Hydrogen-rich saline prevents remifentanil-induced hyperalgesia and inhibits MnSOD nitration via regulation of NR2B-containing NMDA receptor in rats

Molecular hydrogen reduces dermatitis-induced itch, diabetic itch and cholestatic itch by inhibiting spinal oxidative stress and synaptic plasticity via SIRT1-β-catenin pathway in mice

Chronic itch which is primarily associated with dermatologic, systemic, or metabolic disorders is often refractory to most current antipruritic medications, thus highlighting the need for improved therapies. Oxidative damage is a novel determinant of spinal pruriceptive sensitization and synaptic plasticity. The resolution of oxidative insult by molecular hydrogen has been manifested. Herein, we strikingly report that both hydrogen gas (2 %) inhalation and hydrogen-rich saline (5 mL/kg, intraperitoneal) injection prevent and alleviate persistent dermatitis-induced itch, diabetic itch and cholestatic itch. Hydrogen therapy reverses the decrease of spinal SIRT1 expression and antioxidant enzymes (SOD, GPx and CAT) activity after dermatitis, diabetes and cholestasis. Furthermore, hydrogen reduces spinal ROS generation, oxidation products (MDA, 8-OHdG and 3-NT) accumulation, β-catenin acetylation and dendritic spine density in persistent itch models. Spinal SIRT1 inhibition eliminates antipruritic and antioxidative effects of hydrogen, while SIRT1 agonism attenuates chronic itch phenotype, spinal β-catenin acetylation and mitochondrial damage. β-catenin inhibitors are effective against chronic itch via reducing β-catenin acetylation, blocking ERK phosphorylation and elevating antioxidant enzymes activity. Hydrogen treatment suppressed dermatitis and cholestasis mediated spontaneous excitatory postsynaptic currents in vitro. Additionally, hydrogen impairs cholestasis-induced the enhancement of cerebral functional connectivity between the right primary cingulate cortex and bilateral sensorimotor cortex, as well as bilateral striatum. Taken together, this study uncovers that molecular hydrogen protects against chronic pruritus and spinal pruriceptive sensitization by reducing oxidative damage via up-regulation of SIRT1-dependent β-catenin deacetylation in mice, implying a promising strategy in translational development for itch control.

Pharmacological interventions for remifentanil-induced hyperalgesia: A systematic review and network meta-analysis of preclinical trials

BACKGROUND

To improve perioperative pain management, several interventions have been suggested for the prevention of increased pain sensitivity caused by opioids (called opioid-induced hyperalgesia). It is currently unclear which intervention is the most effective or appropriate in preventing opioid-induced hyperalgesia. Remifentanil is the most investigated opioid causing opioid-induced hyperalgesia. Thus, to guide future research, we conducted a systematic review and a network meta-analysis of preclinical trials investigating pharmacological interventions for remifentanil-induced hyperalgesia.

METHODS

To identify relevant articles, electronic database searches were conducted in Embase, PubMed, Web of Science, and Google Scholar. Study characteristics were extracted, and the risk of bias was evaluated. Studies were included in the network meta-analysis if they shared similar characteristics with at least one other study. The interventions were ranked based on P-scores.

RESULTS

Overall, the 62 eligible trials tested 86 individual interventions and 6 combination interventions. Thirty-five studies eligible in the network meta-analysis formed five groups which were further divided into subgroups based on the quantitative sensory tests used. The best-ranked interventions within the subgroups were Anxa12-26, MRS2179, salicylaldehyde isonicotinoyl hydrazone (SIH), ANA-12, TDZD-8, ketamine, dexmedetomidine, JWH015, and the combination of KN93 and ketamine.

DISCUSSION

The current literature is too heterogeneous to produce a clear answer on which intervention is the most effective in preventing remifentanil-induced hyperalgesia. Future research in this field should prioritise finding the most effective intervention over testing the efficacy of new options. The results of our work can be used in planning which comparisons should be included in new trials.

Hydrogen treatment reduces electroencephalographic activity and neuronal death in rats with refractory status epilepticus by inhibiting membrane NR2B phosphorylation and oxidative stress

OBJECTIVE

To investigate the effects of hydrogen therapy on epileptic seizures in rats with refractory status epilepticus and the underlying mechanisms.

METHODS

Status epilepticus was induced using pilocarpine. The effects of hydrogen treatment on epilepsy severity in model rats were then monitored using Racine scores and electroencephalography (EEG), followed by western blot of plasma membrane N-methyl-D-aspartate receptor subtype 2B (NR2B) and phosphorylated NR2B expression. We also generated a cellular epilepsy model using Mg2+-free medium and used polymerase chain reaction to investigate the neuroprotective effects of hydrogen.

RESULTS

There were no significant differences in Racine scores between the hydrogen and control groups. EEG amplitudes were lower in the hydrogen treatment group than in the control group. In epilepsy model rats, hippocampal cell membrane NR2B expression and phosphorylation increased gradually over time. Although hippocampal cell membrane NR2B expression was not significantly different between the two groups, NR2B phosphorylation levels were significantly lower in the hydrogen group. Hydrogen treatment also increased superoxide dismutase, mitochondrial (SOD2) expression.

CONCLUSIONS

Hydrogen treatment reduced EEG amplitudes and NR2B phosphorylation; it also decreased neuronal death by reducing oxidative stress. Hydrogen may thus be a potential treatment for refractory status epilepticus by inhibiting membrane NR2B phosphorylation and oxidative stress.

Nalmefene vs. dexmedetomidine for prevention of postoperative hyperalgesia in patients undergoing laparoscopic gynecological surgery with remifentanil infusion: A randomized double-blind controlled trial

Intraoperative remifentanil infusion may paradoxically induce post-surgical hyperalgesia. Dexmedetomidine reportedly reduces opioid-induced hyperalgesia. Nalmefene selectively reverses several side-effects of opioids without impairing analgesia. Herein, this randomized, double-blind controlled trial investigated whether nalmefene, dexmedetomidine, and both drugs combined prevent remifentanil-induced hyperalgesia. One hundred and fifty patients undergoing elective laparoscopic gynecological surgery under desflurane anesthesia randomly received either intraoperative sufentanil 0.20 μg kg^-1 (Group S), or remifentanil 0.20 μg kg^-1 min^-1 (Group R), or remifentanil and pre-anesthesia nalmefene 0.20 μg kg^-1 (Group N), or remifentanil and pre-anesthesia dexmedetomidine 0.50 μg kg^-1 (Group D), or remifentanil and the combination of dexmedetomidine 0.25 μg kg^-1 and nalmefene 0.10 μg kg^-1 (Group DN). The threshold of postoperative mechanical hyperalgesia (primary outcome) was measured with von Frey filaments. We also recorded pain intensity, analgesic consumptions, hyperalgesic area, and side-effects for 24 h postoperatively. Compared with Group S, remifentanil reduced hyperalgesic threshold on the forearm [mean 89.4 (SD 13.7) vs. 62.2 (10.7) g, p < 0.001] at postoperative 24 h. Pain threshold on the forearm at postoperative 24 h was significantly lower in Group R than in Groups N, D and DN [62.2 (10.7) vs. 71.1 (12.3), 72.4 (12.9) and 78.0 (13.8) g]. Compared with Group R, Postoperative pain intensity, analgesic consumption and hyperalgesic area were lower likewise in Groups D and DN. However, the incidence of intraoperative bradycardia was lower and post-anesthesia recovery time was shorter in Group DN than Group D. Preoperative therapy of dexmedetomidine and nalmefene combined attenuates postoperative hyperalgesia in patients undergoing laparoscopic gynecological surgery under desflurane-remifentanil anesthesia.

New Treatment for the Cognitive and Emotional Deficits Linked with Paclitaxel-Induced Peripheral Neuropathy in Mice

Chemotherapy-provoked peripheral neuropathy and its linked comorbidities severely reduce the quality of a patient's life. Its therapy is not completely resolved and has become an important clinical challenge. The protective actions of molecular hydrogen (H₂) in many neurological disorders have been described, but its effects on memory and the emotional deficits accompanying neuropathic pain induced by chemotherapy remain unknown. In this study, using male mice injected with paclitaxel (PTX), we examined the effects of systemic treatment with hydrogen-rich water (HRW) in: (i) the mechanical and thermal allodynia provoked by PTX and the pathways involved; (ii) the memory deficits, anxiety- and depressive-like behaviors associated with PTX-induced peripheral neuropathy (PIPN); and (iii) the plasticity (p-extracellular signal-regulated protein kinase; p-ERK ½), nociceptive (p-protein kinase B, p-Akt), inflammatory (p-nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha; p-IKBα), and oxidative (4-hydroxynonenal: 4-HNE) alterations provoked by PIPN in the prefrontal cortex (PFC). The results revealed: (1) the antiallodynic actions of HRW administered at one or two times per day during 7 and 3 consecutive days; (2) the participation of Kv7 potassium channels and the Nrf2-heme oxygenase 1-NAD(P)H: quinone oxidoreductase 1 pathway in the painkiller effects of HRW; (3) the inhibition of memory deficits and the anxiodepressive-like behaviors related with PIPN induced by HRW; and (4) the normalization of p-ERK ½, p-Akt and 4-HNE up-regulation and the activation of antioxidant enzymes produced by this treatment in PFC. This study proposes HRW as a possible effective and safe therapy for PIPN and its associated cognitive and emotional deficits.

Pharmacological Interventions for Opioid-Induced Hyperalgesia: A Scoping Review of Preclinical Trials

BACKGROUND

Opioid analgesics are the most effective pharmacological agents for moderate and severe pain. However, opioid use has several limitations such as opioid-induced hyperalgesia (OIH), which refers to the increased pain sensitivity that occurs once analgesia wears off after opioid administration. Several pharmacological interventions have been suggested for OIH, but the current literature does not provide guidelines on which interventions are the most effective and whether they differ depending on the opioid that induces hyperalgesia. This scoping review aimed to identify and describe all the preclinical trials investigating pharmacological interventions for OIH caused by remifentanil, fentanyl, or morphine as the first step towards evaluating whether the most effective OIH interventions are different for different opioids.

METHODS

Electronic database searches were carried out in Embase, PubMed, and Web of Science. Detailed data extraction was conducted on the eligible trials.

RESULTS

72 trials were eligible for the review. Of these, 27 trials investigated remifentanil, 14 trials investigated fentanyl, and 31 trials investigated morphine. A total of 82 interventions were identified. The most studied interventions were ketamine (eight trials) and gabapentin (four trials). The majority of the interventions were studied in only one trial. The most common mechanism suggested for the interventions was inhibition of N-methyl-D-aspartate (NMDA) receptors.

CONCLUSION

This scoping review identified plenty of preclinical trials investigating pharmacological interventions for OIH. Using the current literature, it is not possible to directly compare the effectiveness of the interventions. Hence, to identify the most effective interventions for each opioid, the interventions must be indirectly compared in a meta-analysis.

Hydrogen-Rich Saline Attenuates Chronic Allodynia after Bone Fractures via Reducing Spinal CXCL1/CXCR2-Mediated Iron Accumulation in Mice

PURPOSE

Neuroinflammation often initiates iron overload in the pathogenesis of neurological disorders. Chemokine-driven neuroinflammation is required for central sensitization and chronic allodynia following fractures, but specific molecular modulations are elusive. This present study explored whether hydrogen-rich saline, as one potent anti-inflammatory pharmaceutical, could alleviate fracture-caused allodynia by suppressing chemokine CXCL1 expression and iron overload.

METHODS

A mouse model of tibial fracture with intramedullary pinning was employed for establishing chronic allodynia. Three applications of hydrogen-rich saline (1, 5 or 10 mL/kg) were administrated intraperitoneally on a daily basis from days 4 to 6 following fractures. Spinal CXCL1 and its receptor CXCR2 levels, transferrin receptor 1 (TfR1) expression and iron concentration were examined. Recombinant CXCL1, a selective CXCR2 antagonist and an iron chelator were used for verification of mechanisms.

RESULTS

Repetitive injections of hydrogen-rich saline (5 and 10 mL/kg but not 1 mL/kg) prevent fracture-caused mechanical allodynia and cold allodynia in a dose-dependent manner. Single exposure to hydrogen-rich saline (10 mL/kg) on day 14 after orthopedic surgeries controls the established persistent fracture allodynia. Furthermore, hydrogen-rich saline therapy reduces spinal CXCL1/CXCR2 over-expression and TfR1-mediated iron accumulation in fracture mice. Spinal CXCR2 antagonism impairs allodynia and iron overload following fracture surgery. Intrathecal delivery of recombinant CXCL1 induces acute allodynia and spinal iron overload, which is reversed by hydrogen-rich saline. Moreover, iron chelation alleviates exogenous CXCL1-induced acute pain behaviors.

CONCLUSIONS

These findings identify that hydrogen-rich saline confers protection against fracture-caused chronic allodynia via spinal down-modulation of CXCL1-dependent TfR1-mediated iron accumulation in mice.

Chronic Pain after Bone Fracture: Current Insights into Molecular Mechanisms and Therapeutic Strategies

Bone fracture following traumatic injury or due to osteoporosis is characterized by severe pain and motor impairment and is a major cause of global mortality and disability. Fracture pain often originates from mechanical distortion of somatosensory nerve terminals innervating bones and muscles and is maintained by central sensitization. Chronic fracture pain (CFP) after orthopedic repairs is considered one of the most critical contributors to interference with the physical rehabilitation and musculoskeletal functional recovery. Analgesics available for CFP in clinics not only have poor curative potency but also have considerable side effects; therefore, it is important to further explore the pathogenesis of CFP and identify safe and effective therapies. The typical physiopathological characteristics of CFP are a neuroinflammatory response and excitatory synaptic plasticity, but the specific molecular mechanisms involved remain poorly elucidated. Recent progress has deepened our understanding of the emerging properties of chemokine production, proinflammatory mediator secretion, caspase activation, neurotransmitter release, and neuron-glia interaction in initiating and sustaining synaptogenesis, synaptic strength, and signal transduction in central pain sensitization, indicating the possibility of targeting neuroinflammation to prevent and treat CFP. This review summarizes current literature on the excitatory synaptic plasticity, microgliosis, and microglial activation-associated signaling molecules and discusses the unconventional modulation of caspases and stimulator of interferon genes (STING) in the pathophysiology of CFP. We also review the mechanisms of action of analgesics in the clinic and their side effects as well as promising therapeutic candidates (e.g., specialized pro-resolving mediators, a caspase-6 inhibitor, and a STING agonist) for pain relief by the attenuation of neuroinflammation with the aim of better managing patients undergoing CFP in the clinical setting.

The Involvement of Caspases in Neuroinflammation and Neuronal Apoptosis in Chronic Pain and Potential Therapeutic Targets

Chronic pain is a common, complex and unpleasant sensation following nerve injury, tissue trauma, inflammatory diseases, infection and cancer. It affects up to 25% of adults and is increasingly recognized as the leading cause of distress, disability and disease burden globally. Chronic pain is often refractory to most current analgesics, thus emphasizing the requirement for improved therapeutic medications. It is of great importance to elucidate the specific pathogenesis of chronic pain with different etiologies. Recent progress has advanced our understanding in the contribution of neuroinflammation and glial cells (microglia and astrocyte) activation in the plasticity of excitatory nociceptive synapses and the development of chronic pain phenotypes. Oxidative stress-associated neuronal apoptosis is also identified to be a pivotal step for central pain sensitization. The family of cysteine aspartate specific proteases (Caspases) has been well known to be key signaling molecules for inflammation and apoptosis in several neurological conditions. Recent studies have highlighted the unconventional and emerging role of caspases in microgliosis, astrocytes morphogenesis, chemokines release, cytokines secretion and neuronal apoptosis in initiating and maintaining synaptogenesis, synaptic strength and signal transduction in persistent pain hypersensitivity, suggesting the possibility of targeting caspases pathway for prevention and treatment of chronic pain. In this review, we will discuss and summarize the advances in the distinctive properties of caspases family in the pathophysiology of chronic pain, especially in neuropathic pain, inflammatory pain, cancer pain and musculoskeletal pain, with the aim to find the promising therapeutic candidates for the resolution of chronic pain to better manage patients undergoing chronic pain in clinics.

Pentraxin-3 in the Spinal Dorsal Horn Upregulates Nectin-1 Expression in Neuropathic Pain after Spinal Nerve Damage in Male Mice

Purpose: Neuropathic pain often originates from nerve injury or diseases of the somatosensory nervous system. However, its specific pathogenesis remains unclear. The requirement for excitatory synaptic plasticity in pain-related syndromes has been demonstrated. A recent study reported that pentraxin-3 is important in glutamatergic synaptic formation and function. Meanwhile, nectin-1 mediates synaptogenesis in neurological disorders. The present study aimed to evaluate whether pentraxin-3 and nectin-1 modulate spinal nerve damage-related neuropathic pain in male mice. Methods: L₄ spinal nerve ligation (SNL) in male mice was performed to induce experimental neuropathic pain. Mechanical allodynia and heat hyperalgesia following SNL were based on paw withdrawal (PW) threshold and PW latency, respectively. Spinal pentraxin-3 levels and nectin-1 expression following SNL were examined. Pentraxin-3 and nectin-1 knockdown models were established by the shRNA method. These models were used with a recombinant pentraxin-3 cell model to investigate the underlying mechanisms of SNL. Results: The SNL operation generated persistent decreases in mechanical PW threshold and thermal PW latency, with subsequent long-lasting elevations in spinal pentraxin-3 and nectin-1 expression levels. Pentraxin-3 knockdown reduced SNL-associated neuropathic pain behaviors as well as nectin-1 amounts in the spinal dorsal horn. Nectin-1 deficiency impaired mechanical allodynia and thermal hyperalgesia following spinal nerve injury. The application of recombinant pentraxin-3 in the spinal cord triggered an acute nociception phenotype and induced spinal overexpression of nectin-1. The intrathecal knockdown of nectin-1 prevented exogenous pentraxin-3-evoked pain hypersensitivity. Conclusions: The findings suggest spinal pentraxin-3 is required for SNL-triggered neuropathic pain via nectin-1 upregulation in male mice.

Artesunate Reduces Remifentanil-induced Hyperalgesia and Peroxiredoxin-3 Hyperacetylation via Modulating Spinal Metabotropic Glutamate Receptor 5 in Rats

The experimental investigations on the pathogenesis of remifentanil-induced hyperalgesia (RIH) have been primarily conducted, but the effective treatment of RIH remains unclear. Recent reports highlight the necessity of ionotropic glutamate receptors in oxidative damage in spinal nociceptive transduction. Artesunate, the 1st-line anti-malaria drug, has been identified to be valid in removing superoxide in several pathological conditions. This study evaluated whether artesunate inhibits RIH via regulating metabotropic glutamate receptor 5 (mGluR5) and mitochondrial antioxidant enzyme peroxiredoxin-3 in rats. Artesunate was injected intrathecally 10 min before intravenous infusion of remifentanil (1 μg·kg^-1·min^-1 for 60 min) in rats. The antinociception of artesunate was verified by assessment of paw withdrawal mechanical threshold and paw withdrawal thermal latency. Spinal mGluR5 expression and peroxiredoxin-3 hyperacetylation were examined. Also, both the mGluR5 agonist DHPG and antagonist MPEP were utilized to explore the involvement of mGluR5 in the anti-hyperalgesic property of artesunate. Here, we found that artesunate (10 μg and 100 μg but not 1 μg) prevented RIH in a dose-dependent manner. Artesunate reduced remifentanil-related spinal over-expression of mGluR5 gene and protein, and hyperacetylation of peroxiredoxin-3. Intrathecal application of MPEP (10 nmol and 100 nmol but not 1 nmol) inhibited behavioral RIH and peroxiredoxin-3 acetylation. Moreover, hyperalgesia and peroxiredoxin-3 hyperacetylation were attenuated after the combination of artesunate (1 μg) and MPEP (1 nmol). Additionally, artesunate treatment reversed acute pain and peroxiredoxin-3 hyperacetylation following spinal exposure to DHPG. In conclusion, intrathecal injection of artesunate impairs RIH by down-regulating spinal mGluR5 expression and peroxiredoxin-3 hyperacetylation-mediated oxidative stress in rats.

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.

Neuroprotective Effects of Molecular Hydrogen: A Critical Review

Molecular hydrogen (H2) is a physiologically inert gas. However, during the last 10 years, increasing evidence has revealed its biological functions under pathological conditions. More specifically, H2 has protective effects against a variety of diseases, particularly nervous system disorders, which include ischemia/reperfusion injury, traumatic injury, subarachnoid hemorrhage, neuropathic pain, neurodegenerative diseases, cognitive dysfunction induced by surgery and anesthesia, anxiety, and depression. In addition, H2 plays protective roles mainly through anti-oxidation, anti-inflammation, anti-apoptosis, the regulation of autophagy, and preservation of mitochondrial function and the blood-brain barrier. Further, H2 is easy to use and has neuroprotective effects with no major side-effects, indicating that H2 administration is a potential therapeutic strategy in clinical settings. Here we summarize the H2 donors and their pharmacokinetics. Meanwhile, we review the effectiveness and safety of H2 in the treatment of various nervous system diseases based on preclinical and clinical studies, leading to the conclusion that H2 can be a simple and effective clinical therapy for CNS diseases such as ischemia-reperfusion brain injury, Parkinson's disease, and diseases characterized by cognitive dysfunction. The potential mechanisms involved in the neuroprotective effect of H2 are also analyzed.

TRPV1 and spinal astrocyte activation contribute to remifentanil-induced hyperalgesia in rats

Remifentanil is an ultra-short-acting µ-opioid receptor agonist, which is widely used in general anesthesia. However, comparing with other opioids, remifentanil often induces hyperalgesia. Accumulating evidence suggests that the transient receptor potential (TRP) channels and glial cells activation were involved in the development of neuropathic pain and hyperalgesia. However, whether the TRP channels and glial cells contribute to remifentanil-induced hyperalgesia is still unknown. In this study, we used the hot-plate and Von Frey tests to evaluate the thermal and mechanical hyperalgesia. Protein expressions of TRPV1 and protein kinase C (PKC) in dorsal root ganglion were assayed by western blotting and mRNA level of Trpv1, Trpa1, Trpv4, and Trpm8 were assayed by real-time PCR. TNF-α, IL-1β, and IL-6 levels in spinal cord were measured by ELISA. Immunofluorescence assay was applied to analyze the activation of astrocyte in spinal cord. Continuing infusion of remifentanil induced thermal hyperalgesia and mechanical allodynia, which were accompanied by upregulation of TRPV1 and PKC protein in dorsal root ganglion. Moreover, remifentanil also increased the TNF-α, IL-1β, and IL-6 levels and activates the astrocyte in spinal cord. Our findings suggested that TRPV1 is involved in the TRPV1-PKC signaling pathway, which contributes to the persistence of remifentanil-induced postoperative hyperalgesia. In addition, the spinal astrocyte activation and inflammatory reaction are involved in the remifentanil-induced postoperative hyperalgesia.

Acid-sensing ion channel 3 expression is increased in dorsal root ganglion, hippocampus and hypothalamus in remifentanil-induced hyperalgesia in rats

BACKGROUND

Remifentanil induces hyperalgesia, but the underlying mechanisms are not fully understood. Acid-sensing ion channel 3 (ASIC3) plays a regulatory role in the pain pathway. This study aimed to explore the effect of remifentanil administration on postoperative pain and on ASIC3 expression at the prespinal and supraspinal levels in a rat model.

METHODS

Rats were randomly allocated to the control, incision, remifentanil, and remifentanil + incision groups. Remifentanil was given by a 1-h intravenous infusion prior to plantar incision. Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were measured at different time points before and after incision to evaluate mechanical and thermal hyperalgesia, respectively. The dorsal root ganglion (DRG), hippocampus, and hypothalamus were obtained after sacrifice at 48 h post-incision for determination of the protein expression of ASIC3 using western blot.

RESULTS

Remifentanil administration significantly induced mechanical and thermal hyperalgesia from 2 to 48 h after incision. In addition, remifentanil exposure remarkably stimulated ASIC3 protein expression in DRG, hippocampus, and hypothalamus of rats at 48 h after incision.

CONCLUSION

Remifentanil-induced hyperalgesia is accompanied by increased ASIC3 expression at the DRG and supraspinal levels, implying a possible involvement of ASIC3 in remifentanil-induced hyperalgesia.

Hydrogen-rich Saline Alleviated the Hyperpathia and Microglia Activation via Autophagy Mediated Inflammasome Inactivation in Neuropathic Pain Rats

Neuropathic pain is a complication after a spinal nerve injury. The inflammasomes are now identified to be responsible for triggering inflammation in neuropathic pain. Autophagy participates in the process of neuropathic pain and can regulate the inflammasome activation in different diseases. Our previous research reported that hydrogen exerted a protective effect against neuropathic pain. Therefore, we focused on the mechanism and role of autophagy and inflammasome, by which hydrogen alleviated the hyperpathia induced by neuropathic pain. The results showed that neuropathic pain stimulated activation of inflammasome NLRP3 and autophagy pathway in the microglial cells of the spinal cord. The inhibition of NLRP3 inhibited the hyperpathia induced by spinal nerve litigation surgery. The absence of autophagy aggravated the inflammasome activity and hyperpathia. Hydrogen promoted autophagy related protein expression, inhibited the inflammasome NLRP3 pathway activation, and relieved the hyperpathia induced by neuropathic pain. Hydrogen treatment could alleviate hyperpathia by autophagy-mediated NLRP3 inactivation.

Spinal Protein Kinase Mζ Regulates α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor Trafficking and Dendritic Spine Plasticity via Kalirin-7 in the Pathogenesis of Remifentanil-induced Postincisional Hyperalgesia in Rats

Background

Intraoperative remifentanil anesthesia exaggerates postoperative pain sensitivity. Recent studies recapitulate the significance of protein kinase Mζ in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor–mediated pathologic pain. Kalirin-7, a Rho guanine nucleotide exchange factor, coordinates AMPA receptor trafficking and dendritic spine plasticity. This study examines whether protein kinase Mζ and Kalirin-7 contribute to remifentanil-induced postincisional hyperalgesia via AMPA receptor.

Methods

Plantar incision was performed 10 min after the start of remifentanil infusion (1 µg · kg−1 · min−1 for 60 min). Paw withdrawal threshold (primary outcome), spinal protein kinase Mζ activity, Kalirin-7 expression, AMPA receptor trafficking, and spine morphology were assessed. Protein kinase Mζ inhibitor and Kalirin-7 knockdown by short hairpin RNA elucidated the mechanism and prevention of hyperalgesia. Whole-cell patch-clamp recording analyzed the role of protein kinase Mζ in spinal AMPA receptor–induced current.

Results

Remifentanil reduced postincisional paw withdrawal threshold (mean ± SD, control vs. hyperalgesia, 18.9 ± 1.6 vs. 5.3 ± 1.2 g, n = 7) at postoperative 48 h, which was accompanied by an increase in spinal protein kinase Mζ phosphorylation (97.8 ± 25.1 vs. 181.5 ± 18.3%, n = 4), Kalirin-7 production (101.9 ± 29.1 vs. 371.2 ± 59.1%, n = 4), and number of spines/10 µm (2.0 ± 0.3 vs. 13.0 ± 1.6, n = 4). Protein kinase Mζ inhibitor reduced remifentanil-induced hyperalgesia, Kalirin-7 expression, and GluA1 trafficking. Incubation with protein kinase Mζ inhibitor reversed remifentanil-enhanced AMPA receptor-induced current in dorsal horn neurons. Kalirin-7 deficiency impaired remifentanil-caused hyperalgesia, postsynaptic GluA1 insertion, and spine plasticity. Selective GluA2-lacking AMPA receptor antagonist prevented hyperalgesia in a dose-dependent manner.

Conclusions

Spinal protein kinase Mζ regulation of GluA1-containing AMPA receptor trafficking and spine morphology via Kalirin-7 overexpression is a fundamental pathogenesis of remifentanil-induced hyperalgesia in rats.

Hydrogen-Rich Saline Activated Autophagy via HIF-1α Pathways in Neuropathic Pain Model

Background

Neuropathic pain is a chronic and intractable pain, with very few effective analgesics. It involves an impaired cell autophagy process. Hydrogen-rich saline (HRS) reportedly reduces allodynia and hyperalgesia in a neuropathic pain model; however, it is unknown whether these effects involve autophagy induction.

Methods

We investigated the relationship between HRS and cell autophagy in a neuropathic pain model generated by chronic constriction injury (CCI) in Sprague–Dawley rats. Rats received an intraperitoneal injection of HRS (10 mL/kg daily, from 1 day before until 14 days after CCI), 3MA (autophagy inhibitor), 2ME2 (HIF-1α inhibitor), or EDHB (HIF-1α agonist). The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were tested 1 day before and 1, 3, 7, 10, and 14 days after the operation. HIF-1α and cell autophagy markers in the spinal cord were evaluated by western blotting and real-time PCR assays at 14 days after CCI. Autophagosomes with double membranes were identified by transmission electron microscopy.

Results

CCI caused behavioral hypersensitivity to mechanical and thermal stimulation in the hind-paw of the injured side. HRS improved MWT and TWL, activated autophagy, and increased autophagosomes and autolysosomes in CCI rats. 3-MA aggravated hyperalgesia and allodynia and suppressed autophagy, while EDHB attenuated hyperalgesia and activated the autophagy procedure and the HIF-1α downstream target gene BNIP3. HIF-1α inhibitors reversed the regulatory effects of HRS on autophagy in CCI rats at 14 days after spinal cord injury.

Conclusion

HRS reduced mechanical hyperalgesia and activation of cell autophagy in neuropathic pain through a HIF1-dependent pathway.

Preoperative But Not Postoperative Flurbiprofen Axetil Alleviates Remifentanil-induced Hyperalgesia After Laparoscopic Gynecological Surgery: A Prospective, Randomized, Double-blinded, Trial

BACKGROUND

Acute remifentanil exposure during intraoperative analgesia might enhance sensitivity to noxious stimuli and nociceptive responses to innocuous irritation. Cyclooxygenase inhibition was demonstrated to attenuate experimental remifentanil-induced hyperalgesia (RIH) in rodents and human volunteers. The study aimed to compare the effects of preoperative and postoperative flurbiprofen axetil (FA) on RIH after surgery.

MATERIALS AND METHODS

Ninety patients undergoing elective laparoscopic gynecologic surgery were randomly assigned to receive either intravenous placebo before anesthesia induction (Group C); or intravenous FA (1.0 mg/kg) before anesthesia induction (Group F1) or before skin closure (Group F2). Anesthesia consisted off sevoflurane and remifentanil (0.30 μg/kg/min). Postoperative pain was managed by sufentanil titration in the postanesthetic care unit, followed by sufentanil infusion via patient-controlled analgesia. Mechanical pain threshold (primary outcome), pain scores, sufentanil consumption, and side-effects were documented for 24 hours postoperatively.

RESULTS

Postoperative pain score in Group F1 was lower than Group C. Time of first postoperative sufentanil titration was prolonged in Group F1 than Group C (P=0.021). Cumulative sufentanil consumption in Group F1 was lower than Group C (P<0.001), with a mean difference of 8.75 (95% confidence interval, 5.21-12.29) μg. Mechanical pain threshold on the dominant inner forearm was more elevated in Group F1 than Group C (P=0.005), with a mean difference of 17.7 (95% confidence interval, 5.4-30.0) g. Normalized hyperalgesia area was decreased in Group F1 compared to Group C (P=0.007). No statistically significant difference was observed between Group F2 and Group C.

CONCLUSIONS

Preoperative FA reduces postoperative RIH in patients undergoing laparoscopic gynecologic surgery under sevoflurane-remifentanil anesthesia.

Involvement of Spinal PKMζ Expression and Phosphorylation in Remifentanil-Induced Long-Term Hyperalgesia in Rats

Up-regulation of GluN2B-containing N-methyl-D-aspartate receptors (NMDARs) expression and trafficking is the key mechanism for remifentanil-induced hyperalgesia (RIH), nevertheless, the signaling pathway and pivotal proteins involved in RIH remain equivocal. PKMζ, an isoform of protein kinase C (PKC), maintains pain memory storage in neuropathic pain and inflammatory pain, which plays a parallel role regulated by NMDARs in long-term memory trace. In the present study, Zeta Inhibitory Peptide (ZIP), a PKMζ inhibitor, and a selective GluN2B antagonist Ro-256981 are injected intrathecally before remifentanil infusion (1 μg kg-1 min-1 for 1 h, iv) in order to detect whether GluN2B contributes to RIH through affecting synthesis and activity of PKMζ in spinal dorsal horn. Nociceptive tests are measured by Paw withdrawal mechanical threshold (PWT) and paw withdrawal thermal latency (PWL). The L4-L6 segments of dorsal horn taken from rats with RIH are for determining expression of PKMζ and pPKMζ by Western blot and immunohistochemistry. Our data suggest that remifentanil infusion causes an increase of PKMζ in expression and phosphorylation in rats with nociceptive sensitization, beginning at 2 h, peaked at 2 days, and returned to basal level at 7 days. ZIP (10 ng) could block behavioral sensitization induced by remifentanil. Ro25-6981 dosage-dependently attenuated mechanical and thermal hyperalgesia and reversed expression of PKMζ and pPKMζ, indicating that GluN2B-containing NMDA receptor facilitates development of RIH through mediating expression and activity of spinal PKMζ in rats. Although detailed mechanisms require further comprehensive study, the preventive role of Ro25-6981 and ZIP provide novel options for the effective precaution of RIH in clinics.

Neuroprotection of Ro25-6981 Against Ischemia/Reperfusion-Induced Brain Injury via Inhibition of Autophagy

In this study, we investigated the neuroprotective effect of Ro25-6981 against cerebral ischemia/reperfusion injury. Ro25-6981 alone or in combination with rapamycin was intracerebroventricularly administered to rats which suffered transient forebrain ischemia inducing by 4-vessel occlusion and reperfusion. Nissl staining was used to determine the survival of CA1 pyramidal cells of the hippocampus, while immunohistochemistry was performed to measure neuron-specific enolase (NSE) expression. The expression of autophagy-related proteins, such as microtubule-associated protein l light chain 3 (LC3), Beclin 1, and sequestosome 1 (p62), was assessed by immunoblotting. Nissl staining showed that neuronal damage was reduced in the hippocampal CA1 pyramidal layer in rats that received Ro25-6981. The protective effect of Ro25-6981 was dose-dependent, with a significant effect in the middle-dose range. The expression of NSE increased after Ro25-6981 treatment. Ro25-6981 significantly decreased LC3II (which is membrane bound) and Beclin 1, and increased p62. In addition, Ro25-6981 decreased rapamycin-induced neuronal damage and excessive activation of autophagy after I/R. Taken together, the results suggest that Ro25-6981 could suppress ischemic brain injury by regulating autophagy-related proteins during ischemia/reperfusion.

A comparison of intrathecal magnesium and ketamine in attenuating remifentanil-induced hyperalgesia in rats

Background

Activation of NMDA receptors play an important role in the development of remifentanil-induced hyperalgesia. We hypothesized that in addition to ketamine, intrathecal MgSO₄ could also relieve thermal and mechanical hyperalgesia in rats.

Methods

Initially, 24 Sprague–Dawley rats were divided into control group, remifentanil group, surgical incision group and remifentanil combined with surgical incision group to create an experimental model. Subsequently, 40 rats were divided into control group, model group, model group plus 100 μg MgSO₄, 300 μg MgSO₄ and 10 μg ketamine respectively. Paw withdrawal mechanical thresholds and paw withdrawal thermal latency tests were performed at −24 h, 2 h, 6 h, 24 h, 48 h, 72 h and 7 day after the surgical procedure. After behavior assessment on the 7th day, remifentanil was given again to ascertain whether or not NMDA antagonists could suppress the re-exposure of remifentanil-induced hyperalgesia.

Results

Remifentanil administration plus surgical incision induced significant postoperative hyperalgesia, as indicated by decreased paw withdrawal mechanical thresholds and paw withdrawal thermal latency to mechanical and thermal stimulation. In addition to ketamine, intrathecal MgSO₄ (100, 300 μg) dose-dependently reduced remifentanil-induced mechanical and thermal hyperalgesia. Ketamine had less mechanical hyperalgesia in 6 h (p = 0.018), 24 h (p = 0.014) and 48 h (p = 0.011) than 300 μg MgSO₄. There was no difference in inhibiting thermal hyperalgesia between the group ketamine and group MgSO₄ (300 μg). The rats were given remifentanil again 7 days later after the first exposure of remifentanil. The hyperalgesic effect induced by re-exposure of remifentanil was not reversed in any groups of MgSO₄ or ketamine.

Conclusions

In addition to ketamine, intrathecal administration of MgSO₄ dose-dependently reduced remifentanil-induced hyperalgesia in a surgical incision mode. Re-exposure to remifentanil 1 week later again produced hyperalgesia, and this was not altered by the prior intrathecal treatments in any 4 groups treated with MgSO₄ or ketamine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12871-016-0235-9) contains supplementary material, which is available to authorized users.

Protective effects of hydrogen-rich saline against N-methyl-N-nitrosourea-induced photoreceptor degeneration

The N-methyl-N-nitrosourea (MNU)-treated rat is typically used as an animal model of chemically-induced retinitis pigmentosa (RP). Reactive oxygen species (ROS) have been recognized as the crucial contributor to the retinal photoreceptor apoptosis seen in MNU-treated rats. In the present study, we explored the therapeutic effects of hydrogen-rich saline (HRS), a selective ROS scavenger, on MNU-induced photoreceptor degeneration. Intraperitoneal (IP) administration of HRS ameliorated MNU-induced photoreceptor degeneration in terms of morphology and function: Sharply decreased thickness of the retinal outer nuclear layer (ONL) and flattened photopic and scotopic electroretinogram (ERG) waveforms, typically seen in response to MNU treatment, were substantially rescued in rats cotreated with MNU and HRS (MNU + HRS). Moreover, the terminal deoxyuridine triphosphate nick-end labeling (TUNEL) assay revealed a smaller number of apoptotic photoreceptors in the MNU + HRS group compared that in the MNU group. Compared to MNU-treated rats, retinal malondialdehyde (MDA) content in MNU + HRS rats significantly decreased while superoxide dismutase (SOD) activity significantly increased. Morphological and multi-electrode array (MEA) analyses revealed more efficient preservation of the architecture and field potential waveforms in particularly the peripheral regions of the retinas within the MNU + HRS group, compared to that in the MNU group. However, this enhanced protection of structure and function in the peripheral retina is unlikely the result of site-dependent variation in the efficacy of HRS; rather, it is most likely due to reduced susceptibility of peripheral photoreceptors to MNU-induced degeneration. Inner retinal neuron function in the MNU + HRS rats was better preserved, with fewer apoptotic photoreceptors in the ONL. Collectively, these results support the rationale for future clinical evaluation of HRS as a therapeutic agent for human RP.

The potential utilizations of hydrogen as a promising therapeutic strategy against ocular diseases

Hydrogen, one of the most well-known natural molecules, has been used in numerous medical applications owing to its ability to selectively neutralize cytotoxic reactive oxygen species and ameliorate hazardous inflammations. Hydrogen can exert protective effects on various reactive oxygen species-related diseases, including the transplantation-induced intestinal graft injury, chronic inflammation, ischemia–reperfusion injuries, and so on. Especially in the eye, hydrogen has been used to counteract multiple ocular pathologies in the ophthalmological models. Herein, the ophthalmological utilizations of hydrogen are systematically reviewed and the underlying mechanisms of hydrogen-induced beneficial effects are discussed. It is our hope that the protective effects of hydrogen, as evidenced by these pioneering studies, would enrich our pharmacological knowledge about this natural element and cast light into the discovery of a novel therapeutic strategy against ocular diseases.

Hydrogen-rich Saline is ineffective in oxygen-induced retinopathy

AIMS

Hydrogen-rich saline (HRS) is a novel protection against various oxidative disorders and almost all types of inflammation. Moreover, its toxicity and side effects are rarely reported. We sought to clarify the protective effect of HRS against the oxygen-induced retinopathy (OIR) in C57BL/6 J model.

MAIN METHODS

The OIR in the HRS treated mice and the untreated controls were systematically compared. The retinas of both groups were analyzed using high-molecular-weight FITC-dextran staining of flat-mount preparations, hematoxylin and eosin (H&E) staining of cross-sections. The distribution and expression of the vascular endothelial growth factor (VEGF) were also evaluated by the immunohistochemical measurements between postnatal days 17 (P17) and P21.

KEY FINDING

The leakage and non-perfusion areas of retinal blood vessels were not alleviated in the HRS treatment group. Moreover, the number of preretinal vascular endothelial cell in the HRS treatment group was similar to that in the untreated group after exposure to hyperoxia (P>0.05). The degree of OIR was positively correlated with the expression level of VEGF. Intriguingly, the preretinal vascular endothelial cell count in the retinas of pups reared in room air with HRS treatment was 15.21±2.98. The preretinal vascular endothelial cell count of the HRS treated mice was significantly higher than that of the untreated group reared in room air.

SIGNIFICANCE

In summary, HRS therapy (at the dose of 10ml/day, applied between P12 and P17) did not inhibit retinal neovascularization in OIR; On the contrary, it would induce the retinal neovascularization during the development of normal retinas.

Use of Hydrogen as a Novel Therapeutic Strategy Against Photoreceptor Degeneration in Retinitis Pigmentosa Patients

Retinitis pigmentosa (RP) is a heterogeneous group of inherited retinal dystrophies characterized by progressive photoreceptor apoptosis. Reactive oxygen species (ROS) have been recognized as critical initiators of the photoreceptor apoptosis in RP. Photoreceptor survival in RP mutants will not only require the inhibition of effectors of apoptotic machinery, but also the elimination of the initiating upstream signals, such as ROS. These cytotoxic ROS should be neutralized by the antioxidant defense system, otherwise they would interact with the macromolecules essential for photoreceptor survival. Hydrogen is a promising gaseous agent that has come to the forefront of therapeutic research over the last few years. It has been verified that hydrogen is capable of neutralizing the cytotoxic ROS selectively, rectifying abnormities in the apoptotic cascades, and attenuating the related inflammatory response. Hydrogen is so mild that it does not disturb the metabolic oxidation-reduction reactions or disrupt the physiologic ROS involved in cell signaling. Based on these findings, we hypothesize that hydrogen might be an effective therapeutic agent to slow or prevent photoreceptor degeneration in RP retinas. It is a logical step to test hydrogen for therapeutic use in multiple RP animal models, and ultimately in human RP patients.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Beneficial biological effects and the underlying mechanisms of molecular hydrogen - comprehensive review of 321 original articles

Therapeutic effects of molecular hydrogen for a wide range of disease models and human diseases have been investigated since 2007. A total of 321 original articles have been published from 2007 to June 2015. Most studies have been conducted in Japan, China, and the USA. About three-quarters of the articles show the effects in mice and rats. The number of clinical trials is increasing every year. In most diseases, the effect of hydrogen has been reported with hydrogen water or hydrogen gas, which was followed by confirmation of the effect with hydrogen-rich saline. Hydrogen water is mostly given ad libitum. Hydrogen gas of less than 4 % is given by inhalation. The effects have been reported in essentially all organs covering 31 disease categories that can be subdivided into 166 disease models, human diseases, treatment-associated pathologies, and pathophysiological conditions of plants with a predominance of oxidative stress-mediated diseases and inflammatory diseases. Specific extinctions of hydroxyl radical and peroxynitrite were initially presented, but the radical-scavenging effect of hydrogen cannot be held solely accountable for its drastic effects. We and others have shown that the effects can be mediated by modulating activities and expressions of various molecules such as Lyn, ERK, p38, JNK, ASK1, Akt, GTP-Rac1, iNOS, Nox1, NF-κB p65, IκBα, STAT3, NFATc1, c-Fos, and ghrelin. Master regulator(s) that drive these modifications, however, remain to be elucidated and are currently being extensively investigated.