Thursday, June 10, 2010

pitching in for "itch"!

Earthen sculpture of a bust of a wild-eyed man scratching his back by Robert Arneson, titled Pablo Ruiz with Itch 
Itch (Latin: pruritus) is an unpleasant sensation that causes the desire or reflex to scratch the affected area; and the stimuli that causes it is termed Pruritic stimuli. Chronic itch represents a significant clinical problem resulting from renal diseases and liver diseases, as well as several serious skin
diseases such as atopic dermatitis.
Itch has resisted several attempts to classify it as any one type of sensory experience. Research has shown that though itch has many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response patterns are totally different; while pain creates a withdrawal reflex, itch invariably leads to a scratch reflex. The controversy over their apparent similarity persisted because until now nobody could show that both acted through different neuronal-pathways.
The group headed by Zhou-Feng Chen at the Pain-Center in the Washington University School of Medicine, in a 2007 Nature paper, described the gastrin-releasing peptide receptor (GRPR) and implied it in mediating itch sensation in the dorsal spinal cord. GRPR mutant mice showed comparable thermal, mechanical, inflammatory and neuropathic pain responses relative to wild-type mice. In contrast, induction of scratching behaviour was significantly reduced in GRPR mutant mice in response to pruritogenic stimuli. It was significantly indicative that Gastrin-releasing peptide receptor (GRPR) is an itch-specific gene in the spinal cord. Moreover, direct spinal cerebrospinal fluid injection of a GRPR antagonist significantly inhibited scratching behaviour in three independent itch models.

a. scratching behaviour induced by intradermal injection of compound 48/80; b. PAR2 agonist SLIGRL-NH2; c. chloroquine.

As it turns out, the group has with sufficient evidence, recently shown that Itch and Pain are two different sensations propagated by different pathways, and the mechanism of itch-relief by scratching. But more about in in another post.

A gastrin-releasing peptide receptor mediates the itch sensation in the spinal cord
Yan-Gang Sun and Zhou-Feng Chen.
Nature, Vol 448,9 August 2007.


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