Identification of receptor domains that modify ligand binding to 5-hydroxytryptamine2 and 5-hydroxytryptamine1c serotonin receptors.

Serotonin [5-hydroxytryptamine (5-HT)] receptors are distinguished pharmacologically by their characteristic affinities for agonists and antagonists. Two serotonin receptors, the 5-HT2 and 5-HT1c, share a number of pharmacologic and structural properties while differing in their affinities for certain agonists and antagonists. To identify regions of the 5-HT2 and 5-HT1c receptors important for specifying their unique pharmacology, we constructed six chimeric 5-HT2/5-HT1c receptors in which domains of each receptor were exchanged. The abilities of several drugs to inhibit [3H]mesulergine bound to the chimeric and parent receptors transiently expressed in COS-7 cells were then examined. For spiperone and haloperidol (both butyrophenones), chimeras that exchanged transmembrane (TM) domains I and II or TMs I-III had the greatest effects on binding affinity. The binding affinity of cinanserin (a cinnamanilide) was significantly changed in all the chimeras studied. In contrast, the binding of ketanserin (a 4-fluorobenzoylpiperidine) was strongly influenced by chimeras that exchanged TMs I-III (but not I and II) and by chimeras that exchanged intracellular loop 3 to TM VII. 5-HT binding affinity was greatly altered for chimeras that exchanged domains of intracellular loop 3 to TM VII, with minor effects being noted for chimeras that exchanged TMs I and II and I-III. The affinities of the nonselective drugs mesulergine, mianserin, and m-chlorophenylpiperazine were relatively unaffected when domains of the 5-HT2 and 5-HT1c receptors were exchanged. Taken together, these results imply that structurally diverse 5-HT2 antagonists utilize distinct regions of the 5-HT2 receptor for high affinity binding.