Temporo-Spectral Imaging of Intrinsic Optical Signals during Hypoxia-Induced Spreading Depression-Like Depolarization

Abstract

Spreading depression (SD) is characterized by a sustained near-complete depolarization of neurons, a massive depolarization of glia, and a negative deflection of the extracellular DC potential. These electrophysiological signs are accompanied by an intrinsic optical signal (IOS) which arises from changes in light scattering and absorption. Even though the underlying mechanisms are unclear, the IOS serves as non-invasive tool to define the spatiotemporal dynamics of SD in brain slices. Usually the tissue is illuminated by white light, and light reflectance or transmittance is monitored. Using a polychromatic, fast-switchable light source we now performed temporo-spectral recordings of the IOS associated with hypoxia-induced SD-like depolarization (HSD) in rat hippocampal slices kept in an interface recording chamber. Recording full illumination spectra (320–680 nm) yielded distinct reflectance profiles for the different phases of HSD. Early during hypoxia tissue reflectance decreased within almost the entire spectrum due to cell swelling. HSD was accompanied by a reversible reflectance increase being most pronounced at 400 nm and 460 nm. At 440 nm massive porphyrin absorption (Soret band) was detected. Hypotonic solutions, Ca2+-withdrawal and glial poisoning intensified the reflectance increase during HSD, whereas hypertonic solutions dampened it. Replacement of Cl- inverted the reflectance increase. Inducing HSD by cyanide distorted the IOS and reflectance at 340–400 nm increased irreversibly. The pronounced changes at short wavelengths (380 nm, 460 nm) and their cyanide sensitivity suggest that block of mitochondrial metabolism contributes to the IOS during HSD. For stable and reliable IOS recordings during HSD wavelengths of 460–560 nm are recommended.

Aquaporin-4 regulates the velocity and frequency of cortical spreading depression in mice

“…We conclude that baseline ECS expansion and reduced astrocyte water permeability are necessary and sufficient to account for the experimental observations. This conclusion is supported by studies in hippocampal slices showing that direct ECS expansion with hypertonic saline impairs hypoxia‐induced spreading depression (Huang et al, ; Mane and Muller, ) and that ECS contraction with hypotonic saline increases susceptibility to spreading depression (Chebabo et al, ). The reason(s) for baseline ECS expansion in AQP4 −/− mice are not known, and so it is not possible to extrapolate data here to acute AQP4 inhibition without ECS expansion, as might occur with pharmacological inhibition of AQP4 in humans, if and when suitable AQP4‐selective inhibitors become available.…”

Neuronal and Astroglial Correlates Underlying Spatiotemporal Intrinsic Optical Signal in the Rat Hippocampal Slice

“…Based on the illumination wavelength dependence of IOS [7] we assume that mainly the light scattering component of in vitro afferent evoked IOS was detected at 700 nm. Measuring at this wavelength has the advantage of the deeper penetration into the slice and the reduced contribution of the absorption of the residual hemoglobin (usually measured at 600−660 nm [72], [73], [74]) and the intrinsic cytochromes such porphyrins (maximal absorption wavelengths 440 nm [15]). As dendritic beading and changes of mitochondrial architecture are usually coupled by strong activation (i.e.…”

“…Light absorption changes are mostly attributed to the intrinsic cytochromes like mitochondrial porphyrins and residual hemoglobin [12]. Excitoxicity related light scattering changes are associated with dendritic beading [13], while spreading depression related IOS is ascribed to changes in mitochondrial architecture or metabolic activity [1], [2], [14], [15]. Local changes in light scattering due to activity-dependent cell swelling [1], [16] and alterations of the extracellular volume [17], [18] are regarded as the principal component of the osmotic pressure induced and afferent stimulation evoked IOS [1], [19].…”

Spreading depolarizations in the rat endothelin-1 model of focal cerebellar ischemia

“…The IOS shows an increase during terminal SD which likely results from changes in light scattering and absorption of the parenchyma. 32,[62][63][64] anatomical structure of the cerebellar foliae (Figure 4), the most plausible explanation for the variations in negative DC shift and [K þ ] o signal is that the positions of the electrode tips varied between different experiments.…”