Modality-specific retrograde amnesia of fear

Hippocampus alters visual representation to encode new memory

Hippocampal-cortical interaction is crucial for episodic memory encoding and retaining, but when and how this interaction occurs remains elusive. Here, within the trace eyeblink conditioning paradigm, we found a neuronal ensemble in layer II of mouse visual cortex (VIS) that responded to the paired stimulus of light flash (cue, or conditioned stimulus [CS]) and air puff (unconditioned stimulus [US]), but not discrete stimuli, resembling an associative event during learning. This neuronal representation in VIS is dependent on the hippocampus and contributes to encoding the association. Optogenetic activation of hippocampus can promote the emerging representation to allow the association of separated cues. Mechanistically, fos+ engram cells, modulated by VIP+ neurons, are hubs of the association-activated ensemble in VIS. The hippocampus-modulated memory ensemble emerging in VIS at an early stage implicates the structural organization of the memory network to encode new memory.

Contralesional hippocampal spreading depolarization promotes functional recovery after stroke

Ischemic stroke, brain tissue infarction following obstructed cerebral blood flow, leads to long-term neurological deficits and death. While neocortex is a commonly affected region with established preclinical models, less is known about deeper brain strokes, despite having unique neurological outcomes. We induced focal ischemic stroke while simultaneously monitoring neuronal activity in awake behaving Thy1-GCaMP6f mice by delivering and collecting light through bilateral fiberoptic implants. Unilateral hippocampal stroke resulted in atypical wandering behavior coincident with ipsilesional terminal spreading depolarization (sustained increase in GCaMP6f fluorescence). Ischemia induced seizures that propagated to the contralesional hippocampus triggering a transient spreading depolarization, predominantly in females. Hippocampal stroke impaired contextual fear conditioning acquired pre-stroke. Yet, 7 days post-stroke, contextual fear conditioning was only improved in mice with contralesional spreading depolarization. Blunting peri-stroke contralesional spreading depolarization prevented recovery of hippocampus-dependent learning. Together, we show that regionally isolated deleterious and beneficial spreading depolarizations can occur concurrently in the murine brain during acute stroke.

Auditory fear memory retrieval requires BLA-LS and LS-VMH circuitries via GABAergic and dopaminergic neurons

Fear and associated learning and memory are critical for developing defensive behavior. Excessive fear and anxiety are important components of post-traumatic stress disorder. However, the neurobiology of fear conditioning, especially tone-related fear memory retrieval, has not been clearly defined, which limits specific intervention development for patients with excessive fear and anxiety. Here, we show that auditory fear memory retrieval stimuli activate multiple brain regions including the lateral septum (LS). Inhibition of the LS and the connection between basolateral amygdala (BLA) and LS or between LS and ventromedial nucleus of the hypothalamus (VMH) attenuates tone-related fear conditioning and memory retrieval. Inhibiting GABAergic neurons or dopaminergic neurons in the LS also attenuates tone-related fear conditioning. Our data further show that fear conditioning is inhibited by blocking orexin B signaling in the LS. Our results indicate that the neural circuitries BLA-LS and LS-VMH are critical for tone-related fear conditioning and memory retrieval, and that GABAergic neurons, dopaminergic neurons and orexin signaling in the LS participate in this auditory fear conditioning.

Erasable hippocampal neural signatures predict memory discrimination

Memories involving the hippocampus can take several days to consolidate, challenging efforts to uncover the neuronal signatures underlying this process. Here, we use calcium imaging in freely moving mice to track the hippocampal dynamics underlying memory consolidation across a 10-day contextual fear conditioning task. We find two neural signatures that emerge following learning and predict memory performance: context-specific place field remapping and coordinated neural activity prior to memory recall (freezing). To test whether these signatures support memory consolidation, we pharmacologically induced amnesia in separate mice by administering anisomycin, a protein synthesis inhibitor, immediately following learning. We find that anisomycin paradoxically accelerates cell turnover. Anisomycin also arrests learning-related remapping and blocks coordinated activity predictive of memory-related freezing behavior, effects that are likewise absent in untreated mice that exhibit poor memory expression. We conclude that context-specific place field remapping and the development of coordinated ensemble activity underlie contextual memory consolidation.

Closed-loop modulation of remote hippocampal representations with neurofeedback

Humans can remember specific remote events without acting on them and influence which memories are retrieved based on internal goals. However, animal models typically present sensory cues to trigger memory retrieval and then assess retrieval based on action. Thus, it is difficult to determine whether measured neural activity patterns relate to the cue(s), the memory, or the behavior. We therefore asked whether retrieval-related neural activity could be generated in animals without cues or a behavioral report. We focused on hippocampal "place cells," which primarily represent the animal's current location (local representations) but can also represent locations away from the animal (remote representations). We developed a neurofeedback system to reward expression of remote representations and found that rats could learn to generate specific spatial representations that often jumped directly to the experimenter-defined target location. Thus, animals can deliberately engage remote representations, enabling direct study of retrieval-related activity in the brain.

Associative coding of conditioned fear in the thalamic nucleus reuniens in rodents and humans

The nucleus reuniens (RE) is a midline thalamic structure interconnecting the medial prefrontal cortex (mPFC) and the hippocampus (HPC). Recent work in both rodents and humans implicates the RE in the adaptive regulation of emotional memories, including the suppression of learned fear. However, the neural correlates of aversive learning in the RE of rodents and humans remains unclear. To address this, we recorded RE activity in humans (BOLD fMRI) and rats (fiber photometry) during Pavlovian fear conditioning and extinction. In both rats and humans, we found that conditioned stimulus (CS)-evoked activity in RE reflects the associative value of the CS. In rats, we additionally found that spontaneous neural activity in RE tracks defensive freezing and shows anticipatory increases in calcium activity that precede the termination of freezing behavior. Single-unit recordings in rats confirmed that individual RE neurons index both the associative value of the CS and defensive behavior transitions. Moreover, distinct neuronal ensembles in the RE encode fear versus extinction memories. These findings suggest a conserved role of the RE across species in modulating defensive states and emotional memory processes, providing a foundation for future translational research on fear-related disorders.

Possible effects of radiofrequency electromagnetic radiation on contextual fear conditioning, hippocampal perivascular space, apoptosis and adrenal gland microarchitecture in rats

Whilst the world sees the tremendous growth of mobile phone technology, radiofrequency electromagnetic radiation (RF-EMR) induced possible health effects have emerged as a topic of recent day debate. The current study is designed to test the hypothesis that chronic 900 MHz radiation exposure would potentially dysregulate the stress response system (HPA axis) in vivo, via, its non-thermal mechanisms, leading to alterations in the microarchitecture of the adrenal gland, vulnerable brain regions such as the hippocampus which may results in altered behaviours in rats. Male albino Wistar rats aged four weeks, weighing 50-60 g were subjected to 900 MHz radiation from a mobile phone for four weeks at a rate of one hour per day. On the 29th day, animals from the control, sham exposed and RF-EMR exposed groups were tested for contextual fear conditioning. They were later euthanized to study hippocampal and adrenal gland cytoarchitecture. Bright and dark compartment transitions in the avoidance box were considerably elevated in the RF-EMR exposed group and they exhibited a significant decrease in the latency to enter the dark compartment during the contextual fear conditioning test. Apoptosis was apparent in the CA3 region and perivascular space was significantly increased in the hippocampus of the radiation-exposed group. In addition to lymphocytic infiltrates, congested sinusoids, apoptotic-like changes were evident in the zona fasciculata of the adrenal gland. However, the cytoarchitecture of the adrenal medulla was comparable in all three groups. Chronic RF-EMR exposure caused changes in contextual fear conditioning, enlargement of hippocampal perivascular space, apparent CA3 apoptosis, and apoptotic-like changes in the zona fasciculata of the adrenal gland in rats.

Vagus nerve stimulation (VNS) preventing postoperative cognitive dysfunction (POCD): two potential mechanisms in cognitive function

Postoperative cognitive dysfunction (POCD) impacts a significant number of patients annually, frequently impairing their cognitive abilities and resulting in unfavorable clinical outcomes. Aimed at addressing cognitive impairment, vagus nerve stimulation (VNS) is a therapeutic approach, which was used in many mental disordered diseases, through the modulation of vagus nerve activity. In POCD model, the enhancement of cognition function provided by VNS was shown, demonstrating VNS effect on cognition in POCD. In the present study, we primarily concentrates on elucidating the role of the VNS improving the cognitive function in POCD, via two potential mechanisms: the inflammatory microenvironment and epigenetics. This study provided a theoretical support for the feasibility that VNS can be a potential method to enhance cognition function in POCD.

Hippocampal Pallium Lesion Impairs Transitive Inference in Goldfish

Transitive inference, a process that involves drawing logical conclusions based on preliminary information, is considered a cornerstone of human deductive reasoning. Furthermore, transitive inference is a clear instance of representational flexibility as it implies the novel expression of learned information. In mammals and birds, both episodic memory and transitive inference critically depend on the integrity of the hippocampus. Comparative neurobiological evidence indicates that a hippocampus homologue can also be found in the telencephalic pallium of teleost fish. Here, we investigated whether goldfish demonstrate inferential behavior in a standard transitive inference task, and whether the hippocampal pallium of goldfish, akin to the hippocampus in mammals and birds, plays a role in transitive responding. We trained goldfish with hippocampal pallium lesions and sham-operated controls on a series of overlapping two-item visual premise pairs: A+B-, B+C-, C+D-, D+E-. The sham-operated animals readily learned the premise pair discriminations and responded transitively during the crucial test involving a novel pair of nonadjacent elements (B vs. D). However, hippocampal pallium-lesioned goldfish were impaired in the critical transitive inference test, although they successfully learned to discriminate the premise pairs. These findings suggest that a relational memory function, which supports the novel expression of learned information, could be a primitive feature of the vertebrate hippocampus. Such outcome contributes significantly to the ongoing debate regarding the evolutionary origins of episodic memory in vertebrates.

Cd99l2 regulates excitatory synapse development and restrains immediate-early gene activation

Cd99 molecule-like 2 (Cd99l2) is a type I transmembrane protein that plays a role in the transmigration of leukocytes across vascular endothelial cells. Despite its high expression in the brain, the role of Cd99l2 remains elusive. We find that Cd99l2 is expressed primarily in neurons and positively regulates neurite outgrowth and the development of excitatory synapses. We demonstrate that Cd99l2 inversely regulates the expression of immediate-early genes (IEGs), including Arc, Egr1, and c-Fos, by inhibiting the activity of the transcription factors CREB and SRF. Neuronal inactivation increases the transport of Cd99l2 to the cell surface from recycling endosomes, thereby enhancing Cd99l2-mediated inhibitory signaling. Additionally, Cd99l2 knockout mice exhibit impaired excitatory synaptic transmission and plasticity in the hippocampus, along with deficits in spatial memory and contextual fear conditioning. Based on these findings, we propose that neuronal Cd99l2 functions as a synaptic cell adhesion molecule that inversely controls neuronal activation.

Sex in aging matters: exercise and chronic stress differentially impact females and males across the lifespan

Assessing sex as a biological variable is critical to determining the influence of environmental and lifestyle risks and protective factors mediating behavior and neuroplasticity across the lifespan. We investigated sex differences in affective behavior, memory, and hippocampal neurogenesis following short- or long-term exposure to exercise or chronic mild stress in young and aged mice. Male and female mice were assigned control, running, or chronic stress rearing conditions for 1 month (young) or for 15 months (aged), then underwent a behavioral test battery to assess activity, affective behavior, and memory. Stress exposure into late-adulthood increased hyperactivity in both sexes, and enhanced anxiety-like and depressive-like behavior in aged female, but not male, mice. One month of stress or running had no differential effects on behavior in young males and females. Running increased survival of BrdU-labelled hippocampal cells in both young and aged mice, and enhanced spatial memory in aged mice. These findings highlight the importance of considering sex when determining how aging is differently impacted by modifiable lifestyle factors across the lifespan.

Duration of aversive memory in zebrafish after a single shock

Studies on memory consolidation and reconsolidation, memory loss, and the associated biochemical mechanisms have garnered interest in the past decades due to knowledge of memory performance-affecting factors such as stress, emotions, sleep, age, several neurological diseases, drugs, and chemical pollutants. Memory research has been using animal models, with increased interest in the zebrafish model. This freshwater fish species shows a wide range of behaviors relevant to memory research such as social behavior, aggression, and predator avoidance; however, few studies have investigated the duration of long-term memory. Hence, we designed an experiment to test memory duration by exposing zebrafish to avoidance conditioning using electroshock as the aversive stimulus. Zebrafish were trained to avoid the black side of a black-and-white tank and subsequently tested for aversive memory at 24 h, 48 h, 72 h, 96 h, 168 h, and 240 h. At the 72 h-interval, another zebrafish group was trained and exposed to MK-801(NMDAr antagonist) and then tested. The fish retained memories of the task and avoided the black side of the tank for up to 7 days. At 10 days post-training, the animals could no longer retrieve the aversive memory. Zebrafish treated with MK-801 did not retrieve memory. Knowledge of memory and of long-term memory duration is crucial for optimizing the zebrafish model for use in research investigating cognitive impairments such as memory loss and its ramifications. Additionally, identifying a long-term aversive memory lasting up to 7 days in zebrafish enables further research into the neuronal changes underlying this persistence. Such in-depth investigation could bring valuable insights into memory mechanisms and facilitate targeted interventions for memory-related conditions.

Obesity-associated memory impairment and neuroinflammation precede widespread peripheral perturbations in aged rats

BACKGROUND

Obesity and metabolic syndrome are major public health concerns linked to cognitive decline with aging. Prior work from our lab has demonstrated that short-term high fat diet (HFD) rapidly impairs memory function via a neuroinflammatory mechanism. However, the degree to which these rapid inflammatory changes are unique to the brain is unknown. Moreover, deviations in gut microbiome composition have been associated with obesity and cognitive impairment, but how diet and aging interact to impact the gut microbiome, or how rapidly these changes occur, is less clear. Thus, our study investigated the impact of HFD after two distinct consumption durations: 3 months (to model diet-induced obesity) or 3 days (to detect the rapid changes occurring with HFD) on memory function, anxiety-like behavior, central and peripheral inflammation, and gut microbiome profile in young and aged rats.

RESULTS

Our data indicated that both short-term and long-term HFD consumption impaired memory function and increased anxiety-like behavior in aged, but not young adult, rats. These behavioral changes were accompanied by pro- and anti-inflammatory cytokine dysregulation in the hippocampus and amygdala of aged HFD-fed rats at both time points. However, changes to fasting glucose, insulin, and inflammation in peripheral tissues such as the distal colon and visceral adipose tissue were increased in young and aged rats only after long-term, but not short-term, HFD consumption. Furthermore, while subtle HFD-induced changes to the gut microbiome did occur rapidly, robust age-specific effects were only present following long-term HFD consumption.

CONCLUSIONS

Overall, these data suggest that HFD-evoked neuroinflammation, memory impairment, and anxiety-like behavior in aging develop quicker than, and separately from the peripheral hallmarks of diet-induced obesity.

Memory Development, Configurations, Conjunctions, and the Hippocampal Index

When I began my career, I had no idea that much of it would center around the hippocampus. Here I discuss some of the history of how this happened. I briefly mention my early undergraduate life and the problems it posed for getting into graduate school. I describe the unique circumstances that led me to Allan Wagner's laboratory and changed my career trajectory. My path to the hippocampus began with a decision to study memory development. This led to a collaboration with Rob Sutherland that produced the configural theory of the hippocampus. The idea was that the hippocampus facilitated the construction of representations of the co-occurring stimulus elements currently experienced by the organism. Thus, if two elements, A and B, occurred together, a representation, AB, could be constructed that could be discriminated from its elements, A and B. This idea was partially correct, but we missed an important property of the hippocampal system that was recognized by O'Keefe and Nadel, 1978 that is, that the hippocampus is an unmotivated, rapid learning system. Randy O'Reilly and I addressed this issue in what we called conjunctive representation theory and put forth a detailed cortical-hippocampus computational theory to explain how this could work I later realized that our ideas were remarkably like Tim Teyler's indexing theory of how the hippocampal system supports memory. At a Park City meeting, a chance encounter with Tim (whom I had never met) resulted in the opportunity to write a paper with Tim updating the indexing theory, It is my favorite theoretical paper.

Hippocampal Discoveries: Spatial View Cells, Connectivity, and Computations for Memory and Navigation, in Primates Including Humans

Two key series of discoveries about the hippocampus are described. One is the discovery of hippocampal spatial view cells in primates. This discovery opens the way to a much better understanding of human episodic memory, for episodic memory prototypically involves a memory of where people or objects or rewards have been seen in locations "out there" which could never be implemented by the place cells that encode the location of a rat or mouse. Further, spatial view cells are valuable for navigation using vision and viewed landmarks, and provide for much richer, vision-based, navigation than the place to place self-motion update performed by rats and mice who live in dark underground tunnels. Spatial view cells thus offer a revolution in our understanding of the functions of the hippocampus in memory and navigation in humans and other primates with well-developed foveate vision. The second discovery describes a computational theory of the hippocampal-neocortical memory system that includes the only quantitative theory of how information is recalled from the hippocampus to the neocortex. It is shown how foundations for this research were the discovery of reward neurons for food reward, and non-reward, in the primate orbitofrontal cortex, and representations of value including of monetary value in the human orbitofrontal cortex; and the discovery of face identity and face expression cells in the primate inferior temporal visual cortex and how they represent transform-invariant information. This research illustrates how in order to understand a brain computation, a whole series of integrated interdisciplinary discoveries is needed to build a theory of the operation of each neural system.

The Anatomy of Context

For most of my career, I focused on understanding how and where spatial context, the place where things happen, is represented in the brain. My interest in this began in the early 1990's, during my postdoctoral training with David Amaral, when we defined the rodent homolog of the primate parahippocampal cortex, a region implicated in processing spatial and contextual information. We parceled out the caudal portion of the rat perirhinal cortex (PER) and called it the postrhinal cortex (POR). In my own lab at Brown University, I continued to study the anatomy of the PER, POR, and entorhinal cortices. I also began to characterize and differentiate the functions of these regions, particularly the newly defined POR and the redefined PER. Our electrophysiological and behavioral evidence supports a view of POR function that aligns with our anatomical evidence. Briefly, the POR integrates object and feature information from the PER with spatial information from the retrosplenial, posterior parietal, and secondary visual cortices and the pulvinar and uses this information to represent specific environmental contexts, including the spatial arrangement of objects and features within each context. In addition to maintaining a representation of the current context, the POR plays an attentional role by continually monitoring the context for changes and updating the context representation when changes occur. This context representation is accessible to other regions for cognitive processes, including binding life events with context to form episodic memories, guiding context-relevant behavior, and recognizing objects within scenes and contexts.

Hippocampal Lesions in Male Rats Produce Retrograde Memory Loss for Over-Trained Spatial Memory but Do Not Impact Appetitive-Contextual Memory: Implications for Theories of Memory Organization in the Mammalian Brain

Evidence suggests that hippocampal (HPC) disruption following learning produces retrograde amnesia on a range of tasks. Many of these tasks do not require HPC function in the anterograde direction suggesting that, in the intact brain, the HPC is actively involved during all forms of learning. However, prior work has also demonstrated double dissociations of HPC and amygdala function, which is inconsistent with this view. Here, we aim to understand this discrepancy by assessing the effects of neurotoxic lesions of the HPC on anterograde and retrograde amnesia for conditioned place preference (CPP). This task is dependent on a network centered on the basolateral amygdala and not the HPC. The results show that extensive HPC damage had no impact on the acquisition or expression of CPP. One explanation for this result is that the task requires multiple, distributed training sessions for conditioning to emerge, and it has been proposed that this parameter may eliminate the need for HPC to support memory. To test this, we completed experiments to probe place learning in the Morris water task, which is thought to always require HPC function, but implemented an over-training procedure before HPC damage. We found that rats were severely impaired suggesting that this task parameter does not always allow non-HPC networks to support learning. Finally, an experiment was designed to test whether memories acquired by distinct memory networks on the same days, within hours of one another, would be linked in HPC. The results showed that they remained independent of one another.

Effects of remimazolam and surgery on cognition in a tibia fracture mouse model

BACKGROUND

To investigate the effects of remimazolam and surgery on cognitive function and neuropathology.

METHODS

We performed intramedullary pin fixation of tibial fractures in wild-type male (12-13-week-old) C57BL/6J mice under intraperitoneal anesthesia with remimazolam. Age-matched wild-type control mice received either saline or remimazolam without surgery. Training was performed 1 h before surgery, and the open field test was performed on the third postoperative day, in addition to trace fear conditioning on the third versus the seventh day and the Y maze test on the fourth versus the eighth day. Phosphorylated tau (P-TAU) protein levels in hippocampal tissue, microglial activation, dendritic spine density in neuronal cells, and interleukin-6 (IL-6) levels were determined.

RESULTS

We detected no significant differences in locomotor ability among the three groups in the open field test on the third postoperative day; however, on the conditioned fear test or in the Y-maze, the cognitive related performance of the mice in the surgery group was significantly worse than that of the control group and the remimazolam group. However, there were no differences among the three groups in the behavioural experiments on the seventh and eighth days. In addition, mice in the surgery group had higher levels of P-TAU in their hippocampal tissue, more microglial activation, more significant changes in neuronal dendritic spine density, and higher levels of IL-6 in their hippocampal tissue compared with mice in the other two groups.

CONCLUSIONS

The cognitive dysfunction and neuropathological changes produced by remimazolam-based surgery are mainly of surgical origin and are not related to the use of remimazolam, a general anesthetic agent.

Disruption of hippocampal-prefrontal neural dynamics and risky decision-making in a mouse model of Alzheimer's disease

This study investigates how amyloid pathology influences hippocampal-prefrontal neural dynamics and decision-making in Alzheimer's disease (AD) using 5XFAD mice, a well-established model system characterized by pronounced early amyloid pathology. Utilizing ecologically-relevant "approach food-avoid predator" foraging tasks, we observed that 5XFAD mice exhibited persistent risk-taking behaviors and reduced adaptability to changing threat conditions, indicative of impaired decision-making. Multi-regional neural recordings revealed rigid hippocampal CA1 place-cell fields, decreased sharp-wave ripple (SWR) frequencies, and disrupted medial prefrontal-hippocampal connectivity, all of which corresponded with deficits in behavioral flexibility during spatial risk scenarios. These findings highlight the critical role of SWR dynamics and corticolimbic circuit integrity in adaptive decision-making, with implications for understanding cognitive decline in AD in naturalistic contexts. By identifying specific neural disruptions underlying risky decision-making deficits, this work provides insights into the neural basis of cognitive dysfunction in AD and suggests potential targets for therapeutic intervention.

P2X7 receptors modulate acquisition of cue fear extinction and contextual background memory generalization in male mice

The purinergic P2X7 receptors (P2X7R) are activated by adenosine triphosphate (ATP) in several brain regions, particularly those involved with emotional control and the regulation of fear-related memories. Here, we investigate the role of P2X7R in fear learning memory, specifically in the acquisition and consolidation phases of the cued fear conditioning paradigm. C57Bl/6 wildtype (WT) male mice that received a single i.p. injection of the selective P2X7R antagonist A438079 prior the conditioning session showed generalization of cued fear memory and impaired fear extinction recall in the test session, while those treated prior the extinction session exhibited a similar behavior profile accompanied by resistance in the extinction learning. However, no effects were observed when this drug was administered immediately after the conditioning, extinction, or before the test session. Our results with P2X7R knockout (P2X7 KO) mice showed a behavioral profile that mirrored the collective effects observed across all pharmacological treatment conditions. This suggests that the P2X7R KO model effectively replicates the behavioral changes induced by the pharmacological interventions, demonstrating that we have successfully isolated the role of P2X7R in the fear and extinction phases of memory. These findings highlight the role of P2X7R in the acquisition and recall of extinction memory and supports P2X7R as a promising candidate for controlling abnormal fear processing, with potential applications for stress exposure-related disorders such as post-traumatic stress disorder (PTSD).

Plasticity of Dendritic Spines Underlies Fear Memory

The brain has the powerful ability to transform experiences into anatomic maps and continuously integrate massive amounts of information to form new memories. The manner in which the brain performs these processes has been investigated extensively for decades. Emerging reports suggest that dendritic spines are the structural basis of information storage. The complex orchestration of functional and structural dynamics of dendritic spines is associated with learning and memory. Owing to advancements in techniques, more precise observations and manipulation enable the investigation of dendritic spines and provide clues to the challenging question of how memories reside in dendritic spines. In this review, we summarize the remarkable progress made in revealing the role of dendritic spines in fear memory and the techniques used in this field.

A contextual fear conditioning paradigm in head-fixed mice exploring virtual reality

Contextual fear conditioning is a classical laboratory task that tests associative memory formation and recall. Techniques such as multi-photon microscopy and holographic stimulation offer tremendous opportunities to understand the neural underpinnings of these memories. However, these techniques generally require animals to be head-fixed. There are few paradigms that test contextual fear conditioning in head-fixed mice, and none where the behavioral outcome following fear conditioning is freezing, the most common measure of fear in freely moving animals. To address this gap, we developed a contextual fear conditioning paradigm in head-fixed mice using virtual reality (VR) environments. We designed an apparatus to deliver tail shocks (unconditioned stimulus, US) while mice navigated a VR environment (conditioned stimulus, CS). The acquisition of contextual fear was tested when the mice were reintroduced to the shock-paired VR environment the following day. We tested three different variations of this paradigm and, in all of them, observed an increased conditioned fear response characterized by increased freezing behavior. This was especially prominent during the first trial in the shock-paired VR environment, compared to a neutral environment where the mice received no shocks. Our results demonstrate that head-fixed mice can be fear conditioned in VR, discriminate between a feared and neutral VR context, and display freezing as a conditioned response, similar to freely behaving animals. Furthermore, using a two-photon microscope, we imaged from large populations of hippocampal CA1 neurons before, during, and following contextual fear conditioning. Our findings reconfirmed those from the literature on freely moving animals, showing that CA1 place cells undergo remapping and show narrower place fields following fear conditioning. Our approach offers new opportunities to study the neural mechanisms underlying the formation, recall, and extinction of contextual fear memories. As the head-fixed preparation is compatible with multi-photon microscopy and holographic stimulation, it enables long-term tracking and manipulation of cells throughout distinct memory stages and provides subcellular resolution for investigating axonal, dendritic, and synaptic dynamics in real-time.

Identification of GABAergic subpopulations in the lateral hypothalamus for home-driven behaviors in mice

Home information profoundly influences behavioral states in both humans and animals. However, how "home" is represented in the brain and its role in driving diverse related behaviors remain elusive. Here, we demonstrate that home bedding contains sufficient home information to modulate affective behaviors, including aversion responses, defensive aggression, and mating behaviors. These varied responses to home information are mediated by gama-aminobutyric acid (GABA)ergic neurons in the lateral hypothalamus (LHGABA). Inhibiting LHGABA abolishes, while activating mimics, the effects of home bedding on these behaviors across different contexts. Specifically, projections from LHGABA to the ventral tegmental area (VTA) mediate the relaxation of aversive emotion, while projections to the periaqueductal gray (PAG) initiate defensive concerns. Thus, our data suggest that home information in different contexts converges to activate distinct subgroups of the LHGABA, which, in turn, elicit appropriate affective behaviors in relieving aversion, fighting intruders, or enhancing mating through involving distinct downstream projections.

Sex Differences in Neural Circuits Underlying Fear Processing

Neural circuitry involved in anxiety and fear-related disorders exhibits strong sexual modulation. A limited number of studies integrating female and male data have revealed differences in neural networks, and distinct interconnectivity between these brain areas. Despite the efforts to incorporate female or mixed-sex data, there is compelling evidence that sex, as a biological variable, significantly influences fear processing. This chapter presents primary findings on sex differences in fear circuitry. It is imperative to consider this factor to ensure scientific research's integrity and understand how fear is processed in the central nervous system.

Extinction training suppresses activity of fear memory ensembles across the hippocampus and alters transcriptomes of fear-encoding cells

Contextual fear conditioning has been shown to activate a set of "fear ensemble" cells in the hippocampal dentate gyrus (DG) whose reactivation is necessary and sufficient for expression of contextual fear. We previously demonstrated that extinction learning suppresses reactivation of these fear ensemble cells and activates a competing set of DG cells-the "extinction ensemble." Here, we tested whether extinction was sufficient to suppress reactivation in other regions and used single nucleus RNA sequencing (snRNA-seq) of cells in the dorsal dentate gyrus to examine how extinction affects the transcriptomic activity of fear ensemble and fear recall-activated cells. Our results confirm the suppressive effects of extinction in the dorsal and ventral dentate gyrus and demonstrate that this same effect extends to fear ensemble cells located in the dorsal CA1. Interestingly, the extinction-induced suppression of fear ensemble activity was not detected in ventral CA1. Our snRNA-seq analysis demonstrates that extinction training markedly changes transcription patterns in fear ensemble cells and that cells activated during recall of fear and recall of extinction have distinct transcriptomic profiles. Together, our results indicate that extinction training suppresses a broad portion of the fear ensemble in the hippocampus, and this suppression is accompanied by changes in the transcriptomes of fear ensemble cells and the emergence of a transcriptionally unique extinction ensemble.

Closed-loop modulation of remote hippocampal representations with neurofeedback

Humans can remember specific remote events without acting on them and influence which memories are retrieved based on internal goals. However, animal models typically present sensory cues to trigger memory retrieval and then assess retrieval based on action. Thus, it is difficult to determine whether measured neural activity patterns relate to the cue(s), the memory, or the behavior. We therefore asked whether retrieval-related neural activity could be generated in animals without cues or a behavioral report. We focused on hippocampal "place cells" which primarily represent the animal's current location (local representations) but can also represent locations away from the animal (remote representations). We developed a neurofeedback system to reward expression of remote representations and found that rats could learn to generate specific spatial representations that often jumped directly to the experimenter-defined target location. Thus, animals can deliberately engage remote representations, enabling direct study of retrieval-related activity in the brain.

GDNF facilitates cognitive function recovery following neonatal surgical-induced learning and memory impairment via activation of the RET pathway and modulation of downstream effectors PKMζ and Kalirin in rats

OBJECTIVE

The aim of this study is to elucidate the underlying mechanism through which glial cell line-derived neurotrophic factor (GDNF) improves cognitive deficits in adults resulting from neonatal surgical interventions.

METHODS

Newborn Sprague-Dawley rats, regardless of gender, were randomly allocated into seven groups on postnatal day 7 as follows (n=15): (1) Control group (not subjected to anesthesia, surgery, or any pharmaceutical interventions); (2) GDNF group (received intracerebroventricular injection of GDNF); (3) Surgery group (underwent right carotid artery exposure under anesthesia with 3 % sevoflurane); (4) Surgery plus GDNF group; (5) Surgery plus GDNF and type II JAK inhibitor NVP-BBT594 (BBT594) group (administered intraperitoneal injection of BBT594); (6) BBT group; and (7) Surgery plus BBT group. Starting from postnatal day 33, all rats underwent Barnes maze and fear conditioning tests, followed by decapitation under sevoflurane anesthesia for subsequent analyses. The left hemibrains underwent Golgi staining, while the right hemibrains were used for hippocampal protein extraction to assess Protein kinase Mζ (PKMζ) and Kalirin expression through western blotting.

RESULTS

GDNF demonstrated a mitigating effect on spatial learning and memory impairment, as well as context-related fear memory impairment, reductions in dendritic total lengths, and spinal density within the hippocampus induced by surgical intervention. Notably, all of these ameliorative effects of GDNF were reversed upon administration of the RET inhibitor BBT594. Additionally, GDNF alleviated the downregulation of protein expression of PKMζ and Kalirin in the hippocampus of rats subjected to surgery, subsequently reversed by BBT594.

CONCLUSION

The effective impact of GDNF on learning and memory impairment caused by surgical intervention appears to be mediated through the RET pathway. Moreover, GDNF may exert its influence by upregulating the expression of PKMζ and Kalirin, consequently enhancing the development of dendrites and dendritic spines.

Synapse device based neuromorphic system for biomedical applications

Despite holding valuable information, unstructured data pose challenges for efficient recognition due to the difficulties in feature extraction using traditional Von-Neumann architecture systems, which are limited by power and time bottlenecks. Although biological neural signals offer crucial insights, they require more effective recognition solutions due to inherent noise and the vast volumes of data. Inspired by the human brain, neuromorphic systems have emerged as promising alternatives because of their parallelism, low power consumption, and error tolerance. By leveraging deep neural networks (DNNs), these systems can recognize imprecise data through two key processes: learning (feature extraction) and testing (feature matching and recognition). During the learning phase, DNNs extract and store unique features such as weight changes in synapse units. In the testing phase, new data are compared with the stored features for recognition. The parallelization of the neuromorphic system enables the efficient processing of large, imprecise datasets with minimal energy consumption. Nevertheless, the hardware implementation is essential for determining the full potential of DNNs. This paper focuses on synapse devices, which are the core units for hardware DNN implementations, and presents a biomedical application example: a rat neural signal recognition system implemented using a synapse device-based neuromorphic system.

Brain region-specific action of ketamine as a rapid antidepressant

Ketamine has been found to have rapid and potent antidepressant activity. However, despite the ubiquitous brain expression of its molecular target, the N-methyl-d-aspartate receptor (NMDAR), it was not clear whether there is a selective, primary site for ketamine's antidepressant action. We found that ketamine injection in depressive-like mice specifically blocks NMDARs in lateral habenular (LHb) neurons, but not in hippocampal pyramidal neurons. This regional specificity depended on the use-dependent nature of ketamine as a channel blocker, local neural activity, and the extrasynaptic reservoir pool size of NMDARs. Activating hippocampal or inactivating LHb neurons swapped their ketamine sensitivity. Conditional knockout of NMDARs in the LHb occluded ketamine's antidepressant effects and blocked the systemic ketamine-induced elevation of serotonin and brain-derived neurotrophic factor in the hippocampus. This distinction of the primary versus secondary brain target(s) of ketamine should help with the design of more precise and efficient antidepressant treatments.

Treatment with brain specific estrogen prodrug ameliorates cognitive effects of surgical menopause in mice

Menopause is an endocrine shift leading to increased vulnerability for cognitive impairment and dementia risk factors, in part due to loss of neuroprotective circulating estrogens. Systemic replacement of estrogen post-menopause has limitations, including risk for estrogen-sensitive cancers. A promising therapeutic approach therefore might be to deliver estrogen only to the brain. We examined whether we could enhance cognitive performance by delivering estrogen exclusively to the brain in ovariectomized mice (a surgical menopause model). We treated mice with the prodrug 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED), which can be administered systemically but is converted to 17β-estradiol only in the brain. Young and middle-aged C57BL/6 J mice received ovariectomy and subcutaneous implant containing vehicle or DHED and underwent cognitive testing to assess memory after 1-3.5 months of treatment. Low and medium doses of DHED did not alter metabolic status in middle-aged mice. In both age groups, DHED treatment improved spatial memory in ovariectomized mice. Additional testing in middle-aged mice showed that DHED treatment improved working and recognition memory in ovariectomized mice. These results lay the foundation for future studies determining if this intervention is as efficacious in models of dementia with comorbid risk factors.

Mild neonatal hypoxia disrupts adult hippocampal learning and memory and is associated with CK2-mediated dysregulation of synaptic calcium-activated potassium channel KCNN2

Objective

Although nearly half of preterm survivors display persistent neurobehavioral dysfunction including memory impairment without overt gray matter injury, the underlying mechanisms of neuronal or glial dysfunction, and their relationship to commonly observed cerebral white matter injury are unclear. We developed a mouse model to test the hypothesis that mild hypoxia during preterm equivalence is sufficient to persistently disrupt hippocampal neuronal maturation related to adult cellular mechanisms of learning and memory. Methods: Neonatal (P2) mice were exposed to mild hypoxia (8%O 2 ) for 30 min and evaluated for acute injury responses or survived until adulthood for assessment of learning and memory and hippocampal neurodevelopment.

Results

Neonatal mild hypoxia resulted in clinically relevant oxygen desaturation and tachycardia without bradycardia and was not accompanied by cerebral gray or white matter injury. Neonatal hypoxia exposure was sufficient to cause hippocampal learning and memory deficits and abnormal maturation of CA1 neurons that persisted into adulthood. This was accompanied by reduced hippocampal CA3-CA1 synaptic strength and LTP and reduced synaptic activity of calcium-sensitive SK2 channels, key regulators of spike timing dependent neuroplasticity, including LTP. Structural illumination microscopy revealed reduced synaptic density, but intact SK2 localization at the synapse. Persistent loss of SK2 activity was mediated by altered casein kinase 2 (CK2) signaling.

Interpretation

Clinically relevant mild hypoxic exposure in the neonatal mouse is sufficient to produce morphometric and functional disturbances in hippocampal neuronal maturation independently of white matter injury. Additionally, we describe a novel persistent mechanism of potassium channel dysregulation after neonatal hypoxia. Collectively our findings suggest an unexplored explanation for the broad spectrum of neurobehavioral, cognitive and learning disabilities that paradoxically persist into adulthood without overt gray matter injury after preterm birth.

A comparison of hippocampal and retrosplenial cortical spatial and contextual firing patterns

The hippocampus (HPC) and retrosplenial cortex (RSC) are key components of the brain's memory and navigation systems. Lesions of either region produce profound deficits in spatial cognition and HPC neurons exhibit well-known spatial firing patterns (place fields). Recent studies have also identified an array of navigation-related firing patterns in the RSC. However, there has been little work comparing the response properties and information coding mechanisms of these two brain regions. In the present study, we examined the firing patterns of HPC and RSC neurons in two tasks which are commonly used to study spatial cognition in rodents, open field foraging with an environmental context manipulation and continuous T-maze alternation. We found striking similarities in the kinds of spatial and contextual information encoded by these two brain regions. Neurons in both regions carried information about the rat's current spatial location, trajectories and goal locations, and both regions reliably differentiated the contexts. However, we also found several key differences. For example, information about head direction was a prominent component of RSC representations but was only weakly encoded in the HPC. The two regions also used different coding schemes, even when they encoded the same kind of information. As expected, the HPC employed a sparse coding scheme characterized by compact, high contrast place fields, and information about spatial location was the dominant component of HPC representations. RSC firing patterns were more consistent with a distributed coding scheme. Instead of compact place fields, RSC neurons exhibited broad, but reliable, spatial and directional tuning, and they typically carried information about multiple navigational variables. The observed similarities highlight the closely related functions of the HPC and RSC, whereas the differences in information types and coding schemes suggest that these two regions likely make somewhat different contributions to spatial cognition.

'Nip it in the bud': Low-frequency rTMS of the prefrontal cortex disrupts threat memory consolidation in humans

It is still unclear how the human brain consolidates aversive (e.g., traumatic) memories and whether this process can be disrupted. We hypothesized that the dorsolateral prefrontal cortex (dlPFC) is crucially involved in threat memory consolidation. To test this, we used low-frequency repetitive transcranial magnetic stimulation (LF-rTMS) within the memory stabilization time window to disrupt the expression of threat memory. We combined a differential threat-conditioning paradigm with LF-rTMS targeting the dlPFC in the critical condition, and occipital cortex stimulation, delayed dlPFC stimulation, and sham stimulation as control conditions. In the critical condition, defensive reactions to threat were reduced immediately after brain stimulation, and 1 h and 24 h later. In stark contrast, no decrease was observed in the control conditions, thus showing both the anatomical and temporal specificity of our intervention. We provide causal evidence that selectively targeting the dlPFC within the early consolidation period prevents the persistence and return of conditioned responses. Furthermore, memory disruption lasted longer than the inhibitory window created by our TMS protocol, which suggests that we influenced dlPFC neural activity and hampered the underlying, time-dependent consolidation process. These results provide important insights for future clinical applications aimed at interfering with the consolidation of aversive, threat-related memories.

S-Nitrosoglutathione Attenuates Oxidative Stress and Improves Retention Memory Dysfunctions in Intra-Cerebroventricular-Streptozotocin Rat Model of Sporadic Alzheimer's Disease via Activation of BDNF and Nuclear Factor Erythroid 2-Related Factor-2 Antioxidant Signaling Pathway

INTRODUCTION

The brain-derived neurotrophic factor (BDNF) and transcription nuclear factor erythroid 2-related factor-2 (NRF-2) play an important role in Alzheimer's disease (AD). However, the interactive involvement of BDNF and NRF-2 in respect to antioxidant mechanisms in different parts of the AD brain is still unclear. Considering the above condition, used S-nitrosoglutathione (GSNO) to examine whether it modulates the BDNF and NRF-2 levels to activate signaling pathway to promote antioxidant levels in AD brains.

METHOD

AD was induced by intracerebroventricular infusion of streptozotocin (ICV-STZ, 3 mg/kg) in Wistar rats. The effect of GSNO was analyzed by evaluating the retention of memory in months 1, 2, and 3. After the behavior study, rats were sacrificed and accessed the amyloid beta (Aβ)-40, Aβ42, glutathione (GSH), BDNF, and NRF-2 levels in the hippocampus, cortex, and amygdala tissue.

RESULTS

Pretreatment with GSNO (50 µg/kg/intraperitoneal/day) restored the BDNF, and NRF-2 levels toward normalcy as compared with ICV-STZ + saline-treated animals. Also, GSNO treatment reversed the oxidative stress and increased the GSH levels toward normal levels. Further, reduced Aβ levels and neuronal loss in different brain regions. As a result, GSNO treatment improved the cognitive deficits in ICV-STZ-treated rats.

CONCLUSION

The results showed that endogenous nitric oxide donor GSNO improved the cognitive deficits and ICV-STZ-induced AD pathological conditions, possibly via attenuating the oxidative stress. Hence, the above finding supported that GSNO treatment may activate BDNF and NRF-2 antioxidant signaling pathways in the AD brain to normalize oxidative stress, which is the main causative factor for ICV-STZ-induced AD pathogenesis.

Beyond hippocampus: Thalamic and prefrontal contributions to an evolving memory

The hippocampus has long been at the center of memory research, and rightfully so. However, with emerging technological capabilities, we can increasingly appreciate memory as a more dynamic and brain-wide process. In this perspective, our goal is to begin developing models to understand the gradual evolution, reorganization, and stabilization of memories across the brain after their initial formation in the hippocampus. By synthesizing studies across the rodent and human literature, we suggest that as memory representations initially form in hippocampus, parallel traces emerge in frontal cortex that cue memory recall, and as they mature, with sustained support initially from limbic then diencephalic then cortical circuits, they become progressively independent of hippocampus and dependent on a mature cortical representation. A key feature of this model is that, as time progresses, memory representations are passed on to distinct circuits with progressively longer time constants, providing the opportunity to filter, forget, update, or reorganize memories in the process of committing to long-term storage.

Mystery of the memory engram: History, current knowledge, and unanswered questions

The quest to understand the memory engram has intrigued humans for centuries. Recent technological advances, including genetic labelling, imaging, optogenetic and chemogenetic techniques, have propelled the field of memory research forward. These tools have enabled researchers to create and erase memory components. While these innovative techniques have yielded invaluable insights, they often focus on specific elements of the memory trace. Genetic labelling may rely on a particular immediate early gene as a marker of activity, optogenetics may activate or inhibit one specific type of neuron, and imaging may capture activity snapshots in a given brain region at specific times. Yet, memories are multifaceted, involving diverse arrays of neuronal subpopulations, circuits, and regions that work in concert to create, store, and retrieve information. Consideration of contributions of both excitatory and inhibitory neurons, micro and macro circuits across brain regions, the dynamic nature of active ensembles, and representational drift is crucial for a comprehensive understanding of the complex nature of memory.

'Unpredictable chronic mild stress does not exacerbate memory impairment or altered neuronal and glial plasticity in the hippocampus of middle-aged vitamin D deficient mice'

Vitamin D deficiency is a worldwide health concern, especially in the elderly population. Much remains unknown about the relationship between vitamin D deficiency (VDD), stress-induced cognitive dysfunctions and depressive-like behaviour. In this study, 4-month-old male C57Bl/6J mice were fed with control or vitamin D free diet for 6 months, followed by unpredictable chronic stress (UCMS) for 8 weeks. VDD induced cognitive impairment and reduced grooming behaviour, but did not induce depressive-like behaviour. While UCMS in vitamin D sufficient mice induced expected depressive-like phenotype and impairments in the contextual fear memory, chronic stress did not manifest as an additional risk factor for memory impairments and depressive-like behaviour in VDD mice. In fact, UCMS restored self-care behaviour in VDD mice. At the histopathological level, VDD mice exhibited cell loss in the granule cell layer, reduced survival of newly generated cells, accompanied with an increased number of apoptotic cells and alterations in glial morphology in the hippocampus; however, these effects were not exacerbated by UCMS. Interestingly, UCMS reversed VDD induced loss of microglial cells. Moreover, tyrosine hydroxylase levels decreased in the striatum of VDD mice, but not in stressed VDD mice. These findings indicate that long-term VDD in adulthood impairs cognition but does not augment behavioural response to UCMS in middle-aged mice. While VDD caused cell loss and altered glial response in the DG of the hippocampus, these effects were not exacerbated by UCMS and could contribute to mechanisms regulating altered stress response.

Efficient encoding of aversive location by CA3 long-range projections

Memorizing locations that are harmful or dangerous is a key capability of all organisms and requires an integration of affective and spatial information. In mammals, the dorsal hippocampus mainly processes spatial information, while the intermediate to ventral hippocampal divisions receive affective information via the amygdala. However, how spatial and aversive information is integrated is currently unknown. To address this question, we recorded the activity of hippocampal long-range CA3 axons at single-axon resolution in mice forming an aversive spatial memory. We show that intermediate CA3 to dorsal CA3 (i-dCA3) projections rapidly overrepresent areas preceding the location of an aversive stimulus due to a spatially selective addition of new place-coding axons followed by spatially non-specific stabilization. This sequence significantly improves the encoding of location by the i-dCA3 axon population. These results suggest that i-dCA3 axons transmit a precise, denoised, and stable signal indicating imminent danger to the dorsal hippocampus.

The hippocampus contributes to retroactive stimulus associations during trace fear conditioning

Binding events that occur at different times are essential for memory formation. In trace fear conditioning, animals associate a tone and footshock despite no temporal overlap. The hippocampus is thought to mediate this learning by maintaining a memory of the tone until shock occurrence, however, evidence for sustained hippocampal tone representations is lacking. Here, we demonstrate a retrospective role for the hippocampus in trace fear conditioning. Bulk calcium imaging revealed sustained increases in CA1 activity after footshock that were not observed after tone termination. Optogenetic silencing of CA1 immediately after footshock impaired subsequent memory. Additionally, footshock increased the number of sharp-wave ripples compared to baseline during conditioning. Therefore, post-shock hippocampal activity likely supports learning by reactivating and linking latent tone and shock representations. These findings highlight an underappreciated function of post-trial hippocampal activity in enabling retroactive temporal associations during new learning, as opposed to persistent maintenance of stimulus representations.

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.

Optimizing Concussion Care Seeking: Connecting Care-Seeking Behaviors and Neurophysiological States Through Blood Biomarkers

BACKGROUND

Timely and appropriate medical care after concussion presents a difficult public health problem. Concussion identification and treatment rely heavily on self-report, but more than half of concussions go unreported or are reported after a delay. If incomplete self-report increases exposure to harm, blood biomarkers may objectively indicate this neurobiological dysfunction.

PURPOSE/HYPOTHESIS

The purpose of this study was to compare postconcussion biomarker levels between individuals with different previous concussion diagnosis statuses and care-seeking statuses. It was hypothesized that individuals with undiagnosed concussions and poorer care seeking would show altered biomarker profiles.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

Blood samples were collected from 287 military academy cadets and collegiate athletes diagnosed with concussion in the Advanced Research Core of the Concussion Assessment, Research and Education Consortium. The authors extracted each participant's self-reported previous concussion diagnosis status (no history, all diagnosed, ≥1 undiagnosed) and whether they had delayed or immediate symptom onset, symptom reporting, and removal from activity after the incident concussion. The authors compared the following blood biomarkers associated with neural injury between previous concussion diagnosis status groups and care-seeking groups: glial fibrillary acidic protein, ubiquitin c-terminal hydrolase-L1 (UCH-L1), neurofilament light chain (NF-L), and tau protein, captured at baseline, 24 to 48 hours, asymptomatic, and 7 days after unrestricted return to activity using tests of parallel profiles.

RESULTS

The undiagnosed previous concussion group (n = 21) had higher levels of NF-L at 24- to 48-hour and asymptomatic time points relative to all diagnosed (n = 72) or no previous concussion (n = 194) groups. For those with delayed removal from activity (n = 127), UCH-L1 was lower at 7 days after return to activity than that for athletes immediately removed from activity (n = 131). No other biomarker differences were observed.

CONCLUSION

Individuals with previous undiagnosed concussions or delayed removal from activity showed some different biomarker levels after concussion and after clinical recovery, despite a lack of baseline differences. This may indicate that poorer care seeking can create neurobiological differences in the concussed brain.

Significance of the anterior cingulate cortex in neurogenesis plasticity: Connections, functions, and disorders across postnatal and adult stages

The anterior cingulate cortex (ACC) is a complex and continually evolving brain region that remains a primary focus of research due to its multifaceted functions. Various studies and analyses have significantly advanced our understanding of how the ACC participates in a wide spectrum of memory and cognitive processes. However, despite its strong connections to brain areas associated with hippocampal and olfactory neurogenesis, the functions of the ACC in regulating postnatal and adult neurogenesis in these regions are still insufficiently explored. Investigating the intricate involvement of the ACC in neurogenesis could enhance our comprehension of essential aspects of brain plasticity. This involvement stems from its complex circuitry with other relevant brain regions, thereby exerting both direct and indirect impacts on the neurogenesis process. This review sheds light on the promising significance of the ACC in orchestrating postnatal and adult neurogenesis in conditions related to memory, cognitive behavior, and associated disorders.

Human brain activity and functional connectivity associated with verbal long-term memory consolidation across 1 month

Introduction

Declarative memories are initially dependent on the hippocampus and become stabilized through the neural reorganization of connections between the medial temporal lobe and neocortex. The exact time-course of these neural changes is not well established, although time-dependent changes in retrieval-related brain function can be detected across relatively short time periods in humans (e.g., hours to months).

Methods

In a study involving older adults with normal cognition (N = 24), we investigated changes in brain activity and functional connectivity associated with the long-term memory consolidation of verbal material over one month. Participants studied fact-like, three-word sentences at 1-month, 1-week, 1-day, and 1-hour intervals before a recognition memory test inside an MRI scanner. Old/new recognition with confidence ratings and response times were recorded. We examined whole-brain changes in retrieval-related brain activity, as well as functional connectivity of the hippocampus and ventromedial prefrontal cortex (vmPFC), as memories aged from 1 hour to 1 month. Secondary analyses minimized the effect of confounding factors affected by memory age (i.e., changes in confidence and response time or re-encoding of targets).

Results

Memory accuracy, confidence ratings, and response times changed with memory age. A memory age network was identified where retrieval-related brain activity in cortical regions increased or decreased as a function of memory age. Hippocampal brain activity in an anatomical region of interest decreased with memory age. Importantly, these changes in retrieval-related activity were not confounded with changes in activity related to concomitant changes in behavior or encoding. Exploratory analyses of vmPFC functional connectivity as a function of memory age revealed increased connectivity with the posterior parietal cortex, as well as with the vmPFC itself. In contrast, hippocampal functional connectivity with the vmPFC and orbitofrontal cortex decreased with memory age.

Discussion

The observed changes in retrieval-related brain activity and functional connectivity align with the predictions of standard systems consolidation theory. These results suggest that processes consistent with long-term memory consolidation can be identified over short time periods using fMRI, particularly for verbal material.

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Neural circuits for the adaptive regulation of fear and extinction memory

The regulation of fear memories is critical for adaptive behaviors and dysregulation of these processes is implicated in trauma- and stress-related disorders. Treatments for these disorders include pharmacological interventions as well as exposure-based therapies, which rely upon extinction learning. Considerable attention has been directed toward elucidating the neural mechanisms underlying fear and extinction learning. In this review, we will discuss historic discoveries and emerging evidence on the neural mechanisms of the adaptive regulation of fear and extinction memories. We will focus on neural circuits regulating the acquisition and extinction of Pavlovian fear conditioning in rodent models, particularly the role of the medial prefrontal cortex and hippocampus in the contextual control of extinguished fear memories. We will also consider new work revealing an important role for the thalamic nucleus reuniens in the modulation of prefrontal-hippocampal interactions in extinction learning and memory. Finally, we will explore the effects of stress on this circuit and the clinical implications of these findings.

Post-operative cognitive dysfunction is exacerbated by high-fat diet via TLR4 and prevented by dietary DHA supplementation

Post-operative cognitive dysfunction (POCD) is an abrupt decline in neurocognitive function arising shortly after surgery and persisting for weeks to months, increasing the risk of dementia diagnosis. Advanced age, obesity, and comorbidities linked to high-fat diet (HFD) consumption such as diabetes and hypertension have been identified as risk factors for POCD, although underlying mechanisms remain unclear. We have previously shown that surgery alone, or 3-days of HFD can each evoke sufficient neuroinflammation to cause memory deficits in aged, but not young rats. The aim of the present study was to determine if HFD consumption before surgery would potentiate and prolong the subsequent neuroinflammatory response and memory deficits, and if so, to determine the extent to which these effects depend on activation of the innate immune receptor TLR4, which both insults are known to stimulate. Young-adult (3mo) & aged (24mo) male F344xBN F1 rats were fed standard chow or HFD for 3-days immediately before sham surgery or laparotomy. In aged rats, the combination of HFD and surgery caused persistent deficits in contextual memory and cued-fear memory, though it was determined that HFD alone was sufficient to cause the long-lasting cued-fear memory deficits. In young adult rats, HFD + surgery caused only cued-fear memory deficits. Elevated proinflammatory gene expression in the hippocampus of both young and aged rats that received HFD + surgery persisted for at least 3-weeks after surgery. In a separate experiment, rats were administered the TLR4-specific antagonist, LPS-RS, immediately before HFD onset, which ameliorated the HFD + surgery-associated neuroinflammation and memory deficits. Similarly, dietary DHA supplementation for 4 weeks prior to HFD onset blunted the neuroinflammatory response to surgery and prevented development of persistent memory deficits. These results suggest that HFD 1) increases risk of persistent POCD-associated memory impairments following surgery in male rats in 2) a TLR4-dependent manner, which 3) can be targeted by DHA supplementation to mitigate development of persistent POCD.

Memory encoding and retrieval by retrosplenial parvalbumin interneurons are impaired in Alzheimer's disease model mice

Memory deficits in Alzheimer's disease (AD) show a strong link with GABAergic interneuron dysfunctions.1,2,3,4,5,6,7 The ensemble dynamics of GABAergic interneurons represent memory encoding and retrieval,8,9,10,11,12 but how GABAergic interneuron dysfunction affects inhibitory ensemble dynamics in AD is unknown. As the retrosplenial cortex (RSC) is critical for episodic memory13,14,15,16 and is affected by β-amyloid accumulation in early AD,17,18,19,20,21 we address this question by performing Ca2+ imaging in RSC parvalbumin (PV)-expressing interneurons during a contextual fear memory task in healthy control mice and the 5XFAD mouse model of AD. We found that populations of PV interneurons responsive to aversive electric foot shocks during contextual fear conditioning (shock-responsive) significantly decreased in the 5XFAD mice, indicating dysfunctions in the recruitment of memory-encoding PV interneurons. In the control mice, ensemble activities of shock-responsive PV interneurons were selectively upregulated during the freezing epoch of the contextual fear memory retrieval, manifested by synaptic potentiation of PV interneuron-mediated inhibition. However, such changes in ensemble dynamics during memory retrieval and synaptic plasticity were both absent in the 5XFAD mice. Optogenetic silencing of PV interneurons during contextual fear conditioning in the control mice mimicked the memory deficits in the 5XFAD mice, while optogenetic activation of PV interneurons in the 5XFAD mice restored memory retrieval. These results demonstrate the critical roles of contextual fear memory-encoding PV interneurons for memory retrieval. Furthermore, synaptic dysfunction of PV interneurons may disrupt the recruitment of PV interneurons and their ensemble dynamics underlying contextual fear memory retrieval, subsequently leading to memory deficits in AD.

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.