Cellular physiology of the neonatal rat cerebral cortex: intrinsic membrane properties, sodium and calcium currents

Electrophysiology and Morphology of Human Cortical Supragranular Pyramidal Cells in a Wide Age Range

The basic excitatory neurons of the cerebral cortex, the pyramidal cells, are the most important signal integrators for the local circuit. They have quite characteristic morphological and electrophysiological properties that are known to be largely constant with age in the young and adult cortex. However, the brain undergoes several dynamic changes throughout life, such as in the phases of early development and cognitive decline in the aging brain. We set out to search for intrinsic cellular changes in supragranular pyramidal cells across a broad age range: from birth to 85 years of age and we found differences in several biophysical properties between defined age groups. During the first year of life, subthreshold and suprathreshold electrophysiological properties changed in a way that shows that pyramidal cells become less excitable with maturation, but also become temporarily more precise. According to our findings, the morphological features of the three-dimensional reconstructions from different life stages showed consistent morphological properties and systematic dendritic spine analysis of an infantile and an old pyramidal cell showed clear significant differences in the distribution of spine shapes. Overall, the changes that occur during development and aging may have lasting effects on the properties of pyramidal cells in the cerebral cortex. Understanding these changes is important to unravel the complex mechanisms underlying brain development, cognition and age-related neurodegenerative diseases.

Trem2 expression in microglia is required to maintain normal neuronal bioenergetics during development

Triggering receptor expressed on myeloid cells 2 (Trem2) is a myeloid cell-specific gene expressed in brain microglia, with variants that are associated with neurodegenerative diseases, including Alzheimer's disease. Trem2 is essential for microglia-mediated synaptic refinement, but whether Trem2 contributes to shaping neuronal development remains unclear. Here, we demonstrate that Trem2 plays a key role in controlling the bioenergetic profile of pyramidal neurons during development. In the absence of Trem2, developing neurons in the hippocampal cornus ammonis (CA)1 but not in CA3 subfield displayed compromised energetic metabolism, accompanied by reduced mitochondrial mass and abnormal organelle ultrastructure. This was paralleled by the transcriptional rearrangement of hippocampal pyramidal neurons at birth, with a pervasive alteration of metabolic, oxidative phosphorylation, and mitochondrial gene signatures, accompanied by a delay in the maturation of CA1 neurons. Our results unveil a role of Trem2 in controlling neuronal development by regulating the metabolic fitness of neurons in a region-specific manner.

Synaptic Origin of Early Sensory-evoked Oscillations in the Immature Thalamus

During the critical period of postnatal development, brain maturation is extremely sensitive to external stimuli. Newborn rodents already have functional somatosensory pathways and the thalamus, but the cortex is still forming. Immature thalamic synapses may produce large postsynaptic potentials in immature neurons, while non-synaptic membrane currents remain relatively weak and slow. The thalamocortical system generates spontaneous and evoked early gamma and spindle-burst oscillations in newborn rodents. How relatively strong synapses and weak intrinsic currents interact with each other and how they contribute to early thalamic activities remains largely unknown. Here, we performed local field potential (LFP), juxtacellular, and patch-clamp recordings in the somatosensory thalamus of urethane-anesthetized rat pups at postnatal days 6-7 with one whisker stimulation. We removed the overlying cortex and hippocampus to reach the thalamus with electrodes. Deflection of only one (the principal) whisker induced spikes in a particular thalamic cell. Whisker deflection evoked a group of large-amplitude excitatory events, likely originating from lemniscal synapses and multiple inhibitory postsynaptic events in thalamocortical cells. Large-amplitude excitatory events produced a group of spike bursts and could evoke a depolarization block. Juxtacellular recordings confirmed the partial inactivation of spikes. Inhibitory events prevented inactivation of action potentials and gamma-modulated neuronal firing. We conclude that the interplay of strong excitatory and inhibitory synapses and relatively weak intrinsic currents produces sensory-evoked early gamma oscillations in thalamocortical cells. We also propose that sensory-evoked large-amplitude excitatory events contribute to evoked spindle-bursts.

Axonal connections between S1 barrel, M1, and S2 cortex in the newborn mouse

The development of functionally interconnected networks between primary (S1), secondary somatosensory (S2), and motor (M1) cortical areas requires coherent neuronal activity via corticocortical projections. However, the anatomical substrate of functional connections between S1 and M1 or S2 during early development remains elusive. In the present study, we used ex vivo carbocyanine dye (DiI) tracing in paraformaldehyde-fixed newborn mouse brain to investigate axonal projections of neurons in different layers of S1 barrel field (S1Bf), M1, and S2 toward the subplate (SP), a hub layer for sensory information transfer in the immature cortex. In addition, we performed extracellular recordings in neocortical slices to unravel the functional connectivity between these areas. Our experiments demonstrate that already at P0 neurons from the cortical plate (CP), layer 5/6 (L5/6), and the SP of both M1 and S2 send projections through the SP of S1Bf. Reciprocally, neurons from CP to SP of S1Bf send projections through the SP of M1 and S2. Electrophysiological recordings with multi-electrode arrays in cortical slices revealed weak, but functional synaptic connections between SP and L5/6 within and between S1 and M1. An even lower functional connectivity was observed between S1 and S2. In summary, our findings demonstrate that functional connections between SP and upper cortical layers are not confined to the same cortical area, but corticocortical connection between adjacent cortical areas exist already at the day of birth. Hereby, SP can integrate early cortical activity of M1, S1, and S2 and shape the development of sensorimotor integration at an early stage.

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.

Pax6 limits the competence of developing cerebral cortical cells to respond to inductive intercellular signals

The development of stable specialized cell types in multicellular organisms relies on mechanisms controlling inductive intercellular signals and the competence of cells to respond to such signals. In developing cerebral cortex, progenitors generate only glutamatergic excitatory neurons despite being exposed to signals with the potential to initiate the production of other neuronal types, suggesting that their competence is limited. Here, we tested the hypothesis that this limitation is due to their expression of transcription factor Pax6. We used bulk and single-cell RNAseq to show that conditional cortex-specific Pax6 deletion from the onset of cortical neurogenesis allowed some progenitors to generate abnormal lineages resembling those normally found outside the cortex. Analysis of selected gene expression showed that the changes occurred in specific spatiotemporal patterns. We then compared the responses of control and Pax6-deleted cortical cells to in vivo and in vitro manipulations of extracellular signals. We found that Pax6 loss increased cortical progenitors’ competence to generate inappropriate lineages in response to extracellular factors normally present in developing cortex, including the morphogens Shh and Bmp4. Regional variation in the levels of these factors could explain spatiotemporal patterns of fate change following Pax6 deletion in vivo. We propose that Pax6’s main role in developing cortical cells is to minimize the risk of their development being derailed by the potential side effects of morphogens engaged contemporaneously in other essential functions.

The development of stable specialized cell types in multicellular organisms relies on mechanisms controlling inductive intercellular signals and the competence of cells to respond. This study shows that cortical development is stabilized by the protective actions of the transcription factor Pax6, which adjusts the ability of cortical cells to respond to potentially destabilizing signals present in their local environment.

A direct excitatory projection from entorhinal layer 6b neurons to the hippocampus contributes to spatial coding and memory

The mammalian hippocampal formation (HF) plays a key role in several higher brain functions, such as spatial coding, learning and memory. Its simple circuit architecture is often viewed as a trisynaptic loop, processing input originating from the superficial layers of the entorhinal cortex (EC) and sending it back to its deeper layers. Here, we show that excitatory neurons in layer 6b of the mouse EC project to all sub-regions comprising the HF and receive input from the CA1, thalamus and claustrum. Furthermore, their output is characterized by unique slow-decaying excitatory postsynaptic currents capable of driving plateau-like potentials in their postsynaptic targets. Optogenetic inhibition of the EC-6b pathway affects spatial coding in CA1 pyramidal neurons, while cell ablation impairs not only acquisition of new spatial memories, but also degradation of previously acquired ones. Our results provide evidence of a functional role for cortical layer 6b neurons in the adult brain.

Spontaneous Activity Predicts Survival of Developing Cortical Neurons

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

Modelling the spatial and temporal constrains of the GABAergic influence on neuronal excitability

GABA (γ-amino butyric acid) is an inhibitory neurotransmitter in the adult brain that can mediate depolarizing responses during development or after neuropathological insults. Under which conditions GABAergic membrane depolarizations are sufficient to impose excitatory effects is hard to predict, as shunting inhibition and GABAergic effects on spatiotemporal filtering of excitatory inputs must be considered. To evaluate at which reversal potential a net excitatory effect was imposed by GABA (E_GABA^Thr), we performed a detailed in-silico study using simple neuronal topologies and distinct spatiotemporal relations between GABAergic and glutamatergic inputs.

These simulations revealed for GABAergic synapses located at the soma an E_GABA^Thr close to action potential threshold (E_AP^Thr), while with increasing dendritic distance E_GABA^Thr shifted to positive values. The impact of GABA on AMPA-mediated inputs revealed a complex temporal and spatial dependency. E_GABA^Thr depends on the temporal relation between GABA and AMPA inputs, with a striking negative shift in E_GABA^Thr for AMPA inputs appearing after the GABA input. The spatial dependency between GABA and AMPA inputs revealed a complex profile, with E_GABA^Thr being shifted to values negative to E_AP^Thr for AMPA synapses located proximally to the GABA input, while for distally located AMPA synapses the dendritic distance had only a minor effect on E_GABA^Thr. For tonic GABAergic conductances E_GABA^Thr was negative to E_AP^Thr over a wide range of g_GABA^tonic values. In summary, these results demonstrate that for several physiologically relevant situations E_GABA^Thr is negative to E_AP^Thr, suggesting that depolarizing GABAergic responses can mediate excitatory effects even if E_GABA did not reach E_AP^Thr.

The neurotransmitter GABA mediates an inhibitory action in the mature brain, while it was found that GABA provokes depolarizations in the immature brain or after neurological insults. It is, however, not clear to which extend these GABAergic depolarizations can contribute to an excitatory effect. In the present manuscript we approached this question with a computational model of a simplified neurons to determine what amount of a GABAergic depolarizing effect, which we quantified by the so called GABA reversal potential (E_GABA), was required to turn GABAergic inhibition to excitation. The results of our simulations revealed that if GABA was applied alone a GABAergic excitation was induced when E_GABA was around the action potential threshold. When GABA was applied together with additional excitatory inputs, which is the physiological situation in the brain, only for spatially and temporally correlated inputs E_GABA was close to the action potential threshold. For situations in which the additional excitatory inputs appear after the GABA input or are distant to the GABA input, an excitatory effect of GABA could be observed already at E_GABA substantially negative to the action potential threshold. This results indicate that even slightly depolarizing GABA responses, which may be induced during or after neurological insults, can potentially turn GABAergic inhibition into GABAergic excitation.

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.

Methadone Suppresses Neuronal Function and Maturation in Human Cortical Organoids

Accumulating evidence has suggested that prenatal exposure to methadone causes multiple adverse effects on human brain development. Methadone not only suppresses fetal neurobehavior and alters neural maturation, but also leads to long-term neurological impairment. Due to logistical and ethical issues of accessing human fetal tissue, the effect of methadone on brain development and its underlying mechanisms have not been investigated adequately and are therefore not fully understood. Here, we use human cortical organoids which resemble fetal brain development to examine the effect of methadone on neuronal function and maturation during early development. During development, cortical organoids that are exposed to clinically relevant concentrations of methadone exhibited suppressed maturation of neuronal function. For example, organoids developed from 12th week till 24th week have an about 7-fold increase in AP firing frequency, but only half and a third of this increase was found in organoids exposed to 1 and 10 μM methadone, respectively. We further demonstrated substantial increases in I_Na (4.5-fold) and I_KD (10.8-fold), and continued shifts of Na⁺ channel activation and inactivation during normal organoid development. Methadone-induced suppression of neuronal function was attributed to the attenuated increase in the densities of I_Na and I_KD and the reduced shift of Na⁺ channel gating properties. Since normal neuronal electrophysiology and ion channel function are critical for regulating brain development, we believe that the effect of prolonged methadone exposure contributes to the delayed maturation, development fetal brain and potentially for longer term neurologic deficits.

Subplate Neurons as an Organizer of Mammalian Neocortical Development

Subplate neurons (SpNs) are one of the earliest born and matured neurons in the developing cerebral cortex and play an important role in the early development of the neocortex. It has been known that SpNs have an essential role in thalamocortical axon (TCA) pathfinding and the establishment of the first neural circuit from the thalamus towards cortical layer IV. In addition to this function, it has recently been revealed in mouse corticogenesis that SpNs play an important role in the regulation of radial neuronal migration during the mid-embryonic stage. Moreover, accumulating studies throw light on the possible roles of SpNs in adult brain functions and also their involvement in psychiatric or other neurological disorders. As SpNs are unique to mammals, they may have contributed to the evolution of the mammalian neocortex by efficiently organizing cortical formation during the limited embryonic period of corticogenesis. By increasing our knowledge of the functions of SpNs, we will clarify how SpNs act as an organizer of mammalian neocortical formation.

Electrophysiological properties of neurons grown on soft polymer scaffolds reveal the potential to develop neuromimetic culture environments

Tissue engineering methodologies for various physiological systems are seeing a significant trend towards 3D cell culture in or on 'soft' polymeric hydrogel materials, widely considered to provide a more biomimetic environment for cell growth versus 'hard' materials such as glass or plastic. Progress has been slower with 3D neural cell culture with current studies overwhelmingly reliant on hard substrates. Accordingly, our knowledge of the alterations in electrochemical properties of neurons propagated in soft materials is relatively limited. In this study, primary cortical neurons and glial cells were seeded onto the surface of collagen hydrogels and grown in vitro for 7-8 days. At this time, neurons had formed a complex neurite web interspersed with astrocytes. Neuronal patch clamp recordings revealed voltage-gated Na+ and K+ currents in voltage clamp and action potentials in current clamp. When measured at voltages close to maximum activation, both currents were >1 nA in mean amplitude. When compared to primary cortical neurons cultured on glass coverslips, but otherwise under similar conditions (Evans et al., 2017), the Na+ current from hydrogel neurons was found to be significantly larger although there were no differences in the K+ current amplitude, membrane potential, input resistance or cell capacitance. We speculate that the larger size of the neuronal voltage-dependent Na+ current in the hydrogels is related to the better biomimetic properties of the soft material, being close to values reported for neurons recorded in brain slices. The results highlight the potential benefits offered by neuronal culture on soft and biomimetic polymeric materials for neural tissue engineering studies.

Developmental Aspects of Glucose and Calcium Availability on the Persistence of Memory Function Over the Lifespan

An important aspect concerning the underlying nature of memory function is an understanding of how memories are acquired and lost. The stability, and ultimate demise, of memory over the lifespan of an organism remains a critical topic in determining the neurobiological mechanisms that mediate memory representations. This has important implications for the elucidation and treatment of neurodegenerative diseases such as Alzheimer's disease (AD). One important question in the context of preserving functional plasticity over the lifespan is the determination of the neurobiological structural and functional changes that contribute to the formation of memory during the juvenile time frame that might provide protection against later memory dysfunction by promoting the establishment of redundant neural pathways. The main question being, if memory formation during the juvenile period does strengthen and preserve memory stability over the lifespan, what are the neurobiological structural or functional substrates that mediate this effect? One neural attribute whose function may be altered with early life experience and provide a mechanism to preserve memory through the lifespan is glucose transport-linked calcium (Ca2+) buffering. Because peak increases in glucose utilization overlap with a timeframe during which spatial training can enhance later memory processing, it might be the case that learning-associated changes in glucose utilization would provide an important neural functional change to preserve memory function throughout the lifespan. The glucose transporters are proteins that are reduced in AD pathology and there is evidence that glucose reductions can impair Ca2+ buffering. In the absence of an appropriate supply of ATP, provided via glucose transport and glycolysis, Ca2+ levels can rise leading to neural vulnerability with ensuing pathological outcomes. In this review, we explore the hypothesis that enhancing glucose utilization with spatial training during the preadolescent period will provide a functional enhancement that regulates glucose-dependent Ca2+ signaling during aging or neurodegeneration and provide essential neural resources to preserve functional plasticity and memory function.

Long-range projections from sparse populations of GABAergic neurons in murine subplate

The murine subplate contains some of the earliest generated populations of neurons in the cerebral cortex, which play an important role in the maturation of cortical inhibition. Here we present multiple lines of evidence, that the subplate itself is only very sparsely populated with GABAergic neurons at postnatal day (P)8. We used three different transgenic mouse lines, each of which labels a subset of GABAergic, ganglionic eminence derived neurons. Dlx5/6-eGFP labels the most neurons in cortex (on average 11% of NEUN+ cells across all layers at P8) whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are the sparsest (2% of NEUN+ cells across all layers at P8). There is significant variability in the layer distribution of labeled interneurons, with Dlx5/6-eGFP and Lhx6-Cre::R26R-YFP being expressed most abundantly in Layer 5, whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are least abundant in that layer. All three lines label at most 3% of NEUN+ neurons in the subplate, in contrast to L5, in which up to 30% of neurons are GFP+ in Dlx5/6-eGFP. We assessed all three GABAergic populations for expression of the subplate neuron marker connective tissue growth factor (CTGF). CTGF labels up to two-thirds of NEUN+ cells in the subplate, but was never found to colocalize with labeled GABAergic neurons in any of the three transgenic strains. Despite the GABAergic neuronal population in the subplate being sparse, long-distance axonal connection tracing with carbocyanine dyes revealed that some Gad65-GFP+ subplate cells form long-range axonal projections to the internal capsule or callosum.

Kcnab1 Is Expressed in Subplate Neurons With Unilateral Long-Range Inter-Areal Projections

Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel (Kcnab1) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.

Interactions between Membrane Resistance, GABA-A Receptor Properties, Bicarbonate Dynamics and Cl--Transport Shape Activity-Dependent Changes of Intracellular Cl- Concentration

The effects of ionotropic γ-aminobutyric acid receptor (GABA-A, GABA_A) activation depends critically on the Cl⁻-gradient across neuronal membranes. Previous studies demonstrated that the intracellular Cl⁻-concentration ([Cl⁻]_i) is not stable but shows a considerable amount of activity-dependent plasticity. To characterize how membrane properties and different molecules that are directly or indirectly involved in GABAergic synaptic transmission affect GABA-induced [Cl⁻]_i changes, we performed compartmental modeling in the NEURON environment. These simulations demonstrate that GABA-induced [Cl⁻]_i changes decrease at higher membrane resistance, revealing a sigmoidal dependency between both parameters. Increase in GABAergic conductivity enhances [Cl⁻]_i with a logarithmic dependency, while increasing the decay time of GABA_A receptors leads to a nearly linear enhancement of the [Cl⁻]_i changes. Implementing physiological levels of HCO₃⁻-conductivity to GABA_A receptors enhances the [Cl⁻]_i changes over a wide range of [Cl⁻]_i, but this effect depends on the stability of the HCO₃⁻ gradient and the intracellular pH. Finally, these simulations show that pure diffusional Cl⁻-elimination from dendrites is slow and that a high activity of Cl⁻-transport is required to improve the spatiotemporal restriction of GABA-induced [Cl⁻]_i changes. In summary, these simulations revealed a complex interplay between several key factors that influence GABA-induced [Cl]_i changes. The results suggest that some of these factors, including high resting [Cl⁻]_i, high input resistance, slow decay time of GABA_A receptors and dynamic HCO₃⁻ gradient, are specifically adapted in early postnatal neurons to facilitate limited activity-dependent [Cl⁻]_i decreases.

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.

Putative Role of Taurine as Neurotransmitter During Perinatal Cortical Development

Neurotransmitters and neuronal activity affect neurodevelopmental events like neurogenesis, neuronal migration, apoptosis and differentiation. Beside glutamate and gamma-amino butyric acid, the aminosulfonic acid taurine has been considered as possible neurotransmitter that influences early neuronal development. In this article I review recent studies of our group which demonstrate that taurine can affect a variety of identified neuronal populations in the immature neocortex and directly modulates neuronal activity. These experiments revealed that taurine evoke dose-dependent membrane responses in a variety of neocortical neuron populations, including Cajal-Retzius cells, subplate neurons and GABAergic interneurons. Taurine responses persist in the presence of GABA(A) receptor antagonists and are reduced by the addition of strychnine, suggesting that glycine receptors are involved in taurine-mediated membrane responses. Gramicidin-perforated patch-clamp and cell-attached recordings demonstrated that taurine evokes depolarizing and mainly excitatory membrane responses, in accordance with the high intracellular Cl- concentration in immature neurons. In addition, taurine increases the frequency of postsynaptic GABAergic currents (PSCs) in a considerable fraction of immature pyramidal neurons, indicating a specific activation of presynaptic GABAergic networks projecting toward and exciting pyramidal neurons. In summary, these results suggest that taurine may be critically involved in the regulation of network excitability in the immature neocortex and hippocampus via interactions with glycine receptors.

Cajal-Retzius cells and GABAergic interneurons of the developing hippocampus: Close electrophysiological encounters of the third kind

In contrast to the large number of studies investigating the electrophysiological properties and synaptic connectivity of hippocampal pyramidal neurons, granule cells, and GABAergic interneurons, much less is known about Cajal-Retzius cells. In this review article, we discuss the possible reasons underlying this difference, and review experimental work performed on this cell type in the hippocampus, comparing it with results obtained in the neocortex. Our main emphasis is on data obtained with in vitro electrophysiology. In particular, we address the bidirectional connectivity between Cajal-Retzius cells and GABAergic interneurons, examine their synaptic properties and propose specific functions of Cajal-Retzius cell/GABAergic interneuron microcircuits. Lastly, we discuss the potential involvement of these microcircuits in critical physiological hippocampal functions such as postnatal neurogenesis or pathological scenarios such as temporal lobe epilepsy.

ATP released from astrocytes modulates action potential threshold and spontaneous excitatory postsynaptic currents in the neonatal rat prefrontal cortex

Maternal immune activation during pregnancy is a risk factor for neurodevelopmental disorders, such as schizophrenia; however, a full mechanistic understanding has yet to be established. The activity of a transient cell population, the subplate neurons, is critical for the development of cortical inhibition and functional thalamocortical connections. Sensitivity of these cells to factors released during inflammation, therefore, may offer a link between maternal immune activation and the aberrant cortical development underlying some neuropsychiatric disorders. An elevated extracellular ATP concentration is associated with inflammation and has been shown to have an effect on neuronal activity. Here, we investigated the effect of ATP on the electrophysiological properties of subplate neurons. Exogenous ATP increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) at micromolar concentrations. Further, ATP released by astrocytes activated by the PAR-1 agonist, TFLLR-NH₂, also increased the amplitude and frequency of sEPSCs in subplate neurons. The electrophysiological properties of subplate neurons recorded from prefrontal cortical (PFC) slices from neonatal rats were also disrupted in a maternal immune activation rat model of schizophrenia, with a suramin-sensitive increase in frequency and amplitude of sEPSCs. An alternative neurodevelopmental rat model of schizophrenia, MAM-E17, which did not rely on maternal immune activation, however, showed no change in subplate neuron activity. Both models were validated with behavioral assays, showing schizophrenia-like endophenotypes in young adulthood. The purinergic modulation of subplate neuron activity offers a potential explanatory link between maternal immune activation and disruptions in cortical development that lead to the emergence of neuropsychiatric disorders.

Neuronal activity patterns in the developing barrel cortex

The developing barrel cortex reveals a rich repertoire of neuronal activity patterns, which have been also found in other sensory neocortical areas and in other species including the somatosensory cortex of preterm human infants. The earliest stage is characterized by asynchronous, sparse single-cell firing at low frequencies. During the second stage neurons show correlated firing, which is initially mediated by electrical synapses and subsequently transforms into network bursts depending on chemical synapses. Activity patterns during this second stage are synchronous plateau assemblies, delta waves, spindle bursts and early gamma oscillations (EGOs). In newborn rodents spindle bursts and EGOs occur spontaneously or can be elicited by sensory stimulation and synchronize the activity in a barrel-related columnar network with topographic organization at the day of birth. Interfering with this early activity causes a disturbance in the development of the cortical architecture, indicating that spindle bursts and EGOs influence the formation of cortical columns. Early neuronal activity also controls the rate of programed cell death in the developing barrel cortex, suggesting that spindle bursts and EGOs are physiological activity patterns particularly suited to suppress apoptosis. It remains to be studied in more detail how these different neocortical activity patterns control early developmental processes such as formation of synapses, microcircuits, topographic maps and large-scale networks.

Optogenetics reveal delayed afferent synaptogenesis on grafted human-induced pluripotent stem cell-derived neural progenitors

Reprogramming of somatic cells into pluripotency stem cell state has opened new opportunities in cell replacement therapy and disease modeling in a number of neurological disorders. It still remains unknown, however, to what degree the grafted human-induced pluripotent stem cells (hiPSCs) differentiate into a functional neuronal phenotype and if they integrate into the host circuitry. Here, we present a detailed characterization of the functional properties and synaptic integration of hiPSC-derived neurons grafted in an in vitro model of hyperexcitable epileptic tissue, namely organotypic hippocampal slice cultures (OHSCs), and in adult rats in vivo. The hiPSCs were first differentiated into long-term self-renewing neuroepithelial stem (lt-NES) cells, which are known to form primarily GABAergic neurons. When differentiated in OHSCs for 6 weeks, lt-NES cell-derived neurons displayed neuronal properties such as tetrodotoxin-sensitive sodium currents and action potentials (APs), as well as both spontaneous and evoked postsynaptic currents, indicating functional afferent synaptic inputs. The grafted cells had a distinct electrophysiological profile compared to host cells in the OHSCs with higher input resistance, lower resting membrane potential, and APs with lower amplitude and longer duration. To investigate the origin of synaptic afferents to the grafted lt-NES cell-derived neurons, the host neurons were transduced with Channelrhodopsin-2 (ChR2) and optogenetically activated by blue light. Simultaneous recordings of synaptic currents in grafted lt-NES cell-derived neurons using whole-cell patch-clamp technique at 6 weeks after grafting revealed limited synaptic connections from host neurons. Longer differentiation times, up to 24 weeks after grafting in vivo, revealed more mature intrinsic properties and extensive synaptic afferents from host neurons to the lt-NES cell-derived neurons, suggesting that these cells require extended time for differentiation/maturation and synaptogenesis. However, even at this later time point, the grafted cells maintained a higher input resistance. These data indicate that grafted lt-NES cell-derived neurons receive ample afferent input from the host brain. Since the lt-NES cells used in this study show a strong propensity for GABAergic differentiation, the host-to-graft synaptic afferents may facilitate inhibitory neurotransmitter release, and normalize hyperexcitable neuronal networks in brain diseases, for example, such as epilepsy.

Effects of Atrazine on the Development of Neural System of Zebrafish, Danio rerio

By comparative analysis of histomorphology and AChE activity, the changes of physiological and biochemical parameters were determined in zebrafish embryos and larvae dealt with atrazine (ATR) at different concentrations (0.0001, 0.001, 0.01, 0.1, and 1 mg/L). This study showed that the development of the sarcomere and the arrangement of white muscle myofibers were affected by ATR significantly and the length of sarcomere shortened. Further analysis of the results showed that the AChE activity in juvenile fish which was treated with ATR was downregulated, which can indicate that the innervation efficiency to the muscle was impaired. Conversely, the AChE activity in zebrafish embryos which was treated with ATR was upregulated. A parallel phenomenon showed that embryonic primary sensory neurons (Rohon-Beard cells), principally expressing AChE in embryos, survived the physiological apoptosis. These phenomena demonstrated that the motor integration ability of the zebrafish was damaged by ATR which can disturb the development of sensory neurons and sarcomere and the innervations of muscle.

Control of Spontaneous Ca2+ Transients Is Critical for Neuronal Maturation in the Developing Neocortex

Neural activity plays roles in the later stages of development of cortical excitatory neurons, including dendritic and axonal arborization, remodeling, and synaptogenesis. However, its role in earlier stages, such as migration and dendritogenesis, is less clear. Here we investigated roles of neural activity in the maturation of cortical neurons, using calcium imaging and expression of prokaryotic voltage-gated sodium channel, NaChBac. Calcium imaging experiments showed that postmigratory neurons in layer II/III exhibited more frequent spontaneous calcium transients than migrating neurons. To test whether such an increase of neural activity may promote neuronal maturation, we elevated the activity of migrating neurons by NaChBac expression. Elevation of neural activity impeded migration, and induced premature branching of the leading process before neurons arrived at layer II/III. Many NaChBac-expressing neurons in deep cortical layers were not attached to radial glial fibers, suggesting that these neurons had stopped migration. Morphological and immunohistochemical analyses suggested that branched leading processes of NaChBac-expressing neurons differentiated into dendrites. Our results suggest that developmental control of spontaneous calcium transients is critical for maturation of cortical excitatory neurons in vivo: keeping cellular excitability low is important for migration, and increasing spontaneous neural activity may stop migration and promote dendrite formation.

Cajal-Retzius cells: update on structural and functional properties of these mystic neurons that bridged the 20th century

Cajal-Retzius cells (CRc) represent a mostly transient neuronal cell type localized in the uppermost layer of the developing neocortex. The observation that CRc are a major source of the extracellular matrix protein reelin, which is essential for the laminar development of the cerebral cortex, attracted the interest in this unique cell type. In this review we will (i) describe the morphological and molecular properties of neocortical CRc, with a special emphasize on the question which markers can be used to identify CRc, (ii) summarize reports that identified the different developmental origins of CRc, (iii) discuss the fate of CRc, including recent evidence for apoptotic cell death and a possible persistence of some CRc, (iv) provide a detailed description of the electrical membrane properties and transmitter receptors of CRc, and (v) address the role of CRc in early neuronal circuits and cortical development. Finally, we speculate whether CRc may provide a link between early network activity and the structural maturation of neocortical circuits.

Taurine activates GABAergic networks in the neocortex of immature mice

Although it has been suggested that taurine is the main endogenous neurotransmitter acting on glycine receptors, the implications of glycine receptor-mediated taurine actions on immature neocortical networks have not been addressed yet. To investigate the influence of taurine on the excitability of neuronal networks in the immature neocortex, we performed whole-cell patch-clamp recordings from visually identified pyramidal neurons and interneurons in coronal slices from C57Bl/6 and GAD67-green fluorescent protein (GFP) transgenic mice (postnatal days 2–4). In 46% of the pyramidal neurons bath-application of taurine at concentrations ≥ 300 μM significantly enhanced the frequency of postsynaptic currents (PSCs) by 744.3 ± 93.8% (n = 120 cells). This taurine-induced increase of PSC frequency was abolished by 0.2 μM tetrodotoxin (TTX), 1 μM strychnine or 3 μM gabazine, but was unaffected by the glutamatergic antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and (±) R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP), suggesting that taurine specifically activates GABAergic network activity projecting to pyramidal neurons. Cell-attached recordings revealed that taurine enhanced the frequency of action potentials (APs) in pyramidal neurons, indicating an excitatory action of the GABAergic PSCs. In order to identify the presynaptic targets of taurine we demonstrate that bath application of taurine induced in GAD67-GFP labeled interneurons an inward current that is mainly mediated by glycine receptors and can generate APs in these cells. We conclude from these results that taurine can enhance network excitability in the immature neocortex by selectively activating GABAergic interneurons via interactions with glycine receptors.

Resonance properties of GABAergic interneurons in immature GAD67-GFP mouse neocortex

Subthreshold resonance is a characteristic membrane property of different neuronal classes, is critically involved in the generation of network oscillations, and tunes the integration of synaptic inputs to particular frequency ranges. In order to investigate whether neocortical GABAergic interneurons show resonant behavior already during early postnatal development, we performed whole-cell patch-clamp recordings from visually identified interneurons in supragranular layers of parietal regions in coronal neocortical slices from postnatal day (P) P6-P13 GAD67-GFP knock-in mice. Subthreshold resonance was analyzed by injection of sinusoidal current with varying frequency. About 50% of the investigated GABAergic interneurons showed subthreshold resonance with an average frequency of 2.0±0.2 Hz (n=38). Membrane hyperpolarization to -86 mV attenuated the frequency and strength of subthreshold resonance. In the presence of 1 mM Ni(2+) subthreshold resonance was virtually abolished, suggesting that T-type Ca(2+) currents are critically involved in the generation of resonance. In contrast, subthreshold resonance was not affected by ZD7288, a blocker of HCN channels. Application of TTX suppressed subthreshold resonance at depolarized, but not hyperpolarized membrane potential, suggesting that persistent Na(+) current contribute to the amplification of membrane resonance. In summary, these results demonstrate that GABAergic interneurons express subthreshold resonance at low frequencies, with T-type Ca(2+) and persistent Na(+) currents underlying the generation of membrane resonance. The membrane resonance of immature interneurons may contribute to the generation of slow oscillatory activity pattern in the immature neocortex and enhance the temporal precision of synaptic integration in developing cortical neurons.

Dysfunction of KCNK potassium channels impairs neuronal migration in the developing mouse cerebral cortex

Development of the cerebral cortex depends partly on neural activity, but the identity of the ion channels that might contribute to the activity-dependent cortical development is unknown. KCNK channels are critical determinants of neuronal excitability in the mature cerebral cortex, and a member of the KCNK family, KCNK9, is responsible for a maternally transmitted mental retardation syndrome. Here, we have investigated the roles of KCNK family potassium channels in cortical development. Knockdown of KCNK2, 9, or 10 by RNAi using in utero electroporation impaired the migration of late-born cortical excitatory neurons destined to become Layer II/III neurons. The migration defect caused by KCNK9 knockdown was rescued by coexpression of RNAi-resistant functional KCNK9 mutant. Furthermore, expression of dominant-negative mutant KCNK9, responsible for the disease, and electrophysiological experiments demonstrated that ion channel function was involved in the migration defect. Calcium imaging revealed that KCNK9 knockdown or expression of dominant-negative mutant KCNK9 increased the fraction of neurons showing calcium transients and the frequency of spontaneous calcium transients. Mislocated neurons seen after KCNK9 knockdown stayed in the deep cortical layers, showing delayed morphological maturation. Taken together, our results suggest that dysfunction of KCNK9 causes a migration defect in the cortex via an activity-dependent mechanism.

A role for silent synapses in the development of the pathway from layer 2/3 to 5 pyramidal cells in the neocortex

The integration of neurons within the developing cerebral cortex is a prolonged process dependent on a combination of molecular and physiological cues. To examine the latter we used laser scanning photostimulation (LSPS) of caged glutamate in conjunction with whole-cell patch-clamp electrophysiology to probe the integration of pyramidal cells in the sensorimotor regions of the mouse neocortex. In the days immediately after postnatal day 5 (P5) the origin of the LSPS-evoked AMPA receptor (AMPAR)-mediated synaptic inputs were diffuse and poorly defined with considerable variability between cells. Over the subsequent week this coalesced and shifted, primarily influenced by an increased contribution from layers 2/3 cells, which became a prominent motif of the afferent input onto layer 5 pyramidal cells regardless of cortical region. To further investigate this particular emergent translaminar connection, we alternated our mapping protocol between two holding potentials (-70 and +40 mV) allowing us to detect exclusively NMDA receptor (NMDAR)-mediated inputs. This revealed distal MK-801-sensitive synaptic inputs that predict the formation of the mature, canonical layer 2/3 to 5 pathway. However, these were a transient feature and had been almost entirely converted to AMPAR synapses at a later age (P16). To examine the role of activity in the recruitment of early NMDAR synapses, we evoked brief periods (20 min) of rhythmic bursting. Short intense periods of activity could cause a prolonged augmentation of the total input onto pyramidal cells up until P12; a time point when the canonical circuit has been instated and synaptic integration shifts to a more consolidatory phase.

Resonance properties of different neuronal populations in the immature mouse neocortex

In vivo recordings in the immature neocortex revealed spontaneous and sensory-driven oscillatory activity from delta (0.5-4 Hz) to gamma (30-100 Hz) frequencies. In order to investigate whether the resonance properties of distinct neuronal populations in the immature neocortex contribute to these network oscillations, we performed whole-cell patch-clamp recordings from visually identified neurons in tangential and coronal neocortical slices from postnatal day (P)0-P7 C57Bl/6 mice. Subthreshold resonance was analysed by sinusoidal current injection of varying frequency. All Cajal-Retzius cells showed subthreshold resonance, with an average frequency of 2.6 ± 0.1 Hz (n = 60), which was massively reduced by ZD7288, a blocker of hyperpolarization-activated cation currents. Approximately 65.6% (n = 61) of the supragranular pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.1 Hz (n = 40). Application of Ni(2+) suppressed subthreshold resonance, suggesting that low-threshold calcium currents contribute to resonance in these neurons. Approximately 63.6% (n = 77) of the layer V pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.2 Hz (n = 49), which was abolished by ZD7288. Only 44.1% (n = 59) of the subplate neurons showed subthreshold resonance, with an average frequency of 1.3 ± 0.2 Hz (n = 26) and a small resonance strength. In summary, these results demonstrate that neurons in all investigated layers show resonance behavior, with either hyperpolarization-activated cation or low-threshold calcium currents contributing to the subthreshold resonance. The observed resonance frequencies are in the range of slow activity patterns observed in the immature neocortex, suggesting that subthreshold resonance may support the generation of this activity.

5-HT(3A) receptor-bearing white matter interstitial GABAergic interneurons are functionally integrated into cortical and subcortical networks

In addition to axons and surrounding glial cells, the corpus callosum also contains interstitial neurons that constitute a heterogeneous cell population. There is growing anatomical evidence that white matter interstitial cells (WMICs) comprise GABAergic interneurons, but so far there is little functional evidence regarding their connectivity. The scarcity of these cells has hampered electrophysiological studies. We overcame this hindrance by taking recourse to transgenic mice in which distinct WMICs expressed enhanced green fluorescence protein (EGFP). The neuronal phenotype of the EGFP-labeled WMICs was confirmed by their NeuN positivity. The GABAergic phenotype could be established based on vasoactive intestinal peptide and calretinin expression and was further supported by a firing pattern typical for interneurons. Axons and dendrites of many EGFP-labeled WMICs extended to the cortex, hippocampus, and striatum. Patch-clamp recordings in acute slices showed that they receive excitatory and inhibitory input from cortical and subcortical structures. Moreover, paired recordings revealed that EGFP-labeled WMICs inhibit principal cells of the adjacent cortex, thus providing unequivocal functional evidence for their GABAergic phenotype and demonstrating that they are functionally integrated into neuronal networks.

Hypothesis on the dual origin of the Mammalian subplate

The development of the mammalian neocortex relies heavily on subplate. The proportion of this cell population varies considerably in different mammalian species. Subplate is almost undetectable in marsupials, forms a thin, but distinct layer in mouse and rat, a larger layer in carnivores and big-brained mammals as pig, and a highly developed embryonic structure in human and non-human primates. The evolutionary origin of subplate neurons is the subject of current debate. Some hypothesize that subplate represents the ancestral cortex of sauropsids, while others consider it to be an increasingly complex phylogenetic novelty of the mammalian neocortex. Here we review recent work on expression of several genes that were originally identified in rodent as highly and differentially expressed in subplate. We relate these observations to cellular morphology, birthdating, and hodology in the dorsal cortex/dorsal pallium of several amniote species. Based on this reviewed evidence we argue for a third hypothesis according to which subplate contains both ancestral and newly derived cell populations. We propose that the mammalian subplate originally derived from a phylogenetically ancient structure in the dorsal pallium of stem amniotes, but subsequently expanded with additional cell populations in the synapsid lineage to support an increasingly complex cortical plate development. Further understanding of the detailed molecular taxonomy, somatodendritic morphology, and connectivity of subplate in a comparative context should contribute to the identification of the ancestral and newly evolved populations of subplate neurons.

Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis

The operation of neuronal networks depends on the firing patterns of the network's neurons. When sustained current is injected, some neurons in the central nervous system fire a single action potential and others fire repetitively. For example, in Xenopus laevis tadpoles, primary-sensory Rohon-Beard (RB) neurons fired a single action potential in response to 300-ms rheobase current injections, whereas dorsolateral (DL) interneurons fired repetitively at 10-20 Hz. To investigate the basis for these differences in vivo, we examined drug-induced changes in the firing patterns of Xenopus spinal neurons using whole cell current-clamp recordings. Neuron types were initially separated through cluster analysis, and we compared results produced using different clustering algorithms. We used these results to develop a predictive function to classify subsequently recorded neurons. The potassium channel blocker tetraethylammonium (TEA) converted single-firing RB neurons to low-frequency repetitive firing but reduced the firing frequency of repetitive-firing DL interneurons. Firing frequency in DL interneurons was also reduced by the potassium channel blockers 4-aminopyridine (4-AP), catechol, and margatoxin; 4-AP had the greatest effect. The calcium channel blockers amiloride and nimodipine had few effects on firing in either neuron type but reduced action potential duration in DL interneurons. Muscarine, which blocks M-currents, did not affect RB neurons but reduced firing frequency in DL interneurons. These results suggest that potassium currents may control neuron firing patterns: a TEA-sensitive current prevents repetitive firing in RB neurons, whereas a 4-AP-sensitive current underlies repetitive firing in DL interneurons. The cluster and discriminant analysis described could help to classify neurons in other systems.

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.

Between molecules and experience: role of early patterns of coordinated activity for the development of cortical maps and sensory abilities

Sensory systems processing information from the environment rely on precisely formed and refined neuronal networks that build maps of sensory receptor epithelia at different subcortical and cortical levels. These sensory maps share similar principles of function and emerge according to developmental processes common in visual, somatosensory and auditory systems. Whereas molecular cues set the coarse organization of cortico-subcortical topography, its refinement is known to succeed under the influence of experience-dependent electrical activity during critical periods. However, coordinated patterns of activity synchronize the cortico-subcortical networks long before the meaningful impact of environmental inputs on sensory maps. Recent studies elucidated the cellular and network mechanisms underlying the generation of these early patterns of activity and highlighted their similarities across species. Moreover, the experience-independent activity appears to act as a functional template for the maturation of sensory networks and cortico-subcortical maps. A major goal for future research will be to analyze how this early activity interacts with the molecular cues and to determine whether it is permissive or rather supporting for the establishment of sensory topography.

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.

Cross-species analyses of the cortical GABAergic and subplate neural populations

Cortical GABAergic (gamma-aminobutyric acidergic) neurons include a recently identified subset whose projections extend over relatively long distances in adult rodents and primates. A number of these inhibitory projection neurons are located in and above the conventionally identified white matter, suggesting their persistence from, or a correspondence with, the developmental subplate. GABAergic and subplate neurons share some unique properties unlike those of the more prevalent pyramidal neurons. To better understand the GABAergic and subplate populations, we constructed a database of neural developmental events common to the three species most frequently used in experimental studies: rat, mouse, and macaque, using data from the online database www.translatingtime.net as well as GABAergic and subplate developmental data from the empirical literature. We used a general linear model to test for similarities and differences, a valid approach because the sequence of most neurodevelopmental events is remarkably conserved across mammalian species. Similarities between the two rodent populations are striking, permitting us to identify developmental dates for GABAergic and subplate neural events in rats that were previously identified only in mice, as well as the timing in mouse development for events previously identified in rats. Primate comparative data are also compelling, although slight variability in statistical error measurement indicates differences in primate GABAergic and subplate events when compared to rodents. Although human extrapolations are challenging because fewer empirical data points are available, and because human data display more variability, we also produce estimates of dates for GABAergic and subplate neural events that have not yet been, or cannot be, determined empirically in humans.

Electrical excitability of early neurons in the human cerebral cortex during the second trimester of gestation

Information about development of the human cerebral cortex (proliferation, migration, and differentiation of neurons) is largely based on postmortem histology. Physiological properties of developing human cortical neurons are difficult to access experimentally and therefore remain largely unexplored. Animal studies have shown that information about the arousal of electrical activity in individual cells within fundamental cortical zones (subventricular zone [SVZ], intermediate zone, subplate [SP], and cortical plate [CP]) is necessary for understanding normal brain development. Here we ask where, in what cortical zone, and when, in what gestational week (gw), human neurons acquire the ability to generate nerve impulses (action potentials [APs]). We performed electrical recordings from individual cells in acute brain slices harvested postmortem from the human fetal cerebral cortex (16-22 gw). Tetrodotoxin-sensitive Na(+) current occurs more frequently among CP cells and with significantly greater peak amplitudes than in SVZ. As early as 16 gw, a relatively small population of CP neurons (27%) was able to generate sodium APs upon direct current injection. Neurons located in the SP exhibited the highest level of cellular differentiation, as judged by their ability to fire repetitive APs. At 19 gw, a fraction of human CP and SP neurons possess beta IV spectrin-positive axon initial segments populated with voltage-gated sodium channels (PanNav). These results yield the first physiological characterization of developing human fetal cortical neurons with preserved morphologies in intact surrounding brain tissue.

Increase of intracellular Ca2+ by adenine and uracil nucleotides in human midbrain-derived neuronal progenitor cells

Nucleotides play an important role in brain development and may exert their action via ligand-gated cationic channels or G protein-coupled receptors. Patch-clamp measurements indicated that in contrast to AMPA, ATP did not induce membrane currents in human midbrain derived neuronal progenitor cells (hmNPCs). Various nucleotide agonists concentration-dependently increased [Ca(2+)](i) as measured by the Fura-2 method, with the rank order of potency ATP>ADP>UTP>UDP. A Ca(2+)-free external medium moderately decreased, whereas a depletion of the intracellular Ca(2+) storage sites by cyclopiazonic acid markedly depressed the [Ca(2+)](i) transients induced by either ATP or UTP. Further, the P2Y(1) receptor antagonistic PPADS and MRS 2179, as well as the nucleotide catalyzing enzyme apyrase, allmost abolished the effects of these two nucleotides. However, the P2Y(1,2,12) antagonistic suramin only slightly blocked the action of ATP, but strongly inhibited that of UTP. In agreement with this finding, UTP evoked the release of ATP from hmNPCs in a suramin-, but not PPADS-sensitive manner. Immunocytochemistry indicated the co-localization of P2Y(1,2,4)-immunoreactivities (IR) with nestin-IR at these cells. In conclusion, UTP may induce the release of ATP from hmNPCs via P2Y(2) receptor-activation and thereby causes [Ca(2+)](i) transients by stimulating a P2Y(1)-like receptor.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.