Development and critical period plasticity of the barrel cortex

Opposing role of NMDA receptor GluN2B and GluN2D in somatosensory development and maturation

Development of correct topographical connections between peripheral receptors and central somatosensory stations requires activity-dependent synapse refinement, in which the NMDA type of glutamate receptors plays a key role. Here we compared functional roles of GluN2B (GluRε2 or NR2B) and GluN2D (GluRε4 or NR2D), two major regulatory subunits of neonatal NMDA receptors, in development of whisker-related patterning at trigeminal relay stations. Compared with control littermates, both the appearance of whisker-related patterning and the termination of the critical period, as assessed by unilateral infraorbital nerve transection, were delayed by nearly a day in the somatosensory cortex of GluN2B(+/-) mice but advanced by nearly a day in GluN2D(-/-) mice. Similar temporal shifts were found at subcortical relay stations in the thalamus and brainstem of GluN2B(+/-) and GluN2D(-/-) mice. In comparison, the magnitude of lesion-induced critical period plasticity in the somatosensory cortex, as assessed following row-C whisker removal, was normal in both mutants. Thus, GluN2B and GluN2D play counteractive roles in temporal development and maturation of somatosensory maps without affecting the magnitude of critical period plasticity. To understand the opposing action, we then examined neuronal and synaptic expressions of the two subunits along the trigeminal pathway. At each trigeminal station, GluN2B was predominant at asymmetrical synapses of non-GABAergic neurons, whereas GluN2D was selective to asymmetrical synapses of GABAergic neurons. Together, our findings suggest that GluN2B expressed at glutamatergic synapses on glutamatergic projection neurons facilitates refinement of ascending pathway synapses directly, whereas GluN2D expressed at glutamatergic synapses on GABAergic interneurons delays it indirectly.

Frequency-specific response facilitation of supra and infragranular barrel cortical neurons depends on NMDA receptor activation in rats

Sensory experience has a profound effect on neocortical neurons. Passive stimulation of whiskers or sensory deprivation from whiskers can induce long-lasting changes in neuronal responses or modify the receptive field in adult animals. We recorded barrel cortical neurons in urethane-anesthetized rats in layers 2/3 or 5/6 to determine if repetitive stimulation would induce long-lasting response facilitation. Air-puff stimulation (20-ms duration, 40 pulses at 0.5-8Hz) was applied to a single whisker. This repetitive stimulation increased tactile responses in layers 2/3 and 5/6 for 60min. Moreover, the functional coupling (coherence) between the sensory stimulus and the neural response also increased after the repetitive stimulation in neurons showing response facilitation. The long-lasting response facilitation was due to activation of N-methyl-d-aspartate (NMDA) receptors because it was reduced by APV ((2R)-amino-5-phosphonovaleric acid, (2R)-amino-5-phosphonopentanoate) and MK801 application. Inactivation of layer 2/3 also blocked response facilitation in layer 5/6, suggesting that layer 2/3 may be fundamental in this synaptic plasticity processes. Moreover, i.p. injection of eserine augmented the number of layer 2/3 neurons expressing long-lasting response facilitation; this effect was blocked by atropine, suggesting that muscarinic receptor activation favors the induction of the response facilitation. Our data indicate that physiologically repetitive stimulation of a single whisker at the frequency at which rats move their whiskers during exploration of the environment induces long-lasting response facilitation improving sensory processing.

Classic Cadherins Mediate Selective Intracortical Circuit Formation in the Mouse Neocortex

Understanding the molecular mechanisms underlying the formation of selective intracortical circuitry is one of the important questions in neuroscience research. "Barrel nets" are recently identified intracortical axonal trajectories derived from layer 2/3 neurons in layer 4 of the primary somatosensory (barrel) cortex. Axons of layer 2/3 neurons are preferentially distributed in the septal regions of layer 4 of the barrel cortex, where they show whisker-related patterns. Because cadherins have been viewed as potential candidates that mediate the formation of selective neuronal circuits, here we examined the role of cadherins in the formation of barrel nets. We disrupted the function of cadherins by expressing dominant-negative cadherin (dn-cadherin) using in utero electroporation and found that barrel nets were severely disrupted. Confocal microscopic analysis revealed that expression of dn-cadherin reduced the density of axons in septal regions in layer 4 of the barrel cortex. We also found that cadherins were important for the formation, rather than the maintenance, of barrel nets. Our results uncover an important role of cadherins in the formation of local intracortical circuitry in the neocortex.

Sensory-related neural activity regulates the structure of vascular networks in the cerebral cortex

Neurovascular interactions are essential for proper brain function. While the effect of neural activity on cerebral blood flow has been extensively studied, whether or not neural activity influences vascular patterning remains elusive. Here, we demonstrate that neural activity promotes the formation of vascular networks in the early postnatal mouse barrel cortex. Using a combination of genetics, imaging, and computational tools to allow simultaneous analysis of neuronal and vascular components, we found that vascular density and branching were decreased in the barrel cortex when sensory input was reduced by either a complete deafferentation, a genetic impairment of neurotransmitter release at thalamocortical synapses, or a selective reduction of sensory-related neural activity by whisker plucking. In contrast, enhancement of neural activity by whisker stimulation led to an increase in vascular density and branching. The finding that neural activity is necessary and sufficient to trigger alterations of vascular networks reveals an important feature of neurovascular interactions.

Imprecise Whisker Map in the Neonatal Rat Barrel Cortex

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

Vocal motor changes beyond the sensitive period for song plasticity

Behavior is critically shaped during sensitive periods in development. Birdsong is a learned vocal behavior that undergoes dramatic plasticity during a sensitive period of sensorimotor learning. During this period, juvenile songbirds engage in vocal practice to shape their vocalizations into relatively stereotyped songs. By the time songbirds reach adulthood, their songs are relatively stable and thought to be "crystallized." Recent studies, however, highlight the potential for adult song plasticity and suggest that adult song could naturally change over time. As such, we investigated the degree to which temporal and spectral features of song changed over time in adult Bengalese finches. We observed that the sequencing and timing of song syllables became more stereotyped over time. Increases in the stereotypy of syllable sequencing were due to the pruning of infrequently produced transitions and, to a lesser extent, increases in the prevalence of frequently produced transitions. Changes in song tempo were driven by decreases in the duration and variability of intersyllable gaps. In contrast to significant changes to temporal song features, we found little evidence that the spectral structure of adult song syllables changed over time. These data highlight differences in the degree to which temporal and spectral features of adult song change over time and support evidence for distinct mechanisms underlying the control of syllable sequencing, timing, and structure. Furthermore, the observed changes to temporal song features are consistent with a Hebbian framework of behavioral plasticity and support the notion that adult song should be considered a form of vocal practice.

Map transfer from the thalamus to the neocortex: inputs from the barrel field

Sensory perception relies on the formation of stereotyped maps inside the brain. This feature is particularly well illustrated in the mammalian neocortex, which is subdivided into distinct cortical sensory areas that comprise topological maps, such as the somatosensory homunculus in humans or the barrel field of the large whiskers in rodents. How somatosensory maps are formed and relayed into the neocortex remain essential questions in developmental neuroscience. Here, we will present our current knowledge on whisker map transfer in the mouse model, with the goal of linking embryonic and postnatal studies into a comprehensive framework.

A sensitive period for the impact of hearing loss on auditory perception

Manipulations of the sensory environment typically induce greater changes to the developing nervous system than they do in adulthood. The relevance of these neural changes can be evaluated by examining the age-dependent effects of sensory experience on quantitative measures of perception. Here, we measured frequency modulation (FM) detection thresholds in adult gerbils and investigated whether diminished auditory experience during development or in adulthood influenced perceptual performance. Bilateral conductive hearing loss (CHL) of ≈30 dB was induced either at postnatal day 10 or after sexual maturation. All animals were then trained as adults to detect a 5 Hz FM embedded in a continuous 4 kHz tone. FM detection thresholds were defined as the minimum deviation from the carrier frequency that the animal could reliably detect. Normal-hearing animals displayed FM thresholds of 25 Hz. Inducing CHL, either in juvenile or adult animals, led to a deficit in FM detection. However, this deficit was greater for juvenile onset hearing loss (89 Hz) relative to adult onset hearing loss (64 Hz). The effects could not be attributed to sensation level, nor were they correlated with proxies for attention. The thresholds displayed by CHL animals were correlated with shallower psychometric function slopes, suggesting that hearing loss was associated with greater variance of the decision variable, consistent with increased internal noise. The results show that decreased auditory experience has a greater impact on perceptual skills when initiated at an early age and raises the possibility that altered development of CNS synapses may play a causative role.

Routes to cAMP: shaping neuronal connectivity with distinct adenylate cyclases

cAMP signaling affects a large number of the developmental processes needed for the construction of the CNS, including cell differentiation, axon outgrowth, response to guidance molecules or modulation of synaptic connections. This points to a key role of adenylate cyclases (ACs), the synthetic enzymes of cAMP, for neural development. ACs exist as 10 different isoforms, which are activated by distinct signaling pathways. The implication of specific AC isoforms in neural wiring was only recently demonstrated in mouse mutants, knockout (KO) for different AC isoforms, AC1, AC3, AC5, AC8 and soluble (s)AC/AC10. These studies stressed the importance of three of these isoforms, as sensors of neural activity that could modify the survival of neurons (sAC), axon outgrowth (sAC), or the response of axons to guidance molecules such as ephrins (AC1) or semaphorins (AC3). We summarize here the current knowledge on the role of these ACs for the development of sensory maps, in the somatosensory, visual and olfactory systems, which have been the most extensively studied. In these systems, AC1/AC3 KO revealed targeting mistakes due to the defective pruning and lack of discrimination of incoming axons to signals present in target structures. In contrast, no changes in cell differentiation, survival or axon outgrowth were noted in these mutants, suggesting a specificity of cAMP production routes for individual cellular processes within a given neuron. Further studies indicate that the subcellular localization of ACs could be key to their specific role in axon targeting and may explain their selective roles in neuronal wiring.

Neonatal infraorbital nerve crush-induced CNS synaptic plasticity and functional recovery

Infraorbital nerve (ION) transection in neonatal rats leads to disruption of whisker-specific neural patterns (barrelettes), conversion of functional synapses into silent synapses, and reactive gliosis in the brain stem trigeminal principal nucleus (PrV). Here we tested the hypothesis that neonatal peripheral nerve crush injuries permit better functional recovery of associated central nervous system (CNS) synaptic circuitry compared with nerve transection. We developed an in vitro whisker pad-trigeminal ganglion (TG)-brain stem preparation in neonatal rats and tested functional recovery in the PrV following ION crush. Intracellular recordings revealed that 68% of TG cells innervate the whisker pad. We used the proportion of whisker pad-innervating TG cells as an index of ION function. The ION function was blocked by ∼64%, immediately after mechanical crush, then it recovered beginning after 3 days postinjury and was complete by 7 days. We used this reversible nerve-injury model to study peripheral nerve injury-induced CNS synaptic plasticity. In the PrV, the incidence of silent synapses increased to ∼3.5 times of control value by 2-3 days postinjury and decreased to control levels by 5-7 days postinjury. Peripheral nerve injury-induced reaction of astrocytes and microglia in the PrV was also reversible. Neonatal ION crush disrupted barrelette formation, and functional recovery was not accompanied by de novo barrelette formation, most likely due to occurrence of recovery postcritical period (P3) for pattern formation. Our results suggest that nerve crush is more permissive for successful regeneration and reconnection (collectively referred to as "recovery" here) of the sensory inputs between the periphery and the brain stem.

Neural ECM in regeneration and rehabilitation

Neural extracellular matrix (ECM) is different from the normal ECM in other organs in that it has low fibrous protein content and high carbohydrate content. One of the key carbohydrate components in the brain ECM is chondroitin sulfate proteoglycans (CSPGs). Over the last two decades, the view of CSPGs has changed drastically, from the initial regeneration inhibitor to plasticity regulators present in the perineuronal nets to the most recent view that certain CSPG isoforms may even be growth promoters. In this chapter, we aim to address a few current progresses of CSPGs in regulating plasticity and rehabilitation in various pathological conditions in the central nervous system.

Functional convergence of thalamic and intrinsic projections to cortical layers 4 and 6

Ascending sensory information is conveyed from the thalamus to layers 4 and 6 of sensory cortical areas. Interestingly, receptive field properties of cortical layer 6 neurons are different from those in layer 4. Do such differences reflect distinct inheritance patterns from the thalamus or are they derived instead from local cortical circuits? To distinguish between these possibilities, we utilized in vitro slice preparations containing the thalamocortical pathways in the auditory and somatosensory systems. Responses from neurons in layers 4 and 6 that resided in the same column were recorded using whole-cell patch clamp. Laser-scanning photostimulation via uncaging of glutamate in the thalamus and cortex was used to map the functional topography of thalamocortical and intracortical inputs to each layer. In addition, we assessed the functional divergence of thalamocortical inputs by optical imaging of flavoprotein autofluorescence. We found that the thalamocortical inputs to layers 4 and 6 originated from the same thalamic domain, but the intracortical projections to the same neurons differed dramatically. Our results suggest that the intracortical projections, rather than the thalamic inputs, to each layer contribute more to the differences in their receptive field properties.

Birth regulates the initiation of sensory map formation through serotonin signaling

Although the mechanisms underlying the spatial pattern formation of sensory maps have been extensively investigated, those triggering sensory map formation during development are largely unknown. Here we show that the birth of pups instructively and selectively regulates the initiation of barrel formation in the somatosensory cortex by reducing serotonin concentration. We found that preterm birth accelerated barrel formation, whereas it did not affect either barreloid formation or barrel structural plasticity. We also found that serotonin was selectively reduced soon after birth and that the reduction of serotonin was triggered by birth. The reduction of serotonin was necessary and sufficient for the effect of birth on barrel formation. Interestingly, the regulatory mechanisms described here were also found to regulate eye-specific segregation in the visual system, suggesting that they are utilized in various brain regions. Our results shed light on roles of birth and serotonin in sensory map formation.

The birth of the barrels

Refinement of sensory maps follows a highly reproducible tempo dictated largely by peripheral sensory receptors and neural activity. In this issue of Developmental Cell, Toda et al. (2013) add a new twist by showing that the timing of birth plays a decisive role in setting the clock for pattern emergence.

The many layers of specification and plasticity in the neocortex

In this issue of Neuron, Li et al. (2013) show that transgenically eliminating thalamocortical neurotransmission disrupts the formation of barrel columns in the somatosensory cortex and cortical lamination, providing evidence for the importance of extrinsic activity-dependent factors in cortical development.

Auditory stimuli mimicking ambient sounds drive temporal "delta-brushes" in premature infants

In the premature infant, somatosensory and visual stimuli trigger an immature electroencephalographic (EEG) pattern, "delta-brushes," in the corresponding sensory cortical areas. Whether auditory stimuli evoke delta-brushes in the premature auditory cortex has not been reported. Here, responses to auditory stimuli were studied in 46 premature infants without neurologic risk aged 31 to 38 postmenstrual weeks (PMW) during routine EEG recording. Stimuli consisted of either low-volume technogenic "clicks" near the background noise level of the neonatal care unit, or a human voice at conversational sound level. Stimuli were administrated pseudo-randomly during quiet and active sleep. In another protocol, the cortical response to a composite stimulus ("click" and voice) was manually triggered during EEG hypoactive periods of quiet sleep. Cortical responses were analyzed by event detection, power frequency analysis and stimulus locked averaging. Before 34 PMW, both voice and "click" stimuli evoked cortical responses with similar frequency-power topographic characteristics, namely a temporal negative slow-wave and rapid oscillations similar to spontaneous delta-brushes. Responses to composite stimuli also showed a maximal frequency-power increase in temporal areas before 35 PMW. From 34 PMW the topography of responses in quiet sleep was different for "click" and voice stimuli: responses to "clicks" became diffuse but responses to voice remained limited to temporal areas. After the age of 35 PMW auditory evoked delta-brushes progressively disappeared and were replaced by a low amplitude response in the same location. Our data show that auditory stimuli mimicking ambient sounds efficiently evoke delta-brushes in temporal areas in the premature infant before 35 PMW. Along with findings in other sensory modalities (visual and somatosensory), these findings suggest that sensory driven delta-brushes represent a ubiquitous feature of the human sensory cortex during fetal stages and provide a potential test of functional cortical maturation during fetal development.

Cortical plasticity within and across lifetimes: how can development inform us about phenotypic transformations?

The neocortex is the part of the mammalian brain that is involved in perception, cognition, and volitional motor control. It is a highly dynamic structure that is dramatically altered within the lifetime of an animal and in different lineages throughout the course of evolution. These alterations account for the remarkable variations in behavior that species exhibit. Of particular interest is how these cortical phenotypes change within the lifetime of the individual and eventually evolve in species over time. Because we cannot study the evolution of the neocortex directly we use comparative analysis to appreciate the types of changes that have been made to the neocortex and the similarities that exist across taxa. Developmental studies inform us about how these phenotypic transitions may arise by alterations in developmental cascades or changes in the physical environment in which the brain develops. Both genes and the sensory environment contribute to aspects of the phenotype and similar features, such as the size of a cortical field, can be altered in a variety of ways. Although both genes and the laws of physics place constraints on the evolution of the neocortex, mammals have evolved a number of mechanisms that allow them to loosen these constraints and often alter the course of their own evolution.

Molecular mechanisms at the basis of plasticity in the developing visual cortex: epigenetic processes and gene programs

Neuronal circuitries in the mammalian visual system change as a function of experience. Sensory experience modifies neuronal networks connectivity via the activation of different physiological processes such as excitatory/inhibitory synaptic transmission, neurotrophins, and signaling of extracellular matrix molecules. Long-lasting phenomena of plasticity occur when intracellular signal transduction pathways promote epigenetic alterations of chromatin structure that regulate the induction of transcription factors that in turn drive the expression of downstream targets, the products of which then work via the activation of structural and functional mechanisms that modify synaptic connectivity. Here, we review recent findings in the field of visual cortical plasticity while focusing on how physiological mechanisms associated with experience promote structural changes that determine functional modifications of neural circuitries in V1. We revise the role of microRNAs as molecular transducers of environmental stimuli and the role of immediate early genes that control gene expression programs underlying plasticity in the developing visual cortex.

Functional significance of cortical NMDA receptors in somatosensory information processing

N-methyl-d-aspartate receptor (NMDAR)-mediated activity is required for whisker-related neural patterning in the rodent brain. Deletion of the essential NMDAR subunit NR1 gene in excitatory cortical neurons prevents whisker-specific barrel formation and impairs thalamocortical afferent patterning. We used electrophysiological and voltage-sensitive dye imaging methods to assess synaptic and sensory evoked cortical activity and immunohistochemistry to examine immediate early gene expression following whisker stimulation in cortex-specific NR1 knockout (CxNR1KO) mice. In mutant mice, layer IV neurons lacked NMDAR-mediated excitatory postsynaptic currents, and temporal summation of excitatory postsynaptic potentials (EPSPs) was impaired. Barrel neurons showed both phasic and tonic responses to whisker deflection. The averaged tonic response in CxNR1KO mice was significantly less than that in control mice due to impaired EPSP temporal summation. Electrophysiological estimation of the number of thalamic neurons innervating single barrel neurons indicated a significant increase in CxNR1KO mice. Similarly, voltage-sensitive dye optical signals in response to whisker stimulation were widespread. Immediate early gene expression following whisker stimulation also showed a diffuse expression pattern in the CxNR1KO cortex compared with whisker-specific expression patterns in controls. Thus, when NMDAR function is impaired, spatial discrimination of whisker inputs is severely compromised, and sensory stimulation evokes diffuse, topographically misaligned activity in the barrel cortex.

Adaptive phenotype of microglial cells during the normal postnatal development of the somatosensory "Barrel" cortex

Accumulative evidence indicates that microglial cells influence the normal development of central nervous system (CNS) synapses. Yet, the functional properties of microglia in relation with synapse development remain unclear. We recently showed that in layer 4 of the whisker-related barrel field of the mouse somatosensory cortex, microglial cells are recruited only after postnatal day (P)5 in the center of the barrels where thalamo-cortical synapses are concentrated and begin their maturation. In the present study, we analyzed the phenotype of microglia during this developmental process. We show that between P5 and P7 microglial cells acquire a more ramified morphology with a smaller soma, they express classical markers of microglia (Iba1, CD11b, and CD68) but never markers of activation (Mac-2 and MHCII) and rarely the proliferation marker Ki67. Electrophysiological recordings in acute cortical slices showed that at P5 a proportion of layer 4 microglia transiently express voltage-dependant potassium currents of the delayed rectifier family, mostly mediated by Kv1.3 subunits, which are usually expressed by activated microglia under pathological conditions. This proportion of cells with rectifying properties doubles between P5 and P6, in concomitance with the beginning of microglia invasion of the barrel centers. Finally, analysis of the responses mediated by purinergic receptors indicated that a higher percentage of rectifying microglia expressed functional P2Y6 and P2Y12 receptors, as compared with nonrectifying cells, whereas all cells expressed functional P2X7 receptors. Our results indicate that during normal cortical development distinct microglia properties mature differentially, some of them being exquisitely influenced by the local environment of the maturating neuronal network.

Early gamma oscillations

Gamma oscillations have long been considered to emerge late in development. However, recent studies have revealed that gamma oscillations are transiently expressed in the rat barrel cortex during the first postnatal week, a "critical" period of sensory-dependent barrel map formation. The mechanisms underlying the generation and physiological roles of early gamma oscillations (EGOs) in the development of thalamocortical circuits will be discussed in this review. In contrast to adult gamma oscillations, synchronized through gamma-rhythmic perisomatic inhibition, EGOs are primarily driven through feedforward gamma-rhythmic excitatory input from the thalamus. The recruitment of cortical interneurons to EGOs and the emergence of feedforward inhibition are observed by the end of the first postnatal week. EGOs facilitate the precise synchronization of topographically aligned thalamic and cortical neurons. The multiple replay of sensory input during EGOs supports long-term potentiation at thalamocortical synapses. We suggest that this early form of gamma oscillations, which is mechanistically different from adult gamma oscillations, guides barrel map formation during the critical developmental period.

Sensory cortex limits cortical maps and drives top-down plasticity in thalamocortical circuits

Primary somatosensory cortex (S1) contains a complete body map that mirrors subcortical maps developed by peripheral sensory input projecting to sensory hindbrain, thalamus, then S1. Peripheral changes during development alter these maps through ‘bottom-up’ plasticity. Unknown is how S1 size influences map organization and if an altered S1 map feedbacks to affect subcortical maps. We show in mice that S1 is significantly reduced by cortex-specific deletion of Pax6, resulting in a reduced body map and loss of body representations by exclusion of later-differentiating sensory thalamocortical input. An initially normal sensory thalamus was re-patterned to match the aberrant S1 map by apoptotic deletion of thalamic neurons representing body parts with axons excluded from S1. Deleted representations were rescued by altering competition between thalamocortical axons by sensory deprivation or increasing S1. Thus, S1 size determined resolution and completeness of body maps and engaged ‘top-down’ plasticity that re-patterned sensory thalamus to match S1.

Sensory map transfer to the neocortex relies on pretarget ordering of thalamic axons

Sensory maps, such as the representation of mouse facial whiskers, are conveyed throughout the nervous system by topographic axonal projections that preserve neighboring relationships between adjacent neurons. In particular, the map transfer to the neocortex is ensured by thalamocortical axons (TCAs), whose terminals are topographically organized in response to intrinsic cortical signals. However, TCAs already show a topographic order early in development, as they navigate toward their target. Here, we show that this preordering of TCAs is required for the transfer of the whisker map to the neocortex. Using Ebf1 conditional inactivation that specifically perturbs the development of an intermediate target, the basal ganglia, we scrambled TCA topography en route to the neocortex without affecting the thalamus or neocortex. Notably, embryonic somatosensory TCAs were shifted toward the visual cortex and showed a substantial intermixing along their trajectory. Somatosensory TCAs rewired postnatally to reach the somatosensory cortex but failed to form a topographic anatomical or functional map. Our study reveals that sensory map transfer relies not only on positional information in the projecting and target structures but also on preordering of axons along their trajectory, thereby opening novel perspectives on brain wiring.

In vivo imaging of brain metabolism activity using a phosphorescent oxygen-sensitive probe

Several approaches have been adopted for real-time imaging of neural activity in vivo. We tested a new cell-penetrating phosphorescent oxygen-sensitive probe, NanO2-IR, to monitor temporal and spatial dynamics of oxygen metabolism in the neocortex following peripheral sensory stimulation. Probe solution was applied to the surface of anesthetized mouse brain; optical imaging was performed using a MiCAM-02 system. Trains of whisker stimuli were delivered and associated changes in phosphorescent signal were recorded in the contralateral somatosensory ("barrel") cortex. Sensory stimulation led to changes in oxygenation of activated areas of the barrel cortex. The oxygen imaging results were compared to those produced by the voltage-sensitive dye RH-1691. While the signals emitted by the two probes differed in shape and amplitude, they both faithfully indicated specific whisker evoked cortical activity. Thus, NanO2-IR probe can be used as a tool in visualization and real-time analysis of sensory-evoked neural activity in vivo.

D-serine as a gliotransmitter and its roles in brain development and disease

The development of new techniques to study glial cells has revealed that they are active participants in the development of functional neuronal circuits. Calcium imaging studies demonstrate that glial cells actively sense and respond to neuronal activity. Glial cells can produce and release neurotransmitter-like molecules, referred to as gliotransmitters, that can in turn influence the activity of neurons and other glia. One putative gliotransmitter, D-serine is believed to be an endogenous co-agonist for synaptic N-methyl-D-aspartate receptors (NMDARs), modulating synaptic transmission and plasticity mediated by this receptor. The observation that D-serine levels in the mammalian brain increase during early development, suggests a possible role for this gliotransmitter in normal brain development and circuit refinement. In this review we will examine the data that D-serine and its associated enzyme serine racemase are developmentally regulated. We will consider the evidence that D-serine is actively released by glial cells and examine the studies that have implicated D-serine as a critical player involved in regulating NMDAR-mediated synaptic transmission and neuronal migration during development. Furthermore, we will consider how dysregulation of D-serine may play an important role in the etiology of neurological and psychiatric diseases.

Role of sensory experience in functional development of Drosophila motor circuits

Neuronal circuits are formed according to a genetically predetermined program and then reconstructed in an experience-dependent manner. While the existence of experience-dependent plasticity has been demonstrated for the visual and other sensory systems, it remains unknown whether this is also the case for motor systems. Here we examined the effects of eliminating sensory inputs on the development of peristaltic movements in Drosophila embryos and larvae. The peristalsis is initially slow and uncoordinated, but gradually develops into a mature pattern during late embryonic stages. We tested whether inhibiting the transmission of specific sensory neurons during this period would have lasting effects on the properties of the sensorimotor circuits. We applied Shibire-mediated inhibition for six hours during embryonic development (15–21 h after egg laying [AEL]) and studied its effects on peristalsis in the mature second- and third-instar larvae. We found that inhibition of chordotonal organs, but not multidendritic neurons, led to a lasting decrease in the speed of larval locomotion. To narrow down the sensitive period, we applied shorter inhibition at various embryonic and larval stages and found that two-hour inhibition during 16–20 h AEL, but not at earlier or later stages, was sufficient to cause the effect. These results suggest that neural activity mediated by specific sensory neurons is involved in the maturation of sensorimotor circuits in Drosophila and that there is a critical period for this plastic change. Consistent with a role of chordotonal neurons in sensory feedback, these neurons were activated during larval peristalsis and acute inhibition of their activity decreased the speed of larval locomotion.

EphB2 signaling regulates lesion-induced axon sprouting but not critical period length in the postnatal auditory brainstem

Background

Studies of developmental plasticity may provide insight into plasticity during adulthood, when neural circuitry is less responsive to losses or changes in input. In the mammalian auditory brainstem, globular bushy cell axons of the ventral cochlear nucleus (VCN) innervate the contralateral medial nucleus of the trapezoid body (MNTB) principal neurons. VCN axonal terminations in MNTB, known as calyces of Held, are very large and specialized for high-fidelity transmission of auditory information. Following unilateral deafferentation during postnatal development, VCN axons from the intact side form connections with novel targets, including the ipsilateral MNTB. EphB signaling has been shown to play a role in this process during the first postnatal week, but mechanisms involved in this reorganization during later developmental periods remain unknown.

Results

We found that EphB2 signaling reduces the number of induced ipsilateral projections to the MNTB after unilateral VCN removal at postnatal day seven (P7), but not after removal of the VCN on one side at P10, after the closure of the critical period for lesion-induced innervation of the ipsilateral MNTB.

Conclusions

Results from this study indicate that molecular mechanisms involved in the development of circuitry may also play a part in rewiring after deafferentation during development, but do not appear to regulate the length of critical periods for plasticity.

Postnatal growth defects in mice with constitutive depletion of central serotonin

Although the trophic actions of serotonin (5-HT) are well established, only few developmental defects have been reported in mouse strains with constitutive hyposerotonergia. We analyzed postnatal growth and cortical development in three different mutant mouse strains with constitutive reductions in central 5-HT levels. We compared two previously published mouse strains with severe (-95%) depletions of 5-HT, the tryptophan hydroxylase (Tph) 2(-/-) mouse line and VMAT2(sert-cre) mice, with a new strain, in which VMAT2 deletion is driven by Pet1 (VMAT2(pet1-cre)) in 5-HT raphe neurons leading to partial (-75%) reduction in brain 5-HT levels. We find that normal embryonic growth and postnatal growth retardation are common features of all these mouse strains. Postnatal growth retardation varied from mild to severe according to the extent of the brain 5-HT reduction and gender. Normal growth was reinstated in VMAT2(sert-cre) mice by reconstituting central 5-HT stores. Growth abnormalities could not be linked to altered food intake or temperature control. Morphological study of the cerebral cortex over postnatal development showed a delayed maturation of the upper cortical layers in the VMAT2(sert-cre) and Tph2(-/-) mice, but not in the VMAT2(pet1-cre) mice. No changes in layer-specific gene expression or morphological alterations of barrel cortex development were found. Overall, these observations sustain the notion that central 5-HT signaling is required for the preweaning growth spurt of mouse pups. Brain development appeared to be immune to severe central 5-HT depletion for its overall growth during prenatal life, whereas reduced brain growth and delayed cortical maturation development occurred during postnatal life. Reduced developmental 5-HT signaling during postnatal development might modulate the function and fine structure of neural circuits in ways that affect adult behavior.

Cortical plasticity, excitatory-inhibitory balance, and sensory perception

Experience shapes the central nervous system throughout life. Structural and functional plasticity confers a remarkable ability on the brain, allowing neural circuits to adequately adapt to dynamic environments. This process can require selective adjustment of many excitatory and inhibitory synapses in an organized manner, in such a way as to enhance representations of behaviorally important sensory stimuli while preserving overall network excitability. The rules and mechanisms that orchestrated these changes across different synapses and throughout neuronal ensembles are beginning to be understood. Here, we review the evidence connecting synaptic plasticity to functional plasticity and perceptual learning, focusing on the roles of various neuromodulatory systems in enabling plasticity of adult neural circuits. However, the challenge remains to appropriately leverage these systems and forms of plasticity to persistently improve perceptual abilities and behavioral performance.

Rapid whisker movements in sleeping newborn rats

Spontaneous activity in the sensory periphery drives infant brain activity and is thought to contribute to the formation of retinotopic and somatotopic maps. In infant rats during active (or REM) sleep, brainstem-generated spontaneous activity triggers hundreds of thousands of skeletal muscle twitches each day; sensory feedback from the resulting limb movements is a primary activator of forebrain activity. The rodent whisker system, with its precise isomorphic mapping of individual whiskers to discrete brain areas, has been a key contributor to our understanding of somatotopic maps and developmental plasticity. But although whisker movements are controlled by dedicated skeletal muscles, spontaneous whisker activity has not been entertained as a contributing factor to the development of this system. Here we report in 3- to 6-day-old rats that whiskers twitch rapidly and asynchronously during active sleep; furthermore, neurons in whisker thalamus exhibit bursts of activity that are tightly associated with twitches but occur infrequently during waking. Finally, we observed barrel-specific cortical activity during periods of twitching. This is the first report of self-generated, sleep-related twitches in the developing whisker system, a sensorimotor system that is unique for the precision with which it can be experimentally manipulated. The discovery of whisker twitching will allow us to attain a better understanding of the contributions of peripheral sensory activity to somatosensory integration and plasticity in the developing nervous system.

Insights into the complex influence of 5-HT signaling on thalamocortical axonal system development

The topographic organization of the thalamocortical axons (TCAs) in the barrel field (BF) in the rodent primary somatosensory cortex results from a succession of temporally and spatially precise developmental events. Prenatally, growth and guidance mechanisms enable TCAs to navigate through the forebrain and reach the cortex. Postnatally, TCAs grow into the cortex, and the refinement of their terminal arborization pattern in layer IV creates barrel-like structures. The combined results of studies performed over the past 20 years clearly show that serotonin (5-hydroxytryptamine; 5-HT) signaling modulates these pre- and early postnatal developmental processes. In this context, 5-HT signaling can purposely be described as 'modulating' rather than 'controlling' because developmental alterations of 5-HT synthesis, uptake or degradation either have a dramatic, moderate or no effect at all on TCA pathway and BF formation. In this review we summarize and compare the outcomes of diverse pharmacological and genetic manipulations of 5-HT signaling on TCA pathway and BF formation, in an attempt to understand these discrepancies.

Distinct developmental principles underlie the formation of ipsilateral and contralateral whisker-related axonal patterns of layer 2/3 neurons in the barrel cortex

Axonal organizations with specific patterns underlie the functioning of local intracortical circuitry, but their precise anatomy and development still remain elusive. Here, we selectively visualized layer 2/3 neurons using in utero electroporation and examined their axonal organization in the barrel cortex contralateral to the electroporated side. We found that callosal axons run preferentially in septal regions of layer 4 and showed a whisker-related pattern in the contralateral barrel cortex in rats and mice. In addition, presynaptic marker proteins were found in this whisker-related axonal organization. Although the whisker-related patterns were observed in both the ipsilateral and contralateral barrel cortex, we found a difference in their developmental processes. While the formation of the whisker-related pattern in the ipsilateral cortex consisted of two distinct steps, that in the contralateral cortex did not have the 1st step, in which the axons were diffusely distributed without preference to septal or barrel regions. We also found that these more diffuse axons ran close to radial glial fibers. Together, our results uncovered a whisker-related axonal pattern of callosal axons and two independent developmental processes involved in the formation of the axonal trajectories of layer 2/3 neurons.

Neurotransmitter release at the thalamocortical synapse instructs barrel formation but not axon patterning in the somatosensory cortex

To assess the impact of synaptic neurotransmitter release on neural circuit development, we analyzed barrel cortex formation after thalamic or cortical ablation of RIM1 and RIM2 proteins, which control synaptic vesicle fusion. Thalamus-specific deletion of RIMs reduced neurotransmission efficacy by 67%. A barrelless phenotype was found with a dissociation of effects on the presynaptic and postsynaptic cellular elements of the barrel. Presynaptically, thalamocortical axons formed a normal whisker map, whereas postsynaptically the cytoarchitecture of layer IV neurons was altered as spiny stellate neurons were evenly distributed and their dendritic trees were symmetric. Strikingly, cortex-specific deletion of the RIM genes did not modify barrel development. Adult mice with thalamic-specific RIM deletion showed a lack of activity-triggered immediate early gene expression and altered sensory-related behaviors. Thus, efficient synaptic release is required at thalamocortical but not at corticocortical synapses for building the whisker to barrel map and for efficient sensory function.