Optogenetic activation of parvalbumin and somatostatin interneurons selectively restores theta-nested gamma oscillations and oscillation-induced spike timing-dependent long-term potentiation impaired by amyloid β oligomers

Co-activation of selective nicotinic acetylcholine receptor subtypes is required to reverse hippocampal network dysfunction and prevent fear memory loss in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia with no known cause and cure. Research suggests that a reduction of GABAergic inhibitory interneurons' activity in the hippocampus by beta-amyloid peptide (Aβ) is a crucial trigger for cognitive impairment in AD via hyperexcitability. Therefore, enhancing hippocampal inhibition is thought to be protective against AD. However, hippocampal inhibitory cells are highly diverse, and these distinct interneuron subtypes differentially regulate hippocampal inhibitory circuits and cognitive processes. Moreover, Aβ unlikely affects all subtypes of inhibitory interneurons in the hippocampus equally. Hence, identifying the affected interneuron subtypes in AD to enhance hippocampal inhibition optimally is conceptually and practically challenging. We have previously found that Aβ selectively binds to two of the three major hippocampal nicotinic acetylcholine receptor (nAChR) subtypes, α7- and α4β2-nAChRs, but not α3β4-nAChRs, and inhibits these two receptors in cultured hippocampal inhibitory interneurons to decrease their activity, leading to hyperexcitation and synaptic dysfunction in excitatory neurons. We have also revealed that co-activation of α7- and α4β2-nAChRs is required to reverse the Aβ-induced adverse effects in hippocampal excitatory neurons. Here, we discover that α7- and α4β2-nAChRs predominantly control the nicotinic cholinergic signaling and neuronal activity in hippocampal parvalbumin-positive (PV+) and somatostatin-positive (SST+) inhibitory interneurons, respectively. Furthermore, we reveal that co-activation of these receptors is necessary to reverse hippocampal network dysfunction and fear memory loss in the amyloid pathology model mice. We thus suggest that co-activation of PV+ and SST+ cells is a novel strategy to reverse hippocampal dysfunction and cognitive decline in AD.

Adult-born granule cells modulate CA2 network activity during retrieval of developmental memories of the mother

Adult-born granule cells (abGCs) project to the CA2 region of the hippocampus, but it remains unknown how this circuit affects behavioral function. Here, we show that abGC input to the CA2 of adult mice is involved in the retrieval of remote developmental memories of the mother. Ablation of abGCs impaired the ability to discriminate between a caregiving mother and a novel mother, and this ability returned after abGCs were regenerated. Chemogenetic inhibition of projections from abGCs to the CA2 also temporarily prevented the retrieval of remote mother memories. These findings were observed when abGCs were inhibited at 4-6 weeks old, but not when they were inhibited at 10-12 weeks old. We also found that abGCs are necessary for differentiating features of CA2 network activity, including theta-gamma coupling and sharp wave ripples, in response to novel versus familiar social stimuli. Taken together, these findings suggest that abGCs are necessary for neuronal oscillations associated with discriminating between social stimuli, thus enabling retrieval of remote developmental memories of the mother by their adult offspring.

Cacao consumption improves passive avoidance memory impairment in a rat model of Alzheimer's disease: the role of hippocampal synaptic plasticity and oxidative stress

Introduction: Alzheimer's disease (AD) causes progressive loss of cognitive function and synaptic plasticity, which is the most common form of dementia. The present study was designed to scrutinize the effects of cacao on passive avoidance memory function and to identify the roles of hippocampal synaptic plasticity and oxidative stress in an AD rat model induced by unilateral intracerebroventricular (UICV) injection of amyloid-beta (Aβ). Methods: Oral administration of cacao (500 mg/kg/ day) was given for 2 consecutive months. A memory retention test was conducted 24 h after passive avoidance training was completed. Subsequently, the amplitude of population spike (PS) and slope of field excitatory postsynaptic potentials (fEPSPs) were assessed at hippocampal long-term potentiation (LTP) in perforant pathway-dentate gyrus (PP-DG) synapses. Moreover, total thiol group (TTG) and malondialdehyde (MDA) concentrations were evaluated in the plasma. Furthermore, compact Aβ plaques were detected in the hippocampal DG by performing Congo red staining. Results: As a result of AD induction, passive avoidance memory was impaired; also, reduced fEPSP slopes, PS amplitudes, and content of TTG, and increase in MDA levels in the rats were observed. In contrast, cacao treatment ameliorated passive avoidance memory impairment, improved hippocampal LTP impairment, modulated oxidative-antioxidative status, and delayed Aβ plaques production in AD rats. Disscussion: Conclusively, cacao alleviates Aβ-induced cognitive deficit, probably by the amelioration of hippocampal LTP impairment, modulation of oxidative-antioxidative status, and inhibition of Aβ plaque accumulation.

The Pathophysiological Underpinnings of Gamma-Band Alterations in Psychiatric Disorders

Investigating the biophysiological substrates of psychiatric illnesses is of great interest to our understanding of disorders' etiology, the identification of reliable biomarkers, and potential new therapeutic avenues. Schizophrenia represents a consolidated model of γ alterations arising from the aberrant activity of parvalbumin-positive GABAergic interneurons, whose dysfunction is associated with perineuronal net impairment and neuroinflammation. This model of pathogenesis is supported by molecular, cellular, and functional evidence. Proof for alterations of γ oscillations and their underlying mechanisms has also been reported in bipolar disorder and represents an emerging topic for major depressive disorder. Although evidence from animal models needs to be further elucidated in humans, the pathophysiology of γ-band alteration represents a common denominator for different neuropsychiatric disorders. The purpose of this narrative review is to outline a framework of converging results in psychiatric conditions characterized by γ abnormality, from neurochemical dysfunction to alterations in brain rhythms.

Perturbing cortical networks: in vivo electrophysiological consequences of pan-neuronal chemogenetic manipulations using deschloroclozapine

Introduction

Chemogenetic techniques, specifically the use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), have become invaluable tools in neuroscience research. Yet, the understanding of how Gq- and Gicoupled DREADDs alter local field potential (LFP) oscillations in vivo remains incomplete.

Methods

This study investigates the in vivo electrophysiological effects of DREADD actuation by deschloroclozapine, on spontaneous firing rate and LFP oscillations recorded from the anterior cingulate cortex in lightly anesthetized male rats.

Results

Unexpectedly, in response to the administration of deschloroclozapine, we observed inhibitory effects with pan-neuronal hM3D(Gq) stimulation, and excitatory effects with pan-neuronal hM4D(Gi) stimulation in a significant portion of neurons. These results emphasize the need to account for indirect perturbation effects at the local neuronal network level in vivo, particularly when not all neurons express the chemogenetic receptors uniformly. In the current study, for instance, the majority of cells that were transduced with both hM3D(Gq) and hM4D(Gi) were GABAergic. Moreover, we found that panneuronal cortical chemogenetic modulation can profoundly alter oscillatory neuronal activity, presenting a potential research tool or therapeutic strategy in several neuropsychiatric models and diseases.

Discussion

These findings help to optimize the use of chemogenetic techniques in neuroscience research and open new possibilities for novel therapeutic strategies.

Overexpression of the schizophrenia risk gene C4 in PV cells drives sex-dependent behavioral deficits and circuit dysfunction

Fast-spiking parvalbumin (PV)-positive cells are key players in orchestrating pyramidal neuron activity, and their dysfunction is consistently observed in myriad brain diseases. To understand how immune complement dysregulation - a prevalent locus of brain disease etiology - in PV cells may drive disease pathogenesis, we have developed a transgenic mouse line that permits cell-type specific overexpression of the schizophrenia-associated complement component 4 (C4) gene. We found that overexpression of mouse C4 (mC4) in PV cells causes sex-specific behavioral alterations and concomitant deficits in synaptic connectivity and excitability of PV cells of the prefrontal cortex. Using a computational network, we demonstrated that these microcircuit deficits led to hyperactivity and disrupted neural communication. Finally, pan-neuronal overexpression of mC4 failed to evoke the same deficits in behavior as PV-specific mC4 overexpression, suggesting that C4 perturbations in fast-spiking neurons are more harmful to brain function than pan-neuronal alterations. Together, these results provide a causative link between C4 and the vulnerability of PV cells in brain disease.

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

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

Meta-Analysis in Transgenic Alzheimer's Disease Mouse Models Reveals Opposite Brain Network Effects of Amyloid-β and Phosphorylated Tau Proteins

Background

Cognitive deficits observed in Alzheimer's disease (AD) patients have been correlated with altered hippocampal activity. Although the mechanism remains under extensive study, neurofibrillary tangles and amyloid plaques have been proposed as responsible for brain activity alterations. Aiming to unveil the mechanism, researchers have developed several transgenic models of AD. Nevertheless, the variability in hippocampal oscillatory alterations found in different genetic backgrounds and ages remains unclear.

Objective

To assess the oscillatory alterations in relation to animal developmental age and protein inclusion, amyloid-β (Aβ) load, and abnormally phosphorylated tau (pTau), we reviewed and analyzed the published data on peak power, frequency, and quantification of theta-gamma cross-frequency coupling (modulation index values).

Methods

To ensure that the search was as current as possible, a systematic review was conducted to locate and abstract all studies published from January 2000 to February 2023 that involved in vivo hippocampal local field potential recording in transgenic mouse models of AD.

Results

The presence of Aβ was associated with electrophysiological alterations that are mainly reflected in power increases, frequency decreases, and lower modulation index values. Concomitantly, pTau accumulation was associated with electrophysiological alterations that are mainly reflected in power decreases, frequency decreases, and no significant alterations in modulation index values.

Conclusions

In this study, we showed that electrophysiological parameters are altered from prodromal stages to the late stages of pathology. Thus, we found that Aβ deposition is associated with brain network hyperexcitability, whereas pTau deposition mainly leads to brain network hypoexcitability in transgenic models.

Effects of optogenetic and visual stimulation on gamma activity in the visual cortex

Studying brain functions and activity during gamma oscillations can be a challenge because it requires careful planning to create the necessary conditions for a controlled experiment. Such an experiment consists of placing the brain into a gamma state and investigating cognitive processing with a careful design. Cortical oscillations in the gamma frequency range (30-80 Hz) play an essential role in a variety of cognitive processes, including visual processing and cognition. The present study aims to investigate the effects of a visual stimulus on the primary visual cortex under gamma oscillations. Specifically, we sought to explore the behavior of gamma oscillations triggered by optogenetic stimulation in the II and IV layers of the visual cortex, both with and without concurrent visual stimulation. Our results show that optogenetic stimulation increases the power of gamma oscillation in both layers of the visual cortex. However, the combined stimuli resulted in a reduction of gamma power in layer II and an increase and reinforcement in gamma power in layer IV. Modelling the results with the Wilson-Cowan model suggests changes in the input of the excitatory population due to the combined stimuli. In addition, our analysis of the data using the Lempel-Ziv complexity method supports our interpretations from the modeling. Thus, our results suggest that optogenetic stimulation enhances low gamma power in both layers of the visual cortex, while simultaneous visual stimulation has differing effects on the two layers, reducing gamma power in layer II and increasing it in layer IV.

Gamma oscillations and episodic memory

Enhanced gamma oscillatory activity (30-80 Hz) accompanies the successful formation and retrieval of episodic memories. While this co-occurrence is well documented, the mechanistic contributions of gamma oscillatory activity to episodic memory remain unclear. Here, we review how gamma oscillatory activity may facilitate spike timing-dependent plasticity, neural communication, and sequence encoding/retrieval, thereby ensuring the successful formation and/or retrieval of an episodic memory. Based on the evidence reviewed, we propose that multiple, distinct forms of gamma oscillation can be found within the canonical gamma band, each of which has a complementary role in the neural processes listed above. Further exploration of these theories using causal manipulations may be key to elucidating the relevance of gamma oscillatory activity to episodic memory.

Local and long-range GABAergic circuits in hippocampal area CA1 and their link to Alzheimer's disease

GABAergic inhibitory neurons are the principal source of inhibition in the brain. Traditionally, their role in maintaining the balance of excitation-inhibition has been emphasized. Beyond homeostatic functions, recent circuit mapping and functional manipulation studies have revealed a wide range of specific roles that GABAergic circuits play in dynamically tilting excitation-inhibition coupling across spatio-temporal scales. These span from gating of compartment- and input-specific signaling, gain modulation, shaping input-output functions and synaptic plasticity, to generating signal-to-noise contrast, defining temporal windows for integration and rate codes, as well as organizing neural assemblies, and coordinating inter-regional synchrony. GABAergic circuits are thus instrumental in controlling single-neuron computations and behaviorally-linked network activity. The activity dependent modulation of sensory and mnemonic information processing by GABAergic circuits is pivotal for the formation and maintenance of episodic memories in the hippocampus. Here, we present an overview of the local and long-range GABAergic circuits that modulate the dynamics of excitation-inhibition and disinhibition in the main output area of the hippocampus CA1, which is crucial for episodic memory. Specifically, we link recent findings pertaining to GABAergic neuron molecular markers, electrophysiological properties, and synaptic wiring with their function at the circuit level. Lastly, given that area CA1 is particularly impaired during early stages of Alzheimer's disease, we emphasize how these GABAergic circuits may contribute to and be involved in the pathophysiology.

Early classification of Alzheimer's disease phenotype based on hippocampal electrophysiology in the TgF344-AD rat model

The hippocampus plays a vital role in navigation, learning, and memory, and is affected in Alzheimer's disease (AD). This study investigated the classification of AD-transgenic rats versus wild-type littermates using electrophysiological activity recorded from the hippocampus at an early, presymptomatic stage of the disease (6 months old) in the TgF344-AD rat model. The recorded signals were filtered into low frequency (LFP) and high frequency (spiking activity) signals, and machine learning classifiers were employed to identify the rat genotype (TG vs. WT). By analyzing specific frequency bands in the low frequency signals and calculating distance metrics between spike trains in the high frequency signals, accurate classification was achieved. Gamma band power emerged as a valuable signal for classification, and combining information from both low and high frequency signals improved the accuracy further. These findings provide valuable insights into the early stage effects of AD on different regions of the hippocampus.

Timing to be precise? An overview of spike timing-dependent plasticity, brain rhythmicity, and glial cells interplay within neuronal circuits

In the mammalian brain information processing and storage rely on the complex coding and decoding events performed by neuronal networks. These actions are based on the computational ability of neurons and their functional engagement in neuronal assemblies where precise timing of action potential firing is crucial. Neuronal circuits manage a myriad of spatially and temporally overlapping inputs to compute specific outputs that are proposed to underly memory traces formation, sensory perception, and cognitive behaviors. Spike-timing-dependent plasticity (STDP) and electrical brain rhythms are suggested to underlie such functions while the physiological evidence of assembly structures and mechanisms driving both processes continues to be scarce. Here, we review foundational and current evidence on timing precision and cooperative neuronal electrical activity driving STDP and brain rhythms, their interactions, and the emerging role of glial cells in such processes. We also provide an overview of their cognitive correlates and discuss current limitations and controversies, future perspectives on experimental approaches, and their application in humans.

Pinpointing the locus of GABAergic vulnerability in Alzheimer's disease

The early stages of Alzheimer's disease (AD) have been linked to microcircuit dysfunction and pathophysiological neuronal firing in several brain regions. Inhibitory GABAergic microcircuitry is a critical feature of stable neural-circuit function in the healthy brain, and its dysregulation has therefore been proposed as contributing to AD-related pathophysiology. However, exactly how the critical balance between excitatory and inhibitory microcircuitry is modified by AD pathogenesis remains unclear. Here, we set the current evidence implicating dysfunctional GABAergic microcircuitry as a driver of early AD pathophysiology in a simple conceptual framework. Our framework is based on a generalised reductionist model of firing-rate control by local feedback inhibition. We use this framework to consider multiple loci that may be vulnerable to disruption by AD pathogenesis. We first start with evidence investigating how AD-related processes may impact the gross number of inhibitory neurons in the network. We then move to discuss how pathology may impact intrinsic cellular properties and firing thresholds of GABAergic neurons. Finally, we cover how AD-related pathogenesis may disrupt synaptic connectivity between excitatory and inhibitory neurons. We use the feedback inhibition framework to discuss and organise the available evidence from both preclinical rodent work and human studies in AD patients and conclude by identifying key questions and understudied areas for future investigation.

Theta-gamma phase amplitude coupling in a hippocampal CA1 microcircuit

Phase amplitude coupling (PAC) between slow and fast oscillations is found throughout the brain and plays important functional roles. Its neural origin remains unclear. Experimental findings are often puzzling and sometimes contradictory. Most computational models rely on pairs of pacemaker neurons or neural populations tuned at different frequencies to produce PAC. Here, using a data-driven model of a hippocampal microcircuit, we demonstrate that PAC can naturally emerge from a single feedback mechanism involving an inhibitory and excitatory neuron population, which interplay to generate theta frequency periodic bursts of higher frequency gamma. The model suggests the conditions under which a CA1 microcircuit can operate to elicit theta-gamma PAC, and highlights the modulatory role of OLM and PVBC cells, recurrent connectivity, and short term synaptic plasticity. Surprisingly, the results suggest the experimentally testable prediction that the generation of the slow population oscillation requires the fast one and cannot occur without it.

Alzheimer’s disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression

The synapse has consistently been considered a vulnerable and critical target within Alzheimer’s disease, and synapse loss is, to date, one of the main biological correlates of cognitive decline within Alzheimer’s disease. This occurs prior to neuronal loss with ample evidence that synaptic dysfunction precedes this, in support of the idea that synaptic failure is a crucial stage within disease pathogenesis. The two main pathological hallmarks of Alzheimer’s disease, abnormal aggregates of amyloid or tau proteins, have had demonstrable effects on synaptic physiology in animal and cellular models of Alzheimer’s disease. There is also growing evidence that these two proteins may have a synergistic effect on neurophysiological dysfunction. Here, we review some of the main findings of synaptic alterations in Alzheimer’s disease, and what we know from Alzheimer’s disease animal and cellular models. First, we briefly summarize some of the human evidence to suggest that synapses are altered, including how this relates to network activity. Subsequently, animal and cellular models of Alzheimer’s disease are considered, highlighting mouse models of amyloid and tau pathology and the role these proteins may play in synaptic dysfunction, either in isolation or examining how the two pathologies may interact in dysfunction. This specifically focuses on neurophysiological function and dysfunction observed within these animal models, typically measured using electrophysiology or calcium imaging. Following synaptic dysfunction and loss, it would be impossible to imagine that this would not alter oscillatory activity within the brain. Therefore, this review also discusses how this may underpin some of the aberrant oscillatory patterns seen in animal models of Alzheimer’s disease and human patients. Finally, an overview of some key directions and considerations in the field of synaptic dysfunction in Alzheimer’s disease is covered. This includes current therapeutics that are targeted specifically at synaptic dysfunction, but also methods that modulate activity to rescue aberrant oscillatory patterns. Other important future avenues of note in this field include the role of non-neuronal cell types such as astrocytes and microglia, and mechanisms of dysfunction independent of amyloid and tau in Alzheimer’s disease. The synapse will certainly continue to be an important target within Alzheimer’s disease for the foreseeable future.

Targeting galectin-3 to counteract spike-phase uncoupling of fast-spiking interneurons to gamma oscillations in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a progressive multifaceted neurodegenerative disorder for which no disease-modifying treatment exists. Neuroinflammation is central to the pathology progression, with evidence suggesting that microglia-released galectin-3 (gal3) plays a pivotal role by amplifying neuroinflammation in AD. However, the possible involvement of gal3 in the disruption of neuronal network oscillations typical of AD remains unknown.

METHODS

Here, we investigated the functional implications of gal3 signaling on experimentally induced gamma oscillations ex vivo (20-80 Hz) by performing electrophysiological recordings in the hippocampal CA3 area of wild-type (WT) mice and of the 5×FAD mouse model of AD. In addition, the recorded slices from WT mice under acute gal3 application were analyzed with RT-qPCR to detect expression of some neuroinflammation-related genes, and amyloid-β (Aβ) plaque load was quantified by immunostaining in the CA3 area of 6-month-old 5×FAD mice with or without Gal3 knockout (KO).

RESULTS

Gal3 application decreased gamma oscillation power and rhythmicity in an activity-dependent manner, which was accompanied by impairment of cellular dynamics in fast-spiking interneurons (FSNs) and pyramidal cells. We found that the gal3-induced disruption was mediated by the gal3 carbohydrate-recognition domain and prevented by the gal3 inhibitor TD139, which also prevented Aβ42-induced degradation of gamma oscillations. Furthermore, the 5×FAD mice lacking gal3 (5×FAD-Gal3KO) exhibited WT-like gamma network dynamics and decreased Aβ plaque load.

CONCLUSIONS

We report for the first time that gal3 impairs neuronal network dynamics by spike-phase uncoupling of FSNs, inducing a network performance collapse. Moreover, our findings suggest gal3 inhibition as a potential therapeutic strategy to counteract the neuronal network instability typical of AD and other neurological disorders encompassing neuroinflammation and cognitive decline.

The role of optogenetic stimulations of parvalbumin-positive interneurons in the prefrontal cortex and the ventral hippocampus on an acute MK-801 model of schizophrenia-like cognitive inflexibility

Schizophrenia research has increased in recent decades and focused more on its neural basis. Decision-making and cognitive flexibility are the main cognitive functions that are impaired and considered schizophrenia endophenotypes. Cognitive impairment was recently connected with altered functions of N-methyl-d-aspartate (NMDAR) glutamatergic receptors, which increased cortical activity. Selective NMDAR antagonists, such as MK-801, have been used to model cognitive inflexibility in schizophrenia. Decreased GABAergic inhibitory activity has been shown elsewhere with enhanced cortical activity. This imbalance in the excitatory/inhibitory may reduce the entrainment of prefrontal gamma and hippocampal theta rhythms and result in gamma/theta band de-synchronization. The current study established an acute MK-801 administration model of schizophrenia-like cognitive inflexibility in rats and used the attentional set-shifting task in which rats learned to switch/reverse the relevant rule. During the task, we used in vivo optogenetic stimulations of parvalbumin-positive interneurons at specific light pulses in the prefrontal cortex and ventral hippocampus. The first experiments showed that acute dizocilpine in rats produced schizophrenia-like cognitive inflexibility. The second set of experiments demonstrated that specific optogenetic stimulation at specific frequencies of parvalbumin-positive interneurons in the prefrontal cortex and ventral hippocampus rescued the cognitive flexibility rats that received acute MK-801. These findings advance our knowledge of the pivotal role of parvalbumin interneurons in schizophrenia-like cognitive impairment and may guide further research on this severe psychiatric disorder.

Alterations in theta-gamma coupling and sharp wave-ripple, signs of prodromal hippocampal network impairment in the TgF344-AD rat model

Alzheimer's disease (AD) is a severe neurodegenerative disorder caused by the accumulation of toxic proteins, amyloid-beta (Aβ) and tau, which eventually leads to dementia. Disease-modifying therapies are still lacking, due to incomplete insights into the neuropathological mechanisms of AD. Synaptic dysfunction is known to occur before cognitive symptoms become apparent and recent studies have demonstrated that imbalanced synaptic signaling drives the progression of AD, suggesting that early synaptic dysfunction could be an interesting therapeutic target. Synaptic dysfunction results in altered oscillatory activity, which can be detected with electroencephalography and electrophysiological recordings. However, the majority of these studies have been performed at advanced stages of AD, when extensive damage and cognitive symptoms are already present. The current study aimed to investigate if the hippocampal oscillatory activity is altered at pre-plaque stages of AD. The rats received stereotactic surgery to implant a laminar electrode in the CA1 layer of the right hippocampus. Electrophysiological recordings during two consecutive days in an open field were performed in 4-5-month-old TgF344-AD rats when increased concentrations of soluble Aβ species were observed in the brain, in the absence of Aβ-plaques. We observed a decreased power of high theta oscillations in TgF344-AD rats compared to wild-type littermates. Sharp wave-ripple (SWR) analysis revealed an increased SWR power and a decreased duration of SWR during quiet wake in TgF344-AD rats. The alterations in properties of SWR and the increased power of fast oscillations are suggestive of neuronal hyperexcitability, as has been demonstrated to occur during presymptomatic stages of AD. In addition, decreased strength of theta-gamma coupling, an important neuronal correlate of memory encoding, was observed in the TgF344-AD rats. Theta-gamma phase amplitude coupling has been associated with memory encoding and the execution of cognitive functions. Studies have demonstrated that mild cognitive impairment patients display decreased coupling strength, similar to what is described here. The current study demonstrates altered hippocampal network activity occurring at pre-plaque stages of AD and provides insights into prodromal network dysfunction in AD. The alterations observed could aid in the detection of AD during presymptomatic stages.

Reduced excitatory neuron activity and interneuron-type-specific deficits in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive memory loss and cognitive decline. These impairments correlate with early alterations in neuronal network activity in AD patients. Disruptions in the activity of individual neurons have been reported in mouse models of amyloidosis. However, the impact of amyloid pathology on the spontaneous activity of distinct neuronal types remains unexplored in vivo. Here we use in vivo calcium imaging with multiphoton microscopy to monitor and compare the activity of excitatory and two types of inhibitory interneurons in the cortices of APP/PS1 and control mice under isoflurane anesthesia. We also determine the relationship between amyloid accumulation and the deficits in spontaneous activity in APP/PS1 mice. We show that somatostatin-expressing (SOM) interneurons are hyperactive, while parvalbumin-expressing interneurons are hypoactive in APP/PS1 mice. Only SOM interneuron hyperactivity correlated with proximity to amyloid plaque. These inhibitory deficits were accompanied by decreased excitatory neuron activity in APP/PS1 mice. Our study identifies cell-specific neuronal firing deficits in APP/PS1 mice driven by amyloid pathology. These findings highlight the importance of addressing the complexity of neuron-specific deficits to ameliorate circuit dysfunction in Alzheimer's disease.

Optogenetic neuromodulation with gamma oscillation as a new strategy for Alzheimer disease: a narrative review

The amyloid hypothesis has been considered a major explanation of the pathogenesis of Alzheimer disease. However, failure of phase III clinical trials with anti-amyloid-beta monoclonal antibodies reveals the need for other therapeutic approaches to treat Alzheimer disease. Compared to its relatively short history, optogenetics has developed considerably. The expression of microbial opsins in cells using genetic engineering allows specific control of cell signals or molecules. The application of optogenetics to Alzheimer disease research or clinical approaches is increasing. When applied with gamma entrainment, optogenetic neuromodulation can improve Alzheimer disease symptoms. Although safety problems exist with optogenetics such as the use of viral vectors, this technique has great potential for use in Alzheimer disease. In this paper, we review the historical applications of optogenetic neuromodulation with gamma entrainment to investigate the mechanisms involved in Alzheimer disease and potential therapeutic strategies.

The role of gamma oscillations in central nervous system diseases: Mechanism and treatment

Gamma oscillation is the synchronization with a frequency of 30–90 Hz of neural oscillations, which are rhythmic electric processes of neuron groups in the brain. The inhibitory interneuron network is necessary for the production of gamma oscillations, but certain disruptions such as brain inflammation, oxidative stress, and metabolic imbalances can cause this network to malfunction. Gamma oscillations specifically control the connectivity between different brain regions, which is crucial for perception, movement, memory, and emotion. Studies have linked abnormal gamma oscillations to conditions of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, and schizophrenia. Evidence suggests that gamma entrainment using sensory stimuli (GENUS) provides significant neuroprotection. This review discusses the function of gamma oscillations in advanced brain activities from both a physiological and pathological standpoint, and it emphasizes gamma entrainment as a potential therapeutic approach for a range of neuropsychiatric diseases.

A model of working memory for encoding multiple items and ordered sequences exploiting the theta-gamma code

Recent experimental evidence suggests that oscillatory activity plays a pivotal role in the maintenance of information in working memory, both in rodents and humans. In particular, cross-frequency coupling between theta and gamma oscillations has been suggested as a core mechanism for multi-item memory. The aim of this work is to present an original neural network model, based on oscillating neural masses, to investigate mechanisms at the basis of working memory in different conditions. We show that this model, with different synapse values, can be used to address different problems, such as the reconstruction of an item from partial information, the maintenance of multiple items simultaneously in memory, without any sequential order, and the reconstruction of an ordered sequence starting from an initial cue. The model consists of four interconnected layers; synapses are trained using Hebbian and anti-Hebbian mechanisms, in order to synchronize features in the same items, and desynchronize features in different items. Simulations show that the trained network is able to desynchronize up to nine items without a fixed order using the gamma rhythm. Moreover, the network can replicate a sequence of items using a gamma rhythm nested inside a theta rhythm. The reduction in some parameters, mainly concerning the strength of GABAergic synapses, induce memory alterations which mimic neurological deficits. Finally, the network, isolated from the external environment (“imagination phase”) and stimulated with high uniform noise, can randomly recover sequences previously learned, and link them together by exploiting the similarity among items.

40 Hz Blue LED Relieves the Gamma Oscillations Changes Caused by Traumatic Brain Injury in Rat

Background

Photobiomodulation (PBM) using low-level light-emitting diodes (LEDs) can be rapidly applied to various neurological disorders safely and non-invasively.

Materials and Methods

Forty-eight rats were involved in this study. The traumatic brain injury (TBI) model of rat was set up by a controlled cortical impact (CCI) injury. An 8-channel cortex electrode EEG was fixed to two hemispheres, and gamma oscillations were extracted according to each electrode. A 40 hz blue LED stimulation was set at four points of the frontal and parietal regions for 60 s each, six times per day for 1 week. Modified Neurologic Severity Scores (mNSS) were used to evaluate the level of neurological function.

Results

In the right-side TBI model, the gamma oscillation decreased in electrodes Fp2, T4, C4, and O2; but significantly increased after 1 week of 40 hz Blue LED intervention. In the left-side TBI model, the gamma oscillation decreased in electrodes Fp1, T3, C3, and O1; and similarly increased after 1 week of 40 hz Blue LED intervention. Both left and right side TBI rats performed significantly better in mNSS after 40 hz Blue LED intervention.

Conclusion

TBI causes the decrease of gamma oscillations on the injured side of the brain of rats. The 40 hz Blue LED therapy could relieve the gamma oscillation changes caused by TBI and improve the prognosis of TBI.

Biophysical Kv3 channel alterations dampen excitability of cortical PV interneurons and contribute to network hyperexcitability in early Alzheimer's

In Alzheimer’s disease (AD), a multitude of genetic risk factors and early biomarkers are known. Nevertheless, the causal factors responsible for initiating cognitive decline in AD remain controversial. Toxic plaques and tangles correlate with progressive neuropathology, yet disruptions in circuit activity emerge before their deposition in AD models and patients. Parvalbumin (PV) interneurons are potential candidates for dysregulating cortical excitability as they display altered action potential (AP) firing before neighboring excitatory neurons in prodromal AD. Here, we report a novel mechanism responsible for PV hypoexcitability in young adult familial AD mice. We found that biophysical modulation of K_v3 channels, but not changes in their mRNA or protein expression, were responsible for dampened excitability in young 5xFAD mice. These K⁺ conductances could efficiently regulate near-threshold AP firing, resulting in gamma-frequency-specific network hyperexcitability. Thus, biophysical ion channel alterations alone may reshape cortical network activity prior to changes in their expression levels. Our findings demonstrate an opportunity to design a novel class of targeted therapies to ameliorate cortical circuit hyperexcitability in early AD.

Resilient Hippocampal Gamma Rhythmogenesis and Parvalbumin-Expressing Interneuron Function Before and After Plaque Burden in 5xFAD Alzheimer's Disease Model

Recent studies have implicated impaired Parvalbumin Fast-Spiking Interneuron (PVIN) function as a precipitating factor underlying abnormalities in network synchrony, oscillatory rhythms, and cognition associated with Alzheimer's disease (AD). However, a complete developmental investigation of potential gamma deficits, induced by commonly used carbachol or kainate in ex vivo slice preparations, within AD model mice is lacking. We examined gamma oscillations using field recordings in acute hippocampal slices from 5xFAD and control mice, through the period of developing pathology, starting at 3 months of age, when there is minimal plaque presence in the hippocampus, through to 12+ months of age, when plaque burden is high. In addition, we examined PVIN participation in gamma rhythms using targeted cell-attached recordings of genetically-reported PVINs, in both wild type and mutant mice. In parallel, a developmental immunohistochemical characterisation probing the PVIN-associated expression of PV and perineuronal nets (PNNs) was compared between control and 5xFAD mice. Remarkably, this comprehensive longitudinal evaluation failed to reveal any obvious correlations between PVIN deficits (electrical and molecular), circuit rhythmogenesis (gamma frequency and power), and Aβ deposits/plaque formation. By 6-12 months, 5xFAD animals have extensive plaque formation throughout the hippocampus. However, a deficit in gamma oscillatory power was only evident in the oldest 5xFAD animals (12+ months), and only when using kainate, and not carbachol, to induce the oscillations. We found no difference in PV firing or phase preference during kainate-induced oscillations in younger or older 5xFAD mice compared to control, and a reduction of PV and PNNs only in the oldest 5xFAD mice. The lack of a clear relationship between PVIN function, network rhythmicity, and plaque formation in our study highlights an unexpected resilience in PVIN function in the face of extensive plaque pathology associated with this model, calling into question the presumptive link between PVIN pathology and Alzheimer's progression.

Theta and gamma oscillatory dynamics in mouse models of Alzheimer's disease: A path to prospective therapeutic intervention

Understanding the neural basis of cognitive deficits, a key feature of Alzheimer's disease (AD), is imperative for achieving the therapy of the disease. Rhythmic oscillatory activities in neural systems are a fundamental mechanism for diverse brain functions, including cognition. In several neurological conditions like AD, aberrant neural oscillations have been shown to play a central role. Furthermore, manipulation of brain oscillations in animals has confirmed their impact on cognition and disease. In this article, we review the evidence from mouse models that shows how synchronized oscillatory activity is intricately linked to AD machinery. We primarily focus on recent reports showing abnormal oscillatory activities at theta and gamma frequencies in AD condition and their influence on cellular disturbances and cognitive impairments. A thorough comprehension of the role that neuronal oscillations play in AD pathology should pave the way to therapeutic interventions that can curb the disease.

Involvement of Parvalbumin-Positive Neurons in the Development of Hyperalgesia in a Mouse Model of Fibromyalgia

Fibromyalgia (FM) presents as chronic systemic pain, which might be ascribed to central sensitization, in which pain information processing is amplified in the central nervous system. Since patients with FM display elevated gamma oscillations in the pain matrix and parvalbumin (PV)-positive neurons play a critical role in induction of gamma oscillations, we hypothesized that changes in PV-positive neurons are involved in hyperalgesia in fibromyalgia. In the present study, to investigate a role of PV-positive neurons in neuropathic pain, mice received reserpine administration for 3 consecutive days as an animal model of FM (RES group), while control mice received vehicle injections in the same way (VEH group). The mice were subjected to hot-plate and forced swim tests, and immuno-stained PV-positive neurons were counted in the pain matrix. We investigated relationships between PV-positive neuron density in the pain matrix and pain avoidance behaviors. The results indicated that the mice in the RES group showed transient bodyweight loss and longer immobility time in the forced swim test than the mice in the VEH group. In the hot-plate test, the RES group showed shorter response latencies and a larger number of jumps in response to nociceptive thermal stimulus than the VEH group. Histological examination indicated an increase in the density of PV-positive neurons in the primary somatosensory cortex (S1) in the RES group. Furthermore, response latencies to the hot-plate were significantly and negatively correlated with the density of PV-positive neurons in the S1. These results suggest a critical role for PV-positive neurons in the S1 to develop hyperalgesia in FM.

Optogenetics: implications for Alzheimer's disease research and therapy

Alzheimer’s disease (AD), a critical neurodegenerative condition, has a wide range of effects on brain activity. Synaptic plasticity and neuronal circuits are the most vulnerable in Alzheimer’s disease, but the exact mechanism is unknown. Incorporating optogenetics into the study of AD has resulted in a significant leap in this field during the last decades, kicking off a revolution in our knowledge of the networks that underpin cognitive functions. In Alzheimer's disease, optogenetics can help to reduce and reverse neural circuit and memory impairments. Here we review how optogenetically driven methods have helped expand our knowledge of Alzheimer's disease, and how optogenetic interventions hint at a future translation into therapeutic possibilities for further utilization in clinical settings. In conclusion, neuroscience has witnessed one of its largest revolutions following the introduction of optogenetics into the field.

Somatostatin and Somatostatin-Containing Interneurons—From Plasticity to Pathology

Despite the obvious differences in the pathophysiology of distinct neuropsychiatric diseases or neurodegenerative disorders, some of them share some general but pivotal mechanisms, one of which is the disruption of excitation/inhibition balance. Such an imbalance can be generated by changes in the inhibitory system, very often mediated by somatostatin-containing interneurons (SOM-INs). In physiology, this group of inhibitory interneurons, as well as somatostatin itself, profoundly shapes the brain activity, thus influencing the behavior and plasticity; however, the changes in the number, density and activity of SOM-INs or levels of somatostatin are found throughout many neuropsychiatric and neurological conditions, both in patients and animal models. Here, we (1) briefly describe the brain somatostatinergic system, characterizing the neuropeptide somatostatin itself, its receptors and functions, as well the physiology and circuitry of SOM-INs; and (2) summarize the effects of the activity of somatostatin and SOM-INs in both physiological brain processes and pathological brain conditions, focusing primarily on learning-induced plasticity and encompassing selected neuropsychological and neurodegenerative disorders, respectively. The presented data indicate the somatostatinergic-system-mediated inhibition as a substantial factor in the mechanisms of neuroplasticity, often disrupted in a plethora of brain pathologies.

Prefrontal Cortical Control of Anxiety: Recent Advances

Dysfunction in the prefrontal cortex is commonly implicated in anxiety disorders, but the mechanisms remain unclear. Approach-avoidance conflict tasks have been extensively used in animal research to better understand how changes in neural activity within the prefrontal cortex contribute to avoidance behaviors, which are believed to play a major role in the maintenance of anxiety disorders. In this article, we first review studies utilizing in vivo electrophysiology to reveal the relationship between changes in neural activity and avoidance behavior in rodents. We then review recent studies that take advantage of optical and genetic techniques to test the unique contribution of specific prefrontal cortex circuits and cell types to the control of anxiety-related avoidance behaviors. This new body of work reveals that behavior during approach-avoidance conflict is dynamically modulated by individual cell types, distinct neural pathways, and specific oscillatory frequencies. The integration of these different pathways, particularly as mediated by interactions between excitatory and inhibitory neurons, represents an exciting opportunity for the future of understanding anxiety.

Activation of PLCβ1 enhances endocannabinoid mobilization to restore hippocampal spike-timing-dependent potentiation and contextual fear memory impaired by Alzheimer's amyloidosis

Background

Accumulation of amyloid beta oligomers (AβO) in Alzheimer’s disease (AD) impairs hippocampal long-term potentiation (LTP), leading to memory deficits. Thus, identifying the molecular targets of AβO involved in LTP inhibition is critical for developing therapeutics for AD. Endocannabinoid (eCB) synthesis and release, a process collectively called eCB mobilization by hippocampal CA1 pyramidal cells, is known to facilitate LTP induction. eCB can be mobilized either by postsynaptic depolarization in an intracellular Ca²⁺ concentration ([Ca²⁺]_i)-dependent pathway or by group 1 metabotropic glutamate receptor (mGluR) activation in a phospholipase Cβ (PLCβ)-dependent pathway. Moreover, group 1 mGluR activation during postsynaptic depolarization, which is likely to occur in vivo during memory processing, can cause synergistic enhancement of eCB (S-eCB) mobilization in a PLCβ-dependent pathway. Although AβO has been shown to disrupt [Ca²⁺]_i-dependent eCB mobilization, the effect of AβO on PLCβ-dependent S-eCB mobilization and its association with LTP and hippocampus-dependent memory impairments in AD is unknown.

Methods

We used in vitro whole-cell patch-clamp recordings and western blot analyses to investigate the effect of AβO on PLCβ protein levels, PLCβ-dependent S-eCB mobilization, and spike-timing-dependent potentiation (tLTP) in AβO-treated rat hippocampal slices in vitro. In addition, we assessed the relationship between PLCβ protein levels and hippocampus-dependent memory impairment by performing a contextual fear memory task in vivo in the 5XFAD mouse model of AD.

Results

We found that AβO treatment in rat hippocampal slices in vitro decreased hippocampal PLCβ1 protein levels and disrupted S-eCB mobilization, as measured by western blot analysis and in vitro whole-cell patch-clamp recordings. This consequently led to the impairment of NMDA receptor (NMDAR)-mediated tLTP at CA3-CA1 excitatory synapses in AβO-treated rat hippocampal slices in vitro. Application of the PLCβ activator, m-3M3FBS, in rat hippocampal slices reinstated PLCβ1 protein levels to fully restore S-eCB mobilization and NMDAR-mediated tLTP. In addition, direct hippocampal injection of m-3M3FBS in 5XFAD mice reinstated PLCβ1 protein levels to those observed in wild type control mice and fully restored hippocampus-dependent contextual fear memory in vivo in 5XFAD mice.

Conclusion

We suggest that these results might be the consequence of memory impairment in AD by disrupting S-eCB mobilization. Therefore, we propose that PLCβ-dependent S-eCB mobilization could provide a new therapeutic strategy for treating memory deficits in AD.

Bri2 BRICHOS chaperone rescues impaired fast-spiking interneuron behavior and neuronal network dynamics in an AD mouse model in vitro

Synchronized and properly balanced electrical activity of neurons is the basis for the brain's ability to process information, to learn, and to remember. In Alzheimer's disease (AD), which causes cognitive decline in patients, this synchronization and balance is disturbed by the accumulation of neuropathological biomarkers such as amyloid-beta peptide (Aβ42). Failure of Aβ42 clearance mechanisms as well as desynchronization of crucial neuronal classes such as fast-spiking interneurons (FSN) are root causes for the disruption of the cognition-relevant gamma brain rhythm (30-80 Hz) and consequent cognitive impairment observed in AD. Here we show that recombinant BRICHOS molecular chaperone domains from ProSP-C or Bri2, which interfere with Aβ42 aggregation, can rescue the gamma rhythm. We demonstrate that Aβ42 progressively decreases gamma oscillation power and rhythmicity, disrupts the inhibition/excitation balance in pyramidal cells, and desynchronizes FSN firing during gamma oscillations in the hippocampal CA3 network of mice. Application of the more efficacious Bri2 BRICHOS chaperone rescued the cellular and neuronal network performance from all ongoing Aβ42-induced functional impairments. Collectively, our findings offer critical missing data to explain the importance of FSN for normal network function and underscore the therapeutic potential of Bri2 BRICHOS to rescue the disruption of cognition-relevant brain rhythms in AD.

The Engram's Dark Horse: How Interneurons Regulate State-Dependent Memory Processing and Plasticity

Brain states such as arousal and sleep play critical roles in memory encoding, storage, and recall. Recent studies have highlighted the role of engram neurons-populations of neurons activated during learning-in subsequent memory consolidation and recall. These engram populations are generally assumed to be glutamatergic, and the vast majority of data regarding the function of engram neurons have focused on glutamatergic pyramidal or granule cell populations in either the hippocampus, amygdala, or neocortex. Recent data suggest that sleep and wake states differentially regulate the activity and temporal dynamics of engram neurons. Two potential mechanisms for this regulation are either via direct regulation of glutamatergic engram neuron excitability and firing, or via state-dependent effects on interneuron populations-which in turn modulate the activity of glutamatergic engram neurons. Here, we will discuss recent findings related to the roles of interneurons in state-regulated memory processes and synaptic plasticity, and the potential therapeutic implications of understanding these mechanisms.

Neocortical inhibitory interneuron subtypes are differentially attuned to synchrony- and rate-coded information

Neurons can carry information with both the synchrony and rate of their spikes. However, it is unknown whether distinct subtypes of neurons are more sensitive to information carried by synchrony versus rate, or vice versa. Here, we address this question using patterned optical stimulation in slices of somatosensory cortex from mouse lines labelling fast-spiking (FS) and regular-spiking (RS) interneurons. We used optical stimulation in layer 2/3 to encode a 1-bit signal using either the synchrony or rate of activity. We then examined the mutual information between this signal and the interneuron responses. We found that for a synchrony encoding, FS interneurons carried more information in the first five milliseconds, while both interneuron subtypes carried more information than excitatory neurons in later responses. For a rate encoding, we found that RS interneurons carried more information after several milliseconds. These data demonstrate that distinct interneuron subtypes in the neocortex have distinct sensitivities to synchrony versus rate codes.

In order to address whether distinct subtypes of neurons are more sensitive to information carried by synchrony versus rate, Prince et al. used optical stimulation in slices of somatosensory cortex from mouse lines labelling fast-spiking (FS) and regular-spiking (RS) interneurons. They demonstrated that FS and RS interneurons had differential sensitivity to changes in synchrony and rate, which advances our understanding of neural processing in the neocortex.

Synaptic Plasticity and Oscillations in Alzheimer's Disease: A Complex Picture of a Multifaceted Disease

Brain plasticity is widely accepted as the core neurophysiological basis of memory and is generally defined by activity-dependent changes in synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). By using diverse induction protocols like high-frequency stimulation (HFS) or spike-timing dependent plasticity (STDP), such crucial cognition-relevant plastic processes are shown to be impaired in Alzheimer’s disease (AD). In AD, the severity of the cognitive impairment also correlates with the level of disruption of neuronal network dynamics. Currently under debate, the named amyloid hypothesis points to amyloid-beta peptide 1–42 (Aβ42) as the trigger of the functional deviations underlying cognitive impairment in AD. However, there are missing functional mechanistic data that comprehensively dissect the early subtle changes that lead to synaptic dysfunction and subsequent neuronal network collapse in AD. The convergence of the study of both, mechanisms underlying brain plasticity, and neuronal network dynamics, may represent the most efficient approach to address the early triggering and aberrant mechanisms underlying the progressive clinical cognitive impairment in AD. Here we comment on the emerging integrative roles of brain plasticity and network oscillations in AD research and on the future perspectives of research in this field.

Understanding Changes in Hippocampal Interneurons Subtypes in the Pathogenesis of Alzheimer's Disease: A Systematic Review

Background: It is becoming increasingly recognized that there is significant interneuron degeneration in Alzheimer's disease. As the hippocampus is integral for learning and memory, we performed a systematic review of primary literature focused on the relationship between Alzheimer's and hippocampal interneurons. In this study, we summarize the experimental work performed to date and identify opportunities for future experiments. Objectives: This PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses)-style systematic review seeks to summarize the findings of all accessible research focused on cholecystokinin (CCK), neuropeptide Y (NPY), parvalbumin (PV), and somatostatin (SOM) interneurons in the hippocampal formation. Results: One thousand five hundred ninety-three articles were pulled from PubMed, PsycInfo, and Web of Science, based on three blocks of search terms. There were 45 articles that met all the predetermined inclusion/exclusion criteria. There is strong evidence that PV interneurons are affected early in the disease by toxic amyloid beta (Aβ) fragments; SOM interneurons are affected indirectly while the SOM neuropeptide may act to slowly worsen toxic Aβ fragment accumulation, whereas NPY- and CCK-positive interneurons are affected later in the progression of the disease. Conclusions: Fewer studies have been performed on NPY and CCK interneurons, and there is room for further investigations regarding the role of PV interneurons in Alzheimer's to help resolve contradictory findings. This review found that PV interneurons are affected early in the disease, but only in Alzheimer's precursor protein but not tau models. NPY and CCK interneurons were found to be affected later in the disease, and SOM interneurons vary greatly. Future studies may consider reporting immunohistochemical studies inclusive of either cell location or morphology-as well as marker to give a more robust picture of the disease.

Abnormal theta oscillation aggravated by chronic stress in the CA1 may mediate the deterioration of fear memory impairment induced by lipopolysaccharide

Both environmental stress and immune challenge can induce abnormal neurobehavior. However, the impact of chronic stress on immune challenge-related neurobehavioral abnormalities is still controversial. Hence, we aimed to investigate the effects of chronic stress on immune challenge-related neurobehavioral abnormalities and explore the possible underlying mechanisms. During the first set of experiments, mice were reared under normal condition (NC) or chronic stress (CS) for 4 consecutive weeks. They were allocated to the following four groups: NC + normal saline (NS) group, CS + NS group, NC + lipopolysaccharide (LPS) group, and CS + LPS group. Open field, elevated plus maze, fear conditioning, novel object recognition, and forced swimming tests were performed, and their tissues were harvested. During the second set of experiments, after rearing the mice under the above conditions for 3 weeks, microelectrodes were implanted into the CA1 of the hippocampus. After recovery for 1 week under the respective environmental conditions, the mice were allocated to four groups, as in the first experiments. The basal (home cage) and task (fear conditioning)-related local field potential (LFP) were recorded. In the present study, LPS significantly induced a decrease in the freezing to context and discrimination ratio. However, only the freezing to context was further reduced by prior chronic stress. This suggested that chronic stress worsened fear memory impairment induced by acute LPS challenge. Consistent with the change in fear memory, LPS significantly decreased the expression of PV in the CA1, which was further downregulated by prior chronic stress. On the other hand, LPS inhibited the power of both basal and task-related θ oscillations in the CA1. Only the task-related θ power was further decreased by chronic stress. In conclusion, our study showed that the phenotypic loss of PV interneurons and the decrease in the power of the θ oscillation in the CA1 aggravated by chronic stress may mediate, at least in part, the deterioration of fear memory impairment induced by LPS.

Parvalbumin interneuron-mediated neural disruption in an animal model of postintensive care syndrome: prevention by fluoxetine

Postintensive care syndrome (PICS) is defined as a new or worsening impairment in cognition, mental health, and physical function after critical illness and persisting beyond hospitalization, which is associated with reduced quality of life and increased mortality. Recently, we have developed a clinically relevant animal model of PICS based on two-hit hypothesis. However, the underlying mechanism remains unclear. Accumulating evidence has demonstrated that hippocampal GABAergic interneuron dysfunction is implicated in various mood disorders induced by stress. Thus, this study investigated the role of hippocampal GABAergic interneurons and relevant neural activities in an animal model of PICS. In addition, we tested whether fluoxetine treatment early following combined stress can prevent these anatomical and behavioral pathologies. In the present study, we confirmed our previous study that this PICS model displayed reproducible anxiety- and depression like behavior and cognitive impairments, which resembles clinical features of human PICS. This behavioral state is accompanied by hippocampal neuroinflammation, reduced parvalbumin (PV) expression, and decreased theta and gamma power. Importantly, chronic fluoxetine treatment reversed most of these abnormities. In summary, our study provides additional evidence that PV interneuron-mediated hippocampal network activity disruption might play a key role in the pathology of PICS, while fluoxetine offers protection via modulation of the hippocampal PV interneuron and relevant network activities.

Longitudinal tracing of neurochemical metabolic disorders in working memory neural circuit and optogenetics modulation in rats with vascular cognitive impairment

Chronic cerebral ischemia leads to vascular cognitive impairment (VCI) that exacerbates along with ischemia time and eventually develops into dementia. Recent advances in molecular neuroimaging contribute to understand its pathological characteristics. We previously traced the anisotropic diffusion of water molecules suggests that chronic cerebral ischemia leads to irreversible progressive damage to white matter integrity. However, the abnormalities of gray matter activity following chronic cerebral ischemia remains not entirely understood. In this study, in vivo hydrogen proton magnetic resonance spectroscopy (¹H-MRS) was applied to longitudinally track the neurochemical metabolic disorder of gray matter associated with working memory, and optogenetics modulation of neurochemical metabolism was performed for targeted treatment of VCI. The results showed that the concentration of N-acetylaspartate (NAA) in the right hippocampus, left hippocampus, right medial prefrontal cortex (mPFC) and mediodorsal thalamus was decreased as early as 7 days after chronic cerebral ischemia, subsequently gamma-aminobutyric acid (GABA) declined whereas myo-inositol (mI) and glutamate (Glu) increased at 14 days, as well as choline (Cho) lost at 28 days, concurrently the change of Glu and GABA in the mPFC and hippocampus was ischemia time-dependent manner within 1 month. Behaviorally, working memory and object recognition memory were impaired at 14 days, 28 days that significantly correlated with neurochemical metabolic disorders. Interestingly, using optogenetics modulation of PV neurons in the mPFC, the metabolic abnormalities of NAA and GABA in working memory neural circuit could be repaired after chronic cerebral ischemia, together with behavior improvements. These findings suggested that as early as 1∼4 weeks after chronic cerebral ischemia, the metabolism of NAA, Glu, mI and Cho was synchronously impaired in neural circuit of hippocampus-mediodorsal thalamus-mPFC, and the loss of GABA delayed in the hippocampus, and optogenetics modulation of parvalbumin (PV) neurons in the mPFC can improve the neurochemical metabolism of working memory neural circuit and enhance working memory.

Acute Effects of Two Different Species of Amyloid-β on Oscillatory Activity and Synaptic Plasticity in the Commissural CA3-CA1 Circuit of the Hippocampus

Recent evidence indicates that soluble amyloid-β (Aβ) species induce imbalances in excitatory and inhibitory transmission, resulting in neural network functional impairment and cognitive deficits during early stages of Alzheimer's disease (AD). To evaluate the in vivo effects of two soluble Aβ species (Aβ_25-35 and Aβ_1-40) on commissural CA3-to-CA1 (cCA3-to-CA1) synaptic transmission and plasticity, and CA1 oscillatory activity, we used acute intrahippocampal microinjections in adult anaesthetized male Wistar rats. Soluble Aβ microinjection increased cCA3-to-CA1 synaptic variability without significant changes in synaptic efficiency. High-frequency CA3 stimulation was rendered inefficient by soluble Aβ intrahippocampal injection to induce long-term potentiation and to enhance synaptic variability in CA1, contrasting with what was observed in vehicle-injected subjects. Although soluble Aβ microinjection significantly increased the relative power of γ-band and ripple oscillations and significantly shifted the average vector of θ-to-γ phase-amplitude coupling (PAC) in CA1, it prevented θ-to-γ PAC shift induced by high-frequency CA3 stimulation, opposite to what was observed in vehicle-injected animals. These results provide further evidence that soluble Aβ species induce synaptic dysfunction causing abnormal synaptic variability, impaired long-term plasticity, and deviant oscillatory activity, leading to network activity derailment in the hippocampus.

Ketamine Alters Functional Gamma and Theta Resting-State Connectivity in Healthy Humans: Implications for Schizophrenia Treatment Targeting the Glutamate System

Disturbed functional connectivity is assumed to cause neurocognitive deficits in patients suffering from schizophrenia. A Glutamate N-methyl-D-aspartate receptor (NMDAR) dysfunction has been suggested as a possible mechanism underlying altered connectivity in schizophrenia, especially in the gamma- and theta-frequency range. The present study aimed to investigate the effects of the NMDAR-antagonist ketamine on resting-state power, functional connectivity, and schizophrenia-like psychopathological changes in healthy volunteers. In a placebo-controlled crossover design, 25 healthy subjects were recorded using resting-state 64-channel-electroencephalography (EEG) (eyes closed). The imaginary coherence-based Multivariate Interaction Measure (MIM) was used to measure gamma and theta connectivity across 80 cortical regions. The network-based statistic was applied to identify involved networks under ketamine. Psychopathology was assessed with the Positive and Negative Syndrome Scale (PANSS) and the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC). Ketamine caused an increase in all PANSS (p < 0.001) as well as 5D-ASC scores (p < 0.01). Significant increases in resting-state gamma and theta power were observed under ketamine compared to placebo (p < 0.05). The source-space analysis revealed two distinct networks with an increased mean functional gamma- or theta-band connectivity during the ketamine session. The gamma-network consisted of midline regions, the cuneus, the precuneus, and the bilateral posterior cingulate cortices, while the theta-band network involved the Heschl gyrus, midline regions, the insula, and the middle cingulate cortex. The current source density (CSD) within the gamma-band correlated negatively with the PANSS negative symptom score, and the activity within the gamma-band network correlated negatively with the subjective changed meaning of percepts subscale of the 5D-ASC. These results are in line with resting-state patterns seen in people who have schizophrenia and argue for a crucial role of the glutamate system in mediating dysfunctional gamma- and theta-band-connectivity in schizophrenia. Resting-state networks could serve as biomarkers for the response to glutamatergic drugs or drug development efforts within the glutamate system.

GABAergic Inhibitory Interneuron Deficits in Alzheimer's Disease: Implications for Treatment

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized clinically by severe cognitive deficits and pathologically by amyloid plaques, neuronal loss, and neurofibrillary tangles. Abnormal amyloid β-protein (Aβ) deposition in the brain is often thought of as a major initiating factor in AD neuropathology. However, gamma-aminobutyric acid (GABA) inhibitory interneurons are resistant to Aβ deposition, and Aβ decreases synaptic glutamatergic transmission to decrease neural network activity. Furthermore, there is now evidence suggesting that neural network activity is aberrantly increased in AD patients and animal models due to functional deficits in and decreased activity of GABA inhibitory interneurons, contributing to cognitive deficits. Here we describe the roles played by excitatory neurons and GABA inhibitory interneurons in Aβ-induced cognitive deficits and how altered GABA interneurons regulate AD neuropathology. We also comprehensively review recent studies on how GABA interneurons and GABA receptors can be exploited for therapeutic benefit. GABA interneurons are an emerging therapeutic target in AD, with further clinical trials urgently warranted.

Distinct subtypes of inhibitory interneurons differentially promote the propagation of rate and temporal codes in the feedforward neural network

Sensory information is believed to be encoded in neuronal spikes using two different neural codes, the rate code (spike firing rate) and the temporal code (precisely-timed spikes). Since the sensory cortex has a highly hierarchical feedforward structure, sensory information-carrying neural codes should reliably propagate across the feedforward network (FFN) of the cortex. Experimental evidence suggests that inhibitory interneurons, such as the parvalbumin-positive (PV) and somatostatin-positive (SST) interneurons, that have distinctively different electrophysiological and synaptic properties, modulate the neural codes during sensory information processing in the cortex. However, how PV and SST interneurons impact on the neural code propagation in the cortical FFN is unknown. We address this question by building a five-layer FFN model consisting of a physiologically realistic Hodgkin-Huxley-type models of excitatory neurons and PV/SST interneurons at different ratios. In response to different firing rate inputs (20-80 Hz), a higher ratio of PV over SST interneurons promoted a reliable propagation of all ranges of firing rate inputs. In contrast, in response to a range of precisely-timed spikes in the form of pulse-packets [with a different number of spikes (α, 40-400 spikes) and degree of dispersion (σ, 0-20 ms)], a higher ratio of SST over PV interneurons promoted a reliable propagation of pulse-packets. Our simulation results show that PV and SST interneurons differentially promote a reliable propagation of the rate and temporal codes, respectively, indicating that the dynamic recruitment of PV and SST interneurons may play critical roles in a reliable propagation of sensory information-carrying neural codes in the cortical FFN.

Rodent EEG: Expanding the Spectrum of Analysis

In epilepsy research, the analysis of rodent electroencephalogram (EEG) has been performed by many laboratories with a variety of techniques. However, the acquisition and basic analysis of rodent EEG have only recently been standardized. Since a number of software platforms and increased computational power have become widely available, advanced rodent EEG analysis is now more accessible to investigators working with rodent models of epilepsy. In this review, the approach to the analysis of rodent EEG will be examined, including the evaluation of both epileptiform and background activity. Major caveats when employing these analyses, cellular and circuit-level correlates of EEG changes, and important differences between rodent and human EEG are also reviewed. The currently available techniques show great promise in gaining a deeper understanding of the complexities hidden within the EEG in rodent models of epilepsy.

Dissociation of somatostatin and parvalbumin interneurons circuit dysfunctions underlying hippocampal theta and gamma oscillations impaired by amyloid β oligomers in vivo

Accumulation of amyloid β oligomers (AβO) in Alzheimer’s disease (AD) impairs hippocampal theta and gamma oscillations. These oscillations are important in memory functions and depend on distinct subtypes of hippocampal interneurons such as somatostatin-positive (SST) and parvalbumin-positive (PV) interneurons. Here, we investigated whether AβO causes dysfunctions in SST and PV interneurons by optogenetically manipulating them during theta and gamma oscillations in vivo in AβO-injected SST-Cre or PV-Cre mice. Hippocampal in vivo multi-electrode recordings revealed that optogenetic activation of channelrhodopsin-2 (ChR2)-expressing SST and PV interneurons in AβO-injected mice selectively restored AβO-induced reduction of the peak power of theta and gamma oscillations, respectively, and resynchronized CA1 pyramidal cell (PC) spikes. Moreover, SST and PV interneuron spike phases were resynchronized relative to theta and gamma oscillations, respectively. Whole-cell voltage-clamp recordings in CA1 PC in ex vivo hippocampal slices from AβO-injected mice revealed that optogenetic activation of SST and PV interneurons enhanced spontaneous inhibitory postsynaptic currents (IPSCs) selectively at theta and gamma frequencies, respectively. Furthermore, analyses of the stimulus–response curve, paired-pulse ratio, and short-term plasticity of SST and PV interneuron-evoked IPSCs ex vivo showed that AβO increased the initial GABA release probability to depress SST/PV interneuron’s inhibitory input to CA1 PC selectively at theta and gamma frequencies, respectively. Our results reveal frequency-specific and interneuron subtype-specific presynaptic dysfunctions of SST and PV interneurons’ input to CA1 PC as the synaptic mechanisms underlying AβO-induced impairments of hippocampal network oscillations and identify them as potential therapeutic targets for restoring hippocampal network oscillations in early AD.