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 hu man 5-HT1D beta receptor sites were determined by measuring cAMP respo nses in two permanently transfected cell types: rat C6-glial and Chine se hamster ovary (CHO)-K1 cells. Both transfected cell lines expressed a similar 5-HT1D beta receptor density (361 to 448 fmol/mg protein) a nd 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, (4-methyl-1-piperazinyl)-pyrolo-(1,2-a)quin oxaline (CGS 12066B), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl) 1H-i ndole (RU 24,969), and tryptamine, their maximal effect being comparab le to that of 5-HT; less agonist efficacy with m-trifluoro-phenyl-pipe razine (TFMPP) (it inhibited at most 63% of stimulated cAMP formation) ; and antagonist activity against the 5-CT-mediated agonist response w ith methiothepin, ethyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic a cid -methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), a nd ritanserin. Metergoline and 1-naphtylpiperazine showed different in trinsic activities. In contrast to their pronounced antagonist activit y in the transfected CHO-K1 cell line, the antagonist effect was only partial and absent for metergoline and 1-naphtylpiperazine in the tran sfected C6-glial cell line, respectively. In conclusion, these cell li nes are useful as a tool to measure with high sensitivity differences in intrinsic activities of 5-HT receptor ligands and, therefore, discr iminate between silent antagonists (no intrinsic activity) and antagon ists with intrinsic activity (i.e. partial agonists), even though this intrinsic activity may be relatively weak.