Efficacy and safety of the glycine transporter-1 inhibitor org 25935 for the prevention of relapse in alcohol-dependent patients: a randomized, double-blind, placebo-controlled trial

Reducing the harms of alcohol: nutritional interventions and functional alcohol alternatives

The health risks and harm associated with regular alcohol consumption are well documented. In a recent WHO statement published in The Lancet Public Health alcohol consumption has been estimated to contribute worldwide to 3 million deaths in 2016 while also being responsible for 5·1% of the global burden of disease and injury. The total elimination of alcohol consumption, which has been long imbedded in human culture and society, is not practical and prohibition policies have proved historically ineffective. However, valuable strategies to reduce alcohol harms are already available and improved alternative approaches are currently being developed. Here, we will review and discuss recent advances on two main types of approaches, that is nutritional interventions and functional alcohol alternatives.

AUD in perspective

Alcohol is a major cause of pre-mature death and individual suffering worldwide, and the importance of diagnosing and treating AUD cannot be overstated. Given the global burden and the high attributable factor of alcohol in a vast number of diseases, the need for additional interventions and the development of new medicines is considered a priority by the World Health Organization (WHO). As of today, AUD is severely under-treated with a treatment gap nearing 90%, strikingly higher than that for other psychiatric disorders. Patients often seek treatment late in the progress of the disease and even among those who seek treatment only a minority receive medication, mirroring the still-prevailing stigma of the disease, and a lack of access to effective treatments, as well as a reluctance to total abstinence. To increase adherence, treatment goals should focus not only on maintaining abstinence, but also on harm reduction and psychosocial functioning. A personalised approach to AUD treatment, with a holistic view, and tailored therapy has the potential to improve AUD treatment outcomes by targeting the heterogeneity in genetics and pathophysiology, as well as reason for, and reaction to drinking. Also, the psychiatric co-morbidity rates are high in AUD and dual diagnosis can worsen symptoms and influence treatment response and should be considered in the treatment strategies.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

The GlyT1-inhibitor Org 24598 facilitates the alcohol deprivation abolishing and dopamine elevating effects of bupropion + varenicline in rats

Alcohol Use Disorder (AUD) is a relapsing brain disorder that involves perturbations of brain dopamine (DA) systems, and combined treatment with varenicline + bupropion produces additive effects on accumbal DA output and abolishes the alcohol deprivation effect (ADE) in rats. Also, direct and indirect glycine receptor (GlyR) agonists raise basal DA, attenuate alcohol-induced DA release in the nucleus Accumbens (nAc) and reduce alcohol consumption in rats. This study in rats examines whether the GlyT1-inhibitor Org 24598, an indirect GlyR agonist, enhances the ADE-reducing and DA elevating action of the combined administration of varenicline + bupropion in lower doses than previously applied. Effects on voluntary alcohol consumption, the ADE and extracellular levels of glycine and DA in nAc were examined following treatment with Org 24598 6 and 9 mg/kg i.p., bupropion 3.75 mg/kg i.p. and varenicline 1.5 mg/kg s.c., in monotherapy or combined, using a two-bottle, free-choice alcohol consumption paradigm with an ADE paradigm, and in vivo microdialysis in male Wistar rats. Notably, all treatment regimens appeared to abolish the ADE but only the effect produced by the triple combination (Org24598 + varenicline + bupropion) was significant compared to vehicle. Hence, addition of Org 24598 may enhance the ADE-reducing action of varenicline + bupropion and appears to allow for a dose reduction of bupropion. Treatment with Org 24598 raised accumbal glycine levels but did not significantly alter DA output in monotherapy. Varenicline + bupropion produced a substantial elevation in accumbal DA output that was slightly enhanced following addition of Org 24598. Conceivably, the blockade of the ADE is achieved by the triple combination enhancing accumbal DA transmission in complementary ways, thereby alleviating a hypothesized hypodopaminergia and negative reinforcement to drink. Ultimately, combining an indirect or direct GlyR agonist with varenicline + bupropion may constitute a new pharmacological treatment principle for AUD, although further refinement in dosing and evaluation of other glycinergic compounds are warranted.

The glycine-containing dipeptide leucine-glycine raises accumbal dopamine levels in a subpopulation of rats presenting a lower endogenous dopamine tone

Interventions that elevate glycine levels and target the glycine receptor (GlyR) in the nucleus Accumbens (nAc) reduce ethanol intake in rats, supposedly by acting on the brain reward system via increased basal and attenuated ethanol-induced nAc dopamine release. Glycine transport across the blood brain barrier (BBB) appears inefficient, but glycine-containing dipeptides elevate whole brain tissue dopamine levels in mice. This study explores whether treatment with the glycine-containing dipeptides leucine-glycine (Leu-Gly) and glycine-leucine (Gly-Leu) by means of a hypothesized, facilitated BBB passage, alter nAc glycine and dopamine levels and locomotor activity in two rodent models. The acute effects of Leu-Gly and Gly-Leu (1–1000 mg/kg, i.p.) alone or Leu-Gly in combination with ethanol on locomotion in male NMRI mice were examined in locomotor activity boxes. Striatal and brainstem slices were obtained for ex vivo HPLC analyses of tissue levels of glycine and dopamine. Furthermore, the effects of Leu-Gly i.p. (1–1000 mg/kg) on glycine and dopamine output in the nAc were examined using in vivo microdialysis coupled to HPLC in freely moving male Wistar rats. Leu-Gly and Gly-Leu did not significantly alter locomotion, ethanol-induced hyperlocomotor activity or tissue levels of glycine or dopamine, apart from Gly-Leu 10 mg/kg that slightly raised nAc dopamine. Microdialysis revealed no significant alterations in nAc glycine or dopamine levels when regarding all rats as a homogenous group. In a subgroup of rats defined as dopamine responders, a significant elevation of nAc dopamine (20%) was seen following Leu-Gly 10–1000 mg/kg i.p, and this group of animals presented lower baseline dopamine levels compared to dopamine non-responders. To conclude, peripheral injection of glycine-containing dipeptides appears inefficient in elevating central glycine levels but raises accumbal dopamine levels in a subgroup of rats with a lower endogenous dopamine tone. The tentative relationship between dopamine baseline and ensuing response to glycinergic treatment and presumptive direct interactions between glycine-containing dipeptides and the GlyR bear insights for refinement of the glycinergic treatment concept for alcohol use disorder (AUD).

An Overview of Glycine Transporter Subtype 1 Inhibitors Under Preclinical and Clinical Evaluation for the Treatment of Alcohol Abuse

Abstract:

Being a historical issue that withstands multiple societal control measures, alcohol abuse remains a major healthcare problem. Despite worldwide efforts to limit consumption and educate people about its effects, consumption rates remain unchanged. Alcohol abuse arises from chronic alcohol exposure-caused permanent synaptic plasticity changes in the brain. These manifest in life-threatening withdrawal symptoms and drive relapse even after detoxification and treatment. Since ethanol has multiple targets in the human brain, it warrants a multiapproach therapy; here we introduce the potential therapeutic effects of glycine transporter subtype 1 inhibitors. We have listed the various glycine transporter 1 inhibitors used in studies of alcoholism and how they influenced glycine release from rat hippocampus was demonstrated in a preliminary study. Glycine transporters modulate both glutamatergic and glycinergic pathways: (i) glutamatergic neurotransmission plays an important role in the development of chronic changes in alcoholism as daily alcohol administration was shown to increase N-methyl-D-aspartic acid receptor activity long-term, and (ii) ethanol has access to the dopaminergic reward system via glycine receptors, being an allosteric modulator of glycine receptors. This manuscript summarises the progress and development of glycine transporter 1 inhibitors, characterizing them by their mode of action, adverse effects, and discusses their clinical applicability. Furthermore, we highlight the progress in the latest clinical trials, outline currently applied treatment methods, and offer suggestions for implementing glycine transporter 1 inhibitors into the long-term treatment of alcohol abuse.

The Recent Development of Piperazine and Piperidine Derivatives as Antipsychotic Agents

Schizophrenia is a chronic neuropsychiatric disorder that affects nearly 1% of the global population. There are various anti-psychotic drugs available for the treatment of schizophrenia, but they have certain side effects; therefore, there is a need to explore and develop novel potential lead compounds against schizophrenia. The currently available drugs e.g. typical and atypical antipsychotics act on different dopamine and serotonin receptors and as per literature reports, various piperidine and piperazine derivatives have shown promising activity against these receptors. When different heterocyclic groups are attached to basic piperidine and piperazine rings, the antipsychotic activity is greatly potentiated. In this direction, various antipsychotic drugs have been synthesized at the laboratory level, and few are under clinical trial studies, such as Lu AE58054, PF-04802540, ORG25935, DMXB-A, Bitopertin, and ABT-126. In the present review, we include the studies related to the effect of different substituents on piperidine/piperazine derivatives and their anti-psychotic activity. Various series of synthesized compounds by other researchers with piperidine/piperazine nucleus have been reviewed and diagrammatically represented in the form of SAR (structure-activity relationships), which will help the scientists for the development of potential lead compounds.

Effects of systemic glycine on accumbal glycine and dopamine levels and ethanol intake in male Wistar rats

Approved medications for alcohol use disorder (AUD) display modest effect sizes. Pharmacotherapy aimed at the mechanism(s) by which ethanol activates the dopamine reward pathway may offer improved outcomes. Basal and ethanol-induced accumbal dopamine release in the rat involve glycine receptors (GlyR) in the nucleus accumbens (nAc). Glycine transporter 1 (GlyT-1) inhibitors, which raise extracellular glycine levels, have repeatedly been shown to decrease ethanol intake in the rat. To further explore the rational for elevating glycine levels in the treatment of AUD, this study examined accumbal extracellular glycine and dopamine levels and voluntary ethanol intake and preference in the rat, after systemic treatment with glycine. The effects of three different doses of glycine i.p. on accumbal glycine and dopamine levels were examined using in vivo microdialysis in Wistar rats. In addition, the effects of the intermediate dose of glycine on voluntary ethanol intake and preference were examined in a limited access two-bottle ethanol/water model in the rat. Systemic glycine treatment increased accumbal glycine levels in a dose-related manner, whereas accumbal dopamine levels were elevated in a subpopulation of animals, defined as dopamine responders. Ethanol intake and preference decreased after systemic glycine treatment. These results give further support to the concept of elevating central glycine levels to reduce ethanol intake and indicate that targeting the glycinergic system may represent a pharmacologic treatment principle for AUD.

Alcohol intake reduction for controlling hypertension

BACKGROUND

High blood pressure constitutes one of the leading causes of mortality and morbidity all over the world. At the same time, heavy drinking increases the risk for developing cardiovascular diseases, including cardiomyopathy, hypertension, atrial arrhythmias, or stroke. Several studies have already assessed specifically the relationship between alcohol intake and hypertension. However, the potential effect on blood pressure of alcohol intake reduction interventions is largely unknown.

OBJECTIVES

To assess the effect of any intervention to reduce alcohol intake in terms of blood pressure decrease in hypertensive people with alcohol consumption compared to a control intervention or no intervention at all. To determine additional effects related to mortality, major cardiovascular events, serious adverse events, or quality of life.

SEARCH METHODS

The Cochrane Hypertension Information Specialist searched the following databases for randomised controlled trials up to June 2020: the Cochrane Hypertension Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL) (Issue 5, 2020), MEDLINE Ovid (from 1946), MEDLINE Ovid Epub Ahead of Print, and MEDLINE Ovid In-Process, Embase Ovid (from 1974), ClinicalTrials.gov and the World Health Organization International Clinical Trials Registry Platform. Trial authors were contacted when needed and no language restrictions were applied.

SELECTION CRITERIA

We included randomised controlled trials with minimum 12 weeks duration and including 50 or more subjects per group with quantitative measurement of alcohol consumption and/or biological measurement of the outcomes of interest. Participants were adults (16 years of age or older) with systolic blood pressure (SBP) greater than 140 mmHg and diastolic blood pressure (DBP) greater than 90 mmHg, and SBP ≥ 130 or DBP ≥ 80 mmHg in participants with diabetes. We included any intervention implemented to reduce their alcohol intake.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed search results and extracted data using standard methodological procedures adopted by Cochrane.

MAIN RESULTS

A total of 1210 studies were screened. We included one randomised controlled trial involving a total of 269 participants with a two-year follow-up. Individual patient data for all participants were provided and used in this review. No differences were found between the cognitive-behavioural intervention group and the control group for overall mortality (RR 0.72, 95% CI 0.16 to 3.17; low-certainty evidence), cardiovascular mortality (not estimable) and cardiovascular events (RR 0.80, 95% CI 0.36 to 1.79; very low-certainty evidence). There was no statistical difference in systolic blood pressure (SBP) reduction (Mean Difference (MD) -0.92 mmHg, 95% confidence interval (CI) -5.66 to 3.82 mmHg; very low-certainty evidence) or diastolic blood pressure (DBP) decrease (MD 0.98 mmHg, 95% CI -1.69 to 3.65 mmHg; low-certainty evidence) between the cognitive-behavioural intervention group and the control group. We also did not find any differences in the proportion of subjects with SBP < 140 mmHg and DBP < 90 mmHg (Risk Ratio (RR) 1.21, 95% CI 0.88 to 1.65; very low-certainty evidence). Concerning secondary outcomes, the alcohol intake was significantly reduced in the cognitive-behavioural intervention compared with the control group (MD 191.33 g, 95% CI 85.36 to 297.30 g). We found no differences between the active and control intervention in the proportion of subjects with lower-risk alcohol intake versus higher-risk and extreme drinkers at the end of the study (RR 1.04, 95% CI 0.68 to 1.60). There were no estimable results for the quality of life outcome.

AUTHORS' CONCLUSIONS

An intervention for decreasing alcohol intake consumption did not result in differences in systolic and diastolic blood pressure when compared with a control intervention, although there was a reduction in alcohol intake favouring the active intervention. No differences were found either for overall mortality, cardiovascular mortality or cardiovascular events. No data on serious adverse events or quality of life were available to assess. Adequate randomised controlled trials are needed to provide additional evidence on this specific question.

Activation of glycine receptors in the lateral habenula rescues anxiety- and depression-like behaviors associated with alcohol withdrawal and reduces alcohol intake in rats

The lateral habenula (LHb) is activated by a range of aversive states including those related to alcohol withdrawal and has glycine receptors (GlyRs), a sensitive target of alcohol. However, whether GlyRs in the LHb contribute to alcohol-related behaviors is unknown. Here, we report that rats experiencing withdrawal from chronic alcohol consumption showed higher anxiety and sensitivity to stress compared to their alcohol-naïve counterparts. Intra-LHb injection of glycine attenuated these aberrant behaviors and reduced alcohol intake upon alcohol re-access. Glycine's effect was blocked by strychnine, a GlyR antagonist, indicating that it was mediated by strychnine-sensitive GlyRs. Conversely, intra-LHb strychnine elicited anxiety- and depression-like behaviors in Naïve rats but not in withdrawal rats. Additionally, both the frequency and the amplitude of the spontaneous IPSCs were lower in LHb neurons in slices of withdrawal rats compared to naïve rats. Also, there were sporadic strychnine-sensitive synaptic events in some LHb neurons. Bath perfusion of strychnine induced a depolarizing inward current and increased action potential firings in LHb neurons. By contrast, bath perfusion of glycine or sarcosine, a glycine transporter subtype 1 inhibitor, inhibited LHb activity. Collectively, these data reveal that LHb neurons are under the tonic glycine inhibition both in physiological and pathological conditions. Activation of GlyRs reverses LHb hyperactivity, alleviates aberrant behaviors, and reduces alcohol intake, thus highlighting the GlyRs in the LHb as a potential therapeutic target for alcohol-use disorders.

A Pilot Randomized, Placebo-Controlled Trial of Glycine for Treatment of Schizophrenia and Alcohol Dependence

Objectives: The hypofunctioning of N-methyl-D-aspartate (NMDA) receptors are thought to play an important role in the pathophysiology of schizophrenia. The augmentation of the glutamatergic system through the NMDA receptor may attenuate alcohol craving and use. This study was designed to evaluate the efficacy of glycine, an agonist of the glycine B co-agonist site of the NMDA receptor on alcohol consumption and cravings as well as on negative symptoms in schizophrenia. Methods: Participants (N = 20) were given 0.8 g/kg glycine or matching placebo (provided in bottles with mixed in solution) each week for the duration of the 12-week trial. Primary outcome measures included drinking, craving for alcohol, and symptoms of schizophrenia. Cognitive functioning (attention, concentration, and memory) was also evaluated. Results: Glycine showed no benefit over placebo in the reduction of heavy drinking days or craving for alcohol over a 12-week treatment period. Nor was there an effect on negative symptoms of schizophrenia or on cognitive functioning. Conclusions: Although our study showed no beneficial effect of glycine over placebo, our results are consistent with the largest trial of glycine treatment in schizophrenia. Diagnosed schizophrenia and alcohol dependence might be more difficult to treat because of more severe psychopathology. This is the first study to date to examine an innovative treatment approach with an amino acid, glycine, as potentially acting on both alcohol intake and negative symptoms of schizophrenia.

Glycine Transporters and Its Coupling with NMDA Receptors

Glycine plays two roles in neurotransmission. In caudal areas like the spinal cord and the brainstem, it acts as an inhibitory neurotransmitter, but in all regions of the CNS, it also works as a co-agonist with L-glutamate at N-methyl-D-aspartate receptors (NMDARs). The glycine fluxes in the CNS are regulated by two specific transporters for glycine, GlyT1 and GlyT2, perhaps with the cooperation of diverse neutral amino acid transporters like Asc-1 or SNAT5/SN2. While GlyT2 and Asc-1 are neuronal proteins, GlyT1 and SNAT5 are mainly astrocytic, although neuronal forms of GlyT1 also exist. GlyT1 has attracted considerable interest from the medical community and the pharmaceutical industry since compelling evidence indicates a clear association with the functioning of NMDARs, whose activity is decreased in various psychiatric illnesses. By controlling extracellular glycine, transporter inhibitors might potentiate the activity of NMDARs without activating excitotoxic processes. Physiologically, GlyT1 is a central actor in the cross talk between glutamatergic, glycinergic, dopaminergic, and probably other neurotransmitter systems. Many of these relationships begin to be unraveled by studies performed in recent years using genetic and pharmacological models. These studies are also clarifying the interactions between glycine, glycine transporters, and other co-agonists of the glycine site of NMDARs like D-serine. These findings are also relevant to understand the pathophysiology of devastating diseases like schizophrenia, depression, anxiety, epilepsy, stroke, and chronic pain.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

Further characterization of the GlyT-1 inhibitor Org25935: anti-alcohol, neurobehavioral, and gene expression effects

The glycine transporter-1 inhibitor Org25935 is a promising candidate in a treatment concept for alcohol use disorder targeting the glycine system. Org25935 inhibits ethanol-induced dopamine elevation in brain reward regions and reduces ethanol intake in Wistar rats. This study aimed to further characterise the compound and used ethanol consumption, behavioral measures, and gene expression as parameters to investigate the effects in Wistar rats and, as pharmacogenetic comparison, Alko-Alcohol (AA) rats. Animals were provided limited access to ethanol in a two-bottle free-choice paradigm with daily drug administration. Acute effects of Org25935 were estimated using locomotor activity and neurobehavioral status. Effects on gene expression in Wistar rats were measured with qPCR. The higher but not the lower dose of Org25935 reduced alcohol intake in Wistar rats. Unexpectedly, Org25935 reduced both ethanol and water intake and induced strong CNS-depressive effects in AA-rats (withdrawn from further studies). Neurobehavioral effects by Org25935 differed between the strains (AA-rats towards sedation). Org25935 did not affect gene expression at the mRNA level in the glycine system of Wistar rats. The data indicate a small therapeutic range for the anti-alcohol properties of Org25935, a finding that may guide further evaluations of the clinical utility of GlyT-1 inhibitors. The results point to the importance of pharmacogenetic considerations when developing drugs for alcohol-related medical concerns. Despite the lack of successful clinical outcomes, to date, the heterogeneity of drug action of Org25935 and similar agents and the unmet medical need justify further studies of glycinergic compounds in alcohol use disorder.

Glycinergic signaling in the human nervous system: An overview of therapeutic drug targets and clinical effects

Glycine and related endogenous compounds (d-serine, d-alanine, sarcosine) serve critical roles in both excitatory and inhibitory neurotransmission and are influenced by a multitude of enzymes and transporters, including glycine transporter 1 and 2 (GlyT1 and GlyT2), d-amino acid oxidase (DAAO), serine racemase (SRR), alanine-serine-cysteine transporter 1 (Asc-1), and kynurenine aminotransferase (KAT). MEDLINE, Web of Science, and PsychINFO were searched for relevant human trials of compounds. Many studies utilizing exogenous administration of small molecule agonists of the glycineB site of n-methyl-d-aspartate receptor have been studied as have a growing number of glycine transporter type 1 (GlyT1) inhibitors. The clinical effects of these compounds are reviewed as are the potential effects of newer novel compounds.

Is the Construct of Relapse Heuristic, and Does It Advance Alcohol Use Disorder Clinical Practice?

OBJECTIVE

Alcohol use disorder (AUD) relapse is a construct that has been of major clinical and research interest but has been inconsistently defined. The purpose of this study was to review the definitions of AUD relapse that have been used in clinical research as a basis for drawing conclusions about its heuristic value.

METHOD

A systematic review of the literature was conducted on empirical studies that (a) were published in peer-reviewed journals, (b) were published between 2010 and 2015, (c) were written in English, and (d) provided a definition of alcohol relapse (or lapse) that was used in the study.

RESULTS

The review yielded 139 individual studies that met inclusion criteria. The studies showed wide variability in how relapse was defined and interpreted in the literature, and there was little direct empirical or theoretical rationale provided for the definitions of relapse that were chosen. Furthermore, the concept of AUD relapse as a discrete state is not consistent with the empirical literature on the clinical course of alcohol consumption.

CONCLUSIONS

We conclude that the heuristic value of AUD relapse as currently studied is low. An alternative approach that embeds the construct in theory and data on the clinical course of alcohol consumption and aligns with current trends in healthcare would seem to have a better chance of improving AUD clinical decision-making and knowledge about AUD in general.

Synthesis and Biological Evaluation of N-((1-(4-(Sulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide Inhibitors of Glycine Transporter-1

We previously disclosed the discovery of rationally designed N-((1-(4-(propylsulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide inhibitors of glycine transporter-1 (GlyT-1), represented by analogues 10 and 11. We describe herein further structure-activity relationship exploration of this series via an optimization strategy that primarily focused on the sulfonamide and benzamide appendages of the scaffold. These efforts led to the identification of advanced leads possessing a desirable balance of excellent in vitro GlyT-1 potency and selectivity, favorable ADME and in vitro pharmacological profiles, and suitable pharmacokinetic and safety characteristics. Representative analogue (+)-67 exhibited robust in vivo activity in the cerebral spinal fluid glycine biomarker model in both rodents and nonhuman primates. Furthermore, rodent microdialysis experiments also demonstrated that oral administration of (+)-67 significantly elevated extracellular glycine levels within the medial prefrontal cortex (mPFC).

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Glycine receptors containing α2 or α3 subunits regulate specific ethanol-mediated behaviors

Glycine receptors (GlyRs) are broadly expressed in the central nervous system. Ethanol enhances the function of brain GlyRs, and the GlyRα1 subunit is associated with some of the behavioral actions of ethanol, such as loss of righting reflex. The in vivo role of GlyRα2 and α3 subunits in alcohol responses has not been characterized despite high expression levels in the nucleus accumbens and amygdala, areas that are important for the rewarding properties of drugs of abuse. We used an extensive panel of behavioral tests to examine ethanol actions in mice lacking Glra2 (the gene encoding the glycine receptor alpha 2 subunit) or Glra3 (the gene encoding the glycine receptor alpha 3 subunit). Deletion of Glra2 or Glra3 alters specific ethanol-induced behaviors. Glra2 knockout mice demonstrate reduced ethanol intake and preference in the 24-hour two-bottle choice test and increased initial aversive responses to ethanol and lithium chloride. In contrast, Glra3 knockout mice show increased ethanol intake and preference in the 24-hour intermittent access test and increased development of conditioned taste aversion to ethanol. Mutants and wild-type mice consumed similar amounts of ethanol in the limited access drinking in the dark test. Other ethanol effects, such as anxiolysis, motor incoordination, loss of righting reflex, and acoustic startle response, were not altered in the mutants. The behavioral changes in mice lacking GlyRα2 or α3 subunits were distinct from effects previously observed in mice with knock-in mutations in the α1 subunit. We provide evidence that GlyRα2 and α3 subunits may regulate ethanol consumption and the aversive response to ethanol.

Glycine transporters as novel therapeutic targets in schizophrenia, alcohol dependence and pain

Glycine transporters are endogenous regulators of the dual functions of glycine, which acts as a classical inhibitory neurotransmitter at glycinergic synapses and as a modulator of neuronal excitation mediated by NMDA (N-methyl-D-aspartate) receptors at glutamatergic synapses. The two major subtypes of glycine transporters, GlyT1 and GlyT2, have been linked to the pathogenesis and/or treatment of central and peripheral nervous system disorders, including schizophrenia and related affective and cognitive disturbances, alcohol dependence, pain, epilepsy, breathing disorders and startle disease (also known as hyperekplexia). This Review examines the rationale for the therapeutic potential of GlyT1 and GlyT2 inhibition, and surveys the latest advances in the biology of glycine reuptake and transport as well as the drug discovery and clinical development of compounds that block glycine transporters.