C-fos expression in rat lumbar spinal cord following peripheral stimulation in adjuvant-induced arthritic and normal rats.

Our previous data reported a maximal expression of the c-fos immediate-early gene in the lumbar spinal cord of the non-stimulated polyarthritic rat neurons, three weeks after Freund's adjuvant injection. The present study utilises c-fos expression to judge the reactivity of spinal neurons to calibrated mechanical pressure applied to the ankle joint, in both normal and arthritic rats under ketamine anesthesia. The results indicate that the number of Fos-like immunoreactive neurons (1) is slightly decreased in ketamine-anesthetized non-stimulated arthritic rats as compared to the non-anesthetized non-stimulated ones, (2) is significantly higher in both stimulated normal and arthritic animals as compared to non-stimulated animals, particularly in laminae I, II, V and VI of L3 and L4, and (3) is significantly increased in stimulated arthritic as compared to stimulated normal rats, in all laminae of lumbar spinal segments. The appearance of 'basal' Fos labeling during the adjuvant-induced arthritic disease and the increased number of Fos-like immunoreactive neurons in stimulated arthritic rats compared to stimulated normal animals indirectly suggests that these neurons are abnormally active and thus involved in the hyperalgesia of arthritic disease. Therefore the use of Fos-like immunoreactivity in the chronic pain model seems to be an appropriate tool to study possible effects of various pharmacological compounds, such as analgesics and anti-inflammatory drugs.