Gut Microbiome and Depression – Pathophysiology | Role of Pre and Probiotics
The fetal gut is sterile, and yet in the average human adult, there are over 100 trillion bacteria in the intestinal tract, which is more than 10 times the number of human cells in the entire human body.
In addition, there are estimated to be over 10 million microbial genes, which is 150 times more than the human genome. [Anglin et al. 2015]
The symbiotic connections between the gut microbiota and the brain are evident, and several pathways have been identified: collectively known as the microbiota-gut-brain (MGB) axis. The MGB axis is bidirectional and includes neural, humoral, and immune mechanisms. [Collins et al. 2012]
Experimental evidence from germ-free (GF) mice models has shown that the gut microbiota has an important role in the development and function of the central nervous system; these findings have then been used to make informed hypotheses on the origins of several human psychiatric disorders:
- HPA stress response – GF mice, have an exaggerated HPA stress response, which can be reversed by administering the probiotic, Bifidobacterium infantis. [Sudo et al. 2004]
- Cognition – GF mice have reduced BDNF expression levels in the cortex and hippocampus, which correlate with memory deficits. [Gareau et al. 2011]
- Social development – GF mice have reduced social motivation and a reduced preference for social novelty. Subsequent bacterial colonisation reverses these social avoidance behaviours. [Desbonnet et al. 2014]
- Mood and affect – GF mice have reduced exploratory behaviours and increased levels of anxiety. [Foster et al. 2013]
The pathophysiology of major depressive disorder (MDD) is complex and involves an imbalance in neurotransmitters, a dysfunctional HPA axis, inflammation, and the MGB axis.
Research suggests that the presence of abnormal microbiota or a dysfunctional MGB axis may directly induce psychiatric disorders such as MDD and therefore correcting these disturbances could alleviate depressive symptoms.