Dendritic projections of different types of octopaminergic unpaired median neurons in the locust metathoracic ganglion

Three-dimensional atlas of thoracic ganglia in the praying mantis, Tenodera aridifolia

The praying mantis is a good model for the study of motor control, especially for investigating the transformation from sensory signals into motor commands. In insects, thoracic ganglia (TG) play an important role in motor control. To understand the functional organization of TG, an atlas is useful. However, except for the fruitfly, no three-dimensional atlas of TG has not been reported for insects. In this study, we generated a three-dimensional atlas of prothoracic, mesothoracic, and metathoracic ganglia in the praying mantis (Tenodera aridifolia). First, we observed serial sections of the prothoracic ganglion stained with hematoxylin and eosin to identify longitudinal tracts and transverse commissures. We then visualized neuropil areas by immunostaining whole-mount TG with an anti-synapsin antibody. Before labeling each neuropil area, standardization using the iterative shape averaging method was applied to images to make neuropil contours distinct. Neuropil areas in TG were defined based on their shape and relative position to tracts and commissures. Finally, a three-dimensional atlas was reconstructed from standardized images of the TG. The standard TG are available at the Comparative Neuroscience Platform website (cns.neuroinf.jp/modules/xoonips/detail.php?item_id=11946) and can be used as a common reference map to combine the anatomical data obtained from different individuals.

Epithelial domains and the primordial antennal nervous system of the embryonic grasshopper Schistocerca gregaria

The antenna is a key sensory organ in insects. Factors which pattern its epithelium and the spacing of sensillae will play an important role in shaping its contribution to adaptive behavior. The antenna of the grasshopper S. gregaria has three major articulations: scape, pedicel, and flagellum. During postembryonic development, the flagellum lengthens as segments (so-called meristal annuli) are added at each molt. However, the five most apical annuli do not subdivide; thus, their epithelial domains must already be defined during embryogenesis. We investigated epithelial compartmentalization and its relationship to the developing primordial nervous system of the antenna by simultaneous immunolabeling against the epithelial cell surface molecule Lachesin, against neuron-specific horseradish peroxidase, and against the mitosis marker phospho-histone 3. We found that Lachesin is initially expressed in a highly ordered pattern of "rings" and a "sock" in the apical antennal epithelium of the early embryo. These expression domains appear in a stereotypic order and prefigure later articulations. Proliferative cells segregate into these developing domains and pioneer- and sensory-cell precursors were molecularly identified. Our study allows pioneer neurons, guidepost cells, and the earliest sensory cell clusters of the primordial nervous system to be allocated to their respective epithelial domain. As the apical-most five domains remain stable through subsequent development, lengthening of the flagellum must originate from more basal regions and is likely to be under the control of factors homologous to those which regulate boundary and joint formation in the antenna of Drosophila.

Descending octopaminergic neurons modulate sensory-evoked activity of thoracic motor neurons in stick insects

Neuromodulatory neurons located in the brain can influence activity in locomotor networks residing in the spinal cord or ventral nerve cords of invertebrates. How inputs to and outputs of neuromodulatory descending neurons affect walking activity is largely unknown. With the use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and immunohistochemistry, we show that a population of dorsal unpaired median (DUM) neurons descending from the gnathal ganglion to thoracic ganglia of the stick insect Carausius morosus contains the neuromodulatory amine octopamine. These neurons receive excitatory input coupled to the legs’ stance phases during treadmill walking. Inputs did not result from connections with thoracic central pattern-generating networks, but, instead, most are derived from leg load sensors. In excitatory and inhibitory retractor coxae motor neurons, spike activity in the descending DUM (desDUM) neurons increased depolarizing reflexlike responses to stimulation of leg load sensors. In these motor neurons, descending octopaminergic neurons apparently functioned as components of a positive feedback network mainly driven by load-detecting sense organs. Reflexlike responses in excitatory extensor tibiae motor neurons evoked by stimulations of a femur-tibia movement sensor either are increased or decreased or were not affected by the activity of the descending neurons, indicating different functions of desDUM neurons. The increase in motor neuron activity is often accompanied by a reflex reversal, which is characteristic for actively moving animals. Our findings indicate that some descending octopaminergic neurons can facilitate motor activity during walking and support a sensory-motor state necessary for active leg movements.

NEW & NOTEWORTHY We investigated the role of descending octopaminergic neurons in the gnathal ganglion of stick insects. The neurons become active during walking, mainly triggered by input from load sensors in the legs rather than pattern-generating networks. This report provides novel evidence that octopamine released by descending neurons on stimulation of leg sense organs contributes to the modulation of leg sensory-evoked activity in a leg motor control system.

Ontogeny and development of the tritocerebral commissure giant (TCG): an identified neuron in the brain of the grasshopper Schistocerca gregaria

The tritocerebral commissure giant (TCG) of the grasshopper Schistocerca gregaria is one of the best anatomically and physiologically described arthropod brain neurons. A member of the so-called Ventral Giant cluster of cells, it integrates sensory information from visual, antennal and hair receptors, and synapses with thoracic motor neurons in order to initiate and regulate flight behavior. Its ontogeny, however, remains unclear. In this study, we use bromodeoxyuridine incorporation and cyclin labeling to reveal proliferative neuroblasts in the region of the embryonic brain where the ventral giant cluster is located. Engrailed labeling confirms the deutocerebral identity of this cluster. Comparison of soma locations and initial neurite projections into tracts of the striate deutocerebrum help identify the cells of the ventral cluster in both the embryonic and adult brain. Reconstructions of embryonic cell lineages suggest deutocerebral NB1 as being the putative neuroblast of origin. Intracellular dye injection coupled with immunolabeling against neuron-specific horseradish peroxidase is used to identify the VG1 (TCG) and VG3 neurons from the ventral cluster in embryonic brain slices. Dye injection and backfilling are used to document axogenesis and the progressive expansion of the dendritic arbor of the TCG from mid-embryogenesis up to hatching. Comparative maps of embryonic neuroblasts from several orthopteroid insects suggest equivalent deutocerebral neuroblasts from which the homologous TCG neurons already identified in the adult brain could originate. Our data offer the prospect of identifying further lineage-related neurons from the cluster and so understand a brain connectome from both a developmental and evolutionary perspective.

Whole-Cell Properties of Cerebellar Nuclei Neurons In Vivo

Cerebellar nuclei neurons integrate sensorimotor information and form the final output of the cerebellum, projecting to premotor brainstem targets. This implies that, in contrast to specialized neurons and interneurons in cortical regions, neurons within the nuclei encode and integrate complex information that is most likely reflected in a large variation of intrinsic membrane properties and integrative capacities of individual neurons. Yet, whether this large variation in properties is reflected in a heterogeneous physiological cell population of cerebellar nuclei neurons with well or poorly defined cell types remains to be determined. Indeed, the cell electrophysiological properties of cerebellar nuclei neurons have been identified in vitro in young rodents, but whether these properties are similar to the in vivo adult situation has not been shown. In this comprehensive study we present and compare the in vivo properties of 144 cerebellar nuclei neurons in adult ketamine-xylazine anesthetized mice. We found regularly firing (N = 88) and spontaneously bursting (N = 56) neurons. Membrane-resistance, capacitance, spike half-width and firing frequency all widely varied as a continuum, ranging from 9.63 to 3352.1 MΩ, from 6.7 to 772.57 pF, from 0.178 to 1.98 ms, and from 0 to 176.6 Hz, respectively. At the same time, several of these parameters were correlated with each other. Capacitance decreased with membrane resistance (R2 = 0.12, P<0.001), intensity of rebound spiking increased with membrane resistance (for 100 ms duration R2 = 0.1503, P = 0.0011), membrane resistance decreased with membrane time constant (R2 = 0.045, P = 0.031) and increased with spike half-width (R2 = 0.023, P<0.001), while capacitance increased with firing frequency (R2 = 0.29, P<0.001). However, classes of neuron subtypes could not be identified using merely k-clustering of their intrinsic firing properties and/or integrative properties following activation of their Purkinje cell input. Instead, using whole-cell parameters in combination with morphological criteria revealed by intracellular labelling with Neurobiotin (N = 18) allowed for electrophysiological identification of larger (29.3-50 μm soma diameter) and smaller (< 21.2 μm) cerebellar nuclei neurons with significant differences in membrane properties. Larger cells had a lower membrane resistance and a shorter spike, with a tendency for higher capacitance. Thus, in general cerebellar nuclei neurons appear to offer a rich and wide continuum of physiological properties that stand in contrast to neurons in most cortical regions such as those of the cerebral and cerebellar cortex, in which different classes of neurons operate in a narrower territory of electrophysiological parameter space. The current dataset will help computational modelers of the cerebellar nuclei to update and improve their cerebellar motor learning and performance models by incorporating the large variation of the in vivo properties of cerebellar nuclei neurons. The cellular complexity of cerebellar nuclei neurons may endow the nuclei to perform the intricate computations required for sensorimotor coordination.

A method for immunolabeling neurons in intact cuticularized insect appendages

The antennae of the grasshopper Schistocerca gregaria possess a pair of nerve pathways which are established by so-called pioneer neurons early in embryonic development. Subsequently, sensory cell clusters mediating olfaction, flight, optomotor responses, and phase changes differentiate from the antennal epithelium at stereotypic locations and direct their axons onto those of the pioneers to then project to the brain. Early in embryonic development, before the antennae become cuticularized, immunolabeling can be used to follow axogenesis in these pioneers and sensory cells. At later stages, immunolabeling becomes problematical as the cuticle is laid down and masks internal antigen sites. In order to immunolabel the nervous system of cuticularized late embryonic and first instar grasshopper antennae, we modified a procedure known as sonication in which the appendage is exposed to ultrasound thereby rendering it porous to antibodies. Comparisons of the immunolabeled nervous system of sectioned and sonicated antennae show that the cellular organization of sensory clusters and their axon projections is intact. The expression patterns of neuron-specific, microtubule-specific, and proliferative cell-specific labels in treated antennae are consistent with those reported for earlier developmental stages where sonication is not necessary, suggesting that these molecular epitopes are also conserved. The method ensures reliable immunolabeling in intact, cuticularized appendages so that the peripheral nervous system can be reconstructed directly via confocal microscopy throughout development.

All layers of medial entorhinal cortex receive presubicular and parasubicular inputs

The medial entorhinal cortex (MEC), presubiculum (PrS), and parasubiculum (PaS) are interconnected components of the hippocampal-parahippocampal spatial-representation system. Principal cells in all layers of MEC show signs of directional tuning, overt in head direction cells present in all layers except for layer II, and covert in grid cells, which are the major spatially modulated cell type in layer II. Directional information likely originates in the head direction-vestibular system and PrS and PaS are thought to provide this information to MEC. Efferents from PaS and PrS show a selective laminar terminal distribution in MEC superficial layers II and III, respectively. We hypothesized that this anatomically determined laminar distribution does not preclude monosynaptic interaction with neurons located in deeper layers of MEC in view of the extensive apical dendrites from deeper cells reaching layers II and III. This hypothesis was tested in the rat using tilted in vitro slices in which origins and terminations of PrS and PaS fibers were maintained, as assessed using anterograde anatomical tracing. Based on voltage-sensitive dye imaging, multipatch single-cell recordings, and scanning photostimulation of caged glutamate, we report first that principal neurons in all layers of MEC receive convergent monosynaptic inputs from PrS and PaS and second, that elicited responses show layer-specific decay times and frequency-dependent facilitation. These results indicate that regardless of their selective laminar terminal distribution, PrS and PaS inputs may monosynaptically convey directional information to principal neurons in all layers of MEC through synapses on their extensive dendritic arbors.

Neuromodulatory unpaired median neurons in the New Zealand tree weta, Hemideina femorata

Wetas are ancient Gondwanan orthopterans (Anostostomatidae) with many species endemic to New Zealand. Like all Orthoptera they possess efferent neuromodulatory dorsal unpaired median (DUM) neurons, with bilaterally symmetrical axons, that are important components of motor networks. These neurons produce overshooting action potentials and are easily stimulated by a variety of external mechanosensory stimuli delivered to the body and appendages. In particular, stimulation of the antennae, mouth parts, tarsi and femora of the legs, abdomen, cerci and ovipositor is very effective in activating DUM neurons in the metathoracic ganglion of wetas. In addition, looming visual stimuli or light on-, light off-stimuli excite many metathoracic DUM neurons. These DUM sensory reflex pathways remain viable after the prothoracic to subesophageal connective is cut, whereas in locusts such reflex pathways are interrupted by the ablation. This suggests that, in wetas, sensory reflex pathways for DUM activation are organized in a less centralized fashion than in locusts, and may therefore reflect a plesiomorphic evolutionary state in the weta. In addition, many weta DUM neurons exhibit slow rhythmic bursting which also persists following the connective ablation.

Brain organization in Collembola (springtails)

Arthropoda is comprised of four major taxa: Hexapoda, Crustacea, Myriapoda and Chelicerata. Although this classification is widely accepted, there is still some debate about the internal relationships of these groups. In particular, the phylogenetic position of Collembola remains enigmatic. Some molecular studies place Collembola into a close relationship to Protura and Diplura within the monophyletic Hexapoda, but this placement is not universally accepted, as Collembola is also regarded as either the sister group to Branchiopoda (a crustacean taxon) or to Pancrustacea (crustaceans + hexapods). To contribute to the current debate on the phylogenetic position of Collembola, we examined the brains in three collembolan species: Folsomia candida, Protaphorura armata and Tetrodontophora bielanensis, using antennal backfills, series of semi-thin sections, and immunostaining technique with several antisera, in conjunction with confocal laser scanning microscopy and three-dimensional reconstructions. We identified several neuroanatomical structures in the collembolan brain, including a fan-shaped central body showing a columnar organization, a protocerebral bridge, one pair of antennal lobes with 20-30 spheroidal glomeruli each, and a structure, which we interpret as a simply organized mushroom body. The results of our neuroanatomical study are consistent with the phylogenetic position of Collembola within the Hexapoda and do not contradict the hypothesis of a close relationship of Collembola, Protura and Diplura.

Octopamine neuromodulatory effects on a social behavior decision-making network in Drosophila males

Situations requiring rapid decision-making in response to dynamic environmental demands occur repeatedly in natural environments. Neuromodulation can offer important flexibility to the output of neural networks in coping with changing conditions, but the contribution of individual neuromodulatory neurons in social behavior networks remains relatively unknown. Here we manipulate the Drosophila octopaminergic system and assay changes in adult male decision-making in courtship and aggression paradigms. When the functional state of OA neural circuits is enhanced, males exhibit elevated courtship behavior towards other males in both behavioral contexts. Eliminating the expression of the male form of the neural sex determination factor, Fruitless (Fru(M)), in three OA suboesophageal ganglia (SOG) neurons also leads to increased male-male courtship behavior in these same contexts. We analyzed the fine anatomical structure through confocal examination of labeled single neurons to determine the arborization patterns of each of the three Fru(M)-positive OA SOG neurons. These neurons send processes that display mirror symmetric, widely distributed arbors of endings within brain regions including the ventrolateral protocerebra, the SOG and the peri-esophageal complex. The results suggest that a small subset of OA neurons have the potential to provide male selective modulation of behavior at a single neuron level.

The role of octopamine in locusts and other arthropods

The biogenic amine octopamine and its biological precursor tyramine are thought to be the invertebrate functional homologues of the vertebrate adrenergic transmitters. Octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems and prompts the whole organism to "dynamic action". A growing number of studies suggest a prominent role for octopamine in modulating multiple physiological and behavioural processes in invertebrates, as for example the phase transition in Schistocerca gregaria. Both octopamine and tyramine exert their effects by binding to specific receptor proteins that belong to the superfamily of G protein-coupled receptors. Since these receptors do not appear to be present in vertebrates, they may present very suitable and specific insecticide and acaricide targets.

The cloning, phylogenetic relationship and distribution pattern of two new putative GPCR-type octopamine receptors in the desert locust (Schistocerca gregaria)

The biogenic amine octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems. It plays a prominent role in modulating multiple physiological and behavioural processes in invertebrates. Octopamine exerts its effects by binding to specific receptor proteins that belong to the superfamily of G protein-coupled receptors. We found two partial sequences of putative octopamine receptors in the desert locust Schistocerca gregaria (SgOctalphaR and SgOctbetaR) and investigated their transcript levels in males and females of both phases and during the transition between long-term solitarious and gregarious locusts. The transcript levels of SgOctalphaR are the highest in the central nervous system, whereas those of SgOctbetaR are the highest in the flight muscles, followed by the central nervous system. Both SgOctalphaR and SgOctbetaR show higher transcript levels in long-term gregarious locusts as compared to solitarious ones. The rise of SgOctbetaR transcript levels already appears during the first 4h of gregarisation, during which also the behavioural changes take place.