Prostaglandin levels, vaginal innervation, and cyst innervation as peripheral contributors to endometriosis-associated vaginal hyperalgesia in rodents

Abstract

Endometriosis is a painful condition characterized by growth of endometrial cysts outside the uterus. Here, we tested the hypothesis that peripheral innervation and prostaglandin levels contribute to endometriosis-associated pain. Female Sprague-Dawley rats (n = 16) were surgically instrumented by transplanting uterine tissue onto mesenteric arteries within the peritoneal cavity to create a model of endometriosis which forms extra-uterine endometrial cysts and vaginal hyperalgesia. Our results describe a significant positive correlation between endometriosis-induced vaginal hyperalgesia and cyst innervation density (sensory, r = 0.70, p = 0.003; sympathetic, r = 0.55, p = 0.03), vaginal canal sympathetic innervation density (r = 0.80, p = 0.003), and peritoneal fluid levels of the prostaglandins PGE2 (r = 0.65, p = 0.01) and PGF2α (r = 0.63, p = 0.02). These results support the involvement of cyst innervation and prostaglandins in endometriosis-associated pain. We also describe how sympathetic innervation density of the vaginal canal is an important predictor of vaginal hyperalgesia.

Keywords: Cyst; Endometriosis; Innervation; Pelvic pain; Prostaglandin; Vaginal hyperalgesia.

Fig. 1

For the group (n=16), Vaginal nociception (AUC units) at baseline (black bar) and 4-6wks post-endometriosis (striped bar) analyzed with (A) all estrous stages combined and then (B) divided and analyzed by estrous stage. With all estrous stages combined (A) significant vaginal hyperalgesia developed 4-6wks post-endometriosis compared to baseline. When analyzed by estrous stage (B) no significant differences were found between baseline and post-endometriosis within each estrous stage. ***p = 0.0001 vs. baseline; two-way RM ANOVA and Bonferroni post-test.

Fig. 2

Correlation between hyperalgesic severity and (A) number of cysts and (B) cyst burden at 4-6 wks post-endometriosis. Spearman's correlation found no correlation between hyperalgesic severity (AUC units) and number of cysts or cyst burden.

Fig. 3

Examples of nerve fiber labeling in the ectopic uterus (cyst) with their associated innervation density mean scores. The top panel shows CGRP (sensory) labeling and the bottom panel shows TH (sympathetic) labeling. Calibration bar is 50 μm.

Fig. 4

Correlation between vaginal hyperalgesia and CGRP-sensory (top panel) and TH- sympathetic (bottom panel) innervation density of the (A) ectopic uterus (cysts), (B) eutopic uterus, and (C) vaginal canal at 4-6 wks post-endometriosis. Spearman's correlation found a significant correlation between hyperalgesic severity (AUC units) and the innervation density of the cysts (sympathetic and sensory) and vaginal canal (sympathetic only). Asterisks indicate a significant correlation. *p < 0.05, **p < 0.005, ***p < 0.0005.

Fig. 5

Correlation between hyperalgesic severity and peritoneal fluid levels of (A) PGE2 and (B) PGF2α at 4-6 wks post-endometriosis. Spearman's correlation found a significant correlation between hyperalgesic severity (AUC units) and PGE2 (pmol/g) and PGF2α (pmol/g) concentrations. Asterisks indicate a significant correlation. *p ≤ 0.05.

Fig. 6

Correlation between peritoneal fluid levels of (A) PGE2 and (B) PGF2α and CGRP- sensory (top panel) and TH-sympathetic (bottom panel) innervation density of the endometrial cysts at 4-6 wks post-endometriosis. Spearman's correlation found a significant correlation between PGE2 concentration (pmol/g) and cyst sensory innervation density. Asterisk indicates a significant correlation. *p = 0.05.