Cerebral processing of histamine-induced itch using short-term alternating temperature modulation--an FMRI study

Higher-order sensorimotor circuit of the whole-brain functional network involved in pruritus regulation in atopic dermatitis

BACKGROUND

Little is known about the neural mechanisms underlying pruritus regulation in Atopic dermatitis (AD).

OBJECTIVE

To investigate the functional changes of the resting-state whole brain network of AD participants and the mechanisms by which they were involved in pruritus regulation.

METHOD

Based on the functional magnetic resonance imaging data from 19 AD participants and 37 healthy controls (HC), a graph-theoretical measure of degree centrality (DC) conjoined with a voxel-level seed-based functional connectivity (FC) method was used to identify abnormal higher-order nodes and the functionally relevant circuit in AD participants compared to healthy controls (HC).

RESULTS

Of 64 participants screened, 19 AD participants (12M/7F, median [IQR] age, 27 [14] years) and 36 HCs (13M/23F, median [IQR] age, 20 [1] years) were enrolled. DC values of the left superior frontal gyrus (LSFG) increased in AD participants and exhibited a negative correlation with the SCORAD score (r = -0.561, p = 0.012) compared with HC. In the FC analysis with LSFG as the seed, FC values of several sensory and motor regions increased in AD participants, highly overlapping with the anatomical distribution of the inferior fronto-occipital fascicle (IFOF). AD participants with severe pruritus exhibited lower levels of DC (T = -2.316, p = 0.033) and FC between the LSFG and left insula (T = -2.203, p = 0.042) than those with mild-to- moderate pruritus.

CONCLUSIONS AND RELEVANCE

LSFG was involved in pruritus regulation in AD by forming a high-order sensorimotor circuit through the IFOF, a white matter fascicle that proved to provide multimodal integration in motor control and sensory information processing. These results offer more mechanism-guided treatment targets for severe pruritus in AD.

Human pain ratings to electrical sinusoids increase with cooling through a cold-induced increase in C-fibre excitability

ABSTRACT

Low-frequency sinusoidal current applied to human skin evokes local axon reflex flare and burning pain, indicative of C-fibre activation. Because topical cooling works well as a local analgesic, we examined the effect of cooling on human pain ratings to sinusoidal and rectangular profiles of constant current stimulation. Unexpectedly, pain ratings increased upon cooling the skin from 32 to 18°C. To explore this paradoxical observation, the effects of cooling on C-fibre responses to stimulation with sinusoidal and rectangular current profiles were determined in ex vivo segments of mouse sural and pig saphenous nerve. As expected by thermodynamics, the absolute value of electrical charge required to activate C-fibre axons increased with cooling from 32°C to 20°C, irrespective of the stimulus profile used. However, for sinusoidal stimulus profiles, cooling enabled a more effective integration of low-intensity currents over tens of milliseconds resulting in a delayed initiation of action potentials. Our findings indicate that the paradoxical cooling-induced enhancement of electrically evoked pain in people can be explained by an enhancement of C-fibre responsiveness to slow depolarization at lower temperatures. This property may contribute to symptoms of enhanced cold sensitivity, especially cold allodynia, associated with many forms of neuropathic pain.

Central medial thalamic nucleus dynamically participates in acute itch sensation and chronic itch-induced anxiety-like behavior in male mice

Itch is an annoying sensation consisting of both sensory and emotional components. It is known to involve the parabrachial nucleus (PBN), but the following transmission nodes remain elusive. The present study identified that the PBN-central medial thalamic nucleus (CM)-medial prefrontal cortex (mPFC) pathway is essential for itch signal transmission at the supraspinal level in male mice. Chemogenetic inhibition of the CM-mPFC pathway attenuates scratching behavior or chronic itch-related affective responses. CM input to mPFC pyramidal neurons is enhanced in acute and chronic itch models. Specifically chronic itch stimuli also alter mPFC interneuron involvement, resulting in enhanced feedforward inhibition and a distorted excitatory/inhibitory balance in mPFC pyramidal neurons. The present work underscores CM as a transmit node of the itch signal in the thalamus, which is dynamically engaged in both the sensory and affective dimensions of itch with different stimulus salience.

Dysgranular retrosplenial cortex modulates histaminergic and nonhistaminergic itch processing

Itch is an unpleasant sensation followed by an intense desire to scratch. Previous researches have advanced our understanding about the role of anterior cingulate cortex and prelimbic cortex in itch modulation, whereas little is known about the effects of retrosplenial cortex (RSC) during this process. Here we firstly confirmed that the neuronal activity of dysgranular RSC (RSCd) is significantly elevated during itch-scratching processing through c-Fos immunohistochemistry and fiber photometry recording. Then with designer receptors exclusively activated by designer drugs approaches, we found that pharmacogenetic inhibition of global RSCd neurons attenuated the number of scratching bouts as well as the cumulative duration of scratching bouts elicited by both 5-HT or compound 48/80 injection into rats' nape or cheek; selective inhibition of the pyramidal neurons in RSCd, or of the excitatory projections from caudal anterior cingulate cortex (cACC) to RSCd, demonstrated the similar effects of decreasing itch-related scratching induced by both 5-HT or compound 48/80. Pharmacogenetic intervention of the neuronal or circuitry activities did not affect rats' motor ability. This study presents direct evidence that pyramidal neurons in RSCd, and the excitatory projection from cACC to RSCd are critically involved in central regulation of both histaminergic and nonhistaminergic itch.

The prelimbic cortex regulates itch processing by controlling attentional bias

Itch is a complex and unpleasant sensory experience. Recent studies have begun to investigate the neural mechanisms underlying the modulation of sensory and emotional components of itch in the brain. However, the key brain regions and neural mechanism involved in modulating the attentional processing of itch remain elusive. Here, we showed that the prelimbic cortex (PrL) is associated with itch processing and that the manipulation of itch-responsive neurons in the PrL significantly disrupted itch-induced scratching. Interestingly, we found that increasing attentional bias toward a distracting stimulus could disturb itch processing. We also demonstrated the existence of a population of attention-related neurons in the PrL that drive attentional bias to regulate itch processing. Importantly, itch-responsive neurons and attention-related neurons significantly overlapped in the PrL and were mutually interchangeable in the regulation of itch processing at the cellular activity level. Our results revealed that the PrL regulates itch processing by controlling attentional bias.

Itch-specific neurons in the ventrolateral orbital cortex selectively modulate the itch processing

Itch is a cutaneous sensation that is critical in driving scratching behavior. The long-standing question of whether there are specific neurons for itch modulation inside the brain remains unanswered. Here, we report a subpopulation of itch-specific neurons in the ventrolateral orbital cortex (VLO) that is distinct from the pain-related neurons. Using a Tet-Off cellular labeling system, we showed that local inhibition or activation of these itch-specific neurons in the VLO significantly suppressed or enhanced itch-induced scratching, respectively, whereas the intervention did not significantly affect pain. Conversely, suppression or activation of pain-specific neurons in the VLO significantly affected pain but not itch. Moreover, fiber photometry and immunofluorescence verified that these itch- and pain-specific neurons are distinct in their functional activity and histological location. In addition, the downstream targets of itch- and pain-specific neurons were different. Together, the present study uncovers an important subpopulation of neurons in the VLO that specifically modulates itch processing.

The Role of Temperature Change, Ambient Temperature, and Relative Humidity in Allergic Conjunctivitis in a US Veteran Population

PURPOSE

Studies have implicated temperature and humidity in the pathogenesis of allergic conjunctivitis (AC), as these conditions facilitate air particulate and aeroallergen dispersion and tear film instability. Research also suggests that variation in temperature is associated with risk of asthma, but similar data are limited for AC. This study examined associations between several meteorologic conditions, including temperature variation, and AC visit risk.

DESIGN

Retrospective, case-crossover study.

METHODS

Data on individuals diagnosed with AC (via International Classification of Diseases-Ninth Edition [ICD-9]) at a Veterans Affairs clinic from January 2010-December 2013 was extracted. Local climate data were obtained from the National Climactic Data Center. Utilizing a case-crossover design, all cases were assigned a random control date 90-250 days prior to diagnosis. Daily time-lagged exposures were computed for 30-day lags. The associations between temperature, temperature variation (standard deviation [SD] of temperature), relative humidity (RH), and temperature-RH interaction with visit risk were examined via multivariate logistic regression models both at the national level and across domestic climate regions.

RESULTS

Overall, 74,951 subjects made 116,162 visits for AC. Prevalence was highest in spring (>10% April-May) in the Northeast (NE) and Southeast (SE) (>15%), and lowest in winter (<6.1% December-February) in the Pacific Northwest (PNW) (<5%). AC visit risk was positively associated with temperature (OR 1.028, P < .001), SD of temperature (OR 1.054, P < .01), and temperature-RH interaction (OR 1.0003, P < .01), whereas it was negatively associated with RH (OR 0.998, P < .001). Regionally, the PNW, NE, and Lower Midwest (LMW) accounted for the strongest associations.

CONCLUSION

Temperature, temperature variation, and RH associated with AC visit risk. Observed associations were strongest in northern regions, like the PNW.

Neural Oscillation Associated with Contagious Itch in Patients with Atopic Dermatitis

Objective: Itch is an unpleasant sensation associated with an urge to scratch and is a major health care issue associated with atopic dermatitis (AD). Contagious itch, i.e., subjective feelings of itchiness induced by watching others’ scratching behavior, is common in patients with AD. Using electroencephalography, we examined alpha (8–13 Hz) oscillations in sensorimotor areas associated with the desire to scratch in patients with AD. Methods: Thirty-six patients with AD and 34 healthy controls (HCs) participated in this study. They evaluated their itch levels after watching short videos of a model scratching or tapping parts of his body. Neural oscillations were recorded from nine electrodes, including those placed over sensorimotor areas. Time–frequency analysis was used to compare mu rhythm suppression over the sensorimotor areas in response to these videos between patients with AD and HCs. Results: The behavioral test showed that the visual stimuli induced increased feelings of itchiness in patients with AD relative to HCs under the tapping and scratching conditions. The time–frequency analysis revealed that mu rhythm suppression in response to scratching images was significantly prominent in patients with AD, but not in HCs. Conclusion: Patients with AD exhibited increased susceptibility to contagious itch. This phenomenon might be related to enhanced mu rhythm suppression in sensorimotor areas of the brain in these patients. Our findings provide new insight into the neurophysiological basis of itch sensations in patients with AD.

Increased grey matter volume and associated resting-state functional connectivity in chronic spontaneous urticaria: A structural and functional MRI study

BACKGROUND AND PURPOSE

Chronic itch is one of the most common irritating sensations, yet its mechanisms have not been fully elucidated. Although some studies have revealed relationships between itching and brain function, the structural changes in the brain induced by chronic itching, such as those accompanying chronic spontaneous urticaria (CSU), remain unclear. In this study, we aimed to explore the potential changes in brain structure and the associated functional circuitry in CSU patients to generate insights to aid chronic itch management.

METHODS

Forty CSU patients and forty healthy controls (HCs) were recruited. Seven-day urticaria activity score (UAS7) values were collected to evaluate clinical symptoms. Voxel-based morphometry (VBM) and seed-based resting-state functional connectivity (rs-FC) analysis were used to assess structural changes in the brain and associated changes in functional circuitry.

RESULTS

Compared with HCs, CSU patients had significantly increased grey matter (GM) volume in the right premotor cortex, left fusiform cortex, and cerebellum. UAS7 values were positively associated with GM volume in the left fusiform cortex. In CSU patients relative to HCs, the left fusiform cortex as extracted by VBM analysis demonstrated decreased functional connectivity with the right orbitofrontal cortex, medial prefrontal cortex (mPFC), premotor cortex, primary motor cortex (MI), and cerebellum and increased functional connectivity with the right posterior insular cortex, primary somatosensory cortex (SI), and secondary somatosensory cortex (SII). The left cerebellum as extracted from VBM analysis demonstrated decreased functional connectivity with the right supplementary motor area (SMA) and MI in CSU patients relative to HCs.

CONCLUSIONS

Our findings indicate that patients suffering from chronic itching conditions, such as CSU, are likely to demonstrate altered GM volume in some brain regions. These changes may affect not only the sensorimotor area but also brain regions associated with cognitive function.

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.

Functional and anatomical brain connectivity in psoriasis patients and healthy controls: a pilot brain imaging study after exposure to mentally induced itch

BACKGROUND

Despite the prevalence of psoriasis, the processing of itch in psoriasis and its impact on the central nervous system (CNS) remain unclear.

OBJECTIVE

We studied the influence of psoriasis on the CNS using magnetic resonance imaging techniques (fMRI and DTI, respectively) to investigate whether mentally induced itch can modify the functional connectivity or the white matter microstructure of the brain.

METHODS

Fourteen patients with chronic psoriasis and 15 healthy controls were recruited. Itch was mentally induced in subjects by videos showing others scratching themselves.

RESULTS

The observation of functional connectivity during the viewing the video revealed an interconnected network of brain regions that are more strongly coupled in psoriasis patients than in healthy controls. This network links the cerebellum, the thalami, the anteroposterior cingulum, the inferior parietal lobules, the middle temporal poles and the parahippocampal, hippocampal, lingual and supramarginal gyri. We also found connections with the right precuneus and both left insula and superior temporal gyrus. The DTI analysis showed that chronic itch affects the microstructure of white matter, including the anterior thalamic radiations, the superior and inferior longitudinal fasciculi, the corticospinal tracts, the cingulum, the external capsules, the inferior frontal-occipital fasciculi and both minor and major forceps.

CONCLUSION

Our results indicate that there could exist a network which is more interconnected in psoriasis patients. Among two building blocks of this network, the subnetwork encoding the perception and control of itch sensation is more affected than the subnetwork representing mentalizing and empathy. With an approach consisting of measuring microstructural changes at a local level in the brain, we also contradict the findings obtained with global measures which stated that chronic psoriasis cannot alter the anatomy of the brain. This confirms that itchy pathophysiological conditions have similar effects on functional and structural connectivity as those observed in chronic pain.

Central mechanisms of itch: A systematic literature review and meta-analysis

In recent years, studying the central mechanism of itch has gained momentum. However, a proper meta-analysis has not been conducted in this domain. In this study, we tried to respond to this need. A systematic search and a meta-analysis were carried out to estimate the central mechanism of itch. The itch matrix comprises the thalamus and the parietal, secondary somatosensory, insular and cingulate cortices. We have shown that the basal ganglia (BG) play an important role in itch reduction. Finally, we explored itch processing in AD patients and observed that the itch matrix in these patients was different. In conclusion, this is the first meta-analysis on the central mechanisms of itch perception and processing. Our study demonstrated that different modalities of itch induction can produce a common pattern of activity in the brain and provided further insights into understanding the underlying nature of itch central perception.

The Functional Network Processing Acute Electrical Itch Stimuli in Humans

The posterior insula (pIns) is a major brain region that receives itch-related signals from the periphery and transfers these signals to broad areas in the brain. Previous brain imaging studies have successfully identified brain regions that respond to itch stimuli. However, it is still unknown which brain regions receive and process itch-related signals from the pIns. Addressing this question is important in identifying key functional networks that process itch. Thus, the present study investigated brain regions with significantly increased functional connectivity with the pIns during itch stimuli with 25 healthy subjects by using functional MRI. Electrical itch stimuli was applied to the left wrist. Similar to previous brain imaging studies, many cortical and subcortical areas were activated by itch stimuli. However, not all of these regions showed significant increments of functional connectivity with the pIns during itch stimuli. While the subjects perceived the itch sensation, functional connectivity was significantly increased between the right pIns and the supplementary motor area (SMA), pre-SMA, anterior midcingulate cortex (aMCC), anterior insula (aIns), secondary somatosensory cortex (SII), and basal ganglia (BG), suggesting that this is a key network in processing itch. In particular, intensity of functional connectivity between the pIns and BG was negatively correlated with itch rating. The functional pIns-BG pathway may play an important role in regulation of subjective itch sensation. This study first identified a key brain network to process itch.

The functional and structural alterations of the striatum in chronic spontaneous urticaria

The brain has long been known to be the regulation center of itch, but the neuropathology of chronic itch, such as chronic spontaneous urticaria (CSU), remains unclear. Thus, we aimed to explore the brain areas involved in the pathophysiology of CSU in hopes that our results may provide valuable insights into the treatment of chronic itch conditions. 40 CSU patients and 40 healthy controls (HCs) were recruited. Urticaria activity scores 7 (UAS7) were collected to evaluate patient’s clinical symptoms. Amplitude of low frequency fluctuations (ALFF), voxel-based morphometry (VBM), and seed-based resting-state functional connectivity (rs-FC) analysis were used to assess brain activity and related plasticity. Compared with HCs, CSU patients exhibited 1) higher ALFF values in the right ventral striatum / putamen, which were positively associated with clinical symptoms as measured by UAS7; 2) gray matter volume (GMV) increase in the right ventral striatum and putamen; and 3) decreased rs-FC between the right ventral striatum and the right occipital cortex and between the right putamen and the left precentral gyrus. Using multiple-modality brain imaging tools, we demonstrated the dysfunction of the striatum in CSU. Our results may provide valuable insights into the neuropathology and development of chronic itch.

Functional magnetic resonance imaging reveals differences in brain activation in response to thermal stimuli in diabetic patients with and without diabetic peripheral neuropathy

Introduction

Diabetes affects both the peripheral and central nervous systems. The aim of this study was to explore the changes in brain activity in response to thermal stimuli in diabetic patients with and without diabetic peripheral neuropathy (DPN) using functional magnetic resonance imaging (fMRI).

Methods

A total of 36 right-handed volunteers were enrolled: eight patients with Type-2 diabetes mellitus and DPN, 13 patients with Type-2 diabetes mellitus lacking DPN (NDPN patients), and 15 healthy volunteers (HV). Blood oxygenation level-dependent baseline scans were performed, first without any stimuli, and then with four sessions of thermal stimuli (0, 10, 34, and 44°C, in a random order) applied to the lateral side of the right lower extremity. There was a 240-s rest interval between each thermal stimulation. Each stimulation session consisted of three cycles of 30 s of stimulation followed by 30 s of rest. After each stimuli session, the participant rated pain and itch perception on a visual analog scale. The fMRI data series were analyzed by using Statistical Parametric Mapping 8 and Data Processing Assistant for Resting-State fMRI.

Results

In response to temperature stimuli, DPN patients showed stronger activation than HV and NDPN patients, not only in brain areas that participate in somatosensory pathways (right insula, left caudate nucleus, frontal gyrus, and cingulate cortex), but also in the cognition-related cerebral areas (right temporal lobe, left hippocampus, and left fusiform gyrus). Activation of vermis 1–3 was greater in NDPN patients than in HV in response to 0°C stimulation.

Conclusions

fMRI may be useful for the early detection of central nervous system impairment caused by DPN. Our results indicate that central nervous system impairment related to diabetic neuropathy may not be limited to motion- and sensation-related cortical regions. Cognition-associated cerebral regions such as the hippocampus and fusiform gyrus are also affected by functional changes caused by DPN. This suggests that fMRI can detect the early stages of cognitive impairment in DPN patients before the symptoms become clinically significant.

Nocebo-induced modulation of cerebral itch processing - An fMRI study

It has been shown repeatedly that perceiving itch-related pictures or listening to a lecture on itch can enhance itch sensation and scratching behaviour (Niemeier and Gieler, 2000; Holle et al., 2012; Lloyd et al., 2013), indicating that itch is strongly influenced by expectations. Using fMRI, we investigated the neural correlates of the itch-related nocebo effect in healthy male and female human subjects. Itch sensation on the left forearm was induced by cutaneous histamine application and thermally modulated, with cooling leading to higher itch. Nocebo-induced aggravation of histaminergic itch was achieved by ostensibly treating volunteers with "transcutaneous electrical nerve stimulation (TENS)" about which subjects were instructed that it would increase itch. During a conditioning phase subjects indeed experienced stronger itch due to slightly altered cooling and histamine concentrations, but attributed it to the alleged "TENS stimulation". Importantly, in the subsequent test phase where no "TENS" or electrical stimulation was applied, volunteers significantly reported stronger itch during the nocebo as compared to the control condition. Comparing BOLD responses during nocebo in contrast to control, we observed increased activity in contralateral (right) rolandic operculum. Opercular involvement was repeatedly reported in studies related to the expectation of stimulus intensification and might thus represent an early area integrating expectation information with somatosensory information. Finally, functional coupling between the insula and the periaqueductal gray (PAG) was enhanced specifically in the nocebo condition. This cortex-PAG interaction indicates that context-dependent top-down modulation during itch might represent a shared mechanism with other modalities such as pain.

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]

The imagined itch: brain circuitry supporting nocebo-induced itch in atopic dermatitis patients

BACKGROUND

Psychological factors are known to significantly modulate itch in patients suffering from chronic itch. Itch is also highly susceptible to both placebo and nocebo (negative placebo) effects. Brain activity likely supports nocebo-induced itch, but is currently unknown.

METHODS

We collected functional MRI (fMRI) data from atopic dermatitis (AD) patients, in a within-subject design, and contrast brain response to nocebo saline understood to be allergen vs open-label saline control. Exploratory analyses compared results to real allergen itch response and placebo responsiveness, evaluated in the same patients.

RESULTS

Nocebo saline produced greater itch than open saline control (P < 0.01). Compared to open saline, nocebo saline demonstrated greater fMRI response in caudate, dorsolateral prefrontal cortex (dlPFC), and intraparietal sulcus (iPS) - brain regions important for cognitive executive and motivational processing. Exploratory analyses found that subjects with greater dlPFC and caudate activation to nocebo-induced itch also demonstrated greater dlPFC and caudate activation, respectively, for real allergen itch. Subjects reporting greater nocebo-induced itch also demonstrated greater placebo reduction of allergen-evoked itch, suggesting increased generalized modulation of itch perception.

CONCLUSIONS

Our study demonstrates the capacity of nocebo saline to mimic both the sensory and neural effects of real allergens and provides an insight to the brain mechanisms supporting nocebo-induced itch in AD, thus aiding our understanding of the role that expectations and other psychological factors play in modulating itch perception in chronic itch patients.

Psychophysical measurements of itch and nociceptive sensations in an experimental model of allergic contact dermatitis

Allergic contact dermatitis (ACD) is a common condition that can significantly affect the quality of life. Contact with allergens results in delayed hypersensitivity reactions involving T lymphocytes, with associated skin inflammation and spontaneous itch and nociceptive sensations. However, psychophysical studies of these sensations are lacking. In the present study, we sensitized 8 healthy volunteers to squaric acid dibutyl ester (SADBE). Two weeks later, 1 volar forearm was challenged with SADBE, and the other with acetone vehicle control. Subsequently, participants rated the maximal perceived intensity of spontaneous itch, pricking/stinging, and burning every 6 to 12 hours for 1 week, using the generalized Labeled Magnitude Scale. In the laboratory, they judged stimulus-evoked sensations within and outside the chemically treated area. The SADBE- but not the acetone-treated skin resulted in 1) localized inflammation, with spontaneous itch and nociceptive sensations peaking at 24 to 48 hours after challenge, 2) alloknesis, hyperknesis, and hyperalgesia to mechanical stimuli that were reduced or eliminated by anesthetic cooling of the SADBE-treated area and restored on rewarming, suggesting that sensations and dysesthesias are dependent on ongoing peripheral neural activity, and 3) enhanced itch to intradermal injection of histamine, BAM8-22, or β-alanine. This experimental model of T-cell-mediated inflammation may prove useful in evaluating potential treatments of itch from ACD.

PERSPECTIVE

In a model of allergic contact dermatitis, experimentally applied in humans, psychophysical measurements were obtained of persistent, spontaneous itch and enhanced stimulus-evoked itch and pain sensations. These sensory measurements will be useful in the identification of the neural mechanisms underlying inflammatory itch and pain.

Neurophysiology and itch pathways

As we all can easily differentiate the sensations of itch and pain, the most straightforward neurophysiologic concept would consist of two specific pathways that independently encode itch and pain. Indeed, a neuronal pathway for histamine-induced itch in the peripheral and central nervous system has been described in animals and humans, and recently several non-histaminergic pathways for itch have been discovered in rodents that support a dichotomous concept differentiated into a pain and an itch pathway, with both pathways being composed of different "flavors." Numerous markers and mediators have been found that are linked to itch processing pathways. Thus, the delineation of neuronal pathways for itch from pain pathways seemingly proves that all sensory aspects of itch are based on an itch-specific neuronal pathway. However, such a concept is incomplete as itch can also be induced by the activation of the pain pathway in particular when the stimulus is applied in a highly localized spatial pattern. These opposite views reflect the old dispute between specificity and pattern theories of itch. Rather than only being of theoretic interest, this conceptual problem has key implication for the strategy to treat chronic itch as key therapeutic targets would be either itch-specific pathways or unspecific nociceptive pathways.

[Pruritus: considerable progress in pathophysiology]

Pruritus is defined as "an unpleasant sensation that causes the need to scratch". This is not a small pain. It seems that pruriceptors exist but their level of separation from nociceptive receptors is still debated. Pathways of pruritus were identified from the skin (around the dermo-epidermal junction) to the brain. Many mediators are involved in pruritus but there are at least a histaminergic and a non-histaminergic pathway (PAR-2dependent). Similarly to pain, gate control or peripheral and central sensitization mechanisms have been highlighted in pruritus. These pathophysiological advances are important and anticipate therapeutic advances, that will be very useful for the symptomatic treatment of pruritus (poorly efficient at present).

Evoked itch perception is associated with changes in functional brain connectivity

Chronic itch, a highly debilitating condition, has received relatively little attention in the neuroimaging literature. Recent studies suggest that brain regions supporting itch in chronic itch patients encompass sensorimotor and salience networks, and corticostriatal circuits involved in motor preparation for scratching. However, how these different brain areas interact with one another in the context of itch is still unknown. We acquired BOLD fMRI scans in 14 atopic dermatitis patients to investigate resting-state functional connectivity before and after allergen-induced itch exacerbated the clinical itch perception in these patients. A seed-based analysis revealed decreased functional connectivity from baseline resting state to the evoked-itch state between several itch-related brain regions, particularly the insular and cingulate cortices and basal ganglia, where decreased connectivity was significantly correlated with increased levels of perceived itch. In contrast, evoked itch increased connectivity between key nodes of the frontoparietal control network (superior parietal lobule and dorsolateral prefrontal cortex), where higher increase in connectivity was correlated with a lesser increase in perceived itch, suggesting that greater interaction between nodes of this executive attention network serves to limit itch sensation via enhanced top-down regulation. Overall, our results provide the first evidence of itch-dependent changes in functional connectivity across multiple brain regions.

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Atopic dermatitis patients were subjected to allergen-induced itch.

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Evoked itch reduced functional connectivity between itch-related brain regions.

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Evoked itch increased functional connectivity within frontoparietal control network.

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The above changes in functional connectivity correlated with perceived itch level.

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Itch sensation may be top-down regulated by frontoparietal control network.

Voxel-based morphometry and arterial spin labeling fMRI reveal neuropathic and neuroplastic features of brain processing of itch in end-stage renal disease

Pruritus of end-stage renal disease (ESRD) is a multifactorial symptom of complex etiology not yet fully understood. In this study we have investigated the cerebral perfusion patterns at rest in ESRD patients on hemodialysis, compared with those in healthy volunteers. We have also studied the brain responses evoked by experimental itch induction in ESRD, after stimulating the two distinct histamine and cowhage itch pathways, and compared them with the responses evoked in healthy volunteers. To identify potential structural alterations in ESRD patients compared with a group of age-matched healthy volunteers, we calculated the density of gray matter for the entire brain using a voxel-based morphometric analysis. Our results indicated that gray matter density was significantly reduced in ESRD patients in the frontal, parietal, temporal, and occipital cortices, as well as in the S1, precuneus, and insula, whereas the brain stem, hippocampus, amygdala, midcingulate cortex, and nucleus accumbens displayed an increased gray matter density. Functionally, we found a significantly higher brain perfusion at baseline associated with ESRD pruritus in the anterior cingulate, insula, claustrum, hippocampus, and nucleus accumbens. The brain responses evoked by cowhage itch, which are mediated by protease-activated receptors (PAR2), displayed significant differences compared with responses in healthy individuals and were correlated with perceived itch intensity in a dual, complex manner. The inverse correlations in particular suggested that a negative feedback mechanism modulated itch intensity, when elicited in a preexistent chronic itch background.

Neuropathic pruritus

Pruritus, also known as itch, is a very common, unpleasant sensation that elicits an urge to scratch. Its origin is not always in the skin, and neuropathic itch that is caused by neuronal or glial damage is common, but poorly understood by both dermatologists and neurologists. Although pruritus has not been considered as serious a symptom as pain, it is difficult to treat and--if chronic--can severely impair quality of life. Neuropathic itch is often associated with other clinical symptoms, most commonly neuropathic pain, and hypersensitization to stimuli is present in both pruritus and pain of neuropathic origin. The shared aetiology can aid in finding suitable treatment for itch in some cases, but more detailed knowledge of the mechanisms of itch, along with standardized, well-controlled trials, is needed. Pruritus research is an emerging but currently very active field, and our understanding of this sensation is rapidly increasing. Here, we review new discoveries regarding the role of the nervous system and the contribution of different pathways in pruritus, discuss the different aetiologies of neuropathic itch, and outline currently available and potential strategies for managing neuropathic pruritus.

Acupuncture for allergic disease therapy--the current state of evidence

This review summarizes current evidence for acupuncture treatment of allergies. Several randomized controlled trials have demonstrated a specific effect of acupuncture for allergic rhinitis; while a few studies have shown positive effects for atopic dermatitis, asthma and itch. Specifically for allergic rhinitis and asthma, acupuncture may be cost-effective in terms of money spent per quality-of-life gained. Acupuncture plays an increasingly important role as an evidence-based therapy for allergy relief and can be recommended as adjunct therapy for allergic rhinitis. Future randomized controlled trials need to further explore acupuncture efficacy for the treatment of itch, atopic dermatitis and asthma. More experimental research is also needed to investigate mechanisms of action underlying acupuncture for allergy relief.

Itch relief by mirror scratching. A psychophysical study

Objective

The goal of this study was to test whether central mechanisms of scratching-induced itch attenuation can be activated by scratching the limb contralateral to the itching limb when the participant is made to visually perceive the non-itching limb as the itching limb by means of mirror images.

Methods

Healthy participants were asked to assess the intensity of an experimentally induced itch at their right forearm while they observed externally guided scratch movements either at their right (itching) or left (non-itching) forearm which were either mirrored or not mirrored. In the first experiment, a mirror placed between the participant’s forearms was used to create the visual illusion that the participant’s itching (right) forearm was being scratched while in fact the non-itching (left) forearm was scratched. To control visibility of the left (non-mirrored) forearm, a second experiment was performed in which unflipped and flipped real-time video displays of the participant’s forearms were used to create experimental conditions in which the participant visually perceived scratching either on one forearm only, on both forearms, or no scratching at all.

Results

In both experiments, scratching the non-itching limb attenuated perceived itch intensity significantly and selectively in the mirror condition, i.e., when the non-itching forearm was visually perceived as the itching limb.

Discussion

These data provide evidence that the visual illusion that an itching limb is being scratched while in fact the non-itching limb contralateral to the itching limb is scratched, can lead to significant itch relief. This effect might be due to a transient illusionary intersensory perceptual congruency of visual, tactile and pruriceptive signals. “Mirror scratching” might provide an alternative treatment to reduce itch perception in focal skin diseases with persistent pruritus without causing additional harm to the affected skin and might therefore have significant clinical impact.

Analysis for distinctive activation patterns of pain and itchy in the human brain cortex measured using near infrared spectroscopy (NIRS)

Pain and itch are closely related sensations, yet qualitatively quite distinct. Despite recent advances in brain imaging techniques, identifying the differences between pain and itch signals in the brain cortex is difficult due to continuous temporal and spatial changes in the signals. The high spatial resolution of positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) has substantially advanced research of pain and itch, but these are uncomfortable because of expensiveness, importability and the limited operation in the shielded room. Here, we used near infrared spectroscopy (NIRS), which has more conventional usability. NIRS can be used to visualize dynamic changes in oxygenated hemoglobin and deoxyhemoglobin concentrations in the capillary networks near activated neural circuits in real-time as well as fMRI. We observed distinct activation patterns in the frontal cortex for acute pain and histamine-induced itch. The prefrontal cortex exhibited a pain-related and itch-related activation pattern of blood flow in each subject. Although it looked as though that activation pattern for pain and itching was different in each subject, further cross correlation analysis of NIRS signals between each channels showed an overall agreement with regard to prefrontal area involvement. As a result, pain-related and itch-related blood flow responses (delayed responses in prefrontal area) were found to be clearly different between pain (τ = +18.7 sec) and itch (τ = +0.63 sec) stimulation. This is the first pilot study to demonstrate the temporal and spatial separation of a pain-induced blood flow and an itch-induced blood flow in human cortex during information processing.

Chronic pruritus associated with dermatologic disease in infancy and childhood: update from an interdisciplinary group of dermatologists and pediatricians

An effective treatment strategy for chronic pruritus in children with dermatologic disorders should consider the multidimensional aspects of pruritus, the unique challenges associated with treating pruritic skin disorders in the pediatric population, and evidence-based therapies with demonstrated antipruritic benefits and clinically relevant effects on patient/family quality of life (QoL). The Course of Advanced Learning for the Management of ITch (CALM-IT) Task Force is an interdisciplinary group of experts specializing in core aspects of pruritus treatment, integrating pediatrics, dermatology, psychotherapy, pruritus management, and sleep. CALM-IT recently convened to provide updated guidance on managing chronic pruritus associated with dermatologic diseases in pediatric patients, with a special focus on atopic dermatitis (AD) and chronic spontaneous urticaria (csU). This review highlights the updated concepts and best practices, which were built upon international PRACTALL consensus and modified for children and infants with AD and csU. CALM-IT supports the routine use of basic skin therapy and the escalation of topical medications, according to severity and focused on rapid itch control. Anti-inflammatory agents should be appropriate for infants and children (i.e., with an optimized therapeutic index) and have proven antipruritic properties, such as those demonstrated by methylprednisolone aceponate. New experimental findings do not support the use of non-sedating oral antihistamines as adjuvant antipruritic therapy for AD. In csU, oral H1 -antihistamine use is justified, consistent with the distinct pathophysiologic mechanisms of itch underlying AD and csU. All encompassing QoL assessments should consider the burden of both patient and caregiver and should address outstanding unmet clinical needs of pediatric patients. Future research areas include integrated QoL assessments and multidisciplinary treatment programs with pediatric-targeted pruritic therapies providing rapid itch control.

Recent progress in unraveling central nervous system processing of itch sensation

Itch is the major symptom of many allergic or inflammatory skin diseases, yet it is still difficult to measure objectively. This article shows and updates the development and approaches of central nervous system investigation of itch.

Human neuroimaging studies on the physiology and pathophysiology of itch sensation have been hampered by the lack of a reproducible "on-off" stimulus. Short-term alternating temperature modulation of histamine-induced itch has recently been shown to provide on-off characteristics.

Recent studies with functional magnetic resonance imaging demonstrate that itch sensation in healthy volunteers is processed by a network of brain regions contributing to the encoding of sensory, emotional, attentional, evaluative, and motivational aspects of itch.

The brain circuitry mediating antipruritic effects of acupuncture

Itch is an aversive sensory experience and while systemic therapies, such as acupuncture, have shown promise in alleviating itch in patients suffering from chronic itch, their antipruritic mechanisms are unknown. As several lines of evidence implicate brain-focused mechanisms, we applied functional magnetic resonance imaging and our validated temperature-modulation itch model to evaluate the underlying brain circuitry supporting allergen-induced itch reduction in atopic dermatitis patients by acupuncture, antihistamine, and respective placebo treatments. Brain response to allergen itch demonstrated phase dependency. During an increasing itch phase, activation was localized in anterior insula and striatum, regions associated with salience/interoception and motivation processing. Once itch reached peak plateau, robust activation was noted in prefrontal cognitive and premotor areas. Acupuncture reduced itch and itch-evoked activation in the insula, putamen, and premotor and prefrontal cortical areas. Neither itch sensation nor itch-evoked brain response was altered following antihistamine or placebo acupuncture. Greater itch reduction following acupuncture was associated with greater reduction in putamen response, a region implicated in motivation and habitual behavior underlying the urge to scratch, specifically implicating this region in acupuncture's antipruritic effects. Understanding brain circuitry underlying itch reduction following acupuncture and related neuromodulatory therapies will significantly impact the development and applicability of novel therapies to reduce an itch.

Pruriceptive spinothalamic tract neurons: physiological properties and projection targets in the primate

Itch of peripheral origin requires information transfer from the spinal cord to the brain for perception. Here, primate spinothalamic tract (STT) neurons from lumbar spinal cord were functionally characterized by in vivo electrophysiology to determine the role of these cells in the transmission of pruriceptive information. One hundred eleven STT neurons were identified by antidromic stimulation and then recorded while histamine and cowhage (a nonhistaminergic pruritogen) were sequentially applied to the cutaneous receptive field of each cell. Twenty percent of STT neurons responded to histamine, 13% responded to cowhage, and 2% responded to both. All pruriceptive STT neurons were mechanically sensitive and additionally responded to heat, intradermal capsaicin, or both. STT neurons located in the superficial dorsal horn responded with greater discharge and longer duration to pruritogens than STT neurons located in the deep dorsal horn. Pruriceptive STT neurons discharged in a bursting pattern in response to the activating pruritogen and to capsaicin. Microantidromic mapping was used to determine the zone of termination for pruriceptive STT axons within the thalamus. Axons from histamine-responsive and cowhage-responsive STT neurons terminated in several thalamic nuclei including the ventral posterior lateral, ventral posterior inferior, and posterior nuclei. Axons from cowhage-responsive neurons were additionally found to terminate in the suprageniculate and medial geniculate nuclei. Histamine-responsive STT neurons were sensitized to gentle stroking of the receptive field after the response to histamine, suggesting a spinal mechanism for alloknesis. The results show that pruriceptive information is encoded by polymodal STT neurons in histaminergic or nonhistaminergic pathways and transmitted to the ventrobasal complex and posterior thalamus in primates.

Acupuncture compared with oral antihistamine for type I hypersensitivity itch and skin response in adults with atopic dermatitis: a patient- and examiner-blinded, randomized, placebo-controlled, crossover trial

BACKGROUND

Itch is the major symptom of atopic dermatitis (AD). Acupuncture has been shown to exhibit a significant effect on experimental itch in AD. Our study evaluated acupuncture and antihistamine itch therapy (cetirizine) on type I hypersensitivity itch and skin reaction in AD using a patient and examiner-blinded, randomized, placebo-controlled, crossover trial.

METHODS

Allergen-induced itch was evaluated in 20 patients with AD after several interventions in separate sessions: preventive (preceding) and abortive (concurrent) verum acupuncture (VAp and VAa), cetirizine (10 mg, VC), corresponding placebo interventions (preventive, PAp, and abortive, PAa, placebo acupuncture; placebo cetirizine pill, PC) and a no-intervention control (NI). Itch was induced on the forearm and temperature modulated over 20 min, using our validated model. Outcome parameters included itch intensity, wheal and flare size and the D2 attention test.

RESULTS

Mean itch intensity (SE: 0.31 each) was significantly lower following VAa (31.9) compared with all other groups (PAa: 36.5; VC: 36.8; VAp: 37.6; PC: 39.8; PAp: 39.9; NI: 45.7; P < 0.05). There was no significant difference between VAp and VC (P > 0.1), although both therapies were significantly superior to their respective placebo interventions (P < 0.05). Flare size following VAp was significantly smaller (P = 0.034) than that following PAp. D2 attention test score was significantly lower following VC compared with all other groups (P < 0.001).

CONCLUSIONS

Both VA and cetirizine significantly reduced type I hypersensitivity itch in patients with AD, compared with both placebo and NI. Timing of acupuncture application was important, as VAa had the most significant effect on itch, potentially because of counter-irritation and/or distraction. Itch reduction following cetirizine coincided with reduced attention.

Psychoneuroimmunology of psychological stress and atopic dermatitis: pathophysiologic and therapeutic updates

Atopic dermatitis is a chronic inflammatory skin disease characterized by impaired epidermal barrier function, inflammatory infiltration, extensive pruritus and a clinical course defined by symptomatic flares and remissions. The mechanisms of disease exacerbation are still poorly understood. Clinical occurrence of atopic dermatitis is often associated with psychological stress. In response to stress, upregulation of neuropeptide mediators in the brain, endocrine organs, and peripheral nervous system directly affect immune and resident cells in the skin. Lesional and non-lesional skin of patients with atopic dermatitis demonstrates increased mast cells and mast cell-nerve fiber contacts. In the setting of stress, sensory nerves release neuromediators that regulate inflammatory and immune responses, as well as barrier function. Progress towards elucidating these neuroimmune connections will refine our understanding of how emotional stress influences atopic dermatitis. Moreover, psychopharmacologic agents that modulate neuronal receptors or the amplification circuits of inflammation are attractive options for the treatment of not only atopic dermatitis, but also other stress-mediated inflammatory skin diseases.

A tale of two itches. Common features and notable differences in brain activation evoked by cowhage and histamine induced itch

Previous PET and fMRI brain imaging studies targeting neural networks processing itch sensation have used histamine as the sole itch inducer. In contrast with histamine, cowhage-induced itch is mediated via proteinase activated receptors PAR2 and is transmitted through a separate spinothalamic pathway, therefore imaging the brain activation evoked by cowhage could provide further insight into central processing of itch. We report for the first time a functional MRI Arterial Spin Labeling (ASL) study of neuronal processing of itch induced by cowhage, analyzed in contrast with histamine-induced itch. We also explored the brain responses induced by histamine and cowhage combined in a tight sequence. The results of our analyses obtained in a group of 15 healthy volunteers suggested that cowhage and histamine co-activated a core group of brain structures, while also revealing notable differences. Core areas activated by both stimuli were found in the thalamus, primary and secondary somatosensory cortices, posterior parietal cortex, superior and middle temporal cortices, PCC, ACC, precuneus and cuneus. Cowhage induced a notably distinct and more extensive involvement of the insular cortex, claustrum, basal ganglia, putamen, thalamic nuclei and pulvinar. The differences observed between these two itch modalities were investigated to determine the impact of quantitative versus qualitative factors, and correlations between itch intensity and the patterns in brain activation were explored. Our analysis revealed that the most significant differences between cowhage and histamine itch were not affected by stimulus intensity, although a subset of regions displayed activations which were intensity-dependent. The combined application of cowhage and histamine highlighted the role of insula and claustrum in the processing of both itch modalities in the same time. The present results suggest the existence of overlapping but also distinct neuronal networks processing these two different types of itch.

Pruritus: an overview of current concepts

Pruritus is an integral part of the patient's symptoms in numerous dermatological and systemic diseases in humans and animals. Comparable to chronic pain, pruritus can have a dramatic impact on the quality of life of the patient. In recent years, pruritus has been defined as an autonomous, pain-independent sensation, and itch-specific neurons, mediators, spinal neurons and cortical areas have been identified. These observations have not only improved our understanding of the neurobiology of itch but will also lead to improved diagnosis and to the development of new and more efficient therapeutic options. This article reviews the role of itch fibres and their response to various mediators of pruritus including histamine, vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), and substance P (SP), and opioids. Substances that may be involved in the induction or modulation of itch may be termed pruritogenic mediators and examples discussed include proteases, lipid mediators, neuropeptides, opioids and various cytokines. There is no single, generally accepted clinical classification of chronic pruritus. In the past pruritus has been classified on the basis of the neuroanatomical origin and on the potential underlying disease. Therapeutic options for the management of pruritus are discussed including topical and systemic therapies, assuming that trigger factors have been eliminated where possible. Topical agents may include capsaicin, the calcineurin inhibitors tacrolimus and pimecrolimus, and cannabinoid agonists such as N-palmitoyl ethanolamine. Systemic therapies may include antihistamines, anticonvulsants, opiate receptor antagonist or agonists, antidepressants, ciclosporin, and UV light.

Chronic pruritus in the absence of specific skin disease: an update on pathophysiology, diagnosis, and therapy

Chronic pruritus is a major and distressing symptom of many cutaneous and systemic diseases and can significantly impair the patient's quality of life. Pruritus perception is the final result of a complex network involving dedicated nerve pathways and brain areas, and an increasing number of peripheral and central mediators are thought to be involved. Itch is associated with most cutaneous disorders and, in these circumstances, its management overlaps with that of the skin disease. Itch can also occur without associated skin diseases or primary skin lesions, but only with nonspecific lesions secondary to rubbing or scratching. Chronic itch with no or minimal skin changes can be secondary to important diseases, such as neurologic disorders, chronic renal failure, cholestasis, systemic infections, malignancies, and endocrine disorders, and may also result from exposure to some drugs. The search for the cause of pruritus usually requires a meticulous step-by-step assessment involving careful history taking as well as clinical examination and laboratory investigations. Few evidence-based treatments for pruritus are available. Topical therapy, oral histamine H(1) receptor antagonists, and phototherapy with UV radiation can target pruritus elicitation in the skin, whereas antiepileptic drugs, opioid receptor antagonists, and antidepressants can block signal processing in the CNS.