Sex differences in the traumatic stress response: PTSD symptoms in women recapitulated in female rats

Abstract

Background: Post-traumatic stress disorder (PTSD) affects men and women differently. Not only are women twice as likely as men to develop PTSD, they experience different symptoms and comorbidities associated with PTSD. Yet the dearth of preclinical research on females leaves a notable gap in understanding the underlying neuropathology of this sex difference.

Methods: Using two standard measures of PTSD-like responses in rats, the acoustic startle response (ASR) and dexamethasone suppression test (DST), we tested the effects of traumatic stress in adult male and female rats using two rodent models of PTSD, single prolonged stress and predator exposure. We then examined the neural correlates underlying these responses with cFos and glucocorticoid receptor immunohistochemistry in brain regions implicated in the traumatic stress response.

Results: We now report that adult male and female rats across two models of PTSD show consistent sex-specific responses that recapitulate fundamental differences of PTSD in men and women. Trauma-exposed males showed the well-established hyper-responsive phenotype of enhanced ASR and exaggerated negative feedback control of the hypothalamic-pituitary-adrenal axis, while the same traumatic event had little effect on these same measures in females. Dramatic sex differences in how trauma affected cFos and glucocorticoid receptor expression in the brain lend further support to the idea that the trauma response of male and female rats is fundamentally different.

Conclusions: Two standard measures, ASR and DST, might suggest that females are resilient to the effects of traumatic stress, but other measures make it clear that females are not resilient, but simply respond differently to trauma. The next important question to answer is why. We conclude that males and females show fundamentally different responses to trauma that do not simply reflect differences in resilience. The divergent effects of trauma in the brains of males and females begin to shed light on the neurobiological underpinnings of these sex differences, paving the way for improved diagnostics and therapeutics that effectively treat both men and women.

Keywords: HPA axis; Post-traumatic stress disorder; Predator exposure; Sex differences; Single prolonged stress.

Fig. 1

Timeline of experimental procedures. Experimental timeline begins with daily handling 1 week before (− 7) single prolonged stress (SPS) or predator exposure (PredX) and baseline acoustic startle response (ASR) testing the day before (− 1) SPS or PredX. Rats are left undisturbed for 1 week after SPS or PredX, with post-stress ASR assessed 8 days later, and dexamethasone suppression test (DST) 9 days later

Fig. 2

Single prolonged stress (SPS) affects males and females differently. a Exposure to SPS increased the acoustic startle response (ASR) in male but not in female rats. b Likewise, the dexamethasone (DEX) suppression test (DST) revealed an enhanced sensitivity to DEX only in SPS-exposed males, with DEX significantly lowering CORT levels after acute restraint stress compared to vehicle for SPS-exposed males. DEX treatment of control males failed to significantly reduce the CORT response. Surprisingly, CORT levels of SPS females after restraint stress were comparable, regardless of DEX treatment, suggesting DEX-nonsuppression, a characteristic of depression. Note that DEX lowered baseline CORT levels (0 min) in all groups, demonstrating its effectiveness in both sexes. CORT levels were significantly higher in females compared to males at both time points, as expected. c, d SPS had divergent effects on glucocorticoid receptor (GR) expression in the brain, with GR expression in the paraventricular nucleus of the hypothalamus (PVN) somewhat increased by SPS in males (P = .060) but significantly decreased in females. SPS also affected GR expression in hippocampal CA1/2, decreasing GR in males but increasing GR in females (sex*SPS interaction P = .050). GR expression in the PVN of control females was also higher compared to that of control males. e–g Surprisingly, SPS had no effect on the cFos response of males to restraint stress in the prelimbic (PrL) or infralimbic (IL) subregions of the medial prefrontal cortex (mPFC), basal lateral amygdala (BLA), nor medial amygdala (MeA), but significantly increased cFos responding of females to restraint stress in the PrL, IL, and right BLA of females. The cFos response also showed a sex difference in the mPFC, being lower in control females than control males. Data presented as mean ± SEM. Significance set at P < .05 (indicated by asterisk) for planned pairwise comparisons (Bonferroni). Refer to Additional file 1 for full statistical results

Fig. 3

PredX leads to comparable sex differences in ASR and negative feedback control of CORT. a Only males and not females show an enhanced ASR after PredX exposure, replicating the sex difference found after SPS exposure (Fig. 2a). b Likewise, PredX enhanced HPA negative feedback in males but not females. DEX blocked the stress-induced increase in CORT levels only in PredX males, indicating an enhanced sensitivity to DEX in this group and not in PredX females, paralleling results in the SPS model (Fig. 2b). As expected, CORT levels were significantly higher in female compared to males. Again, DEX lowered baseline CORT levels (0 min) to near zero in all groups, demonstrating the effectiveness of DEX in both sexes. c Unlike SPS, PredX did not affect GR expression in the PVN of either sex, although the baseline sex difference was replicated (see Fig. 2c); females have more GR+ neurons in the PVN than males. These data suggest that GR expression in the PVN may be responsive to only some types of stress. Data are presented as mean ± SEM. Significance set at P < .05 (indicated by asterisk) for planned pairwise comparisons (Bonferroni). Refer to Additional file 2 for full statistical results