Real-time non-invasive in vivo visible light detection of cortical spreading depolarizations in mice

Abstract

Background: Cortical spreading depolarization (CSD) is a phenomenon classically associated with migraine aura. CSDs have also been implicated in secondary injury following ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage, and traumatic brain injury; however, most investigations involving these disease processes do not account for the occurrence of CSDs. A major barrier to detection of CSDs in experimental models is that currently validated methods are invasive and require specialized equipment and a high level of expertise to implement.

New method: We present a low-cost, easy-to-implement approach to the detection of CSDs in the mouse through full-thickness intact skull. Our method uses the optical intrinsic signal from white light illumination (OIS-WL) and allows for real-time in vivo detection of CSDs using readily available USB cameras.

Results: OIS-WL detected 100% of CSDs that were seen with simultaneous electrode recording (69 CSDs in 28 mice), laser Doppler flowmetry (82 CSDs in 10 mice), laser speckle flowmetry (68 CSDs in 25 mice), or combined electrode recording plus laser speckle flowmetry (29 CSDs in 20 mice). OIS-WL detected 1 additional CSD that was missed by laser Doppler flowmetry.

Comparison with existing methods: OIS-WL is less invasive than electrophysiological recordings and easier to implement than laser speckle flowmetry. Moreover, it provides excellent spatial and temporal resolution for dynamic imaging of CSDs in the setting of brain injury.

Conclusions: Detection of CSDs with an inexpensive USB camera and white light source provides a reliable method for the in vivo and non-invasive detection of CSDs through unaltered mouse skull.

Keywords: Cortical spreading depression; Migraine; Non-invasive; Optical intrinsic signal imaging; Stroke.

Figure 1:. Cortical spreading depolarizations (CSD) are reliably detected by optical intrinsic signal imaging with a white light source (OIS-WL) through full thickness intact mouse skull.

OIS- WL detection of CSDs was validated with simultaneous detection using (A) DC electrode recording, (B) laser Doppler flowmetry (LDF), and (C) laser speckle flowmetry (LSF). (A,B) An optical fiber delivers blue light for optogenetic induction of CSDs. Upper panels are full color RGB jpeg images from a USB camera. Bottom panels show grayscale difference images using only the green channel of the jpeg image. Dark areas denote decreased blood volume and lighter areas denote increased blood volume. (A) A DC potential shift during a CSD recorded from an electrode is shown. Images were taken every 2 seconds and difference images where generated by subtracting the prior image. (B) A relative blood flow trace during a CSD from a LDF probe is shown. Images were taken every 10 seconds and the difference images where generated by subtracting the prior image. (C) A cotton ball soaked in 1 M KCI overlying a burr hole induces a CSD. Difference images are in the top panels. Images were taken every 1 second and the difference images were generated by subtracting the image 3 frames prior (i.e. 3 seconds prior). LSF maps are in the bottom panels. A mouse skull schematic is overlaid on a baseline LSF image to provide orientation and a relative LSF color map is shown to indicate relative blood flow. Bregma is indicated with an asterix (*) and lambda with λ on full color visible light images and on the baseline LSF map. A relative scale is indicated by 1 mm graph paper (pink markings, 4 mm shown between bregma and lambda) in the upper panels of (B).