Pro-cognitive effects of the GlyT1 inhibitor Bitopertin in rodents.

N-methyl-D-aspartate-receptor (NMDAR) hypofunction contributes to cognitive impairments in neuropsychiatric disorders such as schizophrenia. Reduced NMDAR signalling can be enhanced by increasing extracellular levels of the NMDAR co-agonist glycine through inhibition of its transporter (GlyT1). This may be one option to improve cognitive deficits or negative symptoms of schizophrenia. In this preclinical study, we aimed at investigating effects of the GlyT1-inhibitor Bitopertin on cognition, social function and motivation. Central target engagement was assessed by Bitopertin-induced changes in glycine levels in rats' cerebrospinal fluid (CSF) and prefrontal cortex (PFC). Behavioural effects of Bitopertin on recognition memory were evaluated using a social-recognition test in rats, while its effects on working memory were tested in a spontaneous alternation task in mice pre-treated with the NMDAR antagonist MK-801. Bitopertin was further investigated using a social interaction test in rats pre-treated with the NMDAR antagonist phencyclidine, and the effects on effortful motivation were explored in progressive ratio tasks in rats. Results show that Bitopertin increased glycine levels in CSF and PFC. Moreover, it enhanced recognition memory and reduced MK-801-induced working memory deficits. By contrast, Bitopertin had no significant effects on PCP-induced social interaction deficits, and it did not alter effort-related responding. Collectively, our data demonstrate that GlyT1 inhibition by Bitopertin increased CSF and extracellular glycine levels and advocated for pro-cognitive effects of GlyT1 inhibition both in intact and NMDAR antagonists-pre-treated rodents. Together, these findings support the use of GlyT1-inhibitors for the treatment of cognitive symptoms in pathologies characterized by NMDR hypofunction, such as schizophrenia.

SLC Neurotransmitter Transporters as Therapeutic Targets for Alcohol Use Disorder: A Narrative Review.

Alcohol use disorder (AUD) is 1 of the most prevalent of all substance use disorders and contributes significantly to global disease burden. Despite its prevalence, <10% of individuals with AUD receive treatment. A significant barrier to receiving treatment is a lack of effective pharmacotherapies. While 3 medications have been approved by the FDA for AUD (disulfiram, acamprosate, naltrexone), their efficacy remains low. Furthermore, a number of undesirable side effects associated with these drugs further reduce patient compliance. Thus, research into new effective pharmacotherapies for AUD is warranted. Due to their involvement in regulating synaptic neurotransmitter levels, solute carrier (SLC) transporters could be targeted for developing effective treatment strategies for AUD. Indeed, a number of studies have shown beneficial reductions in alcohol consumption through the use of drugs that target transporters of dopamine, serotonin, glutamate, glycine, and GABA. The purpose of this narrative review is to summarize preclinical and clinical studies from the last 2 decades targeting SLC neurotransmitter transporters for the treatment of AUD. Limitations, as well as future directions for expanding this field, are also discussed.

The postnatal development of ultrasonic vocalization-associated breathing is altered in glycine transporter 2-deficient mice.

KEY POINTS: Newborn mice produce ultrasonic vocalization to communicate with their mother. The neuronal glycine transporter (GlyT2) is required for efficient loading of synaptic vesicles in glycinergic neurons. Mice lacking GlyT2 develop a phenotype that resembles human hyperekplexia and the mice die in the second postnatal week. In the present study, we show that GlyT2-knockout mice do not acquire adult ultrasonic vocalization-associated breathing patterns. Despite the strong impairment of glycinergic inhibition, they can produce sufficient expiratory airflow to produce ultrasonic vocalization. Because mouse ultrasonic vocalization is a valuable read-out in translational research, these data are highly relevant for a broad range of research fields. ABSTRACT: Mouse models are instrumental with respect to determining the genetic basis and neural foundations of breathing regulation. To test the hypothesis that glycinergic synaptic inhibition is required for normal breathing and proper post-inspiratory activity, we analysed breathing and ultrasonic vocalization (USV) patterns in neonatal mice lacking the neuronal glycine transporter (GlyT2). GlyT2-knockout (KO) mice have a profound reduction of glycinergic synaptic currents already at birth, develop a severe motor phenotype and survive only until the second postnatal week. At this stage, GlyT2-KO mice are smaller, have a reduced respiratory rate and still display a neonatal breathing pattern with active expiration for the production of USV. By contrast, wild-type mice acquire different USV-associated breathing patterns that depend on post-inspiratory control of air flow. Nonetheless, USVs per se remain largely indistinguishable between both genotypes. We conclude that GlyT2-KO mice, despite the strong impairment of glycinergic inhibition, can produce sufficient expiratory airflow to produce ultrasonic vocalization.

[Pathophysiology of the glutamate and the glycine transporters: new therapeutic targets]. / Fisiopatologia de los transportadores de glutamato y de glicina: nuevas dianas terapeuticas.

INTRODUCTION: The amino acids glutamate and glycine, apart from their role in protein synthesis, are two fundamental neurotransmitters in the central nervous system of mammals. The first one is ubiquitous and is involved in excitatory pathways of the neocortex, the retina and the cerebellum, and the second is involved in inhibitory pathways of brain caudal areas. However, both share their way of acting by integrating into the functioning of glutamate receptors of the NMDA type fundamentals in the regulation of motor, sensory and cognitive systems. AIM: To highlight the need for a fine regulation of glutamate and glycine concentrations in the intracellular and extracellular spaces of the nervous system through the action of very specific transporters for both neurotransmitters located in the plasma membrane of neurons and glial cells. DEVELOPMENT: The role of the glutamate and glycine transporters in glutamatergic and glycinergic neurotransmission and in the functioning of the nervous system is described. The pathological consequences of imbalances in these signaling pathways are pointed out. We also describe its involvement in pathologies such as schizophrenia, chronic pain, cerebral ischemia, diseases such as hereditary hyperekplexia and the non-ketotic hyperglycinemia, and neurodegenerative disorders. CONCLUSIONS: The knowledge at molecular level of the way of acting of these transporters for glutamate and glycine is allowing the identification and development of new therapeutic strategies for pathologies such as those described above and the development of new drugs.

TITLE: Fisiopatologia de los transportadores de glutamato y de glicina: nuevas dianas terapeuticas.Introduccion. Los aminoacidos glutamato y glicina, aparte de su papel en la sintesis de proteinas, son dos neurotransmisores fundamentales en el sistema nervioso central de los mamiferos. El primero es ubicuo y esta implicado en vias excitatorias de la neocorteza, la retina y el cerebelo, y el segundo esta asociado a vias inhibitorias de zonas caudales del cerebro. Sin embargo, ambos comparten su manera de actuar al integrarse en el funcionamiento de los receptores de glutamato del tipo NMDA, fundamentales en la regulacion de sistemas motores, sensitivos y cognitivos. Objetivo. Evidenciar la necesidad de una regulacion exquisita de las concentraciones de glutamato y de glicina en los espacios intra y extracelulares del sistema nervioso mediante la actuacion de transportadores muy especificos para ambos neurotransmisores localizados en la membrana plasmatica de las neuronas y de las celulas de la glia. Desarrollo. Se describe el papel de los transportadores de glutamato y glicina en la neurotransmision glutamatergica y glicinergica, y en el funcionamiento del sistema nervioso. Se señalan las consecuencias patologicas de los desequilibrios en estas vias de señalizacion. Tambien se describe su participacion en patologias como la esquizofrenia, el dolor cronico, la isquemia cerebral, la hiperplexia hereditaria, la hiperglicinemia no cetosica o trastornos neurodegenerativos. Conclusiones. El conocimiento de la forma molecular de actuar de los transportadores de glutamato y de glicina esta permitiendo la identificacion y el desarrollo de nuevas estrategias terapeuticas para patologias como las descritas y el desarrollo de nuevos farmacos.

Glycine receptors and glycine transporters: targets for novel analgesics?

Glycinergic neurotransmission has long been known for its role in spinal motor control. During the last two decades, additional functions have become increasingly recognized-among them is a critical contribution to spinal pain processing. Studies in rodent pain models provide proof-of-concept evidence that enhancing inhibitory glycinergic neurotransmission reduces chronic pain symptoms. Apparent strategies for pharmacological intervention include positive allosteric modulators of glycine receptors and modulators or inhibitors of the glial and neuronal glycine transporters GlyT1 and GlyT2. These prospects have led to drug discovery efforts in academia and in industry aiming at compounds that target glycinergic neurotransmission with high specificity. Available data show promising analgesic efficacy. Less is currently known about potential unwanted effects but the presence of glycinergic innervation in CNS areas outside the nociceptive system prompts for a careful evaluation not only of motor function, but also of potential respiratory impairment and addictive properties.

Neurophysiological Effects of Bitopertin in Schizophrenia.

PURPOSE/BACKGROUND: Deficits in N-methyl-D-aspartate receptor (NMDAR) function contribute to symptoms and cognitive dysfunction in schizophrenia and are associated with impaired generation of event-related potential measures including auditory mismatch negativity. Parallel studies of the NMDAR agonist D-serine have suggested that sensitivity of these measures to glutamate-based interventions is related to symptomatic and cognitive response. Bitopertin is a selective inhibitor of glycine transport. This study investigates effects of bitopertin on NMDAR-related event-related potential deficits in schizophrenia. METHODS/PROCEDURES: Patients with schizophrenia/schizoaffective disorder were treated with bitopertin (10 mg, n = 29), in a double-blind, parallel group investigation. Auditory mismatch negativity served as primary outcome measures. Secondary measures included clinical symptoms and neurocognitive performance. FINDINGS/RESULTS: No significant changes were seen with bitopertin for neurophysiological, clinical, or neurocognitive assessments. IMPLICATIONS/CONCLUSIONS: These findings represent the first assessment of the effect of bitopertin on neurophysiological biomarkers. Bitopertin did not significantly affect either symptoms or NMDAR-related biomarkers at the dose tested (10 mg). Mismatch negativity showed high test-retest reliability, supporting its use as a target engagement measure.

Electrophysiological evidence of increased glycine receptor-mediated phasic and tonic inhibition by blockade of glycine transporters in spinal superficial dorsal horn neurons of adult mice.

To understand the synaptic and/or extrasynaptic mechanisms underlying pain relief by blockade of glycine transporter subtypes GlyT1 and GlyT2, whole-cell recordings were made from dorsal horn neurons in spinal slices from adult mice, and the effects of NFPS and ALX-1393, selective GlyT1 and GlyT2 inhibitors, respectively, on phasic evoked or miniature glycinergic inhibitory postsynaptic currents (eIPSCs or mIPSCs) were examined. NFPS and ALX-1393 prolonged the decay phase of eIPSCs without affecting their amplitude. In the presence of tetrodotoxin to record mIPSCs, NFPS and ALX-1393 induced a tonic inward current that was reversed by strychnine. Although NFPS had no statistically significant influences on mIPSCs, ALX-1393 significantly increased their frequency. We then further explored the role of GlyTs in the maintenance of glycinergic IPSCs. To facilitate vesicular release of glycine, repetitive high-frequency stimulation (HFS) was applied at 10 Hz for 3 min during continuous recordings of eIPSCs at 0.1 Hz. Prominent suppression of eIPSCs was evident after HFS in the presence of ALX-1393, but not NFPS. Thus, it appears that phasic and tonic inhibition may contribute to the analgesic effects of GlyT inhibitors. However, reduced glycinergic inhibition due to impaired vesicular refilling could hamper the analgesic efficacy of GlyT2 inhibitors.

Advances in understanding the functions of native GlyT1 and GlyT2 neuronal glycine transporters.

Glycine can be substrate for two transporters: GlyT1, largely expressed by astrocytes but also by some non-glycinergic neurons, and GlyT2, most frequently present in glycine-storing nerve endings. In morphological studies, GlyT2 expression had been found to be restricted to caudal regions, being almost undetectable in neocortex and hippocampus. Here, we compared the uptake activities of GlyT1 and GlyT2 in synaptosomes purified from mouse spinal cord, cerebellum, neocortex and hippocampus. Although, as expected, [(3)H]glycine uptake was significantly lower in telencephalic than in caudal regions, selective GlyT2-mediated uptake could be evaluated in all areas. Appropriately, [(3)H]glycine selectively taken up into hippocampal synaptosomes through GlyT2 could be subsequently released by exocytosis. Native GlyT2, which did not contribute to basal release from cerebellum or spinal cord nerve terminals, could mediate release of [(3)H]glycine by transporter reversal in synaptosomes exposed to veratridine. Moreover, GlyT2 transporters could perform Na(+)-dependent homoexchange in response to externally added glycine. In conclusion, transporters of the GlyT2 type exhibited significant uptake also in telencephalic regions, probably because of the elevated driving force related to their stoichiometry. Although glycine release through GlyT2 had been predicted to be a very difficult process, GlyT2 expressed on isolated glycinergic nerve terminals can perform both release by transporter reversal and homoexchange.

A tale of two neurotransmitters.

Amacrine cells are a diverse set of local circuit neurons of the inner retina, and they all release either GABA or glycine, amino acid neurotransmitters that are generally inhibitory. But some types of amacrine cells have another function besides inhibiting other neurons. One glycinergic amacrine cell, the Aii type, excites a subset of bipolar cells via extensive gap junctions while inhibiting others at chemical synapses. Many types of GABAergic amacrine cells also release monoamines, acetylcholine, or neuropeptides. There is now good evidence that another type of amacrine cell releases glycine at some of its synapses and releases the excitatory amino acid glutamate at others. The glutamatergic synapses are made onto a subset of retinal ganglion cells and amacrine cells and have the asymmetric postsynaptic densities characteristic of central excitatory synapses. The glycinergic synapses are made onto other types of ganglion cells and have the symmetric postsynaptic densities characteristic of central inhibitory synapses. These amacrine cells, which contain vesicular glutamate transporter 3, will be the focus of this brief review.

Inhibition of glycine transporter 1: The yellow brick road to new schizophrenia therapy?

While pharmacological blockade of dopamine D2 receptor can effectively suppress the psychotic or positive symptoms of schizophrenia, there is no satisfactory medication for the negative and cognitive symptoms of schizophrenia in spite of the proliferation of second generation antipsychotic drugs. Excitements over a new class of third generation antipsychotics that might possibly fill this urgent medical need have been prompted by the recent development of glycine transporter 1 (GlyT1) inhibitors. The impetus of this novel pharmacological strategy stems directly from the prevailing hypothesis that negative and cognitive symptoms are attributable to the hypofunction of glutamatergic signalling via the N-methyl-D-aspartate (NMDA) receptor in the brain. Inhibition of GlyT1 reduces clearance of extra-cellular glycine near NMDA receptor-containing synapses, and thereby increases baseline occupancy of the glycine-B site at the NR1 subunit of the NMDA receptor, which is a prerequisite of channel activation upon stimulation by the excitatory neurotransmitter glutamate. Pharmacological inhibition of GlyT1 is expected to boost NMDA receptor function and therefore alleviate persistent negative and cognitive symptoms without excessive risk of excitotoxicity associated with direct NMDA receptor agonists. The recently completed phase III clinical trials of the Roche compound, bitopertin (a.k.a. RG1678 or RO-4917838) had initially raised hope that this new class of drugs might represent the first successful translation of the glutamate hypothesis of schizophrenia to the clinic. However, the outcomes of the multi-centre bitopertin clinical trials have been disappointing. The present review seeks to examine this promise through a critical survey of the latest clinical and preclinical findings on the therapeutic potential of GlyT1 inhibition or down-regulation.

Glycine transporter-1 controls nonsynaptic inhibitory actions of glycine receptors in the neonatal rat hippocampus.

Although functional glycinergic synapses have not been identified in the hippocampus, neurons in this area express Cl(-) permeable extrasynaptic glycine receptors (GlyRs). In experiments on CA3 pyramidal neurons on postnatal day 0-6 rat hippocampal slices, we detected robust GlyR activity as a tonic current and as single-channel events. Glycine release was independent of neuronal activity or extracellular Ca(2+). The endogenous GlyR activity was strongly enhanced by inhibition of the glycine-transporter-1 (GlyT1). Blockade of GlyT1 also caused a profound increase in the baseline current induced by exogenous glycine. Inhibition of GlyT1 reduced the frequency of spontaneous network events known as field giant depolarizing potentials (fGDPs) and of the unit activity in the absence of synaptic transmission. This inhibitory action on fGDPs was mimicked by applying 2 µm glycine or 0.1 µm isoguvacine, a GABAA-receptor agonist. Furthermore, 2 µm glycine suppressed unit spiking in the absence of synaptic transmission. Hence, despite the well known depolarizing Cl(-) equilibrium potential of neonatal hippocampal neurons, physiologically relevant extracellular glycine concentrations can exert an inhibitory action. The present data show that, akin to GABA uptake, GlyT1 exerts a powerful modulatory action on network events in the newborn hippocampus.

Deletion of forebrain glycine transporter 1 enhances conditioned freezing to a reliable, but not an ambiguous, cue for threat in a conditioned freezing paradigm.

Enhanced expression of Pavlovian aversive conditioning but not appetitive conditioning may indicate a bias in the processing of threatening or fearful events. Mice with disruption of glycine transporter 1 (GlyT1) in forebrain neurons exhibit such a bias, but they are at the same time highly sensitive to manipulations that hinder the development of the conditioned response (CR) suggesting that the mutation may modify higher cognitive processes that extract predictive information between environmental cues. Here, we further investigated the development of fear conditioning in forebrain neuronal GlyT1 knockout mice when the predictiveness of a tone stimulus for foot-shock was rendered ambiguous by interspersing [tone→no shock] trials in-between [tone→shock] trials during acquisition. The CR to the ambiguous tone CS (conditioned stimulus) was compared with that generated by an unambiguous CS that was always followed by the shock US (unconditioned stimulus) during acquisition. We showed that rendering the CS ambiguous as described significantly attenuated the CR in the mutants, but it was not sufficient to modify the CR in the control mice. It is concluded that disruption of GlyT1 in forebrain neurons does not increase the risk of forming spurious and potentially maladaptive fear associations.

Relief of cancer pain by glycine transporter inhibitors.

BACKGROUND: Recent studies have revealed the antinociceptive effects of glycine transporter (GlyT) inhibitors in neuropathic pain models such as sciatic nerve-injured and diabetic animals. Bone cancer can cause the most severe pain according to complex mechanisms in which a neuropathic element is included. Bone cancer modifies the analgesic action of opioids and limits their effectiveness, and thus novel medicament for bone cancer pain is desired. METHODS: For the femur bone cancer model, NCTC 2472 tumor cells were injected into the medullary cavity of the distal femur of C3H/HeN mice. Effects of GlyT2 inhibitors, ORG 25543 and ALX 1393, and GlyT1 inhibitors, ORG 25935, and knockdown of the expression of spinal GlyTs protein by GlyTs siRNA on pain-like behaviors, such as allodynia, withdrawal threshold, guarding behavior, and limb-use abnormality, were examined in the femur bone cancer model mice. Effects of morphine in combination with GlyT inhibitor were examined. RESULTS: GlyT2 inhibitors, ORG 25543 and ALX 1393, and GlyT1 inhibitor ORG 25935 by IV or oral administration or knockdown of the expression of spinal GlyTs protein improved pain-like behaviors at 11 days after tumor transplantation. The pain-relief activity was potent and long lasting. Morphine at a dose with no analgesic activity combined with ORG 25543 further promoted the ORG 25543-induced pain-relief activity. Injection of ORG 25543 on the second day after tumor implantation caused 3 phases of pain responses; pain-like behaviors were initially accelerated (at 2-4 days) and subsequently almost disappeared (5-7 days) and then reappeared. Intrathecal injection of strychnine 1 day after injection of ORG 25543 transiently antagonized the pain-relief activity of ORG 25543. In control mice, strychnine improved pain-like behaviors 4 days after tumor implantation and aggravated the behaviors between 4 and 5 days. The evidence suggests that the different mechanisms are phase-dependently involved. CONCLUSIONS: GlyT inhibitors with or without morphine may be a new strategy for the treatment of bone cancer pain and lead to further investigations of the mechanisms underlying the development of bone cancer pain.

Distribution of glycinergic neurons in the brain of glycine transporter-2 transgenic Tg(glyt2:Gfp) adult zebrafish: relationship to brain-spinal descending systems.

We used a Tg(glyt2:gfp) transgenic zebrafish expressing the green fluorescent protein (GFP) under control of the glycine transporter 2 (GLYT2) regulatory sequences to study for the first time the glycinergic neurons in the brain of an adult teleost. We also performed in situ hybridization using a GLYT2 probe and glycine immunohistochemistry. This study was combined with biocytin tract tracing from the spinal cord to reveal descending glycinergic pathways. A few groups of GFP(+) /GLYT2(-) cells were observed in the midbrain and forebrain, including numerous pinealocytes. Conversely, a small nucleus of the midbrain tegmentum was GLYT2(+) but GFP(-) . Most of the GFP(+) and GLYT2(+) neurons were observed in the rhombencephalon and spinal cord, and a portion of these cells showed double GLYT2/GFP labeling. In the hindbrain, GFP/GLYT2(+) populations were observed in the medial octavolateral nucleus; the secondary, magnocellular, and descending octaval nuclei; the viscerosensory lobes; and reticular populations distributed from trigeminal to vagal levels. No glycinergic cells were observed in the cerebellum. Tract tracing revealed three conspicuous pairs of GFP/GLYT2(+) reticular neurons projecting to the spinal cord. In the spinal cord, GFP/GLYT2(+) cells were observed in the dorsal and ventral horns. GFP(+) fibers were observed from the olfactory bulbs to the spinal cord, although their density varied among regions. The Mauthner neurons received very rich GFP(+) innervation, mainly around the axon cap. Comparison of the zebrafish glycinergic system with the glycinergic systems of other adult vertebrates reveals shared patterns but also divergent traits in the evolution of this system.

Glycine transporter-1 inhibition preceding extinction training inhibits reacquisition of cocaine seeking.

Cognitive enhancers that act by increasing glycine transmission might be useful adjuncts to cocaine-cue extinction training to deter relapse. The study investigated the effects of combining treatments of the glycine transporter-1 (GlyT-1) inhibitor, Org24598, with extinction training on the subsequent reacquisition of cocaine self-administration. Squirrel monkeys and rats were trained to self-administer cocaine under a second-order schedule of intravenous drug injection in which responding was maintained by cocaine injections and a cocaine-paired visual stimulus. During three weekly extinction sessions, saline was substituted for cocaine but responding still produced the cocaine-paired stimulus. Subjects were treated with Org24598 or vehicle, either before or after each extinction session. One week later, cocaine injections were restored, and reacquisition of cocaine self-administration was evaluated over 15 sessions. Compared with vehicle, administration of Org24598 (1.0 mg/kg in monkeys; 3.0 or 7.5 mg/kg in rats) before each extinction session significantly inhibited reacquisition of cocaine self-administration in each species. In contrast, administration of Org24598 (1.0 mg/kg in monkeys) following, rather than preceding, each extinction session did not affect reacquisition compared with vehicle. When extinction training was replaced by cocaine self-administration or abstinence control conditions, treatment with the same doses of Org24598 resulted in reacquisition that was significantly more rapid than the reacquisition observed when Org24598 was administered before extinction training sessions. The results support the potential clinical utility of GlyT-1 inhibitor pretreatments combined with cocaine-cue extinction training to inhibit relapse.

Interactions between glycine transporter type 1 (GlyT-1) and some inhibitor molecules - glycine transporter type 1 and its inhibitors (review).

Glycine is a mandatory positive allosteric modulator of N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors in the central nervous system. Elevation of glycine concentrations by inhibition of its reuptake in the vicinity of NMDA receptors may positively influence receptor functions as glycine B binding site on NR1 receptor subunit is not saturated in physiological conditions. Synaptic and extrasynaptic concentrations of glycine are regulated by its type-1 glycine transporter, which is primarily expressed in astroglial and glutamatergic cell membranes. Alteration of synaptic glycine levels may have importance in the treatment of various forms of endogenous psychosis characterized by hypofunctional NMDA receptors. Several lines of evidence indicate that impaired NMDA receptor-mediated glutamatergic neurotransmission is involved in development of the negative (and partly the positive) symptoms and the cognitive deficit in schizophrenia. Inhibitors of glycine transporter type-1 may represent a newly developed therapeutic intervention in treatment of this mental illness. We have synthesized a novel series of N-substituted sarcosines, analogues of the glycine transporter-1 inhibitor NFPS (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine). Of the pyridazinone-containing compounds, SzV-1997 was found to be a potent glycine transporter-1 inhibitor in rat brain synaptosomes and it markedly increased extracellular glycine concentrations in conscious rat striatum. SzV-1997 did not exhibit toxic symptoms such as hyperlocomotion, restless movements, respiratory depression, and lethality, characteristic for NFPS. Besides pyridazinone-based, sarcosine-containing glycine transporter-1 inhibitors, a series of substrate-type amino acid inhibitors was investigated in order to obtain better insight into the ligand-binding characteristics of the substrate binding cavity of the transporter.

Glycine transporter inhibitor attenuates the psychotomimetic effects of ketamine in healthy males: preliminary evidence.

Enhancing glutamate function by stimulating the glycine site of the NMDA receptor with glycine, D-serine, or with drugs that inhibit glycine reuptake may have therapeutic potential in schizophrenia. The effects of a single oral dose of cis-N-methyl-N-(6-methoxy-1-phenyl-1,2,3,4-tetrahydronaphthalen-2-ylmethyl) amino-methylcarboxylic acid hydrochloride (Org 25935), a glycine transporter-1 (GlyT1) inhibitor, and placebo pretreatment on ketamine-induced schizophrenia-like psychotic symptoms, perceptual alterations, and subjective effects were evaluated in 12 healthy male subjects in a randomized, counter-balanced, within-subjects, crossover design. At 2.5 h after administration of the Org 25935 or placebo, subjects received a ketamine bolus and constant infusion lasting 100 min. Psychotic symptoms, perceptual, and a number of subjective effects were assessed repeatedly before, several times during, and after completion of ketamine administration. A cognitive battery was administered once per test day. Ketamine produced behavioral, subjective, and cognitive effects consistent with its known effects. Org 25935 reduced the ketamine-induced increases in measures of psychosis (Positive and Negative Syndrome Scale (PANSS)) and perceptual alterations (Clinician Administered Dissociative Symptoms Scale (CADSS)). The magnitude of the effect of Org 25935 on ketamine-induced increases in Total PANSS and CADSS Clinician-rated scores was 0.71 and 0.98 (SD units), respectively. None of the behavioral effects of ketamine were increased by Org 25935 pretreatment. Org 25935 worsened some aspects of learning and delayed recall, and trended to improve choice reaction time. This study demonstrates for the first time in humans that a GlyT1 inhibitor reduces the effects induced by NMDA receptor antagonism. These findings provide preliminary support for further study of the antipsychotic potential of GlyT1 inhibitors.

Chronically saturating levels of endogenous glycine disrupt glutamatergic neurotransmission and enhance synaptogenesis in the CA1 region of mouse hippocampus.

Glycine serves a dual role in neurotransmission. It is the primary inhibitory neurotransmitter in the spinal cord and brain stem and is also an obligatory coagonist at the excitatory glutamate, N-methyl-D-aspartate receptor (NMDAR). Therefore, the postsynaptic action of glycine should be strongly regulated to maintain a balance between its inhibitory and excitatory inputs. The glycine concentration at the synapse is tightly regulated by two types of glycine transporters, GlyT1 and GlyT2, located on nerve terminals or astrocytes. Genetic studies demonstrated that homozygous (GlyT1-/-) newborn mice display severe sensorimotor deficits characterized by lethargy, hypotonia, and hyporesponsivity to tactile stimuli and ultimately die in their first postnatal day. These symptoms are similar to those associated with the human disease glycine encephalopathy in which there is a high level of glycine in cerebrospinal fluid of affected individuals. The purpose of this investigation is to determine the impact of chronically high concentrations of endogenous glycine on glutamatergic neurotransmission during postnatal development using an in vivo mouse model (GlyT1+/-). The results of our study indicate the following; that compared with wild-type mice, CA1 pyramidal neurons from mutants display significant disruptions in hippocampal glutamatergic neurotransmission, as suggested by a faster kinetic of NMDAR excitatory postsynaptic currents, a lower reduction of the amplitude of NMDAR excitatory postsynaptic currents by ifenprodil, no difference in protein expression for NR2A and NR2B but a higher protein expression for PSD-95, an increase in their number of synapses and finally, enhanced neuronal excitability.

Blockade of glycine transporter 1 by SSR-504734 promotes cognitive flexibility in glycine/NMDA receptor-dependent manner.

Accumulating evidence suggests that cognitive processes may be regulated by glycine concentration in the local environment of glutamate N-methyl-d-aspartate receptor (NMDAR). The concentration of glycine is controlled, among other factors, by the glycine transporter 1 (GlyT1). While GlyT1 inhibitors are developed for a number of indications including cognitive improvement, little is known about their effects in tasks depending on prefrontal cortical function. We examined the effect of GlyT1 inhibitor SSR-504734 on cognitive flexibility assessed in the attentional set-shifting task in rats (ASST). The second goal was to elucidate whether SSR-504734 effect has been due to the compound's action at glycine/NMDAR site. Rats treated with SSR-504734 (3 and 10 mg/kg, IP) required significantly less trials to criteria during extra-dimensional shift (EDs) phase of the ASST. The effect of SSR-504734 (3 mg/kg) was completely prevented by the glycine/NMDAR site antagonist, L-687,414 (30 mg/kg, IP) that by itself exerted no effect on cognitive flexibility. Present study demonstrates that the elevation of glycine concentration through the blockade of its reuptake facilitates cognitive flexibility. As this effect was fully blocked by glycine/NMDAR antagonist, SSR-504734-induced cognitive improvement is likely mediated through glycine action at NMDAR. It is suggested that GlyT1 inhibitors like SSR-504734 may represent a useful pharmacological approach for cognitive enhancement, especially in domains critically affected in schizophrenia.