M.D Tamás Bíró

Balázs I Tóth

Rita Marincsák

Nóra Dobrosi

Tamás Géczy

Ralf Paus

TRP CHANNELS AS NOVEL PLAYERS IN THE PATHOGENESIS AND THERAPY OF ITCH by

Neurophysiology of itch

. This suggests that the neuronal "mechanism" for itch has developed as an evolutionarily ancient system for the removal or avoidance of potentially harmful agents (e.g. skin irritants, allergens, toxic plants) endangering the integrity of the body. Itch-induced scratching can then be interpreted as a goal-directed movement against noxious agents that have successfully passed the epidermal barrier and have already invaded the skin (where, against which, withdrawal would make no sense).

Categories of itch

. In short, "itch" is one of the central themes of medicine and the life sciences.

Summary

Itch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including it ch and pain. In particular, we argue that -due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities -certain thermosensitive TRP channels are key players in pruritus pathogenesis.

We close by proposing a novel, TRP -centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch.

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Both clinicians and neurophysiologists, therefore, must be able to differentiate b etween different types of pruritus. Given that pruritus can be initiated by localized or systemic as well as by peripheral or central stimuli (resulting in either localized or generalized pruritus), the following itch categories have been suggested, based on partly mechanism-based (i.e.

neurophysiologically defined) and partly operational (i.e. clinically relevant) definitions [2,[START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Twycross | Itch: scratching more than the surface[END_REF]:

Ø Pruriceptive itch: peripherally induced pruritus arising from skin diseases such as dry skin, atopic dermatitis (AD), psoriasis, infestations (e.g. scabies, pediculosis), urticaria.

Ø Neurogenic itch: centrally induced pruritus caused by systemic disorders such as chronic liver disease (cholestasis), chronic renal failure, thyroid dysfunction.

Ø Neuropathic itch: pruritus due to a primary neurological disorder of the central or peripheral nervous systems, such as it can be induced by certain brain tumors, multiple sclerosis, peripheral neuropathy (e.g. postherpetic pruritus), nerve compression or irritation (e.g. notalgia paresthetica, brachoradial pruritus).

Ø Psychogenic itch: pruritus is related to psychological or psychiatric disorders such as parasitophobia, obsessive-compulsive disorder, leading to "neurotic excoriations".

This review focuses on the first itch category (pruriceptive itch), i.e. the characteristics of pruritus originating from the skin.

Theories of itch

From a neurophysiological point of view, numerous questions arise when one tries to define the exact mechanism of itch among the plethora of distinct sensory physiological phenomena. These include, for example:

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Ø Is itch a type of pain, or just a "pain in the neck"?

Ø In other words, does pruritus "function" as a sub-modality of pain or is it rather a separate sensory entity?

Ø Actually, what is the exact neurophysiological relationship between itch and pain?

Ø Accordingly, do we really possess a specific sensory pathway for itch, or does itch "use" defined elements of the nociceptive system?

In order to properly answer these questions, it helps to begin by systemically reviewing competing theories of itch.

According to the "intensity theory" of itch, low level activation of nociceptors induces pruritus, whereas higher stimulation rate provokes pain [START_REF] Frey | Zur Physiologie der Juckempfindung[END_REF]. Following this theory, itch represents quasi a submodality of pain and is based on a specific pattern of action potentials running via nociceptive pathways; hence, itch may result from the combination of other primary sensory signals [2,[START_REF] Stander | Pathophysiology of pruritus in atopic dermatitis: and overview[END_REF]. However, application of low concentrations of algogens generally does not cause itch, just less intense pain, and stimulation of afferent nerves induces either pain or pruritus. Moreover, increasing the stimulation frequency elevates the intensity of pain or itch, but no transition from one sensory modality to the other is recognized [START_REF] Stander | Chronic itch and pain -Similarities and differences[END_REF].

These arguments clearly challenge the validity of the "intensity theory".

It is well known, however, that pain inhibits itch [START_REF] Stander | Chronic itch and pain -Similarities and differences[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF][START_REF] Schmelz | A neural pathway for itch[END_REF][START_REF] Andrew | Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch[END_REF]. Actually, the itch-induced scratching response, in itself, is a motoric activity to induce pain and, hence, to alleviate itch.

This has encouraged the development of an itch hypothesis that mirrors the "gate -control theory" of pain [START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]. Indeed, numerous painful signals (such as noxious heat and cold, mechanical, chemical or electrical stimuli) -via the activa tion of specific nociceptive

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7 pathways and spinal cord as well as higher CNS structures -have already been experimentally shown to inhibit itch. This strongly suggests a central mode of action [START_REF] Stander | Chronic itch and pain -Similarities and differences[END_REF][START_REF] Schmelz | A neural pathway for itch[END_REF][START_REF] Ward | A comparison of the effects of noxious and innocuous counterstimuli on experimentally induced itch and pain[END_REF][START_REF] Nilsson | Cutaneous field stimulation (CFS): a new powerful method to combat itch[END_REF][START_REF] Yosipovitch | Noxious heat and scratching decrease histamine-induc ed itch and skin blood flow[END_REF]. Interestingly, certain agents that effectively induce analgesia (e.g. µ -opioid receptor agonists) may indeed reduce the inhibition of itch by pain, thus enhancing pruritus [START_REF] Atanassoff | Enhancement of experimental pruritus and mechanically evoked dysesthesiae with local anesthesia[END_REF][START_REF] Andrew | Itch -mechanisms and mediators[END_REF].

Conversely, the application of µ -opioid antagonists (e.g. to patients with cholestatic itch) can exert antipruritic effects; however, the reduction of itch is accompanied by the induction of pain [START_REF] Mcrae | Pain as a complication of use of opiate antagonists for symptom control in cholestasis[END_REF] and withdrawal-like reactions [START_REF] Jones | Opiate antagonist the rapy for the pruritus of cholestasis: the avoidance of opioid withdrawal-like reactions[END_REF]. It appears, therefore, that besides similar patterns in sensation characteristics, a definite antagonistic interaction does exist between itch and pain.

Recent landmark findings have helped to establish a novel hypothesis for the generation of pruritus; i.e. the "specificity or selectivity theory" of itch [START_REF] Ikoma | The neurobiology of itch[END_REF], and introduced the novel pruriceptive system dedicated for peripheral and central itch processing. As detailed below, the pruriceptive system contains specific as well as non-specific sensory afferents, itchinducing mediators (collectively referred to as pruritogens), and itch-sensitive spinal projection neurons as well as higher CNS struc tures. It is noteworthy, however, that the individual components of the pruriceptive system are strikingly similar (if not identical) to those of the classical, long-established nociceptive system. This provides sound neurophysiological explanations for the intimate relationship between itch and pain, as delineated above.

2.4. The pruriceptive system

Pruriceptive primary afferent fibers

The skin is highly innervated with a dense network of sensory afferents. Among the unmyelinated C -afferent axons ( also referred to as C -nociceptors), approximately 80 % are

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mechano-sensitive polymodal nociceptors, which respond to mechanical, thermal (heat), and chemical stimuli [START_REF] Schmelz | A neural pathway for itch[END_REF][START_REF] Schmidt | Novel classes of responsive and unresponsive C nociceptors in human skin[END_REF][START_REF] Schmidt | Innervation territories of mechanically activated C nociceptor units in human skin[END_REF]. The remaining approximately 20 % do not respond to mechanical stimulation ("silent nociceptors") and are activated by chemical stimuli [START_REF] Schmidt | Novel classes of responsive and unresponsive C nociceptors in human skin[END_REF][START_REF] Schmelz | Encoding of burning pain from capsaicin-treated human skin in two categories of unmyelinated nerve fibres[END_REF].

Among the mechano-insensitive human C -fibers, a certain subset of afferents (comprising approximately 20 % of the mechano-heat-insensitive class of C -fibers) has been discovered

and characterized which have a strong and sustained response to histamine (one of the most "popular" mediators of itch, see below) in parallel to the itch ratings of human subjects [START_REF] Schmelz | Specific Creceptors for itch in human skin[END_REF][START_REF] Schmelz | Chemical response pattern of different classes of C -nociceptors to pruritogens and algogens[END_REF][START_REF] Wahlgren | Two-point discrimination of itch in patients with atopic dermatitis and healthy subjects[END_REF][START_REF] Weidner | Functional attributes discriminating mechano-insensitive and mechanoresponsive C nociceptors in human skin[END_REF][START_REF] Schmelz | Which nerve fibe rs mediate the axon reflex flare in human skin?[END_REF][START_REF] Schmidt | Innervation territories of mechano-insensitive C nociceptors in human skin[END_REF]. However, the histamine -sensitive fiber subclass cannot account for all aspects of itch, especially when pruritus is induced mechanically or without the characteristic flare reaction (axon-reflex erythema). Thus, other subgroups of primary afferents (most probably mechanoheat sensitive ones) must be i nvolved in the generation of pruritus [START_REF] Ikoma | Electrically evoked itch in humans[END_REF]. This diversity of afferent "itch fibers" would nicely explain the various "submodalities" of pruritus experienced by patients, similar to those of pain [START_REF] Darsow | New aspects of itch pathophysiology: component analysis of atopic itch using the 'Eppendorf Itch Questionnaire[END_REF][START_REF] Yosipovitch | Itch characteristics in Chinese patients with atopic dermatitis using a new questionnaire for the assessment of pruritus[END_REF]. Nevertheless, these intriguing data strongly suggest that there is a specific or -since pruriceptors also respond to capsaicin and mustard oil, i.e. characteristic activators of mechano-heat sensitive afferents (for details, see below) [START_REF] Mcmahon | Itching for an explanation[END_REF] -rather a highly selective neuronal pathway for itch.

Spinal pruriceptive projection neurons and higher CNS structures involved in central processing of itch

This concept has been further strengthened by the identification of a distinct, histamineresponsive, and mechano-insensitive class of dorsal horn (lamina I) neurons in the cat [START_REF] Andrew | Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch[END_REF].

Importantly, the time course of these responses was perfectly paralleled to that of itch sensation in humans. Interestingly, in contrast to nociceptive second-order spinothalamic neurons, the itch-specific projection neurons d o not exhibit spontaneous activity. It is

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proposed that the lack of spontaneous activity may be generated by an active (tonic) inhibition exerted by pain -processing neurons [START_REF] Schmelz | A neural pathway for itch[END_REF]. Therefore, suspension of the activity of the "itchinhibitory" nociceptive pathway might provoke pruritus by a pure spinal mechanism -i.e.

without any activation of pruriceptive afferents of the skin (as seen e.g. in neuropathic and neurogenic pruritus) [2][START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Stander | Chronic itch and pain -Similarities and differences[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Stander | Pathophysiology of pruritus in atopic dermatitis: and overview[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF][START_REF] Twycross | Itch: scratching more than the surface[END_REF].

The itch-selective spinal neurons form a distinct pathway projecting f rom lamina I of the spinal cord to the ventrocaudal part of nucleus medialis dorsalis of the thalamus which has projections to higher CNS structures. The various sensory components of itch have been therefore attributed to activation of certain brain areas: the sensory-discriminative component in the primary somatosensory cortex; the motor component of the goal -directed scratching in the premotor and supplementary motor cortex; the affective and motivational components in the anterior cingulate [START_REF] Andrew | Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch[END_REF][START_REF] Craig | How do you feel? Interoception: the sense of the physiological condition of the body[END_REF][START_REF] Hsieh | Urge to scratch represented in the human cerebral cortex during itch[END_REF][START_REF] Drzezga | Central activation by histamine -induced itch: analogies to pain processing: a correlational analysis of O-15 H2O positron emission tomography studies[END_REF][START_REF] Mochizuki | Imaging of central itch modulation in the human brain using positron emission tomography[END_REF][START_REF] Vogt | Cytology and functionally correlated circuits of human posterior cingulate areas[END_REF] and insular cortex. Of great importance, all these brain areas are also involved in pain processing [START_REF] Apkarian | Human brain mechanisms of pain perception and regulation in health and disease[END_REF] suggesting that no clear-cut itch-specific CNS centers exist. Thus, differences between central pain and itch processing -such as the lack of activation of the secondary somatosensory cortex areas and predominant activation of the ipsilateral motor areas in itch -most likely result from a different activation pattern of basically identical CNS centers [START_REF] Mochizuki | Imaging of central itch modulation in the human brain using positron emission tomography[END_REF].

Pruritogens -Mediators of itch

As we have seen above, during the induction of pruriceptive itch by defined noxi or skin diseases, the itch-selective afferent fibers are stimulated by various chemical agents, collectively referred to as pruritogens. These molecules are mostly released from various intracutaneous cell types which are in close proximity to or even in direct physical contact with sensory nerve endings [START_REF] Ansel | Interactions of the skin and nervous system[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF]. The released pruritogens then are able to stimulate and/or

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sensitize the itch-selective sensory afferents resulting in action potential firing of the neurons and hence initiating itch.

However, upon activation by noxious and/or pruritogenic stimuli, certain neuropeptides (such as substance P [SP] and calcitonin gene related peptide [CGRP]) are "anti-dromically" released from cutaneous pruriceptors, which may act on specific neuropeptide receptors expressed on a large variety of non-neuronal cell types [START_REF] Ansel | Interactions of the skin and nervous system[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF]. This serves to profoundly modulate (i.e., mostly to further enhance!) the release of pruritogens, thus generating (positive) regulatory loops. Hence, the established bi-directional sensory neuron -nonneuronal cell network lies at the central core of pruritus pathogenesis [START_REF] Ansel | Interactions of the skin and nervous system[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF][START_REF] Luger | Neuromediators -a crucial component of the skin immune system[END_REF].

In the following sections, we therefore shall briefly discuss a (highly selected) list o f pruritogens and their related pruritogenic mechanisms (Table 1). The key message here is that the "army" of itch-inducing agents appreciated to-date contains many more pivotal players than the usual "suspect", histamine; and that these agents (not surprisingly at all, see above)

are also capable of inducing pain, depending on the actual peripheral or central conditions [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF].

However, due to the "extreme" overlapping of the nociceptive and pruriceptive systems, it is of current debate whether 1) are there a ny pruritogens which have no effect on the nociceptors; 2) can pruritogens activate pruriceptors (without other stimuli) but only sensitize the nociceptors; and 3) why do the below mediators sometimes cause itch but sometimes cause pain?

Histamine

Although histamine is the best-known pruritogen [START_REF] Church | Mediator secretion from human s kin mast cells provoked by immunological and non-immunological stimulation[END_REF][START_REF] Thomsen | Experimental itch in sodium lauryl sulphate-inflamed and normal skin in humans: a randomized, double-blind, placebo-controlled study of histamine and other inducers of itch[END_REF][START_REF] Van Der Kleij | Functional expression of neurokinin 1 receptors on mast cells induced by IL -4 and stem cell factor[END_REF], its pivotal function in the pathogenesis of itch is rather controversial. Endogenously, histamine is mostly released from

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11 activated mast cells and epidermal keratinocytes, and acts on specific H -receptors [START_REF] Ansel | Interactions of the skin and nervous system[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF][START_REF] Luger | Neuromediators -a crucial component of the skin immune system[END_REF][START_REF] Hill | Distribution, properties, and functional characteristics of three clases of histamine receptors[END_REF][START_REF] Steinhoff | Modern aspects of cutaneous neurogenic inflammation[END_REF]. Accordingly, histamine-induced skin reactions (the "classical triad" of itch, wheal and flare) in humans were shown to be efficiently prevented by inhibitors of histamine H 1 receptors, also located on sensory nerves [START_REF] Hill | Distribution, properties, and functional characteristics of three clases of histamine receptors[END_REF][START_REF] Morita | Double -blind, crossover comparison of olopatadine and cetirizine versus placebo: suppressive effects on skin response to histamine iontophoresis[END_REF]. However, small doses of histamine that fail to produce itch are still sufficient to produce edema and erythema upon intracutaneous injection. Furthermore, antagonists of the H 1 (and/or H 2 ) have often found ineffective as antipruritic drugs, for example in the prototypic pruritic disease AD [START_REF] Klein | An evidence-based review of the efficacy of antihistamines in relieving pruritus in atopic dermatitis[END_REF].

Recently, however, additional H receptors have been discovered, and at least one of them (H 4 )

was shown to function as an "itch receptor" in mice [START_REF] Bell | Involvement of histamine H4 and H1 receptors in scratching induced by histamine receptor agonists in Balb C mice[END_REF]. These latter data encourages one to design new clinical trials to revisit the relative importance of histamine in the pathogenesis of pruritus. Similarly, further studies are required to clarify those findings that H 1 and H 2 receptors, which are also expressed on keratinocytes, were implicated in the regulation of epidermal barrier [START_REF] Ashida | Histamine H1 and H2 receptor antagonists accelerate skin barrier repair and prevent epidermal hyperplasia induced by barrier distruption in a dry environment[END_REF], the dysregulation of which contributes to the development of pruritus in dry skin conditions or in such disease as AD and psoriasis [START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF].

SP and other neuropeptides

Upon activation, pruriceptors induce the release of SP [START_REF] Schmelz | Specific Creceptors for itch in human skin[END_REF][START_REF] Schmelz | Itch -mediators and mechanisms[END_REF] which was suggested to indirectly induce pruritus via the activation of non-neuronal skin cells expressing cognate receptors. Indeed, intradermal application of SP activates mast cells to release histamine which, in turn, induces itch [START_REF] Church | Mediator secretion from human s kin mast cells provoked by immunological and non-immunological stimulation[END_REF][START_REF] Thomsen | Experimental itch in sodium lauryl sulphate-inflamed and normal skin in humans: a randomized, double-blind, placebo-controlled study of histamine and other inducers of itch[END_REF][START_REF] Van Der Kleij | Functional expression of neurokinin 1 receptors on mast cells induced by IL -4 and stem cell factor[END_REF]. In addition, mast cells can also release a plethora of (mostly) inflammatory mediators which induce further liberation of SP from peripheral nerve endings, leading to more mast cell activation [START_REF] Ansel | Interactions of the skin and nervous system[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF][START_REF] Luger | Neuromediators -a crucial component of the skin immune system[END_REF][START_REF] Steinhoff | Modern aspects of cutaneous neurogenic inflammation[END_REF]. SP may also trigger the release of other pruritogens from various other cell types such as endothelial cells (kinins, endothelins), keratinocytes (nitric oxide), and immunocytes (cytokines) [START_REF] Abadia -Molina | Increased sensory neuropeptides in nodular prurigo: a quantitative immunohistochemical analysis[END_REF][START_REF] Jiang | Double -labeled immunofluorescence study of cutaneous nerves in psoriasis[END_REF][START_REF] Andoh | Nitric oxide enhances substance P -induced itch-associated responses in mice[END_REF][START_REF] Jarvikallio | Mast cells, nerves and neuropeptides in atopic dermatitis and nummular eczema[END_REF] (see also next paragraphs).

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In contrast, the role of the other main sensory neuron-derived neuropeptide, CGRP, in the induction of itch is still rather controversial. Namely, CGRP was shown to modulate itch and inflammation [START_REF] Averbeck | Interactions of inflammatory mediators stimulating release of calcitonin gene-related peptide, substance P and prostaglandin E(2) from isolated rat skin[END_REF][START_REF] Hagner | Expression and distribution of calcitonin receptor-like receptor in human hairy skin[END_REF]. In contrast, CGRP was found to prolong SP-induced itch latency suggesting an inhibitory effect of CGRP on SP-induced itching [START_REF] Ekblom | Influence of calcitonin gene-related peptide on histamine -and substance P -induced itch, flare and weal in humans[END_REF]. Interestingly, however, increased levels of CGRP were found in nerve fibers of pruritic diseases such as AD, nummular eczema [START_REF] Jarvikallio | Mast cells, nerves and neuropeptides in atopic dermatitis and nummular eczema[END_REF], and prurigo nodularis [START_REF] Liang | CGRP-immunoreactive nerves in prurigo nodularis-an exploration of neurogenic inflammation[END_REF]. 2.4.3.3. Inflammatory mediators as pruritogens -Peripheral sensitization of itch by arachidonic acid derivatives, kinins, and interleukins

It has long been recognized that acute or chronic inflammation of the skin sensitizes the nociceptive sensory afferents resulting in inflammatory hyperalgesia [START_REF] Reeh | Effects of classical algogens[END_REF]. It was also known that numerous inflammatory mediators released from mast cells ( tryptase, tumor necrosis factor-α [TNF-α], prostaglandins, leukotrienes), keratinocytes (prostanoids, nerve growth fact

[NGF] and other neurotrophins), endothelial cells (kinins, endothelins), or immune cells (interleukins) also lower t he threshold for pruritic stimuli, and thus cause peripheral itch sensitization [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bickford | Experiments relating to the itch sensation, its peripheral mechanism and central pathway[END_REF]. These agents may trigger pruritus by either binding to their specific receptors of the pruriceptive sensory afferents or by inducing non-specific mechanisms leading to elevated sensitivity of these neurons to other pruritogens [START_REF] Ständer | Neurophysiology of pruritus: c utaneous elicitation of itch[END_REF].

For example, prostaglandins (especially prostaglandin E 1 and E 2 ) were shown to potentiate histamine -induced itch independently of the liberation of mast cell-derived histamine [START_REF] Hagermark | Pruritogenic activity of prostaglandin E2[END_REF][START_REF] Boss | Lack of effect of the antihistamine drug clemastine on the potentiation of itch by prostaglandin E1[END_REF].

In addition, pretreatment of human skin with prostaglandin E 1 significantly lowered itch threshold [START_REF] Lowell | Burton Prostaglandins and pruritus[END_REF] whereas prostaglandin E2 evoked pruritus both in controls and in AD patients [START_REF] Neisius | Prostaglandin E2 induces vasodilation and pruritus, but no protein extravasation in AD and controls[END_REF]. Similarly, intradermal injection of another eicosanoid, leukotriene B 4 induced scratching

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in mice [START_REF] Andoh | Intradermal leukotriene B4, but not prostaglandin E2, induces itch-associated responses in mice[END_REF]. Furthermore, it was also suggested the SP-evoked itch (at least in part) is due to the production prostaglandins and leukotrienes [START_REF] Andoh | Involvement of leukotriene B4 in substance P -induced itch-associated response in mice[END_REF].

The inflammatory mediator bradykinin (which induces pain via activation of bradykinin B 2 receptors located on the nociceptors) can also induce itch [START_REF] Schmelz | Chemical response pattern of different classes of C -nociceptors to pruritogens and algogens[END_REF][START_REF] Cormia | Proteolytic activity in development of pain and itching, Cutaneous reactions to bradykinin and kallikrein[END_REF][START_REF] Hagermark | Studies on experimental itch induced by kallikrein and bradykinin[END_REF]. Furthermore, bradykinin induces the release of histamine from mast cells [START_REF] Walker | Kinins and kinin receptors in the nervous system[END_REF]), augments the responses to subsequent histamine application [START_REF] Lang | Chemosensitivity of fine afferents from rat skin in vitro[END_REF][START_REF] Koppert | Interactions of histamine and bradykinin on polymodal C-fibres in isolated rat skin[END_REF], and sensitizes sensory afferents for various chemical stimuli [START_REF] Lang | Chemosensitivity of fine afferents from rat skin in vitro[END_REF]. Administration of bradykinin to the skin also elevates the release of SP, CGRP, and prostaglandin E 2 [START_REF] Averbeck | Interactions of inflammatory mediators stimulating release of calcitonin gene-related peptide, substance P and prostaglandin E(2) from isolated rat skin[END_REF] hence may significantly amplify the activity of the pruritogenic cellular network. Finally, a recent report has also shown that bradykinin is a potent pruritogen in AD as well [START_REF] Hosogi | Bradykinin is a potent pruritogen in atopic dermatitis: A switch from pain to itch[END_REF].

Certain members of the large family of inflammatory immune cell-derived interleukins (IL) are also implicated in the pathogenesis of pruritus. For example, recombinant IL-2 (administered to cancer patients) was shown to induce pruritus [START_REF] Gaspari | Dermatologic changes associated with interleukin 2 administration[END_REF]. Moreover, IL -2 induces itch by activation of a subpopulation of cutaneous C -fibers that are chemosensitive to histamine and bradykinin [START_REF] Darsow | Skin testing of the pruritogenic activity of histamine and cytokines (interleukin -2 and tumour necrosis factor-alpha) at the dermal-epidermal junction[END_REF][START_REF] Martin | Interleukin -2 activates a sub-population of cutaneous Cfibre polymodal nociceptors in the rat hairy skin[END_REF] suggesting that bradykinin may enhance the itch-inducing effect of IL-2 [START_REF] Martin | Bradykinin potentiates the chemoresponsiveness of rat cutaneous Cfibre polymodal nociceptors to interleukin-2[END_REF]. Furthermore, transgenic mice overexpressing IL-4 in the epidermis spontaneously developed a pruritic inflammatory skin disease resembling human AD [START_REF] Chan | Expression of interleukin -4 in the epidermis of transgenic mice results in a pruritic inflammatory skin disease: an experimental animal model to study atopic dermatitis[END_REF].

Accordingly, elevated IL -6-like immunoreactivity was found in nerve fibers of patients with prurigo nodularis [START_REF] Nordlind | Immunohistochemical localization of interleukin-6-like immunoreactivity to peripheral nerve-like structures in normal and inflamed human skin[END_REF]. In addition, it was demonstrated that transgenic overexpression of the novel cytokine IL-31 in T-lymphocytes induces severe pruritus and dermatitis in mice [START_REF] Dillon | Interleukin 31, a cytokine produced by activated T cells, induces dermatitis in mice[END_REF]. In good accord with these data, expression IL-31 was found to be increased in an atopic mouse model [START_REF] Takaoka | Expression of IL-31 gene transcripts in NC/Nga mice with atopic dermatitis[END_REF] -as well as in patients with AD [START_REF] Bilsborough | IL-31 is associated with cutaneous lymphocyte antigen-positive skin homing T cells in patients with atopic dermatitis[END_REF] -and was shown to correlate to itch behavior

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14 [START_REF] Takaoka | Involvement of IL-31 on scratching behavior in NC/Nga mice with atopic-like dermatitis[END_REF]. These results unambiguously argue for a special role for IL-31 in pruritus and that IL-31 and its signaling pathway may represent a novel target for antipruritic therapy [START_REF] Sonkoly | Interleukin-31 -A new link between T cells and pruritus in atopic skin inflammation[END_REF]. 2.4.3.4. Proteases and their receptors Serine proteases (trypsin, chymotrypsin, chymase), generally regarded as "weapons of (protein) mass destruction", are capable of inducing inflammation, pain, and pruritus [START_REF] Arthur | The role of proteolytic enzymes in the production of pruritus in man[END_REF][START_REF] Rajka | Latency and duration of pruritus elicited by trypsin in aged patients with itching eczema and psoriasis[END_REF][START_REF] Hagermark | Experimental itch in human skin elicited by rat mast cell chymase[END_REF].

Among their G protein-coupled, metabotropic proteinase-activated receptors (PAR) [START_REF] Vu | Molecular cloning of a functional thrombin receptor reveals a novel proteolytic mechanism of receptor activation[END_REF][START_REF] Steinhoff | Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response[END_REF],

PAR 2 has been shown to play a key role in the pathophysiology of itch [START_REF] Steinhoff | Proteinase-activated receptor-2 mediates itch: a novel pathway for pruritus in human skin[END_REF]. Importantly, functional PAR 2 is expressed on sensory afferents which release neuropeptides upon stimulation by tryptase [START_REF] Vergnolle | Proteinase-activated receptors: novel signals for peripheral nerve[END_REF][START_REF] Steinhoff | Agonists of proteinase-activated receptor 2 induce inflammation by a neurogenic mechanism[END_REF]. With respect to itch, it was also observed that the levels of the endogenous PAR2 agonist tryptase as well as PAR2 were markedly increased on primary afferent fibers of lesional skin of AD patients. Additionally, keratinocyte-derived PAR 2 was also upregulated in the epidermis of AD patients [START_REF] Buddenkotte | Agonists of proteinase-receptor-2 stimulate upregulation of intercellular cell adhesion molecule -1 in primary human keratinocytes via activation of NF-kappa B[END_REF] and hence may mediate pruritus induced by endogenous (trypsins, kallikreins) or exogenous proteases (bacteria, house-dust mite). In perfect agreement with the se findings, intracutaneous injection of specific PAR 2 agonists provoked sustained and prolonged itch in such patients. Moreover, inhibition of tissue kallikreine (tryptic enzyme) suppressed itch [START_REF] Hayashi | Reduction of sodium deoxycholic acid-induced scratching behaviour by bradykinin B2 receptor antagonists[END_REF] and kallikreine activity was also increased in pruritic papular eruptions [START_REF] Reis | Kininogens and kallikrein in pruritic papular eruption[END_REF]. Since the concentration of histamine was not changed in AD, these results suggest that the activation of the novel protease-PAR 2 pathway on cutaneous sensory nerves might be more important than histamine for the transmission of itch responses in AD (and probably in other skin diseases).

NGF and certain neurotrophins -Acute and chronic itch sensitizers

Neurotrophins (such as NGF and neurotrophin-4), which are produced and released by keratinocytes, mast cells, and fibroblasts of the skin [START_REF] Groneberg | Gene expression and regulation of nerve growth factor in atopic dermatitis mast cells and the human mast cell line-1[END_REF], were originally described to play

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crucial roles in cutaneous nerve development and regeneration [START_REF] Botchkarev | Neurotrophins in skin biology and pathology[END_REF]. It was also shown, however, that NGF is highly overexpressed in inflamed and injured tissues and that (as an inflammatory mediator) it initiates acute sensitization and sprouting (leading to chronic sensitization) of C-type afferent fibers via the activation of specific TrkA receptors [START_REF] Botchkarev | Neurotrophins in skin biology and pathology[END_REF][START_REF] Urashima | Cutaneous nerves in atopic dermatitis -a histological, immunohistochemical and electron microscopic study[END_REF][100][101].

Indeed, NGF upregulates the expression of pruritogenic neuropeptides (SP, CGRP) and certain receptors (e.g. transient receptor potential vanilloid-1 [TRPV1], see below) involved in itch, induces degranulation of mast cells, and was found to be pruritogenic when administered therapeutically [START_REF] Rothmann | Physiology of itching[END_REF][START_REF] Schmidt | Innervation territories of mechanically activated C nociceptor units in human skin[END_REF][START_REF] Schmelz | Which nerve fibe rs mediate the axon reflex flare in human skin?[END_REF][START_REF] Ikoma | Electrically evoked itch in humans[END_REF]102,105]. In addition, serum levels of NGF and SP were found to be increased in AD patients [104] and these substances were shown to induce the release of the pruritogenic mediator tryptase [START_REF] Groneberg | Gene expression and regulation of nerve growth factor in atopic dermatitis mast cells and the human mast cell line-1[END_REF]. Also, mast cells and keratinocytes produce high levels of NGF in AD which can be stimulated by histamine [105]. Moreover, increased NGF expression was detected in prurigo nodularis [106] and also in pruritic lesions of patients with psoriasis [107], similarly to the overexpression of neurotrophin -4 in lesional skin of AD patients [108]. Taken together, these results are clearly in favor of important roles of neurotrophins in the pathophysiology of itching.

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TRP channels -"TRiPping" novel players in pruritus pathogenesis

Why TRP channels?

Recently, numerous lines of novel evidence suggest the involvement of new "players" in the pathogenesis of pruriceptive itch. Among these, special attention has been attracted to the transient receptor potential (TRP) superfamily [109][110][111][112], especially to the thermosensitive ones [113]. Here is, why: Ø As shown above, i tch is a characteristic sensory modality with specific/selective CNS processing → TRP channels mostly function as "cellular sensors" of e.g. temperature, osmotic concentration, and taste.

Ø Skin -originating itch requires an orchestration of bidirectional sensory neuron -nonneuronal cell network → TRP channel are ideally suited to this task, since they are equally expressed by both sensory neuronal structures and non-neural cell types of the skin.

Ø The wide array of pruritogens may not only act on specific receptors but also on various new, previously unappreciated, targets → such as the TRP channels.

Ø During the pharmacological treatment of pruritus, certain agents may target novel molecules → such as the TRP channels.

Ø As was revealed during anti-itch therapy using physical methods, the itch pathway is clearly sensitive to temperature changes → such as numerous TRP channels.

On this background, the following paragraphs offer an "exotic TRiP" to the fascinating, sometimes "cloudy", yet steadily unfolding "realm" of the TRP family, and discuss their

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putative roles in itch pathogenesis. Along the way, we provide novel perspectives on the potential use of TRP ligands in itch management (Table 2).

3.2. TRPV1 -A thermosensitive channel with a central role in the pathogenesis and therapy of itch

TRPV1 -A "hot and spicy" target of capsaicin on sensory neurons

TRPV1, as key peripheral integrator of pain sensation, was originally described on C-type nociceptive sensory neurons [114,115] as a molecular target for capsaicin, the pungent vanilloid ingredient of hot chili peppers [116]. The activation of this non-specific cation channel (permeable to e.g. sodium and calcium) first excites these neurons by initiating ionic fluxes and concomitant action potential firin g resulting in the induction of pain in vivo [115,116]. Upon activation by TRPV1 agonists (e.g. during capsaicin-treatment of the skin), these neurons release their neuropeptide content (efferent function of the sensory afferents, see above) which, by acting on the numerous neighboring cell populations, leads to the initiation of neurogenic inflammation [START_REF] Steinhoff | Modern aspects of cutaneous neurogenic inflammation[END_REF][START_REF] Steinhoff | Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response[END_REF]117]. Afterwards, prolonged stimulation of TRPV1 induces desensitization (and, depending on the actual concentration of the agonist, cell death) of t he sensory afferents [114,115] which, at least in part, is due to intracellular calcium accumulation and the depletion of neuropeptides. Hence, prolonged TRPV1 activation results in the suspension of the interplay between skin sensory neurons and nonneuronal cells [2][START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF][START_REF] Twycross | Itch: scratching more than the surface[END_REF]114,115].

"Endovanilloids" -The "itch connection"

In addition to capsaicin, TRPV1 can also be activated and/or sensitized by numerous Taken together, these findings imply that TRPV1 is indeed a central integrator molecule in the pain pathway [115,131].

It is very important to note, however, the above endogenous substances may not only act as algogenic chemical substances but are also recognized as effective pruritogens (see in Table 1). In addition, the itch-selective sensory afferents also respond to the TRPV1-activator capsaicin suggesting that TRPV1 might also be expressed on the pruriceptor subunit of mechano-insensitive fibers [START_REF] Ikoma | The neurobiology of itch[END_REF]. Finally, it has also been shown that increases in skin temperature or dysregulation of the pH of the skin (another significant stimuli of TRPV1) can also effectively modulate itch sensation in human [START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Yosipovitch | Noxious heat and scratching decrease histamine-induc ed itch and skin blood flow[END_REF]132]. Therefore, if one takes a closer look to the above "special features" of TRPV1 in relation to the neurophysiology of itch, one may also propose that TRPV1 can also function as a "central integrator" molecule in the itch pathway [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF].

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TRPV1 on non-neuronal cell types of the skin -Other "itchy" findings to explore

Recent "hot" findings further highlighted the neurophysiological importance of TRPV1 signaling in itch. Namely, TRPV1 channels were described on numerous non-neuronal cells types [133][134][135][136] including, of greatest importance, human skin epidermal keratinocytes, dermal mast cells, dendritic cells, and various keratinocyte populations of the hair follicle [137][138][139][140][141][142][143]. In addition, it was also proven that the activation of TRPV1 -besides markedly affecting proliferation, differentiation, and apoptosis -results in the release of such cytokines and mediators (e.g. various ILs, prostaglandins, growth factors) from these non-neuronal cells that where shown to participate in induction of itch sensation [133][134][135][136]141,142,143].

These novel findings, therefore, invite an attractive hypothesis: According to this, algogenic and pruritogenic substances may therefore not only target TRPV1-expressing sensory neurons but also e.g. TRPV1-expressing mast cells and keratinocytes, and, hence, significantly modulate the proposed neuronal -non-neuronal interaction network to initiate and augment itch [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]. In fact, the importance of keratinocyte -specific TRPV1 was strengthened by presenting that expression o f the receptor was dramatically increased in epidermal keratinocytes of prurigo nodularis patients [141].

Therapeutic implications -

The vanilloid-TRPV1 system to mitigate itch

Capsaicin and related exovanilloids

The generally accepted basis for the therapeutic application of capsaicin to mitigate pain and itch is the well-appreciated densensitizing effect of this vanilloid [115,116,144,145]. Namely, prolonged or repeated vanilloid application results in a depletion of neuropeptides such as SP in the C-type neurons, hence suspending the interplay between skin sensory neurons and other cell populations of the skin [2][START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF][START_REF] Twycross | Itch: scratching more than the surface[END_REF]146]. Indeed, topical capsaicin effectively prevents

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histamine -induced itch under experimental conditions [147]. In addition, capsaicin cream (0.025 -0.075 %) is widely used as an antipruritic agent in several pruritic dermatoses, ranging from prurigo nodularis and notalgia paresthetica via pruritus ani, to hemodialysisrelated and uremic pruritus, where a cascade-like augmentation of the intercellular skin network "into a vicious itch circle" (which can be effectively interrupted by capsaicin administration) is clearly documented [2,[START_REF] Twycross | Itch: scratching more than the surface[END_REF]144,146,148].

The most notorious clinical limitation of capsaicin application is the TRPV1-coupled acute excitation of the sensory C -afferents, which results in a marked burning sensation [2,[START_REF] Twycross | Itch: scratching more than the surface[END_REF]116,144,145]. However, if one takes into account that pain inhibits itch via central mechanisms, one might also speculate: What if the effectiveness of capsaicin to terminate itch is (at least in part) due to its (undesired) pain -inducing properties? Unfortunately, systematic studies that address this intriguing question have not yet been published (e.g. by measuring the itch behavior of TRPV1-KO mice). Nevertheless, at least for now, the need of repeated and prolonged capsaicin application protocols (which fully deplete neuropeptide content of sensory afferents) to efficiently alleviate itch [START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]144,146,148] argues against this hypothesis and suggest a peripheral action of capsaicin in its function as an anti-itch agent.

Considering that pruritic patients usually poorly tolerate capsaicin-induced pain, very often resulting in a greatly reduced patient compliance [149], another important therapeutic achievement, therefore, would be to find and/or synthetically design such TRPV1 agonists which cause only minor receptor excitation (i.e. pain), but still possess a significant desensitization power (thus suppressing itch). Perhaps, the most promising candidate to startoff this important search for better anti-pruritic agents is resiniferatoxin (RTX), another natural product of Euphorbia resinifera (a cactus-like plant) [116,117,150]. Intriguingly, this Collectively, these finding strongly argue for that -just as the TRPV1- It is ancient knowledge among "pruritologists" [2,[START_REF] Ikoma | The neurobiology of itch[END_REF] that whereas cooling inhibits the itch sensation, warming of the skin aggravates pruritus. Recent findings, however, seem to challenge (at least in part) this popular paradigm. Namely, Yosipovitch and coworkers [START_REF] Yosipovitch | Noxious heat and scratching decrease histamine-induc ed itch and skin blood flow[END_REF] have shown that stimulation of human skin by noxious thermal stimuli (close to 50 o C), surprisingly, markedly inhibited the histamine -induced itch (and hyperemia). Although the exact mechanism and its potential therapeutic implications of this intriguing effect remain to be determined, the phenomenon itself strongly suggests that TRPV1 expressed on sensory

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neurons and/or non-neuronal cells (as well as other noxious heat-activated TRP channels such as TRPV2, see below) are directly involved in the antipruritic efficacy of such a "thermal physiotherapy".

Other thermosensitive TRP channels as novel players in pruritus

The profound temperature-dependence of itch sensation has also drawn attention to numerous other thermosensitive TRP channels (besides TRPV1), which have recently been as identified as molecular sensors of temperature changes [109][110][111][112][113]. Each of these distinct cellular temperature sensors, which belong to the TRPV, TRPM, and TRPA subfamilies, is specialized to detect well-defined temperature spectra (some are stimulated by heat, others by cold), thus enabling the body to obtain temperature "readings" over almost the entire relevant temperature range faced by mammals [109-113].

TRPV2, TRPV3, and TRPV4 -More "hot" is coming to fight itch

Similar to TRPV1 (which is activated at =43 o C), the TRPV2, TRPV3, and TRPV4 (originally [ 170,172]. In addition, using a heterologous expression system, it was also postulated that TRPV3 subunits form heteromultimeric structures by interacting with TRPV1 monomers and therefore may act as signal cotransducers and/or regulators of TRPV1-mediated signaling [ 170]. However, to further complicate this issue, another group (using another expression system) failed to identify homo-or heteromeric assembly of TRPV1 and TRPV3 subunits [176].

described
Of great importance, however, previous studies unambiguously argue for that TRPV3 and TRPV4 are highly expressed in skin epidermal keratinocytes [171,172,177,178], which, as was detailed above, also express TRPV1 [137][138][139][140][141]. Moreover, it was shown that these channels expressed by keratinocytes are functional, since they mediate heat-activated membrane currents [177,178]. This is most strongly supported by the finding that mice lacking TRPV3 ha ve prominent deficits in their response to both innocuous (mostly TRPV3 and TRPV4 range) and painful heat (rather TRPV1 range) [179] -underscoring the concept that TRPV3 is crucially involved in cutaneous temperature sensation.

Although we possess only v ery limited data (see below) on whether or not the activation of TRPV3 and TRPV4 results in mediator release from keratinocytes (e.g. of pro-inflammatory mediators, algogens, and pruritogens), the sensitivities of the channels to certain chemicals and pharmacological agents implicate their putative roles in the neurophysiology of itch.

Namely, several plant-derived skin sensitizers and allergens (and, thus, potentially itchinducing substances) that cause a "warm" skin sensation (such as eugenol, thymol, and carvacrol -major components of oregano, savory, clove and thyme) strongly activate and It also deserves further consideration that the activities of both channels (TRPV3, TRPV4) are potentiated by recognized inflammatory pruritogens (similarly to the sensitization of TRPV1, see above). Indeed, TRPV4 is activated by arachidonic acid and its derivative, the endocannabinoid anandamide via the generation of lipid peroxidation products such as eicosanoids [181,182]. Similarly, TRPV3 is sensitized by arachidonic acid and related unsaturated fatty acids [183], while the effects of the culinary ingredients listed above are augmented by histamine and bradykinin in a metabotropic receptor -PLC-dependent manner [180]. Moreover, both TRPV4 and TRPV1 (similar to the pruritogenic histamine H 1 and H 2 receptors [START_REF] Abadia -Molina | Increased sensory neuropeptides in nodular prurigo: a quantitative immunohistochemical analysis[END_REF]) were shown to play important roles in skin permeability barrier homeostasis

[184], the dysregulation of which represents a major stimulus for the onset of itch [START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF].

In light of these data, it is of great clinical significance that camphor (an aromatic and, when applied to human skin, mostly pungent phytoproduct derived from the Cinnamonum camphora tree) is a potent activator of TRPV3 [171,172,179]. Camphor has long been applied to alleviate itch in certain pruritic dermatoses (such as contact irritant dermatitis) [185].

Interestingly, camphor also transiently activates TRPV1 (albeit to lesser extent than capsaicin), but then strongly desensitizes this vanilloid receptor thereby suspending the excitation of TRPV1-expressing sensory afferents [186]. Moreover, camphor also inhibits TRPA1 [187] which data collectively suggest a possible synergistic interplay between TRPV1, TRPV3, and TRPA1 to mediate the antipruritic (as well as analgesic) [188] effect of camphor (see also below).

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Finally, another heat-sensitive channel, TRPV2, deserves mentioning. The molecule was first described on capsaicin-and proton-insensitive, medium to large diameter primary nociceptive sensory neurons and is activated by high-threshold ( =52 o C) noxious thermal stimuli [189].

Keeping in mind the key role of mast cells in the initiation of itch [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Paus | Neuroimmunoendocrine circuitry of the 'brain-skin connection[END_REF], it is important to note mast cells also express TRPV2 (along with TRPV4 and TRPV1) [134,140,141,190].

Intriguingly, the activation of mast cells by physical and thermal stimuli can result in a proinflammatory degranulation event which depends on the activity of protein kinase A-related signaling [190] -one of the chief mechanisms in initiating the sensitization of nociceptors and pruriceptors [123,191]! Furthermore, we have already seen above that TRPV2 (besides TRPV1) may also participate in mediating the antipruritic therapeutic effect of noxious heat stimuli [START_REF] Yosipovitch | Noxious heat and scratching decrease histamine-induc ed itch and skin blood flow[END_REF]. Clearly, further studies are urgently needed to define the itch-related "nonthermosensor" roles of these "hot" channels. However, taken together, the available evidence already suggests that they closely resemble TRPV1 with respect to cell-specific expression (e.g. on sensory neurons and various non-neuronal cell types of the skin), receptor activation, sensitization, and signal transduction characteristics, and warrant careful exploration in pathogenesis and clinical management of itch.

TRPM8 -A "cool" anti-itch agent

TRPM8 is another thermosensitive member of the TRP family [109][110][111][112][113]. In contrast to the heat-sensitivity of the TRPV1-V4 channels, however, this calcium-permeable channel is activated by decreasing temperatures, down to the range of innocuous (i.e. not painful) cold or coolness ( =28 ºC) [192,193]. Moreover, again in contrast to TRPV1 [129], the metabotropic receptor -coupled hydrolysis of PIP2 inhibits the activity of TRPM8 [194].

Consistent with these data, TRPM8 is selectively expressed on a certain subpopulation of A

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and C -type sensory afferents which is distinct from TRPV1-expressing nociceptors [195].

Besides cooling, TRPM8 is highly activated by menthol, eucalyptol, and the "super-cooling agent", icilin [192,193,196]. These agents shift the activation threshold of TRPM8 towards room temperature and, thus, sensitize the TRM8-expressing sensory afferents. This induces the characteristic "cool" sensation when these substances are applied to the skin or mucosal surfaces.

In this context, it is important to recall that skin cooling effectively inhibits itch, as has been demonstrated in numerous trials [START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Yosipovitch | Noxious heat and scratching decrease histamine-induc ed itch and skin blood flow[END_REF]197]. Like physical skin cooling, menthol also alleviates experimental [197] or disease-related [185,198,199] pruritus. Importantly, menthol, besides stimulating TRPM8, also inhibits TRPA1 [187] w hich may also contribute to its effect to mitigate itch (see also below). Moreover, the super-cooling agent icilin (which is 200-400 times more potent than menthol in activating TRPM8) [192,196], when applied as 2 % ointment, reduced the degree of excoriations by 55%-60% in a hairless rat model of scratching, provoked by a magnesium deficient diet [START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]. These findings strongly argue in favor of the concept that TRPM8 offers an excellent target receptor for mediating the antipruritic (as well as the analgesic) effects of the above chemical and physical tools.

TRPA1 -A "pungent ic(e)ing on the cake"

TRPA1 was originally also described as a cold-sensitive, calcium -permeable, non-selective cation channel [200], activated at the noxious cold temperature r ange (=17 o C) [109][110][111][112][113].

Recent findings, however, suggest that -based on its localization, ligand sensitivity, and its relation to inflammation -this "ice-cold" channel is more closely related to the "hot" TRPV channels than to the "cool" TRPM8.

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Namely, TRP A1 was found on TRPV1-positive sensory nociceptive (and pruriceptive)

afferents, yet not on the subset of A and C -fibers which express TRPM8 (see above) [195,200]. Of further importance, pungent human skin irritants (i.e. potential pruritogenic stimuli) such as allyl isothyocyanate (alkaloids contained e.g. in mustard oil, wasabi, and horseradish, which induce skin pain and inflammation similar to the TRPV1-activating capsaicin) and cinnamaldehyde (a substance isolated from cinnamon oil) markedly and specifically activated TRPA1 [201,202]. In addition, recognized algogenic and pruritogenic inflammatory mediators such as bradykinin, arachidonic acid, and prostaglandins as well as activation of PLC were also shown to effectively activate TRPA1 [202]. This must be expected to shift the activation temperature-threshold of TRPA1 to more "physiological" temperature ranges -similar to the effect seen with excitatory actions of these substances on TRPV1, TRPV3, and TRPV4 (see above).

Therefore, in light of the above (i.e. TRPA1 was activated by noxious cold and, at the same time, by agents causing pungent "hot" sensation), Bandell et al. have proposed that TRPA1 may serve as a molecular model explaining how noxious cold may paradoxically cause painful burning [202]. In support of this hypothesis, in animal models using either TRPA1targeted antisense or knockout technologies, it was shown that TRPA1 indeed participates in noxious cold sensation [203] and in the development of cold hyperalgesia induced by either infla mmation or nerve injury [204,205].

Intriguingly, however, in parallel to the above studies, other groups were unable to prove the (noxious) cold -sensitivity of TRPA1 in cell cultures [201,206]. Similarly, Bautista et al. have shown that TRPA1 knockout mice exhibit normal response to cold stimuli [207]. However, these mutant animals displayed pronounced deficits in nociceptor excitation and, of great

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importance, in thermal pain hypersensitivity (heat hyperalgesia) evoked by the inflammatory mediator bradykinin [203,207] -similar to findings observed with TRPV1-deficient mice

[130]! It may be postulated, therefore, that algogenic and (since sensitization also occurs for itch [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF]) pruritogenic inflammation operates as the common activation mechanism for TRPV1 and TRPA1 (and, quite probably, for other heat-sensitive TRPV channels, as well). Indeed, using knockout animals, it was shown that both TRPA1 and TRPV1 are required for the bradykinin-induced thermal hyperalgesia [207] which, at the cellular level, involves the stimulation of TRPA1 following activation/sensititation of TRPV1 [207] or, alternatively, the sequential co-activation of the two channels "the other way around" [168,208].

Naturally, further studies are to be performed to unambiguously reveal the exact role of TRPA1 in temperature sensation and the molecular and functional relationship of TRPA1 and TRPV1. Nevertheless, the "exciting" complexity and controversy of the "story" of above TRP channel may provide an attractive explanation for how the distinct painful "cold" and painful "hot" sensory modalities can travel alongside each other so intimately.

Temperature-sensitive TRP channels establish a basic syntax and molecular substrate of nociception and pruriception -The "molecular psychophysics" of itch and pain sensation

The data presented above, on the one hand, clearly demonstrate the potential neurophysiological roles of defined temperature-sensitive TRP channels in the pathogenesis (and possibly for the therapy) of pruritus. On the other hand, the differential activation pattern of theses molecules by various skin -irritating agents may also help to understand the extreme variability of sensations experienced by patients when such substances are applied to human skin.

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ACCEPTED MANUSCRIPT [START_REF] Darsow | New aspects of itch pathophysiology: component analysis of atopic itch using the 'Eppendorf Itch Questionnaire[END_REF] We propose that temperature-sensitive TRP channels establish a basic syntax and molecular substrate of nociception and -due to the intimate relationship of pain and itch -for pruriception, as well (Table 3 ). Following this novel working hypothesis (which can also be referred to as the "molecular psychophysics" of itch and pain sensation), these sensory categories can, then, be distinguished and re-defined, which will have to be taken into account when analyzing the pathogenesis and management of itch:

Ø "Warm only" sensation (i.e. without pungency) -Stimuli inducing a pure sensation of "warmth" (without any "burning pain" component) may exclusively act on TRPV3

(and, perhaps, also on TRPV4). The prototypic compound for such sensation is eugenol (ingredient of clove oil), which selectively activates TRPV3 without affecting the activity of other thermosensitive TRP channels [180].

Ø "Cool only" sensation (i.e. without pungency) -Stimuli inducing a "pure coolness" sensation (without any "burning pain" component) may exclusively act on TRPM8.

Such a compound is menthol which selectively activates TRPM8 [192,193,200].

However, it also inhibits TRPA1 [187] which suggest that the "pure coolness" sensation induced by menthol (via TRPM8) can be "fine-tuned" by eliminating the "burning" component (via inhibiting TRPA1-mediated signaling).

Ø "Painful burning" sensation (i.e. pungency) -Within this category, based on experimental findings and on purely theoretical speculations, the following subcategories can be defined:

• It is well established that stimuli which may exclusively act on TRPV1 (e.g. capsaicin) induce pungency with a characteristic "hot" component ( "Hot painful burning" sensation).

A C C E P T E D M A N U S C R I P T

• Therefore, it would be feasible to propose that stimuli which may exclusively act on TRPA1 would induce pungency with a characteristic "cold" component.

However, since the "noxious" cold-sensitivity of TRPA1 is very controversially described (see above) and, furthermore, since none of the recognized TRPV1-acting agents such as allyl isothyocyanate and cinnamaldehyde induce cold sensation but rather evoke "Hot painful burning", the existence of the "Cold painful burning " sensation is rather questionable.

• However, it is a common experience that allicin, the pungent ingredient of garlic and capsaicin, the pungent ingredient of hot chili peppers cause distinct pungent sensations upon consumption (yet they both initiate cutaneous irritation and edema). Namely, the ingestion of garlic induces pungency without (or much less) characteristic "hot" (or "cold") component. Since allicin was shown equally activate TRPA1 and TRPV1 [209,210], it can be speculated that simultaneous activation of TRPA1 and TRPV1 would result in a different sensation compared to those experienced upon selective involvement of these c hannels. Therefore, an " Unspecified painful burning only" sensation subcategory is proposed for such agents and/or mechanisms which may co-stimulate TRPV1 and TRPA1.

Naturally, "promiscuous" combinations of TRP channel activation and/or inhibition by certain substances may result in even more complex sensations, as they are sometimes reported by patients [2,[START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]. An intriguing example for such a multimodal (context-dependent?) inducer of itch/pain/burning sensations is the spice-ingredient, carvacrol (contained in oregano) which, besides activating TRPV3 (see above), also stimulates TRPA1 [180]. In perfect agreement with these cellular data, carvacrol, when applied to the skin or mucosal

A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 32
surfaces, not only produces "warm only" sensation but is also described as "pungent" and painful [180] .

In this complex framework of receptor -mediated sensations under the "itch umbrella", camphor is probably the "trickiest" among the better-known, clinically long-employed compounds. This skin irritant potently activates TRPV3 [171,172,179]. Subsequently, however, it first rapidly activates, and then strongly desensitizes TRPV1 [186]. To add a further level of complexity, camphor also inhibits TRPA1-mediated signaling [186,187].

According to our "molecular psychophysics" hypothesis introduced above , these mixed cellular effects on various TRP channel targets may perfectly explain the in vivo sensation "package" induced by skin-applied camphor; i.e. its ability to induce "warm" (TRPV3 activation) as well as transient "burning hot" (activation of TRPV1) sensations followed by the well-known analgesia (desensitization of TRPV1 and inhibition of TRPA1) [188]. Taken together, the presented "triple" mechanism may also provide a rational molecular basis for the well-documented efficacy of camphor to alleviate itch in certain pruritic diseases (such as contact irritant dermatitis) [185]. Furthermore, it could also explain the great inter-individual variations in a patient's response to cutaneous camphor administration which might reflect constitutive differences in the expression levels of and cross-signaling between distinct TRP channels.

Other non-thermosensitive TRP channels -Distant connections to pruritus

Although very little concrete information is as yet available on these non-thermosensitive TRP channels, certain members of the TRPC and TRPM subfamilies [109][110][111][112][113] can be envisaged to contribute to the pathogenesis of pruritus. Moreover, TRPC1 is overexpressed in epidermis of patients with Darier's disease (DD) (or keratosis follicularis), a genodermatosis which is induced by a genetic defect of the sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA)2, and which causes a severe, sometimes intensely pruritic epithelial differentiation disorder [215,216]. TRCP1-mediated Ca 2+ influx is significantly higher in keratinocytes obtained from DD patients. Furthermore, DD keratinocytes show enhanced proliferation and apoptosis resistance, suggesting that TRPC1 is involved in the abnormal keratinization in DD epidermis. Therefore, it is at least conceivable that a TRPC1-mediated keratinization defect contributes to a -puritogenic!disruption of the epidermal barrier, and thus indirectly to itch pathogenesis.

TRPM6 and TRPM7 -Itch and the magnesium connection

Finally, two channels of the TRPM subfamily (i.e. TRPM6 and TRPM7) may also affect itch pathogenesis. These molecules are recognized as key regulators of cellular magnesium homeostasis [109][110][111][112][113]. Since, in the rat, low-dietary magnesium leads to lowered serum Mg 2+ , resulting in universal dermatitis and intense scratching behavior [217], it is tempting to ask whether a signaling dysfunction in the "magnesium -controlling" members of the TRPM A C C E P T E D M A N U S C R I P T

Conclusions, future perspectives

Here, we have reviewed pruritogenic stimuli, pruritogens and selected neural elements and processing centers involved in pruritus pathogenesis and the generation of the itch sensation.

Focusing on recent neurophysiological concepts that explain pruriceptive pruritus and on the role of TRP channels in itch pathogenesis, we have sketched the long path "pruritology" has traveled to move, at long last, much closer to developing intelligent combination strategies for more effective pruritus management [START_REF] Paus | Scratching the brain for more effective "itch" therapy -Frontiers in pruritus research[END_REF][START_REF] Ikoma | The neurobiology of itch[END_REF][START_REF] Steinhoff | Neurophysiological, neuroimmunol ogical and neuroendocrine basis of pruritus[END_REF][START_REF] Bíró | How best to fight that nasty itch -from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches[END_REF]. However, we still lack a single, easily applicable, well-tolerated, and universally effective pharmacological "itch killer".

This underscores the vital clinical importance of identifying novel candidate anti-pruritic agents that target defined molecular itch pathogenesis pathways. In this review, our aim was to specifically highlight the promise that individual members of the fascinating TRP channel family hold in this respect. Though the available information already has invited the proposal of a novel molecular syntax of various itch-related sensory qualities (which border on pain, warm, and cold sensations or incorporate elements thereof), evidently, much more extensive in vitro and in vivo studies are required to explore the exact roles of TRP channels in the neurophysiology and future therapy of itch.

Suffice it, therefore, we conclude this "exotic TRiP" into the "dominion" of experim ental, theoretical, and clinical "pruritology" by defining a few "itchy frontiers" in pruritus research:

Ø Identification of physiological roles of TRP channels expressed on non-neuronal cells of human skin during inflammation and its related production of pruritogens.

Ø Definition of relationship of TRP channels to peripheral and central itch processing (sensitization, desensitization). -"Hot painful burning" sensation (i.e. pungency with a characteristic "hot" component) -Usually followed by analgesia Allyl isothyocyanate, cinnamaldehyde -Activates TRPA1 -"Hot painful burning" sensation (i.e. pungency with a characteristic "hot" component) -The existence of a "Cold painful burning" sensation is to be determined Allicin -Activates TRPA1 -Activates TRPV1

-"Unspecified painful burning only" sensation (i.e. pungency without characteristic "hot" or "cold" components) Carvacrol -Activates TRPV3 -Activates TRPA1

Mixed sensation:

-"Warm only" sensation -"Painful burning" sensation Camphor -Activates TRPV3 -Activates, then desensitizes TRPV1 -Inhibits TRPA1

Mixed sensation:

-"Warm only" sensation -"Painful burning" sensation -Followed by Analgesia

  endogenous substances collectively referred to as "endovanilloids"[118]. The receptor was first shown to be directly stimulated by low-threshold (>43 o C) heat and acidosis [114]. Later, A C C E P T E D M A N U S C R I P T several other molecules were also described to indirectly act on the TRPV1, i.e. via the activation on their specific receptors and initiating various intracellu lar signaling pathways. These stimuli are for example the bradykinin [119,120], ATP [121], lipoxygenase products [120,122], prostaglandins [123], histamine [124], various neurotrophins (such as NGF, neurotrophin -3 and -4) [101,119], TNF-α [125], pro-inflammatory chemokines [126], activation of PAR 2 by proteases [127,128] or activation of metabotropic receptor -coupled hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC) [129]. Most of the above substances and mechanisms, released/activated chiefly during inflammation, shift the activation threshold (>43 o C) of TRPV1 ("to the left") towards more physiological temperatures hence leading to the acute or chronic sensitization of the TRPV1expressing nociceptive C -afferents (inflammatory thermal "heat/hot" hyperalgesia) [115,130].

  a threefold higher potency to induce desensitization (i.e. to treat pain and presumably itch) than excitation (i.e. to induce pain)[116]. Furthermore, future clinical trials should also systemically analyze the in vivo effects of TRPV1 antagonists (such as capsazepine or iodo-RTX)[151,152] to suppress itch.3.2.4.2. Cannabinoids and TRPV1 -The challenge of getting the skin "high", rather than "itchy" Another endogenous, arachidonic acid metabolism-derived substance, anandamide (Narachidonoylethanolamine) connects TRPV1-mediated signaling to the cannabinoids and their receptors (CB-R), another very intriguing endogenous system for the modulation of both pain and itch[153]. CB-Rs and TRPV1 show a marked co-localization pattern on primary afferents[154][155][156][157]. On these sensory structures, the stimulation of CB-Rs by various (endo-and exo-) cannabinoids effectively inhibits pain[158,159] -similar to the effects seen with TRPV1mediated desensitization (see above). The intimate relationship between the two systems is also supported by the observation that the endocannabinoid anandamide, depending on its concentration and other local factors (such as e.g. acidosis in inflammation)[160], may also stimulate TRPV1-mediated signaling (e.g. intracellular calcium elevations) acting as an "endovanilloid"[118,153,158].Various synthetic exocannabinoids, applied either intradermally or topically, also effectively prevent both histamine-induced [161] and disease-related[162,163] pruritus. Since this effect is accompanied by a decreased neuropeptide r elease from the sensory endings[161], this suggests that the TRPV1 pathway may also be involved in mediating the antipruritic action of cannabinoids. Moreover, CB-R1 and CB-R2 have recently been identified on epidermal keratinocytes, along with numerous o ther non-neuronal cell types of human skin (in part, in with TRPV1!)[157,164]. On keratinocytes, the activation of CB-R2 resulted in the release of β-endorphin [165], an analgesic and antipruritic proopiomelanocortin product. This suggests the involvement of CB-R-mediated signaling in the neuronal -nonneuronal cellular networks that generate itch.

  as an osmosensor [169] channels also function as cellular temperature sensor molecules, since all are activated by increasing temperatures, from the innocuous to the noxious range [109-113]. In the context of skin-related sensory modalities, TRPV3 (activated at =33 o C) [170-172] and TRPV4 (with an activation range of 27-42 o C) [173,174] have attracted special attention.However, the expression of these channels on sensory neuronal structures is very controversially described in the literature. Namely, whereas TRPV3 was clearly identified in human and monkey dorsal root ganglia (DRG)[ 170,172], its presence was not found in rat or mouse DRG[171]. In contrast, TRPV4 was described in mouse DRG[169] and on free nerve A C C E P T E D M A N U S C R I P T endings of mouse skin [175]. Moreover, in human and monkey DRG, TRPV3 is expressed o n the small diameter neuron population (similar to TRPV4 in mouse DRG [ 175], and, of importance, in co-localization with TRPV1

  180]. In addition, the activation of TRPV3 by eugenol on mouse keratinocytes results in the release of (pro-inflammatory) IL -1α which may augment the skinsensitizing effect of this alkaloid[180].

1 .

 1 TRPCs -Itch modulation via altered keratinocyte differentiation?TRPC channels are key regulators of intracellular calcium homeostasis in a wide array of different cells types[109][110][111][112][113]. Importantly, numerous TRPC channels (TRPC1, TRPC4, TRPC5, TRPC6, TRPC7) are expressed by epidermal or mucosal keratinocytes[211][212][213].Their expression levels s ignificantly fluctuate in a differentiation-dependent manner, suggesting a role for these TRPC channels in the control of keratinocyte differentiation[211- 213]. Indeed, TRPC1 has already been implicated in the calcium-induced terminal differentiation of human keratinocytes in vitro[214].

  induce pruritogenic pruritus. That this line of thought is not entirely speculative is suggested by the intriguing clinical report that the pruritus of uremic patients completely disappeared after decreasing the concentration of magnesium Mg 2+ in the dialysate[218].

Ø

  Determination of the expression patterns of individual TRP channels in various pruritic dermatoses. Ø Investigation of itch-related behavior of TRP channel gene -deficient mice.Ø In vivo evaluation of "combination therapies", using substances that target different TRP channels.Ø Identification, synthesis, and in vitro as well as in vivo evaluation of novel TRPchannel-acting pharmacological agents, partly based on sensations defined by the concept of "molecular psychophysics".

Table 1 .

 1 Selected pruritogens and their effects*

	Pruritogen	Cellular action	Source	Functions related to itch
	Acetylcholine	Nicotinic and	Nerves, epidermal	-Mediates itch in AD patients
		Muscarinic	keratinocytes,
		acetylcholine	lymphocytes,
		receptors	melanocytes
	Bradykinin	Bradykinin (B 1 , B 2 )	Endothelial cells,	-Induces pain but also pruritus
		receptors	immunocytes	-B 2 receptor antagonists reduce itch
				-Sensitizes sensory afferents for various
				chemical stimuli (e.g. histamine)
				-Pruritogenic in AD
	Calcitonin gene-	CGRP-receptors	Sensory nerve	-Induces pruritus intradermally
	related peptide		fibers	-Prolongs SP-induced itch latency injection
	(CGRP)			(inhibitory effect?)
				-Increased CGRP in afferent fibers in itchy
				skin diseases
	Endothelins	Endothelin (ET)	Endothelium, mast	-Induces burning itch
		receptors	cells	-Degraded by chymase via ETA-receptor
				activation
	Histamine	Histamine (H1-4)	Mast cells,	-Induces itch via H1 in humans and also via H4
		receptors	epidermal	in mice
			keratinocytes	-In humans, H1 antagonists alleviate itch
				-H1 and H2 receptors regulate epidermal barrier
				integrity
	Interleukins (IL)	IL receptors	Immune cells	-IL-2 induces itch in cancer patients
				-IL-2 sensitizes sensory afferents
				-Mice overexpressing IL-4 in the epidermis
				spontaneously develop itch
				-Elevated IL-6-like immunoreacti vity was
				found in nerve fibers of patients with prurigo
				nodularis
				-Mice overexpressing IL-31 in T -cells develop
				itch
				-IL-31 is increased in an atopic mouse model
				as well as in AD
	Leukotrienes	Leukotriene	Mast cells,	-Leukoriene B 4 induces itch in mice
		receptor	epidermal	-Leukoriene B 4 is involved in the SP-and
			keratinocytes	nociceptin-mediated induction of itch
	NGF and	Specific receptors	Epidermal and hair	-NF, NT-3 and NT-4 acutely sensitize sensory
	neurotrophins	TrkA : NGF	follicle	afferents, and up-regulate neuronal
	(NT)	TrkB : NT-4	keratinocytes,	neuropeptides and TRPV1
		TrkC : NT-3	fibroblasts, mas t	-NGF and NT-4 are enhanced in AD
			cells	-NGF induces tryptase release from mast cells
				-NGF and NT-4 induces sprouting of sensory
				afferents resulting in chronic sensitization
	Prostaglandins	Prostanoid	Mast cells,	-PGE1 and PGE2 potentiate the action of
		receptors	epidermal	histamine
			keratinocytes	-PGE1 lowers itch threshold
				-PGE2 evokes pruritus in AD

Table 2 .

 2 Functions and potential therapeutic significance of thermosensitive TRP channels and their endogenous or exogenous modulators in itch

	Receptor	Expression in
		the skin

Effects of endogenous modulators - physiological role in itch Effects of exogenous modulators - potential role in the therapy of itch

		ACCEPTED MANUSCRIPT
		neurons	-Activated by anandamide and
		-Epidermal	arachidonic acid via the productions
		keratinocytes	of lipid peroxidation products as
		-Mast cells	eicosanoids
			-Regulates epidermal barrier integrity
			and homeostasis
	TRPM8 TRPA1	-Sensory neurons	-Activated at T =28 ºC -Metabotropic receptor-coupled hydrolysis of PIP2 inhibits its activity -Activated by bradykinin, arachidonic acid, and prostaglandins -Activated by a PLC -dependent mechanisms -Its noxious (T =17 o C) cold sensitivity is controversial -Its role in inflammatory thermal "cold" hyperalgesia is controversial M A N U S C R I P T -Menthol, eucalyptol, and icilin activate TRPM8 (and induce sensation of cooling) -Cooling, menthol, and icilin inhibit experimental or disease-related itch -Allyl isothyocyanate (alkaloid of e.g. mustard oil, wasabi, and horseradish) and cinnamaldehyde (substance isolated from cinnamon oil) activates TRPA1 -Allicin, the pungent ingredient of garlic, activates TRPA1 -Carvacrol activates TRPA1 -Menthol inhibits TRPA1 -Camphor inhibits TRPA1
	TRPV1	-Sensory neurons -Mast cells -Epidermal keratinocytes -Hair follicle keratinocytes -Langerhans cells -Smooth muscle -Sebocytes A C C E P T E D -Activated at T > 43 o C and acidosis -Activated/sensitized by pruritogenic substances such as histamine, bradykinin, eicosanoids, ATP, TNF-α, chemokines, prostaglandins, NGF, neurotrophins, and tryptases -Activated/sensitized by the PLC -dependent hydrolysis of PIP 2 -Also activated by the endocannabinoid anandamide -Mediates inflammatory thermal "hot" hyperalgesia -Regulates epidermal barrier integrity and homeostasis	-Capsaicin and RTX excites and then desensitizes TRPV1 expressing sensory neurons resulting in depletion of neuropeptides -Capsaicin inhibits experimental and disease-related itch -Camphor, which alleviates itch in pruritic diseases, transiently activates then strongly desensitizes TRPV1 -Allicin, the pungent ingredient of garlic, activates TRPV1 -Noxious thermal stimuli (50 o C) inhibit histamine-induced itch -TRPV1 activation by capsaicin affects epidermal and follicular
				proliferation, differentiation, and
				apoptosis and release of cytokines,
				ILs, and growth factors
				-TRPV1 is increased in epidermal
				keratinocytes of prurigo nodularis
				patients.
	TRPV2	-Sensory	-Activated at T =52 o C	-Noxious thermal stimuli (50 o C)
		neurons	-Activation of TRPV2 on mast cells	inhibit histamine-induced itch
		-Mast cells	by thermal stimuli results in a pro-
			inflammatory degranulation event
	TRPV3	-Sensory	-Activated at T =33 o C, also on	-Eugenol, thymol, and carvacrol
		neurons	epidermal keratinocytes	(from oregano, clove, and thyme)
		-Epidermal	-Sensitized by arachidonic acid and	activate and sensitize TRPV3
		keratinocytes	related unsaturated fatty acids	-Eugenol releases IL-1α from
			-Augmented by histamine and	keratinocytes via TRPV3
			bradykinin in a PLC-dependent	-Camphor activates TRPV3
			manner
	TRPV4	-Sensory	-Activated at T 27-42 o C	-Not known
			65

Table 3 .

 3 Temperature-sensitive TRP channels establish a basic syntax and molecular substrate ("molecular psychophysics") of nociception and pruriception

	Prototypic agent	Effect on TRP channels	Sensory phenomenon
	Eugenol	-Activates TRPV3	-"Warm only" sensation
			(i.e. "pure warmness" without any "burning pain"
			component)
	Menthol	-Activates TRPM8	-"Cool only" sensation
		-Inhibits TRPA1	(i.e. "pure coolness" without any "burning pain"
			component)
	Capsaicin	-Activates, then
		desensitizes TRPV1

Grants, supports: Writing of this article was supported in part by Hungarian research grants (OTKA T049231 and K063153, ETT 480/2006) for T. Bíró and by a grant from Deutsche Forschungsgemeinschaft to R. Paus (Pa 345/11-2).