Background and objective Posttraumatic headache (PTH) is one of the most common, debilitating and difficult symptoms to manage after a mild traumatic brain injury, or concussion. However, the mechanisms underlying PTH remain elusive, in part due to the lack of a clinically relevant animal model. Here, we characterized for the first time, headache and pain-related behaviours in a rat model of concussion evoked by a mild closed head injury (mCHI) - the major type of military and civilian related trauma associated with PTH - and tested responses to current and novel headache therapies. Methods Concussion was induced in adult male rats using a weight-drop device. Characterization of headache and pain related behaviours included assessment of cutaneous tactile pain sensitivity, using von Frey monofilaments, and ongoing pain using the conditioned place preference or aversion (CPP/CPA) paradigms. Sensitivity to headache/migraine triggers was tested by exposing rats to low-dose glyceryl trinitrate (GTN). Treatments included acute systemic administration of sumatriptan and chronic systemic administration of a mouse anti-CGRP monoclonal antibody. Results Concussed rats developed cephalic tactile pain hypersensitivity that was resolved by two weeks post-injury and was ameliorated by treatment with sumatriptan or anti-CGRP monoclonal antibody. Sumatriptan also produced CPP seven days post mCHI, but not in sham animals. Following the resolution of the concussion-evoked cephalic hypersensitivity, administration of GTN produced a renewed and pronounced cephalic pain hypersensitivity that was inhibited by sumatriptan or anti-CGRP antibody treatment as well as a CGRP-dependent CPA. GTN had no effect in sham animals. Conclusions Concussion leads to the development of headache and pain-related behaviours, in particular sustained enhanced responses to GTN, that are mediated through a CGRP-dependent mechanism. Treatment with anti-CGRP antibodies may be a useful approach to treat PTH.

Background Chronic post-traumatic headache (PTH) is one of the most common symptoms of mild traumatic brain injury (mTBI) but its underlying mechanisms remain unknown. Inflammatory degranulation of dural mast cells (MCs) is thought to promote headache, and may play a role in PTH. Whether mTBI is associated with persistent degranulation of dural MCs is yet to be determined. Methods Histochemistry was used to evaluate time course changes in dural MC density and degranulation level in concussive head trauma and blast mouse models of mTBI. The effects of sumatriptan and the MC stabilizer cromolyn sodium on concussion-evoked dural MC degranulation were also investigated. Results Concussive head injury evoked persistent MC degranulation for at least 30 days. Blast trauma gave rise to a delayed MC degranulation response commencing at seven days that also persisted for at least 30 days. Neither sumatriptan nor cromolyn treatment reduced concussion-evoked persistent MC degranulation. Conclusions mTBI evoked by closed head injury or blast exposure is associated with persistent dural MC degranulation. Such a response in mTBI patients may contribute to PTH. Amelioration of PTH by sumatriptan may not involve inhibition of dural MC degranulation. If persistent dural MC degranulation contributes to PTH, then cromolyn treatment may not be effective.

Acute pain following inguinal hernia repair can be difficult to treat. Here we report, for the first time, the development of a novel, anatomically relevant rat model to facilitate improved understanding and treatment of acute postoperative pain following inguinal hernia repair.

Highlights• Administration of an anti-CGRP mAb does not block ongoing meningeal mast cells (MCs) degranulation following a mild closed head injury (mCHI).• mCHI-evoked cephalic mechanical hyperalgesia does not involve acute degranulation of MCs.• Depletion of meningeal MCs content prior to mCHI blocks the development of chronic latent sensitization to the headache trigger glyceryl trintrate (GTN) following recovery from the initial mCHI-evoked acute cranial hyperalgesia• GTN-evoked delayed cephalic and extracephalic mechanical pain hypersensitivity is associated with the latent sensitization is not mediated by GTN-evoked meningeal MC degranulation.. CC-BY-NC-ND 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/392928 doi: bioRxiv preprint first posted online Aug. 16, 2018; AbstractPosttraumatic headache (PTH) is one of the most common, debilitating and difficult symptoms to manage after a mild traumatic brain injury, or concussion. While the mechanisms underlying PTH remain elusive, recent studies suggest the potential involvement of calcitonin gene-related peptide (CGRP), a mediator of neurogenic inflammation, and the ensuing activation of meningeal mast cells (MCs), key pro-algesic resident immune cells that can lead to the activation of the headache pain pathway. The following study investigated the relative contribution of MCs to the development of PTH-like pain behaviors using a recently developed rat model of mild closed head injury (mCHI). We initially employed histological methods and a monoclonal antibody against CGRP to test the relative contribution of peripheral CGRP signaling to the activation of meningeal MCs following mCHI. We then used a prophylactic, pharmacological MC granule depletion protocol, combined with behavioral nociceptive testing, to address the hypotheses that intact meningeal MC granule content is necessary for the development of PTHrelated acute and persistent headache and pain-like behaviors following mCHI. The data suggest that following mCHI, ongoing meningeal MC degranulation does not involve peripheral CGRP signaling, and that the cephalic mechanical pain hypersensitivity that develops following mCHI does not depend upon acute meningeal MC degranulation. Our data, however, also reveals that the development of latent sensitization, a key chronic pain-like phenomenon, manifested as persistent hypersensitivity upon the recovery from mCHI-evoked acute cranial hyperalgesia to the headache trigger glyceryl trinitate (GTN) requires intact MC content during and immediately after mCHI. Collectively, our data implicate the acute activation of meningeal MCs as mediator of chronic pain hypersensitivity following a concussion or mCHI. Targeting MCs may be explored for early prophylactic treatment of PTH.

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Introduction Females are thought to have increased risk of developing post-traumatic headache following a traumatic head injury or concussion. However, the processes underlying this susceptibility remain unclear. We previously demonstrated the development of post-traumatic headache-like pain behaviors in a male rat model of mild closed head injury, along with the ability of sumatriptan and an anti-calcitonin-gene-related peptide monoclonal antibody to ameliorate these behaviors. Here, we conducted a follow-up study to explore the development of post-traumatic headache-like behaviors and the effectiveness of these headache therapies in females subjected to the same head trauma protocol. Methods Adult female Sprague Dawley rats were subjected to a mild closed head injury using a weight-drop device (n = 126), or to a sham procedure (n = 28). Characterization of headache and pain related behaviors included assessment of changes in cutaneous cephalic and extracephalic tactile pain sensitivity, using von Frey monofilaments. Sensitivity to headache/migraine triggers was tested by examining the effect of intraperitoneal administration of a low dose of glyceryl trinitrate (100 µg/kg). Treatments included acute systemic administration of sumatriptan (1 mg/kg) and repeated systemic administration of a mouse anti-calcitonin gene-related peptide monoclonal antibody (30 mg/kg). Serum levels of calcitonin gene-related peptide were measured at baseline and at various time points post head injury in new cohorts of females (n = 38) and males (n = 36). Results Female rats subjected to a mild closed head injury developed cutaneous mechanical hyperalgesia, which was limited to the cephalic region and was resolved 4 weeks later. Cephalic pain hypersensitivity was ameliorated by treatment with sumatriptan but was resistant to an early and prolonged treatment with the anti-calcitonin gene-related peptide monoclonal antibody. Following the resolution of the head injury-evoked cephalic hypersensitivity, administration of glyceryl trinitrate produced a renewed and pronounced cephalic and extracephalic pain hypersensitivity that was inhibited by sumatriptan, but only partially by the anti-calcitonin gene-related peptide treatment. Calcitonin gene-related peptide serum levels were elevated in females but not in males at 7 days post head injury. Conclusions Development of post-traumatic headache-like pain behaviors following a mild closed head injury, and responsiveness to treatment in rats is sexually dimorphic. When compared to the data obtained from male rats in the previous study, female rats display a prolonged state of cephalic hyperalgesia, increased responsiveness to a headache trigger, and a poorer effectiveness of an early and prolonged anti-calcitonin gene-related peptide treatment. The increased risk of females to develop post-traumatic headache may be linked to enhanced responsiveness of peripheral and/or central pain pathways and a mechanism independent of peripheral calcitonin gene-related peptide signaling.

1 Introduction: Females are thought to have increased risk of developing posttraumatic headache 2 (PTH) following a traumatic head injury, or concussion. However, the processes underlying this 3 susceptibility remain unclear. We previously explored the development of PTH-like pain 4 behaviors in a novel rat model of mild closed head injury, along with the ability of sumatriptan 5 and an anti-calcitonin-gene-related peptide monoclonal antibody to ameliorate these behaviors. 6Here, we explored the development of PTH-like behaviors and the effectiveness of these headache 7 therapies in females subjected to the same head trauma protocol. 8 9 Methods: Adult female Sprague Dawley rats were subjected to a mild closed head injury using a 10 weight-drop device. Characterization of headache and pain related behaviors included 11 assessment of changes in cutaneous cephalic and extracephalic tactile pain sensitivity, using von 12 Frey monofilaments. Sensitivity to headache/migraine triggers was tested by examining the 13 effect of systemic administration of a low-dose of glyceryl trinitrate (GTN). Treatments included 14 acute systemic administration of sumatriptan and repeated systemic administration of a mouse 15 anti-calcitonin-gene-related peptide monoclonal antibody. Serum levels of calcitonin-gene-16 related peptide were measured at various time points in females and males after the head injury. 17 18 Results: Female rats subjected to a mild closed head injury developed cutaneous mechanical 19hyperalgesia, that was limited to the cephalic region, and was resolved 4 weeks later. Cephalic 20 pain hypersensitivity was ameliorated by treatment with sumatriptan, but was resistant to an 21 early and prolonged treatment with the anti-CGRP monoclonal antibody. Following the 22 resolution of the head injury-evoked cephalic hypersensitivity, administration of GTN produced 23 a renewed and pronounced cephalic and extracephalic pain hypersensitivity that was inhibited by 1 sumatriptan, but only partially by the anti-CGRP treatment. CGRP serum levels were elevated in 2 females but not in males at 7 days post head injury. 3Conclusions: Development of PTH-like pain behaviors following a mild closed head injury, and 4 responsiveness to treatment in rats is sexually dimorphic. When compared to males, female rats 5 display a prolonged state of cephalic hyperalgesia, increased responsiveness to a headache 6 trigger, and a poorer effectiveness of an early and prolonged anti-CGRP treatment. The increased 7 risk of females to develop PTH may be linked to enhanced responsiveness of peripheral and/or 8 central pain pathways and a mechanism independent of peripheral CGRP signaling. 9

Each of the models will be discussed in terms of their face, construct, and predictive validity as well as overall translational relevance to concussion, mTBI, and post-traumatic headache. Significant contributions to the pathophysiology of PTH garnered from these models are discussed as well as potential contributors to the development of chronic post-traumatic headache. Although post-traumatic headache is one of the most common symptoms following mild head trauma, there remains a disconnect between the study of mild traumatic brain injury and headache in the pre-clinical literature. A greater understanding of the relationship between these phenomena is currently needed to provide more insight into the increasing frequency of this debilitating condition in both military and civilian populations.