Neuroadaptations in Ionotropic and Metabotropic Glutamate Receptor mRNA Produced by Cocaine Treatment

Abstract : The expression of glutamate receptor/subunit mRNAs was examined 3 weeks after discontinuing 1 week of daily injections of saline or cocaine. The level of mRNA for GluR1-4, NMDAR1, and mGluR5 receptors was measured with in situ hybridization and RT-PCR. In nucleus accumbens, acute cocaine treatment significantly reduced the mRNA level for GluR3, GluR4, and NMDAR1 subunits, whereas repeated cocaine reduced the level for GluR3 mRNA. Acute cocaine treatment also reduced the NMDAR1 mRNA level in dorsolateral striatum and ventral tegmental area. In prefrontal cortex, repeated cocaine treatment significantly increased the level of GluR2 mRNA. The GluR2 mRNA level was not changed by acute or repeated cocaine in any other brain regions examined. Repeated cocaine treatment also significantly increased mGluR5 mRNA levels in nucleus accumbens shell and dorsolateral striatum. Functional properties of the ionotropic glutamate receptors are determined by subunit composition. In addition, metabotropic glutamate receptors can modulate synaptic transmission and the response to stimulation of ionotropic receptors. Thus, the observed changes in levels of AMPA and NMDA receptor subunits and the mGluR5 metabotropic receptor may alter excitatory neurotransmission in the mesocorticolimbic dopamine system, which could play a significant role in the enduring biochemical and behavioral effects of cocaine.     

Immunocytochemical localization of metabotropic (mGluR2/3 and mGluR4a) and ionotropic (GluR2/3) glutamate receptors in adrenal medullary ganglion cells

The localization of metabotropic glutamate receptors of groups II (mGluR2/3) and III (mGluR4a) and the subunits 2 and 3 of alfa-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) ionotropic glutamate receptors (GluR2/3) was investigated with immunocytochemical methods in the rat adrenal gland. MGluR2/3, mGluR4a and GluR2/3 immunoreactivities were observed in large-sized, centrally located type I adrenal medullary ganglion neurons. Furthermore, the small-sized type II adrenal ganglion neurons identified by their immunoreactivity to brain nitric oxide synthase (bNOS), also expressed mGluR2/3, mGluR4a and GluR2/3. These cells were disposed in the peripheral portion of the adrenal medulla. None of the type I neurons were positively labeled for bNOS. These morphological observations suggest that activation of glutamate receptors in ganglion neurons may be instrumental in the control of adrenal endocrine systems as well as blood regulation.     

Localization of metabotropic glutamate receptors in the outer plexiform layer of the goldfish retina

We studied the localization of metabotropic glutamate receptors (mGluRs) in the goldfish outer plexiform layer by light-and electron-microscopical immunohistochemistry. The mGluR1α antibody labeled putative ON-type bipolar cell dendrites and horizontal cell processes in both rod spherules and cone triads. Immunolabeling for mGluR2/3 was absent in the rod synaptic complex but was found at horizontal cell dendrites directly opposing the cone synaptic ribbon. The mGluR5 antibody labeled Müller cell processes wrapping rod terminals and horizontal cell somata. The mGluR7 antibody labeled mainly horizontal cell dendrites invaginating rods and cones and some putative bipolar cell dendrites in the cone synaptic complex. The finding of abundant expression of various mGluRs in bipolar and horizontal cell dendrites suggests multiple sites of glutamatergic modulation in the outer retina. Financial support for this work was provided by Conselho Nacional de Pesquisa (CNPq), Brazil (grant 200915/98-3 to C.J.)     

Evidence for low GluR2 AMPA receptor subunit expression at synapses in the rat basolateral amygdala

Fast excitatory synaptic responses in basolateral amygdala (BLA) neurons are mainly mediated by ionotropic glutamate receptors of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtype. AMPA receptors containing an edited GluR2 subunit are calcium impermeable, whereas those that lack this subunit are calcium permeable and also inwardly rectifying. Here, we sought to determine the extent to which synapses in the rat BLA have AMPA receptors with GluR2 subunits. We assessed GluR2 protein expression in the BLA by immunocytochemistry with a GluR2 subunit-specific antiserum at the light and electron microscopic level; for comparison, a parallel examination was carried out in the hippocampus. We also recorded from amygdala brain slices to examine the voltage-dependent properties of AMPA receptor- mediated evoked synaptic currents in BLA principal neurons. At the light microscopic level, GluR2 immunoreactivity was localized to the perikarya and proximal dendrites of BLA neurons; dense labeling was also present over the pyramidal cell layer of hippocampal subfields CA1 and CA3. In electron micrographs from the BLA, most of the synapses were asymmetrical with pronounced postsynaptic densities (PSD). They contained clear, spherical vesicles apposed to the PSD and were predominantly onto spines (86%), indicating that they are mainly with BLA principal neurons. Only 11% of morphological synapses in the BLA were onto postsynaptic elements that showed GluR2 immunoreactivity, in contrast to hippocampal subfields CA1 and CA3 in which 76% and 71% of postsynaptic elements were labeled (p < 0.001). Synaptic staining in the BLA and hippocampus, when it occurred, was exclusively postsynaptic, and particularly heavy over the PSD. In whole-cell voltage clamp recordings, 72% of BLA principal neurons exhibited AMPA receptor-mediated synaptic currents evoked by external capsule stimulation that were inwardly rectifying. Although BLA principal neurons express perikaryal and proximal dendritic GluR2 immunoreactivity, few synapses onto these neurons express GluR2, and a preponderance of principal neurons have inwardly rectifying AMPA-mediated synaptic currents, suggesting that targeting of GluR2 to synapses is restricted. Many BLA synaptic AMPA receptors are likely to be calcium permeable and could play roles in synaptic plasticity, epileptogenesis and excitoxicity.     

大鼠脑内代谢型谷氨酸受体5亚型(mGluR5)定位分布的免疫细胞化学研究

本研究用免疫细胞化学技术观察了大鼠脑内参与兴奋性突触传递的代谢型谷氨酸受体5亚型(mGluR5)的精确定位分布.mGluR5阳性浓染的神经元胞体和纤维密集地分布于大脑皮质浅层、嗅球、伏核、尾壳核、前脑基底部、隔区、苍白球、腹侧苍白球、海马CA1和CA2区、下丘中央核、被盖背侧核和三叉神经脊束核尾侧亚核浅层;淡染而稀疏的mGluR5阳性神经元胞体和纤维见于屏状核、终纹床核、杏仁中央核、丘脑部分核团、上丘浅灰质层、外侧丘系背侧核和延髓中央灰质.     

δ-Glutamate Receptors Are Differentially Distributed at Parallel and Climbing Fiber Synapses on Purkinje Cells

Abstract: Neurons containing multiple excitatory inputs may sort and target glutamate receptor subtypes to subsets of synapses. A good model for testing this hypothesis is the Purkinje cell, which expresses significant levels of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate, N -methyl- d -aspartate, δ-, and metabotropic glutamate receptors. Purkinje cells receive two excitatory inputs, the parallel and climbing fibers; the combined effect of stimulation of these two inputs is to produce long-term depression of parallel fiber/Purkinje cell neurotransmission. Distribution of glutamate receptors in these two synapse populations in rat cerebella was studied using preembedding immunocytochemistry with antibodies to GluR1, GluR2/3, GluR5-7, NR1, δ1/2, and mGluR1α. Moderate/dense postsynaptic staining was most frequent in postsynaptic densities and spines of both parallel and climbing fiber synapses with mGluR1α antibody, was intermediate in frequency with GluR2/3 and GluR5-7 antibodies, and was least frequent with GluR1 and NR1 antibodies. The most striking finding was the absence of significant postsynaptic staining with δ1/2 antibody in climbing fiber synapses in adult animals, even though postsynaptic staining was prevalent in parallel fiber synapses with this antibody. In contrast to adults, moderate/dense postsynaptic immunolabeling of climbing fiber synapses with δ1/2 antibody was common in rats at 10 days postnatal. This study provides direct morphological evidence that δ-glutamate receptors are differentially targeted to synapse populations. Our results support previous suggestions that δ2 is involved in development of parallel and climbing fiber synapses and in long-term depression of parallel fiber/Purkinje synaptic responses in adults.     

Distribution of GluR1 is altered in the olfactory bulb following neonatal naris occlusion

The olfactory system is well suited for studies of glutamate receptor plasticity. The sensory neurons are glutamatergic, and they turn over throughout life, and the olfactory bulb neurons that process their inputs express many of the known glutamate receptor subunits. Neonatal naris occlusion alters olfactory bulb development and the expression of certain neuroactive substances and receptors, at least in part due to loss of the sensory inputs. We therefore postulated that neonatal naris occlusion might alter glutamate receptor expression during postnatal development. Single nares of newborn mice were occluded on postnatal days 1-2, and the distribution of glutamate receptor subunits was evaluated using immunoperoxidase methods. Light microscopic examination on postnatal day 6 failed to reveal adult-like staining of neuronal cell bodies in the olfactory bulbs. By day 12, cell bodies that were immunoreactive (-IR) for the GluR1 subunit were visible in the external plexiform layer (EPL) of both sides. By day 18, many of the GluR1-IR cell bodies could be identified as cell types that had previously been reported to express homomeric GluR1 receptors. Analysis of single, mid-dorsal sections from 18-25-day-old mice showed that the medial EPL of the occluded side had a significantly lower density of these cell bodies. The GluR1 staining of the adjacent mitral cell layer (MCL) was also heavier on the occluded side, but no gross differences in staining for other glutamate receptor subunits were observed. Neonatal naris occlusion therefore appears to provide a new model for studying expression of GluR1 receptors during the development of a discrete population of olfactory bulb neurons.     

A search for colocalization of mglula receptors with CRF or NPY in the rat brain amygdala

Excitatory neurotransmitter glutamate, as well as corticoliberin (CRF) and neuropeptide Y (NPY) play an important role in fear and anxiety. Among the brain structures engaged in these effects the important one is amygdala. In the present study, a single and double immunohistochemical staining techniques were used in order to visualize CRF, NPY and metabotropic glutamate receptors (mGluR1a) in rat amygdala. MGluR1a belongs to class of postsynaptic excitatory receptors and has a preferable somatic localization. CRF and NPY were localized using rabbit polyclonal antibodies, and mGluR1a using a mouse monoclonal one. Then, ABC-peroxidase and DAB or benzidine were used. Upon single immunostaining, NPY and CRF were found in some nerve cell bodies and fibres in the amygdala. The immunoreactivity of mGluR1 a was observed in some nerve cells, processes and fibres, especially on the border between the central and the basolateral nuclei and ventrally to that region. Double staining revealed mGluR1 a-IR on some CRF- and NPY-immunoreactive nerve cell bodies and processes. The obtained results indicate that mGlu1a receptors may control at least some NPY and CRF neurons in the amygdala.     

Postnatal developmental expressions of neurotransmitters and receptors in various brain stem nuclei of rats.

Previously, we reported that the expression of cytochrome oxidase in a number of brain stem nuclei exhibited a plateau or reduction at postnatal day (P) 3-4 and a dramatic decrease at P12, against a general increase with age. The present study examined the expression of glutamate, N-methyl-D-aspartate receptor subunit 1 (NMDAR1), GABA, GABAB receptors, glycine receptors, and glutamate receptor subunit 2 (GluR2) in the ventrolateral subnucleus of the solitary tract nucleus, nucleus ambiguus, hypoglossal nucleus, medial accessory olivary nucleus, dorsal motor nucleus of the vagus, and cuneate nucleus, from P2 to P21 in rats. Results showed that 1) the expression of glutamate increased with age in a majority of the nuclei, whereas that of NMDAR1 showed heterogeneity among the nuclei; 2) GABA and GABAB expressions decreased with age, whereas that of glycine receptors increased with age; 3) GluR2 showed two peaks, at P3-4 and P12; and 4) glutamate and NMDAR1 showed a significant reduction, whereas GABA, GABAB receptors, glycine receptors, and GluR2 exhibited a concomitant increase at P12. These features were present but less pronounced in hypoglossal nucleus and dorsal motor nucleus of the vagus and were absent in the cuneate nucleus. These data suggest that brain stem nuclei, directly or indirectly related to respiratory control, share a common developmental trend with the pre-Botzinger complex in having a transient period of imbalance between inhibitory and excitatory drives at P12. During this critical period, the respiratory system may be more vulnerable to excessive exogenous stressors.     

Glutamate-induced cobalt uptake elicited by kainate receptors in rat taste bud cells

Glutamate-induced cobalt uptake reveals non-N-methyl-D-aspartate (non-NMDA) glutamate receptors (GluRs) in rat taste bud cells. However, it is not known which type of non-NMDA glutamate receptors is involved. We used a cobalt staining technique combined with pharmacological tests for kainate or alpha-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) receptors and/or immunohistochemistry against subunits of GluRs to examine the presence of non-NMDA receptors in rat foliate tastebud cells. Cobalt uptake into taste cells was elicited by treating taste buds with glutamate, kainate or SYM 2081, a kainate receptor agonist. Treating taste buds with AMPA or fluorowillardiine did not stimulate significant cobalt uptake. Moreover, 6-cyano-7-nitro-quinoxaline-2, 3-dione significantly reduced cobalt staining elicited by glutamate or kainate receptor agonists, but SYM 2206, an AMPA receptor antagonist, did not. Immunohistochemistry against subunits of GluRs reveals GluR6 and KA1-like immunoreactivity. Moreover, most glutamate-induced cobalt-stained cells showed GluR6 and KA1-like immunoreactivity. These results suggest that glutamate-induced cobalt uptake in taste cells occurs mainly via kainate type GluRs.     

Glutamate receptor properties of human mesencephalic neural progenitor cells: NMDA enhances dopaminergic neurogenesis in vitro

Human midbrain‐derived neural progenitor cells (NPCs) may serve as a continuous source of dopaminergic neurons for the development of novel regenerative therapies in Parkinson’s disease. However, the molecular and functional characteristics of glutamate receptors in human NPCs are largely unknown. Here, we show that differentiated human mesencepahlic NPCs display a distinct pattern of glutamate receptors. In whole‐cell patch‐clamp recordings, l ‐glutamate and NMDA elicited currents in 93% of NPCs after 3 weeks of differentiation in vitro. The concentration‐response plots of differentiated NPCs yielded an EC₅₀ of 2.2 μM for glutamate and an EC₅₀ of 36 μM for NMDA. Glutamate‐induced currents were markedly inhibited by memantine in contrast to 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) suggesting a higher density of functional NMDA than alpha‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA)/kainate receptors. NMDA‐evoked currents and calcium signals were blocked by the NR2B‐subunit specific antagonist ifenprodil indicating functional expression of NMDA receptors containing subunits NR1 and NR2B. In calcium imaging experiments, the blockade of voltage‐gated calcium channels by verapamil abolished AMPA‐induced calcium responses but only partially reduced NMDA‐evoked transients suggesting the expression of calcium‐impermeable, GluR2‐containing AMPA receptors. Quantitative real‐time PCR showed a predominant expression of subunits NR2A and NR2B (NMDA), GluR2 (AMPA), GluR7 (kainate), and mGluR3 (metabotropic glutamate receptor). Treatment of NPCs with 100 μM NMDA in vitro during proliferation (2 weeks) and differentiation (1 week) increased the amount of tyrosine hydroxylase‐immunopositive cells significantly, which was reversed by addition of memantine. These data suggest that NMDA receptors in differentiating human mesencephalic NPCs are important regulators of dopaminergic neurogenesis in vitro.     

Co-stimulation of mGluR5 and N-methyl-D-aspartate receptors is required for potentiation of excitatory synaptic transmission in hippocampal neurons

In the central nervous system, excitatory synaptic transmission is mediated by the neurotransmitter glutamate and its receptors. Interestingly, stimulation of group I metabotropic glutamate receptors (mGluRs) can either enhance or depress synaptic transmission at CA1 hippocampal synapses. Here we report that co-activation of mGluR5, a member of the group I mGluR family, and N-methyl-d-aspartate receptors (NMDARs) potentiates NMDAR currents and induces a long lasting enhancement of excitatory synaptic transmission in primary cultured hippocampal neurons. Unexpectedly, activation of mGluR5 alone fails to enhance evoked NMDAR currents and synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) AMPAR currents. The observed potentiation requires an mGluR5-induced, inositol 1,4,5-trisphosphate receptor-mediated mobilization of intracellular Ca2+, which acts in concert with a protein kinase C, calcium-activated tyrosine kinase cascade to induce a long lasting enhancement of NMDAR and AMPAR currents.     

Laminar segregation of the cortical plate during corticogenesis is accompanied by changes in glutamate receptor expression

We tested the hypothesis that subtypes of glutamate receptors (GluRs) are differentially expressed during corticogenesis. The neocortex of fetal sheep (term = approximately 145 days) was evaluated by immunoblotting and immunohistochemistry to determine the protein expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (GluR1, GluR2/GluR3 [GluR2/3], and GluR4), kainate (KA) receptors (GluR6/GluR7 [GluR6/7]), and a metabotropic GluR (mGluR5). AMPA/KA receptors and mGluR5 were expressed in neocortex by midgestation. GluR1 and mGluR5 expression increased progressively, with expression being maximal just before birth and then decreasing postnatally. GluR2/3 and GluR6/7 levels increased progressively during corticogenesis to reach adult levels near term. GluR4 was expressed at low levels during corticogenesis and in adult neocortex. The localizations of GluRs in the developing neocortex were distinct. Each GluR had a differential localization within the marginal zone, cortical plate, and subplate. GluR subtypes were expressed in laminar patterns before major cytoarchitectonic segregation occurred based on Nissl staining, although connectional patterns were emergent by midgestation based on labeling of corticostriatal projections with DiI. The GluR localizations changed during cortical plate segregation, resulting in highly differential distributions in the neocortex at term. AMPA/KA receptors were expressed transiently in proliferative zones and in developing white matter. Oligodendrocytes in fetal brain expressed AMPA receptors. The expression of ion channel and metabotropic GluR subtypes is dynamic during corticogenesis, with subtype- and subunit-specific regulation occurring during the laminar segregation of the cortical plate and differentiation of the neocortex.     

Developmental characteristics of AMPA receptors in chick lumbar motoneurons

Ca2+ fluxes through ionotropic glutamate receptors regulate a variety of developmental processes, including neurite outgrowth and naturally occurring cell death. In the CNS, NMDA receptors were originally thought to be the sole source of Ca2+ influx through glutamate receptors; however, AMPA receptors also allow a significant influx of Ca2+ ions. The Ca2+ permeability of AMPA receptors is regulated by the insertion of one or more edited GluR2 subunits. In this study, we tested the possibility that changes in GluR2 expression regulate the Ca2+ permeability of AMPA receptors during a critical period of neuronal development in chick lumbar motoneurons. GluR2 expression is absent between embryonic day (E) 5 and E7, but increases significantly by E8 in the chick ventral spinal cord. Increased GluR2 protein expression is correlated with parallel changes in GluR2 mRNA in the motoneuron pool. Electrophysiological recordings of kainate-evoked currents indicate a significant reduction in the Ca2(+)-permeability of AMPA receptors between E6 and E11. Kainate-evoked currents were sensitive to the AMPA receptor blocker GYKI 52466. Application of AMPA or kainate generates a significant increase in the intracellular Ca2+ concentration in E6 spinal motoneurons, but generates a small response in older neurons. Changes in the Ca(2+)-permeability of AMPA receptors are not mediated by age-dependent changes in the editing pattern of GluR2 subunits. These findings raise the possibility that Ca2+ influx through Ca(2+)-permeable AMPA receptors plays an important role during early embryonic development in chick spinal motoneurons.     

Glutamate and Quisqualate Regulate Expression of Metabotropic Glutamate Receptor mRNA in Cultured Cerebellar Granule Cells

Abstract: Metabotropic glutamate receptor (type 1; mGluR1 ) is expressed predominantly in the hippocampus and the cerebellum. Using cultured cerebellar granule cells, we investigated the regulation of the mGluR1 mRNA expression. Levels of mGluR1 mRNA were decreased to less than half by high potassium stimulation and by glutamate and quisqualate. Although these glutamate receptor agonists tested are also known to cause neuronal cell death in culture, the effect of cell death cannot explain the observed reduction in mGluR1 mRNA because of the following reasons: (a) antagonists of N -methyl-D-aspartate and non- N -methyl-D-aspartate receptors inhibited cell death, but not the reduction of the level of mGluR1 mRNA; (b) mGluR1 mRNA returned to its initial level 48 h after the agonist application; and (c) the mRNA level of one of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptors (GluR1) was not altered by these conditions. Therefore, we conclude that the glutamate or quisqualate stimulation can specifically inhibit the expression of mGluR1 mRNA. The dose response of quisqualate for the reduction in mGluR1 mRNA is consistent with that for inositol phosphate formation stimulated through the cloned mGluR1 . The mRNA reduction did not require extracellular calcium. Desensitization of mGluR1 with phorbol ester abolished the mRNA reduction. These results suggest that the reduction in mGluR1 mRNA is mediated by the activation of the metabotropic receptor itself.     

GluR1 glutamate receptor subunit is regulated differentially in the primate basal ganglia following nigrostriatal dopamine denervation

Nigrostriatal dopaminergic denervation is associated with complex changes in the functional and neurochemical anatomy of the basal ganglia. The excitatory neurotransmitter glutamate mediates neural signaling at crucial points of this circuitry, and glutamate receptors are differentially distributed in the basal ganglia. Available evidence suggests that the glutamatergic corticostriatal and subthalamofugal pathways become overactive after nigrostriatal dopamine depletion. In this study, we have analyzed the regulation of the GluR1 subunit of the a-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptor in the basal ganglia of primates following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopamine denervation. The dopamine denervation resulted in distinct alterations in GluR1 distribution: (1) GluR1 protein expression was markedly increased in caudate and putamen, and this was most pronounced in the striosomes; (2) GluR1 protein was altered minimally in subthalamic nucleus; (3) expression of GluR1 was down-regulated in the globus pallidus by 63% and in the substantia nigra by 57%. The down-regulation of GluR1 expression in the output nuclei of the basal ganglia, the internal segment of the globus pallidus and the substantia nigra pars reticulata, may be a compensation for the overactive glutamatergic input from subthalamic nucleus, which arises after striatal dopamine denervation. Our results indicate that the glutamatergic system undergoes regulatory changes in response to altered basal ganglia activity in a primate model of Parkinson's disease. Targeted manipulation of the glutamatergic system may be a viable approach to the symptomatic treatment of Parkinson's disease.     

GluR2 knockdown reveals a dissociation between [Ca2+]i surge and neurotoxicity

Reduction in GluR2 subunit expression and subsequent increases in AMPA receptor mediated Ca(2+) currents were postulated to exacerbate glutamate neurotoxicity following seizures or global ischemia. To directly test the effects of shifting the GluR1/GluR2 subunit ratio on excitotoxicity, GluR2 antisense deoxyoligonucleotides (AS-ODNs) were applied to dissociated hippocampal cultures for 1-8 days. The GluR1/GluR2 protein ratio was examined immunohistochemically and by Western blotting. [Ca(2+)](i) concentrations were determined by ratiometric imaging of Fura 2-loaded cells. The cultures were exposed to glutamate, AMPA, NMDA or kainic acid (KA) 3 days after GluR2 knockdown and cell viability was determined 1 day later by MTT reduction assay or Trypan blue exclusion. Although GluR2 AS-ODNs increased the GluR1/GluR2 protein ratio in a time dependent manner, neurons and glia appeared healthy and MTT reduction values were similar to untreated and sense controls. Basal [Ca(2+)](i) levels were unchanged but [Ca(2+)](i) was selectively increased by agonist stimulation of AMPA receptors. Unexpectedly, delayed neurotoxicity was attenuated at saturating doses of glutamate while little difference in cell viability was observed at lower doses or with the other excitotoxins at any concentration. Therefore, there was a dissociation between rises in AMPA receptor-mediated Ca(2+) influx and neurotoxicity despite marked decreases in GluR2 but not GluR1 immunoreactivity. It is proposed that a modification of AMPA receptor stochiometry that raises agonist-stimulated Ca(2+) influx during an excitotoxic insult may have eventual neuroprotective effects.     

Differential effects of chronic hyperammonemia on modulation of the glutamate-nitric oxide-cGMP pathway by metabotropic glutamate receptor 5 and low and high affinity AMPA receptors in cerebellum in vivo

Previous studies show that chronic hyperammonemia impairs learning ability of rats by impairing the glutamate-nitric oxide (NO)-cyclic guanosine mono-phosphate (cGMP) pathway in cerebellum. Three types of glutamate receptors cooperate in modulating the NO-cGMP pathway: metabotropic glutamate receptor 5 (mGluR5), (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartic acid (NMDA) receptors. The aim of this work was to assess whether hyperammonemia alters the modulation of this pathway by mGluR5 and AMPA receptors in cerebellum in vivo. The results support that in control rats: (1) low AMPA concentrations (0.1mM) activate nearly completely Ca(2+)-permeable (glutamate receptor subunit 2 (GluR2)-lacking) AMPA receptors and the NO-cGMP pathway; (2) higher AMPA concentrations (0.3 mM) also activate Ca(2+)-impermeable (GluR2-containing) AMPA receptors, leading to activation of NMDA receptors and of NO-cGMP pathway. Moreover, the data support that chronic hyperammonemia: (1) reduces glutamate release and activation of the glutamate-NO-cGMP pathway by activation of mGluR5; (2) strongly reduces the direct activation by AMPA receptors of the NO-cGMP pathway, likely due to reduced entry of Ca(2+) through GluR2-lacking, high affinity AMPA receptors; (3) strongly increases the indirect activation of the NO-cGMP pathway by high affinity AMPA receptors, likely due to increased entry of Na(+) through GluR2-lacking AMPA receptors and NMDA receptors activation; (4) reduces the indirect activation of the NO-cGMP pathway by low affinity AMPA receptors, likely due to reduced activation of NMDA receptors.     

Co-localization of Shaker A-type K+ channel (Kv1.4) and AMPA-glutamate receptor (GluR4) immunoreactivities to dendrites of OFF-bipolar cells of goldfish retina

Immunocytochemical methods were used to determine the comparative distribution of Shaker Kv1.4 and Shal Kv4.2 A-type voltage-gated K⁺ channels and AMPA-type GluR4 glutamate receptors in the goldfish retina. Kv1.4-immunoreactivity (IR) was restricted to a very narrow band of bright puncta and filamentous processes in the outer plexiform layer (OPL), whereas GluR4-IR was found in radial processes of Müller cells in addition to a narrow band in the OPL. Kv4.2-IR was most prominent over cell bodies of horizontal cell, amacrine cells and ganglion cells, with very weak labeling over the synaptic terminal of cone photoreceptors. Double label experiments revealed complete co-localization of Kv1.4-IR and GluR4-IR in the OPL and showed that the Kv1.4 puncta in the OPL appeared enclosed by the Kv4.2-IR cone terminals. Electron microscopical immunocytochemistry showed that Kv1.4-IR and GluR4-IR were restricted to the dendrites of OFF-bipolar cells that innervated cone photoreceptor terminals and thin processes that coursed between the rod and cone terminals in the OPL. These data are consistent with other studies demonstrating the selective clustering of A-type voltage-gated K⁺ channels and ionotropic glutamate receptors. However, they differ from mammalian preparations in which Shal-like Kv4.2 rather than Shaker-like Kv1.4 co-localize postsynaptically with glutamate receptors.     

Expression of NMDAR1, GluR1, GluR7, and KA1 Glutamate Receptor mRNAs Is Decreased in Frontal Cortex of "Neuroleptic-Free" Schizophrenics: Evidence on Reversible Up-Regulation by Typical Neuroleptics

Abstract: Schizophrenics exhibit abnormalities in many memory-associated functions mediated by the frontal cortex. Glutamate receptors play key roles in learning and memory. Hence, abnormalities in glutamate receptors within the frontal cortex may be associated with schizophrenia. In addition, emerging evidence indicates that glutamate receptors may be involved in the actions of antipsychotic drugs. To test these hypotheses, we measured mRNAs encoding the NMDAR1, GluR1, GluR7, and KA1 subunits of glutamate receptor in the left superior frontal gyrus from 21 elderly schizophrenics with varying histories of antipsychotic drug treatment and nine normal drug-free elderly controls. There were significant negative correlations between NMDAR1, GluR1, GluR7, and KA1 mRNA levels and time without neuroleptic medication before death in schizophrenics, indicating that levels of the glutamate receptor mRNAs decline rapidly after drug withdrawal. Further analysis revealed that in "neuroleptic-free" (>6 months) schizophrenics, levels of NMDAR1, GluR1, GluR7, and KA1 mRNAs were significantly lower than in controls. By contrast, in schizophrenics who were receiving neuroleptics until death, levels of NMDAR1, GluR1, GluR7, and KA1 mRNAs did not differ significantly from controls. These findings indicate that decreased levels of NMDAR1, GluR1, GluR7, and KA1 mRNAs may be present in the frontal cortex of some schizophrenics and that typical neuroleptics may reversibly increase levels of these mRNAs.