Subcellular topography of visually driven dendritic activity in the vertebrate visual system

Morphology and synapse topography optimize linear encoding of synapse numbers in Drosophila looming responsive descending neurons

Synapses are often precisely organized on dendritic arbors, yet the role of synaptic topography in dendritic integration remains poorly understood. Utilizing electron microscopy (EM) connectomics we investigate synaptic topography in Drosophila melanogaster looming circuits, focusing on retinotopically tuned visual projection neurons (VPNs) that synapse onto descending neurons (DNs). Synapses of a given VPN type project to non-overlapping regions on DN dendrites. Within these spatially constrained clusters, synapses are not retinotopically organized, but instead adopt near random distributions. To investigate how this organization strategy impacts DN integration, we developed multicompartment models of DNs fitted to experimental data and using precise EM morphologies and synapse locations. We find that DN dendrite morphologies normalize EPSP amplitudes of individual synaptic inputs and that near random distributions of synapses ensure linear encoding of synapse numbers from individual VPNs. These findings illuminate how synaptic topography influences dendritic integration and suggest that linear encoding of synapse numbers may be a default strategy established through connectivity and passive neuron properties, upon which active properties and plasticity can then tune as needed.

DSCAM is differentially patterned along the optic axon pathway in the developing Xenopus visual system and guides axon termination at the target

Background

The Xenopus retinotectal circuit is organized topographically, where the dorsal–ventral axis of the retina maps respectively on to the ventral-dorsal axis of the tectum; axons from the nasal-temporal axis of the retina project respectively to the caudal-rostral axis of the tectum. Studies throughout the last two decades have shown that mechanisms involving molecular recognition of proper termination domains are at work guiding topographic organization. Such studies have shown that graded distribution of molecular cues is important for topographic mapping. However, the complement of molecular cues organizing topography along the developing optic nerve, and as retinal axons cross the chiasm and navigate towards and innervate their target in the tectum, remains unknown. Down syndrome cell adhesion molecule (DSCAM) has been characterized as a key molecule in axon guidance, making it a strong candidate involved in the topographic organization of retinal fibers along the optic path and at their target.

Methods

Using a combination of whole-brain clearing and immunohistochemistry staining techniques we characterized DSCAM expression and the projection of ventral and dorsal retinal fibers starting from the eye, following to the optic nerve and chiasm, and into the terminal target in the optic tectum in Xenopus laevis tadpoles. We then assessed the effects of DSCAM on the establishment of retinotopic maps through spatially and temporally targeted DSCAM knockdown on retinal ganglion cells (RGCs) with axons innervating the optic tectum.

Results

Highest expression of DSCAM was localized to the ventral posterior region of the optic nerve and chiasm; this expression pattern coincides with ventral fibers derived from ventral RGCs. Targeted downregulation of DSCAM expression on ventral RGCs affected the segregation of medial axon fibers from their dorsal counterparts within the tectal neuropil, indicating that DSCAM plays a role in retinotopic organization.

Conclusion

These findings together with previous studies demonstrating cell-autonomous roles for DSCAM during the development of pre- and postsynaptic arbors in the Xenopus retinotectal circuit indicates that DSCAM exerts multiple roles in coordinating axon targeting and structural connectivity in the developing vertebrate visual system.

Emergence of synaptic organization and computation in dendrites

Single neurons in the brain exhibit astounding computational capabilities, which gradually emerge throughout development and enable them to become integrated into complex neural circuits. These capabilities derive in part from the precise arrangement of synaptic inputs on the neurons' dendrites. While the full computational benefits of this arrangement are still unknown, a picture emerges in which synapses organize according to their functional properties across multiple spatial scales. In particular, on the local scale (tens of microns), excitatory synaptic inputs tend to form clusters according to their functional similarity, whereas on the scale of individual dendrites or the entire tree, synaptic inputs exhibit dendritic maps where excitatory synapse function varies smoothly with location on the tree. The development of this organization is supported by inhibitory synapses, which are carefully interleaved with excitatory synapses and can flexibly modulate activity and plasticity of excitatory synapses. Here, we summarize recent experimental and theoretical research on the developmental emergence of this synaptic organization and its impact on neural computations.

Spatiotemporal functional organization of excitatory synaptic inputs onto macaque V1 neurons

The integration of synaptic inputs onto dendrites provides the basis for neuronal computation. Whereas recent studies have begun to outline the spatial organization of synaptic inputs on individual neurons, the underlying principles related to the specific neural functions are not well understood. Here we perform two-photon dendritic imaging with a genetically-encoded glutamate sensor in awake monkeys, and map the excitatory synaptic inputs on dendrites of individual V1 superficial layer neurons with high spatial and temporal resolution. We find a functional integration and trade-off between orientation-selective and color-selective inputs in basal dendrites of individual V1 neurons. Synaptic inputs on dendrites are spatially clustered by stimulus feature, but functionally scattered in multidimensional feature space, providing a potential substrate of local feature integration on dendritic branches. Furthermore, apical dendrite inputs have larger receptive fields and longer response latencies than basal dendrite inputs, suggesting a dominant role for apical dendrites in integrating feedback in visual information processing.

Quadrature demodulation in line-scan focal modulation microscopy for imaging three-dimensional zebrafish neural structure

Visualizing biological processes in neuroscience requires in vivo functional imaging at single-neuron resolution, high image acquisition speed and strong optical sectioning ability. However, due to light scattering of in tissue, very often conventional wide-field fluorescence microscopes are unable to resolve cells in the presence of a strong out-of-focus background. Line-scan focal modulation microscopy enables high temporal resolution and good optical sectioning ability at the same time. Here we demonstrate a quadrature demodulation method to extract the focal information with an extended frequency bandwidth and therefore higher spatial resolution. The performance of the demodulation scheme in line-scan focal modulation microscope has been evaluated by performing imaging experiments with fluorescence beads and zebrafish neural structure. Reduced background, reduced artifacts and more detailed morphological information are evident in the obtained images.

Hypothalamic Projections to the Optic Tectum in Larval Zebrafish

The optic tectum of larval zebrafish is an important model for understanding visual processing in vertebrates. The tectum has been traditionally viewed as dominantly visual, with a majority of studies focusing on the processes by which tectal circuits receive and process retinally-derived visual information. Recently, a handful of studies have shown a much more complex role for the optic tectum in larval zebrafish, and anatomical and functional data from these studies suggest that this role extends beyond the visual system, and beyond the processing of exclusively retinal inputs. Consistent with this evolving view of the tectum, we have used a Gal4 enhancer trap line to identify direct projections from rostral hypothalamus (RH) to the tectal neuropil of larval zebrafish. These projections ramify within the deepest laminae of the tectal neuropil, the stratum album centrale (SAC)/stratum griseum periventriculare (SPV), and also innervate strata distinct from those innervated by retinal projections. Using optogenetic stimulation of the hypothalamic projection neurons paired with calcium imaging in the tectum, we find rebound firing in tectal neurons consistent with hypothalamic inhibitory input. Our results suggest that tectal processing in larval zebrafish is modulated by hypothalamic inhibitory inputs to the deep tectal neuropil.

Emergence of Selectivity to Looming Stimuli in a Spiking Network Model of the Optic Tectum

The neural circuits in the optic tectum of Xenopus tadpoles are selectively responsive to looming visual stimuli that resemble objects approaching the animal at a collision trajectory. This selectivity is required for adaptive collision avoidance behavior in this species, but its underlying mechanisms are not known. In particular, it is still unclear how the balance between the recurrent spontaneous network activity and the newly arriving sensory flow is set in this structure, and to what degree this balance is important for collision detection. Also, despite the clear indication for the presence of strong recurrent excitation and spontaneous activity, the exact topology of recurrent feedback circuits in the tectum remains elusive. In this study we take advantage of recently published detailed cell-level data from tadpole tectum to build an informed computational model of it, and investigate whether dynamic activation in excitatory recurrent retinotopic networks may on its own underlie collision detection. We consider several possible recurrent connectivity configurations and compare their performance for collision detection under different levels of spontaneous neural activity. We show that even in the absence of inhibition, a retinotopic network of quickly inactivating spiking neurons is naturally selective for looming stimuli, but this selectivity is not robust to neuronal noise, and is sensitive to the balance between direct and recurrent inputs. We also describe how homeostatic modulation of intrinsic properties of individual tectal cells can change selectivity thresholds in this network, and qualitatively verify our predictions in a behavioral experiment in freely swimming tadpoles.

An NMDA receptor-dependent mechanism for subcellular segregation of sensory inputs in the tadpole optic tectum

In the vertebrate CNS, afferent sensory inputs are targeted to specific depths or layers of their target neuropil. This patterning exists ab initio, from the very beginning, and therefore has been considered an activity-independent process. However, here we report that, during circuit development, the subcellular segregation of the visual and mechanosensory inputs to specific regions of tectal neuron dendrites in the tadpole optic tectum requires NMDA receptor activity. Blocking NMDARs during the formation of these sensory circuits, or removing the visual set of inputs, leads to less defined segregation, and suggests a correlation-based mechanism in which correlated inputs wire to common regions of dendrites. This can account for how two sets of inputs form synapses onto different regions of the same dendrite. Blocking NMDA receptors during later stages of circuit development did not disrupt segregation, indicating a critical period for activity-dependent shaping of patterns of innervation.

DOI:http://dx.doi.org/10.7554/eLife.20502.001

Fine and distributed subcellular retinotopy of excitatory inputs to the dendritic tree of a collision-detecting neuron

Individual neurons in several sensory systems receive synaptic inputs organized according to subcellular topographic maps, yet the fine structure of this topographic organization and its relation to dendritic morphology have not been studied in detail. Subcellular topography is expected to play a role in dendritic integration, particularly when dendrites are extended and active. The lobula giant movement detector (LGMD) neuron in the locust visual system is known to receive topographic excitatory inputs on part of its dendritic tree. The LGMD responds preferentially to objects approaching on a collision course and is thought to implement several interesting dendritic computations. To study the fine retinotopic mapping of visual inputs onto the excitatory dendrites of the LGMD, we designed a custom microscope allowing visual stimulation at the native sampling resolution of the locust compound eye while simultaneously performing two-photon calcium imaging on excitatory dendrites. We show that the LGMD receives a distributed, fine retinotopic projection from the eye facets and that adjacent facets activate overlapping portions of the same dendritic branches. We also demonstrate that adjacent retinal inputs most likely make independent synapses on the excitatory dendrites of the LGMD. Finally, we show that the fine topographic mapping can be studied using dynamic visual stimuli. Our results reveal the detailed structure of the dendritic input originating from individual facets on the eye and their relation to that of adjacent facets. The mapping of visual space onto the LGMD's dendrites is expected to have implications for dendritic computation.

Visual Stimuli Evoked Action Potentials Trigger Rapidly Propagating Dendritic Calcium Transients in the Frog Optic Tectum Layer 6 Neurons

The superior colliculus in mammals or the optic tectum in amphibians is a major visual information processing center responsible for generation of orientating responses such as saccades in monkeys or prey catching avoidance behavior in frogs. The conserved structure function of the superior colliculus the optic tectum across distant species such as frogs, birds monkeys permits to draw rather general conclusions after studying a single species. We chose the frog optic tectum because we are able to perform whole-cell voltage-clamp recordings fluorescence imaging of tectal neurons while they respond to a visual stimulus. In the optic tectum of amphibians most visual information is processed by pear-shaped neurons possessing long dendritic branches, which receive the majority of synapses originating from the retinal ganglion cells. Since the first step of the retinal input integration is performed on these dendrites, it is important to know whether this integration is enhanced by active dendritic properties. We demonstrate that rapid calcium transients coinciding with the visual stimulus evoked action potentials in the somatic recordings can be readily detected up to the fine branches of these dendrites. These transients were blocked by calcium channel blockers nifedipine CdCl₂ indicating that calcium entered dendrites via voltage-activated L-type calcium channels. The high speed of calcium transient propagation, >300 μm in <10 ms, is consistent with the notion that action potentials, actively propagating along dendrites, open voltage-gated L-type calcium channels causing rapid calcium concentration transients in the dendrites. We conclude that such activation by somatic action potentials of the dendritic voltage gated calcium channels in the close vicinity to the synapses formed by axons of the retinal ganglion cells may facilitate visual information processing in the principal neurons of the frog optic tectum.

General features of the retinal connectome determine the computation of motion anticipation

Motion anticipation allows the visual system to compensate for the slow speed of phototransduction so that a moving object can be accurately located. This correction is already present in the signal that ganglion cells send from the retina but the biophysical mechanisms underlying this computation are not known. Here we demonstrate that motion anticipation is computed autonomously within the dendritic tree of each ganglion cell and relies on feedforward inhibition. The passive and non-linear interaction of excitatory and inhibitory synapses enables the somatic voltage to encode the actual position of a moving object instead of its delayed representation. General rather than specific features of the retinal connectome govern this computation: an excess of inhibitory inputs over excitatory, with both being randomly distributed, allows tracking of all directions of motion, while the average distance between inputs determines the object velocities that can be compensated for.

DOI:http://dx.doi.org/10.7554/eLife.06250.001

The retina is a structure at the back of the eye that converts light into nerve impulses, which are then processed in the brain to produce the images that we see. It normally takes about one-tenth of a second for the retina to send a signal to the brain after an object first moves into view. This is about the same time it takes a tennis ball to travel several meters during a tennis match, yet we are still able to see where the moving tennis ball is in real time. This is because a process called ‘motion anticipation’ is able to compensate for the delay in processing the position of a moving object. However, it was not known precisely how motion anticipation occurs.

Inside the retina, cells called photoreceptors detect light and ultimately send signals (via some intermediate cell types) to nerve cells known as retinal ganglion cells. These signals can either excite a retinal ganglion cell to cause it to send an electrical signal to the brain, or inhibit it, which temporarily prevents electrical activity. Each cell receives signals from several photoreceptors, which each connect to a different site along branch-like structures called dendrites that project out of the retinal ganglion cells.

Johnston and Lagnado have now investigated how motion anticipation occurs in the retina by using electrical recordings of the activity in the retinas of goldfish combined with computer simulations of this activity. This revealed inhibitory signals, sent from photoreceptors to retinal ganglion cells via a type of intermediate cell (called amacrine cells), play a key role in motion anticipation. The ability to track motion effectively in all directions requires more inhibitory signals to be sent to the dendrites of a retinal ganglion cell than excitatory signals. These two types of input must also be randomly distributed across the cell. Furthermore, it is the density of these input sites on a dendrite that determines how well the retina can compensate for the motion of a fast-moving object. The building blocks required for motion anticipation in the retina are also found in visual areas higher in the brain. Therefore, further work may reveal that higher visual areas also use this mechanism to predict the future location of moving objects.

DOI:http://dx.doi.org/10.7554/eLife.06250.002

Qualitatively different effect of repeated stress during adolescence on principal neuron morphology across lateral and basal nuclei of the rat amygdala

Repeated stress can elicit symptoms of depression and anxiety. The amygdala is a significant contributor to the expression of emotion and the basolateral amygdala (BLA) is a major target for the effects of stress on emotion. The adolescent time period may be particularly susceptible to the effects of stress on emotion. While repeated stress has been demonstrated to modify the morphology of BLA neurons in adult rats, little is known about its effects on BLA neurons during adolescence. This study tests the effects of repeated stress during adolescence on BLA neuronal morphology, and whether these are similar to the effects of stress during adulthood. The BLA includes the basal (BA) and lateral (LAT) nuclei, which are differentially responsive to stress in adults. Therefore, effects of stress during adolescence were compared between the BA and LAT nuclei. Morphological features of reconstructed BLA neurons were examined using Golgi-Cox-stained tissue from control or repeated restraint stress-exposed rats. We found subtle dendritic growth coupled with loss of spines after repeated stress during adolescence. The magnitude and dendritic location of these differences varied between the BA and LAT nuclei in strong contrast to the stress-induced increases in spine number seen in adults. These results demonstrate that repeated stress during adolescence has markedly different effects on BLA neuronal morphology, and the extent of these changes is BLA nucleus-dependent. Moreover, altered neuroanatomy was associated with age-dependent effects of repeated stress on generalization of fear, and may point to the necessity for different approaches to target stress-induced changes in adolescents.

Dendritic nonlinearities are tuned for efficient spike-based computations in cortical circuits

Cortical neurons integrate thousands of synaptic inputs in their dendrites in highly nonlinear ways. It is unknown how these dendritic nonlinearities in individual cells contribute to computations at the level of neural circuits. Here, we show that dendritic nonlinearities are critical for the efficient integration of synaptic inputs in circuits performing analog computations with spiking neurons. We developed a theory that formalizes how a neuron's dendritic nonlinearity that is optimal for integrating synaptic inputs depends on the statistics of its presynaptic activity patterns. Based on their in vivo preynaptic population statistics (firing rates, membrane potential fluctuations, and correlations due to ensemble dynamics), our theory accurately predicted the responses of two different types of cortical pyramidal cells to patterned stimulation by two-photon glutamate uncaging. These results reveal a new computational principle underlying dendritic integration in cortical neurons by suggesting a functional link between cellular and systems--level properties of cortical circuits.

Seeing the whole picture: A comprehensive imaging approach to functional mapping of circuits in behaving zebrafish

In recent years, the zebrafish has emerged as an appealing model system to tackle questions relating to the neural circuit basis of behavior. This can be attributed not just to the growing use of genetically tractable model organisms, but also in large part to the rapid advances in optical techniques for neuroscience, which are ideally suited for application to the small, transparent brain of the larval fish. Many characteristic features of vertebrate brains, from gross anatomy down to particular circuit motifs and cell-types, as well as conserved behaviors, can be found in zebrafish even just a few days post fertilization, and, at this early stage, the physical size of the brain makes it possible to analyze neural activity in a comprehensive fashion. In a recent study, we used a systematic and unbiased imaging method to record the pattern of activity dynamics throughout the whole brain of larval zebrafish during a simple visual behavior, the optokinetic response (OKR). This approach revealed the broadly distributed network of neurons that were active during the behavior and provided insights into the fine-scale functional architecture in the brain, inter-individual variability, and the spatial distribution of behaviorally relevant signals. Combined with mapping anatomical and functional connectivity, targeted electrophysiological recordings, and genetic labeling of specific populations, this comprehensive approach in zebrafish provides an unparalleled opportunity to study complete circuits in a behaving vertebrate animal.

Dendritic function in vivo

Dendrites are the predominant entry site for excitatory synaptic potentials in most types of central neurons. There is increasing evidence that dendrites are not just passive transmitting devices but play active roles in synaptic integration through linear and non-linear mechanisms. Frequently, excitatory synapses are formed on dendritic spines. In addition to relaying incoming electrical signals, spines can play important roles in modifying these signals through complex biochemical processes and, thereby, determine learning and memory formation. Here, we review recent advances in our understanding of the function of spines and dendrites in central mammalian neurons in vivo by focusing particularly on insights obtained from Ca(2+) imaging studies.

The horizontal brain slice preparation: a novel approach for visualizing and recording from all layers of the tadpole tectum

The Xenopus tadpole optic tectum is a multisensory processing center that receives direct visual input as well as nonvisual mechanosensory input. The tectal neurons that comprise the optic tectum are organized into layers. These neurons project their dendrites laterally into the neuropil where visual inputs target the distal region of the dendrite and nonvisual inputs target the proximal region of the same dendrite. The Xenopus tadpole tectum is a popular model to study the development of sensory circuits. However, whole cell patch-clamp electrophysiological studies of the tadpole tectum (using the whole brain or in vivo preparations) have focused solely on the deep-layer tectal neurons because only neurons of the deep layer are visible and accessible for whole cell electrophysiological recordings. As a result, whereas the development and plasticity of these deep-layer neurons has been well-studied, essentially nothing has been reported about the electrophysiology of neurons residing beyond this layer. Hence, there exists a large gap in our understanding about the functional development of the amphibian tectum as a whole. To remedy this, we developed a novel isolated brain preparation that allows visualizing and recording from all layers of the tectum. We refer to this preparation as the "horizontal brain slice preparation." Here, we describe the preparation method and illustrate how it can be used to characterize the electrophysiology of neurons across all of the layers of the tectum as well as the spatial pattern of synaptic input from the different sensory modalities.

A synaptic mechanism for network synchrony

Within neural networks, synchronization of activity is dependent upon the synaptic connectivity of embedded microcircuits and the intrinsic membrane properties of their constituent neurons. Synaptic integration, dendritic Ca²⁺ signaling, and non-linear interactions are crucial cellular attributes that dictate single neuron computation, but their roles promoting synchrony and the generation of network oscillations are not well understood, especially within the context of a defined behavior. In this regard, the lamprey spinal central pattern generator (CPG) stands out as a well-characterized, conserved vertebrate model of a neural network (Smith et al., 2013a), which produces synchronized oscillations in which neural elements from the systems to cellular level that control rhythmic locomotion have been determined. We review the current evidence for the synaptic basis of oscillation generation with a particular emphasis on the linkage between synaptic communication and its cellular coupling to membrane processes that control oscillatory behavior of neurons within the locomotor network. We seek to relate dendritic function found in many vertebrate systems to the accessible lamprey central nervous system in which the relationship between neural network activity and behavior is well understood. This enables us to address how Ca²⁺ signaling in spinal neuron dendrites orchestrate oscillations that drive network behavior.

Excitation and inhibition in recurrent networks mediate collision avoidance in Xenopus tadpoles

Information processing in the vertebrate brain is thought to be mediated through distributed neural networks, but it is still unclear how sensory stimuli are encoded and detected by these networks, and what role synaptic inhibition plays in this process. Here we used a collision avoidance behavior in Xenopus tadpoles as a model for stimulus discrimination and recognition. We showed that the visual system of the tadpole is selective for behaviorally relevant looming stimuli, and that the detection of these stimuli first occurs in the optic tectum. By comparing visually guided behavior, optic nerve recordings, excitatory and inhibitory synaptic currents, and the spike output of tectal neurons, we showed that collision detection in the tadpole relies on the emergent properties of distributed recurrent networks within the tectum. We found that synaptic inhibition was temporally correlated with excitation, and did not actively sculpt stimulus selectivity, but rather it regulated the amount of integration between direct inputs from the retina and recurrent inputs from the tectum. Both pharmacological suppression and enhancement of synaptic inhibition disrupted emergent selectivity for looming stimuli. Taken together these findings suggested that, by regulating the amount of network activity, inhibition plays a critical role in maintaining selective sensitivity to behaviorally-relevant visual stimuli.

Hybrid voltage sensor imaging of eGFP-F expressing neurons in chicken midbrain slices

BACKGROUND

Dendritic computation is essential for understanding information processing in single neurons and brain circuits. Optical methods are suited best to investigate function and biophysical properties of cellular compartments at high spatial and temporal resolution. Promising approaches include the use of voltage sensitive dyes, genetically encoded voltage sensors, or hybrid voltage sensors (hVoS) consisting of fluorescent proteins and voltage-dependent quenchers that, so far, are not available in avian neuroscience.

NEW METHOD

We have adapted a hVoS system for a chicken midbrain slice preparation by combining genetically expressed farnesylated eGFP with dipicrylamine (DPA). Depending on the cellular potential, DPA is shifted in the membrane, resulting in quenching of eGFP fluorescence linearly to the membrane potential by Förster resonance electron transfer.

RESULTS

In ovo electroporation resulted in labelled neurons throughout the midbrain with a high level of fine structural detail. After application of DPA, we were able to optically record electrically evoked action potentials with high signal-to-noise ratio and high spatio-temporal resolution.

COMPARISON WITH EXISTING METHODS

Standard methods available for avian neuroscience such as whole-cell patch clamp yield insufficient data for the analysis of dendritic computation in single neurons. The high spatial and temporal resolution of hVoS data overcomes this limitation. The results obtained by our method are comparable to hVoS data published for mammals.

CONCLUSIONS

With the protocol presented here, it is possible to optically record information processing in single avian neurons at such high spatial and temporal resolution, that cellular and subcellular events can be analysed.

An Augmented Two-Layer Model Captures Nonlinear Analog Spatial Integration Effects in Pyramidal Neuron Dendrites

In pursuit of the goal to understand and eventually reproduce the diverse functions of the brain, a key challenge lies in reverse engineering the peculiar biology-based "technology" that underlies the brain's remarkable ability to process and store information. The basic building block of the nervous system is the nerve cell, or "neuron," yet after more than 100 years of neurophysiological study and 60 years of modeling, the information processing functions of individual neurons, and the parameters that allow them to engage in so many different types of computation (sensory, motor, mnemonic, executive, etc.) remain poorly understood. In this paper, we review both historical and recent findings that have led to our current understanding of the analog spatial processing capabilities of dendrites, the major input structures of neurons, with a focus on the principal cell type of the neocortex and hippocampus, the pyramidal neuron (PN). We encapsulate our current understanding of PN dendritic integration in an abstract layered model whose spatially sensitive branch-subunits compute multidimensional sigmoidal functions. Unlike the 1-D sigmoids found in conventional neural network models, multidimensional sigmoids allow the cell to implement a rich spectrum of nonlinear modulation effects directly within their dendritic trees.

Subcellular mapping of dendritic activity in optic flow processing neurons

Dendritic integration is a fundamental element of neuronal information processing. So far, few studies have provided a detailed spatial picture of this process, describing the properties of local dendritic activity and its subcellular organization. Here, we used 2-photon calcium imaging in optic flow processing neurons of the fly Calliphora vicina to determine the preferred location and direction of local motion cues for small branchlets throughout the entire dendrite. We found a pronounced retinotopic mapping on both the subcellular and the cell population level. In addition, dendritic branchlets residing in different layers of the neuropil were tuned to distinct directions of motion. Summing the local receptive fields of all dendritic branchlets reproduced the characteristic properties of these neurons' axonal output receptive fields. Our results corroborate the notion that the dendritic morphology of vertical system cells allows them to selectively collect local motion inputs with particular directional preferences from a spatially organized input repertoire, thus forming filters that match global patterns of optic flow. Furthermore, we suggest that the facet arrangement across the fly's eye shapes the subcellular direction tuning to local motion stimuli. These data illustrate a highly structured circuit organization as an efficient way to hard-wire a complex sensory task.

Mechanisms underlying subunit independence in pyramidal neuron dendrites

Pyramidal neuron (PN) dendrites compartmentalize voltage signals and can generate local spikes, which has led to the proposal that their dendrites act as independent computational subunits within a multilayered processing scheme. However, when a PN is strongly activated, back-propagating action potentials (bAPs) sweeping outward from the soma synchronize dendritic membrane potentials many times per second. How PN dendrites maintain the independence of their voltage-dependent computations, despite these repeated voltage resets, remains unknown. Using a detailed compartmental model of a layer 5 PN, and an improved method for quantifying subunit independence that incorporates a more accurate model of dendritic integration, we first established that the output of each dendrite can be almost perfectly predicted by the intensity and spatial configuration of its own synaptic inputs, and is nearly invariant to the rate of bAP-mediated "cross-talk" from other dendrites over a 100-fold range. Then, through an analysis of conductance, voltage, and current waveforms within the model cell, we identify three biophysical mechanisms that together help make independent dendritic computation possible in a firing neuron, suggesting that a major subtype of neocortical neuron has been optimized for layered, compartmentalized processing under in-vivo-like spiking conditions.

Monitoring synaptic and neuronal activity in 3D with synthetic and genetic indicators using a compact acousto-optic lens two-photon microscope

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We expand the utility of acousto-optic lens (AOL) 3D 2P microscopy.

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We show rapid, simultaneous monitoring of synaptic inputs distributed in 3D.

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First use of genetically encoded indicators with AOL 3D functional imaging.

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Measurement of sensory-evoked neuronal population activity in 3D in vivo.

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Strategies for improving the measurement of the timing of neuronal signals.

Background

Two-photon microscopy is widely used to study brain function, but conventional microscopes are too slow to capture the timing of neuronal signalling and imaging is restricted to one plane. Recent development of acousto-optic-deflector-based random access functional imaging has improved the temporal resolution, but the utility of these technologies for mapping 3D synaptic activity patterns and their performance at the excitation wavelengths required to image genetically encoded indicators have not been investigated.

New method

Here, we have used a compact acousto-optic lens (AOL) two-photon microscope to make high speed [Ca²⁺] measurements from spines and dendrites distributed in 3D with different excitation wavelengths (800–920 nm).

Results

We show simultaneous monitoring of activity from many synaptic inputs distributed over the 3D arborisation of a neuronal dendrite using both synthetic as well as genetically encoded indicators. We confirm the utility of AOL-based imaging for fast in vivo recordings by measuring, simultaneously, visually evoked responses in 100 neurons distributed over a 150 μm focal depth range. Moreover, we explore ways to improve the measurement of timing of neuronal activation by choosing specific regions within the cell soma.

Comparison with existing methods

These results establish that AOL-based 3D random access two-photon microscopy has a wider range of neuroscience applications than previously shown.

Conclusions

Our findings show that the compact AOL microscope design has the speed, spatial resolution, sensitivity and wavelength flexibility to measure 3D patterns of synaptic and neuronal activity on individual trials.

Global hyper-synchronous spontaneous activity in the developing optic tectum

Studies of patterned spontaneous activity can elucidate how the organization of neural circuits emerges. Using in vivo two-photon Ca²⁺ imaging, we studied spatio-temporal patterns of spontaneous activity in the optic tectum of Xenopus tadpoles. We found rhythmic patterns of global synchronous spontaneous activity between neurons, which depends on visual experience and developmental stage. By contrast, synchronous spontaneous activity between non-neuronal cells is mediated more locally. To understand the source of the neuronal spontaneous activity, input to the tectum was systematically removed. Whereas removing input from the visual or mechanosensory system alone had little effect on patterned spontaneous activity, removing input from both systems drastically altered it. These results suggest that either input is sufficient to maintain the intrinsically generated spontaneous activity and that patterned spontaneous activity results from input from multisensory systems. Thus, the amphibian midbrain differs from the mammalian visual system, whose spontaneous activity is controlled by retinal waves.

Modeling human neurodevelopmental disorders in the Xenopus tadpole: from mechanisms to therapeutic targets

The Xenopus tadpole model offers many advantages for studying the molecular, cellular and network mechanisms underlying neurodevelopmental disorders. Essentially every stage of normal neural circuit development, from axon outgrowth and guidance to activity-dependent homeostasis and refinement, has been studied in the frog tadpole, making it an ideal model to determine what happens when any of these stages are compromised. Recently, the tadpole model has been used to explore the mechanisms of epilepsy and autism, and there is mounting evidence to suggest that diseases of the nervous system involve deficits in the most fundamental aspects of nervous system function and development. In this Review, we provide an update on how tadpole models are being used to study three distinct types of neurodevelopmental disorders: diseases caused by exposure to environmental toxicants, epilepsy and seizure disorders, and autism.

Effects of repeated stress on excitatory drive of basal amygdala neurons in vivo

Chronic stress leads to heightened affective behaviors, and can precipitate the emergence of depression and anxiety. These disorders are associated with increased amygdala activity. In animal models, chronic stress leads to increased amygdala-dependent behaviors, as well as hyperactivity of amygdala neurons. However, it is not known whether increased excitatory synaptic drive after chronic stress contributes to hyperactivity of basolateral amygdala (BLA; comprised of basal, lateral, and accessory basal nuclei) neurons. This study tested whether repeated stress causes an increase in excitatory drive of basal amygdala (BA) neurons in vivo, and whether this is correlated with an increase in the number of dendritic spines and a shift in dendritic distribution. Using in vivo intracellular recordings, this study found that repeated restraint stress caused an increase in the frequency of spontaneous excitatory synaptic events in vivo, which correlated with the number of dendritic spines in reconstructed neurons. Furthermore, parallel changes in the kinetics of the synaptic events and the distribution of spines indicated a more prominent functional contribution of synaptic inputs from across the dendritic tree. The shift in spine distribution across the dendritic tree was further confirmed with the examination of Golgi-stained tissue. This abnormal physiological drive of BA neurons after repeated stress may contribute to heightened affective responses after chronic stress. A reduction in the impact of excitatory drive in the BA may therefore be a potential treatment for the harmful effects of chronic stress in psychiatric disorders.

Spatial relationships between GABAergic and glutamatergic synapses on the dendrites of distinct types of mouse retinal ganglion cells across development

Neuronal output requires a concerted balance between excitatory and inhibitory (I/E) input. Like other circuits, inhibitory synaptogenesis in the retina precedes excitatory synaptogenesis. How then do neurons attain their mature balance of I/E ratios despite temporal offset in synaptogenesis? To directly compare the development of glutamatergic and GABAergic synapses onto the same cell, we biolistically transfected retinal ganglion cells (RGCs) with PSD95CFP, a marker of glutamatergic postsynaptic sites, in transgenic Thy1­YFPγ2 mice in which GABA_A receptors are fluorescently tagged. We mapped YFPγ2 and PSD95CFP puncta distributions on three RGC types at postnatal day P12, shortly before eye opening, and at P21 when robust light responses in RGCs are present. The mature I_GABA/E ratios varied among ON-Sustained (S) A-type, OFF-S A-type, and bistratified direction selective (DS) RGCs. These ratios were attained at different rates, before eye-opening for ON-S and OFF-S A-type, and after eye-opening for DS RGCs. At both ages examined, the I_GABA/E ratio was uniform across the arbors of the three RGC types. Furthermore, measurements of the distances between neighboring PSD95CFP and YFPγ2 puncta on RGC dendrites indicate that their local relationship is established early in development, and cannot be predicted by random organization. These close spatial associations between glutamatergic and GABAergic postsynaptic sites appear to represent local synaptic arrangements revealed by correlative light and EM reconstructions of a single RGC's dendrites. Thus, although RGC types have different I_GABA/E ratios and establish these ratios at separate rates, the local relationship between excitatory and inhibitory inputs appear similarly constrained across the RGC types studied.

Two-photon imaging of neural population activity in zebrafish

Rapidly developing imaging technologies including two-photon microscopy and genetically encoded calcium indicators have opened up new possibilities for recording neural population activity in awake, behaving animals. In the small, transparent zebrafish, it is even becoming possible to image the entire brain of a behaving animal with single-cell resolution, creating brain-wide functional maps. In this chapter, we comprehensively review past functional imaging studies in zebrafish, and the insights that they provide into the functional organization of neural circuits. We further offer a basic primer on state-of-the-art methods for in vivo calcium imaging in the zebrafish, including building a low-cost two-photon microscope and highlight possible challenges and technical considerations.

Repeated restraint stress exerts different impact on structure of neurons in the lateral and basal nuclei of the amygdala

Chronic stress exacerbates and can induce symptoms of depression and anxiety disorders. Chronic stress causes amygdala hyperactivity, which may contribute to these detrimental effects. One potential mechanism for amygdala hyperactivity is an increase of excitatory drive after stress. Excitatory inputs to the amygdala predominantly synapse upon dendritic spines, and repeated stress has been demonstrated to increase dendritic spines in the basolateral amygdala (BLA). However, the BLA is comprised of several nuclei, including the lateral nucleus (LAT) and the basal nucleus (BA), which exert functionally distinct roles in amygdala-dependent behaviors. Furthermore, while an increase of dendritic spines can impart significant functional ramifications, a shift of spine distribution can also exert significant impact. However, differences in the effects of repeated stress on LAT and BA have not been examined, nor differential effects on spine distribution. This study examined the effects of repeated restraint stress on dendritic structure of principal neurons from the LAT and BA in Golgi-stained tissue. This study found that repeated stress increased spine number in LAT and BA, but in very distinct patterns, with proximal increases in LAT neurons and non-proximal increases in BA neurons. Furthermore, repeated stress increased dendritic length in the BA, but not the LAT, leading to a global change of spine density in BA, but a focal change in LAT. These distinct effects of repeated stress in the LAT and BA may exert significant functional effects on fear behavior, and may underlie differences in the effects of repeated stress on acquisition, contextual modulation and extinction of fear behavior.

Monitoring of single-cell responses in the optic tectum of adult zebrafish with dextran-coupled calcium dyes delivered via local electroporation

The zebrafish (Danio rerio) has become one of the major animal models for in vivo examination of sensory and neuronal computation. Similar to Xenopus tadpoles neural activity in the optic tectum, the major region controlling visually guided behavior, can be examined in zebrafish larvae by optical imaging. Prerequisites of these approaches are usually the transparency of larvae up to a certain age and the use of two-photon microscopy. This principle of fluorescence excitation was necessary to suppress crosstalk between signals from individual neurons, which is a critical issue when using membrane-permeant dyes. This makes the equipment to study neuronal processing costly and limits the approach to the study of larvae. Thus there is lack of knowledge about the properties of neurons in the optic tectum of adult animals. We established a procedure to circumvent these problems, enabling in vivo calcium imaging in the optic tectum of adult zebrafish. Following local application of dextran-coupled dyes single-neuron activity of adult zebrafish can be monitored with conventional widefield microscopy, because dye labeling remains restricted to tens of neurons or less. Among the neurons characterized with our technique we found neurons that were selective for a certain pattern orientation as well as neurons that responded in a direction-selective way to visual motion. These findings are consistent with previous studies and indicate that the functional integrity of neuronal circuits in the optic tectum of adult zebrafish is preserved with our staining technique. Overall, our protocol for in vivo calcium imaging provides a useful approach to monitor visual responses of individual neurons in the optic tectum of adult zebrafish even when only widefield microscopy is available. This approach will help to obtain valuable insight into the principles of visual computation in adult vertebrates and thus complement previous work on developing visual circuits.

Fast functional imaging of multiple brain regions in intact zebrafish larvae using selective plane illumination microscopy

The optical transparency and the small dimensions of zebrafish at the larval stage make it a vertebrate model of choice for brain-wide in-vivo functional imaging. However, current point-scanning imaging techniques, such as two-photon or confocal microscopy, impose a strong limit on acquisition speed which in turn sets the number of neurons that can be simultaneously recorded. At 5 Hz, this number is of the order of one thousand, i.e., approximately 1–2% of the brain. Here we demonstrate that this limitation can be greatly overcome by using Selective-plane Illumination Microscopy (SPIM). Zebrafish larvae expressing the genetically encoded calcium indicator GCaMP3 were illuminated with a scanned laser sheet and imaged with a camera whose optical axis was oriented orthogonally to the illumination plane. This optical sectioning approach was shown to permit functional imaging of a very large fraction of the brain volume of 5–9-day-old larvae with single- or near single-cell resolution. The spontaneous activity of up to 5,000 neurons was recorded at 20 Hz for 20–60 min. By rapidly scanning the specimen in the axial direction, the activity of 25,000 individual neurons from 5 different z-planes (approximately 30% of the entire brain) could be simultaneously monitored at 4 Hz. Compared to point-scanning techniques, this imaging strategy thus yields a ≃20-fold increase in data throughput (number of recorded neurons times acquisition rate) without compromising the signal-to-noise ratio (SNR). The extended field of view offered by the SPIM method allowed us to directly identify large scale ensembles of neurons, spanning several brain regions, that displayed correlated activity and were thus likely to participate in common neural processes. The benefits and limitations of SPIM for functional imaging in zebrafish as well as future developments are briefly discussed.

Fast non-negative temporal deconvolution for laser scanning microscopy

Laser scanning microscopy (LSM) is a common technique for high resolution fluorescent imaging. Here we describe a fast algorithm for non-negative deconvolution and apply it to readout of LSM detector photocurrents. By broadening photon impulses and deconvolving sampled photocurrent, effective quantum efficiency of the imaging system is increased. Using simulation and imaging with a custom-built two-photon microscope, we demonstrate improved fidelity of images acquired at short dwell times over a wide range of photon rates. Images formed show increased correlation-to-sample equivalent to a 25% increase in photon rate, lower noise, and reduced bleed-through compared to conventional image generation.

Emergence in the central nervous system

"Emergence" is an idea that has received much attention in consciousness literature, but it is difficult to find characterizations of that concept which are both specific and useful. I will precisely define and characterize a type of epistemic ("weak") emergence and show that it is a property of some neural circuits throughout the CNS, on micro-, meso- and macroscopic levels. I will argue that possession of this property can result in profoundly altered neural dynamics on multiple levels in cortex and other systems. I will first describe emergent neural entities (ENEs) abstractly. I will then show how ENEs function specifically and concretely, and demonstrate some implications of this type of emergence for the CNS.

Inter-neuronal correlation distinguishes mechanisms of direction selectivity in cortical circuit models

Direction selectivity is a fundamental physiological property that arises from primary visual cortex (V1) circuitry, yet basic questions of how direction-selective (DS) receptive fields are constructed remain unanswered. We built a set of simple, plausible neuronal circuits that produce DS cells via different mechanisms and tested these circuits to determine how they can be distinguished experimentally. Our models consisted of populations of spiking units representing physiological cell classes ranging from LGN cells to V1 complex DS cells. They differed in network architecture and DS mechanism, including linear summation of non-DS simple-cell inputs or nonlinear pairwise combinations of non-DS inputs. The circuits also varied in the location of the DS time delay and whether the DS interaction was facilitatory or suppressive. We tested the models with visual stimuli often used experimentally, including sinusoidal gratings and flashed bars, and computed shuffle-corrected cross-correlograms (CCGs) of spike trains from pairs of units that would be accessible to extracellular recording. We found that CCGs revealed fundamental features of the DS models, including the location of signal delays in the DS circuit and the sign (facilitatory or suppressive) of DS interactions. We also found that correlation was strongly stimulus-dependent, changing with direction and temporal frequency in a manner that generalized across model architectures. Our models make specific predictions for designing, optimizing, and interpreting electrophysiology experiments aimed at resolving DS circuitry and provide new insights into mechanisms that could underlie stimulus-dependent correlation. The models are available and easy to explore at www.imodel.org.

Location-dependent excitatory synaptic interactions in pyramidal neuron dendrites

Neocortical pyramidal neurons (PNs) receive thousands of excitatory synaptic contacts on their basal dendrites. Some act as classical driver inputs while others are thought to modulate PN responses based on sensory or behavioral context, but the biophysical mechanisms that mediate classical-contextual interactions in these dendrites remain poorly understood. We hypothesized that if two excitatory pathways bias their synaptic projections towards proximal vs. distal ends of the basal branches, the very different local spike thresholds and attenuation factors for inputs near and far from the soma might provide the basis for a classical-contextual functional asymmetry. Supporting this possibility, we found both in compartmental models and electrophysiological recordings in brain slices that the responses of basal dendrites to spatially separated inputs are indeed strongly asymmetric. Distal excitation lowers the local spike threshold for more proximal inputs, while having little effect on peak responses at the soma. In contrast, proximal excitation lowers the threshold, but also substantially increases the gain of distally-driven responses. Our findings support the view that PN basal dendrites possess significant analog computing capabilities, and suggest that the diverse forms of nonlinear response modulation seen in the neocortex, including uni-modal, cross-modal, and attentional effects, could depend in part on pathway-specific biases in the spatial distribution of excitatory synaptic contacts onto PN basal dendritic arbors.

Pyramidal neurons (PNs) are the principal neurons of the cerebral cortex and therefore lie at the heart of the brain's higher sensory, motor, affective, memory, and executive functions. But how do they work? In particular, how do they manage interactions between the classical “driver” inputs that give rise to their basic response properties, and “contextual” inputs that nonlinearly modulate those responses? It is known that PNs are contacted by thousands of excitatory synapses scattered about their dendrites, but despite decades of research, the “rules” that govern how inputs at different locations in the dendritic tree combine to influence the cell's firing rate remain poorly understood. We show here that two excitatory inputs contacting the same dendrite interact in an asymmetric nonlinear way that depends on their absolute and relative locations, where the resulting spectrum of location-dependent synaptic interactions constitutes a previously unknown form of spatial analog computation. In addition to suggesting a possible substrate for classical-contextual interactions in PN dendrites, our results imply that the computing functions of cortical circuits can only be fully understood when the detailed map of synaptic connectivity – the cortical connectome – is known down to the subdendritic level.

Dendritic spines: from structure to in vivo function

Dendritic spines arise as small protrusions from the dendritic shaft of various types of neuron and receive inputs from excitatory axons. Ever since dendritic spines were first described in the nineteenth century, questions about their function have spawned many hypotheses. In this review, we introduce understanding of the structural and biochemical properties of dendritic spines with emphasis on components studied with imaging methods. We then explore advances in in vivo imaging methods that are allowing spine activity to be studied in living tissue, from super-resolution techniques to calcium imaging. Finally, we review studies on spine structure and function in vivo. These new results shed light on the development, integration properties and plasticity of spines.

Probing synaptic function in dendrites with calcium imaging

Calcium imaging has become a widely used technique to probe neuronal activity on the cellular and subcellular levels. In contrast to standard electrophysiological methods, calcium imaging resolves sub- and suprathreshold activation patterns in structures as small as fine dendritic branches and spines. This review highlights recent findings gained on the subcellular level using calcium imaging, with special emphasis on synaptic transmission and plasticity in individual spines. Since imaging allows monitoring activity across populations of synapses, it has recently been adopted to investigate how dendrites integrate information from many synapses. Future experiments, ideally carried out in vivo, will reveal how the dendritic tree integrates and computes afferent signals. For example, it is now possible to directly test the concept that dendritic inputs are clustered and that single dendrites or dendritic stretches act as independent computational units.

Activity-dependent clustering of functional synaptic inputs on developing hippocampal dendrites

During brain development, before sensory systems become functional, neuronal networks spontaneously generate repetitive bursts of neuronal activity, which are typically synchronized across many neurons. Such activity patterns have been described on the level of networks and cells, but the fine-structure of inputs received by an individual neuron during spontaneous network activity has not been studied. Here, we used calcium imaging to record activity at many synapses of hippocampal pyramidal neurons simultaneously to establish the activity patterns in the majority of synapses of an entire cell. Analysis of the spatiotemporal patterns of synaptic activity revealed a fine-scale connectivity rule: neighboring synapses (<16 μm intersynapse distance) are more likely to be coactive than synapses that are farther away from each other. Blocking spiking activity or NMDA receptor activation revealed that the clustering of synaptic inputs required neuronal activity, demonstrating a role of developmentally expressed spontaneous activity for connecting neurons with subcellular precision.

Wnt/PCP proteins regulate stereotyped axon branch extension in Drosophila

Branching morphology is a hallmark feature of axons and dendrites and is essential for neuronal connectivity. To understand how this develops, I analyzed the stereotyped pattern of Drosophila mushroom body (MB) neurons, which have single axons branches that extend dorsally and medially. I found that components of the Wnt/Planar Cell Polarity (PCP) pathway control MB axon branching. frizzled mutant animals showed a predominant loss of dorsal branch extension, whereas strabismus (also known as Van Gogh) mutants preferentially lost medial branches. Further results suggest that Frizzled and Strabismus act independently. Nonetheless, branching fates are determined by complex Wnt/PCP interactions, including interactions with Dishevelled and Prickle that function in a context-dependent manner. Branching decisions are MB-autonomous but non-cell-autonomous as mutant and non-mutant neurons regulate these decisions collectively. I found that Wnt/PCP components do not need to be asymmetrically localized to distinct branches to execute branching functions. However, Prickle axonal localization depends on Frizzled and Strabismus.

Relative input strength rapidly regulates dendritic structure of chick auditory brainstem neurons

Competition between presynaptic inputs has been suggested to shape dendritic form. This hypothesis can be directly tested on bitufted, auditory neurons in chicken nucleus laminaris (NL). Each NL neuron contains two relatively symmetrical dendritic arbors; the dorsal dendrites receive excitatory glutamatergic input from the ipsilateral ear, and the ventral dendrites receive corresponding input from the contralateral ear. To assess the effect of relative synaptic strength on NL dendrites, we used single-cell electroporation; electrophysiology; and live, two-photon laser scanning microscopy to manipulate both the amount and the balance of synaptic input to the two matching sets of dendrites. With simultaneous activation, both sets of dendrites changed together, either growing or retracting over the imaging period. In contrast, stimulation of only one set of dendrites (either dorsal or ventral) resulted in the unstimulated dendrites losing total dendritic branch length, whereas the stimulated dendrites exhibited a tendency to grow. In this system, balanced input leads to balanced changes in the two sets of dendrites, but imbalanced input results in differential changes. Time-lapse imaging revealed that NL dendrites respond to differential stimulation by first decreasing the size of their unstimulated dendrites and then increasing the size of their stimulated dendrites. This result suggests that the relative activity of presynaptic neurons dynamically controls dendritic structure in NL and that dendritic real estate can rapidly be shifted from inactive inputs to active inputs.

Multiple clusters of release sites formed by individual thalamic afferents onto cortical interneurons ensure reliable transmission

Thalamic afferents supply the cortex with sensory information by contacting both excitatory neurons and inhibitory interneurons. Interestingly, thalamic contacts with interneurons constitute such a powerful synapse that even one afferent can fire interneurons, thereby driving feedforward inhibition. However, the spatial representation of this potent synapse on interneuron dendrites is poorly understood. Using Ca imaging and electron microscopy we show that an individual thalamic afferent forms multiple contacts with the interneuronal proximal dendritic arbor, preferentially near branch points. More contacts are correlated with larger amplitude synaptic responses. Each contact, consisting of a single bouton, can release up to seven vesicles simultaneously, resulting in graded and reliable Ca transients. Computational modeling indicates that the release of multiple vesicles at each contact minimally reduces the efficiency of the thalamic afferent in exciting the interneuron. This strategy preserves the spatial representation of thalamocortical inputs across the dendritic arbor over a wide range of release conditions.

Let there be light: zebrafish neurobiology and the optogenetic revolution

Optogenetics has revolutionized the toolbox arsenal that neuroscientists now possess to investigate neuronal circuit function in intact and living animals. With a combination of light emitting 'sensors' and light activated 'actuators', we can monitor and control neuronal activity with minimal perturbation and unprecedented spatiotemporal resolution. Zebrafish neuronal circuits represent an ideal system to apply an optogenetic based analysis owing to its transparency, relatively small size and amenability to genetic manipulation. In this review, we describe some of the most recent advances in the development and applications of optogenetic sensors (i.e., genetically encoded calcium indicators and voltage sensors) and actuators (i.e., light activated ion channels and ion pumps). We focus mostly on the tools that have already been successfully applied in zebrafish and on those that show the greatest potential for the future. We also describe crucial technical aspects to implement optogenetics in zebrafish including strategies to drive a high level of transgene expression in defined neuronal populations, and recent optical advances that allow the precise spatiotemporal control of sample illumination.

Simultaneous two-photon activation of presynaptic cells and calcium imaging in postsynaptic dendritic spines

Background

Dendritic spines of pyramidal neurons are distributed along the complicated structure of the dendritic branches and possess a variety of morphologies associated with synaptic strength. The location and structure of dendritic spines determine the extent of synaptic input integration in the postsynaptic neuron. However, how spine location or size relates to the position of innervating presynaptic cells is not yet known. This report describes a new method that represents a first step toward addressing this issue.

Results

The technique combines two-photon uncaging of glutamate over a broad area (~500 × 250 × 100 μm) with two-photon calcium imaging in a narrow region (~50 × 10 × 1 μm). The former was used for systematic activation of layer 2/3 pyramidal cells in the rat motor cortex, while the latter was used to detect the dendritic spines of layer 5 pyramidal cells that were innervated by some of the photoactivated cells. This technique allowed identification of various sizes of innervated spine located <140 μm laterally from the postsynaptic soma. Spines distal to their parent soma were preferentially innervated by cells on the ipsilateral side. No cluster of neurons innervating the same dendritic branch was detected.

Conclusions

This new method will be a powerful tool for clarifying the microarchitecture of synaptic connections, including the positional and structural characteristics of dendritic spines along the dendrites.

Filtering of visual information in the tectum by an identified neural circuit

The optic tectum of zebrafish is involved in behavioral responses that require the detection of small objects. The superficial layers of the tectal neuropil receive input from retinal axons, while its deeper layers convey the processed information to premotor areas. Imaging with a genetically encoded calcium indicator revealed that the deep layers, as well as the dendrites of single tectal neurons, are preferentially activated by small visual stimuli. This spatial filtering relies on GABAergic interneurons (using the neurotransmitter γ-aminobutyric acid) that are located in the superficial input layer and respond only to large visual stimuli. Photo-ablation of these cells with KillerRed, or silencing of their synaptic transmission, eliminates the size tuning of deeper layers and impairs the capture of prey.

Synchronized neural input shapes stimulus selectivity in a collision-detecting neuron

How higher-order sensory neurons generate complex selectivity from their simpler inputs is a fundamental question in neuroscience. The lobula giant movement detector (LGMD) is such a visual neuron in the locust Schistocerca americana that responds selectively to objects approaching on a collision course or their two-dimensional projections, looming stimuli [1-4]. To study how this selectivity arises, we designed an apparatus allowing us to stimulate, individually and independently, a sizable fraction of the ∼15,000 elementary visual inputs impinging retinotopically onto the LGMD's dendritic fan [5-7] (Figure 1Ai). We then recorded intracellularly in vivo throughout the visual pathway, assessing the LGMD's activity and that of all three successive presynaptic stages conveying local excitatory inputs. Our results suggest that as collision becomes increasingly imminent, the strength of these inputs increases, whereas their latency decreases. This latency decrease favors summation of inputs activated sequentially throughout the looming sequence, making the neuron maximally sensitive to collision-bound trajectories. Thus, the LGMD's selectivity arises partially from presynaptic mechanisms that synchronize a large population of inputs during a looming stimulus and subsequent detection by postsynaptic mechanisms within the neuron itself. Analogous mechanisms are likely to underlie the tuning properties of visual neurons in other species as well.

Focusing on optic tectum circuitry through the lens of genetics

The visual pathway is tasked with processing incoming signals from the retina and converting this information into adaptive behavior. Recent studies of the larval zebrafish tectum have begun to clarify how the 'micro-circuitry' of this highly organized midbrain structure filters visual input, which arrives in the superficial layers and directs motor output through efferent projections from its deep layers. The new emphasis has been on the specific function of neuronal cell types, which can now be reproducibly labeled, imaged and manipulated using genetic and optical techniques. Here, we discuss recent advances and emerging experimental approaches for studying tectal circuits as models for visual processing and sensorimotor transformation by the vertebrate brain.

Foxp1 and lhx1 coordinate motor neuron migration with axon trajectory choice by gating Reelin signalling

During embryonic development of the vertebrate motor system, the same transcription factors that regulate axonal trajectories can also regulate cell body migration, thereby controlling topographic map formation.

Topographic neuronal maps arise as a consequence of axon trajectory choice correlated with the localisation of neuronal soma, but the identity of the pathways coordinating these processes is unknown. We addressed this question in the context of the myotopic map formed by limb muscles innervated by spinal lateral motor column (LMC) motor axons where the Eph receptor signals specifying growth cone trajectory are restricted by Foxp1 and Lhx1 transcription factors. We show that the localisation of LMC neuron cell bodies can be dissociated from axon trajectory choice by either the loss or gain of function of the Reelin signalling pathway. The response of LMC motor neurons to Reelin is gated by Foxp1- and Lhx1-mediated regulation of expression of the critical Reelin signalling intermediate Dab1. Together, these observations point to identical transcription factors that control motor axon guidance and soma migration and reveal the molecular hierarchy of myotopic organisation.

Many areas of our nervous system are organized in a topographic manner, such that the location of a neuron relative to its neighbors is often spatially correlated with its axonal trajectory and therefore target identity. In this study, we focus on the spinal myotopic map, which is characterized by the stereotyped organization of motor neuron cell bodies that is correlated with the trajectory of their axons to limb muscles. An open question for how this map forms is the identity of the molecules that coordinate the expression of effectors of neuronal migration and axonal guidance. Here, we first show that Dab1, a key protein that relays signals directing neuronal migration, is expressed at different concentrations in specific populations of limb-innervating motor neurons and determines the position of their cell bodies in the spinal cord. We then demonstrate that Foxp1 and Lhx1, the same transcription factors that regulate the expression of receptors for motor axon guidance signals, also modulate Dab1 expression. The significance of our findings is that we identify a molecular hierarchy linking effectors of both neuronal migration and axonal projections, and therefore coordinating neuronal soma position with choice of axon trajectory. In general, our findings provide a framework in which to address the general question of how the nervous system is organized.