Therefore, the effects of prebiotics such as fructooligosaccharides and galactooligosaccharides, or probiotics such as Lactobacillus or Bifidobacterium, on behaviour in these animal studies may be a result of the restoration of neuroplasticity brought about by increased serotonin and BDNF levels in key brain regions involved in depression pathophysiology, however colonic serotonin modulation by the gut microbiota appears to be host species-dependent and separate to the CNS, while the mechanism behind the modulation of CNS BDNF levels by the gut microbiota remains to be elucidated. This evidence concerns the gene BDNF and major depressive disorder.