Homeostatic interplay between electrical activity and neuronal apoptosis in the developing neocortex

Mitochondrial transplantation rescues neuronal cells from ferroptosis

Ferroptosis is a type of oxidative cell death that can occur in neurodegenerative diseases and involves damage to mitochondria. Previous studies demonstrated that preventing mitochondrial dysfunction can rescue cells from ferroptotic cell death. However, the complexity of mitochondrial dysfunction and the timing of therapeutic interventions make it difficult to develop an effective treatment strategy against ferroptosis in neurodegeneration conditions. In this study, we explored the use of mitochondrial transplantation as a novel therapeutic approach for preventing ferroptotic neuronal cell death. Our data showed that isolated exogenous mitochondria were incorporated into both healthy and ferroptotic immortalized hippocampal HT-22 cells and primary cortical neurons (PCN). The mitochondrial incorporation was accompanied by increased metabolic activity and cell survival through attenuating lipid peroxidation and mitochondrial superoxide production. Further, the function of mitochondrial complexes I, III and V activities contributed to the neuroprotective activity of exogenous mitochondria. Similarly, we have also showed the internalization of exogenous mitochondria in mouse PCN; these internalized mitochondria were found to effectively preserve the neuronal networks when challenged with ferroptotic stimuli. The administration of exogenous mitochondria into the axonal compartment of a two-compartment microfluidic device induced mitochondrial transportation to the cell body, which prevented fragmentation of the neuronal network in ferroptotic PCN. These findings suggest that mitochondria transplantation may be a promising therapeutic approach for protecting neuronal cells from ferroptotic cell death.

Repetitive and compulsive behavior after Early-Life-Pain associated with reduced long-chain sphingolipid species

BACKGROUND

Pain in early life may impact on development and risk of chronic pain. We developed an optogenetic Cre/loxP mouse model of "early-life-pain" (ELP) using mice with transgenic expression of channelrhodopsin-2 (ChR2) under control of the Advillin (Avil) promoter, which drives expression of transgenes predominantly in isolectin B4 positive non-peptidergic nociceptors in postnatal mice. Avil-ChR2 (Cre +) and ChR2-flfl control mice were exposed to blue light in a chamber once daily from P1-P5 together with their Cre-negative mother.

RESULTS

ELP caused cortical hyperexcitability at P8-9 as assessed via multi-electrode array recordings that coincided with reduced expression of synaptic genes (RNAseq) including Grin2b, neurexins, piccolo and voltage gated calcium and sodium channels. Young adult (8-16 wks) Avil-ChR2 mice presented with nociceptive hypersensitivity upon heat or mechanical stimulation, which did not resolve up until one year of age. The persistent hypersensitivy to nociceptive stimuli was reflected by increased calcium fluxes in primary sensory neurons of aged mice (1 year) upon capsaicin stimulation. Avil-ChR2 mice behaved like controls in maze tests of anxiety, social interaction, and spatial memory but IntelliCage behavioral studies revealed repetitive nosepokes and corner visits and compulsive lickings. Compulsiveness at the behavioral level was associated with a reduction of sphingomyelin species in brain and plasma lipidomic studies. Behavioral studies were done with female mice.

CONCLUSION

The results suggest that ELP may predispose to chronic "pain" and compulsive psychopathology in part mediated by alterations of sphingolipid metabolism, which have been previously described in the context of addiction and psychiatric diseases.

The maturation of aperiodic EEG activity across development reveals a progressive differentiation of wakefulness from sleep

During development, the brain undergoes radical structural and functional changes following a posterior-to-anterior gradient, associated with profound changes of cortical electrical activity during both wakefulness and sleep. However, a systematic assessment of the developmental effects on aperiodic EEG activity maturation across vigilance states is lacking, particularly regarding its topographical aspects. Here, in a population of 160 healthy infants, children and teenagers (from 2 to 17 years, 10 subjects for each year), we investigated the development of aperiodic EEG activity in wakefulness and sleep. Specifically, we parameterized the shape of the aperiodic background of the EEG Power Spectral Density (PSD) by means of the spectral exponent and offset; the exponent reflects the rate of exponential decay of power over increasing frequencies and the offset reflects an estimate of the y-intercept of the PSD. We found that sleep and development caused the EEG-PSD to rotate over opposite directions: during wakefulness the PSD showed a flatter decay and reduced offset over development, while during sleep it showed a steeper decay and a higher offset as sleep becomes deeper. During deep sleep (N2, N3) only the spectral offset decreased over age, indexing a broad-band voltage reduction. As a result, the difference between values in deep sleep and those in both light sleep (N1) and wakefulness increased with age, suggesting a progressive differentiation of wakefulness from sleep EEG activity, most prominent over the frontal regions, the latest to complete maturation. Notably, the broad-band spectral exponent values during deep sleep stages were entirely separated from wakefulness values, consistently across developmental ages and in line with previous findings in adults. Concerning topographical development, the location showing the steepest PSD decay and largest offset shifted from posterior to anterior regions with age. This shift, particularly evident during deep sleep, paralleled the migration of sleep slow wave activity and was consistent with neuroanatomical and cognitive development. Overall, aperiodic EEG activity distinguishes wakefulness from sleep regardless of age; while, during development, it reveals a postero-anterior topographical maturation and a progressive differentiation of wakefulness from sleep. Our study could help to interpret changes due to pathological conditions and may elucidate the neurophysiological processes underlying the development of wakefulness and sleep.

Designing Electrical Stimulation Platforms for Neural Cell Cultivation Using Poly(aniline): Camphorsulfonic Acid

Electrical stimulation is a powerful strategy to improve the differentiation of neural stem cells into neurons. Such an approach can be implemented, in association with biomaterials and nanotechnology, for the development of new therapies for neurological diseases, including direct cell transplantation and the development of platforms for drug screening and disease progression evaluation. Poly(aniline):camphorsulfonic acid (PANI:CSA) is one of the most well-studied electroconductive polymers, capable of directing an externally applied electrical field to neural cells in culture. There are several examples in the literature on the development of PANI:CSA-based scaffolds and platforms for electrical stimulation, but no review has examined the fundamentals and physico-chemical determinants of PANI:CSA for the design of platforms for electrical stimulation. This review evaluates the current literature regarding the application of electrical stimulation to neural cells, specifically reviewing: (1) the fundamentals of bioelectricity and electrical stimulation; (2) the use of PANI:CSA-based systems for electrical stimulation of cell cultures; and (3) the development of scaffolds and setups to support the electrical stimulation of cells. Throughout this work, we critically evaluate the revised literature and provide a steppingstone for the clinical application of the electrical stimulation of cells using electroconductive PANI:CSA platforms/scaffolds.

Malformations-related neocortical circuits in focal seizures

This review article gives an overview on the molecular, cellular and network mechanisms underlying focal seizures in neocortical networks with developmental malformations. Neocortical malformations comprise a large variety of structural abnormalities associated with epilepsy and other neurological and psychiatric disorders. Genetic or acquired disorders of neocortical cell proliferation, neuronal migration and/or programmed cell death may cause pathologies ranging from the expression of dysmorphic neurons and heterotopic cell clusters to abnormal layering and cortical misfolding. After providing a brief overview on the pathogenesis and structure of neocortical malformations in humans, animal models are discussed and how they contributed to our understanding on the mechanisms of neocortical hyperexcitability associated with developmental disorders. State-of-the-art molecular biological and electrophysiological techniques have been also used in humans and on resectioned neocortical tissue of epileptic patients and provide deep insights into the subcellular, cellular and network mechanisms contributing to focal seizures. Finally, a brief outlook is given how novel models and methods can shape translational research in the near future.

Circuit mechanisms underlying embryonic retinal waves

Spontaneous activity is a hallmark of developing neural systems. In the retina, spontaneous activity comes in the form of retinal waves, comprised of three stages persisting from embryonic day 16 (E16) to eye opening at postnatal day 14 (P14). Though postnatal retinal waves have been well characterized, little is known about the spatiotemporal properties or the mechanisms mediating embryonic retinal waves, designated stage 1 waves. Using a custom-built macroscope to record spontaneous calcium transients from whole embryonic retinas, we show that stage 1 waves are initiated at several locations across the retina and propagate across a broad range of areas. Blocking gap junctions reduced the frequency and size of stage 1 waves, nearly abolishing them. Global blockade of nAChRs similarly nearly abolished stage 1 waves. Thus, stage 1 waves are mediated by a complex circuitry involving subtypes of nAChRs and gap junctions. Stage 1 waves in mice lacking the β2 subunit of the nAChRs (β2-nAChR-KO) persisted with altered propagation properties and were abolished by a gap junction blocker. To assay the impact of stage 1 waves on retinal development, we compared the spatial distribution of a subtype of retinal ganglion cells, intrinsically photosensitive retinal ganglion cells (ipRGCs), which undergo a significant amount of cell death, in WT and β2-nAChR-KO mice. We found that the developmental decrease in ipRGC density is preserved between WT and β2-nAChR-KO mice, indicating that processes regulating ipRGC numbers and distributions are not influenced by spontaneous activity.

Altered development of structural MRI connectome hubs at near-term age in very and moderately preterm infants

Preterm infants may exhibit altered developmental patterns of the brain structural network by endogenous and exogenous stimuli, which are quantifiable through hub and modular network topologies that develop in the third trimester. Although preterm brain networks can compensate for white matter microstructural abnormalities of core connections, less is known about how the network developmental characteristics of preterm infants differ from those of full-term infants. We identified 13 hubs and 4 modules and revealed subtle differences in edgewise connectivity and local network properties between 134 preterm and 76 full-term infants, identifying specific developmental patterns of the brain structural network in preterm infants. The modules of preterm infants showed an imbalanced composition. The edgewise connectivity in preterm infants showed significantly decreased long- and short-range connections and local network properties in the dorsal superior frontal gyrus. In contrast, the fusiform gyrus and several nonhub regions showed significantly increased wiring of short-range connections and local network properties. Our results suggested that decreased local network in the frontal lobe and excessive development in the occipital lobe may contribute to the understanding of brain developmental deviances in preterm infants.

Large-scale waves of activity in the neonatal mouse brain in vivo occur almost exclusively during sleep cycles

Spontaneous electrical activity plays major roles in the development of cortical circuitry. This activity can occur highly localized regions or can propagate over the entire cortex. Both types of activity coexist during early development. To investigate how different forms of spontaneous activity might be temporally segregated, we used wide-field trans-cranial calcium imaging over an entire hemisphere in P1-P8 mouse pups. We found that spontaneous waves of activity that propagate to cover the majority of the cortex (large-scale waves; LSWs) are generated at the end of the first postnatal week, along with several other forms of more localized activity. We further found that LSWs are segregated into sleep cycles. In contrast, cortical activity during wake states is more spatially restricted and the few large-scale forms of activity that occur during wake can be distinguished from LSWs in sleep based on their initiation in the motor cortex and their correlation with body movements. This change in functional cortical circuitry to a state that is permissive for large-scale activity may temporally segregate different forms of activity during critical stages when activity-dependent circuit development occurs over many spatial scales. Our data also suggest that LSWs in early development may be a functional precursor to slow sleep waves in the adult, which play critical roles in memory consolidation and synaptic rescaling.

The Phosphofurin Acidic Cluster Sorting Protein 2 (PACS-2) E209K Mutation Responsible for PACS-2 Syndrome Increases Susceptibility to Apoptosis

Phosphofurin acidic cluster sorting protein 2 (PACS-2) is a multifunctional cytosolic membrane trafficking protein with distinct roles in maintaining cellular homeostasis. Recent clinical reports have described 28 individuals possessing a de novo PACS-2 E209K mutation that present with epileptic seizures and cerebellar dysgenesis. As the PACS-2 E209K missense mutation has become a marker for neurodevelopmental disorders, we sought to characterize its biochemical properties. Accordingly, we observed that the PACS-2 E209K protein exhibited a slower turnover rate relative to PACS-2 wild type (WT) upon cycloheximide treatment in 293T cells. The longer half-life of PACS-2 E209K suggests a disruption in its proteostasis, with the potential for altered protein-protein interactions. Indeed, a regulatory protein in neurodevelopment known as 14-3-3ε was identified as having an increased association with PACS-2 E209K. Subsequently, when comparing the effect of PACS-2 WT and E209K expression on the staurosporine-induced apoptosis response, we found that PACS-2 E209K increased susceptibility to staurosporine-induced apoptosis in HCT 116 cells. Overall, our findings suggest PACS-2 E209K alters PACS-2 proteostasis and favors complex formation with 14-3-3ε, leading to increased cell death in the presence of environmental stressors.

Spontaneous Activity Predicts Survival of Developing Cortical Neurons

Spontaneous activity plays a crucial role in brain development by coordinating the integration of immature neurons into emerging cortical networks. High levels and complex patterns of spontaneous activity are generally associated with low rates of apoptosis in the cortex. However, whether spontaneous activity patterns directly encode for survival of individual cortical neurons during development remains an open question. Here, we longitudinally investigated spontaneous activity and apoptosis in developing cortical cultures, combining extracellular electrophysiology with calcium imaging. These experiments demonstrated that the early occurrence of calcium transients was strongly linked to neuronal survival. Silent neurons exhibited a higher probability of cell death, whereas high frequency spiking and burst behavior were almost exclusively detected in surviving neurons. In local neuronal clusters, activity of neighboring neurons exerted a pro-survival effect, whereas on the functional level, networks with a high modular topology were associated with lower cell death rates. Using machine learning algorithms, cell fate of individual neurons was predictable through the integration of spontaneous activity features. Our results indicate that high frequency spiking activity constrains apoptosis in single neurons through sustained calcium rises and thereby consolidates networks in which a high modular topology is reached during early development.

Chromatin compaction precedes apoptosis in developing neurons

While major changes in cellular morphology during apoptosis have been well described, the subcellular changes in nuclear architecture involved in this process remain poorly understood. Imaging of nucleosomes in cortical neurons in vitro before and during apoptosis revealed that chromatin compaction precedes the activation of caspase-3 and nucleus shrinkage. While this early chromatin compaction remained unaffected by pharmacological blockade of the final execution of apoptosis through caspase-3 inhibition, interfering with the chromatin dynamics by modulation of actomyosin activity prevented apoptosis, but resulted in necrotic-like cell death instead. With super-resolution imaging at different phases of apoptosis in vitro and in vivo, we demonstrate that chromatin compaction occurs progressively and can be classified into five stages. In conclusion, we show that compaction of chromatin in the neuronal nucleus precedes apoptosis execution. These early changes in chromatin structure critically affect apoptotic cell death and are not part of the final execution of the apoptotic process in developing cortical neurons.

Single-molecule imaging in developing cortical neurons shows that chromatin compaction precedes apoptosis and is an essential part of it, but can be uncoupled from the following apoptotic process.

Sevoflurane-Induced Apoptosis in the Mouse Cerebral Cortex Follows Similar Characteristics of Physiological Apoptosis

General anesthetics are capable of inducing neuronal apoptosis during the rapid synaptogenesis of immature mammalian brains. In this vulnerable time window, physiological apoptosis also occurs to eliminate excess and inappropriately integrated neurons. We previously showed that physiological and ketamine-induced apoptosis in mouse primary somatosensory cortex (S1) followed similar developmental patterns. However, since sevoflurane is more widely used in pediatric anesthesia, and targets mainly on different receptors, as compared with ketamine, it is important to determine whether sevoflurane-induced apoptosis also follows similar developmental patterns as physiological apoptosis or not. Mice at postnatal days 5 (P5) and P9 were anesthetized with 1.5% sevoflurane for 4 h, and the apoptotic neurons in S1 were quantitated by immunohistochemistry. The results showed that sevoflurane raised the levels of apoptosis in S1 without interfering with the developmental patterns of physiological apoptosis. The cells more vulnerable to both physiological and sevoflurane-induced apoptosis shifted from layer V pyramidal neurons at P5 to layers II–IV GABAergic neurons by P9. The magnitude of both sevoflurane-induced and physiological apoptosis was more attenuated at P9 than P5. To determine whether the Akt-FoxO1-PUMA pathway contributes to the developmental decrease in magnitude of both physiological and sevoflurane-induced apoptosis, Western blot was used to measure the levels of related proteins in S1 of P5 and P9 mice. We observed higher levels of antiapoptotic phosphorylated Akt (p-Akt) and phosphorylated FoxO1 (p-FoxO1), and lower levels of the downstream proapoptotic factor PUMA in control and anesthetized mice at P9 than P5. In addition, the Akt-FoxO1-PUMA pathway may also be responsible for sevoflurane-induced apoptosis. Together, these results suggest that magnitude, lamination pattern and cell-type specificity to sevoflurane-induced apoptosis are age-dependent and follow physiological apoptosis pattern. Moreover, The Akt-FoxO1-PUMA pathway may mediate the developmental decreases in magnitude of both physiological and sevoflurane-induced apoptosis in neonatal mouse S1.

Sleep as a window on the sensorimotor foundations of the developing hippocampus

The hippocampal formation plays established roles in learning, memory, and related cognitive functions. Recent findings also suggest that the hippocampus integrates sensory feedback from self-generated movements to modulate ongoing motor responses in a changing environment. Such findings support the view of Bland and Oddie (Behavioural Brain Research, 2001, 127, 119-136) that the hippocampus is a site of sensorimotor integration. In further support of this view, we review neurophysiological evidence in developing rats that hippocampal function is built on a sensorimotor foundation and that this foundation is especially evident early in development. Moreover, at those ages when the hippocampus is first establishing functional connectivity with distant sensory and motor structures, that connectivity is preferentially expressed during periods of active (or REM) sleep. These findings reinforce the notion that sleep, as the predominant state of early infancy, provides a critical context for sensorimotor development, including development of the hippocampus and its associated network.

Gabaergic Interneurons in Early Brain Development: Conducting and Orchestrated by Cortical Network Activity

Throughout early phases of brain development, the two main neural signaling mechanisms—excitation and inhibition—are dynamically sculpted in the neocortex to establish primary functions. Despite its relatively late formation and persistent developmental changes, the GABAergic system promotes the ordered shaping of neuronal circuits at the structural and functional levels. Within this frame, interneurons participate first in spontaneous and later in sensory-evoked activity patterns that precede cortical functions of the mature brain. Upon their subcortical generation, interneurons in the embryonic brain must first orderly migrate to and settle in respective target layers before they can actively engage in cortical network activity. During this process, changes at the molecular and synaptic level of interneurons allow not only their coordinated formation but also the pruning of connections as well as excitatory and inhibitory synapses. At the postsynaptic site, the shift of GABAergic signaling from an excitatory towards an inhibitory response is required to enable synchronization within cortical networks. Concomitantly, the progressive specification of different interneuron subtypes endows the neocortex with distinct local cortical circuits and region-specific modulation of neuronal firing. Finally, the apoptotic process further refines neuronal populations by constantly maintaining a controlled ratio of inhibitory and excitatory neurons. Interestingly, many of these fundamental and complex processes are influenced—if not directly controlled—by electrical activity. Interneurons on the subcellular, cellular, and network level are affected by high frequency patterns, such as spindle burst and gamma oscillations in rodents and delta brushes in humans. Conversely, the maturation of interneuron structure and function on each of these scales feeds back and contributes to the generation of cortical activity patterns that are essential for the proper peri- and postnatal development. Overall, a more precise description of the conducting role of interneurons in terms of how they contribute to specific activity patterns—as well as how specific activity patterns impinge on their maturation as orchestra members—will lead to a better understanding of the physiological and pathophysiological development and function of the nervous system.

Neurophysiology of the Developing Cerebral Cortex: What We Have Learned and What We Need to Know

This review article aims to give a brief summary on the novel technologies, the challenges, our current understanding, and the open questions in the field of the neurophysiology of the developing cerebral cortex in rodents. In the past, in vitro electrophysiological and calcium imaging studies on single neurons provided important insights into the function of cellular and subcellular mechanism during early postnatal development. In the past decade, neuronal activity in large cortical networks was recorded in pre- and neonatal rodents in vivo by the use of novel high-density multi-electrode arrays and genetically encoded calcium indicators. These studies demonstrated a surprisingly rich repertoire of spontaneous cortical and subcortical activity patterns, which are currently not completely understood in their functional roles in early development and their impact on cortical maturation. Technological progress in targeted genetic manipulations, optogenetics, and chemogenetics now allow the experimental manipulation of specific neuronal cell types to elucidate the function of early (transient) cortical circuits and their role in the generation of spontaneous and sensory evoked cortical activity patterns. Large-scale interactions between different cortical areas and subcortical regions, characterization of developmental shifts from synchronized to desynchronized activity patterns, identification of transient circuits and hub neurons, role of electrical activity in the control of glial cell differentiation and function are future key tasks to gain further insights into the neurophysiology of the developing cerebral cortex.

Optogenetically Controlled Activity Pattern Determines Survival Rate of Developing Neocortical Neurons

A substantial proportion of neurons undergoes programmed cell death (apoptosis) during early development. This process is attenuated by increased levels of neuronal activity and enhanced by suppression of activity. To uncover whether the mere level of activity or also the temporal structure of electrical activity affects neuronal death rates, we optogenetically controlled spontaneous activity of synaptically-isolated neurons in developing cortical cultures. Our results demonstrate that action potential firing of primary cortical neurons promotes neuronal survival throughout development. Chronic patterned optogenetic stimulation allowed to effectively modulate the firing pattern of single neurons in the absence of synaptic inputs while maintaining stable overall activity levels. Replacing the burst firing pattern with a non-physiological, single pulse pattern significantly increased cell death rates as compared to physiological burst stimulation. Furthermore, physiological burst stimulation led to an elevated peak in intracellular calcium and an increase in the expression level of classical activity-dependent targets but also decreased Bax/BCL-2 expression ratio and reduced caspase 3/7 activity. In summary, these results demonstrate at the single-cell level that the temporal pattern of action potentials is critical for neuronal survival versus cell death fate during cortical development, besides the pro-survival effect of action potential firing per se.

Intrinsic burst-firing in lamina I spinoparabrachial neurons during adolescence

A subset of glutamatergic interneurons in the neonatal spinal superficial dorsal horn (SDH) exhibits intrinsic burst-firing (i.e. 'pacemaker' activity), which is tightly regulated by persistent, voltage-gated Na⁺ channels and classic inward-rectifying K⁺ (K_ir2) channels and downregulated over the course of postnatal development. Ascending lamina I projection neurons targeting the parabrachial nucleus (PB) or periaqueductal gray (PAG) can also display pacemaker activity during early life. However, the degree to which the ionic mechanisms driving pacemaker activity are conserved across different cell types in the spinal dorsal horn, as well as whether the intrinsic bursting is restricted to newborn projection neurons, remains to be elucidated. Using in vitro patch clamp recordings from identified lamina I spinoparabrachial neurons in rat spinal cord slices, here we demonstrate that adolescent projection neurons retain their ability to generate pacemaker activity. In contrast to previous findings in lamina I interneurons, pacemaker projection neurons possessed higher membrane capacitance, lower membrane resistance, and a greater K_ir-mediated conductance compared to adjacent spinoparabrachial neurons that lacked intrinsic burst-firing. Nonetheless, as previously seen in interneurons, the bath application of riluzole to block persistent Na⁺ channels significantly dampened pacemaker activity in projection neurons. Collectively, these results suggest that intrinsic burst-firing in the developing dorsal horn can be generated by multiple combinations of ionic conductances, and highlight the need for further investigation into the mechanisms governing pacemaker activity within the major output neurons of the SDH network.

Membrane Depolarization Inhibits BIMEL Upregulation but Prevents Neuronal Apoptosis Primarily by Increasing Cellular GSH Levels

Sympathetic neurons deprived of nerve growth factor (NGF) die by apoptosis. Chronic depolarization with elevated concentrations of extracellular potassium ([K⁺]_E) supports long-term survival of these and other types of neurons in culture. While depolarization has long been used to support neuronal cultures, little is known about the mechanism. We explored how chronic depolarization of NGF-deprived mouse sympathetic neurons in culture blocks apoptotic death. First, we determined the effects of elevated [K⁺]_E on proapoptotic BH3-only proteins reported to be upregulated in sympathetic neurons after NGF withdrawal. Upregulation of BIM_EL was blocked by depolarization while upregulation of PUMA was not. BMF levels did not increase after NGF withdrawal, and elevated [K⁺]_E had no effect on its expression. dp5/HRK was not detectable. A large increase in production of mitochondria-derived reactive species (RS), including reactive oxygen species (ROS), occurs in NGF-deprived sympathetic neurons. Suppressing these RS prevents cytochrome c release from mitochondria and apoptosis. The addition of high [K⁺]_E to cultures rapidly blocked increased RS and cytochrome c release. Elevated [K⁺]_E caused an increase of the cellular antioxidant glutathione (GSH). Preventing this increase prevented the elevated [K⁺]_E from blocking cytochrome c release and death. While suppression of BIM_EL upregulation by elevated [K⁺]_E may contribute to high [K⁺]_E pro-survival effects, we conclude that elevated [K⁺]_E prevents apoptotic death of NGF-deprived sympathetic neurons primarily via an antioxidant mechanism.

A 'Marginal' tale: the development of the neocortical layer 1

The development of neocortical layer 1 is a very dynamic process and the scene of multiple transient events, with Cajal-Retzius cell death being one of the most characteristic ones. Layer 1 is also the route of migration for a substantial number of GABAergic interneurons during embryogenesis and where some of which will ultimately remain in the adult. The two cell types, together with a diverse set of incoming axons and dendrites, create an early circuit that will dramatically change in structure and function in the adult cortex to give prominence to inhibition. Through the engagement of a diverse set of GABAergic inhibitory cells by bottom-up and top-down inputs, adult layer 1 becomes a powerful computational platform for the neocortex.

Can we understand human brain development from experimental studies in rodents?

Animal models are needed to gain an understanding of the genetic, molecular, cellular, and network mechanisms of human brain development. In rodents, a large spectrum of in vitro and in vivo approaches allows detailed analyses and specific experimental manipulations for studying the sequence of developmental steps in corticogenesis. Neurogenesis, neuronal migration, cellular differentiation, programmed cell death, synaptogenesis, and myelination are surprisingly similar in the rodent cortex and the human cortex. Spontaneous EEG activity in the pre- and early postnatal human cortex resembles the activity patterns recorded with intracortical multi-electrode arrays in newborn rodents. This early activity is generated by thalamic activation of a subplate-driven local network coupled via gap junctions, which controls the development of cortical columns and the spatio-temporal pattern of apoptosis. Disturbances of this activity may induce disturbances in cortical structure and function leading to neurological and psychiatric disorders.

Self-Generated Whisker Movements Drive State-Dependent Sensory Input to Developing Barrel Cortex

Cortical development is an activity-dependent process [1-3]. Regarding the role of activity in the developing somatosensory cortex, one persistent debate concerns the importance of sensory feedback from self-generated movements. Specifically, recent studies claim that cortical activity is generated intrinsically, independent of movement [3, 4]. However, other studies claim that behavioral state moderates the relationship between movement and cortical activity [5-7]. Thus, perhaps inattention to behavioral state leads to failures to detect movement-driven activity [8]. Here, we resolve this issue by associating local field activity (i.e., spindle bursts) and unit activity in the barrel cortex of 5-day-old rats with whisker movements during wake and myoclonic twitches of the whiskers during active (REM) sleep. Barrel activity increased significantly within 500 ms of whisker movements, especially after twitches. Also, higher-amplitude movements were more likely to trigger barrel activity; when we controlled for movement amplitude, barrel activity was again greater after a twitch than a wake movement. We then inverted the analysis to assess the likelihood that increases in barrel activity were preceded within 500 ms by whisker movements: at least 55% of barrel activity was attributable to sensory feedback from whisker movements. Finally, when periods with and without movement were compared, 70%-75% of barrel activity was movement related. These results confirm the importance of sensory feedback from movements in driving activity in sensorimotor cortex and underscore the necessity of monitoring sleep-wake states to ensure accurate assessments of the contributions of the sensory periphery to activity in developing somatosensory cortex.

Active Sleep Promotes Coherent Oscillatory Activity in the Cortico-Hippocampal System of Infant Rats

Active sleep (AS) provides a unique developmental context for synchronizing neural activity within and between cortical and subcortical structures. In week-old rats, sensory feedback from myoclonic twitches, the phasic motor activity that characterizes AS, promotes coherent theta oscillations (4-8 Hz) in the hippocampus and red nucleus, a midbrain motor structure. Sensory feedback from twitches also triggers rhythmic activity in sensorimotor cortex in the form of spindle bursts, which are brief oscillatory events composed of rhythmic components in the theta, alpha/beta (8-20 Hz), and beta2 (20-30 Hz) bands. Here we ask whether one or more of these spindle-burst components are communicated from sensorimotor cortex to hippocampus. By recording simultaneously from whisker barrel cortex and dorsal hippocampus in 8-day-old rats, we show that AS, but not other behavioral states, promotes cortico-hippocampal coherence specifically in the beta2 band. By cutting the infraorbital nerve to prevent the conveyance of sensory feedback from whisker twitches, cortical-hippocampal beta2 coherence during AS was substantially reduced. These results demonstrate the necessity of sensory input, particularly during AS, for coordinating rhythmic activity between these two developing forebrain structures.

Self and the Brain. The Immune Metaphor

One of the fundamental questions in neuroscience is how brain activity relates to conscious experience. Even though self-consciousness is considered an emergent property of the brain network, a quantum physics-based theory assigns a momentum of consciousness to the single neuron level. In this work, we present a brain self theory from an evolutionary biological perspective by analogy with the immune self. In this scheme, perinatal reactivity to self inputs would guide the selection of neocortical neurons within the subplate, similarly to T lymphocytes in the thymus. Such self-driven neuronal selection would enable effective discrimination of external inputs and avoid harmful "autoreactive" responses. Multiple experimental and clinical evidences for this model are provided. Based on this self tenet, we outline the postulates of the so-called autophrenic diseases, to then make the case for schizophrenia, an archetypic disease with rupture of the self. Implications of this model are discussed, along with potential experimental verification.

Activity-dependent death of transient Cajal-Retzius neurons is required for functional cortical wiring

Programmed cell death and early activity contribute to the emergence of functional cortical circuits. While most neuronal populations are scaled-down by death, some subpopulations are entirely eliminated, raising the question of the importance of such demise for cortical wiring. Here, we addressed this issue by focusing on Cajal-Retzius neurons (CRs), key players in cortical development that are eliminated in postnatal mice in part via Bax-dependent apoptosis. Using Bax-conditional mutants and CR hyperpolarization, we show that the survival of electrically active subsets of CRs triggers an increase in both dendrite complexity and spine density of upper layer pyramidal neurons, leading to an excitation/inhibition imbalance. The survival of these CRs is induced by hyperpolarization, highlighting an interplay between early activity and neuronal elimination. Taken together, our study reveals a novel activity-dependent programmed cell death process required for the removal of transient immature neurons and the proper wiring of functional cortical circuits.

GABAergic Restriction of Network Dynamics Regulates Interneuron Survival in the Developing Cortex

During neonatal development, sensory cortices generate spontaneous activity patterns shaped by both sensory experience and intrinsic influences. How these patterns contribute to the assembly of neuronal circuits is not clearly understood. Using longitudinal in vivo calcium imaging in un-anesthetized mouse pups, we show that spatially segregated functional assemblies composed of interneurons and pyramidal cells are prominent in the somatosensory cortex by postnatal day (P) 7. Both reduction of GABA release and synaptic inputs onto pyramidal cells erode the emergence of functional topography, leading to increased network synchrony. This aberrant pattern effectively blocks interneuron apoptosis, causing increased survival of parvalbumin and somatostatin interneurons. Furthermore, the effect of GABA on apoptosis is mediated by inputs from medial ganglionic eminence (MGE)-derived but not caudal ganglionic eminence (CGE)-derived interneurons. These findings indicate that immature MGE interneurons are fundamental for shaping GABA-driven activity patterns that balance the number of interneurons integrating into maturing cortical networks.

Spatial quantification of the synaptic activity phenotype across large populations of neurons with Markov random fields

Motivation

The collective and co-ordinated synaptic activity of large neuronal populations is relevant to neuronal development as well as a range of neurological diseases. Quantification of synaptically-mediated neuronal signalling permits further downstream analysis as well as potential application in target validation and in vitro screening assays. Our aim is to develop a phenotypic quantification for neuronal activity imaging data of large populations of neurons, in particular relating to the spatial component of the activity.

Results

We extend the use of Markov random field (MRF) models to achieve this aim. In particular, we consider Bayesian posterior densities of model parameters in Gaussian MRFs to directly model changes in calcium fluorescence intensity rather than using spike trains. The basis of our model is defining neuron 'neighbours' by the relative spatial positions of the neuronal somata as obtained from the image data whereas previously this has been limited to defining an artificial square grid across the field of view and spike binning. We demonstrate that our spatial phenotypic quantification is applicable for both in vitro and in vivo data consisting of thousands of neurons over hundreds of time points. We show how our approach provides insight beyond that attained by conventional spike counting and discuss how it could be used to facilitate screening assays for modifiers of disease-associated defects of communication between cells.

Availability and implementation

We supply the MATLAB code and data to obtain all of the results in the paper.

Supplementary information

Supplementary data are available at Bioinformatics online.

How Early Can a Seizure Happen? Pathophysiological Considerations of Extremely Premature Infant Brain Development

Seizures in neonates represent a neurologic emergency requiring prompt recognition, determination of etiology, and treatment. Yet, the definition and identification of neonatal seizures remain challenging and controversial, in part due to the unique physiology of brain development at this life stage. These issues are compounded when considering seizures in premature infants, in whom the complexities of brain development may engender different clinical and electrographic seizure features at different points in neuronal maturation. In extremely premature infants (< 28 weeks gestational age), seizure pathophysiology has not been explored in detail. This review discusses the physiological and structural development of the brain in this developmental window, focusing on factors that may lead to seizures and their consequences at this early time point. We hypothesize that the clinical and electrographic phenomenology of seizures in extremely preterm infants reflects the specific pathophysiology of brain development in that age window.

The Role of AMPARs in the Maturation and Integration of Caudal Ganglionic Eminence-Derived Interneurons into Developing Hippocampal Microcircuits

In the hippocampal CA1, caudal ganglionic eminence (CGE)-derived interneurons are recruited by activation of glutamatergic synapses comprising GluA2-containing calcium-impermeable AMPARs and exert inhibitory regulation of the local microcircuit. However, the role played by AMPARs in maturation of the developing circuit is unknown. We demonstrate that elimination of the GluA2 subunit (GluA2 KO) of AMPARs in CGE-derived interneurons, reduces spontaneous EPSC frequency coupled to a reduction in dendritic glutamatergic synapse density. Removal of GluA1&2&3 subunits (GluA1-3 KO) in CGE-derived interneurons, almost completely eliminated sEPSCs without further reducing synapse density, but increased dendritic branching. Moreover, in GluA1-3 KOs, the number of interneurons invading the hippocampus increased in the early postnatal period but converged with WT numbers later due to increased apoptosis. However, the CCK-containing subgroup increased in number, whereas the VIP-containing subgroup decreased. Both feedforward and feedback inhibitory input onto pyramidal neurons was decreased in GluA1-3 KO. These combined anatomical, synaptic and circuit alterations, were accompanied with a wide range of behavioural abnormalities in GluA1-3 KO mice compared to GluA2 KO and WT. Thus, AMPAR subunits differentially contribute to numerous aspects of the development and maturation of CGE-derived interneurons and hippocampal circuitry that are essential for normal behaviour.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Alcohol and the Developing Brain: Why Neurons Die and How Survivors Change

The consequences of alcohol drinking during pregnancy are dramatic and usually referred to as fetal alcohol spectrum disorders (FASD). This condition is one of the main causes of intellectual disability in Western countries. The immature fetal brain exposed to ethanol undergoes massive neuron death. However, the same mechanisms leading to cell death can also be responsible for changes of developmental plasticity. As a consequence of such a maladaptive plasticity, the functional damage to central nervous system structures is amplified and leads to permanent sequelae. Here we review the literature dealing with experimental FASD, focusing on the alterations of the cerebral cortex. We propose that the reciprocal interaction between cell death and maladaptive plasticity represents the main pathogenetic mechanism of the alcohol-induced damage to the developing brain.

Cortical developmental death: selected to survive or fated to die

The mature cerebral cortex only contains a fraction of the cells that are generated during embryonic development. Indeed some neuronal populations are produced in excess and later subjected to partial elimination whereas others are almost completely removed during the first two postnatal weeks in mice. Although the identity of cells that disappear, the time course and mechanisms of their death are becoming reasonably well established, the meaning of producing supernumerary cells still remains elusive. In this review, we focus on recent data that shed a new light on the mechanisms involved in adjusting cell numbers and discuss the significance of refinement versus complete elimination of cell populations in the developing cortex.

Active Sleep Promotes Functional Connectivity in Developing Sensorimotor Networks

A ubiquitous feature of active (REM) sleep in mammals and birds is its relative abundance in early development. In rat pups across the first two postnatal weeks, active sleep promotes the expression of synchronized oscillatory activity within and between cortical and subcortical sensorimotor structures. Sensory feedback from self-generated myoclonic twitches - which are produced exclusively during active sleep - also triggers neural oscillations in those structures. We have proposed that one of the functions of active sleep in early infancy is to provide a context for synchronizing developing structures. Specifically, neural oscillations contribute to a variety of neurodevelopmental processes, including synapse formation, neuronal differentiation and migration, apoptosis, and the refinement of topographic maps. In addition, synchronized oscillations promote functional connectivity between distant brain areas. Consequently, any condition or manipulation that restricts active sleep can, in turn, deprive the infant animal of substantial sensory experience, resulting in atypical developmental trajectories.

Modulation of Neocortical Development by Early Neuronal Activity: Physiology and Pathophysiology

Animal and human studies revealed that patterned neuronal activity is an inherent feature of developing nervous systems. This review summarizes our current knowledge about the mechanisms generating early electrical activity patterns and their impact on structural and functional development of the cerebral cortex. All neocortical areas display distinct spontaneous and sensory-driven neuronal activity patterns already at early phases of development. At embryonic stages, intermittent spontaneous activity is synchronized within small neuronal networks, becoming more complex with further development. This transition is accompanied by a gradual shift from electrical to chemical synaptic transmission, with a particular role of non-synaptic tonic currents before the onset of phasic synaptic activity. In this review article we first describe functional impacts of classical neurotransmitters (GABA, glutamate) and modulatory systems (e.g., acetylcholine, ACh) on early neuronal activities in the neocortex with special emphasis on electrical synapses, nonsynaptic and synaptic currents. Early neuronal activity influences probably all developmental processes and is crucial for the proper formation of neuronal circuits. In the second part of our review, we illustrate how specific activity patterns might interfere with distinct neurodevelopmental processes like proliferation, migration, axonal and dendritic sprouting, synapse formation and neurotransmitter specification. Finally, we present evidence that transient alterations in neuronal activity during restricted perinatal periods can lead to persistent changes in functional connectivity and therefore might underlie the manifestation of neurological and neuropsychiatric diseases.

Taurine as an Essential Neuromodulator during Perinatal Cortical Development

A variety of experimental studies demonstrated that neurotransmitters are an important factor for the development of the central nervous system, affecting neurodevelopmental events like neurogenesis, neuronal migration, programmed cell death, and differentiation. While the role of the classical neurotransmitters glutamate and gamma-aminobutyric acid (GABA) on neuronal development is well established, the aminosulfonic acid taurine has also been considered as possible neuromodulator during early neuronal development. The purpose of the present review article is to summarize the properties of taurine as neuromodulator in detail, focusing on the direct involvement of taurine on various neurodevelopmental events and the regulation of neuronal activity during early developmental epochs. The current knowledge is that taurine lacks a synaptic release mechanism but is released by volume-sensitive organic anion channels and/or a reversal of the taurine transporter. Extracellular taurine affects neurons and neuronal progenitor cells mainly via glycine, GABA(A), and GABA(B) receptors with considerable receptor and subtype-specific affinities. Taurine has been shown to directly influence neurogenesis in vitro as well as neuronal migration in vitro and in vivo. It provides a depolarizing signal for a variety of neuronal population in the immature central nervous system, thereby directly influencing neuronal activity. While in the neocortex, taurine probably enhance neuronal activity, in the immature hippocampus, a tonic taurinergic tone might be necessary to attenuate activity. In summary, taurine must be considered as an essential modulator of neurodevelopmental events, and possible adverse consequences on fetal and/or early postnatal development should be evaluated for pharmacological therapies affecting taurinergic functions.

Electrical activity controls area-specific expression of neuronal apoptosis in the mouse developing cerebral cortex

Programmed cell death widely but heterogeneously affects the developing brain, causing the loss of up to 50% of neurons in rodents. However, whether this heterogeneity originates from neuronal identity and/or network-dependent processes is unknown. Here, we report that the primary motor cortex (M1) and primary somatosensory cortex (S1), two adjacent but functionally distinct areas, display striking differences in density of apoptotic neurons during the early postnatal period. These differences in rate of apoptosis negatively correlate with region-dependent levels of activity. Disrupting this activity either pharmacologically or by electrical stimulation alters the spatial pattern of apoptosis and sensory deprivation leads to exacerbated amounts of apoptotic neurons in the corresponding functional area of the neocortex. Thus, our data demonstrate that spontaneous and periphery-driven activity patterns are important for the structural and functional maturation of the neocortex by refining the final number of cortical neurons in a region-dependent manner.