How stress triggers Itch: A preliminary study of the mechanism of stress-induced pruritus using fMRI

Background: Itch is one of the most prevalent and familiar nociceptions in humans and can result in impaired quality of life. The central processing of itch is known to be important in perceptions of itch and strategies for coping with it, and various triggers have been reported to be associated with patterns of brain activation in pruritus. Stress is known to be an important aggravating factor in itch, but the precise mechanism by which brain activation results in stress-induced pruritus remains elusive. Methods: Using functional magnetic resonance imaging (fMRI), we attempted to evaluate if stress-induced pruritus is associated with the activation of certain areas of the brain. Furthermore, we investigated whether sedating antihistamines are more effective than non-sedating antihistamines in decreasing stress-induced pruritus. Memories of stressful events followed by visual stimuli using a series of pictures depicting fearful and stressful conditions, mixed with neutralizing pictures, were used to evoke pruritus. Results: The hippocampus and subcortical structures were found to be involved in stress-associated pruritus. Interestingly, sedating antihistamines were involved in the deactivation of brain regions associated with the relief of itch by active scratching, which suggests that sedating antihistamines may mitigate pruritus by instigating patterns of brain activity similar to those induced by scratching behavior. Conclusions: Itch scratching reflects a complicated pattern of brain activity. Activation of the hippocampus appears to be involved in stress-associated pruritus, and sedating antihistamines may work in a manner similar to that of scratching behavior. To our knowledge, this is the first study to evaluate brain functioning in relation to stress-induced pruritus.