Differential expression of synapsin in visual neurons of the locustSchistocerca gregaria

Organization and neural connections of the lateral complex in the brain of the desert locust

The lateral complexes (LXs) are bilaterally paired neuropils in the insect brain that mediate communication between the central complex (CX), a brain center controlling spatial orientation, various sensory processing areas, and thoracic motor centers that execute locomotion. The LX of the desert locust consists of the lateral accessory lobe (LAL), and the medial and lateral bulb. We have analyzed the anatomical organization and the neuronal connections of the LX in the locust, to provide a basis for future functional studies. Reanalyzing the morphology of neurons connecting the CX and the LX revealed likely feedback loops in the sky compass network of the CX via connections in the gall of the LAL and a newly identified neuropil termed ovoid body. In addition, we characterized 16 different types of neuron that connect the LAL with other areas in the brain. Eight types of neuron provide information flow between both LALs, five types are LAL input neurons, and three types are LAL output neurons. Among these are neurons providing input from sensory brain areas such as the lobula and antennal neuropils. Brain regions most often targeted by LAL neurons are the posterior slope, the wedge, and the crepine. Two descending neurons with dendrites in the LAL were identified. Our data support and complement existing knowledge about how the LAL is embedded in the neuronal network involved in processing of sensory information and generation of appropriate behavioral output for goal-directed locomotion.

Neuroarchitecture of the central complex of the desert locust: Tangential neurons

The central complex (CX) comprises a group of midline neuropils in the insect brain, consisting of the protocerebral bridge (PB), the upper (CBU) and lower division (CBL) of the central body and a pair of globular noduli. It receives prominent input from the visual system and plays a major role in spatial orientation of the animals. Vertical slices and horizontal layers of the CX are formed by columnar, tangential, and pontine neurons. While pontine and columnar neurons have been analyzed in detail, especially in the fruit fly and desert locust, understanding of the organization of tangential cells is still rudimentary. As a basis for future functional studies, we have studied the morphologies of tangential neurons of the CX of the desert locust Schistocerca gregaria. Intracellular dye injections revealed 43 different types of tangential neuron, 8 of the PB, 5 of the CBL, 24 of the CBU, 2 of the noduli, and 4 innervating multiple substructures. Cell bodies of these neurons were located in 11 different clusters in the cell body rind. Judging from the presence of fine versus beaded terminals, the vast majority of these neurons provide input into the CX, especially from the lateral complex (LX), the superior protocerebrum, the posterior slope, and other surrounding brain areas, but not directly from the mushroom bodies. Connections are largely subunit- and partly layer-specific. No direct connections were found between the CBU and the CBL. Instead, both subdivisions are connected in parallel with the PB and distinct layers of the noduli.

Anatomy of the lobula complex in the brain of the praying mantis compared to the lobula complexes of the locust and cockroach

The praying mantis is an insect which relies on vision for capturing prey, avoiding being eaten and for spatial orientation. It is well known for its ability to use stereopsis for estimating the distance of objects. The neuronal substrate mediating visually driven behaviors, however, is not very well investigated. To provide a basis for future functional studies, we analyzed the anatomical organization of visual neuropils in the brain of the praying mantis Hierodula membranacea and provide supporting evidence from a second species, Rhombodera basalis, with particular focus on the lobula complex (LOX). Neuropils were three‐dimensionally reconstructed from synapsin‐immunostained whole mount brains. The neuropil organization and the pattern of γ‐aminobutyric acid immunostaining of the medulla and LOX were compared between the praying mantis and two related polyneopteran species, the Madeira cockroach and the desert locust. The investigated visual neuropils of the praying mantis are highly structured. Unlike in most insects the LOX of the praying mantis consists of five nested neuropils with at least one neuropil not present in the cockroach or locust. Overall, the mantis LOX is more similar to the LOX of the locust than the more closely related cockroach suggesting that the sensory ecology plays a stronger role than the phylogenetic distance of the three species in structuring this center of visual information processing.

Fine structure of synaptic sites and circuits in mushroom bodies of insect brains

In the insect brain, mushroom bodies represent a prominent central neuropil for multisensory integration and, crucially, for learning and memory. For this reason, special attention has been focused on its small chemical synapses. Early studies on synaptic types and their distribution, using conventional electron microscopy, and recent publications have resolved basic features of synaptic circuits. More recent studies, using experimental methods for resolving neurons, such as immunocytochemistry, genetic labelling, high resolution confocal microscopy and more advanced electron microscopy, have revealed many new details about the fine structure and molecular contents of identifiable neurons of mushroom bodies and has led to more refined modelling of functional organisation. Synaptic circuitries have been described in most detail for the calyces. In contrast, the mushroom bodies' columnar peduncle and lobes have been explored to a lesser degree. In dissecting local microcircuits, the scientist is confronted with complex neuronal compartmentalisation and specific synaptic arrangements. This article reviews classical and modern studies on the fine structure of synapses and their networks in mushroom bodies across several insect species.

Changes in Synapsin Levels in the Millipede Gymnostreptus olivaceus Schubart, 1944 Exposed to Different Concentrations of Deltamethrin

Millipedes are ecologically important soil organisms and may also be an economically threatening species in rural and urban areas when population outbreaks occur. In order to control infestations commercial formulations of deltamethrin have been commonly applied, even though there are few studies about the effects of such insecticide on millipedes. This paper describes the effects of this insecticide on millipedes showing neurotoxic effects assessed by synapsin labeling and confocal microscopy. Deltamethrin concentrations related to the DL50 of the active ingredient and a field concentration were applied topically in the diplopod Gymnostreptus olivaceus to evaluate the behavior, mortality rate, and synapsin levels in the brain 12, 24, and 48h after contact with deltamethin. The insecticide caused mortality at the higher concentrations employed, in which no change was observed in neurotransmission in the survivors. In contrast, at field concentrations, deltamethrin did not cause any deaths, but triggered significant changes in synapsin levels. The results obtained form the synapsin labeling provide several interpretations suggesting that the isolated application of this tool must be associated with additional tools in order to evaluate biologically induced effects of deltamethrin in an accurate way. In addition, the feasibility of chemical control of millipedes with deltamethrin is questioned.

Transmedulla Neurons in the Sky Compass Network of the Honeybee (Apis mellifera) Are a Possible Site of Circadian Input

Honeybees are known for their ability to use the sun's azimuth and the sky's polarization pattern for spatial orientation. Sky compass orientation in bees has been extensively studied at the behavioral level but our knowledge about the underlying neuronal systems and mechanisms is very limited. Electrophysiological studies in other insect species suggest that neurons of the sky compass system integrate information about the polarization pattern of the sky, its chromatic gradient, and the azimuth of the sun. In order to obtain a stable directional signal throughout the day, circadian changes between the sky polarization pattern and the solar azimuth must be compensated. Likewise, the system must be modulated in a context specific way to compensate for changes in intensity, polarization and chromatic properties of light caused by clouds, vegetation and landscape. The goal of this study was to identify neurons of the sky compass pathway in the honeybee brain and to find potential sites of circadian and neuromodulatory input into this pathway. To this end we first traced the sky compass pathway from the polarization-sensitive dorsal rim area of the compound eye via the medulla and the anterior optic tubercle to the lateral complex using dye injections. Neurons forming this pathway strongly resembled neurons of the sky compass pathway in other insect species. Next we combined tracer injections with immunocytochemistry against the circadian neuropeptide pigment dispersing factor and the neuromodulators serotonin, and γ-aminobutyric acid. We identified neurons, connecting the dorsal rim area of the medulla to the anterior optic tubercle, as a possible site of neuromodulation and interaction with the circadian system. These neurons have conspicuous spines in close proximity to pigment dispersing factor-, serotonin-, and GABA-immunoreactive neurons. Our data therefore show for the first time a potential interaction site between the sky compass pathway and the circadian clock.

Identification and distribution of SIFamide in the nervous system of the desert locust Schistocerca gregaria

SIFamides are a family of highly conserved arthropod neuropeptides. To date, nine orthocopies from different arthropods, most of them insects, have been identified, all consisting of 11-12 amino acid residues. The striking conservation in sequence is mirrored by highly similar morphologies of SIFamide-immunoreactive neurons: immunolabeling in various insect species revealed four immunopositive neurons with somata in the pars intercerebralis and arborizations extending throughout the brain and ventral nervous system. In contrast, the functional role of these neurons and their neuropeptide SIFamide is largely obscure. To provide an additional basis for functional analysis, we identified, by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry, a SIFamide peptide in the desert locust Schistocerca gregaria and studied its distribution throughout the nervous system. Identification was supported by analysis of transcriptomic data obtained from another grasshopper, Stenobothrus lineatus. Scg-SIFamide, unlike all SIFamides identified so far, is a pentadecapeptide with an extended and highly modified N-terminus (AAATFRRPPFNGSIFamide). As in other insects, pairs of descending neurons with somata in the pars intercerebralis and ramifications in most areas of the nervous system are SIFamide-immunoreactive. In addition, a small number of local interneurons in the brain and ventral ganglia were immunostained. Double-label experiments showed that the SIFamide-immunoreactive descending neurons are identical to previously characterized primary commissure pioneer (PNP) neurons of the locust brain that pioneer the first commissure in the brain. The data suggest that the descending SIFamide-immunoreactive neurons play a developmental role in organizing the insect central nervous system. J. Comp. Neurol. 523:108-125, 2015. © 2014 Wiley Periodicals, Inc.

Topographic organization and possible function of the posterior optic tubercles in the brain of the desert locust Schistocerca gregaria

Migrating desert locusts, Schistocerca gregaria, are able to use the skylight polarization pattern for navigation. They detect polarized light with a specialized dorsal rim area in their compound eye. After multistage processing, polarization signals are transferred to the central complex, a midline-spanning brain area involved in locomotor control. Polarization-sensitive tangential neurons (TB-neurons) of the protocerebral bridge, a part of the central complex, give rise to a topographic arrangement of preferred polarization angles in the bridge, suggesting that the central complex acts as an internal sky compass. TB-neurons connect the protocerebral bridge with two adjacent brain areas, the posterior optic tubercles. To analyze the polarotopic organization of the central complex further, we investigated the number and morphologies of TB-neurons and the presence and colocalization of three neuroactive substances in these neurons. Triple immunostaining with antisera against Diploptera punctata allatostatin (Dip-AST), Manduca sexta allatotropin (Mas-AT), and serotonin (5HT) raised in the same host species revealed three spatially distinct TB-neuron clusters, each consisting of 10 neurons per hemisphere: cluster 1 and 3 showed Dip-AST/5HT immunostaining, whereas cluster 2 showed Dip-AST/Mas-AT immunostaining. Five subtypes of TB-neuron could be distinguished based on ramification patterns. Corresponding to ramification domains in the protocerebral bridge, the neurons invaded distinct but overlapping layers within the posterior optic tubercle. Similarly, neurons interconnecting the tubercles of the two hemispheres also targeted distinct layers of these neuropils. From these data we propose a neuronal circuit that may be suited to stabilize the internal sky compass in the central complex of the locust.

Synaptic connections of first-stage visual neurons in the locust Schistocerca gregaria extend evolution of tetrad synapses back 200 million years

The small size of some insects, and the crystalline regularity of their eyes, have made them ideal for large-scale reconstructions of visual circuits. In phylogenetically recent muscomorph flies, like Drosophila, precisely coordinated output to different motion-processing pathways is delivered by photoreceptors (R cells), targeting four different postsynaptic cells at each synapse (tetrad). Tetrads were linked to the evolution of aerial agility. To reconstruct circuits for vision in the larger brain of a locust, a phylogenetically old, flying insect, we adapted serial block-face scanning electron microscopy (SBEM). Locust lamina monopolar cells, L1 and L2, were the main targets of the R cell pathway, L1 and L2 each fed a different circuit, only L1 providing feedback onto R cells. Unexpectedly, 40% of all locust R cell synapses onto both L1 and L2 were tetrads, revealing the emergence of tetrads in an arthropod group present 200 million years before muscomorph flies appeared, coinciding with the early evolution of flight.

Structural organization of the presynaptic density at identified synapses in the locust central nervous system

In a synaptic active zone, vesicles aggregate around a densely staining structure called the presynaptic density. We focus on its three-dimensional architecture and a major molecular component in the locust. We used electron tomography to study the presynaptic density in synapses made in the brain by identified second-order neuron of the ocelli. Here, vesicles close to the active zone are organized in two rows on either side of the presynaptic density, a level of organization not previously reported in insect central synapses. The row of vesicles that is closest to the density's base includes vesicles docked with the presynaptic membrane and thus presumably ready for release, whereas the outer row of vesicles does not include any that are docked. We show that a locust ortholog of the Drosophila protein Bruchpilot is localized to the presynaptic density, both in the ocellar pathway and compound eye visual neurons. An antibody recognizing the C-terminus of the Bruchpilot ortholog selectively labels filamentous extensions of the presynaptic density that reach out toward vesicles. Previous studies on Bruchpilot have focused on its role in neuromuscular junctions in Drosophila, and our study shows it is also a major functional component of presynaptic densities in the central nervous system of an evolutionarily distant insect. Our study thus reveals Bruchpilot executes similar functions in synapses that can sustain transmission of small graded potentials as well as those relaying large, spike-evoked signals.

The synapsin gene family in basal chordates: evolutionary perspectives in metazoans

Background

Synapsins are neuronal phosphoproteins involved in several functions correlated with both neurotransmitter release and synaptogenesis. The comprehension of the basal role of the synapsin family is hampered in vertebrates by the existence of multiple synapsin genes. Therefore, studying homologous genes in basal chordates, devoid of genome duplication, could help to achieve a better understanding of the complex functions of these proteins.

Results

In this study we report the cloning and characterization of the Ciona intestinalis and amphioxus Branchiostoma floridae synapsin transcripts and the definition of their gene structure using available C. intestinalis and B. floridae genomic sequences. We demonstrate the occurrence, in both model organisms, of a single member of the synapsin gene family. Full-length synapsin genes were identified in the recently sequenced genomes of phylogenetically diverse metazoans. Comparative genome analysis reveals extensive conservation of the SYN locus in several metazoans. Moreover, developmental expression studies underline that synapsin is a neuronal-specific marker in basal chordates and is expressed in several cell types of PNS and in many, if not all, CNS neurons.

Conclusion

Our study demonstrates that synapsin genes are metazoan genes present in a single copy per genome, except for vertebrates. Moreover, we hypothesize that, during the evolution of synapsin proteins, new domains are added at different stages probably to cope up with the increased complexity in the nervous system organization. Finally, we demonstrate that protochordate synapsin is restricted to the post-mitotic phase of CNS development and thereby is a good marker of postmitotic neurons.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.