Determinants of object-in-context and object-place-context recognition in the developing rat

Characterizing Behavioral Effects of Early-Life Stress in an Animal Model of Auditory Processing

Animal models provide significant insight into the development of typical and disordered sensory processing. Such models have been established to take advantage of physical and behavioral characteristics of specific species. For example, the Mongolian gerbil is a well-established model for auditory processing, with a hearing range similar in frequency to that of humans and an easily accessible cochlea. Recently, early-life stress (ELS) has been shown to affect sensory processing in auditory, visual, and somatosensory neural regions. To understand the functional impact of ELS, it is necessary to evaluate the susceptibility of sensory perceptual abilities to this early perturbation. Yet measuring sensory perception - e.g., using operant conditioning - often concurrently involves animal behavioral elements such as attention, memory, learning, and emotion. All of these elements are well-known to be impacted by ELS, and may affect behavioral measurements in ways that could be misconstrued as sensory deficits. Thus, it is critical to characterize which behavioral elements are affected by ELS in any sensory model. Here we induced ELS during a developmental time window for maturation of the auditory cortex in Mongolian gerbils. We conducted behavioral measures in juveniles, a developmental age when ELS is known to impair the auditory pathway. ELS had no effect on overall activity but reduced anxiety-related behavior, impaired recognition memory, and improved spatial memory, with some sex-specific effects. These effects may influence the ability of gerbils to learn and retain operant training, particularly if anxiety-provoking reinforcement is used.

Episodic memory development: Bridging animal and human research

Human episodic memory is not functionally evident until about 2 years of age and continues to develop into the school years. Behavioral studies have elucidated this developmental timeline and its constituent processes. In tandem, lesion and neurophysiological studies in non-human primates and rodents have identified key neural substrates and circuit mechanisms that may underlie episodic memory development. Despite this progress, collaborative efforts between psychologists and neuroscientists remain limited, hindering progress. Here, we seek to bridge human and non-human episodic memory development research by offering a comparative review of studies using humans, non-human primates, and rodents. We highlight critical theoretical and methodological issues that limit cross-fertilization and propose a common research framework, adaptable to different species, that may facilitate cross-species research endeavors.

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.

Cortico-Hippocampal Oscillations Are Associated With the Developmental Onset of Hippocampal-Dependent Memory

Hippocampal-dependent memories emerge late during postnatal development, aligning with hippocampal maturation. During sleep, the two-stage memory formation model states that through hippocampal-neocortical interactions, cortical slow-oscillations (SO), thalamocortical Spindles, and hippocampal sharp-wave ripples (SWR) are synchronized, allowing for the consolidation of hippocampal-dependent memories. However, evidence supporting this hypothesis during development is still lacking. Therefore, we performed successive object-in-place tests during a window of memory emergence and recorded in vivo the occurrence of SO, Spindles, and SWR during sleep, immediately after the memory encoding stage of the task. We found that hippocampal-dependent memory emerges at the end of the 4th postnatal week independently of task overtraining. Furthermore, we observed that those animals with better performance in the memory task had increased Spindle density and duration and lower density of SWR. Moreover, we observed changes in the SO-Spindle and Spindle-SWR temporal-coupling during this developmental period. Our results provide new evidence for the onset of hippocampal-dependent memory and its relationship to the oscillatory phenomenon occurring during sleep that helps us understand how memory consolidation models fit into the early stages of postnatal development.

Postnatal development of projections of the postrhinal cortex to the entorhinal cortex in the rat

The ability to encode and retrieve contextual information is an inherent feature of episodic memory that starts to develop during childhood. The postrhinal cortex, an area of the parahippocampal region, has a crucial role in encoding object-space information and translating egocentric to allocentric representation of local space. The strong connectivity of POR with the adjacent entorhinal cortex, and consequently the hippocampus, suggests that the development of these connections could support the postnatal development of contextual memory. Here, we report that postrhinal cortex projections of the rat develop progressively from the first to the third postnatal week starting in the medial entorhinal cortex before spreading to the lateral entorhinal cortex. The increased spread and complexity of postrhinal axonal distributions is accompanied by an increased complexity of entorhinal dendritic trees and an increase of postrhinal - entorhinal synapses, which supports a gradual maturation in functional activity.SIGNIFICANCE STATEMENTPostrhinal-entorhinal cortical interplay mediates important aspects of encoding and retrieval of contextual information that is important for episodic memory. To better understand the function of the postrhinal interactions with the entorhinal cortex we studied the postnatal development of the connection between the two cortical areas. Our study describes the postnatal development of the postrhinal-to-entorhinal projections as established with neuroanatomical and electrophysiological methods. The projections gradually reach functionally different areas of the entorhinal cortex, reaching the area involved in spatial functions first, followed by the part involved in representing information about objects and sequences of events.

Differential Recruitment of the Infralimbic Cortex in Recent and Remote Retrieval and Extinction of Aversive Memory in Post-Weanling Rats

BACKGROUND

We previously showed that the infralimbic medial prefrontal cortex (IL-mPFC) plays an important role in recent and remote memory retrieval and extinction of conditioned odor aversion (COA) and contextual fear conditioning (CFC) in adult rats. Because the mPFC undergoes maturation during post-weaning, here, we aimed to explore (1) whether post-weanling rats can form recent and remote COA and CFC memory, and (2) the role of the IL-mPFC in mediating these processes.

METHODS

To investigate the retrieval process, we transiently inactivated the IL-mPFC with lidocaine prior to the retrieval test at either recent or remote time points. To target the consolidation process, we applied the protein synthesis inhibitor after the retrieval at recent or remote time points.

RESULTS

Our results show that the post-weanling animals were able to develop both recent and remote memory of both COA and CFC. IL-mPFC manipulations had no effect on retrieval or extinction of recent and remote COA memory, suggesting that the IL has no effect in COA at this developmental stage. In contrast, the IL-mPFC played a role in (1) the extinction of recent, but not remote, CFC memory, and (2) the retrieval of remote, but not recent, CFC memory. Moreover, remote, but not recent, CFC retrieval enhanced c-Fos protein expression in the IL-mPFC.

CONCLUSIONS

Altogether, these results point to a differential role of the IL-mPFC in recent and remote CFC memory retrieval and extinction and further confirm the differences in the role of IL-mPFC in these processes in post-weanling and adult animals.

Object recognition and Morris water maze to detect cognitive impairment from mild hippocampal damage in rats: A reflection based on the literature and experience

Object recognition (OR) and the Morris water maze (MWM) are classical tasks widely used to assess memory parameters and deficits in rodents. Learning processes in both tasks involve integrity of the hippocampus and associated regions, and prefrontal cortex connections. Here, we highlight the idea that these classical tests can be used to indicate memory deficits caused by models of disease that affect hippocampal function in rats, and identify some practical issues of OR and MWM, based on the literature and our experience. Additionally, we have shown that the performance of both tasks does not alter blood levels of corticosterone, considering exposure to a single task. Hence, taking into consideration the difficulties and care required during task execution, the infrastructure needed and the training of the experimenter, we suggest that OR and its variations offer minimal manageable stressful conditions, representing an effective and practical tool for hippocampal-related memory assessment of rats. Thus, OR may provide similar information to that of the MWM, despite controversy regarding hippocampus participation in OR and given due differences in the types of memory evaluated and researchers' objectives. We recommend the observation of some important precautions and details, also based on the literature and our own experience.

Promoting and Optimizing the Use of 3D-Printed Objects in Spontaneous Recognition Memory Tasks in Rodents: A Method for Improving Rigor and Reproducibility

Spontaneous recognition memory tasks are widely used to assess cognitive function in rodents and have become commonplace in the characterization of rodent models of neurodegenerative, neuropsychiatric and neurodevelopmental disorders. Leveraging an animal’s innate preference for novelty, these tasks use object exploration to capture the what, where and when components of recognition memory. Choosing and optimizing objects is a key feature when designing recognition memory tasks. Although the range of objects used in these tasks varies extensively across studies, object features can bias exploration, influence task difficulty and alter brain circuit recruitment. Here, we discuss the advantages of using 3D-printed objects in rodent spontaneous recognition memory tasks. We provide strategies for optimizing their design and usage, and offer a repository of tested, open-source designs for use with commonly used rodent species. The easy accessibility, low-cost, renewability and flexibility of 3D-printed open-source designs make this approach an important step toward improving rigor and reproducibility in rodent spontaneous recognition memory tasks.

Long-term associative memory in rats: Effects of familiarization period in object-place-context recognition test

Spontaneous recognition tests, which utilize rodents’ innate tendency to explore novelty, can evaluate not only simple non-associative recognition memory but also more complex associative memory in animals. In the present study, we investigated whether the length of the object familiarization period (sample phase) improved subsequent novelty discrimination in the spontaneous object, place, and object-place-context (OPC) recognition tests in rats. In the OPC recognition test, rats showed a significant novelty preference only when the familiarization period was 30 min but not when it was 5 min or 15 min. In addition, repeated 30-min familiarization periods extended the significant novelty preference to 72 hours. However, the rats exhibited a successful discrimination between the stayed and replaced objects under 15 min and 30 min familiarization period conditions in the place recognition test and between the novel and familiar objects under all conditions of 5, 15 and 30 min in the object recognition test. Our results suggest that the extension of the familiarization period improves performance in the spontaneous recognition paradigms, and a longer familiarization period is necessary for long-term associative recognition memory than for non-associative memory.

Ontogeny of spontaneous recognition memory in rodents

Spontaneous recognition memory tasks explore thewhat,whereandwhencomponents of recognition memory. These tasks are widely used in rodents to assess cognitive function across the lifespan. While several neurodevelopmental and mental disorders present symptom onset in early life, very little is known about how memories are expressed in early life, and as a consequence how they may be affected in pathological conditions. In this review, we conduct an analysis of the studies examining the expression of spontaneous recognition memory in young rodents. We compiled studies using four different tasks: novel object recognition, object location, temporal order recognition and object place. First, we identify major sources of variability between early life spontaneous recognition studies and classify them for later comparison. Second, we use these classifications to explore the current knowledge on the ontogeny of each of these four spontaneous recognition memory tasks. We conclude by discussing the possible implications of the relative time of onset for each of these tasks and their respective neural correlates. In compiling this research, we hope to advance on establishing a developmental timeline for the emergence of distinct components of recognition memory, while also identifying key areas of focus for future research. Establishing the ontogenetic profile of rodent spontaneous recognition memory tasks will create a necessary blueprint for cognitive assessment in animal models of neurodevelopmental and mental disorders, a first step towards improved and earlier diagnosis as well as novel intervention strategies.

Developmental onset distinguishes three types of spontaneous recognition memory in mice

Spontaneous recognition memory tasks build on an animal’s natural preference for novelty to assess the what, where and when components of episodic memory. Their simplicity, ethological relevance and cross-species adaptability make them extremely useful to study the physiology and pathology of memory. Recognition memory deficits are common in rodent models of neurodevelopmental disorders, and yet very little is known about the expression of spontaneous recognition memory in young rodents. This is exacerbated by the paucity of data on the developmental onset of recognition memory in mice, a major animal model of disease. To address this, we characterized the ontogeny of three types of spontaneous recognition memory in mice: object location, novel object recognition and temporal order recognition. We found that object location is the first to emerge, at postnatal day (P)21. This was followed by novel object recognition (24 h delay), at P25. Temporal order recognition was the last to emerge, at P28. Elucidating the developmental expression of recognition memory in mice is critical to improving our understanding of the ontogeny of episodic memory, and establishes a necessary blueprint to apply these tasks to probe cognitive deficits at clinically relevant time points in animal models of developmental disorders.

Sustained correction of associative learning deficits after brief, early treatment in a rat model of Fragile X Syndrome

Fragile X Syndrome (FXS) is one of the most common monogenic forms of autism and intellectual disability. Preclinical studies in animal models have highlighted the potential of pharmaceutical intervention strategies for alleviating the symptoms of FXS. However, whether treatment strategies can be tailored to developmental time windows that define the emergence of particular phenotypes is unknown. Similarly, whether a brief, early intervention can have long-lasting beneficial effects, even after treatment cessation, is also unknown. To address these questions, we first examined the developmental profile for the acquisition of associative learning in a rat model of FXS. Associative memory was tested using a range of behavioral paradigms that rely on an animal's innate tendency to explore novelty. Fmr1 knockout (KO) rats showed a developmental delay in their acquisition of object-place recognition and did not demonstrate object-place-context recognition paradigm at any age tested (up to 23 weeks of age). Treatment of Fmr1 KO rats with lovastatin between 5 and 9 weeks of age, during the normal developmental period that this associative memory capability is established, prevents the emergence of deficits but has no effect in wild-type animals. Moreover, we observe no regression of cognitive performance in the FXS rats over several months after treatment. This restoration of the normal developmental trajectory of cognitive function is associated with the sustained rescue of both synaptic plasticity and altered protein synthesis. The findings provide proof of concept that the impaired emergence of the cognitive repertoire in neurodevelopmental disorders may be prevented by brief, early pharmacological intervention.

NMDA receptors and the ontogeny of post-shock and retention freezing during contextual fear conditioning

The ontogeny and NMDA-receptor (NMDAR) mechanisms of context conditioning were examined during standard contextual fear conditioning (sCFC) - involving context and context-shock learning in the same trial - as a comparison with our previous reports on the Context Preexposure Facilitation Effect (CPFE), which separates these two types of learning by 24 hr. In Experiment 1, systemic administration of the NMDAR antagonist, MK-801, prior to conditioning disrupted retention but not post-shock freezing during sCFC in PD31 rats. Experiment 2 replicated and extended this effect to PD17 versus PD31 rats. Consistent with Experiment 1, pre-training MK-801 spared post-shock freezing but impaired retention freezing in PD31 rats. In contrast, pre-training MK-801 disrupted post-shock freezing in PD17 rats, which showed no retention freezing regardless of drug. These results reveal developmental differences in the role of NMDAR activity in the acquisition versus retention of a context-shock association during sCFC in pre-weanling and adolescent rats.

Mouse knockout of guanylyl cyclase C: Recognition memory deficits in the absence of activity changes

Guanylyl cyclase C (GC-C) is found in brain regions where dopamine is expressed. We characterized a mouse in which GC-C was knocked out (KO) that was reported to be a model of attention deficit hyperactivity disorder (ADHD). We re-examined this model and controlled for litter effects, used 16 to 23 mice per genotype per sex and assessed an array of behavioral and neurochemical outcomes. GC-C KO mice showed no phenotypic differences from wild-type mice on most behavioral tests, or on striatal or hippocampal monoamines, and notably no evidence of an ADHD-like phenotype. KO mice were impaired on novel object recognition, had decreased tactile startle but not acoustic startle, and females had increased latency on cued training trials in the Morris water maze, but not hidden platform spatial learning trials. Open-field activity showed small differences in females but not males. The data indicate that the GC-C KO mouse with proper controls and sample sizes has a moderate cognitive and startle phenotype but has no ADHD-like phenotype.

The ontogeny of memory persistence and specificity

Interest in the ontogeny of memory blossomed in the twentieth century following the initial observations that memories from infancy and early childhood are rapidly forgotten. The intense exploration of infantile amnesia in subsequent years has led to a thorough characterization of its psychological determinants, although the neurobiology of memory persistence has long remained elusive. By contrast, other phenomena in the ontogeny of memory like infantile generalization have received relatively less attention. Despite strong evidence for reduced memory specificity during ontogeny, infantile generalization is poorly understood from psychological and neurobiological perspectives. In this review, we examine the ontogeny of memory persistence and specificity in humans and nonhuman animals at the levels of behavior and the brain. To this end, we first describe the behavioral phenotypes associated with each phenomenon. Looking into the brain, we then discuss neurobiological mechanisms in the hippocampus that contribute to the ontogeny of memory. Hippocampal neurogenesis and critical period mechanisms have recently been discovered to underlie amnesia during early development, and at the same time, we speculate that similar processes may contribute to the early bias towards memory generalization.

A Barnes maze for juvenile rats delineates the emergence of spatial navigation ability

The neural bases of cognition may be greatly informed by relating temporally defined developmental changes in behavior with concurrent alterations in neural function. A robust improvement in performance in spatial learning and memory tasks occurs at 3 wk of age in rodents. We reported that the developmental increase of spontaneous alternation in a Y-maze was related to changes in temporal dynamics of fast glutamatergic synaptic transmission in the hippocampus. We also showed that, during allothetic behaviors in the Y-maze, network oscillation power increased at frequency bands known to support spatial learning and memory in adults. However, there are no discrete learning and memory phases during free exploration in the Y-maze. Thus, we adapted the Barnes maze for use with juvenile rats. Following a single platform exposure in dim light on the day before training (to encourage exploration), animals were trained on the subsequent 2 d in bright light to find a hidden escape box and then underwent a memory test 24 h later. During escape training, the older animals learned the task in 1 d, while the younger animals required 2 d and did not reach the performance of older animals. Long-term memory performance was also superior in the older animals. Thus, we have validated the use of the Barnes maze for this developmental period and established a timeline for the ontogeny of spatial navigation ability in this maze around 3 wk of age. Subsequent work will pair in vivo recording of hippocampal oscillations and single units with this task to help identify how hippocampal maturation might relate to performance improvements.