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This Article Full Text Full Text (PDF) All Versions of this Article: 93/6/3594    most recent 01075.2004v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Kitagawa, J. Articles by Iwata, K. Search for Related Content PubMed PubMed Citation Articles by Kitagawa, J. Articles by Iwata, K.

Effect of Chronic Inflammation on Dorsal Horn Nociceptive Neurons in Aged Rats

¹Department of Physiology, School of Dentistry, Nihon University, ²Motor and Autonomic Nervous System Integration Research Group, Tokyo Metropolitan Institute of Gerontology, and ³Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo Japan; ⁴Department of Dental Anesthesiology, Osaka University, Graduate School, Faculty of Dentistry, Osaka, ⁵Division of Functional Morphology, Dental Research Center, Nihon University School of Dentistry, Tokyo, ⁶Department of Physiology, Shiga University of Medical Science, Otsu, and ⁷Division of Applied System Neuroscience Advanced Medical Research Center, Nihon University Graduate School of Medical Science, Tokyo, Japan; and ⁸Department of Biomedical Sciences, University of Maryland Dental School, Baltimore, Maryland

Submitted 13 October 2004; accepted in final form 9 January 2005

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.