Development of functional thalamocortical synapses studied with current source-density analysis in whole forebrain slices in the rat

Early cortical GABAergic interneurons determine the projection patterns of L4 excitatory neurons

During cerebral cortex development, excitatory pyramidal neurons (PNs) establish specific projection patterns while receiving inputs from GABAergic inhibitory interneurons (INs). Whether these inhibitory inputs can shape PNs' projection patterns is, however, unknown. While layer 4 (L4) PNs of the primary somatosensory (S1) cortex are all born as long-range callosal projection neurons (CPNs), most of them acquire local connectivity upon activity-dependent elimination of their interhemispheric axons during postnatal development. Here, we demonstrate that precise developmental regulation of inhibition is key for the retraction of S1L4 PNs' callosal projections. Ablation of somatostatin INs leads to premature inhibition from parvalbumin INs onto S1L4 PNs and prevents them from acquiring their barrel-restricted local connectivity pattern. As a result, adult S1L4 PNs retain interhemispheric projections responding to tactile stimuli, and the mice lose whisker-based texture discrimination. Overall, we show that temporally ordered IN activity during development is key to shaping local ipsilateral S1L4 PNs' projection pattern, which is required for fine somatosensory processing.

Layer 6b controls brain state via apical dendrites and the higher-order thalamocortical system

The deepest layer of the cortex (layer 6b [L6b]) contains relatively few neurons, but it is the only cortical layer responsive to the potent wake-promoting neuropeptide orexin/hypocretin. Can these few neurons significantly influence brain state? Here, we show that L6b-photoactivation causes a surprisingly robust enhancement of attention-associated high-gamma oscillations and population spiking while abolishing slow waves in sleep-deprived mice. To explain this powerful impact on brain state, we investigated L6b's synaptic output using optogenetics, electrophysiology, and monoCaTChR ex vivo. We found powerful output in the higher-order thalamus and apical dendrites of L5 pyramidal neurons, via L1a and L5a, as well as in superior colliculus and L6 interneurons. L6b subpopulations with distinct morphologies and short- and long-term plasticities project to these diverse targets. The L1a-targeting subpopulation triggered powerful NMDA-receptor-dependent spikes that elicited burst firing in L5. We conclude that orexin/hypocretin-activated cortical neurons form a multifaceted, fine-tuned circuit for the sustained control of the higher-order thalamocortical system.

Development and Evolution of Thalamocortical Connectivity

Conscious perception in mammals depends on precise circuit connectivity between cerebral cortex and thalamus; the evolution and development of these structures are closely linked. During the wiring of reciprocal thalamus-cortex connections, thalamocortical axons (TCAs) first navigate forebrain regions that had undergone substantial evolutionary modifications. In particular, the organization of the pallial-subpallial boundary (PSPB) diverged significantly between mammals, reptiles, and birds. In mammals, transient cell populations in internal capsule and early corticofugal projections from subplate neurons closely interact with TCAs to guide pathfinding through ventral forebrain and PSPB crossing. Prior to thalamocortical axon arrival, cortical areas are initially patterned by intrinsic genetic factors. Thalamocortical axons then innervate cortex in a topographically organized manner to enable sensory input to refine cortical arealization. Here, we review the mechanisms underlying the guidance of thalamocortical axons across forebrain boundaries, the implications of PSPB evolution for thalamocortical axon pathfinding, and the reciprocal influence between thalamus and cortex during development.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in the adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity," which has been examined during or after the classic "critical period." Because peripheral perturbations can alter the auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters the ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo widefield imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activities in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs, shifting the excitation-inhibition balance toward excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity", which has been examined during or after the classic 'critical period'. Because peripheral perturbations can alter auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the classic critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activity in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs shifting the excitation-inhibition balance towards excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Early brain activity: Translations between bedside and laboratory

Neural activity is both a driver of brain development and a readout of developmental processes. Changes in neuronal activity are therefore both the cause and consequence of neurodevelopmental compromises. Here, we review the assessment of neuronal activities in both preclinical models and clinical situations. We focus on issues that require urgent translational research, the challenges and bottlenecks preventing translation of biomedical research into new clinical diagnostics or treatments, and possibilities to overcome these barriers. The key questions are (i) what can be measured in clinical settings versus animal experiments, (ii) how do measurements relate to particular stages of development, and (iii) how can we balance practical and ethical realities with methodological compromises in measurements and treatments.

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.

Impaired Hearing and Altered Subplate Circuits During the First and Second Postnatal Weeks of Otoferlin-Deficient Mice

Sensory deprivation from the periphery impacts cortical development. Otoferlin deficiency leads to impaired cochlear synaptic transmission and is associated with progressive hearing loss in adults. However, it remains elusive how sensory deprivation due to otoferlin deficiency impacts the early development of the auditory cortex (ACX) especially before the onset of low threshold hearing. To test that, we performed in vivo imaging of the ACX in awake mice lacking otoferlin (Otof-/-) during the first and second postnatal weeks and found that spontaneous and sound-driven cortical activity were progressively impaired. We then characterized the effects on developing auditory cortical circuits by performing in vitro recordings from subplate neurons (SPN), the first primary targets of thalamocortical inputs. We found that in Otof-/- pups, SPNs received exuberant connections from excitatory and inhibitory neurons. Moreover, as a population, SPNs showed higher similarity with respect to their circuit topology in the absence of otoferlin. Together, our results show that otoferlin deficiency results in impaired hearing and has a powerful influence on cortical connections and spontaneous activity in early development even before complete deafness. Therefore, peripheral activity has the potential to sculpt cortical structures from the earliest ages, even before hearing impairment is diagnosed.

Temporal controls over inter-areal cortical projection neuron fate diversity

Interconnectivity between neocortical areas is critical for sensory integration and sensorimotor transformations^1-6. These functions are mediated by heterogeneous inter-areal cortical projection neurons (ICPN), which send axon branches across cortical areas as well as to subcortical targets^7-9. Although ICPN are anatomically diverse^10-14, they are molecularly homogeneous¹⁵, and how the diversity of their anatomical and functional features emerge during development remains largely unknown. Here we address this question by linking the connectome and transcriptome in developing single ICPN of the mouse neocortex using a combination of multiplexed analysis of projections by sequencing^16,17 (MAPseq, to identify single-neuron axonal projections) and single-cell RNA sequencing (to identify corresponding gene expression). Focusing on neurons of the primary somatosensory cortex (S1), we reveal a protracted unfolding of the molecular and functional differentiation of motor cortex-projecting ([Formula: see text]) ICPN compared with secondary somatosensory cortex-projecting ([Formula: see text]) ICPN. We identify SOX11 as a temporally differentially expressed transcription factor in [Formula: see text] versus [Formula: see text] ICPN. Postnatal manipulation of SOX11 expression in S1 impaired sensorimotor connectivity and disrupted selective exploratory behaviours in mice. Together, our results reveal that within a single cortical area, different subtypes of ICPN have distinct postnatal paces of molecular differentiation, which are subsequently reflected in distinct circuit connectivities and functions. Dynamic differences in the expression levels of a largely generic set of genes, rather than fundamental differences in the identity of developmental genetic programs, may thus account for the emergence of intra-type diversity in cortical neurons.

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.

Network Dynamics in the Developing Piriform Cortex of Unanesthetized Rats

The time course of changes in functional cortical activity during early development has been extensively studied in the rodent visual system. A key period in this process is the time of eye opening, which marks the onset of patterned visual input and active vision. However, vision differs from other systems in that it receives limited patterned sensory input before eye opening, and it remains unclear how findings from vision relate to other systems. Here, we focus on the development of cortical network activity in the olfactory system-which is crucial for survival at birth-by recording field potential and spiking activity from piriform cortex of unanesthetized rat pups from birth (P0) to P21. Our results demonstrate that odors evoke stable 10-15 Hz oscillations in piriform cortex from birth to P15, after which cortical responses undergo rapid changes. This transition is coincident with the emergence of gamma oscillations and fast sniffing behavior and preceded by an increase in spontaneous activity. Neonatal network oscillations and their developmental dynamics exhibit striking similarities with those previously observed in the visual, auditory, and somatosensory systems, providing insight into the network-level mechanisms underlying the development of sensory cortex in general and olfactory processing in particular.

In search of common developmental and evolutionary origin of the claustrum and subplate

The human claustrum, a major hub of widespread neocortical connections, is a thin, bilateral sheet of gray matter located between the insular cortex and the striatum. The subplate is a largely transient cortical structure that contains some of the earliest generated neurons of the cerebral cortex and has important developmental functions to establish intra- and extracortical connections. In human and macaque some subplate cells undergo regulated cell death, but some remain as interstitial white matter cells. In mouse and rat brains a compact layer is formed, Layer 6b, and it remains underneath the cortex, adjacent to the white matter. Whether Layer 6b in rodents is homologous to primate subplate or interstitial white matter cells is still debated. Gene expression patterns, such as those of Nurr1/Nr4a2, have suggested that the rodent subplate and the persistent subplate cells in Layer 6b and the claustrum might have similar origins. Moreover, the birthdates of the claustrum and Layer 6b are similarly precocious in mice. These observations prompted our speculations on the common developmental and evolutionary origin of the claustrum and the subplate. Here we systematically compare the currently available data on cytoarchitecture, evolutionary origin, gene expression, cell types, birthdates, neurogenesis, lineage and migration, circuit connectivity, and cell death of the neurons that contribute to the claustrum and subplate. Based on their similarities and differences we propose a partially common early evolutionary origin of the cells that become claustrum and subplate, a likely scenario that is shared in these cell populations across all amniotes.

Transient cortical circuits match spontaneous and sensory-driven activity during development

At the earliest developmental stages, spontaneous activity synchronizes local and large-scale cortical networks. These networks form the functional template for the establishment of global thalamocortical networks and cortical architecture. The earliest connections are established autonomously. However, activity from the sensory periphery reshapes these circuits as soon as afferents reach the cortex. The early-generated, largely transient neurons of the subplate play a key role in integrating spontaneous and sensory-driven activity. Early pathological conditions-such as hypoxia, inflammation, or exposure to pharmacological compounds-alter spontaneous activity patterns, which subsequently induce disturbances in cortical network activity. This cortical dysfunction may lead to local and global miswiring and, at later stages, can be associated with neurological and psychiatric conditions.

The Integrative Function of Silent Synapses on Subplate Neurons in Cortical Development and Dysfunction

The thalamocortical circuit is of central importance in relaying information to the cortex. In development, subplate neurons (SPNs) form an integral part of the thalamocortical pathway. These early born cortical neurons are the first neurons to receive thalamic inputs and excite neurons in the cortical plate. This feed-forward circuit topology of SPNs supports the role of SPNs in shaping the formation and plasticity of thalamocortical connections. Recently it has been shown that SPNs also receive inputs from the developing cortical plate and project to the thalamus. The cortical inputs to SPNs in early ages are mediated by N-methyl-D-aspartate (NMDA)-receptor only containing synapses while at later ages α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptors are present. Thus, SPNs perform a changing integrative function over development. NMDA-receptor only synapses are crucially influenced by the resting potential and thus insults to the developing brain that causes depolarizations, e.g., hypoxia, can influence the integrative function of SPNs. Since such insults in humans cause symptoms of neurodevelopmental disorders, NMDA-receptor only synapses on SPNs might provide a crucial link between early injuries and later circuit dysfunction. We thus here review subplate associated circuits, their changing functions, and discuss possible roles in development and disease.

The Superior Function of the Subplate in Early Neocortical Development

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

Morphological and Physiological Characteristics of Ebf2-EGFP-Expressing Cajal-Retzius Cells in Developing Mouse Neocortex

Cajal-Retzius (CR) cells are one of the earliest populations of neurons in the cerebral cortex of rodents and primates, and they play a critical role in corticogenesis and cortical lamination during neocortical development. However, a comprehensive morphological and physiological profile of CR cells in the mouse neocortex has not yet been established. Here, we systematically investigated the dynamic development of CR cells in Tg(Ebf2-EGFP)58Gsat/Mmcd mice. The morphological complexity, membrane activities and presynaptic inputs of CR cells coordinately increase and reach a plateau at P5–P9 before regressing. Using 3D reconstruction, we delineated a parallel-stratification pattern of the axonal extension of CR cells. Furthermore, we found that the morphological structure and presynaptic inputs of CR cells were disturbed in Reelin-deficient mice. These findings confirm that CR cells undergo a transient maturation process in layer 1 before disappearing. Importantly, Reelin deficiency impairs the formation of synaptic connections onto CR cells. In conclusion, our results provide insights into the rapid maturation and axonal stratification of CR cells in layer 1. These findings suggest that both the electrophysiological activities and the morphology of CR cells provide vital guidance for the modulation of early circuits, in a Reelin-dependent manner.

Molecularly Defined Subplate Neurons Project Both to Thalamocortical Recipient Layers and Thalamus

In mammals, subplate neurons (SPNs) are among the first generated cortical neurons. While most SPNs exist only transiently during development, a number of SPNs persist among adult Layer 6b (L6b). During development, SPNs receive thalamic and intra-cortical input, and primarily project to Layer 4 (L4). SPNs are critical for the anatomical and functional development of thalamocortical connections and also pioneer corticothalamic projections. Since SPNs are heterogeneous, SPN subpopulations might serve different roles. Here, we investigate the connectivity of one subpopulation, complexin-3 (Cplx3)-positive SPNs (Cplx3-SPNs), in mouse whisker somatosensory (barrel) cortex (S1). We find that many Cplx3-SPNs survive into adulthood and become a subpopulation of L6b. Cplx3-SPNs axons project to thalamorecipient layers, that is, L4, 5a, and 1. The L4 projections are biased towards the septal regions between barrels in the second postnatal week. Thus, S1 Cplx3-SPN targets co-localize with the eventual projections of the medial posterior thalamic nucleus (POm). In addition to their cortical targets, Cplx3-SPNs also extend long-range axons to several thalamic nuclei, including POm. Thus, Cplx3-SPN/L6b neurons are associated with paralemniscal pathways and can potentially directly link thalamocortical and corticothalamic circuits. This suggests an additional key role for SPNs in the establishment and maintenance of thalamocortical processing.

Early Somatostatin Interneuron Connectivity Mediates the Maturation of Deep Layer Cortical Circuits

The precise connectivity of somatostatin and parvalbumin cortical interneurons is generated during development. An understanding of how these interneuron classes incorporate into cortical circuitry is incomplete but essential to elucidate the roles they play during maturation. Here, we report that somatostatin interneurons in infragranular layers receive dense but transient innervation from thalamocortical afferents during the first postnatal week. During this period, parvalbumin interneurons and pyramidal neurons within the same layers receive weaker thalamocortical inputs, yet are strongly innervated by somatostatin interneurons. Further, upon disruption of the early (but not late) somatostatin interneuron network, the synaptic maturation of thalamocortical inputs onto parvalbumin interneurons is perturbed. These results suggest that infragranular somatostatin interneurons exhibit a transient early synaptic connectivity that is essential for the establishment of thalamic feedforward inhibition mediated by parvalbumin interneurons.

The serotonin reuptake inhibitor citalopram suppresses activity in the neonatal rat barrel cortex in vivo

Inhibition of serotonin uptake, which causes an increase in extracellular serotonin levels, disrupts the development of thalamocortical barrel maps in neonatal rodents. Previous in vitro studies have suggested that the disruptive effect of excessive serotonin on barrel map formation involves a depression at thalamocortical synapses. However, the effects of serotonin uptake inhibitors on the early thalamocortical activity patterns in the developing barrel cortex in vivo remain largely unknown. Here, using extracellular recordings of the local field potentials and multiple unit activity (MUA) we explored the effects of the selective serotonin reuptake inhibitor (SSRI) citalopram (10-20mg/kg, intraperitoneally) on sensory evoked activity in the barrel cortex of neonatal (postnatal days P2-5) rats in vivo. We show that administration of citalopram suppresses the amplitude and prolongs the delay of the sensory evoked potentials, reduces the power and frequency of the early gamma oscillations, and suppresses sensory evoked and spontaneous neuronal firing. In the adolescent P21-29 animals, citalopram affected neither sensory evoked nor spontaneous activity in barrel cortex. We suggest that suppression of the early thalamocortical activity patterns contributes to the disruption of the barrel map development caused by SSRIs and other conditions elevating extracellular serotonin levels.

Developmental expression of the neuroligins and neurexins in fragile X mice

Neuroligins and neurexins are transsynaptic proteins involved in the maturation of glutamatergic and GABAergic synapses. Research has identified synaptic proteins and function as primary contributors to the development of fragile X syndrome. Fragile X mental retardation protein (FMRP), the protein that is lacking in fragile X syndrome, binds neuroligin-1 and -3 mRNA. Using in situ hybridization, we examined temporal and spatial expression patterns of neuroligin (NLGN) and neurexin (NRXN) mRNAs in the somatosensory (S1) cortex and hippocampus in wild-type (WT) and fragile X knockout (FMR1-KO) mice during the first 5 weeks of postnatal life. Genotype-based differences in expression included increased NLGN1 mRNA in CA1 and S1 cortex, decreased NLGN2 mRNA in CA1 and dentate gyrus (DG) regions of the hippocampus, and increased NRXN3 mRNA in CA1, DG, and S1 cortex between female WT and FMR1-KO mice. In male mice, decreased expression of NRXN3 mRNA was observed in CA1 and DG regions of FMR1-KO mice. Sex differences in hippocampal expression of NLGN2, NRXN1, NRXN2, and NRXN3 mRNAs and in S1 cortex expression of NRXN3 mRNAs were observed WT mice, whereas sex differences in NLGN3, NRXN1, NRXN2, and NRXN3 mRNA expression in the hippocampus and in NLGN1, NRXN2 and NRXN3 mRNA expression in S1 cortex were detected in FMR1-KO mice. These results provide a neuroanatomical map of NLGN and NRXN expression patterns over postnatal development in WT and FMR1-KO mice. The differences in developmental trajectory of these synaptic proteins could contribute to long-term differences in CNS wiring and synaptic function.

Decrease of neocortical paired-pulse depression in GAERS and possible implication of gap junctions

Thalamocortical slices are widely used to study thalamocortical relationships and absence epilepsy. However, it is still not known whether (1) intracortical synaptic transmission, in particular neocortical paired-pulse depression (PPD), is maintained in these slices and (2) whether PPD is altered in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS, a model of absence epilepsy for which cortico-thalamic loops are involved). Furthermore, while the involvement of gap junctions (GJ) in the mechanisms leading to epileptiform discharges has been intensively studied, little is known about their effect on intracortical transmission. We first studied intracortical connection efficacy and PPD in thalamocortical slices from GAERS and non-epileptic rats (NER). We then investigated the effects of GJ blockers (carbenoxolone and quinidine) on intracortical response following single or paired-pulse stimulations in coronal slices from Wistar rats. We show that the efficacy of intracortical connections is not impaired in GAERS. We also show that neocortical PPD is preserved in thalamocortical slices of NER, but that its efficacy is strongly decreased in GAERS. Moreover, a NMDA antagonist strongly reduced the PPD in NER but had no effect in GAERS. Cortical responses to white matter stimulation were not modified by quinidine or carbenoxolone in coronal slices of Wistar rats. PPD was recorded in these slices and was decreased by carbenoxolone but not by quinidine. We hypothesize that the decrease of PPD observed in GAERS might be due to a decrease in function of (1) NMDA receptors and/or (2) astrocytic GJ's.

Imprecise Whisker Map in the Neonatal Rat Barrel Cortex

The somatosensory barrel cortex in rodents contains a topographic map of the facial whiskers where each cortical barrel is tuned to a corresponding whisker. However, exactly when this correspondence is established during development and how precise the functional topography of the whisker protomap is at birth, before the anatomical formation of barrels, are questions that remain unresolved. Here, using extracellular and whole-cell recordings from the barrel cortex of 0- to 7-day-old (P0-7; P0 = day of birth) rat pups in vivo, we report a low level of tuning to the principal whisker at P0-1, with multiple adjacent whiskers evoking large multi- and single-unit responses and excitatory postsynaptic currents in cortical neurons. Additionally, we found broad and largely overlapping projection fields (PFs) for neighboring whiskers in the barrel cortex at P0-1. Starting from P2-3, a segregated whisker map emerged, characterized by preferential single whisker tuning and segregated whisker PFs. These results indicate that the functional whisker protomap in the somatosensory cortex is imprecise at birth, that for 2-3 days after birth, whiskers compete for the cortical target territories, and that formation of a segregated functional whisker map coincides with emergence of the anatomical barrel map.

What brakes the preterm brain? An arresting story

Children surviving premature birth have a high risk of cognitive and learning disabilities and attention deficit. In turn, adverse outcomes are associated with persistent reductions in cerebral growth on magnetic resonance imaging (MRI). It is striking that modern care has been associated with a dramatic reduction in the risk of cystic white matter damage, but modest improvements in terms of neurodevelopmental impairment. This review will explore the hypothesis that the disability is primarily associated with impaired neural connectivity rather than cell death alone. Very preterm infants exhibit reduced thalamocortical connectivity and cortical neuroplasticity compared with term-born controls. In preterm fetal sheep, moderate cerebral ischemia with no neuronal loss, but significant diffuse failure of maturation of cortical pyramidal neurons, was associated with impaired dendritic growth and synapse formation, consistent with altered connectivity. These changes were associated with delayed decline in cortical fractional anisotropy (FA) on MRI. Supporting these preclinical findings, preterm human survivors showed similar enduring impairment of microstructural development of the cerebral cortex defined by FA, consistent with delayed formation of neuronal processes. These findings offer the promise that better understanding of impairment of neural connectivity may allow us to promote normal development and growth of the cortex after preterm birth.

Expression profiling of mouse subplate reveals a dynamic gene network and disease association with autism and schizophrenia

The subplate zone is a highly dynamic transient sector of the developing cerebral cortex that contains some of the earliest generated neurons and the first functional synapses of the cerebral cortex. Subplate cells have important functions in early establishment and maturation of thalamocortical connections, as well as in the development of inhibitory cortical circuits in sensory areas. So far no role has been identified for cells in the subplate in the mature brain and disease association of the subplate-specific genes has not been analyzed systematically. Here we present gene expression evidence for distinct roles of the mouse subplate across development as well as unique molecular markers to extend the repertoire of subplate labels. Performing systematic comparisons between different ages (embryonic days 15 and 18, postnatal day 8, and adult), we reveal the dynamic and constant features of the markers labeling subplate cells during embryonic and early postnatal development and in the adult. This can be visualized using the online database of subplate gene expression at https://molnar.dpag.ox.ac.uk/subplate/. We also identify embryonic similarities in gene expression between the ventricular zones, intermediate zone, and subplate, and distinct postnatal similarities between subplate, layer 5, and layers 2/3. The genes expressed in a subplate-specific manner at some point during development show a statistically significant enrichment for association with autism spectrum disorders and schizophrenia. Our report emphasizes the importance of the study of transient features of the developing brain to better understand neurodevelopmental disorders.

Termination and initial branch formation of SNAP-25-deficient thalamocortical fibres in heterochronic organotypic co-cultures

We are interested in the role of neural activity mediated through regulated vesicular release in the stopping and early branching of the thalamic projections in the cortex. Axon outgrowth, arrival at the cortical subplate, side-branch formation during the waiting period and cortical plate innervation of embryonic thalamocortical projections occurs without major abnormalities in the absence of regulated release in Snap25 (-/-) null mutant mice [Washbourne et al. (2002) Nat. Neurosci. 5:19-26; Molnár et al. (2002) J. Neurosci. 22:10313-10323]. The fact that Snap25 (-/-) null mutant mice die at birth limited our previous experiments to the prenatal period. We therefore investigated the behaviour of thalamic projections in co-culture paradigms by using heterochronic thalamic [embryonic day (E)16-E18] and cortical [postnatal day (P)0-P3] explants, in which the stopping and branching behaviour has been previously documented. Our current co-culture experiments established that thalamic projections from E16-E18 Snap25(+/+) or Snap25 (-/-) explants behaved in an identical fashion in P0-P3 Snap25 (+/+) cortical explants after 7 days in vitro. Thalamic projections from Snap25 (-/-) explants developed similar patterns of fibre ingrowth to the cortex, and stopped and formed branches at a similar depth in the Snap25(+/+) cortical slice as in control cultures. These results imply that thalamic projections can reach their ultimate target cells in layer 4, stop, and start to develop branches in the absence of regulated vesicular transmitter release from their own terminals.

Changing microcircuits in the subplate of the developing cortex

Subplate neurons (SPNs) are a population of neurons in the mammalian cerebral cortex that exist predominantly in the prenatal and early postnatal period. Loss of SPNs prevents the functional maturation of the cerebral cortex. SPNs receive subcortical input from the thalamus and relay this information to the developing cortical plate and thereby can influence cortical activity in a feedforward manner. Little is known about potential feedback projections from the cortical plate to SPNs. Thus, we investigated the spatial distribution of intracortical synaptic inputs to SPNs in vitro in mouse auditory cortex by photostimulation. We find that SPNs fell into two broad classes based on their distinct spatial patterns of synaptic inputs. The first class of SPNs receives inputs from only deep cortical layers, while the second class of SPNs receives inputs from deep as well as superficial layers including layer 4. We find that superficial cortical inputs to SPNs emerge in the second postnatal week and that SPNs that receive superficial cortical input are located more superficially than those that do not. Our data thus suggest that distinct circuits are present in the subplate and that, while SPNs participate in an early feedforward circuit, they are also involved in a feedback circuit at older ages. Together, our results show that SPNs are tightly integrated into the developing thalamocortical and intracortical circuit. The feedback projections from the cortical plate might enable SPNs to amplify thalamic inputs to SPNs.

Functional development of large-scale sensorimotor cortical networks in the brain

Large-scale neuronal networks integrating several cortical areas mediate the complex functions of the brain such as sensorimotor integration. Little is known about the functional development of these networks and the maturational processes by which distant networks become functionally connected. We addressed this question in the postnatal rat sensorimotor system. Using epicranial multielectrode grids that span most of the cortical surface and intracortical electrodes, we show that sensory evoked cortical responses continuously maturate throughout the first 3 weeks with the strongest developmental changes occurring in a very short time around postnatal day 13 (P13). Before P13, whisker stimulation evokes slow, initially surface-negative activity restricted mostly to the lateral parietal area of the contralateral hemisphere. In a narrow time window of ∼48 h around P13, a new early, sharp surface-positive component emerges that coincides with subsequent propagation of activity to sensory and motor areas of both hemispheres. Our data show that this new component developing at the end of the second week corresponds principally to functional maturation of the supragranular cortical layers and appears to be crucial for the functional associations in the large-scale sensorimotor cortical network. It goes along with the onset of whisking behavior, as well as major synaptic and functional changes within the S1 cortex that are known to develop during this period.

Comparative aspects of subplate zone studied with gene expression in sauropsids and mammals

There is currently a debate about the evolutionary origin of the earliest generated cortical preplate neurons and their derivatives (subplate and marginal zone). We examined the subplate with murine markers including nuclear receptor related 1 (Nurr1), monooxygenase Dbh-like 1 (Moxd1), transmembrane protein 163 (Tmem163), and connective tissue growth factor (Ctgf) in developing and adult turtle, chick, opossum, mouse, and rat. Whereas some of these are expressed in dorsal pallium in all species studied (Nurr1, Ctgf, and Tmem163), we observed that the closely related mouse and rat differed in the expression patterns of several others (Dopa decarboxylase, Moxd1, and thyrotropin-releasing hormone). The expression of Ctgf, Moxd1, and Nurr1 in the oppossum suggests a more dispersed subplate population in this marsupial compared with mice and rats. In embryonic and adult chick brains, our selected subplate markers are primarily expressed in the hyperpallium and in the turtle in the main cell dense layer of the dorsal cortex. These observations suggest that some neurons that express these selected markers were present in the common ancestor of sauropsids and mammals.

Development of axonal pathways in the human fetal fronto-limbic brain: histochemical characterization and diffusion tensor imaging

The development of cortical axonal pathways in the human brain begins during the transition between the embryonic and fetal period, happens in a series of sequential events, and leads to the establishment of major long trajectories by the neonatal period. We have correlated histochemical markers (acetylcholinesterase (AChE) histochemistry, antibody against synaptic protein SNAP-25 (SNAP-25-immunoreactivity) and neurofilament 200) with the diffusion tensor imaging (DTI) database in order to make a reconstruction of the origin, growth pattern and termination of the pathways in the period between 8 and 34 postconceptual weeks (PCW). Histological sections revealed that the initial outgrowth and formation of joined trajectories of subcortico-frontal pathways (external capsule, cerebral stalk-internal capsule) and limbic bundles (fornix, stria terminalis, amygdaloid radiation) occur by 10 PCW. As early as 11 PCW, major afferent fibers invade the corticostriatal junction. At 13-14 PCW, axonal pathways from the thalamus and basal forebrain approach the deep moiety of the cortical plate, causing the first lamination. The period between 15 and 18 PCW is dominated by elaboration of the periventricular crossroads, sagittal strata and spread of fibers in the subplate and marginal zone. Tracing of fibers in the subplate with DTI is unsuccessful due to the isotropy of this zone. Penetration of the cortical plate occurs after 24-26 PCW. In conclusion, frontal axonal pathways form the periventricular crossroads, sagittal strata and 'waiting' compartments during the path-finding and penetration of the cortical plate. Histochemistry is advantageous in the demonstration of a growth pattern, whereas DTI is unique for demonstrating axonal trajectories. The complexity of fibers is the biological substrate of selective vulnerability of the fetal white matter.

The subplate and early cortical circuits

The developing mammalian cerebral cortex contains a distinct class of cells, subplate neurons (SPns), that play an important role during early development. SPns are the first neurons to be generated in the cerebral cortex, they reside in the cortical white matter, and they are the first to mature physiologically. SPns receive thalamic and neuromodulatory inputs and project into the developing cortical plate, mostly to layer 4. Thus SPns form one of the first functional cortical circuits and are required to relay early oscillatory activity into the developing cortical plate. Pathophysiological impairment or removal of SPns profoundly affects functional cortical development. SPn removal in visual cortex prevents the maturation of thalamocortical synapses, the maturation of inhibition in layer 4, the development of orientation selective responses and the formation of ocular dominance columns. SPn removal also alters ocular dominance plasticity during the critical period. Therefore, SPns are a key regulator of cortical development and plasticity. SPns are vulnerable to injury during prenatal stages and might provide a crucial link between brain injury in development and later cognitive malfunction.

An evaluation of the conductivity profile in the somatosensory barrel cortex of Wistar rats

Microelectrode arrays used to record local field potentials from the brain are being built with increasingly more spatial resolution, ranging from the initially developed laminar arrays to those with planar and three-dimensional (3D) formats. In parallel with such development in recording techniques, current source density (CSD) analyses have recently been expanded up to the continuous-3D form. Unfortunately, the effect of the conductivity profile on the CSD analysis performed with contemporary microelectrode arrays has not yet been evaluated and most of the studies assumed it was homogeneous and isotropic. In this study, we measured the conductivity profile in the somatosensory barrel cortex of Wistar rats. To that end, we combined multisite electrophysiological data recorded with a homemade assembly of silicon-based probes and a nonlinear least-squares algorithm that implicitly assumed that the cerebral cortex of rodents could be locally approximated as a layered anisotropic spherical volume conductor. The eccentricity of the six cortical layers in the somatosensory barrel cortex was evaluated from postmortem histological images. We provided evidence for the local spherical character of the entire barrels field, with concentric cortical layers. We found significant laminar dependencies in the conductivity values with radial/tangential anisotropies. These results were in agreement with the layer-dependent orientations of myelinated axons, but hardly related to densities of cells. Finally, we demonstrated through simulations that ignoring the real conductivity profile in the somatosensory barrel cortex of rats caused considerable errors in the CSD reconstruction, with pronounced effects on the continuous-3D form and charge-unbalanced CSD. We concluded that the conductivity profile must be included in future developments of CSD analysis, especially for rodents.

Morphological changes and synaptogenesis of corticothalamic neurons in the somatosensory cortex of rat during perinatal development

When rat fetuses grew from embryonic day (E) 18 to the day of birth (P0), the corticothalamic (CT) neurons, as identified by back labeling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI), in the somatosensory cortex underwent gradual changes in the shape of their cell bodies, in their distribution in the cortical plate and in the complexity of dendritic branching. Fluorescence immunocytochemical studies indicated that in the marginal zone (MZ) the apical dendrites of the CT neurons formed contacts with horizontally oriented axons and contained putative glutamatergic, as clusters exhibiting both synaptophysin and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluR1 subunit immunoreactivities, and γ-aminobutyric acid (GABA)-ergic synapses, as clusters exhibiting both synaptophysin and gephyrin immunoreactivities. Quantitative analyses further revealed that during this perinatal period, the proportion of CT neurons containing glutamatergic synapses increased significantly, whereas the proportion of CT neurons containing GABAergic synapses remained virtually unchanged. Our results indicate that glutamatergic and GABAergic synapses between the CT neurons and the axons in the MZ are already formed in rat cortices as early as E18 and further suggest that the activities of the neural networks in the somatosensory cortex could be conveyed to their targets in the thalamus in rat brains at least 3 days before birth.

A conserved switch in sensory processing prepares developing neocortex for vision

Developing cortex generates endogenous activity that modulates the formation of functional units, but how this activity is altered to support mature function is poorly understood. Using recordings from the visual cortex of preterm human infants and neonatal rats, we report a "bursting" period of visual responsiveness during which the weak retinal output is amplified by endogenous network oscillations, enabling a primitive form of vision. This period ends shortly before delivery in humans and eye opening in rodents with an abrupt switch to the mature visual response. The switch is causally linked to the emergence of an activated state of continuous cortical activity dependent on the ascending neuromodulatory systems involved in arousal. This switch is sensory system specific but experience independent and also involves maturation of retinal processing. Thus, the early development of visual processing is governed by a conserved, intrinsic program that switches thalamocortical response properties in anticipation of patterned vision.

"Slow activity transients" in infant rat visual cortex: a spreading synchronous oscillation patterned by retinal waves

A primary feature of the preterm infant electroencephalogram is the presence of large infra-slow potentials containing rapid oscillations called slow activity transients (SATs). Such activity has not been described in animal models, and their generative mechanisms are unknown. Here we use direct-current and multisite extracellular, as well as whole-cell, recording in vivo to demonstrate the existence of regularly repeating SATs in the visual cortex of infant rats before eye opening. Present only in absence of anesthesia, SATs at postnatal day 10-11 were identifiable as a separate group of long-duration (approximately 10 s) events that consisted of large (>1 mV) negative local-field potentials produced by the summation of multiple bursts of rapid oscillatory activity (15-30 Hz). SATs synchronized the vast majority of neuronal activity (87%) in the visual cortex before eye opening. Enucleation eliminated SATs, and their duration, interevent interval, and sub-burst structure matched those of phase III retinal waves recorded in vitro. Retinal waves, however, lacked rapid oscillations, suggesting that they arise centrally. Multielectrode recordings showed that SATs spread horizontally in cortex and synchronize activity at coactive locales via the rapid oscillations. SATs were clearly different from ongoing cortical activity, which was observable as a separate class of short bursts from postnatal day 9. Together, our data suggest that, in vivo, early cortical activity is primarily determined by peripheral inputs-retinal waves in visual cortex-that provide excitatory input, and by thalamocortical circuitry, which transforms this input to beta oscillations. We propose that the synchronous oscillations of SATs participate in the formation of visual circuitry.

Functional excitatory microcircuits in neonatal cortex connect thalamus and layer 4

The functional connectivity of the cerebral cortex is shaped by experience during development, especially during a critical period early in life. In the prenatal and neonatal cortex, transient neuronal circuits are formed by a population of subplate neurons (SPNs). However, SPNs are absent in the adult cortex. While SPNs are crucial for normal development of the cerebral cortex and of thalamocortical synapses, little is known about how they are integrated in the developing thalamocortical circuit. We therefore investigated SPNs in vitro in thalamocortical slices of A1 and medial geniculate nucleus (MGN) in mouse from postnatal day 1 (P1) to P13. We found that SPNs can fire action potentials at P1 and that their intrinsic membrane properties are mature after P5. We find that SPNs receive functional excitatory inputs from the MGN as early as P2. The MGN projections to SPNs strengthen between P2 and P13 and are capable of inducing action potentials in SPNs. Selective activation of SPNs by photostimulation produced EPSCs in layer 4 neurons, demonstrating a functional excitatory connection. Thus, SPNs are tightly integrated into the developing thalamocortical circuit and would be a reliable relay of early spontaneous and sound-evoked activity. The role of SPNs in development likely results from their strong excitatory projection to layer 4, which might function to regulate activity-dependent processes that enable mechanisms required for the functional maturation and plasticity of the developing cortex and thereby contribute to the development of normal cortical organization.

Subplate cells: amplifiers of neuronal activity in the developing cerebral cortex

Due to their unique structural and functional properties, subplate cells are ideally suited to function as important amplifying units within the developing neocortical circuit. Subplate neurons have extensive dendritic and axonal ramifications and relatively mature functional properties, i.e. their action potential firing can exceed frequencies of 40 Hz. At earliest stages of corticogenesis subplate cells receive functional synaptic inputs from the thalamus and from other cortical and non-cortical sources. Glutamatergic and depolarizing GABAergic inputs arise from cortical neurons and neuromodulatory inputs arise from the basal forebrain and other sources. Activation of postsynaptic metabotropic receptors, i.e. muscarinic receptors, elicits in subplate neurons oscillatory burst discharges which are transmitted via electrical and chemical synapses to neighbouring subplate cells and to immature neurons in the cortical plate. The tonic non-synaptic release of GABA from GABAergic subplate cells facilitates the generation of burst discharges. These cellular bursts are amplified by prominent gap junction coupling in the subplate and cortical plate, thereby eliciting 10–20 Hz oscillations in a local columnar network. Thus, we propose that neuronal networks are organized at earliest stages in a gap junction coupled columnar syncytium. We postulate that the subplate does not only serve as a transient relay station for afferent inputs, but rather as an active element amplifying the afferent and intracortical activity.

Subplate neurons: crucial regulators of cortical development and plasticity

The developing cerebral cortex contains a distinct class of cells, subplate neurons, which form one of the first functional cortical circuits. Subplate neurons reside in the cortical white matter, receive thalamic inputs and project into the developing cortical plate, mostly to layer 4. Subplate neurons are present at key time points during development. Removal of subplate neurons profoundly affects cortical development. Subplate removal in visual cortex prevents the maturation of thalamocortical synapse, the maturation of inhibition in layer 4, the development of orientation selective responses in individual cortical neurons, and the formation of ocular dominance columns. In addition, monocular deprivation during development reveals that ocular dominance plasticity is paradoxical in the absence of subplate neurons. Because subplate neurons projecting to layer 4 are glutamatergic, these diverse deficits following subplate removal were hypothesized to be due to lack of feed-forward thalamic driven cortical excitation. A computational model of the developing thalamocortical pathway incorporating feed-forward excitatory subplate projections replicates both normal development and plasticity of ocular dominance as well as the effects of subplate removal. Therefore, we postulate that feed-forward excitatory projections from subplate neurons into the developing cortical plate enhance correlated activity between thalamus and layer 4 and, in concert with Hebbian learning rules in layer 4, allow maturational and plastic processes in layer 4 to commence. Thus subplate neurons are a crucial regulator of cortical development and plasticity, and damage to these neurons might play a role in the pathology of many neurodevelopmental disorders.

Three patterns of oscillatory activity differentially synchronize developing neocortical networks in vivo

Coordinated patterns of electrical activity are important for the early development of sensory systems. The spatiotemporal dynamics of these early activity patterns and the role of the peripheral sensory input for their generation are essentially unknown. We performed extracellular multielectrode recordings in the somatosensory cortex of postnatal day 0 to 7 rats in vivo and observed three distinct patterns of synchronized oscillatory activity. (1) Spontaneous and periphery-driven spindle bursts of 1-2 s in duration and approximately 10 Hz in frequency occurred approximately every 10 s. (2) Spontaneous and sensory-driven gamma oscillations of 150-300 ms duration and 30-40 Hz in frequency occurred every 10-30 s. (3) Long oscillations appeared only every approximately 20 min and revealed the largest amplitude (250-750 microV) and longest duration (>40 s). These three distinct patterns of early oscillatory activity differently synchronized the neonatal cortical network. Whereas spindle bursts and gamma oscillations did not propagate and synchronized a local neuronal network of 200-400 microm in diameter, long oscillations propagated with 25-30 microm/s and synchronized 600-800 microm large ensembles. All three activity patterns were triggered by sensory activation. Single electrical stimulation of the whisker pad or tactile whisker activation elicited neocortical spindle bursts and gamma activity. Long oscillations could be only evoked by repetitive sensory stimulation. The neonatal oscillatory patterns in vivo depended on NMDA receptor-mediated synaptic transmission and gap junctional coupling. Whereas spindle bursts and gamma oscillations may represent an early functional columnar-like pattern, long oscillations may serve as a propagating activation signal consolidating these immature neuronal networks.

GAP-43 is critical for normal targeting of thalamocortical and corticothalamic, but not trigeminothalamic axons in the whisker barrel system

Mice lacking the growth-associated protein GAP-43 (KO) show disrupted cortical topography and no barrels. Whisker-related patterns of cells are normal in the KO brainstem trigeminal complex (BSTC), while the pattern in KO ventrobasal thalamus (VB) is somewhat compromised. To better understand the basis for VB and cortical abnormalities, we used small placements of DiI to trace axonal projections between BSTC, VB, and barrel cortex in wildtype (WT) and GAP-43 KO mice. The trigeminothalamic (TT) pathway consists of axons from cells in the Nucleus Prinicipalis that project to the contralateral VB thalamus. DiI-labeled KO TT axons crossed the midline from BSTC and projected to contralateral VB normally, consistent with normal BSTC cytoarchitecture. By contrast, the KO thalamocortical axons (TCA) projection was highly abnormal. KO TCAs showed delays of 1-2 days in initial ingrowth to cortex. Postnatally, KO TCAs showed multiple pathfinding errors near intermediate targets, and were abnormally fasciculated within the internal capsule (IC). Interestingly, most individually labeled KO TCAs terminated in deep layers instead of in layer IV as in WT. This misprojection is consistent with birthdating analysis in KO mice, which revealed that neurons normally destined for layer IV remain in deep cortical layers. Early outgrowth of KO corticofugal (CF) axons was similar for both genotypes. However, at P7 KO CF fibers remained bundled as they entered the IC, and exhibited few terminal branches in VB. Thus, the establishment of axonal projections between thalamus and cortex are disrupted in GAP-43 KO mice.

Neurophysiologic evaluation of brain function in extremely premature newborn infants

In extremely preterm infants, neonatal brain injury and interruption of the normal maturation of the brain result in functional impairments that appear to manifest in later life. The roots of these impairments may be evaluated in the newborn infant using neurophysiologic techniques, such as evoked potentials and event-related potentials. This paper will review the use of neurophysiologic techniques as a marker of maturational processes in the preterm and newborn brain and as a method of monitoring the development of sensory and cognitive function in preterm infants, focusing on auditory perception, discrimination, and memory. The effects of risk conditions will be reviewed.

Spatio-temporal dynamics of oscillatory network activity in the neonatal mouse cerebral cortex

We used a 60-channel microelectrode array to study in thick (600-1000 microm) somatosensory cortical slices from postnatal day (P)0-P3 mice the spatio-temporal properties of early network oscillations. We recorded local non-propagating as well as large-scale propagating spontaneous oscillatory activity. Both types of activity patterns could never be observed in neocortical slices of conventional thickness (400 microm). Local non-propagating spontaneous oscillations with an average peak frequency of 15.6 Hz, duration of 1.7 s and maximal amplitude of 66.8 microV were highly synchronized in a network of approximately 200 microm in diameter. Spontaneous oscillations of lower frequency (10.4 Hz), longer duration (23.8 s) and larger amplitude (142.9 microV) propagated with 0.11 mm/s in the horizontal direction over at least 1 mm. These propagating oscillations were also synchronized in a columnar manner, but these waves synchronized the activity in a larger neuronal network of 300-400 microm in diameter. Both types of spontaneous network activity could be blocked by the gap junction antagonist carbenoxolone. Electrical stimulation of the subplate (SP) or bath application of the cholinergic agonist carbachol also elicited propagating network oscillations, emphasizing the role of the SP and the cholinergic system in the generation of early cortical network oscillations. Our data demonstrate that a sufficiently large network in thick neocortical slice preparations is capable of generating spontaneous and evoked network oscillations, which are highly synchronized via gap junctions in 200-400-microm-wide columns. These via synchronized oscillations coupled networks may represent a self-organized functional template for the activity-dependent formation of neocortical modules during the earliest stages of development.

Development of structural and functional connectivity in the thalamocortical somatosensory pathway in the wallaby

Neuronal activity is implicated as a driving force in the development of sensory systems. In order for it to play a developmental role, however, the pathways involved must be capable of transmitting this activity. The relationship between afferent arrival, synapse formation and the onset of chemical neurotransmission has been examined using the advantageous model of a marsupial mammal, the wallaby (Macropus eugenii), to determine at what stage activity has the capacity to influence cortical development. It is known that thalamocortical afferents arrive in the somatosensory cortex on postnatal day (P)15 and that their growth cones reach to the base of the compact cell zone of the cortical plate. However, electronmicroscopy showed that thalamocortical synapses were absent at this stage. Glutamatergic responses were recorded in the cortex following stimulation of the thalamus in slices at this time but only in magnesium-free conditions. The responses were mediated entirely by N-methyl-d-aspartate (NMDA) receptors. From P28, responses could be recorded in normal magnesium and comprised a dominant NMDA-mediated component and a non-NMDA mediated component. At this time thalamocortical synapses were first identified and they were in the cortical plate. By P63 the non-NMDA-mediated component had increased relative to the NMDA-mediated component, and by P70 layer IV began to emerge and contained thalamocortical synapses. By P76 a fast non-NMDA-mediated peak dominated the response. This coincides with the appearance of cortical whisker-related patches and the onset in vivo of responses to peripheral stimulation of the whiskers.

Neonatal SEP - back to bedside with basic science

Scalp-recorded somatosensory evoked potentials (SEPs) have been successfully used in neonatal assessment for several decades. The current routine SEP paradigm is markedly predictive for future cerebral palsy (CP) or other neurocognitive sequelae in brain-injured babies. Recent advances in basic science have dramatically increased our knowledge about structural-functional development of SEP-related brain mechanisms. It has thereby become apparent that preterm SEP differs from that in more mature counterparts in that it also comprises responses from transient brain structures, and hence being unique to the preterm period. It is now obvious also that several aspects in the current SEP paradigm, ranging from the type of stimulation to the methods of recording and analysis, are suboptimal for preterm babies. Recent progress in recording and analysis techniques have made it possible to combine SEP studies with EEG recordings, as well as to implement advanced analyses (e.g. time-frequency analysis) into routine practice. This review summarizes literature from relevant areas in basic science, and proposes a novel, integrated approach in neonatal SEP studies in order to significantly increase the fidelity of testing somatosensory system.

Histotypic mouse parietal cortex cultures: Excitation/inhibition ratio and ultrastructural analysis

Primary cultures of mouse parietal cortex, prepared between postnatal day 3 (P3) and P9, were studied using transmission electron microscopy and HPLC of excitatory (aspartate and glutamate) and inhibitory neurotransmitters (glycine, GABA and taurine) to determine their morphological and functional development. Relations between excitation and inhibition (E/I) were contrasted with ultrastructural features over the time course of in vitro development. After 6 days in vitro, cultured parietal cortex neurons prepared from mice at P3 had immature morphological characteristics, whereas P5 cultures showed a more developed histological structure but still with scarce synapses. The acquirement of histotypic characteristics was seen in P7 cultures, which contained numerous symmetric and asymmetric synaptic contacts. On P9, the cultures showed signs of tissue damage. In terms of neurotransmitter levels and E/I ratios, P7 cultures had relatively low E/I ratio as compared with the rest of the cultures prepared before or after P7. These results demonstrated that the development of inhibitory synaptic transmission, as indicated in the fall of E/I ratio, marked the maturation of cerebral cortical tissue and that the critical period to obtain histotypic cultures of mouse parietal cerebral cortex coincides between P5 and P7. This work provides useful information regarding the balance between excitation and inhibition as an indicative parameter for in vitro nerve cell survival, differentiation and maturation and reinforces the great value of histotypic cultures in the study of central nervous system development.

Effects in vitro and in vivo of a gap junction blocker on epileptiform activities in a genetic model of absence epilepsy

We investigated the effects of carbenoxolone (CBX), a gap junctions (GJ) blocker, on epileptiform activities in vivo and in vitro. In a first series of experiments, i.p. CBX decreased the cumulative duration of cortical spike-wave discharges (SWD) in adult Genetic Absence Epilepsy Rats from Strasbourg (GAERS) without reduction in the SW amplitude or frequency. Since SWD are generated in thalamocortical networks, we studied the effect of CBX on thalamic and cortical activities elicited by 4-aminopyridine (4AP) in thalamocortical slices from GAERS or non-epileptic rats (NER). Spontaneous ictal-like activities (ILA) were recorded simultaneously in thalamus and somatosensory cortex. However, experiments where these structures were surgically separated showed that ILA were generated in the cortex and recorded by volume conduction in the thalamus. GABA-dependent negative field potentials were also recorded in the cortex, either isolated or initiating ILA. After bath-applying CBX (100 microM), the frequency and cumulative duration of ILA decreased but less rapidly in GAERS than in NER slices and they disappeared at a time point when GABA-dependent negative potentials remained. These data suggest that GJ do not mediate the 4AP induced interneuronal synchronisation but may be implicated in the spreading of the synchronised activities from interneuronal networks to principal neurones. Our results show that CBX exerts an antiepileptic action in vivo, and that GJ blockers limits spread of synchronised activities in vitro. They may represent an appropriate target for development of new antiepileptic drugs.