Two different mechanisms underlie reversible, intrinsic optical signals in rat hippocampal slices

Intrinsic optical signals (IOSs) induced by synaptic stimulation and moderate hypotonic swelling in brain tissue slices consist of reduced light scattering and are usually attributed to cell swelling. During spreading depression (SD), however, light-scattering increases even though SD has been shown to cause strong cell swelling. To understand this phenomenon, we recorded extracellular voltage, light transmission (LT), which is inversely related to light scattering, and interstitial volume (ISV) simultaneously from the same site (stratum radiatum of CA1) in both interface and submerged hippocampal slices. As expected, moderate lowering of bath osmolarity caused concentration-dependent shrinkage of ISV and increase in LT, while increased osmolarity induced opposite changes in both variables. During severe hypotonia, however, after an initial increase of LT, the direction of the IOS reversed to a progressive decrease in spite of continuing ISV shrinkage. SD caused by hypotonia, by microinjection of high-K+ solution, or by hypoxia, was associated with a pronounced LT decrease, during which ISV shrinkage indicated maximal cell swelling. If most of the extracellular Cl- was substituted by the impermeant anion methylsulfate and also in strongly hypertonic medium, the SD-related decrease in LT was suppressed and replaced by a monotonic increase. Nevertheless, the degree of ISV shrinkage was similar in low and in normal Cl- conditions. The optical signals and ISV changes were qualitatively identical in interface and submerged slices. We conclude that there are at least two mechanisms that underlie reversible optical responses in hippocampal slices. The first mechanism underlies light-scattering decrease (hence enhancing LT) when ISV shrinks (cell swelling) under synaptic stimulation and mild hypotonia. Similarly, as result of this mechanism, expansion of ISV (cell shrinkage) during mild hypertonia leads to an increased light scattering (and decreased LT). Thus optical signals associated with this first mechanism show expected cell-volume changes and are linked to either cell swelling or shrinkage. A different mechanism causes the light-scattering increase (leading to a LT decrease) during severe hypotonia and various forms of SD but with a severely decreased ISV. This second mechanism may be due to organelle swelling or dendritic beading but not to cell-volume increase. These two mechanisms can summate, indicating that they are independent in origin. Suppression of the SD-related light-scattering increase by lowering [Cl-](o) or severe hypertonia unmasks the underlying swelling-related scattering decrease. The simultaneous IOS and ISV measurements clearly distinguish these two mechanisms of optical signal generation.