Development of locustatachykinin immunopositive neurons in the central complex of the beetle Tenebrio molitor

Sequence heterochrony led to a gain of functionality in an immature stage of the central complex: A fly-beetle insight

Animal behavior is guided by the brain. Therefore, adaptations of brain structure and function are essential for animal survival, and each species differs in such adaptations. The brain of one individual may even differ between life stages, for instance, as adaptation to the divergent needs of larval and adult life of holometabolous insects. All such differences emerge during development, but the cellular mechanisms behind the diversification of brains between taxa and life stages remain enigmatic. In this study, we investigated holometabolous insects in which larvae differ dramatically from the adult in both behavior and morphology. As a consequence, the central complex, mainly responsible for spatial orientation, is conserved between species at the adult stage but differs between larvae and adults of one species as well as between larvae of different taxa. We used genome editing and established transgenic lines to visualize cells expressing the conserved transcription factor retinal homeobox, thereby marking homologous genetic neural lineages in both the fly Drosophila melanogaster and the beetle Tribolium castaneum. This approach allowed us for the first time to compare the development of homologous neural cells between taxa from embryo to the adult. We found complex heterochronic changes including shifts of developmental events between embryonic and pupal stages. Further, we provide, to our knowledge, the first example of sequence heterochrony in brain development, where certain developmental steps changed their position within the ontogenetic progression. We show that through this sequence heterochrony, an immature developmental stage of the central complex gains functionality in Tribolium larvae.

The central complex, part of the brain responsible for spatial orientation, differs between insect species and life stages. This study marks and compares the development of homologous neurons between a beetle and a fly, revealing that by heterochronic development an immature form of the central complex becomes functional in beetle larvae.

Neuroarchitecture of the dung beetle central complex

Despite their tiny brains, insects show impressive abilities when navigating over short distances during path integration or during migration over thousands of kilometers across entire continents. Celestial compass cues often play an important role as references during navigation. In contrast to many other insects, South African dung beetles rely exclusively on celestial cues for visual reference during orientation. After finding a dung pile, these animals cut off a piece of dung from the pat, shape it into a ball and roll it away along a straight path until a suitable place for underground consumption is found. To maintain a constant bearing, a brain region in the beetle's brain, called the central complex, is crucially involved in the processing of skylight cues, similar to what has already been shown for path-integrating and migrating insects. In this study, we characterized the neuroanatomy of the sky-compass network and the central complex in the dung beetle brain in detail. Using tracer injections, combined with imaging and 3D modeling, we describe the anatomy of the possible sky-compass network in the central brain. We used a quantitative approach to study the central-complex network and found that several types of neuron exhibit a highly organized connectivity pattern. The architecture of the sky-compass network and central complex is similar to that described in insects that perform path integration or are migratory. This suggests that, despite their different orientation behaviors, this neural circuitry for compass orientation is highly conserved among the insects.

A conserved plan for wiring up the fan-shaped body in the grasshopper and Drosophila

The central complex comprises an elaborate system of modular neuropils which mediate spatial orientation and sensory-motor integration in insects such as the grasshopper and Drosophila. The neuroarchitecture of the largest of these modules, the fan-shaped body, is characterized by its stereotypic set of decussating fiber bundles. These are generated during development by axons from four homologous protocerebral lineages which enter the commissural system and subsequently decussate at stereotypic locations across the brain midline. Since the commissural organization prior to fan-shaped body formation has not been previously analyzed in either species, it was not clear how the decussating bundles relate to individual lineages, or if the projection pattern is conserved across species. In this study, we trace the axonal projections from the homologous central complex lineages into the commissural system of the embryonic and larval brains of both the grasshopper and Drosophila. Projections into the primordial commissures of both species are found to be lineage-specific and allow putatively equivalent fascicles to be identified. Comparison of the projection pattern before and after the commencement of axon decussation in both species reveals that equivalent commissural fascicles are involved in generating the columnar neuroarchitecture of the fan-shaped body. Further, the tract-specific columns in both the grasshopper and Drosophila can be shown to contain axons from identical combinations of central complex lineages, suggesting that this columnar neuroarchitecture is also conserved.

Development of the Neurochemical Architecture of the Central Complex

The central complex represents one of the most conspicuous neuroarchitectures to be found in the insect brain and regulates a wide repertoire of behaviors including locomotion, stridulation, spatial orientation and spatial memory. In this review article, we show that in the grasshopper, a model insect system, the intricate wiring of the fan-shaped body (FB) begins early in embryogenesis when axons from the first progeny of four protocerebral stem cells (called W, X, Y, Z, respectively) in each brain hemisphere establish a set of tracts to the primary commissural system. Decussation of subsets of commissural neurons at stereotypic locations across the brain midline then establishes a columnar neuroarchitecture in the FB which is completed during embryogenesis. Examination of the expression patterns of various neurochemicals in the central complex including neuropeptides, a neurotransmitter and the gas nitric oxide (NO), show that these appear progressively and in a substance-specific manner during embryogenesis. Each neuroactive substance is expressed by neurons located at stereotypic locations in a given central complex lineage, confirming that the stem cells are biochemically multipotent. The organization of axons expressing the various neurochemicals within the central complex is topologically related to the location, and hence birthdate, of the neurons within the lineages. The neurochemical expression patterns within the FB are layered, and so reflect the temporal topology present in the lineages. This principle relates the neuroanatomical to the neurochemical architecture of the central complex and so may provide insights into the development of adaptive behaviors.

Comparative Neuroanatomy of the Lateral Accessory Lobe in the Insect Brain

The lateral accessory lobe (LAL) mediates signals from the central complex to the thoracic motor centers. The results obtained from different insects suggest that the LAL is highly relevant to the locomotion. Perhaps due to its deep location and lack of clear anatomical boundaries, few studies have focused on this brain region. Systematic data of LAL interneurons are available in the silkmoth. We here review individual neurons constituting the LAL by comparing the silkmoth and other insects. The survey through the connectivity and intrinsic organization suggests potential homology in the organization of the LAL among insects.

The insect central complex as model for heterochronic brain development-background, concepts, and tools

The adult insect brain is composed of neuropils present in most taxa. However, the relative size, shape, and developmental timing differ between species. This diversity of adult insect brain morphology has been extensively described while the genetic mechanisms of brain development are studied predominantly in Drosophila melanogaster. However, it has remained enigmatic what cellular and genetic mechanisms underlie the evolution of neuropil diversity or heterochronic development. In this perspective paper, we propose a novel approach to study these questions. We suggest using genome editing to mark homologous neural cells in the fly D. melanogaster, the beetle Tribolium castaneum, and the Mediterranean field cricket Gryllus bimaculatus to investigate developmental differences leading to brain diversification. One interesting aspect is the heterochrony observed in central complex development. Ancestrally, the central complex is formed during embryogenesis (as in Gryllus) but in Drosophila, it arises during late larval and metamorphic stages. In Tribolium, it forms partially during embryogenesis. Finally, we present tools for brain research in Tribolium including 3D reconstruction and immunohistochemistry data of first instar brains and the generation of transgenic brain imaging lines. Further, we characterize reporter lines labeling the mushroom bodies and reflecting the expression of the neuroblast marker gene Tc-asense, respectively.

Novel antennal lobe substructures revealed in the small hive beetle Aethina tumida

The small hive beetle, Aethina tumida, is an emerging pest of social bee colonies. A. tumida shows a specialized life style for which olfaction seems to play a crucial role. To better understand the olfactory system of the beetle, we used immunohistochemistry and 3-D reconstruction to analyze brain structures, especially the paired antennal lobes (AL), which represent the first integration centers for odor information in the insect brain. The basic neuroarchitecture of the A. tumida brain compares well to the typical beetle and insect brain. In comparison to other insects, the AL are relatively large in relationship to other brain areas, suggesting that olfaction is of major importance for the beetle. The AL of both sexes contain about 70 olfactory glomeruli with no obvious size differences of the glomeruli between sexes. Similar to all other insects including beetles, immunostaining with an antiserum against serotonin revealed a large cell that projects from one AL to the contralateral AL to densely innervate all glomeruli. Immunostaining with an antiserum against tachykinin-related peptides (TKRP) revealed hitherto unknown structures in the AL. Small TKRP-immunoreactive spherical substructures are in both sexes evenly distributed within all glomeruli. The source for these immunoreactive islets is very likely a group of about 80 local AL interneurons. We offer two hypotheses on the function of such structures.

Conserved patterns of axogenesis in the panarthropod brain

Neuropils in the cerebral midline of Panarthropoda exhibit a wide spectrum of neuroarchitectures--from rudimentary to highly elaborated--and which at first sight defy a unifying neuroarchitectural principle. Developmental approaches have shown that in model arthropods such as insects, conserved cellular and molecular mechanisms first establish a simple axon scaffold in the brain. However, to be adapted for adult life, this immature ground plan is transformed by a developmental process--known in the grasshopper as "fascicle switching"--in which subsets of neurons systematically redirect their growth cones at stereotypic locations across the brain midline. A topographic system of choice points along the transverse brain axis where axons decussate features in all panarthropods studied even though different modes of neurogenesis and varying degrees of neuropilar elaboration are involved. This suggests that the molecular mechanisms regulating choice point selection may be conserved. In combination with recent cladistic interpretations of arthropod phylogeny based on nuclear protein-coding sequences the data argue for this topographic decussation as having evolved early and being a conserved feature of the Panarthropoda. Differences in elaboration likely reflect both the extent to which neuropilar reorganization has progressed during development and the lifestyle of the individual organism.

Neuroarchitecture and neuroanatomy of the Drosophila central complex: A GAL4-based dissection of protocerebral bridge neurons and circuits

Insects exhibit an elaborate repertoire of behaviors in response to environmental stimuli. The central complex plays a key role in combining various modalities of sensory information with an insect's internal state and past experience to select appropriate responses. Progress has been made in understanding the broad spectrum of outputs from the central complex neuropils and circuits involved in numerous behaviors. Many resident neurons have also been identified. However, the specific roles of these intricate structures and the functional connections between them remain largely obscure. Significant gains rely on obtaining a comprehensive catalog of the neurons and associated GAL4 lines that arborize within these brain regions, and on mapping neuronal pathways connecting these structures. To this end, small populations of neurons in the Drosophila melanogaster central complex were stochastically labeled using the multicolor flip-out technique and a catalog was created of the neurons, their morphologies, trajectories, relative arrangements, and corresponding GAL4 lines. This report focuses on one structure of the central complex, the protocerebral bridge, and identifies just 17 morphologically distinct cell types that arborize in this structure. This work also provides new insights into the anatomical structure of the four components of the central complex and its accessory neuropils. Most strikingly, we found that the protocerebral bridge contains 18 glomeruli, not 16, as previously believed. Revised wiring diagrams that take into account this updated architectural design are presented. This updated map of the Drosophila central complex will facilitate a deeper behavioral and physiological dissection of this sophisticated set of structures. J. Comp. Neurol. 523:997–1037, 2015. © 2014 Wiley Periodicals, Inc.

Organization and functional roles of the central complex in the insect brain

The central complex is a group of modular neuropils across the midline of the insect brain. Hallmarks of its anatomical organization are discrete layers, an organization into arrays of 16 slices along the right-left axis, and precise inter-hemispheric connections via chiasmata. The central complex is connected most prominently with the adjacent lateral complex and the superior protocerebrum. Its developmental appearance corresponds with the appearance of compound eyes and walking legs. Distinct dopaminergic neurons control various forms of arousal. Electrophysiological studies provide evidence for roles in polarized light vision, sky compass orientation, and integration of spatial information for locomotor control. Behavioral studies on mutant and transgenic flies indicate roles in spatial representation of visual cues, spatial visual memory, directional control of walking and flight, and place learning. The data suggest that spatial azimuthal directions (i.e., where) are represented in the slices, and cue information (i.e., what) are represented in different layers of the central complex.

Anatomical basis of sun compass navigation II: the neuronal composition of the central complex of the monarch butterfly

Each fall, eastern North American monarch butterflies in their northern range undergo a long-distance migration south to their overwintering grounds in Mexico. Migrants use a time-compensated sun compass to determine directionality during the migration. This compass system uses information extracted from sun-derived skylight cues that is compensated for time of day and ultimately transformed into the appropriate motor commands. The central complex (CX) is likely the site of the actual sun compass, because neurons in this brain region are tuned to specific skylight cues. To help illuminate the neural basis of sun compass navigation, we examined the neuronal composition of the CX and its associated brain regions. We generated a standardized version of the sun compass neuropils, providing reference volumes, as well as a common frame of reference for the registration of neuron morphologies. Volumetric comparisons between migratory and nonmigratory monarchs substantiated the proposed involvement of the CX and related brain areas in migratory behavior. Through registration of more than 55 neurons of 34 cell types, we were able to delineate the major input pathways to the CX, output pathways, and intrinsic neurons. Comparison of these neural elements with those of other species, especially the desert locust, revealed a surprising degree of conservation. From these interspecies data, we have established key components of a conserved core network of the CX, likely complemented by species-specific neurons, which together may comprise the neural substrates underlying the computations performed by the CX.

Representation of the brain's superior protocerebrum of the flesh fly, Neobellieria bullata, in the central body

The central complex of the insect brain is a system of midline neuropils involved in transforming sensory information into behavioral outputs. Genetic studies focusing on nerve cells supplying the central complex from the protocerebrum propose that such neurons play key roles in circuits involved in learning the distinction of visual cues during operant conditioning. To better identify the possible sites of such circuits we used Bodian and anti-synapsin staining to resolve divisions of the superior protocerebrum into discrete neuropils. Here we show that in the fly Neobellieria bullata, the superior protocerebrum is composed of at least five clearly defined regions that correspond to those identified in Drosophila melanogaster. Intracellular dye fills and Golgi impregnations resolve "tangential neurons" that have intricate systems of branches in two of these regions. The branches are elaborate, decorated with specializations indicative of pre- and postsynaptic sites. The tangentially arranged terminals of these neurons extend across characteristic levels of the central complex's fan-shaped body. In this and another blowfly species, we identify an asymmetric pair of neuropils situated deep in the fan-shaped body, called the asymmetric bodies because of their likely homology with similar elements in Drosophila. One of the pair of bodies receives collaterals from symmetric arrangements of tangential neuron terminals. Cobalt injections reveal that the superior protocerebrum is richly supplied with local interneurons that are likely participants in microcircuitry associated with the distal processes of tangential neurons. Understanding the morphologies and arrangements of these and other neurons is essential for correctly interpreting functional attributes of the central complex.

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.

Embryonic development of the insect central complex: insights from lineages in the grasshopper and Drosophila

The neurons of the insect brain derive from neuroblasts which delaminate from the neuroectoderm at stereotypic locations during early embryogenesis. In both grasshopper and Drosophila, each developing neuroblast acquires an intrinsic capacity for neuronal proliferation in a cell autonomous manner and generates a specific lineage of neural progeny which is nearly invariant and unique. Maps revealing numbers and distributions of brain neuroblasts now exist for various species, and in both grasshopper and Drosophila four putatively homologous neuroblasts have been identified whose progeny direct axons to the protocerebral bridge and then to the central body via an equivalent set of tracts. Lineage analysis in the grasshopper nervous system reveals that the progeny of a neuroblast maintain their topological position within the lineage throughout embryogenesis. We have taken advantage of this to study the pioneering of the so-called w, x, y, z tracts, to show how fascicle switching generates central body neuroarchitecture, and to evaluate the roles of so-called intermediate progenitors as well as programmed cell death in shaping lineage structure. The novel form of neurogenesis involving intermediate progenitors has been demonstrated in grasshopper, Drosophila and mammalian cortical development and may represent a general strategy for increasing brain size and complexity. An analysis of gap junctional communication involving serotonergic cells reveals an intrinsic cellular organization which may relate to the presence of such transient progenitors in central complex lineages.

Developmental expression of neuromodulators in the central complex of the grasshopper Schistocerca gregaria

The central complex is a major integrative region within the insect brain with demonstrated roles in spatial orientation, the regulation of locomotor behavior, and sound production. In the hemimetabolous grasshopper, the central complex comprises the protocerebral bridge, central body (CB), ellipsoid body, noduli, and accessory lobes, and this modular organization develops entirely during embryogenesis. From a biochemical perspective, a range of neuroactive substances has been demonstrated in these modules of the adult central complex, but little is known about their developmental expression. In this study, we use matrix-assisted laser desorption/ionization-imaging mass spectrometry on single brain slices to confirm the presence of several peptide families (tachykinin, allatostatin, periviscerokinin/pyrokinin, FLRFamide, and neuropeptide F) in the adult central complex and then use immunohistochemistry and histology to examine their developmental expression, together with that of the indolamin serotonin, and the endogenous messenger nitric oxide (NO; via its synthesizing enzyme). We find that each neuromodulator is expressed according to a unique, stereotypic, pattern within the various modules making up the central complex. Neuropeptides such as tachykinin (55%) and allatostatin (65%), and the NO-synthesizing enzyme diaphorase (70%), are expressed earlier during embryonic development than the biogenic amine serotonin (80%), whereas periviscerokinin-like peptides and FLRFamide-like peptides begin to be expressed only postembryonically. Within the CB, these neuroactive substances are present in tangential projection neurons before they appear in columnar neurons. There is also no colocalization of serotonin-positive and peptide-positive projections up to the third larval instar during development, consistent with the clear dorsoventral layering of the neuropil we observe. Our results provide the first neurochemical fingerprint of the developing central complex in an hemimetabolous insect.

Structure of the adult central complex in Drosophila: organization of distinct neuronal subsets

The central complex (CX) is a defined set of neuropils located on the midline of the protocerebrum in several arthropods and has been implicated in a number of behaviors. To investigate the function of the CX further it is imperative to know the neuroarchitecture of this structure and to ensure all known neuron types conform to a common nomenclature system. Several types of CX neuron have been identified but it is not known if these exist singly or as components of isomorphic sets. We used an enhancer trap approach to study the adult structure, connectivity, and polarity of CX neurons in Drosophila. We observed several isomorphic sets of small-field neurons including pontine and fb-eb neurons, and also isomorphic sets of large-field neurons including R neurons and F neurons. We found that several types of large-field F neurons existed in isomorphic sets of approximately eight (four per hemisphere) and found evidence for small-field neuron types existing as isomorphic sets of 16. Small-field neurons were observed in clearly organized layers. This study provides a novel insight into CX structure and connectivity and provides a set of characterized enhancer trap lines that will be valuable for future study.

An ontogenetic analysis of locustatachykinin-like expression in the central complex of the grasshopper Schistocerca gregaria

We have investigated the ontogenetic basis of locustatachykinin-like expression in a group of cells located in the pars intercerebralis of the grasshopper midbrain. These cells project fibers to the protocerebral bridge and the central body via a characteristic set of fiber bundles called the w, x, y, z tracts. Lineage analyses associate the immunoreactive cells with one of four neuroblasts (termed W, X, Y, Z) in each protocerebral hemisphere of the early embryo. Locustatachykinin is a ubiquitous myotropic peptide among the insects and its expression in the pars intercerebralis begins at approximately 60-65% of embryogenesis. This coincides with the appearance of the columnar neuroarchitecture characteristic of the central body. The number of immunoreactive cells in a given lineage is initially small, increases significantly in later embryogenesis, and attains the adult situation (about 7% of a lineage) in the first larval instar after hatching. Although each neuroblast generates progeny displaying a spectrum of cell body sizes, there is a clear morphological gradient, which reflects birth order within the lineage. Locustatachykinin expressing cells are located stereotypically at or near the tip of their lineage, which an age profile reveals places them amongst the first born progeny of their respective neuroblasts. Although these neuroblasts begin to generate progeny at approximately 25-27% of embryogenesis, their daughter cells remain quiescent with respect to locustatachykinin expression for over 30% of embryogenesis.

Evolution of the central complex in the arthropod brain with respect to the visual system

Modular midline neuropils, termed arcuate body (Chelicerata, Onychophora) or central body (Myriapoda, Crustacea, Insecta), are a prominent feature of the arthropod brain. In insects and crayfish, the central body is connected to a second midline-spanning neuropil, the protocerebral bridge. Both structures are collectively termed central complex. While some investigators have assumed that central and arcuate bodies are homologous, others have questioned this view. Stimulated by recent evidence for a role of the central complex in polarization vision and object recognition, the architectures of midline neuropils and their associations with the visual system were compared across panarthropods. In chelicerates and onychophorans, second-order neuropils subserving the median eyes are associated with the arcuate body. The central complex of decapods and insects, instead, receives indirect input from the lateral (compound) eye visual system, and connections with median eye (ocellar) projections are present. Together with other characters these data are consistent with a common origin of arcuate bodies and central complexes from an ancestral modular midline neuropil but, depending on the choice of characters, the protocerebral bridge or the central body shows closer affinity with the arcuate body. A possible common role of midline neuropils in azimuth-dependent sensory and motor tasks is discussed.

Building the central complex of the grasshopper Schistocerca gregaria: axons pioneering the w, x, y, z tracts project onto the primary commissural fascicle of the brain

The central complex is a major neuropilar structure in the insect brain whose distinctive, modular, neuroarchitecture in the grasshopper is exemplified by a bilateral set of four fibre bundles called the w, x, y and z tracts. These columns represent the stereotypic projection of axons from the pars intercerebralis into commissures of the central complex. Each column is established separately during early embryogenesis in a clonal manner by the progeny of a subset of four identified protocerebral neuroblasts. We report here that dye injected into identified pioneers of the primary brain commissure between 31 and 37% of embryogenesis couples to cells in the pars intercerebralis which we identify as progeny of the W, X, Y, or Z neuroblasts. These progeny are the oldest within each lineage, and also putatively the first to project an axon into the protocerebral commissure. The axons of pioneers from each tract do not fasciculate with one other prior to entry into the commissure, thereby prefiguring the modular w, x, y, z columns of the adult central complex. Within the commissure, pioneer axons from columnar tracts fasciculate with the growth cones of identified pioneers of the existing primary fascicle and do not pioneer a separate fascicle. The results suggest that neurons pioneering a columnar neuroarchitecture within the embryonic central complex utilize the existing primary commissural scaffold to navigate the brain midline.

Neuronal expression of tachykinin-related peptides and gene transcript during postembryonic development of Drosophila

The gene Dtk, encoding the prohormone of tachykinin-related peptides (TRPs), has been identified from Drosophila. This gene encodes five putative tachykinin-related peptides (DTK-1 to 5) that share the C-terminal sequence FXGXRamide (where X represents variable residues) as well as an extended peptide (DTK-6) with the C-terminus FVAVRamide). By mass spectrometry (MALDI-TOF-MS), we identified ion signals with masses identical to those of DTK-1 to 5 in specific brain regions. We have analyzed the distribution of the Dtk transcript and peptides, by in situ hybridization and immunocytochemistry during postembryonic development of the central nervous system (CNS) of Drosophila. Antiserum against a cockroach TRP that cross-reacts with the DTKs was used for immunocytochemistry. Expression of transcript and peptides was detected from first to third instar larvae, through metamorphosis to adult flies. Throughout postembryonic development, we were able to follow the strong expression of TRPs in a pair of large descending neurons with cell bodies in the brain. The number of TRP-expressing neuronal cell bodies in the brain and ventral nerve cord increases during larval development. In the early pupa (stage P8), the number of TRP-expressing cell bodies is lower than in the third instar larvae. The number drastically increases during later pupal development, and in the adult fly about 200 TRP-expressing neurons can be seen in the CNS. The continuous expression of TRPs in neurons throughout postembryonic development suggests specific functional roles in both larval and imaginal flies and possibly also in some neurons during pupal development.

Early development of the Drosophila brain: IV. Larval neuropile compartments defined by glial septa

In this study, we have analyzed the architecture of the brain neuropile of the Drosophila larva, which is formed by two main structural elements: long axon tracts and terminal axonal/dendritic arborizations carrying synapses. By using several molecular markers expressed in neurons and glial cells, we show that the early larval neuropile is subdivided by glial sheaths into numerous compartments. The three-dimensional layout of these compartments and their relationship to the pattern of long axon tracts described in the accompanying article (Nassif et al. [2003] J. Comp. Neurol 417-434) was modeled by using a three-dimensional illustration computer software. On the basis of their location relative to each other and to long axon tracts, larval brain compartments can be identified with compartments defined by structural and functional criteria for the adult fly brain. We find that small precursors of most of the compartments of the adult central brain can be identified in the early larva. Changes in brain compartmental organization occurring during larval growth are described. Neuropile compartments, representing easily identifiable landmark structures, will assist in future analyses of Drosophila brain development in which the exact location of neurons and their axonal trajectories is of importance.

G Protein-Coupled Receptors in Invertebrates: A State of the Art

G protein-coupled receptors (GPCRs) constitute one of the largest and most ancient superfamilies of membrane-spanning proteins. We focus on neuropeptide GPCRs, in particular on those of invertebrates. In general, such receptors mediate the responses of signaling molecules that constitute the highest hierarchical position in the regulation of physiological processes. Until recently, only a few of these receptors were identified in invertebrates. However, the availability of a plethora of genomic information has boosted the discovery of novel members in several invertebrate species, such as Drosophila, in which 18 neuropeptide GPCRs have been characterized. The finalization of genomic projects in other invertebrates will lead to a similar expansion of GPCR understanding. Many new insights regarding neuropeptide regulation have followed from the discovery of their cognate receptors. Furthermore, information on GPCR signaling is still fragmentary and the elucidation of these pathways in model insects such as Drosophila will lead to further insights in other species, including mammals. In this review we present the current status of what is known about invertebrate GPCRs, discuss some novel perceptions that follow from the identified members, and, finally, present some future prospects.

Fenvalerate treatment affects development of olfactory glomeruli in Manduca sexta

Low doses of fenvalerate, a widely used type-II pyrethroid insecticide, have been shown previously to produce abnormal olfactory centers in the brain and abnormal olfactory-mediated behavior in beetles (Wegerhoff et al. [1998] Neuroreport 9:3241-3245). Here, we use the experimental advantages of the moth Manduca sexta to explore the cellular changes that lead to these abnormalities. Our results indicate that treatment with fenvalerate may affect multiple aspects of the development of the primary olfactory centers, the antennal lobes, in Manduca, including ingrowth of olfactory receptor axons, axon fasciculation, and targeting within the antennal lobe, and intercellular signaling between the receptor axons and the glial cells that ordinarily surround and stabilize the developing olfactory glomeruli.

Cardioacceleratory action of tachykinin-related neuropeptides and proctolin in two coleopteran insect species

Several cardioactive peptides have been identified in insects and most of them are likely to act on the heart as neurohormones. Here we have investigated the cardioactive properties of members of a family of insect tachykinin-related peptides (TRPs) in heterologous bioassays with two coleopteran insects, Tenebrio molitor and Zophobas atratus. Their effects were compared with the action of the pentapeptide proctolin. We tested the cardiotropic activity of LemTRP-4 isolated from the midgut of the cockroach Leucophaea maderae, CavTK-I and CavTK-II isolated from the blowfly Calliphora vomitoria. The semi-isolated hearts of the two coleopteran species were strongly stimulated by proctolin. We observed a dose dependent increase in heartbeat frequency (a positive chronotropic effect) and a decrease in amplitude of contractions (a negative inotropic effect). In both beetles the TRPs are less potent cardiostimulators and exert lower maximal frequency responses than proctolin. LemTRP-4 applied at 10(-9)-10(-6) M was cardiostimulatory in both species inducing an increase of heart beat frequency. The amplitude of contractions was stimulated only in Z. atratus. CavTK-I and CavTK-II also exerted cardiostimulatory effects in Z. atratus at 10(-9)-10(-6) M. Both peptides stimulated the frequency, but only CavTK-II increased the amplitude of the heart beat. In T. molitor, however, the CavTKs induced no significant effect on the heart. Immunocytochemistry with antisera to the locust TRPs LomTK-I and LomTK-II was employed to identify the source of TRPs acting on the heart. No innervation of the heart by TRP immunoreactive axons could detected, instead it is possible that TRPs reach the heart by route of the circulation. The likely sources of circulating TRPs in these insects are TRP-immunoreactive neurosecretory cells of the median neurosecretory cell group in the brain with terminations in the corpora cardiaca and endocrine cells in the midgut. In conclusion, LemTRP-4, CavTK-I and CavTK-II are less potent cardiostimulators than proctolin and also exert stimulatory rather than inhibitory action on amplitude of contractions. The differences in the responses to proctolin and TRPs suggest that the peptides regulate heart activity by different mechanisms.

Characterization of a receptor for insect tachykinin-like peptide agonists by functional expression in a stable Drosophila Schneider 2 cell line

STKR is an insect G protein-coupled receptor, cloned from the stable fly Stomoxys calcitrans. It displays sequence similarity to vertebrate tachykinin [or neurokinin (NK)] receptors. Functional expression of the cloned STKR cDNA was obtained in cultured Drosophila melanogaster Schneider 2 (S2) cells. Insect tachykinin-like peptides or "insectatachykinins," such as Locusta tachykinin (Lom-TK) III, produced dose-dependent calcium responses in stably transfected S2-STKR cells. Vertebrate tachykinins (or neurokinins) did not evoke any effect at concentrations up to 10(-5) M, but an antagonist of mammalian neurokinin receptors, spantide II, inhibited the Lom-TK III-induced calcium response. Further analysis showed that the agonist-induced intracellular release of calcium ions was not affected by pretreatment of the cells with pertussis toxin. The calcium rise was blocked by the phospholipase C inhibitor U73122. In addition, Lom-TK III was shown to have a stimulatory effect on the accumulation of both inositol 1,4,5-trisphosphate and cyclic AMP. These are the same second messengers that are induced in mammalian neurokinin-dependent signaling processes.

Tachykinin-like peptides and their receptors. A review

Tachykinin-like peptides have been identified in many vertebrate and invertebrate species. On the basis of the data reviewed in this paper, these peptides can be classified into two distinct subfamilies, which are recognized by their respective sequence characteristics. All known vertebrate tachykinins and a few invertebrate ones share a common C-terminal sequence motif, -FXGLMa. The insect tachykinins, which have a common -GFX1GX2Ra C-terminus, display about 30% of sequence homology with the first group. Tachykinins are multifunctional brain/gut peptides. In mammals and insects, various isoforms play an important neuromodulatory role in the central nervous system. They are involved in the processing of sensory information and in the control of motor activities. In addition, members of both subfamilies elicit stimulatory responses on a variety of visceral muscles. The receptors for mammalian and insect tachykinins show a high degree of sequence conservation and their functional characteristics are very similar. In both mammals and insects, angiotensin-converting enzyme (ACE) plays a prominent role in tachykinin peptide metabolism.

GABA and serotonin immunoreactivity during postembryonic brain development in the beetle Tenebrio molitor

Analysis of the serotonin immunoreactive neurons in the central brain of the beetle Tenebrio molitor during postembryonic development shows that the basic structural characteristics of larval brain resemble those of the adult. Most, if not all, serotonin immunoreactive central brain neurons persist with metamorphosis. Their fate can be followed during development. GABA immunoreactivity occurs in about 360 neurons assembled in ten different clusters of somata in the larval midbrain. During metamorphosis no additional clusters are formed. However, the number of immunoreactive neurons increases to 450. Their morphological analysis is restricted to location of the somata and the distribution of arborizations within neuropil areas. Metamorphic transition of glomerular sub-units in the antennal lobes as well as ellipsoid body development can be followed by GABA immunohistochemistry. Furthermore, the study of these transitions proved useful in displaying changes during metamorphic pattern formation induced by sublethal application of the pyrethroid insecticide fenvalerate.

The distribution and myotropic activity of locustatachykinin-like peptides in locust midgut

The midgut of the African migratory locust, Locusta migratoria, was found to contain endocrine-like cells that stained positively for locustatachykinin I (Lom TK I)-like immunoreactivity. These cells were distributed in an unequal manner throughout the midgut of the locust, with a greater density of Lom TK I-like immunoreactive endocrine-like cells occurring in the posterior region of the midgut. These singly occurring cells appear elongate with an apical extension projecting toward the midgut lumen and a smaller projection extending towards the midgut basal lamina. No immunoreactive neuronal processes were detected along the midgut wall. Radioimmunoassays revealed that the female midgut contained two to three times more Lom TK I-like material than the male midgut, and radioimmunoassay coupled to high-performance liquid chromatography analysis revealed that at least five locustatachykinin isoforms appear to be present in the midgut. This distribution of Lom TK I-like material suggests possible functional differences in the various regions of the midgut. The role that these cells may play in locust midgut secretory activity and motility remains unknown. However, the addition of synthetic Lom TK I through IV to a ring type midgut muscle preparation stimulated contraction of midgut circular muscles, suggesting a possible physiological role for these peptides. Dose-response curves constructed for Lom TK I-IV revealed that the peptide-induced contractions increased in a dose-dependent manner.

Tachykinin-related peptides in invertebrates: a review

Peptides with sequence similarities to members of the tachykinin family have been identified in a number of invertebrates belonging to the mollusca, echiuridea, insecta and crustacea. These peptides have been designated tachykinin-related peptides (TRPs) and are characterized by the preserved C-terminal pentapeptide FX1GX2Ramide (X1 and X2 are variable residues). All invertebrate TRPs are myostimulatory on insect hindgut muscle, but also have a variety of additional actions: they can induce contractions in cockroach foregut and oviduct and in moth heart muscle, trigger a motor rhythm in the crab stomatogastric ganglion, depolarize or hyperpolarize identified interneurons of locust and the snail Helix and induce release of adipokinetic hormone from the locust corpora cardiaca. Two putative TRP receptors have been cloned from Drosophila; both are G-protein coupled and expressed in the nervous system. The invertebrate TRPs are distributed in interneurons of the CNS of Limulus, crustaceans and insects. In the latter two groups TRPs are also present in the stomatogastric nervous system and in insects endocrine cells of the midgut display TRP-immunoreactivity. In arthropods the distribution of TRPs in neuronal processes of the brain displays similar patterns. Also in coelenterates, flatworms and molluscs TRPs have been demonstrated in neurons. The activity of different TRPs has been explored in several assays and it appears that an amidated C-terminal hexapeptide (or longer) is required for bioactivity. In many invertebrate assays the first generation substance P antagonist spantide I is a potent antagonist of invertebrate TRPs and substance P. Locustatachykinins stimulate adenylate cyclase in locust interneurons and glandular cells of the corpora cardiaca, but in other tissues the putative second messenger systems have not yet been identified. The heterologously expressed Drosophila TRP receptors coupled to the phospholipase C pathway and could induce elevations of inositol triphosphate. The structures, distributions and actions of TRPs in various invertebrates are compared and it is concluded that the TRPs are multifunctional peptides with targets both in the central and peripheral nervous system and other tissues, similar to vertebrate tachykinins. Invertebrate TRPs may also be involved in developmental processes.

The receptor tyrosine kinase gene linotte is required for neuronal pathway selection in the Drosophila mushroom bodies

The linotte (lio) mutant was first isolated as a memory mutant. The lio gene encodes a putative receptor tyrosine kinase (RTK), homologous to the human protein RYK. This gene has been independently identified in a screen for embryonic nervous system axonal guidance defects and called derailed (drl). Here, we report that linotte mutants present structural brain defects in the adult central complex (CX) and mushroom bodies (MB). linotte and derailed are allelic for this phenotype, which can be rescued by a drl+ transgene. The Lio RTK is expressed preferentially in the adult CX and MB. Our results suggest that, analogous to its role within the embryonic nervous system, the Lio RTK is involved in neuronal pathway selection during adult brain development.

Effects of fenvalerate on the development of olfactory perception in a beetle

Low doses of fenvalerate (a Type II pyrethroid) were applied to the beetle Tenebrio molitor at pupation, to ascertain its effects on the developing olfactory system. Doses of fenvalerate that prevent the formation of glomeruli in the primary olfactory neuropil (antennal lobes) also inhibit olfactory orientation behavior for different odors, despite the fact that sensory neurons developed responses to these odors. Even when lower amounts of fenvalerate that allowed glomeruli to develop were applied to pupae, the olfactory behavior was affected. Therefore, the formation of glomerular structures within the antennal lobe is not sufficient to establish olfactory behavior. A possible reason for this developmental effect of fenvalerate is a change in the odotopic arrangement of sensory axons within the glomeruli.

Immunocytochemical demonstration of locustatachykinin-related peptides in the central complex of the locust brain

The central complex, a highly ordered neuropil area in the insect brain, plays a role in motor control and spatial orientation. To further elucidate the neurochemical architecture of this brain area, we have investigated the distribution and morphology of neurons containing locustatachykinin I/II-related substances in the central complex of the locust Schistocerca gregaria. The central complex is innervated by at least 66 locustatachykinin I/II-immunoreactive neurons, which belong to two sets of tangential neurons and four sets of columnar neurons. These neurons give rise to immunostaining in the protocerebral bridge, in several layers of the upper division of the central body, and in all layers except layer 5 of the lower division of the central body. Double-label experiments show colocalization of immunoreactivity for both locustatachykinin I/II and octopamine in tangential neurons of the protocerebral bridge. A pair of tangential neurons of the lower division of the central body exhibits both locustatachykinin I/II and gamma-aminobutyric acid (GABA) immunoreactivity. A set of 16 columnar neurons of the lower division of the central body shows colocalized immunoreactivity for locustatachykinin II, leucokinin, and substance P. This study reveals novel features of the anatomical organization of the locust central complex and suggests a prominent role for locustatachykinin-related peptides as neuromediators and cotransmitters within this brain area.