The involvement of serotonin 5-HT1D beta receptor sites was investigated in the growth of rat C6 glial cells permanently transfected with a gene encoding a human 5-HT1D beta receptor. The 5-HT receptor identity of control and transfected C6 glial/5-HT1D beta cells was determined by reverse transcription-polymerase chain reaction using primers specific for rat 5-HT1A, rat 5-HT1B, rat 5-HT1D alpha, human 5-HT1D beta, and rat 5-HT2A receptor genes. Constitutive mRNA for 5-HT2A receptors was present in control and transfected C6 glial/5-HT1D beta cells, whereas mRNA for 5-HT1D beta receptor sites was only present in the transfected C6 glial/5-HT1D beta cell 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-carbox yli c acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide; IC50 = 97 pM), respectively. No effect on cell growth was measured with the 5-HT2 receptor agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane; 10 microM], suggesting that 5-HT2A receptors are not involved in the 5-HT-stimulated C6 glial/5-HT1D beta cell 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-HT1D beta receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)