Page 36 - JCBP-3-3

Journal of Clinical and
Basic Psychosomatics Microbiota in psychosomatic disorders

and metabolic profiles (Table 1), affecting mood and anxiety and stress responses 57,58 (Table 1). Tryptophan
cognitive functions. For example, alterations in bile acid metabolites produced by gut microbiota also play a
metabolism are associated with changes in serotonin and crucial role in brain function and mood regulation.
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dopamine signaling pathways, both critical for regulating Tryptophan is an essential amino acid that serves as a
mood. Dysregulation of bile acid metabolism has been pre-cursor for several bioactive compounds, including
linked to neurodegenerative diseases and mood disorders, serotonin and kynurenine. Approximately 90% of
further highlighting their role in maintaining neurological serotonin is synthesized in the gut, where gut bacteria
health. Furthermore, bile acids are also associated with modulate the availability of tryptophan for serotonin
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metabolic diseases as they regulate hormone secretion, production by enterochromaffin cells. Serotonin is a key
such as glucagon-like peptide-1 and peptide YY, which neurotransmitter in regulating mood, appetite, and sleep,
regulate appetite and energy balance. 41,42 Emerging and alterations in tryptophan metabolism can directly
evidence also links dysbiosis to cardiovascular diseases, influence its availability. Reduced levels of serotonin
such as atherosclerosis and hypertension. Certain gut are strongly associated with depression and anxiety,
bacteria metabolize dietary nutrients into compounds underscoring the importance of tryptophan metabolism
like trimethylamine-N-oxide, which is associated with a in mental health. In addition to serotonin, tryptophan is
higher risk of cardiovascular events. In addition, dysbiosis- metabolized along the kynurenine pathway, which is also
induced inflammation also plays a role in the development influenced by gut bacteria. Kynurenine and its downstream
of atherosclerosis and other cardiovascular conditions. 47,48 metabolites have neuroactive properties and can either
be neuroprotective or neurotoxic, depending on the
2.3. Neurotransmitter synthesis balance between metabolites. 60,61 For example, kynurenic

The gut microbiota influences brain function and mood acid, a product of this pathway, acts as a neuroprotectant
by producing neurotransmitters such as serotonin, by antagonizing excitatory neurotransmitter receptors,
dopamine, and gamma-aminobutyric acid (GABA) while quinolinic acid, another product, is neurotoxic and
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(Figure 2). For example, Lactobacillus and Bifidobacterium contributes to neuroinflammation. Dysregulation of the
species influence GABA production, helping regulate kynurenine pathway, particularly an overproduction of

Figure 2. Mechanisms of gut microbiota-mediated communication with the brain. The image depicts three key pathways through which gut microbiota
influence brain function. (i) Bacteria releasing active neurotransmitters: Microbial species in the gut lumen directly produce active neurotransmitters,
which can cross the intestinal barrier into the lamina propria and subsequently enter the circulation to potentially influence the brain by crossing the
blood‑brain barrier. (ii) Signaling through the vagus nerve: microbial‑derived neurotransmitters are sensed by the vagus nerve, transmitting signals
from the gut to the brain without systemic circulation. (iii) Precursor‑mediated signal: gut bacteria release precursors of neurotransmitters, which are
absorbed and metabolized by host cells to produce active neurotransmitters. These host-derived neurotransmitters can then act locally or influence the
brain systemically. Each pathway highlights a unique microbiota-brain communication mechanism crucial for maintaining physiological and neurological
health. Image created using BioRender.

Volume 3 Issue 3 (2025) 30 doi: 10.36922/JCBP025040008