Researchers have a growing interest in the gut microbiome, especially its effect on brain health. Scientists have long known the human gut houses trillions of microorganisms, but they thought these bacteria only affected the digestive system. On the contrary, recent research shows a strong link between gut bacteria and neurological disorders, such as autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), and schizophrenia.
A recent study in the journal Cell, by Shelly Buffington and other researchers at Baylor College of Medicine in Texas, shows the alteration of the gut microbiome, from changes in maternal diet, results in offspring with behavioral changes including social interaction problems. The study used germ free (GF) mice, which are commonly used by researchers for experiments of the microbiome because they provide an environment free of bacteria and other microorganisms. These sterile mice offer a controlled base line, which allows researchers to make reliable conclusions about any changes in microbiome. In this study, the researchers controlled the diet of maternal mice, with one group eating a regular diet (MRD)and another group eating a high-fat diet (MHFD).
Interestingly, high fat diet offspring showed decreased levels of oxytocin, a hormone closely associated with social behavior. Compared to regular-diet offspring, the high-fat offspring had fewer neurons reactive to oxytocin in the hypothalamus, a region of the brain where the hormone is synthesized.
In addition to the reduced levels of oxytocin, high-fat offspring showed complications in their dopamine reward system. This reward system is especially important in social interactions, including social attachment and the processing of social cues. Researchers found high-fat offspring did not choose to interact with strangers, whereas regular-diet offspring did. Subsequently, the lack of social interactions led to a decrease in the activity of the dopamine reward pathway. The researchers found this decrease to be long-lasting, suggesting it could be linked to future sociodevelopmental disorders.
In terms of the gut microbiome, genomic sequencing of fecal samples showed high-fat offspring had an extreme reduction of a certain bacterial species, Lactobacillus reuteri. The study’s social results make sense when considering that L. reuteri plays an important role in regulating oxytocin release. In other words, reduced levels of L. reuteri bacteria in the gut of high-fat offspring likely led to decreased levels of oxytocin in the brain, resulting in social problems and an impaired reward system.
Amazingly, when the researchers gave the high-fat offspring a water treatment containing L. reuteri bacteria, it seemed to re-establish social behavior. When the researchers gave the same treatment to regular-diet offspring, there was no change social behavior. This makes sense because they already had sufficient levels of L. reuteri from their mother’s regular, healthy diet.
Another remarkable result from this study was that when high-fat offspring lived in the same cage as regular-diet offspring, they did not display social problems similar to other high-fat offspring. Instead, co-housed high-fat mice showed social behavior similar to their regular-diet counterparts. This can be explained by a normal mouse characteristic: coprophagia. which is feces-eating. While eating the feces of regular-diet offspring, high-fat offspring added L. reuteri to their own guts. Thus, they had normal oxytocin levels, which prevented social problems.
Although this study involved mice, the results have implications for humans since L. reuteri is a probiotic bacterium with known health benefits for humans. According to Probiotic.org, L. reuteri benefits include prevention of infections, treatment of diarrhea, and treatment of urinary tract infection and genital infection. Although the Buffington study does not examine the effect of L. reuteri on the human brain, it demonstrates a striking relationship between the gut microbiome and brain function. The researchers hope that the results of this study can help develop treatments for symptoms of neurodevelopmental disorders, especially problems with social interaction.
Guest article edited by Ross Cummings.