Question: what are the four major categories of neurotransmitters?
Answer: Monoamines – such as dopamine, noradrenaline, adrenaline, histamine, serotonin. Amino acids – such as glutamate, GABA (gamma-aminobutyric acid), glycine, aspartate, D-serine. Peptides – such as opioids, endorphins, somatostatin, oxytocin, vasopressin.