Changes in expression of splice cassettes of NMDA receptor GluN1 subunits within the frontal lobe and memory in mice during aging

Alcohol-drinking during later life by C57BL/6J mice induces sex- and age-dependent changes in hippocampal and prefrontal cortex expression of glutamate receptors and neuropathology markers

Heavy drinking can induce early-onset dementia and increase the likelihood of the progression and severity of Alzheimer's Disease and related dementias (ADRD). Recently, we showed that alcohol-drinking by mature adult C57BL/6J mice induces more signs of cognitive impairment in females versus males without worsening age-related cognitive decline in aged mice. Here, we immunoblotted for glutamate receptors and protein markers of ADRD-related neuropathology within the hippocampus and prefrontal cortex (PFC) of these mice after three weeks of alcohol withdrawal to determine protein correlates of alcohol-induced cognitive decline. Irrespective of alcohol history, age-related changes in protein expression included a male-specific decline in hippocampal glutamate receptors and an increase in the expression of a beta-site amyloid precursor protein cleaving enzyme (BACE) isoform in the PFC as well as a sex-independent increase in hippocampal amyloid precursor protein. Alcohol-drinking was associated with altered expression of glutamate receptors in the hippocampus in a sex-dependent manner, while all glutamate receptor proteins exhibited significant alcohol-related increases in the PFC of both sexes. Expression of BACE isoforms and phosphorylated tau varied in the PFC and hippocampus based on age, sex, and drinking history. The results of this study indicate that withdrawal from a history of alcohol-drinking during later life induces sex- and age-selective effects on glutamate receptor expression and protein markers of ADRD-related neuropathology within the hippocampus and PFC of potential relevance to the etiology, treatment and prevention of alcohol-induced dementia and Alzheimer's Disease.

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.

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.

Spectral blueshift of biophotonic activity and transmission in the ageing mouse brain

The brain is considered to be a complex system with extremely low energy consumption and high-efficiency information transmission and processing, and this system has not been replicated by any artificial systems so far. Several studies indicate that the activity and transmission of biophotons in neural circuits may play an important role in neural information communication, while the biophotonic spectral redshift from lower to higher in animals may be related to the evolution of intelligence. The ageing processes of higher organisms are often accompanied by a decline in brain functions; however, the underlying mechanisms are unclear. Combining an ultraweak biophoton imaging system with the improved biophoton spectral analysis device, we compared and analyzed the spectra of glutamate-induced biophotonic emissions in mouse brain slices at different ages (newborn, 1, 3, 6, 12, 15, and 18 months). We found that the glutamate-induced biophotonic emissions presented a spectral blueshift from young to old mice, suggesting that the brain may transform to use relatively high-energy biophotons for neural information transmission and processing during the ageing process. Such a change may lead to a gradual decrease in the efficiency of the nervous system and provide a new biophysical mechanism for explaining the ageing-related changes in cognitive functions.

Age-related differences in brain activations during spatial memory formation in a well-learned virtual Morris water maze (vMWM) task

The current study applied a rodent-based virtual Morris water maze (vMWM) protocol to an investigation of differences in search performance and brain activations between young and older male human adults. All participants completed in-lab practice and testing before performing the task in the fMRI scanner. Behavioral performance during fMRI scanning - measured in terms of corrected cumulative proximity (CCProx) to the goal - showed that a subgroup of older good performers attained comparable levels of search accuracy to the young while another subgroup of older poor performers exhibited consistently lower levels of search accuracy than both older good performers and the young. With regard to brain activations, young adults exhibited greater activations in the cerebellum and cuneus than all older adults, as well as older poor performers. Older good performers exhibited higher activation than older poor performers in the orbitofrontal cortex (BA 10/11), as well as in the cuneus and cerebellum. Brain-behavior correlations further showed that activations in regions involved in visuomotor control (cerebellum, lingual gyrus) and egocentric spatial processing (premotor cortex, precuneus) correlated positively with search accuracy (i.e., closer proximity to goal) in all participants. Notably, activations in the anterior hippocampus correlated positively with search accuracy (CCProx inversed) in the young but not in the old. Taken together, these findings implicated the orbitofrontal cortex and the cerebellum as playing crucial roles in executive and visuospatial processing in older adults, supporting the proposal of an age-related compensatory shift in spatial memory functions away from the hippocampus toward the prefrontal cortex.

Pathophysiological Role of TRPM2 in Age-Related Cognitive Impairment in Mice

Aging causes various functional changes, including cognitive impairment and inflammatory responses in the brain. Transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable channel expressed abundantly in immune cells, exacerbates inflammatory responses. Previously, we reported that TRPM2 on resident microglia plays a critical role in exacerbating inflammation, white matter injury, and cognitive impairment during chronic cerebral hypoperfusion; however, the physiological or pathophysiological role of TRPM2 during age-associated inflammatory responses remains unclear. Therefore, we examined the effects of TRPM2 deletion in young (2-3 months) and older (12-24 months) mice. Compared with young wild-type (WT) mice, middle-aged (12-16 months) WT mice showed working and cognitive memory dysfunction and aged (20-24 months) WT mice exhibited impaired spatial memory. However, these characteristics were not seen in TRPM2 knockout (TRPM2-KO) mice. Consistent with the finding of cognitive impairment, aged WT mice exhibited white matter injury and hippocampal damage and an increase in the number of Iba1-positive cells and amounts of pro-inflammatory cytokines in the brain; these characteristics were not seen in TRPM2-KO mice. These findings suggest that TRPM2 plays a critical role in exacerbating inflammatory responses and cognitive dysfunction during aging.

Tripchlorolide May Improve Spatial Cognition Dysfunction and Synaptic Plasticity after Chronic Cerebral Hypoperfusion

Chronic cerebral hypoperfusion (CCH) is a common pathophysiological mechanism that underlies cognitive decline and degenerative processes in dementia and other neurodegenerative diseases. Low cerebral blood flow (CBF) during CCH leads to disturbances in the homeostasis of hemodynamics and energy metabolism, which in turn results in oxidative stress, astroglia overactivation, and synaptic protein downregulation. These events contribute to synaptic plasticity and cognitive dysfunction after CCH. Tripchlorolide (TRC) is an herbal compound with potent neuroprotective effects. The potential of TRC to improve CCH-induced cognitive impairment has not yet been determined. In the current study, we employed behavioral techniques, electrophysiology, Western blotting, immunofluorescence, and Golgi staining to investigate the effect of TRC on spatial learning and memory impairment and on synaptic plasticity changes in rats after CCH. Our findings showed that TRC could rescue CCH-induced spatial learning and memory dysfunction and improve long-term potentiation (LTP) disorders. We also found that TRC could prevent CCH-induced reductions in N-methyl-D-aspartic acid receptor 2B, synapsin I, and postsynaptic density protein 95 levels. Moreover, TRC upregulated cAMP-response element binding protein, which is an important transcription factor for synaptic proteins. TRC also prevented the reduction in dendritic spine density that is caused by CCH. However, sham rats treated with TRC did not show any improvement in cognition. Because CCH causes disturbances in brain energy homeostasis, TRC therapy may resolve this instability by correcting a variety of cognitive-related signaling pathways. However, for the normal brain, TRC treatment led to neither disturbance nor improvement in neural plasticity. Additionally, this treatment neither impaired nor further improved cognition. In conclusion, we found that TRC can improve spatial learning and memory, enhance synaptic plasticity, upregulate the expression of some synaptic proteins, and increase the density of dendritic spines. Our findings suggest that TRC may be beneficial in the treatment of cognitive impairment induced by CCH.

Higher Levels of Protein Palmitoylation in the Frontal Cortex across Aging Were Associated with Reference Memory and Executive Function Declines

Cognitive decline with aging is often due to altered levels of protein expression. The NMDA receptor (NMDAR) and the complex of proteins surrounding the receptor are susceptible to age-related changes in expression. In the frontal cortex of aged mice, there is a significant loss of expression of the GluN2B subunit of the NMDAR, an increase in Fyn expression, and no change in PSD-95. Studies have also found that, in the frontal cortex, phosphorylation of GluN2B subunits and palmitoylation of GluN2 subunits and NMDAR complex proteins are affected by age. In this study, we examined some of the factors that may lead to the differences in the palmitoylation levels of NMDAR complex proteins in the frontal cortex of aged animals. The Morris water maze was used to test spatial learning in 3- and 24-month-old mice. The acyl-biotinyl exchange method was used to precipitate palmitoylated proteins from the frontal cortices and hippocampi of the mice. Additionally, brain lysates from old and young mice were probed for the expression of fatty acid transporter proteins. An age-related increase of palmitoylated GluN2A, GluN2B, Fyn, PSD-95, and APT1 (acyl protein thioesterase 1) in the frontal cortex was associated with poorer reference memory and/or executive functions. These data suggest that there may be a perturbation in the palmitoylation cycle in the frontal cortex of aged mice that contributes to age-related cognitive declines.

DISC1 regulates N-methyl-D-aspartate receptor dynamics: abnormalities induced by a Disc1 mutation modelling a translocation linked to major mental illness

The neuromodulatory gene DISC1 is disrupted by a t(1;11) translocation that is highly penetrant for schizophrenia and affective disorders, but how this translocation affects DISC1 function is incompletely understood. N-methyl-D-aspartate receptors (NMDAR) play a central role in synaptic plasticity and cognition, and are implicated in the pathophysiology of schizophrenia through genetic and functional studies. We show that the NMDAR subunit GluN2B complexes with DISC1-associated trafficking factor TRAK1, while DISC1 interacts with the GluN1 subunit and regulates dendritic NMDAR motility in cultured mouse neurons. Moreover, in the first mutant mouse that models DISC1 disruption by the translocation, the pool of NMDAR transport vesicles and surface/synaptic NMDAR expression are increased. Since NMDAR cell surface/synaptic expression is tightly regulated to ensure correct function, these changes in the mutant mouse are likely to affect NMDAR signalling and synaptic plasticity. Consistent with these observations, RNASeq analysis of the translocation carrier-derived human neurons indicates abnormalities of excitatory synapses and vesicle dynamics. RNASeq analysis of the human neurons also identifies many differentially expressed genes previously highlighted as putative schizophrenia and/or depression risk factors through large-scale genome-wide association and copy number variant studies, indicating that the translocation triggers common disease pathways that are shared with unrelated psychiatric patients. Altogether, our findings suggest that translocation-induced disease mechanisms are likely to be relevant to mental illness in general, and that such disease mechanisms include altered NMDAR dynamics and excitatory synapse function. This could contribute to the cognitive disorders displayed by translocation carriers.

Hydrogen Sulfide Reverses Aging-Associated Amygdalar Synaptic Plasticity and Fear Memory Deficits in Rats

As an endogenous neuromodulator, hydrogen sulfide (H₂S) exerts multiple biological effects in the brain. Previous studies have shown that H₂S is involved in the regulation of neural synaptic plasticity and cognition in healthy rodents. It is well known that there is a progressive decline of cognitive function that occurs with increased age. The purpose of this study was to investigate the role of H₂S in aging-associated amygdalar synaptic plasticity and cued fear memory deficits as well as to explore the underlying mechanisms. We found that H₂S levels in the amygdala were significantly lower in aged rats when compared with healthy adult rates, which displayed significant deficits in long-term potentiation (LTP) in the thalamo-lateral amygdala (LA) pathway and amygdala-dependent cued fear memory. Bath application of an H₂S donor, sodium hydrogen sulfide (NaHS), significantly reversed the impaired LTP in brain slices from aged rats, and intra-LA infusion of NaHS restored the cued fear memory in aged rats. Mechanismly, we found that H₂S treatment significantly enhanced NMDAR-mediated synaptic responses in the thalamo-LA pathway of aged rats. Notably, GluN2B-containing NMDARs, but not GluN2A-containing NMDARs, contributed to the effects of H₂S on aging-associated impairments of amygdalar LTP and fear memory, because applying GluN2B antagonist could abolish the beneficial effects of NaHS treatment on amygdalar LTP and cognitive performance in aged rats. Collectively, these results show that H₂S can reverse aging-associated amygdalar synaptic plasticity and fear memory deficits by restoring the function of GluN2B-containing NMDARs, suggesting that supplement of H₂S might be a therapeutic approach for aging-related cognitive disorders.

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.

Disease-associated missense mutations in GluN2B subunit alter NMDA receptor ligand binding and ion channel properties

Genetic and bioinformatic analyses have identified missense mutations in GRIN2B encoding the NMDA receptor GluN2B subunit in autism, intellectual disability, Lennox Gastaut and West Syndromes. Here, we investigated several such mutations using a near-complete, hybrid 3D model of the human NMDAR and studied their consequences with kinetic modelling and electrophysiology. The mutants revealed reductions in glutamate potency; increased receptor desensitisation; and ablation of voltage-dependent Mg2+ block. In addition, we provide new views on Mg2+ and NMDA channel blocker binding sites. We demonstrate that these mutants have significant impact on excitatory transmission in developing neurons, revealing profound changes that could underlie their associated neurological disorders. Of note, the NMDAR channel mutant GluN2BV618G unusually allowed Mg2+ permeation, whereas nearby N615I reduced Ca2+ permeability. By identifying the binding site for an NMDAR antagonist that is used in the clinic to rescue gain-of-function phenotypes, we show that drug binding may be modified by some GluN2B disease-causing mutations.

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.

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.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

[A new strategy for preventive and functional therapeutic methods for dementia--approach using natural products]

Alzheimer's disease (AD) has become a serious social problem in Japan. However, effective preventive and fundamental therapeutic methods for AD have not yet been developed. Using a new strategy in the course of our survey of numerous natural resouces having neurotrophic activity, we isolated a variety of active constituents and proved their pharmacological properties. As a result, we successfully found nobiletin, a compound with anti-dementia activity that comes from citrus peels. Also, we have demonstrated that nobiletin ameliorates cognitive impairment in several dementia model animals such as chronically amyloid β(Aβ) infused rats, amyloid precursor protein transgenic (APPTg) mice, olfactory-bulbectomized (OBX) mice, N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801)-treated mice, senescence-accelated mice and bilaterial common carotid arteries occlusion mice. In a APPTg mouse of AD, nobiletin greatly improved memory impairment, and this was accompanied by a marked decrease in Aβ deposition. Also, in OBX mice memory impairment was markedly recoverd by nobiletin, accompanied by improvement of a decrease indensity of cholinergic neurons. Interestingly, nobiletin improves age-related congnitive impairment and decreased hyperphosphorylation of tau as well as oxidative stress in senescence-accelerated mice. In cultured cells, nobiletin reversed the Aβ-induced inhibition of glutamate-induced increases in cAMP response element binding protein (CREB) phosphorylation and modulated gen expression of thioredoxin-interacting protein and NMDA resceptor subunits. These results suggest that nobiletin prevents memory impairment and exhibits a protecting action against neurodgeneration in AD model animals. Nobiletin and citrus peels thus have potential as functional foods for prevention of dementia.

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.

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.

Soluble epoxide hydrolase inhibitor enhances synaptic neurotransmission and plasticity in mouse prefrontal cortex

Background

The soluble epoxide hydrolase (sEH) is an important enzyme chiefly involved in the metabolism of fatty acid signaling molecules termed epoxyeicosatrienoic acids (EETs). sEH inhibition (sEHI) has proven to be protective against experimental cerebral ischemia, and it is emerging as a therapeutic target for prevention and treatment of ischemic stroke. However, the role of sEH on synaptic function in the central nervous system is still largely unknown. This study aimed to test whether sEH C-terminal epoxide hydrolase inhibitor, 12-(3-adamantan-1-yl-ureido) dodecanoic acid (AUDA) affects basal synaptic transmission and synaptic plasticity in the prefrontal cortex area (PFC). Whole cell and extracellular recording examined the miniature excitatory postsynaptic currents (mEPSCs) and field excitatory postsynaptic potentials (fEPSPs); Western Blotting determined the protein levels of glutamate receptors and ERK phosphorylation in acute medial PFC slices.

Results

Application of the sEH C-terminal epoxide hydrolase inhibitor, AUDA significantly increased the amplitude of mEPSCs and fEPSPs in prefrontal cortex neurons, while additionally enhancing long term potentiation (LTP). Western Blotting demonstrated that AUDA treatment increased the expression of the N-methyl-D-aspartate receptor (NMDA) subunits NR1, NR2A, NR2B; the α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR1, GluR2, and ERK phosphorylation.

Conclusions

Inhibition of sEH induced an enhancement of PFC neuronal synaptic neurotransmission. This enhancement of synaptic neurotransmission is associated with an enhanced postsynaptic glutamatergic receptor and postsynaptic glutamatergic receptor mediated synaptic LTP. LTP is enhanced via ERK phosphorylation resulting from the delivery of glutamate receptors into the PFC by post-synapse by treatment with AUDA. These findings provide a possible link between synaptic function and memory processes.

Anti-dementia Activity of Nobiletin, a Citrus Flavonoid: A Review of Animal Studies

Alzheimer's disease (AD), the most common form of dementia among the elderly, is characterized by the progressive decline of cognitive function and has a detrimental impact worldwide. Despite intensive laboratory and clinical research over the last three decades, pharmacological options for the prevention and effective long-term treatment of AD are not currently available. Consequently, successful therapeutic and preventive treatments for AD are needed. When researching materials from natural resources having anti-dementia drug activity, we identified nobiletin, a polymethoxylated flavone from the peel of Citrus depressa. Nobiletin exhibited memory-improving effects in various animal models of dementia and exerted a wide range of beneficial effects against pathological features of AD including amyloid-β (Aβ) pathology, tau hyperphosphorylation, oxidative stress, cholinergic neurodegeneration and dysfunction of synaptic plasticity-related signaling, suggesting this natural compound could become a novel drug for the treatment and prevention of AD.

Memory in aged mice is rescued by enhanced expression of the GluN2B subunit of the NMDA receptor

The GluN2B subunit of the N-methyl-d-aspartate (NMDA) receptor shows age-related declines in expression across the frontal cortex and hippocampus. This decline is strongly correlated to age-related memory declines. This study was designed to determine if increasing GluN2B subunit expression in the frontal lobe or hippocampus would improve memory in aged mice. Mice were injected bilaterally with either the GluN2B vector, containing cDNA specific for the GluN2B subunit and enhanced green fluorescent protein (eGFP); a control vector or vehicle. Spatial memory, cognitive flexibility, and associative memory were assessed using the Morris water maze. Aged mice, with increased GluN2B subunit expression, exhibited improved long-term spatial memory, comparable to young mice. However, memory was rescued on different days in the Morris water maze; early for hippocampal GluN2B subunit enrichment and later for the frontal lobe. A higher concentration of the GluN2B antagonist, Ro 25-6981, was required to impair long-term spatial memory in aged mice with enhanced GluN2B expression, as compared to aged controls, suggesting there was an increase in the number of GluN2B-containing NMDA receptors. In addition, hippocampal slices from aged mice with increased GluN2B subunit expression exhibited enhanced NMDA receptor-mediated excitatory post-synaptic potentials (EPSP). Treatment with Ro 25-6981 showed that a greater proportion of the NMDA receptor-mediated EPSP was due to the GluN2B subunit in these animals, as compared to aged controls. These results suggest that increasing the production of the GluN2B subunit in aged animals enhances memory and synaptic transmission. Therapies that enhance GluN2B subunit expression within the aged brain may be useful for ameliorating age-related memory declines.

Aging of the NMDA receptor: from a mouse's point of view

Our elderly population is growing and declines in cognitive abilities, such as memory, can be costly, because it can interfere with a person's ability to live independently. The NMDA receptor is very important for many different forms of memory and this receptor is negatively affected by aging. This review examines the progress that has been made recently in characterizing selective vulnerabilities of different subunits and splice variants of the NMDA receptor to normal aging in C57BL/6 mice. Evidence is also presented for changes in the relationships of NMDA receptors to plasticity across aging. Recent interventions show that enhancing NMDA receptors in aged individuals is associated with improvements in memory, but mouse models of neurodegenerative diseases suggest that finding the right balance between too little and too much NMDA receptor activity will be the key to enhancing memory without inducing pathology.