Pharmacological activation of heme oxygenase (HO)-1/carbon monoxide pathway prevents the development of peripheral neuropathic pain in Wistar rats

Recent progress on anti-nociceptive effects of carbon monoxide releasing molecule-2 (CORM-2)

The role of carbon monoxide (CO) has evolved albeit controversial disputes on its toxicity. This biological gasotransmitter participates in the endogenous regulation of neurotransmitters and neuropeptides released in the nervous system. Exogenous CO gas inhalation at a lower concentration has been the subject of investigations, which have revealed its biological homeostatic mechanisms and protective effects against many pathological conditions. This therapeutic procedure of CO is, however, limited due to its immediate release, which favours haemoglobin at a high affinity with the subsequent generation of toxic carboxyhaemoglobin in tissues. In order to address this problem, carbon monoxide releasing molecule-2 (CORM-2) or also known as tricarbonyldichlororuthenium II dimer is developed to liberate a controlled amount of CO in the biological systems. In this review, we examine several potential mechanisms exerted by this therapeutic compound to produce the anti-nociceptive effect that has been demonstrated in previous studies. This review could shed light on the role of CORM-2 to reduce pain, especially in cases of chronic and neuropathic pain.

Umbilical Cord Mesenchymal Stem Cells Overexpressing Heme Oxygenase-1 Promotes Symptoms Recovery in Cystitis Rats by Alleviating Neuroinflammation

Interstitial cystitis/bladder pain syndrome (IC/BPS) seriously reduces the patient's quality of life, yet current therapies only provide partial relief. In the spinal dorsal horn (SDH), neuroinflammation plays a pivotal role in the development of IC. Injection of human umbilical cord mesenchymal stem cells (hUMSCs) to reduce inflammation is an effective strategy, and heme oxygenase-1 (HO-1) exhibits anti-nociceptive effect in neuroinflammatory pain. This study aimed to test the therapeutic effects of hUMSCs overexpressing HO-1 on cyclophosphamide-induced cystitis rat model. Cystitis rats were transplanted with altered cells and then assessed for 3 weeks. A series of behavioral measurements would be trial including suprapubic mechanical allodynia, depressive-like behaviors, micturition frequency, and short-term memory function. Additionally, western blot, immunofluorescence staining, and ELISA kit test for anti-inflammation effect. HUMSCs were capable of being transduced to overexpress HO-1. Injection of hUMSCs overexpressing HO-1 was more effective than hUMSCs alone in alleviating behavioral symptoms in rats. Furthermore, hUMSCs overexpressing HO-1 inhibited the activation of glial and TLR4/p65/NLRP3 pathway, decreased the levels of pro-inflammatory cytokines in the SDH region. Surprisingly, it markedly increased anti-inflammatory cytokine IL-10, reduced MDA content, and protected GSH concentrations in local environment. Our results suggest that injecting hUMSCs overexpressing HO-1 intrathecally can significantly promote functional outcomes in cystitis rats by reducing neuroinflammation, at least, partly through downregulating TLR4/p65/NLRP3 signaling pathway in the SDH region. This cell therapy affords a new strategy for IC/BPS treatment.

Biological and Molecular Docking Evaluation of a Benzylisothiocyanate Semisynthetic Derivative From Moringa oleifera in a Pre-clinical Study of Temporomandibular Joint Pain

Objective

Moringa oleifera possesses multiple biological effects and the 4-[(4′-O-acetyl-α-L- rhamnosyloxy) benzyl] isothiocyanate accounts for them. Based on the original isothiocyanate molecule we obtained a semisynthetic derivative, named 4-[(2′,3′,4′-O-triacetyl-α-L-rhamnosyloxy) N-benzyl] hydrazine carbothioamide (MC-H) which was safe and effective in a temporomandibular joint (TMJ) inflammatory hypernociception in rats. Therefore, considering that there is still a gap in the knowledge concerning the mechanisms of action through which the MC-H effects are mediated, this study aimed to investigate the involvement of the adhesion molecules (ICAM-1, CD55), the pathways heme oxygenase-1 (HO-1) and NO/cGMP/PKG/K⁺_ATP, and the central opioid receptors in the efficacy of the MC-H in a pre-clinical study of TMJ pain.

Methods

Molecular docking studies were performed to test the binding performance of MC-H against the ten targets of interest (ICAM-1, CD55, HO-1, iNOS, soluble cGMP, cGMP-dependent protein kinase (PKG), K⁺_ATP channel, mu (μ), kappa (κ), and delta (δ) opioid receptors). In in vivo studies, male Wistar rats were treated with MC-H 1 μg/kg before TMJ formalin injection and nociception was evaluated. Periarticular tissues were removed to assess ICAM-1 and CD55 protein levels by Western blotting. To investigate the role of HO-1 and NO/cGMP/PKG/K⁺_ATP pathways, the inhibitors ZnPP-IX, aminoguanidine, ODQ, KT5823, or glibenclamide were used. To study the involvement of opioid receptors, rats were pre-treated (15 min) with an intrathecal injection of non-selective inhibitor naloxone or with CTOP, naltrindole, or norbinaltorphimine.

Results

All interactions presented acceptable binding energy values (below −6.0 kcal/mol) which suggest MC-H might strongly bind to its molecular targets. MC-H reduced the protein levels of ICAM-1 and CD55 in periarticular tissues. ZnPP-IX, naloxone, CTOP, and naltrindole reversed the antinociceptive effect of MC-H.

Conclusion

MC-H demonstrated antinociceptive and anti-inflammatory effects peripherally by the activation of the HO-1 pathway, as well as through inhibition of the protein levels of adhesion molecules, and centrally by μ and δ opioid receptors.

A Journey into the Clinical Relevance of Heme Oxygenase 1 for Human Inflammatory Disease and Viral Clearance: Why Does It Matter on the COVID-19 Scene?

Heme oxygenase 1 (HO-1), the rate-limiting enzyme in heme degradation, is involved in the maintenance of cellular homeostasis, exerting a cytoprotective role by its antioxidative and anti-inflammatory functions. HO-1 and its end products, biliverdin, carbon monoxide and free iron (Fe²⁺), confer cytoprotection against inflammatory and oxidative injury. Additionally, HO-1 exerts antiviral properties against a diverse range of viral infections by interfering with replication or activating the interferon (IFN) pathway. Severe cases of coronavirus disease 2019 (COVID-19), an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), are characterized by systemic hyperinflammation, which, in some cases, leads to severe or fatal symptoms as a consequence of respiratory failure, lung and heart damage, kidney failure, and nervous system complications. This review summarizes the current research on the protective role of HO-1 in inflammatory diseases and against a wide range of viral infections, positioning HO-1 as an attractive target to ameliorate clinical manifestations during COVID-19.

Daphnetin inhibits spinal glial activation via Nrf2/HO-1/NF-κB signaling pathway and attenuates CFA-induced inflammatory pain

Daphnetin (7, 8-dihydroxycoumarin, DAPH), a coumarin derivative isolated from Daphne odora var., recently draws much more attention as a promising drug candidate to treat neuroinflammatory diseases due to its protective effects against neuroinflammation. However, itscontribution to chronic inflammatory pain is largely unknown. In the current work, we investigated the effects of DAPH in a murine model of inflammatory pain induced by complete Freund's adjuvant (CFA) and its possible underlying mechanisms. Our results showed that DAPH treatment significantly attenuated mechanical allodynia provoked by CFA. A profound inhibition of spinal glial activation, followed by attenuated expression levels of spinal pro-inflammatory cytokines, was observed in DAPH-treated inflammatory pain mice. Further study demonstrated that DAPH mediated negative regulation of spinal NF-κB pathway, as well as its preferential activation of Nrf2/HO-1 signaling pathway in inflammatory pain mice. This study, for the first time, indicated that DAPH might preventthe development of mechanical allodynia in mice with inflammatory pain. And more importantly, these data provide evidence for the potential application of DAPH in the treatment of chronic inflammatory pain.

The role of carbon monoxide, heme oxygenase 1, and the Nrf2 transcription factor in the modulation of chronic pain and their interactions with opioids and cannabinoids

Chronic pain and its associated comorbidities are difficult to treat, even when the most potent analgesic compounds are used. Thus, research on new strategies to effectively relieve nociceptive and/or emotional disorders accompanying chronic pain is essential. Several studies have demonstrated the anti-inflammatory and antinociceptive effects of different carbon monoxide-releasing molecules (CO-RMs), inducible heme oxygenase 1 (HO-1), and nuclear factor-2 erythroid factor-2 (Nrf2) transcription factor activators in several models of acute and chronic pain caused by inflammation, nerve injury or diabetes. More recently, the antidepressant and/or anxiolytic effects of several Nrf2 transcription factor inducers were demonstrated in a model of chronic neuropathic pain. These effects are mainly produced by inhibition of oxidative stress, inflammation, glial activation, mitogen-activated protein kinases and/or phosphoinositide 3-kinase/phospho-protein kinase B phosphorylation in the peripheral and/or central nervous system. Other studies also demonstrated that the analgesic effects of opioids and cannabinoids are improved when these drugs are coadministered with CO-RMs, HO-1 or Nrf2 activators in different preclinical pain models and that these improvements are generally mediated by upregulation or prevention of the downregulation of µ-opioid receptors, δ-opioid receptors and/or cannabinoid 2 receptors in the setting of chronic pain. We reviewed all these studies as well as studies on the mechanisms of action underlying the effects of CO-RMs, HO-1, and Nrf2 activators in chronic pain. In summary, activation of the Nrf2/HO-1/carbon monoxide signaling pathway alone and/or in combination with the administration of specific analgesics is a valid strategy for the treatment of chronic pain and some associated emotional disorders.

Levo-corydalmine Attenuates Vincristine-Induced Neuropathic Pain in Mice by Upregulating the Nrf2/HO-1/CO Pathway to Inhibit Connexin 43 Expression

Antimicrotubulin chemotherapeutic agents, including plant-derived vincaalkaloids such as vincristine, can cause peripheral neuropathic pain. Exogenously activated heme oxygenase 1 (HO-1) is a potential therapy for chemotherapy-induced neuroinflammation. In this study, we investigated a role for Nrf2/HO-1/CO in mediating vincristine-induced neuroinflammation by inhibiting connexin 43 (Cx43) production in the spinal cord following the intrathecal application of the HO-1 inducer protoporphyrin IX cobalt chloride (CoPP) or inhibitor protoporphyrin IX zinc (ZnPP), and we analyzed the underlying mechanisms by which levo-corydalmine (l-CDL, a tetrahydroprotoberberine) attenuates vincristine-induced pain. Treatment with levo-corydalmine or oxycodone hydrochloride (a semisynthetic opioid analgesic, used as a positive control) attenuated vincristine-induced persistent pain hypersensitivity and degeneration of the sciatic nerve. In addition, the increased prevalence of atypical mitochondria induced by vincristine was ameliorated by l-CDL in both A-fibers and C-fibers. Next, we evaluated whether nuclear factor E2-related factor 2 (Nrf2), an upstream activator of HO-1, directly bound to the HO-1 promoter sequence and degraded heme to produce carbon monoxide (CO) following stimulation with vincristine. Notably, l-CDL dose-dependently increased HO-1/CO expression by activating Nrf2 to inhibit Cx43 expression in both the spinal cord and in cultured astrocytes stimulated with TNF-α, corresponding to decreased Cx43-mediated hemichannel. Furthermore, l-CDL had no effect on Cx43 following the silencing of the HO-1 gene. Taken together, our findings reveal a novel mechanism by which Nrf2/HO-1/CO mediates Cx43 expression in vincristine-induced neuropathic pain. In addition, the present findings suggest that l-CDL likely protects against nerve damage and attenuates vincristine-induced neuroinflammation by upregulating Nrf2/HO-1/CO to inhibit Cx43 expression.

Lion's Mane Mushroom, Hericium erinaceus (Bull.: Fr.) Pers. Suppresses H2O2-Induced Oxidative Damage and LPS-Induced Inflammation in HT22 Hippocampal Neurons and BV2 Microglia

Oxidative stress and inflammation in neuron-glia system are key factors in the pathogenesis of neurodegenerative diseases. As synthetic drugs may cause side effects, natural products have gained recognition for the prevention or management of diseases. In this study, hot water (HE-HWA) and ethanolic (HE-ETH) extracts of the basidiocarps of Hericium erinaceus mushroom were investigated for their neuroprotective and anti-inflammatory activities against hydrogen peroxide (H₂O₂)-induced neurotoxicity in HT22 mouse hippocampal neurons and lipopolysaccharide (LPS)-induced BV2 microglial activation respectively. HE-ETH showed potent neuroprotective activity by significantly (p < 0.0001) increasing the viability of H₂O₂-treated neurons. This was accompanied by significant reduction in reactive oxygen species (ROS) (p < 0.05) and improvement of the antioxidant enzyme catalase (CAT) (p < 0.05) and glutathione (GSH) content (p < 0.01). Besides, HE-ETH significantly improved mitochondrial membrane potential (MMP) (p < 0.05) and ATP production (p < 0.0001) while reducing mitochondrial toxicity (p < 0.001), Bcl-2-associated X (Bax) gene expression (p < 0.05) and nuclear apoptosis (p < 0.0001). However, gene expression of Nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase 1 (HO-1) and NAD(P)H quinone dehydrogenase 1 (NQO1) were unaffected (p > 0.05). HE-ETH also significantly (p < 0.0001) reduced nitric oxide (NO) level in LPS-treated BV2 indicating an anti-inflammatory activity in the microglia. These findings demonstrated HE-ETH maybe a potential neuroprotective and anti-inflammatory agent in neuron-glia environment.

The Effects of Cobalt Protoporphyrin IX and Tricarbonyldichlororuthenium (II) Dimer Treatments and Its Interaction with Nitric Oxide in the Locus Coeruleus of Mice with Peripheral Inflammation

Heme oxygenase 1 (HO-1) and carbon monoxide were shown to normalize oxidative stress and inflammatory reactions induced by neuropathic pain in the central nervous system, but their effects in the locus coeruleus (LC) of animals with peripheral inflammation and their interaction with nitric oxide are unknown. In wild-type (WT) and knockout mice for neuronal (NOS1-KO) or inducible (NOS2-KO) nitric oxide synthases with inflammatory pain induced by complete Freund’s adjuvant (CFA), we assessed: (1) antinociceptive actions of cobalt protoporphyrin IX (CoPP), an HO-1 inducer; (2) effects of CoPP and tricarbonyldichlororuthenium(II) dimer (CORM-2), a carbon monoxide-liberating compound, on the expression of HO-1, NOS1, NOS2, CD11b/c, GFAP, and mitogen-activated protein kinases (MAPK) in the LC. CoPP reduced inflammatory pain in different time-dependent manners in WT and KO mice. Peripheral inflammation activated astroglia in the LC of all genotypes and increased the levels of NOS1 and phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK 1/2) in WT mice. CoPP and CORM-2 enhanced HO-1 and inhibited astroglial activation in all genotypes. Both treatments blocked NOS1 overexpression, and CoPP normalized ERK 1/2 activation. This study reveals an interaction between HO-1 and NOS1/NOS2 during peripheral inflammation and shows that CoPP and CORM-2 improved HO-1 expression and modulated the inflammatory and/or plasticity changes caused by peripheral inflammation in the LC.

Nanocarrier-mediated Delivery of CORM-2 Enhances Anti-allodynic and Anti-hyperalgesic Effects of CORM-2

Neuropathic pain is a devastating chronic condition and effective treatments are still lacking. Carbon monoxide-releasing molecule-2 (CORM-2) as a carbon monoxide (CO) carrier, exerts potent anti-neuropathic pain effects; however, its poor water solubility and short half-life hinder its clinical utility. Therefore, the aim of this study was to investigate whether CORM-2-loaded solid lipid nanoparticles (CORM-2-SLNs) enhance the anti-allodynic and anti-hyperalgesic effects of CORM-2 in a rat chronic constriction injury (CCI) model. CORM-2-SLNs were prepared using a nanotemplate engineering technique with slight modifications. The physiochemical properties of CORM-2-SLNs were characterized and CO release from CORM-2-SLNs was assessed using a myoglobin assay. CO was slowly released from CORM-2-SLNs, was observed, and the half-life of CO release was 50 times longer than that of CORM-2. In vivo results demonstrate that intraperitoneal administration of CORM-2-SLNs (5 and 10 mg/kg/day, ip) once daily for seven consecutive days significantly reduced the mechanical allodynia and mechanical hyperalgesia compared with CORM-2 (10 mg/kg/day, ip). RT-PCR and Western blot analyses on days 7 and 14, revealed that treatment with CORM-2-SLNs resulted in greater reductions in the CCI-elevated levels of heme-oxygenase-2 (HO-2); inducible nitric oxide synthase (iNOS); neuronal NOS (nNOS); and inflammatory mediators (TNF-α, IBA-1, and GFAP) in the spinal cord and dorsal root ganglions compared with treatment with CORM-2. In contrast, HO-1 and IL-10 were significantly increased in the CORM-2-SLN-treated group compared with the group treated with CORM-2. These data indicate that CORM-2-SLNs are superior to CORM-2-S in alleviating mechanical allodynia and mechanical hyperalgesia.

Administration of CORM-2 inhibits diabetic neuropathy but does not reduce dyslipidemia in diabetic mice

The antinociceptive effects of the carbon monoxide-releasing molecule tricarbonyldichlororuthenium (II) dimer (CORM-2) during chronic pain are well documented, but most of its possible side-effects remain poorly understood. In this work, we examine the impact of CORM-2 treatment on the lipoprotein profile and two main atheroprotective functions attributed to high-density lipoprotein (HDL) in a mouse model of type 1 diabetes while analyzing the effect of this drug on diabetic neuropathy. Streptozotocin (Stz)-induced diabetic mice treated with CORM-2 (Stz-CORM-2) or vehicle (Stz-vehicle) were used to evaluate the effect of this drug on the modulation of painful diabetic neuropathy using nociceptive behavioral tests. Plasma and tissue samples were used for chemical and functional analyses, as appropriate. Two main antiatherogenic properties of HDL, i.e., the ability of HDL to protect low-density lipoprotein (LDL) from oxidation and to promote reverse cholesterol transport from macrophages to the liver and feces in vivo (m-RCT), were also assessed. Stz-induced diabetic mice displayed hyperglycemia, dyslipidemia and pain hypersensitivity. The administration of 10 mg/kg CORM-2 during five consecutive days inhibited allodynia and hyperalgesia and significantly ameliorated spinal cord markers (Cybb and Bdkrb1expression) of neuropathic pain in Stz mice, but it did not reduce the combined dyslipidemia shown in Stz-treated mice. Its administration to Stz-treated mice led to a significant increase in the plasma levels of cholesterol (∼ 1.4-fold vs. Ctrl, ∼ 1.3- fold vs. Stz-vehicle; p < 0.05) and was attributed to significant elevations in both non-HDL (∼ 1.8-fold vs. Ctrl; ∼ 1.6-fold vs. Stz-vehicle; p < 0.05) and HDL cholesterol (∼ 1.3-fold vs. Ctrl, ∼ 1.2-fold vs. Stz-vehicle; p < 0.05). The increased HDL in plasma was not accompanied by a commensurate elevation in m-RCT in Stz-CORM-2 compared to Stz-vehicle mice; instead, it was worsened as revealed by decreased [³H]-tracer trafficking into the feces in vivo. Furthermore, the HDL-mediated protection against LDL oxidation ex vivo shown by the HDL isolated from Stz-CORM-2 mice did not differ from that obtained in Stz-vehicle mice. In conclusion, the antinociceptive effects produced by a high dose of CORM-2 were accompanied by antioxidative effects but were without favorable effects on the dyslipidemia manifested in diabetic mice.

H2 Treatment Attenuated Pain Behavior and Cytokine Release Through the HO-1/CO Pathway in a Rat Model of Neuropathic Pain

Neuropathic pain (NP) is characterized by persistent pain, tactile allodynia, or hyperalgesia. Peripheral nerve injury contributes to rapid progress of inflammatory response and simultaneously generates neuropathic pain. Hydrogen (H2) has anti-inflammation, anti-apoptosis, and anti-oxidative stress effects. Therefore, we hypothesized that H2 treatment could alleviate allodynic and hyperalgesic behaviors and the release of inflammatory factors in rats with neuropathic pain. Peripheral neuropathic pain was established by chronic constriction injury of sciatic nerve in rats. H2 was given twice through intraperitoneal injection at a daily dose of 10 mL/kg during days 1-7 after the operation. Hyperalgesia and allodynia were tested, pro-inflammatory factors of dorsal root ganglia (DRG) and the spinal cord were measured by enzyme-linked immunosorbent assay (ELISA) during days 1-14 after the operation, and heme oxygenase (HO)-1 messenger RNA (mRNA) and protein expression and activities were measured at day 14 after sciatic nerve injury in rats. After Sn (IV) protoporphyrin IX dihydrochloride (SnPP)-IX, hemin, and carbon monoxide-releasing molecule (CORM)-2 had been given for chronic constriction injury (CCI) in rats, the above indicators were assessed. We found that H2 clearly inhibited hyperalgesia and allodynia in neuropathic pain and also attenuated the pro-inflammatory cytokines TNF-α, IL-1β, and high-mobility group box (HMGB) 1. H2 improved HO-1 mRNA and protein expression and activities in the process of pain. SnPP-IX reversed the inhibitory effect of H2 on hyperalgesia and allodynia and on pro-inflammatory cytokines in DRG and the spinal cord. The antinociceptive and anti-inflammatory effects of H2 were involved in the activation of HO-1/CO signaling during neuropathic pain in rats.

Gasotransmitters in Vascular Complications of Diabetes

In the past decades three gaseous signaling molecules-so-called gasotransmitters-have been identified: nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S). These gasotransmitters are endogenously produced by different enzymes in various cell types and play an important role in physiology and disease. Despite their specific functions, all gasotransmitters share the capacity to reduce oxidative stress, induce angiogenesis, and promote vasorelaxation. In patients with diabetes, a lower bioavailability of the different gasotransmitters is observed when compared with healthy individuals. As yet, it is unknown whether this reduction precedes or results from diabetes. The increased risk for vascular disease in patients with diabetes, in combination with the extensive clinical, financial, and societal burden, calls for action to either prevent or improve the treatment of vascular complications. In this Perspective, we present a concise overview of the current data on the bioavailability of gasotransmitters in diabetes and their potential role in the development and progression of diabetes-associated microvascular (retinopathy, neuropathy, and nephropathy) and macrovascular (cerebrovascular, coronary artery, and peripheral arterial diseases) complications. Gasotransmitters appear to have both inhibitory and stimulatory effects in the course of vascular disease development. This Perspective concludes with a discussion on gasotransmitter-based interventions as a therapeutic option.

Involvement of heme oxygenase-1 in neuroprotection by sanguinarine against glutamate-triggered apoptosis in HT22 neuronal cells

Sanguinarine is a natural compound isolated from the roots of Macleaya cordata and M. microcarpa, has been reported to possess several biological activities such as anti-inflammatory and anti-oxidant effects. In the present study, we demonstrated that sanguinarine markedly induces the expression of HO-1 which leads to a neuroprotective response in mouse hippocampus-derived neuronal HT22 cells from apoptotic cell death induced by glutamate. Sanguinarine significantly attenuated the loss of mitochondrial function and membrane integrity associated with glutamate-induced neurotoxicity. Sanguinarine protected against glutamate-induced neurotoxicity through inhibition of HT22 cell apoptosis. JC-1 staining, which is a well-established measure of mitochondrial damage, was decreased after treatment with sanguinarine in glutamate-challenged HT22cells. In addition, sanguinarine diminished the intracellular accumulation of ROS and Ca(2+). Sanguinarine also induced HO-1, NQO-1 expression via activation of Nrf2. Additionally, we found that si RNA mediated knock-down of Nrf2 or HO-1 significantly inhibited sanguinarine-induced neuroprotective response. These findings revealed the therapeutic potential of sanguinarine in preventing the neurodegenerative diseases.

Epigenetic modification of DRG neuronal gene expression subsequent to nerve injury: etiological contribution to complex regional pain syndromes (Part I)

DRG is of importance in relaying painful stimulation to the higher pain centers and therefore could be a crucial target for early intervention aimed at suppressing primary afferent stimulation. Complex regional pain syndrome (CRPS) is a common pain condition with an unknown etiology. Recently added new information enriches our understanding of CRPS pathophysiology. Researches on genetics, biogenic amines, neurotransmitters, and mechanisms of pain modulation, central sensitization, and autonomic functions in CRPS revealed various abnormalities indicating that multiple factors and mechanisms are involved in the pathogenesis of CRPS. Epigenetics refers to mitotically and meiotically heritable changes in gene expression that do not affect the DNA sequence. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, neurotransmitter responsiveness, and analgesic sensitivity, they are likely key factors in the development of chronic pain. In this dyad review series, we systematically examine the nerve injury-related changes in the neurological system and their contribution to CRPS. In this part, we first reviewed and summarized the role of neural sensitization in DRG neurons in performing function in the context of pain processing. Particular emphasis is placed on the cellular and molecular changes after nerve injury as well as different models of inflammatory and neuropathic pain. These were considered as the potential molecular bases that underlie nerve injury-associated pathogenesis of CRPS.

Treatment with a carbon monoxide-releasing molecule inhibits chronic inflammatory pain in mice: nitric oxide contribution

RATIONALE

Carbon monoxide synthetized by inducible heme oxygenase (HO-1) exerts potent anti-inflammatory and antinociceptive effects during acute and neuropathic pain, but its role in the modulation of chronic inflammatory pain and the possible involvement of nitric oxide in this action remain unknown.

OBJECTIVES AND METHODS

The antiallodynic and antihyperalgesic effects of a carbon monoxide releasing molecule, tricarbonyldichlororuthenium(II) dimer (CORM-2), daily administered from days 4 to 14 after complete Freund's adjuvant (CFA) injection in wild-type (WT), neuronal (NOS1-KO), and inducible (NOS2-KO) nitric oxide synthases knockout mice, were evaluated using von Frey filaments and plantar tests. Effects of CORM-2 treatment on the expression of HO-1, NOS1, and NOS2 at 14 days after inflammation induction were assessed by Western blot.

RESULTS

Main inflammatory pain symptoms induced by CFA in WT, NOS1-KO, and NOS2-KO mice were significantly reduced in a time-dependent manner by CORM-2 treatment. In all genotypes, inflammation increased the dorsal root ganglia and paw expression of HO-1, but CORM-2 treatment only over-expressed this enzyme in the paw of all genotypes. The increased NOS1 expression induced by inflammation in WT mice was abolished by CORM-2 treatment, while there was no effect of the inflammation in neither CORM-2 treatment in the expression of NOS2 in WT and NOS1-KO mice.

CONCLUSIONS

CORM-2 treatment inhibits inflammatory pain through enhancing HO-1 paw expression in all genotypes and reducing NOS1 over-expression in WT mice. An interaction between HO-1/carbon monoxide and NOS1/nitric oxide systems was also demonstrated. CORM-2 treatment may represent a new approach for management chronic inflammatory pain.