Prefrontal-hippocampal functional connectivity encodes recognition memory and is impaired in intellectual disability

Selective vulnerability of the ventral hippocampus-prelimbic cortex axis parvalbumin interneuron network underlies learning deficits of fragile X mice

High-penetrance mutations affecting mental health can involve genes ubiquitously expressed in the brain. Whether the specific patterns of dysfunctions result from ubiquitous circuit deficits or might reflect selective vulnerabilities of targetable subnetworks has remained unclear. Here, we determine how loss of ubiquitously expressed fragile X mental retardation protein (FMRP), the cause of fragile X syndrome, affects brain networks in Fmr1y/- mice. We find that in wild-type mice, area-specific knockout of FMRP in the adult mimics behavioral consequences of area-specific silencing. By contrast, the functional axis linking the ventral hippocampus (vH) to the prelimbic cortex (PreL) is selectively affected in constitutive Fmr1y/- mice. A chronic alteration in late-born parvalbumin interneuron networks across the vH-PreL axis rescued by VIP signaling specifically accounts for deficits in vH-PreL theta-band network coherence, ensemble assembly, and learning functions of Fmr1y/- mice. Therefore, vH-PreL axis function exhibits a selective vulnerability to loss of FMRP in the vH or PreL, leading to learning and memory dysfunctions in fragile X mice.

Combinatorial expression of neurexin genes regulates glomerular targeting by olfactory sensory neurons

Precise connectivity between specific neurons is essential for the formation of the complex neural circuitry necessary for executing intricate motor behaviors and higher cognitive functions. While trans -interactions between synaptic membrane proteins have emerged as crucial elements in orchestrating the assembly of these neural circuits, the synaptic surface proteins involved in neuronal wiring remain largely unknown. Here, using unbiased single-cell transcriptomic and mouse genetic approaches, we uncover that the neurexin family of genes enables olfactory sensory neuron (OSNs) axons to form appropriate synaptic connections with their mitral and tufted (M/T) cell synaptic partners, within the mammalian olfactory system. Neurexin isoforms are differentially expressed within distinct populations of OSNs, resulting in unique pattern of neurexin expression that is specific to each OSN type, and synergistically cooperate to regulate axonal innervation, guiding OSN axons to their designated glomeruli. This process is facilitated through the interactions of neurexins with their postsynaptic partners, including neuroligins, which have distinct expression patterns in M/T cells. Our findings suggest a novel mechanism underpinning the precise assembly of olfactory neural circuits, driven by the trans -interaction between neurexins and their ligands.

Vulnerability of the Hippocampus to Insults: Links to Blood-Brain Barrier Dysfunction

The hippocampus is a critical brain substrate for learning and memory; events that harm the hippocampus can seriously impair mental and behavioral functioning. Hippocampal pathophysiologies have been identified as potential causes and effects of a remarkably diverse array of medical diseases, psychological disorders, and environmental sources of damage. It may be that the hippocampus is more vulnerable than other brain areas to insults that are related to these conditions. One purpose of this review is to assess the vulnerability of the hippocampus to the most prevalent types of insults in multiple biomedical domains (i.e., neuroactive pathogens, neurotoxins, neurological conditions, trauma, aging, neurodegenerative disease, acquired brain injury, mental health conditions, endocrine disorders, developmental disabilities, nutrition) and to evaluate whether these insults affect the hippocampus first and more prominently compared to other brain loci. A second purpose is to consider the role of hippocampal blood-brain barrier (BBB) breakdown in either causing or worsening the harmful effects of each insult. Recent research suggests that the hippocampal BBB is more fragile compared to other brain areas and may also be more prone to the disruption of the transport mechanisms that act to maintain the internal milieu. Moreover, a compromised BBB could be a factor that is common to many different types of insults. Our analysis indicates that the hippocampus is more vulnerable to insults compared to other parts of the brain, and that developing interventions that protect the hippocampal BBB may help to prevent or ameliorate the harmful effects of many insults on memory and cognition.

Gamma-band-based dynamic functional connectivity in pigeon entopallium during sample presentation in a delayed color matching task

Birds have developed visual cognitions, especially in discriminating colors due to their four types of cones in the retina. The entopallium of birds is thought to be involved in the processing of color information during visual cognition. However, there is a lack of understanding about how functional connectivity in the entopallium region of birds changes during color cognition, which is related to various input colors. We therefore trained pigeons to perform a delayed color matching task, in which two colors were randomly presented in sample stimuli phrases, and the neural activity at individual recording site and the gamma band functional connectivity among local population in entopallium during sample presentation were analyzed. Both gamma band energy and gamma band functional connectivity presented dynamics as the stimulus was presented and persisted. The response features in the early-stimulus phase were significantly different from those of baseline and the late-stimulus phase. Furthermore, gamma band energy showed significant differences between different colors during the early-stimulus phase, but the global feature of the gamma band functional network did not. Further decoding results showed that decoding accuracy was significantly enhanced by adding functional connectivity features, suggesting the global feature of the gamma band functional network did not directly contain color information, but was related to it. These results provided insight into information processing rules among local neuronal populations in the entopallium of birds during color cognition, which is important for their daily life.

Bidirectional Regulation of GABAA Reversal Potential in the Adult Brain: Physiological and Pathological Implications

In physiological conditions, the intracellular chloride concentration is much lower than the extracellular. As GABAA channels are permeable to anions, the reversal potential of GABAA is very close to that of Cl-, which is the most abundant free anion in the intra- and extracellular spaces. Intracellular chloride is regulated by the activity ratio of NKCC1 and KCC2, two chloride-cation cotransporters that import and export Cl-, respectively. Due to the closeness between GABAA reversal potential and the value of the resting membrane potential in most neurons, small changes in intracellular chloride have a major functional impact, which makes GABAA a uniquely flexible signaling system. In most neurons of the adult brain, the GABAA reversal potential is slightly more negative than the resting membrane potential, which makes GABAA hyperpolarizing. Alterations in GABAA reversal potential are a common feature in numerous conditions as they are the consequence of an imbalance in the NKCC1-KCC2 activity ratio. In most conditions (including Alzheimer's disease, schizophrenia, and Down's syndrome), GABAA becomes depolarizing, which causes network desynchronization and behavioral impairment. In other conditions (neonatal inflammation and neuropathic pain), however, GABAA reversal potential becomes hypernegative, which affects behavior through a potent circuit deactivation.

Deficits in neuronal architecture but not over-inhibition are main determinants of reduced neuronal network activity in a mouse model of overexpression of Dyrk1A

In this study, we investigated the impact of Dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) overexpression, a gene associated with Down syndrome, on hippocampal neuronal deficits in mice. Our findings revealed that mice overexpressing Dyrk1A (TgDyrk1A; TG) exhibited impaired hippocampal recognition memory, disrupted excitation-inhibition balance, and deficits in long-term potentiation (LTP). Specifically, we observed layer-specific deficits in dendritic arborization of TG CA1 pyramidal neurons in the stratum radiatum. Through computational modeling, we determined that these alterations resulted in reduced storage capacity and compromised integration of inputs, with decreased high γ oscillations. Contrary to prevailing assumptions, our model suggests that deficits in neuronal architecture, rather than over-inhibition, primarily contribute to the reduced network. We explored the potential of environmental enrichment (EE) as a therapeutic intervention and found that it normalized the excitation-inhibition balance, restored LTP, and improved short-term recognition memory. Interestingly, we observed transient significant dendritic remodeling, leading to recovered high γ. However, these effects were not sustained after EE discontinuation. Based on our findings, we conclude that Dyrk1A overexpression-induced layer-specific neuromorphological disturbances impair the encoding of place and temporal context. These findings contribute to our understanding of the underlying mechanisms of Dyrk1A-related hippocampal deficits and highlight the challenges associated with long-term therapeutic interventions for cognitive impairments.

Rescue of sharp wave-ripples and prevention of network hyperexcitability in the ventral but not the dorsal hippocampus of a rat model of fragile X syndrome

Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by intellectual disability and is related to autism. FXS is caused by mutations of the fragile X messenger ribonucleoprotein 1 gene (Fmr1) and is associated with alterations in neuronal network excitability in several brain areas including hippocampus. The loss of fragile X protein affects brain oscillations, however, the effects of FXS on hippocampal sharp wave-ripples (SWRs), an endogenous hippocampal pattern contributing to memory consolidation have not been sufficiently clarified. In addition, it is still not known whether dorsal and ventral hippocampus are similarly affected by FXS. We used a Fmr1 knock-out (KO) rat model of FXS and electrophysiological recordings from the CA1 area of adult rat hippocampal slices to assess spontaneous and evoked neural activity. We find that SWRs and associated multiunit activity are affected in the dorsal but not the ventral KO hippocampus, while complex spike bursts remain normal in both segments of the KO hippocampus. Local network excitability increases in the dorsal KO hippocampus. Furthermore, specifically in the ventral hippocampus of KO rats we found an increased effectiveness of inhibition in suppressing excitation and an upregulation of α1GABAA receptor subtype. These changes in the ventral KO hippocampus are accompanied by a striking reduction in its susceptibility to induced epileptiform activity. We propose that the neuronal network specifically in the ventral segment of the hippocampus is reorganized in adult Fmr1-KO rats by means of balanced changes between excitability and inhibition to ensure normal generation of SWRs and preventing at the same time derailment of the neural activity toward hyperexcitability.

Correlating MRI-based brain volumetry and cognitive assessment in people with Down syndrome

INTRODUCTION

Down syndrome (DS) is the most common genetic cause of intellectual disability. Children and adults with DS show deficits in language performance and explicit memory. Here, we used magnetic resonance imaging (MRI) on children and adults with DS to characterize changes in the volume of specific brain structures involved in memory and language and their relationship to features of cognitive-behavioral phenotypes.

METHODS

Thirteen children and adults with the DS phenotype and 12 age- and gender-matched healthy controls (age range 4-25) underwent an assessment by MRI and a psychological evaluation for language and cognitive abilities.

RESULTS

The cognitive profile of people with DS showed deficits in different cognition and language domains correlating with reduced volumes of specific regional and subregional brain structures, confirming previous related studies. Interestingly, in our study, people with DS also showed more significant parahippocampal gyrus volumes, in agreement with the results found in earlier reports.

CONCLUSIONS

The memory functions and language skills affected in studied individuals with DS correlate significantly with the reduced volume of specific brain regions, allowing us to understand DS's cognitive-behavioral phenotype. Our results provide an essential basis for early intervention and the design of rehabilitation management protocols.

Altered intestinal barrier contributes to cognitive impairment in old mice with constipation after sevoflurane anesthesia

Background

Elderly patients have a high risk of developing postoperative cognitive dysfunction (POCD). Gastrointestinal disorders, such as constipation, in the elderly population may be involved in the pathogenesis of neurological disorders by promoting inflammatory responses due to a 'leaky gut'. General anesthetic sevoflurane may impair gastrointestinal function in elderly patients to trigger neurological complications following surgery. Therefore, we hypothesized that elderly individuals with gastrointestinal dysfunction may be more vulnerable to sevoflurane and consequently develop POCD.

Methods

Aged mice were randomly divided into four groups: control (CTRL), CTRL+sevoflurane (Sev), slow transit constipation (STC), and STC + Sev. Mice in the STC and STC + Sev groups were intra-gastrically administrated loperamide (3 mg/kg, twice a day for 7 days) to induce a slow transit constipation (STC) model determined with fecal water content and the time of first white fecal pellet, whereas mice in the other groups received the similar volume of saline. One week later, mice in the CTRL+Sev group and STC + Sev group received 2% sevoflurane for 2 h. The gut permeability evaluated with 4-kDa fluorescein isothiocyanate (FITC)-dextran, serum cytokines, microglia density, TLR4/NF-κB signaling expression, and POCD-like behavioral changes were determined accordingly.

Results

The loperamide-induced STC mice had decreased fecal water content and prolonged time of first white fecal pellet. Sevoflurane exposure caused significantly increased gut permeability and serum cytokines, as well as the activation of microglia and the TLR4/NF-κB signaling pathway in the prefrontal cortex of the aged STC mice. Sevoflurane also caused cognitive impairment and emotional phenotype abnormality in aged STC mice.

Conclusion

Aged STC mice were more vulnerable to sevoflurane anesthesia and consequently developed POCD-like behavioral changes. Our data suggest that gastrointestinal disorders including constipation may contribute to the development of POCD.

Perspectives on pain in Down syndrome

Down syndrome (DS) or trisomy 21 is a genetic condition often accompanied by chronic pain caused by congenital abnormalities and/or conditions, such as osteoarthritis, recurrent infections, and leukemia. Although DS patients are more susceptible to chronic pain as compared to the general population, the pain experience in these individuals may vary, attributed to the heterogenous structural and functional differences in the central nervous system, which might result in abnormal pain sensory information transduction, transmission, modulation, and perception. We tried to elaborate on some key questions and possible explanations in this review. Further clarification of the mechanisms underlying such abnormal conditions induced by the structural and functional differences is needed to help pain management in DS patients.

Insights into the potential benefits of triphala polyphenols toward the promotion of resilience against stress-induced depression and cognitive impairment

In response to environmental challenges, stress is a common reaction, but dysregulation of the stress response can lead to neuropsychiatric disorders, including depression and cognitive impairment. Particularly, there is ample evidence that overexposure to mental stress can have lasting detrimental consequences for psychological health, cognitive function, and ultimately well-being. In fact, some individuals are resilient to the same stressor. A major benefit of enhancing stress resilience in at-risk groups is that it may help prevent the onset of stress-induced mental health problems. A potential therapeutic strategy for maintaining a healthy life is to address stress-induced health problems with botanicals or dietary supplements such as polyphenols. Triphala, also known as Zhe Busong decoction in Tibetan, is a well-recognized Ayurvedic polyherbal medicine comprising dried fruits from three different plant species. As a promising food-sourced phytotherapy, triphala polyphenols have been used throughout history to treat a variety of medical conditions, including brain health maintenance. Nevertheless, a comprehensive review is still lacking. Here, the primary objective of this review article is to provide an overview of the classification, safety, and pharmacokinetics of triphala polyphenols, as well as recommendations for the development of triphala polyphenols as a novel therapeutic strategy for promoting resilience in susceptible individuals. Additionally, we summarize recent advances demonstrating that triphala polyphenols are beneficial to cognitive and psychological resilience by regulating 5-hydroxytryptamine (5-HT) and brain-derived neurotrophic factor (BDNF) receptors, gut microbiota, and antioxidant-related signaling pathways. Overall, scientific exploration of triphala polyphenols is warranted to understand their therapeutic efficacy. In addition to providing novel insights into the mechanisms of triphala polyphenols for promoting stress resilience, blood brain barrier (BBB) permeability and systemic bioavailability of triphala polyphenols also need to be improved by the research community. Moreover, well-designed clinical trials are needed to increase the scientific validity of triphala polyphenols' beneficial effects for preventing and treating cognitive impairment and psychological dysfunction.

Deficits in neuronal architecture but not over-inhibition are main determinants of reduced neuronal network activity in a mouse model of overexpression of Dyrk1A

Abnormal dendritic arbors, dendritic spine "dysgenesis" and excitation inhibition imbalance are main traits assumed to underlie impaired cognition and behavioral adaptation in intellectual disability. However, how these modifications actually contribute to functional properties of neuronal networks, such as signal integration or storage capacity is unknown. Here, we used a mouse model overexpressing Dyrk1A (Dual-specificity tyrosine [Y]-regulated kinase), one of the most relevant Down syndrome (DS) candidate genes, to gather quantitative data regarding hippocampal neuronal deficits produced by the overexpression of Dyrk1A in mice (TgDyrk1A; TG). TG mice showed impaired hippocampal recognition memory, altered excitation-inhibition balance and deficits in hippocampal CA1 LTP. We also detected for the first time that deficits in dendritic arborization in TG CA1 pyramidal neurons are layer-specific, with a reduction in the width of the stratum radiatum, the postsynaptic target site of CA3 excitatory neurons, but not in the stratum lacunosum-moleculare, which receives temporo-ammonic projections. To interrogate about the functional impact of layer-specific TG dendritic deficits we developed tailored computational multicompartmental models. Computational modelling revealed that neuronal microarchitecture alterations in TG mice lead to deficits in storage capacity, altered the integration of inputs from entorhinal cortex and hippocampal CA3 region onto CA1 pyramidal cells, important for coding place and temporal context and on connectivity and activity dynamics, with impaired the ability to reach high γ oscillations. Contrary to what is assumed in the field, the reduced network activity in TG is mainly contributed by the deficits in neuronal architecture and to a lesser extent by over-inhibition. Finally, given that therapies aimed at improving cognition have also been tested for their capability to recover dendritic spine deficits and excitation-inhibition imbalance, we also tested the short- and long-term changes produced by exposure to environmental enrichment (EE). Exposure to EE normalized the excitation inhibition imbalance and LTP, and had beneficial effects on short-term recognition memory. Importantly, it produced massive but transient dendritic remodeling of hippocampal CA1, that led to recovery of high γ oscillations, the main readout of synchronization of CA1 neurons, in our simulations. However, those effects where not stable and were lost after EE discontinuation. We conclude that layer-specific neuromorphological disturbances produced by Dyrk1A overexpression impair coding place and temporal context. Our results also suggest that treatments targeting structural plasticity, such as EE, even though hold promise towards improved treatment of intellectual disabilities, only produce temporary recovery, due to transient dendritic remodeling.

Orbitofrontal cortex-hippocampus potentiation mediates relief for depression: A randomized double-blind trial and TMS-EEG study

It has been 15 years since repetitive transcranial magnetic stimulation (rTMS) targeting the dorsolateral prefrontal cortex (DLPFC) was approved by the FDA for clinical depression treatment. Yet, the underlying mechanisms for rTMS-induced depression relief are not fully elucidated. This study analyzes TMS-electroencephalogram (EEG) data from 64 healthy control (HC) subjects and 53 patients with major depressive disorder (MDD) before and after rTMS treatment. Prior to treatment, patients with MDD have lower activity in the DLPFC, the hippocampus (HPC), the orbitofrontal cortex (OFC), and DLPFC-OFC connectivity compared with HCs. Following active rTMS treatment, patients with MDD show a significant increase in the DLPFC, HPC, and OFC. Notably, the increase in HPC activity is specifically associated with amelioration of depressive symptoms but not anxiety or sleep quality. The orbitofrontal-hippocampal pathway plays a crucial role in mediating depression relief following rTMS treatment. These findings suggest potential alternative targets for brain stimulation therapy against depression (chictr.org.cn: ChiCTR2100052007).

Toward the Identification of Neurophysiological Biomarkers for Alzheimer's Disease in Down Syndrome: A Potential Role for Cross-Frequency Phase-Amplitude Coupling Analysis

Cross-frequency coupling (CFC) mechanisms play a central role in brain activity. Pathophysiological mechanisms leading to many brain disorders, such as Alzheimer's disease (AD), may produce unique patterns of brain activity detectable by electroencephalography (EEG). Identifying biomarkers for AD diagnosis is also an ambition among research teams working in Down syndrome (DS), given the increased susceptibility of people with DS to develop early-onset AD (DS-AD). Here, we review accumulating evidence that altered theta-gamma phase-amplitude coupling (PAC) may be one of the earliest EEG signatures of AD, and therefore may serve as an adjuvant tool for detecting cognitive decline in DS-AD. We suggest that this field of research could potentially provide clues to the biophysical mechanisms underlying cognitive dysfunction in DS-AD and generate opportunities for identifying EEG-based biomarkers with diagnostic and prognostic utility in DS-AD.

Innovating Therapies for Down Syndrome: An International Virtual Conference of the T21 Research Society

Research focused on Down syndrome continued to gain momentum in the last several years and is advancing our understanding of how trisomy 21 (T21) modifies molecular and cellular processes. The Trisomy 21 Research Society (T21RS) is the premier scientific organization for researchers and clinicians studying Down syndrome. During the COVID pandemic, T21RS held its first virtual conference program, sponsored by the University of California at Irvine, on June 8-10, 2021 and brought together 342 scientists, families, and industry representatives from over 25 countries to share the latest discoveries on underlying cellular and molecular mechanisms of T21, cognitive and behavioral changes, and comorbidities associated with Down syndrome, including Alzheimer's disease and Regression Disorder. Presentations of 91 cutting-edge abstracts reflecting neuroscience, neurology, model systems, psychology, biomarkers, and molecular and pharmacological therapeutic approaches demonstrate the compelling interest and continuing advancement toward innovating biomarkers and therapies aimed at ameliorating health conditions associated with T21.

Integrative analyses of transcriptome and metabolome reveal comprehensive mechanisms of Epigallocatechin-3-gallate (EGCG) biosynthesis in response to ecological factors in tea plant (Camellia sinensis)

Epigallocatechin-3-gallate (EGCG), a flavoured and healthy compounds in tea, is affected by the ecological factors. However, the biosynthetic mechanisms of EGCG in response to the ecological factors remian unclear. In this study, a response surface method with a Box-Behnken design was used to investigate the relationship between EGCG accumulation and ecological factors; further, integrative transcriptome and metabolome analyses were performed to explore the mechanism underlying EGCG biosynthesis in response to environmental factors. The optimal environmental conditions obtained for EGCG biosynthesis were as follows: 28℃, 70 % relative humidity of the substrate, and 280 µmol·m^-2·s^-1 light intensity; the EGCG content was increased by 86.83 % compared to the control (CK₁). Meanwhile, the order of EGCG content in response to the interaction of ecological factors was as follows: interaction of temperature and light intensity > interaction of temperature and relative humidity of the substrate > interaction of light intensity and relative humidity of the substrate, indicating that temperature was the dominant ecological factors. EGCG biosynthesis in tea plants was found to be comprehensively regulated by a series of structural genes (CsANS, CsF3H, CsCHI, CsCHS, and CsaroDE), miRNAs (miR164, miR396d, miR5264, miR166a, miR171d, miR529, miR396a, miR169, miR7814, miR3444b, and miR5240), and transcription factors (MYB93, NAC2, NAC6, NAC43, WRK24, bHLH30, and WRK70); further, the metabolic flux was regulated and converted from phenolic acid to the flavonoid biosynthesis pathway based on accelerated consumption of phosphoenolpyruvic acid, d-erythrose-4-phosphate, and l-phenylalanine in response to ambient changes in temperature and light intensity. Overall, the results of this study reveal the effect of ecological factors on EGCG biosynthesis in tea plants, providing novel insights for improving tea quality.

Neural substrates of cognitive impairment in a NMDAR hypofunction mouse model of schizophrenia and partial rescue by risperidone

N-methyl D-aspartate receptor (NMDAR) hypofunction is a pathophysiological mechanism relevant for schizophrenia. Acute administration of the NMDAR antagonist phencyclidine (PCP) induces psychosis in patients and animals while subchronic PCP (sPCP) produces cognitive dysfunction for weeks. We investigated the neural correlates of memory and auditory impairments in mice treated with sPCP and the rescuing abilities of the atypical antipsychotic drug risperidone administered daily for two weeks. We recorded neural activities in the medial prefrontal cortex (mPFC) and the dorsal hippocampus (dHPC) during memory acquisition, short-term, and long-term memory in the novel object recognition test and during auditory processing and mismatch negativity (MMN) and examined the effects of sPCP and sPCP followed by risperidone. We found that the information about the familiar object and its short-term storage were associated with mPFC→dHPC high gamma connectivity (phase slope index) whereas long-term memory retrieval depended on dHPC→mPFC theta connectivity. sPCP impaired short-term and long-term memories, which were associated with increased theta power in the mPFC, decreased gamma power and theta-gamma coupling in the dHPC, and disrupted mPFC-dHPC connectivity. Risperidone rescued the memory deficits and partly restored hippocampal desynchronization but did not ameliorate mPFC and circuit connectivity alterations. sPCP also impaired auditory processing and its neural correlates (evoked potentials and MMN) in the mPFC, which were also partly rescued by risperidone. Our study suggests that the mPFC and the dHPC disconnect during NMDAR hypofunction, possibly underlying cognitive impairment in schizophrenia, and that risperidone targets this circuit to ameliorate cognitive abilities in patients.

Cognitive impairments in a Down syndrome model with abnormal hippocampal and prefrontal dynamics and cytoarchitecture

The Dp(10)2Yey mouse carries a ∼2.3-Mb intra-chromosomal duplication of mouse chromosome 10 (Mmu10) that has homology to human chromosome 21, making it an essential model for aspects of Down syndrome (DS, trisomy 21). In this study, we investigated neuronal dysfunction in the Dp(10)2Yey mouse and report spatial memory impairment and anxiety-like behavior alongside altered neural activity in the medial prefrontal cortex (mPFC) and hippocampus (HPC). Specifically, Dp(10)2Yey mice showed impaired spatial alternation associated with increased sharp-wave ripple activity in mPFC during a period of memory consolidation, and reduced mobility in a novel environment accompanied by reduced theta-gamma phase-amplitude coupling in HPC. Finally, we found alterations in the number of interneuron subtypes in mPFC and HPC that may contribute to the observed phenotypes and highlight potential approaches to ameliorate the effects of human trisomy 21.

Alu-minating the Mechanisms Underlying Primate Cortex Evolution

The higher-order cognitive functions observed in primates correlate with the evolutionary enhancement of cortical volume and folding, which in turn are driven by the primate-specific expansion of cellular diversity in the developing cortex. Underlying these changes is the diversification of molecular features including the creation of human and/or primate-specific genes, the activation of specific molecular pathways, and the interplay of diverse layers of gene regulation. We review and discuss evidence for connections between Alu elements and primate brain evolution, the evolutionary milestones of which are known to coincide along primate lineages. Alus are repetitive elements that contribute extensively to the acquisition of novel genes and the expansion of diverse gene regulatory layers, including enhancers, alternative splicing, RNA editing, and microRNA pathways. By reviewing the impact of Alus on molecular features linked to cortical expansions or gyrification or implications in cognitive deficits, we suggest that future research focusing on the role of Alu-derived molecular events in the context of brain development may greatly advance our understanding of higher-order cognitive functions and neurologic disorders.

RNF220 is an E3 ubiquitin ligase for AMPA receptors to regulate synaptic transmission

The accurate expression of postsynaptic AMPA receptors (AMPARs) is critical for information processing in the brain, and ubiquitination is a key regulator for this biological process. However, the roles of E3 ubiquitin ligases in the regulation of AMPARs are poorly understood. Here, we find that RNF220 directly interacts with AMPARs to meditate their polyubiquitination, and RNF220 knockout specifically increases AMPAR protein levels, thereby enhancing basal synaptic activity while impairing synaptic plasticity. Moreover, depending on its E3 ubiquitin ligase activity, RNF220 represses AMPAR-mediated excitatory synaptic responses and their neuronal surface expression. Furthermore, learning and memory are altered in forebrain RNF220-deficient mice. In addition, two neuropathology-related RNF220 variants fail to repress excitatory synaptic activity because of the incapability to regulate AMPAR ubiquitination due to their attenuated interaction. Together, we identify RNF220 as an E3 ubiquitin ligase for AMPARs and establish its substantial role in excitatory synaptic transmission and brain function.

The Role of Aberrant Neural Oscillations in the Hippocampal-Medial Prefrontal Cortex Circuit in Neurodevelopmental and Neurological Disorders

The hippocampus (HPC) and medial prefrontal cortex (mPFC) have well-established roles in cognition, emotion, and sensory processing. In recent years, interests have shifted towards developing a deeper understanding of the mechanisms underlying interactions between the HPC and mPFC in achieving these functions. Considerable research supports the idea that synchronized activity between the HPC and the mPFC is a general mechanism by which brain functions are regulated. In this review, we summarize current knowledge on the hippocampal-medial prefrontal cortex (HPC-mPFC) circuit in normal brain function with a focus on oscillations and highlight several neurodevelopmental and neurological disorders associated with aberrant HPC-mPFC circuitry. We further discuss oscillatory dynamics across the HPC-mPFC circuit as potentially useful biomarkers to assess interventions for neurodevelopmental and neurological disorders. Finally, advancements in brain stimulation, gene therapy and pharmacotherapy are explored as promising therapies for disorders with aberrant HPC-mPFC circuit dynamics.

Reversible Photocontrol of Dopaminergic Transmission in Wild-Type Animals

Understanding the dopaminergic system is a priority in neurobiology and neuropharmacology. Dopamine receptors are involved in the modulation of fundamental physiological functions, and dysregulation of dopaminergic transmission is associated with major neurological disorders. However, the available tools to dissect the endogenous dopaminergic circuits have limited specificity, reversibility, resolution, or require genetic manipulation. Here, we introduce azodopa, a novel photoswitchable ligand that enables reversible spatiotemporal control of dopaminergic transmission. We demonstrate that azodopa activates D₁-like receptors in vitro in a light-dependent manner. Moreover, it enables reversibly photocontrolling zebrafish motility on a timescale of seconds and allows separating the retinal component of dopaminergic neurotransmission. Azodopa increases the overall neural activity in the cortex of anesthetized mice and displays illumination-dependent activity in individual cells. Azodopa is the first photoswitchable dopamine agonist with demonstrated efficacy in wild-type animals and opens the way to remotely controlling dopaminergic neurotransmission for fundamental and therapeutic purposes.

Postnatal environmental enrichment enhances memory through distinct neural mechanisms in healthy and trisomic female mice

Stimulating lifestyles have powerful effects on cognitive abilities, especially when they are experienced early in life. Cognitive therapies are widely used to improve cognitive impairment due to intellectual disability, aging, and neurodegeneration, however the underlying neural mechanisms are poorly understood. We investigated the neural correlates of memory amelioration produced by postnatal environmental enrichment (EE) in diploid mice and the Ts65Dn mouse model of Down syndrome (trisomy 21). We recorded neural activities in brain structures key for memory processing, the hippocampus and the prefrontal cortex, during rest, sleep and memory performance in mice reared in non-enriched or enriched environments. Enriched wild-type animals exhibited enhanced neural synchrony in the hippocampus across different brain states (increased gamma oscillations, theta-gamma coupling, sleep ripples). Trisomic females showed increased theta and gamma rhythms in the hippocampus and prefrontal cortex across different brain states along with enlarged ripples and disrupted circuit gamma signals that were associated with memory deficits. These pathological activities were attenuated in their trisomic EE-reared peers. Our results suggest distinct neural mechanisms for the generation and rescue of healthy and pathological brain synchrony, respectively, by EE and put forward hippocampal-prefrontal hypersynchrony and miscommunication as major targets underlying the beneficial effects of EE in intellectual disability.

Epigallocatechin gallate is a potent inhibitor of cystathionine beta-synthase: Structure-activity relationship and mechanism of action

Epigallocatechin gallate (EGCG) is the main bioactive component of green tea. Through screening of a small library of natural compounds, we discovered that EGCG inhibits cystathionine β-synthase (CBS), a major H₂S-generating enzyme. Here we characterize EGCG's mechanism of action in the context of CBS-derived H₂S production. In the current project, biochemical, pharmacological and cell biology approaches were used to characterize the effect of EGCG on CBS in cellular models of cancer and Down syndrome (DS). The results show that EGCG binds to CBS and inhibits H₂S-producing CBS activity almost 30-times more efficiently than the canonical cystathionine formation (IC₅₀ 0.12 versus 3.3 μM). Through screening structural analogs and building blocks, we identified that gallate moiety of EGCG represents the pharmacophore responsible for CBS inhibition. EGCG is a mixed-mode, CBS-specific inhibitor with no effect on the other two major enzymatic sources of H₂S, CSE and 3-MST. Unlike the prototypical CBS inhibitor aminooxyacetate, EGCG does not bind the catalytic cofactor of CBS pyridoxal-5'-phosphate. Molecular modeling suggests that EGCG blocks a substrate access channel to pyridoxal-5'-phosphate. EGCG inhibits cellular H₂S production in HCT-116 colon cancer cells and in DS fibroblasts. It also exerts effects that are consistent with the functional role of CBS in these cells: in HCT-116 cells it decreases, while in DS cells it improves viability and proliferation. In conclusion, EGCG is a potent inhibitor of CBS-derived H₂S production. This effect may contribute to its pharmacological effects in various pathophysiological conditions.

Hippocampal-medial prefrontal cortex network dynamics predict performance during retrieval in a context-guided object memory task

SignificanceRecovering relevant information, while ignoring the irrelevant, is crucial for episodic memory (remembering a particular event at a specific temporal and spatial context). Information presented at any time could drive the retrieval of more than one memory trace; thus, there should be a mechanism to select the retrieval of the most relevant trace. However, how the brain controls memory interference is not well understood. Here, we analyzed the communication between ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC) during the resolution of an episodic memory task in rats. We found an increased synchronization between the vHPC and mPFC and identified specific mPFC neural subpopulations that selectively respond to object-context associations, and their firing preference correlates with the animals' behavioral responses.

Temporal and brain region-specific elevations of soluble Amyloid-β40-42 in the Ts65Dn mouse model of Down syndrome and Alzheimer's disease

Down syndrome (DS) is a leading cause of intellectual disability that also results in hallmark Alzheimer's disease (AD) pathologies such as amyloid beta (Aβ) plaques and hyperphosphorylated tau. The Ts65Dn mouse model is commonly used to study DS, as trisomic Ts65Dn mice carry 2/3 of the triplicated gene homologues as occur in human DS. The Ts65Dn strain also allows investigation of mechanisms common to DS and AD pathology, with many of these triplicated genes implicated in AD; for example, trisomic Ts65Dn mice overproduce amyloid precursor protein (APP), which is then processed into soluble Aβ_40-42 fragments. Notably, Ts65Dn mice show alterations to the basal forebrain, which parallels the loss of function in this region observed in DS and AD patients early on in disease progression. However, a complete picture of soluble Aβ_40-42 accumulation in a region-, age-, and sex-specific manner has not yet been characterized in the Ts65Dn model. Here, we show that trisomic mice accumulate soluble Aβ_40-42 in the basal forebrain, frontal cortex, hippocampus, and cerebellum in an age-specific manner, with elevation in the frontal cortex and hippocampus as early as 4 months of age. Furthermore, we detected sex differences in accumulation of Aβ_40-42 within the basal forebrain, with females having significantly higher Aβ_40-42 at 7-8 months of age. Lastly, we show that APP expression in the basal forebrain and hippocampus inversely correlates with Aβ_40-42  levels. This spatial and temporal characterization of soluble Aβ_40-42 in the Ts65Dn model allows for further exploration of the role soluble Aβ plays in the progression of other AD-like pathologies in these key brain regions.

How sleep disturbances affect internet gaming disorder: The mediating effect of hippocampal functional connectivity

BACKGROUND

Studies have revealed that sleep disturbances lead to an increased risk of Internet gaming disorder (IGD). However, the neural underpinnings of this feature remain unknown. Exploring this issue would be valuable in understanding the relationship between sleep and psychiatric disorders.

METHODS

Given the impact of sleep on reward circuitry, we examined nucleus accumbens (NAcc) and hippocampal resting-state functional connectivity (rsFC) differences between 41 IGD subjects and 59 healthy controls. Significant connections were determined and used to examine correlations with clinical variables. Finally, we explored the relationship between neuroimaging findings, IGD severity and sleep disturbances through a mediation model.

RESULTS

We observed the connection deviation between the hippocampus and a wide range of cerebral cortexes in IGD subjects, including the prefrontal, parietal and temporal lobes. More importantly, the right posterior hippocampus (pHIP)-left caudate rsFC was positively correlated with both the Pittsburgh Sleep Quality Index (PSQI) and Internet Addiction Test scores and mediated the relationship between the two. For the NAcc, a difference between groups was only observed in the rsFC between the shell partition of the NAcc and the inferior orbitofrontal cortex, but this connectivity was not related to the PSQI score.

CONCLUSIONS

IGD subjects showed a wide range of abnormal connections in the hippocampus, involving memory, reward motivation, and cognitive control. Here we emphasized the potential of the hippocampus in studying sleep disturbances in IGD, especially the coupling between the pHIP and caudate nucleus, which could provide novel insight into how sleep interacts with motivational systems in IGD subjects.

Differential expression of the neuronal CB1 cannabinoid receptor in the hippocampus of male Ts65Dn Down syndrome mouse model

Down syndrome (DS) or Trisomy 21 is the most common genetic cause of mental retardation with severe learning and memory deficits. DS is due to the complete or partial triplication of human chromosome 21 (HSA21) triggering gene overexpression and protein synthesis alterations responsible for a plethora of mental and physical phenotypes. Among the diverse brain target systems that affect hippocampal-dependent learning and memory deficit impairments in DS, the upregulation of the endocannabinoid system (ECS), and notably the overexpression of the cannabinoid type-1 receptor (CB1), seems to play a major role. Combining various protein and gene expression targeted approaches using western blot, qRT-PCR and FISH techniques, we investigated the expression pattern of ECS components in the hippocampus (HPC) of male Ts65Dn mice. Among all the molecules that constitute the ECS, we found that the expression of the CB1 is altered in the HPC of Ts65Dn mice. CB1 distribution is differentially segregated between the dorsal and ventral part of the HPC and within the different cell populations that compose the HPC. CB1 expression is upregulated in GABAergic neurons of Ts65Dn mice whereas it is downregulated in glutamatergic neurons. These results highlight a complex regulation of the CB1 encoding gene (Cnr1) in Ts65Dn mice that could open new therapeutic solutions for this syndrome.

Lithium therapy subdues neuroinflammation to maintain pyramidal cells arborization and rescues neurobehavioural impairments in ovariectomized rats

Oestrogen deprivation as a consequence of menopause alters the brain neuronal circuit and results in the development of neurobehavioural symptoms later. Hormone replacement therapy to some extent helps to overcome these abnormalities but is associated with various adverse events. Lithium therapy is being used to manage multiple neuropsychiatric disorders and is reported to maintain structural synaptic plasticity, suppress neuroinflammation, and promote adult neurogenesis. The present study examined the effect of lithium treatment on the neurobehavioural impairments in ovariectomized rat model mimicking clinical postmenopausal condition. A protective effect of lithium treatment was observed on the reconsolidation of spatial and recognition memory along with depression-like behaviour in ovariectomized rats. The Golgi-Cox staining revealed increased dendritic length and spine density in the pyramidal neurons of the CA1 region of the hippocampus, layer V of the somatosensory cortex, and layer II/III of the prefrontal cortex in the treated group. A significant reduction in pro-inflammatory markers, Il2, II6, and Il1b, was observed in the hippocampus, somatosensory cortex, and prefrontal cortex following lithium treatment. mRNA expression studies of Gfap and Pparg, along with histopathological analysis, suggested reactive astrogliosis to be a major contributor of neuroinflammation in ovariectomized rats that was normalized following lithium treatment. Further, the treatment inhibited Gsk-3β activity and maintained the normal level of β-catenin, CREB, and BDNF. The results revealed a defensive role of lithium against ovariectomy-induced neurobehavioural impairments, thus suggesting it to be a potential therapeutic agent for managing postmenopausal neurological symptoms.

Atypical, but Not Typical, Antipsychotic Drugs Reduce Hypersynchronized Prefrontal-Hippocampal Circuits during Psychosis-Like States in Mice: Contribution of 5-HT2A and 5-HT1A Receptors

Neural synchrony and functional connectivity are disrupted in schizophrenia. We investigated changes in prefrontal-hippocampal neural dynamics during psychosis-like states induced by the NMDAR antagonist phencyclidine and subsequent rescue by two atypical antipsychotic drugs (AAPDs), risperidone and clozapine, and the classical APD haloperidol. The psychotomimetic effects of phencyclidine were associated with prefrontal hypersynchronization, hippocampal desynchronization, and disrupted circuit connectivity. Phencyclidine boosted prefrontal oscillatory power at atypical bands within delta, gamma, and high frequency ranges, while irregular cross-frequency and spike-LFP coupling emerged. In the hippocampus, phencyclidine enhanced delta rhythms but suppressed theta oscillations, theta-gamma coupling, and theta-beta spike-LFP coupling. Baseline interregional theta-gamma coupling, theta phase coherence, and hippocampus-to-cortex theta signals were redirected to delta frequencies. Risperidone and clozapine, but not haloperidol, reduced phencyclidine-induced prefrontal and cortical-hippocampal hypersynchrony. None of the substances restored hippocampal and circuit desynchronization. These results suggest that AAPDs, but not typical APDs, target prefrontal-hippocampal pathways to elicit antipsychotic action. We investigated whether the affinity of AAPDs for serotonin receptors could explain their distinct effects. Serotonin 5-HT2AR antagonism by M100907 and 5-HT1AR agonism by 8-OH-DPAT reduced prefrontal hypersynchronization. Our results point to fundamentally different neural mechanisms underlying the action of atypical versus typical APDs with selective contribution of serotonin receptors.

Social Factors Influence Behavior in the Novel Object Recognition Task in a Mouse Model of Down Syndrome

The use of mouse models has revolutionized the field of Down syndrome (DS), increasing our knowledge about neuropathology and helping to propose new therapies for cognitive impairment. However, concerns about the reproducibility of results in mice and their translatability to humans have become a major issue, and controlling for moderators of behavior is essential. Social and environmental factors, the experience of the researcher, and the sex and strain of the animals can all have effects on behavior, and their impact on DS mouse models has not been explored. Here we analyzed the influence of a number of social and environmental factors, usually not taken into consideration, on the behavior of male and female wild-type and trisomic mice (the Ts65Dn model) in one of the most used tests for proving drug effects on memory, the novel object recognition (NOR) test. Using principal component analysis and correlation matrices, we show that the ratio of trisomic mice in the cage, the experience of the experimenter, and the timing of the test have a differential impact on male and female and on wild-type and trisomic behavior. We conclude that although the NOR test is quite robust and less susceptible to environmental influences than expected, to obtain useful results, the phenotype expression must be contrasted against the influences of social and environmental factors.

Macro- and Microscale Stress-Associated Alterations in Brain Structure: Translational Link With Depression

Major depressive disorder (MDD) is a stress-related disorder associated with many cytoarchitectural and neurochemical changes. However, the majority of these changes cannot be reliably detected in the living brain. The examination of animal stress models and postmortem human brain tissue has significantly contributed to our understanding of the pathophysiology of MDD. Ronald Duman's work in humans and in rodent models was critical to the investigation of the contribution of synaptic deficits to MDD and chronic stress pathology, their role in the development and expression of depressive-like behavior, and reversal by novel drugs. Here, we review evidence from magnetic resonance imaging in humans and animals that suggests that corticolimbic alterations are associated with depression symptomatology. We also discuss evidence of cytoarchitectural alterations affecting neurons, astroglia, and synapses in MDD and highlight how similar changes are described in rodent chronic stress models and are linked to the emotion-related behavioral deficits. Finally, we report on the latest approaches developed to measure the synaptic and astroglial alterations in vivo, using positron emission tomography, and how it can inform on the contribution of MDD-associated cytoarchitectural alterations to the symptomatology and the treatment of stress-related disorders.

Enhanced Hippocampus-Nidopallium Caudolaterale Connectivity during Route Formation in Goal-Directed Spatial Learning of Pigeons

Goal-directed spatial learning is crucial for the survival of animals, in which the formation of the route from the current location to the goal is one of the central problems. A distributed brain network comprising the hippocampus and prefrontal cortex has been shown to support such capacity, yet it is not fully understood how the most similar brain regions in birds, the hippocampus (Hp) and nidopallium caudolaterale (NCL), cooperate during route formation in goal-directed spatial learning. Hence, we examined neural activity in the Hp-NCL network of pigeons and explored the connectivity dynamics during route formation in a goal-directed spatial task. We found that behavioral changes in spatial learning during route formation are accompanied by modifications in neural patterns in the Hp-NCL network. Specifically, as pigeons learned to solve the task, the spectral power in both regions gradually decreased. Meanwhile, elevated hippocampal theta (5 to 12 Hz) connectivity and depressed connectivity in NCL were also observed. Lastly, the interregional functional connectivity was found to increase with learning, specifically in the theta frequency band during route formation. These results provide insight into the dynamics of the Hp-NCL network during spatial learning, serving to reveal the potential mechanism of avian spatial navigation.

Atypical electrophysiological and behavioral responses to diazepam in a leading mouse model of Down syndrome

Mounting evidence implicates dysfunctional GABA_AR-mediated neurotransmission as one of the underlying causes of learning and memory deficits observed in the Ts65Dn mouse model of Down syndrome (DS). The specific origin and nature of such dysfunction is still under investigation, which is an issue with practical consequences to preclinical and clinical research, as well as to the care of individuals with DS and anxiety disorder or those experiencing seizures in emergency room settings. Here, we investigated the effects of GABA_AR positive allosteric modulation (PAM) by diazepam on brain activity, synaptic plasticity, and behavior in Ts65Dn mice. We found Ts65Dn mice to be less sensitive to diazepam, as assessed by electroencephalography, long-term potentiation, and elevated plus-maze. Still, diazepam pre-treatment displayed typical effectiveness in reducing susceptibility and severity to picrotoxin-induced seizures in Ts65Dn mice. These findings fill an important gap in the understanding of GABAergic function in a key model of DS.

High resolution structural and functional MRI of the hippocampus in young adults with Down syndrome

Down syndrome is the phenotypic consequence of trisomy 21, with clinical presentation including both neurodevelopmental and neurodegenerative components. Although the intellectual disability typically displayed by individuals with Down syndrome is generally global, it also involves disproportionate deficits in hippocampally-mediated cognitive processes. Hippocampal dysfunction may also relate to Alzheimer’s disease-type pathology, which can appear in as early as the first decade of life and becomes universal by age 40. Using 7-tesla MRI of the brain, we present an assessment of the structure and function of the hippocampus in 34 individuals with Down syndrome (mean age 24.5 years ± 6.5) and 27 age- and sex-matched typically developing healthy controls. In addition to increased whole-brain mean cortical thickness and lateral ventricle volumes (P < 1.0 × 10⁻⁴), individuals with Down syndrome showed selective volume reductions in bilateral hippocampal subfields cornu Ammonis field 1, dentate gyrus, and tail (P < 0.005). In the group with Down syndrome, bilateral hippocampi showed widespread reductions in the strength of functional connectivity, predominately to frontal regions (P < 0.02). Age was not related to hippocampal volumes or functional connectivity measures in either group, but both groups showed similar relationships of age to whole-brain volume measures (P < 0.05). Finally, we performed an exploratory analysis of a subgroup of individuals with Down syndrome with both imaging and neuropsychological assessments. This analysis indicated that measures of spatial memory were related to mean cortical thickness, total grey matter volume and right hemisphere hippocampal subfield volumes (P < 0.02). This work provides a first demonstration of the usefulness of high-field MRI to detect subtle differences in structure and function of the hippocampus in individuals with Down syndrome, and suggests the potential for development of MRI-derived measures as surrogate markers of drug efficacy in pharmacological studies designed to investigate enhancement of cognitive function.

Prevention of cognitive decline in subjective cognitive decline APOE ε4 carriers after EGCG and a multimodal intervention (PENSA): Study design

INTRODUCTION

Subjects exhibiting subjective cognitive decline (SCD) are at an increased risk for mild cognitive impairment and dementia. Given the delay between risk exposure and disease onset, SCD individuals are increasingly considered a good target population for cost-effective lifestyle-based Alzheimer's disease prevention trials.

METHODS

The PENSA study is a randomized, double-blind, controlled clinical trial that aims to evaluate the efficacy of a personalized multimodal intervention in lifestyle (diet counseling, physical activity, cognitive training, and social engagement) combined with the use of epigallocatechin gallate (EGCG) over 12 months, in slowing down cognitive decline and improving brain connectivity. The study population includes 200 individuals meeting SCD criteria and carrying the apolipoprotein E ε4 allele, who will be randomized into four treatment arms (multimodal intervention + EGCG/placebo, or lifestyle recommendations + EGCG/placebo). The primary efficacy outcome is change in the composite score for cognitive performance measured with the Alzheimer's Disease Cooperative Study Preclinical Alzheimer Cognitive Composite (ADCS-PACC-like) adding to the original version the Interference score from the Stroop Color and Word Test and the Five Digit Test. Secondary efficacy outcomes are (1) change in functional magnetic resonance imaging (fMRI) and structural neuronal connectivity (structural MRI) and (2) the safety assessment of the EGCG compound. This study is framed within the WW-FINGERS consortium.

DISCUSSION

The use of new technologies (i.e., mobile ecological momentary assessments [EMAs], activity tracker) in the PENSA study allows the collection of continuous data on lifestyle behaviors (diet and physical activity) and mood, enabling a personalized design as well as an intensive follow-up of participants. These data will be used to give feedback to participants about their own performance along the intervention, promoting their involvement and adherence. The results of the study may aid researchers on the design of future clinical trials involving preventive lifestyle multicomponent interventions.

Role of trace amine‑associated receptor 1 in the medial prefrontal cortex in chronic social stress-induced cognitive deficits in mice

Emerging evidence supports an essential role of trace amine-associated receptor 1 (TAAR1) in neuropsychiatric disorders such as depression and schizophrenia. Stressful events are critical contributors to various neuropsychiatric disorders. This study examined the role of TAAR1 in mediating the negative outcomes of stressful events. In mice that experienced chronic social defeat stress but not acute stress, a significant reduction in the TAAR1 mRNA level was found in the medial prefrontal cortex (mPFC), a brain region that is known to be vulnerable to stress experience. Conditional TAAR1 knockout in the mPFC mimicked the cognitive deficits induced by chronic stress. In addition, chronic treatment with the selective TAAR1 partial agonist RO5263397 ameliorated chronic stress-induced changes in cognitive function, dendritic arborization, and the synapse number of pyramidal neurons in the mPFC but did not affect chronic stress-induced anxiety-like behaviors. Biochemically, chronic stress reduced the ratio of vesicular transporters of glutamate-1 (VGluT1) / vesicular GABA transporter (VGAT) in the mPFC，most prominently in the prelimbic cortex, and RO5263397 restored the excitatory-inhibitory (E/I) imbalance. Together, the results of this study reveal an essential role of TAAR1 in mediating chronic stress-induced cognitive impairments and suggest that TAAR1 agonists may be uniquely useful to treat MDD-related cognitive impairments.

Alterations of specific cortical GABAergic circuits underlie abnormal network activity in a mouse model of Down syndrome

Down syndrome (DS) results in various degrees of cognitive deficits. In DS mouse models, recovery of behavioral and neurophysiological deficits using GABA_AR antagonists led to hypothesize an excessive activity of inhibitory circuits in this condition. Nonetheless, whether over-inhibition is present in DS and whether this is due to specific alterations of distinct GABAergic circuits is unknown. In the prefrontal cortex of Ts65Dn mice (a well-established DS model), we found that the dendritic synaptic inhibitory loop formed by somatostatin-positive Martinotti cells (MCs) and pyramidal neurons (PNs) was strongly enhanced, with no alteration in their excitability. Conversely, perisomatic inhibition from parvalbumin-positive (PV) interneurons was unaltered, but PV cells of DS mice lost their classical fast-spiking phenotype and exhibited increased excitability. These microcircuit alterations resulted in reduced pyramidal-neuron firing and increased phase locking to cognitive-relevant network oscillations in vivo. These results define important synaptic and circuit mechanisms underlying cognitive dysfunctions in DS.

Down syndrome is a genetic disorder caused by the presence of a third copy of chromosome 21. Affected individuals show delayed growth, characteristic facial features, altered brain development; with mild to severe intellectual disability. The exact mechanisms underlying the intellectual disability in Down syndrome are unclear, although studies in mice have provided clues. Drugs that reduce the inhibitory activity in the brain improve cognition in a mouse model of Down syndrome. This suggests that excessive inhibitory activity may contribute to the cognitive impairments.

Many different neural circuits generate inhibitory activity in the brain. These circuits contain cells called interneurons. Sub-types of interneurons act via different mechanisms to reduce the activity of neurons. Identifying the interneurons that are affected in Down syndrome would thus improve our understanding of the brain basis of the disorder.

Zorrilla de San Martin et al. compared mice with Down syndrome to unaffected control mice. The results revealed an increased activity in two types of inhibitory brain circuits in Down syndrome. The first contains interneurons called Martinotti cells. These help the brain to combine inputs from different sources. The second contains interneurons called parvalbumin-positive basket cells. These help different areas of the brain to synchronize their activity, which in turn makes it easier for those areas to exchange information.

By mapping the changes in inhibitory circuits in Down syndrome, Zorrilla de San Martin et al. have provided new insights into the biological basis of the disorder. Future studies should examine whether targeting specific circuits with pharmacological treatments could ultimately help reduce the associated impairments.