Enhanced long-term and impaired short-term spatial memory in GluA1 AMPA receptor subunit knockout mice: evidence for a dual-process memory model

AMPA receptors in Alzheimer disease: Pathological changes and potential therapeutic targets

Alzheimer disease (AD) is a prevalent neurodegenerative disorder that affects synapses and leads to progressive cognitive decline. The role of N-methyl-D-aspartic acid (NMDA) receptors in the pathogenesis of AD is well-established as they contribute to excitotoxicity and neurodegeneration in the pathological process of extrasynaptic glutamate concentration. However, the therapeutic potential of the NMDA receptor antagonist memantine in rescuing synaptic damage is limited. Research indicates that α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors also play a significant role in AD. Abnormal transcription, expression, and localization of AMPA receptors lead to synaptic dysfunction and damage, contributing to early cognitive impairment in AD patients. Understanding the impact of AMPA receptors on AD pathogenesis and exploring the potential for the development of AMPA receptor-targeting drugs are crucial. This review aims to consolidate recent research findings on AMPA receptors in AD, elucidate the current state of AMPA receptor research and lay the foundation for future basic research and drug development.

Prosapip1 in the dorsal hippocampus mediates synaptic protein composition, long-term potentiation, and spatial memory

Prosapip1 is a brain-specific protein localized to the postsynaptic density, where it promotes dendritic spine maturation in primary hippocampal neurons. However, nothing is known about the role of Prosapip1 in vivo . To examine this, we utilized the Cre-loxP system to develop a Prosapip1 neuronal knockout mouse. We found that Prosapip1 controls the synaptic localization of its binding partner SPAR, along with PSD-95 and the GluN2B subunit of the NMDA receptor (NMDAR) in the dorsal hippocampus (dHP). We next sought to identify the potential contribution of Prosapip1 to the activity and function of the NMDAR and found that Prosapip1 plays an important role in NMDAR-mediated transmission and long-term potentiation (LTP) in the CA1 region of the dHP. As LTP is the cellular hallmark of learning and memory, we examined the consequences of neuronal knockout of Prosapip1 on dHP-dependent memory. We found that global or dHP-specific neuronal knockout of Prosapip1 caused a deficit in learning and memory whereas developmental, locomotor, and anxiety phenotypes were normal. Taken together, Prosapip1 in the dHP promotes the proper localization of synaptic proteins which, in turn, facilitates LTP driving recognition, social, and spatial learning and memory.

Knockout of NMDARs in CA1 and dentate gyrus fails to impair temporal control of conditioned behavior in mice

The hippocampus has been implicated in temporal learning. Plasticity within the hippocampus requires NMDA receptor-dependent glutamatergic neurotransmission. We tested the prediction that hippocampal NMDA receptors are required for learning about time by testing mice that lack postembryonal NMDARs in the CA1 and dentate gyrus (DG) hippocampal subfields on three different appetitive temporal learning procedures. The conditional knockout mice (Grin1ΔDCA1 ) showed normal sensitivity to cue duration, responding at a higher level to a short duration cue than compared to a long duration cue. Knockout mice also showed normal precision and accuracy of response timing in the peak procedure in which reinforcement occurred after 10 s delay within a 30 s cue presentation. Mice were tested on the matching of response rates to reinforcement rates on instrumental conditioning with two levers reinforced on a concurrent variable interval schedule. Pressing on one lever was reinforced at a higher rate than the other lever. Grin1ΔDGCA1 mice showed normal sensitivity to the relative reinforcement rates of the levers. In contrast to the lack of effect of hippocampal NMDAR deletion on measures of temporal sensitivity, Grin1ΔDGCA1 mice showed increased baseline measures of magazine activity and lever pressing. Furthermore, reversal learning was enhanced when the reward contingencies were switched in the lever pressing task, but this was true only for mice trained with a large difference between relative reinforcement rates between the levers. The results failed to demonstrate a role for NMDARs in excitatory CA1 and DG neurons in learning about temporal information.

Hippocampal place code plasticity in CA1 requires postsynaptic membrane fusion

Rapid delivery of glutamate receptors to the postsynaptic membrane via vesicle fusion is a central component of synaptic plasticity. However, it is unknown how this process supports specific neural computations during behavior. To bridge this gap, we combined conditional genetic deletion of a component of the postsynaptic membrane fusion machinery, Syntaxin3 (Stx3), in hippocampal CA1 neurons of mice with population in vivo calcium imaging. This approach revealed that Stx3 is necessary for forming the neural dynamics that support novelty processing, spatial reward memory and offline memory consolidation. In contrast, CA1 Stx3 was dispensable for maintaining aspects of the neural code that exist presynaptic to CA1 such as representations of context and space. Thus, manipulating postsynaptic membrane fusion identified computations that specifically require synaptic restructuring via membrane trafficking in CA1 and distinguished them from neural representation that could be inherited from upstream brain regions or learned through other mechanisms.

Ubiquitination of the GluA1 Subunit of AMPA Receptors Is Required for Synaptic Plasticity, Memory, and Cognitive Flexibility

Activity-dependent changes in the number of AMPA-type glutamate receptors (AMPARs) at the synapse underpin the expression of LTP and LTD, cellular correlates of learning and memory. Post-translational ubiquitination has emerged as a key regulator of the trafficking and surface expression of AMPARs, with ubiquitination of the GluA1 subunit at Lys-868 controlling the post-endocytic sorting of the receptors into the late endosome for degradation, thereby regulating their stability at synapses. However, the physiological significance of GluA1 ubiquitination remains unknown. In this study, we generated mice with a knock-in mutation in the major GluA1 ubiquitination site (K868R) to investigate the role of GluA1 ubiquitination in synaptic plasticity, learning, and memory. Our results reveal that these male mice have normal basal synaptic transmission but exhibit enhanced LTP and deficits in LTD. They also display deficits in short-term spatial memory and cognitive flexibility. These findings underscore the critical roles of GluA1 ubiquitination in bidirectional synaptic plasticity and cognition in male mice.SIGNIFICANCE STATEMENT Subcellular targeting and membrane trafficking determine the precise number of AMPA-type glutamate receptors at synapses, processes that are essential for synaptic plasticity, learning, and memory. Post-translational ubiquitination of the GluA1 subunit marks AMPARs for degradation, but its functional role in vivo remains unknown. Here we demonstrate that the GluA1 ubiquitin-deficient mice exhibit an altered threshold for synaptic plasticity accompanied by deficits in short-term memory and cognitive flexibility. Our findings suggest that activity-dependent ubiquitination of GluA1 fine-tunes the optimal number of synaptic AMPARs required for bidirectional synaptic plasticity and cognition in male mice. Given that increases in amyloid-β cause excessive ubiquitination of GluA1, inhibiting that GluA1 ubiquitination may have the potential to ameliorate amyloid-β-induced synaptic depression in Alzheimer's disease.

Glial modulation of the parallel memory formation

Actions from glial cells could affect the readiness and efficacy of learning and memory. Using a mouse cerebellar-dependent horizontal optokinetic response motor learning paradigm, short-term memory (STM) formation during the online training period and long-term memory (LTM) formation during the offline rest period were studied. A large variability of online and offline learning efficacies was found. The early bloomers with booming STM often had a suppressed LTM formation and late bloomers with no apparent acute training effect often exhibited boosted offline learning performance. Anion channels containing LRRC8A are known to release glutamate. Conditional knockout of LRRC8A specifically in astrocytes including cerebellar Bergmann glia resulted in a complete loss of STM formation while the LTM formation during the rest period remained. Optogenetic manipulation of glial activity by channelrhodopsin-2 or archaerhodopsin-T (ArchT) during the online training resulted in enhancement or suppression of STM formation, respectively. STM and LTM are likely to be triggered simultaneously during online training, but LTM is expressed later during the offline period. STM appears to be volatile and the achievement during the online training is not handed over to LTM. In addition, we found that glial ArchT photoactivation during the rest period resulted in the augmentation of LTM formation. These data suggest that STM formation and LTM formation are parallel separate processes. Strategies to weigh more on the STM or the LTM could depend on the actions of the glial cells.

Extracellular vesicles from mesenchymal stem cells reduce neuroinflammation in hippocampus and restore cognitive function in hyperammonemic rats

Chronic hyperammonemia, a main contributor to hepatic encephalopathy (HE), leads to neuroinflammation which alters neurotransmission leading to cognitive impairment. There are no specific treatments for the neurological alterations in HE. Extracellular vesicles (EVs) from mesenchymal stem cells (MSCs) reduce neuroinflammation in some pathological conditions. The aims were to assess if treatment of hyperammonemic rats with EVs from MSCs restores cognitive function and analyze the underlying mechanisms. EVs injected in vivo reach the hippocampus and restore performance of hyperammonemic rats in object location, object recognition, short-term memory in the Y-maze and reference memory in the radial maze. Hyperammonemic rats show reduced TGFβ levels and membrane expression of TGFβ receptors in hippocampus. This leads to microglia activation and reduced Smad7-IkB pathway, which induces NF-κB nuclear translocation in neurons, increasing IL-1β which alters AMPA and NMDA receptors membrane expression, leading to cognitive impairment. These effects are reversed by TGFβ in the EVs from MSCs, which activates TGFβ receptors, reducing microglia activation and NF-κB nuclear translocation in neurons by normalizing the Smad7-IkB pathway. This normalizes IL-1β, AMPA and NMDA receptors membrane expression and, therefore, cognitive function. EVs from MSCs may be useful to improve cognitive function in patients with hyperammonemia and minimal HE.

Unraveling the mysteries of dendritic spine dynamics: Five key principles shaping memory and cognition

Recent research extends our understanding of brain processes beyond just action potentials and chemical transmissions within neural circuits, emphasizing the mechanical forces generated by excitatory synapses on dendritic spines to modulate presynaptic function. From in vivo and in vitro studies, we outline five central principles of synaptic mechanics in brain function: P1: Stability - Underpinning the integral relationship between the structure and function of the spine synapses. P2: Extrinsic dynamics - Highlighting synapse-selective structural plasticity which plays a crucial role in Hebbian associative learning, distinct from pathway-selective long-term potentiation (LTP) and depression (LTD). P3: Neuromodulation - Analyzing the role of G-protein-coupled receptors, particularly dopamine receptors, in time-sensitive modulation of associative learning frameworks such as Pavlovian classical conditioning and Thorndike's reinforcement learning (RL). P4: Instability - Addressing the intrinsic dynamics crucial to memory management during continual learning, spotlighting their role in "spine dysgenesis" associated with mental disorders. P5: Mechanics - Exploring how synaptic mechanics influence both sides of synapses to establish structural traces of short- and long-term memory, thereby aiding the integration of mental functions. We also delve into the historical background and foresee impending challenges.

Glutamatergic dysfunction leads to a hyper-dopaminergic phenotype through deficits in short-term habituation: a mechanism for aberrant salience

Psychosis in disorders like schizophrenia is commonly associated with aberrant salience and elevated striatal dopamine. However, the underlying cause(s) of this hyper-dopaminergic state remain elusive. Various lines of evidence point to glutamatergic dysfunction and impairments in synaptic plasticity in the etiology of schizophrenia, including deficits associated with the GluA1 AMPAR subunit. GluA1 knockout (Gria1^-/-) mice provide a model of impaired synaptic plasticity in schizophrenia and exhibit a selective deficit in a form of short-term memory which underlies short-term habituation. As such, these mice are unable to reduce attention to recently presented stimuli. In this study we used fast-scan cyclic voltammetry to measure phasic dopamine responses in the nucleus accumbens of Gria1^-/- mice to determine whether this behavioral phenotype might be a key driver of a hyper-dopaminergic state. There was no effect of GluA1 deletion on electrically-evoked dopamine responses in anaesthetized mice, demonstrating normal endogenous release properties of dopamine neurons in Gria1^-/- mice. Furthermore, dopamine signals were initially similar in Gria1^-/- mice compared to controls in response to both sucrose rewards and neutral light stimuli. They were also equally sensitive to changes in the magnitude of delivered rewards. In contrast, however, these stimulus-evoked dopamine signals failed to habituate with repeated presentations in Gria1^-/- mice, resulting in a task-relevant, hyper-dopaminergic phenotype. Thus, here we show that GluA1 dysfunction, resulting in impaired short-term habituation, is a key driver of enhanced striatal dopamine responses, which may be an important contributor to aberrant salience and psychosis in psychiatric disorders like schizophrenia.

Spread of pathological human Tau from neurons to oligodendrocytes and loss of high-firing pyramidal neurons in aging mice

Intracellular aggregation of hyperphosphorylated Tau (pTau) in the brain is associated with cognitive and motor impairments, and ultimately neurodegeneration. We investigate how human pTau affects cells and network activity in the hippocampal formation of the THY-Tau22 tauopathy model mice in vivo. We find that pTau preferentially accumulates in deep-layer pyramidal neurons, leading to neurodegeneration, and we establish that pTau spreads to oligodendrocytes. During goal-directed virtual navigation in aged transgenic mice, we detect fewer high-firing prosubicular pyramidal cells, but the firing population retains its coupling to theta oscillations. Analysis of network oscillations and firing patterns of pyramidal and GABAergic neurons recorded in head-fixed and freely moving mice suggests preserved neuronal coordination. In spatial memory tests, transgenic mice have reduced short-term familiarity, but spatial working and reference memory are surprisingly normal. We hypothesize that unimpaired subcortical network mechanisms maintain cortical neuronal coordination, counteracting the widespread pTau aggregation, loss of high-firing cells, and neurodegeneration.

Reinforcement rate and the balance between excitatory and inhibitory learning: Insights from deletion of the GluA1 AMPA receptor subunit

Conditioned responding is sensitive to reinforcement rate. This rate-sensitivity is impaired in genetically modified mice that lack the GluA1 subunit of the AMPA receptor. A time-dependent application of the Rescorla-Wagner learning rule can be used to derive an account of rate-sensitivity by reflecting the balance of excitatory and inhibitory associative strength over time. By applying this analysis, the impairment in GluA1 knockout mice may be explained by reduced sensitivity to negative prediction error and thus, impaired inhibitory learning, such that excitatory associative strength is not reduced during the nonreinforced periods of a conditioned stimulus. The article describes a test of the role of GluA1 in inhibitory learning that requires summing of the associative strengths of cues presented in compound. Mice were trained on a feature negative discrimination of the form A+/AX-. GluA1 knockout mice acquired the discrimination to a similar extent as controls. The inhibitory properties of cue X were verified in a summation test that included a control for nonassociative, external inhibition. The performance of GluA1 knockout mice was similar to that of controls. However, in line with previous findings, GluA1 deletion impaired the precision of timing of conditioned responding. These results provide further evidence that impaired sensitivity to reinforcement rate is not a consequence of impaired inhibitory learning. The results may more readily fit with accounts of rate sensitivity that propose that it reflects encoding of temporal and numeric information rather than being a consequence of changes in associative strength over time. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Comparative Severity Assessment of Genetic, Stress-Based, and Pharmacological Mouse Models of Depression

The use of animals in neurosciences is pivotal to gaining insights into complex functions and dysfunctions of behavior. For example, various forms of physical and/or psychological stress are inherent to various animal models for psychiatric disorders, e.g., depression. Regarding animal welfare, it would be mandatory to use models that inflict the least amount of stress necessary to address the underlying scientific question. This study compared the severity of different approaches to induce depression in mice: mutagenesis in GluA1 knockout, immobilization stress, and stress-induction via stress hormone treatment. While genetic alterations potentially represent a lifelong burden, the temporary intervention only affects the animals for a limited time. Therefore, we used home cage-based behavioral and physiological parameters, including nest building, burrowing, body weight, and fecal corticosterone metabolites, to determine the well-being of male and female mice. In addition, we performed an evidence-based estimate of severity using a composite score for relative severity assessment (RELSA) with this data. We found that even though restraint stress and supplementation of corticosterone in the diet both aimed at depression-related precipitating stress effects, the latter affected the well-being much stronger, especially in females. Restraint leads to less noticeable well-being impairments but causes depression-associated anhedonic behavior. Mice of both sexes recovered well from the stress treatment. GluA1 KO and their littermates showed diminished well-being, comparable to the immobilization experiments. However, since this is a lifelong condition, this burden is not reversible and potentially accumulative. In line with the 3Rs (Replacement, Reduction, and Refinement), the process of choosing the most suitable model should ideally include an evidence-based severity assessment to be able to opt for the least severe alternative, which still induces the desired effect. Promoting refinement, in our study, this would be the restraint stress.

Identification and functional evaluation of GRIA1 missense and truncation variants in individuals with ID: An emerging neurodevelopmental syndrome

GRIA1 encodes the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, which are ligand-gated ion channels that act as excitatory receptors for the neurotransmitter L-glutamate (Glu). AMPA receptors (AMPARs) are homo- or heteromeric protein complexes with four subunits, each encoded by different genes, GRIA1 to GRIA4. Although GluA1-containing AMPARs have a crucial role in brain function, the human phenotype associated with deleterious GRIA1 sequence variants has not been established. Subjects with de novo missense and nonsense GRIA1 variants were identified through international collaboration. Detailed phenotypic and genetic assessments of the subjects were carried out and the pathogenicity of the variants was evaluated in vitro to characterize changes in AMPAR function and expression. In addition, two Xenopus gria1 CRISPR-Cas9 F₀ models were established to characterize the in vivo consequences. Seven unrelated individuals with rare GRIA1 variants were identified. One individual carried a homozygous nonsense variant (p.Arg377Ter), and six had heterozygous missense variations (p.Arg345Gln, p.Ala636Thr, p.Ile627Thr, and p.Gly745Asp), of which the p.Ala636Thr variant was recurrent in three individuals. The cohort revealed subjects to have a recurrent neurodevelopmental disorder mostly affecting cognition and speech. Functional evaluation of major GluA1-containing AMPAR subtypes carrying the GRIA1 variant mutations showed that three of the four missense variants profoundly perturb receptor function. The homozygous stop-gain variant completely destroys the expression of GluA1-containing AMPARs. The Xenopus gria1 models show transient motor deficits, an intermittent seizure phenotype, and a significant impairment to working memory in mutants. These data support a developmental disorder caused by both heterozygous and homozygous variants in GRIA1 affecting AMPAR function.

Structural Investigation of the Interaction Mechanism between Chlorogenic Acid and AMPA Receptor via In Silico Approaches

Chlorogenic acid (CGA), an important metabolite in natural plant medicines such as honeysuckle and eucommia, has been shown to have potent antinociceptive effects. Nevertheless, the mechanism by which CGA relieves chronic pain remains unclear. α-amino-3-hydroxy-5-methyl-4-isooxazolpropionic acid receptor (AMPAR) is a major ionotropic glutamate receptor that mediates rapid excitatory synaptic transmission and its glutamate ionotropic receptor AMPA type subunit 1 (GluA1) plays a key role in nociceptive transmission. In this study, we used Western blot, surface plasmon resonance (SPR) assay, and the molecular simulation technologies to investigate the mechanism of interaction between CGA and AMPAR to relieve chronic pain. Our results indicate that the protein expression level of GluA1 showed a dependent decrease as the concentration of CGA increased (0, 50, 100, and 200 μM). The SPR assay demonstrates that CGA can directly bind to GluA1 (K_D = 496 μM). Furthermore, CGA forms a stable binding interaction with GluA1, which is validated by molecular dynamics (MD) simulation. The binding free energy between CGA and GluA1 is -39.803 ± 14.772 kJ/mol, where van der Waals interaction and electrostatic interaction are the major contributors to the GluA1-CGA binding, and the key residues are identified (Val-32, Glu-33, Ala-36, Glu-37, Leu-48), which play a crucial role in the binding interaction. This study first reveals the structural basis of the stable interaction between CGA and GluA1 to form a binding complex for the relief of chronic pain. The research provides the structural basis to understand the treatment of chronic pain and is valuable to the design of novel drug molecules in the future.

Distinct contributions of GluA1-containing AMPA receptors of different hippocampal subfields to salience processing, memory and impulse control

Schizophrenia is associated with a broad range of severe and currently pharmacoresistant cognitive deficits. Prior evidence suggests that hypofunction of AMPA-type glutamate receptors (AMPARs) containing the subunit GLUA1, encoded by GRIA1, might be causally related to impairments of selective attention and memory in this disorder, at least in some patients. In order to clarify the roles of GluA1 in distinct cell populations, we investigated behavioural consequences of selective Gria1-knockout in excitatory neurons of subdivisions of the prefrontal cortex and the hippocampus, assessing sustained attention, impulsivity, cognitive flexibility, anxiety, sociability, hyperactivity, and various forms of short-term memory in mice. We found that virally induced reduction of GluA1 across multiple hippocampal subfields impaired spatial working memory. Transgene-mediated ablation of GluA1 from excitatory cells of CA2 impaired short-term memory for conspecifics and objects. Gria1 knockout in CA3 pyramidal cells caused mild impairments of object-related and spatial short-term memory, but appeared to partially increase social interaction and sustained attention and to reduce motor impulsivity. Our data suggest that reduced hippocampal GluA1 expression-as seen in some patients with schizophrenia-may be a central cause particularly for several short-term memory deficits. However, as impulse control and sustained attention actually appeared to improve with GluA1 ablation in CA3, strategies of enhancement of AMPAR signalling likely require a fine balance to be therapeutically effective across the broad symptom spectrum of schizophrenia.

Deletion of AMPA receptor GluA1 subunit gene (Gria1) causes circadian rhythm disruption and aberrant responses to environmental cues

Dysfunction of the glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 subunit and deficits in synaptic plasticity are implicated in schizophrenia and sleep and circadian rhythm disruption. To investigate the role of GluA1 in circadian and sleep behaviour, we used wheel-running, passive-infrared, and video-based home-cage activity monitoring to assess daily rest-activity profiles of GluA1-knockout mice (Gria1-/-). We showed that these mice displayed various circadian abnormalities, including misaligned, fragmented, and more variable rest-activity patterns. In addition, they showed heightened, but transient, behavioural arousal to light→dark and dark→light transitions, as well as attenuated nocturnal-light-induced activity suppression (negative masking). In the hypothalamic suprachiasmatic nuclei (SCN), nocturnal-light-induced cFos signals (a molecular marker of neuronal activity in the preceding ~1-2 h) were attenuated, indicating reduced light sensitivity in the SCN. However, there was no change in the neuroanatomical distribution of expression levels of two neuropeptides-vasoactive intestinal peptide (VIP) and arginine vasopressin (AVP)-differentially expressed in the core (ventromedial) vs. shell (dorsolateral) SCN subregions and both are known to be important for neuronal synchronisation within the SCN and circadian rhythmicity. In the motor cortex (area M1/M2), there was increased inter-individual variability in cFos levels during the evening period, mirroring the increased inter-individual variability in locomotor activity under nocturnal light. Finally, in the spontaneous odour recognition task GluA1 knockouts' short-term memory was impaired due to enhanced attention to the recently encountered familiar odour. These abnormalities due to altered AMPA-receptor-mediated signalling resemble and may contribute to sleep and circadian rhythm disruption and attentional deficits in different modalities in schizophrenia.

Memory suppressor genes: Modulating acquisition, consolidation, and forgetting

The brain has a remarkable but underappreciated capacity to limit memory formation and expression. The term "memory suppressor gene" was coined in 1998 as an attempt to explain emerging reports that some genes appeared to limit memory. At that time, only a handful of memory suppressor genes were known, and they were understood to work by limiting cAMP-dependent consolidation. In the intervening decades, almost 100 memory suppressor genes with diverse functions have been discovered that affect not only consolidation but also acquisition and forgetting. Here we highlight the surprising extent to which biological limits are placed on memory formation through reviewing the literature on memory suppressor genes. In this review, we present memory suppressors within the framework of their actions on different memory operations: acquisition, consolidation, and forgetting. This is followed by a discussion of the reasons why there may be a biological need to limit memory formation.

Lack of Sez6 Family Proteins Impairs Motor Functions, Short-Term Memory, and Cognitive Flexibility and Alters Dendritic Spine Properties

Seizure-related gene 6 (Sez6), Sez6-Like (Sez6L), and Sez6-Like 2 (Sez6L2) comprise a family of homologous proteins widely expressed throughout the brain that have been linked to neurodevelopmental and psychiatric disorders. Here, we use Sez6 triple knockout (TKO) mice, which lack all three Sez6 family proteins, to demonstrate that Sez6 family proteins regulate dendritic spine structure and cognitive functions, motor learning, and maintenance of motor functions across the lifespan. Compared to WT controls, we found that Sez6 TKO mice had impaired motor learning and their motor coordination was negatively affected from 6 weeks old and declined more rapidly as they aged. Sez6 TKO mice had reduced spine density in the hippocampus and dendritic spines were shifted to more immature morphologies in the somatosensory cortex. Cognitive testing revealed that they had enhanced stress responsiveness, impaired working, and spatial short-term memory but intact spatial long-term memory in the Morris water maze albeit accompanied by a reversal deficit. Our study demonstrates that the lack of Sez6 family proteins results in phenotypes commonly associated with neuropsychiatric disorders making it likely that Sez6 family proteins contribute to the complex etiologies of these disorders.

Extreme Glycemic Fluctuations Debilitate NRG1, ErbB Receptors and Olig1 Function: Association with Regeneration, Cognition and Mood Alterations During Diabetes

Neuronal regeneration is crucial for maintaining intact neural interactions for perpetuation of cognitive and emotional functioning. The NRG1-ErbB receptor signaling is a key pathway for regeneration in adult brain and also associated with learning and mood stabilization by modulating synaptic transmission. Extreme glycemic stress is known to affect NRG1-ErbB-mediated regeneration in brain; yet, it remains unclear how the ErbB receptor subtypes are differentially affected due to such metabolic variations. Here, we assessed the alterations in NRG1, ErbB receptor subtypes to study the regenerative potential, both in rodents as well as in neuronal and glial cell models of hyperglycemia and hypoglycemic insults during hyperglycemia. The pro-oxidant and anti-oxidant status leading to degenerative changes in brain regions were determined. The spatial memory and anxiogenic behaviour of experimental rodents were tested using 'T' maze and Elevated Plus Maze. Our data revealed that the extreme glycemic discrepancies during diabetes and recurrent hypoglycemia lead to altered expression of NRG1, ErbB receptor subtypes, Syntaxin1 and Olig1 that shows association with impaired regeneration, synaptic dysfunction, demyelination, cognitive deficits and anxiety.

Genetic manipulations of AMPA glutamate receptors in hippocampal synaptic plasticity

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the principal mediators of fast excitatory synaptic transmission and they are required for various forms of synaptic plasticity, including long-term potentiation (LTP) and depression (LTD), which are key mechanisms of learning and memory. AMPARs are tetrameric complexes assembled from four subunits (GluA1-4), however, the lack of subunit-specific pharmacological tools has made the assessment of individual subunits difficult. The application of genetic techniques, particularly gene targeting, allows for precise manipulation and dissection of each subunit in the regulation of neuronal function and behaviour. In this review, we summarize studies using various mouse models with genetically altered AMPARs and focus on their roles in basal synaptic transmission, LTP, and LTD at the hippocampal CA1 synapse. These studies provide strong evidence that there are multiple forms of LTP and LTD at this synapse which can be induced by various induction protocols, and they are differentially regulated by different AMPAR subunits and domains. We conclude that it is necessary to delineate the mechanism of each of these forms of plasticity and their contribution to memory and brain disorders.

Regulation of Drosophila Long-Term Courtship Memory by Ecdysis Triggering Hormone

Endocrine state is an important determinant of learning and memory in animals. In Drosophila, rejection of male courtship overtures by mated females leads to an aversive response manifested as courtship memory. Here we report that ecdysis triggering hormone (ETH) is an obligatory enabler of long-term courtship memory (LTM). ETH deficiency suppresses LTM, whereas augmented ETH release reduces the minimum training period required for LTM induction. ETH receptor knockdown either in the mushroom body (MB) γ lobe or in octopaminergic dorsal-anterior-lateral (DAL) neurons impairs memory performance, indicating its direct action in these brain areas. Consistent with these findings, brain exposure to ETH mobilizes calcium in MB γ lobe neuropils and DAL neurons. ETH receptor (ETHR) knockdown in the corpus allatum (CA) to create juvenile hormone (JH) deficiency also suppresses LTM, as does knockdown of the JH receptor Met in the MB γ lobe, indicating a convergence of ETH and JH signaling in this region of the brain. Our findings identify endocrine-enabled neural circuit components in the brain that are critical for persistent behavioral changes resulting from aversive social experience.

Delayed-matching-to-position working memory in mice relies on NMDA-receptors in prefrontal pyramidal cells

A hypofunction of N-methyl-D-aspartate glutamate receptors (NMDARs) has been implicated in the pathogenesis of schizophrenia by clinical and rodent studies. However, to what extent NMDAR-hypofunction in distinct cell-types across the brain causes different symptoms of this disease is largely unknown. One pharmaco-resistant core symptom of schizophrenia is impaired working memory (WM). NMDARs have been suggested to mediate sustained firing in excitatory neurons of the prefrontal cortex (PFC) that might underlie WM storage. However, if NMDAR-hypofunction in prefrontal excitatory neurons may indeed entail WM impairments is unknown. We here investigated this question in mice, in which NMDARs were genetically-ablated in PFC excitatory cells. This cell type-selective NMDAR-hypofunction caused a specific deficit in a delayed-matching-to-position (DMTP) 5-choice-based operant WM task. In contrast, T-maze rewarded alternation and several psychological functions including attention, spatial short-term habituation, novelty-processing, motivation, sociability, impulsivity, and hedonic valuation remained unimpaired at the level of GluN1-hypofunction caused by our manipulation. Our data suggest that a hypofunction of NMDARs in prefrontal excitatory neurons may indeed cause WM impairments, but are possibly not accounting for most other deficits in schizophrenia.

Behavioral and Myelin-Related Abnormalities after Blast-Induced Mild Traumatic Brain Injury in Mice

In civilian and military settings, mild traumatic brain injury (mTBI) is a common consequence of impacts to the head, sudden blows to the body, and exposure to high-energy atmospheric shockwaves from blast. In some cases, mTBI from blast exposure results in long-term emotional and cognitive deficits and an elevated risk for certain neuropsychiatric diseases. Here, we tested the effects of mTBI on various forms of auditory-cued fear learning and other measures of cognition in male C57BL/6J mice after single or repeated blast exposure (blast TBI; bTBI). bTBI produced an abnormality in the temporal organization of cue-induced freezing behavior in a conditioned trace fear test. Spatial working memory, evaluated by the Y-maze task performance, was also deleteriously affected by bTBI. Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis for glial markers indicated an alteration in the expression of myelin-related genes in the hippocampus and corpus callosum 1-8 weeks after bTBI. Immunohistochemical and ultrastructural analyses detected bTBI-related myelin and axonal damage in the hippocampus and corpus callosum. Together, these data suggest a possible link between blast-induced mTBI, myelin/axonal injury, and cognitive dysfunction.

Dissociating Representations of Time and Number in Reinforcement-Rate Learning by Deletion of the GluA1 AMPA Receptor Subunit in Mice

Theories of learning differ in whether they assume that learning reflects the strength of an association between memories or symbolic encoding of the statistical properties of events. We provide novel evidence for symbolic encoding of informational variables by demonstrating that sensitivity to time and number in learning is dissociable. Whereas responding in normal mice was dependent on reinforcement rate, responding in mice that lacked the GluA1 AMPA receptor subunit was insensitive to reinforcement rate and, instead, dependent on the number of times a cue had been paired with reinforcement. This suggests that GluA1 is necessary for weighting numeric information by temporal information in order to calculate reinforcement rate. Sample sizes per genotype varied between seven and 23 across six experiments and consisted of both male and female mice. The results provide evidence for explicit encoding of variables by animals rather than implicit encoding via variations in associative strength.

The GluA1 AMPAR subunit is necessary for hedonic responding but not hedonic value in female mice

•

GluA1 containing AMPA receptors are necessary for palatability responses to sucrose.

•

Mice lacking GluA1 showed were sensitive to changes in the hedonic value of sucrose.

•

This suggests that GluA1 is necessary for hedonic responding but not hedonic value.

The GluA1 subunit of the AMPA receptor has been implicated in anhedonia. Mice that lack GluA1 (Gria1 knockout mice) show reduced lick cluster size, a measure of palatability in feeding behaviour. This deficit may reflect a role for GluA1 in encoding the hedonic value of palatable substances or instead a role for GluA1 in the behavioural expression of hedonic value. We tested the role of GluA1 in hedonic value by assessing sensitivity to changes in the rewarding property of sucrose as a consequence of negative/positive contrast effects in female mice. During training, on half of the days consumption of a flavour (CS+) mixed with 4% sucrose was preceded by consumption of 1% sucrose (positive contrast). On the other half of days consumption of a different flavour (CS–) mixed with 4% sucrose was preceded by consumption of 16% sucrose (negative contrast). In the test session both wild-type, controls and Gria1 knockout mice consumed more of the CS+ flavour than the CS– flavour. While Gria1 knockout mice showed reduced lick cluster sizes, both genotypes made larger lick clusters for the CS+ flavour than the CS– flavour suggesting that the CS+ was more palatable than the CS–. A follow up experiment in normal mice demonstrated that the negative contrast procedure resulted in a conditioned reduction of palatability of the CS– in comparison to an associatively neutral, novel flavour. The results failed to demonstrate a role for GluA1 in hedonic value suggesting that, instead, GluA1 is necessary for hedonic responding.

High-Fat and High-Sucrose Diets Impair Time-of-Day Differences in Spatial Working Memory of Male Mice

OBJECTIVE

This study aimed to investigate both the long-term and short-term impacts of high-fat diets (HFD) or high-sucrose diets (HSD) on the normal diurnal pattern of cognitive function, protein expression, and the molecular clock in mice.

METHODS

This study used both 6-month and 4-week feeding strategies by providing male C57BL/6J mice access to either a standard chow, HFD, or HSD. Spatial working memory and synaptic plasticity were assessed both day and night, and hippocampal tissue was measured for changes in NMDA and AMPA receptor subunits (GluN2B, GluA1), as well as molecular clock gene expression.

RESULTS

HFD and HSD both disrupted normal day/night fluctuations in spatial working memory and synaptic plasticity. Mice fed HFD altered their food intake to consume more calories during the day. Both diets disrupted normal hippocampal clock gene expression, and HFD reduced GluN2B levels in hippocampal tissue.

CONCLUSIONS

Taken together, these results suggest that both HFD and HSD induce a loss of day/night performance in spatial working memory and synaptic plasticity as well as trigger a cascade of changes that include disruption to the hippocampal molecular clock.

Emerging role of AMPA receptor subunit GluA1 in synaptic plasticity: Implications for Alzheimer's disease

It is well established that GluA1 mediated synaptic plasticity plays a central role in the early development of AD. The complex cellular and molecular mechanisms that enable GluA1‐related synaptic regulation remain to fully understood. Particularly, understanding the mechanisms that disrupt GluA1 related synaptic plasticity is central to the development of disease‐modifying therapies which are sorely needed as the incidence of AD rises. We surmise that the published evidence establishes deficits in synaptic plasticity as a central factor of AD aetiology. We additionally highlight potential therapeutic strategies for the treatment of AD, and we delve into the roles of GluA1 in learning and memory. Particularly, we review the current understanding of the molecular interactions that confer the actions of this ubiquitous excitatory receptor subunit including post‐translational modification and accessory protein recruitment of the GluA1 subunit. These are proposed to regulate receptor trafficking, recycling, channel conductance and synaptic transmission and plasticity.

Hippocampal Hyperactivity as a Druggable Circuit-Level Origin of Aberrant Salience in Schizophrenia

The development of current neuroleptics was largely aiming to decrease excessive dopaminergic signaling in the striatum. However, the notion that abnormal dopamine creates psychotic symptoms by causing an aberrant assignment of salience that drives maladaptive learning chronically during disease development suggests a therapeutic value of early interventions that correct salience-related neural processing. The mesolimbic dopaminergic output is modulated by several interconnected brain-wide circuits centrally involving the hippocampus and key relays like the ventral and associative striatum, ventral pallidum, amygdala, bed nucleus of the stria terminalis, nucleus reuniens, lateral and medial septum, prefrontal and cingulate cortex, among others. Unraveling the causal relationships between these circuits using modern neuroscience techniques holds promise for identifying novel cellular-and ultimately molecular-treatment targets for reducing transition to psychosis and symptoms of schizophrenia. Imaging studies in humans have implicated a hyperactivity of the hippocampus as a robust and early endophenotype in schizophrenia. Experiments in rodents, in turn, suggested that the activity of its output region-the ventral subiculum-may modulate dopamine release from ventral tegmental area (VTA) neurons in the ventral striatum. Even though these observations suggested a novel circuit-level target for anti-psychotic action, no therapy has yet been developed along this rationale. Recently evaluated treatment strategies-at least in part-target excess glutamatergic activity, e.g. N-acetyl-cysteine (NAC), levetiracetam, and mGluR2/3 modulators. We here review the evidence for the central implication of the hippocampus-VTA axis in schizophrenia-related pathology, discuss its symptom-related implications with a particular focus on aberrant assignment of salience, and evaluate some of its short-comings and prospects for drug discovery.

Meningeal γδ T cell-derived IL-17 controls synaptic plasticity and short-term memory

The notion of "immune privilege" of the brain has been revised to accommodate its infiltration, at steady state, by immune cells that participate in normal neurophysiology. However, the immune mechanisms that regulate learning and memory remain poorly understood. Here, we show that noninflammatory interleukin-17 (IL-17) derived from a previously unknown fetal-derived meningeal-resident γδ T cell subset promotes cognition. When tested in classical spatial learning paradigms, mice lacking γδ T cells or IL-17 displayed deficient short-term memory while retaining long-term memory. The plasticity of glutamatergic synapses was reduced in the absence of IL-17, resulting in impaired long-term potentiation in the hippocampus. Conversely, IL-17 enhanced glial cell production of brain-derived neurotropic factor, whose exogenous provision rescued the synaptic and behavioral phenotypes of IL-17-deficient animals. Together, our work provides previously unknown clues on the mechanisms that regulate short-term versus long-term memory and on the evolutionary and functional link between the immune and nervous systems.

Short-term exposure to dietary cholesterol is associated with downregulation of interleukin-15, reduced thigmotaxis and memory impairment in mice

Alzheimer's disease (AD) is a neurodegenerative condition associated with loss of memory function, depression and anxiety. The etiology of AD is poorly understood, but both cholesterol dyshomeostasis and dysregulation of the immune system are contributing factors. Current evidence is consistent with a detrimental effect of excess cholesterol on neuroinflammation, both in mouse models of memory loss and in dementia in humans. However, whether the impact of cholesterol on neuroinflammation occurs early and contributes to pathogenesis of the disease or simply reflects a pleiotropic impact at advanced stages of disease is unclear. To explore this question, we measured, in 9-13 week-old mice, cognitive status and changes in brain inflammatory mediators in response to a short-term high-cholesterol diet. We hypothesized that short-term exposure to excess dietary cholesterol would alter the early inflammatory responses associated with cognitive and/or behavioral impairment. We report that short-term exposure to a high-cholesterol diet led to decreased thigmotaxis and short-term spatial memory impairment without affecting long-term recognition memory. Furthermore, cognitive and behavioral phenotypes in these mice were associated with a reduction in interleukin-15 levels in the absence of changes in other inflammatory mediators. Our findings indicate that interleukin-15 may play a role in early stages of cognitive impairment secondary to hypercholesterolemia. Consequently, optimization of interleukin-15 signaling may be a viable effective cognitive therapy in the population susceptible to developing dementia due to risk factors associated with cholesterol dysregulation.

In vivo Mechanisms of Antibody-Mediated Neurological Disorders: Animal Models and Potential Implications

Over the last two decades, the discovery of antibodies directed against neuronal surface antigens (NSA-Abs) in patients with different forms of encephalitis has provided a basis for immunotherapies in previously undefined disorders. Nevertheless, despite the circumstantial clinical evidence of the pathogenic role of these antibodies in classical autoimmune encephalitis, specific criteria need to be applied in order to establish the autoimmune nature of a disease. A growing number of studies have begun to provide proof of the pathogenicity of NSA-Abs and insights into their pathogenic mechanisms through passive transfer or, more rarely, through active immunization animal models. Moreover, the increasing evidence that NSA-Abs in the maternal circulation can reach the fetal brain parenchyma during gestation, causing long-term effects, has led to models of antibody-induced neurodevelopmental disorders. This review summarizes different methodological approaches and the results of the animal models of N-methyl-d-aspartate receptor (NMDAR), leucine-rich glioma-inactivated 1 (LGI1), contactin-associated protein 2 (CASPR2), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) antibody-mediated disorders and discuss the results and the limitations. We also summarize recent experiments that demonstrate that maternal antibodies to NMDAR and CASPR2 can alter development in the offspring with potential lifelong susceptibility to neurological or psychiatric disorders.

The role of glutamate receptors in attention-deficit/hyperactivity disorder: From physiology to disease

Attention-deficit hyperactivity disorder (ADHD) is the most common psychiatric disorder in children and adolescents, which is characterized by behavioral problems such as attention deficit, hyperactivity, and impulsivity. As the receptors of the major excitatory neurotransmitter in the mammalian central nervous system (CNS), glutamate receptors (GluRs) are strongly linked to normal brain functioning and pathological processes. Extensive investigations have been made about the structure, function, and regulation of GluR family, describing evidences that support the disruption of these mechanisms in mental disorders, including ADHD. In this review, we briefly described the family and function of GluRs in the CNS, and discussed what is recently known about the role of GluRs in ADHD, that including GluR genes, animal models, and the treatment, which would help us further elucidate the etiology of ADHD.

Born for fairness: evidence of genetic contribution to a neural basis of fairness intuition

Human beings often curb self-interest to develop and enforce social norms, such as fairness, as exemplified in the ultimatum game (UG). Inspired by the dual-system account for the responder's choice during the UG, we investigated whether the neural basis of psychological process induced by fairness is under genetic control using a twin fMRI study (62 monozygotic, 48 dizygotic; mean age: 19.32 ± 1.38 years). We found a moderate genetic contribution to the rejection rate of unfair proposals (24%-35%), independent of stake size or proposer type, during the UG. Using a voxel-level analysis, we found that genetic factors moderately contributed to unfairness-evoked activation in the bilateral anterior insula (AI), regions representing the intuition of fairness norm violations (mean heritability: left 37%, right 40%). No genetic contributions were found in regions related to deliberate, controlled processes in the UG. This study provides the first evidence that evoked brain activity by unfairness in the bilateral AI is influenced by genes and sheds light on the genetic basis of brain processes underlying costly punishment.

Hippocampal-prefrontal coherence mediates working memory and selective attention at distinct frequency bands and provides a causal link between schizophrenia and its risk gene GRIA1

Increased fronto-temporal theta coherence and failure of its stimulus-specific modulation have been reported in schizophrenia, but the psychological correlates and underlying neural mechanisms remain elusive. Mice lacking the putative schizophrenia risk gene GRIA1 (Gria1^-/-), which encodes GLUA1, show strongly impaired spatial working memory and elevated selective attention owing to a deficit in stimulus-specific short-term habituation. A failure of short-term habituation has been suggested to cause an aberrant assignment of salience and thereby psychosis in schizophrenia. We recorded hippocampal-prefrontal coherence while assessing spatial working memory and short-term habituation in these animals, wildtype (WT) controls, and Gria1^-/- mice in which GLUA1 expression was restored in hippocampal subfields CA2 and CA3. We found that beta (20-30 Hz) and low-gamma (30-48 Hz) frequency coherence could predict working memory performance, whereas-surprisingly-theta (6-12 Hz) coherence was unrelated to performance and largely unaffected by genotype in this task. In contrast, in novel environments, theta coherence specifically tracked exploration-related attention in WT mice, but was strongly elevated and unmodulated in Gria1-knockouts, thereby correlating with impaired short-term habituation. Strikingly, reintroduction of GLUA1 selectively into CA2/CA3 restored abnormal short-term habituation, theta coherence, and hippocampal and prefrontal theta oscillations. Although local oscillations and coherence in other frequency bands (beta, gamma), and theta-gamma cross-frequency coupling also showed dependence on GLUA1, none of them correlated with short-term habituation. Therefore, sustained elevation of hippocampal-prefrontal theta coherence may underlie a failure in regulating novelty-related selective attention leading to aberrant salience, and thereby represents a mechanistic link between GRIA1 and schizophrenia.

Attenuation of Novelty-Induced Hyperactivity of Gria1-/- Mice by Cannabidiol and Hippocampal Inhibitory Chemogenetics

Gene-targeted mice with deficient AMPA receptor GluA1 subunits (Gria1-/- mice) show robust hyperlocomotion in a novel environment, suggesting them to constitute a model for hyperactivity disorders such as mania, schizophrenia and attention deficit hyperactivity disorder. This behavioral alteration has been associated with increased neuronal activation in the hippocampus, and it can be attenuated by chronic treatment with antimanic drugs, such as lithium, valproic acid, and lamotrigine. Now we found that systemic cannabidiol strongly blunted the hyperactivity and the hippocampal c-Fos expression of the Gria1-/- mice, while not affecting the wild-type littermate controls. Acute bilateral intra-dorsal hippocampal infusion of cannabidiol partially blocked the hyperactivity of the Gria1-/- mice, but had no effect on wild-types. The activation of the inhibitory DREADD receptor hM4Gi in the dorsal hippocampus by clozapine-N-oxide robustly inhibited the hyperactivity of the Gria1-/- mice, but had no effect on the locomotion of wild-type mice. Our results show that enhanced neuronal excitability in the hippocampus is associated with pronounced novelty-induced hyperactivity of GluA1 subunit-deficient mice. When this enhanced response of hippocampal neurons to novel stimuli is specifically reduced in the hippocampus by pharmacological treatment or by chemogenetic inhibition, Gria1-/- mice recover from behavioral hyperactivity, suggesting a hippocampal dysfunction in hyperactive behaviors that can be treated with cannabidiol.

Bicuculline Reduces Neuroinflammation in Hippocampus and Improves Spatial Learning and Anxiety in Hyperammonemic Rats. Role of Glutamate Receptors

Patients with liver cirrhosis may develop minimal hepatic encephalopathy (MHE) with mild cognitive impairment. Hyperammonemia is a main contributor to cognitive impairment in MHE, which is mediated by neuroinflammation. GABAergic neurotransmission is altered in hyperammonemic rats. We hypothesized that, in hyperammonemic rats, (a) enhanced GABAergic tone would contribute to induce neuroinflammation, which would be improved by reducing GABAergic tone by chronic bicuculline treatment; (b) this would improve spatial learning and memory impairment; and (c) modulation of glutamatergic neurotransmission would mediate this cognitive improvement. The aim of this work was to assess the above hypotheses. Bicuculline was administrated intraperitoneally once a day for 4 weeks to control and hyperammonemic rats. The effects of bicuculline on microglia and astrocyte activation, IL-1β content, on membrane expression of AMPA and NMDA glutamate receptors subunits in the hippocampus and on spatial learning and memory as well as anxiety were assessed. Treatment with bicuculline reduces astrocyte activation and IL-1β but not microglia activation in the hippocampus of hyperammonemic rats. Bicuculline reverses the changes in membrane expression of AMPA receptor subunits GluA1 and GluA2 and of the NR2B (but not NR1 and NR2A) subunit of NMDA receptors. Bicuculline improves spatial learning and working memory and decreases anxiety in hyperammonemic rats. In hyperammonemia, enhanced activation of GABAA receptors in the hippocampus contributes to some but not all aspects of neuroinflammation, to altered glutamatergic neurotransmission and to impairment of spatial learning and memory as well as anxiety, all of which are reversed by reducing activation of GABAA receptors with bicuculline.

Alzheimer's disease

Alzheimer's disease (AD) dementia refers to a particular onset and course of cognitive and functional decline associated with age together with a particular neuropathology. It was first described by Alois Alzheimer in 1906 about a patient whom he first encountered in 1901. Modern clinical diagnostic criteria have been developed, and criteria have also been proposed to recognize preclinical (or presymptomatic) stages of the disease with the use of biomarkers. The primary neuropathology was described by Alzheimer, and in the mid-1980s subsequently evolved into a more specific neuropathologic definition that recognizes the comorbid neuropathologies that frequently contribute to clinical dementia. Alzheimer's disease is now the most common form of neurodegenerative dementia in the United States with a disproportionate disease burden in minority populations. Deficits in the ability to encode and store new memories characterizes the initial stages of the disease. Subsequent progressive changes in cognition and behavior accompany the later stages. Changes in amyloid precursor protein (APP) cleavage and production of the APP fragment beta-amyloid (Aβ) along with hyperphosphorylated tau protein aggregation coalesce to cause reduction in synaptic strength, synaptic loss, and neurodegeneration. Metabolic, vascular, and inflammatory changes, as well as comorbid pathologies are key components of the disease process. Symptomatic treatment offers a modest, clinically measurable effect in cognition, but disease-modifying therapies are desperately needed.

Normal extinction and reinstatement of morphine-induced conditioned place preference in the GluA1-KO mouse line

Extinction and reinstatement of morphine-induced conditioned place preference were studied in glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor GluA1 subunit-deficient mice (global GluA1-KO mice). In line with previous findings, both acquisition and expression of conditioned place preference to morphine (20 mg/kg, subcutaneously) were fully functional in GluA1 KO mice compared with wild-type littermate controls (GluA1-WT), thus enabling the study of extinction. With a 10-session extinction paradigm, the GluA1 KO mice showed complete extinction similar to that of the GluA1-WT mice. Morphine-induced reinstatement (10 mg/kg, subcutaneously) was detected in both mouse lines. GluA1 KO mice moved more during all the phases of the experiment, including the place conditioning trials, extinction sessions, and place preference tests. The results suggest that the GluA1 subunit may be dispensable or prone to compensation at the neural circuitries delineating extinction and reinstatement. The GluA1 KO mice show altered long-term between-session habituation, which extends longer than previously anticipated.

Optogenetic induction of the schizophrenia-related endophenotype of ventral hippocampal hyperactivity causes rodent correlates of positive and cognitive symptoms

Pathological over-activity of the CA1 subfield of the human anterior hippocampus has been identified as a potential predictive marker for transition from a prodromal state to overt schizophrenia. Psychosis, in turn, is associated with elevated activity in the anterior subiculum, the hippocampal output stage directly activated by CA1. Over-activity in these subfields may represent a useful endophenotype to guide translationally predictive preclinical models. To recreate this endophenotype and study its causal relation to deficits in the positive and cognitive symptom domains, we optogenetically activated excitatory neurons of the ventral hippocampus (vHPC; analogous to the human anterior hippocampus), targeting the ventral subiculum. Consistent with previous studies, we found that vHPC over-activity evokes hyperlocomotion, a rodent correlate of positive symptoms. vHPC activation also impaired performance on the spatial novelty preference (SNP) test of short-term memory, regardless of whether stimulation was applied during the encoding or retrieval stage of the task. Increasing dopamine transmission with amphetamine produced hyperlocomotion, but was not associated with SNP impairments. This suggests that short-term memory impairments resulting from hippocampal over-activity likely arise independently of a hyperdopaminergic state, a finding that is consistent with the pharmaco-resistance of cognitive symptoms in patients.

Absent sleep EEG spindle activity in GluA1 (Gria1) knockout mice: relevance to neuropsychiatric disorders

Sleep EEG spindles have been implicated in attention, sensory processing, synaptic plasticity and memory consolidation. In humans, deficits in sleep spindles have been reported in a wide range of neurological and psychiatric disorders, including schizophrenia. Genome-wide association studies have suggested a link between schizophrenia and genes associated with synaptic plasticity, including the Gria1 gene which codes for the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor. Gria1^-/- mice exhibit a phenotype relevant for neuropsychiatric disorders, including reduced synaptic plasticity and, at the behavioural level, attentional deficits leading to aberrant salience. In this study we report a striking reduction of EEG power density including the spindle-frequency range (10-15 Hz) during sleep in Gria1^-/- mice. The reduction of spindle-activity in Gria1^-/- mice was accompanied by longer REM sleep episodes, increased EEG slow-wave activity in the occipital derivation during baseline sleep, and a reduced rate of decline of EEG slow wave activity (0.5-4 Hz) during NREM sleep after sleep deprivation. These data provide a novel link between glutamatergic dysfunction and sleep abnormalities in a schizophrenia-relevant mouse model.

Somatic Accumulation of GluA1-AMPA Receptors Leads to Selective Cognitive Impairments in Mice

The GluA1 subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) plays a crucial, but highly selective, role in cognitive function. Here we analyzed AMPAR expression, AMPAR distribution and spatial learning in mice (Gria1^R/R ), expressing the "trafficking compromised" GluA1(Q600R) point mutation. Our analysis revealed somatic accumulation and reduction of GluA1(Q600R) and GluA2, but only slightly reduced CA1 synaptic localization in hippocampi of adult Gria1^R/R mice. These immunohistological changes were accompanied by a strong reduction of somatic AMPAR currents in CA1, and a reduction of plasticity (short-term and long-term potentiation, STP and LTP, respectively) in the CA1 subfield following tetanic and theta-burst stimulation. Nevertheless, spatial reference memory acquisition in the Morris water-maze and on an appetitive Y-maze task was unaffected in Gria1^R/R mice. In contrast, spatial working/short-term memory during both spontaneous and rewarded alternation tasks was dramatically impaired. These findings identify the GluA1(Q600R) mutation as a loss of function mutation that provides independent evidence for the selective role of GluA1 in the expression of short-term memory.

Dietary supplementation with Allium hirtifolium and/or Astragalus hamosus improved memory and reduced neuro-inflammation in the rat model of Alzheimer’s disease

Allium hirtifolium Boiss and Astragalus hamosus L. are mentioned in Iranian traditional medicine documentation as therapy for a kind of dementia with the features and symptoms similar to those of Alzheimer’s disease (AD). In the present study, the effects of these herbs on neuro-inflammation and memory have been evaluated as new therapies in amyloid beta (Aβ)-induced memory impairment model. Separate groups of rats were fed with A. hirtifolium or A. hamosus extract (both 100 mg/(kg·day)−1) started 1 week before stereotaxic surgery to 24 h before behavioral testing (totally, for 16 successive days). The effects of oral administration of mentioned extracts on the memory and neuro-inflammation were assessed in the Aβ-injected rats. The results of this study showed that oral administration of both A. hirtifolium and A. hamosus improved the memory, examined by using Y-maze test and shuttle box apparatus. Also, Western blotting analysis of cyclooxygenase-2, interleukin-1β, and tumor necrosis factor-α showed that these herbs have ameliorating effects against the neuro-inflammation caused by Aβ. These findings suggest that the use of A. hirtifolium and A. hamosus as herbal therapy may be suitable for decreasing AD-related symptoms and treatment of other neurodegenerative disorders.

The Improving Effect of HL271, a Chemical Derivative of Metformin, a Popular Drug for Type II Diabetes Mellitus, on Aging-induced Cognitive Decline

In recent years, as the aging population grows, aging-induced cognitive impairments including dementia and Alzheimer's disease (AD) have become the biggest challenges for global public health and social care. Therefore, the development of potential therapeutic drugs for aging-associated cognitive impairment is essential. Metabolic dysregulation has been considered to be a key factor that affects aging and dementia. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a primary sensor of cellular energy states and regulates cellular energy metabolism. Metformin (1,1-dimethylbiguanide hydrochloride) is a well-known AMPK activator and has been widely prescribed for type 2 diabetes mellitus (T2DM). Since the incidence of T2DM and dementia increases with aging, metformin has been considered to be one of the most promising drugs to target dementia and its related disorders. To that end, here, we tested the efficacy of metformin and HL271, a novel metformin derivative, in aging-induced cognitive decline. Water (control), metformin (100 mg/kg) or HL271 (50 mg/kg) were orally administered to aged mice for two months; then, the mice were subjected to behavioral tests to measure their cognitive function, particularly their contextual, spatial and working memory. AMPK phosphorylation was also measured in the drug-treated mouse brains. Our results show that oral treatment with HL271 (50 mg/kg) but not metformin (100 mg/kg) improved cognitive decline in aged mice. AMPK activation was correlated with behavior recovery after aging-induced cognitive decline. Taken together, these results suggest that the newly synthesized AMPK activator, HL271, could be a potential therapeutic agent to treat age-related cognitive decline.

Light and Cognition: Roles for Circadian Rhythms, Sleep, and Arousal

Light exerts a wide range of effects on mammalian physiology and behavior. As well as synchronizing circadian rhythms to the external environment, light has been shown to modulate autonomic and neuroendocrine responses as well as regulating sleep and influencing cognitive processes such as attention, arousal, and performance. The last two decades have seen major advances in our understanding of the retinal photoreceptors that mediate these non-image forming responses to light, as well as the neural pathways and molecular mechanisms by which circadian rhythms are generated and entrained to the external light/dark (LD) cycle. By contrast, our understanding of the mechanisms by which lighting influences cognitive processes is more equivocal. The effects of light on different cognitive processes are complex. As well as the direct effects of light on alertness, indirect effects may also occur due to disrupted circadian entrainment. Despite the widespread use of disrupted LD cycles to study the role circadian rhythms on cognition, the different experimental protocols used have subtly different effects on circadian function which are not always comparable. Moreover, these protocols will also disrupt sleep and alter physiological arousal, both of which are known to modulate cognition. Studies have used different assays that are dependent on different cognitive and sensory processes, which may also contribute to their variable findings. Here, we propose that studies addressing the effects of different lighting conditions on cognitive processes must also account for their effects on circadian rhythms, sleep, and arousal if we are to fully understand the physiological basis of these responses.

Glutamatergic synapses are structurally and biochemically complex because of multiple plasticity processes: long-term potentiation, long-term depression, short-term potentiation and scaling

Synapses are complex because they perform multiple functions, including at least six mechanistically different forms of plasticity. Here, I comment on recent developments regarding these processes. (i) Short-term potentiation (STP), a Hebbian process that requires small amounts of synaptic input, appears to make strong contributions to some forms of working memory. (ii) The rules for long-term potentiation (LTP) induction in CA3 have been clarified: induction does not depend obligatorily on backpropagating sodium spikes but, rather, on dendritic branch-specific N-methyl-d-aspartate (NMDA) spikes. (iii) Late LTP, a process that requires a dopamine signal (and is therefore neoHebbian), is mediated by trans-synaptic growth of the synapse, a growth that occurs about an hour after LTP induction. (iv) LTD processes are complex and include both homosynaptic and heterosynaptic forms. (v) Synaptic scaling produced by changes in activity levels are not primarily cell-autonomous, but rather depend on network activity. (vi) The evidence for distance-dependent scaling along the primary dendrite is firm, and a plausible structural-based mechanism is suggested.Ideas about the mechanisms of synaptic function need to take into consideration newly emerging data about synaptic structure. Recent super-resolution studies indicate that glutamatergic synapses are modular (module size 70-80 nm), as predicted by theoretical work. Modules are trans-synaptic structures and have high concentrations of postsynaptic density-95 (PSD-95) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor. These modules function as quasi-independent loci of AMPA-mediated transmission and may be independently modifiable, suggesting a new understanding of quantal transmission.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity.'

Novel Roles for the Insulin-Regulated Glucose Transporter-4 in Hippocampally Dependent Memory

The insulin-regulated glucose transporter-4 (GluT4) is critical for insulin- and contractile-mediated glucose uptake in skeletal muscle. GluT4 is also expressed in some hippocampal neurons, but its functional role in the brain is unclear. Several established molecular modulators of memory processing regulate hippocampal GluT4 trafficking and hippocampal memory formation is limited by both glucose metabolism and insulin signaling. Therefore, we hypothesized that hippocampal GluT4 might be involved in memory processes. Here, we show that, in male rats, hippocampal GluT4 translocates to the plasma membrane after memory training and that acute, selective intrahippocampal inhibition of GluT4-mediated glucose transport impaired memory acquisition, but not memory retrieval. Other studies have shown that prolonged systemic GluT4 blockade causes insulin resistance. Unexpectedly, we found that prolonged hippocampal blockade of glucose transport through GluT4-upregulated markers of hippocampal insulin signaling prevented task-associated depletion of hippocampal glucose and enhanced both working and short-term memory while also impairing long-term memory. These effects were accompanied by increased expression of hippocampal AMPA GluR1 subunits and the neuronal GluT3, but decreased expression of hippocampal brain-derived neurotrophic factor, consistent with impaired ability to form long-term memories. Our findings are the first to show the cognitive impact of brain GluT4 modulation. They identify GluT4 as a key regulator of hippocampal memory processing and also suggest differential regulation of GluT4 in the hippocampus from that in peripheral tissues.

SIGNIFICANCE STATEMENT

The role of insulin-regulated glucose transporter-4 (GluT4) in the brain is unclear. In the current study, we demonstrate that GluT4 is a critical component of hippocampal memory processes. Memory training increased hippocampal GluT4 translocation and memory acquisition was impaired by GluT4 blockade. Unexpectedly, whereas long-term inhibition of GluT4 impaired long-term memory, short-term memory was enhanced. These data further our understanding of the molecular mechanisms of memory and have particular significance for type 2 diabetes (in which GluT4 activity in the periphery is impaired) and Alzheimer's disease (which is linked to impaired brain insulin signaling and for which type 2 diabetes is a key risk factor). Both diseases cause marked impairment of hippocampal memory linked to hippocampal hypometabolism, suggesting the possibility that brain GluT4 dysregulation may be one cause of cognitive impairment in these disease states.

Heteromeric channels formed by TRPC1, TRPC4 and TRPC5 define hippocampal synaptic transmission and working memory

Canonical transient receptor potential (TRPC) channels influence various neuronal functions. Using quantitative high-resolution mass spectrometry, we demonstrate that TRPC1, TRPC4, and TRPC5 assemble into heteromultimers with each other, but not with other TRP family members in the mouse brain and hippocampus. In hippocampal neurons from Trpc1/Trpc4/Trpc5-triple-knockout (Trpc1/4/5^-/-) mice, lacking any TRPC1-, TRPC4-, or TRPC5-containing channels, action potential-triggered excitatory postsynaptic currents (EPSCs) were significantly reduced, whereas frequency, amplitude, and kinetics of quantal miniature EPSC signaling remained unchanged. Likewise, evoked postsynaptic responses in hippocampal slice recordings and transient potentiation after tetanic stimulation were decreased. In vivo, Trpc1/4/5^-/- mice displayed impaired cross-frequency coupling in hippocampal networks and deficits in spatial working memory, while spatial reference memory was unaltered. Trpc1/4/5^-/- animals also exhibited deficiencies in adapting to a new challenge in a relearning task. Our results indicate the contribution of heteromultimeric channels from TRPC1, TRPC4, and TRPC5 subunits to the regulation of mechanisms underlying spatial working memory and flexible relearning by facilitating proper synaptic transmission in hippocampal neurons.

GluA1 AMPAR subunit deletion reduces the hedonic response to sucrose but leaves satiety and conditioned responses intact

The GluA1 subunit of the AMPA receptor has been implicated in schizophrenia. While GluA1 is important for cognition, it is not clear what the role of GluA1 is in hedonic responses that are relevant to the negative symptoms of disorders such as schizophrenia. Here, we tested mice that lack GluA1 (Gria1^−/− mice) on consumption of sucrose solutions using a licking microstructure analysis. GluA1 deletion drastically reduced palatability (as measured by the mean lick cluster size) across a range of sucrose concentrations. Although initial lick rates were reduced, measures of consumption across long periods of access to sucrose solutions were not affected by GluA1 deletion and Gria1^−/− mice showed normal satiety responses to high sucrose concentrations. GluA1 deletion also failed to impair flavour conditioning, in which increased intake of a flavour occurred as a consequence of prior pairing with a high sucrose concentration. These results demonstrate that GluA1 plays a role in responding on the basis of palatability rather than other properties, such as the automatic and learnt post-ingestive, nutritional consequences of sucrose. Therefore, Gria1^−/− mice provide a potential model of anhedonia, adding converging evidence to the role of glutamatergic dysfunction in various symptoms of schizophrenia and related disorders.

Activity-Dependent Gating of Parvalbumin Interneuron Function by the Perineuronal Net Protein Brevican

Activity-dependent neuronal plasticity is a fundamental mechanism through which the nervous system adapts to sensory experience. Several lines of evidence suggest that parvalbumin (PV+) interneurons are essential in this process, but the molecular mechanisms underlying the influence of experience on interneuron plasticity remain poorly understood. Perineuronal nets (PNNs) enwrapping PV+ cells are long-standing candidates for playing such a role, yet their precise contribution has remained elusive. We show that the PNN protein Brevican is a critical regulator of interneuron plasticity. We find that Brevican simultaneously controls cellular and synaptic forms of plasticity in PV+ cells by regulating the localization of potassium channels and AMPA receptors, respectively. By modulating Brevican levels, experience introduces precise molecular and cellular modifications in PV+ cells that are required for learning and memory. These findings uncover a molecular program through which a PNN protein facilitates appropriate behavioral responses to experience by dynamically gating PV+ interneuron function.