Objective　To investigate the effect of estrogen on the pain score

c-Fos and substance P expressions in the dorsal horn of the spinal cord in the mice following formalin stimulation. 　Methods　Fifteen C57/BL6 mice were randomized to 3 groups: control group (intact mice without estrogen treatment)

OVX+V group (ovariectomized mice given vehicle) and OVX+E group (ovariectomized mice with subcutaneous injection of 2 μg/d 17βestradiol for 10 d). Pain score was used to assay the role of estrogen in affecting pain threshold in the mice following formal

in injected into the right hind paw

and expressions of c-Fos and substance P in the dorsal horn of spinal cord (L

3

to L

5

) in 2 h after injection of formalin was tested with immunohistochemisty to evaluate neuron activity and pain afferent fibers. 　Results　Pain score was increased in ovariectomized mice following formalin stimulation

which was inhibited by estrogen especially in the early stage of secondary phase. The number of c-Fos-like immunoreactivity neuron (FLIN

P0.01) and substance P immunoreactivity in the dorsal horn of spinal cord was increased in ovariectomized mice

which was reversed by estrogen. 　Conclusion　Estrogen exerts analgesic effect in OVX mice

which maybe mediate through modulating sensory neurons and afferent fibers in the superficial dorsal horn of the spinal cord.