EFFECTS OF LOCAL-ANESTHESIA ON FORMALIN-INDUCED FOS EXPRESSION IN THERAT DORSAL HORN

A subcutaneous injection of formalin into foot pad of the rat produces a bimodal nociceptive response including an early intense response in the first 5 min and a later moderate response that is exhibited from 20 to 60 min after injection. In this study, we investigated the effec ts of blocking the early phase and late phase input, respectively, on Fos-like immunoreactivity (Fos-LI) expression in dorsal horn neurones. Rats of the early phase block (EB) group were injected with 5% formal in (0.05 mi) into the footpad 5 min after a s.c. injection of 4% lidoc aine (0.15 mi) into the angle. The rats of the late phase block (LB) g roup were injected with 5% formalin into the footpad 10 min before the s.c. injection of 3% prilocaine (0.20 mi) containing felypressin into the ankle. The rats of the control group were given the formalin inje ction alone. Fos-LI was detected in the dorsal horn 2 h after the form alin injection. The numbers of Fos-LI neurones in the dorsal horn of b oth EB and LB group were markedly decreased compared with the control group, being 31.3% (laminae I-III of EB), 37.1% (laminae I-III of LB), 13.9% (laminae IV-VI of EB) and 16.2% (laminae TV-VI of LB) of the co ntrol values. No significant difference was observed between EB and LB group. These findings suggested that the early and late phase contrib ute in concert to the induction of genetic changes in dorsal horn neur ones after formalin injection.