FINAL REPORT: Role of TLR4 and glial cell on histamine and nitric oxide induced migraine mechanisms

Summary

We tested the hypothesis that toll-like receptor 4 (TLR4), an important element of the innate immune system, was involved in the delayed phase headache caused by vascular active substances like histamine and nitric oxide (NO). We found migraine-like symptoms in mice following NO and histamine release. Our initial behavioral assessment measured tactile thresholds near the eyes and cheeks, but we found this to be a relatively insensitive endpoint. We then assessed the development of light aversive behavior in mice as a surrogate of photophobia. We found that a histamine-releasing agent given intravenously caused the mice to show a significant decrease in time spent in the light as compared to the control animals. This aversion to light was observed up to 2 hours, with a complete reversal at 4 hours. Pre-treatment with sumatriptan, an antimigraine agent, significantly blocked the light aversion induced by the histamine agent, thus supporting the validity of this model for predicting migraine pharmacology.

We then assessed the effects of removing the TLR4 receptor. In mice without the receptor, administering histamine no longer produced light aversive behavior. Importantly, the animals continued to show evidence of histamine release, demonstrating that removing the TLR4 receptor did not simply prevent histamine release.

Hypothesis vs. Findings

We hypothesized that both NO and histamine-induced mechanisms of delayed trigeminovascular activation may involve a similar mechanism. And further, that TLR4 may play an important role in initiating and maintaining pain states, as in migraine. So far our hypothesis appears to be true in the case of histamine-induced migraine-like behavior. However, more studies are needed to explore this hypothesis in NO-induced migraine pain mechanisms.

Unanswered Questions

So far we have shown that removing TLR4 receptors in mice reversed the light-aversive behavior (one of the associated symptoms in migraine) induced by the histamine agent. We still need to determine if similar mechanisms exist in NO-induced pain mechanisms. Further, the role of glial cells in maintaining such pain states is yet to be understood. Since TLR4s are present in both neurons and glial cells, we need to study whether both histamine and NO cause long-lasting activations in neurons and glial cells, and whether TLR4 plays a major role in the histamine or GTN-induced neuronal or glial activation. Future studies would examine the effects of TLR4 antagonists to see whether instituting such a blockade could have therapeutic potential.

What This Research Means to You

These observations provide the first evidence of TLR4 involvement in initiating and maintaining migraine-like behavior following the administration of migraine-inducing substances. This suggests that targeting toll receptors, TLR4 in particular, could have novel therapeutic potential.