Fish oil may be important to altering the microbiome, reducing anxiety

Last week we published a blog on the gut-brain axis, and the various associations between brain health and the gut microbiome.  One of the ailments we discussed was depression, which is often studied in mice by inducing early life stress on the mice.  One way to do this is by separating mice from their mothers for hours at a time at a young age.  The Maternal Separation model, as it is known, causes stress and anxiety in these mice, but more importantly, research has shown that it creates a dysbiosis of their gut microbiomes as well.  Many scientists believe the dysbiosis may be implicated in causing some of the stress phenotypes, and so reversing this dysbiosis could have therapeutic value.  Researchers from the University College Cork, in Cork Ireland, experimented with N-3 polyunsaturated fatty acids (PUFAs), like those found in fish oil, in these maternally separated mice, and found they may be important to preventing the dysbiosis.  They published their findings in the journal PLoS ONE.

In the study, the researchers separated mice into two groups, one underwent maternal separation, and the other had a normal upbringing.  Within each group the mice were separated into two more groups, one that received fish oil supplements and the other that didn’t.  Over the course of 17 weeks each groups’ feces were sampled for their microbiomes.  The Maternal separation tended to decrease the bacteroidetes to firmicutes ratio of the mice’s microbiome, which has previously been linked to depression in humans.  Interestingly, supplementation with the fish oil increased this ratio in those maternally separated mice.  In addition, the fish oil also increased the concentration of bacteria that were higher in non-separated mice, such as populations of Rikenella.  Finally, the fish oil increased the amount of butyrate producing bacteria, and as we have seen many times before, butyrate and other short chained fatty acids (SCFAs) are often associated with health.

Overall this study showed that fish oil shifted stressed mice’s microbiome to a more natural state, presumably helping them in the process.  While the scientists did not directly measure stress levels in these mice to support the microbiome connection, hopefully that will be part of a follow up study.  The scientists noted that fish oil is clinically shown to reduce inflammation, and made it a point to connect the stress in the mice to systemic inflammation.  Systemic inflammation is also mediated by the microbiome.  Indeed, people that have inflammation from IBD, for example, do tend to have more stress and anxiety.  In the end, fish oil could make for an interesting prebiotic to shift the microbiome, counteract inflammation, and improve mental health. 

Early life stress implications on the gut microbiome

A growing body of evidence supports the significance of the gut microbiome with respect to behavioral disorders, as mediated by disruption to the gut-brain axis.  Importantly, there is a lack of understanding regarding associations between gut microbiome dysbiosis and behavioral phenotypic outcome.  Traumatic childhood events early-in-life can result in later-in-life behavioral consequences.  Maternal separation (MS) is an example of such an event that is represented with a well-established preclinical (i.e., animals) experimental model for early life stress.  In a recent study, researcher’s sought to investigate the precise role of MS in the induction of changes to the gut microbiomes of mice, and the potential behavior phenotypic consequences brought on by these changes. 

C57 mice were subject to three unique experimental groups: germ-free (GF) mice, specific pathogen-free mice (SPF, i.e. mice with microbiome compositions), and germ-free mice that were eventually subject to recolonization.  Mice were either left alone after birth or exposed to MS 3 days after birth.  Behavior was examined after 8 weeks, and the germ-free group was sacrificed after 9-10 weeks and the SPF group after 16-20 weeks.  The recolonization group was recolonized with microbiota at 12-13 weeks, followed by more behavioral tests and subsequent sacrificed. 

Corticosterone - a major stress hormone - was significantly elevated in both GF and SPF mice that were subjected to MS.  The researchers next wanted to define a relationship between the host microbiome and anxiety-like behaviors weeks after being separated from the mothers.  It was fist observed that MS did indeed alter microbiota composition in SPF mice.  Interestingly, MS-induced anxiety like behavior was observed in SPF mice, but not in GF mice, suggesting that the microbiome played a significant role in the development of these types of behaviors.  The experimenters then recolonized the guts of a subgroup of GF mice.  Indeed, behavior was altered in MS mice whose guts were colonized as compared to the MS mice that remained germ free.  Other physiological indicators demonstrated significant interactions between MS and the presence of gut microbiota, as gut microbiota presence had a significant impact on noradrenaline and serotonin levels. 

All told, the findings in this study suggest microbiome dysbiosis is a critical physiological driving force behind the behavioral phenotypes associated with early life stress events.  It will next be important to begin thinking about translate these preclinical findings in a clinical setting, in hopes of exploring ways to help those in need.

Probiotics may be able to prevent depression

Clinical studies have shown that probiotics can decrease anxiety, improve mental outlook, and induce positive mood changes and outlook.  In fact, some bacteria strains have been shown to reduce anxiety and depression in mice by directly modulating nerve firings associated with these cognitive maladies.  A recent study conducted by Leiden University in The Netherlands explored further in human subjects to assess whether or not probiotics, composed of various strains of Bifidobacteria and Lactobacilli could specifically modulate cognitive reactivity to sad mood, a well-characterized indicator for vulnerability to depression. 

In brief, cognitive reactivity is defined as a series of dysfunctional patterns of thinking prompted by subtle mood changes, such as rumination, aggression, and hopelessness.  It is thought that cognitive reactivity is central in the development, maintenance, and recurrence of depression episodes.  This behavioral reaction is considered to have significant predictive value in detecting vulnerability to developing clinical depression.  Due to this implication, cognitive reactivity is considered a target for therapeutic intervention to prevent depression onset, and was thus analyzed in this study. 

40 healthy, non-depressed adults were selected and split into two groups, each receiving a 4-week regiment of a probiotic or a placebo.  The participants filled out questionnaires before and after the regiment to assess cognitive reactivity and depression symptoms.  Of the several behavioral indicators of cognitive reactivity that were assessed, aggression and rumination were significantly modified according to the behavioral questionnaires.  Specifically, post-regiment scores in the probiotic group were significantly lowered from pre-regiment scores, and this was not observed in the placebo group.  All told, this suggests that a probiotic regiment eased cognitive reactivity to aggressive and ruminative thoughts.

This study is the first to show that probiotics can modulate an important cognitive process that determines vulnerability to clinically diagnosed depression.  These findings are additionally enlightening with respect to the gut microbiome’s role in overall cognitive health.  As is often the case however, there were some limitations.  Specifically, it would have been interesting to investigate biological underpinnings of these interactions in complementary animal models, especially in light of previous findings that indicate probiotics can facilitate microbiota to synthesize and release serotonin.  On Monday we highlighted work done by Professor Diane Hsiao’s group at Cal Tech that stressed the gut microbiome’s role in serotonin production.  As we mentioned, serotonin is implicated in many bodily functions, including a vast range of cognitive mechanisms.  Indeed, serotonin systems have been primary target for therapeutic treatment of depression.  Zoloft, one of the most highly prescribed antidepressants in the world, blocks serotonin metabolism to facilitate its endogenous mode of action in the brain. 

Not only do the current findings complement those from the Cal Tech lab, but they also highlight an exciting new potential toward therapeutic approach.  Conventional therapies that directly target serotonin systems can be ineffective and have many undesired side-effects and limitations.  Understanding the microbiome’s role in serotonin production can give us more insight and perhaps pave a way toward a more organic therapeutic approach aimed at preventing and/or treating depression.  

An obese-type gut microbiome can lead to neurobehavioral pathology

Squirrel on high fat diet. (close enough to a mouse!)

Obesity is a complex condition with an extensive range of health complications.  Among many other issues, neurobehavioral deficits in learning, memory, and executive function are observed in this disorder.  However, the cause behind the manifestations of these deficits remains unclear, and new data suggest that obesity by itself may not be the origin of these neurobehavioral complications.  In other words, neurobehavioral deficits may not be caused by obesity, but rather by the microbiome that develops from the high-fat diet that leads to obesity.  A recent study supports this supposition, demonstrating that an “obese-specific” gut microbiome may be the driving force behind these neurobehavioral complications.

Researchers hypothesized that microbiome communities that develop from a sustained high-fat diet could by themselves induce neurobehavioral maladies, independent of diet, adipose fat accumulation, and/or metabolic dysfunction.  To test this theory, the researchers developed a paradigm in which microbiota taken from the gut of obese mice were recolonized in the gut of non-obese mice.  Specifically, mice were split into two groups, and members from each cohort were administered either a standard chow diet or a high-fat chow diet (to induce obesity).  After 10 weeks on their respective diets, the animals were sacrificed and their microbiota bacteria were harvested from cecal and colonic contents.  A third group of mice were administered an intense antibiotic regimen to wipe out their intestinal microbiota populations.  Microbiotas from either the normal chow diet mice or high-fat chow diet mice were subsequently implanted in the microbial-free guts of third group.  These mice were then subject to behavioral examinations and eventually sacrificed for biochemical analysis to characterize disease markers and pathology indications in the brain and gut. 

Behavioral assessments revealed significant increases in anxiety and anxiety-like behaviors concomitant to decreases in memory in mice administered the high-fat diet-associated microbiota.  To validate that differences in gut microbiomes were the root cause, analysis of cecal and fecal samples from mice indicated that the gut microbiomes in both high fat diet and normal diet groups had distinct phylogenetic profiles, demonstrating that microbiota populations from each group were indeed distinct. 

Researchers next analyzed biological protein markers associated endotoxins and inflammation in the gut, as well as markers for injury and inflammation in the brain.  Several inflammatory-associated markers were significantly upregulated in the high-fat diet group, indicating disruption to intestinal permeability and inflammation.  Furthermore, expression of inflammatory protein markers in the brain was significantly increased in the mice with the high-fat diet microbiota, and two proteins known to maintain integrity of brain vasculature were significantly reduced.  Additionally, a protein known to be present during normal synaptic function was significantly reduced. 

Collectively, these data link disturbances in gut and brain physiology resulting in behavioral dysfunction with obese-specific microbiota rather than the state of obesity.  Importantly, however, this study reveals a potential therapeutic target to remedy behavioral disorders that many have previously perceived as a consequence of simply being obese.  

Obesity, mental illness, and the microbiome

A study published by Biological Psychiatry studied the neurological effects of gut bacteria typically part of the obese microbiome, because obesity, depression, and the microbiome have each been associated with one another.  To do this, 8-week-old male mice were fed either a regular chow diet or a high-fat diet. The microbiomes of these mice were then transplanted into 3-month-old male mice that were on a regular chow diet and antibiotics (the antibiotics were used in place of germ free mice to keep their gut populations low).  16S sequencing eventually showed successful transplantation of the donor microbiome to the recipient mice.

The recipient mice were subjected to anxiety, exploratory, stereotypical behavioral testing, as well as memory testing, all of which are common techniques that test for anxiety and depression in mice. In addition, the mice’s microbiomes and blood were sampled, and the mice’s guts and brains were investigated post-mortem.

Results of the experiment showed that the recipient mice, which were raised conventionally, showed significant disruption of mental behavior after harboring the gut microbiome of obese mice that eat a high-fat diet.  Furthermore, these mice had lower microbiome diversity, higher gut permeability (i.e. leaky guts), and higher levels of overall inflammation and brain inflammation than mice with the normal chow transplants.  It is not understood exactly how gut bacteria affect behavior, but it is further evidence of the importance of the gut-brain axis and the potential value of prebiotic and probiotic therapeutics for mental health.  

The gut-brain axis - microbiome and depression

Today I will be discussing a review from a little more than a year ago that discusses research that links the microbiome with stress, anxiety, and depression.

First, it should be noted that a 2-way communication between the gut and brain has already been established.  The review goes on to mention studies in mice where not only are germ free mice associated with higher stress responses, but stress and anxiety early in life are associated with long-term changes in the microbiome.  There was efficacy in reducing stress by treating stressed mice with probiotics.  Other studies however have shown germ-free mice to have reduced stress when compared to their counterparts.  In addition, multiple studies showed that inflammation of the gut caused by dysbiosis or a pathogenic gut bacteria increased stress and anxiety levels in mice.

There are many mechanisms by which the microbiome communicates with the brain and may affect the stress levels.  I encourage anyone interested to read the paper to learn more.  In the end, more research is needed to discover just how important the microbiome is to our mental health.