Coupling of canine serotonin 5-HT(1B) and 5-HT(1D) receptor subtypes to the formation of inositol phosphates by dual interactions with endogenous G(i/o) and recombinant G(alpha15) proteins.

Molecular cloning and expression of canine (ca) serotonin 5-HT(1B) and ca 5-HT(1D) receptor subtypes showed that besides the lower binding affinity of ketanserin for the ca 5-HT(1D) receptor, the ligand binding profiles were similar to their human homologues. Site-directed mutagenesis studies suggest that a Gln(189) residue in the second extracellular loop of the ca 5-HT(1D) receptor may partially account for the lower binding affinity of ketanserin. The coupling of ca 5-HT(1B) and ca 5-HT(1D) receptor subtypes to the phospholipase C pathway was analyzed by measuring stimulation of inositol phosphate formation in COS-7 cells. Zolmitriptan potently stimulated (EC(50) = 4.9 nM) the inositol phosphate formation at ca 5-HT(1D) receptors in a fully pertussis toxin (PTX)-dependent manner, whereas only a weak PTX-resistant inositol phosphate response (26-29% at 10 microM zolmitriptan) could be detected for the ca 5-HT(1B) receptor at a similar expression level. In contrast, both ca 5-HT(1B) and ca 5-HT(1D) receptor subtypes yielded a similar maximal magnitude of inositol phosphate formation (300-340% at 10 microM zolmitriptan) upon co-expression with a mouse (m) G(alpha15) protein. PTX treatment and co-expression with a beta-adrenergic receptor kinase C-terminal polypeptide partially (20-46%) abolished the m G(alpha15) protein-dependent ca 5-HT(1B) and ca 5-HT(1D) receptor-mediated stimulation of inositol phosphate formation. This study suggests both 5-HT receptor subtypes can activate betagamma subunits of endogenous G(i/o) proteins besides their coupling to recombinant m G(alpha15) protein.