Activation of heterologously expressed D3 dopamine receptors: comparison with D2 dopamine receptors.

Recombinant rat D3 dopamine receptors heterologously expressed in Chinese hamster ovary (CHO) cells are functionally coupled to endogenous G proteins. The affinity of the receptors for agonists is regulated by guanine nucleotides in the same manner as that of other G protein-linked receptors. The magnitude of the change in affinity induced by GTP is much less, however, than what is observed for recombinant rat D2 receptors expressed in CHO cells at similar densities. The striking difference is that the low affinity state (uncoupled D3 receptors) has a much higher affinity for agonists than does the low affinity state (uncoupled) of D2 receptors. Both receptors in the high affinity state (G protein coupled) have similar affinities for dopamine. Three functional responses result from activation of D3 or D2 receptors expressed in CHO cells. Both receptor subtypes mediate inhibition of adenylyl cyclase activity, increases in extracellular acidification rates that are prevented by removal of external Na+ and by amiloride analogs, and stimulation of cell division. However, these three functional results of D3 and D2 receptor activation are both quantitatively and qualitatively different. Dopamine activation of D3 receptors is always 2-5-fold less efficacious than dopamine activation of D2 receptors, despite similar densities of receptors. Both D3 and D2 receptor-mediated increases in extracellular acidification rates are blocked by pertussis toxin; however, the D3 response and not the D2 response is partially attenuated by membrane-soluble cAMP analogs. D3 and D2 receptor-mediated stimulation of mitogenesis is blocked by pertussis toxin and unaffected by cAMP analogs. The results show that D2 and D3 dopamine receptors mediate similar signaling events and are additional examples of G protein-linked receptors that can activate more than one pathway. Having functionally coupled D2 and D3 receptors expressed in the same cell type enabled determinations of agonist potencies at both D2 and D3 receptors. Comparison of the potencies at the two receptors reveals that none of the agonists is as selective for D3 receptors as was previously thought based on radioligand binding data.