Automatic detection of foot-strike onsets in a rhythmic forelimb movement

Rhythmic movement is the fundamental motion dynamics characterized by repetitive patterns. Precisely defining onsets in rhythmic movement is essential for a comprehensive analysis of motor functions. Our study introduces an automated method for detecting rat's forelimb foot-strike onsets using deep learning tools. This method demonstrates high accuracy of onset detection by combining two techniques using joint coordinates and behavioral confidence scale. The analysis extends to neural oscillatory responses in the rat's somatosensory cortex, validating the effectiveness of our combined approach. Our technique streamlines experimentation, demanding only a camera and GPU-accelerated computer. This approach is applicable across various contexts and promotes our understanding of brain functions during rhythmic movements.

Lengthened circadian rhythms in mice with self-controlled ambient light intensity

Laboratory animals are typically maintained under 12-h light and 12-h dark (12:12 LD) conditions with a daytime light intensity of ~ 200 lx. In this study, we designed an apparatus that allowed mice to self-select the room light intensity by nose poking. We measured the behavioral rhythms of the mice under this self-controlled light regimen. The mice quickly learned the relationship between their nose pokes and the resulting changes in the light intensity. Under these conditions, the mice exhibited free-running circadian behavior with a period of 24.5 ± 0.4 h. This circadian period was ~ 1 h longer than that of the same strain of mice when they were kept in constant darkness (DD) after 12:12 LD entrainment, and the lengthened period lasted for at least 30 days. The rhythm of the light intensity controlled by the mice also exhibited a similar period, but the phase of the illuminance rhythm preceded the phase of the locomotor activity rhythm. Mice that did not have access to the light controller were also entrained to the illuminance cycle produced by the mice that did have access to the light controller, but with a slightly delayed phase. The rhythm was likely controlled by the canonical circadian clock because mice with tau mutations in the circadian clock gene CSNK1E exhibited short periods of circadian rhythm under the same conditions. These results indicate that the free-running period of mice in the wild may differ from what they exhibit if they are attuned by forced light cycles in laboratories because mice in their natural habitats can self-control their exposure to ambient light, similar to our experimental conditions.

Microglia induce auditory dysfunction after status epilepticus in mice

Auditory dysfunction and increased neuronal activity in the auditory pathways have been reported in patients with temporal lobe epilepsy, but the cellular mechanisms involved are unknown. Here, we report that microglia play a role in the disinhibition of auditory pathways after status epilepticus in mice. We found that neuronal activity in the auditory pathways, including the primary auditory cortex and the medial geniculate body (MGB), was increased and auditory discrimination was impaired after status epilepticus. We further demonstrated that microglia reduced inhibitory synapses on MGB relay neurons over an 8-week period after status epilepticus, resulting in auditory pathway hyperactivity. In addition, we found that local removal of microglia from the MGB attenuated the increase in c-Fos+ relay neurons and improved auditory discrimination. These findings reveal that thalamic microglia are involved in auditory dysfunction in epilepsy.

Late-spiking retrosplenial cortical neurons are not synchronized with neocortical slow waves in anesthetized mice

Neocortical slow waves are critical for memory consolidation. The retrosplenial cortex is thought to facilitate the slow wave propagation to regions beyond the neocortex. However, it remains unclear which population is responsible for the slow wave propagation. To address this issue, we performed in vivo whole-cell recordings to identify neurons that were synchronous and asynchronous with slow waves. By quantifying their intrinsic membrane properties, we observed that the former exhibited regular spiking, whereas the latter exhibited late spiking. Thus, these two cell types transmit information in different directions between the neocortex and subcortical regions.

Stress-induced vagal activity influences anxiety-relevant prefrontal and amygdala neuronal oscillations in male mice

The vagus nerve crucially affects emotions and psychiatric disorders. However, the detailed neurophysiological dynamics of the vagus nerve in response to emotions and its associated pathological changes remain unclear. In this study, we demonstrated that the spike rates of the cervical vagus nerve change depending on anxiety behavior in an elevated plus maze test, and these changes were eradicated in stress-susceptible male mice. Furthermore, instantaneous spike rates of the vagus nerve were negatively and positively correlated with the power of 2-4 Hz and 20-30 Hz oscillations, respectively, in the prefrontal cortex and amygdala. The oscillations also underwent dynamic changes depending on the behavioral state in the elevated plus maze, and these changes were no longer observed in stress-susceptible and vagotomized mice. Chronic vagus nerve stimulation restored behavior-relevant neuronal oscillations with the recovery of altered behavioral states in stress-susceptible mice. These results suggested that physiological vagal-brain communication underlies anxiety and mood disorders.

Non-canonical interplay between glutamatergic NMDA and dopamine receptors shapes synaptogenesis

Direct interactions between receptors at the neuronal surface have long been proposed to tune signaling cascades and neuronal communication in health and disease. Yet, the lack of direct investigation methods to measure, in live neurons, the interaction between different membrane receptors at the single molecule level has raised unanswered questions on the biophysical properties and biological roles of such receptor interactome. Using a multidimensional spectral single molecule-localization microscopy (MS-SMLM) approach, we monitored the interaction between two membrane receptors, i.e. glutamatergic NMDA (NMDAR) and G protein-coupled dopamine D1 (D1R) receptors. The transient interaction was randomly observed along the dendritic tree of hippocampal neurons. It was higher early in development, promoting the formation of NMDAR-D1R complexes in an mGluR5- and CK1-dependent manner, favoring NMDAR clusters and synaptogenesis in a dopamine receptor signaling-independent manner. Preventing the interaction in the neonate, and not adult, brain alters in vivo spontaneous neuronal network activity pattern in male mice. Thus, a weak and transient interaction between NMDAR and D1R plays a structural and functional role in the developing brain.

Generation of Dopamine Transporter (DAT)-mCherry Knock-in Rats by CRISPR-Cas9 Genome Editing

Midbrain dopaminergic neurons respond to rewards and have a crucial role in positive motivation and pleasure. Electrical stimulation of dopaminergic neurons and/or their axonal fibers and arborization has been often used to motivate animals to perform cognitive tasks. Still, the electrical stimulation is incompatible with electrophysiological recordings. In this light, optical stimulation following artificial expression of channelrhodopsin-2 (ChR2) in the cell membrane has been also used, but the expression level of ChR2 varies among researchers. Thus, we attempted to stably express ChR2 fused with a red fluorescence protein, mCherry, in dopaminergic neurons. Since dopamine transporter (DAT) gene is known as a marker for dopaminergic neurons, we inserted ChR2-mCherry into the downstream of the DAT gene locus of the rat genome by clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (CRISPR-Cas9) genome editing and created DAT-ChR2-mCherry knock-in rats. Immunohistochemistry showed that ChR2-mCherry was expressed in dopaminergic neurons in homozygote knock-in rats, whereas whole-cell recordings revealed that ChR2-mCherry-positive neurons did not fire action potentials upon blue light stimulation, indicating that ChR2 was not functional for optogenetics. Nevertheless, fluorescent labeling of dopaminergic neurons mediated by mCherry could help characterize them physiologically and histologically.

Low Atmospheric Oxygen Attenuates Alpha Oscillations in the Primary Motor Cortex of Awake Rats

Oxygen is pivotal for survival of animals. Their cellular activity and cognitive behavior are impaired when atmospheric oxygen is insufficient, called hypoxia. However, concurrent effects of hypoxia on physiological signals are poorly understood. To address this question, we simultaneously recorded local field potentials in the primary motor cortex, primary somatosensory, and anterior cingulate cortex, electrocardiograms, electroolfactograms, and electromyograms of rats under acute hypoxic conditions (i.e., 5.0% O2). Exposure to acute hypoxia significantly attenuated alpha oscillations alone in the primary motor cortex, while we failed to find any effects of acute hypoxia on the oscillatory power in the somatosensory cortex or anterior cingulate cortex. These area- and frequency-specific effects by hypoxia may be accounted for by neural innervation from the brainstem to each cortical area via thalamic relay nuclei. Moreover, we found that heart rate and respiratory rate were increased during acute hypoxia and high heart rate was maintained even after the oxygen level returned to the baseline. Altogether, our study characterizes a systemic effect of atmospheric hypoxia on neural and peripheral signals from physiological viewpoints, leading to bridging a gap between cellular and behavioral levels.

Postnatal Maturation of Membrane Potential Dynamics during in Vivo Hippocampal Ripples

Sharp-wave ripples (SWRs) are transient high-frequency oscillations of local field potentials (LFPs) in the hippocampus and play a critical role in memory consolidation. During SWRs, CA1 pyramidal cells exhibit rapid spike sequences that often replay the sequential activity that occurred during behavior. This temporally organized firing activity gradually emerges during 2 weeks after the eye opening; however, it remains unclear how the organized spikes during SWRs mature at the intracellular membrane potential (Vm) level. Here, we recorded Vm of CA1 pyramidal cells simultaneously with hippocampal LFPs from anesthetized immature mice of either sex after the developmental emergence of SWRs. On postnatal days 16 and 17, Vm dynamics around SWRs were premature, characterized by prolonged depolarizations without either pre- or post-SWR hyperpolarizations. The biphasic hyperpolarizations, features typical of adult SWR-relevant Vm, formed by approximately postnatal day 30. This Vm maturation was associated with an increase in SWR-associated inhibitory inputs to pyramidal cells. Thus, the development of SWR-relevant inhibition restricts the temporal windows for spikes of pyramidal cells and allows CA1 pyramidal cells to organize their spike sequences during SWRs.SIGNIFICANCE STATEMENT Sharp-wave ripples (SWRs) are prominent hippocampal oscillations and play a critical role in memory consolidation. During SWRs, hippocampal neurons synchronously emit spikes with organized temporal patterns. This temporal structure of spikes during SWRs develops during the third and fourth postnatal weeks, but the underlying mechanisms are not well understood. Here, we recorded in vivo membrane potentials from hippocampal neurons in premature mice and suggest that the maturation of SWR-associated inhibition enables hippocampal neurons to produce precisely controlled spike times during SWRs.

Awake hippocampal synchronous events are incorporated into offline neuronal reactivation

Learning novel experiences reorganizes hippocampal neuronal circuits, represented as coordinated reactivation patterns in post-experience offline states for memory consolidation. This study examines how awake synchronous events during a novel run are related to post-run reactivation patterns. The disruption of awake sharp-wave ripples inhibited experience-induced increases in the contributions of neurons to post-experience synchronous events. Hippocampal place cells that participate more in awake synchronous events are more strongly reactivated during post-experience synchronous events. Awake synchronous neuronal patterns, in cooperation with place-selective firing patterns, determine cell ensembles that undergo pronounced increases and decreases in their correlated spikes. Taken together, awake synchronous events are fundamental for identifying hippocampal neuronal ensembles to be incorporated into synchronous reactivation during subsequent offline states, thereby facilitating memory consolidation.

Effects of theta phase precessing optogenetic intervention on hippocampal neuronal reactivation and spatial maps

As animals explore environments, hippocampal place cells sequentially fire at progressively earlier phases of theta oscillations in hippocampal local field potentials. In this study, we evaluated the network-level significance of theta phase-entrained neuronal activity in organizing place cell spike patterns. A closed-loop system was developed in which optogenetic stimulation with a temporal pattern replicating theta phase precession is delivered to hippocampal CA1 neurons when rats traversed a particular region on a linear track. Place cells that had place fields during phase precessing stimulation, but not random phase stimulation, showed stronger reactivation during hippocampal sharp-wave ripples in a subsequent rest period. After the rest period, place cells with place fields that emerged during phase precessing stimulation showed more stable place fields. These results imply that neuronal reactivation and stability of spatial maps are mediated by theta phase precession in the hippocampus.

Analyzing schizophrenia-related phenotypes in mice caused by autoantibodies against NRXN1α in schizophrenia

The molecular pathological mechanisms underlying schizophrenia remain unclear; however, genomic analysis has identified genes encoding important risk molecules. One such molecule is neurexin 1α (NRXN1α), a presynaptic cell adhesion molecule. In addition, novel autoantibodies that target the nervous system have been found in patients with encephalitis and neurological disorders. Some of these autoantibodies inhibit synaptic antigen molecules. Studies have examined the association between schizophrenia and autoimmunity; however, the pathological data remain unclear. Here, we identified a novel autoantibody against NRXN1α in patients with schizophrenia (n = 2.1%) in a Japanese cohort (n = 387). None of the healthy control participants (n = 362) were positive for anti-NRXN1α autoantibodies. Anti-NRXN1α autoantibodies isolated from patients with schizophrenia inhibited the molecular interaction between NRXN1α and Neuroligin 1 (NLGN1) and between NRXN1α and Neuroligin 2 (NLGN2). Additionally, these autoantibodies reduced the frequency of the miniature excitatory postsynaptic current in the frontal cortex of mice. Administration of anti-NRXN1α autoantibodies from patients with schizophrenia into the cerebrospinal fluid of mice reduced the number of spines/synapses in the frontal cortex and induced schizophrenia-related behaviors such as reduced cognition, impaired pre-pulse inhibition, and reduced social novelty preference. These changes were improved through the removal of anti-NRXN1α autoantibodies from the IgG fraction of patients with schizophrenia. These findings demonstrate that anti-NRXN1α autoantibodies transferred from patients with schizophrenia cause schizophrenia-related pathology in mice. Removal of anti-NRXN1α autoantibodies may be a therapeutic target for a subgroup of patients who are positive for these autoantibodies.

Machine learning-based segmentation of the rodent hippocampal CA2 area from Nissl-stained sections

The hippocampus is a center of learning, memory, and spatial navigation. This region is divided into the CA1, CA2, and CA3 areas, which are anatomically different from each other. Among these divisions, the CA2 area is unique in terms of functional relevance to sociality. The CA2 area is often manually detected based on the size, shape, and density of neurons in the hippocampal pyramidal cell layer, but this manual segmentation relying on cytoarchitecture is impractical to apply to a large number of samples and dependent on experimenters' proficiency. Moreover, the CA2 area has been defined based on expression pattern of molecular marker proteins, but it generally takes days to complete immunostaining for such proteins. Thus, we asked whether the CA2 area can be systematically segmented based on cytoarchitecture alone. Since the expression pattern of regulator of G-protein signaling 14 (RGS14) signifies the CA2 area, we visualized the CA2 area in the mouse hippocampus by RGS14-immunostaining and Nissl-counterstaining and manually delineated the CA2 area. We then established "CAseg," a machine learning-based automated algorithm to segment the CA2 area with the F1-score of approximately 0.8 solely from Nissl-counterstained images that visualized cytoarchitecture. CAseg was extended to the segmentation of the prairie vole CA2 area, which raises the possibility that the use of this algorithm can be expanded to other species. Thus, CAseg will be beneficial for investigating unique properties of the hippocampal CA2 area.

Selective serotonin reuptake inhibitors suppress sharp wave ripples in the ventral hippocampus

Biased memory processing contributes to the development and exacerbation of depression, and thus could represent a potential therapeutic target for stress-induced mental disorders. Synchronized spikes in hippocampal neurons, corresponding to sharp wave ripples (SWRs), may play a crucial role in memory reactivation. In this study, we showed that the frequency of SWRs increased in the ventral hippocampus, but not in the dorsal hippocampus, after stress exposure. Administration of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine and fluvoxamine inhibited the generation of ventral hippocampal SWRs and reduced locomotor activity and local field potential power in the gamma bands. These results suggest that the antidepressant effects of SSRIs may be mediated by the suppression of ventral hippocampal SWRs.

Ramelteon administration enhances novel object recognition and spatial working memory in mice

Ramelteon is used to ameliorate sleep disorders that negatively affect memory performance; however, it remains unknown whether ramelteon strengthens neutral memories, which do not involve reward or punishment. To address this, we monitored behavior of mice treated with vehicle/ramelteon while they performed a novel object recognition task and a spontaneous alternation task. Object memory performance in the novel object recognition task was improved only if ramelteon was injected before training, suggesting that ramelteon specifically enhances the acquisition of object recognition memory. Ramelteon also enhanced spatial working memory in the spontaneous alternation task. Altogether, acute ramelteon treatment enhances memory in quasi-natural contexts.

Mesolimbic dopamine release precedes actively sought aversive stimuli in mice

In some models, animals approach aversive stimuli more than those housed in an enriched environment. Here, we found that male mice in an impoverished and unstimulating (i.e., boring) chamber without toys sought aversive air puffs more often than those in an enriched chamber. Using this animal model, we identified the insular cortex as a regulator of aversion-seeking behavior. Activation and inhibition of the insular cortex increased and decreased the frequencies of air-puff self-stimulation, respectively, and the firing patterns of insular neuron ensembles predicted the self-stimulation timing. Dopamine levels in the ventrolateral striatum decreased with passive air puffs but increased with actively sought puffs. Around 20% of mice developed intense self-stimulation despite being offered toys, which was prevented by administering opioid receptor antagonists. This study establishes a basis for comprehending the neural underpinnings of usually avoided stimulus-seeking behaviors.

Hippocampal sharp wave ripples underlie stress susceptibility in male mice

The ventral hippocampus (vHC) is a core brain region for emotional memory. Here, we examined how the vHC regulates stress susceptibility from the level of gene expression to neuronal population dynamics in male mice. Transcriptome analysis of samples from stress-naïve mice revealed that intrinsic calbindin (Calb1) expression in the vHC is associated with susceptibility to social defeat stress. Mice with Calb1 gene knockdown in the vHC exhibited increased stress resilience and failed to show the increase in the poststress ventral hippocampal sharp wave ripple (SWR) rate. Poststress vHC SWRs triggered synchronous reactivation of stress memory-encoding neuronal ensembles and facilitated information transfer to the amygdala. Suppression of poststress vHC SWRs by real-time feedback stimulation or walking prevented social behavior deficits. Taken together, our results demonstrate that internal reactivation of memories of negative stressful episodes supported by ventral hippocampal SWRs serves as a crucial neurophysiological substrate for determining stress susceptibility.

Theta oscillations represent collective dynamics of multineuronal membrane potentials of murine hippocampal pyramidal cells

Theta (θ) oscillations are one of the characteristic local field potentials (LFPs) in the hippocampus that emerge during spatial navigation, exploratory sniffing, and rapid eye movement sleep. LFPs are thought to summarize multineuronal events, including synaptic currents and action potentials. However, no in vivo study to date has directly interrelated θ oscillations with the membrane potentials (Vm) of multiple neurons, and it remains unclear whether LFPs can be predicted from multineuronal Vms. Here, we simultaneously patch-clamp up to three CA1 pyramidal neurons in awake or anesthetized mice and find that the temporal evolution of the power and frequency of θ oscillations in Vms (θ_Vms) are weakly but significantly correlate with LFP θ oscillations (θ_LFP) such that a deep neural network could predict the θ_LFP waveforms based on the θ_Vm traces of three neurons. Therefore, individual neurons are loosely interdependent to ensure freedom of activity, but they partially share information to collectively produce θ_LFP.

An open-source application to identify the three-dimensional locations of electrodes implanted into the rat brain from computed tomography images

Electrophysiological recordings using metal electrodes implanted into the brains have been widely utilized to evaluate neuronal circuit dynamics related to behavior and external stimuli. The most common method for identifying implanted electrode tracks in the brain tissue has been histological examination following postmortem slicing and staining of the brain tissue, which consumes time and resources and occasionally fails to identify the tracks because the brain preparations have been damaged during processing. Recent studies have proposed the use of a promising alternative method, consisting of computed tomography (CT) scanning that can directly reconstruct the three-dimensional arrangements of electrodes in the brains of living animals. In this study, we developed an open-source Python-based application that estimates the location of an implanted electrode from CT image sequences in a rat. After the user manually sets reference coordinates and an area from a sequence of CT images, this application automatically overlays an estimated location of an electrode tip on a histological template image; the estimates are highly accurate, with less than 135μm of error, irrespective of the depth of the brain region. The estimation of an electrode location can be completed within a few minutes. Our simple and user-friendly application extends beyond currently available CT-based electrode localization methods and opens up the possibility of applying this technique to various electrophysiological recording paradigms.

Specific AAV2/PHP.eB-mediated gene transduction of CA2 pyramidal cells via injection into the lateral ventricle

Given its limited accessibility, the CA2 area has been less investigated compared to other subregions of the hippocampus. While the development of transgenic mice expressing Cre recombinase in the CA2 has revealed unique features of this area, the use of mouse lines has several limitations, such as lack of specificity. Therefore, a specific gene delivery system is required. Here, we confirmed that the AAV-PHP.eB capsid preferably infected CA2 pyramidal cells following retro-orbital injection and demonstrated that the specificity was substantially higher after injection into the lateral ventricle. In addition, a tropism for the CA2 area was observed in organotypic slice cultures. Combined injection into the lateral ventricle and stereotaxic injection into the CA2 area specifically introduced the transgene into CA2 pyramidal cells, enabling us to perform targeted patch-clamp recordings and optogenetic manipulation. These results suggest that AAV-PHP.eB is a versatile tool for specific gene transduction in CA2 pyramidal cells.

[Synaptic phagocytosis by multiple glial cell types]

Neurons in the brain build circuits by synapsing with each other, and glial cells are involved in the formation and elimination of synapses. Glial cells include microglia, astrocytes, and oligodendrocytes, each with distinctive functions supported by different gene expression patterns and morphologies, but all have been shown to regulate the number of synapses in the neuronal circuits through a common function, synaptic phagocytosis. It has also been reported that specific glial cell types phagocytose specific synapses in different brain regions and at different times, and some of the molecular mechanisms involved in each phagocytotic process have been elucidated. For example, microglia, the most frequently reported glial cell type in relation to synaptic phagocytes, are known to recognize various "eat me signals" including complement and phagocytose synapses, contributing to the refinement of neuronal circuits during development. More recently, astrocytes and oligodendrocyte precursor cells have also been shown to be involved in synaptic phagocytosis. Interestingly, there are also reports of different types of glial cells phagocytosing the same types of synapses. And in some cases, it has been suggested that different glial cell types regulate each other's synaptic phagocytosis. In this review, we will discuss the significance of synaptic phagocytosis by multiple types of glial cells by presenting recent studies on synaptic phagocytosis by glial cells.

Replacement of Mouse Microglia With Human Induced Pluripotent Stem Cell (hiPSC)-Derived Microglia in Mouse Organotypic Slice Cultures

Microglia, the major immune cells in the brain, are reported to differ in gene expression patterns among species. Therefore, it would be preferable in some cases to use human microglia rather than mouse microglia in microglia-targeted disease research. In the past half a decade, researchers have developed in vivo transplantation methods in which human induced pluripotent stem cell-derived microglia (hiPSC-MG) are transplanted into a living mouse brain. However, in vivo transplantation methods are not necessarily accessible to all researchers due to the difficulty of obtaining the materials needed and the transplantation technique itself. In addition, for in vivo systems for microglia-targeted drug screening, it is difficult to control the pharmacokinetics, especially blood-brain barrier permeability. Therefore, in addition to existing in vivo transplantation systems, the development of an ex vivo transplantation system would help to further evaluate the properties of hiPSC-MG. In this study, we aimed to establish a method to efficiently transplant hiPSC-MG into cultured mouse hippocampal slices. We found that approximately 80% of the total microglia in a cultured slice were replaced by hiPSC-derived microglia when innate microglia were pharmacologically removed prior to transplantation. Furthermore, when neuronal death was induced by applying Kainic acid (KA) to slice cultures, transplanted hiPSC-MG changed their morphology and phagocytosed cell debris. Thus, this study provides a method to transplant hiPSC-MG into the mouse hippocampal slice cultures with a high replacement rate. Because the transplanted microglia survived and exerted phagocytic functions, this method will be useful for evaluating the properties of hiPSC-MG ex vivo.

Prefrontal-amygdalar oscillations related to social behavior in mice

The medial prefrontal cortex and amygdala are involved in the regulation of social behavior and associated with psychiatric diseases but their detailed neurophysiological mechanisms at a network level remain unclear. We recorded local field potentials (LFPs) from the dorsal medial prefrontal cortex (dmPFC) and basolateral amygdala (BLA) while male mice engaged on social behavior. We found that in wild-type mice, both the dmPFC and BLA increased 4–7 Hz oscillation power and decreased 30–60 Hz power when they needed to attend to another target mouse. In mouse models with reduced social interactions, dmPFC 4–7 Hz power further increased especially when they exhibited social avoidance behavior. In contrast, dmPFC and BLA decreased 4–7 Hz power when wild-type mice socially approached a target mouse. Frequency-specific optogenetic manipulations replicating social approach-related LFP patterns restored social interaction behavior in socially deficient mice. These results demonstrate a neurophysiological substrate of the prefrontal cortex and amygdala related to social behavior and provide a unified pathophysiological understanding of neuronal population dynamics underlying social behavioral deficits.

In Utero Electroporation for Manipulation of Specific Neuronal Populations

The complexity of brain functions is supported by the heterogeneity of brain tissue and millisecond-scale information processing. Understanding how complex neural circuits control animal behavior requires the precise manipulation of specific neuronal subtypes at high spatiotemporal resolution. In utero electroporation, when combined with optogenetics, is a powerful method for precisely controlling the activity of specific neurons. Optogenetics allows for the control of cellular membrane potentials through light-sensitive ion channels artificially expressed in the plasma membrane of neurons. Here, we first review the basic mechanisms and characteristics of in utero electroporation. Then, we discuss recent applications of in utero electroporation combined with optogenetics to investigate the functions and characteristics of specific regions, layers, and cell types. These techniques will pave the way for further advances in understanding the complex neuronal and circuit mechanisms that underlie behavioral outputs.

Weak representation of awake/sleep states by local field potentials in aged mice

Senescence affects various aspects of sleep, and it remains unclear how sleep-related neuronal network activity is altered by senescence. Here, we recorded local field potential signals from multiple brain regions covering the forebrain in young (10-week-old) and aged (2-year-old) mice. Interregional LFP correlations across these brain regions could not detect pronounced differences between awake and sleep states in both young and aged mice. Multivariate analyses with machine learning algorithms with uniform manifold approximation and projection and robust continuous clustering demonstrated that LFP correlational patterns at multiple frequency bands, ranging from delta to high gamma bands, in aged mice less represented awake/sleep states than those in young mice. By housing aged mice in an enriched environment, the LFP patterns were changed to more precisely represent awake/sleep states. Our results demonstrate senescence-induced changes in neuronal activity at the network level and provide insight into the prevention of pathological symptoms associated with sleep disturbance in senescence.

Inhibition allocates spikes during hippocampal ripples

Sets of spikes emitted sequentially across neurons constitute fundamental pulse packets in neural information processing, including offline memory replay during hippocampal sharp-wave ripples (SWRs). The relative timing of neuronal spikes is fine-tuned in each spike sequence but can vary between different sequences. However, the microcircuitry mechanism that enables such flexible spike sequencing remains unexplored. We recorded the membrane potentials of multiple hippocampal CA1 pyramidal cells in mice and found that the neurons were transiently hyperpolarized prior to SWRs. The pre-SWR hyperpolarizations were spatiotemporally heterogeneous, and larger hyperpolarizations were associated with later spikes during SWRs. Intracellular blockade of Cl--mediated inhibition reduced pre-SWR hyperpolarizations and advanced spike times. Single-unit recordings also revealed that the pre-SWR firing rates of inhibitory interneurons predicted the SWR-relevant spike times of pyramidal cells. Thus, pre-SWR inhibitory activity determines the sequential spike times of pyramidal cells and diversifies the repertoire of sequence patterns.

Delayed reinforcement hinders subsequent extinction

In operant conditioning, animals associate their own behavior with a reinforcer, and the probability of the behavioral responses is increased. This form of learning is called reinforcement. In contrast, when the previously reinforced responses are no longer paired with a reinforcer, these responses are eventually extinguished. The effectiveness of reinforcement depends primarily on time intervals between reinforcers and responses, but it is not fully understood how the intervals affect subsequent extinction. To address this question, we performed electrical stimulation of the rat medial forebrain bundle (MFB), a part of the brain reward system, and an operant task in which the MFB was electrically stimulated 0.1 s (immediate condition) or 1 s (delayed condition) after the rat's nose was poked. During the first half of the task period (a reinforcement period), nose pokes were associated with MFB stimulation. In contrast, during the second half (an extinction period), we did not stimulate the MFB irrespective of nose pokes. We found that rats exhibited increased nose-poke behaviors during the reinforcement period under both conditions, whereas during the extinction period, nose pokes were more persistent in the delayed condition than in the immediate condition. The persistent responses in the extinction period were independent of responses in the reinforcement period. Therefore, reinforcement and extinction are driven by independent neural mechanisms.

[Microglial Response to Brain Temperature and Neuronal Circuit Reorganization]

Brain tissue is vulnerable to temperature changes, which are known to affect the structure and function of neural circuits. Owing to their dynamic ramified processes, microglia, which serve as immune cells in the brain, are associated with surveillance of the brain environment and mediate synaptic pruning to reorganize neural circuits. In this section, we discuss the possible role of microglia as temperature sensors in the brain via thermosensitive transient receptor potential channels and their role in reorganization of neural circuits.

Phasic firing of dopaminergic neurons in the ventral tegmental area triggers peripheral immune responses

Dopaminergic neurons in the ventral tegmental area (VTA) play a crucial role in the processing of reward-related information. Recent studies with pharmacological manipulations of VTA neuronal activity demonstrated a VTA-induced immunoenhancement in peripheral organs. Here, to examine the detailed physiological dynamics, we took an optogenetic approach in which VTA dopaminergic neurons were selectively activated with millisecond precision. Optogenetic phasic, rather than tonic, stimulation of VTA dopaminergic neurons increased serum cytokine levels, such as IL-2, IL-4 and TNF-α. These results provide direct evidence to link dopaminergic neuronal phasic firing to peripheral immunity. Next, we tested whether cytokine induction in male mice was boosted by female encounters, a natural condition that induces increased active VTA neurons and gamma power. Female encounters increased serum IL-2 levels, which were abolished by pharmacological inhibition of VTA neuronal activity. Taken together, our results highlight the importance of the brain reward system in the treatment and management of immune-related disorders.

Identification of an exporter that regulates vitamin C supply from blood to the brain

Vitamin C (VC) distribution in our body requires VC transporters. However, mammalian VC exporters are yet to be identified. Herein, to unravel this long-standing mystery, we focused on the pathways whereby VC moves from blood to the brain, which should require a VC entrance and exit system composed of an importer and a latent exporter. Via cell-based transport analyses of VC efflux and using knockout mice generated via the CRISPR-Cas9 system, we identified GLUT12/SLC2A12 as a physiologically important VC efflux protein expressed in the choroid plexus; Glut12/Slc2a12 knockout halved the cerebral VC levels, markedly increased VC accumulation in the choroid plexus, and reduced the cerebrospinal fluid VC levels. These findings facilitate our understanding of VC regulation and the physiological impact of VC in our body.

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A long-standing mystery in vitamin C handling in mammalians was uncovered

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GLUT12 was identified as a physiologically important vitamin C efflux protein—VCEP

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GLUT12 is expressed in the choroid plexus and acts as a vitamin C exporter

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Glut12 knockout halved the cerebral vitamin C levels in mice

Evidence that endpoint feedback facilitates intermanual transfer of visuomotor force learning by a cognitive strategy

Humans continuously adapt their movement to a novel environment by recalibrating their sensorimotor system. Recent evidence, however, shows that explicit planning to compensate for external changes, i.e., a cognitive strategy, can also aid performance. If such a strategy is planned in external space, it should improve performance in an effector-independent manner. We tested this hypothesis by examining whether promoting a cognitive strategy during a visual-force adaptation task performed in one hand can facilitate learning for the opposite hand. Participants rapidly adjusted the height of visual bar on screen to a target level by isometrically exerting force on a handle using their right hand. Visuomotor gain increased during the task and participants learned the increased gain. Visual feedback was continuously provided for one group, whereas for another group only the endpoint of the force trajectory was presented. The latter has been reported to promote cognitive strategy use. We found that endpoint feedback produced stronger intermanual transfer of learning and slower response times than continuous feedback. In a separate experiment, we found evidence that aftereffects are reduced when only endpoint feedback is provided, a finding that has been consistently observed when cognitive strategies are used. The results suggest that intermanual transfer can be facilitated by a cognitive strategy. This indicates that the behavioral observation of intermanual transfer can be achieved either by forming an effector-independent motor representation or by sharing an effector-independent cognitive strategy between the hands.

NEW & NOTEWORTHY The causes and consequences of cognitive strategy use are poorly understood. We tested whether a visuomotor task learned in a manner that may promote cognitive strategy use causes greater generalization across effectors. Visual feedback was manipulated to promote cognitive strategy use. Learning consistent with cognitive strategy use for one hand transferred to the unlearned hand. Our result suggests that intermanual transfer can result from a common cognitive strategy used to control both hands.

Thermosensitive receptors in neural stem cells link stress-induced hyperthermia to impaired neurogenesis via microglial engulfment

Social stress impairs hippocampal neurogenesis and causes psychiatric disorders such as depression. Recent studies have highlighted the significance of increased body temperature in stress responses; however, whether and how social stress–induced hyperthermia affects hippocampal neurogenesis remains unknown. Here, using transgenic mice in which the thermosensitive transient receptor potential vanilloid 4 (TRPV4) is conditionally knocked out in Nestin-expressing neural stem cells (NSCs), we found that social defeat stress (SDS)–induced hyperthermia activates TRPV4 in NSCs in the dentate gyrus and thereby impairs hippocampal neurogenesis. Specifically, SDS activated TRPV4 in NSCs and induced the externalization of phosphatidylserine in NSCs, which was recognized by the brain-resident macrophage, microglia, and promoted the microglial engulfment of NSCs. SDS-induced impairment of hippocampal neurogenesis was ameliorated by NSC-specific knockout of TRPV4 or pharmacological removal of microglia. Thus, this study reveals a previously unknown role of thermosensitive receptors expressed by NSCs in stress responses.

Towards threshold invariance in defining hippocampal ripples

Objective.Hippocampal ripples are transient neuronal features observed in high-frequency oscillatory bands of local field potentials (LFPs), and they occur primarily during periods of behavioral immobility and slow-wave sleep. Ripples have been defined based on mathematically engineered features, such as magnitudes, durations, and cycles per event. However, the 'ripple' could vary from laboratory to laboratory because their definition is subject to human bias, including the arbitrary choice of parameters and thresholds. In addition, LFPs are often influenced by myoelectric noise arising from animal movement, making it difficult to distinguish ripples from high-frequency noises. These problems have to be overcome.Approach.We extracted ripple candidates under few constraints and labeled them as binary or stochastic 'true' or 'false' ripples using Gaussian mixed model clustering and a deep convolutional neural network (CNN) in a weakly supervised fashion.Main results.Our automatic method separated ripples and myoelectric noise and was able to detect ripples even when the animals were moving. Moreover, we confirmed that a CNN detected ripples defined by our method. Leave-one-animal-out cross-validation estimated the accuracy, the area under the precision-recall curve, the receiver operating characteristic curve to be 0.88, 0.99 and 0.96, respectively.Significance.Our automatic ripple detection method will reduce time spent on performing laborious experiments and analyses.

Histamine: A Key Neuromodulator of Memory Consolidation and Retrieval

In pharmacological studies conducted on animals over the last four decades, histamine was determined to be a strong modulator of learning and memory. Activation of histamine signaling enhances memory consolidation and retrieval. Even long after learning and forgetting, it can still restore the retrieval of forgotten memories. These findings based on animal studies led to human clinical trials with histamine H₃ receptor antagonists/inverse agonists, which revealed their positive effects on learning and memory. Therefore, histamine signaling is a promising therapeutic target for improving cognitive impairments in patients with various neuropsychiatric disorders, including Alzheimer's disease. While the memory-modulatory effects of histamine receptor agonists and antagonists have been confirmed by several research groups, the underlying mechanisms remain to be elucidated. This review summarizes how the activation and inhibition of histamine signaling influence memory processes, introduces the cellular and circuit mechanisms, and discusses the relationship between the human histaminergic system and learning and memory.

Multiple states in ongoing neural activity in the rat visual cortex

The brain continuously produces internal activity in the absence of afferently salient sensory input. Spontaneous neural activity is intrinsically defined by circuit structures and associated with the mode of information processing and behavioral responses. However, the spatiotemporal dynamics of spontaneous activity in the visual cortices of behaving animals remain almost elusive. Using a custom-made electrode array, we recorded 32-site electrocorticograms in the primary and secondary visual cortex of freely behaving rats and determined the propagation patterns of spontaneous neural activity. Nonlinear dimensionality reduction and unsupervised clustering revealed multiple discrete states of the activity patterns. The activity remained stable in one state and suddenly jumped to another state. The diversity and dynamics of the internally switching cortical states would imply flexibility of neural responses to various external inputs.

Brain-wide mapping of presynaptic inputs to basolateral amygdala neurons

The basolateral amygdala (BLA), a region critical for emotional processing, is the limbic hub that is connected with various brain regions. BLA neurons are classified into different subtypes that exhibit differential projection patterns and mediate distinct emotional behaviors; however, little is known about their presynaptic input patterns. In this study, we employed projection-specific monosynaptic rabies virus tracing to identify the direct monosynaptic inputs to BLA subtypes. We found that each neuronal subtype receives long-range projection input from specific brain regions. In contrast to their specific axonal projection patterns, all BLA neuronal subtypes exhibited relatively similar input patterns. This anatomical organization supports the idea that the BLA is a central integrator that associates sensory information in different modalities with valence and sends associative information to behaviorally relevant brain regions.

Deep neural network-based classification of cardiotocograms outperformed conventional algorithms

Cardiotocography records fetal heart rates and their temporal relationship to uterine contractions. To identify high risk fetuses, obstetricians inspect cardiotocograms (CTGs) by eye. Therefore, CTG traces are often interpreted differently among obstetricians, resulting in inappropriate interventions. However, few studies have focused on quantitative and nonbiased algorithms for CTG evaluation. In this study, we propose a newly constructed deep neural network model (CTG-net) to detect compromised fetal status. CTG-net consists of three convolutional layers that extract temporal patterns and interrelationships between fetal heart rate and uterine contraction signals. We aimed to classify the abnormal group (umbilical artery pH < 7.20 or Apgar score at 1 min < 7) and the normal group from CTG data. We evaluated the performance of the CTG-net with the F1 score and compared it with conventional algorithms, namely, support vector machine and k-means clustering, and another deep neural network model, long short-term memory. CTG-net showed the area under the receiver operating characteristic curve of 0.73 ± 0.04, which was significantly higher than that of long short-term memory. CTG-net, a quantitative and automated diagnostic aid system, enables early intervention for putatively abnormal fetuses, resulting in a reduction in the number of cases of hypoxic injury.

Concurrent recordings of hippocampal neuronal spikes and prefrontal synaptic inputs from an awake rat

A major challenge in neuroscience is linking synapses to cognition and behavior. Here, we developed an experimental technique to concurrently conduct a whole-cell recording of a prefrontal neuron and a multiunit recording of hippocampal neurons from an awake rat. This protocol includes surgical steps to establish a cranial window and 3D printer-based devices to hold the rat. The data sets allow us to directly compare how subthreshold synaptic transmission is associated with suprathreshold spike patterns of neuronal ensembles.

For complete details on the use and execution of this protocol, please refer to Nishimura et al. (2021).

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A surgical craniotomy is performed on the prefrontal cortex

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A microdrive is implanted on the hippocampus

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A patch-clamp recording is obtained from a prefrontal neuron

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Protocol allows simultaneous multiunit and whole-cell recordings

Phagocytic Glial Cells in Brain Homeostasis

Phagocytosis by glial cells has been shown to play an important role in maintaining brain homeostasis. Microglia are currently considered to be the major phagocytes in the brain parenchyma, and these cells phagocytose a variety of materials, including dead cell debris, abnormally aggregated proteins, and, interestingly, the functional synapses of living neurons. The intracellular signaling mechanisms that regulate microglial phagocytosis have been studied extensively, and several important factors, including molecules known as "find me" signals and "eat me" signals and receptors on microglia that are involved in phagocytosis, have been identified. In addition, recent studies have revealed that astrocytes, which are another major glial cell in the brain parenchyma, also have phagocytic abilities. In this review, we will discuss the roles of microglia and astrocytes in phagocytosis-mediated brain homeostasis, focusing on the characteristics and differences of their phagocytic abilities.

The subiculum sensitizes retrosplenial cortex layer 2/3 pyramidal neurons

KEY POINTS

Neurons in the retrosplenial cortex (RSC), a cerebral region that connects synaptically with various brain regions, are known to increase neuronal activity in accordance with hippocampal sharp wave-ripples. Pyramidal cells in granular RSC (gRSC) layer 2/3, but not layer 5, exhibit slowly ramping depolarization and considerably delayed spikes in response to a step-pulse current injection. The latencies of delayed spikes in RSC layer 2/3 pyramidal neurons were shortened by a preceding current injection. This effect was mimicked by activation of axonal afferents from the subiculum, but not of neocortical afferents. The subiculum is likely to facilitate information processing and flow in the RSC.

ABSTRACT

The retrosplenial cortex (RSC), a cerebral region involved in diverse cognitive functions, is an anatomical hub that forms monosynaptic connections with various brain areas. Here, we report a unique form of short-term intrinsic plasticity in mouse granular RSC layer 2/3 pyramidal cells. These cells exhibited delayed spikes in response to somatic current injection, but the spike latencies were shortened by a preceding brief depolarization (priming). This priming-induced sensitization is distinct from desensitization, which is commonly observed in other cortical neurons. The facilitatory priming effect lasted for more than 3 s, providing a time window for increased sensitivity to RSC inputs. Based on in vitro and in vivo patch-clamp recordings following optogenetic stimulation of axonal fibres, we found that preactivation of subicular afferents replicated the facilitatory priming effect. The results suggest that subicular inputs to RSC layer 2/3 neurons may modulate subsequent information integration in the RSC layer 2/3 circuits.

Molecular Characterization of Superficial Layers of the Presubiculum During Development

The presubiculum, a subarea of the parahippocampal region, plays a critical role in spatial navigation and spatial representation. An outstanding aspect of presubicular spatial codes is head-direction selectivity of the firing of excitatory neurons, called head-direction cells. Head-direction selectivity emerges before eye-opening in rodents and is maintained in adulthood through neurophysiological interactions between excitatory and inhibitory neurons. Although the presubiculum has been physiologically profiled in terms of spatial representation during development, the histological characteristics of the developing presubiculum are poorly understood. We found that the expression of vesicular glutamate transporter 2 (VGluT2) could be used to delimit the superficial layers of the presubiculum, which was identified using an anterograde tracer injected into the anterior thalamic nucleus (ATN). Thus, we immunostained slices from mice ranging in age from neonates to adults using an antibody against VGluT2 to evaluate the VGluT2-positive area, which was identified as the superficial layers of the presubiculum, during development. We also immunostained the slices using antibodies against parvalbumin (PV) and somatostatin (SOM) and found that in the presubicular superficial layers, PV-positive neurons progressively increased in number during development, whereas SOM-positive neurons exhibited no increasing trend. In addition, we observed repeating patch structures in presubicular layer III from postnatal days 12. The abundant expression of VGluT2 suggests that the presubicular superficial layers are regulated primarily by VGluT2-mediated excitatory neurotransmission. Moreover, developmental changes in the densities of PV- and SOM-positive interneurons and the emergence of the VGluT2-positive patch structures during adolescence may be associated with the functional development of spatial codes in the superficial layers of the presubiculum.

Minute-encoding neurons in hippocampal-striatal circuits

Animals process temporal information in an ever-changing environment, but the neuronal mechanisms of this process, especially on timescales longer than seconds, remain unresolved. Here, we designed a hippocampus-dependent task in which rats prospectively increased their reward-seeking behavior over a duration of minutes. During this timing behavior, hippocampal and striatal neurons represented successive time points on the order of minutes by gradually changing their firing rates and transiently increasing their firing rates at specific time points. These minute-encoding patterns progressively developed as the rats learned a time-reward relationship, and the patterns underwent flexible scaling in parallel with timing behavior. These observations suggest a neuronal basis in the hippocampal-striatal circuits that enables temporal processing and formation of episodic memory on a timescale of minutes.

Deep Learning-Based Classification of GAD67-Positive Neurons Without the Immunosignal

Excitatory neurons and GABAergic interneurons constitute neural circuits and play important roles in information processing. In certain brain regions, such as the neocortex and the hippocampus, there are fewer interneurons than excitatory neurons. Interneurons have been quantified via immunohistochemistry, for example, for GAD67, an isoform of glutamic acid decarboxylase. Additionally, the expression level of other proteins varies among cell types. For example, NeuN, a commonly used marker protein for postmitotic neurons, is expressed differently across brain regions and cell classes. Thus, we asked whether GAD67-immunopositive neurons can be detected using the immunofluorescence signals of NeuN and the fluorescence signals of Nissl substances. To address this question, we stained neurons in layers 2/3 of the primary somatosensory cortex (S1) and the primary motor cortex (M1) of mice and manually labeled the neurons as either cell type using GAD67 immunosignals. We then sought to detect GAD67-positive neurons without GAD67 immunosignals using a custom-made deep learning-based algorithm. Using this deep learning-based model, we succeeded in the binary classification of the neurons using Nissl and NeuN signals without referring to the GAD67 signals. Furthermore, we confirmed that our deep learning-based method surpassed classic machine-learning methods in terms of binary classification performance. Combined with the visualization of the hidden layer of our deep learning algorithm, our model provides a new platform for identifying unbiased criteria for cell-type classification.

Subthreshold membrane potential dynamics of posterior parietal cortical neurons coupled with hippocampal ripples

During behavioral states of immobility, sleep, and anesthesia, the hippocampus generates high-frequency oscillations called ripples. Ripples occur simultaneously with synchronous neuronal activity in the neocortex, known as slow waves, and contribute to memory consolidation. During these ripples, various neocortical regions exhibit modulations in spike rates and local field activity irrespective of whether they receive direct synaptic inputs from the hippocampus. However, little is known about the subthreshold dynamics of the membrane potentials of neocortical neurons during ripples. We patch-clamped layer 2/3 pyramidal cells in the posterior parietal cortex (PPC), a neocortical region that is involved in allocentric spatial representation of behavioral exploration and sequential series of relevant action potentials during ripples. We simultaneously monitored the membrane potentials of post hoc-identified PPC neurons and the local field potentials of the hippocampus in anesthetized mice. More than 50% of the recorded PPC neurons exhibited significant depolarizations and/or hyperpolarizations during ripples. Histological inspections of the recorded neurons revealed that the ripple-modulated PPC neurons were distributed in the PPC in a spatially non-biased fashion. These results suggest that hippocampal ripples are widely but selectively associated with the subthreshold dynamics of the membrane potentials of PPC neurons even though there is no monosynaptic connectivity between the hippocampus and the PPC.

Prefrontal synaptic activation during hippocampal memory reactivation

Cooperative reactivation of hippocampal and prefrontal neurons is considered crucial for mnemonic processes. To directly record synaptic substances supporting the interregional interactions, we develop concurrent spike recordings of hippocampal neuronal ensembles and whole-cell patch-clamp recordings of medial prefrontal neurons in awake rats. We find that medial prefrontal neurons depolarize when hippocampal neurons synchronize. The depolarization in medial prefrontal neurons is larger when hippocampal place cells that encoded overlapping place fields and place cells that encoded a novel environment are synchronously reactivated. Our results suggest a functional circuit-synapse association that enables prefrontal neurons to read out specific memory traces from the hippocampus.