Structural subtypes of the dopamine D2 receptor are functionally distinct: expression of the cloned D2A and D2B subtypes in a heterologous cell line.

Dopamine, a major neurotransmitter in the mammalian nervous system, exerts its physiological effects through receptors of the G-protein-coupled receptor superfamily. Two major classes of dopamine receptor, D1 and D2, are distinguishable by both biochemical and pharmacological criteria. D1 receptors activate adenylyl cyclase, whereas the D2 class of receptors inhibits this second messenger system. Two subtypes of the human dopamine D2 receptor are generated by alternate splicing of the RNA transcript of a single gene. These two forms, termed D2A (long) and D2B (short), differ by the insertion of 29 amino acids within the putative third cytoplasmic loop, an intracellular domain thought to have a role in coupling this class of receptors to particular second messenger systems. We report here that the D2A and D2B structural subtypes are also functionally distinct. Expression of the two subtypes in a fibroblast cell line revealed that while occupation of both receptors leads to an increase in cytosolic free calcium concentration, they differ in their capacity to inhibit cAMP production. At physiological dopamine concentrations, the D2B-mediated inhibition of calcitonin gene-related peptide-stimulated cAMP accumulation is almost double the response mediated by the D2A subtype. Furthermore, the D2B subtype can maximally attenuate cAMP accumulation by up to 85%, whereas the D2A subtype is less effective, maximally inhibiting cAMP accumulation by only 64%. The D2A and D2B subtypes, thus, constitute functionally distinct forms of the dopamine receptor that can couple to multiple intracellular signalling pathways.