Spinal nociceptive mechanisms: The effect of chronic inflammation on rat dorsal horn nociceptive neurons with advancing age

Abstract

The present study was designed to elucidate the effect of chronic inflammation on spinal dorsal horn nociceptive neurons with advancing age. Foot temperature, thickness and dorsal horn neural activities were studied in the aged and adult rats with complete Freund's adjuvant (CFA). The changes of foot temperature and thickness were much higher and longer lasting in aged rats than those of adult rats following CFA injection. A total of 150 nociceptive neurons were recorded from the aged (29-34 month old) and adult (9-12 month old) rats. Nociceptive neurons were classified as wide dynamic range (WDR) and nociceptive specific (NS) neurons according to their responsibility to mechanical stimulation of the receptive fields. One hundred twenty four WDR neurons (aged: 59, adult: 65) and 26 NS neurons (aged: 13, adult: 13) were identified. NS neurons from the inflamed adult rats showed significantly higher responses to noxious stimulation following CFA injection. On the other hand, WDR neurons from both inflamed adult and aged rats and NS neurons from the inflamed aged rats did not show higher responses to mechanical stimulation as compare with those of na-e rats. Background activity of WDR neurons from the adult rats was significantly higher than that of na-e rats, whereas that of aged rats and NS neurons was not. The after discharge followed by noxious mechanical stimulation was significantly larger in WDR neurons in both adult and aged rats, whereas no significant differences were observed in NS neurons. These suggest that chronic peripheral inflammation induces differential effects on pain perception in aged and adult rats.