The vagus nerve is necessary for the rapid and widespread neuronal activation in the brain following oral administration of psychoactive bacteria
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There is accumulating evidence that certain gut microbes modulate brain chemistry and have antidepressant-like behavioural effects. However, it is unclear which brain regions respond to bacteria-derived signals or how signals are transmitted to distinct regions. We investigated the role of the vagus in mediating neuronal activation following oral treatment with Lactobacillus rhamnosus (JB-1). Male Balb/c mice were orally administered a single dose of saline or a live or heat-killed preparation of a physiologically active bacterial strain, Lactobacillus rhamnosus (JB-1). 165 min later, c-Fos immunoreactivity in the brain was mapped, and mesenteric vagal afferent fibre firing was recorded. Mice also underwent sub-diaphragmatic vagotomy to investigate whether severing the vagus prevented JB-1-induced c-Fos expression. Finally, we examined the ΔFosB response following acute versus chronic bacterial treatment. While a single exposure to live and heat-killed bacteria altered vagal activity, only live treatment induced rapid neural activation in widespread but distinct brain regions, as assessed by c-Fos expression. Sub-diaphragmatic vagotomy abolished c-Fos immunoreactivity in most, but not all, previously responsive regions. Chronic, but not acute treatment induced a distinct pattern of ΔFosB expression, including in previously unresponsive brain regions. These data identify that specific brain regions respond rapidly to gut microbes via vagal-dependent and independent pathways, and demonstrate that acute versus long-term exposure is associated with differential responses in distinct brain regions.
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