Intrathecal Etanercept Partially Restores Morphine's Antinociception in Morphine-Tolerant Rats via Attenuation of the Glutamatergic Transmission

Effects of swimming exercise on neuropathic pain in a rat model: role of glutamate

OBJECTIVE

The pain-reducing effects of the exercise were exerted through different mechanisms. Knowing more clear mechanisms helps to find more approach that is therapeutic. The objective of the present study is the evaluation of cerebrospinal fluid (CSF) glutamate level alteration in neuropathic pain rats and whether physical activity could modulate it.

METHODS

In the present study 104 male Wistar rats weighing 180-220 g were randomly divided into 4 groups (Sham, Sham + Exe, Neuropathy, and Neuropathy + Exe) which in turn each group subdivided into 4 groups according to time points for behavioral testing and CSF sampling (Baseline, 2 weeks, 3 weeks, and 4 weeks). To induction of neuropathy (by chronic constriction injury,), after anesthetizing with a mixture of ketamine (80 mg/kg) and xylazine (10 mg/kg), the animal's right sciatic nerve was exposed and was ligated using four movable catgut chromic suture 4/0. The exercise protocol included 25 min of daily swimming, 5 days a week for 4 weeks. Thermal hyperalgesia and mechanical tactile threshold were detected using the plantar test and Von Frey filaments, respectively. CSF glutamate level was determined using high-performance liquid chromatography.

RESULTS

Findings indicated that mechanical and thermal thresholds significantly (p < 0.01, p < 0.05 respectively) decreased in the neuropathy group against that in sham groups. On the other hand, exercise significantly increased mechanical tactile threshold (p < 0.0012) and thermal threshold (p < 0.05) compared to the neuropathy group. Moreover, CSF glutamate level prominently (p < 0.01) was increased in the neuropathy group compared to the sham group, and swimming exercise significantly (p < 0.001) reduced it.

IN CONCLUSION

The present findings provide new evidence showing that medium-intensity swimming exercise attenuates pain-like behaviors in neuropathic pain animals, which is possibly due to decreasing CSF glutamate level and its neurotransmission.

Deficiency of SHANK3 isoforms impairs thermal hyperalgesia and dysregulates the expression of postsynaptic proteins in the spinal cord

SHANK3 is one of the scaffolding proteins in the postsynaptic density (PSD). Pain perception and underlying mechanisms were investigated in Shank3 exon 21 deficient (Shank3^△C) mice. Sixty-six mice were attributed according to their genotype to three groups: (1) wild-type (WT), (2) heterozygous Shank3^△C/+, and (3) homozygous Shank3^△C/△C. Complete Freund's adjuvant (CFA) was used to induce inflammatory pain, and thermal hyperalgesia was determined. CFA treatment reduced the thermal threshold in the WT group; groups expressing mutations of Shank3 (^△C/+ and ^△C/△C) had higher thresholds after CFA administration compared to the WT group. Mice with Shank3 mutations (^△C/+ or ^△C/△C) had a lower expression of GluN2A and IP₃R proteins and a higher expression of mGluR5 protein in the PSD compared to WT mice without changes in GluN1, GluN2B, and Homer expression. The crosslinking of Homer-IP₃R, but not Homer-mGluR5, was decreased in the total lysate. Deficit of Shank3 exon 21 may lead to impaired perception of thermal pain in mice under inflammatory conditions. This impairment may result from protein dysregulation in the PSD like downregulation of the GluN2A subunit, which may reduce NMDAR-mediated currents, and/or decreased crosslinking between Homer and IP₃R, which may reduce the release of Ca²⁺ from intracellular stores.

Circular RNA expression profile in the spinal cord of morphine tolerated rats and screen of putative key circRNAs

Morphine tolerance developed after repeated or continuous morphine treatment is a global health concern hindering the control of chronic pain. In our previous research, we have reported that the expression of lncRNAs and microRNAs have been greatly modified in the spinal cord of morphine tolerated rats, and the modulating role of miR-873a-5p, miR-219-5p and miR-365 have already been confirmed. However, whether circular RNAs, another essential kind of non-coding RNA, are involved in the pathogenesis of morphine tolerance is still beyond our knowledge. In this study, we conducted microarray analysis for circRNA profile and found a large number of circRNAs changed greatly in the spinal cord by morphine treatment. Among them, we selected nine circRNAs for validation, and seven circRNAs are confirmed. Gene Ontology/Kyoto Encyclopedia of Genes and Genomes (GO/KEGG) analysis were used for functional annotation. Besides, we confirmed the modified expression of seven circRNAs after validation by real-time PCR, selected 3 most prominently modulated ones among them and predicted their downstream miRNA-mRNA network and analyzed their putative function via circRNA-miRNA-mRNA pathway. Finally, we enrolled the differentially expressed mRNAs derived from the identical spinal cord, these validated circRNAs and their putative miRNA targets for ceRNA analysis and screened a promising circRNA-miRNA-mRNA pathway in the development of morphine tolerance. This study, for the first time, provided valuable information on circRNA profile and gave clues for further study on the circRNA mechanism of morphine tolerance.

Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

Each year, over 50 million Americans suffer from persistent pain, including debilitating headaches, joint pain, and severe back pain. Although morphine is amongst the most effective analgesics available for the management of severe pain, prolonged morphine treatment results in decreased analgesic efficacy (i.e., tolerance). Despite significant headway in the field, the mechanisms underlying the development of morphine tolerance are not well understood. The midbrain ventrolateral periaqueductal gray (vlPAG) is a primary neural substrate for the analgesic effects of morphine, as well as for the development of morphine tolerance. A growing body of literature indicates that activated glia (i.e., microglia and astrocytes) facilitate pain transmission and oppose morphine analgesia, making these cells important potential targets in the treatment of chronic pain. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines and opposition of morphine analgesia. Despite the established role of the vlPAG as an integral locus for the development of morphine tolerance, most studies have examined the role of glia activation within the spinal cord. Additionally, the role of TLR4 in the development of tolerance has not been elucidated. This review attempts to summarize what is known regarding the role of vlPAG glia and TLR4 in the development of morphine tolerance. These data, together, provide information about the mechanism by which central nervous system glia regulate morphine tolerance, and identify a potential therapeutic target for the enhancement of analgesic efficacy in the clinical treatment of chronic pain.

Targeting Cytokines for Morphine Tolerance: A Narrative Review

BACKGROUND

Despite its various side effects, morphine has been widely used in clinics for decades due to its powerful analgesic effect. Morphine tolerance is one of the major side effects, hindering its long-term usage for pain therapy. Currently, the thorough cellular and molecular mechanisms underlying morphine tolerance remain largely uncertain.

METHODS

We searched the PubMed database with Medical subject headings (MeSH) including 'morphine tolerance', 'cytokines', 'interleukin 1', 'interleukin 1 beta', 'interleukin 6', 'tumor necrosis factor alpha', 'interleukin 10', 'chemokines'. Manual searching was carried out by reviewing the reference lists of relevant studies obtained from the primary search. The searches covered the period from inception to November 1, 2017.

RESULTS

The expression levels of certain chemokines and pro-inflammatory cytokines were significantly increased in animal models of morphine tolerance. Cytokines and cytokine receptor antagonist showed potent effect of alleviating the development of morphine tolerance.

CONCLUSION

Cytokines play a fundamental role in the development of morphine tolerance. Therapeutics targeting cytokines may become alternative strategies for the management of morphine tolerance.

In further defense of nonclinical abuse liability testing of biologics

Risk assessment is not a choice. Drug Abuse Liability (DAL) is mandated under international and national drug control statutes for all drugs targeting the CNS. Once administered to humans many biologics may have long-lived or permanent physiological effects that make DAL testing arduous. We respond to premises of a recently published position on DAL testing of biologics by de Zafra et al. (2018). We propose that, at a minimum, Sponsors submitting a Biologics Licensure Application (BLA) must think "outside the box" and include differential study designs for the same three core small NME assays detailed in the current DAL guidelines (self-administration, drug discrimination, and dependence liability). Abuse liability testing for drug scheduling decisions for marketing approval are not excluded or limited from risk assessment analysis simply because the entity is a biologic. In fact, more robust study designs may be necessary to address alterations in the reinforcing and discriminative stimulus effects of common drugs of abuse, as well as the dependence liability of the biologic, itself.

Toll-like Receptor 4 Mediates Morphine-Induced Neuroinflammation and Tolerance via Soluble Tumor Necrosis Factor Signaling

Opioid tolerance and the potential for addiction is a significant burden associated with pain management, yet its precise underlying mechanism and prevention remain elusive. Immune signaling contributes to the decreased efficacy of opioids, and we recently demonstrated that Toll-like receptor 4 (TLR4)-mediated neuroinflammation in the periaqueductal gray (PAG) drives tolerance. Tumor necrosis factor (TNF), a product of TLR4 signaling, promotes inflammation and facilitates glutamatergic signaling, key components of opioid tolerance. Therefore, we hypothesize that TLR4-mediated opioid tolerance requires TNF signaling. By expression of a dominant-negative TNF peptide via lentiviral vector injection in rat PAG to sequester soluble TNF (solTNF), we demonstrate that solTNF mediates morphine tolerance induced by TLR4 signaling, stimulates neuroinflammation (increased IL-1β and TLR4 mRNA), and disrupts glutamate reuptake (decreased GLT-1 and GLAST mRNA). We further demonstrate the efficacy of the brain-permeant PEGylated version of the anti-solTNF peptide, XPro1595, injected systemically, to normalize morphine-induced CNS neuroinflammation and morphine- and endotoxin-induced changes in glutamate transport, effectively preserving the efficacy of morphine analgesia and eliminating tolerance. Our findings provide a novel pharmacological target for the prevention of opioid-induced immune signaling, tolerance, and addiction.

A new pharmacological role for thalidomide: Attenuation of morphine-induced tolerance in rats

OBJECTIVE

Tolerance to the analgesic effect is the main side effect of chronic administration of opioids. Several drugs have been studied to try to find agents to prevent the development of this phenomenon. In the present study we aimed to evaluate the effect of thalidomide on morphine-induced tolerance to the analgesic effect.

METHODS

Groups of male rats were randomly rendered and received daily morphine in combination with thalidomide vehicle or thalidomide (2.5 mg/kg, 5 mg/kg, or 10 mg/kg, intraperitoneally). Nociception was measured using the plantar test apparatus. Latency time was recorded when the animal reacted to the light stimulus; licking or raising its hind paw. Treatments and evaluations continued until completion of tolerance to the analgesic effect of morphine.

RESULTS

Our findings indicated that tolerance was achieved following 11 days of morphine administration, while thalidomide postponed the day of tolerance completion for 4 days (2.5 mg/kg and 5 mg/kg thalidomide) or 10 days (10 mg/kg thalidomide). Moreover, thalidomide prevented the morphine-induced shift to the right of the ED50 in the dose-response curve.

CONCLUSION

It was concluded that thalidomide attenuated the morphine-induced tolerance to the analgesic effect.

Cdk5 inhibitor roscovitine alleviates neuropathic pain in the dorsal root ganglia by downregulating N-methyl-D-aspartate receptor subunit 2A

Cyclin-dependent kinase 5 (Cdk5) is a member of the small proline-directed serine/threonine kinase family. Cdk5 is not involved in cell cycle regulation, but is implicated in neurodegenerative disorders. However, the role of Cdk5 in neuropathic pain remains unclear. This study aimed to evaluate the possibility that Cdk5 is involved in neuropathic pain in the dorsal root ganglia (DRG). We injected intrathecally Cdk5 inhibitor roscovitine in rat model of chronic compression of dorsal root ganglion and examined pain behaviors and the expression of N-methyl-d-aspartate receptor subunit 2A (NR2A) but not NR2B or NR1 in DRG. We found that roscovitine alleviated neuropathic pain, causing decline in paw withdrawal mechanical threshold and paw withdrawal thermal latency. Furthermore, roscovitine inhibited NR2A expression in DRG. These data suggest that Cdk5-NR2A pathway regulates neuropathic pain in DRG, and intrathecal injection of roscovitine could alleviate neuropathic pain. Our findings provide new insight into the analgesic effects of Roscovitine and identify Cdk5-NR2A pathway as a potential target for effective treatment of neuropathic pain.

Pioglitazone prevents morphine antinociceptive tolerance via ameliorating neuroinflammation in rat cerebral cortex

BACKGROUND

Opioid induced neuroinflammation is shown to be implicated in opioid analgesic tolerance development. In the present study the effect of pioglitazone on morphine-induced tolerance and neuroinflammation in the cerebral cortex of the rat was investigated.

MATERIALS AND METHODS

Various groups of rats received morphine (10mg/kg; ip) and vehicle (po), or morphine (10mg/kg) and pioglitazone (20 or 40 mg/kg; po) once a day for 17 days. In order to determine the possible involvement of PPAR-γ in the pioglitazone effect, one group of rats received PPAR-γ antagonist, GW-9662 (2mg/kg; sc), and pioglitazone (40 mg/kg) and morphine once daily for 17 days. Nociception was assessed using a tail flick apparatus and the percentage of the maximal possible effect was calculated as well. On 18th day, 2h after the last morphine injection, the cerebral cortex of the animals were harvested and the tissue levels of tumour necrosis factor alpha, interleukin-1beta, interleukin-6, interleukin-10 and nuclear factor-kappa B activity were determined.

RESULTS

Co-administration of pioglitazone (40 mg/kg) with morphine not only attenuated morphine-induced tolerance, but also prevented the up-regulation of pro-inflammatory cytokines (tumour necrosis factor alpha, interleukin-1beta, interleukin-6) and nuclear factor-kappa B activity in the rat cerebral cortex. Moreover, GW-9662 (2mg/kg) administration 30 min before pioglitazone, antagonized the above mentioned pioglitazone-induced effects.

CONCLUSION

It is concluded that oral administration of pioglitazone attenuates morphine-induced tolerance. This effect of pioglitazone may be, at least in part, due to its anti-inflammatory property which suppressed the cortical pro-inflammatory cytokine and inhibited of nuclear factor-kappa B activity.

Tumor necrosis factor alpha: a link between neuroinflammation and excitotoxicity

Tumor necrosis factor alpha (TNF-α) is a proinflammatory cytokine that exerts both homeostatic and pathophysiological roles in the central nervous system. In pathological conditions, microglia release large amounts of TNF-α; this de novo production of TNF-α is an important component of the so-called neuroinflammatory response that is associated with several neurological disorders. In addition, TNF-α can potentiate glutamate-mediated cytotoxicity by two complementary mechanisms: indirectly, by inhibiting glutamate transport on astrocytes, and directly, by rapidly triggering the surface expression of Ca⁺² permeable-AMPA receptors and NMDA receptors, while decreasing inhibitory GABA_A receptors on neurons. Thus, the net effect of TNF-α is to alter the balance of excitation and inhibition resulting in a higher synaptic excitatory/inhibitory ratio. This review summarizes the current knowledge of the cellular and molecular mechanisms by which TNF-α links the neuroinflammatory and excitotoxic processes that occur in several neurodegenerative diseases, but with a special emphasis on amyotrophic lateral sclerosis (ALS). As microglial activation and upregulation of TNF-α expression is a common feature of several CNS diseases, as well as chronic opioid exposure and neuropathic pain, modulating TNF-α signaling may represent a valuable target for intervention.

A novel role of spinal astrocytic connexin 43: mediating morphine antinociceptive tolerance by activation of NMDA receptors and inhibition of glutamate transporter-1 in rats

AIMS

Connexin 43 (Cx43) has been reported to be involved in neuropathic pain, but whether it contributes to morphine antinociceptive tolerance remains unknown. The present study investigated the role of spinal Cx43 in the development of morphine tolerance and its mechanisms in rats.

METHODS

Morphine tolerance was induced by intrathecal (i.t.) administration of morphine (15 μg) daily for seven consecutive days. The analgesia effect was assessed by hot-water tail-flick test. Expression of proteins was detected by Western blot and immunohistochemistry assay.

RESULTS

Chronic morphine markedly increased the expression of spinal Cx43. Gap26, a specific Cx43 mimic peptide, attenuated not only morphine antinociceptive tolerance, but also the up-regulation of spinal Cx43 expression, the activation of astrocytes, and N-methyl-D-aspartic acid (NMDA) receptors (NR1 and NR2B subunits), as well as the decreased GLT-1 expression induced by chronic morphine. MK-801, a noncompetitive NMDA receptors antagonist, suppressed the chronic morphine-induced spinal Cx43 up-regulation, astrocytes activation and decline of GLT-1 expression.

CONCLUSIONS

The spinal astrocytic Cx43 contributes to the development of morphine antinociceptive tolerance by activating astrocytes and NMDA receptors, and inhibiting GLT-1 expression. We also demonstrate that the role of interaction between the spinal astrocytic Cx43 and neuronal NMDA receptors is important in morphine tolerant rats.

Role of neuroinflammation in morphine tolerance: effect of tumor necrosis factor-α

Opioids have been used as potent analgesics in clinics for decades; however, their long-term administration leads to tolerance. Two possible mechanisms for drug tolerance are postulated as within-system and between-systems adaptation. The within-system tolerance is involved in the signal transduction of opioid receptors, including downregulation of opioid receptors, uncoupling of G-protein from opioid receptors, and β-arrestin recruitment to opioid receptors, which causes receptor desensitization and internalization/endocytosis. The between-systems tolerance comprehends the glutamatergic receptor system and glial activation with the release of proinflammatory cytokines, and thus the analgesic effect of morphine is reduced. Tumor necrosis factor-α (TNF-α) is a vital proinflammatory cytokine and exerts either a neurotoxic or neuroprotective effect on different diseases of the central nervous system. TNF-α has also been demonstrated to correlate with neuronal plasticity via activation of spinal glial cells and enhancement of glutamatergic transmission. Previous studies had revealed an increased expression of TNF-α in morphine tolerance. This review article focuses on the role of TNF-α in neuroinflammation and the glutamatergic receptor system in morphine tolerance. It may provide another adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Selective TNF inhibition for chronic stroke and traumatic brain injury: an observational study involving 629 consecutive patients treated with perispinal etanercept

BACKGROUND

Brain injury from stroke and traumatic brain injury (TBI) may result in a persistent neuroinflammatory response in the injury penumbra. This response may include microglial activation and excess levels of tumour necrosis factor (TNF). Previous experimental data suggest that etanercept, a selective TNF inhibitor, has the ability to ameliorate microglial activation and modulate the adverse synaptic effects of excess TNF. Perispinal administration may enhance etanercept delivery across the blood-CSF barrier.

OBJECTIVE

The objective of this study was to systematically examine the clinical response following perispinal administration of etanercept in a cohort of patients with chronic neurological dysfunction after stroke and TBI.

METHODS

After approval by an independent external institutional review board (IRB), a chart review of all patients with chronic neurological dysfunction following stroke or TBI who were treated open-label with perispinal etanercept (PSE) from November 1, 2010 to July 14, 2012 at a group medical practice was performed.

RESULTS

The treated cohort included 629 consecutive patients. Charts of 617 patients following stroke and 12 patients following TBI were reviewed. The mean age of the stroke patients was 65.8 years ± 13.15 (range 13-97). The mean interval between treatment with PSE and stroke was 42.0 ± 57.84 months (range 0.5-419); for TBI the mean interval was 115.2 ± 160.22 months (range 4-537). Statistically significant improvements in motor impairment, spasticity, sensory impairment, cognition, psychological/behavioural function, aphasia and pain were noted in the stroke group, with a wide variety of additional clinical improvements noted in individuals, such as reductions in pseudobulbar affect and urinary incontinence. Improvements in multiple domains were typical. Significant improvement was noted irrespective of the length of time before treatment was initiated; there was evidence of a strong treatment effect even in the subgroup of patients treated more than 10 years after stroke and TBI. In the TBI cohort, motor impairment and spasticity were statistically significantly reduced.

DISCUSSION

Irrespective of the methodological limitations, the present results provide clinical evidence that stroke and TBI may lead to a persistent and ongoing neuroinflammatory response in the brain that is amenable to therapeutic intervention by selective inhibition of TNF, even years after the acute injury.

CONCLUSION

Excess TNF contributes to chronic neurological, neuropsychiatric and clinical impairment after stroke and TBI. Perispinal administration of etanercept produces clinical improvement in patients with chronic neurological dysfunction following stroke and TBI. The therapeutic window extends beyond a decade after stroke and TBI. Randomized clinical trials will be necessary to further quantify and characterize the clinical response.

The glutamatergic system as a target for neuropathic pain relief

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The understanding of glutamatergic transmission in the nervous system has been greatly expanded with the discovery and investigation of the family of ionotropic and metabotropic glutamate receptors (mGluRs). Metabotropic glutamate receptors are localized at nerve terminals, postsynaptic sites and glial cells and thus, they can influence and modulate the action of glutamate at different levels in the synapse. Moreover, there is substantial evidence of glial participation in glutamate nociceptive processes and neuropathic pain. Metabotropic glutamate receptors have been shown to play a role in neuropathic pain, which is one of the most troublesome illnesses because the therapy is still not satisfactory. Recently, the development of selective mGluR ligands has provided important tools for further investigation of the role of mGluRs in the modulation of chronic pain processing. This paper presents a review of the literature of glutamate receptors in neuropathic pain and the role of glia in these effects. Specifically, pharmacological interventions aimed at inhibiting group I mGluRs and/or potentiating group II and III mGluR-mediated signalling is discussed. Moreover, we introduce data about the role of glutamate transporters. They are responsible for the level of glutamate in the synaptic cleft and thus regulate the effects of all three groups of mGluRs and, in consequence, the activity of this system in nociceptive transmission. Additionally, the question of how the modulation of the glutamatergic system influences the effectiveness of analgesic drugs used in neuropathic pain therapy is addressed.