Neural Circuitry Underlying Nociceptive Escape Behavior in Drosophila

Rapid and efficient escape behaviors in response to noxious sensory stimuli are essential for protection and survival. In Drosophila larvae, the class III (cIII) and class IV (cIV) dendritic arborization (da) neurons detect low-threshold mechanosensory and noxious stimuli, respectively. Their axons project to modality-specific locations in the neuropil, reminiscent of vertebrate dorsal horn organization. Despite extensive characterization of nociceptors across organisms, how noxious stimuli are transformed to the coordinated behaviors that protect animals from harm remains poorly understood. In larvae, noxious mechanical and thermal stimuli trigger an escape behavior consisting of sequential C-shape body bending followed by corkscrew-like rolling, and finally an increase in forward locomotion (escape crawl). The downstream circuitry controlling the sequential coordination of escape responses is largely unknown. This work identifies a population of interneurons in the nerve cord, Down-and-Back (DnB) neurons, that are activated by noxious heat, promote nociceptive behavior, and are required for robust escape responses to noxious stimuli. Activation of DnB neurons can trigger both rolling, and the initial C-shape body bend independent of rolling, revealing modularity in the initial nociceptive responses. Electron microscopic circuit reconstruction shows that DnBs receive direct input from nociceptive and mechanosensory neurons, are presynaptic to pre-motor circuits, and link indirectly to a population of command-like neurons (Goro) that control rolling. DnB activation promotes activity in Goro neurons, and coincident inactivation of Goro neurons prevents the rolling sequence but leaves intact body bending motor responses. Thus, activity from nociceptors to DnB interneurons coordinates modular elements of nociceptive escape behavior. The impact of DnB neurons may not be restricted to synaptic partners, as DnB presynaptic sites accumulate dense-core vesicles, suggesting aminergic or peptidergic signaling. Anatomical analyses show that DnB neurons receive spatially segregated input from cIII mechanosensory and cIV nociceptive neurons. However, DnB neurons do not seem to promote or be required for gentle-touch responses, suggesting a modulatory role for cIII input. Behavioral experiments suggest that cIII input presented prior to cIV input can enhance nociceptive behavior. Moreover, weak co-activation of DnB and cIII neurons can also enhance nociceptive responses, particularly C-shape bending. These results indicate that timing and level of cIII activation might determine its modulatory role. Taken together, these studies describe a novel nociceptive circuit, which integrates nociceptive and mechanosensory inputs, and controls modular motor pathways to promote robust escape behavior. Future work on this circuit could reveal neural mechanisms for sequence transitions, peptidergic modulation of nociception, and developmental mechanisms that control convergence of sensory afferents onto common synaptic partners

From whole-brain data to functional circuit models: the zebrafish optomotor response

Detailed descriptions of brain-scale sensorimotor circuits underlying vertebrate behavior remain elusive. Recent advances in zebrafish neuroscience offer new opportunities to dissect such circuits via whole-brain imaging, behavioral analysis, functional perturbations, and network modeling. Here, we harness these tools to generate a brain-scale circuit model of the optomotor response, an orienting behavior evoked by visual motion. We show that such motion is processed by diverse neural response types distributed across multiple brain regions. To transform sensory input into action, these regions sequentially integrate eye- and direction-specific sensory streams, refine representations via interhemispheric inhibition, and demix locomotor instructions to independently drive turning and forward swimming. While experiments revealed many neural response types throughout the brain, modeling identified the dimensions of functional connectivity most critical for the behavior. We thus reveal how distributed neurons collaborate to generate behavior and illustrate a paradigm for distilling functional circuit models from whole-brain data

Role of serotonergic neurons in the Drosophila larval response to light

<p>Abstract</p> <p>Background</p> <p><it>Drosophila </it>larval locomotion consists of forward peristalsis interrupted by episodes of pausing, turning and exploratory behavior (head swinging). This behavior can be regulated by visual input as seen by light-induced increase in pausing, head swinging and direction change as well as reduction of linear speed that characterizes the larval photophobic response. During 3^rdinstar stage, <it>Drosophila </it>larvae gradually cease to be repelled by light and are photoneutral by the time they wander in search for a place to undergo metamorphosis. Thus, <it>Drosophila </it>larval photobehavior can be used to study control of locomotion.</p> <p>Results</p> <p>We used targeted neuronal silencing to assess the role of candidate neurons in the regulation of larval photobehavior. Inactivation of DOPA decarboxylase (Ddc) neurons increases the response to light throughout larval development, including during the later stages of the 3^rdinstar characterized by photoneutral response. Increased response to light is characterized by increase in light-induced direction change and associated pause, and reduction of linear movement. Amongst Ddc neurons, suppression of the activity of corazonergic and serotonergic but not dopaminergic neurons increases the photophobic response observed during 3^rdinstar stage. Silencing of serotonergic neurons does not disrupt larval locomotion or the response to mechanical stimuli. Reduced serotonin (5-hydroxytryptamine, 5-HT) signaling within serotonergic neurons recapitulates the results obtained with targeted neuronal silencing. Ablation of serotonergic cells in the ventral nerve cord (VNC) does not affect the larval response to light. Similarly, disruption of serotonergic projections that contact the photoreceptor termini in the brain hemispheres does not impact the larval response to light. Finally, pan-neural over-expression of 5-HT1A_Droreceptors, but not of any other 5-HT receptor subtype, causes a significant decrease in the response to light of 3^rdinstar larvae.</p> <p>Conclusion</p> <p>Our data demonstrate that activity of serotonergic and corazonergic neurons contribute to the control of larval locomotion by light. We conclude that this control is carried out by 5-HT neurons located in the brain hemispheres, but does not appear to occur at the photoreceptor level and may be mediated by 5-HT1A_Droreceptors. These findings provide new insights into the function of 5-HT neurons in <it>Drosophila </it>larval behavior as well as into the mechanisms underlying regulation of larval response to light.</p

NBLAST: Rapid, Sensitive Comparison of Neuronal Structure and Construction of Neuron Family Databases.

Neural circuit mapping is generating datasets of tens of thousands of labeled neurons. New computational tools are needed to search and organize these data. We present NBLAST, a sensitive and rapid algorithm, for measuring pairwise neuronal similarity. NBLAST considers both position and local geometry, decomposing neurons into short segments; matched segments are scored using a probabilistic scoring matrix defined by statistics of matches and non-matches. We validated NBLAST on a published dataset of 16,129 single Drosophila neurons. NBLAST can distinguish neuronal types down to the finest level (single identified neurons) without a priori information. Cluster analysis of extensively studied neuronal classes identified new types and unreported topographical features. Fully automated clustering organized the validation dataset into 1,052 clusters, many of which map onto previously described neuronal types. NBLAST supports additional query types, including searching neurons against transgene expression patterns. Finally, we show that NBLAST is effective with data from other invertebrates and zebrafish. VIDEO ABSTRACT.This work was supported by the Medical Research Council [MRC file reference U105188491] and European Research Council Starting and Consolidator Grants to G.S.X.E.J., who is an EMBO Young Investigator.This is the final version of the article. It first appeared from Cell Press via http://dx.doi.org/10.1016/j.neuron.2016.06.01

Developmental organization of central neurons in the adult Drosophila ventral nervous system

We have used MARCM to reveal the adult morphology of the post embryonically produced neurons in the thoracic neuromeres of the Drosophila VNS. The work builds on previous studies of the origins of the adult VNS neurons to describe the clonal organization of the adult VNS. We present data for 58 of 66 postembryonic thoracic lineages, excluding the motor neuron producing lineages (15 and 24) which have been described elsewhere. MARCM labels entire lineages but where both A and B hemilineages survive (e.g., lineages 19, 12, 13, 6, 1, 3, 8, and 11), the two hemilineages can be discriminated and we have described each hemilineage separately. Hemilineage morphology is described in relation to the known functional domains of the VNS neuropil and based on the anatomy we are able to assign broad functional roles for each hemilineage. The data show that in a thoracic hemineuromere, 16 hemilineages are primarily involved in controlling leg movements and walking, 9 are involved in the control of wing movements, and 10 interface between both leg and wing control. The data provide a baseline of understanding of the functional organization of the adult Drosophila VNS. By understanding the morphological organization of these neurons, we can begin to define and test the rules by which neuronal circuits are assembled during development and understand the functional logic and evolution of neuronal networks.</p

The autism protein UBE3A/E6AP regulates remodeling of neuronal dendritic arborization

Autism spectrum disorders (ASDs) are clinically characterized by decreased communication abilities, impaired social interaction, and the occurrence of repetitive behaviors, with high genetic heritability. Ubiquitin protein ligase E3A (UBE3A) is a gene located on human chromosome 15q11-13, a region that has been the focus of genetic studies of susceptibility to ASD AND Angelman syndrome. An increased UBE3A gene dosage and thus an elevated amount of E6AP, the protein product of UBE3A, is associated with ASD. However, the underlying cellular and molecular details remain poorly understood. Normal development of neuronal structure is critical for intercellular connectivity and overall brain function, and abnormal brain development is a commonality amongst ASDs. These studies therefore investigated the role of increased dosage of Ube3A/E6AP in dendritic arborization and synapse maturation during brain development. Increased E6AP expression in vitro led to significant reduction in dendritic arborization by thinning and fragmentation of the distal tip, along with a decrease in spine density and an increase in immature spines in hippocampal neurons. This morphological remodeling effect was mediated by the ubiquitination and subsequent degradation of the X-linked inhibitor of apoptosis protein (XIAP) by E6AP, which led to activation of caspase-3. Furthermore, activated caspases cleaved tubulin, leading to retraction of microtubules from the distal tip of dendrites, dendritic thinning and eventual disappearance. In vivo studies investigated the role of E6AP in ASD-related neuronal development in Ube3A 2X transgenic mice and found that, consistent with our in vitro studies, increased E6AP in the brain lead to decreased XIAP levels, increased active caspase-3, and enhanced tubulin cleavage in hippocampal tissue in Ube3A 2X mice. In accord, Ube3A 2X mice showed a reduction in dendritic growth and branching and spine density. This work elucidated an important role of Ube3A/E6AP in dendritic pruning and identified XIAP as a novel ubiquitination target of E6AP. These findings provide a new insight into the molecular pathways underlying neurodevelopmental defects in Ube3A/E6AP-associated ASDs.2018-07-09T00:00:00

Electrical Compartmentalization in Neurons

The dendritic tree of neurons plays an important role in information processing in the brain. While it is thought that dendrites require independent subunits to perform most of their computations, it is still not understood how they compartmentalize into functional subunits. Here, we show how these subunits can be deduced from the properties of dendrites. We devised a formalism that links the dendritic arborization to an impedance-based tree graph and show how the topology of this graph reveals independent subunits. This analysis reveals that cooperativity between synapses decreases slowly with increasing electrical separation and thus that few independent subunits coexist. We nevertheless find that balanced inputs or shunting inhibition can modify this topology and increase the number and size of the subunits in a context-dependent manner. We also find that this dynamic recompartmentalization can enable branch-specific learning of stimulus features. Analysis of dendritic patch-clamp recording experiments confirmed our theoretical predictions.Peer reviewe

Potential Synaptic Connectivity of Different Neurons onto Pyramidal Cells in a 3D Reconstruction of the Rat Hippocampus

Most existing connectomic data and ongoing efforts focus either on individual synapses (e.g., with electron microscopy) or on regional connectivity (tract tracing). An individual pyramidal cell (PC) extends thousands of synapses over macroscopic distances (∼cm). The contrasting requirements of high-resolution and large field of view make it too challenging to acquire the entire synaptic connectivity for even a single typical cortical neuron. Light microscopy can image whole neuronal arbors and resolve dendritic branches. Analyzing connectivity in terms of close spatial appositions between axons and dendrites could thus bridge the opposite scales, from synaptic level to whole systems. In the mammalian cortex, structural plasticity of spines and boutons makes these “potential synapses” functionally relevant to learning capability and memory capacity. To date, however, potential synapses have only been mapped in the surrounding of a neuron and relative to its local orientation rather than in a system-level anatomical reference. Here we overcome this limitation by estimating the potential connectivity of different neurons embedded into a detailed 3D reconstruction of the rat hippocampus. Axonal and dendritic trees were oriented with respect to hippocampal cytoarchitecture according to longitudinal and transversal curvatures. We report the potential connectivity onto PC dendrites from the axons of a dentate granule cell, three CA3 PCs, one CA2 PC, and 13 CA3b interneurons. The numbers, densities, and distributions of potential synapses were analyzed in each sub-region (e.g., CA3 vs. CA1), layer (e.g., oriens vs. radiatum), and septo-temporal location (e.g., dorsal vs. ventral). The overall ratio between the numbers of actual and potential synapses was ∼0.20 for the granule and CA3 PCs. All potential connectivity patterns are strikingly dependent on the anatomical location of both pre-synaptic and post-synaptic neurons

Statistical connectivity provides a sufficient foundation for specific functional connectivity in neocortical neural microcircuits

It is well-established that synapse formation involves highly selective chemospecific mechanisms, but how neuron arbors are positioned before synapse formation remains unclear. Using 3D reconstructions of 298 neocortical cells of different types (including nest basket, small basket, large basket, bitufted, pyramidal, and Martinotti cells), we constructed a structural model of a cortical microcircuit, in which cells of different types were independently and randomly placed. We compared the positions of physical appositions resulting from the incidental overlap of axonal and dendritic arbors in the model (statistical structural connectivity) with the positions of putative functional synapses (functional synaptic connectivity) in 90 synaptic connections reconstructed from cortical slice preparations. Overall, we found that statistical connectivity predicted an average of 74 ± 2.7% (mean ± SEM) synapse location distributions for nine types of cortical connections. This finding suggests that chemospecific attractive and repulsive mechanisms generally do not result in pairwise-specific connectivity. In some cases, however, the predicted distributions do not match precisely, indicating that chemospecific steering and aligning of the arbors may occur for some types of connections. This finding suggests that random alignment of axonal and dendritic arbors provides a sufficient foundation for specific functional connectivity to emerge in local neural microcircuits