Tuesday, January 18, 2011

Steinhoff's 2006 itch review fascinating, useful

So in advance of meeting Martin Steinhoff, I've been reading a couple of his review papers on itch research. I am well used to reading scientific papers--the slog through the jargon and hard-to-grasp concepts. It's liberating to read a paper about a subject that touches you personally, instead of one about some anonymous molecule with an acronym like ADF46-PGL (I made that up) that is somehow linked to something that might be useful to someone someday. With Steinhoff's itch reviews, there's a fair amount of jargon, yes, but I can relate to virtually every paragraph, and I pay attention because I hope to learn tips that I can apply to my daily life. For example:
Recent results suggest that noxious heat stimuli and scratching produce a stronger itch inhibition than do noxious cold stimuli.
I've always found that a cold pack from the freezer helped kill an itch, at least for a while. Maybe I'll try a hot pack next time.

The main point I take away from the review I've just finished is that itch research is unfortunately far from being able to produce a good target for anti-itch drugs. Decades, maybe--although that review is from 2006, so the current state-of-the-art may be much further ahead; you never know. There's always the hope that an already-approved drug might turn out to have a beneficial side effect against itch. Certainly, one thing that's been discovered is that pain inhibits itch--this was discovered partly because painkiller drugs tend to make people itchy.

A major recent development is that scientists have identified specific neurons that carry itch from the skin to the spinal cord and then to the thalamus in the brain. Some of these neurons are called "histamine-sensitive itch fibers," or "pruriceptors," although there must be other itch fibers because there are many more molecules besides histamine that have been shown to induce itch.
This diversity of primary afferent "itch fibers" would nicely fit to the different submodalities of pruritis ("itch quality") observed in patients.
Exactly! Um...that is, I know I experience different kinds of itch. There's the tickly kind, like when an ant is crawling on my neck; there's a distinct itch I get from a small, active patch of eczema, on my foot, say; and there's the full-on-paroxysm that I occasionally wake up with after I've had the poor judgement to drink eggnog or eat something laced with cayenne pepper. And there are others.

As I just mentioned, there are many more itch-inducing molecules besides histamine. Steinhoff thinks that researchers have focused too much on histamine. Table 1 lists 16 different types of molecules (he calls it the "itching army") that represent the major culprits. In eczema, acetylcholine and mu-opioids appear to be more significant than the other molecules.

On to the next review, which has more gems, I'm sure.

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