Noxious heat threshold temperature and pronociceptive effects of allyl isothiocyanate (mustard oil) in TRPV1 or TRPA1 gene-deleted mice

Hydrogen peroxide production by epidermal dual oxidase 1 regulates nociceptive sensory signals

Keratinocytes of the mammalian skin provide not only mechanical protection for the tissues, but also transmit mechanical, chemical, and thermal stimuli from the external environment to the sensory nerve terminals. Sensory nerve fibers penetrate the epidermal basement membrane and function in the tight intercellular space among keratinocytes. Here we show that epidermal keratinocytes produce hydrogen peroxide upon the activation of the NADPH oxidase dual oxidase 1 (DUOX1). This enzyme can be activated by increasing cytosolic calcium levels. Using DUOX1 knockout animals as a model system we found an increased sensitivity towards certain noxious stimuli in DUOX1-deficient animals, which is not due to structural changes in the skin as evidenced by detailed immunohistochemical and electron-microscopic analysis of epidermal tissue. We show that DUOX1 is expressed in keratinocytes but not in the neural sensory pathway. The release of hydrogen peroxide by activated DUOX1 alters both the activity of neuronal TRPA1 and redox-sensitive potassium channels expressed in dorsal root ganglia primary sensory neurons. We describe hydrogen peroxide, produced by DUOX1 as a paracrine mediator of nociceptive signal transmission. Our results indicate that a novel, hitherto unknown redox mechanism modulates noxious sensory signals.

Nociception and pain in humans lacking a functional TRPV1 channel

BACKGROUNDChronic pain is a debilitating illness with currently limited therapy, in part due to difficulties in translating treatments derived from animal models to patients. The transient receptor potential vanilloid 1 (TRPV1) channel is associated with noxious heat detection and inflammatory pain, and reports of adverse effects in human trials have hindered extensive efforts in the clinical development of TRPV1 antagonists as novel pain relievers.METHODSWe examined 2 affected individuals (A1 and A2) carrying a homozygous missense mutation in TRPV1, rendering the channel nonfunctional. Biochemical and functional assays were used to analyze the mutant channel. To identify possible phenotypes of the affected individuals, we performed psychophysical and medical examinations.RESULTSWe demonstrated that diverse TRPV1 activators, acting at different sites of the channel protein, were unable to open the cloned mutant channel. This finding was not a consequence of impairment in the expression, cellular trafficking, or assembly of protein subunits. The affected individuals were insensitive to application of capsaicin to the mouth and skin and did not demonstrate aversive behavior toward capsaicin. Furthermore, quantitative sensory testing of A1 revealed an elevated heat-pain threshold but also, surprisingly, an elevated cold-pain threshold and extensive neurogenic inflammatory, flare, and pain responses following application of the TRPA1 channel activator mustard oil.CONCLUSIONOur study provides direct evidence in humans for pain-related functional changes linked to TRPV1, which is a prime target in the development of pain relievers.FUNDINGSupported by the Israel Science Foundation (368/19); Teva's National Network of Excellence in Neuroscience grant (no. 0394886) and Teva's National Network of Excellence in Neuroscience postdoctoral fellowship.

The triple function of the capsaicin-sensitive sensory neurons: In memoriam János Szolcsányi

This paper is dedicated to the memory of János Szolcsányi (1938-2018), an outstanding Hungarian scientist. Among analgesics that act on pain receptors, he identified capsaicin as a selective lead molecule. He studied the application of capsaicin and revealed several physiological (pain, thermoregulation) and pathophysiological (inflammation, gastric ulcer) mechanisms. He discovered a new neuroregulatory system without sensory efferent reflex and investigated its pharmacology. The authors of this review are his former Ph.D. students who carried out their doctoral work in Szolcsányi's laboratory between 1985 and 2010 and report on the scientific results obtained under his guidance. His research group provided evidence for the triple function of the peptidergic capsaicin-sensitive sensory neurons including classical afferent function, local efferent responses, and remote, hormone-like anti-inflammatory, and antinociceptive actions. They also proposed somatostatin receptor type 4 as a promising drug target for the treatment of pain and inflammation. They revealed that neonatal capsaicin treatment caused no acute neuronal death but instead long-lasting selective ultrastructural and functional changes in B-type sensory neurons, similar to adult treatment. They described that lipid raft disruption diminished the agonist-induced channel opening of the TRPV1, TRPA1, and TRPM8 receptors in native sensory neurons. Szolcsányi's group has developed new devices for noxious heat threshold measurement: an increasing temperature hot plate and water bath. This novel approach proved suitable for assessing the thermal antinociceptive effects of analgesics as well as for analyzing peripheral mechanisms of thermonociception.

A comprehensive analysis to understand the mechanism of action of balneotherapy: why, how, and where they can be used? Evidence from in vitro studies performed on human and animal samples

Balneotherapy (BT) is one of the most commonly used complementary therapies for many pathological conditions. Its beneficial effects are related to physical and chemical factors, but the exact mechanism of action is not fully understood. Recently, there has been an increased interest in the use of preclinical models to investigate the influence of BT on inflammation, immunity, and cartilage and bone metabolism. The objective of this comprehensive analysis was to summarize the current knowledge about the in vitro studies in BT and to revise the obtained results on the biological effects of mineral waters. Special attention has been paid to the main rheumatological and dermatological conditions, and to the regulation of the immune response. The objective of this review was to summarize the in vitro studies, on human and animal samples, investigating the biological effects of BT. In particular, we analyzed the properties of a thermal water, as a whole, of an inorganic molecule, such as hydrogen sulfide in different cell cultures (keratinocytes, synoviocytes, chondrocytes, and peripheral blood cells), or of the organic component. The results corroborated the scientific value of in vitro studies in demonstrating the anti-inflammatory, antioxidant, chondroprotective, and immunosuppressive role of BT at the cellular level. However, the validity of the cell culture model is limited by several sources of bias, as the differences in experimental procedures, the high heterogeneity among the available researches, and the difficulties in considering all the chemical and physical factors of BT. We would like to stimulate the scientific community to standardize the experimental procedures and enhance in vitro research in the field of BT.

The Operant Plantar Thermal Assay: A Novel Device for Assessing Thermal Pain Tolerance in Mice

Pain is a multidimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the operant plantar thermal assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision-making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of using the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Investigation of Lake Hévíz Mineral Water Balneotherapy and Hévíz Mud Treatment in Murine Osteoarthritis and Rheumatoid Arthritis Models

Arthritic diseases are the most frequent causes of chronic pain and disability. Rheumatoid arthritis (RA) is an autoimmune disease characterized by synovial inflammation and progressive structural joint damage. Osteoarthritis is a degenerative process of the articular cartilage associated with hypertrophic changes in the bone. The aim of the present study was to investigate the anti-inflammatory and analgesic effects of Hévíz thermal water and mud in monosodium iodoacetate- (MIA-) (25 mg/ml, 20 μl i.a.) induced osteoarthritis and Complete Freund's adjuvant- (CFA-) (1 mg/ml, 50-50 μl s.c) induced rheumatoid arthritis murine models. The mechanonociceptive threshold of female NMRI mice (n=6- 8 mice/ group) was measured by aesthesiometry, and paw volume was monitored with plethysmometry, knee joint diameter with digital micrometer, and dynamic weight bearing on the hind limbs with a Bioseb instrument. Periarticular bone destruction was assessed by SkyScan 1176 in vivo micro-CT. Inflammatory cytokines were detected by ELISA in plasma samples. Treatments (30 min, every working day) with tap water, sand, and a combined therapy of tap water and sand served as controls. Hévíz medicinal water and combined treatment with water and mud significantly decreased the mechanical hyperalgesia and knee oedema in MIA-induced osteoarthritis model. However, balneotherapy did not influence mechanical hyperalgesia, weight bearing, or oedema formation induced by CFA. Neither medicinal water nor mud treatment ameliorated deep structural damage of the bones or the joints in the animal models. On the basis of the present findings, we conclude that balneotherapy is an effective complementary treatment to reduce the pain sensation and swelling in degenerative joint diseases such as osteoarthritis. Our experimental data are in agreement with the previous human studies that also confirmed antinociceptive and anti-inflammatory effects of thermal water and Hévíz mud treatments.

TRPA1, substance P, histamine and 5-hydroxytryptamine interact in an interdependent way to induce nociception

BACKGROUND

Although TRPA1, SP, histamine and 5-hydroxytryptamine (5-HT) have recognized contribution to nociceptive mechanisms, little is known about how they interact with each other to mediate inflammatory pain in vivo. In this study we evaluated whether TRPA1, SP, histamine and 5-HT interact, in an interdependent way, to induce nociception in vivo.

METHODS AND RESULTS

The subcutaneous injection of the TRPA1 agonist allyl isothiocyanate (AITC) into the rat's hind paw induced a dose-dependent and short lasting behavioral nociceptive response that was blocked by the co-administration of the TRPA1 antagonist, HC030031, or by the pretreatment with antisense ODN against TRPA1. AITC-induced nociception was significantly decreased by the co-administration of selective antagonists for the NK1 receptor for substance P, the H1 receptor for histamine and the 5-HT_{1A or 3} receptors for 5-HT. Histamine- or 5-HT-induced nociception was decreased by the pretreatment with antisense ODN against TRPA1. These findings suggest that AITC-induced nociception depends on substance P, histamine and 5-HT, while histamine- or 5-HT-induced nociception depends on TRPA1. Most important, AITC interact in a synergistic way with histamine, 5-HT or substance P, since their combination at non-nociceptive doses induced a nociceptive response much higher than that expected by the sum of the effect of each one alone. This synergistic effect is dependent on the H1, 5-HT_{1A or 3} receptors.

CONCLUSION

Together, these findings suggest a self-sustainable cycle around TRPA1, no matter where the cycle is initiated each step is achieved and even subeffective activation of more than one step results in a synergistic activation of the overall cycle.