Anxiety Cells in a Hippocampal-Hypothalamic Circuit

Depression and Anxiety in the Epilepsies: from Bench to Bedside

PURPOSE OF REVIEW

Depression and anxiety substantially contribute to interictal disability in patients with epilepsy (PWE). This review summarizes current studies that shed light on mechanisms of comorbidity.

RECENT FINDINGS

Mounting epidemiological data implicate shared risk factors for anxiety/depression and seizure propensity, but these remain largely elusive and probably vary by epilepsy type. Within PWE, these symptoms appear to be associated with unique genetic, neuropathological, and connectivity profiles. Temporal lobe epilepsy has received enormous emphasis particularly in preclinical studies of comorbidity, where candidate neurobiological mechanisms underlying bidirectionality have been tested without psychopharmacological confounds. Depression and anxiety in epilepsy reflect dysfunction within broadly distributed limbic networks that may be the cause or consequence of epileptogenesis. In refractory epilepsy, seizures and/or certain anticonvulsants may distort central emotional homeostatic mechanisms that perpetually raise seizure risk. Developing future safe and effective combined anticonvulsant-antidepressant treatments will require a detailed understanding of anatomical and molecular nodes that pleiotropically enhance seizure risk and negatively alter emotionality.

Contextual fear memory retrieval by correlated ensembles of ventral CA1 neurons

Ventral hippocampal CA1 (vCA1) projections to the amygdala are necessary for contextual fear memory. Here we used in vivo Ca2+ imaging in mice to assess the temporal dynamics by which ensembles of vCA1 neurons mediate encoding and retrieval of contextual fear memories. We found that a subset of vCA1 neurons were responsive to the aversive shock during context conditioning, their activity was necessary for memory encoding, and these shock-responsive neurons were enriched in the vCA1 projection to the amygdala. During memory retrieval, a population of vCA1 neurons became correlated with shock-encoding neurons, and the magnitude of synchronized activity within this population was proportional to memory strength. The emergence of these correlated networks was disrupted by inhibiting vCA1 shock responses during memory encoding. Thus, our findings suggest that networks of cells that become correlated with shock-responsive neurons in vCA1 are essential components of contextual fear memory ensembles.

The Modulatory Effect of Metformin on Ethanol-Induced Anxiety, Redox Imbalance, and Extracellular Matrix Levels in the Brains of Wistar Rats

The study investigated the potential neuroprotective effects of metformin (MET) on alcohol-induced neurotoxicity in adult Wistar rats. The animals were randomized in four groups (n = 10): control, alcohol (ALC), ALC + MET, and MET. ALC (2 g/kg b.w.) and MET (200 mg/kg b.w.) were orally administered for 21 days, once daily. For the ALC + MET group, MET was administered 2 h after ALC treatment. On day 22, the open field test (OFT) and elevated plus maze (EPM) were performed. MET improved global activity and increased the time spent in unprotected open arms, decreased oxidative stress, both in the frontal lobe and in the hippocampus, and increased neuroglobin expression in the frontal cortex. Histopathologically, an increased neurosecretory activity in the frontal cortex in the ALC + MET group was noticed. Thus, our findings suggest that metformin has antioxidant and anxiolytic effects and may partially reverse the neurotoxic effects induced by ethanol.

An important role of PHRF1 in dendritic architecture and memory formation by modulating TGF-β signaling

PHRF1 is involved in transforming growth factor β (TGF-β) signaling to constrain the formation of acute promyelocytic leukemia (APL) in mouse APL models. PHRF1 also participates in modulating non-homologous end-joining. However, the role of PHRF1 in mammalian dendrite architecture and synaptic plasticity is unclear. Here, we investigated the role of PHRF1 in dendritic formation in the murine hippocampus using Camk2a promoter driven-iCre recombinase to conduct a PHRF1 conditional knockout, namely PHRF1^Δ/Δ, in the forebrain region. PHRF1^Δ/Δ mice developed normally, but exhibited anxiety-like behaviors and displayed defective spatial memory. Alterations of dendritic complexity in apical and basal dendrites of pyramidal neurons were noticed in PHRF1^Δ/Δ mutants. Furthermore, electrical stimulation in the hippocampal CA1 region after the TGF-β1 treatment showed a reduced synaptic plasticity in PHRF1^Δ/Δ mice. Immunoblotting analysis indicated that PHRF1 ablation affected the TGF-β signaling. Collectively, our results demonstrate that PHRF1 is important for the dendritic architecture and required for spatial memory formation in the hippocampus.

Sepsis survivor mice exhibit a behavioral endocrine syndrome with ventral hippocampal dysfunction

Severe acute stressors are known to trigger mood disorders in humans. Sepsis represents one such stressor, and survivors often suffer long term from psychiatric morbidity. We hypothesized that sepsis leads to lasting changes in neural circuits involved in stress integration, altering affective behavior and the stress response. To investigate this hypothesis, sepsis was induced in male C57Bl/6 mice using cecal ligation and puncture (CLP), and control mice underwent sham surgery. Mice recovered from acute illness within 2 weeks, after which they exhibited increased avoidance behavior and behavioral despair compared with sham, with behavioral changes observed more than 5 weeks after recovery. Sepsis survivors also showed evidence of enhanced hypothalamic-pituitary-adrenal (HPA) axis activity, with increased corticosterone after a novel stressor and increased adrenal weight. In the brain, sepsis survivor mice showed decreased stress-induced cfos mRNA and increased glucocorticoid receptor immunoreactivity specifically in the ventral hippocampus, a brain region known to coordinate emotional behavior and HPA axis activity. We conclude that murine sepsis survivors exhibit a behavioral neuroendocrine syndrome of negative affective behavior and HPA axis hyperactivity, which could be explained by ventral hippocampal dysfunction. These findings could contribute to our understanding of the human post-intensive care syndrome.

Expression of serotonin 1A and 2A receptors in molecular- and projection-defined neurons of the mouse insular cortex

The serotonin (5-HT) system is the target of multiple anxiolytics, including Buspirone, which is a partial agonist of the serotonin 1A receptor (5-HT1A). Similarly, ligands of the serotonin 2A receptor (5-HT2A) were shown to alter anxiety level. The 5-HT1A and 2A receptors are widely expressed across the brain, but the target region(s) underlying the influence of those receptors on anxiety remain unknown. Interestingly, recent studies in human and non-human primates have shown that the 5-HT1A and 5-HT2A binding potentials within the insular cortex (insula) are correlated to anxiety. As an initial step to define the function of 5-HT transmission in the insula, we quantified the proportion of specific neuronal populations of the insula expressing 5-HT1A or 5-HT2A. We analyzed seven neural populations, including three defined by a molecular marker (putative glutamate, GABA or parvalbumin), and four defined by their projections to different downstream targets. First, we found that more than 70% of putative glutamatergic neurons, and only 30% of GABAergic neurons express the 5-HT1A. Second, within insular projection neurons, 5-HT1A is highly expressed (75-80%) in the populations targeting one sub-nuclei of the amygdala (central or basolateral), or targeting the rostral or caudal sections of the lateral hypothalamus (LH). Similarly, 70% of putative glutamatergic neurons and only 30% of insular GABAergic neurons contain 5-HT2A. Finally, the 5-HT2A is present in a majority of insula-amygdala and insula-LH projection neurons (73-82%). These observations suggest that most glutamatergic neurons can respond to 5-HT through 5-HT1A or 5-HT2A in the insula, and that 5-HT directly affects a limited number of GABAergic neurons. This study defines a molecular and neuroanatomical map of the 5-HT system within the insular cortex, providing ground knowledge to identify the potential role of serotonergic modulation of selective insular populations in anxiety.

Sparse Activity of Hippocampal Adult-Born Neurons during REM Sleep Is Necessary for Memory Consolidation

The occurrence of dreaming during rapid eye movement (REM) sleep prompts interest in the role of REM sleep in hippocampal-dependent episodic memory. Within the mammalian hippocampus, the dentate gyrus (DG) has the unique characteristic of exhibiting neurogenesis persisting into adulthood. Despite their small numbers and sparse activity, adult-born neurons (ABNs) in the DG play critical roles in memory; however, their memory function during sleep is unknown. Here, we investigate whether young ABN activity contributes to memory consolidation during sleep using Ca²⁺ imaging in freely moving mice. We found that contextual fear learning recruits a population of young ABNs that are reactivated during subsequent REM sleep against a backdrop of overall reduced ABN activity. Optogenetic silencing of this sparse ABN activity during REM sleep alters the structural remodeling of spines on ABN dendrites and impairs memory consolidation. These findings provide a causal link between ABN activity during REM sleep and memory consolidation.

Amyloid β induces interneuron-specific changes in the hippocampus of APPNL-F mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive decline and amyloid-beta (Aβ) depositions generated by the proteolysis of amyloid precursor protein (APP) in the brain. In APPNL-F mice, APP gene was humanized and contains two familial AD mutations, and APP-unlike other mouse models of AD-is driven by the endogenous mouse APP promoter. Similar to people without apparent cognitive dysfunction but with heavy Aβ plaque load, we found no significant decline in the working memory of adult APPNL-F mice, but these mice showed decline in the expression of normal anxiety. Using immunohistochemistry and 3D block-face scanning electron microscopy, we found no changes in GABAA receptor positivity and size of somatic and dendritic synapses of hippocampal interneurons. We did not find alterations in the level of expression of perineuronal nets around parvalbumin (PV) interneurons or in the density of PV- or somatostatin-positive hippocampal interneurons. However, in contrast to other investigated cell types, PV interneuron axons were occasionally mildly dystrophic around Aβ plaques, and the synapses of PV-positive axon initial segment (AIS)-targeting interneurons were significantly enlarged. Our results suggest that PV interneurons are highly resistant to amyloidosis in APPNL-F mice and amyloid-induced increase in hippocampal pyramidal cell excitability may be compensated by PV-positive AIS-targeting cells. Mechanisms that make PV neurons more resilient could therefore be exploited in the treatment of AD for mitigating Aβ-related inflammatory effects on neurons.

Prevention of age-associated neuronal hyperexcitability with improved learning and attention upon knockout or antagonism of LPAR2

Recent studies suggest that synaptic lysophosphatidic acids (LPAs) augment glutamate-dependent cortical excitability and sensory information processing in mice and humans via presynaptic LPAR2 activation. Here, we studied the consequences of LPAR2 deletion or antagonism on various aspects of cognition using a set of behavioral and electrophysiological analyses. Hippocampal neuronal network activity was decreased in middle-aged LPAR2^−/− mice, whereas hippocampal long-term potentiation (LTP) was increased suggesting cognitive advantages of LPAR2^−/− mice. In line with the lower excitability, RNAseq studies revealed reduced transcription of neuronal activity markers in the dentate gyrus of the hippocampus in naïve LPAR2^−/− mice, including ARC, FOS, FOSB, NR4A, NPAS4 and EGR2. LPAR2^−/− mice behaved similarly to wild-type controls in maze tests of spatial or social learning and memory but showed faster and accurate responses in a 5-choice serial reaction touchscreen task requiring high attention and fast spatial discrimination. In IntelliCage learning experiments, LPAR2^−/− were less active during daytime but normally active at night, and showed higher accuracy and attention to LED cues during active times. Overall, they maintained equal or superior licking success with fewer trials. Pharmacological block of the LPAR2 receptor recapitulated the LPAR2^−/− phenotype, which was characterized by economic corner usage, stronger daytime resting behavior and higher proportions of correct trials. We conclude that LPAR2 stabilizes neuronal network excitability upon aging and allows for more efficient use of resting periods, better memory consolidation and better  performance in tasks requiring high selective attention. Therapeutic LPAR2 antagonism may alleviate aging-associated cognitive dysfunctions.

Hippocampal seed connectome-based modeling predicts the feeling of stress

Although the feeling of stress is ubiquitous, the neural mechanisms underlying this affective experience remain unclear. Here, we investigate functional hippocampal connectivity throughout the brain during an acute stressor and use machine learning to demonstrate that these networks can specifically predict the subjective feeling of stress. During a stressor, hippocampal connectivity with a network including the hypothalamus (known to regulate physiological stress) predicts feeling more stressed, whereas connectivity with regions such as dorsolateral prefrontal cortex (associated with emotion regulation) predicts less stress. These networks do not predict a subjective state unrelated to stress, and a nonhippocampal network does not predict subjective stress. Hippocampal networks are consistent, specific to the construct of subjective stress, and broadly informative across measures of subjective stress. This approach provides opportunities for relating hypothesis-driven functional connectivity networks to clinically meaningful subjective states. Together, these results identify hippocampal networks that modulate the feeling of stress.

Interleukin-1α in the ventral hippocampus increases stress vulnerability and inflammation-related processes

Mechanisms of stress vulnerability remain elusive. Previous research demonstrated that inflammation-related processes in the brain play a role in stress vulnerability. Our previous research showed that inflammatory processes in the ventral hippocampus (vHPC) induced a stress vulnerable phenotype. To further understand neuroinflammatory processes in the vHPC in stressed rats, we determined that protein levels of the pro-inflammatory cytokine interleukin-1-α (IL-1α), but not interleukin-1β (IL-1β), were increased in the vHPC of rats vulnerable to the effects of repeated social defeat compared to rats resilient to its effects. Injections of IL-1α into the vHPC increased stress vulnerability as characterized by increases in passive coping during defeat and subsequent decreased social interactions. Conversely, injections of recombinant IL-1 receptor antagonist (IL1-RA) increased latencies to social defeat and decreased anxiety-like behaviors during social interaction, suggesting an reduction in stress vulnerability. Protein analyses revealed increased FosB expression in the vHPC of IL-1α-injected rats, and increased HPA activation following a social encounter. Further analysis of vHPC of IL1-α-injected rats showed increased density of microglia, increased expression of the pro-inflammatory cytokine HMGB1, and increases in a marker for vascular remodeling. Taken together, these data show increasing IL-1α during stress exposure is sufficient to produce a stress vulnerable phenotype potentially by increasing inflammation-related processes in the vHPC.LAY SUMMARYOur previous research demonstrated that inflammation-related processes in the brain play a role in inducing vulnerability to the effects of repeated social stress in rats. Here we demonstrate that a pro-inflammatory cytokine interleukin-1-α (IL-1α) induces inflammatory processes in the vHPC and behavioral vulnerability in stressed rats, whereas blocking IL receptors produces the opposite effects on behavioral vulnerability. Together, these results identify a substrate in the vHPC that produces vulnerability to stress by increasing inflammation-related processes in the vHPC.

PRMT7 deficiency causes dysregulation of the HCN channels in the CA1 pyramidal cells and impairment of social behaviors

HCN channels regulate excitability and rhythmicity in the hippocampal CA1 pyramidal cells. Perturbation in the HCN channel current (I_h) is associated with neuropsychiatric disorders, such as autism spectrum disorders. Recently, protein arginine methyltransferase 7 (PRMT7) was shown to be highly expressed in the hippocampus, including the CA1 region. However, the physiological function of PRMT7 in the CA1 neurons and the relationship to psychiatric disorders are unclear. Here we showed that PRMT7 knockout (KO) mice exhibit hyperactivity and deficits in social interaction. The firing frequency of the CA1 neurons in the PRMT7 KO mice was significantly higher than that in the wild-type (WT) mice. Compared with the WT CA1 neurons, the PRMT7 KO CA1 neurons showed a more hyperpolarized resting potential and a higher input resistance, which were occluded by the I_h-current inhibitor ZD7288; these findings were consistent with the decreased I_h and suggested the contribution of I_h-channel dysfunction to the PRMT7 KO phenotypes. The HCN1 protein level was decreased in the CA1 region of the PRMT7 KO mice in conjunction with a decrease in the expression of Shank3, which encodes a core scaffolding protein for HCN channel proteins. A brief application of the PRMT7 inhibitor DS437 did not reproduce the phenotype of the PRMT7 KO neurons, further indicating that PRMT7 regulates I_h by controlling the channel number rather than the open probability. Moreover, shRNA-mediated PRMT7 suppression reduced both the mRNA and protein levels of SHANK3, implying that PRMT7 deficiency might be responsible for the decrease in the HCN protein levels by altering Shank3 expression. These findings reveal a key role for PRMT7 in the regulation of HCN channel density in the CA1 pyramidal cells that may be amenable to pharmacological intervention for neuropsychiatric disorders.

Disrupted expression of an ion channel that helps stabilize brain cell activity contributes to behavioral symptoms in mice resembling those seen in autism spectrum disorders (ASDs). Nerve cell firing depends on the right balance of ions inside and outside cells, and a channel protein called HCN helps establish ionic conditions that prevent excessive activity. Researchers led by Hana Cho and Ilmin Kwon of the Sungkyunkwan University School of Medicine, Suwon, South Korea have demonstrated that mice lacking another protein called PRMT7 exhibit reduced numbers of HCN channels in brain structures known to be affected in animal models of ASDs. These mice exhibit hyperactivity and social anxiety, presumably as a consequence of poor regulation of nerve cell firing. The authors propose that this PRMT7-HCN pathway may offer a fruitful target for the development of neuropsychiatric therapies.

Potential factors influencing replay across CA1 during sharp-wave ripples

Sharp-wave ripples are complex neurophysiological events recorded along the trisynaptic hippocampal circuit (i.e. from CA3 to CA1 and the subiculum) during slow-wave sleep and awake states. They arise locally but scale brain-wide to the hippocampal target regions at cortical and subcortical structures. During these events, neuronal firing sequences are replayed retrospectively or prospectively and in the forward or reverse order as defined by experience. They could reflect either pre-configured firing sequences, learned sequences or an option space to inform subsequent decisions. How can different sequences arise during sharp-wave ripples? Emerging data suggest the hippocampal circuit is organized in different loops across the proximal (close to dentate gyrus) and distal (close to entorhinal cortex) axis. These data also disclose a so-far neglected laminar organization of the hippocampal output during sharp-wave events. Here, I discuss whether by incorporating cell-type-specific mechanisms converging on deep and superficial CA1 sublayers along the proximodistal axis, some novel factors influencing the organization of hippocampal sequences could be unveiled. This article is part of the Theo Murphy meeting issue 'Memory reactivation: replaying events past, present and future'.

Encoding of contextual fear memory in hippocampal-amygdala circuit

In contextual fear conditioning, experimental subjects learn to associate a neutral context with an aversive stimulus and display fear responses to a context that predicts danger. Although the hippocampal–amygdala pathway has been implicated in the retrieval of contextual fear memory, the mechanism by which fear memory is encoded in this circuit has not been investigated. Here, we show that activity in the ventral CA1 (vCA1) hippocampal projections to the basal amygdala (BA), paired with aversive stimuli, contributes to encoding conditioned fear memory. Contextual fear conditioning induced selective strengthening of a subset of vCA1–BA synapses, which was prevented under anisomycin-induced retrograde amnesia. Moreover, a subpopulation of BA neurons receives stronger monosynaptic inputs from context-responding vCA1 neurons, whose activity was required for contextual fear learning and synaptic potentiation in the vCA1–BA pathway. Our study suggests that synaptic strengthening of vCA1 inputs conveying contextual information to a subset of BA neurons contributes to encoding adaptive fear memory for the threat-predictive context.

Previous studies implicate the hippocampal–amygdala pathway in contextual fear conditioning, in which animals learn to associate a neutral context with an aversive stimulus and display fear responses to dangerous situations. Here the authors show that selective strengthening of hippocampal–amygdala pathway contributes to encoding adaptive fear memory for threat-predictive context.

Genetic Up-Regulation or Pharmacological Activation of the Na+/Ca2+ Exchanger 1 (NCX1) Enhances Hippocampal-Dependent Contextual and Spatial Learning and Memory

The Na⁺/Ca²⁺ exchanger 1 (NCX1) participates in the maintenance of neuronal Na⁺ and Ca²⁺ homeostasis, and it is highly expressed at synapse level of some brain areas involved in learning and memory processes, including the hippocampus, cortex, and amygdala. Furthermore, NCX1 increases Akt1 phosphorylation and enhances glutamate-mediated Ca²⁺ influx during depolarization in hippocampal and cortical neurons, two processes involved in learning and memory mechanisms. We investigated whether the modulation of NCX1 expression/activity might influence learning and memory processes. To this aim, we used a knock-in mouse overexpressing NCX1 in hippocampal, cortical, and amygdala neurons (ncx1.4^over) and a newly synthesized selective NCX1 stimulating compound, named CN-PYB2. Both ncx1.4^over and CN-PYB2-treated mice showed an amelioration in spatial learning performance in Barnes maze task, and in context-dependent memory consolidation after trace fear conditioning. On the other hand, these mice showed no improvement in novel object recognition task which is mainly dependent on non-spatial memory and displayed an increase in the active phosphorylated CaMKIIα levels in the hippocampus. Interestingly, both of these mice showed an increased level of context-dependent anxiety.Altogether, these results demonstrate that neuronal NCX1 participates in spatial-dependent hippocampal learning and memory processes.

CUL3 Deficiency Causes Social Deficits and Anxiety-like Behaviors by Impairing Excitation-Inhibition Balance through the Promotion of Cap-Dependent Translation

Autism spectrum disorders (ASD) are a group of neurodevelopmental disorders with symptoms including social deficits, anxiety, and communication difficulties. However, ASD pathogenic mechanisms are poorly understood. Mutations of CUL3, which encodes Cullin 3 (CUL3), a component of an E3 ligase complex, are thought of as risk factors for ASD and schizophrenia (SCZ). CUL3 is abundant in the brain, yet little is known of its function. Here, we show that CUL3 is critical for neurodevelopment. CUL3-deficient mice exhibited social deficits and anxiety-like behaviors with enhanced glutamatergic transmission and neuronal excitability. Proteomic analysis revealed eIF4G1, a protein for Cap-dependent translation, as a potential target of CUL3. ASD-associated cellular and behavioral deficits could be rescued by pharmacological inhibition of the eIF4G1 function and chemogenetic inhibition of neuronal activity. Thus, CUL3 is critical to neural development, neurotransmission, and excitation-inhibition (E-I) balance. Our study provides novel insight into the pathophysiological mechanisms of ASD and SCZ.

Space, Time, and Fear: Survival Computations along Defensive Circuits

Naturalistic observations show that decisions to avoid or escape predators occur at different spatiotemporal scales and that they are supported by different computations and neural circuits. At their extremes, proximal threats are addressed by a limited repertoire of reflexive and myopic actions, reflecting reduced decision and state spaces and model-free (MF) architectures. Conversely, distal threats allow increased information processing supported by model-based (MB) operations, including affective prospection, replay, and planning. However, MF and MB computations are often intertwined, and under conditions of safety the foundations for future effective reactive execution can be laid through MB instruction of MF control. Together, these computations are associated with distinct population codes embedded within a distributed defensive circuitry whose goal is to determine and realize the best policy.

Posterior basolateral amygdala to ventral hippocampal CA1 drives approach behaviour to exert an anxiolytic effect

The basolateral amygdala (BLA) and ventral hippocampal CA1 (vCA1) are cellularly and functionally diverse along their anterior-posterior and superficial-deep axes. Here, we find that anterior BLA (aBLA) and posterior BLA (pBLA) innervate deep-layer calbindin1-negative (Calb1-) and superficial-layer calbindin1-positive neurons (Calb1+) in vCA1, respectively. Photostimulation of pBLA-vCA1 inputs has an anxiolytic effect in mice, promoting approach behaviours during conflict exploratory tasks. By contrast, stimulating aBLA-vCA1 inputs induces anxiety-like behaviour resulting in fewer approaches. During conflict stages of the elevated plus maze task vCA1Calb1+ neurons are preferentially activated at the open-to-closed arm transition, and photostimulation of vCA1Calb1+ neurons at decision-making zones promotes approach with fewer retreats. In the APP/PS1 mouse model of Alzheimer's disease, which shows anxiety-like behaviour, photostimulating the pBLA-vCA1Calb1+ circuit ameliorates the anxiety in a Calb1-dependent manner. These findings suggest the pBLA-vCA1Calb1+ circuit from heterogeneous BLA-vCA1 connections drives approach behaviour to reduce anxiety-like behaviour.

G Protein-Coupled Estrogen Receptor Immunoreactivity Fluctuates During the Estrous Cycle and Show Sex Differences in the Amygdala and Dorsal Hippocampus

G protein-coupled estrogen receptor (GPER) in the amygdala and the dorsal hippocampus mediates actions of estradiol on anxiety, social recognition and spatial memory. In addition, GPER participates in the estrogenic regulation of synaptic function in the amygdala and in the process of adult neurogenesis in the dentate gyrus. While the distribution of the canonical estrogen receptors α and β in the amygdala and dorsal hippocampus are well characterized, little is known about the regional distribution of GPER in these brain regions and whether this distribution is affected by sex or the stages of the estrous cycle. In this study we performed a morphometric analysis of GPER immunoreactivity in the posterodorsal medial, anteroventral medial, basolateral, basomedial and central subdivisions of the amygdala and in all the histological layers of CA1 and the dentate gyrus of the dorsal hippocampal formation. The number of GPER immunoreactive cells was estimated in these different structures. GPER immunoreactivity was detected in all the assessed subdivisions of the amygdaloid nucleus and dorsal hippocampal formation. The number of GPER immunoreactive cells was higher in males than in estrus females in the central (P = 0.001) and the posterodorsal medial amygdala (P < 0.05); higher in males than in diestrus females in the strata orients (P < 0.01) and radiatum-lacunosum-moleculare (P < 0.05) of CA1-CA3 and in the molecular layer of the dentate gyrus (P < 0.01); higher in diestrus females than in males in the basolateral amygdala (P < 0.05); higher in diestrus females than in estrus females in the central (P < 0.01), posterodorsal medial (P < 0.01) and basolateral amygdala (P < 0.01) and higher in estrus females than in diestrus females in the strata oriens (P < 0.05) and radiatum-lacunosum-moleculare (P < 0.05) of CA1-CA3 and in the molecular layer (P < 0.05) and the hilus of the dentate gyrus (P < 0.05). The findings suggest that estrogenic regulation of the amygdala and hippocampus through GPER may be different in males and in females and may fluctuate during the estrous cycle.

Ventral hippocampus interacts with prelimbic cortex during inhibition of threat response via learned safety in both mice and humans

Heightened fear and inefficient safety learning are key features of fear and anxiety disorders. Evidence-based interventions for anxiety disorders, such as cognitive behavioral therapy, primarily rely on mechanisms of fear extinction. However, up to 50% of clinically anxious individuals do not respond to current evidence-based treatment, suggesting a critical need for new interventions based on alternative neurobiological pathways. Using parallel human and rodent conditioned inhibition paradigms alongside brain imaging methodologies, we investigated neural activity patterns in the ventral hippocampus in response to stimuli predictive of threat or safety and compound cues to test inhibition via safety in the presence of threat. Distinct hippocampal responses to threat, safety, and compound cues suggest that the ventral hippocampus is involved in conditioned inhibition in both mice and humans. Moreover, unique response patterns within target-differentiated subpopulations of ventral hippocampal neurons identify a circuit by which fear may be inhibited via safety. Specifically, ventral hippocampal neurons projecting to the prelimbic cortex, but not to the infralimbic cortex or basolateral amygdala, were more active to safety and compound cues than threat cues, and activity correlated with freezing behavior in rodents. A corresponding distinction was observed in humans: hippocampal-dorsal anterior cingulate cortex functional connectivity-but not hippocampal-anterior ventromedial prefrontal cortex or hippocampal-basolateral amygdala connectivity-differentiated between threat, safety, and compound conditions. These findings highlight the potential to enhance treatment for anxiety disorders by targeting an alternative neural mechanism through safety signal learning.

Dorsal and Ventral Hippocampal Sharp-Wave Ripples Activate Distinct Nucleus Accumbens Networks

Memories of positive experiences link places, events, and reward outcomes. These memories recruit interactions between the hippocampus and nucleus accumbens (NAc). Both dorsal and ventral hippocampus (dH and vH) project to the NAc, but it remains unknown whether dH and vH act in concert or separately to engage NAc representations related to space and reward. We recorded simultaneously from the dH, vH, and NAc of rats during an appetitive spatial task and focused on hippocampal sharp-wave ripples (SWRs) to identify times of memory reactivation across brain regions. Here, we show that dH and vH awake SWRs occur asynchronously and activate distinct and opposing patterns of NAc spiking. Only NAc neurons activated during dH SWRs were tuned to task- and reward-related information. These temporally and anatomically separable hippocampal-NAc interactions point to distinct channels of mnemonic processing in the NAc, with the dH-NAc channel specialized for spatial task and reward information. VIDEO ABSTRACT.

Heterogeneity within classical cell types is the rule: lessons from hippocampal pyramidal neurons

The mechanistic operation of brain regions is often interpreted by partitioning constituent neurons into 'cell types'. Historically, such cell types were broadly defined by their correspondence to gross features of the nervous system (such as cytoarchitecture). Modern-day neuroscientific techniques, enabling a more nuanced examination of neuronal properties, have illustrated a wealth of heterogeneity within these classical cell types. Here, we review the extent of this within-cell-type heterogeneity in one of the simplest cortical regions of the mammalian brain, the rodent hippocampus. We focus on the mounting evidence that the classical CA3, CA1 and subiculum pyramidal cell types all exhibit prominent and spatially patterned within-cell-type heterogeneity, and suggest these cell types provide a model system for exploring the organization and function of such heterogeneity. Given that the hippocampus is structurally simple and evolutionarily ancient, within-cell-type heterogeneity is likely to be a general and crucial feature of the mammalian brain.

Importin α5 Regulates Anxiety through MeCP2 and Sphingosine Kinase 1

Importins mediate transport from synapse to soma and from cytoplasm to nucleus, suggesting that perturbation of importin-dependent pathways should have significant neuronal consequences. A behavioral screen on five importin α knockout lines revealed that reduced expression of importin α5 (KPNA1) in hippocampal neurons specifically decreases anxiety in mice. Re-expression of importin α5 in ventral hippocampus of knockout animals increased anxiety behaviors to wild-type levels. Hippocampal neurons lacking importin α5 reveal changes in presynaptic plasticity and modified expression of MeCP2-regulated genes, including sphingosine kinase 1 (Sphk1). Knockout of importin α5, but not importin α3 or α4, reduces MeCP2 nuclear localization in hippocampal neurons. A Sphk1 blocker reverses anxiolysis in the importin α5 knockout mouse, while pharmacological activation of sphingosine signaling has robust anxiolytic effects in wild-type animals. Thus, importin α5 influences sphingosine-sensitive anxiety pathways by regulating MeCP2 nuclear import in hippocampal neurons.

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Reduced expression of importin α5 in hippocampal neurons decreases anxiety

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Importin α5 is required for nuclear localization of MeCP2 in hippocampal neurons

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Importin α5 knockout increases expression of Sphk1, an MeCP2-regulated gene

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Pharmacological modulation of Sphk1 and the S1P receptor affects anxiety

Cortico-Limbic Interactions Mediate Adaptive and Maladaptive Responses Relevant to Psychopathology

Cortico-limbic circuits provide a substrate for adaptive behavioral and emotional responses. However, dysfunction of these circuits can result in maladaptive responses that are associated with psychopathology. The prefrontal-limbic pathways are of particular interest because they facilitate interactions among emotion, cognition, and decision-making functions, all of which are affected in psychiatric disorders. Regulatory aspects of the prefrontal cortex (PFC) are especially relevant to human psychopathology, as the PFC, in addition to its functions, is more recent from an evolutionary perspective and is considerably more complex in human and nonhuman primates compared with other species. This review provides a neuroanatomical and functional perspective of selected regions of the limbic system, the medial temporal lobe structures-the hippocampus and amygdala as well as regions of the PFC. Beyond the specific brain regions, emphasis is placed on the structure and function of critical PFC-limbic circuits, linking alterations in the processing of information across these pathways to the pathophysiology and psychopathology of various psychiatric illnesses.

Repeated Methylglyoxal Treatment Depletes Dopamine in the Prefrontal Cortex, and Causes Memory Impairment and Depressive-Like Behavior in Mice

Methylglyoxal (MGO) is a highly reactive dicarbonyl molecule that promotes the formation of advanced glycation end products (AGEs), which are believed to play a key role in a number of pathologies, such as diabetes, Alzheimer's disease, and inflammation. Here, Swiss mice were treated with MGO by intraperitoneal injection to investigate its effects on motor activity, mood, and cognition. Acute MGO treatment heavily decreased locomotor activity in the open field test at higher doses (80-200 mg/kg), an effect not observed at lower doses (10-50 mg/kg). Several alterations were observed 4 h after a single MGO injection (10-50 mg/kg): (a) plasma MGO levels were increased, (b) memory was impaired (object location task), (c) anxiolytic behavior was observed in the open field and marble burying test, and (d) depressive-like behavior was evidenced as evaluated by the tail suspension test. Biochemical alterations in the glutathione and glyoxalase systems were not observed 4 h after MGO treatment. Mice were also treated daily with MGO at 0, 10, 25 and 50 mg/kg for 11 days. From the 5th to the 11th day, several behavioral end points were evaluated, resulting in: (a) absence of motor impairment as evaluated in the open field, horizontal bars and pole test, (b) depressive-like behavior observed in the tail suspension test, and (c) cognitive impairments detected on working, short- and long-term memory when mice were tested in the Y-maze spontaneous alternation, object location and recognition tests, and step-down inhibitory avoidance task. An interesting finding was a marked decrease in dopamine levels in the prefrontal cortex of mice treated with 50 mg/kg MGO for 11 days, along with a ~ 25% decrease in the Glo1 content. The MGO-induced dopamine depletion in the prefrontal cortex may be related to the observed memory deficits and depressive-like behavior, an interesting topic to be further studied as a potentially novel route for MGO toxicity.

Acute Chemogenetic Activation of CamKIIα-Positive Forebrain Excitatory Neurons Regulates Anxiety-Like Behaviour in Mice

Anxiety disorders are amongst the most prevalent mental health disorders. Several lines of evidence have implicated cortical regions such as the medial prefrontal cortex, orbitofrontal cortex, and insular cortex along with the hippocampus in the top–down modulation of anxiety-like behaviour in animal models. Both rodent models of anxiety, as well as treatment with anxiolytic drugs, result in the concomitant activation of multiple forebrain regions. Here, we sought to examine the effects of chemogenetic activation or inhibition of forebrain principal neurons on anxiety and despair-like behaviour. We acutely activated or inhibited Ca²⁺/calmodulin-dependent protein kinase II α (CamKIIα)-positive forebrain excitatory neurons using the hM3Dq or the hM4Di Designer Receptor Exclusively Activated by Designer Drug (DREADD) respectively. Circuit activation was confirmed via an increase in expression of the immediate early gene, c-Fos, within both the hippocampus and the neocortex. We then examined the influence of DREADD-mediated activation of forebrain excitatory neurons on behavioural tests for anxiety and despair-like behaviour. Our results indicate that acute hM3Dq DREADD activation of forebrain excitatory neurons resulted in a significant decline in anxiety-like behaviour on the open field, light–dark avoidance, and the elevated plus maze test. In contrast, hM3Dq DREADD activation of forebrain excitatory neurons did not alter despair-like behaviour on either the tail suspension or forced swim tests. Acute hM4Di DREADD inhibition of CamKIIα-positive forebrain excitatory neurons did not modify either anxiety or despair-like behaviour. Taken together, our results demonstrate that chemogenetic activation of excitatory neurons in the forebrain decreases anxiety-like behaviour in mice.

Isolating the key factors defining the magnitude of hippocampal neurogenesis' effects on anxiety, memory and pattern separation

In this paper, I analyze the hypothesis that hippocampal neurogenesis (HN) exerts its effects on behavior via activation of inhibitory circuits in the hippocampus. Using a very simple mathematical model (half-borrowed from biochemistry) to aid the reasoning, I show that the key factors determining the magnitude of HN's effects on behavior are: the baseline levels of HN in the animal, the efficiency of the animal's inhibitory circuits, the strength/intensity of the stimulus presented to the animal and how much accuracy the behavioral task requires from the information contained in the hippocampal representations. Taken together, those factors can help explain patterns observed in the behavioral results for memory, pattern separation and anxiety. The conclusions of the analysis suggest that HN's effects on inhibitory circuits can explain the impact of neurogenesis on both emotion and cognition and provide a framework to interpret future studies about the effects of HN on different behaviors, with animals of different ages and of different species.

Role of adult-born granule cells in the hippocampal functions: Focus on the GluN2B-containing NMDA receptors

Adult-born granule cells constitute a small subpopulation of the dentate gyrus (DG) in the hippocampus. However, they greatly influence several hippocampus-dependent behaviors, suggesting that adult-born granule cells have specific roles that influence behavior. In order to understand how exactly these adult-born granule cells contribute to behavior, it is critical to understand the underlying electrophysiology and neurochemistry of these cells. Here, this review simultaneously focuses on the specific electrophysiological properties of adult-born granule cells, relying on the GluN2B subunit of NMDA glutamate receptors, and how it influences neurochemistry throughout the brain. Especially in a critical age from 4 to 6 weeks post-division during which they modulate hippocampal functions, adult-born granule cells exhibit a higher intrinsic excitability and an enhanced long-term potentiation. Their stimulation decreases the overall excitation/inhibition balance of the DG via recruitment of local interneurons, and in the CA3 region of the hippocampus. However, the link between neurochemical effects of adult-born granule cells and behavior remain to be further examined.

Neuropathologic Correlates of Psychiatric Symptoms in Alzheimer's Disease

Clarifying the relationships between neuropsychiatric symptoms and Alzheimer's disease (AD)-related pathology may open avenues for effective treatments. Here, we investigate the odds of developing neuropsychiatric symptoms across increasing burdens of neurofibrillary tangle and amyloid-β pathology. Participants who passed away between 2004 and 2014 underwent comprehensive neuropathologic evaluation at the Biobank for Aging Studies from the Faculty of Medicine at the University of São Paulo. Postmortem interviews with reliable informants were used to collect information regarding neuropsychiatric and cognitive status. Of 1,092 cases collected, those with any non-Alzheimer pathology were excluded, bringing the cohort to 455 cases. Braak staging was used to evaluate neurofibrillary tangle burden, and the CERAD neuropathology score was used to evaluate amyloid-β burden. The 12-item neuropsychiatric inventory was used to evaluate neuropsychiatric symptoms and CDR-SOB score was used to evaluate dementia status. In Braak I/II, significantly increased odds were detected for agitation, anxiety, appetite changes, depression, and sleep disturbances, compared to controls. Increased odds of agitation continue into Braak III/IV. Braak V/VI is associated with higher odds for delusions. No increased odds for neuropsychiatric symptoms were found to correlate with amyloid-β pathology. Increased odds of neuropsychiatric symptoms are associated with early neurofibrillary tangle pathology, suggesting that subcortical neurofibrillary tangle accumulation with minimal cortical pathology is sufficient to impact quality of life and that neuropsychiatric symptoms are a manifestation of AD biological processes.

Oxytocin and Stress: Neural Mechanisms, Stress-Related Disorders, and Therapeutic Approaches

Clinical reports show that oxytocin (OT) is related to stress-related disorders such as depression, anxiety disorder, and post-traumatic stress disorder. Two key structures in the brain should be paid special attention with regard to stress regulation, namely, the hypothalamus and the hippocampus. The former is the region for central command for most, if not all, of the major endocrine systems, and the latter takes a key position in the regulation of mood and anxiety. There are extensive neural projections between the two structures, and both are functionally intertwined. The hypothalamus projects OTergic neurons to the hippocampus, and the latter possesses high levels of OT receptors. The hippocampus also regulates the secretion of glucocorticoids, a major group of stress hormones. Excessive levels of glucocorticoids in chronic stress cause atrophy of the hippocampus, whereas OT has been shown to protect hippocampal neurons from the toxic effects of glucocorticoids. In this article, we discuss how neural and endocrine mechanisms interplay in stress regulation, with an emphasis on the role of OT, as well as its therapeutic potential in the treatment of stress-related disorders.

Chemogenetic Modulation of Orexin Neurons Reverses Changes in Anxiety and Locomotor Activity in the A53T Mouse Model of Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disease. PD symptomology is recognized as heterogeneous and in addition to motor function decline includes cognitive, mood, sleep, and metabolic disorders. Previous studies showed early reductions in anxiety and locomotion in the A53T mice model of PD. Since inflammation and astrogliosis are an integral part of PD pathology and impair proper neuronal function, we were keen to investigate if behavioral changes in A53T mice are accompanied by increased inflammation and astrogliosis in the hippocampus (Hipp) and motor cortex (mCtx) brain regions involved in the regulation of anxiety and locomotion, respectively. To test this, we used 3-, 5-, and 7-month-old A53T mice to examine anxiety-like behavior, locomotion, and expression of inflammation and astrogliosis markers in the Hipp and mCtx. Further, we examined the presence of alpha-synuclein accumulation in orexin neurons and orexin neuronal loss. The data show early reductions in anxiety-like behavior as well as increased locomotor activity, which was accompanied by inflammation and astrogliosis in the Hipp and mCtx. Due to the persistence of the orexin neuron population in A53T mice and the involvement of orexin in anxiety and locomotor regulation, we hypothesized that chemogenetic modulation of orexin neurons would reverse the observed reductions in anxiety-like behavior and the increases in locomotor activity in these animals. We showed that chemogenetic activation of orexin neurons in A53T mice restores anxiety-like behavior back to control levels without affecting locomotor activity, whereas the inhibition of orexin neurons reverses the elevated locomotor activity without any effects on anxiety-like behavior. This study exemplifies the complex role of orexin neurons in this model of PD and demonstrates the novel finding that changes in locomotor and anxiety-like behavior are accompanied by inflammation and astrogliosis. Together, these data suggest that the orexin system may play a significant role in early and late stages of PD.

Variability in response to severe stress: highly reactive rats exhibit changes in fear and anxiety-like behavior related to distinct neuronal co-activation patterns

There is an important individual variability in development of posttraumatic stress disorder (PTSD) and this feature needs to be better addressed in preclinical studies. Previously we showed that only rats that explored the context before a foot shock (delayed shock group) exhibited long-lasting behavioral changes. In this study the delayed shock group was segregated using the freezing response upon re-exposure to the shock-paired context and we investigated whether higher reactivity would be related to behavioral alterations and to activation of brain regions using Fos immunoreactivity. The latter allowed the analysis of co-activity patterns among brain regions within each group, by creating connectivity maps. High responder rats (HR) displayed heightened freezing response upon context re-exposure, anxiety-like behavior, impaired exploratory behavior and fear sensitization. Fos analysis showed that HR displayed a negative correlation between the medial prefrontal cortex and the ventral hippocampus (vHPC) after the first context re-exposure. After the second context re-exposure, HR displayed reduced Fos expression in vHPC CA1 area, whereas low responders (LR) showed increased Fos in the paraventricular nucleus of the thalamus. Pearson correlation analyses revealed positive associations between freezing and Fos in the dorsal the periaqueductal gray and vHPC after exposure to unfamiliar acoustic stimulus in a novel environment. Thus, assessment of individual variability allowed the identification of a subset of reactive animals that displayed behavioral modifications relevant to PTSD. Fos correlation and network analyses revealed co-activity patterns in HR rats that may point out to associations of brain areas relevant to the behavioral outcomes.

Embracing Complexity in Defensive Networks

The neural basis of defensive behaviors continues to attract much interest, not only because they are important for survival but also because their dysregulation may be at the origin of anxiety disorders. Recently, a dominant approach in the field has been the optogenetic manipulation of specific circuits or cell types within these circuits to dissect their role in different defensive behaviors. While the usefulness of optogenetics is unquestionable, we argue that this method, as currently applied, fosters an atomistic conceptualization of defensive behaviors, which hinders progress in understanding the integrated responses of nervous systems to threats. Instead, we advocate for a holistic approach to the problem, including observational study of natural behaviors and their neuronal correlates at multiple sites, coupled to the use of optogenetics, not to globally turn on or off neurons of interest, but to manipulate specific activity patterns hypothesized to regulate defensive behaviors.

Ultrasonically sculpted virtual relay lens for in situ microimaging

We demonstrate in situ non-invasive relay imaging through a medium without inserting physical optical components. We show that a virtual optical graded-index (GRIN) lens can be sculpted in the medium using in situ reconfigurable ultrasonic interference patterns to relay images through the medium. Ultrasonic wave patterns change the local density of the medium to sculpt a graded refractive index pattern normal to the direction of light propagation, which modulates the phase front of light, causing it to focus within the medium and effectively creating a virtual relay lens. We demonstrate the in situ relay imaging and resolving of small features (22 µm) through a turbid medium (optical thickness = 5.7 times the scattering mean free path), which is normally opaque. The focal distance and the numerical aperture of the sculpted optical GRIN lens can be tuned by changing the ultrasonic wave parameters. As an example, we experimentally demonstrate that the axial focal distance can be continuously scanned over a depth of 5.4 mm in the modulated medium and that the numerical aperture can be tuned up to 21.5%. The interaction of ultrasonic waves and light can be mediated through different physical media, including turbid media, such as biological tissue, in which the ultrasonically sculpted GRIN lens can be used for relaying images of the underlying structures through the turbid medium, thus providing a potential alternative to implanting invasive endoscopes.

Functional Interrogation of a Depression-Related Serotonergic Single Nucleotide Polymorphism, rs6295, Using a Humanized Mouse Model

The serotonin 1A receptor (5-HT1A) system has been extensively implicated in modulating mood and behavior. Notably, 5-HT1A levels in humans display remarkable variation, and differences in receptor levels have been linked with a variety of psychiatric disorders. Further, reduction of receptor levels by 30-50% in mice suggests that changes in receptor levels that model existing human variation are sufficient to drive behavioral alterations. As a result, genetic mechanisms that modulate human 5-HT1A levels may be important for explaining individual differences in mood and behavior, representing a potential source of psychiatric disease risk. One common genetic variant implicated in differential 5-HT1A levels is the G/C single nucleotide polymorphism (SNP) rs6295, located upstream of the human 5-HT1A gene. This SNP differentially binds the transcription factor, NUDR/Deaf1, leading to cell-type specific effects on transcription in vitro. To investigate the direct effects of this SNP in the heterogeneous cellular context of the brain, we generated humanized transgenic mice using a design that maximized the local transcriptional landscape of the human HTR1A gene while also controlling for effects of genomic insertion location. We integrated a 180 kb human bacteria artificial chromosome (BAC) transgene containing G- and C-alleles of rs6295 flanked by FRT or loxP sites. Subsequent deletion of each allele by Cre- or Flp-recombinase resulted in rs6295G and C alleles in the same genomic location. These alleles were bred onto a 5-HT1A null mouse such that the human BAC was the sole source of 5-HT1A in these mice. We generated three separate lines, two of which had detectable human 5-HT1A levels in the brain, although none displayed expression in the raphe. Of these, one line exhibited rs6295-dependent differences in 5-HT1A levels and differences in behavior, even though the overall levels were considerably lower than native expression levels. The line-dependent effect of rs6295 on protein levels and behavior may depend upon differences in background genetic factors or different insertion sites across each line. This work confirms that relatively subtle differences in 5-HT1A levels can contribute to differences in behavior and highlights the challenges of modeling human noncoding genetic variation in mice.

Minimizing threat via heuristic and optimal policies recruits hippocampus and medial prefrontal cortex

Jointly minimizing multiple threats over extended time horizons enhances survival. Consequently, many tests of approach-avoidance conflicts incorporate multiple threats for probing corollaries of animal and human anxiety. To facilitate computations necessary for threat minimization, the human brain may concurrently harness multiple decision policies and associated neural controllers, but it is unclear which. We combine a task that mimics foraging under predation with behavioural modelling and functional neuroimaging. Human choices rely on immediate predator probability-a myopic heuristic policy-and on the optimal policy, which integrates all relevant variables. Predator probability relates positively and the associated choice uncertainty relates negatively to activations in the anterior hippocampus, amygdala and dorsolateral prefrontal cortex. The optimal policy is positively associated with dorsomedial prefrontal cortex activity. We thus provide a decision-theoretic outlook on the role of the human hippocampus, amygdala and prefrontal cortex in resolving approach-avoidance conflicts relevant for anxiety and integral for survival.

Chronic mild stress induced anxiety-like behaviors can Be attenuated by inhibition of NOX2-derived oxidative stress

Chronic stress-induced anxiety disorder is a highly-prevalent, modern social disease in which oxidative stress plays an important role. It is necessary to determine the underlying mechanisms governing this disorder to establish an effective treatment target for anxiety disorders. In this study, we examined the behavioral changes in mice subjected to chronic mild stress (CMS). We found that CMS exposure leads to anxiety-like phenotypes and increased levels of oxidative stress in the ventral hippocampus of mice. Furthermore, CMS increased the excitatory synaptic transmission of pyramidal cells in the ventral CA1 (vCA1). Administration of 4-hydroxy-3-methoxy-acetophenone (apocynin), an inhibitor of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, clearly ameliorated the changes induced by CMS exposure. In addition, our results of behavioral tests and analyses of reactive oxygen species (ROS) using NOX2-deficient mice indicate that CMS-induced enhanced oxidative stress level is primarily caused by the increased expression of NOX2. NOX2-derived oxidative stress can serve as a target for anxiety therapy led by chronic stress.

BSSE: An open-source image processing tool for miniaturized microscopy

Single-photon-excitation-based miniaturized microscope, or miniscope, has recently emerged as a powerful tool for imaging neural ensemble activities in freely moving animals. In the meanwhile, this highly flexible and implantable technology promises great potential for studying a broad range of cells, tissues and organs. To date, however, applications have been largely limited by the properties of the imaging modality. It is therefore highly desirable for a method generally applicable for processing miniscopy images, enabling and extending the applications to diverse anatomical and functional traits, spanning various cell types in the brain and other organs. We report an image processing approach, termed BSSE, for background suppression and signal enhancement for miniscope image processing. The BSSE method provides a simple, automatic solution to the intrinsic challenges of overlapping signals, high background and artifacts in miniscopy images. We validated the method by imaging synthetic structures and various biological samples of brain, tumor, and kidney tissues. The work represents a generally applicable tool for miniscopy technology, suggesting broader applications of the miniaturized, implantable and flexible technology for biomedical research.

Neuroligin-2 Determines Inhibitory Synaptic Transmission in the Lateral Septum to Optimize Stress-Induced Neuronal Activation and Avoidance Behavior

BACKGROUND

Investigations in the neocortex have revealed that the balance of excitatory and inhibitory synaptic transmission (E/I ratio) is important for proper information processing. The disturbance of this balance underlies many neuropsychiatric illnesses, including autism spectrum disorder and schizophrenia. However, little is known about the contribution of E/I balance to the functioning of subcortical brain regions, such as the lateral septum (LS), a structure that plays important roles in regulating anxiety-related behavior.

METHODS

We manipulated E/I balance in the mouse LS by localized conditional deletion of neuroligin-2, a postsynaptic cell adhesion protein located at gamma-aminobutyric acidergic synapses and important for inhibitory synaptic transmission. We then performed analyses of synaptic transmission in the LS, stress-induced expression of immediate early gene c-fos, and anxiety-related and depression-related behavior.

RESULTS

The absence of neuroligin-2 in the LS in the mature mouse brain resulted in postsynaptic impairment of inhibitory synaptic transmission. Importantly, the reduced inhibition and resulting E/I imbalance decreased the responsiveness of LS neurons to stress. Furthermore, this E/I imbalance in the LS was associated with impaired stress-induced activation of downstream hypothalamic nuclei and reduced avoidance behavior of the animals in the elevated plus maze.

CONCLUSIONS

Our results described the synaptic function of neuroligin-2 in the LS, uncovered a positive association between c-Fos-expressing neurons in the LS and downstream hypothalamic areas and avoidance behavior, and demonstrated that intact inhibitory synaptic transmission and proper E/I balance are required for the optimal functioning of this subcortical circuit.

Orchestrated ensemble activities constitute a hippocampal memory engram

The brain stores and recalls memories through a set of neurons, termed engram cells. However, it is unclear how these cells are organized to constitute a corresponding memory trace. We established a unique imaging system that combines Ca²⁺ imaging and engram identification to extract the characteristics of engram activity by visualizing and discriminating between engram and non-engram cells. Here, we show that engram cells detected in the hippocampus display higher repetitive activity than non-engram cells during novel context learning. The total activity pattern of the engram cells during learning is stable across post-learning memory processing. Within a single engram population, we detected several sub-ensembles composed of neurons collectively activated during learning. Some sub-ensembles preferentially reappear during post-learning sleep, and these replayed sub-ensembles are more likely to be reactivated during retrieval. These results indicate that sub-ensembles represent distinct pieces of information, which are then orchestrated to constitute an entire memory.

The brain stores memories through a set of neurons known as engram cells. Here, the authors show that engram cells in the mouse hippocampus are organized into sub-ensembles representing distinct pieces of information, which are then orchestrated to constitute an entire memory.

Acute Disruption of the Dorsal Hippocampus Impairs the Encoding and Retrieval of Trace Fear Memories

A major function of the hippocampus is to link discontiguous events in memory. This process can be studied in animals using Pavlovian trace conditioning, a procedure where the conditional stimulus (CS) and unconditional stimulus (US) are separated in time. While the majority of studies have found that trace conditioning requires the dorsal segment of the hippocampus, others have not. This variability could be due to the use of lesion and pharmacological techniques, which lack cell specificity and temporal precision. More recent studies using optogenetic tools find that trace fear acquisition is disrupted by decreases in dorsal CA1 (dCA1) activity while increases lead to learning enhancements. However, comparing these results is difficult given that some studies manipulated the activity of CA1 pyramidal neurons directly and others did so indirectly (e.g., via stimulation of entorhinal cortex inputs). The goal of the current experiments, therefore, was to compare the effects of direct CA1 excitation and inhibition on the encoding and expression of trace fear memories. Our data indicates that stimulation of ArchT in dCA1 pyramidal neurons reduces activity and impairs both the acquisition and retrieval of trace fear. Unlike previous work, direct stimulation of CA1 with ChR2 increases activity and produces deficits in trace fear learning and expression. We hypothesize that this is due to the artificial nature of optogenetic stimulation, which could disrupt processing throughout the hippocampus and in downstream structures.