Plasma membranes from Chinese hamster ovary (CHO) cells transfected with the serotonin 5-HT1A receptor have been incubated with full or partial receptor agonists and the photoreactive GTP analog, 4-azidoanilido-[alpha-P-32]-GTP ([P-32]-AA-GTP), to characterize the resulting receptor-G-protein interactions. Subsequent solubilization and immunoprecipitation of the membranes with anti-G(i)alpha-2 or anti-G(i)alpha-3 immunoglobulins revealed that full and partial agonists produce concentration-dependent labeling of the respective G-proteins with [P-32]-AA-GTP. Full agonists of the 5-HT1A receptor [serotonin 5-hydroxytryptamine (5-HT) and 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT)] produced a 7-12-fold increase in the labeling of G(i)alpha-2 and G(i)alpha-3, whereas partial agonists (rauwolscine and ipsapirone) produced a smaller incorporation (2-5-fold) of [P-32]-AA-GTP by the same G-proteins. The concentration of agonist producing half-maximal binding of [P-32]-AA-GTP by G(i)alpha-3 [5-HT, 48 +/- 1 nM; 8-OH-DPAT, 28 +/- 1 nM; ipsapirone, 22 +/- 6 nM] compared to G(i)alpha-2 [5-HT, 124 +/- 38 nM; 8-OH-DPAT, 40 +/- 1 nM, ipsapirone, 82 +/- 7 nM] was lower with all agonists except rauwolscine, where the EC50'S were similar (G(i)alpha-2, 604 +/- 145 nM; G(i)alpha-3, 708 +/- 130 nM). Comparison of the rank order of agonist efficacy in producing [P-32]-AA-GTP binding to G(i)alpha-2 or G(i)alpha-3 with the rank order of adenylylcyclase inhibition revealed a significant correlation between the decrements in G(i)alpha-2 labeling and adenylylcyclase inhibition (5-HT > 8-OH-DPAT > rauwolscine > ipsapirone). In contrast, the rank order of agonist efficacy with respect to G(i)alpha-3 labeling (8-OH-DPAT = 5-HT > ipsapirone = rauwolscine) was weakly correlated to the efficacy of adenylylcyclase inhibition. These results are consistent with the suggestion that partial agonists of the 5-HT1A receptor couple to both G(i)alpha-2 and G(i)alpha-3 with a lower efficacy than full agonists of the receptor. Perhaps more importantly, the present findings suggest that some agonists may be capable of inducing relatively selective coupling of the occupied receptor to available G-proteins.
