Use of intrinsic optical signals to monitor physiological changes in brain tissue slices

Optical imaging techniques have the potential to bring a combination of high spatial and temporal resolution to studies of brain function. Many optical techniques require the addition of a dye or fluorescent marker to the tissue, and such methods have proven extremely valuable. It is also known that the intrinsic optical properties of neural tissue are affected by certain physiological changes and that these intrinsic optical signals can provide information not available by other means. Most authors attribute the intrinsic optical change to alterations in cell volume and concomitant change in the concentration of the cytosol. In this article we review the literature on intrinsic optical signals, covering both the mechanisms of the optical change and its use in various branches of neurophysiology. We also discuss technical aspects of the technique as used with hippocampal slices, including illumination methods, cameras, experimental methods, and data collection and analysis procedures. Finally we present data from investigations in which we used intrinsic optical signals in hippocampal slices to study the extent of spread of synaptic activation, propagation of spreading depression, extent and severity of the response to hypoxia, and tissue response to osmotic challenges. We conclude that (1) at least two processes generate intrinsic optical signals in hippocampal slices, one of which causes light scattering to change inversely with cell volume and is related to dilution of the cytoplasm, while the other, opposite in sign, may be due to mitochondrial swelling; and (2) the intrinsic optical signal can be a useful tool for spatial mapping of relatively slow events, but is not suitable for study of fast physiological processes.