Figure 2 Monoamine synthesis and metabolism in neurons Figure 2 | Monoamine synthesis and metabolism in neurons. Tyrosine and tryptophan are the initial substrates in the synthesis pathway. Both substrates enter the brain via the large neutral amino acid transporter, and tyrosine is also produced in neurons by catabolism of phenylalanine. Levels of dopamine and serotonin metabolites can be measured in the cerebrospinal fluid and are crucial to the diagnosis of neurotransmitter disorders. Abbreviations: AADC, aromatic L-amino acid decarboxylase; BH2, dihydrobiopterin; BH4, tetrahydrobiopterin; DHPR, dihydropteridine reductase; PLP, pyridoxal 5′-phosphate; PTP, 6-pyruvoyl tetrahydrobiopterin; PTPS, PTP synthase; qBH2, quinoid BH2; VMAT2, vesicular monoamine transporter 2. Ng, J. et al. (2015) Monoamine neurotransmitter disorders—clinical advances and future perspectives Nat. Rev. Neurol. doi:10.1038/nrneurol.2015.172