CGRP monoclonal antibody prevents the loss of diffuse noxious inhibitory controls (DNIC) in a mouse model of post-traumatic headache

Abstract

Aim

Determine the role of calcitonin-gene related peptide in promoting post-traumatic headache and dysregulation of central pain modulation induced by mild traumatic brain injury in mice.

Methods

Mild traumatic brain injury was induced in lightly anesthetized male C57BL/6J mice by a weight drop onto a closed and unfixed skull, which allowed free head rotation after the impact. We first determined possible alterations in the diffuse noxious inhibitory controls, a measure of net descending pain inhibition called conditioned pain modulation in humans at day 2 following mild traumatic brain injury. Diffuse noxious inhibitory control was assessed as the latency to a thermally induced tail-flick that served as the test stimulus in the presence of right forepaw capsaicin injection that provided the conditioning stimulus. Post-traumatic headache-like behaviors were assessed by the development of cutaneous allodynia in the periorbital and hindpaw regions after mild traumatic brain injury. We then determined if intraperitoneal fremanezumab, an anti-calcitonin-gene related peptide monoclonal antibody or vehicle administered 2 h after sham or mild traumatic brain injury induction could alter cutaneous allodynia or diffuse noxious inhibitory control responses on day 2 post mild traumatic brain injury.

Results

In naïve and sham mice, capsaicin injection into the forepaw elevated the latency to tail-flick, reflecting the antinociceptive diffuse noxious inhibitory control response. Periorbital and hindpaw cutaneous allodynia, as well as a loss of diffuse noxious inhibitory control, was observed in mice 2 days after mild traumatic brain injury. Systemic treatment with fremanezumab blocked mild traumatic brain injury-induced cutaneous allodynia and prevented the loss of diffuse noxious inhibitory controls in mice subjected to a mild traumatic brain injury.

Interpretation

Sequestration of calcitonin-gene related peptide in the initial stages following mild traumatic brain injury blocked the acute allodynia that may reflect mild traumatic brain injury-related post-traumatic headache and, additionally, prevented the loss of net descending inhibition within central pain modulation pathways. As loss of conditioned pain modulation has been linked to multiple persistent pain conditions, dysregulation of descending modulatory pathways may contribute to the persistence of post-traumatic headache. Additionally, evaluation of the conditioned pain modulation/diffuse noxious inhibitory controls response may serve as a biomarker of vulnerability for chronic/persistent pain. These findings suggest that early anti-calcitonin-gene related peptide intervention has the potential to be effective both for the treatment of mild traumatic brain injury-induced post-traumatic headache, as well as inhibiting mechanisms that may promote post-traumatic headache persistence.

Keywords

Mild traumatic brain injury (mTBI)concussion post-traumatic headache (PTH)persistent post-traumatic headache (PPTH)diffuse noxious inhibitory controls (DNIC)conditioned pain modulation (CPM)CGRP fremanezumab

Introduction

Mild traumatic brain injury (mTBI) represents one of the leading causes of disability worldwide, especially among young people (1 –3). mTBI is defined as a transient alteration of brain function usually caused by a direct head impact (1 –3). mTBI, commonly referred to as concussion, is often attributed to falls, motor vehicle collisions, contact or collision sports, exposure to blasts, and other mechanisms of injury that are common in both the civilian and military populations (1,3). Post-traumatic headache (PTH) is the most frequent, and often the most disabling, symptom associated with mTBI. PTH is considered to be “persistent” when it lasts for longer than 3 months (4).

The mechanisms that underlie the persistence of PTH (PPTH) remain to be elucidated. One factor that might contribute to PPTH following mTBI is the neuroplasticity of descending pain modulatory pathways within the central nervous system (CNS) (1,5). The efficiency of descending pain modulation is assessed in humans with the conditioned pain modulation (CPM) procedure and in animals with measures of diffuse noxious inhibitory controls (DNIC) (1,6 –8). The CPM/DNIC response is a pain-inhibits-pain phenomenon that is demonstrated by the simultaneous application of noxious conditioning and test stimuli to different locations on the body (8 –11). Decreased CPM/DNIC responses are associated with many persistent pain conditions, suggesting that the inefficiency of the CPM/DNIC response may contribute to chronic pain and serve as a biomarker of vulnerability for chronic/persistent pain (8 –11). In support of this concept, loss or diminished CPM/DNIC capacity has been reported in patients with chronic migraine (8 –13) and PPTH (5,6,14). Loss of the DNIC response has also been observed in rodent models several weeks after TBI induction (15,16).

Phenotypic overlaps between PPTH and migraine have been noted, suggesting shared biological mechanisms (1 –3). Anti-CGRP antibodies and small molecule CGRP receptor antagonists have been approved for the acute and preventive treatment of migraine (17 –19). There are no approved therapies for PTH (20). In preclinical studies, peripheral CGRP signaling was shown to mediate PTH-like behaviors in a rodent model of mTBI (2,21,22). We have recently reported the presence of initial transient pain-like behaviors and long-lasting latent sensitization in a mouse model of PTH induced by a clinically relevant mTBI (21). Specifically, the mTBI consists of a weight-drop onto a closed and unfixed skull, which reproduces the linear and rotational forces that give rise to concussion (21,23 –25). In our study, early, but not late, treatment of mTBI mice with an anti-CGRP monoclonal antibody (mAb) prevented pain behaviors, as well as latent sensitization (21).

The aim of this study was to determine if CGRP may drive the loss of descending pain modulation that has been observed after mTBI. The DNIC response was assessed in naïve, sham- or mTBI mice at 2 days after mTBI. PTH-like behaviors were confirmed by the development of cutaneous allodynia (CA) at 1 and 2 days after mTBI induction. The role of CGRP was investigated using fremanezumab, an anti-CGRP peptide mAb that was given 2 h after the mTBI, as previously described (21). To our knowledge, the evaluation of CPM/DNIC in the early stages after closed and non-head fixed injury mTBI has not been assessed in preclinical models. Early detection of deficits in CPM/DNIC response might inform therapeutic strategies that prevent the development of PPTH.

Materials and methods

Animals

Studies were performed using 6-week-old male C57BL6/J mice (Jackson Laboratory), housed in a room on a 12/12-hour light/dark cycle (7 am to 7 pm lights on), with controlled temperature and humidity and with free access to food and water in the University of Arizona animal facility. A total of 120 animals were used in these studies with 13–22 animals per group for behavior evaluation. All experimental procedures were performed in accordance with the ARRIVE guidelines, the ethical guidelines of the International Association for the Study of Pain regulations on animal welfare and the National Institutes of Health guidelines for the care and use of laboratory animals. The experimental procedures have been previously approved by the Institutional Animal Care and Use Committee of the University of Arizona. Mice were randomly divided into control and experimental groups and experiments were blinded for the treatment, forepaw injection and behavioral measurements.

Drugs

Capsaicin (TOCRIS, Bristol, UK) was dissolved in 10% Tween 80, 10% ethanol and 80% saline to a 0.25% (0.25 µg/µL) concentration right before the injection and kept on ice. Animals received subcutaneous (s.c.) injection of 10 µL of capsaicin into the right forepaw under light isoflurane anesthesia. The humanized anti-CGRP mAb, fremanezumab, was purchased from the hospital pharmacy at the University of Arizona and was diluted in phosphate-buffered saline PBS immediately prior to use. Animals received intraperitoneal (i.p.) injections of the antibody at 30 mg/kg, 2 h after the mTBI or sham procedures as previously described (21). Control animals received the administration of the respective vehicles.

Mild traumatic brain injury

The mouse model of experimental mTBI was adapted from Kane et al. (26), as reported previously by our group (21,24). Mice were lightly anaesthetized and laid with their ventral surface and in a prone position on an elevated sheet of tissue paper situated over a plexiglass apparatus with soft sponge at the bottom. A metal guide tube was directed to the top of the mouse skull between the ears to ensure standardized placement of the weight drop. The 100 g weight was released from a height of 94 cm onto the closed and unfixed skull, resulting in a concussive impact to the head, pushing the mouse through the tissue paper and flipping it down to land on the soft sponge as previously described (21,24). All mTBI mice experienced both rotational and linear head forces, mimicking to some degree common concussion injuries that involve free head rotation in humans (23 –25). Following the procedure, the righting reflex was recorded, and mice were placed back in their home cages and allowed to recover. Sham animals were anaesthetized and placed on the tissue paper stage but did not receive the weight drop or rotational flip. All mice awoke within 5 min of the procedure.

Periorbital and hindpaw tactile frequency of response

Mice were placed in elevated individual Plexiglass chambers with mesh flooring and allowed to acclimate for 3 days for 2 h each day. Periorbital (cephalic) and hindpaw (extracephalic) tactile frequency of response were measured in the same mice following a 2-h acclimation period. The 0.4 g (3.61) and 0.6 g (3.84) von Frey filaments (Stoelting, Wood Dale, IL, USA) were applied 10 times, with just enough pressure to cause the filament to display a slight arch, to the periorbital and hindpaw region, respectively. Swiping of the face, shaking of the head, and/or turning away from the stimuli were considered positive periorbital responses. Sharp withdrawal, shaking and/or licking the paw were considered positive hindpaw responses. Increased frequency of response represents the development of cutaneous allodynia (CA). Frequency response was calculated as ([number of positive responses × 100%]/10). Tactile allodynia was measured prior to and on days 1 and 2 after the mTBI induction.

Diffuse noxious inhibitory control (DNIC)

The tail-flick response was used as the test stimulus (TS). Animals were gently held by the experimenter, and half of the distal tail was immersed in a 50°C circulating hot-water bath (Neslab Instruments, Inc, Newington, NH). The time required for the animals to flick their tails established the baseline latency (BL). A cut-off of 15 sec was established to avoid tissue damage. Capsaicin (0.25%/10 µL) was the conditioning stimulus (CS) and was injected into the right forepaw. The tail-flick latency (i.e. test latency, TL) was then measured repeatedly as described below. The %DNIC response was calculated 40 min after capsaicin injection, the peak tail-flick antinociception. The following formula was used to calculate %DNIC: 100 × (TL – BL)/(cutoff – BL).

General experimental design overview

The experimental procedures are illustrated in Figures 1 –3. In experiment 1 (Figure 1(a)), the baseline tail-flick response of naïve mice was recorded. Mice then received an injection of capsaicin or vehicle into the right forepaw and the tail-flick latency was assessed again after 20, 40, 60, 90, 120 and 180 min. In experiment 2 (Figure 2(a)), baseline responses for periorbital and hindpaw tactile stimulation with von Frey filaments were collected followed by sham or mTBI induction. Responses to von Frey filaments were evaluated on days 1 and 2 after the sham or mTBI induction. The tail-flick latency was recorded immediately after the tactile evaluation on day 2. Mice then received a capsaicin injection into the right forepaw and tail-flick was re-assessed after an additional 20, 40, 60, 90, 120 and 180 min. The same protocol as experiment 2 was repeated in experiment 3 (Figure 3(a)), with the exception that mice received i.p. anti-CGRP mAb or saline 2 h after sham- or mTBI induction.

Figure 1.

Capsaicin injection induced DNIC response in naïve mice. (a) Timeline of the experimental procedures. Response latency baseline (BL) was assessed using the tail-flick test, followed by s.c. injection of capsaicin or vehicle in the right forepaw of naïve mice. Tail-flick response was performed on indicated minutes after injections. (b) Tail-flick time course and (c) % of DNIC response at 40 min after the injection compared with baseline (BL). Data are plotted as means ± SEM and analysed using (a) two-way ANOVA with Sidak’s multiple comparison test and (b) Student’s t-test with * representing p < 0.05 in comparison to vehicle group (vehicle group, n = 13 and capsaicin group, n = 22).

Figure 2.

mTBI produced CA and loss of DNIC response in mice. (a) Timeline of the experimental procedures. (b) Periorbital and (c) hindpaw frequency of response to tactile stimulation was performed prior (BL) and on days 1 and 2 after mTBI or sham induction. On day 2 after CA evaluation, tail-flick latency baseline (BL) was assessed, followed by s.c. injection of capsaicin in the forepaw. Tail-flick evaluation was performed on indicated minutes after injection. (d) Tail-flick time course and (e) % of DNIC response at 40 min after the injection. Data are plotted as means ± SEM and analysed using ((b), (c) and (d)) Two-way ANOVA with Sidak’s multiple comparison test and (e) Student’s t-test with * representing p < 0.05 in comparison to sham group (sham group, n = 14 and mTBI group, n = 19).

Figure 3.

Anti-CGRP mAb blocked CA and prevented the loss of DNIC response in mTBI-mice. (a) Timeline of the experimental procedures. (b) Periorbital and (c) hindpaw frequency of response to tactile stimulation was performed prior (BL) and on days 1 and 2 after mTBI or sham induction. The anti-CGRP mAb, at 30 mg/kg, or control were administered i.p., 2 h after the mTBI or sham induction. On day 2 after CA evaluation, tail-flick latency baseline (BL) was assessed, followed by s.c. injection of capsaicin in animals’ right forepaw. Tail-flick evaluation was performed on indicated minutes after capsaicin injection. (d) Tail-flick time course and (e) % of DNIC response at 40 min after the forepaw injection. Data are plotted as means ± SEM and analysed using ((b), (c) and (d)) Two- and (e) one-way ANOVA with Tukey’s multiple comparison test with * or # representing p < 0.05 in comparison to sham or mTBI + vehicle groups, respectively (sham group, n = 22; mTBI + vehicle group, n = 17 and mTBI + anti-CGRP mAb group, n = 13).

Data analysis

Sample size was determined using the GPower 3.1 software, with an established significance level of p < 0.05 and statistical power 0,9. Statistical analyses used in these studies were calculated using GraphPad Prism 7 (GraphPad Software, La Jolla, CA). Two-way analysis of variance (ANOVA) followed by the Sidak or Tukey test were used for analysis of the time course of tail-flick and sensory threshold experiments. The unpaired Student’s t-test (two tailed) was used for the evaluation of % of maximal response in the tail-flick test, when only two groups were compared. One-way ANOVA followed by Tukey’s test was used to analyse three groups comparison of % of maximal response in the tail-flick test. The numbers of animals used (n), p-values and F ratios are reported in Table 1. All data are presented as mean ± SEM and statistical significance was set at p < 0.05.

Table 1.

Summary of statistical analyses.

Figure	Analysis	Interaction p	Interaction F	n
1(b)	Two-way ANOVA Sidak	p < 0.0001	F_{(7, 231)} = 5.971	13–22
1(c)	Student’s t-test	p < 0.0001	F = 13.14, 21, 12	13–22
2(b)	Two-way ANOVASidak	p < 0.0001	F_{(2, 62)} = 24.13	14–19
2(c)	Two-way ANOVASidak	p < 0.0001	F_{(2, 62)} = 24	14–19
2(d)	Two-way ANOVASidak	p = 0.0027	F_{(7, 217)} = 3.249	14–19
2(e)	Student’s t-test	p = 0.0010	F = 1.081, 13, 18	14–19
3(b)	Two-way ANOVATukey	p < 0.0001	F_{(4, 98)} = 15.64	13–22
3(c)	Two-way ANOVATukey	p < 0.0001	F_{(4, 98)} = 14.61	13–22
3(d)	Two-way ANOVATukey	p < 0.0001	F_{(14, 343)} = 5.738	13–22
3(e)	One-way ANOVATukey	p = 0.0017	F_{(2, 49)} = 7.274	13–22

Note: p-values, interaction F ratios and n for statistical analyses used in Figures 1 –3.

Results

Experiment 1: DNIC in naïve mice

Subcutaneous injection of capsaicin into the forepaw induced a pronounced DNIC response in naïve mice that was demonstrated by increased tail-flick latency in comparison to vehicle-injected animals (Figure 1(b)). Mice receiving vehicle in the forepaw did not demonstrate significant changes in the tail-flick response latency throughout the experimental time course (Figure 1(b)). The peak of the DNIC-induced antinociception was observed at 40 min after capsaicin (Figure 1(c)).

Experiment 2: mTBI-induced CA and loss of DNIC

Consistent with our previous reports (21), mTBI produced CA, on days 1 and 2, demonstrated by increased frequency of response to tactile stimulation in comparison to sham animals (Figure 2(b),(c)). Sham mice did not demonstrate significant changes in the frequency of response to tactile stimulation throughout the experimental time course (Figure 2(b),(c)). Sham-treated animals showed the expected antinociceptive response following forepaw capsaicin with an increase in tail-flick latency (Figure 2(d)) reaching a maximum of 72% of DNIC-induced antinociception response at 40 min (Figure 2(e)). This increase in tail-flick latency is indicative of DNIC. However, capsaicin-induced antinociception was significantly reduced in animals that had mTBI with a DNIC response of 25%, demonstrating a loss of the DNIC response (Figure 2(e)). Overall, this indicates that mTBI decreased the DNIC antinociceptive response to approximately 47% of sham values.

Experiment 3: Administration of anti-CGRP mAb blocked CA and prevented mTBI-induced loss of DNIC

The control-treated mTBI mice developed CA while the sham group did not show CA (Figure 3(b),(c). Systemic treatment with fremanezumab, 2 h after mTBI, blocked the development of CA in mTBI mice (Figure 3(b),(c)). Following mTBI, vehicle-treated animals demonstrated a loss of DNIC in comparison to sham mice. Treatment with the CGRP mAb prevented the loss of DNIC (Figure 3(d)). Sham-treated mice showed a 62% DNIC while mTBI animals receiving vehicle showed a 17% response. In contrast, mTBI animals receiving the anti-CGRP mAb showed a 60% response demonstrating prevention of loss of DNIC (Figure 3(e)). The DNIC response was reduced by 45% by mTBI in comparison to sham animals, while treatment with fremanezumab maintained the %DNIC response at sham levels. Anti-CGRP mAb did not alter behaviors in sham groups; for this reason, sham animals treated with mAb or vehicle were combined for display (Figure 3).

Discussion

We have used a translational model of PTH to evaluate the role of CGRP in the DNIC response at 2 days after mTBI induction. Our main observations were: i) PTH-like behavior (i.e. cutaneous allodynia) was observed on days 1 and 2 after mTBI induction; ii) mTBI promoted the loss of DNIC responses that were observable at this early post-mTBI time-point; iii) sequestration of CGRP with the anti-CGRP mAb, fremanezumab, administered systemically 2 h after mTBI induction, blocked the development of cutaneous allodynia, and iv) also prevented the loss of DNIC response evaluated on day 2. All together, these data suggest that mTBI induced PTH-like pain behavior and dysregulation of central pain modulatory mechanisms are mediated by CGRP. Neutralization of this peptide in the early post-mTBI period may thus represent an important strategy not only for the treatment of PTH, but also for reducing the risk of pain progression and/or persistence that is associated with reduced net descending pain inhibition.

PTH is the most common symptom reported by individuals experiencing mTBI (1 –3). The development of PTH-like behaviors after TBI has been observed in numerous preclinical studies (2,15,16,21,27). The preclinical model of mTBI adapted for use in our studies was developed to recapitulate the biomechanics associated with common concussion injuries in humans, including unrestrained closed head impact with linear and rotational acceleration (23 –26). We have recently suggested that the periorbital CA and latent sensitization observed after the mTBI may respectively reflect PTH and PPTH (21). In that study, mTBI induced transient CA that resolved by day 14. On day 14, exposure of the mTBI animals to a provocative stimulus such as stress reinstated the CA, reflecting a state of vulnerability termed latent sensitization. A similar timeline for PTH-like behaviors has been observed using a different mTBI model (15,16,28). Central sensitization in pain circuits may therefore significantly contribute to the persistence of pain, including PPTH (15,16,21,27,28).

The symptoms associated with mTBI include headache, hypersensitivity to light and sound, cognitive, mood, sleep, vestibular and autonomic symptoms that resemble migraine (1 –3). The phenotypic overlap between PTH and migraine suggests that these conditions may share common pathophysiological mechanisms, including a prominent role for CGRP (1,3). In addition to our report (21), other preclinical studies have also suggested the potential involvement of CGRP in PTH (1,2,27). These studies have suggested that circulating levels of CGRP might be increased after mTBI, which contributes to the development of PTH and that blockade of CGRP actions might represent an important therapeutic approach for the treatment of acute and persistent PTH. Data from pre-clinical and human studies at least suggest that CGRP might have a more prominent role during the acute phase of PTH versus the persistent phase (1,21,29). Similarly, the current study confirmed the efficacy of early treatment with anti-CGRP mAb in blocking mTBI-induced CA.

Brain images from individuals with mTBI have reported abnormalities in brain structure and connectivity in areas including the PAG and RVM (3,30,31) despite clinical imaging, which is often without visible structural abnormality (3). These areas of abnormality are relevant to central pain modulation pathways, raising the possibility that mTBI may promote dysregulation of these endogenous mechanisms. Healthy individuals typically present efficient CPM/DNIC, in which the application of a noxious conditioning stimulus increases the pain threshold; that is, produces analgesia reflecting pain modulation through engagement of descending inhibition pathway (8 –11). Many chronic pain conditions have been shown to be associated with a loss/deficit of CPM, including primary headache disorders (8 –11) and in individuals with acute PTH and PPTH related to early and later stages after mTBI (5,6,14,23).

Our study demonstrated that the DNIC response of naïve and sham mice was intact and consistent with previous reports, we found that the DNIC response was reduced after mTBI induction. A difference between our study and previous work is that our study demonstrated loss of the DNIC response at 2 days after mTBI, suggesting the rapid emergence of central mechanisms that may be critical in promoting pain. This raises the possibility that disrupted descending modulation of nociceptive signaling observed in early stages after mTBI may be a potential prospective indicator of pain progression and/or persistence (6,8,9,30). In this regard, Yarnitsky and colleagues found that the greatest risk for development of chronic pain in patients scheduled for major surgery (e.g. thoracotomy, abdominal procedures) was a poor CPM response pre-surgery (11,32). The relationship of CPM with chronic pain is complex, so that a relatively weak CPM response may contribute to the development of chronic pain, but additionally, chronic pain may be a factor in promoting decreased CPM (9,11,32). Descending modulation of pain is bidirectional, involving both pain inhibition and pain facilitation (33). Marchand and colleagues reported that while most people respond to the CPM procedure with descending inhibition manifested as analgesia, about 20% of subjects show enhanced descending facilitation that presents as hyperalgesia (34,35). Importantly, when CPM was evaluated in fibromyalgia patients, the proportion of patients responding with hyperalgesia was increased by about twofold, suggesting that the decreased analgesia observed may reflect a loss of descending inhibition, enhanced descending facilitation, or both (34,35). For this reason, preventing dysregulation of these central circuits may be important in preventing the expression of a persistent pain condition. It should be noted that an acute injection of capsaicin does not produce stress-induced analgesia (36,37). Rather, capsaicin produces pain in humans and in rodents and has been used in both human and preclinical studies as a noxious conditioning stimulus in DNIC protocols (12,13,38 –41). The antinociceptive effect of capsaicin observed in our study most likely represents the “pain inhibits pain” DNIC phenomenon.

Whether treatments that can block mTBI-related CA may also have an effect on central pain modulation remains unclear. Irvine and colleagues reported that the histone acetyl transferase inhibitor, anacardic acid (AA), blocked mTBI-induced CA but did not prevent the loss of DNIC produced by mTBI (15). In our studies, we found that administration of the anti-CGRP mAb, given 2 h after mTBI, prevented both CA and the loss of DNIC in mice. This finding suggests that CGRP may be a causal factor in promoting central dysregulation that can promote pain persistence. While these results suggest the potential importance of peripheral CGRP in inducing a net loss of descending inhibition of endogenous pain control mechanisms that could promote PPTH, we note some important limitations. First, the site of action of the CGRP mAb remains uncertain. While it seems logical to assume that the site of action of the anti-CGRP mAb is in the periphery, it is possible that the mTBI produced a transient opening of the blood brain barrier, allowing for access to central CGRP receptors (42 –44). We chose systemic administration of CGRP mAb to increase potential translational relevance of the findings. Additional studies will be required to investigate the site of action for the CGRP mAb in prevention of loss of DNIC. Second, our study used mice treated with CGRP mAb and vehicle controls but did not include an isotype control protein. However, a previous report by our group demonstrated that the control protein has no effect on mTBI-induced CA (21). Third, although females are more susceptible to developing PTH after mTBI (45), the present work evaluated the loss of DNIC response and the effect of sequestration of CGRP after mTBI only in male mice. Whether the same mechanism occurs in females remains to be confirmed. We note that Dussor and colleagues (46) have reported an increase in migraine-like behaviors that is observed preferentially in female mice, suggesting the possibility that CGRP may play an even more prominent role in mTBI-induced PTH and loss of DNIC in female animals. Fourth, we evaluated DNIC on day 2 based on the peak of the transient allodynic, following mTBI induction previously established in our model. Future studies might consider evaluation of the effectiveness of CGRP mAb treatment in modulation of the DNIC responses at later timepoints after mTBI. Finally, an important limitation of our animal model is the use of a brief anesthesia to induce mTBI in mice.

While our data suggest that dysregulation of endogenous pain modulation mechanisms can be observed in the early periods after mTBI in animals, this has yet to be confirmed in humans. If this was demonstrated, a loss of CPM might represent a biomarker or risk factor for PPTH. In support, mTBI patients presented decreased pain inhibition on the conditioned pain modulation test compared with the control group during the acute phase after the injury (14). Regardless of the outcome of such assessments, our data suggest that early treatment with an anti-CGRP mAb might be beneficial in the immediate treatment of PTH, as well as in preventing central dysregulation of pain pathways that likely promote persistent pain conditions including PPTH in humans.

Our data suggest that administration of anti-CGRP mAb at early time points can prevent central dysregulation following mTBI. Administration of pharmacological treatments in humans experiencing mTBI may not be practical at early timepoints. However, the relative time window in which normalization of injury-induced dysregulation of central circuits may be feasible in humans is unknown and may be different from that observed in animals. Additionally, we note that there may be many people who could experience an mTBI while they are already being treated with anti-CGRP mAb for migraine and that, for this reason, there may be a protective preventive effect that is realized. Finally, anti-CGRP mAb treatment might be conceived as a preventive strategy in individuals at higher risk for mTBI events including, for example, active military service members and those active in sports.

Article highlights

mTBI induced PTH-like behaviors and loss of DNIC in male mice.

CGRP contributes to dysregulation of central pain modulation after mTBI.

Anti-CGRP mAb treatment blocked pain-like behaviors and prevented the loss of DNIC in the initial period after mTBI in mice.

Sequestration of CGRP might be a relevant strategy for the treatment of PTH and the prevention of PPTH.

Sequestration of CGRP may be conceived as a preventive therapy for those at higher risk of mTBI.

Footnotes

Declaration of conflicting interests

The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: CMK, JMT, JS, TJW, TA, EN and FP declare that they have no personal, financial, or relational conflicts of interest with this work. TJS reports personal fees from Alder, Allergan, Abbvie, Amgen, Biohaven, Cipla, Click Therapeutics, Dr. Reddys, Eli Lilly, Equinox, Ipsen, Lundbeck, Novartis, Teva, Weber and Weber, and XoC; research grants from American Migraine Foundation, Amgen, Henry Jackson Foundation, National Institutes of Health, Patient Centered Outcomes Research Institute, and United States Department of Defense; stock options from Aural Analytics, Nocira; and royalties from UpToDate. DWD reports the following conflicts within the past 12 months: Consulting: AEON, Amgen, Clexio, Cerecin, Allergan, Alder, Biohaven, Linpharma, Lundbeck, Promius, Eli Lilly, eNeura, Novartis, Impel, Theranica, WL Gore, Nocira, XoC, Zosano, Upjohn (Division of Pfizer), Pieris, Revance, Equinox. Honoraria: CME Outfitters, Curry Rockefeller Group, DeepBench, Global Access Meetings, KLJ Associates, Majallin LLC, Medlogix Communications, Miller Medical Communications, Southern Headache Society (MAHEC), WebMD Health/Medscape, Wolters Kluwer, Oxford University Press, Cambridge University Press. Research Support: Department of Defense, National Institutes of Health, Henry Jackson Foundation, Sperling Foundation, American Migraine Foundation, Patient Centered Outcomes Research Institute (PCORI). Stock Options/Shareholder/Patents/Board of Directors: Aural analytics (options), ExSano (options), Palion (options), Healint (Options), Theranica (Options), Second Opinion/Mobile Health (Options), Epien (Options/Board), Nocira (options), Ontologics (Options/Board), King-Devick Technologies (Options/Board), Precon Health (Options/Board). Patent 17189376.1-1466:vTitle: Botulinum Toxin Dosage Regimen for Chronic Migraine Prophylaxis.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This research was supported in part by NIH1R01NS106902. Coordination for the Improvement of Higher Education Personnel (CAPES) provided doctorate and interuniversity exchange doctorate fellowships for JMT.

ORCID iDs

Caroline M Kopruszinski

Edita Navratilova

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