Cerebral Representation of the Relief of Itch by Scratching

Abstract

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.

Effects of scratching and other counterstimuli on responses of trigeminothalamic tract neurons to itch-inducing stimuli in rats

“…We found that pruriceptive responses did not return to firing levels seen before scratching for the nearly 1-min period that was examined after the termination of 10 s of scratching. Vierow et al (2009) showed that scratching strongly reduced itch induced in human subjects for roughly 10 s and that itch ratings continued to recover for the following 30 s, a time course similar to that of the inhibition of pruriceptive responses seen in this study. Similar effects of scratching, in both intensity and duration, were seen in our previous study of the effect of scratching on pruriceptive responses of primate spinothalamic tract neurons (Davidson et al 2009).…”

“…Scratching the skin is the most commonly employed counterstimulus that reduces or eliminates itch (Greaves and Wall 1996;Vierow et al 2009). Other forms of noxious mechanical stimuli, including pricking or pinching skin, also can reduce or eliminate itch in human subjects (Chapman et al 1960;Graham et al 1951).…”

Functional magnetic resonance imaging in dermatology: The skin, the brain and the invisible

“…[68] Several studies have reported activation of the reward system including the medial prefrontal cortex, anterior cingulate cortex, striatum, midbrain, also referred to as "cortico-striatal circuit" during pleasurable scratching. [90][91][92][93] Interestingly, activation of motor-related areas (eg SMA, PM and cerebellum) correlates with pleasurability of scratching, even if scratched by experimenters, indicating that pleasurable scratching may promote more scratching. [68] This addictive mechanism seems to be reinforced in chronic itch patients shown to have significantly higher activity in the motor and reward areas during scratching than healthy controls and also perceive scratching non-itchy skin as pleasurable.…”

“…In the same study, electroacupuncture significantly reduced both allergen-induced itch intensity and activation of the insula, putamen, premotor and prefrontal cortical areas. [84] Putamen activation has been linked to the urge of scratching [84,93] and electroacupuncture seemed to particularly reduce the putamen response. This is a good example of how the combination of clinical markers and fNI can illuminate unknown modes of actions and scientifically rationalise a treatment strategy.…”

Neural basis of contagious itch and why some people are more prone to it

“…Functional neuroimaging investigations of itch [predominantly functional MRI (fMRI)] typically use an invasive, localized administration of histamine to induce itch (5)(6)(7)(8). This approach has revealed the engagement of a network of regions (the so-called "itch matrix") that includes the anterior insula, cingulate cortex, primary somatosensory cortex, premotor cortex, prefrontal cortex, thalamus, and cerebellum.…”