APUD	amine precursor uptake and decarboxylation hypothesis
BBB	blood–brain barrier
BDNF	brain-derived neurotrophic factor
CNS	central nervous system
CREB	cAMP response element-binding protein
GBA	gut–brain axis
GF	germ-free
HPA	hypothalamic–pituitary–adrenal axis
IgA	immunoglobulin A
iNOS	inducible nitric oxide synthase S
IL	interleukin
MAO-A	monoamine oxidase enzyme A
ROS	reactive oxygen species
SCFAs	short-chain fatty acids
TH	tyrosine hydroxylase enzyme
TNFα	tumour necrosis factor-α
TLR2	toll-like receptor 2
TPH	tryptophan hydroxylase enzyme