Dual role of hydrogen sulfide in mechanical inflammatory hypernociception

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2 ), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2 S), and hydrogen (H2 ) with direct effects, and carbon dioxide (CO2 ) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

Intelligent polymeric hydrogen sulfide delivery systems for therapeutic applications

Hydrogen sulfide (H₂S) plays an important role in regulating various pathological processes such as protecting mammalian cell from harmful injuries, promoting tissue regeneration, and regulating the process of various diseases caused by physiological disorders. Studies have revealed that the physiological effects of H₂S are highly associated with its concentrations. At relatively low concentration, H₂S shows beneficial functions. However, long-time and high-dose donation of H₂S would inhibit regular biological process, resulting in cell dysfunction and apoptosis. To regulate the dosage of H₂S delivery for precision medicine, H₂S delivery systems with intelligent characteristics were developed and a variety of biocompatibility polymers have been utilized to establish intelligent polymeric H₂S delivery systems, with the abilities to specifically target the lesions, smartly respond to pathological microenvironments, as well as real-timely monitor H₂S delivery and lesion conditions by incorporating imaging-capable moieties. In this review, we focus on the design, preparation, and therapeutic applications of intelligent polymeric H₂S delivery systems in cardiovascular therapy, inflammatory therapy, tissue regenerative therapy, cancer therapy and bacteria-associated therapy. Strategies for precise H₂S therapies especially imaging-guided H₂S theranostics are highlighted. Since H₂S donors with stimuli-responsive characters are vital components for establishing intelligent H₂S delivery systems, the development of H₂S donors is also briefly introduced.

•

H₂S is an endogenous gasotransmitter that plays important role in regulating various physiological and pathological pathways.

•

Controlled H₂S delivery is vital since the therapeutic effects of H₂S are highly associated with its concentrations.

•

Intelligent polymeric H₂S delivery systems possess specific targeting, stimuli responsive and imaging guided capabilities, representing a strategic option for next generation of therapies.

Synthesis of 2-(2-(4-thioxo-3H-1,2-dithiole-5-yl) phenoxy)ethyl)isoindole-1,3-thione, a novel hydrogen sulfide-releasing phthalimide hybrid, and evaluation of its activity in models of inflammatory pain

Hydrogen sulfide (H₂S) is a gaseous mediator that modulates several physiological and pathological processes. Phthalimide analogues, substances that have the phthalimide ring in the structure, belong to the group of thalidomide analogues. Both H₂S donors and phthalimide analogues exhibit activities in models of inflammation and pain. As molecular hybridization is an important strategy aiming to develop drugs with a better pharmacological profile, in the present study we synthesized a novel H₂S-releasing phthalimide hybrid, 2-(2-(4-thioxo-3H-1,2-dithiole-5-yl) phenoxy)ethyl)isoindole-1,3-thione (PTD-H₂S), and evaluated its activity in models of inflammatory pain in mice. Per os (p.o.) administration of PTD-H₂S (125 or 250 mg/kg) reduced mechanical allodynia induced by carrageenan and lipopolysaccharide. Intraperitoneal (i.p.) administration of PTD-H₂S (25 mg/kg), but not equimolar doses of its precursors 5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione (14.2 mg/kg) and 2-phthalimidethanol (12 mg/kg), reduced mechanical allodynia induced by lipopolysaccharide. The antiallodynic effect induced by PTD-H₂S (25 mg/kg, i.p.) was more sustained than that induced by the H₂S donor NaHS (8 mg/kg, i.p.). Previous administration of hydroxocobalamin (300 mg/kg, i.p.) or glibenclamide (40 mg/kg, p.o.) attenuated PTD-H₂S antiallodynic activity. In conclusion, we synthesized a novel H₂S-releasing phthalimide hybrid and demonstrated its activity in models of inflammatory pain. PTD-H₂S activity may be due to H₂S release and activation of ATP-sensitive potassium channels. The demonstration of PTD-H₂S activity in models of pain stimulates further studies aiming to evaluate H₂S-releasing phthalimide hybrids as candidates for analgesic drugs.

Dual Sigma-1 receptor antagonists and hydrogen sulfide-releasing compounds for pain treatment: Design, synthesis, and pharmacological evaluation

The development of σ1 receptor antagonists hybridized with a H2S-donor is here reported. We aimed to obtain improved analgesic effects when compared to σ1 receptor antagonists or H2S-donors alone. In an in vivo model of sensory hypersensitivity, thioamide 1a induced analgesia which was synergistically enhanced when associated with the σ1 receptor antagonist BD-1063. The selective σ1 receptor agonist PRE-084 completely reversed this effect. Four thioamide H2S-σ1 receptor hybrids (5a-8a) and their amide derivatives (5b-8b) were synthesized. Compound 7a (AD164) robustly released H2S and showed selectivity for σ1 receptor over σ2 and opioid receptors. This compound induced marked analgesia that was reversed by PRE-084. The amide analogue 7b (AD163) showed only minimal analgesia. Further studies showed that 7a exhibited negligible acute toxicity, together with a favorable pharmacokinetic profile. To the best of our knowledge, compound 7a is the first dual-acting ligand with simultaneous H2S-release and σ1 antagonistic activities.

Emerging analytical tools for the detection of the third gasotransmitter H2S, a comprehensive review

BACKGROUND

Hydrogen sulfide (H2S) is currently considered among the endogenously produced gaseous molecules that exert various signaling effects in mammalian species. It is the third physiological gasotransmitter discovered so far after NO and CO. H2S was originally ranked among the toxic gases at elevated levels to humans. Currently, it is well-known that, in the cardiovascular system, H2S exerts several cardioprotective effects including vasodilation, antioxidant regulation, inhibition of inflammation, and activation of anti-apoptosis. With an increasing interest in monitoring H2S, the development of analysis methods should now follow.

AIM OF REVIEW

This review stages special emphasis on the several analytical technologies used for its determination including spectroscopic, chromatographic, and electrochemical methods. Advantages and limitations with regards to the application of each technique are highlighted with special emphasis on its employment for H2S in vivo measurement i.e., biofluids, tissues.

KEY SCIENTIFIC CONCEPTS AND IMPORTANT FINDINGS OF REVIEW

Fluorescence methods applied for H2S measurement offer an attractive non-invasive and promising approach in addition to its selectivity, however they cannot be considered as H2S-specific probes. On the other hand, colorimetric assays are among the most common methods used for in vitro H2S detection, albeit their employment in vivo H2S measurement has not yet been possible . Separation techniques such as gas or liquid chromatography offer higher selectivity compared to direct spectrophotometric or fluorescence methods especially for suitable for endpoint H2S measurements i.e. plasma or tissue samples. Despite all the developed analytical procedures used for H2S determination, the need for highly selective, much work should be devoted to resolve all the pitfalls of the current methods.

The Role of Hydrogen Sulfide in the Development of Tolerance and Dependence to Morphine in Mice

OBJECTIVE

Hydrogen sulfide is an endogenous gaseous mediator that has been indicated to have a role in pain mechanisms. In this study, we aimed to detect brain and spinal cord hydrogen sulfide levels during different phases of tolerance and dependence to morphine and to determine the effects of inhibition of endogenous hydrogen sulfide production on the development of tolerance and dependence.

METHODS

Morphine tolerance and dependence was developed by subcutaneous injection of morphine (10 mg/kg) twice daily for 12 days. Physical dependence was determined by counting the jumps for 20 min, which is a withdrawal symptom occurring after a single dose of naloxone (5 mg/kg) administered intraperitoneally (i.p.). Propargylglycine (30 mg/kg, i.p.), a cystathionine-γ-lyase inhibitor, and hydroxylamine (12.5 mg/kg, i.p.), a cystathionine-β-synthase inhibitor, were used as hydrogen sulfide synthase inhibitors. The tail-flick and hot-plate tests were used to determine the loss of antinociceptive effects of morphine and development of tolerance.

RESULTS

It was found that chronic and acute uses of both propargylglycine and hydroxylamine prevented the development of tolerance to morphine, whereas they had no effect on morphine dependence. Chronic and acute administrations of hydrogen sulfide synthase inhibitors did not exert any difference in hydrogen sulfide levels in brain and spinal cords of both morphine-tolerant and -dependent animals.

CONCLUSION

It has been concluded that hydrogen sulfide synthase inhibitors may have utility in preventing morphine tolerance.

Role of H2S in pain: Growing evidences of mystification

There have been studies suggesting the pain attenuating as well as pain inducing actions of hydrogen sulfide (H₂S). Exogenous administrated H₂S may be antinociceptive or pronociceptive, while the endogenous H₂S is pronociceptive. Experimental studies have shown that pharmacological inhibitors of H₂S biosynthetic enzymes may attenuate nociceptive as well as neuropathic pain. It suggests that nerve injury or inflammatory agents may induce the expression of H₂S biosynthetic enzymes to increase the endogenous production of H₂S, which acts as a pain neurotransmitter to produce pain. The endogenous H₂S may act through different mechanisms including opening of T-type calcium channels, activation of voltage-gated sodium channels, suppression of potassium channels, activation of TRPA1, TRPV1 and TRPC6 channels, upregulation of spinal NMDA receptors and sensitization of purinergic receptors. Exogenous administration of H₂S/precursors/donors attenuates or facilitates pain. It may be hypothesized that local administration of H₂S may cause pain; while it's systemic administration may attenuate pain. The doses of H₂S may also influence the pain response and H₂S in low doses may contribute in reducing pain, while H₂S in high doses may contribute in relieving pain. Accordingly, enzymatic inhibitors of H₂S synthesis or systemic administration of slow H₂S releasing agents/low dose H₂S donors may be useful in attenuating nociceptive and neuropathic pain. The present review describes the dual role of H₂S in pain attenuation and pain induction along with possible mechanisms.

Treatment with slow-releasing hydrogen sulfide donors inhibits the nociceptive and depressive-like behaviours accompanying chronic neuropathic pain: Endogenous antioxidant system activation

BACKGROUND

Therapies to treat chronic neuropathic pain and its associated comorbidities are limited. Recent studies demonstrated that the administration of slow-releasing hydrogen sulfide (H₂S) donors inhibited chemotherapy-induced neuropathic pain. However, the antidepressant or anxiolytic effects of these compounds and their mechanisms of action during chronic neuropathic pain have not been evaluated.

AIMS

To determine whether the administration of two slow-releasing H₂S donors, allyl isothiocyanate (A-ITC) and phenyl isothiocyanate (P-ITC), inhibits the nociceptive and emotional disorders associated with chronic neuropathic pain.

METHODS

In C57BL/6 male mice with neuropathic pain caused by the chronic constriction of the sciatic nerve, we assessed the effects of intraperitoneal administration of A-ITC and P-ITC in (a) the mechanical allodynia, thermal hyperalgesia and thermal allodynia induced by nerve ligation; (b) the anxiety- and depressive-like behaviours linked with neuropathic pain; (c) glial activation and mitogen-activated protein kinases phosphorylation, and (d) expression of the antioxidant enzymes, heme oxygenase 1 (HO-1), NADPH quinone oxidoreductase1, and glutathione S-transferase mu-1 (GSTM1), and alpha-1 (GSTA1), in hippocampus and prefrontal cortex (PFC).

RESULTS

Both treatments inhibited the allodynia and hyperalgesia, depressive-like behaviours, astroglial activation, and the extracellular signal-regulated kinase 1/2 phosphorylation but were unable to abolish the anxiety-like behaviours accompanying neuropathic pain. A-ITC and P-ITC also augmented the expression of HO-1, GSTM1, and GSTA1 in the hippocampus and/or PFC.

CONCLUSIONS

The administration of slow-releasing H₂S donors might be a promising treatment for the management of chronic neuropathic pain and some associated comorbidities via inhibiting the inflammatory and plasticity changes, and activating the endogenous antioxidant responses.

Hydrogen sulfide attenuates diabetic neuropathic pain through NO/cGMP/PKG pathway and μ-opioid receptor

IMPACT STATEMENT

There are currently approximately 425 million diabetic patients worldwide, of which approximately 90% of patients with diabetes suffer from neuropathy. Diabetic neuropathic pain (DNP) is a common complication of diabetic neuropathy. Nearly half of the patients hospitalized with diabetes have pain symptoms or symptoms related to neurological injury, and the incidence increases with age and diabetic duration. Anti-DNP analgesics have either limited therapeutic effects or serious side effects or lack of clinical trials, which has limited their application. Physiopathological mechanisms and treatment of DNP remain a significant challenge. The present confirmed that inhalation of H₂S may attenuate the diabetic neuropathic pain through NO/cGMP/PKG pathway and μ-opioid receptor. It provides us the animal study foundation for the application of H₂S on the treatment of DNP and clarifies some target molecules in the pain modulation of DNP.

Peripheral nitric oxide signaling directly blocks inflammatory pain

Pain is a classical sign of inflammation, and sensitization of primary sensory neurons (PSN) is the most important mediating mechanism. This mechanism involves direct action of inflammatory mediators such as prostaglandins and sympathetic amines. Pharmacologic control of inflammatory pain is based on two principal strategies: (i) non-steroidal anti-inflammatory drugs targeting inhibition of prostaglandin production by cyclooxygenases and preventing nociceptor sensitization in humans and animals; (ii) opioids and dipyrone that directly block nociceptor sensitization via activation of the NO signaling pathway. This review summarizes basic concepts of inflammatory pain that are necessary to understand the mechanisms of peripheral NO signaling that promote peripheral analgesia; we also discuss therapeutic perspectives based on the modulation of the NO pathway.

The Inhibitory Effects of Slow-Releasing Hydrogen Sulfide Donors in the Mechanical Allodynia, Grip Strength Deficits, and Depressive-Like Behaviors Associated with Chronic Osteoarthritis Pain

Osteoarthritis and its associated comorbidities are important clinical problems that have a negative impact on the quality of life, and its treatment remains unresolved. We investigated whether the systemic administration of slow-releasing hydrogen sulfide (H₂S) donors, allyl isothiocyanate (A-ITC) and phenyl isothiocyanate (P-ITC), alleviates chronic osteoarthritis pain and the associated emotional disorders. In C57BL/6 female mice with osteoarthritis pain induced by the intra-articular injection of monosodium iodoacetate, we evaluated the effects of repeated administration of A-ITC and P-ITC on the (i) mechanical allodynia and grip strength deficits; (ii) emotional conducts; and (iii) glial activity and expression of inducible nitric oxide synthase (NOS2), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), and antioxidant enzymes (heme oxygenase 1, NAD(P)H:quinone oxidoreductase-1, glutathione S-transferase mu 1 and alpha 1) in the hippocampus. The administration of A-ITC and P-ITC inhibited the mechanical allodynia, the grip strength deficits, and the depressive-like behaviors accompanying osteoarthritis. Both treatments inhibited microglial activation, normalized the upregulation of NOS2 and PI3K/p-Akt, and maintained high levels of antioxidant/detoxificant enzymes in the hippocampus. Data suggest that treatment with low doses of slow-releasing H₂S donors might be an interesting strategy for the treatment of nociception, functional disability, and emotional disorders associated with osteoarthritis pain.

The Study of Pain in Rats and Mice

Pain is a clinical syndrome arising from a variety of etiologies in a heterogeneous population, which makes successfully treating the individual patient difficult. Organizations and governments recognize the need for tailored and specific therapies, which drives pain research. This review summarizes the different types of pain assessments currently being used and the various rodent models that have been developed to recapitulate the human pain condition.

The role of neutrophils in neuro-immune modulation

Neutrophils are peripheral immune cells that represent the first recruited innate immune defense against infections and tissue injury. However, these cells can also induce overzealous responses and cause tissue damage. Although the role of neutrophils activating the immune system is well established, only recently their critical implications in neuro-immune interactions are becoming more relevant. Here, we review several aspects of neutrophils in the bidirectional regulation between the nervous and immune systems. First, the role of neutrophils as a diffuse source of acetylcholine and catecholamines is controversial as well as the effects of these neurotransmitters in neutrophil's functions. Second, neutrophils contribute for the activation and sensitization of sensory neurons, and thereby, in events of nociception and pain. In addition, nociceptor activation promotes an axon reflex triggering a local release of neural mediators and provoking neutrophil activation. Third, the recruitment of neutrophils in inflammatory responses in the nervous system suggests these immune cells as innovative targets in the treatment of central infectious, neurological and neurodegenerative disorders. Multidisciplinary studies involving immunologists and neuroscientists are required to define the role of the neurons-neutrophils communication in the pathophysiology of infectious, inflammatory, and neurological disorders.

4-Methylbenzenecarbothioamide, a hydrogen sulfide donor, inhibits tumor necrosis factor-α and CXCL1 production and exhibits activity in models of pain and inflammation

The gasotransmitter hydrogen sulfide (H₂S) is known to regulate many pathophysiological processes. Preclinical assays have demonstrated that H₂S donors exhibit anti-inflammatory and antinociceptive activities, characterized by reduction of inflammatory mediators production, leukocytes recruitment, edema and mechanical allodynia. In the present study, the effects induced by 4-methylbenzenecarbothioamide (4-MBC) in models of pain and inflammation in mice, the mechanisms mediating such effects and the H₂S-releasing property of this compound were evaluated. 4-MBC spontaneously released H₂S in vitro in the absence of organic thiols. Intraperitoneal (i.p.) administration of 4-MBC (100 or 150 mg/kg) reduced the second phase of the nociceptive response induced by formaldehyde and induced a long lasting inhibitory effect on carrageenan mechanical allodynia. 4-MBC antiallodynic effect was not affected by previous administration of naltrexone or glibenclamide. 4-MBC (50, 100 or 150 mg/kg, i.p.) induced a long lasting inhibitory effect on paw edema induced by carrageenan. The highest dose (150 mg/kg, i.p.) of 4-MBC inhibited tumor necrosis factor-α and CXCL1 production and myeloperoxidase activity induced by carrageenan. Mechanical allodynia and paw edema induced by carrageenan were not inhibited by the 4-MBC oxo analogue (p-toluamide). In summary, 4-MBC, an H₂S releasing thiobenzamide, exhibits antinociceptive and anti-inflammatory activities. These activities may be due to reduced cytokine and chemokine production and neutrophil recruitment. The H₂S releasing property is likely essential for 4-MBC activity. Our results indicate that 4-MBC may represent a useful pharmacological tool to investigate the biological roles of H₂S.

[Regulation of Cav3.2-mediated pain signals by hydrogen sulfide]

Hydrogen sulfide (H₂S), an endogenous gasotransmitter, is generated from L-cysteine by 3 distinct enzymes including cystathionine-γ-lyase (CSE), and targets multiple molecules, thereby playing various roles in health and disease. H₂S triggers or accelerates somatic pain and visceral nociceptive signals in the pancreas, colon and bladder by enhancing the activity of Ca_v3.2 T-type calcium channels. H₂S also activates TRPA1, which participates in H₂S-induced somatic pain signaling. However, Ca_v3.2 predominantly mediates colonic nociception by H₂S, because genetic deletion of TRPA1 does not reduce H₂S-induced colonic pain. The functional upregulation of the CSE/H₂S/Ca_v3.2 system is involved in neuropathic pain and visceral pain accompanying pancreatitis and cystitis. Ca_v3.2 also appears to participate in irritable bowel syndrome (IBS), although the role of endogenous H₂S generation by CSE in IBS is still open to question. In this review, we describe how H₂S regulates pain signals, particularly by interacting with Ca_v3.2.

Smoking-induced aggravation of experimental arthritis is dependent of aryl hydrocarbon receptor activation in Th17 cells

BACKGROUND

Epidemiologic studies have highlighted the association of environmental factors with the development and progression of autoimmune and chronic inflammatory diseases. Among the environmental factors, smoking has been associated with increased susceptibility and poor prognosis in rheumatoid arthritis (RA). However, the immune and molecular mechanism of smoking-induced arthritis aggravation remains unclear. The transcription factor aryl hydrocarbon receptor (AHR) regulates the generation of Th17 cells, CD4 T cells linked the development of autoimmune diseases. AHR is activated by organic compounds including polycyclic aromatic hydrocarbons (PAHs), which are environmental pollutants that are also present in cigarette smoke. In this study, we investigated the role of AHR activation in the aggravation of experiment arthritis induced by exposure to cigarette smoke.

METHODS

Mice were exposed to cigarette smoke during the developmental phase of antigen-induced arthritis and collagen-induced arthritis to evaluate the effects of smoking on disease development. Aggravation of articular inflammation was assessed by measuring neutrophil migration to the joints, increase in articular hyperalgesia and changes in the frequencies of Th17 cells. In vitro studies were performed to evaluate the direct effects of cigarette smoke and PAH on Th17 differentiation. We also used mice genetically deficient for AHR (Ahr KO) and IL-17Ra (Il17ra KO) to determine the in vivo mechanism of smoking-induced arthritis aggravation.

RESULTS

We found that smoking induces arthritis aggravation and increase in the frequencies of Th17 cells. The absence of IL-17 signaling (Il17ra KO) conferred protection to smoking-induced arthritis aggravation. Moreover, in vitro experiments showed that cigarette smoke can directly increase Th17 differentiation of T cells by inducing AHR activation. Indeed, Ahr KO mice were protected from cigarette smoke-induced arthritis aggravation and did not display increase in TH17 frequencies, suggesting that AHR activation is an important mechanism for cigarette smoke effects on arthritis. Finally, we demonstrate that PAHs are also able to induce arthritis aggravation.

CONCLUSIONS

Our data demonstrate that the disease-exacerbating effects of cigarette smoking are AHR dependent and environmental pollutants with AHR agonist activity can induce arthritis aggravation by directly enhancing Th17 cell development.

Cystathionine-γ-lyase ameliorates the histone demethylase JMJD3-mediated autoimmune response in rheumatoid arthritis

Cystathionine-γ-lyase (CSE), an enzyme associated with hydrogen sulfide (H₂S) production, is an important endogenous regulator of inflammation. Jumonji domain-containing protein 3 (JMJD3) is implicated in the immune response and inflammation. Here, we investigated the potential contribution of JMJD3 to endogenous CSE-mediated inflammation in rheumatoid arthritis (RA). Upregulated CSE and JMJD3 were identified in synovial fibroblasts (SFs) from RA patients as well as in the joints of arthritic mice. Knocking down CSE augmented inflammation in IL-1β-induced SFs by increasing JMJD3 expression. In addition, CSE^-/- mice with collagen-induced arthritis (CIA) developed severe joint inflammation and bone erosion. Conversely, overexpressing CSE inhibited JMJD3 expression by the transcription factor Sp-1 and was accompanied by reduced inflammation in IL-1β-treated SFs. Furthermore, JMJD3 silencing or the administration of the JMJD3 inhibitor GSK-J4 significantly decreased the inflammatory response in IL-1β-treated SFs, mainly by controlling the methylation status of H3K27me3 at the promoter of its target genes. GSK-J4 markedly attenuated the severity of arthritis in CIA mice. In conclusion, suppressing JMJD3 expression by the transcription factor Sp-1 is likely responsible for the ability of CSE to negatively modulate the inflammatory response and reduce the progression of RA.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

Neonatal Colonic Inflammation Increases Spinal Transmission and Cystathionine β-Synthetase Expression in Spinal Dorsal Horn of Rats with Visceral Hypersensitivity

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder characterized by chronic abdominal pain and alteration of bowel movements. The pathogenesis of visceral hypersensitivity in IBS patients remains largely unknown. Hydrogen sulfide (H₂S) is reported to play an important role in development of visceral hyperalgesia. However, the role of H₂S at spinal dorsal horn level remains elusive in visceral hypersensitivity. The aim of this study is designed to investigate how H₂S takes part in visceral hypersensitivity of adult rats with neonatal colonic inflammation (NCI). Visceral hypersensitivity was induced by neonatal colonic injection of diluted acetic acid. Expression of an endogenous H₂S synthesizing enzyme cystathionine β-synthetase (CBS) was determined by Western blot. Excitability and synaptic transmission of neurons in the substantia gelatinosa (SG) of spinal cord was recorded by patch clamping. Here, we showed that expression of CBS in the spinal dorsal horn was significantly upregulated in NCI rats. The frequency of glutamatergic synaptic activities in SG was markedly enhanced in NCI rats when compared with control rats. Application of NaHS increased the frequency of both spontaneous and miniature excitatory post-synaptic currents of SG neurons in control rats through a presynaptic mechanism. In contrast, application of AOAA, an inhibitor of CBS, dramatically suppressed the frequency of glutamatergic synaptic activities of SG neurons of NCI rats. Importantly, intrathecal injection of AOAA remarkably attenuated visceral hypersensitivity of NCI rats. These results suggest that H₂S modulates pain signaling likely through a presynaptic mechanism in SG of spinal dorsal horn, thus providing a potential therapeutic strategy for treatment for chronic visceral pain in patients with IBS.

International Union of Basic and Clinical Pharmacology. CII: Pharmacological Modulation of H2S Levels: H2S Donors and H2S Biosynthesis Inhibitors

Over the last decade, hydrogen sulfide (H2S) has emerged as an important endogenous gasotransmitter in mammalian cells and tissues. Similar to the previously characterized gasotransmitters nitric oxide and carbon monoxide, H2S is produced by various enzymatic reactions and regulates a host of physiologic and pathophysiological processes in various cells and tissues. H2S levels are decreased in a number of conditions (e.g., diabetes mellitus, ischemia, and aging) and are increased in other states (e.g., inflammation, critical illness, and cancer). Over the last decades, multiple approaches have been identified for the therapeutic exploitation of H2S, either based on H2S donation or inhibition of H2S biosynthesis. H2S donation can be achieved through the inhalation of H2S gas and/or the parenteral or enteral administration of so-called fast-releasing H2S donors (salts of H2S such as NaHS and Na2S) or slow-releasing H2S donors (GYY4137 being the prototypical compound used in hundreds of studies in vitro and in vivo). Recent work also identifies various donors with regulated H2S release profiles, including oxidant-triggered donors, pH-dependent donors, esterase-activated donors, and organelle-targeted (e.g., mitochondrial) compounds. There are also approaches where existing, clinically approved drugs of various classes (e.g., nonsteroidal anti-inflammatories) are coupled with H2S-donating groups (the most advanced compound in clinical trials is ATB-346, an H2S-donating derivative of the non-steroidal anti-inflammatory compound naproxen). For pharmacological inhibition of H2S synthesis, there are now several small molecule compounds targeting each of the three H2S-producing enzymes cystathionine-β-synthase (CBS), cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase. Although many of these compounds have their limitations (potency, selectivity), these molecules, especially in combination with genetic approaches, can be instrumental for the delineation of the biologic processes involving endogenous H2S production. Moreover, some of these compounds (e.g., cell-permeable prodrugs of the CBS inhibitor aminooxyacetate, or benserazide, a potentially repurposable CBS inhibitor) may serve as starting points for future clinical translation. The present article overviews the currently known H2S donors and H2S biosynthesis inhibitors, delineates their mode of action, and offers examples for their biologic effects and potential therapeutic utility.

Role of thiosulfate in hydrogen sulfide-dependent redox signaling in endothelial cells

Recent reports have revealed that hydrogen sulfide (H₂S) exerts critical actions to promote cardiovascular homeostasis and health. Thiosulfate is one of the products formed during oxidative H₂S metabolism, and thiosulfate has been used extensively and safely to treat calcific uremic arteriopathy in dialysis patients. Yet despite its significance, fundamental questions regarding how thiosulfate and H₂S interact during redox signaling remain unanswered. In the present study, we examined the effect of exogenous thiosulfate on hypoxia-induced H₂S metabolite bioavailability in human umbilical vein endothelial cells (HUVECs). Under hypoxic conditions, we observed a decrease of GSH and GSSG levels in HUVECs at 0.5 and 4 h as well as decreased free H₂S and acid-labile sulfide and increased bound sulfide at all time points. Treatment with exogenous thiosulfate significantly decreased the ratio of GSH/GSSG to total sulfide of HUVECs under 0.5 h of hypoxia but significantly increased this ratio in HUVECs under 4 h of hypoxia. These responses reveal that thiosulfate has different effects at low and high doses and under different O₂ tensions. In addition, treatment with thiosulfate also diminished VEGF-induced cystathionine-γ-lyase expression and reduced VEGF-induced HUVEC proliferation under both normoxic and hypoxic conditions. These results indicate that thiosulfate can modulate H₂S metabolites and signaling under various culture conditions that impact angiogenic activity. Thus, thiosulfate may serve as a unique sulfide donor to modulate endothelial responses under pathophysiological conditions involving angiogenesis.NEW & NOTEWORTHY This report provides new evidence that different levels of exogenous thiosulfate dynamically change discrete sulfide biochemical metabolite bioavailability in endothelial cells under normoxia or hypoxia, acting in a slow manner to modulate sulfide metabolites. Moreover, our findings also reveal that thiosulfate surprisingly inhibits VEGF-dependent endothelial cell proliferation associated with a reduction in cystathionine-γ-lyase protein levels.

Role of Gasotransmitters in Oxidative Stresses, Neuroinflammation, and Neuronal Repair

To date, three main gasotransmitters, that is, hydrogen sulfide (H₂S), carbon monoxide (CO), and nitric oxide (NO), have been discovered to play major bodily physiological roles. These gasotransmitters have multiple functional roles in the body including physiologic and pathologic functions with respect to the cellular or tissue quantities of these gases. Gasotransmitters were originally known to have only detrimental and noxious effects in the body but that notion has much changed with years; vast studies demonstrated that these gasotransmitters are precisely involved in the normal physiological functioning of the body. From neuromodulation, oxidative stress subjugation, and cardiovascular tone regulation to immunomodulation, these gases perform critical roles, which, should they deviate from the norm, can trigger the genesis of a number of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). The purpose of this review is to discuss at great length physical and chemical properties and physiological actions of H₂S, NO, and CO as well as shedding light on recently researched molecular targets. We particularly put emphasis on the roles in neuronal inflammation and neurodegeneration and neuronal repair.

Upregulation of spinal NMDA receptors mediates hydrogen sulfide-induced hyperalgesia

Hydrogen sulfide (H2S) is an endogenous neurotransmitter that importantly regulates various physiological and pathological events including pain signal transduction. In this study, we investigated the role of spinal NMDA receptors in the nociception induced by intraplantar injection of NaHS, an H2S donor. Intraplantar injection of NaHS into hindpaw significantly decreased the paw withdrawal threshold (PWT) in contralateral hindpaw. However, intraplantar formalin injection did not produce PWT in contralateral hindpaw. Intrathecal injection of methemoglobin, a H2S scavenger, abolished hyperalgesia induced by NaHS. In addition, NaHS-induced hyperalgesia was partly, but significantly, attenuated by intrathecal injection of hydroxylamine, a cystathionine-β-synthase (CBS) inhibitor. RT-PCR and western blotting analysis revealed that NR2B mRNA and protein levels were increased in the spinal dorsal horn, but not in dorsal root ganglion (DRG) in rats subjected to NaHS intraplantar injection. Collectively, these data suggest that peripheral injection of H2S donor causes hyperalgesia through increase in NR2B expression and production of H2S in the spinal cord.

Peripheral NLCR4 inflammasome participates in the genesis of acute inflammatory pain

Inflammatory hyperalgesia is a complex process that depends on the sensitization of primary nociceptive neurons triggered by proinflammatory mediators, such as interleukin 1β (IL-1β). Recently, the peripheral activation of caspase-1 (previously known as IL-1β-converting enzyme) was implicated in the induction of acute inflammatory pain by promoting the processing of IL-1β from its precursor form, pro-IL-1β. Caspase-1 activation in several systems requires the assembly of an intracellular molecular platform called an inflammasome. Inflammasomes consist of 1 nucleotide-binding oligomerization domain-like receptor (NLR), the adapter molecule apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC), and caspase-1. NLRP3 and NLRC4 inflammasomes are well described. However, the identity of the inflammasome that is involved in the peripheral activation of caspase-1 that accounts for acute inflammatory hyperalgesia has not been described. The present findings demonstrated that mice deficient in NLRC4 or ASC, but not in NLRP3, present reduced mechanical and thermal acute inflammatory hyperalgesia induced by carrageenan. The reduced hyperalgesia was accompanied by significant impairments in the levels of mature forms of IL-1β (p17) and caspase-1 (p20) compared to wild-type mice at the inflammatory site. Therefore, these results identified the inflammasome components NLRC4 and ASC as the molecular platform involved in the peripheral activation of caspase-1 and IL-1β maturation, which are responsible for the induction of acute inflammatory pain. In conclusion, our study provides new therapeutic targets for the control of acute inflammatory pain.

NOSH-aspirin (NBS-1120), a dual nitric oxide and hydrogen sulfide-releasing hybrid, reduces inflammatory pain

The development of nitric oxide (NO)- and hydrogen sulfide (H₂S)-releasing nonsteroidal anti-inflammatory drugs (NSAIDs) has generated more potent anti-inflammatory drugs with increased safety profiles. A new hybrid molecule incorporating both NO and H₂S donors into aspirin (NOSH-aspirin) was recently developed. In the present study, the antinociceptive activity of this novel molecule was compared with aspirin in different models of inflammatory pain. It was found that NOSH-aspirin inhibits acetic acid-induced writhing response and carrageenan (Cg)-induced inflammatory hyperalgesia in a dose-dependent (5–150 μmol/kg, v.o.) manner, which was superior to the effect of the same doses of aspirin. NOSH-aspirin’s antinociceptive effect was also greater and longer compared to aspirin upon complete Freund’s adjuvant (CFA)-induced inflammatory hyperalgesia. Mechanistically, NOSH-aspirin, but not aspirin, was able to reduce the production/release of interleukin-1 beta (IL-1β) during Cg-induced paw inflammation. Furthermore, NOSH-aspirin, but not aspirin, reduced prostaglandin E₂-induced hyperalgesia, which was prevented by treatment with a ATP-sensitive potassium channel (K_ATP) blocker (glibenclamide; glib.). Noteworthy, the antinociceptive effect of NOSH-aspirin was not associated with motor impairment. The present results indicate that NOSH-aspirin seems to present greater potency than aspirin to reduce inflammatory pain in several models. The enhanced effects of NOSH-aspirin seems to be due to its ability to reduce the production of pronociceptive cytokines such as IL-1 β and directly block hyperalgesia caused by a directly acting hyperalgesic mediator in a mechanism dependent on modulation of K_ATP channels. In conclusion, we would like to suggest that NOSH-aspirin represents a prototype of a new class of analgesic drugs with more potent effects than the traditional NSAID, aspirin.

Anti-inflammatory and cytoprotective actions of hydrogen sulfide: translation to therapeutics

SIGNIFICANCE

There is a rapidly expanding body of evidence for important roles of hydrogen sulfide in protecting against tissue injury, reducing inflammation, and promoting repair. There is also growing evidence that H2S can be successfully exploited in drug development.

RECENT ADVANCES

H2S synthesis and degradation are regulated in circumstances of inflammation and injury so as to promote repair and re-establish homeostasis. Novel H2S-releasing drugs exhibit enhanced anti-inflammatory and pro-restorative effects, while having reduced adverse effects in many tissues.

CRITICAL ISSUES

H2S is a pleiotropic mediator, having effects on many elements in the inflammatory cascade and promoting the resolution of inflammation and injury. It also contributes significantly to mucosal defence in the gastrointestinal tract, and in host defence against infection. There is strong evidence that novel, H2S-based therapeutics are safe and effective in animal models, and several are progressing through human trials.

FUTURE DIRECTIONS

A better understanding of the physiological and pathophysiological roles of H2S continues to be restrained by the lack of simple, reliable methods for measurement of H2S synthesis, and the paucity of highly selective inhibitors of enzymes that participate in endogenous H2S synthesis. On the other hand, H2S donors show promise as therapeutics for several important indications.

Anti-inflammatory and cytoprotective properties of hydrogen sulfide

Hydrogen sulfide is an endogenous gaseous mediator that plays important roles in many physiological processes in microbes, plants, and animals. This chapter focuses on the important roles of hydrogen sulfide in protecting tissues against injury, promoting the repair of damage, and downregulating the inflammatory responses. The chapter focuses largely, but not exclusively, on these roles of hydrogen sulfide in the gastrointestinal tract. Hydrogen sulfide is produced throughout the gastrointestinal tract, and it contributes to maintenance of mucosal integrity. Suppression of hydrogen sulfide synthesis renders the tissue more susceptible to injury and it impairs repair. In contrast, administration of hydrogen sulfide donors can increase resistance to injury and accelerate repair. Hydrogen sulfide synthesis is rapidly and dramatically enhanced in the gastrointestinal tract after injury is induced. These increases occur specifically at the site of tissue injury. Hydrogen sulfide also plays an important role in promoting resolution of inflammation, and restoration of normal tissue function. In recent years, these beneficial actions of hydrogen sulfide have provided the basis for development of novel hydrogen sulfide-releasing drugs. Nonsteroidal anti-inflammatory drugs that release small amounts of hydrogen sulfide are among the most advanced of the hydrogen sulfide-based drugs. Unlike the parent drugs, these modified drugs do not cause injury in the gastrointestinal tract, and do not interfere with healing of preexisting damage. Because of the increased safety profile of these drugs, they can be used in circumstances in which the toxicity of the parent drug would normally limit their use, such as in chemoprevention of cancer.

H2S and Pain: A Novel Aspect for Processing of Somatic, Visceral and Neuropathic Pain Signals

Hydrogen sulfide (H2S) formed by multiple enzymes including cystathionine-γ-lyase (CSE) targets Cav3.2 T-type Ca2+ channels (T-channels) and transient receptor potential ankyrin-1 (TRPA1). Intraplantar and intracolonic administration of H2S donors promotes somatic and visceral pain, respectively, via activation of Cav3.2 and TRPA1 in rats and/or mice. Injection of H2S donors into the plantar tissues, pancreatic duct, colonic lumen, or bladder causes T-channel-dependent excitation of nociceptors, determined as phosphorylation of ERK or expression of Fos in the spinal dorsal horn. Electrophysiological studies demonstrate that exogenous and/or endogenous H2S facilitates membrane currents through T-channels in NG108-15 cells and isolated mouse dorsal root ganglion (DRG) neurons that abundantly express Cav3.2 and also in Cav3.2-transfected HEK293 cells. In mice with cerulein-induced pancreatitis and cyclophosphamide-induced cystitis, visceral pain and/or referred hyperalgesia are inhibited by CSE inhibitors and by pharmacological blockade or genetic silencing of Cav3.2, and CSE protein is upregulated in the pancreas and bladder. In rats with neuropathy induced by L5 spinal nerve cutting or by repeated administration of paclitaxel, an anticancer drug, the neuropathic hyperalgesia is reversed by inhibitors of CSE or T-channels and by silencing of Cav3.2. Upregulation of Cav3.2 protein in DRG is detectable in the former, but not in the latter, neuropathic pain models. Thus, H2S appears to function as a nociceptive messenger by facilitating functions of Cav3.2 and TRPA1, and the enhanced function of the CSE/H2S/Cav3.2 pathway is considered to be involved in the pancreatitis- and cystitis-related pain and in neuropathic pain.

Exogenous hydrogen sulfide causes different hemodynamic effects in normotensive and hypertensive rats via neurogenic mechanisms

BACKGROUND

Increasing evidence suggests that disturbances in H2S homeostasis may participate in the development of hypertension. In this study we compared hemodynamic responses to intracerebroventricular (ICV) infusions of sodium hydrosulfide (NaHS), a H2S donor, between normotensive rats (WKY), spontaneously hypertensive rats (SHR) and angiotensin II - induced hypertensive rats (WKY-Ang II).

METHODS

We tested the effects of NaHS on mean arterial blood pressure (MABP) and heart rate (HR) in 12-14-week-old, male rats. MABP and HR were continuously recorded at baseline and during ICV infusion of either vehicle (Krebs-Henseleit buffer) or NaHS.

RESULTS

ICV infusions of the vehicle did not affect MABP and HR. WKY rats infused with 30 nmol/h of NaHS showed a mild decrease in MABP and HR. ICV infusion of 100 nmol/h produced a biphasic response i.e. mild hypotension and bradycardia followed by an increase in MABP and HR, whereas, the infusion of 300 nmol/h of the H2S donor caused a monophasic increases in MABP and HR. In contrast, SHR rats as well as WKY-Ang II rats showed a decrease in MABP and HR during ICV infusions of NaHS.

CONCLUSIONS

The results provide further evidence for the involvement of H2S in the neurogenic regulation of the circulatory system and suggest that alterations in H2S signaling in the brain could be associated with hypertension.

Lipopolysaccharide induces inflammatory hyperalgesia triggering a TLR4/MyD88-dependent cytokine cascade in the mice paw

Inflammatory pain can be triggered by different stimuli, such as trauma, radiation, antigen and infection. In a model of inflammatory pain caused by infection, injection in the mice paw of lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) agonist, produces mechanical hyperalgesia. We identify here the TLR4 linked signaling pathways that elicit this response. Firstly, LPS paw injection in wild type (WT) mice produced mechanical hyperalgesia that was not altered in TRIF^-/- mice. On the other hand, this response was absent in TLR4 mutant and MyD88 null mice and reduced in TNFR1 null mice. Either an IL-1 receptor antagonist, anti-KC/CXCL1 antibody, indomethacin or guanethidine injection also lessened this response. Moreover, LPS-induced time dependent increases in TNF-α, KC/CXCL1 and IL-1β expression in the mice paw, which were absent in TLR4 mutant and MyD88 null mice. Furthermore, in TNFR1 deficient mice, the LPS-induced rises in KC/CXCL1 and IL-1β release were less than in their wild type counterpart. LPS also induced increase of myeloperoxidase activity in the paw skin, which was inhibited in TLR4 mutant and MyD88 null mice, and not altered in TRIF^-/- mice. These results suggest that LPS-induced inflammatory pain in mice is solely dependent on the TLR4/MyD88 rather than the TLR4/TRIF signaling pathway. This pathway triggers pronociceptive cytokine TNF-α release that in turn mediates rises in KC/CXCL1 and IL-1β expression. Finally, these cytokines might be involved in stimulating production of directly-acting hyperalgesic mediators such as prostaglandins and sympathomimetic amine.

A hypothesis: hydrogen sulfide might be neuroprotective against subarachnoid hemorrhage induced brain injury

Gases such as nitric oxide (NO) and carbon monoxide (CO) play important roles both in normal physiology and in disease. Recent studies have shown that hydrogen sulfide (H₂S) protects neurons against oxidative stress and ischemia-reperfusion injury and attenuates lipopolysaccharides (LPS) induced neuroinflammation in microglia, exhibiting anti-inflammatory and antiapoptotic activities. The gas H₂S is emerging as a novel regulator of important physiologic functions such as arterial diameter, blood flow, and leukocyte adhesion. It has been known that multiple factors, including oxidative stress, free radicals, and neuronal nitric oxide synthesis as well as abnormal inflammatory responses, are involved in the mechanism underlying the brain injury after subarachnoid hemorrhage (SAH). Based on the multiple physiologic functions of H₂S, we speculate that it might be a promising, effective, and specific therapy for brain injury after SAH.

Hydrogen sulfide and translational medicine

Hydrogen sulfide (H2S) along with carbon monoxide and nitric oxide is an important signaling molecule that has undergone large numbers of fundamental investigations. H2S is involved in various physiological activities associated with the regulation of homeostasis, vascular contractility, pro- and anti-inflammatory activities, as well as pro- and anti-apoptotic activities etc. However, the actions of H2S are influenced by its concentration, reaction time, and cell/disease types. Therefore, H2S is a signaling molecule without definite effect. The use of existing H2S donors is limited because of the instant release and short lifetime of H2S. Thus, translational medicine involving the sustained and controlled release of H2S is of great value for both scientific and clinical uses. H2S donation can be manipulated by different ways, including where H2S is given, how H2S is donated, or the specific structures of H2S-releasing drugs and H2S donor molecules. This review briefly summarizes recent progress in research on the physiological and pathological functions of H2S and H2S-releasing drugs, and suggests hope for future investigations.

Thrombospondin-1 is a CD47-dependent endogenous inhibitor of hydrogen sulfide signaling in T cell activation

Thrombospondin-1 is a potent suppressor of T cell activation via its receptor CD47. However, the precise mechanism for this inhibition remains unclear. Because H2S is an endogenous potentiator of T cell activation and is necessary for full T cell activation, we hypothesized that thrombospondin-1 signaling through CD47 inhibits T cell activation by antagonizing H2S signaling. Primary T cells from thrombospondin-1 null mice were more sensitive to H2S-dependent activation assessed by proliferation and induction of interleukin-2 and CD69 mRNAs. Exogenous thrombospondin-1 inhibited H2S responses in wild type and thrombospondin-1 null T cells but enhanced the same responses in CD47 null T cells. Fibronectin, which shares integrin and glycosaminoglycan binding properties with thrombospondin-1 but not CD47 binding, did not inhibit H2S signaling. A CD47-binding peptide derived from thrombospondin-1 inhibited H2S-induced activation, whereas two other functional sequences from thrombospondin-1 enhanced H2S signaling. Therefore, engaging CD47 is necessary and sufficient for thrombospondin-1 to inhibit H2S-dependent T cell activation. H2S stimulated T cell activation by potentiating MEK-dependent ERK phosphorylation, and thrombospondin-1 inhibited this signaling in a CD47-dependent manner. Thrombospondin-1 also limited activation-dependent T cell expression of the H2S biosynthetic enzymes cystathionine β-synthase and cystathionine γ-lyase, thereby limiting the autocrine role of H2S in T cell activation. Thus, thrombospondin-1 signaling through CD47 is the first identified endogenous inhibitor of H2S signaling and constitutes a novel mechanism that negatively regulates T cell activation.

The hydrogen sulfide donor, Lawesson's reagent, prevents alendronate-induced gastric damage in rats

Our objective was to investigate the protective effect of Lawesson's reagent, an H2S donor, against alendronate (ALD)-induced gastric damage in rats. Rats were pretreated with saline or Lawesson's reagent (3, 9, or 27 µmol/kg, po) once daily for 4 days. After 30 min, gastric damage was induced by ALD (30 mg/kg) administration by gavage. On the last day of treatment, the animals were killed 4 h after ALD administration. Gastric lesions were measured using a computer planimetry program, and gastric corpus pieces were assayed for malondialdehyde (MDA), glutathione (GSH), proinflammatory cytokines [tumor necrosis factor (TNF)-α and interleukin (IL)-1β], and myeloperoxidase (MPO). Other groups were pretreated with glibenclamide (5 mg/kg, ip) or with glibenclamide (5 mg/kg, ip)+diazoxide (3 mg/kg, ip). After 1 h, 27 µmol/kg Lawesson's reagent was administered. After 30 min, 30 mg/kg ALD was administered. ALD caused gastric damage (63.35 ± 9.8 mm(2)); increased levels of TNF-α, IL-1β, and MDA (2311 ± 302.3 pg/mL, 901.9 ± 106.2 pg/mL, 121.1 ± 4.3 nmol/g, respectively); increased MPO activity (26.1 ± 3.8 U/mg); and reduced GSH levels (180.3 ± 21.9 µg/g). ALD also increased cystathionine-γ-lyase immunoreactivity in the gastric mucosa. Pretreatment with Lawesson's reagent (27 µmol/kg) attenuated ALD-mediated gastric damage (15.77 ± 5.3 mm(2)); reduced TNF-α, IL-1β, and MDA formation (1502 ± 150.2 pg/mL, 632.3 ± 43.4 pg/mL, 78.4 ± 7.6 nmol/g, respectively); lowered MPO activity (11.7 ± 2.8 U/mg); and increased the level of GSH in the gastric tissue (397.9 ± 40.2 µg/g). Glibenclamide alone reversed the gastric protective effect of Lawesson's reagent. However, glibenclamide plus diazoxide did not alter the effects of Lawesson's reagent. Our results suggest that Lawesson's reagent plays a protective role against ALD-induced gastric damage through mechanisms that depend at least in part on activation of ATP-sensitive potassium (KATP) channels.

Promoter demethylation of cystathionine-β-synthetase gene contributes to inflammatory pain in rats

Hydrogen sulfide (H(2)S), an endogenous gas molecule synthesized by cystathionine-β-synthetase (CBS), is involved in inflammation and nociceptive signaling. However, the molecular and epigenetic mechanisms of CBS-H(2)S signaling in peripheral nociceptive processing remain unknown. We demonstrated that peripheral inflammation induced by intraplantar injection of complete Freund adjuvant significantly up-regulated expression of CBS at both protein and mRNA levels in rat dorsal root ganglia (DRG). The CBS inhibitors hydroxylamine and aminooxyacetic acid attenuated mechanical hyperalgesia in a dose-dependent manner and reversed hyperexcitability of DRG neurons in inflamed rats. Intraplantar administration of NaHS (its addition mimics CBS production of H(2)S) or l-cysteine in healthy rats elicited mechanical hyperalgesia. Application of NaHS in vitro enhanced excitability and tetrodotoxin (TTX)-resistant sodium current of DRG neurons from healthy rats, which was attenuated by pretreatment of protein kinase A inhibitor H89. Methylation-specific PCR and bisulfite sequencing demonstrated that promoter region of cbs gene was less methylated in DRG samples from inflamed rats than that from controls. Peripheral inflammation did not alter expression of DNA methyltransferase 3a and 3b, the 2 major enzymes for DNA methylation, but led to a significant up-regulation of methyl-binding domain protein 4 and growth arrest and DNA damage inducible protein 45α, the enzymes involved in active DNA demethylation. Our findings suggest that epigenetic regulation of CBS expression may contribute to inflammatory hyperalgesia. H(2)S seems to increase TTX-resistant sodium channel current, which may be mediated by protein kinase A pathway, thus identifying a potential therapeutic target for the treatment of chronic pain.

Neutrophils recruited by CXCR1/2 signalling mediate post-incisional pain

BACKGROUND

Neutrophil recruitment mediated by the CXCL1/KC chemokine and its receptors CXCR1/CXCR2 plays a critical role in inflammatory diseases. Recently, neutrophil migration and activation triggered by CXCL1-CXCR1/2 signalling was implicated in inflammatory nociception; however, their role in post-surgical pain has not been elucidated. In this study, we addressed the function of neutrophils in the genesis of post-incisional pain in an experimental model of post-surgical pain.

METHODS

Mechanical hyperalgesia was determined with an electronic von Frey test in a mouse hindpaw incisional model. Neutrophil accumulation and the level of CXCL1/KC in the plantar tissue were determined by myeloperoxidase activity assay and enzyme-linked immunosorbent assay, respectively.

RESULTS

An incision in the mouse hindpaw produces long-lasting mechanical hyperalgesia that persists for at least 72 h after surgery. Following surgery, there was an increase in both neutrophil accumulation and the CXCL1/KC level in the incised paws. The depletion of the mouse neutrophils by vinblastine sulphate or anti-neutrophil antibody treatments reduced the mechanical hyperalgesia after paw incision. Furthermore, the treatment of mice with ladarixin, an orally acting CXCR1/2 antagonist, also reduced both the mechanical hyperalgesia and the infiltration of neutrophils in the incised paws.

CONCLUSION

In conclusion, it appears that after surgical processes, neutrophils are recruited by CXCL1-CXCR1/2 signalling and participate in the cascade of events, leading to mechanical hyperalgesia. These results suggest that blocking neutrophil migration through the inhibition of CXCL1-CXCR1/2 signalling might be a target to control post-surgical pain.

Hydrogen sulfide stimulates ischemic vascular remodeling through nitric oxide synthase and nitrite reduction activity regulating hypoxia-inducible factor-1α and vascular endothelial growth factor-dependent angiogenesis

Background

Hydrogen sulfide (H₂S) therapy is recognized as a modulator of vascular function during tissue ischemia with the notion of potential interactions of nitric oxide (NO) metabolism. However, little is known about specific biochemical mechanisms or the importance of H₂S activation of NO metabolism during ischemic tissue vascular remodeling. The goal of this study was to determine the effect of H₂S on NO metabolism during chronic tissue ischemia and subsequent effects on ischemic vascular remodeling responses.

Methods and Results

The unilateral, permanent femoral artery ligation model of hind‐limb ischemia was performed in C57BL/6J wild‐type and endothelial NO synthase–knockout mice to evaluate exogenous H₂S effects on NO bioavailability and ischemic revascularization. We found that H₂S selectively restored chronic ischemic tissue function and viability by enhancing NO production involving both endothelial NO synthase and nitrite reduction mechanisms. Importantly, H₂S increased ischemic tissue xanthine oxidase activity, hind‐limb blood flow, and angiogenesis, which were blunted by the xanthine oxidase inhibitor febuxostat. H₂S treatment increased ischemic tissue and endothelial cell hypoxia‐inducible factor‐1α expression and activity and vascular endothelial growth factor protein expression and function in a NO‐dependent manner that was required for ischemic vascular remodeling.

Conclusions

These data demonstrate that H₂S differentially regulates NO metabolism during chronic tissue ischemia, highlighting novel biochemical pathways to increase NO bioavailability for ischemic vascular remodeling.

TRPA1 has a key role in the somatic pro-nociceptive actions of hydrogen sulfide

Hydrogen sulfide (H₂S), which is produced endogenously from L-cysteine, is an irritant with pro-nociceptive actions. We have used measurements of intracellular calcium concentration, electrophysiology and behavioral measurements to show that the somatic pronociceptive actions of H₂S require TRPA1. A H₂S donor, NaHS, activated TRPA1 expressed in CHO cells and stimulated DRG neurons isolated from Trpa1^+/+ but not Trpa1^−/− mice. TRPA1 activation by NaHS was pH dependent with increased activity at acidic pH. The midpoint of the relationship between NaHS EC₅₀ values and external pH was pH 7.21, close to the expected dissociation constant for H₂S (pK_a 7.04). NaHS evoked single channel currents in inside-out and cell-attached membrane patches consistent with an intracellular site of action. In behavioral experiments, intraplantar administration of NaHS and L-cysteine evoked mechanical and cold hypersensitivities in Trpa1^+/+ but not in Trpa1^−/− mice. The sensitizing effects of L-cysteine in wild-type mice were inhibited by a cystathionine β-synthase inhibitor, D,L-propargylglycine (PAG), which inhibits H₂S formation. Mechanical hypersensitivity evoked by intraplantar injections of LPS was prevented by PAG and the TRPA1 antagonist AP-18 and was absent in Trpa1^−/− mice, indicating that H₂S mediated stimulation of TRPA1 is necessary for the local pronociceptive effects of LPS. The pro-nociceptive effects of intraplantar NaHS were retained in Trpv1^−/− mice ruling out TRPV1 as a molecular target. In behavioral studies, NaHS mediated sensitization was also inhibited by a T-type calcium channel inhibitor, mibefradil. In contrast to the effects of NaHS on somatic sensitivity, intracolonic NaHS administration evoked similar nociceptive effects in Trpa1^+/+ and Trpa1^−/− mice, suggesting that the visceral pro-nociceptive effects of H₂S are independent of TRPA1. In electrophysiological studies, the depolarizing actions of H₂S on isolated DRG neurons were inhibited by AP-18, but not by mibefradil indicating that the primary excitatory effect of H₂S on DRG neurons is TRPA1 mediated depolarization.

Hydrogen sulfide and inflammation: the good, the bad, the ugly and the promising

Hydrogen sulfide is rapidly gaining ground as a physiological mediator of inflammation, but there is no clear consensus as to its precise role in inflammatory signaling. This article discusses the disparate anti-inflammatory ('the good') and proinflammatory ('the bad') effects of endogenous and pharmacological H(2)S in disparate animal model and cell culture systems. We also discuss 'the ugly', such as problems of using wholly specific inhibitors of enzymatic H(2)S synthesis, and the use of pharmacological donor compounds, which release H(2)S too quickly to be physiologically representative of endogenous H(2)S synthesis. Furthermore, recently developed slow-release H(2)S donors, which offer a more physiological approach to understanding the complex role of H(2)S in acute and chronic inflammation ('the promising') are discussed.

Physiological implications of hydrogen sulfide: a whiff exploration that blossomed

The important life-supporting role of hydrogen sulfide (H(2)S) has evolved from bacteria to plants, invertebrates, vertebrates, and finally to mammals. Over the centuries, however, H(2)S had only been known for its toxicity and environmental hazard. Physiological importance of H(2)S has been appreciated for about a decade. It started by the discovery of endogenous H(2)S production in mammalian cells and gained momentum by typifying this gasotransmitter with a variety of physiological functions. The H(2)S-catalyzing enzymes are differentially expressed in cardiovascular, neuronal, immune, renal, respiratory, gastrointestinal, reproductive, liver, and endocrine systems and affect the functions of these systems through the production of H(2)S. The physiological functions of H(2)S are mediated by different molecular targets, such as different ion channels and signaling proteins. Alternations of H(2)S metabolism lead to an array of pathological disturbances in the form of hypertension, atherosclerosis, heart failure, diabetes, cirrhosis, inflammation, sepsis, neurodegenerative disease, erectile dysfunction, and asthma, to name a few. Many new technologies have been developed to detect endogenous H(2)S production, and novel H(2)S-delivery compounds have been invented to aid therapeutic intervention of diseases related to abnormal H(2)S metabolism. While acknowledging the challenges ahead, research on H(2)S physiology and medicine is entering an exponential exploration era.

Hydrogen sulfide: an endogenous mediator of resolution of inflammation and injury

SIGNIFICANCE

Hydrogen sulfide is emerging as an important mediator of many aspects of inflammation, and perhaps most importantly as a factor promoting the resolution of inflammation and repair of injury.

RECENT ADVANCES

In the gastrointestinal tract, H(2)S has been shown to promote healing of ulcers and the resolution of mucosal inflammation. On the other hand, suppression of endogenous H(2)S synthesis impairs mucosal defense and leads to increased granulocyte infiltration. H(2)S has been exploited in the design of more effective and safe anti-inflammatory drugs.

CRITICAL ISSUES

Enteric bacteria can be a significant source of H(2)S, which could affect mucosal integrity; indeed, luminal H(2)S can serve as an alternative to oxygen as a metabolic substrate for mitochondrial respiration in epithelial cells. Enterocytes and colonocytes thereby represent a "metabolic barrier" to the diffusion of bacteria-derived H(2)S into the subepithelial space. A compromise of this barrier could result in modulation of mucosal function and integrity by bacterial H(2)S.

FUTURE DIRECTIONS

Improvements in methods for measurement of H(2)S and development of more selective inhibitors are crucial for gaining a better understanding of the pathophysiological importance of this mediator. Results from animal studies suggest that H(2)S-releasing agents are promising therapeutic agents for many indications, but these compounds need to be assessed in a clinical setting.

Hydrogen sulfide is an endogenous potentiator of T cell activation

H(2)S is an endogenous signaling molecule that may act via protein sulfhydrylation to regulate various physiological functions. H(2)S is also a byproduct of dietary sulfate metabolism by gut bacteria. Inflammatory bowel diseases such as ulcerative colitis are associated with an increase in the colonization of the intestine by sulfate reducing bacteria along with an increase in H(2)S production. Consistent with its increased production, H(2)S is implicated as a mediator of ulcerative colitis both in its genesis or maintenance. As T cells are well established mediators of inflammatory bowel disease, we investigated the effect of H(2)S exposure on T cell activation. Using primary mouse T lymphocytes (CD3+), OT-II CD4+ T cells, and the human Jurkat T cell line, we show that physiological levels of H(2)S potentiate TCR-induced activation. Nanomolar levels of H(2)S (50-500 nM) enhance T cell activation assessed by CD69 expression, interleukin-2 expression, and CD25 levels. Exposure of T cells to H(2)S dose-dependently enhances TCR-stimulated proliferation with a maximum at 300 nM (30% increase, p < 0.01). Furthermore, activation increases the capacity of T cells to make H(2)S via increased expression of cystathionine γ-lyase and cystathionine β-synthase. Disrupting this response by silencing these H(2)S producing enzymes impairs T cell activation, and proliferation and can be rescued by the addition of 300 nM H(2)S. Thus, H(2)S represents a novel autocrine immunomodulatory molecule in T cells.