ACTIVATION OF HETEROLOGOUSLY EXPRESSED D3 DOPAMINE-RECEPTORS - COMPARISON WITH D2 DOPAMINE-RECEPTORS

Recombinant rat D3 dopamine receptors heterologously expressed in Chin ese 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-link ed receptors. The magnitude of the change in affinity induced by GTP i s much less, however, than what is observed for recombinant rat D2 rec eptors expressed in CHO cells at similar densities. The striking diffe rence is that the low affinity state (uncoupled D3 receptors) has a mu ch higher affinity for agonists than does the low affinity state (unco upled) of D2 receptors. Both receptors in the high affinity state (G p rotein coupled) have similar affinities for dopamine. Three functional responses result from activation of D3 or D2 receptors expressed in C HO cells. Both receptor subtypes mediate inhibition of adenylyl cyclas e activity, increases in extracellular acidification rates that are pr evented by removal of external Na+ and by amiloride analogs, and stimu lation 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; how ever, the D3 response and not the D2 response is partially attenuated by membrane-soluble cAMP analogs. D3 and D2 receptor-mediated stimulat ion of mitogenesis is blocked by pertussis toxin and unaffected by cAM P analogs. The results show that D2 and D3 dopamine receptors mediate similar signaling events and are additional examples of G protein-link ed receptors that can activate more than one pathway. Having functiona lly coupled D2 and D3 receptors expressed in the same cell type enable d determinations of agonist potencies at both D2 and D3 receptors. Com parison of the potencies at the two receptors reveals that none of the agonists is as selective for D3 receptors as was previously thought b ased on radioligand binding data.