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.