Capsaicin-sensitive sensory nerve terminals with local and systemic efferent functions: facts and scopes of an unorthodox neuroregulatory mechanism

The Role of Neuroinflammation in Complex Regional Pain Syndrome: A Comprehensive Review

Complex Regional Pain Syndrome (CRPS) is an excess and/or prolonged pain and inflammation condition that follows an injury to a limb. The pathogenesis of CRPS is multifaceted that remains incompletely understood. Neuroinflammation is an inflammatory response in the peripheral and central nervous systems. Dysregulated neuroinflammation plays a crucial role in the initiation and maintenance of pain and nociceptive neuronal sensitization, which may contribute to the transition from acute to chronic pain and the perpetuation of chronic pain in CRPS. The key features of neuroinflammation encompass infiltration and activation of inflammatory cells and the production of inflammatory mediators in both the central and peripheral nervous systems. This article reviews the role of neuroinflammation in the onset and progression of CRPS from six perspectives: neurogenic inflammation, neuropeptides, glial cells, immune cells, cytokines, and keratinocytes. The objective is to provide insights that can inform future research and development of therapeutic targets for CRPS.

Mechanistic insights into the role of the chemokine CCL2/CCR2 axis in dorsal root ganglia to peripheral inflammation and pain hypersensitivity

BACKGROUND

Pain is reported as the leading cause of disability in the common forms of inflammatory arthritis conditions. Acting as a key player in nociceptive processing, neuroinflammation, and neuron-glia communication, the chemokine CCL2/CCR2 axis holds great promise for controlling chronic painful arthritis. Here, we investigated how the CCL2/CCR2 system in the dorsal root ganglion (DRG) contributes to the peripheral inflammatory pain sensitization.

METHODS

Repeated intrathecal (i.t.) administration of the CCR2 antagonist, INCB3344 was tested for its ability to reverse the nociceptive-related behaviors in the tonic formalin and complete Freund's adjuvant (CFA) inflammatory models. We further determined by qPCR the expression of CCL2/CCR2, SP and CGRP in DRG neurons from CFA-treated rats. Using DRG explants, acutely dissociated primary sensory neurons and calcium mobilization assay, we also assessed the release of CCL2 and sensitization of nociceptors. Finally, we examined by immunohistochemistry following nerve ligation the axonal transport of CCL2, SP, and CGRP from the sciatic nerve of CFA-treated rats.

RESULTS

We first found that CFA-induced paw edema provoked an increase in CCL2/CCR2 and SP expression in ipsilateral DRGs, which was decreased after INCB3344 treatment. This upregulation in pronociceptive neuromodulators was accompanied by an enhanced nociceptive neuron excitability on days 3 and 10 post-CFA, as revealed by the CCR2-dependent increase in intracellular calcium mobilization following CCL2 stimulation. In DRG explants, we further demonstrated that the release of CCL2 was increased following peripheral inflammation. Finally, the excitation of nociceptors following peripheral inflammation stimulated the anterograde transport of SP at their peripheral nerve terminals. Importantly, blockade of CCR2 reduced sensory neuron excitability by limiting the calcium mobilization and subsequently decreased peripheral transport of SP towards the periphery. Finally, pharmacological inhibition of CCR2 reversed the pronociceptive action of CCL2 in rats receiving formalin injection and significantly reduced the neurogenic inflammation as well as the stimuli-evoked and movement-evoked nociceptive behaviors in CFA-treated rats.

CONCLUSIONS

Our results provide significant mechanistic insights into the role of CCL2/CCR2 within the DRG in the development of peripheral inflammation, nociceptor sensitization, and pain hypersensitivity. We further unveil the therapeutic potential of targeting CCR2 for the treatment of painful inflammatory disorders.

Prior perineural or neonatal treatment with capsaicin does not alter the development of spinal microgliosis induced by peripheral nerve injury

Peripheral nerve injury is associated with spinal microgliosis which plays a pivotal role in the development of neuropathic pain behavior. Several agents of primary afferent origin causing the microglial reaction have been identified, but the type(s) of primary afferents that release these mediators are still unclear. In this study, specific labeling of C-fiber spinal afferents by lectin histochemistry and selective chemodenervation by capsaicin were applied to identify the type(s) of primary afferents involved in the microglial response. Comparative quantitative morphometric evaluation of the microglial reaction in central projection territories of intact and injured peripheral nerves in the superficial (laminae I and II) and deep (laminae III and IV) spinal dorsal horn revealed a significant, about three-fold increase in microglial density after transection of the sciatic or the saphenous nerve. Prior perineural treatment of these nerves with capsaicin, resulting in a selective defunctionalization of C-fiber afferent fibers failed to affect spinal microgliosis. Similarly, peripheral nerve injury-induced increase in microglial density was unaffected in rats treated neonatally with capsaicin known to result in a near-total loss of C-fiber dorsal root fibers. Perineural treatment with capsaicin per se did not evoke a significant increase in microglial density. These observations indicate that injury-induced spinal microgliosis may be attributed to phenotypic changes in injured myelinated primary afferent neurons, whereas the contribution of C-fiber primary sensory neurons to this neuroimmune response is negligible. Spinal myelinated primary afferents may play a hitherto unrecognized role in regulation of neuroimmune and perisynaptic microenvironments of the spinal dorsal horn.

Complex Regulatory Role of the TRPA1 Receptor in Acute and Chronic Airway Inflammation Mouse Models

The Transient Receptor Potential Ankyrin 1 (TRPA1) cation channel expressed on capsaicin-sensitive afferents, immune and endothelial cells is activated by inflammatory mediators and exogenous irritants, e.g., endotoxins, nicotine, crotonaldehyde and acrolein. We investigated its involvement in acute and chronic pulmonary inflammation using Trpa1 gene-deleted (Trpa1^-/-) mice. Acute pneumonitis was evoked by intranasal Escherichia coli endotoxin (lipopolysaccharide: LPS) administration, chronic bronchitis by daily cigarette smoke exposure (CSE) for 4 months. Frequency, peak inspiratory/expiratory flows, minute ventilation determined by unrestrained whole-body plethysmography were significantly greater, while tidal volume, inspiratory/expiratory/relaxation times were smaller in Trpa1^-/- mice. LPS-induced bronchial hyperreactivity, myeloperoxidase activity, frequency-decrease were significantly greater in Trpa1^-/- mice. CSE significantly decreased tidal volume, minute ventilation, peak inspiratory/expiratory flows in wildtypes, but not in Trpa1^-/- mice. CSE remarkably increased the mean linear intercept (histopathology), as an emphysema indicator after 2 months in wildtypes, but only after 4 months in Trpa1^-/- mice. Semiquantitative histopathological scores were not different between strains in either models. TRPA1 has a complex role in basal airway function regulation and inflammatory mechanisms. It protects against LPS-induced acute pneumonitis and hyperresponsiveness, but is required for CSE-evoked emphysema and respiratory deterioration. Further research is needed to determine TRPA1 as a potential pharmacological target in the lung.

Modulation of Sensory Nerve Function by Insulin: Possible Relevance to Pain, Inflammation and Axon Growth

Insulin, besides its pivotal role in energy metabolism, may also modulate neuronal processes through acting on insulin receptors (InsRs) expressed by neurons of both the central and the peripheral nervous system. Recently, the distribution and functional significance of InsRs localized on a subset of multifunctional primary sensory neurons (PSNs) have been revealed. Systematic investigations into the cellular electrophysiology, neurochemistry and morphological traits of InsR-expressing PSNs indicated complex functional interactions among specific ion channels, proteins and neuropeptides localized in these neurons. Quantitative immunohistochemical studies have revealed disparate localization of the InsRs in somatic and visceral PSNs with a dominance of InsR-positive neurons innervating visceral organs. These findings suggested that visceral spinal PSNs involved in nociceptive and inflammatory processes are more prone to the modulatory effects of insulin than somatic PSNs. Co-localization of the InsR and transient receptor potential vanilloid 1 (TRPV1) receptor with vasoactive neuropeptides calcitonin gene-related peptide and substance P bears of crucial importance in the pathogenesis of inflammatory pathologies affecting visceral organs, such as the pancreas and the urinary bladder. Recent studies have also revealed significant novel aspects of the neurotrophic propensities of insulin with respect to axonal growth, development and regeneration.

TRPA1 and issues relating to animal model selection for extrapolating toxicity data to humans

The transient receptor potential ankyrin 1 (TRPA1) ion channel is a sensor for irritant chemicals, has ancient lineage, and is distributed across animal species including humans, where it features in many organs. Its activation by a diverse panel of electrophilic molecules (TRPA1 agonists) through electrostatic binding and/or covalent attachment to the protein causes the sensation of pain. This article reviews the species differences between TRPA1 channels and their responses, to assess the suitability of different animals to model the effects of TRPA1-activating electrophiles in humans, referring to common TRPA1 activators (exogenous and endogenous) and possible mechanisms of action relating to their toxicology. It concludes that close matching of in vitro and in vivo models will help optimise the identification of relevant biochemical and physiological responses to benchmark the efficacy of potential therapeutic drugs, including TRPA1 antagonists, to counter the toxic effects of those electrophiles capable of harming humans. The analysis of the species issue provided should aid the development of medical treatments to counter poisoning by such chemicals.

A comparison of effects of scalp nerve block and local anesthetic infiltration on inflammatory response, hemodynamic response, and postoperative pain in patients undergoing craniotomy for cerebral aneurysms: a randomized controlled trial

BACKGROUND

The purpose of this study was to compare the effects of scalp nerve block (SNB) and local anesthetic infiltration (LA) with 0.75% ropivacaine on postoperative inflammatory response, intraoperative hemodynamic response, and postoperative pain control in patients undergoing craniotomy.

METHODS

Fifty-seven patients were admitted for elective craniotomy for surgical clipping of a cerebral aneurysm. They were randomly divided into three groups: Group S (SNB with 15 mL of 0.75% ropivacaine), group I (LA with 15 mL of 0.75% ropivacaine) and group C (that only received routine intravenous analgesia). Pro-inflammatory cytokine levels in plasma for 72 h postoperatively, hemodynamic response to skin incision, and postoperative pain intensity were measured.

RESULTS

The SNB with 0.75% ropivacaine not only decreased IL-6 levels in plasma 6 h after craniotomy but also decreased plasma CRP levels and increased plasma IL-10 levels 12 and 24 h after surgery compared to LA and routine analgesia. There were significant increases in mean arterial pressure 2 and 5 mins after the incision and during dura opening in Groups I and C compared with Group S. Group S had lower postoperative pain intensity, longer duration before the first dose of oxycodone, less consumption of oxycodone and lower incidence of PONV through 48 h postoperatively than Groups I and C.

CONCLUSION

Preoperative SNB attenuated inflammatory response to craniotomy for cerebral aneurysms, blunted the hemodynamic response to scalp incision, and controlled postoperative pain better than LA or routine analgesia.

TRIAL REGISTRATION

Clinicaltrials.gov NCT03073889 (PI:Xi Yang; date of registration:08/03/2017).

Skin neurogenic inflammation

The epidermis closely interacts with nerve endings, and both epidermis and nerves produce substances for mutual sustenance. Neuropeptides, like substance P (SP) and calcitonin gene-related protein (CGRP), are produced by sensory nerves in the dermis; they induce mast cells to release vasoactive amines that facilitate infiltration of neutrophils and T cells. Some receptors are more important than others in the generation of itch. The Mas-related G protein-coupled receptors (Mrgpr) family as well as transient receptor potential ankyrin 1 (TRPA1) and protease activated receptor 2(Par2) have important roles in itch and inflammation. The activation of MrgprX1 degranulates mast cells to communicate with sensory nerve and cutaneous cells for developing neurogenic inflammation. Mrgprs and transient receptor potential vanilloid 4 (TRPV4) are crucial for the generation of skin diseases like rosacea, while SP, CGRP, somatostatin, β-endorphin, vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating polypeptide (PACAP) can modulate the immune system during psoriasis development. The increased level of SP, in atopic dermatitis, induces the release of interferon (IFN)-γ, interleukin (IL)-4, tumor necrosis factor (TNF)-α, and IL-10 from the peripheral blood mononuclear leukocytes. We are finally starting to understand the intricate connections between the skin neurons and resident skin cells and how their interaction can be key to controlling inflammation and from there the pathogenesis of diseases like atopic dermatitis, psoriasis, and rosacea.

Evidence for a novel, neurohumoral antinociceptive mechanism mediated by peripheral capsaicin-sensitive nociceptors in conscious rats

Stimulation of capsaicin-sensitive peripheral sensory nerve terminals induces remote anti-inflammatory effects throughout the body of anesthetized rats and guinea-pigs mediated by somatostatin. As somatostatin has also antinociceptive effects, the study aimed at investigating whether similar remote antinociceptive effects can be demonstrated in awake animals. In conscious rats, nociceptive nerve endings of the right hind paw decentralized by cutting the sciatic and saphenous nerves 18h before were chemically stimulated, and drop of the noxious heat threshold (heat hyperalgesia) induced by prior (18h before) plantar incision was measured on the contralateral, left hind paw using an increasing-temperature water bath. 18h after nerve transection, mustard oil-evoked plasma extravasation was not significantly reduced in the right hind paw as tested by in vivo fluorescence imaging. Applying agonist of either transient receptor potential vanilloid 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1) receptor (capsaicin or mustard oil, respectively) to the nerve-transected paw inhibited the plantar incision-induced drop of the noxious heat threshold on the contralateral paw. The onset of these remote antihyperalgesic effects was 10-20min. A similar contralateral inhibitory effect of capsaicin or mustard oil treatment was observed on neuropathic mechanical hyperalgesia evoked by partial sciatic nerve injury 2days before nerve transection and measured by a Randall-Selitto apparatus. The remote thermal antihyperalgesic effect was prevented by chronic (5days) denervation or local capsaicin desensitization of the stimulated paw; reduced by intraperitoneally applied antagonist of somatostatin (cyclosomatostatin) or opioid receptors (naloxone). The response was mimicked by intraperitoneally applied somatostatin and associated with a 72±27% increase in plasma somatostatin-like immunoreactivity that was absent after chronic (5days) denervation. In conclusion, chemical activation of decentralized peripheral capsaicin-sensitive nociceptors evokes remote antihyperalgesic responses initiated outside the central nervous system and mediated by somatostatin and endogenous opioids.

Challenges to develop novel anti-inflammatory and analgesic drugs

Chronic inflammatory diseases and persistent pain of different origin represent common medical, social, and economic burden, and their pharmacotherapy is still an unresolved issue. Therefore, there is a great and urgent need to develop anti-inflammatory and analgesic agents with novel mechanisms of action, but it is a very challenging task. The main problem is the relatively large translational gap between the preclinical experimental data and the clinical results due to characteristics of the models, difficulties with the investigational techniques particularly for pain, as well as species differences in the mechanisms. We summarize here the current state-of-the-art medication and related ongoing strategies, and the novel targets with lead molecules under clinical development. The first members of the gold-standard categories, such as nonsteroidal anti-inflammatory drugs, glucocorticoids, and opioids, were introduced decades ago, and since then very few drugs with novel mechanisms of action have been successfully taken to the clinics despite considerable development efforts. Several biologics targeting different key molecules have provided breakthrough in some autoimmune/inflammatory diseases, but they are expensive, only parenterally available, their long-term side effects often limit their administration, and they do not effectively reduce pain. Some kinase inhibitors and phosphodiesterase-4 blockers have recently been introduced as new directions. There are in fact some promising novel approaches at different clinical stages of drug development focusing on transient receptor potential vanilloid 1/ankyrin 1 channel antagonism, inhibition of voltage-gated sodium/calcium channels, several enzymes (kinases, semicarbazide-sensitive amine oxidases, and matrix metalloproteinases), cytokines/chemokines, transcription factors, nerve growth factor, and modulation of several G protein-coupled receptors (cannabinoids, purinoceptors, and neuropeptides). WIREs Nanomed Nanobiotechnol 2017, 9:e1427. doi: 10.1002/wnan.1427 For further resources related to this article, please visit the WIREs website.

Capsaicin-Sensitive Sensory Nerves Mediate the Cellular and Microvascular Effects of H2S via TRPA1 Receptor Activation and Neuropeptide Release

It is supposed that TRPA1 receptor can be activated by hydrogen sulphide (H2S). Here, we have investigated the role of TRPA1 receptor in H2S-induced [Ca(2+)]i increase in trigeminal ganglia (TRG) neurons, and the involvement of capsaicin-sensitive sensory nerves in H2S-evoked cutaneous vasodilatation. [Ca(2+)]i was measured with ratiometric technique on TRG neurons of TRPA1(+/+) and TRPA1(-/-) mice after NaHS, Na2S, allylisothiocyanate (AITC) or KCl treatment. Microcirculatory changes in the ear were detected by laser Doppler imaging in response to topical NaHS, AITC, NaOH, NaSO3 or NaCl. Mice were either treated with resiniferatoxin (RTX), or CGRP antagonist BIBN4096, or NK1 receptor antagonist CP99994, or K(+) ATP channel blocker glibenclamide. Alpha-CGRP(-/-) and NK1 (-/-) mice were also investigated. NaHS and Na2S increased [Ca(2+)]i in TRG neurons derived from TRPA(+/+) but not from TRPA1(-/-) mice. NaHS increased cutaneous blood flow, while NaOH, NaSO3 and NaCl did not cause significant changes. NaHS-induced vasodilatation was reduced in RTX-treated animals, as well as by pre-treatment with BIBN4096 or CP99994 alone or in combination. NaHS-induced vasodilatation was significantly smaller in alpha-CGRP(-/-) or NK1 (-/-) mice compared to wild-types. H2S activates capsaicin-sensitive sensory nerves through TRPA1 receptors and the resultant vasodilatation is mediated by the release of vasoactive sensory neuropeptides CGRP and substance P.

The Role of Sensory Neurons in Cervical Ripening: Effects of Estrogen and Neuropeptides

Central nervous system nuclei and circuits, such as the medial preoptic, ventromedial and paraventricular nuclei of the hypothalamus, play important roles in reproduction and parturition, and are influenced by estrogen. Peripheral autonomic and sensory neurons also play important roles in pregnancy and parturition. Moreover, the steroid hormone estrogen acts directly, not only on the reproductive tract organs (uterus and cervix), but also on the central and peripheral nerves by regulating expression of various neuronal genes. The peripheral primary afferent neurons innervating the uterine cervix relay mechanical and biochemical sensory information induced by local cervical events and by passage of fetuses, to the spinal cord and supraspinal centers. Consequently, the birth process in mammals is influenced by the combined action of neurons and hormones. Peripheral sensory stimuli, induced physiologically by fetal expulsion or mechanically by vaginocervical stimulation, alter behavior, as well as autonomic and neuroendocrine systems. Recent evidence indicates that primary afferent neurons innervating the cervix, in addition to their sensory effects, likely exert local “efferent” actions on the ripening cervix near term. These efferent effects may involve estrogen-regulated production of such neuropeptides as substance P and calcitonin gene-related peptide in lumbosacral dorsal root ganglia, and their release in the cervix. Collectively, these findings suggest an interrelationship among estrogen, cervix-related sensory neurons, and local cervical events near term.

Role of capsaicin-sensitive nerves and tachykinins in mast cell tryptase-induced inflammation of murine knees

OBJECTIVE, DESIGN

Mast cell tryptase (MCT) is elevated in arthritic joints, but its direct effects are not known. Here, we investigated MCT-evoked acute inflammatory and nociceptive mechanisms with behavioural, in vivo imaging and immunological techniques.

MATERIAL AND SUBJECTS

Neurogenic inflammation involving capsaicin-sensitive afferents, transient receptor potential vanilloid 1 receptor (TRPV1), substance P (SP), neurokinin A (NKA) and their NK1 tachykinin receptor were studied using gene-deleted mice compared to C57Bl/6 wildtypes (n = 5-8/group).

TREATMENT

MCT was administered intraarticularly or topically (20 μl, 12 μg/ml). Capsaicin-sensitive afferents were defunctionalized with the TRPV1 agonist resiniferatoxin (RTX; 30-70-100 μg/kg s.c. pretreatment).

METHODS

Knee diameter was measured with a caliper, synovial perfusion with laser Doppler imaging, mechanonociception with aesthesiometry and weight distribution with incapacitance tester over 6 h. Cytokines and neuropeptides were determined with immunoassays.

RESULTS

MCT induced synovial vasodilatation, oedema, impaired weight distribution and mechanical hyperalgesia, but cytokine or neuropeptide levels were not altered at the 6-h timepoint. Hyperaemia was reduced in RTX-treated and TRPV1-deleted animals, and oedema was absent in NK1-deficient mice. Hyperalgesia was decreased in SP/NKA- and NK1-deficient mice, weight bearing impairment in RTX-pretreated, TRPV1- and NK1-deficient animals.

CONCLUSIONS

MCT evokes synovial hyperaemia, oedema, hyperalgesia and spontaneous pain. Capsaicin-sensitive afferents and TRPV1 receptors are essential for vasodilatation, while tachykinins mediate oedema and pain.

Some like it hot: The emerging role of spicy food (capsaicin) in autoimmune diseases

Autoimmune diseases refer to a spectrum of diseases characterized by an active immune response against the host, which frequently involves increased autoantibody production. The pathogenesis of autoimmune diseases is multifactorial and the exploitation of novel effective treatment is urgent. Capsaicin is a nutritional factor, the active component of chili peppers, which is responsible for the pungent component of chili pepper. As a stimuli, capsaicin selectively activate transient receptor potential vanilloid subfamily 1(TRPV1) and exert various biological effects. This review discusses the effect of capsaicin through its receptor on the development and modulation of autoimmune diseases, which may shed light upon potential therapies in capsaicin-targeted approaches.

Capsaicin and Its Role in Chronic Diseases

A significant number of experimental and clinical studies published in peer-reviewed journals have demonstrated promising pharmacological properties of capsaicin in relieving signs and symptoms of non-communicable diseases (chronic diseases). This chapter provides an overview made from basic and clinical research studies of the potential therapeutic effects of capsaicin, loaded in different application forms, such as solution and cream, on chronic diseases (e.g. arthritis, chronic pain, functional gastrointestinal disorders and cancer). In addition to the anti-inflammatory and analgesic properties of capsaicin largely recognized via, mainly, interaction with the TRPV1, the effects of capsaicin on different cell signalling pathways will be further discussed here. The analgesic, anti-inflammatory or apoptotic effects of capsaicin show promising results in arthritis, neuropathic pain, gastrointestinal disorders or cancer, since evidence demonstrates that the oral or local application of capsaicin reduce inflammation and pain in rheumatoid arthritis, promotes gastric protection against ulcer and induces apoptosis of the tumour cells. Sadly, these results have been paralleled by conflicting studies, which indicate that high concentrations of capsaicin are likely to evoke deleterious effects, thus suggesting that capsaicin activates different pathways at different concentrations in both human and rodent tissues. Thus, to establish effective capsaicin doses for chronic conditions, which can be benefited from capsaicin therapeutic effects, is a real challenge that must be pursued.

Effect of capsaicin on thermoregulation: an update with new aspects

Capsaicin, a selective activator of the chemo- and heat-sensitive transient receptor potential (TRP) V1 cation channel, has characteristic feature of causing long-term functional and structural impairment of neural elements supplied by TRPV1/capsaicin receptor. In mammals, systemic application of capsaicin induces complex heat-loss response characteristic for each species and avoidance of warm environment. Capsaicin activates cutaneous warm receptors and polymodal nociceptors but has no effect on cold receptors or mechanoreceptors. In this review, thermoregulatory features of capsaicin-pretreated rodents and TRPV1-mediated neural elements with innocuous heat sensitivity are summarized. Recent data support a novel hypothesis for the role of visceral warmth sensors in monitoring core body temperature. Furthermore, strong evidence suggests that central presynaptic nerve terminals of TRPV1-expressing cutaneous, thoracic and abdominal visceral receptors are activated by innocuous warmth stimuli and capsaicin. These responses are absent in TRPV1 knockout mice. Thermoregulatory disturbance induced by systemic capsaicin pretreatment lasts for months and is characterized by a normal body temperature at cool environment up to a total dose of 150 mg/kg s.c. Upward differential shift of set points for activation vasodilation, other heat-loss effectors and thermopreference develops. Avoidance of warm ambient temperature (35°C, 40°C) is severely impaired but thermopreference at cool ambient temperatures (Tas) are not altered. TRPV1 knockout or knockdown and genetically altered TRPV1, TRPV2 and TRPM8 knockout mice have normal core temperature in thermoneutral or cool environments, but the combined mutant mice have impaired regulation in warm or cold (4°C) environments. Several lines of evidence support that in the preoptic area warmth sensitive neurons are activated and desensitized by capsaicin, but morphological evidence for it is controversial. It is suggested that these neurons have also integrator function. Fever is enhanced in capsaicin-desensitized rats and the inhibition observed after pretreatment with low i.p. doses does not support in the light of their warmth sensitivity the concept that abdominal TRPV1-expressing nerve terminals serve as nonthermal chemosensors for reference signals in thermoregulation.

Pituitary Adenylate Cyclase-Activating Polypeptide Is Upregulated in Murine Skin Inflammation and Mediates Transient Receptor Potential Vanilloid-1-Induced Neurogenic Edema

Although pituitary adenylate cyclase-activating polypeptide (PACAP) was described as a key vasoregulator in human skin, little is known about its expression in mouse skin. As it is important to investigate PACAP signaling in translational mouse dermatitis models, we determined its presence, regulation, and role in neurogenic and non-neurogenic cutaneous inflammatory mechanisms. The mRNA of PACAP and its specific receptor PAC1 was detected with real-time PCR in several skin regions at comparable levels. PACAP-38-immunoreactivity measured with radioimmunoassay was similar in plantar and dorsal paw skin and the ear but significantly smaller in the back skin. PACAP and PAC1 mRNA, as well as PACAP-38 and PAC1 protein expression, significantly increased in the plantar skin after intraplantar administration of capsaicin (50 μl, 100 μg ml(-1)), an agonist of the transient receptor potential vanilloid 1 (TRPV1) receptor, evoking chiefly neurogenic inflammation without inflammatory cell accumulation. Intraplantar complete Freund's adjuvant (CFA; 50 μl, 1 mg ml(-1)) also increased PACAP/PAC1 mRNA but not the PACAP peptide. Capsaicin-induced neurogenic paw edema, but not CFA-evoked non-neurogenic swelling, was significantly smaller in PACAP-deficient mice throughout a 24-hour period. To our knowledge, we provide previously unreported evidence for PACAP and PAC1 expression upregulation during skin inflammation of different mechanisms and for its pro-inflammatory function in neurogenic edema formation.

Capsaicin-sensitive sensory nerves exert complex regulatory functions in the serum-transfer mouse model of autoimmune arthritis

OBJECTIVE

The K/BxN serum-transfer arthritis is a widely-used translational mouse model of rheumatoid arthritis, in which the immunological components have thoroughly been investigated. In contrast, little is known about the role of sensory neural factors and the complexity of neuro-immune interactions. Therefore, we analyzed the involvement of capsaicin-sensitive peptidergic sensory nerves in autoantibody-induced arthritis with integrative methodology.

METHODS

Arthritogenic K/BxN or control serum was injected to non-pretreated mice or resiniferatoxin (RTX)-pretreated animals where capsaicin-sensitive nerves were inactivated. Edema, touch sensitivity, noxious heat threshold, joint function, body weight and clinical arthritis severity scores were determined repeatedly throughout two weeks. Micro-CT and in vivo optical imaging to determine matrix-metalloproteinase (MMP) and neutrophil-derived myeloperoxidase (MPO) activities, semiquantitative histopathological scoring and radioimmunoassay to measure somatostatin in the joint homogenates were also performed.

RESULTS

In RTX-pretreated mice, the autoantibody-induced joint swelling, arthritis severity score, MMP and MPO activities, as well as histopathological alterations were significantly greater compared to non-pretreated animals. Self-control quantification of the bone mass revealed decreased values in intact female mice, but significantly greater arthritis-induced pathological bone formation after RTX-pretreatment. In contrast, mechanical hyperalgesia from day 10 was smaller after inactivating capsaicin-sensitive afferents. Although thermal hyperalgesia did not develop, noxious heat threshold was significantly higher following RTX pretreatment. Somatostatin-like immunoreactivity elevated in the tibiotarsal joints in non-pretreated, which was significantly less in RTX-pretreated mice.

CONCLUSIONS

Although capsaicin-sensitive sensory nerves mediate mechanical hyperalgesia in the later phase of autoantibody-induced chronic arthritis, they play important anti-inflammatory roles at least partially through somatostatin release.

Upregulation of the transient receptor potential ankyrin 1 ion channel in the inflamed human and mouse colon and its protective roles

Transient Receptor Potential Ankyrin 1 (TRPA1) channels are localized on sensory nerves and several non-neural cells, but data on their functional significance are contradictory. We analysed the presence and alterations of TRPA1 in comparison with TRP Vanilloid 1 (TRPV1) at mRNA and protein levels in human and mouse intact and inflamed colons. The role of TRPA1 in a colitis model was investigated using gene-deficient mice. TRPA1 and TRPV1 expressions were investigated in human colon biopsies of healthy subjects and patients with inflammatory bowel diseases (IBD: ulcerative colitis, Crohn's disease) with quantitative PCR and immunohistochemistry. Mouse colitis was induced by oral 2% dextran-sulphate (DSS) for 10 days. For investigating the functions of TRPA1, Disease Activity Index (weight loss, stool consistency, blood content) was determined in C57BL/6-based Trpa1-deficient (knockout: KO) and wildtype (WT) mice. Sensory neuropeptides, their receptors, and inflammatory cytokines/chemokines were determined with qPCR or Luminex. In human and mouse colons TRPA1 and TRPV1 are located on epithelial cells, macrophages, enteric ganglia. Significant upregulation of TRPA1 mRNA was detected in inflamed samples. In Trpa1 KO mice, Disease Activity Index was significantly higher compared to WTs. It could be explained by the greater levels of substance P, neurokinins A and B, neurokinin 1 receptor, pituitary adenylate-cyclase activating polypeptide, vasoactive intestinal polypeptide, and also interleukin-1beta, macrophage chemoattractant protein-1, monokine induced by gamma interferon-1, tumor necrosis factor-alpha and B-lymphocyte chemoattractant in the distal colon. TRPA1 is upregulated in colitis and its activation exerts protective roles by decreasing the expressions of several proinflammatory neuropeptides, cytokines and chemokines.

The decrease in aqueous tear production associated with pepper spray

PURPOSE

Pepper spray is used both by civilians and by law enforcement. Burning sensation occurs when exposed to skin, pain and temporary blindness occurs when exposed to the eyes. This study focused on the effect of pepper spray on lacrimal tear production and subsequently on corneal sensitivity in a large group after an intense exposure.

METHODS

Ninety-six people who were exposed to pepper spray during the Gezi park protests volunteered. Subjects were asked if they wore any protective goggles and if they irrigated their eyes after exposure. They were asked to record their symptoms regarding dry eye in a standardized questionnaire. Schirmer I and II tests were performed.

RESULTS

Eighty-two people wore protective goggles during exposure, whereas 14 people did not have any protection. Both Schirmer results in unprotected subjects were significantly lower than that in protected subjects. Schirmer I and II results of unprotected subjects were not statistically different, whereas they were statistically different in protected subjects. Thirty-five percent of unprotected subjects and 24% of protected subjects expressed symptoms of dry eye.

DISCUSSION

The active ingredient of pepper spray is oleoresin capsicum. It is randomly diffused to polymodal nerve terminals, leading to opening of non-selective cationic channels and block neuronal transmission. The lower results of both Schirmer in unprotected group emphasize the importance of a protective Google glass around the eyes during exposal, serving as a barrier minimalizing the contact of the spray with the eyes. The combination of the low results and lack of symptoms could suggest that corneal reflex lacrimation in our subjects was not abundant enough. The findings of this study could not fully represent long term findings but it could be assumed that our findings could be indicative of the sensory denervation and alterations demonstrated in studies investigating the long term effects of oleoresin capsicum.

Capsaicin and sensory neurones: a historical perspective

Capsaicin, the pungent ingredient of red pepper has become not only a "hot" topic in neuroscience but its new target-related unique actions have opened the door for the drug industry to introduce a new chapter of analgesics. After several lines of translational efforts with over 1,000 patents and clinical trials, the 8% capsaicin dermal patch reached the market and its long-lasting local analgesic effect in some severe neuropathic pain states is now well established. This introductory chapter outlines on one hand the historical background based on the author's 50 years of experience in this field and on the other hand emphasizes new scopes, fascinating perspectives in pharmaco-physiology, and molecular pharmacology of nociceptive sensory neurons. Evidence for the effect of capsaicin on C-polymodal nociceptors (CMH), C-mechanoinsensitive (CHMi), and silent C-nociceptors are listed and the features of the capsaicin-induced blocking effects of nociceptors are demonstrated. Common and different characteristics of nociceptor-blocking actions after systemic, perineural, local, intrathecal, and in vitro treatments are summarized. Evidence for the misleading conclusions drawn from neonatal capsaicin pretreatment is presented. Perspectives opened from cloning the capsaicin receptor "Transient Receptor Potential Vanilloid 1" (TRPV1) are outlined and potential molecular mechanisms behind the long-lasting functional, ultrastructural, and nerve terminal-damaging effects of capsaicin and other TRPV1 agonists are summarized. Neurogenic inflammation and the long-list of "capsaicin-sensitive" tissue responses are mediated by an unorthodox dual sensory-efferent function of peptidergic TRPV1-expressing nerve terminals which differ from the classical efferent and sensory nerve endings that have a unidirectional role in neuroregulation. Thermoregulatory effects of capsaicin are discussed in detail. It is suggested that since hyperthermia and burn risk due to enhanced noxious heat threshold are the major obstacles of some TRPV1 antagonists, they could be overcome. The special "multisteric" gating function of the TRPV1 cation channel provides the structural ground for blocking chemical activation of TRPV1 without affecting its responsiveness to physical stimuli. A new chapter of potential analgesics targeting nociceptors is now already supported for pain relief in persistent pathological pain states.

Microvascular dysfunction in the context of diabetic neuropathy

Microvascular dysfunction in diabetes plays a crucial role in the development of diabetic complications. The skin, as one of the most accessible organs, serves as a model for the investigation of microvascular dysfunction. Several non-invasive, mostly laser-Doppler-based methods have been developed lately to assess microvascular function in the skin. Microvascular functional changes occur even in the prediabetic state and become more complex with overt diabetes, being exacerbated by the presence of peripheral and/or autonomic diabetic neuropathy. The present article aims at shedding light on the implication of endothelial and neurovascular dysfunction in microvascular changes in diabetes, highlighting the contribution of different forms of diabetic neuropathy.

Role of tachykinin 1 and 4 gene-derived neuropeptides and the neurokinin 1 receptor in adjuvant-induced chronic arthritis of the mouse

Objective

Substance P, encoded by the Tac1 gene, is involved in neurogenic inflammation and hyperalgesia via neurokinin 1 (NK1) receptor activation. Its non-neuronal counterpart, hemokinin-1, which is derived from the Tac4 gene, is also a potent NK1 agonist. Although hemokinin-1 has been described as a tachykinin of distinct origin and function compared to SP, its role in inflammatory and pain processes has not yet been elucidated in such detail. In this study, we analysed the involvement of tachykinins derived from the Tac1 and Tac4 genes, as well as the NK1 receptor in chronic arthritis of the mouse.

Methods

Complete Freund’s Adjuvant was injected intraplantarly and into the tail of Tac1^−/−, Tac4^−/−, Tacr1^−/− (NK1 receptor deficient) and Tac1^−/−/Tac4^−/− mice. Paw volume was measured by plethysmometry and mechanosensitivity using dynamic plantar aesthesiometry over a time period of 21 days. Semiquantitative histopathological scoring and ELISA measurement of IL-1β concentrations of the tibiotarsal joints were performed.

Results

Mechanical hyperalgesia was significantly reduced from day 11 in Tac4^−/− and Tacr1^−/− animals, while paw swelling was not altered in any strain. Inflammatory histopathological alterations (synovial swelling, leukocyte infiltration, cartilage destruction, bone damage) and IL-1β concentration in the joint homogenates were significantly smaller in Tac4^−/− and Tac1^−/−/Tac4^−/− mice.

Conclusions

Hemokinin-1, but not substance P increases inflammation and hyperalgesia in the late phase of adjuvant-induced arthritis. While NK1 receptors mediate its antihyperalgesic actions, the involvement of another receptor in histopathological changes and IL-1β production is suggested.

Tachykinin (NK1, NK2 and NK3) receptor, transient receptor potential vanilloid 1 (TRPV1) and early transcription factor, cFOS, mRNA expression in rat tissues following systemic capsaicin treatment

Capsaicin, the pungent component of chilli pepper, stimulates TRPV1-expressing cells which are followed by desensitisation to subsequent exposure to capsaicin and other TRPV1 activators. At high systemic doses (>125 mg/kg), capsaicin produces long-term changes in both tachykinin receptor and TRPV1 expression and function in rats. However, whether desensitising (low) doses of capsaicin (~50 mg/kg) affect tachykinin receptor and TRPV1 gene expression in the short term has yet to be investigated. The aim of the present study was to compare tachykinin receptor (NK1, NK2 and NK3) and TRPV1 mRNA expression 24h after administration of capsaicin (50 mg/kgs.c.). Tachykinin receptor and TRPV1 mRNA were detected in all tissues studied with expression levels differing by up to 2500-fold between tissues. The highest expression of TRPV1 and NK1 mRNA was observed in the salivary gland, whereas NK2 mRNA expression was highest in the urinary bladder and NK3 mRNA expression in the frontal cortex. In the cervical spinal cord of rats treated with capsaicin, NK1 and NK3 mRNA expression were reduced by 56% and 80%, respectively (P<0.05), whereas NK2 and TRPV1 mRNA expression were increased 2.2- and 1.4-fold, respectively (P<0.05). NK1 and NK2 mRNA expression were decreased (P<0.05) in the urinary bladder and gastric fundus, respectively, following capsaicin treatment. There was a marked 100-fold increase in cFOS mRNA expression and 100-fold decrease in NK2 mRNA expression in the whole blood of capsaicin-treated rats. In conclusion, these studies show that tachykinin receptor and TRPV1 mRNA expression undergo significant changes within 24h of systemic low-dose capsaicin administration.

The contractile effect of anandamide in the guinea-pig small intestine is mediated by prostanoids but not TRPV1 receptors or capsaicin-sensitive nerves

Although exogenous and endogenous cannabinoid receptor agonists have well-documented inhibitory effects on gastrointestinal motility, a TRPV1 receptor-mediated excitatory action of anandamide (arachidonoyl ethanolamide, AEA) in the guinea-pig ileum strip has also been described. We used in vitro capsaicin desensitization for assessing the possible participation of sensory neurons in the contractile effect of anandamide on the guinea-pig whole ileum, as well as autonomic drugs and a cyclooxygenase inhibitor for characterizing this response. Isolated organ experiments were used with isotonic recording. Contractions induced by anandamide (1 or 10 μM) were strongly inhibited by tetrodotoxin, indomethacin or atropine plus a tachykinin NK(1) receptor antagonist, but weakly to moderately reduced by atropine alone and partly diminished by the fatty acid amide hydrolase inhibitor URB 597. Neither capsaicin pre-treatment nor the TRPV1 receptor antagonist BCTC, the ganglionic blocking drug hexamethonium or cannabinoid (CB1 or CB2 ) receptor antagonists, influenced the effect of anandamide. It is concluded that the capsaicin-insensitive, neuronal excitatory effect of anandamide in the intestine is most probably mediated by cyclooxygenase products. Such a mechanism may also play a role at other sites in the mammalian body.

The role of transient receptor potential ankyrin 1 (TRPA1) receptor activation in hydrogen-sulphide-induced CGRP-release and vasodilation

Activation of transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) channels on capsaicin-sensitive sensory neurons causes release of inflammatory neuropeptides, including calcitonin gene-related peptide (CGRP). We investigated whether the hydrogen sulphide (H(2)S)-evoked CGRP release from sensory neurons of isolated rat tracheae and H(2)S-induced increases in the microcirculation of the mouse ear were mediated by TRPA1 receptor activation. Allylisothiocyanate (AITC) or the H(2)S donor sodium hydrogen sulphide (NaHS) were used as stimuli and CGRP release of the rat tracheae was measured by radioimmunoassay. AITC or NaHS were applied to the ears of Balb/c, C57BL/6, TRPA1 and TRPV1 receptor gene knockout mice and blood flow was detected by laser Doppler imaging. Both AITC and NaHS increased CGRP release from isolated rat tracheae, and both responses were inhibited by the TRPA1 antagonist, HC-030031, but was not affected by the TRPV1 receptor blocker, BCTC. Application of AITC or NaHS increased the cutaneous blood flow in the mouse ears. Similarly to the effect of AITC, the vasodilatory response to NaHS was reduced by HC-030031 or in TRPA1 deleted mice. In contrast, genetic deletion of TRPV1 did not affect the increase in the ear blood flow evoked by AITC or NaHS. We conclude that H(2)S activates TRPA1 receptors causing CGRP release from sensory nerves of rat tracheae, as well as inducing cutaneous vasodilatation in the mouse ear. TRPV1 receptors were not involved in these processes. Our results highlight that TRPA1 receptor activation should be considered as a potential mechanism of vasoactive effects of H(2)S.

Agonist-dependent potentiation of vanilloid receptor transient receptor potential vanilloid type 1 function by stilbene derivatives

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel activated by capsaicin, low pH, and noxious heat and plays a key role in nociception. Understanding mechanisms for functional modulation of TRPV1 has important implications. One characteristic of TRPV1 is that channel activity induced by either capsaicin or other activators can be sensitized or modulated by factors involving different cell signaling mechanisms. In this study, we describe a novel mechanism for the modulation of TRPV1 function: TRPV1 function is modulated by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and its analogs. We found that, in rat dorsal root ganglion neurons, although DIDS did not induce the activation of TRPV1 per se but drastically increased the TRPV1 currents induced by either capsaicin or low pH. DIDS also blocked the tachyphylaxis of the low pH-induced TRPV1 currents. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), a DIDS analog, failed to enhance the capsaicin-evoked TRPV1 current but increased the low pH-evoked TRPV1 currents, with an effect comparable with that of DIDS. SITS also blocked the low pH-induced tachyphylaxis. DIDS also potentiated the currents of TRPV1 channels expressed in human embryonic kidney 293 cells, with an effect of left-shifting the concentration-response curve of the capsaicin-induced TRPV1 currents. This study demonstrates that DIDS and SITS, traditionally used chloride channel blockers, can modify TRPV1 channel function in an agonist-dependent manner. The results provide new input for understanding TRPV1 modulation and developing new modulators of TRPV1 function.

Alstonia scholaris R. Br. Significantly Inhibits Retinoid-Induced Skin Irritation In Vitro and In Vivo

Topical retinoids inhibit matrix metalloproteinases and accelerate collagen synthesis, thereby triggering antiaging effects in the skin. However, topical retinoids can cause severe skin reactions, including scaling, erythema, papules, and inflammation. The present study demonstrates that the ethanolic bark extract of Alstonia scholaris R. Br. can significantly inhibit all-trans retinoic acid-induced inflammation in human HaCat keratinocyte cells. Furthermore, two representative retinoid-induced proinflammatory cytokines, monocyte chemoattractant protein-1 and interleukin-8, were significantly suppressed by A. scholaris extract (by 82.1% and 26.3% at 100 ppm, and dose-dependently across the tested concentrations) in vitro. In a cumulative irritation patch test, A. scholaris extract decreased retinol-induced skin irritation, while strengthening the ability of retinoids to inhibit matrix metalloproteinase-1 expression, which is strongly associated with aging effects. These results suggest that A. scholaris is a promising compound that may increase the antiaging function of retinoids while reducing their ability to cause skin irritation.

Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors

There is pharmacological evidence that group II and III metabotropic glutamate receptors (mGluRs) function as activity-dependent autoreceptors, inhibiting transmission in supraspinal sites. These receptors are expressed by peripheral nociceptors. We investigated whether mGluRs function as activity-dependent autoreceptors inhibiting pain transmission to the rat CNS, particularly transient receptor potential vanilloid 1 (TRPV1)-induced activity. Blocking peripheral mGluR activity by intraplantar injection of antagonists LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] (LY) (20, 100 μm, group II/III), APICA [(RS)-1-amino-5-phosphonoindan-1-carboxylic acid] (100 μm, group II), or UBP1112 (α-methyl-3-methyl-4-phosphonophenylglycine) (30 μm, group III) increased capsaicin (CAP)-induced nociceptive behaviors and nociceptor activity. In contrast, group II agonist APDC [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] (0.1 μm) or group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) (10 μm) blocked the LY-induced increase. Ca(2+) imaging in dorsal root ganglion (DRG) cells confirmed LY enhanced CAP-induced Ca(2+) mobilization, which was blocked by APDC and l-AP-4. We hypothesized that excess glutamate (GLU) released by high intensity and/or prolonged stimulation endogenously activated group II/III, dampening nociceptor activation. In support of this, intraplantar GLU + LY produced heat hyperalgesia, and exogenous GLU + LY applied to nociceptors produced enhanced nociceptor activity and thermal sensitization. Intraplantar Formalin, known to elevate extracellular GLU, enhanced pain behaviors in the presence of LY. LY alone produced no pain behaviors, no change in nociceptor discharge rate or heat-evoked responses, and no change in cytosolic Ca(2+) in DRG cells, demonstrating a lack of tonic inhibitory control. Group II/III mGluRs maintain an activity-dependent autoinhibition, capable of significantly reducing TRPV1-induced activity. They are endogenously activated after high-frequency and/or prolonged nociceptor stimulation, acting as built-in negative modulators of TRPV1 and nociceptor function, reducing pain transmission to the CNS.

The role of transient receptor potential vanilloid 1 (TRPV1) receptors in dextran sulfate-induced colitis in mice

The aim of this study was to investigate the involvement of transient receptor potential vanilloid 1 (TRPV1) receptors in oral dextran sulfate sodium-induced (DSS) colitis using TRPV1 knockout mice and their wild-type C57BL/6 counterparts. DSS (2% or 5%) was administered orally ad libitum for 7 days; the controls received tap water. Animal weight, stool consistency, and blood content were scored every day to calculate the disease activity index (DAI). After sacrificing the mice on day 7, the colons were cut into three equal segments (proximal, intermediate, and distal) for histology, myeloperoxidase (MPO), and cytokine measurements. In the 2% DSS-treated group, the lack of TRPV1 receptors decreased the DAI. Each colon segment of wild-type animals showed more than two-fold increase of MPO activity and more severe histological changes compared to the knockouts. This difference was not observed in case of 5% DSS, when extremely severe inflammation occurred in both groups. IL-1beta production was not altered by the absence of TRPV1. In conclusion, activation of TRPV1 channels enhances the clinical symptoms, histopathological changes, and neutrophil accumulation induced by 2% DSS. Elucidating the modulator role of TRPV1 channels in inflammatory bowel diseases may contribute to the development of novel anti-inflammatory drugs for their therapy.

TRPV1: on the road to pain relief

Historically, drug research targeted to pain treatment has focused on trying to prevent the propagation of action potentials in the periphery from reaching the brain rather than pinpointing the molecular basis underlying the initial detection of the nociceptive stimulus: the receptor itself. This has now changed, given that many receptors of nociceptive stimuli have been identified and/or cloned. Transient Receptor Potential (TRP) channels have been implicated in several physiological processes such as mechanical, chemical and thermal stimuli detection. Ten years after the cloning of TRPV1, compelling data has been gathered on the role of this channel in inflammatory and neuropathic states. TRPV1 activation in nociceptive neurons, where it is normally expressed, triggers the release of neuropeptides and transmitters resulting in the generation of action potentials that will be sent to higher CNS areas where they will often be perceived as pain. Its activation also will evoke the peripheral release of pro-inflammatory compounds that may sensitize other neurons to physical, thermal or chemical stimuli. For these reasons as well as because its continuous activation causes analgesia, TRPV1 has become a viable drug target for clinical use in the management of pain. This review will provide a general picture of the physiological and pathophysiological roles of the TRPV1 channel and of its structural, pharmacological and biophysical properties. Finally, it will provide the reader with an overall view of the status of the discovery of potential therapeutic agents for the management of chronic and neuropathic pain.

Sensory denervation reduces visceral hypersensitivity in adult rats exposed to chronic unpredictable stress: evidences of neurogenic inflammation

The purpose of this study is to provide evidence of neurogenic inflammation in chronic unpredictable stressed rats with the changes of visceral sensitivity, number of mast cells, and close proximity among mast cell-nerve-blood vessels. We found that (1) capsaicin denervation blocked stress-induced increase of visceral sensitivity, while doxantrazole presented a partial blocking; (2) capsaicin denervation blocked stress-induced enhancement of the proximity of mast cell-nerve fiber-blood vessels and blood vessel damage, while doxantrazole showed no effects on these; (3) doxantrazole blocked stress-induced increases of the MPO activity, the number and the degranulation of mast cells in the colon; (4) sensory denervation and doxantrazole had no effects on stress-induced behavioral inhibition. These results suggest that capsaicin-sensitive sensory fibers play a key role in stress-induced visceral hypersensitivity and the ultrastructural changes, mast cells play an important role in the generation of stress-induced colon inflammation.

Involvement of transient receptor potential vanilloid 1 receptors in protease-activated receptor-2-induced joint inflammation and nociception

Protease-activated receptor-2 (PAR-2) is a G-protein-coupled receptor activated through proteolytic cleavage. It is localized on epithelial, endothelial and inflammatory cells, as well as on transient receptor potential vanilloid 1 (TRPV1) receptor-expressing neurones. It plays an important role in inflammatory/nociceptive processes. Since there are few reports concerning PAR-2 function in joints, the effects of intraarticular PAR-2 activation on joint pain and inflammation were studied. Secondary hyperalgesia/allodynia, spontaneous weight distribution, swelling and inflammatory cytokine production were measured and the involvement of TRPV1 ion channels was investigated in rats and mice. Injection of the PAR-2 receptor agonist SLIGRL-NH(2) into the knee decreased touch sensitivity and weight bearing of the ipsilateral hindlimb in both species. Secondary mechanical allodynia/hyperalgesia and impaired weight distribution were significantly reduced by the TRPV1 antagonist SB366791 in rats and by the genetic deletion of this receptor in mice. PAR-2 activation did not cause significant joint swelling, but increased IL-1beta concentration which was not influenced by the lack of the TRPV1 channel. For comparison, intraplantar SLIGRL-NH(2) evoked similar primary mechanical hyperalgesia and impaired weight distribution in both WT and TRPV1 deficient mice, but oedema was smaller in the knockouts. The inactive peptide, LRGILS-NH(2), injected into either site did not induce any inflammatory or nociceptive changes. These data provide evidence for a significant role of TRPV1 receptors in secondary mechanical hyperalgesia/allodynia and spontaneous pain induced by PAR-2 receptor activation in the knee joint. Although intraplantar PAR-2 activation-induced oedema is also TRPV1 receptor-mediated, primary mechanical hyperalgesia, impaired weight distribution and IL-1beta production are independent of this channel.

Interdisciplinary review for correlation between the plant origin capsaicinoids, non-steroidal antiinflammatory drugs, gastrointestinal mucosal damage and prevention in animals and human beings

BACKGROUND

The plant origin capsaicinoids (capsaicin, dihydrocapsaicin, norcapsaicin, dihydrocapsaicin, homocapsaicin, homodihydrocapsaicin) are well known and used as nutritional additive agents in the every day nutritional practice from the last 9,500 years; however, we had have a very little scientifically based knowledge on their chemistry, physiology and pharmacology in animal observations, and in humans up to the mid-twentieth century. Our knowledge about their chemistry, physiology, pharmacology entered to be scientifically based evidence from the year 1980, dominantly in animal observations. The human observations with capsaicin (capsaicinoids), in terms of good clinical practice, have been started only in the last 10-year period (from 1997) in randomized, prospective, multiclinical studies. The name of "capsaicin" used only in the physiological and pharmacological research both in animal experiments and in human observation. The "capsaicin" (as a "chemically" used natural compound) modifies the so-called capsaicin-sensitive afferent nerves depending on their applied doses.

AIMS

The specific action of capsaicin (capsaicinoids) on sensory afferent nerves modifying gastrointestinal (GI) function (under very specific conditions) offers a possibility for the production of an orally applicable drug or for other drug combinations, which can be used in the human medical therapy. The production of new drug is based on the critical interdisciplinary review of the results obtained with capsaicinoids.

MATERIALS AND METHODS

This paper gives an interdisciplinary and critical overview on the chemical, physiological, pharmacological and toxicological actions of the natural origin capsaicinoids (from the point of drug production) under conditions of acute, subacute and chronic administration in animal experiments and human observations, toxicology, pharmacokinetics). This interdisciplinary review covers the following main chapters: (1) physiological and pharmacological research tool by capsaicin in the animals and human beings, (2) capsaicin research in animals (including the acute, subacute toxicology and chronic toxicology metabolism, genotoxicology), (3) capsaicin observation with capsaicin in human beings.

CONCLUSION

(1) The capsaicin used in the physiological and pharmacological observations (in animals and human beings) chemically represents different chemical compounds, which can be obtained from the plants (paprika, chilli, etc.), (2) capsaicinoids are able to modify the capsaicin-sensitive afferent nerves, which have principle roles in the defence of different organs (including the gastrointestinal tract [against the different chemicals, heat, strech, chemical millieu-induced damage], (3) the application of capsaicin (capsaicinoids) can be repeated for the beneficial effects on the gastrointestinal tract as those in animal experiments. After this interdisciplinary and critical review, this paper demonstrates the well-planned research pathways of the discoveries of capsaicinoids from plant chemistry, via physiology, pharmacology and toxicology in animal experiments and human observations.

Sex differences in inflammation evoked by noxious chemical, heat and electrical stimulation

Neurogenic inflammation (NI) is a feature of several inflammatory pain conditions in which females are overrepresented. Therefore, we asked if there are sex differences in the inflammatory response evoked by well known neurogenic stimuli. We compared the amount of plasma extravasation (PE), a measure of inflammation, in the hindpaw skin of male and female rats caused by subcutaneous injection of capsaicin, application of noxious heat (51 degrees C water bath) or electrical stimulation of the saphenous nerve. We also compared the amount of PE in males and females evoked by substance P (SP), the principal neurogenic mediator of PE. PE was quantified using a video camera and digital image analysis to measure changes in reflectance (pixel intensity, PI) of skin due to accumulation of extravasated Evans blue (EB) dye. The increase in PI induced by capsaicin was significantly greater in females compared to males (p<0.001) and in estrus, diestrus, and metestrus females compared to proestrus females. The time to reach maximal capsaicin-induced PE was two times longer in estrus, diestrus, and metestrus females compared to males (p<0.05). PE induced by heat was also significantly greater in females compared to males (p<0.001), however, there was no sex-related difference in PE induced by electrical stimulation or by injection of SP. These findings show that females have a greater inflammatory response when inflammation is induced by capsaicin and noxious heat suggesting possible sex-related changes in TRPV-1 receptor mediated mechanisms. These results add to the growing list of sex difference responses to noxious somatic stimulation.

Hot target on nociceptors: perspectives, caveats and unique features

Identification of C-polymodal nociceptors and the selective action of capsaicin on them by acting on a putative receptor, which has been cloned 11 years ago, initiated a burst of interest in pharmacology of nociceptors. Capsaicin receptor transient receptor potential vanilloid-1 (TRPV1) being a noxious heat-gated cation channel gated also by several exogenous and endogenous substances serves as a nocisensor to generate graded receptor potentials in these sense organs. Impressive data on pathways involved in sensitization/desensitization of the channel revealed in isolated cells should also validate at the level of nerve endings and lipid raft around TRPV1 could modify the channel gating. Capsaicin-sensitive nociceptors subserve dual sensory-efferent functions: tachykinins and calcitonin gene-related peptide released from them elicit local tissue responses as neurogenic inflammation and release of somatostatin evokes systemic anti-inflammatory and antihyperalgesic effects. TRPV1 gene-deleted mice show subtle changes in physiological regulations, therefore TRPV1 is a promising but challenging target for drug research.

Neuropeptides, neurogenic inflammation and complex regional pain syndrome (CRPS)

This review explains symptoms and nature of neuropeptide signaling and its importance for clinical symptoms of CRPS. Neurogenic inflammation regularly accompanies excitation of primary afferent nociceptors. It has two major components-plasma extravasation and vasodilatation. The most important mediators are the calcitonin gene-related peptide (CGRP) and substance P (SP). After peripheral trauma immune reaction (e.g. cytokines) and the attempts of the tissue to regenerate (e.g. growth factors) sensitize nociceptors and amplify neurogenic inflammation. This cascade of events has been demonstrated in rat models of CRPS. Clinical findings in these animals strongly resemble clinical findings in CRPS, and can be prevented by anti-cytokine and anti-neuropeptide treatment. In CRPS patients, there is meanwhile also plenty of evidence that neurogenic inflammation contributes to clinical presentation. Increased cytokine production was demonstrated, as well as facilitated neurogenic inflammation. Very recently even "non-inflammatory" signs of CRPS (hyperhidrosis, cold skin) have been linked to neuropeptide signaling. Surprisingly, there was even moderately increased neurogenic inflammation in unaffected body regions. This favors the possibility that CRPS patients share genetic similarities. The future search for genetic commonalities will help us to further unravel the "mystery" CRPS.

The concept of neurogenic inflammation

Neurogenic inflammatory responses have recently been linked to both acute and chronic pathological conditions in the urinary tract. Neurogenic inflammation encompasses a series of vascular and non-vascular inflammatory responses, triggered by the activation of primary sensory neurons and the subsequent release of inflammatory neuropeptides, including substance P and calcitonin gene-related peptide. The reduction of neurogenic inflammatory responses may be key in the mode of action of the adrenergic alpha(1)-adrenoceptor antagonists used to treat lower urinary tract symptoms (LUTS). Indeed, the alpha(1)-adrenoceptor antagonist alfuzosin inhibits expression of the oncogene c-fos- a marker of nociceptive pathway activation - evoked by cyclophosphamide in rats. Capsaicin ameliorates urinary bladder symptoms through its stimulatory action on the transient receptor potential vanilloid 1 (TRPV1) calcium channel, resulting in desensitization of bladder sensory nerve terminals. Involvement of the TRP cation channel, subfamily A, member 1 (TRPA1) has also been reported in models of neurogenic inflammation and nociception promoted by the cyclophosphamide metabolite, acrolein. Blockade by alfuzosin demonstrates the beneficial effects of alpha(1)-adrenoceptor antagonists on neurogenic inflammation via the transient receptor potential family of ionic channels. Consequently, these drugs may have an important role in reducing LUTS.

Investigation of sensory neurogenic components in a bleomycin-induced scleroderma model using transient receptor potential vanilloid 1 receptor- and calcitonin gene-related peptide-knockout mice

OBJECTIVE

Along with their classic afferent function (nociception), capsaicin-sensitive transient receptor potential vanilloid 1 (TRPV1) receptor-expressing sensory nerve terminals exert local and systemic efferent activities. Activation of TRPV1 causes sensory neuropeptide release, which modulates the inflammation process. The aim of the present study was to examine the role of this modulatory role of TRPV1 receptor and that of calcitonin gene-related peptide (CGRP) in bleomycin-induced scleroderma, using transgenic mice.

METHODS

Cutaneous sclerosis was induced with daily subcutaneous injections of bleomycin for 30 days. Control groups were treated with phosphate buffered saline (PBS). TRPV1 receptor gene-deficient (TRPV1(-/-)) mice and CGRP-knockout (CGRP(-/-)) mice and their wild-type (WT) counterparts were investigated. A composite sclerosis score was calculated on the basis of thickening, leukocyte infiltration, and the amount/orientation of collagen bundles. Dermal thickness and the number of alpha-smooth muscle actin (alpha-SMA)-positive cells were also determined. The quantity of the collagen-specific amino acid hydroxyproline was measured by spectrophotometry.

RESULTS

Bleomycin treatment induced marked cutaneous thickening and fibrosis compared with that observed in control mice treated with PBS. The composite sclerosis score was 18% higher, dermal thickness was 19% higher, the number of alpha-SMA-positive cells was 47% higher, and the amount of hydroxyproline was 57% higher in TRPV1(-/-) mice than in their WT counterparts. Similarly, the composite sclerosis score was 47% higher, dermal thickness was 29% higher, the number of alpha-SMA-positive cells was 76% higher, and the amount of hydroxyproline was 30% higher in CGRP(-/-) mice than in the respective WT groups.

CONCLUSION

These results suggest that activation of the TRPV1 receptor by mediators of inflammation induces sensory neuropeptide release, which might exert protective action against fibrosis. We confirmed the protective role of CGRP in the development of cutaneous sclerosis.

Enhanced vascular permeability in rat skin induced by sensory nerve stimulation: evaluation of the time course and appropriate stimulation parameters

Activation of nociceptors causes them to secrete neuropeptides. The binding of these peptides to receptors on blood vessels causes vasodilation and increased vascular permeability that allows loss of proteins and fluid (plasma extravasation, PE); this contributes to inflammation. This study defines the relationship between electrical activation of nociceptors and PE and evaluates the time course of this response in the skin of rats. We measured the time course and extent of PE by digital imaging of changes in skin reflectance caused by leakage of Evans Blue (EB) dye infused in the circulatory system before stimulation. Stimulation of the exclusively sensory saphenous nerve caused the skin to become dark blue within 2 min due to accumulation of EB. While PE is usually measured after 5-15 min of electrical stimulation, we found that stimulation for only 1 min at 4 Hz produced maximum PE. This response was dependent on the number of electrical stimuli at least for 4 Hz and 8 Hz stimulation rates. Since accumulation of EB in the skin is only slowly reversible, to determine the duration of enhanced vascular permeability we administered EB at various times after electrical stimulation of the saphenous nerve. PE was only observed when EB was infused within 5 min of electrical stimulation but could still be observed 50 min after capsaicin (1%, 25 microl) injection into the hind paw. These findings indicate that enhanced vascular permeability evoked by electrical stimulation persists only briefly after release of neuropeptides from nociceptors in the skin. Therefore, treatment of inflammation by blockade of neuropeptide release and receptors may be more effective than treatments aimed at epithelial gaps. We propose, in models of stimulation-induced inflammation, the use of a short stimulus train.

Neural mechanisms of pelvic organ cross-sensitization

Clinical observations of viscerovisceral referred pain in patients with gastrointestinal and genitourinary disorders suggest an overlap of neurohumoral mechanisms underlying both bowel and urinary bladder dysfunctions. Close proximity of visceral organs within the abdominal cavity complicates identification of the exact source of chronic pelvic pain, where it originates, and how it relocates with time. Cross-sensitization among pelvic structures may contribute to chronic pelvic pain of unknown etiology and involves convergent neural pathways of noxious stimulus transmission from two or more organs. Convergence of sensory information from discrete pelvic structures occurs at different levels of nervous system hierarchy including dorsal root ganglia, the spinal cord and the brain. The cell bodies of sensory neurons projecting to the colon, urinary bladder and male/female reproductive organs express a wide range of membrane receptors and synthesize many neurotransmitters and regulatory peptides. These substances are released from nerve terminals following enhanced neuronal excitability and may lead to the occurrence of neurogenic inflammation in the pelvis. Multiple factors including inflammation, nerve injury, ischemia, peripheral hyperalgesia, metabolic disorders and other pathological conditions dramatically alter the function of directly affected pelvic structures as well as organs located next to a damaged domain. Defining precise mechanisms of viscerovisceral cross-sensitization would have implications for the development of effective pharmacological therapies for the treatment of functional disorders with chronic pelvic pain such as irritable bowel syndrome and painful bladder syndrome. The complexity of overlapping neural pathways and possible mechanisms underlying pelvic organ crosstalk are analyzed in this review at both systemic and cellular levels.

Diabetes induced by partial hepatic sensory denervation in conscious rabbits

Exposure of the anterior hepatic plexus to 2% perineurial capsaicin solution over three days caused transient insulin resistance confirmed by hyperinsulinaemic euglycaemic glucose clamping. Three additional perineurial capsaicin treatments divided by 3-month intervals yielded diabetes characterized by an increase in fasting blood glucose and glycated haemoglobin levels. Both insulin sensitivity and glycated haemoglobin level re-normalized over an additional 6-month period. We conclude that chronic partial hepatic sensory denervation produces diabetes in rabbits.

Novel agonistic action of mustard oil on recombinant and endogenous porcine transient receptor potential V1 (pTRPV1) channels

Neurogenic components play a crucial role in inflammation and nociception. Mustard oil (MO) is a pungent plant extract from mustard seed, horseradish and wasabi, the main constituent of which is allylisothiocyanate. We have characterized the action of MO on transient receptor potential V1 (TRPV1), a key receptor of signal transduction pathways in the nociceptive system, using fura-2-based [Ca(2+)](i) imaging and the patch-clamp technique in a heterologous expression system and sensory neurons. In human embryonic kidney (HEK) 293 cells expressing porcine TRPV1 (pTRPV1), MO evoked increases of [Ca(2+)](i) in a concentration-dependent manner. A high concentration of MO elicited irreversible cell swelling. Capsazepine, ruthenium red and iodoresiniferatoxin dose-dependently suppressed the MO-induced [Ca(2+)](i) increase. MO elicited outward rectified currents in pTRPV1-expressing HEK 293 cells with a reversal potential similar to that of capsaicin. [Ca(2+)](i) responses to MO were completely abolished by the removal of external Ca(2+). MO simultaneously elicited an inward current and increase of [Ca(2+)](i) in the same cells, indicating that MO promoted Ca(2+) influx through TRPV1 channels. In cultured porcine dorsal root ganglion (DRG) neurons, MO elicited a [Ca(2+)](i) increase and inward current. Among DRG neurons responding to MO, 85% were also sensitive to capsaicin. The present data indicate that MO is a novel agonist of TRPV1 channels, and suggest that the action of MO in vivo may be partly mediated via TRPV1. These results provide an insight into the TRPV1-mediated effects of MO on inflammation and hyperalgesia.

Role of transient receptor potential vanilloid 1 receptors in endotoxin-induced airway inflammation in the mouse

Airways are densely innervated by capsaicin-sensitive sensory neurons expressing transient receptor potential vanilloid 1 (TRPV1) receptors/ion channels, which play an important regulatory role in inflammatory processes via the release of sensory neuropeptides. The aim of the present study was to investigate the role of TRPV1 receptors in endotoxin-induced airway inflammation and consequent bronchial hyperreactivity with functional, morphological, and biochemical techniques using receptor gene-deficient mice. Inflammation was evoked by intranasal administration of Escherichia coli lipopolysaccharide (60 microl, 167 microg/ml) in TRPV1 knockout (TRPV1(-/-)) mice and their wild-type counterparts (TRPV1(+/+)) 24 h before measurement. Airway reactivity was assessed by unrestrained whole body plethysmography, and its quantitative indicator, enhanced pause (Penh), was calculated after inhalation of the bronchoconstrictor carbachol. Histological examination and spectrophotometric myeloperoxidase measurement was performed from the lung. Somatostatin concentration was measured in the lung and plasma with radioimmunoassay. Bronchial hyperreactivity, histological lesions (perivascular/peribronchial edema, neutrophil/macrophage infiltration, goblet cell hyperplasia), and myeloperoxidase activity were significantly greater in TRPV(-/-) mice. Inflammation markedly elevated lung and plasma somatostatin concentrations in TRPV1(+/+) but not TRPV1(-/-) animals. In TRPV1(-/-) mice, exogenous administration of somatostatin-14 (4 x 100 microg/kg ip) diminished inflammation and hyperreactivity. Furthermore, in wild-type mice, antagonizing somatostatin receptors by cyclo-somatostatin (4 x 250 microg/kg ip) increased these parameters. This study provides the first evidence for a novel counterregulatory mechanism during endotoxin-induced airway inflammation, which is mediated by somatostatin released from sensory nerve terminals in response to activation of TRPV1 receptors of the lung. It reaches the systemic circulation and inhibits inflammation and consequent bronchial hyperreactivity.