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

The effect of psychoactive bacteria, Bifidobacterium longum Rosell®-175 and Lactobacillus rhamnosus JB-1, on brain proteome profiles in mice

RATIONALE

The gut microbiota may play an important role in the development and functioning of the mammalian central nervous system. The assumption of the experiment was to prove that the use of probiotic bacterial strains in the diet of mice modifies the expression of brain proteins involved in metabolic and immunological processes.

OBJECTIVES AND RESULTS

Albino Swiss mice were administered with Bifidobacterium longum Rosell®-175 or Lactobacillus rhamnosus JB-1 every 24 h for 28 days. Protein maps were prepared from hippocampal homogenates of euthanized mice. Selected proteins that were statistically significant were purified and concentrated and identified using MALDI-TOF mass spectrometry. Among the analysed samples, 13 proteins were identified. The mean volumes of calcyon, secreted frizzled-associated protein 3, and catalase in the hippocampus of mice from both experimental groups were statistically significantly higher than in the control group. In mice supplemented with Lactobacillus rhamnosus JB-1, a lower mean volume of fragrance binding protein 2, shadow of prion protein, and glycine receptor α4 subunit was observed compared to the control.

CONCLUSION

The psychobiotics Bifidobacterium longum Rosell®-175 and Lactobacillus rhamnosus JB-1enhances expression of proteins involved in the activation and maturation of nerve cells, as well as myelination and homeostatic regulation of neurogenesis in mice. The tested psychobiotics cause a decrease in the expression of proteins associated with CNS development and in synaptic transmission, thereby reducing the capacity for communication between nerve cells. The results of the study indicate that psychobiotic bacteria can be used in auxiliary treatment of neurological disorders.

Ligand gated receptor interactions: A key to the power of neuronal networks

The discovery of the chemical synapse was a seminal finding in Neurobiology but the large body of microscopic interactions involved in synaptic transmission could hardly have been foreseen at the time of these first discoveries. Characterization of the molecular players at work at synapses and the increased granularity at which we can now analyze electrical and chemical signal processing that occur in even the simplest neuronal system are shining a new light on receptor interactions. The aim of this review is to discuss the complexity of some representative interactions between excitatory and inhibitory ligand-gated ion channels and/or G protein coupled receptors, as well as other key machinery that can impact neurotransmission and to explain how such mechanisms can be an important determinant of nervous system function.

Electrophysiology of ionotropic GABA receptors

GABA_A receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABA_A receptors play in the regulation of neuronal communication and the functioning of the brain. GABA_A receptors have complex electrophysiological properties that enable them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by membrane voltage, phosphorylation and several ions. GABA_A receptors are pentameric and are assembled from a diverse set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure and molecular activity between subtypes. Therefore, the diversity of GABA_A receptors widens the neuronal repertoire of responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.

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.

The synthetic cannabinoid dehydroxylcannabidiol restores the function of a major GABAA receptor isoform in a cell model of hyperekplexia

The functions of the glycine receptor (GlyR) and GABAA receptor (GABAAR) are both impaired in hyperekplexia, a neurological disorder usually caused by GlyR mutations. Although emerging evidence indicates that cannabinoids can directly restore normal GlyR function, whether they affect GABAAR in hyperekplexia remains unknown. Here we show that dehydroxylcannabidiol (DH-CBD), a synthetic nonpsychoactive cannabinoid, restores the GABA- and glycine-activated currents (IGABA and IGly , respectively) in HEK293 cells coexpressing a major GABAAR isoform (α1β2γ2) and GlyRα1 carrying a human hyperekplexia-associated mutation (GlyRα1R271Q). Using coimmunoprecipitation and FRET assays, we found that DH-CBD disrupts the protein interaction between GABAAR and GlyRα1R271Q Furthermore, a point mutation of GlyRα1, changing Ser-296 to Ala-296, which is critical for cannabinoid binding on GlyR, significantly blocked DH-CBD-induced restoration of IGABA and IGly currents. This S296A substitution also considerably attenuated DH-CBD-induced disruption of the interaction between GlyRα1R271Q and GABAAR. These findings suggest that, because it restores the functions of both GlyRα1 and GABAAR, DH-CBD may represent a potentially valuable candidate drug to manage hyperekplexia.

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.

A calcium-dependent pathway underlies activity-dependent plasticity of electrical synapses in the thalamic reticular nucleus

KEY POINTS

Electrical synapses are modified by various forms of activity, including paired activity in coupled neurons and tetanization of the input to coupled neurons. We show that plasticity of electrical synapses that results from paired spiking activity in coupled neurons depends on calcium influx and calcium-initiated signalling pathways. Plasticity that results from tetanization of input fibres does not depend on calcium influx or dynamics. These results imply that electrically coupled neurons have distinct sets of mechanisms for adjusting coupling according to the specific type of activity they experience.

ABSTRACT

Recent results have demonstrated modification of electrical synapse strength by varied forms of neuronal activity. However, the mechanisms underlying plasticity induction in central mammalian neurons are unclear. Here we show that the two established inductors of plasticity at electrical synapses in the thalamic reticular nucleus - paired burst spiking in coupled neurons, and mGluR-dependent tetanization of synaptic input - are separate pathways that converge at a common downstream endpoint. Using occlusion experiments and pharmacology in patched pairs of coupled neurons in vitro, we show that burst-induced depression depends on calcium entry via voltage-gated channels, is blocked by BAPTA chelation, and recruits intracellular calcium release on its way to activation of phosphatase activity. In contrast, mGluR-dependent plasticity is independent of calcium entry or calcium dynamics. Together, these results show that the spiking-initiated mechanisms underlying electrical synapse plasticity are similar to those that induce plasticity at chemical synapses, and offer the possibility that calcium-regulated mechanisms may also lead to alternate outcomes, such as potentiation. Because these mechanistic elements are widely found in mature neurons, we expect them to apply broadly to electrical synapses across the brain, acting as the crucial link between neuronal activity and electrical synapse strength.

Curcumol allosterically modulates GABA(A) receptors in a manner distinct from benzodiazepines

Inhibitory A type γ-aminobutyric acid receptors (GABA_ARs) play a pivotal role in orchestrating various brain functions and represent an important molecular target in neurological and psychiatric diseases, necessitating the need for the discovery and development of novel modulators. Here, we show that a natural compound curcumol, acts as an allosteric enhancer of GABA_ARs in a manner distinct from benzodiazepines. Curcumol markedly facilitated GABA-activated currents and shifted the GABA concentration-response curve to the left in cultured hippocampal neurons. When co-applied with the classical benzodiazepine diazepam, curcumol further potentiated GABA-induced currents. In contrast, in the presence of a saturating concentration of menthol, a positive modulator for GABA_AR, curcumol failed to further enhance GABA-induced currents, suggesting shared mechanisms underlying these two agents on GABA_ARs. Moreover, the benzodiazepine antagonist flumazenil did not alter the enhancement of GABA response by curcumol and menthol, but abolished that by DZP. Finally, mutations at the β2 or γ2 subunit predominantly eliminated modulation of recombinant GABA_ARs by curcumol and menthol, or diazepam, respectively. Curcumol may therefore exert its actions on GABA_ARs at sites distinct from benzodiazepine sites. These findings shed light on the future development of new therapeutics drugs targeting GABA_ARs.

Synaptic Inhibition in Avian Interaural Level Difference Sound Localizing Neurons

Synaptic inhibition plays a fundamental role in the neural computation of the interaural level difference (ILD), an important cue for the localization of high-frequency sound. Here, we studied the inhibitory synaptic currents in the chicken posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), the first binaural level difference encoder of the avian auditory pathway. Using whole-cell recordings in brain slices, we provide the first evidence confirming a monosynaptic inhibition driven by direct electrical and chemical stimulation of the contralateral LLDp, establishing the reciprocal inhibitory connection between the two LLDps, a long-standing assumption in the field. This inhibition was largely mediated by GABAA receptors; however, functional glycine receptors were also identified. The reversal potential for the Cl- channels measured with gramicidin-perforated patch recordings was hyperpolarizing (-88 mV), corresponding to a low intracellular Cl- concentration (5.2 mm). Pharmacological manipulations of KCC2 (outwardly Cl- transporter) activity demonstrate that LLDp neurons can maintain a low intracellular Cl- concentration under a high Cl- load, allowing for the maintenance of hyperpolarizing inhibition. We further demonstrate that hyperpolarizing inhibition was more effective at regulating cellular excitability than depolarizing inhibition in LLDp neurons.

An economic method to build a puffing instrument for drug application in vitro

BACKGROUND

In in vitro electrophysiological studies, a quick application of picoliters of drug within milliseconds is required to avoid the desensitization of membrane receptors. However, conventional gravity-fed drug delivery devices sometime fail to achieve this. Moreover, the high financial cost of the advanced drug delivery system often limits the application of commercial instruments in academic research.

NEW METHOD

Taking advantage of the availability of data acquisition system and software in almost every electrophysiology laboratory, a simple puffing device was designed and assembled using low-cost commercially off-the-shelf components to inject picoliter amounts of drugs.

RESULTS

An optimal drug delivery with precise timing and volume was achieved using the custom made puffing device. The glutamate-evoked currents of cortical neurons recorded with patch-clamp technique were maintained for a prolonged period of time. Similarly, puffed inhibitory transmitters including GABA and glycine also produced satisfactory currents.

COMPARISON WITH EXISTING METHOD(S)

Our custom-made puffing system holds the advantage over conventional gravity-fed systems in operating within milliseconds of time. The channel number of the new device can easily be increased by simply adding more identical modules in parallel, and thus offering more flexibility than commercial puffing devices.

CONCLUSIONS

This custom-made puffing device can be characterized as reliable, modular and inexpensive system for modern drug delivery research and application.

Glycinergic transmission modulates GABAergic inhibition in the avian auditory pathway

For all neurons, a proper balance of synaptic excitation and inhibition is crucial to effect computational precision. Achievement of this balance is remarkable when one considers factors that modulate synaptic strength operate on multiple overlapping time scales and affect both pre- and postsynaptic elements. Recent studies have shown that inhibitory transmitters, glycine and GABA, are co-released in auditory nuclei involved in the computation of interaural time disparities (ITDs), a cue used to process sound source location. The co-release expressed at these synapses is heavily activity dependent, and generally occurs when input rates are high. This circuitry, in both birds and mammals, relies on inhibitory input to maintain the temporal precision necessary for ITD encoding. Studies of co-release in other brain regions suggest that GABA and glycine receptors (GlyRs) interact via cross-suppressive modulation of receptor conductance. We performed in vitro whole-cell recordings in several nuclei of the chicken brainstem auditory circuit to assess whether this cross-suppressive phenomenon was evident in the avian brainstem. We evaluated the effect of pressure-puff applied glycine on synaptically evoked inhibitory currents in nucleus magnocellularis (NM) and the superior olivary nucleus (SON). Glycine pre-application reduced the amplitude of inhibitory postsynaptic currents (IPSCs) evoked during a 100 Hz train stimulus in both nuclei. This apparent glycinergic modulation was blocked in the presence of strychnine. Further experiments showed that this modulation did not depend on postsynaptic biochemical interactions such as phosphatase activity, or direct interactions between GABA and GlyR proteins. Rather, voltage clamp experiments in which we manipulated Cl⁻ flux during agonist application suggest that activation of one receptor will modulate the conductance of the other via local changes in Cl⁻ ion concentration within microdomains of the postsynaptic membrane.

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.

Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

Background

5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices.

Results

Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba²⁺ (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT_2A receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT_2A receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT_2C receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT_2C receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT_2A or 5-HT_2C agonists. A 5-HT₃ receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT₃ receptor antagonist ICS-205,930. The perfusion of 5-HT_2B receptor agonist had no effect on sIPSCs.

Conclusions

Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT_2A and 5-HT_2C receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT₃ receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.

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.

Interaction between taurine and GABA(A)/glycine receptors in neurons of the rat anteroventral cochlear nucleus

Taurine, one of the most abundant endogenous amino acids in the mammalian central nervous system (CNS), is involved in neural development and many physiological functions. In this study, the interaction between taurine and GABA(A)/glycine receptors was investigated in young rat (P13-P15) anteroventral cochlear nucleus (AVCN) neurons using the whole-cell patch-clamp method. We found that taurine at low (0.1mM) and high (1mM) concentrations activated both GABA(A) and glycine receptors, but not AMPA and NMDA receptors. The reversal potentials of taurine-, GABA- or glycine-evoked currents were close to the expected chloride equilibrium potential, indicating that receptors activated by these agonists were mediating chloride conductance. Moreover, our results showed that the currents activated by co-application of GABA and glycine were cross-inhibitive. Sequential application of GABA and glycine or vice versa also reduced the glycine or GABA evoked currents. There was no cross-inhibition when taurine and GABA or taurine and glycine were applied simultaneously, but the response was larger than that evoked by GABA or glycine alone. These results suggest that taurine can serve as a neuromodulator to strengthen GABAergic and glycinergic neurotransmission in the rat AVCN.

Modulation of synaptic input by GABAB receptors improves coincidence detection for computation of sound location

Interaural time disparities (ITDs) are the primary cues for localisation of low-frequency sound stimuli. ITDs are computed by coincidence-detecting neurones in the medial superior olive (MSO) in mammals. Several previous studies suggest that control of synaptic gain is essential for maintaining ITD selectivity as stimulus intensity increases. Using acute brain slices from postnatal day 7 to 24 (P7–P24) Mongolian gerbils, we confirm that activation of GABAB receptors reduces the amplitude of excitatory and inhibitory synaptic currents to the MSO and, moreover, show that the decay kinetics of IPSCs are slowed in mature animals. During repetitive stimuli, activation of GABAB receptors reduced the amount of depression observed, while PSC suppression and the slowed kinetics were maintained. Additionally, we used physiological and modelling approaches to test the potential impact of GABAB activation on ITD encoding in MSO neurones. Current clamp recordings from MSO neurones were made while pharmacologically isolated excitatory inputs were bilaterally stimulated using pulse trains that simulate ITDs in vitro. MSO neurones showed strong selectivity for bilateral delays. Application of both GABAB agonists and antagonists demonstrate that GABAB modulation of synaptic input can sharpen ITD selectivity. We confirmed and extended these results in a computational model that allowed for independent manipulation of each GABAB-dependent effect. Modelling suggests that modulation of both amplitude and kinetics of synaptic inputs by GABAB receptors can improve precision of ITD computation. Our studies suggest that in vivo modulation of synaptic input by GABAB receptors may act to preserve ITD selectivity across various stimulus conditions.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

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.

GABA(A) Receptors: Post-Synaptic Co-Localization and Cross-Talk with Other Receptors

γ-Aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the central nervous system, and importantly contribute to the functional regulation of the nervous system. Several studies in the last few decades have convincingly shown that GABA can be co-localized with other neurotransmitters in the same synapse, and can be co-released with these neurotransmitters either from the same vesicles or from different vesicle pools. The co-released transmitters may act on post-synaptically co-localized receptors resulting in a simultaneous activation of both receptors. Most of the studies investigating such co-activation observed a reduced efficacy of GABA for activating GABA_ARs and thus, a reduced inhibition of the post-synaptic neuron. Similarly, in several cases activation of GABA_ARs has been reported to suppress the response of the associated receptors. Such a receptor cross-talk is either mediated via a direct coupling between the two receptors or via the activation of intracellular signaling pathways and is used for fine tuning of inhibition in the nervous system. Recently, it was demonstrated that a direct interaction of different receptors might already occur in intracellular compartments and might also be used to specifically target the receptors to the cell membrane. In this article, we provide an overview on such cross-talks between GABA_ARs and several other neurotransmitter receptors and briefly discuss their possible physiological and clinical importance.

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.

Functional interactions between nicotinic and P2X receptors in celiac ganglia neurons

Here we characterized the cross-inhibitory interactions between nicotinic and P2X receptors of celiac neurons from the guinea pig by recording whole-cell currents induced by 1mM ACh (I(ACh)), 1mM ATP (I(ATP)) and by the simultaneous application of both agonists (I(ACh)(+ATP)). I(ACh) and I(ATP) were inhibited by hexamethonium (nicotinic channel blocker) and PPADS (P2X receptor antagonist), respectively. The amplitude of I(ACh)(+ATP) was equal to the current induced by the most effective agonist, indicating a current occlusion. Various observations indicate that I(ACh)(+ATP) is carried out through both nicotinic (nACh) and P2X channels: i) I(ACh)(+ATP) desensitisation kinetics were in between that of I(ACh) and I(ATP); ii) application of ATP+ACh, decreased I(ACh) and I(ATP), whereas no cross-desensitisation was observed between nACh and P2X receptors; iii) ATP did not affect I(ACh) in the presence of PPADS or after P2X receptor desensitisation; and iv) ACh did not affect I(ATP) when nACh channels were blocked with hexamethonium or after nACh receptor desensitisation. Current occlusion is not mediated by activation of metabotropic receptors as it is: i) voltage dependent (was not observed at + 5 mV); ii) present at low temperature (10 degrees C) and after inhibition of protein kinase activity (with staurosporine); and iii) absent at 30 microM ATP and 30 microM ACh (concentrations that should activate metabotropic receptors). In conclusion, current occlusion described here is similar to the previously reported myenteric neurons. This occlusion is likely the result of allosteric interactions between these receptors.

Progesterone and allopregnanolone enhance the miniature synaptic release of glycine in the rat hypoglossal nucleus

It is well known that progesterone is synthesised and metabolised within the nervous system, and that one of its metabolites, allopregnanolone, potentiates the activity of GABA receptor anionic channels and modulates GABAergic neurotransmission. Progesterone is now under clinical trial for its neuroprotective properties, but its possible effects on neurotransmission have not yet been fully explored. The present study investigated acute effects of progesterone on the other major type of synaptic inhibition, glycinergic neurotransmission. Spontaneous glycinergic miniature currents were recorded in hypoglossal motoneurons, using the whole-cell patch-clamp technique in rat brainstem slices. A 20-min superfusion with progesterone (1 mum) triggered an increase in the frequency of glycinergic miniatures, whereas no effect of progesterone was observed after block with finasteride (5 mum) of 5alpha -reductase, the first enzymatic step leading from progesterone to allopregnanolone. The effect of progesterone could be mimicked by superfusion with allopregnanolone (0.3 mum), whereas no effect was induced by epiallopregnanolone. Thus, progesterone can increase the synaptic miniature release of glycine and this effect appears to be indirect, resulting from its metabolism into 5alpha-reduced derivatives, in particular into allopregnanolone. A low concentration of an exogenous GABA(A) agonist can also increase the frequency of inhibitory miniature currents in hypoglossal motoneurons. Thus, the effects of progesterone and allopregnanolone on glycine release can be at least partly explained by the potentiation of the activity of depolarizing presynaptic GABA receptor channels. The increase in the tonic synaptic release of a major inhibitory neurotransmitter should reduce the excitability of the neurons and contribute to their protection against excitotoxicity.

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.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

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.

Inhibition of chloride outward transport by gadolinium in cultured rat spinal cord neurons

Gadolinium is a rare-earth lanthanide metal ion and is used as organic gadolinium complexes in magnetic resonance imaging (MRI). Although gadolinium-based MRI agents are thought to be safe in clinical use, the in vivo release of the toxic free inorganic gadolinium (Gd3+) has been reported in some patients with kidney disease. In central nervous system neurons, the inhibitory action of GABA is a consequence of relatively hyperpolarized Cl- equilibrium potential (ECl), which results from the activity of K+-Cl- co-transporter (KCC). The lanthanide ions are reported to affect GABAA receptors. However, little is known about the effect of Gd3+ on GABAA receptor function with intact intracellular Cl- concentration. In the present study, we investigated the effect of Gd3+ on GABAA receptor-mediated currents using gramicidin perforated patch recording method in cultured rat spinal cord neurons. The application of muscimol, a GABAA receptor agonist, caused outward current at a holding potential of -50 mV. Gd3+ inhibited the muscimol-induced outward current in a concentration-dependent and reversible manner. Gd3+ inhibited the maximum muscimol response but had no effect on the half-maximum concentration. The Gd3+ inhibition was accompanied by a depolarizing shift of the reversal potential. The Gd3+ action was blocked by furosemide, a blocker of both KCC and Na+-K+-Cl- co-transporter (NKCC), but not bumetanide, a specific blocker of NKCC. Gd3+ failed to inhibit the muscimol-induced outward currents recorded by conventional whole-cell patch-clamp method which cannot retain intact intracellular Cl- concentration. These results suggest that Gd3+ inhibits a KCC function and gives rise to increase in intracellular Cl- concentration. The reduction of outward chloride transport could be related to the neurotoxic effects of Gd3+.

Frequency-dependent glycinergic inhibition modulates plasticity in hippocampus

Previous studies have demonstrated the presence of functional glycine receptors (GlyRs) in hippocampus. In this work, we examine the baseline activity and activity-dependent modulation of GlyRs in region CA1. We find that strychnine-sensitive GlyRs are open in the resting CA1 pyramidal cell, creating a state of tonic inhibition that "shunts" the magnitude of EPSPs evoked by electrical stimulation of the Schaffer collateral inputs. This GlyR-mediated shunting conductance is independent of the presynaptic stimulation rate; however, pairs of presynaptic and postsynaptic action potentials, repeated at frequencies above 5 Hz, reduce the GlyR-mediated conductance and increase peak EPSP magnitudes to levels at least 20% larger than those seen with presynaptic stimulation alone. We refer to this phenomenon as rate-dependent efficacy (RDE). Exogenous GlyR agonists (glycine, taurine) block RDE by preventing the closure of postsynaptic GlyRs. The GlyR antagonist strychnine blocks postsynaptic GlyRs under all conditions, occluding RDE. During RDE, GlyRs are less responsive to local glycine application, suggesting that a reduction in the number or sensitivity of membrane-inserted GlyRs underlies RDE. By extending the RDE induction protocol to include 500 paired presynaptic and postsynaptic spikes, we can induce long-term synaptic depression (LTD). Manipulations that lead to reduced functionality of GlyRs, either pharmacologically or through RDE, also lead to increased LTD. This result suggests that RDE contributes to long-term synaptic plasticity in the hippocampus.

Interaction of the postsynaptic effects of glycine and GABA on spinal cord neurons in the frog Rana temporaria

Studies were performed on mechanically isolated spinal cord multipolar cells (presumptive motoneurons) from the frog Rana temporaria using patch-clamp methods in the whole-cell configuration. These experiments showed that the amplitudes of transmembrane currents arising in response to simultaneous application of GABA and glycine were smaller than the sums of the amplitudes of the responses of the same neurons to GABA and glycine applied individually. Investigation of the mechanisms of this occlusion showed that superfusion of neurons with glycine solution (0.2 mM) resulted in complete blockade of responses to application of GABA (5 mM) and vice versa. This phenomenon may have resulted from cross-blockade associated with the existence of a single receptor complex sensitive to both GABA and glycine and from the interaction of GABA and glycine receptors.

Extrasynaptic and postsynaptic receptors in glycinergic and GABAergic neurotransmission: a division of labor?

Glycine and GABA mediate inhibitory neurotransmission in the spinal cord and central nervous system. The general concept of neurotransmission is now challenged by the contribution of both phasic activation of postsynaptic glycine and GABA(A) receptors (GlyRs and GABA(A)Rs, respectively) and tonic activity of these receptors located at extrasynaptic sites. GlyR and GABA(A)R kinetics depend on several parameters, including subunit composition, subsynaptic localization and activation mode. Postsynaptic and extrasynaptic receptors display different subunit compositions and are activated by fast presynaptic and slow paracrine release of neurotransmitters, respectively. GlyR and GABA(A)R functional properties also rely on their aggregation level, which is higher at postsynaptic densities than at extrasynaptic loci. Finally, these receptors can co-aggregate at mixed inhibitory postsynaptic densities where they cross-modulate their activity, providing another parameter of functional complexity. GlyR and GABA(A)R density at postsynaptic sites results from the balance between their internalization and insertion in the plasma membrane, but also on their lateral diffusion from and to the postsynaptic loci. The dynamic exchange of receptors between synaptic and extrasynaptic sites and their functional adaptation in terms of kinetics point out a new adaptive process of inhibitory neurotransmission.

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.

GABAA and glycine receptor-mediated transmission in rat lamina II neurones: relevance to the analgesic actions of neuroactive steroids

Analgesic neurosteroids such as 5alpha-pregnan-3alpha-ol-20-one (5alpha3alpha) are potent selective endogenous modulators of the GABA(A) receptor (GABA(A)R) while certain synthetic derivatives (i.e. minaxolone) additionally enhance the function of recombinant glycine receptors (GlyR). Inhibitory transmission within the superficial dorsal horn has been implicated in mediating the analgesic actions of neurosteroids. However, the relative contribution played by synaptic and extrasynaptic receptors is unknown. In this study, we have compared the actions of 5alpha3alpha and minaxolone upon inhibitory transmission mediated by both GABA(A) and strychnine-sensitive GlyRs in lamina II neurones of juvenile (P15-21) rats. At the near physiological temperature of 35 degrees C and at a holding potential of -60 mV we recorded three kinetically distinct populations of miniature IPSCs (mIPSCs): GlyR-mediated, GABA(A)R-mediated and mixed GABA(A)R-GlyR mIPSCs, arising from the corelease of both inhibitory neurotransmitters. In addition, sequential application of strychnine and bicuculline revealed a small (5.2 +/- 1.0 pA) GlyR- but not a GABA(A)R-mediated tonic conductance. 5alpha3alpha (1-10 microm) prolonged GABA(A)R and mixed mIPSCs in a concentration-dependent manner but was without effect upon GlyR mIPSCs. In contrast, minaxolone (1-10 microm) prolonged the decay of GlyR mIPSCs and, additionally, was approximately 10-fold more potent than 5alpha3alpha upon GABA(A)R mIPSCs. However, 5alpha3alpha and minaxolone (1 microm) evoked a similar bicuculline-sensitive inhibitory conductance, indicating that the extrasynaptic GABA(A)Rs do not discriminate between these two steroids. Furthermore, approximately 92% of the effect of 1 microm 5alpha3alpha upon GABAergic inhibition could be accounted for by its action upon the extrasynaptic conductance. These findings are relevant to modulation of inhibitory circuits within spinally mediated pain pathways and suggest that extrasynaptic GABA(A)Rs may represent a relevant molecular target for the analgesic actions of neurosteroids.

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.

Cross-inhibitory interactions between GABAA and P2X channels in myenteric neurones

Inhibitory interactions between GABA(A)[induced by gamma-aminobutyric acid (GABA)] and P2X [activated by adenosine 5'-triphosphate (ATP)] receptors of myenteric neurones from the guinea pig small intestine were characterized using whole-cell recordings. Currents induced by GABA (I(GABA)) or ATP (I(ATP)) were inhibited by picrotoxin or pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, respectively. Currents induced by GABA + ATP (I(GABA+ATP)) were only as large as the current induced by the most effective transmitter, revealing current occlusion. This occlusion requires maximal activation of at least one of these receptors. Sequential applications of neurotransmitters, and kinetic and pharmacological properties of I(GABA+ATP) indicate that they are carried through both GABA(A) and P2X channels. ATP did not affect I(GABA) in neurones: (i) in which P2X channels were not present; (ii) after inhibiting P2X channels with Ca2+ (iii) in the presence of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, a P2X receptor antagonist; (iv) after P2X receptor desensitization or (v) at I(ATP) reversal potential. Similarly, GABA did not affect P2X-mediated currents in neurones: (i) in which GABA(A) channels were not present; (ii) in the presence of picrotoxin, a GABA(A) channel blocker; (iii) after GABA(A) receptor desensitization or (iv) at the I(GABA) reversal potential. Current occlusion occurred as fast as current activation and it was still present in the absence of Ca2+, at 11 degrees C, after adding to the pipette solution a cocktail of protein kinase inhibitors (staurosporine + genistein + K-252a), after substituting the GTP in the pipette with GDP-beta-S and after treating the cells with N-ethylmaleimide. Taken together, all of these results are consistent with a model of cross-inhibition between GABA(A) and P2X.

Cross-talking between 5-HT3 and GABAA receptors in cultured myenteric neurons

We recorded whole-cell ion currents induced by gamma-aminobutyric acid (I(GABA)) and serotonin (I(5-HT)) to investigate and characterize putative interactions between GABA(A) and 5-HT(3) receptors in myenteric neurons from the guinea pig small intestine. I(GABA) and I(5-HT) were inhibited by bicuculline and ondansetron, respectively. Currents induced by the simultaneous application of both, GABA and 5-HT (I(GABA+5-HT)) were significantly lower than the sum of I(GABA) and I(5-HT), indicating the existence of a current occlusion. Such an occlusion was observed when GABA(A) and 5-HT(3) receptors are virtually saturated. Kinetics, and pharmacological properties of I(GABA+5-HT) indicate that they are mediated by activation of both, GABA(A) and 5-HT(3) channels. GABA did not alter I(5-HT) in neurons without GABA(A) channels, in the presence of bicuculline (a GABA(A) receptor antagonist) or at the reversal potential for I(GABA). Similarly, 5-HT did not modify I(GABA) in neurons in which 5-HT(3) channels were absent, after inhibiting 5-HT(3) channels with ondansetron (a 5-HT(3) receptor antagonist) or at the reversal potential for I(5-HT). Current occlusion was observed as soon as GABA(A) and 5-HT(3) channels were being activated, in the absence of Ca(2+), at low temperature (11 degrees C), and after adding staurosporine (a protein kinase inhibitor) to the pipette solution. Our proposal is that GABA(A) and 5-HT(3) channels are organized in clusters and within these, both channels can cross-inhibit each other, likely by allosteric interactions between these proteins.

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.

Modulation of gamma-aminobutyric acid A receptor function by thiopental in the rat spinal dorsal horn neurons

To assess the actions of thiopental at the spinal dorsal horn level, we examined the effects of thiopental using the whole cell patch-clamp technique on mechanically dissociated rat spinal dorsal horn neurons. Thiopental, at large concentrations, elicited a current (I(Thio)) through activation of chloride conductance, and its threshold concentration was approximately 50 microM. I(Thio) was sensitive to bicuculline, a gamma-aminobutyric acid (GABA)A receptor antagonist, but not to strychnine, a glycine receptor antagonist. At a clinically relevant concentration (30 muM), thiopental markedly enhanced the peak amplitude of a subsaturating GABA-induced current (I(GABA)) but not that of a saturating GABA-induced current. Furthermore, thiopental prolonged the time constants of both desensitization and deactivation of I(GABA). At a large concentration (300 muM), it inhibited the peak amplitude of I(GABA), which may be the result of open-channel blockade. In addition, at 30 microM, thiopental increased the duration and decreased the frequency of GABAergic miniature inhibitory postsynaptic currents. These results indicate that thiopental enhances GABAergic inhibitory transmission and suggest that GABA(A) receptors in the spinal cord are a potential target through which thiopental causes immobility and depresses the response to noxious stimuli.

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.

Mechanisms of modulation of pregnanolone on glycinergic response in cultured spinal dorsal horn neurons of rat

The glycine receptors and neurosteroids in spinal cord are both implicated in nociceptive signal processing. However, the modulatory effects of neurosteroid pregnanolone (5beta-pregnan-3alpha-ol-20-one) on native glycine receptors remain unclear. In the present study, we examined the effects of pregnanolone and its three isomers on glycine receptors by using whole-cell patch-clamp technique. Our results showed that pregnanolone reversibly inhibited the amplitude of glycine-induced current mediated by native glycine receptors and recombinant alpha1-, alpha2-, alpha3- and alpha1beta-glycine receptors. In cultured spinal dorsal horn neurons of rats, pregnanolone inhibited the glycine-induced current in dose-dependent manner, with an antagonist concentration inducing half-maximal response of 1.0+/-0.3 microM. The inhibitory effect of pregnanolone on glycine-induced current was voltage-independent and pregnanolone shifted the concentration-response curve for glycine-induced current rightward in a parallel manner without altering the maximal value and Hill coefficient. The isomer of pregnanolone, allopregnanolone (5alpha-pregnan-3alpha-ol-20-one) slightly enhanced glycine-induced current, whereas iso-pregnanolone (5beta-pregnan-3beta-ol-20-one) and iso-allopregnanolone (5alpha-pregnan-3beta-ol-20-one) did not affect the glycine-induced current significantly in cultured spinal dorsal horn neurons. Thus, our results suggest that the inhibitory effect of pregnanolone on glycine-induced current is of a competitive type and depends on the stereo structure of pregnanolone. Furthermore, pregnanolone decreased the amplitude and frequency of the glycinergic miniature inhibitory postsynaptic currents. Through modulating the glycinergic inhibitory neurotransmission, pregnanolone may affect the nociceptive sensory processing under physiological and pathological conditions.

The neurotransmitters glycine and GABA stimulate glucagon-like peptide-1 release from the GLUTag cell line

The incretin hormone, glucagon-like peptide-1 (GLP-1) is released from intestinal L-cells following food ingestion. Its secretion is triggered by a range of nutrients, including fats, carbohydrates and proteins. We reported previously that Na(+)-dependent glutamine uptake triggered electrical activity and GLP-1 release from the L-cell model line GLUTag. However, whereas alanine also triggered membrane depolarization and GLP-1 secretion, the response was Na+ independent. A range of alanine analogues, including d-alanine, beta-alanine, glycine and l-serine, but not d-serine, triggered similar depolarizing currents and elevation of intracellular [Ca2+], a sensitivity profile suggesting the involvement of glycine receptors. In support of this idea, glycine-induced currents and GLP-1 release were blocked by strychnine, and currents showed a 58.5 mV shift in reversal potential per 10-fold change in [Cl-], consistent with the activation of a Cl(-)-selective current. GABA, an agonist of related Cl- channels, also triggered Cl- currents and secretion, which were sensitive to picrotoxin. GABA-triggered [Ca2+]i increments were abolished by bicuculline and partially impaired by (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA), suggesting the involvement of both GABA(A) and GABA(C) receptors. Expression of GABA(A), GABA(C) and glycine receptor subunits was confirmed by RT-PCR. Glycine-triggered GLP-1 secretion was impaired by bumetanide but not bendrofluazide, suggesting that a high intracellular [Cl-] maintained by Na(+)-K(+)-2Cl- cotransporters is necessary for the depolarizing response to glycine receptor ligands. Our results suggest that GABA and glycine stimulate electrical activity and GLP-1 release from GLUTag cells by ligand-gated ion channel activation, a mechanism that might be important in responses to endogenous ligands from the enteric nervous system or dietary sources.

GABA and glycine are protective to mature but toxic to immature rat cortical neurons under hypoxia

Although recent studies suggest that gamma-aminobutyric acid (GABA) and glycine may be 'inhibitory' to mature neurons, but 'excitatory' to immature neurons under normoxia, it is unknown whether inhibitory neurotransmitters are differentially involved in neuronal response to hypoxia in immature and mature neurons. In the present study, we exposed rat cortical neurons to hypoxia (1% O2) and examined the effects of three major inhibitory neurotransmitters (GABA, glycine and taurine) on the hypoxic neurons at different neuronal ages [days in vitro (DIV)4-20]. Our data showed that the cortical neurons expressed both GABA(A) and glycine receptors with differential developmental profiles. GABA (10-2000 microm) was neuroprotective to hypoxic neurons of DIV20, but enhanced hypoxic injury in neurons of <DIV20. Glycine at low concentrations (10-100 microm) exhibited a similar pattern to GABA. However, higher concentrations of glycine (1000-2000 microm) for long-term exposure (48-72 h) displayed neuroprotection at all ages (DIV4-20). Taurine (10-2000 microm), unlike GABA and glycine, displayed protection only in DIV4 neurons, and was slightly toxic to neurons>DIV4. In comparison with delta-opioid receptor (DOR)-induced protection in DIV20 neurons exposed to 72 h of hypoxia, glycine-induced protection was weaker than that of DOR but stronger than that of GABA and taurine. These data suggest that the effects of the inhibitory neurotransmitters on hypoxic cortical neurons are age-dependent, with GABA and glycine being neurotoxic to immature neurons and neuroprotective to mature neurons.

Glycine receptors regulate dopamine release in the rat nucleus accumbens

BACKGROUND

The mesolimbic dopamine (DA) system seems to be centrally involved in regulating reward-related behavior and consequently has been implicated in addictive processes, such as alcoholism and drug addiction. This DA system has also been implicated in psychosis and in regulating hedonia/anhedonia, important components of mania and depression. Given the potentially great importance of the mesolimbic DA system for several psychiatric disorders, it is of major interest to delineate the mechanisms and dynamics underlying DA regulation and release. Recently strychnine-sensitive glycine receptors (GlyR) have attracted some interest in this matter.

METHODS

Western blot and in vivo microdialysis (couplied to high-pressure liquid chromatography with electrochemical detection), as well as reversed microdialysis, in awake, freely moving, adult male Wistar rats.

RESULTS

Here we demonstrate by means of Western blot that alpha GlyR subunit proteins are expressed in the rat nucleus accumbens (nAc), a major target of the mesolimbic DA system. We further show that reversed microdialysis of the competitive GlyR antagonist strychnine into the nAc concentration-dependently (2-200 microM) and in a reversible manner decreases accumbal extracellular DA levels. Conversely, reversed microdialysis of the agonist glycine increases accumbal DA levels in some rats but not others. The strychnine-induced depression of the accumbal DA levels is antagonized by simultaneous local perfusion of glycine.

CONCLUSIONS

The present results indicate that GlyRs in the nAc are tonically activated and of importance for regulating extracellular DA levels. The possibility of pharmacologically interfering with GlyRs to combat psychiatric disorders, in which the mesolimbic DA system is implicated, such as alcoholism, drug addiction, and psychosis, should be explored.

Taurine activates excitatory non-synaptic glycine receptors on dopamine neurones in ventral tegmental area of young rats

The physiological and pharmacological properties of taurine-induced responses were investigated in dopaminergic (DA) neurones from the ventral tegmental area (VTA) of young rats aged 1-13 postnatal days, either in acute brain slices or acutely dissociated neurones. When whole-cell responses were recorded from current-clamped neurones using the gramicidin-perforated technique, the application of taurine (0.01-30 mm) accelerated firings and induced membrane depolarization. In voltage-clamped neurones, taurine induced a current which was antagonized by strychnine and by picrotoxin, but not by bicuculline. In addition, taurine-induced current showed complete cross-desensitization with glycine-activated currents but not with gamma-aminobutyric acid (GABA)-activated currents. Thus, taurine is a full agonist of the glycine receptors (GlyRs) in the VTA. Further studies found that taurine acted mainly on non-synaptic GlyRs. The application of 20 microm bicuculline abolished the spontaneous inhibitory post-synaptic currents (IPSCs) in 40/45 neurones, and 93% of the evoked IPSCs. The addition of 1 microm strychnine completely eliminated the remaining IPSCs. These results suggest that GABAergic IPSCs predominate, and that functional glycinergic synapses are present in a subset of the VTA neurones. The application of 1 mum strychnine alone induced an outward current, suggesting that these neurones were exposed to tonically released taurine/glycine. In conclusion, by activating non-synaptic GlyRs, taurine may act as an excitatory extra-synaptic neurotransmitter in the VTA during early development.

Subunit-specific Coupling between γ-Aminobutyric Acid Type A and P2X2 Receptor Channels

ATP and gamma-aminobutyric acid (GABA) are two fast neurotransmitters co-released at central synapses, where they co-activate excitatory P2X and inhibitory GABAA (GABA type A) receptors. We report here that co-activation of P2X2 and various GABAA receptors, co-expressed in Xenopus oocytes, leads to a functional cross-inhibition dependent on GABAA subunit composition. Sequential applications of GABA and ATP revealed that alphabeta- or alphabetagamma-containing GABAA receptors inhibited P2X2 channels, whereas P2X2 channels failed to inhibit gamma-containing GABAA receptors. This functional cross-talk is independent of membrane potential, changes in current direction, and calcium. Non-additive responses observed between cation-selective GABAA and P2X2 receptors further indicate the chloride independence of this process. Overexpression of minigenes encoding either the C-terminal fragment of P2X2 or the intracellular loop of the beta3 subunit disrupted the functional cross-inhibition. We previously demonstrated functional and physical cross-talk between rho1 and P2X2 receptors, which induced a retargeting of rho1 channels to surface clusters when co-expressed in hippocampal neurons (Boue-Grabot, E., Emerit, M. B., Toulme, E., Seguela, P., and Garret, M. (2004) J. Biol. Chem. 279, 6967-6975). Co-expression of P2X2 and chimeric rho1 receptors with the C-terminal sequences of alpha2, beta3, or gamma2 subunits indicated that only rho1-beta3 and P2X2 channels exhibit both functional cross-inhibition in Xenopus oocytes and co-clustering/retargeting in hippocampal neurons. Therefore, the C-terminal domain of P2X2 and the intracellular loop of beta GABAA subunits are required for the functional interaction between ATP- and GABA-gated channels. This gamma subunit-dependent cross-talk may contribute to the regulation of synaptic activity.

Cellular and subcellular localization of the inhibitory glycine receptor in hippocampal neurons

Inhibitory glycine receptors are most abundant in spinal cord and brainstem, and glycinergic synapses have a well-established role in the regulation of locomotor behavior. Little is known about the function of glycine receptors in cortex and hippocampus, where GABA plays a dominant role in synaptic inhibition. Therefore, we have investigated tissue and cellular expression of glycine receptor alpha-subunits. Western blot and immunohistochemical analyses reveal the presence of glycine receptors in hippocampal tissue. Immunocytochemical experiments in hippocampal cultures show prominent cellular expression of glycine receptors in pyramidal neurons and GAD-positive interneurons similar to the calcium-binding protein VILIP-1 with widespread hippocampal distribution. On the subcellular level we found co-staining of GlyR and the presynaptic marker synapsin I. Furthermore, co-staining with GAD at synaptic terminals indicated partial co-localization of GABA- and glycine receptors.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.