Glutamate Receptor Subunit Composition Research Articles

Remote and Persistent Alterations in Glutamate Receptor Subunit Composition Induced by Spreading Depolarizations in Rat Brain.

Spreading depolarizations (SDs) are massive breakdowns of ion homeostasis in the brain's gray matter and are a necessary pathologic mechanism for lesion development in various injury models. However, injury-induced SDs also propagate into remote, healthy tissue where they do not cause cell death, yet their functional long-term effects are unknown. Here we induced SDs in uninjured cortex and hippocampus of Sprague-Dawley rats to study their impact on glutamate receptor subunit expression after three days. We find that both cortical and hippocampal tissue exhibit changes in glutamate receptor subunit expression, including GluA1 and GluN2B, suggesting that SDs in healthy brain tissue may have a role in plasticity. This study is the first to show prolonged effects of SDs on glutamate signaling and has implications for neuroprotection strategies aimed at SD suppression.

PTPσ Controls Presynaptic Organization of Neurotransmitter Release Machinery at Excitatory Synapses

SummaryLeukocyte common antigen-related receptor tyrosine phosphatases (LAR-RPTPs) are evolutionarily conserved presynaptic organizers. The synaptic role of vertebrate LAR-RPTPs in vivo, however, remains unclear. In the current study, we analyzed the synaptic role of PTPσ using newly generated, single conditional knockout (cKO) mice targeting PTPσ. We found that the number of synapses was reduced in PTPσ cKO cultured neurons in association with impaired excitatory synaptic transmission, abnormal vesicle localization, and abnormal synaptic ultrastructure. Strikingly, loss of presynaptic PTPσ reduced neurotransmitter release prominently at excitatory synapses, concomitant with drastic reductions in excitatory innervations onto postsynaptic target areas in vivo. Furthermore, loss of presynaptic PTPσ in hippocampal CA1 pyramidal neurons had no impact on postsynaptic glutamate receptor responses in subicular pyramidal neurons. Postsynaptic PTPσ deletion had no effect on excitatory synaptic strength. Taken together, these results demonstrate that PTPσ is a bona fide presynaptic adhesion molecule that controls neurotransmitter release and excitatory inputs.

Изменение поведения и экспрессии генов ионотропных рецепторов глутамата в мозге взрослых крыс после неонатальных введений бактериального липополисахарида

A large number of studies indicate the role of early-life experience, in particular neonatal infections, in the formation of a high level of anxiety later in life. Changes in NMDA and AMPA glutamate receptor subunit composition that result in the violation of receptor functional activity may comprise the mechanism behind the behavioral impairments. Our work aimed to investigate exploratory and anxiety-like forms of behavior of adult rats that were treated with lipopolysaccharide (LPS) during the 3rd week of postnatal development at neuroinflammation-inducing doses. Levels of gene expression of NMDA receptor subunits (Grin1, Grin2a, Grin2b) and AMPA receptor subunits (Gria1, Gria2) in the medial prefrontal cortex, ventral and dorsal areas of the hippocampus were assessed by qRT-PCR. Our study on Wistar male rats revealed that the administration of LPS at doses of 25 and 50 μg/kg on P15, P18, P21 increased the expression of proinflammatory cytokine mRNA (interleukin-1β and tumor necrosis factor) in the hippocampus 2 h post treatment. Elevated expression of Grin2b, Gria1, Gria2 genes in the ventral hippocampus was observed 3 months after the administration of 50 μg/kg LPS compared with saline-treated control rats. Gria2 gene expression was found to be increased in the dorsal hippocampus 3 months after 25 μg/kg LPS injections in comparison with the controls. These changes were accompanied by impaired exploratory behavior of adult LPS-treated rats in the Open field test and a decreased level of anxiety in the Elevated plus maze. Taken together, our results substantiate early-life inflammation as a cause of prolonged changes in the hippocampal expression of genes of NMDA and AMPA receptor subunits and have an impact on related forms of behavior.

Age-Dependent, Subunit Specific Action of Hydrogen Sulfide on GluN1/2A and GluN1/2B NMDA Receptors.

Hydrogen sulfide (H2S) is an endogenously produced neuroactive gas implicated in many key processes in the peripheral and central nervous system. Whereas the neuroprotective role of H2S has been shown in adult brain, the action of this messenger in newborns remains unclear. One of the known targets of H2S in the nervous system is the N-methyl-D-aspartate (NMDA) glutamate receptor which can be composed of different subunits with distinct functional properties. In the present study, using patch clamp technique, we compared the effects of the H2S donor sodium hydrosulfide (NaHS, 100 μM) on hippocampal NMDA receptor mediated currents in rats of the first and third postnatal weeks. This was supplemented by testing effects of NaHS on recombinant GluN1/2A and GluN1/2B NMDA receptors expressed in HEK293T cells. The main finding is that NaHS action on NMDA currents is age-dependent. Currents were reduced in newborns but increased in older juvenile rats. Consistent with an age-dependent switch in NMDA receptor composition, in HEK239T cells expressing GluN1/2A receptors, NaHS increased NMDA activated currents associated with acceleration of desensitization and decrease of the deactivation rate. In contrast, in GluN1/2B NMDA receptors, which are prevalent in newborns, NaHS decreased currents and reduced receptor deactivation without effect on the desensitization rate. Adenylate cyclase inhibitor MDL-12330A (10 μM) did not prevent the age-dependent effects of NaHS on NMDA evoked currents in pyramidal neurons of hippocampus. The reducing agent dithiothreitol (DTT, 2 mM) applied on HEK293T cells prevented facilitation induced by NaHS on GluN1/2A NMDA receptors, however in GluN1/2B NMDA receptors the inhibitory effect of NaHS was still observed. Our data indicate age-dependent effect of H2S on NMDA receptor mediated currents determined by glutamate receptor subunit composition. While the inhibitory action of H2 on GluN1/2B receptors could limit the excessive activation in early age, the enhanced functionality of GluN1/2A receptor in the presence of this gasotransmitter can enlarge synaptic efficacy and promote synaptic plasticity in adults.

Acoustic trauma slows AMPA receptor‐mediated EPSCs in the auditory brainstem, reducing GluA4 subunit expression as a mechanism to rescue binaural function

Key points Lateral superior olive (LSO) principal neurons receive AMPA receptor (AMPAR) ‐ and NMDA receptor (NMDAR)‐mediated EPSCs and glycinergic IPSCs.Both EPSCs and IPSCs have slow kinetics in prehearing animals, which during developmental maturation accelerate to sub‐millisecond decay time‐constants. This correlates with a change in glutamate and glycine receptor subunit composition quantified via mRNA levels.The NMDAR‐EPSCs accelerate over development to achieve decay time‐constants of 2.5 ms. This is the fastest NMDAR‐mediated EPSC reported.Acoustic trauma (AT, loud sounds) slow AMPAR‐EPSC decay times, increasing GluA1 and decreasing GluA4 mRNA.Modelling of interaural intensity difference suggests that the increased EPSC duration after AT shifts interaural level difference to the right and compensates for hearing loss.Two months after AT the EPSC decay times recovered to control values.Synaptic transmission in the LSO matures by postnatal day 20, with EPSCs and IPSCs having fast kinetics. AT changes the AMPAR subunits expressed and slows the EPSC time‐course at synapses in the central auditory system. Damaging levels of sound (acoustic trauma, AT) diminish peripheral synapses, but what is the impact on the central auditory pathway? Developmental maturation of synaptic function and hearing were characterized in the mouse lateral superior olive (LSO) from postnatal day 7 (P7) to P96 using voltage‐clamp and auditory brainstem responses. IPSCs and EPSCs show rapid acceleration during development, so that decay kinetics converge to similar sub‐millisecond time‐constants (τ, 0.87 ± 0.11 and 0.77 ± 0.08 ms, respectively) in adult mice. This correlated with LSO mRNA levels for glycinergic and glutamatergic ionotropic receptor subunits, confirming a switch from Glyα2 to Glyα1 for IPSCs and increased expression of GluA3 and GluA4 subunits for EPSCs. The NMDA receptor (NMDAR)‐EPSC decay τ accelerated from >40 ms in prehearing animals to 2.6 ± 0.4 ms in adults, as GluN2C expression increased. In vivo induction of AT at around P20 disrupted IPSC and EPSC integration in the LSO, so that 1 week later the AMPA receptor (AMPAR)‐EPSC decay was slowed and mRNA for GluA1 increased while GluA4 decreased. In contrast, GlyR IPSC and NMDAR‐EPSC decay times were unchanged. Computational modelling confirmed that matched IPSC and EPSC kinetics are required to generate mature interaural level difference functions, and that longer‐lasting EPSCs compensate to maintain binaural function with raised auditory thresholds after AT. We conclude that LSO excitatory and inhibitory synaptic drive matures to identical time‐courses, that AT changes synaptic AMPARs by expression of subunits with slow kinetics (which recover over 2 months) and that loud sounds reversibly modify excitatory synapses in the brain, changing synaptic function for several weeks after exposure.

Impact of synaptic localization and subunit composition of ionotropic glutamate receptors on synaptic function: modeling and simulation studies.

Ionotropic NMDA and AMPA glutamate receptors (iGluRs) play important roles in synaptic function under physiological and pathological conditions. iGluRs sub-synaptic localization and subunit composition are dynamically regulated by activity-dependent insertion and internalization. However, understanding the impact on synaptic transmission of changes in composition and localization of iGluRs is difficult to address experimentally. To address this question, we developed a detailed computational model of glutamatergic synapses, including spine and dendritic compartments, elementary models of subtypes of NMDA and AMPA receptors, glial glutamate transporters, intracellular calcium and a calcium-dependent signaling cascade underlying the development of long-term potentiation (LTP). These synapses were distributed on a neuron model and numerical simulations were performed to assess the impact of changes in composition and localization (synaptic vs extrasynaptic) of iGluRs on synaptic transmission and plasticity following various patterns of presynaptic stimulation. In addition, the effects of various pharmacological compounds targeting NMDARs or AMPARs were determined. Our results showed that changes in NMDAR localization have a greater impact on synaptic plasticity than changes in AMPARs. Moreover, the results suggest that modulators of AMPA and NMDA receptors have differential effects on restoring synaptic plasticity under different experimental situations mimicking various human diseases.

Reconstruction of the molecular interactome of glutamatergic synapses

The subunit composition of ionotropic glutamate receptors is of great significance for synapse functioning. NMDA receptors mediate fast excitatory neurotransmission and are capable of converting specific patterns of neuronal activity into long-term changes in synaptic structure and function. The principal functional properties (ion conductivity, glutamate sensitivity, agonists, and magnesium ions, and deactivation time), spatial location, membrane anchoring, and the response to pharmacological agents depend on subunit composition of the receptors. The analysis of protein–protein interactions in macrocomplexes, including NMDA receptor subunits, is an urgent task, because the results of research in this area will contribute to a better understanding of the principles and molecular mechanisms underlying the basic functions of neurons and the development of pathologies and facilitate the search for pharmacological and therapeutic targets for the correction of pathologies. The goal of the present work consisted of analyzing and reconstructing protein–protein interactions involving NMDA receptor subunits and providing for the mobility of NMDA receptors, the incorporation of these molecules into the membranes, and receptor functions related to modification and maintenance of synaptic transmission efficiency in the hippocampus. Three groups of proteins involved in the formation of NMDA receptor macrocomplexes in hippocampal glutamatergic synapses were identified. The proteins were divided into groups according to their function in the complexes; the functions were inferred from the information presented in various databases and research articles that reported gene and protein structures, expression profiles in the brain, and the role of specific genes and proteins in synaptic plasticity. Particular attention was paid to proteins for which a association with various cognitive disturbances was reported.

Neto-mediated intracellular interactions shape postsynaptic composition at the Drosophila neuromuscular junction.

The molecular mechanisms controlling the subunit composition of glutamate receptors are crucial for the formation of neural circuits and for the long-term plasticity underlying learning and memory. Here we use the Drosophila neuromuscular junction (NMJ) to examine how specific receptor subtypes are recruited and stabilized at synaptic locations. In flies, clustering of ionotropic glutamate receptors (iGluRs) requires Neto (Neuropillin and Tolloid-like), a highly conserved auxiliary subunit that is essential for NMJ assembly and development. Drosophila neto encodes two isoforms, Neto-α and Neto-β, with common extracellular parts and distinct cytoplasmic domains. Mutations that specifically eliminate Neto-β or its intracellular domain were generated. When Neto-β is missing or is truncated, the larval NMJs show profound changes in the subtype composition of iGluRs due to reduced synaptic accumulation of the GluRIIA subunit. Furthermore, neto-β mutant NMJs fail to accumulate p21-activated kinase (PAK), a critical postsynaptic component implicated in the synaptic stabilization of GluRIIA. Muscle expression of either Neto-α or Neto-β rescued the synaptic transmission at neto null NMJs, indicating that Neto conserved domains mediate iGluRs clustering. However, only Neto-β restored PAK synaptic accumulation at neto null NMJs. Thus, Neto engages in intracellular interactions that regulate the iGluR subtype composition by preferentially recruiting and/or stabilizing selective receptor subtypes.

GluN3A promotes dendritic spine pruning and destabilization during postnatal development.

Synaptic rearrangements during critical periods of postnatal brain development rely on the correct formation, strengthening, and elimination of synapses and associated dendritic spines to form functional networks. The correct balance of these processes is thought to be regulated by synapse-specific changes in the subunit composition of NMDA-type glutamate receptors (NMDARs). Among these, the nonconventional NMDAR subunit GluN3A has been suggested to play a role as a molecular brake in synaptic maturation. We tested here this hypothesis using confocal time-lapse imaging in rat hippocampal organotypic slices and assessed the role of GluN3A-containing NMDARs on spine dynamics. We found that overexpressing GluN3A reduced spine density over time, increased spine elimination, and decreased spine stability. The effect of GluN3A overexpression could be further enhanced by using an endocytosis-deficient GluN3A mutant and reproduced by silencing the adaptor protein PACSIN1, which prevents the endocytosis of endogenous GluN3A. Conversely, silencing of GluN3A reduced spine elimination and favored spine stability. Moreover, reexpression of GluN3A in more mature tissue reinstated an increased spine pruning and a low spine stability. Mechanistically, the decreased stability in GluN3A overexpressing neurons could be linked to a failure of plasticity-inducing protocols to selectively stabilize spines and was dependent on the ability of GluN3A to bind the postsynaptic scaffold GIT1. Together, these data provide strong evidence that GluN3A prevents the activity-dependent stabilization of synapses thereby promoting spine pruning, and suggest that GluN3A expression operates as a molecular signal for controlling the extent and timing of synapse maturation.

The ALS gene FUS regulates synaptic transmission at the Drosophila neuromuscular junction

Mutations in the RNA binding protein Fused in sarcoma (FUS) are estimated to account for 5-10% of all inherited cases of amyotrophic lateral sclerosis (ALS), but the function of FUS in motor neurons is poorly understood. Here, we investigate the early functional consequences of overexpressing wild-type or ALS-associated mutant FUS proteins in Drosophila motor neurons, and compare them to phenotypes arising from loss of the Drosophila homolog of FUS, Cabeza (Caz). We find that lethality and locomotor phenotypes correlate with levels of FUS transgene expression, indicating that toxicity in developing motor neurons is largely independent of ALS-linked mutations. At the neuromuscular junction (NMJ), overexpression of either wild-type or mutant FUS results in decreased number of presynaptic active zones and altered postsynaptic glutamate receptor subunit composition, coinciding with a reduction in synaptic transmission as a result of both reduced quantal size and quantal content. Interestingly, expression of human FUS downregulates endogenous Caz levels, demonstrating that FUS autoregulation occurs in motor neurons in vivo. However, loss of Caz from motor neurons increases synaptic transmission as a result of increased quantal size, suggesting that the loss of Caz in animals expressing FUS does not contribute to motor deficits. These data demonstrate that FUS/Caz regulates NMJ development and plays an evolutionarily conserved role in modulating the strength of synaptic transmission in motor neurons.

Subunit composition of glutamate and gamma-aminobutyric acid receptors in status epilepticus

To describe the subunit composition of glutamate and gamma-aminobutyric acid (GABA) receptors in brain tissue from patients with different types of status epilepticus. The subunit composition of glutamate and GABA receptors was analyzed in: (1) surgical brain samples from three patients with refractory convulsive status epilepticus, three patients with electrical status epilepticus in sleep, and six patients with refractory epilepsy, and (2) brain autopsy samples from four controls who died without neurological disorders. Subunit expression was quantified with Western blotting and messenger ribonucleic acid (mRNA) expression was quantified with reverse polymerase chain reaction. Western blot analysis demonstrated the following patterns (as compared to controls): (1) alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: elevated GluA1/GluA2 ratio in electrical status epilepticus in sleep (465%±119) and refractory epilepsy (329%±125; p<0.01); (2) N-methyl-d-aspartate (NMDA) receptors: increased GluN2B/GluN2A ratio in electrical status epilepticus in sleep (3682%±1000) and refractory convulsive status epilepticus (3520%±751; p<0.05); (3) GABA receptors: elevated α2/α1 ratio in refractory epilepsy (321%±138; p<0.05) and refractory convulsive status epilepticus (346%±74; p<0.05); and (4) patients with underlying malformation of cortical development had increased ratios in GluA1/GluA2 (382%±149; p<0.01), GluN2B/GluN2A (3321%±1581; p<0.05) and α2/α1 (303%±86; p<0.01). Quantification of mRNA demonstrated an elevated GABRA2/GABRA1 ratio in refractory epilepsy (712; p<0.05) as compared to controls. The subunit composition of glutamate and GABA receptors in patients with status epilepticus mirrors that found in animal models of refractory status epilepticus and may promote self-sustaining seizures. Receptor subunit changes may provide additional targets for improved treatment.

Unilateral skill acquisition induces bilateral NMDA receptor subunit composition shifts in the rat sensorimotor striatum

The sensorimotor striatum is critical for the acquisition and consolidation of skilled learning-related motor sequences. Excitatory corticostriatal synapses undergo neuroplastic changes that impact signal transmission efficacy. Modification of N-methyl d-aspartate (NMDA) and α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit composition and phosphorylation is critical for bidirectional experience-driven plasticity observed at these synapses. Metaplastic regulation of the ratio of NR2A to NR2B subunits of the NMDA receptor controls the threshold for the induction of subsequent plasticity. However, little is known about how repeated practice effects the differential regulation of glutamate receptors during the acquisition of a unilateral motor skill. Using immunoblot analysis, we assessed changes in NMDA and AMPA receptors during the associative stage of skill acquisition in synaptoneurosome preparations from the rat sensorimotor striatum. We found that the NR2A/B subunit ratio in the striatum contralateral to the trained limb decreased during skill acquisition optimizing the threshold for inducing subsequent synaptic plasticity during learning of the lateralized motor skill. In contrast, there was a significant increase in the NR2A/B subunit ratio in the ipsilateral striatum making the induction of subsequent plasticity more difficult. In addition, there was a selective decrease in AMPAR phosphorylation levels at serine site 831 but not 845 on the GluR1 subunit ipsilaterally with a trend toward a decrease contralaterally. These findings suggest that the successful acquisition of a lateralized motor skill necessitates the integration of motor programs in both striata, each of which reflects unique changes in the NR2A/B ratio that modulate the different task demands on the associated limb.

Expression, subunit composition, and function of AMPA‐type glutamate receptors are changed in activated microglia; possible contribution of GluA2 (GluR‐B)‐deficiency under pathological conditions

Microglia express AMPA (α-amino-hydroxy-5-methyl-isoxazole-4-propionate)-type of glutamate (Glu) receptors (AMPAR), which are highly Ca(2+) impermeable due to the expression of GluA2. However, the functional importance of AMPAR in microglia remains to be investigated, especially under pathological conditions. As low expression of GluA2 was reported in some neurodegenerative diseases, GluA2(-/-) mice were used to show the functional change of microglial AMPARs in response to Glu or kainate (KA). Here we found that Glu-induced currents in the presence of 100 μM cyclothiazide, an inhibitor of AMPAR desensitization, showed time-dependent decrease after activation of microglia with lipopolysaccharide (LPS) in GluA2(+/+) microglia, but not in GluA2(-/-) microglia. Upon activation of microglia, expression level of GluA2 subunits significantly increased, while expression of GluA1, A3 and A4 subunits on membrane surface significantly decreased. These results suggest that nearly homomeric GluA2 subunits were the main reason for low conductance of AMPAR in activated microglia. Increased expression of GluA2 in microglia was also detected partially in brain slices from LPS-injected mice. Cultured microglia from GluA2(-/-) mice showed higher Ca(2+) -permeability, consequently inducing significant increase in the release of proinflammatory cytokine, such as TNF-α. The conditioning medium from KA-treated GluA2(-/-) microglia had more neurotoxic effect on wild type cultured neurons than that from KA-treated GluA2(+/+) microglia. These results suggest that membrane translocation of GluA2-containing AMPARs in activated microglia has functional importance and thus, dysfunction or decreased expression of GluA2 may accelerate Glu neurotoxicity via excess release of proinflammatory cytokines from microglia.

Activity-Mediated AMPA Receptor Remodeling, Driven by Alternative Splicing in the Ligand-Binding Domain

SummaryThe AMPA-type glutamate receptor (AMPAR) subunit composition shapes synaptic transmission and varies throughout development and in response to different input patterns. Here, we show that chronic activity deprivation gives rise to synaptic AMPAR responses with enhanced fidelity. Extrasynaptic AMPARs exhibited changes in kinetics and pharmacology associated with splicing of the alternative flip/flop exons. AMPAR mRNA indeed exhibited reprogramming of the flip/flop exons for GluA1 and GluA2 subunits in response to activity, selectively in the CA1 subfield. However, the functional changes did not directly correlate with the mRNA expression profiles but result from altered assembly of GluA1/GluA2 subunit splice variants, uncovering an additional regulatory role for flip/flop splicing in excitatory signaling. Our results suggest that activity-dependent AMPAR remodeling underlies changes in short-term synaptic plasticity and provides a mechanism for neuronal homeostasis.

Neuroligin 2 Is Required for Synapse Development and Function at theDrosophilaNeuromuscular Junction

Neuroligins belong to a highly conserved family of cell adhesion molecules that have been implicated in synapse formation and function. However, the precise in vivo roles of Neuroligins remain unclear. In the present study, we have analyzed the function of Drosophila neuroligin 2 (dnl2) in synaptic development and function. We show that dnl2 is strongly expressed in the embryonic and larval CNS and at the larval neuromuscular junction (NMJ). dnl2 null mutants are viable but display numerous structural defects at the NMJ, including reduced axonal branching and fewer synaptic boutons. dnl2 mutants also show an increase in the number of active zones per bouton but a decrease in the thickness of the subsynaptic reticulum and length of postsynaptic densities. dnl2 mutants also exhibit a decrease in the total glutamate receptor density and a shift in the subunit composition of glutamate receptors in favor of GluRIIA complexes. In addition to the observed defects in synaptic morphology, we also find that dnl2 mutants show increased transmitter release and altered kinetics of stimulus-evoked transmitter release. Importantly, the defects in presynaptic structure, receptor density, and synaptic transmission can be rescued by postsynaptic expression of dnl2. Finally, we show that dnl2 colocalizes and binds to Drosophila neurexin (dnrx) in vivo. However, whereas homozygous mutants for either dnl2 or dnrx are viable, double mutants are lethal and display more severe defects in synaptic morphology. Altogether, our data show that, although dnl2 is not absolutely required for synaptogenesis, it is required postsynaptically for synapse maturation and function.

Remodeling of Synaptic AMPA Receptor Subtype Alters the Probability and Pattern of Action Potential Firing

Changes in the subunit composition of postsynaptic AMPA-type glutamate receptors can be induced at CNS synapses by neural activity and under certain pathological conditions. Fear-induced incorporation of GluR2-containing receptors at cerebellar synapses selectively prolongs the decay time of synaptic currents, whereas a switch from GluR2-lacking to GluR2-containing receptors induced by parallel fiber stimulation reduces the amplitude in addition to lengthening the duration of EPSCs. Although it is often assumed that these two forms of synaptic plasticity will alter action potential (AP) firing in the postsynaptic neuron, this has not been directly tested. Using a dynamic current-clamp approach, we now show that the fear-induced increase in EPSC duration increases the size of EPSPs and thereby markedly enhances the AP firing probability. In contrast, the parallel fiber stimulation-triggered switch in GluR2 expression reduces the EPSP-AP coupling because of the decrease in the synaptic current amplitude. The switch also abolished the paired-pulse facilitation that arose from an activity and spermine-dependent unblock of GluR2-lacking receptors and hence reduced the ability of paired stimuli to evoke two consecutive APs. Therefore, fear-induced incorporation of GluR2 receptors enhances the EPSP-AP coupling, but the parallel fiber stimulation-triggered switch reduces both the EPSP-AP coupling and evoked AP doublets. In contrast to long-term potentiation and depression, which modify the amplitude of synaptic currents, this activity-induced change in AMPA receptor phenotype alters synaptic conductance waveform and postsynaptic short-term plasticity. These changes modulate both the probability and pattern of evoked AP firing via a fundamentally different mechanism from long-term potentiation and long-term depression.

Expression pattern of membrane-associated guanylate kinases in interneurons of the visual cortex

GABAergic interneurons are key elements regulating the activity of local circuits, and abnormal inhibitory circuits are implicated in certain psychiatric and neurodevelopmental diseases. The glutamatergic input that interneurons receive is a key determinant of their activity, yet its molecular structure and development, which are often distinct from those of glutamatergic input to pyramidal cells, are poorly defined. The membrane-associated guanylate kinase (MAGUK) homologs PSD-95/SAP90, PSD-93/chapsyn110, SAP97, and SAP102 are central organizers of the postsynaptic density at excitatory synapses on pyramidal neurons. We therefore studied the cell-type-specific and developmental expression of MAGUKs in the nonoverlapping parvalbumin (PV)- and somatostatin (SOM)-positive interneurons in the visual cortex. These interneuron subtypes account for the vast majority of interneurons in the cortex and have different functional properties and postsynaptic structures, being either axodendritic (PV(+)) or axospinous (SOM(+)). To study cell-type-specific MAGUK expression, we used DIG-labeled riboprobes against each MAGUK along with antibodies against either PV or SOM and examined tissue from juvenile (P15) and adult mice. Both PV(+) and SOM(+) interneurons express mRNA for PSD-95, PSD-93, and SAP102 in P15 and adult tissue. In contrast, these interneuron subtypes express SAP97 at P15, but for adult visual cortex we found that most PV(+) and SOM(+) interneurons show low or no expression of SAP97. Given the importance of SAP97 in regulating AMPA receptor GluA1 subunit and NMDA receptor subunits at glutamatergic synapses, these results suggest a developmental shift in glutamate receptor subunit composition and regulation of glutamatergic synapses on PV(+) and SOM(+) interneurons.

Linking Local Circuit Inhibition to Olfactory Behavior: A Critical Role for Granule Cells in Olfactory Discrimination

In this issue of Neuron, Abraham et al. report a direct connection between inhibitory function and olfactory behavior. Using molecular methods to alter glutamate receptor subunit composition in olfactory bulb granule cells, the authors found a selective modulation in the time required for difficult, but not simple, olfactory discrimination tasks.

Repeated 4-aminopyridine induced seizures diminish the efficacy of glutamatergic transmission in the neocortex

Systemic administration of the potassium channel blocker 4-aminopyridine (4-AP) elicits acute convulsions. Synchronized tonic–clonic activity develops during the first hour after the treatment. However, subsequent chronic spontaneous seizures do not appear which suggests changes in neuronal excitability. The aim of our present work was to evaluate alterations in the glutamatergic transmission in the somatosensory cortex of rats following daily, brief convulsions elicited by 4-AP treatment. Changes in general neuronal excitability and pharmacological sensitivity of glutamate receptors were tested in ex vivo electrophysiological experiments on brain slices. In parallel studies quantitative changes in subunit composition of glutamate receptors were determined with immunohistoblot technique, together with the analysis of kainate induced Co 2+ uptake. The results of our coordinated electrophysiological, receptor-pharmacological and histoblot studies demonstrated that repeated, daily, short convulsions resulted in a significant decrease of the general excitability of the somatosensory cortex together with changes in ionotropic glutamate receptor subunits. The relative inhibitory effect of the AMPA receptor antagonist, however, did not change. The NMDA receptor antagonist exerted somewhat stronger effect in the slices from convulsing animals. 4-AP pretreatment resulted in the attenuation of kainate induced Co 2+ uptake, which suggests either reduction in non-NMDA receptors numbers or reduction in their Ca 2+ permeability. Repeated seizures decreased GluR1–4 AMPA receptor subunit levels in all cortical layers with a relaitve increase in GluR1 subunits. While the principle NR1 NMDA receptor subunit showed no significant change, the staining density of NR2A subunit increased. These changes in ionotropic glutamate receptors are consistent with reduced excitability at glutamatergic synapses following repeated 4-AP induced seizures.

Ultrastructural analysis of the glutamatergic system in the outer plexiform layer of zebrafish retina

l-Glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF-bipolar cells by ionotropic receptors and hyperpolarizes ON-bipolar cells by metabotropic receptors. Despite extensive light microscopy on the distribution of glutamate receptors in zebrafish retina, there are little ultrastructural data. Given the importance of zebrafish in studies on the genetic manipulation of retinal development and function, precise data on the synaptic neurochemical organization of the zebrafish retina is needed. Immunohistochemical techniques were used to determine the ultrastructural localization of glutamate receptor subunits GluR2, GluR4, NMDA2B (NR2B) and mGluR1α in zebrafish outer plexiform layer (OPL). These antibodies were chosen because of an apparent conservation of localization of GluR2, GluR4 and mGluR1α in the vertebrate OPL, while there is some support for NMDA receptors in the OPL. GluR2-immunoreactivity (IR) was in all horizontal cell dendrites that invaginated cone pedicles and rod spherules. Three arrangements of dendrites contained GluR-IR in rod spherules: classical-type with GluR2-IR on lateral horizontal cell dendrites, a butterfly-shaped horizontal cell dendrite, and a goblet-shaped dendrite, likely of bipolar cell origin. GluR4-IR was restricted to dendrites of OFF-bipolar cells that innervated rod and cone terminals. NR2B-IR was restricted to a subtype of cone ON-bipolar cell. mGluR1α-IR was restricted to ON mixed rod/cone (Mb) bipolar cells whose dendrites innervated rod and cone synaptic terminals. The presence of mGluR1α on Mb bipolar cell dendrites is consistent with a role in retrograde endocannabinoid suppression. The subunit composition of glutamate receptors should affect the kinetics and pharmacology of these cells to glutamate receptor activation.