Differentiation Between Partial and Silent 5-HT1D Beta Receptor Antagonists Using Rat C6-glial and Chinese Hamster Ovary Cell Lines Permanently Transfected with a Cloned Human 5-HT1D Beta Receptor Gene

Intrinsic activities of serotonin (5-HT) receptor ligands at cloned human 5-HT1D beta receptor sites were determined by measuring cAMP responses in two permanently transfected cell types: rat C6-glial and Chinese hamster ovary (CHO)-K1 cells. Both transfected cell lines expressed a similar 5-HT1D beta receptor density (361 to 448 fmol/mg protein) and displayed a number of similar cAMP responses: marked inhibition of forskolin-stimulated cAMP formation by 5-HT; a similar agonist potency and efficacy with 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine, bufotenine, sumatriptan, 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrolo-(1,2-a)quinoxal ine (CGS 12066B), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)1H-indole (RU 24,969), and tryptamine, their maximal effect being comparable to that of 5-HT; less agonist efficacy with m-trifluoro-phenyl-piperazine (TFMPP) (it inhibited at most 63% of stimulated cAMP formation); and antagonist activity against the 5-CT-mediated agonist response with methiothepin, 2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), and ritanserin. Metergoline and 1-naphtylpiperazine showed different intrinsic activities. In contrast to their pronounced antagonist activity in the transfected CHO-K1 cell line, the antagonist effect was only partial and absent for metergoline and 1-naphtylpiperazine in the transfected C6-glial cell line, respectively. In conclusion, these cell lines are useful as a tool to measure with high sensitivity differences in intrinsic activities of 5-HT receptor ligands and, therefore, discriminate between silent antagonists (no intrinsic activity) and antagonists with intrinsic activity (i.e. partial agonists), even though this intrinsic activity may be relatively weak.