what are the four major categories of neurotransmitters?
['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.']