HIGH-AFFINITY AGONIST BINDING TO CLONED 5-HYDROXYTRYPTAMINE(2) RECEPTORS IS NOT SENSITIVE TO GTP ANALOGS

Agonists for GTP-binding protein (G protein)-coupled receptors are tho ught to bind with high affinity to the complex of receptor and G prote in. Nonhydrolyzable GTP analogs have been shown to disrupt this comple x and reduce the binding affinity for many agonists. Antagonists are t hought to bind to the receptor whether or not it is coupled to the G p rotein, and therefore binding remains unchanged in the presence of GTP analogs. The binding of the serotonin 5-hydroxytryptamine (5-HT)2 rec eptor agonists serotonin (5-HT) and 4-bromo-2 5-dimethoxyphenylisoprop ylamine is not affected by the presence of GTP analogs when the cloned 5-HT2 receptor is expressed in the 293 human embryonic kidney cell li ne. The same receptor expressed in mouse NIH3T3 cells is partially sen sitive to GTP analogs. Both cell lines have similar proportions of ago nist and antagonist binding sites. and agonist stimulation of both cel l lines leads to a robust increase in phosphoinositide hydrolysis. Dif ferences in GTP metabolism in 293 cells is not likely to be the cause of the observed difference in inhibition of agonist binding, because t he cloned 5-HT1A serotonin receptor expressed in these cells is sensit ive to GTP analogs. The GTP-insensitive agonist binding is best explai ned by the existence of a G protein-receptor complex in 293 cells that is not sensitive to GTP analogs. Such a G protein-receptor complex ma y explain the fraction of agonist binding in the brain that is not sen sitive to GTP analogs.