RT Journal Article
SR Electronic
T1 Binding Sites for Bilobalide, Diltiazem, Ginkgolide and Picrotoxinin at the 5-HT3 Receptor
JF Molecular Pharmacology
JO Mol Pharmacol
FD American Society for Pharmacology and Experimental Therapeutics
SP mol.111.071415
DO 10.1124/mol.111.071415
A1 Andrew J Thompson
A1 Rujee K Duke
A1 Sarah C R Lummis
YR 2011
UL http://molpharm.aspetjournals.org/content/early/2011/04/19/mol.111.071415.abstract
AB Bilobalide (BB), ginkgolide B (GB), diltiazem (DTZ) and picrotoxinin (PXN) are 5-HT3 receptor antagonists whose principal sites of action are in the channel. To probe their exact binding locations 5-HT3 receptors with substitutions in their pore lining residues were constructed (N-4'Q, E-1'D, S2'A, T6'S, L7'T, L9'V, S12'A, I16'V, D20'E), expressed in Xenopus oocytes, and the effects of the compounds on 5-HT-induced currents examined. EC50s at mutant receptors were less than 6-fold different to wild type, indicating that the mutations were well tolerated. BB, GB, DTZ and PXN had pIC50 values of 3.33, 3.14, 4.67 and 4.97 respectively. Inhibition by BB and GB was abolished in mutant receptors containing T6'S and S12'A substitutions, but their potencies were enhanced (42-fold and 125-fold respectively) in S2'A mutant receptors. S2'A substitution also caused GB ligand trap. PXN potency was modestly enhanced (5-fold) in S2'A, abolished in T6'S, and reduced in L9'V (40-fold) and S12'A (7-fold) receptors. DTZ potency was reduced in L7'T and S12'A receptors (5-fold), and DTZ also displaced [3H]granisetron binding, indicating mixed competitive/non-competitive inhibition We conclude that regions close to the hydrophobic gate of M2 are important for the inhibitory effects of BB, GB, DTZ and PXN at the 5-HT3 receptor; for BB, GB and PXN, the data show that the 6' channel lining residue is their major site of action, with minor roles for 2', 9' and 12' residues, while for DTZ the 7' and 12' sites are important.