Molecular mechanisms underlying activity-dependent AMPA receptor cycling in retinal ganglion cells

On retinal ganglion cells (RGCs) transmit light encoded information to the brain and receive excitatory input from On cone bipolar cells (CBPs). The synaptic CBP input onto On RGCs is mediated by AMPA-type glutamate receptors (AMPARs) that include both those lacking a GluA2 subunit, and are therefore permeable to Ca^2 +, and those that possess at least one GluA2 subunit and are Ca^2 +-impermeable. We have previously demonstrated in electrophysiological studies that periods of low synaptic activity, brought about by housing animals in darkness, enhance the proportion of GluA2-lacking AMPARs at the On CBP–On RGC synapse by mobilizing surface GluA2 containing receptors into a receptor pool that rapidly cycles in and out of the membrane. AMPAR cycling induction by reduced synaptic activity takes several hours. This delay suggests that changes in expression of proteins which regulate AMPAR trafficking may mediate the altered mobility of GluA2 AMPARs in RGCs. In this study, we test the hypothesis that AMPAR trafficking proteins couple synaptic activity to AMPAR cycling in RGCs. Immunocytochemical and biochemical analyses confirmed that darkness decreases surface GluA2 in RGCs and changed the expression levels of three proteins associated with GluA2 trafficking. GRIP was decreased, while PICK1 and Arc were increased. Knockdown of GRIP with siRNA elevated constitutive AMPAR cycling, mimicking effects of reduced synaptic activity, while knockdown of PICK1 and Arc blocked increases in constitutive GluA2 trafficking. Our results support a role for correlated, activity-driven changes in multiple AMPAR trafficking proteins that modulate GluA2 cycling which can in turn affect synaptic AMPAR composition in RGCs.     

Decreased calcium flux in Niemann-Pick type C1 patient-specific iPSC-derived neurons due to higher amount of calcium-impermeable AMPA receptors

Niemann-Pick disease type C1 (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene, resulting mainly in the accumulation of cholesterol and the ganglioside GM2. Recently, we described accumulations of these lipids in neuronal differentiated cells derived from NPC1 patient-specific induced pluripotent stem cells (iPSCs). As these lipids are essential for proper cell membrane composition, we were interested in the expression and function of voltage-gated ion channels and excitatory AMPA receptors (AMPARs) in neurons derived from three patient-specific iPSC lines. By means of patch clamp recordings and microfluorimetric measurements of calcium (Ca^2 +), we examined the expression of voltage-gated ion channels and AMPARs. Cells of the three used cell lines carrying the c.1836A > C/c.1628delC, the c.1180T > C or the c.3182T > C mutation demonstrated a significantly reduced AMPA-induced Ca^2 +-influx, suggesting an altered expression profile of these receptors. RT-qPCR revealed a significant upregulation of mRNA for the AMPA receptor subunits GluA1 and GluA2 and western blot analysis showed increased protein level of GluA2. Thus, we conclude that the observed reduced Ca^2 +-influx is based on an increase of GluA2 containing Ca^2 +-impermeable AMPARs. An attenuated function of GluRs in neurons potentially contributes to the progressive neurodegeneration observed in NPC1 and might represent an objective in regard of the development of new therapeutic approaches in NPC1.     

mGlu1 tonically regulates levels of calcium‐permeable AMPA receptors in cultured nucleus accumbens neurons through retinoic acid signaling and protein translation

In several brain regions, ongoing metabotropic glutamate receptor 1 (mGlu1) transmission has been shown to tonically suppress synaptic levels of Ca²⁺‐permeable AMPA receptors (CP‐AMPARs) while pharmacological activation of mGlu1 removes CP‐AMPARs from these synapses. Consistent with this, we previously showed in nucleus accumbens (NAc) medium spiny neurons (MSNs) that reduced mGlu1 tone enables and mGlu1 positive allosteric modulation reverses the elevation of CP‐AMPAR levels in the NAc that underlies enhanced cocaine craving in the “incubation of craving” rat model of addiction. To better understand mGlu1/CP‐AMPAR interactions, we used a NAc/prefrontal cortex co‐culture system in which NAc MSNs express high CP‐AMPAR levels, providing an in vitro model for NAc MSNs after the incubation of cocaine craving. The non‐specific group I orthosteric agonist dihydroxyphenylglycine (10 min) decreased cell surface GluA1 but not GluA2, indicating CP‐AMPAR internalization. This was prevented by mGlu1 (LY367385) or mGlu5 (MTEP) blockade. However, a selective role for mGlu1 emerged in studies of long‐term antagonist treatment. Thus, LY367385 (24 hr) increased surface GluA1 without affecting GluA2, whereas MTEP (24 hr) had no effect. In hippocampal neurons, scaling up of CP‐AMPARs can occur through a mechanism requiring retinoic acid (RA) signaling and new GluA1 synthesis. Consistent with this, the LY367385‐induced increase in surface GluA1 was blocked by anisomycin (translation inhibitor) or 4‐(diethylamino)‐benzaldehyde (RA synthesis inhibitor). Thus, mGlu1 transmission tonically suppresses cell surface CP‐AMPAR levels, and decreasing mGlu1 tone increases surface CP‐AMPARs via RA signaling and protein translation. These results identify a novel mechanism for homeostatic plasticity in NAc MSNs.     

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 Ca²⁺ 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 Ca²⁺‐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.     

Interactions between N‐Ethylmaleimide‐sensitive factor and GluA2 contribute to effects of glucocorticoid hormones on AMPA receptor function in the rodent hippocampus

Glucocorticoid hormones, via activation of their receptors, promote memory consolidation, but the exact underlying mechanisms remain elusive. We examined how corticosterone regulates AMPA receptor (AMPAR) availability in the synapse, which is important for synaptic plasticity and memory formation. Peptides which specifically block the interaction between N‐Ethylmaleimide‐Sensitive Factor (NSF) and the AMPAR‐subunit GluA2 prevented the increase in synaptic transmission and surface expression of AMPARs known to occur after corticosterone application to hippocampal neurons. Combining a live imaging Fluorescence Recovery After Photobleaching (FRAP) approach with the use of the pH‐sensitive GFP‐AMPAR tagging revealed that this NSF/GluA2 interaction was also essential for the increase of the mobile fraction and reduction of the diffusion of AMPARs after treating hippocampal neurons with corticosterone. We conclude that the interaction between NSF and GluA2 contributes to the effects of corticosterone on AMPAR function. © 2016 Wiley Periodicals, Inc.     

S-SCAM/MAGI-2 is an essential synaptic scaffolding molecule for the GluA2-containing maintenance pool of AMPA receptors

Synaptic plasticity, the cellular basis of learning and memory, involves the dynamic trafficking of AMPA receptors (AMPARs) into and out of synapses. One of the remaining key unanswered aspects of AMPAR trafficking is the mechanism by which synaptic strength is preserved despite protein turnover. In particular, the identity of AMPAR scaffolding molecule(s) involved in the maintenance of GluA2-containing AMPARs is completely unknown. Here we report that the synaptic scaffolding molecule (S-SCAM; also called membrane-associated guanylate kinase inverted-2 and atrophin interacting protein-1) plays the critical role of maintaining synaptic strength. Increasing S-SCAM levels in rat hippocampal neurons led to specific increases in the surface AMPAR levels, enhanced AMPAR-mediated synaptic transmission, and enlargement of dendritic spines, without significantly effecting GluN levels or NMDA receptor (NMDAR) EPSC. Conversely, decreasing S-SCAM levels by RNA interference-mediated knockdown caused the loss of synaptic AMPARs, which was followed by a severe reduction in the dendritic spine density. Importantly, S-SCAM regulated synaptic AMPAR levels in a manner, dependent on GluA2 not GluA1, sensitive to N-ethylmaleimide-sensitive fusion protein interaction, and independent of activity. Further, S-SCAM increased surface AMPAR levels in the absence of PSD-95, while PSD-95 was dependent on S-SCAM to increase surface AMPAR levels. Finally, S-SCAM overexpression hampered NMDA-induced internalization of AMPARs and prevented the induction of long term-depression, while S-SCAM knockdown did not. Together, these results suggest that S-SCAM is an essential AMPAR scaffolding molecule for the GluA2-containing pool of AMPARs, which are involved in the constitutive pathway of maintaining synaptic strength.     

The glutamate receptor GluN2 subunit regulates synaptic trafficking of AMPA receptors in the neonatal mouse brain

The N‐methyl‐d ‐aspartate receptor (NMDAR) plays various physiological and pathological roles in neural development, synaptic plasticity and neuronal cell death. It is composed of two GluN1 and two GluN2 subunits and, in the neonatal hippocampus, most synaptic NMDARs are GluN2B‐containing receptors, which are gradually replaced with GluN2A‐containing receptors during development. Here, we examined whether GluN2A could be substituted for GluN2B in neural development and functions by analysing knock‐in (KI) mice in which GluN2B is replaced with GluN2A. The KI mutation was neonatally lethal, although GluN2A‐containing receptors were transported to the postsynaptic membrane even without GluN2B and functional at synapses of acute hippocampal slices of postnatal day 0, indicating that GluN2A‐containing NMDARs could not be substituted for GluN2B‐containing NMDARs. Importantly, the synaptic α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionic acid receptor (AMPAR) subunit GluA1 was increased, and the transmembrane AMPAR regulatory protein, which is involved in AMPAR synaptic trafficking, was increased in KI mice. Although the regulation of AMPARs by GluN2B has been reported in cultured neurons, we showed here that AMPAR‐mediated synaptic responses were increased in acute KI slices, suggesting differential roles of GluN2A and GluN2B in AMPAR expression and trafficking in vivo. Taken together, our results suggest that GluN2B is essential for the survival of animals, and that the GluN2B–GluN2A switching plays a critical role in synaptic integration of AMPARs through regulation of GluA1 in the whole animal.     

Requirement of the immediate early gene vesl-1S/homer-1a for fear memory formation

Background

Long-term depression (LTD) in the hippocampus can be induced by activation of different types of G-protein coupled receptors, in particular metabotropic glutamate receptors (mGluRs) and muscarinic acethycholine receptors (mAChRs). Since mGluRs and mAChRs activate the same G-proteins and isoforms of phospholipase C (PLC), it would be expected that these two forms of LTD utilise the same molecular mechanisms. However, we find a distinct mechanism of LTD involving GRIP and liprin-α.

Results

Whilst both forms of LTD require activation of tyrosine phosphatases and involve internalisation of AMPARs, they use different molecular interactions. Specifically, mAChR-LTD, but not mGluR-LTD, is blocked by peptides that inhibit the binding of GRIP to the AMPA receptor subunit GluA2 and the binding of GRIP to liprin-α. Thus, different receptors that utilise the same G-proteins can regulate AMPAR trafficking and synaptic efficacy via distinct molecular mechanisms.

Conclusion

Our results suggest that mAChR-LTD selectively involves interactions between GRIP and liprin-α. These data indicate a novel mechanism of synaptic plasticity in which activation of M1 receptors results in AMPAR endocytosis, via a mechanism involving interactions between GluA2, GRIP and liprin-α.     

Transient appearance of Ca2+‐permeable AMPA receptors is crucial for the production of repetitive LTP‐induced synaptic enhancement (RISE) in cultured hippocampal slices

We have previously shown that repetitive induction of long‐term potentiation (LTP) by glutamate (100 μM, 3 min, three times at 24‐hr intervals) provoked long‐lasting synaptic enhancement accompanied by synaptogenesis in rat hippocampal slice cultures, a phenomenon termed RISE (repetitive LTP‐induced synaptic enhancement). Here, we examined the role of Ca²⁺‐permeable (CP) AMPA receptors (AMPARs) in the establishment of RISE. We first found a component sensitive to the Joro‐spider toxin (JSTX), a blocker of CP‐AMPARs, in a field EPSP recorded from CA3‐CA1 synapses at 2–3 days after stimulation, but this component was not found for 9–10 days. We also observed that rectification of AMPAR‐mediated current appeared only 2–3 days after stimulation, using a whole‐cell patch clamp recording from CA1 pyramidal neurons. These findings indicate that CP‐AMPAR is transiently expressed in the developing phase of RISE. The blockade of CP‐AMPARs by JSTX for 24 hr at this developing phase inhibited RISE establishment, accompanied by the loss of small synapses at the ultrastructural level. These results suggest that transiently induced CP‐AMPARs play a critical role in synaptogenesis in the developing phase of long‐lasting hippocampal synaptic plasticity, RISE.     

Calmodulin activity regulates group I metabotropic glutamate receptor‐mediated signal transduction and synaptic depression

Group I metabotropic glutamate receptors (mGluR), including mGluR1 and mGluR 5 (mGluR1/5), are coupled to Gq and modulate activity‐dependent synaptic plasticity. Direct activation of mGluR1/5 causes protein translation‐dependent long‐term depression (LTD). Although it has been established that intracellular Ca²⁺ and the Gq‐regulated signaling molecules are required for mGluR1/5 LTD, whether and how Ca²⁺ regulates Gq signaling and upregulation of protein expression remain unknown. Through pharmacological inhibition, we tested the function of the Ca²⁺ sensor calmodulin (CaM) in intracellular signaling triggered by the activation of mGluR1/5. CaM inhibitor N‐[4‐aminobutyl]‐5‐chloro‐2‐naphthalenesulfonamide hydrochloride (W13) suppressed the mGluR1/5‐stimulated activation of extracellular signal‐regulated kinase 1/2 (ERK1/2) and p70‐S6 kinase 1 (S6K1) in hippocampal neurons. W13 also blocked the mGluR1/5 agonist‐induced synaptic depression in hippocampal slices and in anesthetized mice. Consistent with the function of CaM, inhibiting the downstream targets Ca²⁺/CaM‐dependent protein kinases (CaMK) blocked ERK1/2 and S6K1 activation. Furthermore, disruption of the CaM–CaMK–ERK1/2 signaling cascade suppressed the mGluR1/5‐stimulated upregulation of Arc expression. Altogether, our data suggest CaM as a new Gq signaling component for coupling Ca²⁺ and protein upregulation and regulating mGluR1/5‐mediated synaptic modification. © 2016 Wiley Periodicals, Inc.     

Inhibitory effects of levetiracetam on the high-voltage-activated L-type Ca2+ channels in hippocampal CA3 neurons of spontaneously epileptic rat (SER)

Levetiracetam (LEV) is a widely used antiepileptic agent for partial refractory epilepsy in humans. LEV has unique antiepileptic effects in that it does not inhibit electroshock- or pentylenetetrazol-induced convulsion, but does inhibit seizures in kindling animal and spontaneously epileptic rat (SER: zi/zi, tm/tm) that shows both tonic convulsion and absence-like seizures. LEV also has unique characteristics in terms of its antiepileptic mechanism; it has no activity on Na⁺ and K⁺ channels or on glutamate and GABA_A receptors. Recently, we found that LEV inhibits the depolarization shift and accompanying repetitive firing induced by mossy fiber stimulation in CA3 neurons of SER hippocampal slices. Therefore, this study was performed to determine whether LEV could inhibit the voltage-activated L-type Ca²⁺ current of hippocampal CA3 neurons obtained from SER and the non-epileptic Wistar rat. As previously reported, SER CA3 neurons were classified into type 1 and type 2 neurons. The application of LEV (100 μM) elevated the threshold for activation of the Ca²⁺ current, which was lowered in SER type 1 neurons and reduced the current size. Type 2 neurons of SER have a similar current–voltage relationship to Wistar rat neurons and the decay component of Ca²⁺ current during depolarization pulse in type 2 neurons was found to be smaller than that in Wistar rat neurons. LEV (100 μM) also reduced Ca²⁺ current in SER type 2 neurons. The effects of LEV were examined on such type 2 SER hippocampal CA3 neurons, compared with those on Wistar rat CA3 neurons. Application of LEV (10 μM) produced a significant decrease of amplitude of the Ca²⁺ current in SER neurons, although at this concentration of LEV there was no statistically significant decrease in the amplitude of Ca²⁺ current in Wistar rat neurons. Furthermore, LEV (100 nM–1 mM) reduced the Ca²⁺ current in a concentration-dependent manner in both SER and Wistar rat neurons, but the inhibition was much more potent in the former neurons than in the latter. Under the condition that the Ca²⁺ current had already been inhibited by LEV (10 μM), the addition of nifedipine (10 μM) did not cause further inhibition. Conversely, LEV had no effects on the current that had already been decreased by nifedipine (10 μM) given before LEV treatment (10 μM), indicating that LEV could act on the L-type Ca²⁺channel. LEV elevated the threshold potential level for activation of the Ca²⁺ current and reduced the L-type Ca²⁺ current in type 1 neurons of SER, and the inhibitory action in type 2 neurons was much more potent than that in Wistar rat neurons, suggesting that these effects contribute, at least partly, to the antiepileptic action of LEV.     

mGluR and NMDAR activation internalize distinct populations of AMPARs

Activation of metabotropic- (mGluRs) or NMDA-type glutamate receptors (NMDARs) each can induce long-term depression (LTD) of synaptic transmission in CA1 hippocampal neurons. These two forms of LTD are triggered by diverse signaling pathways yet both are expressed by the internalization of AMPA-type glutamate receptors (AMPARs). An unanswered question remains as to whether the convergence of the mGluR and NMDAR signaling pathways on AMPAR endocytosis renders these two forms of plasticity functionally equivalent, with both pathways inducing endocytosis of the same population of synaptic AMPARs. We now report evidence that these pathways couple to the endocytosis of distinct populations of AMPARs defined by their mobility in the membrane surface. NMDAR activation enhances removal of surface AMPARs that rapidly cycle into and out of the membrane surface, while activation of mGluRs with DHPG results in the internalization of a non-mobile population of AMPARs. Glutamate Receptor Interacting Proteins 1 and 2 (GRIP1/2) play a key role in defining the non-cycling receptor population. GRIP1/2 knockdown with siRNA increases the proportion of rapidly cycling surface AMPARs and inhibits mGluR- but not NMDAR-mediated AMPAR internalization. Additionally, we find that mGluR activation dissociates surface AMPARs from GRIP1/2 while stimulation of NMDARs elicits the loss of membrane receptors not bound to GRIP1/2. We propose that these two receptor pathways can drive the endocytosis of distinct populations of AMPARs: NMDARs activation induces the endocytosis of rapidly cycling surface AMPARs not directly associated with GRIP1/2 while mGluR activation induces the endocytosis of non-cycling GRIP-bound surface AMPARs.     

N-methyl-d-aspartate receptor- and metabotropic glutamate receptor-dependent long-term depression are differentially regulated by the ubiquitin-proteasome system

Long-term depression (LTD) in CA1 pyramidal neurons can be induced by activation of either N -methyl- d -aspartate receptors (NMDARs) or metabotropic glutamate receptors (mGluRs), both of which elicit changes in synaptic efficacy through α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR) endocytosis. To address the role of the ubiquitin-proteasome system in regulating AMPAR endocytosis during these forms of LTD, we examined the effects of pharmacological inhibitors of proteasomal degradation and protein ubiquitination on endocytosis of glutamate receptor 1 (GluR1) -containing AMPARs in dissociated rat hippocampal cultures as well as LTD of excitatory synaptic responses in acute rat hippocampal slices. Our findings suggest that the contribution of the ubiquitin-proteasome system to NMDAR-induced vs. mGluR-induced AMPAR endocytosis and the consequent LTD differs significantly. NMDAR-induced AMPAR endocytosis and LTD occur independently of proteasome function but appear to depend, at least in part, on ubiquitination. In contrast, mGluR-induced AMPAR endocytosis and LTD are enhanced by inhibition of proteasomal degradation, as well as by the inhibitor of protein ubiquitination. Furthermore, the decay of mGluR-induced membrane depolarization and Erk activation is delayed following inhibition of either ubiquitination or proteasomal degradation. These results suggest that, although NMDAR-dependent LTD may utilize ubiquitin as a signal for AMPAR endocytosis, mGluR-induced signaling and LTD are limited by a feedback mechanism that involves the ubiquitin-proteasome system.     

AMPA receptor subunits are differentially expressed in parvalbumin‐ and calretinin‐positive neurons of the rat hippocampus

Recent studies suggest a functional diversity of native α-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor channels (AMPARs). In several types of interneurons, AMPARs are characterized by higher Ca²⁺ permeability and faster kinetics than AMPARs in principal cells. We studied the expression profile of AMPAR subunits in the hippocampal parvalbumin (PV)- and calretinin (CR)-positive cells, which represent different populations of non-principal cells. To this end, non-radioactive in situ hybridization with AMPAR subunit specific cRNAs was combined with immunocytochemistry for PV or CR. Double-immunolabelling using antibodies against AMPAR subunits and PV or CR was also performed. PV-containing neurons represent a fairly homogeneous population of cells expressing high levels of GluR-A and GluR-D mRNAs, moderate levels of GluR-C and low levels of GluR-B mRNAs in all the examined regions of hippocampus. The vast majority of CR-containing cells have a much lower expression of GluR-A, -C and -D mRNA than PV-positive neurons, although similarly featuring low levels of GluR-B mRNA. Only a subpopulation of CR-containing cells, the spiny neurons of the dentate gyrus and CA3 region of the hippocampus were characterized by a strong expression of GluR-A and -D subunit mRNAs. The differential pattern found for the AMPAR subunit mRNA expression was confirmed by immunocytochemistry at protein level. Despite the common feature of low GluR-B subunit expression, PV- and CR-containing interneurons differ with respect to the density and combination of their expressed AMPAR subunits. The different combination of subunits might subserve different properties of the AMPA channels featured by these cell types, with implications for the functioning of the hippocampal network.     

Brain-derived neurotrophic factor rapidly increases AMPA receptor surface expression in rat nucleus accumbens

In the rodent nucleus accumbens (NAc), cocaine elevates levels of brain-derived neurotrophic factor (BDNF). Conversely, BDNF can augment cocaine-related behavioral responses. The latter could reflect enhancement of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) transmission, because AMPARs in the NAc mediate some cocaine-induced behaviors. Furthermore, in vitro studies in other cell types show that BDNF can promote AMPAR synaptic delivery. In this study, we investigated whether BDNF similarly promotes AMPAR trafficking in the adult rat NAc. After unilateral intracranial injection of BDNF into NAc core or shell, rats were killed at post-injection times ranging from 30 min to 3 days. NAc core or shell tissue from both injected and non-injected hemispheres was analysed by Western blotting. A protein cross-linking assay was used to measure AMPAR surface expression. Assessment of tropomyosin receptor kinase B signaling demonstrated that injected BDNF was biologically active. BDNF injection into NAc core, but not NAc shell, led to a protein synthesis- and extracellular signal-regulated kinase-dependent increase in cell surface GluA1 and a trend towards increased total GluA1. This was detected 30 min post-injection but not at longer time-points. GluA2 and GluA3 were unaffected, suggesting an effect of BDNF on homomeric GluA1 Ca(2+) -permeable AMPARs. These results demonstrate that exogenous BDNF rapidly increases AMPAR surface expression in the rat NAc core, raising the possibility of a relationship between increases in endogenous BDNF levels and alterations in AMPAR transmission observed in the NAc of cocaine-experienced rats.     

Exercise modifies α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor expression in striatopallidal neurons in the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine‐lesioned mouse

The α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic‐acid‐type glutamate receptor (AMPAR) plays a critical role in modulating experience‐dependent neuroplasticity, and alterations in AMPAR expression may underlie synaptic dysfunction and disease pathophysiology. Using the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) mouse model of dopamine (DA) depletion, our previous work showed exercise increases total GluA2 subunit expression and the contribution of GluA2‐containing channels in MPTP mice. The purpose of this study was to determine whether exercise‐dependent changes in AMPAR expression after MPTP are specific to the striatopallidal (D₂R) or striatonigral (D₁R) medium spiny neuron (MSN) striatal projection pathways. Drd₂‐eGFP‐BAC transgenic mice were used to delineate differences in AMPAR expression between striatal D₂R‐MSNs and D₁R‐MSNs. Striatal AMPAR expression was assessed by immunohistochemical (IHC) staining, Western immunoblotting (WB) of preparations enriched for postsynaptic density (PSD), and alterations in the current–voltage relationship of MSNs. We found DA depletion results in the emergence of GluA2‐lacking AMPARs selectively in striatopallidal D₂R‐MSNs and that exercise reverses this effect in MPTP mice. Exercise‐induced changes in AMPAR channels observed after DA depletion were associated with alterations in GluA1 and GluA2 subunit expression in postsynaptic protein, D₂R‐MSN cell surface expression, and restoration of corticostriatal plasticity. Mechanisms regulating experience‐dependent changes in AMPAR expression may provide innovative therapeutic targets to increase the efficacy of treatments for basal ganglia disorders, including Parkinson's disease. © 2013 Wiley Periodicals, Inc.     

Hippocampal GluA1-containing AMPA receptors mediate context-dependent sensitization to morphine

Glutamatergic systems, including AMPA receptors (AMPARs), are involved in opiate-induced neuronal and behavioral plasticity, although the mechanisms underlying these effects are not fully understood. In the present study, we investigated the effects of repeated morphine administration on AMPAR expression, synaptic plasticity, and context-dependent behavioral sensitization to morphine. We found that morphine treatment produced changes of synaptic AMPAR expression in the hippocampus, a brain area that is critically involved in learning and memory. These changes could be observed 1 week after the treatment, but only when mice developed context-dependent behavioral sensitization to morphine in which morphine treatment was associated with drug administration environment. Context-dependent behavioral sensitization to morphine was also associated with increased basal synaptic transmission and disrupted hippocampal long-term potentiation (LTP), whereas these effects were less robust when morphine administration was not paired with the drug administration environment. Interestingly, some effects may be related to the prior history of morphine exposure in the drug-associated environment, since alterations of AMPAR expression, basal synaptic transmission, and LTP were observed in mice that received a saline challenge 1 week after discontinuation of morphine treatment. Furthermore, we demonstrated that phosphorylation of GluA1 AMPAR subunit plays a critical role in the acquisition and expression of context-dependent behavioral sensitization, as this behavior is blocked by a viral vector that disrupts GluA1 phosphorylation. These data provide evidence that glutamatergic signaling in the hippocampus plays an important role in context-dependent sensitization to morphine and supports further investigation of glutamate-based strategies for treating opiate addiction.     

A concerted action of L- and T-type Ca2 + channels regulates locus coeruleus pacemaking

Dysfunction of noradrenergic locus coeruleus (LC) neurons is involved in psychiatric and neurodegenerative diseases and is an early hallmark of Parkinson's disease (PD). The analysis of ion channels underlying the autonomous electrical activity of LC neurons, which is ultimately coupled to cell survival signaling pathways, can lead to a better understanding of the vulnerability of these neurons. In LC neurons somatodendritic Ca^2 + oscillations, mediated by L-type Ca^2 + channels, accompany spontaneous spiking and are linked to mitochondrial oxidant stress. However, the expression and functional implication of low-threshold activated T-type Ca^2 + channels in LC neurons were not yet studied. To this end we performed RT-PCR expression analysis in LC neurons. In addition, we utilized slice patch clamp recordings of in vitro brainstem slices in combination with L-type and T-type Ca^2 + channel blockers. We found the expression of a distinct set of L-type and T-type Ca^2 + channel subtypes mediating a pronounced low-threshold activated Ca^2 + current component. Analyzing spike trains, we revealed that neither L-type Ca^2 + channel nor T-type Ca^2 + channel blockade alone leads to a change in firing properties. In contrast, a combined application of antagonists significantly decreased the afterhyperpolarization amplitude, resulting in an increased firing frequency. Hence, we report the functional expression of T-type Ca^2 + channels in LC neurons and demonstrate their role in increasing the robustness of LC pacemaking by working in concert with Cav1 channels.     

Food restriction induces synaptic incorporation of calcium‐permeable AMPA receptors in nucleus accumbens

Chronic food restriction potentiates behavioral and cellular responses to drugs of abuse and D‐1 dopamine receptor agonists administered systemically or locally in the nucleus accumbens (NAc). However, the alterations in NAc synaptic transmission underlying these effects are incompletely understood. AMPA receptor trafficking is a major mechanism for regulating synaptic strength, and previous studies have shown that both sucrose and d‐amphetamine rapidly alter the abundance of AMPA receptor subunits in the NAc postsynaptic density (PSD) in a manner that differs between food‐restricted and ad libitum fed rats. In this study we examined whether food restriction, in the absence of reward stimulus challenge, alters AMPAR subunit abundance in the NAc PSD. Food restriction was found to increase surface expression and, specifically, PSD abundance, of GluA1 but not GluA2, suggesting synaptic incorporation of GluA2‐lacking Ca2+‐permeable AMPARs (CP‐AMPARs). Naspm, an antagonist of CP‐AMPARs, decreased the amplitude of evoked EPSCs in NAc shell, and blocked the enhanced locomotor response to local microinjection of the D‐1 receptor agonist, SKF‐82958, in food‐restricted, but not ad libitum fed, subjects. Although microinjection of the D‐2 receptor agonist, quinpirole, also induced greater locomotor activation in food‐restricted than ad libitum fed rats, this effect was not decreased by Naspm. Taken together, the present findings are consistent with the synaptic incorporation of CP‐AMPARs in D‐1 receptor‐expressing medium spiny neurons in NAc as a mechanistic underpinning of the enhanced responsiveness of food‐restricted rats to natural rewards and drugs of abuse.     

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner

Alzheimer''s disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP₆₁). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP₆₁, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca²⁺-permeable GluA2-lacking AMPA receptors along with higher STEP₆₁ levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.