Developmental trajectory of episodic-like memory in rats.

Introduction: Episodic memory formation requires the binding of multiple associations to a coherent episodic representation, with rich detail of times, places, and contextual information. During postnatal development, the ability to recall episodic memories emerges later than other types of memory such as object recognition. However, the precise developmental trajectory of episodic memory, from weaning to adulthood has not yet been established in rats. Spontaneous object exploration tasks do not require training, and allow repeated testing of subjects, provided novel objects are used on each trial. Therefore, these tasks are ideally suited for the study of the ontogeny of episodic memory and its constituents (e.g., object, spatial, and contextual memory). Methods: In the present study, we used four spontaneous short-term object exploration tasks over two days: object (OR), object-context (OCR), object-place (OPR), and object-place-context (OPCR) recognition to characterise the ontogeny of episodic-like memory and its components in three commonly used outbred rat strains (Lister Hooded, Long Evans Hooded, and Sprague Dawley). Results: In longitudinal studies starting at 3-4 weeks of age, we observed that short term memory for objects was already present at the earliest time point we tested, indicating that it is established before the end of the third week of life (consistent with several other reports). Object-context memory developed during the fifth week of life, while both object-in-place and the episodic-like object-place-context memory developed around the seventh postnatal week. To control for the effects of previous experience in the development of associative memory, we confirmed these developmental trajectories using a cross-sectional protocol. Discussion: Our work provides robust evidence for different developmental trajectories of recognition memory in rats depending on the content and/or complexity of the associations and emphasises the utility of spontaneous object exploration tasks to assess the ontogeny of memory systems with high temporal resolution.

Reducing Glut2 throughout the body does not result in cognitive behaviour differences in aged male mice.

OBJECTIVES: GLUT2 is a major facilitative glucose transporter, expressed from the SLC2A2 gene, with essential roles in the liver. Recent work in mice has shown that preventing Glut2 production in specific neuronal populations increases sugar-seeking behaviour, highlighting the importance of Slc2a2 gene expression in the brain. It implies that reduced GLUT2 in the brain, due to genetic polymorphisms or disease, impacts health through behaviour change. Defects in glucose transport in the brain are observed in conditions including type-2 diabetes and dementia. Few studies have directly examined the effect of modulating neuronal glucose transporter expression on cognitive function. The aim of this study was to investigate whether inactivating one Slc2a2 allele throughout the body had major effects on cognition. Cognitive tests to assess recognition memory, spatial working memory and anxiety were performed in Slc2a2 whole-body heterozygous mice (i.e. reduced Glut2 mRNA and protein), alongside littermates expressing normal levels of the transporter. RESULTS: No significant effects on neurological functions and cognitive capabilities were observed in mice lacking one Slc2a2 allele when fed a chow diet. This suggests that the minor variations in GLUT2 levels that occur in the human population are unlikely to influence behaviour and basic cognition.

Effects of dietary fat manipulation on cognition in mice and rats: protocol for a systematic review and meta-analysis.

INTRODUCTION AND OBJECTIVE: The Western diet that comprises high levels of long-chain saturated fats and sugar is associated not only with metabolic disorders such as obesity and type 2 diabetes but also has been recently linked to brain changes and cognitive dysfunction. However, in animal studies, reported effects are variable, and the mechanisms underlying these effects are unclear. In the proposed review, we aim to summarise the diverse evidence of the effects of so-called 'high-fat' and ketogenic diets on behavioural measures of cognition in postweaning mice and rats, relative to animals on standard diets and to determine potential underlying mechanisms of high-fat diet-induced effects. SEARCH STRATEGY: A comprehensive search strategy was designed to retrieve studies reporting use of a high-fat or ketogenic diet in postweaning mice and rats that included cognitive assessments. Three databases (Medline, SCOPUS and Web of Science) were searched and 4487 unique references were retrieved. SCREENING AND ANNOTATION: Studies were screened for inclusion by two independent reviewers, with 330 studies retained for analysis. Characteristics of disease model choice, experimental design, intervention use and outcome assessment are to be extracted using the Systematic Review Facility (http://syrf.org.uk/) tool. Studies will be assessed for study quality and risk of bias and confidence of mechanistic involvement. DATA MANAGEMENT AND REPORTING: For cognitive outcomes, effect sizes will be calculated using normalised mean difference and summarised using a random effects model. The contribution of potential sources of heterogeneity to the observed effects of diet on cognition will be assessed using multivariable meta-regression, with partitioning of heterogeneity as a sensitivity analysis. A preliminary version of this protocol was published on 9 April 2019 on the Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies website (http://www.dcn.ed.ac.uk/camarades/research.html%23protocols). ETHICS AND DISSEMINATION: No ethical approval is required as there are no subjects in the proposed study.

Early and reversible changes to the hippocampal proteome in mice on a high-fat diet.

BACKGROUND: The rise in global obesity makes it crucial to understand how diet drives obesity-related health conditions, such as premature cognitive decline and Alzheimer's disease (AD). In AD hippocampal-dependent episodic memory is one of the first types of memory to be impaired. Previous studies have shown that in mice fed a high-fat diet (HFD) episodic memory is rapidly but reversibly impaired. METHODS: In this study we use hippocampal proteomics to investigate the effects of HFD in the hippocampus. Mice were fed either a low-fat diet (LFD) or HFD containing either 10% or 60% (Kcal) from fat for 3 days, 1 week or 2 weeks. One group of mice were fed the HFD for 1 week and then returned to the LFD for a further week. Primary hippocampal cultures were challenged with palmitic acid (PA), the most common long-chain saturated FA in the Western diet, and with the anti-inflammatory, n-3 polyunsaturated FA, docosahexaenoic acid (DHA), or a combination of the two to ascertain effects of these fatty acids on dendritic structure. RESULTS: HFD-induced changes occur in hippocampal proteins involved in metabolism, inflammation, cell stress, cell signalling, and the cytoskeleton after 3 days, 1 week and 2 weeks of HFD. Replacement of the HFD after 1 week by a low-fat diet (LFD) for a further week resulted in partial recovery of the hippocampal proteome. Microtubule-associated protein 2 (MAP2), one of the earliest proteins changed, was used to investigate the impact of fatty acids (FAs) on hippocampal neuronal morphology. PA challenge resulted in shorter and less arborised dendrites while DHA had no effect when applied alone but counteracted the effects of PA when FAs were used in combination. Dendritic morphology recovered when PA was removed from the cell culture media. CONCLUSION: This study provides evidence for the rapid and reversible effects of diet on the hippocampal proteome and the impact of PA and DHA on dendritic structure.

A high-fat diet induces rapid changes in the mouse hypothalamic proteome.

BACKGROUND: Prolonged over-consumption of a high-fat diet (HFD) commonly leads to obesity and insulin resistance. However, even 3 days of HFD consumption has been linked to inflammation within the key homeostatic brain region, the hypothalamus. METHODS: Mice were fed either a low-fat diet (LFD) or HFD containing 10% or 60% (Kcal) respectively from fat for 3 days. Mice were weighed, food intake measured and glucose tolerance calculated using intraperitoneal glucose tolerance tests (IPGTT). Proteomic analysis was carried out to determine if hypothalamic proteins were changed by a HFD. The direct effects of dietary fatty acids on mitochondrial morphology and on one of the proteins most changed by a HFD, dihydropyrimidinase-related protein 2 (DRP-2) a microtubule-associated protein which regulates microtubule dynamics, were also tested in mHypoE-N42 (N42) neuronal cells challenged with palmitic acid (PA) and oleic acid (OA). RESULTS: Mice on the HFD, as expected, showed increased adiposity and glucose intolerance. Hypothalamic proteomic analysis revealed changes in 104 spots after 3 days on HFD, which, when identified by LC/MS/MS, were found to represent 78 proteins mainly associated with cytoskeleton and synaptic plasticity, stress response, glucose metabolism and mitochondrial function. Over half of the changed proteins have also been reported to be changed in neurodegenerative conditions such as Alzheimer's disease. Also,in N42 neurons mitochondrial morphology and DRP-2 levels were altered by PA but not by OA. CONCLUSION: These results demonstrate that within 3 days, there is a relatively large effect of HFD on the hypothalamic proteome indicative of cellular stress, altered synaptic plasticity and mitochondrial function, but not inflammation. Changes in N42 cells show an effect of PA but not OA on DRP-2 and on mitochondrial morphology indicating that long-chain saturated fatty acids damage neuronal function.

Metformin reverses early cortical network dysfunction and behavior changes in Huntington's disease.

Catching primal functional changes in early, 'very far from disease onset' (VFDO) stages of Huntington's disease is likely to be the key to a successful therapy. Focusing on VFDO stages, we assessed neuronal microcircuits in premanifest Hdh150 knock-in mice. Employing in vivo two-photon Ca2+ imaging, we revealed an early pattern of circuit dysregulation in the visual cortex - one of the first regions affected in premanifest Huntington's disease - characterized by an increase in activity, an enhanced synchronicity and hyperactive neurons. These findings are accompanied by aberrations in animal behavior. We furthermore show that the antidiabetic drug metformin diminishes aberrant Huntingtin protein load and fully restores both early network activity patterns and behavioral aberrations. This network-centered approach reveals a critical window of vulnerability far before clinical manifestation and establishes metformin as a promising candidate for a chronic therapy starting early in premanifest Huntington's disease pathogenesis long before the onset of clinical symptoms.

Rapid and reversible impairment of episodic memory by a high-fat diet in mice.

Alzheimer's disease is a leading cause of morbidity and mortality with no cure and only limited treatment available. Obesity and type 2 diabetes are positively associated with the development of premature cognitive decline and Alzheimer's disease, linking diet with these conditions. Here we demonstrate that in mice episodic memory, together with spatial and contextual associative memory, is compromised after only one day of high-fat diet. However, object memory remains intact. This shows not only a more rapid effect than previously reported but also that more complex memories are at higher risk of being compromised by a high-fat diet. In addition, we show that these memory deficits are rapidly reversed by switching mice from a high-fat diet back to a low-fat diet. These findings have important implications for the contribution of nutrition to the development of cognitive decline and Alzheimer's disease.

Pharmacological disruption of the MID1/α4 interaction reduces mutant Huntingtin levels in primary neuronal cultures.

Expression of mutant Huntingtin (HTT) protein is central to the pathophysiology of Huntington's Disease (HD). The E3 ubiquitin ligase MID1 appears to have a key role in facilitating translation of the mutant HTT mRNA suggesting that interference with the function of this complex could be an attractive therapeutic approach. Here we describe a peptide that is able to disrupt the interaction between MID1 and the α4 protein, a regulatory subunit of protein phosphatase 2A (PP2A). By fusing this peptide to a sequence from the HIV-TAT protein we demonstrate that the peptide can disrupt the interaction within cells and show that this results in a decrease in levels of ribosomal S6 phosphorylation and HTT expression in cultures of cerebellar granule neurones derived from HdhQ111/Q7 mice. This data serves to validate this pathway and paves the way for the discovery of small molecule inhibitors of this interaction as potential therapies for HD.

Coherence among head direction cells before eye opening in rat pups.

Mammalian navigation is thought to depend on an internal map of space consisting of functionally specialized cells in the hippocampus and the surrounding parahippocampal cortices. Basic properties of this map are present when rat pups explore the world outside of their nest for the first time, around postnatal day 16-18 (P16-P18). One of the first functions to be expressed in navigating animals is the directional tuning of the head direction cells. To determine whether head direction tuning is expressed at even earlier ages, before the start of exploration, and to establish whether vision is necessary for the development of directional tuning, we recorded neural activity in pre- and parasubiculum, or medial entorhinal cortex, from P11 onward, 3-4 days before the eyelids unseal. Head direction cells were present from the first day of recording. Firing rates were lower than in adults, and preferred firing directions were less stable, drifting within trials and changing completely between trials. Yet the cells drifted coherently, i.e., relative firing directions were maintained from one trial to the next. Directional tuning stabilized shortly after eye opening. The data point to a hardwired attractor network for representation of head direction in which directional tuning develops before vision and visual input serves primarily to anchor firing direction to the external world.

Lateral entorhinal cortex is critical for novel object-context recognition.

Episodic memory incorporates information about specific events or occasions including spatial locations and the contextual features of the environment in which the event took place. It has been modeled in rats using spontaneous exploration of novel configurations of objects, their locations, and the contexts in which they are presented. While we have a detailed understanding of how spatial location is processed in the brain relatively little is known about where the nonspatial contextual components of episodic memory are processed. Initial experiments measured c-fos expression during an object-context recognition (OCR) task to examine which networks within the brain process contextual features of an event. Increased c-fos expression was found in the lateral entorhinal cortex (LEC; a major hippocampal afferent) during OCR relative to control conditions. In a subsequent experiment it was demonstrated that rats with lesions of LEC were unable to recognize object-context associations yet showed normal object recognition and normal context recognition. These data suggest that contextual features of the environment are integrated with object identity in LEC and demonstrate that recognition of such object-context associations requires the LEC. This is consistent with the suggestion that contextual features of an event are processed in LEC and that this information is combined with spatial information from medial entorhinal cortex to form episodic memory in the hippocampus.

Ontogeny of neural circuits underlying spatial memory in the rat.

Spatial memory is a well-characterized psychological function in both humans and rodents. The combined computations of a network of systems including place cells in the hippocampus, grid cells in the medial entorhinal cortex and head direction cells found in numerous structures in the brain have been suggested to form the neural instantiation of the cognitive map as first described by Tolman in 1948. However, while our understanding of the neural mechanisms underlying spatial representations in adults is relatively sophisticated, we know substantially less about how this network develops in young animals. In this article we briefly review studies examining the developmental timescale that these systems follow. Electrophysiological recordings from very young rats show that directional information is at adult levels at the outset of navigational experience. The systems supporting allocentric memory, however, take longer to mature. This is consistent with behavioral studies of young rats which show that spatial memory based on head direction develops very early but that allocentric spatial memory takes longer to mature. We go on to report new data demonstrating that memory for associations between objects and their spatial locations is slower to develop than memory for objects alone. This is again consistent with previous reports suggesting that adult like spatial representations have a protracted development in rats and also suggests that the systems involved in processing non-spatial stimuli come online earlier.

The role of hippocampal subregions in memory for stimulus associations.

The hippocampus is hypothesised to be critical for episodic memory in humans and episodic-like memory in animals. Human data regarding the roles of the various subregional networks within the hippocampus is difficult to obtain. In this article we examine the current rodent literature on episodic-like memory and associative recognition and review the roles of the hippocampal subregions in these behavioural tasks. We focus on the large amount of recent data reporting roles for CA3 and CA1 in allocentric spatial and temporal associative memory respectively. Our own recent data are then presented detailing critical roles for CA3 and CA1 in an associative recognition task which does not require allocentric spatial or temporal processing. These data support more generic roles for CA3 and CA1 in episodic-like memory, based on anatomical and theoretical literature on hippocampal function. We also present a novel analysis of our data in which we suggest that the encoding of object, place and context information is unaffected by lesions of the hippocampus and therefore infer that it may be the storage or retrieval phase of this associative memory which is critically dependent on hippocampal function. In conclusion however, more specific anatomically and temporally controlled methods are needed to fully define the role of hippocampal subregions in episodic-like memory.

Development of the spatial representation system in the rat.

In the adult brain, space and orientation are represented by an elaborate hippocampal-parahippocampal circuit consisting of head-direction cells, place cells, and grid cells. We report that a rudimentary map of space is already present when 2 1/2-week-old rat pups explore an open environment outside the nest for the first time. Head-direction cells in the pre- and parasubiculum have adultlike properties from the beginning. Place and grid cells are also present but evolve more gradually. Grid cells show the slowest development. The gradual refinement of the spatial representation is accompanied by an increase in network synchrony among entorhinal stellate cells. The presence of adultlike directional signals at the onset of navigation raises the possibility that such signals are instrumental in setting up networks for place and grid representation.

Associative recognition and the hippocampus: differential effects of hippocampal lesions on object-place, object-context and object-place-context memory.

The hippocampus is thought to be required for the associative recognition of objects together with the spatial or temporal contexts in which they occur. However, recent data showing that rats with fornix lesions perform as well as controls in an object-place task, while being impaired on an object-place-context task (Eacott and Norman (2004) J Neurosci 24:1948-1953), suggest that not all forms of context-dependent associative recognition depend on the integrity of the hippocampus. To examine the role of the hippocampus in context-dependent recognition directly, the present study tested the effects of large, selective, bilateral hippocampus lesions in rats on performance of a series of spontaneous recognition memory tasks: object recognition, object-place recognition, object-context recognition and object-place-context recognition. Consistent with the effects of fornix lesions, animals with hippocampus lesions were impaired only on the object-place-context task. These data confirm that not all forms of context-dependent associative recognition are mediated by the hippocampus. Subsequent experiments suggested that the object-place task does not require an allocentric representation of space, which could account for the lack of impairment following hippocampus lesions. Importantly, as the object-place-context task has similar spatial requirements, the selective deficit in object-place-context recognition suggests that this task requires hippocampus-dependent neural processes distinct from those required for allocentric spatial memory, or for object memory, object-place memory or object-context memory. Two possibilities are that object, place, and context information converge only in the hippocampus, or that recognition of integrated object-place-context information requires a hippocampus-dependent mode of retrieval, such as recollection.

Arbitrary associations in animals: what can paired associate recall in rats tell us about the neural basis of episodic memory? Theoretical comment on Kesner, Hunsaker, & Warthen (2008).

Detailed memories for unique episodes from an individual's past can be triggered, often effortlessly, when that individual is exposed to a stimulus that was present during the original event. The aim of Kesner et al. is to understand the neural basis of memory encoding that supports this cued recall of episodic memories. Kesner and colleagues make novel use of an object-place paired-associate task for rats to provide evidence for a critical role of dorsal CA3 in certain aspects of episodic memory encoding. Using one-trial cued recall versions of the task they show that when rats are cued with an object stimulus, they can be trained to revisit the location in which the object appeared previously. Conversely, when rats are cued with a location, they can learn to choose the object with which it was associated. Rats with dorsal CA3 lesions are severely impaired at these tasks. These data are consistent with the theory that the autoassociative network in CA3 supports the rapid formation of novel associations and may allow pattern completion--the phenomenom whereby a subset of the cues present at an encoding event triggers recall of the whole event. Although flexible recall of arbitrary associations is not fully demonstrated, the study contributes 2 novel behavioral tasks to the previously limited repertoire for studying paired associate recall in rats. It also builds on previous data to specify the role of the hippocampal CA3 subregion in cued recall--a critical aspect of episodic memory.

Exploring the role of context-dependent hippocampal activity in spatial alternation behavior.

In a continuous T-maze spatial alternation task, CA1 place cells fire differentially on the stem of the maze as rats are performing left- and right-turn trials (Wood et al. (2000) Neuron 27:623-633). This context-dependent hippocampal activity provides a potential mechanism by which animals could solve the alternation task, as it provides a cue that could prime the appropriate goal choice. The aim of this study was to examine the relationship between context-dependent hippocampal activity and spatial alternation behavior. We report that rats with complete lesions of the hippocampus learn and perform the spatial alternation task as well as controls if there is no delay between trials, suggesting that the observed context-dependent hippocampal activity does not mediate alternation behavior in this task. However lesioned rats are significantly impaired when delays of 2 or 10 s are interposed. Recording experiments reveal that context-dependent hippocampal activity occurs in both the delay and no-delay versions of the task, but that in the delay version it occurs during the delay period, and not on the stem of the maze. These data are consistent with a role for context-dependent hippocampal activity in delayed spatial alternation, but suggest that, according to specific task demands and memory load, the activity may be generated by different mechanisms and/or in different brain structures.

Schemas and memory consolidation.

Memory encoding occurs rapidly, but the consolidation of memory in the neocortex has long been held to be a more gradual process. We now report, however, that systems consolidation can occur extremely quickly if an associative "schema" into which new information is incorporated has previously been created. In experiments using a hippocampal-dependent paired-associate task for rats, the memory of flavor-place associations became persistent over time as a putative neocortical schema gradually developed. New traces, trained for only one trial, then became assimilated and rapidly hippocampal-independent. Schemas also played a causal role in the creation of lasting associative memory representations during one-trial learning. The concept of neocortical schemas may unite psychological accounts of knowledge structures with neurobiological theories of systems memory consolidation.