DORSAL-ROOT REFLEXES IN ARTICULAR AFFERENTS OCCUR BILATERALLY IN A CHRONIC MODEL OF ARTHRITIS IN RATS

1. Chronic arthritis was produced in rats by the injection of incomple te Freund's adjuvant into one knee joint. By 3-5 days later the rats h ad developed unilateral swelling of the injected knee and demonstrated bilateral hyperalgesia to radiant heat stimuli applied to the foot. 2 . In the same rats anesthetized 3-5 days after the injection, dorsal r oot reflexes could be recorded bilaterally from the proximal ends of t he cut medial articular nerves (MANs) of the knee joint. 3. The dorsal root reflexes consisted of large, medium-sized, and small action pote ntials evoked in response to phasic mechanical stimulation of the late ral aspect of the knee. The activity was greater in the MAN ipsilatera l to the injection than in the contralateral MAN. 4. Local application of capsaicin on the side ipsilateral or contralateral to the arthriti s dramatically reduced the dorsal root reflexes recorded from the cont ralateral MAN, indicating that these dorsal root reflexes depended on activity in fine afferent fibers containing capsaicin receptors, presu mably C fibers. Local application of capsaicin on either side did not significantly change the dorsal root reflexes recorded from the ipsila teral MAN. These dorsal root reflexes were presumably conducted in aff erent fibers that lacked capsaicin receptors, including A beta- and A delta-fibers.