Contributions of the dorsal hippocampus and the dorsal subiculum to processing of idiothetic information and spatial memory

Effects of web-based adapted physical activity on hippocampal plasticity, cardiorespiratory fitness, symptoms, and cardiometabolic markers in patients with schizophrenia: a randomized, controlled study

Among the lifestyle interventions, the physical activity (PA) has emerged as an adjuvant non-pharmacological treatment improving mental and physical health in patients with schizophrenia (SZPs) and increasing the hippocampus (HCP) volume. Previously investigated PA programs have been face-to-face, and not necessary adapted to patients' physiological fitness. We propose an innovative 16-week adapted PA program delivered by real-time videoconferencing (e-APA), allowing SZPs to interact with a coach and to manage their physical condition. The primary goal was to demonstrate a greater increase of total HCP volumes in SZPs receiving e-APA compared to that observed in a controlled group. The secondary objectives were to demonstrate the greater effects of e-APA compared to a controlled group on HCP subfields, cardiorespiratory fitness, clinical symptoms, cognitive functions, and lipidic profile. Thirty-five SZPs were randomized to either e-APA or a controlled group receiving a health education program under the same conditions (e-HE). Variables were assessed at pre- and post-intervention time-points. The dropout rate was 11.4%. Compared to the e-HE group, the e-APA group did not have any effect on the HCP total volumes but increased the left subiculum volume. Also, the e-APA group significantly increased cardiorespiratory fitness (VO2max), improved lipidic profile and negative symptoms but not cognitive functions. This study demonstrated the high feasibility and multiple benefits of a remote e-APA program for SZPs. e-APA may increase brain plasticity and improve health outcomes in SZPs, supporting that PA should be an add-on therapeutic intervention. ClinicalTrial.gov on 25 august 2017 (NCT03261817).

Disrupting direct inputs from the dorsal subiculum to the granular retrosplenial cortex impairs flexible spatial memory in the rat

In a changing environment, animals must process spatial signals in a flexible manner. The rat hippocampal formation projects directly upon the retrosplenial cortex, with most inputs arising from the dorsal subiculum and terminating in the granular retrosplenial cortex (area 29). The present study examined whether these same projections are required for spatial working memory and what happens when available spatial cues are altered. Consequently, injections of iDREADDs were made into the dorsal subiculum of rats. In a separate control group, GFP-expressing adeno-associated virus was injected into the dorsal subiculum. Both groups received intracerebral infusions within the retrosplenial cortex of clozapine, which in the iDREADDs rats should selectively disrupt the subiculum to retrosplenial projections. When tested on reinforced T-maze alternation, disruption of the subiculum to retrosplenial projections had no evident effect on the performance of those alternation trials when all spatial-cue types remained present and unchanged. However, the same iDREADDs manipulation impaired performance on all three alternation conditions when there was a conflict or selective removal of spatial cues. These findings reveal how the direct projections from the dorsal subiculum to the retrosplenial cortex support the flexible integration of different spatial cue types, helping the animal to adopt the spatial strategy that best meets current environmental demands.

Distinct manifold encoding of navigational information in the subiculum and hippocampus

The subiculum (SUB) plays a crucial role in spatial navigation and encodes navigational information differently from the hippocampal CA1 area. However, the representation of subicular population activity remains unknown. Here, we investigated the neuronal population activity recorded extracellularly from the CA1 and SUB of rats performing T-maze and open-field tasks. The trajectory of population activity in both areas was confined to low-dimensional neural manifolds homoeomorphic to external space. The manifolds conveyed position, speed, and future path information with higher decoding accuracy in the SUB than in the CA1. The manifolds exhibited common geometry across rats and regions for the CA1 and SUB and between tasks in the SUB. During post-task ripples in slow-wave sleep, population activity represented reward locations/events more frequently in the SUB than in CA1. Thus, the CA1 and SUB encode information distinctly into the neural manifolds that underlie navigational information processing during wakefulness and sleep.

Hippocampal subfield volumes in mild cognitive impairment and alzheimer's disease: a systematic review and meta-analysis

The hippocampus is a complex structure that consists of several subfields with distinct and specialized functions. Although numerous studies have been performed to explore hippocampal atrophy at the sub-regional level in mild cognitive impairment (MCI) and Alzheimer's disease (AD), the results have been inconsistent especially for whether and which subfields can be served as the most potential biomarkers in MCI and AD. Herein, we used a meta-analytic approach to synthesize the extant literatures on hippocampal subfields in MCI and AD through PubMed, Web of Science, and Embase (PROSPERO CRD42021257586). As a result, a total of twenty studies using Freesurfer 5 and Freesurfer 6 were included in this investigation. These studies revealed that at the sub-regional level, hippocampal subfield volume reductions in MCI and AD were not restricted to specific subfields, and subiculum and presubiculum had the largest z-scores across most comparisons. However, none of the subfield performed much better in discriminating MCI and HC, AD and MCI, AD and HC as compared to whole hippocampus volume. These results suggested that we should explore the changes in the hippocampal subfields in subtypes of MCI or even at an earlier stage, that is subjective cognitive impairment.

The contextual fear conditioning consolidation depends on the functional interaction of the dorsal subiculum and basolateral amygdala in rats

Fear conditioning tasks enable us to explore the neural basis of adaptative and maladaptive behaviors related to aversive memories. Recently, we provided the first evidence of the dorsal subiculum (DSub) involvement in contextual fear conditioning (CFC) consolidation by showing that the post-training bilateral NMDA (N-methyl-D-aspartate) receptor blockade in DSub impaired the performance of animals in the test session. As the memory consolidation process depends on the coordinated engagement of different brain regions, and the DSub share reciprocal projections with the basolateral amygdala (BLA), which is also involved in CFC, it is possible that the functional interaction between these sites can be relevant for the consolidation of this task. In this sense, the present study aimed to explore the effects of the functional disconnection of the DSub and BLA in the CFC consolidation after NMDA post-training blockade. In addition, to verify if the observed effects were due to spatial representation processes mediated by the DSub, we employed a hippocampal-independent procedure: tone fear conditioning (TFC). Results showed that the functional disconnection of these regions by post-training NMDA blockade impaired CFC consolidation, whereas there was no impairment in TFC. Altogether, the present data suggest that the DSub and BLA would functionally interact through NMDA-related synaptic plasticity to support CFC consolidation probably due to DSub-related spatial processing showing that the TFC consolidation was not disrupted. This work contributes to filling a gap of studies exploring the DSub involvement in fear conditioning by providing a broad framework of the subicular-amygdaloid connection functionality.

Maternal Immune Activation and Enriched Environments Impact B2 SINE Expression in Stress Sensitive Brain Regions of Rodent Offspring

Early life stress (ELS) can have wide-spread neurodevelopmental effects with support accumulating for the idea that genomic mechanisms may induce lasting physiological and behavioral changes following stress exposure. Previous work found that a sub-family of transposable elements, SINEs, are repressed epigenetically after acute stress. This gives support to the concept that the mammalian genome may be regulating retrotransposon RNA expression allowing for adaptation in response to environmental challenges, such as maternal immune activation (MIA). Transposon (TE) RNAs are now thought to work at the epigenetic level and to have an adaptive response to environmental stressors. Abnormal expression of TEs has been linked to neuropsychiatric disorders like schizophrenia, which is also linked to maternal immune activation. Environmental enrichment (EE), a clinically utilized intervention, is understood to protect the brain, enhance cognitive performance, and attenuate responses to stress. This study examines the effects of MIA on offspring B2 SINE expression and further, the impact that EE, experienced throughout gestation and early life, may have in conjunction with MIA during development. Utilizing RT-PCR to quantify the expression of B2 SINE RNA in the juvenile brain of MIA exposed rat offspring, we found dysregulation of B2 SINE expression associated with MIA in the prefrontal cortex. For offspring experiencing EE, the prefrontal cortex exhibited an attenuation of the MIA response observed in standard housed animals. Here, the adaptive nature of B2 is observed and thought to be aiding in the animal's adaptation to stress. The present changes indicate a wide-spread stress-response system adaptation that impacts not only changes at the genomic level but potentially observable behavioral impacts throughout the lifespan, with possible translational relevance to psychotic disorders.

Subicular neurons represent multiple variables of a hippocampal-dependent task by using theta rhythm

The subiculum is positioned at a critical juncture at the interface of the hippocampus with the rest of the brain. However, the exact roles of the subiculum in most hippocampal-dependent memory tasks remain largely unknown. One obstacle to make comparisons of neural firing patterns between the subiculum and hippocampus is the broad firing fields of the subicular cells. Here, we used spiking phases in relation to theta rhythm to parse the broad firing field of a subicular neuron into multiple subfields to find the unique functional contribution of the subiculum while male rats performed a hippocampal-dependent visual scene memory task. Some of the broad firing fields of the subicular neurons were successfully divided into multiple subfields similar to those in the CA1 by using the theta phase precession cycle. The new paradigm significantly improved the detection of task-relevant information in subicular cells without affecting the information content represented by CA1 cells. Notably, we found that multiple fields of a single subicular neuron, unlike those in the CA1, carried heterogeneous task-related information such as visual context and choice response. Our findings suggest that the subicular cells integrate multiple task-related factors by using theta rhythm to associate environmental context with action.

Deconstructing the Direct Reciprocal Hippocampal-Anterior Thalamic Pathways for Spatial Learning

The hippocampus is essential for normal memory but does not act in isolation. The anterior thalamic nuclei may represent one vital partner. Using DREADDs, the behavioral consequences of transiently disrupting anterior thalamic function were examined, followed by inactivation of the dorsal subiculum. Next, the anterograde transport of an adeno-associated virus expressing DREADDs was paired with localized intracerebral infusions of a ligand to target specific input pathways. In this way, the direct projections from the anterior thalamic nuclei to the dorsal hippocampal formation were inhibited, followed by separate inhibition of the dorsal subiculum projections to the anterior thalamic nuclei. To assay spatial working memory, all animals performed a reinforced T-maze alternation task, then a more challenging version that nullifies intramaze cues. Across all four experiments, deficits emerged on the spatial alternation task that precluded the use of intramaze cues. Inhibiting dorsal subiculum projections to the anterior thalamic nuclei produced the severest spatial working memory deficit. This deficit revealed the key contribution of dorsal subiculum projections to the anteromedial and anteroventral thalamic nuclei for the processing of allocentric information, projections not associated with head-direction information. The overall pattern of results provides consistent causal evidence of the two-way functional significance of direct hippocampal-anterior thalamic interactions for spatial processing. At the same time, these findings are consistent with hypotheses that these same, reciprocal interactions underlie the common core symptoms of temporal lobe and diencephalic anterograde amnesia.

SIGNIFICANCE STATEMENT It has long been conjectured that the anterior thalamic nuclei might be key partners with the hippocampal formation and that, respectively, they are principally responsible for diencephalic and temporal lobe amnesia. However, direct causal evidence for this functional relationship is lacking. Here, we examined the behavioral consequences of transiently silencing the direct reciprocal interconnections between these two brain regions on tests of spatial learning. Disrupting information flow from the hippocampal formation to the anterior thalamic nuclei and vice versa impaired performance on tests of spatial learning. By revealing the conjoint importance of hippocampal-anterior thalamic pathways, these findings help explain why pathology in either the medial diencephalon or the medial temporal lobes can result in profound anterograde amnesic syndromes.

Chronic Intermittent Ethanol Exposure Selectively Increases Synaptic Excitability in the Ventral Domain of the Rat Hippocampus

Many studies have implicated hippocampal dysregulation in the pathophysiology of alcohol use disorder (AUD). However, over the past twenty years, a growing body of evidence has revealed distinct functional roles of the dorsal (dHC) and ventral (vHC) hippocampal subregions, with the dHC being primarily involved in spatial learning and memory and the vHC regulating anxiety- and depressive-like behaviors. Notably, to our knowledge, no rodent studies have examined the effects of chronic ethanol exposure on synaptic transmission along the dorsal/ventral axis. To that end, we examined the effects of the chronic intermittent ethanol vapor exposure (CIE) model of AUD on dHC and vHC synaptic excitability. Adult male Long-Evans rats were exposed to CIE or AIR for 10 days (12 h/day; targeting blood ethanol levels of 175-225 mg%) and recordings were made 24 h into withdrawal. As expected, this protocol increased anxiety-like behaviors on the elevated plus-maze and successive alleys test. Extracellular recordings revealed marked CIE-associated increases in synaptic excitation in the CA1 region that were exclusively restricted to the ventral domain of the hippocampus. Western blot analysis of synaptoneurosomal fractions revealed that the expression of two proteins that regulate synaptic strength, GluA2 and SK2, were dysregulated in the vHC, but not the dHC, following CIE. Together, these findings suggest that the ventral CA1 region may be particularly sensitive to the maladaptive effects of chronic ethanol exposure and provide new insight into some of the neural substrates that may contribute to the negative affective state that develops during withdrawal.

Regional Specific Evidence for Memory-Load Dependent Activity in the Dorsal Subiculum and the Lateral Entorhinal Cortex

The subiculum and the lateral entorhinal cortex (LEC) are the main output areas of the hippocampus which contribute to spatial and non-spatial memory. The proximal part of the subiculum (bordering CA1) receives heavy projections from the perirhinal cortex and the distal part of CA1 (bordering the subiculum), both known for their ties to object recognition memory. However, the extent to which the proximal subiculum contributes to non-spatial memory is still unclear. Comparatively, the involvement of the LEC in non-spatial information processing is quite well known. However, very few studies have investigated its role within the frame of memory function. Thus, it is not known whether its contribution depends on memory load. In addition, the deep layers of the EC have been shown to be predictive of subsequent memory performance, but not its superficial layers. Hence, here we tested the extent to which the proximal part of the subiculum and the superficial and deep layers of the LEC contribute to non-spatial memory, and whether this contribution depends on the memory load of the task. To do so, we imaged brain activity at cellular resolution in these areas in rats performing a delayed nonmatch to sample task based on odors with two different memory loads (5 or 10 odors). This imaging technique is based on the detection of the RNA of the immediate-early gene Arc, which is especially tied to synaptic plasticity and behavioral demands, and is commonly used to map activity in the medial temporal lobe. We report for the first time that the proximal part of the subiculum is recruited in a memory-load dependent manner and the deep layers of the LEC engaged under high memory load conditions during the retrieval of non-spatial memory, thus shedding light on the specific networks contributing to non-spatial memory retrieval.

Neural Activity Patterns Underlying Spatial Coding in the Hippocampus

The hippocampus is well known as a central site for memory processing-critical for storing and later retrieving the experiences events of daily life so they can be used to shape future behavior. Much of what we know about the physiology underlying hippocampal function comes from spatial navigation studies in rodents, which have allowed great strides in understanding how the hippocampus represents experience at the cellular level. However, it remains a challenge to reconcile our knowledge of spatial encoding in the hippocampus with its demonstrated role in memory-dependent tasks in both humans and other animals. Moreover, our understanding of how networks of neurons coordinate their activity within and across hippocampal subregions to enable the encoding, consolidation, and retrieval of memories is incomplete. In this chapter, we explore how information may be represented at the cellular level and processed via coordinated patterns of activity throughout the subregions of the hippocampal network.

The subiculum: the heart of the extended hippocampal system

While descriptions of the subiculum often emphasize its role as a recipient of hippocampal inputs, the area also has particular importance as a source of hippocampal projections. The extrinsic projections from the subiculum not only parallel those from hippocampal fields CA1-4 but also terminate in sites that do not receive direct inputs from the rest of the hippocampus. Both electrophysiological and lesion studies reveal how, despite its very dense CA1 inputs, the subiculum has functional properties seemingly independent from the rest of the hippocampus. In understanding the subiculum, it is necessary to appreciate that its connections are topographically organized along all three planes (longitudinal, transverse, and depth). These topographies may enable the subiculum to separate multiple information types and, hence, support multiple functions. The particular significance of the subiculum for learning and memory is underlined by its importance as a source of hippocampal projections to nuclei in the medial diencephalon, which are themselves vital for human memory and rodent spatial learning. Of particular note are its reciprocal connections with the anterior thalamic nuclei, which are not shared by the rest of the hippocampus (CA1-4). These thalamosubiculum connections may be of especial significance for resolving memory problems that suffer high interference and require the flexible use of stimulus representations.

Behavioral testing-related changes in the expression of Synapsin I and glucocorticoid receptors in standard and enriched aged Wistar rats

Our aim was to assess the changes in the Synapsin I and glucocorticoid receptor (GR) expression induced by behavioral testing in the dorsal and ventral hippocampi of standard and enriched aged Wistar rats. The environmental enrichment (EE) was carried out 3h/day over a period of two months and then, the rats were tested in the elevated zero-maze (EZM) and radial-arm water maze (RAWM). Behavioral results showed that, even at an advanced age, EE was able to reduce anxiety-related behaviors and improve the performance in the RAWM. Regarding the neurobiological data, Synapsin I expression in the dorsal CA3, but not in the ventral, was enhanced both in enriched and standard rats when they performed the behavioral testing. Interestingly, the EE exposure was enough to increase Synapsin I in the ventral CA3. The analysis of GR in the dorsal hippocampus showed an increase of this receptor in the dDG both in enriched and standard rats when they performed the behavioral testing, whereas in the dCA1 and dCA3, the effect of the testing depended on the previous housing condition. In the ventral region, we found that the effects of EE were higher because on the one hand, the GR expression induced by the behavioral testing was enhanced in the dSUB, vCA1 and vCA3 when the rats were previously enriched and on the other hand, EE, regardless of the behavioral testing, increased the GR expression in the vDG and vSUB. Therefore, our results suggest that the effect of the behavioral testing on the neurobiological mechanisms studied is different depending on the previous housing condition of aged rats.

Relationship between cortisol level and prevalent/incident cognitive impairment and its moderating factors in older adults

BACKGROUND

The objectives of this study were to examine the factors modifying the relationship between cortisol level and prevalent/incident cognitive impairment in older adults and to verify whether these relationships were non-linear.

METHODS

Data were collected from 1,226 individuals aged 65 and older by two in-home interviews separated by 12 months. Cortisol level was measured using saliva samples taken at the beginning of the baseline interview before cognitive, mental, and physical health evaluations. Prevalent and incident cognitive impairment were defined using the Mini-Mental State Examination scores according to normative data for age, education level, and sex.

RESULTS

High morning cortisol level increased the risk of incident cognitive impairment in participants with anxiety or depressive episode while low cortisol level increased the risk in participants without anxiety or depressive episode. In high educated participants, but not in low educated participants, high morning cortisol level was associated with prevalent cognitive impairment and high afternoon cortisol level increased the risk of incident cognitive impairment. The results also suggested that lower morning cortisol values could increase the risk of incident cognitive impairment in individuals with few chronic diseases. A curvilinear relationship was observed between morning cortisol and the probability of incident cognitive impairment, but further analyses suggested that it was likely explained by anxiety and depressive episode.

CONCLUSIONS

These results suggest that cognitive impairment in older adults is linked to higher or lower cortisol level depending on characteristics such as anxiety, depressive episode, education level, and physical health.

A role for dorsal and ventral hippocampus in response learning

The hippocampus and the striatum have been traditionally considered as part of different and independent memory systems despite growing evidence supporting that both brain regions may even compete for behavioral control in particular learning tasks. In this regard, it has been reported that the hippocampus could be necessary for the use of idiothetic cues in several types of spatial learning tasks. Accordingly, the ventral striatum receives strong anatomical projections from the hippocampus, suggesting a participation of both regions in goal-directed behavior. Our work examined the role of the dorsal and ventral hippocampus on a response learning task. Cytochrome c oxidase (C.O.) quantitative histochemistry was used as an index of brain oxidative metabolism. In addition, determination of C.O. subunit I levels in the hippocampus by western blot analysis was performed to assess the contribution of this subunit to overall C.O. activity. Increased brain oxidative metabolism was found in most of the studied hippocampal subregions when experimental group was compared with a swim control group. However, no differences were found in the amount of C.O. subunit I expressed in the hippocampus by western blot analysis. Our results support that both the dorsal and ventral hippocampus are associated with the use of response strategies during response learning.

Divergent effects of age on performance in spatial associative learning and real idiothetic memory in humans

This study focuses on age-related differences concerning two kinds of spatial memory assessed by: (1) Paired Associates Learning (PAL) test from the CANTAB and (2) a test of Real Idiothetic Memory (RIM) using real-life settings. Despite a clear age-related drop in PAL that is reported in existing studies, age-related differences in idiothetic navigation still remain unclear. In our study we tested 80 healthy volunteers classified according to their age into two groups, i.e. young (aged from 20 to 29 years of life; n=40; 20M/20F) and elderly (from 64 to 77 years; n=40; 20M/20F) healthy volunteers. They were asked in the PAL test to remember the spatial location of visual patterns presented on a computer screen, and in the RIM test to walk on the arena in darkness in order to find a cue place and then to return to the start/exit point. A white noise was switched on at entering the cue place and switched off at leaving this place. Elderly subjects indicated poorer performance than their younger counterparts on the PAL test, as evidenced by all tested outcome measures. In contrast, for the RIM test no clear age effect was evidenced. In both tests no gender effect was observed. A dissociation in age-related changes for these two tests indicates that visuo-spatial associative learning and idiothetic navigation may have different cognitive control which is probably rooted in an interplay of different brain structures.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.