Recent heuristic models of schizophrenia propose that abnormalities in the gamma frequency cerebral oscillations may be closely tied to the pathophysiology of the disorder, with hypofunction of N-methyl-d-aspartate receptors (NMDAr) implicated as having a crucial role. Prepulse inhibition (PPI) is a behavioural measure of sensorimotor gating that is disrupted in schizophrenia. We tested the ability for antipsychotic drugs with diverse pharmacological actions to (1) ameliorate NMDAr antagonist-induced disruptions to gamma oscillations and (2) attenuate NMDAr antagonist-induced disruptions to PPI. We hypothesized that antipsychotic-mediated improvement of PPI deficits would be accompanied by a normalization of gamma oscillatory activity. Wistar rats were implanted with extradural electrodes to facilitate recording of electroencephalogram during PPI behavioural testing. In each session, the rats were administered haloperidol (0.25 mg kg(-1)), clozapine (5 mg kg(-1)), olanzapine (5 mg kg(-1)), LY379268 (3 mg kg(-1)), NFPS (sarcosine, 1 mg kg(-1)), d-serine (1800 mg kg(-1)) or vehicle, followed by the NMDAr antagonists MK-801(0.16 mg kg(-1)), ketamine (5 mg kg(-1)) or vehicle. Outcome measures were auditory-evoked, as well as ongoing, gamma oscillations and PPI. Although treatment with all the clinically validated antipsychotic drugs reduced ongoing gamma oscillations, clozapine was the only compound that prevented the sensory-evoked gamma deficit produced by ketamine and MK-801. In addition, clozapine was also the only antipsychotic that attenuated the disruption to PPI produced by the NMDAr antagonists. We conclude that disruptions to evoked, but not ongoing, gamma oscillations caused by NMDAr antagonists are functionally relevant, and suggest that compounds, which restore sensory-evoked gamma oscillations may improve sensory processing in patients with schizophrenia.
That said, I really want to avoid using clozapine again! Would have been interesting to see if other SGAs augmented with NMDAR glycine site agonists etc performed any better.
Deficits in N-methyl-D-aspartate receptor (NMDAR) function are increasingly linked to persistent negative symptoms and cognitive deficits in schizophrenia. Accordingly, clinical studies have been targeting the modulatory site of the NMDA receptor, based on the decreased function of NMDA receptor, to see whether increasing NMDA function can potentially help treat the negative and cognitive deficits seen in the disease. Glycine and D-serine are endogenous ligands to the NMDA modulatory site, but since high doses are needed to affect brain levels, related compounds are being developed, for example glycine transport (GlyT) inhibitors to potentially elevate brain glycine or targeting enzymes, such as D-amino acid oxidase (DAAO) to slow the breakdown and increase the brain level of D-serine. In the present study we further evaluated the effect of DAAO inhibitors 5-chloro-benzo[d]isoxazol-3-ol (CBIO) and sodium benzoate (NaB) in a phencyclidine (PCP) rodent mouse model to see if the inhibitors affect PCP-induced locomotor activity, alter brain D-serine level, and thereby potentially enhance D-serine responses. D-Serine dose-dependently reduced the PCP-induced locomotor activity at doses above 1000 mg/kg. Acute CBIO (30 mg/kg) did not affect PCP-induced locomotor activity, but appeared to reduce locomotor activity when given with D-serine (600 mg/kg); a dose that by itself did not have an effect. However, the effect was also present when the vehicle (Trappsol®) was tested with D-serine, suggesting that the reduction in locomotor activity was not related to DAAO inhibition, but possibly reflected enhanced bioavailability of D-serine across the blood brain barrier related to the vehicle. With this acute dose of CBIO, D-serine level in brain and plasma were not increased. Another weaker DAAO inhibitor NaB (400 mg/kg), and NaB plus D-serine also significantly reduced PCP-induced locomotor activity, but without affecting plasma or brain D-serine level, arguing against a DAAO-mediated effect. However, NaB reduced plasma L-serine and based on reports that NaB also elevates various plasma metabolites, for example aminoisobutyric acid (AIB), a potential effect via the System A amino acid carrier may be involved in the regulation of synaptic glycine level to modulate NMDAR function needs to be investigated. Acute ascorbic acid (300 mg/kg) also inhibited PCP-induced locomotor activity, which was further attenuated in the presence of D-serine (600 mg/kg). Ascorbic acid may have an action at the dopamine membrane carrier and/or altering redox mechanisms that modulate NMDARs, but this needs to be further investigated. The findings support an effect of D-serine on PCP-induced hyperactivity. They also offer suggestions on an interaction of NaB via an unknown mechanism, other than DAAO inhibition, perhaps through metabolomic changes, and find unexpected synergy between D-serine and ascorbic acid that supports combined NMDA glycine- and redox-site intervention. Although mechanisms of these specific agents need to be determined, overall it supports continued glutamatergic drug development.
Objectives Hypofunction of NMDA receptor is implicated in the pathophysiology, particularly cognitive impairment, of schizophrenia. Sarcosine, a glycine transporter I (GlyT-1) inhibitor, and sodium benzoate, a d-amino acid oxidase (DAAO) inhibitor, can both enhance NMDA receptor-mediated neurotransmission. We proposed simultaneously inhibiting DAAO and GlyT-1 may be more effective than inhibition of either in improving the cognitive and global functioning of schizophrenia patients. Methods This study compared add-on sarcosine (2 g/day) plus benzoate (1 g/day) vs. sarcosine (2 g/day) for the clinical symptoms, as well as the cognitive and global functioning, of chronic schizophrenia patients in a 12-week, double-blind, randomised, placebo-controlled trial. Participants were measured with the Positive and Negative Syndrome Scale and the Global Assessment of Functioning Scale every 3 weeks. Seven cognitive domains, recommended by the Measurement and Treatment Research to Improve Cognition in Schizophrenia Committee, were measured at weeks 0 and 12. Results Adjunctive sarcosine plus benzoate, but not sarcosine alone, improved the cognitive and global functioning of patients with schizophrenia, even when their clinical symptoms had not improved. Conclusions This finding suggests N-methyl-d-aspartate receptor-enhancement therapy can improve the cognitive function of patients with schizophrenia, further indicating this pro-cognitive effect can be primary without improvement in clinical symptoms
GlyT1 inhibitors may be useful for the treatment of cognitive dysfunction and the negative symptoms of schizophrenia without having undesirable central nervous system side effects . Similarly, GlyT1 inhibition is a potential therapeutic strategy in addressing the symptom domains of autism spectrum conditions .
Based on an animal model (reduced Sp4 expression in mice), GlyT1 inhibition may have promise in treating attentional deficits in neuropsychiatric patients but not learning or motivational deficits :
“…by using this model relevant to schizophrenia, we demonstrated that GlyT-1 inhibition (Org 24598) significantly reversed the attentional but not learning or motivational deficits of these mice. Interestingly, GlyT-1 inhibition impaired the attention of WT littermate mice without affecting their learning or motivation. These data support GlyT-1 inhibition as a potential treatment for attentional but not positive valence deficiencies (negative symptoms) related to schizophrenia as well as a U-shape dose response of optimal synaptic glycine levels for attentional performance.”
“…the inability of GlyT1-inhibition to remediate behaviors relevant to negative symptoms is surprising given that: (1) lower plasma and cerebrospinal fluid glycine level of patients with schizophrenia is linked to negative symptoms (Hashimoto et al, 2003); and (2) glycine treatment (although chronic) modestly lowers negative symptom ratings (Heresco-Levy et al, 1999; Javitt et al, 1994). However, such positive findings have not always been reproduced (Buchanan et al, 2007). In fact, Roche recently stopped trials testing a GlyT-1 inhibitor for the improvement of negative symptoms in schizophrenia, perhaps due in part to the sole reliance on clinical rating scales rather than objective translational laboratory tests as primary outcome measures. Testing negative symptoms using laboratory-based measures with relevance to those presented here may provide more relevant cross-species findings (Barnes et al, 2014; Der-Avakian et al, 2013; Young et al, 2013b) and greater sensitivity to the effects of GlyT-1 inhibitors. Constitutively reducing Sp4 expression in mice resulted in impaired attention that was remediated by GlyT-1 inhibition. The finding that this treatment did not remediate motivational deficits suggests that the attentional deficits of Sp4 mice are unlikely a result of altered motivation or learning. The lack of effect on motivation could be because of a requirement of longer treatment duration, although it improved attention acutely. Alternatively, more direct NMDAR1 activation may be required. Furthermore, the mechanism(s) underlying impaired motivation and learning resulting from reduced Sp4 expression have yet to be delineated. As SP4 regulates the transcription of NMDA receptor subunits GluN1, GluN2A, and GluN2B (Priya et al, 2014; Priya et al, 2013), further investigation of other mechanisms using these mice is warranted.
In conclusion, reduced Sp4 expression in mice largely recreates the attentional deficits observed in patients with schizophrenia as measured by the 5C-CPT (Young et al, 2013a). These data support pairing attentional assessment with evidence of reduced SP4 levels in peripheral blood mononuclear cells, as seen in first-episode patients with schizophrenia (Fuste et al, 2013). Such an approach might provide a useful personalized biomarker for predicting whether GlyT-1 inhibition may remediate attentional deficits in individual patients. However, such treatment would unlikely treat impaired positive valence related to reward anticipation. SP4 rare copy number variations and reduced protein levels are linked to several psychiatric disorders that exhibit attentional deficits including schizophrenia, bipolar disorder, and major depressive disorder (Pinacho et al, 2011; Shi et al, 2011; Tam et al, 2010; Zhou et al, 2009). Hence, identifying the mechanism(s) of how reducing Sp4 levels negatively affect attention and the neurobiology underlying GlyT-1 inhibition-induced reversal of these effects will prove vital. Importantly though, the present work using this model organism provides opportunities for personalized medicine for the treatment of attentional deficits in neuropsychiatric patients having low SP4 levels (McMahon and Insel, 2012).”
Sarcosine, a simple GlyT-1 inhibitor, has demonstrated some promising potential [4, 5]. Some worsening of symptoms has been noted at doses in excess of 2000mg  and a case of hypomania has been reported . Antidepressant effects have also been reported .
Regulation of GlyTs:
BDNF, acting on TrkB receptors, inhibits glycine uptake in astrocytes by promoting GlyT internalization through a Rho-GTPase activity dependent mechanism . GSK3β is important for stabilising and/or controlling the expression of functional GlyTs on the neural cell surface .
A soon to be published article may be of interest:
While pharmacological blockade of dopamine D2 receptor can effectively suppress the psychotic or positive symptoms of schizophrenia, there are 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 [link out]
While the dopamine hypothesis has dominated schizophrenia research for several decades, more recent studies have highlighted the role of fast synaptic transmitters and their receptors in schizophrenia etiology. Here we review evidence that schizophrenia is associated with a reduction in N-methyl-d-aspartate receptor (NMDAR) function. By highlighting postmortem, neuroimaging and electrophysiological studies, we provide evidence for preferential disruption of GABAergic circuits in the context of NMDAR hypo-activity states. The functional relationship between NMDARs and GABAergic neurons is realized at the molecular, cellular, microcircuit and systems levels. A synthesis of findings across these levels explains how NMDA-mediated inhibitory dysfunction may lead to aberrant interactions among brain regions, accounting for key clinical features of schizophrenia. This synthesis of schizophrenia unifies observations from diverse fields and may help chart pathways for developing novel diagnostics and therapeutics.
Targeting NMDARs and interneurons as a potential therapeutic strategy
“While current pharmacologic management of schizophrenia is dependent on D2 blockers, the evolving understanding of NMDAR and GABA interactions in schizophrenia holds promise for future therapeutics. As subunit-specific positive and negative allosteric modulators become available, this approach will increasingly be guided by selective targeting of subpopulations of NMDARs in an approach consistent with their neurodevelopmental expression.
Several drugs acting at the glycine binding site of NMDARs have been tested with mixed results over the past 20 years (Tuominen et al., 2005). Increasing glycine via dietary supplementation (Rosse et al., 1989, Costa et al., 1990, Javitt et al., 1994 and Heresco-Levy et al., 1999) or by inhibiting glycine reuptake with the competitive inhibitors sarcosine (Tsai et al., 2004) or bitopertin (Umbricht et al., 2014) has been shown to ameliorate negative symptoms of schizophrenia, although results have been inconsistent (Buchanan et al., 2007 and Goff, 2014). Other trials have supported the efficacy of high dose d-serine (Tsai et al., 1998, Heresco-Levy et al., 2005 and Kantrowitz et al., 2010) and low dose d-cycloserine (Goff et al., 1999b) but again, negative results have also been reported (Goff et al., 2005, Buchanan et al., 2007 and Weiser et al., 2012). The difficulty in replicating early positive findings may reflect the larger problem of heterogeneity in schizophrenia and the unreliability of clinical trials in this population. In addition, clozapine and possibly other second generation antipsychotics may enhance glutamatergic transmission, thereby complicating pharmacologic add-on strategies (Goff et al., 1999a, Goff et al., 2002, Wittmann et al., 2005 and Fumagalli et al., 2008). Repeated dosing with glycine site agonists may produce tachyphylaxis via endocytosis of NMDARs (Nong et al., 2003 and Parnas et al., 2005) which has led to intermittent dosing strategies (Goff et al., 2008 and Cain et al., 2014).
Intracellular pathways downstream of NMDARs may also present targets for pharmacologic intervention, as exemplified by nitric oxide augmentation by nitroprusside infusion (Hallak et al., 2013).
Of note, clozapine reverses the loss of PV in interneurons produced by repeated administration of NMDAR antagonists in adult mice (Cochran et al., 2003) and differs from other antipsychotics in showing efficacy for the glycine site of the NMDAR (Schwieler et al., 2008).
Another promising new pharmacologic approach targets the Kv3.1 channel which is primarily localized on PV + interneurons (Yanagi et al., 2014).
It remains to be established whether newer strategies, such as interneuron precursor transplants (Gilani et al., 2014) and transcranial electrical stimulation (Filmer et al., 2014) will prove effective in correcting interneuron functional deficits.
Given the many genetic links between schizophrenia and NMDAR pathways, a personalized medicine approach may produce larger and more consistent therapeutic benefits which could fundamentally advance our understanding of the illness and expand our available therapeutic options.”
A recent study by found “for the first time, an in vivo impairment in GABA transmission in schizophrenia, most prominent in antipsychotic-naive individuals. The impairment in GABA transmission appears to be linked to clinical symptoms, disturbances in cortical oscillations, and cognition.” 
A 2013 article reviews the following NMDAR-based therapeutics (direct and indirect) which have also been somewhat covered on this site:
The evidence for the use of adjunctive glutamate modulators in schizophrenia is reviewed here and in a meta-analysis here.
Also reviewed in the article are:
mGlu5 receptor PAMs
“Numerous mGlu5 PAMs have been reported to be efficacious in various preclinical antipsychotic and cognition models. However, despite the mounting evidence that mGlu5 PAMs show promise as potential novel antipsychotics, drug discovery efforts face various challenges. In addition to ‘flat’ SAR and challenging activity landscapes, multiple series have exhibited ‘molecular switches’, whereby minor structural changes dramatically alter the pharmacological profile of a series. ‘Molecular switches’ may also apply to metabolites generated, rendering the development of potential candidates considerably more difficult. In addition, despite the potential therapeutic advantages of allosteric modulation, mechanism-based neurotoxic effects were recently observed in rats orally dosed with mGlu5 PAMs. The study was conducted with a small set of four structurally similar modulators, and the effects observed include convulsion-like behaviors, abnormal mouth movements, decreased activity, and neuronal cell death; similar behaviors elicited by i.c.v. administration of (S)-3,5-DHPG. Additional studies are required to ascertain what factors contributed to these adverse effects and to ensure that other structural classes of mGlu5 PAM scaffolds do not share these liabilities. These questions will likely need to be addressed prior to clinical evaluation of mGlu5 PAMs.”
Positive and negative allosteric modulators (PAMs and NAMs, respectively) of type 5 metabotropic glutamate receptors (mGluR5) are currently being investigated as novel treatments for neuropsychiatric diseases including autism spectrum disorders, drug addiction, schizophrenia, and Fragile X syndrome. There is strong support for the hypothesis that mGluR5 is involved in the pathology of schizophrenia, and that alterations to mGluR5 trafficking might contribute to the hippocampal-dependent cognitive dysfunctions associated with this disorder . mGluR5 PAMs may have therapeutic utility in targeting specific aspects of impulsivity and executive dysfunction , resilience to chronic stress [4, 5] and in treating disorders of sociability [6, 7]. A mGluR5 potentiator induces a pro-vigilant profile that is distinct from that of amphetamine, caffeine and modafinil . mGluR5 signalling is important for synapse formation, neuroplasticity and long term potentiation as well as neuroprotection and has been shown to have a regulatory role in neuroinflammation . mGluR5 PAMs and NAMs differentially affect mPFC dendritic spine structural plasticity .
mGlu5-GABAB interplay in animal models of positive, negative and cognitive symptoms of schizophrenia has been investigated and may open up new avenues for therapeutics :
“Both mGlu5 and GABAB receptor modulators [GABAB (GS39783 and CGP7930), mGlu5 (CDPPB)] effectively reversed MK-801-induced deficits in behavioral models of schizophrenia. Moreover, the concomitant administration of sub-effective doses of CDPPB and GS39783, induced a clear antipsychotic-like effect in all the procedures used, except DOI-induced head twitches.”
GABAB agonism has been proposed to be a novel strategy for modifying the regulatory role of prefrontal and striatal glutamate on striatal dopamine levels :
“There has been substantial progress in translating animal models to human research focusing on schizophrenia (Modinos et al., 2015). It was shown that hippocampal electrophysiological activity enhances phasic firing of midbrain dopamine neurons (Grace et al., 2007), indicating a potential excitatory effect of glutamatergic input on midbrain dopamine firing via the hippocampus. Such glutamatergic input was shown to act locally at striatal presynaptic dopamine terminals via ionotropic (e.g., NMDA) receptors to facilitate tonic and impulse-independent phasic dopamine release (Borland and Michael, 2004), but glutamate may also indirectly enhance striatal dopamine via reuptake inhibition (Whitton, 1997). Regarding prefrontal glutamate, there is support that glutamatergic projections from the PFC influence dopaminergic projections to the striatum via GABA interneurons (Mora et al., 2002). Interestingly, infusion of the GABA(B) receptor agonists C4H12NO2P and baclofen into the PFC and striatum reduced dopamine levels, and this effect was reversed by a GABA antagonist (Balla et al., 2009). However, more research regarding specific receptor interactions potentially mediating the presented findings is needed.”
To conclude “mGlu5 receptor PAMs are effective in several animal models predictive of antipsychotic activity, and are currently tested in phase2 clinical trials” 
Kynurenine aminotransferase II (KATII) inhibitors
“Kynurenine aminotransferase II (KAT II) is involved in the KYNA biosynthetic pathway, and it is speculated that inhibition of KAT II would lower endogenous central KYNA levels and enhance NMDA receptor function. In fact, KAT II knockout mice showed reduced hippocampal KYNA levels (up to 70% reduction) and enhanced performance in cognition models relative to wild-type controls. The early KAT II inhibitor tool compounds (S)-ESBA and BFF-122 were poorly CNS penetrant and required central administration to assess antipsychotic activity. However, recent KAT II inhibitors with improved oral bioavailability and CNS penetration are in various stages of pre-clinical development. Pfizer’s pre-clinical candidate PF-04859989 was reported to produce a dose-dependent reduction in brain KYNA levels in rats (up to 80% reduction in the prefrontal cortex), and it demonstrated in vivo efficacy in rodent and nonhuman primate cognition models. The inhibitor also rapidly reversed anhedonia in a rodent chronic mild stress model, suggesting it may also improve negative symptoms.”
GluN2 subtype selective NMDA receptor modulators
“…the NMDA receptor is a heterotetrameric complex composed of two GluN1 and two GluN2 (GluN2A–D) sub-units. NMDA receptors may be comprised of the same or two different GluN2 sub-units, and the GluN2 sub-unit composition influences the expression and biophysical characteristics (e.g., open probability and channel gating kinetics) of the receptor. The GluN2A/B sub-units are found extensively throughout the forebrain, whereas GluN2C/D subunits are largely expressed in the cerebellum, basal ganglia, and on hippocampal and cortical interneurons.
Sub-unit selective GluN2B negative allosteric modulators (NAMs), such as ifenprodil, have been studied to assess therapeutic potential for various CNS disorders (e.g., depression, neuropathic pain, cerebral ischemia, Alzheimer’s disease, and Parkinson’s disease) and they have been used extensively as pharmacological tools to elucidate the role GluN2B NMDA receptors play in synaptic plasticity and cognition. Mutagenesis, molecular modeling, and X-ray crystallographic studies have shown that ifenprodil and similar analogues bind to a distinct allosteric site located on the GluN2B ATD.The GluN2B NAM traxoprodil (CP-101,606) was reported to induce a dose-dependent impairment of cognitive function and memory as well as cause psychomimetic effects in Phase II clinical trials for traumatic brain injury and major depressive disorder.These data lend further support to the glutamate hypothesis and suggest subtype selective GluN2B PAMs may offer an approach to treat schizophrenia.
Despite considerable efforts in the area of GluN2B NAMs, drug-like and selective GluN2B PAMs have yet to be reported. The endogenous polyamine spermine was found to potentiate NMDA receptor-mediated responses at GluN2B by increasing glycine affinity and reducing proton-induced inhibition of the receptor. However, the compound is weakly potent (163 μM) and does not possess drug-like characteristics for further development. The neurosteroid pregnenolone sulfate (PS) was also found to potentiate GluN2B NMDA receptors, but it does not exhibit GluN2 subtype selectivity.
With the exception of GluN2B NAMs, GluN2 subtype selective modulation is a relatively nascent field. Selective modulation of NMDA receptor subtypes holds considerable promise for the treatment of various CNS disorders, including schizophrenia, and a better understanding of the distinct pharmacological roles of the GluN2 subtypes is to be gained as additional tool compounds exhibiting good selectivity and improved drug-like characteristics are identified.”
An approach not mentioned in the article is α5 GABAA receptor modulation. Recently it was found that negative modulation of α5 GABAA receptors may partially prevent memory impairment induced by MK-801, but not amphetamine- or MK-801-elicited hyperlocomotion :
“Reportedly, negative modulation of α5 GABAA receptors may improve cognition in normal and pharmacologically-impaired animals, and such modulation has been proposed as an avenue for treatment of cognitive symptoms in schizophrenia. This study assessed the actions of PWZ-029, administered at doses (2, 5, and 10 mg/kg) at which it reached micromolar concentrations in brain tissue with estimated free concentrations adequate for selective modulation of α5 GABAA receptors, in three cognitive tasks in male Wistar rats acutely treated with the noncompetitive N-methyl-d-aspartate receptor antagonist, MK-801 (0.1 mg/kg), as well in tests of locomotor activity potentiated by MK-801 (0.2 mg/kg) or amphetamine (0.5 mg/kg). In a hormetic-like manner, only 5 mg/kg PWZ-029 reversed MK-801-induced deficits in novel object recognition test (visual recognition memory), whereas in the Morris water maze, the 2 mg/kg dose of PWZ-029 exerted partial beneficial effects on spatial learning impairment. PWZ-029 did not affect recognition memory deﬁcits in social novelty discrimination procedure. Motor hyperactivity induced with MK-801 or amphetamine was not preventable by PWZ-029. Our results show that certain MK-801-induced memory deficits can be ameliorated by negative modulation of α5 GABAA receptors, and point to the need for further elucidation of their translational relevance to cognitive deterioration in schizophrenia.”
“There have been more than 70 placebo-controlled clinical trials of glycine modulatory site (GMS) agonists in schizophrenia, including d-serine, glycine, d-cycloserine (DCS), and d-alanine. The results have been mixed; many studies reported significant improvements over multiple symptom domains while others did not. Other than intrinsic differences in efficacy among these GMS agonists, methodological factors likely contributed to the variability in results among these trials, most notably small sample sizes, differences in concomitant typical and atypical antipsychotic use, and subject compliance.”
Glycine has poor efficacy and requires high doses (at least 60 g/day).
The more CNS penetrant co-agonist at the glycine site of the NMDAR, D-Serine, has also shown benefit. When analysed alone, d-serine improved total psychopathology, negative symptoms, and cognitive symptoms. In healthy adults, D-serine (2.1g/day orally) “reduces subjective feelings of sadness and anxiety and has procognitive effects that are overall opposed to the known effects of NMDAR antagonists.”  It may aid cognitive remediation in schizophrenia 
“Although d-serine is substantially metabolized, daily doses of 30–120 mg/kg were effective.”
“…there is concern about nephrotoxicity at higher doses, although no significant adverse events have yet been observed at doses of ≤4 g/day”
“The use of DSR as NMDAR-GLY site probe has several cardinal advantages. DSR penetrates better the blood-brain-barrier and has a stronger affinity versus GLY at the NMDAR-GLY site at which, unlike DCS, acts as a specific and potent full agonist. DSR is thought to represent the primary forebrain NMDR co-agonist and unlike GLY is known to act exclusively at the NMDAR-GLY site. Moreover, in contrast to GLY, DSR may preferentially act at synaptic NMDARs which are involved in signal transduction of cell survival pathways. In animal models, DSR was shown to have procognitive and antidepressant/anxiolytic effects and reduced DSR serum and cerebrospinal fluid levels were documented in schizophrenia”
“DSR may induce improvements in cognitive parameters and reduction of subjective feelings of sadness and anxiety. Although the effects were overall modest and did not extend to all the measurements employed, the findings are remarkable given that the study: 1) used a single dose paradigm and 2) included a young population composed mainly of university students.”
“…an apparently contradictory body of data advocates in favor of NMDAR agonists efficacy as antidepressants. NMDAR-GLY site agonists improve depressive symptoms in schizophrenia patients and DSR is efficient in depression and anxiety animal models. Moreover, the GLYT1 inhibitor sarcosine and the d-amino acid oxidase inhibitor sodium benzoate were recently reported to be beneficial in depressed patients who were drug naïve for at least 3 months and had no history of treatment-resistance.”
“Depression feelings are improved in both schizophrenia and normal subjects by DSR; and in schizophrenia and non-refractory depression by sarcosine.”
D-cycloserine, a glycine site partial agonist may be effective, particularly in combination with cognitive remediation. Research is conflicting:
“Glycine, d-serine, and sarcosine significantly improved multiple symptom domains, whereas no symptom domain was improved by d-cycloserine. Furthermore, glycine, d-serine, and sarcosine were found to be superior to d-cycloserine in improving overall psychopathology”
NOTE: Chronic d-serine promotes NMDAR internalization; chronic d-cycloserine results in desensitization; exogenous glycine may preferentially act at extra-synaptic NMDARs
Sodium benzoate, a DAAO inhibitor, offered a significant improvement in a single study with 500mg b.i.d  Attempts to replicate these findings in a single patient (1000mg mane, patient stabilised on clozapine) led to some worsening of positive symptoms.
“…sodium benzoate (SB), a prototype competitive d-amino acid oxidase inhibitor, was effective in the treatment of several symptoms, such as positive and negative symptoms, and cognitive impairment in medicated patients with schizophrenia.”
Sodium benzoate administration has been studied in a NMDA antagonist animal model of schizophrenia.
These findings may not extend to use with clozapine:
“Relative to effects in combination with typical or newer atypical antipsychotics, glycine site agonists have proved less effective when combined with clozapine. In double blind, placebo-controlled studies in which glycine (Evins et al., 2000) or D-serine (Tsai et al., 1999) has been added to clozapine, no significant beneficial response has been observed, while D-cycloserine is reported to lead to the worsening of symptoms when used in combination with clozapine (Goff et al., 1996).”
Other findings and different routes of administration:
“…intravenous administration of 100 and 200 mg/kg (D’Souza et al., 2000), and 200 mg/kg (Neumeister et al., 2006) GLY did not result in significant behavioral or neuropsychological effects. Significant cognitive effects were neither observed following acute oral administration of 0.8 g/kg GLY (Palmer et al., 2008) or 50 mg d-cycloserine DCS (D’Souza et al., 2000) which acts as a partial NMDAR agonist. In contrast, 100 mg GLY sublingually (File et al., 1999) and oral administration of 1.2 g of the GLY precursor milacemide (Schwartz et al., 1991) were reported to improve performance of memory tasks in healthy subjects. Moreover, Onur et al. (2010) and Kuriyama et al. (2011) have recently shown that DCS (250 mg and 100 mg single doses) improve declarative and procedural learning, respectively. Feld et al. (2013) demonstrated that DCS (175 mg) can enhance sleep-dependent declarative memory consolidation.”
Trials have been conducted with allosteric AMPA receptor modulators (‘‘AMPAkines’’), as well as metabotropic receptor agonists and antagonists, including a recent trial of the mGluR2/3 agonist LY-2140023.
“Piracetam augmentation of haloperidol was capable of improving psychotic symptoms in schizophrenia, but had no effect on PANSS. Because only 30 patients (all receiving haloperidol) completed the placebo-controlled trial, more scientific evidence is needed to support such an effect.
Trials with CX156 led to controversial results. In a small study, CX156 improved cognitive functions and negative symptoms in schizophrenic patients when compared to patients treated with clozapine. However, a larger study was unable to show any effect of CX516 on cognition or negative symptoms when compared to controls. Taken together, there is only little evidence about these molecules and their therapeutic effect.”
“Metanalysis of clinical studies showed that pomeglumetad was effective in schizophrenic patients that were early-in-disease and had not been treated with atypical antipsychotics inhibiting 5-HT2A receptors (S. Stauffer, presented at the 8th international meeting on mGlu receptors, Taormina, Italy; September 28–October 3, 2014). This is in line with the evidence that treatment with atypical antipsychotics causes an epigenetic down-regulation of mGlu2 receptors in the prefrontal cortex. Thus, there is still optimism for the use of mGlu2/3 agonists or mGlu2 PAMs in the treatment of schizophrenia in targeted subpopulations of patients.
mGlu5 receptor PAMs are the other major candidate drugs in the treatment of schizophrenia. mGlu5 receptors are functional partners of NMDA receptors at many synapses of the CNS. mGlu5 receptor PAMs are effective in several animal models predictive of antipsychotic activity, and are currently tested in phase2 clinical trials”
“Despite the progress that has been made, many questions still exist, including the effective and safe window of mGluR2/3 agonists in different developmental periods, the difference between mGluR2/3 agonists and more selective mGluR2 agonists and PAMs, as well as the comparison between mGluR2/3 agonists and other antipsychotic drugs. More specifically, it is unknown whether the opposite effects of mGluR2/3 on pre-and postsynaptic sites could explain the failed action in improving SZ symptoms in clinical trials. Are the effects of mGluR2/3 agonist cell-type (excitatory pyramidal neurons versus inhibitory GABAergic interneurons) and brain region (PFC, hippocampus, ventral tegmental area, and other limbic areas) specific? Are the effects of mGluR2/3 agonist age- and/or SZ stage-dependent? Answering these questions in the future will certainly not only enhance our understanding of the mGluR2/3 effects in brain function but also provide better insights into the development of novel compounds targeting mGluR2/3 for clinical intervention of SZ. Only by solving these mechanistic puzzles can we put mGluR2/3 agonists into clinical trials and provide the optimal therapeutic benefit for individuals afflicted with SZ or other related psychiatric disorders.”
“In summary, as a potential non-dopaminergic “antipsychotic” drug, pharmacological activation of mGluR2 or mGluR3 still holds promise in alleviating symptoms of SZ. Especially, given the low side effects and high tolerability profile, selective mGluR2 agonist and mGluR2 PAM are still considered promising candidates for pharmacogenomic treatment or as an adjunct to current antipsychotic drugs in the treatment of SZ, particularly for cognitive and negative symptoms. The failure of current clinical trials should not be interpreted as a default termination of all potential studies on group II mGluRs because as discussed above, the precise manner of the execution of a clinical trial can have profound consequences for the ultimate outcome of the trial. Based on the literature review, we argue that until the mechanisms associated with mGluR2 or mGluR3, individually or jointly as mGluR2/3, are clearly elucidated, it is important to continue the study of mGluR2/3 as a therapeutic target for SZ.”
N-Acetylcysteine (NAC) enhances the endogenous activation of mGlu2/3 receptors by activating the glial glutamate:cystine membrane exchanger.
Sunifiram, via modulation of the glycine site of NMDAR (although less efficacious as an agonist than glycine) alongside AMPA receptor mediated effects, is anecdotally effective for both cognitive and positive symptoms. Further studies are required to evaluate its effectiveness but estimated human doses are low: 5-15mg orally.
For a hypothetical review of the promise of novel nootropics in the treatment of schizophrenia, see here
“…memantine [is] a weak noncompetitive NMDAR partial antagonist of low affinity (Gardoni and Di Luca, 2006) which at therapeutically relevant plasma concentrations produces only 30% NMDAR occupancy (Mor’e et al., 2008).”
Low-dose lamotrigine augmentation therapy seems to improve residual symptoms in some cases of treatment resistant schizophrenia.
The weak uncompetitive NMDA antagonist and putative immunomodulator dextromethorphan is yet another readily available augmentative strategy . Dextromethorphan binds to several neurotransmitter receptors, notably and in rank order, to the serotonin transporter > sigma-1 receptor = norepinephrine transporter > NMDA receptor = mu opiate receptor . Very high doses, leading to significant NMDA antagonism and related psychotomimetic effects, are likely to exacerbate positive symptoms. The study found that 60mg/day (doses low enough for insignificant NMDA antagonism via the parent compound, the potential role of the active metabolite is discussed later) improved immunological parameters such as TNF-α, IL-6 and IL-1β and inflammation in risperidone treated patients and lead to greater reductions in positive and negative symptoms, along with a faster treatment response over risperidone alone. Memantine has been proposed as another uncompetitive NMDA antagonist with therapeutic potential (mentioned earlier) and paradoxically, is hypothesised to be capable of reducing cortical and prefrontal signal-to-noise patterns, thus improving symptoms [see review]. Research concludes that there is a “low but significant block of NMDA receptors by memantine at nontoxic therapeutic doses (~20mg/day)” and at clinically relevant concentrations memantine can promote synaptic plasticity, protect against excitotoxicity and preserve or enhance memory whilst lacking cognition impairing and psychotomimetic properties . Whether higher doses of other low-affinity uncompetitive NMDA antagonists such as dextromethorphan (or the more potent O-desmethyl metabolite, which would begin to antagonise the NMDA receptor more so than the parent compound) can also achieve similar outcomes needs to be verified. Potential interactions with other serotonergics and metabolism to the more potent NMDA antagonist by CYP2D6 necessitate caution with regard to potential drug-drug interactions. Nonetheless, a combination of dextromethorphan and quinidine (which inhibits CYP2D6, leading to higher serum levels) has been proposed as a potential treatment for bipolar disorder and MDDIs there any evidence for glutamate modulators as an adjunctive treatment in schizophrenia?
High quality evidence suggests a medium effect of adjunctive glycine, D-cycloserine, D–serine or CX516 for improving overall and negative symptoms, but no benefit for cognitive function.
D-serine is most effective for overall symptoms when added to non-clozapine antipsychotics, and CX516 when added to clozapine.
For glycine, there is also moderate to low quality evidence of a small benefit for improved positive and depressive symptoms when compared to placebo.
For D-serine, there is moderate to low quality evidence for a small benefit for total psychopathology, negative, cognitive and depressive symptoms when compared to placebo.
For sarcosine, there is moderate to low quality evidence for small improvements in total symptoms, general psychopathology and negative symptoms when compared to placebo.
For memantine, there is moderate to low quality evidence for improving negative symptoms.
There is limited (low quality) evidence for the use of D-alanine or N-acetyl cysteine for improving outcomes in schizophrenia.
For patients with insufficient response to clozapine, moderate quality evidence suggests no improvements with glycine augmentation and low quality evidence is unable to determine any benefits of CX516, D-cycloserine, D-serine or sarcosine augmentation.