Altered neuropathic pain behaviour in a rat model of depression is associated with changes in inflammatory gene expression in the amygdala

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.

Targeting the JAK2/STAT3 signaling pathway for chronic pain

Chronic pain is a notable health concern because of its prevalence, persistence, and associated mental stress. Drugs targeting chronic pain with potent abirritation, and minimal side effects remain unidentified. Substantial evidence indicates that the Janus Kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway plays a distinct and critical role in different stages of chronic pain. Aberrant activation of the JAK2/STAT3 signaling pathway is evident in multiple chronic pain models. Moreover, an increasing number of studies have demonstrated that the downregulation of JAK2/STAT3 can attenuate chronic pain in different animal models. In this review, we investigated the mechanism and role of the JAK2/STAT3 signaling pathway in modulating chronic pain. The aberrant activation of JAK2/STAT3 can trigger chronic pain by interacting with microglia and astrocytes, releasing proinflammatory cytokines, inhibiting anti-inflammatory cytokines, and regulating synaptic plasticity. We also retrospectively reviewed current reports on JAK2/STAT3 pharmacological inhibitors that demonstrated their significant therapeutic potential in different types of chronic pain. In summary, our results provide strong evidence that the JAK2/STAT3 signaling pathway is a promising therapeutic target for chronic pain.

Glial Cells of the Central Nervous System: A Potential Target in Chronic Prostatitis/Chronic Pelvic Pain Syndrome

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is one of the most common diseases of the male urological system while the etiology and treatment of CP/CPPS remain a thorny issue. Cumulative research suggested a potentially important role of glial cells in CP/CPPS. This narrative review retrospected literature and grasped the research process about glial cells and CP/CPPS. Three types of glial cells showed a crucial connection with general pain and psychosocial symptoms. Microglia might also be involved in lower urinary tract symptoms. Only microglia and astrocytes have been studied in the animal model of CP/CPPS. Activated microglia and reactive astrocytes were found to be involved in both pain and psychosocial symptoms of CP/CPPS. The possible mechanism might be to mediate the production of some inflammatory mediators and their interaction with neurons. Glial cells provide a new insight to understand the cause of complex symptoms of CP/CPPS and might become a novel target to develop new treatment options. However, the activation and action mechanism of glial cells in CP/CPPS needs to be further explored.

Novel Histone Modifications in Microglia Derived from a Mouse Model of Chronic Pain

As the resident immune cells in the central nervous system, microglia play an important role in the maintenance of its homeostasis. Dysregulation of microglia has been associated with the development and maintenance of chronic pain. However, the relevant molecular pathways remain poorly defined. In this study, we used a mass spectrometry-based proteomic approach to screen potential changes of histone protein modifications in microglia isolated from the brain of control and cisplatin-induced neuropathic pain adult C57BL/6J male mice. We identified several novel microglial histone modifications associated with pain including statistically significantly decreased histone H3.1 lysine 27 mono-methylation (H3.1K27me1, 54.8% of control) and lysine 56 tri-methylation (7.5% of control), as well as a trend suggesting increased histone 3 tyrosine 41 nitration. We further investigated the functional role of H3.1K27me1 and found that treatment of cultured microglial cells for 4 consecutive days with 1-10 μM of NCDM-64, a potent and selective inhibitor of lysine demethylase 7A, an enzyme responsible for the demethylation of H3K27me1, dose-dependently elevated its levels with a greater than a 2-fold increase observed at 10 μM compared to vehicle-treated control cells. Moreover, pre-treatment of mice with NCDM-64 (10 or 25 mg/kg/day, i.p.) prior to cisplatin treatment prevented the development of neuropathic pain in mice. The identification of specific chromatin marks in microglia associated with chronic pain may yield critical insight into the contribution of microglia to the development and maintenance of pain, and opens new avenues for the development of novel non-opioid therapeutics for the effective management of chronic pain. This article is protected by copyright. All rights reserved.

Pre-treatment Pain Symptoms Influence Antidepressant Response to Ketamine in Depressive Patients

BACKGROUND

Pain strongly coexists with depression. Ketamine has great analgesic and antidepressant effects, acting as a promising role in treating depression with pain. Few studies have evaluated impact of pain symptoms on antidepressant effect of ketamine infusions. Thus, present study investigated whether pain symptoms in individuals with depression moderate response to ketamine.

METHODS

One hundred and four individuals with major depressive disorder and bipolar depression received six intravenous infusions of ketamine. The Montgomery-Åsberg Depression Rating Scale (MADRS) was administered at baseline, the next morning after each infusion and 2 weeks (Day 26) after the last infusion. Pain symptoms were collected at baseline using the short-form McGill Pain Questionnaire (SF-MPQ).

RESULTS

The prevalence of pain in patients with depression was 48.8%. Mix model analyses showed that pre-treatment pain symptoms assessed by each domain of SF-MPQ significantly moderated antidepressant response to six infusions of ketamine from baseline to day 26 (all p < 0.05). Then follow-up simple slopes analyses suggested that all patients across groups showed a significant symptomatic improvement after ketamine infusions (all p < 0.05), and patients with severe pain (across all domains of SF-MPQ) had greater improvement in depressive symptoms than those with mild pain or non-pain (all p < 0.05).

CONCLUSION

A significant and rapid improvement in depressive symptoms was observed in patients with depression and pain after ketamine treatment. Ketamine may be a novel and promising antidepressant preferentially for the therapy of depression with severe pain.

Reactive Astrocytes: Critical Players in the Development of Chronic Pain

Chronic pain is associated with long term plasticity of nociceptive pathways in the central nervous system. Astrocytes can profoundly affect synaptic function and increasing evidence has highlighted how altered astrocyte activity may contribute to the pathogenesis of chronic pain. In response to injury, astrocytes undergo a shift in form and function known as reactive astrogliosis, which affects their release of cytokines and gliotransmitters. These neuromodulatory substances have been implicated in driving the persistent changes in central nociceptive activity. Astrocytes also release lactate which neurons can use to produce energy during synaptic plasticity. Furthermore, recent research has provided insight into lactate's emerging role as a signaling molecule in the central nervous system, which may be involved in directly modulating neuronal and astrocytic activity. In this review, we present evidence for the involvement of astrocyte-derived tumor necrosis factor alpha in pain-associated plasticity, in addition to research suggesting the potential involvement of gliotransmitters D-serine and adenosine-5'-triphosphate. We also discuss work implicating astrocyte-neuron metabolic coupling, and the possible role of lactate, which has been sparsely studied in the context of chronic pain, in supporting pathological changes in central nociceptive activity.

Electroacupuncture Attenuates Neuropathic Pain and Comorbid Negative Behavior: The Involvement of the Dopamine System in the Amygdala

Neuropathic pain (NeuP) is an important clinical problem accompanying negative mood symptoms. Neuroinflammation in the amygdala is critically involved in NeuP, and the dopamine (DA) system acts as an important endogenous anti-inflammatory pathway. Electroacupuncture (EA) can improve the clinical outcomes in NeuP, but the underlying mechanisms have not been fully elucidated. This study was designed to assess the effectiveness of EA on pain and pain-related depressive-like and anxiety-like behaviors and explore the role of the DA system in the effects of EA. Male Sprague-Dawley rats were subjected to the chronic constrictive injury (CCI) model to induce NeuP. EA treatment was carried out for 30 min once every other day for 3 weeks. The results showed that CCI caused mechanical hyperalgesia and depressive and anxiety-like behaviors in rats and neuroinflammation in the amygdala, such as an increased protein level of TNFα and IL-1β and activation of astrocytes. EA treatment significantly improved mechanical allodynia and the emotional dysfunction induced by CCI. The effects of EA were accompanied by markedly decreased expression of TNFα, IL-1β, and glial fibrillary acid protein (GFAP) in the amygdala. Moreover, EA treatment reversed CCI-induced down-regulation of DA concentration, tyrosine hydroxylase (TH) expression, and DRD1 and DRD2 receptors. These results suggest that EA-ameliorated NeuP may possibly be associated with the DA system to inhibit the neuroinflammation in the amygdala.

Effect of acetyl-L-carnitine on hypersensitivity in acute recurrent caerulein-induced pancreatitis and microglial activation along the brain’s pain circuitry

BACKGROUND

Acute pancreatitis (AP) and recurring AP are serious health care problems causing excruciating pain and potentially lethal outcomes due to sepsis. The validated caerulein- (CAE) induced mouse model of acute/recurring AP produces secondary persistent hypersensitivity and anxiety-like behavioral changes for study.

AIM

To determine efficacy of acetyl-L-carnitine (ALC) to reduce pain-related behaviors and brain microglial activation along the pain circuitry in CAE-pancreatitis.

METHODS

Pancreatitis was induced with 6 hly intraperitoneal (i.p.) injections of CAE (50 µg/kg), 3 d a week for 6 wk in male C57BL/6J mice. Starting in week 4, mice received either vehicle or ALC until experiment’s end. Mechanical hyper-sensitivity was assessed with von Frey filaments. Heat hypersensitivity was determined with the hotplate test. Anxiety-like behavior was tested in week 6 using elevated plus maze and open field tests. Microglial activation in brain was quantified histologically by immunostaining for ionized calcium-binding adaptor molecule 1 (Iba1).

RESULTS

Mice with CAE-induced pancreatitis had significantly reduced mechanical withdrawal thresholds and heat response latencies, indicating ongoing pain. Treatment with ALC attenuated inflammation-induced hypersensitivity, but hypersensitivity due to abdominal wall injury caused by repeated intraperitoneal injections persisted. Animals with pancreatitis displayed spontaneous anxiety-like behavior in the elevated plus maze compared to controls. Treatment with ALC resulted in increased numbers of rearing activity events, but time spent in “safety” was not changed. After all the abdominal injections, pancreata were translucent if excised at experiment’s end and opaque if excised on the subsequent day, indicative of spontaneous healing. Post mortem histopathological analysis performed on pancreas sections stained with Sirius Red and Fast Green identified wide-spread fibrosis and acinar cell atrophy in sections from mice with CAE-induced pancreatitis that was not rescued by treatment with ALC. Microglial Iba1 immunostaining was significantly increased in hippocampus, thalamus (intralaminar nuclei), hypothalamus, and amygdala of mice with CAE-induced pancreatitis compared to naïve controls but unchanged in the primary somatosensory cortex compared to naïves.

CONCLUSION

CAE-induced pancreatitis caused increased pain-related behaviors, pancreatic fibrosis, and brain microglial changes. ALC alleviated CAE-induced mechanical and heat hypersensitivity but not abdominal wall injury-induced hypersensitivity caused by the repeated injections.

Time-restricted feeding prevents depressive-like and anxiety-like behaviors in male rats exposed to an experimental model of shift-work

Individuals who regularly shift their sleep timing, like night and/or shift-workers suffer from circadian desynchrony and are at risk of developing cardiometabolic diseases and cancer. Also, shift-work is are suggested to be a risk factor for the development of mood disorders such as the burn out syndrome, anxiety, and depression. Experimental and clinical studies provide evidence that food intake restricted to the normal activity phase is a potent synchronizer for the circadian system and can prevent the detrimental health effects associated with circadian disruption. Here, we explored whether adult male Wistar rats exposed to an experimental model of shift-work (W-AL) developed depressive and/or anxiety-like behaviors and whether this was associated with neuroinflammation in brain areas involved with mood regulation. We also tested whether time-restricted feeding (TRF) to the active phase could ameliorate circadian disruption and therefore would prevent depressive and anxiety-like behaviors as well as neuroinflammation. In male Wistar rats, W-AL induced depressive-like behavior characterized by hypoactivity and anhedonia and induced increased anxiety-like behavior in the open field test. This was associated with increased number of glial fibrillary acidic protein and IBA-1-positive cells in the prefrontal cortex and basolateral amygdala. Moreover W-AL caused morphological changes in the microglia in the CA3 area of the hippocampus indicating microglial activation. Importantly, TRF prevented behavioral changes and decreased neuroinflammation markers in the brain. Present results add up evidence about the importance that TRF in synchrony with the light-dark cycle can prevent neuroinflammation leading to healthy mood states in spite of circadian disruptive conditions.

Preclinical Considerations about Affective Disorders and Pain: A Broadly Intertwined, yet Often Under-Explored, Relationship Having Major Clinical Implications

Background: Pain, a distinctive undesirable experience, encompasses several different and fluctuating presentations across varying mood disorders. Therefore, the present narrative review aimed to shed further light on the matter, accounting for both experimental animal models and clinical observations about major depressive disorder (MDD) pathology. Method: Major databases were inquired from inception until April 2016 for records about MDD and pain. Results: Pain and MDD are tightly associated with each other in a bi-directional fashion. Several cross-sectional and retrospective studies indicated a high presence of pain in the context of mood disorders, including MDD (up to 65%), but also increased prevalence rates in the case of mood disorders documented among people with a primary diagnosis of either psychological or somatic pain (prevalence rates exceeding 45%). The clinical implications of these observations suggest the need to account for mood and pain manifestations as a whole rather than distinct entities in order to deliver more effective interventions. Limitations: Narrative review, lack of systematic control groups (e.g., people with the primary diagnosis at review, but not the associated comorbidity as a study) to allow reliable comparisons. Prevalence rates and clinical features associated with pain varied across different studies as corresponding operational definitions did. Conclusions: Pain may have a detrimental effect on the course of mood disorders-the opposite holds. Promoting a timely recognition and management of such an often neglected comorbidity would therefore represent a primary goal toward the delivery of effective, multi-disciplinary care.

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.

Neuroinflammation, Pain and Depression: An Overview of the Main Findings

Chronic pain is a serious public health problem with a strong affective-motivational component that makes it difficult to treat. Most patients with chronic pain suffer from severe depression; hence, both conditions coexist and exacerbate one another. Brain inflammatory mediators are critical for maintaining depression-pain syndrome and could be substrates for it. The goal of our paper was to review clinical and preclinical findings to identify the neuroinflammatory profile associated with the cooccurrence of pain and depression. In addition, we aimed to explore the regulatory effect of neuronal reorganization on the inflammatory response in pain and depression. We conducted a quantitative review supplemented by manual screening. Our results revealed inflammatory signatures in different preclinical models and clinical articles regarding depression-pain syndrome. We also identified that improvements in depressive symptoms and amelioration of pain can be modulated through direct targeting of inflammatory mediators, such as cytokines and molecular inhibitors of the inflammatory cascade. Additionally, therapeutic targets that improve and regulate the synaptic environment and its neurotransmitters may act as anti-inflammatory compounds, reducing local damage-associated molecular patterns and inhibiting the activation of immune and glial cells. Taken together, our data will help to better elucidate the neuroinflammatory profile in pain and depression and may help to identify pharmacological targets for effective management of depression-pain syndrome.

Mu and delta opioid receptors play opposite nociceptive and behavioural roles on nerve-injured mice

BACKGROUND AND PURPOSE

Mu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations.

EXPERIMENTAL APPROACH

We generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC).

KEY RESULTS

Constitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL.

CONCLUSIONS AND IMPLICATIONS

MOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.

The Genetics of Neuropathic Pain from Model Organisms to Clinical Application

Neuropathic pain (NeuP) arises due to injury of the somatosensory nervous system and is both common and disabling, rendering an urgent need for non-addictive, effective new therapies. Given the high evolutionary conservation of pain, investigative approaches from Drosophila mutagenesis to human Mendelian genetics have aided our understanding of the maladaptive plasticity underlying NeuP. Successes include the identification of ion channel variants causing hyper-excitability and the importance of neuro-immune signaling. Recent developments encompass improved sensory phenotyping in animal models and patients, brain imaging, and electrophysiology-based pain biomarkers, the collection of large well-phenotyped population cohorts, neurons derived from patient stem cells, and high-precision CRISPR generated genetic editing. We will discuss how to harness these resources to understand the pathophysiological drivers of NeuP, define its relationship with comorbidities such as anxiety, depression, and sleep disorders, and explore how to apply these findings to the prediction, diagnosis, and treatment of NeuP in the clinic.

Insights on Nutrients as Analgesics in Chronic Pain

Many serious inflammatory disorders and nutrient deficiencies induce chronic pain, and anti-inflammatory diets have been applied successfully to modify the inflammatory symptoms causing chronic pain. Numerous scientific data and clinical investigations have demonstrated that long-term inflammation could lead to an inappropriate or exaggerated sensibility to pain. In addition, some Non-steroidal Anti-inflammatory Drugs (NSAID), which directly act on the many enzymes involved in pain and inflammation, including cyclooxygenases, are used to dampen the algesic signal to the central nervous system, reducing the responses of soft C-fibers to pain stimuli. On the other hand, there are a few reports from both health authorities and physicians, reporting that decreased transmission of pain signals can be achieved and improved, depending on the patient's dietary habit. Many nutrients, as well as a suitable level of exercise (resistance training), are the best methods for improving the total mitochondrial capacity in muscle cells, which can lead to a reduction in sensitivity to pain, particularly by lowering the inflammatory signaling to C-fibers. According to the current literature, it could be proposed that chronic pain results from the changed ratio of neuropeptides, hormones, and poor nutritional status, often related to an underlying inflammatory disorder. The current review also evaluates the effective role of nutrition-related interventions on the severity of chronic pain. This review pointed out that nutritional interventions can have a positive effect on pain experience through the indirect inhibitory effect on prostaglandin E2 and attenuation of mitochondrial dysfunction caused by ischemia/reperfusion in skeletal muscle, improving the intracellular antioxidant defense system. These data highlight the need for more nutrition studies where chronic pain is the primary outcome, using accurate interventions. To date, no nutritional recommendation for chronic pain has been officially proposed. Therefore, the goal of this article is to explore pain management and pain modulation, searching for a mode of nutrition efficient in reducing pain.

Electroacupuncture treatment upregulates α7nAChR and inhibits JAK2/STAT3 in dorsal root ganglion of rat with spared nerve injury

Background

Neuropathic pain with complicated mechanism severely disrupts patient quality of life. The novel approaches and more effective management should be further investigated. It was reported that alpha-7 nicotinic acetylcholine receptor (α7nAChR) and janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling in dorsal root ganglion (DRG) contributed to the pathogenesis of neuropathic pain. Our previous study has shown that electroacupuncture (EA) alleviated neuropathic pain via activating α7nAChR in the spinal cord. However, whether the effect of 2 Hz EA on spared nerve injury (SNI)-induced neuropathic pain is mediated through modulation of α7nAChR and JAK2/STAT3 pathway in the DRG remains unclear.

Materials and methods

The SNI-induced neuropathic pain rat model was used in this study. After application of 2 Hz EA treatment to SNI rats on day 3, 7, 14 and 21 post-surgery, the expression levels of α7nAChR, JAK2/STAT3 and some cytokines in DRG were determined by qRT-PCR and Western blot analysis.

Results

We found that SNI induced significant down-regulation of α7nAChR mRNA and protein expression. SNI also obviously elicited the decrease in anti-inflammatory cytokine IL-10 protein expression. The enhancement of p-JAK2, p-STAT3, pro-inflammatory cytokines IL-1β and IL-6 protein levels induced by SNI were also observed. However, 2 Hz EA treatment to SNI rats distinctly improved α7nAChR and IL-10 levels and reduced p-JAK2, p-STAT3, IL-1β and IL-6 expression in the DRG.

Conclusion

Our present study suggested that 2 Hz EA treatment indeed activated α7nAChR, suppressed JAK2/STAT3 signaling and re-balanced the relationship between pro-inflammatory and anti-inflammatory cytokines in DRG of SNI rat, which provided insight into our understanding of the mechanism for 2 Hz EA to attenuate neuropathic pain.

Microglial activation and neurobiological alterations in experimental autoimmune prostatitis-induced depressive-like behavior in mice

BACKGROUND

Patients with chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) frequently show depressive symptoms clinically and increasing evidence indicates a correlation between CP/CPPS and depression. However, the underlying mechanisms of CP/CPPS-related depression remain poorly understood. Here, we sought to determine the role of hippocampal microglial activation and neurobiological changes in a mouse model of experimental autoimmune prostatitis (EAP)-induced depression and the treatment efficacy of Chinese herb extract baicalein.

METHODS

EAP was induced through intradermal injection of prostate antigen and adjuvant twice. Then, mice were assessed for affective behaviors in the open field test, elevated plus maze, forced swim test, and tail suspension test. The morphology and function of microglia and astrocytes were detected by immunofluorescence, Western blotting, and transmission electron microscopy. Proinflammatory mediators along with serotonin transporter (SLC6A4/SERT) and indoleamine 2,3-dioxygenase (IDO) were quantified with reverse transcription-polymerase chain reaction (RT‑PCR), and serum serotonin concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Proton magnetic resonance spectroscopy (¹H-MRS) was performed to measure hippocampal glutamate levels. In addition, baicalein was used in a subset of EAP mice to test its anti-depressant action.

RESULTS

EAP was successfully established and induced depressive- and anxiety-like behavior in mice. Increasing levels of co-expressed ionized calcium-binding adapter molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) and ultrastructural observations suggested microglial activation and reactive astrocytosis in the hippocampus. These activated microglia resulted in increased expressions of multiple proinflammatory cytokines. Simultaneously, EAP mice showed higher gene expressions of SLC6A4 and IDO and lower concentrations of serotonin. ¹H-MRS indicated a decrease in the glutamate + glutamine (Glx)/total creatine (tCr) ratio in EAP mice. Furthermore, baicalein treatment alleviated the depressive-like behavior and neuroinflammation by suppressing the nuclear factor-kappa B (NF-κB) pathway.

CONCLUSION

Our data indicate that EAP-induced depressive-like behavior is linked to microglia activation and subsequent neurotransmitter metabolism. Moreover, baicalein attenuates behavioral changes by inhibiting neuroinflammation via downregulation of the NF-κB pathway.

Activation of trigeminal ganglion satellite glial cells in CFA-induced tooth pulp pain in rats

This study further investigated the mechanisms underlying the rat model of tooth pulp inflammatory pain elicited by complete Freund's adjuvant (CFA), in comparison to other pulpitis models. Pulps of the left maxillary first molars were accessed. In the CFA group, the pulps were exposed, and CFA application was followed by dental sealing. In the open group, the pulps were left exposed to the oral cavity. For the closed group, the pulps were exposed, and the teeth were immediately sealed. Naïve rats were used as negative controls. Several parameters were evaluated at 1, 2, 3 and 8 days. There was no statistical significant difference among the groups when body weight variation, food or water consumption were compared. Analysis of serum cytokines (IL-1β, TNF or IL-6) or differential blood cell counts did not reveal any evidence of systemic inflammation. The CFA group displayed a significant reduction in the locomotor activity (at 1 and 3 days), associated with an increased activation of satellite glial cells in the ipsilateral trigeminal ganglion (TG; for up to 8 days). Amygdala astrocyte activation was unaffected in any experimental groups. We provide novel evidence indicating that CFA-induced pulp inflammation impaired the locomotor activity, with persistent activation of ipsilateral TG satellite cells surrounding sensory neurons, without any evidence of systemic inflammation or amygdala astrogliosis.

Gunn rats with glial activation in the hippocampus show prolonged immobility time in the forced swimming test and tail suspension test

INTRODUCTION

Recent studies imply that glial activation plays a role in the pathogenesis of psychiatric disorders, such as schizophrenia and major depression. We previously demonstrated that Gunn rats with hyperbilirubinemia show congenital gliosis and schizophrenia-like behavior.

METHODS

As it has been suggested that major depression involves glial activation associated with neuroinflammation, we examined whether Gunn rats show depression-like behavior using the forced swimming test (FST) and the tail suspension test (TST). In addition, we quantitatively evaluated both microgliosis and astrogliosis in the hippocampus of Gunn rats using immunohistochemistry analysis of the microglial marker ionized calcium-binding adaptor molecule (Iba) 1 and the astrocytic marker S100B.

RESULTS

Both the FST and TST showed that immobility time of Gunn rats was significantly longer than that of normal control Wistar rats, indicating that Gunn rats are somewhat helpless, a sign of depression-like behavior. In the quantification of immunohistochemical analysis, Iba1immunoreactivity in the dentate gyrus (DG), cornu ammonis (CA) 1, and CA3 and the number of Iba1-positive cells in the CA1 and CA3 were significantly increased in Gunn rats compared to Wistar rats. S100B immunoreactivity in the DG, CA1, and CA3 and the number of S100B-positive cells in the DG and CA3 were significantly increased in Gunn rats compared to Wistar rats.

CONCLUSION

Our findings suggest that both microglia and astrocyte are activated in Gunn rats and their learned helplessness could be related to glial activation.

Impaired Spinal Glucocorticoid Receptor Signaling Contributes to the Attenuating Effect of Depression on Mechanical Allodynia and Thermal Hyperalgesia in Rats with Neuropathic Pain

Although depression-induced altered pain perception has been described in several laboratory and clinical studies, its neurobiological mechanism in the central nervous system (CNS), particularly in the spinal dorsal horn, remains unclear. Therefore, in this study, we aimed to clarify whether nociceptive sensitivity of neuropathic pain is altered in the olfactory bulbectomy (OB) model of depression and whether glucocorticoid receptor (GR), which is involved in the etio-pathologic mechanisms of both major depression and neuropathic pain, contributes to these processes in the spinal dorsal horn of male Sprague-Dawley rats. The results showed that mechanical allodynia and thermal hyperalgesia induced by spinal nerve ligation (SNL) were attenuated in OB-SNL rats with decreased spinal GR expression and nuclear translocation, whereas non-olfactory bulbectomy (NOB)-SNL rats showed increased spinal GR nuclear translocation. In addition, decreased GR nuclear translocation with normal mechanical nociception and hypoalgesia of thermal nociception were observed in OB-Sham rats. Intrathecal injection (i.t.) of GR agonist dexamethasone (Dex; 4 μg/rat/day for 1 week) eliminated the attenuating effect of depression on nociceptive hypersensitivity in OB-SNL rats and aggravated neuropathic pain in NOB-SNL rats, which was associated with the up-regulation of brain-derived neurotrophic factor (BDNF), TrkB and NR2B expression in the spinal dorsal horn. The present study shows that depression attenuates the mechanical allodynia and thermal hyperalgesia of neuropathic pain and suggests that altered spinal GR-BDNF-TrkB signaling may be one of the reasons for depression-induced hypoalgesia.

Bupivacaine effectively relieves inflammation-induced pain by suppressing activation of the NF-κB signalling pathway and inhibiting the activation of spinal microglia and astrocytes

The pain induced by local acute inflammation results in mild to severe discomfort, in addition to the possibility of physiological dysfunction and psychiatric disorders, such as sleep disorders and depression. However, the pathogenesis of pain is yet to be fully elucidated. In the present study, the effects of bupivacaine were explored in rat models inflammatory pain in order to investigate the anti-pain mechanism of bupivacaine. Complete Freund's adjuvant (CFA) was injected into the right rear foot of the rats to establish a model of transient inflammation-induced pain. Rats were randomly divided into four groups (n=8): CFA, CFA plus bupivacaine, CFA plus saline and untreated. The mechanical withdrawal threshold (MWT) of the rats was detected prior to and following CFA injection, and the results demonstrated that the MWT in the right rear foot significantly decreased from the 1st day of CFA injection (P<0.01; n=8), as compared with the untreated controls. Bupivacaine treatment was demonstrated to significantly increase the MWT of rats treated with CFA stimulation, as compared with the CFA group (P<0.01). Rotarod testing was performed to assess the motor activity of the rats, and the results demonstrated no significant differences among the four groups (P>0.05). Furthermore, the respective body weights of the rats were determined every two days before and after CFA injection, and no significant differences were detected among the four groups (P>0.05). Western blot analysis was performed to analyze expression levels of IκB and nuclear factor (NF)-κB, and the results demonstrated that bupivacaine increased the expression of IκB and decreased the expression levels of NF-κB, as compared with the rats with CFA-induced inflammatory responses, suggesting that bupivacaine inhibited NF-κB activation in the dorsal horn of the lumbar spinal cord of the model rats. Furthermore, reverse transcription-quantitative polymerase chain reaction analysis was performed to analyze the expression levels of inflammatory cytokines, which demonstrated that bupivacaine significantly inhibited the expression of TNF-α, IL-1β and IL-6, as compared with the untreated group (P<0.01). Moreover, bupivacaine treatment significantly decreased the expression of spinal microglial marker OX42 and astrocyte marker-glial fibrillary acidic protein, as compared with the rats in the CFA group (P<0.01). The present findings demonstrated that treatment with bupivacaine significantly decreased the activation of microglia and astrocytes in rat models of inflammatory pain. Therefore, the present results may provide clarification of the pathogenesis and mechanism of inflammation-induced pain and may provide novel therapeutic strategies for the clinical treatment of pain.

Enhanced c-Fos expression in the central amygdala correlates with increased thigmotaxis in rats with peripheral nerve injury

Background

Pain is associated with affective, cognitive and sensory dysfunction. Animal models can be used to observe ethologically relevant behaviours such as thigmotaxis, giving insight into how ongoing sensory abnormalities influence natural rodent behaviours. The amygdala is a complex group of nuclei implicated in the integration and generation of emotional behavioural responses, including those associated with pain, and a region known as the central amygdala is particularly associated with generation of behavioural responses, due to its links to the descending pain modulation pathways; as such, study of amygdalar c‐Fos immunoreactivity can help identify the neuronal circuits involved.

Method

This study investigated changes in both nociceptive evoked responses and open field behaviour following spinal nerve transection (SNT) in male Wistar rats, and attempted to correlate these with changes in central amygdala c‐Fos immunoreactivity.

Results

Fourteen days after SNT, mechanical hypersensitivity was present in the hind paw ipsilateral to site of injury. Thigmotactic behaviour was significantly increased in both SNT and sham surgery animals, with c‐Fos immunoreactivity in the central amygdala significantly greater in SNT animals compared to both sham and naive groups. Activation was greatest in the capsular and lateral subnuclei of the central amygdala, and in the caudal‐most regions. There was a strong correlation between thigmotactic behaviour and central amygdala activation following SNT surgery not seen in sham animals suggesting a role for the amygdala in behavioural responses to peripheral nerve injury.

Conclusions

This study provides evidence to support the role of the amygdala in thigmotactic open field behaviour following SNT.

What does this study add?

Thigmotaxis and amygdala activation are positively correlated in rats following spinal nerve transection.

Behavioural changes seen in sham animals did not correlate with amygdala activation, suggesting amygdala activation is related to nociceptive input.

Evoked measures, such as hindpaw withdrawal, are not correlated with either thigmotaxis or amygdala activation, emphasizing the importance of complex behaviours when studying pain.

Impaired recognition memory and cognitive flexibility in the rat L5-L6 spinal nerve ligation model of neuropathic pain

BACKGROUND AND AIMS

Although neuropathic pain is known to negatively affect cognition, the neural mechanisms involved are poorly understood. Chronic pain is associated with changes in synaptic plasticity in the brain which may impact on cognitive functioning. The aim of this study was to model neuropathic pain in mid-aged rats using spinal nerve ligation (SNL). Following establishment of allodynia and hyperalgesia, behaviour was assessed in a battery of cognitive tests. Expression of the presynaptic protein, synaptophysin, and its colocalisation with the vesicular GABA and glutamate transporters (vGAT and vGLUT, respectively), was investigated in the medial prefrontal cortex (mPFC) and hippocampus.

METHODS

Nine month old male Sprague Dawley rats underwent L5-L6 spinal nerve ligation or a sham procedure. Mechanical and cold allodynia and thermal hyperalgesia were assessed using von Frey, acetone and Hargreaves tests, respectively. Cognition was assessed in the novel-object recognition, air-puff passive avoidance and Morris water maze behavioural tasks. Immunohistochemistry was used to examine the expression of synaptophysin in the mPFC and CA1 region of the hippocampus and double labelling of synaptophysin and the vesicular transporters vGAT and vGlut was used to investigate the distribution of synaptophysin on GABAergic and glutamatergic neurons.

RESULTS

SNL rats displayed impaired performance in the novel-object recognition task. Passive-avoidance responding, and spatial learning and memory in the Morris water maze, were unaffected by SNL surgery. However, in the water maze reversal task, pain-related impairments were evident during training and probe trials. SNL surgery was not associated with any differences in the expression of synaptophysin or its colocalisation with vGAT or vGLUT in the mPFC or the hippocampal CA1 region.

CONCLUSIONS

These results suggest that the SNL model of neuropathic pain is associated with deficits in recognition memory and cognitive flexibility, but these deficits are not associated with altered synaptophysin expression or distribution in the mPFC and CA1.

IMPLICATIONS

Cognitive complaints are common amongst chronic pain patients. Here we modelled cognitive impairment in a well-established animal model of neuropathic pain and investigated the neural mechanisms involved. A better understanding of this phenomenon is an important prerequisite for the development of improved treatment of patients affected.

High Times for Painful Blues: The Endocannabinoid System in Pain-Depression Comorbidity

Depression and pain are two of the most debilitating disorders worldwide and have an estimated cooccurrence of up to 80%. Comorbidity of these disorders is more difficult to treat, associated with significant disability and impaired health-related quality of life than either condition alone, resulting in enormous social and economic cost. Several neural substrates have been identified as potential mediators in the association between depression and pain, including neuroanatomical reorganization, monoamine and neurotrophin depletion, dysregulation of the hypothalamo-pituitary-adrenal axis, and neuroinflammation. However, the past decade has seen mounting evidence supporting a role for the endogenous cannabinoid (endocannabinoid) system in affective and nociceptive processing, and thus, alterations in this system may play a key role in reciprocal interactions between depression and pain. This review will provide an overview of the preclinical evidence supporting an interaction between depression and pain and the evidence supporting a role for the endocannabinoid system in this interaction.

Short-term effect of acute and repeated urinary bladder inflammation on thigmotactic behaviour in the laboratory rat

Understanding the non-sensory components of the pain experience is crucial to developing effective treatments for pain conditions. Chronic pain is associated with increased incidence of anxio-depressive disorders, and patients often report feelings of vulnerability which can decrease quality of life. In animal models of pain, observation of behaviours such as thigmotaxis can be used to detect such affective disturbances by exploiting the influence of nociceptive stimuli on the innate behavioural conflict between exploration of a novel space and predator avoidance behaviour. This study investigates whether acute and repeated bladder inflammation in adult female Wistar rats increases thigmotactic behaviour in the open field paradigm, and aims to determine whether this correlates with activation in the central amygdala, as measured by c-Fos immunoreactivity. Additionally, up-regulation of inflammatory mediators in the urinary bladder was measured using RT-qPCR array featuring 92 transcripts to examine how local mediators change under experimental conditions. We found acute but not repeated turpentine inflammation of the bladder increased thigmotactic behaviour (decreased frequency of entry to the inner zone) in the open field paradigm, a result that was also observed in the catheter-only instrumentation group. Decreases in locomotor activity were also observed in both models in turpentine and instrumentation groups. No differences were observed in c-Fos activation, although a general increased in activation along the rostro-caudal axis was seen. Inflammatory mediator up-regulation was greatest following acute inflammation, with CCL12, CCL7, and IL-1β significantly up-regulated in both conditions when compared to naïve tissue. These results suggest that acute catheterisation, with or without turpentine inflammation, induces affective alterations detectable in the open field paradigm accompanied by up-regulation of multiple inflammatory mediators.

Pain and depression: a neurobiological perspective of their relationship

Remarkable progresses have been achieved regarding the understanding of the neurobiological bases of pain and depression. The principal role of neurotransmitters, neuromodulators, and neurohormones has been proposed in the development of pain and depression. With the progression of molecular biology, an intricate interaction among biological factors accountable to the development and management of pain and depression has been also shown in a numerous preclinical and clinical researches. This mini-review will briefly describe the current issues and future research direction for better understanding of the relationship between pain and depression.

Pain and depression comorbidity: a preclinical perspective

Pain and depression are two highly prevalent and deleterious disorders with significant socioeconomic impact to society. Clinical observations have long recognized the co-existence and interactions of pain and depression. However, the underlying mechanisms of pain-depression comorbidity and their dynamic interactions remain largely unknown. Preclinical animal studies may provide critical information for the understanding of this important comorbidity. This review analyzed the current preclinical evidence of interactions between pain and depression, which generally supports the causative relationship of the two conditions. In addition, the analysis proposed to apply domain interplay concept in future model development of pain-depression comorbidity and mechanism studies. The application of spectrum-centered animal models will better the understanding of pain-depression dyad and foster the development of more effective therapeutic strategies.

An altered spinal serotonergic system contributes to increased thermal nociception in an animal model of depression

The olfactory bulbectomized (OB) rat, an animal model of chronic depression with comorbid anxiety, exhibits a profound dysregulation of the brain serotonergic signalling, a neurotransmission system involved in pain transmission and modulation. We here report an increased nociceptive response of OB rats in the tail flick test which is reverted after chronic, but not acute, administration of fluoxetine. Autoradiographic studies demonstrated down-regulation of 5-HT transporters ([(3)H]citalopram binding) and decreased functionality of 5-HT1A receptors (8-OH-DPAT-stimulated [(35)S]GTPγS binding) in the dorsal horn of the lumbar spinal cord in OB rats. Acute administration of fluoxetine (5-40 mg/kg i.p.) did not modify tail flick latencies in OB rats. However, chronic fluoxetine (10 mg/kg/day s.c., 14 days; osmotic minipumps) progressively attenuated OB-associated thermal hyperalgesia, and a total normalization of the nociceptive response was achieved at the end of the treatment with the antidepressant. In these animals, autoradiographic studies revealed further down-regulation of 5-HT transporters and normalization in the functionality of 5-HT1A receptors on the spinal cord. On the other hand, acute morphine (0.5-10 mg/kg s.c.) produced a similar analgesic effect in OB and sham and OB rats, and no changes were detected in the density ([(3)H]DAMGO binding) and functionality (DAMGO-stimulated [(35)S]GTPγS binding) of spinal μ-opioid receptors in OB rats before and after chronic fluoxetine. Our findings demonstrate the participation of the spinal serotonergic system in the increased thermal nociception exhibited by the OB rat and the antinociceptive effect of chronic fluoxetine in this animal model of depression.