Endometriosis-induced vaginal hyperalgesia in the rat: effect of estropause, ovariectomy, and estradiol replacement

Abstract

Endometriosis (ENDO) is a painful disorder defined by extrauteral endometrial growths. It is created in rats by autotransplanting pieces of uterus (which form cysts), or, for shamENDO, fat (no cysts). ENDO induces vaginal hyperalgesia, likely via central sensitization. The severity of this hyperalgesia correlates with estradiol levels during the estrous cycle, suggesting the hyperalgesia is estradiol-modulated. If so, then hyperalgesic severity should track estradiol changes during reproductive senescence (estropause) when estradiol levels initially decrease, then increase. Using psychophysical methods to assess vaginal nociception, we found that the severity of ENDO-induced hyperalgesia paralleled estradiol changes during estropause: hyperalgesia first decreased, then returned. Furthermore, the return occurred regardless of the presence of the cysts (excised in some rats). This finding provides further support for ENDO's likely centrally-mediated effects. Additionally, the results suggest that elimination of estradiol via ovariectomy (OVX) should alleviate ENDO-induced hyperalgesia and estradiol replacement should restore it. However, in healthy and shamENDO rats, OVX produces a vaginal hyperalgesia that is alleviated by estradiol, likely via estradiol's peripheral influences on the vagina. Hence, we tested the hypothesis that OVX in ENDO rats would trigger a different type of vaginal hyperalgesia dependent on the loss of estradiol. We predicted that the opposing influences of estradiol on ENDO- and OVX-induced hyperalgesia would cancel each other. As predicted, OVX had no effect on ENDO-induced hyperalgesia and estradiol replacement alleviated it. These results suggest that, in intact rats, ENDO-induced vaginal hyperalgesia is exacerbated by estradiol, and that different mechanisms underlie ENDO-induced versus OVX-induced vaginal hyperalgesia.

Figure 1

Changes in cycling patterns and plasma E2 levels as rats age through estropause. A and B are modified from Lu et al. (1979), with permission. A shows changes in cycling patterns of Long-Evans rats, whereas B shows how E2 levels change from regular four-day cycles of high and low E2 levels to moderate levels after the Long-Evans rats have stopped cycling and have entered into a constant estrus (CE) condition. Note that E2 levels then increase after several months of CE. Not shown here is the pattern of E2 changes during the transitional phase of irregular cycling. During irregular cycling, ovarian E2 levels are somewhat higher than they are in corresponding parts of the cycle in regularly cycling rats (Peluso et al., 1979). The E2 levels D1, D2, and E are, however, still lower than those in P in regularly cycling rats (Peluso et al., 1979). The table in C lists the pattern of vaginal cytological changes and ages of the rats in the present experiment as they progressed through estropause. Note two features of these rats. First, they progress through estropause at ages about 2 months younger than the rats in the Lu et al. (1979) study. Thus, estradiol levels in the rats in this study during their “late senescence” period are likely higher than their levels during “early senescence.” Second, during the transition period for the rats in this study, the predominant stage was E, indicating that overall the estradiol level in these rats during the transition period was less than it was when they were cycling. P, proestrus; E, estrus; D1, diestrus 1 (sometimes called metestrus); D2, diestrus 2 (sometimes called diestrus).

Figure 2

Percent escape response (A,B,C) and vaginal pressure (D,E,F) to different volumes of distention of the vaginal canal. Results are shown from the three different experimental groups named at the top of the graphs and described in the text. Arrows in A-C indicate vaginal nociceptive thresholds for each condition. The other symbols indicate significant (P < 0.05) differences between conditions for each volume, as follows: *, baseline vs post-ENDO; #, baseline vs OVX; ≈, post-shamENDO vs OVX; †, baseline vs early senescence; @, baseline vs late senescence; ¶, transition vs late senescence.

Figure 3

Percent escape responses for individual ENDO rats in the group that progressed through estropause (A,C) and in the group that had OVX, then OVX+E2 (B,D). A and C show results from two rats. One rat (B, #3480) received no treatment during estropause. The other rat (D, #3533) underwent cyst-removal surgery at the end of her transition period. Note that there are no differences between these two rats in the pattern of change in escape responses as the rats progress through estropause. In both, the hyperalgesia produced by ENDO is reduced during the transition phase, but then returns and increases during early and late senescence. B and D show results from two rats who had been treated with E2 for 6 months. Note that the responses of both rats (#3061 and #3064) increased from baseline after the ENDO surgery (i.e., they developed vaginal hyperalgesia) and that OVX had no influence on the responses. E2 replacement in the first two months reduced their responses to baseline levels (i.e., their hyperalgesia was completely reduced), and this reduction continued for another four months.