Abstract
Cell volume changes are associated with alterations of intrinsic optical signals (IOS). In submerged brain slices in vitro, afferent stimulation induces an increase in light transmission. As assessed by measurement of the largely membrane impermeant ion tetramethylammonium (TMA) in the extracellular space, these IOS correlate with the extent and time course of the change of the extracellular space size. They have a high signal to noise ratio and allow measurements of IOS changes in the order of a few percent. Under conditions of reduced net KCl uptake (low Cl solution) a directed spatial buffer mechanism (K syphoning) can be demonstrated in the neocortex with widening of the extracellular space in superficial layers associated with a reduced light transmission and an increase of extracellular K concentration. The nature of the IOS under pathophysiological conditions is less clear. Spreading depressions first cause an increase of light transmission, then a decrease. Such a decrease has also been observed following application of NMDA where it was associated with structural damage. Pharmacological analyses suggest that under physiological conditions changes of extracellular space size are mainly caused by astrocytic volume changes while with strong stimuli and under pathophysiological conditions also neuronal swelling occurs. With reflected light usually signals opposite to those observed with transmitted light are seen. Recording of IOS from interface slices gives very complex signals since under these conditions an increase of light transmission has been reported to be superimposed by a decrease of the signal due to mechanical lensing effects of the slice surface. Depending on the method of measurement and the exact conditions, several mechanisms may contribute to IOS. Under well defined conditions IOS are a useful supplementary tool to monitor changes of extracellular volume both in space and time.
Highlights
The extracellular space is an important pathway for signal transmission in the brain
Intrinsic optical signals may be recorded from transmitted or reflected light. These signals give in principle the same results though with different polarity (Aitken et al 1999): with transmitted light, the signal intensity increases when the preparation is superfused with a hypotonic solution causing cell swelling, while it decreases with absorbed light
The activation columns were associated with increases of extracellular potassium activity from a baseline of 3mM up to peak levels of 10mM
Summary
The extracellular space is an important pathway for signal transmission in the brain. Intrinsic optical signals may be recorded from transmitted or reflected light These signals give in principle the same results though with different polarity (Aitken et al 1999): with transmitted light, the signal intensity increases when the preparation is superfused with a hypotonic solution causing cell swelling, while it decreases with absorbed light. Intrinsic optical signals may be recorded from interface chambers The differences between these two configurations have elegantly been investigated by Kreisman et al (1995) who measured light transmittance in interface and submerged slices. Superfusion of a submerged slice with hypotonic solution causing a cell swelling results in increased light transmission, while the same slice displays a decreased light transmission when brought into the interface configuration They explained this effect in the following way. The actual size and direction of the signal may depend even on the distance between detector and slice surface
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