the hypothesis of nmda receptor hypofunction for …cent hypothesis of schizophrenia as a...

Taiwanese Journal of Psychiatry (Taipei) Vol. 26 No. 3 2012 • 147 •Overview

Schizophrenia, a multifactorial mental disorder with polygenic inheritance as well as environmental infl uences, encompasses a characteristic group of symp-toms and neurocognitive defi cits. Cognitive function, a major determinant of qual-ity of life and overall function in schizophrenia, contributes more to the prognosis of the disease than positive symptoms, such as delusions or hallucinations. Al-though its exact etiological mechanisms remain relatively unknown, extensive studies are ongoing to explore. Among them, one of the primary causal factors is dysfunction of the N-methyl-D-aspartate (NMDA)-type glutamate receptors. This article reviews the clinical limitations of current antipsychotics in treating the core symptoms of schizophrenia and the trend in the reconceptualization of the disease nature and treatment modalities. The NMDA receptor model plays a critical role in the revolution of pharmaceutical industry as a new set of drug targets in addition to those based on the traditional monoaminergic models is proposed. The evidence of NMDA receptor hypofunction in schizophrenia is accumulating from the inves-tigations on the modulation of glutamatergic system, particularly the intrinsic NMDA/glycine site, through genetic research and various clinical trials. A group of “NMDA-enhancing agents,” being used either as adjuncts to typical/atypical antipsychotics or as monotherapy, in schizophrenic patients, particularly those with refractory negative and cognitive symptoms, offer effi cacy in preclinical and early clinical trials. Novel therapeutic agents acting as NMDA enhancers show promise as the next wave of drug development for schizophrenia.

The Hypothesis of NMDA Receptor Hypofunction for Schizophrenia

Huey-Jen Chang, M.D.1,2, Hsien-Yuan Lane, M.D., Ph.D.1,3,

Guochuan E. Tsai, M.D., Ph.D.4*

Key words: schizophrenia, NMDA receptor hypofunction, negative symptoms, cognitive function

(Taiwanese Journal of Psychiatry [Taipei] 2012; 26: 147-61)

1 Institute of Clinical Medical Science, China Medical University, Taichung, Taiwan 2 Department of Psychiatry, Chiayi and Wanq-iao Branch, Taichung Veterans General Hospital, Chiayi, Taiwan 3 Department of Psychiatry, China Medical University Hospital, Taichung, Taiwan 4 Department of Psychiatry, Harbor-UCLA Medical Center, Torrance, CA, U.S.A.Received: August 21, 2012; revised: August 23, 2012; accepted: August 24, 2012*Corresponding author. No. 1000, West Carson Street, Torrance, CA 90509, U.SA.E-mail address: Guochuan E. Tsai <[email protected]>

• 148 • The NMDA Hypofunction in Schizophrenia

Introduction

Schizophrenia, affecting about 1% of the population worldwide, is a devastating and costly illness due to its resistance to treatment, the con-sequences of relapse, and substantial economic burden. Clinical symptoms of schizophrenia have three (positive symptoms, negative symptoms, and neurocognitive defi cits) main categories. The latter two possess high predictive value for clini-cal outcomes [1] and account for much of the long-term morbidity of this illness. Optimizing the treatment of schizophrenia will be an impor-tant goal in the early era of 21st century, particu-larly on negative symptoms, and neurocognitive defi cits.

Drug models have been applied extensively to study the pathophysiology of schizophrenia and thus provide a better insight into the neurobiology of the disorder. Hypofunction of N-methyl-D-aspartate receptor (NMDAR) mediated neuro-transmission is implicated in the critical defi cits associated with many brain disorders, especially schizophrenia [2]. This is evidenced by observa-tions of the clinical simulation exerted by the non-competitive antagonists of NMDAR, phencycli-dine (PCP) and ketamine on nonpsychotic individuals and schizophrenic patients [3]. Therefore, in addition to the dopamine and sero-tonin hypotheses, the NMDA hypofunction model of schizophrenia has recently gained extensive attention.

Enhancing NMDAR neurotransmission has been considered as a novel treatment approach, in particular through the glycine “modulatory” com-ponent (the coagonist site) at these receptors to avoid the excitotoxicity mediated through the glu-tamate binding site [4]. Recent advances in under-standing the function, pharmacology, genetics and

structure of NMDAR have promoted a search for new compounds that could be therapeutically ben-efi cial. These compounds act on the coagonist binding sites, either directly or indirectly (Figure 1). Various NMDA-enhancing agents have been proposed and they have been or currently are un-der extensive studies. Many clinical trials on NMDA-enhancing agents have revealed encour-aging results.

Glutamatergic Hypothesis

The classical dopamine hypothesis of schizo-phrenia [5] postulates that dopaminergic hyperac-tivity is responsible for the psychotic symptoms of this disorder. First-generation antipsychotic drugs ([FGAs], typical antipsychotics), which were developed in the 1950s with blockade of D2 receptor as a necessary therapeutic action [5], treat positive symptoms of schizophrenia effec-tively. In addition to D2 receptor blockade, the 5-HT2A receptor blockade plays a contributory role in the actions of the second-generation anti-psychotic drugs ([SGAs], atypical antipsychot-ics), thus has advantages over typical antipsychot-ics in terms of greater effi cacy for improving positive and negative symptoms, equivocal bene-fi cial effects on cognitive function, and less extra-pyramidal side effects and tardive dyskinesia. SGAs have been gradually replacing the FGAs since 1990s and became the fi rst line treatments for schizophrenia. Nevertheless, these medica-tions still show limited effi cacy on negative and cognitive symptoms of schizophrenia as well as qualities of life, and they are associated with se-vere side effects, including agranulocytosis, sud-den cardiac death, stroke, diabetes mellitus, hy-percholesterolemia and signifi cant weight gain.

Carlsson et al. [6] have pointed out that it is likely the dopaminergic system is not the only

Chang HJ, Lane HY, Tsai GE • 149 •

dysfunctional system in schizophrenia as post-mortem examination of the brains of patients with schizophrenia did not reveal alterations in the lev-els of dopamine or dopamine receptors. Interactions between dopamine and several other neurotransmitters in complex neural networks have been revealed, thus other neurotransmitter systems are likely involved in the pathophysiolo-gy of schizophrenia.

Several studies have provided evidence that a dysfunction in glutamatergic neurotransmission

might be involved in the pathophysiology of schizophrenia. Kim et al. [7] were among the pio-neers in proposing a glutamatergic hypothesis of schizophrenia, based on fi ndings of signifi cantly reduced cerebrospinal fl uid (CSF) levels of gluta-mate in patients with schizophrenia compared with normal controls. Studies of post-mortem brain and CSF have revealed a lower density of glutamatergic receptors and lower level of gluta-mate in schizophrenic patients than in healthy comparison subjects [8]. Lower indices of gluta-

Figure 1. Subunits and binding sites of NMDAR receptors. The NMDARs form heteromeric receptor-ion channels comprising two copies of NR1 and NR2 (A-D) subunits. The subunits contain binding sites for the agonist glutamate and the co-agonist glycine (or D-serine, D-alanine, D-cycloserine). Mg2+, magnesium, is a channel blocker which blocks the channel at resting membrane potential and is released upon depo-larization. The NMDAR channel is highly permeable to Ca2+ and Na+, and its open-ing requires simultaneous binding of glutamate/glycine and postsynaptic mem-brane depolarization. Zn2+, zinc, blocks the channel in a voltage-independent manner. The polyamine site binds compounds, which either potentiate or inhibit the activity of the receptor, depending on the combination of subunits forming each NMDAR. The PCP receptor site, where non-competitive antagonists such as PCP and PCP-like compounds (ketamine, and MK-801) bind.


matergic neurotransmission are in correlation to more prominent thought disorder as well as ven-tricular enlargement [9]. Among these fi ndings, the most compelling and more direct evidence is provided by the psychomimetic effects of the NMDA antagonists, phencyclidine (PCP) and ket-amine [8].

Glutamate and dopamine have been reported to exhibit reciprocal actions at subcortical struc-tures. The mechanisms underlying hyperdopami-nergic function in schizophrenia may involve cor-tical glutamatergic projections to dopamine neurons in the midbrain [6]. Investigators further raised the hypothesis that NMDARs which regu-late mesolimbic and mesocortical dopamine path-ways may be hypoactive in schizophrenia [10].

Dopaminergic neurons are mastered either directly by corticofugal glutamatergic neurons or indirectly through γ-aminobutyric acid (GABA) interneurons, which act as accelerators or brakes, respectively [11]. A descending glutamatergic pathway projecting from cortical pyramidal neu-rons to dopaminergic neurons in the ventral teg-mental area (VTA) normally functions as a brake on the mesolimbic dopaminergic pathway through an inhibitory GABAergic interneuron in the VTA, resulting in tonic inhibition of dopamine release from the mesolimbic pathway. Hypoactive NMDAR, as in schizophrenics or induced by ket-amine, in the VTA may fail to tonically inhibit me-solimbic dopaminergic neurons; this would cause mesolimbic dopamine hyperactivity and thus the positive symptoms. Different from the actions of cortico-brainstem glutamatergic neurons on me-solimbic dopaminergic neurons, separate cortico-brainstem glutamatergic neurons synapse directly upon those dopaminergic neurons in the VTA that project to the cortex, so-called mesocortical dopa-minergic neurons, and normally act to tonically excite them [10]. Therefore, hypoactive NMDAR

in the VTA would cause mesocortical dopamine hypoactivity and thus the cognitive defi cits and/or negative symptoms.

Although a focus on cognitive defi cits and/or negative symptoms has come up with a more re-cent hypothesis of schizophrenia as a “glutamate disorder” [12], the glutamatergic hypofunction hypothesis is not in confl ict with a role for dopa-mine in the pathogenesis of schizophrenia or with the action of currently available antipsychotics. This notion is supported by a rodent study which demonstrated that persistent NMDAR blockade produced a rapid and profound decrease in the levels of D2 receptor mRNA and receptor density, which suggests that NMDAR play an important role in the expression of D2 receptors in basal ganglia. Therefore, the interaction between gluta-mate and dopamine regulate the functions of the basal ganglia [13]. It was further proposed that dopamine receptor blockade might act secondari-ly to balance glutamatergic insuffi ciency [6]. Consequently, enhancement of NMDAR-medi-ated neurotransmission has been proposed as hav-ing the therapeutic potential at a fundamental pathophysiological level. Supporting the hypoth-esis, accumulating evidences from ongoing and forthcoming clinical trials, using drugs acting on NMDAR, give rise to optimism.

NMDA hypofunction model of schizophre-nia complements the dopamine and serotonin hy-potheses and its role has gained extensive atten-tion. Over the last two decades, the relationship of NMDA function and schizophrenia is supported by the evidences of the effects of the noncompeti-tive antagonists of NMDAR. Anis et al. [14] fi rst-ly reported that PCP and ketamine blocked the action of NMDA in ion fl ow through the NMDAR in the brain. A PCP receptor site was soon identi-fi ed and characterized [15], where non-competi-tive antagonists, such as PCP, ketamine and MK-


801 bind, within the NMDA ion channel which is composed of both NR1 and NR2 subunits. These NMDA antagonists induce psychiatric and physi-ological changes that closely resemble schizo-phrenia [3]. In contrast to amphetamine/dopamine model, which implies increased dopaminergic ac-tivity in the brain, PCP induces not only positive symptoms similar to amphetamine, but also nega-tive symptoms and cognitive defi cits seen in schizophrenia [3]. The physiologic manifestations of schizophrenia such as hypofrontality, disrup-tion of prepulse inhibition (PPI), enhanced sub-cortical dopamine release, and increased metabo-lism and extracellular glutamate levels in defi ned limbic circuits [16] are demonstrated by these an-tagonists as well. These fi ndings suggest that schizophrenic symptoms could possibly arise from attenuated NMDAR-mediated neurotrans-mission. It was further postulated that the psy-chotomimetic effects may not be exerted by non-competitive antagonists alone but could be an outcome of any dysfunctional attenuation of the NMDAR-mediated neurotransmission [17].

NMDAR, a subtype of the ionotropic gluta-mate receptor with a family of subunits identifi ed thus far including NR1, NR2A, NR2B, NR2C, and NR2D [18], plays an important role in neuro-development and cognition. A functional NMDAR is composed of multiple subunits including NR1 and one of four NR2 subunits (A-D), which con-tain binding sites for glycine and glutamate, re-spectively, to form heteromeric receptor-ion chan-nels [18] (Figure 1). Some studies offer the evidence of developmental regulation of specifi c components of the NMDAR unit [18]. An animal study of mRNAs encoding NMDA receptor sub-units in the developing rat CNS provided evidence that the NR1 gene is expressed in virtually all neo-cortical neurons at all stages, whereas the four NR2 transcripts display dissimilar developmental

expression pattern [18]. Therefore, defi cits in NMDA neurotransmission can potentially ac-count for developmental risk factors and cognitive impairments in schizophrenia.

Evidence of NMDA Receptor Hypofunction in Schizophrenia

Challenging tests with NMDA receptor antagonists

PCP and ketamine function primarily by binding to a site within the ion channel of the NMDAR that blocks cations infl ux, thereby acting as noncompetitive antagonist [19] (Figure 1). Binding studies in brain tissue from schizophren-ics on PCP sites with PCP ligands (3H-MK-801, 3H-TCP) show signifi cant differences between schizophrenics and healthy comparisons. The re-gional distribution of PCP sites in controls is the highest in frontal cortex, followed by entorhinal cortex, hippocampus and amygdale, while the fewest number of receptors is in the substantia nigra and the nucleus dentatus [20]. In the post-mortem brains of schizophrenia, PCP sites are more abundant in regions including orbital frontal cortex, amygdala, entorhinal area, hippocampus and putamen [20].

Nonpsychotic volunteersStudies have been conducted to demonstrate

that the administration of subanesthetic doses of PCP to nonpsychotic subjects induced neuropsy-chological and behavioral psychopathology simi-lar to that associated with schizophrenia while acutely PCP-intoxicated individuals were almost indistinguishable from symptomatic schizophren-ic patients [21]. But placebo-controlled, double-blind studies of the effects of NMDA antagonists in humans have been limited for safety concern as highly potent PCP can induce pathomorphologi-


cal changes in rat brain as well as prolonged toxic psychosis and severe medical problems in humans [21].

Ketamine, with lower binding affi nity to NMDAR and lower potency than PCP, produces minimal cardiac and respiratory adverse effects and its anesthetic and behavioral effects mitigate soon after administration [22]. When subanesthet-ic dose (0.3-0.5 mg/kg) was infused intravenously to nonpsychotic subjects, ketamine produces an avolitional state characterized by blunted affect, withdrawal, and psychomotor retardation [23], psychotic symptoms in the form of suspicious-ness, disorganization, and perceptual alterations [24], and cognition defi cits demonstrated by im-paired performance on tests of vigilance, verbal memory, verbal fl uency, and the Wisconsin Card Sorting Test [3, 23]. Signifi cant increase in the scores of Brief Psychiatric Rating Scale (BPRS), Scale for the Assessment of Negative Symptoms (SANS) and Scale for the Assessment of Positive Symptoms (SAPS) were observed [24]. Dissociative symptoms were prominent, with de-personalization in particular similar to the early feature of the schizophrenia prodrome [3]. In ad-dition, during smooth pursuit eye tracking task, ketamine induces nystagmus and oculomotor dys-function that were similar to some of the abnor-malities seen in schizophrenia [8].

Patients with schizophreniaFurther studies were conducted in patients

with schizophrenia to elucidate the possible un-derlying etiology of schizophrenia. Chronic PCP abusers have commonly been misdiagnosed as schizophrenics, whereas PCP administration ex-acerbates symptoms in chronic stabilized schizo-phrenic patients [3, 21]. Dramatic exacerbation of psychotic symptoms was observed in chronic schizophrenics under administration of subanes-

thetic doses of PCP (0.1 mg/kg) [25]. These pa-tients became more assertive, hostile, and unman-ageable, and these changes lasted not only a few hours as in nonpsychotic volunteers but around four to six weeks [25], suggesting the substantial NMDA vulnerability of this population that is eas-ily to deteriorate further. Similar to PCP fi ndings, overall, patients with schizophrenia receiving subanesthetic dose of ketamine experienced a brief worsening of positive and negative symp-toms as well as further impairment in recall and recognition memory [8].

Novel Therapy: Modulation of NMDA Receptors

NMDA glycine-site agonistsNMDAR is unique as it consists of a co-ago-

nist site that binds the endogenous full agonists, in addition to the glutamate recognition site. Glycine, D-serine, or D-alanine acts as an obligatory en-dogenous co-agonist for activation of the NMDAR complex through a strychnine-insensitive site on the NR1 subunit [26] (Figure 1). Several ap-proaches have emerged aiming towards modulat-ing this co-agonist site. D-serine is a more potent agonist than glycine at the coagonist site and has a greater ability to penetrate the blood brain barrier [27], indicating that D-serine administration may be more effi cacious for the symptoms of schizo-phrenia. D-serine is synthesized in protoplasmic astrocytes by SR that reversibly converts L-to D-serine and is degraded by D-amino acid oxi-dase (DAAO) into hydroxylpyruvate [27]. Reduction of central and peripheral D-serine lev-els in schizophrenic patients resulting in impaired D-serine function could contribute to NMDAR hypofunction [27]. Several controlled clinical tri-als have shown that co-administration of D-serine or glycine with antipsychotics can ameliorate


some symptoms of the disorder [9, 10, 28]. D-alanine, another endogenous full agonist of the coagonist site, might also have benefi cial effects on schizophrenia [26]. D-cycloserine, an anti-tu-berculosis drug, is a partial agonist at the coago-nist site. Its clinical effi cacy is less than the full agonists [29] as the intrinsic activity of D-cycloserine is only about half that of full ago-nists (i.e., glycine, D-serine) in potentiating NMDAR activation. D-cycloserine may even function as a partial antagonist in the presence of high levels of glycine and D-serine, leading to de-creased NMDA neurotransmission [29].

Glycine transport inhibitorsModulation of the NMDAR function can be

done through a number of sites other than the co-agonist site. Extracellular glycine levels are regu-lated through uptake by two types of high affi nity glycine transporters, GlyT-1 and GlyT-2. GlyT-2 has a more limited distribution, predominantly in brain stem and spinal cord neurons, and is thought to provide the principal glycine uptake mecha-nism at inhibitory glycinergic synapses [30], whereas GlyT-1 is widely expressed in glial cells of the hippocampus, cortex, and cerebellum, as well as the brain stem and spinal cord in associa-tion with NMDAR [30] and it has been proposed to function mainly at excitatory synapses by regu-lating glycine levels at the coagonist binding site of NMDAR.

As evidence suggests that synaptic glycine may be effi ciently regulated at a subsaturating level by the GlyT-1, a rational approach to en-hance NMDA neurotransmission might be through blocking the reuptake of glycine by GlyT-1. This mechanism is analogous to that of using a sero-tonin reuptake inhibitor to potentiate serotonergic neurotransmission [17]. In support of this hypoth-esis, it was demonstrated that N [3-(4′-fl uoro-

phenyl)-3-(4′-phenylphenoxy) propyl] sarcosine (NFPS), a specifi c inhibitor of GlyT-1, potentiated NMDAR-mediated responses, such as increased LTP in the dentate gyrus and enhanced PPI of acoustic startle, in vivo [31]. GlyT-1 knockdown mutation further demonstrated that the effects ex-erted by NFPS were indeed mediated by GlyT-1 [32]. Sarcosine, which is an endogenous inhibitor of GlyT-1, has shown clinical effi cacy while being administered as add-on therapy to FGAs and SGAs or as monotherapy, thereby supporting its NMDA-enhancing and antipsychotic functions [4, 33, 34] ( Figure 2).

D-amino acid oxidase inhibitorsDAAO is a fl avoenzyme that catalyzes the

oxidative deamination of D-amino acids. D-serine, the endogenous NMDAR co-agonist, is the pre-dominant D-amino acid in mammalian CNS. DAAO might have the potential in modulating NMDAR function via D-serine degradation [35], contributing to the reduction in NMDAR-mediated neurotransmission, thus increased DAAO is proposed to be in association with sus-ceptibility to schizophrenia. Consistent with the altered D-serine metabolism in schizophrenia, lower serum levels of D-serine were found in schizophrenic patients as compared to healthy controls [36].

Several lines of evidence support the above hypothesis: in postmortem studies, the mean DAAO activity is two-fold higher in the schizo-phrenia group compared with the control group and there is increased DAAO expression in the bilateral hippocampal CA4 of the schizophrenia group [37]; in genetic studies, DAAO shows as-sociations to schizophrenia in several, though not all, studies [35]. Collectively, DAAO inhibition and D-serine elevation in combination is sugges-tive of potential therapeutic benefi ts.


Figure 2. Schematic illustration of glutamatergic system. The ionotropic glutamate receptors, including N-methyl-D-aspartate (NMDA),

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate sub-types, are located in the postsynaptic membrane. They participate in synaptic transmission by directly opening ion channels upon glutamate binding, allowing ion infl ux (Na+, Ca2+) and causing excitatory post-synaptic current (EPSC) and mainly function to mediate fast synaptic transmission. Among them, NMDARs are the subtype with strongest implication in the pathophysiology of schizophrenia. Glutamate is agonist, which is synthesized and stored in high concentration within presynaptic nerve terminals and released from the nerve terminal into the synaptic cleft; glycine and D-serine are co-agonists of the NMDAR.

D-serine is synthesized by serine racemase from L-serine. D-serine is localized to both neurons and astrocytes and is uptaken by ASC-1 in the presynaptic mem-brane. D-serine is metabolized by DAAO into hydroxyl pyruvate. Role of DAOA (G72) as an activator or inhibitor is unclear. Glycine is uptaken by GlyT-1 and me-tabolized to L-serine by glycine cleavage system (GCS).

Sarcosine inhibits glycine uptake through GlyT-1. DAAO inhibitors act to reduce D-amino acids degradation. Other potential regulators and drug targets of NMDA synapse include the “glycine” co-agonist site, serine racemase, D-amino acid oxi-dase activator (DAOA, G72), and arginine-serine-cysteine transporter-1(ASC-1).

AMPA receptors have the characteristic of being mobile and they function coopera-tively with NMDARs to maintain overall integrity of glutamatergic synapses. Acti-vation of AMPA receptor depolarizes the synaptic membrane allowing Ca2+ infl ux through unblocked NMDA channels in a voltage-dependent manner.


Therapeutic Effects Exerted by NMDA-enhancing Agents

Tsai and Lin [38] performed a meta-analysis which is the most comprehensive review of NMDA-enhancing agents. It included all pub-lished, randomized, double-blind trials of the NMDA-enhancing agents and reviewed 26 stud-ies with about 800 patients with schizophrenia. The clinical effi cacy among different NMDA-enhancing agents as adjuncts to different concom-itant antipsychotic agents on different symptom domains, effi cacy, the dose-response and the side effects were examined.

GlycineGlycine added to FGAs

The effi cacy of glycine as adjuvant therapy to FGAs in treating negative symptoms and cog-nitive defi cits has been investigated in several small-scale double-blind and open-label clinical trials since 1990. One study [39] with 6 patients found no benefi cial effects regarding the negative symptoms of schizophrenia. However, the limited penetrability of glycine across the blood-brain barrier and a relative low dose of glycine (10.8 g/day) administered orally might explain the lack of effectiveness in this study. Three studies revealed signifi cant improvement in negative symptoms [40-42]. In 2007, the Cognitive and Negative Symptoms in Schizophrenia Trial (CONSIST), a randomized double-blind study with a duration of 16 weeks and with the participation of 4 sites in the United States and one site in Israel, was pub-lished [43]. A total of 157 patients were randomly assigned to glycine, D-cycloserine and placebo groups. This study suggested that glycine is not an effective therapeutic option for treating negative symptoms or cognitive defi cits as there was no

signifi cant SANS total score differences between glycine and placebo groups. Moreover, greater re-ductions in negative symptoms for both the gly-cine and D-cycloserine groups in comparison to the placebo group were detected in inpatients, but not in outpatients. Furthermore, there was no sig-nifi cant glycine/placebo between-group differ-ence on the cognitive measure. In terms of adverse effect, more new or worsened nausea was ob-served in glycine subjects than in placebo subjects.

Three of the above mentioned trials [41, 42, 44] also evaluated the cognitive effects of glycine based on PANSS cognitive subscale, showing a benefi cial effect in cognitive function. But it is noteworthy that these studies were conducted on relatively small sample sizes and over short-term periods.

Glycine added to SGAs A 6-week double-blind placebo-controlled

crossover trial with high-dose glycine (0.8 g/kg body weight per day) added to olanzapine and ris-peridone [45] revealed signifi cant improvements in negative symptoms and cognitive function. Another short-term trial [42] also showed a sig-nifi cant reduction in negative symptoms and an improvement in cognitive function based on PANSS cognitive subscale. However, these clini-cal trials were conducted on small samples under short-term treatment. In 2007, the CONSIST [43] suggested that glycine is not an effective thera-peutic option for patients with negative symptoms or cognitive impairments not benefi ted by SGAs as no signifi cant difference in change in the SANS total score and in the average cognitive domain Z scores between glycine and placebo groups was observed.


Glycine added to clozapineAbout one third to two thirds of treatment-

resistant schizophrenic patients fail to benefi t from clozapine therapy or are partial responders despite adequate dosage and duration. During the last two decades, several clozapine adjunctive agents have come into clinical practice in order to maximize the effi cacy of clozapine, including FGAs, SGAs, mood stabilizers, other anticonvul-sants, selective serotonin reuptake inhibitors (SSRI), and glycinergic agents. Since 1990’s the effi cacy of glycine as clozapine adjuncts has been evaluated in several clinical trials. Seven trials [41, 42, 44, 46-49] were published. Five of them [42, 44, 46, 48, 49] did not fi nd a signifi cant im-provement in positive symptoms. One study [47] found worsening of positive symptoms. Four tri-als [41, 42, 44, 46] showed signifi cant reductions in negative symptoms and the improvement per-sisted after discontinuation of glycine. Cognitive functioning was assessed in only four trials [41, 42, 44, 48] and three of them found positive change based on cognitive subscale of PANSS.

D-serineFive clinical trials with D-serine as adjuvant

therapy to antipsychotic treatment [9, 28, 34, 50, 51] were reported. First trial evaluated the addi-tion of D-serine to either FGAs or SGAs, with sig-nifi cant improvements in positive, negative and cognitive symptoms [9]. Five trials [4, 28, 34, 50, 51] evaluated the addition of D-serine to SGAs with three trials showed encouraging results. One study [51] published in 2010 performed a 4-week, double-blind investigation of adjunctive D-serine at dose-escalation (30, 60 or 120 mg/kg body weight/day) and the fi ndings suggested benefi cial effects in treatment of positive symptoms, nega-tive symptoms, and cognitive defi cits at doses of 60 and 120 mg/kg/day, and signifi cant dose-de-

pendent increase of plasma D-serine levels corre-lated with improved symptomatic and neuropsy-chological function. Renal side effect was observed at high D-serine dosage, which resolved upon D-serine discontinuation. The other study [28] also demonstrated signifi cant improvements in positive, negative and cognitive symptoms. But one study [34] with D-serine and risperidone co-treatment in patients with acute exacerbation of symptoms and another study [50] with addition of D-serine to clozapine did not fi nd benefi cial ef-fects in any of the three core symptoms of schizophrenia.

D-alanineThere is only one clinical trial [26] examin-

ing the effi cacy of D-alanine as adjuncts to anti-psychotics. Thirty-two schizophrenic patients were enrolled in this 6-week double-blind, place-bo-controlled trial, in which D-alanine (100 mg/kg/day) was added to their stable antipsychotic regimens, including various typical antipsychotics and risperidone. Signifi cant improvements in pos-itive and negative symptoms were the fi ndings. Cognitive symptoms assessed by the cognitive subscale of PANSS also revealed improvement. D-alanine was, moreover, a well-tolerated com-pound, and no signifi cant side effect was noted [26]. The positive fi ndings of D-alanine as add-on therapy for schizophrenia, particularly the nega-tive and cognitive symptoms, further support the hypothesis that augmentation of NMDA neuro-transmission through the NMDA coagonist site is a promising approach for schizophrenia.

D-cycloserineD-cycloserine added to FGAs

Nine trials evaluated the addition of D-cycloserine to FGAs in chronic schizophrenic patients [29, 43, 55-58]. The fi rst study [52] re-


ported that an exacerbation of positive symptoms was observed in about 70% of the patients receiv-ing adjunctive high-dose D-cycloserine (dosages between 500 mg/day and 3,000 mg/day). The sec-ond study [53] reported that 250 mg D-cycloserine daily aggravated psychotic symptoms in four of seven patients and only one patient exhibited a slight improvement. Three trials [55-57] exhibit signifi cant reductions in negative symptoms, while three others [29, 43, 58] did not fi nd signifi -cant changes in negative symptoms. In addition, one trial [55] demonstrated equivocal improve-ment of one cognitive task at the dose of 50 mg/day, while four other trials [29, 43, 56, 58] did not fi nd signifi cant changes in cognitive symptoms. One study [54] with doses of 100 mg/day showed worsening of positive symptoms and failed to im-prove negative symptoms. These unexpected fi nd-ings may be explained by the antagonistic effects of higher-dose D-cycloserine at the coagonist site of the NMDAR due to competition with the en-dogenous agonist glycine and/or D-serine or its interaction with antipsychotics which have endog-enous activity on NMDAR-mediated neurotrans-mission [54].

D-cycloserine added to SGAsFour trials evaluated the effect of

D-cycloserine added to SGAs [43, 57, 59, 60]. None of them detected any signifi cant change in positive symptoms. Three of these studies [57, 59, 60] demonstrated a signifi cant improvement in negative symptoms at the dose of 50 mg/day. CONSIST trial [43] showed no effect of D-cycloserine at 50 mg/day on negative symp-toms. Two trials [43, 60] examined the effects on cognitive function and revealed no signifi cant impact.

D-cycloserine added to clozapineThree trials evaluated the effect of

D-cycloserine as add-on thearpy to clozapine [56, 59, 61]. No signifi cant effect on cognitive or posi-tive symptoms was observed in any of the trial. One trial with D-cycloserine given at a dose of 50 mg/day [59] found improvement in negative symptoms. However, the other two trials [56, 61] with D-cycloserine given at the same dose showed worsened negative symptoms.

SarcosineFour double-blind placebo-controlled clini-

cal trials investigated the effects of sarcosine as adjuncts to stable antipsychotic regimens. The fi rst trial [33]evaluated the addition of sarcosine (2 g/day) to typical antipsychotics and to risperi-done, revealing signifi cant improvements in the positive, negative and cognitive symptoms. The second trial [34] evaluated the addition of sarco-sine (2 g/day) or D-serine (2 g/day) to risperidone in patients with acute exacerbation of schizophre-nia, revealing signifi cant improvements in posi-tive, negative, and cognitive symptoms and the therapeutic effects of sarcosine are superior to those of D-serine. Previous studies found no ben-efi cial effects of glycine, D-serine, or D-cycloserine as add-on therapy to clozapine, Lane et al. [62] thus further examined the com-bined effect of sarcosine (2 g/day) and clozapine, which exhibited no improvement in the core symptoms.

Addition of sarcosine to an existing antipsy-chotic regimen other than clozapine has shown its effi cacy for both chronically stable and acutely ill patients. However, the effi cacy of NMDA agents as a primary antipsychotic agent has not yet been demonstrated. Therefore, Lane et al. [63] evaluat-ed the effect of sarcosine monotherapy on 20 acutely symptomatic schizophrenic patients, who


were randomly assigned to receive either 1 or 2 g of sarcosine daily. Patients receiving the 2 g daily dose were more likely to respond, particularly the antipsychotic-naïve patients. However, in order to fully assess the therapeutic effect of sarcosine, placebo-or active-controlled, larger-sized studies are needed. Recently, Lane et al. [4] conducted a 6-week double-blind, placebo-controlled trial with enrollment of 60 chronic schizophrenic pa-tients comparing the effects of sarccosine (2 g/day) and D-serine as adjuncts to existing stable antipsychotics. Greater effi cacy of sarcosine ther-apy than D-serine for all outcome measures was the fi nding whereas D-serine treatment has better effi cacy than placebo group.

Perspectives

D-amino acid oxidase inhibitorsExperimental evidence supports that D-se-

rine, D-cycloserine, and D-alanine as adjuncts to FGAs or SGAs are effi cacious approaches in treating negative symptoms and cognitive impair-ments. Findings have been most encouraging in studies that have used full agonists of the NMDA receptor (i.e., glycine, D-serine, D-alanine) as op-posed to the partial antagonist D-cycloserine. Furthermore, effects have generally been more powerful when these agents were used in combi-nation with antipsychotics other than clozapine.

In addition to the coagonist site and the GlyT-1, another novel drug target to enhance NMDA function is DAAO which degrades D-amino acids including D-serine, D-alanine and other D-amino acids. Overactive DAAO may con-tribute to NMDA hypofunction, thus inhibition of DAAO is a plausible approach to enhance NMDAR function in schizophrenia. The thera-peutic value of DAAO inhibitors is relatively un-explored and remains at preclinical stage, there-

fore future extensive study is expected to establish their effi cacy, tolerability, and mechanism. Nevertheless, our recent fi ndings revealed that a DAAO inhibitor is much more effi cacious than other NMDA-enhancing agents in improving the symptoms of schizophrenia. Furthermore, it im-proves the cognition as measured by a compre-hensive MATRICS-like cognitive battery.

Financial Disclosure

Intellectual property right of benzoate treat-ment is under US2010/0189818, WO 2010/085452 and Taiwan patent application, No.100101995.

Fundings and Supports

This work was supported by the National Science Council, Taiwan (NSC-97-2314-B-039-006-MY3, NSC-100-2627-B-039-001, NSC-101-2314-B-039-030-MY3, and NSC-101-2627-B- 039-001), National Health Research Institutes, Taiwan (NHRI-EX-101-9904NI), Taiwan Depart-ment of Health Clinical Trial and Research Center of Excellence (DOH101-TD-B-111-004), and China Medical University Hospital, Taichung, Taiwan (DMR-99-153 and DMR-99-117).

References

1. Tamminga CA, Buchanan RW, Gold JM: The role of negative symptoms and cognitive dysfunction in schizophrenia outcome. Int Clin Psychopharmacol 1998; 13 Suppl 3: S21-6.

2. Javitt DC, Zukin SR: Recent advances in the phency-clidine model of schizophrenia. Am J Psychiatry 1991; 148: 1301-8.

3. Krystal JH, Karper LP, Seibyl JP, et al.: Subanesthetic effects of the noncompetitive NMDA antagonist, keta-mine, in humans: psychotomimetic, perceptual, cog-nitive, and neuroendocrine responses. Arch Gen


Psychiatry 1994; 51: 199-214.4. Lane HY, Lin CH, Huang YJ, Liao CH, Chang YC,

Tsai GE: A randomized, double-blind, placebo-con-trolled comparison study of sarcosine (N-methyl-glycine) and D-serine add-on treatment for schizo-phrenia. Int J Neuropsychopharmacol 2010; 13: 451-60.

5. Carlsson A, Lindqvist M: Effect of chlorpromazine or haloperidol on formation of 3-methoxytyramine and normetanephrine in mouse brain. Acta Pharmacol Toxicol (Copenh) 1963; 20: 140-4.

6. Carlsson A, Hansson LO, Waters N, Carlsson ML: Neurotransmitter aberrations in schizophrenia: new perspectives and therapeutic implications. Life Sci 1997; 61: 75-94.

7. Kim JS, Kornhuber HH, Holzmüller B, Schmid-Burgk W, Mergner T, Krzepinski G: Reduction of cerebrospinal fl uid glutamic acid in Huntington’s chorea and in schizophrenic patients. Arch Psychiatr Nervenkr 1980; 228: 7-10.

8. Mechri A, Saoud M, Khiari G, d’Amato T, Dalery J, Gaha L: Glutaminergic hypothesis of schizophrenia: clinical research studies with ketamine. Encephale 2001; 27: 53-9.

9. Tsai G, Yang P, Chung LC, Lange N, Coyle JT: D-serine added to antipsychotics for the treatment of schizophrenia. Biol Psychiatry 1998; 44: 1081-9.

10. Goff DC, Coyle JT: The emerging role of glutamate in the pathophysiology and treatment of schizophre-nia. Am J Psychiatry 2001; 158: 1367-77.

11. Tsapakis EM, Travis MJ: Glutamate and psychiatric disorders. Adv Psychiatr Treatment 2002; 8: 189-97.

12. Insel TR: Rethinking schizophrenia. Nature 2010; 468: 187-93.

13. Qin ZH, Zhou LW, Weiss B: D2 dopamine receptor messenger RNA is altered to a greater extent by blockade of glutamate receptors than by blockade of dopamine receptors. Neuroscience 1994; 60: 97-114.

14. Anis NA, Berry SC, Burton NR, Lodge D: The dis-sociative anesthetics ketamine and phencyclidine se-lectively reduce excitation of central mammalian neurons by N-methyl-D-aspartate. Br J Pharmacol 1983; 79: 565-75.

15. Lodge D, Johnson KM: Noncompetitive excitatory amino acid receptor antagonists. Trends Pharmacol Sci 1990;11: 81-6.

16. Coyle JT, Tsai G, Goff DC: Ionotropic glutamate re-ceptors as therapeutic targets in schizophrenia. Curr Drug Targets CNS Neurol Disord 2002; 1: 183-9.

17. Tsai G: A New Class of Antipsychotic Drugs: en-hancing Neurotransmission Mediated by NMDA Receptors. Psychiatric Times 2008: 25.

18. Monyer H, Burnashev N, Laurie DJ, Sakmann B, Seeburg PH: Developmental and regional expression in the rat brain and functional properties of four NMDA receptors. Neuron 1994; 12: 529-40.

19. Jentsch J, Roth R: The neuropsychopharmacology of phencyclidine: from NMDA receptor hypofunction to the dopamine hypothesis of schizophrenia. Neuropsychopharmacology 1999; 20: 201-25.

20. Kornhuber J, Mack-Burkhardt F, Riederer P: Regional distribution of 3H-MK-801 binding sites in the human brain. Brain Res 1989; 489: 397-9.

21. Allen RM, Young SJ: Phencyclidine-induced psy-chosis. Am J Psychiatry 1978; 135: 1081-4.

22. Moretti RJ, Hassan SZ, Goodman LI, Meltzer HY: Comparison of ketamine and thiopental in healthy volunteers: Effects on mental status, mood, and per-sonality. Anesth Analg 1984; 63: 1087-96.

23. Cohen B, Rosenbaum G, Luby E, Gottlieb J: Comparison of phencyclidine hydrochloride (sernyl) with other drugs: simulation of schizophrenic perfor-mance with phencyclidine hydrochloride (sernyl), lysergic acid diethylamide (LSD-25), and amobarbi-tal (Amytal) sodium, II: symbolic and sequential thinking. Arch Gen Psychiatry 1962; 6: 79-85.

24. van Berckel BN, Oranje B, van Ree JM, Verbaten MN, Kahn RS: The effects of low dose ketamine on sensory gating, neuroendocrine secretion and behav-ior in healthy human subjects. Psychopharmacology (Berl) 1998; 137: 271-81.

25. Luby ED, Cohen BD, Rosenbaum G, Gottlieb JS, Kelly R: Study of a new schizophrenic-like drug: Sernyl. Sernyl Archives of Neurological Psychiatry 1959; 81: 363-9.

26. Tsai GE, Yang P, Chang YC, Chong MY: D-alanine


added to antipsychotics for the treatment of schizo-phrenia. Biol Psychiatry 2006; 59: 230-4.

27. Hashimoto A, Oka T: Free D-aspartate and D-serine in the mammalian brain and periphery. Prog Neurobiol 1997; 52: 325-53.

28. Heresco-Levy U, Javitt DC, Ebstein R, et al.: D-serine effi cacy as add-on pharmacotherapy to ris-peridone and olanzapine for treatment-refractory schizophrenia. Biol Psychiatry 2005; 57: 577-85.

29. Goff DC, Herz L, Posever T, et al.: A six-month, placebo-controlled trial of D-cycloserine co-admin-istered with conventional antipsychotics in schizo-phrenia patients. Psychopharmacology 2005; 179: 144-50.

30. Zafra F, Aragon C, Olivares L, Danbolt NC, Gimenez C, Storm-Mathisen J: Glycine transporters are differ-entially expressed among CNS cells. J Neurosci 1995; 15: 3952-69.

31. Kinney GG, Sur C, Burno M, et al.: The glycine transporter type 1 inhibitor N-[3-(4′-fl uorophenyl)-3-(4′-phenylphenoxy) propyl] sarcosine potentiates NMDA receptor-mediated responses in vivo and pro-duces an antipsychotic profile in rodent behavior. J Neurosci 2003; 23: 7586-91.

32. Tsai G, Ralph-Williams RJ, Martina M, et al.: Gene knockout of glycine transporter 1: characterization of the behavioral phenotype. Proc Natl Acad Sci USA 2004; 101: 8485-90.

33. Tsai G, Lane HY, Yang P, Chong MY, Lange N: Glycine transporter I inhibitor, N-methylglycine (sar-cosine) added to antipsychotics for the treatment of schizophrenia. Biol Psychiatry 2004; 55: 452-6.

34. Lane HY, Chang YC, Liu YC, Chiu CC, Tsai GE: Sarcosine or D-serine add-on treatment for acute ex-acerbation of schizophrenia: a randomized, double-blind, placebo-controlled study. Arch Gen Psychiatry 2005; 62: 1196-204.

35. Verrall L, Burnet PWJ, Betts JF, Harrison PJ: The neurobiology of D-amino acid oxidase (DAO) and its involvement in schizophrenia. Mol Psychiatry 2010; 15: 122-37.

36. Hashimoto K, Fukushima T, Shimizu E, et al.: Decreased serum levels of D-serine in patients with

schizophrenia: evidence in support of the N-methyl-D-aspartate receptor hypofunction hypothesis of schizophrenia. Arch Gen Psychiatry 2003; 60: 572-6.

37. Habl G, Zink M, Petroianu G, et al.: Increased D-amino acid oxidase expression in the bilateral hip-pocampal CA4 of schizophrenic patients: a post-mortem study. J Neural Transm 2009; 116: 1657-65.

38. Tsai G, Lin PY: Strategies to enhance N-methyl-D-aspartate receptor-mediated neurotransmission in schizophrenia, a critical review and meta-analysis. Curr Pharm Des 2010; 16: 522-37.

39. Rosse RB, Theut SK, Banay-Schwartz M, et al.: Glycine adjuvant therapy to conventional neuroleptic treatment in schizophrenia: an open-label, pilot study. Clin Neuropharmacol 1989; 12: 416-24.

40. Javitt DC, Zylberman I, Zukin SR, Heresco-Levy U, Lindenmayer JP: Amelioration of negative symp-toms in schizophrenia by glycine. Am J Psychiatry 1994; 151: 1234-6.

41. Heresco-Levy U, Javitt DC, Ermilov M, Mordel C, Silipo G, Lichtenstein M: Effi cacy of high-dose gly-cine in the treatment of enduring negative symptoms of schizophrenia. Arch Gen Psychiatry 1999; 56: 29-36.

42. Javitt DC, Silipo G, Cienfuegos A, et al.: Adjunctive high-dose glycine in the treatment of schizophrenia. Int J Neuropsychopharmacol 2001; 4: 385-91.

43. Buchanan RW, Javitt DC, Marder SR, et al.: The cog-nitive and negative symptoms in schizophrenia trial (CONSIST): the effi cacy of glutamatergic agents for negative symptoms and cognitive impairments. Am J Psychiatry 2007; 164: 1593-602.

44. Heresco-Levy U, Javitt DC, Ermilov M, Mordel C, Horowitz A, Kelly D: Double-blind, placebo-con-trolled, crossover trial of glycine adjuvant therapy for treatment-resistant schizophrenia. Br J Psychiatry 1996; 169: 610-7.

45. Heresco-Levy U, Ermilov M, Lichtenberg P, Bar G, Javitt DC: High-dose glycine added to olanzapine and risperidone for the treatment of schizophrenia. Biol Psychiatry 2004; 55: 165-71.

46. Leiderman E, Zylberman I, Zukin SR, Cooper TB,


Javitt DC: Preliminary investigation of high-dose oral glycine on serum levels and negative symptoms in schizophrenia: an open label trial. Biol Psychiatry 1996; 39: 213-5.

47. Potkin SG, Jin Y, Bunney BG, Costa J, Gulasekaram B: Effect of clozapine and adjunctive high-dose gly-cine in treatment-resistant schizophrenia. Am J Psychiatry 1999; 156: 145-7.

48. Evins AE, Fitzgerald SM, Wine L, Rosselli R, Goff DC: Placebo-controlled trial of glycine added to clo-zapine in schizophrenia. Am J Psychiatry 2000; 157: 826-8.

49. Diaz P, Bhaskara S, Dursun SM, Deakin B: Double-blind, placebo-controlled, crossover trial of clozap-ine plus glycine in refractory schizophrenia negative results. J Clin Psychopharmacol 2005; 25: 277-8.

50. Tsai GE, Yang P, Chung LC, Tsai IC, Tsai CW, Coyle JT: D-serine added to clozapine for the treatment of schizophrenia. Am J Psychiatry 1999; 156: 1822-5.

51. Kantrowitz, JT, Malhotra AK, Cornblatt B, et al.: High dose D-serine in the treatment of schizophrenia. Schizophr Res 2010; 121: 125-30.

52. Simeon J, Fink M, Itil TM, Ponce D: D-Cycloserine therapy of psychosis by symptom provocation. Comp Psychiatry 1970; 11: 80-8.

53. Cascella NG, Macciardi F, Cavallini C, Smeraldi E: D-Cycloserine adjuvant therapy to conventional neu-roleptic treatment in schizophreania: an open-label study. J Neural Transm-Gen Sect 1994; 95: 105-11.

54. van Berckel BN, Evenblij CN, van Loon BJ, et al.: D-cycloserine increases positive symptoms in chron-ic schizophrenic patients when administered in addi-tion to antipsychotics: a double-blind, parallel, place-bo-controlled study. Neuropsychopharmacology 1999; 21: 203-10.

55. Goff DC, Tsai G, Manoach DS, Coyle JT: Dose-

fi nding trial of D-cycloserine added to neuroleptics for negative symptoms in schizophrenia. Am J Psychiatry 1995; 152: 1213-5.

56. Goff DC, Tsai G, Levitt J, et al.: A placebo-controlled trial of D-cycloserine added to conventional neuro-leptics in patients with schizophrenia. Arch Gen Psychiatry 1999; 56: 21-7.

57. Heresco-Levy U, Ermilov M, Shimoni J, Shapira B, Silipo G, Javit DC: Placebo-controlled trial of D-cycloserine added to conventional neuroleptics, olanzapine, or risperidone in schizophrenia. Am J Psychiatry 2002; 159: 480-2.

58. Duncan EJ, Szilagyi S, Schwartz MP, et al.: Effects of D-cycloserine on negative symptoms in schizo-phrenia. Schizophr Res 2004; 71: 239-48.

59. Heresco-Levy U, Javitt DC, Ermilov M, Silipo G, Shimoni J: Double-blind, placebo-controlled, cross-over trial of D-cycloserine adjuvant therapy for treat-ment-resistant schizophrenia. Int J Neuropsycho-pharmacol 1998; 1: 131-5.

60. Evins AE, Amico E, Posever TA, Toker R, Goff DC: D-cycloserine added to risperidone in patients with primary negative symptoms of schizophrenia. Schizophr Res 2002; 56: 19-23.

61. Goff DC, Tsai G, Manoach DS, Flood J, Darby DG, Coyle JT: D-cycloserine added to clozapine for pa-tients with schizophrenia. Am J Psychiatry 1996; 153: 1628-30.

62. Lane HY, Huang CL, Wu PL, et al.: Glycine trans-porter I inhibitor, N-methylglycine (sarcosine), add-ed to clozapine for the treatment of schizophrenia. Biol Psychiatry 2006; 60: 645-9.

63. Lane HY, Liu YC, Huang CL, et al.: Sarcosine (N-methylglycine) treatment for acute schizophrenia: a randomized, double-blind study. Biol Psychiatry 2008; 63: 9-12.

the hypothesis of nmda receptor hypofunction for …cent hypothesis of schizophrenia as a...

Documents