Long-term potentiation in isolated dendritic spines

Alterations in the Proteome of Developing Neocortical Synaptosomes in the Absence of MET Signaling Revealed by Comparative Proteomics

Alterations in the expression of genes encoding proteins involved in synapse formation, maturation, and function are a hallmark of many neurodevelopmental and psychiatric disorders. For example, there is reduced neocortical expression of the MET receptor tyrosine kinase (MET) transcript and protein in Autism Spectrum Disorder (ASD) and Rett syndrome. Preclinical in vivo and in vitro models manipulating MET signaling reveal that the receptor modulates excitatory synapse development and maturation in select forebrain circuits. The molecular adaptations underlying the altered synaptic development remain unknown. We performed a comparative mass spectrometry analysis of synaptosomes generated from the neocortex of wild type and Met null mice during the peak of synaptogenesis (postnatal day 14; data are available from ProteomeXchange with identifier PXD033204). The analyses revealed broad disruption of the developing synaptic proteome in the absence of MET, consistent with the localization of MET protein in pre- and postsynaptic compartments, including proteins associated with the neocortical synaptic MET interactome and those encoded by syndromic and ASD risk genes. In addition to an overrepresentation of altered proteins associated with the SNARE complex, multiple proteins in the ubiquitin-proteasome system and associated with the synaptic vesicle, as well as proteins that regulate actin filament organization and synaptic vesicle exocytosis/endocytosis, were disrupted. Taken together, the proteomic changes are consistent with structural and functional changes observed following alterations in MET signaling. We hypothesize that the molecular adaptations following Met deletion may reflect a general mechanism that produces circuit-specific molecular changes due to loss or reduction of synaptic signaling proteins.

p140Cap Regulates the Composition and Localization of the NMDAR Complex in Synaptic Lipid Rafts

The NMDARs are key players in both physiological and pathologic synaptic plasticity because of their involvement in many aspects of neuronal transmission as well as learning and memory. The contribution in these events of different types of GluN2A-interacting proteins is still unclear. The p140Cap scaffold protein acts as a hub for postsynaptic complexes relevant to psychiatric and neurologic disorders and regulates synaptic functions, such as the stabilization of mature dendritic spine, memory consolidation, LTP, and LTD. Here we demonstrate that p140Cap directly binds the GluN2A subunit of NMDAR and modulates GluN2A-associated molecular network. Indeed, in p140Cap KO male mice, GluN2A is less associated with PSD95 both in ex vivo synaptosomes and in cultured hippocampal neurons, and p140Cap expression in KO neurons can rescue GluN2A and PSD95 colocalization. p140Cap is crucial in the recruitment of GluN2A-containing NMDARs and, consequently, in regulating NMDARs' intrinsic properties. p140Cap is associated to synaptic lipid-raft (LR) and to soluble postsynaptic membranes, and GluN2A and PSD95 are less recruited into synaptic LR of p140Cap KO male mice. Gated-stimulated emission depletion microscopy on hippocampal neurons confirmed that p140Cap is required for embedding GluN2A clusters in LR in an activity-dependent fashion. In the synaptic compartment, p140Cap influences the association between GluN2A and PSD95 and modulates GluN2A enrichment into LR. Overall, such increase in these membrane domains rich in signaling molecules results in improved signal transduction efficiency.SIGNIFICANCE STATEMENT Here we originally show that the adaptor protein p140Cap directly binds the GluN2A subunit of NMDAR and modulates the GluN2A-associated molecular network. Moreover, we show, for the first time, that p140Cap also associates to synaptic lipid rafts and controls the selective recruitment of GluN2A and PSD95 to this specific compartment. Finally, gated-stimulated emission depletion microscopy on hippocampal neurons confirmed that p140Cap is required for embedding GluN2A clusters in lipid rafts in an activity-dependent fashion. Overall, our findings provide the molecular and functional dissection of p140Cap as a new active member of a highly dynamic synaptic network involved in memory consolidation, LTP, and LTD, which are known to be altered in neurologic and psychiatric disorders.

Molecular Characterization of AMPA-Receptor-Containing Vesicles

Regulated delivery of AMPA receptors (AMPARs) to the postsynaptic membrane is an essential step in synaptic strength modification, and in particular, long-term potentiation (LTP). While LTP has been extensively studied using electrophysiology and light microscopy, several questions regarding the molecular mechanisms of AMPAR delivery via trafficking vesicles remain outstanding, including the gross molecular make up of AMPAR trafficking organelles and identification and location of calcium sensors required for SNARE complex-dependent membrane fusion of such trafficking vesicles with the plasma membrane. Here, we isolated AMPA-containing vesicles (ACVs) from whole mouse brains via immunoisolation and characterized them using immunoelectron microscopy, immunoblotting, and liquid chromatography–tandem mass spectrometry (LC–MS/MS). We identified several proteins on ACVs that were previously found to play a role in AMPAR trafficking, including synaptobrevin-2, Rabs, the SM protein Munc18-1, the calcium-sensor synaptotagmin-1, as well as several new candidates, including synaptophysin and synaptogyrin on ACV membranes. Additionally, we identified two populations of ACVs based on size and molecular composition: small-diameter, synaptobrevin-2- and GluA1-containing ACVs, and larger transferrin- receptor-, GluA1-, GluA2-, and GluA3-containing ACVs. The small-diameter population of ACVs may represent a fusion-capable population of vesicles due to the presence of synaptobrevin-2. Because the fusion of ACVs may be a requisite of LTP, this population could represent trafficking vesicles related to LTP.

Chemical Stimulation of Rodent and Human Cortical Synaptosomes: Implications in Neurodegeneration

Synaptic plasticity events, including long-term potentiation (LTP), are often regarded as correlates of brain functions of memory and cognition. One of the central players in these plasticity-related phenomena is the α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR). Increased levels of AMPARs on postsynaptic membranes thus constitute a biochemical measure of LTP. Isolated synaptic terminals (synaptosomes) are an excellent ex vivo tool to monitor synaptic physiology in healthy and diseased brains, particularly in human research. We herein describe three protocols for chemically-induced LTP (cLTP) in synaptosomes from both rodent and human brain tissues. Two of these chemical stimulation protocols are described for the first time in synaptosomes. A pharmacological block of synaptosomal actin dynamics confirmed the efficiency of the cLTP protocols. Furthermore, the study prototypically evaluated the deficiency of cLTP in cortical synaptosomes obtained from human cases of early-onset Alzheimer’s disease (EOAD) and frontotemporal lobar degeneration (FLTD), as well as an animal model that mimics FLTD.

Exosomal tau with seeding activity is released from Alzheimer's disease synapses, and seeding potential is associated with amyloid beta

Synaptic transfer of tau has long been hypothesized from the human pathology pattern and has been demonstrated in vitro and in vivo, but the precise mechanisms remain unclear. Extracellular vesicles such as exosomes have been suggested as a mechanism, but not all tau is exosomal. The present experiments use a novel flow cytometry assay to quantify depolarization of synaptosomes by KCl after loading with FM2-10, which induces a fluorescence reduction associated with synaptic vesicle release; the degree of reduction in cryopreserved human samples equaled that seen in fresh mouse synaptosomes. Depolarization induced the release of vesicles in the size range of exosomes, along with tetraspanin markers of extracellular vesicles. A number of tau peptides were released, including tau oligomers; released tau was primarily unphosphorylated and C-terminal truncated, with Aβ release just above background. When exosomes were immunopurified from release supernatants, a prominent tau band showed a dark smeared appearance of SDS-stable oligomers along with the exosomal marker syntenin-1, and these exosomes induced aggregation in the HEK tau biosensor assay. However, the flow-through did not seed aggregation. Size exclusion chromatography of purified released exosomes shows faint signals from tau in the same fractions that show a CD63 band, an exosomal size signal, and seeding activity. Crude synaptosomes from control, tauopathy, and AD cases demonstrated lower seeding in tauopathy compared to AD that is correlated with the measured Aβ42 level. These results show that AD synapses release exosomal tau that is C-terminal-truncated, oligomeric, and with seeding activity that is enhanced by Aβ. Taken together with previous findings, these results are consistent with a direct prion-like heterotypic seeding of tau by Aβ within synaptic terminals, with subsequent loading of aggregated tau onto exosomes that are released and competent for tau seeding activity.

Cocaine-Induced Synaptic Redistribution of NMDARs in Striatal Neurons Alters NMDAR-Dependent Signal Transduction

The consequence of repeated cocaine exposure and prolonged abstinence on glutamate receptor expression in the nucleus accumbens has been extensively studied. However, the early effects of cocaine on NMDAR signaling remain unknown. NMDAR signaling depends on the subunit composition, subcellular localization, and the interaction with proteins at the postsynaptic density (PSD), where NMDARs and other proteins form supercomplexes that are responsible for the signaling pathways activated by NMDAR-induced Ca²⁺ influx. Here, we investigated the effect of cocaine on NMDAR subunit composition and subcellular localization after both intraperitoneal non-contingent cocaine and response-contingent intravenous cocaine self-administration in mice. We found that repeated cocaine exposure, regardless of the route or contingency of drug administration, decreases NMDAR interactions with the PSD and synaptic lipid rafts in the accumbens shell and dorsal striatum. We provide evidence that cocaine triggers an early redistribution of NMDARs from synaptic to extrasynaptic sites, and that this adaptation has implications in the activation of downstream signaling pathways. Thus, consistent with a loss of NMDAR function, cocaine-induced ERK phosphorylation is attenuated. Because early NMDAR activity contributes to the initiation of lasting addiction-relevant neuroadaptations, these data may hold clues into cellular mechanisms responsible for the development of cocaine addiction.

Glutamate treatment mimics LTP- and LTD-like biochemical activity in viable synaptosome preparation

Long-term potentiation (LTP) and long-term depression (LTD) are considered to be the cellular mechanisms behind the increase or decrease of synaptic strength respectively. Electrophysiologically induced LTP/LTD is associated with the activation of glutamate receptors in the synaptic terminals resulting in the initiation of biochemical processes in the postsynaptic terminals and thus propagation of synaptic activity. Isolated nerve endings i.e. synaptosome preparation was used to study here, the biochemical phenotypes of LTP and LTD, and glutamate treatment in varying concentration for different time was used to induce those biochemical phenomena. Treatment with 200 μM glutamate showed increased GluA1 phosphorylation at serine 831 and activation of CaMKIIα by phosphorylation at threonine 286 like LTP, whereas 100 μM glutamate treatment showed decrease in GluA1 phosphorylation level at both pGluA1(S831) and pGluA1(S845), and activation of GSK3β by de-phosphorylating pGSK3β at serine 9 like LTD. The 200 μM glutamate treatment was associated with an increase in the local translation of Arc, BDNF, CaMKIIα and Homer1, whereas 100 μM glutamate treatments resulted in decrease in the level of the said synaptic proteins and the effect was blocked by the proteasomal inhibitor, Lactasystin. Both, the local translation and local degradation was sensitive to the Ca²⁺ chellator, Bapta-AM, indicating that both the phenomena were dependent on the rise in intra-synaptosomal Ca²⁺, like LTP and LTD. Overall the results of the present study suggest that synaptosomal preparations can be a viable alternative to study mechanisms underlying the biochemical activities of LTP/LTD in short term.

Spatial Distribution and Expression of Ectonucleotidases in Rat Hippocampus After Removal of Ovaries and Estradiol Replacement

Purinergic signaling is the main synaptic and non-synaptic signaling system in brain. ATP acts as a fast excitatory transmitter, while adenosine sets a global inhibitory tone within hippocampal neuronal networks. ATP and adenosine are interconnected by ectonucleotidase enzymes, which convert ATP to adenosine. Existing data point to the converging roles of ovarian steroids and purinergic signaling in synapse formation and refinement and synapse activity in the hippocampus. Therefore, in the present study, we have used enzyme histochemistry and expression analysis to obtain data on spatial distribution and expression of ecto-enzymes NTPDase1, NTPDase2, and ecto-5'-nucleotidase (eN) after removal of ovaries (OVX) and estradiol replacement (E2) in female rat hippocampus. The results show that target ectonucleotidases are predominantly localized in synapse-rich hippocampal layers. The most represented NTPDase in the hippocampal tissue is NTPDase2, being at the same time the mostly affected ectonucleotidase by OVX and E2. Specifically, OVX decreases the expression of NTPDase2 and eN, whereas E2 restores their expression to control level. Impact of OVX and E2 on ectonucleotidase expression was also examined in purified synaptosome (SYN) and gliosome (GLIO) fractions. Data reveal that SYN expresses NTPDase1 and NTPDase2, both of which are reduced following OVX and restored with E2. GLIO exhibits NTPDase2-mediated ATP hydrolysis, which falls in OVX, and recovers by E2. These changes in the activity occur without parallel changes in NTPDase2-protein abundance. The same holds for eN. The lack of correlation between NTPDase2 and eN activities and their respective protein abundances suggest a non-genomic mode of E2 action, which is studied further in primary astrocyte culture. Since ovarian steroids shape hippocampal synaptic networks and regulate ectonucleotidase activities, it is possible that cognitive deficits seen after ovary removal may arise from the loss of E2 modulatory actions on ectonucleotidase expression in the hippocampus.

Tau exacerbates excitotoxic brain damage in an animal model of stroke

Neuronal excitotoxicity induced by aberrant excitation of glutamatergic receptors contributes to brain damage in stroke. Here we show that tau-deficient (tau^−/−) mice are profoundly protected from excitotoxic brain damage and neurological deficits following experimental stroke, using a middle cerebral artery occlusion with reperfusion model. Mechanistically, we show that this protection is due to site-specific inhibition of glutamate-induced and Ras/ERK-mediated toxicity by accumulation of Ras-inhibiting SynGAP1, which resides in a post-synaptic complex with tau. Accordingly, reducing SynGAP1 levels in tau^−/− mice abolished the protection from pharmacologically induced excitotoxicity and middle cerebral artery occlusion-induced brain damage. Conversely, over-expression of SynGAP1 prevented excitotoxic ERK activation in wild-type neurons. Our findings suggest that tau mediates excitotoxic Ras/ERK signaling by controlling post-synaptic compartmentalization of SynGAP1.

Excitotoxicity contributes to neuronal injury following stroke. Here the authors show that tau promotes excitotoxicity by a post-synaptic mechanism, involving site-specific control of ERK activation, in a mouse model of stroke.

Cytokines and cytokine networks target neurons to modulate long-term potentiation

Cytokines play crucial roles in the communication between brain cells including neurons and glia, as well as in the brain-periphery interactions. In the brain, cytokines modulate long-term potentiation (LTP), a cellular correlate of memory. Whether cytokines regulate LTP by direct effects on neurons or by indirect mechanisms mediated by non-neuronal cells is poorly understood. Elucidating neuron-specific effects of cytokines has been challenging because most brain cells express cytokine receptors. Moreover, cytokines commonly increase the expression of multiple cytokines in their target cells, thus increasing the complexity of brain cytokine networks even after single-cytokine challenges. Here, we review evidence on both direct and indirect-mediated modulation of LTP by cytokines. We also describe novel approaches based on neuron- and synaptosome-enriched systems to identify cytokines able to directly modulate LTP, by targeting neurons and synapses. These approaches can test multiple samples in parallel, thus allowing the study of multiple cytokines simultaneously. Hence, a cytokine networks perspective coupled with neuron-specific analysis may contribute to delineation of maps of the modulation of LTP by cytokines.

Pharmacological Rescue of Long-Term Potentiation in Alzheimer Diseased Synapses

Long-term potentiation (LTP) is an activity-dependent and persistent increase in synaptic transmission. Currently available techniques to measure LTP are time-intensive and require highly specialized expertise and equipment, and thus are not well suited for screening of multiple candidate treatments, even in animal models. To expand and facilitate the analysis of LTP, here we use a flow cytometry-based method to track chemically induced LTP by detecting surface AMPA receptors in isolated synaptosomes: fluorescence analysis of single-synapse long-term potentiation (FASS-LTP). First, we demonstrate that FASS-LTP is simple, sensitive, and models electrically induced LTP recorded in intact circuitries. Second, we conducted FASS-LTP analysis in two well-characterized Alzheimer's disease (AD) mouse models (3xTg and Tg2576) and, importantly, in cryopreserved human AD brain samples. By profiling hundreds of synaptosomes, our data provide the first direct evidence to support the idea that synapses from AD brain are intrinsically defective in LTP. Third, we used FASS-LTP for drug evaluation in human synaptosomes. Testing a panel of modulators of cAMP and cGMP signaling pathways, FASS-LTP identified vardenafil and Bay-73-6691 (phosphodiesterase-5 and -9 inhibitors, respectively) as potent enhancers of LTP in synaptosomes from AD cases. These results indicate that our approach could provide the basis for protocols to study LTP in both healthy and diseased human brains, a previously unattainable goal.

SIGNIFICANCE STATEMENT

Learning and memory depend on the ability of synapses to strengthen in response to activity. Long-term potentiation (LTP) is a rapid and persistent increase in synaptic transmission that is thought to be affected in Alzheimer's disease (AD). However, direct evidence of LTP deficits in human AD brain has been elusive, primarily due to methodological limitations. Here, we analyze LTP in isolated synapses from AD brain using a novel approach that allows testing LTP in cryopreserved brain. Our analysis of hundreds of synapses supports the idea that AD-diseased synapses are intrinsically defective in LTP. Further, we identified pharmacological agents that rescue LTP in AD, thus opening up a new avenue for drug screening and evaluation of strategies for alleviating memory impairments.

17β-Estradiol-Induced Synaptic Rearrangements Are Accompanied by Altered Ectonucleotidase Activities in Male Rat Hippocampal Synaptosomes

17β-Estradiol (E2) rapidly, by binding to membrane estrogen receptors, activates cell signaling cascades which induce formation of new dendritic spines in the hippocampus of males as in females, but the interaction with other metabolic processes, such as extracellular adenine nucleotides metabolism, are currently unknown. Extracellular adenine nucleotides play significant roles, controlling excitatory glutamatergic synapses and development of neural circuits and synaptic plasticity. Their precise regulation in the synaptic cleft is tightly controlled by ecto-nucleoside triphosphate diphosphohydrolase (NTPDase)/ecto-5'-nucleotidase (eN) enzyme chain. Therefore, we sought to clarify whether a single systemic injection of E2 in male rats is accompanied by changes in the expression of the pre- and postsynaptic proteins and downstream kinases linked to E2-induced synaptic rearrangement as well as alterations in NTPDase/eN pathway in the hippocampal synaptosomes. Obtained data showed activation of mammalian target of rapamycin and upregulation of key synaptic proteins necessary for spine formation, 24 h after systemic E2 administration. In E2-mediated conditions, we found downregulation of NTPDase1 and NTPDase2 and attenuation of adenine nucleotide hydrolysis by NTPDase/eN enzyme chain, without changes in NTPDase3 properties and augmentation of synaptic tissue-nonspecific alkaline phosphatase (TNAP) activity. Despite reduced NTPDase activities, increased TNAP activity probably prevents toxic accumulation of ATP in the extracellular milieu and also hydrolyzes accumulated ADP due to unchanged NTPDase3 activity. Thus, our initial evaluation supports idea of specific roles of different ectonucleotidases and their coordinated actions in E2-mediated spine remodeling and maintenance.

Increased Activity of Src Homology 2 Domain Containing Phosphotyrosine Phosphatase 2 (Shp2) Regulates Activity-dependent AMPA Receptor Trafficking

Long term synaptic plasticity, such as long term potentiation (LTP), has been widely accepted as a cellular mechanism underlying memory. Recently, it has been unraveled that Shp2 plays a role in synaptic plasticity and memory in Drosophila and mice, revealing significant and conserved effects of Shp2 in cognitive function. However, the exact mechanism underlying this function of Shp2 in synaptic plasticity and memory still remains elusive. Here, we examine the regulation of Shp2 in hippocampal LTP and contextual fear conditioning. We find that Shp2 is rapidly recruited into spines after LTP induction. Furthermore, the phosphorylation level of Shp2 at Tyr-542 is elevated after LTP stimuli either in cultured hippocampal neurons or acute slices. Notably, contextual fear conditioning also regulates the phosphorylation level of Shp2 at Tyr-542, suggesting fine-tuned regulation of Shp2 in LTP and memory formation. By using a Shp2-specific inhibitor and adeno-associated virus-Cre mediated Shp2 knock-out in cultured neurons, we provide evidence that the phosphatase activity of Shp2 is critical for activity-dependent AMPA receptor surface trafficking. Collectively, our results have revealed a regulatory mechanism of Shp2 underlying LTP and memory, broadening our understanding of Shp2 in cognitive function.

Synapse-specific IL-1 receptor subunit reconfiguration augments vulnerability to IL-1β in the aged hippocampus

In the aged brain, synaptic plasticity and memory show increased vulnerability to impairment by the inflammatory cytokine interleukin 1β (IL-1β). In this study, we evaluated the possibility that synapses may directly undergo maladaptive changes with age that augment sensitivity to IL-1β impairment. In hippocampal neuronal cultures, IL-1β increased the expression of the IL-1 receptor type 1 and the accessory coreceptor AcP (proinflammatory), but not of the AcPb (prosurvival) subunit, a reconfiguration that potentiates the responsiveness of neurons to IL-1β. To evaluate whether synapses develop a similar heightened sensitivity to IL-1β with age, we used an assay to track long-term potentiation (LTP) in synaptosomes. We found that IL-1β impairs LTP directly at the synapse and that sensitivity to IL-1β is augmented in aged hippocampal synapses. The increased synaptic sensitivity to IL-1β was due to IL-1 receptor subunit reconfiguration, characterized by a shift in the AcP/AcPb ratio, paralleling our culture data. We suggest that the age-related increase in brain IL-1β levels drives a shift in IL-1 receptor configuration, thus heightening the sensitivity to IL-1β. Accordingly, selective blocking of AcP-dependent signaling with Toll-IL-1 receptor domain peptidomimetics prevented IL-1β-mediated LTP suppression and blocked the memory impairment induced in aged mice by peripheral immune challenge (bacterial lipopolysaccharide). Overall, this study demonstrates that increased AcP signaling, specifically at the synapse, underlies the augmented vulnerability to cognitive impairment by IL-1β that occurs with age.

AMPA receptor trafficking in inflammation-induced dorsal horn central sensitization

Activity-dependent postsynaptic receptor trafficking is critical for long-term synaptic plasticity in the brain, but it is unclear whether this mechanism actually mediates the spinal cord dorsal horn central sensitization (a specific form of synaptic plasticity) that is associated with persistent pain. Recent studies have shown that peripheral inflammation drives changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit trafficking in the dorsal horn and that such changes contribute to the hypersensitivity that underlies persistent pain. Here, we review current evidence to illustrate how spinal cord AMPARs participate in the dorsal horn central sensitization associated with persistent pain. Understanding these mechanisms may allow the development of novel therapeutic strategies for treating persistent pain.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

Gestational nicotine exposure regulates expression of AMPA and NMDA receptors and their signaling apparatus in developing and adult rat hippocampus

Untimely activation of nicotinic acetylcholine receptors (nAChRs) by nicotine results in short- and long-term consequences on learning and behavior. In this study, the aim was to determine how prenatal nicotine exposure affects components of glutamatergic signaling in the hippocampus during postnatal development. We investigated regulation of both nAChRs and glutamate receptors for AMPA and N-methyl-D-aspartate (NMDA), from postnatal day 1 (P1) to P63 after a temporally restricted exposure to saline or nicotine for 14 days in utero. We analyzed postsynaptic density components associated with AMPA receptor (AMPAR) and NMDA receptor (NMDAR) signaling: calmodulin (CaM), CaM Kinase II alpha (CaMKIIα), and postsynaptic density-95 (PSD95), as well as presynaptically localized synaptosomal-associated protein 25 (SNAP25). At P1, there was significantly heightened expression of AMPAR subunit GluR1 but not GluR2, and of NMDAR subunits NR1, NR2a, and NR2d but not NR2b. NR2c was not detectable. CaM, CaMKIIα, and PSD95 were also significantly upregulated at P1, together with presynaptic SNAP25. This enhanced expression of glutamate receptors and signaling proteins was concomitant with elevated levels of [³H]epibatidine (³H]EB) binding in prenatal nicotine-exposed hippocampus, indicating that α4β2 nAChR may influence glutamatergic function in the hippocampus at P1. By P14, neither [³H]EB binding nor the expression levels of subunits GluR1, GluR2, NR1, NR2a, NR2b, NR2c, or NR2d seemed changed with prenatal nicotine. However, CaMKIIα was significantly upregulated with nicotine treatment while CaM showed downregulation at P14. The effects of nicotine persisted in P63 young adult brains which exhibited significantly downregulated GluR2, NR1, and NR2c expression levels in hippocampal homogenates and a considerably muted overall distribution of [³H]AMPA binding in areas CA1, CA2 and CA3, and the dentate gyrus. Our results suggest that prenatal nicotine exposure can regulate the glutamatergic signaling system throughout postnatal development by enhancing or inhibiting availability of AMPAR and NMDAR or their signaling components. The persistent depression, in adults, of the requisite NR1 subunit for NMDAR assembly, and of GluR2, important for assembly, trafficking, and biophysical properties of AMPAR, indicates that nicotine may alter ionotropic glutamate receptor stoichiometry and functional properties in adults after prenatally restricted nicotine exposure.

Routes, destinations and delays: recent advances in AMPA receptor trafficking

Postsynaptic AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission and are crucial for many aspects of brain function, including learning, memory and cognition. The number, synaptic localization and subunit composition of synaptic AMPARs are tightly regulated by network activity and by the history of activity at individual synapses. Furthermore, aberrant AMPAR trafficking is implicated in neurodegenerative diseases. AMPARs therefore represent a prime target for drug development and the mechanisms that control their synaptic delivery, retention and removal are the subject of extensive research. Here, we review recent findings that have provided new insights into AMPAR trafficking and that might lead to the development of novel therapeutic strategies.