Itchy and scratchy

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Ezine

  • Published: Jan 1, 2007
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Itchy and scratchy

Researchers are only just beginning to scratch the surface of the brain with functional MRI. Now, a study of perception in both allergen- and histamine-induced itch has revealed how different parts of the brain are activated in response to stimulation from each type.

Allergens, such as pollen and dust, and histamine released by allergy cells as a result of activation by foods, drugs, or infection often lead to a vicious itch-scratch cycle as any allergy sufferer will tell you. However, researchers at Oxford University have demonstrated that the brain responds differently to itchiness caused by allergens and histamine.

Siri Leknes, Susanna Bantick, Richard Wise, and Irene Tracey at Oxford have worked with Carolyn Willis and John Wilkinson of the Department of Dermatology, at Amersham Hospital to try to understand the nature of itch-cycle with a view to improving outcomes for allergy sufferers and people with certain chronic skin conditions.

The team enlisted sixteen males and females who were either allergy sufferers or not atopic or non-atopic individuals. A standard whole-brain MRI was carried out using a gradient echo-planar imaging sequence to obtain the functional scans. The volunteers received a skin "challenge" to the toe area in the form of a skinprick with allergen, histamine, or saline as a control. They volunteers rated the resulting itchiness on a scale of 0 to 5.

In a parallel experiment, the team enlisted twenty eight female volunteers fourteen of whom tested positive for type I allergens, such as grass pollen and house dust mite. The other fourteen were non-atopic. The researchers then "challenged" the volunteers with either their specific allergen or histamine, and a control group with just saline solution applied via a skin prick to the forearm. The volunteers were asked to rate the itchiness produced on a scale of 0 to 10 and blood flow was determined using a laser Doppler effect.

Analysis of the results from both experiments revealed that there is much in common between allergen- and histamine-induced itch. In particular, there is extensive involvement of the brain's motivation circuitry in response to both types of itches. However, there were numerous significant differences.

For instance, allergen-induced itch intensity ratings were higher compared to histamine, perception of itch and changes in blood flow were significantly greater in allergen-induced itch. The perception of itchiness and the changes in blood flow were found to occur significantly later in response to allergen but to exist for much longer. The team also found that itch caused by allergens triggered activity in different parts of the brain.

It is the revelation of differences in the orbifrontal regions of the brain linked to compulsion that have the biggest implications for eczema sufferers, for instance. The compulsion to scratch is very strong in the allergen group, a feeling with which sufferers will be very familiar. This, the researchers say, might help to explain why eczema sufferers scratch to the point of harm because they are compelled to do so and cannot help themselves. In other words, there's no point in telling sufferers to "stop scratching!", they simply cannot.


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