Anti-superoxide and anti-peroxynitrite strategies in pain suppression

Boronate-Based Oxidant-Responsive Derivatives of Acetaminophen as Proinhibitors of Myeloperoxidase

Myeloperoxidase (MPO) is an important component of the human innate immune system and the main source of a strong oxidizing and chlorinating species, hypochlorous acid (HOCl). Inadvertent, misplaced, or excessive generation of HOCl by MPO is associated with multiple human inflammatory diseases. Therefore, there is a considerable interest in the development of MPO inhibitors. Here, we report the synthesis and characterization of a boronobenzyl derivative of acetaminophen (AMBB), which can function as a proinhibitor of MPO and release acetaminophen, the inhibitor of chlorination cycle of MPO, in the presence of inflammatory oxidants, i.e., hydrogen peroxide, hypochlorous acid, or peroxynitrite. We demonstrate that the AMBB proinhibitor undergoes conversion to acetaminophen by all three oxidants, with the involvement of the primary phenolic product intermediate, with relatively long half-life at pH 7.4. The determined rate constants of the reaction of the AMBB proinhibitor with hydrogen peroxide, hypochlorous acid, or peroxynitrite are equal to 1.67, 1.6 × 104, and 1.0 × 106 M-1 s-1, respectively. AMBB showed lower MPO inhibitory activity (IC50 > 0.3 mM) than acetaminophen (IC50 = 0.14 mM) toward MPO-dependent HOCl generation. Finally, based on the determined reaction kinetics and the observed inhibitory effects of two plasma components, uric acid and albumin, on the extent of AMBB oxidation by ONOO- and HOCl, we conclude that ONOO- is the most likely potential activator of AMBB in human plasma.

Melatonin Mitigates iNOS-Related Effects of HEMA and Camphorquinone in Human Dental Pulp Cells: Relevance for Postoperative Sensitivity Mechanism in Type 2 Diabetes

High elution and diffusion of 2-hydroxylethyl methacrylate (HEMA) and camphorquinone (CQ) through dentinal tubules may induce pulp injury and postoperative sensitivity. We aimed to investigate the melatonin protective effect in HEMA- and CQ-treated human dental pulp cells (hDPCs) as well as its relevance in a mechanism for postoperative sensitivity in diabetic patients. hDPCs were exposed to HEMA (5 mM) and/or CQ (1 mM) in the absence and presence of melatonin (MEL) (0.1 mM and 1 mM). Heme oxygenase-1 (HMOX1), NADPH oxidase-4 (NOX4), BCL-2-associated X-protein (BAX), B-cell lymphoma-2 (BCL-2) and caspase-3 (CASP3) gene expression levels, and superoxide dismutase (SOD) activity were measured in hDPCs while inducible nitric oxide synthase (iNOS) and melatonin protein expression were measured in human dental pulp as well, by RT-PCR, by ELISA, and spectrophotometrically. Bioinformatic analyses were performed by using the ShinyGO (v.0.75) application. Type 2 diabetic patients showed a higher incidence of postoperative sensitivity and lower melatonin and higher iNOS content in dental pulp tissue compared with non-diabetic patients. Melatonin, when co-added in hDPC culture, reverses HEMA and CQ cytotoxic effects via anti-apoptotic and anti-inflammatory/antioxidant iNOS-related effects. Enrichment analyses showed that genes/proteins, altered by HEMA and CQ and normalized by melatonin, are the most prominently overrepresented in type 2 diabetes mellitus pathways and that they share subcellular localization in different oligomeric protein complexes consisting of anti- and pro-apoptotic regulators. This is the first evidence of the ability of melatonin to counteract iNOS-mediated inflammatory and stress effects in HEMA- and CQ-treated hDPCs, which could be of significance for the modulation of presently observed immediate postoperative sensitivity after composite restoration in type 2 diabetic patients.

Effects of Different Opioid Drugs on Oxidative Status and Proteasome Activity in SH-SY5Y Cells

Opioids are the most effective drugs used for the management of moderate to severe pain; however, their chronic use is often associated with numerous adverse effects. Some results indicate the involvement of oxidative stress as well as of proteasome function in the development of some opioid-related side effects including analgesic tolerance, opioid-induced hyperalgesia (OIH) and dependence. Based on the evidence, this study investigated the impact of morphine, buprenorphine or tapentadol on intracellular reactive oxygen species levels (ROS), superoxide dismutase activity/gene expression, as well as β2 and β5 subunit proteasome activity/biosynthesis in SH-SY5Y cells. Results showed that tested opioids differently altered ROS production and SOD activity/biosynthesis. Indeed, the increase in ROS production and the reduction in SOD function elicited by morphine were not shared by the other opioids. Moreover, tested drugs produced distinct changes in β2(trypsin-like) and β5(chymotrypsin-like) proteasome activity and biosynthesis. In fact, while prolonged morphine exposure significantly increased the proteolytic activity of both subunits and β5 mRNA levels, buprenorphine and tapentadol either reduced or did not alter these parameters. These results, showing different actions of the selected opioid drugs on the investigated parameters, suggest that a low µ receptor intrinsic efficacy could be related to a smaller oxidative stress and proteasome activation and could be useful to shed more light on the role of the investigated cellular processes in the occurrence of these opioid drug side effects.

Novel Non-Opioid Based Therapeutics for Chronic Neuropathic Pain

Chronic neuropathic pain is currently a major health issue in U.S. complicated by the lack of non-opioid analgesic alternatives. Our investigations led to the discovery of major signaling pathways involved in the transition of acute to chronic neuropathic pain and the identification of several targets for therapeutic intervention. Our translational approach has facilitated the advancement of novel medicines for chronic neuropathic pain that are in advanced clinical development and clinical trials.

The Lectin Isolated from the Alga Hypnea cervicornis Promotes Antinociception in Rats Subjected to Zymosan-Induced Arthritis: Involvement of cGMP Signalization and Cytokine Expression

This study investigated the effects of the alga lectin Hypnea cervicornis agglutinin (HCA) on rat zymosan-induced arthritis (ZyA). Zymosan (50-500 μg/25 μL) or sterile saline (Sham) was injected into the tibio-tarsal joint of female Wistar rats (180-200 g). Arthritic animals received morphine (4 mg/kg, intraperitoneal), indomethacin (5 mg/kg, intraperitoneal), or 2% lidocaine (100 μL, subcutaneous). HCA (0.3-3 mg/kg) was administered by intravenous route 30 min before or 2 h after zymosan. 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ, 4 μg, intra-articular) was given 30 min prior HCA. Hypernociception was measured every hour until 6 h, time in which animals were sacrificed for evaluation of leukocytes of the intra articular fluid and gene expression of TNF-α, IL-1, IL-10, and iNOS in the joint tissues using PCR techniques. Hypernociception was responsive to morphine and indomethacin, and its threshold was not altered by lidocaine. The post-treatment of HCA reduced both hypernociception and leukocyte influx. This antinociceptive effect was abolished either by ODQ and glibenclamide. HCA also reduced gene expression of iNOS and TNF-α. In conclusion, the antinociceptive effect of HCA in ZyA involves cyclic GMP signalization and selective modulation of cytokine expression.

The antioxidant N-(2-mercaptopropionyl)-glycine (tiopronin) attenuates expression of neuropathic allodynia and hyperalgesia

The current pharmacotherapy of neuropathic pain is inadequate as neuropathic pain involves varied clinical manifestations with multifactorial etiology, modulated by a cascade of physical and molecular events leading to different clinical presentations of pain. There is an accumulating evidence of the involvement of oxidative stress in neuropathy, and antioxidants have shown promise in mitigating neuropathic pain syndromes. To explore the evidence supporting this beneficial proclivity of antioxidants, this study investigated the antinociceptive effectiveness of N-(2-mercaptopropionyl)glycine or tiopronin, a well-recognized aminothiol antioxidant, in a refined chronic constriction injury (CCI) rat model of neuropathic pain. Tiopronin (10, 30, and 90 mg/kg, i.p.) and pregabalin (30 mg/kg, i.p.) were administered daily after CCI surgery. The neuropathic paradigms of mechanical/cold allodynia and mechanical/heat hyperalgesia were assessed on days 3, 7, 14, and 21 post-nerve ligation. At the end of study, malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH) levels were estimated in the sciatic nerve, dorsal root ganglion, and spinal cord for assessing the extent of oxidative stress. The expression of neuropathic nociception was attenuated by tiopronin which was observed as a significant attenuation of CCI-induced allodynia and hyperalgesia. Tiopronin reversed the neuronal oxidative stress by significantly reducing MDA, and increasing SOD, CAT, and GSH levels. Pregabalin also showed similar beneficial propensity on CCI-induced neuropathic aberrations. These findings suggest prospective neuropathic pain attenuating efficacy of tiopronin and further corroborated the notion that antioxidants are effective in mitigating the development and expression of neuropathic pain and underlying neuronal oxidative stress.

Nonhormonal Treatment for Endometriosis Focusing on Redox Imbalance

The aim of this review is to investigate the oxidant/antioxidant status and its regulatory mechanisms in patients with endometriosis and to summarize the antioxidant therapy as an alternative to hormonal therapy for endometriosis. Each keyword alone or in combination was used to search from PubMed and Embase by applying the filters of the title and the publication years between January 2000 and March 2020. Endometriosis is a chronic inflammatory disease characterized by repeated episodes of hemorrhage. Methemoglobin in repeated hemorrhage produces large amounts of superoxide anion via the autoxidation of hemoglobin. Excessive free-radical production causes redox imbalance, leading to inadequate antioxidant defenses and damage to endometrial cells, but may contribute to endometrial cell growth and survival through activation of various signaling pathways. In addition, to overcome excessive oxidative stress, estradiol participates in the induction of antioxidants such as superoxide dismutase in mitochondria. Several antioxidants that suppress free radicals may be effective in endometriosis-related pain. We searched for 23 compounds and natural substances that could reduce the pain caused by superoxide/reactive oxygen species in basic research and animal models. Next, we built a list of 16 drugs that were suggested to be effective against endometriosis other than hormone therapy in preclinical studies and clinical trials. Of the 23 and 16 drugs, 4 overlapping drugs could be potential candidates for clinically reducing endometriosis-related pain caused by superoxide anion/reactive oxygen species. These drugs include polyphenols (resveratrol and polydatin), dopamine agonists (cabergoline), and statins (simvastatin). However, no randomized controlled trials have evaluated the efficacy of these drugs. In conclusion, this review summarizes the following 2 points: superoxide anion generation by methemoglobin is enhanced in endometriosis, resulting in redox imbalance; and some compounds and natural substances that can suppress free radicals may be effective in endometriosis-related pain. Further randomized clinical trials based on larger series are mandatory to confirm the promising role of antioxidants in the nonhormonal management of endometriosis.

Protective Effect of Antioxidants in Nitric Oxide/COX-2 Interaction during Inflammatory Pain: The Role of Nitration

In clinical practice, inflammatory pain is an important, unresolved health problem, despite the utilization of non-steroidal anti-inflammatory drugs (NSAIDs). In the last decade, different studies have proven that reactive oxygen species (ROS) and reactive nitrogen species (RNS) are involved in the development and maintenance of inflammatory pain and hyperalgesia via the post-translation modification of key proteins, such as manganese superoxide dismutase (MnSOD). It is well-known that inducible cyclooxygenase 2 (COX-2) plays a crucial role at the beginning of the inflammatory response by converting arachidonic acid into proinflammatory prostaglandin PGE₂ and then producing other proinflammatory chemokines and cytokines. Here, we investigated the impact of oxidative stress on COX-2 and prostaglandin (PG) pathways in paw exudates, and we studied how this mechanism can be reversed by using antioxidants during hyperalgesia in a well-characterized model of inflammatory pain in rats. Our results reveal that during the inflammatory state, induced by intraplantar administration of carrageenan, the increase of PGE₂ levels released in the paw exudates were associated with COX-2 nitration. Moreover, we showed that the inhibition of ROS with Mn (III) tetrakis (4-benzoic acid) porphyrin(MnTBAP) antioxidant prevented COX-2 nitration, restored the PGE₂ levels, and blocked the development of thermal hyperalgesia.

The Neuroimmunology of Chronic Pain: From Rodents to Humans

Chronic pain, encompassing conditions, such as low back pain, arthritis, persistent post-surgical pain, fibromyalgia, and neuropathic pain disorders, is highly prevalent but remains poorly treated. The vast majority of therapeutics are directed solely at neurons, despite the fact that signaling between immune cells, glia, and neurons is now recognized as indispensable for the initiation and maintenance of chronic pain. This review highlights recent advances in understanding fundamental neuroimmune signaling mechanisms and novel therapeutic targets in rodent models of chronic pain. We further discuss new technological developments to study, diagnose, and quantify neuroimmune contributions to chronic pain in patient populations.

Emerging Treatment Targets for Migraine and Other Headaches

Migraine is a complex disorder that is characterized by an assortment of neurological and systemic effects. While headache is the most prominent feature of migraine, a host of symptoms affecting many physiological functions are also observed before, during, and after an attack. Furthermore, migraineurs are heterogeneous and have a wide range of responses to migraine therapies. The recent approval of calcitonin gene-related-peptide based therapies has opened up the treatment of migraine and generated a renewed interest in migraine research and discovery. Ongoing advances in migraine research have identified a number of other promising therapeutic targets for this disorder. In this review, we highlight emergent treatments within the following biological systems: pituitary adenylate cyclase activating peptdie, 2 non-mu opioid receptors that have low abuse liability - the delta and kappa opioid receptors, orexin, and nitric oxide-based therapies. Multiple mechanisms have been identified in the induction and maintenance of migraine symptoms; and this divergent set of targets have highly distinct biological effects. Increasing the mechanistic diversity of the migraine tool box will lead to more treatment options and better patient care.

Repurposing of the Nootropic Drug Vinpocetine as an Analgesic and Anti-Inflammatory Agent: Evidence in a Mouse Model of Superoxide Anion-Triggered Inflammation

Clinically active drugs for the treatment of acute pain have their prescription limited due to the significant side effects they induce. An increase in reactive oxygen species (ROS) has been linked to several conditions, including inflammation and pain processing. Therefore, new or repurposed drugs with the ability of reducing ROS-triggered responses are promising candidates for analgesic drugs. Vinpocetine is a clinically used nootropic drug with antioxidant, anti-inflammatory, and analgesic properties. However, the effects of vinpocetine have not been investigated in a model with a direct relationship between ROS, inflammation, and pain. Based on that, we aimed to investigate the effects of vinpocetine in a model of superoxide anion-induced pain and inflammation using potassium superoxide (KO₂) as a superoxide anion donor to trigger inflammation and pain. In the KO₂ model, vinpocetine dose-dependently reduced pain-like behaviors (spontaneous pain and hyperalgesia), paw edema, and neutrophil and mononuclear cell recruitment to the paw skin (assessed by H&E staining, fluorescence, and enzymatic assays) and to the peritoneal cavity. Vinpocetine also restored tissue endogenous antioxidant ability and Nrf2 and Ho-1 mRNA expression and reduced superoxide anion production and gp91^phox mRNA expression. We also observed the inhibition of IκBα degradation by vinpocetine, which demonstrates a reduction in the activation of NF-κB explaining the diminished production of IL-33, IL-1β, and TNF-α. Collectively, our data show that vinpocetine alleviates pain and inflammation induced by KO₂, which is a mouse model with a direct role of ROS in triggering pain and other inflammatory phenomena. Thus, the results suggest the repurposing of vinpocetine as an anti-inflammatory and analgesic drug.

15d-PGJ2-loaded nanocapsules ameliorate experimental gout arthritis by reducing pain and inflammation in a PPAR-gamma-sensitive manner in mice

Gout arthritis (GA) is a painful inflammatory disease in response to monosodium urate (MSU) crystals in the joints. 15deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂) is a natural activator of PPAR-γ with analgesic, anti-inflammatory, and pro-resolution properties. Thus, we aimed to evaluate the effect and mechanisms of action of 15d-PGJ₂ nanocapsules (NC) in the model of GA in mice, since a reduction of 33-fold in the dose of 15d-PGJ₂ has been reported. Mice were treated with 15d-PGJ₂-loaded NC, inert NC, free 15d-PGJ₂ (without NC), or 15d-PGJ₂-loaded NC+ GW9662, a PPAR-γ inhibitor. We show that 15d-PGJ₂-loaded NC provided analgesic effect in a dose that the free 15d-PGJ₂ failed to inhibiting pain and inflammation. Hence, 15d-PGJ₂-loaded NC reduced MSU-induced IL-1β, TNF-α, IL-6, IL-17, and IL-33 release and oxidative stress. Also, 15d-PGJ₂-loaded NC decreased the maturation of IL-1β in LPS-primed BMDM triggered by MSU. Further, 15d-PGJ₂-loaded NC decreased the expression of the components of the inflammasome Nlrp3, Asc, and Pro-caspase-1, as consequence of inhibiting NF-κB activation. All effects were PPAR-γ-sensitive. Therefore, we demonstrated that 15d-PGJ₂-loaded NC present analgesic and anti-inflammatory properties in a PPAR-γ-dependent manner inhibiting IL-1β release and NF-κB activation in GA. Concluding, 15d-PGJ₂-loaded NC ameliorates MSU-induced GA in a PPAR-γ-sensitive manner.

Osthole ameliorates neurogenic and inflammatory hyperalgesia by modulation of iNOS, COX-2, and inflammatory cytokines in mice

BACKGROUND

Osthole is a bioactive component reported in medicinal plants such as Angelica pubescens and Cnidium monnieri, known for analgesic activity. However, the toxicity, median effective dose (ED₅₀), and dual modulation of nitric oxide and cyclooxygenase pathways along with inflammatory cytokines of osthole are yet to be determined.

METHODS

The animals (mice) were assessed for general behaviour and mortality in varying doses (50, 300, and 2000 mg kg^-1) of osthole for acute toxicity over 14 days. The analgesic activity was investigated using acetic acid and formalin-induced hyperalgesia, and anti-inflammatory activity was explored in carrageenan-induced paw oedema. ED₅₀ of osthole was calculated using Design Expert software. Involvement of nitric oxide and cyclooxygenase pathways was investigated by agonist challenges with L-arginine and substance P, respectively. The expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) was determined in spinal sections by immunohistochemical analysis. Lipopolysaccharide (LPS) challenge was used to assess in vivo effect on inflammatory cytokines (TNFα and IL-6).

RESULTS

Acute toxicity studies revealed no behavioural abnormality or mortality on osthole treatment and unremarkable histological findings. Osthole was found to significantly decrease acetic acid and formalin-induced hyperalgesia (ED₅₀ = 5.43 mg kg^-1) and carrageenan-induced paw oedema with no toxicity symptoms. Osthole produced a marked decrease in iNOS and COX-2 expression as well as TNFα and IL-6. The findings corroborate to modulation of iNOS and COX-2 and inflammatory cytokines by osthole. This study provides promising insights and prospects for application of osthole in pain management.

The Neurotoxin DSP-4 Induces Hyperalgesia in Rats that is Accompanied by Spinal Oxidative Stress and Cytokine Production

Central neuropathic pain (CNP) a significant problem for many people, is not well-understood and difficult to manage. Dysfunction of the central noradrenergic system originating in the locus coeruleus (LC) may be a causative factor in the development of CNP. The LC is the major noradrenergic nucleus of the brain and plays a significant role in central modulation of nociceptive neurotransmission. Here, we examined CNS pathophysiological changes induced by intraperitoneal administration of the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). Administration of DSP-4 decreased levels of norepinephrine in spinal tissue and cerebrospinal fluid (CSF) and led to the development of thermal and mechanical hyperalgesia over 21 days, that was reversible with morphine. Hyperalgesia was accompanied by significant increases in noradrenochrome (oxidized norepinephrine) and expression of 4-hydroxynonenal in CSF and spinal cord tissue respectively at day 21, indicative of oxidative stress. In addition, spinal levels of pro-inflammatory cytokines (interleukins 6 and 17A, tumor necrosis factor-α), as well as the anti-inflammatory cytokine interleukin10 were also significantly elevated at day 21, indicating that an inflammatory response occurred. The inflammatory effect of DSP-4 presented in this study that includes oxidative stress may be particularly useful in elucidating mechanisms of CNP in inflammatory disease states.

Forced exercise attenuates neuropathic pain in chronic constriction injury of male rat: an investigation of oxidative stress and inflammation

BACKGROUND AND OBJECTIVE

Initial peripheral/central nerve injuries, such as chronic constriction injury (CCI)/spinal cord injury, are often compounded by secondary mechanisms, including inflammation and oxidative stress, which may lead to chronic neuropathic pain characterized by hyperalgesia or allodynia. On the other hand, exercise as a behavioral and non-pharmacological treatment has been shown to alleviate chronic neuropathic pain. Therefore, this study was conducted to examine whether or not exercise reduces neuropathic pain through modifying oxidative stress and inflammation in chronic constriction injury of the sciatic nerve.

MATERIALS AND METHODS

Wistar male rats weighing 200±20 g were randomly divided into five groups (normal, sham, CCI, pre-CCI exercise, and post-CCI exercise group). Sciatic nerve of anesthetized rats was loosely ligated to induce CCI, and they were then housed in separate cages. The rats ran on treadmill at a moderate speed for 3 weeks. Mechanical allodynia and thermal hyperalgesia were determined using von Frey filament and plantar test, respectively. Tumor necrosis factor-alpha (TNF-α) assayed in the cerebrospinal fluid, malondialdehyde, and total antioxidant capacity were measured in the serum using Western blot test, thiobarbituric acid, and ferric reducing ability of plasma (FRAP), respectively.

RESULTS

The mechanical allodynia (P=0.024) and thermal hyperalgesia (P=0.002) in the CCI group were higher than those in the sham group. Exercise after CCI reduced (P=0.004) mechanical allodynia and thermal hyperalgesia (P=0.025) compared with the CCI group. Moreover, the level of FRAP in the CCI group was (P=0.001) lower than that in the sham group, and post-CCI exercise reversed FRAP amount toward the control level (P=0.019). The amount of malondialdehyde did not differ between groups. Level of TNF-α increased in the CCI group (P=0.0002) compared with sham group and post-CCI exercise could reverse it toward the level of control (P=0.005).

CONCLUSION

Post CCI-exercise but not pre CCI-exercise reduces CCI-induced neuropathic pain. One of the possible involved mechanisms is increasing the total antioxidant capacity and reducing the amount of TNF-α.

Nitroxidative Signaling Mechanisms in Pathological Pain

Tissue injury can initiate bidirectional signaling between neurons, glia, and immune cells that creates and amplifies pain. While the ability for neurotransmitters, neuropeptides, and cytokines to initiate and maintain pain has been extensively studied, recent work has identified a key role for reactive oxygen and nitrogen species (ROS/RNS; nitroxidative species), including superoxide, peroxynitrite, and hydrogen peroxide. In this review we describe how nitroxidative species are generated after tissue injury and the mechanisms by which they enhance neuroexcitability in pain pathways. Finally, we discuss potential therapeutic strategies for normalizing nitroxidative signaling, which may also enhance opioid analgesia, to help to alleviate the enormous burden of pathological pain.

Intrathecal Administration of Tempol Reduces Chronic Constriction Injury-Induced Neuropathic Pain in Rats by Increasing SOD Activity and Inhibiting NGF Expression

We investigate the antinociceptive effect of intrathecal and intraperitoneal tempol administration in a rat model of chronic constriction injury (CCI)-induced neuropathic pain and explore the underlying antinociceptive mechanisms of tempol. Rats were randomly assigned to four groups (n = 8 per group): sham group, CCI group, Tem1 group (intrathecal injection of tempol), and Tem2 group (intraperitoneal injection of tempol). Neuropathic pain was induced by CCI of the sciatic nerve. Tempol was intrathecally or intraperitoneally administered daily for 7 days beginning on postoperative day one. The mechanical withdrawal threshold and thermal withdrawal latency were tested on preoperative day 3 and postoperative days 1, 3, 5, 7, 10, 14, and 21. Structural changes were examined by hematoxylin and eosin staining, toluidine blue staining, and electron microscopy. Malondialdehyde (MDA) and superoxide dismutase (SOD) levels were determined using the thiobarbituric acid and nitroblue tetrazolium methods, respectively. Nerve growth factor (NGF) expression levels were determined by immunohistochemistry and Western blot. Intrathecal, but not intraperitoneal, injection of tempol produced a persistent antinociceptive effect. Intraperitoneal injection of tempol did not result in high enough concentration of tempol in the cerebrospinal fluid. Intrathecal, but not intraperitoneal, injection of tempol inhibited CCI-induced structural damage in the spinal cord reduced MDA levels, and increased SOD activities in the spinal cord. Furthermore, intrathecal, but not intraperitoneal, injection of tempol further downregulated the expression of NGF in the spinal cord following CCI, and this effect was blocked by p38MAPK inhibitor. Intrathecal injection of tempol produces antinociceptive effects and reduces CCI-induced structural damage in the spinal cord by increasing SOD activities and downregulating the expression of NGF via the p38MAPK pathway. Intraperitoneal administration of tempol does not exhibit antinociceptive effects.

Pyrrolidine dithiocarbamate inhibits superoxide anion-induced pain and inflammation in the paw skin and spinal cord by targeting NF-κB and oxidative stress

We evaluated the effect of pyrrolidine dithiocarbamate (PDTC) in superoxide anion-induced inflammatory pain. Male Swiss mice were treated with PDTC and stimulated with an intraplantar or intraperitoneal injection of potassium superoxide, a superoxide anion donor. Subcutaneous PDTC treatment attenuated mechanical hyperalgesia, thermal hyperalgesia, paw oedema and leukocyte recruitment (neutrophils and macrophages). Intraplantar injection of superoxide anion activated NF-κB and increased cytokine production (IL-1β, TNF-α and IL-10) and oxidative stress (nitrite and lipid peroxidation levels) at the primary inflammatory foci and in the spinal cord (L4-L6). PDTC treatment inhibited superoxide anion-induced NF-κB activation, cytokine production and oxidative stress in the paw and spinal cord. Furthermore, intrathecal administration of PDTC successfully inhibited superoxide anion-induced mechanical hyperalgesia, thermal hyperalgesia and inflammatory response in peripheral foci (paw). These results suggest that peripheral stimulus with superoxide anion activates the local and spinal cord oxidative- and NF-κB-dependent inflammatory nociceptive mechanisms. PDTC targets these events, therefore, inhibiting superoxide anion-induced inflammatory pain in mice.

AMPKα1 knockout enhances nociceptive behaviors and spinal glutamatergic synaptic activities via production of reactive oxygen species in the spinal dorsal horn

Emerging studies have shown that pharmacological activation of adenosine monophosphate-activated protein kinase (AMPK) produces potent analgesic effects in different animal pain models. Currently, the spinal molecular and synaptic mechanism by which AMPK regulates the pain signaling system remains unclear. To address this issue, we utilized the Cre-LoxP system to conditionally knockout the AMPKα1 gene in the nervous system of mice. We demonstrated that AMPKα1 is imperative for maintaining normal nociception, and mice deficient for AMPKα1 exhibit mechanical allodynia. This is concomitantly associated with increased glutamatergic synaptic activities in neurons located in the superficial spinal dorsal horn, which results from the increased glutamate release from presynaptic terminals and function of ligand-gated glutamate receptors at the postsynaptic neurons. Additionally, AMPKα1 knockout mice have increased activities of extracellular signal-regulated kinases (ERK) and p38 mitogen-activated protein kinases (p38), as well as elevated levels of interleukin-1β (IL-1β), reactive oxygen species (ROS), and heme oxygenase 1 (HO-1) in the spinal dorsal horn. Systemic administration of a non-specific ROS scavenger (phenyl-N-tert-butylnitrone, PBN) or a HO-1 activator (Cobalt protoporphyrin IX, CoPP) attenuated allodynia in AMPKα1 knockout mice. Bath-perfusion of the ROS scavenger or HO-1 activator effectively attenuated the increased ROS levels and glutamatergic synaptic activities in the spinal dorsal horn. Our findings suggest that ROS are the key down-stream signaling molecules mediating the behavioral hypersensitivity in AMPKα1 knockout mice. Thus, targeting AMPKα1 may represent an effective approach for the treatment of pathological pain conditions associated with neuroinflammation at the spinal dorsal horn.

The bifunctional μ opioid agonist/antioxidant [Dmt(1)]DALDA is a superior analgesic in an animal model of complex regional pain syndrome-type i

Reactive oxygen species (ROS) play an important role in the development of complex regional pain syndrome-Type I (CRPS-I), as also demonstrated with the chronic post ischemia pain (CPIP) animal model of CRPS-I. We show that morphine and the antioxidant N-acetylcysteine (NAC) act synergistically to reduce mechanical allodynia in CPIP rats. The tetrapeptide amide [Dmt(1)]DALDA (H-Dmt-d-Arg-Phe-Lys-NH2) is a potent and selective μ opioid receptor (MOR) agonist with favorable pharmacokinetic properties and with antioxidant activity due to its N-terminal Dmt (2',6'-dimethyltyrosine) residue. In the CPIP model, [Dmt(1)]DALDA was 15-fold more potent than morphine in reversing mechanical allodynia and 4.5-fold more potent as analgesic in the heat algesia test. The results indicate that bifunctional compounds with MOR agonist/antioxidant activity have therapeutic potential for the treatment of CRPS-I.

Ameliorative potential of pioglitazone and ceftriaxone alone and in combination in rat model of neuropathic pain: Targeting PPARγ and GLT-1 pathways

BACKGROUND

The relation between glutamate homeostasis and PPAR gamma has got tremendous importance in nerve trauma and pain. Present study has been designed to elucidate the interaction between the GLT-1 activator (ceftriaxone) and PPAR gamma agonist (pioglitazone) in the spinal nerve ligation induced neuropathic pain.

METHODS

Male SD rats were subjected to spinal nerve ligation to induce neuropathic pain. Pioglitazone, ceftriaxone and their combination treatments were given for 28 days. Various behavioral, biochemical, neuroinflammatory and apoptotic mediators were assessed subsequently.

RESULTS

In the present study, ligation of L5 and L6 spinal nerves resulted in marked hyperalgesia and allodynia to different mechanical and thermal stimuli. In addition there is marked increase in oxidative-nitrosative stress parameters, inflammatory and apoptotic markers in spinal cord of spinal nerve ligated rats. Treatment with pioglitazone and ceftriaxone significantly prevented these behavioral, biochemical, mitochondrial and cellular alterations in rats. Further, combination of pioglitazone (10mg/kg, ip) with ceftriaxone (100mg/kg, ip) significantly potentiated the protective effects as compared to their effects per se.

CONCLUSION

Based on these results we propose that possible interplay between the neuroprotective effects of pioglitazone and ceftriaxone exists in suppressing the behavioral, biochemical, mitochondrial, neuroinflammatory and apoptotic cascades in spinal nerve ligation induced neuropathic pain in rats.

Vinpocetine reduces carrageenan-induced inflammatory hyperalgesia in mice by inhibiting oxidative stress, cytokine production and NF-κB activation in the paw and spinal cord

Vinpocetine is a safe nootropic agent used for neurological and cerebrovascular diseases. The anti-inflammatory activity of vinpocetine has been shown in cell based assays and animal models, leading to suggestions as to its utility in analgesia. However, the mechanisms regarding its efficacy in inflammatory pain treatment are still not completely understood. Herein, the analgesic effect of vinpocetine and its anti-inflammatory and antioxidant mechanisms were addressed in murine inflammatory pain models. Firstly, we investigated the protective effects of vinpocetine in overt pain-like behavior induced by acetic acid, phenyl-p-benzoquinone (PBQ) and formalin. The intraplantar injection of carrageenan was then used to induce inflammatory hyperalgesia. Mechanical and thermal hyperalgesia were evaluated using the electronic von Frey and the hot plate tests, respectively, with neutrophil recruitment to the paw assessed by a myeloperoxidase activity assay. A number of factors were assessed, both peripherally and in the spinal cord, including: antioxidant capacity, reduced glutathione (GSH) levels, superoxide anion, tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β) levels, as well as nuclear factor kappa B (NF-κB) activation. Vinpocetine inhibited the overt pain-like behavior induced by acetic acid, PBQ and formalin (at both phases), as well as the carrageenan-induced mechanical and thermal hyperalgesia and associated neutrophil recruitment. Both peripherally and in the spinal cord, vinpocetine also inhibited: antioxidant capacity and GSH depletion; increased superoxide anion; IL-1β and TNF-α levels; and NF-κB activation. As such, vinpocetine significantly reduces inflammatory pain by targeting oxidative stress, cytokine production and NF-κB activation at both peripheral and spinal cord levels.

The ruthenium nitric oxide donor, [Ru(HEDTA)NO], inhibits acute nociception in mice by modulating oxidative stress, cytokine production and activating the cGMP/PKG/ATP-sensitive potassium channel signaling pathway

Nitric oxide plays an important role in various biological processes including antinociception. The control of its local concentration is crucial for obtaining the desired effect and can be achieved with exogenous nitric oxide-carriers such as ruthenium complexes. Therefore, we evaluated the analgesic effect and mechanism of action of the ruthenium nitric oxide donor [Ru(HEDTA)NO] focusing on the role of cytokines, oxidative stress and activation of the cyclic guanosine monophosphate/protein kinase G/ATP-sensitive potassium channel signaling pathway. It was observed that [Ru(HEDTA)NO] inhibited in a dose-dependent (1-10 mg/kg) manner the acetic acid-induced writhing response. At the dose of 1 mg/kg, [Ru(HEDTA)NO] inhibited the phenyl-p-benzoquinone-induced writhing response, and formalin- and complete Freund's adjuvant-induced licking and flinching responses. Systemic and local treatments with [Ru(HEDTA)NO] also inhibited the carrageenin-induced mechanical hyperalgesia and increase of myeloperoxidase activity in paw skin samples. Mechanistically, [Ru(HEDTA)NO] inhibited carrageenin-induced production of the hyperalgesic cytokines tumor necrosis factor-α and interleukin-1β, and decrease of reduced glutathione levels. Furthermore, the inhibitory effect of [Ru(HEDTA)NO] in the carrageenin-induced hyperalgesia and myeloperoxidase activity was prevented by the treatment with ODQ (soluble guanylyl cyclase inhibitor), KT5823 (protein kinase G inhibitor) and glybenclamide (ATP-sensitive potassium channel inhibitor), indicating that [Ru(HEDTA)NO] inhibits inflammatory hyperalgesia by activating the cyclic guanosine monophosphate/protein kinase G/ATP-sensitive potassium channel signaling pathway, respectively. These results demonstrate that [Ru(HEDTA)NO] exerts its analgesic effect in inflammation by inhibiting pro-nociceptive cytokine production, oxidative imbalance and activation of the nitric oxide/cyclic guanosine monophosphate/protein kinase G/ATP-sensitive potassium channel signaling pathway in mice.

Molecular hydrogen attenuates neuropathic pain in mice

Neuropathic pain remains intractable and the development of new therapeutic strategies are urgently required. Accumulating evidence indicates that overproduction of oxidative stress is a key event in the pathogenesis of neuropathic pain. However, repeated intra-peritoneal or intrathecal injections of antioxidants are unsuitable for continuous use in therapy. Here we show a novel therapeutic method against neuropathic pain: drinking water containing molecular hydrogen (H2) as antioxidant. The effect of hydrogen on neuropathic pain was investigated using a partial sciatic nerve ligation model in mice. As indicators of neuropathic pain, temporal aspects of mechanical allodynia and thermal hyperalgesia were analysed for 3 weeks after ligation. Mechanical allodynia and thermal hyperalgesia were measured using the von Frey test and the plantar test, respectively. When mice were allowed to drink water containing hydrogen at a saturated level ad libitum after ligation, both allodynia and hyperalgesia were alleviated. These symptoms were also alleviated when hydrogen was administered only for the induction phase (from day 0 to 4 after ligation). When hydrogen was administered only for the maintenance phase (from day 4 to 21 after ligation), hyperalgesia but not allodynia was alleviated. Immunohistochemical staining for the oxidative stress marker, 4-hydroxy-2-nonenal and 8-hydroxydeoxyguanosine, showed that hydrogen administration suppressed oxidative stress induced by ligation in the spinal cord and the dorsal root ganglion. In conclusion, oral administration of hydrogen water may be useful for alleviating neuropathic pain in a clinical setting.

Evaluation of antinociceptive and antioxidant properties of 3-[4-(3-trifluoromethyl-phenyl)-piperazin-1-yl]-dihydrofuran-2-one in mice

The aim of this study was to evaluate the influence of 3-[4-(3-trifluoromethyl-phenyl)-piperazin-1-yl]-dihydrofuran-2-one (LPP1) on nociceptive thresholds in mouse models of persistent pain. Influence of LPP1 on motor coordination and its antioxidant capacity in mouse brain tissue homogenates were also assessed. Pain sensitivity thresholds in animals treated with LPP1 were established using 5 % formalin solution in normoglycemic mice and in streptozotocin (STZ)-treated diabetic mice in the von Frey, hot plate, innocuous, and noxious cold water tests (water at 10 °C and 4 °C, respectively). Motor deficits were assessed in the rotarod test, whereas antioxidant capacities were evaluated using ferric reducing ability of plasma (FRAP) assay, catalase (CAT), and superoxide dismutase (SOD) activities. LPP1was antinociceptive in both phases of the formalin test, in particular, in the late phase (at doses 0.9-30 mg/kg for 66-99 % vs. control normoglycemic mice) and in a statistically significant manner increased nociceptive thresholds in response to mechanical, heat, and noxious cold stimulation in neuropathic mice (at 30 mg/kg for 274, 192, and 316 %, respectively vs. diabetic control). LPP1 did not impair motor coordination of mice in the rotarod revolving at 6 or 18 rpm. In brain tissue homogenates, it demonstrated antioxidant capacity in FRAP assay and increased SOD activity for 63 % (acute administration) and 28 % (chronic administration) vs. control. No influence on CAT activity was observed. LPP1 has significant antinociceptive properties in the formalin model and elevates pain thresholds in neuropathic mice. It has antioxidant capacity and is devoid of negative influence on animals' motor coordination.

NADPH oxidase-4 maintains neuropathic pain after peripheral nerve injury

Reactive oxygen species (ROS) contribute to sensitization of pain pathways during neuropathic pain, but little is known about the primary sources of ROS production and how ROS mediate pain sensitization. Here, we show that the NADPH oxidase isoform Nox4, a major ROS source in somatic cells, is expressed in a subset of nonpeptidergic nociceptors and myelinated dorsal root ganglia neurons. Mice lacking Nox4 demonstrated a substantially reduced late-phase neuropathic pain behavior after peripheral nerve injury. The loss of Nox4 markedly attenuated injury-induced ROS production and dysmyelination processes of peripheral nerves. Moreover, persisting neuropathic pain behavior was inhibited after tamoxifen-induced deletion of Nox4 in adult transgenic mice. Our results suggest that Nox4 essentially contributes to nociceptive processing in neuropathic pain states. Accordingly, inhibition of Nox4 may provide a novel therapeutic modality for the treatment of neuropathic pain.

Antioxidant, antinociceptive, and anti-inflammatory effects of carotenoids extracted from dried pepper (Capsicum annuum L.)

Carotenoids extracted from dried peppers were evaluated for their antioxidant, analgesic, and anti-inflammatory activities. Peppers had a substantial carotenoid content: guajillo 3406 ± 4 μg/g, pasilla 2933 ± 1 μg/g, and ancho 1437 ± 6 μg/g of sample in dry weight basis. A complex mixture of carotenoids was discovered in each pepper extract. The TLC analysis revealed the presence of chlorophylls in the pigment extract from pasilla and ancho peppers. Guajillo pepper carotenoid extracts exhibited good antioxidant activity and had the best scavenging capacity for the DPPH(+) cation (24.2%). They also exhibited significant peripheral analgesic activity at 5, 20, and 80 mg/kg and induced central analgesia at 80 mg/kg. The results suggest that the carotenoids in dried guajillo peppers have significant analgesic and anti-inflammatory benefits and could be useful for pain and inflammation relief.