# Gut Microbiota as an Endocrine Organ: Unveiling Its Role in Human Physiology and Health
## 3 Neurohormones and Their Role in Microbial Effects
### Neurohormones: Bridging the Gap between the Brain and Microbial Effects
#### 3.2.1 Gut Microbiota Modulation of Serotonin Dynamics  
The human gastrointestinal tract, bustling with diverse microbial inhabitants, orchestrates a symphony of interactions profoundly shaping serotonin (5-HT) dynamics. Most intestinal serotonin has an endogenous origin (i.e., by enterochromaffin cells) [47], although the gut microbiota significantly modulates serotonin levels, contributing to the intricate network of the gut-brain axis. Studies have revealed alterations in serotonin synthesis and metabolism in germ-free rodents [48]. While postnatal reconstitution of gut microbiota corrects serotonin deficiency, underlining the microbiota's essential role in serotonin regulation [49]. Additionally, specific microbial genera, such as _Erysipelotrichaceae_, _Clostriidales_, and _Terrisporobacter_, have been correlated with tryptophan concentrations in the gut, influencing serotonin pathway metabolites [50]. Disturbances and imbalances in the gut microbiota, which play a role in gastrointestinal symptoms observed in conditions like autism spectrum disorder (ASD) and functional gastrointestinal disorders, have been linked to disturbances in serotonin levels [51]. The Serotonin Transporter (SERT), encoded by the SLC6A4 gene, is a crucial player in serotonin reuptake. The activity and expression of SERT are dynamically regulated by the gut microbiota, influencing serotonin availability in both the gut and other body areas. Research indicates that germ-free conditions increase SERT's colonic expression, potentially as a compensatory response to deficient serotonin synthesis [52]. Spore-forming bacteria colonisation in germ-free mice corrects 5-HT deficiency, promoting TPH1-mediated 5-HT biosynthesis by colonic enterochromaffin cells (ECs) [53]. Furthermore, the microbiota's impact on SERT extends beyond the gut, affecting systemic serotonin levels, emphasising the comprehensive influence of gut microbiota on serotonin transport and availability [48]. Outside the gastrointestinal tract, the gut microbiota regulates systemic serotonin levels, highlighting the far-reaching impact of the gut-brain axis. Studies demonstrate that gut microbiota-induced changes in serotonin levels have implications for neurological and metabolic functions [54]. For instance, the gut microbiota has been implicated in regulating serotonin synthesis in the liver, impacting hepatic steatosis [55]. Understanding the influence of gut microbiota on systemic serotonin levels provides insights into the intricate connections between the gut and various organs, opening avenues for therapeutic interventions targeting microbiota to mitigate disorders associated with serotonin dysregulation. Diets enriched with Chlorogenic Acid (CGA) (for example, coffee, green tea, fruits like citrus, apples, and pears, berries such as blackberries and raspberries, arichokes, tomatoes, and sweet potatoes) contribute to alterations in gut microbial composition, thereby influencing serotonin outflow [56]. CGA supplementation enhances microbiota diversity and affects tryptophan and 5-HT levels, emphasising the bidirectional relationship between diet, microbiota, and serotonin regulation. For instance, high levels of CGA have been associated with an increased abundance of specific genera, such as _Streptococcus_, _Blautia_, _Citrobacter_, and _Lactococcus_, affecting serotonin metabolism [49; 57]. These dietary interventions exemplify the intricate interplay between nutritional components, gut microbiota, and serotonin regulation, showcasing the potential for targeted dietary strategies in modulating serotonin homeostasis. Antibiotic-induced alterations in gut microbiota composition led to changes in gut motility, impacting the spontaneous phasic contractions of colonic longitudinal muscle [49]. The microbiota's role in modifying the development and function of enteroendocrine cells (ECs) further influences gut motility.