Glycine Receptor-mediated Currents Research Articles

Resveratrol noncompetitively inhibits glycine receptor-mediated currents in neurons of rat central auditory neurons

Resveratrol, a naturally occurring stilbene found in red wine, is known to modulate the activity of several types of ion channels and membrane receptors, including Ca2+, K+, and Na+ ion channels. However, little is known about the effects of resveratrol on some important receptors, such as glycine receptors and GABAA receptors, in the central nervous system (CNS). In the present study, the effects of resveratrol on glycine receptor or GABAA receptor-mediated currents in cultured rat inferior colliculus (IC) and auditory cortex (AC) neurons were studied using whole-cell voltage-clamp recordings. Resveratrol itself did not evoke any currents in IC neurons but it reversibly decreased the amplitude of glycine-induced current (IGly) in a concentration-dependent manner. Resveratrol did not change the reversal potential of IGly but it shifted the concentration-response relationship to the right without changing the Hill coefficient and with decreasing the maximum response of IGly. Interestingly, resveratrol inhibited the amplitude of IGly but not that of GABA-induced current (IGABA) in AC neurons. More importantly, resveratrol inhibited GlyR-mediated but not GABAAR-mediated inhibitory postsynaptic currents in IC neurons using brain slice recordings. Together, these results demonstrate that resveratrol noncompetitively inhibits IGly in auditory neurons by decreasing the affinity of glycine to its receptor. These findings suggest that the native glycine receptors but not GABAA receptors in central neurons are targets of resveratrol during clinical administrations.

Long-term potentiation of glycinergic synapses by semi-natural stimulation patterns during tonotopic map refinement

Before the onset of hearing, cochlea-generated patterns of spontaneous spike activity drive the maturation of central auditory circuits. In the glycinergic sound localization pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) this spontaneous activity guides the strengthening and silencing of synapses which underlies tonotopic map refinement. However, the mechanisms by which patterned activity regulates synaptic refinement in the MNTB-LSO pathway are still poorly understood. To address this question, we recorded from LSO neurons in slices from prehearing mice while stimulating MNTB afferents with stimulation patterns that mimicked those present in vivo. We found that these semi-natural stimulation patterns reliably elicited a novel form of long-term potentiation (LTP) of MNTB-LSO synapses. Stimulation patterns that lacked the characteristic high-frequency (200 Hz) component of prehearing spike activity failed to elicit potentiation. LTP was calcium dependent, required the activation of both g-protein coupled GABAB and metabotropic glutamate receptors and involved an increase in postsynaptic glycine receptor-mediated currents. Our results provide a possible mechanism linking spontaneous spike bursts to tonotopic map refinement and further highlight the importance of the co-release of GABA and glutamate from immature glycinergic MNTB terminals.

Signaling mechanism for modulation by ATP of glycine receptors on rat retinal ganglion cells.

ATP modulates voltage- and ligand-gated channels in the CNS via the activation of ionotropic P2X and metabotropic P2Y receptors. While P2Y receptors are expressed in retinal neurons, the function of these receptors in the retina is largely unknown. Using whole-cell patch-clamp techniques in rat retinal slice preparations, we demonstrated that ATP suppressed glycine receptor-mediated currents of OFF type ganglion cells (OFF-GCs) dose-dependently, and the effect was in part mediated by P2Y1 and P2Y11, but not by P2X. The ATP effect was abolished by intracellular dialysis of a Gq/11 protein inhibitor and phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor, but not phosphatidylcholine (PC)-PLC inhibitor. The ATP effect was accompanied by an increase in [Ca2+]i through the IP3-sensitive pathway and was blocked by intracellular Ca2+-free solution. Furthermore, the ATP effect was eliminated in the presence of PKC inhibitors. Neither PKA nor PKG system was involved. These results suggest that the ATP-induced suppression may be mediated by a distinct Gq/11/PI-PLC/IP3/Ca2+/PKC signaling pathway, following the activation of P2Y1,11 and other P2Y subtypes. Consistently, ATP suppressed glycine receptor-mediated light-evoked inhibitory postsynaptic currents of OFF-GCs. These results suggest that ATP may modify the ON-to-OFF crossover inhibition, thus changing action potential patterns of OFF-GCs.

Activation of 5-HT2A/C receptor reduces glycine receptor-mediated currents in cultured auditory cortical neurons.

Glycine receptors (GlyRs) permeable to chloride only mediate tonic inhibition in the cerebral cortex where glycinergic projection is completely absent. The functional modulation of GlyRs was largely studied in subcortical brain regions with glycinergic transmissions, but the function of cortical GlyRs was rarely addressed. Serotonin could broadly modulate many ion channels through activating 5-HT2 receptor, but whether cortical GlyRs are subjected to serotonergic modulation remains unexplored. The present study adopted patch clamp recordings to examine functional regulation of strychnine-sensitive GlyRs currents in cultured cortical neurons by DOI (2,5-Dimethoxy-4-iodoamphetamine), a 5-HT2A/C receptor agonist. DOI caused a concentration-dependent reduction of GlyR currents with unchanged reversal potential. This reduction was blocked by the selective receptor antagonists (ritanserin and risperidone) and G protein inhibitor (GDP-β-s) demonstrated that the reducing effect of DOI on GlyR current required the activation of 5-HT2A/C receptors. Strychnine-sensitive tonic currents revealed the inhibitory tone mediated by nonsynaptic GlyRs, and DOI similarly reduced the tonic inhibition. The impaired microtube-dependent trafficking or clustering of GlyRs was thought to be involved in that nocodazole as a microtube depolymerizing drug largely blocked the inhibition mediated by 5-HT2A/C receptors. Our results suggested that activation of 5-HT2A/C receptors might suppress cortical tonic inhibition mediated by GlyRs, and the findings would provide important insight into serotonergic modulation of tonic inhibition mediated by GlyRs, and possibly facilitate to develop the therapeutic treatment of neurological diseases such as tinnitus through regulating cortical GlyRs.

Modulation of inhibitory glycine receptors in cultured embryonic mouse hippocampal neurons by zinc, thiol containing redox agents and carnosine

Modulation of inhibitory glycine receptors by zinc (Zn 2+) and endogenous redox agents such as glutathione may alter inhibition in the mammalian brain. Despite the abundance of Zn 2+ in the hippocampus and its ability to modulate glycine receptors, few studies have examined Zn 2+ modulation of hippocampal glycine receptors. Whether redox agents modulate hippocampal glycine receptors also remains unknown. This study examined Zn 2+ and redox modulation of glycine receptor-mediated currents in cultured embryonic mouse hippocampal neurons using whole-cell recordings. Zn 2+ concentrations below 10 μM potentiated currents elicited by low glycine, β-alanine, and taurine concentrations by 300–400%. Zn 2+ concentrations above 300 μM produced nearly complete inhibition. Potentiating Zn 2+ concentrations shifted the dose-response curves for the three agonists to the left and decreased the Hill coefficient for glycine and β-alanine but not taurine. Inhibiting Zn 2+ concentrations shifted the dose-response curves for glycine and β-alanine to the right but reduced the maximum taurine response. Histidine residues may participate in potentiation because diethyl pyrocarbonate and pH 5.4 diminished Zn 2+ enhancement of glycine currents. pH 5.4 diminished Zn 2+ block of glycine currents, but diethyl pyrocarbonate did not. These findings indicate that separate sites mediate Zn 2+ potentiation and inhibition. The redox agents glutathione, dithiothreitol, tris(2-carboxyethyl)phosphine, and 5,5′-dithiobis(2-nitrobenzoic acid) did not alter glycine currents by a redox mechanism. However, glutathione and dithiothreitol interfered with the effects of Zn 2+ on glycine currents by chelating it. Carnosine had similar effects. Thus, Zn 2+ and thiol containing redox agents that chelate Zn 2+ modulate hippocampal glycine receptors with the mechanism of Zn 2+ modulation being agonist dependent.

Does Acetaldehyde Mediate Ethanol Action in the Central Nervous System?

Some of the effects of ethanol in the central nervous system are due to changes in function of ligand-gated ion channels. Production of detectable amounts of acetaldehyde, a primary metabolite of ethanol, has been demonstrated in brain homogenates. The aim of this study was to determine whether central actions that are often attributed to ethanol may actually be mediated by acetaldehyde. The effects of acetaldehyde (1-1000 microM) were tested by two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes expressing 10 different ligand-gated ion channel receptors [alpha1 glycine; alpha1beta2gamma2Sgamma-aminobutyric acid (GABA)A; rho1 GABAc; 5-hydroxytryptamine-3A; NR1a/NR2A NMDA; GluR1/GluR2 AMPA; GluR6/KA2 kainate; and alpha4beta2, alpha4beta4, and alpha2beta4 nicotinic-acetylcholine] and the G-protein-coupled inward rectifying potassium channel GIRK2. We also investigated the effect of acetaldehyde on the dopamine transporter (DAT), performing dopamine uptake assays in oocytes expressing DAT. Acetaldehyde (1 and 10 microM) significantly enhanced alpha1 glycine receptor-mediated currents. Acetaldehyde did not affect the function of any of the other receptors tested or the potassium currents measured in GIRK2 channels. Moreover, acetaldehyde did not alter the DAT-mediated dopamine uptake. Our results suggest a potential minor role for acetaldehyde in the glycine receptor-mediated effects of ethanol. Otherwise, acetaldehyde does not modulate function of the neuronal receptors tested in this study, in GIRK channels or DAT, when expressed recombinantly in Xenopus laevis oocytes.