Editor’s note: The research described below comes from a recipient of a 2023 MSC Travel Grant supporting travel to the 65th Annual Scientific Meeting of the American Headache Society. These grants reimburse travel expenses for those who have had their abstract for a presentation or poster accepted at a meeting.
By Sarah Louise Christenson, PhD, postdoctoral researcher, Beth Israel Deaconess Medical Center, Harvard Medical School, US.
What is the research gap that your study addresses?
There is a gap in our knowledge about migraine pain. Where does it hurt and how? This study addresses one aspect of the molecular mechanisms of migraine pain generation. HCN ion channels have recently been proposed as mediators of migraine pain. These results were derived from experiments in rat and mouse models. Therefore, we wanted to explore these HCN channels in another model to further test and validate their relevance as novel drug targets for migraine pain.
What is your research hypothesis?
We hypothesized that opening of a specific potassium ion channel (KATP channel) causes migraine pain because it leads to activation of HCN channels, which may activate neurons to signal pain.
What methodology did you use to address your research hypothesis?
We applied an experimental mouse model that was previously developed to study mechanisms of migraine pain triggered by the KATP channel opener levcromakalim. Mice were injected with levcromakalim every other day for a total of 4 times. To detect the migraine-inducing effect of levcromakalim, we measured the hind paw withdrawal threshold where the skin is pricked with thin nylon filaments of different strengths. It is well established that migraine-provoking substances lower the withdrawal thresholds of mice and rats, similar to what is seen in some migraine patients (allodynia). To find out if the HCN channels are involved in the development of migraine pain induced by levcromakalim, we injected ivabradine (a blocker of HCN activity) prior to levcromakalim and measured the effect. We also tested if ivabradine could reverse already-established allodynia. Control measures were taken to rule out any adverse effects of ivabradine alone. All substances were given in doses that previous studies have shown to be effective.
What are the main results of your study?
The HCN channel blocker ivabradine did not prevent the development of allodynia induced by levcromakalim. Also, ivabradine failed to alleviate already-established allodynia. We found no adverse effect of ivabradine on the allodynia measure nor on general motor coordination and well-being of the mice.
What conclusions did you reach based on your results?
We found that HCN channels were not involved in allodynia induced by levcromakalim. Thus, we conclude that migraine pain can be induced without activation of HCN channels and that HCN channels are therefore not a universal target for alleviation of migraine pain.
What are the limitations of your study?
We used an experimental mouse model that was developed based on migraine provocation experiments in humans. As mice cannot tell us if they experience migraine pain, we are dependent on other measurements to know. Here, we measured sensitivity on the skin of the hind paw. The test is well validated to measure migraine-relevant effects – but it is different from the human experience of migraine pain. Even though patients report genuine migraine attacks after levcromakalim, these provoked attacks may be different in biochemical nature than spontaneous attacks.
What is the relevance of your study to migraine?
We could not confirm the importance of the HCN channel in the applied animal model, and therefore question the relevance of the HCN channel as a drug target for migraine pain. The present study favors resources to perhaps be better spent exploring other possible drug targets. Importantly, the contrast of our results to those of others highlight the importance of recognizing the differences between the animal models used in different studies; despite all the models aiming to mimic human migraine, they provide different results. This is an important aspect of translational science that the field should bear in mind.