Bravo et al investigated the effect of administering Lactobacillus rhamnosus (JB-1 strain), typically found as part of the rich culture of microbial gut flora, to untreated mice or to mice that underwent vagotomy (i.e. severing the Nervus vagus below the diaphragm). A possible effect by the bacteria on the expression of the GABA (γ-aminobutyric acid, a neurotransmitter that inhibits signaling when activated) receptor subtypes A and B was assessed.
The study showed that vagotomy of or administration of L. rhamnosus to wild type mice elicited anxiolytic effects: in the open-field test and on the elevated plus maze, mice became more explorative and were thus considered less fearful and anxious.
mRNA levels of GABA_B1b and GABA_Aα2 in the prefrontal cortex amygdala and hippocampus were altered.
- For GABA_B1b, mRNA levels were increased in the cingulate cortex (emotion formation and processing, learning and memory) as well as in the prelimbic cortex (expression of learned fears) and decreased in the hippocampus (memory), amygdala (memory, emotion, decision making) and locus coeruleus (stress and panic).
- For GABA_Aα2, mRNA levels were decreased in the prefrontal cortex and amygdala, and increased in the hippocampus.
Further experiments where L. rhamnosus was administered to mice who had undergone vagotomy were no longer sensitive to the bacteria its anxiolytic effects or expression on the GABA receptor expression. Overall, this illustrates that the overall signaling induced by L. rhamnosus, occurs via the Nervus vagus and involves the hypothalamic–pituitary–adrenal (HPA) axis.
The article emphasized the importance of this finding as there is a relationship between gastrointestinal disorders and stress-related psychiatric disorders such as anxiety and depression: The fact that the microbial flora can affect the response to stress and anxiety through interfering with the HPA seems in agreement with the multitude of recent data that find a role for the microbial flora in the (symptomatic amelioration of) psychiatric disorders.