State-dependent cross-inhibition between anionic GABA(A) and glycine ionotropic receptors in rat hippocampal CA1 neurons

Advanced progress of vestibular compensation in vestibular neural networks

Vestibular compensation is the natural process of recovery that occurs with acute peripheral vestibular lesion. Here, we summarize the current understanding of the mechanisms underlying vestibular compensation, focusing on the role of the medial vestibular nucleus (MVN), the central hub of the vestibular system, and its associated neural networks. The disruption of neural activity balance between the bilateral MVNs underlies the vestibular symptoms after unilateral vestibular damage, and this balance disruption can be partially reversed by the mutual inhibitory projections between the bilateral MVNs, and their top-down regulation by other brain regions via different neurotransmitters. However, the detailed mechanism of how MVN is involved in vestibular compensation and regulated remains largely unknown. A deeper understanding of the vestibular neural network and the neurotransmitter systems involved in vestibular compensation holds promise for improving treatment outcomes and developing more effective interventions for vestibular disorders.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

Cross-talk pattern between GABAA- and glycine-receptors in CNS neurons is shaped by their relative expression levels

GABA_A receptors (GABA_ARs) and glycine receptors (GlyRs) are two principal inhibitory chloride ion channels in the central nervous system. The two receptors do not function independently but cross-talk to each other, i.e., the activation of one receptor would inhibit the other. This cross-talk is present in different patterns across various regions in the central nervous system; however, the factor that determines these patterns is not understood. Here, we show that the pattern of cross-talk between the two receptors is shaped by their relative expression level in a neuron: a higher expression level correlates with louder talk. In line with a tendency of decrease in expression level of GlyRs and increase in expression level of GABA_ARs from the spinal cord, the brainstem to the neocortex, GlyRs talked much louder (i.e. produced greater inhibition) than GABA_ARs (one-way pattern) in spinal cord neurons, about equally loud as GABA_ARs (symmetric pattern) in inferior colliculus neurons and less loud (i.e. less inhibition) than GABA_ARs (asymmetric pattern) in auditory cortex neurons. Overexpression of GlyRs in inferior colliculus neurons produced an asymmetric pattern that should otherwise have been observed in spinal cord neurons. These expression level-dependent patterns of cross-talk between the two receptors may suggest how the central nervous system uses an alternative mechanism to maintain a delicate level of inhibition through adjusting the proportion of the two receptors in a neuron along its pathway.

Distinct Co-Modulation Rules of Synapses and Voltage-Gated Currents Coordinate Interactions of Multiple Neuromodulators

Multiple neuromodulators act in concert to shape the properties of neural circuits. Different neuromodulators usually activate distinct receptors but can have overlapping targets. Therefore, circuit output depends on neuromodulator interactions at shared targets, a poorly understood process. We explored quantitative rules of co-modulation of two principal targets of neuromodulation: synapses and voltage-gated ionic currents. In the stomatogastric ganglion of the male crab Cancer borealis, the neuropeptides proctolin (Proc) and the crustacean cardioactive peptide (CCAP) modulate synapses of the pyloric circuit and activate a voltage-gated current (I _MI) in multiple neurons. We examined the validity of a simple dose-dependent quantitative rule, that co-modulation by Proc and CCAP is predicted by the linear sum of the individual effects of each modulator up to saturation. We found that this rule is valid for co-modulation of synapses, but not for the activation of I _MI, in which co-modulation was sublinear. The predictions for the co-modulation of I _MI activation were greatly improved if we assumed that the intracellular pathways activated by two peptide receptors inhibit one another. These findings suggest that the pathways activated by two neuromodulators could have distinct interactions, leading to distinct co-modulation rules for different targets even in the same neuron. Given the evolutionary conservation of neuromodulator receptors and signaling pathways, such distinct rules for co-modulation of different targets are likely to be common across neuronal circuits.SIGNIFICANCE STATEMENT We examine the quantitative rules of co-modulation at multiple shared targets, the first such characterization to our knowledge. Our results show that dose-dependent co-modulation of distinct targets in the same cells by the same two neuromodulators follows different rules: co-modulation of synaptic currents is linearly additive up to saturation, whereas co-modulation of the voltage-gated ionic current targeted in a single neuron is nonlinear, a mechanism that is likely generalizable. Given that all neural systems are multiply modulated and neuromodulators often act on shared targets, these findings and the methodology could guide studies to examine dynamic actions of neuromodulators at the biophysical and systems level in sensory and motor functions, sleep/wake regulation, and cognition.

Glycine Receptor Activation Impairs ATP-Induced Calcium Transients in Cultured Cortical Astrocytes

In central nervous system, glycine receptor (GlyR) is mostly expressed in the spinal cord and brainstem, but glycinergic transmission related elements have also been identified in the brain. Astrocytes are active elements at the tripartite synapse, being responsible for the maintenance of brain homeostasis and for the fine-tuning of synaptic activity. These cells communicate, spontaneously or in response to a stimulus, by elevations in their cytosolic calcium (calcium transients, Ca2+T) that can be propagated to other cells. How these Ca2+T are negatively modulated is yet poorly understood. In this work, we evaluated GlyR expression and its role on calcium signaling modulation in rat brain astrocytes. We first proved that GlyR, predominantly subunits α2 and β, was expressed in brain astrocytes and its localization was confirmed in the cytoplasm and astrocytic processes by immunohistochemistry assays. Calcium imaging experiments in cultured astrocytes showed that glycine (500 μM), a GlyR agonist, caused a concentration-dependent reduction in ATP-induced Ca2+T, an effect abolished by the GlyR antagonist, strychnine (0.8 μM), as well as by nocodazole (1 μM), known to impair GlyR anchorage to the plasma membrane. This effect was mimicked by activation of GABAAR, another Cl--permeable channel. In summary, we demonstrated that GlyR activation in astrocytes mediates an inhibitory effect upon ATP induced Ca2+T, which most probably involves changes in membrane permeability to Cl- and requires GlyR anchorage at the plasma membrane. GlyR in astrocytes may thus be part of a mechanism to modulate astrocyte-to-neuron communication.

Expression of glycine receptors and gephyrin in rat medial vestibular nuclei and flocculi following unilateral labyrinthectomy

The medial vestibular nucleus (MVN) and the cerebellar flocculus have been known to be the key areas involved in vestibular compensation (VC) following unilateral labyrinthectomy (UL). In this study, we examined the role of gephyrin and glycine receptor (GlyR) in VC using Sprague-Dawley rats, in an aim to gain deeper insight into the mechanisms responsible for VC. The expression of the α1 and β subunits of GlyR and gephyrin was immunohistochemically localized in rat MVN and flocculi. The mRNA and protein expression of GlyR (α1 and β subunits) and gephyrin was quantitatively determined by RT-qPCR and western blot analysis at 8 h, and at 1, 3 and 7 days following UL. It was found that in the ipsilateral MVN, the mRNA and protein expression of the β subunit of GlyR was significantly increased in comparison to the sham-operated (P<0.01) rats, and in comparison to the contralateral side (P<0.01) at 8 h following UL. In the ipsilateral flocculi, GlyR β protein expression was significantly elevated (P<0.01 for all), as compared to the sham-operated rats at 8 h, and at 1 and 3 days and to the contralateral side 8 h, 1 and 3 days following UL. No significant differences were observed in the mRNA and protein expression of GlyR α1 and gephyrin in the MVN or flocculi between the two sides (ipsilateral and contralateral) in the UL group, and between the sham-operated group and the UL group at any time point. The findings of our study thus suggest that GlyR plays a major role in the recovery of the resting discharge of the deafferented MVN neurons in the central vestibular system.

Effects of propofol on glycinergic neurotransmission in a single spinal nerve synapse preparation

The effects of the intravenous anesthetic, propofol, on glycinergic transmission and on glycine receptor-mediated whole-cell currents (IGly) were examined in the substantia gelatinosa (SG) neuronal cell body, mechanically dissociated from the rat spinal cord. This "synaptic bouton" preparation, which retains functional native nerve endings, allowed us to evaluate glycinergic inhibitory postsynaptic currents (IPSCs) and whole-cell currents in a preparation in which experimental solution could rapidly access synaptic terminals. Synaptic IPSCs were measured as spontaneous (s) and evoked (e) IPSCs. The eIPSCs were elicited by applying paired-pulse focal electrical stimulation, while IGly was evoked by a bath application of glycine. A concentration-dependent enhancement of IGly was observed for ≥10µM propofol. Propofol (≥3µM) significantly increased the frequency of sIPSCs and prolonged the decay time without altering the current amplitude. However, propofol (≥3µM) also significantly increased the mean amplitude of eIPSCs and decreased the failure rate (Rf). A decrease in the paired-pulse ratio (PPR) was noted at higher concentrations (≥10µM). The decay time of eIPSCs was prolonged only at the maximum concentration tested (30µM). Propofol thus acts at both presynaptic glycine release machinery and postsynaptic glycine receptors. At clinically relevant concentrations (<1μM) there was no effect on IGly, sIPSCs or eIPSCs suggesting that at anesthetic doses propofol does not affect inhibitory glycinergic synapses in the spinal cord.

Curcumol from Rhizoma Curcumae suppresses epileptic seizure by facilitation of GABA(A) receptors

Rhizoma Curcumae is a common Chinese dietary spice used in South Asia and China for thousands of years. As the main extract, Rhizoma Curcumae oil has attracted a great interest due to its newly raised therapeutic activities including its pharmacological effects upon central nervous system such as neuroprotection, cognitive enhancement, and anti-seizure efficacy; however the molecular mechanisms and the target identification remain to be established. Here we characterize an inhibitory effect of curcumol, a major bioactive component of Rhizoma Curcumae oil, on the excitability of hippocampal neurons in culture, the basal locomotor activity of freely moving animals, and the chemically induced seizure activity in vivo. Electrophysiological recording showed that acute application of curcumol significantly facilitated the γ-aminobutyric acid (GABA)-activated current in cultured mouse hippocampal neurons and in human embryonic kidney cells expressing α1- or α5-containing A type GABA (GABAA) receptors in a concentration-dependent manner. Measurement of tonic and miniature inhibitory postsynaptic GABAergic currents in hippocampal slices indicated that curcumol enhanced both forms of inhibition. In both pentylenetetrazole and kainate seizure models, curcumol suppressed epileptic activity in mice by prolonging the latency to clonic and tonic seizures and reducing the mortality as well as the susceptibility to seizure, presumably by facilitating the activation of GABAA receptors. Taken together, our results identified curcumol as a novel anti-seizure agent which inhibited neuronal excitability through enhancing GABAergic inhibition.

Glycine transporters as novel therapeutic targets in schizophrenia, alcohol dependence and pain

Glycine transporters are endogenous regulators of the dual functions of glycine, which acts as a classical inhibitory neurotransmitter at glycinergic synapses and as a modulator of neuronal excitation mediated by NMDA (N-methyl-D-aspartate) receptors at glutamatergic synapses. The two major subtypes of glycine transporters, GlyT1 and GlyT2, have been linked to the pathogenesis and/or treatment of central and peripheral nervous system disorders, including schizophrenia and related affective and cognitive disturbances, alcohol dependence, pain, epilepsy, breathing disorders and startle disease (also known as hyperekplexia). This Review examines the rationale for the therapeutic potential of GlyT1 and GlyT2 inhibition, and surveys the latest advances in the biology of glycine reuptake and transport as well as the drug discovery and clinical development of compounds that block glycine transporters.

Glycine receptors and brain development

Glycine receptors (GlyRs) are ligand-gated chloride ion channels that mediate fast inhibitory neurotransmission in the spinal cord and the brainstem. There, they are mainly involved in motor control and pain perception in the adult. However, these receptors are also expressed in upper regions of the central nervous system, where they participate in different processes including synaptic neurotransmission. Moreover, GlyRs are present since early stages of brain development and might influence this process. Here, we discuss the current state of the art regarding GlyRs during embryonic and postnatal brain development in light of recent findings about the cellular and molecular mechanisms that control brain development.

Subunit-specific inhibition of glycine receptors by curcumol

Emerging evidence has suggested that inhibitory glycine receptors (GlyRs) are an important molecular target in the treatment of numerous neurological disorders. Rhizoma curcumae is a medicinal plant with positive neurological effects. In this study, we showed that curcumol, a major bioactive component of R. curcumae, reversibly and concentration-dependently inhibited the glycine-activated current (IGly) in cultured rat hippocampal neurons. The inhibitory effect was neither voltage- nor agonist concentration-dependent. Moreover, curcumol selectively inhibited homomeric α2-containing, but not α1- or α3-containing, GlyRs. The addition of β subunit conferred the curcumol sensitivity of α3-containing, but not α1-containing, GlyRs. Site-directed mutagenesis analysis revealed that a threonine at position 59 of the α2 subunit is critical for the susceptibility of GlyRs to curcumol-mediated inhibition. Furthermore, paralleling a decline of α2 subunit expression during spinal cord development, the degree of IGly inhibition by curcumol decreased with prolonged culture of rat spinal dorsal horn neurons. Taken together, our results suggest that the GlyRs are novel molecular targets of curcumol, which may underlie its pharmaceutical effects in the central nervous system.

Cl(-) concentration changes and desensitization of GABA(A) and glycine receptors

Desensitization of ligand-gated ion channels plays a critical role for the information transfer between neurons. The current view on γ-aminobutyric acid (GABA)_A and glycine receptors includes significant rapid components of desensitization as well as cross-desensitization between the two receptor types. Here, we analyze the mechanism of apparent cross-desensitization between native GABA_A and glycine receptors in rat central neurons and quantify to what extent the current decay in the presence of ligand is a result of desensitization versus changes in intracellular Cl⁻ concentration ([Cl⁻]_i). We show that apparent cross-desensitization of currents evoked by GABA and by glycine is caused by changes in [Cl⁻]_i. We also show that changes in [Cl⁻]_i are critical for the decay of current in the presence of either GABA or glycine, whereas changes in conductance often play a minor role only. Thus, the currents decayed significantly quicker than the conductances, which decayed with time constants of several seconds and in some cells did not decay below the value at peak current during 20-s agonist application. By taking the cytosolic volume into account and numerically computing the membrane currents and expected changes in [Cl⁻]_i, we provide a theoretical framework for the observed effects. Modeling diffusional exchange of Cl⁻ between cytosol and patch pipettes, we also show that considerable changes in [Cl⁻]_i may be expected and cause rapidly decaying current components in conventional whole cell or outside-out patch recordings. The findings imply that a reevaluation of the desensitization properties of GABA_A and glycine receptors is needed.

Characteristics and interaction of GABAergic and glycinergic processes in frog spinal cord neurons

Whole-cell patch clamp recordings from isolated spinal cord neurons from the frog Rana temporaria were made to study the interaction of processes induced by application of GABA and glycine. The amplitudes of currents evoked by application of glycine did not change with time, while the amplitudes of GABA-mediated currents decreased two-fold during the first 15 min of the experiment and stabilized at the new level. Neuron responses to simultaneous application of GABA and glycine were always smaller than the sum of the responses to separate application of these neurotransmitters. On application of GABA and glycine at the same concentration (5 mM), the amplitude of the response to simultaneous application decreased with time, reaching the level of the glycine-mediated response. A mixture of glycine and GABA at 8 microM and 5 mM, respectively, gave settled responses which were larger than the largest individual response by more than obtained with other mixtures. These data provide evidence that frog motoneurons may express receptors activated by both GABA and glycine.

Differences in the activation of inhibitory motoneuron receptors in the frog Rana ridibunda by GABA and glycine and their interaction

Intracellular recording of potentials was used in isolated spinal cord segments from the frog Rana ridibunda to compare the inhibitory effects of GABA and glycine on the motoneuron membrane. At equal concentrations, the response (a change in membrane potential) to application of glycine was 1.5-2 times greater than the response to GABA in terms of amplitude, and EC(50) values were 0.75 and 1.57 mM, respectively. The response to simultaneous application of GABA and glycine averaged 79.1 +/- 2.4% (n = 19) of the sum of the individual responses and 130.1 +/- 1.5% (n = 19) of the glycine response (partial occlusion). Preliminary application of glycine decreased the GABA response by 85.3 +/- 0.2% (n = 10), while preapplication of GABA decreased the glycine response by only 52.9 +/- 0.3% (n = 11). The glycine and GABA responses were specifically suppressed by strychnine and bicuculline. These results provide evidence that as in mammals, amphibian motoneurons have both glycine (predominantly) and GABA(A) receptors; they also show that asymmetrical cross inhibition can occur.

Glycine receptor in rat hippocampal and spinal cord neurons as a molecular target for rapid actions of 17-beta-estradiol

Glycine receptors (GlyRs) play important roles in regulating hippocampal neural network activity and spinal nociception. Here we show that, in cultured rat hippocampal (HIP) and spinal dorsal horn (SDH) neurons, 17-β-estradiol (E₂) rapidly and reversibly reduced the peak amplitude of whole-cell glycine-activated currents (I_Gly). In outside-out membrane patches from HIP neurons devoid of nuclei, E₂ similarly inhibited I_Gly, suggesting a non-genomic characteristic. Moreover, the E₂ effect on I_Gly persisted in the presence of the calcium chelator BAPTA, the protein kinase inhibitor staurosporine, the classical ER (i.e. ERα and ERβ) antagonist tamoxifen, or the G-protein modulators, favoring a direct action of E₂ on GlyRs. In HEK293 cells expressing various combinations of GlyR subunits, E₂ only affected the I_Gly in cells expressing α2, α2β or α3β subunits, suggesting that either α2-containing or α3β-GlyRs mediate the E₂ effect observed in neurons. Furthermore, E₂ inhibited the GlyR-mediated tonic current in pyramidal neurons of HIP CA1 region, where abundant GlyR α2 subunit is expressed. We suggest that the neuronal GlyR is a novel molecular target of E₂ which directly inhibits the function of GlyRs in the HIP and SDH regions. This finding may shed new light on premenstrual dysphoric disorder and the gender differences in pain sensation at the CNS level.

Glycine uptake regulates hippocampal network activity via glycine receptor-mediated tonic inhibition

Functional glycine receptors (GlyRs) are enriched in the hippocampus, but their role in hippocampal function remains unclear. Since the concentration of ambient glycine is determined by the presence of powerful glycine transporter (GlyT), we blocked the reuptake of glycine in hippocampal slices to examine the role of GlyRs. Antagonists of GlyT type 1 (GlyT1) but not that of GlyT type 2 (GlyT2) induced excitatory postsynaptic potential (EPSP)-spike depression, which was reversed by the specific GlyR antagonist strychnine. Moreover, endogenously elevating the glycine concentration with the GlyT1 antagonists facilitated NMDA receptor-dependent long-term potentiation induction, and elicited a strychnine-sensitive chloride current. In addition, impairment of glial function with fluoroacetate blocked the effect of GlyT1 antagonists on the EPSP-spike curve. Furthermore, pretreatment with sarcosine was effective in controlling pentylenetetrazol-induced seizures. These results indicate an essential role of GlyTs in fine-tuning tonic activation of GlyRs and suggest a potential role of GlyR-dependent EPSP-spike depression in hippocampal network stability.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

An intracellular motif of P2X(3) receptors is required for functional cross-talk with GABA(A) receptors in nociceptive DRG neurons

Functional cross-talk between structurally unrelated P2X ATP receptors and members of the 'cys-loop' receptor-channel superfamily represents a recently-discovered mechanism for rapid modulation of information processing. The extent and the mechanism of the inhibitory cross-talks between these two classes of ionotropic receptors remain poorly understood, however. Both ionic and molecular coupling were proposed to explain cross-inhibition between P2X subtypes and GABA(A) receptors, suggesting a P2X subunit-dependent mechanism. We show here that cross-inhibition between neuronal P2X(3) or P2X(2+3) and GABA(A) receptors does not depend on chloride and calcium ions. We identified an intracellular QST(386-388) motif in P2X(3) subunits which is required for the functional coupling with GABA(A) receptors. Moreover the cross-inhibition between native P2X(3) and GABA receptors in cultured rat dorsal root ganglia (DRG) neurons is abolished by infusion of a peptide containing the QST motif as well as by viral expression of the main intracellular loop of GABA(A)beta3 subunits. We provide evidence that P2X(3) and GABA(A) receptors are colocalized in the soma and central processes of nociceptive DRG neurons, suggesting that specific intracellular P2X(3)-GABA(A) subunit interactions underlie a pre-synaptic cross-talk that might contribute to the regulation of sensory synaptic transmission in the spinal cord.

Chloride homeostasis differentially affects GABA(A) receptor- and glycine receptor-mediated effects on spontaneous circuit activity in hippocampal cell culture

The potassium-chloride cotransporter 2 (KCC2)-dependent intracellular chloride level determines whether neurons respond to GABA and/or glycine by depolarization or hyperpolarization. However, still unknown is the role of KCC2-dependent chloride homeostasis in regulating the spontaneous activity of neuronal circuits via GABA(A) receptor (GABA(A)R) and the glycine receptor (GlyR). In this study, patch-clamp recordings were performed to measure the change of spontaneous neuronal activity in cultured hippocampal neurons. Our results showed that inhibition of KCC2 with furosemide, as well as blockade of GABA(A)R with bicuculline, significantly enhanced circuit activity. Perfusion with bicuculline further enhanced the effects of furosemide on spontaneous circuit activity, while furosemide did not alter the effects of bicuculline. Surprisingly, blockade of GlyR not only induced obvious tonic currents, but also significantly decreased spontaneous synaptic activity. Moreover, inhibition of KCC2 did not change the depressive effect of strychnine on neuronal circuits. Our findings suggest that KCC2-dependent chloride homeostasis is mainly involved in GABA(A)R-mediated synaptic inhibition whereas GlyR-mediated tonic action plays a totally different role in regulating hippocampal circuit activity.

Glycine receptor activation regulates short-term plasticity in CA1 area of hippocampal slices of rats

Functional glycine receptors (GlyRs) are enriched in the hippocampus, but their roles in synaptic transmission are unclear. In this study, we examined the effect of GlyR activation on paired-pulse stimulation of the whole-cell postsynaptic currents (PSCs) in the Schaffer-CA1 synapses in rat hippocampal slices. Bath application of glycine reduced the amplitude of PSCs, accompanied by an increase in holding current and resting conductance. Moreover, glycine application increased the paired-pulse ratio (PPR) of PSCs significantly, an effect largely abolished by the GlyR specific antagonist strychnine. Interestingly, glycine application had no significant effect on either the amplitude or the PPR of excitatory postsynaptic currents (EPSCs). Our findings suggest that GlyR activation regulates hippocampal short-term plasticity by altering GABAergic neurotransmission.

Thiopental inhibits glycine receptor function in acutely dissociated rat spinal dorsal horn neurons

Whole-cell patch-clamp was used to assess the modulatory effect of thiopental (Thio) on glycine (Gly) receptor in mechanically dissociated rat spinal dorsal horn neurons. It was found that Thio inhibited the amplitude, accelerated the desensitization and prolonged the deactivation of Gly-induced currents (IGly) in a concentration-dependent manner. In addition, a rebound current occurred after washout of the co-application of Gly and Thio in most neurons tested. Moreover, the inhibitory effect of Thio was not the result of cross-inhibition between Gly and GABAA receptors. Furthermore, taurine-induced currents, a low-affinity agonist for Gly receptors, were also markedly inhibited by Thio in a similar way to IGly. These results indicate that Thio suppresses Gly receptor function and suggest that Thio anesthetic actions might not be mediated by Gly receptors. We speculate that the weak muscle relaxation and the limited analgesic effects observed during Thio anesthesia may attribute to its inhibitory effects on Gly receptors.

Characterization of the inhibitory glycine receptor on entorhinal cortex neurons

In addition to the well-established functional description of the glycine receptor (GlyR) in the spinal cord, GlyR expression has recently been found in higher brain regions, such as the striatum or hippocampus. In this study we have investigated the electrophysiological response of glycine in the rat entorhinal cortex slice. In all recorded cells we found significant current responses to glycine with an EC(50) value of about 100 micro m. Most importantly, we detected a cross-inhibition of glycine responses by GABA but not vice versa. These findings are in line with recent published data of cross-talks between GABA(A)R and GlyR but indicate a novel type of cross-inhibition of these receptors in the entorhinal cortex.

Gephyrin is critical for glycine receptor clustering but not for the formation of functional GABAergic synapses in hippocampal neurons

The role of the scaffolding protein gephyrin at hippocampal inhibitory synapses is not well understood. A previous study (Kneussel et al., 1999) reported a complete loss of synaptic clusters of the major GABA(A)R subunits alpha2 and gamma2 in hippocampal neurons lacking gephyrin. In contrast, we show here that GABA(A)R alpha2 and gamma2 subunits do cluster at pyramidal synapses in hippocampal cultures from gephyrin-/- mice, albeit at reduced levels compared with control neurons. Synaptic aggregation of GABA(A)R alpha1 on interneurons was identical between the culture types. Furthermore, we recorded miniature IPSCs (mIPSCs) from gephyrin-/- neurons. Although the mean mIPSC amplitude was reduced (by 23%) compared with control, the frequency of these events was unchanged. Cell surface labeling experiments indicated that gephyrin contributes, in part, to aggregation but not to insertion or stabilization of GABA(A)R alpha2 and gamma2 in the plasma membrane. Thus, a major gephyrin-independent component of hippocampal inhibitory synapse development must exist. We also report that glycine receptors cluster at GABAergic synapses in a subset of hippocampal interneurons and pyramidal neurons. Unlike GABA(A)Rs, synaptic clustering of glycine receptors was completely abolished in gephyrin-/- neurons. Finally, artificial extrasynaptic aggregation of GABA(A)R was able to redistribute and cocluster gephyrin by a mechanism requiring a neuron-specific modification or intermediary protein. We propose a model of hippocampal inhibitory synapse development in which some GABA(A)Rs cluster at synapses by a gephyrin-independent mechanism and recruit gephyrin. This clustered gephyrin may then recruit glycine receptors, additional GABA(A)Rs, and other signal-transducing components.

Taurine activates strychnine-sensitive glycine receptors in neurons freshly isolated from nucleus accumbens of young rats

Although functional glycine receptors (GlyRs) are present in the mature nucleus accumbens (NAcc), an important area of the mesolimbic dopamine system involved in drug addiction, their role has been unclear because the NAcc contains little glycine. However, taurine, an agonist of GlyRs, is abundant throughout the brain, especially during early development. In the present study on freshly dissociated NAcc neurons from young Sprague-Dawley rats (12- to 21-day old), we found that both glycine and taurine can strongly depolarize NAcc neurons and modulate their excitability. In voltage-clamped NAcc neurons, glycine and taurine elicited chloride currents (IGly and ITau) with an EC50 of 0.12 and 1.25 mM, respectively. The reversal potential of IGly or ITau was 0 mV in conventional whole cell mode and -30 mV in gramicidin-perforated mode. At concentrations <1 mM, both glycine and taurine were very effectively antagonized by strychnine and by picrotoxin (with an IC50 of 60 nM and 36.5 microM for IGly, and 40 nM and 42.2 microM for ITau) but were insensitive to 10 microM bicuculline. The currents elicited by taurine (< or =1 mM) showed complete cross-desensitization with IGly, but none with gamma-aminobutyric acid (GABA)-induced currents (IGABA). However, ITau elicited by very concentrated taurine (10 mM) showed partial cross-desensitization with IGABA, and it was substantially antagonized by 10 microM bicuculline. These results indicate that taurine binds mainly to GlyRs in NAcc, but it could be a partial agonist of GABAA receptors. By activating GlyRs, taurine may play an important physiological role in the control of NAcc function, especially during development.

Asymmetric cross-inhibition between GABAA and glycine receptors in rat spinal dorsal horn neurons

Presynaptic nerve terminals of inhibitory synapses in the dorsal horn of the spinal cord and brain stem can release both GABA and glycine, leading to coactivation of postsynaptic GABAA and glycine receptors. In the present study we have analyzed functional interactions between GABAA and glycine receptors in acutely dissociated neurons from rat sacral dorsal commissural nucleus. Although the application of GABA and glycine activates pharmacologically distinct receptors, the current induced by a simultaneous application of these two transmitters was less than the sum of currents induced by applying two transmitters separately. Sequential application of glycine and GABA revealed that the GABA-evoked current is more affected by glycine than glycine-evoked responses by GABA. Activation of glycine receptors decreased the amplitude and accelerated the rate of desensitization of GABA-induced currents. This asymmetric cross-inhibition is reversible, dependent on the agonist concentration applied, but independent of both membrane potential and intracellular calcium concentration or changes in the chloride equilibrium potential. During sequential applications, the asymmetric cross-inhibition was prevented by selective GABAA or glycine receptor antagonists, suggesting that occupation of binding sites did not suffice to induce glycine and GABAA receptors functional interaction, and receptor channel activation is required. Furthermore, inhibition of phosphatase 2B, but not phosphatase 1 or 2A, prevented GABAA receptor inhibition by glycine receptor activation, whereas inhibition of phosphorylation pathways rendered cross-talk irreversible. Taken together, our results demonstrated that there is an asymmetric cross-inhibition between glycine and GABAA receptors and that a selective modulation of the state of phosphorylation of GABAA receptor and/or mediator proteins underlies the asymmetry in the cross-inhibition.

Inhibition of glycine response by amiloride in rat spinal neurons

The modulatory effect of amiloride on glycine-activated current (I(Gly)) was investigated in acutely dissociated rat spinal dorsal horn neurons using the whole-cell patch clamp technique. Amiloride inhibited I(Gly) reversibly in a concentration-dependent manner. It shifted the concentration-response relationship to the right without altering the maximum response and Hill coefficient of the I(Gly). Amiloride did not change the ion selectivity of glycine receptor either. In addition, Na(+) - or Ca(2+) -free extracellular solutions and intracellular application of amiloride did not alter the amiloride inhibition of I(Gly). These results indicate that amiloride directly inhibited the glycine receptor response by decreasing the affinity of glycine to its receptor.

The general anesthetic pentobarbital slows desensitization and deactivation of the glycine receptor in the rat spinal dorsal horn neurons

Although many general anesthetics have been found to produce anesthetic and analgesic effects by augmenting GABA(A) receptor (GABA(A)R) function, the role of the glycine receptor (GlyR) in this process is not fully understood at the neuronal level in the spinal cord. We investigated the effects of a barbiturate general anesthetic, pentobarbital (PB), on the glycinergic miniature inhibitory postsynaptic currents (mIPSCs) and the responses to exogenously applied glycine, or taurine, a low affinity GlyR agonist, by using the whole-cell patch-clamp technique in the rat spinal dorsal horn neurons isolated using a novel mechanical method. Bath application of 30 microm PB significantly prolonged the decay time constant of the spontaneous glycinergic mIPSC without changing its amplitude and frequency. Co-application of 0.3 mm PB reduced the peak amplitude, affected the macroscopic desensitization and deactivation of the response to externally applied Gly in a concentration-dependent manner. In addition, the recovery of Gly response from desensitization was also prolonged by PB. However, PB did not change the desensitization and deactivation kinetics of the taurine-induced response. The GABA(A)R antagonist bicuculline (10 microm) did not affect the effect of PB on the Gly response. Thus, PB prolonged the spinal glycinergic mIPSCs by slowing desensitization and deactivation of GlyR. Two other structurally different intravenous anesthetics, i.e. propofol (10 microm) and etomidate (3 microm), prolonged the duration of the glycinergic mIPSC in the rat spinal dorsal horn neurons. In conclusion, on GlyR-Cl(-) channel complexes there may exist action site(s) of intravenous general anesthetics. GlyR and glycinergic neurotransmission may play an important role in the modulation of general anesthesia in the mammalian spinal cord.

The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat

The spinal cord plays an important role in modulating anesthetic-induced suppression of nociceptive transmission. To gain some insight into the anesthetic mechanisms of propofol at the spinal level, we investigated the direct action of propofol and its modulation on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol induced Cl(-) currents (I(Cl)), which were sensitive to bicuculline and, to a lesser extent, to strychnine. The activation, desensitization, and deactivation of propofol-induced I(Cl) were slower than those of GABA- and glycine-induced I(Cl). In addition, this study revealed similar modulatory actions of propofol on GABA(A)R and GlyR. Propofol potentiated both GABA- and glycine-induced I(Cl) at small con-centrations and inhibited both GABA- and glycine-induced I(Cl) at large concentrations. The potentiation of propofol on I(Cl) was caused by slowing current desensitization and deactivation, whereas the inhibition actions might be involved in the cross-desensitization between GABA- and propofol-induced I(Cl) and the cross-inhibition between the GABA(A)R and GlyR. The results suggest that propofol facilitation of GABA(A)R and GlyR at the spinal level could contribute significantly to general anesthetic-induced analgesia and anesthesia.

IMPLICATIONS

The actions of propofol on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) were investigated in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol was found to potentiate the functions of GABA(A)R and GlyR at the spinal level, which might contribute to propofol-induced analgesia and anesthesia.