Akihiko Ikoma

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

A sensory neuron–expressed IL-31 receptor mediates T helper cell–dependent itch: Involvement of TRPV1 and TRPA1

BACKGROUND Although the cytokine IL-31 has been implicated in inflammatory and lymphoma-associated itch, the cellular basis for its pruritic action is yet unclear. OBJECTIVE We sought to determine whether immune cell-derived IL-31 directly stimulates sensory neurons and to identify the molecular basis of IL-31-induced itch. METHODS We used immunohistochemistry and quantitative real-time PCR to determine IL-31 expression levels in mice and human subjects. Immunohistochemistry, immunofluorescence, quantitative real-time PCR, in vivo pharmacology, Western blotting, single-cell calcium imaging, and electrophysiology were used to examine the distribution, functionality, and cellular basis of the neuronal IL-31 receptor α in mice and human subjects. RESULTS Among all immune and resident skin cells examined, IL-31 was predominantly produced by TH2 and, to a significantly lesser extent, mature dendritic cells. Cutaneous and intrathecal injections of IL-31 evoked intense itch, and its concentrations increased significantly in murine atopy-like dermatitis skin. Both human and mouse dorsal root ganglia neurons express IL-31RA, largely in neurons that coexpress transient receptor potential cation channel vanilloid subtype 1 (TRPV1). IL-31-induced itch was significantly reduced in TRPV1-deficient and transient receptor channel potential cation channel ankyrin subtype 1 (TRPA1)-deficient mice but not in c-kit or proteinase-activated receptor 2 mice. In cultured primary sensory neurons IL-31 triggered Ca(2+) release and extracellular signal-regulated kinase 1/2 phosphorylation, inhibition of which blocked IL-31 signaling in vitro and reduced IL-31-induced scratching in vivo. CONCLUSION IL-31RA is a functional receptor expressed by a small subpopulation of IL-31RA(+)/TRPV1(+)/TRPA1(+) neurons and is a critical neuroimmune link between TH2 cells and sensory nerves for the generation of T cell-mediated itch. Thus targeting neuronal IL-31RA might be effective in the management of TH2-mediated itch, including atopic dermatitis and cutaneous T-cell lymphoma.

Painful stimuli evoke itch in patients with chronic pruritus Central sensitization for itch

Background: Central sensitization for pain is important for patients with chronic pain. The authors investigated a possible role of central sensitization for itch in patients with chronic pruritus. Methods: Noxious stimuli were applied in lesional and visually nonlesional skin areas of 25 patients with atopic dermatitis, in lesional skin areas of 9 patients with psoriasis vulgaris, and in 20 healthy subjects. The stimuli included mechanical pinpricks, electrical stimuli, contact heat, and injection of low-pH solution. Intensities of itch and pain were assessed separately on a numeric rating scale. Results: All the noxious stimuli primarily evoked pain in control subjects and patients with psoriasis vulgaris. In patients with atopic dermatitis, however, itch was evoked instead of burning pain. In their lesional skin, itch was the predominant sensation. Chemical stimuli evoked intense itch in lesional and visually healthy skin areas (the area under the curve of itch rating compared with the control, mean ± SEM, 668 ± 166 and 625 ± 192 vs 38 ± 23; p < 0.001; p < 0.01). Chemically induced itch also was observed in healthy subjects after a conditioning histamine stimulus of 15 minutes, but not after a conditioning histamine stimulus of 2 minutes. Conclusion: The chronic barrage of pruriceptive input may elicit central sensitization for itch so that nociceptive input no longer inhibits itch but on the contrary is perceived as itch. In contrast to the well-known A-fiber-mediated alloknesis and hyperknesis, this type of central sensitization appears to be elicited by C-nociceptors.

Bradykinin is a potent pruritogen in atopic dermatitis: a switch from pain to itch.

&NA; Histamine, substance P, serotonin and bradykinin were applied by iontophoresis to lesional and visually non‐lesional skin of 14 patients with atopic dermatitis, and normal skin of 15 healthy volunteers. Itch could be evoked by light stroking of skin with a cotton swab (alloknesis) in all lesional skin sites, but not in non‐lesional or normal skin. Substances were applied in the same skin area before and 3 h after administration of placebo or antihistamine (olopatadine hydrochloride: H1‐receptor‐blocker). Intensities of itch and pain sensation and areas of flare and wheal were measured. All the substances induced significantly more intense itch in lesional skin than in non‐lesional skin of patients. Even bradykinin, which evoked only weak itch and pain of similar intensities in non‐lesional skin of patients and in healthy volunteers, induced intense itch in lesional skin, while the simultaneously increased pain did not suppress the itch sensation, indicating central sensitization. Histamine‐ and substance P‐induced itch was almost completely suppressed by antihistamines, whereas bradykinin‐ and serotonin‐induced itch was not. This suggests that substance P is a histamine‐dependent pruritogen also in lesional skin under sensitized conditions but that bradykinin and serotonin are histamine‐independent pruritogens in lesional skin. It is concluded that serotonin and bradykinin, classic endogenous algogens, can turn into potent histamine‐independent pruritogens in lesional skin of atopic dermatitis.

Electrically evoked itch in humans.

&NA; We compared itch sensations and axon reflex flare induced by transcutaneous electrical (0.08–8 ms, 2–200 Hz) and chemical (histamine iontophoresis; 100 μC) stimulation. Stimuli were applied to non‐lesional volar wrist skin in 20 healthy human subjects and 10 patients with atopic dermatitis. Intensity of evoked itch and pain sensations were rated on a numerical rating scale (NRS) of 0 (no sensation) to 10 (the maximum sensation imaginable). The axon reflex erythema was measured by laser Doppler imager and areas of alloknesis (itch evoked by light brushing) and hyperknesis (itch evoked by pricking) were assessed psychophysically. Electrical stimulation was most effective for stimulus durations ≥2 ms and frequencies ≥50 Hz. It evoked pure itch as threshold sensation in 80% of the subjects that was perceived with a delay of approximately 1 s. Itch intensities of up to 7/10 were not accompanied by an axon reflex flare. In contrast, histamine provoked a massive increase of axon reflex erythema and maximum itch ratings of 3.1±0.2. The extention of alloknesis areas (2.3±0.5 cm) evoked by electrical stimulation clearly exceeded those induced by histamine (0.7±0.3 cm). Healthy subjects and patients with atopic dermatitis did not differ significantly in their response to either stimulation. We conclude that C‐fiber activation underlies the electrically evoked itch sensation. The low electrical thresholds and the absence of an axon reflex flare suggest that these fibers are not identical with the previously described mechano‐insensitive histamine responsive C fibers, but represent a separate peripheral neuronal system for the induction of itch.

Upregulation of Aquaporin-3 Is Involved in Keratinocyte Proliferation and Epidermal Hyperplasia

Aquaporin-3 (AQP3) is a water/glycerol-transporting protein expressed in keratinocytes of the epidermis. We previously showed that AQP3-mediated transport of water and glycerol is involved in keratinocyte migration and proliferation, respectively. However, the involvement of AQP3 in epidermal hyperplasia in skin diseases, such as atopic dermatitis (AD), is unknown. In this study, we found significantly increased AQP3 transcript and protein expression in the epidermis of human AD lesions. The upregulation of AQP3 expression in human keratinocytes by transfection with human AQP3 DNA plasmid was associated with increased cellular glycerol and ATP, as well as increased cell proliferation. Among several cytokines and chemokines produced in the skin, CCL17, which is highly expressed in AD, was found to be a strong inducer of AQP3 expression and enhanced keratinocyte proliferation. In mouse AD models, AQP3 was strongly overexpressed in the epidermis in wild-type mice. Epidermal hyperplasia was reduced in AQP3-deficient mice, with a decreased number of proliferating keratinocytes. These results suggest the involvement of AQP3 in epidermal hyperplasia by a mechanism involving upregulated AQP3 expression and consequent enhancement of keratinocyte proliferation.

Sustained Exogenous Expression of Therapeutic Levels of IFN-γ Ameliorates Atopic Dermatitis in NC/Nga Mice via Th1 Polarization

The short in vivo half-life of IFN-γ can prevent the cytokine from inducing immunological changes that are favorable for the treatment of Th2-dominant diseases, such as atopic dermatitis. To examine whether a sustained supply of IFN-γ is effective in regulating the balance of Th lymphocyte subpopulations, plasmid vector encoding mouse IFN-γ, pCpG-Muγ, or pCMV-Muγ was injected into the tail vein of NC/Nga mice, a model for human atopic dermatitis. A single hydrodynamic injection of a CpG motif reduced pCpG-Muγ at a dose of 0.14 μg/mouse resulted in a sustained concentration of IFN-γ in the serum, and the concentration was maintained at >300 pg/ml over 80 d. The pCpG-Muγ–mediated IFN-γ gene transfer was associated with an increase in the serum concentration of IL-12, reduced production of IgE, and inhibition of mRNA expression of IL-4, -5, -10, -13, and -17 and thymus and activation-regulated chemokine in the spleen. These immunological changes were not clearly observed in mice receiving two injections of 20 μg pCMV-Muγ, a CpG-replete plasmid DNA, because of the transient nature of the expression from the vector. The mice receiving pCpG-Muγ showed a significant reduction in the severity of skin lesions and in the intensity of their scratching behavior. Furthermore, high transepidermal water loss, epidermal thickening, and infiltration of lymphocytes and eosinophils, all of which were obvious in the untreated mice, were significantly inhibited. These results indicate that an extraordinary sustained IFN-γ expression induces favorable immunological changes, leading to a Th1-dominant state in the atopic dermatitis model.

Mouse Model of Touch-Evoked Itch (Alloknesis)

Lightly touching normal skin near a site of itch can elicit itch sensation, a phenomenon known as alloknesis. To investigate the neural mechanisms of alloknesis, we have developed an animal model. Low-threshold mechanical stimulation of the skin normally does not elicit any response in naïve C57/BL6 mice. Following acute intradermal (id) injection of histamine in the rostral back, mechanical stimulation 7 mm from the injection site elicited discrete hindlimb scratch bouts directed toward the stimulus. This began at 10 min and peaked 20–40 min post-histamine, declining over the next hour. Histamine itself elicited bouts of scratching not associated with the mechanical stimulus, that ceased after 30 min. Histamine- and touch-evoked scratching was inhibited by the μ-opiate antagonist naltrexone. Touch-evoked scratching was observed following id 5-HT, a PAR-4 agonist and a MrgprC11 agonist BAM8-22, but not chloroquine or a PAR-2 agonist. The histamine H1 receptor antagonist terfenadine prevented scratching and alloknesis evoked by histamine, but not 5-HT, a PAR-4 agonist or a MrgprC11 agonist. In mice with experimental dry skin, there was a time-dependent increase in spontaneous and touch-evoked scratching. This animal model, which to our knowledge is previously unreported, appears to be useful to investigate neural mechanisms of itch and alloknesis.

Role of Spinal Neurotransmitter Receptors in Itch: New Insights into Therapies and Drug Development

Targets for antipruritic therapies are now expanding from the skin to the central nervous system. Recent studies demonstrate that various neuronal receptors in the spinal cord are involved in pruritus. The spinal opioid receptor is one of the best‐known examples. Spinal administration of morphine is frequently accompanied by segmental pruritus. In addition to μ‐opioid receptor antagonists, κ‐opioid receptor agonists have recently come into usage as novel antipruritic drugs, and are expected to suppress certain subtypes of itch such as hemodialysis‐ and cholestasis‐associated itch that are difficult to treat with antihistamines. The gastrin‐releasing peptide receptor in the superficial dorsal horn of the spinal cord has also received recent attention as a novel pathway of itch‐selective neural transmission. The NMDA glutamate receptor appears to be another potential target for the treatment of itch, especially in terms of central sensitization. The development of NMDA receptor antagonists with less undesirable side effects on the central nervous system might be beneficial for antipruritic therapies. Drugs suppressing presynaptic glutamate‐release such as gabapentin and pregabalin also reportedly inhibit certain subtypes of itch such as brachioradial pruritus. Spinal receptors of other neuromediators such as bradykinin, substance P, serotonin, and histamine may also be potential targets for antipruritic therapies, given that most of these molecules interfere not only with pain, but also with itch transmission or regulation. Thus, the identification of itch‐specific receptors and understanding itch‐related circuits in the spinal cord may be innovative strategies for the development of novel antipruritic drugs.

Cerebral representation of the relief of itch by scratching.

Cerebral processing of itch-scratching cycles was studied with functional magnetic resonance imaging (fMRI) in healthy volunteers. The back of the hand was repetitively scratched in the absence and presence of itch induced by histamine applied close to the scratched site. Blood-oxygenation-level-dependent (BOLD) effects were assessed in predefined cortical and subcortical brain regions of interest. Scratch-related activation clusters were found in cortical and subcortical areas which had been associated before with pain processing, namely S1, S2, parietal association cortex, motor and premotor cortex, anterior and posterior insula, anterior and medial cingulum, lateral and medial frontal areas, ipsilateral cerebellum and contralateral putamen. Cortical activations were generally stronger in the contralateral hemisphere. General linear model (GLM) analysis and GLM contrast analysis revealed stronger activations during itch-related trials in the motor and premotor cortex, in lateral frontal fields of both sides, and in a left medial frontal cluster. Subcortically, stronger activation during itch-related scratching trials was found in the contralateral putamen and in the ipsilateral cerebellum. Time course analysis showed significantly higher BOLD levels during the last 3-6 s before the start of scratching when the itch intensity was strongest. This effect was found in frontal areas, in the putamen, and in the somatosensory projection areas. During the scratching, no significant differences were found between itch and control conditions with the exception of the putamen, which showed stronger activations during itch-related scratch bouts. We interpret these itch-related activations anticipating the scratching as possible cerebral correlates of the itch processing and the craving for scratch.