Developmental study of afferented and deafferented bee antennal lobes

Comparative Development of the Ant Chemosensory System

The insect antennal lobe (AL) contains the first synapses of the olfactory system, where olfactory sensory neurons (OSNs) contact second-order projection neurons (PNs). In Drosophila melanogaster, OSNs expressing specific receptor genes send stereotyped projections to one or two of about 50 morphologically defined glomeruli [1-3]. The mechanisms for this precise matching between OSNs and PNs have been studied extensively in D. melanogaster, where development is deterministic and independent of neural activity [4-6]. However, a number of insect lineages, most notably the ants, have receptor gene repertoires many times larger than D. melanogaster and exhibit more structurally complex antennal lobes [7-12]. Moreover, perturbation of OSN function via knockout of the odorant receptor (OR) co-receptor, Orco, results in drastic AL reductions in ants [13, 14], but not in Drosophila [15]. Here, we characterize AL development in the clonal raider ant, Ooceraea biroi. We find that, unlike in Drosophila, ORs and Orco are expressed before the onset of glomerulus formation, and Orco protein is trafficked to developing axon terminals, raising the possibility that ORs play a role during ant AL development. Additionally, ablating ant antennae at the onset of pupation results in AL defects that recapitulate the Orco mutant phenotype. Thus, early loss of functional OSN innervation reveals latent structure in the AL that develops independently of peripheral input, suggesting that the AL is initially pre-patterned and later refined in an OSN-dependent manner. This two-step process might increase developmental flexibility and thereby facilitate the rapid evolution and expansion of the ant chemosensory system.

Structural aspects of plasticity in the nervous system of Drosophila

Neurons extend and retract dynamically their neurites during development to form complex morphologies and to reach out to their appropriate synaptic partners. Their capacity to undergo structural rearrangements is in part maintained during adult life when it supports the animal's ability to adapt to a changing environment or to form lasting memories. Nonetheless, the signals triggering structural plasticity and the mechanisms that support it are not yet fully understood at the molecular level. Here, we focus on the nervous system of the fruit fly to ask to which extent activity modulates neuronal morphology and connectivity during development. Further, we summarize the evidence indicating that the adult nervous system of flies retains some capacity for structural plasticity at the synaptic or circuit level. For simplicity, we selected examples mostly derived from studies on the visual system and on the mushroom body, two regions of the fly brain with extensively studied neuroanatomy.

An Early Sensitive Period Induces Long-Lasting Plasticity in the Honeybee Nervous System

The effect of early experiences on the brain during a sensitive period exerts a long-lasting influence on the mature individual. Despite behavioral and neural plasticity caused by early experiences having been reported in the honeybee Apis mellifera, the presence of a sensitive period in which associative experiences lead to pronounced modifications in the adult nervous system is still unclear. Laboratory-reared bees were fed with scented food within specific temporal windows and were assessed for memory retention, in the regulation of gene expression related to the synaptic formation and in the olfactory perception of their antennae at 17 days of age. Bees were able to retain a food-odor association acquired 5–8 days after emergence, but not before, and showed better retention than those exposed to an odor at 9–12 days. In the brain, the odor-rewarded experiences that occurred at 5–8 days of age boosted the expression levels of the cell adhesion proteins neurexin 1 (Nrx1) and neuroligin 2 (Nlg2) involved in synaptic strength. At the antennae, the experiences increased the electrical response to a novel odor but not to the one experienced. Therefore, a sensitive period that induces long-lasting behavioral, functional and structural changes is found in adult honeybees.

orco Mutagenesis Causes Loss of Antennal Lobe Glomeruli and Impaired Social Behavior in Ants

Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here, we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the ∼500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain. VIDEO ABSTRACT.

Age-associated increase of the active zone protein Bruchpilot within the honeybee mushroom body

In honeybees, age-associated structural modifications can be observed in the mushroom bodies. Prominent examples are the synaptic complexes (microglomeruli, MG) in the mushroom body calyces, which were shown to alter their size and density with age. It is not known whether the amount of intracellular synaptic proteins in the MG is altered as well. The presynaptic protein Bruchpilot (BRP) is localized at active zones and is involved in regulating the probability of neurotransmitter release in the fruit fly, Drosophila melanogaster. Here, we explored the localization of the honeybee BRP (Apis mellifera BRP, AmBRP) in the bee brain and examined age-related changes in the AmBRP abundance in the central bee brain and in microglomeruli of the mushroom body calyces. We report predominant AmBRP localization near the membrane of presynaptic boutons within the mushroom body MG. The relative amount of AmBRP was increased in the central brain of two-week old bees whereas the amount of Synapsin, another presynaptic protein involved in the regulation of neurotransmitter release, shows an increase during the first two weeks followed by a decrease. In addition, we demonstrate an age-associated modulation of AmBRP located near the membrane of presynaptic boutons within MG located in mushroom body calyces where sensory input is conveyed to mushroom body intrinsic neurons.

We discuss that the observed age-associated AmBRP modulation might be related to maturation processes or to homeostatic mechanisms that might help to maintain synaptic functionality in old animals.

Behavioral and neurophysiological study of olfactory perception and learning in honeybees

The honeybee Apis mellifera has been a central insect model in the study of olfactory perception and learning for more than a century, starting with pioneer work by Karl von Frisch. Research on olfaction in honeybees has greatly benefited from the advent of a range of behavioral and neurophysiological paradigms in the Lab. Here I review major findings about how the honeybee brain detects, processes, and learns odors, based on behavioral, neuroanatomical, and neurophysiological approaches. I first address the behavioral study of olfactory learning, from experiments on free-flying workers visiting artificial flowers to laboratory-based conditioning protocols on restrained individuals. I explain how the study of olfactory learning has allowed understanding the discrimination and generalization ability of the honeybee olfactory system, its capacity to grant special properties to olfactory mixtures as well as to retain individual component information. Next, based on the impressive amount of anatomical and immunochemical studies of the bee brain, I detail our knowledge of olfactory pathways. I then show how functional recordings of odor-evoked activity in the brain allow following the transformation of the olfactory message from the periphery until higher-order central structures. Data from extra- and intracellular electrophysiological approaches as well as from the most recent optical imaging developments are described. Lastly, I discuss results addressing how odor representation changes as a result of experience. This impressive ensemble of behavioral, neuroanatomical, and neurophysiological data available in the bee make it an attractive model for future research aiming to understand olfactory perception and learning in an integrative fashion.

Roles of glial cells in neural circuit formation: insights from research in insects

Investigators over the years have noted many striking similarities in the structural organization and function of neural circuits in higher invertebrates and vertebrates. In more recent years, the discovery of similarities in the cellular and molecular mechanisms that guide development of these circuits has driven a revolution in our understanding of neural development. Cellular mechanisms discovered to underlie axon pathfinding in grasshoppers have guided productive studies in mammals. Genes discovered to play key roles in the patterning of the fruitfly's central nervous system have subsequently been found to play key roles in mice. The diversity of invertebrate species offers to investigators numerous opportunities to conduct experiments that are harder or impossible to do in vertebrate species, but that are likely to shed light on mechanisms at play in developing vertebrate nervous systems. These experiments elucidate the broad suite of cellular and molecular interactions that have the potential to influence neural circuit formation across species. Here we focus on what is known about roles for glial cells in some of the important steps in neural circuit formation in experimentally advantageous insect species. These steps include axon pathfinding and matching to targets, dendritic patterning, and the sculpting of synaptic neuropils. A consistent theme is that glial cells interact with neurons in two-way, reciprocal interactions. We emphasize the impact of studies performed in insects and explore how insect nervous systems might best be exploited next as scientists seek to understand in yet deeper detail the full repertory of functions of glia in development.

Early olfactory experience modifies neural activity in the antennal lobe of a social insect at the adult stage

In the antennal lobe (AL), the first olfactory centre of the insect brain, odorants are represented as spatiotemporal patterns of glomerular activity. Whether and how such patterns are modified in the long term after precocious olfactory experiences (i.e. in the first days of adulthood) remains unknown. To address this question, we used in vivo optical imaging of calcium activity in the antennal lobe of 17-day-old honeybees which either experienced an odorant associated with sucrose solution 5-8 days after emergence or were left untreated. In both cases, we imaged neural responses to the learned odor and to three novel odors varying in functional group and carbon-chain length. Two different odor concentrations were used. We also measured behavioral responses of 17-day-old honeybees, treated and untreated, to these stimuli. We show that precocious olfactory experience increased general odor-induced activity and the number of activated glomeruli in the adult AL, but also affected qualitative odor representations, which appeared shifted in the neural space of treated animals relative to control animals. Such effects were not limited to the experienced odor, but were generalized to other perceptually similar odors. A similar trend was found in behavioral experiments, in which increased responses to the learned odor extended to perceptually similar odors in treated bees. Our results show that early olfactory experiences have long-lasting effects, reflected in behavioral responses to odorants and concomitant neural activity in the adult olfactory system.

Recognition, presence, and survival of locust central nervous glia in situ and in vitro

Insect glial cells serve functions for the formation, maintenance, and performance of the central nervous system in ways similar to their vertebrate counterparts. Characterization of physiological mechanisms that underlie the roles of glia in invertebrates is largely incomplete, partly due to the lack of markers that universally label all types of glia throughout all developmental stages in various species. Studies on primary cell cultures from brains of Locusta migratoria demonstrated that the absence of anti-HRP immunoreactivity, which has previously been used to identify glial cells in undissociated brains, can also serve as a reliable glial marker in vitro, but only in combination with a viability test. As cytoplasmic membranes of cultured cells are prone to degradation when they lose viability, only cells that are both anti-HRP immunonegative and viable should be regarded as glial cells, whereas the lack of anti-HRP immunoreactivity alone is not sufficient. Cell viability can be assessed by the pattern of nuclear staining with DAPI (4',6-diamidino-2-phenylindole), a convenient, sensitive labeling method that can be used in combination with other immunocytochemical cellular markers. We determined the glia-to-neuron ratio in central brains of fourth nymphal stage of Locusta migratoria to be 1:2 both in situ and in dissociated primary cell cultures. Analysis of primary cell cultures revealed a progressive reduction of glial cells and indicated that dead cells detach from the substrate and vanish from the analysis. Such changes in the composition of cell cultures should be considered in future physiological studies on cell cultures from insect nervous systems.

Glial investment of the adult and developing antennal lobe of Drosophila

In recent years the Drosophila olfactory system, with its unparalleled opportunities for genetic dissection of development and functional organization, has been used to study the development of central olfactory neurons and the molecular basis of olfactory coding. The results of these studies have been interpreted in the absence of a detailed understanding of the steps in maturation of glial cells in the antennal lobe. Here we present a high-resolution study of the glia associated with olfactory glomeruli in adult and developing antennal lobes. The study provides a basis for comparison of findings in Drosophila with those in the moth Manduca sexta that indicate a critical role for glia in antennal lobe development. Using flies expressing GFP under a Nervana2 driver to visualize glia for confocal microscopy, and probing at higher resolution with the electron microscope, we find that glial development in Drosophila differs markedly from that in moths: glial cell bodies remain in a rind around the glomerular neuropil; glial processes ensheathe axon bundles in the nerve layer but likely contribute little to axonal sorting; their processes insinuate between glomeruli only very late and then form only a sparse, open network around each glomerulus; and glial processes invade the synaptic neuropil. Taking our results in the context of previous studies, we conclude that glial cells in the developing Drosophila antennal lobe are unlikely to play a strong role in either axonal sorting or glomerulus stabilization and that in the adult, glial processes do not electrically isolate glomeruli from their neighbors.

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.

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.

Maturation of odor representation in the honeybee antennal lobe

The antennal lobe (AL) is the first center for processing odors in the insect brain, as is the olfactory bulb (OB) in vertebrates. Both the AL and the OB have a characteristic glomerular structure; odors sensed by olfactory receptor neurons are represented by patterns of glomerular activity. Little is known about when and how an odor begins to be perceived in a developing brain. We address this question by using calcium imaging to monitor odor-evoked neural activity in the ALs of bees of different ages. We find that odor-evoked neural activity already occurs in the ALs of bees as young as 1 or 2 days. In young bees, the responses to odors are relatively weak and restricted to a small number of glomeruli. However, different odors already evoke responses in different combinations of glomeruli. In mature bees, the responses are stronger and are evident in more glomeruli, but continue to have distinct odor-dependent signatures. Our findings indicate that the specific glomerular patterns for odors are conserved during the development, and that odor representations are fully developed in the AL during the first 2 weeks following emergence.

Influence of season and environment on adult neurogenesis in the central olfactory pathway of the shore crab, Carcinus maenas

In most vertebrates hitherto examined including humans, certain brain areas retain the capacity to build new neurons during adult life. In some arthropods, above all in crustaceans, continuous genesis of brain neurons has also been shown, namely for soma clusters of the olfactory brain. Several factors as, e.g., sensory input, living conditions, or stress, are known to influence the rate of cell proliferation, survival, and cell differentiation. The present study was undertaken to test whether seasonal changes and/or captivity would influence the proliferation of cells in the lateral cluster (LC) of the olfactory lobe (OL) and in the cluster of the hemiellipsoid body (HB) of the eyestalk of shore crabs. During a period of more than a year, 5-bromo-deoxyuridine (BrdU) injections were administered to freshly caught animals and to animals kept for 12 weeks after capture under artificial conditions. Counts of BrdU-labeled cells showed that animal size, seasonal changes as well as captivity had an influence on the number of proliferating cells. Further, in the lateral soma cluster and the soma cluster of the hemiellipsoid body, cell proliferation is most likely regulated independently. While the lateral soma cluster showed two peaks of cell proliferation (spring and late summer), the soma cluster of the hemiellipsoid body had only one peak in early summer. Furthermore, proliferation decreased with size and hence age of the animal only in the lateral soma cluster but not in the soma cluster of the hemiellipsoid body. Although captivity reduced the number of newborn cells in general, cell proliferation remained synchronous with the seasons of the year, indicating that an endogenous circannual rhythm regulates neurogenesis.

Differential expression of voltage-sensitive K+ and Ca2+ currents in neurons of the honeybee olfactory pathway

In order to understand the neuronal processes underlying olfactory learning, biophysical properties such as ion channel activity need to be analysed within neurons of the olfactory pathway. This study analyses voltage-sensitive ionic currents of cultured antennal lobe projection neurons and mushroom body Kenyon cells in the brain of the honeybee Apis mellifera. Rhodamine-labelled neurons were identified in vitro prior to recording, and whole-cell K(+) and Ca(2+) currents were measured. All neurons expressed transient and sustained outward K(+) currents, but Kenyon cells expressed higher relative amounts of transient A-type K(+) (I(K,A)) currents than sustained delayed rectifier K(+) current (I(K,V)). The current density of the I(K,V) was significantly higher in projection neurons than in Kenyon cells. The voltage-dependency of K(+) currents at positive membrane potentials was linear in Kenyon cells, but N-shaped in projection neurons. Blocking of voltage-sensitive Ca(2+) currents transformed the N-shaped voltage-dependency into a linear one, indicating activation of calcium-dependent K(+) currents (I(K,Ca)). The densities of currents through voltage-sensitive Ca(2+) channels did not differ between the two neuron classes and the voltage-dependency of current activation was similar. Projection neurons thus express higher calcium-dependent K(+) currents. These analyses revealed that the various neurons of the honeybee olfactory pathway in vitro have different current phenotypes, which may reflect functional differences between the neuron types in vivo.

Adult-like complexity of the larval antennal lobe of D. melanogaster despite markedly low numbers of odorant receptor neurons

We provide a detailed analysis of the larval head chemosensory system of Drosophila melanogaster, based on confocal microscopy of cell-specific reporter gene expression in P[GAL4] enhancer trap lines. In particular, we describe the neuronal composition of three external and three pharyngeal chemosensory organs, the nerve tracts chosen by their afferents, and their central target regions. With a total of 21 olfactory and 80 gustatory neurons, the sensory level is numerically much simpler than that of the adult. Moreover, its design is different than in the adult, showing an association between smell and taste sensilla. In contrast, the first-order relay of the olfactory afferents, the larval antennal lobe (LAL), exhibits adult-like features both in terms of structure and cell number. It shows a division into approximately 30 subunits, reminiscent of glomeruli in the adult antennal lobe. Taken together, the design of the larval chemosensory system is a "hybrid," with larval-specific features in the periphery and central characteristics in common with the adult. The largely reduced numbers of afferents and the similar architecture of the LAL and the adult antennal lobe, render the larval chemosensory system of Drosophila a valuable model system, both for studying smell and taste and for examining the development of its adult organization.

Glia in development, function, and neurodegeneration of the adult insect brain

Glial cells have long been viewed as a passive framework for neurons but in the meanwhile were shown to play a much more active role in brain function and development. Several reviews have described the function of glia in the insect embryo. The focus of this review is the role of glial cells in the development and function of the normal and diseased adult brain. In different insect species, a considerable variety of central nervous system glia has been described indicating adaptation to different functional requirements. In the development of the adult visual and olfactory system, glial cells guide incoming axons acting as intermediate targets. Glia are part of the insect blood-brain barrier, provide nourishment for neurons, and help to regulate the extracellular concentration of ions and neurotransmitters. To fulfill these tasks insect glial cells, like vertebrate glia, interact with each other and with neurons, thus influencing neural activity. The examples presented suggest that crosstalk between all brain cells is necessary not only to develop and maintain the complex insect brain but also to endow it with the capacity to respond and adapt to the changing environment.

Neurogenesis in the central olfactory pathway of the adult shore crab Carcinus maenas is controlled by sensory afferents

The number of olfactory projection neurons (OPNs) in the brain of the juvenile and adult shore crab Carcinus maenas continues to increase during the life of the animal. In vivo labeling of adult crabs with the proliferation marker bromodeoxyuridine (BrdU) revealed a group of proliferating neuronal precursor cells in the lateral soma clusters (LCs) and in the soma clusters of the hemiellipsoid bodies (HBCs). The LCs contain the cell bodies of the olfactory projection neurons and the HBCs house the cell bodies to which the OPNs project. The aim of the present study was to examine whether the input from primary olfactory afferents has any influence on the rate of proliferation and survival of the neuronal precursors in the central olfactory pathways of C. maenas. Different sets of experiments involving BrdU injection and its immunocytochemical detection combined with unilateral amputation of the antennule that houses the olfactory organ were carried out. Our results show that the missing olfactory sensory input affects the rate of proliferation and the survival of postmitotic cells in the LC and in the HBC compared with control animals. The effect on the survival of postmitotic cells tested by BrdU injection followed by unilateral ablation is lateralized. Proliferation of neuronal precursor cells tested by the reversed experimental order was drastically impaired bilaterally. We conclude that the olfactory sensory input is necessary for a normal rate of proliferation of neuronal precursors and the survival of their progeny in the LC and in the HBC of C. maenas.

Odor exposure causes central adaptation and morphological changes in selected olfactory glomeruli in Drosophila

In an attempt to correlate behavioral and neuronal changes, we examined the structural and functional effects of odor exposure in Drosophila. Young adult flies were exposed to a high concentration of the selected odor, usually benzaldehyde or isoamyl acetate, for 4 d and subsequently tested for their olfactory response to a variety of odorants and concentrations. The behavioral response showed specific adaptation to the exposed odor. By contrast, olfactory transduction, as measured in electroantennograms, remained normal. In vivo volume measurements were performed on olfactory glomeruli, the anatomical and functional units involved in odor processing. Pre-exposed flies exhibited volume reduction of certain glomeruli, in an odor-selective manner. Of a sample of eight glomeruli measured, dorsal medial (DM) 2 and ventral (V) were affected by benzaldehyde exposure, whereas DM6 was affected by isoamyl acetate. Estimation of the number of synapses indicates that volume reduction involves synapse loss that can reach 30% in the V glomerulus of flies adapted to benzaldehyde. Additional features of odorant-induced adaptation, including concentration dependence and perdurance, also show correlation, because both effects are elicited by high odor concentrations and are long-lasting (>1 week). Finally, the dunce mutant fails to develop behavioral adaptation as well as morphological changes in the olfactory glomeruli after exposure. These neural changes thus appear to require the cAMP signaling pathway.

Formation of antennal lobe and mushroom body neuropils during metamorphosis in the honeybee, apis mellifera

The projections to the mushroom bodies (mbs) have been clearly described in the brain of adult honeybees (Apis mellifera). Olfactory projection neurons arborize in the lip of the calyceal neuropil, whereas visual projection neurons project to the collar. To study the maturation of this pattern of innervation, as well as the development of uniglomerular projection neurons within the antennal lobes (als), we conducted the following three studies focused on the first four stages of pupal development: mass staining of olfactory projection neurons, single cell labeling of olfactory projection neurons, and simultaneous labeling of olfactory projection neurons and visual projection neurons. Examination of whole-mount preparations with the confocal laser scanning microscope revealed that the olfactory projection neurons achieved their adult arborization pattern within their main output region, the lip of the mb calyces, earlier during development (pupal stage 1) than their dendritic processes within their main input region, the al (pupal stage 2). Simultaneous labeling experiments showed further that the fiber terminals of olfactory projection neurons and visual projection neurons did not overlap but instead occupied their respective projection areas within the mb calyces as early as pupal stage 1. We conclude that selective innervation of different subregions of the calycal neuropil precedes the segregation of glomerular units within the antennal lobe neuropil, and that the Kenyon cells themselves provide a template for the innervation of olfactory and visual projection neurons.

Deafferentation-induced changes in the olfactory bulb of adult zebrafish

The influence of the olfactory organ on maintenance of olfactory bulb structure was examined in zebrafish, using peripheral deafferentation. This fish provides a model in which the olfactory organ is easily accessible for removal, the animals easily survive the surgery, and the olfactory bulbs are small enough to allow rigorous analysis of the resulting effects. Unilateral olfactory organ ablations were performed on anesthetized adult zebrafish using a small-vessel cautery iron. Fish were allowed to survive for 1, 3, or 6 weeks following the procedure. Analysis of deafferented animals revealed that most, if not all, of the olfactory organ was missing on the ablated side, and the structure did not regenerate. The morphology of the olfactory bulb was affected notably by the removal of its primary afferent innervation. The olfactory nerve layer was diminished at 1 week and absent by 3 weeks post-deafferentation. At all of the survival times the deafferented bulb appeared significantly smaller at the gross level, and there was a statistically significant effect on bulb size and cell number after 6 weeks. Tyrosine hydroxylase expression, as revealed by immunohistochemistry, was decreased noticeably on the ablated side. In conclusion, the olfactory organ is important in the preservation of normal olfactory bulb anatomy and neurochemistry in adult zebrafish. Thus, the influence of the periphery does not end with the formation of the mature olfactory bulb.

Dendritic pattern development of the honeybee antennal lobe neurons: a laser scanning confocal microscopic study

The processing of odorant signals is performed, in the olfactory bulb of vertebrates or in the antennal lobe of insects, by different types of neurons which display specific morphological and functional features. The present work characterizes the morphogenesis of the main neuronal types which participate in olfactory discrimination in the adult honeybee (Apis mellifera). Neurons were stained intracellularly with Lucifer yellow at different stages of pupal development and in the adult, and imaged by laser scanning confocal microscopy. Attending to branching patterns, all pupal neurons could be attributed to morphological types previously established in the adult. Given the functional importance of intraglomerular dendritic arbors in the processing of olfactory information, the study focused on their development. The two main classes, dense and sparse intraglomerular arbors, display adultlike features as early as the second day of pupal development. However, morphometric measurements and confocal observations show that their general pattern undergoes continuous maturation processes until late pupal stages and after emergence of the adult. Among these, the results point out a pruning of dendritic branches in sparse arbors, but not in dense arbors.

Axons of olfactory receptor cells of transsexually grafted antennae induce development of sexually dimorphic glomeruli in Manduca sexta

The influence of olfactory receptor cell (ORC) axons from transsexually grafted antennae on the development of glomeruli in the antennal lobes (ALs), the primary olfactory centers, was studied in the moth Manduca sexta. Normally during metamorphic adult development, the pheromone-specific macroglomerular complex (MGC) forms only in the ALs of males, whereas two lateral female-specific glomeruli (LFGs) develop exclusively in females. A female AL innervated by ORC axons from a grafted male antenna developed an MGC with three glomeruli, like the MGC of a normal male AL. Conversely, a male AL innervated by ORC axons from a grafted female antenna lacked the MGC but exhibited LFGs. ORC axons from grafted male antenna terminated in the MGC-specific target area, even in cases when the antennal nerve (AN) entered the AL via an abnormal route. Within ectopic neuromas formed by ANs that had become misrouted and failed to enter the brain, male-specific axons were not organized and formed terminal branches in many areas. The results suggest the presence of guidance cues within the AL for male-specific ORC axons. Depending on the sex of the antennal innervation, glial borders formed in a pattern characteristic of the MGC or LFGs. The sex-specific number of projection neurons (PNs) in the medial group of AL neurons remained unaffected by the antennal graft, but significant changes occurred in the organization of PN arborizations. In gynandromorphic females, LFG-specific PNs extended processes into the induced MGC, whereas in gynandromorphic males, PNs became restricted to the LFGs. The results indicate that male-and female-specific ORC axons play important roles in determining the position, anatomical features, and innervation of sexually dimorphic glomeruli.

Olfactory development in invertebrates. On the scent of central developmental issues

Invertebrate olfactory systems offer many advantages for cellular and molecular studies of development and for functional studies of developmental plasticity. For example, nematodes have chemical senses that can be studied using genetic approaches. Arthropods, which include insects and crustacea, have the advantages that certain neurons can be reliably identified from one individual to another, and that olfactory receptor neurons are located on peripheral appendages and thus can be manipulated independently of their brain targets even very early in development. Among the insects, olfactory learning can be displayed and used as a basis for studying olfactory plasticity in bees; genes are especially tractable in flies; individual growth cones can be visualized in the grasshopper embryo; and receptor neurons and glomeruli of known olfactory specificity and behavioral significance can be followed during early development in moths. In addition, many insect nervous systems are amenable to organ culture and dissociated-cell culture, opening the door to experimental studies of cellular interactions that can not be performed in situ. Recent research in the moth Manduca sexta attempts to identify the nature of the interactions between olfactory sensory axons, olfactory neurons of the brain, and glial cells in the creation of the array of glomeruli that underlie olfaction in the adult. Results indicate that timing of the ingrowth of olfactory receptor axons is critical for normal glomerulus development, that a subset of axons expresses a fasciclin II-like molecule that may play a role in guidance of their growth, and that glial cells must surround developing glomeruli in order to stabilize the 'protoglomerular' template made by receptor axon terminals. Moreover, glial cells are dye-coupled to each other early in glomerulus development and gradually become uncoupled. Electrical activity in neurons is not necessary for glomerulus formation; and some intercellular interactions, perhaps involving soluble factors, appear to involve tyrosine phosphorylation. In sum, a detailed picture is emerging of the cellular interactions that lead to the formation of glomeruli.

Growth-related and antennular amputation-induced changes in the olfactory centers of crayfish brain

Freshwater crayfish increase in size throughout their lives, and this growth is accompanied by an increase in the length of the appendages and number of mechanoreceptive and chemoreceptive sensilla on them. We find that in the Australian freshwater crayfish Cherax destructor, neuropil volumes of the olfactory centers increase linearly with body size over the entire size range of animals found in their natural habitat. The number of cell somata of two groups of interneurons associated with the olfactory centers (projection neurons and small local neurons) also increases linearly with the size of the animals. In contrast, axon counts of interneurons that represent a nonolfactory input to the olfactory centers show that these reach a total number in the very early adult stages that then remains constant regardless of the size of the animal. Only the axon diameter of these interneurons increases linearly with body size. Amputation of the antennule and olfactory sensilla reduces the number of projection and local interneurons on the amputated side. No change in the size of the olfactory centers occurs on the unamputated side. Amputation of the olfactory receptor neurons in crayfish therefore leads not only to a degeneration of the receptor cell endings in the olfactory lobe but also to a trans-synaptic response in which the number of higher order neurons decreases. Reconstitution of the antennule and olfactory receptor neurons in small adult crayfish is accompanied by the reestablishment of the normal number of interneurons and neuropil volume in the olfactory centers.

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.

Influence of receptor axons on the formation of olfactory glomeruli in a hemimetabolous insect, the cockroach Periplaneta americana

The embryonic development of the hemimetabolous insect Periplaneta americana requires approximately 31 days. Deafferentation experiments were used to investigate the role of ingrowing receptor axons during embryogenesis, specifically their influence 1) on the subdivision of the antennal lobe neuropil into glomeruli, 2) on the morphology and number of glial cells, and 3) on the arborization pattern of central neurons. The flagellum of one antenna was removed from embryos at different developmental stages starting with day 10. Subsequently, they were raised in culture until a total age of 26 days. At day 10, the deutocerebrum has received only a very small number (ca. 0.4%) of antennal receptor axons; deafferentation at this stage allowed us to deprive the deutocerebrum of approximately 99% of its normal antennal input. Deafferentation has marked effects on the organization of the antennal lobe neuropil. The deafferented lobe is reduced in volume compared to the control side; the characteristic glomeruli are missing. During normal development glomeruli are formed between day 19 and 22, first in dorsal and then in ventral antennal lobe regions. By removing the antenna before day 20, their formation is disturbed in all parts of the antennal lobe. If deafferentation is performed after stage 20, glomeruli persist in dorsal regions, but are missing in ventral regions. On day 24 or later, glomeruli in both dorsal and ventral regions are unaffected by deafferentation. Glial cells continue to extend fine processes into the neuropil in the absence of ingrowing receptor axons. The number of glial cells is reduced compared to control lobes. Multiglomerular local interneurons and other gamma-amino butyric acid-immunoreactive neurons, as well as projection neurons, fail to develop glomerular arborization patterns in antennal lobes deprived of sensory axons.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.

Morphology of neuroglia in the antennal lobes and mushroom bodies of the brain of the honeybee

We investigated the distribution and anatomical organization of glial cells in the antennal lobes and mushroom bodies of the honeybee. Reconstructions from serial sections, prepared according to the ethyl gallate method, revealed the entire morphology of glial cells in neuropiles, tracts, and the soma rind. The distribution of the glial cell bodies in the neuropiles was derived from the staining of cell nuclei with a fluorescent dye. There are glial cells of different shape in the soma rind which are wrapped around the neuronal cell bodies of the antennal lobes and the Kenyon cells of the mushroom bodies. Glial cells surround neuropilar areas such as the external and lateral sides of the glomeruli of the antennal lobes. Whereas we could not detect glia in the glomerular neuropile, glial cells with long processes are located in the core of the antennal lobe. Extensions of these glial cells also invade tracts containing the olfactory projection neurons. A layer of glial cells separates the mushroom body neuropile from the surrounding protocerebral neuropile. The neuropile of the mushroom bodies is clearly compartmented by glial cells. There is a high density of astrocyte-like glia in a column of the pedunculus which can be followed to the ventral part of the alpha-lobe. A network of mushroom body intrinsic glial cells separates the alpha-lobe from the beta-lobe and the pedunculus. This anatomical description of glial cell types in olfactory information processing pathways of an insect brain provides a framework for further physiological studies of neuroglia in dissociated cell culture.

Structural plasticity of identified glomeruli in the antennal lobes of the adult worker honey bee

Adult worker honey bees alter their behaviour with age but retain a strong reliance on sensory information from the antennae. The antennae house a diverse array of receptors, including mechanoreceptors, hygroreceptors, olfactory receptors, and contact chemoreceptors, which relay information to the brain. Antennal sensory neurons that project to the antennal lobes of the brain converge onto second-order interneurones to form discrete spheres of neuropil, called glomeruli. The spatial organisation of glomeruli in the antennal lobes of the honey bee is constant, but the central distribution of information from receptors tuned to different sensory modalities is unknown. Here we show that the glomerular neuropil of the antennal lobes undergoes constant modification during the lifetime of the adult worker bee. Changes in morphology are site specific and highly predictable. The total volume of the glomerular neuropil of the antennal lobe increased significantly during the first 4 days of adult life. Each of the five readily identifiable glomeruli examined in this study exhibited a unique pattern of growth. The growth of two of the five glomeruli changed dramatically with the shift to foraging duties. Furthermore, significant differences were identified between the antennal lobes of bees performing nectar- and pollen-foraging tasks. The highly compartmentalized nature of the antennal lobes, the ease with which specific glomeruli can be identified, and the predictability of changes to the antennal lobe neuropil make this an ideal system for examining the mechanisms and behavioural consequences of structural plasticity in primary sensory centres of the brain.

Embryonic development of the antennal lobes of a hemimetabolous insect, the cockroach Periplaneta americana: light and electron microscopic observations

In the hemimetabolous insect Periplaneta americana, the adult-like organization of the primary olfactory centers, the antennal lobes, is established during the approximately 31 days of embryogenesis. This report describes the temporal sequence of developmental events as viewed in the light and electron microscope by means of histological stains and by DiI labeling of antennal receptor axons with subsequent photoconversion. Glomeruli, characteristic differentiations of the antennal lobe neuropil, are first observed on day 19; their development, which is not synchronous in the various parts of the antennal lobe, lasts until about day 22. From day 10 on, glial cells begin to form a narrow boundary layer between the soma cortex and the central neuropil. They exhibit a lengthening of their processes in parallel with the formation of glomeruli. Marked proliferation or migration of these glial cells into the neuropil between glomeruli has not been observed. Antennal receptor axons could be labeled from stage 15 on. They terminate in an elongated growth cone with numerous filopodia. From day 18 on, some of these become bent or show an initial bifurcation. From day 22 on, the first afferent axons develop an adult-like arborization pattern. Synaptic contacts between receptor axons and unidentified neurons were observed as early as stages 16 and 19, in which the axons still have a growth cone-like form. In stage 27, in which the fibers have adult-like arborizations, many output contacts and few input contacts were found.

Uniglomerular projection neurons participate in early development of olfactory glomeruli in the moth Manduca sexta

Glomerular organization of the antennal (olfactory) lobe is initiated by the arrival of sensory axons from the antenna. Bundles of axon terminals coalesce into spheroidal knots of neuropil called protoglomeruli. Previous studies have suggested that the protoglomeruli form a template for the mature glomerular array, but an early role for projection neurons in establishing the template has not been excluded. We examined with the confocal laser scanning microscope the morphological development of the uniglomerular projection neurons during the stages in which glomeruli are constructed. Groups of projection neurons were stained with the lipophilic dye DiI to assess the development of the population as a whole; individual neurons were filled intracellularly with Lucifer Yellow to examine in detail the development of shape. In some preparations, sensory axons and glial cells also were labeled by using different fluorescent dyes to reveal possible interactions between projection neuron dendrites and sensory axons or glial cells. Protoglomeruli form in a wave beginning at the entry point of the antennal nerve and proceeding across the lobe to the opposite pole. A second wave follows in which projection neurons become tufted and innervate the newly formed glomeruli, sometimes extending into the glial border surrounding the protoglomeruli. In animals deprived of sensory axons, some projection neurons still form tufted dendritic trees and, in one region of the neuropil, a glomerulus-like structure. The early presence of projection neuron processes in the protoglomeruli and the formation of at least one glomerulus-like structure in unafferented lobes suggest that uniglomerular projection neurons play an active role in the construction of olfactory glomeruli.

In vitro analyses of neurite outgrowth indicate a potential role for tenascin-like molecules in the development of insect olfactory glomeruli

Tenascin-like material is associated with glial cells that form borders around developing glomerular units in the olfactory (antennal) lobe of the moth Manduca sexta and is present at critical stages of glomerulus formation (Krull et al., 1994, J. Neurobiol. 25:515-534). Tenascin-like immunoreactivity declines in the mature lobe, coincident with a wave of synapse formation within the glomeruli and glomerulus stabilization. Tenascin-like molecules associated with neuropilar glia are in the correct position to influence the branching patterns of growing neurites by constraining them to glomeruli. In this study, we examine the growth of cultured moth antennal-lobe neurons in response to mouse CNS tenascin. Uniform tenascin provides a poor substrate for cell-body attachment and neurite outgrowth. Neuronal cell bodies provided with a striped substratum consisting of tenascin and concanavalin-A (con-A)/laminin attach preferentially to con-A/laminin lanes. Most neurons restrict their branching to con-A/laminin lanes both at early and later times in culture but others send processes across multiple tenascin and con-/laminin lanes in an apparently indiscriminate manner. Tenascin can inhibit the neuritic outgrowth of most antennal-lobe neurons, and this raises the possibility that the tenascin-like molecules associated with neuropilar glia in vivo act to constrain growing neurites to glomeruli. Thus, glial cells, acting in concert with olfactory axons, might act to promote glomerular patterns of branching by antennal-lobe neurons.

Perturbed glial scaffold formation precedes axon tract malformation in Drosophila mutants

The longitudinal glia (LG), progeny of a single glioblast, form a scaffold that presages the formation of longitudinal tracts in the ventral nerve cord (VNC) of the Drosophila embryo. The LG are used as a substrate during the extension of the first axons of the longitudinal tract. I have examined the differentiation of the LG in six mutations in which the longitudinal tracts were absent, displaced, or interrupted to determine whether the axon tract malformations may be attributable to disruptions in the LG scaffold. Embryos mutant for the gene prospero had no longitudinal tracts, and glial differentiation remained arrested at a preaxonogenic state. Two mutants of the Polycomb group also lacked longitudinal tracts; here the glia failed to form an oriented scaffold, but cytological differentiation of the LG was unperturbed. The longitudinal tracts in embryos mutant for slit fused at the VNC midline and scaffold formation was normal, except that it was medially displaced. Longitudinal tracts had intersegmental interruptions in embryos mutant for hindsight and midline. In hindsight, there were intersegmental gaps in the glial scaffold. In midline, the glial scaffold retracted after initial extension. LG morphogenesis during axonogenesis was abnormal in midline. Commitment to glial identity and glial differentiation also occurred before scaffold formation. In all mutants examined, the early distribution of the glycoprotein neuroglian was perturbed. This was indicative of early alterations in VNC pattern present before LG scaffold formation began. Therefore, some changes in scaffold formation may have reflected changes in the placement and differentiation of other cells of the VNC. In all mutants, alterations in scaffold formation preceded longitudinal axon tract formation.