Tonic inhibition of allergic itch signaling by the descending noradrenergic system in mice

Basic mechanisms of itch

Pruritus (or itch) is an unpleasant sensation leading to a desire to scratch. In the epidermis, there are selective C or Aδ epidermal nerve endings that are pruriceptors. At their other ends, peripheral neurons form synapses with spinal neurons and interneurons. Many areas in the central nervous system are involved in itch processing. Although itch does not occur solely because of parasitic, allergic, or immunologic diseases, it is usually the consequence of neuroimmune interactions. Histamine is involved in a minority of itchy conditions, and many other mediators play a role: cytokines (eg, IL-4, IL-13, IL-31, IL-33, and thymic stromal lymphopoietin), neurotransmitters (eg, substance P, calcitonin gene-related peptide, vasoactive intestinal peptide, neuropeptide Y, NBNP, endothelin 1, and gastrin-releasing peptide), and neurotrophins (eg, nerve growth factor and brain-derived neurotrophic factor). Moreover, ion channels such as voltage-gated sodium channels, transient receptor potential vanilloid 1, transient receptor ankyrin, and transient receptor potential cation channel subfamily M (melastatin) member 8 play a crucial role. The main markers of nonhistaminergic pruriceptors are PAR-2 and MrgprX2. A notable phenomenon is the sensitization to pruritus, in which regardless of the initial cause of pruritus, there is an increased responsiveness of peripheral and central pruriceptive neurons to their normal or subthreshold afferent input in the context of chronic itch.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Circuit Mechanisms of Itch in the Brain

Itch is an unpleasant somatic sensation with the desire to scratch, and it consists of sensory, affective, and motivational components. Acute itch serves as a critical protective mechanism because an itch-evoked scratching response will help to remove harmful substances invading the skin. Recently, exciting progress has been made in deciphering the mechanisms of itch at both the peripheral nervous system and the CNS levels. Key neuronal subtypes and circuits have been revealed for ascending transmission and the descending modulation of itch. In this review, we mainly summarize the current understanding of the central circuit mechanisms of itch in the brain.

Chronic itch: emerging treatments following new research concepts

Until recently, itch pathophysiology was poorly understood and treatments were poorly effective in relieving itch. Current progress in our knowledge of the itch processing, the numerous mediators and receptors involved has led to a large variety of possible therapeutic pathways. Currently, inhibitors of IL-31, IL-4/13, NK₁ receptors, opioids and cannabinoids, JAK, PDE4 or TRP are the main compounds involved in clinical trials. However, many new targets, such as Mas-related GPCRs and unexpected new pathways need to be also explored.

Intrinsic braking role of descending locus coeruleus noradrenergic neurons in acute and chronic itch in mice

Itch is defined as an unpleasant sensation that provokes a desire to scratch. Our understanding of neuronal circuits for itch information transmission and processing in the spinal dorsal horn (SDH) has progressively advanced following the identification of SDH neuron subsets that are crucial for scratching behavior in models of itch. However, little is known about the control of acute and chronic itch by descending signals from the brain to the SDH. In this study, using genetic approaches that enable cell-type and circuit-specific functional manipulation, we reveal an intrinsic potential of locus coeruleus (LC)-noradrenergic (NAergic) neurons that project to the SDH to control acute and chronic itch. Activation and silencing of SDH-projecting LC-NAergic neurons reduced and enhanced scratching behavior, respectively, in models of histamine-dependent and -independent acute itch. Furthermore, in a model of chronic itch associated with contact dermatitis, repetitive scratching behavior was suppressed by the activation of the descending LC-NAergic pathway and by knocking out NA transporters specific to descending LC-NAergic neurons using a CRISPR-Cas9 system. Moreover, patch-clamp recording using spinal slices showed that noradrenaline facilitated inhibitory synaptic inputs onto gastrin-releasing peptide receptor-expressing SDH neurons, a neuronal subset known to be essential for itch transmission. Our findings suggest that descending LC-NAergic signaling intrinsically controls acute and chronic itch and provide potential therapeutic strategies for the treatment of acute and chronic itch.

Neural Mechanisms of Itch

Itch is a unique sensation that helps organisms scratch away external threats; scratching itself induces an immune response that can contribute to more itchiness. Itch is induced chemically in the peripheral nervous system via a wide array of receptors. Given the superficial localization of itch neuron terminals, cells that dwell close to the skin contribute significantly to itch. Certain mechanical stimuli mediated by recently discovered circuits also contribute to the itch sensation. Ultimately, in the spinal cord, and likely in the brain, circuits that mediate touch, pain, and itch engage in cross modulation. Much of itch perception is still a mystery, but we present in this review the known ligands and receptors associated with itch. We also describe experiments and findings from investigations into the spinal and supraspinal circuitry responsible for the sensation of itch.

Central circuit mechanisms of itch

Itch, in particular chronic forms, has been widely recognized as an important clinical problem, but much less is known about the mechanisms of itch in comparison with other sensory modalities such as pain. Recently, considerable progress has been made in dissecting the circuit mechanisms of itch at both the spinal and supraspinal levels. Major components of the spinal neural circuit underlying both chemical and mechanical itch have now been identified, along with the circuits relaying ascending transmission and the descending modulation of itch. In this review, we summarize the progress in elucidating the neural circuit mechanism of itch at spinal and supraspinal levels.

Clinical practice guidelines for the management of atopic dermatitis 2018

Atopic dermatitis (AD) is a disease characterized by relapsing eczema with pruritus as a primary lesion. The current strategies to treat AD in Japan from the perspective of evidence-based medicine consist of three primary measures: (i) the use of topical corticosteroids and tacrolimus ointment as the main treatment for the inflammation; (ii) topical application of emollients to treat the cutaneous barrier dysfunction; and (iii) avoidance of apparent exacerbating factors, psychological counseling and advice about daily life. The guidelines present recommendations to review clinical research articles, evaluate the balance between the advantages and disadvantages of medical activities, and optimize medical activity-related patient outcomes with respect to several important points requiring decision-making in clinical practice.

Peripheral and Central Mechanisms of Itch

Itch is a unique sensory experience that is encoded by genetically distinguishable neurons both in the peripheral nervous system (PNS) and central nervous system (CNS) to elicit a characteristic behavioral response (scratching). Itch interacts with the other sensory modalities at multiple locations, from its initiation in a particular dermatome to its transmission to the brain where it is finally perceived. In this review, we summarize the current understanding of the molecular and neural mechanisms of itch by starting in the periphery, where itch is initiated, and discussing the circuits involved in itch processing in the CNS.

Selective deficiencies in descending inhibitory modulation in neuropathic rats: implications for enhancing noradrenergic tone

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Descending noradrenergic pathways modulate spontaneous but not evoked thalamic neuronal hyperexcitability in neuropathic pain states. Spinal clonidine inhibits evoked and spontaneous firing, whereas reboxetine selectively inhibits evoked firing.

Pontine noradrenergic neurones form part of a descending inhibitory system that influences spinal nociceptive processing. Weak or absent descending inhibition is a common feature of chronic pain patients. We examined the extent to which the descending noradrenergic system is tonically active, how control of spinal neuronal excitability is integrated into thalamic relays within sensory-discriminative projection pathways, and how this inhibitory control is altered after nerve injury. In vivo electrophysiology was performed in anaesthetised spinal nerve–ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus (VPL). In sham rats, spinal block of α₂-adrenoceptors with atipamezole resulted in enhanced stimulus-evoked and spontaneous firing in the VPL, and produced conditioned place avoidance. However, in SNL rats, these conditioned avoidance behaviours were absent. Furthermore, inhibitory control of evoked neuronal responses was lost, but spinal atipamezole markedly increased spontaneous firing. Augmenting spinal noradrenergic tone in neuropathic rats with reboxetine, a selective noradrenergic reuptake inhibitor, modestly reinstated inhibitory control of evoked responses in the VPL but had no effect on spontaneous firing. By contrast, clonidine, an α₂ agonist, inhibited both evoked and spontaneous firing, and exhibited increased potency in SNL rats compared with sham controls. These data suggest descending noradrenergic inhibitory pathways are tonically active in sham rats. Moreover, in neuropathic states, descending inhibitory control is diminished, but not completely absent, and distinguishes between spontaneous and evoked neuronal activity. These observations may have implications for how analgesics targeting the noradrenergic system provide relief.

Dynamics and Functional Role of Dopaminergic Neurons in the Ventral Tegmental Area during Itch Processing

Itchiness triggers a strong urge to engage in scratching behavior, which could lead to severe skin or tissue damage in patients with chronic itch. This process is dynamically modulated. However, the neural mechanisms underlying itch modulation remain largely unknown. Here, we report that dopaminergic (DA) neurons in the ventral tegmental area (VTA) play a critical role in modulating itch-induced scratching behavior. We found that the activity of VTA DA neurons was increased during pruritogen-induced scratching behavior in freely moving male mice. Consistently, individual VTA DA neurons mainly exhibited elevated neural activity during itch-induced scratching behavior as demonstrated by in vivo extracellular recording. In behavioral experiments, the transient suppression of VTA DA neurons with the optogenetic approach shortened the pruritogen-induced scratching train. Furthermore, the DA projection from the VTA to the lateral shell of the nucleus accumbens exhibited strong activation as measured with fiber photometry during itch-elicited scratching behavior. These results revealed the dynamic activity of VTA DA neurons during itch processing and demonstrated the modulatory role of the DA system in itch-induced scratching behavior.SIGNIFICANCE STATEMENT Itchiness is an unpleasant sensation that evokes a scratching response for relief. However, the neural mechanism underlying the modulation of itch-evoked scratching in the brain remains elusive. Here, by combining fiber photometry, extracellular recording, and optogenetic manipulation, we show that the dopaminergic neurons in the ventral tegmental area play a modulatory role in itch-evoked scratching behavior. These results reveal a potential target for suppressing excessive scratching responses in patients with chronic itch.

Opposing effects of cervical spinal cold block on spinal itch and pain transmission

Inactivation of descending pathways enhanced responses of spinal dorsal horn neurons to noxious stimuli, but little is known regarding tonic descending modulation of spinal itch transmission. To study effects of cervical spinal cold block on responses of dorsal horn neurons to itch-evoking and pain-evoking stimuli, single-unit recordings were made from superficial dorsal horn wide dynamic range and nociceptive-specific-type neurons in pentobarbital-anesthetized mice. Intradermal histamine excited 17 units. Cold block starting 1 minute after intradermal injection of histamine caused a marked decrease in firing. The histamine-evoked response during and following cold block was significantly lower compared with control histamine-evoked responses in the absence of cold block. A similar but weaker depressant effect of cold block was observed for dorsal horn unit responses to chloroquine. Twenty-six units responded to mustard oil allyl isothiocyanate (AITC), with a further significant increase in firing during the 1-minute period of cold block beginning 1 minute after AITC application. Activity during cold block was significantly greater compared with the same time period of control responses to AITC in the absence of cold block. Ten units' responses to noxious heat were significantly enhanced during cold block, while 6 units' responses were reduced and 18 unaffected. Cold block had no effect on mechanically evoked responses. These results indicate that spinal chemonociceptive transmission is under tonic descending inhibitory modulation, while spinal pruriceptive transmission is under an opposing, tonic descending facilitatory modulation.

Peripheral gabapentin regulates mosquito allergy-induced itch in mice

The antipruritic activity of gabapentin, an anticonvulsant, was studied in a mouse model of allergic itch. In mice sensitized by an extract of the salivary glands of the mosquito (ESGM), an intradermal injection of ESGM elicited scratching and increased peripheral nerve firing. Oral or intradermal administration of gabapentin at the ESGM injection site inhibited ESGM-induced scratching and peripheral nerve firing. However, gabapentin did not affect histamine-induced scratching. The distributions of immunoreactivity to the voltage-dependent calcium channel α₂δ-1 subunit, a site of gabapentin action, and the histamine H₁ receptor differed in the mouse dorsal root ganglia. The α₂δ-1 subunit was mainly found in neurons that were 15-20 µm in diameter, whereas the H₁ receptor was mainly in 20-30 µm neurons. In addition, α₂δ-1 subunit immunoreactivity co-localized with that of transient receptor potential vanilloid 1 (TRPV1). These results suggest that gabapentin regulates allergic itch by acting on the calcium channel α₂δ-1 subunit in peripheral TRPV1-positive neurons.

The peripheral and central mechanisms underlying itch

Itch is one of the most distressing sensations that substantially impair quality of life. It is a cardinal symptom of many skin diseases and is also caused by a variety of systemic disorders. Unfortunately, currently available itch medications are ineffective in many chronic itch conditions, and they often cause undesirable side effects. To develop novel therapeutic strategies, it is essential to identify primary afferent neurons that selectively respond to itch mediators as well as the central nervous system components that process the sensation of itch and initiate behavioral responses. This review summarizes recent progress in the study of itch, focusing on itch-selective receptors, signaling molecules, neuronal pathways from the primary sensory neurons to the brain, and potential decoding mechanisms based on which itch is distinguished from pain. [BMB Reports 2016; 49(9): 474-487]

Exacerbating factors of itch in atopic dermatitis

Atopic dermatitis (AD) displays different clinical symptoms, progress, and response to treatment during early infancy and after childhood. After the childhood period, itch appears first, followed by formation of well-circumscribed plaque or polymorphous dermatoses at the same site. When accompanied with dermatitis and dry skin, treatment of skin lesions should be prioritized. When itch appears first, disease history, such as causes and time of appearance of itch should be obtained by history taking. In many cases, itch increases in the evening when the sympathetic nerve activity decreased. Treatment is provided considering that hypersensitivity to various external stimulations can cause itch. Heat and sweating are thought to especially exacerbate itch. Factors causing itch, such as cytokines and chemical messengers, also induce itch mainly by stimulating the nerve. Scratching further aggravates dermatitis. Skin hypersensibility, where other non-itch senses, such as pain and heat, are felt as itch, sometimes occurs in AD. Abnormal elongation of the sensory nerve into the epidermis, as well as sensitizing of the peripheral/central nerve, are possible causes of hypersensitivity, leading to itch. To control itch induced by environmental factors such as heat, treatment for dermatitis is given priority. In the background of itch exacerbated by sweating, attention should be given to the negative impact of sweat on skin homeostasis due to 1) leaving excess sweat on the skin, and 2) heat retention due to insufficient sweating. Excess sweat on the skin should be properly wiped off, and dermatitis should be controlled so that appropriate amount of sweat can be produced. Not only stimulation from the skin surface, but also visual and auditory stimulation can induce new itch. This "contagious itch" can be notably observed in patients with AD. This article reviews and introduces causes of aggravation of itch and information regarding how to cope with such causes.

Descending serotonergic and noradrenergic systems do not regulate the antipruritic effects of cannabinoids

BACKGROUND

For centuries, cannabinoids have been known to be effective in pain states. Itch and pain are two sensations sharing a lot in common.

OBJECTIVE

The goal of this research was to observe whether the cannabinoid agonist WIN 55,212-2 reduces serotonin-induced scratching behaviour and whether neurotoxic destruction of descending serotonergic and noradrenergic pathways mediate the antipruritic effect of WIN 55,212-2. Material and methods Scratching behaviour was induced by intradermal injection of serotonin (50 µg/50 µl/mouse) to Balb/c mice. The neurotoxins 5,7-dihydroxytryptamine (5,7-DHT, 50 μg/mouse) and 6-hydroxydopamine (6-OHDA, 20 μg/mouse) are applied intrathecally to deplete serotonin and noradrenaline in the spinal cord. WIN 55,212-2 (1, 3, 10 mg/kg, i.p.) dose-dependently attenuated serotonin-induced scratches. Neurotoxic destruction of neither the serotonergic nor the noradrenergic systems by 5,7-DHT and 6-OHDA, respectively, had any effect on the antipruritic action of WIN 55,212-2.

CONCLUSION

Our findings indicate that cannabinoids dose-dependently reduce serotonin-induced scratching behaviour and neurotoxic destruction of descending inhibitory pathways does not mediate this antipruritic effect.

Methods for preclinical assessment of antipruritic agents and itch mechanisms independent of mast-cell histamine

Itch is a sensation that provokes a desire to scratch. Mast-cell histamine was thought to be a key itch mediator. However, histamine and mast-cell degranulation were reported not to elicit scratching in animals. It was difficult to investigate the pathophysiology of itching and to evaluate the antipruritic efficacy of chemical agents in the early 1990 s. We showed that hind-paw scratching and biting were elicited by stimulation with pruritogenic agents in mice. Those results demonstrated for the first time that cutaneous itching could be evaluated behaviorally in animals. We established various animal models of pathological itch of the skin (dry skin, mosquito allergy, surfactant-induced pruritus, and herpes zoster) and mucus membranes (pollen allergy). Mast-cell histamine did not play a key role in itching in any animal model examined except for the pollen allergy model. Histamine is not an exclusive itch mediator of mast cells; tryptase and leukotriene B4 released from mast cells also act as itch mediators. Epidermal keratinocytes release several itch mediators, such as leukotriene B4, sphingosylphosphorylcholine, thromboxane A2, nociceptin, nitric oxide, and histamine, which may play important roles in pathological itching. Appropriate animal models of pathological itching are needed for pharmacological evaluation of the antipruritic efficacy of chemical agents.

Noradrenergic modulation of itch transmission in the spinal cord

Inhibition of both itching and scratching is important in the treatment of chronic pruritic diseases, because itching has a negative impact on quality of life and vigorous scratching worsens skin conditions. Pharmacological modulation of itch transmission in the dorsal horn is an effective way to inhibit both itching and scratching in pruritic diseases. Pruriceptive transmission in the spinal dorsal horn undergoes inhibitory modulation by the descending noradrenergic system. The noradrenergic inhibition is mediated by excitatory α₁-adrenoceptors located on inhibitory interneurons and inhibitory α₂-adrenoceptors located on central terminals of primary sensory neurons. The descending noradrenergic system and α-adrenoceptors in the dorsal horn are potential targets for antipruritic drugs.

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.

[A memoir of my research on pain and analgesia for 39 years]

This review describes my research for the past 39 years regarding the pharmacology of pain and analgesia. We have demonstrated that the descending noradrenergic system is involved in the analgesic effect of morphine injected into the nucleus reticularis gigantocellularis, and that noradrenaline exerts antinociception mediated by α-adrenoceptors. We have found that noxious mechanical and thermal stimuli to the skin increase the release of substance P and somatostatin, respectively, from the dorsal horn in situ, and that noradrenaline inhibits the release of substance P and glutamate from primary afferents. We developed an animal model of cancer pain using melanoma cells. We have shown that the suppression of cancer pain results in the inhibition of tumor growth and lung metastasis, and that melanoma cells release several algogenic substances including ATP, endothelin-1, and bradykinin. We investigated neuropathic allodynia induced by the chemotherapeutic drugs paciltaxel, oxaliplatin, vincristine, and bortezomib. Single administration of these drugs caused allodynia with similar time-courses. However, antiallodynic actions of adjuvant analgesics, including gabapentin and limaprost, were dependent on the chemotherapeutic drugs used. Limaprost experiments have revealed that a decrease in peripheral blood flow is involved in allodynia exacerbation after the administration of paciltaxel and oxaliplatin. We have developed animal models of herpetic pain and postherpetic neuralgia using herpes simplex virus 1. We have demonstrated that nitric oxide, prostaglandin E2, and galectin-3 are involved in herpetic allodynia, that risk factors associated with postherpetic allodynia include severe herpetic pain, nociceptin, and major histocompatibility complex, and that deafferentation and nitric oxide are involved in postherpetic allodynia.

Gabapentin inhibits bortezomib-induced mechanical allodynia through supraspinal action in mice

Bortezomib, an inhibitor of proteasome holoenzyme, is used to treat relapsed and refractory multiple myeloma. Peripheral neuropathy is a treatment-limiting adverse effect of bortezomib and is very difficult to control. In this study, we examined the efficacy of gabapentin in inhibiting bortezomib-induced peripheral neuropathy. Single intravenous injections of bortezomib (0.03 - 0.3 mg/kg) dose-dependently induced mechanical allodynia with a peak effect 12 days after injection. Bortezomib (0.3 mg/kg) also caused mechanical hyperalgesia, but neither affected thermal nociception nor induced cold allodynia. Bortezomib increased the response of the saphenous nerve to weak punctate stimulation but not response to cool stimulation of the skin. When administered 12 days after bortezomib injection, oral and intracisternal gabapentin markedly inhibited mechanical allodynia. Intrathecal, but not intraplantar, gabapentin had a tendency to reduce mechanical allodynia. The antiallodynic activity of orally administered gabapentin was suppressed by noradrenaline, but not serotonin, depletion in the spinal cord. Bortezomib did not affect the expression levels of the calcium channel α₂δ-1 subunit, a high-affinity binding site of gabapentin, in the plantar skin, spinal cord, medulla oblongata, and pons. These results suggest that gabapentin inhibits bortezomib-induced mechanical allodynia, most likely through the activation of the descending noradrenergic system.

The development of a murine model for Forcipomyia taiwana (biting midge) allergy

BACKGROUND

Forcipomyia taiwana (biting midge) allergy is the most prevalent biting insect allergy in Taiwan. An animal model corresponding to the human immuno-pathologic features of midge allergy is needed for investigating the mechanisms and therapies. This study successfully developed a murine model of Forcipomyia taiwana allergy.

METHODS

BALB/c mice were sensitized intra-peritoneally with midge extract on days 0, 7, 14, 21 then intra-dermally on days 28, 31 and 35. Serum midge-specific IgE, IgG1, and IgG2a were measured every 14 days by indirect ELISA. The mice were challenged intradermally with midge extract at day 40 and then sacrificed. Proliferation and cytokine production of splenocytes after stimulation with midge extract were determined by MTT assay and ELISA, respectively. The cytokine mRNA expression in response to midge stimulation was analyzed by RT-PCR.

RESULTS

Serum IgE, total IgG, and IgG1 antibody levels against midge extract were significantly higher in the midge-sensitized mice than in the control mice. After the two-step sensitization, all mice in the midge-sensitized group displayed immediate itch and plasma extravasation reactions in response to challenge with midge extract. Skin histology from midge-sensitized mice showed marked eosinophil and lymphocyte infiltrations similar to that observed in humans. Stimulation of murine splenocytes with midge extract elicited significant proliferation, IL-4, IL-10, IL-13 and IFN-γ protein production, and up-regulation of mRNA in a dose-dependent manner in the midge-sensitized group, but not in the control group.

CONCLUSIONS

A murine model of midge bite allergy has been successfully developed using a two-step sensitization protocol. The sensitized mice have very similar clinical and immunologic reactions to challenge with midge proteins as the reactions of human to midge bites. This murine model may be a useful platform for future research and the development of treatment strategies for insect bite allergy.

Case report of atopic dermatitis with refractory pruritus markedly improved with the novel use of clonidine and trimeprazine

We report a 6-year-old boy with severe atopic dermatitis and refractory pruritus. The novel use of clonidine, an adrenergic agonist, along with trimeprazine, led to dramatic improvement. This represents the first case report of clonidine's effect in relieving pruritus in atopic dermatitis.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Milnacipran inhibits itch-related responses in mice through the enhancement of noradrenergic transmission in the spinal cord

We investigated whether milnacipran, a serotonin-noradrenaline reuptake inhibitor, exhibits an antipruritic effect through the spinal action in mice. Intrathecal injections of milnacipran (0.1 - 10 μg/site) significantly suppressed serotonin-induced biting, which is an itch-related response. However, such an effect was not observed with fluvoxamine (10 μg/site), which is a selective serotonin reuptake inhibitor. Furthermore, an intraperitoneal injection of milnacipran (10 mg/kg) inhibited serotonin-induced biting. When phentolamine (1.0 μg/site), a non-selective α-adrenoceptor antagonist, was intrathecally injected, it inhibited the above response of milnacipran. These results suggest that milnacipran suppresses itching through the inhibition of noradrenaline reuptake in the spinal cord.

Transmitters and pathways mediating inhibition of spinal itch-signaling neurons by scratching and other counterstimuli

Scratching relieves itch, but the underlying neural mechanisms are poorly understood. We presently investigated a role for the inhibitory neurotransmitters GABA and glycine in scratch-evoked inhibition of spinal itch-signaling neurons in a mouse model of chronic dry skin itch. Superficial dorsal horn neurons ipsilateral to hindpaw dry skin treatment exhibited a high level of spontaneous firing that was significantly attenuated by cutaneous scratching, pinch and noxious heat. Scratch-evoked inhibition was nearly abolished by spinal delivery of the glycine antagonist, strychnine, and was markedly attenuated by respective GABA_A and GABA_B antagonists bicuculline and saclofen. Scratch-evoked inhibition was also significantly attenuated (but not abolished) by interruption of the upper cervical spinal cord, indicating the involvement of both segmental and suprasegmental circuits that engage glycine- and GABA-mediated inhibition of spinal itch-signaling neurons by noxious counterstimuli.