title: Licking decreases phosphorylation of extracellular signal-regulated kinase in the dorsal horn of the spinal cord after a formalin test.
abstract: BACKGROUND
  Nociceptive behaviors might attenuate pain sensation. Phosphorylation of extracellular signal-regulated kinase (pERK) was recently reported to be induced by noxious stimuli in dorsal horn neurons. We investigated, in a formalin test, whether pERK of the dorsal horn is affected by licking.
  
  
  METHODS
  Twenty-four adult male rats were divided into four groups: control, formalin test, restricted control, and restricted formalin test. Ten percent formalin was injected subcutaneously into the left rear paw of the formalin test and restricted formalin test groups. The control and formalin test group rats were kept in a clear plastic chamber, whereas the restricted control and restricted formalin test group rats were kept in a modified-restraint, pipe-shaped chamber. All rats were killed after 25 min. Twelve sections of the lumbar spinal cord were processed for p-ERK immunohistochemistry using the avidin-biotin peroxidase method.
  
  
  RESULTS
  The number of p-ERK positive cells in the restricted formalin test group was significantly higher than in the other three groups in the ipsilateral-side superficial dorsal horn (P < 0.05). However, there was no significant difference between the formalin test group and the two control groups in pERK expression.
  
  
  CONCLUSIONS
  Licking decreased pERK of the spinal cord of the formalin test group. The findings suggested that licking attenuated the pain of the formalin test.
authors.affiliation: Department of Anesthesiology, Institute of Clinical Medicine, Graduate School of Comprehensive Human Sciences, Tsukuba University, Tsukuba-city, Ibaraki 305-8575, Japan. taekof@md.tsukuba.ac.jp
authors.forename: Taeko
authors.initials: T
authors.lastname: Fukuda
authors.forename: Setsuji
authors.initials: S
authors.lastname: Hisano
authors.forename: Makoto
authors.initials: M
authors.lastname: Tanaka
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D004195:Disease Models, Animal; D015536:Down-Regulation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D005557:Formaldehyde; D007150:Immunohistochemistry; D008161:Lumbosacral Region; D008297:Male; D040542:Mechanotransduction, Cellular; D010146:Pain / Q000139:chemically induced / Q000201:enzymology / Q000503:physiopathology*; D010147:Pain Measurement; D017288:Pain Threshold; D010766:Phosphorylation; D020671:Posterior Horn Cells / Q000201:enzymology*; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014059:Tongue / Q000503:physiopathology*
chemical_list: D016760:Proto-Oncogene Proteins c-fos; D005557:Formaldehyde; D048049:Extracellular Signal-Regulated MAP Kinases