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Résumé : A brief overview of the dopamine pathway The life history of a dopamine molecule begins in the liver, with the synthesis of the precursor tyrosine, and ends in the kidney, with the elimination of the conjugated dopamine metabolites to the urine. Only during a brief and specific interval in its life can a dopamine molecule engage in its proper function, which is the mediation of signaling via activation of dopamine receptors. The chemical structures of molecules in the dopamine biosynthesis and catabolic pathway are illustrated in Figure 1.1. This scheme does not include the catecholamines noradrenaline and adrenaline, for which dopamine is a precursor, since these substances might properly serve as the topic of another book. As presented in Figure 1.1, all the reactants and enzymes in the dopamine pathway seem to be present in the same space. However, in the living organism, molecules and chemical reactions normally occur within strictly segregated spaces, known as metabolic compartments. Transfer of a molecule in the dopamine pathway from one compartment to another may be strictly impeded by diffusion barriers, or may occur via carrier-mediated facilitated diffusion or by ATP-driven active transport. Thus, the schematic pathway for dopamine synthesis in the living organism should be projected onto a model containing cellular compartments. The model proposed by Carlsson (1966) illustrates the blood, extracellular space, intracellular space, and vesicles as distinct compartments (Figure 1.2). Enzymes and transporters conduct the transfer of mass from one compartment to another, here represented as arrows.