BACKGROUND: Not many recent studies have shown that morphine antinociception may be directly expressed in forebrain structures. It is generally accepted that c-fos is a marker of neuronal activity and its expresson is correlated with nerve pathway activated by nociceptive stimuli. The aim of this study is to examine the effect of morphine on c-fos expression in the incisional pain rat brain.

METHODS: A 1 cm longitudinal incision was made through the skin, fascia and muscle of the plantar aspect of the hindpaw in enflurane-anesthetized rats. 10 mg/kg of morphine was injected intraperitoneally 1 hour before (pre-morphine group; n = 15) and 30 minutes after surgery (post-morphine group; n = 15). The same amount of saline was injected 30 minutes after surgery (control group; n = 15). Two hours later c-fos protein expressions in the thalamus, hypothalamus, cerebral cortex and amygdala were examined by immunohistochemistry using a specific antibody.

RESULTS: Numerous c-fos positive cells were observed in thalamus, hypothalamus, cerebral cortex, and amygdala in all groups. There were no significant differences in c-fos expression between pre-morphine, post-morphine and control group (P <0.05).

CONCLUSIONS: In this study we expected to decreased c-fos expression in incisional pain rat brain by morphine injection. But no differences were observed compared to control group in thalamus and cortex which transmitting pain to CNS, also in hypothalamus and amygdale which transmitting emotional stress to CNS. These results suggests that intraperitoneal morphine can not protect the c-fos expression of ascending pathway to thalamus, hypothalmus, amygdala and cerebral cortex. Also we can not support the effect of morphine on the descending pathway of pain. So we thought for the more information, additional study, for example, behavior test, PCR (polymerase chain reaction)study, may be needed.