Effect Of Intrathecal Clonidine On C-fos Expression In Dorsal Horn Of Spinal Cord In Formalin Inducing Rats Pain

Objective To observe the nociceptive behavioral responses of intrathecal clonidine against formalin inducing rats pain and the expression of c-fos in dorsal horn of spinal cord, and to investigate the function and the possible mechanism of clonidine in a rat model of formalin-induced pain.Methods Thirty-two male SD rats were randomly divided into 4 groups(n=8 each):Control group(C), Induced-pain group(NS), Medication administration team before inducing pain(Clo1), Medication administration team after inducing pain(Clo2). Peripheral pain was induced by injection of 5%formalin 100μ1 into the plantar surface of left hindpaw in group NS,Clo1 and Clo2.Group NS intrathecal injection with normal saline 20μ1 at 10 min before the formalin-induced pain; Group Clo1 intrathecal injection with clonidine 20μg at 10 min before the formalin-induced pain; Group Clo2 intrathecal injection with clonidine 20μg at 10 min after the formalin-induced pain; 100μ1 Normal saline(NS) was injected into the plantar surface of left hindpaw in group C and injected normal saline 20μ1 intrathecally in group C. Pain behavior was assessed using mean flinching and licking time of the influenced limb,and all the animals were killed at 2 hours after injecting 5% formalin into planta, and taked some tissue in the enlarged lumbar spinal cord, and expression of c-fos in dorsal horn of spinal cord was determinated by immunohistochemistry.Result The flinching and licking time of phaseⅡi n group NS,group Clo1 and Clo2 was much longer than that in group C(p<0.01); The flinching and licking time of phaseⅡin group Clo1 and Clo2 was shorter than that in group NS(p<0.01) and the time of phaseⅡin group Clo1 was shorter than that in group Clo2 .(p<0.01). formalin -induced fos immunoreactive neurons were mainly located in the superficial dorsal horn of spinal cord (laminaeⅠandⅡ)of pain-affected side. The c-fos expression in dorsal horn of spinal cord was significantly higher in group NS than in group C; but was significantly lower in group Clo1 and group Clo2 (intrathecal clonidine)than that in group NS(P<0.01). The c-fos expression In group Clo2 was higher than in group Clo1 (P<0.01).Conclusion The formalin -induced nociceptive behavior and Fos protein are suppressed by Intrathecal clonidine , Mechanism of clonidine may inhibit the the dorsal horn of the spinal cord fos immunoreactive neuron activity which transmit and regulate the pain signal, to stop the conduction of the pain signal in the spinal cord and result in the anti-injury function. Clonidine has preemptive analgesic effect at the spinal level.