Advertisement

Neurovascular Aspects of Skin Neurogenic Inflammation

  • Aisah A. Aubdool
    Affiliations
    Cardiovascular Division and Centre for Integrative Biomedicine, BHF King's College London Cardiovascular Centre of Excellence, King's College London, Waterloo Campus, London, UK
    Search for articles by this author
  • Susan D. Brain
    Correspondence
    Cardiovascular Division and Centre for Integrative Biomedicine, BHF King's College London Cardiovascular Centre of Excellence, King's College London, Franklin-Wilkins Building, Waterloo Campus, London, SE1 9NH, UK
    Affiliations
    Cardiovascular Division and Centre for Integrative Biomedicine, BHF King's College London Cardiovascular Centre of Excellence, King's College London, Waterloo Campus, London, UK
    Search for articles by this author
      Neurogenic inflammation is involved in skin inflammation. It is hypothesized that it is involved in the pathogenesis of the common chronic cutaneous vascular disorder rosacea, but the exact mechanism of action is currently unknown. Transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) are widely expressed on primary sensory neuron endings and non-neuronal cells such as keratinocytes. Here we describe the potential for TRPV1 and TRPA1 receptors to be involved in the pathophysiology of rosacea due to their polymodal activation, including cold and hot temperature, pungent products from vegetable and spices, reactive oxygen species, and mechanical stimuli. We discuss the role of both receptors and the sensory neuropeptides that they release in inflammation and pain sensation and evidence suggesting that both TRPV1 and TRPA1 receptors may be promising therapeutic targets for the treatment of the inflammatory symptoms of rosacea.

      Abbreviations:

      CGRP
      calcitonin gene-related peptide
      KC
      keratinocyte
      ROS
      reactive oxygen species
      TRPA1
      transient receptor potential (TRP) ankyrin 1
      TRPV1
      TRP vanilloid receptor 1

      Background to Rosacea and Treatment

      Rosacea, a common and chronic cutaneous vascular disorder, currently affects approximately 45 million people worldwide, over the age of 30 years (
      • Abram K.
      • Silm H.
      • Oona M.
      Prevalence of rosacea in an Estonian working population using a standard classification.
      ). Although women are commonly more affected than men in the early stages of this disorder (
      • Berg M.
      • Liden S.
      An epidemiological study of rosacea.
      ), once affected, men can progress to the advanced stages more often than women (
      • Buechner S.A.
      Rosacea: an update.
      ). The National Rosacea Society committee has developed a classification system that is based on four stages of the lesion morphology of rosacea: erythematotelangiectatic, papulopustular, phymatous, or ocular (
      • Wilkin J.
      • Dahl M.
      • Detmar M.
      • et al.
      Standard classification of rosacea: report of the national rosacea society expert committee on the classification and staging of rosacea.
      ). Although the underlying cause of this disorder is currently unknown, genetic and environmental factors are thought to contribute to the pathogenesis of rosacea (
      • Bamford J.T.
      Rosacea: current thoughts on origin.
      ). Rosacea primarily affects the face and is characterized by persistent redness, inflammation, and lesions of the skin, which may occur because of dysfunctional regulation in the neurovascular system. This skin disorder has multiple associated signs and symptoms, such as increased frequency of skin flushing, itching, persistent intensification of erythema, facial edema, swelling papules, pustules, and fibrosis. The pathophysiology of rosacea is multifactorial, and triggering factors include stress, menopause, and alcohol consumption, environmental exposures such as temperature extremes and sun exposure, certain foods such as spices, wind, and temperature extremes (
      • Crawford G.H.
      • Pelle M.T.
      • James W.D.
      Rosacea: I. Etiology, pathogenesis, and subtype classification.
      ).
      It is well established that rosacea is a recurrent disease that may require long-term therapy such as topical and oral antibiotics, but this may lead to the development of antibiotic-resistant organisms (
      • Berman B.
      • Perez O.A.
      • Zell D.
      Update on rosacea and anti-inflammatory-dose doxycycline.
      ). Inflammation has a central role in the pathogenesis of rosacea and is usually treated with noninflammatory agents; flushing episodes are with vasoconstrictor agents and telangiectasias with laser and light therapy (
      • Baldwin H.E.
      Systemic therapy for rosacea.
      ). There is currently no cure for rosacea, but the combinations of medical and psychological approaches can improve the symptoms of this skin disorder. As there is no laboratory benchmark test, and the basic pathophysiology and etiopathogenesis remain unclear (
      • Crawford G.H.
      • Pelle M.T.
      • James W.D.
      Rosacea: I. Etiology, pathogenesis, and subtype classification.
      ), the diagnosis of rosacea relies on the basis of recognizing morphological characteristics of the disorder. This includes clinical manifestations, histology, and multiple factors initiating or worsening the skin disorder.
      Primary sensory neurons are known to innervate the skin, and their stimulation in response to temperature changes, mechanical or chemical stimuli leads to release of vasoactive inflammatory neuropeptides. The role of these neuropeptides in inducing inflammation qualifies them for a role in the pathogenesis of rosacea. For example, flushing has been suggested to be controlled by two vasodilatory mechanisms including humoral substances and neuronal stimuli (
      • Wilkin J.K.
      Why is flushing limited to a mostly facial cutaneous distribution?.
      ), but the exact mechanism of action is unknown. The recent discovery of the novel class of nonselective cation channels, the transient receptor potential (TRP) channels, has increased our understanding of how sensory nerve endings are activated to release vasoactive neuropeptides, and hence may enhance our knowledge on the pathophysiology of the inflammatory process in different stages of rosacea. This review discusses the pathogenesis of rosacea from current clinical observations and laboratory research, as well as the proposed role of TRP channels such as TRPV1 and TRPA1 in this common skin disorder.

      TRP Channels and Observational Links to the Pathophysiology of Rosacea

      The TRP channel receptor family is currently composed of 28 channels with seven subfamilies based on the sequence homology (
      • Nilius B.
      • Owsianik G.
      • Voets T.
      • et al.
      Transient receptor potential cation channels in disease.
      ). The TRP channels are ubiquitously expressed throughout the body on neuronal and non-neuronal tissues, and are predicted to have six transmembrane proteins, with a pore loop domain between the fifth and sixth transmembrane (
      • Nilius B.
      • Owsianik G.
      • Voets T.
      • et al.
      Transient receptor potential cation channels in disease.
      ). It is well established that TRP channels are polymodal and functionally diverse. Recent studies have focused on vanilloid 1 (TRPV1), also known as the “capsaicin receptor” or the VR1, and ankyrin 1 (TRPA1) channels. Both channels have multiple functions as sensors in cells, and have a prominent role in pain sensation and inflammation (
      • Nilius B.
      • Owsianik G.
      • Voets T.
      • et al.
      Transient receptor potential cation channels in disease.
      ). They respond to a great number of stimuli such as natural chemical compounds, cold or hot temperature extremes, mechanical stimuli (Figure 1), and changes in lipid bilayer (
      • Nilius B.
      • Owsianik G.
      • Voets T.
      • et al.
      Transient receptor potential cation channels in disease.
      ). There are contrasting results in the literature on whether mammalian TRPA1 is mechanosensitive, and this remains an important hypothesis for future research in this field.
      Figure thumbnail gr1
      Figure 1The structure and activators of transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) receptors in primary sensory neuron with a link to rosacea. Both receptors have similar structure, with each subunit composed of six putative transmembrane domains with a pore loop domain between the fifth and sixth transmembrane. TRPA1 and TRPV1 have distinct thermal activation and can be stimulated by pungent products from vegetables and spices. Reactive oxygen species (ROS) can activate TRPA1 and induce vasodilatation. Mediators, such as proteases, released in inflammation function on protease-activated receptor 2 (PAR2) and enhance activation of both receptors. TRPV1 and TRPA1 are colocalized in primary sensory neuron and there is suggestion of a potential interaction between them that regulates their activity.

      TRP Channels as Thermosensors in Rosacea?

      Both TRPA1 and TRPV1 channels display distinct thermal activation threshold; interestingly, a survey conducted by the National Rosacea Society in 2002 showed that hot and cold weather is a common aggravating factor for 53% and 36% of rosacea patients, respectively, (
      • Blount B.W.
      • Pelletier A.L.
      Rosacea: a common, yet commonly overlooked, condition.
      ). It is well known that temperature changes can be linked to pain and burning sensation, as experienced in rosacea patients. Indeed, earlier studies by
      • Brain S.D.
      • Petty R.G.
      • Lewis J.D.
      • et al.
      Cutaneous blood flow responses in the forearms of Raynaud's patients induced by local cooling and intradermal injections of CGRP and histamine.
      have shown that brief exposure of the forearm to cold temperature (∼5 °C) triggers a localized reactive hyperemia in humans, which may involve microvascular mechanisms. Noxious cold temperatures (∼17 °C) can directly activate TRPA1 channels in heterologous expression systems (
      • Story G.M.
      • Peier A.M.
      • Reeve A.J.
      • et al.
      ANKTM1, a TRP-like channel expressed in nociceptive neurons, is activated by cold temperatures.
      ). However, there is significant debate regarding the role of TRPA1 as a “cold sensor” channel, as studies from other groups failed to reproduce cold responsiveness in TRPA1 (
      • Jordt S.E.
      • Bautista D.M.
      • Chuang H.H.
      • et al.
      Mustard oils and cannabinoids excite sensory nerve fibres through the TRP channel ANKTM1.
      ;
      • Nagata K.
      • Duggan A.
      • Kumar G.
      • et al.
      Nociceptor and hair cell transducer properties of TRPA1, a channel for pain and hearing.
      ). Although there is increasing evidence from cellular and behavioral studies in TRPA1-deficient mice that TRPA1 is not required for acute cold-induced pain in vivo (
      • Bautista D.M.
      • Jordt S.E.
      • Nikai T.
      • et al.
      TRPA1 mediates the inflammatory actions of environmental irritants and proalgesic agents.
      ), other groups are showing that pharmacological blockade of TRPA1 channels in primary sensory neurons can reverse cold-induced hyperalgesia caused by inflammation and nerve injury (
      • Obata K.
      • Katsura H.
      • Mizushima T.
      • et al.
      TRPA1 induced in sensory neurons contributes to cold hyperalgesia after inflammation and nerve injury.
      ).
      • Karashima Y.
      • Talavera K.
      • Everaerts W.
      • et al.
      TRPA1 acts as a cold sensor in vitro and in vivo.
      also recently showed that TRPA1 functions as a sensor for noxious cold both in vivo and in vitro. Using whole-cell patch recordings, they showed that TRPA1-expressing Chinese Hamster ovary cells were activated by cold temperature (10 °C). Furthermore, they demonstrated that cold-induced nociceptive behavior was dependent on TRPA1 in mice in vivo (
      • Karashima Y.
      • Talavera K.
      • Everaerts W.
      • et al.
      TRPA1 acts as a cold sensor in vitro and in vivo.
      ).
      Interestingly, another TRP channel (melastin) TRPM8 is also activated at temperatures below ∼23 °C (
      • Colburn R.W.
      • Lubin M.L.
      • Stone Jr, D.J.
      • et al.
      Attenuated cold sensitivity in TRPM8 null mice.
      ). Using TRPM8/TRPA1 double-knockout mice,
      • Knowlton W.M.
      • Bifolck-Fisher A.
      • Bautista D.M.
      • et al.
      TRPM8, but not TRPA1, is required for neural and behavioral responses to acute noxious cold temperatures and cold-mimetics in vivo.
      recently showed that noxious cold signaling is exclusive to TRPM8 only, mediating neural and behavioral responses to cold and cold mimetics. Nevertheless, the results from
      • del Camino D.
      • Murphy S.
      • Heiry M.
      • et al.
      TRPA1 contributes to cold hypersensitivity.
      also support the findings that TRPA1 is likely to have a comparatively minor role in acute cold sensation. They showed that once TRPA1 receptors are activated by an agonist in vitro and in vivo, they have a significant role in producing cold allodynia; hence, TRPA1 is now considered a key mediator of cold hypersensitivity in disease states where endogenous proinflammatory mediators of the channel are present. It remains to be determined whether cold-induced inflammatory responses in rosacea are directly linked to TRPA1 or TRPM8 channels. Both channels may have therapeutic potential.
      Sun exposure and hot baths, or exercises are two common precipitating factors that develop or aggravate the symptoms of rosacea as they dilate cutaneous vessels because of increase in core temperature (
      • Bae Y.I.
      • Yun S.J.
      • Lee J.B.
      • et al.
      Clinical evaluation of 168 Korean patients with rosacea: the sun exposure correlates with the erythematotelangiectatic subtype.
      ). Facial skin temperature has also been reported to be higher in rosacea patients, and this increases the growth of bacteria (
      • Dahl M.V.
      • Ross A.J.
      • Schlievert P.M.
      Temperature regulates bacterial protein production: possible role in rosacea.
      ). Although the skin microcirculation is used to regulate body temperature, the exact mechanism of action is not clear. Interestingly, using patch–clamp methods,
      • Caterina M.J.
      • Schumacher M.A.
      • Tominaga M.
      • et al.
      The capsaicin receptor: a heat-activated ion channel in the pain pathway.
      showed that upon exposure to a rapid increase in temperature (22–48 °C in 25 seconds), transfected human embryonic kidney cells expressing TRPV1 produced similar large inward currents evoked by the application of a TRPV1 agonist. These findings confirm TRPV1 as a thermal transducer and that TRPV1-lacking mice are deficient in responses to noxious heat and acute thermal stimuli (
      • Caterina M.J.
      • Leffler A.
      • Malmberg A.B.
      • et al.
      Impaired nociception and pain sensation in mice lacking the capsaicin receptor.
      ). These results highlight that TRPV1 channels have an essential role in thermal hypersensitivity in the setting in inflammation.
      Using quantitative thermal (32–50 °C) sensory testing,
      • Guzman-Sanchez D.A.
      • Ishiuji Y.
      • Patel T.
      • et al.
      Enhanced skin blood flow and sensitivity to noxious heat stimuli in papulopustular rosacea.
      showed that the skin area affected by rosacea has a significantly lower heat pain threshold as compared with non-affected area in rosacea patients or control subjects. The pathophysiology of this cutaneous hypersensitivity is not fully understood but is suggested to involve a neurogenic etiology and may involve TRPV1 activation. Sensitive skin is known to be characterized by various symptoms including prickling, burning, flushing, and pain. Numerous studies in the past decade have focused on developing novel selective and chemically diverse TRPV1 antagonists, as well as investigating their effects in preclinical models of pain and inflammation. Acute pharmacological antagonism of TRPV1 channels can lead to pronounced hyperthermia in mouse and humans, but not in TRPV1 knockout mice (
      • Gavva N.R.
      • Bannon A.W.
      • Hovland Jr, D.N.
      • et al.
      Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade.
      ,
      • Gavva N.R.
      • Treanor J.J.
      • Garami A.
      • et al.
      Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans.
      ). However, repeated dosing of TRPV1 antagonists to rats, dogs, and monkeys attenuated this hyperthermia response (
      • Gavva N.R.
      • Bannon A.W.
      • Hovland Jr, D.N.
      • et al.
      Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade.
      ); this finding suggests that TRPV1 can have a pivotal role in core body temperature homeostasis at the receptor level (
      • Gunthorpe M.J.
      • Chizh B.A.
      Clinical development of TRPV1 antagonists: targeting a pivotal point in the pain pathway.
      ). The mechanisms are unknown; however, it has been suggested that TRPV1 may regulate temperature via peripheral mechanisms in the abdominal cavity (
      • Steiner A.A.
      • Turek V.F.
      • Almeida M.C.
      • et al.
      Nonthermal activation of transient receptor potential vanilloid-1 channels in abdominal viscera tonically inhibits autonomic cold-defence effectors.
      ).
      From the above information, one may hypothesize that hot and cold temperatures may directly activate TRPV1 and TRPA1, respectively, and mediate the release of neuropeptides that enhance inflammation in rosacea. Moreover, we hypothesize that if these channels are involved in the onset of rosacea, selective antagonists may function as beneficial therapeutic agents.

      Spices Influence Rosacea via Activation of TRP Channels?

      Rosacea patients often experience increased skin sensitivity and frequent episodes of burning sensation after the consumption of spices (
      • Crawford G.H.
      • Pelle M.T.
      • James W.D.
      Rosacea: I. Etiology, pathogenesis, and subtype classification.
      ). Interestingly, exogenous agonists for both TRPA1 and TRPV1 channels are compounds derived from natural products such as vegetables and spices (Figure 1). The activation of TRPV1 by capsaicin, the pungent chemical from chilli peppers, and TRPA1 by cinnamaldehyde, the main constituent of cinnamon, lead to intense and acute painful burning sensation (
      • Nilius B.
      • Owsianik G.
      • Voets T.
      • et al.
      Transient receptor potential cation channels in disease.
      ), which may be similar to that observed in rosacea.
      Apart from quantitative sensory testing, blood flow is another sensitive marker of C-fiber neurovascular dysfunction, and hence may account for the significant burning and flushing in rosacea (
      • Guzman-Sanchez D.A.
      • Ishiuji Y.
      • Patel T.
      • et al.
      Enhanced skin blood flow and sensitivity to noxious heat stimuli in papulopustular rosacea.
      ). Interestingly, prolonged flushing, which lasts longer than 10 minutes (
      • Crawford G.H.
      • Pelle M.T.
      • James W.D.
      Rosacea: I. Etiology, pathogenesis, and subtype classification.
      ) in response to spices, alcohol, food intolerance (
      • Wilkin J.K.
      Why is flushing limited to a mostly facial cutaneous distribution?.
      ), menopausal, blushing, or vasodilator therapy (
      • Wilkin J.K.
      Vasodilator rosacea.
      ), is known to be the earliest apparent component of rosacea. Studies have shown that rosacea patients have dilated microvasculature and there is an increase in blood flow in skin lesions of individuals with papulopustular rosacea as compared with control subjects (
      • Guzman-Sanchez D.A.
      • Ishiuji Y.
      • Patel T.
      • et al.
      Enhanced skin blood flow and sensitivity to noxious heat stimuli in papulopustular rosacea.
      ).
      There is also some evidence to suggest that TRPA1 is expressed in the skin (
      • Atoyan R.
      • Shander D.
      • Botchkareva N.V.
      Non-neuronal expression of transient receptor potential type A1 (TRPA1) in human skin.
      ;
      • Denda M.
      • Tsutsumi M.
      • Goto M.
      • et al.
      Topical application of TRPA1 agonists and brief cold exposure accelerate skin permeability barrier recovery.
      ).
      • Atoyan R.
      • Shander D.
      • Botchkareva N.V.
      Non-neuronal expression of transient receptor potential type A1 (TRPA1) in human skin.
      used real-time PCR and western blot analysis to show TRPA1 mRNA and protein levels, respectively, in primary cultures of human epidermal keratinocytes (KCs). Furthermore, using immunofluorescence analysis, they showed that TRPA1 was located in the basal layer of the epidermis, in the dermis, and in the epithelium of the hair follicle (
      • Atoyan R.
      • Shander D.
      • Botchkareva N.V.
      Non-neuronal expression of transient receptor potential type A1 (TRPA1) in human skin.
      ). Activation of TRPA1 by icilin, a pungent chemical from garlic, in primary human epidermal KCs caused an increase in the expression of proinflammatory cytokine IL-1, which is a key contributor to skin inflammation, and this finding suggests a possible functional role for TRPA1 in the human KCs (
      • Atoyan R.
      • Shander D.
      • Botchkareva N.V.
      Non-neuronal expression of transient receptor potential type A1 (TRPA1) in human skin.
      ). Immunoreactivity to TRPA1 antibody was also observed throughout the mouse epidermis, and topical application of TRPA1 agonist (mustard oil or cinnamaldehyde) or brief cold exposure (10–15 °C) accelerated the epidermal permeability barrier recovery rate by a TRPA1-dependent mechanism (
      • Denda M.
      • Tsutsumi M.
      • Goto M.
      • et al.
      Topical application of TRPA1 agonists and brief cold exposure accelerate skin permeability barrier recovery.
      ). Further studies are needed for a better understanding of the function of TRPA1 in epidermal KCs. Interestingly, our group has shown that capsaicin, the classical exogenous TRPV1 agonist, stimulates an increase in mouse skin blood flow, which is dependent on the release of vasodilator neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP;
      • Grant A.D.
      • Gerard N.P.
      • Brain S.D.
      Evidence of a role for NK1 and CGRP receptors in mediating neurogenic vasodilatation in the mouse ear.
      ;
      • Starr A.
      • Graepel R.
      • Keeble J.
      • et al.
      A reactive oxygen species-mediated component in neurogenic vasodilatation.
      ), and that the exogenous TRPA1 agonist mustard oil has a similar effect (
      • Grant A.D.
      • Pinter E.
      • Salmon A.M.
      • et al.
      An examination of neurogenic mechanisms involved in mustard oil-induced inflammation in the mouse.
      ). Our most recent study showed that another TRPA1 agonist, cinnamaldehyde, can also cause neurogenic vasodilatation in mouse skin, which is TRPA1 dependent (
      • Pozsgai G.
      • Bodkin J.V.
      • Graepel R.
      • et al.
      Evidence for the pathophysiological relevance of TRPA1 receptors in the cardiovascular system in vivo.
      ). These findings highlight that TRPA1 and TRPV1 may be activated by the triggers of rosacea, such as cold or hot temperatures and spices, to mediate flushing or burning sensation episodes of rosacea.
      TRPA1 is co-expressed in approximately 50% of all TRPV1-positive sensory neurons (
      • Andersson D.A.
      • Gentry C.
      • Moss S.
      • et al.
      Transient receptor potential A1 is a sensory receptor for multiple products of oxidative stress.
      ), and this suggests a potential interaction between the two receptors (Figure 2), although the potential mechanisms are under debate.
      • Salas M.M.
      • Hargreaves K.M.
      • Akopian A.N.
      TRPA1-mediated responses in trigeminal sensory neurons: interaction between TRPA1 and TRPV1.
      showed that mustard oil-gated currents showed faster kinetics activation in TRPA1-expressing than in TRPA1/TRPV1-co-expressing Chinese Hamster ovary cells in vitro. They suggested that there might be a downregulation of TRPA1 expression in TRPV1 knockout mice. Our recent study in vivo showed that TRPA1-mediated vasodilatation following topical application of mustard oil was significantly potentiated in TRPV1 knockout mice or CD1 mice pretreated with selective antagonist SB366791 (
      • Aubdool A.A.
      • Graepel R.
      • Brain S.D.
      A potential link between the TRPA1 and TRPV1 receptors in vivo.
      ). Hence, there may be a potential link between receptors function and TRPV1 may be involved in regulating TRPA1 receptor-mediated responses. It remains unknown whether TRPA1–TRPV1 co-expression is exclusive on sensory neurons or whether it is also seen in non-neuronal tissues. Future studies investigating TRPA1 and TRPV1 expression profile and their interactions would provide details on the pharmacological properties and function of these receptors, and may potentially help in understanding how they may be involved in the pathogenesis of rosacea.
      Figure thumbnail gr2
      Figure 2Potential mechanisms of transient receptor potential vanilloid 1 (TRPV1)- and ankyrin 1 (TRPA1)-mediated inflammatory responses in rosacea. TRPA1 and TRPV1 receptors are expressed on neuronal and non-neuronal tissues. Their activation leads to opening of the nonselective cation channels, increasing the intracellular calcium concentrations and mediating the release of neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP). SP can mediate an increase in vasodilatation and vascular permeability, leading to edema formation, and can also stimulate mast cells to release inflammatory mediators such as histamine. SP can also prime leukocytes to induce the release of proteases and reactive oxygen species (ROS). CGRP relaxes arteries and also mediates an increase in cutaneous blood flow. These inflammatory processes potentially lead to pain and itch.

      Substance P and CGRP in Rosacea

      Our studies have revealed that both activation of TRPV1 by capsaicin and TRPA1 by mustard oil mediates the release of neuropeptides (
      • Grant A.D.
      • Gerard N.P.
      • Brain S.D.
      Evidence of a role for NK1 and CGRP receptors in mediating neurogenic vasodilatation in the mouse ear.
      ;
      • Starr A.
      • Graepel R.
      • Keeble J.
      • et al.
      A reactive oxygen species-mediated component in neurogenic vasodilatation.
      ).
      • Chizh B.A.
      • O'Donnell M.B.
      • Napolitano A.
      • et al.
      The effects of the TRPV1 antagonist SB-705498 on TRPV1 receptor-mediated activity and inflammatory hyperalgesia in humans.
      demonstrated that topical application of capsaicin cream evoked a robust flare in human skin, which was ultimately blocked by administration of the TRPV1 antagonist SB705498. In addition,
      • Sinclair S.R.
      • Kane S.A.
      • Van der Schueren B.J.
      • et al.
      Inhibition of capsaicin-induced increase in dermal blood flow by the oral CGRP receptor antagonist, telcagepant (MK-0974).
      recently demonstrated that the novel oral CGRP antagonist, telcagepant, was able to inhibit capsaicin-induced dermal blood flow on the human forearm. These results highlight that the axon reflex flare following TRPV1 activation is due to the release of CGRP. CGRP is one of the most potent microvascular vasodilator found in human skin, and it enhances local inflammation as a consequence of increased blood flow and its ability to modulate cell activities (
      • Brain S.D.
      • Williams T.J.
      • Tippins J.R.
      • et al.
      Calcitonin gene-related peptide is a potent vasodilator.
      ,
      • Brain S.D.
      • Tippins J.R.
      • Morris H.R.
      • et al.
      Potent vasodilator activity of calcitonin gene-related peptide in human skin.
      ; Figure 2). Although both CGRP and SP have properties closely related to the pathogenesis of rosacea, the role of CGRP in rosacea remains unknown.
      Flushing results from the transient filling of the dilated microvasculature (
      • Marks R.
      The enigma of rosacea.
      ), and this may potentially also involve neuropeptides such as SP, known to have neuromediated vasodilator properties, functioning via neurokinin-1 receptors (
      • Powell F.C.
      • Corbally N.
      • Powell D.
      Substance P and rosacea.
      ). Earlier studies by
      • Kurkcuoglu N.
      • Alaybeyi F.
      Substance P immunoreactivity in rosacea.
      showed that there is an increase in SP-immunoreactive nerves around the papillary dermal blood vessels in erythematous papules of rosacea patients and, interestingly, the serum level of this neuropeptide was shown to be elevated in rosacea patients as compared with control subjects (
      • Powell F.C.
      • Corbally N.
      • Powell D.
      Substance P and rosacea.
      ). There is evidence that nerve fibers in healthy human skin express TRP channels such as TRPV1 and TRPM8 that co-localize with SP and CGRP (
      • Axelsson H.E.
      • Minde J.K.
      • Sonesson A.
      • et al.
      Transient receptor potential vanilloid 1, vanilloid 2 and melastatin 8 immunoreactive nerve fibers in human skin from individuals with and without norrbottnian congenital insensitivity to pain.
      ). Thus, activation of the TRPV1 receptor may enhance SP and CGRP release. It would be interesting to investigate whether the increase in serum level of SP is due to the activation of TRPV1, and whether the level of expression of TRPV1 channels is altered in rosacea.
      • Lonne-Rahm S.
      • Nordlind K.
      • Edstrom D.W.
      • et al.
      Laser treatment of rosacea - a pathoetiological study.
      showed that laser treatment reduced facial sensitivity, sensory nerve marker, and SP-positive nerve fibers in the papillary dermis of 31 rosacea patients. There is evidence that SP can also induce skin edema formation and, interestingly, genetic deletion or pharmacological blockade of neurokinin-1-receptor reduces polymorphonuclear leukocyte accumulation in inflamed skin of mice (
      • Cao T.
      • Gerard N.P.
      • Brain S.D.
      Analysis of neurokinin-1 (NK1) receptor-mediated oedema formation: use of NK1 knockout mice.
      ;
      • Costa S.K.P.
      • Yshii L.M.
      • Poston R.N.
      • et al.
      Pivotal role of endogenous tachykinins and the NK1 receptor in mediating leukocyte accumulation, in the absence of oedema formation, in response to TNF alpha in the cutaneous microvasculature.
      ). Inflammatory edema is known to appear rapidly if facial cutaneous lymphatic vessels are acutely damaged. Furthermore, there is a proliferation of widely dilated irregular vessels in the disorganized dermis of facial skin biopsies taken from patients experiencing an inflammatory episode in rosacea (
      • Marks R.
      • Harcourt-Webster J.N.
      Histopathology of rosacea.
      ). Telangiectasia and hyperplasia of sebaceous glands represent the later phase of vascular rosacea (
      • Bamford J.T.
      Rosacea: current thoughts on origin.
      ), and telangiectasia results because of massive dilation of the most superficial capillaries. Interestingly, experiments conducted by
      • Foreman J.C.
      • Jordan C.C.
      • Oehme P.
      • et al.
      Structure activity relationships for some substance-P-related peptides that cause wheal and flare reactions in human-skin.
      suggested that SP functions additionally to stimulate mast cells to release histamine and 5-hydroxytryptamine (
      • Brain S.D.
      • Cox H.M.
      Neuropeptides and their receptors: innovative science providing novel therapeutic targets.
      ). These mediators bind to histamine H1 receptors and serotonin 5-hydroxytryptamine-1 receptors, respectively, causing nerve-mediated vasodilatation and protein exudation. Moreover, earlier studies showed that CGRP could potentiate edema formation if SP is active, and that this induces increased vascular permeability in a certain vascular bed of rabbit dorsal skin (
      • Brain S.D.
      • Williams T.J.
      Inflammatory edema induced by synergism between calcitonin gene-related peptide (CGRP) and mediators of increased vascular permeability.
      ). Hence, CGRP may function synergistically with other inflammatory mediators such as SP to cause local edema. In addition, CGRP also inhibits the degradation of SP, and hence it facilitates its activity (
      • Brain S.D.
      • Cox H.M.
      Neuropeptides and their receptors: innovative science providing novel therapeutic targets.
      ). These findings suggest that the blockade of neuropeptide-driven inflammatory processes resulting from activation of TRPV1 and TRPA1 receptors may prove to be a novel therapeutic approach to treating rosacea.

      Reactive Oxygen Species in Rosacea

      Substance P can prime polymorphonuclear neutrophils in humans by functioning on neurokinin-1 receptors when exposed to cytokines such as IL-8 (
      • Dianzani C.
      • Lombardi G.
      • Collino M.
      • et al.
      Priming effects of substance P on calcium changes evoked by interleukin-8 in human neutrophils.
      ), and it can also induce the release of neutrophil-derived mediators such as reactive oxygen species (ROS;
      • Sterner-Kock A.
      • Braun R.K.
      • van der Vliet A.
      • et al.
      Substance P primes the formation of hydrogen peroxide and nitric oxide in human neutrophils.
      ). There is evidence in the literature suggesting that inflammation in the early stage of rosacea may be linked with ROS such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, which are released by inflammatory cells such as neutrophils (
      • Miyachi Y.
      Pharmacologic modulation of neutrophil functions.
      ).
      • Bakar O.
      • Demircay Z.
      • Yuksel M.
      • et al.
      The effect of azithromycin on reactive oxygen species in rosacea.
      showed that the level of ROS was significantly higher in the facial skin of patients with rosacea compared with healthy control subjects or the same rosacea patients after being treated with the antibiotic azithromycin. NADPH oxidase is an enzyme involved in the production of ROS in neutrophils, and is known to be involved in microbicidal activity (
      • Nabeebaccus A.
      • Zhang M.
      • Shah A.M.
      NADPH oxidases and cardiac remodelling.
      ). Previous studies have shown that cathelicidin, a potent inflammatory peptide that is upregulated in rosacea (
      • Yamasaki K.
      • Di Nardo A.
      • Bardan A.
      • et al.
      Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea.
      ), can stimulate ROS generation by activation of NADPH oxidase and intracellular Ca2+ mobilization (
      • Zheng Y.
      • Niyonsaba F.
      • Ushio H.
      • et al.
      Cathelicidin LL-37 induces the generation of reactive oxygen species and release of human alpha-defensins from neutrophils.
      ). It is known that ROS are produced endogenously in inflammation and hence may have a pivotal role in mediating inflammatory hyperalgesia (
      • Keeble J.E.
      • Bodkin J.V.
      • Liang L.
      • et al.
      Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms.
      ). Interestingly, a link between ROS and TRPV1 receptor activation has been previously established in our group (
      • Starr A.
      • Graepel R.
      • Keeble J.
      • et al.
      A reactive oxygen species-mediated component in neurogenic vasodilatation.
      ;
      • Keeble J.E.
      • Bodkin J.V.
      • Liang L.
      • et al.
      Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms.
      ). TRPV1-mediated increase in cutaneous blood flow was shown to be attenuated in the presence of the ROS-degrading enzymes, superoxide dismutase, and the hydrogen peroxidase scavenger, catalase, and most importantly with treatment of selective NADPH inhibitor, apocynin, as well as in mice in which gp91phox gene was deleted (
      • Starr A.
      • Graepel R.
      • Keeble J.
      • et al.
      A reactive oxygen species-mediated component in neurogenic vasodilatation.
      ). The proposed mechanism of action is that activation of the TRPV1 receptor by capsaicin mediates the release of vasodilatory neuropeptides such as SP and CGRP. This in turn functions on vascular receptors to stimulate NADPH oxidase and hence mediate ROS-dependent relaxation (Figure 2). These results suggest that endogenous inflammatory mediators such as ROS released in rosacea may be involved in mediating TRPV1-mediated neurogenic vasodilatation, and there is also recent evidence that TRPA1 may function as an oxidant sensor for vasodilator responses in vivo (
      • Graepel R.
      • Fernandes E.S.
      • Aubdool A.A.
      • et al.
      4-Oxo-2-nonenal (4-ONE): evidence of transient receptor potential ankyrin 1-dependent and -independent nociceptive and vasoactive responses in vivo.
      ). The removal of ROS production by antagonism of TRPV1 or TRPA1 receptors may be a novel therapeutic target for anti-inflammatory drugs in rosacea.

      Proteases in Rosacea

      It is also known that protease activity is higher in the facial skin of rosacea patients, and that antibiotics such as tetracyclines can indirectly inhibit serine proteases (
      • Acharya M.R.
      • Venitz J.
      • Figg W.D.
      • et al.
      Chemically modified tetracyclines as inhibitors of matrix metalloproteinases.
      ).
      • Yamasaki K.
      • Di Nardo A.
      • Bardan A.
      • et al.
      Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea.
      demonstrated that there is an increase in KC serine protease activity in facial skin where symptoms of rosacea aggravate compared with non-affected sites. Endogenous proteases released by neutrophils may also degrade elastin and collagen, which further leads to a compromised lymphatic system in the skin (
      • Miyachi Y.
      Pharmacologic modulation of neutrophil functions.
      ). They also activate protease-activated receptors (PARs), which are a group of seven transmembrane protein receptors. There are currently four members of PAR (PAR1–4), and PAR2 has been shown to be widely expressed in the human skin, and it is activated by trypsin and chymotrypsin, which contribute to inflammatory responses (
      • Steinhoff M.
      • Buddenkotte J.
      • Shpacovitch V.
      • et al.
      Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response.
      ). PAR2 activation has also been shown to induce neurogenic inflammation by stimulating the release of SP and CGRP from the spinal afferent neurons (
      • Steinhoff M.
      • Vergnolle N.
      • Young S.H.
      • et al.
      Agonists of proteinase-activated receptor 2 induce inflammation by a neurogenic mechanism.
      ). The proinflammatory effects of PAR2 activation was also shown to be mediated by nitric oxide, E-selectin, intercellular adhesion molecule-1, and IL-6 (
      • Seeliger S.
      • Derian C.K.
      • Vergnolle N.
      • et al.
      Proinflammatory role of proteinase-activated receptor-2 in humans and mice during cutaneous inflammation in vivo.
      ) during cutaneous inflammation in vivo (
      • Seeliger S.
      • Derian C.K.
      • Vergnolle N.
      • et al.
      Proinflammatory role of proteinase-activated receptor-2 in humans and mice during cutaneous inflammation in vivo.
      ).
      Interestingly, several studies have suggested that PAR2 activation can enhance the activation of TRPV1 receptors (see
      • Grant A.
      • Amadesi S.
      • Bunnett N.W.
      Protease-activated receptors: mechanisms by which proteases sensitize TRPV channels to induce neurogenic inflammation and pain.
      ), through a protein kinase C-dependent pathway (
      • Dai Y.
      • Moriyama T.
      • Higashi T.
      • et al.
      Proteinase-activated receptor 2-mediated potentiation of transient receptor potential vanilloid subfamily 1 activity reveals a mechanism for proteinase-induced inflammatory pain.
      ).
      • Amadesi S.
      • Nie J.
      • Vergnolle N.
      • et al.
      Protease-activated receptor 2 sensitizes the capsaicin receptor transient receptor potential vanilloid receptor 1 to induce hyperalgesia.
      showed that PAR2 agonist can sensitize TRPV1-mediated calcium mobilization in vitro, and that PAR2-induced thermal hyperalgesia is abolished in TRPV1 knockout mice. There is also evidence that PAR2 and TRPA1 are colocalized in rat DRG neurons, and PAR2 activation can increase TRPA1 currents upon activation on HEK293 cells transfected with TRPA1 (
      • Dai Y.
      • Wang S.
      • Tominaga M.
      • et al.
      Sensitization of TRPA1 by PAR2 contributes to the sensation of inflammatory pain.
      ). Thus, it may be possible that PAR2 is activated by proteases released in inflammatory processes in rosacea, and this in turn leads to sensitization of TRPV1 and TRPA1, which further results in exacerbated TRPV1- and TRPA1-mediated inflammatory responses.

      Conclusion

      This review has discussed a range of mechanisms by which TRPV1 and TRPA1 may contribute to the pathogenesis of rosacea. Although our knowledge is incomplete, the central role in cellular skin reactivity to various triggers of rosacea might be attributed to both TRPA1 and TRPV1 receptors, as they are located on neuronal and non-neuronal cells. The activation of both receptors particularly in stimulating sensory neuronal C-fibers to release vasoactive peptides such as CGRP and SP may aggravate the symptoms of rosacea. The role of TRPA1 and TRPV1 in inflammation and pain sensation has been extensively studied. New evidence also suggests that the co-expression of the receptors on the sensory neuron may regulate the activity of each other as a result of interactions between them at the receptor level. Understanding the role of TRPV1 and TRPA1 in experimental models of rosacea and in human studies will enhance our knowledge on the pathogenesis of rosacea. Evidence in the literature so far strongly suggests that both TRPV1 and TRPA1 may be involved in the inflammatory stages of this skin disorder. The development of potent and highly selective dual TRPV1/TRPA1 antagonist may reduce the neurogenic-induced inflammatory symptoms of rosacea and may be a beneficial therapeutic treatment for rosacea patients.

      ACKNOWLEDGMENTS

      We thank JV Bodkin and E-S Fernandes for editorial assistance while preparing the manuscript. This work was supported by a Capacity Building Award in Integrative Mammalian Biology funded by the BBSRC, BPS, HEFCE, KTN, MRC, and SFC.

      REFERENCES

        • Abram K.
        • Silm H.
        • Oona M.
        Prevalence of rosacea in an Estonian working population using a standard classification.
        Acta Derm Venereol. 2010; 90: 269-273
        • Acharya M.R.
        • Venitz J.
        • Figg W.D.
        • et al.
        Chemically modified tetracyclines as inhibitors of matrix metalloproteinases.
        Drug Resist Updat. 2004; 7: 195-208
        • Amadesi S.
        • Nie J.
        • Vergnolle N.
        • et al.
        Protease-activated receptor 2 sensitizes the capsaicin receptor transient receptor potential vanilloid receptor 1 to induce hyperalgesia.
        J Neurosci. 2004; 24: 4300-4312
        • Andersson D.A.
        • Gentry C.
        • Moss S.
        • et al.
        Transient receptor potential A1 is a sensory receptor for multiple products of oxidative stress.
        J Neurosci. 2008; 28: 2485-2494
        • Atoyan R.
        • Shander D.
        • Botchkareva N.V.
        Non-neuronal expression of transient receptor potential type A1 (TRPA1) in human skin.
        J Invest Dermatol. 2009; 129: 2312-2315
        • Aubdool A.A.
        • Graepel R.
        • Brain S.D.
        A potential link between the TRPA1 and TRPV1 receptors in vivo.
        Inflamm Res. 2011; 60: S203
        • Axelsson H.E.
        • Minde J.K.
        • Sonesson A.
        • et al.
        Transient receptor potential vanilloid 1, vanilloid 2 and melastatin 8 immunoreactive nerve fibers in human skin from individuals with and without norrbottnian congenital insensitivity to pain.
        Neuroscience. 2009; 162: 1322-1332
        • Bae Y.I.
        • Yun S.J.
        • Lee J.B.
        • et al.
        Clinical evaluation of 168 Korean patients with rosacea: the sun exposure correlates with the erythematotelangiectatic subtype.
        Ann Dermatol. 2009; 21: 243-249
        • Bakar O.
        • Demircay Z.
        • Yuksel M.
        • et al.
        The effect of azithromycin on reactive oxygen species in rosacea.
        Clin Exp Dermatol. 2007; 32: 197-200
        • Baldwin H.E.
        Systemic therapy for rosacea.
        Skin Therapy Lett. 2007; 12 (9): 1-5
        • Bamford J.T.
        Rosacea: current thoughts on origin.
        Semin Cutan Med Surg. 2001; 20: 199-206
        • Bautista D.M.
        • Jordt S.E.
        • Nikai T.
        • et al.
        TRPA1 mediates the inflammatory actions of environmental irritants and proalgesic agents.
        Cell. 2006; 124: 1269-1282
        • Berg M.
        • Liden S.
        An epidemiological study of rosacea.
        Acta Derm Venereol. 1989; 69: 419-423
        • Berman B.
        • Perez O.A.
        • Zell D.
        Update on rosacea and anti-inflammatory-dose doxycycline.
        Drugs Today (Barc). 2007; 43: 27-34
        • Blount B.W.
        • Pelletier A.L.
        Rosacea: a common, yet commonly overlooked, condition.
        Am Fam Physician. 2002; 66: 435-440
        • Brain S.D.
        • Cox H.M.
        Neuropeptides and their receptors: innovative science providing novel therapeutic targets.
        Br J Pharmacol. 2006; 147: S202-S211
        • Brain S.D.
        • Petty R.G.
        • Lewis J.D.
        • et al.
        Cutaneous blood flow responses in the forearms of Raynaud's patients induced by local cooling and intradermal injections of CGRP and histamine.
        Br J Clin Pharmacol. 1990; 30: 853-859
        • Brain S.D.
        • Tippins J.R.
        • Morris H.R.
        • et al.
        Potent vasodilator activity of calcitonin gene-related peptide in human skin.
        J Invest Dermatol. 1986; 87: 533-536
        • Brain S.D.
        • Williams T.J.
        • Tippins J.R.
        • et al.
        Calcitonin gene-related peptide is a potent vasodilator.
        Nature. 1985; 313: 54-56
        • Brain S.D.
        • Williams T.J.
        Inflammatory edema induced by synergism between calcitonin gene-related peptide (CGRP) and mediators of increased vascular permeability.
        Br J Pharmacol. 1985; 86: 855-860
        • Buechner S.A.
        Rosacea: an update.
        Dermatology. 2005; 210: 100-108
        • Cao T.
        • Gerard N.P.
        • Brain S.D.
        Analysis of neurokinin-1 (NK1) receptor-mediated oedema formation: use of NK1 knockout mice.
        Br J Pharmacol. 1999; 126: U79
        • Caterina M.J.
        • Leffler A.
        • Malmberg A.B.
        • et al.
        Impaired nociception and pain sensation in mice lacking the capsaicin receptor.
        Science. 2000; 288: 306-313
        • Caterina M.J.
        • Schumacher M.A.
        • Tominaga M.
        • et al.
        The capsaicin receptor: a heat-activated ion channel in the pain pathway.
        Nature. 1997; 389: 816-824
        • Chizh B.A.
        • O'Donnell M.B.
        • Napolitano A.
        • et al.
        The effects of the TRPV1 antagonist SB-705498 on TRPV1 receptor-mediated activity and inflammatory hyperalgesia in humans.
        Pain. 2007; 132: 132-141
        • Colburn R.W.
        • Lubin M.L.
        • Stone Jr, D.J.
        • et al.
        Attenuated cold sensitivity in TRPM8 null mice.
        Neuron. 2007; 54: 379-386
        • Costa S.K.P.
        • Yshii L.M.
        • Poston R.N.
        • et al.
        Pivotal role of endogenous tachykinins and the NK1 receptor in mediating leukocyte accumulation, in the absence of oedema formation, in response to TNF alpha in the cutaneous microvasculature.
        J Neuroimmunol. 2006; 171: 99-109
        • Crawford G.H.
        • Pelle M.T.
        • James W.D.
        Rosacea: I. Etiology, pathogenesis, and subtype classification.
        J Am Acad Dermatol. 2004; 51: 327-341
        • Dahl M.V.
        • Ross A.J.
        • Schlievert P.M.
        Temperature regulates bacterial protein production: possible role in rosacea.
        J Am Acad Dermatol. 2004; 50: 266-272
        • Dai Y.
        • Moriyama T.
        • Higashi T.
        • et al.
        Proteinase-activated receptor 2-mediated potentiation of transient receptor potential vanilloid subfamily 1 activity reveals a mechanism for proteinase-induced inflammatory pain.
        J Neurosci. 2004; 24: 4293-4299
        • Dai Y.
        • Wang S.
        • Tominaga M.
        • et al.
        Sensitization of TRPA1 by PAR2 contributes to the sensation of inflammatory pain.
        J Clin Invest. 2007; 117: 1979-1987
        • Denda M.
        • Tsutsumi M.
        • Goto M.
        • et al.
        Topical application of TRPA1 agonists and brief cold exposure accelerate skin permeability barrier recovery.
        J Invest Derm. 2010; 130: 1942-1945
        • del Camino D.
        • Murphy S.
        • Heiry M.
        • et al.
        TRPA1 contributes to cold hypersensitivity.
        J Neurosci. 2010; 30: 15165-15174
        • Dianzani C.
        • Lombardi G.
        • Collino M.
        • et al.
        Priming effects of substance P on calcium changes evoked by interleukin-8 in human neutrophils.
        J Leukoc Biol. 2001; 69: 1013-1018
        • Foreman J.C.
        • Jordan C.C.
        • Oehme P.
        • et al.
        Structure activity relationships for some substance-P-related peptides that cause wheal and flare reactions in human-skin.
        J Physiol. 1983; 335: 449-465
        • Gavva N.R.
        • Bannon A.W.
        • Hovland Jr, D.N.
        • et al.
        Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade.
        J Pharmacol Exp Ther. 2007; 323: 128-137
        • Gavva N.R.
        • Treanor J.J.
        • Garami A.
        • et al.
        Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans.
        Pain. 2008; 136: 202-210
        • Graepel R.
        • Fernandes E.S.
        • Aubdool A.A.
        • et al.
        4-Oxo-2-nonenal (4-ONE): evidence of transient receptor potential ankyrin 1-dependent and -independent nociceptive and vasoactive responses in vivo.
        J Pharmacol Exp Ther. 2011; 337: 117-124
        • Grant A.
        • Amadesi S.
        • Bunnett N.W.
        Protease-activated receptors: mechanisms by which proteases sensitize TRPV channels to induce neurogenic inflammation and pain.
        in: Liedkte W.B. Heller S. TRP Ion Channel Function in Sensory Transduction and Cellular Signalling Cascades. CRC Press, Boca Raton, Chapter 312007
        • Grant A.D.
        • Gerard N.P.
        • Brain S.D.
        Evidence of a role for NK1 and CGRP receptors in mediating neurogenic vasodilatation in the mouse ear.
        Br J Pharmacol. 2002; 135: 356-362
        • Grant A.D.
        • Pinter E.
        • Salmon A.M.
        • et al.
        An examination of neurogenic mechanisms involved in mustard oil-induced inflammation in the mouse.
        Eur J Pharmacol. 2005; 507: 273-280
        • Gunthorpe M.J.
        • Chizh B.A.
        Clinical development of TRPV1 antagonists: targeting a pivotal point in the pain pathway.
        Drug Discov Today. 2009; 14: 56-67
        • Guzman-Sanchez D.A.
        • Ishiuji Y.
        • Patel T.
        • et al.
        Enhanced skin blood flow and sensitivity to noxious heat stimuli in papulopustular rosacea.
        J Am Acad Dermatol. 2007; 57: 800-805
        • Jordt S.E.
        • Bautista D.M.
        • Chuang H.H.
        • et al.
        Mustard oils and cannabinoids excite sensory nerve fibres through the TRP channel ANKTM1.
        Nature. 2004; 427: 260-265
        • Karashima Y.
        • Talavera K.
        • Everaerts W.
        • et al.
        TRPA1 acts as a cold sensor in vitro and in vivo.
        Proc Natl Acad Sci USA. 2009; 106: 1273-1278
        • Keeble J.E.
        • Bodkin J.V.
        • Liang L.
        • et al.
        Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms.
        Pain. 2009; 141: 135-142
        • Knowlton W.M.
        • Bifolck-Fisher A.
        • Bautista D.M.
        • et al.
        TRPM8, but not TRPA1, is required for neural and behavioral responses to acute noxious cold temperatures and cold-mimetics in vivo.
        Pain. 2010; 150: 340-350
        • Kurkcuoglu N.
        • Alaybeyi F.
        Substance P immunoreactivity in rosacea.
        J Am Acad Dermatol. 1991; 25: 725-726
        • Lonne-Rahm S.
        • Nordlind K.
        • Edstrom D.W.
        • et al.
        Laser treatment of rosacea - a pathoetiological study.
        Arch Dermatol. 2004; 140: 1345-1349
        • Marks R.
        The enigma of rosacea.
        J Dermatolog Treat. 2007; 18: 326-328
        • Marks R.
        • Harcourt-Webster J.N.
        Histopathology of rosacea.
        Arch Dermatol. 1969; 100: 683-691
        • Miyachi Y.
        Pharmacologic modulation of neutrophil functions.
        Clin Dermatol. 2000; 18: 369-373
        • Nabeebaccus A.
        • Zhang M.
        • Shah A.M.
        NADPH oxidases and cardiac remodelling.
        Heart Fail Rev. 2011; 16: 5-12
        • Nagata K.
        • Duggan A.
        • Kumar G.
        • et al.
        Nociceptor and hair cell transducer properties of TRPA1, a channel for pain and hearing.
        J Neurosci. 2005; 25: 4052-4061
        • Nilius B.
        • Owsianik G.
        • Voets T.
        • et al.
        Transient receptor potential cation channels in disease.
        Physiol Rev. 2007; 87: 165-217
        • Obata K.
        • Katsura H.
        • Mizushima T.
        • et al.
        TRPA1 induced in sensory neurons contributes to cold hyperalgesia after inflammation and nerve injury.
        J Clin Invest. 2005; 115: 2393-2401
        • Powell F.C.
        • Corbally N.
        • Powell D.
        Substance P and rosacea.
        J Am Acad Dermatol. 1993; 28: 132-133
        • Pozsgai G.
        • Bodkin J.V.
        • Graepel R.
        • et al.
        Evidence for the pathophysiological relevance of TRPA1 receptors in the cardiovascular system in vivo.
        Cardiovasc Res. 2010; 87: 760-768
        • Salas M.M.
        • Hargreaves K.M.
        • Akopian A.N.
        TRPA1-mediated responses in trigeminal sensory neurons: interaction between TRPA1 and TRPV1.
        Eur J Neurosci. 2009; 29: 1568-1578
        • Seeliger S.
        • Derian C.K.
        • Vergnolle N.
        • et al.
        Proinflammatory role of proteinase-activated receptor-2 in humans and mice during cutaneous inflammation in vivo.
        FASEB J. 2003; 17: 1871-1885
        • Sinclair S.R.
        • Kane S.A.
        • Van der Schueren B.J.
        • et al.
        Inhibition of capsaicin-induced increase in dermal blood flow by the oral CGRP receptor antagonist, telcagepant (MK-0974).
        Br J Clin Pharmacol. 2010; 69: 15-22
        • Starr A.
        • Graepel R.
        • Keeble J.
        • et al.
        A reactive oxygen species-mediated component in neurogenic vasodilatation.
        Cardiovasc Res. 2008; 78: 139-147
        • Steiner A.A.
        • Turek V.F.
        • Almeida M.C.
        • et al.
        Nonthermal activation of transient receptor potential vanilloid-1 channels in abdominal viscera tonically inhibits autonomic cold-defence effectors.
        J Neurosci. 2007; 27: 7459-7468
        • Steinhoff M.
        • Buddenkotte J.
        • Shpacovitch V.
        • et al.
        Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response.
        Endocr Rev. 2005; 26: 1-43
        • Steinhoff M.
        • Vergnolle N.
        • Young S.H.
        • et al.
        Agonists of proteinase-activated receptor 2 induce inflammation by a neurogenic mechanism.
        Nat Med. 2000; 6: 151-158
        • Sterner-Kock A.
        • Braun R.K.
        • van der Vliet A.
        • et al.
        Substance P primes the formation of hydrogen peroxide and nitric oxide in human neutrophils.
        J Leukoc Biol. 1999; 65: 834-840
        • Story G.M.
        • Peier A.M.
        • Reeve A.J.
        • et al.
        ANKTM1, a TRP-like channel expressed in nociceptive neurons, is activated by cold temperatures.
        Cell. 2003; 112: 819-829
        • Wilkin J.
        • Dahl M.
        • Detmar M.
        • et al.
        Standard classification of rosacea: report of the national rosacea society expert committee on the classification and staging of rosacea.
        J Am Acad Dermatol. 2002; 46: 584-587
        • Wilkin J.K.
        Vasodilator rosacea.
        Arch Dermatol. 1980; 116: 598
        • Wilkin J.K.
        Why is flushing limited to a mostly facial cutaneous distribution?.
        J Am Acad Dermatol. 1988; 19: 309-313
        • Yamasaki K.
        • Di Nardo A.
        • Bardan A.
        • et al.
        Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea.
        Nat Med. 2007; 13: 975-980
        • Zheng Y.
        • Niyonsaba F.
        • Ushio H.
        • et al.
        Cathelicidin LL-37 induces the generation of reactive oxygen species and release of human alpha-defensins from neutrophils.
        Br J Dermatol. 2007; 157: 1124-1131