An itchy question
How the sensation of itch arises might seem purely academic for those whose experience is confined to mosquito bites, but it is much more important for patients suffering from therapy-resistant itch, a frequent consequence of atopic dermatitis, liver disease, chronic renal disease and HIV infection. The simplest idea to explain itching is that there are ‘itch-specific’ neurons, and these have been sought for almost a century. In this issue (Nature Neuroscience, Vol. 4, Issue 1, 01 Jan 2001), Andrew and Craig finally report the identification of itch-specific neurons in cat spinal cord.
The best-known inducer of itch in human skin is histamine. Andrew and Craig discovered neurons in the dorsal horn of the anesthetized cat that respond when histamine is applied to their innervation territory in the skin. The pathway for itch -- like that for pain -- involves the spinothalamic tract, which connects the dorsal horn of the spinal cord to the thalamus. The itch-specific neurons are distinct from conventional pain-sensing neurons in several ways, including their anatomical connections and their physiological response patterns. Similar primary afferent neurons were found in 1997 by extracellular single-fiber recording in humans. These fibers responded to histamine with a time course similar to that of the itch sensation and had properties similar to those of the newly described second-order neurons (which receive axonal contacts from primary afferent neurons). Thus, it seems that the histamine-responsive neurons in cat and human are very similar, strongly suggesting that these histamine-sensitive central neurons are distinct from conventional pain-sensing neurons in both species, and instead constitute a separate system for the processing of itch.
Martin Schmelz discusses this work in an accompanying News & Views article.
Dr. David Andrew
Atkinson Pain Research Lab
Divison of Neurosurgery
Barrow Neurological Institute
350 W. Thomas Rd.
Phoenix, Arizona 85013
tel: +1 602 406 3148
fax: +1 602 406 4121
Dr. Martin Schmelz
Institut für Physiologie I
D 91054 Erlangen
tel: +49 9131852 2692
fax: +49 9131852 2497
(C) Nature Neuroscience press release.
For general questions about Nature Neuroscience, please contact the editor, Charles Jennings, at +1-212-726-9319 or by e-mail (firstname.lastname@example.org).
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