Cortical astrocyte activation triggers meningeal nociception and migraine-like pain

Introduction

Migraine headaches are a very common cause of disabling pain, particularly for women (1). Specialized sensory cells called trigeminal ganglia neurons (located just under the skull) are key players in migraine pain. Astrocytes, star-shaped glial cells located in the brain and spinal cord, support the function of (among others) the trigeminal ganglia neurons. However, it’s unclear if astrocyte activation could play a role in migraine pathophysiology. Bree and colleagues, the authors of this preprint, set out to determine whether hyperactivity within the cortical astrocyte-to-trigeminal ganglia network drives migraine pain, and the exact circumstances necessary to do so.

Key findings

Activation of cortical astrocytes sensitizes trigeminal ganglia neurons

To address their hypothesis, the authors used a chemogenetic tool called DREADDs (Designer Receptors Exclusively Activated by Designer Drugs (2)) in the rat primary visual cortex that allowed them to selectively activate astrocytes in this region of the brain (see Fig. 1). One of their first experiments showed that selective visual cortex astrocyte activation using DREADDs led to increased baseline electrical activity of trigeminal ganglia nociceptive neurons, which are widely known to contribute to pain. Importantly, DREADD activation of cortical astrocytes also triggered mechanical sensitization of trigeminal ganglia nociceptors, suggesting that heightened activity of visual cortex astrocytes could underlie touch sensitivity during migraine.

Fig. 1 from the preprint: Chemogenetic activation of astrocytes in visual cortex drives activity in trigeminal nociceptors. (A) Illustration showing visual cortex (V1) injection of virus encoding an excitatory DREADD (hM3Dq) coupled with astrocyte promoter (GFAP) and mCherry fluorescent tag. (B-C) Viral infection demonstrated a high degree of specificity and penetrance in visual cortex (V1) between 4-6 weeks following virus injection, with >95% of virus-infected cells (mCherry+) being astrocytes (co-labeled with GFAP), and >88% of astrocytes (GFAP label) also expressing virus (mCherry+). (D-E) Rats were injected with hM3Dq virus targeted to astrocytes in the visual cortex. This allowed the authors to chemogenetically activate this cell population using systemic clozapine N-oxide (CNO) injection, an exogenous ligand to the hM3Dq receptor. Such activation in (E) resulted in elevated electrical activity of trigeminal nociceptor neurons between 60 min and 120 min post-CNO injection. Image made available under a CC-BY-NC-ND 4.0 International license.

Cortical astrocyte-trigeminal network activation drives overt behavioral migraine-like pain

Next, the authors wanted to examine how sensitization of the astrocyte-trigeminal neuron network might affect behavioral sensitivity of rats – that is, does trigeminal nociceptor mechanical sensitization translate to behavioral indicators of migraine pain? For this, visual cortex astrocytes of rats were chemogenetically activated before testing for evoked facial mechanical sensitivity using von Frey filaments (long flexible wires), or non-evoked pain-like behavior using an open field test. Activating these cortical astrocytes in rats triggered increased withdrawal and escape/attack behaviors in response to von Frey stimulation of the face and suppressed overall exploration in a novel open field. The authors therefore concluded that cortical astrocyte activation provokes overt behavioral migraine-like pain.

CGRP mediates cortical astrocyte activation-induced migraine-like pain behavior

Since other migraine research strongly implicates calcitonin gene-related peptide (CGRP) in migraine pain (3), the authors next wanted to ask whether cortical astrocyte-driven sensitization of trigeminal nociceptors and overt migraine pain-like behaviors are mediated by CGRP? The authors repeated their experiments, but this time used an antibody against CGRP to block its function. While the authors found that astrocyte activation-induced trigeminal nociceptor electrical activity was reversed by anti-CGRP treatment, mechanical sensitization remained intact. Despite this, rats given anti-CGRP treatment showed attenuated migraine pain-like behavior following astrocyte activation – rats were less likely to respond to von Frey facial stimulation and displayed more overall movement in the open field. Importantly, anti-CGRP treatment also attenuated astrocyte activation-induced anxiety-like behavior in female rats.

Overall takeaways

1. Activation of visual cortex astrocytes drives trigeminal nociceptor activation and mechanical sensitization, as well as overt pain-like behaviors, strongly implicating this network in migraine pain.
2. Although systemic pretreatment with a CGRP antibody is sufficient to suppress some of the electrical activity within the astrocyte-to-trigeminal network, it doesn’t block mechanical sensitization. This is consistent with other work showing roughly half of migraine patients see relief with prophylactic CGRP antibody treatment (4), and that migraine symptoms are not completely alleviated by inhibition of CGRP signaling (5).
3. Despite this caveat, pretreating rats with an anti-CGRP antibody suppresses pain-like behaviors in rats when their astrocytes are chemogenetically activated. This treatment also increases the amount of time that female rats spend in the center of an open field box, suggesting anxiety-like behavior in female rats might be driven by CGRP-dependent signaling in the astrocyte-to-trigeminal network.

Why I like this study

I chose this study because migraine pain is debilitating for many people and it’s unclear exactly which treatments will work for each patient. For these reasons, this research is super important to the pain field. I also loved how the authors were completely transparent about sex differences and showed their results separately for males and females. Since women are more susceptible to migraine, it is extremely important that basic science research using rodent models includes the role of sex in mechanisms of migraine pain. Finally, the authors cleverly showed that manipulation of a central nervous system mechanism (cortical astrocyte activation) directly affects peripheral nervous system mechanisms (trigeminal nociceptor sensitization). This kind of innovative investigation into cross-nervous-system networks is super important in a field where studying isolated mechanisms is the norm.

Future directions/questions for the authors

1. Given your manipulations were targeted to visual cortex astrocytes, what, if any, implications do you think your research has on visual disturbances and visual aura in migraine? Are there ways to determine whether cortical astrocyte activation alters vision in rats (or if they might have other sensory disturbances)?
2. In this study, a G_q DREADD was used to activate cortical astrocytes. Do you think that suppressing cortical astrocyte activity, using a G_i DREADD for example, either before or after migraine onset might alleviate migraine pain?

References

1. Steiner, T., Stovner, L., Jensen, R., Uluduz, D., Katsarava, Z. (2020). Migraine remains second among the world’s causes of disability, and first among young women: findings from GBD2019. The journal of headache and pain, 21(1), 137. https://doi.org/10.1186/s10194-020-01208-0
2. Roth, B. (2016). DREADDs for Neuroscientists. Neuron, 89(40, 683-694. https://doi.org/10.1016/j.neuron.2016.01.040
3. Levy, D., Moskowitz, M. (2023). Meningeal mechanisms and the migraine connection.Annual review of neuroscience, 46, 39–58. https://doi.org/10.1146/annurev-neuro-080422-105509
4. Wattiez, A., Sowers, L., Russo, A. (2020). Calcitonin gene-related peptide (CGRP): role in migraine pathophysiology and therapeutic targeting. Expert opinion on therapeutic targets, 24(2), 91–100. https://doi.org/10.1080/14728222.2020.1724285
5. Do, T., Deligianni, C., Amirguliyev, S., Snellman, J., Lopez Lopez C., Al-Karagholi, M., Guo, S., Ashina, M. (2023). Second messenger signalling bypasses CGRP receptor blockade to provoke migraine attacks in humans. Brain, 146(12), 5224-5234. https://doi.org/10.1093/brain/awad261

 

doi: https://doi.org/10.1242/prelights.40504

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