Estradiol replacement in ovariectomized rats is antihyperalgesic in the formalin test

A subcutaneous implant of 17beta-estradiol or progesterone provides steady-state serum hormone levels from 7 to 24 days after implantation and allows the evaluation of the effects of the replacement with these hormones on phase 1 and phase 2 formalin-induced behaviors in ovariectomized (OVX) rats. Graded doses of 17beta-estradiol (5% to 40%) reduce formalin-induced behavior by 35% to 49% during phase 2 but not during phase 1, as measured with an automated formalin apparatus. The maximal response is seen with 20% 17beta-estradiol. The antihyperalgesic effect of 20% 17beta-estradiol is significant at 8 days after implantation and persists at 21 days. In contrast, graded doses of progesterone have no effect on either phase of formalin. The estrogen receptor antagonist tamoxifen completely prevents the antihyperalgesic effect of the 20% 17beta-estradiol implant. Formalin-induced behaviors during phase 2 are significantly less in proestrus females and OVX rats given 20% 17beta-estradiol compared with OVX control rats. Also, the formalin-induced increase in serum corticosterone is attenuated in OVX control rats compared with proestrus females and OVX rats given 20% 17beta-estradiol. These results indicate that estrogen replacement in OVX rats restores the maximal corticosterone response to tonic pain and, by an estrogen receptor-mediated process, inhibits tonic pain.

Hormone replacement (HR) therapy remains a widely used modality. We used a pharmacokinetically based rat HR model that results in continuous physiological levels of 17beta-estradiol to demonstrate the analgesic (antihyperalgesic) effects of estrogen replacement in an inflammatory pain model (formalin). These results suggest a potentially important consequence of HR therapy.