A Canadian study showed that one week of oral treatment using several antibiotics in adult mice induced modifications in the microbiota, a state of anxiety and a rise in the levels of the protein implicated in growth and survival of neurons of the hippocampus and the tonsils. Arrest of the antibiotic enabled the rodents to resume their normal behavior.

This same team observed that mice that did not have this same spontaneous behavior differed in their gut composition. Some were shy and anxious, while others showed a strong tendency to explore their environment. Reversing the gut flora by transplantation caused their behavior to be modified: the shy mice became brave explorers, and vice-versa!

Active substances in the brain seem to be produced by the microbiota.

But how does the brain interact with the flora composition?

In mice that were rendered anxious and sensitive to stress (by ablation of the olfactory bulbs), scientists demonstrated a change in the microbiota as well as an increase in the intracerebral rate of a stress neuromediator (corticotrophin-releasing factor, or CRF) released by the hypothalamus. Injection of this same neurotransmitter into normal mice created the same effects on their flora. In both cases, a change in colon motility was observed.

Numerous questions remain and much work lies ahead (but is also currently in progress). What are these neuroactive substances? From which microorganisms do they originate? What are their targets? How do the microbiota and certain neuropsychiatric diseases

get tangled up ? And can we manipulate the gut flora for therapeutic reasons?

Much remains to be learned about the gut microbiota.