How to study anxiety and depression in rodent models of chronic pain?

Hindbrain networks: Exploring the hidden anxiety circuits in rodents

Anxiety disorders are multifaceted conditions that engage numerous brain regions and circuits. While the hindbrain is pivotal in fundamental biological functions, its role in modulating emotions has been underappreciated. This review will uncover critical targets and circuits within the hindbrain that are essential for both anxiety and anxiolytic effects, expanding on research obtained through behavioral tests. The bidirectional neural pathways between the hindbrain and other brain regions, with a spotlight on vagal afferent signaling, provide a crucial framework for unraveling the neural mechanisms underlying anxiety. Exploring neural circuits within the hindbrain can help to unravel the neurobiological mechanisms of anxiety and elucidate differences in the expression of these circuits between genders, thereby providing valuable insights for the development of future anxiolytic drugs.

Epidural injection of varying doses of capsaicin alleviates inflammatory pain in rats via the TLR4/AKT/NF-κB pathway

BACKGROUND

Capsaicin (CAP) induces transient pain sensation by activating transient receptor potential vanilloid-1 (TRPV1). However, the initial neuronal excitation induced by CAP is followed by a prolonged refractory period, resulting in long-lasting analgesia. Although the effects of CAP on microglia in the dorsal root ganglion of neuropathic pain disorders have been reported, the regulatory pathways of CAP on microglia remain poorly defined.

METHODS

A chronic pain model was established via plantar injection of complete Freund's adjuvant (CFA), and different doses of CAP were administered to rats. Pain behavior, expression of pain-related factors, protein expression of TRPV1 in nerve cells, and the inflammatory activation of microglia were evaluated. In vitro experiments were conducted to explore the activation and migration ability of microglia, expression of inflammatory cytokines and pathway proteins, TRPV1 expression in nerve cells, and intracellular calcium concentration under different doses of CAP.

RESULTS

Different doses of CAP alleviated chronic pain in rats, reduced TRPV1 expression in nerve cells, and inhibited the activation of microglia; however, high doses of CAP were particularly effective in improving chronic pain. In vitro experiments confirmed that CAP reduces the secretion of inflammatory cytokines by microglia via inhibition of the TLR4/AKT/NF-κB signaling pathway. This mechanism reduced the injury and apoptosis of nerve cells, the expression of TRPV1, and the influx of calcium ions in nerve cells.

CONCLUSIONS

CAP reduced inflammatory responses in microglia in a dose-dependent manner by inhibiting the TLR4/AKT/NF-κB signaling pathway, which consequently reduced TRPV1 expression on neuronal cells and reduced chronic pain.

Neuromolecular and behavioral effects of cannabidiol on depressive-associated behaviors and neuropathic pain conditions in mice

BACKGROUND AND AIMS

Neuropathic pain (NP) has a high incidence in the general population, is closely related to anxiety disorders, and has a negative impact on the quality of life. Cannabidiol (CBD), as a natural product, has been extensively studied for its potential therapeutic effects on symptoms such as pain and depression (DP). However, the mechanism of CBD in improving NP with depression is not fully understood.

METHODS

First, we used bioinformatics tools to deeply mine the intersection genes associated with NP, DP, and CBD. Secondly, the core targets were screened by Protein-protein interaction network, Gene Ontology, Kyoto Encyclopedia of Genes and Genomes analysis, molecular docking and molecular dynamics simulation. Next, the effects of CBD intervention on pain and depressive behaviors in the spinal nerve ligation (SNL) mouse model were evaluated using behavioral tests, and dose-response curves were plotted. After the optimal intervention dose was determined, the core targets were verified by Western blot (WB) and Quantitative Polymerase Chain Reaction (qPCR). Finally, we investigated the potential mechanism of CBD by Nissl staining, Immunofluorescence (IF) and Transmission Electron Microscopy (TEM).

RESULTS

A total of five core genes of CBD most associated with NP and DP were screened by bioinformatics analysis, including PTGS2, GPR55, SOD1, CYP1A2 and NQO1. Behavioral test results showed that CBD by intraperitoneal administration 5 mg/kg can significantly improve the pain behavior and depressive state of SNL mice. WB, qPCR, IF, and TEM experiments further confirmed the regulatory effects of CBD on key molecules.

CONCLUSION

In this study, we found five targets of CBD in the treatment of NP with DP. These findings provide further theoretical and experimental basis for CBD as a potential therapeutic agent.

Anxiogenic-like effects of coumarin, possibly through the GABAkine interaction pathway: Animal studies with in silico approaches

BACKGROUND

Anxiety disorder is the most common mental illness and a major contributor to impairment. Thus, there is an urgent need to find novel lead compounds to mitigate anxiety. It is widely recognized that the neurobiology of anxiety-related behavior involves GABAergic systems.

OBJECTIVES

This research aimed to examine the anxiogenic action of coumarin (CMN), a natural benzopyrone derived from plants, and determine its underlying mechanism through in vivo and in silico investigations.

METHODS

This was accomplished by using a variety of behavioral procedures, including open field, swing, hole cross, and light-dark tests, on male and female Swiss albino mice that had been orally administered three experimental doses of CMN (1, 2, and 4 mg/kg). The CMN group was also examined with the GABAA receptor agonist diazepam (DZP, 2 mg/kg) and flumazenil antagonist (FLU, 0.1 mg/kg). Furthermore, CMN and standards were subjected to a molecular docking analysis to determine their binding affinities for the GABAA receptor subunits (α1, α4, β2, γ2, and δ). Several software programs were used to visualize the ligand-receptor interaction and analyze the pharmacokinetic profile.

RESULTS

Compared to typical treatments, our results show that CMN (1 mg/kg) significantly (p < 0.05) increases the locomotor activity of animals. Furthermore, CMN exerted the highest binding affinity (-6.5 kcal/mol) with the GABA-α1 receptor compared to conventional DZP. Along with FLU, CMN displayed several hydrophobic and hydrogen bonds with GABAA receptor subunits. The pharmacokinetic and drug-like properties of CMN are also remarkable. In animal studies, CMN worked synergistically with FLU to provide anxiogenic-like effects.

CONCLUSION

We conclude that, based on in vivo and in silico data, CMN, alone or in combination with FLU, may be employed in future neurological clinical studies. However, further research is needed to confirm this behavioral activity and elucidate the possible mechanism of action.

Induction of orofacial pain potentiates fibromyalgia symptoms in mice: Relevance of nociceptin system

AIMS

Fibromyalgia patients might experience temporomandibular disorder (TMD) as a comorbidity. However, the connection between these two syndromes is not fully understood. Nociceptin (N/OFQ) and NOP receptors are implicated in both conditions, but their relevance in the comorbidity needs investigation. This study featured a comorbidity model of fibromyalgia plus TMD in mice, attempting to evaluate the significance of the N/OFQ-NOP receptor in this paradigm.

MATERIALS AND METHODS

Female CF-1 mice were submitted to the fibromyalgia model induced by three daily consecutive injections of reserpine (0.25 mg/kg) and received an intra-masseter injection of complete Freund's adjuvant (CFA; 10 μl; diluted 1:1) on day four.

KEY FINDINGS

There was a rise in nocifensive and depression-like behaviors in the comorbidity group, as evaluated by the Grimace scores and the tail suspension test (TST). This group displayed anxiogenic-like effects in the hole board and the elevated plus maze tests. The comorbidity group showed an increment of c-Fos immunopositivity in the ipsilateral side of CFA injection, in the trigeminal ganglion (TG) and thalamus. The administration of N/OFQ (1 nmol/kg, i.p.) boosted the Grimace scores in the comorbidity group, with no effect for the NOP receptor antagonist UFP-101 (1 nmol/kg, i.p.). Either NOP ligand failed to alter depression or anxiety behavioral changes. Alternatively, pregabalin (30 mg/kg; i.p.) reduced the nociceptive responses and the number of head dips in the hole board.

SIGNIFICANCE

Data reveal new evidence suggesting that inducing TMD with CFA may worsen fibromyalgia symptoms in reserpine-treated mice, an effect partially regulated by systemic N/OFQ.

A Systematic Review and Meta-Analysis of Anxiety- and Depressive-Like Behaviors in Rodent Models of Neuropathic Pain

Background

Epidemiological studies have frequently shown the concurrence of chronic pain with symptoms of anxiety and depression, particularly in women. Animal models are useful to understand the complex mechanisms underlying comorbidities, but the wide range of methods employed and the wealth of evidence sometimes impedes effective translation and reproducibility. In this systematic review and meta-analysis, we aimed to synthesize the evidence regarding the influence of variables such as sex and species on anxiety- and depressive-like behaviors in rodent models of neuropathic pain.

Methods

Following PROSPERO registration, we searched EMBASE, Scopus, and the Web of Science from their inception to November 24, 2023, identifying 126 studies that met the inclusion criteria. The Hedges' g value for each experiment and study was calculated, and further subgroup and meta-regression analyses were performed.

Results

Neuropathic pain significantly reduced the time that rats and mice spent in the open arms of the elevated plus and zero mazes (g = -1.14), time spent in the center of the open field (g = -1.12), sucrose consumption in the sucrose preference test (g = -1.43), and grooming time in the splash test (g = -1.37) while increasing latency to feed in the novelty-suppressed feeding test (g = 1.59) and immobility in the forced swimming (g = 1.85) and tail suspension (g = 1.91) tests. Sex differences were observed, with weaker effects in female than in male rodents for several behavioral paradigms, and funnel plots identified positive publication bias in the literature.

Conclusions

This meta-analysis emphasizes the effect of neuropathic pain on anxiety- and depressive-like behaviors in rodents, highlighting the importance of investigating sex differences in future experimental studies.

Causal impacts of smoking on pain conditions and the mediating pathways: a mendelian randomization study

Smoking is a risk factor for multiple diseases. We performed mendelian randomization (MR) analyses to investigate the causal association of smoking initiation on pain conditions and the potential mediating pathways. Genetic associated with smoking initiation at the genome-wide significance level were selected as instrumental variables. Genetic associations with 10 pain conditions were derived from the FinnGen and UK Biobank study. Multivariable MR analysis was conducted to explore the mediation effects of depression, insomnia and sedentary behavior. A series of sensitivity analyses were conducted to assess the stability of our research findings. Genetic liability to smoking initiation was associated with an increased risk of angina pectoris, dorsalgia, low back pain, pain in limb, pain in joint, pain in thoracic spine and sciatica in both FinnGen and UK Biobank study. These causal associations were largely mediated by major depression (2.9- 39.5%), sedentary behavior (13.0- 31.2%), insomnia (10.3- 33.1%) and combination of all three mediators (30.2- 65.3%). The effects of smoking on outcomes were partly attenuated after adjusting for depression, sedentary behavior and insomnia respectively, and the direct effect of smoking initiation on pain was diminished toward null after adjusting for combined three mediators. These results were robust to sensitivity analyses. Our findings illustrated the causal effect of smoking and a broad range of pain conditions, and major depression, sedentary behavior and insomnia mediate many of these associations.

Neuroinflammation in osteoarthritis: From pain to mood disorders

Osteoarthritis (OA) is the most common form of musculoskeletal disease, and its prevalence is increasing due to the aging of the population. Chronic pain is the most burdensome symptom of OA that significantly lowers patients' quality of life, also due to its frequent association with emotional comorbidities, such as anxiety and depression. In recent years, both chronic pain and mood alterations have been linked to the development of neuroinflammation in the peripheral nervous system, spinal cord and supraspinal brain areas. Thus, mechanisms at the basis of the development of the neuroinflammatory process may indicate promising targets for novel treatment for pain and affective comorbidities that accompany OA. In order to assess the key role of neuroinflammation in the maintenance of chronic pain and its potential involvement in development of psychiatric components, the monoiodoacetate (MIA) model of OA in rodents has been used and validated. In the present commentary article, we aim to summarize up-to-date results achieved in this experimental model of OA, focusing on glia activation and cytokine production in the sciatic nerve, dorsal root ganglia (DRGs), spinal cord and brain areas. The association of a neuroinflammatory state with the development of pain and anxiety- and depression-like behaviors are discussed. Results suggest that cells and molecules involved in neuroinflammation may represent novel targets for innovative pharmacological treatments of OA pain and mood comorbidities.

Neurobehavioral Impairments Predict Specific Cerebral Damage in Rat Model of Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a severe form of stroke that can cause unpredictable and diffuse cerebral damage, which is difficult to detect until it becomes irreversible. Therefore, there is a need for a reliable method to identify dysfunctional regions and initiate treatment before permanent damage occurs. Neurobehavioral assessments have been suggested as a possible tool to detect and approximately localize dysfunctional cerebral regions. In this study, we hypothesized that a neurobehavioral assessment battery could be a sensitive and specific method for detecting damage in discrete cerebral regions following SAH. To test this hypothesis, a behavioral battery was employed at multiple time points after SAH induced via an endovascular perforation, and brain damage was confirmed via postmortem histopathological analysis. Our results demonstrate that impairment of sensorimotor function accurately predict damage in the cerebral cortex (AUC 0.905; sensitivity 81.8%; specificity 90.9%) and striatum (AUC 0.913; sensitivity 90.1%; specificity 100%), while impaired novel object recognition is a more accurate indicator of damage to the hippocampus (AUC 0.902; sensitivity 74.1%; specificity 83.3%) than impaired reference memory (AUC 0.746; sensitivity 72.2%; specificity 58.0%). Tests for anxiety-like and depression-like behaviors predict damage to the amygdala (AUC 0.900; sensitivity 77.0%; specificity 81.7%) and thalamus (AUC 0.963; sensitivity 86.3%; specificity 87.8%), respectively. This study suggests that recurring behavioral testing can accurately predict damage in specific brain regions, which could be developed into a clinical battery for early detection of SAH damage in humans, potentially improving early treatment and outcomes.

Gene discovery and biological insights into anxiety disorders from a large-scale multi-ancestry genome-wide association study

We leveraged information from more than 1.2 million participants, including 97,383 cases, to investigate the genetics of anxiety disorders across five continental groups. Through ancestry-specific and cross-ancestry genome-wide association studies, we identified 51 anxiety-associated loci, 39 of which were novel. In addition, polygenic risk scores derived from individuals of European descent were associated with anxiety in African, admixed American and East Asian groups. The heritability of anxiety was enriched for genes expressed in the limbic system, cerebral cortex, cerebellum, metencephalon, entorhinal cortex and brain stem. Transcriptome-wide and proteome-wide analyses highlighted 115 genes associated with anxiety through brain-specific and cross-tissue regulation. Anxiety also showed global and local genetic correlations with depression, schizophrenia and bipolar disorder and widespread pleiotropy with several physical health domains. Overall, this study expands our knowledge regarding the genetic risk and pathogenesis of anxiety disorders, highlighting the importance of investigating diverse populations and integrating multi-omics information.

Distinct roles of general anesthesia activated CeA neurons in acute versus late phase of neuropathic pain

A previous study discovered a distinct population of GABAergic neurons in the ce ntral a mygdala (CeA) that can be activated by g eneral a nesthesia (CeA GA ) and exert analgesic functions (Hua et al., 2020). To independently reproduce these prior findings and to investigate the electrophysiological properties of CeA GA neurons, we first used 1.2% isoflurane to induce c-Fos activation in the mouse brain and validated the Fos expression by RNAscope in situ hybridization. Indeed, isoflurane induced robust Fos expression in CeA and these Fos + CeA GA neurons are GABAergic neurons (Vgat + ). We next used Fos-TRAP2 method (different from the CANE method used in the prior study) to label CeA GA neurons (tdTomato + ). Our ex vivo electrophysiological recordings in brain slices revealed that compared to Fos-negative CeA neurons, CeA GA neurons had significantly higher excitability and exhibited distinct patterns of action potentials. Chemogenetic activation of Fos-TRAPed CeA GA neurons was effective at increasing pain thresholds in naïve mice and mice with early-phase neuropathic pain 2 weeks after spared nerve injury (SNI). However, the same chemogenetic activation of CeA GA neurons only had modest analgesia in the late phase of SNI at 8 weeks, although it was highly effective in reducing chronic pain-associated anxiety behaviors at this stage. We found that Fos-negative CeA neurons, but not CeA GA neurons, exhibited increased excitability in the late-phase of SNI, suggesting that chronic pain causes a shift in the relative activity of the CeA microcircuit. Interestingly, Fos-negative neurons exhibited much higher expression of K + -Cl - cotransporter-2 (KCC2), and KCC2 expression was downregulated in the CeA in the late-phase of neuropathic pain. These results support the idea that targeting CeA GA neurons may provide therapeutic benefits for pain relief and chronic pain-associated anxiety. Our findings also suggest distinct roles of CeA GA neurons in regulating physiological pain, acute pain, and chronic pain with a possible involvement of KCC2.

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.

Environmental complexity impacts anxiety in broiler chickens depending on genetic strain and body weight

The objective was to assess the impact of environmental complexity on affective state (anxiety) in fast- and slow-growing broilers (Gallus gallus domesticus) as they gain weight. Six hundred fast-growing broilers (Ross 708; "fast-growers") and 600 slow-growing broilers (Hubbard Redbro Mini; "slow-growers") were raised in 24 pens with simple (standard; SE) or complex (permanent and temporary enrichments; CE) environments. Six birds/pen underwent the attention bias test on day 23 (fast-growers only), 28-29, 35-36, 42-43, and 56-57 (slow-growers only), with individuals only tested once (n = 576). Proportion of birds feeding, time spent vigilant and latencies to eat and step were recorded. Greater vigilance and longer latencies indicate more anxiety. Slow-growers fed more (p = 0.001), were less vigilant (p = 0.003), and stepped sooner than fast-growers (p = 0.007). For both strains, likelihood of feeding was unrelated to weight in SE, but decreased with increasing weight in CE (p = 0.048). Birds in CE stepped sooner than birds in SE (p = 0.030). Vigilance increased with body weight (p = 0.024). These results indicate that affective state (anxiety) can change as birds gain weight, depending on environmental complexity and genetic strain. Overall, slow-growers showed reduced anxiety compared to fast-growers, across housing treatments or weights.

Hyperexcitation of the glutamatergic neurons in lateral hypothalamus induced by chronic pain contributes to depression-like behavior and learning and memory impairment in male mice

Chronic pain can induce mood disorders and cognitive dysfunctions, such as anxiety, depression, and learning and memory impairment in humans. However, the specific neural network involved in anxiety- and depression-like behaviors and learning and memory impairment caused by chronic pain remains poorly understood. In this study, behavioral test results showed that chronic pain induced anxiety- and depression-like behaviors, and learning and memory impairment in male mice. c-Fos immunofluorescence and fiber photometry recording showed that glutamatergic neurons in the LH of mice with chronic pain were selectively activated. Next, the glutamatergic neurons of LH in normal mice were activated using optogenetic and chemogenetic methods, which recapitulates some of the depressive-like behaviors, as well as memory impairment, but not anxiety-like behavior. Finally, inhibition of glutamatergic neurons in the LH of mice with chronic pain, effectively relieved anxiety- and depression-like behaviors and learning and memory impairment. Taken together, our findings suggest that hyperexcitation of glutamatergic neurons in the LH is involved in depression-like behavior and learning and memory impairment induced by chronic pain.

Impaired neuronal macroautophagy in the prelimbic cortex contributes to comorbid anxiety-like behaviors in rats with chronic neuropathic pain

A large proportion of patients with chronic pain experience co-morbid anxiety. The medial prefrontal cortex (mPFC) is proposed to underlie this comorbidity, but the molecular and neuronal mechanisms are not fully understood. Here, we reported that impaired neuronal macroautophagy in the prelimbic cortical (PrL) subregion of the mPFC paralleled the occurrence of anxiety-like behaviors in rats with chronic spared nerve injury (SNI). Intriguingly, such macroautophagy impairment was mainly observed in a FOS/c-Fos+ neuronal subpopulation in the PrL. Chemogenetic inactivation of this comorbid anxiety-related neuronal ensemble relieved pain-induced anxiety-like behaviors. Rescuing macroautophagy impairment in this neuronal ensemble relieved chronic pain-associated anxiety and mechanical allodynia and restored synaptic homeostasis at the molecular level. By contrast, artificial disruption of macroautophagy induced early-onset co-morbid anxiety in neuropathic rats, but not general anxiety in normal rats. Taken together, our work identifies causal linkage between PrL neuronal macroautophagy dysfunction and comorbid anxiety in neuropathic pain and provides novel insights into the role of PrL by differentiating its contribution in pain-induced comorbid anxiety from its modulation over general anxiety-like behaviors.Abbreviation: AAV: adeno-associated viruses; ACC: anterior cingulate cortex; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CNO: clozapine-N-oxide; CQ: chloroquine; DIA: data independent acquisition; DIO: double floxed inverse orf; DLG4/PSD-95: discs large MAGUK scaffold protein 4; Dox: doxycycline; GABA: γ-aminobutyric acid; GFP: green fluorescent protein; GO: gene ontology; Gi: inhibitory guanine nucleotide-binding proteins; HsCHRM4/M4D: human cholinergic receptor muscarinic 4; HsSYN: human synapsin; KEGG: Kyoto encyclopedia of genes and genomes; LAMP1: lysosomal-associated membrane protein 1; LC3-II: PE conjugated microtubule-associated protein 1 light chain3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mPFC: medial prefrontal cortex; P2A: 2A self-cleaving peptide; PPI: protein-protein interaction networks; PrL: prelimbic cortex; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; rtTA: reverse tetracycline-transactivator; SDS-PAGE: sodium dodecylsulfate-polyacrylamide gel electrophoresis; SHANK3: SH3 and multiple ankyrin repeat domains 3; SLC1A1/EAAC1: solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, systemXag), member 1; SNAP23: synaptosomal-associated protein 23; SNI:spared nerve injury; SQSTM1/p62: sequestosome 1; SYT3: synaptotagmin 3; TRE: tetracycline-responsive element; TRE3G: third-generation tetracycline-responsive element.

Inhibition of cortical synaptic transmission, behavioral nociceptive, and anxiodepressive-like responses by arecoline in adult mice

Areca nut, the seed of Areca catechu L., is one of the most widely consumed addictive substances in the world after nicotine, ethanol, and caffeine. The major effective constituent of A. catechu, arecoline, has been reported to affect the central nervous system. Less is known if it may affect pain and its related emotional responses. In this study, we found that oral application of arecoline alleviated the inflammatory pain and its induced anxiolytic and anti-depressive-like behavior. Arecoline also increased the mechanical nociceptive threshold and alleviated depression-like behavior in naïve mice. In the anterior cingulate cortex (ACC), which acts as a hinge of nociception and its related anxiety and depression, by using the multi-electrode field potential recording and whole-cell patch-clamp recording, we found that the evoked postsynaptic transmission in the ACC of adult mice has been inhibited by the application of arecoline. The muscarinic receptor is the major receptor of the arecoline in the ACC. Our results suggest that arecoline alleviates pain, anxiety, and depression-like behavior in both physiological and pathological conditions, and this new mechanism may help to treat patients with chronic pain and its related anxiety and disorder in the future.

Spared nerve injury decreases motivation in long-access homecage-based operant tasks in mice

ABSTRACT

Neuropathic pain causes both sensory and emotional maladaptation. Preclinical animal studies of neuropathic pain-induced negative affect could result in novel insights into the mechanisms of chronic pain. Modeling pain-induced negative affect, however, is variable across research groups and conditions. The same injury may or may not produce robust negative affective behavioral responses across different species, strains, and laboratories. Here, we sought to identify negative affective consequences of the spared nerve injury model on C57BL/6J male and female mice. We found no significant effect of spared nerve injury across a variety of approach-avoidance conflict, hedonic choice, and coping strategy assays. We hypothesized these inconsistencies may stem in part from the short test duration of these assays. To test this hypothesis, we used the homecage-based Feeding Experimentation Device version 3 to conduct 12-hour, overnight progressive ratio testing to determine whether mice with chronic spared nerve injury had decreased motivation to earn palatable food rewards. Our data demonstrate that despite equivalent task learning, spared nerve injury mice are less motivated to work for a sugar pellet than sham controls. Furthermore, when we normalized behavioral responses across all the behavioral assays we tested, we found that a combined normalized behavioral score is predictive of injury state and significantly correlates with mechanical thresholds. Together, these results suggest that homecage-based operant behaviors provide a useful platform for modeling nerve injury-induced negative affect and that valuable pain-related information can arise from agglomerative data analyses across behavioral assays-even when individual inferential statistics do not demonstrate significant mean differences.

Neuropathic pain, mood, and stress-related disorders: A literature review of comorbidity and co-pathogenesis

Neuropathic pain can be caused by multiple factors, and its prevalence can reach 10% of the global population. It is becoming increasingly evident that limited or short-lasting response to treatments for neuropathic pain is associated with psychological factors, which include psychiatric comorbidities known to affect quality of life. It is estimated that 60% of patients with neuropathic pain also experience depression, anxiety, and stress symptoms. Altered mood, including stress, can be a consequence of several painful conditions but can also favor pain chronicization when preexisting. Despite the apparent tight connection between clinical pain and mood/stress disorders, the exact physiological mechanisms remain unclear. This review aims to provide an overview of state-of-the-art research on the mechanisms of pain related to the pathophysiology of depression, anxiety, and stress disorders.

Eagle syndrome: A rare neuropathic disorder affecting head and neck

To investigate the clinical characteristics of Eagle syndrome (ES) and evaluate psychological distress of the patients. Ten cases of ES were enrolled, clinical characteristics and management were analyzed. Psychological disorders of the patients were assessed by the test of self-rating anxiety scale (SAS) and self-rating depression scale (SDS). There were 8 females and 2 males varying from 34 to 56 years with a mean age of 44.86 ± 8.38 years. The main complaints included foreign body sensation of pharynx, odynophagia, vertigo with turning of the head-neck, pain of anterolateral neck, and toothache. Three cases were right-side affected, 6 cases were left-sided and 1 case was bilateral. Radiographic examination showed the elongated styloid process of bilateral in all cases, however, hypertrophy, improper inclination, abnormal angulation of styloid process and more complete calcification of stylohoid ligament of the complained side were observed compared to the opposite side. Eight cases suffered from anxiety and/or depression. A surgical intervention was carried out on 6 patients to resect the elongated styloid process, the symptoms and mental distress disappeared after the operation and no recurrence was found in their follow-ups. Meticulous interrogation of illness history, proper examination, and radiological studies may be valuable in diagnostic confirmation of ES. It is the hyperostosis, abnormal angulation of the styloid process rather than the simple elongation which is more likely to be attributed to the development of ES. Psychological disorders in ES patients were observed in our study and should be paid more attention in the future research.

Prokineticin-2 Participates in Chronic Constriction Injury-Triggered Neuropathic Pain and Anxiety via Regulated by NF-κB in Nucleus Accumbens Shell in Rats

Neuropathic pain (NP) is an intractable pain that results from primary nervous system injury and dysfunction. Herein, we demonstrated in animal models that peripheral nerve injury induced enhanced pain perception and anxiety-like behaviors. According to previous reports, nucleus accumbens (NAc) shell is required for complete expression of neuropathic pain behaviors and mood alternations, we found the elevated mRNA and protein level of Prokineticin-2 (Prok2) in the NAc shell after Chronic Constriction Injury (CCI). Prok2 knockdown in the NAc shell reversed NP and anxiety-like behaviors in rats, indicating that Prok2 might play a fundamental role in NP and anxiety co-morbidity. CCI significantly enhanced Prok2 co-expression with NF-κB P-p65 in comparison with control animals. In addition to reversing the established nociceptive hypersensitivities and anxiety simultaneously, NAc microinjection of NF-κB siRNA or specific inhibitor PDTC reversed Prok2 upregulation. Besides, Prok2 was significantly decreased in vitro when co-transfected with si-NF-κB. Dual-Luciferase assay showed NF-κB directly activated Prok2 gene transcriptional activity. Overall, these findings provide new insights into the neurobiological mechanisms behind NP and comorbid anxiety. The NF-κB/Prok2 pathway could be a potential therapeutic target for NP and anxiety disorders.

[Management of chronic non-oncologic pain by multicomponent programs using non-pharmacologic therapies: A systematic review of the literature]

Chronic pain is a public health problem suffered by 20% of the world's population. Pharmacological approaches are insufficient, so a multi-therapeutic approach that also includes non-pharmacological therapies (psychological therapies, meditation, physical exercise, healthy habits, etc.) is proposed. The aim of this review was to review the existing scientific evidence on the effect of multicomponent programs with non-pharmacological therapies in people with chronic non-oncologic pain. To this end, a search for scientific articles was carried out in three databases (PubMed, Web of Science and PsycINFO) and 17 articles were selected, following the PRISMA recommendations. The patients who participated in these programs were mostly women, aged 18 to 80years, working or on sick leave due to pain, with secondary education or less and married. The most frequent pain was musculoskeletal, mainly low back pain. All the articles studied the effectiveness of two or more therapies, highlighting psychological therapies, physical exercise and education. Positive results were obtained in the reduction of different variables such as pain, pain catastrophizing, anxiety and depression, in addition to improving functionality and quality of life. It has also been shown that patients' prior expectations regarding the intervention influence its effectiveness. Although throughout the review there was great heterogeneity in the interventions, in the evaluation methods and in the results themselves, it can be concluded that multicomponent programs show positive results in the management of chronic pain, and should therefore be incorporated as a routine therapeutic treatment.

Anxiolytic-like effects of YL-IPA08, a potent ligand for the translocator protein (18 kDa) via regulating the synaptic plasticity in hippocampus

TSPO, translocator protein (18 kDa) ligands have demonstrated consistent antidepression and anxiolytic effects in several preclinical studies. This study aimed to examine whether YL-IPA08[N-ethyl-N-(2-pyridinylmethyl)-2-(3,4-ichlorophenyl) -7-methylimidazo [1,2-a] pyridine-3-acetamide hydrochloride], a potent and selective TSPO ligand synthesized by our institute, could alleviate anxiety-related behaviors induced by electric shock (ES) and investigate its underlying mechanism. As expected, we showed that chronic treatment with YL-IPA08 significantly reversed anxiety-related behaviors induced by electrical stimulation (0.5 mA, 12 times, duration 1s, interval 10s) exposure. Using the analysis of RNA-sequencing (RNA-seq) technology, it was found that the differential genes associated with the anxiolytic effect of YL-IPA08 were mainly related to synaptic plasticity. Furthermore, YL-IPA08 restored the decreased levels of brain-derived neurotrophic factor (BDNF), synapse-related protein (e.g. synapsin-1 and post-synaptic density95, PSD95), and the number of doublecortin (DCX) + neurons in the hippocampus of post-ES mice. In addition, YL-IPA08 also enhanced the dendritic complexity and dendritic spine density of hippocampal dentate gyrus (DG) granule neurons. Meanwhile, the induction of long-term potentiation (LTP) was significantly enhanced by YL-IPA08. In summary, the findings from the current study showed that YL-IPA08 exerted a clear anxiolytic effect, which might be partially mediated by promoting hippocampal neuroplasticity.

A neural circuit associated with anxiety-like behaviors induced by chronic inflammatory pain and the anxiolytic effects of electroacupuncture

AIMS

Negative emotions induced by chronic pain are a serious clinical problem. Electroacupuncture (EA) is a clinically proven safe and effective method to manage pain-related negative emotions. However, the circuit mechanisms underlying the effect of EA treatment on negative emotions remain unclear.

METHODS

Plantar injection of complete Freund's adjuvant (CFA) was performed to establish a rat model of chronic inflammatory pain-induced anxiety-like behaviors. Adeno-associated virus (AAV) tracing was used to identify excitatory synaptic transmission from the rostral anterior cingulate cortex (rACC) to the dorsal raphe nucleus (DRN). Employing chemogenetic approaches, we examined the role of the rACC-DRN circuit in chronic pain-induced anxiety-like behaviors and investigated whether EA could reverse chronic pain-induced dysfunctions of the rACC-DRN circuit and anxiety-like behaviors.

RESULTS

We found that chemogenetic activation of the rACC-DRN circuit alleviated CFA-induced anxiety-like behaviors, while chemogenetic inhibition of the rACC-DRN circuit resulted in short-term CFA-induced anxiety-like behaviors. Further research revealed that the development of CFA-induced anxiety-like behaviors was attributed to the dysfunction of rACC CaMKII neurons projecting to DRN serotonergic neurons (rACCCaMKII-DRN5-HT neurons) but not rACC CaMKII neurons projecting to DRN GABAergic neurons (rACCCaMKII-DRNGABA neurons). This is supported by the findings that chemogenetic activation of the rACCCaMKII-DRN5-HT circuit alleviates anxiety-like behaviors in rats with chronic pain, whereas neither chemogenetic inhibition nor chemogenetic activation of the rACCCaMKII-DRNGABA circuit altered CFA chronic pain-evoked anxiety-like behaviors in rats. More importantly, we found that EA could reverse chronic pain-induced changes in the activity of rACC CaMKII neurons and DRN 5-HTergic neurons and that chemogenetic inhibition of the rACCCaMKII-DRN5-HT circuit blocked the therapeutic effects of EA on chronic pain-induced anxiety-like behaviors.

CONCLUSIONS

Our data suggest that the reversal of rACCCaMKII-DRN5-HT circuit dysfunction may be a mechanism underlying the therapeutic effect of EA on chronic pain-induced anxiety-like behaviors.

MHC-I in the hippocampus promotes comorbid depressive symptoms in bone cancer pain via the upregulation of microglial TREM2/DAP12 signaling

Pain and depression comorbidity affects patients' physical and mental health, as well as quality of life. Comorbid depressive symptoms in cancer pain have a severe impact on the recognition and treatment of pain. Similarly, cancer pain patients with depression are inclined towards more despair and greater impairment. The mechanisms responsible for the comorbid depressive symptoms in bone cancer pain (BCP) have not been fully delineated. Here, it was reported that the implantation of carcinoma cells into the femoral cavity of mice led to the upregulation of major histocompatibility complex class I (MHC-I) in the hippocampus. This was associated with the activation of microglial signaling pathway mediated by the triggering receptor expressed on myeloid cells 2 protein (TREM2) and DNAX-activating protein of 12 kDa (DAP12). Pain and depression-like behaviors were reversed by the knockdown of hippocampal MHC-I via a lentiviral vector harboring ribonucleic acid interference (RNAi) sequence. Moreover, MHC-I knockdown exhibited a marked reduction in the expression of TREM2 and DAP12. These results suggested that hippocampal MHC-I was involved in BCP and depression comorbidity via upregulating the signals mediated by TREM2/DAP12 in microglia. The suppression of MHC-I could be a potential therapeutic target for BCP.

Association between long-term sedentary behavior and depressive symptoms in U.S. adults

The study aimed to investigate the association between long-term sedentary behavior (LTSB) and depressive symptoms within a representative sample of the U.S. adult population. Data from NHANES 2017-2018 were used, encompassing information on demographics, depressive symptoms, physical activity (PA), and LTSB. Depressive symptoms were identified using the Patient Health Questionnaire (PHQ-9), with "depressive symptoms" defined as a PHQ-9 score of ≥ 5, and "moderate to severe depressive symptoms (MSDS)" defined as a PHQ-9 score of ≥ 10. PA and LTSB were assessed through the Global Physical Activity Questionnaire, where LTSB was interpreted as sedentary time ≥ 600 min. Restricted Cubic Spline (RCS) curves were utilized to observe potential nonlinear relationships. Binary Logistic regressions were conducted to analyze the associations. A total of 4728 participants (mean age 51.00 ± 17.49 years, 2310 males and 2418 females) were included in the study. Among these individuals, 1194 (25.25%) displayed depressive symptoms, with 417 (8.82%) exhibiting MSDS. RCS curves displayed increased risk of depressive symptoms with prolonged sedentary duration. Logistic regression models indicated significant associations between LTSB and depressive symptoms (OR 1.398, 95% CI 1.098-1.780), and LTSB and MSDS (OR 1.567, 95% CI 1.125-2.183), after adjusting for covariates. These findings suggest that LTSB may act as a potential risk factor for both depressive symptoms and MSDS in the studied population.

Environmental factors and their impact on chronic pain development and maintenance

It is more than recognized and accepted that the environment affects the physiological responses of all living things, from bacteria to superior vertebrates, constituting an important factor in the evolution of all species. Environmental influences range from natural processes such as sunlight, seasons of the year, and rest to complex processes like stress and other mood disorders, infections, and air pollution, being all of them influenced by how each creature deals with them. In this chapter, it will be discussed how some of the environmental elements affect directly or indirectly neuropathic pain, a type of chronic pain caused by a lesion or disease of the somatosensory nervous system. For that, it was considered the edge of knowledge in translational research, thus including data from human and experimental animals as well as the applicability of such findings.

Gene Discovery and Biological Insights into Anxiety Disorders from a Multi-Ancestry Genome-wide Association Study of >1.2 Million Participants

We leveraged information from more than 1.2 million participants to investigate the genetics of anxiety disorders across five continental ancestral groups. Ancestry-specific and cross-ancestry genome-wide association studies identified 51 anxiety-associated loci, 39 of which are novel. Additionally, polygenic risk scores derived from individuals of European descent were associated with anxiety in African, Admixed-American, and East Asian groups. The heritability of anxiety was enriched for genes expressed in the limbic system, the cerebral cortex, the cerebellum, the metencephalon, the entorhinal cortex, and the brain stem. Transcriptome- and proteome-wide analyses highlighted 115 genes associated with anxiety through brain-specific and cross-tissue regulation. We also observed global and local genetic correlations with depression, schizophrenia, and bipolar disorder and putative causal relationships with several physical health conditions. Overall, this study expands the knowledge regarding the genetic risk and pathogenesis of anxiety disorders, highlighting the importance of investigating diverse populations and integrating multi-omics information.

Sex-dependent effect of inflammatory pain on negative affective states is prevented by kappa opioid receptors blockade in the nucleus accumbens shell

Pain comorbidities include several psychological disorders, such as anxiety and anhedonia. However, the way pain affects male and female individuals and by which mechanism is not well understood. Previous research shows that pain induces alterations in the dynorphinergic pathway within the mesocorticolimbic system (MCLS), together with a relationship between corticotropin-releasing system and dynorphin release in the MCLS. Here, we analyse the sex and time course-dependent effects of pain on negative affect. Additionally, we study the implication of dynorphinergic and corticotropin releasing factor in these pain related behaviours. We used behavioural pharmacology and biochemical tools to characterise negative affective states induced by inflammatory pain in male and female rats, and the alterations in the dynorphinergic and the corticotropin systems within the MCLS. Female rats showed persistent anxiety-like and reversible anhedonia-like behaviours derived from inflammatory pain. Additionally, we found alterations in dynorphin and corticotropin releasing factor in NAc and amygdala, which suggests sex-dependent dynamic adaptations. Finally blockade on the kappa opioid receptor in the NAc confirmed its role in pain-induced anxiety-like behaviour in female rats. Our results show sex and time-dependent anxiety- and anhedonia-like behaviours induced by the presence of pain in female rats. Furthermore, we replicated previous data, pointing to the KOR/DYN recruitment in the NAc as a key neurological substrate mediating pain-induced behavioural alterations. This research studies the mechanisms underlying these behaviours, to better understand the emotional dimension of pain.

Research hotspots and trends on neuropathic pain-related mood disorders: a bibliometric analysis from 2003 to 2023

Introduction

Neuropathic Pain (NP) is often accompanied by mood disorders, which seriously affect the quality of life of patients. This study aimed to analyze the hotspots and trends in NP-related mood disorder research using bibliometric methods and to provide valuable predictions for future research in this field.

Methods

Articles and review articles on NP-related mood disorders published from January 2003 to May 2023 were retrieved from the Web of Science Core Collection. We used CiteSpace to analyze publications, countries, institutions, authors, cited authors, journals, cited journals, references, cited references, and keywords. We also analyzed collaborative network maps and co-occurrence network maps.

Results

A total of 4,540 studies were collected for analysis. The number of publications concerning NP-related mood disorders every year shows an upward trend. The United States was a major contributor in this field. The University of Toronto was the most productive core institution. C GHELARDINI was the most prolific author, and RH DWORKIN was the most frequently cited author. PAIN was identified as the journal with the highest productivity and citation rate. The current research hotspots mainly included quality of life, efficacy, double-blind methodology, gabapentin, pregabalin, postherpetic neuralgia, and central sensitization. The frontiers in research mainly focused on the mechanisms associated with microglia activation, oxidative stress, neuroinflammation, and NP-related mood disorders.

Discussion

In conclusion, the present study provided insight into the current state and trends in NP-related mood disorder research over the past 20 years. Consequently, researchers will be able to identify new perspectives on potential collaborators and cooperative institutions, hot topics, and research frontiers in this field.

Research Trends of Acupuncture Therapy for Chronic Pain-Related Depression or Anxiety from 2003 to 2023: A Bibliometric Analysis

Background

Chronic pain (CP) as a long-lasting stressor can often cause mood disorders, such as depression or anxiety. The comorbidity of CP and mood disorders poses challenges for treatment and increases healthcare costs. Acupuncture has emerged as a widely utilized approach to alleviate both CP and mood disorders. However, there is a lack of well-established bibliometric analyses in this area of research. Therefore, this study aimed to identify the current hotspots and research trends regarding the use of acupuncture for treating CP-related depression or anxiety.

Methods

We searched the Web of Science Core Collection spanning from 2003 to 2023 to identify relevant literature about the use of acupuncture for treating CP-related depression or anxiety. Bibliometric and visualization analyses were performed using CiteSpace 5.7.R5 and Vosviewer 1.6.19 software.

Results

A total of 254 articles published between 2003 and 2023 were included, revealing an upward trajectory with some fluctuations in publication numbers over the past two decades. China and the Beijing University of Chinese Medicine were the most productive country and institution in this field. Fang JQ and Vickers AJ ascended as the most prolific and influential authors, respectively. Trials was the journal with the highest number of publications, while Pain and BMJ-British Medical Journal exhibited the highest citation and centrality, respectively. "Acupuncture", "depression", and "chronic pain" were the top three keywords. The hotspots in this domain encompass types of chronic pain that predispose to negative mood, including fibromyalgia, irritable bowel syndrome (IBS), and neuropathic pain. Randomized controlled trials (RCTs), mechanisms, and evidence-based evaluations are the main research directions.

Conclusion

This study uses bibliometric techniques to analyze the research hotspots and forefronts of acupuncture as a therapeutic approach for CP-related depression or anxiety. Our objective is to provide researchers with valuable references and identify research focal points for future investigations.

Electroacupuncture promotes synaptic plasticity in rats with chronic inflammatory pain-related depression by upregulating BDNF/TrkB/CREB signaling pathway

BACKGROUND

Chronic inflammatory pain (CIP) frequently coincides with depression among patients. The onset and development of pain and depression are associated with altered neural synaptic plasticity. Electroacupuncture (EA) can effectively relieve CIP and depression. However, the underlying mechanisms have not been fully illustrated.

OBJECTIVE

To explore whether EA can relieve CIP and depression by regulating hippocampal synaptic plasticity, and the present study offers foundational evidence for the efficacy of EA in treating CIP-related depression (CIPD).

METHODS

Rats were divided into four groups: 0.9% normal saline group, complete Freund's adjuvant (CFA) group, CFA + duloxetine group, and CFA + EA group. Pain hypersensitivity was detected by mechanical withdrawal threshold and thermal paw withdrawal latency, and the depression level was gauged using the open field test, the sucrose preference test, and the forced swimming test. The morphology of the hippocampal neurons was observed using Nissl staining. The protein expression levels of synuclein (Syn), postsynaptic density protein-95 (PSD-95), brain-derived neurotrophic factors (BDNFs), tyrosine-protein kinase B (TrKB), p-TrkB, cAMP response element binding protein (CREB), and p-CREB were measured by western blotting and immunofluorescence staining. BDNF and TrkB mRNA expression were detected using quantitative real-time polymerase chain reaction (PCR) (qRT-PCR). The content of 5-hydroxytryptamine (5-HT) and γ-aminobutyric acid (GABA) was detected using enzyme-linked immunosorbent assay, and the glutamic acid (Glu) content was determined using the ultraviolet colorimetry method. The hippocampal neuron ultrastructure was observed using transmission electron microscopy.

RESULTS

EA could alleviate CIP and related depressive behaviors as well as protect the hippocampal neuronal structure from damage and regulate 5-HT/GABA/Glu levels in the hippocampus. Additionally, EA could significantly increase the expression of synapse-associated proteins such as PSD-95 and Syn by activating the BDNF/TrKB/CREB signaling pathway.

CONCLUSION

EA improves pain and depressive behaviors in CIPD rats, and the mechanism may be related to synaptic plasticity mediated by the BDNF/TrKB/CREB signaling pathway.

The potent analgesia of intrathecal 2R, 6R-HNK via TRPA1 inhibition in LF-PENS-induced chronic primary pain model

BACKGROUND

Chronic primary pain (CPP) is an intractable pain of unknown cause with significant emotional distress and/or dysfunction that is a leading factor of disability globally. The lack of a suitable animal model that mimic CPP in humans has frustrated efforts to curb disease progression. 2R, 6R-hydroxynorketamine (2R, 6R-HNK) is the major antidepressant metabolite of ketamine and also exerts antinociceptive action. However, the analgesic mechanism and whether it is effective for CPP are still unknown.

METHODS

Based on nociplastic pain is evoked by long-term potentiation (LTP)-inducible high- or low-frequency electrical stimulation (HFS/LFS), we wanted to develop a novel CPP mouse model with mood and cognitive comorbidities by noninvasive low-frequency percutaneous electrical nerve stimulation (LF-PENS). Single/repeated 2R, 6R-HNK or other drug was intraperitoneally (i.p.) or intrathecally (i.t.) injected into naïve or CPP mice to investigate their analgesic effect in CPP model. A variety of behavioral tests were used to detect the changes in pain, mood and memory. Immunofluorescent staining, western blot, reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) and calcium imaging of in cultured dorsal root ganglia (DRG) neurons by Fluo-8-AM were used to elucidate the role and mechanisms of 2R, 6R-HNK in vivo or in vitro.

RESULTS

Intrathecal 2R, 6R-HNK, rather than intraperitoneal 2R, 6R-HNK or intrathecal S-Ketamine, successfully mitigated HFS-induced pain. Importantly, intrathecal 2R, 6R-HNK displayed effective relief of bilateral pain hypersensitivity and depressive and cognitive comorbidities in a dose-dependent manner in LF-PENS-induced CPP model. Mechanically, 2R, 6R-HNK markedly attenuated neuronal hyperexcitability and the upregulation of calcitonin gene-related peptide (CGRP), transient receptor potential ankyrin 1 (TRPA1) or vanilloid-1 (TRPV1), and vesicular glutamate transporter-2 (VGLUT2) in peripheral nociceptive pathway. In addition, 2R, 6R-HNK suppressed calcium responses and CGRP overexpression in cultured DRG neurons elicited by the agonists of TRPA1 or/and TRPV1. Strikingly, the inhibitory effects of 2R, 6R-HNK on these pain-related molecules and mechanical allodynia were substantially occluded by TRPA1 antagonist menthol.

CONCLUSIONS

In the newly designed CPP model, our findings highlighted the potential utility of intrathecal 2R, 6R-HNK for preventing and therapeutic modality of CPP. TRPA1-mediated uprgulation of CGRP and neuronal hyperexcitability in nociceptive pathways may undertake both unique characteristics and solving process of CPP.

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.

Bone marrow-derived mesenchymal stem cells attenuate complete Freund's adjuvant-induced inflammatory pain by inhibiting the expression of P2X3

Bone marrow-derived mesenchymal stem cells (BMSCs) show a good property for pain treatment by modulating inflammatory response. However, the underlying therapeutic effect and related mechanism of BMSCs on inflammatory pain remain unclear. Therefore, we explored the function and potential mechanism of BMSCs performing in a complete Freund's adjuvant (CFA)-induced inflammatory pain model in this study. Here, BMSCs were injected into the CFA-treated rats, and we used behavioural tests to evaluate the changes in hypersensitivity. High-throughput sequencing was used to screen out the hub genes. Molecular biology experiments were performed to detect the level of P2X3 or inflammatory mediators in rats and observed the distribution of P2X3 in neural cells. Furthermore, the function of the P2X3 was explored via inhibitor and activator experiments. Finally, we found that BMSCs alleviated hyperalgesia and spinal levels of pro-inflammatory factors in CFA-treated rats. High-throughput sequencing showed that P2X3 and P2X7 were identified as hub genes, and only the expression level of P2X3 was significantly down-regulated after BMSCs treatment. Immunohistochemistry showed that P2X3 mainly colocalized with microglia and astrocytes. The levels of P2X3 and pro-inflammatory factors were all significantly reduced after BMSC injection. Moreover, similar attenuation was found in the CFA-treated rats after injecting the P2X3 inhibitor, and a P2X3 antagonist reversed the attenuation induced by the BMSCs. These findings suggest that BMSCs exerted a therapeutic effect on inflammatory pain by inhibiting the expression of P2X3 and the excessive production of inflammatory mediators was associated with an increased P2X3 level and BMSC therapy reverse these effects.

Janus effect of the anterior cingulate cortex: Pain and emotion

Over the past 20 years, clinical and preclinical studies point to the anterior cingulate cortex (ACC) as a site of interest for several neurological and psychiatric conditions. The ACC plays a critical role in emotion, autonomic regulation, pain processing, attention, memory and decision making. An increasing number of studies have demonstrated the involvement of the ACC in the emotional component of pain and its comorbidity with emotional disorders such as anxiety and depression. Thanks to the development of animal models combined with state-of-the-art technologies, we now have a better mechanistic understanding of the functions of the ACC. Hence, the primary aim of this review is to compile the most recent preclinical studies on the role of ACC in the emotional component and consequences of chronic pain. Herein, we thus thoroughly describe the pain-induced electrophysiological, molecular and anatomical alterations in the ACC and in its related circuits. Finally, we discuss the next steps that are needed to strengthen our understanding of the involvement of the ACC in emotional and pain processing.

Prokineticin System Is a Pharmacological Target to Counteract Pain and Its Comorbid Mood Alterations in an Osteoarthritis Murine Model

Osteoarthritis (OA) is the most prevalent joint disease associated with chronic pain. OA pain is often accompanied by mood disorders. We addressed the role of the Prokineticin (PK) system in pain and mood alterations in a mice OA model induced with monosodium iodoacetate (MIA). The effect of a PK antagonist (PC1) was compared to that of diclofenac. C57BL/6J male mice injected with MIA in the knee joint were characterized by allodynia, motor deficits, and fatigue. Twenty-eight days after MIA, in the knee joint, we measured high mRNA of PK2 and its receptor PKR1, pro-inflammatory cytokines, and MMP13. At the same time, in the sciatic nerve and spinal cord, we found increased levels of PK2, PKR1, IL-1β, and IL-6. These changes were in the presence of high GFAP and CD11b mRNA in the sciatic nerve and GFAP in the spinal cord. OA mice were also characterized by anxiety, depression, and neuroinflammation in the prefrontal cortex and hippocampus. In both stations, we found increased pro-inflammatory cytokines. In addition, PK upregulation and reactive astrogliosis in the hippocampus and microglia reactivity in the prefrontal cortex were detected. PC1 reduced joint inflammation and neuroinflammation in PNS and CNS and counteracted OA pain and emotional disturbances.

Electroacupuncture attenuates chronic inflammatory pain and depression comorbidity by inhibiting hippocampal neuronal apoptosis via the PI3K/Akt signaling pathway

In chronic inflammatory pain (CIP) and depression, neuroapoptosis has been identified as a contributing factor. Electroacupuncture (EA) shows promise as an alternative therapy for this comorbidity. However, the underlying mechanism remains unclear. This study aimed to investigate the effects of EA on hippocampal neuronal apoptosis in rats with CIP and depression. Rats received plantar injections of complete Freund's adjuvant (CFA) on days 0 and 14. They were then divided into groups: sham operation, model, EA, and duloxetine. EA was administered at Hegu (LI4) and Taichong (LR3) from days 15 to 28, while the duloxetine group received duloxetine and distilled water daily (0.1 mg/ml). Pain behavior was assessed using the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) tests. Depression-like behavior was evaluated through the sucrose preference test (SPT), open-field test (OFT), and forced swim test (FST). Hematoxylin and eosin (HE) staining was employed to assess pathological changes in the hippocampus. Nerve cell apoptosis was determined using TUNEL fluorescence staining. Western blot analysis was conducted to measure the protein expression of Bcl-2, Bax, p-PI3K/PI3K, and p-Akt/Akt. EA demonstrated significant pain intensity reduction and alleviation of pain-related depressive symptoms. Our findings from the HE staining confirmed that CIP induced by CFA led to morphological changes in the hippocampus, while EA effectively reversed these pathological alterations. Moreover, EA intervention remarkably reduced neuronal apoptosis and exhibited an upregulation of Bcl-2 protein expression accompanied by a decrease in Bax expression. Additionally, EA activated the PI3K/Akt signaling pathway. Overall, our study suggests that EA holds the potential to improve pain and depressive behaviors in rats with CIP and depression comorbidity, potentially mediated through the activation of the PI3K/Akt pathway, leading to a reduction in hippocampal neuronal apoptosis.

The impact of fibromyalgia syndrome on obstructive sleep apnea syndrome in terms of pain threshold, daytime symptoms, anxiety, depression, disease severity, and sleep quality: a polysomnographic study

BACKGROUND

Current studies have focused on the association of fibromyalgia syndrome (FMS) and obsctructive sleep apnea syndrome (OSAS). Results of these studies on the effect of this association have been inconsistent. The current study aimed to investigate the effect of FMS on OSAS regarding sleep quality, pressure pain threshold, fatigue, daytime symptoms, anxiety, and depression, and also to determine the relationship between OSAS severity and FMS.

METHODS

In a cross-sectional design, patients diagnosed with OSAS were evaluated in two groups comparing those with and those without FMS. Data on demographics, headache, morning fatigue, and chronic pain duration were collected. Questionnaires including the Fatigue Severity Scale (FSS), Fibromyalgia Impact Questionnaire (FIQ), Beck Depression Inventory (BDI), and Beck Anxiety Inventory (BAI) were completed. Pressure pain threshold, tender points, and polysomnographic data were recorded.

RESULTS

Of 69 patients, 27 were diagnosed with FMS + OSAS and 42 were diagnosed as OSAS only. Statistically significant differences were found between the two groups in VAS, pain duration, morning fatigue, headache, BAI, tender point count, FIQ and FSS scores, and algometer measurements. All polysomnografic data were compared, and no statistically significant differences were found between the two groups. There were no statistically significant differences in the algometer, BDI, BAI, FIQ, and FSS scores when analyzed according to the severity of OSAS.

CONCLUSION

The findings suggest that FMS has no effect on polysomnographic parameters of OSAS. Headache, daytime fatigue, anxiety, depression, pain duration, and pain intensity are higher while the pressure pain threshold is lower when FMS is present. No correlation was found between OSAS severity and FMS, fatigue, pressure pain threshold, depression, and anxiety.

CLINICAL TRIAL REGISTRATION NUMBER

NCT05367167/date: April 8, 2022.

The pro-resolving lipid mediator Maresin 1 ameliorates pain responses and neuroinflammation in the spared nerve injury-induced neuropathic pain: A study in male and female mice

Specialized pro-resolving mediators (SPMs) have recently emerged as promising therapeutic approaches for neuropathic pain (NP). We evaluated the effects of oral treatment with the SPM Maresin 1 (MaR1) on behavioral pain responses and spinal neuroinflammation in male and female C57BL/6J mice with spared nerve injury (SNI)-induced NP. MaR1, or vehicle, was administered once daily, on post-surgical days 3 to 5, by voluntary oral intake. Sensory-discriminative and affective-motivational components of pain were evaluated with von Frey and place escape/avoidance paradigm (PEAP) tests, respectively. Spinal microglial and astrocytic activation were assessed by immunofluorescence, and the spinal concentration of cytokines IL-1β, IL-6, IL-10, and macrophage colony-stimulating factor (M-CSF) were evaluated by multiplex immunoassay. MaR1 treatment reduced SNI-induced mechanical hypersensitivity on days 7 and 11 in both male and female mice, and appeared to ameliorate the affective component of pain in males on day 11. No definitive conclusions could be drawn about the impact of MaR1 on the affective-motivational aspects of pain in female mice, since repeated suprathreshold mechanical stimulation of the affected paw in the dark compartment did not increase the preference of vehicle-treated SNI females for the light side, during the PEAP test session (a fundamental assumption for PAEP's validity). MaR1 treatment also reduced ipsilateral spinal microglial and astrocytic activation in both sexes and marginally increased M-CSF in males, while not affecting cytokines IL-1β, IL-6 and IL-10 in either sex. In summary, our study has shown that oral treatment with MaR1 (i) produces antinociception even in an already installed peripheral NP mouse model, and (ii) this antinociception may extend for several days beyond the treatment time-frame. These therapeutic effects are associated with attenuated microglial and astrocytic activation in both sexes, and possibly involve modulation of M-CSF action in males.

Neurobehavioral impairments predict specific cerebral damage in rat model of subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a severe form of stroke that can cause unpredictable and diffuse cerebral damage, which is difficult to detect until it becomes irreversible. Therefore, there is a need for a reliable method to identify dysfunctional regions and initiate treatment before permanent damage occurs. Neurobehavioral assessments have been suggested as a possible tool to detect and approximately localize dysfunctional cerebral regions. In this study, we hypothesized that a neurobehavioral assessment battery could be a sensitive and specific early warning for damage in discrete cerebral regions following SAH. To test this hypothesis, a behavioral battery was employed at multiple time points after SAH induced via an endovascular perforation, and brain damage was confirmed via postmortem histopathological analysis. Our results demonstrate that impairment of sensorimotor function accurately predict damage in the cerebral cortex (AUC: 0.905; sensitivity: 81.8%; specificity: 90.9%) and striatum (AUC: 0.913; sensitivity: 90.1%; specificity: 100%), while impaired novel object recognition is a more accurate indicator of damage to the hippocampus (AUC: 0.902; sensitivity: 74.1%; specificity: 83.3%) than impaired reference memory (AUC: 0.746; sensitivity: 72.2%; specificity: 58.0%). Tests for anxiety-like and depression-like behaviors predict damage to the amygdala (AUC: 0.900; sensitivity: 77.0%; specificity: 81.7%) and thalamus (AUC: 0.963; sensitivity: 86.3%; specificity: 87.8%), respectively. This study suggests that recurring behavioral testing can accurately predict damage in specific brain regions, which could be developed into a clinical battery for early detection of SAH damage in humans, potentially improving early treatment and outcomes.

Anxiety and depression are associated with poor outcomes in open elbow arthrolysis

BACKGROUND

Open elbow arthrolysis (OEA) is an established treatment for posttraumatic elbow stiffness (PTES); however, its efficacy is debatable for some patients. Poor surgical outcomes have been associated with anxiety and depression in other orthopedic conditions, but no studies have examined this association in OEA. In this study, we aimed to determine whether a high preoperative anxiety and depression score is associated with a worse functional outcome in OEA for PTES.

METHODS

A retrospective review of prospectively collected data was carried out in patients undergoing OEA between April 2021 and March 2022. Mental state evaluated by Hospital Anxiety and Depression Scale (HADS), subjective elbow function valued by Disabilities of the Arm, Shoulder, and Hand (DASH) score, objective elbow function valued by Mayo Elbow Performance Score (MEPS), pain score measured by visual analog scale (VAS) and the flexion-extension range of motion (ROM) of the affected elbow were collected before and after surgery in outpatient clinic follow-up at 3 months and 6 months. Patient satisfaction was only recorded 6 months postoperatively. All patients were divided into 2 groups based on the preoperative HADS score for analysis: Group A was the nonanxiety-depression group, and Group B was the anxiety-depression group.

RESULTS

A total of 49 patients were included. Both groups improved in DASH, MEPS and ROM at 3 months and at 6 months. The HADS score in Group B decreased significantly at 6 months, showing that the mental state of patients in Group B improved after surgery. Group A had a lower DASH at 3 months and 6 months, larger 6-month ROM and higher satisfaction rate than Group B. Comparing the differences between preoperative and postoperative measurements, Group A improved more in ROM at 6 months. There was no significant difference in other outcome measures between the two groups.

CONCLUSIONS

OEA is a safe and effective treatment for PTES, and can achieve good clinical outcomes in the short-term follow-up, regardless of whether the patients suffer from anxiety or depression. Patients with a HADS score ≥11 before OEA, however, have worse outcomes than those with a HADS score <11.

LEVEL OF EVIDENCE

Level II; Retrospective Design; Prognosis Study.

Environmental enrichment promotes resilience to neuropathic pain-induced depression and correlates with decreased excitability of the anterior cingulate cortex

Depression is a common comorbidity of chronic pain with many patients being affected. However, efficient pharmacological treatment strategies are still lacking. Therefore, it is desirable to find additional alternative approaches. Environmental enrichment has been suggested as a method to alleviate pain-induced depression. However, the neuronal mechanisms of its beneficial effects are still elusive. The anterior cingulate cortex (ACC) plays a central role in processing pain-related negative affect and chronic pain-induced plasticity in this region correlates with depressive symptoms. We studied the consequences of different durations of environmental enrichment on pain sensitivity and chronic pain-induced depression-like behaviors in a mouse model of neuropathic pain. Furthermore, we correlated the behavioral outcomes to the activity levels of pyramidal neurons in the ACC by analyzing their electrophysiological properties ex vivo. We found that early exposure to an enriched environment alone was not sufficient to cause resilience against pain-induced depression-like symptoms. However, extending the enrichment after the injury prevented the development of depression and reduced mechanical hypersensitivity. On the cellular level, increased neuronal excitability was associated with the depressive phenotype that was reversed by the enrichment. Therefore, neuronal excitability in the ACC was inversely correlated to the extended enrichment-induced resilience to depression. These results suggest that the improvement of environmental factors enhanced the resilience to developing chronic pain-related depression. Additionally, we confirmed the association between increased neuronal excitability in the ACC and depression-like states. Therefore, this non-pharmacological intervention could serve as a potential treatment strategy for comorbid symptoms of chronic pain.

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.

Modulation of Heart Rate Variability following PAP Ion Magnetic Induction Intervention in Subjects with Chronic Musculoskeletal Pain: A Pilot Randomized Controlled Study

Heart rate variability (HRV) analysis has emerged as a simple and non-invasive technique to indirectly evaluate the autonomic nervous system (ANS), and it is considered a sensible and advanced index of health status. Pulsed electromagnetic fields (PEMFs) are widely used in clinical settings for improving the health status of individuals with chronic musculoskeletal pain. The aim of the present single-blind, randomized, placebo-controlled parallel pilot study was to investigate the acute effect of a single session of PEMFs stimulation by a PAP ion magnetic induction (PAPIMI) device on ANS activity, as measured by HRV, in patients with chronic musculoskeletal pain, and compare such effect with that induced by a sham (control) PAPIMI inductor. Thirty-two patients were randomized into two groups: PAPIMI intervention (PAP) (n = 17) and sham PAPIMI intervention (SHAM-PAP) (n = 15). HRV was assessed before and following the interventions. The PAP group showed a significant increase in all values of the time-domain parameters (SDNN, RMSSD, NN50, and pNN50) and the HF component of HRV, suggesting a parasympathetic effect. In contrast, the SHAM-PAP group showed no significant differences in all HRV indices following the intervention. Preliminary findings suggested that PAPIMI inductor could influence ANS activity and provided initial evidence of the potential physiological response induced by the PAPIMI device.

Drug repositioning: diacerein as a new therapeutic approach in a mice model of sciatic nerve injury

BACKGROUND

Peripheral nerve injuries negatively impact the quality of life of patients, with no effective treatment available that accelerates sensorimotor recovery and promotes functional improvement and pain relief. The aim of this study was to evaluate the effects of diacerein (DIA) in an experimental mice model of sciatic nerve crush.

METHOD

In this study, male Swiss mice were used, randomly separated into six groups as follows: FO (false-operated + vehicle); FO + DIA (false-operated + diacerein 30 mg/kg); SNI (sciatic nerve injury + vehicle); SNI + DIA in doses of 3, 10 and 30 mg/kg (sciatic nerve injury + treatment with diacerein in doses of 3-30 mg/kg). DIA or vehicle was administered 24 h after the surgical procedure, intragastrically, twice a day. The lesion of the right sciatic nerve was generated by crush.

RESULTS

We found that the treatment of animals with DIA accelerated sensorimotor recovery of the animal. In addition, animals in the sciatic nerve injury + vehicle (SNI) group showed hopelessness, anhedonia, and lack of well-being, which were significantly inhibited by DIA treatment. The SNI group showed a reduction in the diameters of nerve fibers, axons, and myelin sheaths, while DIA treatment recovered all these parameters. In addition, the treatment of animals with DIA prevented an increase the levels of interleukin (IL)-1β and a reduction in the levels of the brain-derived growth factor (BDNF).

CONCLUSIONS

Treatment with DIA reduces hypersensitivity and depression like behaviors in animals. Furthermore, DIA promotes functional recovery and regulates IL-1β and BDNF concentrations.

Experimenter familiarization is a crucial prerequisite for assessing behavioral outcomes and reduces stress in mice not only under chronic pain conditions

Rodent behavior is affected by different environmental conditions. These do not only comprise experimental and housing conditions but also familiarization with the experimenter. However, specific effects on pain-related behavior and chronic pain conditions have not been examined. Therefore, we aimed to investigate the impact of different housing conditions, using individually ventilated and standard open top cages, inverted day-night cycles, and experimenter familiarization on male mice following peripheral neuropathy using the spared nerve injury (SNI) model. Using a multimodal approach, we evaluated evoked pain-related- using von Frey hair filaments, measured gait pattern with the CatWalk system, assessed anxiety- and depression-like behavior with the Elevated plus maze and tail suspension test, measured corticosterone metabolite levels in feces and utilized an integrative approach for relative-severity-assessment. Mechanical sensitivity differed between the cage systems and experimenter familiarization and was affected in both sham and SNI mice. Experimenter familiarization and an inverted day-night cycle reduced mechanical hypersensitivity in SNI and sham mice. SNI mice of the inverted day-night group displayed the slightest pronounced alterations in gait pattern in the Catwalk test. Anxiety-related behavior was only found in SNI mice of experimenter-familiarized mice compared to the sham controls. In addition, familiarization reduced the stress level measured by fecal corticosteroid metabolites caused by the pain and the behavioral tests. Although no environmental condition significantly modulated the severity in SNI mice, it influenced pain-affected phenotypes and is, therefore, crucial for designing and interpreting preclinical pain studies. Moreover, environmental conditions should be considered more in the reporting guidelines, described in more detail, and discussed as a potential influence on pain phenotypes.

Magel2 truncation alters select behavioral and physiological outcomes in a rat model of Schaaf-Yang syndrome

Previous studies in mice have utilized Magel2 gene deletion models to examine the consequences of its absence. We report the generation, molecular validation and phenotypic characterization of a novel rat model with a truncating Magel2 mutation modeling variants associated with Schaaf-Yang syndrome-causing mutations. Within the hypothalamus, a brain region in which human MAGEL2 is paternally expressed, we demonstrated, at the level of transcript and peptide detection, that rat Magel2 exhibits a paternal, parent-of-origin effect. In evaluations of behavioral features across several domains, juvenile Magel2 mutant rats displayed alterations in anxiety-like behavior and sociability measures. Moreover, the analysis of peripheral organ systems detected alterations in body composition, cardiac structure and function, and breathing irregularities in Magel2 mutant rats. Several of these findings are concordant with reported mouse phenotypes, indicating the conservation of MAGEL2 function across rodent species. Our comprehensive analysis revealing impairments across multiple domains demonstrates the tractability of this model system for the study of truncating MAGEL2 mutations.

Wellness of patients with chronic pain is not only about pain intensity

OBJECTIVE

Attaining good outcomes in the management of chronic pain remains a clinical challenge. This study aimed to investigate the relationships between - and the contribution of - pain and related conditions to the wellness of these patients.

DESIGN

A secondary analysis of database of patients with chronic pain treated with medical cannabis (MC) to carry out a one-year prospective follow-up study was conducted. Questionnaires were completed before (T₀ ), six (T₆ ), and twelve (T₁₂ ) months after MC initiation. Data included patients' demographics and questionnaires related to three latent factors: pain intensity measures, related conditions (catastrophizing, sleep disturbance, anxiety, and depression), and wellness parameters (quality-of-life, disability, subjective-health-state). Weighted average of the observed variables (WOBs) were calculated for each latent factor. Longitudinal structural equation modeling (SEM) and mediation analyses were performed to identify predictors and interrelations between the WOBs, respectively.

RESULTS

Participants included 510 patients. All variables were significantly improved from T₀ to T₆ and T₁₂ . SEM revealed that related conditions, and to a lesser extent pain, predicted wellness at T₀ , T₆ , and T₁₂ (related conditions: β₀  = 0.55, p < 0.001; β₆  = 0.54, p < 0.001; and β₁₂  = 0.51, p < 0.001; pain: β₀  = 0.42, p < 0.001; β₆  = 0.18, p < 0.001; and β₁₂  = 0.25, p < 0.001). Mediation analyses demonstrated that the effect of WOB-related conditions was greater than WOB-pain on wellness.

CONCLUSION

Wellness of patients with chronic pain can be determined not only by pain itself but even more so by the severity of related conditions. Thus, considering a broad spectrum of pain measures and related conditions seems relevant for improving the wellness of patients with chronic pain.

Effects of NADPH Oxidase Isoform-2 (NOX2) Inhibition on Behavioral Responses and Neuroinflammation in a Mouse Model of Neuropathic Pain

NADPH oxidase isoform-2 (NOX2) has been implicated in the pathophysiology of neuropathic pain (NP), mostly through the modulation of neuroinflammation. Since it is also accepted that some neuroimmune mechanisms underlying NP are sex-dependent, we aimed to evaluate the effects of early systemic treatment with the NOX2-selective inhibitor (NOX2i) GSK2795039 on behavioral responses and spinal neuroinflammation in spared nerve injury (SNI)-induced NP in male and female mice. Mechanical sensitivity was evaluated with the von Frey test, while general well-being and anxiety-like behavior were assessed with burrowing and light/dark box tests. Spinal microglial activation and cytokines IL-1β, IL-6, and IL-10, as well as macrophage colony-stimulating factor (M-CSF) were evaluated by immunofluorescence and multiplex immunoassay, respectively. NOX2i treatment reduced SNI-induced mechanical hypersensitivity and early SNI-induced microglial activation in both sexes. SNI-females, but not males, showed a transient reduction in burrowing activity. NOX2i treatment did not improve their burrowing activity, but tendentially reduced their anxiety-like behavior. NOX2i marginally decreased IL-6 in females, and increased M-CSF in males. Our findings suggest that NOX2-selective inhibition may be a potential therapeutic strategy for NP in both male and female individuals, with particular interest in females due to its apparent favorable impact in anxiety-like behavior.

Probing pain aversion in rats with the "Heat Escape Threshold" paradigm

The aversive aspect of pain constitutes a major burden faced by pain patients. This has been recognized by the pain research community, leading to the development of novel methods focusing on affective-motivational behaviour in pain model animals. The most common tests used to assess pain aversion in animals require cognitive processes, such as associative learning, complicating the interpretation of results. To overcome this issue, studies in recent years have utilized unconditioned escape as a measure of aversion. However, the vast majority of these studies quantify jumping - a common escape behaviour in mice, but not in adult rats, thus limiting its use. Here, we present the "Heat Escape Threshold" (HET) paradigm for assessing heat aversion in rats. We demonstrate that this method can robustly and reproducibly detect the localized effects of an inflammatory pain model (intraplantar carrageenan) in male and female Sprague-Dawley rats. In males, a temperature that evoked unconditioned escape following carrageenan treatment also induced real-time place avoidance (RTPA). Systemic morphine more potently alleviated carrageenan-induced heat aversion (as measured by the HET and RTPA methods), as compared to reflexive responses to heat (as measured by the Hargreaves test), supporting previous findings. Next, we examined how blocking of excitatory transmission to the lateral parabrachial nucleus (LPBN), a key node in the ascending pain system, affects pain behaviour. Using the HET and Hargreaves tests, we show that intra-LPBN application of glutamate antagonists reverses the effects of carrageenan on both affective and reflexive pain behaviour, respectively. Finally, we employed the HET paradigm in a generalized opioid-withdrawal pain model. Withdrawal from a brief systemic administration of remifentanil resulted in a long-lasting and robust increase in heat aversion, but no change in reflexive responses to heat. Taken together, these data demonstrate the utility of the HET paradigm as a novel tool in preclinical pain research.