Alterations In Glutamate Receptors Research Articles

801. Developmental Nicotine Exposure Induces Persistent Alterations in Accumbens Glutamatergic Circuitry

Developmental nicotine exposure (DNE) is used to model human smoking during pregnancy, and has been shown to cause alterations in glutamate receptors, dendritic morphology, and neuronal function as well as increase nicotine (NIC) self-administration in adult rats. The underlying neurobiological mechanisms leading to increased nicotine addiction vulnerability following DNE, however, are unknown.

Metabolomics and mitochondrial dysfunction in Alzheimer’s disease

Alzheimer’s disease (AD) is characterized by cognitive impairment, progressive neurodegeneration, and Aβ accumulation. Aβ oligomers can lead to synaptic damage via alterations in glutamate receptors and excitotoxicity, as well as mitochondrial dysfunction. AD is associated with various biological indicators, including (1) predisposing factors such as genetic risk factors, (2) laboratory markers such as Aβ and tau protein, and (3) diagnostic markers such as MRI and PET findings. However, these markers are not confirmed, invasive, or expensive. In the present study, we employed nuclear magnetic resonance (NMR) methods that are inexpensive, time-efficient, and can be performed using samples obtained from various easily accessible sources such as cerebrospinal fluid, plasma, and peripheral tissue, thus highlighting the clinical utility of this approach. NMR analyses of blood metabolites showed that glutamine, glutamate, leucine, oxaloacetate, aspartate, isoleucine, and 3-hydroxyisovalerate are increased in patients with AD compared with control individuals. These metabolites seem to be related to mitochondrial dysfunction. Our data indicated that 3-hydroxyisovalerate, which is linked to known pathologic processes associated with mitochondrial dysfunction and accelerated neurodegeneration, was increased in the blood samples of patients with AD.

Region-specific alterations in glutamate receptor 1 phosphorylation during context-induced drug seeking after withdrawal from morphine self-administration

Re-exposure to morphine-associated environments elicits morphine-seeking behavior after a long period of withdrawal in rats with a history of morphine dependence. Adaptations in glutamate receptor 1 (GluR1) phosphorylation in limbic brain regions have been shown to occur during withdrawal from addictive drugs, such as cocaine, methamphetamine, and heroin. However, whether similar adaptations exist after spontaneous withdrawal from repeated morphine intake has not been studied. In our present study, alterations in GluR1 expression and GluR1 phosphorylation at serine 845 (Ser845) and serine 831 (Ser831) in multiple limbic brain regions of rats were measured following context-induced drug craving after 1 or 10 days of withdrawal from intravenous morphine self-administration. Phosphorylation of GluR1 at Ser845, but not Ser831, increases in the nucleus accumbens and central amygdala from 1 to 10 days of withdrawal, and there were no changes in GluR1 phosphorylation at Ser845 or Ser831 in the hippocampal CA1 subregion from 1 to 10 days of withdrawal. Significant positive correlations between numbers of drug-seeking responses and GluR1 phosphorylation at Ser845 in the nucleus accumbens were found in individual animals. These results suggest that time-dependent and region-specific changes in phosphorylation of GluR1 at Ser845, but not Ser831, are involved in the drug-seeking behavior elicited by re-exposure to the morphine-associated context.

Developmental Modification of Synaptic NMDAR Composition and Maturation of Glutamatergic Synapses: Matching Postsynaptic Slots With Receptor Pegs

The numbers and types of ionotropic glutamate receptors at most vertebrate central excitatory synapses are altered as a function of changes in input activity patterns that occur during postnatal development. Activity-dependent developmental alterations in glutamate receptors underlie lasting changes in synaptic efficacy (plasticity) and metaplasticity (the plasticity of synaptic plasticity), which are critical elements of normal brain maturation. Understanding the specific involvement of glutamate receptors in synaptic development and function is made multiplicatively complex by the existence of a large number of glutamate receptor subunits, numerous subunit-specific amino acid sequences that regulate receptor function, and subunit-specific synaptic insertion restrictions imposed by associated anchoring proteins. Many receptor properties are altered when subunits are switched, so it is unclear which individual receptor property or properties underlie changes in synaptic function and plasticity during postnatal development. As a result, a more detailed understanding of the factors that regulate synaptic and cognitive development will involve mutations in glutamate receptor subunits that separate individual receptor properties and permit synaptic insertion at both immature and mature synapses in genetically modified organisms. This position paper focuses on structural modifications in N-methyl-d-aspartate receptors (NMDARs) that occur during postnatal forebrain development and attempts to provide a method for pursuing a more complete understanding of the functional ramifications of developmental alterations in NMDAR subunit composition.

Multiple sclerosis and glutamate excitotoxicity

The previous understanding of multiple sclerosis was solely related to neuroinflammation and its harmful effects; however, countless data indicate the importance of some inflammation-independent, neurodegenerative mechanisms associated with mitochondria malfunction, iron deposition and oxidative stress. Recently, it has been postulated that glutamate excitotoxicity, a phenomenon that takes place when an excessive amount of glutamate overactivates its cellular receptors and induces cell death, could be a missing link between inflammatory and neurodegenerative processes evident in multiple sclerosis. Glutamate is the major excitatory neurotransmitter of the central nervous system, which has been proven to have a central role in a complex communication network established between all residential brain cells, including neurons, astrocytes, oligodendrocytes and microglia. Thus, the disturbance of glutamate homeostasis could affect practically all physiological functions and interactions of brain cells, leading to heterogeneity of pathological events. The understanding of glutamate excitotoxicity as a valid mechanism of central nervous system damage in multiple sclerosis, requires the revision of the current knowledge about a source of elevated extracellular glutamate, glutamate receptor alterations, alterations of glutamate transporters and metabolizing enzymes, as well as molecular mechanism of excitotoxic damage.

The nicotine metabolite, cotinine, attenuates glutamate (NMDA) antagonist-related effects on the performance of the five choice serial reaction time task (5C-SRTT) in rats

Cotinine, the most predominant metabolite of nicotine in mammalian species, has a pharmacological half-life that greatly exceeds its precursor. However, until recently, relatively few studies had been conducted to systematically characterize the behavioral pharmacology of cotinine. Our previous work indicated that cotinine improves prepulse inhibition of the auditory startle response in rats in pharmacological impairment models and that it improves working memory in non-human primates. Here we tested the hypothesis that cotinine improves sustained attention in rats and attenuates behavioral alterations induced by the glutamate (NMDA) antagonist MK-801. The effects of acute subcutaneous (dose range 0.03–10.0mg/kg) and chronic oral administration (2.0mg/kg/day in drinking water) of cotinine were evaluated in fixed and variable stimulus duration (VSD) as well as variable intertrial interval (VITI) versions of a five choice serial reaction time task (5C-SRTT). The results indicated only subtle effects of acute cotinine (administered alone) on performance of the 5C-SRTT (e.g., decreases in timeout responses). However, depending on dose, acute treatment with cotinine attenuated MK-801-related impairments in accuracy and elevations in timeout responses, and it increased the number of completed trials. Moreover, chronic cotinine attenuated MK-801-related impairments in accuracy and it reduced premature and timeout responses when the demands of the task were increased (i.e., by presenting VSDs or VITIs in addition to administering MK-801). These data suggest that cotinine may represent a prototype for compounds that have therapeutic potential for neuropsychiatric disorders (i.e., by improving sustained attention and decreasing impulsive and compulsive behaviors), especially those characterized by glutamate receptor alterations.

Molecular mechanisms of neurodegeneration in Alzheimer's disease.

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Comprehensive Gene Expression Analysis in Bipolar Disorder

To review recent findings by DNA microarray in bipolar disorder (BD). A literature search was performed. Comprehensive gene expression analysis in the brain, peripheral blood cells, and olfactory neuroepithelium would be a promising strategy for the research of BD. To date, alterations in glutamate receptors (GR), mitochondria-related genes, chaperone genes, oligodendrocyte genes, and markers of gamma amino butyric acidergic (GABAergic) neurons in postmortem brains are replicated by several different strategies. However, alterations in mitochondria-related genes are associated with agonal factors, sample pH, and effects of drugs. Analysis of blood cells showed altered endoplasmic reticulum stress pathway and other molecular cascades. Analysis of olfactory epithelium showed altered expression of genes associated with apoptosis. These findings warrant that comprehensive gene expression analysis by DNA microarray will be useful to identify the molecular cascades responsible for BD.

Elevated GRIA1 mRNA expression in Layer II/III and V pyramidal cells of the DLPFC in schizophrenia

The functional integrity of the dorsolateral prefrontal cortex (DLPFC) is altered in schizophrenia leading to profound deficits in working memory and cognition. Growing evidence indicates that dysregulation of glutamate signaling may be a significant contributor to the pathophysiology mediating these effects; however, the contribution of NMDA and AMPA receptors in the mediation of this deficit remains unclear. The equivocality of data regarding ionotropic glutamate receptor alterations of subunit expression in the DLPFC of schizophrenics is likely reflective of subtle alterations in the cellular and molecular composition of specific neuronal populations within the region. Given previous evidence of Layer II/III and V pyramidal cell alterations in schizophrenia and the significant influence of subunit composition on NMDA and AMPA receptor function, laser capture microdissection combined with quantitative PCR was used to examine the expression of AMPA (GRIA1-4) and NMDA (GRIN1, 2A and 2B) subunit mRNA levels in Layer II/III and Layer V pyramidal cells in the DLPFC. Comparisons were made between individuals diagnosed with schizophrenia, bipolar disorder, major depressive disorder and controls ( n = 15/group). All subunits were expressed at detectable levels in both cell populations for all diseases as well as for the control group. Interestingly, GRIA1 mRNA was significantly increased in both cell types in the schizophrenia group compare to controls, while similar trends were observed in major depressive disorder (Layers II/III and V) and bipolar disorder (Layer V). These data suggest that increased GRIA1 subunit expression may contribute to schizophrenia pathology.

Long-term alterations in glutamate receptor and transporter expression following early-life seizures are associated with increased seizure susceptibility.

Prolonged seizures in early childhood are associated with an increased risk of development of epilepsy in later life. The mechanism(s) behind this susceptibility to later development of epilepsy is unclear. Increased synaptic activity during development has been shown to permanently alter excitatory neurotransmission and could be one of the mechanisms involved in this increased susceptibility to the development of epilepsy. In the present study we determine the effect of status-epilepticus induced by lithium/pilocarpine at postnatal day 10 (P10 SE) on the expression of glutamate receptor and transporter mRNAs in hippocampal dentate granule cells and protein levels in dentate gyrus of these animals in adulthood. The results revealed a decrease in glutamate receptor 2 (GluR2) mRNA expression and protein levels as well as an increase in protein levels for the excitatory amino acid carrier 1 (EAAC1) in P10 SE rats compared to controls. Expression of glutamate receptor 1 (GluR1) mRNA was decreased in both P10 SE rats and identically handled, lithium-injected littermate controls compared to naive animals, and GluR1 protein levels were significantly lower in lithium-controls than in naive rats, suggesting an effect of either the handling or the lithium on GluR1 expression. These changes in EAA receptors and transporters were accompanied by an increased susceptibility to kainic acid induced seizures in P10 SE rats compared to controls. The current data suggest that early-life status-epilepticus can result in permanent alterations in glutamate receptor and transporter gene expression, which may contribute to a lower seizure threshold.

Levodopa-induced motor complications are associated with alterations of glutamate receptors in Parkinson's disease

Glutamate receptors were studied in the brains of controls and Parkinson's disease (PD) patients, of which 10 of 14 developed motor complications (dyskinesias and/or wearing-off) following levodopa therapy. 125I-RTI binding to the dopamine transporter and dopamine concentrations show comparable nigrostriatal denervation between the subgroups of PD patients. 3H-Ro 25-6981 binding to the NR1/NR2B NMDA receptor was increased in the putamen of PD patients experiencing motor complications compared to those who did not (+53%) and compared to controls (+18%) whereas binding remained unchanged in the caudate nucleus. 3H-AMPA binding was increased in the lateral putamen (+23%) of PD patients with motor complications compared to those without whereas it was decreased in the caudate nucleus of the PD patients (−16%) compared to controls. Caudate and putamen 3H-CGP39653 binding to NR1/NR2A NMDA receptor and NR1 subunit mRNA levels measured by in situ hybridization were unchanged in subgroups of PD patients compared to controls. These findings suggest that glutamate receptor supersensitivity in the putamen plays a role in the development of motor complications (both wearing-Off and dyskinesias) following long-term levodopa therapy in PD.

Glutamatergic mechanisms in schizophrenia.

Schizophrenia is a chronic, severely disabling brain disorder with symptomatic onset in early adulthood. Typical antipsychotic medications that block dopamine D2 receptors are most effective in treating the psychosis but have limited effects on the negative symptoms and cognitive impairments. Considerable research has demonstrated that noncompetitive NMDA receptor antagonists, the dissociative anaesthetic like phencyclidine and ketamine, reproduce the cardinal symptomatic features of schizophrenia. Postmortem studies reveal variable alterations in glutamate receptors and their modulators in schizophrenia. Several clinical trials indicate agents that enhance NMDA receptor function via the glycine modulatory site reduce negative and variably improve cognitive function in schizophrenics receiving typical antipsychotics. Thus, hypofunction of a subpopulation of cortico-limbic NMDA receptors may participate in the pathophysiology of schizophrenia.

Topology of ionotropic glutamate receptors in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of a G-protein inwardly rectifying K(+) channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, ionotropic glutamate receptors sensitive to N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA) were examined by autoradiography with [(3)H]MK-801, [(3)H]AMPA, and [(3)H]KA. These surveys were carried out in selected areas of cerebral cortex, hippocampus and related limbic regions, basal ganglia, thalamus, hypothalamus, brainstem, and cerebellum from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild-type (+/+) mice. In wv/+ and wv/wv mutants, NMDA receptor levels were lower in cortical areas, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus, and in the cerebellar granular layer. Densities of KA receptors were lower in cortical areas, hippocampus, limbic system structures, neostriatum, nucleus accumbens, thalamus and hypothalamus, superior and inferior colliculi, and cerebellar cortex of wv/wv mutants. Levels of AMPA receptors in the weaver were higher than in +/+ mice, particularly in somatosensory and piriform cortices and periaqueductal gray of wv/+, and in somatosensory cortex, CA1 field of Ammon's horn and cerebellar granular layer of wv/wv. Abnormal developmental signals, aberrant cellular responses, or a distorted balance between neurotransmitter interactions may underlie such widespread and reciprocal glutamate receptor alterations, while in the case of cerebellar cortex, NMDA receptors are lacking due to a massive disappearance of cerebellar granule cells and some loss of Purkinje neurons.

Cell Death and Synaptic Reorganizations Produced by Seizures

The events that follow epilepsy seizures are not restricted to the immediate period. A series of long-term alterations occurs, including synaptic rearrangements, which have an impact on the brain circuit's mode of operation. With models of temporal lobe epilepsy, seizures have been shown to generate long-lasting changes in synaptic efficacy (epileptic long-term potentiation) because of removal of the magnesium block, activation of N-methyl-D-aspartate receptors, and an increase in intracellular calcium. This novel form of synaptic plasticity provides a link between memory effects and pathologic processes. Additionally, high-affinity kainate autoradiography, Timm stain, intraventricular injection of kainic acid, and 3D reconstruction experiments clearly indicate that even brief seizures produce changes in synaptic efficacy, followed 2-3 weeks later by aberrant neosynapse formation. Several key steps have been identified in the cascade leading from transient hyperactivity episodes to long-lasting, quasi-permanent modification of the neuronal circuit organization. These include the activation of immediate-early genes, activation of growth factor genes within hours, alterations in glutamate receptors, glial hypertrophy, and cytoskeletal protein changes. The cascade is activated by the increase in intracellular calcium and leads to axonal growth and neosynapse formation, which in turn participates in the etiology of the syndrome by reducing the threshold for further seizures. In summary, study data imply that the mature epileptic circuit has unique features in comparison with those present before a seizure episode, including new receptors, ionic channels, and other proteins. It is therefore essential to develop novel strategies based on the unique mode of operation of the mature epileptic circuit, rather than on acute models of epilepsy.

Reduction in Phosphorylated Heavy Neurofilament in the Cerebellum in HIV Disease

In the absence of significant neuronal infection HIV induces neuronal damage and death. The pathogenesis of this process is not clear and can only be assessed in the HIV infected brain by examining surviving neuronal populations. Cerebellar Purkinje cells are a model population. We have already demonstrated glutamate receptor alterations in these neurons in AIDS, and in the current study we have investigated the phosphorylation status of heavy neurofilament (NF-H), which is under the control of various intracellular kinases. While the number of Purkinje cells expressing non-phosphorylated NF-H was unchanged, the number of Purkinje cells expressing phosphorylated NF-H was decreased by 36% in the HIV group. This may be a marker of neuronal damage, and possibly indicate alteration in the activity of various intracellular signalling kinase pathways in the HIV infected brain.

Chapter 24 Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Alterations in glutamate receptor 2/3 subunits and amyloid precursor protein expression during the course of Alzheimer's disease and Lewy body variant.

Alterations in the processing and patterns of trophic and/or toxic factors might lead to the increased neuronal vulnerability in the entorhinal cortex in Alzheimer's disease (AD) and Lewy body variant (LBV). Therefore, patterns and levels of amyloid precursor protein (APP) and glutamate receptor (gluR) expression in the entorhinal cortex and hippocampus in relation to disease severity were investigated. Sections from the hippocampus and entorhinal cortex were single and double immunolabeled for APP, gluR2/3, and N-methyl-D-aspartate receptor (NMDA-R). Within the hippocampus and entorhinal cortex, image analysis revealed progressively decreased APP and gluR2/3 levels during the course of AD and LBV, whereas levels of NMDA-R were unaltered. Furthermore, the present study showed a positive correlation and close co-localization of APP and gluR2/3 immunoreactivity in neurons, suggesting a possible interaction between these two factors. In conclusion, these data imply that alterations in neuronal APP and gluR2/3 expression in the entorhinal cortex lead to increased susceptibility to neurodegeneration and might be markers of vulnerability.

Altered distribution of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2(4) and the N-methyl-D-aspartate receptor subunit NMDAR1 in the hippocampus of patients with temporal lobe epilepsy.

In patients with therapy-refractory temporal lobe epilepsy (TLE), alterations of glutamate receptors have been proposed as a mechanism for enhanced excitability. Using commercially available monoclonal antibodies specific for the N-methyl-D-aspartate (NMDA) receptor subunit NMDAR1 and for the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2(4), we have examined the distribution of these polypeptides in human hippocampal tissue that was surgically removed from patients with intractable TLE. Surgical specimens were classified according to the presence of Ammon's horn sclerosis (AHS) or a focal lesion in the temporal lobe. Cell counts and a densitometric analysis of the immunoreactivity patterns were carried out for all hippocampal subfields. NMDAR1 and GluR2(4) levels were markedly reduced in patients with AHS, primarily in those subfields with substantial neuronal cell loss (in particular CA1, CA4 and CA3), compared to those seen in patients with focal lesions and in control specimens obtained at autopsy. In contrast, the molecular layer of the dentate gyrus (DG-ML) showed significantly higher levels of GluR2(4) immunoreactivity in AHS compared to control tissue, while NMDAR1 showed no significant up-regulation in this sublayer. When the receptor staining intensity was normalized for alterations in neuronal density, no significant alterations could be detected except for an increase in GluR2(4) in the DG-ML of patients with AHS. These changes may reflect synaptic reorganization observed in the DG-ML of specimens from patients with chronic intractable TLE.

Unilateral frontal cortex ablation producing neglect causes time-dependent changes in striatal glutamate receptors

This study's goal is to identify adaptations involving striatal glutamate (GLU) or dopamine (DA) receptors that may contribute to recovery of function following cortical injury. Unilateral aspiration of the medial agranular region of frontal cortex (AGm) in rats produces neglect of contralateral stimuli. Pharmacological and immunocytochemical studies suggest that glutamatergic and dopaminergic processes within striatum may contribute to spontaneous recovery from this neglect. This study examined by autoradiography radioligand binding to striatal GLU and DA receptor subfamilies in AGm-ablated rats surviving 5 days (unrecovered) or 3 or more weeks (recovered) postsurgery. Density of radioligand binding was quantified in striatal subregions by computerized image analysis. Compared to striatal binding densities in the intact hemisphere, [3H]kainate binding and [3H]GLU binding to NMDA receptors were decreased in the lesioned hemisphere of unrecovered AGm-ablated rats, but normalized (for kainate) or increased (for NMDA) in the lesioned hemisphere of recovered rats. Ablation of AGm did not affect [3H]AMPA binding or the binding of [3H]SCH23390, [3H]spiperone, or [3H]mazindol to dopaminergic D1 or D2 receptor subfamilies, or to DA uptake sites, respectively. The results suggest that a small percentage of NMDA and kainate receptors are located on corticostriatal axon terminals, and that over time an upregulation of striatal NMDA and/or kainate receptors may offset the loss of cortical glutamatergic input caused by cortical injury. These time-dependent alterations in GLU receptors may contribute to the recovery of function and normalizations of immediate early gene expression seen weeks after AGm ablation. Upregulation of striatal dopamine receptors was not evident, and thus is unlikely to mediate recovery from neglect produced by cortical injury.