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Résumé : Pharmacology and biochemistry The two classes of dopamine receptors, D 1 and D 2 , were first distinguished on the basis of classical pharmacology (Kebabian & Calne 1979). In the late 1980s, the dopamine receptor genes were cloned from rodent brain expression libraries by hybridization with the genes for homologous GTP-binding protein-linked receptors (Neve & Neve 1997). These techniques subsequently revealed a further subdivision of the two classes of dopamine receptor. The D 1 -like receptors consist of D 1 and D 5 , while the D 2 -like receptors consist of D 2 , D 3 , and D 4 receptors. In addition, alternate splicing of the D 2 gene product permits the expression of a short form of the D 2 receptor (D 2 S), and a long form with an additional 29 amino acids (D 2 L) at the third cytoplasmic loop (Giros et al . 1989). Members of these two dopamine receptor classes have considerable sequence homology and broadly similar pharmacology. All of the dopamine receptors possess the 7-transmembrane domains which are characteristic of members of the large family of receptors that are coupled to GTP-binding proteins (G-proteins). Pharmacological specificity of dopamine receptors is imparted by the amino acid sequence of the agonist binding site, and transductional specificity is imparted by the nature of the intracellular G-protein binding domain. Thus, the dopamine D 1 -like receptors are coupled to a G-protein activating cytosolic adenylate cyclase (G s ), whereas dopamine D 2 -like receptors generally inhibit the activity of adenylate cyclase (G i /G o ), although other signaling pathways have also been implicated.