title: Involvement of spinal 5-HT1A receptors in isolation rearing-induced hypoalgesia in mice.
abstract: BACKGROUND
  Isolation rearing in rodents causes not only abnormal behaviors which resemble the clinical symptoms of schizophrenia but also hypoalgesia in thermal nociception models. However, the mechanism of the hypoalgesia is not known.
  
  
  OBJECTIVE
  The present study investigated the effect of isolation rearing on acute pain and the descending pain inhibitory pathways in mice.
  
  
  RESULTS
  Rearing in isolation for 6 weeks from post-weaning reduced pain sensitivity in the hot plate test and acetic acid-induced writhing test. Isolation rearing also reduced the intraplantar capsaicin-induced licking behavior. Capsaicin increased c-Fos expression, a neuronal activity marker, in the spinal cord and primary somatosensory cortex both in group- and isolation-reared mice, but this effect did not differ between groups. On the other hand, c-Fos expression in the anterior cingulate cortex, periaqueductal gray matter, and rostral ventromedial medulla, but not in the spinal cord or somatosensory cortex, was enhanced by isolation rearing. Systemic administration of WAY100635 (serotonin (5-HT)1A receptor antagonist), but not of ketanserin (5-HT2 receptor antagonist), prazosin (α1-adrenoceptor antagonist), or yohimbine (α2-adrenoceptor antagonist), attenuated isolation rearing-induced hypoalgesia in capsaicin-induced licking behavior. Attenuation of isolation rearing-induced hypoalgesia was also observed following the intrathecal injection of WAY100635. Naloxone, an opioid receptor antagonist, did not affect the hypoalgesia in isolation-reared mice.
  
  
  CONCLUSIONS
  These findings suggest that isolation rearing causes hypoalgesia in mouse models of acute pain and imply that the spinal 5-HT1A receptor activation probably through descending serotonergic inhibitory pathway is involved in isolation rearing-induced hypoalgesia.
authors.affiliation: Laboratory of Medicinal Pharmacology, Graduate School of Pharmaceutical Sciences, Osaka University, 1-6 Yamada-oka, Suita, 565-0871, Osaka, Japan.
authors.forename: Naotaka
authors.initials: N
authors.lastname: Horiguchi
authors.forename: Yukio
authors.initials: Y
authors.lastname: Ago
authors.forename: Kazuki
authors.initials: K
authors.lastname: Asada
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kita
authors.forename: Naoki
authors.initials: N
authors.lastname: Hiramatsu
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Takuma
authors.forename: Toshio
authors.initials: T
authors.lastname: Matsuda
mesh_terms: D019342:Acetic Acid / Q000633:toxicity; D059787:Acute Pain / Q000503:physiopathology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D002211:Capsaicin / Q000494:pharmacology; D004195:Disease Models, Animal; D008297:Male; D051379:Mice; D010147:Pain Measurement; D017288:Pain Threshold; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D044282:Receptor, Serotonin, 5-HT1A / Q000378:metabolism*; D012934:Social Isolation; D013003:Somatosensory Cortex / Q000378:metabolism; D013116:Spinal Cord / Q000378:metabolism*
chemical_list: D016760:Proto-Oncogene Proteins c-fos; D044282:Receptor, Serotonin, 5-HT1A; D019342:Acetic Acid; D002211:Capsaicin