among neuronal regions. C ell swelling is a known consequence of excessive neuronal discharge. T herefore, the imaging of changes in light transmittance across brain s lices should prove useful in monitoring epileptiform and excitotoxic s tates.