Impact of Psychological Stress on Pain Perception in an Animal Model of Endometriosis

Abstract

Purpose: Pain in patients with endometriosis is considered a significant source of stress but does not always correlate with severity of the condition. We have demonstrated that stress can worsen endometriosis in an animal model. Here, we tested the impact of a psychological stress protocol on pain thresholds and pain receptors.

Methods: Endometriosis was induced in female rats by suturing uterine horn tissue next to the intestinal mesentery. Sham rats had sutures only. Rats were exposed to water avoidance stress for 7 consecutive days or handled for 5 minutes (no stress). Fecal pellets and serum corticosterone (CORT) levels were measured as an index of anxiety. Pain perception was assessed using hot plate and Von Frey tests. Substance P, enkephalin, endomorphin-2, Mu opioid receptor (MOR), and neurokinin-1 receptor expression in the spinal cord were measured by immunohistochemistry.

Results: Fecal pellets and CORT were significantly higher in the endo-stress (ES) group than endo-no stress (ENS; P < .01) and sham-no stress groups (SNS; P < .01). The ES rats had more colonic damage ( P < .001 vs SNS; P < .05 vs ENS), vesicle mast cell infiltration ( P < .01 vs ENS), and more severe vesicles than ENS. The ES developed significant hyperalgesia ( P < .05) but stress reversed the allodynic effect caused by endo ( P < .001). The MOR expression was significantly reduced in ENS versus SNS ( P < .05) and more enkephalin expression was found in endo groups.

Conclusion: Animals subjected to stress develop more severe symptoms but interestingly stress seems to have beneficial effects on abdominal allodynia, which could be a consequence of the stress-induced analgesia phenomenon.

Keywords: endometriosis; opioids; pain; stress.

Figure 1.

Stress increases anxiety levels. A, All animals were subjected to psychological stress for 7 consecutive days after endometriosis induction. HPT indicates hot plate test; HPT^C, hot plate test cold; VFT, Von Frey filaments. B, Endo-stress animals had increased fecal pellet counts (**P < .01, ***P < .001 vs endo-no stress animals, ^## P < .01, ^### P < .001 vs sham-no stress animals). C, Endo-stress animals had increased levels of corticosterone during the first (^# P < .05 vs sham-no stress animals) and day 4 of stress (**P < .01 vs endo-no stress animals, ^## P < .01 vs sham-no stress animals), which decreased over time (n = 12 ± standard error of the mean [SEM]).

Figure 2.

Effect of stress on nociception. A, There was a decrease in latency of paw withdrawal from before surgery to after surgery/before stress in the sham-no stress group and from before surgery to after stress in the endo-stress group (^% P < .05 vs before surgery). B, Endo-no stress and sham-no stress groups decreased their tolerance to mechanical testing from their first exposure (^% P < .05, ^%% P < .01, ^%%% P < .001 vs day 1). C, Sham-no stress and endo-stress groups demonstrated a decrease in sensitivity after surgery (^%% P < .01 vs day 1), but only the endo-stress group recovered from the allodynic effect after the stress protocol (^%%% P < .001 vs day 1; n = 12 ± standard error of the mean [SEM]).

Figure 3.

Stress increased the size of the vesicles. A, Endometriosis animals exposed to stress developed vesicles with a higher grade of severity. The endo-stress group developed more vesicles with grade 5 (where the vesicle was >6.0 mm; 22.92%) when compared to the endo-no stress group (12.50%). B, There was no significant difference in overall average volume of the vesicles per animal although this tended to be higher in those receiving stress (n = 12 ± standard error of the mean [SEM]).

Figure 4.

Stress worsens colonic damage and increases mast cell infiltration in the vesicles. A, Endo groups have more colonic damage when compared to sham-no stress group (^### P < .001 vs sham-no stress group). The endo-stress group has more damage than the endo-no stress group (n = 12 ± SEM, *P < .05 vs endo-no stress group). B, There was a significant increase in mast cell numbers in the vesicles of endo-stress animals when compared to the endo-no stress animals (n = 30-34 vesicles ± standard error of the mean [SEM], **P < .01 vs endo-no stress animals).

Figure 5.

Endometriosis decreases Mu opioid receptor (MOR) expression in the spinal cord. A, Regardless of exposing the animals to stress, the endometriosis by itself decreased MORs cells labeling in the endo animals compared to the sham-no stress animals reaching statistical significance (*P < .05 vs sham-no stress animals). Representative MOR immunofluorescent staining of the lumbar region in (B) sham-no stress animals and (C) endo-no stress animals (1:100 at 400×; n = 11-12 ± standard error of the mean [SEM]).

Figure 6.

Endogenous opioid and substance P (SP) expression in the spinal cord of rats exposed to stress. A, There were no significant differences in endomorphin-2 expression between groups (n = 10-12 ± standard error of the mean [SEM]). B, Regardless of stress endometriosis by itself tended to increase enkephalin expression in the endo animals compared to the sham-no stress animals (n = 10-12 ± SEM). C, There were no significant differences between groups (n = 10-12 ± SEM).