FUNCTIONAL COUPLING OF HUMAN D-2, D-3, AND D-4 DOPAMINE-RECEPTORS IN HEK293 CELLS

The D-2 dopamine receptor is known to be functionally coupled to the inhibition of adenylate cyclase when expressed in a number of mammalian cell lines. However, functional coupling of the recently discovered D-3 and D-4 dopamine receptor subtypes has been more difficult to demonstrate. In this study, human D-2, D-3 and D-4 receptors were stably expressed separately in human embryonic kidney cells (HEK 293). In these cells, activation of D-2, D-3 or D-4 receptors resulted in the inhibition of forskolin-stimulated adenylate cyclase activity in a dose responsive manner. This activation was prevented by pre-incubation of the cells expressing these receptors with the dopaminergic antagonist haloperidol. Radioligand binding studies using [H-3]spiperone confirmed that the atypical neuroleptic clozapine has higher affinity for the human D-4 receptor than the D-3 or D-4 receptors, although only 6-fold higher than the D-2 receptor in this study. In addition, ribonuclease protection studies demonstrated the presence of D-4 dopamine receptor mRNA in human brain regions.