Intrathecal injection of spironolactone attenuates radicular pain by inhibition of spinal microglia activation in a rat model

Modelling the Interplay Between Neuron-Glia Cell Dysfunction and Glial Therapy in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complicated, interpersonally defined, static condition of the underdeveloped brain. Although the aetiology of autism remains unclear, disturbance of neuronglia interactions has lately been proposed as a significant event in the pathophysiology of ASD. In recent years, the contribution of glial cells to autism has been overlooked. In addition to neurons, glial cells play an essential role in mental activities, and a new strategy that emphasises neuron-glia interactions should be applied. Disturbance of neuron-glia connections has lately been proposed as a significant event in the pathophysiology of ASD because aberrant neuronal network formation and dysfunctional neurotransmission are fundamental to the pathology of the condition. In ASD, neuron and glial cell number changes cause brain circuits to malfunction and impact behaviour. A study revealed that reactive glial cells result in the loss of synaptic functioning and induce autism under inflammatory conditions. Recent discoveries also suggest that dysfunction or changes in the ability of microglia to carry out physiological and defensive functions (such as failure in synaptic elimination or aberrant microglial activation) may be crucial for developing brain diseases, especially autism. The cerebellum, white matter, and cortical regions of autistic patients showed significant microglial activation. Reactive glial cells result in the loss of synaptic functioning and induce autism under inflammatory conditions. Replacement of defective glial cells (Cell-replacement treatment), glial progenitor cell-based therapy, and medication therapy (inhibition of microglia activation) are all utilised to treat glial dysfunction. This review discusses the role of glial cells in ASD and the various potential approaches to treating glial cell dysfunction.

P2Y1 purinergic receptor inhibition attenuated remifentanil-induced postoperative hyperalgesia via decreasing NMDA receptor phosphorylation in dorsal root ganglion

BACKGROUND

Remifentanil-induced postoperative hyperalgesia is an intractable side effect of the clinical use of remifentanil, the mechanism of which remains obscure, especially in the peripheral nervous system. N-methyl-D-aspartate receptor (NMDAR) phosphorylation in dorsal root ganglion (DRG) plays a pronociceptive role in neuropathic pain. The contribution of the P2Y1 purinergic receptor (P2Y1R) in DRG to pain hypersensitivity derived from various origins and P2Y1R upregulation-induced NMDAR activation in neurons have also been uncovered. This study aimed to investigate whether P2Y1R participates in nociceptive processing in the DRG and spinal cord in remifentanil-induced postoperative hyperalgesia.

METHODS

Rats with remifentanil-induced postoperative hyperalgesia were intrathecally injected with NMDAR antagonist MK801 or P2Y1R antagonist MRS2179 at 10 min prior to remifentanil infusion. Mechanical allodynia, heat hyperalgesia, and cold hyperalgesia were measured at -24 h, 2 h, 6 h, 24 h, and 48 h following remifentanil infusion. The P2Y1R expression and NMDAR expression and phosphorylation in DRG ipsilateral to the incision were detected by Western blot and immunofluorescence.

RESULTS

Incision and remifentanil induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia accompanied by upregulated P2Y1R expression, increased NMDAR subunit NR1 expression and phosphorylation at Ser896, and NR2B expression and phosphorylation at Tyr1472 in DRG. Inhibition of NMDAR phosphorylation by MK801 effectively attenuated remifentanil-induced postoperative hyperalgesia. Furthermore, P2Y1R blockade by MRS2179 not only lessened remifentanil-evoked postoperative hypersensitivity to mechanical, heat, and cold stimuli, but also suppressed the increases in NR1 and NR2B expression and phosphorylation in DRG induced by incision and remifentanil.

CONCLUSION

The process by which P2Y1R mediates NMDAR expression and phosphorylation represents a mechanism of remifentanil-induced postoperative hyperalgesia in the DRG and/or spinal cord.

The Effect of Spironolactone on β-amyloid-Induced Memory Impairment in Male Rats: The Role of Microglial Inhibition

Purpose: Neuroinflammation was indicated in the pathophysiology of Alzheimer’s disease (AD). Previous reports have also signified that spironolactone has anti-inflammatory effects. Therefore, the aim of this study was to assess the modulatory effects of spironolactone on neuroinflammation and memory loss in a rat model of AD.

Methods: The β-amyloid protein fragment 25-35 (Aβ) was injected in the dorsal hippocampus (5 μg/2.5 μL each side) of male Sprague-Dawley rats for four consecutive days to induce memory impairment. Animals have intraperitoneally received spironolactone (10, 25, or 50 mg/kg, N = 6/ group) or vehicle for 14 days. The passive inhibitory avoidance and the novel recognition tests were used for memory evaluation. Neuroinflammation was assessed by measuring the level of Iba1 protein, a marker of microglial activation, using western immunoblotting.

Results: Different doses of spironolactone showed no significant changes in latency times and discriminations ratios in passive inhibitory avoidance and novel recognition tests, respectively, as compared to vehicle. However, spironolactone-treated groups showed significantly lower Iba1 protein levels in comparison to the vehicle-treated group (P < 0.01).

Conclusion: Spironolactone had a modulatory effect on neuroinflammation through a repressive effect on microglial activation with no valuable effect on memory improvement in a rat model of AD. The findings of this study suggest that Aβ-induced memory loss may not be directly linked to microglial activation. Spironolactone may be a potential candidate to be examined in other neuroinflammatory disorders.

P2Y1 Purinergic Receptor Contributes to Remifentanil-Induced Cold Hyperalgesia via Transient Receptor Potential Melastatin 8-Dependent Regulation of N-methyl-d-aspartate Receptor Phosphorylation in Dorsal Root Ganglion

BACKGROUND

Remifentanil can induce postinfusion cold hyperalgesia. N-methyl-d-aspartate receptor (NMDAR) activation and upregulation of transient receptor potential melastatin 8 (TRPM8) membrane trafficking in dorsal root ganglion (DRG) are critical to cold hyperalgesia derived from neuropathic pain, and TRPM8 activation causes NMDAR-dependent cold response. Contribution of P2Y1 purinergic receptor (P2Y1R) activation in DRG to cold pain hypersensitivity and NMDAR activation induced by P2Y1R upregulation in neurons are also unraveled. This study explores whether P2Y1R contributes to remifentanil-induced cold hyperalgesia via TRPM8-dependent regulation of NMDAR phosphorylation in DRG.

METHODS

Rats with remifentanil-induced cold hyperalgesia were injected with TRPM8 antagonist or P2Y1R antagonist at 10 minutes before remifentanil infusion. Cold hyperalgesia (paw lift number and withdrawal duration on cold plate) was measured at -24, 2, 6, 24, and 48 hours following remifentanil infusion. After the last behavioral test, P2Y1R expression, TRPM8 expression and membrane trafficking, and NMDAR subunit (NR1 and NR2B) expression and phosphorylation in DRG were detected by western blot, and colocalization of P2Y1R with TRPM8 was determined by double-labeling immunofluorescence. Two-way repeated measures analysis of variance (ANOVA) or 2 × 2 factorial design ANOVA with repeated measures was used to analyze behavioral data of cold hyperalgesia. One-way ANOVA followed by Bonferroni post hoc comparisons was used to analyze the data in western blot and immunofluorescence.

RESULTS

Remifentanil infusion (1 μg·kg-1·min-1 for 60 minutes) induced cold hyperalgesia (hyperalgesia versus control, paw lift number and withdrawal duration on cold plate at 2-48 hours, P < .0001) with upregulated NR1 (hyperalgesia versus naive, 48 hours, mean ± standard deviation [SD], 114.00% ± 12.48% vs 41.75% ± 5.20%, P < .005) and NR2B subunits expression (104.13% ± 8.37% vs 24.63% ± 4.87%, P < .005), NR1 phosphorylation at Ser896 (91.88% ± 7.08% vs 52.00% ± 7.31%, P < .005) and NR2B phosphorylation at Tyr1472 (115.75% ± 8.68% vs 59.75% ± 7.78%, P < .005), TRPM8 expression (115.38% ± 9.27% vs 40.50% ± 4.07%, P < .005) and membrane trafficking (112.88% ± 5.62% vs 48.88% ± 6.49%, P < .005), and P2Y1R expression (128.25% ± 14.86% vs 45.13% ± 7.97%, P < .005) in DRG. Both TRPM8 and P2Y1R antagonists attenuated remifentanil-induced cold hyperalgesia and downregulated increased NR1 and NR2B expression and phosphorylation induced by remifentanil (remifentanil + RQ-00203078 versus remifentanil + saline, NR1 phosphorylation, 69.38% ± 3.66% vs 92.13% ± 4.85%; NR2B phosphorylation, 72.25% ± 6.43% vs 111.75% ± 11.00%, P < .0001). NMDAR activation abolished inhibition of TRPM8 and P2Y1R antagonists on remifentanil-induced cold hyperalgesia. P2Y1R antagonist inhibited remifentanil-evoked elevations in TRPM8 expression and membrane trafficking and P2Y1R-TRPM8 coexpression (remifentanil + 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate [MRS2179] versus remifentanil + saline, coexpression, 8.33% ± 1.33% vs 22.19% ± 2.15%, P < .0001).

CONCLUSIONS

Attenuation of remifentanil-induced cold hyperalgesia by P2Y1R inhibition is attributed to downregulations in NMDAR expression and phosphorylation via diminishing TRPM8 expression and membrane trafficking in DRG.

Aldosterone Synthase in Peripheral Sensory Neurons Contributes to Mechanical Hypersensitivity during Local Inflammation in Rats

Background

Recent emerging evidence suggests that extra-adrenal synthesis of aldosterone occurs (e.g., within the failing heart and in certain brain areas). In this study, the authors investigated evidence for a local endogenous aldosterone production through its key processing enzyme aldosterone synthase within peripheral nociceptive neurons.

Methods

In male Wistar rats (n = 5 to 8 per group) with Freund’s complete adjuvant hind paw inflammation, the authors examined aldosterone, aldosterone synthase, and mineralocorticoid receptor expression in peripheral sensory neurons using quantitative reverse transcriptase–polymerase chain reaction, Western blot, immunohistochemistry, and immunoprecipitation. Moreover, the authors explored the nociceptive behavioral changes after selective mineralocorticoid receptor antagonist, canrenoate-K, or specific aldosterone synthase inhibitor application.

Results

In rats with Freund’s complete adjuvant–induced hind paw inflammation subcutaneous and intrathecal application of mineralocorticoid receptor antagonist, canrenoate-K, rapidly and dose-dependently attenuated nociceptive behavior (94 and 48% reduction in mean paw pressure thresholds, respectively), suggesting a tonic activation of neuronal mineralocorticoid receptors by an endogenous ligand. Indeed, aldosterone immunoreactivity was abundant in peptidergic nociceptive neurons of dorsal root ganglia and colocalized predominantly with its processing enzyme aldosterone synthase and mineralocorticoid receptors. Moreover, aldosterone and its synthesizing enzyme were significantly upregulated in peripheral sensory neurons under inflammatory conditions. The membrane mineralocorticoid receptor consistently coimmunoprecipitated with endogenous aldosterone, confirming a functional link between mineralocorticoid receptors and its endogenous ligand. Importantly, inhibition of endogenous aldosterone production in peripheral sensory neurons by a specific aldosterone synthase inhibitor attenuated nociceptive behavior after hind paw inflammation (a 32% reduction in paw pressure thresholds; inflammation, 47 ± 2 [mean ± SD] vs. inflammation + aldosterone synthase inhibitor, 62 ± 2).

Conclusions

Local production of aldosterone by its processing enzyme aldosterone synthase within peripheral sensory neurons contributes to ongoing mechanical hypersensitivity during local inflammation via intrinsic activation of neuronal mineralocorticoid receptors.

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Bone-Marrow-Derived Mononuclear Cells Relieve Neuropathic Pain after Spinal Nerve Injury in Mice

Treating neuropathic pain is a critical clinical issue. Although numerous therapies have been proposed, effective treatments have not been established. Therefore, safe and feasible treatment methods are urgently needed. In this study, we investigated the therapeutic effects of autologous intrathecal administration of bone-marrow-derived mononuclear cells (MNCs) on neuropathic pain. We generated a mouse model of neuropathic pain by transecting the spinal nerve and evaluated neuropathic pain by measuring the mechanical threshold in the following 14 days. Mice in the MNC injection group had a higher mechanical threshold than those in the buffer group. We assessed the effect of MNC treatment on the dorsal root ganglia and spinal cord by immunohistochemistry, mRNA expression, and cytokine assay. The migration and accumulation of microglia were significantly suppressed in the MNC group, and the mRNA expression of inflammatory cytokines such as interleukin (IL)-6, IL-1β, and tumor necrosis factor alpha (TNF-α) was markedly downregulated. Furthermore, MNC administration tended to suppress various cytokines in the cerebrospinal fluid of the model mice. In conclusion, our results suggest that intrathecal injection of MNCs relieves neuropathic pain and might be a promising cell therapy for the treatment of this condition.

Regulatory mechanisms and therapeutic potential of microglial inhibitors in neuropathic pain and morphine tolerance

Microglia are important cells involved in the regulation of neuropathic pain (NPP) and morphine tolerance. Information on their plasticity and polarity has been elucidated after determining their physiological structure, but there is still much to learn about the role of this type of cell in NPP and morphine tolerance. Microglia mediate multiple functions in health and disease by controlling damage in the central nervous system (CNS) and endogenous immune responses to disease. Microglial activation can result in altered opioid system activity, and NPP is characterized by resistance to morphine. Here we investigate the regulatory mechanisms of microglia and review the potential of microglial inhibitors for modulating NPP and morphine tolerance. Targeted inhibition of glial activation is a clinically promising approach to the treatment of NPP and the prevention of morphine tolerance. Finally, we suggest directions for future research on microglial inhibitors.

Microglia as possible therapeutic targets for autism spectrum disorders

Malfunctions of the nervous and immune systems are now recognized to be fundamental causes of autism spectrum disorders (ASDs). Studies have suggested that the brain's resident immune cells, microglia are possible key players in ASDs. Specifically, deficits in synaptic pruning by microglia may underlie the pathogenesis of ASDs, in which excess synapses are occasionally reported. This idea has driven researchers to investigate causal links between microglial dysfunction and ASDs. In this review, we first introduce the characteristics of microglia in ASD brains and discuss their possible roles in the pathogenesis of ASDs. We also refer to immunomodulatory agents that could be potentially used as symptomatic therapies for ASDs in light of their ability to modify microglial functions. Finally, we will mention a possible strategy to radically cure some of the symptoms reported in ASDs through reorganizing neural circuits via microglia-dependent synaptic pruning.

Puerarin alleviate radicular pain from lumbar disc herniation by inhibiting ERK-dependent spinal microglia activation

Lumbar disc herniation is a common cause of radicular pain, but the mechanism remains ambiguous and the treatment stays unsatisfied. Many studies revealed a traditional Chinese medicine puerarin may moderate chronic pain from diabetes and nerve injury. Thus far, the role and mechanism of puerarin in radicular pain is still unknown. In this study, by using a rat model of lumbar disc herniation, which was induced by autologous nucleus pulposus (NP) implantation, the analgesic effect of puerarin on radicular pain was tested. Puerarin was delivered intraperitoneally form 1 h before surgery, and once daily for 7 days. The results demonstrated that NP implantation induced long-lasting pain, characterized by decrease of paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) in ipsilateral hindpaws, as long as day 20 after surgery. Spinal phosphorylated extracellular signal-regulated kinase (p-ERK) was up-regulated from day 5 to day 20 after surgery in ipsilateral but not contralateral side, and p-ERK was mainly co-localized with microglia. Puerarin decreased p-ERK expression from day 7 to day 20 after surgery. Puerarin or ERK inhibitor PD98059 alleviated pain behaviors, decreased expression of microglia marker ionized calcium-binding adaptor molecule 1 (Iba-1) in rats with NP implantation. The results suggested puerarin may alleviate radicular pain by inhibiting ERK-dependent or accompanied spinal microglia activation.

Pre-treatment with Meloxicam Prevents the Spinal Inflammation and Oxidative Stress in DRG Neurons that Accompany Painful Cervical Radiculopathy

Painful neuropathic injuries are accompanied by robust inflammatory and oxidative stress responses that contribute to the development and maintenance of pain. After neural trauma the inflammatory enzyme cyclooxygenase-2 (COX-2) increases concurrent with pain onset. Although pre-treatment with the COX-2 inhibitor, meloxicam, before a painful nerve root compression prevents the development of pain, the pathophysiological mechanisms are unknown. This study evaluated if pre-treatment with meloxicam prior to painful root injury prevents pain by reducing spinal inflammation and peripheral oxidative stress. Glial activation and expression of the inflammatory mediator secreted phospholipase A2 (sPLA2) in the spinal cord were assessed at day 7 using immunohistochemistry. The extent of oxidative damage was measured using the oxidative stress marker, 8-hydroxyguanosine (8-OHG) and localization of 8-OHG with neurons, microglia and astrocytes in the spinal cord and peripherally in the dorsal root ganglion (DRG) at day 7. In addition to reducing pain, meloxicam reduced both spinal microglial and astrocytic activation at day 7 after nerve root compression. Spinal sPLA2 was also reduced with meloxicam treatment, with decreased production in neurons, microglia and astrocytes. Oxidative damage following nerve root compression was found predominantly in neurons rather than glial cells. The expression of 8-OHG in DRG neurons at day 7 was reduced with meloxicam. These findings suggest that meloxicam may prevent the onset of pain following nerve root compression by suppressing inflammation and oxidative stress both centrally in the spinal cord and peripherally in the DRG.

Activated Glia Increased the Level of Proinflammatory Cytokines in a Resiniferatoxin-Induced Neuropathic Pain Rat Model

BACKGROUND AND OBJECTIVES

Administration of resiniferatoxin (RTX) can mimic the clinical symptoms of postherpetic neuralgia. However, it is unclear whether activated glia contribute to the pathogenesis of RTX-induced neuropathic pain; furthermore, the relationship between p38, N-methyl-D-aspartate receptor type 2B (NR2B) as well as proinflammatory cytokines and activated glia remains unknown.

METHODS

Intraperitoneal injection of RTX was performed to induce neuropathic pain in rats. Mechanical allodynia and thermal hyperalgesia were assessed by von Frey filaments and a radiant heat stimulus, respectively. Western blot and immunofluorescence labeling examined the expression of NR2B, activated glia markers, p38, and proinflammatory cytokines in the spinal cord. We further investigated the effect of the glial inhibitors, fluorocitrate and minocycline, on nociceptive behaviors and expression of p38, NR2B, and proinflammatory cytokines.

RESULTS

Resiniferatoxin leads to an increase of paw withdrawal latency to a heat stimulus and caused a mechanical allodynia within 2 weeks. The expression of tumor necrosis factor α, IL-1β, p38, and NR2B was up-regulated in RTX-induced neuropathic pain rat model and lasted for at least 49 days. Microglia were activated at the early phase of the disease, whereas activated astrocytes were detected in the sustainment phase. Both minocycline and fluorocitrate attenuated the nociceptive behaviors and expression of related proteins.

CONCLUSIONS

Activated glia participate in the pathogenesis of RTX-induced neuropathic pain and are likely to be the source of proinflammatory cytokines. Inhibition of glia contributes to an analgesic effect. These findings provide a novel strategy for the treatment of postherpetic neuralgia.

The Src family kinase inhibitor dasatinib delays pain-related behaviour and conserves bone in a rat model of cancer-induced bone pain

Pain is a severe and debilitating complication of metastatic bone cancer. Current analgesics do not provide sufficient pain relief for all patients, creating a great need for new treatment options. The Src kinase, a non-receptor protein tyrosine kinase, is implicated in processes involved in cancer-induced bone pain, including cancer growth, osteoclastic bone degradation and nociceptive signalling. Here we investigate the role of dasatinib, an oral Src kinase family and Bcr-Abl tyrosine kinase inhibitor, in an animal model of cancer-induced bone pain. Daily administration of dasatinib (15 mg/kg, p.o.) from day 7 after inoculation of MRMT-1 mammary carcinoma cells significantly attenuated movement-evoked and non-evoked pain behaviour in cancer-bearing rats. Radiographic - and microcomputed tomographic analyses showed significantly higher relative bone density and considerably preserved bone micro-architecture in the dasatinib treated groups, suggesting a bone-preserving effect. This was supported by a significant reduction of serum TRACP 5b levels in cancer-bearing rats treated with 15 mg/kg dasatinib. Furthermore, immunoblotting of lumbar spinal segments showed an increased activation of Src but not the NMDA receptor subunit 2B. These findings support a role of dasatinib as a disease modifying drug in pain pathologies characterized by increased osteoclast activity, such as bone metastases.

Spironolactone

Therapeutic Reviews aim to provide essential independent information for health professionals about drugs used in palliative and hospice care. Additional content is available on www.palliativedrugs.com. Country-specific books (Hospice and Palliative Care Formulary USA, and Palliative Care Formulary, British and Canadian editions) are also available and can be ordered from www.palliativedrugs.com. The series editors welcome feedback on the articles (hq@palliativedrugs.com).

Src-family kinases activation in spinal microglia contributes to central sensitization and chronic pain after lumbar disc herniation

Background Lumbar disc herniation is a major cause of radicular pain, but the underlying mechanisms remain largely unknown. Spinal activation of src-family kinases are involved in the development of chronic pain from nerve injury, inflammation, and cancer. In the present study, the role of src-family kinases activation in lumbar disc herniation-induced radicular pain was investigated. Results Lumbar disc herniation was induced by implantation of autologous nucleus pulposus, harvest from tail, in lumbar 4/5 spinal nerve roots of rat. Behavior test and electrophysiologic data showed that nucleus pulposus implantation induced persistent mechanical allodynia and thermal hyperalgesia and increased efficiency of synaptic transmission in spinal dorsal horn which underlies central sensitization of pain sensation. Western blotting and immunohistochemistry staining revealed that the expression of phosphorylated src-family kinases was upregulated mainly in spinal microglia of rats with nucleus pulposus. Intrathecal delivery of src-family kinases inhibitor PP2 alleviated pain behaviors, decreased efficiency of spinal synaptic transmission, and reduced phosphorylated src-family kinases expression. Furthermore, we found that the expression of ionized calcium-binding adapter molecule 1 (marker of microglia), tumor necrosis factor-α, interleukin 1 -β in spinal dorsal horn was increased in rats with nucleus pulposus. Therapeutic effect of PP2 may be related to its capacity in reducing the expression of these factors. Conclusions These findings suggested that central sensitization was involved in radicular pain from lumbar disc herniation; src-family kinases-mediated inflammatory response may be responsible for central sensitization and chronic pain after lumbar disc herniation.

Do Diuretics have Antinociceptive Actions: Studies of Spironolactone, Eplerenone, Furosemide and Chlorothiazide, Individually and with Oxycodone and Morphine

Spironolactone, eplerenone, chlorothiazide and furosemide are diuretics that have been suggested to have antinociceptive properties, for example via mineralocorticoid receptor antagonism. In co-administration, diuretics might enhance the antinociceptive effect of opioids via pharmacodynamic and pharmacokinetic mechanisms. Effects of spironolactone (100 mg/kg, i.p.), eplerenone (100 mg/kg, i.p.), chlorothiazide (50 mg/kg, i.p.) and furosemide (100 mg/kg, i.p.) were studied on acute oxycodone (0.75 mg/kg, s.c.)- and morphine (3 mg/kg, s.c.)-induced antinociception using tail-flick and hot plate tests in male Sprague Dawley rats. The diuretics were administered 30 min. before the opioids, and behavioural tests were performed 30 and 90 min. after the opioids. Concentrations of oxycodone, morphine and their major metabolites in plasma and brain were quantified by mass spectrometry. In the hot plate test at 30 and 90 min., spironolactone significantly enhanced the antinociceptive effect (% of maximum possible effect) of oxycodone from 10% to 78% and from 0% to 50%, respectively, and that of morphine from 12% to 73% and from 4% to 83%, respectively. The brain oxycodone and morphine concentrations were significantly increased at 30 min. (oxycodone, 46%) and at 90 min. (morphine, 190%). We did not detect any independent antinociceptive effects with the diuretics. Eplerenone and chlorothiazide did not enhance the antinociceptive effect of either opioid. The results suggest that spironolactone enhances the antinociceptive effect of both oxycodone and morphine by increasing their concentrations in the central nervous system.

Intrathecal Injection of JWH-015 Attenuates Bone Cancer Pain Via Time-Dependent Modification of Pro-inflammatory Cytokines Expression and Astrocytes Activity in Spinal Cord

Cannabinoid receptor type 2 (CB2) agonists display potential analgesic effects in acute and neuropathic pain. However, its complex cellular and molecular mechanisms in bone cancer pain remain unclear. And less relevant reports concerned its time-dependent effects on the long-lasting modifications of behavior, spinal inflammatory cytokines levels, astrocytes activity induced by bone cancer pain. A rat model of bone cancer pain induced by intra-tibia inoculation of Walker 256 mammary gland carcinoma cells was utilized. Pain behaviors at different time points were assessed by ambulatory pain scores and paw withdrawal mechanical threshold (PWMT). Pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, IL-18, and tumor necrosis factor alpha (TNF-α), were quantitated by Western blots. Glial activity was assessed by immunohistochemistry. Intra-tibia inoculation of Walker 256 mammary gland carcinoma cells induced progressive bone cancer pain; a long-term up-regulation of IL-1β, IL-6, IL-18, and TNF-α; and the activation of glia in spinal cord. Activation of microglia was first evident on day 4 after surgery and reached to a peak on day 7 while activation of astrocytes was on day 10. A single intrathecal injection of JWH-015 attenuated bone cancer induced spontaneous pain and mechanical allodynia, reduced the expression of pro-inflammatory cytokines, and inhibited the activity of astrocytes. All the modifications were transient and peaked at 24 h after JWH-015 administration. Furthermore, the protective effects of JWH-015 were reversed in the presence of CB2-selective antagonist AM630. Overall, our results provided evidences for the persistent participation of inflammation reaction in the progression of bone cancer pain, and demonstrated that JWH-015 reduced the expression of IL-1β, IL-6, IL-18, and TNF-α and inhibited astrocytes activation in a time-dependent manner, thereby displaying an analgesic effect.

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

Clinically, preoperative anxiety adversely affected postoperative hyperalgesia. As stress-induced glucocorticoids (GCs) were reported to sensitize the activation of microglia, the present study investigated whether and how GCs and microglia played in the process of preoperative anxiety-induced postoperative hyperalgesia. The study used an animal model that exposed rats to single prolonged stress (SPS) procedure to induce preoperative anxiety-like behaviors 24 h before the plantar incisional surgery. Behavioral testing revealed that preoperative SPS enhanced the mechanical allodynia induced by plantar incision. SPS was also found to induce elevated circulating corticosterone levels, potentiate the activation of spinal microglia, and increase the expression of spinal proinflammatory cytokines. Inhibition of microglia by pretreatment with minocycline attenuated the SPS-enhanced mechanical allodynia, and this was accompanied by decreased activation of spinal microglia and expression of proinflammatory cytokines. Another experiment was conducted by administering RU486, the GC receptor (GR) antagonist, to rats. The results showed that RU486 suppressed SPS-induced and SPS-potentiated proinflammatory activation of spinal microglia and revealed analgesic effects. Together, these data indicated that inhibition of stress-induced GR activation attenuated the preoperative anxiety-induced exacerbation of postoperative pain, and the suppression of spinal microglia activation may underlie this anti-hyperalgesia effect. Pending further studies, these findings suggested that GR and spinal microglia may play important roles in the development of preoperative anxiety-induced postoperative hyperalgesia and may serve as novel targets to prevent this phenomenon.

Acute mechanical sensitization of peripheral nociceptors by aldosterone through non-genomic activation of membrane bound mineralocorticoid receptors in naive rats

Recently, there is increasing interest in the role of peripheral mineralocorticoid receptors (MR) to modulate pain, but their localization in neurons and glia of the periphery and their distinct involvement in pain control remains elusive. In naive Wistar rats our double immunofluorescence confocal microscopy of the spinal cord, dorsal root ganglia, sciatic nerve and innervated skin revealed that MR predominantly colocalized with calcitonin-gene-related peptide (CGRP)- and trkA-immunoreactive (IR) nociceptive neurons and only marginally with myelinated trkB-IR mechanoreceptive and trkC-IR proprioreceptive neurons underscoring a pivotal role for MR in the modulation of pain. MR could not be detected in Schwann cells, satellite cells, and astrocytes and only scarcely in spinal microglia cells excluding a relevant functional role of glia-derived MR at least in naïve rats. Intrathecal (i.t.) and intraplantar (i.pl.) application of increasing doses of the MR selective agonist aldosterone acutely increased nociceptive behavior which was reversible by a MR selective antagonist and most likely due to non-genomic effects. This was further substantiated by the first identification of membrane bound MR specific binding sites in sensory neurons of dorsal root ganglia and spinal cord. Therefore, a crucial role of MR on nociceptive neurons but not on glia cells and their impact on nociceptive behavior most likely due to immediate non-genomic effects has to be considered under normal but more so under pathological conditions in future studies.

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.

Blocking the mineralocorticoid receptor improves effectiveness of steroid treatment for low back pain in rats

BACKGROUND

Localized inflammation of lumbar dorsal root ganglia (DRG) may contribute to low back pain. Local injections of corticosteroids used for low back pain are sometimes ineffective. Many corticosteroids activate not only the target glucocorticoid receptor (GR) but also the mineralocorticoid receptor (MR), which may have proinflammatory effects countering the effects of GR activation.

METHODS

A low back pain model was implemented in rats (n = 6 to 10 per group) by locally inflaming the L5 DRG. Sensory neuron excitability and mechanical hypersensitivity of the hind paws were measured. Tested steroids were applied locally to the inflamed DRG or orally.

RESULTS

The selective MR blocker eplerenone reduced pain behaviors when given orally starting at the time of surgery, or starting 7 days later. The highly GR-selective agonist fluticasone, applied locally to the inflamed DRG, was much more effective in reducing mechanical hypersensitivity. The MR/GR agonist 6-α methylprednisolone, commonly injected for low back pain, reduced mechanical hypersensitivity when applied locally to the DRG but was less effective than fluticasone. Its effectiveness was improved by combining it with local eplerenone. All tested steroids reduced hyperexcitability of myelinated sensory neurons (n = 71 to 220 cells per group) after inflammation, particularly abnormal spontaneous activity.

CONCLUSIONS

This preclinical study indicates the MR may play an important role in low back pain involving inflammation. Some MR effects may occur at the level of the sensory neuron. It may be useful to consider the action of clinically used steroids at the MR as well as at the GR.

Effects of salicylate on the inflammatory genes expression and synaptic ultrastructure in the cochlear nucleus of rats

Aspirin (salicylate), as a common drug that is frequently used for long-term treatment in a clinical setting, has the potential to cause reversible tinnitus. However, few reports have examined the inflammatory cytokines expression and alteration of synaptic ultrastructure in the cochlear nucleus (CN) in a rat model of tinnitus. The tinnitus-like behavior of rats were detected by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. We investigated the expression levels of the tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), N-methyl D-aspartate receptor subunit 2A (NR2A) mRNA and protein in the CN and compared synapses ultrastructure in the CN of tinnitus rats with normal ones. GPIAS showed that rats with long-term administration of salicylate were experiencing tinnitus, and the mRNA and protein expression levels of TNF-α and NR2A were up-regulated in chronic treatment groups, and they returned to baseline 14 days after cessation of treatment. Furthermore, compared to normal rats, repetitive salicylate-treated rats showed a greater number of presynaptic vesicles, thicker and longer postsynaptic densities, increased synaptic interface curvature. These data revealed that chronic salicylate administration markedly, but reversibly, induces tinnitus possibly via augmentation of the expression of TNF-α and NR2A and cause changes in synaptic ultrastructure in the CN. Long-term administration of salicylate causes neural plasticity changes at the CN level.

Stress-induced pain: a target for the development of novel therapeutics

Although current therapeutics provide relief from acute pain, drugs used for treatment of chronic pain are typically less efficacious and limited by adverse side effects, including tolerance, addiction, and gastrointestinal upset. Thus, there is a significant need for novel therapies for the treatment of chronic pain. In concert with chronic pain, persistent stress facilitates pain perception and sensitizes pain pathways, leading to a feed-forward cycle promoting chronic pain disorders. Stress exacerbation of chronic pain suggests that centrally acting drugs targeting the pain- and stress-responsive brain regions represent a valid target for the development of novel therapeutics. This review provides an overview of how stress modulates spinal and central pain pathways, identifies key neurotransmitters and receptors within these pathways, and highlights their potential as novel targets for therapeutics to treat chronic pain.

The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception

BACKGROUND

Spironolactone, a commonly used mineralocorticoid receptor antagonist, has been reported to potentiate the effect of morphine in the rat. The aim of this study was to investigate the effects of spironolactone on morphine antinociception and tissue distribution.

METHODS

The effects of spironolactone on acute morphine-induced antinociception, induction of morphine tolerance and established morphine tolerance were studied with tail-flick and hot plate tests in male Sprague-Dawley rats. Serum, brain, and liver morphine and its metabolite concentrations were quantified using high-pressure liquid chromatography-tandem mass spectrometry. Spironolactone was also administered with the peripherally acting, P-glycoprotein (P-gp) substrate loperamide to test whether spironolactone allows loperamide to pass the blood-brain barrier.

RESULTS

Spironolactone (50 mg/kg, i.p.) had no antinociceptive effects of its own, but it enhanced the antinociceptive effect of morphine in both thermal tests. Two doses of spironolactone enhanced the maximum possible effect (MPE) from 19.5% to 100% in the hot plate test 90 min after administration of 4 mg/kg morphine. Morphine concentrations in the brain were increased fourfold at 90 min by spironolactone. Spironolactone did not inhibit the formation of morphine-3-glucuronide. Acute spironolactone restored morphine antinociception in morphine-tolerant rats but did not inhibit the development of tolerance. The peripherally restricted opioid, loperamide (10 mg/kg), had no antinociceptive effects when administered alone, but co-administration with spironolactone produced a 40% MPE in the hot plate test.

CONCLUSIONS

Spironolactone has no antinociceptive effects in thermal models of pain, but it enhances the antinociceptive effects of morphine mainly by increasing morphine central nervous system concentrations, probably by inhibiting P-gp.

Preclinical studies of low back pain

Chronic low back pain is a major cause of disability and health care costs. Current treatments are inadequate for many patients. A number of preclinical models have been developed that attempt to mimic aspects of clinical conditions that contribute to low back pain. These involve application of nucleus pulposus material near the lumbar dorsal root ganglia (DRG), chronic compression of the DRG, or localized inflammation of the DRG. These models, which are primarily implemented in rats, have many common features including behavioral hypersensitivity of the hindpaw, enhanced excitability and spontaneous activity of sensory neurons, and locally elevated levels of inflammatory mediators including cytokines. Clinically, epidural injection of steroids (glucocorticoids) is commonly used when more conservative treatments fail, but clinical trials evaluating these treatments have yielded mixed results. There are relatively few preclinical studies of steroid effects in low back pain models. One preclinical study suggests that the mineralocorticoid receptor, also present in the DRG, may have pro-inflammatory effects that oppose the activation of the glucocorticoid receptor. Although the glucocorticoid receptor is the target of anti-inflammatory steroids, many clinically used steroids activate both receptors. This could be one explanation for the limited effects of epidural steroids in some patients. Additional preclinical research is needed to address other possible reasons for limited efficacy of steroids, such as central sensitization or presence of an ongoing inflammatory stimulus in some forms of low back pain.