FINAL REPORT: Examining the role of astrocytes in migraine using optogenetic methods

Published in the Journal of Cerebral Blood Flow and Metabolism, April 2016

Awards: 2014 International Headache Society Best Basic Science Presentation

Summary

Cortical spreading depression (CSD) is believed to be the physiological substrate of the migraine aura, and CSD has been used as a model of migraine to test new therapies. The mechanisms by which CSD starts and spreads remain poorly understood, in part because the methods for studying it are invasive and non-physiological.

We have successfully characterized CSD using optogenetic techniques (using genetically modified cells to express light-sensitive ion channels to enable measurement of the effects). By targeting both astrocytes and neurons, we were able to show that the optogenetic approach works in brain slices, in anesthetized animals, in awake and head-fixed animals, and in awake and freely moving animals. We found that we could both start and track CSD without breaching the skull, which should make the model much more useful for long term behavioral and mechanistic investigations.

Hypothesis vs. Findings

We hypothesized that the activation of astrocytes would start CSD, and we confirmed this by using optogenetics to specifically activate cortical astrocytes that were engineered to express channelrhodopsin receptors and respond to blue light. We showed that this was true in both in vitro (brain-slice) and in vivo preparations (anesthetized and awake/behaving mice).

We also have observed that disrupting astrocyte glutamate processing can result in spontaneous and large amplitude CSDs, which further suggests that astrocytes can play important roles in both the start and the termination of and recovery from CSD. We noted several changes in behavior following CSD — such as increases and decreases in spontaneous running on a treadmill—that were dependent on the phase of CSD.

Additionally, we observed that neurons targeted in the same way could also start CSD. All indications are that CSDs produced by neuronal or astrocyte targeting are very similar, although CSD is harder to elicit when targeting astrocytes. By targeting neurons optogenetically in awake animals, we found that grip strength is reduced on the opposite side of the hemisphere where CSD occurs and only when the wave of CSD passes over the motor cortex.

Unanswered Questions

We still need to perform parallel behavioral investigations following CSD induced by astrocyte activation. Our investigations using the glutamate biosensor are also preliminary, and we hope to eventually extend them into in vivo experiments. Careful investigations on the differences between the spread of CSD when elicited by different cell types remains unexplored.

What this Research Means to You

We are excited to continue to use optogenetics to non-invasively target different cell types to elicit CSD for further studies. Besides different cell types, we can also begin to target different brain areas thought to be involved in headache and migraine, and we can embark on longitudinal studies to examine how animal behavior and physiology might be affected by multiple CSDs spread out over longer periods of time.

The improved and minimally invasive translational model we developed avoids many of the problems associated with previous invasive methods and should be useful for drug screening and for a basic understanding of the contributions of different cell types to CSD. The simplified procedure for eliciting CSDs also makes the model easier to use in research.