The time course of transmitter at glycinergic synapses onto motoneurons

Dynamics of receptor activation by agonists

How do agonists turn on receptors? The model system we have used to address this question is the adult-type skeletal muscle nicotinic acetylcholine receptor. This ligand-gated ion channel has two orthosteric sites (for neurotransmitters) in the extracellular domain linked to an allosteric site (a gate) in the transmembrane domain. The goal of this perspective is to summarize how measurements of agonist binding energy reveal the dynamics of the neurotransmitter sites and the fundamental link between binding and gating.

Glycine: The Smallest Anti-Inflammatory Micronutrient

Glycine is a non-essential amino acid with many functions and effects. Glycine can bind to specific receptors and transporters that are expressed in many types of cells throughout an organism to exert its effects. There have been many studies focused on the anti-inflammatory effects of glycine, including its abilities to decrease pro-inflammatory cytokines and the concentration of free fatty acids, to improve the insulin response, and to mediate other changes. However, the mechanism through which glycine acts is not clear. In this review, we emphasize that glycine exerts its anti-inflammatory effects throughout the modulation of the expression of nuclear factor kappa B (NF-κB) in many cells. Although glycine is a non-essential amino acid, we highlight how dietary glycine supplementation is important in avoiding the development of chronic inflammation.

GLRA2 gene mutations cause high myopia in humans and mice

BACKGROUND

High myopia (HM) is a leading cause of blindness that has a strong genetic predisposition. However, its genetic and pathogenic mechanisms remain largely unknown. Thus, this study aims to determine the genetic profile of individuals from two large Chinese families with HM and 200 patients with familial/sporadic HM. We also explored the pathogenic mechanism of HM using HEK293 cells and a mouse model.

METHODS

The participants underwent genome-wide linkage analysis and exome sequencing. Visual acuity, electroretinogram response, refractive error, optical parameters and retinal rod cell genesis were measured in knockout mice. Immunofluorescent staining, biotin-labelled membrane protein isolation and electrophysiological characterisation were conducted in cells transfected with overexpression plasmids.

RESULTS

A novel HM locus on Xp22.2-p11.4 was identified. Variant c.539C>T (p.Pro180Leu) in GLRA2 gene was co-segregated with HM in the two families. Another variant, c.458G>A (p.Arg153Gln), was identified in a sporadic sample. The Glra2 knockout mice showed myopia-related phenotypes, decreased electroretinogram responses and impaired retinal rod cell genesis. Variants c.458G>A and c.539C>T altered the localisation of GlyRα2 on the cell membrane and decreased agonist sensitivity.

CONCLUSION

GLRA2 was identified as a novel HM-causing gene. Its variants would cause HM through altered visual experience by impairing photoperception and visual transmission.

MOLECULAR MECHANISMS DEFINING APPLICATION OF GLYCINE AND ZINC COMBINATIONIN CORRECTION OF STRESS AND ANXIETY MAIN MANIFESTATIONS

The aim of the work was to carry out a systematic analysis of the molecular mechanisms that determine the possibility of a combined use of amino acid glycine and zinc compounds for the treatment of patients with manifestations of stress and anxiety.

Materials and methods. Information retrieval (Scopus, PubMed) and library (eLibrary) databases were used as research tools. In some cases, the ResearchGate application was applied for a semantic search. The analysis and generalization of references was carried out on the research topic, covering the period from 2000 to the present time.

Results. It has been shown that amino acid glycine, along with gamma-aminobutyric acid (GABA), is a key neurotransmitter that regulates physiological inhibition processes in the central nervous system (CNS) by increasing transmembrane conductance in specific pentameric ligand-gated ion channels. The introduction of zinc ions can potentiate the opening of these receptors by increasing their affinity for glycine, resulting in an inhibitory processes increase in CNS neurons. The replenishment of the glycine and zinc combined deficiency is an important element in the correction of a post-stress dysfunction of the central nervous system. A balanced intake of zinc and glycine is essential for most people who experience daily effects of multiple stresses and anxiety. This combination is especially useful for the people experiencing a state of chronic psycho-emotional stress and maladaptation, including those who have a difficulty in falling asleep.

Conclusion. A balanced maintenance of the zinc and glycine concentration in the body of a healthy person leads to the development of a stable anti-anxiety effect, which is accompanied by the normalization of the sleep-wake rhythm, which makes it possible to have a good rest without any loss of working efficiency after waking up.

Acidic pH reduces agonist efficacy and responses to synaptic-like glycine applications in zebrafish α1 and rat α1β recombinant glycine receptors

Many pentameric ligand-gated ion channels are modulated by extracellular pH. Glycine receptors (GlyRs) share this property, but it is not well understood how they are affected by pH changes. Whole cell experiments on HEK293 cells expressing zebrafish homomeric α1 GlyR confirmed previous reports that acidic pH (6.4) reduces GlyR sensitivity to glycine, whereas alkaline pH (8.4) has small or negligible effects. In addition to that, at pH 6.4 we observed a reduction in the maximum responses to the partial agonists β-alanine and taurine relative to the full agonist glycine. In cell-attached single-channel recording, low pH reduced agonist efficacy, as the maximum open probability decreased from 0.97, 0.91 and 0.66 to 0.93, 0.57 and 0.34 for glycine, β-alanine and taurine, respectively, reflecting a threefold decrease in efficacy equilibrium constants for all three agonists. We also tested the effect of pH 6.4 in conditions that replicate those at the native synapse, recording outside-out currents elicited by fast application of millisecond pulses of agonists on α1 and α1β GlyR, at a range of intracellular chloride concentrations. Acidic pH reduced the area under the curve of the currents, by reducing peak amplitude, slowing activation and speeding deactivation. Our results show that acidification of the extracellular pH by one unit, as may occur in pathological conditions such as ischaemia, impairs GlyR gating and is likely to reduce the effectiveness of glycinergic synaptic inhibition. KEY POINTS: Extracellular pH in the central nervous system (CNS) is known to shift towards acidic values during pathophysiological conditions such as ischaemia and seizures. Acidic extracellular pH is known to affect GABAergic inhibitory synapses, but its effect on signals mediated by glycine receptors (GlyR) is not well characterised. Moderate acidic conditions (pH 6.4) reduce the maximum single channel open probability of recombinant homomeric GlyRs produced by the neurotransmitter glycine or other agonists, such as β-alanine and taurine. When glycine was applied with a piezoelectric stepper to outside out patches, to simulate its fast rise and short duration at the synapse, responses became shorter and smaller at pH 6.4. The effect was also observed with physiologically low intracellular chloride and in mammalian heteromeric GlyRs. This suggests that acidic pH is likely to reduce the strength of inhibitory signalling at glycinergic synapses.

Identification of a stereotypic molecular arrangement of endogenous glycine receptors at spinal cord synapses

Precise quantitative information about the molecular architecture of synapses is essential to understanding the functional specificity and downstream signaling processes at specific populations of synapses. Glycine receptors (GlyRs) are the primary fast inhibitory neurotransmitter receptors in the spinal cord and brainstem. These inhibitory glycinergic networks crucially regulate motor and sensory processes. Thus far, the nanoscale organization of GlyRs underlying the different network specificities has not been defined. Here, we have quantitatively characterized the molecular arrangement and ultra-structure of glycinergic synapses in spinal cord tissue using quantitative super-resolution correlative light and electron microscopy. We show that endogenous GlyRs exhibit equal receptor-scaffold occupancy and constant packing densities of about 2000 GlyRs µm^-2 at synapses across the spinal cord and throughout adulthood, even though ventral horn synapses have twice the total copy numbers, larger postsynaptic domains, and more convoluted morphologies than dorsal horn synapses. We demonstrate that this stereotypic molecular arrangement is maintained at glycinergic synapses in the oscillator mouse model of the neuromotor disease hyperekplexia despite a decrease in synapse size, indicating that the molecular organization of GlyRs is preserved in this hypomorph. We thus conclude that the morphology and size of inhibitory postsynaptic specializations rather than differences in GlyR packing determine the postsynaptic strength of glycinergic neurotransmission in motor and sensory spinal cord networks.

GABAergic Mechanisms Can Redress the Tilted Balance between Excitation and Inhibition in Damaged Spinal Networks

Correct operation of neuronal networks depends on the interplay between synaptic excitation and inhibition processes leading to a dynamic state termed balanced network. In the spinal cord, balanced network activity is fundamental for the expression of locomotor patterns necessary for rhythmic activation of limb extensor and flexor muscles. After spinal cord lesion, paralysis ensues often followed by spasticity. These conditions imply that, below the damaged site, the state of balanced networks has been disrupted and that restoration might be attempted by modulating the excitability of sublesional spinal neurons. Because of the widespread expression of inhibitory GABAergic neurons in the spinal cord, their role in the early and late phases of spinal cord injury deserves full attention. Thus, an early surge in extracellular GABA might be involved in the onset of spinal shock while a relative deficit of GABAergic mechanisms may be a contributor to spasticity. We discuss the role of GABA A receptors at synaptic and extrasynaptic level to modulate network excitability and to offer a pharmacological target for symptom control. In particular, it is proposed that activation of GABA A receptors with synthetic GABA agonists may downregulate motoneuron hyperexcitability (due to enhanced persistent ionic currents) and, therefore, diminish spasticity. This approach might constitute a complementary strategy to regulate network excitability after injury so that reconstruction of damaged spinal networks with new materials or cell transplants might proceed more successfully.

Glycinergic Transmission in the Presence and Absence of Functional GlyT2: Lessons From the Auditory Brainstem

Synaptic transmission is controlled by re-uptake systems that reduce transmitter concentrations in the synaptic cleft and recycle the transmitter into presynaptic terminals. The re-uptake systems are thought to ensure cytosolic concentrations in the terminals that are sufficient for reloading empty synaptic vesicles (SVs). Genetic deletion of glycine transporter 2 (GlyT2) results in severely disrupted inhibitory neurotransmission and ultimately to death. Here we investigated the role of GlyT2 at inhibitory glycinergic synapses in the mammalian auditory brainstem. These synapses are tuned for resilience, reliability, and precision, even during sustained high-frequency stimulation when endocytosis and refilling of SVs probably contribute substantially to efficient replenishment of the readily releasable pool (RRP). Such robust synapses are formed between MNTB and LSO neurons (medial nucleus of the trapezoid body, lateral superior olive). By means of patch-clamp recordings, we assessed the synaptic performance in controls, in GlyT2 knockout mice (KOs), and upon acute pharmacological GlyT2 blockade. Via computational modeling, we calculated the reoccupation rate of empty release sites and RRP replenishment kinetics during 60-s challenge and 60-s recovery periods. Control MNTB-LSO inputs maintained high fidelity neurotransmission at 50 Hz for 60 s and recovered very efficiently from synaptic depression. During 'marathon-experiments' (30,600 stimuli in 20 min), RRP replenishment accumulated to 1,260-fold. In contrast, KO inputs featured severe impairments. For example, the input number was reduced to ~1 (vs. ~4 in controls), implying massive functional degeneration of the MNTB-LSO microcircuit and a role of GlyT2 during synapse maturation. Surprisingly, neurotransmission did not collapse completely in KOs as inputs still replenished their small RRP 80-fold upon 50 Hz | 60 s challenge. However, they totally failed to do so for extended periods. Upon acute pharmacological GlyT2 inactivation, synaptic performance remained robust, in stark contrast to KOs. RRP replenishment was 865-fold in marathon-experiments, only ~1/3 lower than in controls. Collectively, our empirical and modeling results demonstrate that GlyT2 re-uptake activity is not the dominant factor in the SV recycling pathway that imparts indefatigability to MNTB-LSO synapses. We postulate that additional glycine sources, possibly the antiporter Asc-1, contribute to RRP replenishment at these high-fidelity brainstem synapses.

Subunit-Specific Photocontrol of Glycine Receptors by Azobenzene-Nitrazepam Photoswitcher

Photopharmacology is a unique approach that through a combination of photochemistry methods and advanced life science techniques allows the study and control of specific biological processes, ranging from intracellular pathways to brain circuits. Recently, a first photochromic channel blocker of anion-selective GABA_A receptors, the azobenzene-nitrazepam-based photochromic compound (Azo-NZ1), has been described. In the present study, using patch-clamp technique in heterologous system and in mice brain slices, site-directed mutagenesis and molecular modeling we provide evidence of the interaction of Azo-NZ1 with glycine receptors (GlyRs) and determine the molecular basis of this interaction. Glycinergic synaptic neurotransmission determines an important inhibitory drive in the vertebrate nervous system and plays a crucial role in the control of neuronal circuits in the spinal cord and brain stem. GlyRs are involved in locomotion, pain sensation, breathing, and auditory function, as well as in the development of such disorders as hyperekplexia, epilepsy, and autism. Here, we demonstrate that Azo-NZ1 blocks in a UV-dependent manner the activity of α2 GlyRs (GlyR2), while being barely active on α1 GlyRs (GlyR1). The site of Azo-NZ1 action is in the chloride-selective pore of GlyR at the 2’ position of transmembrane helix 2 and amino acids forming this site determine the difference in Azo-NZ1 blocking activity between GlyR2 and GlyR1. This subunit-specific modulation is also shown on motoneurons of brainstem slices from neonatal mice that switch during development from expressing “fetal” GlyR2 to “adult” GlyR1 receptors.

A System for Assessing Dual Action Modulators of Glycine Transporters and Glycine Receptors

Reduced inhibitory glycinergic neurotransmission is implicated in a number of neurological conditions such as neuropathic pain, schizophrenia, epilepsy and hyperekplexia. Restoring glycinergic signalling may be an effective method of treating these pathologies. Glycine transporters (GlyTs) control synaptic and extra-synaptic glycine concentrations and slowing the reuptake of glycine using specific GlyT inhibitors will increase glycine extracellular concentrations and increase glycine receptor (GlyR) activation. Glycinergic neurotransmission can also be improved through positive allosteric modulation (PAM) of GlyRs. Despite efforts to manipulate this synapse, no therapeutics currently target it. We propose that dual action modulators of both GlyTs and GlyRs may show greater therapeutic potential than those targeting individual proteins. To show this, we have characterized a co-expression system in Xenopus laevis oocytes consisting of GlyT1 or GlyT2 co-expressed with GlyRα1. We use two electrode voltage clamp recording techniques to measure the impact of GlyTs on GlyRs and the effects of modulators of these proteins. We show that increases in GlyT density in close proximity to GlyRs diminish receptor currents. Reductions in GlyR mediated currents are not observed when non-transportable GlyR agonists are applied or when Na+ is not available. GlyTs reduce glycine concentrations across different concentration ranges, corresponding with their ion-coupling stoichiometry, and full receptor currents can be restored when GlyTs are blocked with selective inhibitors. We show that partial inhibition of GlyT2 and modest GlyRα1 potentiation using a dual action compound, is as useful in restoring GlyR currents as a full and potent single target GlyT2 inhibitor or single target GlyRα1 PAM. The co-expression system developed in this study will provide a robust means for assessing the likely impact of GlyR PAMs and GlyT inhibitors on glycine neurotransmission.

The startle disease mutation α1S270T predicts shortening of glycinergic synaptic currents

KEY POINTS

Loss-of-function mutations in proteins found at glycinergic synapses, most commonly in the α1 subunit of the glycine receptor (GlyR), cause the startle disease/hyperekplexia channelopathy in man. It was recently proposed that the receptors responsible are presynaptic homomeric GlyRs, rather than postsynaptic heteromeric GlyRs (which mediate glycinergic synaptic transmission), because heteromeric GlyRs are less affected by many startle mutations than homomers. We examined the α1 startle mutation S270T, at the extracellular end of the M2 transmembrane helix. Recombinant heteromeric GlyRs were less impaired than homomers by this mutation when we measured their response to equilibrium applications of glycine. However, currents elicited by synaptic-like millisecond applications of glycine to outside-out patches were much shorter (7- to 10-fold) in all mutant receptors, both homomeric and heteromeric. Thus, the synaptic function of heteromeric receptors is likely to be impaired by the mutation.

ABSTRACT

Human startle disease is caused by mutations in glycine receptor (GlyR) subunits or in other proteins associated with glycinergic synapses. Many startle mutations are known, but it is hard to correlate the degree of impairment at molecular level with the severity of symptoms in patients. It was recently proposed that the disease is caused by disruption in the function of presynaptic homomeric GlyRs (rather than postsynaptic heteromeric GlyRs), because homomeric GlyRs are more sensitive to loss-of-function mutations than heteromers. Our patch-clamp recordings from heterologously expressed GlyRs characterised in detail the functional consequences of the α1S270T startle mutation, which is located at the extracellular end of the pore lining M2 transmembrane segment (18'). This mutation profoundly decreased the maximum single-channel open probability of homomeric GlyRs (to 0.16; cf. 0.99 for wild type) but reduced only marginally that of heteromeric GlyRs (0.96; cf. 0.99 for wild type). However, both heteromeric and homomeric mutant GlyRs became less sensitive to the neurotransmitter glycine. Responses evoked by brief, quasi-synaptic pulses of glycine onto outside-out patches were impaired in mutant receptors, as deactivation was approximately 10- and 7-fold faster for homomeric and heteromeric GlyRs, respectively. Our data suggest that the α1S270T mutation is likely to affect the opening step in GlyR activation. The faster decay of synaptic currents mediated by mutant heteromeric GlyRs is expected to reduce charge transfer at the synapse, despite the high equilibrium open probability of these mutant channels.

Effect of Glycine on BV-2 Microglial Cells Treated with Interferon-γ and Lipopolysaccharide

Microglia are first-line defense antigen-presenting phagocytes in the central nervous system. Activated microglial cells release pro-inflammatory cytokines and can trigger an oxidative burst. The amino acid glycine exerts anti-inflammatory, immunomodulatory and cytoprotective effects and influences cell volume regulation. This study aimed to investigate the role of glycine in the modulation of inflammatory processes in mouse BV-2 microglial cells. Inflammatory stress was induced by lipopolysaccharide/interferon-γ (LPS/IFN-γ) treatment for 24 h in the absence or presence of 1 or 5 mM glycine. Cells were analyzed by flow cytometry for cell volume, side scatter, apoptosis/necrosis and expression of activation-specific surface markers. Apoptosis progression was monitored by life cell imaging. Reduced glutathione/oxidized glutathione (GSH/GSSG) ratios and release of the pro-inflammatory cytokines IL-6 and TNF-α were measured using luminescence-based assays and ELISA, respectively. We found that LPS/IFN-γ-induced apoptosis was decreased and the fraction of living cells was increased by glycine. Expression of the surface markers CD11b, CD54 and CD80 was dose-dependently increased, while IL-6 and TNF-α release was not altered compared to LPS/IFN-γ-treated cells. We showed that in BV-2 microglial cells glycine improves viability and counteracts deleterious responses to LPS/IFN-γ, which might be relevant in neurodegenerative processes associated with inflammation, like Alzheimer’s or Parkinson’s disease.

Thermodynamic first law efficiency of membrane proteins

Proteins are nature's biomolecular machines. Proteins, such as transporters, pumps and motors, have complex function/operating-machinery/mechanisms, comparable to the macro-scaled machines that we encounter in our daily life. These proteins, as it is for their macro-scaled counterparts, convert (part of) other/various forms of energy into work. In this study, we are performing the first law analysis on a set of proteins, including the dopamine transporter, glycine transporters I and II, glutamate transporter, sodium-potassium pump and Ca²⁺ ATPase. Each of these proteins operates on a thermodynamic/mechanic cycle to perform their function. In each of these cycles, they receive energy from a source, convert part of this energy into work and reject the remaining part of the energy to the environment. Conservation of energy principle was applied to the thermodynamic/mechanic cycle of each protein, and thermodynamic first law efficiency was evaluated for each cycle, which shows how much of the energy input per cycle was converted into useful work. Interestingly, calculations based on experimental data indicate that proteins can operate under a range of efficiencies, which vary based on the extracellular and intracellular ion and substrate concentrations. The lowest observed first law efficiency was 50%, which is a very high value if compared to the efficiency of the macro-scaled heat engines we encounter in our daily lives.Communicated by Ramaswamy H. Sarma.

Photolysis of a Caged, Fast-Equilibrating Glutamate Receptor Antagonist, MNI-Caged γ-D-Glutamyl-Glycine, to Investigate Transmitter Dynamics and Receptor Properties at Glutamatergic Synapses

Fast uncaging of low affinity competitive receptor antagonists can in principle measure the timing and concentration dependence of transmitter action at receptors during synaptic transmission. Here, we describe the development, synthesis and characterization of MNI-caged γ-D-glutamyl-glycine (γ-DGG), which combines the fast photolysis and hydrolytic stability of nitroindoline cages with the well-characterized fast-equilibrating competitive glutamate receptor antagonist γ-DGG. At climbing fiber-Purkinje cell (CF-PC) synapses MNI-caged-γ-DGG was applied at concentrations up to 5 mM without affecting CF-PC transmission, permitting release of up to 1.5 mM γ-DGG in 1 ms in wide-field flashlamp photolysis. In steady-state conditions, photoreleased γ-DGG at 0.55–1.7 mM inhibited the CF first and second paired EPSCs by on average 30% and 60%, respectively, similar to reported values for bath applied γ-DGG. Photolysis of the L-isomer MNI-caged γ-L-glutamyl-glycine was ineffective. The time-course of receptor activation by synaptically released glutamate was investigated by timed photolysis of MNI-caged-γ-DGG at defined intervals following CF stimulation in the second EPSCs. Photorelease of γ-DGG prior to the stimulus and up to 3 ms after showed strong inhibition similar to steady-state inhibition; in contrast γ-DGG applied by a flash at 3–4 ms post-stimulus produced weaker and variable block, suggesting transmitter-receptor interaction occurs mainly in this time window. The data also show a small and lasting component of inhibition when γ-DGG was released at 4–7 ms post stimulus, near the peak of the CF-PC EPSC, or at 10–11 ms. This indicates that competition for binding and activation of AMPA receptors occurs also during the late phase of the EPSC, due to either delayed transmitter release or persistence of glutamate in the synaptic region. The results presented here first show that MNI-caged-γ-DGG has properties suitable for use as a synaptic probe at high concentration and that its photolysis can resolve timing and extent of transmitter activation of receptors in glutamatergic transmission.

Heterogeneous Signaling at GABA and Glycine Co-releasing Terminals

The corelease of several neurotransmitters from a single synaptic vesicle has been observed at many central synapses. Nevertheless, the signaling synergy offered by cotransmission and the mechanisms that maintain the optimal release and detection of neurotransmitters at mixed synapses remain poorly understood, thus limiting our ability to interpret changes in synaptic signaling and identify molecules important for plasticity. In the brainstem and spinal cord, GABA and glycine cotransmission is facilitated by a shared vesicular transporter VIAAT (also named VGAT), and occurs at many immature inhibitory synapses. As sensory and motor networks mature, GABA/glycine cotransmission is generally replaced by either pure glycinergic or GABAergic transmission, and the functional role for the continued corelease of GABA and glycine is unclear. Whether or not, and how, the GABA/glycine content is balanced in VIAAT-expressing vesicles from the same terminal, and how loading variability effects the strength of inhibitory transmission is not known. Here, we use a combination of loose-patch (LP) and whole-cell (WC) electrophysiology in cultured spinal neurons of GlyT2:eGFP mice to sample miniature inhibitory post synaptic currents (mIPSCs) that originate from individual GABA/glycine co-releasing synapses and develop a modeling approach to illustrate the gradual change in mIPSC phenotypes as glycine replaces GABA in vesicles. As a consistent GABA/glycine balance is predicted if VIAAT has access to both amino-acids, we test whether vesicle exocytosis from a single terminal evokes a homogeneous population of mixed mIPSCs. We recorded mIPSCs from 18 individual synapses and detected glycine-only mIPSCs in 4/18 synapses sampled. The rest (14/18) were co-releasing synapses that had a significant proportion of mixed GABA/glycine mIPSCs with a characteristic biphasic decay. The majority (9/14) of co-releasing synapses did not have a homogenous phenotype, but instead signaled with a combination of mixed and pure mIPSCs, suggesting that there is variability in the loading and/or storage of GABA and glycine at the level of individual vesicles. Our modeling predicts that when glycine replaces GABA in synaptic vesicles, the redistribution between the peak amplitude and charge transfer of mIPSCs acts to maintain the strength of inhibition while increasing the temporal precision of signaling.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Structure-Function Analysis of the GlyR α2 Subunit Autism Mutation p.R323L Reveals a Gain-of-Function

Glycine receptors (GlyRs) containing the α2 subunit regulate cortical interneuron migration. Disruption of the GlyR α2 subunit gene (Glra2) in mice leads to disrupted dorsal cortical progenitor homeostasis, leading to a depletion of projection neurons and moderate microcephaly in newborn mice. In humans, rare variants in GLRA2, which is located on the X chromosome, are associated with autism spectrum disorder (ASD) in the hemizygous state in males. These include a microdeletion (GLRA2∆ex8-9) and missense mutations in GLRA2 (p.N109S and p.R126Q) that impair cell-surface expression of GlyR α2, and either abolish or markedly reduce sensitivity to glycine. We report the functional characterization of a third missense variant in GLRA2 (p.R323L), associated with autism, macrocephaly, epilepsy and hypothyroidism in a female proband. Using heterosynapse and macroscopic current recording techniques, we reveal that GlyR α2^R323L exhibits reduced glycine sensitivity, but significantly increased inhibitory postsynaptic current (IPSC) rise and decay times. Site-directed mutagenesis revealed that the nature of the amino acid switch at position 323 is critical for impairment of GlyR function. Single-channel recordings revealed that the conductance of α2^R323Lβ channels was higher than α2β channels. Longer mean opening durations induced by p.R323L may be due to a change in the gating pathway that enhances the stability of the GlyR open state. The slower synaptic decay times, longer duration active periods and increase in conductance demonstrates that the GlyR α2 p.R323L mutation results in an overall gain of function, and that GlyR α2 mutations can be pathogenic in the heterozygous state in females.

A Chemogenetic Receptor That Enhances the Magnitude and Frequency of Glycinergic Inhibitory Postsynaptic Currents without Inducing a Tonic Chloride Flux

The gene transfer-mediated expression of inhibitory ion channels in nociceptive neurons holds promise for treating intractable pain. Chemogenetics, which involves expressing constructs activated by biologically inert molecules, is of particular interest as it permits tunable neuromodulation. However, current chloride-permeable chemogenetic constructs are problematic as they mediate a tonic chloride influx which over time would deplete the chloride electrochemical gradient and reduce inhibitory efficacy. Inflammatory pain sensitization can be caused by prostaglandin E2-mediated inhibition of glycinergic inhibitory postsynaptic currents in spinal nociceptive neurons. We developed a highly conducting (100 pS) inhibitory chemogenetic construct based on a human glycine receptor (α1^Y279F,A288G) with high ivermectin sensitivity. When virally infected into spinal neurons, 10 nM ivermectin increased the magnitude and frequency of glycinergic postsynaptic currents without activating a tonic chloride flux. The construct should thus produce analgesia. Its human origin and the well-established biocompatibility of its ligand suggest it may be suited to human use.

Deactivation kinetics of acid-sensing ion channel 1a are strongly pH-sensitive

Acid-sensing ion channels (ASICs) are trimeric cation-selective ion channels activated by protons in the physiological range. Recent reports have revealed that postsynaptically localized ASICs contribute to the excitatory postsynaptic current by responding to the transient acidification of the synaptic cleft that accompanies neurotransmission. In response to such brief acidic transients, both recombinant and native ASICs show extremely rapid deactivation in outside-out patches when jumping from a pH 5 stimulus to a single resting pH of 8. Given that the resting pH of the synaptic cleft is highly dynamic and depends on recent synaptic activity, we explored the kinetics of ASIC1a and 1a/2a heteromers to such brief pH transients over a wider [H⁺] range to approximate neuronal conditions better. Surprisingly, the deactivation of ASICs was steeply dependent on the pH, spanning nearly three orders of magnitude from extremely fast (<1 ms) at pH 8 to very slow (>300 ms) at pH 7. This study provides an example of a ligand-gated ion channel whose deactivation is sensitive to agonist concentrations that do not directly activate the receptor. Kinetic simulations and further mutagenesis provide evidence that ASICs show such steeply agonist-dependent deactivation because of strong cooperativity in proton binding. This capacity to signal across such a large synaptically relevant bandwidth enhances the response to small-amplitude acidifications likely to occur at the cleft and may provide ASICs with the ability to shape activity in response to the recent history of the synapse.

The Startle Disease Mutation E103K Impairs Activation of Human Homomeric α1 Glycine Receptors by Disrupting an Intersubunit Salt Bridge across the Agonist Binding Site

Glycine receptors (GlyR) belong to the pentameric ligand-gated ion channel (pLGIC) superfamily and mediate fast inhibitory transmission in the vertebrate CNS. Disruption of glycinergic transmission by inherited mutations produces startle disease in man. Many startle mutations are in GlyRs and provide useful clues to the function of the channel domains. E103K is one of few startle mutations found in the extracellular agonist binding site of the channel, in loop A of the principal side of the subunit interface. Homology modeling shows that the side chain of Glu-103 is close to that of Arg-131, in loop E of the complementary side of the binding site, and may form a salt bridge at the back of the binding site, constraining its size. We investigated this hypothesis in recombinant human α1 GlyR by site-directed mutagenesis and functional measurements of agonist efficacy and potency by whole cell patch clamp and single channel recording. Despite its position near the binding site, E103K causes hyperekplexia by impairing the efficacy of glycine, its ability to gate the channel once bound, which is very high in wild type GlyR. Mutating Glu-103 and Arg-131 caused various degrees of loss-of-function in the action of glycine, whereas mutations in Arg-131 enhanced the efficacy of the slightly bigger partial agonist sarcosine (N-methylglycine). The effects of the single charge-swapping mutations of these two residues were largely rescued in the double mutant, supporting the possibility that they interact via a salt bridge that normally constrains the efficacy of larger agonist molecules.

The Free Zinc Concentration in the Synaptic Cleft of Artificial Glycinergic Synapses Rises to At least 1 μM

Zn²⁺ is concentrated into presynaptic vesicles at many central synapses and is released into the synaptic cleft by nerve terminal stimulation. There is strong evidence that synaptically released Zn²⁺ modulates glutamatergic neurotransmission, although there is debate concerning the peak concentration it reaches in the synaptic cleft. Glycine receptors (GlyRs), which mediate inhibitory neurotransmission in the spinal cord and brainstem, are potentiated by low nanomolar Zn²⁺ and inhibited by micromolar Zn²⁺. Mutations that selectively ablate Zn²⁺ potentiation result in hyperekplexia phenotypes suggesting that Zn²⁺ is a physiological regulator of glycinergic neurotransmission. There is, however, little evidence that Zn²⁺ is stored presynaptically at glycinergic terminals and an alternate possibility is that GlyRs are modulated by constitutively bound Zn²⁺. We sought to estimate the peak Zn²⁺ concentration in the glycinergic synaptic cleft as a means of evaluating whether it is likely to be synaptically released. We employed 'artificial' synapses because they permit the insertion of engineered α1β GlyRs with defined Zn²⁺ sensitivities into synapses. By comparing the effect of Zn²⁺ chelation on glycinergic IPSCs with the effects of defined Zn²⁺ and glycine concentrations applied rapidly to the same recombinant GlyRs in outside-out patches, we inferred that synaptic Zn²⁺ rises to at least 1 μM following a single presynaptic stimulation. Moreover, using the fast, high-affinity chelator, ZX1, we found no evidence for tonic Zn²⁺ bound constitutively to high affinity GlyR binding sites. We conclude that diffusible Zn²⁺ reaches 1 μM or higher and is therefore likely to be phasically released in artificial glycinergic synapses.

Investigating the Mechanism by Which Gain-of-function Mutations to the α1 Glycine Receptor Cause Hyperekplexia

Hyperekplexia is a rare human neuromotor disorder caused by mutations that impair the efficacy of glycinergic inhibitory neurotransmission. Loss-of-function mutations in the GLRA1 or GLRB genes, which encode the α1 and β glycine receptor (GlyR) subunits, are the major cause. Paradoxically, gain-of-function GLRA1 mutations also cause hyperekplexia, although the mechanism is unknown. Here we identify two new gain-of-function mutations (I43F and W170S) and characterize these along with known gain-of-function mutations (Q226E, V280M, and R414H) to identify how they cause hyperekplexia. Using artificial synapses, we show that all mutations prolong the decay of inhibitory postsynaptic currents (IPSCs) and induce spontaneous GlyR activation. As these effects may deplete the chloride electrochemical gradient, hyperekplexia could potentially result from reduced glycinergic inhibitory efficacy. However, we consider this unlikely as the depleted chloride gradient should also lead to pain sensitization and to a hyperekplexia phenotype that correlates with mutation severity, neither of which is observed in patients with GLRA1 hyperekplexia mutations. We also rule out small increases in IPSC decay times (as caused by W170S and R414H) as a possible mechanism given that the clinically important drug, tropisetron, significantly increases glycinergic IPSC decay times without causing motor side effects. A recent study on cultured spinal neurons concluded that an elevated intracellular chloride concentration late during development ablates α1β glycinergic synapses but spares GABAergic synapses. As this mechanism satisfies all our considerations, we propose it is primarily responsible for the hyperekplexia phenotype.

Acid-sensing ion channels are tuned to follow high-frequency stimuli

KEY POINTS

Acid-sensing ion channels (ASICs) act as neurotransmitter receptors by responding to synaptic cleft acidification. We investigated how ASIC1a homomers and ASIC1a/2a heteromers respond to brief stimuli, jumping from pH 8.0 to 5.0, approximating the time course of neurotransmitter in the cleft. We find that ASICs deactivate surprisingly fast in response to such brief stimuli from pH 8.0 to 5.0, whereas they desensitize comparatively slowly to prolonged activation. The combination of unusually fast deactivation with slow desensitzation enables recombinant ASIC1a homomers and ASIC1a/2a heteromers, as well as native ASICs of sensory neurons, to follow trains of such brief pH 8.0 to 5.0 stimuli at high frequencies. This capacity for high-frequency signalling persists under a physiological pH of 7.4 with ASIC1a/2a heteromers, suggesting that they may sustain postsynaptic responses when other receptors desensitize.

ABSTRACT

The neurotransmitter-gated ion channels that underlie rapid synaptic transmission are often subjected to bursts of very brief neurotransmitter release at high frequencies. When challenged with such short duration high-frequency stimuli, neurotransmitter-gated ion channels generally exhibit the common response of desensitization. Recently, acid-sensing ion channels (ASICs) were shown to act as neurotransmitter-gated ion channels because postsynaptic ASICs can be activated by the transient acidification of the synaptic cleft accompanying neurotransmission. In the present study, we examined the responses of recombinant ASIC1a homomers, ASIC1a/2a heteromers and native ASICs from sensory neurons to 1 ms acidification stimuli, switching from pH 8.0 to 5.0, as either single pulses or trains of pulses at physiologically relevant frequencies. We found that ASIC deactivation is extremely fast and, in contrast to most other neurotransmitter-gated ion channels, ASICs show no desensitization during high-frequency stimulus trains under these conditions. We also found that accelerating ASIC desensitization by anion substitution can induce depression during high-frequency trains. When using a baseline physiological pH of 7.4, the ASIC1a responses were too small to reliably measure, presumably as a result of steady-state desensitization. However, ASIC1a/2 heteromers gave robust responses when using a baseline pH of 7.4 and were also able to sustain these responses during high-frequency stimulus trains. In conclusion, we report that the slow desensitization and fast deactivation of ASIC1a/2a heteromers enables them to sustain postsynaptic responses to bursts at high frequencies at a physiological pH that may desensitize other receptors.

Synaptic Connectivity between Renshaw Cells and Motoneurons in the Recurrent Inhibitory Circuit of the Spinal Cord

Renshaw cells represent a fundamental component of one of the first discovered neuronal circuits, but their function in motor control has not been established. They are the only central neurons that receive collateral projections from motor outputs, yet the efficacy of the excitatory synapses from single and converging motoneurons remains unknown. Here we present the results of dual whole-cell recordings from identified, synaptically connected Renshaw cell-motoneuron pairs in the mouse lumbar spinal cord. The responses from single Renshaw cells demonstrate that motoneuron synapses elicit large excitatory conductances with few or no failures. We show that the strong excitatory input from motoneurons results from a high probability of neurotransmitter release onto multiple postsynaptic contacts. Dual current-clamp recordings confirm that single motoneuron inputs were sufficient to depolarize the Renshaw cell beyond threshold for firing. Reciprocal connectivity was observed in approximately one-third of the paired recordings tested. Ventral root stimulation was used to evoke currents from Renshaw cells or motoneurons to characterize responses of single neurons to the activation of their corresponding presynaptic cell populations. Excitatory or inhibitory synaptic inputs in the recurrent inhibitory loop induced substantial effects on the excitability of respective postsynaptic cells. Quantal analysis estimates showed a large number of converging inputs from presynaptic motoneuron and Renshaw cell populations. The combination of considerable synaptic efficacy and extensive connectivity within the recurrent circuitry indicates a role of Renshaw cells in modulating motor outputs that may be considerably more important than has been previously supposed.

SIGNIFICANCE STATEMENT

We have recently shown that Renshaw cells mediate powerful shunt inhibition on motoneuron excitability. Here we complete a quantitative description of the recurrent circuit using recordings of excitatory synapses between identified motoneuron and Renshaw cell pairs. We show that the excitation is highly effective as a result of a high probability of neurotransmitter release onto multiple release sites and that efficient neurotransmission is maintained at physiologically relevant firing rates in motoneurons. Our results also show that both excitatory and inhibitory connections exhibit considerable convergence of inputs. Because evaluation of the determinants of synaptic strength and the extent of connectivity constitute fundamental parameters affecting the operation of the recurrent circuit, our findings are critical for informing any future models of motor control.

Glycine transporter2 inhibitors: Getting the balance right

Neurotransmitter transporters are targets for a wide range of therapeutically useful drugs. This is because they have the capacity to selectively manipulate the dynamics of neurotransmitter concentrations and thereby enhance or diminish signalling through particular brain pathways. High affinity glycine transporters (GlyTs) regulate extracellular concentrations of glycine and provide novel therapeutic targets for neurological disorders.

Disturbances of Ligand Potency and Enhanced Degradation of the Human Glycine Receptor at Affected Positions G160 and T162 Originally Identified in Patients Suffering from Hyperekplexia

Ligand-binding of Cys-loop receptors is determined by N-terminal extracellular loop structures from the plus as well as from the minus side of two adjacent subunits in the pentameric receptor complex. An aromatic residue in loop B of the glycine receptor (GlyR) undergoes direct interaction with the incoming ligand via a cation-π interaction. Recently, we showed that mutated residues in loop B identified from human patients suffering from hyperekplexia disturb ligand-binding. Here, we exchanged the affected human residues by amino acids found in related members of the Cys-loop receptor family to determine the effects of side chain volume for ion channel properties. GlyR variants were characterized in vitro following transfection into cell lines in order to analyze protein expression, trafficking, degradation and ion channel function. GlyR α1 G160 mutations significantly decrease glycine potency arguing for a positional effect on neighboring aromatic residues and consequently glycine-binding within the ligand-binding pocket. Disturbed glycinergic inhibition due to T162 α1 mutations is an additive effect of affected biogenesis and structural changes within the ligand-binding site. Protein trafficking from the ER toward the ER-Golgi intermediate compartment, the secretory Golgi pathways and finally the cell surface is largely diminished, but still sufficient to deliver ion channels that are functional at least at high glycine concentrations. The majority of T162 mutant protein accumulates in the ER and is delivered to ER-associated proteasomal degradation. Hence, G160 is an important determinant during glycine binding. In contrast, T162 affects primarily receptor biogenesis whereas exchanges in functionality are secondary effects thereof.

Not GABA but glycine mediates segmental, propriospinal, and bulbospinal postsynaptic inhibition in adult mouse spinal forelimb motor neurons

The general view is that both glycine (Eccles, 1964) and GABA (Curtis and Felix, 1971) evoke postsynaptic inhibition in spinal motor neurons. In newborn or juvenile animals, there are conflicting results showing postsynaptic inhibition in motor neurons by corelease of GABA and glycine (Jonas et al., 1998) or by glycine alone (Bhumbra et al., 2012). To resolve the relative contributions of GABA and glycine to postsynaptic inhibition, we performed in vivo intracellular recordings from forelimb motor neurons in adult mice. Postsynaptic potentials evoked from segmental, propriospinal, and bulbospinal systems in motor neurons were compared across four different conditions: control, after gabazine, gabazine followed by strychnine, and strychnine alone. No significant differences were observed in the proportion of IPSPs and EPSPs between control and gabazine conditions. In contrast, EPSPs but not IPSPs were recorded after adding strychnine with gabazine or administering strychnine alone, suggesting an exclusive role for glycine in postsynaptic inhibition. To test whether the injected (intraperitoneal) dose of gabazine blocked GABAergic inhibitory transmission, we evoked GABAA receptor-mediated monosynaptic IPSPs in deep cerebellar nuclei neurons by stimulation of Purkinje cell fibers. No monosynaptic IPSPs could be recorded in the presence of gabazine, showing the efficacy of gabazine treatment. Our results demonstrate that, in the intact adult mouse, the postsynaptic inhibitory effects in spinal motor neurons exerted by three different systems, intrasegmental and intersegmental as well as supraspinal, are exclusively glycinergic. These findings emphasize the importance of glycinergic postsynaptic inhibition in motor neurons and challenge the view that GABA also contributes.

Activity-dependent modulation of inhibitory synaptic kinetics in the cochlear nucleus

Spherical bushy cells (SBCs) in the anteroventral cochlear nucleus respond to acoustic stimulation with discharges that precisely encode the phase of low-frequency sound. The accuracy of spiking is crucial for sound localization and speech perception. Compared to the auditory nerve input, temporal precision of SBC spiking is improved through the engagement of acoustically evoked inhibition. Recently, the inhibition was shown to be less precise than previously understood. It shifts from predominantly glycinergic to synergistic GABA/glycine transmission in an activity-dependent manner. Concurrently, the inhibition attains a tonic character through temporal summation. The present study provides a comprehensive understanding of the mechanisms underlying this slow inhibitory input. We performed whole-cell voltage clamp recordings on SBCs from juvenile Mongolian gerbils and recorded evoked inhibitory postsynaptic currents (IPSCs) at physiological rates. The data reveal activity-dependent IPSC kinetics, i.e., the decay is slowed with increased input rates or recruitment. Lowering the release probability yielded faster decay kinetics of the single- and short train-IPSCs at 100 Hz, suggesting that transmitter quantity plays an important role in controlling the decay. Slow transmitter clearance from the synaptic cleft caused prolonged receptor binding and, in the case of glycine, spillover to nearby synapses. The GABAergic component prolonged the decay by contributing to the asynchronous vesicle release depending on the input rate. Hence, the different factors controlling the amount of transmitters in the synapse jointly slow the inhibition during physiologically relevant activity. Taken together, the slow time course is predominantly determined by the receptor kinetics and transmitter clearance during short stimuli, whereas long duration or high frequency stimulation additionally engage asynchronous release to prolong IPSCs.

The recurrent case for the Renshaw cell

Although Renshaw cells (RCs) were discovered over half a century ago, their precise role in recurrent inhibition and ability to modulate motoneuron excitability have yet to be established. Indirect measurements of recurrent inhibition have suggested only a weak modulatory effect but are limited by the lack of observed motoneuron responses to inputs from single RCs. Here we present dual recordings between connected RC-motoneuron pairs, performed on mouse spinal cord. Motoneuron responses demonstrated that Renshaw synapses elicit large inhibitory conductances and show short-term potentiation. Anatomical reconstruction, combined with a novel method of quantal analysis, showed that the strong inhibitory input from RCs results from the large number of synaptic contacts that they make onto individual motoneurons. We used the NEURON simulation environment to construct realistic electrotonic models, which showed that inhibitory conductances from Renshaw inputs exert considerable shunting effects in motoneurons and reduce the frequency of spikes generated by excitatory inputs. This was confirmed experimentally by showing that excitation of a single RC or selective activation of the recurrent inhibitory pathway to generate equivalent inhibitory conductances both suppress motoneuron firing. We conclude that recurrent inhibition is remarkably effective, in that a single action potential from one RC is sufficient to silence a motoneuron. Although our results may differ from previous indirect observations, they underline a need for a reevaluation of the role that RCs perform in one of the first neuronal circuits to be discovered.

Functional analysis of the inhibitory neurotransmitter transporters GlyT1, GAT-1, and GAT-3 in astrocytes of the lateral superior olive

Neurotransmitter clearance from the synaptic cleft is a major function of astrocytes and requires neurotransmitter transporters. In the rodent lateral superior olive (LSO), a conspicuous auditory brainstem center, both glycine and GABA mediate synaptic inhibition. However, the main inhibitory input from the medial nucleus of the trapezoid body (MNTB) appears to be glycinergic by postnatal day (P) 14, when circuit maturation is almost accomplished. Using whole-cell patch-clamp recordings at P3-20, we analyzed glycine transporters (GlyT1) and GABA transporters (GAT-1, GAT-3) in mouse LSO astrocytes, emphasizing on their developmental regulation. Application of glycine or GABA induced a dose- and age-dependent inward current and a respective depolarization. The GlyT1-specific inhibitor sarcosine reduced the maximal glycine-induced current (IGly (max) ) by about 60%. The GAT-1 and GAT-3 antagonists NO711 and SNAP5114, respectively, reduced the maximal GABA-induced current (IGABA (max) ) by about 35%. Furthermore, [Cl(-) ]o reduction decreased IGly (max) and IGABA (max) by about 85 to 95%, showing the Cl(-) dependence of GlyT and GAT. IGABA (max) was stronger than IGly (max) , and the ratio increased developmentally from 1.6-fold to 3.7-fold. Together, our results demonstrate the functional presence of the three inhibitory neurotransmitter transporters GlyT1, GAT-1, and GAT-3 in LSO astrocytes. Furthermore, the uptake capability for GABA was higher than for glycine, pointing toward eminent GABAergic signaling in the LSO. GABA may originate from another source than the MNTB-LSO synapses, namely from another projection or from reversal of astrocytic GATs. Thus, neuronal signaling in the LSO appears to be more versatile than previously thought. GLIA 2014;62:1992-2003.

The timing of dopamine- and noradrenaline-mediated transmission reflects underlying differences in the extent of spillover and pooling

Metabotropic transmission typically occurs through the spillover activation of extrasynaptic receptors. This study examined the mechanisms underlying somatodendritic dopamine and noradrenaline transmission and found that the extent of spillover and pooling varied dramatically between these two transmitters. In the mouse ventral tegmental area, the time course of D2-receptor-mediated IPSCs (D2-IPSCs) was consistent between cells and was unaffected by altering stimulation intensity, probability of release, or the extent of diffusion. Blocking dopamine reuptake with cocaine extended the time course of D2-IPSCs and suggested that transporters strongly limited spillover. As a result, individual release sites contributed independently to the duration of D2-IPSCs. In contrast, increasing the release of noradrenaline in the rat locus ceruleus prolonged the duration of α2-receptor-mediated IPSCs even when reuptake was intact. Spillover and subsequent pooling of noradrenaline activated distal α2-receptors, which prolonged the duration of α2-IPSCs when multiple release sites were activated synchronously. By using the rapid application of agonists onto large macropatches, we determined the concentration profile of agonists underlying the two IPSCs. Incorporating the results into a model simulating extracellular diffusion predicted that the functional range of noradrenaline diffusion was nearly fivefold greater in the locus ceruleus than dopamine in the midbrain. This study demonstrates that catecholamine synapses differentially regulate the extent of spillover and pooling to control the timing of local inhibition and suggests diversity in the roles of uptake and diffusion in governing metabotropic transmission.

Rapid, activity-independent turnover of vesicular transmitter content at a mixed glycine/GABA synapse

The release of neurotransmitter via the fusion of transmitter-filled, presynaptic vesicles is the primary means by which neurons relay information. However, little is known regarding the molecular mechanisms that supply neurotransmitter destined for vesicle filling, the endogenous transmitter concentrations inside presynaptic nerve terminals, or the dynamics of vesicle refilling after exocytosis. We addressed these issues by recording from synaptically coupled pairs of glycine/GABA coreleasing interneurons (cartwheel cells) of the mouse dorsal cochlear nucleus. We find that the plasma membrane transporter GlyT2 and the intracellular enzyme glutamate decarboxylase supply the majority of glycine and GABA, respectively. Pharmacological block of GlyT2 or glutamate decarboxylase led to rapid and complete rundown of transmission, whereas increasing GABA synthesis via intracellular glutamate uncaging dramatically potentiated GABA release within 1 min. These effects were surprisingly independent of exocytosis, indicating that prefilled vesicles re-equilibrated upon acute changes in cytosolic transmitter. Titration of cytosolic transmitter with postsynaptic responses indicated that endogenous, nonvesicular glycine/GABA levels in nerve terminals are 5-7 mm, and that vesicular transport mechanisms are not saturated under basal conditions. Thus, cytosolic transmitter levels dynamically set the strength of inhibitory synapses in a release-independent manner.

The kinetic properties of the α3 rat glycine receptor make it suitable for mediating fast synaptic inhibition

Glycine receptors mediate fast synaptic inhibition in spinal cord and brainstem. Two α subunits are present in adult neurones, α1, which forms most of the synaptic glycine receptors, and α3. The physiological role of α3 is not known, despite the fact that α3 expression is concentrated in areas involved in nociceptive processing, such as the superficial dorsal horn. In the present study, we characterized the kinetic properties of rat homomeric α3 glycine receptors heterologously expressed in HEK293 cells. We analysed steady state single channel activity at a range of different glycine concentrations by fitting kinetic schemes and found that α3 channels resemble α1 receptors in their high maximum open probability (99.1% cf. 98% for α1), but differ in that maximum open probability is reached when all five binding sites are occupied by glycine (cf. three out of five sites for α1). α3 activation was best described by kinetic schemes that allow the channel to open also when partially liganded and that contain more than the minimum number of shut states, either as desensitized distal states (Jones and Westbrook scheme) or as pre-open gating intermediates (flip scheme). We recorded also synaptic-like α3 currents elicited by the rapid application of 1 ms pulses of high concentration glycine to outside-out patches. These currents had fast deactivation, with a time constant of decay of 9 ms. Thus, if native synaptic currents can be mediated by α3 glycine receptors, they are likely to be very close in their kinetics to α1-mediated synaptic events.

Target-specific IPSC kinetics promote temporal processing in auditory parallel pathways

The acoustic environment contains biologically relevant information on timescales from microseconds to tens of seconds. The auditory brainstem nuclei process this temporal information through parallel pathways that originate in the cochlear nucleus from different classes of cells. Although the roles of ion channels and excitatory synapses in temporal processing have been well studied, the contribution of inhibition is less well understood. Here, we show in CBA/CaJ mice that the two major projection neurons of the ventral cochlear nucleus, the bushy and T-stellate cells, receive glycinergic inhibition with different synaptic conductance time courses. Bushy cells, which provide precisely timed spike trains used in sound localization and pitch identification, receive slow inhibitory inputs. In contrast, T-stellate cells, which encode slower envelope information, receive inhibition that is eightfold faster. Both types of inhibition improved the precision of spike timing but engage different cellular mechanisms and operate on different timescales. Computer models reveal that slow IPSCs in bushy cells can improve spike timing on the scale of tens of microseconds. Although fast and slow IPSCs in T-stellate cells improve spike timing on the scale of milliseconds, only fast IPSCs can enhance the detection of narrowband acoustic signals in a complex background. Our results suggest that target-specific IPSC kinetics are critical for the segregated parallel processing of temporal information from the sensory environment.

Differential distribution of glycine receptor subtypes at the rat calyx of Held synapse

The properties of glycine receptors (GlyRs) depend upon their subunit composition. While the prevalent adult forms of GlyRs are heteromers, previous reports suggested functional α homomeric receptors in mature nervous tissues. Here we show two functionally different GlyRs populations in the rat medial nucleus of trapezoid body (MNTB). Postsynaptic receptors formed α1/β-containing clusters on somatodendritic domains of MNTB principal neurons, colocalizing with glycinergic nerve endings to mediate fast, phasic IPSCs. In contrast, presynaptic receptors on glutamatergic calyx of Held terminals were composed of dispersed, homomeric α1 receptors. Interestingly, the parent cell bodies of the calyces of Held, the globular bushy cells of the cochlear nucleus, expressed somatodendritic receptors (α1/β heteromers) and showed similar clustering and pharmacological profile as GlyRs on MNTB principal cells. These results suggest that specific targeting of GlyR β-subunit produces segregation of GlyR subtypes involved in two different mechanisms of modulation of synaptic strength.

Reliable evaluation of the quantal determinants of synaptic efficacy using Bayesian analysis

Communication between neurones in the central nervous system depends on synaptic transmission. The efficacy of synapses is determined by pre- and postsynaptic factors that can be characterized using quantal parameters such as the probability of neurotransmitter release, number of release sites, and quantal size. Existing methods of estimating the quantal parameters based on multiple probability fluctuation analysis (MPFA) are limited by their requirement for long recordings to acquire substantial data sets. We therefore devised an algorithm, termed Bayesian Quantal Analysis (BQA), that can yield accurate estimates of the quantal parameters from data sets of as small a size as 60 observations for each of only 2 conditions of release probability. Computer simulations are used to compare its performance in accuracy with that of MPFA, while varying the number of observations and the simulated range in release probability. We challenge BQA with realistic complexities characteristic of complex synapses, such as increases in the intra- or intersite variances, and heterogeneity in release probabilities. Finally, we validate the method using experimental data obtained from electrophysiological recordings to show that the effect of an antagonist on postsynaptic receptors is correctly characterized by BQA by a specific reduction in the estimates of quantal size. Since BQA routinely yields reliable estimates of the quantal parameters from small data sets, it is ideally suited to identify the locus of synaptic plasticity for experiments in which repeated manipulations of the recording environment are unfeasible.

Evaluation of glutamate concentration transient in the synaptic cleft of the rat calyx of Held

Establishing the spatiotemporal concentration profile of neurotransmitter following synaptic vesicular release is essential for our understanding of inter-neuronal communication. Such profile is a determinant of synaptic strength, short-term plasticity and inter-synaptic crosstalk. Synaptically released glutamate has been suggested to reach a few millimolar in concentration and last for <1 ms. The synaptic cleft is often conceived as a single concentration compartment, whereas a huge gradient likely exists. Modelling studies have attempted to describe this gradient, but two key parameters, the number of glutamate in a vesicle (N(Glu)) and its diffusion coefficient (D(Glu)) in the extracellular space, remained unresolved. To determine this profile, the rat calyx of Held synapse at postnatal day 12-16 was studied where diffusion of glutamate occurs two-dimensionally and where quantification of AMPA receptor distribution on individual postsynaptic specialization on medial nucleus of the trapezoid body principal cells is possible using SDS-digested freeze-fracture replica labelling. To assess the performance of these receptors as glutamate sensors, a kinetic model of the receptors was constructed from outside-out patch recordings. From here, we simulated synaptic responses and compared them with the EPSC recordings. Combinations of N(Glu) and D(Glu) with an optimum of 7000 and 0.3 μm(2) ms(-1) reproduced the data, suggesting slow diffusion. Further simulations showed that a single vesicle does not saturate the synaptic receptors, and that glutamate spillover does not affect the conductance amplitude at this synapse. Using the estimated profile, we also evaluated how the number of multiple vesicle releases at individual active zones affects the amplitude of postsynaptic signals.

The α1K276E startle disease mutation reveals multiple intermediate states in the gating of glycine receptors

Loss-of-function mutations in human glycine receptors cause hyperekplexia, a rare inherited disease associated with an exaggerated startle response. We have studied a human disease mutation in the M2-M3 loop of the glycine receptor α1 subunit (K276E) using direct fitting of mechanisms to single-channel recordings with the program HJCFIT. Whole-cell recordings from HEK293 cells showed the mutation reduced the receptor glycine sensitivity. In single-channel recordings, rat homomeric α1 K276E receptors were barely active, even at 200 mM glycine. Coexpression of the β subunit partially rescued channel function. Heteromeric mutant channels opened in brief bursts at 300 μM glycine (a concentration that is near-maximal for wild type) and reached a maximum one-channel open probability of about 45% at 100 mm glycine (compared to 96% for wild type). Distributions of apparent open times contained more than one component in high glycine and, therefore, could not be described by mechanisms with only one fully liganded open state. Fits to the data were much better with mechanisms in which opening can also occur from more than one fully liganded intermediate (e.g., "primed" models). Brief pulses of glycine (∼3 ms, 30 mM) applied to mutant channels in outside-out patches activated currents with a slower rise time (1.5 ms) than those of wild-type channels (0.2 ms) and a much faster decay. These features were predicted reasonably well by the mechanisms obtained from fitting single-channel data. Our results show that, by slowing and impairing channel gating, the K276E mutation facilitates the detection of closed reaction intermediates in the activation pathway of glycine channels.

Inhibitory synaptic regulation of motoneurons: a new target of disease mechanisms in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the third most common adult-onset neurodegenerative disease. It causes the degeneration of motoneurons and is fatal due to paralysis, particularly of respiratory muscles. ALS can be inherited, and specific disease-causing genes have been identified, but the mechanisms causing motoneuron death in ALS are not understood. No effective treatments exist for ALS. One well-studied theory of ALS pathogenesis involves faulty RNA editing and abnormal activation of specific glutamate receptors as well as failure of glutamate transport resulting in glutamate excitotoxicity; however, the excitotoxicity theory is challenged by the inability of anti-glutamate drugs to have major disease-modifying effects clinically. Nevertheless, hyperexcitability of upper and lower motoneurons is a feature of human ALS and transgenic (tg) mouse models of ALS. Motoneuron excitability is strongly modulated by synaptic inhibition mediated by presynaptic glycinergic and GABAergic innervations and postsynaptic glycine receptors (GlyR) and GABA(A) receptors; yet, the integrity of inhibitory systems regulating motoneurons has been understudied in experimental models, despite findings in human ALS suggesting that they may be affected. We have found in tg mice expressing a mutant form of human superoxide dismutase-1 (hSOD1) with a Gly93 → Ala substitution (G93A-hSOD1), causing familial ALS, that subsets of spinal interneurons degenerate. Inhibitory glycinergic innervation of spinal motoneurons becomes deficient before motoneuron degeneration is evident in G93A-hSOD1 mice. Motoneurons in these ALS mice also have insufficient synaptic inhibition as reflected by smaller GlyR currents, smaller GlyR clusters on their plasma membrane, and lower expression of GlyR1α mRNA compared to wild-type motoneurons. In contrast, GABAergic innervation of ALS mouse motoneurons and GABA(A) receptor function appear normal. Abnormal synaptic inhibition resulting from dysfunction of interneurons and motoneuron GlyRs is a new direction for unveiling mechanisms of ALS pathogenesis that could be relevant to new therapies for ALS.

Zinc modulation of glycine receptors

Glycine receptors are widely expressed in the mammalian central nervous system, and previous studies have demonstrated that glycine receptors are modulated by endogenous zinc. Zinc is concentrated in synaptic vesicles in several brain regions but is particularly abundant in the hippocampus and olfactory bulb. In the present study, we used patch-clamp electrophysiology of rat hippocampal and olfactory bulb neurons in primary culture to examine the effects of zinc on glycine receptors. Although glycine has been reported to reach millimolar concentrations during synaptic transmission, most previous studies on the effects of zinc on glycine receptors have used relatively low concentrations of glycine. High concentrations of glycine cause receptor desensitization. Our current results extend our previous demonstration that the modulatory actions of zinc are largely prevented when co-applied with desensitizing concentrations of glycine (300 μM), suggesting that the effects of zinc are dependent on the state of the receptor. In contrast, pre-application of 300 μM zinc, prior to glycine (300 μM) application, causes a slowly developing inhibition with a slow rate of recovery, suggesting that the timing of zinc and glycine release also influences the effects of zinc. Furthermore, previous evidence suggests that synaptically released zinc can gain intracellular access, and we provide the first demonstration that low concentrations of intracellular zinc can potentiate glycine receptors. These results support the notion that zinc has complex effects on glycine receptors and multiple factors may interact to influence the efficacy of glycinergic transmission.

Impact of synaptic neurotransmitter concentration time course on the kinetics and pharmacological modulation of inhibitory synaptic currents

The time course of synaptic currents is a crucial determinant of rapid signaling between neurons. Traditionally, the mechanisms underlying the shape of synaptic signals are classified as pre- and post-synaptic. Over the last two decades, an extensive body of evidence indicated that synaptic signals are critically shaped by the neurotransmitter time course which encompasses several phenomena including pre- and post-synaptic ones. The agonist transient depends on neurotransmitter release mechanisms, diffusion within the synaptic cleft, spill-over to the extra-synaptic space, uptake, and binding to post-synaptic receptors. Most estimates indicate that the neurotransmitter transient is very brief, lasting between one hundred up to several hundreds of microseconds, implying that post-synaptic activation is characterized by a high degree of non-equilibrium. Moreover, pharmacological studies provide evidence that the kinetics of agonist transient plays a crucial role in setting the susceptibility of synaptic currents to modulation by a variety of compounds of physiological or clinical relevance. More recently, the role of the neurotransmitter time course has been emphasized by studies carried out on brain slice models that revealed a striking, cell-dependent variability of synaptic agonist waveforms ranging from rapid pulses to slow volume transmission. In the present paper we review the advances on studies addressing the impact of synaptic neurotransmitter transient on kinetics and pharmacological modulation of synaptic currents at inhibitory synapses.

Comparative posthearing development of inhibitory inputs to the lateral superior olive in gerbils and mice

Interaural intensity differences are analyzed in neurons of the lateral superior olive (LSO) by integration of an inhibitory input from the medial nucleus of the trapezoid body (MNTB), activated by sound from the contralateral ear, with an excitatory input from the ipsilateral cochlear nucleus. The early postnatal refinement of this inhibitory MNTB-LSO projection along the tonotopic axis of the LSO has been extensively studied. However, little is known to what extent physiological changes at these inputs also occur after the onset of sound-evoked activity. Using whole-cell patch-clamp recordings of LSO neurons in acute brain stem slices, we analyzed the developmental changes of inhibitory synaptic currents evoked by MNTB fiber stimulation occurring after hearing onset. We compared these results in gerbils and mice, two species frequently used in auditory research. Our data show that neither the number of presumed input fibers nor the conductance of single fibers significantly changed after hearing onset. Also the amplitude of miniature inhibitory currents remained constant during this developmental period. In contrast, the kinetics of inhibitory synaptic currents greatly accelerated after hearing onset. We conclude that tonotopic refinement of inhibitory projections to the LSO is largely completed before the onset of hearing, whereas acceleration of synaptic kinetics occurs to a large part after hearing onset and might thus be dependent on proper auditory experience. Surprisingly, inhibitory input characteristics, as well as basic membrane properties of LSO neurons, were rather similar in gerbils and mice.

Glycine receptor channels in spinal motoneurons are abnormal in a transgenic mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a rapidly evolving and fatal adult-onset neurological disease characterized by progressive degeneration of motoneurons. Our previous study showed that glycinergic innervation of spinal motoneurons is deficient in an ALS mouse model expressing a mutant form of human superoxide dismutase-1 with a Gly93→Ala substitution (G93A-SOD1). In this study, we have examined, using whole-cell patch-clamp recordings, glycine receptor (GlyR)-mediated currents in spinal motoneurons from these transgenic mice. We developed a dissociated spinal cord culture model using embryonic transgenic mice expressing enhanced green fluorescent protein (eGFP) driven by the Hb9 promoter. Motoneurons were identified as Hb9-eGFP-expressing (Hb9-eGFP(+)) neurons with a characteristic morphology. To examine GlyRs in ALS motoneurons, we bred G93A-SOD1 mice to Hb9-eGFP mice and compared glycine-evoked currents in cultured Hb9-eGFP(+) motoneurons prepared from G93A-SOD1 embryos and from their nontransgenic littermates. Glycine-evoked current density was significantly smaller in the G93A-SOD1 motoneurons compared with control. Furthermore, the averaged current densities of spontaneous glycinergic miniature IPSCs (mIPSCs) were significantly smaller in the G93A-SOD1 motoneurons than in control motoneurons. No significant differences in GABA-induced currents and GABAergic mIPSCs were observed between G93A-SOD1 and control motoneurons. Quantitative single-cell reverse transcription-PCR found lower GlyRα1 subunit mRNA expression in G93A-SOD1 motoneurons, indicating that the reduction of GlyR current may result from the downregulation of GlyR mRNA expression in motoneurons. Immunocytochemistry demonstrated a decrease of surface postsynaptic GlyR on G93A-SOD1 motoneurons. Our study suggests that selective alterations in GlyR function contribute to inhibitory insufficiency in motoneurons early in the disease process of ALS.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Hypothalamic hypocretin/orexin (hcrt/orx) neurons promote arousal and reward seeking, while reduction in their activity has been linked to narcolepsy, obesity and depression. However, the mechanisms influencing the activity of hcrt/orx networks in situ are not fully understood. Here we show that glycine, a neurotransmitter best known for its actions in the brainstem and spinal cord, elicits dose dependent postsynaptic Cl⁻ currents in hcrt/orx cells in acute mouse brain slices. The effect was blocked by the glycine receptor (GLyR) antagonist strychnine and mimicked by the GlyR agonist alanine. Postsynaptic GlyRs on hcrt/orx cells remained functional during both early postnatal and adult periods, and gramicidin-perforated patch-clamp recordings revealed that they progressively switch from excitatory to inhibitory during the first two postnatal weeks. The pharmacological profile of the glycine response suggested that developed hcrt/orx neurons contain α/β-heteromeric GlyRs that lack α2-subunits, whereas α2-subunits, whereas α2-subunits are present in early postnatal hcrt/orx neurons. All postsynaptic currents (PSCs) in developed hcrt/orx cells were blocked by inhibitors of GABA and glutamate receptors, with no evidence of GlyR-mediated PSCs. However, the frequency but not amplitude of miniature PSCs was reduced by strychnine and increased by glycine in ~50% of hcrt/orx neurons. Together, these results provide the first evidence for functional GlyRs in identified hcrt/orx circuits and suggest that the activity of developed hcrt/orx cells is regulated by two GlyR pools: inhibitory extrasynaptic GlyRs located on all hcrt/orx cells and excitatory GlyRs located on presynaptic terminals contacting some hcrt/orx cells.

N-arachidonyl-glycine modulates synaptic transmission in superficial dorsal horn

BACKGROUND AND PURPOSE

The arachidonyl-amino acid N-arachidonyl-glycine (NAGly) is an endogenous lipid, generated within the spinal cord and producing spinally mediated analgesia via non-cannabinoid mechanisms. In this study we examined the actions of NAGly on neurons within the superficial dorsal horn, a key site for the actions of many analgesic agents.

EXPERIMENTAL APPROACH

Whole cell patch clamp recordings were made from lamina II neurons in rat spinal cord slices to examine the effect of NAGly on glycinergic and NMDA-mediated synaptic transmission.

KEY RESULTS

N-arachidonyl-glycine prolonged the decay of glycine, but not β-alanine induced inward currents and decreased the amplitude of currents induced by both glycine and β-alanine. NAGly and ALX-1393 (inhibitor of the glycine transporter, GLYT2), but not the GLYT1 inhibitor, ALX-5407, produced a strychnine-sensitive inward current. ALX-5407 and ALX-1393, but not NAGly prolonged the decay phase of glycine receptor-mediated miniature inhibitory postsynaptic currents (IPSCs). NAGly prolonged the decay phase of evoked IPSCs, although to a lesser extent than ALX-5407 and ALX-1393. In the presence of ALX-1393, NAGly shortened the decay phase of evoked IPSCs. ALX-5407 increased and NAGly decreased the amplitude of evoked NMDA-mediated excitatory postsynaptic currents.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that NAGly enhanced inhibitory glycinergic synaptic transmission within the superficial dorsal horn by blocking glycine uptake via GLYT2. In addition, NAGly decreased excitatory NMDA-mediated synaptic transmission. Together, these findings provide a cellular explanation for the spinal analgesic actions of NAGly.

Slow GABA transient and receptor desensitization shape synaptic responses evoked by hippocampal neurogliaform cells

The kinetics of GABAergic synaptic currents can vary by an order of magnitude depending on the cell type. The neurogliaform cell (NGFC) has recently been identified as a key generator of slow GABA(A) receptor-mediated volume transmission in the isocortex. However, the mechanisms underlying slow GABA(A) receptor-mediated IPSCs and their use-dependent plasticity remain unknown. Here, we provide experimental and modeling data showing that hippocampal NGFCs generate an unusually prolonged (tens of milliseconds) but low-concentration (micromolar range) GABA transient, which is responsible for the slow response kinetics and which leads to a robust desensitization of postsynaptic GABA(A) receptors. This strongly contributes to the use-dependent synaptic depression elicited by various patterns of NGFC activity including the one detected during theta network oscillations in vivo. Synaptic depression mediated by NGFCs is likely to play an important modulatory role in the feedforward inhibition of CA1 pyramidal cells provided by the entorhinal cortex.