Intrinsic Optical Signals in Rat Hippocampal Slices During Hypoxia-Induced Spreading Depression-Like Depolarization

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Abstract
In interfaced rat hippocampal slices spreading depression (SD) and hypoxia-induced SD-like depolarization are associated with increased light reflectance and decreased light transmittance, indicating increased light scattering. By contrast, mild hypotonicity or electrical stimulation decrease light scattering, which is usually taken to be caused by cell swelling. This difference has been attributed to experimental conditions, but in our laboratory moderate osmotic challenge and SD produced opposite intrinsic optical signals (IOSs) in the same slice under identical conditions. To decide whether the SD-induced IOS is related to cell swelling, we investigated the effects of Cl− transport inhibitors and Cl−withdrawal on both light reflectance and transmittance, as well as on changes in interstitial volume and tissue electrical resistance. In normal [Cl−]o, early during hypoxia, there was a slight decrease in light reflectance paired with increase in transmittance. At the onset of hypoxic SD, coincident with the onset of cell swelling (restriction of TMA+ space), the IOS signals suddenly inverted, indicating sharply increased scattering. The SD-related IOSs started in a single spot and spread out over the entire CA1 region without invading CA3. Application of 2 mM furosemide decreased IOS intensity. When [Cl−]o was substituted by methylsulfate or gluconate, the SD-related reflectance increase and transmittance decrease were suppressed and replaced by opposite signals, indicating scattering decrease. Yet Cl−withdrawal did not prevent cell swelling measured as shrinkage of TMA+ space. The SD-related increase of tissue electrical resistance was reduced when bath Cl− was replaced by methylsulfate and almost eliminated when replaced by gluconate. The TMA+ signal is judged to be a more reliable indicator of interstitial space than tissue resistance. Neither application of cyclosporin A nor raising [Mg2+]o depressed the SD-related reflectance increase, suggesting that Cl−flux through mitochondrial “megachannels” may not be a major factor in its generation. Fluoroacetate poisoning of glial cells (5 mM) accelerated SD onset and enhanced the SD-induced reflectance increase threefold. This suggests, first, that glial cells normally moderate the SD process and, second, that neurons are the predominant generators of the light-scattering increase. We conclude that light scattering by cerebral tissue can be changed by at least two different physical processes. Cell swelling decreases light scattering, whereas a second process increases scattering. During hypoxic SD the scattering increase masks the swelling-induced scattering decrease, but the latter is revealed when Cl− is removed. The scattering increase is Cl− dependent, nevertheless it is apparently not related to cell volume changes. Its underlying mechanism is as yet not clear; possible factors are discussed.