5-HT	5-Hydroxytryptamine (serotonin)
5-HTP	5-Hydroxytryptophan
5-MT	5-Methoxytryptamine
AAAD	Aromatic amino acid decarboxylase
AAAH	Aromatic amino acid hydroxylase
ADHD	Attention deficit hyperactivity disorder
ASD	Autism spectrum disorders
ASMT	N-Acetylserotonin-O-methyltransferase
BA	Biogenic amine
BBB	Blood–brain barrier
BP	Bipolar disorder
CD	Crohn’s disease
CNS	Central nervous system
COMT	Caffeic acid O-methyltransferase
EC	Enterochromaffin cells
ENS	Enteric nervous system
HPLC-MS	High performance liquid chromatography mass spectrometry
IBD	Inflammatory bowel disease
IBS	Irritable bowel syndrome
L-DOPA	L-Dihydroxyphenylalanine
MH4	Tetrahydromonapterin
mRNA	Messenger ribonucleic acid
NADPH2	Nicotinamide-adenine dinucleotide phosphate (reduced)
OCD	Obsessive compulsive disorder
PEA	Phenylethylamine
PheH	Phenylalanine hydroxylase
PLP	Pyridoxal 5′-phosphate
ROS	Reactive oxygen species
SCFA	Short-chain fatty acids
SNAT	Serotonin-N-acetyltransferase
T5H	Tryptamine 5-hydroxylase
TrpD	Tryptophan decarboxylase
TrpH	Tryptophan hydroxylase
TyrD	Tyrosine decarboxylase
TyrH	Tyrosine hydroxylase
UC	Ulcerative colitis