title: Site-dependent and state-dependent inhibition of pruritogen-responsive spinal neurons by scratching.
abstract: The relief of itch by scratching is thought to involve inhibition of pruritogen-responsive neurons in the spinal cord. We recorded the responses of superficial dorsal horn neurons in mice to intradermal injection of the pruritogens chloroquine and histamine. Scratching within an area 5-17 mm distant from the injection site, outside of the units' mechanoreceptive fields (off-site), significantly inhibited chloroquine-evoked and histamine-evoked responses without affecting capsaicin-evoked firing. This is consistent with observations that scratching at a distance from a site of itch is antipruritic. In contrast, scratching directly at the injection site (within the receptive field; on-site) had no effect on chloroquine-evoked neuronal firing, but enhanced the same neurons' responses to intradermal injection of the algogen capsaicin. Moreover, neuronal responses to histamine were enhanced during on-site scratching, and this was followed by suppression of firing below baseline levels after termination of scratching. Scratching thus inhibits pruritogen-responsive neurons in a manner that depends on the input modality (i.e. pain vs. histamine-dependent or histamine-independent itch) and skin location.
authors.affiliation: Department of Neurobiology, Physiology & Behavior, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA.
authors.forename: Tasuku
authors.initials: T
authors.lastname: Akiyama
authors.forename: Mitsutoshi
authors.initials: M
authors.lastname: Tominaga
authors.forename: Mirela Iodi
authors.initials: MI
authors.lastname: Carstens
authors.forename: Earl E
authors.initials: EE
authors.lastname: Carstens
mesh_terms: D000818:Animals; D002211:Capsaicin / Q000494:pharmacology; D002738:Chloroquine / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology*; D006632:Histamine / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D020671:Posterior Horn Cells / Q000503:physiopathology*; D011537:Pruritus / Q000139:chemically induced / Q000503:physiopathology*; D012018:Reflex / Q000187:drug effects / Q000502:physiology; D014110:Touch / Q000502:physiology*
chemical_list: D006632:Histamine; D002738:Chloroquine; D002211:Capsaicin