Developmental sculpting of dendritic morphology of layer 4 neurons in visual cortex: influence of retinal input

From active affordance to active inference: vertical integration of cognition in the cerebral cortex through dual subcortical control systems

In previous papers, we proposed that the dorsal attention system's top-down control is regulated by the dorsal division of the limbic system, providing a feedforward or impulsive form of control generating expectancies during active inference. In contrast, we proposed that the ventral attention system is regulated by the ventral limbic division, regulating feedback constraints and error-correction for active inference within the neocortical hierarchy. Here, we propose that these forms of cognitive control reflect vertical integration of subcortical arousal control systems that evolved for specific forms of behavior control. The feedforward impetus to action is regulated by phasic arousal, mediated by lemnothalamic projections from the reticular activating system of the lower brainstem, and then elaborated by the hippocampus and dorsal limbic division. In contrast, feedback constraint-based on environmental requirements-is regulated by the tonic activation furnished by collothalamic projections from the midbrain arousal control centers, and then sustained and elaborated by the amygdala, basal ganglia, and ventral limbic division. In an evolutionary-developmental analysis, understanding these differing forms of active affordance-for arousal and motor control within the subcortical vertebrate neuraxis-may help explain the evolution of active inference regulating the cognition of expectancy and error-correction within the mammalian 6-layered neocortex.

Short-Term Dendritic Dynamics of Neonatal Cortical Neurons Revealed by In Vivo Imaging with Improved Spatiotemporal Resolution

Individual neurons in sensory cortices exhibit specific receptive fields based on their dendritic patterns. These dendritic morphologies are established and refined during the neonatal period through activity-dependent plasticity. This process can be visualized using two-photon in vivo time-lapse imaging, but sufficient spatiotemporal resolution is essential. We previously examined dendritic patterning from spiny stellate (SS) neurons, the major type of layer 4 (L4) neurons, in the mouse primary somatosensory cortex (barrel cortex), where mature dendrites display a strong orientation bias toward the barrel center. Longitudinal imaging at 8 h intervals revealed the long-term dynamics by which SS neurons acquire this unique dendritic pattern. However, the spatiotemporal resolution was insufficient to detect the more rapid changes in SS neuron dendrite morphology during the critical neonatal period. In the current study, we imaged neonatal L4 neurons hourly for 8 h and improved the spatial resolution by uniform cell surface labeling. The improved spatiotemporal resolution allowed detection of precise changes in dendrite morphology and revealed aspects of short-term dendritic dynamics unique to the neonatal period. Basal dendrites of barrel cortex L4 neurons were highly dynamic. In particular, both barrel-inner and barrel-outer dendrites (trees and branches) emerged/elongated and disappeared/retracted at similarly high frequencies, suggesting that SS neurons acquire biased dendrite patterns through rapid trial-and-error emergence, elongation, elimination, and retraction of dendritic trees and branches. We also found correlations between morphology and behavior (elongation/retraction) of dendritic tips. Thus, the current study revealed short-term dynamics and related features of cortical neuron dendrites during refinement.

Thalamocortical control of cell-type specificity drives circuits for processing whisker-related information in mouse barrel cortex

Excitatory spiny stellate neurons are prominently featured in the cortical circuits of sensory modalities that provide high salience and high acuity representations of the environment. These specialized neurons are considered developmentally linked to bottom-up inputs from the thalamus, however, the molecular mechanisms underlying their diversification and function are unknown. Here, we investigated this in mouse somatosensory cortex, where spiny stellate neurons and pyramidal neurons have distinct roles in processing whisker-evoked signals. Utilizing spatial transcriptomics, we identified reciprocal patterns of gene expression which correlated with these cell-types and were linked to innervation by specific thalamic inputs during development. Genetic manipulation that prevents the acquisition of spiny stellate fate highlighted an important role for these neurons in processing distinct whisker signals within functional cortical columns, and as a key driver in the formation of specific whisker-related circuits in the cortex.

Golgi polarity shift instructs dendritic refinement in the neonatal cortex by mediating NMDA receptor signaling

Dendritic refinement is a critical component of activity-dependent neuronal circuit maturation, through which individual neurons establish specific connectivity with their target axons. Here, we demonstrate that the developmental shift of Golgi polarity is a key process in dendritic refinement. During neonatal development, the Golgi apparatus in layer 4 spiny stellate (SS) neurons in the mouse barrel cortex lose their original apical positioning and acquire laterally polarized distributions. This lateral Golgi polarity, which is oriented toward the barrel center, peaks on postnatal days 5-7 (P5-P7) and disappears by P15, which aligns with the developmental time course of SS neuron dendritic refinement. Genetic ablation of N-methyl-D-aspartate (NMDA) receptors, key players in dendritic refinement, disturbs the lateral Golgi polarity. Golgi polarity manipulation disrupts the asymmetric dendritic projection pattern and the primary-whisker-specific response of SS neurons. Our results elucidate activity-dependent Golgi dynamics and their critical role in developmental neuronal circuit refinement.

Optogenetic stimulation shapes dendritic trees of infragranular cortical pyramidal cells

Spontaneous or experimentally evoked activity can lead to changes in length and/or branching of neocortical pyramidal cell dendrites. For instance, an early postnatal overexpression of certain AMPA or kainate glutamate receptor subunits leads to larger amplitudes of depolarizing events driven by spontaneous activity, and this increases apical dendritic complexity. Whether stimulation frequency has a role is less clear. In this study, we report that the expression of channelrhodopsin2-eYFP was followed by a 5-day optogenetic stimulation from DIV 5-10 or 11-15 in organotypic cultures of rat visual cortex-evoked dendritic remodeling. Stimulation at 0.05 Hz, at a frequency range of spontaneous calcium oscillations known to occur in the early postnatal neocortex in vivo until eye opening, had no effect. Stimulation with 0.5 Hz, a frequency at which the cortex in vivo adopts after eye opening, unexpectedly caused shorter and somewhat less branched apical dendrites of infragranular pyramidal neurons. The outcome resembles the remodeling of corticothalamic and callosal projection neurons of layers VI and V, which in the adult have apical dendrites no longer terminating in layer I. Exposure to 2.5 Hz, a frequency not occurring naturally during the time windows, evoked dendritic damage. The results suggested that optogenetic stimulation at a biologically meaningful frequency for the selected developmental stage can influence dendrite growth, but contrary to expectation, the optogenetic stimulation decreased dendritic growth.

Calcium and activity-dependent signaling in the developing cerebral cortex

Calcium influx can be stimulated by various intra- and extracellular signals to set coordinated gene expression programs into motion. As such, the precise regulation of intracellular calcium represents a nexus between environmental cues and intrinsic genetic programs. Mounting genetic evidence points to a role for the deregulation of intracellular calcium signaling in neuropsychiatric disorders of developmental origin. These findings have prompted renewed enthusiasm for understanding the roles of calcium during normal and dysfunctional prenatal development. In this Review, we describe the fundamental mechanisms through which calcium is spatiotemporally regulated and directs early neurodevelopmental events. We also discuss unanswered questions about intracellular calcium regulation during the emergence of neurodevelopmental disease, and provide evidence that disruption of cell-specific calcium homeostasis and/or redeployment of developmental calcium signaling mechanisms may contribute to adult neurological disorders. We propose that understanding the normal developmental events that build the nervous system will rely on gaining insights into cell type-specific calcium signaling mechanisms. Such an understanding will enable therapeutic strategies targeting calcium-dependent mechanisms to mitigate disease.

Thalamocortical axons regulate neurogenesis and laminar fates in the early sensory cortex

This study addresses how the cerebral cortex is partitioned into specialized areas during development. Although both early embryonic patterning and postnatal synaptic input from sensory thalamic nuclei are known to be critical, early roles of thalamic axons in area-specific regulation of cortical neurogenesis are poorly understood. We examined this by developing a genetic mouse model in which thalamocortical projections fail to properly form during embryogenesis, and found these axons are required not only for an enhanced production of superficial layer neurons but also for promoting the layer 4 cell fate, a hallmark of the primary sensory cortex. These findings provide a mechanism by which thalamocortical axons complement the intrinsic programs of neurogenesis and early fate specification.

Area-specific axonal projections from the mammalian thalamus shape unique cellular organization in target areas in the adult neocortex. How these axons control neurogenesis and early neuronal fate specification is poorly understood. By using mutant mice lacking the majority of thalamocortical axons, we show that these axons are required for the production and specification of the proper number of layer 4 neurons in primary sensory areas by the neonatal stage. Part of these area-specific roles is played by the thalamus-derived molecule, VGF. Our work reveals that extrinsic cues from sensory thalamic projections have an early role in the formation of cortical cytoarchitecture by enhancing the production and specification of layer 4 neurons.

A binocular synaptic network supports interocular response alignment in visual cortical neurons

In visual cortex, signals from the two eyes merge to form a coherent binocular representation. Here we investigate the synaptic interactions underlying the binocular representation of stimulus orientation in ferret visual cortex with in vivo calcium imaging of layer 2/3 neurons and their dendritic spines. Individual neurons with aligned somatic responses received a mixture of monocular and binocular synaptic inputs. Surprisingly, monocular pathways alone could not account for somatic alignment because ipsilateral monocular inputs poorly matched somatic preference. Binocular inputs exhibited different degrees of interocular alignment, and those with a high degree of alignment (congruent) had greater selectivity and somatic specificity. While congruent inputs were similar to others in measures of strength, simulations show that the number of active congruent inputs predicts aligned somatic output. Our study suggests that coherent binocular responses derive from connectivity biases that support functional amplification of aligned signals within a heterogeneous binocular intracortical network.

Dynamic interplay between thalamic activity and Cajal-Retzius cells regulates the wiring of cortical layer 1

Cortical wiring relies on guidepost cells and activity-dependent processes that are thought to act sequentially. Here, we show that the construction of layer 1 (L1), a main site of top-down integration, is regulated by crosstalk between transient Cajal-Retzius cells (CRc) and spontaneous activity of the thalamus, a main driver of bottom-up information. While activity was known to regulate CRc migration and elimination, we found that prenatal spontaneous thalamic activity and NMDA receptors selectively control CRc early density, without affecting their demise. CRc density, in turn, regulates the distribution of upper layer interneurons and excitatory synapses, thereby drastically impairing the apical dendrite activity of output pyramidal neurons. In contrast, postnatal sensory-evoked activity had a limited impact on L1 and selectively perturbed basal dendrites synaptogenesis. Collectively, our study highlights a remarkable interplay between thalamic activity and CRc in L1 functional wiring, with major implications for our understanding of cortical development.

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Prenatal thalamic waves of activity regulate CRc density in L1

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Prenatal and postnatal CRc manipulations alter specific interneuron populations

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Postnatal CRc shape L5 apical dendrite structural and functional properties

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Early sensory activity selectively regulates L5 basal dendrite spine formation

Structural and functional plasticity in the dorsolateral geniculate nucleus of mice following bilateral enucleation

Within the nervous system, plasticity mechanisms attempt to stabilize network activity following disruption by injury, disease, or degeneration. Optic nerve injury and age-related diseases can induce homeostatic-like responses in adulthood. We tested this possibility in the thalamocortical (TC) neurons in the dorsolateral geniculate nucleus (dLGN) using patch-clamp electrophysiology, optogenetics, immunostaining, and single-cell dendritic analysis following loss of visual input via bilateral enucleation. We observed progressive loss of vGlut2-positive retinal terminals in the dLGN indicating degeneration post-enucleation that was coincident with changes in microglial morphology indicative of microglial activation. Consistent with the decline of vGlut2 puncta, we also observed loss of retinogeniculate (RG) synaptic function assessed using optogenetic activation of RG axons while performing whole-cell voltage clamp recordings from TC neurons in brain slices. Surprisingly, we did not detect any significant changes in the frequency of miniature post-synaptic currents (mEPSCs) or corticothalamic feedback synapses. Analysis of TC neuron dendritic structure from single-cell dye fills revealed a gradual loss of dendrites proximal to the soma, where TC neurons receive the bulk of RG inputs. Finally, analysis of action potential firing demonstrated that TC neurons have increased excitability following enucleation, firing more action potentials in response to depolarizing current injections. Our findings show that degeneration of the retinal axons/optic nerve and loss of RG synaptic inputs induces structural and functional changes in TC neurons, consistent with neuronal attempts at compensatory plasticity in the dLGN.

Defining neuroplasticity

Neuroplasticity, i.e., the modifiability of the brain, is different in development and adulthood. The first includes changes in: (i) neurogenesis and control of neuron number; (ii) neuronal migration; (iii) differentiation of the somato-dendritic and axonal phenotypes; (iv) formation of connections; (v) cytoarchitectonic differentiation. These changes are often interrelated and can lead to: (vi) system-wide modifications of brain structure as well as to (vii) acquisition of specific functions such as ocular dominance or language. Myelination appears to be plastic both in development and adulthood, at least, in rodents. Adult neuroplasticity is limited, and is mainly expressed as changes in the strength of excitatory and inhibitory synapses while the attempts to regenerate connections have met with limited success. The outcomes of neuroplasticity are not necessarily adaptive, but can also be the cause of neurological and psychiatric pathologies.

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.

Folding brains: from development to disease modeling

The human brain is characterized by the large size and intricate folding of its cerebral cortex, which are fundamental for our higher cognitive function and frequently altered in pathological dysfunction. Cortex folding is not unique to humans, nor even to primates, but is common across mammals. Cortical growth and folding are the result of complex developmental processes that involve neural stem and progenitor cells and their cellular lineages, the migration and differentiation of neurons, and the genetic programs that regulate and fine-tune these processes. All these factors combined generate mechanical stress and strain on the developing neural tissue, which ultimately drives orderly cortical deformation and folding. In this review we examine and summarize the current knowledge on the molecular, cellular, histogenic and mechanical mechanisms that are involved in and influence folding of the cerebral cortex, and how they emerged and changed during mammalian evolution. We discuss the main types of pathological malformations of human cortex folding, their specific developmental origin, and how investigating their genetic causes has illuminated our understanding of key events involved. We close our review by presenting the state-of-the-art animal and in vitro models of cortex folding that are currently used to study these devastating developmental brain disorders in children, and what are the main challenges that remain ahead of us to fully understand brain folding.

From Cell Types to an Integrated Understanding of Brain Evolution: The Case of the Cerebral Cortex

With the discovery of the incredible diversity of neurons, Cajal and coworkers laid the foundation of modern neuroscience. Neuron types are not only structural units of nervous systems but also evolutionary units, because their identities are encoded in the genome. With the advent of high-throughput cellular transcriptomics, neuronal identities can be characterized and compared systematically across species. The comparison of neurons in mammals, reptiles, and birds indicates that the mammalian cerebral cortex is a mosaic of deeply conserved and recently evolved neuron types. Using the cerebral cortex as a case study, this review illustrates how comparing neuron types across species is key to reconciling observations on neural development, neuroanatomy, circuit wiring, and physiology for an integrated understanding of brain evolution.

Von Economo Neurons - Primate-Specific or Commonplace in the Mammalian Brain?

The pioneering work by von Economo in 1925 on the cytoarchitectonics of the cerebral cortex revealed a specialized and unique cell type in the adult human fronto-insular (FI) and anterior cingulate cortex (ACC). In modern studies, these neurons are termed von Economo neurons (VENs). In his work, von Economo described them as stick, rod or corkscrew cells because of their extremely elongated and relatively thin cell body clearly distinguishable from common oval or spindle-shaped infragranular principal neurons. Before von Economo, in 1899 Cajal depicted the unique somato-dendritic morphology of such cells with extremely elongated soma in the FI. However, although VENs are increasingly investigated, Cajal’s observation is still mainly being neglected. On Golgi staining in humans, VENs have a thick and long basal trunk with horizontally oriented terminal branching (basilar skirt) from where the axon arises. They are clearly distinguishable from a spectrum of modified pyramidal neurons found in infragranular layers, including oval or spindle-shaped principal neurons. Spindle-shaped cells with highly elongated cell body were also observed in the ACC of great apes, but despite similarities in soma shape, their dendritic and axonal morphology has still not been described in sufficient detail. Studies identifying VENs in non-human species are predominantly done on Nissl or anti-NeuN staining. In most of these studies, the dendritic and axonal morphology of the analyzed cells was not demonstrated and many of the cells found on Nissl or anti-NeuN staining had a cell body shape characteristic for common oval or spindle-shaped cells. Here we present an extensive literature overview on VENs, which demonstrates that human VENs are specialized elongated principal cells with unique somato-dendritic morphology found abundantly in the FI and ACC of the human brain. More research is needed to properly evaluate the presence of such specialized cells in other primates and non-primate species.

Non-canonical role for Lpar1-EGFP subplate neurons in early postnatal mouse somatosensory cortex

Subplate neurons (SPNs) are thought to play a role in nascent sensory processing in neocortex. To better understand how heterogeneity within this population relates to emergent function, we investigated the synaptic connectivity of Lpar1-EGFP SPNs through the first postnatal week in whisker somatosensory cortex (S1BF). These SPNs comprise of two morphological subtypes: fusiform SPNs with local axons and pyramidal SPNs with axons that extend through the marginal zone. The former receive translaminar synaptic input up until the emergence of the whisker barrels, a timepoint coincident with significant cell death. In contrast, pyramidal SPNs receive local input from the subplate at early ages but then - during the later time window - acquire input from overlying cortex. Combined electrical and optogenetic activation of thalamic afferents identified that Lpar1-EGFP SPNs receive sparse thalamic innervation. These data reveal components of the postnatal network that interpret sparse thalamic input to direct the emergent columnar structure of S1BF.

Development of Auditory Cortex Circuits

The ability to process and perceive sensory stimuli is an essential function for animals. Among the sensory modalities, audition is crucial for communication, pleasure, care for the young, and perceiving threats. The auditory cortex (ACtx) is a key sound processing region that combines ascending signals from the auditory periphery and inputs from other sensory and non-sensory regions. The development of ACtx is a protracted process starting prenatally and requires the complex interplay of molecular programs, spontaneous activity, and sensory experience. Here, we review the development of thalamic and cortical auditory circuits during pre- and early post-natal periods.

NMDA Receptor Enhances Correlation of Spontaneous Activity in Neonatal Barrel Cortex

Correlated spontaneous activity plays critical role in the organization of neocortical circuits during development. However, cortical mechanisms regulating activity correlation are still elusive. In this study, using two-photon calcium imaging of the barrel cortex layer 4 (L4) in living neonatal mice, we found that NMDA receptors (NMDARs) in L4 neurons are important for enhancement of spontaneous activity correlation. Disruption of GluN1 (Grin1), an obligatory NMDAR subunit, in a sparse population of L4 neurons reduced activity correlation between GluN1 knock-out (GluN1KO) neuron pairs within a barrel. This reduction in activity correlation was even detected in L4 neuron pairs in neighboring barrels and most evident when either or both of neurons are located on the barrel edge. Our results provide evidence for the involvement of L4 neuron NMDARs in spatial organization of the spontaneous firing activity of L4 neurons in the neonatal barrel cortex.SIGNIFICANCE STATEMENT Precise wiring of the thalamocortical circuits is necessary for proper sensory information processing, and thalamus-derived correlated spontaneous activity is important for thalamocortical circuit formation. The molecular mechanisms involved in the correlated activity transfer from the thalamus to the neocortex are largely unknown. In vivo two-photon calcium imaging of the neonatal barrel cortex revealed that correlated spontaneous activity between layer four neurons is reduced by mosaic knock-out (KO) of the NMDA receptor (NMDAR) obligatory subunit GluN1. Our results suggest that the function of NMDARs in layer four neurons is necessary for the communication between presynaptic and postsynaptic partners during thalamocortical circuit formation.

Aberrant development of excitatory circuits to inhibitory neurons in the primary visual cortex after neonatal binocular enucleation

The development of GABAergic interneurons is important for the functional maturation of cortical circuits. After migrating into the cortex, GABAergic interneurons start to receive glutamatergic connections from cortical excitatory neurons and thus gradually become integrated into cortical circuits. These glutamatergic connections are mediated by glutamate receptors including AMPA and NMDA receptors and the ratio of AMPA to NMDA receptors decreases during development. Since previous studies have shown that retinal input can regulate the early development of connections along the visual pathway, we investigated if the maturation of glutamatergic inputs to GABAergic interneurons in the visual cortex requires retinal input. We mapped the spatial pattern of glutamatergic connections to layer 4 (L4) GABAergic interneurons in mouse visual cortex at around postnatal day (P) 16 by laser-scanning photostimulation and investigated the effect of binocular enucleations at P1/P2 on these patterns. Gad2-positive interneurons in enucleated animals showed an increased fraction of AMPAR-mediated input from L2/3 and a decreased fraction of input from L5/6. Parvalbumin-expressing (PV) interneurons showed similar changes in relative connectivity. NMDAR-only input was largely unchanged by enucleation. Our results show that retinal input sculpts the integration of interneurons into V1 circuits and suggest that the development of AMPAR- and NMDAR-only connections might be regulated differently.

The regulation of cortical neurogenesis

The mammalian cerebral cortex is the pinnacle of brain evolution, reaching its maximum complexity in terms of neuron number, diversity and functional circuitry. The emergence of this outstanding complexity begins during embryonic development, when a limited number of neural stem and progenitor cells manage to generate myriads of neurons in the appropriate numbers, types and proportions, in a process called neurogenesis. Here we review the current knowledge on the regulation of cortical neurogenesis, beginning with a description of the types of progenitor cells and their lineage relationships. This is followed by a review of the determinants of neuron fate, the molecular and genetic regulatory mechanisms, and considerations on the evolution of cortical neurogenesis in vertebrates leading to humans. We finish with an overview on how dysregulation of neurogenesis is a leading cause of human brain malformations and functional disabilities.

Elucidating mechanisms of neuronal circuit formation in layer 4 of the somatosensory cortex via intravital imaging

The cerebral cortex has complex yet perfectly wired neuronal circuits that are important for high-level brain functions such as perception and cognition. The rodent's somatosensory system is widely used for understanding the mechanisms of circuit formation during early developmental periods. In this review, we summarize the developmental processes of circuit formation in layer 4 of the somatosensory cortex, and we describe the molecules involved in layer 4 circuit formation and neuronal activity-dependent mechanisms of circuit formation. We also introduce the dynamic mechanisms of circuit formation in layer 4 revealed by intravital two-photon imaging technologies, which include time-lapse imaging of neuronal morphology and calcium imaging of neuronal activity in newborn mice.

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.

New insights into cortical development and plasticity: from molecules to behavior

The human brain contains 100 billion neurons, and each neuron can have up to 200,000 connections to other neurons. Recent advancements in neuroscience-ranging from molecular studies in animal models to behavioral studies in humans-have given us deeper insights into the development of this extraordinarily intricate system. Studies show a complex interaction between biological predispositions and environment; while the gross neuroanatomy and low-level functions develop early prior to receiving environmental inputs, functional selectivity is shaped through experience, governed by the maturation of local excitatory and inhibitory circuits and synaptic plasticity during sensitive periods early in development. Plasticity does not end with the closing of the early sensitive period - the environment continues to play an important role in learning throughout the lifespan. Recent work delineating the cascade of events that initiates, controls and ends sensitive periods, offers new hope of eventually being able to remediate various clinical conditions by selectively reopening plasticity.

Transient Deregulation of Canonical Wnt Signaling in Developing Pyramidal Neurons Leads to Dendritic Defects and Impaired Behavior

During development, the precise implementation of molecular programs is a key determinant of proper dendritic development. Here, we demonstrate that canonical Wnt signaling is active in dendritic bundle-forming layer II pyramidal neurons of the rat retrosplenial cortex during dendritic branching and spine formation. Transient downregulation of canonical Wnt transcriptional activity during the early postnatal period irreversibly reduces dendritic arbor architecture, leading to long-lasting deficits in spatial exploration and/or navigation and spatial memory in the adult. During the late phase of dendritogenesis, canonical Wnt-dependent transcription regulates spine formation and maturation. We identify neurotrophin-3 as canonical Wnt target gene in regulating dendritogenesis. Our findings demonstrate how temporary imbalance in canonical Wnt signaling during specific time windows can result in irreversible dendritic defects, leading to abnormal behavior in the adult.

Spatiotemporal Differences in the Regional Cortical Plate and Subplate Volume Growth during Fetal Development

The regional specification of the cerebral cortex can be described by protomap and protocortex hypotheses. The protomap hypothesis suggests that the regional destiny of cortical neurons and the relative size of the cortical area are genetically determined early during embryonic development. The protocortex hypothesis suggests that the regional growth rate is predominantly shaped by external influences. In order to determine regional volumes of cortical compartments (cortical plate (CP) or subplate (SP)) and estimate their growth rates, we acquired T2-weighted in utero MRIs of 40 healthy fetuses and grouped them into early (<25.5 GW), mid- (25.5-31.6 GW), and late (>31.6 GW) prenatal periods. MRIs were segmented into CP and SP and further parcellated into 22 gyral regions. No significant difference was found between periods in regional volume fractions of the CP or SP. However, during the early and mid-prenatal periods, we found significant differences in relative growth rates (% increase per GW) between regions of cortical compartments. Thus, the relative size of these regions are most likely conserved and determined early during development whereas more subtle growth differences between regions are fine-tuned later, during periods of peak thalamocortical growth. This is in agreement with both the protomap and protocortex hypothesis.

Distinct Neocortical Progenitor Lineages Fine-tune Neuronal Diversity in a Layer-specific Manner

How the variety of neurons that organize into neocortical layers and functional areas arises is a central question in the study of cortical development. While both intrinsic and extrinsic cues are known to influence this process, whether distinct neuronal progenitor groups contribute to neuron diversity and allocation is poorly understood. Using in vivo genetic fate-mapping combined with whole-cell patch clamp recording, we show that the firing pattern and apical dendritic morphology of excitatory neurons in layer 4 of the barrel cortex are specified in part by their neural precursor lineage. Further, we show that separate precursors contribute to unique features of barrel cortex topography including the intralaminar position and thalamic innervation of the neurons they generate. Importantly, many of these lineage-specified characteristics are different from those previously measured for pyramidal neurons in layers 2-3 of the frontal cortex. Collectively, our data elucidate a dynamic temporal program in neuronal precursors that fine-tunes the properties of their progeny according to the lamina of destination.

Layer 4 pyramidal neuron dendritic bursting underlies a post-stimulus visual cortical alpha rhythm

Alpha rhythms (9–11 Hz) are a dominant feature of EEG recordings, particularly over occipital cortex on cessation of a visual stimulation. Little is known about underlying neocortical mechanisms so here we constructed alpha rhythm models that follow cessation of cortical stimulation. The rhythm manifests following a period of gamma frequency activity in local V1 networks in layer 4. It associates with network level bias of excitatory synaptic activity in favour of NMDA- rather than AMPA-mediated signalling and reorganisation of synaptic inhibition in favour of fast GABA_A receptor-mediated events. At the cellular level the alpha rhythm depended upon the generation of layer 4 pyramidal neuron dendritic bursting mediated primarily by PPDA-sensitive NR2C/D-containing NMDA receptors, which lack the magnesium-dependent open channel block. Subthreshold potassium conductances are also critical. The rhythm dynamically filters outputs from sensory relay neurons (stellate neurons in layer 4) such that they become temporally uncoupled from downstream population activity.

The authors combine computational and electrophysiological approaches to study the neocortical mechanisms underlying alpha rhythms generated after visual stimuli cessation. They show that layer 4 pyramidal neuron bursting, as well as a shift towards NMDA- and GABA_A- receptor transmission is critical for the generation of these alpha oscillations.

RORα Coordinates Thalamic and Cortical Maturation to Instruct Barrel Cortex Development

The retinoic acid-related orphan receptor alpha (RORα) is well-known for its role in cerebellar development and maturation as revealed in staggerer mice. However, its potential involvement in the development of other brain regions has hardly been assessed. Here, we describe a new role of RORα in the development of primary somatosensory maps. Staggerer mice showed a complete disruption of barrels in the somatosensory cortex and of barreloids in the thalamus. This phenotype results from a severe reduction of thalamocortical axon (TCA) branching and a defective maturation of layer IV cortical neurons during postnatal development. Conditional deletion of RORα was conducted in the thalamus or the cortex to determine the specific contribution of RORα in each of these structures to these phenotypes. This showed that RORα is cell-autonomously required in the thalamus for the organization of TCAs into periphery-related clusters and in the somatosensory cortex for the dendritic maturation of layer IV neurons. Microarray analyses revealed that Sema7a, Neph, and Adcy8 are RORα regulated genes that could be implicated in TCA and cortical maturation. Overall, our study outlines a new role of RORα for the coordinated maturation of the somatosensory thalamus and cortex during the assembly of columnar barrel structures.

Effects of Sexual Experience and Puberty on Mouse Genital Cortex revealed by Chronic Imaging

The topographic map in layer 4 of somatosensory cortex is usually specified early postnatally and stable thereafter. Genital cortex, however, undergoes a sex-hormone- and sexual-touch-dependent pubertal expansion. Here, we image pubertal development of genital cortex in Scnn1a-Tg3-Cre mice, where transgene expression has been shown to be restricted to layer 4 neurons with primary sensory cortex identity. Interestingly, during puberty, the number of Scnn1a+ neurons roughly doubled within genital cortex. The increase of Scnn1a+ neurons was gradual and rapidly advanced by initial sexual experience. Neurons that gained Scnn1a expression comprised stellate and pyramidal neurons in layer 4. Unlike during neonatal development, pyramids did not retract their apical dendrites during puberty. Calcium imaging revealed stronger genital-touch responses in Scnn1a+ neurons in males versus females and a developmental increase in responsiveness in females. The first sexual interaction is a unique physical experience that often creates long-lasting memories. We suggest such experience uniquely alters somatosensory body maps.

Dynamic postnatal development of the cellular and circuit properties of striatal D1 and D2 spiny projection neurons

KEY POINTS

Imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs) are thought to contribute to many basal ganglia disorders, including early-onset neurodevelopmental disorders such as obsessive-compulsive disorder, attention deficit hyperactivity disorder and Tourette's syndrome. This study provides the first detailed quantitative investigation of development of D1 and D2 SPNs, including their cellular properties and connectivity within neural circuits, during the first postnatal weeks. This period is highly dynamic with many properties changing, but it is possible to make three main observations: many aspects of D1 and D2 SPNs progressively mature in parallel; there are notable exceptions when they diverge; and many of the defining properties of mature striatal SPNs and circuits are already established by the first and second postnatal weeks, suggesting guidance through intrinsic developmental programmes. These findings provide an experimental framework for future studies of striatal development in both health and disease.

ABSTRACT

Many basal ganglia neurodevelopmental disorders are thought to result from imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs). Insight into these disorders is reliant on our understanding of normal D1 and D2 SPN development. Here we provide the first detailed study and quantification of the striatal cellular and circuit changes occurring for both D1 and D2 SPNs in the first postnatal weeks using in vitro whole-cell patch-clamp electrophysiology. Characterization of their intrinsic electrophysiological and morphological properties, the excitatory long-range inputs coming from cortex and thalamus, as well their local gap junction and inhibitory synaptic connections reveals this period to be highly dynamic with numerous properties changing. However it is possible to make three main observations. Firstly, many aspects of SPNs mature in parallel, including intrinsic membrane properties, increases in dendritic arbours and spine densities, general synaptic inputs and expression of specific glutamate receptors. Secondly, there are notable exceptions, including a transient stronger thalamic innervation of D2 SPNs and stronger cortical NMDA receptor-mediated inputs to D1 SPNs, both in the second postnatal week. Thirdly, many of the defining properties of mature D1 and D2 SPNs and striatal circuits are already established by the first and second postnatal weeks, including different electrophysiological properties as well as biased local inhibitory connections between SPNs, suggesting this is guided through intrinsic developmental programmes. Together these findings provide an experimental framework for future studies of D1 and D2 SPN development in health and disease.

Development and Arealization of the Cerebral Cortex

Adult cortical areas consist of specialized cell types and circuits that support unique higher-order cognitive functions. How this regional diversity develops from an initially uniform neuroepithelium has been the subject of decades of seminal research, and emerging technologies, including single-cell transcriptomics, provide a new perspective on area-specific molecular diversity. Here, we review the early developmental processes that underlie cortical arealization, including both cortex intrinsic and extrinsic mechanisms as embodied by the protomap and protocortex hypotheses, respectively. We propose an integrated model of serial homology whereby intrinsic genetic programs and local factors establish early transcriptomic differences between excitatory neurons destined to give rise to broad "proto-regions," and activity-dependent mechanisms lead to progressive refinement and formation of sharp boundaries between functional areas. Finally, we explore the potential of these basic developmental processes to inform our understanding of the emergence of functional neural networks and circuit abnormalities in neurodevelopmental disorders.

Layer 4 of mouse neocortex differs in cell types and circuit organization between sensory areas

Layer 4 (L4) of mammalian neocortex plays a crucial role in cortical information processing, yet a complete census of its cell types and connectivity remains elusive. Using whole-cell recordings with morphological recovery, we identified one major excitatory and seven inhibitory types of neurons in L4 of adult mouse visual cortex (V1). Nearly all excitatory neurons were pyramidal and all somatostatin-positive (SOM+) non-fast-spiking interneurons were Martinotti cells. In contrast, in somatosensory cortex (S1), excitatory neurons were mostly stellate and SOM+ interneurons were non-Martinotti. These morphologically distinct SOM+ interneurons corresponded to different transcriptomic cell types and were differentially integrated into the local circuit with only S1 neurons receiving local excitatory input. We propose that cell type specific circuit motifs, such as the Martinotti/pyramidal and non-Martinotti/stellate pairs, are used across the cortex as building blocks to assemble cortical circuits.

How Forces Fold the Cerebral Cortex

Improved understanding of the factors that govern folding of the cerebral cortex is desirable for many reasons. The existence of consistent patterns in folding within and between species suggests a fundamental role in brain function. Abnormal folding patterns found in individuals affected by a diverse array of neurodevelopmental disorders underline the clinical relevance of understanding the folding process. Recent experimental and computational efforts to elucidate the biomechanical forces involved in cerebral cortical folding have converged on a consistent approach. Brain growth is modeled with two components: an expanding outer zone, destined to become the cerebral cortex, is mechanically coupled to an inner zone, destined to become white matter, that grows at a slower rate, perhaps in response to stress induced by expansion from the outer layer. This framework is consistent with experimentally observed internal forces in developing brains, and with observations of the folding process in physical models. In addition, computational simulations based on this foundation can produce folding patterns that recapitulate the characteristics of folding patterns found in gyroencephalic brains. This perspective establishes the importance of mechanical forces in our current understanding of how brains fold, and identifies realistic ranges for specific parameters in biophysical models of developing brain tissue. However, further refinement of this approach is needed. An understanding of mechanical forces that arise during brain development and their cellular-level origins is necessary to interpret the consequences of abnormal brain folding and its role in functional deficits as well as neurodevelopmental disease.Dual Perspectives Companion Paper: How Cells Fold the Cerebral Cortex, by Víctor Borrell.

Differential dynamics of cortical neuron dendritic trees revealed by long-term in vivo imaging in neonates

Proper neuronal circuit function relies on precise dendritic projection, which is established through activity-dependent refinement during early postnatal development. Here we revealed dynamics of dendritic refinement in the mammalian brain by conducting long-term imaging of the neonatal mouse barrel cortex. By “retrospective” analyses, we identified “prospective” barrel-edge spiny stellate (SS) neurons in early neonates, which had an apical dendrite and primitive basal dendrites (BDs). These neurons retracted the apical dendrite gradually and established strong BD orientation bias through continuous “dendritic tree” turnover. A greater chance of survival was given to BD trees emerged in the barrel-center side, where thalamocortical axons (TCAs) cluster. When the spatial bias of TCA inputs to SS neurons was lost, BD tree turnover was suppressed, and most BD trees became stable and elaborated mildly. Thus, barrel-edge SS neurons could establish the characteristic BD projection pattern through differential dynamics of dendritic trees induced by spatially biased inputs.

Layer 4 stellate neurons in barrel cortex have a characteristic dendritic pattern. Here, the authors conduct long-term imaging from postnatal day 3–6 to show that an orientation bias is established through dendritic tree turnover and selective elaboration, which may be induced by biased thalamocortical inputs.

Progenitor Hyperpolarization Regulates the Sequential Generation of Neuronal Subtypes in the Developing Neocortex

During corticogenesis, ventricular zone progenitors sequentially generate distinct subtypes of neurons, accounting for the diversity of neocortical cells and the circuits they form. While activity-dependent processes are critical for the differentiation and circuit assembly of postmitotic neurons, how bioelectrical processes affect nonexcitable cells, such as progenitors, remains largely unknown. Here, we reveal that, in the developing mouse neocortex, ventricular zone progenitors become more hyperpolarized as they generate successive subtypes of neurons. Experimental in vivo hyperpolarization shifted the transcriptional programs and division modes of these progenitors to a later developmental status, with precocious generation of intermediate progenitors and a forward shift in the laminar, molecular, morphological, and circuit features of their neuronal progeny. These effects occurred through inhibition of the Wnt-beta-catenin signaling pathway by hyperpolarization. Thus, during corticogenesis, bioelectric membrane properties are permissive for specific molecular pathways to coordinate the temporal progression of progenitor developmental programs and thus neocortical neuron diversity.

Blindness and Human Brain Plasticity

Early blindness causes fundamental alterations of neural function across more than 25% of cortex-changes that span the gamut from metabolism to behavior and collectively represent one of the most dramatic examples of plasticity in the human brain. The goal of this review is to describe how the remarkable behavioral and neuroanatomical compensations demonstrated by blind individuals provide insights into the extent, mechanisms, and limits of human brain plasticity.

A Complex Code of Extrinsic Influences on Cortical Progenitor Cells of Higher Mammals

Development of the cerebral cortex depends critically on the regulation of progenitor cell proliferation and fate. Cortical progenitor cells are remarkably diverse with regard to their morphology as well as laminar and areal position. Extrinsic factors, such as thalamic axons, have been proposed to play key roles in progenitor cell regulation, but the diversity, extent and timing of interactions between extrinsic elements and each class of cortical progenitor cell in higher mammals remain undefined. Here we use the ferret to demonstrate the existence of a complex set of extrinsic elements that may interact, alone or in combination, with subpopulations of progenitor cells, defining a code of extrinsic influences. This code and its complexity vary significantly between developmental stages, layer of residence and morphology of progenitor cells. By analyzing the spatial-temporal overlap of progenitor cell subtypes with neuronal and axonal populations, we show that multiple sets of migrating neurons and axon tracts overlap extensively with subdivisions of the Subventricular Zones, in an exquisite lamina-specific pattern. Our findings provide a framework for understanding the feedback influence of both intra- and extra-cortical elements onto progenitor cells to modulate their dynamics and fate decisions in gyrencephalic brains.

Development of tactile sensory circuits in the CNS

Molecular identification of neuronal types and genetic and imaging approaches to characterize their properties reveal morphological, physiological and dynamic aspects of sensory circuit development. Here we focus on the mouse tactile sensory circuitry, with particular emphasis on the main trigeminal pathway that connects the whiskers, the major tactile organ in rodents, to the neocortex. At each level of this pathway, neurogenesis, axonal elongation, pathfinding, target recognition and circuit reorganization including dendritic refinement of cortical layer 4 neurons occur contemporaneously and a multitude of molecular signals are used in differing combinations. We highlight recent advances in development of tactile circuitry and note gaps in our understanding.

The Mouse Pulvinar Nucleus Links the Lateral Extrastriate Cortex, Striatum, and Amygdala

The pulvinar nucleus is a large thalamic structure involved in the integration of visual and motor signals. The pulvinar forms extensive connections with striate and extrastriate cortical areas, but the impact of these connections on cortical circuits has not previously been directly tested. Using a variety of anatomical, optogenetic, and in vitro physiological techniques in male and female mice, we show that pulvinocortical terminals are densely distributed in the extrastriate cortex where they form synaptic connections with spines and small-diameter dendrites. Optogenetic activation of these synapses in vitro evoked large excitatory postsynaptic responses in the majority of pyramidal cells, spiny stellate cells, and interneurons within the extrastriate cortex. However, specificity in pulvinar targeting was revealed when recordings were targeted to projection neuron subtypes. The neurons most responsive to pulvinar input were those that project to the striatum and amygdala (76% responsive) or V1 (55%), whereas neurons that project to the superior colliculus were rarely responsive (6%). Because the pulvinar also projects directly to the striatum and amygdala, these results establish the pulvinar nucleus as a hub linking the visual cortex with subcortical regions involved in the initiation and control of movement. We suggest that these circuits may be particularly important for coordinating body movements and visual perception.SIGNIFICANCE STATEMENT We found that the pulvinar nucleus can strongly influence extrastriate cortical circuits and exerts a particularly strong impact on the activity of extrastriate neurons that project to the striatum and amygdala. Our results suggest that the conventional hierarchical view of visual cortical processing may not apply to the mouse visual cortex. Instead, our results establish the pulvinar nucleus as a hub linking the visual cortex with subcortical regions involved in the initiation and control of movement, and predict that the execution of visually guided movements relies on this network.

mGluR5 Exerts Cell-Autonomous Influences on the Functional and Anatomical Development of Layer IV Cortical Neurons in the Mouse Primary Somatosensory Cortex

Glutamate neurotransmission refines synaptic connections to establish the precise neural circuits underlying sensory processing. Deleting metabotropic glutamate receptor 5 (mGluR5) in mice perturbs cortical somatosensory map formation in the primary somatosensory (S1) cortex at both functional and anatomical levels. To examine the cell-autonomous influences of mGluR5 signaling in the morphological and functional development of layer IV spiny stellate glutamatergic neurons receiving sensory input, mGluR5 genetic mosaic mice were generated through in utero electroporation. In the S1 cortex of these mosaic brains, we found that most wild-type neurons were located in barrel rings encircling thalamocortical axon (TCA) clusters while mGluR5 knock-out (KO) neurons were placed in the septal area, the cell-sparse region separating barrels. These KO neurons often displayed a symmetrical dendritic morphology with increased dendritic complexity, in contrast to the polarized pattern of wild-type neurons. The dendritic spine density of mGluR5 KO spiny stellate neurons was significantly higher than in wild-type neurons. Whole-cell electrophysiological recordings detected a significant increase in the frequencies of spontaneous and miniature excitatory postsynaptic events in mGluR5 KO neurons compared with neighboring wild-type neurons. Our mosaic analysis provides strong evidence supporting the cell-autonomous influence of mGluR5 signaling on the functional and anatomical development of cortical glutamatergic neurons. Specifically, mGluR5 is required in cortical glutamatergic neurons for the following processes: (1) the placement of cortical glutamatergic neurons close to TCA clusters; (2) the regulation of dendritic complexity and outgrowth toward TCA clusters; (3) spinogenesis; and (4) tuning of excitatory inputs.

SIGNIFICANCE STATEMENT

Glutamatergic transmission plays a critical role in cortical circuit formation. Its dysfunction has been proposed as a core factor in the etiology of many neurological diseases. Here we conducted mosaic analysis to reveal the cell-autonomous role of the metabotropic glutamate receptor 5 (mGluR5). We found that mGluR5 is required for several key steps in wiring up the thalamocortical connections to form the cortical somatosensory map. mGluR5-dependent processes during early postnatal brain development affect the following: (1) placement of activity-directed cortical neurons; (2) regulation of polarized dendritic outgrowth toward thalamocortical axons relaying sensory information, (3) synaptogenesis; and (4) development of functional connectivity in spiny stellate neurons. Perturbing mGluR5 expression could lead to abnormal neuronal circuits, which may contribute to neurological and psychiatric disease.

A Transient Translaminar GABAergic Interneuron Circuit Connects Thalamocortical Recipient Layers in Neonatal Somatosensory Cortex

GABAergic activity is thought to influence developing neocortical sensory circuits. Yet the late postnatal maturation of local layer (L)4 circuits suggests alternate sources of GABAergic control in nascent thalamocortical networks. We show that a population of L5b, somatostatin (SST)-positive interneuron receives early thalamic synaptic input and, using laser-scanning photostimulation, identify an early transient circuit between these cells and L4 spiny stellates (SSNs) that disappears by the end of the L4 critical period. Sensory perturbation disrupts the transition to a local GABAergic circuit, suggesting a link between translaminar and local control of SSNs. Conditional silencing of SST+ interneurons or conversely biasing the circuit toward local inhibition by overexpression of neuregulin-1 type 1 results in an absence of early L5b GABAergic input in mutants and delayed thalamic innervation of SSNs. These data identify a role for L5b SST+ interneurons in the control of SSNs in the early postnatal neocortex.

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Early postnatal thalamic synaptic input onto L5b somatostatin interneurons

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Transient reciprocal connectivity between L5b INs and L4 spiny stellate cells

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Sensory activity is required for the transition to a local L4 GABAergic circuit

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Molecular bias toward early local IN synapses delays thalamic innervation of SSNs

Effects of developmental alcohol and valproic acid exposure on play behavior of ferrets

Exposure to alcohol and valproic acid (VPA) during pregnancy can lead to fetal alcohol spectrum disorders and fetal valproate syndrome, respectively. Altered social behavior is a hallmark of both these conditions and there is ample evidence showing that developmental exposure to alcohol and VPA affect social behavior in rodents. However, results from rodent models are somewhat difficult to translate to humans owing to the substantial differences in brain development, morphology, and connectivity. Since the cortex folding pattern is closely related to its specialization and that social behavior is strongly influenced by cortical structures, here we studied the effects of developmental alcohol and VPA exposure on the play behavior of the ferret, a gyrencephalic animal known for its playful nature. Animals were injected with alcohol (3.5g/kg, i.p.), VPA (200mg/kg, i.p.) or saline (i.p) every other day during the brain growth spurt period, between postnatal days 10 and 30. The play behavior of pairs of the same experimental group was evaluated 3 weeks later. Both treatments induced significant behavioral differences compared to controls. Alcohol and VPA exposed ferrets played less than saline treated ones, but while animals from the alcohol group displayed a delay in start playing with each other, VPA treated ones spent most of the time close to one another without playing. These findings not only extend previous results on the effects of developmental exposure to alcohol and VPA on social behavior, but make the ferret a great model to study the underlying mechanisms of social interaction.