Molecular biology of glycinergic neurotransmission

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly through a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory post synaptic currents (eIPSCs) onto hippocampal principal cells. Importantly O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude, mimicking forskolin. Our findings show under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is accessible to agonists, permitting strengthening of synaptic inhibition.

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.

Synergistic Control of Transmitter Turnover at Glycinergic Synapses by GlyT1, GlyT2, and ASC-1

In addition to being involved in protein biosynthesis and metabolism, the amino acid glycine is the most important inhibitory neurotransmitter in caudal regions of the brain. These functions require a tight regulation of glycine concentration not only in the synaptic cleft, but also in various intracellular and extracellular compartments. This is achieved not only by confining the synthesis and degradation of glycine predominantly to the mitochondria, but also by the action of high-affinity large-capacity glycine transporters that mediate the transport of glycine across the membranes of presynaptic terminals or glial cells surrounding the synapses. Although most cells at glycine-dependent synapses express more than one transporter with high affinity for glycine, their synergistic functional interaction is only poorly understood. In this review, we summarize our current knowledge of the two high-affinity transporters for glycine, the sodium-dependent glycine transporters 1 (GlyT1; SLC6A9) and 2 (GlyT2; SLC6A5) and the alanine–serine–cysteine-1 transporter (Asc-1; SLC7A10).

Developmental Formation of the GABAergic and Glycinergic Networks in the Mouse Spinal Cord

Gamma-aminobutyric acid (GABA) and glycine act as inhibitory neurotransmitters. Three types of inhibitory neurons and terminals, GABAergic, GABA/glycine coreleasing, and glycinergic, are orchestrated in the spinal cord neural circuits and play critical roles in regulating pain, locomotive movement, and respiratory rhythms. In this study, we first describe GABAergic and glycinergic transmission and inhibitory networks, consisting of three types of terminals in the mature mouse spinal cord. Second, we describe the developmental formation of GABAergic and glycinergic networks, with a specific focus on the differentiation of neurons, formation of synapses, maturation of removal systems, and changes in their action. GABAergic and glycinergic neurons are derived from the same domains of the ventricular zone. Initially, GABAergic neurons are differentiated, and their axons form synapses. Some of these neurons remain GABAergic in lamina I and II. Many GABAergic neurons convert to a coreleasing state. The coreleasing neurons and terminals remain in the dorsal horn, whereas many ultimately become glycinergic in the ventral horn. During the development of terminals and the transformation from radial glia to astrocytes, GABA and glycine receptor subunit compositions markedly change, removal systems mature, and GABAergic and glycinergic action shifts from excitatory to inhibitory.

Pre-steady-state Kinetic Analysis of Amino Acid Transporter SLC6A14 Reveals Rapid Turnover Rate and Substrate Translocation

SLC6A14 (solute carrier family 6 member 14) is an amino acid transporter, driven by Na⁺ and Cl⁻ co-transport, whose structure, function, and molecular and kinetic mechanism have not been well characterized. Its broad substrate selectivity, including neutral and cationic amino acids, differentiates it from other SLC6 family members, and its proposed involvement in nutrient transport in several cancers suggest that it could become an important drug target. In the present study, we investigated SLC6A14 function and its kinetic mechanism after expression in human embryonic kidney (HEK293) cells, including substrate specificity and voltage dependence under various ionic conditions. We applied rapid solution exchange, voltage jumps, and laser photolysis of caged alanine, allowing sub-millisecond temporal resolution, to study SLC6A14 steady state and pre-steady state kinetics. The results highlight the broad substrate specificity and suggest that extracellular chloride enhances substrate transport but is not required for transport. As in other SLC6 family members, Na⁺ binding to the substrate-free transporter (or conformational changes associated with it) is electrogenic and is likely rate limiting for transporter turnover. Transient current decaying with a time constant of <1ms is also observed after rapid amino acid application, both in forward transport and homoexchange modes, indicating a slightly electrogenic, but fast and not rate-limiting substrate translocation step. Our results, which are consistent with kinetic modeling, suggest rapid transporter turnover rate and substrate translocation with faster kinetics compared with other SLC6 family members. Together, these results provided novel information on the SLC6A14 transport cycle and mechanism, expanding our understanding of SLC6A14 function.

Glycine Metabolism and Its Alterations in Obesity and Metabolic Diseases

Glycine is the proteinogenic amino-acid of lowest molecular weight, harboring a hydrogen atom as a side-chain. In addition to being a building-block for proteins, glycine is also required for multiple metabolic pathways, such as glutathione synthesis and regulation of one-carbon metabolism. Although generally viewed as a non-essential amino-acid, because it can be endogenously synthesized to a certain extent, glycine has also been suggested as a conditionally essential amino acid. In metabolic disorders associated with obesity, type 2 diabetes (T2DM), and non-alcoholic fatty liver disease (NAFLDs), lower circulating glycine levels have been consistently observed, and clinical studies suggest the existence of beneficial effects induced by glycine supplementation. The present review aims at synthesizing the recent advances in glycine metabolism, pinpointing its main metabolic pathways, identifying the causes leading to glycine deficiency-especially in obesity and associated metabolic disorders-and evaluating the potential benefits of increasing glycine availability to curb the progression of obesity and obesity-related metabolic disturbances. This study focuses on the importance of diet, gut microbiota, and liver metabolism in determining glycine availability in obesity and associated metabolic disorders.

Distinct development of the glycinergic terminals in the ventral and dorsal horns of the mouse cervical spinal cord

In the spinal cord, glycine and γ-amino butyric acid (GABA) are inhibitory neurotransmitters. However, the ontogeny of the glycinergic network remains unclear. To address this point, we examined the developmental formation of glycinergic terminals by immunohistochemistry for glycine transporter 2 (GlyT2), a marker of glycinergic terminals, in developing mouse cervical spinal cord. Furthermore, the developmental localization of GlyT2 was compared with that of glutamic acid decarboxylase (GAD), a marker of GABAergic terminals, and vesicular GABA transporter (VGAT), a marker of inhibitory terminals, by single and double immunolabeling. GlyT2-positive dots (glycinergic terminals) were first detected in the marginal zone on embryonic day 14 (E14). In the ventral horn, they were detected at E16 and increased in observed density during postnatal development. Until postnatal day 7 (P7), GAD-positive dots (GABAergic terminals) were dominant and GlyT2 immunolabeling was localized at GAD-positive dots. During the second postnatal week, GABAergic terminals markedly decreased and glycinergic terminals became dominant. In the dorsal horn, glycinergic terminals were detected at P0 in lamina IV and P7 in lamina III and developmentally increased. GlyT2 was also localized at GAD-positive dots, and colocalizing dots were dominant at P21. VGAT-positive dots (inhibitory terminals) continued to increase until P21. These results suggest that GABAergic terminals first appear during embryonic development and may often change to colocalizing terminals throughout the gray matter during development. The colocalizing terminals may remain in the dorsal horn, whereas in the ventral horn, colocalizing terminals may give rise to glycinergic terminals.

D-Serine in neurobiology: CNS neurotransmission and neuromodulation

Homochirality is fundamental for life. L-Amino acids are exclusively used as substrates for the polymerization and formation of peptides and proteins in living systems. However, D- amino acids were recently detected in various living organisms, including mammals. Of these D-amino acids, D-serine has been most extensively studied. D-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl- D-aspartate receptor (NMDAr). D-Serine binds with high affinity to a co-agonist site at the NMDAr and, along with glutamate, mediates several vital physiological and pathological processes, including NMDAr transmission, synaptic plasticity and neurotoxicity. Therefore, a key role for D-serine as a determinant of NMDAr mediated neurotransmission in mammalian CNS has been suggested. In this context, we review the known functions of D-serine in human physiology, such as CNS development, and pathology, such as neuro-psychiatric and neurodegenerative diseases related to NMDAr dysfunction.

Multiple functions of neuronal plasma membrane neurotransmitter transporters

Removal from receptors of neurotransmitters just released into synapses is one of the major steps in neurotransmission. Transporters situated on the plasma membrane of nerve endings and glial cells perform the process of neurotransmitter (re)uptake. Because the density of transporters in the membranes can fluctuate, transporters can determine the transmitter concentrations at receptors, thus modulating indirectly the excitability of neighboring neurons. Evidence is accumulating that neurotransmitter transporters can exhibit multiple functions. Being bidirectional, neurotransmitter transporters can mediate transmitter release by working in reverse, most often under pathological conditions that cause ionic gradient dysregulations. Some transporters reverse to release transmitters, like dopamine or serotonin, when activated by 'indirectly acting' substrates, like the amphetamines. Some transporters exhibit as one major function the ability to capture transmitters into nerve terminals that perform insufficient synthesis. Transporter activation can generate conductances that regulate directly neuronal excitability. Synaptic and non-synaptic transporters play different roles. Cytosolic Na(+) elevations accompanying transport can interact with plasmalemmal or/and mitochondrial Na(+)/Ca(2+) exchangers thus generating calcium signals. Finally, neurotransmitter transporters can behave as receptors mediating releasing stimuli able to cause transmitter efflux through multiple mechanisms. Neurotransmitter transporters are therefore likely to play hitherto unknown roles in multiple therapeutic treatments.

Complex formation of Sn(II) with glycine: an IR, DTA/TGA and DFT investigation

The novel Sn(Gly)2⋅H2O complex compound has been synthesized and characterized by TGA, IR and Raman spectroscopy. Molecular spectroscopy and ab initio simulation have given the evidence of glycine molecule being coordinated to Sn(II) as bidentate chelating ligand by oxygen atom of carboxyl group and nitrogen atom of amino group. Water molecule is bonded with amino and carboxylic groups by hydrogen bonds in the out sphere. The M06, TPSS, TPSSm, TPSSh and revTPSS density functionals have been tested for calculation of structural and vibrational data. The vibrational assignment of experimental IR and Raman and simulated spectra has been carried out. The TPSS and TPSSm density functionals and Def2-TZVP basis set have provided the most accurate results.

The relationship between glycine transporter 1 occupancy and the effects of the glycine transporter 1 inhibitor RG1678 or ORG25935 on object retrieval performance in scopolamine impaired rhesus monkey

Reduced NMDA receptor functioning is hypothesized to underlie the cognitive and negative symptoms associated with schizophrenia. However, because direct activation of the NMDA receptor is accompanied by neurotoxicity, mechanisms that activate the glycine co-agonist site on the NMDA receptor could carry greater therapeutic potential. In the current study, the effects of two glycine transporter 1 (GlyT1) inhibitors, RG1678 and ORG25935, were characterized in the object-retrieval detour (ORD) task in scopolamine-impaired rhesus monkeys and, using positron emission tomography (PET), the GlyT1 occupancy to efficacy relationship of each compound was established. Scopolamine exerted a significant decrease in accuracy in the ORD task. Lower doses of RG1678 (0.3 and 1.0 mg/kg, p.o.) significantly attenuated the impact of scopolamine, whereas the highest dose tested (1.8 mg/kg) did not. The predicted GlyT1 occupancies of RG1678 at the effective doses were ~10 and 30 %. ORG25935 (0.1, 0.3, and 1 mg/kg, p.o.) also significantly attenuated the impact of scopolamine on the ORD task, whereas 3 mg/kg did not. The predicted GlyT1 occupancies of ORG25935 at the effective doses ranged from 16 to 80 %. These data suggest that GlyT1 inhibitors have the potential to improve performance on prefrontal cortex-dependent tests such as the ORD task, but that efficacy is lost when higher occupancies are achieved. Importantly, recent Ph2B data published by Roche suggests that low but not high doses of RG1678 improved negative symptoms in patients with schizophrenia, highlighting the potential translational nature of the current preclinical findings.

A novel liquid chromatography/tandem mass spectrometry method for the quantification of glycine as biomarker in brain microdialysis and cerebrospinal fluid samples within 5min

Glycine is an important amino acid neurotransmitter in the central nervous system (CNS) and a useful biomarker to indicate biological activity of drugs such as glycine reuptake inhibitors (GRI) in the brain. Here, we report how a liquid chromatography/tandem mass spectrometry (LC-MS/MS) method for the fast and reliable analysis of glycine in brain microdialysates and cerebrospinal fluid (CSF) samples has been established. Additionally, we compare this method with the conventional approach of high performance liquid chromatography (HPLC) coupled to fluorescence detection (FD). The present LC-MS/MS method did not require any derivatisation step. Fifteen microliters of sample were injected for analysis. Glycine was detected by a triple quadrupole mass spectrometer in the positive electrospray ionisation (ESI) mode. The total running time was 5min. The limit of quantitation (LOQ) was determined as 100nM, while linearity was given in the range from 100nM to 100μM. In order to demonstrate the feasibility of the LC-MS/MS method, we measured glycine levels in striatal in vivo microdialysates and CSF of rats after administration of the commercially available glycine transporter 1 (GlyT1) inhibitor LY 2365109 (10mg/kg, p.o.). LY 2365109 produced 2-fold and 3-fold elevated glycine concentrations from 1.52μM to 3.6μM in striatal microdialysates and from 10.38μM to 36μM in CSF, respectively. In conclusion, we established a fast and reliable LC-MS/MS method, which can be used for the quantification of glycine in brain microdialysis and CSF samples in biomarker studies.

In Vivo Evaluation of (11)C-labeled Three Radioligands for Glycine Transporter 1 in the Mouse Brain

Objective

Glycine transporter 1 (GlyT-1) is one of the most attractive therapeutic targets for schizophrenia. There is great interest in developing radioligands for in vivo imaging of GlyT-1 in the brain using positron emission tomography. Here, we report the properties of three novel non-sarcosine-based radioligands [¹¹C]CHIBA-3007, [¹¹C]CHIBA-3009, and [¹¹C]CHIBA-3011, for GlyT-1 imaging in the mouse brain in vivo.

Methods

The three radioligands were synthesized by N-[¹¹C] methylation of the corresponding desmethyl precursor. A pharmacological characterization of these radioligands for in vivo imaging of GlyT-1 in the brain was conducted using male ddY mice.

Results

[¹¹C]CHIBA-3009 and [¹¹C]CHIBA-3011 were scarcely incorporated into the brain, whereas [¹¹C]CHIBA-3007 showed slight but considerable brain uptake. Regional brain uptake of [¹¹C]CHIBA-3007 (medulla oblongata>cerebellum>cortex) was similar to the distribution of the GlyT-1 protein. However, pretreatment with CHIBA-3007 (1 mg/kg) or the GlyT-1 selective inhibitor ALX5407 (N-[(3R)-3-([1,1'-Biphenyl]-4-yloxy)-3-(4-fluorophenyl)propyl]-N-methylglycine) (30 mg/kg) did not significantly decrease brain uptake of [¹¹C]CHIBA-3007, suggesting low specific binding to GlyT-1. Pretreatment with cyclosporin A significantly increased brain uptake of [¹¹C]CHIBA-3009 and [¹¹C]CHIBA-3011, suggesting a role for P-glycoprotein in the brain uptake of these ligands. All three radioligands were rapidly degraded intact forms were 3-18% in plasma and 15-74% in the brain at 15 min after injection.

Conclusion

The results suggest that these three radioligands are not suitable for in vivo imaging of GlyT-1 in the brain because of low brain uptake and rapid metabolism. Further structural refinement is necessary to enhance brain uptake.

Mechanisms of glycine release, which build up synaptic and extrasynaptic glycine levels: the role of synaptic and non-synaptic glycine transporters

Glycine is an amino acid neurotransmitter that is involved in both inhibitory and excitatory neurochemical transmission in the central nervous system. The role of glycine in excitatory neurotransmission is related to its coagonist action at glutamatergic N-methyl-D-aspartate receptors. The glycine levels in the synaptic cleft rise many times higher during synaptic activation assuring that glycine spills over into the extrasynaptic space. Another possible origin of extrasynaptic glycine is the efflux of glycine occurring from astrocytes associated with glutamatergic synapses. The release of glycine from neuronal or glial origins exhibits several differences compared to that of biogenic amines or other amino acid neurotransmitters. These differences appear in an external Ca(2+)- and temperature-dependent manner, conferring unique characteristics on glycine as a neurotransmitter. Glycine transporter type-1 at synapses may exhibit neural and glial forms and plays a role in controlling synaptic glycine levels and the spill over rate of glycine from the synaptic cleft into the extrasynaptic biophase. Non-synaptic glycine transporter type-1 regulates extrasynaptic glycine concentrations, either increasing or decreasing them depending on the reverse or normal mode operation of the carrier molecule. While we can, at best, only estimate synaptic glycine levels at rest and during synaptic activation, glycine concentrations are readily measurable via brain microdialysis technique applied in the extrasynaptic space. The non-synaptic N-methyl-D-aspartate receptor may obtain glycine for activation following its spill over from highly active synapses or from its release mediated by the reverse operation of non-synaptic glycine transporter-1. The sensitivity of non-synaptic N-methyl-D-aspartate receptors to glutamate and glycine is many times higher than that of synaptic N-methyl-D-aspartate receptors making the former type of receptor the primary target for drug action. Synaptic and non-synaptic N-methyl-D-aspartate receptors mediate different neural functions, many of which are not clearly defined at present. Non-synaptic glycine transporter-1 and its blockade by inhibitory drugs may be important in drug therapy interventions, such as for reducing negative symptoms of schizophrenia.

Proteomic and metabolomic profiling of a trait anxiety mouse model implicate affected pathways

Depression and anxiety disorders affect a great number of people worldwide. Whereas singular factors have been associated with the pathogenesis of psychiatric disorders, growing evidence emphasizes the significance of dysfunctional neural circuits and signaling pathways. Hence, a systems biology approach is required to get a better understanding of psychiatric phenotypes such as depression and anxiety. Furthermore, the availability of biomarkers for these disorders is critical for improved diagnosis and monitoring treatment response. In the present study, a mouse model presenting with robust high versus low anxiety phenotypes was subjected to thorough molecular biomarker and pathway discovery analyses. Reference animals were metabolically labeled with the stable (15)N isotope allowing an accurate comparison of protein expression levels between the high anxiety-related behavior versus low anxiety-related behavior mouse lines using quantitative mass spectrometry. Plasma metabolomic analyses identified a number of small molecule biomarkers characteristic for the anxiety phenotype with particular focus on myo-inositol and glutamate as well as the intermediates involved in the tricarboxylic acid cycle. In silico analyses suggested pathways and subnetworks as relevant for the anxiety phenotype. Our data demonstrate that the high anxiety-related behavior and low anxiety-related behavior mouse model is a valuable tool for anxiety disorder drug discovery efforts.

PKCβ-dependent phosphorylation of the glycine transporter 1

The extracellular levels of the neurotransmitter glycine in the brain are tightly regulated by the glycine transporter 1 (GlyT1) and the clearance rate for glycine depends on its rate of transport and the levels of cell surface GlyT1. Over the years, it has been shown that PKC tightly regulates the activity of several neurotransmitter transporters. In the present work, by stably expressing three N-terminus GlyT1 isoforms in porcine aortic endothelial cells and assaying for [(32)P]-orthophosphate metabolic labeling, we demonstrated that the isoforms GlyT1a, GlyT1b, and GlyT1c were constitutively phosphorylated, and that phosphorylation was dramatically enhanced, in a time dependent fashion, after PKC activation by phorbol ester. The phosphorylation was PKC-dependent, since pre-incubation of the cells with bisindolylmaleimide I, a selective PKC inhibitor, abolished the phorbol ester-induced phosphorylation. Blotting with specific anti-phospho-tyrosine antibodies did not yield any signal that could correspond to GlyT1 tyrosine phosphorylation, suggesting that the phosphorylation occurs at serine and/or threonine residues. In addition, a 23-40%-inhibition on V(max) was obtained by incubation with phorbol ester without a significant change on the apparent Km value. Furthermore, pre-incubation of the cells with the selective PKCα/β inhibitor Gö6976 abolished the downregulation effect of phorbol ester on uptake and phosphorylation, whereas the selective PKCβ inhibitors (PKCβ inhibitor or LY333531) prevented the phosphorylation without affecting glycine uptake, defining a specific role of classical PKC on GlyT1 uptake and phosphorylation. Taken together, these data suggest that conventional PKCα/β regulates the uptake of glycine, whereas PKCβ is responsible for GlyT1 phosphorylation.

Pharmacological characterization of [³H]CHIBA-3007 binding to glycine transporter 1 in the rat brain

Glycine transporter-1 (GlyT-1) in glial cells regulates extracellular levels of glycine, which acts as an obligatory co-agonist at the N-methyl-D-aspartate (NMDA) receptors in the brain. In the present study, we developed a novel radioligand, [³H]3-chloro-N-((S)-((R)-1-methylpiperidin-2-yl)(thiophen- 3-yl)methyl)-4- (trifluoromethyl)picolinamide ([³H]CHIBA-3007), for studying GlyT-1 in the brain. The presence of a single saturable high-affinity binding component for [³H]CHIBA-3007 binding to the rat brain membranes was detected. Scatchard analysis revealed an apparent equilibrium dissociation constant (K(d)) of 1.61±0.16 nM and a maximal number of binding sites (B(max)) of 692.8±22.8 fmol/mg protein (mean ± SEM, n = 3). The specific binding of [³H]CHIBA-3007 was inhibited by a number of GlyT-1 inhibitors, such as CHIBA-3007, desmethyl-CHIBA-3007, CHIBA-3008, SSR504734, NFPS/ALX5407, LY2365109 and Org24598, consistent with the pharmacological profiles of GlyT-1 inhibitors. Interestingly, the potency of eight GlyT-1 inhibitors (CHIBA-3007, desmethyl-CHIBA-3007, NFPS/ALX5407, LY2365109, Org24598, SSR504734, sarcosine, and glycine) for blocking in vitro specific binding of [³H]CHIBA-3007 was significantly correlated with the potency of these inhibitors for inhibiting [¹⁴C]glycine uptake in the rat brain membranes. In contrast, the GlyT-2 inhibitor ALX1393 exhibited very weak for [³H]CHIBA-3007 binding. Furthermore, the regional distribution of [³H]CHIBA-3007 binding in the rat brain was similar to the previously reported distribution of GlyT-1. The present findings suggest that [³H]CHIBA-3007 would be a useful new radioligand for studying GlyT-1 in the brain.

D-serine: the right or wrong isoform?

Only recently, d-amino acids have been identified in mammals. Of these, d-serine has been most extensively studied. d-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl-d-aspartate receptor (NMDAr), similar to glycine. Therefore, d-serine may well play a role in all physiological and pathological processes in which NMDArs have been implied. In this review, we discuss the findings implying an important role for d-serine in human physiology (CNS development and memory and learning) and pathology (excitotoxicity, perinatal asphyxia, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, epilepsy, schizophrenia and bipolar disorder). We will debate on the relative contribution of d-serine versus glycine and conclude with clinical applications derived from these results and future directions to progress in this field. In general, adequate concentrations of d-serine are required for normal CNS development and function, while both decreased and increased concentrations can lead to CNS pathology. Therefore, d-serine appears to be the right isoform when present in the right concentrations.

Prophylactic glycine administration attenuates pancreatic damage and inflammation in experimental acute pancreatitis

BACKGROUND/AIMS

Acute pancreatitis (AP) is characterized by premature zymogen activation, systemic inflammatory response resulting in inflammatory infiltrates, sustained intracellular calcium, neurogenic inflammation and pain. The inhibitory neurotransmitter and cytoprotective amino acid glycine exerts a direct inhibitory effect on inflammatory cells, inhibits calcium influx and neuronal activation and therefore represents a putative therapeutic agent in AP.

METHODS

To explore the impact of glycine, mild AP was induced in rats by supramaximal cerulein stimulation (10 μg/kg BW/h) and severe AP by retrograde injection of sodium taurocholate solution (3%) into the common biliopancreatic duct. 100/300 mmol glycine was administered intravenously before induction of AP. To elucidate the effect of glycine on AP, we determined pathomorphology, pancreatic cytokines as well as proteases, serum lipase and amylase, pancreatic and lung MPO activity and pain sensation.

RESULTS

Glycine administration resulted in a noticeable improvement of pathomorphological alterations in AP, such as a reduction of necrosis, inflammatory infiltrates and cytoplasmic vacuoles in cerulein pancreatitis. In taurocholate pancreatitis, glycine additionally diminished pancreatic cytokines and MPO activity, as well as serum lipase and amylase levels.

CONCLUSIONS

Glycine reduced the severity of mild and much more of severe AP by attenuating the intrapancreatic and systemic inflammatory response. Therefore, glycine seems to be a promising tool for prophylactic treatment of AP. and IAP.

GlyT2+ neurons in the lateral cerebellar nucleus

The deep cerebellar nuclei (DCN) are a major hub in the cerebellar circuitry but the functional classification of their neurons is incomplete. We have previously characterized three cell groups in the lateral cerebellar nucleus: large non-GABAergic neurons and two groups of smaller neurons, one of which express green fluorescence protein (GFP) in a GAD67/GFP mouse line and is therefore GABAergic. However, as a substantial number of glycinergic and glycine/GABA co-expressing neurons have been described in the DCN, this classification needed to be refined by considering glycinergic neurons. To this end we took advantage of a glycine transporter isoform 2 (GlyT2)-eGFP mouse line that allows identification of GlyT2-expressing, presumably glycinergic neurons in living cerebellar slices and compared their electrophysiological properties with previously described DCN neuron populations. We found two electrophysiologically and morphologically distinct sets of GlyT2-expressing neurons in the lateral cerebellar nucleus. One of them showed electrophysiological similarity to the previously characterized GABAergic cell group. The second GlyT2+ cell population, however, differed from all other so far described neuron types in DCN in that the cells (1) are intrinsically silent in slices and only fire action potentials upon depolarizing current injection and (2) have a projecting axon that was often seen to leave the DCN and project in the direction of the cerebellar cortex. Presence of this so far undescribed DCN neuron population in the lateral nucleus suggests a direct inhibitory pathway from the DCN to the cerebellar cortex.

Glycine transport inhibitors for the treatment of schizophrenia

Multiple lines of evidence indicate that hypofunction of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors might be implicated in the pathophysiology of schizophrenia, suggesting that increasing NMDA receptor function via pharmacological manipulation could provide a new strategy for the management of schizophrenia. Currently, the glycine modulatory sites on NMDA receptors present the most attractive therapeutic targets for the treatment of schizophrenia. One means of enhancing NMDA receptor neurotransmission is to increase the availability of the obligatory co-agonist glycine at modulatory sites on the NMDA receptors through the inhibition of glycine transporter-1 (GlyT-1) on glial cells. Clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methyl glycine) shows antipsychotic activity in patients with schizophrenia. Accordingly, a number of pharmaceutical companies have developed novel and selective GlyT-1 inhibitors for the treatment of schizophrenia. This paper provides an overview of the various GlyT-1 inhibitors and their therapeutic potential.

Transmitter-phenotypes of commissural interneurons in the lumbar spinal cord of newborn mice

Commissural interneurons (CINs) are a necessary component of central pattern generators (CPGs) for locomotion because they mediate the coordination of left and right muscle activity. The projection patterns and relative locations of different classes of CINs in the ventromedial part of the rodent lumbar cord have been described (Eide et al. [1999] J Comp Neurol 403:332-345; Stokke et al. [2002] J Comp Neurol 446:349-359; Nissen et al. [2005] J Comp Neurol 483:30-47). However, the distribution and relative prevalence of different CIN neurotransmitter phenotypes in the ventral region of the mammalian spinal cord where the locomotor CPG is localized is unknown. In this study we describe the relative proportions and anatomical locations of putative inhibitory and excitatory CINs in the lumbar spinal cord of newborn mice. To directly visualize potential neurotransmitter phenotypes we combined retrograde labeling of CINs with in situ hybridization against the glycine transporter, GlyT2, or the vesicular glutamate transporter, vGluT2, in wildtype mice and in transgenic mice expressing eGFP driven by the promoters of glutamic acid decarboxylase (GAD) 65, GAD67, or GlyT2. Our study shows that putative glycinergic, GABAergic, and glutamatergic CINs are expressed in almost equal numbers, with a small proportion of CINs coexpressing GlyT2 and GAD67::eGFP, indicating a putative combined glycinergic/GABAergic phenotype. These different CIN phenotypes were intermingled in laminas VII and VIII. Our results suggest that glycinergic, GABAergic, and glutamatergic CINs are the principal CIN phenotypes in the CPG region of the lumbar spinal cord in the newborn mouse. We compare these results to descriptions of CIN neurotransmitter phenotypes in other vertebrate species.

Glycinergic innervation of motoneurons is deficient in amyotrophic lateral sclerosis mice: a quantitative confocal analysis

Altered motoneuron excitability is involved in amyotrophic lateral sclerosis pathobiology. To test the hypothesis that inhibitory interneuron innervation of spinal motoneurons is abnormal in an amyotrophic lateral sclerosis mouse model, we measured GABAergic, glycinergic, and cholinergic immunoreactive terminals on spinal motoneurons in mice expressing a mutant form of human superoxide dismutase-1 with a Gly93-->Ala substitution (G93A-SOD1) and in controls at different ages. Glutamic acid decarboxylase, glycine transporter-2, and choline acetyltransferase were used as markers for GABAergic, glycinergic, and cholinergic terminals, respectively. Triple immunofluorescent labeling of boutons contacting motoneurons was visualized by confocal microscopy and analyzed quantitatively. Glycine transporter-2-bouton density on lateral motoneurons was decreased significantly in G93A-SOD1 mice compared with controls. This reduction was absent at 6 weeks of age but present in asymptomatic 8-week-old mice and worsened with disease progression from 12 to 14 weeks of age. Motoneurons lost most glycinergic innervation by 16 weeks of age (end-stage) when there was a significant decrease in the numbers of motoneurons and choline acetyltransferase-positive boutons. No significant differences in glutamic acid decarboxylase-bouton densities were found in G93A-SOD1 mice. Reduction of glycinergic innervation preceded mitochondrial swelling and vacuolization. Calbindin-positive Renshaw cell number was decreased significantly at 12 weeks of age in G93A-SOD1 mice. Thus, either the selective loss of inhibitory glycinergic regulation of motoneuron function or glycinergic interneuron degeneration contributes to motoneuron degeneration in amyotrophic lateral sclerosis.

Phenotype of V2-derived interneurons and their relationship to the axon guidance molecule EphA4 in the developing mouse spinal cord

The ventral spinal cord consists of interneuron groups arising from distinct, genetically defined, progenitor domains along the dorsoventral axis. Many of these interneuron groups settle in the ventral spinal cord which, in mammals, contains the central pattern generator for locomotion. In order to better understand the locomotor networks, we have used different transgenic mice for anatomical characterization of one of these interneuron groups, called V2 interneurons. Neurons in this group are either V2a interneurons marked by the postmitotic expression of the transcription factor Chx10, or V2b interneurons which express the transcription factors Gata2 and Gata3. We found that all V2a and most V2b interneurons were ipsilaterally projecting in embryos as well as in newborns. V2a interneurons were for the most part glutamatergic while V2b interneurons were mainly GABAergic or glycinergic. Furthermore, we demonstrated that a large proportion of V2 interneurons expressed the axon guidance molecule EphA4, a molecule previously shown to be important for correct organization of locomotor networks. We also showed that V2 interneurons and motor neurons alone did not account for all EphA4-expressing neurons in the spinal cord. Together, these findings enable a better interpretation of neural networks underlying locomotion, and open up the search for as yet unknown components of the mammalian central pattern generator.

The synaptic and nonsynaptic glycine transporter type-1 inhibitors Org-24461 and NFPS alter single neuron firing rate in the rat dorsal raphe nucleus. Further evidence for a glutamatergic-serotonergic interaction and its role in antipsychotic action

Single neuron firing rate was recorded from dorsal raphe nucleus of anesthetized rats. The firing rate of raphe neurons varied from 4 to 8 discharge per second before drug administration and this neuronal activity was decreased by L-701,324 (2 mg/kg i.v. injection), a competitive antagonist of glycineB binding site of N-methyl-D-aspartate (NMDA) receptors. The glycine transporter type-1 (GlyT1) antagonists Org-24461 (10 mg/kg i.v.) and NFPS (3 mg/kg i.v.) reversed the inhibitory effect of L-701,324 on single neuron activity recorded from dorsal raphe nucleus of the rat. Org-24461 and NFPS both tended to increase the raphe neuronal firing rate also when given alone but their effect was not significant. This finding serves further evidence that glutamate released from axon terminals of the cortico-striatal projection neurons stimulates serotonergic neurons in the raphe nuclei and this effect is mediated at least in part by postsynaptic NMDA receptors. Thus, GlyT1 inhibitors are able to reverse the hypofunctional state of NMDA receptors, suggesting that these drugs may have beneficial therapeutic effects in neurological and psychiatric disorders characterized with impaired NMDA receptor-mediated transmission.

A screen for neurotransmitter transporters expressed in the visual system of Drosophila melanogaster identifies three novel genes

The fly eye provides an attractive substrate for genetic studies, and critical transport activities for synaptic transmission and pigment biogenesis in the insect visual system remain unknown. We therefore screened for transporters in Drosophila melanogaster that are down-regulated by genetically ablating the eye. Using a large panel of transporter specific probes on Northern blots, we identified three transcripts that are down-regulated in flies lacking eye tissue. Two of these, CG13794 and CG13795, are part of a previously unknown subfamily of putative solute carriers within the neurotransmitter transporter family. The third, CG4476, is a member of a related subfamily that includes characterized nutrient transporters expressed in the insect gut. Using imprecise excision of a nearby transposable P element, we have generated a series of deletions in the CG4476 gene. In fast phototaxis assays, CG4476 mutants show a decreased behavioral response to light, and the most severe mutant behaves as if it were blind. These data suggest an unforeseen role for the "nutrient amino acid transporter" subfamily in the nervous system, and suggest new models to study transport function using the fly eye.

Direct correlation between ischemic injury and extracellular glycine concentration in mice with genetically altered activities of the glycine cleavage multienzyme system

BACKGROUND AND PURPOSE

Ischemia elicits the rapid release of various amino acid neurotransmitters. A glutamate surge activates N-methyl-d-aspartate (NMDA) glutamate receptors, triggering deleterious processes in neurons. Although glycine is a coagonist of the NMDA receptor, the effect of extracellular glycine concentration on ischemic injury remains controversial. To approach this issue, we examined ischemic injury in mice with genetically altered activities of the glycine cleavage multienzyme system (GCS), which plays a fundamental role in maintaining extracellular glycine concentration.

METHODS

A mouse line with increased GCS activity (340% of C57BL/6 control mice) was generated by transgenic expression of glycine decarboxylase, a key GCS component (high-GCS mice). Another mouse line with reduced GCS activity (29% of controls) was established by transgenic expression of a dominant-negative mutant of glycine decarboxylase (low-GCS mice). We examined neuronal injury after transient occlusion of the middle cerebral artery in these mice by measuring extracellular amino acid concentrations in microdialysates.

RESULTS

High-GCS and low-GCS mice had significantly lower and higher basal concentrations of extracellular glycine than did controls, respectively. In low-GCS mice, the extracellular glycine concentration reached 2-fold of control levels during ischemia, and infarct volume was significantly increased by 69% with respect to controls. In contrast, high-GCS mice had a significantly smaller infarct volume (by 21%). No significant difference was observed in extracellular glutamate concentrations throughout the experiments. An antagonist for the NMDA glycine site, SM-31900, attenuated infarct size, suggesting that glycine operated via the NMDA receptor.

CONCLUSIONS

There is a direct correlation between ischemic injury and extracellular glycine concentration maintained by the GCS.

Comprehensive mutation analysis ofGLDC,AMT, andGCSHin nonketotic hyperglycinemia

Nonketotic hyperglycinemia (NKH) is an inborn error of metabolism characterized by accumulation of glycine in body fluids and various neurological symptoms. NKH is caused by deficiency of the glycine cleavage multi-enzyme system with three specific components encoded by GLDC, AMT, and GCSH. We undertook the first comprehensive screening for GLDC, AMT, and GCSH mutations in 69 families (56, six, and seven families with neonatal, infantile, and late-onset type NKH, respectively). GLDC or AMT mutations were identified in 75% of neonatal and 83% of infantile families, but not in late-onset type NKH. No GCSH mutation was identified in this study. GLDC mutations were identified in 36 families, and AMT mutations were detected in 11 families. In 16 of the 36 families with GLDC mutations, mutations were identified in only one allele despite sequencing of the entire coding regions. The GLDC gene consists of 25 exons. Seven of the 32 GLDC missense mutations were clustered in exon 19, which encodes the cofactor-binding site Lys754. A large deletion involving exon 1 of the GLDC gene was found in Caucasian, Oriental, and black families. Multiple origins of the exon 1 deletion were suggested by haplotype analysis with four GLDC polymorphisms. This study provides a comprehensive picture of the genetic background of NKH as it is known to date.

The importance of glutamate, glycine, and γ-aminobutyric acid transport and regulation in manganese, mercury and lead neurotoxicity

Historically, amino acids were studied in the context of their importance in protein synthesis. In the 1950s, the focus of research shifted as amino acids were recognized as putative neurotransmitters. Today, many amino acids are considered important neurochemicals. Although many amino acids play a role in neurotransmission, glutamate (Glu), glycine (Gly), and gamma-aminobutyric acid (GABA) are among the more prevalent and better understood. Glu, the major excitatory neurotransmitter, and Gly and GABA, the major inhibitory neurotransmitters, in the central nervous system, are known to be tightly regulated. Prolonged exposure to environmental toxicants, such as manganese (Mn), mercury (Hg), or lead (Pb), however, can lead to dysregulation of these neurochemicals and subsequent neurotoxicity. While the ability of these metals to disrupt the regulation of Glu, Gly and GABA have been studied, few articles have examined the collective role of these amino acids in the respective metal's mechanism of toxicity. For each of the neurotransmitters above, we will provide a brief synopsis of their regulatory function, including the importance of transport and re-uptake in maintaining their optimal function. Additionally, the review will address the hypothesis that aberrant homeostasis of any of these amino acids, or a combination of the three, plays a role in the neurotoxicity of Mn, Hg, or Pb.

Glycinergic neurons expressing enhanced green fluorescent protein in bacterial artificial chromosome transgenic mice

Although glycine is a major inhibitory transmitter in the mammalian CNS, the role of glycinergic neurons in defined neuronal circuits remains ill defined. This is due in part to difficulties in identifying these cells in living slice preparations for electrophysiological recordings and visualizing their axonal projections. To facilitate the morphological and functional analysis of glycinergic neurons, we generated bacterial artificial chromosome (BAC) transgenic mice, which specifically express enhanced green fluorescent protein (EGFP) under the control of the promotor of the glycine transporter (GlyT) 2 gene, which is a reliable marker for glycinergic neurons. Neurons expressing GlyT2-EGFP were intensely fluorescent, and their dendrites and axons could be visualized in great detail. Numerous positive neurons were detected in the spinal cord, brainstem, and cerebellum. The hypothalamus, intralaminar nuclei of the thalamus, and basal forebrain also received a dense GlyT2-EGFP innervation, whereas in the olfactory bulb, striatum, neocortex, hippocampus, and amygdala positive fibers were much less abundant. No GlyT2-EGFP-positive cell bodies were seen in the forebrain. On the subcellular level, GlyT2-EGFP fluorescence was colocalized extensively with glycine immunoreactivity in somata and dendrites and with both glycine and GlyT2 immunoreactivity in axon terminals, as shown by triple staining at all levels of the neuraxis, confirming the selective expression of the transgene in glycinergic neurons. In slice preparations of the spinal cord, no difference between the functional properties of EGFP-positive and negative neurons could be detected, confirming the utility of visually identifying glycinergic neurons to investigate their functional role in electrophysiological studies.

Detection of amino acid and peptide transmitters in physiologically identified brainstem cardiorespiratory neurons

Most of the CNS neurons that regulate circulation and respiration reside in regions of the brain characterized by extreme cellular heterogeneity (nucleus of the solitary tract, reticular formation, parabrachial nuclei, periaqueductal gray matter, hypothalamus, etc.). The chemical neuroanatomy of these regions is correspondingly complex and teasing out specific circuits in their midst remains a problem that is usually very difficult if not impossible to solve by conventional tract-tracing methods, Fos methodology or electrophysiology in slices. In addition, identifying the type of amino acid or peptide transmitter used by electrophysiologically recorded neurons has been until recently an especially difficult task either for lack of a specific marker or because such markers (many peptides for example) are exported to synaptic terminals and thus undetectable in neuronal cell bodies. In this review, we describe a general purpose method that solves many of these problems. The approach combines juxtacellular labeling in vivo with the histological identification of mRNAs that provide definitive neurochemical phenotypic identification (e.g. vesicular glutamate transporter 1 or 2, glutamic acid decarboxylase). The results obtained with this method are discussed in the general context of amino acid transmission in brainstem cardiorespiratory pathways. The presence of markers of amino acid transmission in specific aminergic pre-sympathetic neurons is especially emphasized as is the extensive co-localization of markers of GABAergic and glycinergic transmission in the brainstem reticular formation.

An immunogold investigation of the distribution of GABA and glycine in nerve terminals on the somata of spherical bushy cells in the anteroventral cochlear nucleus of guinea pig

Spherical bushy neurons in the anteroventral cochlear nucleus receive glutamatergic primary terminals from the cochlear nerve and terminals of noncochlear (i.e. nonprimary) origin, many of which colocalize gamma-aminobutyric acid (GABA) and glycine. Here the relationship between GABA and glycine in these terminals has been investigated using postembedding immunogold labelling. A significant negative correlation was found between the density of terminal labelling for GABA and for glycine in four guinea pigs. Terminals could be divided into three categories, GABA-only, glycine-only, or colocalizing depending on whether they had a significantly higher labelling density for either amino acid than the primary terminals. The overall labelling density in all four animals was significantly greater for GABA in GABA-only terminals than colocalizing ones but similar for glycine in both. Within the terminals, the labelling density over synaptic vesicles, nonvesicular regions of cytoplasm and mitochondria was also investigated. No significant difference was detected in the labelling density of vesicles compared with nonvesicular regions for either amino acid. However, a significant difference was found between the overall labelling density over mitochondria and nonvesicular regions for both. There was also significantly more mitochondrial GABA labelling in GABA-only terminals compared to colocalizing terminals but mitochondrial glycine labelling was similar in glycine-only and colocalizing terminals. Thus the level of GABA is higher in single than in colocalizing terminals, particularly in the mitochondria, but similar for glycine in both. It is possible therefore that the presence of glycine affects the level of GABA in the nonprimary terminals but that the presence of GABA does not affect the level of glycine.

Inactivation of the glycine transporter 1 gene discloses vital role of glial glycine uptake in glycinergic inhibition

The glycine transporter subtype 1 (GlyT1) is widely expressed in astroglial cells throughout the mammalian central nervous system and has been implicated in the regulation of N-methyl-D-aspartate (NMDA) receptor activity. Newborn mice deficient in GlyT1 are anatomically normal but show severe motor and respiratory deficits and die during the first postnatal day. In brainstem slices from GlyT1-deficient mice, in vitro respiratory activity is strikingly reduced but normalized by the glycine receptor (GlyR) antagonist strychnine. Conversely, glycine or the GlyT1 inhibitor sarcosine suppress respiratory activity in slices from wild-type mice. Thus, during early postnatal life, GlyT1 is essential for regulating glycine concentrations at inhibitory GlyRs, and GlyT1 deletion generates symptoms found in human glycine encephalopathy.

Distribution of prospective glutamatergic, glycinergic, and GABAergic neurons in embryonic and larval zebrafish

Zebrafish are an excellent model for studies of the functional organization of neuronal circuits, but little is known regarding the transmitter phenotypes of the neurons in their nervous system. We examined the distribution in spinal cord and hindbrain of neurons expressing markers of transmitter phenotype, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD65/67) for GABAergic neurons. All three markers were expressed in a large domain in the dorsal two-thirds of spinal cord, with additional, more ventral expression domains for VGLUT2 and GAD/GABA. In the large dorsal domain, dual in situ staining showed that GLYT2-positive cells were intermingled with VGLUT2 cells, with no dual-stained neurons. Many of the neurons in the dorsal expression domain that were positive for GABA markers at embryonic stages were also positive for GLYT2, suggesting that the cells might use both GABA and glycine, at least early in their development. The intermingling of neurons expressing inhibitory and excitatory markers in spinal cord contrasted markedly with the organization in hindbrain, where neurons expressing a particular marker were clustered together to form stripes that were visible running from rostral to caudal in horizontal sections and from dorsomedial to ventrolateral in cross sections. Dual labeling showed that the stripes of neurons labeled with one transmitter marker alternated with stripes of cells labeled for the other transmitter phenotypes. The differences in the distribution of excitatory and inhibitory neurons in spinal cord versus hindbrain may be tied to differences in their patterns of development and functional organization.

Glycine tranporter-1 blockade potentiates NMDA-mediated responses in rat prefrontal cortical neurons in vitro and in vivo

The N-methyl-D-aspartate (NMDA) receptor (NMDA-R) has pivotal roles in neural development, learning, memory, and synaptic plasticity. Functional impairment of NMDA-R has been implicated in schizophrenia. NMDA-R activation requires glycine to act on the glycine-B (GlyB) site of the NMDA-R as an obligatory co-agonist with glutamate. Extracellular glycine near NMDA-R is regulated effectively by a glial glycine transporter (GlyT1). Using whole-cell voltage-clamp recordings in prefrontal cortex (PFC) slices, we have shown that exogenous GlyB site agonists glycine and D-serine, or a specific GlyT1 inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) in the presence of exogenous glycine (10 microM), potentiated synaptically evoked NMDA excitatory postsynaptic currents (EPSCs) in vitro. Furthermore, in urethan-anesthetized rats, microiontophoretic NMDA pulses excite single PFC neurons. When these responses were blocked by approximately 50% to approximately 90% on continuous iontophoretic application of the GlyB site, antagonist (+)HA-966, intravenous NFPS (5 mg/kg), or a GlyB site agonist D-serine (50 mg/kg iv) reversed this (+)HA-966 block. NFPS may elevate endogenous glycine levels sufficiently to displace (+)HA-966 from the GlyB sites of the NMDA-R, thus enabling reactivation of the NMDA-Rs by iontophoretic NMDA applications. D-Serine (50-100 mg/kg iv) or NFPS (1-2 mg/kg iv) alone also augmented NMDA-evoked excitatory responses. These data suggest that direct GlyB site stimulation by D-serine, or blockade of GLYT1 to elevate endogenous glycine to act on unsaturated GlyB sites on NMDA-Rs, potentiated NMDA-R-mediated firing responses in rat PFC. Hence, blockade of GlyT1 to elevate glycine near the NMDA-R may activate hypofunctional NMDA-R, which has been implicated to play a critical role in the pathophysiology of schizophrenia.

Why glycine transporters have different stoichiometries

In the brain, neurons and glial cells compete for the uptake of the fast neurotransmitters, glutamate, GABA and glycine, through specific transporters. The relative contributions of glia and neurons to the neurotransmitter uptake depend on the kinetic properties, thermodynamic coupling and density of transporters but also on the intracellular metabolization or sequestration of the neurotransmitter. In the case of glycine, which is both an inhibitory transmitter and a neuromodulator of the excitatory glutamatergic transmission as a co-agonist of N-methyl D-aspartate receptors, the glial (GlyT1b) and neuronal (GlyT2a) transporters differ at least in three aspects: (i) stoichiometries, (ii) reverse uptake capabilities and (iii) pre-steady-state kinetics. A 3 Na(+)/1 Cl(-)/gly stoichiometry was established for GlyT2a on the basis of a 2 charges/glycine flux ratio and changes in the reversal potential of the transporter current as a function of the extracellular glycine, Na(+) and Cl(-) concentrations. Therefore, the driving force available for glycine uphill transport in neurons is about two orders of magnitude larger than for glial cells. In addition, GlyT2a shows a severe limitation for reverse uptake, which suggests an essential role of GlyT2a in maintaining a high intracellular glycine pool, thus facilitating the refilling of synaptic vesicles by the low affinity, low specificity vesicular transporter VGAT/VIAAT. In contrast, the 2 Na(+)/1 Cl(-)/gly stoichiometry and bi-directional transport properties of GlyT1b are appropriate for the control of the extracellular glycine concentration in a submicromolar range that can modulate N-methyl D-aspartate receptors effectively. Finally, analysis of the pre-steady-state kinetics of GlyT1b and GlyT2a revealed that at the resting potential neuronal transporters are preferentially oriented outward, ready to bind glycine, which suggests a kinetic advantage in the uptake contest.

Structure and expression of the glycine cleavage system in rat central nervous system

The glycine cleavage system (GCS) is a mitochondrial multienzyme system consisting of four individual proteins, three specific components (P-, T-, and H-proteins) and one house-keeping enzyme, dihydrolipoamide dehydrogenase. Inherited deficiency of the GCS causes nonketotic hyperglycinemia (NKH), an inborn error of glycine metabolism. NKH is characterized by massive accumulation of glycine in serum and cerebrospinal fluids and severe neuronal dysfunction in neonates. To elucidate the neuropathogenesis of NKH, we cloned cDNAs encoding three specific components of the GCS and studied the gene expression in rat central nervous system. P-, T-, and H-protein cDNAs encoded 1024, 403, and 170 amino acids, respectively. In situ hybridization analysis revealed that P-protein mRNA was expressed mainly in glial-like cells, including Bergmann glias in the cerebellum, while T- and H-protein mRNAs were detected in both glial-like cells and neurons. T- and H-protein mRNAs, but not P-protein mRNA, were expressed in the spinal cord. Primary astrocyte cultures established from cerebral cortex had higher GCS activities than hepatocytes whereas those from spinal cord expressed only H-protein mRNA and had no enzymatic activity. An important role of glycine as inhibitory neurotransmitter has been established in the brainstem and spinal cord and another role of glycine as an excitation modulator of N-methyl-D-aspartate receptor is suggested in the hippocampus, cerebral cortex, olfactory bulbus, and cerebellum. Our results suggest that the GCS plays a major role in the forebrain and cerebellum rather than in the spinal cord, and that N-methyl-D-aspartate receptor may participate in neuropathogenesis of NKH.

Expression of glycine receptors in rat sensory neurons vs. HEK293 cells yields different functional properties

Many structure-function studies of the glycine receptor (GlyR), and other ligand-gated ion channels, use somatic cell lines or Xenopus oocytes as expression systems. Using a polyethylenimine-based technique, we transfected GlyR cDNA into primary cultures of rat dorsal root ganglion (DRG) neurons. We then compared the functional properties of wildtype and a mutant GlyR expressed in DRG neurons with HEK 293 cells. The glycine sensitivity of the wildtype GlyR was nearly identical for the two cell types. The mutant GlyR has an arginine for glutamine substitution at position 271 (R271Q), which results in low glycine sensitivity relative to wildtype receptors expressed in HEK cells. This point mutation is associated with startle disease (hyperekplexia) in humans. Mutant GlyR expression in DRG neurons resulted in a significantly lower glycine sensitivity than was seen in HEK cells. This supports the idea that neuron-specific post-translational modifications may be important for determining receptor function.

Discovery and SAR of org 24598-a selective glycine uptake inhibitor

The discovery of Org 24598, one of the first potent and selective inhibitors of the glycine transporter is discussed. In vitro structure-activity relationships (SARs) data for interaction of a ligand with this system is discussed.

Pharmacological assessment of the role of the glycine transporter GlyT-1 in mediating high-affinity glycine uptake by rat cerebral cortex and cerebellum synaptosomes

Two distinct types of glycine transporter, GlyT-1 and GlyT-2, have been characterised. GlyT-1 and GlyT-2 are known to be differentially expressed amongst CNS areas, but direct functional evidence for their relative contributions to high-affinity glycine uptake by brain tissues is lacking. In the present study, we have used the selective GlyT-1 inhibitor N[3-(4"-fluorophenyl)-3-(4"-phenylphenoxy)propyl]sarcosine (NFPS) to investigate the role of GlyT-1 in mediating glycine uptake. HEK293 cells expressing human GlyT-1c or GlyT-2 showed high levels of Na(+)-dependent glycine uptake, with K(m) values of 117+/-13 and 200+/-22 microM, respectively. NFPS potently inhibited uptake in GlyT-1c cells (IC(50) value 0.22+/-0.03 microM), being around 500-fold more potent than glycine or sarcosine, but had no effect on uptake in GlyT-2 cells (IC(50) >10 microM). Efflux of pre-loaded [3H]-glycine from GlyT-1c cells was increased by glycine or sarcosine, whereas NFPS had no effect on its own but blocked the effects of glycine or sarcosine. These results confirm that NFPS is a potent, selective and non-transportable GlyT-1 inhibitor. Rat cortex and cerebellum synaptosomes also showed a high-affinity Na(+)-dependent component of glycine uptake, with affinities similar to those observed for uptake in GlyT-1c or GlyT-2 cells. In cortex synaptosomes, NFPS and sarcosine produced the same maximal inhibition of uptake as glycine itself. However, in cerebellum synaptosomes, the maximal inhibition produced by NFPS and sarcosine was only half that produced by glycine. In both tissues NFPS was around 1000-fold more potent than glycine or sarcosine. Overall, our findings indicate that high-affinity glycine uptake in cerebral cortex occurs predominantly via GlyT-1. However, in cerebellum, only a part of the high-affinity uptake is mediated by GlyT-1, with the remaining NFPS-insensitive component most likely mediated by GlyT-2.

Neurokinin-1 receptor-immunoreactive neurons of the ventral respiratory group in the rat

The rostral end of the ventral respiratory group (VRG) contains neurons that are intensely neurokinin-1 receptor (NK1R) immunoreactive (ir). It has been theorized that some of these cells might be critical to respiratory rhythmogenesis (Gray et al. [1999] Science 286:1566-1568). In the present study we determined what major transmitter these NK1R-ir cells make and whether they are bulbospinal or propriomedullary. NK1R-ir neurons were found in the VRG between Bregma levels -11.7 and -13.6 mm. The highest concentration was found between Bregma -12.3 and -13.0 mm. This region overlaps with the pre-Bötzinger complex (pre-BötC) as it was found to contain many pre-inspiratory neurons, few E2-expiratory neurons, and no I-incremental neurons. VRG NK1R-ir neurons contain neither tyrosine hydroxylase (TH) nor choline acetyl-transferase (ChAT) immunoreactivity, although dual-labeled neurons were found elsewhere within the rostral medulla. GAD67 mRNA was commonly detected in the ventrolateral medulla (VLM) but rarely in the NK1R-ir neurons of the pre-BötC region (6 % of somatic profiles). GlyT2 mRNA was commonly found in the pre-BötC region but rarely within NK1R-ir neurons (1.3 %). Up to 40% of VRG NK1R-ir neurons were retrogradely labeled by Fluoro-Gold (FG) injected in the contralateral pre-BötC region. Some NK1R-ir VRG neurons located caudal to Bregma -12.6 mm were retrogradely labeled by FG injected in the spinal cord (C4-C5, T2-T4). In sum, NK1R immunoreactivity is present in many types of ventral medullary neurons. Within the VRG proper, NK1R-ir neurons are concentrated in an area that overlaps with the pre-BötC. Within this limited region of the VRG, NK1R-ir neurons are neither cholinergic nor catecholaminergic, and very few are gamma-aminobutyric acid (GABA)ergic or glycinergic. The data suggest that most NK1R-ir neurons of the pre-BötC region are excitatory. Furthermore, the more rostral NK1R-ir cells are propriomedullary, whereas some of the caudal ones project to the spinal cord.