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Remarkable advances have been made in schizophrenia (SCZ) GWAS, but gleaning biological insight from these loci is challenging. Genetic influences on gene expression (e.g., eQTLs) are cell type-specific, but most studies that attempt to clarify GWAS loci's influence on gene expression have employed tissues with mixed cell compositions that can obscure cell-specific effects. Furthermore, enriched SCZ heritability in the fetal brain underscores the need to study the impact of SCZ risk loci in specific developing neurons. MGE-derived cortical interneurons (cINs) are consistently affected in SCZ brains and show enriched SCZ heritability in human fetal brains. We identified SCZ GWAS risk genes that are dysregulated in iPSC-derived homogeneous populations of developing SCZ cINs. These SCZ GWAS loci differential expression (DE) genes converge on the PKC pathway. Their disruption results in PKC hyperactivity in developing cINs, leading to arborization deficits. We show that the fine-mapped GWAS locus in the ATP2A2 gene of the PKC pathway harbors enhancer marks by ATACseq and ChIPseq, and regulates ATP2A2 expression. We also generated developing glutamatergic neurons (GNs), another population with enriched SCZ heritability, and confirmed their functionality after transplantation into the mouse brain. Then, we identified SCZ GWAS risk genes that are dysregulated in developing SCZ GNs. GN-specific SCZ GWAS loci DE genes converge on the ion transporter pathway, distinct from those for cINs. Disruption of the pathway gene CACNA1D resulted in deficits of Ca

Shape-shifting, a phenomenon wide-spread in folklore, refers to the ability to physically change from one identity to another, typically from an innocuous entity to a destructive one. The amino acid D-serine over the last 25 years has "shape-shifted" into several identities: a purported glial transmitter activating N-methyl-D-aspartate receptors (NMDARs), a co-transmitter concentrated in excitatory glutamatergic neurons, an autocrine that is released at dendritic spines to prime their post-synaptic NMDARs for an instantaneous response to glutamate and an excitotoxic moiety released from inflammatory (A1) astrocytes. This article will review evidence in support of these scenarios and the artifacts that misled investigators of the true identity of D-serine.

International audience; Prefrontal control of cognitive functions critically depends upon glutamatergic transmission and N-methyl D-aspartate (NMDA) receptors, the activity of which is regulated by dopamine. Yet whether the NMDA receptor coagonist d -serine is implicated in the dopamine–glutamate dialogue in the prefrontal cortex (PFC) and other brain areas remains unexplored. Here, using electrophysiological recordings, we show that d -serine is required for the fine-tuning of glutamatergic neurotransmission, neuronal excitability, and synaptic plasticity in the PFC through the actions of dopamine at D 1 and D 3 receptors. Using in vivo microdialysis, we show that D 1 and D 3 receptors exert a respective facilitatory and inhibitory influence on extracellular levels and activity of d -serine in the PFC, with actions expressed primarily via the cAMP/protein kinase A (PKA) signaling cascade. Further, using functional magnetic resonance imaging (fMRI) and behavioral assessment, we show that d -serine is required for the potentiation of cognition by D 3 R blockade as revealed in a test of novel object recognition memory. Collectively, these results unveil a key role for d -serine in the dopaminergic neuromodulation of glutamatergic transmission and PFC activity, findings with clear relevance to the pathogenesis and treatment of diverse brain disorders involving alterations in dopamine–glutamate cross-talk.

IntroductionAbnormalities in electroencephalographic (EEG) biomarkers occur in patients with schizophrenia and those clinically at high risk for transition to psychosis and are associated with cognitive impairment. While the pathophysiology of schizophrenia remains poorly understood, converging evidence suggests N-methyl-D-aspartate receptor (NMDAR) hypofunction plays a central role and likely contributes to biomarker impairments. Thus, the characterization of such biomarkers is of significant interest for both the early diagnosis of schizophrenia and the development of novel treatments.MethodsWe utilized an established model of chronic NMDAR hypofunction, serine racemase knockout (SRKO) mice. In vivo EEG recording and behavioral analyses were performed on adult male and female SRKO mice and wild-type littermates to determine the impact of chronic NMDAR hypofunction on a battery of translationally-relevant electrophysiological biomarkers.ResultsSRKO mice displayed impairments in investigation-elicited gamma power that corresponded with reduced short-term social recognition. This impairment was associated with enhanced background (pre-investigation) broadband gamma activity that only appeared during social task performance. Additionally, SRKO mice exhibited sensory gating impairments, in both gamma power and event-related potential amplitude. However, other biomarkers such as the auditory steady-state response, sleep spindles, and state-specific power spectral density were generally neurotypical.ConclusionsSRKO mice provide a useful model to understand how chronic NMDAR hypofunction contributes to deficits in a subset of translationally-relevant EEG biomarkers that are altered in schizophrenia. Importantly, our gamma band findings support the hypothesis that an aberrant signal-to-noise ratio impairing cognition occurs with NMDAR hypofunction, which may be tied to impaired taskdependent alteration in functional connectivity.

d-serine is the primary NMDAR coagonist at mature forebrain synapses and is synthesized by the enzyme serine racemase (SR). However, our understanding of the mechanisms regulating the availability of synapticd-serine remains limited. Though early studies suggestedd-serine is synthesized and released from astrocytes, more recent studies have demonstrated a predominantly neuronal localization of SR. More specifically, recent work intriguingly suggests that SR may be found at the postsynaptic density, yet the functional implications of postsynaptic SR on synaptic transmission are not yet known. Here, we show an age-dependent dendritic and postsynaptic localization of SR andd-serine by immunohistochemistry and electron microscopy in mouse CA1 pyramidal neurons. In addition, using a single-neuron genetic approach in SR conditional KO mice from both sexes, we demonstrate a cell-autonomous role for SR in regulating synaptic NMDAR function at Schaffer collateral (CA3)-CA1 synapses. Importantly, single-neuron genetic deletion of SR resulted in the elimination of LTP at 1 month of age, which could be rescued by exogenousd-serine. Interestingly, there was a restoration of LTP by 2 months of age that was associated with an upregulation of synaptic GluN2B. Our findings support a cell-autonomous role for postsynaptic neuronal SR in regulating synaptic NMDAR function and suggests a possible autocrine mode ofd-serine action.SIGNIFICANCE STATEMENTNMDARs are key regulators of neurodevelopment and synaptic plasticity and are unique in their requirement for binding of a coagonist, which isd-serine at most forebrain synapses. However, our understanding of the mechanisms regulating synapticd-serine availability remains limited.d-serine is synthesized in the brain by the neuronal enzyme serine racemase (SR). Here, we show dendritic and postsynaptic localization of SR andd-serine in CA1 pyramidal neurons. In addition, using single-neuron genetic deletion of SR, we establish a role of postsynaptic SR in regulating NMDAR function. These results support an autocrine mode ofd-serine action at synapses.

d -Serine plays an important role in modulating N-methyl- d -aspartate receptor (NMDAR) neurotransmission in the mammalian brain by binding to the receptor’s glycine modulatory site (GMS). The cytosolic enzyme serine racemase (SR) converts L-serine to d -serine, while the peroxisomal enzyme d -amino acid oxidase (DAAO) catalyzes the breakdown of d -serine. Although it is important to understand how the activities of SR and DAAO regulate d -serine levels, very little is known about the mechanisms that regulate the expression of SR and DAAO. In this study, we investigated whether the different centrally active drugs affect the expression of SR and DAAO in adult mouse brain. We found that the NMDAR antagonist, MK801, and cocaine, psychotropic drugs that both augment glutamate release, reduce the expression of SR and DAAO. This regulation is brain region selective, and in the case of cocaine, is reversed in part byα-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX). However, d -serine and antipsychotics do not regulate SR and DAAO protein levels. In a genetic model of SR disruption, we found that DAAO expression was unaltered in SR conditional knockout mice, in which tissue d -serine content remains fairly stable despite marked reduction in SR expression. This study reveals a new mechanism by which AMPAR activity could regulate NMDAR function via d -serine availability.

d-serine is synthesized by serine racemase (SR) and is a co-agonist at forebrain N-methyl-d-aspartate receptors (NMDARs). d-serine and SR are expressed primarily in neurons, but not in quiescent astrocytes. In this study, we examined the localization of d-serine and SR in the mouse striatum and the effects of genetically silencing SR expression in GABAergic interneurons (iSR−/−). iSR−/− mice had substantially reduced SR expression almost exclusively in striatum, but only exhibited marginal d-serine reduction. SR positive cells in the striatum showed strong co-localization with dopamine- and cyclic AMP-regulated neuronal phosphoprotein (DARPP32) in wild type mice. Transgenic fluorescent reporter mice for either the D1 or D2 dopamine receptors exhibited a 65:35 ratio for co-localization with D1and D2 receptor positive cells, respectively. These results indicate that GABAergic medium spiny neurons receiving dopaminergic inputs in striatum robustly and uniformly express SR. In behavioral tests, iSR−/− mice showed a blunted response to the hedonic and stimulant effects of cocaine, without affecting anxiety-related behaviors. Because the cocaine effects have been shown in the constitutive SR−/− mice, the restriction of the blunted response to cocaine to iSR−/− mice reinforces the conclusion that d-serine in striatal GABAergic neurons plays an important role in mediating dopaminergic stimulant effects. Results in this study suggest that SR in striatal GABAergic neurons is synthesizing d-serine, not as a glutamatergic co-transmitter, but rather as an autocrine whereby the GABAergic neurons control the excitability of their NMDARs by determining the availability of the co-agonist, d-serine.

Deep layer III pyramidal cells in the dorsolateral prefrontal cortex (DLPFC) from subjects with schizophrenia and bipolar disorder previously were shown to exhibit dendritic arbor pathology. This study sought to determine whether MARCKS, its regulatory protein dysbindin-1, and two proteins, identified using microarray data, CDC42BPA and ARHGEF6, were associated with dendritic arbor pathology in the DLPFC from schizophrenia and bipolar disorder subjects. Using western blotting, relative protein expression was assessed in the DLPFC (BA 46) grey matter from subjects with schizophrenia (n = 19), bipolar disorder (n = 17) and unaffected control subjects (n = 19). Protein expression data were then correlated with dendritic parameter data obtained previously. MARCKS and dysbindin-1a expression levels did not differ among the three groups. Dysbindin-1b expression was 26% higher in schizophrenia subjects (p = 0.01) and correlated inversely with basilar dendrite length (r = -0.31, p = 0.048) and the number of spines per basilar dendrite (r = -0.31, p = 0.048), but not with dendritic spine density (r = -0.16, p = 0.32). The protein expression of CDC42BPA was 33% higher in schizophrenia subjects (p = 0.03) but, did not correlate with any dendritic parameter (p > 0.05). ARHGEF6 87 kDa isoform expression did not differ among the groups. CDC42BPA expression was not altered in frontal cortex from rats chronically administered haloperidol or clozapine. Dysbindin-1b appears to play a role in dendritic arbor pathology observed previously in the DLPFC in schizophrenia.

Background The amygdala is a central component of the neural circuitry that underlies fear learning. N-methyl-D-aspartate receptor–dependent plasticity in the amygdala is required for pavlovian fear conditioning and extinction. N-methyl-D-aspartate receptor activation requires the binding of a coagonist, D-serine, which is synthesized from L-serine by the neuronal enzyme serine racemase (SR). However, little is known about SR and D-serine function in the amygdala. Methods We used immunohistochemical methods to characterize the cellular localization of SR and D-serine in the mouse and human amygdala. Using biochemical and molecular techniques, we determined whether trace fear conditioning and extinction engages the SR/D-serine system in the brain. D-serine was administered systemically to mice to evaluate its effect on fear extinction. Finally, we investigated whether the functional single nucleotide polymorphism rs4523957, which is an expression quantitative trait locus of the human serine racemase (SRR) gene, was associated with fear-related phenotypes in a highly traumatized human cohort. Results We demonstrate that approximately half of the neurons in the amygdala express SR, including both excitatory and inhibitory neurons. We find that the acquisition and extinction of fear memory engages the SR/D-serine system in the mouse amygdala and that D-serine administration facilitates fear extinction. We also demonstrate that the SRR single nucleotide polymorphism, rs4523957, is associated with posttraumatic stress disorder in humans, consistent with the facilitatory effect of D-serine on fear extinction. Conclusions These new findings have important implications for understanding D-serine–mediated N-methyl-D-aspartate receptor plasticity in the amygdala and how this system could contribute to disorders with maladaptive fear circuitry.

cAMP-response-element-binding protein (CREB) is a transcription factor ubiquitously expressed in the brain that regulates neuroplasticity by modulating gene expression. The influx of calcium through N-methyl-d-aspartate receptors (NMDARs) is a well-defined mechanism that leads to the increased expression of CREB-dependent genes, including brain derived neurotrophic factor (BDNF), microRNA-132, and activity-regulated cytoskeleton-associated protein (Arc). These molecules are implicated in the pathophysiology of schizophrenia. We previously demonstrated that serine racemase knockout (SR-/-) mice, which exhibit NMDAR hypofunction due to a lack of the forebrain NMDAR co-agonist d-serine, also have reduced expression of CREB-dependent genes in the hippocampus. Using chromatin immunoprecipitation, we show here that, in SR-/- mice, there is less CREB bound to the promoter regions of BDNF, microRNA-132, and Arc. These data suggest that NMDAR hypofunction in SR-/- mice leads to reduced CREB binding on known activity-dependent genes, in turn contributing to their reduced expression.

Schizophrenia is a chronic disabling mental disorder that affects about 1% population world-wide, for which there is a desperate need to develop more effective treatments. In this minireview, we summarize the findings from recent studies using induced pluripotent stem cells to model the developmental pathogenesis of schizophrenia and discuss what we have learned from these studies. We also discuss what are the important next steps and key issues to be addressed to move the field forward.

Although β-amyloid plaques are a well-recognized hallmark of Alzheimer's disease (AD) neuropathology, no drugs reducing amyloid burden have shown efficacy in clinical trials, suggesting that once AD symptoms emerge, disease progression becomes independent of Aβ production. Reactive astrocytes are another neuropathological feature of AD, where there is an emergence of neurotoxic (A1) reactive astrocytes. We find that serine racemase (SR), the neuronal enzyme that produces the N-methyl-d-aspartate receptor (NMDAR) co-agonist d-serine, is robustly expressed in A1-reactive neurotoxic astrocytes in the hippocampus and entorhinal cortex of AD subjects and an AD rat model. Furthermore, we observe intracellular signaling changes consistent with increased extra-synaptic NMDAR activation, excitotoxicity and decreased neuronal survival. Thus, reducing neurotoxic d-serine release from A1 inflammatory astrocytes could have therapeutic benefit for mild to advanced AD, when anti-amyloid strategies are ineffective.

The last two decades have witnessed remarkable advance in our understanding the role of d-amino acids in the mammalian nervous system: from the unknown, to known molecules with unknown functions, to potential central players in health and disease. d-Amino acids have emerged as an important class of signaling molecules. In particular, the exploration of the roles of d-serine in brain physiopathology is a vibrant field that is growing at an accelerating pace. However, disentangling the functions of a chiral molecule in a complex chemical matrice as the brain requires specific measurement and detection methods but is also a challenging task as many molecular tools and models investigators are using can lead to confounded observations. Thus, study of d-amino acids demands accurate methodologies and specific controls, and these have often been lacking. Here we outline best practices for d-amino acid research, with a special emphasis on d-serine. We hope these concepts help move the field to greater rigor and reproducibility, allowing the field to advance.

Traumatic brain injury (TBI), ranging from mild concussion to severe penetrating wounds, can involve brain regions that contain damaged or lost synapses in the absence of neuronal death. These affected regions significantly contribute to sensory, motor and/or cognitive deficits. Thus, studying the mechanisms responsible for synaptic instability and dysfunction is important for protecting the nervous system from the consequences of progressive TBI. Our controlled cortical impact (CCI) injury produces ~20% loss of synapses and mild changes in synaptic protein levels in the CA3-CA1 hippocampus without neuronal losses. These synaptic changes are associated with functional deficits, indicated by >50% loss in synaptic plasticity and impaired learning behavior. We show that the receptor tyrosine kinase EphB3 participates in CCI injury-induced synaptic damage, where EphB3−/− mice show preserved long-term potentiation and hippocampal-dependent learning behavior as compared with wild type (WT) injured mice. Improved synaptic function in the absence of EphB3 results from attenuation in CCI injury-induced synaptic losses and reduced d-serine levels compared with WT injured mice. Together, these findings suggest that EphB3 signaling plays a deleterious role in synaptic stability and plasticity after TBI.

Recent developments in precision gene editing have led to the emergence of the marmoset as an experimental subject of considerable interest and translational value. A better understanding of behavioral phenotypes of the common marmoset will inform the extent to which forthcoming transgenic mutants are cognitively intact. Therefore, additional information regarding their learning, inhibitory control, and motivational abilities is needed. The present studies used touchscreen-based repeated acquisition and discrimination reversal tasks to examine basic dimensions of learning and response inhibition. Marmosets were trained daily to respond to one of the two simultaneously presented novel stimuli. Subjects learned to discriminate the two stimuli (acquisition) and, subsequently, with the contingencies switched (reversal). In addition, progressive ratio performance was used to measure the effort expended to obtain a highly palatable reinforcer varying in magnitude and, thereby, provide an index of relative motivational value. Results indicate that rates of both acquisition and reversal of novel discriminations increased across successive sessions, but that rate of reversal learning remained slower than acquisition learning, i.e., more trials were needed for mastery. A positive correlation was observed between progressive ratio break point and reinforcement magnitude. These results closely replicate previous findings with squirrel monkeys, thus providing evidence of similarity in learning processes across nonhuman primate species. Moreover, these data provide key information about the normative phenotype of wild-type marmosets using three relevant behavioral endpoints.

There is substantial evidence that NMDA receptor (NMDAR) hypofunction contributes to the pathophysiology of schizophrenia (SCZ). A recent large-scale genome-wide association study identified serine racemase (SR), the enzyme that produces the NMDAR co-agonist D-serine, as a risk gene for SCZ. Serine racemase knockout (SR-/-) mice, which lack D-serine, exhibit many of the neurochemical and behavioral abnormalities observed in SCZ. Metabotropic glutamate receptor 5 (mGlu5)-positive allosteric modulators (PAMs) are currently being developed to treat cognitive dysfunction. We used in vitro electrophysiology to determine whether the mGlu5 PAM VU0409551 directly enhances NMDAR function in hippocampal slices from adult male SR-/- mice. We administered VU0409551 systemically for 5 days to adult male wild-type C57BL/6 animals to determine the optimal dose to test in SR-/- mice. We used western blot analyses and trace-fear conditioning to determine whether 5 days of VU0409551 treatment could reverse the neuroplasticity and learning deficits, respectively, in SR-/- mice. We show that VU0409551 enhances NMDAR function and rescues long-term potentiation in hippocampal slices obtained from SR-/- mice. Systemic treatment with VU0409551 (10 and 30 mg/kg) to wild-type mice causes a dose-dependent increase in the Akt/GS3Kα/β signaling pathway, which is reduced in SR-/- mice and in SCZ. Furthermore, the administration of VU0409551 to SR-/- mice reverses their deficits in several neuroplasticity signaling pathways and improves their contextual fear memory. These results support positive allosteric modulation of mGlu5, particularly with VU0409551, as a viable mechanism to reverse the deficits in NMDAR function, synaptic plasticity, and memory that are known to be impaired in SCZ.

Purpose Mesopic flash electroretinography (fERG) as a tool to identify N-methyl-d-aspartate receptor (NMDAR) hypofunction in subjects with schizophrenia shows great potential. We report the first fERG study in a genetic mouse model of schizophrenia characterized by NMDAR hypofunction from gene silencing of serine racemase (SR) expression (SR-/-), an established risk gene for schizophrenia. We analyzed fERG parameters under various background light adaptations to determine the most significant variables to allow for early identification of people at risk for schizophrenia, prior to onset of psychosis. SR is a risk gene for schizophrenia, and negative and cognitive symptoms antedate the onset of psychosis that is required for diagnosis. Methods The scotopic, photopic, and mesopic fERGs were analyzed in male and female mice in both SR-/- and wild-type (WT) mice and also analyzed for sex differences. Amplitude and implicit time of the a- and b-wave components, b-/a-wave ratio, and Fourier transform analysis were analyzed. Results Mesopic a- and b-wave implicit times were significantly delayed, and b-wave amplitudes, b/a ratios, and Fourier transform were significantly decreased in the male SR-/- mice compared to WT, but not in female SR-/- mice. No significant differences were observed in photopic or scotopic fERGs between genotype. Conclusions The fERG prognostic capability may be improved by examination of background light adaptation, a larger array of light intensities, considering sex as a variable, and performing Fourier transform analyses of all waveforms. This should improve the ability to differentiate between controls and subjects with schizophrenia characterized by NMDAR hypofunction.

Schizophrenia is a chronic and severe psychiatric disorder that has profound impact on an individual's life and on society. Thus, developing more effective therapeutic interventions is essential. Over the past quarter-century, an abundance of evidence from pharmacologic challenges, post-mortem studies, brain imaging, and genetic studies supports the role of glutamatergic dysregulation in the pathophysiology of schizophrenia, and the results of recent randomized clinical trials based on this evidence have yielded promising results. In this article, we review the evidence that alterations in glutamatergic neurotransmission, especially focusing on the N-methyl-d-aspartate receptor (NMDAR) function, may be a critical causative feature of schizophrenia, how this contributes to pathologic circuit function in the brain, and how these insights are revealing whole new avenues for treatment development that could reduce treatment-resistant symptoms, which account for persistent disability.

The N-methyl-D-aspartate receptor (NMDAR) is unique in requiring two agonists to bind simultaneously to open its cation channel: the neurotransmitter, glutamate, and the coagonists, glycine, or D-serine. The Snyder laboratory was the first to clone serine racemase (SR), the enzyme that synthesizes D-serine, and to localize it immunocytochemically. Our laboratory has focused on the role of D-serine in brain disorders. Silencing the expression of SR, a risk gene for schizophrenia (SCZ), in mice (SR−/−), results in a phenotype that closely resembles SCZ including: cortical atrophy, reduced dendritic spine density and complexity, downregulation of parvalbumin-positive cortical GABAergic neurons, and cognitive impairments. This pathology can be reversed by treatment of SR−/− mice with D-serine in adulthood. SR−/− mice also exhibit abnormal response toward abusable substances, such as stimulants. They show reduced behavioral sensitization to D-amphetamine, but fail to extinguish it. Place preference to cocaine is altered, and the hedonic response to it is profoundly impaired as assessed by intracranial self-stimulation. D-cycloserine, a partial agonist at the NMDAR glycine modulatory site, shows therapeutic benefit for treating pathologic anxiety in combination with behavioral therapies. Studies in vitro with cortical culture and in vivo with middle cerebral artery occlusion show that silencing SR provides substantial protection against ischemic neuronal death. Finally, the switch of SR expression from neurons to reactive astrocytes after closed head trauma accounts for the reduced in vivo neuro-plasticity, electroencephalogram abnormalities, and cognitive impairments.

In the hippocampus of mice lacking the gene for serine racemase (SR), a D-serine synthesizing enzyme, in the CaMKIIα-expressing neurons, we observed a significant decrease in the extracellular concentration of D-serine, a coagonist for the N-methyl-D-aspartate type glutamate receptor (NMDAR), and NMDAR hypofunction as revealed by diminished extracellular taurine concentrations after an intra-hippocampal NMDA infusion when compared to the wild type controls. Therefore, the neuronal SR could regulate the extracellular D-serine signaling responsible for NMDAR activation in the hippocampus.

d-Serine is a co-agonist at forebrain N-methyl-d-aspartate receptors (NMDAR) and is synthesized by serine racemase (SR). Although d-serine and SR were originally reported to be localized to glia, recent studies have provided compelling evidence that under healthy physiologic conditions both are localized primarily in neurons. However, in pathologic conditions, reactive astrocytes can also express SR and synthesize d-serine. Since cultured astrocytes exhibit features of reactive astrocytes, we have characterized d-serine synthesis and the expression of enzymes involved in its disposition in primary glial cultures. The levels of SR were quite low early in culture and increased markedly in all astrocytes with the duration in vitro. The concentration of d-serine in the culture medium increased in parallel with SR expression in the astrocytes. Microglia, identified by robust expression of Iba1, did not express SR. While the levels of glial fibrillary acidic protein (GFAP), glycine decarboxylase (GLDC) and phosphoglycerate dehydrogenase (PHGDH), the initial enzyme in the pathway converting glycine to l-serine, remained constant in culture, the expression of lipocalin-2, a marker for pan-reactive astrocytes, increased several-fold. The cultured astrocytes also expressed Complement-3a, a marker for a subpopulation of reactive astrocytes (A1). Astrocytes grown from mice with a copy number variant associated with psychosis, which have four copies of the GLDC gene, showed a more rapid production of d-serine and a reduction in glycine in the culture medium. These results substantiate the conclusion that A1 reactive astrocytes express SR and release d-serine under pathologic conditions, which may contribute to their neurotoxic effects by activating extra-synaptic NMDARs.

Both schizophrenia (SZ) and substance abuse (SA) exhibit significant heritability. Moreover, N-methyl-d-aspartate receptors (NMDARs) have been implicated in the pathophysiology of both SZ and SA. We hypothesize that the high prevalence of comorbid SA in SZ is due to dysfunction of NMDARs caused by shared risk genes. We used transgenic mice with a null mutation of the gene encoding serine racemase (SR), the enzyme that synthesizes the NMDAR co-agonist d-serine and an established risk gene for SZ, to recreate the pathology of SZ. We determined the effect of NMDAR hypofunction resulting from the absence of d-serine on motivated behavior by using intracranial self-stimulation and neurotransmitter release in the nucleus accumbens by using in vivo microdialysis. Compared with wild-type mice, SR-/- mice exhibited similar baseline intracranial self-stimulation thresholds but were less sensitive to the threshold-lowering (rewarding) and the performance-elevating (stimulant) effects of cocaine. While basal dopamine (DA) and glutamate release were elevated in the nucleus accumbens of SR-/- mice, cocaine-induced increases in DA and glutamate release were blunted. γ-Amino-butyric acid efflux was unaffected in the SR-/- mice. Together, these findings suggest that the impaired NMDAR function and a consequent decrease in sensitivity to cocaine effects on behavior are mediated by blunted DA and glutamate responses normally triggered by the drug. Projected to humans, NMDAR hypofunction due to mutations in SR or other genes impacting glutamatergic function in SZ may render abused substances less potent and effective, thus requiring higher doses to achieve a hedonic response, resulting in elevated drug exposure and increased dependence/addiction.

Schizophrenia is a chronic severe mental disorder characterized by psychosis, cognitive impairments, and social and motivational deficits. It is associated with a progressive loss of cortical volume after onset of psychosis; nevertheless, cortical atrophy correlates with the cognitive impairments and the negative symptoms but not with the psychosis. The cortical atrophy is not primarily due to neuronal degeneration but rather to neuronal atrophy and loss of glutamatergic synapses. A downregulation of the presynaptic markers for the parvalbumin-expressing GABAergic interneurons that provide recurrent inhibition to cortical pyramidal neurons is another consistent pathologic feature. Antipsychotic drugs continue after 50 years to be the mainstay of treatment although these drugs, with the possible exception of clozapine, have negligible effects on cognition and negative symptoms. Pharmacologic challenge studies, postmortem analyses and a recent sufficiently powered genome-wide association study and copy number variant studies provide compelling evidence that NMDA receptor hypofunction is an important pathophsysiologic feature of schizophrenia. Silencing the gene encoding serine racemase, the enzyme that synthesizes the cortical-limbic NMDA receptor co-agonist, D-serine, replicates the dendritic and GABAergic pathology and cognitive deficits of schizophrenia in mice. Pharmacologic strategies to overcome NMDA receptor hypofunction hold promise of treating the disabling cognitive and negative symptoms.

d-Serine modulates N-methyl d-aspartate receptors (NMDARs) and regulates synaptic plasticity, neurodevelopment, and learning and memory. However, the primary site of d-serine synthesis and release remains controversial, with some arguing that it is a gliotransmitter and others defining it as a neuronal cotransmitter. Results from several laboratories using different strategies now show that the biosynthetic enzyme of d-serine, serine racemase (SR), is expressed almost entirely by neurons, with few astrocytes appearing to contain d-serine. Cell-selective suppression of SR expression demonstrates that neuronal, rather than astrocytic d-serine, modulates synaptic plasticity. Here, we propose an alternative conceptualization whereby astrocytes affect d-serine levels by synthesizing l-serine that shuttles to neurons to fuel the neuronal synthesis of d-serine.

The molecular pathogenesis of bipolar disorder (BPD) is poorly understood. Using human-induced pluripotent stem cells (hiPSCs) to unravel such mechanisms in polygenic diseases is generally challenging. However, hiPSCs from BPD patients responsive to lithium offered unique opportunities to discern lithium's target and hence gain molecular insight into BPD. By profiling the proteomics of BDP-hiPSC-derived neurons, we found that lithium alters the phosphorylation state of collapsin response mediator protein-2 (CRMP2). Active non-phosphorylated CRMP2, which binds cytoskeleton, is present throughout the neuron; inactive phosphorylated CRMP2, which dissociates from cytoskeleton, exits dendritic spines. CRMP2 elimination yields aberrant dendritogenesis with diminished spine density and lost lithium responsiveness (LiR). The "set-point" for the ratio of pCRMP2: CRMP2 is elevated uniquely in hiPSC-derived neurons from LiR BPD patients, but not with other psychiatric (including lithium-nonresponsive BPD) and neurological disorders. Lithium (and other pathway modulators) lowers pCRMP2, increasing spine area and density. Human BPD brains show similarly elevated ratios and diminished spine densities; lithium therapy normalizes the ratios and spines. Consistent with such "spine-opathies," human LiR BPD neurons with abnormal ratios evince abnormally steep slopes for calcium flux; lithium normalizes both. Behaviorally, transgenic mice that reproduce lithium's postulated site-of-action in dephosphorylating CRMP2 emulate LiR in BPD. These data suggest that the " lithium response pathway" in BPD governs CRMP2's phosphorylation, which regulates cytoskeletal organization, particularly in spines, modulating neural networks. Aberrations in the posttranslational regulation of this developmentally critical molecule may underlie LiR BPD pathogenesis. Instructively, examining the proteomic profile in hiPSCs of a functional agent-even one whose mechanism-of-action is unknown-might reveal otherwise inscrutable intracellular pathogenic pathways., Funding Agencies:NIH's Library of Integrated Network-based Cellular Signatures Program Viterbi Foundation Neuroscience Initiative Stanley Medical Research Institute R21MH093958 R33MH087896 R01MH095088 Tau Consortium California Institute of Regenerative Medicine training grants University of California, San Diego T32 training grant in psychiatry California Bipolar Foundation International Bipolar Foundation Creation of Innovation Centers for Advanced Interdisciplinary Research Areas Program in the Project for Developing Innovation Systems from the Ministry of Education, Science, Sports and Culture in Japan 42890001 RC2MH090011

Glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 are incretins secreted by respective K and L enteroendocrine cells after eating and amplify glucose-stimulated insulin secretion (GSIS). This amplification has been termed the “incretin response.” To determine the role(s) of K cells for the incretin response and type 2 diabetes mellitus (T2DM), diphtheria toxin–expressing (DT) mice that specifically lack GIP-producing cells were backcrossed five to eight times onto the diabetogenic NONcNZO10/Ltj background. As in humans with T2DM, DT mice lacked an incretin response, although GLP-1 release was maintained. With high-fat (HF) feeding, DT mice remained lean but developed T2DM, whereas wild-type mice developed obesity but not diabetes. Metabolomics identified biochemicals reflecting impaired glucose handling, insulin resistance, and diabetes complications in prediabetic DT/HF mice. β-Hydroxypyruvate and benzoate levels were increased and decreased, respectively, suggesting β-hydroxypyruvate production from d-serine. In vitro, β-hydroxypyruvate altered excitatory properties of myenteric neurons and reduced islet insulin content but not GSIS. β-Hydroxypyruvate–to–d-serine ratios were lower in humans with impaired glucose tolerance compared with normal glucose tolerance and T2DM. Earlier human studies unmasked a neural relay that amplifies GIP-mediated insulin secretion in a pattern reciprocal to β-hydroxypyruvate–to–d-serine ratios in all groups. Thus, K cells may maintain long-term function of neurons and β-cells by regulating β-hydroxypyruvate levels.

Both the serotonin and glutamate systems have been implicated in the pathophysiology of schizophrenia, as well as in the mechanism of action of antipsychotic drugs. Psychedelic drugs act through the serotonin 2A receptor (5-HT2AR), and elicit a head-twitch response (HTR) in mice, which directly correlates to 5-HT2AR activation and is absent in 5-HT2AR knockout mice. The precise mechanism of this response remains unclear, but both an intrinsic cortico-cortical pathway and a thalamo-cortical pathway involving glutamate release have been proposed. Here, we used a genetic model of NMDAR hypofunction, a serine racemase knockout (SRKO) mouse, to explore the role of glutamatergic transmission in regulating the 5-HT2AR-mediated cellular and behavioral responses. SRKO mice treated with the 5-HT2AR agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished HTR and lower induction of c-fos mRNA. These altered functional responses in SRKO mice were not associated with changes in cortical or hippocampal 5-HT levels or in 5-HT2AR and metabotropic glutamate-2 receptor (mGluR2) mRNA and protein expression. Together, these findings suggest that D-serine-dependent NMDAR activity is involved in mediating the cellular and behavioral effects of 5-HT2AR activation.

d-Serine, a co-agonist at the NMDA receptor (NMDAR), is synthesized from l-serine by the enzyme serine racemase (SR), which is heavily expressed in the forebrain. Although SR was originally reported to be localized exclusively to astrocytes, recent conditional knock out results demonstrate that little SR is expressed in forebrain astrocytes. As a consequence, the cellular location of its product, d-serine, in the brain is also uncertain. Immunocytochemistry now indicates that SR is expressed primarily in forebrain glutamatergic neurons with the remainder in GABAergic interneurons. We utilized SR deficient (SR–/–) mice, which have

Schizophrenia is a severe, persistent, and fairly common mental illness. Haloperidol is widely used and is effective against the symptoms of psychosis seen in schizophrenia. Chronic oral haloperidol administration decreased the number of astrocytes in the parietal cortex of macaque monkeys (Konopaske et al., Biol Psych 63:759-765, 2008). Since astrocytes play a key role in glutamate metabolism, chronic haloperidol administration was hypothesized to modulate astrocyte metabolic function and glutamate homeostasis.This study investigated the effects of chronic haloperidol administration on astrocyte metabolic activity and glutamate, glutamine, and GABA homeostasis.We used ex vivo ¹³C magnetic resonance spectroscopy along with high-performance liquid chromatography after [1-¹³C]glucose and [1,2-¹³C]acetate administration to analyze forebrain tissue from rats administered oral haloperidol for 1 or 6 months.Administration of haloperidol for 1 month produced no changes in ¹³C labeling of glutamate, glutamine, or GABA, or in their total levels. However, a 6-month haloperidol administration increased ¹³C labeling of glutamine by [1,2-¹³C]acetate. Moreover, total GABA levels were also increased. Haloperidol administration also increased the acetate/glucose utilization ratio for glutamine in the 6-month cohort.Chronic haloperidol administration in rats appears to increase forebrain GABA production along with astrocyte metabolic activity. Studies exploring these processes in subjects with schizophrenia should take into account the potential confounding effects of antipsychotic medication treatment.

After traumatic brain injury (TBI), glial cells have both beneficial and deleterious roles in injury progression and recovery. However, few studies have examined the influence of reactive astrocytes in the tripartite synapse following TBI. Here, we have demonstrated that hippocampal synaptic damage caused by controlled cortical impact (CCI) injury in mice results in a switch from neuronal to astrocytic d-serine release. Under nonpathological conditions, d-serine functions as a neurotransmitter and coagonist for NMDA receptors and is involved in mediating synaptic plasticity. The phasic release of neuronal d-serine is important in maintaining synaptic function, and deficiencies lead to reductions in synaptic function and plasticity. Following CCI injury, hippocampal neurons downregulated d-serine levels, while astrocytes enhanced production and release of d-serine. We further determined that this switch in the cellular source of d-serine, together with the release of basal levels of glutamate, contributes to synaptic damage and dysfunction. Astrocyte-specific elimination of the astrocytic d-serine-synthesizing enzyme serine racemase after CCI injury improved synaptic plasticity, brain oscillations, and learning behavior. We conclude that the enhanced tonic release of d-serine from astrocytes after TBI underlies much of the synaptic damage associated with brain injury.

Parvalbumin inhibitory interneurons (PVIs) are crucial for maintaining proper excitatory/inhibitory balance and high-frequency neuronal synchronization. Their activity supports critical developmental trajectories, sensory and cognitive processing, and social behavior. Despite heterogeneity in the etiology across schizophrenia and autism spectrum disorder, PVI circuits are altered in these psychiatric disorders. Identifying mechanism(s) underlying PVI deficits is essential to establish treatments targeting in particular cognition. On the basis of published and new data, we propose oxidative stress as a common pathological mechanism leading to PVI impairment in schizophrenia and some forms of autism. A series of animal models carrying genetic and/or environmental risks relevant to diverse etiological aspects of these disorders show PVI deficits to be all accompanied by oxidative stress in the anterior cingulate cortex. Specifically, oxidative stress is negatively correlated with the integrity of PVIs and the extracellular perineuronal net enwrapping these interneurons. Oxidative stress may result from dysregulation of systems typically affected in schizophrenia, including glutamatergic, dopaminergic, immune and antioxidant signaling. As convergent end point, redox dysregulation has successfully been targeted to protect PVIs with antioxidants/redox regulators across several animal models. This opens up new perspectives for the use of antioxidant treatments to be applied to at-risk individuals, in close temporal proximity to environmental impacts known to induce oxidative stress.

Although glutamate was first hypothesized to be involved in the pathophysiology of schizophrenia in the 1980s, it was the demonstration that N-methyl-D-aspartate (NMDA) receptor antagonists, the dissociative anesthetics, could replicate the full range of psychotic, negative, cognitive, and physiologic features of schizophrenia in normal subjects that placed the "NMDA receptor hypofunction hypothesis" on firm footing. Additional support came from the demonstration that a variety of agents that enhanced NMDA receptor function at the glycine modulatory site significantly reduced negative symptoms and variably improved cognition in patients with schizophrenia receiving antipsychotic drugs. Finally, persistent blockade of NMDA receptors recreates in experimental animals the critical pathologic features of schizophrenia including downregulation of parvalbumin-positive cortical GABAergic neurons, pyramidal neuron dendritic dysgenesis, and reduced spine density.

d -Serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N -methyl- d -aspartate receptor (NMDAR). In an animal model of NMDAR hypofunction, the constitutive SR knockout (SR−/−) mouse, pyramidal neurons in primary somatosensory cortex (S1) have reductions in the complexity, total length, and spine density of apical and basal dendrites. We wondered whether the dendritic pathology required deprivation of d -serine throughout development or reflected the loss of d -serine only in adulthood. To address this question, we used mice homozygous for floxed SR in which we bred CaMKIICre2834, which is expressed in forebrain glutamatergic neurons starting at 3–4 weeks post-partum (nSR−/−). Our prior studies demonstrated that the majority of cortical SR is expressed in glutamatergic neurons. We found that similar to SR−/− mice, pyramidal neurons in S1 of nSR−/− also had significantly reduced dendritic arborization and spine density, albeit to a lesser degree. S1 neurons of nSR−/− mice had reduced total basal dendritic length that was accompanied by less complex arborization. These characteristics were unaltered in the apical dendritic compartment. In contrast, spine density on S1 neurons was significantly reduced on apical, but not basal dendrites of nSR−/− mice. These results demonstrate that in adulthood neuronally derived d -serine, which is required for optimal activation of post-synaptic NMDAR activity, regulates pyramidal neuron dendritic arborization and spine density. Moreover, they highlight the glycine modulatory site (GMS) of the NMDAR as a potential target for therapeutic intervention in diseases characterized by synaptic deficits, like schizophrenia.

At central synapses, glutamate is the main excitatory neurotransmitter. Once released from presynaptic terminals, glutamate activates a number of different glutamatergic receptors one of which is the ligand gated ionophore glutamatergic subtype N-methyl-D-aspartate receptors (NMDARs). NMDARs play a crucial role in controlling various determinants of synaptic function. N-acetylaspartylglutamate (NAAG) is the most prevalent peptide transmitter in the mammalian central nervous system. NAAG is released upon neuronal depolarization by a calcium-dependent process from glutamatergic and GABAergic neurons. It is cleaved by a specific peptidase located on astrocytes, glutamate carboxypeptidase type II (GCP-II), to N-acetylaspartate (NAA) and glutamate. Current evidence supports the hypothesis that NAAG is an endogenous agonist at G protein coupled mGluR3 receptors and an antagonist at NMDAR. In several disorders and animal models of human diseases, the levels of NAAG and the activity of GCP-II are altered in ways that are consistent with NAAG's role in regulation of glutamatergic neurotransmission. Several lines of evidence suggest that a dysfunction in glutamatergic via the NMDAR might be involved in schizophrenia. This hypothesis has evolved from findings that NMDAR antagonists such as phencyclidine (PCP or "angel dust"), produces a syndrome in normal individuals that closely resembles schizophrenia and exacerbates psychotic symptoms in patients with chronic schizophrenia. Recent postmortem, metabolic and genetic studies have provided evidence that hypofunction of discrete populations of NMDAR can contribute to the symptoms of schizophrenia, at least in some patients. The review outlines the role of endogenous NAAG at NMDAR neurotransmission and its putative role in the pathophysiology of schizophrenia.

D-Serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N-methyl-D-aspartate receptor (NMDAR). Crucial to an understanding of the signaling functions of D-serine is defining the sites responsible for its synthesis and release. In order to quantify the contributions of astrocytes and neurons to SR and D-serine localization, we used recombinant DNA techniques to effect cell type selective suppression of SR expression in astrocytes (aSRCKO) and in forebrain glutamatergic neurons (nSRCKO). The majority of SR is expressed in neurons: SR expression was reduced by ~65% in nSRCKO cerebral cortex and hippocampus, but only ~15% in aSRCKO as quantified by western blots. In contrast, nSRCKO is associated with only modest decreases in D-serine levels as quantified by HPLC, whereas D-serine levels were unaffected in aSRCKO mice. Liver expression of SR was increased by 35% in the nSRCKO, suggesting a role for peripheral SR in the maintenance of brain D-serine. Electrophysiologic studies of long-term potentiation (LTP) at the Schaffer collateral–CA1 pyramidal neuron synapse revealed no alterations in the aSRCKO mice versus wild-type. LTP induced by a single tetanic stimulus was reduced by nearly 70% in the nSRCKO mice. Furthermore, the mini-excitatory post-synaptic currents mediated by NMDA receptors but not by AMPA receptors were significantly reduced in nSRCKO mice. Our findings indicate that in forebrain, where D-serine appears to be the endogenous co-agonist at NMDA receptors, SR is predominantly expressed in glutamatergic neurons, and co-release of glutamate and D-serine is required for optimal activation of post-synaptic NMDA receptors.

There is substantial evidence, both pharmacological and genetic, that hypofunction of the N-methyl-d-aspartate receptor (NMDAR) is a core pathophysiological feature of schizophrenia. There are morphological brain changes associated with schizophrenia, including perturbations in the dendritic morphology of cortical pyramidal neurons and reduction in cortical volume. Our experiments investigated whether these changes in dendritic morphology could be recapitulated in a genetic model of NMDAR hypofunction, the serine racemase knockout (SR-/-) mouse. Pyramidal neurons in primary somatosensory cortex (S1) of SR-/- mice had reductions in the complexity, total length, and spine density of apical and basal dendrites. In accordance with reduced cortical neuropil, SR-/- mice also had reduced cortical volume as compared to wild type mice. Analysis of S1 mRNA by DNA microarray and gene expression analysis revealed gene changes in SR-/- that are associated with psychiatric and neurologic disorders, as well as neurodevelopment. The microarray analysis also identified reduced expression of brain derived neurotrophic factor (BDNF) in SR-/- mice. Follow-up analysis by ELISA confirmed a reduction of BDNF protein levels in the S1 of SR-/- mice. Finally, S1 pyramidal neurons in glycine transporter heterozygote (GlyT1+/-) mutants, which display enhanced NMDAR function, had increased dendritic spine density. These results suggest that proper NMDAR function is important for the arborization and spine density of pyramidal neurons in cortex. Moreover, they suggest that NMDAR hypofunction might, in part, be contributing to the dendritic and synaptic changes observed in schizophrenia and highlight this signaling pathway as a potential target for therapeutic intervention.

Schizophrenia is a severe mental illness that afflicts nearly 1% of the world's population. One of the cardinal pathological features of schizophrenia is perturbation in synaptic connectivity. Although the etiology of schizophrenia is unknown, it appears to be a developmental disorder involving the interaction of a potentially large number of risk genes, with no one gene producing a strong effect except rare, highly penetrant copy number variants. The purpose of this review is to detail how putative schizophrenia risk genes (DISC-1, neuregulin/ErbB4, dysbindin, Akt1, BDNF, and the NMDA receptor) are involved in regulating neuroplasticity and how alterations in their expression may contribute to the disconnectivity observed in schizophrenia. Moreover, this review highlights how many of these risk genes converge to regulate common neurotransmitter systems and signaling pathways. Future studies aimed at elucidating the functions of these risk genes will provide new insights into the pathophysiology of schizophrenia and will likely lead to the nomination of novel therapeutic targets for restoring proper synaptic connectivity in the brain in schizophrenia and related disorders.

There is substantial evidence implicating N-methyl-d-aspartate receptors (NMDARs) in memory and cognition. It has also been suggested that NMDAR hypofunction might underlie the cognitive deficits observed in schizophrenia since morphological changes, including alterations in the dendritic architecture of pyramidal neurons in the prefrontal cortex (PFC), have been reported in the schizophrenic brain post mortem. Here, we used a genetic model of NMDAR hypofunction, a serine racemase knockout (SR−/−) mouse in which the first coding exon of the mouse serine racemase gene has been deleted, to explore the role of d-serine in regulating cognitive functions as well as dendritic architecture. SR −/− mice exhibited a significantly disrupted representation of the order of events in distinct experiences as revealed by object recognition and odor sequence tests; however, SR −/− animals were unimpaired in the detection of novel objects and in spatial displacement, and showed intact relational memory in a test of transitive inference. In addition, SR −/− mice exhibited normal sociability and preference for social novelty. Neurons in the medial PFC of SR−/− mice displayed reductions in the complexity, total length, and spine density of apical dendrites. These findings demonstrate that d-serine is important for specific aspects of cognition, as well as in regulating dendritic morphology of pyramidal neurons in the mPFC. Moreover, they suggest that NMDAR hypofunction might, in part, be responsible for the cognitive deficits and synaptic changes associated with schizophrenia, and highlight this signaling pathway as a potential target for therapeutic intervention.

All current drugs approved to treat schizophrenia appear to exert their antipsychotic effects through blocking the dopamine D2 receptor. Recent meta-analyses and comparative efficacy studies indicate marginal differences in efficacy of newer atypical antipsychotics and the older drugs, and little effects on negative and cognitive symptoms. This review integrates findings from postmortem, imaging, and drug-challenge studies to elucidate a corticolimbic "pathologic circuit" in schizophrenia that may be particularly relevant to the negative symptoms and cognitive impairments of schizophrenia. Potential sites for pharmacologic intervention targeting glutatatergic, GABAergic, and cholinergic neurotransmission to treat these symptoms of schizophrenia are discussed.

Enhancement of N-methyl-D: -aspartate receptor (NMDAR) activity through its glycine modulatory site (GMS) is a novel therapeutic approach in schizophrenia. Brain concentrations of endogenous GMS agonist D: -serine and antagonist N-acetyl-aspartylglutamate are regulated by serine racemase (SR) and glutamic acid decarboxylase 2 (GCP2), respectively. Using mice genetically, under-expressing these enzymes may clarify the role of NMDAR-mediated neurotransmission in schizophrenia.We investigated the behavioral effects of two psychotomimetic drugs, the noncompetitive NMDAR antagonist, phencyclidine (PCP; 0, 1.0, 3.0, or 6.0 mg/kg), and the indirect dopamine receptor agonist, amphetamine (AMPH; 0, 1.0, 2.0, or 4.0 mg/kg), in SR -/- and GCP2 -/+ mice. Outcome measures were locomotor activity and prepulse inhibition (PPI) of the acoustic startle reflex. Acute effects of an exogenous GMS antagonist, gavestinel (0, 3.0, or 10.0 mg/kg), on PCP-induced behaviors were examined in wild-type mice for comparison to the mutants with reduced GMS activity.PCP-induced hyperactivity was increased in GCP2 -/+ mice, and PCP-enhanced startle reactivity was increased in SR -/- mice. PCP disruption of PPI was unaffected in either mutant. In contrast, gavestinel attenuated PCP-induced PPI disruption without effect on baseline PPI or locomotor activity. AMPH effects were similar to controls in both mutant strains.The results of the PCP experiments demonstrate that convergence of pharmacological and genetic manipulations at NMDARs may confound the predictive validity of these preclinical assays for the effects of GMS activation in schizophrenia. The AMPH data provide additional evidence that hyperdopaminergia in schizophrenia may be distinct from NMDAR hypofunction.

d-Serine, formed froml-serine by serine racemase (SR), is a physiologic coagonist at NMDA receptors. Using mice with targeted deletion of SR, we demonstrate a role ford-serine in NMDA receptor-mediated neurotoxicity and stroke. Brain cultures of SR-deleted mice display markedly diminished nitric oxide (NO) formation and neurotoxicity. In intact SR knock-out mice, NO formation and nitrosylation of NO targets are substantially reduced. Infarct volume following middle cerebral artery occlusion is dramatically diminished in several regions of the brains of SR mutant mice despite evidence of increased NMDA receptor number and sensitivity.

Progress in regenerative medicine seems likely to produce new treatments for neurologic conditions that use human cells as therapeutic agents; at least one trial for such an intervention is already under way. The development of cell-based interventions for neurologic conditions (CBI-NCs) will likely include preclinical studies using animals as models for humans with conditions of interest. This paper explores predictive validity challenges and the proper role for animal models in developing CBI-NCs. In spite of limitations, animal models are and will remain an essential tool for gathering data in advance of first-in-human clinical trials. The goal of this paper is to provide a realistic lens for viewing the role of animal models in the context of CBI-NCs and to provide recommendations for moving forward through this challenging terrain. © 2009 ISCBFM All rights reserved.

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

This review examines the large body of toxicological and epidemiological information on human exposures to chlorpyrifos, with an emphasis on the controversial potential for chlorpyrifos to induce neurodevelopmental effects at low doses. The results of this review demonstrate that the use of urinary 3,5,6-trichlorpyridinol (TCPy), a metabolite of chlorpyrifos as a biomarker of nonoccupational exposure is problematic and may overestimate nonoccupational exposures to chlorpyrifos by 10-to 20-fold because of the widespread presence of both TCPy and chlorpyrifos-methyl in the food supply. Current "background" (nonoccupational) levels of exposure to chlorpyrifos are several orders of magnitude lower than those required to inhibit plasma cholinesterase activity, which is a more sensitive target than nervous system cholinesterase. However, several in vitro studies have identified putative neurodevelopmental mechanisms that are altered at concentrations of chlorpyrifos below those that inhibit cholinesterases. Although one human cohort study reported an association between maternal and cord blood chlorpyrifos levels and several measures of neurodevelopment, two other cohort studies that utilized urinary TCPy as a surrogate for chlorpyrifos exposure did not demonstrate an association. Although the weight of the scientific evidence demonstrates that current levels of chlorpyrifos exposure will not have any adverse effects on neurodevelopment that might result from inhibition of nervous system cholinesterases, several recent studies propose alternative mechanisms. Thus, further in vivo investigation on neurodevelopment in an appropriate animal model is needed; additional epidemiological studies may be warranted if a suitable, chlorpyrifos-exposed cohort can be identified and more rigorous measures of exposure are utilized.

Galantamine is an approved treatment for mild to moderate Alzheimer's disease, with demonstrated benefits for cognition and functional ability in human studies. The mechanism of action that is most generally recognized as underlying the clinical benefits of galantamine is inhibition of brain acetylcholinesterase (AChE). However, an increasing body of evidence suggests that an additional mechanism, most likely allosteric modulation of nicotinic acetylcholine receptors (nAChRs), may contribute to the therapeutic effects of galantamine. This review summarizes the research on this additional mechanism, with emphasis on data derived from in vivo animal studies and open-label hypothesis-generating studies in humans. In general, these studies provide evidence of effects beyond those of AChE inhibition alone, most notably in comparisons with other AChE inhibitors, in which galantamine produced similar or greater effects at doses that provided lower levels of brain AChE inhibition. The use of nAChR agonists and antagonists in some of these studies lends support to the proposed allosteric potentiating ligand activity of galantamine at nAChRs. This dual action of galantamine may account for its therapeutic profile.

N-Acetylaspartylglutamate (NAAG) is a neuropeptide found in high concentrations in the brain. Using whole-cell recordings of CA1 pyramidal neurons in acute hippocampal slices, we found that either (i) the application of exogenous NAAG or (ii) an increase of endogenous extracellular NAAG, caused by the inhibition of its catabolic enzyme glutamate carboxypeptidase II (GCP II), resulted in a significant reduction in the amplitude of the isolated NMDA receptor (NMDAR) component of the evoked excitatory postsynaptic current (EPSC). Conversely, reduction of endogenous extracellular NAAG caused by either (i) perfusion with a soluble form of pure human GCP II or (ii) affinity purified antibodies against NAAG, enhanced the amplitude of the isolated NMDAR current. Bath application of GCP II inhibitor induced a progressive loss of spontaneous NMDAR miniatures. Furthermore, NAAG blocked the induction of long-term potentiation at Schaffer collateral axons-CA1 pyramidal neuron synapses. All together, these results suggest that NAAG acts as an endogenous modulator of NMDARs in the CA1 area of the hippocampus.

Schizophrenia is associated with high prevalence of substance abuse. Recent research suggests that dysregulation of N-methyl-d-aspartate receptor (NMDAR) function may play a role in the pathophysiology of both schizophrenia and drug addiction, and thus, may account for this high comorbidity. Our laboratory has developed two transgenic mouse lines that exhibit contrasting NMDAR activity based on the availability of the glycine modulatory site (GMS) agonists d-serine and glycine. Glycine transporter 1 knockdowns (GlyT1(+/-)) exhibit NMDAR hyperfunction, whereas serine racemase knockouts (SR(-/-)) exhibit NMDAR hypofunction. We characterized the behavior of these lines in a cocaine-induced (20 mg/kg) conditioned place preference (CPP) and locomotor sensitization paradigm. Compared with wild-type mice, GlyT1(+/-) mice displayed hastened extinction of CPP and robust cocaine-induced reinstatement. SR(-/-) mice appeared to immediately "forget" the learned preference, because they did not exhibit cocaine-induced reinstatement and also displayed attenuated locomotor sensitization. Treatment of GlyT1(+/-) mice with gavestinel (10 mg/kg on day 1; 5 mg/kg on days 2-17), a GMS antagonist, attenuated cocaine-induced CPP and caused them to immediately "forget" the learned preference. Treatment of SR(-/-) mice with d-serine (300 mg/kg on day 1; 150 mg/kg on days 2-17) to normalize brain levels caused them to avoid the cocaine-paired side of the chamber during extinction. These results highlight NMDAR dysfunction as a possible neural mechanism underlying comorbid schizophrenia and substance abuse. Also, these findings suggest drugs that directly or indirectly activate the NMDAR GMS could be an effective treatment of cocaine abuse.

NMDA receptor (NMDAR) hypofunction is a compelling hypothesis for the pathophysiology of schizophrenia, because in part, NMDAR antagonists cause symptoms in healthy adult subjects that resemble schizophrenia. Therefore, NMDAR antagonists have been used as a method to induce NMDAR hypofunction in animals as a pharmacological model of schizophrenia. Serine racemase-null mutant (SR −/−) mice display constitutive NMDAR hypofunction due to the lack of d -serine. SR −/− mice have deficits in tropomyosin-related kinase receptor (TrkB)/Akt signaling and activity regulated cytoskeletal protein (Arc) expression, which mirror what is observed in schizophrenia. Thus, we analyzed these signaling pathways in MK801 sub-chronically (0.15 mg/kg; 5 days) treated adult wild-type mice. We found that in contrast to SR −/− mice, the activated states of downstream signaling molecules, but not TrkB, increased in MK801 treated mice. Furthermore, there is an age-dependent change in the behavioral reaction of people to NMDAR antagonists. We therefore administered the same dosing regimen of MK801 to juvenile mice and compared them to juvenile SR −/− mice. Our findings demonstrate that pharmacological NMDAR antagonism has different effects on TrkB/Akt signaling than genetically-induced NMDAR hypofunction. Given the phenotypic disparity between the MK801 model and schizophrenia, our results suggest that SR −/− mice more accurately reflect NMDAR hypofunction in schizophrenia.

Schizophrenia is a chronic, disabling psychiatric disorder that genetic studies have shown to be highly heritable. Although the dopamine hypothesis has dominated the thinking about the cause of schizophrenia for 40 years, post-mortem and genetic studies have provided little support for it. Rather, post-mortem studies point to hypofunction of subsets of GABAergic interneurons in the prefrontal cortex and the hippocampus. Furthermore, clinical pharmacologic, post-mortem and genetic studies have provided compelling evidence of hypofunction of a subpopulation of NMDA receptors in schizophrenia. In support of this inference, agents that directly or indirectly activate the glycine modulatory site on the NMDA receptor (the Glycine B receptor) reduce symptoms in chronic schizophrenia, especially negative symptoms and cognitive impairments. Electrophysiologic and pharmacologic studies suggest that the vulnerable NMDA receptors in schizophrenia may be concentrated on cortico-limbic GABAergic interneurons, thereby linking these two neuropathologic features of the disorder.