Dopamine is a catecholamine neurotransmitter that functions in the CNS to control locomotor, cognitive, emotional and neurendocrine processes, and in the periphery to modulate cardiovascular, renal and gastrointestinal processes. The biological activities of dopamine are mediated by a family of five GPCRs. The D1 and D5 subtypes couple to Gs to increase intracellular cAMP, whereas the D2, D3 and D4 subtypes couple to Gi to reduce cAMP (Missale et al., 1998). The D2 dopamine receptors have been of particular clinical interest due to their regulation of prolactin secretion and their affinity for antipsychotic drugs. The D2 receptor exists as two alternatively spliced isoforms differing in the insertion of a stretch of 29 amino acids in the third intracellular loop (D2S and D2L) (Giros et al., 1989; Grandy et al., 1989). The cloned human D2L-expressing cell line is made in the Chem-7 host, which supports high levels of recombinant D2 expression on the cell surface and contains high levels of the promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between D2 and its ligands