CHARACTERIZATION OF RECOMBINANT SEROTONIN 5-HT1A RECEPTORS EXPRESSED IN CHINESE-HAMSTER OVARY CELLS - THE AGONIST [H-3] LISURIDE LABELS FREE RECEPTOR AND RECEPTOR-COUPLED TO G-PROTEIN

Serotonin 5-HT1A receptors expressed stably in recombinant Chinese ham ster ovary cells have been studied using radioligand binding with the radiolabelled agonist [H-3]lisuride. Competition studies with a range of antagonists versus [H-3]lisuride confirmed that all of the specific [H-3]lisuride binding was to 5-HT1A receptors on the cells. Competiti on studies with the antagonist spiperone and several agonists gave dat a that fitted best to two-binding-site models. The affinities of these competing ligands at the two classes of sites were generally in agree ment with their corresponding affinities determined in previous work w ith either 8-[H-3]hydroxydipropylaminotetralin ([H-3]8-OH-DPAT; labels receptor coupled to G protein) or [H-3]spiperone (labels free recepto r). Saturation analyses with [H-3]lisuride showed that this radioligan d labels a single class of binding sites, but the level of radioligand binding was approximately twice that seen when either [H-3]8-OH-DPAT or [H-3]spiperone was used, [H-3]Lisuride binding was partially inhibi ted by addition of guanine nucleotides, and the extent of inhibition d ecreased as the [H-3]lisuride concentration was increased. This inhibi tion was due to the effect of guanine nucleotide to decrease slightly the affinity of [H-3]lisuride for binding to the 5-HT1A receptors on t he cells. It is concluded that [H-3]lisuride can label both the free r eceptor and the receptor coupled to G proteins but with slightly diffe rent affinities and that these two states of the receptor exist in rou ghly equal amounts in the cells. Agonists generally have a higher affi nity for the receptor coupled to G protein, whereas antagonists, with the exception of spiperone (which has a higher affinity for the free r eceptor), have roughly equal affinities for the free receptor and the receptor coupled to G proteins.