BASAL AND TOUCH-EVOKED FOS-LIKE IMMUNOREACTIVITY DURING EXPERIMENTAL INFLAMMATION IN THE RAT

Fos-immunoreactivity can readily be induced in spinal cord neurones by noxious, but to a much more limited extent, by innocuous peripheral s timuli. The present study has investigated whether low intensity stimu li and electrical stimulation of A beta afferents elicit greater c-fos expression during the behavioural sensory hypersensitivity generated by experimental peripheral inflammation. We have examined the time-cou rse of c-fos expression after inflammation produced by either an intra -plantar injection of the irritant turpentine oil or of complete Freun d's adjuvant (CFA). In the former case, a significant initial expressi on in all dorsal horn laminae was followed by a gradual decrease, wher eas after CFA injection, an initial expression limited to the superfic ial laminae subsequently extended into the deep laminae, with a decrea se at 24 h and an increase in labelling at later times. Low intensity touch stimuli repeated for 10 min, when applied at 24 h and 48 h after CFA injection, elicited a significant increase in the number of Fos-i mmunoreactive neurons in both the superficial and deep laminae of the dorsal horn compared to non-inflamed animals. Electrical stimulation o f the sciatic nerve 24 h post-CFA injection, at a strength sufficient only to activate A beta-afferents fibres (100 mu A, 50 mu s, 10 min), also elicited a significant increase in labelling relative to the same stimuli applied in control animals, especially in laminae V-VI. The p resent results demonstrate that low intensity cutaneous stimuli elicit a significantly greater increase in c-fos expression in dorsal horn n eurons during peripheral inflammation and that A beta-afferent input c ontributes to this, a finding that may relate to the allodynia experie nced during inflammation.