Estradiol-induced antinociceptive responses on formalin-induced nociception are independent of COX and HPA activation

Abstract

Estrogen modulates pain perception but how it does so is not fully understood. The aim of this study was to determine if estradiol reduces nociceptive responses in part via hypothalamic-pituitary-adrenal (HPA) axis regulation of cyclooxygenase (COX)-1/COX-2 activity. The first study examined the effects of estradiol (20%) or vehicle with concurrent injection nonsteroidal antiinflammatory drugs (NSAIDs) on formalin-induced nociceptive responding (flinching) in ovariectomized (OVX) rats. The drugs were ibuprofen (COX-1 and COX-2 inhibitor), SC560 (COX-1 inhibitor), or NS398 (COX-2 inhibitor). In a second study, estradiol's effects on formalin-induced nociception were tested in adrenalectomized (ADX), OVX, and ADX+OVX rats. Serum levels of prostaglandins (PG) PGE(2) and corticosterone were measured. Estradiol significantly decreased nociceptive responses in OVX rats with effects during both the first and the second phase of the formalin test. The nonsteroidal antiinflammatory drugs (NSAIDs) did not alter nociception at the doses used here. Adrenalectomy neither altered flinching responses in female rats nor reversed estradiol-induced antinociceptive responses. Estradiol alone had no effect on corticosterone (CORT) or prostaglandin levels after the formalin test, dissociating the effects of estradiol on behavior and these serum markers. Ibuprofen and NS398 significantly reduced PGE2 levels. CORT was not decreased by OVX surgery or by estradiol below that of ADX. Only IBU significantly increased corticosterone levels. Taken together, our results suggest that estradiol-induced antinociception in female rats is independent of COX activity and HPA axis activation.

Figure 1. Effects of estradiol

A. Estradiol significantly reduced flinching when totaled over the entire session (F(1,89)=13.17, p<.001). B. Effects of hormone treatment over bins. In addition to a significant bin effect (denoted by horizontal line with asterisk in all figures), there was a bin X hormone interaction (F(14,1256)=3.264, p<.001). Asterisks above the curves denote differences within the bin. N=41 and 50 for the estradiol (EB) and cholesterol (Chol) groups respectively.

Figure 2. Effects of ibuprofen and estradiol

A. There was an overall effect of experimental condition. In this and subsequent figures, the same letter above pairs of bars identifies significant differences in the posthoc analyses. The difference between veh- estradiol and 40-estradiol showed a trend (p=.052). B. There was a significant effect of bins but no bin by experimental group interaction. N = 9–11 per group.

Figure 3. Effects of NS-398 and estradiol

A. There was an overall effect of experimental condition. The reduction in flinching induced by NS-398 in the cholesterol (Chol) treated animals was a trend (p=.054). B. There was a significant effect of bins and only a trend for a bin by experimental group interaction (p=.057). N= 13–15 per group.

Figure 4. Effects of SC-560 and estradiol

A. There was a significant effect of experimental condition [F (1, 54) =3.78; p=.005]. B. There was a significant effect of bins but no bin by experimental group interaction. N= 17–18 per group.

Figure 5. Effects of adrenalectomy, ovariectomy and estradiol

A. There was a significant effect of experimental condition [F (5,52) =4.912; p=0.001] but that was entirely due to effects of estradiol. Only the comparison between same surgical groups given cholesterol or estradiol are shown. In addition to the significant difference between OVX and the ADX groups with and without estradiol (“a” and “b”), the two OVX/ADX groups with and without estradiol, differed with a .068 probability (denoted by the letter “c”). B. There was a significant effect of bins and a significant bin by experimental group interaction (F(70,728)=1.462, p=.011). The major effect was of estradiol during the second phase. N= 10 except the ADX/OVX/EB group which had N=8.