Brain's reward circuits mediate itch relief. a functional MRI study of active scratching.

Alexandru D P Papoiu,Gil Yosipovitch,Robert C Coghill,Leigh A Nattkemper,Robert A Kraft,Kristen M Sanders,Yiong-Huak Chan,Linda Chao

doi:10.1371/journal.pone.0082389

Abstract

Previous brain imaging studies investigating the brain processing of scratching used an exogenous intervention mimicking scratching, performed not by the subjects themselves, but delivered by an investigator. In real life, scratching is a conscious, voluntary, controlled motor response to itching, which is directed to the perceived site of distress. In this study we aimed to visualize in real-time by brain imaging the core mechanisms of the itch-scratch cycle when scratching was performed by subjects themselves. Secondly, we aimed to assess the correlations between brain patterns of activation and psychophysical ratings of itch relief or pleasurability of scratching. We also compared the patterns of brain activity evoked by self-scratching vs. passive scratching. We used a robust tridimensional Arterial Spin Labeling fMRI technique that is less sensitive to motion artifacts: 3D gradient echo and spin echo (GRASE) - Propeller. Active scratching was accompanied by a higher pleasurability and induced a more pronounced deactivation of the anterior cingulate cortex and insula, in comparison with passive scratching. A significant involvement of the reward system including the ventral tegmentum of the midbrain, coupled with a mechanism deactivating the periaqueductal gray matter (PAG), suggests that itch modulation operates in reverse to the mechanism known to suppress pain. Our findings not only confirm a role for the central networks processing reward in the pleasurable aspects of scratching, but also suggest they play a role in mediating itch relief.

Highlights

Scratching is the natural response to itch and, by definition, inseparable from it
The itch-scratch cycle is a complex phenomenon involving sensory, motor and emotional components. It is well-known that the urge to scratch can be remarkably intense, since the reward provided by scratching brings itch relief and associated feelings of pleasurability
In the current study, we used fMRI to visualize the stages of the itch-scratch cycle, when scratching was performed by study subjects themselves

Summary

Introduction

Scratching is the natural response to itch and, by definition, inseparable from it. The act of scratching diminishes itch, but can be rewarding and addictive. The itch-scratch cycle is a complex phenomenon involving sensory, motor and emotional components. It is well-known that the urge to scratch can be remarkably intense, since the reward provided by scratching brings itch relief and associated feelings of pleasurability. Previous brain imaging studies have employed BOLD fMRI to examine the effects evoked by scratching. These studies used “passive scratching” performed by an investigator using a device, such as a cytology brush [3] or a copper wire [4]. In real life, scratching is a conscious, controlled motor response, targeted to the perceived site of itch, and continuously adjusted by sensory feedback in a manner that seeks to provide relief.

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