RESUMEN
PURPOSE: The effects of pain on brain is not well known. Also, differences between somatic and visceral pains have not been fully elucidated. This study was conducted to investigate changes in the expression of c-Fos protein after somatic and visceral pains were induced by formalin. METHODS: Male rats(n=65) were underwent one of three procedures : (i) Control group, rats were left undisturbed in their cages; (ii) Somatic pain group, rats were injected subcutaneously with 0.1 ml of 10% formalin in the plantar surface of right hindpaw; (iii) Visceral pain group, rats were administered with same amount of formalin, as described above, in the rectum. Rats were sacrificed at increasing times(30 minutes, 1 hour, 2 hours, 6 hours, 1 day, 3 days and 7 days) after noxious formalin stimuli to hindpaws and rectums. Rat brains were removed and sliced in rat brain matrix. Brain slices were coronal sectioned at interaural 5.70-6.70mm. Serial sections were immunohistochemically reacted with polyclonal c-Fos antibody. The numbers of c-Fos protein immunoreactive neurons in cingulate cortex, primary somatosensory area, and hippocampus were examined and analyzed statistically with Mann-Whitney U test. RESULTS: 1) The numbers of c-For protein immunoreactive neurons in cingulate cortex, primary somatosensory area and hippocampus peaked at 2 hours after somatic pain stimuli and reached almost normal conditions at 7 days. 2) The numbers of c-Fos protein immunoreactive neurons in cingulate cortex, primary somatosensory area and hippocampus peaked at 1 day after visceral pain stimuli and reached almost normal conditions at 7 days. 3) The numbers of c-Fos protein immunoreactive neurons of somatic pain groups were higher than that of visceral groups at all times and the difference of numbers peaked at 2 hours after pain stimuli. CONCLUSION: Reactions of somatic pain stimuli influenced more changable than visceral pain stimuli to brain. Conduction velocities of somatic pain were more faster than those of visceral pain. Higher numbers of c-Fos protein immunoreactive neurons were found in specific regions. These results provide some basic knowledge in understanding the mechanism and control of pain.