Distinct Dorsal and Ventral Hippocampal CA3 Outputs Govern Contextual Fear Discrimination

Dorsolateral septum GLP-1R neurons regulate feeding via lateral hypothalamic projections

OBJECTIVE

Although glucagon-like peptide 1 (GLP-1) is known to regulate feeding, the central mechanisms contributing to this function remain enigmatic. Here, we aim to test the role of neurons expressing GLP-1 receptors (GLP-1R) in the dorsolateral septum (dLS; dLSGLP-1R) that project to the lateral hypothalamic area (LHA) on food intake and determine the relationship with feeding regulation.

METHODS

Using chemogenetic manipulations, we assessed how activation or inhibition of dLSGLP-1R neurons affected food intake in Glp1r-ires-Cre mice. Then, we used channelrhodopsin-assisted circuit mapping, chemogenetics, and electrophysiological recordings to identify and assess the role of the pathway from dLSGLP-1R →LHA projections in regulating food intake.

RESULTS

Chemogenetic inhibition of dLSGLP-1R neurons increases food intake. LHA is a major downstream target of dLSGLP-1R neurons. The dLSGLP-1R→LHA projections are GABAergic, and chemogenetic inhibition of this pathway also promotes food intake. While chemogenetic activation of dLSGLP-1R→LHA projections modestly decreases food intake, optogenetic stimulation of the dLSGLP-1R→LHA projection terminals in the LHA rapidly suppresses feeding behavior. Finally, we demonstrate that the GLP-1R agonist, Exendin 4 enhances dLSGLP-1R →LHA GABA release.

CONCLUSIONS

Together, these results demonstrate that dLS-GLP-1R neurons and the inhibitory pathway to LHA can regulate feeding behavior, which might serve as a potential therapeutic target for the treatment of eating disorders or obesity.

GADD45B in the ventral hippocampal CA1 modulates aversive memory acquisition and spatial cognition

AIMS

This study was designed to investigate the role of growth arrest and DNA damage-inducible β (GADD45B) in modulating fear memory acquisition and elucidate its underlying mechanisms.

MAIN METHODS

Adeno-associated virus (AAV) that knockdown or overexpression GADD45B were injected into ventral hippocampal CA1 (vCA1) by stereotactic, and verified by fluorescence and Western blot. The contextual fear conditioning paradigm was employed to examine the involvement of GADD45B in modulating aversive memory acquisition. The Y-maze and novel location recognition (NLR) tests were used to examine non-aversive cognition. The synaptic plasticity and electrophysiological properties of neurons were measured by slice patch clamp.

KEY FINDINGS

Knockdown of GADD45B in the vCA1 significantly enhanced fear memory acquisition, accompanied by an upregulation of long-term potentiation (LTP) expression and intrinsic excitability of vCA1 pyramidal neurons (PNs). Conversely, overexpression of GADD45B produced the opposite effects. Notably, silencing the activity of vCA1 neurons abolished the impact of GADD45B knockdown on fear memory development. Moreover, mice with vCA1 GADD45B overexpression exhibited impaired spatial cognition, whereas mice with GADD45B knockdown did not display such impairment.

SIGNIFICANCE

These results provided compelling evidence for the crucial involvement of GADD45B in the formation of aversive memory and spatial cognition.

GABAergic interneurons in the hippocampal CA1 mediate contextual fear generalization in PTSD rats

Fear overgeneralization is widely accepted as a pathogenic marker of post-traumatic stress disorder (PTSD). Recently, GABAergic interneurons have been regarded as key players in the regulation of fear memory. The role of hippocampal GABAergic interneurons in contextual fear generalization of PTSD remains incompletely understood. In the present study, we established a rat model of PTSD with inescapable foot shocks (IFS) and observed the loss of GABAergic interneuron phenotype in the hippocampal cornu ammonis-1 (CA1) subfield. To determine whether the loss of GABAergic interneuron phenotype was associated with fear generalization in PTSD rats, we used adeno-associated virus (AAV) to reduce the expression of GAD67 in CA1 and observed its effect on fear generalization. The results showed that the reduction of GAD67 in CA1 enhanced contextual fear generalization in rats. We investigated whether the PERK pathway was involved in the GABAergic interneuron injury. Increased expression of p-PERK, CHOP, and Caspase12 in GABAergic interneurons of PTSD rats was observed. Then, we used salubrinal, an endoplasmic reticulum stress inhibitor, to modulate the PERK pathway. The salubrinal treatment significantly protected the GABAergic interneurons and relieved fear generalization in PTSD rats. In addition, the results showed that salubrinal down-regulated the expression of CHOP and Caspase12 in GABAergic interneurons of PTSD rats. In conclusion, this study provided evidence that the loss of GABAergic interneuron phenotype in CA1 may contribute to contextual fear generalization in PTSD. The PERK pathway is involved in the GABAergic interneuron injury of PTSD rats and modulating it can protect GABAergic interneurons and constrain contextual fear generalization.

Dorsolateral septum GLP-1R neurons regulate feeding via lateral hypothalamic projections

Objective

Although glucagon-like peptide 1 (GLP-1) is known to regulate feeding, the central mechanisms contributing to this function remain enigmatic. Here, we aim to test the role of neurons expressing GLP-1 receptors (GLP-1R) in the dorsolateral septum (dLS; dLS GLP-1R ) and their downstream projections on food intake and determine the relationship with feeding regulation.

Methods

Using chemogenetic manipulations, we assessed how activation or inhibition of dLS GLP-1R neurons affected food intake in Glp1r-ires-Cre mice. Then, we used channelrhodopsin-assisted circuit mapping, chemogenetics, and electrophysiological recordings to identify and assess the role of the pathway from dLS GLP-1R neurons to the lateral hypothalamic area (LHA) in regulating food intake.

Results

Chemogenetic inhibition of dLS GLP-1R neurons increases food intake. LHA is a major downstream target of dLS GLP-1R neurons. The dLS GLP-1R →LHA projections are GABAergic, and chemogenetic inhibition of this pathway also promotes food intake. While chemogenetic activation of dLS GLP-1R →LHA projections modestly decreases food intake, optogenetic stimulation of the dLS GLP-1R →LHA projection terminals in the LHA rapidly suppressed feeding behavior. Finally, we demonstrate that the GLP-1R agonist, Exendin 4 enhances dLS GLP-1R →LHA GABA release.

Conclusions

Together, these results demonstrate that dLS-GLP-1R neurons and the inhibitory pathway to LHA can regulate feeding behavior, which might serve as a potential therapeutic target for the treatment of eating disorders or obesity.

Highlights

Chemogenetic inhibition of dLS GLP-1R neurons boosts food intake in mice dLS GLP-1R neuron activation does not alter feeding, likely by collateral inhibition dLS GLP-1R neurons project to LHA and release GABA Activation of dLS GLP-1R →LHA axonal terminals suppresses food intake GLP-1R agonism enhances dLS GLP-1R →LHA GABA release via a presynaptic mechanism.

TrkB-dependent regulation of molecular signaling across septal cell types

The lateral septum (LS), a GABAergic structure located in the basal forebrain, is implicated in social behavior, learning, and memory. We previously demonstrated that expression of tropomyosin kinase receptor B (TrkB) in LS neurons is required for social novelty recognition. To better understand molecular mechanisms by which TrkB signaling controls behavior, we locally knocked down TrkB in LS and used bulk RNA-sequencing to identify changes in gene expression downstream of TrkB. TrkB knockdown induces upregulation of genes associated with inflammation and immune responses, and downregulation of genes associated with synaptic signaling and plasticity. Next, we generated one of the first atlases of molecular profiles for LS cell types using single nucleus RNA-sequencing (snRNA-seq). We identified markers for the septum broadly, and the LS specifically, as well as for all neuronal cell types. We then investigated whether the differentially expressed genes (DEGs) induced by TrkB knockdown map to specific LS cell types. Enrichment testing identified that downregulated DEGs are broadly expressed across neuronal clusters. Enrichment analyses of these DEGs demonstrated that downregulated genes are uniquely expressed in the LS, and associated with either synaptic plasticity or neurodevelopmental disorders. Upregulated genes are enriched in LS microglia, associated with immune response and inflammation, and linked to both neurodegenerative disease and neuropsychiatric disorders. In addition, many of these genes are implicated in regulating social behaviors. In summary, the findings implicate TrkB signaling in the LS as a critical regulator of gene networks associated with psychiatric disorders that display social deficits, including schizophrenia and autism, and with neurodegenerative diseases, including Alzheimer's.

Pathway specific interventions reveal the multiple roles of ventral hippocampus projections in cognitive functions

Since the 1950s study of Scoville and Milner on the case H.M., the hippocampus has attracted neuroscientists' attention. The hippocampus has been traditionally divided into dorsal and ventral parts, each of which projects to different brain structures and mediates various functions. Despite a predominant interest in its dorsal part in animal models, especially regarding episodic-like and spatial cognition, recent data highlight the role of the ventral hippocampus (vHPC), as the main hippocampal output, in cognitive processes. Here, we review recent studies conducted in rodents that have used advanced in vivo functional techniques to specifically monitor and manipulate vHPC efferent pathways and delineate the roles of these specific projections in learning and memory processes. Results highlight that vHPC projections to basal amygdala are implicated in emotional memory, to nucleus accumbens in social memory and instrumental actions and to prefrontal cortex in all the above as well as in object-based memory. Some of these hippocampal projections also modulate feeding and anxiety-like behaviours providing further evidence that the "one pathway-one function" view is outdated and future directions are proposed to better understand the role of hippocampal pathways and shed further light on its connectivity and function.

Abnormal accumulation of extracellular vesicles in hippocampal dystrophic axons and regulation by the primary cilia in Alzheimer's disease

Dystrophic neurites (DNs) are abnormal axons and dendrites that are swollen or deformed in various neuropathological conditions. In Alzheimer's disease (AD), DNs play a crucial role in impairing neuronal communication and function, and they may also contribute to the accumulation and spread of amyloid beta (Aβ) in the brain of AD patients. However, it is still a challenge to understand the DNs of specific neurons that are vulnerable to Aβ in the pathogenesis of AD. To shed light on the development of radiating DNs, we examined enriched dystrophic hippocampal axons in a mouse model of AD using a three-dimensional rendering of projecting neurons. We employed the anterograde spread of adeno-associated virus (AAV)1 and conducted proteomic analysis of synaptic compartments obtained from hippocampo-septal regions. Our findings revealed that DNs were formed due to synaptic loss at the axon terminals caused by the accumulation of extracellular vesicle (EV). Abnormal EV-mediated transport and exocytosis were identified in association with primary cilia, indicating their involvement in the accumulation of EVs at presynaptic terminals. To further address the regulation of DNs by primary cilia, we conducted knockdown of the Ift88 gene in hippocampal neurons, which impaired EV-mediated secretion of Aβ and promoted accumulation of axonal spheroids. Using single-cell RNA sequencing, we identified the septal projecting hippocampal somatostatin neurons (SOM) as selectively vulnerable to Aβ with primary cilia dysfunction and vesicle accumulation. Our study suggests that DNs in AD are initiated by the ectopic accumulation of EVs at the neuronal axon terminals, which is affected by neuronal primary cilia.

TrkB-dependent regulation of molecular signaling across septal cell types

The lateral septum (LS), a GABAergic structure located in the basal forebrain, is implicated in social behavior, learning and memory. We previously demonstrated that expression of tropomyosin kinase receptor B (TrkB) in LS neurons is required for social novelty recognition. To better understand molecular mechanisms by which TrkB signaling controls behavior, we locally knocked down TrkB in LS and used bulk RNA-sequencing to identify changes in gene expression downstream of TrkB. TrkB knockdown induces upregulation of genes associated with inflammation and immune responses, and downregulation of genes associated with synaptic signaling and plasticity. Next, we generated one of the first atlases of molecular profiles for LS cell types using single nucleus RNA-sequencing (snRNA-seq). We identified markers for the septum broadly, and the LS specifically, as well as for all neuronal cell types. We then investigated whether the differentially expressed genes (DEGs) induced by TrkB knockdown map to specific LS cell types. Enrichment testing identified that downregulated DEGs are broadly expressed across neuronal clusters. Enrichment analyses of these DEGs demonstrated that downregulated genes are uniquely expressed in the LS, and associated with either synaptic plasticity or neurodevelopmental disorders. Upregulated genes are enriched in LS microglia, associated with immune response and inflammation, and linked to both neurodegenerative disease and neuropsychiatric disorders. In addition, many of these genes are implicated in regulating social behaviors. In summary, the findings implicate TrkB signaling in the LS as a critical regulator of gene networks associated with psychiatric disorders that display social deficits, including schizophrenia and autism, and with neurodegenerative diseases, including Alzheimer's.

Mechanisms underlying the effect of voluntary running on adult hippocampal neurogenesis

Adult hippocampal neurogenesis is important for preserving learning and memory-related cognitive functions. Physical exercise, especially voluntary running, is one of the strongest stimuli to promote neurogenesis and has beneficial effects on cognitive functions. Voluntary running promotes exit of neural stem cells (NSCs) from the quiescent stage, proliferation of NSCs and progenitors, survival of newborn cells, morphological development of immature neuron, and integration of new neurons into the hippocampal circuitry. However, the detailed mechanisms driving these changes remain unclear. In this review, we will summarize current knowledge with respect to molecular mechanisms underlying voluntary running-induced neurogenesis, highlighting recent genome-wide gene expression analyses. In addition, we will discuss new approaches and future directions for dissecting the complex cellular mechanisms driving change in adult-born new neurons in response to physical exercise.

Prefrontal modulation of anxiety through a lens of noradrenergic signaling

Anxiety disorders are the most common class of mental illness in the U.S., affecting 40 million individuals annually. Anxiety is an adaptive response to a stressful or unpredictable life event. Though evolutionarily thought to aid in survival, excess intensity or duration of anxiogenic response can lead to a plethora of adverse symptoms and cognitive dysfunction. A wealth of data has implicated the medial prefrontal cortex (mPFC) in the regulation of anxiety. Norepinephrine (NE) is a crucial neuromodulator of arousal and vigilance believed to be responsible for many of the symptoms of anxiety disorders. NE is synthesized in the locus coeruleus (LC), which sends major noradrenergic inputs to the mPFC. Given the unique properties of LC-mPFC connections and the heterogeneous subpopulation of prefrontal neurons known to be involved in regulating anxiety-like behaviors, NE likely modulates PFC function in a cell-type and circuit-specific manner. In working memory and stress response, NE follows an inverted-U model, where an overly high or low release of NE is associated with sub-optimal neural functioning. In contrast, based on current literature review of the individual contributions of NE and the PFC in anxiety disorders, we propose a model of NE level- and adrenergic receptor-dependent, circuit-specific NE-PFC modulation of anxiety disorders. Further, the advent of new techniques to measure NE in the PFC with unprecedented spatial and temporal resolution will significantly help us understand how NE modulates PFC function in anxiety disorders.

Social trauma engages lateral septum circuitry to occlude social reward

In humans, traumatic social experiences can contribute to psychiatric disorders1. It is suggested that social trauma impairs brain reward function such that social behaviour is no longer rewarding, leading to severe social avoidance2,3. In rodents, the chronic social defeat stress (CSDS) model has been used to understand the neurobiology underlying stress susceptibility versus resilience following social trauma, yet little is known regarding its impact on social reward4,5. Here we show that, following CSDS, a subset of male and female mice, termed susceptible (SUS), avoid social interaction with non-aggressive, same-sex juvenile C57BL/6J mice and do not develop context-dependent social reward following encounters with them. Non-social stressors have no effect on social reward in either sex. Next, using whole-brain Fos mapping, in vivo Ca2+ imaging and whole-cell recordings, we identified a population of stress/threat-responsive lateral septum neurotensin (NTLS) neurons that are activated by juvenile social interactions only in SUS mice, but not in resilient or unstressed control mice. Optogenetic or chemogenetic manipulation of NTLS neurons and their downstream connections modulates social interaction and social reward. Together, these data suggest that previously rewarding social targets are possibly perceived as social threats in SUS mice, resulting from hyperactive NTLS neurons that occlude social reward processing.

Lateral Septum Somatostatin Neurons are Activated by Diverse Stressors

The lateral septum (LS) is a forebrain structure that has been implicated in a wide range of behavioral and physiological responses to stress. However, the specific populations of neurons in the LS that mediate stress responses remain incompletely understood. Here, we show that neurons in the dorsal lateral septum (LSd) that express the somatostatin gene (hereafter, LSd^Sst neurons) are activated by diverse stressors. Retrograde tracing from LSd^Sst neurons revealed that these neurons are directly innervated by neurons in the locus coeruleus (LC), the primary source of norepinephrine well-known to mediate diverse stress-related functions in the brain. Consistently, we found that norepinephrine increased excitatory synaptic transmission onto LSd^Sst neurons, suggesting the functional connectivity between LSd^Sst neurons and LC noradrenergic neurons. However, optogenetic stimulation of LSd^Sst neurons did not affect stress-related behaviors or autonomic functions, likely owing to the functional heterogeneity within this population. Together, our findings show that LSd^Sst neurons are activated by diverse stressors and suggest that norepinephrine released from the LC may modulate the activity of LSd^Sst neurons under stressful circumstances.

A neural circuit from the dorsal CA3 to the dorsomedial hypothalamus mediates balance between risk exploration and defense

An appropriate balance between explorative and defensive behavior is essential for the survival and reproduction of prey animals in risky environments. However, the neural circuit and mechanism that allow for such a balance remains poorly understood. Here, we use a semi-naturalistic predator threat test (PTT) to observe and quantify the defense-exploration balance, especially risk exploration behavior in mice. During the PTT, the activity of the putative dorsal CA3 glutamatergic neurons (dCA3^Glu) is suppressed by predatory threat and risk exploration, whereas the neurons are activated during contextual exploration. Moreover, optogenetic excitation of these neurons induces a significant increase in risk exploration. A circuit, comprising the dorsal CA3, dorsal lateral septal, and dorsomedial hypothalamic (dCA3^Glu-dLS^GABA-DMH) areas, may be involved. Moreover, activation of the dCA3^Glu-dLS^GABA-DMH circuit promotes the switch from defense to risk exploration and suppresses threat-induced increase in arousal.

Parallel ventral hippocampus-lateral septum pathways differentially regulate approach-avoidance conflict

The ability to resolve an approach-avoidance conflict is critical to adaptive behavior. The ventral CA3 (vCA3) and CA1 (vCA1) subfields of the ventral hippocampus (vHPC) have been shown to facilitate avoidance and approach behavior, respectively, in the face of motivational conflict, but the neural circuits by which this subfield-specific regulation is implemented is unknown. We demonstrate that two distinct pathways from these subfields to lateral septum (LS) contribute to this divergent control. In Long-Evans rats, chemogenetic inhibition of the vCA3- LS caudodorsal (cd) pathway potentiated approach towards a learned conflict-eliciting stimulus, while inhibition of the vCA1-LS rostroventral (rv) pathway potentiated approach non-specifically. Additionally, vCA3-LScd inhibited animals were less hesitant to explore food during environmental uncertainty, while the vCA1- LSrv inhibited animals took longer to initiate food exploration. These findings suggest that the vHPC influences multiple behavioral systems via differential projections to the LS, which in turn send inhibitory projections to motivational centres of the brain.

Reversible Suppression of Fear Memory Recall by Transient Circadian Arrhythmia

We tested the hypothesis that a temporary period of circadian arrhythmia would transiently impair recall of an aversive memory in Siberian hamsters (Phodopus sungorus). Unlike mice or rats, circadian arrhythmia is easily induced in this species by a one-time manipulation of their ambient lighting [i.e., the disruptive phase shift (DPS) protocol]. Hamsters were conditioned to associate footshocks with a shock chamber (context) and with a predictive auditory tone (cue), and then exposed to the DPS protocol. Following DPS, animals either became arrhythmic (ARR), reentrained to the light-dark cycle (ENT), or became arrhythmic for < 14 days before their circadian locomotor rhythms spontaneously recovered and reentrained (ARR-ENT). Tests for contextual memory showed that freezing was decreased 9–10 days post-DPS when both ARR and ARR-ENT groups were arrhythmic. Once ARR-ENT animals reentrained (day 41), however, freezing was elevated back to Pre-DPS levels and did not differ from those observed in ENT hamsters. ENT animals maintained high levels of freezing at both time points, whereas, freezing remained low in ARR hamsters. In contrast to contextual responses, cued responses were unaffected by circadian arrhythmia; all three groups exhibited elevated levels of freezing in response to the tones. The differential impact of circadian arrhythmia on contextual versus cued associative memory suggests that arrhythmia preferentially impacts memory processes that depend on the hippocampus.

Hippocampus mediates nocebo impairment of opioid analgesia through changes in functional connectivity

The neural mechanisms underlying placebo analgesia have attracted considerable attention over the recent years. In contrast, little is known about the neural underpinnings of a nocebo-induced increase in pain. We previously showed that nocebo-induced hyperalgesia is accompanied by increased activity in the hippocampus that scaled with the perceived level of anxiety. As a key node of the neural circuitry of perceived threat and fear, the hippocampus has recently been proposed to coordinate defensive behaviour in a context-dependent manner. Such a role requires close interactions with other regions involved in the detection of and responses to threat. Here, we investigated the functional connectivity of the hippocampus during nocebo-induced hyperalgesia. Our results show an increase in functional connectivity between hippocampus and brain regions implicated in the processing of sensory-discriminative aspects of pain (posterior insula and primary somatosensory/motor cortex) as well as the periaqueductal gray (PAG). This nocebo-induced increase in connectivity scaled with an individual's increase in anxiety. Moreover, hippocampus connectivity with the amygdala was negatively correlated with the pain intensity reported during nocebo hyperalgesia relative to the placebo condition. Our findings suggest that the hippocampus links nocebo-induced anxiety to a heightened responsiveness to nociceptive input through changes in its crosstalk with pain-modulatory brain areas.

Dysfunction of the Hippocampal-Lateral Septal Circuit Impairs Risk Assessment in Epileptic Mice

Temporal lobe epilepsy, a chronic disease of the brain characterized by degeneration of the hippocampus, has impaired risk assessment. Risk assessment is vital for survival in complex environments with potential threats. However, the underlying mechanisms remain largely unknown. The intricate balance of gene regulation and expression across different brain regions is related to the structure and function of specific neuron subtypes. In particular, excitation/inhibition imbalance caused by hyperexcitability of glutamatergic neurons and/or dysfunction of GABAergic neurons, have been implicated in epilepsy. First, we estimated the risk assessment (RA) by evaluating the behavior of mice in the center of the elevated plus maze, and found that the kainic acid-induced temporal lobe epilepsy mice were specifically impaired their RA. This experiment evaluated approach-RA, with a forthcoming approach to the open arm, and avoid-RA, with forthcoming avoidance of the open arm. Next, results from free-moving electrophysiological recordings showed that in the hippocampus, ∼7% of putative glutamatergic neurons and ∼15% of putative GABAergic neurons were preferentially responsive to either approach-risk assessment or avoid-risk assessment, respectively. In addition, ∼12% and ∼8% of dorsal lateral septum GABAergic neurons were preferentially responsive to approach-risk assessment and avoid-risk assessment, respectively. Notably, during the impaired approach-risk assessment, the favorably activated dorsal dentate gyrus and CA3 glutamatergic neurons increased (∼9%) and dorsal dentate gyrus and CA3 GABAergic neurons decreased (∼7%) in the temporal lobe epilepsy mice. Then, we used RNA sequencing and immunohistochemical staining to investigate which subtype of GABAergic neuron loss may contribute to excitation/inhibition imbalance. The results show that temporal lobe epilepsy mice exhibit significant neuronal loss and reorganization of neural networks. In particular, the dorsal dentate gyrus and CA3 somatostatin-positive neurons and dorsal lateral septum cholecystokinin-positive neurons are selectively vulnerable to damage after temporal lobe epilepsy. Optogenetic activation of the hippocampal glutamatergic neurons or chemogenetic inhibition of the hippocampal somatostatin neurons directly disrupts RA, suggesting that an excitation/inhibition imbalance in the dHPC dorsal lateral septum circuit results in the impairment of RA behavior. Taken together, this study provides insight into epilepsy and its comorbidity at different levels, including molecular, cell, neural circuit, and behavior, which are expected to decrease injury and premature mortality in patients with epilepsy.

The role of the lateral septum in neuropsychiatric disease

The lateral septum (LS) is a structure in the midline of the brain that is interconnected with areas associated with stress and feeding. This review highlights the role of the LS in anxiety, depression, and eating disorders and their comorbidity. There is a prevailing view that the LS is anxiolytic. This review finds that the LS is both anxiolytic and anxiogenic. Furthermore, the LS can promote and inhibit feeding. Given these shared roles, the LS represents a common site for the comorbidity of neuropsychiatric disorders, and therefore a potential pharmacological target. This is crucial since currently available treatments are not always effective. Corticotrophin-releasing factor 2 antagonists are potential drugs for the treatment of anxiety and anorexia and require further research. Furthermore, other drugs currently in trials for binge eating, such as alpha-adrenergic agonists, may in fact promote food intake. It is hoped that the advancements in chemo- and optogenetic techniques will allow future studies to profile the specific neural connections of the LS and their function. This information could facilitate our understanding of the underlying mechanisms, and therefore pharmacological targets, of these psychiatric conditions.

Subcellular localization of hippocampal ryanodine receptor 2 and its role in neuronal excitability and memory

Ryanodine receptor 2 (RyR2) is abundantly expressed in the heart and brain. Mutations in RyR2 are associated with both cardiac arrhythmias and intellectual disability. While the mechanisms of RyR2-linked arrhythmias are well characterized, little is known about the mechanism underlying RyR2-associated intellectual disability. Here, we employed a mouse model expressing a green fluorescent protein (GFP)-tagged RyR2 and a specific GFP probe to determine the subcellular localization of RyR2 in hippocampus. GFP-RyR2 was predominantly detected in the soma and dendrites, but not the dendritic spines of CA1 pyramidal neurons or dentate gyrus granular neurons. GFP-RyR2 was also detected within the mossy fibers in the stratum lucidum of CA3, but not in the presynaptic terminals of CA1 neurons. An arrhythmogenic RyR2-R4496C^+/− mutation downregulated the A-type K⁺ current and increased membrane excitability, but had little effect on the afterhyperpolarization current or presynaptic facilitation of CA1 neurons. The RyR2-R4496C^+/− mutation also impaired hippocampal long-term potentiation, learning, and memory. These data reveal the precise subcellular distribution of hippocampal RyR2 and its important role in neuronal excitability, learning, and memory.

A mouse model containing a GFP-tagged ryanodine receptor 2 (RyR2) has shed light on the precise subcellular localization of hippocampal RyR2 and mechanisms underlying neuronal excitability, learning, and memory.

Disrupted Mitochondrial Network Drives Deficits of Learning and Memory in a Mouse Model of FOXP1 Haploinsufficiency

Reduced cognitive flexibility, characterized by restricted interests and repetitive behavior, is associated with atypical memory performance in autism spectrum disorder (ASD), suggesting hippocampal dysfunction. FOXP1 syndrome is a neurodevelopmental disorder characterized by ASD, language deficits, global developmental delay, and mild to moderate intellectual disability. Strongly reduced Foxp1 expression has been detected in the hippocampus of Foxp1+/- mice, a brain region required for learning and memory. To investigate learning and memory performance in these animals, fear conditioning tests were carried out, which showed impaired associative learning compared with wild type (WT) animals. To shed light on the underlying mechanism, we analyzed various components of the mitochondrial network in the hippocampus. Several proteins regulating mitochondrial biogenesis (e.g., Foxo1, Pgc-1α, Tfam) and dynamics (Mfn1, Opa1, Drp1 and Fis1) were significantly dysregulated, which may explain the increased mitophagy observed in the Foxp1+/- hippocampus. The reduced activity of complex I and decreased expression of Sod2 most likely increase the production of reactive oxygen species and the expression of the pre-apoptotic proteins Bcl-2 and Bax in this tissue. In conclusion, we provide evidence that a disrupted mitochondrial network and the resulting oxidative stress in the hippocampus contribute to the altered learning and cognitive impairment in Foxp1+/- mice, suggesting that similar alterations also play a major role in patients with FOXP1 syndrome.

Top-down regulation of motivated behaviors via lateral septum sub-circuits

How does cognition regulate innate behaviors? While the cognitive functions of the cortex have been extensively studied, we know much less about how cognition can regulate innate motivated behaviors to fulfill physiological, safety and social needs. Selection of appropriate motivated behaviors depends on external stimuli and past experiences that helps to scale priorities. With its abundant inputs from neocortical and allocortical regions, the lateral septum (LS) is ideally positioned to integrate perception and experience signals in order to regulate the activity of hypothalamic and midbrain nuclei that control motivated behaviors. In addition, LS receives numerous subcortical modulatory inputs, which represent the animal internal states and also participate in this regulation. In this perspective, we argue that LS sub-circuits regulate distinct motivated behaviors by integrating neural activity from neocortical, allocortical and neuromodulatory inputs. In addition, we propose that lateral inhibition between LS sub-circuits may allow the emergence of functional units that orchestrates competing motivated behaviors.

Adult hippocampal neurogenesis shapes adaptation and improves stress response: a mechanistic and integrative perspective

Adult hippocampal neurogenesis (AHN) represents a remarkable form of neuroplasticity that has increasingly been linked to the stress response in recent years. However, the hippocampus does not itself support the expression of the different dimensions of the stress response. Moreover, the main hippocampal functions are essentially preserved under AHN depletion and adult-born immature neurons (abGNs) have no extrahippocampal projections, which questions the mechanisms by which abGNs influence functions supported by brain areas far from the hippocampus. Within this framework, we propose that through its computational influences AHN is pivotal in shaping adaption to environmental demands, underlying its role in stress response. The hippocampus with its high input convergence and output divergence represents a computational hub, ideally positioned in the brain (1) to detect cues and contexts linked to past, current and predicted stressful experiences, and (2) to supervise the expression of the stress response at the cognitive, affective, behavioral, and physiological levels. AHN appears to bias hippocampal computations toward enhanced conjunctive encoding and pattern separation, promoting contextual discrimination and cognitive flexibility, reducing proactive interference and generalization of stressful experiences to safe contexts. These effects result in gating downstream brain areas with more accurate and contextualized information, enabling the different dimensions of the stress response to be more appropriately set with specific contexts. Here, we first provide an integrative perspective of the functional involvement of AHN in the hippocampus and a phenomenological overview of the stress response. We then examine the mechanistic underpinning of the role of AHN in the stress response and describe its potential implications in the different dimensions accompanying this response.

A circuit of COCH neurons encodes social-stress-induced anxiety via MTF1 activation of Cacna1h

The hippocampus is a temporal lobe structure critical for cognition, such as learning, memory, and attention, as well as emotional responses. Hippocampal dysfunction can lead to persistent anxiety and/or depression. However, how millions of neurons in the hippocampus are molecularly and structurally organized to engage their divergent functions remains unknown. Here, we genetically target a subset of neurons expressing the coagulation factor c homolog (COCH) gene. COCH-expressing neurons or COCH neurons are topographically segregated in the distal region of the ventral CA3 hippocampus and express Mtf1 and Cacna1h. MTF1 activation of Cacna1h transcription in COCH neurons encodes the ability of COCH neurons to burst action potentials and cause social-stress-induced anxiety-like behaviors by synapsing directly with a subset of GABAergic inhibitory neurons in the lateral septum. Together, this study provides a molecular and circuitry-based framework for understanding how COCH neurons in the hippocampus are assembled to engage social behavior.

Putting Together Pieces of the Lateral Septum: Multifaceted Functions and Its Neural Pathways

The lateral septum (LS) is implicated as a hub that regulates a variety of affects, such as reward, feeding, anxiety, fear, sociability, and memory. However, it remains unclear how the LS, previously treated as a structure of homogeneity, exhibits such multifaceted functions. Emerging evidence suggests that different functions of the LS are mediated largely by its diverse input and output connections. It has also become clear that the LS is a heterogeneous region, where its dorsal and ventral poles play dissociable and often opposing roles. This functional heterogeneity can often be explained by distinct dorsal and ventral hippocampal inputs along the LS dorsoventral axis, as well as antagonizing connections between LS subregions. Similarly, outputs from LS subregions to respective downstream targets, such as hypothalamic, preoptic, and tegmental areas, also account for this functional heterogeneity. In this review, we provide an updated perspective on LS subregion classification, connectivity, and functions. We also identify key questions that have yet to be addressed in the field.

Exercise hormone irisin is a critical regulator of cognitive function

Identifying secreted mediators that drive the cognitive benefits of exercise holds great promise for the treatment of cognitive decline in ageing or Alzheimer's disease (AD). Here, we show that irisin, the cleaved and circulating form of the exercise-induced membrane protein FNDC5, is sufficient to confer the benefits of exercise on cognitive function. Genetic deletion of Fndc5/irisin (global Fndc5 knock-out (KO) mice; F5KO) impairs cognitive function in exercise, ageing and AD. Diminished pattern separation in F5KO mice can be rescued by delivering irisin directly into the dentate gyrus, suggesting that irisin is the active moiety. In F5KO mice, adult-born neurons in the dentate gyrus are morphologically, transcriptionally and functionally abnormal. Importantly, elevation of circulating irisin levels by peripheral delivery of irisin via adeno-associated viral overexpression in the liver results in enrichment of central irisin and is sufficient to improve both the cognitive deficit and neuropathology in AD mouse models. Irisin is a crucial regulator of the cognitive benefits of exercise and is a potential therapeutic agent for treating cognitive disorders including AD.

Territorial blueprint in the hippocampal system

As we skillfully navigate through familiar places, neural computations of distances and coordinates escape our attention. However, we perceive clearly the division of space into socially meaningful territories. 'My space' versus 'your space' is a distinction familiar to all of us. Spatial frontiers are social in nature since they regulate individuals' access to utilities in space depending on hierarchy and affiliation. How does the brain integrate spatial geometry with social territory? We propose that the action of oxytocin (OT) in the entorhinal-hippocampal regions supports this process. Grounded on the functional role of the hypothalamic neuropeptide in the hippocampal system, we show how OT-induced plasticity may bias the geometrical coding of place and grid cells to represent social territories.

Hippocampal-Dependent Cognitive Dysfunction following Repeated Diffuse Rotational Brain Injury in Male and Female Mice

Cognitive dysfunction is a common, often long-term complaint following acquired traumatic brain injury (TBI). Cognitive deficits suggest dysfunction in hippocampal circuits. The goal of the studies described here is to phenotype in both male and female mice the hippocampal-dependent learning and memory deficits resulting from TBI sustained by the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) device—a model that delivers both a contact–concussion injury as well as unrestrained rotational head movement. Mice sustained either sham procedures or four injuries (0.7 J, 24-h intervals). Spatial learning and memory skills assessed in the Morris water maze (MWM) approximately 3 weeks following injuries were significantly impaired by brain injuries; however, slower swimming speeds and poor performance on visible platform trials suggest that measurement of cognitive impairment with this test is confounded by injury-induced motor and/or visual impairments. A separate experiment confirmed hippocampal-dependent cognitive deficits with trace fear conditioning (TFC), a behavioral test less dependent on motor and visual function. Male mice had greater injury-induced deficits on both the MWM and TFC tests than female mice. Pathologically, the injury was characterized by white matter damage as observed by silver staining and glial fibrillary acidic protein (astrogliosis) in the optic tracts, with milder damage seen in the corpus callosum, and fimbria and brainstem (cerebral peduncles) of some animals. No changes in the density of GABAergic parvalbumin-expressing cells in the hippocampus, amygdala, or parietal cortex were found. This experiment confirmed significant sexually dimorphic cognitive impairments following a repeated, diffuse brain injury.

Arbitration of Approach-Avoidance Conflict by Ventral Hippocampus

When environmental cues or stimuli that represent both rewarding and aversive outcomes are presented, complex computations must be made in order to determine whether approach or avoidance is the better behavioral strategy. In many neuropsychiatric illnesses these computations can be skewed. In some instances, circumstances that may normally warrant avoidance instead promote approach, thus producing compulsive-like behavioral strategies that are inflexible in response to new or conflicting information. Alternatively, high sensitivity to aversion or low sensitivity to reward can result in the failure to achieve goals and loss of resilience that characterizes depressive disorders. Increases in compulsive-like behavior have been found to be associated with disrupted signaling in regions that regulate response to conflicting stimuli, including the hippocampus. Classic behavioral inhibition theories of hippocampus function in anxiety suggest that the hippocampus blocks aberrant behavior in response to anxiety related cues or stimuli. The hippocampus may act to block approach in the face of conflicting stimuli. Dysregulations of hippocampal function, as may be present in neuropsychiatric disorders, may therefore promote aberrant approach behavior. The ventral hippocampus (vHPC) subregion is key for coordinating this approach/avoidance conflict resolution, likely through its participation with cortico-striatal and mesolimbic circuits. We revisit Gray's behavioral inhibition theory of HPC function, first posited in the 1980s, and interpret in the context of new knowledge on vHPC function gained through modern technology. Taken together with the extant, classical literature on hippocampal function, we propose that these new findings suggest that vHPC circuits balance behavioral response to conflicting stimuli in a manner that is both state- and context-dependent and, further, that disruption of specific vHPC circuits tips the balance in favor of biased approach or avoidance behavioral strategies.

New boundaries and dissociation of the mouse hippocampus along the dorsal-ventral axis based on glutamatergic, GABAergic and catecholaminergic receptor densities

In rodents, gene-expression, neuronal tuning, connectivity and neurogenesis studies have postulated that the dorsal, the intermediate and the ventral hippocampal formation (HF) are distinct entities. These findings are underpinned by behavioral studies showing a dissociable role of dorsal and ventral HF in learning, memory, stress and emotional processing. However, up to now, the molecular basis of such differences in relation to discrete boundaries is largely unknown. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA, kainate and mGluR_2/3 , GABAergic GABA_A (including benzodiazepine binding sites), GABA_B , dopaminergic D_1/5 and noradrenergic α₁ and α₂ receptors as key modulators for signal transmission in hippocampal functions, using quantitative in vitro receptor autoradiography along the dorsal-ventral axis of the mouse HF. Beside general different receptor profiles of the dentate gyrus (DG) and Cornu Ammonis fields (CA1, CA2, CA3, CA4/hilus), we detected substantial differences between dorsal, intermediate and ventral subdivisions and individual layers for all investigated receptor types, except GABA_B . For example, striking higher densities of α₂ receptors were detected in the ventral DG, while the dorsal DG possesses higher numbers of kainate, NMDA, GABA_A and D_1/5 receptors. CA1 dorsal and intermediate subdivisions showed higher AMPA, NMDA, mGluR_2/3 , GABA_A , D_1/5 receptors, while kainate receptors are higher expressed in ventral CA1, and noradrenergic α₁ and α₂ receptors in the intermediate region of CA1. CA2 dorsal was distinguished by higher kainate, α₁ and α₂ receptors in the intermediate region, while CA3 showed a more complex dissociation. Our findings resulted not only in a clear segmentation of the mouse hippocampus along the dorsal-ventral axis, but also provides insights into the neurochemical basis and likely associated physiological processes in hippocampal functions. Therein, the presented data has a high impact for future studies modeling and investigating dorsal, intermediate and ventral hippocampal dysfunction in relation to neurodegenerative diseases or psychiatric disorders.

Enhancing adult neurogenesis promotes contextual fear memory discrimination and activation of hippocampal-dorsolateral septal circuits

Hippocampal circuitry is continuously modified by integration of adult-born dentate granule cells (DGCs). Prior work has shown that enhancing adult hippocampal neurogenesis decreases interference or overlap or conflict between ensembles of similar contexts and promotes discrimination of a shock-associated context from a similar, neutral context. However, the impact of enhanced integration of adult-born neurons on hippocampal network activity or downstream circuits such as the dorsolateral septum that mediate defensive behavioral responses is poorly understood. Here, we first replicated our finding that genetic expansion of the population of adult-born dentate granule cells (8 weeks and younger) promotes contextual fear discrimination. We found that enhanced contextual fear discrimination is associated with greater c-Fos expression in discrete hippocampal subfields along the proximo-distal and dorsoventral axis. Examination of the dorsolateral septum revealed an increase in activation of somatostatin expressing neurons consistent with recent characterization of these cells as calibrators of defensive behavior. Together, these findings begin to shed light on how genetically enhancing adult hippocampal neurogenesis affects activity of hippocampal-dorsolateral septal circuits.

Topographic heterogeneity of intrinsic excitability in mouse hippocampal CA3 pyramidal neurons

Area CA3 in the hippocampus is traditionally thought to act as a homogeneous neural circuit that is vital for spatial navigation and episodic memories. However, recent studies have revealed that CA3 pyramidal neurons in dorsal hippocampus display marked anatomic and functional heterogeneity along the proximodistal (transverse) axis. The hippocampus is also known to be functionally segregated along the dorsoventral (longitudinal) axis, with dorsal hippocampus strongly involved in spatial navigation and ventral hippocampus associated with emotion and anxiety. Surprisingly, however, relatively little is known about CA3 functional heterogeneity along the dorsoventral axis. Here, we carried out mouse-brain-slice patch-clamp recordings and morphological analyses to examine the heterogeneity of CA3 cellular properties along both proximodistal and dorsoventral axes. We find that CA3 pyramidal neurons exhibit considerable heterogeneity of somatodendritic morphology and intrinsic membrane properties, with ventral CA3 (vCA3) displaying more elaborate somatodendritic morphology, lower intrinsic excitability, smaller input resistance, greater cell capacitance, and more prominent hyperpolarization-activated current than dorsal CA3 (dCA3). Furthermore, although both dCA3 and vCA3 exhibit proximal-to-distal gradients in intrinsic properties and neuronal morphology, these proximal-to-distal gradients in vCA3 are more moderate than those in dCA3. Taken together, our results extend previous findings on the proximodistal heterogeneity of dCA3 function and uncover a complex, yet orderly, pattern of topographic organization of CA3 neuronal features that extends to multiple anatomic dimensions and may contribute to its in vivo functional diversity.NEW & NOTEWORTHY Area CA3 is a major hippocampal region that is classically thought to act as a homogeneous neural network vital for spatial navigation and episodic memories. Here, we report that CA3 pyramidal neurons exhibit marked heterogeneity of somatodendritic morphology and cellular electrical properties along both proximodistal and dorsoventral axes. These new results uncover a complex, yet orderly, pattern of topographic organization of CA3 neuronal features that may contribute to its in vivo functional diversity.

An Integrated Index: Engrams, Place Cells, and Hippocampal Memory

The hippocampus and its extended network contribute to encoding and recall of episodic experiences. Drawing from recent anatomical, physiological, and behavioral studies, we propose that hippocampal engrams function as indices to mediate memory recall. We broaden this idea to discuss potential relationships between engrams and hippocampal place cells, as well as the molecular, cellular, physiological, and circuit determinants of engrams that permit flexible routing of information to intra- and extrahippocampal circuits for reinstatement, a feature critical to memory indexing. Incorporating indexing into frameworks of memory function opens new avenues of study and even therapies for hippocampal dysfunction.

Activation of astrocytes in hippocampus decreases fear memory through adenosine A1 receptors

Astrocytes respond to and regulate neuronal activity, yet their role in mammalian behavior remains incompletely understood. Especially unclear is whether, and if so how, astrocyte activity regulates contextual fear memory, the dysregulation of which leads to pathological fear-related disorders. We generated GFAP-ChR2-EYFP rats to allow the specific activation of astrocytes in vivo by optogenetics. We found that after memory acquisition within a temporal window, astrocyte activation disrupted memory consolidation and persistently decreased contextual but not cued fear memory accompanied by reduced fear-related anxiety behavior. In vivo microdialysis experiments showed astrocyte photoactivation increased extracellular ATP and adenosine concentrations. Intracerebral blockade of adenosine A₁ receptors (A₁Rs) reversed the attenuation of fear memory. Furthermore, intracerebral or intraperitoneal injection of A₁R agonist mimicked the effects of astrocyte activation. Therefore, our findings provide a deeper understanding of the astrocyte-mediated regulation of fear memory and suggest a new and important therapeutic strategy against pathological fear-related disorders.

Memory is the record of what we learn over time and is essential to our survival. But not all memories are helpful. Repeatedly recalling a traumatic event – such as an assault – can be harmful. About 1 in 3 people who experience severe trauma go on to develop post-traumatic stress disorder (PTSD), in which they re-live the traumatic event in the form of flashbacks and nightmares. Others develop panic disorder, phobias or depression.

Preventing this chain of events is challenging because fear memories form rapidly and last a long time. Current treatments involve re-exposing individuals to the traumatic event. This could be real-life exposure in the case of a phobia. Or it could involve visualizing the event, in the case of PTSD. Controlled re-exposure can help individuals learn new coping strategies. But it does not erase the initial fear memory.

A better approach might be to take advantage of the fact that new memories are unstable. To form a long-lasting memory trace, newly acquired information must go through a process called consolidation to stabilize it. This process takes place in an area of the brain called the hippocampus. If consolidation does not occur, new memory traces can fade away.

Li, Li et al. now show that preventing consolidation in the rat brain stops the animals from forming lasting memories of a stressful event, namely a foot shock. In the study, the rats first learned to associate a foot shock with a tone. This training took place inside a specific chamber. After learning the association, the rats began to freeze – a sign of fear – whenever they entered the chamber. This happened even if the tone was not played. But Li, Li et al. showed that they could reduce this fear response by activating cells in the hippocampus known as astrocytes, shortly after the learning episode.

Activating the astrocytes made them release a substance called adenosine. Molecules of adenosine then bound to and activated proteins called adenosine A₁ receptors. Administering a drug that activated these receptors directly had the same effect as activating the astrocytes themselves. This suggests that drugs of this type could one day help patients with fear-related disorders such as PTSD and phobias. For this to become a reality, new studies must test different drugs and find the best ways of administering them. After testing in animal models, the next step will be preliminary clinical trials in people.

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

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