Molecular determinants of picrotoxin inhibition of 5-hydroxytryptamine type 3 receptors

Previously, we reported that the GABAA receptor antagonist picrotoxin also antagonizes serotonin (5-HT)3 receptors and that its effects are subunit-dependent. Here, we sought to identify amino acids involved in picrotoxin inhibition of 5-HT3 receptors. Mutation of serine to alanine at the transmembrane domain 2 (TM2) 2 position did not affect picrotoxin (PTX) sensitivity in murine 5-HT3A receptors. However, mutation of the 6 TM2 threonine to phenylalanine dramatically reduced PTX sensitivity. Mutation of 6 asparagine to threonine in the 5-HT3B subunit enhanced PTX sensitivity in heteromeric 5-HT3A/3B receptors. Introduction of serine (native to the human 3B subunit) at the 6 position also increased PTX sensitivity, suggesting a species-specific effect. Mutation of the 7 leucine to threonine in 5-HT3A receptors increased PTX sensitivit