Antagonistic interplay between necdin and Bmi1 controls proliferation of neural precursor cells in the embryonic mouse neocortex

Neural precursor cells (NPCs) in the neocortex exhibit a high proliferation capacity during early embryonic development and give rise to cortical projection neurons after maturation. Necdin, a mammal-specific MAGE (melanoma antigen) family protein that possesses anti-mitotic and pro-survival activities, is expressed abundantly in postmitotic neurons and moderately in tissue-specific stem cells or progenitors. Necdin interacts with E2F transcription factors and suppresses E2F1-dependent transcriptional activation of the cyclin-dependent kinase Cdk1 gene. Here we show that necdin serves as a suppressor of NPC proliferation in the embryonic neocortex. Necdin is moderately expressed in the ventricular zone of mouse embryonic neocortex, in which proliferative cell populations are significantly increased in necdin-null mice. In the neocortex of necdin-null embryos, expression of Cdk1 and Sox2, a stem cell marker, is significantly increased, whereas expression of p16, a cyclin-dependent kinase inhibitor, is markedly diminished. Cdk1 and p16 expression levels are also significantly increased and decreased, respectively, in primary NPCs prepared from necdin-null embryos. Intriguingly, necdin interacts directly with Bmi1, a Polycomb group protein that suppresses p16 expression and promotes NPC proliferation. In HEK293A cells transfected with luciferase reporter constructs, necdin relieves Bmi1-dependent repression of p16 promoter activity, whereas Bmi1 counteracts necdin-mediated repression of E2F1-dependent Cdk1 promoter activity. In lentivirus-infected primary NPCs, necdin overexpression increases p16 expression, suppresses Cdk1 expression, and inhibits NPC proliferation, whereas Bmi1 overexpression suppresses p16 expression, increases Cdk1 expression, and promotes NPC proliferation. Our data suggest that embryonic NPC proliferation in the neocortex is regulated by the antagonistic interplay between necdin and Bmi1.

[Neuronal and glial antigen distribution in the columns of somatosensory cortex of rat brain (an immunohistochemical study)]

The aim of the study was to detect the neocortical columns in the S1 field on frontal sections of brain of albino rats using the method of immunohistochemistry and the antibodies against neuronal (synaptophysin, neurofilament) and gliocyte (glial fibrillary acidic protein--GFAP, myelin basic protein) proteins. The examination of the expression of the major neurospecific antigens revealed that on thin sections (4 micromin) a column could be identified due to accumulations of the astrocytes and neuronal processes--axons and dendrites. GFAP expression study also showed that cortical layer I usually contained multiple large astrocytes with branching processes, as well as numerous smaller processes with high intensity of expression. Synaptophysin content was high in all the layers of the cortex, but the most intense reaction was detected in the molecular layer, similarly with the intensity of GFAP reaction. The expression of myelin basic protein was detected in accordance with the radially extending myelinated processes of the neurons in the cortex.

Developmental changes in the spatial organization of neurons in the neocortex of humans and common chimpanzees

In adult humans the prefrontal cortex possesses wider minicolumns and more neuropil space than other cortical regions. These aspects of prefrontal cortex architecture, furthermore, are increased in comparison to chimpanzees and other great apes. In order to determine the developmental appearance of this human cortical specialization, we examined the spatial organization of neurons in four cortical regions (frontal pole [Brodmann's area 10], primary motor [area 4], primary somatosensory [area 3b], and prestriate visual cortex [area 18]) in chimpanzees and humans from birth to approximately the time of adolescence (11 years of age). Horizontal spacing distance (HSD) and gray level ratio (GLR) of layer III neurons were measured in Nissl-stained sections. In both human and chimpanzee area 10, HSD was significantly higher in the postweaning specimens compared to the preweaning ones. No significant age-related differences were seen in the other regions in either species. In concert with other recent studies, the current findings suggest that there is a relatively slower maturation of area 10 in both humans and chimpanzees as compared to other cortical regions, and that further refinement of the spatial organization of neurons within this prefrontal area in humans takes place after the postweaning periods included here.

Increased dosage of DYRK1A and DSCR1 delays neuronal differentiation in neocortical progenitor cells

Down's syndrome (DS), a major genetic cause of mental retardation, arises from triplication of genes on human chromosome 21. Here we show that DYRK1A (dual-specificity tyrosine-phosphorylated and -regulated kinase 1A) and DSCR1 (DS critical region 1), two genes lying within human chromosome 21 and encoding for a serine/threonine kinase and calcineurin regulator, respectively, are expressed in neural progenitors in the mouse developing neocortex. Increasing the dosage of both proteins in neural progenitors leads to a delay in neuronal differentiation, resulting ultimately in alteration of their laminar fate. This defect is mediated by the cooperative actions of DYRK1A and DSCR1 in suppressing the activity of the transcription factor NFATc. In Ts1Cje mice, a DS mouse model, dysregulation of NFATc in conjunction with increased levels of DYRK1A and DSCR1 was observed. Furthermore, counteracting the dysregulated pathway ameliorates the delayed neuronal differentiation observed in Ts1Cje mice. In sum, our findings suggest that dosage of DYRK1A and DSCR1 is critical for proper neurogenesis through NFATc and provide a potential mechanism to explain the neurodevelopmental defects in DS.

The Convallis rule for unsupervised learning in cortical networks

The phenomenology and cellular mechanisms of cortical synaptic plasticity are becoming known in increasing detail, but the computational principles by which cortical plasticity enables the development of sensory representations are unclear. Here we describe a framework for cortical synaptic plasticity termed the "Convallis rule", mathematically derived from a principle of unsupervised learning via constrained optimization. Implementation of the rule caused a recurrent cortex-like network of simulated spiking neurons to develop rate representations of real-world speech stimuli, enabling classification by a downstream linear decoder. Applied to spike patterns used in in vitro plasticity experiments, the rule reproduced multiple results including and beyond STDP. However STDP alone produced poorer learning performance. The mathematical form of the rule is consistent with a dual coincidence detector mechanism that has been suggested by experiments in several synaptic classes of juvenile neocortex. Based on this confluence of normative, phenomenological, and mechanistic evidence, we suggest that the rule may approximate a fundamental computational principle of the neocortex.

Dendritic morphology of pyramidal neurons in the chimpanzee neocortex: regional specializations and comparison to humans

The primate cerebral cortex is characterized by regional variation in the structure of pyramidal neurons, with more complex dendritic arbors and greater spine density observed in prefrontal compared with sensory and motor cortices. Although there are several investigations in humans and other primates, virtually nothing is known about regional variation in the morphology of pyramidal neurons in the cerebral cortex of great apes, humans' closest living relatives. The current study uses the rapid Golgi stain to quantify the dendritic structure of layer III pyramidal neurons in 4 areas of the chimpanzee cerebral cortex: Primary somatosensory (area 3b), primary motor (area 4), prestriate visual (area 18), and prefrontal (area 10) cortex. Consistent with previous studies in humans and macaque monkeys, pyramidal neurons in the prefrontal cortex of chimpanzees exhibit greater dendritic complexity than those in other cortical regions, suggesting that prefrontal cortical evolution in primates is characterized by increased potential for integrative connectivity. Compared with chimpanzees, the pyramidal neurons of humans had significantly longer and more branched dendritic arbors in all cortical regions.

Morphology, input-output relations and synaptic connectivity of Cajal-Retzius cells in layer 1 of the developing neocortex of CXCR4-EGFP mice

Layer 1 (L1) neurons, in particular Cajal–Retzius (CR) cells are among the earliest generated neurons in the neocortex. However, their role and that of L1 GABAergic interneurons in the establishment of an early cortical microcircuit are still poorly understood. Thus, the morphology of whole-cell recorded and biocytin-filled CR cells was investigated in postnatal day (P) 7–11 old CXCR4-EGFP mice where CR cells can be easily identified by their fluorescent appearance. Confocal-, light- and subsequent electron microscopy was performed to investigate their developmental regulation, morphology, synaptic input–output relationships and electrophysiological properties. CR cells reached their peak in occurrence between P4 to P7 and from thereon declined to almost complete disappearance at P14 by undergoing selective cell death through apoptosis. CR cells formed a dense and long-range horizontal network in layer 1 with a remarkable high density of synaptic boutons along their axons. They received dense GABAergic and non-GABAergic synaptic input and in turn provided synaptic output preferentially with spines or shafts of terminal tuft dendrites of pyramidal neurons. Interestingly, no dye-coupling between CR cells with other cortical neurons was observed as reported for other species, however, biocytin-labeling of individual CR cells leads to co-staining of L1 end foot astrocytes. Electrophysiologically, CR cells are characterized by a high input resistance and a characteristic firing pattern. Increasing depolarizing currents lead to action potential of decreasing amplitude and increasing half width, often terminated by a depolarization block. The presence of membrane excitability, the high density of CR cells in layer 1, their long-range horizontal axonal projection together with a high density of synaptic boutons and their synaptic input–output relationship suggest that they are an integral part of an early cortical network important not only in layer 1 but also for the establishment and formation of the cortical column.

Diversity of neural precursor cell types in the prenatal macaque cerebral cortex exists largely within the astroglial cell lineage

The germinal zones of the embryonic macaque neocortex comprise the ventricular zone (VZ) and the subventricular zone (SVZ). The mammalian SVZ is subdivided into an inner SVZ and an outer SVZ, with the outer SVZ being particularly large in primates. The existence of distinct precursor cell types in the neocortical proliferative zones was inferred over 100 years ago and recent evidence supports this concept. Precursor cells exhibiting diverse morphologies, patterns of transcription factor expression, and fate potential have been identified in the neocortical proliferative zones. Neurogenic precursor cells are thought to exhibit characteristics of glial cells, but the existence of neurogenic precursor cells that do not share glial specific properties has also been proposed. Therefore, one question that remains is whether neural precursor cells in the prenatal neocortex belong within the astroglial cell class, as they do in neurogenic regions of the adult neocortex, or instead include a diverse collection of precursor cells belonging to distinct cell classes. We examined the expression of astroglial markers by mitotic precursor cells in the telencephalon of prenatal macaque and human. We show that in the dorsal neocortex all mitotic cells at the surface of the ventricle, and all Pax6+ and Tbr2+ mitotic cells in the proliferative zones, express the astroglial marker GFAP. The majority of mitotic cells undergoing division away from the ventricle express GFAP, and many of the GFAP-negative mitoses express markers of cells derived from the ventral telencephalon or extracortical sites. In contrast, a markedly lower proportion of precursor cells express GFAP in the ganglionic eminence. In conclusion, we propose that the heterogeneity of neural precursor cells in the dorsal cerebral cortex develops within the GFAP+ astroglial cell class.

Phenytoin inhibits the persistent sodium current in neocortical neurons by modifying its inactivation properties

The persistent Na⁺ current (I_NaP) is important for neuronal functions and can play a role in several pathologies, although it is small compared to the transient Na⁺ current (I_NaT). Notably, I_NaP is not a real persistent current because it undergoes inactivation with kinetics in the order of tens of seconds, but this property has often been overlooked. Na⁺ channel blockers, drugs used for treating epilepsy and other diseases, can inhibit I_NaP, but the mechanism of this action and the conditions in which I_NaP can be actually inhibited have not been completely clarified yet. We evaluated the action of phenytoin (PHT), a prototype anti-epileptic Na⁺ channel blocker, on I_NaP inactivation in pyramidal neurons of rat sensorimotor cortical slices at different concentrations, from 5 to 100 µM. PHT did not modify I_NaP evoked with depolarizing voltage ramps of 50 or 100 mVs⁻¹, but decreased I_NaP evoked by slower voltage ramps (10 mVs⁻¹). However, at all of the tested concentrations, PHT decreased I_NaP evoked by faster ramps when they were preceded by inactivating pre-pulses. Moreover, PHT shifted towards negative potentials the voltage-dependence of I_NaP inactivation and accelerated its kinetics of development also at depolarized potentials (+40 mV), not consistently with a simple inactivated state stabilizer. Therefore, our study shows a prominent PHT effect on I_NaP inactivation rather than an open channel block, which is instead often implied. I_NaP is inhibited by PHT only in conditions that induce major I_NaP inactivation. These results highlight the importance of I_NaP inactivation not only for physiological functions but also as drug target, which could be shared by other therapeutic drugs. Through this action PHT can reduce I_NaP-induced long-lasting pathological depolarisations and intracellular sodium overload, whereas shorter I_NaP actions should not be modified. These properties set the conditions of efficacy and the limits of PHT as I_NaP inhibitor.

Morphometric and histochemical characteristics of the neocortex and hippocampus of experimentally enlarged rat brain during the suckling period of ontogeny

Enlargement of the brain weight was attained in rats by reducing the number of neonates in the litter. The brain of 14- and 30-day-old animals was examined (controls: intact animals from litters of average numbers). Difference between the brain weight at 14 and 30 days was about 200 mg. The parameter in the experimental rats was higher than in control: by 19.9 and 15.8% at the age of 14 and 30 days, respectively. The neocortex and hippocampus of the experimental rat brain differed by the size of the neuron cytoplasm and nucleus, these cells' count in a visual field, and activities of NADH- and NADPH-dehydrogenases in the neuron cytoplasm. During the suckling period, the differences in the weights of experimentally enlarged brain were paralleled by specific features of the cortex, morphometric and histochemical characteristics of the neurons.

[Phenotypic characteristics of factor expression induced by hypoxia and redox status of the rat neocortical cells at different stages of adaptation to hypoxia]

Hypoxic preconditioning induces two-phase increase of HIF-1alpha expression in the neocortex of low-resistance rats. The first, brief phase appears after each hypoxic episode and rapidly disappears in normoxic conditions. The second increase in of HIF-1alpha expression occurs in 24 hours after the hypoxic episode. The phase-nature of HIF-1alpha expression corresponds to the dynamics of urgent and long-term resistance in low-resistance rats, which suggests the HIF-1alpha involvement in mechanisms of urgent and long-term adaptation. It was found that in the mode of preconditioning, hypoxic treatments mobilized the anti-oxidant system (activated Cu, Zn-SOD) and had no effect on the intensity of lipid peroxidation processes in neocortex (INH, 10% O2) or even decreased the content of lipid peroxidation products and oxidized glutathione in neocortical cells in the early post-hypoxic period (HBH-5000, 10.5% O2). Thus, ROS do not play a key role in the induction of HIF-1alpha expression and fast-response/long-term adaptation to O2 deficiency in hypoxia-sensitive animals. In high-resistance rats, hypoxia preconditioning does not influence the HIF-1alpha protein expression and the adaptation. Severe hypoxic modes (HBH-7000, 8% O2) caused activation of lipid peroxidation processes in neocortex of hypoxia-sensitive rats. With the pro-oxidant systems dominating over the anti-oxidant ones, the neocortical expression of HIF-1alpha was found to decrease, which was accompanied by the impairment of the mechanisms of fast-response/long-term adaptation to hypoxia.

A modular attractor associative memory with patchy connectivity and weight pruning

An important research topic in neuroscience is the study of mechanisms underlying memory and the estimation of the information capacity of the biological system. In this report we investigate the performance of a modular attractor network with recurrent connections similar to the cortical long-range connections extending in the horizontal direction. We considered a single learning rule, the BCPNN, which implements a kind of Hebbian learning and we trained the network with sparse random patterns. The storage capacity was measured experimentally for networks of size between 500 and 46 K units with a constant activity level, gradually diluting the connectivity. We show that the storage capacity of the modular network with patchy connectivity is comparable with the theoretical values estimated for simple associative memories and furthermore we introduce a new technique to prune the connectivity, which enhances the storage capacity up to the asymptotic value.

[The effect of mild hypobaric hypoxia regime on expression of factor induced by hypoxia in the rat neocortex]

Using quantitative immunohistochemistry, modifications of HIF-1alpha expression in neocortex of rats exposed to various modes of mild hypobaric hypoxia (MHH) (1,3 and 6 episodes) differed in their neuroprotective efficacy have been studied. It has been shown that three-trial MHH being the most effective neuroprotective mode when used as a preconditioning produces most considerable changes in HIF-1 by substantial up-regulation of its regulatory alpha-subunit expression in the rat neocortex. Present findings support the hypothesis on important roles of HIF-1 in the mechanisms of brain hypoxic tolerance induced by the hypoxic preconditioning with three-trial MHH.

[Development of dissociated cells of various rat cns primordia after transplantation into the damaged nerve]

The purpose of this paper was to examine the possibilities of engraftment, and to study the differentiation of the dissociated cells from the embryonic primordia of the spinal cord and the neocortex of Wistar rats, after their transplantation into the sciatic nerve of adult animals. The cell suspension obtained as a result of a dissociation of fragments of the cervical spinal cord and the anterior cerebral vesicle from rat fetuses at day 15 of development, was injected into the proximal segment of a previously damaged sciatic nerve. Using the immunocytochemichal marker of neural stem/progenitor cells (Msi-1) the transplanted cells were identified in the nerve trunks after 1 day after the operation. After 21 day some of these cells underwent differentiation into NeuN-immunopositive neurons, however their number was small. Thus, dissociated precursor cells from embryonic rat spinal cord and neocortex survive for three weeks under conditions of transplantation into the damaged nerve and retain the ability to differentiate into neurons, but the number is small. Most of the cells in the neocortex transplants, unlike those from spinal cord transplants, within 21 days after the operation were represented by the ependymocytes.

Slit/Robo signaling modulates the proliferation of central nervous system progenitors

Neurogenesis relies on a delicate balance between progenitor maintenance and neuronal production. Progenitors divide symmetrically to increase the pool of dividing cells. Subsequently, they divide asymmetrically to self-renew and produce new neurons or, in some brain regions, intermediate progenitor cells (IPCs). Here we report that central nervous system progenitors express Robo1 and Robo2, receptors for Slit proteins that regulate axon guidance, and that absence of these receptors or their ligands leads to loss of ventricular mitoses. Conversely, production of IPCs is enhanced in Robo1/2 and Slit1/2 mutants, suggesting that Slit/Robo signaling modulates the transition between primary and intermediate progenitors. Unexpectedly, these defects do not lead to transient overproduction of neurons, probably because supernumerary IPCs fail to detach from the ventricular lining and cycle very slowly. At the molecular level, the role of Slit/Robo in progenitor cells involves transcriptional activation of the Notch effector Hes1. These findings demonstrate that Robo signaling modulates progenitor cell dynamics in the developing brain.

Target-specific expression of presynaptic NMDA receptors in neocortical microcircuits

Traditionally, NMDA receptors are located postsynaptically; yet, putatively presynaptic NMDA receptors (preNMDARs) have been reported. Although implicated in controlling synaptic plasticity, their function is not well understood and their expression patterns are debated. We demonstrate that, in layer 5 of developing mouse visual cortex, preNMDARs specifically control synaptic transmission at pyramidal cell inputs to other pyramidal cells and to Martinotti cells, while leaving those to basket cells unaffected. We also reveal a type of interneuron that mediates ascending inhibition. In agreement with synapse-specific expression, we find preNMDAR-mediated calcium signals in a subset of pyramidal cell terminals. A tuned network model predicts that preNMDARs specifically reroute information flow in local circuits during high-frequency firing, in particular by impacting frequency-dependent disynaptic inhibition mediated by Martinotti cells, a finding that we experimentally verify. We conclude that postsynaptic cell type determines presynaptic terminal molecular identity and that preNMDARs govern information processing in neocortical columns.

Cell-type homologies and the origins of the neocortex

The six-layered neocortex is a uniquely mammalian structure with evolutionary origins that remain in dispute. One long-standing hypothesis, based on similarities in neuronal connectivity, proposes that homologs of the layer 4 input and layer 5 output neurons of neocortex are present in the avian forebrain, where they contribute to specific nuclei rather than to layers. We devised a molecular test of this hypothesis based on layer-specific gene expression that is shared across rodent and carnivore neocortex. Our findings establish that the layer 4 input and the layer 5 output cell types are conserved across the amniotes, but are organized into very different architectures, forming nuclei in birds, cortical areas in reptiles, and cortical layers in mammals.

Layer-specific CREB target gene induction in human neocortical epilepsy

Epilepsy is a disorder of recurrent seizures that affects 1% of the population. To understand why some areas of cerebral cortex produce seizures and others do not, we identified differentially expressed genes in human epileptic neocortex compared with nearby regions that did not produce seizures. The transcriptome that emerged strongly implicates MAPK signaling and CREB-dependent transcription, with 74% of differentially expressed genes containing a cAMP response element (CRE) in their proximal promoter, more than half of which are conserved. Despite the absence of recent seizures in these patients, epileptic brain regions prone to seizures showed persistent activation of ERK and CREB. Persistent CREB activation was directly linked to CREB-dependent gene transcription by chromatin immunoprecipitation that showed phosphorylated CREB constitutively associated with the proximal promoters of many of the induced target genes involved in neuronal signaling, excitability, and synaptic plasticity. A distinct spatial pattern of ERK activation was seen in superficial axodendritic processes of epileptic neocortex that colocalized with both CREB phosphorylation and CREB target gene induction in well demarcated populations of layer 2/3 neurons. These same neuronal lamina showed a marked increase in synaptic density. The findings generated in this study generate a robust and spatially restricted pattern of epileptic biomarkers and associated synaptic changes that could lead to new mechanistic insights and potential therapeutic targets for human epilepsy.

Promoted neuronal differentiation after activation of alpha4/beta2 nicotinic acetylcholine receptors in undifferentiated neural progenitors

BACKGROUND

Neural progenitor is a generic term used for undifferentiated cell populations of neural stem, neuronal progenitor and glial progenitor cells with abilities for proliferation and differentiation. We have shown functional expression of ionotropic N-methyl-D-aspartate (NMDA) and gamma-aminobutyrate type-A receptors endowed to positively and negatively regulate subsequent neuronal differentiation in undifferentiated neural progenitors, respectively. In this study, we attempted to evaluate the possible functional expression of nicotinic acetylcholine receptor (nAChR) by undifferentiated neural progenitors prepared from neocortex of embryonic rodent brains.

METHODOLOGY/PRINCIPAL FINDINGS

Reverse transcription polymerase chain reaction analysis revealed mRNA expression of particular nAChR subunits in undifferentiated rat and mouse progenitors prepared before and after the culture with epidermal growth factor under floating conditions. Sustained exposure to nicotine significantly inhibited the formation of neurospheres composed of clustered proliferating cells and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide reduction activity at a concentration range of 1 µM to 1 mM without affecting cell survival. In these rodent progenitors previously exposed to nicotine, marked promotion was invariably seen for subsequent differentiation into cells immunoreactive for a neuronal marker protein following the culture of dispersed cells under adherent conditions. Both effects of nicotine were significantly prevented by the heteromeric α4β2 nAChR subtype antagonists dihydro-β-erythroidine and 4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine, but not by the homomeric α7 nAChR subtype antagonist methyllycaconitine, in murine progenitors. Sustained exposure to nicotine preferentially increased the expression of Math1 among different basic helix-loop-helix proneural genes examined. In undifferentiated progenitors from embryonic mice defective of NMDA receptor subunit-1, nicotine was still effective in significantly inhibiting the proliferation.

CONCLUSIONS/SIGNIFICANCE

Functional α4β2 nAChR subtype would be constitutively expressed to play a role in the mechanism underlying the determination of proliferation and subsequent differentiation fate into a neuronal lineage in association with preferential promotion of Math1 expression in undifferentiated neural progenitors of developing rodent neocortex independently of NMDA receptor activation.

Hippocampal theta modulation of neocortical spike times and gamma rhythm: a biophysical model study

The hippocampal theta and neocortical gamma rhythms are two prominent examples of oscillatory neuronal activity. The hippocampus has often been hypothesized to influence neocortical networks by its theta rhythm, and, recently, evidence for such a direct influence has been found. We examined a possible mechanism for this influence by means of a biophysical model study using conductance-based model neurons. We found, in agreement with previous studies, that networks of fast-spiking GABA -ergic interneurons, coupled with shunting inhibition, synchronize their spike activity at a gamma frequency and are able to impose this rhythm on a network of pyramidal cells to which they are coupled. When our model was supplied with hippocampal theta-modulated input fibres, the theta rhythm biased the spike timings of both the fast-spiking and pyramidal cells. Furthermore, both the amplitude and frequency of local field potential gamma oscillations were influenced by the phase of the theta rhythm. We show that the fast-spiking cells, not pyramidal cells, are essential for this latter phenomenon, thus highlighting their crucial role in the interplay between hippocampus and neocortex.

Development of coherent neuronal activity patterns in mammalian cortical networks: common principles and local hetereogeneity

Many mammals are born in a very immature state and develop their rich repertoire of behavioral and cognitive functions postnatally. This development goes in parallel with changes in the anatomical and functional organization of cortical structures which are involved in most complex activities. The emerging spatiotemporal activity patterns in multi-neuronal cortical networks may indeed form a direct neuronal correlate of systemic functions like perception, sensorimotor integration, decision making or memory formation. During recent years, several studies--mostly in rodents--have shed light on the ontogenesis of such highly organized patterns of network activity. While each local network has its own peculiar properties, some general rules can be derived. We therefore review and compare data from the developing hippocampus, neocortex and--as an intermediate region--entorhinal cortex. All cortices seem to follow a characteristic sequence starting with uncorrelated activity in uncoupled single neurons where transient activity seems to have mostly trophic effects. In rodents, before and shortly after birth, cortical networks develop weakly coordinated multineuronal discharges which have been termed synchronous plateau assemblies (SPAs). While these patterns rely mostly on electrical coupling by gap junctions, the subsequent increase in number and maturation of chemical synapses leads to the generation of large-scale coherent discharges. These patterns have been termed giant depolarizing potentials (GDPs) for predominantly GABA-induced events or early network oscillations (ENOs) for mostly glutamatergic bursts, respectively. During the third to fourth postnatal week, cortical areas reach their final activity patterns with distinct network oscillations and highly specific neuronal discharge sequences which support adult behavior. While some of the mechanisms underlying maturation of network activity have been elucidated much work remains to be done in order to fully understand the rules governing transition from immature to mature patterns of network activity.

Spike-timing dependence of structural plasticity explains cooperative synapse formation in the neocortex

Structural plasticity governs the long-term development of synaptic connections in the neocortex. While the underlying processes at the synapses are not fully understood, there is strong evidence that a process of random, independent formation and pruning of excitatory synapses can be ruled out. Instead, there must be some cooperation between the synaptic contacts connecting a single pre- and postsynaptic neuron pair. So far, the mechanism of cooperation is not known. Here we demonstrate that local correlation detection at the postsynaptic dendritic spine suffices to explain the synaptic cooperation effect, without assuming any hypothetical direct interaction pathway between the synaptic contacts. Candidate biomolecular mechanisms for dendritic correlation detection have been identified previously, as well as for structural plasticity based thereon. By analyzing and fitting of a simple model, we show that spike-timing correlation dependent structural plasticity, without additional mechanisms of cross-synapse interaction, can reproduce the experimentally observed distributions of numbers of synaptic contacts between pairs of neurons in the neocortex. Furthermore, the model yields a first explanation for the existence of both transient and persistent dendritic spines and allows to make predictions for future experiments.

Structural plasticity has been observed even in the adult mammalian neocortex – in seemingly static neuronal circuits structural remodeling is continuously at work. Still, it has been shown that the connection patterns between pairs of neurons are not random. In contrast, there is evidence that the synaptic contacts between a pair of neurons cooperate: several experimental studies report either zero or about 3–6 synapses between neuron pairs. The mechanism by which the synapses cooperate, however, has not yet been identified. Here we propose a model for structural plasticity that relies on local processes at the dendritic spine. We combine and extend the previous models and determine the equilibrium probability distribution of synaptic contact numbers of the model. By optimizing the parameters numerically for each of three reference datasets, we obtain equilibrium contact number distributions that fit the references very well. We conclude that the local dendritic mechanisms that we assume suffice to explain the cooperative synapse formation in the neocortex.

An anatomically comprehensive atlas of the adult human brain transcriptome

Neuroanatomically precise, genome-wide maps of transcript distributions are critical resources to complement genomic sequence data and to correlate functional and genetic brain architecture. Here we describe the generation and analysis of a transcriptional atlas of the adult human brain, comprising extensive histological analysis and comprehensive microarray profiling of ∼900 neuroanatomically precise subdivisions in two individuals. Transcriptional regulation varies enormously by anatomical location, with different regions and their constituent cell types displaying robust molecular signatures that are highly conserved between individuals. Analysis of differential gene expression and gene co-expression relationships demonstrates that brain-wide variation strongly reflects the distributions of major cell classes such as neurons, oligodendrocytes, astrocytes and microglia. Local neighbourhood relationships between fine anatomical subdivisions are associated with discrete neuronal subtypes and genes involved with synaptic transmission. The neocortex displays a relatively homogeneous transcriptional pattern, but with distinct features associated selectively with primary sensorimotor cortices and with enriched frontal lobe expression. Notably, the spatial topography of the neocortex is strongly reflected in its molecular topography-the closer two cortical regions, the more similar their transcriptomes. This freely accessible online data resource forms a high-resolution transcriptional baseline for neurogenetic studies of normal and abnormal human brain function.

Electrophysiological and morphological properties of neurons in layer 5 of the rat postrhinal cortex

The postrhinal (POR) cortex of the rat is homologous to the parahippocampal cortex of the primate based on connections and other criteria. POR provides the major visual and visuospatial input to the hippocampal formation, both directly to CA1 and indirectly through connections with the medial entorhinal cortex. Although the cortical and hippocampal connections of the POR cortex are well described, the physiology of POR neurons has not been studied. Here, we examined the electrical and morphological characteristics of layer 5 neurons from POR cortex of 14- to 16-day-old rats using an in vitro slice preparation. Neurons were subjectively classified as regular-spiking (RS), fast-spiking (FS), or low-threshold spiking (LTS) based on their electrophysiological properties and similarities with neurons in other regions of neocortex. Cells stained with biocytin included pyramidal cells and interneurons with bitufted or multipolar dendritic patterns. Similarity analysis using only physiological data yielded three clusters that corresponded to FS, LTS, and RS classes. The cluster corresponding to the FS class was composed entirely of multipolar nonpyramidal cells, and the cluster corresponding to the RS class was composed entirely of pyramidal cells. The third cluster, corresponding to the LTS class, was heterogeneous and included both multipolar and bitufted dendritic arbors as well as one pyramidal cell. We did not observe any intrinsically bursting pyramidal cells, which is similar to entorhinal cortex but unlike perirhinal cortex. We conclude that POR includes at least two major classes of neocortical inhibitory interneurons, but has a functionally restricted cohort of pyramidal cells.

Reliable in vivo identification of both GABAergic and glutamatergic neurons using Emx1-Cre driven fluorescent reporter expression

The development of genetically modified mice in which subpopulations of cortical neurons are labelled by fluorescent proteins has greatly facilitated single-cellular imaging and electrophysiology studies in vitro and in vivo. However, the parallel visualization of both inhibitory and excitatory neocortical neurons remains problematic. We here provide an alternative approach to identify GABAergic neurons in the context of in vivo calcium imaging. The method relies on the Emx1(IREScre) recombinase driven expression of a red fluorescent protein in excitatory neurons and glia. We quantitatively examined the upper layers of the visual neocortex in vivo and found that due to pronounced neuropil staining Emx1(IREScre)-negative and Emx1(IREScre)-positive neurons can be reliably differentiated based on negative and positive contrast, respectively. Immunohistochemical analyses confirmed that the entire population of GABAergic interneurons is represented by Emx1(IREScre)-negative cells. The potential usefulness of the method is exemplified by calcium imaging of sensory-evoked responses in the primary visual cortex. We conclude that the proposed method extends the repertoire of strategies aimed at discriminating two major neocortical neuron populations in situ.