Promotion of cell growth by stimulation of cloned human 5-HT1D receptor sites in transfected C6-glial cells is highly sensitive to intrinsic activity at 5-HT1D receptors.

5-Hydroxytryptamine (serotonin, 5-HT), essentially known as a neurotransmitter and vasoactive agent, also functions as a mitogen in various cell types through several different second messenger systems. Stimulation of cloned human 5-HT1D receptor sites by sumatriptan in stably transfected rat C6-glial/5-HT1D cells promotes cell growth (Pauwels et al. (1996) Naunyn-Schmiedeberg's Arch Pharmacol 353:144-156). In the present study, the pharmacology of this growth response was investigated using a broad series of 5-HT receptor ligands. The data were compared with the responses obtained by measuring inhibition of forskolin-stimulated cAMP formation. 5-HT (EC50: 25 nM) promoted cell growth of C6-glial/5HT1D cells, and this in contrast to the absence of any measurable effect in pcDNA3-plasmid transfected and non-transfected C6-glial cells. The 5-HT effect could be mimicked by the following compounds (EC50 in nM): zolmitriptan (0.41), 2'-methyl-4'-(-methyl[1,2,4] oxadiazol-3-yl)biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amid (GR 127,935;0.86), naratriptan (0.92), metergoline (1.9), sumatriptan (2.9), (N,N-dimethyl-2-[5-(1,2,4-triazol-1-ylmethyl)-1H-indol-3-yl] ethylamine (MK-462; 3.0), and R(+)-8-(hydroxy-2-(di-n-propylamino)tetralin (R(+)-8-OH-DPAT; 30.7). These EC50 -values correspond to the compounds binding affinities at the human 5-HT1D receptor site and, with the exception of GR 127,935 and metergoline also to the EC50-values found by measuring over 5 min inhibition of forskolin (100 microM)-stimulated cAMP formation. Prolonged exposure of GR 127,935(3 h) and metergoline (30 min) to cells yielded EC50 values in the cAMP assay more close to those measured in the mitogenic response. The growth response to sumatriptan, 5-HT, GR 127,935 and metergoline was blocked by the apparently silent antagonist methiothepin, ritanserin and ketanserin with potencies similar to blockade of inhibition of stimulated cAMP formation. The 8-OH-DPAT effect also is likely mediated by 5-HT1D receptors; stereoselectivity was found with its enantiomers at this receptor site and the effect was blocked by ketanserin (1 microM) but not by spiperone (1 microM). Micromolar concentrations of the 5-HT1B receptor agonist 3-(1,2,5,6-tetrahydro)-4-pyridil-5-pyrrolo[3,2-b]pyril-5-one (CP 93,129) and of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl_-2-aminopropane (DOI) induced cell growth with a potency that accorded with the affinity of these compounds for the human 5-HT1D receptor site. These effects were sensitive to ketanserin (1 microM) antagonism, but not to blockade by beta-adrenergic blockers and the 5-HT2 receptor antagonist 2-anilino-N-[2-(3-chlorophenoxy)-propyl] acetamidine hydroiodide (BW 501-C-67). The findings suggest that 5-HT1A, 5-HT1B and 5-HT2 receptors are not implicated in 5-HT-stimulated C6-glial/5-HT1D cell growth. In conclusion, human 5-HT1D receptors are involved in the growth of C6-glial/5-HT1D cells. This cellular response is highly sensitive to the intrinsic activity of compounds at 5-HT1D receptors.