Pharmacological properties of the naturally occurring Phe-124-Cys variant of the human 5-HGT1B receptor:: changes in ligand binding, G-protein coupling and second messenger formation

The aim of this study was to analyse whether substitution of phenylalanine in postion 124 of the human (h) 5-HT1B receptor by cysteine, a naturally occurring variant of this receptor, modifies not only ligand binding, but also G-protein coupling and second messenger formation. Stably transfected rat C6 glioma cells, which express either the h5-HT1B variant receptor (VR) or the wild-type receptor (WTR) were used. In saturation experiments with [H-3]5-carboxamidotryptamine ([H-3] 5-CT), the maximum binding (B-max) of the VR amounted to only 60% of that to WTR, In competition experiments with 1 nM [H-3]5-CT. the following 5-HT receptor ligands exhibited a higher affinity for the mutant receptor than for the WTR: L-694,247, 5-CT, 5-HT, sumatriptan (agonists listed at decreasing order of potency) and SB-224289 (a selective h5-HT1B receptor inverse agonist with competitive antagonistic properties). In contrast, the mixed 5-HT1B/1D receptor antagonist GR-127935 exhibited equal affinity for both isoforms. The efficacy of L-694,247, 5-CT, 5-HT and sumatriptan in stimulating [S-35]GTP gamma S binding (a measure of G protein coupling) to membranes of cells expressing the Vp. was approximately 50-65% lower compared to membranes of cells expressing the WTR, but their potency was 2.8-3.6-fold higher, SB-224289, which decreased [S-35]GTP gamma S binding when given alone, but not GR-127935, was more potent in antagonizing the stimulatory effect of 5-CT on [S-35]GTP gamma S binding to membranes expressing the VR compared to membranes expressing the WTR. In whole cells expressing the VR, 5-CT and sumatriptan inhibited the forskolin-stimulated cAMP accumulation 3.2-fold more potently than in cells expressing the WTR. In conclusion, our data suggest that the Phe-124-Cys mutation modifies the pharmacological properties of the h5-HT1B receptor and may account for pharmacogenetic differences in the action of h5-HT1B receptor ligands, Thus, the sumatriptan-induced vasospasm which occurs at low incidence as a side-effect in migraine therapy may be related to the expression of the (124-Cys)h5-HT1B receptor in patients with additional pathogenetic factors such as coronary heart disease. Pharmacogenetics 10:655-666 (C) 2000 Lippincott Williams & Wilkins.