Toward a conceptualization of retrohippocampal contributions to learning and memory

Brain-wide neuron quantification toolkit reveals strong sexual dimorphism in the evolution of fear memory

Fear responses are functionally adaptive behaviors that are strengthened as memories. Indeed, detailed knowledge of the neural circuitry modulating fear memory could be the turning point for the comprehension of this emotion and its pathological states. A comprehensive understanding of the circuits mediating memory encoding, consolidation, and retrieval presents the fundamental technological challenge of analyzing activity in the entire brain with single-neuron resolution. In this context, we develop the brain-wide neuron quantification toolkit (BRANT) for mapping whole-brain neuronal activation at micron-scale resolution, combining tissue clearing, high-resolution light-sheet microscopy, and automated image analysis. The robustness and scalability of this method allow us to quantify the evolution of activity patterns across multiple phases of memory in mice. This approach highlights a strong sexual dimorphism in recruited circuits, which has no counterpart in the behavior. The methodology presented here paves the way for a comprehensive characterization of the evolution of fear memory.

Sex differences in contextual fear learning and generalization: a behavioral and computational analysis of hippocampal functioning

There are sex differences in anxiety disorders with regard to occurrence and severity of episodes such that females tend to experience more frequent and more severe episodes. Contextual fear learning and generalization are especially relevant to anxiety disorders, which are often defined by expressing fear and/or anxiety in safe contexts. In contextual fear conditioning, a representation of the context must first be created, and then that representation must be paired with an aversive consequence. With some variation, the experiments presented here use a 3-d procedure in which day 1 consists of pre-exposure to the to-be-shocked context, day 2 consists of a single context-shock pairing after some placement-to-shock interval (PSI), and day 3 consists of testing in either the same or a novel context. With shorter pre-exposure periods, male rats showed more contextual fear, consistent with previous literature; however, after longer pre-exposure periods, female rats showed greater contextual fear. Additionally, while pre-exposure and PSI are both periods of time prior to the shock, it was found that they were not equivalent to each other. Animals with 120 sec of pre-exposure and a 30-sec PSI show a differential level and time course of fear expression than animals who received no pre-exposure and a 150-sec PSI, and this further depended on sex of the rat. Additionally, an experiment comparing recently versus remotely acquired contextual fear was run. Males were again shown to have greater contextual fear at both time points, and this contextual fear incubated/increased over time in males but not females. To facilitate identification of what processes caused sex differences, we used BaconX, a conceptual and computational model of hippocampal contextual learning. Computational simulations using this model predicted many of our key findings. Furthermore, these simulations suggest potential mechanisms with regard to hippocampal computation; namely, an increased feature sampling rate in males, which may account for the sex differences presented here and in prior literature.

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.

Retrosplenial cortex in spatial memory: focus on immediate early genes mapping

The ability to form, retrieve and update autobiographical memories is one of the most fascinating features of human behavior. Spatial memory, the ability to remember the layout of the external environment and to navigate within its boundaries, is closely related to the autobiographical memory domain. It is served by an overlapping brain circuit, centered around the hippocampus (HPC) where the cognitive map index is stored. Apart from the hippocampus, several cortical structures participate in this process. Their relative contribution is a subject of intense research in both humans and animal models. One of the most widely studied regions is the retrosplenial cortex (RSC), an area in the parietal lobe densely interconnected with the hippocampal formation. Several methodological approaches have been established over decades in order to investigate the cortical aspects of memory. One of the most successful techniques is based on the analysis of brain expression patterns of the immediate early genes (IEGs). The common feature of this diverse group of genes is fast upregulation of their mRNA translation upon physiologically relevant stimulus. In the central nervous system they are rapidly triggered by neuronal activity and plasticity during learning. There is a widely accepted consensus that their expression level corresponds to the engagement of individual neurons in the formation of memory trace. Imaging of the IEGs might therefore provide a picture of an emerging memory engram. In this review we present the overview of IEG mapping studies of retrosplenial cortex in rodent models. We begin with classical techniques, immunohistochemical detection of protein and fluorescent in situ hybridization of mRNA. We then proceed to advanced methods where fluorescent genetically encoded IEG reporters are chronically followed in vivo during memory formation. We end with a combination of genetic IEG labelling and optogenetic approach, where the activity of the entire engram is manipulated. We finally present a hypothesis that attempts to unify our current state of knowledge about the function of RSC.

The retrosplenial cortex as a possible "sensory integration" area: A neural network modeling approach of the differential outcomes effect in negative patterning

We explored the hypothesis that learning a Pavlovian negative patterning task would be facilitated when training with differential, as opposed to non-differential, reinforcing outcomes. Two groups of rats received pairings of one visual and one auditory stimulus with food reward when these stimuli were presented on separate training trials, but without reward when both stimuli were presented on simultaneous stimulus compound trials (V+, A+, AV-; similar to an XOR problem). For Group Differential, each stimulus was separately paired with distinctively tasting food rewards, whereas for Group Non-Differential each stimulus was randomly paired with both food reward types across different stimulus element trials. We observed that rats learned the negative patterning task more rapidly and effectively when trained with differential outcomes. These data support a multi-layered connectionist model introduced by Delamater (2012) in which a multi-modal processing structure plays the role of a "sensory integration" area like that hypothesized for the retrosplenial cortex by Dave Bucci and his colleagues (e.g., Todd, Fournier, & Bucci, 2019). We discuss how such a region may develop different "negative occasion setting" and "configural inhibition" mechanisms in solving negative patterning and related tasks.

Context value updating and multidimensional neuronal encoding in the retrosplenial cortex

The retrosplenial cortex (RSC) has diverse functional inputs and is engaged by various sensory, spatial, and associative learning tasks. We examine how multiple functional aspects are integrated on the single-cell level in the RSC and how the encoding of task-related parameters changes across learning. Using a visuospatial context discrimination paradigm and two-photon calcium imaging in behaving mice, a large proportion of dysgranular RSC neurons was found to encode multiple task-related dimensions while forming context-value associations across learning. During reversal learning requiring increased cognitive flexibility, we revealed an increased proportion of multidimensional encoding neurons that showed higher decoding accuracy for behaviorally relevant context-value associations. Chemogenetic inactivation of RSC led to decreased behavioral context discrimination during learning phases in which context-value associations were formed, while recall of previously formed associations remained intact. RSC inactivation resulted in a persistent positive behavioral bias in valuing contexts, indicating a role for the RSC in context-value updating.

Sexually dimorphic muscarinic acetylcholine receptor modulation of contextual fear learning in the dentate gyrus

Contextual fear conditioning, where the prevailing situational cues become associated with an aversive unconditional stimulus such as electric shock, is sexually dimorphic. Males typically show higher levels of fear than females. There are two components to contextual fear conditioning. First the multiple cues that encompass the context must be integrated into a coherent representation, a process that requires the hippocampus. The second is that representation must be communicated to the basolateral amygdala where it can be associated with shock. If there is inadequate time for forming the representation prior to shock poor conditioning results and this is called the immediate shock deficit. One can isolate the contextual processing component, as well as alleviate the deficit, by providing an opportunity to explore the context without shock prior to the conditioning session. The purpose of the present study was to determine the extent to which cholinergic processes within the dentate gyrus of the hippocampus during contextual processing contribute to the sexual dimorphism. Clozapine-n-oxide (CNO) is a putatively inactive compound that acts only upon synthetic genetically engineered receptors. However, we found that CNO infused into the dentate gyrus prior to exploration eliminated the sexual dimorphism by selectively decreasing freezing in males to the level of females. Biological activity of CNO is usually attributed to metabolism of CNO to clozapine and we found that clozapine, and the muscarinic cholinergic antagonist, scopolamine, produced results similar to CNO, preferentially affecting males. On the other hand, the muscarinic agonist oxotremorine selectively impaired conditioning in females. Overall, the current experiments reveal significant off-target effects of CNO and implicate muscarinic cholinergic receptors in the dentate gyrus as a significant mediator of the sexual dimorphism in contextual fear conditioning.

Higher-Order Conditioning and Dopamine: Charting a Path Forward

Higher-order conditioning involves learning causal links between multiple events, which then allows one to make novel inferences. For example, observing a correlation between two events (e.g., a neighbor wearing a particular sports jersey), later helps one make new predictions based on this knowledge (e.g., the neighbor's wife's favorite sports team). This type of learning is important because it allows one to benefit maximally from previous experiences and perform adaptively in complex environments where many things are ambiguous or uncertain. Two procedures in the lab are often used to probe this kind of learning, second-order conditioning (SOC) and sensory preconditioning (SPC). In second-order conditioning (SOC), we first teach subjects that there is a relationship between a stimulus and an outcome (e.g., a tone that predicts food). Then, an additional stimulus is taught to precede the predictive stimulus (e.g., a light leads to the food-predictive tone). In sensory preconditioning (SPC), this order of training is reversed. Specifically, the two neutral stimuli (i.e., light and tone) are first paired together and then the tone is paired separately with food. Interestingly, in both SPC and SOC, humans, rodents, and even insects, and other invertebrates will later predict that both the light and tone are likely to lead to food, even though they only experienced the tone directly paired with food. While these processes are procedurally similar, a wealth of research suggests they are associatively and neurobiologically distinct. However, midbrain dopamine, a neurotransmitter long thought to facilitate basic Pavlovian conditioning in a relatively simplistic manner, appears critical for both SOC and SPC. These findings suggest dopamine may contribute to learning in ways that transcend differences in associative and neurological structure. We discuss how research demonstrating that dopamine is critical to both SOC and SPC places it at the center of more complex forms of cognition (e.g., spatial navigation and causal reasoning). Further, we suggest that these more sophisticated learning procedures, coupled with recent advances in recording and manipulating dopamine neurons, represent a new path forward in understanding dopamine's contribution to learning and cognition.

Cortical Contributions to Higher-Order Conditioning: A Review of Retrosplenial Cortex Function

In higher-order conditioning paradigms, such as sensory preconditioning or second-order conditioning, discrete (e.g., phasic) or contextual (e.g., static) stimuli can gain the ability to elicit learned responses despite never being directly paired with reinforcement. The purpose of this mini-review is to examine the neuroanatomical basis of high-order conditioning, by selectively reviewing research that has examined the role of the retrosplenial cortex (RSC) in sensory preconditioning and second-order conditioning. For both forms of higher-order conditioning, we first discuss the types of associations that may occur and then review findings from RSC lesion/inactivation experiments. These experiments demonstrate a role for the RSC in sensory preconditioning, suggesting that this cortical region might contribute to higher-order conditioning via the encoding of neutral stimulus-stimulus associations. In addition, we address knowledge gaps, avenues for future research, and consider the contribution of the RSC to higher-order conditioning in relation to related brain structures.

Appetitive and aversive sensory preconditioning in rats is impaired by disruption of the postrhinal cortex

Episodic memory involves binding stimuli and/or events together in time and place. Furthermore, memories become more complex when new experiences influence the meaning of stimuli within the original memory. Thus collectively, complex episodic memory formation and maintenance involves processes such as encoding, storage, retrieval, updating and reconsolidation, which can be studied using animal models of higher-order conditioning. In the present study aversive and appetitive sensory preconditioning paradigms were used to test the hypothesis that the postrhinal cortex (POR), which is a component of the hippocampal memory system, is involved in higher-order conditioning. Drawing on the known role of the POR in contextual learning, Experiment 1 employed a four-phase sensory preconditioning task that involved fear learning and context discrimination in rats with or without permanent lesions of the POR. In parallel, to examine POR function during higher-order conditioning in the absence of a particular spatial arrangement, Experiments 2 and 3 used a three-phase sensory preconditioning paradigm involving phasic stimuli. In Experiment 2, bilateral lesions of the POR were made and in Experiment 3, a chemogenetic approach was used to temporarily inactivate POR neurons during each phase of the paradigm. Evidence of successful sensory preconditioning was observed in sham rats which, during the critical context discrimination test, demonstrated higher levels of freezing behavior when re-exposed to the paired versus the unpaired context, whereas POR-lesioned rats did not. Data from the appetitive sensory preconditioning paradigm also confirmed the hypothesis in that during the critical auditory discrimination test, sham rats showed greater food cup responding following presentations of the paired compared to the unpaired auditory stimulus, whereas POR-lesioned rats did not. Lastly, in Experiment 3, when the POR was inactivated only during preconditioning or only during conditioning, discrimination during the critical auditory test was impaired. Thus, regardless of whether stimulus-stimulus associations were formed between static or phasic stimuli or whether revaluation of the paired stimulus occurred through association with an aversive or an appetitive unconditioned stimulus, the effects were the same; POR lesions disrupted the ability to use higher-order conditioned stimuli to guide prospective behavior.

Configural Cues Associated with Reward Elicit Theta Oscillations of Rat Retrosplenial Cortical Neurons Phase-Locked to LFP Theta Cycles

Previous behavioral studies implicated the retrosplenial cortex (RSC) in stimulus-stimulus associations, and also in the retrieval of remote associative memory based on EEG theta oscillations. However, neural mechanisms involved in the retrieval of stored information of such associations and memory in the RSC remain unclear. To investigate the neural mechanisms underlying these processes, RSC neurons and local field potentials (LFPs) were simultaneously recorded from well-trained rats performing a cue-reward association task. In the task, simultaneous presentation of two multimodal conditioned stimuli (configural CSs) predicted a reward outcome opposite to that associated with the individual presentation of each elemental CS. Here, we show neurophysiological evidence that the RSC is involved in stimulus-stimulus association where configural CSs are discriminated from each elementary CS that is a constituent of the configural CSs, and that memory retrieval of rewarding CSs is associated with theta oscillation of RSC neurons during CS presentation, which is phase-locked to LFP theta cycles. The results suggest that cue (elementary and configural CSs)-reinforcement associations are stored in the RSC neural circuits, and are retrieved in synchronization with LFP theta rhythm.

Functional connectivity of the retrosplenial cortex in rats with ischemic stroke is improved by electroacupuncture

BACKGROUND

The aim of this study was to investigate the central mechanism underlying the putative beneficial effects of electroacupuncture (EA) on learning and memory ability of rats with ischemic stroke-induced cognitive deficits by resting-state functional magnetic resonance imaging (fMRI).

METHODS

A rat model of middle cerebral artery occlusion (MCAO)-induced cognitive deficit (MICD) was established. Rats were randomly assigned into a sham-operated control group (SC group, n = 12), untreated MICD model group (MICD group, n = 12), and MICD group receiving EA treatment at GV20 and GV24 (MICD + EA group, n = 12).

RESULTS

Compared to the MICD group, rats in the MICD + EA group receiving EA at GV20 and GV24 exhibited significantly shortened escape latency times and crossed the position of the platform a significantly increased number of times during the Morris water maze test on the 14th day after EA, which suggested EA could significantly improve spatial learning and memory ability. Furthermore, compared to the MICD group, functional connectivity of the left retrosplenial cortex (RSC) with the left hippocampus, left RSC, right RSC, left cingulate gyrus, right cingulate gyrus, right tegmentum of midbrain, and right visual cortex was increased in the MICD + EA group; the MICD group showed decreased functional connectivity of the left RSC with the left hippocampus, right hippocampus, left RSC, right RSC, right amygdaloid body, left visual cortex, and right visual cortex.

CONCLUSION

These findings suggest that EA at GV20 and GV24 might improve the learning and memory ability of MICD rats by increasing the functional connectivity between the RSC and hippocampus, cingulate gyrus and midbrain, which is encouraging for the potential treatment for cognitive impairment secondary to ischemia stroke.

Retrosplenial cortex damage impairs unimodal sensory preconditioning

The retrosplenial cortex (RSC) is positioned at the interface between cortical sensory regions and the structures that compose the medial temporal lobe memory system. It has recently been suggested that 1 functional role of the RSC involves the formation of associations between cues in the environment (stimulus-stimulus [S-S] learning; Bucci & Robinson, 2014). This suggestion is based, in part, on the finding that lesions or temporary inactivation of the RSC impair sensory preconditioning. However, all prior studies examining the role of the RSC in sensory preconditioning have used cues from multiple modalities (both visual and auditory stimuli). The purpose of the present experiment was to determine whether the RSC contributes to unimodal sensory preconditioning. In the present study we found that both electrolytic and neurotoxic lesions of the RSC impaired sensory preconditioning with auditory cues. Together with previous experiments, these findings indicate that the RSC contributes to both multisensory and unimodal sensory integration, which suggests a general role for the RSC in linking sensory cues in the environment. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

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.

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.

Retrosplenial cortex and its role in cue-specific learning and memory

The retrosplenial cortex (RSC) contributes to spatial navigation, as well as contextual learning and memory. However, a growing body of research suggests that the RSC also contributes to learning and memory for discrete cues, such as auditory or visual stimuli. In this review, we summarize and assess the Pavlovian and instrumental conditioning experiments that have examined the role of the RSC in cue-specific learning and memory. We use the term cue-specific to refer to these putatively non-spatial conditioning paradigms that involve discrete cues. Although these paradigms emphasize behavior related to cue presentations, we note that cue-specific learning and memory always takes place against a background of contextual stimuli. We review multiple ways by which contexts can influence responding to discrete cues and suggest that RSC contributions to cue-specific learning and memory are intimately tied to contextual learning and memory. Indeed, although the RSC is involved in several forms of cue-specific learning and memory, we suggest that many of these can be linked to processing of contextual stimuli.

Permanent damage or temporary silencing of retrosplenial cortex impairs the expression of a negative patterning discrimination

The retrosplenial cortex (RSC) is positioned at the interface between cortical sensory regions and the hippocampal/parahippocampal memory system. As such, it has been theorized that RSC may have a fundamental role in linking sensory stimuli together in the service of forming complex representations. To test this, three experiments were carried out to determine the effects of RSC damage or temporary inactivation on learning or performing a negative patterning discrimination. In this procedure, two conditioned stimuli are reinforced when they are presented individually (i.e., stimulus elements) but are non-reinforced when they are presented simultaneously as a compound stimulus. Normal rats successfully discriminate between the two types of trials as evidenced by more responding to the elements compared to the compound stimulus. This is thought to reflect the formation of a configural representation of the compound stimulus; that is, the two cues are linked together in such a fashion that the compound stimulus is a wholly different, unique stimulus. Permanent lesions of RSC made prior to training (Experiment 1) had no effect on learning the discrimination. However, lesions (Experiment 2) or temporary chemogenetic inactivation (Experiment 3) of RSC made after training impaired subsequent performance of the discrimination. We argue that this pattern of results indicates that RSC may normally be involved in forming the configural representations manifested in negative patterning, but that absent the RSC, other brain systems or structures can compensate sufficiently to result in normal behavior.

Distribution of HAP1-immunoreactive Cells in the Retrosplenial-retrohippocampal Area of Adult Rat Brain and Its Application to a Refined Neuroanatomical Understanding of the Region

Huntingtin-associated protein 1 (HAP1) is a neural interactor of huntingtin in Huntington's disease and interacts with gene products in a number of other neurodegenerative diseases. In normal brains, HAP1 is expressed abundantly in the hypothalamus and limbic-associated regions. These areas tend to be spared from neurodegeneration while those with little HAP1 are frequently neurodegenerative targets, suggesting its role as a protective factor against apoptosis. In light of the relationship between neurodegenerative diseases and deterioration of higher nervous activity, it is important to definitively clarify HAP1 expression in a cognitively important brain region, the retrosplenial-retrohippocampal area. Here, HAP1 expression was evaluated immunohistochemically over the retrosplenial cortex, the subicular complex, and the entorhinal and perirhinal cortices. HAP1-immunoreactive (ir) cells were classified into five discrete groups: (1) a distinct retrosplenial cell cluster exclusive to the superficial layers of the granular cortex, (2) a conspicuous, thin line of cells in layers IV/V of the "subiculum-backing cortex," (3) a group of highly immunoreactive cells associated with the medial entorhinal-subicular corner, (4) pericallosal cells just below layer VI and adjacent to the white matter, and (5) other sporadic, widely-disseminated HAP1-immunoreactive cells. HAP1 was found to be the first marker for the complex subiculum-backing cortex and a precise marker for several subfields in the retrosplenial-retrohippocampal area, verified through comparative staining with other neurochemicals. HAP1 may play an important role in protecting these cortical structures and functions for higher nervous activity by increasing the threshold to neurodegeneration and decreasing vulnerability to stress or aging.

Functional connectivity in category-selective brain networks after encoding predicts subsequent memory

Activity in category selective regions of the temporal and parietal lobes during encoding has been associated with subsequent memory for face and scene stimuli. Reactivation theories of memory consolidation predict that after encoding connectivity between these category-selective regions and the hippocampus should be modulated and predict recognition memory. However, support for this proposal has been limited in humans. Here, participants completed a resting-state functional MRI (fMRI) scan, followed by face- and place-encoding tasks, followed by another resting-state fMRI scan during which they were asked to think about the stimuli they had previously encountered. Individual differences in face recognition memory were predicted by the degree to which connectivity between face-responsive regions of the fusiform gyrus and perirhinal cortex increased following the face-encoding task. In contrast, individual differences in scene recognition were predicted by connectivity between the hippocampus and a scene-selective region of the retrosplenial cortex before and after the place-encoding task. Our results provide novel evidence for category specificity in the neural mechanisms supporting memory consolidation.

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.

What's in a context? Cautions, limitations, and potential paths forward

The purpose of memory is to guide current and future behavior based on previous experiences. Part of this process involves either discriminating between or generalizing across similar experiences that contain overlapping conditions (such as space, time, or internal state), which we often conceptualize as "contexts". In this review, we highlight major challenges facing the field as we attempt a neuroscience-based approach to the study of context and its impact on learning and memory. Here, we review some of the methodologies and approaches used to investigate context in both animals and humans, including the neurobiological mechanisms involved. Finally, we propose three tenets for operationalizing context in the experimental setting: 1) contexts must be stable over time along an experiential dimension; 2) contexts must be at least moderately complex in nature and their representations must be modifiable or adaptable, and 3) contexts must have some behavioral relevance (be it overt or incidental) so that its role can be measured.

Putting fear in context: Elucidating the role of the retrosplenial cortex in context discrimination in rats

The retrosplenial cortex (RSC), which receives visuo-spatial sensory input and interacts with numerous hippocampal memory system structures, has a well-established role in contextual learning and memory. While it has been demonstrated that RSC function is necessary to learn to recognize a single environment that is directly paired with an aversive event, the role of the RSC in discriminating between two different contexts remains largely unknown. To address this, first order (Experiment 1) and higher order (Experiment 2) fear conditioning paradigms were conducted with sham and RSC-lesioned rats. In Experiment 1 rats were exposed to one context in which shock was delivered and to a second, distinct context without shock. Their ability to discriminate between the contexts was assessed during a re-exposure test. In a second experiment, a new cohort of RSC-lesioned rats was exposed to two contexts made distinct through visual, olfactory and auditory stimuli. In a subsequent conditioning phase, the salience of one of the auditory stimuli was paired to an aversive footshock while the other was not. Similar to Experiment 1, freezing behavior was analyzed upon re-exposure to the contexts in the absence of both the auditory cue and the footshock. The results revealed that RSC is not necessary for rats to use contextual information to successfully discriminate between two contexts under the relatively simple demands involved in this first order conditioning paradigm but that context discrimination is impaired when the processing of complex and/or ambiguous contextual stimuli is required to select appropriate behavioral responses.

Protective effect of aspirin treatment on mouse behavior in the acute phase of experimental infection with Trypanosoma cruzi

Chagas disease is a potentially fatal disease caused by the parasite Trypanosoma cruzi, which can in some cases affect the central nervous system. The objective was to evaluate the effect of aspirin (ASA) in the behavior of mice infected with T. cruzi during the acute phase. This was an experimental study with random assignation. Twenty four BALB/c mice were divided into four groups of six animals each as follows: only ASA (OA), ASA before infection (BI), ASA after infection (AI) and only infection (OI). The strain used for infection was M/HOM/Bra/53/Y. An ASA dose of 100 mg/kg per day was administered 72 h before infection to BI group and the same dose 48 h after infection to AI group. Mice behavior in the open field test, mortality, and brain histopathology was evaluated. Data were analyzed using ANOVA, chi square test, and Kaplan-Meier with long-rank for survival analysis. In the open field test, the OA group has similar results with the BI group, in the variables of immobility and escape. Also, the OA group displayed significantly higher rates of micturition (p < 0.001) and defecation (p < 0.001) compared to infected groups. Mortality was higher in BI group (p = 0.02). The presence of T. cruzi amastigotes were higher in brain tissues of the AI and OI groups (p = 0.008). In conclusion, the administration of ASA before infection seemed to prevent behavioral changes induced by the acute infection, but it led to accelerated mortality. The study highlighted the potential importance of the pathways inhibited by ASA in the early hours of acute infection with T. cruzi.

A GluN2B-Selective NMDAR Antagonist Reverses Synapse Loss and Cognitive Impairment Produced by the HIV-1 Protein Tat

HIV-associated neurocognitive disorder (HAND) affects approximately half of HIV-infected patients. Loss of synaptic connections is a hallmark of many neurocognitive disorders, including HAND. The HIV-1 protein transactivator of transcription (Tat) disrupts synaptic connections both in vitro and in vivo and has been linked to impaired neurocognitive function in humans. In vitro studies have shown that ifenprodil, an antagonist selective for GluN2B-containing NMDARs, reverses synapse loss when applied after Tat. Here, we tested the hypothesis that Tat-induced loss and ifenprodil-mediated rescue of synaptic spines in vivo would predict impairment and rescue of cognitive function. Using intracranial multiphoton imaging, we found that infusion of 100 ng of HIV-1 Tat into the lateral ventricle of yellow fluorescent protein-expressing transgenic mice produced a 17 ± 1% loss of dendritic spines in layer 1 of retrosplenial cortex. Repeated imaging of the same dendrites over 3 weeks enabled longitudinal experiments that demonstrated sustained spine loss after Tat infusion and transient rescue after ifenprodil administration (10 mg/kg, i.p.). Parallel trace fear conditioning experiments showed that spine loss predicted learning deficits and that the time course of ifenprodil-induced rescue of spine density correlated with restoration of cognitive function. These results show for the first time that, during exposure to an HIV-1 neurotoxin in vivo, alteration of GluN2B-containing NMDAR signaling suppresses spine density and impairs learning. Pharmacological inhibition of these NMDARs rescued spines and restored cognitive function. Drugs that rescue synapses may improve neurocognitive function in HAND.SIGNIFICANCE STATEMENT Synaptodendritic damage correlates with cognitive decline in HIV-associated neurocognitive disorder (HAND) patients. We developed an in vivo imaging approach for longitudinal tracking of spine density that enabled correlation of synaptic changes with behavioral outcomes in a model of HAND. We show for the first time that spine loss after exposure to an HIV-1 protein can be reversed pharmacologically and that loss and recovery of dendritic spines predict impairment and restoration of cognitive function, respectively. Therefore, synapse loss, the hallmark of cognitive decline in HAND, is reversible. Drugs that restore spine density may have broad application for improving cognitive function during the early phases of neurodegenerative diseases.

The influence of low-level stimulus features on the representation of contexts, items, and their mnemonic associations

Since the earliest attempts to characterize the "receptive fields" of neurons, a central aim of many neuroscience experiments is to elucidate the information that is represented in various regions of the brain. Recent studies suggest that, in the service of memory, information is represented in the medial temporal lobe in a conjunctive or associative form with the contextual aspects of the experience being the primary factor or highest level of the conjunctive hierarchy. A critical question is whether the information that has been observed in these studies reflects notions such as a cognitive representation of context or whether the information reflects the low-level sensory differences between stimuli. We performed two functional magnetic resonance imaging experiments to address this question and we found that associative representations observed between context and item (and order) in the human brain can be highly influenced by low-level sensory differences between stimuli. Our results place clear constraints on the experimental design of studies that aim to investigate the representation of contexts and items during performance of associative memory tasks. Moreover, our results raise interesting theoretical questions regarding the disambiguation of memory-related representations from processing-related representations.

Medial temporal pathways for contextual learning: Network c-fos mapping in rats with or without perirhinal cortex lesions

BACKGROUND

In the rat brain, context information is thought to engage network interactions between the postrhinal cortex, medial entorhinal cortex, and the hippocampus. In contrast, object information is thought to be more reliant on perirhinal cortex and lateral entorhinal cortex interactions with the hippocampus.

METHOD

The 'context network' was explored by mapping expression of the immediate-early gene, c-fos, after exposure to a new spatial environment.

RESULTS

Structural equation modelling of Fos counts produced networks of good fit that closely matched prior predictions based on anatomically-grounded functional models. These same models did not, however, fit the Fos data from home-cage controls nor did they fit the corresponding data from a previous study exploring object recognition. These additional analyses highlight the specificity of the context network. The home-cage controls, meanwhile, showed raised levels of inter-area Fos correlations between the many sites examined, i.e., their changes in Fos levels lacked anatomical specificity. Two additional groups of rats received perirhinal cortex lesions. While the loss of perirhinal cortex reduced lateral entorhinal c-fos activity, it did not affect mean levels of hippocampal c-fos expression. Similarly, overall c-fos expression in the prelimbic cortex, retrosplenial cortex and nucleus reuniens of the thalamus appeared unaffected by the perirhinal cortex lesions.

CONCLUSION

The perirhinal cortex lesions disrupted network interactions involving the medial entorhinal cortex and the hippocampus, highlighting ways in which perirhinal cortex might affect specific aspects of context learning.

Reentrant Information Flow in Electrophysiological Rat Default Mode Network

Functional MRI (fMRI) studies have demonstrated that the rodent brain shows a default mode network (DMN) activity similar to that in humans, offering a potential preclinical model both for physiological and pathophysiological studies. However, the neuronal mechanism underlying rodent DMN remains poorly understood. Here, we used electrophysiological data to analyze the power spectrum and estimate the directed phase transfer entropy (dPTE) within rat DMN across three vigilance states: wakeful rest (WR), slow-wave sleep (SWS), and rapid-eye-movement sleep (REMS). We observed decreased gamma powers during SWS compared with WR in most of the DMN regions. Increased gamma powers were found in prelimbic cortex, cingulate cortex, and hippocampus during REMS compared with WR, whereas retrosplenial cortex showed a reverse trend. These changed gamma powers are in line with the local metabolic variation of homologous brain regions in humans. In the analysis of directional interactions, we observed well-organized anterior-to-posterior patterns of information flow in the delta band, while opposite patterns of posterior-to-anterior flow were found in the theta band. These frequency-specific opposite patterns were only observed in WR and REMS. Additionally, most of the information senders in the delta band were also the receivers in the theta band, and vice versa. Our results provide electrophysiological evidence that rat DMN is similar to its human counterpart, and there is a frequency-dependent reentry loop of anterior-posterior information flow within rat DMN, which may offer a mechanism for functional integration, supporting conscious awareness.

Retrograde amnesia of contextual fear conditioning: Evidence for retrosplenial cortex involvement in configural processing

It has been suggested that contextual fear conditioning can be supported by either an elemental system, where individual features of the environment are associated with shock, or a configural system, where environmental features are bound together and associated with shock. Although the retrosplenial cortex (RSC) is known to be involved in contextual fear conditioning, it is not clear whether it contributes to the elemental or configural system. To isolate the role of the RSC in contextual fear conditioning, the current experiments examined the influence of RSC lesions on the context preexposure facilitation effect, a procedure known to produce conditioning to a configural representation of context. In Experiment 1, rats that were preexposed to the conditioning context froze more compared to rats that were not, replicating the context preexposure facilitation effect. Although pretraining lesions of the RSC had no impact on the context preexposure facilitation effect (Experiment 2a), posttraining lesions attenuated the effect (Experiment 2b), suggesting that the RSC normally contributes to a configural context representation. Retrohippocampal contributions to contextual fear conditioning are discussed. (PsycINFO Database Record

Intact renewal after extinction of conditioned suppression with lesions of either the retrosplenial cortex or dorsal hippocampus

Extinction of fear to a Pavlovian conditioned stimulus (CS) is known to be context-specific. When the CS is tested outside the context of extinction, fear returns, or renews. Several studies have demonstrated that renewal depends upon the hippocampus, although there are also studies where renewal was not impacted by hippocampal damage, suggesting that under some conditions context encoding and/or retrieval of extinction depends upon other regions. One candidate region is the retrosplenial cortex (RSC), which is known to contribute to contextual and spatial learning and memory. Using a conditioned-suppression paradigm, Experiment 1 tested the impact of pre-training RSC lesions on renewal of extinguished fear. Consistent with previous studies, lesions of the RSC did not impact acquisition or extinction of conditioned fear to the CS. Further, there was no evidence that RSC lesions impaired renewal, indicating that contextual encoding and/or retrieval of extinction does not depend upon the RSC. In Experiment 2, post-extinction lesions of either the RSC or dorsal hippocampus (DH) also had no impact on renewal. However, in Experiment 3, both RSC and DH lesions did impair performance in an object-in-place procedure, an index of place memory. RSC and DH contributions to extinction and renewal are discussed.

The BDNF Val66Met Polymorphism Influences Reading Ability and Patterns of Neural Activation in Children

Understanding how genes impact the brain’s functional activation for learning and cognition during development remains limited. We asked whether a common genetic variant in the BDNF gene (the Val⁶⁶Met polymorphism) modulates neural activation in the young brain during a critical period for the emergence and maturation of the neural circuitry for reading. In animal models, the bdnf variation has been shown to be associated with the structure and function of the developing brain and in humans it has been associated with multiple aspects of cognition, particularly memory, which are relevant for the development of skilled reading. Yet, little is known about the impact of the Val⁶⁶Met polymorphism on functional brain activation in development, either in animal models or in humans. Here, we examined whether the BDNF Val⁶⁶Met polymorphism (dbSNP rs6265) is associated with children’s (age 6–10) neural activation patterns during a reading task (n = 81) using functional magnetic resonance imaging (fMRI), genotyping, and standardized behavioral assessments of cognitive and reading development. Children homozygous for the Val allele at the SNP rs6265 of the BDNF gene outperformed Met allele carriers on reading comprehension and phonological memory, tasks that have a strong memory component. Consistent with these behavioral findings, Met allele carriers showed greater activation in reading–related brain regions including the fusiform gyrus, the left inferior frontal gyrus and left superior temporal gyrus as well as greater activation in the hippocampus during a word and pseudoword reading task. Increased engagement of memory and spoken language regions for Met allele carriers relative to Val/Val homozygotes during reading suggests that Met carriers have to exert greater effort required to retrieve phonological codes.

Higher-order conditioning and the retrosplenial cortex

The retrosplenial cortex (RSC) is known to contribute to contextual and spatial learning and memory. This is consistent with its well-established connectivity; the RSC is located at the interface of visuo-spatial association areas and the parahippocampal-hippocampal memory system. However, the RSC also contributes to learning and memory for discrete cues. For example, both permanent lesions and temporary inactivation of the RSC have been shown to impair sensory preconditioning, a form of higher-order conditioning. The purpose of the present experiment was to examine the role of the RSC in a closely related higher-order conditioning paradigm: second-order conditioning. Sham and RSC lesioned rats received first-order conditioning in which one visual stimulus (V1) was paired with footshock and one visual stimulus (V2) was not. Following first-order conditioning, one auditory stimulus (A1) was then paired with V1 and a second auditory stimulus (A2) was paired with V2. Although lesions of the RSC impaired the first-order discrimination, they had no impact on the acquisition of second-order conditioning. Thus, the RSC does not appear necessary for acquisition/expression of second-order fear conditioning. The role of the RSC in higher-order conditioning, as well as a possible dissociation from the hippocampus, is discussed.

Retrosplenial cortex is required for the retrieval of remote memory for auditory cues

The restrosplenial cortex (RSC) has a well-established role in contextual and spatial learning and memory, consistent with its known connectivity with visuo-spatial association areas. In contrast, RSC appears to have little involvement with delay fear conditioning to an auditory cue. However, all previous studies have examined the contribution of the RSC to recently acquired auditory fear memories. Since neocortical regions have been implicated in the permanent storage of remote memories, we examined the contribution of the RSC to remotely acquired auditory fear memories. In Experiment 1, retrieval of a remotely acquired auditory fear memory was impaired when permanent lesions (either electrolytic or neurotoxic) were made several weeks after initial conditioning. In Experiment 2, using a chemogenetic approach, we observed impairments in the retrieval of remote memory for an auditory cue when the RSC was temporarily inactivated during testing. In Experiment 3, after injection of a retrograde tracer into the RSC, we observed labeled cells in primary and secondary auditory cortices, as well as the claustrum, indicating that the RSC receives direct projections from auditory regions. Overall our results indicate the RSC has a critical role in the retrieval of remotely acquired auditory fear memories, and we suggest this is related to the quality of the memory, with less precise memories being RSC dependent.

Brain plasticity and cognitive functions after ethanol consumption in C57BL/6J mice

Acute or chronic administrations of high doses of ethanol in mice are known to produce severe cognitive deficits linked to hippocampal damage. However, we recently reported that chronic and moderate ethanol intake in C57BL/6J mice induced chromatin remodeling within the Bdnf promoters, leading to both enhanced brain-derived neurotrophic factor (BDNF) expression and hippocampal neurogenesis under free-choice protocol. We performed here a series of cellular and behavioral studies to analyze the consequences of these modifications. We showed that a 3-week chronic free-choice ethanol consumption in C57BL/6J mice led to a decrease in DNA methylation of the Bdnf gene within the CA1 and CA3 subfields of the hippocampus, and upregulated hippocampal BDNF signaling pathways mediated by ERK, AKT and CREB. However, this activation did not affect long-term potentiation in the CA1. Conversely, ethanol intake impaired learning and memory capacities analyzed in the contextual fear conditioning test and the novel object recognition task. In addition, ethanol increased behavioral perseveration in the Barnes maze test but did not alter the mouse overall spatial capacities. These data suggested that in conditions of chronic and moderate ethanol intake, the chromatin remodeling leading to BDNF signaling upregulation is probably an adaptive process, engaged via epigenetic regulations, to counteract the cognitive deficits induced by ethanol.

Retrosplenial Cortex and Long-Term Memory: Molecules to Behavior

The retrosplenial cortex (RSC) is reciprocally connected with the hippocampus and various parahippocampal cortical regions, suggesting that RSC is well-positioned to contribute to hippocampal-dependent memory. Consistent with this, substantial behavioral evidence indicates that RSC is essential for consolidating and/or retrieving contextual and spatial memories. In addition, there is growing evidence that RSC neurons undergo activity-dependent plastic changes during memory formation and retrieval. In this paper we review both the behavioral and cellular/molecular data and posit that the RSC has a particularly important role in the storage and retrieval of spatial and contextual memories perhaps due its involvement in binding together multiple cues in the environment. We identify remaining questions and avenues for future research that take advantage of emerging methods to selectively manipulate RSC neurons both spatially and temporally and to image the RSC in awake, behaving animals.

The retrosplenial cortex is involved in the formation of memory for context and trace fear conditioning

The retrosplenial cortex (RSC) is known to play a role in the retrieval of context memory, but its involvement in memory formation and consolidation is unclear. To better characterize the role of the RSC, we tested its involvement in the formation and retrieval of memory for trace fear conditioning, a task that requires the association of two cues separated by an empty period of time. We have previously shown that trace fear extinction requires the RSC (Kwapis, Jarome, Lee, Gilmartin, & Helmstetter, 2014) and have hypothesized that trace memory may be stored in a distributed cortical network that includes prelimbic and retrosplenial cortices (Kwapis, Jarome, & Helmstetter, 2015). Whether the RSC participates in acquiring and storing cued trace fear, however, is currently unknown. Here, we demonstrate that blocking protein synthesis in the RSC before, but not after acquisition impairs rats' memory for trace CS and context fear without affecting memory for the CS in standard delay fear conditioning. We also show that NMDA receptor blockade in the RSC transiently impairs memory retrieval for trace, but not delay memory. The RSC therefore appears to critically contribute to formation of trace and context fear memory in addition to its previously recognized role in context memory retrieval.

Cortical connectivity maps reveal anatomically distinct areas in the parietal cortex of the rat

A central feature of theories of spatial navigation involves the representation of spatial relationships between objects in complex environments. The parietal cortex has long been linked to the processing of spatial visual information and recent evidence from single unit recording in rodents suggests a role for this region in encoding egocentric and world-centered frames. The rat parietal cortex can be subdivided into four distinct rostral-caudal and medial-lateral regions, which includes a zone previously characterized as secondary visual cortex. At present, very little is known regarding the relative connectivity of these parietal subdivisions. Thus, we set out to map the connectivity of the entire anterior-posterior and medial-lateral span of this region. To do this we used anterograde and retrograde tracers in conjunction with open source neuronal segmentation and tracer detection tools to generate whole brain connectivity maps of parietal inputs and outputs. Our present results show that inputs to the parietal cortex varied significantly along the medial-lateral, but not the rostral-caudal axis. Specifically, retrosplenial connectivity is greater medially, but connectivity with visual cortex, though generally sparse, is more significant laterally. Finally, based on connection density, the connectivity between parietal cortex and hippocampus is indirect and likely achieved largely via dysgranular retrosplenial cortex. Thus, similar to primates, the parietal cortex of rats exhibits a difference in connectivity along the medial-lateral axis, which may represent functionally distinct areas.

Contribution of the retrosplenial cortex to temporal discrimination learning

The retrosplenial cortex (RSC) has an important role in contextual learning and memory. While the majority of experiments have focused on the physical context, the present study asked whether the RSC is involved in processing the temporal context. Rats were trained in a temporal discrimination procedure where the duration of the intertrial interval (ITI) signaled whether or not the next tone conditioned stimulus would be paired with food pellet reinforcement. When the tone was presented after a 16-min ITI it was reinforced, but when it was presented after a 4-min ITI it was not. Rats demonstrated successful discrimination in this procedure by responding more to the tone on reinforced trials than on non-reinforced trials. Pre-training electrolytic lesions of the RSC attenuated acquisition of the temporal discrimination. The results are the first to demonstrate a role for the RSC in processing temporal information and in turn extend the role of the RSC beyond the physical context to now include the temporal context.