Bidirectional modulation of AMPA receptor properties by exogenous phospholipase A2 in the hippocampus

The synaptic modifications underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission in various brain structures may result from changes in the properties of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors. In the present study, we report that treatment of rat synaptoneurosomes with increasing concentrations of phospholipase A₂ (PLA₂) produces a biphasic effect on AMPA receptor binding, with low concentrations causing a decrease and high concentrations an increase in agonist binding. Analysis of the saturation kinetics of ³H-AMPA binding revealed that the biphasic effect of PLA₂ was due to modifications in receptor affinity and not to changes in the maximum number of binding sites for AMPA receptors. The 12-lipoxygenase inhibitors preferentially reduced PLA₂-induced decrease in AMPA binding and treatment of hippocampal synaptoneurosomes with arachidonic acid (AA) or 12-HPETE, the first metabolite generated from the hydrolysis of AA by 12-lipoxygenases, decreased ³H-AMPA binding. Moreover, electrophysiological experiments indicated that the 12-lipoxygenase inhibitor baicalein totally blocked LTD formation in area CA₁ of hippocampal slices. The decrease in ³H-AMPA binding elicited by low concentrations of PLA₂, as well as the level of LTD, were partially reduced by AA-861, a 5-lipoxygenase inhibitor, while the cyclooxygenase inhibitor indomethacin did not prevent LTD formation or the effects of PLA₂ on ³H-AMPA binding. Our results provide evidence for a possible involvement of lipoxygenase metabolites in the regulation of AMPA receptor during synaptic depression. In addition, they strongly support the idea that the same biochemical pathway, i.e., NMDA receptor activation and endogenous PLA₂ stimulation, may represent a common mechanism resulting in AMPA receptor alterations for both LTP and LTD formation. Hippocampus 1998;8:299–309. © 1998 Wiley-Liss, Inc.     

Spatial compartmentalization of AMPA glutamate receptor subunits at the calyx of Held synapse

The mature calyx of Held ending on principal neurons of the medial nucleus of the trapezoid body (MNTB) has very specialized morphological and molecular features that make it possible to transmit auditory signals with high fidelity. In a previous work we described an increased localization of the ionotropic α‐amino‐3‐hydroxy‐5‐methyl‐4 isoxazolepropionic acid (AMPA) glutamate receptor (GluA) subunits at postsynaptic sites of the calyx of Held‐principal cell body synapses from postnatal development to adult. The aim of the present study was to investigate whether the pattern of the synaptic distribution of GluA2/3/4c and ‐4 in adult MNTB principal cell bodies correlated with preferential subcellular domains (stalks and swellings) of the calyx. We used a postembedding immunocytochemical method combined with specific antibodies to GluA2/3/4c and GluA4 subunits. We found that the density of GluA2/3/4c in calyceal swellings (19 ± 1.54 particles/μm) was higher than in stalks (10.93 ± 1.37 particles/μm); however, the differences for GluA4 were not statistically significant (swellings: 13.84 ± 1.39 particles/μm; stalks: 10.42 ± 1.24 particles/μm). Furthermore, GluA2/3/4c and GluA4 labeling co‐localized to some extent in calyceal stalks and swellings. Taking these data together, the distribution pattern of GluA subunits in postsynaptic specializations are indicative of a spatial compartmentalization of AMPA subunits in mature calyx‐principal neuron synapses that may support the temporally precise transmission required for sound localization in the auditory brainstem. J. Comp. Neurol. 518:163–174, 2010. © 2009 Wiley‐Liss, Inc.     

Sulfate- and size-dependent polysaccharide modulation of AMPA receptor properties

Previous work found evidence that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors interact with and are functionally regulated by the glycosaminoglycan heparin. The present study tested whether dextran species affect ligand binding, channel kinetics, and calcium permeability of AMPA receptors. Dextran sulfate of 500 kDa markedly reduced high affinity [3H]AMPA binding in solubilized hippocampal membranes. In isolated receptors reconstituted in a lipid bilayer, the same dextran sulfate prolonged the lifetime of open states exhibited by AMPA-induced channel fluctuations. The large polysaccharide further changed the single channel kinetics by increasing the open channel probability five- to sixfold. Such modulation of channel activity corresponded with enhanced levels of calcium influx as shown in hippocampal neurons loaded with Fluo3AM dye. With an exposure time of <1 min, AMPA produced a dose-dependent increase in intracellular calcium that was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX). Dextran sulfate, at the same concentration range that modified ligand binding (EC50 of 5-10 nM), enhanced the AMPA-induced calcium influx by as much as 60%. The enhanced influx was blocked by CNQX, although unchanged by the N-methyl-D-aspartate (NMDA) receptor antagonist AP5. Confocal microscopy showed that the increase in calcium occurred in neuronal cell bodies and their processes. Interestingly, smaller 5-8-kDa dextran sulfate and a non-sulfated dextran of 500 kDa had little or no effect on the binding, channel, and calcium permeability properties. Together, these findings suggest that synaptic polysaccharide species modulate hippocampal AMPA receptors in a sulfate- and size-dependent manner.     

Heterogeneous distribution of AMPA glutamate receptor subunits at the photoreceptor synapses of rodent retina

In the retina the segregation of different aspects of visual information starts at the first synapse in signal transfer from the photoreceptors to the second-order neurons, via the neurotransmitter glutamate. We examined the distribution of the four AMPA glutamate receptor subunits GluR1-GluR4 at the photoreceptor synapses in mouse and rat retinae by light and immunoelectron microscopy and serial section reconstructions. On the dendrites of OFF-cone bipolar cells, which make flat, noninvaginating contacts postsynaptic at cone synaptic terminals, the subunits GluR1 and GluR2 were predominantly found. Horizontal cell processes postsynaptic at both rod and cone synaptic terminals preferentially expressed the subunits GluR2, GluR2/3 and GluR4. An intriguing finding was the presence of GluR2/3 and GluR4 subunits on dendrites of putative rod bipolar cells, which are thought to signal through the sign-inverting metabotropic glutamate receptor 6, mGluR6. Furthermore, at the rod terminals, horizontal cell processes and rod bipolar cell dendrites showed labelling for the AMPA receptor subunits at the ribbon synaptic site or perisynaptically at their site of invagination into the rod terminal. The wide distribution of AMPA receptor subunits at the photoreceptor synapses suggests that AMPA receptors play an important role in visual signal transfer from the photoreceptors to their postsynaptic partners.     

Expression of NR1 and NR2A/B subunits of the NMDA receptor in cortical astrocytes

Ionotropic glutamate (Glu) receptors of the N-methyl-D-aspartate type (NMDA) play a fundamental role in many cortical functions. Native NMDA receptors are composed of a heteromeric assembly of different subunits belonging to two classes: NMDAR1 (NR1) and NMDAR2 (NR2). To date, NMDA receptors are believed to be expressed only in neurons, although electrophysiological and in situ hybridization studies have suggested that this class of Glu receptors might be also expressed by some astrocytes. In this study, we have investigated in the cerebral cortex of adult rats the presence of astrocytes expressing NR1 and NR2A/B subunits by immunocytochemistry with specific antibodies, and we show that some distal astrocytic processes, but only rarely astrocytic cell bodies, contain immunoreaction product indicative of NR1 and NR2A/B expression. These findings suggest that at least part of the role NMDA has in cortical functions might depend on the activation of astrocytic NMDA receptors; the subcellular localization of NR1 and NR2A/B subunits in distal processes suggests that NMDA receptors contribute to monitoring Glu levels in the extracellular space. © 1996 Wiley-Liss, Inc.     

Dopamine modulation of excitatory currents in the striatum is dictated by the expression of D1 or D2 receptors and modified by endocannabinoids

Striatal medium‐sized spiny neurons (MSSNs) receive glutamatergic inputs modulated presynaptically and postsynaptically by dopamine. Mice expressing the gene for enhanced green fluorescent protein as a reporter gene to identify MSSNs containing D1 or D2 receptor subtypes were used to examine dopamine modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in slices and postsynaptic N‐methyl‐d ‐aspartate (NMDA) and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) currents in acutely isolated cells. The results demonstrated dopamine receptor‐specific modulation of sEPSCs. Dopamine and D1 agonists increased sEPSC frequency in D1 receptor‐expressing MSSNs (D1 cells), whereas dopamine and D2 agonists decreased sEPSC frequency in D2 receptor‐expressing MSSNs (D2 cells). These effects were fully (D1 cells) or partially (D2 cells) mediated through retrograde signaling via endocannabinoids. A cannabinoid 1 receptor (CB1R) agonist and a blocker of anandamide transporter prevented the D1 receptor‐mediated increase in sEPSC frequency in D1 cells, whereas a CB1R antagonist partially blocked the decrease in sEPSC frequency in D2 cells. At the postsynaptic level, low concentrations of a D1 receptor agonist consistently increased NMDA and AMPA currents in acutely isolated D1 cells, whereas a D2 receptor agonist decreased these currents in acutely isolated D2 cells. These results show that both glutamate release and postsynaptic excitatory currents are regulated in opposite directions by activation of D1 or D2 receptors. The direction of this regulation is also specific to D1 and D2 cells. We suggest that activation of postsynaptic dopamine receptors controls endocannabinoid mobilization, acting on presynaptic CB1Rs, thus modulating glutamate release differently in glutamate terminals projecting to D1 and D2 cells.     

Activity regulates the expression of AMPA receptor subunit GluR4 in developing visual cortex

In the developing visual cortex, the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit GluR4 precedes that of the other AMPAR subunits GluR1-3, and then declines to become almost absent in adults. The current study shows that the neuronal activity regulates the expression of GluR4 by a culture system in vitro and a dark-rearing (DR) system in vivo. Membrane depolarization by treatment of cultured neurons of the visual cortex with a high concentration of KCl (35 mm; HK) promoted a decline in the expression of GluR4. This effect of HK on the expression of GluR4 was significantly blocked by the addition of an N-methyl-d-aspartate receptor (NMDAR) antagonist, (D)-2-amino-5-phosphonovaleric acid (APV), but not by the voltage-sensitive calcium channel antagonist nifedipine. Moreover, the Ca(2+)-calmodulin-dependent kinase (CaMKII) inhibitor KN62 and the cAMP-dependent protein kinase A (PKA) inhibitor H-89 blocked this effect, which suggests the involvement of Ca(2+) influx via NMDAR and the subsequent activation of CaMKII and PKA. Conversely, the MAP kinase inhibitor PD98059 promoted the effect of HK on the expression of GluR4. Significantly, APV, KN62, H-89 and PD98059 either promoted or inhibited the expression of GluR4 even in normal KCl (5 mm) conditions. The developmental change in the expression of GluR4 was significantly attenuated in DR in vivo, and the results suggest that neuronal activity such as visual experience may be involved in the mechanism of the expression of GluR4, which is mediated by NMDAR and tuned by certain protein kinases at an early developmental stage in the visual cortex.     

AMPA receptor phosphorylation is selectively regulated by constitutive phospholipase A(2) and 5-lipoxygenase activities

The present investigation provides the first indication that constitutive, calcium-independent phospholipase A2 activity (iPLA2) modulates phosphorylation of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtype of glutamate receptors. Preincubation of frozen-thawed brain sections with two iPLA2 inhibitors, bromoenol lactone (BEL) or palmitoyl trifluoromethyl ketone (PACO), produced a dose-dependent enhancement in phosphorylation at both Ser831 and Ser845 sites on the GluR1 subunit of AMPA receptors. This effect was not associated with changes in phosphorylation at the Ser sites of either the GluR2/3 subunits of AMPA receptors or the NR1 subunits of N-methyl-D-aspartate (NMDA) receptors, nor was it reproduced by inhibition of the calcium-dependent form of PLA2 activity. These results suggest that the effects of these inhibitors are selective to GluR1 subunits and that they are dependent on iPLA2 activity. The ability of iPLA2 inhibitors to increase GluR1 phosphorylation was mimicked by the 5-lipoxygenase (5-LO) inhibitor MK-886, but not by blockers of 12-lipoxygenase (12-LO) or cyclooxygenase. Additional experiments indicated that calcium-mediated truncation of GluR1 subunits was reduced by iPLA2 inhibitors, an effect that was not correlated with overall changes in the distribution of AMPA receptors between intracellular and membrane compartments prepared from whole brain sections. However, quantitative autoradiographic analysis indicated enhanced 3H-AMPA binding to the CA1 stratum radiatum of the hippocampus in BEL-treated sections. Saturation kinetics experiments demonstrated that this binding augmentation was due to an increase in the maximal number of AMPA binding sites. Altogether, our results point to the conclusion that basal iPLA2 activity, through the generation of 5-LO metabolites, regulates AMPA receptor phosphorylation of GluR1 subunits, an effect that might selectively influence the number of membrane receptors in area CA1 of the hippocampus.     

The inhibitory input to mouse cerebellar Purkinje cells is reciprocally modulated by Bergmann glial P2Y1 and AMPA receptor signaling

Synaptic transmission has been shown to be modulated by glial functions, but the modes of specific glial action may vary in different neural circuits. We have tested the hypothesis, if Bergmann GLIA (BG) are involved in shaping neuronal communication in the mouse cerebellar cortex, using acutely isolated cerebellar slices of wild‐type (WT) and of glia‐specific receptor knockout mice. Activation of P2Y1 receptors by ADP (100 µM) or glutamatergic receptors by AMPA (0.3 µM) resulted in a robust, reversible and repeatable rise of evoked inhibitory input in Purkinje cells by 80% and 150%, respectively. The ADP‐induced response was suppressed by prior application of AMPA, and the AMPA‐induced response was suppressed by prior application of ADP. Genetic deletion or pharmacological blockade of either receptor restored the response to the other receptor agonist. Both ADP and AMPA responses were sensitive to Rose Bengal, which blocks vesicular glutamate uptake, and to the NMDA receptor antagonist D‐AP5. Our results provide strong evidence that activation of both ADP and AMPA receptors, located on BGs, results in the release of glutamate, which in turn activates inhibitory interneurons via NMDA‐type glutamate receptors. This infers that BG cells, by means of metabotropic signaling via their AMPA and P2Y1 receptors, which mutually suppress each other, would interdependently contribute to the fine‐tuning of Purkinje cell activity in the cerebellar cortex. GLIA 2016. GLIA 2016;64:1265–1280     

Low expression of GluR2 AMPA receptor subunit protein by human motor neurons

Disturbance of glutamate neurotransmission may contribute to motor neuron injury in amyotrophic lateral sclerosis. There is evidence that human motor neurons may express a specific profile of glutamate receptors, with low or absent expression of mRNA for the GluR2 AMPA receptor subunit, which has a crucial role in controlling calcium permeability. This study, using an immunocytochemical approach with a GluR2 specific antibody, shows that human upper and lower motor neurons have a very low/absent expression of GluR2 protein, in contrast to many other neuronal groups. Thus, it is likely that human motor neurons express a high proportion of atypical, calcium permeable AMPA receptors which may contribute to selective vulnerability and may allow cell-specific modulation of the actions of glutamate.     

Evaluation of GluR2 subunit involvement in AMPA receptor function of neonatal rat hypoglossal motoneurons

AMPA receptors (AMPAr) mediate fast synaptic responses to glutamate and, when they lack the GluR2 subunit, are strongly Ca2+ permeable and may increase intracellular Ca2+ levels. Because hypoglossal motoneurons possess restricted ability to buffer internal Ca2+ and are vulnerable to Ca2+ excitotoxicity, we wondered if, in these cells, any significant Ca2+ influx could be generated via AMPAr activity. Using whole cell patch-clamp recording from neonatal rat hypoglossal motoneurons, we tested the AMPAr properties conferred by GluR2 subunits, namely Ca2+ permeability, current rectification and sensitivity to pentobarbital or to the subunit-specific channel blockers, IEM-1460 and IEM-1925. We recorded membrane currents generated by the agonist, kainate, and compared them with those obtained from hippocampal pyramidal neurons (expressing GluR2-containing AMPAr) and from striatal giant aspiny or hippocampal interneurons (with GluR2-lacking AMPAr). Ca2+ vs. Na+ permeability of motoneuron AMPAr was relatively low (0.25 +/- 0.05), although higher than that of pyramidal neurons. With intracellularly applied spermine, significant inward rectification was absent from motoneurons. These data indicated the prevalence of functional GluR2 subunits. However, the sensitivity of motoneuron AMPAr to pentobarbital did not differ from that of GluR2-lacking AMPAr on interneurons. Motoneurons possessed sensitivity to IEM-1460 (IC50 = 90 +/- 10 microm) approximately 10-fold lower than striatal interneurons, although 10-fold higher than hippocampal pyramidal cells. IEM-1925 also reduced the amplitude of excitatory synaptic currents in brainstem slice motoneurons. We hypothesize that hypoglossal motoneuron AMPAr (moderately Ca2+ permeable because they contain few GluR2 subunits) may contribute to intracellular Ca2+ rises especially if persistent AMPAr activation (or the pathological GluR2 down-regulation) occurs.     

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.     

AMPA GluR‐A receptor subunit mediates hippocampal responsiveness in mice exposed to stress

Because stress represents a major precipitating event for psychiatric disorders, it is important to identify molecular mechanisms that may be altered in vulnerable individuals when exposed to stress. Here, we studied GluR‐A^?/? mice, animals with compromised AMPA receptor signaling, and characterized by a schizophrenic as well as depressive phenotype to investigate changes occurring in response to an acute stress. Wild‐type and GluR‐A^?/? mice were exposed to a single immobilization stress and sacrificed immediately after the end of the stress for the analysis of activity regulated genes and of glutamatergic synapse responsiveness. The acute stress produced a marked increase in the hippocampal expression of Arc (activity‐regulated cytoskeletal‐associated protein) in GluR‐A^?/?, but not in wild‐type mice, which was associated with a similar increase of phospho‐CaMKII, a partner in the action of Arc. When looking at the glutamatergic response to stress in wild‐type animals, we found that stress increased GluR‐A phosphorylation on serine831, an effect that was paralleled by a significant increase of the phosphorylation of the main NMDA receptor subunits, that is, NR‐1 and NR‐2B. Conversely, the stress‐induced modulation of NMDA receptor subunits was not observed in GluR‐A^?/? mice. We suggest that enhanced stress responsiveness in GluR‐A^?/? mice may be due, at least in part, to their inability to activate NMDA‐mediated glutamatergic neurotransmission, suggesting that the integrity of AMPA/NMDA receptor function may be important for successful coping under stressful conditions. © 2010 Wiley‐Liss, Inc.     

Alterations in GluR2 AMPA receptor phosphorylation at serine 880 following group I metabotropic glutamate receptor stimulation in the rat dorsal striatum

Phosphorylation of ionotropic glutamate receptors in the brain plays a crucial role in the regulation of synaptic plasticity. In this study, we investigated the regulation of α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) receptor phosphorylation by thestimulation of group I metabotropic glutamate receptors (mGluRs) in the dorsal striatum in vivo. The results showed that intrastriatal infusion of the group I mGluR agonist, (RS)‐3,5‐dihydroxyphenylglycine (DHPG, 250 nmol), enhanced the sensitivity of GluR2 subunit in its phosphorylation at serine 880 (S880) in the dorsal striatum. This enhancement of the sensitivity of GluR2‐S880 phosphorylation was reduced by blocking group I mGluRs and N‐methyl‐D‐aspartate (NMDA) receptors. Similar reduction of the enhancement was also induced by inhibiting phospholipase C (PLC), calcium/calmodulin‐dependent protein kinase (CaMK), c‐Jun N‐terminal kinase (JNK), and protein kinase C (PKC). Inhibition of protein phosphatase (PP) 1/2A and calcineurin (PP2B) alone enhanced GluR2‐S880 phosphorylation in the dorsal striatum, whereas inhibition of these phosphatases did not further enhance the S880 phosphorylation by DHPG stimulation. In addition, inhibition of PP1/2A or PP2B also enhanced the phosphorylation of CaMKII, JNK and PKC. These data suggest that the phosphorylation of AMPA receptor GluR2 subunit at S880 is subject to the upregulation by the stimulation of group I mGluRs. Interactions among glutamate receptors, protein kinases, and PPs participate in this upregulation. © 2009 Wiley‐Liss, Inc.     

Synapse-specific localization of NMDA and GABAA receptor subunits revealed by antigen-retrieval immunohistochemistry

Conventional immunohistochemistry provides little evidence for the synaptic localization of ionotropic neurotransmitter receptors, suggesting that their epitopes are not readily accessible in situ. Here, we have adapted antigen retrieval procedures based on microwave irradiation to enhance the immunohistochemical staining of γ-aminobutyric acid type A (GABA_A) and N-methyl-D-aspartate (NMDA) receptor subunits in rat brain tissue. Microwave irradiation of fixed tissue produced a marked reduction of nonspecific staining, allowing an improved detection of GABA_A receptor subunits. However, staining of NMDA receptor subunits remained suboptimal. In contrast, microwave irradiation of cryostat sections prepared from fresh tissue resulted in a major enhancement of both NMDA and GABA_A receptor subunit staining. The diffuse, partially intracellular signals were largely replaced by numerous, intensely immunoreactive puncta outlining neuronal somata and dendrites, highly suggestive of synaptic receptors. In hippocampus CA1–CA3 fields, the NR2A and NR2B subunit positive puncta exhibited an extensive colocalization in the stratum oriens and radiatum, whereas pyramidal cell bodies, which receive no excitatory synapses, were unstained. In addition, the NR2A subunit, but not the NR2B subunit, was selectively detected on pyramidal cell dendrites in the stratum lucidum of CA3, suggesting a selective targeting to sites of mossy fiber input. For the GABA_A receptor subunits, the most striking change induced by this protocol was the selective staining of the axon initial segment of cortical and hippocampal pyramidal cells. The α2 subunit immunoreactivity was particularly prominent in these synapses. In control experiments, the staining of cytoskeletal proteins (neurofilaments, glial fibrillary acid protein) was not influenced by prior microwave irradiation. The enhancement of cell-surface–associated staining is therefore strongly suggestive of an ‘unmasking’ of subunit epitopes by the microwave treatment. These results reveal a remarkable specificity in the synaptic targeting of NMDA and GABA_A receptor subunits in hippocampal and neocortical neurons, suggesting that individual neurons can express multiple receptor subtypes in functionally distinct synapses. J. Comp. Neurol. 390:194–210, 1998. © 1998 Wiley-Liss, Inc.     

Subunit selective decrease of AMPA and metabotropic glutamate receptor mRNA expression in rat brain by systemic administration of the NMDA receptor blocker MK-801

Glutamate mediates its effects in mammals through both ionotropic and metabotropic receptors. Antagonists of ionotropic N-methyl-d-aspartate (NMDA) glutamate receptors elicit neuroprotective and neurotropic effects that have been attributed to Ca²⁺ block through the membrane ion channel. Nonetheless, molecular and biochemical effects of NMDA receptor antagonism on other glutamate receptor subunits remain poorly understood. We investigated the effects of acute administration of the noncompetitive NMDA receptor antagonist MK-801 on the mRNA expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and metabotropic glutamate receptor (mGluR) subunits to determine the contribution of different glutamate receptors in response to blockade of NMDA receptor channels. In situ hybridization to rat brain sections revealed that AMPA receptor subunits GluR3 and GluR4, and mGluR3 were modestly but significantly decreased ∼10–20%, 8 h following 5 mg/kg MK-801 administration. A time course and dose response study revealed that the effect on mGluR3 was reversed by 24 h and occurred significantly at a dose range from 1 to 5 mg/kg. These results indicate that selected AMPA and mGluR subunit mRNAs respond at the RNA level to the blockade of NMDA receptors.