Acute but not chronic stimulation of glial cells in rat spinal cord by systemic injection of lipopolysaccharide is associated with hyperalgesia

Mild inflammation impairs acute intermittent hypoxia-induced phrenic long-term facilitation by a spinal adenosine-dependent mechanism

Inflammation undermines neuroplasticity, including serotonin-dependent phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (mAIH: 3, 5-min episodes, arterial Po2: 40-50 mmHg; 5-min intervals). Mild inflammation elicited by a low dose of the TLR-4 receptor agonist, lipopolysaccharide (LPS; 100 µg/kg, ip), abolishes mAIH-induced pLTF by unknown mechanisms. In the central nervous system, neuroinflammation primes glia, triggering ATP release and extracellular adenosine accumulation. As spinal adenosine 2 A (A2A) receptor activation impairs mAIH-induced pLTF, we hypothesized that spinal adenosine accumulation and A2A receptor activation are necessary in the mechanism whereby LPS impairs pLTF. We report that 24 h after LPS injection in adult male Sprague Dawley rats: 1) adenosine levels increase in ventral spinal segments containing the phrenic motor nucleus (C3-C5; P = 0.010; n = 7/group) and 2) cervical spinal A2A receptor inhibition (MSX-3, 10 µM, 12 µL intrathecal) rescues mAIH-induced pLTF. In LPS vehicle-treated rats (saline, ip), MSX-3 enhanced pLTF versus controls (LPS: 110 ± 16% baseline; controls: 53 ± 6%; P = 0.002; n = 6/group). In LPS-treated rats, pLTF was abolished as expected (4 ± 6% baseline; n = 6), but intrathecal MSX-3 restored pLTF to levels equivalent to MSX-3-treated control rats (120 ± 14% baseline; P < 0.001; n = 6; vs. LPS controls with MSX-3: P = 0.539). Thus, inflammation abolishes mAIH-induced pLTF by a mechanism that requires increased spinal adenosine levels and A2A receptor activation. As repetitive mAIH is emerging as a treatment to improve breathing and nonrespiratory movements in people with spinal cord injury or ALS, A2A inhibition may offset undermining effects of neuroinflammation associated with these neuromuscular disorders.NEW & NOTEWORTHY Mild inflammation undermines motor plasticity elicited by mAIH. In a model of mAIH-induced respiratory motor plasticity (phrenic long-term facilitation; pLTF), we report that inflammation induced by low-dose lipopolysaccharide undermines mAIH-induced pLTF by a mechanism requiring increased cervical spinal adenosine and adenosine 2 A receptor activation. This finding advances the understanding of mechanisms impairing neuroplasticity, potentially undermining the ability to compensate for the onset of lung/neural injury or to harness mAIH as a therapeutic modality.

Gabapentinoids Suppress Lipopolysaccharide-Induced Interleukin-6 Production in Primary Cell Cultures of the Rat Spinal Dorsal Horn

INTRODUCTION

Gabapentin and pregabalin are drugs to treat neuropathic pain. Several studies highlighted effects on presynaptic terminals of nociceptors. Via binding to α2δ subunits of voltage-gated calcium channels, gabapentinoids modulate the synaptic transmission of nociceptive information. However, recent studies revealed further properties of these substances. Treatment with gabapentin or pregabalin in animal models of neuropathic pain resulted not only in reduced symptoms of hyperalgesia but also in an attenuated activation of glial cells and decreased production of pro-inflammatory mediators in the spinal dorsal horn.

METHODS

In the present study, we aimed to investigate the impact of gabapentinoids on the inflammatory response of spinal dorsal horn cells, applying the established model of neuro-glial primary cell cultures of the superficial dorsal horn (SDH). We studied effects of gabapentin and pregabalin on lipopolysaccharide (LPS)-induced cytokine release (bioassays), expression of inflammatory marker genes (RT-qPCR), activation of transcription factors (immunocytochemistry), and Ca2+ responses of SDH neurons to stimulation with substance P and glutamate (Ca2+-imaging).

RESULTS

We detected an attenuated LPS-induced expression and release of interleukin-6 by SDH cultures in the presence of gabapentinoids. In addition, a significant main effect of drug treatment was observed for mRNA expression of microsomal prostaglandin E synthase 1 and the inhibitor of nuclear factor kappa B. Nuclear translocation of inflammatory transcription factors in glial cells was not significantly affected by gabapentinoid treatment. Moreover, both substances did not modulate neuronal responses upon stimulation with substance P or glutamate.

CONCLUSION

Our results provide evidence for anti-inflammatory capacities of gabapentinoids on the acute inflammatory response of SDH primary cultures upon LPS stimulation. Such effects may contribute to the pain-relieving effects of gabapentinoids.

Dexmedetomidine Alleviates Microglia-Induced Spinal Inflammation and Hyperalgesia in Neonatal Rats by Systemic Lipopolysaccharide Exposure

Noxious stimulus and painful experience in early life can induce cognitive deficits and abnormal pain sensitivity. As a major component of the outer membrane of gram-negative bacteria, lipopolysaccharide (LPS) injection mimics clinical symptoms of bacterial infections. Spinal microglial activation and the production of pro-inflammatory cytokines have been implicated in the pathogenesis of LPS-induced hyperalgesia in neonatal rats. Dexmedetomidine (DEX) possesses potent anti-neuroinflammatory and neuroprotective properties through the inhibition of microglial activation and microglial polarization toward pro-inflammatory (M1) phenotype and has been widely used in pediatric clinical practice. However, little is known about the effects of DEX on LPS-induced spinal inflammation and hyperalgesia in neonates. Here, we investigated whether systemic LPS exposure has persistent effects on spinal inflammation and hyperalgesia in neonatal rats and explored the protective role of DEX in adverse effects caused by LPS injection. Systemic LPS injections induced acute mechanical hyperalgesia, increased levels of pro-inflammatory cytokines in serum, and short-term increased expressions of pro-inflammatory cytokines and M1 microglial markers in the spinal cord of neonatal rats. Pretreatment with DEX significantly decreased inflammation and alleviated mechanical hyperalgesia induced by LPS. The inhibition of M1 microglial polarization and microglial pro-inflammatory cytokines expression in the spinal cord may implicate its neuroprotective effect, which highlights a new therapeutic target in the treatment of infection-induced hyperalgesia in neonates and preterm infants.

Inducing inflammation following subacute spinal cord injury in female rats: A double-edged sword to promote motor recovery

The inflammatory response following spinal cord injury is associated with increased tissue damage and impaired functional recovery. However, inflammation can also promote plasticity and the secretion of growth-promoting substances. Previously we have shown that inducing inflammation with a systemic injection of lipopolysaccharide in the chronic (8 weeks) stage of spinal cord injury enhances neuronal sprouting and the efficacy of rehabilitative training in rats. Here, we tested whether administration of lipopolysaccharide in female rats in the subacute (10 days) stage of spinal cord injury would have a similar effect. Since the lesioned environment is already in a pro-inflammatory state at this earlier time after injury, we hypothesized that triggering a second immune response may not be beneficial for recovery. Contrary to our hypothesis, we found that eliciting an inflammatory response 10 days after spinal cord injury enhanced the recovery of the ipsilesional forelimb in rehabilitative training. Compared to rats that received rehabilitative training without treatment, rats that received systemic lipopolysaccharide showed restored motor function without the use of compensatory strategies that translated beyond the trained task. Furthermore, lipopolysaccharide treatment paradoxically promoted the resolution of chronic neuroinflammation around the lesion site. Unfortunately, re-triggering a systemic immune response after spinal cord injury also resulted in a long-term increase in anxiety-like behaviour.

Batroxobin inhibits astrocyte activation following nigrostriatal pathway injury

Batroxobin is a thrombin-like serine protease from the venom of the Bothrops atrox and Bothrops moojeni snake species. Sirtuin 1 (Sirt1) has been shown to play an important role in neuroprotection after traumatic brain injury. However, its underlying mechanism of action remains poorly understood. The purpose of this study was to investigate whether the mechanism by which batroxobin participates in the activation of astrocytes is associated with Sirt1. Mouse models of nigrostriatal pathway injury were established. Immediately after modeling, mice were intraperitoneally administered 39 U/kg batroxobin. Batroxobin significantly reduced the expression of cleaved caspase-3 in both the substantia nigra and striatum, inhibited neuronal apoptosis, and promoted the recovery of rat locomotor function. These changes coincided with a remarkable reduction in astrocyte activation. Batroxobin also reduced Sirt1 expression and extracellular signal-regulated kinase activation in brain tissue. Intraperitoneal administration of the Sirt1-specific inhibitor EX527 (5 mg/kg) 30 minutes prior to injury could inhibit the abovementioned effects. In mouse astrocyte cultures, 1 ng/mL batroxobin attenuated interleukin-1β-induced activation of astrocytes and extracellular signal-regulated kinase. EX527 could also inhibit the effects of batroxobin. These findings suggest that batroxobin inhibits astrocyte activation after nigrostriatal pathway injury through the Sirt1 pathway. This study was approved by the Animal Ethics Committee of China Medical University, China (approval No. CMU2020037) on July 19, 2015.

Interleukin-1 receptor antagonist ameliorates the pain hypersensitivity, spinal inflammation and oxidative stress induced by systemic lipopolysaccharide in neonatal rats

Perinatal inflammation-induced reduction in pain threshold may alter pain sensitivity to hyperalgesia or allodynia which may persist into adulthood. In this study, we investigated the anti-inflammatory protective effect of interleukin-1 receptor antagonist (IL-1ra), an anti-inflammatory cytokine, on systemic lipopolysaccharide (LPS)-induced spinal cord inflammation and oxidative stress, thermal hyperalgesia, and mechanical allodynia in neonatal rats. Intraperitoneal (i.p.) injection of LPS (2 mg/kg) or sterile saline was performed in postnatal day 5 (P5) rat pups, and IL-1ra (100 mg/kg) or saline was administered (i.p.) 5 min after LPS injection. Pain reflex behavior, spinal cord inflammation and oxidative stress were examined 24 h after LPS administration. Systemic LPS exposure led to a reduction of tactile threshold in the von Frey filament tests (mechanical allodynia) and pain response latency in the tail-flick test (thermal hyperalgesia) of P6 neonatal rats. Spinal cord inflammation was indicated by the increased numbers of activated glial cells including microglia (Iba1+) and astrocytes (GFAP+), and elevated levels of pro-inflammatory cytokine interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), and prostaglandin E2 (PGE2) 24 h after LPS treatment. LPS treatment induced spinal oxidative stress as evidenced by the increase in thiobarbituric acid reactive substances (TBARS) content in the spinal cord. LPS exposure also led to a significant increase in oligodendrocyte lineage population (Olig2+) and mature oligodendrocyte cells (APC+) in the neonatal rat spinal cord. IL-1ra treatment significantly reduced LPS-induced effects including hyperalgesia, allodynia, the increased number of activated microglia, astrocytes and oligodendrocytes, and elevated levels of IL-1β, COX-2, PGE2, and lipid peroxidation (TBARS) in the neonatal rat spinal cord. These data suggest that IL-1ra provides a protective effect against the development of pain hypersensitivity, spinal cord inflammation and oxidative stress in the neonatal rats following LPS exposure, which may be associated with the blockade of LPS-induced pro-inflammatory cytokine IL-1β.

Spinal Cord Microglia in Health and Disease

The review summarizes data of recent experimental studies on spinal microglia, the least explored cells of the spinal cord. It focuses on the origin and function of microglia in mammalian spinal cord embryogenesis. The main approaches to the classification of microgliocytes based on their structure, function, and immunophenotypic characteristics are analyzed. We discuss the results of studies conducted on experimental models of spinal cord diseases such as multiple sclerosis, amyotrophic lateral sclerosis, systemic inflammation, and some others, with special emphasis on the key role of microglia in the pathogenesis of these diseases. The review highlights the need to detect the new microglia-specific marker proteins expressed at all stages of ontogeny. New sensitive and selective microglial markers are necessary in order to improve identification of spinal cord microgliocytes in normal and pathological conditions. Possible morphometric methods to assess the functional activity of microglial cells are presented.

Differential effect of LPS and paclitaxel on microglial functional phenotypes and circulating cytokines: the possible role of CX3CR1 and IL-4/10 in blocking persistent inflammation

Neuroinflammation plays a role in cancer chemotherapy-induced chronic pain. Thus far, most studies have focused on neuroinflammation suppression. However, there are limited reports of which factor is involved in the transition from acute inflammation to chronic inflammation, resulting in neuroinflammation and chronic pain. Here, we compared the inflammatory reaction and pain response induced by LPS and paclitaxel. LPS (0.5 mg/kg) or paclitaxel (2 mg/kg/day for 5 days) was administered intraperitoneally to mice, and mechanical allodynia was examined by von Frey test. LPS induced transient mechanical allodynia, whereas paclitaxel induced persistent mechanical allodynia. The CD86/CX3CR1 ratio remained unchanged due to CX3CR1 elevation following LPS injection, whereas the ratio was increased on day 1 after paclitaxel injection. LPS also increased CD45, CCL2, and CCL5 mRNA in the spinal cord and circulating pro- and anti-inflammatory cytokines 1 day after injection; however, the pattern was not consistent. Paclitaxel gradually increased inflammatory cytokines in the spinal cord. CX3CR1 might be involved in blocking the transition from acute pain to persistent pain in the LPS group. In addition, serum IL-4 and IL-10 elevation in the LPS group may be associated with chronic pain prevention. Therefore, targeting CX3CR1, IL-4, and IL-10 might be an alternative therapeutic strategy.

Activation of Microglyocytes in the Anterior Horns of Rat Spinal Cord after Administration of Bacterial Lipopolysaccharide

We studied the reaction of the microglia of the anterior horns of the rat spinal cord to intraperitoneal administration of bacterial LPS. Immunohistochemical analysis showed that acute systemic inflammation leads to activation of more than half of microglial cells as soon as in 24 h after LPS injection, while the total number of microglial cells does not change significantly. It was hypothesized that activated microglial cells are involved in the reorganization of synaptic connections, but do not have a neurotoxic effect.

Polymorphisms in CAMKK2 may predict sensory neuropathy in African HIV patients

HIV-associated sensory neuropathy (HIV-SN) is the most common neurological condition associated with HIV. HIV-SN has characteristics of an inflammatory pathology caused by the virus itself and/or by antiretroviral treatment (ART). Here, we assess the impact of single-nucleotide polymorphisms (SNPs) in a cluster of three genes that affect inflammation and neuronal repair: P2X7R, P2X4R and CAMKK2. HIV-SN status was assessed using the Brief Peripheral Neuropathy Screening tool, with SN defined by bilateral symptoms and signs. Forty-five SNPs in P2X7R, P2X4R and CAMKK2 were genotyped using TaqMan fluorescent probes, in DNA samples from 153 HIV(+) black Southern African patients exposed to stavudine. Haplotypes were derived using the fastPHASE algorithm, and SNP genotypes and haplotypes associated with HIV-SN were identified. Optimal logistic regression models included demographics (age and height), with SNPs (model p < 0.0001; R (2) = 0.19) or haplotypes (model p < 0.0001; R (2) = 0.18, n = 137 excluding patients carrying CAMKK2 haplotypes perfectly associated with SN). Overall, CAMKK2 exhibited the strongest associations with HIV-SN, with two SNPs and six haplotypes predicting SN status in black Southern Africans. This gene warrants further study.

Transdermal cannabidiol reduces inflammation and pain-related behaviours in a rat model of arthritis

BACKGROUND

Current arthritis treatments often have side-effects attributable to active compounds as well as route of administration. Cannabidiol (CBD) attenuates inflammation and pain without side-effects, but CBD is hydrophobic and has poor oral bioavailability. Topical drug application avoids gastrointestinal administration, first pass metabolism, providing more constant plasma levels.

METHODS

This study examined efficacy of transdermal CBD for reduction in inflammation and pain, assessing any adverse effects in a rat complete Freund's adjuvant-induced monoarthritic knee joint model. CBD gels (0.6, 3.1, 6.2 or 62.3 mg/day) were applied for 4 consecutive days after arthritis induction. Joint circumference and immune cell invasion in histological sections were measured to indicate level of inflammation. Paw withdrawal latency (PWL) in response to noxious heat stimulation determined nociceptive sensitization, and exploratory behaviour ascertained animal's activity level.

RESULTS

Measurement of plasma CBD concentration provided by transdermal absorption revealed linearity with 0.6-6.2 mg/day doses. Transdermal CBD gel significantly reduced joint swelling, limb posture scores as a rating of spontaneous pain, immune cell infiltration and thickening of the synovial membrane in a dose-dependent manner. PWL recovered to near baseline level. Immunohistochemical analysis of spinal cord (CGRP, OX42) and dorsal root ganglia (TNFα) revealed dose-dependent reductions of pro-inflammatory biomarkers. Results showed 6.2 and 62 mg/day were effective doses. Exploratory behaviour was not altered by CBD indicating limited effect on higher brain function.

CONCLUSIONS

These data indicate that topical CBD application has therapeutic potential for relief of arthritis pain-related behaviours and inflammation without evident side-effects.

Minocycline blocks lipopolysaccharide induced hyperalgesia by suppression of microglia but not astrocytes

Systemic injection of lipopolysaccharide (LPS) induces a robust immune response as well as thermal and mechanical hyperalgesia. Spinal and peripheral glial cells have been implicated as important mediators in this hyperalgesia but the specific contributions of microglia versus astrocytes are not entirely clear. To better define these mechanisms, this study examined the febrile response, nociceptive sensitivity, glial cell reactivity and cytokine production in the dorsal root ganglion (DRG) and spinal cord in rats following systemic treatment with LPS and the effects of minocycline in countering these responses. Intraperitoneal LPS injection resulted in an increase in core body temperature and produced hyperalgesia to heat and mechanical stimuli. Western blot studies revealed increased expression of microgial cell, macrophage and satellite cell markers in DRG and microglial and astrocyte markers in spinal cord following LPS treatment. Real-time RT-PCR indicated that LPS treatment increased cytokine mRNA expression levels in both the DRG and the spinal cord. Minocycline suppressed all LPS-induced behavioral effects but not the febrile response. Moreover, minocycline prevented LPS-induced microglia/macrophage activation and cytokine responses in spinal cord and DRG, but did not affect the activation of astrocytes/satellite cells. These data demonstrate that LPS-induced changes in nociceptive sensitivity are likely mediated by activation of microglial cells and/or macrophages in the spinal cord and DRG.

Glial cell activity is maintained during prolonged inflammatory challenge in rats

We evaluated the expression of glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), ionized calcium binding adaptor protein-1 (Iba-1), and ferritin in rats after single or repeated lipopolysaccharide (LPS) treatment, which is known to induce endotoxin tolerance and glial activation. Male Wistar rats (200-250 g) received ip injections of LPS (100 µg/kg) or saline for 6 days: 6 saline (N = 5), 5 saline + 1 LPS (N = 6) and 6 LPS (N = 6). After the sixth injection, the rats were perfused and the brains were collected for immunohistochemistry. After a single LPS dose, the number of GFAP-positive cells increased in the hypothalamic arcuate nucleus (ARC; 1 LPS: 35.6 ± 1.4 vs control: 23.1 ± 2.5) and hippocampus (1 LPS: 165.0 ± 3.0 vs control: 137.5 ± 2.5), and interestingly, 6 LPS injections further increased GFAP expression in these regions (ARC = 52.5 ± 4.3; hippocampus = 182.2 ± 4.1). We found a higher GS expression only in the hippocampus of the 6 LPS injections group (56.6 ± 0.8 vs 46.7 ± 1.9). Ferritin-positive cells increased similarly in the hippocampus of rats treated with a single (49.2 ± 1.7 vs 28.1 ± 1.9) or repeated (47.6 ± 1.1 vs 28.1 ± 1.9) LPS dose. Single LPS enhanced Iba-1 in the paraventricular nucleus (PVN: 92.8 ± 4.1 vs 65.2 ± 2.2) and hippocampus (99.4 ± 4.4 vs 73.8 ± 2.1), but had no effect in the retrochiasmatic nucleus (RCA) and ARC. Interestingly, 6 LPS increased the Iba-1 expression in these hypothalamic and hippocampal regions (RCA: 57.8 ± 4.6 vs 36.6 ± 2.2; ARC: 62.4 ± 6.0 vs 37.0 ± 2.2; PVN: 100.7 ± 4.4 vs 65.2 ± 2.2; hippocampus: 123.0 ± 3.8 vs 73.8 ± 2.1). The results suggest that repeated LPS treatment stimulates the expression of glial activation markers, protecting neuronal activity during prolonged inflammatory challenges.

Lipopolysaccharide induces paired immunoglobulin-like receptor B (PirB) expression, synaptic alteration, and learning-memory deficit in rats

Some typical immune proteins are expressed in the nervous system, among which the paired-immunoglobulin-like receptor B (PirB) is a receptor for major histocompatibility complex class I antigen (MHC-I), but may play a physiological role in the brain for neuronal circuitry stability by inhibiting synaptic plasticity. Chronic neuroinflammation is common to many neurodegenerative diseases and is often associated with neuronal/synaptic damage and dysfunction. Here we examined the expression of PirB in the rat brain following intracerebral application of lipopolysaccharide (LPS), which has been shown to induce proinflammatory changes and cognitive deficits in rodents. One month after unilateral intrahippocampal LPS injection (10 μg in 4 μl phosphate-buffered saline, PBS), increased protein levels and immunoreactivity of PirB were detected in the ipsilateral hippocampal formation and cortex of the experimental group relative to vehicle (PBS) control. The increased PirB labeling was localized to astrocytes and neurons. Reduced synaptophysin protein levels and immunoreactivity were also found in the ipsilateral hippocampal formation and cortex in LPS-treated rats relative to controls. Morris water maze tests indicated that hippocampus-dependent spatial learning and memory were impaired in LPS-treated animals. Our findings add new experimental data for an upregulation of immune proteins in neuronal and glial cells in the brain in a model of endotoxin-induced neuroinflammation, synaptic alteration, and cognitive decline. The results suggest that PirB modulation may be involved in the pathological process under neurodegenerative conditions.

P2X receptors in neuroglia

Different types of ionotropic P2X purinoceptors are expressed in all major types of neuroglia, where they mediate a variety of physiological and pathological signaling. Cortical astrocytes express specific P2X_1/5 heteromeric receptors that are activated by ongoing synaptic transmission and can trigger fast local signaling through elevation in cytoplasmic Ca²⁺ and Na⁺ concentrations. Oligodendrocytes express several types of P2X receptors that may control their development and mediate axonal-glial interactions. In microglia, P2X₄ and P2X₇ receptors regulate numerous events associated with microglial activation, motility, and release of proinflammatory factors.

Pain intensity and duration can be enhanced by prior challenge: initial evidence suggestive of a role of microglial priming

Activation of spinal microglia and consequent release of proinflammatory mediators facilitate pain. Under certain conditions, responses of activated microglia can become enhanced. Enhanced microglial production of proinflammatory products may result from priming (sensitization), similar to macrophage priming. We hypothesized that if spinal microglia were primed by an initial inflammatory challenge, subsequent challenges may create enhanced pain. Here, we used a "two-hit" paradigm using 2 successive challenges, which affect overlapping populations of spinal microglia, presented 2 weeks apart. Mechanical allodynia and/or activation of spinal glia were assessed. Initially, laparotomy preceded systemic lipopolysaccharide (LPS). Prior laparotomy caused prolonged microglial (not astrocyte) activation plus enhanced LPS-induced allodynia. In this "two-hit" paradigm, minocycline, a microglial activation inhibitor, significantly reduced later exaggerated pain induced by prior surgery when minocycline was administered intrathecally for 5 days starting either at the time of surgery or 5 days before LPS administration. To test generality of the priming effect, subcutaneous formalin preceded intrathecal HIV-1 gp120, which activates spinal microglia and causes robust allodynia. Prior formalin enhanced intrathecal gp120-induced allodynia, suggesting that microglial priming is not limited to laparotomy and again supporting a spinal site of action. Therefore, spinal microglial priming may increase vulnerability to pain enhancement.

PERSPECTIVE

Spinal microglia may become "primed" (sensitized) following their activation by disparate forms of peripheral trauma/inflammation. As a result, such primed microglia may overrespond to subsequent challenges, thereby enhancing pain intensity and duration.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Long-term changes of spine dynamics and microglia after transient peripheral immune response triggered by LPS in vivo

Background

An episode of peripheral immune response may create long-lasting alterations in the neural network. Recent studies indicate a glial involvement in synaptic remodeling. Therefore it is postulated that both synaptic and glial changes could occur under the peripheral inflammation.

Results

We tested this possibility by in vivo two-photon microscopy of dendritic spines after induction of a peripheral immune response by lipopolysaccharide (LPS) treatment of mice.

We observed that the spines were less stable in LPS-treated mice. The accumulation of spine changes gradually progressed and remained low over a week after LPS treatment but became significantly larger at four weeks. Over eight weeks after LPS treatment, the fraction of eliminated spines amounted to 20% of the initial population and this persistent destabilization resulted in a reduction of the total spine density.

We next evaluated glial activation by LPS administration. Activation of microglia was confirmed by a persistent increase of Iba1 immunoreactivity. Morphological changes in microglia were observed two days after LPS administration and were partially recovered within one week but sustained over a long time period.

Conclusions

These results indicate long-lasting aggravating effects of a single transient peripheral immune response on both spines and microglia. The parallel persistent alterations of both spine turnover and the state of microglia in vivo suggest the presence of a pathological mechanism that sustains the enhanced remodeling of neural networks weeks after peripheral immune responses. This pathological mechanism may also underlie long-lasting cognitive dysfunctions after septic encephalopathy in human patients.

Lipopolysaccharide attenuates phrenic long-term facilitation following acute intermittent hypoxia

Lipopolysaccharide (LPS) induces inflammatory responses, including microglial activation in the central nervous system. Since LPS impairs certain forms of hippocampal and spinal neuroplasticity, we hypothesized that LPS would impair phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) in outbred Sprague-Dawley (SD) and inbred Lewis (L) rats. Approximately 3h following a single LPS injection (i.p.), the phrenic response during hypoxic episodes is reduced in both rat strains versus vehicle treated, control rats (SD: 84 ± 7% vs. 128 ± 14% baseline for control, p < 0.05; L: 62 ± 10% vs. 90 ± 9% baseline for control, p < 0.05). At 60 min post-AIH, pLTF is also diminished by LPS in both strains: (SD: 22 ± 5% vs. 73.5 ± 14% baseline for control, p < 0.05; L: 18 ± 15% vs. 56 ± 8% baseline for control, p < 0.05). LPS alone does not affect phrenic burst frequency in either rat strain, suggesting that acute LPS injection has minimal effect on brainstem respiratory rhythm generation. Thus, systemic LPS injections and (presumptive) inflammation impair pLTF, a form of spinal neuroplasticity in respiratory motor control. These results suggest that ongoing infection or inflammation must be carefully considered in studies of respiratory plasticity, or during attempts to harness spinal plasticity as a therapeutic tool in the treatment of respiratory insufficiency, such as spinal cord injury.

Low rather than high dose lipopolysaccharide 'priming' of muscle provides an animal model of persistent elevated mechanical sensitivity for the study of chronic pain

Experimental animal pain models involving peripheral nerve lesions have expanded the understanding of the pathological changes caused by nerve damage. However models for the pathogenesis of chronic pain patients lacking obvious nerve injuries have not been developed to the same extent. Guided by clinical observations, we focused on the initiating noxious event, the context when applying nociceptive stimulation targeting long-lasting pain elicited by muscle insult. The administration of a nociceptive agent (6% hypertonic saline: HS; 5-time repeated-injection: HS5) after pretreatment with an immuno-inflammatory agent (lipopolysaccharide: LPS, 2 μg/kg) into one gastrocnemius muscle produced markedly long-persisting biphasic sustained mechanical hypersensitivity on the plantar surface of both hindpaws. In the acute phase, the blockade of afferent inputs from the injected-site was effective in returning the contralateral enhanced-responses to baseline levels. In contrast, similar blockade during the chronic phase did not affect the contralateral enhanced-responses, indicating that the hypersensitivity in the two phases was probably induced by different mechanisms. However, increasing the dose of LPS (20 μg/kg) before applying HS5 eliminated the development of mechanical hypersensitivity in the chronic phase, while the hypersensitivity in the acute phase was significantly more severe than with low-dose LPS-pretreatment. In this model, the development of hypersensitivity could be modulated by manipulating LPS-doses prior to noxious stimulation. This novel chronic pain model based on a preceding 'priming' myalgic stimulus provides an intriguing means for studying the pathogenesis of chronic pain.

Glia-induced reversible disruption of blood-brain barrier integrity and neuropathological response of the neurovascular unit

The blood-brain barrier (BBB) is the regulated interface that mediates selective transcellular transport of nutrients and essential components from the blood into the brain parenchyma. Many neurodegenerative diseases including stroke, multiple sclerosis, rheumatoid arthritis, and AIDS dementia exhibit loss of BBB integrity. Despite the increasing body of evidence for the involvement of glia in maintaining the BBB, few studies have addressed glial/endothelial/extracellular matrix interactions. A chemically induced astrocyte lesion provides a noninvasive model to study reversible BBB dysfunction in vivo. Blood-brain barrier integrity was assessed with fluorescent dextran tracers (3-70 kDa) and magnetic resonance imaging, in parallel with confocal and electron microscopy imaging of the neurovascular unit. These studies demonstrated modified tight-junction protein expression with loss of vascular integrity. We propose that adherens junction proteins and extracellular matrix remodeling provide a temporary size-selective barrier, whereas astrocyte and microglia activation direct tight-junction proteins to paracellular domains and restore BBB integrity. Morphological comparisons were made with the area postrema, a circumventricular organ with a naturally porous BBB. Further studies into cellular mechanisms of glial/endothelial/extracellular matrix interactions may identify novel glial-based therapeutic targets and innovate therapies for modulating diseases in which gliosis and raised levels of pro-inflammatory mediators are central components.

Enhancement of antinociception by coadministration of minocycline and a non-steroidal anti-inflammatory drug indomethacin in naïve mice and murine models of LPS-induced thermal hyperalgesia and monoarthritis

BACKGROUND

Minocycline and a non-steroidal anti-inflammatory drug (NSAID) indomethacin, have anti-inflammatory activities and are both used in the management of rheumatoid arthritis. However, there are no reports on whether coadministration of these drugs could potentiate each other's activities in alleviating pain and weight bearing deficits during arthritis.

METHODS

LPS was injected to BALB/c mice intraperitoneally (i.p.) to induce thermal hyperalgesia. The hot plate test was used to study thermal nociception in naïve BALB/c and C57BL/6 mice and BALB/c mice with LPS-induced thermal hyperalgesia and to evaluate antinociceptive effects of drugs administered i.p. Monoarthritis was induced by injection of LPS intra-articularly into the right hind (RH) limb ankle joint of C57BL/6 mice. Weight bearing changes and the effect of i.p. drug administration were analyzed in freely moving mice using the video-based CatWalk gait analysis system.

RESULTS

In naïve mice indomethacin (5 to 50 mg/kg) had no significant activity, minocycline (25 to 100 mg/kg) produced hyperalgesia to thermal nociception, however, coadministration of minocycline 50 mg/kg with indomethacin 5 or 10 mg/kg produced significant antinociceptive effects in the hot plate test. A selective inhibitor of COX-1, FR122047 (10 mg/kg) and a selective COX-2 inhibitor, CAY10404 (10 mg/kg) had no significant antinociceptive activities to thermal nociception in naïve mice, however, coadministration of minocycline, with CAY10404 but not FR122047 produced significant antinociceptive effects. In mice with LPS-induced hyperalgesia vehicle, indomethacin (10 mg/kg) or minocycline (50 mg/kg) did not produce significant changes, however, coadministration of minocycline plus indomethacin resulted in antinociceptive activity. LPS-induced RH limb monoarthritis resulted in weight bearing (RH/left hind (LH) limb paw pressure ratios) and RH/LH print area ratios deficits. Treatment with indomethacin (1 mg/kg) or minocycline (50 mg/kg) had no effects on the weight bearing and print area ratios deficits of monoarthritic mice. However, combination of minocycline plus indomethacin restored weight bearing and paw print area ratios of monoarthritic mice similar to that observed in non-arthritic control mice.

CONCLUSIONS

Coadministration of indomethacin or a selective COX-2 inhibitor, CAY10404 with minocycline potentiates their effects and results in antinociception against thermal nociception, reduction of thermal hyperalgesia and alleviation of weight bearing deficits in monoarthritic mice at doses where either drug alone has no significant activity. Thus, the coadministration of lower doses of a NSAID or a selective COX-2 inhibitor plus minocycline could be useful in the management of inflammatory pain and arthritis.

Glycogen synthase kinase-3 regulates inflammatory tolerance in astrocytes

Inflammatory tolerance is the down-regulation of inflammation upon repeated stimuli, which is well-established to occur in peripheral immune cells. However, less is known about inflammatory tolerance in the brain although it may provide an important protective mechanism from detrimental consequences of prolonged inflammation, which appears to occur in many psychiatric and neurodegenerative conditions. Array analysis of 308 inflammatory molecules produced by mouse primary astrocytes after two sequential stimulations with lipopolysaccharide (LPS) distinguished three classes, tolerant, sensitized and unaltered groups. For many of these inflammatory molecules, inhibition of glycogen synthase kinase-3 (GSK3) increased tolerance and reduced sensitization. Focusing on LPS-tolerance in interleukin-6 (IL-6) production, we found that microglia exhibited a strong tolerance response that matched that of macrophages, whereas astrocytes exhibited only partial tolerance. The astrocyte semi-tolerance was found to be regulated by GSK3. GSK3 inhibitors or knocking down GSK3 levels promoted LPS-tolerance and astrocytes expressing constitutively active GSK3 did not develop LPS-tolerance. These findings identify the critical role of GSK3 in counteracting IL-6 inflammatory tolerance in cells of the CNS, supporting the therapeutic potential of GSK3 inhibitors to reduce neuroinflammation by promoting tolerance.