Science magazine explores the gut-brain connection further with a study that finds gut bacteria can help us get over our fear responses… unless antibiotics have wiped them out:
The research used a classic Pavlovian test: Shock a mouse on the foot while playing a tone and the rodent will quickly learn to associate the noise with pain, flinching whenever it hears the sound. But the association doesn’t last forever. After several sessions of hearing the tone but not getting the shock, the mouse will forget the association, and the sound will have no effect. This “forgetting” is important for people as well; it’s impaired, for example, in those with chronic anxiety and post-traumatic stress disorder.
David Artis, an immunologist and microbiologist at Weill Cornell Medicine in New York City, wondered whether gut bacteria played any role in the learning and forgetting responses. He and colleagues treated mice with antibiotics to totally rid them of the bacteria in their gut, collectively known as the microbiome. They then played a tone and right after gave the mouse a mild shock, doing this multiple times.
All of the animals quickly learned to associate the noise with pain, freezing when they heard the sound. But only mice with normal microbiomes eventually forgot the connection: By 3 days, the noise no longer affected them most of them, whereas the antibiotic-treated mice still reacted, the team reports today in Nature.
They discovered that a region of the brain known as the medial prefrontal cortex was involved in the differences between the treated and untreated mice. Some of the region’s so-called excitatory neurons, which are involved in learning and memory, appeared to be key. When gut microbes were missing, these neurons failed to appropriately form and absorb spines that stick out of these cells, which help with learning and forgetting, the researchers report.