A

L-DOPA and 5-hydroxytryptophan are used therapeutically because, unlike dopamine or serotonin themselves, their amino-acid precursors can be actively transported across the blood–brain barrier and then decarboxylated to the respective neurotransmitters within the CNS [Ahlskog, 2001, PMID 11532645; Birdsall, 1998, PMID 9727088].