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1. AGamma-aminobutyric acid (GABA), the principal inhibitory amino acid neurotransmitter rather than a metabolite
2. BHomovanillic acid (HVA), the principal central breakdown product of dopamine released from nigrostriatal neurons
3. C3-methoxy-4-hydroxyphenylglycol (MHPG), the principal central breakdown product of noradrenaline
4. DVanillylmandelic acid (VMA), a predominantly peripheral breakdown product of adrenal catecholamine release
5. E5-hydroxyindoleacetic acid (5-HIAA), the principal central breakdown product of serotonin released from raphe neurons