Rapid development and plasticity of layer 2/3 maps in rat barrel cortex in vivo

Network state transitions during cortical development

Mammalian cortical networks are active before synaptogenesis begins in earnest, before neuronal migration is complete, and well before an animal opens its eyes and begins to actively explore its surroundings. This early activity undergoes several transformations during development. The most important of these is a transition from episodic synchronous network events, which are necessary for patterning the neocortex into functionally related modules, to desynchronized activity that is computationally more powerful and efficient. Network desynchronization is perhaps the most dramatic and abrupt developmental event in an otherwise slow and gradual process of brain maturation. In this Review, we summarize what is known about the phenomenology of developmental synchronous activity in the rodent neocortex and speculate on the mechanisms that drive its eventual desynchronization. We argue that desynchronization of network activity is a fundamental step through which the cortex transitions from passive, bottom-up detection of sensory stimuli to active sensory processing with top-down modulation.

Early-life maturation of the somatosensory cortex: sensory experience and beyond

Early life experiences shape physical and behavioral outcomes throughout lifetime. Sensory circuits are especially susceptible to environmental and physiological changes during development. However, the impact of different types of early life experience are often evaluated in isolation. In this mini review, we discuss the specific effects of postnatal sensory experience, sleep, social isolation, and substance exposure on barrel cortex development. Considering these concurrent factors will improve understanding of the etiology of atypical sensory perception in many neuropsychiatric and neurodevelopmental disorders.

In vivo whole-cortex marker of excitation-inhibition ratio indexes cortical maturation and cognitive ability in youth

A balanced excitation-inhibition ratio (E/I ratio) is critical for healthy brain function. Normative development of cortex-wide E/I ratio remains unknown. Here, we noninvasively estimate a putative marker of whole-cortex E/I ratio by fitting a large-scale biophysically plausible circuit model to resting-state functional MRI (fMRI) data. We first confirm that our model generates realistic brain dynamics in the Human Connectome Project. Next, we show that the estimated E/I ratio marker is sensitive to the gamma-aminobutyric acid (GABA) agonist benzodiazepine alprazolam during fMRI. Alprazolam-induced E/I changes are spatially consistent with positron emission tomography measurement of benzodiazepine receptor density. We then investigate the relationship between the E/I ratio marker and neurodevelopment. We find that the E/I ratio marker declines heterogeneously across the cerebral cortex during youth, with the greatest reduction occurring in sensorimotor systems relative to association systems. Importantly, among children with the same chronological age, a lower E/I ratio marker (especially in the association cortex) is linked to better cognitive performance. This result is replicated across North American (8.2 to 23.0 y old) and Asian (7.2 to 7.9 y old) cohorts, suggesting that a more mature E/I ratio indexes improved cognition during normative development. Overall, our findings open the door to studying how disrupted E/I trajectories may lead to cognitive dysfunction in psychopathology that emerges during youth.

In-vivo whole-cortex marker of excitation-inhibition ratio indexes cortical maturation and cognitive ability in youth

A balanced excitation-inhibition ratio (E/I ratio) is critical for healthy brain function. Normative development of cortex-wide E/I ratio remains unknown. Here we non-invasively estimate a putative marker of whole-cortex E/I ratio by fitting a large-scale biophysically-plausible circuit model to resting-state functional MRI (fMRI) data. We first confirm that our model generates realistic brain dynamics in the Human Connectome Project. Next, we show that the estimated E/I ratio marker is sensitive to the GABA-agonist benzodiazepine alprazolam during fMRI. Alprazolam-induced E/I changes are spatially consistent with positron emission tomography measurement of benzodiazepine receptor density. We then investigate the relationship between the E/I ratio marker and neurodevelopment. We find that the E/I ratio marker declines heterogeneously across the cerebral cortex during youth, with the greatest reduction occurring in sensorimotor systems relative to association systems. Importantly, among children with the same chronological age, a lower E/I ratio marker (especially in association cortex) is linked to better cognitive performance. This result is replicated across North American (8.2 to 23.0 years old) and Asian (7.2 to 7.9 years old) cohorts, suggesting that a more mature E/I ratio indexes improved cognition during normative development. Overall, our findings open the door to studying how disrupted E/I trajectories may lead to cognitive dysfunction in psychopathology that emerges during youth.

Improvement of sensory deficits in fragile X mice by increasing cortical interneuron activity after the critical period

Changes in the function of inhibitory interneurons (INs) during cortical development could contribute to the pathophysiology of neurodevelopmental disorders. Using all-optical in vivo approaches, we find that parvalbumin (PV) INs and their immature precursors are hypoactive and transiently decoupled from excitatory neurons in postnatal mouse somatosensory cortex (S1) of Fmr1 KO mice, a model of fragile X syndrome (FXS). This leads to a loss of parvalbumin INs (PV-INs) in both mice and humans with FXS. Increasing the activity of future PV-INs in neonatal Fmr1 KO mice restores PV-IN density and ameliorates transcriptional dysregulation in S1, but not circuit dysfunction. Critically, administering an allosteric modulator of Kv3.1 channels after the S1 critical period does rescue circuit dynamics and tactile defensiveness. Symptoms in FXS and related disorders could be mitigated by targeting PV-INs.

Arousal state transitions occlude sensory-evoked neurovascular coupling in neonatal mice

In the adult sensory cortex, increases in neural activity elicited by sensory stimulation usually drive vasodilation mediated by neurovascular coupling. However, whether neurovascular coupling is the same in neonatal animals as adults is controversial, as both canonical and inverted responses have been observed. We investigated the nature of neurovascular coupling in unanesthetized neonatal mice using optical imaging, electrophysiology, and BOLD fMRI. We find in neonatal (postnatal day 15, P15) mice, sensory stimulation induces a small increase in blood volume/BOLD signal, often followed by a large decrease in blood volume. An examination of arousal state of the mice revealed that neonatal mice were asleep a substantial fraction of the time, and that stimulation caused the animal to awaken. As cortical blood volume is much higher during REM and NREM sleep than the awake state, awakening occludes any sensory-evoked neurovascular coupling. When neonatal mice are stimulated during an awake period, they showed relatively normal (but slowed) neurovascular coupling, showing that that the typically observed constriction is due to arousal state changes. These result show that sleep-related vascular changes dominate over any sensory-evoked changes, and hemodynamic measures need to be considered in the context of arousal state changes.

Arousal state transitions occlude sensory-evoked neurovascular coupling in neonatal mice

In the adult sensory cortex, increases in neural activity elicited by sensory stimulation usually drives vasodilation mediated by neurovascular coupling. However, whether neurovascular coupling is the same in neonatal animals as adults is controversial, as both canonical and inverted responses have been observed. We investigated the nature of neurovascular coupling in unanesthetized neonatal mice using optical imaging, electrophysiology, and BOLD fMRI. We find in neonatal (postnatal day 15, P15) mice, sensory stimulation induces a small increase in blood volume/BOLD signal, often followed by a large decrease in blood volume. An examination of arousal state of the mice revealed that neonatal mice were asleep a substantial fraction of the time, and that stimulation caused the animal to awaken. As cortical blood volume is much higher during REM and NREM sleep than the awake state, awakening occludes any sensory-evoked neurovascular coupling. When neonatal mice are stimulated during an awake period, they showed relatively normal (but slowed) neurovascular coupling, showing that that the typically observed constriction is due to arousal state changes. These result show that sleep-related vascular changes dominate over any sensory-evoked changes, and hemodynamic measures need to be considered in the context of arousal state changes.

Significance Statement

In the adult brain, increases in neural activity are often followed by vasodilation, allowing activity to be monitored using optical or magnetic resonance imaging. However, in neonates, sensory stimulation can drive vasoconstriction, whose origin was not understood. We used optical and magnetic resonance imaging approaches to investigate hemodynamics in neonatal mice. We found that sensory-induced vasoconstriction occurred when the mice were asleep, as sleep is associated with dilation of the vasculature of the brain relative to the awake state. The stimulus awakens the mice, causing a constriction due to the arousal state change. Our study shows the importance of monitoring arousal state, particularly when investigating subjects that may sleep, and the dominance arousal effects on brain hemodynamics.

Juvenile depletion of microglia reduces orientation but not high spatial frequency selectivity in mouse V1

Microglia contain multiple mechanisms that shape the synaptic landscape during postnatal development. Whether the synaptic changes mediated by microglia reflect the developmental refinement of neuronal responses in sensory cortices, however, remains poorly understood. In postnatal life, the development of increased orientation and spatial frequency selectivity of neuronal responses in primary visual cortex (V1) supports the emergence of high visual acuity. Here, we used the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to rapidly and durably deplete microglia in mice during the juvenile period in which increased orientation and spatial frequency selectivity emerge. Excitatory and inhibitory tuning properties were measured simultaneously using multi-photon calcium imaging in layer II/III of mouse V1. We found that microglia depletion generally increased evoked activity which, in turn, reduced orientation selectivity. Surprisingly, microglia were not required for the emergence of high spatial frequency tuned responses. In addition, microglia depletion did not perturb cortical binocularity, suggesting normal depth processing. Together, our finding that orientation and high spatial frequency selectivity in V1 are differentially supported by microglia reveal that microglia are required normal sensory processing, albeit selectively.

Schizophrenia risk-gene Crmp2 deficiency causes precocious critical period plasticity and deteriorated binocular vision

Brain-specific loss of a microtubule-binding protein collapsin response mediator protein-2 (CRMP2) in the mouse recapitulates many schizophrenia-like behaviors of human patients, possibly resulting from associated developmental deficits in neuronal differentiation, path-finding, and synapse formation. However, it is still unclear how the Crmp2 loss affects neuronal circuit function and plasticity. By conducting in vivo and ex vivo electrophysiological recording in the mouse primary visual cortex (V1), we reveal that CRMP2 exerts a key regulation on the timing of postnatal critical period (CP) for experience-dependent circuit plasticity of sensory cortex. In the developing V1, the Crmp2 deficiency induces not only a delayed maturation of visual tuning functions but also a precocious CP for visual input-induced ocular dominance plasticity and its induction activity - coincident binocular inputs right after eye-opening. Mechanistically, the Crmp2 deficiency accelerates the maturation process of cortical inhibitory transmission and subsequently promotes an early emergence of balanced excitatory-inhibitory cortical circuits during the postnatal development. Moreover, the precocious CP plasticity results in deteriorated binocular depth perception in adulthood. Thus, these findings suggest that the Crmp2 deficiency dysregulates the timing of CP for experience-dependent refinement of circuit connections and further leads to impaired sensory perception in later life.

The critical period: neurochemical and synaptic mechanisms shared by the visual cortex and the brain stem respiratory system

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in the normal rat. In both regions, the critical period is a time of (i) heightened inhibition; (ii) reduced expression of brain-derived neurotrophic factor (BDNF); and (iii) synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period is necessary to enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

An increase in dendritic plateau potentials is associated with experience-dependent cortical map reorganization

Here we describe a mechanism for cortical map plasticity. Classically, representational map changes are thought to be driven by changes within cortico-cortical circuits, e.g., Hebbian plasticity of synaptic circuits that lost vs. maintained an excitatory drive from the first-order thalamus, possibly steered by neuromodulatory forces from deep brain regions. Our work provides evidence for an additional gating mechanism, provided by plateau potentials, which are driven by higher-order thalamic feedback. Higher-order thalamic neurons are characterized by broad receptive fields, and the plateau potentials that they evoke strongly facilitate long-term potentiation and elicit spikes. We show that these features combined constitute a powerful driving force for the fusion or expansion of sensory representations within cortical maps.

The organization of sensory maps in the cerebral cortex depends on experience, which drives homeostatic and long-term synaptic plasticity of cortico-cortical circuits. In the mouse primary somatosensory cortex (S1) afferents from the higher-order, posterior medial thalamic nucleus (POm) gate synaptic plasticity in layer (L) 2/3 pyramidal neurons via disinhibition and the production of dendritic plateau potentials. Here we address whether these thalamocortically mediated responses play a role in whisker map plasticity in S1. We find that trimming all but two whiskers causes a partial fusion of the representations of the two spared whiskers, concomitantly with an increase in the occurrence of POm-driven N-methyl-D-aspartate receptor-dependent plateau potentials. Blocking the plateau potentials restores the archetypical organization of the sensory map. Our results reveal a mechanism for experience-dependent cortical map plasticity in which higher-order thalamocortically mediated plateau potentials facilitate the fusion of normally segregated cortical representations.

Spatial organization and transitions of spontaneous neuronal activities in the developing sensory cortex

The sensory cortex underlies our ability to perceive and interact with the external world. Sensory perceptions are controlled by specialized neuronal circuits established through fine-tuning, which relies largely on neuronal activity during the development. Spontaneous neuronal activity is an essential driving force of neuronal circuit refinement. At early developmental stages, sensory cortices display spontaneous activities originating from the periphery and characterized by correlated firing arranged spatially according to the modality. The firing patterns are reorganized over time and become sparse, which is typical for the mature brain. This review focuses mainly on rodent sensory cortices. First, the features of the spontaneous activities during early postnatal stages are described. Then, the developmental changes in the spatial organization of the spontaneous activities and the transition mechanisms involved are discussed. The identification of the principles controlling the spatial organization of spontaneous activities in the developing sensory cortex is essential to understand the self-organization process of neuronal circuits.

Micropipet-Based Navigation in a Microvascular Model for Imaging Endothelial Cell Topography Using Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) has enabled cell surface topography at a high resolution with low invasiveness. However, SICM has not been applied to the observation of cell surfaces in hydrogels, which can serve as scaffolds for three-dimensional cell culture. In this study, we applied SICM for imaging a cell surface in a microvascular lumen reconstructed in a hydrogel. To achieve this goal, we developed a micropipet navigation technique using ionic current to detect the position of a microvascular lumen. Combining this navigation technique with SICM, endothelial cells in a microvascular model and blebs were visualized successfully at the single-cell level. To the best of our knowledge, this is the first report on visualizing cell surfaces in hydrogels using a SICM. This technique will be useful for furthering our understanding of the mechanism of intravascular diseases.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Sensory input drives rapid homeostatic scaling of the axon initial segment in mouse barrel cortex

The axon initial segment (AIS) is a critical microdomain for action potential initiation and implicated in the regulation of neuronal excitability during activity-dependent plasticity. While structural AIS plasticity has been suggested to fine-tune neuronal activity when network states change, whether it acts in vivo as a homeostatic regulatory mechanism in behaviorally relevant contexts remains poorly understood. Using the mouse whisker-to-barrel pathway as a model system in combination with immunofluorescence, confocal analysis and electrophysiological recordings, we observed bidirectional AIS plasticity in cortical pyramidal neurons. Furthermore, we find that structural and functional AIS remodeling occurs in distinct temporal domains: Long-term sensory deprivation elicits an AIS length increase, accompanied with an increase in neuronal excitability, while sensory enrichment results in a rapid AIS shortening, accompanied by a decrease in action potential generation. Our findings highlight a central role of the AIS in the homeostatic regulation of neuronal input-output relations.

Critical period regulation across multiple timescales

Brain plasticity is dynamically regulated across the life span, peaking during windows of early life. Typically assessed in the physiological range of milliseconds (real time), these trajectories are also influenced on the longer timescales of developmental time (nurture) and evolutionary time (nature), which shape neural architectures that support plasticity. Properly sequenced critical periods of circuit refinement build up complex cognitive functions, such as language, from more primary modalities. Here, we consider recent progress in the biological basis of critical periods as a unifying rubric for understanding plasticity across multiple timescales. Notably, the maturation of parvalbumin-positive (PV) inhibitory neurons is pivotal. These fast-spiking cells generate gamma oscillations associated with critical period plasticity, are sensitive to circadian gene manipulation, emerge at different rates across brain regions, acquire perineuronal nets with age, and may be influenced by epigenetic factors over generations. These features provide further novel insight into the impact of early adversity and neurodevelopmental risk factors for mental disorders.

Two-Photon Voltage Imaging of Spontaneous Activity from Multiple Neurons Reveals Network Activity in Brain Tissue

Recording the electrical activity of multiple neurons simultaneously would greatly facilitate studies on the function of neuronal circuits. The combination of the fast scanning by random-access multiphoton microscopy (RAMP) and the latest two-photon-compatible high-performance fluorescent genetically encoded voltage indicators (GEVIs) has enabled action potential detection in deep layers in in vivo brain. However, neuron connectivity analysis on optically recorded action potentials from multiple neurons in brain tissue has yet to be achieved. With high expression of a two-photon-compatible GEVI, ASAP3, via in utero electroporation and RAMP, we achieved voltage recording of spontaneous activities from multiple neurons in brain slice. We provide evidence for the developmental changes in intralaminar horizontal connections in somatosensory cortex layer 2/3 with a greater sensitivity than calcium imaging. This method thus enables investigation of neuronal network connectivity at the cellular resolution in brain tissue.

Encoding schemes in somatosensation: From micro- to meta-topography

Encoding schemes are systematic large-scale arrangements that convert incoming sensory information into a format required for further information processing. The increased spatial resolution of brain images obtained with ultra-high field magnetic resonance imaging at 7 T (7T-MRI) and above increases the granularity and precision of processing units that mediate the link between neuronal encoding and functional readouts. Here, these new developments are reviewed with a focus on human tactile encoding schemes derived from small-scale processing units (in the order of 0.5-5 mm) that are relevant for theoretical and practical concepts of somatosensory encoding and cortical plasticity. Precisely, we review recent approaches to characterize meso-scale maps, layer units, and cortical fields in the sensorimotor cortex of the living human brain and discuss their impact on theories of perception, motor control, topographic encoding, and cortical plasticity. Finally, we discuss concepts on the integration of small-scale processing units into functional networks that span multiple topographic maps and multiple cortical areas. Novel research areas are highlighted that may help to bridge the gap between cortical microstructure and meta-topographic models on brain anatomy and function.

Novel synaptic plasticity enhancer drug to augment functional recovery with rehabilitation

Purpose of review

Stroke is a devastating illness which severely attenuates quality of life because of paralysis. Despite recent advances in therapies during acute phase such as thrombolytic therapy, clinical option to intervene the process of rehabilitation is limited. No pharmacological intervention that could enhance the effect of rehabilitation has not been established. Recent articles, which are summarized in the review article, reported novel small compound which accelerates training-dependent motor function recovery after brain damage.

Recent findings

A novel small compound, edonerpic maleate, binds to collapsin response mediator protein 2 (CRMP2) and enhance synaptic plasticity leading to the acceleration of rehabilitative training-dependent functional recovery after brain damage in rodent and nonhuman primate. The clinical trial to test this effect in human is now ongoing. Future preclinical and clinical studies will delineate the potentials of this compound.

Summary

A novel CRMP2-binding small compound, edonerpic maleate, accelerates motor function recovery after brain damage in rodent and nonhuman primate.

The cellular and molecular basis of in vivo synaptic plasticity in rodents

Plasticity within the neuronal networks of the brain underlies the ability to learn and retain new information. The initial discovery of synaptic plasticity occurred by measuring synaptic strength in vivo, applying external stimulation and observing an increase in synaptic strength termed long-term potentiation (LTP). Many of the molecular pathways involved in LTP and other forms of synaptic plasticity were subsequently uncovered in vitro. Over the last few decades, technological advances in recording and imaging in live animals have seen many of these molecular mechanisms confirmed in vivo, including structural changes both pre- and postsynaptically, changes in synaptic strength, and changes in neuronal excitability. A well-studied aspect of neuronal plasticity is the capacity of the brain to adapt to its environment, gained by comparing the brains of deprived and experienced animals in vivo, and in direct response to sensory stimuli. Multiple in vivo studies have also strongly linked plastic changes to memory by interfering with the expression of plasticity and by manipulating memory engrams. Plasticity in vivo also occurs in the absence of any form of external stimulation, i.e., during spontaneous network activity occurring with brain development. However, there is still much to learn about how plasticity is induced during natural learning and how this is altered in neurological disorders.

Temporal refinement of sensory-evoked activity across layers in developing mouse barrel cortex

Rhythmic whisking behavior in rodents fully develops during a critical period about 2 weeks after birth, in parallel with the maturation of other sensory modalities and the onset of exploratory locomotion. How whisker-related sensory processing develops during this period in the primary somatosensory cortex (S1) remains poorly understood. Here, we characterized neuronal activity evoked by single- or dual-whisker stimulation patterns in developing S1, before, during and after the occurrence of active whisking. Employing multi-electrode recordings in all layers of barrel cortex in urethane-anesthetized mice, we find layer-specific changes in multi-unit activity for principal and neighboring barrel columns. While whisker stimulation evoked similar early responses (0-50 ms post-stimulus) across development, the late response (50-150 ms post-stimulus) decreased in all layers with age. Furthermore, peak onset times and the duration of the late response decreased in all layers across age groups. Responses to paired-pulse stimulation showed increases in spiking precision and in paired-pulse ratios in all cortical layers during development. Sequential activation of two neighboring whiskers with varying stimulus intervals evoked distinct response profiles in the activated barrel columns, depending on the direction and temporal separation of the stimuli. In conclusion, our findings indicate that the temporal sharpening of sensory-evoked activity coincides with the onset of active whisking.

Early postnatal development of pyramidal neurons across layers of the mouse medial prefrontal cortex

Mammalian neocortex is a highly layered structure. Each layer is populated by distinct subtypes of principal cells that are born at different times during development. While the differences between principal cells across layers have been extensively studied, it is not known how the developmental profiles of neurons in different layers compare. Here, we provide a detailed morphological and functional characterisation of pyramidal neurons in mouse mPFC during the first postnatal month, corresponding to known critical periods for synapse and neuron formation in mouse sensory neocortex. Our data demonstrate similar maturation profiles of dendritic morphology and intrinsic properties of pyramidal neurons in both deep and superficial layers. In contrast, the balance of synaptic excitation and inhibition differs in a layer-specific pattern from one to four postnatal weeks of age. Our characterisation of the early development and maturation of pyramidal neurons in mouse mPFC not only demonstrates a comparable time course of postnatal maturation to that in other neocortical circuits, but also implies that consideration of layer- and time-specific changes in pyramidal neurons may be relevant for studies in mouse models of neuropsychiatric and neurodevelopmental disorders.

A Hypothetical Model Concerning How Spike-Timing-Dependent Plasticity Contributes to Neural Circuit Formation and Initiation of the Critical Period in Barrel Cortex

Spike timing is an important factor in the modification of synaptic strength. Various forms of spike timing-dependent plasticity (STDP) occur in the brains of diverse species, from insects to humans. In unimodal STDP, only LTP or LTD occurs at the synapse, regardless of which neuron spikes first; the magnitude of potentiation or depression increases as the time between presynaptic and postsynaptic spikes decreases. This from of STDP may promote developmental strengthening or weakening of early projections. In bidirectional Hebbian STDP, the magnitude and the sign (potentiation or depression) of plasticity depend, respectively, on the timing and the order of presynaptic and postsynaptic spikes. In the rodent barrel cortex, multiple forms of STDP appear sequentially during development, and they contribute to network formation, retraction, or fine-scale functional reorganization. Hebbian STDP appears at L4-L2/3 synapses starting at postnatal day (P) 15; the synapses exhibit unimodal "all-LTP STDP" before that age. The appearance of Hebbian STDP at L4-L2/3 synapses coincides with the maturation of parvalbumin-containing GABA interneurons in L4, which contributes to the generation of L4-before-L2/3 spiking in response to thalamic input by producing fast feedforward suppression of both L4 and L2/3 cells. After P15, L4-L2/3 STDP mediates fine-scale circuit refinement, essential for the critical period in the barrel cortex. In this review, we first briefly describe the relevance of STDP to map plasticity in the barrel cortex, then look over roles of distinct forms of STDP during development. Finally, we propose a hypothesis that explains the transition from network formation to the initiation of the critical period in the barrel cortex.

A databank for intracellular electrophysiological mapping of the adult somatosensory cortex

Background

Neurons in the supragranular layers of the somatosensory cortex integrate sensory (bottom-up) and cognitive/perceptual (top-down) information as they orchestrate communication across cortical columns. It has been inferred, based on intracellular recordings from juvenile animals, that supragranular neurons are electrically mature by the fourth postnatal week. However, the dynamics of the neuronal integration in adulthood is largely unknown. Electrophysiological characterization of the active properties of these neurons throughout adulthood will help to address the biophysical and computational principles of the neuronal integration.

Findings

Here, we provide a database of whole-cell intracellular recordings from 315 neurons located in the supragranular layers (L2/3) of the primary somatosensory cortex in adult mice (9–45 weeks old) from both sexes (females, N = 195; males, N = 120). Data include 361 somatic current-clamp (CC) and 476 voltage-clamp (VC) experiments, recorded using a step-and-hold protocol (CC, N = 257; VC, N = 46), frozen noise injections (CC, N = 104) and triangular voltage sweeps (VC, 10 (N = 132), 50 (N = 146) and 100 ms (N = 152)), from regular spiking (N = 169) and fast-spiking neurons (N = 66).

Conclusions

The data can be used to systematically study the properties of somatic integration and the principles of action potential generation across sexes and across electrically characterized neuronal classes in adulthood. Understanding the principles of the somatic transformation of postsynaptic potentials into action potentials will shed light onto the computational principles of intracellular information transfer in single neurons and information processing in neuronal networks, helping to recreate neuronal functions in artificial systems.

Functional Relevance of Endocannabinoid-Dependent Synaptic Plasticity in the Central Nervous System

The endocannabinoid (eCB) signaling system plays a key role in short-term and long-term synaptic plasticity in brain regions involved in various neural functions ranging from action selection to appetite control. This review will explore the role of eCBs in shaping neural circuit function to regulate behaviors. In particular, we will discuss the behavioral consequences of eCB mediated long-term synaptic plasticity in different brain regions. This review brings together evidence from in vitro and ex vivo studies and points out the need for more in vivo studies.

Layer I Interneurons Sharpen Sensory Maps during Neonatal Development

The neonatal mammal faces an array of sensory stimuli when diverse neuronal types have yet to form sensory maps. How these inputs interact with intrinsic neuronal activity to facilitate circuit assembly is not well understood. By using longitudinal calcium imaging in unanesthetized mouse pups, we show that layer I (LI) interneurons, delineated by co-expression of the 5HT3a serotonin receptor (5HT3aR) and reelin (Re), display spontaneous calcium transients with the highest degree of synchrony among cell types present in the superficial barrel cortex at postnatal day 6 (P6). 5HT3aR Re interneurons are activated by whisker stimulation during this period, and sensory deprivation induces decorrelation of their activity. Moreover, attenuation of thalamic inputs through knockdown of NMDA receptors (NMDARs) in these interneurons results in expansion of whisker responses, aberrant barrel map formation, and deficits in whisker-dependent behavior. These results indicate that recruitment of specific interneuron types during development is critical for adult somatosensory function. VIDEO ABSTRACT.

The pial vasculature of the mouse develops according to a sensory-independent program

The cerebral vasculature is organized to supply the brain’s metabolic needs. Sensory deprivation during the early postnatal period causes altered neural activity and lower metabolic demand. Neural activity is instructional for some aspects of vascular development, and deprivation causes changes in capillary density in the deprived brain region. However, it is not known if the pial arteriole network, which contains many leptomeningeal anastomoses (LMAs) that endow the network with redundancy against occlusions, is also affected by sensory deprivation. We quantified the effects of early-life sensory deprivation via whisker plucking on the densities of LMAs and penetrating arterioles (PAs) in anatomically-identified primary sensory regions (vibrissae cortex, forelimb/hindlimb cortex, visual cortex and auditory cortex) in mice. We found that the densities of penetrating arterioles were the same across cortical regions, though the hindlimb representation had a higher density of LMAs than other sensory regions. We found that the densities of PAs and LMAs, as well as quantitative measures of network topology, were not affected by sensory deprivation. Our results show that the postnatal development of the pial arterial network is robust to sensory deprivation.

Layer-Specific Refinement of Sensory Coding in Developing Mouse Barrel Cortex

Rodent rhythmic whisking behavior matures during a critical period around 2 weeks after birth. The functional adaptations of neocortical circuitry during this developmental period remain poorly understood. Here, we characterized stimulus-evoked neuronal activity across all layers of mouse barrel cortex before, during, and after the onset of whisking behavior. Employing multi-electrode recordings and 2-photon calcium imaging in anesthetized mice, we tested responses to rostro-caudal whisker deflections, axial "tapping" stimuli, and their combination from postnatal day 10 (P10) to P28. Within this period, whisker-evoked activity of neurons displayed a general decrease in layer 2/3 (L2/3) and L4, but increased in L5 and L6. Distinct alterations in neuronal response adaptation during the 2-s period of stimulation at ~5 Hz accompanied these changes. Moreover, single-unit analysis revealed that response selectivity in favor of either lateral deflection or axial tapping emerges in deeper layers within the critical period around P14. For superficial layers we confirmed this finding using calcium imaging of L2/3 neurons, which also exhibited emergence of response selectivity as well as progressive sparsification and decorrelation of evoked responses around P14. Our results demonstrate layer-specific development of sensory responsiveness and response selectivity in mouse somatosensory cortex coinciding with the onset of exploratory behavior.

Activity-Dependent Downscaling of Subthreshold Synaptic Inputs during Slow-Wave-Sleep-like Activity In Vivo

Activity-dependent synaptic plasticity is critical for cortical circuit refinement. The synaptic homeostasis hypothesis suggests that synaptic connections are strengthened during wake and downscaled during sleep; however, it is not obvious how the same plasticity rules could explain both outcomes. Using whole-cell recordings and optogenetic stimulation of presynaptic input in urethane-anesthetized mice, which exhibit slow-wave-sleep (SWS)-like activity, we show that synaptic plasticity rules are gated by cortical dynamics in vivo. While Down states support conventional spike timing-dependent plasticity, Up states are biased toward depression such that presynaptic stimulation alone leads to synaptic depression, while connections contributing to postsynaptic spiking are protected against this synaptic weakening. We find that this novel activity-dependent and input-specific downscaling mechanism has two important computational advantages: (1) improved signal-to-noise ratio, and (2) preservation of previously stored information. Thus, these synaptic plasticity rules provide an attractive mechanism for SWS-related synaptic downscaling and circuit refinement.

Sensory Deprivation Triggers Synaptic and Intrinsic Plasticity in the Hippocampus

Hippocampus, a temporal lobe structure involved in learning and memory, receives information from all sensory modalities. Despite extensive research on the role of sensory experience in cortical map plasticity, little is known about whether and how sensory experience regulates functioning of the hippocampal circuits. Here, we show that 9 ± 2 days of whisker deprivation during early mouse development depresses activity of CA3 pyramidal neurons by several principal mechanisms: decrease in release probability, increase in the fraction of silent synapses, and reduction in intrinsic excitability. As a result of deprivation-induced presynaptic inhibition, CA3-CA1 synaptic facilitation was augmented at high frequencies, shifting filtering properties of synapses. The changes in the AMPA-mediated synaptic transmission were accompanied by an increase in NR2B-containing NMDA receptors and a reduction in the AMPA/NMDA ratio. The observed reconfiguration of the CA3-CA1 connections may represent a homeostatic adaptation to augmentation in synaptic activity during the initial deprivation phase. In adult mice, tactile disuse diminished intrinsic excitability without altering synaptic facilitation. We suggest that sensory experience regulates computations performed by the hippocampus by tuning its synaptic and intrinsic characteristics.

Somatosensory maps

Somatosensory areas containing topographic maps of the body surface are a major feature of parietal cortex. In primates, parietal cortex contains four somatosensory areas, each with its own map, with the primary cutaneous map in area 3b. Rodents have at least three parietal somatosensory areas. Maps are not isomorphic to the body surface, but magnify behaviorally important skin regions, which include the hands and face in primates, and the whiskers in rodents. Within each map, intracortical circuits process tactile information, mediate spatial integration, and support active sensation. Maps may also contain fine-scale representations of touch submodalities, or direction of tactile motion. Functional representations are more overlapping than suggested by textbook depictions of map topography. The whisker map in rodent somatosensory cortex is a canonic system for studying cortical microcircuits, sensory coding, and map plasticity. Somatosensory maps are plastic throughout life in response to altered use or injury. This chapter reviews basic principles and recent findings in primate, human, and rodent somatosensory maps.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

Defining a critical period for inhibitory circuits within the somatosensory cortex

Although experience-dependent changes in brain inhibitory circuits are thought to play a key role during the “critical period” of brain development, the nature and timing of these changes are poorly understood. We examined the role of sensory experience in sculpting an inhibitory circuit in the primary somatosensory cortex (S1) of mice by using optogenetics to map the connections between parvalbumin (PV) expressing interneurons and layer 2/3 pyramidal cells. Unilateral whisker deprivation decreased the strength and spatial range of inhibitory input provided to pyramidal neurons by PV interneurons in layers 2/3, 4 and 5. By varying the time when sensory input was removed, we determined that the critical period closes around postnatal day 14. This yields the first precise time course of critical period plasticity for an inhibitory circuit.

A Critical Period for the Rapid Modification of Synaptic Properties at the VPm Relay Synapse

In addition to cortical areas, the thalamus also displays plasticity during a critical period in early life. Since most sensory information is transmitted to the cortex via the thalamus, it will be of significant interest to understand the precise time window and underlying mechanisms of this critical period in the thalamus. By using in vitro whole-cell patch recording in acute brain slices, we found that VPm relay synapses were only sensitive to whisker deprivation from postnatal day 11 (P11) to P14. Whisker deprivation initiated within the P11 to P14 window significantly reduced the amplitude of AMPAR-EPSCs, but not NMDAR-EPSCs when recorded 24 h after whisker removal. From P10 to P11, the timing for entry into the critical period and the kinetics underlying NMDAR-EPSCs function were significantly altered. At P11, NMDAR-EPSCs were less sensitive to ifenprodil, a selective blocker of NR2B-containing NMDAR, and the protein level of NR2A was significantly increased compared to those at P10. At the end of the critical period there were no obvious changes in synaptic properties when compared between P14 and P15. Using calcium imaging, we found that fewer P15 VPm neurons could be excited by the GABAa receptor agonist, muscimol, when compared to P14 VPm neurons; this correlated to an increase in KCC2 expression. Our studies revealed a precise critical period of sensory experience-dependent plasticity in the thalamus featuring distinct molecular mechanisms which occur at the start and end of this critical window.

Activity-dependent axonal plasticity in sensory systems

The rodent whisker-to-barrel cortex pathway is a classic model to study the effects of sensory experience and deprivation on neuronal circuit formation, not only during development but also in the adult. Decades of research have produced a vast body of evidence highlighting the fundamental role of neuronal activity (spontaneous and/or sensory-evoked) for circuit formation and function. In this context, it has become clear that neuronal adaptation and plasticity is not just a function of the neonatal brain, but persists into adulthood, especially after experience-driven modulation of network status. Mechanisms for structural remodeling of the somatodendritic or axonal domain include microscale alterations of neurites or synapses. At the same time, functional alterations at the nanoscale such as expression or activation changes of channels and receptors contribute to the modulation of intrinsic excitability or input-output relationships. However, it remains elusive how these forms of structural and functional plasticity come together to shape neuronal network formation and function. While specifically somatodendritic plasticity has been studied in great detail, the role of axonal plasticity, (e.g. at presynaptic boutons, branches or axonal microdomains), is rather poorly understood. Therefore, this review will only briefly highlight somatodendritic plasticity and instead focus on axonal plasticity. We discuss (i) the role of spontaneous and sensory-evoked plasticity during critical periods, (ii) the assembly of axonal presynaptic sites, (iii) axonal plasticity in the mature brain under baseline and sensory manipulation conditions, and finally (iv) plasticity of electrogenic axonal microdomains, namely the axon initial segment, during development and in the mature CNS.

Treadmill exercise suppressed stress-induced dendritic spine elimination in mouse barrel cortex and improved working memory via BDNF/TrkB pathway

Stress-related memory deficit is correlated with dendritic spine loss. Physical exercise improves memory function and promotes spinogenesis. However, no studies have been performed to directly observe exercise-related effects on spine dynamics, in association with memory function. This study utilized transcranial two-photon in vivo microscopy to investigate dendritic spine formation and elimination in barrel cortex of mice under physical constrain or naive conditions, followed by memory performance in a whisker-dependent novel texture discrimination task. We found that stressed mice had elevated spine elimination rate in mouse barrel cortex plus deficits in memory retrieval, both of which can be rescued by chronic exercise on treadmill. Exercise also elevated brain-derived neurotrophic factor (BDNF) expression in barrel cortex. The above-mentioned rescuing effects for both spinognesis and memory function were abolished after inhibiting BDNF/tyrosine kinase B (TrkB) pathway. In summary, this study demonstrated the improvement of stress-associated memory function by exercise via facilitating spine retention in a BDNF/TrkB-dependent manner.

Plasticity of Visual Pathways and Function in the Developing Brain: Is the Pulvinar a Crucial Player?

The pulvinar is the largest of the thalamic nuclei in the primates, including humans. In the primates, two of the three major subdivisions, the lateral and inferior pulvinar, are heavily interconnected with a significant proportion of the visual association cortex. However, while we now have a better understanding of the bidirectional connectivity of these pulvinar subdivisions, its functions remain somewhat of an enigma. Over the past few years, researchers have started to tackle this problem by addressing it from the angle of development and visual cortical lesions. In this review, we will draw together literature from the realms of studies in nonhuman primates and humans that have informed much of the current understanding. This literature has been responsible for changing many long-held opinions on the development of the visual cortex and how the pulvinar interacts dynamically with cortices during early life to ensure rapid development and functional capacity Furthermore, there is evidence to suggest involvement of the pulvinar following lesions of the primary visual cortex (V1) and geniculostriate pathway in early life which have far better functional outcomes than identical lesions obtained in adulthood. Shedding new light on the pulvinar and its role following lesions of the visual brain has implications for our understanding of visual brain disorders and the potential for recovery.

Spontaneous Neuronal Network Persistent Activity in the Neocortex: A(n) (Endo)phenotype of Brain (Patho)physiology

Abnormal synaptic homeostasis in the cerebral cortex represents a risk factor for both psychiatric and neurodegenerative disorders, from autism and schizophrenia to Alzheimer's disease. Neurons via synapses form recurrent networks that are intrinsically active in the form of oscillating activity, visible at increasingly macroscopic neurophysiological levels: from single cell recordings to the local field potentials (LFPs) to the clinically relevant electroencephalography (EEG). Understanding in animal models the defects at the level of neural circuits is important in order to link molecular and cellular phenotypes with behavioral phenotypes of neurodevelopmental and/or neurodegenerative brain disorders. In this study we introduce the novel idea that recurring persistent network activity (Up states) in the neocortex at the reduced level of the brain slice may be used as an endophenotype of brain disorders that will help us understand not only how local microcircuits of the cortex may be affected in brain diseases, but also when, since an important issue for the design of successful treatment strategies concerns the time window available for intervention.

Experience-Dependent and Differential Regulation of Local and Long-Range Excitatory Neocortical Circuits by Postsynaptic Mef2c

Development of proper cortical circuits requires an interaction of sensory experience and genetic programs. Little is known of how experience and specific transcription factors interact to determine the development of specific neocortical circuits. Here, we demonstrate that the activity-dependent transcription factor, Myocyte enhancer factor-2C (Mef2c), differentially regulates development of local versus long-range excitatory synaptic inputs onto layer 2/3 neurons in the somatosensory neocortex in vivo. Postnatal, postsynaptic deletion of Mef2c in a sparse population of L2/3 neurons suppressed development of excitatory synaptic connections from all local input pathways tested. In the same cell population, Mef2c deletion promoted the strength of excitatory inputs originating from contralateral neocortex. Both the synapse promoting and synapse suppressing effects of Mef2c deletion required normal whisking experience. These results reveal a role of Mef2c in experience-dependent development of specific sensory neocortical circuits.

Oxytocin is implicated in social memory deficits induced by early sensory deprivation in mice

Early-life sensory input plays a crucial role in brain development. Although deprivation of orofacial sensory input at perinatal stages disrupts the establishment of the barrel cortex and relevant callosal connections, its long-term effect on adult behavior remains elusive. In this study, we investigated the behavioral phenotypes in adult mice with unilateral transection of the infraorbital nerve (ION) at postnatal day 3 (P3). Although ION-transected mice had normal locomotor activity, motor coordination, olfaction, anxiety-like behaviors, novel object memory, preference for social novelty and sociability, they presented deficits in social memory and spatial memory compared with control mice. In addition, the social memory deficit was associated with reduced oxytocin (OXT) levels in the hypothalamus and could be partially restored by intranasal administration of OXT. Thus, early sensory deprivation does result in behavioral alterations in mice, some of which may be associated with the disruption of oxytocin signaling.

Anti-correlations in the degree distribution increase stimulus detection performance in noisy spiking neural networks

Neuronal circuits in the rodent barrel cortex are characterized by stable low firing rates. However, recent experiments show that short spike trains elicited by electrical stimulation in single neurons can induce behavioral responses. Hence, the underlying neural networks provide stability against internal fluctuations in the firing rate, while simultaneously making the circuits sensitive to small external perturbations. Here we studied whether stability and sensitivity are affected by the connectivity structure in recurrently connected spiking networks. We found that anti-correlation between the number of afferent (in-degree) and efferent (out-degree) synaptic connections of neurons increases stability against pathological bursting, relative to networks where the degrees were either positively correlated or uncorrelated. In the stable network state, stimulation of a few cells could lead to a detectable change in the firing rate. To quantify the ability of networks to detect the stimulation, we used a receiver operating characteristic (ROC) analysis. For a given level of background noise, networks with anti-correlated degrees displayed the lowest false positive rates, and consequently had the highest stimulus detection performance. We propose that anti-correlation in the degree distribution may be a computational strategy employed by sensory cortices to increase the detectability of external stimuli. We show that networks with anti-correlated degrees can in principle be formed by applying learning rules comprised of a combination of spike-timing dependent plasticity, homeostatic plasticity and pruning to networks with uncorrelated degrees. To test our prediction we suggest a novel experimental method to estimate correlations in the degree distribution.

HDAC1 negatively regulates Bdnf and Pvalb required for parvalbumin interneuron maturation in an experience-dependent manner

During early postnatal development, neuronal circuits are sculpted by sensory experience provided by the external environment. This experience-dependent regulation of circuitry development consolidates the balance of excitatory-inhibitory (E/I) neurons in the brain. The cortical barrel-column that innervates a single principal whisker is used to provide a clear reference frame for studying the consolidation of E/I circuitry. Sensory deprivation of S1 at birth disrupts the consolidation of excitatory-inhibitory balance by decreasing inhibitory transmission of parvalbumin interneurons. The molecular mechanisms underlying this decrease in inhibition are not completely understood. Our findings show that epigenetic mechanisms, in particular histone deacetylation by histone deacetylases, negatively regulate the expression of brain-derived neurotrophic factor (Bdnf) and parvalbumin (Pvalb) genes during development, which are required for the maturation of parvalbumin interneurons. After whisker deprivation, increased histone deacetylase 1 expression and activity led to increased histone deacetylase 1 binding and decreased histone acetylation at Bdnf promoters I-IV and Pvalb promoter, resulting in the repression of Bdnf and Pvalb gene transcription. The decrease in Bdnf expression further affected parvalbumin interneuron maturation at layer II/III in S1, demonstrated by decreased parvalbumin expression, a marker for parvalbumin interneuron maturation. Knockdown of HDAC1 recovered Bdnf and Pvalb gene transcription and also prevented the decrease of inhibitory synapses accompanying whisker deprivation.

Hypergravity within a critical period impacts on the maturation of somatosensory cortical maps and their potential for use-dependent plasticity in the adult

We investigated experience-dependent plasticity of somatosensory maps in rat S1 cortex during early development. We analyzed both short- and long-term effects of exposure to 2G hypergravity (HG) during the first 3 postnatal weeks on forepaw representations. We also examined the potential of adult somatosensory maps for experience-dependent plasticity after early HG rearing. At postnatal day 22, HG was found to induce an enlargement of cortical zones driven by nail displacements and a contraction of skin sectors of the forepaw map. In these remaining zones serving the skin, neurons displayed expanded glabrous skin receptive fields (RFs). HG also induced a bias in the directional sensitivity of neuronal responses to nail displacement. HG-induced map changes were still found after 16 wk of housing in normogravity (NG). However, the glabrous skin RFs recorded in HG rats decreased to values similar to that of NG rats, as early as the end of the first week of housing in NG. Moreover, the expansion of the glabrous skin area and decrease in RF size normally induced in adults by an enriched environment (EE) did not occur in the HG rats, even after 16 wk of EE housing in NG. Our findings reveal that early postnatal experience critically and durably shapes S1 forepaw maps and limits their potential to be modified by novel experience in adulthood.

Whisker Deprivation Drives Two Phases of Inhibitory Synapse Weakening in Layer 4 of Rat Somatosensory Cortex

Inhibitory synapse development in sensory neocortex is experience-dependent, with sustained sensory deprivation yielding fewer and weaker inhibitory synapses. Whether this represents arrest of synapse maturation, or a more complex set of processes, is unclear. To test this, we measured the dynamics of inhibitory synapse development in layer 4 of rat somatosensory cortex (S1) during continuous whisker deprivation from postnatal day 7, and in age-matched controls. In deprived columns, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) and evoked IPSCs developed normally until P15, when IPSC amplitude transiently decreased, recovering by P16 despite ongoing deprivation. IPSCs remained normal until P22, when a second, sustained phase of weakening began. Delaying deprivation onset by 5 days prevented the P15 weakening. Both early and late phase weakening involved measurable reduction in IPSC amplitude relative to prior time points. Thus, deprivation appears to drive two distinct phases of active IPSC weakening, rather than simple arrest of synapse maturation.

Expression of Npas4 mRNA in Telencephalic Areas of Adult and Postnatal Mouse Brain

The transcription factor neuronal PAS domain-containing protein 4 (Npas4) is an inducible immediate early gene which regulates the formation of inhibitory synapses, and could have a significant regulatory role during cortical circuit formation. However, little is known about basal Npas4 mRNA expression during postnatal development. Here, postnatal and adult mouse brain sections were processed for isotopic in situ hybridization using an Npas4 specific cRNA antisense probe. In adults, Npas4 mRNA was found in the telencephalon with very restricted or no expression in diencephalon or mesencephalon. In most telencephalic areas, including the anterior olfactory nucleus (AON), piriform cortex, neocortex, hippocampus, dorsal caudate putamen (CPu), septum and basolateral amygdala nucleus (BLA), basal Npas4 expression was detected in scattered cells which exhibited strong hybridization signal. In embryonic and neonatal brain sections, Npas4 mRNA expression signals were very low. Starting at postnatal day 5 (P5), transcripts for Npas4 were detected in the AON, CPu and piriform cortex. At P8, additional Npas4 hybridization was found in CA1 and CA3 pyramidal layer, and in primary motor cortex. By P13, robust mRNA expression was located in layers IV and VI of all sensory cortices, frontal cortex and cingulate cortex. After onset of expression, postnatal spatial mRNA distribution was similar to that in adults, with the exception of the CPu, where Npas4 transcripts became gradually restricted to the most dorsal part. In conclusion, the spatial distribution of Npas4 mRNA is mostly restricted to telencephalic areas, and the temporal expression increases with developmental age during postnatal development, which seem to correlate with the onset of activity-driven excitatory transmission.

Adaptive Pulvinar Circuitry Supports Visual Cognition

The pulvinar is the largest thalamic nucleus in primates and one of the most mysterious. Endeavors to understand its role in vision have focused on its abundant connections with the visual cortex. While its connectivity mapping in the cortex displays a broad topographic organization, its projections are also marked by considerable convergence and divergence. As a result, the pulvinar is often regarded as a central forebrain hub. Moreover, new evidence suggests that its comparatively modest input from structures such as the retina and superior colliculus may critically shape the functional organization of the visual cortex, particularly during early development. Here we review recent studies that cast fresh light on how the many convergent pathways through the pulvinar contribute to visual cognition.

NEURODEVELOPMENT. Adult cortical plasticity depends on an early postnatal critical period

Development of the cerebral cortex is influenced by sensory experience during distinct phases of postnatal development known as critical periods. Disruption of experience during a critical period produces neurons that lack specificity for particular stimulus features, such as location in the somatosensory system. Synaptic plasticity is the agent by which sensory experience affects cortical development. Here, we describe, in mice, a developmental critical period that affects plasticity itself. Transient neonatal disruption of signaling via the C-terminal domain of "disrupted in schizophrenia 1" (DISC1)—a molecule implicated in psychiatric disorders—resulted in a lack of long-term potentiation (LTP) (persistent strengthening of synapses) and experience-dependent potentiation in adulthood. Long-term depression (LTD) (selective weakening of specific sets of synapses) and reversal of LTD were present, although impaired, in adolescence and absent in adulthood. These changes may form the basis for the cognitive deficits associated with mutations in DISC1 and the delayed onset of a range of psychiatric symptoms in late adolescence.

A Developmental Switch for Hebbian Plasticity

Hebbian forms of synaptic plasticity are required for the orderly development of sensory circuits in the brain and are powerful modulators of learning and memory in adulthood. During development, emergence of Hebbian plasticity leads to formation of functional circuits. By modeling the dynamics of neurotransmitter release during early postnatal cortical development we show that a developmentally regulated switch in vesicle exocytosis mode triggers associative (i.e. Hebbian) plasticity. Early in development spontaneous vesicle exocytosis (SVE), often considered as 'synaptic noise', is important for homogenization of synaptic weights and maintenance of synaptic weights in the appropriate dynamic range. Our results demonstrate that SVE has a permissive, whereas subsequent evoked vesicle exocytosis (EVE) has an instructive role in the expression of Hebbian plasticity. A timed onset for Hebbian plasticity can be achieved by switching from SVE to EVE and the balance between SVE and EVE can control the effective rate of Hebbian plasticity. We further show that this developmental switch in neurotransmitter release mode enables maturation of spike-timing dependent plasticity. A mis-timed or inadequate SVE to EVE switch may lead to malformation of brain networks thereby contributing to the etiology of neurodevelopmental disorders.

Neurotransmitter release is the principal form of chemical communication in the brain. When an action potential reaches a synapse, calcium influx activates the machinery for neurotransmitter release. During early neuronal development this machinery matures such that neurotransmitter release becomes time-locked to action potentials. By modeling this change in neurotransmitter release, we mechanistically show that the maturation process can be solely responsible for switching on associative (i.e. Hebbian) plasticity in the brain. The relevant proteins of the release machinery can thereby regulate the rate at which neural circuits represent sensory input, providing a novel mechanism to control the learning rate and onset. Appropriately timing of the onset of Hebbian plasticity is important because during early development sensory experience fine-tunes, often irreversibly, the neural wiring in our brain.

NG2-glia and their functions in the central nervous system

In the central nervous system, NG2-glia represent a neural cell population that is distinct from neurons, astrocytes, and oligodendrocytes. While in the past the main role ascribed to these cells was that of progenitors for oligodendrocytes, in the last years it has become more obvious that they have further functions in the brain. Here, we will discuss some of the most current and highly debated issues regarding NG2-glia: Do these cells represent a heterogeneous population? Can they give rise to different progenies, and does this change under pathological conditions? How do they respond to injury or pathology? What is the role of neurotransmitter signaling between neurons and NG2-glia? We will first give an overview on the developmental origin of NG2-glia, and then discuss whether their distinct properties in different brain regions are the result of environmental influences, or due to intrinsic differences. We will then review and discuss their in vitro differentiation potential and in vivo lineage under physiological and pathological conditions, together with their electrophysiological properties in distinct brain regions and at different developmental stages. Finally, we will focus on their potential to be used as therapeutic targets in demyelinating and neurodegenerative diseases. Therefore, this review article will highlight the importance of NG2-glia not only in the healthy, but also in the diseased brain.