Cellular and molecular basis for the formation of lamina-specific thalamocortical projections

Postnatal development of subfields in the core region of the mouse auditory cortex

The core region of the rodent auditory cortex has two subfields: the primary auditory area (A1) and the anterior auditory field (AAF). Although the postnatal development of A1 has been studied in several mammalian species, few studies have been conducted on the postnatal development of AAF. Using a voltage-sensitive-dye-based imaging method, we examined and compared the postnatal development of AAF and A1 in mice from postnatal day 11 (P11) to P40. We focused on the postnatal development of tonotopy, the relative position between A1 and AAF, and the properties of tone-evoked responses in the subfields. Tone-evoked responses in the mouse auditory cortex were first observed at P12, and tonotopy was found in both A1 and AAF at this age. Quantification of tonotopy using the cortical magnification factor (CMF; octave difference per unit cortical distance) revealed a rapid change from P12 to P14 in both A1 and AAF, and a stable level from P14. A similar time course of postnatal development was found for the distance between the 4 kHz site in A1 and AAF, the distance between the 16 kHz site in A1 and AAF, and the angle between the frequency axis of A1 and AAF. The maximum amplitude and rise time of tone-evoked signals in both A1 and AAF showed no significant change from P12 to P40, but the latency of the responses to both the 4 kHz and 16 kHz tones decreased during this period, with a more rapid decrease in the latency to 16 kHz tones in both subfields. The duration of responses evoked by 4 kHz tones in both A1 and AAF showed no significant postnatal change, but the duration of responses to 16 kHz tones decreased exponentially in both subfields. The cortical area activated by 4 kHz tones in AAF was always larger than that in A1 at all ages (P12-P40). Our results demonstrated that A1 and AAF developed in parallel postnatally, showing a rapid maturation of tonotopy, slow maturation of response latency and response duration, and a dorsal-to-ventral order (high-frequency site to low-frequency site) of functional maturation.

Percolation Model of Sensory Transmission and Loss of Consciousness Under General Anesthesia

Neurons communicate with each other dynamically; how such communications lead to consciousness remains unclear. Here, we present a theoretical model to understand the dynamic nature of sensory activity and information integration in a hierarchical network, in which edges are stochastically defined by a single parameter p representing the percolation probability of information transmission. We validate the model by comparing the transmitted and original signal distributions, and we show that a basic version of this model can reproduce key spectral features clinically observed in electroencephalographic recordings of transitions from conscious to unconscious brain activities during general anesthesia. As p decreases, a steep divergence of the transmitted signal from the original was observed, along with a loss of signal synchrony and a sharp increase in information entropy in a critical manner; this resembles the precipitous loss of consciousness during anesthesia. The model offers mechanistic insights into the emergence of information integration from a stochastic process, laying the foundation for understanding the origin of cognition.

Pre-synaptic and post-synaptic neuronal activity supports the axon development of callosal projection neurons during different post-natal periods in the mouse cerebral cortex

Callosal projection neurons, one of the major types of projection neurons in the mammalian cerebral cortex, require neuronal activity for their axonal projections [H. Mizuno et al. (2007) J. Neurosci., 27, 6760-6770; C. L. Wang et al. (2007) J. Neurosci., 27, 11334-11342]. Here we established a method to label a few callosal axons with enhanced green fluorescent protein in the mouse cerebral cortex and examined the effect of pre-synaptic/post-synaptic neuron silencing on the morphology of individual callosal axons. Pre-synaptic/post-synaptic neurons were electrically silenced by Kir2.1 potassium channel overexpression. Single axon tracing showed that, after reaching the cortical innervation area, green fluorescent protein-labeled callosal axons underwent successive developmental stages: axon growth, branching, layer-specific targeting and arbor formation between post-natal day (P)5 and P9, and the subsequent elaboration of axon arbors between P9 and P15. Reducing pre-synaptic neuronal activity disturbed axon growth and branching before P9, as well as arbor elaboration afterwards. In contrast, silencing post-synaptic neurons disturbed axon arbor elaboration between P9 and P15. Thus, pre-synaptic neuron silencing affected significantly earlier stages of callosal projection neuron axon development than post-synaptic neuron silencing. Silencing both pre-synaptic and post-synaptic neurons impaired callosal axon projections, suggesting that certain levels of firing activity in pre-synaptic and post-synaptic neurons are required for callosal axon development. Our findings provide in-vivo evidence that pre-synaptic and post-synaptic neuronal activities play critical, and presumably differential, roles in axon growth, branching, arbor formation and elaboration during cortical axon development.

Many paths to synaptic specificity

The most impressive structural feature of the nervous system is the specificity of its synaptic connections. Even after axons have navigated long distances to reach target areas, they must still choose appropriate synaptic partners from the many potential partners within easy reach. In many cases, axons also select a particular domain of the postsynaptic cell on which to form a synapse. Thus, synapse formation is selective at both cellular and subcellular levels. Unsurprisingly, the nervous system uses multiple mechanisms to ensure proper connectivity; these include complementary labels, coordinated growth of synaptic partners, sorting of afferents, prohibition or elimination of inappropriate synapses, respecification of targets, and use of short-range guidance mechanisms or intermediate targets. Specification of any circuit is likely to involve integration of multiple mechanisms. Recent studies of vertebrate and invertebrate systems have led to the identification of molecules that mediate a few of these interactions.

The regulative role of neurite mechanical tension in network development

A bewildering series of dynamical processes take part in the development of the nervous system. Neuron branching dynamics, the continuous formation and elimination of neural interconnections, are instrumental in constructing distinct neuronal networks, which are the functional building blocks of the nervous system. In this study, we investigate and validate the important regulative role of mechanical tension in determining the final morphology of neuronal networks. To single out the mechanical effect, we cultured relatively large invertebrate neurons on clean quartz surfaces. Applied to these surfaces were isolated anchoring sites consisting of carbon nanotube islands to which the cells and the neurites could mechanically attach. Inspection of branching dynamics and network wiring upon development revealed an innate selection mechanism in which one axon branch wins over another. The apparent mechanism entails the build-up of mechanical tension in developing axons. The tension is maintained by the attachment of the growth cone to the substrate or, alternatively, to the neurites of a target neuron. The induced tension promotes the stabilization of one set of axon branches while causing retraction or elimination of axon collaterals. We suggest that these findings represent a crucial, early step that precedes the formation of synapses and regulates neuronal interconnections. Mechanical tension serves as a signal for survival of the axonal branch and perhaps for the subsequent formation of synapses.

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

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

Commissure formation in the mammalian forebrain

Commissural formation in the mammalian brain is highly organised and regulated both by the cell-autonomous expression of transcription factors, and by non-cell-autonomous mechanisms including the formation of midline glial structures and their expression of specific axon guidance molecules. These mechanisms channel axons into the correct path and enable the subsequent connection of specific brain areas to their appropriate targets. Several key findings have been made over the past two years, including the discovery of novel mechanisms of action that 'classical' guidance factors such as the Slits, Netrins, and their receptors have in axon guidance. Moreover, novel guidance factors such as members of the Wnt family, and extracellular matrix components such as heparan sulphate proteoglycans, have been shown to be important for mammalian brain commissure formation. Additionally, there have been significant discoveries regarding the role of FGF signalling in the formation of midline glial structures. In this review, we discuss the most recent advances in the field that have contributed to our current understanding of commissural development in the telencephalon.

The role of neural activity in cortical axon branching

Axonal branching is an important process for establishing the final pattern of connections between a neuron and its target cells. Cortical connections between upper-layer cells in the neocortex have provided insights into the cellular mechanisms by which electrical activity regulates neural connectivity, including branch formation. Recent evidence further indicates that spontaneous firing and synaptic transmission contribute to axonal branching of cortical neurons through postsynaptic activation.

Activity dependence of cortical axon branch formation: a morphological and electrophysiological study using organotypic slice cultures

The influence of neuronal activity on cortical axon branching was studied by imaging axons of layer 2/3 neurons in organotypic slice cultures of rat visual cortex. Upper layer neurons labeled by electroporation of plasmid encoding yellow fluorescent protein were observed by confocal microscopy. Time-lapse observation of single-labeled axons showed that axons started to branch after 8-10 d in vitro. Over the succeeding 7-10 d, branch complexity gradually increased by both growth and retraction of branches, resulting in axon arbors that morphologically resembled those observed in 2- to 3-week-old animals. Electrophysiological recordings of neuronal activity in the upper layers, made using multielectrode dishes, showed that the frequency of spontaneous firing increased dramatically approximately 10 d in vitro and remained elevated at later stages. To examine the involvement of spontaneous firing and synaptic activity in branch formation, various blockers were applied to the culture medium. Cultures were silenced by TTX or by a combination of APV and DNQX but exhibited a homeostatic recovery of spontaneous activity over several days in the presence of blockers of either NMDA-type or non-NMDA-type glutamate receptors alone. Axonal branching was suppressed by TTX and AMPA receptor blockade but not by NMDA receptor blockade. We conclude that cortical axon branching is highly dynamic and that neural activity regulates the early developmental branching of upper layer cortical neurons through the activation of AMPA-type glutamate receptors.

Exuberant thalamocortical axon arborization in cortex-specific NMDAR1 knockout mice

Development of whisker-specific neural patterns in the rodent somatosensory system requires NMDA receptor (NMDAR)-mediated activity. In cortex-specific NR1 knockout (CxNR1KO) mice, while thalamocortical afferents (TCAs) develop rudimentary whisker-specific patterns in the primary somatosensory (barrel) cortex, layer IV cells do not develop barrels or orient their dendrites towards TCAs. To determine the role of postsynaptic NMDARs in presynaptic afferent development and patterning in the barrel cortex, we examined the single TCA arbors in CxNR1KO mice between postnatal days (P) 1-7. Sparsely branched TCAs invade the cortical plate on P1 in CxNR1KO mice as in control mice. In control animals, TCAs progressively elaborate patchy terminals, mostly restricted to layer IV. In CxNR1KO mice, TCAs develop far more extensive arbors between P3-7. Their lateral extent is twice that of controls from P3 onwards. By P7, CxNR1KO TCAs have significantly fewer branch points and terminal endings in layers IV and VI but more in layers II/III and V than control mouse TCAs. Within expansive terminal arbors, CxNR1KO TCAs develop focal terminal densities in layer IV, corresponding to the rudimentary whisker-specific patches. Given that thalamic NMDARs are spared in CxNR1KO mice, the present results show that postsynaptic NMDARs play an important role in refinement of presynaptic afferent arbors and whisker-specific patterning in the developing barrel cortex.

Gene expression analysis of the late embryonic mouse cerebral cortex using DNA microarray: identification of several region- and layer-specific genes

The mammalian neocortex develops layer organizations with regional differences represented by expression of multiple genes at embryonic stages. These genes could play important roles in the formation of areal cyto-architecture, yet, the number of genes identified so far is not sufficient to explain such intricate processes. Here we collected five regions--the medial, dorsal, lateral, rostral and occipital--from the dissected E16.5 mouse cerebral cortex and performed extensive gene expression analysis using the Affymetrix U74Av2 array with probes for 12,500 genes. After relative quantitative analysis, 34, 33 and 15 genes were selected as highly expressed genes in the medial, dorsal and lateral regions, respectively. The combination of GeneChip system, real-time quantitative reverse transcription polymerase chain reaction and in situ hybridization analyses allowed the successful identification of seven genes from the dorsal region (Neuropeptide Y, Wnt7b, TGF-beta RI, Nrf3, Bcl-6, MT4-MMP and Rptp kappa), three genes from the medial region (Hop-pending, HtrA and Crystallin), and three genes from the lateral region (Somatostatin, Ngef and Fxyd7). Particularly, all seven genes identified in the dorsal region demarcated the future somatosensory and auditory areas in the cortical plate with high rostrolateral-low caudomedial gradation. Their expression patterns were not uniform, but delineated either the superficial or the deep layer in the cortical plate. Furthermore, the regional expression pattern of Neuropeptide Y was shifted rostrally and the layer specificity was disorganized in the Pax6-deficient mice. Our results provide new information about a subclass of regionally expressed genes in the cortical plate at the late embryonic stage, which may help understand the molecular mechanisms of neocortical arealization.

N-cadherin regulates ingrowth and laminar targeting of thalamocortical axons

Thalamocortical axons are precisely targeted to cortical layer IV, but the identity of specific molecules that govern the establishment of laminar specificity in the thalamocortical projection has been elusive. In this study, we test the role of N-cadherin, a homophilic cell adhesion molecule, in laminar targeting of thalamocortical axons using cocultured thalamic and cortical slice explants exposed to N-cadherin function-blocking antibodies or inhibitory peptides. In untreated cocultures, labeled thalamocortical axons normally grow to and stop in layer IV, forming terminal-like arbors. In the N-cadherin-blocked cocultures, thalamic axons reach layer IV by growing through deep layers at the same rate as those in the untreated cocultures, but instead of terminating in layer IV, they continue growing uninterruptedly through layer IV and extend into supragranular layers to reach the outermost cortical edge, where some form terminal-like arbors in this aberrant laminar position. In cocultures in which the cortical slice is taken at an earlier maturational stage, one that corresponds to a time when thalamic axons are normally growing through deep layers before the emergence of layer IV from the cortical plate, thalamic axon ingrowth through deep layers is significantly attenuated by N-cadherin blocking reagents. These data indicate that N-cadherin has multifaceted roles in establishing the thalamocortical projection, governing aspects of both thalamic axon ingrowth and laminar targeting by acting as a layer IV stop signal, which progressively change in parallel with the maturational state of the cortex.