Retrosplenial Cortex and Long-Term Memory: Molecules to Behavior

The Granular Retrosplenial Cortex Is Necessary in Male Rats for Object-Location Associative Learning and Memory, But Not Spatial Working Memory or Visual Discrimination and Reversal, in the Touchscreen Operant Chamber

The retrosplenial cortex (RSC) is a hub of diverse afferent and efferent projections thought to be involved in associative learning. RSC shows early pathology in mild cognitive impairment and Alzheimer's disease (AD), which impairs associative learning. To understand and develop therapies for diseases such as AD, animal models are essential. Given the importance of human RSC in object-location associative learning and the success of object-location associative paradigms in human studies and in the clinic, it would be of considerable value to establish a translational model of object-location learning for the rodent. For this reason, we sought to test the role of RSC in object-location learning in male rats using the object-location paired-associates learning (PAL) touchscreen task. First, increased cFos immunoreactivity was observed in granular RSC following PAL training when compared with extended pretraining controls. Following this, RSC lesions following PAL acquisition were used to explore the necessity of the RSC in object-location associative learning and memory and two tasks involving only one modality: trial-unique nonmatching-to-location for spatial working memory and pairwise visual discrimination/reversal. RSC lesions impaired both memory for learned paired-associates and learning of new object-location associations but did not affect performance in either the spatial or visual single-modality tasks. These findings provide evidence that RSC is necessary for object-location learning and less so for learning and memory involving the individual modalities therein.

Longitudinal Neuropathological Consequences of Extracranial Radiation Therapy in Mice

Cancer-related cognitive impairment (CRCI) is a consequence of chemotherapy and extracranial radiation therapy (ECRT). Our prior work demonstrated gliosis in the brain following ECRT in SKH1 mice. The signals that induce gliosis were unclear. Right hindlimb skin from SKH1 mice was treated with 20 Gy or 30 Gy to induce subclinical or clinical dermatitis, respectively. Mice were euthanized at 6 h, 24 h, 5 days, 12 days, and 25 days post irradiation, and the brain, thoracic spinal cord, and skin were collected. The brains were harvested for spatial proteomics, immunohistochemistry, Nanostring nCounter® glial profiling, and neuroinflammation gene panels. The thoracic spinal cords were evaluated by immunohistochemistry. Radiation injury to the skin was evaluated by histology. The genes associated with neurotransmission, glial cell activation, innate immune signaling, cell signal transduction, and cancer were differentially expressed in the brains from mice treated with ECRT compared to the controls. Dose-dependent increases in neuroinflammatory-associated and neurodegenerative-disease-associated proteins were measured in the brains from ECRT-treated mice. Histologic changes in the ECRT-treated mice included acute dermatitis within the irradiated skin of the hindlimb and astrocyte activation within the thoracic spinal cord. Collectively, these findings highlight indirect neuronal transmission and glial cell activation in the pathogenesis of ECRT-related CRCI, providing possible signaling pathways for mitigation strategies.

Spatially resolved cell atlas of the teleost telencephalon and deep homology of the vertebrate forebrain

The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking variations in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely "everted" telencephalon, which has confounded comparisons of their brain regions to other vertebrates. Here we combine spatial transcriptomics and single nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the Mchenga conophorus cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell-types in the fish telencephalon and subpallial, hippocampal, and cortical cell-types in tetrapods, and find support for partial eversion of the teleost telencephalon. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.

Psilocybin-enhanced fear extinction linked to bidirectional modulation of cortical ensembles

The serotonin 2 receptor (5HT2R) agonist psilocybin displays rapid and persistent therapeutic efficacy across neuropsychiatric disorders characterized by cognitive inflexibility. However, the impact of psilocybin on patterns of neural activity underlying sustained changes in behavioral flexibility has not been characterized. To test the hypothesis that psilocybin enhances behavioral flexibility by altering activity in cortical neural ensembles, we performed longitudinal single-cell calcium imaging in the retrosplenial cortex across a five-day trace fear learning and extinction assay. A single dose of psilocybin induced ensemble turnover between fear learning and extinction days while oppositely modulating activity in fear- and extinction- active neurons. The acute suppression of fear-active neurons and delayed recruitment of extinction-active neurons were predictive of psilocybin-enhanced fear extinction. A computational model revealed that acute inhibition of fear-active neurons by psilocybin is sufficient to explain its neural and behavioral effects days later. These results align with our hypothesis and introduce a new mechanism involving the suppression of fear-active populations in the retrosplenial cortex.

Impact of high-fat diet on cognitive behavior and central and systemic inflammation with aging and sex differences in mice

Aging and age-related diseases are associated with cellular stress, metabolic imbalance, oxidative stress, and neuroinflammation, accompanied by cognitive impairment. Lifestyle factors such as diet, sleep fragmentation, and stress can potentiate damaging cellular cascades and lead to an acceleration of brain aging and cognitive impairment. High-fat diet (HFD) has been associated with obesity, metabolic disorders like diabetes, and cardiovascular disease. HFD also induces neuroinflammation, impairs learning and memory, and may increase anxiety-like behavior. Effects of a HFD may also vary between sexes. The interaction between Age- and Sex- and Diet-related changes in neuroinflammation and cognitive function is an important and poorly understood area of research. This study was designed to examine the effects of HFD on neuroinflammation, behavior, and neurodegeneration in mice in the context of aging or sex differences. In a series of studies, young (2-3 months) or old (12-13 months) C57BL/6J male mice or young male and female C57Bl/6J mice were fed either a standard diet (SD) or a HFD for 5-6 months. Behavior was assessed in Activity Chamber, Y-maze, Novel Place Recognition, Novel Object Recognition, Elevated Plus Maze, Open Field, Morris Water Maze, and Fear Conditioning. Post-mortem analyses assessed a panel of inflammatory markers in the plasma and hippocampus. Additionally, proteomic analysis of the hypothalamus, neurodegeneration, neuroinflammation in the locus coeruleus, and neuroinflammation in the hippocampus were assessed in a subset of young and aged male mice. We show that HFD increased body weight and decreased locomotor activity across groups compared to control mice fed a SD. HFD altered anxiety-related exploratory behavior. HFD impaired spatial learning and recall in young male mice and impaired recall in cued fear conditioning in young and aged male mice, with no effects on spatial learning or fear conditioning in young female mice. Effects of Age and Sex were observed on neuroinflammatory cytokines, with only limited effects of HFD. HFD had a more significant impact on systemic inflammation in plasma across age and sex. Aged male mice had induction of microglial immunoreactivity in both the locus coeruleus (LC) and hippocampus an effect that HFD exacerbated in the hippocampal CA1 region. Proteomic analysis of the hypothalamus revealed changes in pathways related to metabolism and neurodegeneration with both aging and HFD in male mice. Our findings suggest that HFD induces widespread systemic inflammation and limited neuroinflammation. In addition, HFD alters exploratory behavior in male and female mice, and impairs learning and memory in male mice. These results provide valuable insight into the impact of diet on cognition and aging pathophysiology.

Repetition Suppression Reveals Cue-Specific Spatial Representations for Landmarks and Self-Motion Cues in the Human Retrosplenial Cortex

The efficient use of various spatial cues within a setting is crucial for successful navigation. Two fundamental forms of spatial navigation, landmark-based and self-motion-based, engage distinct cognitive mechanisms. The question of whether these modes invoke shared or separate spatial representations in the brain remains unresolved. While nonhuman animal studies have yielded inconsistent results, human investigation is limited. In our previous work (Chen et al., 2019), we introduced a novel spatial navigation paradigm utilizing ultra-high field fMRI to explore neural coding of positional information. We found that different entorhinal subregions in the right hemisphere encode positional information for landmarks and self-motion cues. The present study tested the generalizability of our previous finding with a modified navigation paradigm. Although we did not replicate our previous finding in the entorhinal cortex, we identified adaptation-based allocentric positional codes for both cue types in the retrosplenial cortex (RSC), which were not confounded by the path to the spatial location. Crucially, the multi-voxel patterns of these spatial codes differed between the cue types, suggesting cue-specific positional coding. The parahippocampal cortex exhibited positional coding for self-motion cues, which was not dissociable from path length. Finally, the brain regions involved in successful navigation differed from our previous study, indicating overall distinct neural mechanisms recruited in our two studies. Taken together, the current findings demonstrate cue-specific allocentric positional coding in the human RSC in the same navigation task for the first time and that spatial representations in the brain are contingent on specific experimental conditions.

Remote memory in a Bayesian model of context fear conditioning (BaconREM)

Here, we propose a model of remote memory (BaconREM), which is an extension of a previously published Bayesian model of context fear learning (BACON) that accounts for many aspects of recently learned context fear. BaconREM simulates most known phenomenology of remote context fear as studied in rodents and makes new predictions. In particular, it predicts the well-known observation that fear that was conditioned to a recently encoded context becomes hippocampus-independent and shows much-enhanced generalization ("hyper-generalization") when systems consolidation occurs (i.e., when memory becomes remote). However, the model also predicts that there should be circumstances under which the generalizability of remote fear may not increase or even decrease. It also predicts the established finding that a "reminder" exposure to a feared context can abolish hyper-generalization while at the same time making remote fear again hippocampus-dependent. This observation has in the past been taken to suggest that reminders facilitate access to detail memory that remains permanently in the hippocampus even after systems consolidation is complete. However, the present model simulates this result even though it totally moves all the contextual memory that it retains to the neo-cortex when context fear becomes remote.

Sleep-wake dependent hippocampal regulation of fear memory

The hippocampus (HPC) plays a pivotal role in fear learning and memory. Our two recent studies suggest that rapid eye movement (REM) sleep via the HPC downregulates fear memory consolidation and promotes fear extinction. However, it is not clear whether and how the dorsal and the ventral HPC regulates fear memory differently; and how the HPC in wake regulates fear memory. By chemogenetic stimulating in the HPC directly and its afferent entorhinal cortex that selectively activated the HPC in REM sleep for 3-6 h post-fear-acquisition, we found that HPC activation in REM sleep consolidated fear extinction memory. In particular, dorsal HPC (dHPC) stimulation in REM sleep virtually eliminated fear memory by enhancing fear extinction and reducing fear memory consolidation. By contrast, chemogenetic stimulating HPC afferent the supramammillary nucleus (SUM) induced 3-hr wake with HPC activation impaired fear extinction. Finally, desipramine (DMI) injection that selectively eliminated REM sleep for >6 h impaired fear extinction. Our results demonstrate that the HPC is critical for fear memory regulation; and wake HPC and REM sleep HPC have an opposite role in fear extinction of respective impairment and consolidation.

Generalization and discrimination of inhibitory avoidance differentially engage anterior and posterior retrosplenial subregions

Introduction

In a variety of behavioral procedures animals will show selective fear responding in shock-associated contexts, but not in other contexts. However, several factors can lead to generalized fear behavior, where responding is no longer constrained to the conditioning context and will transfer to novel contexts.

Methods

Here, we assessed memory generalization using an inhibitory avoidance paradigm to determine if generalized avoidance behavior engages the retrosplenial cortex (RSC). Male and female Long Evans rats received inhibitory avoidance training prior to testing in the same context or a shifted context in two distinct rooms; one room that had fluorescent lighting (Light) and one that had red LED lighting (Dark).

Results

We found that animals tested in a light context maintained context-specificity; animals tested in the same context as training showed longer latencies to cross and animals tested in the shifted context showed shorter latencies to cross. However, animals tested in the dark generalized their avoidance behavior; animals tested in the same context and animals tested in the shifted context showed similarly-high latencies to cross. We next examined expression of the immediate early gene zif268 and perineuronal nets (PNNs) following testing and found that while activity in the basolateral amygdala corresponded with overall levels of avoidance behaviors, anterior RSC (aRSC) activity corresponded with learned avoidance generally, but posterior RSC (pRSC) activity seemed to correspond with generalized memory. PNN reduction in the RSC was associated with memory formation and retrieval, suggesting a role for PNNs in synaptic plasticity. Further, PNNs did not reduce in the RSC in animals who showed a generalized avoidance behavior, in line with their hypothesized role in memory consolidation.

Discussion

These findings suggest that there is differential engagement of retrosplenial subregions along the rostrocaudal axis to generalization and discrimination.

Single-cell RNA sequencing uncovers the cell type-dependent transcriptomic changes in the retrosplenial cortex after peripheral nerve injury

The retrosplenial cortex (RSC) is a vital area for storing remote memory and has recently been found to undergo broad changes after peripheral nerve injury. However, little is known about the role of RSC in pain regulation. Here, we examine the involvement of RSC in the pain of mice with nerve injury. Notably, reducing the activities of calcium-/calmodulin-dependent protein kinase type II-positive splenial neurons chemogenetically increases paw withdrawal threshold and extends thermal withdrawal latency in mice with nerve injury. The single-cell or single-nucleus RNA-sequencing results predict enhanced excitatory synaptic transmissions in RSC induced by nerve injury. Local infusion of 1-naphthyl acetyl spermine into RSC to decrease the excitatory synaptic transmissions relieves pain and induces conditioned place preference. Our data indicate that RSC is critical for regulating physiological and neuropathic pain. The cell type-dependent transcriptomic information would help understand the molecular basis of neuropathic pain.

Rethinking retrosplenial cortex: Perspectives and predictions

The last decade has produced exciting new ideas about retrosplenial cortex (RSC) and its role in integrating diverse inputs. Here, we review the diversity in forms of spatial and directional tuning of RSC activity, temporal organization of RSC activity, and features of RSC interconnectivity with other brain structures. We find that RSC anatomy and dynamics are more consistent with roles in multiple sensorimotor and cognitive processes than with any isolated function. However, two more generalized categories of function may best characterize roles for RSC in complex cognitive processes: (1) shifting and relating perspectives for spatial cognition and (2) prediction and error correction for current sensory states with internal representations of the environment. Both functions likely take advantage of RSC's capacity to encode conjunctions among sensory, motor, and spatial mapping information streams. Together, these functions provide the scaffold for intelligent actions, such as navigation, perspective taking, interaction with others, and error detection.

Retrosplenial and Hippocampal Synchrony during Retrieval of Old Memories in Macaques

Memory for events from the distant past relies on multiple brain regions, but little is known about the underlying neural dynamics that give rise to such abilities. We recorded neural activity in the hippocampus and retrosplenial cortex of two female rhesus macaques as they visually selected targets in year-old and newly acquired object-scene associations. Whereas hippocampal activity was unchanging with memory age, the retrosplenial cortex responded with greater magnitude alpha oscillations (10-15 Hz) and greater phase locking to memory-guided eye movements during retrieval of old events. A similar old-memory enhancement was observed in the anterior cingulate cortex but in a beta2/gamma band (28-35 Hz). In contrast, remote retrieval was associated with decreased gamma-band synchrony between the hippocampus and each neocortical area. The increasing retrosplenial alpha oscillation and decreasing hippocampocortical synchrony with memory age may signify a shift in frank memory allocation or, alternatively, changes in selection among distributed memory representations in the primate brain.SIGNIFICANCE STATEMENT Memory depends on multiple brain regions, whose involvement is thought to change with time. Here, we recorded neuronal population activity from the hippocampus and retrosplenial cortex as nonhuman primates searched for objects embedded in scenes. These memoranda were either newly presented or a year old. Remembering old material drove stronger oscillations in the retrosplenial cortex and led to a greater locking of neural activity to search movements. Remembering new material revealed stronger oscillatory synchrony between the hippocampus and retrosplenial cortex. These results suggest that with age, memories may come to rely more exclusively on neocortical oscillations for retrieval and search guidance and less on long-range coupling with the hippocampus.

β Bursting in the Retrosplenial Cortex Is a Neurophysiological Correlate of Environmental Novelty Which Is Disrupted in a Mouse Model of Alzheimer's Disease

The retrosplenial cortex (RSC) plays a significant role in spatial learning and memory and is functionally disrupted in the early stages of Alzheimer's disease (AD). In order to investigate neurophysiological correlates of spatial learning and memory in this region we employed in vivo electrophysiology in awake and freely moving male mice, comparing neural activity between wild-type and J20 mice, a transgenic model of AD-associated amyloidopathy. To determine the response of the RSC to environmental novelty local field potentials (LFPs) were recorded while mice explored novel and familiar recording arenas. In familiar environments we detected short, phasic bursts of β (20-30 Hz) oscillations (β bursts), which arose at a low but steady rate. Exposure to a novel environment rapidly initiated a dramatic increase in the rate, size and duration of β bursts. Additionally, θ-α/β cross-frequency coupling was significantly higher during novelty, and spiking of neurons in the RSC was significantly enhanced during β bursts. Finally, excessive β bursting was seen in J20 mice, including increased β bursting during novelty and familiarity, yet a loss of coupling between β bursts and spiking activity. These findings support the concept that β bursting may be responsible for the activation and reactivation of neuronal ensembles underpinning the formation and maintenance of cortical representations, and that disruptions to this activity in J20 mice may underlie cognitive impairments seen in these animals.SIGNIFICANCE STATEMENT The retrosplenial cortex (RSC) is thought to be involved in the formation, recall and consolidation of contextual memory. The discovery of bursts of β oscillations in this region, which are associated with increased neuronal spiking and strongly upregulated while mice explore novel environments, provides a potential mechanism for the activation of neuronal ensembles, which may underlie the formation of cortical representations of context. Excessive β bursting in the RSC of J20 mice, a mouse model of Alzheimer's disease (AD), alongside the disassociation of β bursting from neuronal spiking, may underlie spatial memory impairments previously shown in these mice. These findings introduce a novel neurophysiological correlate of spatial learning and memory, and a potentially new form of AD-related cortical dysfunction.

Unique roles for the anterior and posterior retrosplenial cortices in encoding and retrieval of memory for context

The rat retrosplenial cortex (RSC) makes critical contributions to learning and memory but these contributions may not be uniform along its rostro-caudal axis. Previous work suggests that event-related and context-related information are differentially encoded by anterior and posterior RSC subregions. Here, we further test this idea using a procedure in which spatial/environmental cues (context) and discrete event memories are acquired separately. All animals received a 5-min pre-exposure to the training context 24 h before contextual fear conditioning where shock was delivered immediately upon being placed in the chamber. Rats were tested for memory for the context the next day. We found that optogenetic inhibition of cells in only the posterior RSC during the pre-exposure phase, when spatial information is encoded, reduced behavioral responding during the subsequent memory test. However, similar inhibition of either the anterior or posterior RSC during shock delivery, when information about both the context and the shock become integrated, impaired memory. Finally, inhibiting cellular activity in only the posterior RSC during memory retrieval during testing reduced responding. Together, these results suggest that while activity in both subregions is needed during the period in which the event-related information becomes integrated with the context representation, the posterior RSC is important for both memory formation and retrieval or expression of memory for information about the context. These results add to a growing literature demonstrating a role for the RSC in integration of multiple aspects of memory, and provide information on how spatial representations reliant on the retrosplenial cortex interact with associative learning.

Association between walking speed and cognitive domain functions in Chinese suburban-dwelling older adults

Objective

To examine the relationship among walking speed, cognitive impairment, and cognitive domain functions in older men and women living in a Chinese suburban community.

Methods

In total, 625 elderly (72.54 ± 5.80 years old) men (n = 258) and women (n = 367) from the Chongming district of Shanghai participated in this study. All participants had Mini-Mental State Examination (MMSE), 4-m walking test, medical history questionnaire, and physical examination. They were grouped according to walking speed (>0.8 vs. ≤ 0.8 m/s) with the stratification of sex. The odds ratio (OR) and the 95% confidence interval (CI) were assessed using the chi-square test and logistic regression analysis.

Results

Around 11.6% of men and 14.2% of women had slow walking speeds. After adjusting for age, body mass index (BMI), education level, spouse, faller, the Geriatric Depression Scale (GDS) score, heart disease, stroke, arthritis, and low back pain, walking speed was negatively related to cognitive impairment in men (OR 0.11 [95% CI: 0.01, 0.94]; p = 0.043). In addition, the relationship between walking speed and impaired orientation was significant in both men (OR 0.003 [95% CI: 0.001, 0.05]; p < 0.001) and women (OR 0.15 [95% CI: 0.03, 0.75]; p = 0.021).

Conclusion

The relationship between walking speed and cognitive impairment was only significant in men, but the association with impaired orientation was found in both men and women. Assessing the walking speed of the elderly is beneficial, which may help with early detection and early therapeutic prevention of cognitive impairment.

Effect of Escitalopram on the Number of DCX-Positive Cells and NMUR2 Receptor Expression in the Rat Hippocampus under the Condition of NPSR Receptor Blockade

BACKGROUND

Neuropeptide S (NPS) is a multifunctional regulatory factor that exhibits a potent anxiolytic activity in animal models. However, there are no reports dealing with the potential molecular interactions between the activity of selective serotonin reuptake inhibitors (SSRIs) and NPS signaling, especially in the context of adult neurogenesis and the expression of noncanonical stress-related neuropeptides such as neuromedin U (NMU). The present work therefore focused on immunoexpression of neuromedin U receptor 2 (NMUR2) and doublecortin (DCX) in the rat hippocampus after acute treatment with escitalopram and in combination with selective neuropeptide S receptor (NPSR) blockade.

METHODS

Studies were carried out on adult, male Sprague-Dawley rats that were divided into five groups: animals injected with saline (control) and experimental individuals treated with escitalopram (at single dose 10 mg/kg daily), escitalopram + SHA-68, a selective NPSR antagonist (at single dose 40 mg/kg), SHA-68 alone, and corresponding vehicle control. All animals were sacrificed under halothane anaesthesia. The whole hippocampi were quickly excised, fixed, and finally sliced for general qualitative immunohistochemical assessment of the NPSR and NMUR2 expression. The number of immature neurons was enumerated using immunofluorescent detection of doublecortin (DCX) expression within the subgranular zone (SGZ).

RESULTS

Acute escitalopram administration affects the number of DCX and NMUR2-expressing cells in the adult rat hippocampus. A decreased number of DCX-expressing neuroblasts after treatment with escitalopram was augmented by SHA-68 coadministration.

CONCLUSIONS

Early pharmacological effects of escitalopram may be at least partly connected with local NPSR-related alterations of neuroblast maturation in the rat hippocampus. Escitalopram may affect neuropeptide and DCX-expression starting even from the first dose. Adult neurogenesis may be regulated via paracrine neuropeptide S and NMU-related signaling.

Functional neuroanatomy of allocentric remote spatial memory in rodents

Successful spatial cognition involves learning, consolidation, storage, and later retrieval of a spatial memory trace. The functional contributions of specific brain areas and their interactions during retrieval of past spatial events are unclear. This systematic review collects studies about allocentric remote spatial retrieval assessed at least two weeks post-acquisition in rodents. Results including non-invasive interventions, brain lesion and inactivation experiments, pharmacological treatments, chemical agent administration, and genetic manipulations revealed that there is a normal forgetting when time-periods are close to or exceed one month. Moreover, changes in the morphology and functionality of neocortical areas, hippocampus, and other subcortical structures, such as the thalamus, have been extensively observed as a result of spatial memory retrieval. In conclusion, apart from an increasingly neocortical recruitment in remote spatial retrieval, the hippocampus seems to participate in the retrieval of fine spatial details. These results help to better understand the timing of memory maintenance and normal forgetting, outlining the underlying brain areas implicated.

Examining a role for the retrosplenial cortex in age-related memory impairment

Aging is often characterized by changes in the ability to form and accurately recall episodic memories, and this is especially evident in neuropsychiatric conditions including Alzheimer's disease and dementia. Memory impairments and cognitive decline associated with aging mirror the impairments observed following damage to the retrosplenial cortex, suggesting that this region might be important for continued cognitive function throughout the lifespan. Here, we review lines of evidence demonstrating that degeneration of the retrosplenial cortex is critically involved in age-related memory impairment and suggest that preservation of function in this region as part of a larger circuit that supports memory maintenance will decrease the deleterious effects of aging on memory processing.

Loss of efferent projections of the hippocampal formation in the mouse intrahippocampal kainic acid model

Unilateral intrahippocampal injection of kainic acid is used as a model of medial temporal lobe epilepsy and provides a platform to study the mechanisms of epilepsy. Here, we used an AAV-9 EYFP-tagged viral vector as an anterograde tracer, injected into the dorsal and ventral hippocampus after kainic acid injection, to map out the efferent hippocampal projections after the development of spontaneous seizures in this model. The purpose of the study was to identify the extent of changes in hippocampal efferent system in several brain regions that receive significant inputs from the hippocampus. Loss of efferent hippocampal fibers was greatest in the retrosplenial cortex where neuronal loss was also observed. Loss of fibers was also observed in the fornix without any specific effect in the lateral mammillary nuclei. Although expected, these observations provide further evidence of the broader network effects as a result of hippocampal cell loss.

The limbic memory circuit and the neural basis of contextual memory

The hippocampus, retrosplenial cortex and anterior thalamus are key components of a neural circuit known to be involved in a variety of memory functions, including spatial, contextual and episodic memory. In this review, we focus on the role of this circuit in contextual memory processes. The background environment, or context, is a powerful cue for memory retrieval, and neural representations of the context provide a mechanism for efficiently retrieving relevant memories while avoiding interference from memories that belong to other contexts. Data from experimental lesions and neural manipulation techniques indicate that each of these regions is critical for contextual memory. Neurophysiological evidence from the hippocampus and retrosplenial cortex suggest that contextual information is represented within this circuit by population-level neural firing patterns that reliably differentiate each context a subject encounters. These findings indicate that encoding contextual information to support context-dependent memory retrieval is a key function of this circuit.

Correlation between Alteration of Sharp-wave Ripple Coupled Cortical Oscillation and Long-term Memory Deficit in Alzheimer Disease Model Mice

Alzheimer’s disease (AD) is the most common cause of dementia, characterized by prominent episodic memory dysfunction. Recent studies have suggested that there is a sequential mechanism in the memory deficit, with long-term ones preceding short-term ones. However, there is lack of explanation for these symptoms. Interaction between the hippocampus and retrosplenial cortex (RSC) during slow-wave sleep (SWS) is a crucial step for successful long-term memory formation. In particular, sharp-wave ripple (SWR) is a principal hippocampus oscillation that coordinates with RSC activity. To determine the relationship between memory dysfunction and SWR-related oscillation changes in AD, we implanted local field potential electrodes in the hippocampus and RSC of AD model mice (APP/PS1). We found that the SWR-coupled ripple wave increased in the RSC, while the amplitude of the SWR was preserved. In addition, the corresponding delta power in hippocampus and RSC was elevated, together with altered delta synchrony in AD mice. All these findings showed a significant correlation with long-term memory deficits measured in contextual fear conditions. Our study suggests that altered SWR-coupled oscillations are a possible underlying mechanism of episodic memory dysfunction in AD mice.

Functional connectivity of anterior retrosplenial cortex in object recognition memory

Recognition memory can rely on three components: "what", "where" and "when". Recently we demonstrated that the anterior retrosplenial cortex (aRSC), like the perirhinal cortex (PRH) and unlike the hippocampus (HP), is required for consolidation of the "what" component. Here, we aimed at studying which brain structures interact with the aRSC to process object recognition (OR) memory in rats. We studied the interaction of six brain structures that are connected to the aRSC during OR memory processing: PRH, medial prefrontal cortex (mPFC), anteromedial thalamic nuclei (AM), medial entorhinal cortex (MEC), anterior cingulate cortex (ACC) and the dorsal HP (dHP). We previously described the role of the PRH and dHP, so we first studied the participation of the mPFC, AM, MEC and ACC in OR memory consolidation by bilateral microinfusions of the GABA_A receptor agonist muscimol. We observed an impairment in OR long-term memory (LTM) when inactivating the mPFC, the AM and the MEC, but not the ACC. Then, we studied the functional connections by unilateral inactivation of the aRSC and each one of the six structures in the same (ipsilateral) or the opposite (contralateral) hemisphere. Our results showed an amnesic LTM effect in rats with ipsilateral inactivations of aRSC-PRH, aRSC-mPFC, aRSC-AM, or aRSC-MEC. On the other hand, we observed memory impairment when aRSC-ACC were inactivated in opposite hemispheres, and no effect when the aRSC-dHP connection was inactivated. Thus, our ipsilateral inactivation findings reveal that the aRSC and, at least one brain region required in OR LTM processing are essential to consolidate OR memory. In conclusion, our results show that several cortico-cortical and cortico-thalamic pathways are important for OR memory consolidation.

Optogenetic inhibition of either the anterior or posterior retrosplenial cortex disrupts retrieval of a trace, but not delay, fear memory

Previous work investigating the role of the retrosplenial cortex (RSC) in memory formation has demonstrated that its contributions are not uniform throughout the rostro-caudal axis. While the anterior region was necessary for encoding CS information in a trace conditioning procedure, the posterior retrosplenial cortex was needed to encode contextual information. Using the same behavioral procedure, we tested if there was a similar dissociation during memory retrieval. First, we found that memory retrieval following trace conditioning results in increased neural activity in both the anterior and posterior retrosplenial cortex, measured using the immediate early gene zif268. Similar increases were not found in either RSC subregion using a delay conditioning task. We then found that optogenetic inhibition of neural activity in either subregion impairs retrieval of a trace, but not delay, memory. Together these results add to a growing literature showing a role for the retrosplenial cortex in memory formation and retention. Further, they suggest that following formation, memory storage becomes distributed to a wider network than is needed for its initial consolidation.

Contributions of the rodent cingulate-retrosplenial cortical axis to associative learning and memory: A proposed circuit for persistent memory maintenance

While the anterior cingulate (ACC) and retrosplenial (RSC) cortices have been extensively studied for their role in spatial navigation, less is known about how they contribute to associative learning and later memory recall. The limited work that has been conducted on this topic suggests that each of these cortical regions makes distinct, but similar contributions to associative learning and memory. Here, we review evidence from the rodent literature demonstrating that while ACC activity seems to be necessary at remote time points associated with imprecise or generalized memories, the role of the RSC seems to be uniform over time. Together, the lines of evidence reviewed here suggest that the ACC and RSC likely function together to support memory formation and maintenance following associative learning.

The separate and combined properties of the granular (area 29) and dysgranular (area 30) retrosplenial cortex

Retrosplenial cortex contains two principal subdivisions, area 29 (granular) and area 30 (dysgranular). Their respective anatomical connections in the rat brain reveal that area 29 is the primary recipient of hippocampal and parahippocampal spatial and contextual information while area 30 is the primary interactor with current visual information. Lesion studies and measures of neuronal activity in rodents indicate that retrosplenial cortex helps to integrate space from different perspectives, e.g., egocentric and allocentric, providing landmark and heading cues for navigation and spatial learning. It provides a repository of scene information that, over time, becomes increasingly independent of the hippocampus. These processes, reflect the interactive actions between areas 29 and 30, along with their convergent influences on cortical and thalamic targets. Consequently, despite their differences, both areas 29 and 30 are necessary for an array of spatial and learning problems.

The anterior retrosplenial cortex encodes event-related information and the posterior retrosplenial cortex encodes context-related information during memory formation

The retrosplenial cortex (RSC) is extensively interconnected with the dorsal hippocampus and has several important roles in learning and memory. Recent work has demonstrated that certain types of context-dependent learning are selectively impaired when the posterior, but not the anterior, region of the RSC is damaged, suggesting that the role of the RSC in memory formation may not be uniform along its rostro-caudal axis. The current experiments tested the idea that the anterior and posterior portions of the rat RSC contribute to different aspects of memory formation. We first confirmed that brief optogenetic inhibition of either the anterior or posterior RSC resulted in decreased local cellular activity as indexed by immediate early gene zif268 expression and that this decrease was restricted to the target region within RSC. We then found that silencing the anterior or posterior RSC during trace fear training trials had different effects on memory: While inhibiting neural activity in the anterior RSC had a selective impact on behavior evoked by the auditory CS, inhibition of the posterior RSC selectively impaired memory for the context in which training was conducted. These results contribute to a growing literature that supports functionally distinct roles in learning and memory for subregions of the RSC.

Memories are not written in stone: Re-writing fear memories by means of non-invasive brain stimulation and optogenetic manipulations

The acquisition of fear associative memory requires brain processes of coordinated neural activity within the amygdala, prefrontal cortex (PFC), hippocampus, thalamus and brainstem. After fear consolidation, a suppression of fear memory in the absence of danger is crucial to permit adaptive coping behavior. Acquisition and maintenance of fear extinction critically depend on amygdala-PFC projections. The robust correspondence between the brain networks encompassed cortical and subcortical hubs involved into fear processing in humans and in other species underscores the potential utility of comparing the modulation of brain circuitry in humans and animals, as a crucial step to inform the comprehension of fear mechanisms and the development of treatments for fear-related disorders. The present review is aimed at providing a comprehensive description of the literature on recent clinical and experimental researches regarding the noninvasive brain stimulation and optogenetics. These innovative manipulations applied over specific hubs of fear matrix during fear acquisition, consolidation, reconsolidation and extinction allow an accurate characterization of specific brain circuits and their peculiar interaction within the specific fear processing.

Hyperexcitable Neurons Enable Precise and Persistent Information Encoding in the Superficial Retrosplenial Cortex

The retrosplenial cortex (RSC) is essential for memory and navigation, but the neural codes underlying these functions remain largely unknown. Here, we show that the most prominent cell type in layers 2/3 (L2/3) of the mouse granular RSC is a hyperexcitable, small pyramidal cell. These cells have a low rheobase (LR), high input resistance, lack of spike frequency adaptation, and spike widths intermediate to those of neighboring fast-spiking (FS) inhibitory neurons and regular-spiking (RS) excitatory neurons. LR cells are excitatory but rarely synapse onto neighboring neurons. Instead, L2/3 is a feedforward, not feedback, inhibition-dominated network with dense connectivity between FS cells and from FS to LR neurons. Biophysical models of LR but not RS cells precisely and continuously encode sustained input from afferent postsubicular head-direction cells. Thus, the distinct intrinsic properties of LR neurons can support both the precision and persistence necessary to encode information over multiple timescales in the RSC.

Hippocampal Ripple Coordinates Retrosplenial Inhibitory Neurons during Slow-Wave Sleep

The hippocampus and retrosplenial cortex (RSC) play indispensable roles in memory formation, and importantly, a hippocampal oscillation known as ripple is key to consolidation of new memories. However, it remains unclear how the hippocampus and RSC communicate and the role of ripple oscillation in coordinating the activity between these two brain regions. Here, we record from the dorsal hippocampus and RSC simultaneously in freely behaving mice during sleep and reveal that the RSC displays a pre-ripple activation associated with slow and fast oscillations. Immediately after ripples, a subpopulation of RSC putative inhibitory neurons increases firing activity, while most RSC putative excitatory neurons decrease activity. Consistently, optogenetic stimulation of this hippocampus-RSC pathway activates and suppresses RSC putative inhibitory and excitatory neurons, respectively. These results suggest that the dorsal hippocampus mainly inhibits RSC activity via its direct innervation of RSC inhibitory neurons, which overshadows the RSC in supporting learning and memory functions.

Converging evidence suggests that hippocampal ripple oscillations and their interaction with the neocortex are critical for memory consolidation. By combining electrophysiology and optogenetic techniques in freely behaving mice, Opalka et al. provide direct evidence that hippocampal ripples communicate with retrosplenial cortex (RSC) interneurons and inhibit RSC population activity during sleep-associated memory consolidation.

Brain functional networks associated with social bonding in monogamous voles

Previous studies have related pair-bonding in Microtus ochrogaster, the prairie vole, with plastic changes in several brain regions. However, the interactions between these socially relevant regions have yet to be described. In this study, we used resting-state magnetic resonance imaging to explore bonding behaviors and functional connectivity of brain regions previously associated with pair-bonding. Thirty-two male and female prairie voles were scanned at baseline, 24 hr, and 2 weeks after the onset of cohabitation. By using network-based statistics, we identified that the functional connectivity of a corticostriatal network predicted the onset of affiliative behavior, while another predicted the amount of social interaction during a partner preference test. Furthermore, a network with significant changes in time was revealed, also showing associations with the level of partner preference. Overall, our findings revealed the association between network-level functional connectivity changes and social bonding.

Structural and functional alterations in the retrosplenial cortex following neuropathic pain

Human and animal imaging studies demonstrated that chronic pain profoundly alters the structure and the functionality of several brain regions. In this article, we conducted a longitudinal and multimodal study to assess how chronic pain affects the brain. Using the spared nerve injury model which promotes both long-lasting mechanical and thermal allodynia/hyperalgesia but also pain-associated comorbidities, we showed that neuropathic pain deeply modified the intrinsic organization of the brain functional network 1 and 2 months after injury. We found that both functional metrics and connectivity of the part A of the retrosplenial granular cortex (RSgA) were significantly correlated with the development of neuropathic pain behaviours. In addition, we found that the functional RSgA connectivity to the subiculum and the prelimbic system are significantly increased in spared nerve injury animals and correlated with peripheral pain thresholds. These brain regions were previously linked to the development of comorbidities associated with neuropathic pain. Using a voxel-based morphometry approach, we showed that neuropathic pain induced a significant increase of the gray matter concentration within the RSgA, associated with a significant activation of both astrocytes and microglial cells. Together, functional and morphological imaging metrics of the RSgA could be used as a predictive biomarker of neuropathic pain.

Hippocampal efferents to retrosplenial cortex and lateral septum are required for memory acquisition

Learning and memory involves a large neural network of many brain regions, including the notable hippocampus along with the retrosplenial cortex (RSC) and lateral septum (LS). Previous studies have established that the dorsal hippocampus (dHPC) plays a critical role during the acquisition and retrieval/expression of episodic memories. However, the role of downstream circuitry from the dHPC, including the dHPC-to-RSC and dHPC-to-LS pathways, has come under scrutiny only recently. Here, we used an optogenetic approach with contextual fear conditioning in mice to determine whether the above two pathways are involved in acquisition and expression of contextual fear memory. We found that a selective inhibition of the dHPC neuronal terminals in either the RSC or LS during acquisition impaired subsequent memory performance, suggesting that both the dHPC-to-RSC and dHPC-to-LS pathways play a critical role in memory acquisition. We also selectively inhibited the two dHPC efferent pathways during memory retrieval and found a differential effect on memory performance. These results indicate the intricacies of memory processing and that hippocampal efferents to cortical and subcortical regions may be differentially involved in aspects of physiological and cognitive memory processes.

Equipment for Repetitive Transcranial Magnetic Stimulation

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive neuromodulation technique used for the treatment of a great variety of neurological disorders. The technique involves applying a magnetic field in certain areas of the cerebral cortex in order to modify neuronal excitability outside the skull. However, the exact brain mechanisms underlying rTMS effects are not completely elucidated. For that purpose, and in order to generate a pulsed magnetic field, a half-bridge converter controlled by a microcontroller has been designed to apply rTMS in small animals. Moreover, the small size of the rodent head makes it necessary to design a magnetic transducer, with the aim of focusing the magnetic field on selected brain areas using a specific and a small magnetic head. Using such devices, our purpose was to compare the effects of five different rTMS dosages on rat brain metabolic activity. The experimental results showed that one day of stimulation leads to an enhancement of brain metabolic activity in cortical areas, meanwhile with three days of stimulation it is possible to also modify subcortical zones, results that were not found when extending the number of rTMS applications up to seven days. In consequence, the number of pulses delivered might be an important parameter in rTMS protocols, highlighting its importance in rTMS impact.

Recognition memory impairments and amyloid-beta deposition of the retrosplenial cortex at the early stage of 5XFAD mice

Early diagnosis and treatment of AD are critical for delaying its progression. The present study, therefore, examined the cognitive status and neuropathological characteristics of 4-month-old 5X familial AD (5XFAD) transgenic (Tg) mice, as an early stage of AD animal model. The novel object recognition task was performed with retention tests at varying intervals (i.e., 10 min, 1 h, 4 h, and 24 h) to measure the retention capacity of recognition memory of 5XFAD mice. At the 4h retention interval, 5XFAD mice exhibited worse performances than non-Tg control mice. Therefore, using amyloid-beta (Aβ) 42- and 4G8-immunoreactive plaques, the accumulation of Aβ was examined in the gray and white matter of the system that was necessary for the retention of recognition memory, with a focus on the hippocampus and retrosplenial cortex. The expression of ionized calcium-binding adapter molecule-1 (Iba-1) was also examined to measure microglial activation. The immunohistological analysis of Aβ and Iba-1 revealed that the retrosplenial cortex was the most affected region in the brains of 4-month-old 5XFAD mice. These findings indicate that the cognitive and neuropathological characteristics of 4-month-old 5XFAD mice would provide a research platform for studying early diagnosis and treatment of AD.

A2A R inhibition in alleviating spatial recognition memory impairment after TBI is associated with improvement in autophagic flux in RSC

Spatial recognition memory impairment is an important complication after traumatic brain injury (TBI). We previously found that spatial recognition memory impairment can be alleviated in adenosine A_2A receptor knockout (A_2AR KO) mice after TBI, but the mechanism remains unclear. In the current study, we used manganese‐enhanced magnetic resonance imaging and the Y‐maze test to determine whether the electrical activity of neurons in the retrosplenial cortex (RSC) was reduced and spatial recognition memory was impaired in wild‐type (WT) mice after moderate TBI. Furthermore, spatial recognition memory was damaged by optogenetically inhibiting the electrical activity of RSC neurons in WT mice. Additionally, the electrical activity of RSC neurons was significantly increased and spatial recognition memory impairment was reduced in A_2AR KO mice after moderate TBI. Specific inhibition of A_2AR in the ipsilateral RSC alleviated the impairment in spatial recognition memory in WT mice. In addition, A_2AR KO improved autophagic flux in the ipsilateral RSC after injury. In primary cultured neurons, activation of A_2AR reduced lysosomal‐associated membrane protein 1 and cathepsin D (CTSD) levels, increased phosphorylated protein kinase A and phosphorylated extracellular signal‐regulated kinase 2 levels, reduced transcription factor EB (TFEB) nuclear localization and impaired autophagic flux. These results suggest that the impairment of spatial recognition memory after TBI may be associated with impaired autophagic flux in the RSC and that A_2AR activation may reduce lysosomal biogenesis through the PKA/ERK2/TFEB pathway to impair autophagic flux.

Satb2 is required for the regionalization of retrosplenial cortex

The retrosplenial cortex (Rsp) is a transitional cortex located between the neocortex and archicortex, but the molecular mechanism specifying Rsp from the archicortex remains elusive. We here report that the transcription factor Satb2 is required for specifying Rsp identity during its morphogenesis. In Satb2 CKO mice, the boundary between the Rsp and archicortex [i.e., subiculum (SubC)] disappears as early as E17.5, and Rsp efferent projection is aberrant. Rsp-specific genes are lost, whereas SubC-specific genes are ectopically expressed in Rsp of Satb2 CKO mice. Furthermore, cell-autonomous role of Satb2 in maintaining Rsp neuron identity is revealed by inactivation of Satb2 in Rsp neurons. Finally, Satb2 represses the transcription of Nr4a2. The misexpression of Nr4a2 together with Ctip2 induces expression of SubC-specific genes in wild-type Rsp, and simultaneous knockdown of these two genes in Rsp Satb2-mutant cells prevents their fate transition to SubC identity. Thus, Satb2 serves as a determinant gene in the Rsp regionalization by repressing Nr4a2 and Ctip2 during cortical development.

Donepezil attenuates vascular dementia in rats through increasing BDNF induced by reducing HDAC6 nuclear translocation

Vascular dementia (VD) is the second most common dementia disease after Alzheimer's diseases (AD) in the world. Donepezil is used to treat mild to moderate AD, and it has been shown to treat cognitive impairment and memory deficits caused by VD. However, the action mechanism of donepezil against VD has not been clarified. In this study, a bilateral common carotid artery occlusion (BCCAO) model was established in rats to simulate the pathology of VD. Two weeks after the surgery, the rats were administered donepezil (10 mg · kg^-1 · d^-1, ig) for 3 weeks, and then subjected to behavioral tests. We showed that donepezil treatment significantly improved the performance of BCCAO rats in Morris Water Mazes test and Step-down test. Furthermore, we showed that donepezil treatment significantly attenuated neurodegeneration and restored the synapse dendritic spines density in cortex and hippocampus. We revealed that donepezil treatment significantly increased BDNF expression in cortex and hippocampus. Interestingly, donepezil treatment significantly decreased nuclear translocation of HDAC6 and the binding between HDAC6 and BDNF promoter IV in cortex, but not in the hippocampus. The attenuated neurodegeneration by donepezil in cortex and hippocampus might due to the reduced ROS levels and increased phosphorylation of AMPK, whereas increased phosphorylation of AKT was only detected in cortex. In conclusion, our results demonstrate that donepezil attenuates neurodegeneration in cortex and hippocampus via increasing BDNF expression; the regulation of donepezil on HDAC6 occurred in cortex, but not in the hippocampus. This study further clarifies the pharmacological mechanism of donepezil, while also emphasizes the promising epigenetic regulation of HDAC6.

Heterogeneity of neuronal firing type and morphology in retrosplenial cortex of male F344 rats

The rodent granular retrosplenial cortex (gRSC) has reciprocal connections to the hippocampus to support fear memories. Although activity-dependent plasticity occurs within the RSC during memory formation, the intrinsic and morphological properties of RSC neurons are poorly understood. The present study used whole-cell recordings to examine intrinsic neuronal firing and morphology of neurons in layer 2/3 (L2/3) and layer 5 (L5) of the gRSC in adult male rats. Five different classifications were observed: regular-spiking (RS), regular-spiking afterdepolarization (RS_ADP), late-spiking (LS), burst-spiking (BS), and fast-spiking (FS) neurons. RS_ADP neurons were the most commonly observed neuronal class, identified by their robust spike frequency adaptation and pronounced afterdepolarization (ADP) following an action potential (AP). They also had the most extensive dendritic branching compared with other cell types. LS neurons were predominantly found in L2/3 and exhibited a long delay before onset of their initial AP. They also had reduced dendritic branching compared with other cell types. BS neurons were limited to L5 and generated an initial burst of two or more APs. FS neurons demonstrated sustained firing and little frequency adaptation and were the only nonpyramidal firing type. Relative to adults, RS neurons from juvenile rats (PND 14-30) lacked an ADP and were less excitable. Bath application of group 1 mGluR blockers attenuated the ADP in adult neurons. In other fear-related brain structures, the ADP has been shown to enhance excitability and synaptic plasticity. Thus, understanding cellular mechanisms of the gRSC will provide insight regarding its precise role in memory-related processes across the lifespan.NEW & NOTEWORTHY This is the first study to demonstrate that granular retrosplenial cortical (gRSC) neurons exhibit five distinctive firing types: regular spiking (RS), regular spiking with an afterdepolarization (RS_ADP), late spiking (LS), burst spiking (BS), and fast spiking (FS). RS_ADP neurons were the most frequently observed cell type in adult gRSC neurons. Interestingly, RS neurons without an ADP were most common in gRSC neurons of juvenile rats (PND 14-30). Thus, the ADP property, which was previously shown to enhance neuronal excitability, emerges during development.

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.

Optogenetic reactivation of memory ensembles in the retrosplenial cortex induces systems consolidation

The neural circuits underlying memory change over prolonged periods after learning, in a process known as systems consolidation. Postlearning spontaneous reactivation of memory-related neural ensembles is thought to mediate this process, although a causal link has not been established. Here we test this hypothesis in mice by using optogenetics to selectively reactivate neural ensembles representing a contextual fear memory (sometimes referred to as engram neurons). High-frequency stimulation of these ensembles in the retrosplenial cortex 1 day after learning produced a recent memory with features normally observed in consolidated remote memories, including higher engagement of neocortical areas during retrieval, contextual generalization, and decreased hippocampal dependence. Moreover, this effect was only present if memory ensembles were reactivated during sleep or light anesthesia. These results provide direct support for postlearning memory ensemble reactivation as a mechanism of systems consolidation, and show that this process can be accelerated by ensemble reactivation in an unconscious state.

When is the rat retrosplenial cortex required for stimulus integration?

The rodent retrosplenial cortex is known to be vital for spatial cognition, but evidence has also pointed to a role in processing nonspatial information. It has been suggested that the retrosplenial cortex may serve as a site of integration of incoming sensory information. To examine this proposal, the current set of experiments assessed the impact of excitotoxic lesions in the retrosplenial cortex on two behavioral tasks that tax animals' ability to process multiple and overlapping environmental stimuli. In Experiment 1, rats with retrosplenial lesions acquired a negative patterning discrimination, a form of configural learning that can be solved only by learning the conjunction of cues. Subsequent transfer tests confirmed that both the lesion and control animals had solved the task by using configural representations. Furthermore, in Experiment 2, a 2nd cohort of retrosplenial lesion animals successfully acquired conditioned inhibition. Nevertheless, the same animals failed a subsequent summation test that assesses the ability to transfer what has been learned about one stimulus to another stimulus in the absence of reinforcement. Taken together, these results suggest that in the nonspatial domain, the retrosplenial cortex is not required for forming associations between multiple or overlapping environmental stimuli and, consequently, retrosplenial engagement in such processes is more selective than was previously envisaged. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

The retrosplenial cortical role in encoding behaviorally significant cues

The retrosplenial cortex (RSC) has recently begun to gain widespread interest because of its anatomical connectivity with other well-known memory structures, such as the hippocampus and anterior thalamus, and its role in spatial, contextual, and episodic memory. Although much of the current work on the RSC is focused on spatial cognition, there is also an extensive literature that shows that the RSC plays a critical role in a variety of conditioning tasks that have no obvious spatial component. Many of these studies suggest that the RSC is involved in identifying and encoding behaviorally significant cues, particularly those cues that predict reinforcement or the need for a behavioral response. Consistent with this idea, recent studies have shown that RSC neurons also encode cues in spatial navigation tasks. In this article, we review these findings and suggest that the encoding of cues is an important component of the RSC contribution to many forms of learning. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Network supporting contextual fear learning after dorsal hippocampal damage has increased dependence on retrosplenial cortex

Hippocampal damage results in profound retrograde, but no anterograde amnesia in contextual fear conditioning (CFC). Although the content learned in the latter have been discussed, alternative regions supporting CFC learning were seldom proposed and never empirically addressed. Here, we employed network analysis of pCREB expression quantified from brain slices of rats with dorsal hippocampal lesion (dHPC) after undergoing CFC session. Using inter-regional correlations of pCREB-positive nuclei between brain regions, we modelled functional networks using different thresholds. The dHPC network showed small-world topology, equivalent to SHAM (control) network. However, diverging hubs were identified in each network. In a direct comparison, hubs in both networks showed consistently higher centrality values compared to the other network. Further, the distribution of correlation coefficients was different between the groups, with most significantly stronger correlation coefficients belonging to the SHAM network. These results suggest that dHPC network engaged in CFC learning is partially different, and engage alternative hubs. We next tested if pre-training lesions of dHPC and one of the new dHPC network hubs (perirhinal, Per; or disgranular retrosplenial, RSC, cortices) would impair CFC. Only dHPC-RSC, but not dHPC-Per, impaired CFC. Interestingly, only RSC showed a consistently higher centrality in the dHPC network, suggesting that the increased centrality reflects an increased functional dependence on RSC. Our results provide evidence that, without hippocampus, the RSC, an anatomically central region in the medial temporal lobe memory system might support CFC learning and memory.

Estradiol Protects White Matter of Male C57BL6J Mice against Experimental Chronic Cerebral Hypoperfusion

BACKGROUND AND PURPOSE

Estradiol is a sex steroid hormone known to protect the brain against damage related to transient and global cerebral ischemia. In the present study, we leverage an experimental murine model of bilateral carotid artery stenosis (BCAS) to examine the putative effects of estradiol therapy on chronic cerebral hypoperfusion. We hypothesize that long-term estradiol therapy protects against white matter injury and declarative memory deficits associated with chronic cerebral hypoperfusion.

METHODS

Adult male C57BL/6J mice underwent either surgical BCAS or sham procedures. Two days after surgery, the mice were given oral estradiol (Sham+E, BCAS+E) or placebo (Sham+P, BCAS+P) treatments daily for 31-34 days. All mice underwent Novel Object Recognition (NOR) testing 31-34 days after the start of oral treatments. Following sacrifice, blood was collected and brains fixed, sliced, and prepared for histological examination of white matter injury and extracellular signal-regulated kinase (ERK) expression.

RESULTS

Animals receiving long-term oral estradiol therapy (BCAS-E2 and Sham-E2) had higher plasma estradiol levels than those receiving placebo treatment (BCAS-P and Sham-P). BCAS-E2 mice demonstrated less white matter injury (Klüver-Barrera staining) and performed better on the NOR task compared to BCAS-P mice. ERK expression in the brain was increased in the BCAS compared to sham cohorts. Among the BCAS mice, the BCAS-E2 cohort had a greater number of ERK + cells.

CONCLUSION

This study demonstrates a potentially protective role for oral estradiol therapy in the setting of white matter injury and declarative memory deficits secondary to murine chronic cerebral hypoperfusion.

Role of retrosplenial cortex in processing stress-related context memories

This work summarizes evidence for the role of RSC in processing fear-inducing context memories. Specifically, we discuss molecular, cellular, and network mechanisms by which RSC might contribute the processing of contextual fear memories. We focus on glutamatergic and cholinergic mechanisms underlying encoding, retrieval, and extinction of context-dependent fear. RSC mechanisms underlying retrieval of recently and remotely acquired memories are compared to memory mechanisms of anterior cortices. Due to the strong connectivity between hippocampus and RSC, we also compare the extent to which their mechanisms of encoding, retrieval, and extinction show overlap. At a theoretical level, we discuss the role of RSC in the framework of systems consolidation as well as retrieval-induced memory modulation. Lastly, we emphasize the implication of these findings for psychopathologies associated with neurological and psychiatric disorders. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Retrosplenial cortex has a time-dependent role in memory for visual stimuli

Although the retrosplenial cortex (RSC) is critically involved in spatial learning and memory, it appears to have more selective contributions to learning and memory for discrete cues. For example, damage to the RSC does not impair Pavlovian delay fear conditioning to a discrete auditory cue (e.g., tone), when RSC manipulation occurs just prior to, or shortly after, conditioning. In contrast, when lesions of the RSC occur following a substantial retention interval (e.g., 28 days), the RSC is necessary for retrieval of fear to the tone. Thus, the RSC makes time-dependent contributions to memory retrieval for discrete auditory cues. The purpose of the current experiment was to assess if the time-dependent involvement of the RSC in cue-specific fear memory extended to cues of other sensory modalities. Rats firsts underwent fear conditioning to a visual stimulus, and lesions of the RSC subsequently occurred 1 or 28 days later. Lesions of the RSC impaired fear expression when made 28 days after conditioning, but not when made 1 day following conditioning. Coupled with previous findings, the current results suggest the RSC is necessary for retrieval of remotely acquired cued fear memories across multiple modalities. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Cerebral Responses to Acupuncture at GV24 and Bilateral GB13 in Rat Models of Alzheimer's Disease

Acupuncture has been widely used in China to treat neurological diseases including Alzheimer's disease (AD). However, its mechanism remains unclear. In the present study, eighty healthy Wistar rats were divided into a normal control group (n = 15) and premodel group (n = 65). Forty-five rats that met the criteria for the AD model were then randomly divided into the model group (MG), the nonacupoint group (NG), and the acupoint group (AG). All rats received positron emission tomography (PET) scanning, and the images were analyzed with Statistical Parametric Mapping 8.0. MG exhibited hypometabolism in the olfactory bulb, insular cortex, orbital cortex, prelimbic cortex, striatum, parietal association cortex, visual cortex, cingulate gyrus, and retrosplenial cortex. AG exhibited prominent and extensive hypermetabolism in the thalamus, hypothalamus, bed nucleus of the stria terminalis, cerebral peduncle, midbrain tegmentum, and pontine tegmentum compared to NG. These results demonstrated that acupuncturing at GV24 and bilateral GB13 acupoints may improve the learning and memory abilities of the AD rats, probably via altering cerebral glucose metabolism (CGM) in the hypothalamus, thalamus, and brain stem. The observed effects of acupuncture may be caused by regulating the distribution of certain kinds of neurotransmitters and enhancing synaptic plasticity.