Structure, function and regulation of glycine neurotransporters

The physiological roles of vesicular GABA transporter during embryonic development: a study using knockout mice

Background

The vesicular GABA transporter (VGAT) loads GABA and glycine from the neuronal cytoplasm into synaptic vesicles. To address functional importance of VGAT during embryonic development, we generated global VGAT knockout mice and analyzed them.

Results

VGAT knockouts at embryonic day (E) 18.5 exhibited substantial increases in overall GABA and glycine, but not glutamate, contents in the forebrain. Electrophysiological recordings from E17.5-18.5 spinal cord motoneurons demonstrated that VGAT knockouts presented no spontaneous inhibitory postsynaptic currents mediated by GABA and glycine. Histological examination of E18.5 knockout fetuses revealed reductions in the trapezius muscle, hepatic congestion and little alveolar spaces in the lung, indicating that the development of skeletal muscle, liver and lung in these mice was severely affected.

Conclusion

VGAT is fundamental for the GABA- and/or glycine-mediated transmission that supports embryonic development. VGAT knockout mice will be useful for further investigating the roles of VGAT in normal physiology and pathophysiologic processes.

The effects of the glycine reuptake inhibitor R213129 on the central nervous system and on scopolamine-induced impairments in psychomotor and cognitive function in healthy subjects

In this study the effects of R213129, a selective glycine transporter 1 inhibitor, on central nervous system function were investigated in healthy males in the absence and presence of scopolamine. This was a double-blind, placebo-controlled, 4-period crossover ascending dose study evaluating the following endpoints: body sway, saccadic and smooth pursuit eye movements, pupillometry, electroencephalography, visual analogue scales for alertness, mood, calmness and psychedelic effects, adaptive tracking, finger tapping, Visual and Verbal Learning Task, Stroop test, hormone levels and pharmacokinetics. R213129 dose levels were selected based on exposure levels that blocked the GlyT1 sites >50% in preclinical experiments. Forty-three of the 45 included subjects completed the study. Scopolamine significantly affected almost every central nervous system parameter measured in this study. R213129 alone compared with placebo did not elicit pharmacodynamic changes. R213129 had some small effects on scopolamine-induced central nervous system impairments. Scopolamine-induced finger tapping impairment was further enhanced by 3 mg R213129 with 2.0 taps/10 seconds (95% CI -4.0, -0.1), electroencephalography alpha power was increased by 10 mg R213129 with respectively 12.9% (0.7, 26.6%), scopolamine-induced impairment of the Stroop test was partly reversed by 10 mg R213129 with 59 milliseconds (-110, -7). Scopolamine produced robust and consistent effects in psychomotor and cognitive function in healthy volunteers. The most logical reason for the lack of R213129 effects seems to be that the central nervous system concentrations were too low. The effects of higher doses in healthy volunteers and the clinical efficacy in patients remain to be established.

Effects of sarcosine, a glycine transporter type 1 inhibitor, in two mouse seizure models

Sarcosine, a natural amino acid found in muscles and other body tissues, is an endogenous glycine transporter type 1 inhibitor that increases the glycine concentration, resulting in an indirect potentiation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Sarcosine, similar to other NMDA receptor-activating agents, is an effective adjuvant in the treatment of schizophrenia. It is widely accepted that increased glutamatergic neurotransmission is involved in the initiation and propagation of seizures. Because sarcosine facilitates NMDA receptor function, it may affect the seizure threshold. Therefore, we examined the effects of sarcosine on the seizure threshold in two different mouse seizure models: the timed intravenous (iv) pentylenetetrazole (PTZ) infusion test and the maximal electroshock seizure threshold test. In the iv PTZ test, sarcosine did not exert a significant effect on the seizure threshold at any of the doses tested (100, 200, 400 and 800 mg/kg, ip). However, at doses of 400 and 800 mg/kg, sarcosine significantly raised the threshold for electroconvulsions (p < 0.01). The present findings indicate that sarcosine did not lower the seizure threshold. Conversely, sarcosine showed weak anticonvulsant properties by increasing the threshold current for the induction of tonic seizures. Therefore, sarcosine may be considered as a safe adjuvant treatment for schizophrenia without proconvulsant risk. In addition, the compound may serve as an interesting addition to epilepsy treatment.

Adenoviral-mediated Cre expression effectively suppresses GlyT1 binding in the thalamic area of GlyT1 conditional knock-out mice

To properly understand the function of genes of neurological interest, in vivo manipulation in the adult is essential, particularly when the target gene is involved in brain development. Moreover, since the physiological effects of target protein may be region-specific, targeting a distinct brain region could be required to dissect these effects in specific brain locations. Infection of somatic tissues of transgenic mice bearing loxP-flanked gene sequences with a viral vector expressing Cre recombinase provides a means of allowing flexible spatio-temporal control of target gene expression. Viral vector-mediated Cre expression could be used to mediate localized gene modulation in a specific brain region. In the present study this technology was applied to the glycine transporter type-1 (GlyT1) protein which is responsible for the uptake of synaptic glycine in the forebrain and has been implicated as a therapeutic target for the treatment of schizophrenia. Since GlyT1 is widely expressed in glial cells, we employed an adenoviral-based vector (Ad5) to deliver Cre protein, due to the preferentially transduction of glial cells by adenoviral vectors in rodent brain. We show significant reduced GlyT1 binding specifically in the thalamic area of conditional GlyT1 (GlyT1c) transgenic mice injected with Ad5-Cre virus, as measured by GlyT1 autoradiography. In conclusion, we demonstrated the validity of viral vector-mediated delivery of Cre to loxP targeted transgenic mice as a novel strategy to investigate target gene function in selected subregions of the adult brain, which provides a valuable technique to investigate gene function both in normal physiology and in disease models.

Alterations in brain extracellular dopamine and glycine levels following combined administration of the glycine transporter type-1 inhibitor Org-24461 and risperidone

The most dominant hypotheses for the pathogenesis of schizophrenia have focused primarily upon hyperfunctional dopaminergic and hypofunctional glutamatergic neurotransmission in the central nervous system. The therapeutic efficacy of all atypical antipsychotics is explained in part by antagonism of the dopaminergic neurotransmission, mainly by blockade of D(2) dopamine receptors. N-methyl-D-aspartate (NMDA) receptor hypofunction in schizophrenia can be reversed by glycine transporter type-1 (GlyT-1) inhibitors, which regulate glycine concentrations at the vicinity of NMDA receptors. Combined drug administration with D(2) dopamine receptor blockade and activation of hypofunctional NMDA receptors may be needed for a more effective treatment of positive and negative symptoms and the accompanied cognitive deficit in schizophrenia. To investigate this type of combined drug administration, rats were treated with the atypical antipsychotic risperidone together with the GlyT-1 inhibitor Org-24461. Brain microdialysis was applied in the striatum of conscious rats and determinations of extracellular dopamine, DOPAC, HVA, glycine, glutamate, and serine concentrations were carried out using HPLC/electrochemistry. Risperidone increased extracellular concentrations of dopamine but failed to influence those of glycine or glutamate measured in microdialysis samples. Org-24461 injection reduced extracellular dopamine concentrations and elevated extracellular glycine levels but the concentrations of serine and glutamate were not changed. When risperidone and Org-24461 were added in combination, a decrease in extracellular dopamine concentrations was accompanied with sustained elevation of extracellular glycine levels. Interestingly, the extracellular concentrations of glutamate were also enhanced. Our data indicate that coadministration of an antipsychotic with a GlyT-1 inhibitor may normalize hypofunctional NMDA receptor-mediated glutamatergic neurotransmission with reduced dopaminergic side effects characteristic for antipsychotic medication.

The neuronal glycine transporter GLYT2 associates with membrane rafts: functional modulation by lipid environment

The neuronal glycine transporter GLYT2 is a plasma membrane protein that removes the neurotransmitter glycine from the synaptic cleft, thereby aiding the pre-synaptic terminal reloading and the termination of the glycinergic signal. Missense mutations in the gene encoding GLYT2 (SLC6A5) cause hyperekplexia in humans. The activity of GLYT2 seems to be highly regulated. In this report, we demonstrate that GLYT2 is associated with membrane rafts in the plasma membrane of brainstem terminals and neurons. The transporter is localized to Triton X-100-insoluble light synaptosomal membranes together with flotillin-1, a marker protein for membrane rafts, in a methyl-beta-cyclodextrin (MbetaCD)-sensitive manner. In brainstem primary neurons, the GLYT2 punctuate pattern visualized by confocal microscopy was modified by cholesterol depletion with MbetaCD, unlike other non-raft neuronal markers. GLYT2-associated gold particles were observed by electron microscopy on purified rafts from brainstem synaptosomes. Furthermore, either in brainstem terminals and cultured neurons, the pharmacological reduction of the levels of raft components, cholesterol and sphingomyelin, impairs both the association of GLYT2 with membrane rafts and its transport activity. Thus, GLYT2 may require membrane raft location for optimal function, and therefore the lipid environment may constitute a new mechanism to modulate GLYT2.

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: implications for co-transmission

Immunoreactive structures visualised with antibodies to glycine were prominent in areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius, but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher density, more homogenous distribution of structures labelled for gamma-aminobutyric acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated a close correspondence between cells and puncta labelled by glycine and GABA antisera in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic terminals with high, presumed transmitter levels of glycine were discriminated from terminals containing low, metabolic levels by quantitative analysis of gold particle labelling densities. In a random sample of terminals, 28.5% qualified on this basis as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses. Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone, with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity. No evidence for co-localisation of glycine and glutamate was found. Light, confocal and electron microscopic labelling with antibodies to proteins specific for glycine and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing GABA are found predominantly in more lateral areas of NTS, despite glycine receptors and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of the nucleus. The data suggest that synaptic terminals in certain functionally distinct areas of NTS co-release both inhibitory amino acids, which may account for the previously reported differential inhibitory effects of glycine and GABA on NTS neurones.

Blockade of glycine transporter (GlyT) 2, but not GlyT1, ameliorates dynamic and static mechanical allodynia in mice with herpetic or postherpetic pain

Glycine is an inhibitory neurotransmitter in the spinal dorsal horn and its extracellular concentration is regulated by glial glycine transporter (GlyT) 1 and neuronal GlyT2. This study was conducted to elucidate the effects of intrathecal injections of GlyT1 and GlyT2 inhibitors on two distinct types of mechanical allodynia, dynamic and static allodynia, in mice with herpetic or postherpetic pain. The GlyT2 inhibitor ALX1393, but not the GlyT1 inhibitor sarcosine, suppressed dynamic and static allodynia at the herpetic and postherpetic stages. Intrathecal ALX1393 suppressed dynamic allodynia induced by intrathecal strychnine and N-methyl-D-aspartate (NMDA). Intrathecal sarcosine suppressed dynamic allodynia induced by intrathecal strychnine, but not NMDA. Expression level of GlyT1, but not GlyT2, mRNA in the lumbar dorsal horn was decreased at the herpetic and postherpetic stages. Glycine receptor alpha1-subunit mRNA was decreased in the lumbar dorsal horn at the herpetic, but not postherpetic stage, without alteration in alpha3-subunit mRNA. The results suggest that GlyT2 is a potential target for treatment of dynamic and static allodynia in patients with herpes zoster and postherpetic neuralgia. The lack of efficacy of GlyT1 inhibitor may be explained by activation of NMDA receptors and the down-regulation of GlyT1 in the lumbar dorsal horn.

Transient currents in the glycine cotransporter GlyT1 reveal different steps in transport mechanism

The relation between presteady-state (transient) currents elicited by voltage steps in the absence of organic substrate and transport-associated currents in the presence of glycine was investigated in Xenopus oocytes expressing the neuronal glycine transporter GlyT1b. Saturating amounts of glycine converted the transient currents in steady transport currents. Analysis of the transient currents abolished by the substrate confirmed the intramembrane nature of the underlying charge movement process. The sigmoidal Q/V relationship had a moderate slope consistent with the known GlyT1b stoichiometry. The transient currents were best fitted by the sum of two exponentials, with the slow time constant (tau (slow)) being in the order of tens of milliseconds. The apparent affinity for glycine was in the micromolar range and voltage-dependent, slightly decreasing at positive potentials. Numerical simulations show that a simplified, three-state model is sufficient to explain the main features of GlyT1b operation.

Glycine and glycine receptor signalling in non-neuronal cells

Glycine is an inhibitory neurotransmitter acting mainly in the caudal part of the central nervous system. Besides this neurotransmitter function, glycine has cytoprotective and modulatory effects in different non-neuronal cell types. Modulatory effects were mainly described in immune cells, endothelial cells and macroglial cells, where glycine modulates proliferation, differentiation, migration and cytokine production. Activation of glycine receptors (GlyRs) causes membrane potential changes that in turn modulate calcium flux and downstream effects in these cells. Cytoprotective effects were mainly described in renal cells, hepatocytes and endothelial cells, where glycine protects cells from ischemic cell death. In these cell types, glycine has been suggested to stabilize porous defects that develop in the plasma membranes of ischemic cells, leading to leakage of macromolecules and subsequent cell death. Although there is some evidence linking these effects to the activation of GlyRs, they seem to operate in an entirely different mode from classical neuronal subtypes.

Interactions between the glycine transporter 1(GlyT1) inhibitor SSR504734 and psychoactive drugs in mouse motor behaviour

The specific glycine transporter 1 (GlyT1) inhibitor, SSR504734, is highly effective in enhancing N-methyl-D-aspartate receptor (NMDAR) function by elevating the availability of the NMDAR co-agonist, glycine, in the vicinity of NMDAR-containing glutamatergic synapses. According to the glutamatergic hypofunction hypothesis of schizophrenia, SSR504734 may therefore possess antipsychotic potential. Here, we evaluated the effects of SSR504734 in response to three psychomimetic drugs: phencyclidine, amphetamine, and apomorphine in male C57BL/6 mice. SSR504734 attenuated phencyclidine-induced (5 mg/kg, i.p.) hyperlocomotion, but potentiated the motor stimulant and motor depressant effects of amphetamine (2.5 mg/kg, i.p.) and apomorphine (0.75 mg/kg, s.c.), respectively. Hence, SSR504734 not only confers resistance to NMDAR blockade, but also enhances the locomotor response to dopaminergic stimulation. The latter finding adds to reports that SSR504734 may modulate dopamine-mediated behaviour by interference with normal glutamate-dopamine interaction. The specificity of this action of SSR504734 will be highly relevant to its potential application as an antipsychotic agent.

Subcellular localization of the neuronal glycine transporter GLYT2 in brainstem

The neuronal glycine transporter GLYT2 belongs to the neurotransmitter:sodium:symporter (NSS) family and removes glycine from the synaptic cleft, thereby aiding the termination of the glycinergic signal and achieving the reloading of the presynaptic terminal. The task fulfilled by this transporter is fine tuned by regulating both transport activity and intracellular trafficking. Different stimuli such as neuronal activity or protein kinase C (PKC) activation can control GLYT2 surface levels although the intracellular compartments where GLYT2 resides are largely unknown. Here, by biochemical and immunological techniques in combination with electron and confocal microscopy, we have investigated the subcellular distribution of GLYT2 in rat brainstem tissue, and characterized the vesicles that contain the transporter. GLYT2 is shown to be present in small and larger vesicles that contain the synaptic vesicle protein synaptophysin, the recycling endosome small GTPase Rab11, and in the larger vesicle population, the vesicular inhibitory amino acid transporter VIAAT. Rab5A, the GABA transporter GAT1, synaptotagmin2 and synaptobrevin2 (VAMP2) were not present. Coexpression of a Rab11 dominant negative mutant with recombinant GLYT2 impaired transporter trafficking and glycine transport. Dual immunogold labeling of brainstem synaptosomes showed a very close proximity of GLYT2 and Rab11. Therefore, the intracellular GLYT2 resides in a subset of endosomal membranes and may traffic around several compartments, mainly Rab11-positive endosomes.

Trafficking properties and activity regulation of the neuronal glycine transporter GLYT2 by protein kinase C

The neuronal glycine transporter GLYT2 controls the availability of the neurotransmitter in glycinergic synapses, and the modulation of its function may influence synaptic transmission. The active transporter is located in membrane rafts and reaches the cell surface through intracellular trafficking. In the present study we prove that GLYT2 constitutively recycles between the cell interior and the plasma membrane by means of a monensin-sensitive trafficking pathway. Also, a regulated trafficking can be triggered by PMA. We demonstrate that PMA inhibits GLYT2 transport by causing net accumulation of the protein in internal compartments through an increase of the internalization rate. In addition, a small increase of plasma membrane delivery and a redistribution of the transporter to non-raft domains is triggered by PMA. A previously identified phorbol-ester-resistant mutant (K422E) displaying an acidic substitution in a regulatory site, exhibits constitutive traffic but, in contrast with the wild-type, fails to show glycine uptake inhibition, membrane raft redistribution and trafficking modulation by PMA. We prove that the action of PMA on GLYT2 involves PKC (protein kinase C)-dependent and -independent pathways, although an important fraction of the effects are PKC-mediated. We show the additional participation of signalling pathways triggered by the small GTPase Rac1 on PMA action. GLYT2 inhibition by PMA and monensin also take place in brainstem primary neurons and synaptosomes, pointing to a GLYT2 trafficking regulation in the central nervous system.

Spinal antiallodynia action of glycine transporter inhibitors in neuropathic pain models in mice

Neuropathic pain is refractory against conventional analgesics, and thus novel medicaments are desired for the treatment. Glycinergic neurons are localized in specific brain regions, including the spinal cord, where they play an important role in the regulation of pain signal transduction. Glycine transporter (GlyT)1, present in glial cells, and GlyT2, located in neurons, play roles in modulating glycinergic neurotransmission by clearing synaptically released glycine or supplying glycine to the neurons and thus could modify pain signal transmission in the spinal cord. In this study, we demonstrated that i.v. or intrathecal administration of GlyT1 inhibitors, cis-N-methyl-N-(6-methoxy-1-phenyl-1,2,3,4-tetrahydronaphthalen-2-yl methyl)amino methylcarboxylic acid (ORG25935) or sarcosine, and GlyT2 inhibitors, 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)-methyl]benzamide (ORG25543) and (O-[(2-benzyloxyphenyl-3-fluorophenyl)methyl]-L-serine) (ALX1393), or knockdown of spinal GlyTs by small interfering RNA of GlyTs mRNA produced a profound antiallodynia effect in a partial peripheral nerve ligation model and other neuropathic pain models in mice. The antiallodynia effect is mediated through spinal glycine receptor alpha3. These results established GlyTs as the target molecules for the development of medicaments for neuropathic pain. However, these manipulations to stimulate glycinergic neuronal activity were without effect during the 4 days after nerve injury, whereas manipulations to inhibit glycinergic neuronal activity protected against the development of allodynia in this phase. The results implied that the timing of medication with their inhibitors should be considered, because glycinergic control of pain was reversed in the critical period of 3 to 4 days after surgery. This may also provide important information for understanding the underlying molecular mechanisms of the development of neuropathic pain.

The Neurotransmitter Cycle and Quantal Size

Changes in the response to release of a single synaptic vesicle have generally been attributed to postsynaptic modification of receptor sensitivity, but considerable evidence now demonstrates that alterations in vesicle filling also contribute to changes in quantal size. Receptors are not saturated at many synapses, and changes in the amount of transmitter per vesicle contribute to the physiological regulation of release. On the other hand, the presynaptic factors that determine quantal size remain poorly understood. Aside from regulation of the fusion pore, these mechanisms fall into two general categories: those that affect the accumulation of transmitter inside a vesicle and those that affect vesicle size. This review will summarize current understanding of the neurotransmitter cycle and indicate basic, unanswered questions about the presynaptic regulation of quantal size.

Real-time measurement of free Ca2+ changes in CNS myelin by two-photon microscopy

Here we describe a technique for measuring changes in Ca2+ in the cytosolic domain of mature compact myelin of live axons in the central nervous system (CNS). We label the myelin sheath of optic nerve and dorsal column axons by using the Ca2+ indicator X-rhod-1 coupled with DiOC6(3) to produce bright myelin counterstaining, thereby providing unambiguous identification of the myelin sheath for analysis of two-photon excited fluorescence. We present evidence for localization of the Ca2+ reporter to the cytosolic domain of myelin, obtained by using fluorescence lifetime, spectral measurements and Mn2+ quenching. Chemical ischemia increased myelinic X-rhod-1 fluorescence (approximately 50% after 30 min) in a manner dependent on extracellular Ca2+. Inhibiting Na+-dependent glutamate transporters (with TBOA) or glycine transporters (with sarcosine and ALX-1393) reduced the ischemia-induced increase in Ca2+. We show that myelinic N-methyl-D-aspartate (NMDA) receptors are activated by the two conventional coagonists glutamate and glycine, which are released by specific transporters under conditions of cellular Na+ loading and depolarization in injured white matter. This new technique facilitates detailed studies of living myelin, a vital component of the mammalian CNS.

Glycine-induced long-term synaptic potentiation is mediated by the glycine transporter GLYT1

The negative symptoms of schizophrenia are reverted by treatment with glycine or other agonists of the glycine-B site which facilitate NMDA receptor function. On the other hand, there are experimental observations showing that exogenous application of glycine (0.5-10mM) results in a long-lasting potentiation of glutamatergic synaptic transmission (LTP-GLY). The characterization of the mechanisms underlying LTP-GLY could be useful to develop new therapies for schizophrenia. The main goal of this work is to deepen the understanding of this potentiation phenomenon. The present study demonstrates in rat hippocampal slices that superfusion of glycine 1mM during 30 min produces a potentiation of excitatory postsynaptic potentials in CA3-CA1 pathway lasting at least 1h. Glycine application does not modify neither presynaptic fiber volley nor paired-pulse facilitation of synaptic potentials. This LTP-GLY is independent of both strychnine-sensitive glycine receptors and nifedipine-sensitive calcium channels. Interestingly, LTP-GLY is not inhibited but strengthened by NMDA receptors antagonists such as AP-5 or MK-801. In contrast, LTP-GLY is partially or totally blocked with the antagonists of glycine transporter GLYT1, sarcosine or ALX-5407, respectively. These results indicate that LTP-GLY requires the activation of GLYT1, a glycine transporter co-localized and associated to NMDA receptors. In addition, the fact that NMDA receptor inhibition increases LTP-GLY magnitude, opens the possibility that these receptors could have a negative control on GLYT1 activity.

High-affinity glycine and glutamate transport in pig forebrain white and gray matter: A quantitative study

High-affinity uptake of glycine and glutamate modulates glutamatergic neurotransmission in gray matter. N-Methyl-D-aspartate (NMDA) receptors were recently described on white matter oligodendrocytes, therefore uptake of glutamate and glycine in white matter may also modulate NMDA receptor function. We found that glycine uptake in white structures of pig forebrain (corpus callosum, fimbria, subcortical pyramidal tracts, and occipital subcortical white matter) was similar to that in gray structures (frontal and temporal cortices and hippocampus), and that it was sensitive to sarcosine, a GLYT1 inhibitor (IC(50) 15 microM). Glutamate uptake in white matter was approximately 10% of that in gray; it was sensitive to dihydrokainate, an EAAT2 inhibitor. The levels of glycine and its precursor serine were similar in white and gray matter: approximately 2 and 1 nmol/mg tissue, respectively. The white matter level of glutamate was approximately 7.6 nmol/mg tissue, or approximately 74% of gray matter levels. The activity of serine hydroxymethyl transferase, which converts serine into glycine, was similar in white and gray matter (11-18 pmol/(mg tissue)min), whereas the white matter activity of phosphate-activated glutaminase, which converts glutamine into glutamate, was approximately 100 pmol/(mg tissue)min, or approximately 34% of gray matter activity. The white matter activity of glutamine synthetase, the glial enzyme that converts glutamate into glutamine, was 20-40 nmol/(mg tissue)min in neocortex and 5-6 nmol/(mg tissue)min in white matter. The data show that forebrain white matter is equipped to regulate extracellular levels of glycine and glutamate, functions that may modulate white matter NMDA receptor function.

A novel, non-substrate-based series of glycine type 1 transporter inhibitors derived from high-throughput screening

The synthesis and structure-activity relationships (SAR) of a series of indane and tetralin inhibitors of the type 1 glycine transporter, derived from a high-throughput screening (HTS) hit, are described. Key modifications that reduced the 5HT1B receptor affinity of the HTS hit and the P450 2D6 inhibition of subsequent analogues are delineated. While these modifications led to potent and selective GlyT1 inhibitors, HERG affinity and human microsomal clearance remain an issue for this series of compounds.

Absence of GABA type A signaling in adult medial habenular neurons

Neural inhibition in the brain is mainly mediated by ionotropic GABA type A receptors. Apart from the GABA type A receptors, both K(+)-Cl(-) cotransporter isoform 2 and the GABA-synthesizing enzyme, glutamic acid decarboxylase, are essential determinants for GABA type A receptor-mediated inhibition. By using immunofluorescent staining, we observed that K(+)-Cl(-) cotransporter isoform 2, GABA type A receptor beta2/3 subunits and a presynaptically localized glutamic acid decarboxylase isoform, glutamic acid decarboxylase 65, were all absent in adult Sprague-Dawley rat medial habenular nucleus, while immunopositive staining for glutamic acid decarboxylase 67, GABA and GABA type B receptor type 2 subunit were present in the medial habenular nucleus. Consistent with the lack of GABA type A signaling as detected by immunohistochemistry, GABA (100 muM) evoked no measurable currents in the medial habenular nucleus but induced bicuculline-sensitive currents in the lateral habenular nucleus and in the CA1 area of hippocampus. We also failed to record miniature inhibitory postsynaptic currents in medial habenular nucleus neurons. These results support the idea that GABAergic transmission in medial habenular nucleus is probably not mediated by any of the most common GABA type A receptor subtypes. Our data suggest that GABA type B receptor-mediated inhibition may play a role in balancing neuronal excitation in this special region. Further exploration for factors determining medial habenular nucleus neural inhibition will lead to a more complete understanding of control of synaptic balance in the CNS.

The synthesis and SAR of 2-arylsulfanylphenyl-1-oxyalkylamino acids as GlyT-1 inhibitors

Elevation of glycine levels by inhibition of the glycine transporter-1 (GlyT-1) and activation of the NMDA receptor is a potential strategy for the treatment of schizophrenia. A novel series of 2-arylsulfanylphenyl-1-oxyalkyl amino acids have been identified. The most prominent member of this series S-1-{2-[3-(3-fluoro-phenylsulfanyl)biphenyl-4-yloxy]ethyl}pyrrolidine-2-carboxylic acid (38) is a potent GlyT-1 inhibitor (IC50=59 nM). In vitro and in vivo assessment of CNS exposure indicates this compound is a likely substrate for active efflux transporters.

Molecular identification and characterisation of the glycine transporter (GLYT1) and the glutamine/glutamate transporter (ASCT2) in the rat lens

Glutathione (GSH) is an essential antioxidant required for the maintenance of lens transparency. In the lens, GSH is maintained at unusually high concentrations as a result of direct GSH uptake and/or intracellular de novo synthesis from its precursor amino acids; cysteine, glycine and glutamine/glutamate. With increasing age, the levels of GSH, particularly in the core of the lens, are significantly reduced. It has been proposed that alterations in the transport of GSH and/or its precursor amino acids may contribute to the changes in GSH levels in older lenses. As considerable uncertainty exists about the molecular identity of GSH transporters in the lens, we have focused on identifying transporters involved in the uptake of the precursor amino acids required for GSH synthesis. Previously, we identified an uptake system for cyst(e)ine mediated by the Xc(-) exchanger and the Excitatory Amino Acid Transporters (EAATs) in the rat lens. In this current study, we have identified and localised additional uptake systems that contribute to GSH synthesis. Transcripts for GLYT1 (glycine transporter) and ASCT2 (glutamine/glutamate transporter) were detected in rat lens fiber cells using the reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis confirmed the expression of both GLYT1 and ASCT2 at the protein level. Immunocytochemistry revealed GLYT1 expression to be restricted to cortical regions of the lens. Labelling was predominantly cytoplasmic with some labelling of the membrane. In contrast, ASCT2 was expressed throughout the lens extending from the outer cortex through to the core. In the outer cortex, ASCT2 expression was predominantly cytoplasmic. However, with deeper distance into the lens, labelling became more membraneous indicating insertion of ASCT2 into the membranes of mature fiber cells of the lens core. The molecular identification and localisation of GLYT1 and ASCT2 in the lens suggests that these transporters may be responsible for the uptake of the precursor amino acids, glycine and glutamine, which are involved in GSH synthesis. Moreover, the presence of ASCT2 in the centre of the lens raises the possibility that ASCT2 may work with the Xc(-) exchanger to accumulate cysteine where it can potentially act as a low molecular mass antioxidant.

Distinct effects of D-serine on spinal nociceptive responses in normal and carrageenan-injected rats

Single unit extracellular recordings from dorsal horn neurons were performed with glass micropipettes in pentobarbital-anesthetized rats. A total of 60 wide dynamic range (WDR) neurons were obtained from 34 rats. In normal rats (20/34), spinally administered D-serine (10 nmol), a putative endogenous agonist of glycine site of NMDA receptors, significantly enhanced the C- but not Abeta-, and Adelta-fiber responses of WDR neurons in the spinal dorsal horn. When 1 nmol of the glycine site antagonist 7-chlorokynurenic acid (7-CK) was co-administered with 10 nmol D-serine, the facilitation of D-serine on C-fiber response was completely blocked. 7-CK (1 nmol) alone failed to influence Abeta-, Adelta-, and C-fiber responses of WDR neurons. In contrast, in carrageenan-injected rats (14/34), 10 nmol D-serine had no effect on C-fiber response, while 1 nmol 7-CK per se markedly depressed C-fiber response of WDR neurons. These findings suggest that under physiological conditions, glycine sites in the spinal cord were available but became saturated following peripheral inflammation. Thus, increased endogenous d-serine or glycine may be involved in nociceptive transmission by modulating NMDA receptor activities. The glycine site of NMDA receptors may become a target for the prevention of inflammatory pain.

The zebrafish shocked gene encodes a glycine transporter and is essential for the function of early neural circuits in the CNS

shocked (sho) is a zebrafish mutation that causes motor deficits attributable to CNS defects during the first2dof development. Mutant embryos display reduced spontaneous coiling of the trunk, diminished escape responses when touched, and an absence of swimming. A missense mutation in the slc6a9 gene that encodes a glycine transporter (GlyT1) was identified as the cause of the sho phenotype. Antisense knock-down of GlyT1 in wild-type embryos phenocopies sho, and injection of wild-type GlyT1 mRNA into mutants rescues them. A comparison of glycine-evoked inward currents in Xenopus oocytes expressing either the wild-type or mutant protein found that the missense mutation results in a nonfunctional transporter. glyt1 and the related glyt2 mRNAs are expressed in the hindbrain and spinal cord in nonoverlapping patterns. The fact that these regions are known to be required for generation of early locomotory behaviors suggests that the regulation of extracellular glycine levels in the CNS is important for proper function of neural networks. Furthermore, physiological analysis after manipulation of glycinergic activity in wild-type and sho embryos suggests that the mutant phenotype is attributable to elevated extracellular glycine within the CNS.

NMDA receptors mediate calcium accumulation in myelin during chemical ischaemia

Central nervous system myelin is a specialized structure produced by oligodendrocytes that ensheaths axons, allowing rapid and efficient saltatory conduction of action potentials. Many disorders promote damage to and eventual loss of the myelin sheath, which often results in significant neurological morbidity. However, little is known about the fundamental mechanisms that initiate myelin damage, with the assumption being that its fate follows that of the parent oligodendrocyte. Here we show that NMDA (N-methyl-d-aspartate) glutamate receptors mediate Ca2+ accumulation in central myelin in response to chemical ischaemia in vitro. Using two-photon microscopy, we imaged fluorescence of the Ca2+ indicator X-rhod-1 loaded into oligodendrocytes and the cytoplasmic compartment of the myelin sheath in adult rat optic nerves. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptor antagonist NBQX completely blocked the ischaemic Ca2+ increase in oligodendroglial cell bodies, but only modestly reduced the Ca2+ increase in myelin. In contrast, the Ca2+ increase in myelin was abolished by broad-spectrum NMDA receptor antagonists (MK-801, 7-chlorokynurenic acid, d-AP5), but not by more selective blockers of NR2A and NR2B subunit-containing receptors (NVP-AAM077 and ifenprodil). In vitro ischaemia causes ultrastructural damage to both axon cylinders and myelin. NMDA receptor antagonism greatly reduced the damage to myelin. NR1, NR2 and NR3 subunits were detected in myelin by immunohistochemistry and immunoprecipitation, indicating that all necessary subunits are present for the formation of functional NMDA receptors. Our data show that the mature myelin sheath can respond independently to injurious stimuli. Given that axons are known to release glutamate, our finding that the Ca2+ increase was mediated in large part by activation of myelinic NMDA receptors suggests a new mechanism of axo-myelinic signalling. Such a mechanism may represent a potentially important therapeutic target in disorders in which demyelination is a prominent feature, such as multiple sclerosis, neurotrauma, infections (for example, HIV encephalomyelopathy) and aspects of ischaemic brain injury.

Targeting prefrontal cortical dopamine D1 and N-methyl-D-aspartate receptor interactions in schizophrenia treatment

The prefrontal cortex plays a principal role in higher cognition and particularly in the fast online manipulation of appropriate information to guide forthcoming behavior. Dysfunction of this process represents a main feature in the pathophysiology of schizophrenia. Both dopamine D1 and N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex play a critical role in synaptic plasticity, memory mechanisms, and cognition. Recent data have shown that D1 and NMDA receptors interact bidirectionally and may greatly influence the output of the prefrontal cortex. Hypofunction of these receptor systems in the prefrontal cortex is found in schizophrenia. This review attempts to summarize some of the latest findings on the cellular mechanisms that underlie D1-NMDA receptor interactions. These findings have provided potential therapeutic strategies that aim to functionally up-regulate D1 and/or NMDA receptor safely via selective activation of D1 receptors or coagonist activation of NMDA receptors through blockade of the glycine transporter-1.

Modulation of striatal dopamine release in vitro by agonists of the glycineB site of NMDA receptors; interaction with antipsychotics

The N-methyl-D-aspartate (NMDA) glutamate receptor possesses an obligatory co-agonist site for D-serine and glycine, named the glycineB site. Several clinical trials indicate that glycineB agonists can improve negative and cognitive symptoms of schizophrenia when co-administered with antipsychotics. In the present study we have investigated the effects of glycineB agonists on the endogenous release of dopamine from preparations of rat striatal tissue prisms in static conditions. The glycineB agonists glycine (1 mM) and D-serine (10 microM), but not D-cycloserine (10 microM), substantially increased the spontaneous release of dopamine, but significantly reduced the release of dopamine evoked by NMDA. The effect of glycine on spontaneous release was abolished by the non-competitive NMDA antagonists 5R,10S-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801, 10 microM) and ifenprodil (5 microM), but was only partially suppressed by the competitive antagonist 4-(3-phosphonopropyl)-piperazine-2-carboxylic acid (CPP, 10 microM). The selective inhibitor of the glial glycine transporter GlyT1 N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS, 10 microM) significantly increased the release of dopamine in an MK-801-sensitive manner. Interestingly, haloperidol (1 microM), but not clozapine (10 microM), prevented the effects of glycine. This study shows that glycineB modulators can control dopamine release by interacting with a distinctive NMDA receptor subtype with which some typical antipsychotics can interfere.

Inhibitors of GlyT1 and GlyT2 differentially modulate inhibitory transmission

The chronic effects of glycine transporter 1 and 2 inhibitors (sarcosine and ALX-1393, respectively) on miniature inhibitory postsynaptic currents were studied in cultured spinal neurons. We found that sarcosine increased the frequency of overall miniature inhibitory postsynaptic currents without affecting the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents, whereas ALX-1393 changed the ratio by increasing the proportions of GABAergic and mixed miniature inhibitory postsynaptic currents without affecting overall mIPSC frequency. We propose that inhibition of glycine transporter 1 by sarcosine increased overall mIPSC frequency via the activation of presynaptic glycine receptors, while inhibition of glycine transporter 2 by ALX-1393 changed the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents by shifting the balance of inhibitory transmitters in vesicles towards gamma-aminobutyric acid.

Identification and selective inhibition of the channel mode of the neuronal GABA transporter 1

The function of GAT1, the transporter for the inhibitory neurotransmitter GABA, is characterized by expression in Xenopus laevis oocytes and measurements of GABA-induced uptake of [3H]GABA, 22Na+, and 36Cl-, and GABA-evoked currents under voltage-clamp conditions. N-[4,4-Diphenyl-3-butenyl]-nipecotic acid (SKF-89976-A), a specific inhibitor of GAT1, is used in our system as a pharmacological tool. The GABA-evoked current can be decomposed into a transport current, which is coupled to the GABA uptake, and a transmitter-gated current, which is uncoupled from the GABA uptake. The transport current results from a fixed stoichiometry of 1 GABA/2 Na+/1 Cl- transported during each cycle, as determined by radioactive tracer flux measurements. The transmitter-gated current is mediated by an Na+-conductance pathway. As a competitive inhibitor for GABA uptake, SKF-89976-A can separate the two current components. The GABA uptake is blocked with a K(I) value of approximately 7 microM, whereas the uncoupled transmitter-gated current is inhibited with a K(I) value of approximately 0.03 microM. Thus, the results of this study not only identify the transport mode and the channel mode of GAT1 but also raise the possibility of separating these components in a physiological environment.

Transmembrane domains 1 and 3 of the glycine transporter GLYT1 contain structural determinants of N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine specificity

The neurotransmitter glycine is removed from the synaptic cleft by two Na(+)-and Cl(-)-dependent transporters: GLYT1 and GLYT2. GLYT1, expressed in glial processes of glycinergic areas and in glia and neurons of glutamatergic pathways that contain N-methyl-d-aspartate (NMDA) receptors, is essential for regulating glycine levels both at glycinergic and NMDA-containing synapses. GLYT2 is the transporter present in glycinergic neurons and provides cytosolic glycine for vesicular release from glycinergic terminals. GLYT1 is selectively inhibited by the sarcosine derivative N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine (NFPS). In the present report, GLYT1-GLYT2 chimeric transporters have been generated and their inhibition by NFPS has been studied. The introduction of GLYT2 transmembrane domains (TMs) 1 or 3, but not 2, on GLYT1 structure reduced the inhibition potency of NFPS and sarcosine. Binding studies and kinetic analysis of NFPS inhibition indicate lower affinity and smaller sensitivity of the chimeras to the compound. Opposite chimeras containing TM1 or TM3 of GLYT1 on GLYT2 structure became sensitive to NFPS. Individual substitution mutants of GLYT2 TM1 residues on GLYT1 and opposite GLYT1 TM1 residues on GLYT2 indicate that the more N-terminal portion of GLYT1 including residue E40 contributes to NFPS specificity. Our results demonstrate that TM1 and TM3, but not TM2, contain residues involved in the specific action of NFPS on GLYT1.

Glycine--an important neurotransmitter and cytoprotective agent

BACKGROUND

Glycine, the simplest of the amino acids, is an essential component of important biological molecules, a key substance in many metabolic reactions, the major inhibitory neurotransmitter in the spinal cord and brain stem, and an anti-inflammatory, cytoprotective, and immune modulating substance.

MATERIAL AND METHODS

Based on available literature, we discuss some of the important biological properties of glycine. In addition, we describe some clinical disorders where glycine plays a central role, either as an essential structural element, or through its metabolism or receptors.

RESULTS

The past few years have witnessed a broadening of glycine research. The traditional prime interest in aspects related to its role as an inhibitory neurotransmitter in the central nervous system has been expanded to equally emphasize other organs and tissues. With the demonstration of glycine-gated chloride channels on neurons in the central nervous system, on most leukocytes, and subsequently on other cells as well, a unifying mechanism of action accounting for many of the widespread effects of glycine has been found.

CONCLUSIONS

Glycine is a simple, easily available, and inexpensive substance with few and innocuous side-effects. The diversity of biological activities is well documented in the literature. Despite this, glycine has only gained a modest place in clinical medicine.

Glycine transporters: crucial roles of pharmacological interest revealed by gene deletion

The functions of the high-affinity glycine transporters (GLYTs) in vivo have been revealed recently using gene-deletion studies. Results from studies of homozygous knockout mice have reinforced the idea that GLYTs might be specific clinical targets to modulate inhibitory glycine-mediated neurotransmission. In addition, molecular and behavioural analysis of heterozygous mice has confirmed the therapeutic potential of GLYT1 inhibitors in the treatment of several neurological and psychiatric disorders.

Cellular localization and subcellular distribution of Unc-33-like protein 6, a brain-specific protein of the collapsin response mediator protein family that interacts with the neuronal glycine transporter 2

Unc-33-like protein (Ulip)6, a brain-specific phosphoprotein of the Ulip/collapsin response mediator protein family, was originally identified in our laboratory by yeast two-hybrid screening using the cytoplasmic N-terminal domain of the neuronal glycine transporter, glycine transporter (GlyT) 2, as a bait. Here, the interaction of Ulip6 with the N-terminal domain of GlyT2 was found to be specific for this member of the Ulip/collapsin response mediator protein family and to involve amino acids 135-184 of GlyT2. In pull-down assays and coimmunoprecipitation experiments with rat spinal cord extract, the presence of phosphatase inhibitors significantly enhanced binding of Ulip6 to GlyT2. Subcellular fractionation of spinal cord and retina homogenates at different developmental stages showed Ulip6 immunoreactivity to be associated with light vesicles that were distinct from GlyT2-containing and synaptic vesicles. Immunocytochemistry revealed punctate Ulip6 immunoreactivity in both somatic regions and processes of cultured spinal neurones; no colocalization with GlyT2 or other synaptic marker proteins was found. In retina, which expresses only GlyT1 but not GlyT2, Ulip6 was detected in the inner plexiform layer and along the somata and processes of selected bipolar, amacrine and ganglion cells. Our data support a model in which Ulip6 transiently interacts with GlyT2 in a phosphorylation-dependent manner.

Novel glycine transporter type-2 reuptake inhibitors. Part 2: beta- and gamma-amino acid derivatives

Several beta- and gamma-amino acid derivatives were prepared as glycine transport inhibitors and their ability to block the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter-2 (hGlyT-2) were evaluated. A range of lipophilic side chains were tolerated in the beta-amino acid series (i.e., Ph, CH(2)Ph, CH(CH(3))(2), and CH(2)CH(CH(3))(2)). In the gamma-amino acid series, minimal differences in potency were observed between the alpha,beta-unsaturated analogs and the corresponding saturated derivatives. In both series, a 4-biphenyl or 4-phenoxyphenyl substituent appended to the urea or cyanogunaidine moiety was necessary for in vitro activity.

Inhibitors of the glycine transporter type-2 (GlyT-2): synthesis and biological activity of benzoylpiperidine derivatives

A series of benzoylpiperidine analogs related to 4a was prepared, and their ability to inhibit the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter type-2 (GlyT-2) was evaluated. Small structural changes to the benzoylpiperidine region of the molecule led to a significant decrease in GlyT-2 inhibitory activity. In contrast, the distal aryl ring was more tolerant to functional group modifications and could accommodate a variety of substitutes at the C-2 or C-3 positions. Comparable activities to 4a were obtained by replacing the anilino nitrogen with an ether linkage 27 or by exchanging the isopropoxy ether moiety with an isopropyl amino group 15. A distinct preference for a 2-carbon tether (n=1) was observed relative to the corresponding 3-carbon homolog (n=2).

Novel glycine transporter type-2 reuptake inhibitors. Part 1: alpha-amino acid derivatives

A variety of alpha-amino acid derivatives were prepared as glycine transport inhibitors and their ability to block the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter-2 (hGlyT-2) was evaluated. An array of substituents at the chiral center was studied and overall, L-phenylalanine was identified as the preferred amino acid residue. Compounds prepared from l-amino acids were more potent GlyT-2 inhibitors than analogs derived from the corresponding d-amino acids. Introducing an achiral amino acid such as glycine, or incorporating geminal substitution in the alpha-position, led to a significant reduction in GlyT-2 inhibitory properties.

Overall distribution of GLYT2 mRNA-containing versus GAD67 mRNA-containing neurons and colocalization of both mRNAs in midbrain, pons, and cerebellum in rats

We aimed to clarify the overall distribution of glycinergic neurons in the midbrain, pons, and cerebellum in rats, using in situ hybridization for mRNA encoding glycine transporter 2 (GLYT2), which reliably detects glycinergic cell bodies. We combined this method with in situ hybridization for mRNA encoding glutamic acid decarboxylase isoform 67 (GAD67), and have presented for the first time global and detailed views of the distribution of glycinergic neurons in relation to GABAergic neurons. In addition to this single-detection study, we performed double-detection of GLYT2 mRNA and GAD67 mRNA to determine the distribution of neurons co-expressing these mRNAs. We have shown that many areas of the brainstem and cerebellum, not only areas where previous immunohistochemical studies have specified, involve double-labeled neurons with GLYT2 and GAD67 mRNAs. In particular, when lightly labeled GLYT2 mRNA-positive neurons were distributed within the area of GAD67 mRNA-positive neurons, almost all such GLYT2 mRNA-positive neurons were GAD67 mRNA-positive. Areas or neuron groups expressing exclusively GLYT2 mRNA or GAD67 mRNA were rather limited, such as the superior colliculus, nucleus of the trapezoid body, and Purkinje cells. The present study suggests that the corelease of glycine and GABA from single neurons is more widespread than has been reported.

The Second Intracellular Loop of the Glycine Transporter 2 Contains Crucial Residues for Glycine Transport and Phorbol Ester-induced Regulation

Na+ and Cl(-)-coupled glycine transporters control the availability of glycine neurotransmitter in the synaptic cleft of inhibitory glycinergic pathways. In this report, we have investigated the involvement of the second intracellular loop of the neuronal glycine transporter 2 (GLYT2) on the protein conformational equilibrium and the regulation by 4alpha-phorbol 12 myristate 13-acetate (PMA). By substituting several charged (Lys-415, Lys-418, and Lys-422) and polar (Thr-419 and Ser-420) residues for different amino acids and monitoring plasma membrane expression and kinetic behavior, we found that residue Lys-422 is crucial for glycine transport. The introduction of a negative charge in 422, and to a lower extent in neighboring N-terminal residues, dramatically increases transporter voltage dependence as assessed by response to high potassium depolarizing conditions. In addition, [2-(trimethylammonium)ethyl] methanethiosulfonate accessibility revealed a conformational connection between Lys-422 and the glycine binding/permeation site. Finally, we show that the mutation of positions Thr-419, Ser-420, and mainly Lys-422 to acidic residues abolishes the PMA-induced inhibition of transport activity and the plasma membrane transporter internalization. Our results establish a new structural basis for the action of PMA on GLYT2 and suggest a complex nature of the PMA action on this glycine transporter.

Gene structure and alternative splicing of the mouse glycine transporter type-2

The type-2 glycine transporter GLYT2 is expressed in glycinergic neurons and is involved in the termination of inhibitory neurotransmission at strychnine-sensitive glycinergic synapses. We isolated cDNA of a GLYT2 isoform, GLYT2a, from mouse brain, and found that it contains a coding sequence of 798 amino acids. We also isolated and characterized the mouse GLYT2 (mGLYT2) gene, which was found to be divided into 18 exons and spread over 55 kb. 5'-rapid amplification of cDNA ends analyses demonstrated the existence of another two isoforms, mGLYT2b and mGLYT2c, in addition to mGLYT2a. Both mGLYT2b and mGLYT2c would produce a protein eight amino acids shorter than mGLYT2a. Analysis of the genomic clones encompassing the 5'-exons revealed that the three transcripts arose from a single gene by alternative splicing. RT-PCR analysis indicated that all three mGLYT2 isoforms were expressed at high levels in brain stem and spinal cord. These data will be useful for investigating the function of GLYT2 proteins and glycinergic neurons by gene targeting experiments.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.