An increase in the association of GluN2B containing NMDA receptors with membrane scaffolding proteins was related to memory declines during aging

Action of GABAB receptor on local network oscillation in somatosensory cortex of oral part: focusing on NMDA receptor

The balance of activity between glutamatergic and GABAergic networks is particularly important for oscillatory neural activities in the brain. Here, we investigated the roles of GABAB receptors in network oscillation in the oral somatosensory cortex (OSC), focusing on NMDA receptors. Neural oscillation at the frequency of 8-10 Hz was elicited in rat brain slices after caffeine application. Oscillations comprised a non-NMDA receptor-dependent initial phase and a later NMDA receptor-dependent oscillatory phase, with the oscillator located in the upper layer of the OSC. Baclofen was applied to investigate the actions of GABAB receptors. The later NMDA receptor-dependent oscillatory phase completely disappeared, but the initial phase did not. These results suggest that GABAB receptors mainly act on NMDA receptor, in which metabotropic actions of GABAB receptors may contribute to the attenuation of NMDA receptor activities. A regulatory system for network oscillation involving GABAB receptors may be present in the OSC.

N-methyl-D-aspartate receptor hypofunction as a potential contributor to the progression and manifestation of many neurological disorders

N-methyl-D-aspartate receptors (NMDA) are glutamate-gated ion channels critical for synaptic transmission and plasticity. A slight variation of NMDAR expression and function can result in devastating consequences, and both hyperactivation and hypoactivation of NMDARs are detrimental to neural function. Compared to NMDAR hyperfunction, NMDAR hypofunction is widely implicated in many neurological disorders, such as intellectual disability, autism, schizophrenia, and age-related cognitive decline. Additionally, NMDAR hypofunction is associated with the progression and manifestation of these diseases. Here, we review the underlying mechanisms of NMDAR hypofunction in the progression of these neurological disorders and highlight that targeting NMDAR hypofunction is a promising therapeutic intervention in some neurological disorders.

Cognitive Flexibility in Mice: Effects of Puberty and Role of NMDA Receptor Subunits

Cognitive flexibility refers to the ability to adapt flexibly to changing circumstances. In laboratory mice, we investigated whether cognitive flexibility is higher in pubertal mice than in adult mice, and whether this difference is related to the expression of distinct NMDA receptor subunits. Using the attentional set shifting task as a measure of cognitive flexibility, we found that cognitive flexibility was increased during puberty. This difference was more pronounced in female pubertal mice. Further, the GluN2A subunit of the NMDA receptor was more expressed during puberty than after puberty. Pharmacological blockade of GluN2A reduced the cognitive flexibility of pubertal mice to adult levels. In adult mice, the expression of GluN2A, GluN2B, and GluN2C in the orbitofrontal cortex correlated positively with performance in the attentional set shifting task, whereas in pubertal mice this was only the case for GluN2C. In conclusion, the present study confirms the observation in humans that cognitive flexibility is higher during puberty than in adulthood. Future studies should investigate whether NMDA receptor subunit-specific agonists are able to rescue deficient cognitive flexibility, and whether they have the potential to be used in human diseases with deficits in cognitive flexibility.

GluN2B-NMDAR subunit contribution on synaptic plasticity: A phenomenological model for CA3-CA1 synapses

Synaptic plasticity is believed to be a key mechanism underlying learning and memory. We developed a phenomenological N-methyl-D-aspartate (NMDA) receptor-based voltage-dependent synaptic plasticity model for synaptic modifications at hippocampal CA3-CA1 synapses on a hippocampal CA1 pyramidal neuron. The model incorporates the GluN2A-NMDA and GluN2B-NMDA receptor subunit-based functions and accounts for the synaptic strength dependence on the postsynaptic NMDA receptor composition and functioning without explicitly modeling the NMDA receptor-mediated intracellular calcium, a local trigger of synaptic plasticity. We embedded the model into a two-compartmental model of a hippocampal CA1 pyramidal cell and validated it against experimental data of spike-timing-dependent synaptic plasticity (STDP), high and low-frequency stimulation. The developed model predicts altered learning rules in synapses formed on the apical dendrites of the detailed compartmental model of CA1 pyramidal neuron in the presence of the GluN2B-NMDA receptor hypofunction and can be used in hippocampal networks to model learning in health and disease.

GluN2A-Selective NMDA Receptor Antagonists: Mimicking the U-Shaped Bioactive Conformation of TCN-201 by a [2.2]Paracyclophane System

Under physiological conditions, N-Methyl-D-Aspartate (NMDA) receptors play a crucial role for synaptic plasticity, long-term potentiation and long-term depression. However, overactivation of NMDA receptors can result in excitotoxicity, which is associated with various neurological and neurodegenerative diseases. The physiological properties of NMDA receptors are strongly dependent on the GluN2 subunit incorporated into the heterotetrameric NMDA receptor. Therefore, subtype selective NMDA receptor modulators are of high interest. Since prototypical GluN2A-NMDA receptor antagonists TCN-201 and its MPX-analogs adopt a U-shaped conformation within the binding pocket, paracyclophanes were designed containing the phenyl rings in an already parallel orientation. Docking studies of the designed paracyclophanes show a similar binding pose as TCN-201. [2.2]Paracyclophanes with a benzoate or benzamide side chain were prepared in four-step synthesis, respectively, starting with a radical bromination in benzylic 1-position of [2.2]paracyclophane. In two-electrode voltage clamp experiments using Xenopus laevis oocytes transfected with cRNAs for the GluN1-4a and GluN2A subunits, the esters and amides (conc. 10 μM) did not show considerable inhibition of ion flux. It can be concluded that the GluN2A-NMDA receptor does not accept ligands with a paracyclophane scaffold functionalized in benzylic 1-position, although docking studies had revealed promising binding poses for benzoic acid esters and benzamides.

Effects of Aging on the Structure and Expression of NMDA Receptors of Somatostatin Expressing Neurons in the Mouse Hippocampus

Changes in the physiology, neurochemistry and structure of neurons, particularly of their dendritic spines, are thought to be crucial players in age-related cognitive decline. One of the most studied brain structures affected by aging is the hippocampus, known to be involved in different essential cognitive processes. While the aging-associated quantitative changes in dendritic spines of hippocampal pyramidal cells have already been studied, the relationship between aging and the structural dynamics of hippocampal interneurons remains relatively unknown. Spines are not a frequent feature in cortical inhibitory neurons, but these postsynaptic structures are abundant in a subpopulation of somatostatin expressing interneurons, particularly in oriens-lacunosum moleculare (O-LM) cells in the hippocampal CA1. Previous studies from our laboratory have shown that the spines of these interneurons are highly plastic and influenced by NMDA receptor manipulation. Thus, in the present study, we have investigated the impact of aging on this interneuronal subpopulation. The analyses were performed in 3−, 9−, and 16-month-old GIN mice, a strain in which somatostatin positive interneurons express GFP. We studied the changes in the density of dendritic spines, en passant boutons, and the expression of NMDA receptors (GluN1 and GluN2B) using confocal microscopy and image analysis. We observed a significant decrease in dendritic spine density in 9-month-old animals when compared with 3-month-old animals. We also observed a decrease in the expression of the GluN2B subunit in O-LM cells, but not of that of GluN1, during aging. These results will constitute the basis for more advanced studies of the structure and connectivity of interneurons during aging and their contribution to cognitive decline.

C9orf72 hexanucleotide repeat expansion leads to altered neuronal and dendritic spine morphology and synaptic dysfunction

Frontotemporal lobar degeneration (FTLD) comprises a heterogenous group of progressive neurodegenerative syndromes. To date, no validated biomarkers or effective disease-modifying therapies exist for the different clinical or genetic subtypes of FTLD. The most common genetic cause underlying FTLD and amyotrophic lateral sclerosis (ALS) is a hexanucleotide repeat expansion in the C9orf72 gene (C9-HRE). FTLD is accompanied by changes in several neurotransmitter systems, including the glutamatergic, GABAergic, dopaminergic, and serotonergic systems and many clinical symptoms can be explained by disturbances in these systems. Here, we aimed to elucidate the effects of the C9-HRE on synaptic function, molecular composition of synapses, and dendritic spine morphology. We overexpressed the pathological C9-HRE in cultured E18 mouse primary hippocampal neurons and characterized the pathological, morphological, and functional changes by biochemical methods, confocal microscopy, and live cell calcium imaging. The C9-HRE-expressing neurons were confirmed to display the pathological RNA foci and DPR proteins. C9-HRE expression led to significant changes in dendritic spine morphologies, as indicated by decreased number of mushroom-type spines and increased number of stubby and thin spines, as well as diminished neuronal branching. These morphological changes were accompanied by concomitantly enhanced susceptibility of the neurons to glutamate-induced excitotoxicity as well as augmented and prolonged responses to excitatory stimuli by glutamate and depolarizing potassium chloride as compared to control neurons. Mechanistically, the hyperexcitation phenotype in the C9-HRE-expressing neurons was found to be underlain by increased activity of extrasynaptic GluN2B-containing N-methyl-d-aspartate (NMDA) receptors. Our results are in accordance with the idea suggesting that C9-HRE is associated with enhanced excitotoxicity and synaptic dysfunction. Thus, therapeutic interventions targeted to alleviate synaptic disturbances might offer efficient avenues for the treatment of patients with C9-HRE-associated FTLD.

Differential effects of memory enhancing and impairing doses of methylphenidate on serotonin metabolism and 5-HT1A, GABA, glutamate receptor expression in the rat prefrontal cortex

Methylphenidate (MPD), a psychostimulant, is a prescription medicine for treating attention deficit hyperactivity disorder (ADHD). Previously we have shown that moderate doses of MPD enhanced learning and memory while higher doses impaired it. To understand neurochemical mechanisms and receptors involved in memory enhancing and impairing effects of MPD, the present study concerns the effects of these doses of MPD on serotonin, 5-HT1A, GABA, and NMDA receptor mRNA expression in the prefrontal cortex (PFC). We found that low doses (2.5 mg/kg) of MPD improved performance in the water-maze test but higher doses (5 mg/kg) impaired memory retention. Animals showing improved performance had high 5-HT metabolism in the PFC while these levels were not affected in the group treated with higher MPD doses and exhibiting impaired memory. There was downregulation of 5-HT1A receptors in the PFC of rats treated with higher dose MPD, which didn't occur in low dose of MPD treated animals. Further, a decrease in GABA_Areceptor mRNA expression occurred in low doses of MPD treated animals and GluN2A expression was reduced in higher doses of MPD treated animals. The findings suggest that memory enhancing doses of MPD increase 5-HT and reduce GABA_A receptor mRNA expression in the PFC to release excitatory glutamate neurons from the inhibitory influence of GABA. Conversely, higher dose of MPD downregulates 5-HT1A receptor mRNA expression to enhance inhibitory GABA influence on glutamate neurons and impair cognitive performance. The findings show an important role of 5-HT1A heteroreceptors in the PFC for improving therapeutic use of MPD and developing novel cognitive enhancers.

Flexibility as a marker of early cognitive decline in humanized apolipoprotein E ε4 (ApoE4) mice

To test the hypothesis that ApoE4 may be involved in cognitive deficits associated with aging, we investigated the impact of APOE4 status and aging on the flexibility and memory components of spatial learning in mice. Young adult (6 months) and middle-aged (14 months) ApoE4, ApoE3 and C57BL/6 male mice were tested for flexibility in an aquatic Y-maze, and for spatio-temporal memory acquisition in the Starmaze. Our results revealed a flexibility deficit of the 6-month-old ApoE4 mice compared to controls. However, this deficit was not associated with spatio-temporal memory deficit at the same age. Importantly, the ApoE4 flexibility deficit did not increase with age, nor turn into memory deficit, or was able to predict individual variations of memory performance at 14 months. By contrast, control ApoE3 mice showed a decline of flexibility at 14 months resulting in performance similar to that of ApoE4. Overall, our results suggest that ApoE4 could be associated with an acceleration of the flexibility decrease otherwise observed in normal aging.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

Effects of Mephedrone and Amphetamine Exposure during Adolescence on Spatial Memory in Adulthood: Behavioral and Neurochemical Analysis

A synthetic cathinone, mephedrone is widely abused by adolescents and young adults. Despite its widespread use, little is known regarding its long-term effects on cognitive function. Therefore, we assessed, for the first time, whether (A) repeated mephedrone (30 mg/kg, i.p., 10 days, once a day) exposure during adolescence (PND 40) induces deleterious effects on spatial memory and reversal learning (Barnes maze task) in adult (PND 71-84) rats and whether (B) these effects were comparable to amphetamine (2.5 mg/kg, i.p.). Furthermore, the influence of these drugs on MMP-9, NMDA receptor subunits (GluN1, GluN2A/2B) and PSD-95 protein expression were assessed in adult rats. The drug effects were evaluated at doses that per se induce rewarding/reinforcing effects in rats. Our results showed deficits in spatial memory (delayed effect of amphetamine) and reversal learning in adult rats that received mephedrone/amphetamine in adolescence. However, the reversal learning impairment may actually have been due to spatial learning rather than cognitive flexibility impairments. Furthermore, mephedrone, but not amphetamine, enhanced with delayed onset, MMP-9 levels in the prefrontal cortex and the hippocampus. Mephedrone given during adolescence induced changes in MMP-9 level and up-regulation of the GluN2B-containing NMDA receptor (prefrontal cortex and hippocampus) in young adult (PND 63) and adult (PND 87) rats. Finally, in adult rats, PSD-95 expression was increased in the prefrontal cortex and decreased in the hippocampus. In contrast, in adult rats exposed to amphetamine in adolescence, GluN2A subunit and PSD-95 expression were decreased (down-regulated) in the hippocampus. Thus, in mephedrone-but not amphetamine-treated rats, the deleterious effects on spatial memory were associated with changes in MMP-9 level. Because the GluN2B-containing NMDA receptor dominates in adolescence, mephedrone seems to induce more harmful effects on cognition than amphetamine does during this period of life.

Age-related impairment of declarative memory: linking memorization of temporal associations to GluN2B redistribution in dorsal CA1

GluN2B subunits of NMDA receptors have been proposed as a target for treating age-related memory decline. They are indeed considered as crucial for hippocampal synaptic plasticity and hippocampus-dependent memory formation, which are both altered in aging. Because a synaptic enrichment in GluN2B is associated with hippocampal LTP in vitro, a similar mechanism is expected to occur during memory formation. We show instead that a reduction of GluN2B synaptic localization induced by a single-session learning in dorsal CA1 apical dendrites is predictive of efficient memorization of a temporal association. Furthermore, synaptic accumulation of GluN2B, rather than insufficient synaptic localization of these subunits, is causally involved in the age-related impairment of memory. These challenging data identify extra-synaptic redistribution of GluN2B-containing NMDAR induced by learning as a molecular signature of memory formation and indicate that modulating GluN2B synaptic localization might represent a useful therapeutic strategy in cognitive aging.

Effects of exercise on proactive interference in memory: potential neuroplasticity and neurochemical mechanisms

Proactive interference occurs when consolidated memory traces inhibit new learning. This kind of interference decreases the efficiency of new learning and also causes memory errors. Exercise has been shown to facilitate some types of cognitive function; however, whether exercise reduces proactive interference to enhance learning efficiency is not well understood. Thus, this review discusses the effects of exercise on proactive memory interference and explores potential mechanisms, such as neurogenesis and neurochemical changes, mediating any effect.

Glycogen synthase kinase-3: The missing link to aberrant circuit function in disorders of cognitive dysfunction?

Elevated GSK-3 activity has been implicated in cognitive dysfunction associated with various disorders including Alzheimer's disease, schizophrenia, type 2 diabetes, traumatic brain injury, major depressive disorder and bipolar disorder. Further, aberrant neural oscillatory activity in, and between, cortical regions and the hippocampus is consistently present within these same cognitive disorders. In this review, we will put forth the idea that increased GSK-3 activity serves as a pathological convergence point across cognitive disorders, inducing similar consequent impacts on downstream signaling mechanisms implicated in the maintenance of processes critical to brain systems communication and normal cognitive functioning. In this regard we suggest that increased activation of GSK-3 and neuronal oscillatory dysfunction are early pathological changes that may be functionally linked. Mechanistic commonalities between these disorders of cognitive dysfunction will be discussed and potential downstream targets of GSK-3 that may contribute to neuronal oscillatory dysfunction identified.

Age-Dependent Modification of Intracellular Zn2+ Buffering in the Hippocampus and Its Impact

The basal concentrations of extracellular Zn²⁺ and intracellular Zn²⁺, which are approximately 10 nM and 100 pM, respectively, in the brain, are markedly lower than those of extracellular Ca²⁺ (1.3 mM) and intracellular Ca²⁺ (100 nM), respectively, resulting in much less attention paid to Zn²⁺ than to Ca²⁺. However, intracellular Zn²⁺ dysregulation, which is closely linked with glutamate- and amyloid β-mediated extracellular Zn²⁺ influx, is more critical for cognitive decline and neurodegeneration than intracellular Ca²⁺ dysregulation. It is estimated that the age-dependent increase in the basal concentration of extracellular Zn²⁺ in the hippocampus plays a key role in cognitive decline and neurodegeneration. The characteristics of extracellular Zn²⁺ influx in the hippocampus may be modified age-dependently, probably followed by modification of intracellular Zn²⁺ buffering that is closely linked with age-related cognitive decline and neurodegeneration. Reduction of intracellular Zn²⁺-buffering capacity may be linked with the pathophysiology of progressive neurodegeneration such as Alzheimer's disease. This paper deals with age-dependent modification of intracellular Zn²⁺ buffering in the hippocampus and its impact. On the basis of the estimated impact, we propose a potential defense strategy against Zn²⁺-mediated neurodegeneration, i.e., metallothionein induction in the hippocampus.

Protein expression changes of HCN1 and HCN2 in hippocampal subregions of gerbils during the normal aging process

OBJECTIVES

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play essential roles in various hippocampal functions, including regulation of long-term potentiation, synaptic plasticity, and hippocampal-dependent cognitive process. The objective of this study was to investigate age-related changes in HCN1 and HCN2 protein expressions in gerbil hippocampus at various ages.

MATERIALS AND METHODS

In this study, the protein expressions of HCN1 and HCN2 were compared in the hippocampus at the ages of 1, 3, 12, and 24 months using Western blot analysis and immunohistochemistry.

RESULTS

Immunoreactivity of both HCN1 and HCN2 was shown primarily in cells of the pyramidal cell layer in the hippocampus proper and in cells of the granule cell layer in the dentate gyrus. HCN1 and HCN2 protein expression levels and immunoreactivity were significantly increased at three months (3 M) of age compared with those at 1 M of age. After that, both HCN1 and HCN2 expression levels in the hippocampus were gradually decreased with age.

CONCLUSION

Our results show that the normal aging process affects the expression levels of HCN1 and HCN2 in hippocampal cells in gerbils. There are marked reductions in HCN1 and HCN2 expressions in the aged hippocampus compared to the young hippocampus. Such reductions might be related to aging in the hippocampus.

The application of a rodent-based Morris water maze (MWM) protocol to an investigation of age-related differences in human spatial learning

The current study applied a rodent-based Morris water maze (MWM) protocol to an investigation of search performance differences between young and older adult humans. To investigate whether similar age-related decline in search performance could be seen in humans based on the rodent-based protocol, we implemented a virtual MWM (vMWM) that has characteristics similar to those of the MWM used in previous studies of spatial learning in mice. Through the use of a proximity to platform measure, robust differences were found between healthy young and older adults in search performance. After dividing older adults into good and poor performers based on a median split of their corrected cumulative proximity values, the age effects in place learning were found to be largely related to search performance differences between the young and poor-performing older adults. When compared with the young, poor-performing older adults exhibited significantly higher proximity values in 83% of 24 place trials and overall in the probe trials that assessed spatial learning in the absence of the hidden platform. In contrast, good-performing older adults exhibited patterns of search performance that were comparable with that of the younger adults in most place and probe trials. Taken together, our findings suggest that the low search accuracy in poor-performing older adults stemmed from potential differences in strategy selection, differences in assumptions or expectations of task demands, as well as possible underlying functional and/or structural changes in the brain regions involved in vMWM search performance. (PsycINFO Database Record

Age-dependent decrease of Nurr1 protein expression in the gerbil hippocampus

Nuclear receptor related-1 protein (Nurr1) serves important roles in hippocampal-dependent cognitive process. In the present study, the protein expression of Nurr1 was compared in the hippocampi of young [postnatal month 3 (PM 3)], adult (PM 12) and aged (PM 24) gerbils using western blot analysis and immunohistochemistry. Results indicated that the protein level of Nurr1 was significantly and gradually decreased in the gerbil hippocampus with increasing age. In addition, strong Nurr1 immunoreactivity was primarily observed in pyramidal neurons and granule cells of the hippocampus in the young group, which was determined to be reduced in the adult group and to a greater extent in the aged group. Collectively the data demonstrated that Nurr1 immunoreactivity was gradually and markedly decreased during normal aging. These results indicate that gradual decrease of Nurr1 expression in the hippocampus may be associated with the normal aging process and a decline in hippocampus-dependent cognitive function.

Long-term cilostazol administration prevents age-related decline of hippocampus-dependent memory in mice

Phosphodiesterases (PDEs) are enzymes that hydrolyze and inactivate 3', 5'-cyclic adenosine monophosphate (cAMP) and/or 3', 5'-cyclic guanosine monophosphate (cGMP). The regulation of intracellular signaling pathways mediated by cyclic nucleotides is imperative to synaptic plasticity and memory in animals. Because PDEs play an important role in this regulation, PDE inhibitors are considered as candidate compounds for treating cognitive and memory disorders. In the present study, we tested whether cilostazol, a selective PDE3 inhibitor, prevents the cognitive deterioration that occurs during the course of normal aging in mice. Ten months of cilostazol administration (1.5%) in 13-month-old mice improved spatial memory when tested at 23 months of age. First, it prevented the decline in the ability of these aged mice to recognize a change in an object's location in the object recognition task. Second, spatial memory of these cilostazol-treated aged mice in the Morris water maze was comparable to that of untreated middle-aged mice (13 months old). Cilostazol administration had no effect on the emotional states and physical ability of aged mice. Thus, long-term cilostazol administration prevented hippocampus-dependent memory decline in aged mice, allowing them to achieve a level of cognitive performance similar to middle-aged mice and without negative behavioral side effects. Considering its well-established safety in other medical contexts, cilostazol may be a potential therapeutic candidate drug for staving off cognitive decline in the aging human population.

Proteomic Analysis of Postsynaptic Protein Complexes Underlying Neuronal Plasticity

Normal neuronal communication and synaptic plasticity at glutamatergic synapses requires dynamic regulation of postsynaptic molecules. Protein expression and protein post-translational modifications regulate protein interactions that underlie this organization. In this Review, we highlight data obtained over the last 20 years that have used qualitative and quantitative proteomics-based approaches to identify postsynaptic protein complexes. Herein, we describe how these proteomics studies have helped lay the foundation for understanding synaptic physiology and perturbations in synaptic signaling observed in different pathologies. We also describe emerging technologies that can be useful in these analyses. We focus on protein complexes associated with the highly abundant and functionally critical proteins: calcium/calmodulin-dependent protein kinase II, the N-methyl-d-aspartate, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors, and postsynaptic density protein of 95 kDa.

Cognitive deficit in hippocampal-dependent tasks in Werner syndrome mouse model

Mammalian aging is often characterized by metabolic disturbances, cognitive declines and DNA repairs deficiency, but the underlying molecular mechanisms are still not well understood. Alterations in DNA repair can significantly exacerbate aging. Mammalian neuronal cells which accumulate unrepaired DNA damage over time could potentially lead to brain functions disorders. Focusing on the ATP-dependent RecQ-type DNA helicase, an enzyme involved in repair of double strand DNA, a mouse model of Werner syndrome (WS) had been proposed as a model of accelerated aging. Until now, no study has investigated the impact of this premature aging syndrome on learning and memory. Spatial memory and cognitive flexibility are particularly affected by the aging process in both men and rodents. Studies have shown that aged mice exhibited similar performance than young adult mice on non-hippocampus dependent memory whereas their performances were decreased in hippocampus-dependent tasks. In this study, we have submitted 3, 5 and 8 month-old WS mice to several behavioral paradigms to evaluate hippocampus-dependent (spatial object location, Morris water maze and fear conditioning) and non hippocampus-dependent (object recognition) memories. No effect on the locomotion activity and anxiety level has been observed in adult WS mice. Interestingly, the 8 month-old WS mice exhibit long-term memory impairment similar to aged mice, suggesting that adult WS mice do develop some aspects of cognitive aging.

Effects of ibuprofen on cognition and NMDA receptor subunit expression across aging

Age-related declines in long- and short-term memory show relationships to decreases in N-methyl-d-aspartate (NMDA) receptor expression, which may involve inflammation. This study was designed to determine effects of an anti-inflammatory drug, ibuprofen, on cognitive function and NMDA receptor expression across aging. Male C57BL/6 mice (ages 5, 14, 20, and 26months) were fed ibuprofen (375ppm) in NIH31 diet or diet alone for 6weeks prior to testing. Behavioral testing using the Morris water maze showed that older mice performed significantly worse than younger in spatial long-term memory, reversal, and short-term memory tasks. Ibuprofen enhanced overall performance in the short-term memory task, but this appeared to be more related to improved executive function than memory. Ibuprofen induced significant decreases over all ages in the mRNA densities for GluN2B subunit, all GluN1 splice variants, and GluN1-1 splice forms in the frontal cortex and in protein expression of GluN2A, GluN2B and GluN1 C2' cassettes in the hippocampus. GluN1-3 splice form mRNA and C2' cassette protein were significantly increased across ages in frontal lobes of ibuprofen-treated mice. Ibuprofen did not alter expression of pro-inflammatory cytokines IL-1β and TNFα, but did reduce the area of reactive astrocyte immunostaining in frontal cortex of aged mice. Enhancement in executive function showed a relationship to increased GluN1-3 mRNA and decreased gliosis. These findings suggest that inflammation may play a role in executive function declines in aged animals, but other effects of ibuprofen on NMDA receptors appeared to be unrelated to aging or inflammation.

Differential Regulation of N-Methyl-D-Aspartate Receptor Subunits is an Early Event in the Actions of Soluble Amyloid-β(1-40) Oligomers on Hippocampal Neurons

Synaptic dysfunction during early stages of Alzheimer's disease (AD) is triggered by soluble amyloid-β (Aβ) oligomers that interact with NMDA receptors (NMDARs). We previously showed that Aβ induces synaptic protein loss through NMDARs, albeit through undefined mechanisms. Accordingly, we here examined the contribution of individual NMDAR subunits to synaptotoxicity and demonstrate that Aβ exerts differential effects on the levels and distribution of GluN2A and GluN2B subunits of NMDAR in dendrites. Treatment of cultured hippocampal neurons with Aβ1-40 (10 μM, 1 h) induced a significant increase of dendritic and synaptic GluN2B puncta densities with parallel decreases in the puncta densities of denritic and synaptic pTyr1472-GluN2B. Conversely, Aβ significantly decreased dendritic and synaptic GluN2A and dendritic pTyr1325-GluN2A puncta densities and increased synaptic pTyr1325-GluN2A puncta densities. Unexpectedly, Aβ treatment resulted in a significant reduction of GluN2B and pTyr1472-GluN2B protein levels but did not influence GluN2A and pTyr1325-GluN2A levels. These results show that Aβ exerts complex and distinct regulatory effects on the trafficking and phosphorylation of GluN2A and GluN2B, as well as on their localization within synaptic and non-synaptic sites. Increased understanding of these early events in Aβ-induced synaptic dysfunction is likely to be important for the development of timely preventive and therapeutic interventions.

NR2A-Containing NMDARs in the Prefrontal Cortex Are Required for Working Memory and Associated with Age-Related Cognitive Decline

Working memory, the ability to temporarily maintain representational knowledge, is a foundational cognitive process that can become compromised in aging and neuropsychiatric disease. NMDA receptor (NMDAR) activation in prefrontal cortex (PFC) is necessary for the pyramidal neuron activity believed to enable working memory; however, the distinct biophysical properties and localization of NMDARs containing NR2A and NR2B subunits suggest unique roles for NMDAR subtypes in PFC neural activity and working memory. Experiments herein show that working memory depends on NR2A- but not NR2B-NMDARs in PFC of rats and that NR2A-NMDARs mediate the majority of evoked NMDAR currents on layer 2/3 PFC pyramidal neurons. Moreover, attenuated expression of the NR2A but not the NR2B subunit in PFC associates with naturally occurring working memory impairment in aged rats. Finally, NMDAR currents and working memory are enhanced in aged rats by promoting activation of the NR2A-enriched synaptic pool of PFC NMDARs. These results implicate NR2A-NMDARs in normal working memory and suggest novel treatment strategies for improving working memory in cognitive disorders.

SIGNIFICANCE STATEMENT

Working memory, the ability to hold information "in mind," requires persistent activity of pyramidal neurons in prefrontal cortex (PFC) mediated by NMDA receptor (NMDAR) activation. NMDAR loss in PFC may account for working memory impairments in aging and psychiatric disease. Our studies demonstrate that NMDARs containing the NR2A subunit, but not the NR2B subunit, are required for working memory and that loss of NR2A predicts severity of age-related working memory impairment. The importance of NR2A to working memory is likely due its abundant contribution to pyramidal neuron activity and location at synaptic sites in PFC. This information is useful in designing new therapies to treat working memory impairments by enhancing the function of NR2A-containing NMDARs.

Higher levels of phosphorylated Y1472 on GluN2B subunits in the frontal cortex of aged mice are associated with good spatial reference memory, but not cognitive flexibility

The N-methyl-D-aspartate receptor (NMDAr) is particularly vulnerable to aging. The GluN2B subunit of the NMDAr, compared to other NMDAr subunits, suffers the greatest losses of expression in the aging brain, especially in the frontal cortex. While expression levels of GluN2B mRNA and protein in the aged brain are well documented, there has been little investigation into age-related posttranslational modifications of the subunit. In this study, we explored some of the mechanisms that may promote differences in the NMDAr complex in the frontal cortex of aged animals. Two ages of mice, 3 and 24 months, were behaviorally tested in the Morris water maze. The frontal cortex and hippocampus from each mouse were subjected to differential centrifugation followed by solubilization in Triton X-100. Proteins from Triton-insoluble membranes, Triton-soluble membranes, and intracellular membranes/cytosol were examined by Western blot. Higher levels of GluN2B tyrosine 1472 phosphorylation in frontal cortex synaptic fractions of old mice were associated with better reference learning but poorer cognitive flexibility. Levels of GluN2B phosphotyrosine 1336 remained steady, but there were greater levels of the calpain-induced 115 kDa GluN2B cleavage product on extrasynaptic membranes in these old good learners. There was an age-related increase in calpain activity, but it was not associated with better learning. These data highlight a unique aging change for aged mice with good spatial learning that might be detrimental to cognitive flexibility. This study also suggests that higher levels of truncated GluN2B on extrasynaptic membranes are not deleterious to spatial memory in aged mice.

Major Histocompatibility Complex class I proteins are critical for maintaining neuronal structural complexity in the aging brain

Major histocompatibility complex class I (MHCI) proteins have been implicated in neuronal function through the modulation of neuritogenesis, synaptogenesis, synaptic plasticity, and memory consolidation during development. However, the involvement of MHCI in the aged brain is unclear. Here we demonstrate that MHCI deficiency results in significant dendritic atrophy along with an increase in thin dendritic spines and a reduction in stubby spines in the hippocampus of aged (12 month old) mice. Ultrastructural analyses revealed a decrease in spine head diameter and post synaptic density (PSD) area, as well as an increase in overall synapse density, and non-perforated, small spines. Interestingly, we found that the changes in synapse density and morphology appear relatively late (after the age of 6 months). Finally, we found a significant age dependent increase in the levels of the glutamate receptor, GluN2B in aged MHCI knockout mice, with no change in GluA2/3, VGluT1, PSD95 or synaptophysin. These results indicate that MHCI may be also be involved in maintaining brain integrity at post-developmental stages notably in the modulation of neuronal and spine morphology and synaptic function during non-pathological aging which could have significant implications for cognitive function.

NMDA Receptor Function During Senescence: Implication on Cognitive Performance

N-methyl-D-aspartate (NMDA) receptors, a family of L-glutamate receptors, play an important role in learning and memory, and are critical for spatial memory. These receptors are tetrameric ion channels composed of a family of related subunits. One of the hallmarks of the aging human population is a decline in cognitive function; studies in the past couple of years have demonstrated deterioration in NMDA receptor subunit expression and function with advancing age. However, a direct relationship between impaired memory function and a decline in NMDA receptors is still ambiguous. Recent studies indicate a link between an age-associated NMDA receptor hypofunction and memory impairment and provide evidence that age-associated enhanced oxidative stress might be contributing to the alterations associated with senescence. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between age-associated impaired cognitive faculties and NMDA receptor hypofunction. The current review intends to present an overview of the research findings regarding changes in expression of various NMDA receptor subunits and deficits in NMDA receptor function during senescence and its implication in age-associated impaired hippocampal-dependent memory function.

Moderate prenatal alcohol exposure enhances GluN2B containing NMDA receptor binding and ifenprodil sensitivity in rat agranular insular cortex

Prenatal exposure to alcohol affects the expression and function of glutamatergic neurotransmitter receptors in diverse brain regions. The present study was undertaken to fill a current gap in knowledge regarding the regional specificity of ethanol-related alterations in glutamatergic receptors in the frontal cortex. We quantified subregional expression and function of glutamatergic neurotransmitter receptors (AMPARs, NMDARs, GluN2B-containing NMDARs, mGluR1s, and mGluR5s) by radioligand binding in the agranular insular cortex (AID), lateral orbital area (LO), prelimbic cortex (PrL) and primary motor cortex (M1) of adult rats exposed to moderate levels of ethanol during prenatal development. Increased expression of GluN2B-containing NMDARs was observed in AID of ethanol-exposed rats compared to modest reductions in other regions. We subsequently performed slice electrophysiology measurements in a whole-cell patch-clamp preparation to quantify the sensitivity of evoked NMDAR-mediated excitatory postsynaptic currents (EPSCs) in layer II/III pyramidal neurons of AID to the GluN2B negative allosteric modulator ifenprodil. Consistent with increased GluN2B expression, ifenprodil caused a greater reduction in NMDAR-mediated EPSCs from prenatal alcohol-exposed rats than saccharin-exposed control animals. No alterations in AMPAR-mediated EPSCs or the ratio of AMPARs/NMDARs were observed. Together, these data indicate that moderate prenatal alcohol exposure has a significant and lasting impact on GluN2B-containing receptors in AID, which could help to explain ethanol-related alterations in learning and behaviors that depend on this region.

Xanthohumol improved cognitive flexibility in young mice

The protein palmitoylation cycle has been shown to be important for protein signaling and synaptic plasticity. Data from our lab showed a change in the palmitoylation status of certain proteins with age. A greater percentage of the NMDA receptor subunits GluN2A and GluN2B, along with Fyn and PSD95 proteins, were palmitoylated in the old mice. The higher level of protein palmitoylation was also associated with poorer learning scores. Xanthohumol is a prenylated flavonoid that has been shown to increase beta-oxidation in the livers of rodents, decreasing circulating free fatty acids in the serum. What is not known is whether the application of xanthohumol could influence the palmitoylation status of proteins. In this study, young and old mice were fed a diet supplemented with xanthohumol for 8 weeks. Spatial memory was assessed with the Morris water maze and protein palmitoylation quantified. The young xanthohumol-treated mice showed a significant improvement in cognitive flexibility. However, this appeared to be associated with the young control mice, on a defined, phytoestrogen-deficient diet, performing as poorly as the old mice and xanthohumol reversing this effect. The old mice receiving xanthohumol did not significantly improve their learning scores. Xanthohumol treatment was unable to affect the palmitoylation of NMDA receptor subunits and associated proteins assessed in this study. This evidence suggests that xanthohumol may play a role in improving cognitive flexability in young animals, but it appears to be ineffective in adjusting the palmitoylation status of neuronal proteins in aged individuals.