Normal patterns of spontaneous activity are required for correct motor axon guidance and the expression of specific guidance molecules

A role for correlated spontaneous activity in the assembly of neural circuits

Before the onset of sensory transduction, developing neural circuits spontaneously generate correlated activity in distinct spatial and temporal patterns. During this period of patterned activity, sensory maps develop and initial coarse connections are refined, which are critical steps in the establishment of adult neural circuits. Over the last decade, there has been substantial evidence that altering the pattern of spontaneous activity disrupts refinement, but the mechanistic understanding of this process remains incomplete. In this review, we discuss recent experimental and theoretical progress toward the process of activity-dependent refinement, focusing on circuits in the visual, auditory, and motor systems. Although many outstanding questions remain, the combination of several novel approaches has brought us closer to a comprehensive understanding of how complex neural circuits are established by patterned spontaneous activity during development.

Characterization of retinoic acid-induced neurobehavioral effects in developing zebrafish

Retinoic signaling plays an important role in cell proliferation and differentiation. Disruption of retinoic signaling via excessive or deficient retinoic acid can cause teratogenic effects on developing embryos. Similar to retinoic acid, many xenobiotic environmental pollutants have been found to disrupt retinoic signaling through binding and eliciting agonistic activity on retinoic acid receptors. Currently, studies of retinoic acid or retinoic acid-like compounds in aquatic organisms have mainly focused on teratogenicity and few studies have explored their neurobehavioral toxicity. In the present study, the authors used retinoic acid as an example to explore the neurobehavioral toxicity associated with developmental exposure of retinoic acid-like compounds in zebrafish. The findings confirmed retinoic acid's teratogenic effects such as bent spine, malformed tail, and pericardial edema in developing zebrafish with a median effective concentration of 2.47 nM. Retinoic acid-induced cell apoptosis at 24 h postfertilization was consistently found in the eye and tail regions of embryos. Spontaneous movement as characterized by tail bend frequency was significantly increased in zebrafish embryos following exposure to 2 nM and 8 nM retinoic acid. Relatively low-dose retinoic acid exposure of 2 nM led to fast locomotion behavior in the dark period and hyperactivity during light-dark photoperiod stimulation. The 2-nM retinoic acid exposure also led to alterations of neurobehavior- and optic nerve-related genes, with the transforming growth factor-β signal transduction inhibitor noggin (nog) and the spinal cord marker homeobox c3a (hox) being underexpressed and the retinal G protein-coupled receptor a (rgr), the photoreceptor cell marker rhodopsin (rho), and the short wave-sensitive cone pigment opsin 1 (opn1sw1) being overexpressed. Increased expression of opn1sw1 and rho was confirmed by whole-mount in situ hybridization. Whether the misexpression of these genes leads to the neurobehavioral changes merits further study. The findings demonstrated that low-dose retinoic acid exposure perturbed the visual system and optic nerve development and caused hyperactivity in developing zebrafish.

Uncoupling of EphA/ephrinA signaling and spontaneous activity in neural circuit wiring

Classic studies have proposed that genetically encoded programs and spontaneous activity play complementary but independent roles in the development of neural circuits. Recent evidence, however, suggests that these two mechanisms could interact extensively, with spontaneous activity affecting the expression and function of guidance molecules at early developmental stages. Here, using the developing chick spinal cord and the mouse visual system to ectopically express the inwardly rectifying potassium channel Kir2.1 in individual embryonic neurons, we demonstrate that cell-intrinsic blockade of spontaneous activity in vivo does not affect neuronal identity specification, axon pathfinding, or EphA/ephrinA signaling during the development of topographic maps. However, intrinsic spontaneous activity is critical for axon branching and pruning once axonal growth cones reach their correct topographic position in the target tissues. Our experiments argue for the dissociation of spontaneous activity from hard-wired developmental programs in early phases of neural circuit formation.

Functional regeneration of intraspinal connections in a new in vitro model

Regeneration in the adult mammalian spinal cord is limited due to intrinsic properties of mature neurons and a hostile environment, mainly provided by central nervous system myelin and reactive astrocytes. Recent results indicate that propriospinal connections are a promising target for intervention to improve functional recovery. To study this functional regeneration in vitro we developed a model consisting of two organotypic spinal cord slices placed adjacently on multi-electrode arrays. The electrodes allow us to record the spontaneously occurring neuronal activity, which is often organized in network bursts. Within a few days in vitro (DIV), these bursts become synchronized between the two slices due to the formation of axonal connections. We cut them with a scalpel at different time points in vitro and record the neuronal activity 3 weeks later. The functional recovery ability was assessed by calculating the percentage of synchronized bursts between the two slices. We found that cultures lesioned at a young age (7-9 DIV) retained the high regeneration ability of embryonic tissue. However, cultures lesioned at older ages (>19 DIV) displayed a distinct reduction of synchronized activity. This reduction was not accompanied by an inability for axons to cross the lesion site. We show that functional regeneration in these old cultures can be improved by increasing the intracellular cAMP level with Rolipram or by placing a young slice next to an old one directly after the lesion. We conclude that co-cultures of two spinal cord slices are an appropriate model to study functional regeneration of intraspinal connections.

Retinoic acid induces changes in electrical properties of adult neurons in a dose- and isomer-dependent manner

The electrical activity of neurons is known to play a role in neuronal development, as well as repair of adult nervous tissue. For example, the extension of neurites and motility of growth cones can be modulated by changes in the electrical firing of neurons. The vitamin A metabolite retinoic acid also plays a critical role during nervous system development and is also known to elicit regenerative responses, namely the induction, enhancement, and directionality of neurite outgrowth. However, no studies have previously reported the ability of retinoic acid to modify the electrical activity of neurons. In this study, we determined whether retinoic acid might exert effects on the nervous system by altering the electrical properties of neurons. Using cultured adult neurons from Lymnaea stagnalis, we showed that acute application of retinoic acid can rapidly elicit changes in neuronal firing properties. Retinoic acid caused the presence of atypical firing behavior such as rhythmic bursting and altered the shape of action potentials, causing increases in half-amplitude duration and decay time. Retinoic acid also caused cell silencing, whereby neuronal activity was halted within an hour. These effects of retinoic acid were shown to be both dose and isomer dependent. We then showed that the effects of retinoic acid on cell firing (but not silencing) were significantly reduced in the presence of an retinoid X receptor pan-antagonist HX531. This study suggests that some of the effects of retinoic acid during neuronal development or regeneration might possibly occur as a result of changes in electrical activity of neurons.

Role of electrical activity in horizontal axon growth in the developing cortex: a time-lapse study using optogenetic stimulation

During development, layer 2/3 neurons in the neocortex extend their axons horizontally, within the same layers, and stop growing at appropriate locations to form branches and synaptic connections. Firing and synaptic activity are thought to be involved in this process, but how neuronal activity regulates axonal growth is not clear. Here, we studied axonal growth of layer 2/3 neurons by exciting cell bodies or axonal processes in organotypic slice cultures of the rat cortex. For neuronal stimulation and morphological observation, plasmids encoding channelrhodopsin-2 (ChR2) and DsRed were coelectroporated into a small number of layer 2/3 cells. Firing activity induced by photostimulation (475 nm) was confirmed by whole-cell patch recording. Axonal growth was observed by time-lapse confocal microscopy, using a different excitation wavelength (560 nm), at 10–20-min intervals for several hours. During the first week in vitro, when spontaneous neuronal activity is low, DsRed- and ChR2-expressing axons grew at a constant rate. When high-frequency photostimulation (4 or 10 Hz) for 1 min was applied to the soma or axon, most axons paused in their growth. In contrast, lower-frequency stimulation did not elicit this pause behavior. Moreover, in the presence of tetrodotoxin, even high-frequency stimulation did not cause axonal growth to pause. These results indicate that increasing firing activity during development suppresses axon growth, suggesting the importance of neuronal activity for the formation of horizontal connections.

Sensory input is required for callosal axon targeting in the somatosensory cortex

Background

Sensory input is generally thought to be necessary for refining and consolidating neuronal connections during brain development. We here report that cortical callosal axons in somatosensory cortex require sensory input for their target selection in contralateral cortex.

Results

Eliminating sensory input to either hemisphere by unilateral transection of infraorbital nerve (ION) prevents target selection of callosal axons in contralateral cortex. Strikingly, blocking sensory input bilaterally, by simultaneously transecting both IONs, results in rescued callosal projection. In contrast, non-simultaneous bilateral ION transection has the same effect as unilateral transection. Similar results are obtained by lesion of whisker hair follicles. c-Fos-positive neurons in brain slices treated with KCl is decreased more in contralateral cortex with unilateral removal of sensory input, but decreased similarly in both cortices in mice with simultaneous bilateral removal of sensory input. Frequency of sEPSC of cortical neurons is also reduced in contralateral cortex with the unilateral removal of sensory input, but equally reduced on both sides with the bilateral removal of sensory input, suggesting that unbalanced bilateral sensory input might lead to mismatched neuronal activity between the two cortices and contribute to the formation of callosal projection.

Conclusion

Our data demonstrate a critical role of balanced bilateral somatosensory input in the formation of callosal connections, and thus reveal a new role of sensory input in wiring brain circuits.

Induced current pharmacological split stimulation system for neuronal networks

Magnetic stimulation noninvasively modulates neuronal activity through a magnetically induced current. However, despite the usefulness and popularity of this method, the effects of neuronal activity in the nonstimulated regions on the stimulus responses are unknown. Here, we report that the induced current-evoked responses were affected by neuronal activities in the nonstimulated regions. Our experiment used a Mu-metal-based localized induced current stimulation (LICS) system combined with the microfabricated cell culture chamber system and a microelectrode array (MEA). The cell culture chamber system has radiating microtunnels connecting one central and eight outer chambers, which were fabricated using soft lithography and a replica modeling technique with SU-8 photoresist and polydimethylsiloxane (PDMS). Rat cortical neurons were separately cultured in the chambers and formed functional synaptic connections through the microtunnels. By applying a biphasic alternating pulsed magnetic field to the Mu-metal located in the central chamber, induced currents were mainly generated near the cultured neurons and modified the neuronal activities, which were recorded through MEA. Furthermore, we confirmed that the evoked responses were modified by localized pharmacological stimulation (LPS) in the outer chambers. These results suggest that our system would be promising tool for analyzing the effect of magnetic stimulation on interacting neuronal activity.

Optogenetic-mediated increases in in vivo spontaneous activity disrupt pool-specific but not dorsal-ventral motoneuron pathfinding

Rhythmic waves of spontaneous electrical activity are widespread in the developing nervous systems of birds and mammals, and although many aspects of neural development are activity-dependent, it has been unclear if rhythmic waves are required for in vivo motor circuit development, including the proper targeting of motoneurons to muscles. We show here that electroporated channelrhodopsin-2 can be activated in ovo with light flashes to drive waves at precise intervals of approximately twice the control frequency in intact chicken embryos. Optical monitoring of associated axial movements ensured that the altered frequency was maintained. In embryos thus stimulated, motor axons correctly executed the binary dorsal-ventral pathfinding decision but failed to make the subsequent pool-specific decision to target to appropriate muscles. This observation, together with the previous demonstration that slowing the frequency by half perturbed dorsal-ventral but not pool-specific pathfinding, shows that modest changes in frequency differentially disrupt these two major pathfinding decisions. Thus, many drugs known to alter early rhythmic activity have the potential to impair normal motor circuit development, and given the conservation between mouse and avian spinal cords, our observations are likely relevant to mammals, where such studies would be difficult to carry out.

Hox genes: choreographers in neural development, architects of circuit organization

The neural circuits governing vital behaviors, such as respiration and locomotion, are comprised of discrete neuronal populations residing within the brainstem and spinal cord. Work over the past decade has provided a fairly comprehensive understanding of the developmental pathways that determine the identity of major neuronal classes within the neural tube. However, the steps through which neurons acquire the subtype diversities necessary for their incorporation into a particular circuit are still poorly defined. Studies on the specification of motor neurons indicate that the large family of Hox transcription factors has a key role in generating the subtypes required for selective muscle innervation. There is also emerging evidence that Hox genes function in multiple neuronal classes to shape synaptic specificity during development, suggesting a broader role in circuit assembly. This Review highlights the functions and mechanisms of Hox gene networks and their multifaceted roles during neuronal specification and connectivity.

GABAA receptor-mediated tonic depolarization in developing neural circuits

The activation of GABAA receptors (the type A receptors for γ-aminobutyric acid) produces two distinct forms of responses, phasic (i.e., transient) and tonic (i.e., persistent), that are mediated by synaptic and extrasynaptic GABAA receptors, respectively. During development, the intracellular chloride levels are high so activation of these receptors causes a net outward flow of anions that leads to neuronal depolarization rather than hyperpolarization. Therefore, in developing neural circuits, tonic activation of GABAA receptors may provide persistent depolarization. Recently, it became evident that GABAA receptor-mediated tonic depolarization alters the structure of patterned spontaneous activity, a feature that is common in developing neural circuits and is important for neural circuit refinement. Thus, this persistent depolarization may lead to a long-lasting increase in intracellular calcium level that modulates network properties via calcium-dependent signaling cascades. This article highlights the features of GABAA receptor-mediated tonic depolarization, summarizes the principles for discovery, reviews the current findings in diverse developing circuits, examines the underlying molecular mechanisms and modulation systems, and discusses their functional specializations for each developing neural circuit.

Retinal input regulates the timing of corticogeniculate innervation

Neurons in layer VI of visual cortex represent one of the largest sources of nonretinal input to the dorsal lateral geniculate nucleus (dLGN) and play a major role in modulating the gain of thalamic signal transmission. However, little is known about how and when these descending projections arrive and make functional connections with dLGN cells. Here we used a transgenic mouse to visualize corticogeniculate projections to examine the timing of cortical innervation in dLGN. Corticogeniculate innervation occurred at postnatal ages and was delayed compared with the arrival of retinal afferents. Cortical fibers began to enter dLGN at postnatal day 3 (P3) to P4, a time when retinogeniculate innervation is complete. However, cortical projections did not fully innervate dLGN until eye opening (P12), well after the time when retinal inputs from the two eyes segregate to form nonoverlapping eye-specific domains. In vitro thalamic slice recordings revealed that newly arriving cortical axons form functional connections with dLGN cells. However, adult-like responses that exhibited paired pulse facilitation did not fully emerge until 2 weeks of age. Finally, surgical or genetic elimination of retinal input greatly accelerated the rate of corticogeniculate innervation, with axons invading between P2 and P3 and fully innervating dLGN by P8 to P10. However, recordings in genetically deafferented mice showed that corticogeniculate synapses continued to mature at the same rate as controls. These studies suggest that retinal and cortical innervation of dLGN is highly coordinated and that input from retina plays an important role in regulating the rate of corticogeniculate innervation.

Spontaneous calcium waves in granule cells in cerebellar slice cultures

Multiple regions in the CNS display propagating correlated activity during embryonic and postnatal development. This activity can be recorded as waves of increased calcium concentrations in spiking neurons or glia cells, and have been suggested to be involved in patterning, axonal guidance and establishment of synaptic transmission. Here, we used calcium imaging in slice cultures of the postnatal cerebellum, and observe spontaneous propagating calcium waves in NeuN-positive granule-like cells. Wave formation was blocked by TTX and the AMPA antagonist NBQX, but persisted after NMDA receptor blockade with MK-801. Whole-cell recordings during wave formation showed cyclic EPSP barrages with an amplitude of 10-20 mV concurrent with wave activity. Local non-propagating putative transglial waves were also present in the cultures, and could be reproduced by pressure application of ATP. We hypothesize, that the propagating wave activity is carried through the tissue by axonal collaterals formed by neighboring granule cells, and further suggest that the correlated activity may be related to processes that ensure correct postnatal wiring of the cerebellar circuits.

The cell-autonomous role of excitatory synaptic transmission in the regulation of neuronal structure and function

The cell-autonomous role of synaptic transmission in the regulation of neuronal structural and electrical properties is unclear. We have now employed a genetic approach to eliminate glutamatergic synaptic transmission onto individual CA1 pyramidal neurons in a mosaic fashion in vivo. Surprisingly, while electrical properties are profoundly affected in these neurons, as well as inhibitory synaptic transmission, we found little perturbation of neuronal morphology, demonstrating a functional segregation of excitatory synaptic transmission from neuronal morphological development.

Role of EphA/ephrin--a signaling in the development of topographic maps in mouse corticothalamic projections

Corticothalamic (CT) feedback outnumbers thalamocortical projections and regulates sensory information processing at the level of the thalamus. It is well established that EphA7, a member of EphA receptor family, is involved in the topographic mapping of CT projections. The aim of the present study was to dissect the precise impact of EphA7 on each step of CT growth. We used in utero electroporation-mediated EphA7 overexpression in developing somatosensory CT axons to dissect EphA7/ephrin-A-dependent mechanisms involved in regulating both initial targeting and postnatal growth of the CT projections. Our data revealed that topographic maps of cortical afferents in the ventrobasal complex and medial part of the posterior complex in the thalamus become discernible shortly after birth and are fully established by the second postnatal week. This process starts with the direct ingrowth of the CT axons to the designated areas within target thalamic nuclei and by progressive increase of axonal processes in the terminal zones. Large-scale overproduction and elimination of exuberant widespread axonal branches outside the target zone was not observed. Each developmental event was coordinated by spatially and temporally different responsiveness of CT axons to the ephrin-A gradient in thalamic nuclei, as well as by the matching levels of EphA7 in CT axons and ephrin-As in thalamic nuclei. These results support the concept that the topographic connections between the maps in the cerebral cortex and corresponding thalamic nuclei are genetically prespecified to a large extent, and established by precise spatiotemporal molecular mechanisms that involve the Eph family of genes.

Homeostatic synaptic plasticity in developing spinal networks driven by excitatory GABAergic currents

Homeostatic plasticity refers to mechanisms that the cell or network engage in order to homeostatically maintain a preset level of activity. These mechanisms include compensatory changes in cellular excitability, excitatory and inhibitory synaptic strength and are typically studied at a developmental stage when GABA or glycine is inhibitory. Here we focus on the expression of homeostatic plasticity in the chick embryo spinal cord at a stage when GABA is excitatory. When spinal activity is perturbed in the living embryo there are compensatory changes in postsynaptic AMPA receptors and in the driving force for GABAergic currents. These changes are triggered by reduced GABAA receptor signaling, which appears to be part of the sensing machinery for triggering homeostatic plasticity. We compare and contrast these findings to homeostatic plasticity expressed in spinal systems at different stages of development, and to the developing retina at a stage when GABA is depolarizing. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Neuronal correlates of the dominant role of GABAergic transmission in the developing mouse locomotor circuitry

GABA and glycine are the primary fast inhibitory neurotransmitters in the mammalian spinal cord, but they differ in their regulatory functions, balancing neuronal excitation in the locomotor circuitry in the mammalian spinal cord. This review focuses on the unique role of GABAergic transmission during the assembly of the locomotor circuitry, from early embryonic stages when GABA(A) receptor-activated membrane depolarizations increase network excitation, to the period of early postnatal development, when GABAergic inhibition plays a primary role in coordinating the patterns of locomotor-like motor activity. To gain insight into the mechanisms that underlie the dominant contribution of GABAergic transmission to network activity during that period, we examined the morphological and electrophysiological properties of a subpopulation of GABAergic commissural interneurons that fit well with their putative function as integrated components of the rhythm-coordinating networks in the mouse spinal cord.

Large-scale synchronized activity in the embryonic brainstem and spinal cord

In the developing central nervous system, spontaneous activity appears well before the brain responds to external sensory inputs. One of the earliest activities is observed in the hindbrain and spinal cord, which is detected as rhythmic electrical discharges of cranial and spinal motoneurons or oscillations of Ca(2+)- and voltage-related optical signals. Shortly after the initial expression, the spontaneous activity appearing in the hindbrain and spinal cord exhibits a large-scale correlated wave that propagates over a wide region of the central nervous system, maximally extending to the lumbosacral cord and to the forebrain. In this review, we describe several aspects of this synchronized activity by focusing on the basic properties, development, origin, propagation pattern, pharmacological characteristics, and possible mechanisms underlying the generation of the activity. These profiles differ from those of the respiratory and locomotion pattern generators observed in the mature brainstem and spinal cord, suggesting that the wave is primordial activity that appears during a specific period of embryonic development and plays some important roles in the development of the central nervous system.

Axon guidance mechanisms for establishment of callosal connections

Numerous studies have investigated the formation of interhemispheric connections which are involved in high-ordered functions of the cerebral cortex in eutherian animals, including humans. The development of callosal axons, which transfer and integrate information between the right/left hemispheres and represent the most prominent commissural system, must be strictly regulated. From the beginning of their growth, until reaching their targets in the contralateral cortex, the callosal axons are guided mainly by two environmental cues: (1) the midline structures and (2) neighboring? axons. Recent studies have shown the importance of axona guidance by such cues and the underlying molecular mechanisms. In this paper, we review these guidance mechanisms during the development of the callosal neurons. Midline populations express and secrete guidance molecules, and "pioneer" axons as well as interactions between the medial and lateral axons are also involved in the axon pathfinding of the callosal neurons. Finally, we describe callosal dysgenesis in humans and mice, that results from a disruption of these navigational mechanisms.

Activity-dependent competition regulates motor neuron axon pathfinding via PlexinA3

The role of electrical activity in axon guidance has been extensively studied in vitro. To better understand its role in the intact nervous system, we imaged intracellular Ca(2+) in zebrafish primary motor neurons (PMN) during axon pathfinding in vivo. We found that PMN generate specific patterns of Ca(2+) spikes at different developmental stages. Spikes arose in the distal axon of PMN and were propagated to the cell body. Suppression of Ca(2+) spiking activity in single PMN led to stereotyped errors, but silencing all electrical activity had no effect on axon guidance, indicating that an activity-based competition rule regulates this process. This competition was not mediated by synaptic transmission. Combination of PlexinA3 knockdown with suppression of Ca(2+) activity in single PMN produced a synergistic increase in the incidence of pathfinding errors. However, expression of PlexinA3 transcripts was not regulated by activity. Our results provide an in vivo demonstration of the intersection of spontaneous electrical activity with the PlexinA3 guidance molecule receptor in regulation of axon pathfinding.

Tonic and transient endocannabinoid regulation of AMPAergic miniature postsynaptic currents and homeostatic plasticity in embryonic motor networks

Endocannabinoid signaling has been shown to mediate synaptic plasticity by retrogradely inhibiting presynaptic transmitter release in several systems. We found that endocannabinoids act tonically to regulate AMPA miniature postsynaptic current (mPSC) frequency in embryonic motor circuits of the chick spinal cord. Further, strong postsynaptic depolarizations also induced a short-lived endocannabinoid-mediated suppression of mEPSC frequency. Unlike many previous studies, endocannabinoid signaling was not found to influence evoked transmitter release. The results suggest a special role for spontaneous glutamatergic mPSCs and their control by endocannabinoids in the developing spinal cord. We determined that blocking endocannabinoid signaling, which increases spontaneous glutamatergic release, increased spontaneous network activity in vitro and in vivo. Previous work in spinal motoneurons had shown that reducing spontaneous network activity (SNA) chronically in vivo led to homeostatic increases in AMPA and GABA mPSC amplitude (homeostatic synaptic plasticity). Blocking endocannabinoid signaling in vivo, and thus increasing SNA, triggered compensatory decreases of both AMPA and GABA mPSC amplitudes. These findings, combined with previous results, are consistent with the idea that this form of homeostatic synaptic plasticity is a bidirectional process in the living embryo. Together, our results suggest a role for tonic signaling of endocannabinoids as a potential mechanism to regulate the level of SNA, which is known to be critical for synaptic maturation in the embryonic spinal cord.

Activity-dependent callosal axon projections in neonatal mouse cerebral cortex

Callosal axon projections are among the major long-range axonal projections in the mammalian brain. They are formed during the prenatal and early postnatal periods in the mouse, and their development relies on both activity-independent and -dependent mechanisms. In this paper, we review recent findings about the roles of neuronal activity in callosal axon projections. In addition to the well-documented role of sensory-driven neuronal activity, recent studies using in utero electroporation demonstrated an essential role of spontaneous neuronal activity generated in neonatal cortical circuits. Both presynaptic and postsynaptic neuronal activities are critically involved in the axon development. Studies have begun to reveal intracellular signaling pathway which works downstream of neuronal activity. We also review several distinct patterns of neuronal activity observed in the developing cerebral cortex, which might play roles in activity-dependent circuit construction. Such neuronal activity during the neonatal period can be disrupted by genetic factors, such as mutations in ion channels. It has been speculated that abnormal activity caused by such factors may affect activity-dependent circuit construction, leading to some developmental disorders. We discuss a possibility that genetic mutation in ion channels may impair callosal axon projections through an activity-dependent mechanism.

Timing of developmental sequences in different brain structures: physiological and pathological implications

The developing brain is not a small adult brain. Voltage- and transmitter-gated currents, like network-driven patterns, follow a developmental sequence. Studies initially performed in cortical structures and subsequently in subcortical structures have unravelled a developmental sequence of events in which intrinsic voltage-gated calcium currents are followed by nonsynaptic calcium plateaux and synapse-driven giant depolarising potentials, orchestrated by depolarizing actions of GABA and long-lasting NMDA receptor-mediated currents. The function of these early patterns is to enable heterogeneous neurons to fire and wire together rather than to code specific modalities. However, at some stage, behaviourally relevant activities must replace these immature patterns, implying the presence of programmed stop signals. Here, we show that the developing striatum follows a developmental sequence in which immature patterns are silenced precisely when the pup starts locomotion. This is mediated by a loss of the long-lasting NMDA-NR2C/D receptor-mediated current and the expression of a voltage-gated K(+) current. At the same time, the descending inputs to the spinal cord become fully functional, accompanying a GABA/glycine polarity shift and ending the expression of developmental patterns. Therefore, although the timetable of development differs in different brain structures, the g sequence is quite similar, relying first on nonsynaptic events and then on synaptic oscillations that entrain large neuronal populations. In keeping with the 'neuroarcheology' theory, genetic mutations or environmental insults that perturb these developmental sequences constitute early signatures of developmental disorders. Birth dating developmental disorders thus provides important indicators of the event that triggers the pathological cascade leading ultimately to disease.

Expression patterns of Ephs and ephrins throughout retinotectal development in Xenopus laevis

The Eph family of receptor tyrosine kinases and their ligands the ephrins play an essential role in the targeting of retinal ganglion cell axons to topographically correct locations in the optic tectum during visual system development. The African claw-toed frog Xenopus laevis is a popular animal model for the study of retinotectal development because of its amenability to live imaging and electrophysiology. Its visual system undergoes protracted growth continuing beyond metamorphosis, yet little is known about ephrin and Eph expression patterns beyond stage 39 when retinal axons first arrive in the tectum. We used alkaline phosphatase fusion proteins of EphA3, ephrin-A5, EphB2, and ephrin-B1 as affinity probes to reveal the expression patterns of ephrin-As, EphAs, ephrin-Bs, and EphBs, respectively. Analysis of brains from stage 40 to adult frog revealed that ephrins and Eph receptors are expressed throughout development. As observed in other species, staining for ephrin-As displayed a high caudal to low rostral expression pattern across the tectum, roughly complementary to the expression of EphAs. In contrast with the prevailing model, EphBs were found to be expressed in the tectum in a high dorsal to low ventral gradient in young animals. In animals with induced binocular tectal innervation, ocular dominance bands of alternating input from the two eyes formed in the tectum; however, ephrin-A and EphA expression patterns were unmodulated and similar to those in normal frogs, confirming that the segregation of axons into eye-specific stripes is not the consequence of a respecification of molecular guidance cues in the tectum.

Brief transvertebral electrical stimulation of the spinal cord improves the specificity of femoral nerve reinnervation

BACKGROUND

Functional outcomes are generally poor following peripheral nerve injury (PNI). The reason is multifactorial but includes the misdirection of regenerating axons to inappropriate end organs. It has been shown that brief electrical stimulation (Estim) of nerves has the potential to improve the accuracy and rate of peripheral axon regeneration.

OBJECTIVE

The present study explores a novel percutaneous transvertebral approach to Estim, which was tested in the mouse femoral nerve model.

METHODS

Inspired by the protocol of Gordon and colleagues (ie, 20 Hz, for 1 hour), we applied Estim to the cervicothoracic spinal cord (SC-Estim) to remotely activate lumbar motor neurons following transection and repair of the femoral nerve. Fluorescent dyes were applied to the distal nerve to label reinnervating cells. Sections of nerve were taken to quantify the numbers of reinnervating axons as well as to stain for a known femoral axon guidance molecule-polysialylated neural cell adhesion molecule (PSA-NCAM).

RESULTS

In comparison to sham treatment, SC-Estim led to significantly greater expression of PSA-NCAM as well as improved the specificity of motor reinnervation. Interestingly, although SC-Estim did not alter the number of early reinnervating (ie, pioneer) axons, there was a reduction in the number of retrogradely labeled neurons at 2 weeks postrepair. However, by 6 weeks postrepair, there was no difference in the number of neurons that had reinnervated the femoral nerve.

CONCLUSIONS

The present findings support the development of SC-Estim as a novel approach to enhance the specificity of reinnervation and potentially improve functional outcomes following PNI.

Shaping brain connections through spontaneous neural activity

An overwhelming number of observations demonstrate that neural activity and genetic programs interact to specify the composition and organization of neural circuits during all stages of development. Spontaneous neuronal activities have been documented in several developing neural regions in both invertebrates and vertebrates, and their roles are mostly conserved among species. Among these roles, Ca(2+) spikes and levels of electrical activity have been shown to regulate neurite growth, axon extension and axon branching. Here, we review selected findings concerning the role of spontaneous activity on circuit development.

Interplay between electrical activity and bone morphogenetic protein signaling regulates spinal neuron differentiation

A gradient of bone morphogenetic proteins (BMPs) along the dorsoventral axis of the spinal cord is necessary for the specification of dorsal neurons. Concurrently, a gradient of calcium-mediated electrical activity is present in the developing spinal cord but in an opposing ventrodorsal direction. Whether BMPs and electrical activity interact in embryonic spinal neurons remains unknown. We show that BMP decreases electrical activity by enhancing p38 MAPK-mediated negative modulation of voltage-gated sodium channels. In turn, electrical activity affects the phosphorylation status and nuclear level of activated Smads, the canonical components of BMP signaling. This interaction between calcium spike activity and BMP signaling regulates the specification of the dorsal commissural spinal neuron phenotype. The present study identifies an unexpected interplay between BMPs and electrical activity that is critical for decoding the morphogen gradient during spinal neuron differentiation.

Developmental lead acetate exposure induces embryonic toxicity and memory deficit in adult zebrafish

Lead is a persistent metal and commonly present in our living environment. The present study was aimed to investigate lead-induced embryonic toxicity, behavioral responses, and adult learning/memory deficit in zebrafish. Lead acetate (PbAc) induced malformations such as uninflated swim bladder, bent spine and yolk-sac edema with an EC₅₀ of 0.29 mg/L at 120 h post fertilization (hpf). Spontaneous movement as characterized by tail bend frequency was significantly altered in zebrafish embryos following exposure to PbAc. Behavior assessment demonstrated that lead exposure changed behavioral responses in zebrafish larvae, as hyperactivity was detected within the first minute of light-to-dark transition in the fish exposed to PbAc from 6 to 96 hpf, and a different dose-dependent change was found in swimming speeds in the dark and in the light at 120 hpf following lead exposure. Learning/memory task assay showed that embryos exposed to PbAc from 6 to 120 hpf developed learning/memory deficit at adulthood as exhibited by a significant decrease in accuracy rate to find the food and a significant increase in finding time. Overall, our results suggested that low dose of developmental lead exposure resulted in embryonic toxicity, behavioral alteration, and adult learning/memory deficit in zebrafish.

Spontaneous activity regulates Robo1 transcription to mediate a switch in thalamocortical axon growth

Developing axons must control their growth rate to follow the appropriate pathways and establish specific connections. However, the regulatory mechanisms involved remain elusive. By combining live imaging with transplantation studies in mice, we found that spontaneous calcium activity in the thalamocortical system and the growth rate of thalamocortical axons were developmentally and intrinsically regulated. Indeed, the spontaneous activity of thalamic neurons governed axon growth and extension through the cortex in vivo. This activity-dependent modulation of growth was mediated by transcriptional regulation of Robo1 through an NF-κB binding site. Disruption of either the Robo1 or Slit1 genes accelerated the progression of thalamocortical axons in vivo, and interfering with Robo1 signaling restored normal axon growth in electrically silent neurons. Thus, modifications to spontaneous calcium activity encode a switch in the axon outgrowth program that allows the establishment of specific neuronal connections through the transcriptional regulation of Slit1 and Robo1 signaling.

Spinocerebellar ataxia type 13 mutation that is associated with disease onset in infancy disrupts axonal pathfinding during neuronal development

Spinocerebellar ataxia type 13 (SCA13) is an autosomal dominant disease caused by mutations in the Kv3.3 voltage-gated potassium (K⁺) channel. SCA13 exists in two forms: infant onset is characterized by severe cerebellar atrophy, persistent motor deficits and intellectual disability, whereas adult onset is characterized by progressive ataxia and progressive cerebellar degeneration. To test the hypothesis that infant- and adult-onset mutations have differential effects on neuronal development that contribute to the age at which SCA13 emerges, we expressed wild-type Kv3.3 or infant- or adult-onset mutant proteins in motor neurons in the zebrafish spinal cord. We characterized the development of CaP (caudal primary) motor neurons at ∼36 and ∼48 hours post-fertilization using confocal microscopy and 3D digital reconstruction. Exogenous expression of wild-type Kv3.3 had no significant effect on CaP development. In contrast, CaP neurons expressing the infant-onset mutation made frequent pathfinding errors, sending long, abnormal axon collaterals into muscle territories that are normally innervated exclusively by RoP (rostral primary) or MiP (middle primary) motor neurons. This phenotype might be directly relevant to infant-onset SCA13 because interaction with inappropriate synaptic partners might trigger cell death during brain development. Importantly, pathfinding errors were not detected in CaP neurons expressing the adult-onset mutation. However, the adult-onset mutation tended to increase the complexity of the distal axonal arbor. From these results, we speculate that infant-onset SCA13 is associated with marked changes in the development of Kv3.3-expressing cerebellar neurons, reducing their health and viability early in life and resulting in the withered cerebellum seen in affected children.

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.

Eye-specific projections of retinogeniculate axons are altered in albino mice

The divergence of retinal ganglion cell (RGC) axons into ipsilateral and contralateral projections at the optic chiasm and the subsequent segregation of retinal inputs into eye-specific domains in their target, the dorsal lateral geniculate nucleus (dLGN), are crucial for binocular vision. In albinism, affected individuals exhibit a lack or reduction of pigmentation in the eye and skin, a concomitant reduced ipsilateral projection, and diverse visual defects. Here we investigate how such altered decussation affects eye-specific retinogeniculate targeting in albino mice using the C57BL/6 Tyr(c-2J/c-2J) strain, in which tyrosinase, necessary for melanogenesis, is mutated. In albino mice, fewer RGCs from the ventrotemporal (VT) retina project ipsilaterally, reflected in a decrease in cells expressing ipsilateral markers. In addition, a population of RGCs from the VT retina projects contralaterally and, within the dLGN, their axons cluster into a patch separated from the contralateral termination area. Furthermore, eye-specific segregation is not complete in the albino dLGN and, upon perturbing postnatal retinal activity with epibatidine, the ipsilateral projection fragments and the aberrant contralateral patch disappears. These results suggest that the defects in afferent targeting and activity-dependent refinement in the albino dLGN arise from RGC misspecification together with potential perturbations of early activity patterns in the albino retina.

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

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

PlexinA4 distribution in the adult rat spinal cord and dorsal root ganglia

PlexinsA1-A4 participate in class 3 semaphorin signaling as co-receptors to neuropilin 1 and 2, PlexinA4 being the latest member of the PlexinA subfamily to be identified. Little is known about the cellular distribution of PlexinA4 in the spinal cord and dorsal root ganglion (DRG). Here, immunohistochemical studies using antibodies to PlexinA4 revealed immunolabeling in neurons in both dorsal and, to a greater extent, ventral horns of the spinal cord. Ventral horn PlexinA4 positive neurons exhibited morphology, size, and location consistent with both motor neurons and interneurons. Labeling was found in motor axons exiting through the ventral roots, and more widespread labeling was observed in ascending and descending white matter tracts. Within the DRG, immunostaining was observed in neuronal cell bodies as well as the central and peripheral processes of these cells. PlexinA4 is expressed in the peripheral nervous system where its expression is regulated upon nerve injury. This is the first detailed description of the cellular and subcellular distribution of PlexinA4 in the adult spinal cord and DRG, and it will set the basis for future studies on the potential role of PlexinA4 in regeneration and repair of the adult central and peripheral nervous system.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Spontaneous depolarization wave in the mouse embryo: origin and large-scale propagation over the CNS identified with voltage-sensitive dye imaging

Spontaneous embryonic movements, called embryonic motility, are produced by correlated spontaneous activity in the cranial and spinal nerves, which is driven by brainstem and spinal networks. Using optical imaging with a voltage-sensitive dye, we have revealed previously that this correlated activity is a widely propagating wave of neural depolarization, which we termed the depolarization wave. We have observed in the chick and rat embryos that the activity spread over an extensive region of the CNS, including the spinal cord, hindbrain, cerebellum, midbrain and forebrain. One important consideration is whether a depolarization wave with similar characteristics occurs in other species, especially in different mammals. Here, we provide evidence for the existence of the depolarization wave in the mouse embryo by showing that the widely propagating wave appeared independently of the localized spontaneous activity detected previously with Ca(2+) imaging. Furthermore, we mapped the origin of the depolarization wave and revealed that the wave generator moved from the rostral spinal cord to the caudal cord as development proceeded, and was later replaced with mature rhythmogenerators. The present study, together with an accompanying paper that describes pharmacological properties of the mouse depolarization wave, shows that a synchronized wave with common characteristics is expressed in different species, suggesting fundamental roles in neural development.

Bilateral enucleation alters gene expression and intraneocortical connections in the mouse

Background

Anatomically and functionally distinct sensory and motor neocortical areas form during mammalian development through a process called arealization. This process is believed to be reliant on both activity-dependent and activity-independent mechanisms. Although both mechanisms are thought to function concurrently during arealization, the nature of their interaction is not understood. To examine the potential interplay of extrinsic activity-dependent mechanisms, such as sensory input, and intrinsic activity-independent mechanisms, including gene expression in mouse neocortical development, we performed bilateral enucleations in newborn mice and conducted anatomical and molecular analyses 10 days later. In this study, by surgically removing the eyes of the newborn mouse, we examined whether early enucleation would impact normal gene expression and the development of basic anatomical features such as intraneocortical connections and cortical area boundaries in the first 10 days of life, before natural eye opening. We examined the acute effects of bilateral enucleation on the lateral geniculate nucleus of the thalamus and the neocortical somatosensory-visual area boundary through detailed analyses of intraneocortical connections and gene expression of six developmentally regulated genes at postnatal day 10.

Results

Our results demonstrate short-term plasticity on postnatal day 10 resulting from the removal of the eyes at birth, with changes in nuclear size and gene expression within the lateral geniculate nucleus as well as a shift in intraneocortical connections and ephrin A5 expression at the somatosensory-visual boundary. In this report, we highlight the correlation between positional shifts in ephrin A5 expression and improper refinement of intraneocortical connections observed at the somatosensory-visual boundary in enucleates on postnatal day 10.

Conclusions

Bilateral enucleation induces a positional shift of both ephrin A5 expression and intraneocortical projections at the somatosensory-visual border in only 10 days. These changes occur prior to natural eye opening, suggesting a possible role of spontaneous retinal activity in area border formation within the neocortex. Through these analyses, we gain a deeper understanding of how extrinsic activity-dependent mechanisms, particularly input from sensory organs, are integrated with intrinsic activity-independent mechanisms to regulate neocortical arealization and plasticity.

Homeostatic synaptic plasticity: local and global mechanisms for stabilizing neuronal function

Neural circuits must maintain stable function in the face of many plastic challenges, including changes in synapse number and strength, during learning and development. Recent work has shown that these destabilizing influences are counterbalanced by homeostatic plasticity mechanisms that act to stabilize neuronal and circuit activity. One such mechanism is synaptic scaling, which allows neurons to detect changes in their own firing rates through a set of calcium-dependent sensors that then regulate receptor trafficking to increase or decrease the accumulation of glutamate receptors at synaptic sites. Additional homeostatic mechanisms may allow local changes in synaptic activation to generate local synaptic adaptations, and network-wide changes in activity to generate network-wide adjustments in the balance between excitation and inhibition. The signaling pathways underlying these various forms of homeostatic plasticity are currently under intense scrutiny, and although dozens of molecular pathways have now been implicated in homeostatic plasticity, a clear picture of how homeostatic feedback is structured at the molecular level has not yet emerged. On a functional level, neuronal networks likely use this complex set of regulatory mechanisms to achieve homeostasis over a wide range of temporal and spatial scales.

Knockdown of ephrin-A5 expression by 40% does not affect motor axon growth or migration into the chick hindlimb

Bidirectional signaling between Eph receptor tyrosine kinases and their cell-surface protein signals, the ephrins, comprises one mechanism for guiding motor axons to their proper targets. During projection of motor axons from the lateral motor column (LMC) motor neurons of the spinal cord to the hindlimb muscles in chick embryos, ephrin-A5 has been shown to be expressed in the LMC motor axons until they reach the base of the limb bud and initiate sorting into their presumptive dorsal and ventral nerve trunks, at which point expression is extinguished. We tested the hypothesis that this dynamic pattern of ephrin-A5 expression in LMC motor axons is important for the growth and guidance of the axons to, and into, the hindlimb by knocking down endogenous ephrin-A5 expression in the motor neurons and their axons. No perturbation of LMC motor axon projections was observed in response to this treatment, suggesting that ephrin-A5 is not needed for LMC motor axon growth or guidance.

Renshaw cell interneuron specialization is controlled by a temporally restricted transcription factor program

The spinal cord contains a diverse array of physiologically distinct interneuron cell types that subserve specialized roles in somatosensory perception and motor control. The mechanisms that generate these specialized interneuronal cell types from multipotential spinal progenitors are not known. In this study, we describe a temporally regulated transcriptional program that controls the differentiation of Renshaw cells (RCs), an anatomically and functionally discrete spinal interneuron subtype. We show that the selective activation of the Onecut transcription factors Oc1 and Oc2 during the first wave of V1 interneuron neurogenesis is a key step in the RC differentiation program. The development of RCs is additionally dependent on the forkhead transcription factor Foxd3, which is more broadly expressed in postmitotic V1 interneurons. Our demonstration that RCs are born, and activate Oc1 and Oc2 expression, in a narrow temporal window leads us to posit that neuronal diversity in the developing spinal cord is established by the composite actions of early spatial and temporal determinants.

How is the olfactory map formed and interpreted in the mammalian brain?

Odor signals received by odorant receptors (ORs) expressed by olfactory sensory neurons (OSNs) in the olfactory epithelium (OE) are represented as an odor map in the olfactory bulb (OB). In the mouse, there are ~1,000 different OR species, and each OSN expresses only one functional OR gene in a monoallelic manner. Furthermore, OSN axons expressing the same type of OR converge on a specific target site in the OB, forming a glomerular structure. Because each glomerulus represents a single OR species, and a single odorant can interact with multiple OR species, odor signals received in the OE are converted into a topographic map of multiple glomeruli activated with varying magnitudes. Here we review recent progress in the study of the mammalian olfactory system, focusing on the formation of the olfactory map and the transmission of topographical information in the OB to the olfactory cortex to elicit various behaviors.

Eph and ephrin signaling: lessons learned from spinal motor neurons

In nervous system assembly, Eph/ephrin signaling mediates many axon guidance events that shape the formation of precise neuronal connections. However, due to the complexity of interactions between Ephs and ephrins, the molecular logic of their action is still being unraveled. Considerable advances have been made by studying the innervation of the limb by spinal motor neurons, a series of events governed by Eph/ephrin signaling. Here, we discuss the contributions of different Eph/ephrin modes of interaction, downstream signaling and electrical activity, and how these systems may interact both with each other and with other guidance molecules in limb muscle innervation. This simple model system has emerged as a very powerful tool to study this set of molecules, and will continue to be so by virtue of its simplicity, accessibility and the wealth of pioneering cellular studies.

Artificial CSF motion ensures rhythmic activity in the developing CNS ex vivo: a mechanical source of rhythmogenesis?

Spontaneous rhythmic activity is a ubiquitous feature of developing neural structures that has been shown to be essential for the establishment of functional CNS connectivity. However, the primordial origin of these rhythms remains unknown. Here, we describe two types of rhythmic activity in distinct parts of the developing CNS isolated ex vivo on microelectrode arrays, the expression of which was found to be strictly dependent upon the movement of the artificial CSF (aCSF) flowing over the inner wall of the ventricles or over the outer surface of the CNS. First, whole embryonic mouse hindbrain-spinal cord preparations (stages E12.5-E15.5) rhythmically expressed waves of activity originating in the hindbrain and propagating in the spinal cord. Interestingly enough, the frequency of this rhythm was completely determined by the speed of the aCSF flow. In particular, at all stages considered, hindbrain activity was abolished when the perfusion was stopped. Immature rhythmic activity was also recorded in the isolated newborn (P0-P8) mouse cortex under normal aCSF perfusion. Again, this rhythm was abolished when the perfusion flow was stopped. In both structures, this phenomenon was not due to changes in temperature, oxygen level, or pH of the bath, but to the movement itself of the aCSF. These observations challenge the so-called "spontaneous" nature of rhythmic activity in immature neural networks and suggest that the movement of CSF in the ventricles and around the brain in vivo may mechanically drive rhythmogenesis in the developing CNS.

In vivo calcium dynamics during neural crest cell migration and patterning using GCaMP3

Examining calcium dynamics within the neural crest (NC) has the potential to shed light on mechanisms that regulate complex cell migration and patterning events during embryogenesis. Unfortunately, typical calcium indicators are added to culture media or have low signal to noise after microinjection into tissue that severely limit analyses to cultured cells or superficial events. Here, we studied in vivo calcium dynamics during NC cell migration and patterning, using a genetically encoded calcium sensor, GCaMP3. We discovered that trunk NC cells displayed significantly more spontaneous calcium transients than cranial NC cells, and during cell aggregation versus cell migration events. Spontaneous calcium transients were more prevalent during NC cell aggregation into discrete sympathetic ganglia (SG). Blocking of N-cadherin activity in trunk NC cells near the presumptive SG led to a dramatic decrease in the frequency of spontaneous calcium transients. Detailed analysis and mathematical modeling of cell behaviors during SG formation showed NC cells aggregated into clusters after displaying a spontaneous calcium transient. This approach highlights the novel application of a genetically encoded calcium indicator to study subsets of cells during ventral events in embryogenesis.