Molecular cloning, functional expression, and pharmacological characterization of a novel serotonin receptor...

Using the polymerase chain reaction amplification technique in conjunction with conventional cloning techniques, we have isolated a novel member of the serotonin [5-hydroxytryptamine (5-HT)] 1C/2 receptor subfamily (designated 5-HT2F) from rat stomach fundus. Two DNA fragments were amplified from cDNA synthesized from rat stomach fundus poly(A)+ RNA using the polymerase chain reaction technique with degenerate oligonucleotide primers derived from sequence comparisons of the second, third, and sixth putative transmembrane domains of known 5-HT receptors. These fragments were used as hybridization probes to isolate full length cDNA clones from rat stomach fundus cDNA libraries. Full length cDNA clones contained one open reading frame encoding a 479-amino acid protein with seven hydrophobic domains, characteristic of members of the guanine nucleotide-binding protein-coupled receptor superfamily. Within these seven putative transmembrane domains, the 5-HT2F receptor shared greatest homology with the rat 5-HT1C and 5-HT2 receptor subtypes (70% and 68%, respectively). Cell lines stably expressing the 5-HT2F receptor were established and demonstrated functional coupling to phosphatidylinositol hydrolysis upon 5-HT stimulation analagous to that observed for the 5-HT1C and 5-HT2 receptors. Membranes from the stably transfected cell lines (but not the untransfected parental lines) exhibited high affinity (Kd = 7.9 nM), saturable binding of [3H]5-HT. Maximum binding ranged from 0.1 to 2.4 pmol/mg of protein, depending on the clonal isolate. Using [3H]5-HT as the basis for a radioligand binding assay, the relative affinities of several tryptamine and piperazine derivatives for the cloned 5-HT2F receptor correlated with their relative potencies to contract the rat stomach fundus. These data suggest a probable relationship between this novel 5-HT2F receptor and the serotonin contractile receptor of the rat stomach fundus.