Mechanisms and Therapeutic Strategies for Post-traumatic Headache

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Post-traumatic headache (PTH) commonly occurs following mild traumatic brain injury (mTBI), also known as concussion. PTH is a secondary headache that often presents with a migraine-like phenotype and is subdivided as acute or persistent (PPTH) depending on whether it resolves within 3 months after injury. The pathophysiology of PTH and PPTH is not understood and no evidence-based treatments exist for these conditions. Critically, PPTH might differ from PTH, not only in the duration but also in underlying mechanisms and responsiveness to treatment. The reasons for emergence of PPTH in some patients remain unclear but may be related to risk factors including pre-existing migraine and the experience of a previous mTBI. We have developed an approach to investigate the mechanisms of PTH and PPTH as well as potential strategies for treatment. Using a weight drop method in male and female mice that recapitulates biomechanical properties and clinical features of mTBI, we have shown that a single mTBI is sufficient to induce clinically relevant PTH symptoms including an acute period of allodynia, elevated CGRP blood levels and lowered thresholds for induction of cortical spreading depression (CSD). Additionally, we have explored the concept that the transition from acute to chronic pain states may rely on a “pain memory” that can be studied using the “two-hit” model of hyperalgesic priming where a prior insult confers vulnerability to a subsequent provocative stimulus. Thus, following resolution of acute allodynia, mTBI mice transition into a long-lasting persistent phase (PPTH) where, remarkably, allodynia can be reinstated by physiologically relevant and common migraine triggers, including stress. CGRP is established in migraine pathogenesis and our data also suggest an important role in promoting PTH. Treatment with either a CGRP antibody or with onabotulinum toxin A (botox) prevents mTBI-related allodynia (PTH) as well as subsequent provoked allodynia representative of PPTH. However, blockade of CGRP after mTBI sensitization is established is ineffective in blocking provoked allodynia, while botox still maintains efficacy. We have hypothesized that mTBI results in CGRP release from meningeal afferents promoting PTH and central sensitization that underlies the development of PPTH, but that PPTH may be maintained in a CGRP-independent fashion. Additionally, we hypothesize that existing sensitization prior to a mTBI event will promote vulnerability to the development of CGRP-independent PPTH. We explore these hypotheses with two related but, independent, aims using behavioral, neurochemical, immunohistochemical and electrophysiolgical analyses. Aim 1 will determine whether, and when currently available therapies can block mTBI-related outcomes relevant to PTH and if these treatments can prevent the expression of PPTH. Aim 2 will determine if prior sensitization promotes more severe, long-lasting and CGRP-resistant PPTH. Our studies will fill in significant knowledge gaps about the role of CGRP in promoting PTH and the importance of pre-existing sensitization in establishing CGRP- independent PPTH. Such information will influence treatment as well as guide the discovery of new therapies.