What is already known
Various neuropsychiatric illnesses differ between sexes in terms of occurrence, symptoms, seriousness. Women tend to have higher rates of stress and fear-related psychopathologies, such as anxiety disorders, depression, and post-traumatic stress disorder. Studies exploring the reasons behind these gender differences have highlighted the impact of reproductive hormones in both humans and animal models. However, it is also likely that the gut microbial communities play a role, given that they vary between males and females.
What this research adds
A recent review examined the role of gut microbiota-brain connections in psychiatric disorders related to stress and fear; the interactions between gut microbiota and sex hormones, specifically focusing on estrogen; and how these interactions impact fear extinction, to identify potential targets for psychiatric treatment.
Conclusions
The gut microbiota-brain axis is described as a bidirectional communication system, linking intestinal functions with sex, hormones, and the emotional and cognitive centres of the brain, to influence both host behaviour and neurological disease pathogenesis. Indeed, changes in fear-related mental health conditions are observed when the gut microbiota is altered or removed.
Anxiety, post-traumatic stress disorder (PTSD), and other stress/fear-related disorders are more prevalent in women with emerging evidence that sex hormones and sexually dimorphic gut microbiota may play a role. However, the mechanisms through which gut-brain interactions might influence pathophysiology and treatment of mental illness are not clear.
A recent review, published in the International journal of psychophysiology, by Maeng and Beumer, highlights the presence of a critical crosstalk between sex hormone status and gut microbiota which might play a role in modulating extinction processes and their neural correlates.
The authors focused on estradiol, which is associated with mental health conditions and has been shown to enhance fear extinction in both women and female rodents.
The gut microbiota-brain axis in mental illness
Multiple studies have noted specific bacterial differences between major depressive disorder (MDD) patients and healthy controls. Differences in gut bacterial composition in anxiety and trauma-related disorders, have also been described, such as generalized anxiety disorder (GAD) and PTSD. Compared to healthy controls, overall reduced bacterial richness and diversity was observed in GAD patients. While members of the phylum Firmicutes were decreased in patients with GAD, both Fusobacteria and Bacteroidetes were increased compared to healthy controls. Other studies revealed that gut microbiota composition disruptions, specifically in the genus Bacteroides, were linked with both depression and anxiety.
Sex differences in the gut microbiota associated with mental illness
Sex differences in the gut microbiome have been described, such as lower levels of Bacteroidetes and higher levels of Actinobacteria and Firmicutes in women compared to men. Sex differences in gut microbiota appear to be associated with mental illnesses that also show a sex bias in prevalence, but there are still few studies examining these differences across neuropsychiatric disorders.
Women with MDD display higher levels of Actinobacteria compared to healthy women, whereas men with MDD exhibited lower levels of Bacteroidetes. Alterations in the gut microbiome are also linked with anxiety disorders, which are more prevalent in women. It must be noted that differences in gut microbiota appear during puberty, suggesting a role for sex hormones, such as estrogen, as critical players underlying these differences.
The “estrobolome”: estrogen and gut microbiota
Estrogen is a steroid-derived sex hormone that drives reproductive cycling, and regulates immunity, stress, anxiety, and fear across a number of species. The main driver of these effects in reproductively mature mammals is estradiol (E2), which is produced mainly by the ovaries but also in adipose tissue and other local sites, including the brain.
Estrogens can modulate and be modulated by gut microbiota and the gene products of gut microbiota that can metabolize estrogens are known as the “estrobolome”. Estrogens are usually excreted from the body via the bile. Gut bacteria, specifically of the genera Bifidobacterium, Clostridium, and Lactobacillus, can produce enzymes such as β-glucuronidases that modify excreted estrogens, causing them to be retained in the body via reabsorption into circulation. This leads to increased estrogen levels which can cause several hormone-linked diseases. Gut microbiota can also produce dehydrogenases that can transform biologically inert estrogens into active ones to be reintroduced into circulation.
Preclinical and human studies in the estrobolome
Animal studies have shown a bidirectionality of estrobolome interactions. Indeed, germ-free female mice appeared to have lower levels of estradiol compared to conventionally raised mice, probably due to the absence of bacterial β-glucuronidases enzymes. Moreover, removal of ovaries in rats produced changes in the gut microbiota with a concomitant decrease of bacterially derived metabolites and a reduction in the richness of the gut microbiota, suggesting that the loss of estrogen induces dysbiosis.
Human studies have provided evidence for a relationship between microbial diversity and estrogen level, with gut microbial diversity increasing with estrogen metabolites, particularly in men and postmenopausal women, whereas there is less evidence in premenopausal women. Women with high levels of estradiol, mostly premenopausal women, showed increased microbial diversity compared to women with low and medium estradiol levels. Increasing estradiol was correlated with an increase in Bacteroidetes and a decrease in Firmicutes, leading women with low estradiol to have a higher Firmicutes to Bacteroidetes ratio. Moreover, women with low estradiol were significantly enriched of Actinobacteria and Bacteroidetes, indicating the correlation between specific taxa and varying estrogen levels.
As observed in mice, removal of ovaries in women, increased the abundance of Firmicutes and women taking hormonal contraceptives also showed significant differences in the abundance of microbial species and functional pathways.
In a small study of men and women, a short course of antibiotics led to a decrease in urinary estrogens, probably reflecting an altered activity of microbial β-glucuronidases, which modify estrogens thus preventing them from being excreted. Β-glucuronidase activity is found extensively in the gut microbiota, particularly in Clostridia, but also other taxa including Bacteroides and Bifidobacteria.
Diet in conjunction with gut microbiota can also affect estrogen levels; phytoestrogens, estrogen-like compounds naturally found in plants we eat, can be converted to active estrogen metabolites by the gut microbiota. Gut microbiota modulate serum, fecal and urinary estrogens in various ways; however, a more complete understanding of the metabolic pathways and enzymes involved in the estrobolome is necessary.
Gut microbiota influences in fear extinction
In animals, the conditioned fear response can be observed as escape, darting, or freezing behavior in responses to the stimulus. In humans, measures of fear include skin conductance response as a psychophysiological readout of sympathetic nervous system activation, and self-reports of threat or anxiety. In fear extinction learning, the cue is presented without the expected stimulus, and with repeated presentations of this, the subject reduces its fear response.
The role of the gut-brain axis is increasingly highlighted in fear-related processes for influencing behavior and modulating critical fear neurocircuitry. Interestingly, the neurocircuitry underlying fear conditioning and extinction also appears to be affected in anxiety and PTSD. One study found that mice administered with an antibiotic mixture had reduced bacterial community and exhibited deficits in extinction learning but no differences in fear acquisition, suggesting that the presence and diversity of gut microbiota is necessary for successful extinction of conditioned fear, but not for fear acquisition. Moreover, recolonization of germ-free mice with microbiota from healthy control mice before weaning, rescued extinction learning, demonstrating that a diverse and functioning gut microbiome is necessary for successful extinction.
Probiotics, which are live, commensal bacteria that can treat dysbiosis and improve gut microbial diversity, have also helped elucidate the role of the gut microbiome on fear behaviors. Probiotic-treated animals exhibited more fear compared to controls during fear memory recall, but there were no group differences during fear conditioning or extinction.
A study in humans examined skin conductance responses in a healthy cohort of men and women using a fear learning and extinction task, and found no relationship between gut microbial composition and extinction learning. However, the authors found significant associations between Firmicutes, Bacteroidetes, and fear learning. As with the animal studies, this study did not investigate sex differences or consider sex hormones, and some of these differences may have influenced the associations (or lack of associations) observed.
The estrogen-gut microbiota-brain connection and fear extinction
Sex hormones can modulate behaviors outside of those related to reproduction, such as learning and memory processes. Estradiol has been reported to influence fear and anxiety behaviors, and enhanced fear extinction memory recall has been observed in periods of higher estrogen concentration or when estradiol was administered exogenously, in both female mice and women. Hormonal contraceptives, which reduce estradiol levels, have also been shown to influence fear extinction, though it is not clear whether they impair or enhance extinction.
It is unclear where and how the gut microbiome may contribute to estradiol’s extinction memory enhancement, although there may be a possible link with its receptor activity. Although the significant bidirectional communication between the gut and brain, including via the vagus nerve, enteric nervous system, and products of gut microbial activity, how estradiol might impact these signalling pathways to regulate fear is also unclear and may hold important sex-specific clinical implications.
Progesterone in the gut microbiome and fear extinction
Progesterone, another ovarian hormone which fluctuates across the menstrual cycle and during pregnancy, also appears to have a bidirectional relationship with, and can be metabolized by, gut microbiota. Estradiol and progesterone are found to increase oral Prevotella, which has also been found increased in the gut microbiota during pregnancy. Furthermore, progesterone administration in female mice with removed ovaries, increased gut bacteria Lactobacillus reuteri, which was associated with improvements in behavioral tests for anxiety and depression.
Divergent studies show that progesterone can improve fear extinction memory recall similar to estradiol in rodents, but can also impair extinction recall, depending on how soon after administration extinction training occurred. Studies have supported the facilitatory effects of ovarian hormones on extinction memory recall in female rodents; while findings in women showed that increased estradiol induced these effects, levels of progesterone did not. This suggests that estradiol might be the critical factor for improved extinction memory recall.
Given the covariation of the estradiol and progesterone fluctuations during the rodent estrous and human menstrual cycles, it may be essential, though difficult, for future studies to draw a distinction between the effects of the two ovarian hormones and the interactions between them.
In conclusion, the standard treatments for anxiety and other disorders related to stress and fear typically involve exposure-based methods and concentrate on acquiring the skills to control irrational or exaggerated fear reactions. The efficacy of these treatments is increasingly associated with fear extinction learning and memory, with changes in symptom severity paralleling changes in fear responding during extinction pre- and post-treatment.
Examination of gut microbiota before and after psychiatric interventions in individuals with PTSD, revealed abnormal gut microbial environment and activity, associated with an increase in lipopolysaccharide (LPS) as a marker for intestinal barrier dysfunction compared to people without PTSD. Following a cognitive processing therapy-based treatment program, PTSD symptom improvement post-therapy was not reflected in changes in microbial metabolites and LPS, suggesting that adjunctive treatment targeting the gut microbiota would support existing PTSD treatments.
Assessment of gut microbiota of MDD patients before and after treatment with antidepressant, revealed an improvement in depressive symptoms after treatment and comparable gut bacterial composition compared to healthy controls.
The significant role of gut microbiome-brain interactions in stress- and fear-related psychiatric disorders provide insight into additional targets that may be considered in psychiatric treatment. However, much of the work has focused exclusively on males, although these disorders occur with nearly twice the frequency in women. As the role(s) of female sex hormones, such as estradiol, play is limited, future studies must be conducted for the development of more personalized treatments that take sex hormone status into account.
A precise understanding of the role the gut microbiome plays in both estrogenic signaling and neuropsychiatric disorders could be immensely impactful for women’s health, as well as those individuals who rely on gonadal hormone-altering treatments.
