Figure 2:

Dopamine synthesis relies on the conversion of tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA) by tyrosine hydroxylase (TH). Phenylalanine is converted to tyrosine by phenylalanine hydroxylase (PAH). Both TH and PAH require tetrahydrobiopterin (BH4) as a cofactor. Although inflammatory cytokines induce GTP-cyclohydrolase I expression, an enzyme necessary for BH4 synthesis, they also increase free radicals. Reactive oxygen species (ROS) contribute to oxidative stress and reduction of BH4. Inflammatory cytokines can both increase and decrease dopamine synthesis through different pathways. Red arrows indicate inhibitory effects and green arrows indicate excitatory effects on enzymes.