Metaplastic Effects of Ketamine and MK-801 on Glutamate Receptors Expression in Rat Medial Prefrontal Cortex and Hippocampus

Ketamine and MK-801 by blocking NMDA receptors may induce reinforcing effects as well as schizophrenia-like symptoms. Recent results showed that ketamine can also effectively reverse depressive signs in patients' refractory to standard therapies. This evidence clearly points to the need of characterization of effects of these NMDARs antagonists on relevant brain areas for mood disorders. The aim of the present study was to investigate the molecular changes occurring at glutamatergic synapses 24 h after ketamine or MK-801 treatment in the rat medial prefrontal cortex (mPFC) and hippocampus (Hipp). In particular, we analyzed the levels of the glutamate transporter-1 (GLT-1), NMDA receptors, AMPA receptors subunits, and related scaffolding proteins. In the homogenate, we found a general decrease of protein levels, whereas their changes in the post-synaptic density were more complex. In fact, ketamine in the mPFC decreased the level of GLT-1 and increased the level of GluN2B, GluA1, GluA2, and scaffolding proteins, likely indicating a pattern of enhanced excitability. On the other hand, MK-801 only induced sparse changes with apparently no correlation to functional modification. Differently from mPFC, in Hipp, both substances reduced or caused no changes of glutamate receptors and scaffolding proteins expression. Ketamine decreased NMDA receptors while increased AMPA receptors subunit ratios, an effect indicative of permissive metaplastic modulation; conversely, MK-801 only decreased the latter, possibly representing a blockade of further synaptic plasticity. Taken together, these findings indicate a fine tuning of glutamatergic synapses by ketamine compared to MK-801 both in the mPFC and Hipp.

An emerging map of glutamate delta 1 receptors in the forebrain

Glutamate delta 1 (GluD1) and glutamate delta 2 (GluD2) form the delta family of ionotropic glutamate receptors; these proteins plays widespread roles in synaptic architecture, motor behavior, and cognitive function. Though the role of GluD2 at cerebellar parallel fiber-Purkinje cell synapses is well established, attention now turns to the function of GluD receptors in the forebrain. GluD1 regulates synaptic assembly and modulation in multiple higher brain regions, acting as a postsynaptic cell adhesion molecule with effects on both excitatory and inhibitory transmission. Furthermore, variations and mutations in the GRID1 gene, which codes for GluD1, and in genes which code for proteins functionally linked to GluD1, are associated with mental disorders including autism, schizophrenia, bipolar disorder, and major depression. Cerebellin (Cbln) family proteins, the primary binding partners of delta receptors, are secreted C1q-like proteins which also bind presynaptic neurexins (NRXNs), forming a tripartite synaptic bridge. Published research explores this bridge's function in regions including the striatum, hippocampus, cortex, and cerebellum. In this review, we summarize region- and circuit-specific functions and expression patterns for GluD1 and its related proteins, and their implications for behavior and disease.

Psychosocial Crowding Stress-Induced Changes in Synaptic Transmission and Glutamate Receptor Expression in the Rat Frontal Cortex

This study demonstrates how exposure to psychosocial crowding stress (CS) for 3, 7, and 14 days affects glutamate synapse functioning and signal transduction in the frontal cortex (FC) of rats. CS effects on synaptic activity were evaluated in FC slices of the primary motor cortex (M1) by measuring field potential (FP) amplitude, paired-pulse ratio (PPR), and long-term potentiation (LTP). Protein expression of GluA1, GluN2B mGluR1a/5, VGLUT1, and VGLUT2 was assessed in FC by western blot. The body’s response to CS was evaluated by measuring body weight and the plasma level of plasma corticosterone (CORT), adrenocorticotropic hormone (ACTH), and interleukin 1 beta (IL1B). CS 3 14d increased FP and attenuated LTP in M1, while PPR was augmented in CS 14d. The expression of GluA1, GluN2B, and mGluR1a/5 was up-regulated in CS 3d and downregulated in CS 14d. VGLUTs expression tended to increase in CS 7d. The failure to blunt the effects of chronic CS on FP and LTP in M1 suggests the impairment of habituation mechanisms by psychosocial stressors. PPR augmented by chronic CS with increased VGLUTs level in the CS 7d indicates that prolonged CS exposure changed presynaptic signaling within the FC. The CS bidirectional profile of changes in glutamate receptors’ expression seems to be a common mechanism evoked by stress in the FC.

Differential effects between one week and four weeks exposure to same mass of SO2 on synaptic plasticity in rat hippocampus

Sulfur dioxide (SO2 ) is a ubiquitous air pollutant. The previous studies have documented the adverse effects of SO2 on nervous system health, suggesting that acutely SO2 inhalation at high concentration may be associated with neurotoxicity and increase risk of hospitalization and mortality of many brain disorders. However, the remarkable features of air pollution exposure are lifelong duration and at low concentration; and it is rarely reported that whether there are different responses on synapse when rats inhaled same mass of SO2 at low concentration with a longer term. In this study, we evaluated the synaptic plasticity in rat hippocampus after exposure to same mass of SO2 at various concentrations and durations (3.5 and 7 mg/m(3) , 6 h/day, for 4 weeks; and 14 and 28 mg/m(3) , 6 h/day, for 1 week). The results showed that the mRNA level of synaptic plasticity marker Arc, glutamate receptors (GRIA1, GRIA2, GRIN1, GRIN2A, and GRIN2B) and the protein expression of memory related kinase p-CaMKпα were consistently inhibited by SO2 both in 1 week and 4 weeks exposure cases; the protein expression of presynaptic marker synaptophysin, postsynaptic density protein 95 (PSD-95), protein kinase A (PKA), and protein kinase C (PKC) were increased in 1 week exposure case, and decreased in 4 weeks exposure case. Our results indicated that SO2 inhalation caused differential synaptic injury in 1 week and 4 weeks exposure cases, and implied the differential effects might result from different PKA- and/or PKC-mediated signal pathway. © 2014 Wiley Periodicals, Inc. Environ Toxicol 31: 820-829, 2016.

Cell-specific cre recombinase expression allows selective ablation of glutamate receptors from mouse horizontal cells

In the mouse retina, horizontal cells form an electrically coupled network and provide feedback signals to photoreceptors and feedforward signals to bipolar cells. Thereby, horizontal cells contribute to gain control at the first visual synapse and to the antagonistic organization of bipolar and ganglion cell receptive fields. However, the nature of horizontal cell output remains a matter of debate, just as the exact contribution of horizontal cells to center-surround antagonism. To facilitate studying horizontal cell function, we developed a knockin mouse line which allows ablating genes exclusively in horizontal cells. This knockin line expresses a Cre recombinase under the promoter of connexin57 (Cx57), a gap junction protein only expressed in horizontal cells. Consistently, in Cx57+/Cre mice, Cre recombinase is expressed in almost all horizontal cells (>99%) and no other retinal neurons. To test Cre activity, we crossbred Cx57+/Cre mice with a mouse line in which exon 11 of the coding sequence for the ionotropic glutamate receptor subunit GluA4 was flanked by two loxP sites (GluA4fl/fl). In GluA4fl/fl:Cx57+/Cre mice, GluA4 immunoreactivity was significantly reduced (∼50%) in the outer retina where horizontal cells receive photoreceptor inputs, confirming the functionality of the Cre/loxP system. Whole-cell patch-clamp recordings from isolated horizontal cell somata showed a reduction of glutamate-induced inward currents by ∼75%, suggesting that the GluA4 subunit plays a major role in mediating photoreceptor inputs. The persistent current in GluA4-deficient cells is mostly driven by AMPA and to a very small extent by kainate receptors as revealed by application of the AMPA receptor antagonist GYKI52466 and concanavalin A, a potentiator of kainate receptor-mediated currents. In summary, the Cx57+/Cre mouse line provides a versatile tool for studying horizontal cell function. GluA4fl/fl:Cx57+/Cre mice, in which horizontal cells receive less excitatory input, can thus be used to analyze the contribution of horizontal cells to retinal processing.

An investigation of interactions between hypocretin/orexin signaling and glutamate receptor surface expression in the rat nucleus accumbens under basal conditions and after cocaine exposure

Hypocretin peptides are critical for the effects of cocaine on excitatory synaptic strength in the ventral tegmental area (VTA). However, little is known about their role in cocaine-induced synaptic plasticity in the nucleus accumbens (NAc). First, we tested whether hypocretin-1 by itself could acutely modulate glutamate receptor surface expression in the NAc, given that hypocretin-1 in the VTA reproduces cocaine's effects on glutamate transmission. We found no effect of hypocretin-1 infusion on AMPA or NMDA receptor surface expression in the NAc, measured by biotinylation, either 30 min or 3h after the infusion. Second, we were interested in whether changes in hypocretin receptor-2 (Hcrtr-2) expression contribute to cocaine-induced plasticity in the NAc. As a first step towards addressing this question, Hcrtr-2 surface expression was compared in the NAc after withdrawal from extended-access self-administration of saline (control) versus cocaine. We found that surface Hcrtr-2 levels remain unchanged following 14, 25 or 48 days of withdrawal from cocaine, a time period in which high conductance GluA2-lacking AMPA receptors progressively emerge in the NAc. Overall, our results fail to support a role for hypocretins in acute modulation of glutamate receptor levels in the NAc or a role for altered Hcrtr-2 expression in withdrawal-dependent synaptic adaptations in the NAc following cocaine self-administration.

Drosophila glutamate receptor mRNA expression and mRNP particles

The processes controlling glutamate receptor expression early in synaptogenesis are poorly understood. Here, we examine glutamate receptor (GluR) subunit mRNA expression and localization in Drosophila embryonic/larval neuromuscular junctions (NMJs). We show that postsynaptic GluR subunit gene expression is triggered by contact from the presynaptic nerve, approximately halfway through embryogenesis. After contact, GluRIIA and GluRIIB mRNA abundance rises quickly approximately 20-fold, then falls within a few hours back to very low levels. Protein abundance, however, gradually increases throughout development. At the same time that mRNA levels decrease following their initial spike, GluRIIA, GluRIIB, and GluRIIC subunit mRNA aggregates become visible in the cytoplasm of postsynaptic muscle cells. These mRNA aggregates do not colocalize with eIF4E, but nevertheless presumably represent mRNP particles of unknown function. Multiplex FISH shows that different GluR subunit mRNAs are found in different mRNPs. GluRIIC mRNPs are most common, followed by GluRIIA and then GluRIIB mRNPs. GluR mRNP density is not increased near NMJs, for any subunit; if anything, GluR mRNP density is highest away from NMJs and near nuclei. These results reveal some of the earliest events in postsynaptic development and provide a foundation for future studies of GluR mRNA biology.

Regulation of GABA(A) and glutamate receptor expression, synaptic facilitation and long-term potentiation in the hippocampus of prion mutant mice

Background

Prionopathies are characterized by spongiform brain degeneration, myoclonia, dementia, and periodic electroencephalographic (EEG) disturbances. The hallmark of prioniopathies is the presence of an abnormal conformational isoform (PrP^sc) of the natural cellular prion protein (PrP^c) encoded by the Prnp gene. Although several roles have been attributed to PrP^c, its putative functions in neuronal excitability are unknown. Although early studies of the behavior of Prnp knockout mice described minor changes, later studies report altered behavior. To date, most functional PrP^c studies on synaptic plasticity have been performed in vitro. To our knowledge, only one electrophysiological study has been performed in vivo in anesthetized mice, by Curtis and coworkers. They reported no significant differences in paired-pulse facilitation or LTP in the CA1 region after Schaffer collateral/commissural pathway stimulation.

Methodology/Principal Findings

Here we explore the role of PrP^c expression in neurotransmission and neural excitability using wild-type, Prnp −/− and PrP^c-overexpressing mice (Tg20 strain). By correlating histopathology with electrophysiology in living behaving mice, we demonstrate that both Prnp −/− mice but, more relevantly Tg20 mice show increased susceptibility to KA, leading to significant cell death in the hippocampus. This finding correlates with enhanced synaptic facilitation in paired-pulse experiments and hippocampal LTP in living behaving mutant mice. Gene expression profiling using Illumina™ microarrays and Ingenuity pathways analysis showed that 129 genes involved in canonical pathways such as Ubiquitination or Neurotransmission were co-regulated in Prnp −/− and Tg20 mice. Lastly, RT-qPCR of neurotransmission-related genes indicated that subunits of GABA_A and AMPA-kainate receptors are co-regulated in both Prnp −/− and Tg20 mice.

Conclusions/Significance

Present results demonstrate that PrP^c is necessary for the proper homeostatic functioning of hippocampal circuits, because of its relationships with GABA_A and AMPA-Kainate neurotransmission. New PrP^c functions have recently been described, which point to PrP^c as a target for putative therapies in Alzheimer's disease. However, our results indicate that a “gain of function” strategy in Alzheimer's disease, or a “loss of function” in prionopathies, may impair PrP^c function, with devastating effects. In conclusion, we believe that present data should be taken into account in the development of future therapies.

Injury and repair mechanisms in ischemic stroke: considerations for the development of novel neurotherapeutics

Ischemic stroke triggers a complex and highly interconnected cascade of cellular and molecular events. Early events induced following ischemic injury, including excitotoxicity, calcium overload and oxidative stress, rapidly result in cell death in the infarct core. Later events, such as neuroinflammation and apoptosis, are relevant to the death of the ischemic penumbra. Drugs that limit delayed-injury events have a wide therapeutic window for protection; however, the damaging events of the ischemic cascade will eventually prevail if reperfusion is not achieved within minutes after ischemia. The combination of thrombolytics with protective drugs may provide a promising therapy in the management of stroke. Targeting all components of the neurovascular unit, rather than just the neuron, should be a priority in stroke research, and agents that block multiple events of the injury cascade are more likely to provide cerebroprotection. Understanding when the brain begins the transition from injury to repair could have important implications for stroke therapy. Several ischemic mediators have dual roles, with detrimental acute effects, but beneficial effects in the repair phase; therefore, extending experimental stroke investigations to an analysis of the long-term outcome is important. This review provides a critical evaluation of promising therapeutic strategies for ischemic stroke, and a translational perspective on how to improve success in the development of novel pharmaceuticals for cerebral ischemia.

The glutamatergic system expression in human PC-3 and LNCaP prostate cancer cells

BACKGROUND

The glutamatergic system (Glu system) comprises the Glu receptors (GluRs), the Glu transporters (GluTs) and glutamine synthetase (GS).

MATERIALS AND METHODS

Using PCR-based detection and Western blot analysis, the expression of Glu system components was assessed in human androgen-independent PC-3 and androgen-dependent LNCaP prostate cancer cells.

RESULTS

iGluRs, such as NR1, NR2A, NR2C, NR2D and NR3B; mGLuRs such as mGluR1, mGluR2, mGluR3, mGluR4 and mGluR5; GluTs such as EAAT1, EAAT2, EAAT3 and EAATS; and GS mRNA were steadily expressed in both cell lines. In addition, NR3A, mGluR6, mGluR8 and EAAT4 mRNA were differentially expressed in PC-3 and LNCaP cells, mGluR7 and EAAT4 mRNA expression was induced and mGluR8 was silenced by dihydrotestosterone (DHT) treatment in LNCaP cells. GS, EAAT1 and mGLuR5 were also detected at the protein level in both PC-3 and LNCAP cells.

CONCLUSION

These data suggest that the Glu system could be an important regulator of prostate cancer cell biology.

Down-regulation of early ionotrophic glutamate receptor subunit developmental expression as a mechanism for observed plasticity deficits following gestational exposure to benzo(a)pyrene

The focus of this study was to characterize the impact of gestational exposure to benzo(a)pyrene [B(a)P] on modulation of glutamate receptor subunit expression that is critical for the maintenance of synaptic plasticity mechanisms during hippocampal or cortical development in offspring. Previous studies have demonstrated that hippocampal and/or cortical synaptic plasticity (as measured by long-term potentiation and S1-cortex spontaneous/evoked neuronal activity) and learning behavior (as measured by fixed-ratio performance operant testing) is significantly impaired in polycyclic aromatic or halogenated aromatic hydrocarbon-exposed offspring as compared to controls. These previous studies have also revealed that brain to body weight ratios are greater in exposed offspring relative to controls indicative of intrauterine growth retardation which has been shown to manifest as low birth weight in offspring. Recent epidemiological studies have identified an effect of prenatal exposure to airborne polycyclic aromatic hydrocarbons on neurodevelopment in the first 3 years of life among inner-city children [Perera FP, Rauh V, Whyatt RM, Tsai WY, Tang D, Diaz D, et al. Effect of prenatal exposure to airborne polycyclic aromatic hydrocarbons on neurodevelopment in the first 3 years of life among inner-city children. Environ Health Perspect 2006;114:1287-92]. The present study utilizes a well-characterized animal model to test the hypothesis that gestational exposure to B(a)P causes dysregulation of developmental ionotropic glutamate receptor subunit expression, namely the N-methyl-d-aspartate receptor (NMDAR) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptor (AMPAR) both critical to the expression of synaptic plasticity mechanisms. To mechanistically ascertain the basis of B(a)P-induced plasticity perturbations, timed pregnant Long-Evans rats were exposed in an oral subacute exposure regimen to 0, 25 and 150mug/kg BW B(a)P on gestation days 14-17. The first sub-hypothesis tested whether gestational exposure to B(a)P would result in significant disposition in offspring. The second sub-hypothesis tested whether gestational exposure to B(a)P would result in down-regulation of early developmental expression of NMDA and AMPA receptor subunits in the hippocampus of offspring as well as in primary neuronal cultures. The results of these studies revealed significant: (1) disposition to the hippocampus and cortex, (2) down-regulation of developmental glutamate receptor mRNA and protein subunit expression and (3) voltage-dependent decreases in the amplitude of inward currents at negative potentials in B(a)P-treated cortical neuronal membranes. These results suggest that plasticity and behavioral deficits produced as a result of gestational B(a)P exposure are at least, in part, a result of down-regulation of early developmental glutamate receptor subunit expression and function at a time when excitatory synapses are being formed for the first time in the developing central nervous system. The results also predict that in B(a)P-exposed offspring with reduced early glutamate receptor subunit expression, a parallel deficit in behaviors that depend on normal hippocampal or cortical functioning will be observed and that these deficits will be present throughout life.

Glutamate receptor expression and chronic glutamate toxicity in rat motor cortex

In addition to the loss of spinal motor neurons, amyotrophic lateral sclerosis (ALS) is also associated with degeneration of corticospinal layer V pyramidal neurons and decreased glutamate transport in the cortex. We characterized the glutamate receptors on corticospinal neurons in acutely isolated rat motor cortex slices and found that the synaptic inputs to the corticospinal layer V neurons had a lesser proportional contribution from NMDA receptors relative to AMPA receptors than did layer II/III pyramidal neurons. The synaptic I(AMPA) was also more inwardly rectified, indicating a greater Ca(2+)-permeable component, in layer V. In a cortical organotypic slice culture model, blockade of glutamate transporters elevated glutamate in the media and led to pyramidal neuron loss in both layers. The loss of layer V pyramidal neurons was attenuated by antagonists of AMPA/kainate or Ca(2+)-permeable AMPA receptors, suggesting their therapeutic potential in the protection of the motor cortex in ALS.

Phosphorylation of glutamate receptor interacting protein 1 regulates surface expression of glutamate receptors

The number of synaptic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors (AMPARs) controls the strength of excitatory transmission. AMPARs cycle between internal endosomal compartments and the plasma membrane. Interactions between the AMPAR subunit GluR2, glutamate receptor interacting protein 1 (GRIP1), and the endosomal protein NEEP21 are essential for correct GluR2 recycling. Here we show that an about 85-kDa protein kinase phosphorylates GRIP1 on serine 917. This kinase is present in NEEP21 immunocomplexes and is activated in okadaic acid-treated neurons. Pulldown assays and atomic force microscopy indicate that phosphorylated GRIP shows reduced binding to NEEP21. AMPA or N-methyl-D-aspartate stimulation of hippocampal neurons induces delayed phosphorylation of the same serine 917. A wild type carboxy-terminal GRIP1 fragment expressed in hippocampal neurons interferes with GluR2 surface expression. On the contrary, a S917D mutant fragment does not interfere with GluR2 surface expression. Likewise, coexpression of GluR2 together with full-length wild type GRIP1 enhances GluR2 surface expression in fibroblasts, whereas full-length GRIP1-S917D had no effect. This indicates that this serine residue is implicated in AMPAR cycling. Our results identify an important regulatory mechanism in the trafficking of AMPAR subunits between internal compartments and the plasma membrane.

Glutamate receptors and endoplasmic reticulum quality control: looking beneath the surface

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. The cellular regulation of glutamate receptor (GluR) ion channel function and expression is important for maintaining or adjusting target cell excitability to meet ever-changing demands, for example, in relation to developmental or use-dependent synaptic plasticity. Dysregulation of GluR function or expression may be a contributing factor in certain forms of epilepsy, stroke/ischemia, head trauma, cognitive impairments, and neurodegenerative disease. Recent years have seen substantial progress in understanding how GluRs operate in terms of their structural and functional properties, their synaptic targeting and membrane anchoring by PDZ-domain proteins, and their activity-dependent cycling at the plasma membrane. Yet precious little is known about the earliest events in GluR biogenesis or the mechanisms in place to ensure the GluRs that reach the cell surface are processed, folded, and oligomerized in an appropriate manner. Indeed, only a minor fraction of the GluR content of cells is expressed at any given time on the cell surface, whereas most of the remaining receptors exist in the endoplasmic reticulum (ER). The functional competence and significance of the ER fraction of receptors are presently unknown, but they are generally thought to represent immature, unassembled, or improperly assembled subunits. Some are ultimately destined for insertion in the plasma membrane. Others may be targeted for proteosomal degradation. Still others might provide a latent pool of fully functional receptors that can be recruited to enhance cell excitability in response to specific signals or under pathological conditions. This review will explore the structural and functional elements that regulate GluR assembly and export from the ER.

Lateral components in the cone terminals of the rabbit retina: horizontal cell origin and glutamate receptor expression

We examined the identities of horizontal cell (HC) lateral components in cone terminals and the expression of glutamate receptors on the tips of HC dendrites. We injected A-type horizontal cells (AHCs) with neurobiotin and demonstrated that neurobiotin labeled completely all AHCs within a patch of retina. We converted neurobiotin by using diaminobenzidine and considered labeled processes to be from AHCs and unlabeled processes to be from B-type horizontal cells (BHCs). Three possible combinations of HC dendrites could exist in cone pedicles: both lateral components originating from AHCs, both from BHCs, or one from an AHC and the other from a BHC. EM observations revealed that a majority of cone terminals contained about equal numbers of lateral components originating from each of the two types of HCs and that each of the three possible combinations was present in equal numbers. Localization of different types of glutamate receptors on HC dendritic tips showed that 55% of AHC dendritic tips expressed AMPA receptors and 30% expressed kainate receptors, whereas, in the case of BHCs, 22% of dendritic tips expressed AMPA receptors and 33% expressed kainate receptors. This study suggests that cone photoreceptors feed the light signal equally into networks of AHCs and BHCs and that differential expression of AMPA/kainate receptors by different HCs could account for different functions.

Experience-dependent modification of mechanisms of long-term depression

Mechanisms of long-term potentiation and depression (LTP and LTD) change considerably during development, but the importance of these changes and the factors that control them is not clear. We found that visual experience triggered a switch in mechanisms of LTD in rat perirhinal cortex, an area critical for visual recognition memory. Thus, changes in synaptic plasticity mechanisms were correlated with the changing physiological demands on the CNS.

The glutamatergic system outside the CNS and in cancer biology

Glutamate is a major excitatory neurotransmitter in the CNS. The signalling machinery consists of: glutamate receptors, which are responsible for signal input; plasma glutamate transporters, which are responsible for signal termination; and vesicular glutamate transporters for signal output through exocytic release. Recently, data have suggested that the glutamatergic system plays an important role in non-neuronal tissues. In addition, the expression of glutamatergic system has been implicated in tumour biology. This review outlines the evidence, which suggests that the glutamatergic system may have an important role in cancer biology.

The C. elegans nuclear receptor gene fax-1 and homeobox gene unc-42 coordinate interneuron identity by regulating the expression of glutamate receptor subunits and other neuron-specific genes

The fax-1 gene of the nematode C. elegans encodes a conserved nuclear receptor that is the ortholog of the human PNR gene and functions in the specification of neuron identities. Mutations in fax-1 result in locomotion defects. FAX-1 protein accumulates in the nuclei of 18 neurons, among them the AVA, AVB, and AVE interneuron pairs that coordinate body movements. The identities of AVA and AVE interneurons are defective in fax-1 mutants; neither neuron expresses the NMDA receptor subunits nmr-1 and nmr-2. Other ionotropic glutamate receptor subunits are expressed normally in the AVA and AVE neurons. The unc-42 homeobox gene also regulates AVA and AVE identity; however, unc-42 mutants display the complementary phenotype: NMDA receptor subunit expression is normal, but some non-NMDA glutamate receptor subunits are not expressed. These observations support a combinatorial role for fax-1 and unc-42 in specifying AVA and AVE identity. However, in four other neuron types, fax-1 is regulated by unc-42, and both transcriptional regulators function in the regulation of the opt-3 gene in the AVE neurons and the flp-1 and ncs-1 genes in the AVK neurons. Therefore, while fax-1 and unc-42 act in complementary parallel pathways in some cells, they function in overlapping or linear pathways in other cellular contexts, suggesting that combinatorial relationships among transcriptional regulators are complex and cannot be generalized from one neuron type to another.

Rat gustatory neurons in the geniculate ganglion express glutamate receptor subunits

Taste receptor cells are innervated by primary gustatory neurons that relay sensory information to the central nervous system. The transmitter(s) at synapses between taste receptor cells and primary afferent fibers is (are) not yet known. By analogy with other sensory organs, glutamate might a transmitter in taste buds. We examined the presence of AMPA and NMDA receptor subunits in rat gustatory primary neurons in the ganglion that innervates the anterior tongue (geniculate ganglion). AMPA and NMDA type subunits were immunohistochemically detected with antibodies against GluR1, GluR2, GluR2/3, GluR4 and NR1 subunits. Gustatory neurons were specifically identified by retrograde tracing with fluorogold from injections made into the anterior portion of the tongue. Most gustatory neurons in the geniculate ganglion were strongly immunoreactive for GluR2/3 (68%), GluR4 (78%) or NR1 (71%). GluR1 was seen in few cells (16%). We further examined if glutamate receptors were present in the peripheral terminals of primary gustatory neurons in taste buds. Many axonal varicosities in fungiform and vallate taste buds were immunoreactive for GluR2/3 but not for NR1. We conclude that gustatory neurons express glutamate receptors and that glutamate receptors of the AMPA type are likely targeted to synapses within taste buds.

The cellular mRNA expression of GABA and glutamate receptors in spinal motor neurons of SOD1 mice

ALS is a fatal neurodegenerative disorder characterized by a selective loss of upper motor neurons in the motor cortex and lower motor neurons in the brain stem and spinal cord. About 10% of ALS cases are familial, in 10-20% of these, mutations in the gene coding for superoxide dismutase 1 (SOD1) can be detected. Overexpression of mutated SOD1 in mice created animal models which clinically resemble ALS. Abnormalities in glutamatergic and GABAergic neurotransmission presumably contribute to the selective motor neuron damage in ALS. By in situ hybridization histochemistry (ISH), we investigated the spinal mRNA expression of the GABAA and AMPA type glutamate receptor subunits at different disease stages on spinal cord sections of mutant SOD1 mice and control animals overexpressing wild-type SOD1 aged 40, 80, 120 days and at disease end-stage, i.e. around 140 days) (n=5, respectively). We detected a slight but statistically significant decrease of the AMPA receptor subunits GluR3 and GluR4 only in end stage disease animals.

Differential long-term neuroadaptations of glutamate receptors in the basolateral and central amygdala after withdrawal from cocaine self-administration in rats

Humans and laboratory animals remain highly vulnerable to relapse to cocaine-seeking after prolonged periods of withdrawal from the drug. It has been hypothesized that this persistent cocaine relapse vulnerability involves drug-induced alterations in glutamatergic synapses within the mesolimbic dopamine reward system. Previous studies have shown that cocaine self-administration induces long-lasting neuroadaptations in glutamate neurons of the ventral tegmental area and nucleus accumbens. Here, we determined the effect of cocaine self-administration and subsequent withdrawal on glutamate receptor expression in the amygdala, a component of the mesolimbic dopamine system that is involved in cocaine seeking and craving induced by drug-associated cues. Rats were trained for 10 days to self-administer intravenous cocaine (6 h/day) or saline (a control condition) and were killed after one or 30 withdrawal days. Basolateral and central amygdala tissues were assayed for protein expression of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits (GluR1 and GluR2) and the NMDA receptor subunits (NR1, NR2A and NR2B). In the basolateral amygdala, GluR1 but not GluR2 levels were increased on days 1 and 30, NR2A levels were increased on day 1, and NR2B levels were decreased on day 30 of withdrawal from cocaine. In the central amygdala, GluR2 but not GluR1 levels were increased on days 1 and 30, NR1 levels were increased on day 30 and NR2A or NR2B levels were not altered after withdrawal from cocaine. These results indicate that cocaine self-administration and subsequent withdrawal induces long-lasting and differential neuroadaptations in basolateral and central amygdala glutamate receptors.

Mechanical loading modulates glutamate receptor subunit expression in bone

The cellular mechanisms coupling mechanical loading with bone remodeling remain unclear. In the CNS, the excitatory amino acid glutamate (Glu) serves as a potent neurotransmitter exerting its effects via various membrane Glu receptors (GluR). Nerves containing Glu exist close to bone cells expressing functional GluRs. Demonstration of a mechanically sensitive glutamate/aspartate transporter protein and the ability of glutamate to stimulate bone resorption in vitro suggest a role for glutamate linking mechanical load and bone remodeling. We used immunohistochemical techniques to identify the expression of N-methyl-d-aspartate acid (NMDA) and non-NMDA (AMPA or kainate) ionotropic GluR subunits on bone cells in vivo. In bone sections from young adult rats, osteoclasts expressed numerous GluR subunits including AMPA (GluR2/3 and GluR4), kainic acid (GluR567) and NMDA (NMDAR2A, NMDAR2B and NMDAR2C) receptor subtypes. Bone lining cells demonstrated immunoexpression for NMDAR2A, NMDAR2B, NMDAR2C, GluR567, GluR23, GluR2 and GluR4 subunits. Immunoexpression was not evident on osteocytes, chondrocytes or vascular channels. To investigate the effects of mechanical loading on GluR expression, we used a Materials Testing System (MTS) to apply 10 N sinusoidal axial compressive loads percutaneously to the right limbs (radius/ulna, tibia/fibula) of rats. Each limb underwent 300-load cycles/day (cycle rate, 1 Hz) for 4 consecutive days. Contralateral, non-loaded limbs served as controls. Mechanically loaded limbs revealed a load-induced loss of immunoexpression for GluR2/3, GluR4, GluR567 and NMDAR2A on osteoclasts and NMDAR2A, NMDAR2B, GluR2/3 and GluR4 on bone lining cells. Both neonatal rabbit and rat osteoclasts were cultured on bone slices to investigate the effect of the NMDA receptor antagonist, MK801, and the AMPA/kainic acid receptor antagonist, NBQX, on osteoclast resorptive activity in vitro. The inhibition of resorptive function seen suggested that both NMDAR and kainic acid receptor function are required for normal osteoclast function. While the exact role of ionotropic GluRs in skeletal tissue remains unclear, the modulation of GluR subunit expression by mechanical loading lends further support for participation of Glu as a mechanical loading effector. These ionotropic receptors appear to be functionally relevant to normal osteoclast resorptive activity.

Genes involved in Drosophila glutamate receptor expression and localization

Background

A clear picture of the mechanisms controlling glutamate receptor expression, localization, and stability remains elusive, possibly due to an incomplete understanding of the proteins involved. We screened transposon mutants generated by the ongoing Drosophila Gene Disruption Project in an effort to identify the different types of genes required for glutamate receptor cluster development.

Results

To enrich for non-silent insertions with severe disruptions in glutamate receptor clustering, we identified and focused on homozygous lethal mutants in a collection of 2185 BG and KG transposon mutants generated by the BDGP Gene Disruption Project. 202 lethal mutant lines were individually dissected to expose glutamatergic neuromuscular junctions, stained using antibodies that recognize neuronal membrane and the glutamate receptor subunit GluRIIA, and viewed using laser-scanning confocal microscopy. We identified 57 mutants with qualitative differences in GluRIIA expression and/or localization. 84% of mutants showed loss of receptors and/or clusters; 16% of mutants showed an increase in receptors. Insertion loci encode a variety of protein types, including cytoskeleton proteins and regulators, kinases, phosphatases, ubiquitin ligases, mucins, cell adhesion proteins, transporters, proteins controlling gene expression and protein translation, and proteins of unknown/novel function. Expression pattern analyses and complementation tests, however, suggest that any single mutant – even if a mutant gene is uniquely tagged – must be interpreted with caution until the mutation is validated genetically and phenotypically.

Conclusion

Our study identified 57 transposon mutants with qualitative differences in glutamate receptor expression and localization. Despite transposon tagging of every insertion locus, extensive validation is needed before one can have confidence in the role of any individual gene. Alternatively, one can focus on the types of genes identified, rather than the identities of individual genes. This genomic approach, which circumvents many technical caveats in favor of a wider perspective, suggests that glutamate receptor cluster formation involves many cellular processes, including: 1) cell adhesion and signaling, 2) extensive and relatively specific regulation of gene expression and RNA, 3) the actin and microtubule cytoskeletons, and 4) many novel/unexplored processes, such as those involving mucin/polycystin-like proteins and proteins of unknown function.

Effects of electroacupuncture with different frequencies on spinal ionotropic glutamate receptor expression in complete Freund's adjuvant-injected rat

We investigated expressional changes of spinal glutamate receptors by electroacupuncture (EA) in an inflammatory animal model. Inflammation was induced by an intraplantar injection of complete Freund's adjuvant (CFA) into the hindpaw of male Sprague-Dawley rats. Bilateral EA stimulation at 2, 15 and 120 Hz was applied at those acupoints corresponding to Zusanli and Sanyinjiao in man using needles with 3-day intervals for 30 days. Paw edema and mechanical thresholds were measured by a water displacement plethysmometer and Analgesy-Meter, respectively. Edema and mechanical sensitivity of the hindpaw induced by CFA-injection were strongly inhibited by EA stimulation. At 30 days after CFA-injection, effects of EA on ionotropic glutamate receptor (NR-1, NR-2A, GluR-1 and GluR-2/3) expression in association with c-fos and calcitonin gene-related peptide (CGRP) expression were observed in the dorsal horn of the spinal cord using immunohistochemistry. The number of c-fos-like immunostained cells was decreased significantly in the superficial laminae of the dorsal horn by 2Hz EA, but CGRP expression also showed a marked decrease in the same region using the other types of EA stimulation. N-methyl-D-aspartate receptor (NR-1 and NR-2A) expression was attenuated in all regions of the dorsal horn by all types of EA. Of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (GluR-1 and -2/3), only GluR-1 expression was prevented by EA treatment in the superficial laminae and the neck of the dorsal horn. It is concluded that EA treatment can attenuate inflammatory edema and mechanical thresholds in CFA-injected rats through modulating expression of ionotropic glutamate receptors, and especially N-methyl-D-aspartate receptors, in the dorsal horn of the spinal cord.

Dopamine depletion alters responses to glutamate and GABA in the rat subthalamic nucleus

We used whole-cell recordings to compare currents evoked by glutamate and GABA receptor agonists in subthalamic nucleus neurons located ipsilateral and contralateral to unilateral 6-hydroxydopamine (6-OHDA) injections into the substantia nigra zona compacta. The ratio of currents evoked by AMPA (0.6 microM) and NMDA (20 microM) was significantly greater in neurons recorded ipsilateral to 6-OHDA lesions compared with the ratio of currents recorded in control (contralateral) neurons. Both the GABA(A) agonist isoguvacine (20 microM) and the GABA(B) agonist baclofen (10 microM) evoked significantly greater outward currents in subthalamic nucleus neurons ipsilateral to the lesion compared to contralateral neurons. We conclude that chronic dopamine depletion up-regulates expression of GABA receptors and shifts the functional expression of ionotropic glutamate receptor subtype from NMDA to AMPA receptors in subthalamic nucleus.

Effect of chronic glutamate administration to pregnant rats during gestation on metabotropic glutamate receptors from mothers and full-term fetuses brain

Chronic glutamate treatment during gestational period caused a significant decrease in total metabotropic glutamate receptors (mGluR) number. Similar results were observed on the steady-state level of mGlu(1) receptor detected by immunoblotting assays, suggesting that this is the main receptor subtype modulated by agonist exposure. Furthermore, no variations on mRNA coding mGlu(1) receptor were found, suggesting post-transcriptional modulation as a possible mechanism of the lost of receptor detected at the membrane surface. On the other hand, western-blotting to determine level of G(q/11) protein and phospholipase C beta(1) revealed a significant decrease of both proteins in mothers brain. This decrease was associated with significant variation in glutamate and DHPG-stimulated phospholipase C activity. No significant differences on mGluR transduction pathway components were observed in fetuses brain. These results suggest that glutamate intake during pregnancy causes a down-regulation of different proteins involved in glutamate response mediated by mGluR only in mothers brain without significantly affecting fetuses brain.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunit expression in rat olfactory bulb

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (AMPARs) mediate rapid responses at most central excitatory synapses, including those in the olfactory bulb (OB). These receptors are composed of the glutamate subunits GluR1-4, which each has two splice variant (flip/flop) forms. We recently showed that AMPARs on OB neurons are kinetically and pharmacologically diverse. Here, we explored whether this functional heterogeneity reflects a diverse expression of AMPAR subunits and/or splice variants. Total RNA from rat OBs was amplified by RT-PCR. Digestion of the panGluR PCR product with subunit-specific restriction enzymes revealed that the OB expresses mRNAs for GluR1-4 but in different relative amounts i.e., GluR2 (61 +/- 2.4%), GluR1 (31 +/- 3.5%), GluR4 (6.3 +/- 1.4%), GluR3 (1.4 +/- 0.7%). Furthermore, GluR2 and GluR4 transcripts were composed of similar amounts of flip and flop, whereas GluR1 and GluR3 transcripts consisted mostly of flip. If similar to other brain regions, this heterogeneity in patterns of expression may facilitate information processing.

Response of Purkinje neurons to hypobaric hypoxic exposure as shown by alteration in expression of glutamate receptors, nitric oxide synthases and calcium binding proteins

Hypobaric hypoxia is known to impair muscular coordination. It is not known whether hypobaric hypoxia causes any damage to the Purkinje neurons which may be responsible for impairment of muscular coordination. Expression of ionotropic glutamate receptors N-methyl-d-aspartate receptor subunit 1, amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid GluR2/3, calcium binding proteins and nitric oxide synthases in the Purkinje neurons was examined in rats exposed to hypobaric hypoxia. The mRNA expression of N-methyl-d-aspartate receptor subunit 1, GluR2, GluR3 and nitric oxide synthases [neuronal, endothelial and inducible] was upregulated at 3 h peaking at 24 h after the exposure. This was sustained up to 3 days; thereafter, it was comparable to the controls. Immunohistochemical analysis confirmed a marked expression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 at the above time intervals. Immunoexpression of calbindin-D28k (calbindin) and parvalbumin was intense in the soma of Purkinje neurons in the control rats. It was, however, drastically downregulated up to 3 days after exposure. At 3 days the neuronal dendrites showed intense expression of calbindin which returned to control levels at 7 days. Expression of neuronal nitric oxide synthase and inducible nitric oxide synthase was markedly upregulated from 3 h to 3 days whereas endothelial nitric oxide synthase expression, localized in the blood vessels and Purkinje neurons, remained elevated up to 24 h after the exposure. A progressive darkening of the Purkinje neuron cell bodies was observed at ultrastructural level up to 3 days but degenerating cells were not observed. A salient alteration was the dilation and stacking of smooth endoplasmic reticulum in the dendrites up to 14 days after the exposure. The present results suggest that hypobaric hypoxia leads to overexpression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 in Purkinje neurons that may be responsive to altered calcium levels as manifested by decreased expression of calcium binding proteins. This together with excess nitric oxide production may have led to transient ultrastructural changes. We propose that the functions of the Purkinje neurons may be altered in response to an acute exposure to hypobaric hypoxia resulting in impairment of motor coordination.

Ions and amino acid analysis of Cyperus articulatus L. (Cyperaceae) extracts and the effects of the latter on oocytes expressing some receptors

Extracts from rhizomes of Cyperus articulatus L. (Cyperaceae) used in Africa and Amazonia to treat many diseases has been shown to possess sedative and anticonvulsant properties. The aim of this study is to determine the mechanism of action of Cyperus articulatus extracts. In Xenopus oocytes expressing receptors, using electrophysiological measurement, extracts of rhizomes of Cyperus articulatus (300 microg/ml) inhibited 50% of the EC(50) and EC(80) of glutamate (1.3 and 2.9 microM, respectively) induced inward current through hNMDAR1A/2A receptors. Extracts induced very small current through rGluR3 receptors. The largest current induced by the extract (30 mg/ml) represents 128% of the EC(100) of glutamate induced inward current, through rGluR3 receptors. The excess 28% current could be induced by aspartate and/or glutamate in the extracts. The effect on Xenopus oocytes expressing heteromeric GABA(B)R1b/R2 receptors and rectifying potassium channels (Kir3) is clear. A decoction and water extract of Cyperus articulatus induced a large inward current that represented 71 and 57% (respectively) of the EC(100) of gaba (30 microM) induced inward current. The water extract induced also a large current through rectifying potassium channels (Kir3). Part of the current induced through GABA(B) receptors could be related to rectifying potassium channels and GABA(B) site receptors. Cyperus articulatus extracts possessed components that could decrease excitation (NMDA receptor antagonists) and increase inhibition (GABA(B) receptor agonists) in the central nervous system.

UPREGULATION OF GLUTAMATE RECEPTOR SUBTYPES DURING ALCOHOL WITHDRAWAL IN RATS

AIMS

To investigate glutamate receptor subtypes during alcohol withdrawal.

METHODS

Rats were exposed to severe alcohol intoxication for 84 h and then decapitated at 0, 12 and 36 h after the last alcohol dose (n = 7 per group). Alcohol was administered five times a day by intragastric intubation. The densities of N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors were studied in membranes from the forebrain by using the specific ligands [3H]MK-801 and [3H]AMPA, respectively.

RESULTS

Although no change in the maximal density (B(max)) of [3H]MK-801 binding sites was observed at the time of withdrawal, [3H]MK-801 binding was increased by 49% 12 h into the withdrawal reaction compared with the control group. At 36 h post alcohol the B(max) of the [3H]MK-801 binding was still increased by 24% compared with the control group; however, this difference was not statistically significant. When investigated at the time of withdrawal from chronic alcohol intoxication, no significant alterations in the B(max) of the [3H]AMPA binding was detected, but 12 h into the withdrawal reaction the [3H]AMPA binding was markedly increased by 94%. At 36 h post alcohol the [3H]AMPA binding had returned to control levels. No significant alterations in the dissociation constant (K(D)) of either [3H]MK-801 or [3H]AMPA binding was observed at any time point.

CONCLUSIONS

NMDA and AMPA receptors are involved in the cerebral hyperactivity of alcohol withdrawal.

Peripubertal environmental enrichment reverses the effects of maternal care on hippocampal development and glutamate receptor subunit expression

Maternal care in the rat influences the development of cognitive function in the offspring through neural systems known to mediate activity-dependent synaptic plasticity. The offspring of mothers that exhibit increased levels of pup licking/grooming (high-LG mothers) show increased hippocampal N-methyl-D-aspartate (NMDA) subunit mRNA expression, enhanced synaptogenesis and improved hippocampal-dependent spatial learning in comparison with animals reared by low-LG mothers. The effects of reduced maternal care on cognitive function are reversed with peripubertal environmental enrichment; however, the neural mechanisms mediating this effect are not known. In these studies we exposed the offspring of high- and low-LG mothers to environmental enrichment from days 22 to 70 of life, and measured the expression of genes encoding for glutamate receptor subunits and synaptophysin expression as a measure of synaptic density. Environmental enrichment reversed the effects of maternal care on synaptic density and this effect was, in turn, associated with a reversal of the effect of maternal care on the NR2A and NR2B subunits of the NMDA receptor, as well as effects on (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits. Finally, direct infusion of an NR2B-specific NMDA receptor antagonist into the hippocampus eliminated the effects of maternal care on spatial learning/memory in the Morris water maze. These findings suggest that: (1) the effects of maternal care are mediated by changes in NR2B gene expression; and (2) that environmental enrichment reverses the effects of reduced maternal care through the same genomic target, the NR2B gene, and possibly effects on other subunits of the NMDA and AMPA receptors.

Differential localization of glutamate receptor subunits at the Drosophila neuromuscular junction

The subunit composition of postsynaptic neurotransmitter receptors is a key determinant of synaptic physiology. Two glutamate receptor subunits, Drosophila glutamate receptor IIA (DGluRIIA) and DGluRIIB, are expressed at the Drosophila neuromuscular junction and are redundant for viability, yet differ in their physiological properties. We now identify a third glutamate receptor subunit at the Drosophila neuromuscular junction, DGluRIII, which is essential for viability. DGluRIII is required for the synaptic localization of DGluRIIA and DGluRIIB and for synaptic transmission. Either DGluRIIA or DGluRIIB, but not both, is required for the synaptic localization of DGluRIII. DGluRIIA and DGluRIIB compete with each other for access to DGluRIII and subsequent localization to the synapse. These results are consistent with a model of a multimeric receptor in which DGluRIII is an essential component. At single postsynaptic cells that receive innervation from multiple motoneurons, DGluRIII is abundant at all synapses. However, DGluRIIA and DGluRIIB are differentially localized at the postsynaptic density opposite distinct motoneurons. Hence, innervating motoneurons may regulate the subunit composition of their receptor fields within a shared postsynaptic cell. The capacity of presynaptic inputs to shape the subunit composition of postsynaptic receptors could be an important mechanism for synapse-specific regulation of synaptic function and plasticity.

Chronic nicotine causes functional upregulation of ionotropic glutamate receptors mediating hippocampal noradrenaline and striatal dopamine release

It has been proposed that (-)-nicotine can activate release-stimulating presynaptic nicotinic acetylcholine receptors (nAChRs) on glutamatergic nerve terminals to release glutamate, which in turn stimulates the release of noradrenaline (NA) and dopamine (DA) via presynaptic ionotropic glutamate receptors on catecholaminergic terminals. The objective of this study was to compare the function of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazide-4-propionic acid (AMPA) glutamate receptors in synaptosomes of rat hippocampus and striatum following acute and chronic (-)-nicotine administration. In hippocampal synaptosomes, prelabeled with [3H]NA, both the NMDA- and AMPA-evoked releases were higher in (-)-nicotine-treated (10 days) than in (-)-nicotine-treated (1 day) or vehicle-treated (1 or 10 days) rats. In striatal synaptosomes prelabeled with [3H]DA, the NMDA-evoked, but not the AMPA-evoked, release of [3H]DA was higher in (-)-nicotine-treated (10 days) than in nicotine-treated (1 day) or vehicle-treated (1 or 10 days) animals. Chronic (-)-nicotine did not affect catecholamine uptake, basal release and release evoked by high-K+ depolarization. Thus, chronic exposure to nicotine enhances the function of ionotropic glutamate receptors mediating noradrenaline release in the hippocampus and dopamine release in the striatum.

Loss of GLUR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor subunit differentially affects remaining synaptic glutamate receptors in cerebellum and cochlear nuclei

The alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) type of ionotropic glutamate receptor is the major mediator of fast neurotransmission in the brain and spinal cord. Most AMPA receptors are impermeable to calcium because they contain the GluR2 subunit. However, some AMPA receptors lack GluR2 and pass calcium which can mediate synaptic plasticity and, in excess, neurotoxicity. Previously, we showed a decrease in the density of synaptic AMPA receptors in the hippocampus of mice lacking GluR2. In this study, using these GluR2-lacking mice, we examined other areas of the brain that differ in the amount of GluR2 normally present. Like hippocampal spines, cerebellar Purkinje spines normally express AMPA receptors with high GluR2 and showed a decrease in synaptic AMPA receptors in mutant mice. In contrast, neurons that normally express AMPA receptors with little or no GluR2, such as in the anteroventral cochlear nucleus, showed no decrease in AMPA receptors and even showed an increase in one AMPA receptor subunit. These two different patterns may relate to preadaptations to prevent calcium neurotoxicity; such mechanisms might be absent in Purkinje and hippocampal spines so that these neurons must decrease their total expression of synaptic AMPA receptors (calcium permeable in mutant mice) to prevent calcium neurotoxicity. In addition, we found that another glutamate receptor, GluRdelta2, which is abundant only in parallel fibre synapses on Purkinje cells and in the dorsal cochlear nucleus, is up-regulated at these synapses in mutant mice; this probably reflects some change in GluRdelta2 targeting to these synapses.

Functional excitatory synapses in HEK293 cells expressing neuroligin and glutamate receptors

The discovery that neuroligin is a key protein involved in synapse formation offers the unprecedented opportunity to induce functional synapses between neurons and heterologous cells. We took this opportunity recording for the first-time synaptic currents in human embryonic kidney 293 (HEK293) cells transfected with neuroligin and the N-methyl-d-aspartate or AMPA receptor subunits in a co-culture with rat cerebellar granule cells. These currents were similar to synaptic currents recorded in neurons, and their decay kinetics was determined by the postsynaptic subunit combination. Although neuroligin expression was sufficient to detect functional synapses, cotransfection of HEK293 cells with Postsynaptic density-95/synapse-associated protein-90 (PSD-95) significantly increased current frequency. Our results support the central role of neuroligin in the formation of CNS synapses, validate the proposal that PSD-95 allows synaptic maturation, and provide a unique experimental model to study how molecular components determine functional properties of excitatory synapses.

EAAT4 mRNA expression is preserved in the cerebellum of prion protein-deficient mice

To study the mechanism underlying the selective degeneration of Purkinje cells in the cerebellum of the Nagasaki (Ngsk) prion protein-deficient (PrP(-/-)) mice, the mRNA levels of glutamate transporter EAAT4, the marker highly specific for Purkinje cell synapses, were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. EAAT4 mRNA was expressed in the cerebellum of PrP(-/-) mice presenting with cerebellar ataxia, at the levels identical to those in the cerebellum of non-ataxic PrP(+/-) mice. Furthermore, EAAT4 mRNA was identified in the cerebrum of both PrP(-/-) and PrP(+/-) mice, although its levels were much lower than those in the cerebellum. These results indicate that Purkinje cell degeneration found in the cerebellum of PrP(-/-) mice is not primarily caused by glutamate neurotoxicity, although it remains to be investigated whether preserved expression of EAAT4 might represent a compensatory mechanism for protecting against Purkinje cell degeneration in the PrP(-/-) mice cerebellum.

Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats

Previous studies have demonstrated age-related decreases in the transmitters glycine and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with declining levels of binding for glycine receptors. The purpose of this study was to evaluate structural correlates to the transmitter and receptor losses that accompany aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old F344 rats. Montages were constructed from electron micrographs taken of several sites in each AVCN section. The presynaptic terminals were classified by vesicle type and postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry software. The calibers of all dendritic profiles were also measured, and cell counts were performed on semi-thin sections. The data were compared among the three age groups using analysis of variance followed by Tukey's Honestly Significant Difference for pairwise comparisons. There were significant age-related decreases in the size of terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month olds when compared to the 3-month olds. On the other hand, the densities and numbers of synaptic terminals and dendritic profiles did not differ among age groups, and no neuronal losses were evident in the older animals. Also, there were no detectable changes in synaptic area among groups. The decrease in terminal size may be related to age-associated reductions in neurotransmitter levels previously described in the F344 CN. The observations presented here contrast with those previously described in the inferior colliculus (IC), in which there were significant age-related losses of synaptic terminals and dendrites, but no change in the size of synaptic terminals. The lack of synaptic and dendritic losses suggests that the structural connectivity of the rat AVCN remains relatively intact during aging, which is interesting in light of the synaptic and dendritic changes evident in the IC, a major target of its projections.

Effects of chronic dizocilpine on acute pain and on mRNA expression of neuropeptides and the dopamine and glutamate receptors

The mesocorticolimbic circuitry has been implicated in the pathophysiology of several neuropsychiatric syndromes like chronic pain and addiction. The aim of this study was to evaluate the effects of dizocilpine (MK-801), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, on sensorimotor behaviors and the consequent changes in the dopamine, glutamate, and opiate systems in rats. Five groups of rats were subjected to acute tests for nociception (hot plate and paw pressure) before and after MK-801 (0.05, 0.1, 0.2 and 0.4 mg/kg, i.p.) or saline. Another two groups received daily i.p. saline or MK-801 (0.4 mg/kg) for 15 days. The nociceptive tests were performed on days 1, 7, and 14. On day 15 the rats received the last injection and were immediately sacrificed. We measured the mRNA expression, by in situ hybridization (ISH), of various dopamine and glutamate receptors, and enkephalin (Enk), dynorphin (Dyn), and substance P (SP) in the striatum, nucleus accumbens (NAC), piriform and cingulate cortex. Acute MK-801, dose-dependently, resulted in hyperalgesia. The chronic effects of 0.4 mg/kg MK-801 showed an extinction of the acute hyperalgesic effects especially with the hot plate test. The ISH studies revealed a decrease in mRNA expression of Enk and SP in the striatum and NAC. Our results indicate that the reversal of acute MK-801-induced hyperalgesia, with repeated exposure to systemic MK-801, is not directly related to changes in dopamine and glutamate receptors and might involve alteration of the striatal neuropeptide system.

Altered glutamate receptor and corticoliberin gene expression in brain regions related to hedonic behavior in rats

Present experiments in rats were aimed to verify the hypothesis that glutamatergic neurotransmission and stress hormones play a role in impairment of hedonic behavior, a sign of depression-like state. On the basis of individual variability in sucrose preference, test rats were divided into anhedonic and hedonic groups. Anhedonic animals showed higher basal concentrations of adrenocorticotropin and corticosterone but reduced hormonal responses during novelty stress compared to hedonic animals. Acute administration of citalopram (10 mg/kg ip) induced similar effects in both groups. Corticotropin-releasing hormone (CRH) mRNA levels in hypothalamic paraventricular nucleus (PVN) were higher in anhedonic rats. Oxytocin (OT) and vasopressin gene expression in the PVN and proopiomelanocortin (POMC) expression in the anterior pituitary failed to show any significant differences. Gene expression of NR1 receptor subunit of N-methyl-D-aspartate (NMDA) glutamate receptor in the ventral tegmental area (VTA) was found to be lower in anhedonic rats. In the nucleus accumbens (NAc) and the hippocampus of anhedonic animals, higher mRNA levels of NR2A subunit compared to those of hedonic rats were detected. Thus, low sucrose preference is associated with altered HPA axis activity, NMDA receptor subunits and CRH gene expression in selected brain regions. These mechanisms may operate in the disposition to develop hedonic deficit in some mental disorders.

Lurcher GRID2-induced death and depolarization can be dissociated in cerebellar Purkinje cells

The Lurcher mutation transforms the GRID2 receptor into a constitutively opened channel. In Lurcher heterozygous mice, cerebellar Purkinje cells are permanently depolarized, a characteristic that has been thought to be the primary cause of their death, which occurs from the second postnatal week onward. The more dramatic phenotype of Lurcher homozygotes is thought to be due to a simple gene dosage effect of the mutant allele. We have analyzed the phenotype of Lurcher/hotfoot heteroallelic mutants bearing only one copy of the Lurcher allele and no wild-type Grid2. Our results show that the absence of wild-type GRID2 receptors in these heteroallelic mutants induces an early and massive Purkinje cell death that is correlated with early signs of autophagy. This neuronal death is independent of depolarization and can be explained by the direct activation of autophagy by Lurcher GRID2 receptors through the recently discovered signaling pathway formed by GRID2, n-PIST, and Beclin1.

Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function

We previously reported that 1 month following unilateral loss (>95%) of striatal dopamine, there is an increase in striatal glutamate function as measured by in vivo microdialysis and quantitative immuno-gold electron microscopy, Neuroscience 88, 1-16). The goal of this study was to determine the effect of bilateral loss of striatal dopamine on striatal glutamate function following acute or subchronic administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57/B6J mice. Animals were administered either single injections (ip) of 30 mg/kg/day for 7 days (subchronically treated group) or 20 mg/kg x 4 doses every 2 h (acutely treated group) of the toxin or saline. One month following the first injection, there was a 44 and 65% loss in the relative density of tyrosine hydroxylase (TH) immunolabeling within the dorsolateral striatum in the subchronically and acutely MPTP-treated groups compared to the saline group, respectively. There was a decrease in the basal level of extracellular glutamate within the striatum in the subchronically MPTP-treated animals compared to an increase in the acutely treated group in relationship to the saline group. Ultrastructurally, only in the acutely MPTP-treated group was there a decrease in the density of glutamate immunolabeling within nerve terminals associated with an asymmetrical synaptic contact in the dorsolateral striatum compared to either the subchronic or saline groups. In addition, there was a decrease in the relative density of GluR-2/3 subunit immunolabeling within the dorsolateral striatum in the acute MPTP compared to the saline group. These data indicate that differences in striatal glutamate function appear to be associated with the dosing interval of MPTP administration and the variable loss of striatal TH immunolabeling.

The 'glial' glutamate transporter, EAAT2 (Glt-1) accounts for high affinity glutamate uptake into adult rodent nerve endings

The excitatory amino acid transporters (EAAT) removes neurotransmitters glutamate and aspartate from the synaptic cleft. Most CNS glutamate uptake is mediated by EAAT2 into glia, though nerve terminals show evidence for uptake, through an unknown transporter. Reverse-transcriptase PCR identified the expression of EAAT1, EAAT2, EAAT3 and EAAT4 mRNAs in primary cultures of mouse cortical or striatal neurones. We have used synaptosomes and glial plasmalemmal vesicles (GPV) from adult mouse and rat CNS to identify the nerve terminal transporter. Western blotting showed detectable levels of the transporters EAAT1 (GLAST) and EAAT2 (Glt-1) in both synaptosomes and GPVs. Uptake of [3H]D-aspartate or [3H]L-glutamate into these preparations revealed sodium-dependent uptake in GPV and synaptosomes which was inhibited by a range of EAAT blockers: dihydrokainate, serine-o-sulfate, l-trans-2,4-pyrrolidine dicarboxylate (PDC) (+/-)-threo-3-methylglutamate and (2S,4R )-4-methylglutamate. The IC50 values found for these compounds suggested functional expression of the 'glial, transporter, EAAT2 in nerve terminals. Additionally blockade of the majority EAAT2 uptake sites with 100 micro m dihydrokainate, failed to unmask any functional non-EAAT2 uptake sites. The data presented in this study indicate that EAAT2 is the predominant nerve terminal glutamate transporter in the adult rodent CNS.

Expression of glutamate receptor subtypes in the spinal cord of control and mnd mice, a model of motor neuron disorder

We studied the expression and distribution of glutamate receptor subtypes in the spinal cord of mnd mice, a model of motor neuron disorders and neuronal ceroid lipofuscinosis, and control mice using immunocytochemistry and in situ hybridization. The constitutive subunit of the NMDA ionotropic glutamate receptor, NMDAR1, was expressed in all neurons of the grey matter and was not modified in the spinal cord of mnd mice in either its normal or phosphorylated form. The immunoreactivity of GluR2, but not its mRNA, was increased mainly in the substantia gelatinosa both in presymptomatic and in 8-month-old symptomatic mice, suggesting compensatory changes aimed at reducing the Ca2+ permeability of the receptor channel. In spinal cord of mnd mice, mRNA, and protein levels of GluR3 were low only at the symptomatic stage, possibly as a consequence of motor neuron dysfunction. This was not due to motoneuron degeneration, because the number of choline acetyltransferase (ChAT) immunopositive lumbar motor neurons and the ChAT activity in the spinal cord and hind leg muscles of symptomatic mnd mice were no different from control mice. GluR4 mRNA was increased throughout the grey matter, presumably in relation to the marked microglia activation reported in the grey matter of the lumbar spinal cord in mnd mice. These changes in ionotropic glutamate receptors may alter glutamatergic neurotransmission and play some role in the pathology of mnd mice.

Chronic dexamethasone-treatment alters mineralocorticoid receptor, truncated trkB and selected glutamate receptor subunit mRNA expression in the porcine hippocampus

Prepubertal boars (n = 4/treatment) were killed 24 h after a 5 day course of intravenous injections of dexamethasone (Dex, 1 and 5 mg kg(-1)), or saline vehicle. Gene expression was quantified in brain sections following in situ hybridisation histochemistry. The objective was to determine whether chronic glucocorticoid treatment would alter the expression of mRNAs for gluco- and mineralocorticoid receptors (GR and MR), brain-derived neurotrophic factor (BDNF), its receptor, trkB, and selected ionotropic glutamate receptor (iGluR) subunits in the hippocampus. Although Dex did not alter GR message, the higher dose reduced MR mRNA in all hippocampal subfields studied. There was no effect of Dex on the expression of BDNF, or the full-length form of its receptor but there was evidence to suggest that mRNA for the truncated form of trkB was increased. Expression of mRNA for glutamate receptor subunits was either unaffected (NR1) or decreased (GluR2 and GluR3). These findings indicate that acute and chronic glucocorticoid treatment has differential effects on hippocampal gene expression in the porcine brain.

Analysis of glutamate receptor surface expression in acute hippocampal slices

Trafficking of receptors to and from the cell surface is a powerful mechanism for regulating neuronal excitability. To date, the majority of studies concerning glutamate receptor trafficking have been performed in neuronal cultures in which surface expression can be readily assayed by immunofluorescence techniques. Results from such studies have had important implications in the field of synaptic plasticity. However, cultured neurons are, by necessity, prepared from very young animals. Moreover, although an enhancement of excitatory neurotransmission can be induced in such systems, classic long-term potentiation (LTP) can be produced only in acute slices or in vivo. To study trafficking in adult tissues, we have adapted two biochemical techniques, proteolysis and cross-linking. These techniques help define surface-expressed and intracellular pools of native receptors in acute hippocampal slices.

Morphological changes in immunopositive cells of ionotropic glutamate receptor subunits during the development of transplanted fetal ventral mesencephalic neurons

To elucidate the morphological changes in immunopositive cells of ionotropic glutamate receptors within intrastriatal 'developing' grafts of fetal ventral mesencephalon (VM) in 6-hydroxydopamine-lesioned rats, immunohistochemistry was performed to detect cells expressing N-methyl-D-aspartate (NMDA) receptor subunit 1 (NR1), the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits (GluR1, GluR2/3, and GluR4), or tyrosine hydroxylase (TH) in the intrastriatal VM grafts at 1, 4, and 12 weeks following transplantation. One week after transplantation, TH-positive cells were detected without any immunoreactivity of the NMDA and AMPA receptor subunits in the grafts. Four weeks after transplantation, TH-positive cells, distributed homogeneously in the grafts, appeared to be multipolar and larger compared to those at 1 week post-grafting. At this stage, we could observe immunopositive cells of NMDA and AMPA receptors distributed homogeneously in the grafts. Twelve weeks after transplantation, the numbers of NR1- and GluR1-positive cells were smaller than that at 4 weeks post-grafting, whereas TH-positive cells appeared to be more matured in shape and size. On the other hand, the numbers of GluR2/3- and GluR4-positive cells were not changed as compared with those at 4 weeks post-grafting. These results suggest that the ionotropic glutamate receptors have differential roles during the developmental period of the intrastriatal VM grafts.

Control of neuronal subtype identity by the C. elegans ARID protein CFI-1

The Caenorhabditis elegans hermaphrodite nervous system is composed of 302 neurons that fall into at least 118 diverse classes. Here we describe cfi-1, a gene that contributes to the development of neuronal diversity. cfi-1 promotes appropriate differentiation of the URA sensory neurons and inhibits URA from expressing the male-specific CEM neuronal fate. The UNC-86 POU homeodomain protein is present in CEM and URA neurons, and can promote expression of CEM-specific genes in both CEM and URA, but CFI-1 inhibits expression of these genes in the URA cells. cfi-1 also promotes appropriate differentiation and glutamate receptor expression in the AVD and PVC interneurons. cfi-1 encodes a conserved neuron- and muscle-restricted DNA-binding protein containing an A/T rich interaction domain (ARID). ARID proteins regulate early patterning and muscle fate in Drosophila, but they have not previously been implicated in the control of neuronal subtype identity.

Striatal excitatory amino acid transporter transcript expression in schizophrenia, bipolar disorder, and major depressive disorder

Because abnormalities of glutamatergic neurotransmission in psychiatric illness are likely not limited to glutamate receptor expression, we investigated expression of excitatory amino acid transporters (EAATs) in the striatum. The EAATs, normally expressed in both glia (EAAT1 and EAAT2) and neurons (EAAT3 and EAAT4), have previously been implicated in Huntington's disease, amyotrophic lateral sclerosis, and schizophrenia. In this study, we investigated striatal expression of transcripts encoding EAATs in tissue from mood disordered and schizophrenic subjects. With probes designed for the human EAAT1, EAAT2, EAAT3, and EAAT4 transcripts, we performed in situ hybridization and detected decreased expression of EAAT3 and EAAT4 transcripts in the striatum in bipolar disorder. We also detected decreased EAAT3 transcript expression in schizophrenia and decreased EAAT4 transcript expression in major depressive disorder. These results suggest that changes in striatal transporter mRNA expression are restricted to neuronal EAATs and extend the body of evidence implicating abnormal glutamatergic neurotransmission in schizophrenia and mood disorders.

Enhanced auditory reversal learning by genetic activation of protein kinase C in small groups of rat hippocampal neurons

The hippocampus has a central role in specific types of learning, but there is only limited evidence identifying the requisite molecular changes in ensembles of hippocampal neurons. To investigate the role of protein kinase C (PKC) pathways in hippocampal mediated learning, a constitutively active, catalytic domain of rat PKC betaII was delivered into hippocampal dentate granule neurons using a Herpes Simplex Virus (HSV-1) vector. This PKC causes a long-lasting, activation-dependent increase in neurotransmitter release from cultured cells. Activation of PKC pathways in a small percentage (< or =0.26%) of dentate granule neurons was sufficient to enhance rat auditory discrimination reversal learning. The affected neurons altered hippocampal physiology as revealed by elevated NMDA receptor densities in specific hippocampal areas. Thus, these results directly suggest that activation of PKC pathways in a specific hippocampal area alters rat auditory discrimination reversal learning. Because each rat may contain a unique pattern of affected neurons, there appears to be considerable flexibility and/or redundancy in the groups of neurons that can modify learning.