The Adjustment Mechanism Of Quercetin On The Glycine Receptor In The Central Nervous System And Functional Study

Ligand-gated ion channels permit cells to respond rapidly to changes in their externalenvironment. They are particularly well known for mediating fastneurotransmission in the nervous system. The glycine receptor channels (GlyRs)belong to the ligand-gated ion channel superfamily. GlyR is a pentamer made of twoglycosylated integral membrane proteins: subunitα(48kD) and subunitβ(58kD).Fourαsubunits have been discovered. Theαsubunit alone can both bind glycine andproduce a chloride current. Theβsubunit alone does not form functional GlyR, but itis important in the assembly and modulation of the heteromeric receptor.In spinal cord, GlyRs participates in mediating the fast inhibitory synapticneurotransmission and is involved in sensory information processing (includingnociceptive signal transduction). In hippocampus, GlyRs is widely expressed, but noglycinergic neurotransmission has been found in this region and the roles of GlyRs inhippocampal function are unclear. It had been reported that GlyRs in thehippocampus may be tonically activated and suppress hyperexcitability. Manystudies suggest that the tonic activation of GlyRs contributes to the activity in thesynaptic network, the cross-inhibition ofγ-aminobutyric acid type A receptors(GABA_ARs), and the short-term plasticity. Thus, in this study, the effects ofquercetin on native GlyRs in cultured rat hippocampal neurons and spinal neuronswere investigated by using whole-cell patch-clamp technique. We also investigatedthe effects of quercetin on glycine-induced current (I_Gly) in recombinant GlyRsexpressed in HEK293T cells.Flavonoids are nearly ubiquitous in plants and are recognized as the pigmentsresponsible for the colors of leaves. They are rich in seeds, citrus fruits, olive oil, tea,and red wine. They are low molecular weight compounds composed of a three-ringstructure with various substitutions. This basic structure is shared by tocopherols(vitamin E). Among the many different flavonoids present in plants, quercetin is themost abundant one. Certain plants and spices containing quercetin have been used for thousands of years in traditional Eastern medicine. Suggestions are made where suchpossibilities may be worth pursuingQuercetin has important effects in plant biochemistry and physiology, acting asantioxidants, enzyme inhibitors, precursors of toxic substances, and pigments.Quercetin displays a remarkable array of biochemical and pharmacological actions,some of which suggest that it may significantly affect the function of variousmammalian cellular systems. Quercetin has long been recognized topossess anti-inflammatory, antioxidant, antiallergic, hepatoprotective, antithrombotic,antiviral, and anticarcinogenic activities. Queretin is typical phenolic compound and,therefore, act as potent metal chelators and free radical scavengers.In the present study, we found that quercetin reversibly andconcentration-dependently depressed I_Gly, with IC₅₀ of 10.7±0.24μM. Quercetindepressed maximum I_Gly and significant changed the EC₅₀ for glycine and Hillcoefficient. Kinetic analysis indicated that quercetin accelerated the rates ofdesensitization. Interestingly, after the end of glycine with quercetin co-application,a transient rebound occurred. The quercetin effects also displayedvoltage-dependence, being greater at positive membrane potentials. These effectssuggested that it may act as an open channel blocker. Furthermore, in the sequentialapplication protocol, quercetin inhibited the peak amplitude of I_Gly to a macroscopicdegree while slowing the GlyR desensitization. These effects implied that quercetinhas a depressant effect independent of GlyR channel's opening which maybe causedby an allosteric mechanism. Strikingly, we found that quercetin inhibited theamplitude of recombinant-induced current mediated byα2-,α2β,α3- andα3β-GlyRs but had no effects onα1- andα1β-GlyRs which were expressed inHEK293T cells. We also investigated the effects of quercetin on I_Gly in spinal cordneurons during development in vitro. The extent of blockade by quercetin on I_Gly wasslighter in spinal cord neurons than in hippocampal neurons in adevelopment-dependent manner.Taken together, our results suggest that quercetin has possible effects ininformation processing within a neuronal network by inhibition of I_Gly and may be useful as pharmacological probe for identifying the subunit types of GlyRs.