Rapid erasure of hippocampal memory following inhibition of dentate gyrus granule cells

Selective knockdown of GABAA-α2 subunit in the dorsal dentate gyrus in adulthood induces anxiety, learning and memory deficits and impairs synaptic plasticity

Animal studies and clinical trials suggest that maintenance of gamma-aminobutyric acid (GABA)-ergic activity may be crucial in coping with stressful conditions, anxiety and mood disorders. Drugs highly efficient in promoting anxiolysis were shown to activate this system, particularly via the α2-subunit of type A receptors (GABAA α2). Given the high expression of GABAA α2 in the dentate gyrus (DG) sub-field of the hippocampus, we sought to examine whether manipulation of the α2 subunit in this area will evoke changes in emotional behaviour, memory and learning as well as in synaptic plasticity. We found that knockdown of GABAAα2 receptor specifically in the dorsal DG of rats caused increased anxiety without affecting locomotor activity. Spatial memory and learning in the Morris water maze were also impaired in GABAAα2 receptor knocked down rats, an effect accompanied by alterations in synaptic plasticity, as assessed by long-term potentiation in the DG. Our findings provide further support to the notion that emotional information processing in the hippocampus may be controlled, at least in part, via the inhibitory GABAA α2 receptor subunit, opening a potential avenue for early interventions from pre- puberty into adulthood, as a strategy for controlling anxiety-related psychopathology.

5-HT1A Receptors on Dentate Gyrus Granule Cells Confer Stress Resilience

BACKGROUND

Hyperactivity of granule cells in the ventral dentate gyrus (vDG) promotes vulnerability to chronic stress. However, which receptors in the vDG could be targeted to inhibit this hyperactivity and confer stress resilience is not known. The serotonin 1A receptor (5-HT1AR) is a Gi protein-coupled inhibitory receptor that has been implicated in stress adaptation, anxiety, depression, and antidepressant responses. 5-HT1ARs are highly expressed in the DG, but their potential to promote stress resilience by regulating granule cell activity has never been examined.

METHODS

We exposed male and female mice expressing 5-HT1ARs only in DG granule cells to 10 days of chronic social defeat stress (CSDS) and treated them with the 5-HT1AR agonist 8-OH-DPAT every day 30 minutes before each defeat throughout the CSDS paradigm. We then used whole-cell current clamp recordings, immunohistochemistry for the immediate early gene cFos, corticosterone immunoassays, and behavioral testing to determine how activating 5-HT1ARs on granule cells affects DG activity, neuroendocrine stress responses, and avoidance behavior.

RESULTS

We found that activating 5-HT1ARs hyperpolarized DG granule cells and reduced cFos+ granule cells in the vDG following CSDS, indicating that 5-HT1AR activation rescued stress-induced vDG hyperactivity. Moreover, 5-HT1AR activation dampened corticosterone responses to CSDS and prevented the development of stress-induced avoidance in the social interaction test and in the open field test.

CONCLUSIONS

Our findings show that activating 5-HT1ARs on DG granule cells can prevent stress-induced neuronal hyperactivity of the vDG and confer resilience to chronic stress.

Dentate gyrus is needed for memory retrieval

The hippocampus is crucial for acquiring and retrieving episodic and contextual memories. In previous studies, the inactivation of dentate gyrus (DG) neurons by chemogenetic- and optogenetic-mediated hyperpolarization led to opposing conclusions about DG's role in memory retrieval. One study used Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-mediated clozapine N-oxide (CNO)-induced hyperpolarization and reported that the previously formed memory was erased, thus concluding that denate gyrus is needed for memory maintenance. The other study used optogenetic with halorhodopsin induced hyperpolarization and reported and dentate gyrus is needed for memory retrieval. We hypothesized that this apparent discrepancy could be due to the length of hyperpolarization in previous studies; minutes by optogenetics and several hours by DREADD/CNO. Since hyperpolarization interferes with anterograde and retrograde neuronal signaling, it is possible that the memory engram in the dentate gyrus and the entorhinal to hippocampus trisynaptic circuit was erased by long-term, but not with short-term hyperpolarization. We developed and applied an advanced chemogenetic technology to selectively silence synaptic output by blocking neurotransmitter release without hyperpolarizing DG neurons to explore this apparent discrepancy. We performed in vivo electrophysiology during trace eyeblink in a rabbit model of associative learning. Our work shows that the DG output is required for memory retrieval. Based on previous and recent findings, we propose that the actively functional anterograde and retrograde neuronal signaling is necessary to preserve synaptic memory engrams along the entorhinal cortex to the hippocampal trisynaptic circuit.

Senktide blocks aberrant RTN3 interactome to retard memory decline and tau pathology in social isolated Alzheimer's disease mice

Sporadic or late-onset Alzheimer's disease (LOAD) accounts for more than 95% of Alzheimer's disease (AD) cases without any family history. Although genome-wide association studies have identified associated risk genes and loci for LOAD, numerous studies suggest that many adverse environmental factors, such as social isolation, are associated with an increased risk of dementia. However, the underlying mechanisms of social isolation in AD progression remain elusive. In the current study, we found that 7 days of social isolation could trigger pattern separation impairments and presynaptic abnormalities of the mossy fibre-CA3 circuit in AD mice. We also revealed that social isolation disrupted histone acetylation and resulted in the downregulation of 2 dentate gyrus (DG)-enriched miRNAs, which simultaneously target reticulon 3 (RTN3), an endoplasmic reticulum protein that aggregates in presynaptic regions to disturb the formation of functional mossy fibre boutons (MFBs) by recruiting multiple mitochondrial and vesicle-related proteins. Interestingly, the aggregation of RTN3 also recruits the PP2A B subunits to suppress PP2A activity and induce tau hyperphosphorylation, which, in turn, further elevates RTN3 and forms a vicious cycle. Finally, using an artificial intelligence-assisted molecular docking approach, we determined that senktide, a selective agonist of neurokinin3 receptors (NK3R), could reduce the binding of RTN3 with its partners. Moreover, application of senktide in vivo effectively restored DG circuit disorders in socially isolated AD mice. Taken together, our findings not only demonstrate the epigenetic regulatory mechanism underlying mossy fibre synaptic disorders orchestrated by social isolation and tau pathology but also reveal a novel potential therapeutic strategy for AD.

Evolutionary conservation of hippocampal mossy fiber synapse properties

Various specialized structural/functional properties are considered essential for contextual memory encoding by hippocampal mossy fiber (MF) synapses. Although investigated to exquisite detail in model organisms, synapses, including MFs, have undergone minimal functional interrogation in humans. To determine the translational relevance of rodent findings, we evaluated MF properties within human tissue resected to treat epilepsy. Human MFs exhibit remarkably similar hallmark features to rodents, including AMPA receptor-dominated synapses with small contributions from NMDA and kainate receptors, large dynamic range with strong frequency facilitation, NMDA receptor-independent presynaptic long-term potentiation, and strong cyclic AMP (cAMP) sensitivity of release. Array tomography confirmed the evolutionary conservation of MF ultrastructure. The astonishing congruence of rodent and human MF core features argues that the basic MF properties delineated in animal models remain critical to human MF function. Finally, a selective deficit in GABAergic inhibitory tone onto human MF postsynaptic targets suggests that unrestrained detonator excitatory drive contributes to epileptic circuit hyperexcitability.

Increasing NPYergic transmission in the hippocampus rescues aging-related deficits of long-term potentiation in the mouse dentate gyrus

Loss of neuropeptide Y (NPY)-expressing interneurons in the hippocampus and decaying cholinergic neuromodulation are thought to contribute to impaired cognitive function during aging. However, the interaction of these two neuromodulatory systems in maintaining hippocampal synaptic plasticity during healthy aging has not been explored so far. Here we report profound sex differences in the Neuropeptide-Y (NPY) levels in the dorsal dentate gyrus (DG) with higher NPY concentrations in the male mice compared to their female counterparts and a reduction of NPY levels during aging specifically in males. This change in aged males is accompanied by a deficit in theta burst-induced long-term potentiation (LTP) in the medial perforant path-to-dorsal DG (MPP-DG) synapse, which can be rescued by enhancing cholinergic activation with the acetylcholine esterase blocker, physostigmine. Importantly, NPYergic transmission is required for this rescue of LTP. Moreover, exogenous NPY application alone is sufficient to recover LTP induction in aged male mice, even in the absence of the cholinergic stimulator. Together, our results suggest that in male mice NPYergic neurotransmission is a critical factor for maintaining dorsal DG LTP during aging.

Memory discrimination is promoted by the expression of the transcription repressor WT1 in the dentate gyrus

The hippocampus is critical for the precise formation of contextual memories. Overlapping inputs coming from the entorhinal cortex are processed by the trisynaptic pathway to form distinct memories. Disruption in any step of the circuit flow can lead to a lack of memory precision, and to memory interference. We have identified the transcriptional repressor Wilm's Tumor 1 (WT1) as an important regulator of synaptic plasticity involved in memory discrimination in the hippocampus. In male mice, using viral and transgenic approaches, we showed that WT1 deletion in granule cells of the dentate gyrus (DG) disrupts memory discrimination. With electrophysiological methods, we then identified changes in granule cells' excitability and DG synaptic transmission indicating that WT1 knockdown in DG granule cells disrupts the inhibitory feedforward input from mossy fibers to CA3 by decreasing mIPSCs and shifting the normal excitatory/inhibitory (E/I) balance in the DG → CA3 circuit in favor of excitation. Finally, using a chemogenetic approach, we established a causal link between granule cell hyperexcitability and memory discrimination impairments. Our results suggest that WT1 enables a circuit-level computation that drives pattern discrimination behavior.

Role of mucin glycosylation in the gut microbiota-brain axis of core 3 O-glycan deficient mice

Alterations in intestinal mucin glycosylation have been associated with increased intestinal permeability and sensitivity to inflammation and infection. Here, we used mice lacking core 3-derived O-glycans (C3GnT-/-) to investigate the effect of impaired mucin glycosylation in the gut-brain axis. C3GnT-/- mice showed altered microbial metabolites in the caecum associated with brain function such as dimethylglycine and N-acetyl-L-tyrosine profiles as compared to C3GnT+/+ littermates. In the brain, polysialylated-neural cell adhesion molecule (PSA-NCAM)-positive granule cells showed an aberrant phenotype in the dentate gyrus of C3GnT-/- mice. This was accompanied by a trend towards decreased expression levels of PSA as well as ZO-1 and occludin as compared to C3GnT+/+. Behavioural studies showed a decrease in the recognition memory of C3GnT-/- mice as compared to C3GnT+/+ mice. Combined, these results support the role of mucin O-glycosylation in the gut in potentially influencing brain function which may be facilitated by the passage of microbial metabolites through an impaired gut barrier.

Assessments of dentate gyrus function: discoveries and debates

There has been considerable speculation regarding the function of the dentate gyrus (DG) - a subregion of the mammalian hippocampus - in learning and memory. In this Perspective article, we compare leading theories of DG function. We note that these theories all critically rely on the generation of distinct patterns of activity in the region to signal differences between experiences and to reduce interference between memories. However, these theories are divided by the roles they attribute to the DG during learning and recall and by the contributions they ascribe to specific inputs or cell types within the DG. These differences influence the information that the DG is thought to impart to downstream structures. We work towards a holistic view of the role of DG in learning and memory by first developing three critical questions to foster a dialogue between the leading theories. We then evaluate the extent to which previous studies address our questions, highlight remaining areas of conflict, and suggest future experiments to bridge these theories.

Impaired synaptic transmission in dorsal dentate gyrus increases impulsive alcohol seeking

Both human and animal studies indicate that the dentate gyrus (DG) of the hippocampus is highly exploited by drug and alcohol abuse. Yet, it is poorly understood how DG dysfunction affects addiction-related behaviors. Here, we used an animal model of alcohol use disorder (AUD) in automated IntelliCages and performed local genetic manipulation to investigate how synaptic transmission in the dorsal DG (dDG) affects alcohol-related behaviors. We show that a cue light induces potentiation-like plasticity of dDG synapses in alcohol-naive mice. This process is impaired in mice trained to drink alcohol. Acamprosate (ACA), a drug that reduces alcohol relapse, rescues the impairment of dDG synaptic transmission in alcohol mice. A molecular manipulation that reduces dDG synaptic AMPAR and NMDAR levels increases impulsive alcohol seeking during cue relapse (CR) in alcohol mice but does not affect alcohol reward, motivation or craving. These findings suggest that hindered dDG synaptic transmission specifically underlies impulsive alcohol seeking induced by alcohol cues, a core symptom of AUD.

Locus coeruleus input-modulated reactivation of dentate gyrus opioid-withdrawal engrams promotes extinction

Extinction training during the reconsolidation window following memory recall is an effective behavioral pattern for promoting the extinction of pathological memory. However, promoted extinction by recall-extinction procedure has not been universally replicated in different studies. One potential reason for this may relate to whether initially acquired memory is successfully activated. Thus, the methods for inducing the memory into an active or plastic condition may contribute to promoting its extinction. The aim of this study is to find and demonstrate a manipulatable neural circuit that engages in the memory recall process and where its activation improves the extinction process through recall-extinction procedure. Here, naloxone-precipitated conditioned place aversion (CPA) in morphine-dependent mice was mainly used as a pathological memory model. We found that the locus coeruleus (LC)-dentate gyrus (DG) circuit was necessary for CPA memory recall and that artificial activation of LC inputs to the DG just prior to initiating a recall-extinction procedure significantly promoted extinction learning. We also found that activating this circuit caused an increase in the ensemble size of DG engram cells activated during the extinction, which was confirmed by a cFos targeted strategy to label cells combined with immunohistochemical and in vivo calcium imaging techniques. Collectively, our data uncover that the recall experience is important for updating the memory during the reconsolidation window; they also suggest a promising neural circuit or target based on the recall-extinction procedure for weakening pathological aversion memory, such as opioid withdrawal memory and fear memory.

Functional properties of eyelid conditioned responses and involved brain centers

For almost a century the classical conditioning of nictitating membrane/eyelid responses has been used as an excellent and feasible experimental model to study how the brain organizes the acquisition, storage, and retrieval of new motor abilities in alert behaving mammals, including humans. Lesional, pharmacological, and electrophysiological approaches, and more recently, genetically manipulated animals have shown the involvement of numerous brain areas in this apparently simple example of associative learning. In this regard, the cerebellum (both cortex and nuclei) has received particular attention as a putative site for the acquisition and storage of eyelid conditioned responses, a proposal not fully accepted by all researchers. Indeed, the acquisition of this type of learning implies the activation of many neural processes dealing with the sensorimotor integration and the kinematics of the acquired ability, as well as with the attentional and cognitive aspects also involved in this process. Here, we address specifically the functional roles of three brain structures (red nucleus, cerebellar interpositus nucleus, and motor cortex) mainly involved in the acquisition and performance of eyelid conditioned responses and three other brain structures (hippocampus, medial prefrontal cortex, and claustrum) related to non-motor aspects of the acquisition process. The main conclusion is that the acquisition of this motor ability results from the contribution of many cortical and subcortical brain structures each one involved in specific (motor and cognitive) aspects of the learning process.

Aberrant ventral dentate gyrus structure and function in trauma susceptible mice

Post-traumatic stress disorder (PTSD) is a psychiatric disorder vulnerable individuals can develop following a traumatic event, whereas others are resilient. Enhanced insight into the mechanistic underpinnings contributing to these inter-individual differences in trauma susceptibility is key to improved treatment and prevention. Aberrant function of the hippocampal dentate gyrus (DG) may contribute to its psychopathology, with the dorsal DG potentially encoding trauma memory generalization and the ventral DG anxiety. Using a mouse model, we hypothesized that susceptibility to develop PTSD-like symptoms following trauma will be underpinned by aberrant DG structure and function. Mice were exposed to a traumatic event (unpredictable, inescapable foot shocks) and tested for PTSD-like symptomatology following recovery. In four independent experiments, DG neuronal morphology, synaptic protein gene and protein expression, and neuronal activity during trauma encoding and recall were assessed. Behaviorally, trauma-susceptible animals displayed increased anxiety-like behavior already prior to trauma, increased novelty-induced freezing, but no clear differences in remote trauma memory recall. Comparison of the ventral DG of trauma susceptible vs resilient mice revealed lower spine density, reduced expression of the postsynaptic protein homer1b/c gene and protein, a larger population of neurons active during trauma encoding, and a greater presence of somatostatin neurons. In contrast, the dorsal DG of trauma-susceptible animals did not differ in terms of spine density or gene expression but displayed more active neurons during trauma encoding and a lower amount of somatostatin neurons. Collectively, we here report on specific structural and functional changes in the ventral DG in trauma susceptible male mice.

Calcium signaling in individual APP/PS1 mouse dentate gyrus astrocytes increases ex vivo with Aβ pathology and age without affecting astrocyte network activity

Astrocytes are critical for healthy brain function. In Alzheimer's disease, astrocytes become reactive, which affects their signaling properties. Here, we measured spontaneous calcium transients ex vivo in hippocampal astrocytes in brain slices containing the dentate gyrus of 6- (6M) and 9-month-old (9M) APPswe/PSEN1dE9 (APP/PS1) mice. We investigated the frequency and duration of calcium transients in relation to aging, amyloid-β (Aβ) pathology, and the proximity of the astrocyte to Aβ plaques. The 6M APP/PS1 astrocytes showed no change in spontaneous calcium-transient properties compared to wild-type (WT) astrocytes. 9M APP/PS1 astrocytes, however, showed more hyperactivity compared to WT, characterized by increased spontaneous calcium transients that were longer in duration. Our data also revealed an effect of aging, as 9M astrocytes overall showed an increase in calcium activity compared to 6M astrocytes. Subsequent calcium-wave analysis showed an increase in sequential calcium transients (i.e., calcium waves) in 9M astrocytes, suggesting increased network activity ex vivo. Further analysis using null models revealed that this network effect is caused by chance, due to the increased number of spontaneous transients. Our findings show that alterations in calcium signaling in individual hippocampal astrocytes of APP/PS1 mice are subject to both aging and Aβ pathology but these do not lead to a change in astrocyte network activity. These alterations in calcium dynamics of astrocytes may help to understand changes in neuronal physiology leading to cognitive decline and ultimately dementia.

Transsynaptic Long-Term Potentiation in the Hippocampus of Behaving Mice

Long-term potentiation (LTP) is an experimental procedure that shares certain mechanisms with neuronal learning and memory processes and represents a well-known example of synaptic plasticity. LTP consists of an increase of the synaptic response to a control stimulus following the presentation of a high-frequency stimulation (HFS) train to an afferent pathway. This technique is studied mostly in the hippocampus due to the latter’s high susceptibility and its laminar nature which facilitates the location of defined synapses. Although most preceding studies have been performed in vitro, we have developed an experimental approach to carry out these experiments in alert behaving animals. The main goal of this study was to confirm the existence of synaptic changes in strength in synapses that are post-synaptic to the one presented with the HFS. We recorded field excitatory post-synaptic potentials (fEPSPs) evoked in five hippocampal synapses, from both hemispheres, of adult male mice. HFS was presented to the perforant pathway (PP). We characterized input/output curves, paired-pulse stimulation, and LTP of these synapses. We also performed depth-profile recordings to determine differences in fEPSP latencies. Collected data indicate that the five selected synapses have similar basic electrophysiological properties, a fact that enables an easier comparison of LTP characteristics. Importantly, we observed the presence of significant LTP in the contralateral CA1 (cCA1) area following the control stimulation of non-HFS-activated pathways. These results indicate that LTP appears as a physiological process present in synapses located far away from the HFS-stimulated afferent pathway.

A role for cerebral cortex in the suppression of innate defensive behaviour

The cerebral cortex is widely accepted to be involved in the control of cognition and the processing of learned information. However, data suggest that it may also have a role in the regulation of innate responses because rodents, cats or primates with surgical removal of cortical regions show excessive aggression and rage elicited by threatening stimuli. Nevertheless, the imprecision and chronic nature of these lesions leave open the possibility that compensatory processes may underlie some of these phenotypes. In the present study we applied a precise, rapid and reversible inhibition approach to examine the contribution of the cerebral cortex to defensive behaviours elicited by a variety of innately aversive stimuli in laboratory mice. Pharmacological treatment of mice carrying the pharmacogenetic inhibitory receptor hM4D selectively in neocortex, archicortex and related dorsal telencephalon-derived structures resulted in the rapid inhibition of cerebral cortex neural activity. Cortical inhibition was associated with a selective increase in defensive behaviours elicited by an aggressive conspecific, a novel prey and a physically stressful stimulus. These findings are consistent with a role for cortex in the acute inhibition of innate defensive behaviours.

G-Protein-Gated Inwardly Rectifying Potassium (Kir3/GIRK) Channels Govern Synaptic Plasticity That Supports Hippocampal-Dependent Cognitive Functions in Male Mice

The G-protein-gated inwardly rectifying potassium (Kir3/GIRK) channel is the effector of many G-protein-coupled receptors (GPCRs). Its dysfunction has been linked to the pathophysiology of Down syndrome, Alzheimer's and Parkinson's diseases, psychiatric disorders, epilepsy, drug addiction, or alcoholism. In the hippocampus, GIRK channels decrease excitability of the cells and contribute to resting membrane potential and inhibitory neurotransmission. Here, to elucidate the role of GIRK channels activity in the maintenance of hippocampal-dependent cognitive functions, their involvement in controlling neuronal excitability at different levels of complexity was examined in C57BL/6 male mice. For that purpose, GIRK activity in the dorsal hippocampus CA3-CA1 synapse was pharmacologically modulated by two drugs: ML297, a GIRK channel opener, and Tertiapin-Q (TQ), a GIRK channel blocker. Ex vivo, using dorsal hippocampal slices, we studied the effect of pharmacological GIRK modulation on synaptic plasticity processes induced in CA1 by Schaffer collateral stimulation. In vivo, we performed acute intracerebroventricular (i.c.v.) injections of the two GIRK modulators to study their contribution to electrophysiological properties and synaptic plasticity of dorsal hippocampal CA3-CA1 synapse, and to learning and memory capabilities during hippocampal-dependent tasks. We found that pharmacological disruption of GIRK channel activity by i.c.v. injections, causing either function gain or function loss, induced learning and memory deficits by a mechanism involving neural excitability impairments and alterations in the induction and maintenance of long-term synaptic plasticity processes. These results support the contention that an accurate control of GIRK activity must take place in the hippocampus to sustain cognitive functions.SIGNIFICANCE STATEMENT Cognitive processes of learning and memory that rely on hippocampal synaptic plasticity processes are critically ruled by a finely tuned neural excitability. G-protein-gated inwardly rectifying K+ (GIRK) channels play a key role in maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Here, we demonstrate that modulation of GIRK channels activity, causing either function gain or function loss, transforms high-frequency stimulation (HFS)-induced long-term potentiation (LTP) into long-term depression (LTD), inducing deficits in hippocampal-dependent learning and memory. Together, our data show a crucial GIRK-activity-mediated mechanism that governs synaptic plasticity direction and modulates subsequent hippocampal-dependent cognitive functions.

Microglial synaptic pruning on axon initial segment spines of dentate granule cells: Sexually dimorphic effects of early-life stress and consequences for adult fear response

Axon initial segments (AIS) of dentate granule cells in the hippocampus exhibit prominent spines (AISS) during early development that are associated with microglial contacts. In the present study, we investigated whether developmental changes in AISS could be modified by early-life stress (ELS), specifically neonatal maternal separation (MS), through stress hormones and microglial activation and examined the potential behavioural consequences. We examined AISS at postnatal day (PND)5, 15 and 50, using Golgi-Cox staining and anatomical analysis. Neurone-microglial interaction was assessed using antibodies against ankyrin-G, PSD-95 and Iba1, for AIS, AISS and microglia visualisation, respectively, in normally reared and neonatal maternally separated male and female rats. We observed a higher density of AISS in ELS rats at both PND15 and PND50 compared to controls. Effects were more pronounced in females than males. AIS-associated microglia in ELS rats showed a hyper-ramified morphology and less co-localisation with PSD-95 compared to controls at PND15. ELS-associated alteration in microglial morphology and synaptic pruning was mimicked by treatment of acute hippocampal slices of normally reared rats with vasopressin. ELS rats exhibited increased freezing behaviour during auditory fear memory testing, which was more pronounced in female subjects and corresponded with increased Fos expression in dorsal and ventral dentate granule cells. Thus, microglial synaptic pruning in dentate AIS of hippocampus is influenced by ELS, with demonstrable sex bias regarding its anatomical characteristics and subsequent fear-induced defensive behaviours.

Recovery of post-stroke cognitive and motor deficiencies by Shuxuening injection via regulating hippocampal BDNF-mediated Neurotrophin/Trk Signaling

A mild ischemic stroke may cause both debilitating locomotor and cognitive decline, for which the mechanism is not fully understood, and no therapies are currently available. In this study, a nonfatal stroke model was constructed in mice by a modified middle cerebral artery occlusion (MCAO) procedure, allowing an extended recovery period up to 28 days. The extended MCAO model successfully mimicked phenotypes of a recovery phase post-stroke, including locomotor motor and cognitive deficiencies, which were effectively improved after Shuxuening injection (SXNI) treatment. Tissue slices staining showed that SXNI repaired brain injury and reduced neuronal apoptosis, especially in the hippocampus CA3 region. Transcriptomics sequencing study revealed 565 differentially expressed genes (DEGs) in the ischemic brain after SXNI treatment. Integrated network pharmacological analysis identified Neurotrophin/Trk Signaling was the most relevant pathway, which involves 15 key genes. Related DEGs were further validated by RT-PCR. Western-blot analysis showed that SXNI reversed the abnormal expression of BDNF, TrkB, Mek3 and Jnk1after stroke. ELISA found that SXNI increased brain level of p-Erk and Creb. At sub-brain level, the expression of BDNF and TrkB was decreased and GFAP was increased on the hippocampal CA3 region in the post-stroke recovery phase and this abnormality was improved by SXNI. In vitro experiments also found that oxygen glucose deprivation reduced the expression of BDNF and TrkB, which was reversed by SXNI. In summary, we conclude that SXNI facilitates the recovery of cognitive and locomotor dysfunction by modulating Neurotrophin/Trk Signaling in a mouse model for the recovery phase of post-ischemic stroke.

Aberrant Synaptic PTEN in Symptomatic Alzheimer's Patients May Link Synaptic Depression to Network Failure

In Alzheimer’s disease (AD), Amyloid β (Aβ) impairs synaptic function by inhibiting long-term potentiation (LTP), and by facilitating long-term depression (LTD). There is now evidence from AD models that Aβ provokes this shift toward synaptic depression by triggering the access to and accumulation of PTEN in the postsynaptic terminal of hippocampal neurons. Here we quantified the PTEN in 196,138 individual excitatory dentate gyrus synapses from AD patients at different stages of the disease and from controls with no neuropathological findings. We detected a gradual increase of synaptic PTEN in AD brains as the disease progresses, in conjunction with a significant decrease in synaptic density. The synapses that remain in symptomatic AD patients are more likely to be smaller and exhibit fewer AMPA receptors (AMPARs). Hence, a high Aβ load appears to strongly compromise human hippocampal synapses, as reflected by an increase in PTEN, inducing a loss of AMPARs that may eventually provoke synaptic failure and loss.

Synchronous activity patterns in the dentate gyrus during immobility

The hippocampal dentate gyrus is an important relay conveying sensory information from the entorhinal cortex to the hippocampus proper. During exploration, the dentate gyrus has been proposed to act as a pattern separator. However, the dentate gyrus also shows structured activity during immobility and sleep. The properties of these activity patterns at cellular resolution, and their role in hippocampal-dependent memory processes have remained unclear. Using dual-color in vivo two-photon Ca²⁺ imaging, we show that in immobile mice dentate granule cells generate sparse, synchronized activity patterns associated with entorhinal cortex activation. These population events are structured and modified by changes in the environment; and they incorporate place- and speed cells. Importantly, they are more similar than expected by chance to population patterns evoked during self-motion. Using optogenetic inhibition, we show that granule cell activity is not only required during exploration, but also during immobility in order to form dentate gyrus-dependent spatial memories.

Dopamine D2R is Required for Hippocampal-dependent Memory and Plasticity at the CA3-CA1 Synapse

Dopamine receptors play an important role in motivational, emotional, and motor responses. In addition, growing evidence suggests a key role of hippocampal dopamine receptors in learning and memory. It is well known that associative learning and synaptic plasticity of CA3-CA1 requires the dopamine D1 receptor (D1R). However, the specific role of the dopamine D2 receptor (D2R) on memory-related neuroplasticity processes is still undefined. Here, by using two models of D2R loss, D2R knockout mice (Drd2-/-) and mice with intrahippocampal injections of Drd2-small interfering RNA (Drd2-siRNA), we aimed to investigate how D2R is involved in learning and memory as well as in long-term potentiation of the hippocampus. Our studies revealed that the genetic inactivation of D2R impaired the spatial memory, associative learning, and the classical conditioning of eyelid responses. Similarly, deletion of D2R reduced the activity-dependent synaptic plasticity in the hippocampal CA1-CA3 synapse. Our results demonstrate the first direct evidence that D2R is essential in behaving mice for trace eye blink conditioning and associated changes in hippocampal synaptic strength. Taken together, these results indicate a key role of D2R in regulating hippocampal plasticity changes and, in consequence, acquisition and consolidation of spatial and associative forms of memory.

Nicotine abolishes memory-related synaptic strengthening and promotes synaptic depression in the neurogenic dentate gyrus of miR-132/212 knockout mice

Micro-RNAs (miRNAs) are highly evolutionarily conserved short-length/noncoding RNA molecules that modulate a wide range of cellular functions in many cell types by regulating the expression of a variety of targeted genes. miRNAs have also recently emerged as key regulators of neuronal genes mediating the effects of psychostimulant drugs and memory-related neuroplasticity processes. Smoking is a predominant addictive behaviour associated with millions of deaths worldwide, and nicotine is a potent natural psychoactive agonist of cholinergic receptors, highly abundant in cigarettes. The influence of miRNAs modulation on cholinergic signalling in the nervous system remains however poorly explored. Using miRNA knockout mice and biochemical, electrophysiological and pharmacological approaches, we examined the effects of miR-132/212 gene disruption on the levels of hippocampal nicotinic acetylcholine receptors, total ERK and phosphorylated ERK (pERK) and MeCP2 protein levels, and studied the impact of nicotine stimulation on hippocampal synaptic transmission and synaptic depression and strengthening. miR-132/212 deletion significantly altered α7-nAChR and pERK protein levels, but not total ERK or MeCP2, and resulted in both exacerbated synaptic depression and virtually abolished memory-related synaptic strengthening upon nicotine stimulation. These observations reveal a functional miRNAs/nicotinergic signalling interplay critical for nicotinic-receptor expression and neuroplasticity in brain structures relevant for drug addiction and learning and memory functions.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Metaplastic contribution of neuropeptide Y receptors to spatial memory acquisition

Neuropeptide Y (NPY) is highly abundant in the brain and is released as a co-transmitter with plasticity-related neurotransmitters such as glutamate, GABA and noradrenaline. Functionally, its release is associated with appetite, anxiety, and stress regulation. NPY acting on Y2 receptors (Y₂R), facilitates fear extinction, suggesting a role in associative memory. Here, we explored to what extent NPY action at Y₂R contributes to hippocampus-dependent spatial memory and found that dorsal intrahippocampal receptor antagonism improved spatial reference memory acquired in a water maze in rats, without affecting anxiety levels, or spontaneous motor activity. Water maze training resulted in an increase of Y₂R, but not Y₁R expression in the hippocampus. By contrast, in the prefrontal cortex there was a decrease in Y₂R, and an increase of Y₁R expression. Our results indicate that neuropeptide Y₂R are significantly involved in hippocampus-dependent spatial memory and that receptor expression is dynamically regulated by this learning experience. Effects are consistent with a metaplastic contribution of NPY receptors to cumulative spatial learning.

Dentate gyrus circuits for encoding, retrieval and discrimination of episodic memories

The dentate gyrus (DG) has a key role in hippocampal memory formation. Intriguingly, DG lesions impair many, but not all, hippocampus-dependent mnemonic functions, indicating that the rest of the hippocampus (CA1-CA3) can operate autonomously under certain conditions. An extensive body of theoretical work has proposed how the architectural elements and various cell types of the DG may underlie its function in cognition. Recent studies recorded and manipulated the activity of different neuron types in the DG during memory tasks and have provided exciting new insights into the mechanisms of DG computational processes, particularly for the encoding, retrieval and discrimination of similar memories. Here, we review these DG-dependent mnemonic functions in light of the new findings and explore mechanistic links between the cellular and network properties of, and the computations performed by, the DG.

Genetic Ablation of Neural Progenitor Cells Impairs Acquisition of Trace Eyeblink Conditioning

Adult-born neurons are believed to play a role in memory formation by providing enhanced plasticity to the hippocampus. Past studies have demonstrated that reduction of neurogenesis impairs associative learning, but these experiments used irradiation or neurotoxic substances, which may have had unintended off-target effects. Therefore, to investigate the role of these adult-born neurons more precisely, we used nestin-HSV-TK transgenic mice (Nes-TK) to selectively ablate newborn neurons. Nes-TK mice were fed a chow infused with valganciclovir to induce the ablation of neural progenitor cells. After being on this diet for 4 weeks, mice were trained on trace eyeblink conditioning, a hippocampus-dependent temporal associative memory task. Following the completion of training, brain sections from these animals were stained for doublecortin, a marker for immature neurons, to quantify levels of neurogenesis. We found that male transgenic mice on valganciclovir had significantly decreased amounts of doublecortin relative to male control animals, indicating a successful reduction in levels of neurogenesis. In conjunction with this reduction in neurogenesis, the male transgenic mice on valganciclovir learned at a significantly slower rate than male control mice. The female Nes-TK mice on valganciclovir showed no significant decrease in neurogenesis and no behavioral impairment relative to female control mice. Ultimately, the results are consistent with, and expand upon, prior studies that demonstrated that adult-born neurons are involved in the formation of associative memories. This study also provides a foundation to continue to explore the physiological role of newborn neurons with in vivo recordings during behavioral training.

Lack of Neuronal Glycogen Impairs Memory Formation and Learning-Dependent Synaptic Plasticity in Mice

Since brain glycogen is stored mainly in astrocytes, the role of this polysaccharide in neurons has been largely overlooked. To study the existence and relevance of an active neuronal glycogen metabolism in vivo, we generated a mouse model lacking glycogen synthase specifically in the Camk2a-expressing postnatal forebrain pyramidal neurons (GYS1^Camk2a–KO), which include the prefrontal cortex and the CA3 and CA1 cell layers of the hippocampus. The latter are involved in memory and learning processes and participate in the hippocampal CA3-CA1 synapse, the function of which can be analyzed electrophysiologically. Long-term potentiation evoked in the hippocampal CA3-CA1 synapse was decreased in alert behaving GYS1^Camk2a–KO mice. They also showed a significant deficiency in the acquisition of an instrumental learning task – a type of associative learning involving prefrontal and hippocampal circuits. Interestingly, GYS1^Camk2a–KO animals did not show the greater susceptibility to hippocampal seizures and myoclonus observed in animals completely depleted of glycogen in the whole CNS. These results unequivocally demonstrate the presence of an active glycogen metabolism in neurons in vivo and reveal a key role of neuronal glycogen in the proper acquisition of new motor and cognitive abilities, and in the changes in synaptic strength underlying such acquisition.

Fear renewal activates cyclic adenosine monophosphate signaling in the dentate gyrus

BACKGROUND

Fear renewal, the context-specific relapse of a conditioned fear after extinction, is a widely pursued model of post-traumatic stress disorder and phobias. However, its cellular and molecular mechanisms remain poorly understood. The dentate gyrus (DG) has emerged as a critical locus of plasticity with relevance to memory, anxiety disorders, and depression, and it contributes to fear memory retrieval. Here, we have identified the role of the DG in fear renewal and its molecular mechanism.

MATERIALS AND METHODS

Muscimol (MUS), activator of cyclic adenosine monophosphate (cAMP) forskolin (FSK), inhibitor of protein kinase A (PKA), Rip-cAMP, and a phosphodiesterase inhibitor rolipram were infused into DG of standard deviation rats before renewal testing. cAMP levels after fear renewal was measured by enzyme-linked immunosorbent assay. The protein levels of phosphodiesterase 4 (PDE4) isoforms were tested by western blot. At last, the roles of cAMP signaling were also tested in the acquisition of fear conditioning, fear retrieval, and extinction.

RESULTS

Intra-DG treatment of MUS and Rp-cAMP impaired fear renewal. FSK and rolipram exhibited the opposite effect, which also occurred in the retrieval of original fear memory. This change in fear renewal was regulated by PDE4 isoforms PDE4A, PDE4A5, and PDE4D. In addition, FSK and rolipram facilitated the acquisition of fear conditioning in long-term memory, but not short-term memory, while Rp-cAMP impaired long-term memory. For extinction, FSK and rolipram inhibited extinction process, while Rp-cAMP facilitated fear extinction.

CONCLUSION

These findings demonstrated that fear renewal activated cAMP signaling in the DG through decreased PDE4 activity. Because of the role of cAMP signaling in the acquisition or retrieval of fear conditioning and encoding of extinction, it is speculated that initial learning and extinction may have similarities in molecular mechanism, especially fear retrieval and fear renewal may share cAMP signaling pathway in the DG.

Artificially Enhancing and Suppressing Hippocampus-Mediated Memories

Emerging evidence indicates that distinct hippocampal domains differentially drive cognition and emotion [1, 2]; dorsal regions encode spatial, temporal, and contextual information [3-5], whereas ventral regions regulate stress responses [6], anxiety-related behaviors [7, 8], and emotional states [8-10]. Although previous studies demonstrate that optically manipulating cells in the dorsal hippocampus can drive the behavioral expression of positive and negative memories, it is unknown whether changes in cellular activity in the ventral hippocampus can drive such behaviors [11-14]. Investigating the extent to which distinct hippocampal memories across the longitudinal axis modulate behavior could aid in the understanding of stress-related psychiatric disorders known to affect emotion, memory, and cognition [15]. Here, we asked whether tagging and stimulating cells along the dorsoventral axis of the hippocampus could acutely, chronically, and differentially promote context-specific behaviors. Acute reactivation of both dorsal and ventral hippocampus cells that were previously active during memory formation drove freezing behavior, place avoidance, and place preference. Moreover, chronic stimulation of dorsal or ventral hippocampal fear memories produced a context-specific reduction or enhancement of fear responses, respectively, thus demonstrating bi-directional and context-specific modulation of memories along the longitudinal axis of the hippocampus. Fear memory suppression was associated with a reduction in hippocampal cells active during retrieval, while fear memory enhancement was associated with an increase in basolateral amygdala activity. Together, our data demonstrate that discrete sets of cells throughout the hippocampus provide key nodes sufficient to bi-directionally reprogram both the neural and behavioral expression of memory.

Distinct hippocampal engrams control extinction and relapse of fear memory

Learned fear often relapses after extinction, suggesting that extinction training generates a new memory that coexists with the original fear memory; however, the mechanisms governing expression of competing fear and extinction memories remain unclear. We used activity-dependent neural tagging to investigate representations of fear and extinction memories in the dentate gyrus (DG). We demonstrate that extinction training suppresses reactivation of context fear engram cells, while activating a second ensemble, a putative extinction engram. Optogenetic inhibition of neurons that were active during extinction training increased fear after extinction training, whereas silencing neurons that were active during fear training reduced spontaneous recovery of fear. Optogenetic stimulation of fear acquisition neurons increased fear, while stimulation of extinction neurons suppressed fear and prevented spontaneous recovery. Our results indicate the hippocampus generates a fear extinction representation and that interactions between hippocampal fear and extinction representations govern suppression and relapse of fear after extinction.

Neuropharmacological Insight from Allosteric Modulation of mGlu Receptors

The metabotropic glutamate (mGlu) receptors are a family of G-protein-coupled receptors (GPCRs) that regulate cell physiology throughout the nervous system. The potential of mGlu receptors as therapeutic targets has been bolstered by current research that has provided insight into the diverse modes of mGlu activation and signaling. In particular, the allosteric modulation of mGlu receptors represents a major area of focus in studies of basic pharmacology as well as drug development, largely due to the high subtype specificity achievable by targeting allosteric sites on mGlu receptors. These provide sophisticated regulation of neuronal excitability and synaptic transmission to influence behavioral output. Here, we review how these allosteric mechanisms have been leveraged preclinically to demonstrate the therapeutic potential of allosteric modulators for neurological and neuropsychiatric disorders, such as autism, cognitive impairment, Parkinson's disease (PD), stress, and schizophrenia.

Intrahippocampal 5-HT1A receptor antagonist inhibits the improving effect of low-frequency stimulation on memory impairment in kindled rats

In addition to its anticonvulsant effect, low frequency stimulation (LFS) improves learning and memory in kindled animals. In the present study, the role of 5-HT_1A receptors in mediating LFS' improving effect on spatial learning and memory was investigated in amygdala-kindled rats. Amygdala kindling was conducted in a semi-rapid kindling stimulations (12 stimulations per day) in male Wistar rats. LFS (4 trains of 0.1 ms pulse duration at 1 Hz, 200 pulses, 50-150 μA, at 5 min intervals) was applied after termination of kindling stimulations. NAD-299 (a selective 5-HT_1A receptor antagonist; 2.5 and 5 μg/μl) was microinjected into the hippocampal CA1 before applying LFS. The Morris water maze, and novel object recognition tests were conducted after the last kindling stimulation. Hippocampal samples were also prepared, and 5-HT_1A receptor gene expression levels were assessed using quantitative RT-PCR. In kindled animals, LFS reduced impairments in spatial learning and memory in the Morris water maze and novel object recognition tests. Microinjection of NAD doses of 5 μg/μl reduced the effects of LFS on learning and memory. The gene expression level of 5-HT_1A receptors increased significantly in the hippocampus of amygdala-kindled rats. However, LFS applied after kindling stimulations inhibited this effect. It seems that activation of 5-HT_1A receptors in the CA1 field is necessary for LFS' improving effects on spatial learning and memory in kindled animals; although surprisingly, LFS application prevented the elevation in gene expression of 5-HT_1A receptors in kindled animals.

Dentate spikes and learning: disrupting hippocampal function during memory consolidation can improve pattern separation

Hippocampal dentate spikes (DSs) are short-duration, large-amplitude fluctuations in hilar local field potentials and take place while resting and sleeping. During DSs, dentate gyrus granule cells increase firing while CA1 pyramidal cells decrease firing. Recent findings suggest DSs play a significant role in memory consolidation after training on a hippocampus-dependent, nonspatial associative learning task. Here, we aimed to find out whether DSs are important in other types of hippocampus-dependent learning tasks as well. To this end, we trained adult male Sprague-Dawley rats in a spatial reference memory task, a fixed interval task, and a pattern separation task. During a rest period immediately after each training session, we either let neural activity to take place as usual, timed electrical stimulation of the ventral hippocampal commissure (vHC) to immediately follow DSs, or applied the vHC stimulation during a random neural state. We found no effect of vHC stimulation on performance in the spatial reference memory task or in the fixed interval task. Surprisingly, vHC stimulation, especially contingent on DSs, improved performance in the pattern separation task. In conclusion, the behavioral relevance of hippocampal processing and DSs seems to depend on the task at hand. It could be that in an intact brain, offline memory consolidation by default involves associating neural representations of temporally separate but related events. In some cases this might be beneficial for adaptive behavior in the future (associative learning), while in other cases it might not (pattern separation).

NEW & NOTEWORTHY The behavioral relevance of dentate spikes seems to depend on the learning task at hand. We suggest that dentate spikes are related to associating neural representations of temporally separate but related events within the dentate gyrus. In some cases this might be beneficial for adaptive behavior in the future (associative learning), while in other cases it might not (pattern separation).

Monoamines and their Derivatives on GPCRs: Potential Therapy for Alzheimer's Disease

Albeit cholinergic depletion remains the key event in Alzheimer's Disease (AD), recent information describes stronger links between monoamines (trace amines, catecholamines, histamine, serotonin, and melatonin) and AD than those known in the past century. Therefore, new drug design strategies focus efforts to translate the scope on these topics and to offer new drugs which can be applied as therapeutic tools in AD. In the present work, we reviewed the state-of-art regarding genetic, neuropathology and neurochemistry of AD involving monoamine systems. Then, we compiled the effects of monoamines found in the brain of mammals as well as the reported effects of their derivatives and some structure-activity relationships. Recent derivatives have triggered exciting effects and pharmacokinetic properties in both murine models and humans. In some cases, the mechanism of action is clear, essentially through the interaction on G-protein-coupled receptors as revised in this manuscript. Additional mechanisms are inhibition of enzymes for their biotransformation, regulation of free-radicals in the central nervous system and others for the effects on Tau phosphorylation or amyloid-beta accumulation. All these data make the monoamines and their derivatives attractive potential elements for AD therapy.

Gq-Coupled Muscarinic Receptor Enhancement of KCNQ2/3 Channels and Activation of TRPC Channels in Multimodal Control of Excitability in Dentate Gyrus Granule Cells

KCNQ (Kv7, "M-type") K⁺ channels and TRPC (transient receptor potential, "canonical") cation channels are coupled to neuronal discharge properties and are regulated via G_q/11-protein-mediated signals. Stimulation of G_q/11-coupled receptors both consumes phosphatidylinositol 4,5-bisphosphate (PIP₂) via phosphalipase Cβ hydrolysis and stimulates PIP₂ synthesis via rises in Ca²⁺ _i and other signals. Using brain-slice electrophysiology and Ca²⁺ imaging from male and female mice, we characterized threshold K⁺ currents in dentate gyrus granule cells (DGGCs) and CA1 pyramidal cells, the effects of G_q/11-coupled muscarinic M₁ acetylcholine (M₁R) stimulation on M current and on neuronal discharge properties, and elucidated the intracellular signaling mechanisms involved. We observed disparate signaling cascades between DGGCs and CA1 neurons. DGGCs displayed M₁R enhancement of M-current, rather than suppression, due to stimulation of PIP₂ synthesis, which was paralleled by increased PIP₂-gated G-protein coupled inwardly rectifying K⁺ currents as well. Deficiency of KCNQ2-containing M-channels ablated the M₁R-induced enhancement of M-current in DGGCs. Simultaneously, M₁R stimulation in DGGCs induced robust increases in [Ca²⁺]_i, mostly due to TRPC currents, consistent with, and contributing to, neuronal depolarization and hyperexcitability. CA1 neurons did not display such multimodal signaling, but rather M current was suppressed by M₁R stimulation in these cells, similar to the previously described actions of M₁R stimulation on M-current in peripheral ganglia that mostly involves PIP₂ depletion. Therefore, these results point to a pleiotropic network of cholinergic signals that direct cell-type-specific, precise control of hippocampal function with strong implications for hyperexcitability and epilepsy.SIGNIFICANCE STATEMENT At the neuronal membrane, protein signaling cascades consisting of ion channels and metabotropic receptors govern the electrical properties and neurotransmission of neuronal networks. Muscarinic acetylcholine receptors are G-protein-coupled metabotropic receptors that control the excitability of neurons through regulating ion channels, intracellular Ca²⁺ signals, and other second-messenger cascades. We have illuminated previously unknown actions of muscarinic stimulation on the excitability of hippocampal principal neurons that include M channels, TRPC (transient receptor potential, "canonical") cation channels, and powerful regulation of lipid metabolism. Our results show that these signaling pathways, and mechanisms of excitability, are starkly distinct between peripheral ganglia and brain, and even between different principal neurons in the hippocampus.

Characterization of the Glucagonlike Peptide-1 Receptor in Male Mouse Brain Using a Novel Antibody and In Situ Hybridization

Glucagonlike peptide 1 (GLP-1) is a physiological regulator of appetite, and long-acting GLP-1 receptor (GLP-1R) agonists lower food intake and body weight in both human and animal studies. The effects are mediated through brain GLP-1Rs, and several brain nuclei expressing the GLP-1R may be involved. To date, the mapping of the complete location of GLP-1R protein in the brain has been challenged by lack of good antibodies and the discrepancy between mRNA and protein, especially relevant in neuronal axonal processes. Here, we present a specific monoclonal GLP-1R antibody for immunohistochemistry with murine tissue and show detailed distribution of GLP-1R expression, as well as mapping of GLP-1R mRNA by nonradioactive in situ hybridization. Semiautomated image analysis was performed to map the GLP-1R distribution to atlas plates from the Allen Institute for Brain Science. The GLP-1R was abundantly expressed in numerous regions, including the septal nucleus, hypothalamus, and brain stem. GLP-1R protein expression was also observed on neuronal projections in brain regions devoid of any mRNA that has not been observed in earlier reports. Taken together, these findings provide knowledge on GLP-1R expression in neuronal cell bodies and neuronal projections.

(E)-2-Methoxy-4-(3-(4-methoxyphenyl) prop-1-en-1-yl) Phenol Ameliorates LPS-Mediated Memory Impairment by Inhibition of STAT3 Pathway

Alzheimer’s disease (AD) is pathologically characterized by an excessive accumulation of amyloid-beta (Aβ) fibrils within the brain. We tested the anti-inflammatory and anti-amyloidogenic effects of (E)-2-methoxy-4-(3-(4-methoxyphenyl) prop-1-en-1-yl) phenol (MMPP), a selective signal transducer and activator of transcription 3 (STAT3) inhibitor. We examined whether MMPP (5 mg/kg in drinking water for 1 month) prevents amyloidogenesis and cognitive impairment on AD model mice induced by intraperitoneal LPS (250 μg/kg daily 7 times) injections. Additionally, we investigated the anti-neuroinflammatory and anti-amyloidogenic effect of MMPP (1, 5, and 10 μg/mL) in LPS (1 μg/mL)-treated cultured astrocytes and microglial BV-2 cells. MMPP treatment reduced LPS-induced memory loss. This memory recovery effect was associated with the reduction of LPS-induced inflammatory proteins; cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) as well as activation of microglial cells and astrocytes in the brain. Furthermore, MMPP reduced LPS-induced β-secretase and Aβ generation. In in vitro study, LPS-induced expression of inflammatory proteins and amyloidogenic proteins was decreased in microglial BV-2 cells and cultured astrocytes by MMPP treatment. Moreover, MMPP treatment suppressed DNA binding activities of the activation of STAT3 in in vivo and in vitro. These results indicated that MMPP inhibits LPS-induced amyloidogenesis and neuroinflammation via inhibition of STAT3.

Habituation based synaptic plasticity and organismic learning in a quantum perovskite

A central characteristic of living beings is the ability to learn from and respond to their environment leading to habit formation and decision making. This behavior, known as habituation, is universal among all forms of life with a central nervous system, and is also observed in single-cell organisms that do not possess a brain. Here, we report the discovery of habituation-based plasticity utilizing a perovskite quantum system by dynamical modulation of electron localization. Microscopic mechanisms and pathways that enable this organismic collective charge-lattice interaction are elucidated by first-principles theory, synchrotron investigations, ab initio molecular dynamics simulations, and in situ environmental breathing studies. We implement a learning algorithm inspired by the conductance relaxation behavior of perovskites that naturally incorporates habituation, and demonstrate learning to forget: a key feature of animal and human brains. Incorporating this elementary skill in learning boosts the capability of neural computing in a sequential, dynamic environment.Habituation is a learning mechanism that enables control over forgetting and learning. Zuo, Panda et al., demonstrate adaptive synaptic plasticity in SmNiO₃ perovskites to address catastrophic forgetting in a dynamic learning environment via hydrogen-induced electron localization.

HIPP neurons in the dentate gyrus mediate the cholinergic modulation of background context memory salience

Cholinergic neuromodulation in the hippocampus controls the salience of background context memory acquired in the presence of elemental stimuli predicting an aversive reinforcement. With pharmacogenetic inhibition we here demonstrate that hilar perforant path-associated (HIPP) cells of the dentate gyrus mediate the devaluation of background context memory during Pavlovian fear conditioning. The salience adjustment is sensitive to reduction of hilar neuropeptide Y (NPY) expression via dominant negative CREB expression in HIPP cells and to acute blockage of NPY-Y1 receptors in the dentate gyrus during conditioning. We show that NPY transmission and HIPP cell activity contribute to inhibitory effects of acetylcholine in the dentate gyrus and that M1 muscarinic receptors mediate the cholinergic activation of HIPP cells as well as their control of background context salience. Our data provide evidence for a peptidergic local circuit in the dentate gyrus that mediates the cholinergic encoding of background context salience during fear memory acquisition.

Intra-hippocampal circuits are essential for associating a background context with behaviorally salient stimuli and involve cholinergic modulation at SST⁺ interneurons. Here the authors show that the salience of the background context memory is modulated through muscarinic activation of NPY⁺ hilar perforant path associated interneurons and NPY signaling in the dentate gyrus.

Dentate Gyrus Contributes to Retrieval as well as Encoding: Evidence from Context Fear Conditioning, Recall, and Extinction

Dentate gyrus (DG) is widely thought to provide a teaching signal that enables hippocampal encoding of memories, but its role during retrieval is poorly understood. Some data and models suggest that DG plays no role in retrieval; others encourage the opposite conclusion. To resolve this controversy, we evaluated the effects of optogenetic inhibition of dorsal DG during context fear conditioning, recall, generalization, and extinction in male mice. We found that (1) inhibition during training impaired context fear acquisition; (2) inhibition during recall did not impair fear expression in the training context, unless mice had to distinguish between similar feared and neutral contexts; (3) inhibition increased generalization of fear to an unfamiliar context that was similar to a feared one and impaired fear expression in the conditioned context when it was similar to a neutral one; and (4) inhibition impaired fear extinction. These effects, as well as several seemingly contradictory published findings, could be reproduced by BACON (Bayesian Context Fear Algorithm), a physiologically realistic hippocampal model positing that acquisition and retrieval both involve coordinated activity in DG and CA3. Our findings thus suggest that DG contributes to retrieval and extinction, as well as to the initial establishment of context fear.SIGNIFICANCE STATEMENT Despite abundant evidence that the hippocampal dentate gyrus (DG) plays a critical role in memory, it remains unclear whether the role of DG relates to memory acquisition or retrieval. Using contextual fear conditioning and optogenetic inhibition, we show that DG contributes to both of these processes. Using computational simulations, we identify specific mechanisms through which the suppression of DG affects memory performance. Finally, we show that DG contributes to fear extinction learning, a process in which learned fear is attenuated through exposures to a fearful context in the absence of threat. Our data resolve a long-standing question about the role of DG in memory and provide insight into how disorders affecting DG, including aging, stress, and depression, influence cognitive processes.

Transcription factor NRF2 controls the fate of neural stem cells in the subgranular zone of the hippocampus

Neural stem/progenitor cells (NSPCs) located at the subgranular zone (SGZ) of the hippocampus participate in the maintenance of synaptic networks that ensure cognitive functions during life. Although it is known that this neurogenic niche losses activity with oxidative stress and ageing, the molecular events involved in its regulation are largely unknown. Here, we studied the role of transcription factor Nuclear Factor-Erythroid 2-Related Factor 2 (NRF2) in the control of NSPCs destinies in the SGZ. We first describe that NRF2-knockout (Nrf2^-/-) mice exhibit impaired long term potentiation, a function that requires integrity of the SGZ, therefore suggesting a cognitive deficit that might be linked to hippocampal neurogenesis. Then, we found a reduction in NSCs from birth to adulthood that was exacerbated in Nrf2^-/- vs. Nrf2^+/+ mice. The clonogenic and proliferative capacity of SGZ-derived NSPCs from newborn and 3-month-old Nrf2^-/- mice was severely reduced as determined in neurosphere cultures. Nrf2-deficiency also impaired neuronal differentiation both the SGZ, and in neurosphere differentiation assays, leading to an abnormal production of astrocytes and oligodendrocytes vs. neurons. Rescue of Nrf2^-/- NSPCs by ectopic expression of NRF2 attenuated the alterations in clonogenic, proliferative and differentiating capacity of hippocampal NSPCs. In turn, knockdown of the NRF2 gene in wild type NSPCs reproduced the data obtained with Nrf2^-/- NSPCs. Our findings demonstrate the importance of NRF2 in the maintenance of proper proliferation and differentiation rates of hippocampal NSPCs and suggest that interventions to up-regulate NRF2 might provide a mechanism to preserve the neurogenic functionality of the hippocampus.

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NRF2-knockout mice show impaired hippocampal long term potentiation.

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NRF2-lack impairs the proliferative and clonogenic capacity of neural stem cells.

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The neuronal to glial differentiation ratio is altered in the absence of NRF2.

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NRF2 is required for homeostasis of the hippocampal neurogenic niche.

Transient cell assembly networks encode stable spatial memories

One of the mysteries of memory is that it can last despite changes in the underlying synaptic architecture. How can we, for example, maintain an internal spatial map of an environment over months or years when the underlying network is full of transient connections? In the following, we propose a computational model for describing the emergence of the hippocampal cognitive map in a network of transient place cell assemblies and demonstrate, using methods of algebraic topology, how such a network can maintain spatial memory over time.

Adult hippocampal neurogenesis and cognitive flexibility - linking memory and mood

Adult hippocampal neurogenesis has been implicated in cognitive processes, such as pattern separation, and in the behavioural effects of stress and antidepressants. Young adult-born neurons have been shown to inhibit the overall activity of the dentate gyrus by recruiting local interneurons, which may result in sparse contextual representations and improved pattern separation. We propose that neurogenesis-mediated inhibition also reduces memory interference and enables reversal learning both in neutral situations and in emotionally charged ones. Such improved cognitive flexibility may in turn help to decrease anxiety-like and depressive-like behaviour.

Memory Retrieval Re-Activates Erk1/2 Signaling in the Same Set of CA1 Neurons Recruited During Conditioning

The hippocampus enables a range of behaviors through its intrinsic circuits and concerted actions with other brain regions. One such important function is the retrieval of episodic memories. How hippocampal cells support retrieval of contextual fear memory remains largely unclear. Here we monitored phospho-activation of extracellular-regulated kinase (Erk1/2) across neuronal populations of the hippocampus to find that CA1 pyramidal neurons, but not cells in CA3 or dentate gyrus, specifically respond to retrieval of an aversive context. In contrast, retrieval of a neutral context that fails to elicit a threat response did not activate Erk1/2. Moreover, retrieval preferentially re-activated Erk1/2 in the same set of CA1 neurons previously activated during conditioning in a context-specific manner. By confining drug inhibition within dorsal CA1, we established the crucial role for Erk1/2 activity in retrieval of long-term memory, as well as in amygdala activation associated with fear expression. These data provide functional evidence that Erk1/2 signaling in CA1 encodes a specific neural representation of contextual memory with emotional value.

Adult Neurogenesis in the Songbird: Region-Specific Contributions of New Neurons to Behavioral Plasticity and Stability

Our understanding of the role of new neurons in learning and encoding new information has been largely based on studies of new neurons in the mammalian dentate gyrus and olfactory bulb - brain regions that may be specialized for learning. Thus the role of new neurons in regions that serve other functions has yet to be fully explored. The song system provides a model for studying new neuron function in brain regions that contribute differently to song learning, song auditory discrimination, and song motor production. These regions subserve learning as well as long-term storage of previously learned information. This review examines the differences between learning-based and activity-based retention of new neurons and explores the potential contributions of new neurons to behavioral stability in the song motor production pathway.