HIGH-AFFINITY AGONIST BINDING IS NOT SUFFICIENT FOR AGONIST EFFICACY AT 5-HYDROXYTRYPTAMINE(2A) RECEPTORS - EVIDENCE IN FAVOR OF A MODIFIEDTERNARY COMPLEX MODEL

In this study, the relationship between high-affinity agonist binding and second messenger production was examined at native and mutant 5-hy droxytryptamine(2A). receptors. At native 5-hydroxytryptamine(2A) rece ptors all agonists, with the exception of quipazine, discriminated bet ween high- and low-affinity states of the receptor, as determined by a nalysis of competition binding assays. There was no correlation betwee n the ability of selected agonists to label the high-affinity agonist state and to augment phosphoinositide hydrolysis. Quipazine, which did not discriminate between the affinity states of the receptor, behaved as a full agonist. Similar results were obtained when a point mutatio n (F340L) of a highly conserved phenylalanine located in transmembrane domain VI was examined. With the F340L mutant, most of the agonists t ested labeled significantly fewer high-affinity sites, compared with t he native receptor. There was no significant relationship between high -affinity agonist binding and second messenger production. Bufotenine and 4-iodo-3,5-dimethoxyphenylisopropylamine labeled similar percentag es of high-affinity agonist binding sites (22% vs. 26%), but 4-iodo-3, 5-dimethoxyphenylisopropylamine behaved as a full agonist, whereas buf otenine was devoid of detectable agonist activity. The inability of se lected agonists to activate phosphoinositide hydrolysis was not due so lely to lower agonist affinity for the mutant receptor, because the bi nding affinity of quipazine was unchanged by the F340L mutation but qu ipazine had no detectable agonist activity at the mutant receptor. Our results demonstrate that the ability of an agonist to promote the hig h-affinity state of the 5-hydroxytryptamine(2A) receptor is not correl ated with its ability to augment second messenger production. These re sults are consistent with recent models of G protein-receptor function ing (e.g., modified ternary complex model) that predict that additiona l transition states of the receptor-ligand complex are essential for a gonist efficacy.