Inhaled hydrogen sulfide prevents neuropathic pain after peripheral nerve injury in mice

Hydrogen Sulfide Signaling in the Tumor Microenvironment: Implications in Cancer Progression and Therapy

Significance: Cancer is a complex and heterotypic structure with a spatial organization that contributes to challenges in therapeutics. Enzymes associated with producing the gasotransmitter hydrogen sulfide (H2S) are differentially expressed in tumors. Indeed, critical and paradoxical roles have been attributed to H2S in cancer-promoting characteristics by targeting both cancer cells and their milieu. This review focuses on the evidence and knowledge gaps of H2S on the tumor redox microenvironment and the pharmacological effects of H2S donors on cancer biology. Recent Advances: Endogenous and pharmacological concentrations of H2S evoke different effects on the same cell type: physiological H2S concentrations have been associated with tumor development and progression. In contrast, pharmacological concentrations have been associated with anticancer effects. Critical Issues: The exact threshold between the promotion and inhibition of tumorigenesis by H2S is largely unknown. The main issues covered in this review include H2S-modulated signaling pathways that are critical for cancer cells, the potential effects of H2S on cellular components of the tumor microenvironment, temporal modulation of H2S in promoting or inhibiting tumor progression (similar to observed for inflammation), and pharmacological agents that modulate H2S and which could play a role in antineoplastic therapy. Future Directions: Given the complexity and heterogeneity of tumor composition, mechanistic studies on context-dependent pharmacological effects of H2S donors for cancer therapy are necessary. These studies must determine the critical signaling pathways and the cellular components involved to allow advances in the rational use of H2S donors as antineoplastic agents. Antioxid. Redox Signal. 40, 250-271.

Respiratory and nervous system effects of a hydrogen sulfide crisis in Carson, California

BACKGROUND

In October 2021, many residents in Carson, California experienced malodors, headaches, and respiratory symptoms. Hydrogen sulfide (H2S), a toxic odorous gas, was measured in Carson at concentrations up to 7000 parts per billion (ppb) and remained above California's acute air quality standard of 30 ppb for about a month. Research on how low- and medium-level H2S exposure affects the respiratory and nervous systems has yielded conflicting results, and few studies have examined the effects of subacute H2S exposure.

METHODS

We calculated daily rates of emergency department (ED) visits with various respiratory and nervous systems diagnosis codes in Carson area ZIP codes (≤6 km from event's epicenter) and in Los Angeles County ZIP codes >15 km from event's epicenter (control area). Using controlled interrupted time series, we compared ED visit rates during the month of the H2S crisis in Carson to the predicted rates had the incident not occurred, based on 2018-2021 ED trends, and controlling for ED visit rate changes in the control area.

RESULTS

We observed a 24 % increase in ED visit rate for all respiratory system diseases (rate ratio = 1.24, 95 % CI: 1.16, 1.32), a 38 % increase for asthma (RR = 1.38, 95 % CI: 1.26, 1.50), a 26 % increase for acute upper respiratory infections (RR = 1.26, 95 % CI: 1.13, 1.38), a 21 % increase for dizziness (RR = 1.21, 95 % CI: 1.04, 1.38), and a 25 % increase for migraines and headaches (RR = 1.25, 95 % CI: 1.13, 1.36) in the Carson area during the first month of the H2S event compared to the expected rates.

CONCLUSIONS

This H2S crisis was associated with increased ED visit rates for multiple respiratory and nervous system outcomes. Reducing H2S exposure and improving to response during H2S episodes may improve public health.

T-type calcium channel modulation by hydrogen sulfide in neuropathic pain conditions

Neuropathic pain can appear as a direct or indirect nerve damage lesion or disease that affects the somatosensory nervous system. If the neurons are damaged or indirectly stimulated, immune cells contribute significantly to inflammatory and neuropathic pain. After nerve injury, peripheral macrophages/spinal microglia accumulate around damaged neurons, producing endogenous hydrogen sulfide (H₂S) through the cystathionine-γ-lyase (CSE) enzyme. H₂S has a pronociceptive modulation on the Ca_v3.2 subtype, the predominant Ca_v3 isoform involved in pain processes. The present review provides relevant information about H₂S modulation on the Ca_v3.2 T-type channels in neuropathic pain conditions. We have discussed that the dual effect of H₂S on T-type channels is concentration-dependent, that is, an inhibitory effect is seen at low concentrations of 10 µM and an augmentation effect on T-current at 100 µM. The modulation mechanism of the Ca_v3.2 channel by H₂S involves the direct participation of the redox/Zn²⁺ affinity site located in the His191 in the extracellular loop of domain I of the channel, involving a group of extracellular cysteines, comprising C114, C123, C128, and C1333, that can modify the local redox environment. The indirect interaction pathways involve the regulation of the Ca_v3.2 channel through cytokines, kinases, and post-translational regulators of channel expression. The findings conclude that the CSE/H₂S/Ca_v3.2 pathway could be a promising therapeutic target for neuropathic pain disorders.

The Role of Hydrogen Sulfide in Regulation of Cell Death following Neurotrauma and Related Neurodegenerative and Psychiatric Diseases

Injuries of the central (CNS) and peripheral nervous system (PNS) are a serious problem of the modern healthcare system. The situation is complicated by the lack of clinically effective neuroprotective drugs that can protect damaged neurons and glial cells from death. In addition, people who have undergone neurotrauma often develop mental disorders and neurodegenerative diseases that worsen the quality of life up to severe disability and death. Hydrogen sulfide (H₂S) is a gaseous signaling molecule that performs various cellular functions in normal and pathological conditions. However, the role of H₂S in neurotrauma and mental disorders remains unexplored and sometimes controversial. In this large-scale review study, we examined the various biological effects of H₂S associated with survival and cell death in trauma to the brain, spinal cord, and PNS, and the signaling mechanisms underlying the pathogenesis of mental illnesses, such as cognitive impairment, encephalopathy, depression and anxiety disorders, epilepsy and chronic pain. We also studied the role of H₂S in the pathogenesis of neurodegenerative diseases: Alzheimer's disease (AD) and Parkinson's disease (PD). In addition, we reviewed the current state of the art study of H₂S donors as neuroprotectors and the possibility of their therapeutic uses in medicine. Our study showed that H₂S has great neuroprotective potential. H₂S reduces oxidative stress, lipid peroxidation, and neuroinflammation; inhibits processes associated with apoptosis, autophagy, ferroptosis and pyroptosis; prevents the destruction of the blood-brain barrier; increases the expression of neurotrophic factors; and models the activity of Ca²⁺ channels in neurotrauma. In addition, H₂S activates neuroprotective signaling pathways in psychiatric and neurodegenerative diseases. However, high levels of H₂S can cause cytotoxic effects. Thus, the development of H₂S-associated neuroprotectors seems to be especially relevant. However, so far, all H₂S modulators are at the stage of preclinical trials. Nevertheless, many of them show a high neuroprotective effect in various animal models of neurotrauma and related disorders. Despite the fact that our review is very extensive and detailed, it is well structured right down to the conclusions, which will allow researchers to quickly find the proper information they are interested in.

H2S Alleviates Neuropathic Pain in Mice by Nrf2 Signaling Pathway Activation

Neuropathic pain is a chronic pain caused by direct damage to the peripheral or central nervous system, characterized by hyperalgesia, allodynia, and spontaneous pain. Hydrogen sulfide (H2S) therapy has been applied for neuropathic pain treatment, although the underlying mechanisms remain unknown. In this study, we sought to ascertain whether H2S therapy could alleviate neuropathic pain in a model of chronic constriction injury (CCI) and, if so, the potential mechanism. A CCI model was established in mice through a spinal nerve ligation method. Intrathecal injection of NaHS was used to treat CCI model mice. The thermal paw withdrawal latency (TPWL) and mechanical paw withdrawal threshold (MPWT) were used for pain threshold evaluation in mice. A series of experiments including immunofluorescence, enzyme-linked immunosorbent assay, electrophysiological test, mitochondrial DNA (mtDNA) quantification, measurement of ATP content, demethylase activity, and western blot were performed to investigate the specific mechanism of H2S treatment in neuropathic pain. Mice with CCI exposure exhibited a decrease in MPWT and TPWL, an increase in IL-1β and TNF-α expressions, elevated eEPSP amplitude, an upregulation of mtDNA, and a reduction in ATP production, whereas H2S treatment significantly reversed these changes. Furthermore, CCI exposure induced a remarkable increase in vGlut2- and c-fos-positive as well as vGlut2- and Nrf2-positive cells, an increase in Nrf2 located in the nucleus, and an upregulation of H3K4 methylation, and H2S treatment further enhanced these changes. In addition, ML385, a selective Nrf2 inhibitor, reversed the neuroprotective effects of H2S. H2S treatment mitigates CCI-induced neuropathic pain in mice. This protective mechanism is possibly linked to the activation of the Nrf2 signaling pathway in vGlut2-positive cells.

Progress and perspective on hydrogen sulfide donors and their biomedical applications

Following the discovery of nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂ S) has been identified as the third gasotransmitter in humans. Increasing evidence have shown that H₂ S is of preventive or therapeutic effects on diverse pathological complications. As a consequence, it is of great significance to develop suitable approaches of H₂ S-based therapeutics for biomedical applications. H₂ S-releasing agents (H₂ S donors) play important roles in exploring and understanding the physiological functions of H₂ S. More importantly, accumulating studies have validated the theranostic potential of H₂ S donors in extensive repertoires of in vitro and in vivo disease models. Thus, it is imperative to summarize and update the literatures in this field. In this review, first, the background of H₂ S on its chemical and biological aspects is concisely introduced. Second, the studies regarding the H₂ S-releasing compounds are categorized and described, and accordingly, their H₂ S-donating mechanisms, biological applications, and therapeutic values are also comprehensively delineated and discussed. Necessary comparisons between related H₂ S donors are presented, and the drawbacks of many typical H₂ S donors are analyzed and revealed. Finally, several critical challenges encountered in the development of multifunctional H₂ S donors are discussed, and the direction of their future development as well as their biomedical applications is proposed. We expect that this review will reach extensive audiences across multiple disciplines and promote the innovation of H₂ S biomedicine.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

The Effects of Nuclear Factor Erythroid 2 (NFE2)-Related Factor 2 (Nrf2) Activation in Preclinical Models of Peripheral Neuropathic Pain

Oxidative stress, resulting from an imbalance between the formation of damaging free radicals and availability of protective antioxidants, can contribute to peripheral neuropathic pain conditions. Reactive oxygen and nitrogen species, as well as products of the mitochondrial metabolism such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, are common free radicals. Nuclear factor erythroid 2 (NFE2)-related factor 2 (Nrf2) is a transcription factor encoded by the NFE2L2 gene and is a member of the cap 'n' collar subfamily of basic region leucine zipper transcription factors. Under normal physiological conditions, Nrf2 remains bound to Kelch-like ECH-associated protein 1 in the cytoplasm that ultimately leads to proteasomal degradation. During peripheral neuropathy, Nrf2 can translocate to the nucleus, where it heterodimerizes with muscle aponeurosis fibromatosis proteins and binds to antioxidant response elements (AREs). It is becoming increasingly clear that the Nrf2 interaction with ARE leads to the transcription of several antioxidative enzymes that can ameliorate neuropathy and neuropathic pain in rodent models. Current evidence indicates that the antinociceptive effects of Nrf2 occur via reducing oxidative stress, neuroinflammation, and mitochondrial dysfunction. Here, we will summarize the preclinical evidence supporting the role of Nrf2 signaling pathways and Nrf2 inducers in alleviating peripheral neuropathic pain.

Glucoraphanin Increases Intracellular Hydrogen Sulfide (H2S) Levels and Stimulates Osteogenic Differentiation in Human Mesenchymal Stromal Cell

Osteopenia and osteoporosis are among the most prevalent consequences of ageing, urging the promotion of healthy nutritional habits as a tool in preventing bone fractures. Glucosinolates (GLSs) are organosulfur compounds considered relatively inert precursors of reactive derivatives isothiocyanates (ITCs). Recent evidence suggests that GLSs may exert biological properties based on their capacity to release hydrogen sulfide (H₂S). H₂S-donors are known to exert anabolic function on bone cells. Here, we investigated whether a GLS, glucoraphanin (GRA) obtained from Tuscan black kale, promotes osteogenesis in human mesenchymal stromal cells (hMSCs). H₂S release in buffer and intracellular H₂S levels were detected by amperometric measurements and fluorimetric/cytofluorimetric analyses, respectively. Alizarin red staining assay and real-time PCR were performed to evaluate mineral apposition and mRNA expression of osteogenic genes. Using an in vitro cell culture model, our data demonstrate a sulforaphane (SFN)-independent osteogenic stimulation of GRA in hMSCs, at least partially attributable to H₂S release. In particular, GRA upregulated the expression of osteogenic genes and enhanced mineral apposition while increasing intracellular concentrations of H₂S. Overall, this study suggests the feasibility of using cruciferous derivatives as natural alternatives to chemical H₂S-donors as adjuvant therapies in the treatment of bone-wasting diseases.

Dietary organosulfur compounds: Emerging players in the regulation of bone homeostasis by plant-derived molecules

The progressive decline of bone mass and the deterioration of bone microarchitecture are hallmarks of the bone aging. The resulting increase in bone fragility is the leading cause of bone fractures, a major cause of disability. As the frontline pharmacological treatments for osteoporosis suffer from low patients' adherence and occasional side effects, the importance of diet regimens for the prevention of excessive bone fragility has been increasingly recognized. Indeed, certain diet components have been already associated to a reduced fracture risk. Organosulfur compounds are a broad class of molecules containing sulfur. Among them, several molecules of potential therapeutic interest are found in edible plants belonging to the Allium and Brassica botanical genera. Polysulfides derived from Alliaceae and isothiocyanates derived from Brassicaceae hold remarkable nutraceutical potential as anti-inflammatory, antioxidants, vasorelaxant and hypolipemic. Some of these effects are linked to the ability to release the gasotrasmitter hydrogen sulfide (H₂S). Recent preclinical studies have investigated the effect of organosulfur compounds in bone wasting and metabolic bone diseases, revealing a strong potential to preserve skeletal health by exerting cytoprotection and stimulating the bone forming activity by osteoblasts and attenuating bone resorption by osteoclasts. This review is intended for revising evidence from preclinical and epidemiological studies on the skeletal effects of organosulfur molecules of dietary origin, with emphasis on the direct regulation of bone cells by plant-derived polysulfides, glucosinolates and isothiocyanates. Moreover, we highlight the potential molecular mechanisms underlying the biological role of these compounds and revise the importance of the so-called 'H₂S-system' on the regulation of bone homeostasis.

The Anxiolytic and Antidepressant Effects of Diallyl Disulfide and GYY4137 in Animals with Chronic Neuropathic Pain

When neuropathic pain is maintained long term, it can also lead to the development of emotional disorders that are even more intense than pain perception and difficult to treat. Hydrogen sulfide (H₂S) donors relieve chronic pain, but their effects on the associated mood disorders are not completely elucidated. We evaluated if treatment with DADS (diallyl disulfide) or GYY4137 (morpholin-4-ium 4-methoxyphenyl(morpholino) phosphinodithioate dichloromethane complex), two slow-releasing H₂S donors, inhibits the anxiety- and depressive-like behaviors that concur with chronic neuropathic pain generated by sciatic nerve injury in mice. The modulatory role of these drugs in the inflammatory, apoptotic, and oxidative processes implicated in the development of the affective disorders was assessed. Our results revealed the anxiolytic, antidepressant, and antinociceptive properties of DADS and GYY4137 during neuropathic pain by inhibiting microglial activation and the up-regulation of phosphoinositide 3-kinase/phosphorylated protein kinase B and BAX in the amygdala (AMG) and/or periaqueductal gray matter (PAG). Both treatments also normalized and/or activated the endogenous antioxidant system, but only DADS blocked ERK 1/2 phosphorylation. Both H₂S donors decreased allodynia and hyperalgesia in a dose-dependent manner by activating the Kv7 potassium channels and heme oxygenase 1 signaling pathways. This study provides evidence of the anxiolytic and antidepressant properties of DADS and GYY4137 during neuropathic pain and reveals their analgesic actions, suggesting that these therapeutic properties may result from the inhibition of the inflammatory, apoptotic, and oxidative responses in the AMG and/or PAG. These findings support the use of these treatments for the management of affective disorders accompanying chronic neuropathic pain.

Enhanced Analgesic Effects and Gastrointestinal Safety of a Novel, Hydrogen Sulfide-Releasing Anti-Inflammatory Drug (ATB-352): A Role for Endogenous Cannabinoids

Aims: The covalent linking of nonsteroidal anti-inflammatory drugs to a hydrogen sulfide (H₂S)-releasing moiety has been shown to dramatically reduce gastrointestinal (GI) damage and bleeding, as well as increase anti-inflammatory and analgesic potency. We have tested the hypothesis that an H₂S-releasing derivative of ketoprofen (ATB-352) would exhibit enhanced efficacy without significant GI damage in a mouse model of allodynia/hyperalgesia. Results: ATB-352 was significantly more potent and effective as an analgesic than ketoprofen and did not elicit GI damage. Pretreatment with an antagonist of the CB1 cannabinoid receptor (AM251) significantly reduced the analgesic effects of ATB-352. The CB1 antagonist exacerbated GI damage when coadministered with ketoprofen, but GI damage was not induced by the combination of ATB-352 and the CB1 antagonist. In vitro, ATB-352 was substantially more potent than ketoprofen as an inhibitor of fatty acid amide hydrolase, consistent with a contribution of endogenous cannabinoids to the analgesic effects of this drug. Blood anandamide levels were significantly depressed by ketoprofen, but remained unchanged after treatment with ATB-352. Innovation: Ketoprofen is a potent analgesic, but its clinical use, even in the short term, is significantly limited by its propensity to cause significant ulceration and bleeding in the GI tract. Covalently linking an H₂S-releasing moiety to ketoprofen profoundly reduces the GI toxicity of the drug, while boosting analgesic effectiveness. Conclusion: This study demonstrates a marked enhancement of the potency and effectiveness of ATB-352, an H₂S-releasing derivative of ketoprofen, in part, through the involvement of the endogenous cannabinoid system. This may have significant advantages for the control and management of pain, such as in a postoperative setting.

Protective Effects of Hydrogen Sulfide Against the ATP-Induced Meningeal Nociception

We previously showed that extracellular ATP and hydrogen sulfide (H₂S), a recently discovered gasotransmitter, are both triggering the nociceptive firing in trigeminal nociceptors implicated in migraine pain. ATP contributes to meningeal nociception by activating the P2X3 subunit-containing receptors whereas H₂S operates mainly via TRP receptors. However, H₂S was also proposed as a neuroprotective and anti-nociceptive agent. This study aimed to test the effect of H₂S on ATP-mediated nociceptive responses in rat meningeal afferents and trigeminal neurons and on ATP-induced degranulation of dural mast cells. Electrophysiological recording of trigeminal nerve activity in meninges was supplemented by patch-clamp and calcium imaging studies of isolated trigeminal neurons. The H₂S donor NaHS induced a mild activation of afferents and fully suppressed the subsequent ATP-induced firing of meningeal trigeminal nerve fibers. This anti-nociceptive effect of H₂S was specific as an even stronger effect of capsaicin did not abolish the action of ATP. In isolated trigeminal neurons, NaHS decreased the inward currents and calcium transients evoked by activation of ATP-gated P2X3 receptors. Moreover, NaHS prevented ATP-induced P2X7 receptor-mediated degranulation of meningeal mast cells which emerged as triggers of migraine pain. Finally, NaHS decreased the concentration of extracellular ATP in the meningeal preparation. Thus, H₂S exerted the multiple protective actions against the nociceptive effects of ATP. These data highlight the novel pathways to reduce purinergic mechanisms of migraine with pharmacological donors or by stimulation production of endogenous H₂S.

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.

Organic Isothiocyanates as Hydrogen Sulfide Donors

Significance: Hydrogen sulfide (H₂S), the "new entry" in the series of endogenous gasotransmitters, plays a fundamental role in regulating the biological functions of various organs and systems. Consequently, the lack of adequate levels of H₂S may represent the etiopathogenetic factor of multiple pathological alterations. In these diseases, the use of H₂S donors represents a precious and innovative opportunity. Recent Advances: Natural isothiocyanates (ITCs), sulfur compounds typical of some botanical species, have long been investigated because of their intriguing pharmacological profile. Recently, the ITC moiety has been proposed as a new H₂S-donor chemotype (with a l-cysteine-mediated reaction). Based on this recent discovery, we can clearly observe that almost all the effects of natural ITCs can be explained by the H₂S release. Consistently, the ITC function was also used as an original H₂S-releasing moiety for the design of synthetic H₂S donors and original "pharmacological hybrids." Very recently, the chemical mechanism of H₂S release, resulting from the reaction between l-cysteine and some ITCs, has been elucidated. Critical Issues: Available literature gives convincing demonstration that H₂S is the real player in ITC pharmacology. Further, countless studies have been carried out on natural ITCs, but this versatile moiety has been used only rarely for the design of synthetic H₂S donors with optimal drug-like properties. Future Directions: The development of more ITC-based synthetic H₂S donors with optimal drug-like properties and selectivity toward specific tissues/pathologies seem to represent a stimulating and indispensable prospect of future experimental activities.

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.

Efficacy of isothiocyanate-based compounds on different forms of persistent pain

Purpose

Current pharmacotherapy for persistent pain related to neuropathy or articular diseases is unsatisfactory, due to the large number of unresponsive patients and side effects. Isothiocyanates (ITCs) are a class of natural or synthetic compounds characterized by the general formula R–NCS. ITCs show antihyperalgesic effects in models of central and peripheral nervous tissue injury and anti-inflammatory properties. The pharmacodynamics are strictly related to the release of the gasotransmitter hydrogen sulfide (H₂S) from their moiety. In particular, phenyl ITC (PITC) and 3-carboxyphenyl ITC (3C-PITC) exhibit interesting slow H₂S-release properties suitable for treating painful pathology. The aim of the present work was to evaluate the efficacy of PITC and 3C-PITC against mechanical hyperalgesia and spontaneous pain induced by nerve injury and osteoarthritis.

Methods

Nerve injury and osteoarthritis were induced in rats by ligation of the sciatic nerve (chronic constriction injury) and intra-articular injection of monoiodoacetate, respectively. Behavioral tests were performed 14 days after damage induction.

Results

Single subcutaneous administrations of PITC, 3C-PITC (4.43 and 13.31 µmol kg⁻¹, respectively) were able to completely reverse hypersensitivity to noxious stimuli in both models of neuropathic and osteoarticular pain. The effect of ITCs was compared with that of NaHS, the prototypical H₂S donor, showing similar efficacy and higher potency. ITCs and NaHS also reduced spontaneous pain.

Conclusion

ITCs offer a promising novel approach to counteract persistent, drug-resistant painful pathology.

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.

Effects of natural and synthetic isothiocyanate-based H2S-releasers against chemotherapy-induced neuropathic pain: Role of Kv7 potassium channels

Hydrogen sulfide (H₂S) is a crucial signaling molecule involved in several physiological and pathological processes. Nonetheless, the role of this gasotransmitter in the pathogenesis and treatment of neuropathic pain is controversial. The aim of the present study was to investigate the pain relieving profile of a series of slow releasing H₂S donors (the natural allyl-isothiocyanate and the synthetics phenyl- and carboxyphenyl-isothiocyanate) in animal models of neuropathic pain induced by paclitaxel or oxaliplatin, anticancer drugs characterized by a dose-limiting neurotoxicity. The potential contribution of Kv7 potassium channels modulation was also studied. Mice were treated with paclitaxel (2.0 mg kg^-1) i.p. on days 1, 3, 5 and 7; oxaliplatin (2.4 mg kg^-1) was administered i.p. on days 1-2, 5-9, 12-14. Behavioral tests were performed on day 15. In both models, single subcutaneous administrations of H₂S donors (1.33, 4.43, 13.31 μmol kg^-1) reduced the hypersensitivity to cold non-noxious stimuli (allodynia-related measurement). The prototypical H₂S donor NaHS was also effective. Activity was maintained after i.c.v. administrations. On the contrary, the S-lacking molecule allyl-isocyanate did not increase pain threshold; the H₂S-binding molecule hemoglobin abolished the pain-relieving effects of isothiocyanates and NaHS. The anti-neuropathic properties of H₂S donors were reverted by the Kv7 potassium channel blocker XE991. Currents carried by Kv7.2 homomers and Kv7.2/Kv7.3 heteromers expressed in CHO cells were potentiated by H₂S donors. Sistemically- or centrally-administered isothiocyanates reduced chemotherapy-induced neuropathic pain by releasing H₂S. Activation of Kv7 channels largely mediate the anti-neuropathic effect.

Physiological Importance of Hydrogen Sulfide: Emerging Potent Neuroprotector and Neuromodulator

Hydrogen sulfide (H2S) is an emerging neuromodulator that is considered to be a gasotransmitter similar to nitrogen oxide (NO) and carbon monoxide (CO). H2S exerts universal cytoprotective effects and acts as a defense mechanism in organisms ranging from bacteria to mammals. It is produced by the enzymes cystathionine β-synthase (CBS), cystathionine ϒ-lyase (CSE), 3-mercaptopyruvate sulfurtransferase (MST), and D-amino acid oxidase (DAO), which are also involved in tissue-specific biochemical pathways for H2S production in the human body. H2S exerts a wide range of pathological and physiological functions in the human body, from endocrine system and cellular longevity to hepatic protection and kidney function. Previous studies have shown that H2S plays important roles in peripheral nerve regeneration and degeneration and has significant value during Schwann cell dedifferentiation and proliferation but it is also associated with axonal degradation and the remyelination of Schwann cells. To date, physiological and toxic levels of H2S in the human body remain unclear and most of the mechanisms of action underlying the effects of H2S have yet to be fully elucidated. The primary purpose of this review was to provide an overview of the role of H2S in the human body and to describe its beneficial effects.

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.