Korean J Physiol Pharmacol.  2011 Feb;15(1):17-22.

Quercetin Inhibits alpha3beta4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

• Lee BH
• Hwang SH
• Choi SH
• Shin TJ
• Kang J
• Lee SM
• Nah SY

Abstract

Quercetin mainly exists in the skin of colored fruits and vegetables as one of flavonoids. Recent studies show that quercetin, like other flavonoids, has diverse pharmacological actions. However, relatively little is known about quercetin effects in the regulations of ligand-gated ion channels. In the previous reports, we have shown that quercetin regulates subsets of homomeric ligand-gated ion channels such as glycine, 5-HT3A and alpha7 nicotinic acetylcholine receptors. In the present study, we examined quercetin effects on heteromeric neuronal alpha3beta4 nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal alpha3 and beta4 subunits. Treatment with acetylcholine elicited an inward peak current (IACh) in oocytes expressing alpha3beta4 nicotinic acetylcholine receptor. Co-treatment with quercetin and acetylcholine inhibited IACh in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors. The inhibition of IACh by quercetin was reversible and concentration-dependent. The half-inhibitory concentration (IC50) of quercetin was 14.9+/-0.8 microM in oocytes expressing alpha3beta4 nicotinic acetylcholine receptor. The inhibition of IACh by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate alpha3beta4 nicotinic acetylcholine receptor and this regulation might be one of the pharmacological actions of quercetin in nervous systems.

Keyword

Flavonoids; Quercetin; alpha3beta4 nicotinic acetylcholine receptor; Xenopus oocyte

MeSH Terms

Acetylcholine
Flavonoids
Fruit
Glycine
Ligand-Gated Ion Channels
Nervous System
Neurons
Oocytes
Quercetin
Receptors, Nicotinic
RNA, Complementary
Skin
Social Control, Formal
Vegetables
Xenopus
Acetylcholine
Flavonoids
Glycine
Ligand-Gated Ion Channels
Quercetin
RNA, Complementary
Receptors, Nicotinic

Figure

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  Fig. 1. Chemical structure of quercetin (A) and effect of quercetin (Que) in oocytes expressing α3β4 nicotinic acetylcholine receptors. Quercetin itself had no effect on IACh in oocytes expressing α3β4 nicotinic acetylcholine receptors (B).

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  Fig. 2. Effect of quercetin (Que) on IACh in oocytes expressing α3β4 nicotinic acetylcholine receptors. (A) Acetylcholine (ACh, 100 μM) was first applied and then acetylcholine was co- or pre-applied with quercetin (Que, 30 μM). Thus, co- and pre-application of quercetin with acetylcholine inhibited IACh. The resting membrane potential of oocytes was about –35 mV and oocytes were voltage-clamped at a holding potential of –80 mV prior to drug application. Traces are representative of six separate oocytes from three different frogs. (B) Co- or pre-application of quercetin did not affect differently on IACh. (C) IACh in oocytes expressing α3β4 nicotinic acetylcholine receptors was elicited at –80 mV holding potential with indicated time in the presence of 100 μM acetylcholine and then the indicated concentration of quercetin was co-applied with acetylcholine. (D) % Inhibition by quercetin of IACh was calculated from the average of the peak inward current elicited by acetylcholine alone before quercetin and the peak inward current elicited by acetylcholine alone after co-application of quercetin with acetylcholine. The continuous line shows the curve fitted according to the equation. y/ymax=[Quercetin]/[Quercetin] + K1/2), where ymax, the maximum inhibition (97.8±1.7%, mean±S.E.M.) and K1/2 is the concentration for half-maximum inhibition (14.9±0.8 μM, mean±S.E.M.), and [Quercetin] is the concentration of quercetin. Each point represents the mean±S.E.M. (n=9∼12 from three different frogs).

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  Fig. 3. Current-voltage relationship and voltage-independent inhibition by quercetin. (A) Current-voltage relationships of IACh inhibition by quercetin (Que) in α3β4 nicotinic acetylcholine receptors. Representative current-voltage relationships were obtained using voltage ramps of –100 to +60 mV for 300 ms at a holding potential of –80 mV. Voltage steps were applied before and after application of 100 μM acetylcholine in the absence or presence of 20 μM quercetin. (B) Voltage-independent inhibition of IACh in the α3β4 nicotinic acetylcholine receptors by quercetin. Inset; the values were obtained from the receptors in the absence or presence of 20 μM quercetin at the indicated membrane holding potentials.

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  Fig. 4. Concentration-dependent effects of acetylcholine on quercetin-mediated inhibition of IACh. (A) The representative traces were obtained from α3β4 nicotinic acetylcholine receptors expressed in oocytes. IACh of the upper and lower panels were elicited from concentration of 30 μM ACh and 1 mM ACh at a holding potential of –80 mV, respectively. (B) Concentration-response relationships for ACh in the α3β4 nicotinic acetylcholine receptors treated with ACh (3∼1,000 μM) alone or with ACh plus co-application of 20 μM quercetin. The IACh of oocytes expressing the α3β4 nicotinic acetylcholine receptors was measured using the indicated concentration of ACh in the absence (☐) or presence (❍) of 20 μM quercetin (Que). Oocytes were exposed to ACh alone or to ACh with quercetin. Oocytes were voltage-clamped at a holding potential of –80 mV. Each point represents the mean± S.E.M. (n=9∼12/group).

Reference

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