Up-regulation of spinal microglial Iba-1 expression persists after resolution of neuropathic pain hypersensitivity

Implication of system xc- in neuroinflammation during the onset and maintenance of neuropathic pain

BACKGROUND

Despite the high prevalence of neuropathic pain, treating this neurological disease remains challenging, given the limited efficacy and numerous side effects associated with current therapies. The complexity in patient management is largely attributed to an incomplete understanding of the underlying pathological mechanisms. Central sensitization, that refers to the adaptation of the central nervous system to persistent inflammation and heightened excitatory transmission within pain pathways, stands as a significant contributor to persistent pain. Considering the role of the cystine/glutamate exchanger (also designated as system xc-) in modulating glutamate transmission and in supporting neuroinflammatory responses, we investigated the contribution of this exchanger in the development of neuropathic pain.

METHODS

We examined the implication of system xc- by evaluating changes in the expression/activity of this exchanger in the dorsal spinal cord of mice after unilateral partial sciatic nerve ligation. In this surgical model of neuropathic pain, we also examined the consequence of the genetic suppression of system xc- (using mice lacking the system xc- specific subunit xCT) or its pharmacological manipulation (using the pharmacological inhibitor sulfasalazine) on the pain-associated behavioral responses. Finally, we assessed the glial activation and the inflammatory response in the spinal cord by measuring mRNA and protein levels of GFAP and selected M1 and M2 microglial markers.

RESULTS

The sciatic nerve lesion was found to upregulate system xc- at the spinal level. The genetic deletion of xCT attenuated both the amplitude and the duration of the pain sensitization after nerve surgery, as evidenced by reduced responses to mechanical and thermal stimuli, and this was accompanied by reduced glial activation. Consistently, pharmacological inhibition of system xc- had an analgesic effect in lesioned mice.

CONCLUSION

Together, these observations provide evidence for a role of system xc- in the biochemical processes underlying central sensitization. We propose that the reduced hypersensitivity observed in the transgenic mice lacking xCT or in sulfasalazine-treated mice is mediated by a reduced gliosis in the lumbar spinal cord and/or a shift in microglial M1/M2 polarization towards an anti-inflammatory phenotype in the absence of system xc-. These findings suggest that drugs targeting system xc- could contribute to prevent or reduce neuropathic 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.

Sexually Dimorphic Pattern of Pain Mitigation Following Prophylactic Regenerative Peripheral Nerve Interface (RPNI) in a Rat Neuroma Model

BACKGROUND

Treating neuroma pain is a clinical challenge. Identification of sex-specific nociceptive pathways allows a more individualized pain management. The Regenerative Peripheral Nerve Interface (RPNI) consists of a neurotized autologous free muscle using a severed peripheral nerve to provide physiological targets for the regenerating axons.

OBJECTIVE

To evaluate prophylactic RPNI to prevent neuroma pain in male and female rats.

METHODS

F344 rats of each sex were assigned to neuroma, prophylactic RPNI, or sham groups. Neuromas and RPNIs were created in both male and female rats. Weekly pain assessments including neuroma site pain and mechanical, cold, and thermal allodynia were performed for 8 weeks. Immunohistochemistry was used to evaluate macrophage infiltration and microglial expansion in the corresponding dorsal root ganglia and spinal cord segments.

RESULTS

Prophylactic RPNI prevented neuroma pain in both sexes; however, female rats displayed delayed pain attenuation when compared with males. Cold allodynia and thermal allodynia were attenuated exclusively in males. Macrophage infiltration was mitigated in males, whereas females showed a reduced number of spinal cord microglia.

CONCLUSION

Prophylactic RPNI can prevent neuroma site pain in both sexes. However, attenuation of both cold allodynia and thermal allodynia occurred in males exclusively, potentially because of their sexually dimorphic effect on pathological changes of the central nervous system.

Study of nerve cell regeneration on nanofibers containing cerium oxide nanoparticles in a spinal cord injury model in rats

Since the CNS is unable to repair itself via neuronal regeneration in adult mammals, alternative therapies need to be found. The use of cerium oxide nanoparticles to repair nerve damage could be a promising approach for spinal cord reconstruction. In this study, we constructed a scaffold containing cerium oxide nanoparticles (Scaffold-CeO₂) and investigated the rate of nerve cell regeneration in a rat model of spinal cord injury. The scaffold of gelatin and polycaprolactone was synthesized, and a gelatin solution containing cerium oxide nanoparticles was attached to the scaffold. For the animal study, 40 male Wistar rats were randomly divided into 4 groups (n = 10): (a) Control; (b) Spinal cord injury (SCI); (c) Scaffold (SCI + scaffold without CeO₂ nanoparticles); (d) Scaffold-CeO₂ (SCI + scaffold containing CeO₂ nanoparticles). After creation of a hemisection SCI, scaffolds were placed at the site of injury in groups c and d, and after 7 weeks the rats were subjected to behavioral tests and then sacrificed for preparation of the spinal cord tissue to measure the expression of G-CSF, Tau and Mag proteins by Western blotting and Iba-1 protein by immunohistochemistry. The result of behavioral tests confirmed motor improvement and pain reduction in the Scaffold-CeO₂ group compared to the SCI group. Decreased expression of Iba-1 and higher expression of Tau and Mag in the Scaffold-CeO₂ group compared to the SCI group could be the result of nerve regeneration caused by the scaffold containing CeONPs as well as relief of pain symptoms.

Voluntary exercise ameliorates neuropathic pain by suppressing calcitonin gene-related peptide and ionized calcium-binding adapter molecule 1 overexpression in the lumbar dorsal horns in response to injury to the cervical spinal cord

BACKGROUND

Neuropathic pain (NP) is a frequent finding in patients diagnosed with spinal cord injuries (SCIs). To improve our understanding of the maladaptive changes taking place in the lumbar spinal cord that can lead to the development of NP and to find alternative options to treat this condition, we aimed to investigate the effects of voluntary exercise on NP after SCI and to elucidate its potential mechanisms.

METHODS

A rat model of post-SCI NP induced by compression of the posterior or lateral cervical spinal cord was used to evaluate the effects of voluntary exercise by measuring the bilateral withdrawal of the hind paws using the Von Frey filament and Hargreaves tests. The place escape/avoid paradigm was used to evaluate supraspinal pain processing and somatosensory evoked potentials (SEPs) were used to examine disturbances in proprioception. Locomotor function was evaluated using Basso, Beattie, and Bresnahan (BBB) scoring. Pathologic findings in hematoxylin and eosin-stained tissue and magnetic resonance imaging were used to evaluate the morphological changes after SCI. The lesion size within the cervical spinal cord was evaluated by staining with Eriochrome cyanine R. Quantitative polymerase chain reaction and immunohistochemistry were used to assess the expression of calcitonin gene-related peptide (CGRP) and ionized calcium-binding adapter molecule 1 (Iba-1) in the lumbar dorsal horns.

RESULTS

All injured rats developed mechanical hypersensitivity, hyposensitivity, and thermal hyperalgesia in the contralateral hind paws at 1 week post-injury. Rats that underwent lateral compression injury developed NP in the ipsilateral hind paws 1 week later than rats with a posterior compression injury. Our findings revealed that voluntary exercise ameliorated mechanical allodynia and thermal hyperalgesia, and significantly improved proprioception as measured by SEP, but had no impact on mechanical hypoalgesia or motor recovery and provided no significant neuroprotection after recovery from an acute SCI. SCI-induced NP was accompanied by increased expression of CGRP and Iba-1 in the lumbar dorsal horn. These responses were reduced in rats that underwent voluntary exercise.

CONCLUSIONS

Voluntary exercise ameliorates NP that develops in rats after compression injury. Increased expression of CGRP and Iba-1 in the lumbar dorsal horns of rats exhibiting symptoms of NP suggests that microglial activation might play a crucial role in its development. Collectively, voluntary exercise may be a promising therapeutic modality to treat NP that develops clinically in response to SCI.

The analgesic effects of β-elemene in rats with neuropathic pain by inhibition of spinal astrocytic ERK activation

Neuropathic pain takes a heavy toll on individual well-being, while current therapy is far from desirable. Herein, we assessed the analgesic effect of β-elemene, a chief component in the traditional Chinese medicine Curcuma wenyujin, and explored the underlying mechanisms at the level of spinal dorsal horn (SDH) under neuropathic pain. A spared nerve injury (SNI)-induced neuropathic pain model was established in rats. Intraperitoneal injection (i.p.) of β-elemene was administered for 21 consecutive days. Mechanical allodynia was explored by von Frey filaments. The activation of the mitogen-activated protein kinase (MAPK) family (including ERK, p38, and JNK) in spinal neurons, astrocytes, and microglia was evaluated using immunostaining 29 days after SNI surgery. The expression of GFAP, Iba-1, p-ERK, p-JNK, and p-p38 within the SDH was measured using immunoblotting. The levels of proinflammatory cytokines (including TNF-α, IL-1β, and IL-6) were measured with ELISA. The levels of oxidative stress indicators (including MDA, SOD, and GSH-PX) were detected using biochemical tests. Consecutive i.p. administration of β-elemene relieved SNI-induced mechanical allodynia (with an EC50 of 16.40 mg/kg). SNI significantly increased the expression of p-ERK in spinal astrocytes but not microglia on day 29. β-elemene reversed spinal astrocytic ERK activation and subsequent upregulation of proinflammatory cytokines in SNI rats, with no effect on the expression of p38 and JNK in spinal glia. β-elemene also exerted antioxidative effects by increasing the levels of SOD and GSH-PX and decreasing the level of MDA. Our results suggest that SNI induces robust astrocytic ERK activation within the SDH in the late phase of neuropathic pain. β-elemene exerts remarkable analgesic effects on neuropathic pain, possibly by inhibiting spinal astrocytic ERK activation and subsequent neuroinflammatory processes. Our findings suggest that β-elemene might be a promising analgesic for the treatment of chronic pain.

Long noncoding RNA LOC100911498 is a novel regulator of neuropathic pain in rats

INTRODUCTION

Neuropathic pain (NP) is the most debilitating of all clinical pain syndromes and may be a consequence of dysfunction in the somatosensory nervous system. Unfortunately, the pathogenesis of NP is not fully understood yet and it cannot be cured totally. Long noncoding RNA (lncRNA) is a type of RNA molecule greater than 200 nucleotides, and dysregulated expression of lncRNAs play a critical role in the facilitation of NP. Previous study showed the expression level of LOC100911498 in the spinal cords of spared nerve injury (SNI) rats were increased. This research was aimed at exploring what role LOC100911498 plays in the pathophysiological process of NP.

METHODS

The mechanical withdrawal threshold (MWT) of rats was measured by the von Frey test. The expression levels of P2X4 receptor (P2X4R), ionized calcium-binding adaptor molecule 1 (Iba-1), p-p38 and brain-derived neurotrophic factor (BDNF) in spinal cords were detected, respectively.

RESULTS

Our results suggested that the level of LOC100911498 in SNI rats was markedly higher than that in the sham group; the MWT values in rats were treated with LOC100911498siRNA were increased, and the expression levels of P2X4R, Iba-1, p-p38 and BDNF in SNI+ LOC100911498siRNA group were reduced compared with those in the SNI group.

CONCLUSION

Our study indicated the effects lncRNA LOC100911498 siRNA exerted on NP were mediated by P2X4R on microglia in the spinal cords of rats. Further, LOC100911498 may be a novel positive regulator of NP by regulating the expression and function of the P2X4R.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Neurophysiological assessment of joint nociceptors in the rat medial meniscus transection model of post-traumatic osteoarthritis

OBJECTIVE

Meniscal injury is a common prelude to post-traumatic osteoarthritis (PTOA). Joint nerves can become damaged in arthritic joints leading to the manifestation of neuropathic pain. Both PTOA and neuropathic pain are more common in females; however, it is unknown whether the neural processing of joint pain is sex-specific.

DESIGN

Male and female Wistar rats (230-286g) underwent unilateral medial meniscus transection (MMT) and allowed to recover for 28 days. Pain development was assessed over the time course by von Frey hair algesiometry and dynamic weight bearing. Recordings from joint primary afferents was carried out by electrophysiology at end-stage disease. Nerve damage and β-endorphin levels were also compared between MMT and sham operated animals.

RESULTS

Male MMT rats exhibited significant pain behaviour compared to sham control. Evoked afferent firing rate was heightened in male MMT animals. Female PTOA rats did not show signs of pain behaviour on each of the test days and the neurophysiological properties of their nociceptors was not different from control. Peripheral neuropathy was observed in about 30% of axons from male MMT animals compared to 15% in females. Systemic β-endorphin levels in female PTOA rats was 91.0 ± 10.4 pg/mL and only 49.0 ± 5.0 pg/mL in males.

CONCLUSIONS

Secondary allodynia and joint pain were observed in male but not female MMT rats. Joint nociceptors were sensitized in PTOA males but not in females. This lack of pain in females may be due to the absence of a peripheral neuropathy and greater endogenous opioid production.

Effect of two-week continuous epidural administration of 2% lidocaine on mechanical allodynia induced by spinal nerve ligation in rats

Background

Lidocaine is an effective against certain types of neuropathic pain. This study aimed to investigate whether timing of initiating continuous epidural infusion of lidocaine affected the glial activation and development of neuropathic pain induced by L5/6 spinal nerve ligation (SNL) in rats.

Methods

Following L5/6 SNL, rats were epidurally infused 2% lidocaine (drug infusion initiated on days 1, and 7 post SNL model establishment) or saline (saline infusion initiated on day 1 post SNL model establishment) continuously for 14 days. Mechanical allodynia of the hind paw to von Frey filament stimuli was determined prior to surgery, postoperative day 3, and once weekly after SNL model establishment. At 7 days after the infusion of saline or lidocaine ended, spinal activation of proinflammatory cytokines and astrocytes was evaluated immunohistochemically, using antibodies to interleukin-6 (IL-6) and glial fibrillary acidic protein (GFAP).

Results

Continuous epidural administration of 2% lidocaine for 14 days increased the mechanical withdrawal threshold regardless of the difference in timing of initiating lidocaine administration. Epidurally infusing 2% lidocaine inhibited nerve ligation-induced IL-6 and GFAP activation. In the 2% lidocaine infusion group, rats maintained the increased mechanical withdrawal threshold even at 7 days after the discontinuation of 2% lidocaine infusion.

Conclusions

Continuous epidural administration of 2% lidocaine inhibited the development of SNL-induced mechanical allodynia and suppressed IL-6 and GFAP activation regardless of the difference in timing of initiating lidocaine administration.

The blockade of CC chemokine receptor type 1 influences the level of nociceptive factors and enhances opioid analgesic potency in a rat model of neuropathic pain

A growing body of evidence has indicated that the release of nociceptive factors, such as interleukins and chemokines, by activated immune and glial cells has crucial significance for neuropathic pain generation and maintenance. Moreover, changes in the production of nociceptive immune factors are associated with low opioid efficacy in the treatment of neuropathy. Recently, it has been suggested that CC chemokine receptor type 1 (CCR1) signaling is important for nociception. Our study provides evidence that the development of hypersensitivity in rats following chronic constriction injury (CCI) of the sciatic nerve is associated with significant up-regulation of endogenous CCR1 ligands, namely, CCL2, CCL3, CCL4, CCL6, CCL7 and CCL9 in the spinal cord and CCL2, CCL6, CCL7 and CCL9 in dorsal root ganglia (DRG). We showed that single and repeated intrathecal administration of J113863 (an antagonist of CCR1) attenuated mechanical and thermal hypersensitivity. Moreover, repeated administration of a CCR1 antagonist enhanced the analgesic properties of morphine and buprenorphine after CCI. Simultaneously, repeated administration of J113863 reduced the protein levels of IBA-1 in the spinal cord and MPO and CD4 in the DRG and, as a consequence, the level of pronociceptive factors, such as interleukin-1β (IL-1β), IL-6 and IL-18. The data obtained provide evidence that CCR1 blockade reduces hypersensitivity and increases opioid-induced analgesia through the modulation of neuroimmune interactions.

Early life stress by repeated maternal separation induces long-term neuroinflammatory response in glial cells of male rats

Childhood maltreatment and neglect lead to a wide range of mental disorders highlighted by hormone and immune alterations in neglected children. This social-health challenge has led to the creation of early stress models such as maternal separation (MS) in rodents. We performed a MS model (4 h per day, 21 days; n = 16 MS and n = 16 control), and then measured three parameters in adult male rat brains, in order to look for long-term effects of early life stress. We used immunocytochemistry to mark glial fibrillary acidic protein (GFAP)-positive cells, which indicates changes in astroglia, and ionized calcium binding adaptor molecule 1 (Iba-1)-positive cells, which inform about reactive microglia. In order to study mRNA levels of some immune mediators, interleukin determination (interleukin-6, IL-6; tumor necrosis factor, TNFα) mRNAs were evaluated by real-time polymerase chain reaction (rt-PCR) in discrete brain regions. Measurements of numbers of GFAP-positive cells, and expression of Iba-1, IL-6 and TNFα mRNAs were performed in prefrontal cortex (PFC): cingulate cortex (CG), prelimbic cortex (PL) and infralimbic cortex (IL), striatal areas (dorsal striatum, STD; and nucleus accumbens, ACC), and dorsal hippocampus (HC: CA1, CA3 and dentate gyrus (DG)). We found that MS produces a dramatic and sustained decrease in the astroglial population in all the areas measured (from -25% in CA1 to -85.7% in ACC), whereas increased numbers of microglia were found, in more restricted regions: STD (72.6%), ACC (31%) and CA3 (33.3%) areas. Regarding mRNA measurements, we found increased IL-6 mRNA expression in HC (104.2%), and after MS.

Inhibition of Spinal Microglia and Astrocytes Contributes to the Anti-Allodynic Effect of Electroacupuncture in Neuropathic Pain Induced by Spinal Nerve Ligation

Objective

Besides neurons, activated microglia and astrocytes in the spinal cord dorsal horn (SCDH) contribute to the pathogenesis of chronic pain. Electroacupuncture (EA) has been used widely to treat various chronic pain diseases, however, the underlying mechanisms of EA are still not fully understood.

Methods

Male Sprague-Dawley rats were randomly divided into four groups, including an untreated healthy Control group (n=14), a True-spinal nerve ligation (SNL) group that underwent SNL and remained untreated (n=25), a True-SNL+EA group that underwent SNL followed by EA treatment (n=25), and a Sham-SNL group that underwent sham surgery and remained untreated (n=15). SNL was performed unilaterally at L5 and EA was applied to ST36 and BL60 bilaterally once per day. Paw withdrawal thresholds (PWTs) were measured ipsilaterally at baseline and 1, 3, 7, and 14 days after ligation. Activation of microglia and astrocytes in the SCDH were examined bilaterally by immunofluorescence staining, and concentrations of interleukin-1β (IL-1β) and interleukin (IL-6) were measured in the ipsilateral SCDH by ELISA.

Results

SNL significantly decreased PWTs and activated glial cells in the superficial laminae of the ipsilateral SCDH. In rats with SNL, glial fibrillary acidic protein (GFAP) immunoreactivity peaked at 7 days and was maintained until 14 days post-ligation, while anti-integrin alphaM (OX-42) immunoreactivity peaked at 3 days and declined gradually. EA significantly alleviated SNL-induced mechanical allodynia. Furthermore, EA reduced microglial activation (OX-42 positive ratios) in the lumbar SCDH at 3 days post-ligation and suppressed astrocyte activation (GFAP positive ratios) at all time points observed.

Conclusions

EA stimulation alleviates SNL-induced neuropathic pain, at least in part through inhibition of spinal glial activation. Moreover, inhibition of spinal microglia and astrocyte activation may contribute to the immediate effects and maintenance of EA analgesia, respectively.

A new model of nerve injury in the rat reveals a role of Regulator of G protein Signaling 4 in tactile hypersensitivity

Tactile hypersensitivity is one of the most debilitating symptoms of neuropathic pain syndromes. Clinical studies have suggested that its presence at early postoperative stages may predict chronic (neuropathic) pain after surgery. Currently available animal models are typically associated with consistent tactile hypersensitivity and are therefore limited to distinguish between mechanisms that underlie tactile hypersensitivity as opposed to mechanisms that protect against it. In this study we have modified the rat model of spared nerve injury, restricting the surgical lesion to a single peripheral branch of the sciatic nerve. This modification reduced the prevalence of tactile hypersensitivity from nearly 100% to approximately 50%. With this model, we here also demonstrated that the Regulator of G protein Signaling 4 (RGS4) was specifically up-regulated in the lumbar dorsal root ganglia and dorsal horn of rats developing tactile hypersensitivity. Intrathecal delivery of the RGS4 inhibitor CCG63802 was found to reverse tactile hypersensitivity for a 1h period. Moreover, tactile hypersensitivity after modified spared nerve injury was most frequently persistent for at least four weeks and associated with higher reactivity of glial cells in the lumbar dorsal horn. Based on these data we suggest that this new animal model of nerve injury represents an asset in understanding divergent neuropathic pain outcomes, so far unravelling a role of RGS4 in tactile hypersensitivity. Whether this model also holds promise in the study of the transition from acute to chronic pain will have to be seen in future investigations.

A critical time window for the analgesic effect of central histamine in the partial sciatic ligation model of neuropathic pain

Background

It is known that histamine participates in pain modulation. However, the effect of central histamine on neuropathic pain is not fully understood. Here, we report a critical time window for the analgesic effect of central histamine in the partial sciatic nerve ligation model of neuropathic pain.

Methods

Neuropathic pain was induced by partial sciatic nerve ligation (PSL) in rats, wild-type (C57BL/6J) mice and HDC^−/− (histidine decarboxylase gene knockout) and IL-1R^−/− (interleukin-1 receptor gene knockout) mice. Histidine, a precursor of histamine that can increase the central histamine levels, was administered intraperitoneally (i.p.). Histidine decarboxylase (HDC) enzyme inhibitor α-fluoromethylhistidine was administered intracerebroventricularly (i.c.v.). Histamine H₁ receptor antagonist mepyramine and H₂ receptor antagonist cimetidine were given intrathecally (i.t.) and intracisternally (i.c.). Withdrawal thresholds to tactile and heat stimuli were measured with a set of von Frey hairs and infrared laser, respectively. Immunohistochemistry and Western blot were carried out to evaluate the morphology of microglia and IL-1β production, respectively.

Results

Histidine (100 mg/kg, i.p.) administered throughout days 0–3, 0–7, or 0–14 postoperatively (PO) alleviated mechanical allodynia and thermal hyperalgesia in the hindpaw following PSL in rats. Intrathecal histamine reversed PSL-induced thermal hyperalgesia in a dose-dependent manner and intracisternal histamine alleviated both mechanical allodynia and thermal hyperalgesia. Moreover, α-fluoromethylhistidine (i.c.v.) abrogated the analgesic effect of histidine. However, histidine treatment initiated later than the first postoperative day (treatment periods included days 2–3, 4–7, and 8–14 PO) did not show an analgesic effect. In addition, histidine treatment initiated immediately, but not 3 days after PSL, inhibited microglial activation and IL-1β upregulation in the lumbar spinal cord, in parallel with its effects on behavioral hypersensitivity. Moreover, the inhibitory effects on pain hypersensitivity and spinal microglial activation were absent in HDC^−/− mice and IL-1R^−/− mice. H₁ receptor antagonist mepyramine (200 ng/rat i.t. or i.c.), but not H₂ receptor antagonist cimetidine (200, 500 ng/rat i.t. or 500 ng/rat i.c.), blocked the effects of histidine on pain behavior and spinal microglia.

Conclusions

These results demonstrate that central histamine is analgesic within a critical time window in the PSL model of neuropathic pain via histamine H₁ receptors. This effect may partly relate to the inhibition of microglial activation and IL-1β production in the spinal cord following nerve injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0637-0) contains supplementary material, which is available to authorized users.

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.

Double Roles of Macrophages in Human Neuroimmune Diseases and Their Animal Models

Macrophages are important immune cells of the innate immune system that are involved in organ-specific homeostasis and contribute to both pathology and resolution of diseases including infections, cancer, obesity, atherosclerosis, and autoimmune disorders. Multiple lines of evidence point to macrophages as a remarkably heterogeneous cell type. Different phenotypes of macrophages exert either proinflammatory or anti-inflammatory roles depending on the cytokines and other mediators that they are exposed to in the local microenvironment. Proinflammatory macrophages secrete detrimental molecules to induce disease development, while anti-inflammatory macrophages produce beneficial mediators to promote disease recovery. The conversion of the phenotypes of macrophages can regulate the initiation, development, and recovery of autoimmune diseases. Human neuroimmune diseases majorly include multiple sclerosis (MS), neuromyelitis optica (NMO), myasthenia gravis (MG), and Guillain-Barré syndrome (GBS) and macrophages contribute to the pathogenesis of these neuroimmune diseases. In this review, we summarize the double roles of macrophage in neuroimmune diseases and their animal models to further explore the mechanisms of macrophages involved in the pathogenesis of these disorders, which may provide a potential therapeutic approach for these disorders in the future.

Inhibition of the regulator of G protein signalling RGS4 in the spinal cord decreases neuropathic hyperalgesia and restores cannabinoid CB1 receptor signalling

BACKGROUND AND PURPOSE

Regulators of G protein signalling (RGS) are major determinants of metabotropic receptor activity, reducing the lifespan of the GTP-bound state of G proteins. Because the reduced potency of analgesic agents in neuropathic pain may reflect alterations in RGS, we assessed the effects of CCG 63802, a specific RGS4 inhibitor, on pain hypersensitivity and signalling through cannabinoid receptors, in a model of neuropathic pain.

EXPERIMENTAL APPROACH

The partial sciatic nerve ligation (PSNL) model in male Sprague Dawley rats was used to measure paw withdrawal thresholds to mechanical (von Frey hairs) or thermal (Hargreaves method) stimuli, during and after intrathecal injection of CCG 63802. HEK293 cells expressing CB1 receptors and conditional expression of RGS4 were used to correlate cAMP production and ERK phosphorylation with receptor activation and RGS4 action.

KEY RESULTS

Treatment of PSNL rats with CCG 63802, twice daily for 7 days after nerve injury, attenuated thermal hyperalgesia during treatment. Spinal levels of anandamide were higher in PSNL animals, irrespective of the treatment. Although expression of CB1 receptors was unaffected, HU210-induced CB1 receptor signalling was inhibited in PSNL rats and restored after intrathecal CCG 63802. In transfected HEK cells expressing CB1 receptors and RGS4, inhibition of cAMP production, a downstream effect of CB1 receptor signalling, was blunted after RGS4 overexpression. RGS4 expression also attenuated the CB1 receptor-controlled activation of ERK1/2.

CONCLUSIONS AND IMPLICATIONS

Inhibition of spinal RGS4 restored endogenous analgesic signalling pathways and mitigated neuropathic pain. Signalling through CB1 receptors may be involved in this beneficial effect.

Activity-triggered tetrapartite neuron-glial interactions following peripheral injury

Recent studies continue to support the proposition that non-neuronal components of the nervous system, mainly glial cells and associated chemical mediators, contribute to the development of neuronal hyperexcitability that underlies persistent pain conditions. In the event of peripheral injury, enhanced or abnormal nerve input is likely the most efficient way to activate simultaneously central neurons and glia. Injury induces phenotypic changes in glia and triggers signaling cascades that engage reciprocal interactions between presynaptic terminals, postsynaptic neurons, microglia and astrocytes. While some responses to peripheral injury may help the nervous system to adapt positively to counter the disastrous effect of injury, the net effect often leads to long-lasting sensitization of pain transmission pathways and chronic pain.

Modulation of spinal glial reactivity by intrathecal PPF is not sufficient to inhibit mechanical allodynia induced by nerve crush

Spinal glial reactivity has been strongly implicated in pain that follows peripheral nerve injury. Among the many therapeutic agents that have been tested for anti-allodynia through immune modulation is the atypical methylxanthine propentofylline. While propentofylline shows a potent anti-allodynia effect after nerve transection injury, we here demonstrate that, when propentofylline is used intrathecally at the effective immune-modulatory dose, allodynia after rat nerve crush injury is completely preserved. Microglial/macrophage Iba-1 and astrocytic GFAP expression, increased in the dorsal horn of nerve crushed animals, was, however, effectively attenuated by propentofylline. Effective modulation of spinal glial reactivity is, thus, no assurance for anti-allodynia.

Cannabinoid receptor type 1 antagonist, AM251, attenuates mechanical allodynia and thermal hyperalgesia after burn injury

BACKGROUND

Burn injury causes nociceptive behaviors, and inflammation-related pathologic pain can lead to glial cell activation. This study tested the hypothesis that burn injury activates glial cells, and cannabinoid receptor 1 (CB1R) antagonist, AM251, will decrease burn pain.

METHODS

Anesthetized rats received 0.75-cm third-degree burn on dorsal hind paw. Vehicle or AM251 30 μg intrathecally (older rats, n=6 per group) or, either vehicle, 0.1 or 1.0 mg/kg intraperitoneally (younger rats, n=6 per group), started immediate postburn, was administered for 7 days. Mechanical allodynia and thermal hyperalgesia were tested on ventral paw for 14 days. Microglial and astroglial activity was assessed by immunocytochemistry.

RESULTS

Allodynia, observed on burn side from day 1 to 14, was significantly (P<0.05) attenuated by intrathecal and intraperitoneal AM251 (1 mg/kg) starting from 3 to 14 days. Hyperalgesia, observed from day 3 to 12, was completely (P<0.05) reversed by intrathecal and intraperitoneal AM251 (1 mg/kg). AM251 0.1 mg/kg had no effect. Microglial activity (n=3 per time point) increased (P<0.05) 18.5±7.5 and 12.3±1.6 (mean±SD) fold at 7 and 14 days, respectively. Astroglial activity (n=4 per time point) increased 2.9±0.3 fold at day 7 only. Glial activities were unaltered by AM251.

CONCLUSIONS

AM251 inhibited nociceptive behaviors after burn even beyond 7-day period of administration. Although many studies have documented the utility of CB1R agonists, this study indicates that endogenous cannabinoids may have an unexpected pronociceptive effect during development of burn pain, explaining why CB1R antagonist, AM251, improves nociceptive behaviors. The decreased nociception with AM251 without altering glial activity indicates that AM251 acts further downstream of activated glial cells.

Experimental autoimmune prostatitis induces microglial activation in the spinal cord

BACKGROUND

The pathogenesis of chronic prostatitis/chronic pelvic pain syndrome is unknown and factors including the host's immune response and the nervous system have been attributed to the development of CP/CPPS. We previously demonstrated that mast cells and chemokines such as CCL2 and CCL3 play an important role in mediating prostatitis. Here, we examined the role of neuroinflammation and microglia in the CNS in the development of chronic pelvic pain.

METHODS

Experimental autoimmune prostatitis (EAP) was induced using a subcutaneous injection of rat prostate antigen. Sacral spinal cord tissue (segments S14-S5) was isolated and utilized for immunofluorescence or QRT-PCR analysis. Tactile allodynia was measured at baseline and at various points during EAP using Von Frey fibers as a function for pelvic pain. EAP mice were treated with minocycline after 30 days of prostatitis to test the efficacy of microglial inhibition on pelvic pain.

RESULTS

Prostatitis induced the expansion and activation of microglia and the development of inflammation in the spinal cord as determined by increased expression levels of CCL3, IL-1β, Iba1, and ERK1/2 phosphorylation. Microglial activation in mice with prostatitis resulted in increased expression of P2X4R and elevated levels of BDNF, two molecular markers associated with chronic pain. Pharmacological inhibition of microglia alleviated pain in mice with prostatitis and resulted in decreased expression of IL-1β, P2X4R, and BDNF.

CONCLUSION

Our data show that prostatitis leads to inflammation in the spinal cord and the activation and expansion of microglia, mechanisms that may contribute to the development and maintenance of chronic pelvic pain.

The contribution of immune and glial cell types in experimental autoimmune encephalomyelitis and multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system characterised by widespread areas of focal demyelination. Its aetiology and pathogenesis remain unclear despite substantial insights gained through studies of animal models, most notably experimental autoimmune encephalomyelitis (EAE). MS is widely believed to be immune-mediated and pathologically attributable to myelin-specific autoreactive CD4+ T cells. In recent years, MS research has expanded beyond its focus on CD4+ T cells to recognise the contributions of multiple immune and glial cell types to the development, progression, and amelioration of the disease. This review summarises evidence of T and B lymphocyte, natural killer cell, macrophage/microglial, astrocytic, and oligodendroglial involvement in both EAE and MS and the intercommunication and influence of each cell subset in the inflammatory process. Despite important advances in the understanding of the involvement of these cell types in MS, many questions still remain regarding the various subsets within each cell population and their exact contribution to different stages of the disease.