Characterisation of recombinant serotonin 5-HT1A receptors expressed in Chinese hamster ovary cells: the agonist [3H]lisuride labels free receptor and receptor coupled to G protein.

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