Reactive oxygen species (ROS) have a key role in different etiologies of pain. At sub-cellular level, mitochondria and plasma membranes have been identified as endogenous sources of ROS required for pain generation. NADPH oxidase (NOX) is the main contributor of membrane associated ROS generation. Out of 7 isozymes, NOX1, NOX2 and NOX4 are reported to be associated with nociceptive sensitization. Therefore, it has been hypothesized that specific inhibition of the NOX isozymes could be putative strategy for treatment of pain. However, unavailability of specific inhibitors was the biggest obstacle to test this hypothesis. Here, we investigated anti-nociceptive potential of a newly identified specific NOX1 inhibitor ML171 in formalin induced inflammatory pain. ML171 administration decreased the paw lickings and flinching response in both phases of formalin test. Behavioral response was supported with decreased activation of c-Fos in spinal dorsal horn. The increased level of total NOX activity, ROS and pERK1/2 in dorsal root ganglion (DRG) and spinal dorsal horn of formalin induced nociception were reversed by ML171 administration. ML171 also inhibited the upregulated Tumor necrosis factor receptor 1 (TNFR1) expression in DRG, whereas did not show any effect in spinal dorsal horn which was unaltered after formalin insult. The study for the first time depicts anti-nociceptive potential of ML171 via regulation of ROS mediated ERK1/2 signaling by inhibition of NOX1 activity.

INTRODUCTION:Pain is an unpleasant sensation triggered by noxious stimulation. It is one of the most prevalent conditions, limiting productivity and diminishing quality of life. Non steroidal anti inflammatory drugs (NSAIDs) are widely used as pain relievers in present day practice as pain is mostly initiated due to inflammation. However, due to potentially serious side effects, long term use of these antihyperalgesic drugs raises concern. Therefore there is a demand to search novel medicines with least side effects. Herbal products have been used for centuries to reduce pain and inflammation, and phytochemicals are known to cause fewer side effects. However, identification of active phytochemicals of herbal medicines and clear understanding of the molecular mechanism of their action is needed for clinical acceptance.MATERIALS AND METHODS:In this review, we have briefly discussed the cellular and molecular changes during hyperalgesia via inflammatory mediators and neuro-modulatory action involved therein. The review includes 54 recently reported phytochemicals with antihyperalgesic action, as per the literature available with PubMed, Google Scholar and Scopus.CONCLUSION:Compounds of high interest as potential antihyperalgesic agents are: curcumin, resveratrol, capsaicin, quercetin, eugenol, naringenin and epigallocatechin gallate (EGCG). Current knowledge about molecular targets of pain and their regulation by these phytochemicals is elaborated and the scope of further research is discussed.

Impact of ROS in development of hyperalgesia has recently motivated scientists to focus on ROS as novel target of anti-hyperalgesic interventions. However, role of ROS in molecular signaling of hyperalgesia is still poorly understood. The present study is aimed to analyze the effect of dietary antioxidant resveratrol on antioxidant defense system, ROS level and TNFR1–ERK signaling pathway during early and late phase of inflammatory hyperalgesia.

Concomitant generation of reactive oxygen species during tissue inflammation has been recognised as a major factor for the development and the maintenance of hyperalgesia, out of which H2O2 is the major player. However, molecular mechanism of H2O2 induced hyperalgesia is still obscure. The aim of present study is to analyse the mechanism of H2O2-induced hyperalgesia in rats. Intraplantar injection of H2O2 (5, 10 and 20 mu moles/paw) induced a significant thermal hyperalgesia in the hind paw, confirmed by increased c-Fos activity in dorsal horn of spinal cord. Onset of hyperalgesia was prior to development of oxidative stress and inflammation. Rapid increase in phosphorylation of extracellular signal regulated kinase (ERK) was observed in neurons of dorsal root ganglia after 20 min of H2O2 (10 mu moles/paw) administration, which gradually returned towards normal level within 24h, following the pattern of thermal hyperalgesia. The expression of TNFR1 followed the same pattern and colocalised with pERK. ERK phosphorylation was observed in NF-200-positive and -negative neurons, indicating the involvement of ERK in C-fibres as well as in A-fibres. Intrathecal preadministration of Src family kinases (SFKs) inhibitor (PP1) and MEK inhibitor (PD98059) prevented H2O2 induced augmentation of ERK phosphorylation and thermal hyperalgesia. Pretreatment of protein tyrosine phosphatases (PTPs) inhibitor (sodium orthovanadate) also diminished hyperalgesia, although it further increased ERK phosphorylation. Combination of orthovanadate with PP1 or PD98059 did not exhibit synergistic antihyperalgesic effect. The results demonstrate SFKs-mediated ERK activation and increased TNFR1 expression in nociceptive neurons during H2O2 induced hyperalgesia. However, the role of PTPs in hyperalgesic behaviour needs further molecular analysis.

Sensitization of nociceptive neurons by inflammatory mediators leads to hypersensitivity for normal painful stimuli which is termed hyperalgesia. Oxidative stress is an essential factor in pathological pain; therefore, antioxidants qualify as potential anti-hyperalgesic agents. The present study examines the efficacy of the natural antioxidant resveratrol in complete Freund's adjuvant (CFA) induced hyperalgesic rats. Thermal hyperalgesia was measured at different time points by paw withdrawal latency test and confirmed by c-Fos expression in spinal dorsal horn. The impact of resveratrol treatment on inflammatory mediators at peripheral (paw skin) and central (spinal cord) sites was determined during early (6h) as well as late phase (48h) of hyperalgesia. Intraplanter injection of CFA increased the level of cytokines IL-1, TNF- and IL-6 as well as inflammatory enzymes COX-2 and iNOS in paw skin in both phases. In case of spinal cord, the level of COX-2 was found to be elevated in both phases, whereas iNOS could not be detected. The cytokines were found to be elevated only in late phase in spinal cord. Administration of resveratrol (20mg/kg) shifted the level of all inflammatory mediators towards normal, except cytokines in paw skin. The present study suggests that the anti-nociceptive effect of resveratrol is implicated at both peripheral and central sites in a tissue specific manner. Copyright (c) 2016 John Wiley & Sons, Ltd.

Reactive oxygen species are signaling mediators of nociceptive pathways. Exogenous administrations of antioxidants show anti-hyperalgesic effect. However, very little is known about the role of endogenous antioxidant defense system in pain pathology. Curcumin is a dietary antioxidant which shows ameliorative effect on thermal hypersensitivity, however detailed study is lacking. Present study was aimed to analyze the changes in oxidative stress, modulation of antioxidant enzymes and pro-inflammatory cytokines in complete Freund's adjuvant induced inflammatory hyperalgesia and the effect of curcumin on antioxidant defense system and pro-inflammatory cytokines. Anti-hyperalgesic activity of curcumin was evidenced after 6 h of treatment. Oxidative stress was evidenced in paw skin and spinal cord of hyperalgesic rats by high level of lipid peroxidation. A decrease in activity of antioxidant enzymes like catalase, superoxide dismutase, glutathione peroxidase, glutathione reductase and an increase in level of pro-inflammatory cytokines TNF-α, IL-1β and IL-6 in paw skin was observed as compared to normal rats. However, activity of antioxidant enzymes was enhanced in spinal cord. The changes were brought towards normal level after curcumin treatment. The results suggest that modulation of antioxidant defense system is early event in initiation of inflammatory hyperalgesia which might lead to initiation of other signaling pathways mediated by lipid peroxide, TNF-α, IL-1β and IL-6. Decrease in oxidative stress and down regulation of these cytokines by curcumin is suggested to be involved in its anti-hyperalgesic effect.

Weak noncovalent interactions are the basic forces in crystal engineering. Polymorphism in flexible molecules is very common, leading to the development of the crystals of same organic compounds with different medicinal and material properties. Crystallization of 2,2′-{[1,2-phenylenebis(methylene)]bis(sulfanediyl)}bis(4,6-dimethylnicotinonitrile) by evaporation at room temperature from ethyl acetate and hexane and from methanol and ethyl acetate gave stable polymorphs 4a and 4b, respectively, while in acetic acid, it gave metastable polymorph 4c. The polymorphic behavior of the compound has been visualized through single-crystal X-ray and Hirshfeld analysis. These polymorphs are tested for anti-inflammatory activity via the complete Freund's adjuvant-induced rat paw model, and compounds have exhibited moderate activities. Studies of docking in the catalytic site of cyclooxygenase-2 were used to identify potential anti-inflammatory lead compounds. These results suggest that the supramolecular aggregate structure, which is formed in solution, influences the solid state structure and the biological activity obtained upon crystallization.