New role of serotonin as a biomarker of gut–brain interaction

Serotonin, often dubbed the feel-good neurotransmitter, is widely recognised for its role in mood regulation within the brain. However, a significant majority—approximately 90%—of the body's serotonin is synthesised in the gastrointestinal (GI) tract, underscoring its pivotal influence on digestive health (Gershon, 2013). This intricate relationship between serotonin and the gut has profound implications for both physical and mental well-being.

Serotonin's role in gastrointestinal function

Within the GI system, serotonin is primarily produced by enterochromaffin cells lining the intestinal walls (Furness, 2012). Upon the presence of food or other stimuli, these cells release serotonin, which then binds to specific receptors, orchestrating a series of actions:

• regulation of motility: serotonin modulates peristalsis—the rhythmic contractions that propel contents through the digestive tract (Terry & Margolis, 2017). An optimal balance ensures smooth transit, while imbalances can lead to conditions such as irritable bowel syndrome (IBS). Research indicates that altered serotonin signalling is implicated in IBS, affecting bowel habits and sensitivity (Gershon, 2013).

• secretion control: it influences the secretion of digestive enzymes and fluids, facilitating efficient nutrient breakdown and absorption (Furness, 2012).

• sensory signalling: serotonin communicates sensory information from the gut to the central nervous system, playing a role in sensations like nausea and fullness (Spohn & Mawe, 2017).

The gut-brain axis: a bidirectional communication

The concept of the gut-brain axis highlights the continuous dialogue between the gastrointestinal system and the brain (Cryan et al., 2019). Serotonin serves as a crucial mediator in this communication network:

• mood and mental health: emerging studies suggest that gut-derived serotonin can influence mood disorders (O'Mahony et al., 2015). For instance, research involving mouse models has shown that increasing serotonin levels in the gut epithelium can alleviate symptoms of anxiety and depression. This finding opens avenues for potential treatments targeting gut serotonin to manage mental health conditions (Zhang & Li, 2021).

• impact of antidepressants: selective serotonin reuptake inhibitors (SSRIs), commonly prescribed for depression, increase serotonin levels. However, their influence isn't confined to the brain. Studies have found that SSRIs can affect the gut microbiota, leading to changes in its composition and function (Clarke et al., 2013). This interaction may explain some gastrointestinal side effects experienced by patients on these medications.

Serotonin, gut microbiota and immune function

The gut microbiome—the diverse community of microorganisms residing in the intestines—plays a significant role in modulating serotonin levels:

• microbial influence: certain gut bacteria can influence serotonin production (Yano et al., 2015). An imbalance in these microbial populations may disrupt serotonin synthesis, potentially contributing to mood disorders and gastrointestinal issues.

• immune modulation: serotonin has been shown to interact with the immune system within the gut. Alterations in serotonin signalling can affect immune responses, which may have implications for conditions like inflammatory bowel disease (IBD) (Reigstad et al., 2015).

Clinical implications and future directions

Understanding the multifaceted role of serotonin in the gut offers promising avenues for therapeutic interventions:

• targeted therapies: developing treatments that modulate serotonin levels specifically within the gut could provide relief for both gastrointestinal and mood disorders, potentially reducing systemic side effects associated with current medications (Terry & Margolis, 2017).

• diet and lifestyle: dietary choices and lifestyle factors that promote a healthy gut microbiome may enhance serotonin production, contributing to improved digestive and mental health (Zhang & Li, 2021).

Conclusion

In conclusion, serotonin's influence extends far beyond mood regulation, playing a central role in gastrointestinal function and the intricate communication between the gut and brain. Ongoing research continues to unravel these connections, paving the way for innovative approaches to health and wellness.

References

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