Recombinant saphenous vein 5-HT1B receptors of the rabbit: Comparative pharmacology with human 5-HT1B receptors

1 The rabbit recombinant saphenous vein 5-hydroxytryptamine,B (rb 5-HT1B) receptor stably transfected in rat C6-glial cells was characterized by measuring adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation upon exposure to various 5-HT receptor ligands. The effects of agonists and antagonists were compared with their effects determined previously at the human cloned 5-HT1B (h 5- HT1B) receptor under similar experimental conditions. 2 Intact C6-glial cells expressing rb 5-HT1B receptors exhibited [H-3]-5-carboxamidotryptamine (5-CT) binding sites with a K-d of 0.80 +/- 0.13 nM and a B-max between 225 to 570 fmol mg(-1) protein. The binding affinities of a series of 5-HT receptor ligands determined in a membrane preparation with [H-3]-5-CT or [H-3]-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide (GR 125,743) were similar. With the exception of ketanserin, ligand affinities were comparable to those determined at the cloned h 5-HT1B receptor site. 3 rb 5-HT1B receptors were negatively coupled to cyclic AMP formation upon stimulation with 5-HT agonists. Of the several 5-HT agonists tested, 5-CT was the most potent, the potency rank order being: 5-CT > 5-HT > zolmitriptan > naratriptan > rizatriptan > sumatriptan > R(+)-8-(hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The maximal responses of these agonists were similar to those induced by 5-HT. The potency of these agonists showed a positive correlation (r(2)=0.87; P<0.002) with their potency at the cloned h 5-HT1B receptor subtype. 4 2'-Methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), methiothepin and ketanserin each behaved as silent, competitive antagonists at rb 5-HT1B receptors; pK(B) values were 8.41, 8.32 and 7.05, respectively when naratriptan was used as an agonist. These estimates accorded with their binding affinities and the potencies found on 5-HT and/or sumatriptan-mediated contraction of isolated rabbit saphenous vein segments. 5 In conclusion, the recombinant saphenous vein 5-HT1B receptor of the rabbit shares important pharmacological similarities with the cloned h 5-HT1B receptor. However, ketanserin is a more potent antagonist of rb 5-HT1B receptors.