Nociceptive neuropeptide increases and periorbital allodynia in a model of traumatic brain injury

Oxytocin shortens spreading depolarization-induced periorbital allodynia

BACKGROUND

Migraine is among the most prevalent and burdensome neurological disorders in the United States based on disability-adjusted life years. Cortical spreading depolarization (SD) is the most likely electrophysiological cause of migraine aura and may be linked to trigeminal nociception. We previously demonstrated, using a minimally invasive optogenetic approach of SD induction (opto-SD), that opto-SD triggers acute periorbital mechanical allodynia that is reversed by 5HT1B/1D receptor agonists, supporting SD-induced activation of migraine-relevant trigeminal pain pathways in mice. Recent data highlight hypothalamic neural circuits in migraine, and SD may activate hypothalamic neurons. Furthermore, neuroanatomical, electrophysiological, and behavioral data suggest a homeostatic analgesic function of hypothalamic neuropeptide hormone, oxytocin. We, therefore, examined the role of hypothalamic paraventricular nucleus (PVN) and oxytocinergic (OXT) signaling in opto-SD-induced trigeminal pain behavior.

METHODS

We induced a single opto-SD in adult male and female Thy1-ChR2-YFP transgenic mice and quantified fos immunolabeling in the PVN and supraoptic nucleus (SON) compared with sham controls. Oxytocin expression was also measured in fos-positive neurons in the PVN. Periorbital mechanical allodynia was tested after treatment with selective OXT receptor antagonist L-368,899 (5 to 25 mg/kg i.p.) or vehicle at 1, 2, and 4 h after opto-SD or sham stimulation using von Frey monofilaments.

RESULTS

Opto-SD significantly increased the number of fos immunoreactive cells in the PVN and SON as compared to sham stimulation (p < 0.001, p = 0.018, respectively). A subpopulation of fos-positive neurons also stained positive for oxytocin. Opto-SD evoked periorbital mechanical allodynia 1 h after SD (p = 0.001 vs. sham), which recovered quickly within 2 h (p = 0.638). OXT receptor antagonist L-368,899 dose-dependently prolonged SD-induced periorbital allodynia (p < 0.001). L-368,899 did not affect mechanical thresholds in the absence of opto-SD.

CONCLUSIONS

These data support an SD-induced activation of PVN neurons and a role for endogenous OXT in alleviating acute SD-induced trigeminal allodynia by shortening its duration.

Low-Dose Interleukin-2 Reverses Traumatic Brain Injury-Induced Cognitive Deficit and Pain in a Murine Model

OBJECTIVE

Despite the high prevalence, mild traumatic brain injury (mTBI)-induced chronic headache and cognitive deficits are poorly understood and lack effective treatments. Low-dose interleukin-2 (LD-IL-2) treatment soon after mTBI or overexpressing IL-2 in brain astrocytes prior to injury protects mice from developing post-traumatic headache (PTH)-related behaviors and cognitive decline. The present study addresses a clinically relevant knowledge gap: whether LD-IL-2 treatment long after the initial injury is still effective for chronic PTH and cognitive deficits.

METHODS

mTBI was induced by a noninvasive closed-head weight drop method. LD-IL-2 was administered 4-6 weeks post-mTBI to assess its effects on chronic PTH-related facial mechanical hypersensitivity as well as mTBI-induced impairment in novel object recognition and object location tests. Endogenous regulatory T (Treg) cells were depleted to investigate the mechanism of action of LD-IL-2.

RESULTS

Delayed LD-IL-2 treatment abolished chronic PTH-related behaviors. It also completely reversed mTBI-induced cognitive impairment in both male and female mice. Treg cell depletion not only prolonged PTH-related behaviors but also abolished the effects of LD-IL-2. Interestingly, LD-IL-2 treatment significantly increased the number of Treg cells in dura but not in brain tissues.

INTERPRETATION

These results suggest that the beneficial effects of LD-IL-2 treatment are mediated through the expansion of meningeal Treg cells. Collectively, our study identifies Treg as a cellular target and LD-IL-2 as a promising therapy for both chronic PTH and mTBI-induced cognitive impairment for both males and females, with a wide therapeutic time window and the potential of reducing polypharmacy in mTBI treatment. ANN NEUROL 2024;96:508-525.

Inhibition of 2-AG hydrolysis alleviates posttraumatic headache attributed to mild traumatic brain injury

BACKGROUND

Posttraumatic headache (PTH) is a common and debilitating symptom following repetitive mild traumatic brain injury (rmTBI), and it mainly resembles a migraine-like phenotype. While modulation of the endocannabinoid system (ECS) is effective in treating TBI and various types of pain including migraine, the role of augmentation of endocannabinoids in treating PTH has not been investigated.

METHODS

Repetitive mild TBI was induced in male C57BL/6J mice using the non-invasive close-head impact model of engineered rotational acceleration (CHIMERA). Periorbital allodynia was assessed using von Frey filaments and determined by the "Up-Down" method. Immunofluorescence staining was employed to investigate glial cell activation and calcitonin gene-related peptide (CGRP) expression in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC) of the rmTBI mice. Levels of 2-arachidonoyl glycerol (2-AG), anandamide (AEA), and arachidonic acid (AA) in the TG, medulla (including TNC), and periaqueductal gray (PAG) were measured by mass spectrometry. The therapeutic effect of endocannabinoid modulation on PTH was also assessed.

RESULTS

The rmTBI mice exhibited significantly increased cephalic pain hypersensitivity compared to the sham controls. MJN110, a potent and selective inhibitor of the 2-AG hydrolytic enzyme monoacylglycerol lipase (MAGL), dose-dependently attenuated periorbital allodynia in the rmTBI animals. Administration of CGRP at 0.01 mg/kg reinstated periorbital allodynia in the rmTBI animals on days 33 and 45 post-injury but had no effect in the sham and MJN110 treatment groups. Activation of glial cells along with increased production of CGRP in the TG and TNC at 7 and 14 days post-rmTBI were attenuated by MJN110 treatment. The anti-inflammatory and anti-nociceptive effects of MJN110 were partially mediated by cannabinoid receptor activation, and the pain-suppressive effect of MJN110 was completely blocked by co-administration of DO34, an inhibitor of 2-AG synthase. The levels of 2-AG in TG, TNC and PAG were decreased in TBI animals, significantly elevated and further reduced by the selective inhibitors of 2-AG hydrolytic and synthetic enzymes, respectively.

CONCLUSION

Enhancing endogenous levels of 2-AG appears to be an effective strategy for the treatment of PTH by attenuating pain initiation and transmission in the trigeminal pathway and facilitating descending pain inhibitory modulation.

Exploring cerebral structural and functional abnormalities in a mouse model of post-traumatic headache induced by mild traumatic brain injury

Mild traumatic brain injury (mTBI)-induced post-traumatic headache (PTH) is a pressing public health concern and leading cause of disability worldwide. Although PTH is often accompanied by neurological disorders, the exact underlying mechanism remains largely unknown. Identifying potential biomarkers may prompt the diagnosis and development of effective treatments for mTBI-induced PTH. In this study, a mouse model of mTBI-induced PTH was established to investigate its effects on cerebral structure and function during short-term recovery. Results indicated that mice with mTBI-induced PTH exhibited balance deficits during the early post-injury stage. Metabolic kinetics revealed that variations in neurotransmitters were most prominent in the cerebellum, temporal lobe/cortex, and hippocampal regions during the early stages of PTH. Additionally, variations in brain functional activities and connectivity were further detected in the early stage of PTH, particularly in the cerebellum and temporal cortex, suggesting that these regions play central roles in the mechanism underlying PTH. Moreover, our results suggested that GABA and glutamate may serve as potential diagnostic or prognostic biomarkers for PTH. Future studies should explore the specific neural circuits involved in the regulation of PTH by the cerebellum and temporal cortex, with these two regions potentially utilized as targets for non-invasive stimulation in future clinical treatment.

Extrasynaptic δGABAA receptors mediate resistance to migraine-like phenotype in rats

BACKGROUND

GABA, a key inhibitory neurotransmitter, has synaptic and extrasynaptic receptors on the postsynaptic neuron. Background GABA, which spills over from the synaptic cleft, acts on extrasynaptic delta subunit containing GABAA receptors. The role of extrasynaptic GABAergic input in migraine is unknown. We investigated the susceptibility to valid migraine-provoking substances with clinically relevant behavioral readouts in Genetic Absence Epilepsy of Rats Strasbourg (GAERS), in which the GABAergic tonus was altered. Subsequently, we screened relevant GABAergic mechanisms in Wistar rats by pharmacological means to identify the mechanisms.

METHODS

Wistar and GAERS rats were administered nitroglycerin (10 mg/kg) or levcromakalim (1 mg/kg). Mechanical allodynia and photophobia were assessed using von Frey monofilaments and a dark-light box. Effects of GAT-1 blocker tiagabine (5 mg/kg), GABAB receptor agonist baclofen (2 mg/kg), synaptic GABAA receptor agonist diazepam (1 mg/kg), extrasynaptic GABAA receptor agonists gaboxadol (4 mg/kg), and muscimol (0.75 mg/kg), T-type calcium channel blocker ethosuximide (100 mg/kg) or synaptic GABAA receptor antagonist flumazenil (15 mg/kg) on levcromakalim-induced migraine phenotype were screened.

RESULTS

Unlike Wistar rats, GAERS exhibited no reduction in mechanical pain thresholds or light aversion following nitroglycerin or levcromakalim injection. Ethosuximide did not reverse the resistant phenotype in GAERS, excluding the role of T-type calcium channel dysfunction in this phenomenon. Tiagabine prevented levcromakalim-induced mechanical allodynia in Wistar rats, suggesting a key role in enhanced GABA spillover. Baclofen did not alleviate mechanical allodynia. Diazepam failed to mitigate levcromakalim-induced migraine phenotype. Additionally, the resistant phenotype in GAERS was not affected by flumazenil. Extrasynaptic GABAA receptor agonists gaboxadol and muscimol inhibited periorbital allodynia in Wistar rats.

CONCLUSION

Our study introduced a rat strain resistant to migraine-provoking agents and signified a critical involvement of extrasynaptic δGABAergic receptors. Extrasynaptic δ GABAA receptors, by mediating constant background inhibition on the excitability of neurons, stand as a novel drug target with a therapeutic potential in migraine.

Recovery from Traumatic Brain Injury Is Nociceptin/Orphanin FQ Peptide Receptor Genotype-, Sex-, and Injury Severity-Dependent

Traumatic brain injury (TBI) is a leading cause of death and disability in the United States, and survivors often experience mental and physical health consequences that reduce quality of life. We previously reported that blockade of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor reduced tissue damage markers produced by blast TBI. The goal of this study was to determine the extent to which N/OFQ and NOP receptor levels change following mild (mTBI) and moderate TBI (modTBI) and whether the absence of the NOP receptor attenuates TBI-induced sequelae. Male and female NOP receptor knockout (KO) or wild-type (WT) rats received craniotomy-only (sham) or craniotomy plus mTBI, or modTBI impact to the left cerebral hemisphere. Neurologic and vestibulomotor deficits and nociceptive hyperalgesia and allodynia found in WT male and female rats following mTBI and modTBI were greatly reduced or absent in NOP receptor KO rats. NOP receptor levels increased in brain tissue from injured males but remained unchanged in females. Neurofilament light chain (NF-L) and glial fibrillary acidic protein (GFAP) expression were reduced in NOP receptor KO rats compared with WT following TBI. Levels of N/OFQ in injured brain tissue correlated with neurobehavioral outcomes and GFAP in WT males, but not with KO male or WT and KO female rats. This study reveals a significant contribution of the N/OFQ-NOP receptor system to TBI-induced deficits and suggests that the NOP receptor should be regarded as a potential therapeutic target for TBI. SIGNIFICANCE STATEMENT: This study revealed that nociceptin/orphanin FQ peptide (NOP) receptor knockout animals experienced fewer traumatic brain injury (TBI)-induced deficits than their wild-type counterparts in a sex- and injury severity-dependent manner, suggesting that NOP receptor antagonists may be a potential therapy for TBI.

Neuroinflammation drives sex-dependent effects on pain and negative affect in a murine model of repeated mild traumatic brain injury

ABSTRACT

The Center for Disease Control and Prevention estimates that 75% of reported cases of traumatic brain injury (TBI) are mild, where chronic pain and depression are 2 of the most common symptoms. In this study, we used a murine model of repeated mild TBI to characterize the associated pain hypersensitivity and affective-like behavior and to what extent microglial reactivity contributes to these behavioral phenotypes. Male and female C57BL/6J mice underwent sham or repeated mild traumatic brain injury (rmTBI) and were tested for up to 9 weeks postinjury, where an anti-inflammatory/neuroprotective drug (minocycline) was introduced at 5 weeks postinjury in the drinking water. Repeated mild traumatic brain injury mice developed cold nociceptive hypersensitivity and negative affective states, as well as increased locomotor activity and risk-taking behavior. Minocycline reversed negative affect and pain hypersensitivities in male but not female mice. Repeated mild traumatic brain injury also produced an increase in microglial and brain-derived neurotropic factor mRNA transcripts in limbic structures known to be involved in nociception and affect, but many of these changes were sex dependent. Finally, we show that the antiepileptic drug, gabapentin, produced negative reinforcement in male rmTBI mice that was prevented by minocycline treatment, whereas rmTBI female mice showed a place aversion to gabapentin. Collectively, pain hypersensitivity, increased tonic-aversive pain components, and negative affective states were evident in both male and female rmTBI mice, but suppression of microglial reactivity was only sufficient to reverse behavioral changes in male mice. Neuroinflammation in limbic structures seems to be a contributing factor in behavioral changes resulting from rmTBI.

Trigeminal Sensitization in a Closed Head Model for Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is often accompanied by neurological and ocular symptoms that involve trigeminal nerve pathways. Laser-induced shock wave (LISW) was applied to the skull of male rats as a model for mTBI, while behavioral and neural recording methods were used to assess trigeminal function. The LISW caused greater eye wiping behavior to ocular instillation of hypertonic saline (Sham = 4.83 ± 0.65 wipes/5 min, LISW = 12.71 ± 1.89 wipes/5 min, p < 0.01) and a marked reduction in the time spent in bright light consistent with enhanced periocular and intraocular hypersensitivity, respectively (Sham = 16.3 ± 5.6 s, LISW = 115.5 ± 27.3 s, p < 0.01). To address the early neural mechanisms of mTBI, single trigeminal brainstem neurons, identified by activation to corneal or dural mechanical stimulation, were recorded in trigeminal subnucleus interpolaris/caudalis (Vi/Vc) and trigeminal subnucleus caudalis/upper cervical cord (Vc/C1) regions. The LISW caused marked sensitization to hypertonic saline and to exposure to bright light in neurons of both regions (p < 0.05). Laser speckle imaging revealed an increase in meningeal arterial blood flow to bright light after LISW (Sham = 4.7 ± 2.0 s, LISW = 469.0 ± 37.9 s, p < 0.001). Local inhibition of synaptic activity at Vi/Vc, but not at Vc/C1, by microinjection of CoCl2, prevented light-evoked increases in meningeal blood flow in LISW-treated rats. By contrast, topical meningeal application of phenylephrine significantly reduced light-evoked responses of Vi/Vc and Vc/C1 neurons. These data suggested that neurons in both regions became sensitized after LISW and were responsive to changes in meningeal blood flow. Neurons at the Vi/Vc transition and at Vc/C1, however, likely serve different roles in mediating the neurovascular and sensory aspects of mTBI.

Migraine is a risk factor for pseudophakic positive dysphotopsia following monofocal lens implantation

OBJECTIVE

To identify neuroadaptation-related risk factors for persistent positive dysphotopsia (>6 months) following monofocal lens implantation.

DESIGN

Retrospective cohort study.

PARTICIPANTS

Patients of an academic institution and a private practice in Saint Louis, Missouri. Inclusion criteria were adults with cataract extraction between January 2010 and April 2021 with monofocal intraocular lens implantation. Exclusion criteria included dementia, <20/40 acuity, visual pathway damage, visual field loss, and significant pathology causing photopsia.

METHODS

Participants were surveyed via telephone.

RESULTS

There were 385 participants (385 eyes), of whom 66 had persistent dysphotopsia (58 positive), 298 had none, and 21 had nonpersistent dysphotopsia. Among the 58 who had positive persistent dysphotopsia, mean Pseudophakic Dysphotopsia Questionnaire 6 (PDQ-6) score was 14.11 (SD, 8.46). There were no significant differences in sex or race. Migraine prevalence was greater among those with dysphotopsia (21.2%) than among those without (11.4%; p = 0.054). History of migraine was associated with an increase in PDQ-6 score of 2.76 points (p = 0.006). Six people in each group had Visual Aura Rating Scale (VARS) scores greater than zero. Mean VARS score was 0.48 for those with dysphotopsia and 0.14 for those without (p = 0.03). History of migraine or increased VARS score, younger age, and female sex were associated with lower satisfaction.

CONCLUSION

History of migraine was associated with increased dysphotopsia severity and decreased patient satisfaction. Although further study with a larger sample size is warranted, these preliminary results highlight the potential of simple questions to individualize lens choice, reduce the risk of dysphotopsia, and improve patient satisfaction.

Inflammation, brain connectivity, and neuromodulation in post-traumatic headache

Post-traumatic headache (PTH) is a debilitating condition that affects individuals with different levels of traumatic brain injury (TBI) severity. The difficulties in developing an effective treatment are related to a lack of understanding the complicated mechanisms and neurobiological changes in brain function after a brain injury. Preclinical studies have indicated that peripheral and central sensitization of the trigeminal nociceptive pathways contributes to PTH. While recent brain imaging studies have uncovered widespread changes in brain functional connectivity following trauma, understanding exactly how these networks contribute to PTH after injury remains unknown. Stimulation of peripheral (trigeminal or vagus) nerves show promising efficacies in PTH experimental animals, likely mediated by influencing TBI-induced pathological plasticity by decreasing neuroinflammation and neuronal apoptosis. Non-invasive brain stimulations, such as transcranial magnetic or direct current stimulations, show analgesia for multiple chronic pain conditions, including PTH. Better mechanistic understanding of analgesia achieved by neuromodulations can define peripheral and central mechanisms involved in the development, the resolution, and the management of PTH.

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

A female-specific role for trigeminal dynorphin in orofacial pain comorbidity

ABSTRACT

Migraine is commonly reported in patients with temporomandibular disorders (TMDs), but little is known about the mechanisms underlying the comorbid condition. Here, we prepared a mouse model to investigate this comorbidity, in which masseter muscle tendon ligation (MMTL) was performed to induce a myogenic TMD, and the pre-existing TMD enabled a subthreshold dose of nitroglycerin (NTG) to produce migraine-like pain in mice. RNA sequencing followed by real-time quantitative polymerase chain reaction confirmation showed that MMTL plus NTG treatment increased prodynorphin ( Pdyn ) mRNA expression in the spinal trigeminal nucleus caudalis (Sp5C) of female mice but not in male mice. Chemogenetic inhibition of Pdyn -expressing neurons or microinjection of antidynorphin antiserum in the Sp5C alleviated MMTL-induced masseter hypersensitivity and diminished the MMTL-enabled migraine-like pain in female mice but not in male mice. Moreover, chemogenetic activation of Pdyn -expressing neurons or microinjection of dynorphin A (1-17) peptide in the Sp5C enabled a subthreshold dose of NTG to induce migraine-like pain in female mice but not in male mice. Taken together, our results suggest that trigeminal dynorphin has a female-specific role in the modulation of comorbid TMDs and migraine.

Inhibition of persistent sodium current reduces spreading depression-evoked allodynia in a mouse model of migraine with aura

ABSTRACT

We investigated the efficacy of inhibiting persistent Na + currents (I NaP ) in acute rodent models of migraine with aura. Cortical spreading depression (SD) is a slow wave of neuronal and glial depolarization that underlies the migraine aura. Minimally invasive optogenetic SD (opto-SD) causes periorbital mechanical allodynia in mice, suggesting SD activates trigeminal nociceptors. Persistent Na + currents contribute to neuronal intrinsic excitability and have been implicated in peripheral and cortical excitation. We examined a preferential inhibitor of I NaP, GS-458967, on SD-induced periorbital allodynia, SD susceptibility, and formalin-induced peripheral pain. Periorbital mechanical allodynia was tested in male and female Thy1-ChR2-YFP mice after a single opto-SD event using manual von Frey monofilaments. GS-458967 (1 mg/kg, s.c.) or vehicle was dosed immediately after opto-SD induction, and allodynia was tested 1 hour later. The electrical SD threshold and KCl-induced SD frequency were examined in the cortex in male Sprague-Dawley rats after 1 hour pretreatment with GS-458967 (3 mg/kg, s.c.) or vehicle. Effects of GS-458967 (0.5-5 mg/kg, p.o.) on spontaneous formalin hind paw behavior and locomotion were also examined in male CD-1 mice. GS-458967 suppressed opto-SD-induced periorbital allodynia and decreased susceptibility to SD. GS-458967 also diminished early and late phase formalin-induced paw-licking behavior with early phase paw licking responding to lower doses. GS-458967 up to 3 mg/kg had no impact on locomotor activity. These data provide evidence that I NaP inhibition can reduce opto-SD-induced trigeminal pain behavior and support I NaP inhibition as an antinociceptive strategy for both abortive and preventive treatment of migraine.

Aryl hydrocarbon receptors improve migraine-like pain behaviors in rats through the regulation of regulatory T cell/T-helper 17 cell-related homeostasis

OBJECTIVE

To investigate the effect of the aryl hydrocarbon receptor (AHR)/regulatory T cell (Treg)/T-helper 17 (Th17) cell pathway on the pathogenesis of migraine.

BACKGROUND

Migraine is a disabling neurovascular disease that imposes an enormous burden on both individuals and society. The pathophysiological mechanisms of migraine remain controversial. Recent studies have suggested that immune dysfunction may be involved in the pathogenesis of migraine. The AHR, a receptor expressed on most immune cells, has been implicated in the occurrence of many autoimmune diseases; however, whether it is involved in the pathogenesis of migraine is unclear.

METHODS

A chronic migraine rat model was established through repeated intraperitoneal injection of nitroglycerin (NTG). The mechanical and thermal pain thresholds were assessed using von Frey filaments and radiant heat. Next, the protein expression levels of AHR in the trigeminal nucleus caudalis (TNC) region of chronic migraine (CM)-like rats were quantified and the changes in Treg/Th17-related transcription factors and inflammatory factors in the TNC were explored. To determine the role of AHR in CM, we examined the effects of the AHR agonist 2-(1'-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), and AHR antagonist CH-223191 on pain behavior, c-Fos, calcitonin gene-related peptide (CGRP), AHR, and Treg/Th17-related factor expression in CM-like rats.

RESULTS

Repeated administration of NTG significantly enhanced nociceptive hypersensitivity and increased expression of c-Fos and CGRP in rats, while AHR was significantly decreased in the TNC. In addition, the expression of the transcription factor forkhead box protein P3 and the signal transducer and activator of transcription 5 decreased significantly. In contrast, the expression of the transcription factor retinoic acid receptor-related orphan receptor γ t and signal transducer and activator of transcription 3 were significantly increased. Moreover, the mRNA level of transforming growth factor beta-1 was decreased, while that of interleukin (IL)-10 and IL-22 was increased in the TNC. The AHR agonist ITE alleviated migraine-like pain behaviors in rats, activated the AHR signaling pathway, and improved the imbalance of Treg/Th17-related transcription factors and inflammatory factors. Conversely, the AHR antagonist CH-223191 did not alleviate migraine-like pain behaviors in rats; and even exacerbated them.

CONCLUSIONS

The AHR participates in the development of CM by regulating Treg/Th17-related homeostasis. Therefore, treatments targeting the AHR/Treg/Th17 signaling pathway could be new effective interventions for CM treatment.

The effect of sex and estrus cycle stage on optogenetic spreading depression induced migraine-like pain phenotypes

BACKGROUND

Migraine is more prevalent in females, raising the possibility that sex and gonadal hormones modulate migraine. We recently demonstrated that minimally invasive optogenetic spreading depolarization (opto-SD) elicits robust periorbital allodynia. The objective of this study was to test the hypothesis that opto-SD induced migraine-like pain behavior is worse in females and varies during the estrus cycle.

METHODS

Single or repeated opto-SDs were induced in male and female adult Thy1-ChR2-YFP transgenic mice. Von Frey monofilaments were used to test periorbital mechanical allodynia. Mouse grimace was also examined under increasing light intensity to quantify spontaneous discomfort and light-aversive behavior. Vaginal smears were obtained for estrus cycle staging at the end of behavioral testing.

RESULTS

A multi-variable regression analysis was performed using a male and female cohort to test the effect of independent variables on periorbital allodynia. Opto-SD predicted lower periorbital thresholds as compared with sham stimulation (p < 0.0001). Additionally, female sex predicted lower periorbital thresholds compared with males (p = 0.011). There were significant interactions between opto-SD and time (interaction p = 0.030) as animals tended to recover from opto-SD allodynia over time, and between sex and time (p = 0.020) as females tended to take longer to recover. Proestrus, estrus (PE) and metestrus, diestrus (MD) stages were combined to represent high versus low circulating estradiol relative to progesterone, respectively. Multi-variable regression revealed an effect of estrus cycle (p = 0.015) on periorbital thresholds. In the sham group, PE had lower thresholds than MD. However, there was no interaction between opto-SD and the estrus cycle (p = 0.364). Grimace scores were also examined at incremental light intensities. There was an effect of opto-SD (p < 0.0001), light intensity (p = 0.001) and estrus cycle (p = 0.024) on grimace without interaction among them (three-way ANOVA).

CONCLUSIONS

Female sex and estrus stages with high circulating estradiol relative to progesterone lower trigeminal pain thresholds and augment photosensitivity. In females, opto-SD increased pain behavior and photosensitivity irrespective of the estrus stage.

Neuropeptide Y in the medial habenula alleviates migraine-like behaviors through the Y1 receptor

BACKGROUND

Migraine is a highly disabling health burden with multiple symptoms; however, it remains undertreated because of an inadequate understanding of its neural mechanisms. Neuropeptide Y (NPY) has been demonstrated to be involved in the modulation of pain and emotion, and may play a role in migraine pathophysiology. Changes in NPY levels have been found in patients with migraine, but whether and how these changes contribute to migraine is unknown. Therefore, the purpose of this study was to investigate the role of NPY in migraine-like phenotypes.

METHODS

Here, we used intraperitoneal injection of glyceryl trinitrate (GTN, 10 mg/kg) as a migraine mouse model, which was verified by light-aversive test, von Frey test, and elevated plus maze test. We then performed whole-brain imaging with NPY-GFP mice to explore the critical regions where NPY was changed by GTN treatment. Next, we microinjected NPY into the medial habenula (MHb), and further infused Y1 or Y2 receptor agonists into the MHb, respectively, to detect the effects of NPY in GTN-induced migraine-like behaviors.

RESULTS

GTN effectively triggered allodynia, photophobia, and anxiety-like behaviors in mice. After that, we found a decreased level of GFP⁺ cells in the MHb of GTN-treated mice. Microinjection of NPY attenuated GTN-induced allodynia and anxiety without affecting photophobia. Furthermore, we found that activation of Y1-but not Y2-receptors attenuated GTN-induced allodynia and anxiety.

CONCLUSIONS

Taken together, our data support that the NPY signaling in the MHb produces analgesic and anxiolytic effects through the Y1 receptor. These findings may provide new insights into novel therapeutic targets for the treatment of migraine.

Acetaldehyde via CGRP receptor and TRPA1 in Schwann cells mediates ethanol-evoked periorbital mechanical allodynia in mice: relevance for migraine

BACKGROUND

Ingestion of alcoholic beverages is a known trigger of migraine attacks. However, whether and how ethanol exerts its pro-migraine action remains poorly known. Ethanol stimulates the transient receptor potential vanilloid 1 (TRPV1) channel, and its dehydrogenized metabolite, acetaldehyde, is a known TRP ankyrin 1 (TRPA1) agonist.

METHODS

Periorbital mechanical allodynia following systemic ethanol and acetaldehyde was investigated in mice after TRPA1 and TRPV1 pharmacological antagonism and global genetic deletion. Mice with selective silencing of the receptor activated modifying protein 1 (RAMP1), a component of the calcitonin gene-related peptide (CGRP) receptor, in Schwann cells or TRPA1 in dorsal root ganglion (DRG) neurons or Schwann cells, were used after systemic ethanol and acetaldehyde.

RESULTS

We show in mice that intragastric ethanol administration evokes a sustained periorbital mechanical allodynia that is attenuated by systemic or local alcohol dehydrogenase inhibition, and TRPA1, but not TRPV1, global deletion, thus indicating the implication of acetaldehyde. Systemic (intraperitoneal) acetaldehyde administration also evokes periorbital mechanical allodynia. Importantly, periorbital mechanical allodynia by both ethanol and acetaldehyde is abrogated by pretreatment with the CGRP receptor antagonist, olcegepant, and a selective silencing of RAMP1 in Schwann cells. Periorbital mechanical allodynia by ethanol and acetaldehyde is also attenuated by cyclic AMP, protein kinase A, and nitric oxide inhibition and pretreatment with an antioxidant. Moreover, selective genetic silencing of TRPA1 in Schwann cells or DRG neurons attenuated periorbital mechanical allodynia by ethanol or acetaldehyde.

CONCLUSIONS

Results suggest that, in mice, periorbital mechanical allodynia, a response that mimics cutaneous allodynia reported during migraine attacks, is elicited by ethanol via the systemic production of acetaldehyde that, by releasing CGRP, engages the CGRP receptor in Schwann cells. The ensuing cascade of intracellular events results in a Schwann cell TRPA1-dependent oxidative stress generation that eventually targets neuronal TRPA1 to signal allodynia from the periorbital area.

Preclinical Studies of Posttraumatic Headache and the Potential Therapeutics

Posttraumatic headache (PTH) attributed to traumatic brain injury (TBI) is a secondary headache developed within 7 days after head injury, and in a substantial number of patients PTH becomes chronic and lasts for more than 3 months. Current medications are almost entirely relied on the treatment of primary headache such as migraine, due to its migraine-like phenotype and the limited understanding on the PTH pathogenic mechanisms. To this end, increasing preclinical studies have been conducted in the last decade. We focus in this review on the trigeminovascular system from the animal studies since it provides the primary nociceptive sensory afferents innervating the head and face region, and the pathological changes in the trigeminal pathway are thought to play a key role in the development of PTH. In addition to the pathologies, PTH-like behaviors induced by TBI and further exacerbated by nitroglycerin, a general headache inducer through vasodilation are reviewed. We will overview the current pharmacotherapies including calcitonin gene-related peptide (CGRP) monoclonal antibody and sumatriptan in the PTH animal models. Given that modulation of the endocannabinoid (eCB) system has been well-documented in the treatment of migraine and TBI, the therapeutic potential of eCB in PTH will also be discussed.

Selective inhibition of soluble TNF using XPro1595 improves hippocampal pathology to promote improved neurological recovery following traumatic brain injury in mice

AIMS

To determine the efficacy of XPro1595 to improve pathophysiological and functional outcomes in a mouse model of traumatic brain injury (TBI).

BACKGROUND

Symptoms associated with TBI can be debilitating, and treatment without off-target side effects remains a challenge. This study aimed to investigate the efficacy of selectively inhibiting the soluble form of TNF (solTNF) using the biologic XPro1595 in a mouse model of TBI.

OBJECTIVES

Use XPro1595 to determine whether injury-induced solTNF promotes hippocampal inflammation and dendritic plasticity, and associated functional impairments.

METHODS

Mild-to-moderate traumatic brain injury (CCI model) was induced in adult male C57Bl/6J WT and Thy1-YFPH mice, with XPro1595 (10 mg/kg, S.C.) or vehicle being administered in a clinically relevant window (60 minutes post-injury). The animals were assessed for differences in neurological function, and hippocampal tissue was analyzed for inflammation and glial reactivity, as well as neuronal degeneration and plasticity.

RESULTS

We report that unilateral CCI over the right parietal cortex in mice promoted deficits in learning and memory, depressive-like behavior, and neuropathic pain. Using immunohistochemical and Western blotting techniques, we observed the cortical injury promoted a set of expected pathophysiology's within the hippocampus consistent with the observed neurological outcomes, including glial reactivity, enhanced neuronal dendritic degeneration (dendritic beading), and reduced synaptic plasticity (spine density and PSD-95 expression) within the DG and CA1 region of the hippocampus, that were prevented in mice treated with XPro1595.

CONCLUSION

Overall, we observed that selectively inhibiting solTNF using XPro1595 improved the pathophysiological and neurological sequelae of brain-injured mice, which provides support for its use in patients with TBI.

Uncontrolled mitochondrial calcium uptake underlies the pathogenesis of neurodegeneration in MICU1-deficient mice and patients

Dysregulation of mitochondrial Ca²⁺ homeostasis has been linked to neurodegenerative diseases. Mitochondrial Ca²⁺ uptake is mediated via the calcium uniporter complex that is primarily regulated by MICU1, a Ca²⁺-sensing gatekeeper. Recently, human patients with MICU1 loss-of-function mutations were diagnosed with neuromuscular and cognitive impairments. While studies in patient-derived cells revealed altered mitochondrial calcium signaling, the neuronal pathogenesis was difficult to study. To fill this void, we created a neuron-specific MICU1-KO mouse model. These animals show progressive, abnormal motor and cognitive phenotypes likely caused by the degeneration of motor neurons in the spinal cord and the cortex. We found increased susceptibility to mitochondrial Ca²⁺ overload-induced excitotoxic insults and cell death in MICU1-KO neurons and MICU1-deficient patient-derived cells, which can be blunted by inhibiting the mitochondrial permeability transition pore. Thus, our study identifies altered neuronal mitochondrial Ca²⁺ homeostasis as causative in the clinical symptoms of MICU1-deficient patients and highlights potential therapeutic targets.

Schwann cell endosome CGRP signals elicit periorbital mechanical allodynia in mice

Efficacy of monoclonal antibodies against calcitonin gene-related peptide (CGRP) or its receptor (calcitonin receptor-like receptor/receptor activity modifying protein-1, CLR/RAMP1) implicates peripherally-released CGRP in migraine pain. However, the site and mechanism of CGRP-evoked peripheral pain remain unclear. By cell-selective RAMP1 gene deletion, we reveal that CGRP released from mouse cutaneous trigeminal fibers targets CLR/RAMP1 on surrounding Schwann cells to evoke periorbital mechanical allodynia. CLR/RAMP1 activation in human and mouse Schwann cells generates long-lasting signals from endosomes that evoke cAMP-dependent formation of NO. NO, by gating Schwann cell transient receptor potential ankyrin 1 (TRPA1), releases ROS, which in a feed-forward manner sustain allodynia via nociceptor TRPA1. When encapsulated into nanoparticles that release cargo in acidified endosomes, a CLR/RAMP1 antagonist provides superior inhibition of CGRP signaling and allodynia in mice. Our data suggest that the CGRP-mediated neuronal/Schwann cell pathway mediates allodynia associated with neurogenic inflammation, contributing to the algesic action of CGRP in mice.

Endogenous Opioid Dynorphin Is a Potential Link between Traumatic Brain Injury, Chronic Pain, and Substance Use Disorder

Traumatic brain injury (TBI) is a serious public health problem associated with numerous physical and neuropsychiatric comorbidities. Chronic pain is prevalent and interferes with post-injury functioning and quality of life, whereas substance use disorder (SUD) is the third most common neuropsychiatric diagnosis after TBI. Neither of these conditions has a clear mechanistic explanation based on the known pathophysiology of TBI. Dynorphin is an endogenous opioid neuropeptide that is significantly dysregulated after TBI. Both dynorphin and its primary receptor, the ĸ-opioid receptor (KOR), are implicated in the neuropathology of chronic pain and SUD. Here, we review the known roles of dynorphin and KORs in chronic pain and SUDs. We synthesize this information with our current understanding of TBI and highlight potential mechanistic parallels between and across conditions that suggest a role for dynorphin in long-term sequelae after TBI. In pain studies, dynorphin/KOR activation has either antinociceptive or pro-nociceptive effects, and there are similarities between the signaling pathways influenced by dynorphin and those underlying development of chronic pain. Moreover, the dynorphin/KOR system is considered a key regulator of the negative affective state that characterizes drug withdrawal and protracted abstinence in SUD, and molecular and neurochemical changes observed during the development of SUD are mirrored by the pathophysiology of TBI. We conclude by proposing hypotheses and directions for future research aimed at elucidating the potential role of dynorphin/KOR in chronic pain and/or SUD after TBI.

Chondroitin sulfate produces antinociception and neuroprotection in chronic constriction injury-induced neuropathic pain in rats by increasing anti-inflammatory molecules and reducing oxidative stress

OBJECTIVES

Damage to the peripheral and central nervous system lead to Neuropathic pain (NP) which is a widespread and devitalizing condition. chondroitin sulfate (CS), has been used in managing joint pain and osteoarthritis. In this study, the effectiveness of CS on NP induced by chronic constriction injury (CCI) is examined.

METHODS

Thirty Wistar rats were distributed at random into six groups (n = 5). Sciatic nerve ligation was carried out by encircling the nerve with four loose ligatures to induce NP. Allodynia (cold and mechanical) and heat hyperalgesia were assessed using Acetone, von Frey filament and Hot plate tests. CCI induction resulted to NP, prominent from the 3rd day after surgery. Structural architecture of sciatic nerves was evaluated via histological examination of the transverse section of the nerves.

RESULTS

Oral administration of CS (600 mg/kg and 900 mg/kg for 21 days) resulted in significant (P < 0.05) inhibition of allodynia (cold and mechanical) and thermal hyperalgesia. Lipid peroxidation, tumor necrosis factor-α (TNF-α), calcitonin gene related peptide (CGRP), C reactive protein (CRP), and oxidative stress were attenuated by CS. CS also improved interleukin (IL)-6, nitric oxide (NO), total antioxidant capacity (TAC).

CONCLUSION

These findings suggest that CS attenuates allodynia, and thermal hyperalgesia induced by CCI by downregulating TNF-α, CRP, CGRP, oxidative enzymes, and upregulating IL-6, NO, and TAC. Nociceptive behavioral studies and histological findings showed significant improvement in the CS treated groups compared to CCI rats. These findings are responsible for the beneficial effect of CS in NP.

Photophobia: shared pathophysiology underlying dry eye disease, migraine and traumatic brain injury leading to central neuroplasticity of the trigeminothalamic pathway

BACKGROUND

Photophobia is a potentially debilitating symptom often found in dry eye disease (DE), migraine and traumatic brain injury (TBI).

METHODS

We conducted a review of the literature via a PubMed search of English language articles with a focus on how photophobia may relate to a shared pathophysiology across DE, migraine and TBI.

RESULTS

DE, migraine and TBI are common conditions in the general population, are often comorbid, and share photophobia as a symptom. Across the three conditions, neural dysregulation of peripheral and central nervous system components is implicated in photophobia in various animal models and in humans. Enhanced activity of the neuropeptide calcitonin gene-related peptide (CGRP) is closely linked to photophobia. Current therapies for photophobia include glasses which shield the eyes from specific wavelengths, botulinum toxin, and inhibition of CGRP and its receptor. Many individuals have persistent photophobia despite the use of these therapies, and thus, development of new therapies is needed.

CONCLUSIONS

The presence of photophobia in DE, migraine and TBI suggests shared trigeminothalamic pathophysiologic mechanisms, as explained by central neuroplasticity and hypersensitivity mediated by neuropeptide CGRP. Treatment strategies which target neural pathways (ie, oral neuromodulators, transcutaneous nerve stimulation) should be considered in patients with persistent photophobia, specifically in individuals with DE whose symptoms are not controlled with traditional therapies.

Photophobia and allodynia in persistent post-traumatic headache are associated with higher disease burden

OBJECTIVE

To assess photophobia and allodynia in subjects with post-traumatic headache and examine how these sensory hypersensitivities associate with clinical measures of disease burden.

BACKGROUND

Post-traumatic headache is the most frequent and disabling long-term consequence of mild traumatic brain injury. There is evidence of sensory dysfunction in acute post-traumatic headache, and it is known from other headache conditions that sensory amplifications correlate with more severe disease. However, systematic studies in post-traumatic headache are surprisingly scarce.

METHODS

We tested light and tactile sensitivity, along with measures of disease burden, in 30 persistent post-traumatic headache subjects and 35 controls.

RESULTS

In all, 79% of post-traumatic headache subjects exhibited sensory hypersensitivity based on psychophysical assessment. Of those exhibiting hypersensitivity, 54% exhibited both light and tactile sensitivity. Finally, sensory thresholds were correlated across modalities, as well as with headache attack frequency.

CONCLUSIONS

In this study, post-traumatic headache subjects with both light and tactile sensitivity had significantly higher headache frequencies and lower sensitivity thresholds to both modalities, compared to those with single or no sensory hypersensitivity. This pattern suggests that hypersensitivity across multiple modalities may be functionally synergistic, reflect a higher disease burden, and may serve as candidate markers of disease.

Impact of abuse on migraine-related sensory hypersensitivity symptoms: Results from the American Registry for Migraine Research

BACKGROUND AND OBJECTIVES

Prior studies have established an association between a history of abuse and the development of migraine. This cross-sectional observational study explored the relationship between self-reported abuse history with migraine-related sensory hypersensitivity symptoms.

METHODS

In total, 588 adult patients with migraine from the American Registry for Migraine Research completed questionnaires: Generalized Anxiety Disorder-7, Patient Health Questionnaire-2 for depression, Photosensitivity Assessment Questionnaire, Hyperacusis Questionnaire, and Allodynia Symptom Checklist. Using four binary screening questions, patients were asked to self-report if they believed they had suffered emotional, physical, or sexual abuse in their lifetime. Differences in questionnaire scores between groups with and without a history of abuse were determined. Regression models adjusted for age, sex, and basic headache features analyzed the relationship between abuse history and sensory hypersensitivity symptoms. Moderation analysis explored the role of headache frequency in this relationship. Mediation analysis assessed the indirect (Mediated) effect (IE) of abuse on sensory hypersensitivity through depression or anxiety. Additional models analyzed relationships between sensory hypersensitivity symptoms and abuse subtypes or the number of abuse subtypes.

RESULTS

Of 588 participants, 222 (38%) reported a history of abuse. Patients with a history of abuse reported statistically significantly greater average headache frequency (7.6 vs. 4.7 days, p = 0.030). Patients with a history of abuse also reported higher average or median questionnaire scores: anxiety (7.6 vs. 4.7, p < 0.001, d = 0.56), depression (1.7 vs. 1.3, p = 0.009, d = 0.24), photophobia (0.54 vs. 0.44, p < 0.001, d = 0.32), hyperacusis (19.6 vs. 14.9, p < 0.001, d = 0.49), ictal allodynia (6.0 vs. 3.0, p < 0.001, d = 0.46), and interictal allodynia (1.0 vs. 0.0, p < 0.001, d = 0.30). After controlling for patient age, sex and years lived with headache, abuse maintained a significant association with every sensory hypersensitivity measure. Headache frequency significantly moderated the relationship between a history of abuse with increased ictal allodynia (p = 0.036). Anxiety significantly mediated the relationships between abuse with photophobia (IE = 0.03, 95% CI = 0.01-0.04), hyperacusis (IE = 1.51, 95% CI = 0.91-2.24), ictal allodynia (IE = 0.02, 95% CI = 0.01-0.04), and interictal allodynia (IE = 0.02, 95% CI = 0.01-0.06). Depression significantly mediated the relationship between abuse with photophobia (IE = 0.02, 95% CI = 0.01-0.03) and with hyperacusis (IE = 0.45, 95% CI = 0.11-0.88). The association between the individual subtypes of abuse and the number of subtypes of abuse with sensory hypersensitivity symptoms varied.

CONCLUSION

A history of abuse is associated with greater migraine-related sensory hypersensitivity symptoms. To reduce the impact of abuse on migraine symptoms, future studies should explore mechanistic connections between abuse and migraine-associated symptoms.

An underrepresented majority: A systematic review utilizing allodynic criteria to examine the present scarcity of discrete animal models for episodic migraine

BACKGROUND

Despite increasing evidence differentiating episodic and chronic migraine, little work has determined how currently utilized animal models of migraine best represent each distinct disease state.

AIM

In this review, we seek to characterize accepted preclinical models of migraine-like headache by their ability to recapitulate the clinical allodynic features of either episodic or chronic migraine.

METHODS

From a search of the Pu bMed database for "animal models of migraine", "headache models" and "preclinical migraine", we identified approximately 80 recent (within the past 20 years) publications that utilized one of 10 different models for migraine research. Models reviewed fit into one of the following categories: Dural KCl application, direct electrical stimulation, nitroglycerin administration, inflammatory soup injection, CGRP injection, medication overuse, monogenic animals, post-traumatic headache, specific channel activation, and hormone manipulation. Recapitulation of clinical features including cephalic and extracephalic hypersensitivity were evaluated for each and compared.

DISCUSSION

Episodic migraineurs comprise over half of the migraine population, yet the vast majority of current animal models of migraine appear to best represent chronic migraine states. While some of these models can be modified to reflect episodic migraine, there remains a need for non-invasive, validated models of episodic migraine to enhance the clinical translation of migraine research.

Post-traumatic headache due to mild traumatic brain injury: Current knowledge and future directions

BACKGROUND/OBJECTIVE

Post-traumatic headache is one of the most common and persistent symptoms following mild traumatic brain injury. The objective of this narrative review is to provide an update on the diagnostic criteria, clinical presentation, epidemiology, pathophysiology, and treatment of post-traumatic headache, and to identify future research priorities.

METHODS

This is a narrative review of the literature regarding post-traumatic headache attributed to mild traumatic brain injury.

RESULTS

Onset of post-traumatic headache within 7 days of injury is the only evidence for a causal relationship between the injury and the headache included in the diagnostic criteria. Post-traumatic headache often resolves within the first few days of onset, whereas it persists for at least 3 months in 30-50%. The majority of insights into post-traumatic headache pathophysiology come from pre-clinical animal studies and human imaging studies, which implicate structural, functional, metabolic, and neuroinflammatory mechanisms for post-traumatic headache. There is a paucity of quality evidence for how to best treat post-traumatic headache.

CONCLUSIONS

Although meaningful progress has been made in the post-traumatic headache field, priorities for future research are numerous, including the optimization of diagnostic criteria, a greater understanding of post-traumatic headache pathophysiology, identifying mechanisms and predictors for post-traumatic headache persistence, and identifying safe, well-tolerated, effective therapies.

Optogenetic Spreading Depression Elicits Trigeminal Pain and Anxiety Behavior

OBJECTIVE

Cortical spreading depression (SD) is an intense depolarization underlying migraine aura. Despite the weight of evidence linking SD to the pain phase of migraine, controversy remains over a causal role of SD in cephalgia because of the invasive nature of previous SD induction methods. To overcome this problem, we used a novel minimally invasive optogenetic SD induction method and examined the effect of SD on behavior.

METHODS

Optogenetic SD was induced as a single event or repeatedly every other day for 2 weeks. End points, including periorbital and hindpaw mechanical allodynia, mouse grimace, anxiety, and working memory, were examined in male and female mice.

RESULTS

A single SD produced bilateral periorbital mechanical allodynia that developed within 1 hour and resolved within 2 days. Sumatriptan prevented periorbital allodynia when administered immediately after SD. Repeated SDs also produced bilateral periorbital allodynia that lasted 4 days and resolved within 2 weeks after the last SD. In contrast, the hindpaw withdrawal thresholds did not change after repeated SDs suggesting that SD-induced allodynia was limited to the trigeminal region. Moreover, repeated SDs increased mouse grimace scores 2 days after the last SD, whereas a single SD did not. Repeated SDs also increased thigmotaxis scores as a measure of anxiety. In contrast, neither single nor repeated SDs affected visuospatial working memory. We did not detect sexual dimorphism in any end point.

INTERPRETATION

Altogether, these data show a clinically congruent causal relationship among SD, trigeminal pain, and anxiety behavior, possibly reflecting SD modulation of hypothalamic, thalamic, and limbic mechanisms. ANN NEUROL 2021;89:99-110.

Capsaicin-sensitive fibers mediate periorbital allodynia and activation of inflammatory cells after traumatic brain injury in rats: Involvement of TRPV1 channels in post-traumatic headache

Post-traumatic headache (PTH) is a condition that frequently affects individuals after traumatic brain injury (TBI). Inflammation is one of the major causes of this disability. However, little is known about the trigger for, and endurance of, this painful process. Thus, the involvement of fibers containing the transient receptor potential vanilloid 1 (TRPV1) channels on the PTH and inflammation after TBI through neonatal treatment with capsaicin are investigated. Fluid percussion injury (FPI) in adult male Wistar rats caused periorbital allodynia in one, three and seven days after injury, and the neonatal treatment reversed the painful sensation in seven days. The lack of TRPV1 channels reduced the activation of macrophages and glial cells induced by TBI in the trigeminal system, which were characterized by glial fibrillary acidic protein (GFAP) and ionized calcium binding adapter molecule-1 (IBA-1) immune content in the ipsilateral trigeminal ganglion, brainstem, and perilesional cortex. Immunofluorescence analyses of the ipsilateral Sp5C nucleus demonstrated a hypertrophic astrocytes profile after TBI which was reduced with treatment. Moreover, effects of succinate sumatriptan (SUMA - 1 mg/kg), TRPV1 selective antagonist capsazepine (CPZ - 2 mg/kg), and TRP non-selective antagonist ruthenium red (RR - 3 mg/kg) were evaluated. Although all mentioned drugs reduced the painful sensation, SUMA and CPZ demonstrated a stronger effect compared to the RR treatment, reinforcing the involvement of TRPV1 channels in periorbital allodynia after TBI. Hence, this report suggests that TRPV1-containing fibers and TRPV1 channels are able to induce inflammation of the trigeminal system and maintain the painful sensation after TBI.

Persistent post-traumatic headache: a migrainous loop or not? The preclinical evidence

Background

According to the International Classification of Headache Disorders 3, post-traumatic headache (PTH) attributed to traumatic brain injury (TBI) is a secondary headache reported to have developed within 7 days from head injury, regaining consciousness following the head injury, or discontinuation of medication(s) impairing the ability to sense or report headache following the head injury. It is one of the most common secondary headache disorders, and it is defined as persistent when it lasts more than 3 months.

Main body

Currently, due to the high prevalence of this disorder, several preclinical studies have been conducted using different animal models of mild TBI to reproduce conditions that engender PTH. Despite representing a simplification of a complex disorder and displaying different limitations concerning the human condition, animal models are still a mainstay to study in vivo the mechanisms of PTH and have provided valuable insight into the pathophysiology and possible treatment strategies. Different models reproduce different types of trauma and have been ideated in order to ensure maximal proximity to the human condition and optimal experimental reproducibility.

Conclusion

At present, despite its high prevalence, PTH is not entirely understood, and the differential contribution of pathophysiological mechanisms, also observed in other conditions like migraine, has to be clarified. Although facing limitations, animal models are needed to improve understanding of PTH. The knowledge of currently available models is necessary to all researchers who want to investigate PTH and contribute to unravel its mechanisms.

A Pre-Existing Myogenic Temporomandibular Disorder Increases Trigeminal Calcitonin Gene-Related Peptide and Enhances Nitroglycerin-Induced Hypersensitivity in Mice

Migraine is commonly reported among patients with temporomandibular disorders (TMDs), especially myogenic TMD. The pathophysiologic mechanisms related to the comorbidity of the two conditions remain elusive. In the present study, we combined masseter muscle tendon ligation (MMTL)-produced myogenic TMD with systemic injection of nitroglycerin (NTG)-induced migraine-like hypersensitivity in mice. Facial mechanical allodynia, functional allodynia, and light-aversive behavior were evaluated. Sumatriptan, an FDA-approved medication for migraine, was used to validate migraine-like hypersensitivity. Additionally, we examined the protein level of calcitonin gene-related peptide (CGRP) in the spinal trigeminal nucleus caudalis using immunohistochemistry. We observed that mice with MMTL pretreatment have a prolonged NTG-induced migraine-like hypersensitivity, and MMTL also enabled a non-sensitizing dose of NTG to trigger migraine-like hypersensitivity. Systemic injection of sumatriptan inhibited the MMTL-enhanced migraine-like hypersensitivity. MMTL pretreatment significantly upregulated the protein level of CGRP in the spinal trigeminal nucleus caudalis after NTG injection. Our results indicate that a pre-existing myogenic TMD can upregulate NTG-induced trigeminal CGRP and enhance migraine-like hypersensitivity.

Low-dose interleukin-2 reverses behavioral sensitization in multiple mouse models of headache disorders

Headache disorders are highly prevalent and debilitating, with limited treatment options. Previous studies indicate that many proinflammatory immune cells contribute to headache pathophysiology. Given the well-recognized role of regulatory T (Treg) cells in maintaining immune homeostasis, we hypothesized that enhancing Treg function may be effective to treat multiple headache disorders. In a mouse model of chronic migraine, we observed that repeated nitroglycerin (NTG, a reliable trigger of migraine in patients) administration doubled the number of CD3 T cells in the trigeminal ganglia without altering the number of Treg cells, suggesting a deficiency in Treg-mediated immune homeostasis. We treated mice with low-dose interleukin-2 (ld-IL2) to preferentially expand and activate endogenous Treg cells. This not only prevented the development of NTG-induced persistent sensitization but also completely reversed the established facial skin hypersensitivity resulting from repeated NTG administration. The effect of ld-IL2 was independent of mouse sex and/or strain. Importantly, ld-IL2 treatment did not alter basal nociceptive responses, and repeated usage did not induce tolerance. The therapeutic effect of ld-IL2 was abolished by Treg depletion and was recapitulated by Treg adoptive transfer. Furthermore, treating mice with ld-IL2 1 to 7 days after mild traumatic brain injury effectively prevented as well as reversed the development of behaviors related to acute and chronic post-traumatic headache. In a model of medication overuse headache, Ld-IL2 completely reversed the cutaneous hypersensitivity induced by repeated administration of sumatriptan. Collectively, this study identifies ld-IL2 as a promising prophylactic for multiple headache disorders with a mechanism distinct from the existing treatment options.

Posttraumatic headache: recent progress

PURPOSE OF REVIEW

Posttraumatic headache (PTH) attributed to mild traumatic brain injury is common and debilitating. In up to one-half of those with acute PTH, the PTH becomes persistent (PTH), enduring for longer than 3 months. The high incidence and persistence of PTH necessitate research into PTH pathophysiology and treatment. In this review, recent developments regarding the diagnostic criteria for PTH, the pathophysiology of PTH, and PTH treatment are discussed.

RECENT FINDINGS

International Classification of Headache Disorders 3 diagnostic criteria for PTH attributed to head trauma require that 'a headache of any type' starts within 7 days of a head injury. PTH is considered 'persistent' when it endures for more than 3 months. Preclinical and human PTH research suggest multiple pathophysiologic mechanisms including genetic influences, neuroinflammation, increased release and inadequate clearance of neuropeptides and neurotransmitters, mast cell degranulation, and brain structural and functional remodeling. Even when it has a phenotype similar to a primary headache, data suggest that PTH is distinct from primary headaches. There is a lack of high-quality evidence for the acute or preventive treatment of PTH. However, results from published studies of conventional headache therapies and newer therapies, such as calcitonin gene-related peptide mAbs and transcranial magnetic stimulation, justify the current and future randomized controlled trials.

SUMMARY

Evidence points towards a complex pathophysiology for PTH that is at least partially distinct from the primary headaches. Although properly conducted clinical trials of PTH treatment are needed, existing work has provided important data that help to plan these clinical trials. Current and future investigations will help to identify PTH mechanisms, predictors for PTH persistence, therapeutic targets, and evidence-based treatment options.

CGRP-dependent and independent mechanisms of acute and persistent post-traumatic headache following mild traumatic brain injury in mice

Background

Acute and persistent post-traumatic headache are often debilitating consequences of traumatic brain injury. Underlying physiological mechanisms of post-traumatic headache and its persistence remain unknown, and there are currently no approved therapies for these conditions. Post-traumatic headache often presents with a migraine-like phenotype. As calcitonin-gene related peptide promotes migraine headache, we explored the efficacy and timing of intervention with an anti- calcitonin-gene related peptide monoclonal antibody in novel preclinical models of acute post-traumatic headache and persistent post-traumatic headache following a mild traumatic brain injury event in mice.

Methods

Male, C57Bl/6 J mice received a sham procedure or mild traumatic brain injury resulting from a weight drop that allowed free head rotation while under minimal anesthesia. Periorbital and hindpaw tactile stimulation were used to assess mild traumatic brain injury-induced cutaneous allodynia. Two weeks after the injury, mice were challenged with stress, a common aggravator of migraine and post-traumatic headache, by exposure to bright lights (i.e. bright light stress) and cutaneous allodynia was measured hourly for 5 hours. A murine anti- calcitonin-gene related peptide monoclonal antibody was administered after mild traumatic brain injury at different time points to allow evaluation of the consequences of either early and sustained calcitonin-gene related peptide sequestration or late administration only prior to bright light stress.

Results

Mice with mild traumatic brain injury, but not a sham procedure, exhibited both periorbital and hindpaw cutaneous allodynia that resolved by post-injury day 13. Following resolution of injury-induced cutaneous allodynia, exposure to bright light stress re-instated periorbital and hindpaw cutaneous allodynia in injured, but not sham mice. Repeated administration of anti-calcitonin-gene related peptide monoclonal antibody at 2 hours, 7 and 14 days post mild traumatic brain injury significantly attenuated the expression of cutaneous allodynia when evaluated over the 14-day post injury time course and also prevented bright light stress-induced cutaneous allodynia in injured mice. Administration of anti-calcitonin-gene related peptide monoclonal antibody only at 2 hours and 7 days after mild traumatic brain injury blocked injury-induced cutaneous allodynia and partially prevented bright light stress-induced cutaneous allodynia. A single administration of anti-calcitonin-gene related peptide monoclonal antibody after the resolution of the peak injury-induced cutaneous allodynia, but prior to bright light stress challenge, did not prevent bright light stress-induced cutaneous allodynia.

Conclusions

We used a clinically relevant mild traumatic brain injury event in mice along with a provocative stimulus as novel models of acute post-traumatic headache and persistent post-traumatic headache. Following mild traumatic brain injury, mice demonstrated transient periorbital and hindpaw cutaneous allodynia suggestive of post-traumatic headache-related pain and establishment of central sensitization. Following resolution of injury-induced cutaneous allodynia, exposure to bright light stress re-established cutaneous allodynia, suggestive of persistent post-traumatic headache-related pain. Continuous early sequestration of calcitonin-gene related peptide prevented both acute post-traumatic headache and persistent post-traumatic headache. In contrast, delayed anti-calcitonin-gene related peptide monoclonal antibody treatment following establishment of central sensitization was ineffective in preventing persistent post-traumatic headache. These observations suggest that mechanisms involving calcitonin-gene related peptide underlie the expression of acute post-traumatic headache, and drive the development of central sensitization, increasing vulnerability to headache triggers and promoting persistent post-traumatic headache. Early and continuous calcitonin-gene related peptide blockade following mild traumatic brain injury may represent a viable treatment option for post-traumatic headache and for the prevention of post-traumatic headache persistence.

Abbreviations

CA Cutaneous allodynia CGRP Calcitonin gene-related peptide mTBI Mild traumatic brain injury PTH Post-traumatic headache APTH Acute post-traumatic headache PPTH Persistent post-traumatic headache

Pioglitazone treatment prior to transplantation improves the efficacy of human mesenchymal stem cells after traumatic brain injury in rats

Traumatic brain injury is a leading cause of death and disability around the world. So far, drugs are not available to repair brain damage. Human mesenchymal stem cell (hMSC) transplantation therapy is a promising approach, although the inflammatory microenvironment of the injured brain affects the efficacy of transplanted hMSCs. We hypothesize that reducing the inflammation in the cerebral microenvironment by reducing pro-inflammatory chemokines prior to hMSC administration will improve the efficacy of hMSC therapy. In a rat model of lateral fluid percussion injury, combined pioglitazone (PG) and hMSC (combination) treatment showed less anxiety-like behavior and improved sensorimotor responses to a noxious cold stimulus. Significant reduction in brain lesion volume, neurodegeneration, microgliosis and astrogliosis were observed after combination treatment. TBI induced expression of inflammatory chemokine CCL20 and IL1-β were significantly decreased in the combination treatment group. Combination treatment significantly increased brain-derived neurotrophic factor (BDNF) level and subventricular zone (SVZ) neurogenesis. Taken together, reducing proinflammatory cytokine expression in the cerebral tissues after TBI by PG administration and prior to hMSC therapy improves the outcome of the therapy in which BDNF could have a role.

TRESK K+ Channel Activity Regulates Trigeminal Nociception and Headache

Although TWIK-related spinal cord K⁺ (TRESK) channel is expressed in all primary afferent neurons in trigeminal ganglia (TG) and dorsal root ganglia (DRG), whether TRESK activity regulates trigeminal pain processing is still not established. Dominant-negative TRESK mutations are associated with migraine but not with other types of pain in humans, suggesting that genetic TRESK dysfunction preferentially affects the generation of trigeminal pain, especially headache. Using TRESK global knock-out mice as a model system, we found that loss of TRESK in all TG neurons selectively increased the intrinsic excitability of small-diameter nociceptors, especially those that do not bind to isolectin B4 (IB4^-). Similarly, loss of TRESK resulted in hyper-excitation of the small IB4^- dural afferent neurons but not those that bind to IB4 (IB4⁺). Compared with wild-type littermates, both male and female TRESK knock-out mice exhibited more robust trigeminal nociceptive behaviors, including headache-related behaviors, whereas their body and visceral pain responses were normal. Interestingly, neither the total persistent outward current nor the intrinsic excitability was altered in adult TRESK knock-out DRG neurons, which may explain why genetic TRESK dysfunction is not associated with body and/or visceral pain in humans. We reveal for the first time that, among all primary afferent neurons, TG nociceptors are the most vulnerable to the genetic loss of TRESK. Our findings indicate that endogenous TRESK activity regulates trigeminal nociception, likely through controlling the intrinsic excitability of TG nociceptors. Importantly, we provide evidence that genetic loss of TRESK significantly increases the likelihood of developing headache.

TRPA1/NOX in the soma of trigeminal ganglion neurons mediates migraine-related pain of glyceryl trinitrate in mice

Glyceryl trinitrate is administered as a provocative test for migraine pain. Glyceryl trinitrate causes prolonged mechanical allodynia in rodents, which temporally correlates with delayed glyceryl trinitrate-evoked migraine attacks in patients. However, the underlying mechanism of the allodynia evoked by glyceryl trinitrate is unknown. The proalgesic transient receptor potential ankyrin 1 (TRPA1) channel, expressed by trigeminal nociceptors, is sensitive to oxidative stress and is targeted by nitric oxide or its by-products. Herein, we explored the role of TRPA1 in glyceryl trinitrate-evoked allodynia. Systemic administration of glyceryl trinitrate elicited in the mouse periorbital area an early and transient vasodilatation and a delayed and prolonged mechanical allodynia. The systemic, intrathecal or local administration of selective enzyme inhibitors revealed that nitric oxide, liberated from the parent drug by aldehyde dehydrogenase 2 (ALDH2), initiates but does not maintain allodynia. The central and the final phases of allodynia were respectively associated with generation of reactive oxygen and carbonyl species within the trigeminal ganglion. Allodynia was absent in TRPA1-deficient mice and was reversed by TRPA1 antagonists. Knockdown of neuronal TRPA1 by intrathecally administered antisense oligonucleotide and selective deletion of TRPA1 from sensory neurons in Advillin-Cre; Trpa1^fl/fl mice revealed that nitric oxide-dependent oxidative and carbonylic stress generation is due to TRPA1 stimulation, and resultant NADPH oxidase 1 (NOX1) and NOX2 activation in the soma of trigeminal ganglion neurons. Early periorbital vasodilatation evoked by glyceryl trinitrate was attenuated by ALDH2 inhibition but was unaffected by TRPA1 blockade. Antagonists of the calcitonin gene-related peptide receptor did not affect the vasodilatation but partially inhibited allodynia. Thus, although both periorbital allodynia and vasodilatation evoked by glyceryl trinitrate are initiated by nitric oxide, they are temporally and mechanistically distinct. While vasodilatation is due to a direct nitric oxide action in the vascular smooth muscle, allodynia is a neuronal phenomenon mediated by TRPA1 activation and ensuing oxidative stress. The autocrine pathway, sustained by TRPA1 and NOX1/2 within neuronal cell bodies of trigeminal ganglia, may sensitize meningeal nociceptors and second order trigeminal neurons to elicit periorbital allodynia, and could be of relevance for migraine-like headaches evoked by glyceryl trinitrate in humans.

Acute Post-Traumatic Sleep May Define Vulnerability to a Second Traumatic Brain Injury in Mice

Chronic neurological impairments can manifest from repetitive traumatic brain injury (rTBI), particularly when subsequent injuries occur before the initial injury completely heals. Herein, we apply post-traumatic sleep as a physiological biomarker of vulnerability, hypothesizing that a second TBI during post-traumatic sleep worsens neurological and histological outcomes compared to one TBI or a second TBI after post-traumatic sleep subsides. Mice received sham or diffuse TBI by midline fluid percussion injury; brain-injured mice received one TBI or rTBIs at 3- or 9-h intervals. Over 40 h post-injury, injured mice slept more than shams. Functional assessments indicated lower latencies on rotarod and increased Neurological Severity Scores for mice with rTBIs within 3 h. Anxiety-like behaviors in the open field task were increased for mice with rTBIs at 3 h. Based on pixel density of silver accumulation, neuropathology was greater at 28 days post-injury (DPI) in rTBI groups than sham and single TBI. Cortical microglia morphology was quantified and mice receiving rTBI were de-ramified at 14 DPI compared to shams and mice receiving a single TBI, suggesting robust microglial response in rTBI groups. Orexin-A-positive cells were sustained in the lateral hypothalamus with no loss detected, indicating that loss of wake-promoting neurons did not contribute to post-traumatic sleep. Thus, duration of post-traumatic sleep is a period of vulnerability that results in exacerbated injury from rTBI. Monitoring individual post-traumatic sleep is a potential clinical tool for personalized TBI management, where regular sleep patterns may inform rehabilitative strategies and return-to-activity guidelines.

Migraine-provoking substances evoke periorbital allodynia in mice

Background

Administration of endogenous mediators or exogenous chemicals in migraine patients provoke early headaches and delayed migraine-like attacks. Although migraine provoking substances are normally vasodilators, dilation of arterial vessels does not seem to be the sole contributing factor, and the underlying mechanisms of the delayed migraine pain are mostly unknown. Sustained mechanical allodynia is a common response associated with the local administration of various proalgesic substances in experimental animals and humans. Here, we investigated the ability of a series of endogenous mediators which provoke or do not provoke migraine in patients, to cause or not cause mechanical allodynia upon their injection in the mouse periorbital area.

Methods

Mechanical allodynia was assessed with the von Frey filament assay. Stimuli were given by subcutaneous injection in the periorbital area of C57BL/6J mice; antagonists were administered by local and systemic injections.

Results

Calcitonin gene related peptide (CGRP), but not adrenomedullin and amylin, pituitary adenylyl cyclase activating peptide (PACAP), but not vasoactive intestinal polypeptide (VIP), histamine, prostaglandin E₂ (PGE₂) and prostacyclin (PGI₂), but not PGF_2α, evoked a dose-dependent periorbital mechanical allodynia. The painful responses were attenuated by systemic or local (periorbital) administration of antagonists for CGRP (CLR/RAMP1), PACAP (PAC-1), histamine H₁, PGE₂ (EP₄), and PGI₂ (IP) receptors, respectively.

Conclusions

The correspondence between substances that provoke (CGRP; PACAP, histamine, PGE₂, PGI₂), or do not provoke (VIP and PGF_2α), migraine-like attacks in patients and periorbital allodynia in mice suggests that the study of allodynia in mice may provide information on the proalgesic mechanisms of migraine-provoking agents in humans. Results underline the ability of migraine-provoking substances to initiate mechanical allodynia by acting on peripheral terminals of trigeminal afferents.

Bioactive Lipids in Cancer, Inflammation and Related Diseases : Acute and Chronic Mild Traumatic Brain Injury Differentially Changes Levels of Bioactive Lipids in the CNS Associated with Headache

Headache is a common complaint after mild traumatic brain injury (mTBI). Changes in the CNS lipidome were previously associated with acrolein-induced headache in rodents. mTBI caused similar headache-like symptoms in rats; therefore, we tested the hypothesis that mTBI might likewise alter the lipidome. Using a stereotaxic impactor, rats were given either a single mTBI or a series of 4 mTBIs 48 h apart. 72 h later for single mTBI and 7 days later for repeated mTBI, the trigeminal ganglia (TG), trigeminal nucleus (TNC), and cerebellum (CER) were isolated. Using HPLC/MS/MS, ~80 lipids were measured in each tissue and compared to sham controls. mTBI drove widespread alterations in lipid levels. Single mTBI increased arachidonic acid and repeated mTBI increased prostaglandins in all 3 tissue types. mTBI affected multiple TRPV agonists, including N-arachidonoyl ethanolamine (AEA), which increased in the TNC and CER after single mTBI. After repeated mTBI, AEA increased in the TG, but decreased in the TNC. Common to all tissue types in single and repeated mTBI was an increase the AEA metabolite, N-arachidonoyl glycine, a potent activator of microglial migration. Changes in the CNS lipidome associated with mTBI likely play a role in headache and in long-term neurodegenerative effects of repeated mTBI.

The development of a mouse model of mTBI-induced post-traumatic migraine, and identification of the delta opioid receptor as a novel therapeutic target

BACKGROUND

Post-traumatic headache is the most common and long-lasting impairment observed following mild traumatic brain injury, and frequently has migraine-like characteristics. The mechanisms underlying progression from mild traumatic brain injury to post-traumatic headache are not fully understood. The aim of this study was to develop a mouse model of post-traumatic headache and identify mechanisms and novel targets associated with this disorder.

METHODS

We combined the closed head weight-drop method and the nitroglycerin chronic migraine model. To induce mild traumatic brain injury, a weight was dropped onto intact crania of mildly anesthetized mice, and mechanical responses to chronic-intermittent administration of nitroglycerin, a human migraine trigger, were determined at multiple time points post-injury.

RESULTS

Low dose nitroglycerin (0.1 mg/kg) evoked acute periorbital and hind paw allodynia in both mild traumatic brain injury and sham animals. However, only mild traumatic brain injury mice developed chronic hypersensitivity to low dose nitroglycerin. Migraine medications, sumatriptan and topiramate, inhibited post-traumatic headache-associated allodynia. In addition, the delta opioid receptor agonist, SNC80, also blocked post-traumatic headache-associated allodynia. Finally, we examined the expression of calcitonin gene-related peptide within this model and found that it was increased in trigeminal ganglia two weeks post-mild traumatic brain injury.

CONCLUSIONS

Overall, we have established a mouse model of post-traumatic headache and identified the delta opioid receptor as a novel therapeutic target for this disorder.

Narrative Review of the Pathophysiology of Headaches and Photosensitivity in Mild Traumatic Brain Injury and Concussion

The most common symptom of post-concussive syndrome (PCS) is post-traumatic headache (PTH) accompanied by photophobia. Post-traumatic headache is currently categorized as a secondary headache disorder with a clinical phenotype described by its main features and resembling one of the primary headache disorders: tension, migraine, migraine-like cluster. Although PTH is often treated with medication used for primary headache disorders, the underlying mechanism for PTH has yet to be elucidated. The goal of this narrative literature review is to determine the current level of knowledge of these PTHs and photophobia in mild traumatic brain injury (mTBI) in order to guide further research and attempt to discover the underlying mechanism to both symptoms. The ultimate purpose is to better understand the pathophysiology of these symptoms in order to provide better and more targeted care to afflicted patients. A review of the literature was conducted using the databases CINAHL, EMBASE, PubMed. All papers were screened for sections on pathophysiology of PTH or photophobia in mTBI patients. Our paper summarizes current hypotheses. Although the exact pathophysiology of PTH and photophobia in mTBI remains to be determined, we highlight several interesting findings and avenues for future research, including central and peripheral explanations for PTH, neuroinflammation, cortical spreading depolarization and the role of glutamate excitotoxicity. We discuss the possible neuroanatomical pathways for photophobia and hypothesize a possible common pathophysiological basis between PTH and photophobia.

Chronic Upregulation of Cleaved-Caspase-3 Associated with Chronic Myelin Pathology and Microvascular Reorganization in the Thalamus after Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) is associated with long-term disabilities and devastating chronic neurological complications including problems with cognition, motor function, sensory processing, as well as behavioral deficits and mental health problems such as anxiety, depression, personality change and social unsuitability. Clinical data suggest that disruption of the thalamo-cortical system including anatomical and metabolic changes in the thalamus following TBI might be responsible for some chronic neurological deficits following brain trauma. Detailed mechanisms of these pathological processes are not completely understood. The goal of this study was to evaluate changes in the thalamus following TBI focusing on cleaved-caspase-3, a specific effector of caspase pathway activation and myelin and microvascular pathologies using immuno- and histochemistry at different time points from 24 h to 3 months after controlled cortical impact (CCI) in adult Sprague-Dawley rats. Significant increases in cleaved-caspase-3 immunoreactivity in the thalamus were observed starting one month and persisting for at least three months following experimental TBI. Further, the study demonstrated an association of cleaved-caspase-3 with the demyelination of neuronal processes and tissue degeneration in the gray matter in the thalamus, as reflected in alterations of myelinated fiber integrity (luxol fast blue) and decreases in myelin basic protein (MBP) immunoreactivity. The immunofluorescent counterstaining of cleaved-caspase-3 with endothelial barrier antigen (EBA), a marker of blood-brain barrier, revealed limited direct and indirect associations of cleaved caspase-3 with blood-brain barrier damage. These results demonstrate for the first time a significant chronic upregulation of cleaved-caspase-3 in selected thalamic regions associated with cortical regions directly affected by CCI injury. Further, our study is also the first to report that significant upregulation of cleaved-caspase-3 in selected ipsilateral thalamic regions is associated with microvascular reorganization reflected in the significant increases in the number of microvessels with blood-brain barrier alterations detected by EBA staining. These findings provide new insights into potential mechanisms of TBI cell death involving chronic activation of caspase-3 associated with disrupted cortico-thalamic and thalamo-cortical connectivity. Moreover, this study offers the initial evidence that this upregulation of activated caspase-3, delayed degeneration of myelinated nerve fibers and microvascular reorganization with impaired blood-brain barrier integrity in the thalamus might represent reciprocal pathological processes affecting neuronal networks and brain function at the chronic stages of TBI.

Long-term deficits in risky decision-making after traumatic brain injury on a rat analog of the Iowa gambling task

Traumatic brain injury (TBI) affects 2.8 million people annually in the United States, with significant populations suffering from ongoing cognitive dysfunction. Impairments in decision-making can have major implications for patients and their caregivers, often enduring for years to decades, yet are rarely explored in experimental TBI. In the current study, the Rodent Gambling Task (RGT), an Iowa Gambling Task analog, was used to assess risk-based decision-making and motor impulsivity after TBI. During testing, rats chose between options associated with different probabilities of reinforcement (sucrose) or punishment (timeout). To determine effects of TBI on learned behaviors versus the learning process, rats were trained either before, or after, a bilateral frontal controlled cortical impact TBI, and then assessed for 12 weeks. To evaluate the degree to which monoamine systems, such as dopamine, were affected by TBI, rats were given an amphetamine challenge, and behavior recorded. Injury immediately and chronically decreased optimal decision-making, and biased rats towards both riskier, and safer (but suboptimal) choices, regardless of prior learning history. TBI also increased motor impulsivity across time, reflecting ongoing neural changes. Despite these similarities in trained and acquisition rats, those that learned the task after injury demonstrated reduced effects of amphetamine on optimal decision-making, suggesting a lesser role of monoamines in post-injury learning. Amphetamine also dose-dependently reduced motor impulsivity in injured rats. This study opens up the investigation of psychiatric-like dysfunction in animal models of TBI and tasks such as the RGT will be useful in identifying therapeutics for the chronic post-injury period.