Effect Of Tramadol On Formaldehyde-induced Fos-protein, Nitric Oxide Synthase And Somatostatin Of Spinal Cord In Rats

The dose-dependent analgesic activity of tramadol has been demonstrated in mice and rats by using various tests of analgesia including hot plaU tail-flick response > abodominal constriction and pain induced by formalin. Opioid and nonopioid mechanisms of action of tramadol are thought to act synergistically on descending inhitory pathways in the central nervous system, resulting in the modulation of nociceptive neurons in the spinal cord.Objective: The genetic product of c-fos proto-oncogene has been accepted as a morphological marker to assess the effectiveness of analgesia. In the present study, we used Fos expression as a biochemical correlate of behavioural indices to test whether or not tramadol suppressed spinal sensitization in the rat formalin model and to observe the possible mechanism.Method: Awake rats received a subcutaneous 100 microliters injection of 5% formalin into the plantar region of the right hindpaw. 10 min prior to the formalin injection, the rats received tramadol (2.5, 5, 10, 20mg/kg s.c. ) or saline vehicle. To test the preemptive analgesic effect oftramadol, a postinjury group was set which tramadol (lOmg/kg) was administered 10 min after the formalin injection. In the behavioral study, licking /bitting behavior was evaluated 0-60 min after hindpaw formalin injection. Two hours late, rats were killed and perfused, L3-5 sects of spinal cords were dissected, sliced at 30 u m , and processed by immunostained with a rabbit polyclonal antiserum directed aganist c-fos. The changes of FLI positive neurons in the dorsal horn were studied. To observe the possible mechanism of tramadol, NADPH-d histochemistry^ Fos/NADPH-d double-labeling method and Somatostatin(Som) immuno-cytochemistry were used to study their changes in the dorsal horn.Result: (1) Formalin-induced Fos-LI positive neurons were mainly located in the superficial dorsal horn (laminae I and II) and in the neck of the dorsal horn (laminae V and VI) ipsilateral to the formalin injection. The results were consistent with the previous reports. (2) Tramadol treatment markedly reduced formalin-evoked nociceptive behavior and decreased the numbers of FLI positive neurons of the dorsal horn in a dose-dependent manner. The total number of fos-like immunoreactivity neurons (FLIN) of the preinjury group (Tramadol 5, 10, 20mg/kg) are 86.09% (PO.05), 60.75% (P<0.01), 38.04% (PO.01) of control respectively. (3) The effect of tramadol preinjury is superior to tramadol postinjury both in behavioral experiments and spinal Fos suppression (P<0.05). (4) lOmg/kg and 20mg/kg tramadol preinjury decreasedNADPH-d positive neurons > Fos/NADPH-d double-labeled neurons and Somatostatin-like immunoreactivity (Som-LI) positive neurons in the ipsilateral spinal cord and the differences were significant compared to saline vehicle (P<0.05).Conclusion: (1) The formaldehyde-evoked nociceptive behaviors and Fos expression were suppressed by tramadol administration in a dose-dependent manner; (2) The effect of tramadol given before pain stimulation was better than that given after formalin injection; (3) Tramadol enhanced the activity of brainstem descending pain modulation system, and inhibited the inputs of peripheral pain afferent message at spinal cord level.