# Gut Microbiota as an Endocrine Organ: Unveiling Its Role in Human Physiology and Health
## 4 Gut Microbiota Metabolites and Hormonal Alterations in Human Physiology
### Gut Microbiota: Behavioural Influences  
Various studies indicate that the gut microbiota is critical in influencing social behaviour [171]. This influence may involve the horizontal transmission of microbes between individuals of the same species, such as in specific _Battodea_, through events like copropha-gia and social bees. _Bifdobacterium_ and _Lacobacillus_ in the gut are essential for SCFA production and nutrition during starvation [172]. The convergence of core gut microbial taxa in Baboons affects social behaviours like grooming [173].  
Mice born from mothers on a high-fat diet exhibit altered microbiota composition, affecting their ability to discriminate between familiar and unknown conspecifics. This defect can be rectified by _Lacobacillus reuteri_, leading to increased oxytocin and improved social conduct [174]. Alterations in the gut microbiota, associated with toll-like receptor (TLR) expression, contribute to the altered response to pathogens. For example, a TLR4-knockout mouse shows no response to lipopolysaccharide-acrylamide (LPS) from gram-negative bacteria [175].  
The Griesofulvin Mouse model, under stressful conditions, exhibits elevated corticosterone and adrenocortic hormone levels, partially reversible by faecal microbial transplant or single _Bifdobacterium infantis_[176]. Timing of microbiota modelling is critical for the precocious maturation of the hypothalamus-pituitary-adrenal axis, with a gender-specific response [177]. Long-term stress leads to significant differences in the gut microbiota composition, with reduced _Bacteroides_ and increased _Clostridium_.  
Experimental models of germ-free and antibiotic-treated animals show macroscopic alterations in neurotransmitter turnover, neuronal morphology, and neuroinflammation, depending on the time of microbiota onset. Microbiota substitution drastically modifies behaviour in rodents. Conversely, supplementation with _Bifidobacterium_ and _Lacobacillus_ improves social behaviour [178].  
The concept of animals as \"holobionts\" [179], dynamic ecosystems comprising a host and associated microorganisms, emphasises the role of the microbiota in stimulating the immune system and affecting phenotypes subject to natural selection.  
There is a suggested interface between the gut microbiota and sleep regulation. The gut microbiome activates intestinal macrophages, inducing cytokine production. The relationship between TNF-\(\alpha\), IL-18, and non-rapid eye movement (NREM) sleep is established [180]. Cortisol inhibits cytokine synthesis, and LPS administration in humans increases inflammatory markers, impacting mood and memory.  
In a depression rat model induced by social defeat stress, changes in faecal gut microbiota were observed, with altered levels of specific bacterial classes. Alcoholics with elevated intestinal permeability exhibit higher depression, anxiety, and alcohol craving, along with changes in gut microbial populations [181].  
The gut microbiota significantly impacts animal and human behaviour, influencing cognitive function, memory, stress response, anxiety, and social behaviour. Conditions like stress-related irritable bowel syndrome (IBS) and autism note connections between the gut microbiota and emotional states. These findings highlight the microbiota's ability to modulate host behaviour, prompting further exploration of underlying functional mechanisms.