Traumatic Brain Injury Disrupts Pain Signaling in the Brainstem and Spinal Cord

Linking severe traumatic brain injury to pulmonary Infections: Translocation of intestinal bacteria mediated by nociceptor neurons

The prevalence of bacterial infections significantly increases among patients with severe traumatic brain injury (STBI), leading to a notable rise in mortality rates. While immune dysfunctions are linked to the incidence of pneumonia, our observations indicate that endogenous pathogens manifest in the lungs post-STBI due to the migration of gut commensal bacteria. This translocation involves gut-innervating nociceptor sensory neurons, which are crucial for host defense. Following STBI, the expression of transient receptor potential vanilloid 1 (TRPV1) in dorsal root ganglion (DRG) neurons significantly decreases, despite an initial brief increase. The timing of TRPV1 defects coincides with the occurrence of pulmonary infections post-STBI. This alteration in TRPV1+ neurons diminishes their ability to signal bacterial injuries, weakens defense mechanisms against intestinal bacteria, and increases susceptibility to pulmonary infections via bacterial translocation. Experimental evidence demonstrates that pulmonary infections can be successfully replicated through the chemical ablation and gene interference of TRPV1+ nociceptors, and that these infections can be mitigated by TRPV1 activation, thereby confirming the crucial role of nociceptor neurons in controlling intestinal bacterial migration. Furthermore, TRPV1+ nociceptors regulate the immune response of microfold cells by releasing calcitonin gene-related peptide (CGRP), thereby influencing the translocation of gut bacteria to the lungs. Our study elucidates how changes in nociceptive neurons post-STBI impact intestinal pathogen defense. This new understanding of endogenous risk factors within STBI pathology offers novel insights for preventing and treating pulmonary infections.

Animal models of neuropathic pain

Animal models continue to be crucial to developing our understanding of the molecular, cellular, and neurophysiological mechanisms that lead to neuropathic pain. The overwhelming majority of animal studies use rodent models, ranging from surgical and trauma-induced models to those induced by metabolic diseases, genetic mutations, viruses, neurotoxic drugs, and cancer. We discuss the clinical relevance of the available models and the pain behavior tests commonly used as outcome measures. Finally, we summarize the refinements that have been proposed to improve the ability of animal model studies to predict clinical efficacy.

The impact of neuroinflammation on neuronal integrity

Neuroinflammation, characterized by a complex interplay among innate and adaptive immune responses within the central nervous system (CNS), is crucial in responding to infections, injuries, and disease pathologies. However, the dysregulation of the neuroinflammatory response could significantly affect neurons in terms of function and structure, leading to profound health implications. Although tremendous progress has been made in understanding the relationship between neuroinflammatory processes and alterations in neuronal integrity, the specific implications concerning both structure and function have not been extensively covered, with the exception of perspectives on glial activation and neurodegeneration. Thus, this review aims to provide a comprehensive overview of the multifaceted interactions among neurons and key inflammatory players, exploring mechanisms through which inflammation influences neuronal functionality and structural integrity in the CNS. Further, it will discuss how these inflammatory mechanisms lead to impairment in neuronal functions and architecture and highlight the consequences caused by dysregulated neuronal functions, such as cognitive dysfunction and mood disorders. By integrating insights from recent research findings, this review will enhance our understanding of the neuroinflammatory landscape and set the stage for future interventions that could transform current approaches to preserve neuronal integrity and function in CNS-related inflammatory conditions.

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Mechanisms and treatments of chronic pain after traumatic brain injury

While pain after trauma generally resolves, some trauma patients experience pain for months to years after injury. An example, relevant to both combat and civilian settings, is chronic pain after traumatic brain injury (TBI). Headache as well as pain in the back and extremities are common locations for TBI-related chronic pain to be experienced. TBI-related pain can exist alone or can exacerbate pain from other injuries long after healing has occurred. Consequences of chronic pain in these settings include increased suffering, higher levels of disability, serious emotional problems, and worsened cognitive deficits. The current review will examine recent evidence regarding dysfunction of endogenous pain modulatory mechanisms, neuroplastic changes in the trigeminal circuitry and alterations in spinal nociceptive processing as contributors to TBI-related chronic pain. Key pain modulatory centers including the locus coeruleus, periaqueductal grey matter, and rostroventromedial medulla are vulnerable to TBI. Both the rationales and existing evidence for the use of monoamine reuptake inhibitors, CGRP antagonists, CXCR2 chemokine receptor antagonists, and interventional therapies will be presented. While consensus guidelines for the management of chronic post-traumatic TBI-related pain are lacking, several approaches to this clinically challenging situation deserve focused evaluation and may prove to be viable therapeutic options.

Anacardic acid improves neurological deficits in traumatic brain injury by anti-ferroptosis and anti-inflammation

BACKGROUND

Traumatic brain injury (TBI) is an important cause of disability and death. TBI leads to multiple forms of nerve cell death including ferroptosis due to iron-dependent lipid peroxidation. Anacardic acid (AA) is a natural component extracted from cashew nut shells, which has been reported to have neuroprotective effects in traumatic brain injury. We investigated whether AA has an anti-ferroptosis effect in TBI.

METHODS

We used the Feeney free-fall impact method to construct a TBI model to investigate the effect of AA on ferroptosis caused by TBI, in which Ferrostatin-1 (Fer-1), a ferroptosis inhibitor, served as a positive control group. We first identified the therapeutic effect of AA on TBI through modified neurological severity score (mNSS) and determined the appropriate concentration. Secondly, we investigated the effect of AA on the expression level of the key protein of ferroptosis by Western blotting and immunohistochemistry. Then the effect of AA on nerve tissue injury and nerve function improvement was verified. Finally, enzym-linked immunosorbent assay (ELISA) was used to verify that AA could reduce inflammation after TBI.

RESULTS

We found the intensely inhibitory effect of AA on ferroptosis, which is in parallel with the results obtained after Fer-1 treatment. In addition, AA and Fer-1 mitigated TBI-mediated tissue defects, destruction of the blood-brain barrier, and neurodegeneration. Novel object recognition (NOR), mNSS and water maze test showed that AA could significantly reduce the impairment of neural function and behavioral cognitive ability caused by TBI. Finally, we also demonstrated that AA has not only an anti-ferroptosis effect, but also an anti-inflammation effect.

CONCLUSIONS

AA can reduce the neurological impairment and behavioral cognitive impairment caused by TBI through the dual effect of anti-ferroptosis and anti-inflammation.

Effect of Voluntary Exercise on Endogenous Pain Control Systems and Post-traumatic Headache in Mice

Traumatic brain injury (TBI) can cause acute and chronic pain along with motor, cognitive, and emotional problems. Although the mechanisms are poorly understood, previous studies suggest disruptions in endogenous pain modulation may be involved. Voluntary exercise after a TBI has been shown to reduce some consequences of injury including cognitive impairment. We hypothesized, therefore, that voluntary exercise could augment endogenous pain control systems in a rodent model of TBI. For these studies, we used a closed-head impact procedure in male mice modeling mild TBI. We investigated the effect of voluntary exercise on TBI-induced hindpaw nociceptive sensitization, diffuse noxious inhibitory control failure, and periorbital sensitization after bright light stress, a model of post-traumatic headache. Furthermore, we investigated the effects of exercise on memory, circulating markers of brain injury, neuroinflammation, and spinal cord gene expression. We observed that exercise significantly reduced TBI-induced hindpaw allodynia and periorbital allodynia in the first week following TBI. We also showed that exercise improved the deficits associated with diffuse noxious inhibitory control and reduced bright light stress-induced allodynia up to 2 months after TBI. In addition, exercise preserved memory and reduced TBI-induced increases in spinal BDNF, CXCL1, CXCL2, and prodynorphin expression, all genes previously linked to TBI-induced nociceptive sensitization. Taken together, our observations suggest that voluntary exercise may reduce pain after TBI by reducing TBI-induced changes in nociceptive signaling and preserving endogenous pain control systems. PERSPECTIVE: This article evaluates the effects of exercise on pain-related behaviors in a preclinical model of traumatic brain injury (TBI). The findings show that exercise reduces nociceptive sensitization, loss of diffuse noxious inhibitory control, memory deficits, and spinal nociception-related gene expression after TBI. Exercise may reduce or prevent pain after TBI.

Persistent Changes in Mechanical Nociception in Rats With Traumatic Brain Injury Involving Polytrauma

Traumatic brain injury (TBI) survivors often experience debilitating consequences. Due to the high impact nature of TBI, patients often experience concomitant peripheral injuries (ie, polytrauma). A common, yet often overlooked, comorbidity of TBI is chronic pain. Therefore, this study investigated how common concomitant peripheral injuries (ie, femoral fracture and muscle crush) can affect long-term behavioral and structural TBI outcomes with a particular focus on nociception. Rats were randomly assigned to 1 of 4 groups: polytrauma (POLY; ie, fracture + muscle crush + TBI), peripheral injury (PERI; ie, fracture + muscle crush + sham TBI), TBI (ie, sham fracture + sham muscle crush + TBI), and sham-injured (SHAM; ie, sham fracture + sham muscle crush + sham TBI). Rats underwent behavioral testing at 3-, 6-, and 11-weeks postinjury, and were then euthanized for postmortem magnetic resonance imaging (MRI). POLY rats had a persisting increase in pain sensitivity compared to all groups on the von Frey test. MRI revealed that POLY rats also had abnormalities in the cortical and subcortical brain structures involved in nociceptive processing. These findings have important implications and provide a foundation for future studies to determine the underlying mechanisms and potential treatment strategies for chronic pain in TBI survivors. PERSPECTIVE: Rats with TBI and concomitant peripheral trauma displayed chronic nociceptive pain and MRI images also revealed damaged brain structures/pathways that are involved in chronic pain development. This study highlights the importance of polytrauma and the affected brain regions for developing chronic pain.

Repeat mild traumatic brain injuries (RmTBI) modify nociception and disrupt orexinergic connectivity within the descending pain pathway

Repeat mild traumatic brain injuries (RmTBI) result in substantial burden to the public health system given their association with chronic post-injury pathologies, such as chronic pain and post-traumatic headache. Although this may relate to dysfunctional descending pain modulation (DPM), it is uncertain what mechanisms drive changes within this pathway. One possibility is altered orexinergic system functioning, as orexin is a potent anti-nociceptive neuromodulator. Orexin is exclusively produced by the lateral hypothalamus (LH) and receives excitatory innervation from the lateral parabrachial nucleus (lPBN). Therefore, we used neuronal tract-tracing to investigate the relationship between RmTBI and connectivity between lPBN and the LH, as well as orexinergic projections to a key site within the DPM, the periaqueductal gray (PAG). Prior to injury induction, retrograde and anterograde tract-tracing surgery was performed on 70 young-adult male Sprague Dawley rats, targeting the lPBN and PAG. Rodents were then randomly assigned to receive RmTBIs or sham injuries before undergoing testing for anxiety-like behaviour and nociceptive sensitivity. Immunohistochemical analysis identified distinct and co-localized orexin and tract-tracing cell bodies and projections within the LH. The RmTBI group exhibited altered nociception and reduced anxiety as well as a loss of orexin cell bodies and a reduction of hypothalamic projections to the ventrolateral nucleus of the PAG. However, there was no significant effect of injury on neuronal connectivity between the lPBN and orexinergic cell bodies within the LH. Our identification of structural losses and the resulting physiological changes in the orexinergic system following RmTBI begins to clarify acute post-injury mechanistic changes that drive may drive the development of post-traumatic headache and the chronification of pain.

Intrathecal dexmedetomidine attenuates mechanical allodynia through the downregulation of brain-derived neurotrophic factor in a mild traumatic brain injury rat model

BACKGROUND

This study evaluated the effects of dexmedetomidine and propofol on brain-derived neurotrophic factor level in the cerebrospinal fluid (c-BDNF) and mechanical allodynia in a mild traumatic brain injury (TBI) rat model.

METHODS

After fixing the rat's skull on a stereotactic frame under general anesthesia, craniotomy was performed. After impact, 10 µl of drug was injected into the cisterna magna (group S: sham, group D: dexmedetomidine 5 μg/kg, group P: propofol 500 μg/kg, and group T: untreated TBI). The 50% mechanical withdrawal threshold (50% MWT) and c-BDNF level were measured on postoperative days (PODs) 1, 7, and 14.

RESULTS

The 50% MWT measured on PODs 1, 7, and 14 was lower and the c-BDNF level on POD 1 was higher in group T than in group S. In group D, the c-BDNF level on POD 1 was lower than that in group T and was comparable with that in group S during the whole study period. The 50% MWT of group D was higher than that of group T throughout the postoperative period. In group P, there were no significant differences in the 50% MWT during the entire postoperative period compared with group T; the c-BDNF level was higher than that in group T on POD 1.

CONCLUSIONS

Intrathecal administration of dexmedetomidine may attenuate TBI-induced mechanical allodynia for up to two weeks post-injury through immediate suppression of c-BDNF in mild TBI rats. The inhibition of c-BDNF expression in the acute phase reduced the occurrence of TBI-induced chronic neuropathic pain.

Multidimensional pain phenotypes after Traumatic Brain Injury

More than 50% of individuals develop chronic pain following traumatic brain injury (TBI). Research suggests that a significant portion of post-TBI chronic pain conditions is neuropathic in nature, yet the relationship between neuropathic pain, psychological distress, and somatosensory function following TBI is not fully understood. This study evaluated neuropathic pain symptoms, psychological and somatosensory function, and psychosocial factors in individuals with TBI (TBI, N = 38). A two-step cluster analysis was used to identify phenotypes based on the Neuropathic Pain Symptom Inventory and Beck's Anxiety Inventory scores. Phenotypes were then compared on pain characteristics, psychological and somatosensory function, and psychosocial factors. Our analyses resulted in two different neuropathic pain phenotypes: (1) Moderate neuropathic pain severity and anxiety scores (MNP-AS, N = 11); and (2) mild or no neuropathic pain symptoms and anxiety scores (LNP-AS, N = 27). Furthermore, the MNP-AS group exhibited greater depression, PTSD, pain severity, and affective distress scores than the LNP-AS group. In addition, thermal somatosensory function (difference between thermal pain and perception thresholds) was significantly lower in the MNP-AS compared to the LNP-AS group. Our findings suggest that neuropathic pain symptoms are relatively common after TBI and are not only associated with greater psychosocial distress but also with abnormal function of central pain processing pathways.

Activation of the Locus Coeruleus Mediated by Designer Receptor Exclusively Activated by Designer Drug Restores Descending Nociceptive Inhibition after Traumatic Brain Injury in Rats

Disruption of endogenous pain control mechanisms including descending pain inhibition has been linked to several forms of pain including chronic pain after traumatic brain injury (TBI). The locus coeruleus (LC) is the principal noradrenergic (NA) nucleus participating in descending pain inhibition. We therefore hypothesized that selectively stimulating LC neurons would reduce nociception after TBI. All experiments used a well-characterized rat lateral fluid percussion model of TBI. NA neurons were stimulated by administering clozapine N-oxide (CNO) to rats selectively expressing a designer receptor exclusively activated by designer drug (DREADD) viral construct in their LC's. Mechanical nociceptive thresholds were measured using von Frey fibers. The efficacy of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, was assessed using the hindpaw administration of capsaicin. Immunohistochemical analyses demonstrated the selective expression of the DREADD construct in LC neurons after stereotactic injection. During the 1st week after TBI, when rats demonstrated hindlimb (HL) nociceptive sensitization, CNO administration provided transient anti-allodynia in DREADD-expressing rats but not in rats injected with control virus. Seven weeks after TBI we observed a complete loss of DNIC in response to capsaicin. However, CNO administration largely restored DNIC in TBI DREADD-expressing rats but not those injected with control virus. Unexpectedly, the effects of LC activation in the DREADD-expressing rats were blocked by the α-1 adrenergic receptor antagonist prazosin, but not the α-2 adrenergic receptor antagonist atipamezole. These results suggest that directly stimulating the LC after TBI can reduce both early and late manifestations of dysfunctional endogenous pain regulation. Clinical approaches to activating descending pain circuits may reduce suffering in those with pain after TBI.

Preoperative dynamic quantitative sensory testing in remote pain-free areas is associated with axial pain after posterior cervical spinal surgeries

BACKGROUND

Postoperative axial pain (PAP), characterized by pain and/or stiffness around the posterior neck, periscapular areas and/or shoulder region, is a vexing complication affecting 5-60% of patients undergoing posterior cervical decompression. Given its relatively high frequency and negative impact on patients' physical and mental status, efforts preoperatively to confirm patients at risk of developing PAP to offer more efficient pain management to minimize this complication have a high priority. The aim of this study is to investigate the role of preoperative dynamic quantitative sensory testing (QST) in predicting the PAP after posterior cervical decompression.

METHODS

This longitudinal observational study included 122 patients with degenerative cervical myelopathy undergoing laminoplasty or laminectomy. Preoperatively, all patients underwent the assessment of pressure pain thresholds (PPTs) at local and remote pain-free areas and both temporal summation (TS) and conditioned pain modulation (CPM) at remote pain free-areas. These patients underwent further pain-related, psychosocial and clinical function assessments before and/or after operation.

RESULTS

In the present study, 21 patients (21/122, 17.2%) developed PAP, and the 6-month postoperative follow-up demonstrated that 8 of these 21 patients developed chronic PAP (CPAP). All preoperative covariates with significant differences between the PAP and non-PAP groups were subjected to multivariate logistic regression, and the presence of preoperative axial pain, surgical plan including C2 decompression, total international physical activity questionnaire score (cutoff value [CV]: 2205.5, sensitivity: 82.4%; specificity: 61.1%) and TS value (CV: 2.5, sensitivity: 42.9%; specificity: 83.2%) were independently associated with PAP (P < 0.05). Logistic regression further revealed that the presence of preoperative axial pain, TS value (CV: 2.5, sensitivity: 62.5%; specificity: 83.2%) and CPM value (CV: 0.65, sensitivity: 87.5%; specificity: 61.4%) were significant predictors of CPAP (P < 0.05).

CONCLUSIONS

The findings of this study support the hypothesis that preoperative endogenous pain modulation efficiency may be associated with axial pain after posterior cervical decompression. Clinically, preoperative estimation of both TS and CPM in remote pain-free areas may provide additional useful information for identifying patients who may be at risk of developing both PAP and CPAP, which may be beneficial in enabling stratification in the perioperative period of patients based on individual vulnerabilities to avoid/reduce this complication.

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.

Exposure to Gulf War Illness-related agents leads to the development of chronic pain and fatigue

AIMS

A substantial contingent of veterans from the first Gulf War continues to suffer from a number of Gulf War-related illnesses (GWI) affecting the neurological and musculoskeletal systems; the most common symptoms include chronic pain and fatigue. Although animal models have recapitulated several aspects of cognitive impairments in GWI, the pain and fatigue symptoms have not been well documented to allow examination of potential pathogenic mechanisms.

MAIN METHODS

We used a mouse model of GWI by exposing mice repeatedly to a combination of Gulf War chemicals (pyridostigmine bromide, permethrin, DEET, and chlorpyrifos) and mild immobilization stress, followed by investigating their pain susceptibilities and fatigue symptoms. To assess whether enhanced antioxidant capacity can counter the effects of GW agents, transgenic mice overexpressing extracellular superoxide dismutase (SOD3OE) were also examined.

KEY FINDINGS

The mouse model recapitulated several aspects of the human illness, including hyperalgesia, impaired descending inhibition of pain, and increased tonic pain. There is a close association between chronic pain and fatigue in GWI patients. Consistent with this observation, the mouse model showed a significant reduction in physical endurance on the treadmill. Examination of skeletal muscles suggested reduction in mitochondrial functions may have contributed to the fatigue symptoms. Furthermore, the negative impacts of GW agents in pain susceptibilities were largely diminished in SOD3OE mice, suggesting that increased oxidative stress was associated with the emergence of these Gulf War symptoms.

SIGNIFICANCE

the mouse model will be suitable for delineating specific defects in the pain pathways and mechanisms of fatigue in GWI.

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

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.

Loss of diffuse noxious inhibitory control after traumatic brain injury in rats: A chronic issue

Chronic pain is one of the most challenging and debilitating symptoms to manage after traumatic brain injury (TBI), yet the underlying mechanisms remain elusive. The disruption of normal endogenous pain control mechanisms has been linked to several forms of chronic pain and may play a role in pain after TBI. We hypothesized therefore that dysfunctional descending noradrenergic and serotonergic pain control circuits may contribute to the loss of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, weeks to months after TBI. For these studies, the rat lateral fluid percussion model of mild TBI was used along with a DNIC paradigm involving a capsaicin-conditioning stimulus. We observed sustained failure of the DNIC response up to 180-days post injury. We confirmed, that descending α₂ adrenoceptor-mediated noradrenergic signaling was critical for endogenous pain inhibition in uninjured rats. However, augmenting descending noradrenergic signaling using reboxetine, a selective noradrenaline reuptake inhibitor, failed to restore DNIC after TBI. Furthermore, blocking serotonin-mediated descending signaling using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine was also unsuccessful at restoring endogenous pain modulation after TBI. Unexpectedly, increasing descending serotonergic signaling using the selective serotonin reuptake inhibitor escitalopram and the serotonin-norepinephrine reuptake inhibitor duloxetine restored the DNIC response in TBI rats at both 49- and 180- days post injury. Consistent with these observations, spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine eliminated the effects of escitalopram. Intact α₂ adrenoceptor signaling, however, was not required for the serotonin-mediated restoration of DNIC after TBI. These results suggest that TBI causes maladaptation of descending nociceptive signaling mechanisms and changes in the function of both adrenergic and serotonergic circuits. Such changes could predispose those with TBI to chronic pain.

Mild Traumatic Brain Injury Causes Nociceptive Sensitization through Spinal Chemokine Upregulation

High rates of acute and chronic pain are associated with traumatic brain injury (TBI), but mechanisms responsible for the association remain elusive. Recent data suggest dysregulated descending pain modulation circuitry could be involved. Based on these and other observations, we hypothesized that serotonin (5-HT)-dependent activation of spinal CXC Motif Chemokine Receptor 2 (CXCR2) may support TBI-related nociceptive sensitization in a mouse model of mild TBI (mTBI). We observed that systemic 5-HT depletion with p-chlorophenylalanine attenuated mechanical hypersensitivity seen after mTBI. Likewise, selective spinal 5-HT fiber depletion with 5,7-dihydroxytryptamine (5,7-DHT) reduced hypersensitivity after mTBI. Consistent with a role for spinal 5-HT₃ serotonin receptors, intrathecal ondansetron administration after TBI dose-dependently attenuated nociceptive sensitization. Also, selective CXCR2 antagonist SCH527123 treatment attenuated mechanical hypersensitivity after mTBI. Furthermore, spinal CXCL1 and CXCL2 mRNA and protein levels were increased after mTBI as were GFAP and IBA-1 markers. Spinal 5,7-DHT application reduced both chemokine expression and glial activation. Our results suggest dual pathways for nociceptive sensitization after mTBI, direct 5-HT effect through 5-HT₃ receptors and indirectly through upregulation of chemokine signaling. Designing novel clinical interventions against either the 5-HT₃ mediated component or chemokine pathway may be beneficial in treating pain frequently seen in patients after mTBI.

Enhanced descending pain facilitation in acute traumatic brain injury

Acute and persistent pain are recognized consequences of TBI that can enhance suffering and significantly impair rehabilitative efforts. Both experimental models and clinical studies suggest that TBI may result in an imbalance between descending pain facilitatory and inhibitory pathways. The aim of this study was to assess the role of enhanced descending serotonin-mediated pain facilitation in a rat TBI model using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine (DHT). We observed significant hindpaw allodynia in TBI rats that was reduced after DHT but not vehicle treatment. Immunohistochemical studies demonstrated profound spinal serotonin depletion in DHT-treated rats. Furthermore, lumbar intrathecal administration of the 5-HT₃ receptor antagonist ondansetron at 7 days post-injury (DPI), when hindpaw allodynia was maximal, also attenuated nociceptive sensitization. Additional immunohistochemical analyses of the lumbar spinal cord at 7 DPI revealed a robust bilateral microglial response in the superficial dorsal horns that was significantly reduced with DHT treatment. Furthermore, serotonin depletion also prevented the TBI-induced bilateral increase in c-Fos positive cells within the Rexed laminae I and II of the dorsal horns. These results indicate that in the weeks following TBI, pain may be responsive to 5-HT₃ receptor antagonists or other measures which rebalance descending pain modulation.

Descending Control Mechanisms and Chronic Pain

PURPOSE OF REVIEW

The goal of the review was to highlight recent advances in our understanding of descending pain-modulating systems and how these contribute to persistent pain states, with an emphasis on the current state of knowledge around "bottom-up" (sensory) and "top-down" (higher structures mediating cognitive and emotional processing) influences on pain-modulating circuits.

RECENT FINDINGS

The connectivity, physiology, and function of these systems have been characterized extensively over the last 30 years. The field is now beginning to ask how and when these systems are engaged to modulate pain. A recent focus is on the parabrachial complex, now recognized as the major relay of nociceptive information to pain-modulating circuits, and plasticity in this circuit and its connections to the RVM is marked in persistent inflammatory pain. Top-down influences from higher structures, including hypothalamus, amygdala, and medial prefrontal areas, are also considered. The challenge will be to tease out mechanisms through which a particular behavioral context engages distinct circuits to enhance or suppress pain, and to understand how these mechanisms contribute to chronic pain.

Metabolic features and regulation of the healing cycle-A new model for chronic disease pathogenesis and treatment

Without healing, multicellular life on Earth would not exist. Without healing, one injury predisposes to another, leading to disability, chronic disease, accelerated aging, and death. Over 60% of adults and 30% of children and teens in the United States now live with a chronic illness. Advances in mass spectrometry and metabolomics have given scientists a new lens for studying health and disease. This study defines the healing cycle in metabolic terms and reframes the pathophysiology of chronic illness as the result of metabolic signaling abnormalities that block healing and cause the normal stages of the cell danger response (CDR) to persist abnormally. Once an injury occurs, active progress through the stages of healing is driven by sequential changes in cellular bioenergetics and the disposition of oxygen and carbon skeletons used for fuel, signaling, defense, repair, and recovery. >100 chronic illnesses can be organized into three persistent stages of the CDR. One hundred and two targetable chemosensory G-protein coupled and ionotropic receptors are presented that regulate the CDR and healing. Metabokines are signaling molecules derived from metabolism that regulate these receptors. Reframing the pathogenesis of chronic illness in this way, as a systems problem that maintains disease, rather than focusing on remote trigger(s) that caused the initial injury, permits new research to focus on novel signaling therapies to unblock the healing cycle, and restore health when other approaches have failed.