Abstract:The involvement of serotonin 5‐HT1Dβreceptor sites was investigated in the growth of rat C6 glial cells permanently transfected with a gene encoding a human 5‐HT1Dβreceptor. The 5‐HT receptor identity of control and transfected C6 glial/5‐HT1Dβcells was determined by reverse transcription‐polymerase chain reaction using primers specific for rat 5‐HT1A, rat 5‐HT1B, rat 5‐HT1Dα, human 5‐HT1Dβ, and rat 5‐HT2Areceptor genes. Constitutive mRNA for 5‐HT2Areceptors was present in control and transfected C6 glial/5‐HT1Dβcells, whereas mRNA for 5‐HT1Dβreceptor sites was only present in the transfected C6 glial/5‐HT1Dβcell line. 5‐HT inhibited forskolin‐stimulated cyclic AMP formation and promoted cell growth, in contrast to the absence of any measurable effect in pcDNA3 plasmid‐transfected and nontransfected C6 glial cells. The 5‐HT effects could be mimicked by sumatriptan (EC50= 44–76 nM) and were totally and partially blocked by methiothepin (IC50= 9 nM) and GR 127,935 {2′‐methyl‐4′‐(5‐methyl[1,2,4]oxadiazol‐3‐yl)‐biphenyl‐4‐carboxylic acid [4‐methoxy‐3‐(4‐methylpiperazin‐1‐yl)phenyl]amide; IC50= 97 pM}, respectively. No effect on cell growth was measured with the 5‐HT2receptor agonist DOI [1‐(2,5‐dimethoxy‐4‐iodophenyl)‐2‐aminopropane; 10 µM], suggesting that 5‐HT2Areceptors are not involved in the 5‐HT‐stimulated C6 glial/5‐HT1Dβcell growth. Dibutyryl‐cyclic AMP (0.3 mM)‐treated cultures did not show sumatriptan‐promoted cell growth, indicating an inhibitory role for cyclic AMP in the cell growth mediated by 5‐HT1Dβreceptor sites. In conclusion, 5‐HT promotes cell proliferation in transfected C6 glial/5‐HT1Dβcells by