Ionotropic glutamate receptors and expression of N-methyl-D-aspartate receptor subunits in subregions of human hippocampus: effects of schizophrenia

Phenotype of schizophrenia: a review and formulation

The discovery of the pathophysiology(ies) for schizophrenia is necessary to direct rational treatment directions for this brain disorder. Firm knowledge about this illness is limited to areas of phenomenology, clinical electrophysiology, and genetic risk; some aspects of dopamine pharmacology, cognitive symptoms, and risk genes are known. Basic questions remain about diagnostic heterogeneity, tissue neurochemistry, and in vivo brain function. It is an illness ripe for molecular characterization using a rational approach with a confirmatory strategy; drug discovery based on knowledge is the only way to advance fully effective treatments. This paper reviews the status of general knowledge in this area and proposes an approach to discovery, including identifying brain regions of dysfunction and subsequent localized, hypothesis-driven molecular screening.

Gene expression profiles in microdissected neurons from human hippocampal subregions

Pyramidal neurons in hippocampal subregions are selectively vulnerable in certain disease states. To investigate, we tested the hypothesis that selective vulnerability in human hippocampus is related to regional differences in neuronal cell death and cell receptor gene expression in CA1 vs. CA3 subregions. We used laser capture microdissection to remove approximately 600 CA1 and 600 CA3 pyramidal neurons each from five fresh-frozen normal post-mortem brains, extracted total RNA and double-amplified mRNA. This was reverse transcribed and labeled for hybridization onto human cDNA array chips containing probes to 10,174 genes and unknown ESTs. RNA from additional microdissections was pooled for replicate hybridizations and quantitative RT-PCR validation. Gene expression differences were few (< 1%). We found 43 enriched genes in CA1 neuronal samples that included peripheral benzodiazipine receptor-associated protein, nicotinic cholinergic receptor, two chemokine receptors (CCR1 and CCR5) and several transcriptional factors. We found 17 enriched genes in the CA3 neuronal samples that included fibroblast growth factor receptor and prostaglandin-endoperoxide synthase 1. We found no differential gene expression for 23 calcium channel proteins; nine transporter proteins; 55 cell death and apoptotic regulator proteins; and an additional 497 cell receptors, including 24 glutamate receptors. Quantitative RT-PCR of four differentially expressed genes confirmed the microarray data. The results confirm the ability to examine gene expression profiles in microdissected neurons from human autopsy brain. They show only minor gene expression differences between two distinct neuronal populations in the hippocampus and suggest that selective hippocampal vulnerability is due to factors other than intrinsic differential expression in glutamate receptors and cell death genes.

Selective subunit antagonists suggest an inhibitory relationship between NR2B and NR2A-subunit containing N-methyl-d-aspartate receptors in hippocampal slices

Glutamate receptors responding to N-methyl-D: -aspartate (NMDA) are involved in neural development, excitotoxicity and neuronal plasticity. Each receptor includes at least two NR2 subunits. Here, we have examined the effects of selective antagonists of NR2A and NR2B subunits (NVP-AAM07 and Ro25-6981 respectively) on the effects of NMDA in the CA1 field of rat hippocampal slices. We have observed that Ro25-6981 potentiates, rather than blocks, the effects of NMD on field EPSPs and paired-pulse interactions (indicators of presynaptic effects) and on postsynaptic depolarisation in hippocampal slices. The NR2A subunit antagonist NVP-AAM077 blocks the effects of NMDA alone, or after potentiation by Ro25-6981. The potentiation of NMDA by Ro25-6981 was not prevented by staurosporine (protein kinase inhibitor), okadaic acid (an inhibitor of serine/threonine protein phosphatases) or anisomycin (protein synthesis inhibitor), but was prevented by cyclosporin A, which inhibits Ca2+/calmodulin-dependent phosphatase 2B [calcineurin]. NMDA-dependent long-term potentiation (LTP) induced by electrical stimulation was not prevented by Ro25-6981 but was prevented by selective blockade of the NR2A subunit. The results suggest that, at both presynaptic and postsynaptic sites in the rat hippocampus, NR2B-subunit-containing receptors limit NMDA receptor function by inhibitory restraint over NR2A-subunit-containing receptors, via calcineurin activation, and that LTP induction critically involves primarily receptors containing the NR2A subunit. Endogenous factors or drugs that modify this NR2B/NR2A interaction could have a major influence on synaptic transmission and plasticity in the brain.

NR3A NMDA receptor subunit mRNA expression in schizophrenia, depression and bipolar disorder

Growing evidence suggests that NMDA receptor (NMDAR) dysfunction may be involved in schizophrenia. The NMDAR is a multimeric assembly derived from seven different genes (NR1, NR2A-2D and NR3A-3B). While region-specific changes in the expression of most NMDAR subunits have been reported in schizophrenia, possible abnormalities of NR3A expression have not been investigated. Both electrophysiological and anatomical data in rodents, however, suggest that NR3A subunits could play a role in this disorder. In this study, we measured NR3A transcript levels in the dorsolateral prefrontal cortex (DLPFC) and inferior temporal neocortex in the brains of people with schizophrenia, bipolar disorder, depression, and a comparison group. This transcript was elevated by 32% in schizophrenia relative to controls, but only in the DLPFC and not inferior temporal cortical regions. Interestingly, this effect was restricted to gyral aspects of the DLPFC and did not involve sulcal areas. NR3A mRNA was significantly decreased by 12% in bipolar disorder relative to the comparison group in DLPFC, although there were no gyral versus sulcal differences. As was the case in schizophrenia, no changes in NR3A expression were observed in the inferior temporal cortex in bipolar disorder. These data indicate that the NR3A subunit is abnormally expressed in both schizophrenia and bipolar disorder.

Neuregulin genotype and medication response in Finnish patients with schizophrenia

Neuregulin 1 is involved both in neurodevelopment and neurotransmitter mechanisms in the brain. There is evidence of an association between neuregulin 1 genotype and schizophrenia. We compared neuregulin 1 genotypes in patients with schizophrenia (n=94) and control subjects (n=395) of Finnish origin by using one SNP (SNP8NRG221533) as a genetic marker. We also analyzed NRG1 genotype with regard to age at onset and between responders and non-responders to conventional antipsychotics. The NRG1 genotype or allele frequencies showed similar distributions between patient and control groups. Age at onset was not associated with NRG1 genotype. The TT genotype was overrepresented in the non-responders group compared with the responders (p=0.013). Further studies are needed to ascertain the significance of neuregulin genotype in medication response to schizophrenia.

Glutamate receptor subunits are altered in forebrain and cerebellum in rats chronically exposed to the NMDA receptor antagonist phencyclidine

Phencyclidine (PCP) is a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype. It produces transient psychoses in normal individuals and exacerbates psychoses in schizophrenics. When administered to rodents, PCP elicits stereotypic behaviors including unrelenting head swaying, hyperlocomotion, and social withdrawal. In this study, we examined the relative distribution of the NMDA receptor subunits, as well as the subunits of its modulating receptor, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) in the forebrain, hippocampus, and cerebellum of rats chronically exposed to PCP. Rats were injected for 30 days with PCP (10 mg/kg) and age/sex-matched controls were injected for 30 days with saline vehicle. Brain NMDA and AMPA receptor subunit distribution patterns and protein levels were then analyzed by immunocytochemistry and Western blot analysis. Chronic PCP-treated animals showed significant alterations in glutamate receptor subunits, particularly for the NR1, NR2B, NR2C, and NR2D components of the NMDA receptor. AMPA receptor subunits demonstrated few significant changes in subunit availabilities. Western blot analysis largely confirmed the immunocytochemical findings. These results support the conclusion that subunits of the NMDA receptor are selectively altered by chronic PCP antagonism, with minimal to no changes observed in AMPA receptor subunits. Our findings are consistent with the interpretation that a dysfunctional NMDA receptor complex may mediate abnormal glutamatergic neurotransmission and potentially contribute to the complex etiology of cognitive disorders.

Neuroplasticity and schizophrenia

This article's title is also the name of a workshop sponsored by the International Congress on Schizophrenia Research that was focused on an appraisal of the potential role of neuroplastic processes in the etiology or course of schizophrenia. The workshop brought together clinical investigators of schizophrenia and basic scientists who study various aspects of neuroplasticity, including central nervous system (CNS) development, learning and memory, and drug action. The goal was to identify special opportunities to advance knowledge and understanding of schizophrenia pathology, treatment, or prevention by applying neuroplasticity concepts as a framework to theories of the illness. Although the focus of this workshop was schizophrenia, the phenomena considered are pertinent to other disorders, such as depression and drug abuse.

Thalamic dysfunction in schizophrenia: neurochemical, neuropathological, and in vivo imaging abnormalities

While abnormalities of the prefrontal cortex and temporal lobe structures have typically been associated with the pathophysiology of schizophrenia, recent findings implicate thalamic dysfunction in this illness as well. The thalamus plays a critical role in processing and integrating sensory information relevant to emotional and cognitive functions. Neuropathological and in vivo imaging studies in schizophrenia have identified several structural and metabolic abnormalities in the thalamus, which may contribute to a deficit in sensory processing and be related to psychotic symptomatology. In addition to these postmortem and in vivo imaging studies indicating structural and metabolic changes in the thalamus in schizophrenia, more recent studies have examined the neurochemical substrates that accompany these changes. Much of this work to date has focused on glutamatergic abnormalities in the thalamus, in part because it is a predominant neurotransmitter used in the thalamus, and because glutamatergic dysfunction has been hypothesized to be involved in schizophrenia. Several studies, however, have also examined markers of gamma-aminobutyric acid (GABA) and dopaminergic neurotransmission in the thalamus in schizophrenia. We review these neurochemical findings, as well as the growing body of postmortem and in vivo imaging evidence that supports the hypothesis of thalamic dysfunction in schizophrenia.

Variations in the NMDA receptor subunit 2B gene (GRIN2B) and schizophrenia: a case-control study

A well established model for the pathophysiology of schizophrenia postulates a role for the NMDA-mediated glutamate transmission. The human gene coding for the 2B subunit of the NMDA receptor (GRIN2B) is considered a candidate based on its selective expression in brain. To evaluate the hypothesis that GRIN2B acts as a major gene in determining susceptibility to schizophrenia, a case-control association study was performed. Five single nucleotide polymorphisms (SNPs) were genotyped in 188 Italian patients and 156 control subjects. The association study showed a marginally significant excess of homozygosity for the polymorphism located in the 3'UTR region (P = 0.04). No other difference in genotype and allele frequencies was found in schizophrenics as compared to the control series. The case-control study was also carried out on estimated haplotypes, confirming a trend for association (P = 0.04). These results suggest that GRIN2B variations might be linked with susceptibility to schizophrenia. Replication studies on larger samples are warranted to further test this hypothesis.

Abnormalities of the NMDA Receptor and Associated Intracellular Molecules in the Thalamus in Schizophrenia and Bipolar Disorder

Several lines of investigation support a hypothesis of glutamatergic dysfunction in schizophrenia, including our recent reports of altered NMDA receptor subunit and associated intracellular protein transcripts in the thalamus of elderly patients with schizophrenia. In the present study, we used in situ hybridization to measure the expression of NMDA subunits (NR1, NR2A-D), and associated intracellular proteins (NF-L, PSD95, and SAP102) in a second, younger cohort from the Stanley Foundation Neuropathology Consortium, which included patients with both schizophrenia and affective disorders. We wanted to determine whether glutamatergic abnormalities in the thalamus in schizophrenia are present at younger ages, and whether these abnormalities occur in other psychiatric illnesses. In the present work, we observed increased expression of NMDA NR2B subunit transcripts, and decreased expression of all three associated postsynaptic density protein transcripts in schizophrenia. We also found evidence of glutamatergic dysfunction in the thalamus in affective disorders, particularly in bipolar disorder. In particular, we found decreased NF-L, PSD95, and SAP102 transcripts in bipolar disorder, and decreased SAP102 levels in major depression. Interestingly, one of the most consistent findings across diagnostic groups was an abnormality of intracellular signaling molecules that are linked to the NMDA receptor, rather than changes in the receptor subunits themselves. PSD95 and similar scaffolding molecules link the NMDA receptor with intracellular enzymes that mediate signaling, and also provide a physical link between different neurotransmitter systems to coordinate and integrate information from multiple effector systems. Abnormalities of PSD95-like molecules and other intracellular signaling machinery may contribute to dysregulated communication between multiple neurotransmitter systems (such as glutamatergic and dopaminergic systems) that are potentially involved in the neurobiology of schizophrenia and affective disorders.

The hippocampus in schizophrenia: a review of the neuropathological evidence and its pathophysiological implications

This paper puts the case for the hippocampus as being central to the neuropathology and pathophysiology of schizophrenia. The evidence comes from a range of approaches, both in vivo (neuropsychology, structural and functional imaging) and post mortem (histology, morphometry, gene expression, and neurochemistry). Neuropathologically, the main positive findings concern neuronal morphology, organisation, and presynaptic and dendritic parameters. The results are together suggestive of an altered synaptic circuitry or "wiring" within the hippocampus and its extrinsic connections, especially with the prefrontal cortex. These changes plausibly represent the anatomical component of the aberrant functional connectivity that underlies schizophrenia. Glutamatergic pathways are prominently but not exclusively affected. Changes appear somewhat greater in the left hippocampus than the right, and CA1 is relatively uninvolved compared to other subfields. Hippocampal pathology in schizophrenia may be due to genetic factors, aberrant neurodevelopment, and/or abnormal neural plasticity; it is not due to any recognised neurodegenerative process. Hippocampal involvement is likely to be associated with the neuropsychological impairments of schizophrenia rather than with its psychotic symptoms.

Regulation of dopamine D1 receptor function by physical interaction with the NMDA receptors

Functional interactions between dopamine D1-like receptors and NMDA subtype glutamate receptors have been implicated in the maintenance of normal brain activity and neurological dysfunction. Although modulation of NMDA receptor functions by D1 receptor activation has been the subject of extensive investigation, little is known as to how the activation of NMDA receptors alters D1 function. Here we report that NMDA receptors regulate D1 receptor function via a direct protein-protein interaction mediated by the carboxyl tail regions of both receptors. In both cotransfected cells and cultured hippocampal neurons the activation of NMDA receptors increases the number of D1 receptors on the plasma membrane surface and enhances D1 receptor-mediated cAMP accumulation via a SNARE-dependent mechanism. Furthermore, overexpression of mini-genes encoding either NR1 or D1 carboxyl tail fragments disrupts the D1-NR1 direct protein-protein interaction and abolishes NMDA-induced changes in both D1 cell surface expression and D1-mediated cAMP accumulation. Our results demonstrate that the D1-NR1 physical interaction enables NMDA receptors to increase plasma membrane insertion of D1 receptors and provides a novel mechanism by which the activation of NMDA receptors upregulates D1 receptor function. Understanding the molecular mechanisms by which D1 and NMDA receptors functionally interact may provide insight toward elucidating the molecular neurobiological mechanisms involved in many neuropsychiatric illnesses, such as schizophrenia.

Glutamate receptors and transporters in the hippocampus in schizophrenia

Postmortem studies, using various methods and directed at several molecular targets, have provided increasing evidence that glutamatergic neurotransmission is affected in schizophrenia. The bulk of the data are in the hippocampus, wherein there is reduced expression of one or more subunits for all three ionotropic receptors (NMDA, AMPA, and kainate). Presynaptic glutamatergic markers, notably the vesicular glutamate transporter VGLUT1, may also be decreased in schizophrenia, especially in older subjects. CA1 appears less affected than other subfields, and the decrements may be greater in the left than in the right hippocampus. The recently described susceptibility genes for schizophrenia all act upon glutamatergic synaptic transmission, which may, therefore, be part of the core pathophysiology of the disorder.

Expression of the ionotropic glutamate receptor subunits and NMDA receptor-associated intracellular proteins in the substantia nigra in schizophrenia

Multiple neurotransmitter systems have been implicated in the pathophysiology of schizophrenia. Dopamine hyperactivity has often been implicated in this illness. More recently, the glutamate hypothesis of schizophrenia suggests that NMDA receptor (NMDAR) hypofunction may also play a role in this illness. This is based primarily on studies showing that phencyclidine, an NMDAR antagonist, can induce a schizophreniform psychosis. While NMDAR dysfunction is most often implicated in schizophrenia, other components of the glutamate system, such as the AMPA and kainate receptors, as well as NMDAR-associated intracellular proteins, may also play a role in regulating NMDA receptor activity and glutamate neurotransmission. There is growing interest in the hypothesis that the pathophysiology of schizophrenia involves alterations in dopamine-glutamate interactions. The glutamate system is anatomically and functionally linked to the dopamine system, and glutamate can modulate dopaminergic activity and release by stimulating various glutamate receptor subtypes expressed by dopaminergic neurons in the substantia nigra/ventral tegmental area. In this study, we investigated dopamine-glutamate interactions by measuring the expression of transcripts encoding the subunits for the ionotropic glutamate receptors (NMDA, AMPA and kainate) and five NMDAR-associated intracellular proteins, PSD-93, PSD-95, SAP102, NF-L and yotiao in the dopaminergic neurons in the substantia nigra pars compacta (SNc) of subjects with schizophrenia and a comparison group. Tyrosine hydroxylase (TH, a marker of dopamine-synthesizing cells), NR1 (an NMDA receptor subunit) and GluR5 (a kainate subunit) transcript levels were significantly increased in the SNc in schizophrenia. These data support the hypothesis that schizophrenia may involve alterations in dopamine-glutamate interactions.

Biochemical and molecular studies of NMDA receptor subunits NR1/2A/2B in hippocampal subregions throughout progression of Alzheimer's disease pathology

Alzheimer's disease (AD) is characterized by loss of specific cell populations within selective subregions of the hippocampus. Excitotoxicity, mediated via ionotropic glutamate receptors, may play a crucial role in this selective neuronal vulnerability. We investigated whether alterations in NMDA receptor subunits occurred during AD progression. Employing biochemical and in situ hybridization techniques in subjects with a broad range of AD pathology, protein levels, and mRNA expression of NR1/2A/2B subunits were assayed. With increasing AD neuropathology, protein levels and mRNA expression for NR1/2B subunits were significantly reduced, while the NR2A subunit mRNA expression and protein levels were unchanged. Cellular analysis of neuronal mRNA expression revealed a significant increase in the NR2A subunit in subjects with moderate neurofibrillary tangle neuropathology. This investigation supports the hypothesis that alterations occur in the expression of specific NMDA receptor subunits with increasing AD pathologic severity, which is hypothesized to contribute to the vulnerability of these neurons.

Neuregulin 1 and schizophrenia

We discuss in this review the role of the neuregulin (NRG1) gene in schizophrenia. NRG1 contributes to the genetics of schizophrenia in both Icelandic and Scottish schizophrenia patients. NRG1 participates in glutamatergic signaling by regulating the N-methyl-D-aspartate (NMDA) receptor through the interaction of the NRG1 protein and its receptors. NRG1 plays a central role in neural development and is most likely involved in regulating synaptic plasticity, or how the brain responds or adapts to the environment. The discovery that defects in NRG1 signaling may be involved in some cases of schizophrenia, not only implicates NRG1, but suggests that its biological pathway, active both at developing and mature synapses, is worth inspecting further in a search for other schizophrenia genes possibly in epistasis with NRG1.

Neuregulin1 downregulates postsynaptic GABAa receptors at the hippocampal inhibitory synapse

The growth factor neuregulin 1 (NRG1) has been proposed to contribute to the formation and maturation of neuromuscular and interneuronal synapses by upregulating the expression of specific neurotransmitter receptor subunits. In the present report, we show that, in the hippocampus, NRG1 is expressed in a pattern suggesting that it regulates synapse development in the CA1 region. However, in contrast to what has been shown in other synapses, NRG1 reduces the expression of gamma-aminobutyric acid (GABA)A receptors alpha subunits in hippocampal slices, and the mean amplitude of GABAergic miniature inhibitory postsynaptic currents (IPSCs) in hippocampal CA1 pyramidal neurons, without affecting IPSC kinetics or frequency. These effects of NRG1 occur without concomitant changes in glutamate receptors and other synaptic proteins. We propose that the role of NRG1 in the formation and maturation in the hippocampal inhibitory synapse is downregulation, rather than upregulation, of receptor subunit expression. These results suggest that NRG1 may contribute to the reduction in GABAergic synaptic activity in hippocampal CA1 pyramidal neurons that normally occurs during early postnatal development, and that alterations in NRG1 signaling in the hippocampus may contribute to schizophrenia and epilepsy.

Effects of birth insult and stress at adulthood on excitatory amino acid receptors in adult rat brain

Birth complications involving fetal hypoxia and stress at adulthood, which are risk factors for schizophrenia, can produce alterations in subcortical dopamine (DA) function in rat models. As adults, rats born either by cesarean section (C-section) or by C-section with added global anoxia show increased stress-induced DA release from nucleus accumbens and increased amphetamine-induced locomotion, compared to vaginally born controls. Moreover, stress at adulthood interacts with these birth insults to modulate DA receptor and transporter levels. Glutamatergic transmission at the level of the nucleus accumbens, prefrontal cortex, and hippocampus are known to modulate subcortical DA activity. Thus, altered excitatory amino acid (EAA) function might contribute to the dopaminergic changes observed in rats after birth insult and/or stress at adulthood. To test this possibility, rats born vaginally, by C-section, or by C-section with 15 min of anoxia, were either repeatedly stressed (15 min of tail pinch daily for 5 days) at adulthood or received no stress, and levels of EAA receptor binding were measured by ligand autoradiography in limbic brain regions. As adults, rats born by C-section showed increases in AMPA receptor binding in nucleus accumbens shell, NMDA receptor binding in cingulate cortex, and kainate receptor binding in the hippocampal CA1 region. Anoxic rats showed increases in CA1 kainate receptor and anterior olfactory NMDA receptor binding. Stress at adulthood increased AMPA receptor binding in several regions of prefrontal cortex and reduced NMDA receptor binding in infralimbic cortex and dentate gyrus, across all birth groups. Two instances of interactions between birth insult and stress at adulthood were observed. Stress reduced cingulate cortex NMDA receptor binding and increased olfactory tubercle kainate receptor binding only in C-sectioned animals, but not in controls. The possibility that the observed EAA receptor changes contribute to dopaminergic dysfunction in these animal models is discussed, in light of known glutamate-DA interactions.

Decrease of serotonin receptor 2C in schizophrenia brains identified by high-resolution mRNA expression analysis

BACKGROUND

RNA expression profiling can provide hints for the selection of candidate susceptibility genes, for formulation of hypotheses about the development of a disease, and/or for selection of candidate gene targets for novel drug development. We measured messenger RNA expression levels of 16 candidate genes in brain samples from 55 schizophrenia patients and 55 controls. This is the largest sample so far used to identify genes differentially expressed in schizophrenia brains.

METHODS

We used a sensitive real-time polymerase chain reaction methodology and a novel statistical approach, including the development of a linear model of analysis of covariance type.

RESULTS

We found two genes differentially expressed: monoamine oxidase B was significantly increased in schizophrenia brain (p =.001), whereas one of the serotonin receptor genes, serotonin receptor 2C, was significantly decreased (p =.001). Other genes, previously proposed to be differentially expressed in schizophrenia brain, were invariant in our analysis.

CONCLUSIONS

The differential expression of serotonin receptor 2C is particularly relevant for the development of new atypical antipsychotic drugs. The strategy presented here is useful to evaluate hypothesizes for the development of the disease proposed by other investigators.

Prepulse inhibition deficits of the startle reflex in neonatal ventral hippocampal-lesioned rats: reversal by glycine and a glycine transporter inhibitor

BACKGROUND

Neonatal ventral hippocampal (NVH) lesions in rats induce behavioral abnormalities at adulthood thought to simulate some aspects of the positive, negative, and cognitive deficits classically observed in schizophrenic patients. Such lesions induce a postpubertal emergence of prepulse inhibition (PPI) deficits of the startle reflex reminiscent of the sensorimotor gating deficits observed in a majority of schizophrenic patients. To study the potential involvement of the glycinergic neurotransmission in such deficits, we investigated the capacity of glycine (an obligatory N-methyl-D-aspartate [NMDA] receptor co-agonist) and ORG 24598 (a selective glycine transporter 1 inhibitor) to reverse NVH lesion-induced PPI deficits in rats.

METHODS

Ibotenic acid was injected bilaterally into the ventral hippocampus of 7-day-old pups. Prepulse inhibition of the startle reflex was measured at adulthood.

RESULTS

Glycine (.8 and 1.6 g/kg IP) and ORG 24598 (10 mg/kg IP) fully and partially reversed lesion-induced PPI deficits, respectively.

CONCLUSIONS

These findings confirm that an impaired glutamatergic neurotransmission may be responsible for PPI deficits exhibited by NVH-lesioned rats and support the hypoglutamatergic hypothesis of schizophrenia. They also suggest that drugs acting either directly at the NMDA receptor glycine site or indirectly on the glycine transporter 1 could offer promising targets for the development of novel therapies for schizophrenia.

Peeling the onion: NMDA dysfunction as a unifying model in schizophrenia

N-methyl-d-aspartate receptor (NMDAR) dysfunction plays a crucial role in schizophrenia, leading to impairments in cognitive coordination. NMDAR agonists (e.g., glycine) ameliorate negative and cognitive symptoms, consistent with NMDAR models. However, not all types of cognitive coordination use NMDAR. Further, not all aspects of cognitive coordination are impaired in schizophrenia, suggesting the need for specificity in applying the cognitive coordination construct.

High-frequency synchronisation in schizophrenia: Too much or too little?

Phillips & Silverstein's focus on schizophrenia as a failure of “cognitive coordination” is welcome. They note that a simple hypothesis of reduced Gamma synchronisation subserving impaired coordination does not fully account for recent observations. We suggest that schizophrenia reflects a dynamic compensation to a core deficit of coordination, expressed either as hyper- or hyposynchronisation, with neurotransmitter systems and arousal as modulatory mechanisms.

No blind schizophrenics: Are NMDA-receptor dynamics involved?

Numerous searches have failed to identify a single co-occurrence of total blindness and schizophrenia. Evidence that blindness causes loss of certain NMDA-receptor functions is balanced by reports of compensatory gains. Connections between visual and anterior cingulate NMDA-receptor systems may help to explain how blindness could protect against schizophrenia.

Setting domain boundaries for convergence of biological and psychological perspectives on cognitive coordination in schizophrenia

The claim that the disorganized subtype of schizophrenia results from glutamate hypofunction is enhanced by consideration of current subtypology of schizophrenia, symptom definition, interdependence of neurotransmitters, and the nature of the data needed to support the hypothesis. Careful specification clarifies the clinical reality of disorganization as a feature of schizophrenia and increases the utility of the subtype.

Schizophrenia: Putting context in context

Although context-processing deficits may be core features of schizophrenia, context remains a poorly defined concept. To test Phillips & Silverstein's model, we need to operationalize context more precisely. We offer several useful ways of framing context and discuss enhancing or facilitating schizophrenic patients' performance under different contextual situations. Furthermore, creativity may be a byproduct of cognitive uncoordination.

Phenomenology, context, and self-experience in schizophrenia

Impairments in cognitive coordination in schizophrenia are supported by phenomenological data that suggest deficits in the processing of visual context. Although the target article is sympathetic to such a phenomenological perspective, we argue that the relevance of phenomenological data for a wider understanding of consciousness in schizophrenia is not sufficiently addressed by the authors.

Guarding against over-inclusive notions of “context”: Psycholinguistic and electrophysiological studies of specific context functions in schizophrenia

Phillips & Silverstein offer an exciting synthesis of ongoing efforts to link the clinical and cognitive manifestations of schizophrenia with cellular accounts of its pathophysiology. We applaud their efforts but wonder whether the highly inclusive notion of “context” adequately captures some important details regarding schizophrenia and NMDA/glutamate function that are suggested by work on language processing and cognitive electrophysiology.

Mechanisms of disrupted language comprehension in schizophrenia

Mechanisms that contribute to perceptual processing dysfunction in schizophrenia were examined by Phillips & Silverstein, and formulated as involving disruptions in both local and higher-level coordination of signals. We agree that dysfunction in the coordination of cognitive functions (disconnection) is also indicated for many of the linguistic processing deficits documented for schizophrenia. We suggest, however, that it may be necessary to add a timing mechanism to the theoretical account.

Theory of mind in schizophrenia: Damaged module or deficit in cognitive coordination?

Schizophrenics exhibit a deficit in theory of mind (ToM), but an intact theory of biology (ToB). One explanation is that ToM relies on an independent module that is selectively damaged. Phillips & Silverstein's analyses suggest an alternative: ToM requires the type of coordination that is impaired in schizophrenia, whereas ToB is spared because this type of coordination is not involved.

Cognitive coordination and its neurobiological bases: A new continent to explore

The additional arguments and evidence supplied by the commentaries strengthen the hypothesis that underactivity of NMDA receptors produces impaired cognitive coordination in schizophrenia. This encourages the hope that though the distance from molecules to mind is great, it can nevertheless be traversed. We therefore predict that in this decade or the next molecular psychology will be seen to be as fundamental to our understanding of mind as molecular biology is to our understanding of life.

Context rules

It is proposed that cortical activity is normally coordinated across synaptically connected areas and that this coordination supports cognitive coherence relations. This view is consistent with the NMDA- hypoactivity hypothesis of the target article in regarding disorganization symptoms in schizophrenia as arising from disruption of normal interareal coordination. This disruption may produce abnormal contextual effects in the cortex that lead to anomalous cognitive coherence relations.

The ketamine model for schizophrenia

This commentary compares clinical aspects of ketamine with the amphetamine model of schizophrenia. Hallucinations and loss of insight, associated with amphetamine, seem more schizophrenia-like. Flat affect encountered with ketamine is closer to the clinical presentation in schizophrenia. We argue that flat affect is not a sign of schizophrenia, but rather, arisk factorfor chronic schizophrenia.

Cortical connectivity in high-frequency beta-rhythm in schizophrenics with positive and negative symptoms

In chronic schizophrenic patients with both positive and negative symptoms (see Table 1), interhemispheric connections at the high frequency beta2-rhythm are absent during cognitive tasks, in contrast to normal controls, who have many interhemispheric connections at this frequency in the same situation. Connectivity is a fundamental brain feature, evidently greatly promoted by the NMDA system. It is a more reliable measure of brain function than the spectral power of this rhythm.

Expression of NMDA receptor NR1, NR2A and NR2B subunit mRNAs during development of the human hippocampal formation

The N-methyl-d-aspartate receptor plays a critical role in the formation and maintenance of synapses during brain development. In the rodent, changes in subunit expression and assembly of the heteromeric receptor complex accompany these maturational processes. However, little is known about N-methyl-d-aspartate receptor subunit expression during human brain development. We used in situ hybridization to examine the distribution and relative abundance of NR1, NR2A and NR2B subunit messenger ribonucleic acids in the hippocampal formation and adjacent cortex of 34 human subjects at five stages of life (neonate, infant, adolescent, young adult and adult). At all ages, the three messenger ribonucleic acids were expressed in all subfields, predominantly by pyramidal neurons, granule cells and polymorphic hilar cells. However, their abundance varied across ontogeny. Levels of NR1 messenger ribonucleic acid in CA4, CA3 and CA2 subfields were significantly lower in the neonate than all other age groups. In the dentate gyrus, subiculum and parahippocampal gyrus, NR2B messenger ribonucleic acid levels were higher in the neonate than in older age groups. NR2A messenger ribonucleic acid levels remained constant, leading to an age-related increase in NR2A/2B transcript ratio. We conclude that N-methyl-d-aspartate receptor subunit messenger ribonucleic acids are differentially expressed during postnatal development of the human hippocampus, with a pattern similar but not identical to that seen in the rodent. Changes in subunit composition may thus contribute to maturational differences in human hippocampal N-methyl-d-aspartate receptor function, and to their role in the pathophysiology of schizophrenia and other neurodevelopmental disorders.

Long-term effects of olanzapine, risperidone, and quetiapine on ionotropic glutamate receptor types: implications for antipsychotic drug treatment

Levels of ionotropic glutamate (Glu) N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainic acid (KA) receptors in rat forebrain regions were compared by quantitative in vitro receptor autoradiography after continuous treatment for 28 days with the atypical antipsychotics olanzapine, risperidone, and quetiapine, or vehicle controls. All three treatments significantly decreased NMDA binding in caudate-putamen (CPu; by 30, 34, and 26%, respectively) but increased AMPA receptor levels in same region (by 22, 30, and 28%). Olanzapine and risperidone, but not quetiapine, also reduced NMDA receptor labeling in hippocampal CA1 (21 and 19%) and CA3 (23 and 22%) regions. KA receptors were unaltered by any treatment in the brain regions examined. These findings suggest that the antipsychotic effects of olanzapine and risperidone may be mediated in part by NMDA receptors in hippocampus, and perhaps AMPA receptors in CPu. The findings also support the hypothesis that down-regulation of NMDA receptors by atypical antipsychotic agents in CPu contributes to their low risk of extra-pyramidal side effects. Inability of olanzapine, risperidone, and quetiapine to alter KA receptors suggests their minimal role in mediating the central nervous system actions of these drugs.

Corticosterone alters N-methyl-D-aspartate receptor subunit mRNA expression before puberty

Stress and stress hormones alter the expression of mRNA for the NR1, NR2A and NR2B subunits of the N-methyl-D-aspartate (NMDA) receptor in brain regions associated with the stress response. Early life stress contributes to the risk and pathophysiology of mental illness. Examining how stress hormones modulate NMDA receptor subunit gene expression before and after pubertal onset will further contribute to the understanding of how stress during adolescence relates to adult mental illness. Using in situ hybridization histochemistry, we measured NR1, NR2A and NR2B mRNA expression in the hippocampus and in the hypothalamic paraventricular nucleus (PVN) of rats that had undergone adrenalectomy (ADX) or sham surgery before or after puberty. Some ADX rats received corticosterone pellets that released either normal or stress levels of corticosterone for 14 days prior to sacrifice. There was a significant increase in NR1 subunit mRNA expression throughout the subfields of the hippocampus and in the PVN of ADX prepubertal rats. However, similar changes in hippocampal NR1 expression were not observed in postpubertal ADX rats. Pre- and postpubertal ADX rats implanted with a high-dose corticosterone pellet had decreased expression of PVN NR1 mRNA. Only prepubertal rats had an increase in dentate gyrus NR2A mRNA and CA3 region NR2B mRNA following high-dose replacement. These results provide evidence that glucocorticoids have differential effects on the regional expression of mRNA NMDA receptor subunits and elucidate a window during adolescence in which the NR1, NR2A and NR2B genes are responsive to glucocorticoids.

Loss and altered spatial distribution of oligodendrocytes in the superior frontal gyrus in schizophrenia

BACKGROUND

Brain imaging, molecular genetic, and ultrastructural evidence indicate the existence of pathologic alterations in the cortical and subcortical white matter of schizophrenic patients.

METHODS

We performed a stereologic analysis of numbers, densities, and spatial distribution of oligodendrocytes in layer III and in the gyral white matter of Brodmann's area 9 in the superior frontal gyrus to assess whether these cells are affected in schizophrenia. Counts were obtained on Nissl-stained materials and on sections immunolabeled for the oligodendrocyte marker 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) in seven schizophrenic and seven age-matched control cases.

RESULTS

A 28% decrease in total numbers (or densities) of cortical layer III oligodendrocytes and a 27% decrease in the white matter were detected in schizophrenic compared with control cases based on CNPase immunostaining. Nissl and CNPase immunohistochemistry yielded comparable results. The spatial distribution of oligodendrocytes in area 9 white matter exhibited a less clustered arrangement in schizophrenic cases.

CONCLUSIONS

These results suggest a severe pathology of oligodendrocytes in schizophrenia and provide a quantitative cellular correlate of the white matter changes observed by brain imaging in vivo, showing reduced fractional anisotropy in schizophrenia. The data support recent evidence that several genes encoding myelin-related proteins consistently exhibit reduced expression in schizophrenia.

SNAP-25 reduction in the hippocampus of patients with schizophrenia

In this study, the authors sought to replicate the findings of reduced synaptosomal associated protein 25 kDa (SNAP-25) immunoreactivity in the hippocampus of patients with schizophrenia. The authors also measured N-methyl-D-aspartate (NMDA) receptor 1 (NR1) receptor subunit to determine if glutamatergic synapses were involved with the loss of SNAP-25. We found 49% less SNAP-25 immunointensity in the schizophrenic group (n=7) compared to the control (n=8) or bipolar groups (n=4) (P=.004). There was no change in NMDA NR1 levels in the three groups. The authors confirm the previous report of less SNAP-25 immunoreactivity in the hippocampus using a different cohort of patients with schizophrenia. It also appears that NMDA NR1 was unchanged, indicating that the overall level of NMDA glutamatergic synapses in hippocampus is normal. These data add to evidence suggesting that in schizophrenia the molecular pathology of the hippocampus involves presynaptic components.

Early maternal deprivation reduces the expression of BDNF and NMDA receptor subunits in rat hippocampus

It is well accepted that events that interfere with the normal program of neuronal differentiation and brain maturation may be relevant for the etiology of psychiatric disorders, setting the stage for synaptic disorganization that becomes functional later in life. In order to investigate molecular determinants for these events, we examined the modulation of the neurotrophin brain-derived neurotrophic factor (BDNF) and the glutamate NMDA receptor following 24 h maternal separation (MD) on postnatal day 9. We found that in adulthood the expression of BDNF as well as of NR-2A and NR-2B, two NMDA receptor forming subunits, were significantly reduced in the hippocampus of MD rats whereas, among other structures, a slight reduction of NR-2A and 2B was detected only in prefrontal cortex. These changes were not observed acutely, nor in pre-weaning animals. Furthermore we found that in MD rats the modulation of hippocampal BDNF in response to an acute stress was altered, indicating a persistent functional impairment in its regulation, which may subserve a specific role for coping with challenging situations. We propose that adverse events taking place during brain maturation can modulate the expression of molecular players of cellular plasticity within selected brain regions, thus contributing to permanent alterations in brain function, which might ultimately lead to an increased vulnerability for psychiatric diseases.

Where the rubber meets the road: The importance of implementation

Phillips & Silverstein argue that a range of cognitive disturbances in schizophrenia result from a deficit in cognitive coordination attributable to NMDA receptor dysfunction. We suggest that the viability of this hypothesis would be further supported by explicit implementation in a computational framework that can produce quantitative estimates of the behavior of both healthy individuals and individuals with schizophrenia.

Context, connection, and coordination: The need to switch

Context, connection, and coordination (CCC) describe well where the problems that apply to thought-disordered patients with schizophrenia lie. But they may be part of the experience of those with other symptom constellations. Switching is an important mechanism to allow context to be applied appropriately to changing circumstances. In some cases, NMDA-voltage modulations may be central, but gain and shift are also functions that monoaminergic systems express in CCC.

Synchronous dynamics for cognitive coordination: But how?

Although interesting, the hypotheses proposed by Phillips & Silverstein lack unifying structure both in specific mechanisms and in cited evidence. They provide little to support the notion that low-level sensory processing and high-level cognitive coordination share dynamic grouping by synchrony as a common processing mechanism. We suggest that more realistic large-scale modeling at multiple levels is needed to address these issues.

A wide-spectrum coordination model of schizophrenia

The target article presents a model for schizophrenia extending four levels of abstraction: molecules, cells, cognition, and syndrome. An important notion in the model is that of coordination, applicable to both the level of cells and of cognition. The molecular level provides an “implementation” of the coordination at the cellular level, which in turn underlies the coordination at the cognitive level, giving rise to the clinical symptoms.

Linking brain to mind in normal behavior and schizophrenia

To understand schizophrenia, a linking hypothesis is needed that shows how brain mechanisms lead to behavioral functions in normals, and also how breakdowns in these mechanisms lead to behavioral symptoms of schizophrenia. Such a linking hypothesis is now available that complements the discussion offered by Phillips & Silverstein (P&S).

Spatial integration in perception and cognition: An empirical approach to the pathophysiology of schizophrenia

Evidence for a dysfunction in cognitive coordination in schizophrenia is emerging, but it is not specific enough to prove (or disprove) this long-standing hypothesis. Many aspects of the external world are spatially mapped in the brain. A comprehensive internal representation relies on integration of information across space. Focus on spatial integration in the perceptual and cognitive processes will generate empirical data that shed light on the pathophysiology of schizophrenia.

Inferring contextual field interactions from scalp EEG

This commentary highlights methods for using scalp EEG to make inferences about contextual field interactions, which, in view of the target article, may be specially relevant to the study of schizophrenia. Although scalp EEG has limited spatial resolution, prior knowledge combined with experimental manipulations may be used to strengthen inferences about underlying brain processes. Both spatial and temporal context are discussed within the framework of nonlinear interactions. Finally, results from a visual contour integration EEG pilot study are summarized in view of a hypothesis that relates receptive field and contextual field processing to evoked and induced activity, respectively.

Reconciling schizophrenic deficits in top-down and bottom-up processes: Not yet

This commentary challenges the authors to use their computational modeling techniques to support one of their central claims: that schizophrenic deficits in bottom-up (Gestalt-type tasks) and top-down (cognitive control tasks) context processing tasks arise from the same dysfunction. Further clarification about the limits of cognitive coordination would also strengthen the hypothesis.