Social behavior is common to all animal species and refers to interactions between individuals of a species. Studying social behavior is crucial to understanding how organisms evolve and contribute to their fitness. In humans, social behavior is affected by neurological diseases such as schizophrenia, cerebral palsy, and autism spectrum disorders (ASD). Recently, studies connecting the gut microbiome to brain function and development have shed light on how gut microbial composition might directly impact social behavior.
A new study has revealed the underlying mechanism of this connection by understanding how gut bacteria can modify neuronal circuits in the brain. The study used zebrafish as a model system to observe social behavior. Zebrafish are naturally social animals that exist and move together in groups of 4-12. This social grouping behavior is commonly referred to as shoaling and is a straightforward measure of social behavior. Moreover, the transparent nature of the zebrafish body helps scientists visualize their concomitant development easily.
Scientists in the study generated a “germ-free” experimental group of zebrafish and compared it to a control group containing normal gut bacteria. The group containing normal gut bacteria exhibited shoaling behavior about 15 days after birth, whereas the germ-free mice did not. Interestingly, scientists also observed structural differences in brain development. The neuronal connections in the brain were different between the two groups. In germ-free mice, the neurons responsible for social development appear to become more tangled and make unproductive connections.
How can bacteria physically present in the gut influence neuron growth and connections in the brain? One possibility is that bacteria might send “signals” through chemicals or other signal molecules that may modify how these neurons develop in early life.
Can human social behavior be similarly affected by gut bacteria during brain development? The existence of a so-called gut-brain axis is well-studied in humans. Studies have shown that gut bacteria can impact mood, depression, and other mental and cognitive functions. Hence, the same social neurons influenced by bacterial signals in zebrafish could also be affected during human brain development. However, it remains to be seen if gut bacteria can alter human brain development as it does in zebrafish. Establishing this connection will significantly impact the prognosis of neurodevelopmental disorders like autism spectrum disorder (ASD) and ADHD. No interventions currently exist for these conditions soon after birth because a correct diagnosis could happen as late as seven years after birth. While early intervention seems exciting and powerful, scientists in the study note that the human microbiome is complex and poorly understood. However, this study brings us closer to understanding the complexities of the gut microbiome and its impacts.
Sources: Quanta magazine, PLOS Biology