Differential pharmacology between the guinea-pig and the gorilla 5-HT1D receptor as probed with isochromans (5-HT1D-selective ligands)

1 Both the 5-HT1D and 5-HT1B receptors are implicated in migraine pathophys iology. Recently isochromans have been discovered to bind primate 5-HT1D re ceptors with much higher affinity than 5-HT1B receptors. In the guinea-pig, a primary animal model for anti-migraine drug testing, however, isochroman s bound the 5-HT1D receptor with lower affinity than the gorilla receptor. 2 This species-specific pharmacology was investigated, using site-directed mutagenesis on cloned guinea-pig receptors heterologously expressed in huma n embryonic kidney 293 cells. Mutations of threonine 100 and arginine 102 a t the extracellular side of transmembrane II of the guinea-pig 5-HT1D recep tor to the corresponding primate residues, isoleucine and histidine, respec tively, enhanced its affinity for isochromans to that of the gorilla recept or, with little effects on its affinities for serotonin, sumatriptan and me tergoline. Free energy change from the R102M mutation was about twice as mu ch as that from the T100I mutation. 3 For G protein-coupling, serotonin marginally enhanced GTP gamma(35)S bind ing in membranes expressing the guinea-pig 5-HT1D receptor and its mutants, but robustly in membranes expressing the gorilla receptor. Sumatriptan enh anced GTP gamma(35)S binding in the latter nearly as much as serotonin. and several isochromans by 30-60% of serotonin. 4 We discovered key differences in the function and binding properties of g uinea-pig and gorilla 5-HT1D receptors, and identified contributions of I10 0 and H102 of primate 5-HT1D receptors to isochroman binding. Among common experimental animals, only the rabbit shares I100 and H102 with primates, a nd could be useful for studying isochroman actions in vivo.