Are the emergence of affective disturbances in neuropathic pain states contingent on supraspinal neuroinflammation?

18 kDa Translocator protein (TSPO) is upregulated in rat brain after peripheral nerve injury and downregulated by diroximel fumarate

Neuroimmune signaling is a key process underlying neuropathic pain. Clinical studies have demonstrated that 18 kDa translocator protein (TSPO), a putative marker of neuroinflammation, is upregulated in discrete brain regions of patients with chronic pain. However, no preclinical studies have investigated TSPO dynamics in the brain in the context of neuropathic pain and in response to analgesic treatments. We used positron emission tomography-computed tomography (PET-CT) and [18F]-PBR06 radioligand to measure TSPO levels in the brain across time after chronic constriction injury (CCI) of the sciatic nerve in both male and female rats. Up to 10 weeks post-CCI, TSPO expression was increased in discrete brain regions, including medial prefrontal cortex, somatosensory cortex, insular cortex, anterior cingulate cortex, motor cortex, ventral tegmental area, amygdala, midbrain, pons, medulla, and nucleus accumbens. TSPO was broadly upregulated across these regions at 4 weeks post CCI in males, and 10 weeks in females, though there were regional differences between the sexes. Using immunohistochemistry, we confirmed TSPO expression in these regions. We further demonstrated that TSPO was upregulated principally in microglia in the nucleus accumbens core, and astrocytes and endothelial cells in the nucleus accumbens shell. Finally, we tested whether TSPO upregulation was sensitive to diroximel fumarate, a drug that induces endogenous antioxidants via nuclear factor E2-related factor 2 (Nrf2). Diroximel fumarate alleviated neuropathic pain and reduced TSPO upregulation. Our findings indicate that TSPO is upregulated over the course of neuropathic pain development and is resolved by an antinociceptive intervention in animals with peripheral nerve injury.

Strategies for measuring non-evoked pain in preclinical models of neuropathic pain: Systematic review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

Microglia as a Game Changer in Epilepsy Comorbid Depression

As one of the most common neurological diseases, epilepsy is often accompanied by psychiatric disorders. Depression is the most universal comorbidity of epilepsy, especially in temporal lobe epilepsy (TLE). Therefore, it is urgently needed to figure out potential mechanisms and the optimization of therapeutic strategies. Microglia play a pivotal role in the coexistent relationship between epilepsy and depression. Activated microglia released cytokines like IL-6 and IL-1β, orchestrating neuroinflammation especially in the hippocampus, worsening both depression and epilepsy. The decrease of intracellular K+ is a common part in various molecular changes. The P2X7-NLRP3-IL-1β is a major inflammatory pathway that disrupts brain network. Extra ATP and CX3CL1 also lead to neuronal excitotoxicity and blood-brain barrier (BBB) disruption. Regulating neuroinflammation aiming at microglia-related molecules is capable of suspending the vicious mutual aggravating circle of epilepsy and depression. Other overlaps between epilepsy and depression lie in transcriptomic, neuroimaging, diagnosis and treatment. Hippocampal sclerosis (HS) and amygdala enlargement (AE) may be the underlying macroscopic pathological changes according to current studies. Extant evidence shows that cognitive behavioral therapy (CBT) and antidepressants like selective serotonin-reuptake inhibitors (SSRIs) are safe, but the effect is limited. Improvement in depression is likely to reduce the frequency of seizure. More comprehensive experiments are warranted to better understand the relationship between them.

High mobility group box-1: A therapeutic target for analgesia and associated symptoms in chronic pain

The number of patients with chronic pain continues to increase against the background of an ageing society and a high incidence of various epidemics and disasters. One factor contributing to this situation is the absence of truly effective analgesics. Chronic pain is a persistent stress for the organism and can trigger a variety of neuropsychiatric symptoms. Hence, the search for useful analgesic targets is currently being intensified worldwide, and it is anticipated that the key to success may be molecules involved in emotional as well as sensory systems. High mobility group box-1 (HMGB1) has attracted attention as a therapeutic target for a variety of diseases. It is a very unique molecule having a dual role as a nuclear protein while also functioning as an inflammatory agent outside the cell. In recent years, numerous studies have shown that HMGB1 acts as a pain inducer in primary sensory nerves and the spinal dorsal horn. In addition, HMGB1 can function in the brain, and is involved in the symptoms of depression, anxiety and cognitive dysfunction that accompany chronic pain. In this review, we will summarize recent research and discuss the potential of HMGB1 as a useful drug target for chronic pain.

Activation of LXRs alleviates neuropathic pain-induced cognitive dysfunction by modulation of microglia polarization and synaptic plasticity via PI3K/AKT pathway

OBJECTIVE

Cognitive dysfunction is a common comorbidity in patients with chronic pain. Activation of Liver X receptors (LXRs) plays a potential role in improving cognitive disorders in central nervous diseases. In this study, we investigated the role of LXRs in cognitive deficits induced by neuropathic pain.

METHODS

We established the spared nerve injury (SNI) model to investigate pain-induced memory dysfunction. Pharmacological activation of LXRs with T0901317 or inhibition with GSK2033 was applied. PI3K inhibitor LY294002 was administered to explore the underlying mechanism of LXRs. Changes in neuroinflammation, microglia polarization, and synaptic plasticity were assessed using biochemical technologies.

RESULTS

We found that SNI-induced cognitive impairment was associated with reduced LXRβ expression, increased M1-phenotype microglia, decreased synaptic proteins, and inhibition of PI3K/AKT signaling pathway in the hippocampus. Activation of LXRs using T0901317 effectively alleviated SNI-induced cognitive impairment. Additionally, T0901317 promoted the polarization of microglia from M1 to M2, reduced pro-inflammatory cytokines, and upregulated synaptic proteins in the hippocampus. However, administration of GSK2033 or LY294002 abolished these protective effects of T0901317 in SNI mice.

CONCLUSIONS

LXRs activation alleviates neuropathic pain-induced cognitive impairment by modulating microglia polarization, neuroinflammation, and synaptic plasticity, at least partly via activation of PI3K/AKT signaling in the hippocampus. LXRs may be promising targets for addressing pain-related cognitive deficits.

Anti-inflammatory effect of dental pulp stem cells

Dental pulp stem cells (DPSCs) have received a lot of attention as a regenerative medicine tool with strong immunomodulatory capabilities. The excessive inflammatory response involves a variety of immune cells, cytokines, and has a considerable impact on tissue regeneration. The use of DPSCs for controlling inflammation for the purpose of treating inflammation-related diseases and autoimmune disorders such as supraspinal nerve inflammation, inflammation of the pulmonary airways, systemic lupus erythematosus, and diabetes mellitus is likely to be safer and more regenerative than traditional medicines. The mechanism of the anti-inflammatory and immunomodulatory effects of DPSCs is relatively complex, and it may be that they themselves or some of the substances they secrete regulate a variety of immune cells through inflammatory immune-related signaling pathways. Most of the current studies are still at the laboratory cellular level and animal model level, and it is believed that through the efforts of more researchers, DPSCs/SHED are expected to be transformed into excellent drugs for the clinical treatment of related diseases.

The Inhibition of Neuropathic Pain Incited by Nerve Injury and Accompanying Mood Disorders by New Heme Oxygenase-1 Inducers: Mechanisms Implicated

Neuropathic pain is a type of pain that persists for a long time and becomes pathological. Additionally, the anxiodepressive disorders derived from neuropathic pain are difficult to palliate with the current treatments and need to be resolved. Then, using male mice with neuropathic pain provoked by chronic constriction of the sciatic nerve (CCI), we analyzed and compared the analgesic actions produced by three new heme oxygenase 1 (HO-1) inducers, 1m, 1b, and 1a, with those performed by dimethyl fumarate (DMF). Their impact on the anxiety- and depressive-like comportments and the expression of the inflammasome NLRP3, Nrf2, and some antioxidant enzymes in the dorsal root ganglia (DRG) and amygdala (AMG) were also investigated. Results revealed that the administration of 1m, 1b, and DMF given orally for four days inhibited the allodynia and hyperalgesia caused by CCI, while 1a merely reduced the mechanical allodynia. However, in the first two days of treatment, the antiallodynic effects produced by 1m were higher than those of 1a and DMF, and its antihyperalgesic actions were greater than those produced by 1b, 1a, and DMF, revealing that 1m was the most effective compound. At four days of treatment, all drugs exerted anxiolytic and antidepressant effects, decreased the NLRP3 levels, and increased/normalized the Nrf2, HO-1, and superoxide dismutase 1 levels in DRG and AMG. Data indicated that the dual modulation of the antioxidant and inflammatory pathways produced by these compounds, especially 1m, is a new promising therapeutic approach for neuropathic pain and related emotional illnesses.

Brain nuclei and neural circuits in neuropathic pain and brain modulation mechanisms of acupuncture: a review on animal-based experimental research

Neuropathic pain (NP) is known to be associated with abnormal changes in specific brain regions, but the complex neural network behind it is vast and complex and lacks a systematic summary. With the help of various animal models of NP, a literature search on NP brain regions and circuits revealed that the related brain nuclei included the periaqueductal gray (PAG), lateral habenula (LHb), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC); the related brain circuits included the PAG-LHb and mPFC-ACC. Moreover, acupuncture and injurious information can affect different brain regions and influence brain functions via multiple aspects to play an analgesic role and improve synaptic plasticity by regulating the morphology and structure of brain synapses and the expression of synapse-related proteins; maintain the balance of excitatory and inhibitory neurons by regulating the secretion of glutamate, γ-aminobutyric acid, 5-hydroxytryptamine, and other neurotransmitters and receptors in the brain tissues; inhibit the overactivation of glial cells and reduce the release of pro-inflammatory mediators such as interleukins to reduce neuroinflammation in brain regions; maintain homeostasis of glucose metabolism and regulate the metabolic connections in the brain; and play a role in analgesia through the mediation of signaling pathways and signal transduction molecules. These factors help to deepen the understanding of NP brain circuits and the brain mechanisms of acupuncture analgesia.

Cannabidiol modulates chronic neuropathic pain aversion behavior by attenuation of neuroinflammation markers and neuronal activity in the corticolimbic circuit in male Wistar rats

Chronic neuropathic pain (CNP) is a vast world health problem often associated with the somatosensory domain. This conceptualization is problematic because, unlike most other sensations that are usually affectively neutral and may present emotional, affective, and cognitive impairments. Neuronal circuits that modulate pain can increase or decrease painful sensitivity based on several factors, including context and expectation. The objective of this study was to evaluate whether subchronic treatment with Cannabidiol (CBD; 0.3, 3, and 10 mg/kg intraperitoneal route - i.p., once a day for 3 days) could promote pain-conditioned reversal, in the conditioned place preference (CPP) test, in male Wistar rats submitted to chronic constriction injury (CCI) of the sciatic nerve. Then, we evaluated the expression of astrocytes and microglia in animals treated with CBD through the immunofluorescence technique. Our results demonstrated that CBD promoted the reversal of CPP at 3 and 10 mg/kg. In CCI animals, CBD was able to attenuate the increase in neuronal hyperactivity, measured by FosB protein expression, in the regions of the corticolimbic circuit: anterior cingulate cortex (ACC), complex basolateral amygdala (BLA), granular layer of the dentate gyrus (GrDG), and dorsal hippocampus (DH) - adjacent to subiculum (CA1). CBD also prevented the increased expression of GFAP and IBA-1 in CCI animals. We concluded that CBD effects on CNP are linked to the modulation of the aversive component of pain. These effects decrease chronic neuronal activation and inflammatory markers in regions of the corticolimbic circuit.

People's experiences living with peripheral neuropathy: a qualitative study

Introduction

Peripheral neuropathy is a neurological disorder characterised by pain, numbness, or tingling due to nerve damage. Peripheral neuropathy is one of the main health issues in Kuwait and is a rising concern which affects a large proportion of the population, therefore the lived experience needs to be explored to identify areas for improvement in care. This qualitative study explored the experiences of people living with peripheral neuropathy in Kuwait.

Methods

Semi-structured interviews were conducted with 25 participants recruited from the Neurology Outpatient Clinic of the Ibn Sina Hospital in Kuwait. The interview questions explored their experiences and understanding of pain along with the impact on their daily life. The interviews were audio recorded, transcribed and translated into English then coded using NVivo 12. Thematic analysis was conducted to identify patterns and themes in the data.

Results

Three major themes were identified including treatment beliefs (perceived effectiveness of treatment and seeking alternative treatments), the barriers to pain management (medication side effects, relationships with healthcare professionals and lack of information and access to healthcare), and the impact on quality of life (impact on work and social, physical, and psychological consequences). Self-efficacy was a key construct and over-arching theme that was discussed in all aspects, which finds reflection in the protection motivation theory.

Discussion

This paper presents the experiences of people living with peripheral neuropathy and highlights there is scope for improvement of current treatments in Kuwait. Self-management strategies are recommended alongside prescribed medication and healthcare professionals are encouraged to use a patient-centered approach. More importantly, information and support on the condition to promote coping strategies and self-efficacy should be adopted to improve quality of life. These findings can be implemented locally and globally to improve the quality of care provided to people living with peripheral neuropathy.

Cerebrolysin Attenuates Exacerbation of Neuropathic Pain, Blood-spinal Cord Barrier Breakdown and Cord Pathology Following Chronic Intoxication of Engineered Ag, Cu or Al (50-60 nm) Nanoparticles

Neuropathic pain is associated with abnormal sensations and/or pain induced by non-painful stimuli, i.e., allodynia causing burning or cold sensation, pinching of pins and needles like feeling, numbness, aching or itching. However, no suitable therapy exists to treat these pain syndromes. Our laboratory explored novel potential therapeutic strategies using a suitable composition of neurotrophic factors and active peptide fragments-Cerebrolysin (Ever Neuro Pharma, Austria) in alleviating neuropathic pain induced spinal cord pathology in a rat model. Neuropathic pain was produced by constrictions of L-5 spinal sensory nerves for 2-10 weeks period. In one group of rats cerebrolysin (2.5 or 5 ml/kg, i.v.) was administered once daily after 2 weeks until sacrifice (4, 8 and 10 weeks). Ag, Cu and Al NPs (50 mg/kg, i.p.) were delivered once daily for 1 week. Pain assessment using mechanical (Von Frey) or thermal (Hot-Plate) nociceptive showed hyperalgesia from 2 weeks until 10 weeks progressively that was exacerbated following Ag, Cu and Al NPs intoxication in nerve lesioned groups. Leakage of Evans blue and radioiodine across the blood-spinal cord barrier (BSCB) is seen from 4 to 10 weeks in the rostral and caudal cord segments associated with edema formation and cell injury. Immunohistochemistry of albumin and GFAP exhibited a close parallelism with BSCB leakage that was aggravated by NPs following nerve lesion. Light microscopy using Nissl stain exhibited profound neuronal damages in the cord. Transmission electron microcopy (TEM) show myelin vesiculation and synaptic damages in the cord that were exacerbated following NPs intoxication. Using ELISA spinal tissue exhibited increased albumin, glial fibrillary acidic protein (GFAP), myelin basic protein (MBP) and heat shock protein (HSP 72kD) upregulation together with cytokines TNF-α, IL-4, IL-6, IL-10 levels in nerve lesion that was exacerbated following NPs intoxication. Cerebrolysin treatment significantly reduced hyperalgesia and attenuated BSCB disruption, edema formation and cellular changes in nerve lesioned group. The levels of cytokines were also restored near normal levels with cerebrolysin treatment. Albumin, GFAP, MABP and HSP were also reduced in cerebrolysin treated group and thwarted neuronal damages, myelin vesiculation and cell injuries. These neuroprotective effects of cerebrolysin with higher doses were also effective in nerve lesioned rats with NPs intoxication. These observations suggest that cerebrolysin actively protects spinal cord pathology and hyperalgesia following nerve lesion and its exacerbation with metal NPs, not reported earlier.

Differential activation of spinal and parabrachial glial cells in a neuropathic pain model

The clinical burden faced by chronic pain patients is compounded by affective comorbidities, such as depression and anxiety disorders. Emerging evidence suggests that reactive glial cells in the spinal cord dorsal horn play a key role in the chronification of pain, while supraspinal glia are important for psychological aspects of chronic pain. The lateral parabrachial nucleus (LPBN) in the brainstem is a key node in the ascending pain system, and is crucial for the emotional dimension of pain. Yet, whether astrocytes and microglia in the LPBN are activated during chronic pain is unknown. Here, we evaluated the occurrence of glial activation in the LPBN of male Sprague-Dawley rats 1, 4, and 7 weeks after inducing a chronic constriction injury (CCI) of the sciatic nerve, a prevalent neuropathic pain model. CCI animals developed mechanical and thermal hypersensitivity that persisted for at least 4 weeks, and was mostly reversed after 7 weeks. Using immunohistochemical staining and confocal imaging, we found that CCI caused a strong increase in the expression of the astrocytic marker GFAP and the microglial marker Iba1 in the ipsilateral spinal dorsal horn, with peak expression observed 1 week post-injury. Moreover, morphology analysis revealed changes in microglial phenotype, indicative of microglia activation. In contrast, CCI did not induce any detectable changes in either astrocytes or microglia in the LPBN, at any time point. Thus, our results indicate that while neuropathic pain induces a robust glial reaction in the spinal dorsal horn, it fails to activate glial cells in the LPBN.

Pain-resolving immune mechanisms in neuropathic pain

Interactions between the immune and nervous systems are of central importance in neuropathic pain, a common and debilitating form of chronic pain caused by a lesion or disease affecting the somatosensory system. Our understanding of neuroimmune interactions in pain research has advanced considerably. Initially considered as passive bystanders, then as culprits in the pathogenesis of neuropathic pain, immune responses in the nervous system are now established to underpin not only the initiation and progression of pain but also its resolution. Indeed, immune cells and their mediators are well-established promoters of neuroinflammation at each level of the neural pain pathway that contributes to pain hypersensitivity. However, emerging evidence indicates that specific subtypes of immune cells (including antinociceptive macrophages, pain-resolving microglia and T regulatory cells) as well as immunoresolvent molecules and modulators of the gut microbiota-immune system axis can reduce the pain experience and contribute to the resolution of neuropathic pain. This Review provides an overview of the immune mechanisms responsible for the resolution of neuropathic pain, including those involved in innate, adaptive and meningeal immunity as well as interactions with the gut microbiome. Specialized pro-resolving mediators and therapeutic approaches that target these neuroimmune mechanisms are also discussed.

Transient Reflexive Pain Responses and Chronic Affective Nonreflexive Pain Responses Associated with Neuroinflammation Processes in Both Spinal and Supraspinal Structures in Spinal Cord-Injured Female Mice

Central neuropathic pain is not only characterized by reflexive pain responses, but also emotional or affective nonreflexive pain responses, especially in women. Some pieces of evidence suggest that the activation of the neuroimmune system may be contributing to the manifestation of mood disorders in patients with chronic pain conditions, but the mechanisms that contribute to the development and chronicity of CNP and its associated disorders remain poorly understood. This study aimed to determine whether neuroinflammatory factor over-expression in the spinal cord and supraspinal structures may be associated with reflexive and nonreflexive pain response development from acute SCI phase to 12 weeks post-injury in female mice. The results show that transient reflexive responses were observed during the SCI acute phase associated with transient cytokine overexpression in the spinal cord. In contrast, increased nonreflexive pain responses were observed in the chronic phase associated with cytokine overexpression in supraspinal structures, especially in mPFC. In addition, results revealed that besides cytokines, the mPFC showed an increased glial activation as well as CX3CL1/CX3CR1 upregulation in the neurons, suggesting the contribution of neuron-glia crosstalk in the development of nonreflexive pain responses in the chronic spinal cord injury phase.

Anti-interleukin-6 receptor antibody improves allodynia and cognitive impairment in mice with neuropathic pain following partial sciatic nerve ligation

Neuropathic pain caused by nerve injury presents with severe spontaneous pain and a range of comorbidities, including deficits in higher executive functioning, none of which are adequately treated with current analgesics. Interleukin-6 (IL-6), a proinflammatory cytokine, is critically involved in the development and maintenance of central sensitization. However, the roles of IL-6 in neuropathic pain and related comorbidities have yet to be fully clarified. The present study examined the effect of MR16-1, an anti-IL-6 receptor antibody and inhibits IL-6 activity, on allodynia and cognitive impairment in mice with neuropathic pain following partial sciatic nerve ligation (PSNL). Significant upregulation of IL-6 expression was observed in the hippocampus in PSNL mice. Intranasal administration of MR16-1 significantly improved cognitive impairment but not allodynia in PSNL mice. Intranasal MR16-1 blocked PSNL-induced degenerative effects on hippocampal neurons. Intraperitoneal administration of MR16-1 suppressed allodynia but not cognitive impairment of PSNL mice. The findings suggest that cognitive impairment associated with neuropathic pain is mediated through changes in hippocampus induced by IL-6. These data also suggest that IL-6 mediated peripheral inflammation underlies allodynia, and IL-6 mediated inflammation in the central nervous system underlies cognitive impairment associated with neuropathic pain, and further suggest the therapeutic potential of blocking IL-6 functioning by blocking its receptor.

Transcriptome Profiles of IncRNA and mRNA Highlight the Role of Ferroptosis in Chronic Neuropathic Pain With Memory Impairment

Background: Chronic neuropathic pain is commonly associated with memory loss, which increases the risk of dementia, lowers life quality and spending. On the other hand, the molecular processes are unknown, and effective therapies have yet to be discovered. Long non-coding RNAs (lncRNAs) are emerging potential therapeutic targets for chronic pain, but their role in chronic pain-induced memory impairment is unknown. Methods: We established a CCI-induced memory impairment rat model. To investigate and validate the gene expression alterations in the hippocampus of CCI-induced memory impairment, we used RNA-Seq, bioinformatics analysis, qRT-PCR, western blot, immunostaining, Nissl staining, and Diaminobenzidine-enhanced Perls' stain. Results: CCI rats displayed long-term memory deficits in the Y maze and novel objective recognition tests, and chronic mechanical and thermal pain hypersensitivity in the hind paws. We found a total of 179 differentially expressed mRNAs (DEmRNAs) (81 downregulated and 98 upregulated) and 191 differentially expressed long noncoding RNAs (DElncRNAs) (87 downregulated and 105 upregulated) between the hippocampus CA1 of CCI-induced memory impairment model and the sham control, using RNA-Seq expression profiles. The most enriched pathways involving oxidation and iron metabolism were explored using a route and function pathway analysis of DEmRNAs and DElncRNAs. We also discovered that ATF3 was considerably overexpressed in the hippocampal CA1 area, and gene markers of ferroptosis, such as GPX4, SLC7A11, SLC1A5, and PTGS2, were dysregulated in the CCI-induced memory impairment paradigm. Furthermore, in the hippocampus CA1 of CCI-induced memory impairment, lipid peroxidation and iron overload were considerably enhanced. Fer-1 treatment reversed ferroptosis damage of CCI with memory impairment model. Finally, in CCI-induced memory impairment, a competing RNA network analysis of DElncRNAs and DEmRNAs was performed to investigate the putative regulatory link of DElncRNAs on DEmRNAs via miRNA sponging. Conclusion: Using RNA-Seq, we created a genome-wide profile of the whole hippocampus of a rat model of CCI-induced memory impairment. In the hippocampus, pathways and function analyses revealed numerous intriguing genes and pathways involved in ferroptosis and memory impairment in response to chronic pain stress. As a result, our research may aid in the identification of potential and effective treatments for CCI-induced memory impairment.

Neural correlates of co-occurring pain and depression: an activation-likelihood estimation (ALE) meta-analysis and systematic review

The relationship between pain and depression is thought to be bidirectional and the underlying neurobiology 'shared' between the two conditions. However, these claims are often based on qualitative comparisons of brain regions implicated in pain or depression, while focused quantitative studies of the neurobiology of pain-depression comorbidity are lacking. Particularly, the direction of comorbidity, i.e., pain with depression vs. depression with pain, is rarely addressed. In this systematic review (PROSPERO registration CRD42020219876), we aimed to delineate brain correlates associated with primary pain with concomitant depression, primary depression with concurrent pain, and equal pain and depression comorbidity, using activation likelihood estimation (ALE) meta-analysis. Neuroimaging studies published in English until the 28th of September 2021 were evaluated using PRISMA guidelines. A total of 70 studies were included, of which 26 reported stereotactic coordinates and were analysed with ALE. All studies were assessed for quality by two authors, using the National Institute of Health Quality Assessment Tool. Our results revealed paucity of studies that directly investigated the neurobiology of pain-depression comorbidity. The ALE analysis indicated that pain with concomitant depression was associated with the right amygdala, while depression with concomitant pain was related primarily to the left dorsolateral prefrontal cortex (DLPFC). We provide evidence that pain and depression have a cumulative negative effect on a specific set of brain regions, distinct for primary diagnosis of depression vs. pain.

Resolvin D1 alleviates mechanical allodynia via ALX/FPR2 receptor targeted NLRP3/ERK signaling in a neuropathic pain model

The current study aimed to investigate the role and underlying mechanism of Resolvin D1 (RvD1) alleviating spinal nerve ligation (SNL)-induced neuropathic pain (NP) and its interplay with regulatory cascades of NLRP3 inflammasome. Sprague-Dawley male rat model of SNL-stimulated NP was established, which were pre-treated with different doses of RvD1, WRW4 (ALX/FPR2 inhibitor) or U0126 (ERK inhibitor) for three successive days following the operation. Pain behavior was assessed by measuring changes in the mechanical sensitivity of the hind paws during an observation period of 7 consecutive days. The spinal cord (SC) and dorsal root ganglions (DRGs) tissues were collected on postoperative day 7. Immunohistochemistry (IHC) and western blot were performed to determine the expression levels of NLRP3 inflammasome complex, ALX/FPR2 receptor and extracellular signal-related kinase (ERK). The pro-inflammatory mediators (IL-1β and IL-18) were measured by enzyme-linked immunosorbent assay (ELISA). The results showed that RvD1 could alleviate mechanical allodynia significantly in the SNL-induced NP rat model. Also, RvD1 inhibited the expression of p-ERK, the NLRP3 inflammasomes complex and its corresponding downstream pro-inflammatory mediators which were significantly enhanced in the SC and DRGs of the rat of SNL model. While these changes were partially reversed by pre-administration of WRW4 and further strengthened by co-treated with U0126. Our results suggest that RvD1 dependent on ALX/FPR2 may have an analgesic and anti-inflammatory influence on SNL-induced NP driven by inhibiting NLRP3 inflammasome via ERK signaling pathway. These data also provide strong support for the recent modulation of neuro-inflammatory priming and highlight the potential for specialized pro-resolving mediators (SPMs) as novel therapeutic avenues for NP.

Altered basal ganglia infraslow oscillation and resting functional connectivity in complex regional pain syndrome

Complex regional pain syndrome (CRPS) is a painful condition commonly accompanied by movement disturbances and often affects the upper limbs. The basal ganglia motor loop is central to movement, however, non-motor basal ganglia loops are involved in pain, sensory integration, visual processing, cognition, and emotion. Systematic evaluation of each basal ganglia functional loop and its relation to motor and non-motor disturbances in CRPS has not been investigated. We recruited 15 upper limb CRPS and 45 matched healthy control subjects. Using functional magnetic resonance imaging, infraslow oscillations (ISO) and resting-state functional connectivity in motor and non-motor basal ganglia loops were investigated using putamen and caudate seeds. Compared to controls, CRPS subjects displayed increased ISO power in the putamen contralateral to the CRPS affected limb, specifically, in contralateral putamen areas representing the supplementary motor area hand, motor hand, and motor tongue. Furthermore, compared to controls, CRPS subjects displayed increased resting connectivity between these putaminal areas as well as from the caudate body to cortical areas such as the primary motor cortex, supplementary and cingulate motor areas, parietal association areas, and the orbitofrontal cortex. These findings demonstrate changes in basal ganglia loop function in CRPS subjects and may underpin motor disturbances of CRPS.

Structure-guided discovery of potent and oral soluble epoxide hydrolase inhibitors for the treatment of neuropathic pain

Soluble epoxide hydrolase (sEH) is related to arachidonic acid cascade and is over-expressed in a variety of diseases, making sEH an attractive target for the treatment of pain as well as inflammatory-related diseases. A new series of memantyl urea derivatives as potent sEH inhibitors was obtained using our previous reported compound 4 as lead compound. A preferential modification of piperidinyl to 3-carbamoyl piperidinyl was identified for this series via structure-based rational drug design. Compound A20 exhibited moderate percentage plasma protein binding (88.6%) and better metabolic stability in vitro. After oral administration, the bioavailability of A20 was 28.6%. Acute toxicity test showed that A20 was well tolerated and there was no adverse event encountered at dose of 6.0 g/kg. Inhibitor A20 also displayed robust analgesic effect in vivo and dose-dependently attenuated neuropathic pain in rat model induced by spared nerve injury, which was better than gabapentin and sEH inhibitor (±)-EC-5026. In one word, the oral administration of A20 significantly alleviated pain and improved the health status of the rats, demonstrating that A20 was a promising candidate to be further evaluated for the treatment of neuropathic pain.

Activación de la microglía en el hipocampo asociada con lesión del nervio facial

Introducción. Las lesiones del nervio facial afectan la plasticidad a largo plazo en el hipocampo, así como la memoria de reconocimiento de objetos y la memoria espacial, dos procesos dependientes de esta estructura. Objetivo. Caracterizar en ratas el efecto de la lesión unilateral del nervio facial sobre la activación de células de la microglía en el hipocampo contralateral.Materiales y métodos. Se hicieron experimentos de inmunohistoquímica para detectar células de la microglía en el hipocampo de ratas sometidas a lesión irreversible del nervio facial. Los animales se sacrificaron en distintos momentos después de la lesión, para evaluar la evolución de la proliferación (densidad de células) y la activación (área celular) de la microglía en el tejido del hipocampo. Los tejidos cerebrales de los animales de control se compararon con los de animales lesionados sacrificados en los días 1, 3, 7, 21 y 35 después de la lesión.Resultados. Las células de la microglía en el hipocampo de animales con lesión del nervio facial mostraron signos de proliferación y activación a los 3, 7 y 21 días después de la lesión. Sin embargo, al cabo de cinco semanas, estas modificaciones se revirtieron, a pesar de que no hubo recuperación funcional de la parálisis facial.Conclusiones. La lesión irreversible del nervio facial produce proliferación y activación temprana y transitoria de las células de la microglía en el hipocampo. Estos cambios podrían estar asociados con las modificaciones electrofisiológicas y las alteraciones comportamentales dependientes del hipocampo descritas recientemente.

HDAC6 inhibitor ACY-1215 improves neuropathic pain and its comorbidities in rats of peripheral nerve injury by regulating neuroinflammation

The fact that neuropathic pain (NP) has no effective therapy and is frequently accompanied by psychiatric comorbidities is well established. Aberrant neuroinflammation plays an important role in the development and maintenance of NP. HDAC6 inhibitors have been demonstrated to ameliorate mechanical allodynia brought on by chemotherapy and peripheral nerve damage. However, its pharmacological mechanisms and its effects on NP-related mental disorders have not been fully elucidated. The present study was dedicated to exploring the effects of ACY-1215 (a specific HDAC6 inhibitor) on neuroinflammation and behavioral abnormalities associated with NP. In this work, spinal nerve ligation (SNL) was performed as an NP model on rats. Mechanical allodynia, cognitive impairment, and depressive-like behavior caused by SNL were attenuated by continuous intraperitoneal injection of ACY-1215. Moreover, ACY-1215 administration suppressed SNL-induced neuroinflammatory responses (including microgliosis, the elevation of pro-inflammatory factors IL-1β and TNF-α) in ligation of the ipsilateral spinal dorsal horn (iSDH), hippocampus (HPC) and prefrontal cortex (PFC). Mechanistically, MyD88-dependent pro-inflammatory pathways (MyD88/NF-κB and MyD88/ERK) were activated in the iSDH following SNL and were inhibited by ACY-1215. Moreover, ACY-1215 enhanced the acetylation modification of MyD88 and inhibited the SNL-induced elevation of MyD88 without affecting its transcription in the iSDH. These findings suggest that pharmacological inhibition of HDAC6 can ameliorate NP and its psychiatric complications through modulating neuroinflammation, in part by blocking the MyD88-mediated pro-inflammatory pathways. The possible mechanism is that ACY-1215 prevents the elevation of MyD88 reactivity by increasing its acetylation level. Notably, neither SNL nor ACY-1215 significantly altered MyD88 expression in HPC and PFC, indicating differentiated pro-inflammatory mechanisms in the supraspinal neural regions.

An overlooked nerve in neuropathies associated with intragluteal injections: the posterior femoral cutaneous nerve

OBJECTIVE

The aim of this study was to investigate the frequency of posterior femoral cutaneous nerve (PFCN) lesions in patients referred to the electrophysiology laboratory with an initial diagnosis of sciatic nerve lesion following injection, and to create awareness that PFCN lesions can occur following intramuscular injections administered to the gluteal region.

METHODS

Fifty-seven patients who were referred to the electrophysiology laboratory because of injection neuropathy were identified from the hospital records. In addition to the routine electrophysiological examination, PFCN sensory conduction study was performed according to the technique of Dumitru and Nelson. The scores of the Hospital Anxiety and Depression Scale (HADS) and the Leeds Assessment of Neuropathic Symptoms and Signs Pain Scale were recorded for all participants.

RESULTS

Of the 21 participants who agreed to participate in the study, 2 patients were diagnosed with PFCN lesions, one of them had isolated complete PFCN lesion, and another had it accompanied by sciatic nerve lesion. Patients with PFCN lesions had a lower body mass index and a higher HADS score than patients with sciatic nerve lesions (p = 0.01, p = 0.04, respectively).

CONCLUSIONS

As correct diagnosis is the priority starting point for successful treatment, clinicians should plan examinations taking into consideration the fact that PFCN lesions can occur following gluteal region injection.

Neurodynamics: is tension contentious?

Tensioning techniqueswere the first neurodynamic techniques used therapeutically in the management of people with neuropathies. This article aims to provide a balanced evidence-informed view on the effects of optimal tensile loading on peripheral nerves and the use of tensioning techniques. Whilst the early use of neurodynamics was centered within a mechanical paradigm, research into the working mechanisms of tensioning techniques revealed neuroimmune, neurophysiological, and neurochemical effects. In-vitro and ex-vivo research confirms that tensile loading is required for mechanical adaptation of healthy and healing neurons and nerves. Moreover, elimination of tensile load can have detrimental effects on the nervous system. Beneficial effects of tensile loading and tensioning techniques, contributing to restored homeostasis at the entrapment site, dorsal root ganglia and spinal cord, include neuronal cell differentiation, neurite outgrowth and orientation, increased endogenous opioid receptors, reduced fibrosis and intraneural scar formation, improved nerve regeneration and remyelination, increased muscle power and locomotion, less mechanical and thermal hyperalgesia and allodynia, and improved conditioned pain modulation. However, animal and cellular models also show that ‘excessive’ tensile forces have negative effects on the nervous system. Although robust and designed to withstand mechanical load, the nervous system is equally a delicate system. Mechanical loads that can be easily handled by a healthy nervous system, may be sufficient to aggravate clinical symptoms in patients. This paper aims to contribute to a more balanced view regarding the use of neurodynamics and more specifically tensioning techniques.

Research progress in immune microenvironment regulation of muscle atrophy induced by peripheral nerve injury

Denervated skeletal muscular atrophy is primarily characterized by loss of muscle strength and mass and an unideal functional recovery of the muscle after extended denervation. This review emphasizes the interaction between the immune system and the denervated skeletal muscle. Immune cells such as neutrophils, macrophages and T-cells are activated and migrate to denervated muscle, where they release a high concentration of cytokines and chemokines. The migration of these immune cells, the transformation of different functional immune cell subtypes, and the cytokine network in the immune microenvironment may be involved in the regulatory process of muscle atrophy or repair. However, the exact mechanisms of the interaction between these immune cells and immune molecules in skeletal muscles are unclear. In this paper, the immune microenvironment regulation of muscle atrophy induced by peripheral nerve injury is reviewed.

Role of neuroglia in neuropathic pain and depression

Patients with neuropathic pain induced by nerve injury usually present with co-morbid affective changes, such as depression. Neuroglia was reported to play an important role in the development and maintenance of neuropathic pain both centrally and peripherally. Meanwhile, there have been studies showing that neuroglia participated in the development of depression. However, the specific role of neuroglia in neuropathic pain and depression has not been reviewed comprehensively. Therefore, we summarized the recent findings on the role of neuroglia in neuropathic pain and depression. Based on this review, we found a bridge-like role of neuroglia in neuropathic pain co-morbid with depression. This review may provide therapeutic implications in the treatment of neuropathic pain and offer potential help in the studies of mechanisms in the future.

Correlation between Hyperalgesia and Upregulation of TNF-α and IL-1β in Aqueous Humor and Blood in Second Eye Phacoemulsification: Clinical and Experimental Investigation

The aim of this study was to explore the correlation between intraoperative hyperalgesia of the second eye and the dynamic changes of tumor necrosis factor (TNF)-α and interleukin (IL)-1β levels in aqueous humor (AH) of the second eye and whole blood after the first eye cataract surgery. A rabbit model of monocular phacoemulsification was established by administration of 0.3% levofloxacin. Whole blood and AH samples from non-surgical eyes in the experimental group (n =25) and second eye in the blank control group (n =15) were obtained and corneal sensitivity was examined after surgery (1, 3, 7, 14, and 21 days postoperatively). TNF-α and IL-1β levels in AH and TNF-α mRNA and IL-1β mRNA levels in whole blood were measured. In a clinical study, 30 patients who underwent bilateral phacoemulsification within 1 month were divided into six groups in accordance with the operation intervals (1, 3, 7, 10, 14, and 21days). TNF-α and IL-1β levels in AH were measured at the beginning of surgery and intraoperative pain was assessed immediately after surgery. Corneal sensitivity (F =244.910, P <0.05), TNF-α and IL-1β levels in AH (F =184.200, 82.900, P <0.05) of non-surgical eyes and in whole blood (F =272.800, 193.530, P <0.05) in the experimental group were significantly higher than the baseline levels after phacoemulsification. In the clinical study, NRS scores of second eye surgery were higher than those of the first eye(P =0.0025) and 19 (63.3%) patients reported more pain during the second eye surgery. TNF-α and IL-1β concentrations in AH of the second eye were significantly higher than those of the first eye (F =123.60, P <0.05; F =59.60, P <0.05). In conclusion, within 1 month after the first eye phacoemulsification, higher pain sensitivity (hyperalgesia) exists in the second eye, which may be related to dynamic changes in TNF-α, IL-1β levels in AH or whole blood.

Impact of Dose, Sex, and Strain on Oxaliplatin-Induced Peripheral Neuropathy in Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose limiting, and long-lasting side effect of chemotherapy treatment. Unfortunately, no treatment has proven efficacious for this side effect. Rodent models play a crucial role in the discovery of new mechanisms underlying the initiation, progression, and recovery of CIPN and the potential discovery of new therapeutics. However, there is limited consistency in the dose, the sex, age, and genetic background of the animal used in these studies and the outcome measures used in evaluation of CIPN rely primarily on noxious and reflexive measures. The main objective of this study was to provide a comprehensive and systematic characterization of oxaliplatin-induced peripheral neuropathy in mice by using a battery of behavioral, sensory, electrophysiological, and morphometric measures in both sexes of the two widely used strains of mice, C57BL/6J and BALB/cJ. Mice received intraperitoneal injections of 3 or 30 mg/kg cumulative doses of oxaliplatin over the course of 2 weeks. Both doses induced long-term and time-dependent mechanical and cold hypersensitivity. Our results show that 30 mg/kg oxaliplatin reduced the locomotor activity in C57BL/6J mice, and C57BL/6J females showed anxiety-like behavior one-week post completion of treatment. In the same dose group, BALB/cJ males and females sustained a larger decrease in sucrose preference than either male or female C57BL/6J mice. Both strains failed to show significant changes in burrowing and nesting behaviors. Two clinically relevant assessments of changes to the peripheral nerve fibers, nerve conduction and intraepidermal nerve fiber density (IENFD) were evaluated. Only BALB/cJ females showed significant reduction in the nerve conduction amplitude 1 week after 30 mg/kg oxaliplatin regimen. Moreover, this dose of the chemo agent reduced the IENF density in both sexes and strains. Our findings suggest that mouse strain, sex, and assay type should be carefully considered when assessing the effects of oxaliplatin and potential therapeutic interventions.

Disruption to normal excitatory and inhibitory function within the medial prefrontal cortex in people with chronic pain

BACKGROUND

Growing evidence indicates a link between changes in the medial prefrontal cortex and the pathophysiology of chronic pain. In particular, chronic pain is associated with altered medial prefrontal anatomy and biochemistry. Due to the comorbid affective disorders seen across all pain conditions, the medial prefrontal cortex is a region of significance as it is involved in emotional processing. We have recently reported that a decrease in medial prefrontal N-acetylaspartate and glutamate is associated with increased emotional dysregulation, indicating there are neurotransmitter imbalances in chronic pain. Therefore, we compared medial prefrontal neurochemistry in 24 people with chronic pain conditions to 24 age and sex-matched healthy controls with no history of chronic pain.

METHOD

GABA-edited MEGA-PRESS was used to measure GABA⁺ levels, and short TE PRESS was used to measure glutamate levels in the medial prefrontal cortex. Psychometric measures regarding pain intensity a week before scanning, during the scan and the total duration of chronic pain, were also recorded and compared to measured GABA⁺ and glutamate levels.

RESULTS

This study reveals that the presence of chronic pain is associated with significant decreases in medial prefrontal GABA⁺ and glutamate. These findings support the hypothesis that chronic pain is associated with altered medial prefrontal biochemistry.

CONCLUSION

The dysregulation of glutamatergic and GABAergic neurotransmitter systems supports a model of disinhibition of chronic pain, which may play a key role in both the experience of persistent pain and its associated affective disturbances.

SIGNIFICANCE

This study reveals a significant reduction in γ-aminobutyric acid (GABA⁺ ) and glutamate within the medial prefrontal cortex in chronic pain sufferers. While the current findings should be considered with reference to a small sample size, the disruption to normal excitatory and inhibitory medial prefrontal cortex function may be key in the development and maintenance of chronic pain and comorbid mental health disorders.

Decreased connexin43 expression in the hippocampus is related to the antidepressant effect of amitriptyline in neuropathic pain mice

Downregulation of astrocytic connexin43 (Cx43) has been observed in several brain regions in rodents and patients with depression. However, its specific role in this effect remains unknown. Moreover, chronic pain can induce depressive disorders. Therefore, the current study examined the relationship between Cx43 expression and depressive-like behavior in a neuropathic pain model. Neuropathic pain was induced by spared nerve injury (SNI) in mice. Depressive-like behavior was evaluated using the forced swim test. Expression of Cx43 in the hippocampus was evaluated using Western blotting and real-time PCR. SNI downregulated Cx43 protein in the contralateral hippocampus of mice, whereas expression of hippocampal Cx43 mRNA was not altered following SNI. Although SNI mice showed longer immobility time compared with sham mice during the forced swim test, duration of depressive-like behavior was not correlated with the expression of Cx43 in the hippocampus of SNI mice. Repeated intraperitoneal administration of amitriptyline ameliorated SNI-induced depressive-like behavior. Furthermore, the antidepressant effect of amitriptyline was correlated with decreased hippocampal Cx43 expression in SNI mice. The current findings suggest that the alteration of Cx43 expression in the hippocampus may not be involved in the induction of depressive disorder but may influence the efficacy of antidepressants. Therefore, the level of Cx43 expression in the hippocampus could be a key parameter to evaluate individual differences in antidepressant effects in patients with depressive disorder.

Efficacy of the iDBT-Pain skills training intervention to reduce emotional dysregulation and pain intensity in people with chronic pain: protocol for a single-case experimental design with multiple baselines

INTRODUCTION

Difficulties in emotional regulation are key to the development and maintenance of chronic pain. Recent evidence shows internet-delivered dialectic behaviour therapy (iDBT) skills training can reduce emotional dysregulation and pain intensity. However, further studies are needed to provide more definitive evidence regarding the efficacy of iDBT skills training in the chronic pain population.

METHODS AND ANALYSIS

A single-case experimental design (SCED) with multiple baselines will be used to examine the efficacy of a 4-week iDBT-Pain skills training intervention (iDBT-Pain intervention) to reduce emotional dysregulation and pain intensity in individuals with chronic pain. The iDBT-Pain intervention encompasses two components: (1) iDBT-Pain skills training sessions (iDBT-Pain sessions) and (2) the iDBT-Pain skills training web application (iDBT-Pain app). Three individuals with chronic pain will be recruited and randomly allocated to different baseline phases (5, 9 or 12 days). Following the baseline phase, participants will receive six 60-90 min iDBT-Pain sessions approximately 4 or 5 days apart, delivered by a psychologist via Zoom. To reinforce learnings from the iDBT-Pain sessions, participants will have unlimited use of the iDBT-Pain app. A 7-day follow-up phase (maintenance) will follow the intervention, whereby the iDBT-Pain sessions cease but the iDBT-Pain app is accessible. Emotional regulation, as the primary outcome measure, will be assessed using the Difficulties in Emotion Regulation Scale. Pain intensity, as the secondary outcome measure, will be assessed using a visual analogue scale. Generalisation measures will assess psychological state factors (depression, anxiety and coping behaviour), alongside sleep quality, well-being and harm avoidance. SCEDs are increasingly considered effective designs for internet-delivered psychological interventions because SCED enables the investigation of interindividual variability in a heterogeneous population such as chronic pain.

ETHICS AND DISSEMINATION

This trial was approved by the University of New South Wales (HC200199). Results will be published in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

ACTRN12620000604909.

Neuropathic Low Back Pain and Burnout among Hungarian Workers

Burnout is an increasingly prevalent syndrome mainly involving those working in human services. Although it is categorized as an occupational phenomenon and not as a medical condition, it seems to be strongly associated with several diseases such as pain syndromes. However, no studies examined the association between neuropathic low back pain and burnout. This questionnaire-based study was carried out between April 2019 and March 2020 in three main sites among teachers, social workers and healthcare workers. Demographic criteria included age, gender, marital status, number of children, type of work, years spent with work, work schedule, legal relation, secondary employment. Included diseases were diabetes, hypertension, ischemic heart disease, generalized pain (pain involving more than one area of the body) and depression. Low back pain was assessed by the painDETECT questionnaire, burnout was measured with the Maslach Burnout Inventory (MBI) and depression was measured by the Beck Depression Inventory. Dysfunctional attitudes were also recorded. Overall, 1500 questionnaires were successfully delivered and 1141 responses received (response rate of 76%). Three hundred social workers, 399 teachers, 339 paramedics, 35 doctors and 68 medical attendants have completed our survey. In a multivariate analysis including of all factors (demographic criteria, burnout, depression, dysfunctional attitudes, comorbidity etc.) neuropathic low back pain was associated with age > 62 (OR = 3.981, p = 0.01), number of children ≥ 2 (OR = 2.638, p = 0.003), job type (being a social worker) (OR = 6.654, p < 0.001), burnout (OR = 2.577, p < 0.001), current depression (OR = 2.397, p < 0.001), and suffering from generalized pain (OR= 4.076, p < 0.001). This is the first study showing the association of burnout and neuropathic low back pain, which is the most common cause of disability. Based on our results neuropathic low back pain and burnout have similar risk factors and consequences which raises the possibility of similar pathophysiology.

Sigma-1 receptor: A drug target for the modulation of neuroimmune and neuroglial interactions during chronic pain

Immune and glial cells play a pivotal role in chronic pain. Therefore, it is possible that the pharmacological modulation of neurotransmission from an exclusively neuronal perspective may not be enough for adequate pain management, and the modulation of complex interactions between neurons and other cell types might be needed for successful pain relief. In this article, we review the current scientific evidence for the modulatory effects of sigma-1 receptors on communication between the immune and nervous systems during inflammation, as well as the influence of this receptor on peripheral and central neuroinflammation. Several experimental models of pathological pain are considered, including peripheral and central neuropathic pain, osteoarthritic, and cancer pain. Sigma-1 receptor inhibition prevents peripheral (macrophage infiltration into the dorsal root ganglion) and central (activation of microglia and astrocytes) neuroinflammation in several pain models, and enhances immune-driven peripheral opioid analgesia during painful inflammation, maximizing the analgesic potential of peripheral immune cells. Therefore, sigma-1 antagonists may constitute a new class of analgesics with an unprecedented mechanism of action and potential utility in several painful disorders.

Skimmetin/osthole mitigates pain-depression dyad via inhibiting inflammatory and oxidative stress-mediated neurotransmitter dysregulation

Pain and depression are often co-existing pathological states that promote mutual severity resulting in limited efficacy of current treatment strategies. Thus, there is a need to develop an efficacious alternate treatment regimen for pain-depression dyad. Skimmetin and osthole are molecules of natural origin that have been explored for an anti-hyperglycemic, anti-bacterial, anti-fungal, and anti-diabetic activities in preclinical studies. in animal models. The current study has been designed to explore the beneficial effect of skimmetin/osthole in reserpine-induced pain-depression dyad in mice. Female Swiss albino mice (n = 6) were challenged with reserpine (0.5 mg/kg s.c.) for the first 3 days to induce a pain-depression dyad-like state. Skimmetin (10 mg/kg i.p.) and osthole (10 mg/kg i.p.) were administered for 5 days consecutively, starting from the first day of study. Reserpine treatment significantly reduced the pain threshold in the pressure application measurement (PAM) and electronic von frey (eVF) test. In forced swim test (FST) and Morris water maze (MWM) test mice displayed an increased immobility time and latency to reach platform respectively. Biochemical results showed an increased level of TNF-α, IL-1β, TBARS, glutamate, and reduced level of GSH, norepinephrine, and serotonin in the reserpine treated group. Reserpine treatment also increased brain MAO-A activity. Skimmetin/osthole treatment was found to attenuate the behavioral and biochemical alterations induced by reserpine. The results of the current investigation delineated that skimmetin/osthole may exert anti-nociceptive, anti-depressant, and improved cognition via inhibiting inflammatory and oxidative stress-mediated neurotransmitter dysregulation.

Ibudilast for prevention of oxaliplatin-induced acute neurotoxicity: a pilot study assessing preliminary efficacy, tolerability and pharmacokinetic interactions in patients with metastatic gastrointestinal cancer

PURPOSE

This prospective, open-label, sequential 'before vs. after' pilot study was conducted to provide preliminary efficacy and tolerability data for ibudilast in the prevention of oxaliplatin-induced neurotoxicity in patients with metastatic upper gastrointestinal or colorectal cancer. Any potential impact of ibudilast on oxaliplatin and 5-fluorouracil pharmacokinetics was also explored.

METHODS

Participants were administered a chemotherapy cycle (FOLFOX or CapeOx), followed by a chemotherapy cycle with co-administration of ibudilast 30 mg b.i.d. p.o. Efficacy was assessed on Day 3 and end of cycle using the Oxaliplatin-Specific Neurotoxicity Scale (OSNS) and additional clinical/patient-reported neurotoxicity measures. A population pharmacokinetic approach was used to determine oxaliplatin and 5-fluorouracil pharmacokinetics with and without ibudilast.

RESULTS

Sixteen participants consented; 14 completed both chemotherapy cycles. Across all measures, the majority of participants experienced either an improvement or no worsening of neurotoxicity with ibudilast treatment. Based on OSNS assessments, acute neurotoxicity was unchanged in 12/14 participants and improved in 2/14 participants. The 90% confidence interval (CI) of the dose-normalised ratio of oxaliplatin AUC (90% CI 95.0-109%) and 5-fluorouracil AUC (90% CI 66.5-173%) indicated no significant impact of ibudilast on systemic exposure.

CONCLUSION

This pilot study indicated ibudilast co-administration may improve or stabilise oxaliplatin-induced neurotoxicity. Given the expected worsening of symptoms in patients with continued chemotherapy, this represents a signal of effect that warrants further investigation. Pharmacokinetic analysis indicates ibudilast has no significant effect on oxaliplatin pharmacokinetics, and is unlikely to influence pharmacokinetics of 5-fluorouracil.

CLINICAL TRIAL REGISTRATION

Trial registration number: UTN U1111-1209-0075 and ANZCTRN12618000232235 (registered 13/02/2018).

Persistent pain induces mood problems and memory loss by the involvement of cytokines, growth factors, and supraspinal glial cells

Lesions of peripheral nerves lead to pain, hyperalgesia, and psychological comorbidities. However, the relationship between mood disorders and neuropathic pain is unclear, as well as the underlying mechanisms related to these disorders. Therefore, we investigated if nerve injury induces depression, anxiety, and cognitive impairment and if there were changes in cytokines, growth factors, and glial cell activation in cortical sites involved in processing pain and mood in animals with nerve injury. Nerve injury was induced by partial sciatic nerve ligation (PSNL) in male Swiss mice and compared to sham-operated animals. Nociceptive behavioral tests to mechanical and thermal (heat and cold) stimuli confirmed the development of hyperalgesia. We further examined mood disorders and memory behaviors. We show nerve injury induces a decrease in mechanical withdrawal thresholds and thermal latency to heat and cold. We also show that nerve injury causes depressive-like and anxiety-like behaviors as well as impairment in short-term memory in mice. There were increases in proinflammatory cytokines as well as Brain-Derived Neurotrophic Factor (BDNF) in the injured nerve. In the spinal cord, there were increases in both pro and anti-inflammatory cytokines, as well as of BDNF and Nerve Growth Factor (NGF). Further, in our data was a decrease in the density of microglia and astrocytes in the hippocampus and increased microglial density in the prefrontal cortex, areas associated with neuropathic pain conditions.

Novel immune biomarkers in complex regional pain syndrome

We investigated serum levels of 29 cytokines and immune-activated kynurenine and tetrahydrobiopterin pathway metabolites in 15 complex regional pain syndrome (CRPS) subjects and 14 healthy controls. Significant reductions in interleukin-37 and tryptophan were found in CRPS subjects, along with positive correlations between kynurenine/tryptophan ratio and TNF-α levels with kinesiophobia, tetrahydrobiopterin levels with McGill pain score, sRAGE, and xanthurenic acid and neopterin levels with depression, anxiety and stress scores. Using machine learning, we identified a set of binary variables, including IL-37 and GM-CSF, capable of distinguishing controls from established CRPS subjects. These results suggest possible involvement of various inflammatory markers in CRPS pathogenesis.

The Medial Prefrontal Cortex as a Central Hub for Mental Comorbidities Associated with Chronic Pain

Chronic pain patients frequently develop and suffer from mental comorbidities such as depressive mood, impaired cognition, and other significant constraints of daily life, which can only insufficiently be overcome by medication. The emotional and cognitive components of pain are processed by the medial prefrontal cortex, which comprises the anterior cingulate cortex, the prelimbic, and the infralimbic cortex. All three subregions are significantly affected by chronic pain: magnetic resonance imaging has revealed gray matter loss in all these areas in chronic pain conditions. While the anterior cingulate cortex appears hyperactive, prelimbic, and infralimbic regions show reduced activity. The medial prefrontal cortex receives ascending, nociceptive input, but also exerts important top-down control of pain sensation: its projections are the main cortical input of the periaqueductal gray, which is part of the descending inhibitory pain control system at the spinal level. A multitude of neurotransmitter systems contributes to the fine-tuning of the local circuitry, of which cholinergic and GABAergic signaling are particularly emerging as relevant components of affective pain processing within the prefrontal cortex. Accordingly, factors such as distraction, positive mood, and anticipation of pain relief such as placebo can ameliorate pain by affecting mPFC function, making this cortical area a promising target region for medical as well as psychosocial interventions for pain therapy.

The analgesic action of larixyl acetate, a potent TRPC6 inhibitor, in rat neuropathic pain model induced by spared nerve injury

Background

Neuropathic pain is a debilitating status that is insusceptible to the existing analgesics. It is important to explore the underlying pathophysiological changes and search for new pharmacological approaches. Transient receptor potential canonical 6 (TRPC6) is a mechanosensitive channel that is expressed by dorsal root ganglia and glial cells. It has been demonstrated that this channel in dorsal root ganglia plays essential roles in the formation of mechanical hyperalgesia in neuropathic pain. Recent pharmacological screening suggests that larixyl acetate (LA), a main constituent of larch resin, is able to selectively inhibit TRPC6 function. But whether LA is effective in treating neuropathic pain remains unknown. We investigated the efficacy of LA in rat neuropathic pain model, examined its effects on central neuroinflammation, and explored the possible molecular mechanisms by targeting the spinal dorsal horn.

Methods

Spared nerve injury (SNI) was conducted in Sprague-Dawley rats. Mechanical hypersensitivity and cold allodynia before and after single and multiple i.t. applications of LA at the dose of 3, 10, and 30 μM were evaluated by von Frey filament and acetone tests, respectively. Western blot, immunohistochemical, and immunocytochemical stainings were employed to examine the level and expression feature of ionized calcium-binding adaptor molecule 1 (Iba-1), glial fibrillary acidic protein (GFAP), TRPC6, and phosphorylated p38 kinase. The changes of cytokine concentrations, including that of TNF-α, IL-1β, IL-6, and IL-10, were also assessed by multiplex analysis. TRPC6 antisense strategy was finally adopted to investigate the action mechanisms of LA.

Results

Single application of LA on day 5 post injury caused dose-dependent inhibition of mechanical allodynia with the ED₅₀ value of 13.43 μM. Multiple applications of LA at 30 μM not only enhanced the analgesic efficacy but also elongated the effective duration without obvious influences on animal locomotor activities. Single and multiple administrations of LA at 30 μM played similar but weaker inhibitory effects on cold allodynia. In addition to behavioral improvements, multiple applications of LA for 6 days dose-dependently inhibited the upregulation of Iba-1, TNF-α, IL-1β, and IL-6, whereas had no obvious effects on the levels of GFAP and IL-10. Combined Western blot and immunostaining assays revealed that the expression of TRPC6 was significantly increased in both spinal dorsal horn after nerve injury and the cultured microglia challenged by LPS, which was however suppressed by the addition of LA at 30 μM or 10 μM, respectively. Further knockdown of TRPC6 with antisense oligodeoxynucleotide produced prominent analgesic effects in rats with SNI, accompanied by the reduced phosphorylation level of p38 in the microglia.

Conclusions

These data demonstrate that i.t. applied LA exhibits analgesic and anti-inflammatory action in neuropathic pain. The action of LA involves the suppression of TRPC6 and p38 signaling in the microglia. LA may be thus a promising pharmacological candidate for the treatment of intractable chronic pain.

CD3+ T cells are critical for the resolution of comorbid inflammatory pain and depression-like behavior

•

T cells are necessary for resolution of CFA-induced mechanical allodynia and spontaneous pain.

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T cells are required for the resolution of inflammation-induced depression-like behavior.

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T cells did not contribute to onset or severity of indicators of pain and depression-like behavior.

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T cells did not affect cytokine expression in the paw, spinal cord and brain.

Background

Chronic pain and depression often co-occur. The mechanisms underlying this comorbidity are incompletely understood. Here, we investigated the role of CD3⁺ T cells in an inflammatory model of comorbid persistent mechanical allodynia, spontaneous pain, and depression-like behavior in mice.

Methods

C57Bl/6 wt and Rag2^−/− mice were compared in their response to intraplantar administration of complete Freund’s adjuvant (CFA). Mechanical allodynia, spontaneous pain and depression-like behavior were assessed by von Frey, conditioned place preference and forced swim test respectively.

Results

Resolution of mechanical allodynia, spontaneous pain, and depression-like behavior was markedly delayed in Rag2^−/− mice that are devoid of adaptive immune cells. Reconstitution of Rag2^−/− mice with CD3⁺ T cells from WT mice before CFA injection normalized the resolution of indicators of pain and depression-like behavior. T cells did not contribute to onset or severity of indicators of pain and depression-like behavior. The lack of T cells did not affect cytokine expression in the paw, spinal cord and brain, indicating that the delayed resolution was not resulting from prolonged (neuro)inflammation.

Conclusions

Our findings show that T cells are critical for the natural resolution of mechanical allodynia, spontaneous pain, and depression-like behavior after an inflammatory challenge. Dysregulation of this T cell-mediated resolution pathway could contribute to the comorbidity of chronic pain and depression.

Significance

Chronic pain and depression are frequently associated with signs of inflammation. However, general immunosuppression is not sufficient to resolve comorbid pain and depression. Here we demonstrate that T cells are required for resolution of comorbid persistent mechanical allodynia, spontaneous pain, and depression in a model of peripheral inflammation, indicating the immune system can contribute to both onset and resolution of these comorbidities. Enhancing pro-resolution effects of T cells may have a major impact to treat patients with comorbid persistent pain and depression.

A Systematic Review of Efficacy, Safety, and Tolerability of Duloxetine

Duloxetine is a serotonin-norepinephrine reuptake inhibitor approved for the treatment of patients affected by major depressive disorder (MDD), generalized anxiety disorder (GAD), neuropathic pain (NP), fibromyalgia (FMS), and stress incontinence urinary (SUI). These conditions share parallel pathophysiological pathways, and duloxetine treatment might be an effective and safe alternative. Thus, a systematic review was conducted following the 2009 Preferred Reporting Items (PRISMA) recommendations and Joanna Briggs Institute Critical (JBI) Appraisals guidelines. Eighty-five studies focused on efficacy, safety, and tolerability of duloxetine were included in our systematic review. Studies were subdivided by clinical condition and evaluated individually. Thus, 32 studies of MDD, 11 studies of GAD, 19 studies of NP, 9 studies of FMS, and 14 studies of SUI demonstrated that the measured outcomes indicate the suitability of duloxetine in the treatment of these clinical conditions. This systematic review confirms that the dual mechanism of duloxetine benefits the treatment of comorbid clinical conditions, and supports the efficacy, safety, and tolerability of duloxetine in short- and long-term treatments.

Reduced Glutamate in the Medial Prefrontal Cortex Is Associated With Emotional and Cognitive Dysregulation in People With Chronic Pain

A decrease in glutamate in the medial prefrontal cortex (mPFC) has been extensively found in animal models of chronic pain. Given that the mPFC is implicated in emotional appraisal, cognition and extinction of fear, could a potential decrease in glutamate be associated with increased pessimistic thinking, fear and worry symptoms commonly found in people with chronic pain? To clarify this question, 19 chronic pain subjects and 19 age- and gender-matched control subjects without pain underwent magnetic resonance spectroscopy. Both groups also completed the Temperament and Character, the Beck Depression and the State Anxiety Inventories to measure levels of harm avoidance, depression, and anxiety, respectively. People with chronic pain had significantly higher scores in harm avoidance, depression and anxiety compared to control subjects without pain. High levels of harm avoidance are characterized by excessive worry, pessimism, fear, doubt and fatigue. Individuals with chronic pain showed a significant decrease in mPFC glutamate levels compared to control subjects without pain. In people with chronic pain mPFC glutamate levels were significantly negatively correlated with harm avoidance scores. This means that the lower the concentration of glutamate in the mPFC, the greater the total scores of harm avoidance. High scores are associated with fearfulness, pessimism, and fatigue-proneness. We suggest that chronic pain, particularly the stress-induced release of glucocorticoids, induces changes in glutamate transmission in the mPFC, thereby influencing cognitive, and emotional processing. Thus, in people with chronic pain, regulation of fear, worry, negative thinking and fatigue is impaired.

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

Chronic pain, including inflammatory, neuropathic pain, is a serious clinical issue. There are increasing numbers of patients with chronic pain due to the growing number of elderly and it is estimated that about 25% of the global population will develop chronic pain. Chronic pain patients are refractory to medications used to treat acute pain such as opioids and non-steroidal anti-inflammatory drugs. Furthermore, the complexity and diversity of chronic pain mechanisms hinder the development of new analgesics. Thus, a better understanding of the mechanism of chronic pain is needed, which would facilitate the development of novel analgesics based on novel mechanisms. With this goal, connexins (Cxs) could be targeted for the development of new analgesics. Connexins are proteins with 20 subtypes, and function as channels, gap junctions between cells, and hemichannels that sample the extracellular space and release molecules such as neurotransmitters. Furthermore, Cxs could have functions independent of channel activity. Recent studies have shown that Cxs could be crucial in the induction and maintenance of chronic pain, and modulation of the activity or the expression of Cxs ameliorates nociceptive hypersensitivity in multiple chronic pain models. This review will cite novel findings on the role of of Cxs in the nociceptive transduction pathway under the chronic pain state and antinociceptive effects of various molecules modulating activity or expression of Cxs. Also, the potential of Cx modulation as a therapeutic strategy for intractable chronic pain will be discussed.

Environmental enrichment improves pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain: role of NPAS4

Patients suffering from neuropathic pain have a higher incidence of depression and cognitive decline. Although environment enrichment (EE) may be effective in the treatment of neuropathic pain, the precise mechanisms underlying its actions remain determined. The aim of the study was to examine the molecular mechanisms underlying the EE's beneficial effects in mice with neuropathic pain. EE attenuated the pain threshold reduction, depression-like phenotype, and memory deficit in mice after chronic constriction injury (CCI). Furthermore, EE attenuated decreased neurogenesis and increased inflammation in the hippocampus of mice with neuropathic pain after CCI. Moreover, the suppression of adult hippocampal neurogenesis by temozolomide antagonized the beneficial effects of EE on depression-like phenotype and cognitive deficit in the mice with neuropathic pain. In addition, lipopolysaccharide-induced increase in tumor necrosis factor-α (TNF-α) in the hippocampus antagonized the beneficial effects of EE for these behavioral abnormalities in mice with neuropathic pain. Knock-down of NPAS4 (neuronal PAS domain protein 4) in the hippocampus by lentivirus targeting NPAS4 blocked these beneficial effects of EE in the mice with neuropathic pain. These all findings suggest that hippocampal NPAS4 plays a key role in the beneficial effects of EE on the pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain. Therefore, it is likely that NPAS4 would be a new therapeutic target for perceptional, affective, and cognitive dimensions in patients with chronic pain.

Characterizing the Structural Pattern Predicting Medication Response in Herpes Zoster Patients Using Multivoxel Pattern Analysis

Herpes zoster (HZ) can cause a blistering skin rash with severe neuropathic pain. Pharmacotherapy is the most common treatment for HZ patients. However, most patients are usually the elderly or those that are immunocompromised, and thus often suffer from side effects or easily get intractable post-herpetic neuralgia (PHN) if medication fails. It is challenging for clinicians to tailor treatment to patients, due to the lack of prognosis information on the neurological pathogenesis that underlies HZ. In the current study, we aimed at characterizing the brain structural pattern of HZ before treatment with medication that could help predict medication responses. High-resolution structural magnetic resonance imaging (MRI) scans of 14 right-handed HZ patients (aged 61.0 ± 7.0, 8 males) with poor response and 15 (aged 62.6 ± 8.3, 5 males) age- (p = 0.58), gender-matched (p = 0.20) patients responding well, were acquired and analyzed. Multivoxel pattern analysis (MVPA) with a searchlight algorithm and support vector machine (SVM), was applied to identify the spatial pattern of the gray matter (GM) volume, with high predicting accuracy. The predictive regions, with an accuracy higher than 79%, were located within the cerebellum, posterior insular cortex (pIC), middle and orbital frontal lobes (mFC and OFC), anterior and middle cingulum (ACC and MCC), precuneus (PCu) and cuneus. Among these regions, mFC, pIC and MCC displayed significant increases of GM volumes in patients with poor response, compared to those with a good response. The combination of sMRI and MVPA might be a useful tool to explore the neuroanatomical imaging biomarkers of HZ-related pain associated with medication responses.

Expansion and activation of distinct central memory T lymphocyte subsets in complex regional pain syndrome

BACKGROUND

Complex regional pain syndrome (CRPS) is a debilitating condition where trauma to a limb results in devastating persistent pain that is disproportionate to the initial injury. The pathophysiology of CRPS remains unknown; however, accumulating evidence suggests it is an immunoneurological disorder, especially in light of evidence of auto-antibodies in ~ 30% of patients. Despite this, a systematic assessment of all circulating leukocyte populations in CRPS has never been performed.

METHODS

We characterised 14 participants as meeting the Budapest clinical criteria for CRPS and assessed their pain ratings and psychological state using a series of questionnaires. Next, we performed immunophenotyping on blood samples from the 14 CRPS participants as well as 14 healthy pain-free controls using mass cytometry. Using a panel of 38 phenotypic and activation markers, we characterised the numbers and intracellular activation status of all major leukocyte populations using manual gating strategies and unsupervised cluster analysis.

RESULTS

We have shown expansion and activation of several distinct populations of central memory T lymphocytes in CRPS. The number of central memory CD8+ T cells was increased 2.15-fold; furthermore, this cell group had increased phosphorylation of NFkB and STAT1 compared to controls. Regarding central memory CD4+ T lymphocytes, the number of Th1 and Treg cells was increased 4.98-fold and 2.18-fold respectively, with increased phosphorylation of NFkB in both populations. We also found decreased numbers of CD1c+ myeloid dendritic cells, although with increased p38 phosphorylation. These changes could indicate dendritic cell tissue trafficking, as well as their involvement in lymphocyte activation.

CONCLUSIONS

These findings represent the first mass cytometry immunophenotyping study in any chronic pain state and provide preliminary evidence of an antigen-mediated T lymphocyte response in CRPS. In particular, the presence of increased numbers of long-lived central memory CD4+ and CD8+ T lymphocytes with increased activation of pro-inflammatory signalling pathways may indicate ongoing inflammation and cellular damage in CRPS.

Suberoylanilide Hydroxamic Acid Triggers Autophagy by Influencing the mTOR Pathway in the Spinal Dorsal Horn in a Rat Neuropathic Pain Model

Histone acetylation levels can be upregulated by treating cells with histone deacetylase inhibitors (HDACIs), which can induce autophagy. Autophagy flux in the spinal cord of rats following the left fifth lumber spinal nerve ligation (SNL) is involved in the progression of neuropathic pain. Suberoylanilide hydroxamic acid (SAHA), one of the HDACIs can interfere with the epigenetic process of histone acetylation, which has been shown to ease neuropathic pain. Recent research suggest that SAHA can stimulate autophagy via the mammalian target of rapamycin (mTOR) pathway in some types of cancer cells. However, little is known about the role of SAHA and autophagy in neuropathic pain after nerve injury. In the present study, we aim to investigate autophagy flux and the role of the mTOR pathway on spinal cells autophagy activation in neuropathic pain induced by SNL in rats that received SAHA treatment. Autophagy-related proteins and mTOR or its active form were assessed by using western blot, immunohistochemistry, double immunofluorescence staining and transmission electron microscopy (TEM). We found that SAHA decreased the paw mechanical withdrawal threshold (PMWT) of the lower compared with SNL. Autophagy flux was mainly disrupted in the astrocytes and neuronal cells of the spinal cord dorsal horn on postsurgical day 28 and was reversed by daily intrathecal injection of SAHA (n = 100 nmol/day or n = 200 nmol/day). SAHA also decreased mTOR and phosphorylated mTOR (p-mTOR) expression, especially p-mTOR expression in astrocytes and neuronal cells of the spinal dorsal horn. These results suggest that SAHA attenuates neuropathic pain and contributes to autophagy flux in astrocytes and neuronal cells of the spinal dorsal horn via the mTOR signaling pathway.

Neuropathic-Like Ocular Pain and Nonocular Comorbidities Correlate With Dry Eye Symptoms

OBJECTIVE

To evaluate the association between dry eye (DE) symptoms and neuropathic-like ocular pain (NOP) features, chronic pain conditions, depression, and anxiety in patients presenting for routine ophthalmic examinations.

METHODS

Two hundred thirty-three consecutive patients ≥18 years of age presenting to a comprehensive eye clinic between January and August 2016 were included in this study. Information on demographics, chronic pain conditions, medication use, DE symptoms (dry eye questionnaire, DEQ5), NOP complaints (burning; wind, light, and temperature sensitivity), depression, and anxiety indices (patient health questionnaire 9, PHQ-9 and symptom checklist 90-revised, SCL-90-R) were collected for each individual. Pearson correlation was used to evaluate strengths of association. Logistic regression analysis examined risk factors for any (DEQ5≥6) and severe (DEQ5≥12) DE symptoms.

RESULTS

The mean age of the population was 46.3 years (±13.0); 67.8% (n=158) were female. Per the DEQ5, 40.3% (n=94) had mild or greater DE symptoms and 12% (n=24) had severe symptoms. Severity of DE symptoms correlated with NOP complaints: burning (Pearson r=0.37, P<0.001); sensitivity to wind (r=0.37, P<0.001), sensitivity to light (r=0.34, P<0.001), and sensitivity to temperature (r=0.30, P<0.001). Sex, race, and ethnicity were not significant risk factors for DE symptoms. Risk factors for mild or greater DE symptoms included a greater number of chronic nonocular pain conditions (odds ratio [OR]=1.38, P<0.001), arthritic pain (OR=6.34, P<0.001), back pain (OR=2.47, P=0.004), headaches (OR=2.14, P=0.02), depression (OR=1.17, P<0.001), and anxiety (OR=1.13, P=0.02).

CONCLUSION

Dry eye severity positively associated with NOP complaints, comorbid chronic pain conditions, and symptoms of depression and anxiety.