Glucocorticoid-Potentiated Spinal Microglia Activation Contributes to Preoperative Anxiety-Induced Postoperative Hyperalgesia

Blocking brown adipocyte β3-adrenoceptor attenuates blood-spinal cord barrier impairment and chronic postsurgical pain in a rat model of preoperative stress

Preoperative stress has been recognized as an independent risk factor for chronic postsurgical pain (CPSP). However, the underlying mechanisms of CPSP influenced by preoperative stress remain elusive. Previous studies indicated that excessive stress could induce disruption of the blood-spinal cord barrier (BSCB). We wondered whether and how BSCB involves in CPSP by using a single prolonged stress (SPS) combining plantar incision model in male rats to mimic preoperative stress-related postsurgical pain. Here, we observed that preoperative SPS-exposed rats exhibited relentless incisional pain, which was accompanied by impairment of BSCB and persistent elevation of serum IL-6. Intraperitoneal injections of Tocilizumab (an IL-6 receptor monoclonal antibody) not only mitigated BSCB breakdown but also alleviated pain behaviors. In addition, intervening β3-adrenoceptor (ADRB3) signaling in brown adipocytes by SR59230a (a specific ADRB3 antagonist) treatment or removal of brown adipose tissues could effectively decrease serum IL-6 levels, ameliorate BSCB disruption, and alleviate incisional pain. Further results displayed that SI-exposed rats also showed markedly spinal microglia activation. And exogenous His-tagged IL-6 could pass through the disrupted BSCB, which might contribute to microglia activation. Injection of SR59230a or ablation of brown adipose tissues could effectively reduce the activation of spinal microglia. Thus, our findings suggest that serum IL-6 induced by brown adipocyte ADRB3 signaling contributed to BSCB disruption and spinal microglia activation, which might be involved in preoperative stress mediated CPSP. This work indicates a promising treatment strategy for preoperative stress induced CPSP by blocking ADRB3.

Stress-induced changes in nociceptive responding post-surgery in preclinical rodent models

Chronic post-surgical pain affects up to 85% of individuals depending on the type of surgery, the extent of inflammation, tissue and/or nerve damage. Pre-surgical stress is associated with greater pain intensity, prolonged recovery and is one of the main risk factors for the development of chronic post-surgical pain. Clinically valid animal models provide an important means of examining the mechanisms underlying the effects of stress on post-surgical pain and identifying potential novel therapeutic targets. This review discusses the current data from preclinical animal studies examining the effect of stress on post-surgical pain, the potential underlying mechanisms and gaps in the knowledge that require further investigation.

Spinal microglial activation promotes perioperative social defeat stress-induced prolonged postoperative pain in a sex-dependent manner

Prolonged postsurgical pain, which is associated with multiple risk factors in the perioperative stage, is a common medical and social problem worldwide. Suitable animal models should be established to elucidate the mechanisms underlying the perioperative prolonged postsurgical pain. In this study, standard and modified social defeat stress mice models, including chronic social defeat stress (CSDS), chronic nondiscriminatory social defeat stress (CNSDS) and vicarious social defeat stress (VSDS), were applied to explore the effect of perioperative social defeat stress on postsurgical pain in male and female mice. Our results showed that exposure to preoperative CSDS could induce prolonged postsurgical pain in defeated mice regardless of susceptibility or resilience differentiated by the social interaction test. Similar prolongation of incision-induced mechanical hypersensitivity was also observed in both sexes upon exposing to CNSDS or VSDS in the preoperative period. Moreover, we found that using the modified CNSDS or VSDS models at different recovery stages after surgery could still promote abnormal pain without sex differences. Further studies revealed the key role of spinal microglial activation in the stress-induced transition from acute to prolonged postoperative pain in male but not female mice. Together, these data indicate that perioperative social defeat stress is a vital risk factor for developing prolonged postoperative pain in both sexes, but the promotion of stress-induced prolonged postoperative pain by spinal microglial activation is sexually dimorphic in mice.

Angiotensin II type 1 receptor blockade attenuates posttraumatic stress disorder-related chronic pain by inhibiting glial activation in the spinal cord

Clinically, posttraumatic stress disorder (PTSD) and chronic pain are highly comorbid conditions, but the underlying mechanisms of and therapeutic strategies against PTSD-related pain remain unclear. Our previous studies suggested that dysregulation of neuroinflammation contributes to the development of stress-induced hyperalgesia. Recent studies reported that angiotensin II was a 'stress-related hormone', and could induce glial activation by stimulating the type 1 receptor (AT1R). In the present study, we aimed to investigate whether AT1R blockade could attenuate mechanical allodynia induced by PTSD-like stress. Adult male rats were exposed to single prolonged stress (SPS) to establish a model of PTSD-pain comorbidity. Our results showed that SPS exposure increased the levels of angiotensin II in the hippocampus, prefrontal cortex (PFC) and spinal cord; intraperitoneal injection of losartan attenuated SPS-induced mechanical allodynia, and suppressed SPS-induced glial activation (both microglia and astrocytes) and proinflammatory cytokine expression in the PFC and spinal cord, but not in the hippocampus. We further showed that intrathecal injection of losartan also exerted anti-hyperalgesic effect and suppressed SPS-induced glial activation and proinflammatory cytokine expression in the spinal cord. These results indicated that AT1R blockade by losartan attenuated mechanical allodynia induced by PTSD-like stress, and this may be attributed to the suppression of glial activation and proinflammatory cytokine expression in the spinal cord. Although further research is warranted to verify our findings in female rodents and to assess pharmacological effects of AT1R blockade in PFC and hippocampus, our study suggested the therapeutic potential of targeting AT1R in the treatment of PTSD-related chronic pain.

Emerging Role of Serum Glucocorticoid-Regulated Kinase 1 in Pathological Pain

Currently, the management of acute and chronic pain in clinical practice remains unsatisfactory due to the existence of limited effective treatments, and novel therapeutic strategies for pathological pain are urgently needed. In the past few decades, the role of serum and glucocorticoid-inducible kinase 1 (SGK1) in the development of pain and diurnal rhythms has been implicated in numerous studies. The expression levels of SGK1 mRNA and protein were found to be elevated in the spinal cord and brain in various pathological pain models. Blocking SGK1 significantly attenuated pain-like responses and the development of pathological pain. These studies provide strong evidence that SGK1 plays a role in the development of various types of pathological pain and that targeting SGK1 may be a novel therapeutic strategy for pain management. In this review article, we provide evidence from animal models for the potential role of SGK1 in the regulation of pathological pain caused by inflammation, nerve injury, psychiatric disorders, and chronic opioid exposure.

TNF-Alpha as an Initiator of Allodynia and Anxiety-Like Behaviors in a Preclinical Model of PTSD and Comorbid Pain

Post-Traumatic Stress Disorder (PTSD) is a debilitating mental health disorder that occurs after exposure to a traumatic event. Patients with comorbid chronic pain experience affective distress, worse quality of life, and poorer responses to treatments for pain or PTSD than those with either condition alone. FDA-approved PTSD treatments are often ineffective analgesics, requiring additional drugs to treat co-morbid symptoms. Therefore, development of new treatment strategies necessitate a better understanding of the pathophysiology of PTSD and comorbid pain. The single prolonged stress (SPS) model of PTSD induces the development of persistent mechanical allodynia and thermal hyperalgesia. Increased Nociceptin/Orphanin FQ (N/OFQ) levels in serum and CSF accompany these exaggerated nociceptive responses, as well as increased serum levels of the pro-inflammatory cytokine tumor necrosis factor (TNF-α). Therefore, the primary goal was to determine the role of TNF-α in the development of SPS-induced allodynia/hyperalgesia and elevated serum and CNS N/OFQ using two approaches: TNF-α synthesis inhibition, and blockade with anti-TNF-α antibody that acts primarily in the periphery. Administration of TNF-α synthesis blocker, thalidomide (THL), immediately after SPS prevented increased TNF-α and development of allodynia and hyperalgesia. The THL effect lasted at least 21 days, well after thalidomide treatment ended (day 5). THL also prevented SPS-induced increases in serum N/OFQ and reversed regional N/OFQ mRNA expression changes in the CNS. Serum TNF-α increases detected at 4 and 24 h post SPS were not accompanied by blood brain barrier disruption. A single injection of anti-TNF-α antibody to male and female rats during the SPS procedure prevented the development of allodynia, hyperalgesia, and elevated serum N/OFQ, and reduced SPS-induced anxiety-like behaviors in males. Anti-TNFα treatment also blocked development of SPS-induced allodynia in females, and blocked increased hypothalamic N/OFQ in males and females. This suggests that a peripheral TNF-α surge is necessary for the initiation of allodynia associated with SPS, as well as the altered central and peripheral N/OFQ that maintains nociceptive sensitivity. Therefore, early alleviation of TNF-α provides new therapeutic options for investigation as future PTSD and co-morbid pain treatments.

Acute exposure of rats to a severe stressor alters the circadian pattern of corticosterone and sensitizes to a novel stressor: Relationship to pre-stress individual differences in resting corticosterone levels

Traumatic events have been proposed to be associated with hypo-activity of the hypothalamic-pituitary-adrenal (HPA) axis, but data in animal models exposed to severe stressors are controversial and have important methodological concerns. Individual differences in resting or stress levels of corticosterone might explain some of the inconsistencies. We then studied this issue in male rats exposed to 2 h immobilization on boards (IMO), a severe stressor. Thirty-six rats were blood sampled under resting conditions four times a day on three non-consecutive days. Then, they were assigned to control (n = 14) or IMO (n = 22) to study the HPA response to IMO, the stressor-induced alterations in the circadian pattern of corticosterone (CPCORT), and the behavioral and HPA responsiveness to an open-field. Individual differences in pre-IMO resting corticosterone were inconsistent, but averaging data markedly improved consistency. The CPCORT was markedly altered on day 1 post-IMO (higher trough and lower peak levels), less altered on day 3 and apparently normal on day 7. Importantly, when rats were classified in low and high resting corticosterone groups (LCORT and HCORT, respectively), on the basis of the area under the curve (AUC) of the averaged pre-IMO data, AUC differences between LCORT and HCORT groups were maintained in controls but disappeared in IMO rats during the post-IMO week. Open-field hypo-activity and corticosterone sensitization were similar in LCORT and HCORT groups nine days after IMO. A single IMO exposure causes long-lasting HPA alterations, some of them dependent on pre-stress resting corticosterone levels, with no evidence for post-IMO resting corticosterone hypo-activity.

Significance and Mechanisms of P-glycoprotein in Central Nervous System Diseases

P-glycoprotein (P-gp) is a member of ATP-Binding Cassette (ABC) transporter family. Because of its characteristic luminal surface location, high transport potency and structural specificity, Pgp is regarded as a selective gatekeeper of the Blood Brain Barrier (BBB) to prevent the entry of toxins or unwanted substances into the brain. In recent years, increasing evidence has shown that P-gp is involved in the immune inflammatory response in the Central Nervous System (CNS) disorders by regulating microglia activation, and mediating immune cell migration. Furthermore, Glucocorticoid Receptor (GR) may play a crucial role in P-gp-mediated microglia activation and immune cell migration via GR-mediated mRNA decay. In this article, we will review P-gp structure, distribution, function, regulatory mechanisms, inhibitors and effects of P-gp in the pathogenesis of several CNS diseases and will discuss the role of P-gp in microglia activation, immune cell migration and the relationship with cytokine secretion.

Preoperative anxiety-induced glucocorticoid signaling reduces GABAergic markers in spinal cord and promotes postoperative hyperalgesia by affecting neuronal PAS domain protein 4

Preoperative anxiety is common in patients undergoing elective surgery and is closely related to postoperative hyperalgesia. In this study, a single prolonged stress model was used to induce preoperative anxiety-like behavior in rats 24 h before the surgery. We found that single prolonged stress exacerbated the postoperative pain and elevated the level of serum corticosterone. Previous studies have shown that glucocorticoid is associated with synaptic plasticity, and decreased spinal GABAergic activity can cause hyperalgesia in rodents. Here, single prolonged stress rats’ lumbar spinal cord showed reduced glutamic acid decarboxylase-65, glutamic acid decarboxylase-67, GABA type A receptor alpha 1 subunit, and GABA type A receptor gamma 2 subunit, indicating an impairment of GABAergic system. Furthermore, neuronal PAS domain protein 4 was also reduced in rats after single prolonged stress stimulation, which has been reported to promote GABAergic synapse development. Then, intraperitoneal injection of RU486 (a glucocorticoid receptor antagonist) rather than spironolactone (a mineralocorticoid receptor antagonist) was found to relieve single prolonged stress-induced hyperalgesia and reverse neuronal PAS domain protein 4 reduction and the impairment of GABAergic system. Furthermore, overexpressing neuronal PAS domain protein 4 could also restore the damage of GABAergic system caused by single prolonged stress while interfering with neuronal PAS domain protein 4 caused an opposite effect. Finally, after stimulation of rat primary spinal cord neurons with exogenous corticosterone in vitro, neuronal PAS domain protein 4 and GABAergic markers were also downregulated, and RU486 reversed that. Together, our results demonstrated that preoperative anxiety led to GABAergic system impairment in spinal cord and thus caused hyperalgesia due to glucocorticoid-induced downregulation of neuronal PAS domain protein 4.

GPR30 receptor promotes preoperative anxiety-induced postoperative hyperalgesia by up-regulating GABAA-α4β1δ subunits in periaqueductal gray in female rats

BACKGROUND

G-protein coupled estrogen receptor 30 (GPR30) was proved the specific estrogen receptor relating to mechanical hyperalgesia. Studies have shown that the GABA_A receptor subunits α4, β1, and δ in the periaqueductal gray (PAG) neurons promote the descending facilitation system. This study inquired into whether and how GPR30 and GABA_A-α4β1δ in the PAG promote preoperative anxiety-induced postoperative hyperalgesia in female rats.

METHODS

All the female rats were subjected to the single prolonged stress (SPS) to stimulate preoperative anxiety. Subsequently, mechanical allodynia was evaluated before and after the incision, based on the paw withdrawal mechanical threshold (PWMT). The selective GPR30 agonist G1 and antagonist G15 were locally microinjected into the PAG. The expression of GPR30, protein kinase A (PKA), and GABA_A receptor subunits α4, β1, and δ in the PAG neurons were detected using western blotting and immunofluorescence.

RESULTS

Behavioral testing revealed that Group S and Group I decreased the nociceptive threshold levels of PWMT in female rats. PWMT in Group S + I decreased more than that of Group S and Group I. Further, results of western blotting showed the expression of GPR30, PKA, and GABA_A α4, β1, and δ subunits significantly up-regulated in Group S + I, and immunofluorescence indicated that the neurons of PAG in Group S + I appeared simultaneously immunopositive for GPR30 and GABA_A α4, β1, and δ receptors. After microinjection of G1 into the PAG, female rats with plantar incision continued to exhibit significant hyperalgesia until postoperative 48 h. On the other hand, microinjection of G15 with SPS and plantar incision procedure relieved postoperative hyperalgesia in female rats. Western blotting demonstrated that intra-PAG injection of G15 markedly decreased the GPR30, PKA, and GABA_A α4, β1, and δ levels in Group G15 + I.

CONCLUSIONS

Our results indicate that the GPR30-PKA-GABA_Aα4β1δ pathway in the PAG promotes preoperative anxiety-induced postoperative hyperalgesia in female rats. This mechanism might be a potential novel therapeutic target for hyperalgesia in females.

The GCs-SGK1-ATP Signaling Pathway in Spinal Astrocytes Underlied Presurgical Anxiety-Induced Postsurgical Hyperalgesia

BACKGROUND

Patients undergoing surgery often feel anxious. Accumulating evidence indicated that presurgical anxiety was related to the more severe postsurgical pain. An animal model was established that exposed Sprague-Dawley rats to a single-prolonged stress (SPS) procedure to induce presurgical anxiety-like behaviors. The experiment revealed that presurgical anxiety not only aggravated but also prolonged postsurgical pain. However, the underlying mechanisms were unknown.

METHODS

The rats in group C + Cort, group I + Cort, group A + Cort, and group AI + Cort were injected with corticosterone. The rats in group C + RU486, group I + RU486, group A + RU486, and group AI + RU486 were injected with mifepristone (RU486). The rats in group C + GSK650394 and group AI + GSK650394 were injected with GSK650394. The rats in group C + FC1 and group AI + FC1 were injected with fluorocitrate (FC) 30 minutes before SPS, 30 minutes before incision, and on postoperative days 1, 2, 3, 4, and 5. The rats in group C + FC2 and group AI + FC2 were injected with FC on postoperative days 7, 8, 9, 10, 11, 12, and 13. The paw withdrawal mechanical threshold was assessed 24 hours before SPS and from postoperative days 1 to 28. The level of corticosterone was determined by enzyme-linked immunosorbent assay. The expression of serum/glucocorticoid regulated kinase 1 (SGK1), interleukin-1β, and tumor necrosis factor-α was visualized by Western blot. The concentrations of adenosine triphosphate (ATP) were measured by ATP assay kit.

RESULTS

This study showed SPS elevated plasma glucocorticoids and ATP release from astrocytes, which meant the mechanical pain hypersensitivity in presurgical anxiety-induced postsurgical hyperalgesia was dependent on GCs-SGK1-ATP signaling pathway. SGK1 protein level in astrocytes was increased in response to the glucocorticoid stimuli and enhanced the extracellular release of ATP. Furthermore, spinal astrocytes played a key role in the maintenance. Targeting spinal astrocytes in maintenance phase prevented the pathological progression.

CONCLUSIONS

These data suggested an important signaling pathway that affected the pain sensitivity after operation caused by presurgical anxiety.

Neuroinflammation and glial cell activation in mental disorders

Mental disorders (MDs) are highly prevalent and potentially debilitating complex disorders which causes remain elusive. Looking into deeper aspects of etiology or pathophysiology underlying these diseases would be highly beneficial, as the scarce knowledge in mechanistic and molecular pathways certainly represents an important limitation. Association between MDs and inflammation/neuroinflammation has been widely discussed and accepted by many, as high levels of pro-inflammatory cytokines were reported in patients with several MDs, such as schizophrenia (SCZ), bipolar disorder (BD) and major depression disorder (MDD), among others. Correlation of pro-inflammatory markers with symptoms intensity was also reported. However, the mechanisms underlying the inflammatory dysfunctions observed in MDs are not fully understood yet. In this context, microglial dysfunction has recently emerged as a possible pivotal player, as during the neuroinflammatory response, microglia can be over-activated, and excessive production of pro-inflammatory cytokines, which can modify the kynurenine and glutamate signaling, is reported. Moreover, microglial activation also results in increased astrocyte activity and consequent glutamate release, which are both toxic to the Central Nervous System (CNS). Also, as a result of increased microglial activation in MDs, products of the kynurenine pathway were shown to be changed, influencing then the dopaminergic, serotonergic, and glutamatergic signaling pathways. Therefore, in the present review, we aim to discuss how neuroinflammation impacts on glutamate and kynurenine signaling pathways, and how they can consequently influence the monoaminergic signaling. The consequent association with MDs main symptoms is also discussed. As such, this work aims to contribute to the field by providing insights into these alternative pathways and by shedding light on potential targets that could improve the strategies for pharmacological intervention and/or treatment protocols to combat the main pharmacologically unmatched symptoms of MDs, as the SCZ.

Hypertrophic Scar Severity at Autograft Sites Is Associated With Increased Pain and Itch After Major Thermal Burn Injury

Approximately three quarters of major thermal burn injury (MThBI) survivors suffer from hypertrophic scarring (HTS) and over half experience chronic pain or itch. In survivors of MThBI, HTS and chronic pain or itch are considered one of the greatest unmet challenges of postburn injury care and psychosocial reintegration. Although scarring, itch, and pain have been clinically associated, there are no prospective, multisite studies examining tissue autograft site pain or itch and scar outcomes. The authors collected a representative cohort (n = 56) of MThBI survivors who received autografting within 14 days of injury and evaluated graft-site pain or itch severity (0-10 Numeric Rating Scale) and HTS using a validated scar photograph assessment scale 6 months following MThBI. Given that stress is known to influence wound healing, the authors also assessed the relationship between previous trauma exposure, peritraumatic stress, preburn overall health (SF-12), scarring, and chronic pain or itch severity using Spearman's correlation. Association between HTS and chronic pain or itch was significant in a linear regression model adjusted for age, sex, and ethnicity (β = 0.2, P = .033 for pain, β = 0.2, P = .019 for itch). Results indicate that prior trauma exposure is inversely correlated (r = -.363, P = .030) with scar severity, but not pain or itch severity 6 months after MThBI. Study results suggest that preburn chronic pain or itch is associated with pathological scarring 6 months following MThBI. Results also indicate that stress may improve scarring after MThBI. Further work to understand the mechanisms that underlie both HTS and chronic pain or itch and their relationship to chronic stress is critical to the development of novel therapies to assist burn survivors recover.

Corticosterone-mediated microglia activation affects dendritic spine plasticity and motor learning functions in minimal hepatic encephalopathy

Minimal hepatic encephalopathy (MHE) is characterized as cognitive deficits including memory and learning dysfunctions after liver injuries or hepatic diseases. Our understandings of neurological mechanisms of MHE-associated cognitive syndromes, however, are far from complete. In the current study we generated a mouse MHE model by repetitive administrations of thioacetamide (TAA), which induced hyperammonemia plus elevated proinflammatory cytokines in both the general circulation and motor cortex. MHE mice presented prominent motor learning deficits, which were associated with excess dendritic spine pruning in the motor cortex under 2-photon in vivo microscopy. The pharmaceutical blockade of glucocorticoid receptor or suppression of its biosynthesis further rescued motor learning deficits and synaptic protein loss. Moreover, MHE mice presented microglial activation, which can be alleviated after glucocorticoid pathway inhibition. In sum, our data demonstrates corticosterone-induced microglial activation, synaptic over-pruning and motor learning impairments in MHE, providing new insights for MHE pathogenesis and potential targets of clinical interventions.

Glucocorticoids mediate stress induction of the alarmin HMGB1 and reduction of the microglia checkpoint receptor CD200R1 in limbic brain structures

Exposure to stressors primes neuroinflammatory responses to subsequent immune challenges and stress-induced glucocorticoids (GCs) play a mediating role in this phenomenon of neuroinflammatory priming. Recent evidence also suggests that the alarmin high-mobility group box-1 (HMGB1) and the microglial checkpoint receptor CD200R1 serve as proximal mechanisms of stress-induced neuroinflammatory priming. However, it is unclear whether stress-induced GCs play a causal role in these proximal mechanisms of neuroinflammatory priming; this forms the focus of the present investigation. Here, we found that exposure to a severe acute stressor (inescapable tailshock) induced HMGB1 and reduced CD200R1 expression in limbic brain regions and pharmacological blockade of GC signaling (RU486) mitigated these effects of stress. To confirm these effects of RU486, adrenalectomy (ADX) with basal corticosterone (CORT) replacement was used to block the stress-induced increase in GCs as well as effects on HMGB1 and CD200R1. As with RU486, ADX mitigated the effects of stress on HMGB1 and CD200R1. Subsequently, exogenous CORT was administered to determine whether GCs are sufficient to recapitulate the effects of stress. Indeed, exogenous CORT induced expression of HMGB1 and reduced expression of CD200R1. In addition, exposure of primary microglia to CORT also recapitulated the effects of stress on CD200R1 suggesting that CORT acts directly on microglia to reduce expression of CD200R1. Taken together, these findings suggest that GCs mediate the effects of stress on these proximal mechanisms of neuroinflammatory priming.

The Selective Glucocorticoid Receptor Modulator Cort 113176 Reduces Neurodegeneration and Neuroinflammation in Wobbler Mice Spinal Cord

Wobbler mice are experimental models for amyotrophic lateral sclerosis. As such they show motoneuron degeneration, motor deficits, and astrogliosis and microgliosis of the spinal cord. Additionally, Wobbler mice show increased plasma, spinal cord and brain corticosterone levels and focal adrenocortical hyperplasia, suggesting a pathogenic role for glucocorticoids in this disorder. Considering this endocrine background, we examined whether the glucocorticoid receptor (GR) modulator CORT 113176 prevents spinal cord neuropathology of Wobblers. CORT 113176 shows high affinity for the GR, with low or null affinity for other steroid receptors. We employed five-month-old genotyped Wobbler mice that received s.c. vehicle or 30 mg/kg/day for 4 days of CORT 113176 dissolved in sesame oil. The mice were used on the 4th day, 2 h after the last dose of CORT 113176. Vehicle-treated Wobbler mice presented vacuolated motoneurons, increased glial fibrillary acidic protein (GFAP)+ astrocytes and decreased glutamine synthase (GS)+ cells. There was strong neuroinflammation, shown by increased staining for IBA1+ microglia and CD11b mRNA, enhanced expression of tumor necrosis factor-α, its cognate receptor TNFR1, toll-like receptor 4, the inducible nitric oxide synthase, NFkB and the high-mobility group box 1 protein (HMGB1). Treatment of Wobbler mice with CORT 113176 reversed the abnormalities of motoneurons and down-regulated proinflammatory mediators and glial reactivity. Expression of glutamate transporters GLT1 and GLAST mRNAs and GLT1 protein was significantly enhanced over untreated Wobblers. In summary, antagonism of GR with CORT 113176 prevented neuropathology and showed anti-inflammatory and anti-glutamatergic effects in the spinal cord of Wobbler mice.

Psychosocial Stress Delays Recovery of Postoperative Pain Following Incisional Surgery in the Rat

Psychosocial factors such as anxiety, depression and catastrophizing, commonly associated with established chronic pain, also may be associated with an increased risk of chronic postsurgical pain (CPSP) when present preoperatively. We used a repeat social defeat (RSD) paradigm to induce psychosocial stress in rodents prior to incisional surgery of the paw. Mixed effects growth curve models were utilized to examine resolution of mechanical hypersensitivity in rats for four weeks following surgery. Eight days following surgery, immunohistochemistry was conducted to examine glial activation as well as evoked neuronal activation in the spinal cord. Here we document that RSD resulted in reduced weight gain and increased depressive symptoms prior to surgery. Rats exposed to RSD displayed delayed resolution of mechanical hypersensitivity in the ipsilateral paw following surgery compared to non-defeated rats. Prior exposure to RSD significantly increased microglial activation and neuronal sensitization (pERK-IR) within the ipsilateral spinal cord. In conclusion, we found that chronic social stress alters the neurobiological response to surgical injury, resulting in slowed recovery. This model maybe useful for future interventional studies examining the mechanistic interactions between depression and risk of CPSP.

What role does multiple sclerosis play in the development of untreatable painful conditions?

Clinical data outline the high incidence of pain syndromes in patients with multiple sclerosis, with a significant prevalence of craniofacial manifestations, including trigeminal neuralgia and migraine, which are very difficult to be managed pharmacologically. The common explanation of a localization of demyelinating plaques in areas devoted to pain modulation and integration as a trigger for pain development seems now partially unsatisfactory, since pain is often manifested well before the clinical signs of the pathology and its severity does not correlate with disease progression. This review focuses on additional mechanisms which could be at the basis of pain development in multiple sclerosis, whose identification will help identifying new targets to design more effective analgesic strategies.

Single-Prolonged Stress: A Review of Two Decades of Progress in a Rodent Model of Post-traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a common, costly, and often debilitating psychiatric condition. However, the biological mechanisms underlying this disease are still largely unknown or poorly understood. Considerable evidence indicates that PTSD results from dysfunction in highly-conserved brain systems involved in stress, anxiety, fear, and reward. Pre-clinical models of traumatic stress exposure are critical in defining the neurobiological mechanisms of PTSD, which will ultimately aid in the development of new treatments for PTSD. Single prolonged stress (SPS) is a pre-clinical model that displays behavioral, molecular, and physiological alterations that recapitulate many of the same alterations observed in PTSD, illustrating its validity and giving it utility as a model for investigating post-traumatic adaptations and pre-trauma risk and protective factors. In this manuscript, we review the present state of research using the SPS model, with the goals of (1) describing the utility of the SPS model as a tool for investigating post-trauma adaptations, (2) relating findings using the SPS model to findings in patients with PTSD, and (3) indicating research gaps and strategies to address them in order to improve our understanding of the pathophysiology of PTSD.

Sex Differences in Nociceptin/Orphanin FQ Peptide Receptor-Mediated Pain and Anxiety Symptoms in a Preclinical Model of Post-traumatic Stress Disorder

Nociceptin/Orphanin FQ (N/OFQ) is a neuropeptide that modulates pain transmission, learning/memory, stress, anxiety, and fear responses via activation of the N/OFQ peptide (NOP or ORL1) receptor. Post-traumatic stress disorder (PTSD) is an anxiety disorder that may arise after exposure to a traumatic or fearful event, and often is co-morbid with chronic pain. Using an established animal model of PTSD, single-prolonged stress (SPS), we were the first to report that NOP receptor antagonist treatment reversed traumatic stress-induced allodynia, thermal hyperalgesia, and anxiety-like behaviors in male Sprague-Dawley rats. NOP antagonist treatment also reversed SPS-induced serum and CSF N/OFQ increase and circulating corticosterone decrease. The objective of this study was to examine the role of the NOP receptor in male and female rats subjected to traumatic stress using Wistar wild type (WT) and NOP receptor knockout (KO) rats. The severity of co-morbid allodynia was assessed as change in paw withdrawal threshold (PWT) to von Frey and paw withdrawal latency (PWL) to radiant heat stimuli, respectively. PWT and PWL decreased in male and female WT rats within 7 days after SPS, and remained decreased through day 28. Baseline sensitivity did not differ between genotypes. However, while male NOP receptor KO rats were protected from SPS-induced allodynia and thermal hypersensitivity, female NOP receptor KO rats exhibited tactile allodynia and thermal hypersensitivity to the same extent as WT rats. Male NOP receptor KO rats had a lower anxiety index (AI) than WT, but SPS did not increase AI in WT males. In contrast, SPS significantly increased AI in WT and NOP receptor KO female rats. SPS increased circulating N/OFQ levels in male WT, but not in male NOP receptor KO, or WT or KO female rats. These results indicate that the absence of the NOP receptor protects males from traumatic-stress-induced allodynia and hyperalgesia, consistent with our previous findings utilizing a NOP receptor antagonist. However, female NOP receptor KO rats experience allodynia, hyperalgesia and anxiety-like symptoms to the same extent as WT females following SPS. This suggests that endogenous N/OFQ-NOP receptor signaling plays an important, but distinct, role in males and females following exposure to traumatic stress.

Chronic unpredictable stress exacerbates surgery-induced sickness behavior and neuroinflammatory responses via glucocorticoids secretion in adult rats

Accumulated evidence indicates that stress sensitizes neuroinflammatory responses to a subsequent peripheral immune challenge. The present study investigated whether chronic unpredictable stress (CUS) aggravated surgery-induced sickness behavior and neuroinflammatory processes via glucocorticoids secretion in the adult brain.

Saturated fatty acids recognition by the CD14-TLR4-MD2 complex may engage in the presurgical anxiety-induced persistent postsurgical pain

It has been proved that presurgical anxiety can induce the development and progression of persistent postsurgical pain through elevating circulating corticosterone levels and activating the glucocorticoids receptor. Under stressful circumstances, the concentration of blood saturated fatty acids (SFAs) increases rapidly to provide enough ATP for individuals' survival owing to stress hormones such as glucocorticoid, catecholamine and glucagon. It is reported that SFAs can trigger an inflammatory response through CD14-TLR4-MD2 complex. The role of TLR4 and its downstream signaling pathway has been confirmed in the pathogenesis of cancer pain and inflammatory pain. Furthermore, a newly clinical research uncovers that the children with recurrent abdominal pain, which is triggered by early psychosomatic stress, have higher levels of SFAs than healthy individuals. We therefore put forward the hypothesis that the elevated level of SFAs induced by stress hormones may be engaged in the presurgical anxiety-induced persistent postsurgical pain. If established, it's of important clinical significance, which will make great contributions to the prevention and treatment of the presurgical anxiety-induced persistent postsurgical pain.

Spinal activation of alpha7-nicotinic acetylcholine receptor attenuates posttraumatic stress disorder-related chronic pain via suppression of glial activation

The high prevalence of chronic pain in posttraumatic stress disorder (PTSD) individuals has been widely reported by clinical studies, which emphasized an urgent need to uncover the underlying mechanisms and identify potential therapeutic targets. Recent studies suggested that targeting activated glia and their pro-inflammatory products may provide a novel and effective therapy for the stress-related pain. In this study, we investigated whether activation of alpha-7 nicotinic acetylcholine receptor (α7 nAChR), a novel anti-inflammatory target, could attenuate PTSD-related chronic pain. The experiments were conducted in a rat model of single prolonged stress (SPS), an established model of PTSD-pain comorbidity. We found that SPS exposure produced persistent mechanical allodynia. Immunohistochemical and enzyme-linked immuno sorbent assay analysis showed that SPS also induced elevated activation of glia cells (including microglia and astrocytes) and accumulation of pro-inflammatory cytokines in spinal cord. In another experiment, we found that intrathecal injection of PHA-543613, a selective α7 nAchR agonist, attenuated the SPS-evoked allodynia in a dose dependent manner. However, this anti-hyperalgesic effect was blocked by pretreatment with methyllycaconitine (MLA), a selective α7 nAchR antagonist. Further analyses showed that PHA-543613 suppressed SPS-induced spinal glial activation and SPS-elevated spinal pro-inflammatory cytokines, and these were abolished by MLA. Taken together, the present study showed that spinal activation of α7 nAChR by PHA-543613 attenuated mechanical allodynia induced by PTSD-like stress, and the suppression of spinal glial activation may underlie this anti-hyperalgesic effect. Our study demonstrated the therapeutic potential of targeting α7 nAChR in the treatment of PTSD-related chronic pain.

Hippocampal activation of microglia may underlie the shared neurobiology of comorbid posttraumatic stress disorder and chronic pain

The high comorbidity rates of posttraumatic stress disorder and chronic pain have been widely reported, but the underlying mechanisms remain unclear. Emerging evidence suggested that an excess of inflammatory immune activities in the hippocampus involved in the progression of both posttraumatic stress disorder and chronic pain. Considering that microglia are substrates underlying the initiation and propagation of the neuroimmune response, we hypothesized that stress-induced activation of hippocampal microglia may contribute to the pathogenesis of posttraumatic stress disorder-pain comorbidity. We showed that rats exposed to single prolonged stress, an established posttraumatic stress disorder model, exhibited persistent mechanical allodynia and anxiety-like behavior, which were accompanied by increased activation of microglia and secretion of pro-inflammatory cytokines in the hippocampus. Correlation analyses showed that hippocampal activation of microglia was significantly correlated with mechanical allodynia and anxiety-like behavior. Our data also showed that both intraperitoneal and intra-hippocampal injection of minocycline suppressed single prolonged stress-induced microglia activation and inflammatory cytokines accumulation in the hippocampus, and attenuated both single prolonged stress-induced mechanical allodynia and anxiety-like behavior. Taken together, the present study suggests that stress-induced microglia activation in the hippocampus may serve as a critical mechanistic link in the comorbid relationship between posttraumatic stress disorder and chronic pain. The novel concept introduces the possibility of cotreating chronic pain and posttraumatic stress disorder.