Quercetin Enhances Human α7 Nicotinic Acetylcholine Receptor-Mediated Ion Current through Interactions with Ca2+ Binding Sites

Abstract

The flavonoid quercetin is a low molecular weight substance found in fruits and vegetables. Aside from its anti-oxidative effect, quercetin, like other flavonoids, has a wide range of neuropharmacological actions. The α7 nicotinic acetylcholine receptor (α7 nAChR) has a Ca(2+)-binding site, is highly permeable to the Ca(2+) ion, and plays important roles in Ca(2+)-related normal brain functions. Dysfunctions of α7 nAChR are associated with a variety of neurological disorders. In the present study, we investigated the effects of quercetin on the ACh-induced inward peak current (I(ACh)) in Xenopus oocytes that heterologously express human α7 nAChR. I(ACh) was measured with the two-electrode voltage clamp technique. In oocytes injected with α7 nAChR cRNA, the effects of the co-application of quercetin on I(ACh) were concentration-dependent and reversible. The ED(50) was 36.1 + 6.1 μM. Quercetin-mediated enhancement of I(ACh) caused more potentiation when quercetin was pre-applied. The degree of I(ACh) potentiation by quercetin pre-application was time-dependent and saturated after 1 min. Quercetin-mediated I(ACh) enhancement was not affected by ACh concentration and was voltage-independent. However, quercetin-mediated I(ACh) enhancement was dependent on extracellular Ca(2+) concentrations and was specific to the Ca(2+) ion, since the removal of extracellular Ca(2+) or the addition of Ba(2+) instead of Ca(2+) greatly diminished quercetin enhancement of I(ACh). The mutation of Glu195 to Gln195, in the Ca(2+)-binding site, almost completely diminished quercetin-mediated I(ACh) enhancement. These results indicate that quercetin-mediated I(ACh) enhancement human α7 nAChR heterologously expressed in Xenopus oocytes could be achieved through interactions with the Ca(2+)-binding site of the receptor.

Quercetin Inhibits α3β4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed inXenopusOocytes

“…However, the underlying cellular mechanisms of quercetin actions are relatively unknown, especially with regards to possible regulation of receptors or ionic channels involved in synaptic transmissions. Recently, we demonstrated that quercetin differentially regulates subsets of nicotinic acetylcholine receptor such as homomeric glycine, 5-HT3A, α7 and α9α10 nicotinic acetylcholine receptors [17][18][19][20]. In the present study, we examined the effects of quercetin on heteromeric α3β4 nicotine acetylcholine receptor channel activity and report here that quercetin inhibits heteromeric α3β4 nicotine acetylcholine receptor channel activity with voltage-independent and non-competitive manner.…”

“…Electrophysiological experiments were performed three to five days after oocyte isolation. For α3β4 nicotine acetylcholine receptor experiments, α3 and β4 nicotine acetylcholine receptor subunit-encoding cRNAs (40 nl) were co-injected into the animal or vegetal pole of each oocyte 1 day after isolation using a 10-μl microdispenser (VWR Scientific, West Chester, PA, USA) fitted with a tapered glass pipette tip (15∼20 μm in diameter) [19].…”

“…A custom-made Plexiglas net chamber was used for two-electrode voltage-clamp recordings, as previously reported [19]. A single oocyte was constantly superfused with a recording solution (96 mM NaCl, 2 mM KCl, 1.8 mM CaCl2, and 10 mM HEPES, pH 7.5) in the absence or presence of glutamate or quercetin during recording.…”

Inhibitory Effects of Quercetin on Muscle-type of Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

“…In previous studies, we demonstrated that quercetin regulates α3β4, α7 or α9α10 nicotinic acetylcholine receptor-gated ion currents [22][23][24]. For example, quercetin inhibited heteromeric α3β4 and α9α10 nicotinic acetylcholine receptor channel activities, whereas quercetin enhanced homomeric α7 nicotinic acetylcholine receptors, indicating a differential effect of quercetin on nicotinic acetylcholine receptor regulations.…”

“…In previous reports, we have shown that quercetin regulates the Cys-loop family of ligand-gated ion channels, such as 5-HT3A, human glycine α1, α7, α9α10 and α3β4 nicotine acetylcholine receptors [20][21][22][23][24]. However, relatively little is known about the effects of quercetin on the muscle-type nicotinic acetylcholine receptor channel activity.…”