Figure 1.

Synthesis, release, catabolism, and postsynaptic action of dopamine. Synthesis: The TH enzyme converts L-tyrosine to L-DOPA; then, the AADC enzyme allows the production of dopamine, which is loaded into synaptic vesicles by VMAT-2. Release and recycling: once released in the synaptic cleft, the DAT transporter reuptakes dopamine, which is recycled into synaptic vesicles. Catabolism: Dopamine is degraded by specialized enzymes; the MAO enzyme breaks down dopamine to DOPAC and DOPET. In the synaptic cleft, the COMT enzyme catalyzes dopamine to HVA, which is the main end-product of dopamine degradation. At the post-synapse, dopamine binds with D1-like and D2-like receptors. The D1-like receptor activates the G_αs/olf subunit, which stimulates adenylyl cyclase protein, increasing protein phosphorylation. D2-like receptor, by activating the G_αi/o subunit, inhibits the protein adenylyl cyclase, generating a decrease in protein phosphorylation. TH: tyrosine hydroxylase, L-DOPA: L-3,4-dihydroxyphenylalanine, AADC: L-amino acid decarboxylase, VMAT-2: vesicular monoamine transporter 2, DAT: dopamine transporter, MAO: monoamine oxidase, DOPAL: 3,4-dihydroxyphenylacetaldehyde, ALDH: aldehyde dehydrogenase, DOPAC: 3,4-dihydroxyphenylacetic acid, ADH: alcohol dehydrogenase, DOPET: 3,4-dihydroxyphenylethanol, COMT: catechol O-methyl-transferase, HMPAL: 3-methoxy-4-hydroxyphenylacetaldehyde, HVA: homovanillic acid.