Control of central synaptic specificity in insect sensory neurons

Measuring conduction velocity distributions in peripheral nerves using neurophysiological techniques

OBJECTIVE

To determine how long it takes for neural impulses to travel along peripheral nerve fibers in living humans.

METHODS

A collision test was performed to measure the conduction velocity distribution of the ulnar nerve. Two stimuli at the distal and proximal sites were used to produce the collision. Compound muscle or nerve action potentials were recorded to perform the measurements on the motor or mixed nerve, respectively. Interstimulus interval was set at 1-5 ms. A quadri-pulse technique was used to measure the refractory period and calibrate the conduction time.

RESULTS

Compound muscle action potential produced by the proximal stimulation started to emerge at the interstimulus interval of about 1.5 ms and increased with the increment in interstimulus interval. Two groups of motor nerve fibers with different conduction velocities were identified. The mixed nerve showed a wider conduction velocity distribution with identification of more subgroups of nerve fibers than the motor nerve.

CONCLUSIONS

The conduction velocity distributions in high resolution on a peripheral motor and mixed nerve are different and this can be measured with the collision test.

SIGNIFICANCE

We provided ground truth data to verify the neuroimaging pipelines for the measurements of latency connectome in the peripheral nervous system.

Neuronal Innervation of the Subgenual Organ Complex and the Tibial Campaniform Sensilla in the Stick Insect Midleg

Mechanosensory organs in legs play are crucial receptors in the feedback control of walking and in the detection of substrate-borne vibrations. Stick insects serve as a model for the physiological role of chordotonal organs and campaniform sensilla. This study documents, by axonal tracing, the neural innervation of the complex chordotonal organs and groups of campaniform sensilla in the proximal tibia of the midleg in Sipyloidea sipylus. In total, 6 nerve branches innervate the different sensory structures, and the innervation pattern associates different sensilla types by their position. Sensilla on the anterior and posterior tibia are innervated from distinct nerve branches. In addition, the variation in innervation is studied for five anatomical branching points. The most common variation is the innervation of the subgenual organ sensilla by two nerve branches rather than a single one. The fusion of commonly separated nerve branches also occurred. However, a common innervation pattern can be demonstrated, which is found in >75% of preparations. The variation did not include crossings of nerves between the anterior and posterior side of the leg. The study corrects the innervation of the posterior subgenual organ reported previously. The sensory neuroanatomy and innervation pattern can guide further physiological studies of mechanoreceptor organs and allow evolutionary comparisons to related insect groups.

Development of connectivity in a motoneuronal network in Drosophila larvae

Background

Much of our understanding of how neural networks develop is based on studies of sensory systems, revealing often highly stereotyped patterns of connections, particularly as these diverge from the presynaptic terminals of sensory neurons. We know considerably less about the wiring strategies of motor networks, where connections converge onto the dendrites of motoneurons. Here, we investigated patterns of synaptic connections between identified motoneurons with sensory neurons and interneurons in the motor network of the Drosophila larva and how these change as it develops.

Results

We find that as animals grow, motoneurons increase the number of synapses with existing presynaptic partners. Different motoneurons form characteristic cell-type-specific patterns of connections. At the same time, there is considerable variability in the number of synapses formed on motoneuron dendrites, which contrasts with the stereotypy reported for presynaptic terminals of sensory neurons. Where two motoneurons of the same cell type contact a common interneuron partner, each postsynaptic cell can arrive at a different connectivity outcome. Experimentally changing the positioning of motoneuron dendrites shows that the geography of dendritic arbors in relation to presynaptic partner terminals is an important determinant in shaping patterns of connectivity.

Conclusions

In the Drosophila larval motor network, the sets of connections that form between identified neurons manifest an unexpected level of variability. Synapse number and the likelihood of forming connections appear to be regulated on a cell-by-cell basis, determined primarily by the postsynaptic dendrites of motoneuron terminals.

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Growing motoneurons consolidate synapses with existing presynaptic partners

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Motoneuron dendritic arbors are active parties in setting connectivity patterns

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Cell-type-specific features coexist with variations at the individual cell level

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Motoneuron wiring strategies may contrast with those of sensory neurons

Engrailed alters the specificity of synaptic connections of Drosophila auditory neurons with the giant fiber

We show that a subset of sound-detecting Johnston's Organ neurons (JONs) in Drosophila melanogaster, which express the transcription factors Engrailed (En) and Invected (Inv), form mixed electrical and chemical synaptic inputs onto the giant fiber (GF) dendrite. These synaptic connections are detected by trans-synaptic Neurobiotin (NB) transfer and by colocalization of Bruchpilot-short puncta. We then show that misexpressing En postmitotically in a second subset of sound-responsive JONs causes them to form ectopic electrical and chemical synapses with the GF, in turn causing that postsynaptic neuron to redistribute its dendritic branches into the vicinity of these afferents. We also introduce a simple electrophysiological recording paradigm for quantifying the presynaptic and postsynaptic electrical activity at this synapse, by measuring the extracellular sound-evoked potentials (SEPs) from the antennal nerve while monitoring the likelihood of the GF firing an action potential in response to simultaneous subthreshold sound and voltage stimuli. Ectopic presynaptic expression of En strengthens the synaptic connection, consistent with there being more synaptic contacts formed. Finally, RNAi-mediated knockdown of En and Inv in postmitotic neurons reduces SEP amplitude but also reduces synaptic strength at the JON-GF synapse. Overall, these results suggest that En and Inv in JONs regulate both neuronal excitability and synaptic connectivity.

Auditory responses of engrailed and invected-expressing Johnston's Organ neurons in Drosophila melanogaster

The roles of the transcription factor Engrailed (En), and its paralogue Invected (Inv), in adult Drosophila Johnston’s Organ sensory neurons are unknown. We used en-GAL4 driven CD8-GFP and antibody staining to characterize these neurons in the pedicel (second antennal segment). The majority of En and Inv-expressing Johnston’s Organ neurons (En-JONs) are located in the ventral part of the posterior group of JONs, with only a few in the medial group. Anatomical classification of En-JON axon projections shows they are mainly type A and E, with a few type B. Extracellular recording of sound-evoked potentials (SEPs) from the antennal nerve was used along with Kir2.1 silencing to assess the contribution that En-JONs make to the auditory response to pure-tone sound stimuli. Silencing En-JONs reduces the SEP amplitude at the onset of the stimulus by about half at 100, 200 and 400 Hz, and also reduces the steady-state response to 200 Hz. En-JONs respond to 82 dB and 92 dB sounds but not 98 dB. Despite their asymmetrical distribution in the Johnston’s Organ they respond equally strongly to both directions of movement of the arista. This implies that individual neurons are excited in both directions, a conclusion supported by reanalysis of the morphology of the pedicel-funicular joint. Other methods of silencing the JONs were also used: RNAi against the voltage-gated Na⁺ channel encoded by the para gene, expression of attenuated diphtheria toxin, and expression of a modified influenza toxin M2(H37A). Only the latter was found to be more effective than Kir2.1. Three additional JON subsets were characterized using Flylight GAL4 lines. inv-GAL4 88B12 and Gycβ100B-GAL4 12G03 express in different subsets of A group neurons and CG12484-GAL4 91G04 is expressed in B neurons. All three contribute to the auditory response to 200 Hz tones.

Midline signalling systems direct the formation of a neural map by dendritic targeting in the Drosophila motor system

A fundamental strategy for organising connections in the nervous system is the formation of neural maps. Map formation has been most intensively studied in sensory systems where the central arrangement of axon terminals reflects the distribution of sensory neuron cell bodies in the periphery or the sensory modality. This straightforward link between anatomy and function has facilitated tremendous progress in identifying cellular and molecular mechanisms that underpin map development. Much less is known about the way in which networks that underlie locomotion are organised. We recently showed that in the Drosophila embryo, dendrites of motorneurons form a neural map, being arranged topographically in the antero-posterior axis to represent the distribution of their target muscles in the periphery. However, the way in which a dendritic myotopic map forms has not been resolved and whether postsynaptic dendrites are involved in establishing sets of connections has been relatively little explored. In this study, we show that motorneurons also form a myotopic map in a second neuropile axis, with respect to the ventral midline, and they achieve this by targeting their dendrites to distinct medio-lateral territories. We demonstrate that this map is "hard-wired"; that is, it forms in the absence of excitatory synaptic inputs or when presynaptic terminals have been displaced. We show that the midline signalling systems Slit/Robo and Netrin/Frazzled are the main molecular mechanisms that underlie dendritic targeting with respect to the midline. Robo and Frazzled are required cell-autonomously in motorneurons and the balance of their opposite actions determines the dendritic target territory. A quantitative analysis shows that dendritic morphology emerges as guidance cue receptors determine the distribution of the available dendrites, whose total length and branching frequency are specified by other cell intrinsic programmes. Our results suggest that the formation of dendritic myotopic maps in response to midline guidance cues may be a conserved strategy for organising connections in motor systems. We further propose that sets of connections may be specified, at least to a degree, by global patterning systems that deliver pre- and postsynaptic partner terminals to common "meeting regions."

Synapse formation in developing neural circuits

The nervous system consists of hundreds of billions of neurons interconnected into the functional neural networks that underlie behaviors. The capacity of a neuron to innervate and function within a network is mediated via specialized cell junctions known as synapses. Synapses are macromolecular structures that regulate intercellular communication in the nervous system, and are the main gatekeepers of information flow within neural networks. Where and when synapses form determines the connectivity and functionality of neural networks. Therefore, our knowledge of how synapse formation is regulated is critical to our understanding of the nervous system and how it goes awry in neurological disorders. Synapse formation involves pairing of the pre- and postsynaptic partners at a specific neurospatial coordinate. The specificity of synapse formation requires the precise execution of multiple developmental events, including cell fate specification, cell migration, axon guidance, dendritic growth, synaptic target selection, and synaptogenesis (Juttner and Rathjen in Cell. Mol. Life Sci. 62:2811, 2005; Salie et al., in Neuron 45:189, 2005; Waites et al., in Annu. Rev. Neurosci. 28:251, 2005). Remarkably, during the development of the vertebrate nervous system, these developmental processes occur almost simultaneously in billions of neurons, resulting in the formation of trillions of synapses. How this remarkable specificity is orchestrated during development is one of the outstanding questions in the field of neurobiology, and the focus of discussion of this chapter. We center the discussion of this chapter on the early developmental events that orchestrate the process of synaptogenesis prior to activity-dependent mechanisms. We have therefore limited the discussion of important activity-dependent synaptogenic events, which are discussed in other chapters of this book. Moreover, our discussion is biased toward lessons we have learned from invertebrate systems, in particular from C. elegans and Drosophila. We did so to complement the discussions from other chapters in this book, which focus on the important findings that have recently emerged from the vertebrate literature. The chapter begins with a brief history of the field of synaptic biology. This serves as a backdrop to introduce some of the historically outstanding questions of synaptic development that have eluded us during the past century, and which are the focus of this review. We then discuss some general features of synaptic structure as it relates to its function. In particular, we will highlight evolutionarily conserved traits shared by all synaptic structures, and how these features have helped optimize these ancient cellular junctions for interneural communication. We then discuss the regulatory signals that orchestrate the precise assembly of these conserved macromolecular structures. This discussion will be framed in the context of the neurodevelopmental process. Specifically, much of our discussion will focus on how the seemingly disparate developmental processes are intimately linked at a molecular level, and how this relationship might be crucial in the developmental orchestration of circuit assembly. We hope that the discussion of the multifunctional cues that direct circuit development provides a conceptual framework into understanding how, with a limited set of signaling molecules, precise neural wiring can be coordinated between synaptic partners.

Multifunctional role of protein kinase C in regulating the formation and maturation of specific synapses

Target-dependent increases in axon growth and varicosities accompany the formation of functional synapses between Aplysia sensory neurons and specific postsynaptic neurons (L7 and not L11). The enhanced growth is regulated in part by a target-dependent increase in the secretion of sensorin, the sensory neuron neuropeptide. We report here that protein kinase C (PKC) activity is required for synapse formation by sensory neurons with L7 and for the target-dependent increases in sensorin synthesis and secretion. Blocking PKC activity reversibly blocked synapse formation and axon growth of sensory neurons contacting L7, but did not affect axon growth of sensory neurons contacting L11 or axon growth of the postsynaptic targets. Blocking PKC activity also blocked the target-induced increase in sensorin synthesis and secretion. Sensorin then activates additional signaling pathways required for synapse maturation and synapse-associated growth. Exogenous anti-sensorin antibody blocked target-induced activation and translocation into sensory neuron nuclei of p42/44 mitogen-activated protein kinase (MAPK), attenuated synapse maturation, and curtailed growth of sensory neurons contacting L7, but not the growth of sensory neurons contacting L11. Inhibitors of MAPK or phosphoinositide 3-kinase also attenuated synapse maturation and curtailed growth and varicosity formation of sensory neurons contacting L7, but not growth of sensory neurons contacting L11. These results suggest that PKC activity regulated by specific cell-cell interactions initiates the formation of specific synapses and the subsequent synthesis and release of a neuropeptide to activate additional signaling pathways required for synapse maturation.

Projections of Drosophila multidendritic neurons in the central nervous system: links with peripheral dendrite morphology

Neurons establish diverse dendritic morphologies during development, and a major challenge is to understand how these distinct developmental programs might relate to, and influence, neuronal function. Drosophila dendritic arborization (da) sensory neurons display class-specific dendritic morphology with extensive coverage of the body wall. To begin to build a basis for linking dendrite structure and function in this genetic system, we analyzed da neuron axon projections in embryonic and larval stages. We found that multiple parameters of axon morphology, including dorsoventral position, midline crossing and collateral branching, correlate with dendritic morphological class. We have identified a class-specific medial-lateral layering of axons in the central nervous system formed during embryonic development, which could allow different classes of da neurons to develop differential connectivity to second-order neurons. We have examined the effect of Robo family members on class-specific axon lamination, and have also taken a forward genetic approach to identify new genes involved in axon and dendrite development. For the latter, we screened the third chromosome at high resolution in vivo for mutations that affect class IV da neuron morphology. Several known loci, as well as putative novel mutations, were identified that contribute to sensory dendrite and/or axon patterning. This collection of mutants, together with anatomical data on dendrites and axons, should begin to permit studies of dendrite diversity in a combined developmental and functional context, and also provide a foundation for understanding shared and distinct mechanisms that control axon and dendrite morphology.

PlexinA1 signaling directs the segregation of proprioceptive sensory axons in the developing spinal cord

As different classes of sensory neurons project into the CNS, their axons segregate and establish distinct trajectories and target zones. One striking instance of axonal segregation is the projection of sensory neurons into the spinal cord, where proprioceptive axons avoid the superficial dorsal horn-the target zone of many cutaneous afferent fibers. PlexinA1 is a proprioceptive sensory axon-specific receptor for sema6C and sema6D, which are expressed in a dynamic pattern in the dorsal horn. The loss of plexinA1 signaling causes the shafts of proprioceptive axons to invade the superficial dorsal horn, disrupting the organization of cutaneous afferents. This disruptive influence appears to involve the intermediary action of oligodendrocytes, which accompany displaced proprioceptive axon shafts into the dorsal horn. Our findings reveal a dedicated program of axonal shaft positioning in the mammalian CNS and establish a role for plexinA1-mediated axonal exclusion in organizing the projection pattern of spinal sensory afferents.