Opposing Roles of Estradiol and Testosterone on Stress-Induced Visceral Hypersensitivity in Rats

Abstract

Chronic stress produces maladaptive pain responses, manifested as alterations in pain processing and exacerbation of chronic pain conditions including irritable bowel syndrome. Female predominance, especially during reproductive years, strongly suggests a role of gonadal hormones. However, gonadal hormone modulation of stress-induced pain hypersensitivity is not well understood. In the present study, we tested the hypothesis that estradiol is pronociceptive and testosterone is antinociceptive in a model of stress-induced visceral hypersensitivity (SIVH) in rats by recording the visceromotor response to colorectal distention after a 3-day forced swim (FS) stress paradigm. FS induced visceral hypersensitivity that persisted at least 2 weeks in female, but only 2 days in male rats. Ovariectomy blocked and orchiectomy facilitated SIVH. Furthermore, estradiol injection in intact male rats increased SIVH and testosterone in intact female rats attenuated SIVH. Western blot analyses indicated estradiol increased excitatory glutamate ionotropic receptor NMDA type subunit 1 expression and decreased inhibitory metabotropic glutamate receptor 2 expression after FS in male thoracolumbar spinal cord. In addition, the presence of estradiol during stress increased spinal brain-derived neurotrophic factor (BDNF) expression independent of sex. In contrast, testosterone blocked the stress-induced increase in BDNF expression in female rats. These data suggest that estradiol facilitates and testosterone attenuates SIVH by modulating spinal excitatory and inhibitory glutamatergic receptor expression.

Perspective: SIVH is more robust in female rats. Estradiol facilitates whereas testosterone dampens the development of SIVH. This could partially explain the greater prevalence of certain chronic visceral pain conditions in women. An increase in spinal BDNF is concomitant with increased stress-induced pain. Pharmaceutical interventions targeting this molecule could provide promising alleviation of SIVH in women.

Keywords: Stress; estradiol; hyperalgesia; testosterone; visceral pain.

Figure 1

A: experimental timeline for all groups of rats used in behavioral experiments. B: Struggle time for intact female and male rats during first 3 min of FS each day. There was an overall significant interaction effect. Intact females showed greater struggle time than intact males on day 1. ** p<0.01 vs. males; #### p<0.0001 vs. days 2,3; $$ p<0.01 vs. day 3. n=12–14 per group. Not all rats evaluated for struggle time were tested for the VMR.

Figure 2

Effect of sex on stress-induced visceral hypersensitivity. A,B: Stress increased visceral sensitivity for at least 18 days following the end of the 3 day forced swim paradigm in female rats. N=15. C,D: There was a shorter duration visceral hypersensitivity in males. N=17. *,** p<0.05, 0.01 vs. baseline; #### p<0.0001 vs. other times except 10 days at 40 mmHg (p<0.01); $, $$$$ p<0.05. 0.0001 vs. 2 days; ^^,^^^ p<0.01,0.001 vs. 18 days; ¥ p<0.05 vs. baseline (t-test).

Figure 3

Effects of hormone depletion on SIVH. A,B: Stress failed to induce visceral hypersensitivity following ovariectomy. N=8. C,D: Stress induced significant visceral hypersensitivity following orchiectomy. ^ p<0.05 (10 d), p<0.001 (6d); $ p<0.05 (18d), p<0.001 (2d), p<0.0001 (6,10d); & p<0.001 (6,18d), p<0.0001 (2,10d); * p<0.05 vs. baseline. N=8.

Figure 4

Effect of hormone addition on stress-induced visceral hypersensitivity. A,B: testosterone administration to intact female rats blocked SIVH. N=10. C: serum testosterone concentration in females + testosterone vs. intact males. D,E: E2 administration to intact males facilitated SIVH. N=7. *,** p<0.05, 0.01 vs. baseline; #### p<0.0001 vs. other times except 2 days at 40 mmHg (p<0.01); $ p<0.05 vs. 10d; $$ p<0.01 vs. 6d; $$$ p<0.0001 vs. 6,10d). F: serum E2 concentration in males + E2 vs. intact females.

Figure 5

Summary of hormonal modulation of SIVH; data are the mean response for post-stress days 2–18. Control (C): intact female or male in the absence of stress. Intact: female or male following stress. T, E2: addition of hormone; OVx, Orch: hormone depletion. *,*** p<0.05, 0.001 vs. control; ## p<0.01 vs. male + E2.

Figure 6

Western blots from TL and LS spinal cord segments 6 days following the end of FS. Two-way ANOVA compared the effect of stress and hormone treatment separately in the TL and LS spinal cord of male and female rats. Significant ANOVAs are reported in the text. Significant comparisons are shown. *, ** p<0.05, 0.01. # p<0.05 vs. oil without stress.