Ketamine depresses toll-like receptor 3 signaling in spinal microglia in a rat model of neuropathic pain

Towards Novel Potential Molecular Targets for Antidepressant and Antipsychotic Pharmacotherapies

Depression and schizophrenia are two highly prevalent and severely debilitating neuropsychiatric disorders. Both conventional antidepressant and antipsychotic pharmacotherapies are often inefficient clinically, causing multiple side effects and serious patient compliance problems. Collectively, this calls for the development of novel drug targets for treating depressed and schizophrenic patients. Here, we discuss recent translational advances, research tools and approaches, aiming to facilitate innovative drug discovery in this field. Providing a comprehensive overview of current antidepressants and antipsychotic drugs, we also outline potential novel molecular targets for treating depression and schizophrenia. We also critically evaluate multiple translational challenges and summarize various open questions, in order to foster further integrative cross-discipline research into antidepressant and antipsychotic drug development.

Toll-like receptors and their role in neuropathic pain and migraine

Migraine is a complex neurological disease of unknown etiology involving both genetic and environmental factors. It has previously been reported that persistent pain may be mediated by the immune and inflammatory systems. Toll-like receptors (TLRs) play a significant role in immune and inflammatory responses and are expressed by microglia and astrocytes. One of the fundamental mechanisms of the innate immune system in coordinating inflammatory signal transduction is through TLRs, which protect the host organism by initiating inflammatory signaling cascades in response to tissue damage or stress. TLRs reside at the neuroimmune interface, and accumulating evidence has suggested that the inflammatory consequences of TLR activation on glia (mainly microglia and astrocytes), sensory neurons, and other cell types can influence nociceptive processing and lead to pain. Several studies have shown that TLRs may play a key role in neuropathic pain and migraine etiology by activating the microglia. The pathogenesis of migraine may involve a TLR-mediated crosstalk between neurons and immune cells. Innate responses in the central nervous system (CNS) occur during neuroinflammatory phenomena, including migraine. Antigens found in the environment play a crucial role in the inflammatory response, causing a broad range of diseases, including migraines. These can be recognized by several innate immune cells, including macrophages, microglia, and dendritic cells, and can be activated through TLR signaling. Given the prevalence of migraine and the insufficient efficacy and safety of current treatment options, a deeper understanding of TLRs is expected to provide novel therapies for managing chronic migraine. This review aimed to justify the view that TLRs may be involved in migraine.

β-Hydroxybutyrate Attenuates Painful Diabetic Neuropathy via Restoration of the Aquaporin-4 Polarity in the Spinal Glymphatic System

Waste removal is essential for maintaining homeostasis and the normal function of the central nervous system (CNS). The glymphatic system based on aquaporin-4 (AQP4) water channels on the endfeet of astrocytes is recently discovered as the excretion pathway for metabolic waste products of CNS. In the CNS, α-syntrophin (SNTA1) directly or indirectly anchors AQP4 in astrocyte membranes facing blood vessels. Studies have indicated that β-hydroxybutyrate (BHB) can raise the expression of SNTA1 and thus restoring AQP4 polarity in mice models with Alzheimer’s disease. The study aims to evaluate the neuroprotective mechanism of BHB in rats with painful diabetic neuropathy (PDN). PDN rats were modeled under a high-fat and high-glucose diet with a low dose of streptozotocin. Magnetic resonance imaging (MRI) was applied to observe the clearance of contrast to indicate the functional variability of the spinal glymphatic system. Mechanical allodynia was assessed by paw withdrawal threshold. The expressions of SNTA1 and AQP4 were tested, and the polarity reversal of AQP4 protein was measured. As demonstrated, PDN rats were manifested with deceased contrast clearance of the spinal glymphatic system, enhanced mechanical allodynia, lower expression of SNTA1, higher expression of AQP4, and reversed polarity of AQP4 protein. An opposite change in the above characteristics was observed in rats being treated with BHB. This is the first study that demonstrated the neuroprotective mechanism of BHB to attenuate PDN via restoration of the AQP4 polarity in the spinal glymphatic system and provides a promising therapeutic strategy for PDN.

Plasticity of the spinal glymphatic system in male SD rats with painful diabetic neuropathy induced by type 2 diabetes mellitus

The glymphatic system is a recently discovered glial‐dependent macroscopic interstitial waste clearance system that promotes the efficient elimination of soluble proteins and metabolites from the central nervous system. Its anatomic foundation is the astrocytes and aquaporin‐4 (AQP4) water channels on the endfeet of astrocytes. The aim of this study is to evaluate the plasticity of the spinal glymphatic system in male SD rats with painful diabetic neuropathy (PDN) induced by type 2 diabetes mellitus. PDN rats were modeled under a high‐fat and high‐glucose diet with a low dose of streptozotocin. MRI was applied to observe the infiltration and clearance of contrast to indicate the functional variability of the glymphatic system at the spinal cord level. The paw withdrawal threshold was used to represent mechanical allodynia. The numerical change of glial fibrillary acidic protein (GFAP) positive astrocytes was assessed and the polarity reversal of AQP4 protein was measured by immunofluorescence. As a result, deceased contrast infiltration and clearance, enhanced mechanical allodynia, increased number of GFAP positive astrocytes, and reversed polarity of AQP4 protein were found in the PDN rats. The above molecular level changes may contribute to the impairment of the spinal glymphatic system in PDN rats. This study revealed the molecular and functional variations of the spinal glymphatic system in PDN rats and for the first time indicated that there might be a correlation between the impaired spinal glymphatic system and PDN rats.

Anesthetics mediated the immunomodulatory effects via regulation of TLR signaling

Anesthetics have been widely used in surgery and found to suppress inflammatory injury and affect the outcomes of the surgery and diseases. In contrast, anesthetics are also found to induce neuronal injury and inflammation. However, the immune-modulation mechanism of anesthetics is still not clear. Recent studies have shown that the immune-modulation of anesthetics is associated with the regulation of toll-like receptor (TLR)-mediated signaling. Moreover, the regulation of anesthetics in TLR signaling is related to modulations of non-coding RNAs (nc RNAs). Consistently, nc RNAs are mainly divided into micro RNAs (miRs) and long non-coding RNAs (lnc RNAs), which have been found to exert regulatory effects on the immune system. In this review, we summarize the immunomodulatory functions of the widely used anesthetic agents, which are associated with regulation of TLR signaling. In addition, we also focus on the roles of nc RNAs induced by anesthetics in regulations of TLR signaling.

Efficacy of ketamine in relieving neuropathic pain: a systematic review and meta-analysis of animal studies

ABSTRACT

In humans, proof of long-term efficacy of ketamine treatment in neuropathic pain is lacking. To improve our understanding of ketamine behavior under various administration conditions, we performed a systematic review and meta-analyses of controlled studies on the efficacy of ketamine in mice and rats with a disease model of nerve injury on relief of allodynia. Searches in PubMed and EMBASE identified 31 unique studies. Four meta-analyses were conducted. The first analysis included 19 comparisons on a single ketamine dose and measurement of effect within 3 hours of dosing and showed an appreciable effect (standardized mean difference 1.6, 95% confidence interval 1.1-2.1). Subgroup analyses showed no effect of species, administration route, or dose. A single administration was insufficient to sustain relief of allodynia at 24 or 72 hours after dosing, as observed in our second analysis (7 comparisons) with similar effects in ketamine-treated and control animals. Chronic ketamine administration (9 comparisons) caused profound relief of allodynia when tested during ketamine exposure (effect size 5.1, 3.7-6.5). The final analysis (6 comparisons) showed that chronic administration caused a slow loss of relief of allodynia with 70% loss of effect 24 days after end of treatment. No subgroups analyses were possible in the last 3 meta-analyses due to small group sizes. These results indicate long-term ketamine anti-allodynic effects after chronic exposure (>3 days) but not after a single administration. Given several limitations, extrapolation of the animal data to the human condition is tenuous.

The effects of anaesthetics and sedatives on brain inflammation

Microglia are involved in many dynamic processes in the central nervous system (CNS) including the development of inflammatory processes and neuromodulation. Several sedative, analgesic or anaesthetic drugs, such as opioids, ∝2-adrenergic agonists, ketamine, benzodiazepines and propofol can cause both neuroprotective and harmful effects on the brain. The purpose of this review is to present the main findings on the use of these drugs and the mechanisms involved in microglial activation. Alpha 2-adrenergic agonists, propofol and benzodiazepines have several pro- or anti-inflammatory effects on microglia. Long-term use of benzodiazepines and propofol causes neuroapoptotic effects and α2-adrenergic agonists may attenuate these effects. Conversely, morphine and fentanyl may have proinflammatory effects, causing behavioural changes in patients and changes in cell viability in vitro. Conversely, chronic administration of morphine induces CCL5 chemokine expression in microglial cells that promotes their survival.

Ketamine Use for Cancer and Chronic Pain Management

Ketamine, an N-methyl-D-aspartate receptor antagonist, is widely known as a dissociative anesthetic and phencyclidine derivative. Due to an undesirable adverse event profile when used as an anesthetic it had widely fallen out of human use in favor of more modern agents. However, it has recently been explored for several other indications such as treatment resistant depression and chronic pain. Several recent studies and case reports compiled here show that ketamine is an effective analgesic in chronic pain conditions including cancer-related neuropathic pain. Of special interest is ketamine’s opioid sparing ability by counteracting the central nervous system sensitization seen in opioid induced hyperalgesia. Furthermore, at the sub-anesthetic concentrations used for analgesia ketamine’s safety and adverse event profiles are much improved. In this article, we review both the basic science and clinical evidence regarding ketamine’s utility in chronic pain conditions as well as potential adverse events.

Microglia in depression: current perspectives

Major depressive disorder (MDD) is a prevalent psychiatric disease that involves malfunctions of different cell types in the brain. Accumulating studies started to reveal that microglia, the primary resident immune cells, play an important role in the development and progression of depression. Microglia respond to stress-triggered neuroinflammation, and through the release of proinflammatory cytokines and their metabolic products, microglia may modulate the function of neurons and astrocytes to regulate depression. In this review, we focused on the role of microglia in the etiology of depression. We discussed the dynamic states of microglia; the correlative and causal evidence of microglial abnormalities in depression; possible mechanisms of how microglia sense depression-related stress and modulate depression state; and how antidepressive therapies affect microglia. Understanding the role of microglia in depression may shed light on developing new treatment strategies to fight against this devastating mental illness.

TLR3 promotes hepatocyte proliferation after partial hepatectomy by stimulating uPA expression and the release of tissue-bound HGF

TLR3 is implicated in anti-viral immune responses, but may also act as a sensor of tissue damage in the absence of infection. Here, we provide evidence for an essential role of TLR3 in liver regeneration after an acute loss of tissue due to partial hepatectomy. Mice lacking TLR3 had a severe and sustained defect in the restoration of liver tissue with reduced liver-to-body weight ratios even after an extended recovery period of 2 weeks. Hepatocyte cell cycle progression into S phase was impaired in TLR3-deficient mice. Mechanistic analyses revealed that TLR3-deficient mice had markedly reduced systemic levels of active HGF, but had increased amounts of inactive tissue-bound HGF. Importantly, expression of uPA, which orchestrates the processing and release of HGF from the hepatic extracellular matrix, was reduced in regenerating livers of TLR3-deficient mice. In addition, expression of the HGF maturation factor HGFAC was transiently diminished in TLR3-deficient mice. In vitro, engagement of TLR3 directly stimulated expression of uPA by hepatic stellate cells. Thus, TLR3 supports liver regeneration through upregulation of uPA, which promotes the release of preformed HGF from extracellular matrix stores.

Penehyclidine hydrochloride suppressed peripheral nerve injury-induced neuropathic pain by inhibiting microglial MAPK/p-p38/IL-1 pathway activation

Background

Millions of people suffered from neuropathic pain, which is related to neuroinflammation in the central nervous system. Penehyclidine hydrochloride is a premedication of general anesthesia, which has been confirmed possessing neuroprotective effects against various neurodegenerative or neuroinflammatory diseases. However, it is not clear that whether penehyclidine hydrochloride could suppress neuropathic pain through its anti-neuroinflammatory effects.

Methods

This study investigated the effects of penehyclidine hydrochloride on rat spinal nerve ligation injury-induced neuropathic pain with behavioral, morphological, and molecular biological methods in animals.

Results

The results indicated that penehyclidine hydrochloride could attenuate spinal nerve ligation-induced neuropathic pain without any motor impairment and had no effect on sham-operated animals after repeated intraperitoneal administration. Intraperitoneal penehyclidine hydrochloride could suppress spinal nerve ligation-induced ipsilateral spinal dorsal horn microglial activation with downregulation of OX42 expression. Moreover, intraperitoneal penehyclidine hydrochloride inhibited spinal nerve ligation-induced spinal p-p38 mitogen-activated protein kinase expression, which was specially colocalized with the spinal dorsal horn microglia. Furthermore, intraperitoneal penehyclidine hydrochloride could depress spinal neuroinflammation by suppressing spinal nerve ligation-induced interleukin (IL)-1β over-expression.

Conclusion

These results indicated that the anti-allodynic effects of penehyclidine hydrochloride on spinal nerve ligation-induced neuropathic pain did not rely on motor impairment. Inhibiting spinal microglial p-p38/IL-1β pathway activation might contribute to the anti-allodynic effect of penehyclidine hydrochloride on nerve injury-induced neuropathic pain.

Spinal circRNA-9119 Suppresses Nociception by Mediating the miR-26a-TLR3 Axis in a Bone Cancer Pain Mouse Model

Altered expression of circular RNA (circRNA) is recognized as a contributor to malignant pain where microRNA (miRNA) exerts an essential effect. We generated a murine model for bone malignancy pain in which 2472 osteolytic sarcoma cells were injected into the femurs of mice. CircRNA microarray and quantitative PCR (qPCR) and revealed that circ9119 expression was repressed in the spinal cord of bone malignancy pain model mice, which is the first relay site involved in the transmission of nociceptive information to the cerebrum of mice that receive spinal analgesics for malignancy pain. Overexpression of circ9119 by plasmid injection in the model mice reduced progressive thermal hyperalgesia and mechanical hyperalgesia. Bioinformatics prediction and dual-luciferase reporter assay showed that circ9119 functions as a sponge of miR-26a, which targets the TLR3 3'-untranslated region. Furthermore, expression of miR-26a was elevated and TLR3 level was repressed in bone malignancy pain model mice, which were counteracted by circ9119 in the spinal cord of tumor-bearing mice. Moreover, excessive expression of miR-26a was involved in the recovery of mice from progressive thermal hyperalgesia and mechanical hyperalgesia triggered via circ9119. TLR3 knockdown in bone malignancy pain model mice thoroughly impaired pain in the initial stages and reduced the effects of circ9119 on hyperalgesia. Our research findings indicate that targeting the circ9119-miR-26a-TLR3 axis may be a promising analgesic strategy to manage malignancy pain.

In vivo Two-Photon Imaging of Anesthesia-Specific Alterations in Microglial Surveillance and Photodamage-Directed Motility in Mouse Cortex

Two-photon imaging of fluorescently labeled microglia in vivo provides a direct approach to measure motility of microglial processes as a readout of microglial function that is crucial in the context of neurodegenerative diseases, as well as to understand the neuroinflammatory response to implanted substrates and brain-computer interfaces. In this longitudinal study, we quantified surveilling and photodamage-directed microglial processes motility in both acute and chronic cranial window preparations and compared the motility under isoflurane and ketamine anesthesia to an awake condition in the same animal. The isoflurane anesthesia increased the length of surveilling microglial processes in both acute and chronic preparations, while ketamine increased the number of microglial branches in acute preparation only. In chronic (but not acute) preparation, the extension of microglial processes toward the laser-ablated microglial cell was faster under isoflurane (but not ketamine) anesthesia than in awake mice, indicating distinct effects of anesthetics and of preparation type. These data reveal potentiating effects of isoflurane on microglial response to damage, and provide a framework for comparison and optimal selection of experimental conditions for quantitative analysis of microglial function using two-photon microscopy in vivo.

Metformin attenuates increase of synaptic number in the rat spinal dorsal horn with painful diabetic neuropathy induced by type 2 diabetes: a stereological study

In our previous study, we have shown that number of synapses in the L5 segment of spinal dorsal horn increased significantly in a rat model of painful diabetic neuropathy (PDN) induced by high-dose of streptozotocin (an animal model of type 1 diabetes). The aims of this study were: (1) to determine whether high fat diet/low dose streptozotocin-diabetes, a rat model for type 2 diabetes, related PDN was also associated with this synaptic plasticity, (2) to reveal the range of this synaptic plasticity change occurred (in the whole length of spinal dorsal horn or only in the L5 lumbar segment of spinal dorsal horn) and (3) to discover whether treatment with metformin had effect on this synaptic plasticity. Male adult Sprague-Dawley rats were randomly allocated into the control group (n = 7), the PDN group (n = 6) and the PDN treated with metformin (PDN + M) group (n = 7), respectively. 28 days after medication, synaptic and neuronal numbers in the whole length of spinal dorsal horn or in 1 mm length of the L5 segment of spinal dorsal horn were estimated by the optical disector (a stereological technique). Compared to the control group and the PDN + M group, number of synapses in the L5 segment of spinal dorsal horn increased significantly in the PDN group (P < 0.05). There was no significant change between the control group and the PDN + M group in terms of the parameters in the L5 segment of the spinal dorsal horn (P > 0.05). Parameters of the whole length of spinal dorsal horn showed no significant changes (P > 0.05). Our results suggest that high fat diet/low dose streptozotocin diabetes related PDN is also associated with a numerical increase of synapses in the L5 segment of spinal dorsal horn but not in the whole length of spinal dorsal horn. Furthermore, the analgesic effect of metformin against PDN is related to its inhibition of numerical increase of synaptic number in the rat spinal dorsal horn.

FAAH inhibition attenuates TLR3-mediated hyperthermia, nociceptive- and anxiety-like behaviour in female rats

Aberrant activation of toll-like receptor (TLR)s results in persistent and prolonged neuroinflammation and has been implicated in the pathogenesis and exacerbation of psychiatric and neurodegenerative disorders. TLR3 coordinates the innate immune response to viral infection and recent data have demonstrated that inhibiting fatty acid amide hydrolase (FAAH), the enzyme that primarily metabolizes anandamide, modulates TLR3-mediated neuroinflammation. However, the physiological and behavioural consequences of such modulation are unknown. The present study examined the effect of URB597, a selective FAAH inhibitor, on neuroinflammation, physiological and behavioural alterations following administration of the TLR3 agonist and viral mimetic poly I:C to female rats. URB597 attenuated TLR3-mediated fever, mechanical and cold allodynia, and anxiety-like behaviour in the elevated plus maze and open field arena. There was no effect of URB597 on TLR3-mediated decreases in body weight and no effect in the sucrose preference or forced swim tests. URB597 attenuated the TLR3-mediated increase in the expression of CD11b and CD68, markers of microglia/macrophage activation. In summary, these data demonstrate that enhancing FAAH substrate levels suppresses TLR3-mediated microglia/macrophage activation and associated changes in fever, nociceptive responding and anxiety-related behaviour. These data provide further support for FAAH as a novel therapeutic target for neuroinflammatory disorders.

Microglial Inhibition Influences XCL1/XCR1 Expression and Causes Analgesic Effects in a Mouse Model of Diabetic Neuropathy

BACKGROUND

Recent studies indicated the involvement of some chemokines in the development of diabetic neuropathy; however, participation of the chemokine-C-motif ligand (XCL) subfamily remains unknown. The goal of this study was to examine how microglial inhibition by minocycline hydrochloride (MC) influences chemokine-C-motif ligand 1 (XCL1)-chemokine-C-motif receptor 1 (XCR1)/G protein-coupled receptor 5 expression and the development of allodynia/hyperalgesia in streptozotocin-induced diabetic neuropathy.

METHODS

The studies were performed on streptozotocin (200 mg/kg, intraperitoneally)-induced mouse diabetic neuropathic pain model and primary glial cell cultures. The MC (30 mg/kg, intraperitoneally) was injected two times daily until day 21. XCL1 and its neutralizing antibody were injected intrathecally, and behavior was evaluated with von Frey and cold plate tests. Quantitative analysis of protein expression of glial markers, XCL1, and/or XCR1 was performed by Western blot and visualized by immunofluorescence.

RESULTS

MC treatment diminished allodynia (0.9 ± 0.1 g; n = 7 vs. 3.8 ± 0.7 g; n = 7) and hyperalgesia (6.5 ± 0.6 s; n = 7 vs. 16.5 ± 1 s; n = 7) in the streptozotocin-induced diabetes. Repeated MC administration prevented microglial activation and inhibited the up-regulation of the XCL1/XCR1 levels. XCL1 administration (10 to 500 ng/5 μl; n = 9) in naive mice enhanced nociceptive transmission, and injections of neutralizing XCL1 (4 to 8 μg/5 μl; n = 10) antibody into the mice with diabetic neuropathic pain diminished allodynia/hyperalgesia. Microglia activation evoked in primary microglial cell cultures resulted in enhanced XCL1 release and XCR1 expression. Additionally, double immunofluorescence indicated the widespread coexpression of XCR1-expressing cells with spinal neurons.

CONCLUSIONS

In diabetic neuropathy, declining levels of XCL1 evoked by microglia inhibition result in the cause of analgesia. The putative mechanism corroborating this finding can be related to lower spinal expression of XCR1 together with the lack of stimulation of these XCR1 receptors, which are localized on neurons.

Decreased Endomorphin-2 and μ-Opioid Receptor in the Spinal Cord Are Associated with Painful Diabetic Neuropathy

Painful diabetic neuropathy (PDN) is one of the most common complications in the early stage of diabetes mellitus (DM). Endomorphin-2 (EM2) selectively activates the μ-opioid receptor (MOR) and subsequently induces antinociceptive effects in the spinal dorsal horn. However, the effects of EM2-MOR in PDN have not yet been clarified in the spinal dorsal horn. Therefore, we aimed to explore the role of EM2-MOR in the pathogenesis of PDN. The main findings were the following: (1) streptozotocin (STZ)-induced diabetic rats exhibited hyperglycemia, body weight loss and mechanical allodynia; (2) in the spinal dorsal horn, the expression levels of EM2 and MOR decreased in diabetic rats; (3) EM2 protein concentrations decreased in the brain, lumbar spinal cord and cerebrospinal fluid (CSF) in diabetic rats but were unchanged in the plasma; (4) the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) was significantly higher in diabetic rats than in control rats; and (5) intrathecal injection of EM2 for 14 days in the early stage of PDN partially alleviated mechanical allodynia and reduced MOR expression in diabetic rats. Our results demonstrate that the EM2-MOR signal may be involved in the early stage of PDN.

Activation of corticotropin-releasing factor neurons and microglia in paraventricular nucleus precipitates visceral hypersensitivity induced by colorectal distension in rats

Visceral hypersensitivity is a major contributor to irritable bowel syndrome and other disorders with visceral pain. Substantial evidence has established that glial activation and neuro-glial interaction play a key role in the establishment and maintenance of visceral hypersensitivity. We recently demonstrated that activation of spinal microglial toll-like receptor 4 (TLR4)/myeloid differentiation factor 88 (MyD88)/nuclear factor κB (NF-κB) signaling facilitated the development of visceral hypersensitivity in a rat model developed by neonatal and adult colorectal distensions (CRDs). Hypothalamic paraventricular nucleus (PVN) plays a pivotal role in the pathogenesis of chronic pain. In this study, we examined the mechanism by which microglia and neurons in PVN establish and maintain visceral hypersensitivity and the involvement of TLR4 signaling. Visceral hypersensitivity was precipitated by adult colorectal distension (CRD) only in rats that experienced neonatal CRDs. Visceral hypersensitivity was associated with an increase in the expression of c-fos, corticotropin-releasing factor (CRF) protein and mRNA in PVN, which could be prevented by intra-PVN infusion of lidocaine or small interfering RNA targeting the CRF gene. These results suggest PVN CRF neurons modulate visceral hypersensitivity. Adult CRD induced an increase in the expression of Iba-1 (a microglial marker), TLR4 protein, and its downstream effectors MyD88, NF-κB, as well as proinflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) only in rats that experienced neonatal CRDs. Intra-PVN infusion of minocycline, a nonselective microglial inhibitor, attenuated the hyperactivity of TLR4 signaling cascade, microglial activation, and visceral hypersensitivity. Taken together, these data suggest that neonatal CRDs induce a glial activation in PVN. Adult CRD potentiates the glial and CRF neuronal activity, and precipitates visceral hypersensitivity and pain. TLR4 signaling and proinflammatory cytokines TNF-α and IL-1β may participate in neuro-glial interaction during the pathogenesis of visceral hypersensitivity.

Inhibition of microglial activation contributes to propofol-induced protection against post-cardiac arrest brain injury in rats

It has been suggested that propofol can modulate microglial activity and hence may have potential roles against neuroinflammation following brain ischemic insult. However, whether and how propofol can inhibit post-cardiac arrest brain injury via inhibition of microglia activation remains unclear. A rat model of asphyxia cardiac arrest (CA) was created followed by cardiopulmonary resuscitation. CA induced marked microglial activation in the hippocampal CA1 region, revealed by increased OX42 and P2 class of purinoceptor 7 (P2X7R) expression, as well as p38 MAPK phosphorylation. Morris water maze showed that learning and memory deficits following CA could be inhibited or alleviated by pre-treatment with the microglial inhibitor minocycline or propofol. Microglial activation was significantly suppressed likely via the P2X7R/p-p38 pathway by propofol. Moreover, hippocampal neuronal injuries after CA were remarkably attenuated by propofol. In vitro experiment showed that propofol pre-treatment inhibited ATP-induced microglial activation and release of tumor necrosis factor-α and interleukin-1β. In addition, propofol protected neurons from injury when co-culturing with ATP-treated microglia. Our data suggest that propofol pre-treatment inhibits CA-induced microglial activation and neuronal injury in the hippocampus and ultimately improves cognitive function. We proposed a possible mechanism of propofol-mediated brain protection after cardiac arrest (CA). CA induces P2X7R upregulation and p38 phosphorylation in microglia, which induces release of TNF-α and IL-1β and consequent neuronal injury. Propofol could inhibit microglial activation and alleviate neuronal damage. Our results suggest propofol-induced anti-inflammatory treatment as a plausible strategy for therapeutic intervention in post-CA brain injury.

Efficacy of an intrathecal multidrug infusion for pain control in older adults and in end-stage malignancies: A report of three cases

The aim of the present study was to explore the effectiveness of an alternative method to manage pain based on a time-limited intrathecal (IT) infusion of an analgesic medication mixture. Three patients (69, 64 and 94 years of age) with intractable and poorly controlled pain due to bed sores, pelvic metastatic mass, and thoracic vertebra and rib fractures, respectively, were treated. Daily doses of opioids could not be increased due to side effects. An IT catheter (20 G) was placed by percutaneous approach in the lumbar area while advancing toward the thoracic region, and was then tunnelled and fixed subcutaneously. It was connected to an external infusion pump with a mixture of bupivacaine 1 mg⁄mL, naloxone 0.02 ng⁄mL, ketamine 100 µg⁄mL, morphine 0.01 mg⁄mL and clonidine 0.75 µg⁄mL. The starting rate was 1 mL⁄h. The pain was mostly controlled at a rate of <1 mL⁄h. Opioid consumption was reduced dramatically. The catheter was kept in place for one month in the first and third patients, and for six months in the second patient, until his death. Major side effects, such as hypotension, constipation, muscle weakness, sphincter dysfunction, and cognitive or mood deterioration, were not observed with this approach. One patient experienced a urinary tract infection followed by sepsis and meningitis, which was cured by antibiotics. The catheter was removed in this patient. IT infusion with a low-concentration multidrug mixture could be considered as an alternative modality for intractable pain relief in older adults or in malignancies.

Toll-like receptors in central nervous system injury and disease: a focus on the spinal cord

Toll-like receptors (TLRs) are best known for recognizing pathogens and initiating an innate immune response to protect the host. However, they also detect tissue damage and induce sterile inflammation upon the binding of endogenous ligands released by stressed or injured cells. In addition to immune system-related cells, TLRs have been identified in central nervous system (CNS) neurons and glial subtypes including microglia, astrocytes and oligodendrocytes. Direct and indirect effects of TLR ligands on neurons and glial subtypes have been documented in vitro. Likewise, the effects of TLR ligands have been demonstrated in vivo using animal models of CNS trauma and disease including spinal cord injury (SCI), amyotrophic lateral sclerosis (ALS) and neuropathic pain. The indirect effects are most likely mediated via microglia or immune system cells that infiltrate the diseased or injured CNS. Despite considerable progress over the past decade, the role of TLRs in the physiological and pathological function of the spinal cord remains inadequately defined. Published reports collectively highlight TLRs as promising targets for therapeutic interventions in spinal cord pathology. The findings also underscore the complexity of TLR-mediated mechanisms and the necessity for further research in this field. The goals of the current review are to recapitulate the studies that investigated the role of TLRs in the spinal cord, to discuss potential future research directions, and to examine some of the challenges associated with pre-clinical studies pertinent to TLRs in the injured or diseased spinal cord.

Inhibiting spinal neuron-astrocytic activation correlates with synergistic analgesia of dexmedetomidine and ropivacaine

Background

This study aims to identify that intrathecal (i.t.) injection of dexmedetomidine (Dex) and ropivacaine (Ropi) induces synergistic analgesia on chronic inflammatory pain and is accompanied with corresponding “neuron-astrocytic” alterations.

Methods

Male, adult Sprague-Dawley rats were randomly divided into sham, control and i.t. medication groups. The analgesia profiles of i.t. Dex, Ropi, and their combination detected by Hargreaves heat test were investigated on the subcutaneous (s.c.) injection of complete Freund adjuvant (CFA) induced chronic pain in rat and their synergistic analgesia was confirmed by using isobolographic analysis. During consecutive daily administration, pain behavior was daily recorded, and immunohistochemical staining was applied to investigate the number of Fos-immunoreactive (Fos-ir) neurons on hour 2 and day 1, 3 and 7, and the expression of glial fibrillary acidic protein (GFAP) within the spinal dorsal horn (SDH) on day 1, 3, 5 and 7 after s.c. injection of CFA, respectively, and then Western blot to examine spinal GFAP and β-actin levels on day 3 and 7.

Results

i.t. Dex or Ropi displayed a short-term analgesia in a dose-dependent manner, and consecutive daily administrations of their combination showed synergistic analgesia and remarkably down-regulated neuronal and astrocytic activations indicated by decreases in the number of Fos-ir neurons and the GFAP expression within the SDH, respectively.

Conclusion

i.t. co-delivery of Dex and Ropi shows synergistic analgesia on the chronic inflammatory pain, in which spinal “neuron-astrocytic activation” mechanism may play an important role.

Alterations in the neural circuits from peripheral afferents to the spinal cord: possible implications for diabetic polyneuropathy in streptozotocin-induced type 1 diabetic rats

Diabetic polyneuropathy (DPN) presents as a wide variety of sensorimotor symptoms and affects approximately 50% of diabetic patients. Changes in the neural circuits may occur in the early stages in diabetes and are implicated in the development of DPN. Therefore, we aimed to detect changes in the expression of isolectin B4 (IB4, the marker for nonpeptidergic unmyelinated fibers and their cell bodies) and calcitonin gene-related peptide (CGRP, the marker for peptidergic fibers and their cell bodies) in the dorsal root ganglion (DRG) and spinal cord of streptozotocin (STZ)-induced type 1 diabetic rats showing alterations in sensory and motor function. We also used cholera toxin B subunit (CTB) to show the morphological changes of the myelinated fibers and motor neurons. STZ-induced diabetic rats exhibited hyperglycemia, decreased body weight gain, mechanical allodynia and impaired locomotor activity. In the DRG and spinal dorsal horn, IB4-labeled structures decreased, but both CGRP immunostaining and CTB labeling increased from day 14 to day 28 in diabetic rats. In spinal ventral horn, CTB labeling decreased in motor neurons in diabetic rats. Treatment with intrathecal injection of insulin at the early stages of DPN could alleviate mechanical allodynia and impaired locomotor activity in diabetic rats. The results suggest that the alterations of the neural circuits between spinal nerve and spinal cord via the DRG and ventral root might be involved in DPN.

Tramadol and propentofylline coadministration exerted synergistic effects on rat spinal nerve ligation-induced neuropathic pain

Neuropathic pain is an intractable clinical problem. Drug treatments such as tramadol have been reported to effectively decrease neuropathic pain by inhibiting the activity of nociceptive neurons. It has also been reported that modulating glial activation could also prevent or reverse neuropathic pain via the administration of a glial modulator or inhibitor, such as propentofylline. Thus far, there has been no clinical strategy incorporating both neuronal and glial participation for treating neuropathic pain. Therefore, the present research study was designed to assess whether coadministration of tramadol and propentofylline, as neuronal and glial activation inhibitors, respectively, would exert a synergistic effect on the reduction of rat spinal nerve ligation (SNL)-induced neuropathic pain. Rats underwent SNL surgery to induce neuropathic pain. Pain behavioral tests were conducted to ascertain the effect of drugs on SNL-induced mechanical allodynia with von-Frey hairs. Proinflammatory factor interleukin-1β (IL-1β) expression was also detected by Real-time RT-PCR. Intrathecal tramadol and propentofylline administered alone relieved SNL-induced mechanical allodynia in a dose-dependent manner. Tramadol and propentofylline coadministration exerted a more potent effect in a synergistic and dose dependent manner than the intrathecal administration of either drug alone. Real-time RT-PCR demonstrated IL-1β up-expression in the ipsilateral spinal dorsal horn after the lesion, which was significantly decreased by tramadol and propentofylline coadministration. Inhibiting proinflammatory factor IL-1β contributed to the synergistic effects of tramadol and propentofylline coadministration on rat peripheral nerve injury-induced neuropathic pain. Thus, our study provided a rationale for utilizing a novel strategy for treating neuropathic pain by blocking the proinflammatory factor related pathways in the central nervous system.

Toll-like receptor 2 mediates peripheral nerve injury-induced NADPH oxidase 2 expression in spinal cord microglia

We have previously reported that NADPH oxidase 2 (Nox2) is up-regulated in spinal cord microglia after spinal nerve injury, demonstrating that it is critical for microglia activation and subsequent pain hypersensitivity. However, the mechanisms and molecules involved in Nox2 induction have not been elucidated. Previous studies have shown that Toll-like receptors (TLRs) are involved in nerve injury-induced spinal cord microglia activation. In this study, we investigated the role of TLR in Nox2 expression in spinal cord microglia after peripheral nerve injury. Studies using TLR knock-out mice have shown that nerve injury-induced microglial Nox2 up-regulation is abrogated in TLR2 but not in TLR3 or -4 knock-out mice. Intrathecal injection of lipoteichoic acid, a TLR2 agonist, induced Nox2 expression in spinal cord microglia both at the mRNA and protein levels. Similarly, lipoteichoic acid stimulation induced Nox2 expression and reactive oxygen species production in primary spinal cord glial cells in vitro. Studies on intracellular signaling pathways indicate that NF-κB and p38 MAP kinase activation is required for TLR2-induced Nox2 expression in glial cells. Conclusively, our data show that TLR2 mediates nerve injury-induced Nox2 gene expression in spinal cord microglia via NF-κB and p38 activation and thereby may contribute to spinal cord microglia activation.

Emerging role of Toll-like receptors in the control of pain and itch

Toll-like receptors (TLRs) are germline-encoded pattern-recognition receptors that initiate innate immune responses by recognizing molecular structures shared by a wide range of pathogens, known as pathogen-associated molecular patterns (PAMPs). After tissue injury or cellular stress, TLRs also detect endogenous ligands known as danger-associated molecular patterns (DAMPs). TLRs are expressed in both non-neuronal and neuronal cell types in the central nervous system (CNS) and contribute to both infectious and non-infectious disorders in the CNS. Following tissue insult and nerve injury, TLRs (such as TLR2, TLR3, and TLR4) induce the activation of microglia and astrocytes and the production of the proinflammatory cytokines in the spinal cord, leading to the development and maintenance of inflammatory pain and neuropathic pain. In particular, primary sensory neurons, such as nociceptors, express TLRs (e.g., TLR4 and TLR7) to sense exogenous PAMPs and endogenous DAMPs released after tissue injury and cellular stress. These neuronal TLRs are new players in the processing of pain and itch by increasing the excitability of primary sensory neurons. Given the prevalence of chronic pain and itch and the suffering of affected people, insights into TLR signaling in the nervous system will open a new avenue for the management of clinical pain and itch.

Temporal distribution of p300/CBP immunoreactivity in the adult rat spinal dorsal horn following chronic constriction injury (CCI)

p300 and its homolog cyclic AMP response element binding protein (CBP) are coactivators that were identified to participate in many biological processes including neural development and cognition. Their roles within the rodent spinal cord have not been reported systematically; in this study, their spatiotemporal distribution in the spinal cord of adult rat following chronic constriction injury (CCI) was studied. p300 and CBP expressed predominantly in nuclei in the gray matter of rat spinal cord. Rats undergoing CCI surgery showed increased p300/CBP immunoreactivity (IR) compared with normal control and sham-operated rats. The number of IR cells reached the peak at day 14 following CCI compared with those on day 3, 7, and 21, accompanied with significant behavioral changes of neuropathic pain. Cell-type determination by immunofluorescence at day 14 following CCI revealed that p300 and CBP expressed in neurons, but not in astrocytes or microglial cells. These results suggest that p300 and CBP are probably involved in the maintenance of neuropathic pain on spinal cord level. Furthermore, p300 and CBP may serve as a sensor only in neurons but not in astrocytes or microglia cells in the adult rat spinal cord.