Small, But Mighty: The Role of the Gut Microbiome in Social Behaviors

This week's papers give insight into an emerging field in behavioral neuroscience- the role of the gut microbiome in mental health and social behaviors. Both papers acknowledge that stress can cause an imbalance in the gut microbiome, which can lead to gastrointestinal disorders and altered social behavior. However, these papers found in their own research that an existing imbalance in the gut microbiome can lead to susceptibility to stress. So which is it? This is truly a "chicken or egg" matter. To me, it appears to be a vicious cycle. Luckily, these issues can be attacked at the behavioral and gastrointestinal levels.

Buffington et al found that in their model of maternal high fat diets, reinstating only one bacterial population in the offspring was enough to relieve the neural effects of an imbalanced gut microbiome. If this were applied to humans, could a simple probiotic cocktail resolve dozens of cases of depression, autism, and other mental disorders? It seems far fetched considering that the bacterial treatment did not resolve some symptoms such as the repetitive and persevering behavior of marble burying. Then again, a little extra probiotic couldn't hurt to try. In combination with stress management techniques, could this be a new and highly efficient way to treat mental health disorders?

On a different note, I noticed one thing in the Reber et al paper of particular interest. In the "Stress Promotes Colitogenic Dysbiosis" section, they mention that beta diversity, or diversity between samples, increased with chronic subordinate colony (CSC) housing. They seem to attribute this to the effect of stress, but I thought that it would make more sense that this change in diversity across many sampled time points was actually from being exposed to different mice throughout the behavioral paradigm. Buffington et al says that families that co-habitate are known to share gut microbiota. Although the CSC housing in the Reber et al paper is hardly a loving family, isn't it possible that the mice are still sharing important gut microbiota? Furthermore, the paradigm involves changing the dominant mouse at several intervals so that the experimental mice don't habituate. Wouldn't this change introduce novel gut microbiota to their environment? And could this bring a minor beneficial effect to the experimental animals?

11/28: Buffington et al. and Reber et al.

The MHFD offspring have many qualities of ASD, it shouldn’t be thought of as a complete model of ASD. Although obese mothers are 1.5 times more likely to have an autistic child, not all children of obese mothers are autistic. Furthermore, not all of these children have dysbiosis of gut microbiota. So, this is model is relevant for some cases of ASD, but not all. Also, only the social deficits are rescued through restoring gut microbiota. A different treatment would be required to rescue the repetitive behaviors and anxiety associated with ASD.

Since L. reuteri administration to MHFD offspring can rescue the social deficits in the reciprocal social interaction test and the three chamber test, I was wondering if administering the bacteria to the HFD mothers would change the gut microbiota of the offspring. Since no negative effects were seen when MRD offspring were given L. reuteri, it doesn’t seem like there would be any negative effects for the mothers. It would be interesting to do an experiment comparing the social behaviors and oxytocin levels of offspring of RD mothers, HFD mothers, and HFD mothers that had been administered L. reuteri.

Reber et al. found that there was a decrease in reactive coping measures and anxiety when mice were administered their final m. vaccae immunization one week prior to the beginning of CSC housing. They found similar results when the interval between immunization and the start of CSC housing was two weeks. To see if the results are even more long-lasting, I think that a longer interval of time between immunization and CSC housing would be more convincing. In the introduction, they claim that the effects can last up to 12 weeks after administration, so I was confused about why they only extended the interval by one week for the second test.

When the stress coping behaviors were assessed on day 15, it seemed like the effects of m. vaccae seemed to be wearing off. The dominance status and submissive behavior scores were the same for vehicle and m. vaccae treated CSC mice. This made me think that the effects of the m. vaccae bacteria were not strong enough to rescue mice from the stress they were going through, since the results were similar for the one week and two week injection to CSC interval.

However, these results have good translational value for patients that suffer from colitis and stress-related disorders. M. vaccae administration prevented colitis in stressful situations. It would be viable to administer m. vaccae to a human patient suffering from these symptoms, and there have been other human studies in the past that have had promising results in immunoregulation studies.

11/28 - Microbiota

          I found this week's papers to be the most interesting ones we have read this entire semester. They discuss how critical microorganisms are in immunity, the gut, and stress-related behaviors (Reber et al., 2016), as well as in social behaviors (Buffington et al., 2016). I am currently taking a Microbiology course and obviously I was aware of how important microbes are throughout our human body and lifestyle, but I was not aware they were involved to this extent, how strongly they can affect our social lives in addition to our physical being. One of, if not the most important, aspect of these papers is how much this research can help humans in the clinical setting suffering from not only colon disorders like colitis, but also psychiatric diseases like PTSD and other fear/anxiety illnesses. Since microorganisms are found in all areas of the body, and since the brain has pathways projecting to all areas of the body, I wonder how many other diseases in different areas of the body can be treated by probiotics and how far researchers are in this.
          Both papers left me with a lot of questions regarding how exactly the brain modulates immunity and the gut. That being said, it appears to be extremely complex and thus would have made these papers even longer than they already are, I would like to see a whole nother paper (or few) discussing these processes. Overall, I thought both of them had a very well-rounded approach in testing their hypotheses, although they left me wondering why Reber et al. did not use any control groups that included live Mycobacterium vaccae (they just used heat-killed preparations). They mentioned that heat-killed is involved in dendritic function and anti-inflammatory secretions, but they did not mention any disadvantages (or advantages) of using live preparations.

microbiome - joe

Buffington et al vs. Reber et al

Seminar in BioPsych

Fall 2016

Professor Shansky

The most powerful implication of this week’s papers is the readily translatable conclusions, and moreover the fact that the authors addressed such a prominent issue in Western civilization. Normally, the abstract of papers includes a few sentences about the significance of this research. The great thing about these papers is that they were so different from the rest of papers we have read this semester; they spoke to the changes in Western culture (i.e. increases in obesity and detachment from certain bacterial environments in high-income families) and how those changes directly affected the gut — and indirectly the brain. It’s bizarre to think that the brain receives input from and is regulated by things as far detached as the gut. But indeed, the brain is an amazing piece of machinery.

I appreciated that both papers used very many measures to prove that this manipulation had far-reaching effects. Buffington et al used behavioral (social tests), biochemical (oxytocin and DA level characterization), genetic (gene sequencing for dysbiosis), and electrophysiological (recording firing patterns in reward centers) approaches, while Reber et al used similar approaches along with testing to see if this bacterium had other effects on the colon and immunity in general. This is an impressive set of experiments!

One thing I wondered was why Buffington et al showed that the effect they noticed was limited to reconstitution of live and not heat-killed bacteria, while Reber et al used heat-killed bacteria without addressing the potential effects of using live bacteria instead. I tried googling the difference in using either type of bacteria but I could not find a definitive answer. Furthermore, neither paper addressed the entire communication pathway between the gut and the brain, and so it was difficult for me to ascertain the gut’s modulatory effects on the brain, other than to see the implicit effects it was having on, say, LTP in the VTA (Buffington et al) or biosynthetic markers of 5-HT (Reber et al). The craziest — and most mysterious — element of these studies is that one change in the constitution of the microbiome can affect so many different things in the brain (from biogenic amine synthesis to inflammatory microglial response to firing pattern changes to downstream behavioral changes).

Reber et al remarked, when discussing the preventative effects of M. vaccae on stress-induced colitis, that “…immunization with M. vaccae, or similar bioimmunomodulatory approaches, may be useful for prevention of chronic stress/repeated trauma-induced inflammation and subsequent development of somatic and mental disorders,” to which I reply, “the future!” The papers we have gone through in the past few months have gradually opened my eyes to the magnitude of questions there still are to be addressed regarding the brain and its functions. Specifically, I have found that the multitude of ways that the brain can regulate itself as well as ways the environment can regulate it brings a plethora of questions that will keep the curiosity of scientists fueled for as long as my mind can conceive.

cocaine - joe

Holly et al vs Vassoler et al

Seminar in BioPsych

Fall 2016

Professor Shansky

I thought it was interesting that the intro of Holly et al alluded to a 1995 paper by Haney et al regarding sex differences in cocaine self administration, and the paper ended up following up on it by using different measures to assess sex and stress differences to cocaine administration. Effectively, they asked, “what is happening to the dopamine system to lead to these changes in cocaine use and what about the female biological system leads to higher cocaine administration?” I think that they’re really interesting questions and probe the infrastructure of addiction; however, the finding was modest and did not answer fully any fundamental questions. I thought it was super interesting that DA levels remained strikingly high for so much longer in stressed females. This says that there is something happening to the dopaminergic system in response to stress; however, this response only occurs in females. Isn’t it crazy the differences in phenotype that can occur from one chromosomal change? Even more bizarre is the fact that these differences are highly neglected in research. It was also interesting to see that despite the higher levels of dopamine in stressed females, those animals were also less satiated (as indicated by their long binges of cocaine infusion). Quick note: did the stressed females have a higher number of total infusions because they binged for longer? Or is that adjusted for binge time? Would it more correctly be titled infusion rate then?

Anyway, where is this dopamine going if so much more is needed for them? And is there desensitization occurring more quickly on the post synapse of DA neurons? Is there a broken negative feedback in these stressed mice? Is cocaine not being biotransformed as well in them? I’ve been told many times that good science addresses a question and is able to prompt a bunch of new questions that can be addressed because of that finding. For example, this paper took differences in cocaine administration in stressed vs non stressed and male vs female mice and asked if there might be some dopaminergic differences between them, and whether that precipitates the behavioral changes. One can follow up on this by addressing any of the above questions regarding cocaine drug action or DA modulation.

Regarding the second paper, I think epigenetics is the coolest thing since sliced ham; it adds a very interesting level of complexity to what the genetic code means. I thought it was interesting that they addressed the transgenerational effects of cocaine administration in sires only because of potential confounds of in utero and maternal behavior effects — I think these should both be addressed. I think this especially because they had to account for maternal behavior with their experimental design anyway (because of the differential allocation hypothesis)!

I appreciated that they took into account the potential confound of learning deficits in the offspring with a sucrose learning task. But I also think that it was lucky that the differential male offspring effect that they found was well coupled with the BDNF exon IV transcript expression patterns. But then it was really cool that stopping BDNF signaling in the offspring was sufficient to abolish this tolerance, and furthermore that BDNF increase was accompanied by AcH3 level increases in the sperm of the sires.

11/21 Holly et al. and Vassoler et al.

Holly et al. examined the effects of cocaine in male and female rats after social defeat. In experiment 1, I would have liked to see data presented from more than just 5-10 minutes and 25-30 minutes. I think it would have been more interesting to see when the differences in walk duration occurred across the entire experiment. Also, statistical significance markers seemed confusing and crowded in the figure. It might have looked cleaner if they broke it up into multiple figures. In experiment 2, when tonic dopamine was being analyzed, estrous and non-estrous females weren’t separated. I think that separating the females by estrous cycle would strengthen the data because they talked about how their research is important because not much work has been done in the past about cocaine addiction and circulating levels of estradiol in females.

In the discussion, the authors addressed that the social defeat paradigm in males is different than females because of the body locations that the aggressors attacked. They said that there was no difference in stress response. However, I was wondering if the differences in cocaine response were due to the differences in social defeat. In the future, it would be interesting for another set of experiments like these to be done with a different stressor to see if the differences in response to cocaine were from the differences in social defeat stress.

Vassoler et al. examined the effects of paternal cocaine self-administration on offspring. The day following 60 days of cocaine use, rat fathers mated with females. The researchers investigated the differences in cocaine-sired male and female offspring’s response to cocaine. In the discussion, the authors mention that circulating gonadal hormones, including estrogen, may play a role in the differences. The Holly et al. paper showed that higher levels of estradiol enhance the effects of cocaine, so some of the sex differences found in this experiment may be because of that.

Also, it would be interesting to see what effects paternal cocaine use would have if the fathers had been abstinent from cocaine use for a period of time before mating. This would show if the harmful effects of cocaine on paternal sperm are chronic or acute.

Nature vs. Nuture- Transgenic and Environmental Factors in Cocaine use

 This week's papers address sex differences in both transgenic and environmental factors on cocaine usage. I found it interesting that while one paper found results consistent with human epidemiological studies, the other paper's results were counter intuitive.

Holly et al used episodic social defeat stress to explore the effects of stress on cocaine usage and sensitization. One issue that is addressed with the use of social defeat stress is that male and female aggressors attack in slightly different ways and therefore, the experimental animals may experience different levels of stress. The researchers claim that there were no differences in stress responses such as weight gain, corticosterone levels, and behavioral changes. However, this data was not shown and it appears to be anecdotal. An additional experiment that they could have included would be a social interaction test to see if the experimental rat did in fact learn to be afraid of the aggressor. In order to completely avoid the confound of sex differences in aggressive behavior, they could have also used chronic immobilization stress instead of episodic social defeat stress.

Vassoler et al took a transgenic approach to measuring the effects of cocaine resistance and sensitivity. They specifically focus on a pathway of increased BDNF in the mPFC which is at least partially a result of histone H3 acetylation. These transgenic changes cause male offspring, but not female offspring, to have resistance to cocaine addiction. However, because these rats are not part of a complex society, nor did they experience the environmental factors relevant to humans, the results were counter intuitive to what one may expect. In human epidemiology studies the offspring of cocaine users were actually more likely to become users themselves. I am curious to know if the increased BDNF levels found by Vassoler et al altered dopamine levels in the nucleus accumbens. Because dopamine is highly important for reward mechanisms, I wonder if the increased BDNF in the mPFC causes a decrease in dopamine that would cause cocaine to be less rewarding.