The pyramidal neuron of the cerebral cortex: morphological and chemical characteristics of the synaptic inputs

Looking at Developmental Neurotoxicity Testing from the Perspective of an Invertebrate Embryo

Developmental neurotoxicity (DNT) of chemical compounds disrupts the formation of a normal brain. There is impressive progress in the development of alternative testing methods for DNT potential in chemicals, some of which also incorporate invertebrate animals. This review briefly touches upon studies on the genetically tractable model organisms of Caenorhabditis elegans and Drosophila melanogaster about the action of specific developmental neurotoxicants. The formation of a functional nervous system requires precisely timed axonal pathfinding to the correct cellular targets. To address this complex key event, our lab developed an alternative assay using a serum-free culture of intact locust embryos. The first neural pathways in the leg of embryonic locusts are established by a pair of afferent pioneer neurons which use guidance cues from membrane-bound and diffusible semaphorin proteins. In a systematic approach according to recommendations for alternative testing, the embryo assay quantifies defects in pioneer navigation after exposure to a panel of recognized test compounds for DNT. The outcome indicates a high predictability for test-compound classification. Since the pyramidal neurons of the mammalian cortex also use a semaphorin gradient for neurite guidance, the assay is based on evolutionary conserved cellular mechanisms, supporting its relevance for cortical development.

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.

Mechanisms underlying dorsolateral prefrontal cortex contributions to cognitive dysfunction in schizophrenia

Kraepelin, in his early descriptions of schizophrenia (SZ), characterized the illness as having "an orchestra without a conductor." Kraepelin further speculated that this "conductor" was situated in the frontal lobes. Findings from multiple studies over the following decades have clearly implicated pathology of the dorsolateral prefrontal cortex (DLPFC) as playing a central role in the pathophysiology of SZ, particularly with regard to key cognitive features such as deficits in working memory and cognitive control. Following an overview of the cognitive mechanisms associated with DLPFC function and how they are altered in SZ, we review evidence from an array of neuroscientific approaches addressing how these cognitive impairments may reflect the underlying pathophysiology of the illness. Specifically, we present evidence suggesting that alterations of the DLPFC in SZ are evident across a range of spatial and temporal resolutions: from its cellular and molecular architecture, to its gross structural and functional integrity, and from millisecond to longer timescales. We then present an integrative model based upon how microscale changes in neuronal signaling in the DLPFC can influence synchronized patterns of neural activity to produce macrocircuit-level alterations in DLPFC activation that ultimately influence cognition and behavior. We conclude with a discussion of initial efforts aimed at targeting DLPFC function in SZ, the clinical implications of those efforts, and potential avenues for future development.

Analysis of P2X7-Induced Neuronal Branching

P2X7 receptors regulate different aspects of neuronal development, including neurogenesis, dendritic outgrowth, and axonal elongation. Primary neuronal culture is a widely used model system in neuroscience as it enables to study molecular and cellular events caused by the activation of different ion channels, receptors, and transporters under controlled conditions. Primary neuronal cultures derived from normal and genetically modified mouse models can be used with a wide array of molecular biological, anatomical, and functional techniques such as RNA sequencing, western blots, immunostaining, Ca²⁺ imaging, and electrophysiology. In addition, they can also be genetically manipulated relatively easily. Moreover, cells can survive for multiple weeks if they are properly maintained and thus the development and maturation of individual neurons and their morphological properties can be studied under different conditions. Here, we present a protocol for the isolation and culturing of primary hippocampal cells from embryonic mouse hippocampal tissue (embryonic days 17.5-18.5). The neurons are plated in poly-L-lysine/laminin coated coverslips, where astroglia proliferation is controlled for the proper study of individual primary neurons. To investigate the development of dendrites and axons, as a good correlate of neuron morphology, we present a transfection protocol, which allows us to fill the whole neuron with a fluorescent protein. Subsequently, we perform tracing and analysis of dendritic branching by Sholl analysis using Neurolucida tracing Software (MBF Bioscience).

The Developmental Changes in Intrinsic and Synaptic Properties of Prefrontal Neurons Enhance Local Network Activity from the Second to the Third Postnatal Weeks in Mice

The prefrontal cortex (PFC) is characterized by protracted maturation. The cellular mechanisms controlling the early development of prefrontal circuits are still largely unknown. Our study delineates the developmental cellular processes in the mouse medial PFC (mPFC) during the second and the third postnatal weeks and characterizes their contribution to the changes in network activity. We show that spontaneous inhibitory postsynaptic currents (sIPSC) are increased, whereas spontaneous excitatory postsynaptic currents (sEPSC) are reduced from the second to the third postnatal week. Drug application suggested that the increased sEPSC frequency in mPFC at postnatal day 10 (P10) is due to depolarizing γ-aminobutyric acid (GABA) type A receptor function. To further validate this, perforated patch-clamp recordings were obtained and the expression levels of K-Cl cotransporter 2 (KCC2) protein were examined. The reversal potential of IPSCs in response to current stimulation was significantly more depolarized at P10 than P20 while KCC2 expression is decreased. Moreover, the number of parvalbumin-expressing GABAergic interneurons increases and their intrinsic electrophysiological properties significantly mature in the mPFC from P10 to P20. Using computational modeling, we show that the developmental changes in synaptic and intrinsic properties of mPFC neurons contribute to the enhanced network activity in the juvenile compared with neonatal mPFC.

A Kalirin missense mutation enhances dendritic RhoA signaling and leads to regression of cortical dendritic arbors across development

Normally, dendritic size is established prior to adolescence and then remains relatively constant into adulthood due to a homeostatic balance between growth and retraction pathways. However, schizophrenia is characterized by accelerated reductions of cerebral cortex gray matter volume and onset of clinical symptoms during adolescence, with reductions in layer 3 pyramidal neuron dendritic length, complexity, and spine density identified in multiple cortical regions postmortem. Nogo receptor 1 (NGR1) activation of the GTPase RhoA is a major pathway restricting dendritic growth in the cerebral cortex. We show that the NGR1 pathway is stimulated by OMGp and requires the Rho guanine nucleotide exchange factor Kalirin-9 (KAL9). Using a genetically encoded RhoA sensor, we demonstrate that a naturally occurring missense mutation in Kalrn, KAL-PT, that was identified in a schizophrenia cohort, confers enhanced RhoA activitation in neuronal dendrites compared to wild-type KAL. In mice containing this missense mutation at the endogenous locus, there is an adolescent-onset reduction in dendritic length and complexity of layer 3 pyramidal neurons in the primary auditory cortex. Spine density per unit length of dendrite is unaffected. Early adult mice with these structural deficits exhibited impaired detection of short gap durations. These findings provide a neuropsychiatric model of disease capturing how a mild genetic vulnerability may interact with normal developmental processes such that pathology only emerges around adolescence. This interplay between genetic susceptibility and normal adolescent development, both of which possess inherent individual variability, may contribute to heterogeneity seen in phenotypes in human neuropsychiatric disease.

Vesicular glutamate transporter 2-containing neurons of the centrally-projecting Edinger-Westphal nucleus regulate alcohol drinking and body temperature

Previous studies in rodents have repeatedly demonstrated that the centrally-projecting Edinger-Westphal nucleus (EWcp) is highly sensitive to alcohol and is also involved in regulating alcohol intake and body temperature. Historically, the EWcp has been known as the main site of Urocortin 1 (Ucn1) expression, a corticotropin-releasing factor-related peptide, in the brain. However, the EWcp also contains other populations of neurons, including neurons that express the vesicular glutamate transporter 2 (Vglut2). Here we transduced the EWcp with adeno-associated viruses (AAVs) encoding Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to test the role of the EWcp in alcohol drinking and in the regulation of body temperature. Activation of the EWcp with excitatory DREADDs inhibited alcohol intake in a 2-bottle choice procedure in male C57BL/6J mice, whereas inhibition of the EWcp with DREADDs had no effect. Surprisingly, analysis of DREADD expression indicated Ucn1-containing neurons of the EWcp did not express DREADDs. In contrast, AAVs transduced non-Ucn1-containing EWcp neurons. Subsequent experiments showed that the inhibitory effect of EWcp activation on alcohol intake was also present in male Ucn1 KO mice, suggesting that a Ucn1-devoid population of EWcp regulates alcohol intake. A final set of chemogenetic experiments showed that activation of Vglut2-expressing EWcp neurons inhibited alcohol intake and induced hypothermia in male and female mice. These studies expand on previous literature by indicating that a glutamatergic, Ucn1-devoid subpopulation of the EWcp regulates alcohol consumption and body temperature.

Proteomic Profiling of Astrocytic O-GlcNAc Transferase-Related Proteins in the Medial Prefrontal Cortex

The medial prefrontal cortex (mPFC), a key part of the brain networks that are closely related to the regulation of behavior, acts as a key regulator in emotion, social cognition, and decision making. Astrocytes are the majority cell type of glial cells, which play a significant role in a number of processes and establish a suitable environment for the functioning of neurons, including the brain energy metabolism. Astrocyte’s dysfunction in the mPFC has been implicated in various neuropsychiatric disorders. Glucose is a major energy source in the brain. In glucose metabolism, part of glucose is used to convert UDP-GlcNAc as a donor molecule for O-GlcNAcylation, which is controlled by a group of enzymes, O-GlcNAc transferase enzyme (OGT), and O-GlcNAcase (OGA). However, the role of O-GlcNAcylation in astrocytes is almost completely unknown. Our research showed that astrocytic OGT could influence the expression of proteins in the mPFC. Most of these altered proteins participate in metabolic processes, transferase activity, and biosynthetic processes. GFAP, an astrocyte maker, was increased after OGT deletion. These results provide a framework for further study on the role of astrocytic OGT/O-GlcNAcylation in the mPFC.

Inhibitory control of synaptic signals preceding locomotion in mouse frontal cortex

The frontal cortex is essential for organizing voluntary movement. The secondary motor cortex (MOs) is a frontal subregion thought to integrate internal and external inputs before motor action. However, how excitatory and inhibitory synaptic inputs to MOs neurons are integrated preceding movement remains unclear. Here, we address this question by performing in vivo whole-cell recordings from MOs neurons of head-fixed mice moving on a treadmill. We find that principal neurons produce slowly increasing membrane potential and spike ramps preceding spontaneous running. After goal-directed training, ramps show larger amplitudes and accelerated kinetics. Chemogenetic suppression of interneurons combined with modeling suggests that the interplay between parvalbumin-positive (PV+) and somatostatin-positive (SOM+) interneurons, along with principal neuron recurrent connectivity, shape ramping signals. Plasticity of excitatory synapses on SOM+ interneurons can explain the ramp acceleration after training. Altogether, our data reveal that local interneurons differentially control task-dependent ramping signals when MOs neurons integrate inputs preceding movement.

Burst control: Synaptic conditions for burst generation in cortical layer 5 pyramidal neurons

The output of neocortical layer 5 pyramidal cells (L5PCs) is expressed by a train of single spikes with intermittent bursts of multiple spikes at high frequencies. The bursts are the result of nonlinear dendritic properties, including Na+, Ca2+, and NMDA spikes, that interact with the ~10,000 synapses impinging on the neuron's dendrites. Output spike bursts are thought to implement key dendritic computations, such as coincidence detection of bottom-up inputs (arriving mostly at the basal tree) and top-down inputs (arriving mostly at the apical tree). In this study we used a detailed nonlinear model of L5PC receiving excitatory and inhibitory synaptic inputs to explore the conditions for generating bursts and for modulating their properties. We established the excitatory input conditions on the basal versus the apical tree that favor burst and show that there are two distinct types of bursts. Bursts consisting of 3 or more spikes firing at < 200 Hz, which are generated by stronger excitatory input to the basal versus the apical tree, and bursts of ~2-spikes at ~250 Hz, generated by prominent apical tuft excitation. Localized and well-timed dendritic inhibition on the apical tree differentially modulates Na+, Ca2+, and NMDA spikes and, consequently, finely controls the burst output. Finally, we explored the implications of different burst classes and respective dendritic inhibition for regulating synaptic plasticity.

Comprehensive characterization of oscillatory signatures in a model circuit with PV- and SOM-expressing interneurons

Neural circuits contain a wide variety of interneuron types, which differ in their biophysical properties and connectivity patterns. The two most common interneuron types, parvalbumin-expressing and somatostatin-expressing cells, have been shown to be differentially involved in many cognitive functions. These cell types also show different relationships with the power and phase of oscillations in local field potentials. The mechanisms that underlie the emergence of different oscillatory rhythms in neural circuits with more than one interneuron subtype, and the roles specific interneurons play in those mechanisms, are not fully understood. Here, we present a comprehensive analysis of all possible circuit motifs and input regimes that can be achieved in circuits comprised of excitatory cells, PV-like fast-spiking interneurons and SOM-like low-threshold spiking interneurons. We identify 18 unique motifs and simulate their dynamics over a range of input strengths. Using several characteristics, such as oscillation frequency, firing rates, phase of firing and burst fraction, we cluster the resulting circuit dynamics across motifs in order to identify patterns of activity and compare these patterns to behaviors that were generated in circuits with one interneuron type. In addition to the well-known PING and ING gamma oscillations and an asynchronous state, our analysis identified three oscillatory behaviors that were generated by the three-cell-type motifs only: theta-nested gamma oscillations, stable beta oscillations and theta-locked bursting behavior, which have also been observed in experiments. Our characterization provides a map to interpret experimental activity patterns and suggests pharmacological manipulations or optogenetics approaches to validate these conclusions.

Influence of Implantation Depth on the Performance of Intracortical Probe Recording Sites

Microelectrode arrays (MEAs) enable the recording of electrical activity from cortical neurons which has implications for basic neuroscience and neuroprosthetic applications. The design space for MEA technology is extremely wide where devices may vary with respect to the number of monolithic shanks as well as placement of microelectrode sites. In the present study, we examine the differences in recording ability between two different MEA configurations: single shank (SS) and multi-shank (MS), both of which consist of 16 recording sites implanted in the rat motor cortex. We observed a significant difference in the proportion of active microelectrode sites over the 8-week indwelling period, in which SS devices exhibited a consistent ability to record activity, in contrast to the MS arrays which showed a marked decrease in activity within 2 weeks post-implantation. Furthermore, this difference was revealed to be dependent on the depth at which the microelectrode sites were located and may be mediated by anatomical heterogeneity, as well as the distribution of inhibitory neurons within the cortical layers. Our results indicate that the implantation depth of microelectrodes within the cortex needs to be considered relative to the chronic performance characterization.

Three-Dimensional Synaptic Organization of Layer III of the Human Temporal Neocortex

In the present study, we have used focused ion beam/scanning electron microscopy (FIB/SEM) to perform a study of the synaptic organization of layer III of Brodmann's area 21 in human tissue samples obtained from autopsies and biopsies. We analyzed the synaptic density, 3D spatial distribution, and type (asymmetric/symmetric), as well as the size and shape of each synaptic junction of 4945 synapses that were fully reconstructed in 3D. Significant differences in the mean synaptic density between autopsy and biopsy samples were found (0.49 and 0.66 synapses/μm3, respectively). However, in both types of samples (autopsy and biopsy), the asymmetric:symmetric ratio was similar (93:7) and most asymmetric synapses were established on dendritic spines (75%), while most symmetric synapses were established on dendritic shafts (85%). We also compared several electron microscopy methods and analysis tools to estimate the synaptic density in the same brain tissue. We have shown that FIB/SEM is much more reliable and robust than the majority of the other commonly used EM techniques. The present work constitutes a detailed description of the synaptic organization of cortical layer III. Further studies on the rest of the cortical layers are necessary to better understand the functional organization of this temporal cortical region.

Enhanced phosphorylation of S6 protein in mouse cortical layer V and subplate neurons

The mammalian neocortex is composed of six major layers of neurons. Each group of neurons in the cortical layers has distinct characteristics based on the expression of specific genes and connectivity patterns of neural circuits. Neuronal subtype transition and regional identity acquisition are established by temporal cues and interaction between several transcription factors during neurogenesis. The impairment of cortical lamination or neural circuits results in a wide range of neurodevelopmental disorders such as autism, schizophrenia, and certain forms of childhood epilepsy. Despite continuous efforts to classify neurons with the aid of genetic and epigenetic analyses, the neuron-specific properties associated with post-transcriptional modification remain unclear. In the present study, the distribution of phosphorylated S6-positive layers across the neocortex was examined using several layer markers. The development of pS6 S235/236 layers in layer V and the subplate was spatiotemporally regulated in the mouse brain. In addition, enhanced phosphorylation of ribosomal protein S6 in Ctip2-positive layer V neurons in vivo was sustained under in-vitro conditions using a culture of primary cortical neurons.

Neuronal excitation/inhibition imbalance: core element of a translational perspective on Alzheimer pathophysiology

Our incomplete understanding of the link between Alzheimer's Disease pathology and symptomatology is a crucial obstacle for therapeutic success. Recently, translational studies have begun to connect the dots between protein alterations and deposition, brain network dysfunction and cognitive deficits. Disturbance of neuronal activity, and in particular an imbalance in underlying excitation/inhibition (E/I), appears early in AD, and can be regarded as forming a central link between structural brain pathology and cognitive dysfunction. While there are emerging (non-)pharmacological options to influence this imbalance, the complexity of human brain dynamics has hindered identification of an optimal approach. We suggest that focusing on the integration of neurophysiological aspects of AD at the micro-, meso- and macroscale, with the support of computational network modeling, can unite fundamental and clinical knowledge, provide a general framework, and suggest rational therapeutic targets.

Age-Related Changes in the Primary Motor Cortex of Newborn to Adult Domestic Pig Sus scrofa domesticus

The pig has been increasingly used as a suitable animal model in translational neuroscience. However, several features of the fast-growing, immediately motor-competent cerebral cortex of this species have been adequately described. This study analyzes the cytoarchitecture of the primary motor cortex (M1) of newborn, young and adult pigs (Sus scrofa domesticus). Moreover, we investigated the distribution of the neural cells expressing the calcium-binding proteins (CaBPs) (calretinin, CR; parvalbumin, PV) throughout M1. The primary motor cortex of newborn piglets was characterized by a dense neuronal arrangement that made the discrimination of the cell layers difficult, except for layer one. The absence of a clearly recognizable layer four, typical of the agranular cortex, was noted in young and adult pigs. The morphometric and immunohistochemical analyses revealed age-associated changes characterized by (1) thickness increase and neuronal density (number of cells/mm2 of M1) reduction during the first year of life; (2) morphological changes of CR-immunoreactive neurons in the first months of life; (3) higher density of CR- and PV-immunopositive neurons in newborns when compared to young and adult pigs. Since most of the present findings match with those of the human M1, this study strengthens the growing evidence that the brain of the pig can be used as a potentially valuable translational animal model during growth and development.

Islet1 Precursors Contribute to Mature Interneuron Subtypes in Mouse Neocortex

Cortical interneurons (GABAergic cells) arise during embryogenesis primarily from the medial and caudal ganglionic eminences (MGE and CGE, respectively) with a small population generated from the preoptic area (POA). Progenitors from the lateral ganglionic eminence (LGE) are thought to only generate GABAergic medium spiny neurons that populate the striatum and project to the globus pallidus. Here, we report evidence that neuronal precursors that express the LGE-specific transcription factor Islet1 (Isl1) can give rise to a small population of cortical interneurons. Lineage tracing and homozygous deletion of Nkx2.1 in Isl1 fate-mapped mice showed that neighboring MGE/POA-specific Nkx2.1 cells and LGE-specific Isl1 cells make both common and distinct lineal contributions towards cortical interneuron fate. Although the majority of cells had overlapping transcriptional domains between Nkx2.1 and Isl1, a population of Isl1-only derived cells also contributed to the adult cerebral cortex. The data indicate that Isl1-derived cells may originate from both the LGE and the adjacent LGE/MGE boundary regions to generate diverse neuronal progeny. Thus, a small population of neocortical interneurons appear to originate from Isl-1-positive precursors.

Variation in Pyramidal Cell Morphology Across the Human Anterior Temporal Lobe

Pyramidal neurons are the most abundant and characteristic neuronal type in the cerebral cortex and their dendritic spines are the main postsynaptic elements of cortical excitatory synapses. Previous studies have shown that pyramidal cell structure differs across layers, cortical areas, and species. However, within the human cortex, the pyramidal dendritic morphology has been quantified in detail in relatively few cortical areas. In the present work, we performed intracellular injections of Lucifer Yellow at several distances from the temporal pole. We found regional differences in pyramidal cell morphology, which showed large inter-individual variability in most of the morphological variables measured. However, some values remained similar in all cases. The smallest and least complex cells in the most posterior temporal region showed the greatest dendritic spine density. Neurons in the temporal pole showed the greatest sizes with the highest number of spines. Layer V cells were larger, more complex, and had a greater number of dendritic spines than those in layer III. The present results suggest that, while some aspects of pyramidal structure are conserved, there are specific variations across cortical regions, and species.

The Lamprey Forebrain - Evolutionary Implications

The forebrain plays a critical role in a broad range of neural processes encompassing sensory integration and initiation/selection of behaviour. The forebrain functions through an interaction between different cortical areas, the thalamus, the basal ganglia with the dopamine system, and the habenulae. The ambition here is to compare the mammalian forebrain with that of the lamprey representing the oldest now living group of vertebrates, by a review of earlier studies. We show that the lamprey dorsal pallium has a motor, a somatosensory, and a visual area with retinotopic representation. The lamprey pallium was previously thought to be largely olfactory. There is also a detailed similarity between the lamprey and mammals with regard to other forebrain structures like the basal ganglia in which the general organisation, connectivity, transmitters and their receptors, neuropeptides, and expression of ion channels are virtually identical. These initially unexpected results allow for the possibility that many aspects of the basic design of the vertebrate forebrain had evolved before the lamprey diverged from the evolutionary line leading to mammals. Based on a detailed comparison between the mammalian forebrain and that of the lamprey and with due consideration of data from other vertebrate groups, we propose a compelling account of a pan-vertebrate schema for basic forebrain structures, suggesting a common ancestry of over half a billion years of vertebrate evolution.

Coherence and cognition in the cortex: the fundamental role of parvalbumin, myelin, and the perineuronal net

The calcium binding protein parvalbumin is expressed in interneurons of two main morphologies, the basket and chandelier cells, which target perisomatic domains on principal cells and are extensively interconnected in laminar networks by synapses and gap junctions. Beyond its utility as a convenient cellular marker, parvalbumin is an unambiguous identifier of the key role that these interneurons play in the fundamental functions of the cortex. They provide a temporal framework for principal cell activity by propagating gamma oscillation, providing coherence for cortical information processing and the basis for timing-dependent plasticity processes. As these parvalbumin networks mature, they are physically and functionally stabilised by axonal myelination and development of the extracellular matrix structure termed the perineuronal net. This maturation correlates with the emergence of high-speed, highly energetic activity and provides a coherent foundation for the unique ability of the cortex to cross-correlate activity across sensory modes and internal representations.

Differential Structure of Hippocampal CA1 Pyramidal Neurons in the Human and Mouse

Pyramidal neurons are the most common cell type and are considered the main output neuron in most mammalian forebrain structures. In terms of function, differences in the structure of the dendrites of these neurons appear to be crucial in determining how neurons integrate information. To further shed light on the structure of the human pyramidal neurons we investigated the geometry of pyramidal cells in the human and mouse CA1 region-one of the most evolutionary conserved archicortical regions, which is critically involved in the formation, consolidation, and retrieval of memory. We aimed to assess to what extent neurons corresponding to a homologous region in different species have parallel morphologies. Over 100 intracellularly injected and 3D-reconstructed cells across both species revealed that dendritic and axonal morphologies of human cells are not only larger but also have structural differences, when compared to mouse. The results show that human CA1 pyramidal cells are not a stretched version of mouse CA1 cells. These results indicate that there are some morphological parameters of the pyramidal cells that are conserved, whereas others are species-specific.

Comparative Ultrastructural Analysis of Thalamocortical Innervation of the Primary Motor Cortex and Supplementary Motor Area in Control and MPTP-Treated Parkinsonian Monkeys

The synaptic organization of thalamic inputs to motor cortices remains poorly understood in primates. Thus, we compared the regional and synaptic connections of vGluT2-positive thalamocortical glutamatergic terminals in the supplementary motor area (SMA) and the primary motor cortex (M1) between control and MPTP-treated parkinsonian monkeys. In controls, vGluT2-containing fibers and terminal-like profiles invaded layer II-III and Vb of M1 and SMA. A significant reduction of vGluT2 labeling was found in layer Vb, but not in layer II-III, of parkinsonian animals, suggesting a potential thalamic denervation of deep cortical layers in parkinsonism. There was a significant difference in the pattern of synaptic connectivity in layers II-III, but not in layer Vb, between M1 and SMA of control monkeys. However, this difference was abolished in parkinsonian animals. No major difference was found in the proportion of perforated versus macular post-synaptic densities at thalamocortical synapses between control and parkinsonian monkeys in both cortical regions, except for a slight increase in the prevalence of perforated axo-dendritic synapses in the SMA of parkinsonian monkeys. Our findings suggest that disruption of the thalamic innervation of M1 and SMA may underlie pathophysiological changes of the motor thalamocortical loop in the state of parkinsonism.

Developmental Changes in Pyramidal Cell Morphology in Multiple Visual Cortical Areas Using Cluster Analysis

Neuronal morphology is characterized by salient features such as complex axonal and dendritic arbors. In the mammalian brain, variations in dendritic morphology among cell classes, brain regions, and animal species are thought to underlie known differences in neuronal function. In this work, we obtained a large dataset from http://neuromorpho.org/ comprising layer III pyramidal cells in different cortical areas of the ventral visual pathway (V1, V2, V4, TEO, and TE) of the macaque monkey at different developmental stages. We performed an in depth quantitative analysis of pyramidal cell morphology throughout development in an effort to determine which aspects mature early in development and which features require a protracted period of maturation. We were also interested in establishing if developmental changes in morphological features occur simultaneously or hierarchically in multiple visual cortical areas. We addressed these questions by performing principal component analysis (PCA) and hierarchical clustering analysis on relevant morphological features. Our analysis indicates that the maturation of pyramidal cell morphology is largely based on early development of topological features in most visual cortical areas. Moreover, the maturation of pyramidal cell morphology in V1, V2, V4, TEO, and TE is characterized by unique developmental trajectories.

3D Analysis of the Synaptic Organization in the Entorhinal Cortex in Alzheimer's Disease

The entorhinal cortex (EC) is especially vulnerable in the early stages of Alzheimer's disease (AD). In particular, cognitive deficits have been linked to alterations in the upper layers of EC. In the present report, we examined Layers II and III from eight human brain autopsies (four subjects with no recorded neurologic alterations and four AD cases). We used stereological methods to assess cortical atrophy of the EC and possible changes in the volume occupied by different cortical elements (neuronal and glial cell bodies; blood vessels; and neuropil). We performed 3D ultrastructural analyses of synapses using focused ion beam/scanning electron microscopy (FIB/SEM) to examine possible alterations related to AD. At the light microscope level, we found a significantly lower volume fraction occupied by neuronal bodies in Layer III and a higher volume fraction occupied by glial cell bodies in Layer II in AD cases. At the ultrastructural level, we observed that (1) there was a significantly lower synaptic density in both layers in AD cases; (2) synapses were larger and more complex in Layer II in AD cases; and (3) there was a greater proportion of small and simple synapses in Layer III in AD cases than in control individuals. These structural differences may play a role in the anatomic basis for the impairment of cognitive functions in AD.

Pyramidal cell subtype-dependent cortical oscillatory activity regulates motor learning

The cortex processes information through intricate circuitry and outputs to multiple brain areas by different sets of pyramidal cells (PCs). PCs form intra- and inter-laminar subnetworks, depending on PC projection subtypes. However, it remains unknown how individual PC subtypes are involved in cortical network activity and, thereby, in distinct brain functions. Here, we examined the effects of optogenetic manipulations of specific PC subtypes on network activity in the motor cortex. In layer V, the beta/gamma frequency band of oscillation was evoked by photostimulation, depending on PC subtypes. Our experimental and simulation results suggest that oscillatory activity is generated in reciprocal connections between pyramidal tract (PT) and fast-spiking cells. A similar frequency band was also observed in local field potentials during a pattern learning task. Manipulation of PT cell activity affected beta/gamma band power and learning. Our results suggest that PT cell-dependent oscillations play important roles in motor learning.

Otsuka and Kawaguchi investigate how manipulation of pyramidal cell subtypes in the motor cortex affects cortical network activity. Their findings suggest that pyramidal cell type cell-dependent oscillatory activity play an important role in motor learning.

Morphological alterations of the pyramidal and stellate cells of the visual cortex in schizophrenia

Schizophrenia is a severe brain disorder characterized by certain types of delusion, hallucination and thought disorder. Studies have revealed impaired synaptic plasticity and reduced gamma-aminobutyric acid levels of the visual cortex in patients with schizophrenia. While previous work established a critical role for interneurons and cortical connectivity in the generation of hallucinations, the present study set out to examine the morphology of pyramidal cells and interneurons from layers 3 and 4 in the primary visual cortex from schizophrenic brains and to identify any dendritic and spinal alterations in comparison to normal control brains. The morphological and morphometric changes of the pyramidal cells and the interneurons of the visual cortices of 10 brains obtained from patients with schizophrenia, in comparison to 10 age-matched controls, were studied using the Golgi method and 3D neuronal reconstruction techniques. Analysis using the Golgi impregnation technique revealed a significant loss of distal dendritic segments, tortuous branches and varicosities and an overall restriction of the dendritic field in the brains of schizophrenic patients in both pyramidal cells and in aspiny interneurons. The present results may explain certain clinical phenomena associated with the visual cortex usually encountered in schizophrenia.

Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

The disproportionate evolutionary expansion of the human cerebral cortex with reinforcement of cholinergic innervations warranted a major rise in the functional and metabolic load of the conserved basal forebrain (BF) cholinergic system. Given that acetylcholine (ACh) regulates properties of the microtubule-associated protein (MAP) tau and promotes non-amyloidogenic processing of amyloid precursor protein (APP), growing neocortex predicts higher demands for ACh, while the emerging role of BF cholinergic projections in Aβ clearance infers greater exposure of source neurons and their innervation fields to amyloid pathology. The higher exposure of evolutionary most recent cortical areas to the amyloid pathology of Alzheimer's disease (AD) with synaptic impairments and atrophy, therefore, might involve attenuated homeostatic effects of BF cholinergic projections, in addition to fall-outs of inherent processes of expanding association areas. This unifying model, thus, views amyloid pathology and loss of cholinergic cells as a quid pro quo of the allometric evolution of the human brain, which in combination with increase in life expectancy overwhelm the fine homeostatic balance and trigger the disease process.

3D Ultrastructure of Synaptic Inputs to Distinct GABAergic Neurons in the Mouse Primary Visual Cortex

Synapses are the fundamental elements of the brain's complicated neural networks. Although the ultrastructure of synapses has been extensively studied, the difference in how synaptic inputs are organized onto distinct neuronal types is not yet fully understood. Here, we examined the cell-type-specific ultrastructure of proximal processes from the soma of parvalbumin-positive (PV+) and somatostatin-positive (SST+) GABAergic neurons in comparison with a pyramidal neuron in the mouse primary visual cortex (V1), using serial block-face scanning electron microscopy. Interestingly, each type of neuron organizes excitatory and inhibitory synapses in a unique way. First, we found that a subset of SST+ neurons are spiny, having spines on both soma and dendrites. Each of those spines has a highly complicated structure that has up to eight synaptic inputs. Next, the PV+ and SST+ neurons receive more robust excitatory inputs to their perisoma than does the pyramidal neuron. Notably, excitatory synapses on GABAergic neurons were often multiple-synapse boutons, making another synapse on distal dendrites. On the other hand, inhibitory synapses near the soma were often single-targeting multiple boutons. Collectively, our data demonstrate that synaptic inputs near the soma are differentially organized across cell types and form a network that balances inhibition and excitation in the V1.

Prenatal disruption of D1R-SynGAP complex causes cognitive deficits in adulthood

γ-aminobutyric acid (GABA)-ergic interneurons are essential for the physiological function of the mammalian central nervous system. Dysregulated GABAergic interneuron function has been implicated in the pathophysiology of a number of neurodevelopmental disorders including schizophrenia and autism spectrum disorder. Tangential migration is an important process to ensure the proper localization of GABAergic interneurons. Previously we found that disrupting the interaction between dopamine D1 receptor (D1R) and synaptic Ras GTPase- activating protein (SynGAP) using an interfering peptide (TAT-D₁R_pep) during embryonic development impaired tangential migration. Here, we assessed the effects of prenatal disruption of D1R-SynGAP complex with the TAT-D₁R_pep on the expression of several behaviours during adulthood. Mice with prenatal D1R-SynGAP disruption exhibited transiently reduced locomotor activity, abnormal sensorimotor gating, impaired sociability and deficits in visual discrimination associative learning compared to their control counterparts. Our findings reinforce the importance of GABAergic interneuron migration in the manifestation of normal motor, sensory, and cognitive behaviours of animals during adulthood.

Structural Brain Changes during the Neonatal Period in a Rabbit Model of Intrauterine Growth Restriction

BACKGROUND

Intrauterine growth restriction (IUGR) is associated with abnormal neurodevelopment, but the associated structural brain changes are poorly documented. The aim of this study was to describe in an animal model the brain changes at the cellular level in the gray and white matter induced by IUGR during the neonatal period.

METHODS

The IUGR model was surgically induced in pregnant rabbits by ligating 40-50% of the uteroplacental vessels in 1 horn, whereas the uteroplacental vessels of the contralateral horn were not ligated. After 5 days, IUGR animals from the ligated horn and controls from the nonligated were delivered. On the day of delivery, perinatal data and placentas were collected. On postnatal day 1, functional changes were first evaluated, and thereafter, neuronal arborization in the frontal cortex and density of pre-oligodendrocytes, astrocytes, and microglia in the corpus callosum were evaluated.

RESULTS

Higher stillbirth in IUGR fetuses together with a reduced birth weight as compared to controls was evidenced. IUGR animals showed poorer functional results, an altered neuronal arborization pattern, and a decrease in the pre-oligodendrocytes, with no differences in microglia and astrocyte densities.

CONCLUSIONS

Overall, in the rabbit model used, IUGR is related to functional and brain changes evidenced already at birth, including changes in the neuronal arborization and abnormal oligodendrocyte maturation.

Transcriptome-scale spatial gene expression in the human dorsolateral prefrontal cortex

We used the 10x Genomics Visium platform to define the spatial topography of gene expression in the six-layered human dorsolateral prefrontal cortex. We identified extensive layer-enriched expression signatures and refined associations to previous laminar markers. We overlaid our laminar expression signatures on large-scale single nucleus RNA-sequencing data, enhancing spatial annotation of expression-driven clusters. By integrating neuropsychiatric disorder gene sets, we showed differential layer-enriched expression of genes associated with schizophrenia and autism spectrum disorder, highlighting the clinical relevance of spatially defined expression. We then developed a data-driven framework to define unsupervised clusters in spatial transcriptomics data, which can be applied to other tissues or brain regions in which morphological architecture is not as well defined as cortical laminae. Last, we created a web application for the scientific community to explore these raw and summarized data to augment ongoing neuroscience and spatial transcriptomics research ( http://research.libd.org/spatialLIBD ).

A Systematic Review of Long-Interval Intracortical Inhibition as a Biomarker in Neuropsychiatric Disorders

Long-interval intracortical inhibition (LICI) is a paired-pulse transcranial magnetic stimulation (TMS) paradigm mediated in part by gamma-aminobutyric acid receptor B (GABA_B) inhibition. Prior work has examined LICI as a putative biomarker in an array of neuropsychiatric disorders. This review conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) sought to examine existing literature focused on LICI as a biomarker in neuropsychiatric disorders. There were 113 articles that met the inclusion criteria. Existing literature suggests that LICI may have utility as a biomarker of GABA_B functioning but more research with increased methodologic rigor is needed. The extant LICI literature has heterogenous methodology and inconsistencies in findings. Existing findings to date are also non-specific to disease. Future research should carefully consider existing methodological weaknesses and implement high-quality test-retest reliability studies.

Laminar Differences in the Targeting of Dendritic Spines by Cortical Pyramidal Neurons and Interneurons in Human Dorsolateral Prefrontal Cortex

Activation of specific neural circuits in different layers of the primate dorsolateral prefrontal cortex (DLPFC) is essential for working memory, a core cognitive function. Recurrent excitation between pyramidal neurons in middle and deep layers of the DLPFC contributes to the laminar-specific activity associated with different working memory subprocesses. Excitation between cortical pyramidal neurons is mediated by glutamatergic synapses on dendritic spines, but whether the relative abundance of spines receiving cortical inputs differs between middle and deep cortical layers in human DLPFC is unknown. Additionally, GABAergic inputs to spines sculpt pyramidal neuron activity. Whether dendritic spines that receive a glutamatergic input from a cortical pyramidal neuron are targeted by GABAergic interneurons in the human DLPFC is unknown. Using triple-label fluorescence confocal microscopy, we found that 1) the density of spines receiving an input from a cortical pyramidal neuron is greater in the middle than in the deep laminar zone, 2) dendritic spines dually innervated by a cortical pyramidal neuron and an interneuron are present in the human DLPFC, and 3) the density of spines dually innervated by a cortical pyramidal neuron and an interneuron is also greater in the middle than in the deep laminar zone. Ultrastructural analyses support the presence of spines that receive a cortical pyramidal neuron synapse and an interneuron synapse in human and monkey DLPFC. These data support the notion that the DLPFC middle laminar zone is particularly endowed with a microcircuit structure that supports the gating, integrating and fine-tuning of synaptic information in recurrent excitatory microcircuits.

Fhod3 Controls the Dendritic Spine Morphology of Specific Subpopulations of Pyramidal Neurons in the Mouse Cerebral Cortex

Changes in the shape and size of the dendritic spines are critical for synaptic transmission. These morphological changes depend on dynamic assembly of the actin cytoskeleton and occur differently in various types of neurons. However, how the actin dynamics are regulated in a neuronal cell type-specific manner remains largely unknown. We show that Fhod3, a member of the formin family proteins that mediate F-actin assembly, controls the dendritic spine morphogenesis of specific subpopulations of cerebrocortical pyramidal neurons. Fhod3 is expressed specifically in excitatory pyramidal neurons within layers II/III and V of restricted areas of the mouse cerebral cortex. Immunohistochemical and biochemical analyses revealed the accumulation of Fhod3 in postsynaptic spines. Although targeted deletion of Fhod3 in the brain did not lead to any defects in the gross or histological appearance of the brain, the dendritic spines in pyramidal neurons within presumptive Fhod3-positive areas were morphologically abnormal. In primary cultures prepared from the Fhod3-depleted cortex, defects in spine morphology were only detected in Fhod3 promoter-active cells, a small population of pyramidal neurons, and not in Fhod3 promoter-negative pyramidal neurons. Thus, Fhod3 plays a crucial role in dendritic spine morphogenesis only in a specific population of pyramidal neurons in a cell type-specific manner.

Inhibitory stabilization and cortical computation

Neuronal networks with strong recurrent connectivity provide the brain with a powerful means to perform complex computational tasks. However, high-gain excitatory networks are susceptible to instability, which can lead to runaway activity, as manifested in pathological regimes such as epilepsy. Inhibitory stabilization offers a dynamic, fast and flexible compensatory mechanism to balance otherwise unstable networks, thus enabling the brain to operate in its most efficient regimes. Here we review recent experimental evidence for the presence of such inhibition-stabilized dynamics in the brain and discuss their consequences for cortical computation. We show how the study of inhibition-stabilized networks in the brain has been facilitated by recent advances in the technological toolbox and perturbative techniques, as well as a concomitant development of biologically realistic computational models. By outlining future avenues, we suggest that inhibitory stabilization can offer an exemplary case of how experimental neuroscience can progress in tandem with technology and theory to advance our understanding of the brain.

A locust embryo as predictive developmental neurotoxicity testing system for pioneer axon pathway formation

Exposure to environmental chemicals during in utero and early postnatal development can cause a wide range of neurological defects. Since current guidelines for identifying developmental neurotoxic chemicals depend on the use of large numbers of rodents in animal experiments, it has been proposed to design rapid and cost-efficient in vitro screening test batteries that are mainly based on mixed neuronal/glial cultures. However, cell culture tests do not assay correct wiring of neuronal circuits. The establishment of precise anatomical connectivity is a key event in the development of a functional brain. Here, we expose intact embryos of the locust (Locusta migratoria) in serum-free culture to test chemicals and visualize correct navigation of identified pioneer axons by fluorescence microscopy. We define separate toxicological endpoints for axonal elongation and navigation along a stereotyped pathway. To distinguish developmental neurotoxicity (DNT) from general toxicity, we quantify defects in axonal elongation and navigation in concentration-response curves and compare it to the biochemically determined viability of the embryo. The investigation of a panel of recognized DNT-positive and -negative test compounds supports a rather high predictability of this invertebrate embryo assay. Similar to the semaphorin-mediated guidance of neurites in mammalian cortex, correct axonal navigation of the locust pioneer axons relies on steering cues from members of this family of cell recognition molecules. Due to the evolutionary conserved mechanisms of neurite guidance, we suggest that our pioneer axon paradigm might provide mechanistically relevant information on the DNT potential of chemical agents on the processes of axon elongation, navigation, and fasciculation.

Cortical Presynaptic Boutons Progressively Engulf Spinules as They Mature

Despite decades of discussion in the neuroanatomical literature, the role of the synaptic "spinule" in synaptic development and function remains elusive. Canonically, spinules are finger-like projections that emerge from postsynaptic spines and can become enveloped by presynaptic boutons. When a presynaptic bouton encapsulates a spinule in this manner, the membrane apposition between the spinule and surrounding bouton can be significantly larger than the membrane interface at the synaptic active zone. Hence, spinules may represent a mechanism for extrasynaptic neuronal communication and/or may function as structural "anchors" that increase the stability of cortical synapses. Yet despite their potential to impact synaptic function, we have little information on the percentages of developing and adult cortical bouton populations that contain spinules, the percentages of these cortical spinule-bearing boutons (SBBs) that contain spinules from distinct neuronal/glial origins, or whether the onset of activity or cortical plasticity are correlated with increased prevalence of cortical SBBs. Here, we employed 2D and 3D electron microscopy to determine the prevalence of spinules in excitatory presynaptic boutons at key developmental time points in the primary visual cortex (V1) of female and male ferrets. We find that the prevalence of SBBs in V1 increases across postnatal development, such that ∼25% of excitatory boutons in late adolescent ferret V1 contain spinules. In addition, we find that a majority of spinules within SBBs at later developmental time points emerge from postsynaptic spines and adjacent boutons/axons, suggesting that synaptic spinules may enhance synaptic stability and allow for axo-axonal communication in mature sensory cortex.

Maternal Spirulina supplementation during pregnancy and lactation partially prevents oxidative stress, glial activation and neuronal damage in protein malnourished F1 progeny

Protein malnutrition (PMN) is a global health issue but most prevalent in Africa and Asia. It exerts detrimental effect on structural and physiological aspects of hippocampal circuitry. Despite accumulating evidence for PMN induced changes in nervous system, relatively very little is known about how maternal nutritional supplementation during malnutrition affects glial cells and neurons. Herein, we aimed to investigate the effects of maternal Spirulina supplementation against PMN induced oxidative stress, reactive gliosis and neuronal damage in hippocampus of F1 progeny. Three months old healthy Sprague Dawley females (n = 24) were shifted to normoprotein (NC; 20% protein) and low protein (LP; 8% protein) diets 15 days before conception. The NC and LP group females were subdivided into two groups according to Spirulina supplementation (400 mg/kg/b.wt. orally throughout gestation and lactation period): normal control with Spirulina (NC SPI) and low protein with Spirulina supplemented group (LP SPI). F1 progeny born were used in present study. Thus, building on earlier results of ameliorated neurobehavioral and cognitive abilities in Spirulina supplemented protein deprived rats, the present study incorporates neurochemical and morphometric analysis of glial cells and neurons and revealed that maternal Spirulina consumption partially prevented the PMN associated neuropathological alterations in terms of attenuated oxidative brain damage, reduced reactive gliosis and apoptotic cell population, improved dendritic branch complexity with few damaged neurons and enhanced mushroom shaped spine density. The results suggest that cellular changes in hippocampus after PMN are partially restored after maternal Spirulina supplementation and one could envision intervention approaches using Spirulina against malnutrition.

Novel Therapeutic Approach for Excitatory/Inhibitory Imbalance in Neurodevelopmental and Neurodegenerative Diseases

Excitatory/inhibitory (E/I) balance, defined as the balance between excitation and inhibition of synaptic activity in a neuronal network, accounts in part for the normal functioning of the brain, controlling, for example, normal spike rate. In many pathological conditions, this fine balance is perturbed, leading to excessive or diminished excitation relative to inhibition, termed E/I imbalance, reflected in network dysfunction. E/I imbalance has emerged as a contributor to neurological disorders that occur particularly at the extremes of life, including autism spectrum disorder and Alzheimer's disease, pointing to the vulnerability of neuronal networks at these critical life stages. Hence, it is important to develop approaches to rebalance neural networks. In this review, we describe emerging therapies that can normalize the E/I ratio or the underlying abnormality that contributes to the imbalance in electrical activity, thus improving neurological function in these maladies.

DSCAM Mutation Impairs Motor Cortex Network Dynamic and Voluntary Motor Functions

While it is well known that netrin-1 and its receptors UNC5 and UNC40 family members are involved in the normal establishment of the motor cortex and its corticospinal tract, less is known about its other receptor Down syndrome cell adherence molecule (DSCAM). DSCAM is expressed in the developing motor cortex, regulates axonal outgrowth of cortical neurons, and its mutation impairs the dendritic arborization of cortical neurons, thus suggesting that it might be involved in the normal development and functioning of the motor cortex. In comparison to WT littermates, DSCAM2J mutant mice slipped and misplaced their paw while walking on the rungs of a horizontal ladder, and exhibited more difficulties in stepping over an obstacle while walking at slow speed. Anterograde tracing showed a normal pyramidal decussation and corticospinal projection, but a more dorsal distribution of their axonal terminals in the spinal gray matter. Intracortical microstimulations showed a reduced corticospinal and intracortical efficacy, whereas stimulations of the pyramidal tract revealed a normal spinal efficacy and excitability of corticospinal tract axons, thus arguing for a dysfunctional cortical development. Our study reveals impairment of the network dynamics within the motor cortex, reducing corticospinal drive and impairing voluntary locomotor functions upon DSCAM2J mutation.

3D Ultrastructural Study of Synapses in the Human Entorhinal Cortex

The entorhinal cortex (EC) is a brain region that has been shown to be essential for memory functions and spatial navigation. However, detailed three-dimensional (3D) synaptic morphology analysis and identification of postsynaptic targets at the ultrastructural level have not been performed before in the human EC. In the present study, we used Focused Ion Beam/Scanning Electron Microscopy to perform a 3D analysis of the synapses in the neuropil of medial EC in layers II and III from human brain autopsies. Specifically, we studied synaptic structural parameters of 3561 synapses, which were fully reconstructed in 3D. We analyzed the synaptic density, 3D spatial distribution, and type (excitatory and inhibitory), as well as the shape and size of each synaptic junction. Moreover, the postsynaptic targets of synapses could be clearly determined. The present work constitutes a detailed description of the synaptic organization of the human EC, which is a necessary step to better understand the functional organization of this region in both health and disease.

Firing rate homeostasis counteracts changes in stability of recurrent neural networks caused by synapse loss in Alzheimer's disease

The impairment of cognitive function in Alzheimer's disease is clearly correlated to synapse loss. However, the mechanisms underlying this correlation are only poorly understood. Here, we investigate how the loss of excitatory synapses in sparsely connected random networks of spiking excitatory and inhibitory neurons alters their dynamical characteristics. Beyond the effects on the activity statistics, we find that the loss of excitatory synapses on excitatory neurons reduces the network's sensitivity to small perturbations. This decrease in sensitivity can be considered as an indication of a reduction of computational capacity. A full recovery of the network's dynamical characteristics and sensitivity can be achieved by firing rate homeostasis, here implemented by an up-scaling of the remaining excitatory-excitatory synapses. Mean-field analysis reveals that the stability of the linearised network dynamics is, in good approximation, uniquely determined by the firing rate, and thereby explains why firing rate homeostasis preserves not only the firing rate but also the network's sensitivity to small perturbations.

DNA Methyltransferase 1 (DNMT1) Function Is Implicated in the Age-Related Loss of Cortical Interneurons

Increased life expectancy in modern society comes at the cost of age-associated disabilities and diseases. Aged brains not only show reduced excitability and plasticity, but also a decline in inhibition. Age-associated defects in inhibitory circuits likely contribute to cognitive decline and age-related disorders. Molecular mechanisms that exert epigenetic control of gene expression contribute to age-associated neuronal impairments. Both DNA methylation, mediated by DNA methyltransferases (DNMTs), and histone modifications maintain neuronal function throughout lifespan. Here we provide evidence that DNMT1 function is implicated in the age-related loss of cortical inhibitory interneurons. Dnmt1 deletion in parvalbumin-positive interneurons attenuates their age-related decline in the cerebral cortex. Moreover, conditional Dnmt1-deficient mice show improved somatomotor performance and reduced aging-associated transcriptional changes. A decline in the proteostasis network, responsible for the proper degradation and removal of defective proteins, is implicated in age- and disease-related neurodegeneration. Our data suggest that DNMT1 acts indirectly on interneuron survival in aged mice by modulating the proteostasis network during life-time.

Maturation of the Human Cerebral Cortex During Adolescence: Myelin or Dendritic Arbor?

Previous in vivo studies revealed robust age-related variations in structural properties of the human cerebral cortex during adolescence. Neurobiology underlying these maturational phenomena is largely unknown. Here we employ a virtual-histology approach to gain insights into processes associated with inter-regional variations in cortical microstructure and its maturation, as indexed by magnetization transfer ratio (MTR). Inter-regional variations in MTR correlate with inter-regional variations in expression of genes specific to pyramidal cells (CA1) and ependymal cells; enrichment analyses indicate involvement of these genes in dendritic growth. On the other hand, inter-regional variations in the change of MTR during adolescence correlate with inter-regional profiles of oligodendrocyte-specific gene expression. Complemented by a quantitative hypothetical model of the contribution of surfaces associated with dendritic arbor (1631 m2) and myelin (48 m2), these findings suggest that MTR signals are driven mainly by macromolecules associated with dendritic arbor while maturational changes in the MTR signal are associated with myelination.

Ca2+ imaging of neurons in freely moving rats with automatic post hoc histological identification

BACKGROUND

Cognitive neuroscientists aim to understand behavior often based on the underlying activity of individual neurons. Recently developed miniaturized epifluorescence microscopes allow recording of cellular calcium transients, resembling neuronal activity, of individual neurons even in deep brain areas in freely behaving animals. At the same time, molecular markers allow the characterization of diverse neuronal subtypes by post hoc immunohistochemical labeling. Combining both methods would allow researchers to increase insights into how individual neuronal activity and entities contribute to behavior.

NEW METHOD

Here, we present a novel method for identifying the same neurons, recorded with calcium imaging using a miniaturized epifluorescence microscope, post hoc in fixed histological sections. This allows immunohistochemical investigations to detect the molecular signature of in vivo recorded neurons. Our method utilizes the structure of blood vessels for aligning in vivo acquired 2D images with a reconstructed 3D histological model.

RESULTS

We automatically matched, 60 % of all in vivo recorded cells post hoc in histology. Across all animals, we successfully matched 43 % to 89 % of the recorded neurons. We provide a measure for the confidence of matched cells and validated our method by multiple simulation studies.

COMPARISON WITH EXISTING METHODS

To our knowledge, we present the first method for matching cells, recorded with a miniaturized epifluorescence microscope in freely moving animals, post hoc in histological sections.

CONCLUSIONS

Our method allows a comprehensive analysis of how cortical circuits relate to freely moving animal behavior by combining functional activity of individual neurons with their underlying histological profiles.

Modelling unitary fields and the single-neuron contribution to local field potentials in the hippocampus

Key points

We simulate the unitary local field potential (uLFP) generated in the hippocampus CA3, using morphologically detailed models.

The model suggests that cancelling effects between apical and basal dendritic synapses explain the low amplitude of excitatory uLFPs.

Inhibitory synapses around the soma do not cancel and could explain the high‐amplitude inhibitory uLFPs.

These results suggest that somatic inhibition constitutes a strong component of LFPs, which may explain a number of experimental observations.

Abstract

Synaptic currents represent a major contribution to the local field potential (LFP) in brain tissue, but the respective contribution of excitatory and inhibitory synapses is not known. Here, we provide estimates of this contribution by using computational models of hippocampal pyramidal neurons, constrained by in vitro recordings. We focus on the unitary LFP (uLFP) generated by single neurons in the CA3 region of the hippocampus. We first reproduce experimental results for hippocampal basket cells, and in particular how inhibitory uLFP are distributed within hippocampal layers. Next, we calculate the uLFP generated by pyramidal neurons, using morphologically reconstructed CA3 pyramidal cells. The model shows that the excitatory uLFP is of small amplitude, smaller than inhibitory uLFPs. Indeed, when the two are simulated together, inhibitory uLFPs mask excitatory uLFPs, which might create the illusion that the inhibitory field is generated by pyramidal cells. These results provide an explanation for the observation that excitatory and inhibitory uLFPs are of the same polarity, in vivo and in vitro. These results suggest that somatic inhibitory currents are large contributors to the LFP, which is important information for interpreting this signal. Finally, the results of our model might form the basis of a simple method to compute the LFP, which could be applied to point neurons for each cell type, thus providing a simple biologically grounded method for calculating LFPs from neural networks.

In conclusion, computational models constrained by in vitro recordings suggest that: (1) Excitatory uLFPs are of smaller amplitude than inhibitory uLFPs. (2) Inhibitory uLFPs form the major contribution to LFPs. (3) uLFPs can be used as a simple model to generate LFPs from spiking networks.

We simulate the unitary local field potential (uLFP) generated in the hippocampus CA3, using morphologically detailed models.

The model suggests that cancelling effects between apical and basal dendritic synapses explain the low amplitude of excitatory uLFPs.

Inhibitory synapses around the soma do not cancel and could explain the high‐amplitude inhibitory uLFPs.

These results suggest that somatic inhibition constitutes a strong component of LFPs, which may explain a number of experimental observations.

Unanticipated Stressful and Rewarding Experiences Engage the Same Prefrontal Cortex and Ventral Tegmental Area Neuronal Populations

Brain networks that mediate motivated behavior in the context of aversive and rewarding experiences involve the prefrontal cortex (PFC) and ventral tegmental area (VTA). Neurons in both regions are activated by stress and reward, and by learned cues that predict aversive or appetitive outcomes. Recent studies have proposed that separate neuronal populations and circuits in these regions encode learned aversive versus appetitive contexts. But how about the actual experience? Do the same or different PFC and VTA neurons encode unanticipated aversive and appetitive experiences? To address this, we recorded unit activity and local field potentials (LFPs) in the dorsomedial PFC (dmPFC) and VTA of male rats as they were exposed, in the same recording session, to reward (sucrose) or stress (tail pinch) spaced 1 h apart. As expected, experience-specific neuronal responses were observed. Approximately 15–25% of single units in each region responded by excitation or inhibition to either stress or reward, and only stress increased LFP theta oscillation power in both regions and coherence between regions. But the largest number of responses (29% dmPFC and 30% VTA units) involved dual-valence neurons that responded to both stress and reward exposure. Moreover, the temporal profile of neuronal population activity in dmPFC and VTA as assessed by principal component analysis (PCA) were similar during both types of experiences. These results reveal that aversive and rewarding experiences engage overlapping neuronal populations in the dmPFC and the VTA. These populations may provide a locus of vulnerability for stress-related disorders, which are often associated with anhedonia.