Oxygen saturation (sO2) and blood perfusion in brain tissue have been known to be modulated with cellular activity in the brain. A single fiber system (SFS) has previously been shown to enable sO2 measurements from localized deep brain regions in freely moving animals. Reflectance spectra (RSF) obtained through the SFS can be used to understand changes in blood perfusion and fit to an empirical model to extract sO2. The sO2 extracted is dependent on the shape of RSF and thus relatively resistant to noise as compared to blood perfusion which is dependent on the magnitude of RSF at specific wavelengths. While slow changes in sO2 have been shown to be robust, sources of certain relatively rapid temporal variations observed in the sO2 signal remains unclear. Potential sources could be variations in cellular activity in the brain or noise due to motion artifacts. In this work, we have described the design of new experiments focused to investigate the effects of motion artifacts on RSF and sO2. Computer simulations and mathematical modelling have been used to explain the experimental findings. Results suggest that the motion artifacts mainly arise from the fiber/brain interface and appear to offset RSF. Using the interpretation from a mathematical model, we also propose a motion artifact correction algorithm which can potentially be used for comparison of perfusion signals.
KEYWORDS: Luminescence, In vivo imaging, Photometry, Optical fibers, Tissue optics, Signal to noise ratio, Signal detection, Calcium, Modulation, Neurons
Fiber photometry uses genetically encoded optical reporters to link specific cellular activity in stereotaxically targeted brain structures to specific behaviors. There are still a number of barriers that have hindered the widespread adoption of this approach. This includes cost, but also the high-levels of light required to excite the fluorophore, limiting commercial systems to the investigation of short-term transients in neuronal activity to avoid damage of tissue by light. Here, we present a cost-effective optoelectronic system for in vivo fiber photometry that achieves high-sensitivity to changes in fluorescence intensity, enabling detection of optical transients of a popular calcium reporter with excitation powers as low as 100 nW. By realizing a coherent detection scheme and by using a photomultiplier tube as a detector, the system demonstrates reliable study of in vivo neuronal activity, positioning it for future use in the experiments inquiring into learning and memory processes. The system was applied to study stress-evoked calcium transients in corticotropin-releasing hormone neurons in the mouse hypothalamus.
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