The transition from acute to chronic pain is critically important but not well understood. Here, we investigated the pathophysiological mechanisms underlying the transition from acute to chronic low back pain (LBP) and performed transcriptome-wide analysis in peripheral immune cells of 98 participants with acute LBP, followed for 3 months. Transcriptomic changes were compared between patients whose LBP was resolved at 3 months with those whose LBP persisted. We found thousands of dynamic transcriptional changes over 3 months in LBP participants with resolved pain but none in those with persistent pain. Transient neutrophil-driven up-regulation of inflammatory responses was protective against the transition to chronic pain. In mouse pain assays, early treatment with a steroid or nonsteroidal anti-inflammatory drug (NSAID) also led to prolonged pain despite being analgesic in the short term; such a prolongation was not observed with other analgesics. Depletion of neutrophils delayed resolution of pain in mice, whereas peripheral injection of neutrophils themselves, or S100A8/A9 proteins normally released by neutrophils, prevented the development of long-lasting pain induced by an anti-inflammatory drug. Analysis of pain trajectories of human subjects reporting acute back pain in the UK Biobank identified elevated risk of pain persistence for subjects taking NSAIDs. Thus, despite analgesic efficacy at early time points, the management of acute inflammation may be counterproductive for long-term outcomes of LBP sufferers.

Chemotherapy-induced peripheral neuropathy (CIPN) accompanied by chronic neuropathic pain is a major dose-limiting side effect of a large number of antitumoral agents including paclitaxel (Taxol). Thus, CIPN is one of most common causes of dose reduction and discontinuation of what is otherwise a life-saving therapy. Neuropathological changes in spinal cord are linked to CIPN, but the causative mediators and mechanisms remain poorly understood. We report that formation of peroxynitrite (PN) in response to activation of nitric oxide synthases and NADPH oxidase in spinal cord contributes to neuropathological changes through two mechanisms. The first involves modulation of neuroexcitatory and proinflammatory (TNF-α and IL-1β) and anti-inflammatory (IL-10 and IL-4) cytokines in favor of the former. The second involves post-translational nitration and modification of glia-derived proteins known to be involved in glutamatergic neurotransmission (astrocyte-restricted glutamate transporters and glutamine synthetase). Targeting PN with PN decomposition catalysts (PNDCs) not only blocked the development of paclitaxel-induced neuropathic pain without interfering with antitumor effects, but also reversed it once established. Herein, we describe our mechanistic study on the role(s) of PN and the prevention of neuropathic pain in rats using known PNDCs (FeTMPyP⁵⁺and MnTE-2-PyP⁵⁺). We also demonstrate the prevention of CIPN with our two new orally active PNDCs, SRI6 and SRI110. The improved chemical design of SRI6 and SRI110 also affords selectivity for PN over other reactive oxygen species (such as superoxide). Our findings identify PN as a critical determinant of CIPN, while providing the rationale toward development of superoxide-sparing and “PN-targeted” therapeutics.

The clinical efficacy of opiates for pain control is severely limited by analgesic tolerance and hyperalgesia. Herein we show that chronic morphine upregulates both the sphingolipid ceramide in spinal astrocytes and microglia, but not neurons, and spinal sphingosine-1-phosphate (S1P), the end-product of ceramide metabolism. Coadministering morphine with intrathecal administration of pharmacological inhibitors of ceramide and S1P blocked formation of spinal S1P and development of hyperalgesia and tolerance in rats. Our results show that spinally formed S1P signals at least in part by (1) modulating glial function because inhibiting S1P formation blocked increased formation of glial-related proinflammatory cytokines, in particular tumor necrosis factor-α, interleukin-1βα, and interleukin-6, which are known modulators of neuronal excitability, and (2) peroxynitrite-mediated posttranslational nitration and inactivation of glial-related enzymes (glutamine synthetase and the glutamate transporter) known to play critical roles in glutamate neurotransmission. Inhibitors of the ceramide metabolic pathway may have therapeutic potential as adjuncts to opiates in relieving suffering from chronic pain.

Activation of the N-methyl-D-aspartate receptor (NMDAR) is fundamental in the development of hyperalgesia. Overactivation of this receptor releases superoxide and nitric oxide that, in turn, forms peroxynitrite (PN). All of these events have been linked to neurotoxicity. The receptors and enzymes involved in the handling of glutamate pathway—specifically NMDARs, glutamate transporter, and glutamine synthase (GS)—have key tyrosine residues which are targets of the nitration process causing subsequent function modification. Our results demonstrate that the thermal hyperalgesia induced by intrathecal administration of NMDA is associated with spinal nitration of GluN1 and GluN2B receptor subunits, GS, that normally convert glutamate into nontoxic glutamine, and glutamate transporter GLT1. Intrathecal injection of PN decomposition catalyst FeTM-4-PyP⁵⁺prevents nitration and overall inhibits NMDA-mediated thermal hyperalgesia. Our study supports the hypothesis that nitration of key proteins involved in the regulation of glutamate transmission is a crucial pathway used by PN to mediate the development and maintenance of NMDA-mediated thermal hyperalgesia. The broader implication of our findings reinforces the notion that free radicals may contribute to various forms of pain events and the importance of the development of new pharmacological tool that can modulate the glutamate transmission without blocking its actions directly.

Citrus Bergamia Risso, commonly known as Bergamot, is a fruit whose Essential Oil and Bergamot Polyphenolic Fraction have numerous medicinal properties. It is also an excellent antioxidant and in this study, for the first time, its potential effect on morphine induced tolerance in mice has been investigated. Our studies revealed that development of antinociceptive tolerance to repeated doses of morphine in mice is consistently associated with increased formation of superoxide, malondialdehyde and tyrosine-nitrated proteins in the dorsal horn of the spinal cord such as the enzyme glutamine synthase. Nitration of this protein is intimately linked to inactivation of its biological function and resulting increase of glutamate levels in the spinal cord. Administration of Bergamot Polyphenolic Fraction (5–50 mg/kg) attenuated tolerance development. This effect was accompanied by reduction of superoxide and malondialdehyde production, prevention of GS nitration, re-establishment of its activity and of glutamate levels. Our studies confirmed the main role of free radicals during the cascade of events induced by prolonged morphine treatment and the co-administration of natural derivatives antioxidant such as Bergamot Polyphenolic Fraction can be an important therapeutic approach to restore opioids analgesic efficacy.

Background: Opioids are widely used for chronic low back pain (CLBP); however, it is still unclear how to predict their effectiveness and safety. Codeine, tramadol and oxycodone are metabolized by CYP/CYP450 2D6 (CYP2D6), a highly polymorphic enzyme linked to allele-specific related differences in metabolic activity. Purpose: CYP2D6 genetic polymorphisms could potentially help to predict the effectiveness and safety of opioid-based drugs in clinical practice, especially in the treatment of CLBP. Patients and methods: A cohort of 224 Italian patients with CLBP treated with codeine or oxycodone was retrospectively evaluated to determine whether adverse reactions and effectiveness were related to CYP2D6 single-nucleotide polymorphisms. CYP2D6 genotyping was performed using the xTAG^® CYP2D6 Kit v3 (Luminex) to determine CYP2D6 metabolizer phenotype (poor, intermediate, rapid and ultrarapid). Subjects from the cohort were categorized into two groups according to the occurrence of side effects (Case) or benefit (Control) after chronic analgesic treatment. The impact of CYP2D6 polymorphism on treatment outcome was tested at the metabolizer phenotype, diplotype and haplotype levels. Results: CYP2D6 polymorphism was significantly associated with opioid treatment outcome (Omnibus P=0.018, for both global haplotype and diplotype distribution test). CYP2D6*6 and *9 carriers, alleles characterized by a reduced (*9) or absent (*6) enzymatic activity, were significantly (P<0.05) associated with therapeutic failure. CYP2D6 ultrarapid metabolizers (CYP2D6*2N patients) showed an increased risk of side effects, as would be predicted. Despite their low frequency, CYP2D6 *1/*11, *4/*6 and *41/* 2N diplotypes showed significant (P<0.05) associations of efficacy and side effects with chronic opioid treatment. Conclusion: Our results showed that reduced CYP2D6 activity is correlated with lack of therapeutic effect. We found that the pharmacogenetic analysis of CYP2D6 could be helpful in foreseeing the safety and effectiveness of codeine or oxycodone treatment in CLBP.