Differential pharmacology between the guinea-pig and the gorilla 5-HT1Dreceptor as probed with isochromans (5-HT1D-selective ligands)

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Abstract
1. Both the 5-HT1D and 5-HT1B receptors are implicated in migraine pathophysiology. Recently isochromans have been discovered to bind primate 5-HT1D receptors with much higher affinity than 5-HT1B receptors. In the guinea-pig, a primary animal model for anti-migraine drug testing, however, isochromans 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 human embryonic kidney 293 cells. Mutations of threonine 100 and arginine 102 at the extracellular side of transmembrane II of the guinea-pig 5-HT1D receptor to the corresponding primate residues, isoleucine and histidine, respectively, enhanced its affinity for isochromans to that of the gorilla receptor, with little effects on its affinities for serotonin, sumatriptan and metergoline. Free energy change from the R102H mutation was about twice as much as that from the T100I mutation. 3. For G protein-coupling, serotonin marginally enhanced GTPgamma35S binding in membranes expressing the guinea-pig 5-HT1D receptor and its mutants, but robustly in membranes expressing the gorilla receptor. Sumatriptan enhanced GTPgamma35S 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 guinea-pig and gorilla 5-HT1D receptors, and identified contributions of I100 and H102 of primate 5-HT1D receptors to isochroman binding. Among common experimental animals, only the rabbit shares I100 and H102 with primates, and could be useful for studying isochroman actions in vivo.