FMRFamide-like immunoreactivity in the brain of the honeybee (Apis mellifera). A light-and electron microscopical study

Single-cell transcriptomic analysis of honeybee brains identifies vitellogenin as caste differentiation-related factor

The honeybee (Apis mellifera) is a well-known eusocial insect. In honeybee colonies, thousands of sterile workers, including nurse and forager bees, perform various tasks within or outside the hive, respectively. The queen is the only fertile female and is responsible for reproduction. The queen and workers share similar genomes but occupy different caste statuses. We established single-cell transcriptomic atlases of brains from queens and worker subcastes and identified five major cell groups: Kenyon, optic lobe, olfactory projection, glial, and hemocyte cells. By dividing Kenyon and glial cells into multiple subtypes based on credible markers, we observed that vitellogenin (vg) was highly expressed in specific glial-cell subtypes in brains of queens. Knockdown of vg at the early larval stage significantly suppressed the development into adult queens. We demonstrate vg expression as a "molecular signature" for the queen caste and suggest involvement of vg in regulating caste differentiation.

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scRNA-seq revealed distinct gene expression in the brains of queens and workers

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Vitellogenin (vg) may represent a "molecular signature" of the queen caste

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Knockdown of vg at early larval stage suppressed development into adult queens

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Vg may be involved in regulating caste differentiation

A three-dimensional atlas of the honeybee central complex, associated neuropils and peptidergic layers of the central body

The central complex (CX) in the brain of insects is a highly conserved group of midline-spanning neuropils consisting of the upper and lower division of the central body, the protocerebral bridge, and the paired noduli. These neuropils are the substrate for a number of behaviors, most prominently goal-oriented locomotion. Honeybees have been a model organism for sky-compass orientation for more than 70 years, but there is still very limited knowledge about the structure and function of their CX. To advance and facilitate research on this brain area, we created a high-resolution three-dimensional atlas of the honeybee's CX and associated neuropils, including the posterior optic tubercles, the bulbs, and the anterior optic tubercles. To this end, we developed a modified version of the iterative shape averaging technique, which allowed us to achieve high volumetric accuracy of the neuropil models. For a finer definition of spatial locations within the central body, we defined layers based on immunostaining against the neuropeptides locustatachykinin, FMRFamide, gastrin/cholecystokinin, and allatostatin and included them into the atlas by elastic registration. Our honeybee CX atlas provides a platform for future neuroanatomical work.

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.

Neuropeptidome of Tribolium castaneum antennal lobes and mushroom bodies

Neuropeptides are a highly diverse group of signaling molecules that affect a broad range of biological processes in insects, including development, metabolism, behavior, and reproduction. In the central nervous system, neuropeptides are usually considered to act as neuromodulators and cotransmitters that modify the effect of "classical" transmitters at the synapse. The present study analyzes the neuropeptide repertoire of higher cerebral neuropils in the brain of the red flour beetle Tribolium castaneum. We focus on two integrative neuropils of the olfactory pathway, the antennal lobes and the mushroom bodies. Using the technique of direct peptide profiling by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, we demonstrate that these neuropils can be characterized by their specific neuropeptide expression profiles. Complementary immunohistological analyses of selected neuropeptides revealed neuropeptide distribution patterns within the antennal lobes and the mushroom bodies. Both approaches revealed consistent differences between the neuropils, underlining that direct peptide profiling by mass spectrometry is a fast and reliable method to identify neuropeptide content.

Short neuropeptide F acts as a functional neuromodulator for olfactory memory in Kenyon cells of Drosophila mushroom bodies

In insects, many complex behaviors, including olfactory memory, are controlled by a paired brain structure, the so-called mushroom bodies (MB). In Drosophila, the development, neuroanatomy, and function of intrinsic neurons of the MB, the Kenyon cells, have been well characterized. Until now, several potential neurotransmitters or neuromodulators of Kenyon cells have been anatomically identified. However, whether these neuroactive substances of the Kenyon cells are functional has not been clarified yet. Here we show that a neuropeptide precursor gene encoding four types of short neuropeptide F (sNPF) is required in the Kenyon cells for appetitive olfactory memory. We found that activation of Kenyon cells by expressing a thermosensitive cation channel (dTrpA1) leads to a decrease in sNPF immunoreactivity in the MB lobes. Targeted expression of RNA interference against the sNPF precursor in Kenyon cells results in a highly significant knockdown of sNPF levels. This knockdown of sNPF in the Kenyon cells impairs sugar-rewarded olfactory memory. This impairment is not due to a defect in the reflexive sugar preference or odor response. Consistently, knockdown of sNPF receptors outside the MB causes deficits in appetitive memory. Altogether, these results suggest that sNPF is a functional neuromodulator released by Kenyon cells.

Neuropeptides in insect mushroom bodies

Owing to their experimental amenability, insect nervous systems continue to be in the foreground of investigations into information processing in - ostensibly - simple neuronal networks. Among the cerebral neuropil regions that hold a particular fascination for neurobiologists are the paired mushroom bodies, which, despite their function in other behavioral contexts, are most renowned for their role in learning and memory. The quest to understand the processes that underlie these capacities has been furthered by research focusing on unraveling neuroanatomical connections of the mushroom bodies and identifying key players that characterize the molecular machinery of mushroom body neurons. However, on a cellular level, communication between intrinsic and extrinsic mushroom body neurons still remains elusive. The present account aims to provide an overview on the repertoire of neuropeptides expressed in and utilized by mushroom body neurons. Existing data for a number of insect representatives is compiled and some open gaps in the record are filled by presenting additional original data.

Immunolocalization of the short neuropeptide F receptor in queen brains and ovaries of the red imported fire ant (Solenopsis invicta Buren)

Background

Insect neuropeptides are involved in diverse physiological functions and can be released as neurotransmitters or neuromodulators acting within the central nervous system, and as circulating neurohormones in insect hemolymph. The insect short neuropeptide F (sNPF) peptides, related to the vertebrate neuropeptide Y (NPY) peptides, have been implicated in the regulation of food intake and body size, and play a gonadotropic role in the ovaries of some insect species. Recently the sNPF peptides were localized in the brain of larval and adult Drosophila. However, the location of the sNPF receptor, a G protein-coupled receptor (GPCR), has not yet been investigated in brains of any adult insect. To elucidate the sites of action of the sNPF peptide(s), the sNPF receptor tissue expression and cellular localization were analyzed in queens of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera), an invasive social insect.

Results

In the queen brains and subesophageal ganglion about 164 cells distributed in distinctive cell clusters (C1-C9 and C12) or as individual cells (C10, C11) were immuno-positive for the sNPF receptor. Most of these neurons are located in or near important sensory neuropils including the mushroom bodies, the antennal lobes, the central complex, and in different parts of the protocerebrum, as well as in the subesophageal ganglion. The localization of the sNPF receptor broadly links the receptor signaling pathway with circuits regulating learning and feeding behaviors. In ovaries from mated queens, the detection of sNPF receptor signal at the posterior end of oocytes in mid-oogenesis stage suggests that the sNPF signaling pathway may regulate processes at the oocyte pole.

Conclusions

The analysis of sNPF receptor immunolocalization shows that the sNPF signaling cascade may be involved in diverse functions, and the sNPF peptide(s) may act in the brain as neurotransmitter(s) or neuromodulator(s), and in the ovaries as neurohormone(s). To our knowledge, this is the first report of the cellular localization of a sNPF receptor on the brain and ovaries of adult insects.

Revisiting the anatomy of the central nervous system of a hemimetabolous model insect species: the pea aphid Acyrthosiphon pisum

Aphids show a marked phenotypic plasticity, producing asexual or sexual and winged or wingless morphs depending on environmental conditions and season. We describe here the general structure of the brain of various morphs of the pea aphid Acyrthosiphon pisum. This is the first detailed anatomical study of the central nervous system of an aphid by immunocytochemistry (synapsin, serotonin, and several neuropeptides), ethyl-gallate staining, confocal laser scanning microscopy, and three-dimensional reconstructions. The study has revealed well-developed optic lobes composed of lamina, medulla, and lobula complex. Ocelli are only present in males and winged parthenogenetic females. The central complex is well-defined, with a central body divided into two parts, a protocerebral bridge, and affiliated lateral accessory lobes. The mushroom bodies are ill-defined, lacking calyces, and only being visualized by using an antiserum against the neuropeptide orcokinin. The antennal lobes contain poorly delineated glomeruli but can be clearly visualized by performing antennal backfills. On the basis of our detailed description of the brain of winged and wingless parthenogenetic A. pisum females, an anatomical map is now available that should improve our knowledge of the way that these structures are involved in the regulation of phenotypic plasticity.

Allatostatin immunoreactivity in the honeybee brain

Information transmission and processing in the brain is achieved through a small family of chemical neurotransmitters and neuromodulators and a very large family of neuropeptides. In order to understand neural networks in the brain it will be necessary, therefore, to understand the connectivity, morphology, and distribution of peptidergic neurons, and to elucidate their function in the brain. In this study we characterize the distribution of substances related to Dip-allatostatin I in the honeybee brain, which belongs to the allatostatin-A (AST) peptide family sharing the conserved c-terminal sequence -YXFGL-NH(2). We found about 500 AST-immunoreactive (ASTir) neurons in the brain, scattered in 18 groups that varied in their precise location across individuals. Almost all areas of the brain were innervated by ASTir fibers. Most ASTir neurites formed networks within functionally distinct areas, e.g., the antennal lobes, the mushroom bodies, or the optic lobes, indicating local functions of the peptide. A small number of very large neurons had widespread arborizations and neurites were found in the corpora cardiaca and in the cervical connectives, suggesting that AST also has global functions. We double-stained AST and GABA and found that a subset of ASTir neurons were GABA-immunoreactive (GABAir). Double staining AST with backfills of olfactory receptor neurons or mass fills of neurons in the antennal lobes and in the mushroom bodies allowed a more fine-grained description of ASTir networks. Together, this first comprehensive description of AST in the bee brain suggests a diverse functional role of AST, including local and global computational tasks.

Stratification and synaptogenesis in the mushroom body of the honeybee, Apis mellifera

Stratification is a basic anatomical feature of central brain in both vertebrates and many invertebrates. The aim of this study was to investigate the relationship between stratification and synaptogenesis in the developing mushroom bodies of the honeybee. During metamorphosis, the vertical lobe of mushroom body shows progressive stratification with three thick primary strata and more secondary strata and laminae. Three primary strata are formed at the metamorphic stage P1, before the youngest generation of the mushroom body intrinsic neurons, Kenyon cells, is produced. Thus, the primary strata within the lobe are unlikely to represent three major subpopulations of the Kenyon cells sequentially produced in the mushroom bodies. Formation of laminae starts at the stage P2 and culminates at the end of metamorphosis. The laminae appear within the lobe rather than being added sequentially from the ingrowth stratum. Alternating dark and light lamina (lamina doublets) are formed in the vertical lobe in late metamorphosis (stages P6-P9), but they are not visible in adults. The pattern of stratification is not continuous along the vertical lobe at the same developmental stage, and resorting of axons of the Kenyon cells is likely to occur within dark laminae. In the developing vertical lobe, dark laminae show lower synaptic density and exhibit an ultra structure that is indicative for a delay in synaptogenesis relative to the primary strata. A local transient block of synaptogenesis within the dark laminae may provide correct targeting of Kenyon cells by extrinsic mushroom body neurons.

Modular subdivision of mushroom bodies by Kenyon cells in the silkmoth

In insects, olfactory information in the glomeruli of the antennal lobe, the first olfactory center, is transmitted to the lateral protocerebrum and the calyx of the mushroom body via projection neurons. In male silkmoths (Bombyx mori), arborization patterns in the calyx differ markedly between projection neurons that respond to sex pheromones and those that respond to general odors. However, little is known about the organization of the mushroom body's intrinsic neurons, called Kenyon cells (KCs), which receive the inputs from the projection neurons. We investigated the silkmoth mushroom body and identified four parallel subdivisions in the lobes and pedunculus by immunolabeling with antibodies against the catalytic subunit of protein kinase A in Drosophila melanogaster (DC0) and the neuromodulatory peptide FMRFamide. To further understand the detailed organization of the mushroom body, we performed extensive labeling of individual KCs. We identified four morphological types whose axonal projections corresponded to the subdivisions in the lobes, and found that each type of KC had a characteristic dendritic morphology in the calyx. These results show a correlation between the axonal projections of KCs in the lobes and dendritic morphology in the calyx, and indicate different functional roles for the subdivisions.

Mass spectrometric profiling of (neuro)-peptides in the worker honeybee, Apis mellifera

The honeybee is the economically most important beneficial insect and a model for studying immunity, development and social behavior. Hence, this species was selected for genome sequencing and annotation. An intensive interplay between bioinformatics and mass spectrometry (MS) resulted in the annotation of 36 neuropeptide genes (Hummon et al., 2006). Exactly 100 peptides were demonstrated by a variety of MS techniques. In this follow-up study we dissected and analysed separately all ganglia of the central nervous system (CNS) of adult worker bees in three repeats. The combined MALDI-TOF spectra enabled the accurate mapping of 67 peptides, encoded by 20 precursors. We also demonstrated the expression of an additional but already predicted peptide. In addition to putative bioactive peptides we also list and discuss spacer peptides, propeptides and truncated peptides. The majority of such peptides have a more restricted distribution pattern. Their presence provides some information on the precursor turnover and/or the location of neural cell bodies in which they are produced. Of a given precursor, the (neuro)-peptides with the widest distribution pattern are likely to be the best candidates to interact with receptors. The separate analysis of a neuroendocrine complex and the mushroom body yields suggestions as to which (neuro)-peptides might act as hormones and which neuropeptides might be involved in the complex spectrum of non-hormone driven honeybee behaviour, at these sites. Our data complement immunohistochemical studies of (neuro)-peptides in the honeybee, and form a reference for comparative studies in other insect or arthropod models, in particular in the light of recent or upcoming genome projects. Finally, they also form a firm basis for physiological, functional and/or differential peptidomics studies in the honeybee.

Brain architecture in the terrestrial hermit crab Coenobita clypeatus (Anomura, Coenobitidae), a crustacean with a good aerial sense of smell

Background

During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

Results

The primary olfactory centers in this species dominate the brain and are composed of many elongate olfactory glomeruli. The secondary olfactory centers that receive an input from olfactory projection neurons are almost equally large as the olfactory lobes and are organized into parallel neuropil lamellae. The architecture of the optic neuropils and those areas associated with antenna two suggest that C. clypeatus has visual and mechanosensory skills that are comparable to those of marine Crustacea.

Conclusion

In parallel to previous behavioral findings of a good sense of aerial olfaction in C. clypeatus, our results indicate that in fact their central olfactory pathway is most prominent, indicating that olfaction is a major sensory modality that these brains process. Interestingly, the secondary olfactory neuropils of insects, the mushroom bodies, also display a layered structure (vertical and medial lobes), superficially similar to the lamellae in the secondary olfactory centers of C. clypeatus. More detailed analyses with additional markers will be necessary to explore the question if these similarities have evolved convergently with the establishment of superb aerial olfactory abilities or if this design goes back to a shared principle in the common ancestor of Crustacea and Hexapoda.

Intrinsic neurons of Drosophila mushroom bodies express short neuropeptide F: relations to extrinsic neurons expressing different neurotransmitters

Mushroom bodies constitute prominent paired neuropils in the brain of insects, known to be involved in higher olfactory processing and learning and memory. In Drosophila there are about 2,500 intrinsic mushroom body neurons, Kenyon cells, and a large number of different extrinsic neurons connecting the calyx, peduncle, and lobes to other portions of the brain. The neurotransmitter of the Kenyon cells has not been identified in any insect. Here we show expression of the gene snpf and its neuropeptide products (short neuropeptide F; sNPFs) in larval and adult Drosophila Kenyon cells by means of in situ hybridization and antisera against sequences of the precursor and two of the encoded peptides. Immunocytochemistry displays peptide in intrinsic neuronal processes in most parts of the mushroom body structures, except for a small core in the center of the peduncle and lobes and in the alpha'- and beta'-lobes. Weaker immunolabeling is seen in Kenyon cell bodies and processes in the calyx and initial peduncle and is strongest in the more distal portions of the lobes. We used different antisera and Gal4-driven green fluorescent protein to identify Kenyon cells and different populations of extrinsic neurons defined by their signal substances. Thus, we display neurotransmitter systems converging on Kenyon cells: neurons likely to utilize dopamine, tyramine/octopamine, glutamate, and acetylcholine. Attempts to identify other neurotransmitter components (including vesicular glutamate transporter) in Kenyon cells failed. However, it is likely that the Kenyon cells utilize an additional neurotransmitter, yet to be identified, and that the neuropeptides described here may represent cotransmitters.

Central gustatory projections and side-specificity of operant antennal muscle conditioning in the honeybee

Gustatory stimuli to the antennae, especially sucrose, are important for bees and are employed in learning paradigms as unconditioned stimulus. The present study identified primary antennal gustatory projections in the bee brain and determined the impact of stimulation of the antennal tip on antennal muscle activity and its plasticity. Central projections of antennal taste hairs contained axons of two morphologies projecting into the dorsal lobe, which is also the antennal motor centre. Putative mechanosensory axons arborised in a dorso-lateral area. Putative gustatory axons projected to a ventro-medial area. Bees scan gustatory and mechanical stimuli with their antennae using variable strategies but sensory input to the motor system has not been investigated in detail. Mechanical, gustatory, and electrical stimulation of the ipsilateral antennal tip were found to evoke short-latency responses in an antennal muscle, the fast flagellum flexor. Contralateral gustatory stimulation induced smaller responses with longer latency. The activity of the fast flagellum flexor was conditioned operantly by pairing high muscle activity with ipsilateral antennal sucrose stimulation. A proboscis reward was unnecessary for learning. With contralateral antennal sucrose stimulation, conditioning was unsuccessful. Thus, muscle activity induced by gustatory stimulation was important for learning success and conditioning was side-specific.

Neuropeptides in interneurons of the insect brain

A large number of neuropeptides has been identified in the brain of insects. At least 35 neuropeptide precursor genes have been characterized in Drosophila melanogaster, some of which encode multiple peptides. Additional neuropeptides have been found in other insect species. With a few notable exceptions, most of the neuropeptides have been demonstrated in brain interneurons of various types. The products of each neuropeptide precursor seem to be co-expressed, and each precursor displays a unique neuronal distribution pattern. Commonly, each type of neuropeptide is localized to a relatively small number of neurons. We describe the distribution of neuropeptides in brain interneurons of a few well-studied insect species. Emphasis has been placed upon interneurons innervating specific brain areas, such as the optic lobes, accessory medulla, antennal lobes, central body, and mushroom bodies. The functional roles of some neuropeptides and their receptors have been investigated in D. melanogaster by molecular genetics techniques. In addition, behavioral and electrophysiological assays have addressed neuropeptide functions in the cockroach Leucophaea maderae. Thus, the involvement of brain neuropeptides in circadian clock function, olfactory processing, various aspects of feeding behavior, and learning and memory are highlighted in this review. Studies so far indicate that neuropeptides can play a multitude of functional roles in the brain and that even single neuropeptides are likely to be multifunctional.

Functional division of intrinsic neurons in the mushroom bodies of male Spodoptera littoralis revealed by antibodies against aspartate, taurine, FMRF-amide, Mas-allatotropin and DC0

The aim of this study was to further reveal the organization of Kenyon cells in the mushroom body calyx and lobes of the male moth Spodoptera littoralis, by using immunocytochemical labeling. Subdivisions of the mushroom bodies were identified employing antisera raised against the amino acids taurine and aspartate, the neuropeptides FMRF-amide and Mas-allatotropin, and against the protein kinase A catalytic subunit DC0. These antisera have previously been shown to label subsets of Kenyon cells in other species. The present results show that the organization of the mushroom body lobes into discrete divisions, described from standard neuroanatomical methods, is confirmed by immunocytology and shown to be further elaborated. Anti-taurine labels the accessory Y-tract, the gamma division of the lobes, and a thin subdivision of the most posterior component of the lobes. Aspartate antiserum labels the entire mushroom body. FMRF-amide-like immunolabeling is pronounced in the gamma division and in the anterior perimeter of the alpha/beta and alpha'/beta' divisions. Mas-allatotropin-like immunolabeling shows the opposite of FMRF-amide-like and taurine-like immunolabeling: the gamma division and the accessory Y-system is immunonegative whereas strong labeling is seen in both the alpha/beta and alpha'/beta' divisions. The present results agree with findings from other insects that mushroom bodies are anatomically divided into discrete parallel units. Functional and developmental implications of this organization are discussed.

Structure of the mushroom bodies of the insect brain

The past decade has produced an explosion of new information on the development, neuroanatomy, and possible functions of the mushroom bodies. This review provides a concise, contemporary overview of the structure of the mushroom bodies. Two topics are highlighted: the volume plasticity of mushroom body neuropils evident in the brains of some adult insects and a possible essential role for the gamma lobe in olfactory memory.

Synaptogenesis in the mushroom body calyx during metamorphosis in the honeybeeApis mellifera: An electron microscopic study

The goals of this study are to determine relationships between synaptogenesis and morphogenesis within the mushroom body calyx of the honeybee Apis mellifera and to find out how the microglomerular structure characteristic for the mature calyx is established during metamorphosis. We show that synaptogenesis in the mushroom body calycal neuropile starts in early metamorphosis (stages P1-P3), before the microglomerular structure of the neuropile is established. The initial step of synaptogenesis is characterized by the rare occurrence of distinct synaptic contacts. A massive synaptogenesis starts at stage P5, which coincides with the formation of microglomeruli, structural units of the calyx that are composed of centrally located presynaptic boutons surrounded by spiny postsynaptic endings. Microglomeruli are assembled either via accumulation of fine postsynaptic processes around preexisting presynaptic boutons or via ingrowth of thin neurites of presynaptic neurons into premicroglomeruli, tightly packed groups of spiny endings. During late pupal stages (P8-P9), addition of new synapses and microglomeruli is likely to continue. Most of the synaptic appositions formed there are made by boutons (putative extrinsic mushroom body neurons) into small postsynaptic profiles that do not exhibit presynaptic specializations (putative intrinsic mushroom body neurons). Synapses between presynaptic boutons characteristic of the adult calyx first appear at stage P8 but remain rare toward the end of metamorphosis. Our observations are consistent with the hypothesis that most of the synapses established during metamorphosis provide the structural basis for afferent information flow to calyces, whereas maturation of local synaptic circuitry is likely to occur after adult emergence.

Development of cricket mushroom bodies

Mushroom bodies are recognized as a multimodal integrator for sensorial stimuli. The present study analyzes cricket mushroom body development from embryogenesis to adulthood. In the house cricket, Kenyon cells were born from a group of neuroblasts located at the apex of mushroom bodies. Our results demonstrate the sequential generation of Kenyon cells: The more external they are, the earlier they were produced. BrdU treatment on day 8 (57% stage) of embryonic life results, at the adult stage, in the labelling of the large Kenyon cells at the periphery of the mushroom body cortex. These cells have specific projections into the posterior calyx, the gamma lobe, and an enlargement at the inner part of the vertical lobe; they represent a part of mushroom bodies of strictly embryonic origin. The small Kenyon cells were formed from day 9 (65% stage) of the embryonic stage onward, and new interneurons are produced throughout the entire life of the insect. They send their projections into the anterior calyx and into the vertical and medial lobes. Mushroom body development of Acheta should be considered as a primitive template, and cross-taxonomic comparisons of the mushroom body development underscore the precocious origin of the gamma lobe. As a result of continuous neurogenesis, cricket mushroom bodies undergo remodeling throughout life, laying the foundation for future studies of the functional role of this developmental plasticity.

Segregation of visual input to the mushroom bodies in the honeybee (Apis mellifera)

Insect mushroom bodies are brain regions that receive multisensory input and are thought to play an important role in learning and memory. In most neopteran insects, the mushroom bodies receive direct olfactory input. In addition, the calyces of Hymenoptera receive substantial direct input from the optic lobes. We describe visual inputs to the calyces of the mushroom bodies of the honeybee Apis mellifera, the neurons' dendritic fields in the optic lobes, the medulla and lobula, and the organization of their terminals in the calyces. Medulla neurons terminate in the collar region of the calyx, where they segregate into five layers that receive alternating input from the dorsal or ventral medulla, respectively. A sixth, innermost layer of the collar receives input from lobula neurons. In the basal ring region of the calyx, medulla neuron terminals are restricted to a small, distal part. Lobula neurons are more prominent in the basal ring, where they terminate in its outer half. Although the collar and basal ring layers generally receive segregated input from both optic neuropils, some overlap occurs at the borders of the layers. At least three different types of mushroom body input neurons originate from the medulla: (a) neurons with narrow dendritic fields mainly restricted to the vicinity of the medulla's serpentine layer and found throughout the medulla; (b) neurons restricted to the ventral half of the medulla and featuring long columnar dendritic branches in the outer medulla; and (c) a group of neurons whose dendrites are restricted to the most ventral part of the medulla and whose axons form the anterior inferior optic tract. Most medulla neurons (groups a and b) send their axons via the anterior superior optic tract to the mushroom bodies. Neurons connecting the lobula with the mushroom bodies have their dendrites in a defined dorsal part of the lobula. Their axons form a third tract to the mushroom bodies, here referred to as the lobula tract. Our findings match the anatomy of intrinsic mushroom body neurons (Strausfeld, 2002) and together indicate that the mushroom bodies may be composed of many more functional subsystems than previously suggested.

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

Organization of the honey bee mushroom body: representation of the calyx within the vertical and gamma lobes

Studies of the mushroom bodies of Drosophila melanogaster have suggested that their gamma lobes specifically support short-term memory, whereas their vertical lobes are essential for long-term memory. Developmental studies have demonstrated that the Drosophila gamma lobe, like its equivalent in the cockroach Periplaneta americana, is supplied by a special class of intrinsic neuron-the clawed Kenyon cells-that are the first to differentiate during early development. To date, however, no account identifies a corresponding lobe in the honey bee, another taxon used extensively for learning and memory research. Received opinion is that, in this taxon, each of the mushroom body lobes comprises three parallel divisions representing one of three concentric zones of the calyces, called the lip, collar, and basal ring. The present account shows that, although these zones are represented in the lobes, they occupy only two thirds of the vertical lobe. Its lowermost third receives the axons of the clawed class II Kenyon cells, which are the first to differentiate during early development and which represent the whole calyx. This component of the lobe is anatomically and developmentally equivalent to the gamma lobe of Drosophila and has been here named the gamma lobe of the honey bee. A new class of intrinsic neurons, originating from perikarya distant from the mushroom body, provides a second system of parallel fibers from the calyx to the gamma lobe. A region immediately beneath the calyces, called the neck, is invaded by these neurons as well as by a third class of intrinsic cell that provides connections within the neck of the pedunculus and the basal ring of the calyces. In the honey bee, the gamma lobe is extensively supplied by afferents from the protocerebrum and gives rise to a distinctive class of efferent neurons. The terminals of these efferents target protocerebral neuropils that are distinct from those receiving efferents from divisions of the vertical lobe that represent the lip, collar, and basal ring. The identification of a gamma lobe unites the mushroom bodies of evolutionarily divergent taxa. The present findings suggest the need for critical reinterpretation of studies that have been predicated on early descriptions of the mushroom body's lobes.

Synaptic organization of the mushroom body calyx in Drosophila melanogaster

The calyx neuropil of the mushroom body in adult Drosophila melanogaster contains three major neuronal elements: extrinsic projection neurons, presumed cholinergic, immunoreactive to choline acetyltransferase (ChAT-ir) and vesicular acetylcholine transporter (VAChT-ir) antisera; presumed gamma-aminobutyric acid (GABA)ergic extrinsic neurons with GABA-like immunoreactivity; and local intrinsic Kenyon cells. The projection neurons connecting the calyx with the antennal lobe via the antennocerebral tract are the only source of cholinergic elements in the calyces. Their terminals establish an array of large boutons 2-7 microm in diameter throughout all calycal subdivisions. The GABA-ir extrinsic neurons, different in origin, form a network of fine fibers and boutons codistributed in all calycal regions with the cholinergic terminals and with tiny profiles, mainly Kenyon cell dendrites. We have investigated the synaptic circuits of these three neuron types using preembedding immuno-electron microscopy. All ChAT/VAChT-ir boutons form divergent synapses upon multitudinous surrounding Kenyon cell dendrites. GABA-ir elements also regularly contribute divergent synaptic input onto these dendrites, as well as occasional inputs to boutons of projection neurons. The same synaptic microcircuits involving these three neuron types are repeatedly established in glomeruli in all calycal regions. Each glomerulus comprises a large cholinergic bouton at its core, encircled by tiny vesicle-free Kenyon cell dendrites as well as by a number of GABAergic terminals. A single dendritic profile may thereby receive synaptic input from both cholinergic and GABAergic elements in close vicinity at presynaptic sites with T-bars typical of fly synapses. ChAT-ir boutons regularly have large extensions of the active zones. Thus, Kenyon cells may receive major excitatory input from cholinergic boutons and considerable postsynaptic inhibition from GABAergic terminals, as well as, more rarely, presynaptic inhibitory signaling. The calycal glomeruli of Drosophila are compared with the cerebellar glomeruli of vertebrates. The cholinergic boutons are the largest identified cholinergic synapses in the Drosophila brain and an eligible prospect for studying the genetic regulation of excitatory presynaptic function.

Embryonic and larval development of the Drosophila mushroom bodies: concentric layer subdivisions and the role of fasciclin II

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the arthropod brain. In order to understand the cellular and genetic processes that control the early development of MBs, we have performed high-resolution neuroanatomical studies of the embryonic and post-embryonic development of the Drosophila MBs. In the mid to late embryonic stages, the pioneer MB tracts extend along Fasciclin II (FAS II)-expressing cells to form the primordia for the peduncle and the medial lobe. As development proceeds, the axonal projections of the larval MBs are organized in layers surrounding a characteristic core, which harbors bundles of actin filaments. Mosaic analyses reveal sequential generation of the MB layers, in which newly produced Kenyon cells project into the core to shift to more distal layers as they undergo further differentiation. Whereas the initial extension of the embryonic MB tracts is intact, loss-of-function mutations of fas II causes abnormal formation of the larval lobes. Mosaic studies demonstrate that FAS II is intrinsically required for the formation of the coherent organization of the internal MB fascicles. Furthermore, we show that ectopic expression of FAS II in the developing MBs results in severe lobe defects, in which internal layers also are disrupted. These results uncover unexpected internal complexity of the larval MBs and demonstrate unique aspects of neural generation and axonal sorting processes during the development of the complex brain centers in the fruit fly brain.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

GABA-immunoreactive neurons in the mushroom bodies of the honeybee: an electron microscopic study

Synaptic contacts of gamma-aminobutyric acid (GABA) -immunoreactive neurons in honeybee mushroom bodies were studied by using electron microscopic immunocytochemistry. In the lip region of the calyx neuropil, GABA-immunoreactive profiles formed synapses onto both small postsynaptic profiles (76%) and large immunonegative boutons (4%), which were likely to belong to the intrinsic and extrinsic mushroom body neurons, respectively. Three morphologic types of the large immunonegative boutons were distinguished: "light," "dark," and "dense core"; all of them received synaptic inputs from the GABA-immunoreactive profiles. A significant proportion of the synapses formed by the GABA-immunoreactive neurons in the lip region (20%) were input synapses from immunonegative neurons. Analysis of thin serial sections showed that the output and input synapses formed microcircuits in which both large immunonegative boutons and small postsynaptic profiles were involved. We interpret these findings to show that negative feedforward and feedback loops exist within the microcircuits of the lip region.

Representation of the calyces in the medial and vertical lobes of cockroach mushroom bodies

Previous studies of honey bee and cockroach mushroom bodies have proposed that afferent terminals and intrinsic neurons (Kenyon cells) in the calyces are arranged according to polar coordinates. It has been suggested that there is a transformation by Kenyon cell axons of the polar arrangements of their dendrites in the calyces to laminar arrangements of their terminals in the lobes. Findings presented here show that cellular organization in the calyx of an evolutionarily basal neopteran, Periplaneta americana, is instead rectilinear, as it is in the lobes. It is shown that each calyx is divided into two halves (hemicalyces), each supplied by its own set of Kenyon cells. Each calyx is separately represented in the medial lobe where the dendritic trees of some efferent neurons receive inputs from one calyx only. Kenyon cell dendrites are arranged as narrow elongated fields, organized as rows in each hemicalyx. Dendritic fields arise from 14 to 16 sheets of Kenyon cell axons stacked on top of each other lining the inner surface of the calyx cup. A sheet consists of approximately 60 small bundles, each containing 5-15 axons that converge from the rim of the calyx to its neck. Each sheet contributes to a pair oflaminae, one dark one pale, called a doublet, that extends through the mushroom body. Dark laminae contain Kenyon cell axons packed with synaptic vesicles. Axons in pale laminae are sparsely equipped with vesicles. By analogy with photoreceptors, and with reference to field potential recordings, it is speculated that dark laminae are continuously active, being modulated by odor stimuli, whereas pale laminae are intermittently activated. Timm's silver staining and immunocytology reveal a second type of longitudinal division of the lobes. Five layers extend through the pedunculus and lobes, each composed of subsets of doublets. Four layers represent zones of afferent endings in the calyces. A fifth (the y layer) represents a specific type of Kenyon cell. It is concluded that the mushroom bodies comprise two independent modular systems, doublets and layers. Developmental studies show that new doublets are added at each instar to layers that are already present early in second instar nymphs. There are profound similarities between the mushroom bodies of Periplaneta, an evolutionarily basal taxon, and those of Drosophila melanogaster and the honey bee.

Synaptic organization in the lamina of the superposition eye of a skipper butterfly, Parnara guttata

The first optic neuropil of the compound eye, the lamina, of the skipper butterfly Parnara guttata, was examined by light microscopy after Golgi-impregnation and by electron microscopy (EM) to clarify the cellular and synaptic organization. In the lamina, five different types of lamina neurons (L neurons) were characterized by using Golgi-impregnation. By EM, each cartridge was found to contain all nine receptor axons from an ommatidium, five L neurons, and a few putative centrifugal elements. Axons from photoreceptors (retinula cells) R2, R3, R4, R6, R7, and R8 terminate as short visual fibers (svfs) in the lamina cartridge. Those from R1, R5, and R9 penetrate the lamina and terminate in the medulla as long visual fibers (lvfs). In the cartridges, the synaptic contacts were formed from svfs onto L neurons, from the lvfs of R1 and/or R5 to the lvf of R9 and L neurons, and from the lvf of R9 to L neurons. The putative centrifugal fibers also make synapses to svfs and L neurons. At the most distal level of the cartridge, one of the centrifugal fibers containing dense-core vesicles makes presynaptic contacts to the putative long collaterals of the L neuron. A novel characteristic feature of this lamina is that svfs of R3 and R7 and the lvfs of R1 or R5 have long collaterals extending into neighboring cartridges. Presynaptic contacts were confirmed in such long collaterals from the svf. These results imply that receptor axons provide direct intercartridge connections as well as providing indirect connections to neighboring cartridges by way of their input upon L neurons.

Histochemistry of classical neurotransmitters in antennal lobes and mushroom bodies of the honeybee

This paper summarizes histochemical and immunocytochemical investigations of cholinergic, GABAergic, and glutamatergic pathways in the central brain and suboesophageal ganglion of the honeybee. Acetylcholinesterase histochemistry, immunocytochemical staining for nicotinic acetylcholine receptors, and mapping for alpha-bungarotoxin binding sites indicate cholinergic synaptic interactions in the antennal lobe and a cholinergic pathway via a subset of olfactory projection neurons into the mushroom bodies. Calcium imaging experiments in cell cultures prepared from mushroom bodies demonstrate the expression of nicotinic cholinergic receptors on Kenyon cells. Neurons synthesizing GABA and glutamate are stained with well-defined polyclonal antisera against the amino acids. GABA-immunoreactivity is mainly localized in local interneurons of the antennal lobe and in extrinsic neurons innervating the mushroom bodies. High levels of glutamate-immunoreactivity are found in motoneurons of the suboesophageal ganglion, the dorsal lobe, and also in interneurons. A subgroup of the Kenyon cells shows distinct but weaker levels of glutamate-immunoreactivity. The detailed knowledge about the chemical neuroanatomy of the bee provides a framework for behavioral pharmacological approaches, which implicate the involvement of cholinergic mechanisms in olfactory learning and GABAergic mechanisms in odor discrimination.

Topography of modular subunits in the mushroom bodies of the cockroach

The mushroom body (MB), a conspicuous neuropil structure in the insect brain, is implicated in associative memory and in some aspects of motor control. Intrinsic neurons of the MB (Kenyon cells) extend dendrites into the calyx, and their axons run through the pedunculus and then bifurcate to form the alpha and the beta lobes. At the pedunculus and the lobes, Kenyon cells make synaptic connections with dendrites of extrinsic (output) neurons. Previously, we reported that the alpha lobe of the cockroach MB consists of repetitive modular subunits (Mizunami et al. [1997] Neurosci. Lett. 229:153-156). Each subunit is composed of a dark layer and a light layer, and the layers are refereed to as slabs. Each slab is composed of axons of a specific subset of Kenyon cells. In the present study, we examined serial sections of reduced silver preparations and found that each dark and light slab continues throughout the length of the pedunculus and the alpha and beta lobes. We also found that Golgi-impregnated Kenyon cells often exhibit a characteristic grouping, forming a thin sheet interlaced by dozens or hundreds of axons. The sheet is much thinner than the slab, and each sheet remains within a particular slab throughout the length of the pedunculus and the lobes. Thus, the sheet is a component forming the slab. In the pedunculus and the beta lobe, a class of Golgi-impregnated extrinsic neurons exhibit segmented dendritelike arbors that interact with every other slab, i.e., either with only dark or light slabs. Because each neuron of this class interacts with each particular set of dark or light slabs, we conclude that the slabs are units for transmitting output signals from the MB.

Evolution, Discovery, and Interpretations of Arthropod Mushroom Bodies

Mushroom bodies are prominent neuropils found in annelids and in all arthropod groups except crustaceans. First explicitly identified in 1850, the mushroom bodies differ in size and complexity between taxa, as well as between different castes of a single species of social insect. These differences led some early biologists to suggest that the mushroom bodies endow an arthropod with intelligence or the ability to execute voluntary actions, as opposed to innate behaviors. Recent physiological studies and mutant analyses have led to divergent interpretations. One interpretation is that the mushroom bodies conditionally relay to higher protocerebral centers information about sensory stimuli and the context in which they occur. Another interpretation is that they play a central role in learning and memory. Anatomical studies suggest that arthropod mushroom bodies are predominately associated with olfactory pathways except in phylogenetically basal insects. The prominent olfactory input to the mushroom body calyces in more recent insect orders is an acquired character. An overview of the history of research on the mushroom bodies, as well as comparative and evolutionary considerations, provides a conceptual framework for discussing the roles of these neuropils.

Modular structures in the mushroom body of the cockroach

The mushroom body (MB) is a higher center of the insect brain and is critical to olfactory and other forms of associative memory. Here, we report that repetitive modular subunits, which we refer to as slabs, are present in the internal matrix of the alpha lobe, a major output neuropil of the MB in the cockroach. The methods employed were osmium-ethyl gallate, Bodian-reduced silver, and Golgi staining procedures. A total of 15 dark and 15 pale slabs, each consisting of specific subsets of intrinsic neurons (Kenyon cells), alternate throughout the length of the alpha lobe. One of the major classes of MB output neurons, which are postsynaptic to Kenyon cells, exhibited segmented dendritic arbors that interact with every other slabs, i.e. only either dark or pale slabs. As each output neuron interacts with each specific set of dark or pale slabs, the slab likely functions as a unit for transmitting MB output signals.

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

Locustatachykinin-immunoreactive (LomTK-IR) interneurons were found to be associated with the central complex, a prominent neuropil region of the insect brain. The structures and development of this set of brain interneurons was studied from the embryo onward in the beetle Tenebrio molitor, showing individual neurons that persist from the late embryo to the adult stage. Their essential structural characteristics were already present in the late embryo, but distinct parts of their arborization patterns became newly formed throughout development. Using a combination of immunohistochemistry and single-cell injection, we demonstrated minute structural changes, allowing a characterization of structural plasticity of identifiable, persistent, neuropeptidergic neurons throughout ontogenesis. Furthermore, this study has provided new information about basic principles of central brain neuroanatomy and the development of a distinct midbrain region of the insect brain, the central complex. The development of its basic connections, the connections between the fan-shaped body and the protocerebral bridge, and the compartmentation of these neuropil regions were shown, using LomTK-IR neurons as marker structures. These basic features of the central complex-associated LomTK-immunopositive neurons were formed in the embryonic brain, whereas in metamorphosis, reorganization of these persistent interneurons was restricted to the formation of a precisely defined projection of their side branches.

Expression of the surface antigen A2B7 in adult and developing honeybee olfactory pathway

In order to identify molecules involved in the development of the honeybee olfactory pathway, hybridoma technology has been used. Among different cell lines, A2B7 has been selected. It produces a specific antibody for a surface glycoprotein of 91 kDa. This protein is mainly expressed by both the antennal receptor cells and mushroom body neurons. Based on (i) the spatio-temporal pattern of expression during pupal development; (ii) the cell surface location of the antigen; and (iii) the partial molecular characterization of the antigen, a putative role for this protein in axonal fasciculation and guidance is discussed.

Putative neurohemal areas in the peripheral nervous system of an insect, Gryllus bimaculatus, revealed by immunocytochemistry

The morphology and position of putative neurohemal areas in the peripheral nervous system (ventral nerve cord and retrocerebral complex) of the cricket Gryllus bimaculatus are described. By using antisera to the amines dopamine, histamine, octopamine, and serotonin, and the neuropeptides crustacean cardioactive peptide, FMRFamide, leucokinin 1, and proctolin, an extensive system of varicose fibers has been detected throughout the nerves of all neuromeres, except for nerve 2 of the prothoracic ganglion. Immunoreactive varicose fibers occur mainly in a superficial position at the neurilemma, indicating neurosecretory storage and release of neuroactive compounds. The varicose fibers are projections from central or peripheral neurons that may extend over more than one segment. The peripheral fiber varicosities show segment-specific arrangements for each of the substances investigated. Immunoreactivity to histamine and octopamine is mainly found in the nerves of abdominal segments, whereas serotonin immunoreactivity is concentrated in subesophageal and terminal ganglion nerves. Immunoreactivity to FMRFamide and crustacean cardioactive peptide is widespread throughout all segments. Structures immunoreactive to leucokinin 1 are present in abdominal nerves, and proctolin immunostaining is found in the terminal ganglion and thoracic nerves. Codistribution of peripheral varicose fiber plexuses is regularly seen for amines and peptides, whereas the colocalization of substances in neurons has not been detected for any of the neuroactive compounds investigated. The varicose fiber system is regarded as complementary to the classical neurohemal organs.

Descending neurons with dopamine-like or with substance P/FMRFamide-like immunoreactivity target the somata of olfactory interneurons in the brain of the spiny lobster, Panulirus argus

Two sets of descending neurons primarily target the somata of neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus. Hundreds to thousands of dopamine-like immunoreactive fibers originate in the lateral protocerebrum and terminate among the clustered somata of the olfactory deutocerebrum projection neurons (lateral soma cluster) and those of the olfactory deutocerebrum local interneurons (medial soma cluster). A pair of giant neurons with substance P- and FMRFamide-like immunoreactivity from the median protocerebrum terminate primarily in the lateral soma cluster, but also branch in the core of the olfactory lobe itself. Neurons of both types terminate in numerous bouton-like swellings. The terminals in the lateral cluster at least contain numerous, large, dense-core and small, clear vesicles. The terminals contact the somata and the primary neurites through both traditional chemical synapses and large zones of direct membrane appositions. In most instances, a vesicle-containing profile forms a triadic arrangement with a neurite and a soma the latter being frequently connected via large gap-junction-like structures. Rosette-like arrangements formed by a vesicle-containing profile surrounded by up to eight neurites are also common. Dissociated lateral cluster somata support both fast inward and sustained outward voltage-activated currents. Substance P, but not dopamine or FMRFamide-related peptides, alters the fast inward current. The somata of the olfactory projection neurons, and possibly those of the olfactory local interneurons, appear to serve an integrative, and not merely a supportive role in these invertebrate central neurons.

Intracellular dye injection of previously immunolabeled insect neurons in fixed brain slices

Our method combines intracellular dye injection and immunohistochemistry. Under optical control, Lucifer Yellow was injected into immunohistochemically identified neurons that reside in fixed tissue. The technique allows visualization of the complete arborization patterns of immunostained neurons. Injections were performed on small neurons (somata < 10 microns in diameter). The technique works on microslices of insect brain. Standard immunohistochemical procedures have only been varied slightly, omitting Triton X-100 treatment. Anti-Lucifer Yellow immunohistochemistry, or alternatively the photoconversion technique, enables extension of the morphological analysis of these cells to the electron microscopic level. In the present study, Lucifer Yellow injections were performed on immunohistochemically pretreated brain microslices (anti-Locusta tachykinin II antiserum) of the beetle Tenebrio molitor.