Effects of previous noxious stimulus applied to remote areas on noxious stimulus-evoked c-Fos expression in the rat trigeminal nucleus caudalis

Noxious stimulus-evoked c-Fos expression in the spinal dorsal horn is modulated by noxious stimuli applied previously to remote areas of the body. To confirm the existence of such modulation in c-Fos expression in the trigeminal system, changes in c-Fos expression in the trigeminal nucleus caudalis induced by formalin injection into the rat whisker pad were examined by previously injecting formalin into different areas (contralateral whisker pad, ipsilateral or contralateral forepaw) of the body. Formalin injection-evoked c-Fos expression in this nucleus was significantly reduced by previous formalin injection into the contralateral whisker pad or ipsilateral forepaw but not into the contralateral forepaw. The interval between the two injections of formalin that produced a maximal reduction of formalin injection-evoked c-Fos expression was 1 h, and the reduction of c-Fos expression was less when the interval of the two noxious stimuli was longer or shorter than 1 h. These results suggested that noxious stimulus-evoked c-Fos expression in the trigeminal nucleus caudalis is reduced by noxious stimulus applied previously to remote areas, and the reduction is dependent on the area of previous noxious stimulation and interval between the two noxious stimuli.