Ca2+ responses in Chinese hamster ovary-K1 cells demonstrate an atypical pattern of ligand-induced 5-HT1A receptor activation

Little experimental evidence has been reported for diverse signaling via 5-hydroxytryptamine (5-HT)(1A) receptors despite the fact that agonists seem to be more efficacious at dorsal raphe somatodendritic 5-HT1A autoreceptors than at postsynaptic 5-HT1A receptors. The present study investigated Ca2+ responses in Chinese hamster ovary (CHO)-K1 cells expressing a human 5-HT1A receptor by 5-HT, prototypical 5-HT1A agonists, N-(3-chloro-4-fluorobenzoyl)-4-fluoro-4-[(5-methyl-6-; methyl-aminopyridin-2-yl)-methylaminomethyl]-piperidine (F 14679), and especially N-(3-chloro-4-fluorobenzoyl)-4-fluoro-4-[(5-methylpyridin-2-yl)-; methylaminomethyl]piperidine (F 13640) as representative ligands of a new chemical class (methyl-amino-pyridine) that combines both high efficacy and selectivity for 5-HT1A receptors. 5-HT (pEC(50) = 6.70 +/- 0.02) induced a pertussis toxin-sensitive, transient high-magnitude Ca2+ response. High-magnitude Ca2+ responses (E-max, percentage versus 5-HT) were also found with F 13640 (107 +/- 4), 5-carboxamidotryptamine 100 +/- 3), and F 14679 (87 +/- 3). In contrast, the prototypical 5-HT1A receptor agonists buspirone, ipsapirone, and 8-(hydroxy-2-(di-n-propylamino) tetralin, and also flesinoxan and eptapirone, were virtually inactive (less than or equal to5). This atypical pattern of 5-HT1A receptor activation contrasts with the broad spectrum of the ligands' partial agonist properties as observed by measuring guanosine 5'-O-(3-[S-35]thio)triphosphate ([S-35]GTPgammaS) binding responses with membranes of either CHO-K1 or C6-glial cells stably expressing a human 5-HT1A receptor. Remarkably, differences between ligands that seem small in the [S-35]GTPgammaS binding assay translate into huge differences in the magnitude of Ca2+ responses. Therefore, some of these 5-HT1A ligands (i.e., F13640) may in a selective way induce responses that may be not at all be achieved with other ligands (i.e., buspirone). In conclusion, the pharmacology of 5-HT1A receptor ligands seems to be codetermined by the effector pathway.