Reference Memory Deficits Research Articles

Class 1 histone deacetylases differentially modulate memory and synaptic genes in a spatial and temporal manner in aged and APP/PS1 mice

Epigenetics plays a vital role in aging and Alzheimer’s disease (AD); however, whether epigenetic alterations during aging can initiate AD and exacerbate AD progression remains unclear. In this study, using 3-, 12- and 18- month-old APP/PS1 mice and age matched WT littermates, we conducted a series of memory tests, measured synapse-related gene expression, class 1 histone deacetylases (HDACs) abundance, and H3K9ac levels at target gene promoters in the hippocampus and prefrontal cortex (PFC). Our results showed impaired recognition and long-term spatial memory in 18-month-old WT mice and impaired recognition, short-term working, and long-term spatial reference memory in 12-and 18- month-old APP/PS1 mice. These memory impairments are associated with changes of synapse-related gene (nr2a, glur1, glur2, psd95) expression, HDAC abundance, and H3K9ac levels; more specifically, increased HDAC2 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during aging and AD progression in the hippocampus. Conversely, increased HDAC3 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during AD progression in the PFC. These findings suggest memory impairments in aging and AD may associated with a differential HDAC modulation of synapse-related gene expression in the brain.

Search strategy analysis reveals sex‐specific deficits in spatial reference memory in 5xFAD mice

AbstractBackgroundSpatial disorientation is one of the earliest symptoms in Alzheimer’s disease. Search strategy classification systems to study navigational deficits in Alzheimer disease have been developed but aren’t widely available. Hence, we studied the navigational strategies in the 5xFAD murine model and evaluated differences between sexes.MethodWe studied spatial reference memory in 3‐month‐, 7‐month‐ and 12‐month‐old female and male 5xFAD mice and their aged‐matched controls in the Morris water maze. Furthermore, a detailed analysis of the swimming strategies using “Pathfinder” was performed. Navigational strategies were classified in spatial (“direct path”, “directed search”, “focal search”, “indirect search”) and non‐spatial categories (“chaining”, “scanning”, “random search”, thigmotaxis”).ResultEscape latency was prolonged in 5xFAD mice compared to WT, only at 7 and 12 months, with greater magnitude in females than males. Regarding search strategies during acquisition training, the most common spatial strategy was “indirect search” whereas “random search” was the most frequent non‐spatial strategy. At 3 months, no clear difference was seen between 5xFAD and WT animals. However, at 7 months, 5xFAD mice used more frequently non‐spatial strategies. Although transgenic mice progressively shifted to more spatial strategies as they learned (mainly “indirect search”), WT animals applied more frequently spatial strategies with higher cognitive value such as “direct path”, “directed search” and “focal search”. In comparison, 12 month‐old 5xFAD mice used more non‐spatial strategies, mainly “random search”, than the WT and their transition to “indirect search” occurred more rarely than in younger mice. Overall, all animals increased the use of non‐spatial strategies as they aged but this increment was greater and earlier in 5xFAD females than in 5xFAD males. Lastly, in the probe trial, 5xFAD female mice at 3 and 12 months applied “random search” more often, while the WT chose “indirect search” with higher frequency. No difference was seen in males during probe trial.ConclusionConventional behavior analysis of escape latencies revealed spatial reference memory deficits in 7m and 12m 5XFAD mice. Deficits in spatial memory were more pronounced in female 5XFAD mice than in male mice. 5XFAD mice, regardless of sex, used a qualitatively and quantitatively different search strategy pattern than WT animals.

Astrocytic A2A receptors silencing negatively impacts hippocampal synaptic plasticity and memory of adult mice.

Astrocytes are wired to bidirectionally communicate with neurons namely with synapses, thus shaping synaptic plasticity, which in the hippocampus is considered to underlie learning and memory. Adenosine A2A receptors (A2A R) are a potential candidate to modulate this bidirectional communication, since A2A R regulate synaptic plasticity and memory and also control key astrocytic functions. Nonetheless, little is known about the role of astrocytic A2A R in synaptic plasticity and hippocampal-dependent memory. Here, we investigated the impact of genetic silencing astrocytic A2A R on hippocampal synaptic plasticity and memory of adult mice. The genetic A2A R silencing in astrocytes was accomplished by a bilateral injection into the CA1 hippocampal area of a viral construct (AAV5-GFAP-GFP-Cre) that inactivate A2A R expression in astrocytes of male adult mice carrying "floxed" A2A R gene, as confirmed by A2A R binding assays. Astrocytic A2A R silencing alters astrocytic morphology, typified by an increment of astrocytic arbor complexity, and led to deficits in spatial reference memory and compromised hippocampal synaptic plasticity, typified by a reduction of LTP magnitude and a shift of synaptic long-term depression (LTD) toward LTP. These data indicate that astrocytic A2A R control astrocytic morphology and influence hippocampal synaptic plasticity and memory of adult mice in a manner different from neuronal A2A R.

A Comparative Study of the Impact of NO-Related Agents on MK-801- or Scopolamine-Induced Cognitive Impairments in the Morris Water Maze

Learning and memory deficits accompany numerous brain dysfunctions, including schizophrenia and Alzheimer's disease (AD), and many studies point to the role of nitric oxide (NO) in these processes. The present investigations constitute the follow-up of our previous research, in which we investigated the activity of NO releasers and a selective inhibitor of neuronal NO synthase (nNOS) to prevent short-term memory deficits in novel object recognition and T-maze. Here, the ability of the compounds to prevent the induction of long-term memory deficits by MK-801 or scopolamine administration was investigated. The Morris Water Maze test, a reliable and valid test of spatial learning and memory, was used, in which escape latency in the acquisition phase and nine different parameters in the retention phase were measured. A fast NO releaser (spermine NONOate), a slow NO releaser (DETA NONOate), and a nNOS inhibitor, N(ω)-propyl-L-arginine (NPLA), were used. The compounds were administered i.p. at a dose range of 0.05-0.5 mg/kg. All compounds prevented learning deficits in the acquisition phase and reversed reference memory deficits in the retention phase of the scopolamine-treated mice. Spermine NONOate was the least effective. In contrast, the drugs poorly antagonised MK-801-induced deficits, and only the administration of DETA NONOate induced some improvements in the retention trial.

Search Strategy Analysis of 5xFAD Alzheimer Mice in the Morris Water Maze Reveals Sex- and Age-Specific Spatial Navigation Deficits

Spatial disorientation and navigational impairments are not only some of the first memory deficits in Alzheimer's disease, but are also very disease-specific. In rodents, the Morris Water Maze is used to investigate spatial navigation and memory. Here, we examined the spatial memory in the commonly used 5xFAD Alzheimer mouse model in a sex- and age-dependent manner. Our findings show first spatial learning deficits in 7-month-old female 5xFAD and 12-month-old male 5xFAD mice, respectively. While the assessment of spatial working memory using escape latencies provides a global picture of memory performance, it does not explain how an animal solves a spatial task. Therefore, a detailed analysis of swimming strategies was performed to better understand the behavioral differences between 5xFAD and WT mice. 5xFAD mice used a qualitatively and quantitatively different search strategy pattern than wildtype animals that used more non-spatial strategies and showed allocentric-specific memory deficits. Furthermore, a detailed analysis of swimming strategies revealed allocentric memory deficits in the probe trial in female 3-month-old and male 7-month-old 5xFAD animals before the onset of severe reference memory deficits. Overall, we could demonstrate that spatial navigation deficits in 5xFAD mice are age- and sex-dependent, with female mice being more severely affected. In addition, the implementation of a search strategy classification system allowed an earlier detection of behavioral differences and therefore could be a powerful tool for preclinical drug testing in the 5xFAD model.

Isoniazid improves cognitive performance, clears Aβ plaques, and protects dendritic synapses in APP/PS1 transgenic mice.

Alzheimer's disease (AD) is characterized by amyloid β (Aβ) aggregation and neuroinflammation. This study aimed to investigate the therapeutic effect of isoniazid (INH) against AD. The APP/PS1 transgenic mouse model of AD was adopted. The APP/PS1 mice received oral INH (45 mg/kg/d) for 14 days. The cognitive capability was assessed by the Morris Water Maze test. Amyloid plaques and Aβ levels were determined by immunohistochemistry and ELISA assay. The dendritic spines were analyzed by DiOlistic labeling. Immunofluorescence staining was used to observe the microglia and astrocytes. The Morris Water Maze test suggested that INH administration can effectively attenuate the reference memory deficit and improve the working memory of the APP/PS1 mice compared to the untreated mice (all p < 0.001). INH significantly decreased the Aβ plaques in the hippocampus and cortex and reduced the levels of Aβ1-40 and Aβ1-42 in the brain homogenates, cerebrospinal fluid, and serum (all p < 0.001). INH also inhibited enzyme activities of β-site amyloid precursor protein cleaving enzyme 1 (BACE1, p < 0.05) and monoamine oxidase B (Mao-b, p < 0.01). INH significantly increased the protrusion density in the hippocampus (p < 0.01). Immunofluorescence staining revealed that INH significantly reduced the number of activated microglia and astrocytes around the Aβ plaques (both p < 0.01). Isoniazid administration effectively improved cognitive performance, cleared Aβ plaques, protected dendritic synapses, and reduced innate immune cells around the Aβ plaques, suggesting that INH could be a potential drug for AD treatment.

Acetylcholine and noradrenaline differentially regulate hippocampus-dependent spatial learning and memory.

Severe loss of cholinergic neurons in the basal forebrain nuclei and of noradrenergic neurons in the locus coeruleus are almost invariant histopathological hallmarks of Alzheimer's disease. However, the role of these transmitter systems in the spectrum of cognitive dysfunctions typical of the disease is still unclear, nor is it yet fully known whether do these systems interact and how. Selective ablation of either neuronal population, or both of them combined, were produced in developing animals to investigate their respective and/or concurrent contribution to spatial learning and memory, known to be severely affected in Alzheimer's disease. Single or double lesions were created in 4-8 days old rats by bilateral intraventricular infusion of two selective immunotoxins. At about 16 weeks of age, the animals underwent behavioural tests specifically designed to evaluate reference and working memory abilities, and their brains were later processed for quantitative morphological analyses. Animals with lesion to either system alone showed no significant reference memory deficits which, by contrast, were evident in the double-lesioned subjects. These animals could not adopt an efficient search strategy on a given testing day and were unable to transfer all relevant information to the next day, suggesting deficits in acquisition, storage and/or recall. Only animals with single noradrenergic or double lesions exhibited impaired working memory. Interestingly, ablation of cholinergic afferents to the hippocampus stimulated a robust ingrowth of thick fibres from the superior cervical ganglion which, however, did not appear to have contributed to the observed cognitive performance. Ascending cholinergic and noradrenergic afferents to the hippocampus and neocortex appear to be primarily involved in the regulation of different cognitive domains, but they may functionally interact, mainly at hippocampal level, for sustaining normal learning and memory. Moreover, these transmitter systems are likely to compensate for each other, but apparently not via ingrowing sympathetic fibres.

Effects of Histone Deacetylase Inhibitors on Different Memory Domains in an Animal Model of Aging and Alzheimer’s Disease

AbstractBackgroundHistone deacetylases (HDACs) have been involved in regulation of memory and previous work from our group and others have shown promising beneficial effects of HDAC inhibitors (HDACis) in aging and neurodegenerative disorders, including Alzheimer’s disease. However, currently, there are no reports on cross comparison of none‐selective and selective HDACis for their beneficial effects on memory and AD‐like pathologies in both aging and AD models.MethodIn this study, 3‐, 12‐, and 18‐months of age WT and APP/PS1 mice were administered a broad acting HDACi valproic acid (VPA), and two selective Class 1 HDACis entinostat (MS‐275, targeting HDAC 1 and 3) and tacedinaline (CI‐994, targeting HDAC 1, 2 and 3) for 30 days. After HDACi administration, mice underwent three memory tests: the novel object recognition (NOR) for executive memory, Y‐maze for short‐term working memory, and Morris water maze (MWM) for long‐term spatial reference memory (n = 8‐10). After memory assessment, mouse prefrontal cortex and hippocampus were collected for biochemical studies.ResultWe found a decrease in executive memory, and long‐term spatial reference memory in WT mice at 18 months of age and, severe memory deficits of three memory domains in APP/PS1 mice at 12‐ and 18‐months of age, and most importantly, we found an age and genotype interaction in the severity of executive and long‐term spatial reference memory deficits in APP/PS1 mice. Both MS‐275 and CI‐994 rescued executive memory, and short‐term spatial reference memory, however, only CI‐994 was sufficient to rescue long‐term spatial reference memory in APP/PS1 mice. We also found a decrease in H3K9ac in APP/PS1 mice with aging at the gene promoters related to memory and synaptic plasticity, however, only CI‐994 rescued H3K9ac levels in APP/PS1 mice.ConclusionOur results suggest that selective HDACis could rescue memory function in APP/PS1 mice, and HDAC2 may be an important modulator for hippocampal dependent memory. We are continuing our investigation of changes of specific HDAC and histone acetylation marks on genes related to memory and synaptic plasticity in the hippocampus, but also in the PFC to identify regional specific of HDAC modulation across aging and AD.

A Role for PGC-1a in the Control of Abnormal Mitochondrial Dynamics in Alzheimer's Disease.

Emerging evidence suggests that the proper control of mitochondrial dynamics provides a window for therapeutic intervention for Alzheimer’s disease (AD) progression. The transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator 1 (PGC-1a) has been shown to regulate mitochondrial biogenesis in neurons. Thus far, the roles of PGC-1a in Alzheimer’s disease and its potential value for restoring mitochondrial dysfunction remain largely unknown. In the present study, we explored the impacts of PGC-1a on AD pathology and neurobehavioral dysfunction and its potential mechanisms with a particular focus on mitochondrial dynamics. Paralleling AD-related pathological deposits, neuronal apoptosis, abnormal mitochondrial dynamics and lowered membrane potential, a remarkable reduction in the expression of PGC-1a was shown in the cortex of APP/PS1 mice at 6 months of age. By infusing AAV-Ppargc1α into the lateral parietal association (LPtA) cortex of the APP/PS1 brain, we found that PGC-1a ameliorated AD-like behavioral abnormalities, such as deficits in spatial reference memory, working memory and sensorimotor gating. Notably, overexpressed PGC-1a in LPtA rescued mitochondrial swelling and damage in neurons, likely through correcting the altered balance in mitochondrial fission–fusion and its abnormal distribution. Our findings support the notion that abnormal mitochondrial dynamics is likely an important mechanism that leading to mitochondrial dysfunction and AD-related pathological and cognitive impairments, and they indicate the potential value of PGC-1a for restoring mitochondrial dynamics as an innovative therapeutic target for AD.

Electroacupuncture modulates gut microbiota in mice: A potential target in postoperative cognitive dysfunction.

The detailed mechanism of inflammation in postoperative cognitive dysfunction (POCD) is unclear. This study aimed to determine whether electroacupuncture (EA) ameliorates POCD by modulating gut microbial dysbiosis. Compared to the control group, mice in the EA group were treated at the acupoints Zusanli (ST36), Quchi (L111), Baihui (GV20), and Dazhui (GV14) 1 week before appendectomy. Novel object recognition and the Morris water maze tests were used to assess learning and spatial reference memory deficits, whereas hippocampus samples and stool samples were collected for central inflammatory tests and 16S-rRNA sequencing of intestinal flora, respectively. In amyloid precursor protein/presenilin 1 (APP/PS1) mice, EA enhanced spatial memory and learning deficits. The fecal microbial community was altered in APP/PS1 mice in the absence of EA following surgery. Among them, Coprococcus and Bacteroidetes were more abundant in the EA groups than in the control groups; however, Actinobacteriota, Helicobacteraceae, and Escherichia/shigella constitute the minor bacterial colonization in the EA groups. Furthermore, we found a significant negative correlation between Firmicutes and escape latency (Pearson correlation coefficient - 0.551, p < 0.01) and positive correlation between Proteobacteria and escape latency (Pearson correlation coefficient 0.462, p < 0.05). Electron microscopy revealed signs of blood-brain barrier (BBB) impairments and immunofluorescence images showed glial cells activated in the hippocampus of APP/PS mice without EA, and serum diamine oxidase levels were increased in these mice; whereas EA treatment significantly relieved the above pathological changes. Our findings implied that EA decreases hippocampal inflammation ofAPP/PS1 by upregulating benificial gut microbiota, reducing BBB and intestinal barrier dysfunction, thus alleviates postoperative cognitive dysfunction. This may provide a novel target in POCD management.

Search strategy analysis of Tg4-42 Alzheimer Mice in the Morris Water Maze reveals early spatial navigation deficits

Spatial disorientation is one of the earliest symptoms in Alzheimer’s disease and allocentric deficits can already be detected in the asymptomatic preclinical stages of the disease. The Morris Water Maze (MWM) is used to study spatial learning in rodent models. Here we investigated the spatial memory of female 3, 7 and 12 month-old Alzheimer Tg4-42 mice in comparison to wild-type control animals. Conventional behavior analysis of escape latencies and quadrant preference revealed spatial memory and reference memory deficits in female 7 and 12 month-old Tg4-42 mice. In contrast, conventional analysis of the MWM indicated an intact spatial memory in 3 month-old Tg4-42 mice. However, a detailed analysis of the swimming strategies demonstrated allocentric-specific memory deficits in 3 month-old Tg4-42 mice before the onset of severe memory deficits. Furthermore, we could show that the spatial reference memory deficits in aged Tg4-42 animals are caused by the lack of allocentric and spatial strategies. Analyzing search strategies in the MWM allows to differentiate between hippocampus-dependent allocentric and hippocampus-independent egocentric search strategies. The spatial navigation impairments in young Tg4-42 mice are well in line with the hypometabolism and synaptic deficits in the hippocampus. Therefore, analyzing search strategies in the Tg4-42 model can be a powerful tool for preclinical drug testing and identifying early therapeutic successes.

Postnatal GABAA Receptor Activation Alters Synaptic Plasticity and Cognition in Adult Wistar Rats.

Early life alteration in the activity of gamma-aminobutyric acid (GABA) receptors is associated with long-lasting developmental effects on the brain and behavior. GABAA receptors act as excitatory rather than inhibitory in neonates. Excessive activation of GABAA receptors during the early postnatal period may affect cognitive functions later in life. In this study, we sought to determine whether neonatal activation of GABAA receptors with muscimol can alter the electrophysiology profile of hippocampal CA1 neurons and spatial learning and memory in adult rats. Male and female Wistar rat pups received a subcutaneous injection of either saline or muscimol (500µg/kg) on postnatal days (PND) 7, 9, and 11 and then underwent different electrophysiology and behavioral experiments in adulthood. Early life treatment with muscimol did not alter the basic synaptic transmission but significantly reduced the paired-pulse facilitation (PPF) in the CA1 area. Neonatal application of muscimol led to a pronounced decrease in long-term potentiation (LTP) and long-term depression (LTD) in CA1 neurons along with a declined theta-burst responses in both sexes. We obtained some evidence that neonatal GABAA activation leads to reduced brain-derived neurotrophic factor (BDNF) in the hippocampus and prefrontal cortex. Our electrophysiology data was supported with spatial reference and working memory deficits in rats. This study provides the first detailed description of altered electrophysiology in hippocampal CA1 neurons in adult rats undergone GABAA activation early in life.

Microglial Inflammation and Cognitive Dysfunction in Comorbid Rat Models of Striatal Ischemic Stroke and Alzheimer’s Disease: Effects of Antioxidant Catalase-SKL on Behavioral and Cellular Pathology

Ischemic stroke often co-occurs with Alzheimer’s disease (AD) leading to a worsened clinical outcome. Neuroinflammation is a critical process implicated in AD and ischemic pathology, associated with cognitive decline. We sought to investigate the combined effects of ischemic stroke induced by endothelin-1 injection in two AD rat models, using motor function, memory and microglial inflammation in the basal forebrain and striatum as readouts. In addition, we sought to determine the effectiveness of the antioxidant biologic CAT-SKL in one of the models. The early AD model employed the bilateral intracerebroventricular injections of the toxic β-amyloid peptide Aβ25–35, the prodromal AD model used the transgenic Fischer 344 rat overexpressing a pathological mutant human amyloid precursor protein. Motor function was assessed using a cylinder, modified sticky tape and beam-walk tasks; learning and memory were tested in the Morris water maze. Microglial activation was examined using immunohistochemistry. Aβ25–35 toxicity and stroke combination greatly increased microglial inflammation in the basal forebrain. Prodromal AD-pathology coupled with ischemia in the transgenic rat resulted in a greater microgliosis in the striatum. Combined transgenic rats showed balance alterations, comorbid Aβ25–35 rats showed a transient sensorimotor deficit, and both demonstrated spatial reference memory deficit. CAT-SKL treatment ameliorated memory impairment and basal forebrain microgliosis in Aβ25–35 rats with stroke. Our results suggest that neuroinflammation could be one of the early processes underlying the interaction of AD with stroke and contributing to the cognitive impairment, and that therapies such as antioxidant CAT-SKL could be a potential therapeutic strategy.

Abstract WMP18: Protection Against Dementia In Mouse VCID By Repetitive Hypoxic Conditioning: Cortical Proteomics Defines Disease And Disease Resilient Phenotypes

Introduction: Vascular cognitive impairment and dementia (VCID) exerts huge morbidity costs, and efficacious therapies are lacking. In a well-established mouse model of VCID, we recently documented in vivo an epigenetically-induced resilience to reference memory deficits by 2 mo of repetitive hypoxic conditioning (RHC) treatment prior to bilateral carotid artery stenosis-induced chronic cerebral hypoperfusion (CCH). The present study was undertaken to define the phenotypic basis of this cognition-protective effect. Methods: Samples of prefrontal cortex from untreated and RHC-treated mice were obtained after 4 mo of CCH, along with samples from age-matched sham-CCH controls, and analyzed by mass spectrometry using TMTpro labels for quantification. Differentially expressed proteins were bioinformatically analyzed using IPA (Qiagen), with rigid thresholds/filters for fold-change, z-scores, and statistical significance applied. Results: 5071 proteins were identified - 3975 of which were quantified - in the 3 experimental groups. Relative to the cortical proteome of controls, the cortex of memory-impaired CCH mice exhibited a downregulation of proteins involved in Nrf2 signaling and synaptogenesis signaling pathways, and an increase of proteins and related proinflammatory mediators participating in leukocyte extravasation signaling. In addition, gamma secretase levels were increased. Treatment with RHC prior to CCH abrogated the aforementioned Nrf2, synaptogenesis, and leukocyte signaling changes, increased proteins driving oxidative phosphorylation, Rho-mediated CXCR4 signaling, DNA repair, and diverse cell survival networks, and reduced protein enrichments in both apoptosis and necrosis signaling, and reactive oxygen species generating, pathways. Levels of both amyloid precursor protein and amyloid-beta levels, as well as tau, were also lower in RHC-treated, dementia-resilient mice. Conclusions: These novel findings reveal potential mechanistic insights into relatively early changes in phenotype that characterize VCID and, in turn, how this phenotype is modified by an efficacious epigenetic therapeutic. Causal studies designed to confirm these multifactorial mechanisms of cognitive protection are warranted.

Mulberry fruit extract alleviates the intracellular amyloid-β oligomer-induced cognitive disturbance and oxidative stress in Alzheimer's disease model mice.

Intracellular amyloid-β (Aβ) oligomers are key therapeutic targets because they are strongly cytotoxic and play crucial roles in the cognitive function in Alzheimer's disease (AD). Anthocyanins, polyphenolic flavonoids with antioxidant and neuroprotective properties, are potential therapeutic candidates for AD. Here, we investigated the effects of anthocyanin-enriched extracts from fruits of mulberry (Morus alba Linn.) in Thailand against the neurotoxicity of Aβ oligomers. Using the monitoring system for Aβ aggregation, we showed that the extract induced the dissociation of Aβ in cultured HEK293T cells. To investigate the effects on cognitive function, we orally administered the extract to Aβ-GFP transgenic mice (Aβ-GFP Tg), a mouse model that expresses Aβ oligomers inside neurons, and performed the novel object recognition test and passive avoidance test. Aβ-GFP Tg usually showed deficits in novel object recognition memory and reference memory compared with non-Tg, but administration of the extract improved both compared with vehicle-treated Aβ-GFP Tg. Aβ-GFP Tg exhibited lower superoxide dismutase (SOD) activity than non-Tg. However, after the administration of the extract, the SOD activity was restored. These results suggest that Thai mulberry fruit extract ameliorates cytotoxicity induced by the intracellular Aβ oligomers and may be an effective therapeutic or preventive candidate for AD.

Norepinephrine as a spatial memory reset signal.

Contextual information is represented in the hippocampus (HPC) partially through the recruitment of distinct neuronal ensembles. It is believed that reactivation of these ensembles underlies memory retrieval processes. Recently, we showed that norepinephrine input from phasic locus coeruleus activation induces hippocampal plasticity resulting in the recruitment of new neurons and disengagement from previously established representations. We hypothesize that norepinephrine may provide a neuromodulatory mnemonic switch signaling the HPC to move from a state of retrieval to encoding in the presence of novelty, and therefore, plays a role in memory updating. Here, we tested whether bilateral dorsal dentate gyrus (dDG) infusions of the β-adrenergic receptor (BAR) agonist isoproterenol (ISO), administered prior to encoding or retrieval, would impair spatial working and reference memory by reverting, the system to encoding (thereby recruiting new neurons) potentially interfering with the retrieval of the previously established spatial ensemble. We also investigated whether dDG infusions of ISO could promote cognitive flexibility by switching the system to encoding when it is adaptive (ie, when new information is presented, eg, reversal learning). We found that intra-dDG infusions of ISO given prior to retrieval caused deficits in working and reference memory which was blocked by pretreatment with the BAR-antagonist, propranolol (PRO). In contrast, ISO administered prior to reversal learning led to improved performance. These data support our hypothesis that norepinephrine serves as a novelty signal to update HPC contextual representations via BAR activation-facilitated recruitment of new neurons. This can be both maladaptive and adaptive depending on the situation.

Aneurysmal subarachnoid hemorrhage survivors show long-term deficits in spatial reference memory in a pilot study of a virtual water maze paradigm

BackgroundLimited data exists on the long-term effects of aneurysmal subarachnoid hemorrhage (SAH) on spatial memory. Herein, we used a computerized virtual water maze to evaluate the feasibility of spatial memory testing in pilot cohort of ten patients who survived previous SAH. MethodsTen SAH survivors (5.8 ± 5.1 years after initial hemorrhage) and 7 age-matched controls underwent testing in a virtual water maze computer program. Additional subgroup analyses were performed to evaluate spatial reference memory correlation for ventricular size on admission, placement of an external ventricular drain and placement of a shunt. ResultsWith respect to the spatial memory acquisition phase, there was no significant difference of pathway length traveled to reach the platform between SAH survivors and control subjects. During the probe trial, control subjects spent significantly longer time in target quadrants compared to SAH survivors (F(3, 24) = 10.32, p = 0.0001; Target vs. Right: Mean percent difference 0.16 [0–0.32], p = 0.045; Target vs. Across: Mean percent difference 0.35 [0.19–0.51], p < 0.0001; Target vs. Left: Mean percent difference 0.21 [0.05–0.37], p = 0.0094). Furthermore, patients who initially presented with smaller ventricles performed worse that those patients who had ventriculomegaly and/or required surgical management of hydrocephalus. ConclusionsOur data demonstrate that SAH survivors have persistent spatial reference memory deficits years after the hemorrhage. Hydrocephalus at presentation and external ventricular drainage were not found to be associated with poor spatial memory outcomes in this pilot cohort. Therefore, other causes such as global cerebral edema or magnitude of initial ICP spike, need to be considered to be examined as root cause as well in subsequent studies. The protocol described in this manuscript is able to demonstrate a spatial reference memory deficit and can be used to study risk factors for spatial memory impairment on a larger scale.

Investigating the Effect of Noise Exposure on Hippocampal‐Dependent Spatial Learning and Memory in a Rat Model

Hearing loss is one of the most prevalent chronic health conditions worldwide, with excessive exposure to loud noise as a leading cause of hearing loss. Beyond the devastating effects of hearing impairment itself, epidemiological studies have identified hearing loss as a major risk factor for cognitive decline. Furthermore, preclinical studies on rodents have identified that the hippocampus—a brain region outside of the classical auditory pathway which subserves spatial navigation—appears to be vulnerable to noise exposure. For example, two hours of exposure to very loud noise (e.g., 123 dB sound pressure level, SPL) has been shown to cause long-term impairment in spatial learning and memory, as well as suppress hippocampal neurogenesis (i.e., the processes by which new neurons are generated from neural stem cells in the adult brain). That said, because these past studies used noise levels that far exceed those frequently experienced by humans who work in noisy environments, it remains uncertain whether a more modest degree of hearing loss, consistent with that caused by daily, occupational noise exposure, is also problematic for normal hippocampal function. In the present study, we are using a rat model to study the effect of daily noise exposure on spatial learning, memory and hippocampal neurogenesis. Following baseline hearing testing using the auditory brainstem response (i.e., an electrophysiologically-measured evoked potential from the brain in response to sound), 6-month old Fischer 344 rats were exposed to 100 dB SPL white noise (or silence) for 4 hours/day for 30 days. Separate cohorts of noise- and sham-exposed rats then underwent behavioral testing at 7, 10, or 13 months old using a Morris water maze (MWM) protocol to assess hippocampal-dependent spatial acquisition learning and reference memory. After completion of the behavioral testing and post-exposure hearing assessments, the rats were sacrificed, and their brains harvested for tissue processing. As predicted, the noise-exposed rats had a mild degree of high-frequency hearing loss, evidenced by a ~25 dB increase in the rats’ hearing threshold to a 20 kHz acoustic stimulus. Overall, both treatment groups demonstrated spatial acquisition learning over the 5 training days on the MWM hidden platform task; however, the noise-exposed rats performed slightly, albeit significantly worse than the shams at 10 months of age. Moreover, in contrast to past studies that used very loud noise exposures, none of the cohorts of noise-exposed rats in the present study were impaired during the MWM probe task; findings which identify that deficits in spatial reference memory are not an inevitable consequence of repeated exposure to sounds that cause a mild degree of hearing loss. Histological analysis of hippocampal neurogenesis is ongoing. Ultimately, given that not all individuals exposed to occupational noise suffer the same degree of hearing loss, it will be important that we correlate each rat's hearing sensitivity with its’ cognitive-behavioral performance and extent of hippocampal neurogenesis, so as to better understand the complex relationship between noise-induced hearing loss, cognitive impairment and neuropathology.

Role of mTOR-Regulated Autophagy in Synaptic Plasticity Related Proteins Downregulation and the Reference Memory Deficits Induced by Anesthesia/Surgery in Aged Mice.

Postoperative cognitive dysfunction increases mortality and morbidity in perioperative patients and has become a major concern for patients and caregivers. Previous studies demonstrated that synaptic plasticity is closely related to cognitive function, anesthesia and surgery inhibit synaptic function. In central nervous system, autophagy is vital to synaptic plasticity, homeostasis of synapticproteins, synapse elimination, spine pruning, proper axon guidance, and when dysregulated, is associated with behavioral and memory functions disorders. The mammalian target of rapamycin (mTOR) negatively regulates the process of autophagy. This study aimed to explore whether rapamycin can ameliorate anesthesia/surgery-induced cognitive deficits by inhibiting mTOR, activating autophagy and rising synaptic plasticity-related proteins in the hippocampus. Aged C57BL/6J mice were used to establish POCD models with exploratory laparotomy under isoflurane anesthesia. The Morris Water Maze (MWM) was used to measure reference memory after anesthesia and surgery. The levels of mTOR phosphorylation (p-mTOR), Beclin-1 and LC3-II were examined on postoperative days 1, 3 and 7 by western blotting. The levels of synaptophysin (SYN) and postsynaptic density protein 95 (PSD-95) in the hippocampus were also examined by western blotting. Here we showed that anesthesia/surgery impaired reference memory and induced the activation of mTOR, decreased the expression of autophagy-related proteins such as Beclin-1 and LC3-II. A corresponding decline in the expression of neuronal/synaptic, plasticity-related proteins such as SYN and PSD-95 was also observed. Pretreating mice with rapamycin inhibited the activation of mTOR and restored autophagy function, also increased the expression of SYN and PSD-95. Furthermore, anesthesia/surgery-induced learning and memory deficits were also reversed by rapamycin pretreatment. In conclusion, anesthesia/surgery induced mTOR hyperactivation and autophagy impairments, and then reduced the levels of SYN and PSD-95 in the hippocampus. An mTOR inhibitor, rapamycin, ameliorated anesthesia/surgery-related cognitive impairments by inhibiting the mTOR activity, inducing activation of autophagy, enhancing SYN and PSD-95 expression.

Serine Racemase Deletion Affects the Excitatory/Inhibitory Balance of the Hippocampal CA1 Network.

d-serine is the major co-agonist of N-methyl-D-aspartate receptors (NMDAR) at CA3/CA1 hippocampal synapses, the activation of which drives long-term potentiation (LTP). The use of mice with targeted deletion of the serine racemase (SR) enzyme has been an important tool to uncover the physiological and pathological roles of D-serine. To date, some uncertainties remain regarding the direction of LTP changes in SR-knockout (SR-KO) mice, possibly reflecting differences in inhibitory GABAergic tone in the experimental paradigms used in the different studies. On the one hand, our extracellular recordings in hippocampal slices show that neither isolated NMDAR synaptic potentials nor LTP were altered in SR-KO mice. This was associated with a compensatory increase in hippocampal levels of glycine, another physiologic NMDAR co-agonist. SR-KO mice displayed no deficits in spatial learning, reference memory and cognitive flexibility. On the other hand, SR-KO mice showed a weaker LTP and a lower increase in NMDAR potentials compared to controls when GABAA receptors were pharmacologically blocked. Our results indicate that depletion of endogenous D-serine caused a reduced inhibitory activity in CA1 hippocampal networks, altering the excitatory/inhibitory balance, which contributes to preserve functional plasticity at synapses and to maintain related cognitive abilities.