Noxious tooth pulp stimulation suppresses c-fos expression in the rat hippocampal formation

Changes in the expression of immediate early gene c-fas by noxious mechanic al stimulation to the mandibular incisor pulp of rats were immunohistochemi cally examined in the hippocampus (Ammon's horn and dentate gyrus) and the retrohippocampus (subiculum, presubiculum, parasubiculum and entorhinal cor tex). The highest control levels were found in subiculum, CAI, dentate and deep medial entorhinal cortex. Lower, but substantial levels were present i n the other areas. Whereas weak dentinal stimulation caused increases in c- fos expression in some regions which were not statistically significant, st rong tooth pulp stimulation caused a bilateral decrease in c-fos expression in every region except contralateral subiculum. These decreases reached st atistical significance in superficial layer parasubiculum bilaterally (p < 0.01), bilateral CA1 and ipsilateral side of superficial layer of medial en torhinal cortex (p < 0.05). We suggest that inhibitory circuitry in hippoca mpal formation regions may be activated by peripheral noxious somatosensory inputs and this change in activity is accompanied by a change in the expre ssion of the immediate early gene, c-fos. (C) 1999 Elsevier Science B.V. Al l rights reserved.