Quercetin inhibits the 5-hydroxytryptamine type 3 receptor-mediated ion current by interacting with pre-transmembrane domain I

The flavonoid, quercetin, is a low molecular weight substance found in apple, tomato and other fruit. Besides its antioxidative effect, quercetin, like other flavonoids, has a wide range of neuropharmacological actions including analgesia, and motility, sleep, anticonvulsant, sedative and anxiolytic effects. In the present study, we investigated its effect on mouse 5-hydroxytryptamine type 3 (5-HT_3A) receptor channel activity, which is involved in pain transmission, analgesia, vomiting, and mood disorders. The 5-HT_3A receptor was expressed in Xenopus oocytes, and the current was measured with the two-electrode voltage clamp technique. In oocytes injected with 5-HT _3A receptor cRNA, quercetin inhibited the 5-HT-induced inward peak current (I_5-HT) with an IC₅₀ of 64.7 ± 2.2 μM. Inhibition was competitive and voltage-independent. Point mutations of pretransmembrane domain 1 (pre-TM1) such as R222T and R222A, but not R222D, R222E and R222K, abolished inhibition, indicating that quercetin interacts with the pre-TM1 of the 5-HT_3A receptor.