Human glycine α1 receptor inhibition by quercetin is abolished or inversed by α267 mutations in transmembrane domain 2

Quercetin, one of the flavonoids, is a compound of low molecular weight found in fruits and vegetables. Besides its antioxidative effect, quercetin also shows a wide range of diverse neuropharmacological actions. However, the cellular mechanisms of quercetin's actions, especially onligand-gated ion channels and synaptic transmissions, are not well studied. We investigated the effect of quercetin on the human glycine alpha l receptor channel expressed in Xenopus oocytes using a two-electrode voltage clamp technique. Application of quercetin reversibly inhibited glycine-induced current (I-Gly). Quercetin's inhibition depends on its dose, with an IC50 of 21.5 +/- .2 mu M. The inhibition was sensitive to membrane voltages. Site-directed mutations of S267 to S267Y but not S267A, S267F, S267G, S267K, S267L and S267T at transmembrane domain 2 (TM2) nearly abolished quercetin-induced inhibition of I-Gly. In contrast, in site-directed mutant receptors such as S267 to S267I, S267R and S267V, quercetin enhanced I-GIy compared to the wild-type receptor. The EC50 was 22.6 +/- 1.4, 25.5 +/- 4.2, and 14.5 +/- 3.1 pM for S267I, S267R and S267V, respectively. These results indicate that quercetin might regulate the human glycine a, receptor via interaction with amino acid residue alpha 267 and that alpha 267 plays a key role in determining the regulatory consequences of the human glycine al receptor by quercetin.