title: Effect of chronic inflammation on dorsal horn nociceptive neurons in aged rats.
abstract: To elucidate the effect of chronic inflammation on spinal nociceptive neurons in the elderly, we compared nocifensive behavior, peripheral inflammatory responses, and spinal dorsal horn neuronal activities between the aged (29-34 mo) and adult (7-12 mo) male rats after injection of complete Freund's adjuvant (CFA) into the hind paw. Aged rats exhibited a significantly lower mechanical paw withdrawal threshold before inflammation. However, after CFA injection mechanical allodynia developed in both adult and aged rats after CFA injection. The changes of foot temperature and thickness after CFA injection were greater and lasted longer in aged than in adult rats. Sets of 124 wide dynamic range (WDR) neurons (aged: 59, adult: 65) and 26 nociceptive specific (NS) neurons (aged: 13, adult: 13) were recorded from the lumber spinal dorsal horn. NS neurons from the inflamed adult rats showed significantly higher responses to noxious mechanical stimulation than those in aged rats, whereas WDR neurons from inflamed adult and aged rats were similar. Background activity of WDR neurons from the adult rats increased after CFA, whereas WDR neurons of aged rats and NS neurons from either group were not. The afterdischarge followed by noxious mechanical stimulation was significantly greater for WDR neurons in both adult and aged rats, whereas no significant differences were observed in NS neurons. Two days after CFA injection, Fos expression increased similarly in aged and adult rats. Thus the aged rats showed enhanced peripheral inflammatory responses to CFA injection with only a slight change in dorsal horn neuronal activity. Together with our previous finding that nociceptive neurons in aged rats exhibit hyperexcitability, these results suggest that the dorsal horn nociceptive system becomes sensitized with advancing age and its excitability cannot be further increased by inflammation.
authors.affiliation: Department of Physiology, School of Dentistry, Nihon University, 1-8-13 Kandasurugadai, Chiyoda-ku Tokyo, 101-8310, Japan.
authors.forename: Junichi
authors.initials: J
authors.lastname: Kitagawa
authors.forename: Kenro
authors.initials: K
authors.lastname: Kanda
authors.forename: Miho
authors.initials: M
authors.lastname: Sugiura
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Tsuboi
authors.forename: Akiko
authors.initials: A
authors.lastname: Ogawa
authors.forename: Kohei
authors.initials: K
authors.lastname: Shimizu
authors.forename: Natsu
authors.initials: N
authors.lastname: Koyama
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kamo
authors.forename: Tatsuhisa
authors.initials: T
authors.lastname: Watanabe
authors.forename: Ke
authors.initials: K
authors.lastname: Ren
authors.forename: Koichi
authors.initials: K
authors.lastname: Iwata
mesh_terms: D000200:Action Potentials / Q000502:physiology / Q000528:radiation effects; D000367:Age Factors; D000375:Aging / Q000502:physiology*; D000818:Animals; D001522:Behavior, Animal; D001831:Body Temperature / Q000502:physiology; D001931:Brain Mapping; D002908:Chronic Disease; D004307:Dose-Response Relationship, Radiation; D004487:Edema / Q000473:pathology / Q000503:physiopathology; D004558:Electric Stimulation / Q000379:methods; D005121:Extremities / Q000473:pathology / Q000503:physiopathology; D005620:Freund's Adjuvant; D007839:Functional Laterality; D007249:Inflammation / Q000139:chemically induced / Q000473:pathology / Q000503:physiopathology*; D008297:Male; D009619:Nociceptors / Q000503:physiopathology*; D010146:Pain / Q000473:pathology / Q000503:physiopathology*; D010147:Pain Measurement / Q000379:methods; D017288:Pain Threshold; D010812:Physical Stimulation / Q000009:adverse effects; D020671:Posterior Horn Cells / Q000503:physiopathology*; D051381:Rats; D011916:Rats, Inbred F344; D013116:Spinal Cord / Q000166:cytology*; D013997:Time Factors
chemical_list: D005620:Freund's Adjuvant