OBJECTIVETo investigate whether formalin inflammatory pain can induce the change in heme oxygenase-1 (HO-1) expression in the spinal cord of rats or not and the time course character.

METHODS42 SD rats were divided into 7 groups (n = 6): control formalin 6 h, formalin 12 h, formalin 1 d, formalin 2 d, formalin 3 d and formalin 7 d groups. Rats were subcutaneously injected with 0.2 ml 0.5% formalin into the ventral surface of right hind paw to induce periphery inflammatory pain. The immunohistochemistry was used to observe the expression of HO-1 protein in laminae I - II of the spinal cord dorsal horn and the area around canalis centralis of the I5 spinal segment of rats.

RESULTSThere are rare HO-1 immunoreactive cells in the laminae I - II of the dorsal horn and the area around canalis centralis of the I5 spinal segment of rats of control group and HO-1 immunoreactive cells were light in staining degree. Comparing with control group, the numbers of HO-1 immunoreactive cells in the I - II laminae of dorsal horn and area around canalis centralis were increased in the rats at 6 h after formalin injection. The number and staining degree of HO-1 immunoreactive cells were further increased at 12 h and peaked at 1 d after formalin injection. They didn't return to normal level at the 7th day. There were no difference in right and left dorsal horn in the number and staining degree of HO-1 immunoreactive cells at the same time after formalin injection.

CONCLUSIONFormalin inflammatory pain induced increased expression of HO-1 in the spinal cord dorsal horn and the area around canalis centralis of rats. At 1 d after injection of formalin, the increased expression of HO-1 was the most obviously.