Retroactive interference of object-in-context long-term memory: role of dorsal hippocampus and medial prefrontal cortex

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during early life developmental periods is associated with impaired memory function, particularly for hippocampus (HPC)-dependent processes. We developed an early life WD rodent model associated with long-lasting HPC dysfunction to investigate the neurobiological mechanisms mediating these effects. Rats received either a cafeteria-style WD (ad libitum access to various high-fat/high-sugar foods; CAF) or standard healthy chow (CTL) during the juvenile and adolescent stages (postnatal days 26-56). Behavioral and metabolic assessments were performed both before and after a healthy diet intervention period beginning at early adulthood. Results revealed HPC-dependent contextual episodic memory impairments in CAF rats that persisted despite the healthy diet intervention. Given that dysregulated HPC acetylcholine (ACh) signaling is associated with memory impairments in humans and animal models, we examined protein markers of ACh tone in the dorsal HPC (HPCd) in CAF and CTL rats. Results revealed significantly lower protein levels of vesicular ACh transporter in the HPCd of CAF vs. CTL rats, indicating chronically reduced ACh tone. Using intensity-based ACh sensing fluorescent reporter (iAChSnFr) in vivo fiber photometry targeting the HPCd, we next revealed that ACh release during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Neuropharmacological results showed that alpha 7 nicotinic ACh receptor agonist infusion in the HPCd during training rescued memory deficits in CAF rats. Overall, these findings reveal a functional connection linking early life WD intake with long-lasting dysregulation of HPC ACh signaling, thereby identifying an underlying mechanism for WD-associated memory impairments.

Early- but not late-adolescent Western diet consumption programs for long-lasting memory impairments in male but not female rats

Early life Western diet (WD) consumption leads to impaired memory function, particularly for processes mediated by the hippocampus. However, the precise critical developmental window(s) during which WD exposure negatively impacts hippocampal function are unknown. Here, we exposed male and female rats to a WD model involving free access to a variety of high-fat and/or high-sugar food and drink items during either the early-adolescent period (postnatal days [PN] 26-41; WD-EA) or late-adolescent period (PN 41-56; WD-LA). Control (CTL) rats were given healthy standard chow throughout both periods. To evaluate long-lasting memory capacity well beyond the early life WD exposure periods, we performed behavioral assessments after both a short (4 weeks for WD-EA, 2 weeks for WD-LA) and long (12 weeks for WD-EA, 10 weeks for WD-LA) period of healthy diet intervention. Results revealed no differences in body weight or body composition between diet groups, regardless of sex. Following the shorter period of healthy diet intervention, both male and female WD-EA and WD-LA rats showed deficits in hippocampal-dependent memory compared to CTL rats. Following the longer healthy diet intervention period, memory impairments persisted in male WD-EA but not WD-LA rats. In contrast, in female rats the longer healthy diet intervention reversed the initial memory impairments in both WD-EA and WD-LA rats. Collectively, these findings reveal that early-adolescence is a critical period of long-lasting hippocampal vulnerability to dietary insults in male but not female rats, thus highlighting developmental- and sex-specific effects mediating the relationship between the early life nutritional environment and long-term cognitive health.

Early- but not late-adolescent Western diet consumption programs for long-lasting memory impairments in male but not female rats

Early life Western diet (WD) consumption leads to impaired memory function, particularly for processes mediated by the hippocampus. However, the precise critical developmental window(s) during which WD exposure negatively impacts hippocampal function are unknown. Here, we exposed male and female rats to a WD model involving free access to a variety of high-fat and/or high-sugar food and drink items during either the early-adolescent period (postnatal days [PN] 26-41; WD-EA) or late-adolescent period (PN 41-56; WD-LA). Control (CTL) rats were given healthy standard chow throughout both periods. To evaluate long-lasting memory capacity well beyond the early life WD exposure periods, we performed behavioral assessments after both a short (4 weeks for WD-EA, 2 weeks for WD-LA) and long (12 weeks for WD-EA, 10 weeks for WD-LA) period of healthy diet intervention. Results revealed no differences in body weight or body composition between diet groups, regardless of sex. Following the shorter period of healthy diet intervention, both male and female WD-EA and WD-LA rats showed deficits in hippocampal-dependent memory compared to CTL rats. Following the longer healthy diet intervention period, memory impairments persisted in male WD-EA but not WD-LA rats. In contrast, in female rats the longer healthy diet intervention reversed the initial memory impairments in both WD-EA and WD-LA rats. Collectively, these findings reveal that early-adolescence is a critical period of long-lasting hippocampal vulnerability to dietary insults in male but not female rats, thus highlighting developmental- and sex-specific effects mediating the relationship between the early life nutritional environment and long-term cognitive health.

Separate orexigenic hippocampal ensembles shape dietary choice by enhancing contextual memory and motivation

The hippocampus (HPC), traditionally known for its role in learning and memory, has emerged as a controller of food intake. While prior studies primarily associated the HPC with food intake inhibition, recent research suggests a critical role in appetitive processes. We hypothesized that orexigenic HPC neurons differentially respond to fats and/or sugars, potent natural reinforcers that contribute to obesity development. Results uncover previously-unrecognized, spatially-distinct neuronal ensembles within the dorsal HPC (dHPC) that are responsive to separate nutrient signals originating from the gut. Using activity-dependent genetic capture of nutrient-responsive HPC neurons, we demonstrate a causal role of both populations in promoting nutrient-specific preference through different mechanisms. Sugar-responsive neurons encode an appetitive spatial memory engram for meal location, whereas fat-responsive neurons selectively enhance the preference and motivation for fat intake. Collectively, these findings uncover a neural basis for the exquisite specificity in processing macronutrient signals from a meal that shape dietary choices.

Adaptive expression of engrams by retroactive interference

Long-term memories are stored as configurations of neuronal ensembles, termed engrams. Although investigation of engram cell properties and functionality in memory recall has been extensive, less is known about how engram cells are affected by forgetting. We describe a form of interference-based forgetting using an object memory behavioral paradigm. By using activity-dependent cell labeling, we show that although retroactive interference results in decreased engram cell reactivation during recall trials, optogenetic stimulation of the labeled engram cells is sufficient to induce memory retrieval. Forgotten engrams may be reinstated via the presentation of similar or related environmental information. Furthermore, we demonstrate that engram activity is necessary for interference to occur. Taken together, these findings indicate that retroactive interference modules engram expression in a manner that is both reversible and updatable. Inference may constitute a form of adaptive forgetting where, in everyday life, new perceptual and environmental inputs modulate the natural forgetting process.

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during development yields long-lasting memory impairments, yet the underlying neurobiological mechanisms remain elusive. Here we developed an early life WD rodent model to evaluate whether dysregulated hippocampus (HPC) acetylcholine (ACh) signaling, a pathology associated with memory impairment in human dementia, is causally-related to WD-induced cognitive impairment. Rats received a cafeteria-style WD (access to various high-fat/high-sugar foods; CAF) or healthy chow (CTL) during the juvenile and adolescent periods (postnatal days 26-56). Behavioral, metabolic, and microbiome assessments were performed both before and after a 30-day healthy diet intervention beginning at early adulthood. Results revealed CAF-induced HPC-dependent contextual episodic memory impairments that persisted despite healthy diet intervention, whereas CAF was not associated with long-term changes in body weight, body composition, glucose tolerance, anxiety-like behavior, or gut microbiome. HPC immunoblot analyses after the healthy diet intervention identified reduced levels of vesicular ACh transporter in CAF vs. CTL rats, indicative of chronically reduced HPC ACh tone. To determine whether these changes were functionally related to memory impairments, we evaluated temporal HPC ACh binding via ACh-sensing fluorescent reporter in vivo fiber photometry during memory testing, as well as whether the memory impairments could be rescued pharmacologically. Results revealed dynamic HPC ACh binding during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Further, HPC alpha-7 nicotinic receptor agonist infusion during consolidation rescued memory deficits in CAF rats. Overall, these findings identify dysregulated HPC ACh signaling as a mechanism underlying early life WD-associated memory impairments.

Differential involvement of hippocampal subfields in the relationship between Alzheimer's pathology and memory interference in older adults

Introduction

We tested whether Alzheimer's disease (AD) pathology predicts memory deficits in non-demented older adults through its effects on medial temporal lobe (MTL) subregional volume.

Methods

Thirty-two, non-demented older adults with cerebrospinal fluid (CSF) (amyloid-beta [Aβ]42/Aβ40, phosphorylated tau [p-tau]181, total tau [t-tau]), positron emission tomography (PET; 18F-florbetapir), high-resolution structural magnetic resonance imaging (MRI), and neuropsychological assessment were analyzed. We examined relationships between biomarkers and a highly granular measure of memory consolidation, retroactive interference (RI).

Results

Biomarkers of AD pathology were related to RI. Dentate gyrus (DG) and CA3 volume were uniquely associated with RI, whereas CA1 and BA35 volume were related to both RI and overall memory recall. AD pathology was associated with reduced BA35, CA1, and subiculum volume. DG volume and Aβ were independently associated with RI, whereas CA1 volume mediated the relationship between AD pathology and RI.

Discussion

Integrity of distinct hippocampal subfields demonstrate differential relationships with pathology and memory function, indicating specificity in vulnerability and contribution to different memory processes.

Early life Western diet-induced memory impairments and gut microbiome changes in female rats are long-lasting despite healthy dietary intervention

OBJECTIVE

Western diet consumption during adolescence results in hippocampus (HPC)-dependent memory impairments and gut microbiome dysbiosis. Whether these adverse outcomes persist in adulthood following healthy dietary intervention is unknown. Here we assessed the short- and long-term effects of adolescent consumption of a Western diet enriched with either sugar or both sugar and fat on metabolic outcomes, HPC function, and gut microbiota.

METHODS

Adolescent female rats (PN 26) were fed a standard chow diet (CHOW), chow with access to 11% sugar solution (SUG), or a junk food cafeteria-style diet (CAF) containing various foods high in fat and/or sugar. During adulthood (PN 65+), metabolic outcomes, HPC-dependent memory, and gut microbial populations were evaluated. In a subsequent experiment, these outcomes were evaluated following a 5-week dietary intervention where CAF and SUG groups were maintained on standard chow alone.

RESULTS

Both CAF and SUG groups demonstrated impaired HPC-dependent memory, increased adiposity, and altered gut microbial populations relative to the CHOW group. However, impaired peripheral glucose regulation was only observed in the SUG group. When examined following a healthy dietary intervention in a separate experiment, metabolic dysfunction was not observed in either the CAF or SUG group, whereas HPC-dependent memory impairments were observed in the CAF but not the SUG group. In both groups the composition of the gut microbiota remained distinct from CHOW rats after the dietary intervention.

CONCLUSIONS

While the metabolic impairments associated with adolescent junk food diet consumption are not present in adulthood following dietary intervention, the HPC-dependent memory impairments and the gut microbiome dysbiosis persist.

Early-life low-calorie sweetener consumption disrupts glucose regulation, sugar-motivated behavior, and memory function in rats

Low-calorie sweetener (LCS) consumption in children has increased dramatically due to its widespread presence in the food environment and efforts to mitigate obesity through sugar replacement. However, mechanistic studies on the long-term impact of early-life LCS consumption on cognitive function and physiological processes are lacking. Here, we developed a rodent model to evaluate the effects of daily LCS consumption (acesulfame potassium, saccharin, or stevia) during adolescence on adult metabolic, behavioral, gut microbiome, and brain transcriptomic outcomes. Results reveal that habitual early-life LCS consumption impacts normal postoral glucose handling and impairs hippocampal-dependent memory in the absence of weight gain. Furthermore, adolescent LCS consumption yielded long-term reductions in lingual sweet taste receptor expression and brought about alterations in sugar-motivated appetitive and consummatory responses. While early-life LCS consumption did not produce robust changes in the gut microbiome, brain region-specific RNA-Seq analyses reveal LCS-induced changes in collagen- and synaptic signaling-related gene pathways in the hippocampus and nucleus accumbens, respectively, in a sex-dependent manner. Collectively, these results reveal that habitual early-life LCS consumption has long-lasting implications for glucoregulation, sugar-motivated behavior, and hippocampal-dependent memory in rats, which may be based in part on changes in nutrient transporter, sweet taste receptor, and central gene pathway expression.

Two distinct ways to form long-term object recognition memory during sleep and wakefulness

Memory consolidation is promoted by sleep. However, there is also evidence for consolidation into long-term memory during wakefulness via processes that preferentially affect nonhippocampal representations. We compared, in rats, the effects of 2-h postencoding periods of sleep and wakefulness on the formation of long-term memory for objects and their associated environmental contexts. We employed a novel-object recognition (NOR) task, using object exploration and exploratory rearing as behavioral indicators of these memories. Remote recall testing (after 1 wk) confirmed significant long-term NOR memory under both conditions, with NOR memory after sleep predicted by the occurrence of EEG spindle-slow oscillation coupling. Rats in the sleep group decreased their exploratory rearing at recall testing, revealing successful recall of the environmental context. By contrast, rats that stayed awake after encoding showed equally high levels of rearing upon remote testing as during encoding, indicating that context memory was lost. Disruption of hippocampal function during the postencoding interval (by muscimol administration) suppressed long-term NOR memory together with context memory formation when animals slept, but enhanced NOR memory when they were awake during this interval. Testing remote recall in a context different from that during encoding impaired NOR memory in the sleep condition, while exploratory rearing was increased. By contrast, NOR memory in the wake rats was preserved and actually superior to that after sleep. Our findings indicate two distinct modes of long-term memory formation: Sleep consolidation is hippocampus dependent and implicates event-context binding, whereas wake consolidation is impaired by hippocampal activation and strengthens context-independent representations.

Hippocampal Estrogen Signaling Mediates Sex Differences in Retroactive Interference

Despite being a crucial physiological function of the brain, the mechanisms underlying forgetting are still poorly understood. Estrogens play a critical role in different brain functions, including memory. However, the effects of sex hormones on forgetting vulnerabilitymediated by retroactive interference (RI), a phenomenon in which newly acquired information interferes with the retrieval of already stored information, are still poorly understood. The aim of our study was to characterize the sex differences in interference-mediated forgetting and identify the underlying molecular mechanisms. We found that adult male C57bl/6 mice showed a higher susceptibility to RI-dependent memory loss than females. The preference index (PI) in the NOR paradigm was 52.7 ± 5.9% in males and 62.3 ± 13.0% in females. The resistance to RI in female mice was mediated by estrogen signaling involving estrogen receptor α activation in the dorsal hippocampus. Accordingly, following RI, females showed higher phosphorylation levels (+30%) of extracellular signal-regulated kinase1/2 (ERK1/2) in the hippocampus. Pharmacological inhibition of ERK1/2 made female mice prone to RI. The PI was 70.6 ± 11.0% in vehicle-injected mice and 47.4 ± 10.8% following PD98059 administration. Collectively, our data suggest that hippocampal estrogen α receptor-ERK1/2 signaling is critically involved in a pattern separation mechanism that inhibits object-related RI in female mice.

Persistence of spatial memory induced by spaced training involves a behavioral-tagging process

Spaced training, which involves long inter-trial intervals, has positive effects on memories. One of the main attributes of long-term memories (LTM) is persistence. Here, to identify the process that promotes LTM persistence by spaced learning, we used the spatial object recognition (SOR) task. The protocol consisted of a first strong training session that induced LTM formation (tested 1 day after training), but not LTM persistence (tested 7 or 14 days after training); and a second weak training session that promoted memory persistence when applied 1 day, but not 7 days, after the first training. We propose that the promotion of memory persistence is based on the Behavioral Tagging (BT) mechanism operating when the memory trace is retrieved. BT involves the setting of a tag induced by learning which gives rise to input selectivity, and the use of plasticity-related proteins (PRPs) to establish the mnemonic trace. We postulate that retraining will mainly retag the sites initially activated by the original learning, where the PRPs needed for memory expression and/or induced by retrieval would be used to maintain a persistent mnemonic trace. Our results suggest that the mechanism of memory expression, but not those of memory reinforcement or reconsolidation, is necessary to promote memory persistence after retraining. The molecular mechanisms involve ERKs1/2 activity to set the SOR learning tag, and the availability of GluA2-containing AMPA receptor. In conclusion, both the synthesis of PRPs and the setting of a learning tag are key processes triggered by retraining that allow SOR memory persistence.

Schizophrenia-associated SAP97 mutations increase glutamatergic synapse strength in the dentate gyrus and impair contextual episodic memory in rats

Mutations in the putative glutamatergic synapse scaffolding protein SAP97 are associated with the development of schizophrenia in humans. However, the role of SAP97 in synaptic regulation is unclear. Here we show that SAP97 is expressed in the dendrites of granule neurons in the dentate gyrus but not in the dendrites of other hippocampal neurons. Schizophrenia-related perturbations of SAP97 did not affect CA1 pyramidal neuron synapse function. Conversely, these perturbations produce dramatic augmentation of glutamatergic neurotransmission in granule neurons that can be attributed to a release of perisynaptic GluA1-containing AMPA receptors into the postsynaptic densities of perforant pathway synapses. Furthermore, inhibiting SAP97 function in the dentate gyrus was sufficient to impair contextual episodic memory. Together, our results identify a cell-type-specific synaptic regulatory mechanism in the dentate gyrus that, when disrupted, impairs contextual information processing in rats.

The effects of SAP97 mutations associated with schizophrenia on synaptic function are unclear. Here, the authors show that schizophrenia-related SAP97 mutations enhance glutamatergic synapse strength in the dentate gyrus, impairing contextual episodic memory in rats.

The Black Box effect: sensory stimulation after learning interferes with the retention of long-term object location memory in rats

Reducing sensory experiences during the period that immediately follows learning improves long-term memory retention in healthy humans, and even preserves memory in patients with amnesia. To date, it is entirely unclear why this is the case, and identifying the neurobiological mechanisms underpinning this effect requires suitable animal models, which are currently lacking. Here, we describe a straightforward experimental procedure in rats that future studies can use to directly address this issue. Using this method, we replicated the central findings on quiet wakefulness obtained in humans: We show that rats that spent 1 h alone in a familiar dark and quiet chamber (the Black Box) after exploring two objects in an open field expressed long-term memory for the object locations 6 h later, while rats that instead directly went back into their home cage with their cage mates did not. We discovered that both visual stimulation and being together with conspecifics contributed to the memory loss in the home cage, as exposing rats either to light or to a cage mate in the Black Box was sufficient to disrupt memory for object locations. Our results suggest that in both rats and humans, everyday sensory experiences that normally follow learning in natural settings can interfere with processes that promote long-term memory retention, thereby causing forgetting in form of retroactive interference. The processes involved in this effect are not sleep-dependent because we prevented sleep in periods of reduced sensory experience. Our findings, which also have implications for research practices, describe a potentially useful method to study the neurobiological mechanisms that might explain why normal sensory processing after learning impairs memory both in healthy humans and in patients suffering from amnesia.

Behavioral tagging as a mechanism for aversive-memory formation under acute stress

The behavioral tagging (BT) hypothesis postulates that a weak learning experience, which only induces short-term memory, may benefit from another event that provides plasticity-related proteins (PRPs) to establish a long-lasting memory. According to BT, the weak experience sets a transient learning tag at specific activated sites, and its temporal and spatial convergence with the PRPs allows the long-term memory (LTM) formation. In this work, rats were subjected to a weak inhibitory avoidance (IAw) training and we observed that acute stress (elevated platform, EP) experienced 1 hr before IAw promoted IA-LTM formation. This effect was dependent on glucocorticoid-receptor activity as well as protein synthesis in the dorsal hippocampus. However, the same stress has negative effects on IA-LTM formation when training is strong, probably by competing for necessary PRPs. Furthermore, our experiments showed that EP immediately after training did not impair the setting of the learning tag and even facilitated IA-LTM formation. These findings reveal different impacts of a given acute stressful experience on the formation of an aversive memory that could be explained by BT processes.

Chronic administration of pharmacological doses of angiotensin 1-7 and iodoangiotensin 1-7 has minimal effects on blood pressure, heart rate, and cognitive function of spontaneously hypertensive rats

Cardiovascular diseases are the principal cause of death worldwide, with hypertension being the most common cardiovascular disease risk factor. High blood pressure (BP) is also associated with an increased risk of poor cognitive performance and dementia including Alzheimer's disease. Angiotensin 1–7 (Ang 1‐7), a product of the renin‐angiotensin system (RAS), exhibits central and peripheral actions to reduce BP. Recent data from our lab reveals that the addition of a non‐radioactive iodine molecule to the tyrosine in position 4 of Ang 1‐7 (iodoAng 1‐7) makes it ~1000‐fold more potent than Ang 1‐7 in competing for the ¹²⁵I‐Ang 1‐7 binding site (Stoyell‐Conti et al., 2020). Moreover, the addition of the non‐radioactive iodine molecule increases (~4‐fold) iodoAng 1‐7’s ability to bind to the AT1 receptor (AT1R), the primary receptor for Ang II. Preliminary data indicates that iodoAng 1‐7 can also compete for the ¹²⁵I‐Ang IV binding site with a low micromolar IC50. Thus, our aims were to compare the effects of chronic treatment of the Spontaneously Hypertensive Rat (SHR) with iodoAng 1‐7 (non‐radioactive iodine isotope) and Ang 1‐7 on arterial pressure, heart rate, and cognitive function. For this study, male SHRs were divided into three groups and treated with Saline, Ang 1‐7, or iodoAng 1‐7 administrated subcutaneously using a 28‐day osmotic mini pump. Systolic BP was measured non‐invasively by the tail‐cuff technique. Cognitive function was assessed by Y‐Maze test and novel object recognition (NOR) test. We have demonstrated in SHRs that subcutaneous administration of high doses of iodoAng 1‐7 prevented the increase in heart rate with age, while Ang 1‐7 showed a trend toward preventing the increase in heart rate, possibly by improving baroreflex control of the heart. Conversely, neither Ang 1‐7 nor iodoAng 1‐7 administered subcutaneously affected BP nor cognitive function.

Gut microbial taxa elevated by dietary sugar disrupt memory function

Emerging evidence highlights a critical relationship between gut microbiota and neurocognitive development. Excessive consumption of sugar and other unhealthy dietary factors during early life developmental periods yields changes in the gut microbiome as well as neurocognitive impairments. However, it is unclear whether these two outcomes are functionally connected. Here we explore whether excessive early life consumption of added sugars negatively impacts memory function via the gut microbiome. Rats were given free access to a sugar-sweetened beverage (SSB) during the adolescent stage of development. Memory function and anxiety-like behavior were assessed during adulthood and gut bacterial and brain transcriptome analyses were conducted. Taxa-specific microbial enrichment experiments examined the functional relationship between sugar-induced microbiome changes and neurocognitive and brain transcriptome outcomes. Chronic early life sugar consumption impaired adult hippocampal-dependent memory function without affecting body weight or anxiety-like behavior. Adolescent SSB consumption during adolescence also altered the gut microbiome, including elevated abundance of two species in the genus Parabacteroides (P. distasonis and P. johnsonii) that were negatively correlated with hippocampal function. Transferred enrichment of these specific bacterial taxa in adolescent rats impaired hippocampal-dependent memory during adulthood. Hippocampus transcriptome analyses revealed that early life sugar consumption altered gene expression in intracellular kinase and synaptic neurotransmitter signaling pathways, whereas Parabacteroides microbial enrichment altered gene expression in pathways associated with metabolic function, neurodegenerative disease, and dopaminergic signaling. Collectively these results identify a role for microbiota "dysbiosis" in mediating the detrimental effects of early life unhealthy dietary factors on hippocampal-dependent memory function.

Early life stress and brain plasticity: from molecular alterations to aberrant memory and behavior

Early life stress (ELS) is one of the most critical factors that could modify brain plasticity, memory and learning abilities, behavioral reactions, and emotional response in adulthood leading to development of different mental disorders. Prenatal and early postnatal periods appear to be the most sensitive periods of brain development in mammals, thereby action of various factors at these stages of brain development might result in neurodegeneration, memory impairment, and mood disorders at later periods of life. Deciphering the processes underlying aberrant neurogenesis, synaptogenesis, and cerebral angiogenesis as well as deeper understanding the effects of ELS on brain development will provide novel approaches to prevent or to cure psychiatric and neurological deficits caused by stressful conditions at the earliest stages of ontogenesis. Neuropeptide oxytocin serves as an amnesic, anti-stress, pro-angiogenic, and neurogenesis-controlling molecule contributing to dramatic changes in brain plasticity in ELS. In the current review, we summarize recent data on molecular mechanisms of ELS-driven changes in brain plasticity with the particular focus on oxytocin-mediated effects on neurogenesis and angiogenesis, memory establishment, and forgetting.

Computational cognitive modeling and validation of Dp140 induced alteration of working memory in Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy has emerged as a model to assess cognitive domains. The DMD gene variant location and its association with variable degrees of cognitive impairment necessitate identification of a common denominator. Computer architectures provide a framework to delineate the mechanisms involved in the cognitive functioning of the human brain. Copy number variations in the 79 exons of DMD gene were screened in 84 DMD subjects by Multiplex Ligation-dependent Probe Amplification (MLPA). DMD subjects were categorized based on the presence or absence of DP140 isoform. The cognitive and neuropsychological assessments were carried out as per inclusion criteria using standard scales. Instance-based learning theory (IBLT) based on the partial matching process was developed to mimic Stroop Color and Word Task (SCWT) performance on Adaptive Control of Thought-Rational (ACT-R) cognitive architecture based on IBLT. Genotype-phenotype correlation was conducted based on the mutation location in DMD gene. Assessment of specific cognitive domains in DP140 - ve group corresponded to the involvement of multiple brain lobes including temporal (verbal and visual learning and memory), parietal (visuo-conceptual and visuo-constructive abilities) and frontal (sustained and focused attention, verbal fluency, cognitive control). Working memory axis was found to be the central domain through tasks including RAVLT trial 1, recency effect, digit span backward, working memory index, arithmetic subtests in the Dp140 - ve group. IBLT validated the non-reliance of DMD subjects on recency indicating affected working memory domain. Modeling strategy revealed altered working memory processes in DMD cases with affected Dp140 isoform. DMD brain was observed to rely on primacy than the recency suggesting alterations in working memory capacity. Modeling revealed lowered activation of DMD brain with Dp140 - ve in order to retrieve the instances.

Combined effects of acute exercise and hypoxia on memory

No previous studies have evaluated the potential combined effects of acute exercise and acute hypoxia exposure on memory function, which was the purpose of this study. Twenty-five participants (Mage = 21.2 years) completed two laboratory visits in a counterbalanced order, involving 1) acute exercise (a 20-min bout of moderate-intensity exercise) and then 30 min of exposure to hypoxia (FIO2 = 0.12), and 2) exposure to hypoxia alone (FIO2 = 0.12) for 30 min. Following this, participants completed a cued-recall and memory interference task (AB/AC paradigm), assessing cued-recall memory (recall 1 and recall 2) and memory interference (proactive and retroactive interference). For cued-recall memory, we observed a significant main effect for condition, with Exercise + Hypoxia condition having significantly greater cued-recall performance than Hypoxia alone. Memory interference did not differ as a function of the experimental condition. This experiment demonstrates that engaging in an acute bout of exercise prior to acute hypoxia exposure had an additive effect in enhancing cued-recall memory performance.

Anterior retrosplenial cortex is required for long-term object recognition memory

The retrosplenial cortex (RSC) is implicated on navigation and contextual memory. Lesions studies showed that the RSC shares functional similarities with the hippocampus (HP). Here we evaluated the role of the anterior RSC (aRSC) in the “what” and “where” components of recognition memory and contrasted it with that of the dorsal HP (dHP). Our behavioral and molecular findings show functional differences between the aRSC and the dHP in recognition memory. The inactivation of the aRSC, but not the dHP, impairs the consolidation and expression of the “what” memory component. In addition, object recognition task is accompanied by c-Fos levels increase in the aRSC. Interestingly, we found that the aRSC is recruited to process the “what” memory component only if it is active during acquisition. In contrast, both the aRSC and dHP are required for encoding the “where” component, which correlates with c-Fos levels increase. Our findings introduce a novel role of the aRSC in recognition memory, processing not only the “where”, but also the “what” memory component.

Simulating Emotions: An Active Inference Model of Emotional State Inference and Emotion Concept Learning

The ability to conceptualize and understand one's own affective states and responses - or "Emotional awareness" (EA) - is reduced in multiple psychiatric populations; it is also positively correlated with a range of adaptive cognitive and emotional traits. While a growing body of work has investigated the neurocognitive basis of EA, the neurocomputational processes underlying this ability have received limited attention. Here, we present a formal Active Inference (AI) model of emotion conceptualization that can simulate the neurocomputational (Bayesian) processes associated with learning about emotion concepts and inferring the emotions one is feeling in a given moment. We validate the model and inherent constructs by showing (i) it can successfully acquire a repertoire of emotion concepts in its "childhood", as well as (ii) acquire new emotion concepts in synthetic "adulthood," and (iii) that these learning processes depend on early experiences, environmental stability, and habitual patterns of selective attention. These results offer a proof of principle that cognitive-emotional processes can be modeled formally, and highlight the potential for both theoretical and empirical extensions of this line of research on emotion and emotional disorders.

The Effects of Acute Exercise on Retroactive Memory Interference

PURPOSE

Retroactive interference involves the disruption of previously encoded information from newly learned information and thus may impair the consolidation of long-term memory. The objective of this study was to evaluate whether acute exercise can attenuate retroactive memory interference.

DESIGN

Three experimental studies were employed. Experiment 1 employed a between-subject randomized control trial (RCT) involving moderate-intensity walking (15 minutes). Experiment 2 employed a between-subject RCT involving high-intensity jogging (15 minutes). Experiment 3 employed a within-subject RCT involving moderate-intensity walking (15 minutes).

SETTING

University setting.

PARTICIPANTS

One hundred twelve young adults.

MEASURES

After exercise, memory interference was evaluated from an episodic word-list memory task, involving the recall of 2 word lists.

RESULTS

The pooled effect size (standard difference in means: -0.35; 95% confidence interval: -0.64 to -0.06) across the 3 experiments was statistically significant (P = .01).

CONCLUSION

We provide suggestive evidence that acute, short-duration exercise may help attenuate a retroactive memory interference effect. Implications of these findings for exercise to improve memory and attenuate memory decay are discussed.

Remembering to Forget: A Dual Role for Sleep Oscillations in Memory Consolidation and Forgetting

It has been known since the time of patient H. M. and Karl Lashley's equipotentiality studies that the hippocampus and cortex serve mnestic functions. Current memory models maintain that these two brain structures accomplish unique, but interactive, memory functions. Specifically, most modeling suggests that memories are rapidly acquired during waking experience by the hippocampus, before being later consolidated into the cortex for long-term storage. Sleep has been shown to be critical for the transfer and consolidation of memories in the cortex. Like memory consolidation, a role for sleep in adaptive forgetting has both historical precedent, as Francis Crick suggested in 1983 that sleep was for "reverse-learning," and recent empirical support. In this article I review the evidence indicating that the same brain activity involved in sleep replay associated memory consolidation is responsible for sleep-dependent forgetting. In reviewing the literature, it became clear that both a cellular mechanism for systems consolidation and an agreed upon general, as well as cellular, mechanism for sleep-dependent forgetting is seldom discussed or is lacking. I advocate here for a candidate cellular systems consolidation mechanism wherein changes in calcium kinetics and the activation of consolidative signaling cascades arise from the triple phase locking of non-rapid eye movement sleep (NREMS) slow oscillation, sleep spindle and sharp-wave ripple rhythms. I go on to speculatively consider several sleep stage specific forgetting mechanisms and conclude by discussing a notional function of NREM-rapid eye movement sleep (REMS) cycling. The discussed model argues that the cyclical organization of sleep functions to first lay down and edit and then stabilize and integrate engrams. All things considered, it is increasingly clear that hallmark sleep stage rhythms, including several NREMS oscillations and the REMS hippocampal theta rhythm, serve the dual function of enabling simultaneous memory consolidation and adaptive forgetting. Specifically, the same sleep rhythms that consolidate new memories, in the cortex and hippocampus, simultaneously organize the adaptive forgetting of older memories in these brain regions.

Spatial object recognition memory formation under acute stress

Stress is known to have a critical impact on memory processes. In the present work, we focus on the effects of an acute stress event closely associated to an unrelated learning task. Here, we show that acute stress (elevated platform [EP] session) experienced 1 hr after a weak spatial object recognition (SOR) training, which only induces a short-term memory (STM), promoted the formation of SOR-long term memory (SOR-LTM) in rats. The effect induced by stress was dependent on the activation of glucocorticoid- and mineralocorticoid-receptors, brain-derived neurotrophic factor (BDNF) and protein synthesis in the dorsal hippocampus. In contrast, EP after a strong SOR impaired SOR-LTM probably by interfering with the use of necessary resources. Moreover, we show that the EP session before training induced anterograde interference, which it was not reversed by a subsequent exposure to an open field. Our findings provide novel insights into the impact of stress on LTM formation in rodents and they are discussed under the behavioral analogue of the synaptic tagging and capture hypothesis.

Gut vagal sensory signaling regulates hippocampus function through multi-order pathways

The vagus nerve is the primary means of neural communication between the gastrointestinal (GI) tract and the brain. Vagally mediated GI signals activate the hippocampus (HPC), a brain region classically linked with memory function. However, the endogenous relevance of GI-derived vagal HPC communication is unknown. Here we utilize a saporin (SAP)-based lesioning procedure to reveal that selective GI vagal sensory/afferent ablation in rats impairs HPC-dependent episodic and spatial memory, effects associated with reduced HPC neurotrophic and neurogenesis markers. To determine the neural pathways connecting the gut to the HPC, we utilize monosynaptic and multisynaptic virus-based tracing methods to identify the medial septum as a relay connecting the medial nucleus tractus solitarius (where GI vagal afferents synapse) to dorsal HPC glutamatergic neurons. We conclude that endogenous GI-derived vagal sensory signaling promotes HPC-dependent memory function via a multi-order brainstem–septal pathway, thereby identifying a previously unknown role for the gut–brain axis in memory control.

Feeding-relevant vagal signaling occurs between the gastrointestinal tract and the brain, but it is unclear if this pathway influences cognitive processes. This study shows that endogenous gastrointestinal derived vagal sensory signaling promotes hippocampal-dependent memory function via a multi-order brainstem–septal pathway.

Early-life sugar consumption has long-term negative effects on memory function in male rats

OBJECTIVES

Added dietary sugars contribute substantially to the diet of children and adolescents in the USA, and recent evidence suggests that consuming sugar-sweetened beverages (SSBs) during early life has deleterious effects on hippocampal-dependent memory function. Here, we test whether the effects of early-life sugar consumption on hippocampal function persist into adulthood when access to sugar is restricted to the juvenile/adolescent phase of development.

METHODS

Male rats were given ad libitum access to an 11% weight-by-volume sugar solution (made with high fructose corn syrup-55) throughout the adolescent phase of development (post-natal day (PN) 26-56). The control group received a second bottle of water instead, and both groups received ad libitum standard laboratory chow and water access throughout the study. At PN 56 sugar solutions were removed and at PN 175 rats were subjected to behavioral testing for hippocampal-dependent episodic contextual memory in the novel object in context (NOIC) task, for anxiety-like behavior in the Zero maze, and were given an intraperitoneal glucose tolerance test.

RESULTS

Early-life exposure to SSBs conferred long-lasting impairments in hippocampal-dependent memory function later in life- yet had no effect on body weight, anxiety-like behavior, or glucose tolerance. A second experiment demonstrated that NOIC performance was impaired at PN 175 even when SSB access was limited to 2 hours daily from PN 26-56.

DISCUSSION

Our data suggest that even modest SSB consumption throughout early life may have long-term negative consequences on memory function during adulthood.

Recognition memory is selectively impaired in adult rats exposed to binge-like doses of ethanol during early postnatal life

Exposure to alcohol in utero can induce a variety of physical and mental impairments, collectively known as fetal alcohol spectrum disorders (FASD). This study explores the persistent cognitive consequences of ethanol administration in rat pups over postnatal days (PD) 4-9, modeling human third trimester consumption. Between PD65-70, ethanol-exposed (5E) and control rats were evaluated in two variants of recognition memory, the spontaneous novel object recognition (NOR) task, using 20 and 240 min sample-to-test delays, and the associative object-in-context (OIC) task, using a 20 min delay. No treatment group differences were observed in object exploration during the sample session for any task. In the 20 min NOR test session the 5E rats explored the novel object significantly less than controls, relative to the total time exploring both objects. Postnatal ethanol exposure is hypothesized to impede object memory consolidation in the perirhinal cortex of 5E rats, hindering their ability to discriminate between familiar and novel objects at short delays. The 5E rats performed as well or better than control rats in the 240 min NOR and the 20 min OIC tasks, indicating developmental ethanol exposure selectively impairs the retention and expression of recognition memories in young adult rats.

Novelty during a late postacquisition time window attenuates the persistence of fear memory

Learning to avoid threats in the environment is highly adaptive. However, sometimes a dysregulation of fear memories processing may underlie fear-related disorders. Despite recent advances, a major question of how to effectively attenuate persistent fear memories in a safe manner remains unresolved. Here we show experiments employing a behavioural tool to target a specific time window after training to limit the persistence of a fear memory in rats. We observed that exposure to a novel environment 11 h after an inhibitory avoidance (IA) training that induces a long-lasting memory, attenuates the durability of IA memory but not its formation. This effect is time-restricted and not seen when the environment is familiar. In addition, novelty-induced attenuation of IA memory durability is prevented by the intrahippocampal infusion of the CaMKs inhibitor KN-93. This new behavioural approach which targets a specific time window during late memory consolidation, might represent a new tool for reducing the durability of persistent fear memories.

Neuropsychological Assessment of Hippocampal Integrity

Finding methods to describe subcortical processes assisting cognition is an important concern for clinical neuropsychological practice. In this study, we reviewed the literature concerning the relationship between a neuropsychological instrument and the underlying neural substructure. We examined evidence indicating that one of the oldest neuropsychological tests still in use, the Rey Auditory Verbal Learning Test (RAVLT), includes reliable indicators of hippocampal integrity. We reviewed studies investigating the neural structures underlying seven tasks generated by the RAVLT, from the perspective of whether the performance of these tasks is dependent on the hippocampus. We found support for our hypothesis in five cases: learning capacity, proactive interference, immediate recall, delayed recall, and delayed recognition. No support for our hypothesis was found with regard to short-term memory and retroactive interference. The RAVLT appears to be a reliable tool for assessing the integrity of the hippocampus and for the early detection of dysfunction. There is a need for such assessments, due to the crucial role of the hippocampus in cognition, for instance, in terms of predicting future outcomes.

Dizocilpine (MK-801) impairs learning in the active place avoidance task but has no effect on the performance during task/context alternation

The prevention of engram interference, pattern separation, flexibility, cognitive coordination and spatial navigation are usually studied separately at the behavioral level. Impairment in executive functions is often observed in patients suffering from schizophrenia. We have designed a protocol for assessing these functions all together as behavioral separation. This protocol is based on alternated or sequential training in two tasks testing different hippocampal functions (the Morris water maze and active place avoidance), and alternated or sequential training in two similar environments of the active place avoidance task. In Experiment 1, we tested, in adult rats, whether the performance in two different spatial tasks was affected by their order in sequential learning, or by their day-to-day alternation. In Experiment 2, rats learned to solve the active place avoidance task in two environments either alternately or sequentially. We found that rats are able to acquire both tasks and to discriminate both similar contexts without obvious problems regardless of the order or the alternation. We used two groups of rats, controls and a rat model of psychosis induced by a subchronic intraperitoneal application of 0.08mg/kg of dizocilpine (MK-801), a non-competitive antagonist of NMDA receptors. Dizocilpine had no selective effect on parallel/sequential learning of tasks/contexts. However, it caused hyperlocomotion and a significant deficit in learning in the active place avoidance task regardless of the task alternation. Cognitive coordination tested by this task is probably more sensitive to dizocilpine than spatial orientation because no hyperactivity or learning impairment was observed in the Morris water maze.

The consolidation of inhibitory avoidance memory in mice depends on the intensity of the aversive stimulus: The involvement of the amygdala, dorsal hippocampus and medial prefrontal cortex

Several studies using inhibitory avoidance models have demonstrated the importance of limbic structures, such as the amygdala, dorsal hippocampus and medial prefrontal cortex, in the consolidation of emotional memory. However, we aimed to investigate the role of the amygdala (AMG), dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) of mice in the consolidation of step-down inhibitory avoidance and whether this avoidance would be conditioned relative to the intensity of the aversive stimulus. To test this, we bilaterally infused anisomycin (ANI-40μg/μl, a protein synthesis inhibitor) into one of these three brain areas in mice. These mice were then exposed to one of two different intensities (moderate: 0.5mA or intense: 1.5mA) in a step-down inhibitory avoidance task. We found that consolidation of both of the aversive experiences was mPFC dependent, while the AMG and DH were only required for the consolidation of the intense experience. We suggest that in moderately aversive situations, which do not represent a severe physical risk to the individual, the consolidation of aversive experiences does not depend on protein synthesis in the AMG or the DH, but only the mPFC. However, for intense aversive stimuli all three of these limbic structures are essential for the consolidation of the experience.

Tonic Inhibitory Control of Dentate Gyrus Granule Cells by α5-Containing GABAA Receptors Reduces Memory Interference

Interference between similar or overlapping memories formed at different times poses an important challenge on the hippocampal declarative memory system. Difficulties in managing interference are at the core of disabling cognitive deficits in neuropsychiatric disorders. Computational models have suggested that, in the normal brain, the sparse activation of the dentate gyrus granule cells maintained by tonic inhibitory control enables pattern separation, an orthogonalization process that allows distinct representations of memories despite interference. To test this mechanistic hypothesis, we generated mice with significantly reduced expression of the α5-containing GABAA (α5-GABAARs) receptors selectively in the granule cells of the dentate gyrus (α5DGKO mice). α5DGKO mice had reduced tonic inhibition of the granule cells without any change in fast phasic inhibition and showed increased activation in the dentate gyrus when presented with novel stimuli. α5DGKO mice showed impairments in cognitive tasks characterized by high interference, without any deficiencies in low-interference tasks, suggesting specific impairment of pattern separation. Reduction of fast phasic inhibition in the dentate gyrus through granule cell-selective knock-out of α2-GABAARs or the knock-out of the α5-GABAARs in the downstream CA3 area did not detract from pattern separation abilities, which confirms the anatomical and molecular specificity of the findings. In addition to lending empirical support to computational hypotheses, our findings have implications for the treatment of interference-related cognitive symptoms in neuropsychiatric disorders, particularly considering the availability of pharmacological agents selectively targeting α5-GABAARs.

SIGNIFICANCE STATEMENT

Interference between similar memories poses a significant limitation on the hippocampal declarative memory system, and impaired interference management is a cognitive symptom in many disorders. Thus, understanding mechanisms of successful interference management or processes that can lead to interference-related memory problems has high theoretical and translational importance. This study provides empirical evidence that tonic inhibition in the dentate gyrus (DG), which maintains sparseness of neuronal activation in the DG, is essential for management of interference. The specificity of findings to tonic, but not faster, more transient types of neuronal inhibition and to the DG, but not the neighboring brain areas, is presented through control experiments. Thus, the findings link interference management to a specific mechanism, proposed previously by computational models.

Behavioral Tagging: A Translation of the Synaptic Tagging and Capture Hypothesis

Similar molecular machinery is activated in neurons following an electrical stimulus that induces synaptic changes and after learning sessions that trigger memory formation. Then, to achieve perdurability of these processes protein synthesis is required for the reinforcement of the changes induced in the network. The synaptic tagging and capture theory provided a strong framework to explain synaptic specificity and persistence of electrophysiological induced plastic changes. Ten years later, the behavioral tagging hypothesis (BT) made use of the same argument, applying it to learning and memory models. The hypothesis postulates that the formation of lasting memories relies on at least two processes: the setting of a learning tag and the synthesis of plasticity related proteins, which once captured at tagged sites allow memory consolidation. BT explains how weak events, only capable of inducing transient forms of memories, can result in lasting memories when occurring close in time with other behaviorally relevant experiences that provide proteins. In this review, we detail the findings supporting the existence of BT process in rodents, leading to the consolidation, persistence, and interference of a memory. We focus on the molecular machinery taking place in these processes and describe the experimental data supporting the BT in humans.

Ontogeny of object-in-context recognition in the rat

The object-in-context recognition (OiC) task [19] is a spontaneous exploration task that serves as an index of incidental contextual learning and memory. During the test phase, rats prefer to explore the object mismatched to the testing context based on previous object-context pairings experienced during training. The mechanisms of OiC memory have been explored in adult rats [12,35]; however, little is known about its determinants during development. Thus, the present study examined the ontogeny of the OiC task in preweanling through adolescent rats. We demonstrate that postnatal day (PD) 17, 21, 26, and 31 rats can perform the OiC task (Experiment 1) and that preference for the novel target is eliminated when rats are tested in an alternate context not encountered during training (Experiment 2). Lastly, we show that PD26 but not PD17 rats can perform the OiC task when the training contexts only differed by distal spatial cues (Experiment 3). These data demonstrate for the first time that PD17 rats can acquire and retain short-term OiC memory, which involves associative learning of object and context information. However, we also provide evidence that preweanling rats' ability to utilize certain aspects of a context (i.e., distal spatial cues) in the OiC task is not equivalent to that of their older counterparts. Implications for the development of contextual memory and its related neural substrates are discussed.