Parallel organization in honey bee mushroom bodies by peptidergic kenyon cells

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.

Categorizing Visual Information in Subpopulations of Honeybee Mushroom Body Output Neurons

Multisensory integration plays a central role in perception, as all behaviors usually require the input of different sensory signals. For instance, for a foraging honeybee the association of a food source includes the combination of olfactory and visual cues to be categorized as a flower. Moreover, homing after successful foraging using celestial cues and the panoramic scenery may be dominated by visual cues. Hence, dependent on the context, one modality might be leading and influence the processing of other modalities. To unravel the complex neural mechanisms behind this process we studied honeybee mushroom body output neurons (MBON). MBONs represent the first processing level after olfactory-visual convergence in the honeybee brain. This was physiologically confirmed in our previous study by characterizing a subpopulation of multisensory MBONs. These neurons categorize incoming sensory inputs into olfactory, visual, and olfactory-visual information. However, in addition to multisensory units a prominent population of MBONs was sensitive to visual cues only. Therefore, we asked which visual features might be represented at this high-order integration level. Using extracellular, multi-unit recordings in combination with visual and olfactory stimulation, we separated MBONs with multisensory responses from purely visually driven MBONs. Further analysis revealed, for the first time, that visually driven MBONs of both groups encode detailed aspects within this individual modality, such as light intensity and light identity. Moreover, we show that these features are separated by different MBON subpopulations, for example by extracting information about brightness and wavelength. Most interestingly, the latter MBON population was tuned to separate UV-light from other light stimuli, which were only poorly differentiated from each other. A third MBON subpopulation was neither tuned to brightness nor to wavelength and encoded the general presence of light. Taken together, our results support the view that the mushroom body, a high-order sensory integration, learning and memory center in the insect brain, categorizes sensory information by separating different behaviorally relevant aspects of the multisensory scenery and that these categories are channeled into distinct MBON subpopulations.

Mushroom body evolution demonstrates homology and divergence across Pancrustacea

Descriptions of crustacean brains have focused mainly on three highly derived lineages of malacostracans: the reptantian infraorders represented by spiny lobsters, lobsters, and crayfish. Those descriptions advocate the view that dome- or cap-like neuropils, referred to as 'hemiellipsoid bodies,' are the ground pattern organization of centers that are comparable to insect mushroom bodies in processing olfactory information. Here we challenge the doctrine that hemiellipsoid bodies are a derived trait of crustaceans, whereas mushroom bodies are a derived trait of hexapods. We demonstrate that mushroom bodies typify lineages that arose before Reptantia and exist in Reptantia thereby indicating that the mushroom body, not the hemiellipsoid body, provides the ground pattern for both crustaceans and hexapods. We show that evolved variations of the mushroom body ground pattern are, in some lineages, defined by extreme diminution or loss and, in others, by the incorporation of mushroom body circuits into lobeless centers. Such transformations are ascribed to modifications of the columnar organization of mushroom body lobes that, as shown in Drosophila and other hexapods, contain networks essential for learning and memory.

Mushroom bodies in Reptantia reflect a major transition in crustacean brain evolution

Brain centers possessing a suite of neuroanatomical characters that define mushroom bodies of dicondylic insects have been identified in mantis shrimps, which are basal malacostracan crustaceans. Recent studies of the caridean shrimp Lebbeus groenlandicus further demonstrate the existence of mushroom bodies in Malacostraca. Nevertheless, received opinion promulgates the hypothesis that domed centers called hemiellipsoid bodies typifying reptantian crustaceans, such as lobsters and crayfish, represent the malacostracan cerebral ground pattern. Here, we provide evidence from the marine hermit crab Pagurus hirsutiusculus that refutes this view. P. hirsutiusculus, which is a member of the infraorder Anomura, reveals a chimeric morphology that incorporates features of a domed hemiellipsoid body and a columnar mushroom body. These attributes indicate that a mushroom body morphology is the ancestral ground pattern, from which the domed hemiellipsoid body derives and that the "standard" reptantian hemiellipsoid bodies that typify Astacidea and Achelata are extreme examples of divergence from this ground pattern. This interpretation is underpinned by comparing the lateral protocerebrum of Pagurus with that of the crayfish Procambarus clarkii and Orconectes immunis, members of the reptantian infraorder Astacidea.

Evolution of brain elaboration

Large, complex brains have evolved independently in several lineages of protostomes and deuterostomes. Sensory centres in the brain increase in size and complexity in proportion to the importance of a particular sensory modality, yet often share circuit architecture because of constraints in processing sensory inputs. The selective pressures driving enlargement of higher, integrative brain centres has been more difficult to determine, and may differ across taxa. The capacity for flexible, innovative behaviours, including learning and memory and other cognitive abilities, is commonly observed in animals with large higher brain centres. Other factors, such as social grouping and interaction, appear to be important in a more limited range of taxa, while the importance of spatial learning may be a common feature in insects with large higher brain centres. Despite differences in the exact behaviours under selection, evolutionary increases in brain size tend to derive from common modifications in development and generate common architectural features, even when comparing widely divergent groups such as vertebrates and insects. These similarities may in part be influenced by the deep homology of the brains of all Bilateria, in which shared patterns of developmental gene expression give rise to positionally, and perhaps functionally, homologous domains. Other shared modifications of development appear to be the result of homoplasy, such as the repeated, independent expansion of neuroblast numbers through changes in genes regulating cell division. The common features of large brains in so many groups of animals suggest that given their common ancestry, a limited set of mechanisms exist for increasing structural and functional diversity, resulting in many instances of homoplasy in bilaterian nervous systems.

Insect societies and the social brain

The 'social brain hypothesis,' the relationship between social behavior and brain size, does not apply to insects. In social insects, especially those of the Order Hymenoptera (ants, bees and wasps), sociality has not always increased individual behavioral repertoires and is associated with only subtle variation in the size of a higher brain center, the mushroom bodies. Rather than sociality, selection for novel visual behavior, perhaps spatial learning, has led to the acquisition of novel visual inputs and profound increases in mushroom body size. This occurred in nonsocial ancestors suggesting that the sensory and cognitive advantages of large mushroom bodies may be preadaptations to sociality. Adaptations of the insect mushroom bodies are more reliably associated with sensory ecology than social behavior.

Honey Bee Allatostatins Target Galanin/Somatostatin-Like Receptors and Modulate Learning: A Conserved Function?

Sequencing of the honeybee genome revealed many neuropeptides and putative neuropeptide receptors, yet functional characterization of these peptidic systems is scarce. In this study, we focus on allatostatins, which were first identified as inhibitors of juvenile hormone synthesis, but whose role in the adult honey bee (Apis mellifera) brain remains to be determined. We characterize the bee allatostatin system, represented by two families: allatostatin A (Apime-ASTA) and its receptor (Apime-ASTA-R); and C-type allatostatins (Apime-ASTC and Apime-ASTCC) and their common receptor (Apime-ASTC-R). Apime-ASTA-R and Apime-ASTC-R are the receptors in bees most closely related to vertebrate galanin and somatostatin receptors, respectively. We examine the functional properties of the two honeybee receptors and show that they are transcriptionally expressed in the adult brain, including in brain centers known to be important for learning and memory processes. Thus we investigated the effects of exogenously applied allatostatins on appetitive olfactory learning in the bee. Our results show that allatostatins modulate learning in this insect, and provide important insights into the evolution of somatostatin/allatostatin signaling.

A new antigenic marker specifically labels a subpopulation of the class II Kenyon cells in the brain of the European honeybee Apis mellifera

The mushroom bodies are the higher-order integration center in the insect brain and are involved in higher brain functions such as learning and memory. In the social hymenopteran insects such as honeybees, the mushroom bodies are the prominent brain structures. The mushroom bodies are composed of lobed neuropils formed by thousands of parallel-projecting axons of intrinsic neurons, and the lobes are divided into parallel subdivisions. In the present paper, we report a new antigenic marker to label a single layer in the vertical lobes of the European honeybee Apis mellifera. In the brain of A. mellifera, a monoclonal antibody (mAb) 15C3, which was originally developed against an insect ecdysone receptor (EcR) protein, immunolabels a single layer of the vertical lobes that correspond to the most dorsal layer of the γ-lobe. The 15C3 mAb recognizes a single ~200 kDa protein expressed in the adult honeybee brain. In addition, the 15C3 mAb immunoreactivity was also observed in the lobes of the developing pupal mushroom bodies. Since γ-lobe is well known to their extensive reorganization that occurs during metamorphosis in Drosophila, the novel antigenic marker for the honeybee γ-lobe allows us to investigate morphological changes of the mushroom bodies during metamorphosis.

Evolution of complex higher brain centers and behaviors: behavioral correlates of mushroom body elaboration in insects

Large, complex higher brain centers have evolved many times independently within the vertebrates, but the selective pressures driving these acquisitions have been difficult to pinpoint. It is well established that sensory brain centers become larger and more structurally complex to accommodate processing of a particularly important sensory modality. When higher brain centers such as the cerebral cortex become greatly expanded in a particular lineage, it is likely to support the coordination and execution of more complex behaviors, such as those that require flexibility, learning, and social interaction, in response to selective pressures that made these new behaviors advantageous. Vertebrate studies have established a link between complex behaviors, particularly those associated with sociality, and evolutionary expansions of telencephalic higher brain centers. Enlarged higher brain centers have convergently evolved in groups such as the insects, in which multimodal integration and learning and memory centers called the mushroom bodies have become greatly elaborated in at least four independent lineages. Is it possible that similar selective pressures acting on equivalent behavioral outputs drove the evolution of large higher brain centers in all bilaterians? Sociality has greatly impacted brain evolution in vertebrates such as primates, but it has not been a major driver of higher brain center enlargement in insects. However, feeding behaviors requiring flexibility and learning are associated with large higher brain centers in both phyla. Selection for the ability to support behavioral flexibility appears to be a common thread underlying the evolution of large higher brain centers, but the precise nature of these computations and behaviors may vary.

Effects of sublethal dose of fipronil on neuron metabolic activity of Africanized honeybees

Fipronil is a neurotoxic insecticide that inhibits the gamma-aminobutyric acid receptor and can affect gustative perception, olfactory learning, and motor activity of the honeybee Apis mellifera. This study determined the lethal dose (LD50) and the lethal concentration (LC50) for Africanized honeybee and evaluated the toxicity of a sublethal dose of fipronil on neuron metabolic activity by way of histochemical analysis using cytochrome oxidase detection in brains from worker bees of different ages. In addition, the present study investigated the recovery mechanism by discontinuing the oral exposure to fipronil. The results showed that mushroom bodies of aged Africanized honeybees are affected by fipronil, which causes changes in metabolism by increasing the respiratory activity of mitochondria. In antennal lobes, the sublethal dose of fipronil did not cause an increase in metabolic activity. The recovery experiments showed that discontinued exposure to a diet contaminated with fipronil did not lead to recovery of neural activity. Our results show that even at very low concentrations, fipronil is harmful to honeybees and can induce several types of injuries to honeybee physiology.

Long-term memory and response generalization in mushroom body extrinsic neurons in the honeybee Apis mellifera

Honeybees learn to associate an odor with sucrose reward under conditions that allow the monitoring of neural activity by imaging Ca(2+) transients in morphologically identified neurons. Here we report such recordings from mushroom body extrinsic neurons - which belong to a recurrent tract connecting the output of the mushroom body with its input, potentially providing inhibitory feedback - and other extrinsic neurons. The neurons' responses to the learned odor and two novel control odors were measured 24 h after learning. We found that calcium responses to the learned odor and an odor that was strongly generalized with it were enhanced compared with responses to a weakly generalized control. Thus, the physiological responses measured in these extrinsic neurons accurately reflect what is observed in behavior. We conclude that the recorded recurrent neurons feed information back to the mushroom body about the features of learned odor stimuli. Other extrinsic neurons may signal information about learned odors to different brain regions.

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.

The evolution of "deformed" brains in ant-like stone beetles (Scydmaeninae, Staphylinidae)

We present the first study of the central nervous system of adult representatives of Scydmaeninae. Histological staining, scanning electron microscopy and computer-based 3D reconstruction techniques were used to document the shape and configuration of the major cephalic elements of the central nervous system and to explain its anomalies compared to other Coleoptera. For the first time we report the presence of cephalic glands in ant-like stone beetles: in Scydmaenus (Cholerus) hellwigii openings of voluminous glands are located near the occipital constriction and their secretion accumulates in a large cavity of the dorsal head region. In Scydmaenus (Cholerus) perrisi the proto-, deuto-, tritocerebrum and the suboesophageal ganglion together form a large and compact ganglionic mass around the anterior foregut in the retracted neck region of the head. We exclude miniaturization as the driving force of the observed modifications. Comparative study of the head anatomy of S. perrisi, S. hellwigii, Scydmaenus (s. str.) tarsatus, Scydmaenus (Parallomicrus) rufus and Neuraphes elongatulus suggests a possible evolutionary scenario. We propose an evolutionary reversal hypothesis, involving a) the displacement and concentration of the cephalic central nervous system induced by the development of glandular cavities of the head, followed by b) a reduction of the glandular structures, without a secondary relocation of the cephalic CNS. The interpretation of head modifications in Scydmaeninae in the light of such a scenario may turn out as important for the reconstruction of the phylogeny and evolution of this highly successful group of beetles.

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.

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.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera)

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems. Here, we report on the characterization of Eph receptor and ephrin orthologs in the honeybee, Apis mellifera (Am), and their role in learning and memory. In situ hybridization for mRNA expression showed a uniform distribution of expression of both genes across the developing pupal and adult brain. However, in situ labeling with Fc fusion proteins indicated that the AmEphR and Amephrin proteins were differentially localized to cell body regions in the mushroom bodies and the developing neuropiles of the antennal and optic lobes. In adults, AmEphR protein was localized to regions of synaptic contacts in optic lobes, in the glomeruli of antennal lobes, and in the medial lobe of the mushroom body. The latter two regions are involved in olfactory learning and memory in the honeybee. Injections of EphR-Fc and ephrin-Fc proteins into the brains of adult bees, 1 h before olfactory conditioning of the proboscis extension reflex, significantly reduced memory 24 h later. Experimental amnesia in the group injected with ephrin-Fc was apparent 1 h post-training. Experimental amnesia was also induced by post-training injections with ephrin-Fc suggesting a role in recall. This is the first demonstration that Eph molecules function to regulate the formation of memory in insects.

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.

Analysis of Dscam diversity in regulating axon guidance in Drosophila mushroom bodies

Dscam is an immunoglobulin (Ig) superfamily member that regulates axon guidance and targeting in Drosophila. Alternative splicing potentially generates 38,016 isoforms differing in their extracellular Ig and transmembrane domains. We demonstrate that Dscam mediates the sorting of axons in the developing mushroom body (MB). This correlates with the precise spatiotemporal pattern of Dscam protein expression. We demonstrate that MB neurons express different arrays of Dscam isoforms and that single MB neurons express multiple isoforms. Two different Dscam isoforms differing in their extracellular domains introduced as transgenes into single mutant cells partially rescued the mutant phenotype. Expression of one isoform of Dscam in a cohort of MB neurons induced dominant phenotypes, while expression of a single isoform in a single cell did not. We propose that different extracellular domains of Dscam share a common function and that differences in isoforms expressed on the surface of neighboring axons influence interactions between them.

Current- and Voltage-Clamp Recordings and Computer Simulations of Kenyon Cells in the Honeybee

The mushroom body of the insect brain is an important locus for olfactory information processing and associative learning. The present study investigated the biophysical properties of Kenyon cells, which form the mushroom body. Current- and voltage-clamp analyses were performed on cultured Kenyon cells from honeybees. Current-clamp analyses indicated that Kenyon cells did not spike spontaneously in vitro. However, spikes could be elicited by current injection in approximately 85% of the cells. Of the cells that produced spikes during a 1-s depolarizing current pulse, approximately 60% exhibited repetitive spiking, whereas the remaining approximately 40% fired a single spike. Cells that spiked repetitively showed little frequency adaptation. However, spikes consistently became broader and smaller during repetitive activity. Voltage-clamp analyses characterized a fast transient Na+current ( INa), a delayed rectifier K+current ( IK,V), and a fast transient K+current ( IK,A). Using the neurosimulator SNNAP, a Hodgkin–Huxley-type model was developed and used to investigate the roles of the different currents during spiking. The model led to the prediction of a slow transient outward current ( IK,ST) that was subsequently identified by reevaluating the voltage-clamp data. Simulations indicated that the primary currents that underlie spiking are INaand IK,V, whereas IK,Aand IK,STprimarily determined the responsiveness of the model to stimuli such as constant or oscillatory injections of current.

Digital three-dimensional models of Drosophila development

Digital models of organs, cells and subcellular structures have become important tools in biological and medical research. Reaching far beyond their traditional widespread use as didactic tools, computer-generated models serve as electronic atlases to identify specific elements in complex patterns, and as analytical tools that reveal relationships between such pattern elements that would remain obscure in two-dimensional sections. Digital models also offer the unique opportunity to store and display gene-expression patterns, and pilot studies have been made in several genetic model organisms, including mouse, Drosophila and Caenorhabditis elegans, to construct digital graphic databases intended as repositories for gene-expression data.

Development and morphology of Class II Kenyon cells in the mushroom bodies of the honey bee,Apis mellifera

Class II Kenyon cells, defined by their early birthdate and unique dendritic arborizations, have been observed in the mushroom bodies of evolutionarily divergent insects. In the fruit fly Drosophila melanogaster, Class II (also called clawed) Kenyon cells are well known for their extensive reorganization that occurs during metamorphosis. The present account reports for the first time the occurrence of mushroom body reorganization during metamorphosis in holometabolous insect species outside of the Diptera. In the honey bee, Apis mellifera, Class II Kenyon cells show signs of degeneration and undergo a subtle reshaping of their axons during metamorphosis. Unlike in Drosophila, reorganization of Class II Kenyon cells in the honey bee does not involve the loss of axon branches. In contrast, the mushroom bodies of closely related hymenopteran species, the polistine wasps, undergo a much more dramatic restructuring near the end of metamorphosis. Immunohistochemistry, dextran fills, and Golgi impregnations illuminate the heterogeneous nature of Class II Kenyon cells in the developing and adult honey bee brain, with subpopulations differing in the location of dendritic arbors within the calyx, and branching pattern in the lobes. Furthermore, polyclonal antibodies against the catalytic subunit of Drosophila protein kinase A (anti-DC0) label an unusual and previously undescribed trajectory for these neurons. The observed variations in morphology indicate that subpopulations of Class II Kenyon cells in the honey bee can likely be further defined by significant differences in their specific connections and functions within the mushroom bodies.

Patterns of PERIOD and pigment-dispersing hormone immunoreactivity in the brain of the European honeybee (Apis mellifera): age- and time-related plasticity

We explored the neural basis of age- and task-related plasticity in circadian patterns of activity in the honeybee. To identify putative circadian pacemakers in the bee brain, we used antibodies against Drosophila melanogaster and Antheraea pernyi PERIOD and an antiserum to crustacean pigment-dispersing hormone (PDH) known to cross-react with insect pigment-dispersing factors (PDFs). In contrast to previous results from Drosophila, PDH and PER immunoreactivity (-ir) were not colocalized in bee neurons. The most intense PER-ir was cytoplasmic, in two groups of large neurons in the protocerebrum. The number of protocerebral PER-ir neurons and PER-ir intensity within individual cells were highest in brains collected during subjective night and higher in old bees than in young bees. These results are consistent with previous analyses of brain per mRNA in honeybees. Nuclear PER-ir was found throughout the brain, including the optic and antennal lobes. A single group of PDH-ir neurons (approximately 20/optic lobe) was consistently and intensely labeled at the medial margin of the medulla, independent of age or time of day. The processes of these neurons extended to specific neuropils in the protocerebrum and the optic lobes but not to the deutocerebrum. The patterns displayed by PER- and PDH-ir do not completely match any patterns previously described. This suggests that, although clock proteins are conserved across insect groups, there is no universal pattern of coexpression that allows ready identification of pacemaker neurons within the insect brain.

Developmental changes in expression patterns of two dopamine receptor genes in mushroom bodies of the honeybee,Apis mellifera

The expression patterns of two dopamine receptor genes, Amdop1 and Amdop2, in the developing mushroom bodies of the honeybee brain were determined by using in situ hybridisation. Both genes were expressed throughout pupal development, but their patterns of expression in the three major divisions of mushroom body intrinsic neurons (outer compact cells, noncompact cells, and inner compact cells) were quite distinct. Amdop1 expression could be detected in all three mushroom body cell groups throughout development. Staining for Amdop1 mRNA was particularly intense in newly born Kenyon cells, suggesting that levels of Amdop1 expression are higher in newborn cells than in more mature mushroom body neurons. This was not the case for Amdop2. Amdop2 expression in the mushroom bodies was restricted to inner and outer compact cells during most of pupal development, appearing in noncompact cells only late in metamorphosis or at adult eclosion. In contrast to the case with Amdop1, staining for Amdop2 mRNA was observed in glial cells. Expression of Amdop2 in glial cells was detected only at early stages of glial cell development, when the cells are reported to be actively dividing. This study not only implicates dopamine in the development of honeybee mushroom bodies but also suggests different roles for the two dopamine receptors investigated.

Development and evolution of the insect mushroom bodies: towards the understanding of conserved developmental mechanisms in a higher brain center

The insect mushroom bodies are prominent higher order neuropils consisting of thousands of approximately parallel projecting intrinsic neurons arising from the minute basophilic perikarya of globuli cells. Early studies described these structures as centers for intelligence and other higher functions; at present, the mushroom bodies are regarded as important models for the neural basis of learning and memory. The insect mushroom bodies share a similar general morphology, and the same basic sequence of developmental events is observed across a wide range of insect taxa. Globuli cell progenitors arise in the embryo and proliferate throughout the greater part of juvenile development. Discrete morphological and functional subpopulations of globuli cells (or Kenyon cells, as they are called in insects) are sequentially produced at distinct periods of development. Kenyon cell somata are arranged by age around the center of proliferation, as are their processes in the mushroom body neuropil. Other aspects of mushroom body development are more variable from species to species, such as the origin of specific Kenyon cell populations and neuropil substructures, as well as the timing and pace of the general developmental sequence.

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.

Nitric oxide synthase histochemistry in insect nervous systems: Methanol/formalin fixation reveals the neuroarchitecture of formaldehyde-sensitive NADPH diaphorase in the cockroach Periplaneta americana

Formaldehyde-insensitive NADPH diaphorase (NADPHd) activity is used widely as a histochemical marker for neuronal nitric oxide synthase (NOS). However, in several insects including the cockroach Periplaneta americana, NOS is apparently formaldehyde-sensitive; NADPHd fails to reveal neuron morphology and results in faint generalized staining. Here we have used a novel fixative, methanol/ formalin (MF), to reveal for the first time the neuroarchitecture of NADPHd in the cockroach, with intense selective staining occurring in neurons throughout the brain and thoracic ganglia. Immunocytochemical and histochemical analysis of cockroach and locust nervous systems indicated that neuronal NADPHd after MF fixation can be attributed to NOS. However, NADPHd in locust glial and perineurial cells was histochemically different from that in neurons and may thus be due to enzymes other than NOS. Histochemical implications of species-specific enzyme properties and of the transcriptional complexity of the NOS gene are discussed. The present findings suggest that MF fixation is a valuable new tool for the comparative analysis of the neuroarchitecture of NO signaling in insects. The Golgi-like definition of the staining enabled analysis of the NADPHd architecture in the cockroach and comparison with that in the locust. NADPHd in the tactile neuropils of the thoracic ganglia showed a similar organization in the two species. The olfactory glomeruli of the antennal lobes were in both species densely innervated by NADPHd-positive local interneurons that correlated in number with the number of glomeruli. Thus, the NADPHd architectures appear highly conserved in primary sensory neuropils. In the cockroach mushroom bodies, particularly dense staining in the gamma-layer of the lobes was apparently derived from Kenyon cells, whereas extrinsic arborizations were organized in domains across the lobes, an architecture that contrasts with the previously described tubular compartmentalization of locust mushroom bodies. These divergent architectures may result in different spatiotemporal dynamics of NO diffusion and suggest species differences in the role of NO in the mushroom bodies.

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.

Neurotransmitters and neuropeptides in the brain of the locust

As part of continuous research on the neurobiology of the locust, the distribution and functions of neurotransmitter candidates in the nervous system have been analyzed particularly well. In the locust brain, acetylcholine, glutamate, gamma-aminobutyric acid (GABA), and the biogenic amines serotonin, dopamine, octopamine, and histamine most likely serve a transmitter function. Increasing evidence, furthermore, supports a signalling function for the gaseous molecule nitric oxide, but a role for neuroptides is so far suggested only by immunocytochemistry. Acetylcholine, glutamate, and GABA appear to be present in large numbers of interneurons. As in other insects, antennal sensory afferents might be cholinergic, while glutamate is the transmitter candidate of antennal motoneurons. GABA is regarded as the principle inhibitory transmitter of the brain, which is supported by physiological studies in the antennal lobe. The cellular distribution of biogenic amines has been analyzed particularly well, in some cases down to physiologically characterized neurons. Amines are present in small numbers of interneurons, often with large branching patterns, suggesting neuromodulatory roles. Histamine, furthermore, is the transmitter of photoreceptor neurons. In addition to these "classical transmitter substances," more than 60 neuropeptides were identified in the locust. Many antisera against locust neuropeptides label characteristic patterns of neurosecretory neurons and interneurons, suggesting that these peptides have neuroactive functions in addition to hormonal roles. Physiological studies supporting a neuroactive role, however, are still lacking. Nitric oxide, the latest addition to the list of neurotransmitter candidates, appears to be involved in early stages of sensory processing in the visual and olfactory systems.

Visualization of neuropeptide expression, transport, and exocytosis in Drosophila melanogaster

Neuropeptides affect an extremely diverse set of physiological processes. Neuropeptides are often coreleased with neurotransmitters but, unlike neurotransmitters, the neuropeptide target cells may be distant from the site(s) of secretion. Thus, it is often difficult to measure the amount of neuropeptide release in vivo by electrophysiological methods. Here we establish an in vivo system for studying the developmental expression, processing, transport, and release of neuropeptides. A GFP-tagged atrial natriuretic factor fusion (preproANF-EMD) was expressed in the Drosophila nervous system with the panneural promoter, elav. During embryonic development, proANF-EMD was first seen to accumulate in synaptic regions of the CNS in stage 17 embryos. By the third instar larval stage, highly fluorescent neurons were evident throughout the CNS. In the adult, fluorescence was pronounced in the mushroom bodies, antennal lobe, and the central complex. At the larval neuromuscular junction, proANF-EMD was concentrated in nerve terminals. We compared the release of proANF-EMD from synaptic boutons of NMJ 6/7, which contain almost exclusively glutamate-containing clear vesicles, to those of NMJ 12, which include the peptidergic type III boutons. Upon depolarization, approximately 60% of the tagged neuropeptide was released from NMJs of both muscles in 15 min, as assayed by decreased fluorescence. Although the elav promoter was equally active in the motor neurons that innervate both NMJs 6/7 and 12, NMJ 12 contained 46-fold more neuropeptide and released much more proANF-EMD during stimulation than did NMJ 6/7. Our results suggest that peptidergic neurons have an enhanced ability to accumulate and/or release neuropeptides as compared to neurons that primarily release classical neurotransmitters.

Effect of juvenile hormone on short-term olfactory memory in young honeybees (Apis mellifera)

Reliable retention of olfactory learning following a 1-trial classical conditioning of the proboscis extension reflex (PER) is not achieved in honeybees until they are 6-7 days old. Here we show that treatment of newly emerged honeybees with juvenile hormone (JH) has a profound effect on the maturation of short-term olfactory memory. JH-treated individuals display excellent short-term (1 h) memory of associative learning at times as early as 3 days of age and perform consistently better than untreated bees for at least the first week of their lives. By contrast, the retention of long-term (24 h) memory following a 3-trial conditioning of the PER is not significantly improved in JH-treated bees. Our study also shows that experience and (or) chemosensory activation are not essential to improve learning performance in olfactory tasks. The lack of accelerated development of long-term retention of olfactory memories in JH-treated honeybees is discussed in the context of neural circuits suspected to mediate memory formation and retrieval in the honeybee 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.

Development of laminar organization in the mushroom bodies of the cockroach: Kenyon cell proliferation, outgrowth, and maturation

The mushroom bodies of the insect brain are lobed integration centers made up of tens of thousands of parallel-projecting axons of intrinsic (Kenyon) cells. Most of the axons in the medial and vertical lobes of adult cockroach mushroom bodies derive from class I Kenyon cells and are organized into regular, alternating pairs (doublets) of pale and dark laminae. Organization of Kenyon cell axons into the adult pattern of laminae occurs gradually over the course of nymphal development. Newly hatched nymphs possess tiny mushroom bodies with lobes containing a posterior lamina of ingrowing axons, followed by a single doublet, which is flanked anteriorly by a gamma layer composed of class II Kenyon cells. Golgi impregnations show that throughout nymphal development, regardless of the number of doublets present, the most posterior lamina serves as the "ingrowth lamina" for axons of newborn Kenyon cells. Axons of the ingrowth lamina are taurine- and synaptotagmin-immunonegative. They produce fine growth cone tipped filaments and long perpendicularly oriented collaterals along their length. The maturation of these Kenyon cells and the formation of a new lamina are marked by the loss of filaments and collaterals, as well as the onset of taurine and synaptotagmin expression. Class I Kenyon cells thus show plasticity in both morphology and transmitter expression during development. In a hemimetabolous insect such as the cockroach, juvenile stages are morphologically and behaviorally similar to the adult. The mushroom bodies of these insects must be functional from hatching onward, while thousands of new neurons are added to the existing structure. The observed developmental plasticity may serve as a mechanism by which extensive postembryonic development of the mushroom bodies can occur without disrupting function. This contrasts with the more evolutionarily derived holometabolous insects, such as the honey bee and the fruit fly, in which nervous system development is accomplished in a behaviorally simple larval stage and a quiescent pupal stage.

Subdivisions of hymenopteran mushroom body calyces by their afferent supply

The mushroom bodies are regions in the insect brain involved in processing complex multimodal information. They are composed of many parallel sets of intrinsic neurons that receive input from and transfer output to extrinsic neurons that connect the mushroom bodies with the surrounding neuropils. Mushroom bodies are particularly large in social Hymenoptera and are thought to be involved in the control of conspicuous orientation, learning, and memory capabilities of these insects. The present account compares the organization of sensory input to the mushroom body's calyx in different Hymenoptera. Tracer and conventional neuronal staining procedures reveal the following anatomic characteristics: The calyx comprises three subdivisions, the lip, collar, and basal ring. The lip receives antennal lobe afferents, and these olfactory input neurons can terminate in two or more segregated zones within the lip. The collar receives visual afferents that are bilateral with equal representation of both eyes in each calyx. Visual inputs provide two to three layers of processes in the collar subdivision. The basal ring is subdivided into two modality-specific zones, one receiving visual, the other antennal lobe input. Some overlap of modality exists between calycal subdivisions and within the basal ring, and the degree of segregation of sensory input within the calyx is species-specific. The data suggest that the many parallel channels of intrinsic neurons may each process different aspects of sensory input information.