Extrasynaptic NMDA Receptors Bidirectionally Modulate Intrinsic Excitability of Inhibitory Neurons.

The NMDA subtype glutamate receptors (NMDARs) play important roles in both physiological and pathologic processes in the brain. Compared with their critical roles in synaptic modifications and excitotoxicity in excitatory neurons, much less is understood about the functional contributions of NMDARs to the inhibitory GABAergic neurons. By using selective NMDAR inhibitors and potentiators, we here show that NMDARs bidirectionally modulate the intrinsic excitability (defined as spontaneous/evoked spiking activity and EPSP-spike coupling) in inhibitory GABAergic neurons in adult male and female mice. This modulation depends on GluN2C/2D- but not GluN2A/2B-containing NMDARs. We further show that NMDAR modulator EU1794-4 mostly enhances extrasynaptic NMDAR activity, and by using it we demonstrate a significant contribution of extrasynaptic NMDARs to the modulation of intrinsic excitability in inhibitory neurons. Together, this bidirectional modulation of intrinsic excitability reveals a previously less appreciated importance of NMDARs in the second-to-second functioning of inhibitory GABAergic neurons.SIGNIFICANCE STATEMENT NMDA subtype of glutamate receptors (NMDARs) have important roles in brain functions, including both physiological and pathologic ones. The role of NMDARs in inhibitory neurons has been less elucidated compared with that in excitatory neurons. Our results demonstrate the importance of GluN2C/GluN2D-containing but not GluN2A/GluN2B-containing extrasynaptic NMDARs in modulating the intrinsic excitability of inhibitory neurons. These results further suggest distinct contributions of subsynaptic locations and subunit compositions of NMDARs to their functions in excitatory and inhibitory neurons. The above findings have implications for better understanding of brain diseases, such as schizophrenia.

Hippocampal interlamellar cell-cell connectome that counts.

The hippocampus is regarded as a cognition hub, particularly for learning and memory. Previously, neuronal mechanisms underlying various cognitive functions are delineated with the lamellar hippocampal circuitry, dentate gyrus-CA3 or CA2-CA1, within the transverse plane. More recently, interlamellar (often referred to as longitudinal) projections have received intensive attention to help understand signal convergence and divergence in cognition and behavior. Signal propagation along the longitudinal axis is evidenced by axonal arborization patterns and synaptic responses to electro- and photo-stimulation, further demonstrating that information flow is more enriched in the longitudinal plane than the transverse plane. Here, we review the significance of longitudinal connections for cognition, discuss a putative circuit mechanism of place coding, and suggest the reconceptualization of the hippocampal circuitry.

Rapid and sustained restoration of astrocytic functions by ketamine in depression model mice.

Molecules with fast-acting antidepressant effects have potentials to become new antidepressants. Here we report the fast-acting (1hr) antidepressant effects of ketamine (10 mg/kg, i.p.) in chronic adreno-cortico-tropic-hormone (ACTH)-induced and chronic unpredictable mild stress (CUMS)-induced depression mouse models. These behavioral anti-depressant effects are associated with normalized expression of glutamate transporter-1(GLT-1), glial fibrillary acidic protein (GFAP), brain-derived neurotrophic factor (BDNF) and eukaryotic elongation factor 2 phosphorylation (p-eEF2) in the prelimbic prefrontal cortex (PrL-PFC). Excitatory neurons in PrL also showed reduced ambient glutamate responses to synaptic stimulation, and reduced ambient NMDA receptor responses after ketamine injection. Interestingly, ketamine induced biochemical and electrophysiological changes still occurred with GLT-1 knockdown in PrL, suggesting that elevated GLT-1 level is not required for ketamine to exert its antidepressant effect. At the same time, ketamine did not elevate GLT-1 level in the isolated astrocytes, suggesting distinct contributions from neurons and astrocytes to ketamine-induced changes.

Neuroinflammation mediates noise-induced synaptic imbalance and tinnitus in rodent models.

Hearing loss is a major risk factor for tinnitus, hyperacusis, and central auditory processing disorder. Although recent studies indicate that hearing loss causes neuroinflammation in the auditory pathway, the mechanisms underlying hearing loss-related pathologies are still poorly understood. We examined neuroinflammation in the auditory cortex following noise-induced hearing loss (NIHL) and its role in tinnitus in rodent models. Our results indicate that NIHL is associated with elevated expression of proinflammatory cytokines and microglial activation-two defining features of neuroinflammatory responses-in the primary auditory cortex (AI). Genetic knockout of tumor necrosis factor alpha (TNF-α) or pharmacologically blocking TNF-α expression prevented neuroinflammation and ameliorated the behavioral phenotype associated with tinnitus in mice with NIHL. Conversely, infusion of TNF-α into AI resulted in behavioral signs of tinnitus in both wild-type and TNF-α knockout mice with normal hearing. Pharmacological depletion of microglia also prevented tinnitus in mice with NIHL. At the synaptic level, the frequency of miniature excitatory synaptic currents (mEPSCs) increased and that of miniature inhibitory synaptic currents (mIPSCs) decreased in AI pyramidal neurons in animals with NIHL. This excitatory-to-inhibitory synaptic imbalance was completely prevented by pharmacological blockade of TNF-α expression. These results implicate neuroinflammation as a therapeutic target for treating tinnitus and other hearing loss-related disorders.

Distinct Firing Activities of the Hypothalamic Arcuate Nucleus Neurons to Appetite Hormones.

The hypothalamic arcuate nucleus (Arc) is a central unit that controls the appetite through the integration of metabolic, hormonal, and neuronal afferent inputs. Agouti-related protein (AgRP), proopiomelanocortin (POMC), and dopaminergic neurons in the Arc differentially regulate feeding behaviors in response to hunger, satiety, and appetite, respectively. At the time of writing, the anatomical and electrophysiological characterization of these three neurons has not yet been intensively explored. Here, we interrogated the overall characterization of AgRP, POMC, and dopaminergic neurons using genetic mouse models, immunohistochemistry, and whole-cell patch recordings. We identified the distinct geographical location and intrinsic properties of each neuron in the Arc with the transgenic lines labelled with cell-specific reporter proteins. Moreover, AgRP, POMC, and dopaminergic neurons had different firing activities to ghrelin and leptin treatments. Ghrelin led to the increased firing rate of dopaminergic and AgRP neurons, and the decreased firing rate of POMC. In sharp contrast, leptin resulted in the decreased firing rate of AgRP neurons and the increased firing rate of POMC neurons, while it did not change the firing rate of dopaminergic neurons in Arc. These findings demonstrate the anatomical and physiological uniqueness of three hypothalamic Arc neurons to appetite control.

Dynamics of longitudinal dentate gyrus axons associated with seizure.

KEY POINTS: Axon collaterals of DG granule neurons project towards neighbouring DG granule cell layer Longitudinal axons in the DG-DG circuit possess denser synapses than transverse axons in the DG-CA3 circuit The size of varicosities of the longitudinal axons, but not transverse ones, is regulated by seizures as measured behaviourally Varicosity size of DG-DG axons can be a symptomatic marker of DG-related brain diseases ABSTRACT: The hippocampus network has captured the attention of neuroscientists as a model for understanding cognition and behaviour. Previously, we have identified interlamellar, namely longitudinal, axons between CA1 pyramidal neurons analogous to recurrent connections between CA3 pyramidal neurons. Currently it is unknown whether longitudinal axons of DG granule neurons are present and how they are associated with the behavioural symptoms of seizure. We found longitudinal axons projections from DG granule cells extending to neighbouring DG granule cell layers. These DG-DG axons have more numerous varicosities and are thinner than the DG-CA3 axons, suggesting heavy synaptic formation along a longitudinal axis. Furthermore, the size of varicosities in the DG-DG but not DG-CA3 axons is regulated by seizures as measured behaviourally. The results suggest that the dynamics of longitudinal DG axons is a symptomatic marker of DG-related brain diseases.

Tinnitus Correlates with Downregulation of Cortical Glutamate Decarboxylase 65 Expression But Not Auditory Cortical Map Reorganization.

Hearing loss is the biggest risk factor for tinnitus, and hearing-loss-related pathological changes in the auditory pathway have been hypothesized as the mechanism underlying tinnitus. However, due to the comorbidity of tinnitus and hearing loss, it has been difficult to differentiate between neural correlates of tinnitus and consequences of hearing loss. In this study, we dissociated tinnitus and hearing loss in FVB mice, which exhibit robust resistance to tinnitus following monaural noise-induced hearing loss. Furthermore, knock-down of glutamate decarboxylase 65 (GAD65) expression in auditory cortex (AI) by RNA interference gave rise to tinnitus in normal-hearing FVB mice. We found that tinnitus was significantly correlated with downregulation of GAD65 in the AI. By contrast, cortical map distortions, which have been hypothesized as a mechanism underlying tinnitus, were correlated with hearing loss but not tinnitus. Our findings suggest new strategies for the rehabilitation of tinnitus and other phantom sensation, such as phantom pain.SIGNIFICANCE STATEMENT Hearing loss is the biggest risk factor for tinnitus in humans. Most animal models of tinnitus also exhibit comorbid hearing loss, making it difficult to dissociate the mechanisms underlying tinnitus from mere consequences of hearing loss. Here we show that, although both C57BL/6 and FVB mice exhibited similar noise-induced hearing threshold increase, only C57BL/6, but not FVB, mice developed tinnitus following noise exposure. Although both strains showed frequency map reorganization following noise-induced hearing loss, only C57BL/6 mice had reduced glutamate decarboxylase 65 (GAD65) expression in the auditory cortex (AI). Knocking down GAD65 expression in the AI resulted in tinnitus in normal-hearing FVB mice. Our results suggest that reduced inhibitory neuronal function, but not sensory map reorganization, underlies noise-induced tinnitus.

Epidural Electrotherapy for Epilepsy.

Penetrating electronics have been used for treating epilepsy, yet their therapeutic effects are debated largely due to the lack of a large-scale, real-time, and safe recording/stimulation. Here, the proposed technology integrates ultrathin epidural electronics into an electrocorticography array, therein simultaneously sampling brain signals in a large area for diagnostic purposes and delivering electrical pulses for treatment. The system is empirically tested to record the ictal-like activities of the thalamocortical network in vitro and in vivo using the epidural electronics. Also, it is newly demonstrated that the electronics selectively diminish epileptiform activities, but not normal signal transduction, in live animals. It is proposed that this technology heralds a new generation of diagnostic and therapeutic brain-machine interfaces. Such an electronic system can be applicable for several brain diseases such as tinnitus, Parkinson's disease, Huntington's disease, depression, and schizophrenia.

Interlamellar CA1 network in the hippocampus.

To understand the cellular basis of learning and memory, the neurophysiology of the hippocampus has been largely examined in thin transverse slice preparations. However, the synaptic architecture along the longitudinal septo-temporal axis perpendicular to the transverse projections in CA1 is largely unknown, despite its potential significance for understanding the information processing carried out by the hippocampus. Here, using a battery of powerful techniques, including 3D digital holography and focal glutamate uncaging, voltage-sensitive dye, two-photon imaging, electrophysiology, and immunohistochemistry, we show that CA1 pyramidal neurons are connected to one another in an associational and well-organized fashion along the longitudinal axis of the hippocampus. Such CA1 longitudinal connections mediate reliable signal transfer among the pyramidal cells and express significant synaptic plasticity. These results illustrate a need to reconceptualize hippocampal CA1 network function to include not only processing in the transverse plane, but also operations made possible by the longitudinal network. Our data will thus provide an essential basis for future computational modeling studies on information processing operations carried out in the full 3D hippocampal network that underlies its complex cognitive functions.

Impaired development and competitive refinement of the cortical frequency map in tumor necrosis factor-α-deficient mice.

Early experience shapes sensory representations in a critical period of heightened plasticity. This adaptive process is thought to involve both Hebbian and homeostatic synaptic plasticity. Although Hebbian plasticity has been investigated as a mechanism for cortical map reorganization, less is known about the contribution of homeostatic plasticity. We investigated the role of homeostatic synaptic plasticity in the development and refinement of frequency representations in the primary auditory cortex using the tumor necrosis factor-α (TNF-α) knockout (KO), a mutant mouse with impaired homeostatic but normal Hebbian plasticity. Our results indicate that these mice develop weaker tonal responses and incomplete frequency representations. Rearing in a single-frequency revealed a normal expansion of cortical representations in KO mice. However, TNF-α KOs lacked homeostatic adjustments of cortical responses following exposure to multiple frequencies. Specifically, while this sensory over-stimulation resulted in competitive refinement of frequency tuning in wild-type controls, it broadened frequency tuning in TNF-α KOs. Our results suggest that homeostatic plasticity plays an important role in gain control and competitive interaction in sensory cortical development.

Homeostatic plasticity drives tinnitus perception in an animal model.

Hearing loss often results in tinnitus and auditory cortical map changes, leading to the prevailing view that the phantom perception is associated with cortical reorganization. However, we show here that tinnitus is mediated by a cortical area lacking map reorganization. High-frequency hearing loss results in two distinct cortical regions: a sensory-deprived region characterized by a decrease in inhibitory synaptic transmission and a normal hearing region showing increases in inhibitory and excitatory transmission and map reorganization. Hearing-lesioned animals displayed tinnitus with a pitch in the hearing loss range. Furthermore, drugs that enhance inhibition, but not those that reduce excitation, reversibly eliminated the tinnitus behavior. These results suggest that sensory deprivation-induced homeostatic down-regulation of inhibitory synapses may contribute to tinnitus perception. Enhancing sensory input through map reorganization may plausibly alleviate phantom sensation.

Neurodiagnostic and neurotherapeutic potential of graphene nanomaterials.

Graphene has emerged as a highly promising nanomaterial for a variety of advanced technologies, including batteries, energy, electronics, and biotechnologies. Its recent contribution to neurotechnology is particularly noteworthy because its superior conductivity, chemical resilience, biocompatibility, thermal stability, and scalable nature make it well-suited for measuring brain activity and plasticity in health and disease. Graphene-mediated compounds are microfabricated in two central methods: chemical processes with natural graphite and chemical vapor deposition of graphene in a film form. They are widely used as biosensors and bioelectronics for neurodiagnostic and neurotherapeutic purposes in several brain disorders, such as Parkinson's disease, stroke, glioma, epilepsy, tinnitus, and Alzheimer's disease. This review provides an overview of studies that have demonstrated the technical advances of graphene nanomaterials in neuroscientific and clinical applications. We also discuss current limitations and future demands in relation to the clinical application of graphene, highlighting its potential technological and clinical significance for treating brain disorders. Our review underscores the potential of graphene nanomaterials as powerful tools for advancing the understanding of the brain and developing new therapeutic strategies.

Injectable 2D Material-Based Sensor Array for Minimally Invasive Neural Implants.

Intracranial implants for diagnosis and treatment of brain diseases have been developed over the past few decades. However, the platform of conventional implantable devices still relies on invasive probes and bulky sensors in conjunction with large-area craniotomy and provides only limited biometric information. Here, an implantable multi-modal sensor array that can be injected through a small hole in the skull and inherently spread out for conformal contact with the cortical surface is reported. The injectable sensor array, composed of graphene multi-channel electrodes for neural recording and electrical stimulation and MoS2-based sensors for monitoring intracranial temperature and pressure, is designed based on a mesh structure whose elastic restoring force enables the contracted device to spread out. It is demonstrated that the sensor array injected into a rabbit's head can detect epileptic discharges on the surface of the cortex and mitigate it by electrical stimulation while monitoring both intracranial temperature and pressure. This method provides good potential for implanting a variety of functional devices via minimally invasive surgery.

Electrophysiological measurements of synaptic connectivity and plasticity in the longitudinal dentate gyrus network from mouse hippocampal slices.

Longitudinal synaptic connections between dentate gyrus (DG) granule neurons in the hippocampus have been found to be correlated with increased anxiety. Here, we present a protocol to assess synaptic connectivity and plasticity in the longitudinal DG network. We detail the steps for (1) obtaining acute mouse hippocampal slices that contain longitudinal DG-DG connections, (2) measuring excitatory postsynaptic potentials using whole-cell patch clamp recording combined with two-photon microscopy and glutamate uncaging, and (3) assessing synaptic plasticity using extracellular field recording. For complete details on the use and execution of this protocol, please refer to Pak et al. (2022).1.

Short-term postsynaptic plasticity facilitates predictive tracking in continuous attractors.

Introduction: The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic transmission and is associated with various neurological and psychiatric disorders. Recently, a novel form of postsynaptic plasticity known as NMDAR-based short-term postsynaptic plasticity (STPP) has been identified. It has been suggested that long-lasting glutamate binding to NMDAR allows for the retention of input information in brain slices up to 500 ms, leading to response facilitation. However, the impact of STPP on the dynamics of neuronal populations remains unexplored. Methods: In this study, we incorporated STPP into a continuous attractor neural network (CANN) model to investigate its effects on neural information encoding in populations of neurons. Unlike short-term facilitation, a form of presynaptic plasticity, the temporally enhanced synaptic efficacy resulting from STPP destabilizes the network state of the CANN by increasing its mobility. Results: Our findings demonstrate that the inclusion of STPP in the CANN model enables the network state to predictively respond to a moving stimulus. This nontrivial dynamical effect facilitates the tracking of the anticipated stimulus, as the enhanced synaptic efficacy induced by STPP enhances the system's mobility. Discussion: The discovered STPP-based mechanism for sensory prediction provides valuable insights into the potential development of brain-inspired computational algorithms for prediction. By elucidating the role of STPP in neural population dynamics, this study expands our understanding of the functional implications of NMDAR-related plasticity in information processing within the brain. Conclusion: The incorporation of STPP into a CANN model highlights its influence on the mobility and predictive capabilities of neural networks. These findings contribute to our knowledge of STPP-based mechanisms and their potential applications in developing computational algorithms for sensory prediction.

Therapeutic effects of phlorotannins in the treatment of neurodegenerative disorders.

Phlorotannins are natural polyphenolic compounds produced by brown marine algae and are currently found in nutritional supplements. Although they are known to cross the blood-brain barrier, their neuropharmacological actions remain unclear. Here we review the potential therapeutic benefits of phlorotannins in the treatment of neurodegenerative diseases. In mouse models of Alzheimer's disease, ethanol intoxication and fear stress, the phlorotannin monomer phloroglucinol and the compounds eckol, dieckol and phlorofucofuroeckol A have been shown to improve cognitive function. In a mouse model of Parkinson's disease, phloroglucinol treatment led to improved motor performance. Additional neurological benefits associated with phlorotannin intake have been demonstrated in stroke, sleep disorders, and pain response. These effects may stem from the inhibition of disease-inducing plaque synthesis and aggregation, suppression of microglial activation, modulation of pro-inflammatory signaling, reduction of glutamate-induced excitotoxicity, and scavenging of reactive oxygen species. Clinical trials of phlorotannins have not reported significant adverse effects, suggesting these compounds to be promising bioactive agents in the treatment of neurological diseases. We therefore propose a putative biophysical mechanism of phlorotannin action in addition to future directions for phlorotannin research.

Cortical surface plasticity promotes map remodeling and alleviates tinnitus in adult mice.

Tinnitus induced by hearing loss is caused primarily by irreversible damage to the peripheral auditory system, which results in abnormal neural responses and frequency map disruption in the central auditory system. It remains unclear whether and how electrical rehabilitation of the auditory cortex can alleviate tinnitus. We hypothesize that stimulation of the cortical surface can alleviate tinnitus by enhancing neural responses and promoting frequency map reorganization. To test this hypothesis, we assessed and activated cortical maps using our newly designed graphene-based electrode array with a noise-induced tinnitus animal model. We found that cortical surface stimulation increased cortical activity, reshaped sensory maps, and alleviated hearing loss-induced tinnitus behavior in adult mice. These effects were likely due to retained long-term synaptic potentiation capabilities, as shown in cortical slices from the mice model. These findings suggest that cortical surface activation can be used to facilitate practical functional recovery from phantom percepts induced by sensory deprivation. They also provide a working principle for various treatment methods that involve electrical rehabilitation of the cortex.

Prelimbic Cortical Stimulation with L-methionine Enhances Cognition through Hippocampal DNA Methylation and Neuroplasticity Mechanisms.

Declining global DNA methylation and cognitive impairment are reported to occur in the normal aging process. It is not known if DNA methylation plays a role in the efficacy of memory-enhancing therapies. In this study, aged animals were administered prelimbic cortical deep brain stimulation (PrL DBS) and/or L-methionine (MET) treatment. We found that PrL DBS and MET (MET-PrL DBS) co-administration resulted in hippocampal-dependent spatial memory enhancements in aged animals. Molecular data suggested MET-PrL DBS induced DNA methyltransferase DNMT3a-dependent methylation, robust synergistic upregulation of neuroplasticity-related genes, and simultaneous inhibition of the memory-suppressing gene calcineurin in the hippocampus. We further found that MET-PrL DBS also activated the PKA-CaMKIIα-BDNF pathway, increased hippocampal neurogenesis, and enhanced dopaminergic and serotonergic neurotransmission. We next inhibited the activity of DNA methyltransferase (DNMT) by RG108 infusion in the hippocampus of young animals to establish a causal relationship between DNMT activity and the effects of PrL DBS. Hippocampal DNMT inhibition in young animals was sufficient to recapitulate the behavioral deficits observed in aged animals and abolished the memory-enhancing and molecular effects of PrL DBS. Our findings implicate hippocampal DNMT as a therapeutic target for PrL DBS and pave way for the potential use of non-invasive neuromodulation modalities against dementia.

Long-term, but not transient, threshold shifts alter the morphology and increase the excitability of cortical pyramidal neurons.

Partial hearing loss often results in enlarged representations of the remaining hearing frequency range in primary auditory cortex (AI). Recent studies have implicated certain types of synaptic plasticity in AI map reorganization in response to transient and long-term hearing loss. How changes in neuronal excitability and morphology contribute to cortical map reorganization is less clear. In the present study, we exposed adult rats to a 4-kHz tone at 123 dB, which resulted in increased thresholds over their entire hearing range. The threshold shift gradually recovered in the lower-frequency, but not the higher-frequency, range. As reported previously, two distinct zones were observed 10 days after the noise exposure, an enlarged lower-characteristic frequency (CF) zone displaying normal threshold and enhanced cortical responses and a higher-CF zone showing higher threshold and a disorganized tonotopic map. Membrane excitability of layer II/III pyramidal neurons increased only in the higher-CF, but not the lower-CF, zone. In addition, dendritic morphology and spine density of the pyramidal neurons were altered in the higher-CF zone only. These results indicate that membrane excitability and neuronal morphology are altered by long-term, but not transient, threshold shift. They also suggest that these changes may contribute to tinnitus but are unlikely to be involved in map expansion in the lower-CF zone.

Altered synaptic plasticity of the longitudinal dentate gyrus network in noise-induced anxiety.

Anxiety is characteristic comorbidity of noise-induced hearing loss (NIHL), which causes physiological changes within the dentate gyrus (DG), a subfield of the hippocampus that modulates anxiety. However, which DG circuit underlies hearing loss-induced anxiety remains unknown. We utilize an NIHL mouse model to investigate short- and long-term synaptic plasticity in DG networks. The recently discovered longitudinal DG-DG network is a collateral of DG neurons synaptically connected with neighboring DG neurons and displays robust synaptic efficacy and plasticity. Furthermore, animals with NIHL demonstrate increased anxiety-like behaviors similar to a response to chronic restraint stress. These behaviors are concurrent with enhanced synaptic responsiveness and suppressed short- and long-term synaptic plasticity in the longitudinal DG-DG network but not in the transverse DG-CA3 connection. These findings suggest that DG-related anxiety is typified by synaptic alteration in the longitudinal DG-DG network.