Activity blockade does not prevent the construction of olfactory glomeruli in the moth Manduca sexta

Temperature regulates synaptic subcellular specificity mediated by inhibitory glutamate signaling

Environmental factors such as temperature affect neuronal activity and development. However, it remains unknown whether and how they affect synaptic subcellular specificity. Here, using the nematode Caenorhabditis elegans AIY interneurons as a model, we found that high cultivation temperature robustly induces defects in synaptic subcellular specificity through glutamatergic neurotransmission. Furthermore, we determined that the functional glutamate is mainly released by the ASH sensory neurons and sensed by two conserved inhibitory glutamate-gated chloride channels GLC-3 and GLC-4 in AIY. Our work not only presents a novel neurotransmission-dependent mechanism underlying the synaptic subcellular specificity, but also provides a potential mechanistic insight into high-temperature-induced neurological defects.

Evolution, developmental expression and function of odorant receptors in insects

Animals rely on their chemosensory system to discriminate among a very large number of attractive or repulsive chemical cues in the environment, which is essential to respond with proper action. The olfactory sensory systems in insects share significant similarities with those of vertebrates, although they also exhibit dramatic differences, such as the molecular nature of the odorant receptors (ORs): insect ORs function as heteromeric ion channels with a common Orco subunit, unlike the G-protein-coupled olfactory receptors found in vertebrates. Remarkable progress has recently been made in understanding the evolution, development and function of insect odorant receptor neurons (ORNs). These studies have uncovered the diversity of olfactory sensory systems among insect species, including in eusocial insects that rely extensively on olfactory sensing of pheromones for social communication. However, further studies, notably functional analyses, are needed to improve our understanding of the origins of the Orco-OR system, the mechanisms of ORN fate determination, and the extraordinary diversity of behavioral responses to chemical cues.

Embryonic origin of olfactory circuitry in Drosophila: contact and activity-mediated interactions pattern connectivity in the antennal lobe

The first study of the embryonic development of the Drosophila olfactory network reveals unexpected similarities with vertebrate systems.

Olfactory neuropiles across different phyla organize into glomerular structures where afferents from a single olfactory receptor class synapse with uniglomerular projecting interneurons. In adult Drosophila, olfactory projection interneurons, partially instructed by the larval olfactory system laid down during embryogenesis, pattern the developing antennal lobe prior to the ingrowth of afferents. In vertebrates it is the afferents that initiate and regulate the development of the first olfactory neuropile. Here we investigate for the first time the embryonic assembly of the Drosophila olfactory network. We use dye injection and genetic labelling to show that during embryogenesis, afferent ingrowth pioneers the development of the olfactory lobe. With a combination of laser ablation experiments and electrophysiological recording from living embryos, we show that olfactory lobe development depends sequentially on contact-mediated and activity-dependent interactions and reveal an unpredicted degree of similarity between the olfactory system development of vertebrates and that of the Drosophila embryo. Our electrophysiological investigation is also the first systematic study of the onset and developmental maturation of normal patterns of spontaneous activity in olfactory sensory neurons, and we uncover some of the mechanisms regulating its dynamics. We find that as development proceeds, activity patterns change, in a way that favours information transfer, and that this change is in part driven by the expression of olfactory receptors. Our findings show an unexpected similarity between the early development of olfactory networks in Drosophila and vertebrates and demonstrate developmental mechanisms that can lead to an improved coding capacity in olfactory neurons.

The mechanisms underlying the patterning of connectivity in the insect olfactory system are radically different from those found in vertebrates, but to date most studies in insects have focused on the development of the adult olfactory network. Here, for the first time, we report how larval olfactory circuitry is formed in the embryo of the fruitfly Drosophila. By labelling developing sensory neurons and interneurons from the earliest stages to maturity, we find that the patterning of the antennal lobe in Drosophila, like the olfactory bulb in mouse, is pioneered by ingrowing sensory afferents, and that interneuronal development depends on the terminals of these pioneering afferents. We also find that antennal lobe patterning depends on contact and activity-mediated interactionsbetween its component cells, as it does in vertebrates. Finally, we report the results of electrophysiological recordings in developing embryos, the first of their kind in any developing olfactory network. We conclude that fundamental mechanisms of circuit assembly and patterning are conserved between Drosophila and vertebrates.

Direct and glia-mediated effects of GABA on development of central olfactory neurons

Previously studied for its role in processing olfactory information in the antennal lobe, GABA also may shape development of the olfactory pathway, acting either through or on glial cells. Early in development, the dendrites of GABAergic neurons extend to the glial border that surrounds the nascent olfactory lobe neuropil. These neuropil glia express both GABAA and GABAB receptors, about half of the glia in acute cultures responded to GABA with small outward currents, and about a third responded with small transient increases in intracellular calcium. The neuronal classes that express GABA in vivo, the local interneurons and a subset of projection neurons, also do so in culture. Exposure to GABA in culture increased the size and complexity of local interneurons, but had no effect on glial morphology. The presence of glia alone did not affect neuronal morphology, but in the presence of both glia and GABA, the growth-enhancing effects of GABA on cultured antennal lobe neurons were eliminated. Contact between the glial cells and the neurons was not necessary. Operating in vivo, these antagonistic effects, one direct and one glia mediated, could help to sculpt the densely branched, tufted arbors that are characteristic of neurons innervating olfactory glomeruli.

Development of a glial network in the olfactory nerve: role of calcium and neuronal activity

In adult olfactory nerves of mammals and moths, a network of glial cells ensheathes small bundles of olfactory receptor axons. In the developing antennal nerve (AN) of the moth Manduca sexta, the axons of olfactory receptor neurons (ORNs) migrate from the olfactory sensory epithelium toward the antennal lobe. Here we explore developmental interactions between ORN axons and AN glial cells. During early stages in AN glial-cell migration, glial cells are highly dye coupled, dividing glia are readily found in the nerve and AN glial cells label strongly for glutamine synthetase. By the end of this period, dye-coupling is rare, glial proliferation has ceased, glutamine synthetase labeling is absent, and glial processes have begun to extend to enwrap bundles of axons, a process that continues throughout the remainder of metamorphic development. Whole-cell and perforated-patch recordings in vivo from AN glia at different stages of network formation revealed two potassium currents and an R-like calcium current. Chronic in vivo exposure to the R-type channel blocker SNX-482 halted or greatly reduced AN glial migration. Chronically blocking spontaneous Na-dependent activity by injection of tetrodotoxin reduced the glial calcium current implicating an activity-dependent interaction between ORNs and glial cells in the development of glial calcium currents.

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.

Activity-dependent modulation of neural circuit synaptic connectivity

In many nervous systems, the establishment of neural circuits is known to proceed via a two-stage process; (1) early, activity-independent wiring to produce a rough map characterized by excessive synaptic connections, and (2) subsequent, use-dependent pruning to eliminate inappropriate connections and reinforce maintained synapses. In invertebrates, however, evidence of the activity-dependent phase of synaptic refinement has been elusive, and the dogma has long been that invertebrate circuits are “hard-wired” in a purely activity-independent manner. This conclusion has been challenged recently through the use of new transgenic tools employed in the powerful Drosophila system, which have allowed unprecedented temporal control and single neuron imaging resolution. These recent studies reveal that activity-dependent mechanisms are indeed required to refine circuit maps in Drosophila during precise, restricted windows of late-phase development. Such mechanisms of circuit refinement may be key to understanding a number of human neurological diseases, including developmental disorders such as Fragile X syndrome (FXS) and autism, which are hypothesized to result from defects in synaptic connectivity and activity-dependent circuit function. This review focuses on our current understanding of activity-dependent synaptic connectivity in Drosophila, primarily through analyzing the role of the fragile X mental retardation protein (FMRP) in the Drosophila FXS disease model. The particular emphasis of this review is on the expanding array of new genetically-encoded tools that are allowing cellular events and molecular players to be dissected with ever greater precision and detail.

Acetylcholine-mediated axon-glia signaling in the developing insect olfactory system

In the olfactory system of the sphinx moth Manduca sexta, migration of neuropil glial cells is triggered by olfactory receptor axons and depends on intraglial Ca(2+) signaling. It is not known, however, how receptor axons and glial cells communicate and whether Ca(2+) signaling is a consequence of this communication. We studied Ca(2+) increases in glial cells in vivo and in situ, evoked by electrical stimulation of olfactory receptor axons in pupae and by odor stimulation of receptor neurons in adult moths. Axonal activity leads to Ca(2+) increases in neuropil glial cells that are blocked by nicotinic acetylcholine receptor antagonists and can be mimicked by acetylcholine and carbachol application. In addition, Ca(2+) transients were abolished by removal of external Ca(2+) and blockage of voltage-gated Ca(2+) channels. During development, acetylcholine-mediated Ca(2+) signaling could first be elicited at stage 6, the time when neuropil glial cells start to migrate. Glial migration was reduced after injection of nicotinic antagonists into pupae. The results show that Ca(2+) signaling can be induced by acetylcholine release from olfactory receptor axons, which activates nicotinic acetylcholine receptors and leads to voltage-gated Ca(2+) influx. The results further suggest that cholinergic signaling in the olfactory system is required for glial cell migration in Manduca.

Manganese-enhanced 3D MRI of established and disrupted synaptic activity in the developing insect brain in vivo

The antennal lobe of the sphinx moth Manduca sexta serves as a model for the development of the olfactory system. Here, the establishment of the glomerular synaptic network formed by the olfactory receptor axons and antennal lobe neurons at pupal stage P12 was followed by transection of the right antenna and - within 24 h - by injection of MnCl2 into the hemolymph. In vivo 3D MRI at 100 and 60 microm isotropic resolution was then performed at P13 to P17. Whereas the left antennal lobe revealed a pronounced increase of the signal-to-noise ratio (SNR) reflecting normal synaptic activity, the observation of only a small SNR increase within the right antennal lobe indicated the disruption of pertinent activity after antennal transection. The accumulation of manganese in the intact antennal system became observable within 3 h and lasted for at least 2 days after injection. Intra-individual comparisons between the right and left side yielded a statistically significant differential SNR increase in the left antennal lobe. Because such an effect was not observed in younger animals studied at pupal stages P10/P11, the MRI findings confirm the development of functional synapses in the antennal lobe of Manduca sexta by P13.

Blockage of voltage-gated calcium signaling impairs migration of glial cells in vivo

Migration of glial cells is an essential step in the development of the antennal lobe, the primary olfactory center of insects, to establish well-defined borders between olfactory glomeruli required for odor discrimination. In the present study, we used two-photon microscopy to visualize calcium signaling in developing antennal lobe glial cells of the sphinx moth Manduca sexta. We found a correlation between the upregulation of functional voltage-gated calcium channels and the onset of glial cell migration. In addition, glial cells migrating into the center of the antennal lobe express larger voltage-gated calcium transients than glial cells that remain at the periphery. Migration behavior and calcium signaling of glial cells in vivo were manipulated either by deafferentation, by injection of the calcium channel blockers diltiazem, verapamil, and flunarizine, or by injection of the calcium chelators BAPTA-AM and Fluo-4-AM. In deafferented antennal lobes, glial cells failed to express functional voltage-gated calcium channels and did not migrate. Calcium channel blockage or reducing glial calcium signals by calcium chelators prevented glial cell migration and resulted in antennal lobes lacking glial borders around glomeruli, indicating that voltage-gated calcium signaling is required for the migration of antennal lobe glial cells and the development of mature olfactory glomeruli.

Plateau Potentials in Developing Antennal-Lobe Neurons of the Moth,Manduca sexta

Using whole cell recordings from antennal-lobe (AL) neurons in vitro and in situ, in semi-intact brain preparations, we examined membrane properties that contribute to electrical activity exhibited by developing neurons in primary olfactory centers of the brain of the sphinx moth, Manduca sexta. This activity is characterized by prolonged periods of membrane depolarization that resemble plateau potentials. The presence of plateau potential–generating mechanisms was confirmed using a series of tests established earlier. Brief depolarizing current pulses could be used to trigger a plateau state. Once triggered, plateau potentials could be terminated by brief pulses of hyperpolarizing current. Both triggering and terminating of firing states were threshold phenomena, and both conditions resulted in all-or-none responses. Rebound excitation from prolonged hyperpolarizing pulses could also be used to generate plateau potentials in some cells. These neurons were found to express a hyperpolarization-activated inward current. Neither the generation nor the maintenance of plateau potentials was affected by removal of Na+ions from the extracellular medium or by blockade of Na+currents with TTX. However, blocking of Ca2+currents with Cd2+(5 × 10−4M) inhibited the generation of plateau potentials, indicating that, in Manduca AL neurons, plateau potentials depend on Ca2+. Examining Ca2+currents in isolation revealed that activation of these currents occurs in the absence of experimentally applied depolarizing stimuli. Our results suggest that this activity underlies the generation of plateau potentials and characteristic bursts of electrical activity in developing AL neurons of M. sexta.

Monitoring neuronal calcium signalling using a new method for ratiometric confocal calcium imaging

Ca2+ signalling influences many processes in the adult and developing nervous system like exocytosis, synaptic plasticity, and growth cone motility. Optical techniques in combination with fluorescent Ca2+ indicators are the most frequently used methods to measure Ca2+ signalling in cells. In the present study, a new method for ratiometric confocal Ca2+ imaging was developed, and the usefulness of the system was tested with two different neuronal preparations. Developing Manduca sexta antennal lobe neurons were loaded with the Ca2+-sensitive dye Fura Red-AM, and the ratio of fluorescence excited at 457 and 488nm was measured with a confocal laser scanning microscope. During pupal stages 4-12, the antennal lobe neuropil is restructured which includes the ingrowth of olfactory receptor axons, dendritic outgrowth of antennal lobe neurons, and synaptogenesis. In antennal lobe neurons, application of the AChR agonist carbachol induced Ca2+ oscillations the amplitude and frequency of which changed during stages 4-9, while at the end of synaptogenesis, at stages 11 and 12, only single Ca2+ transients were elicited. The Ca2+ oscillations were blocked by D-tubocurarine and Cd2+, indicating that they were due to Ca2+ influx through voltage-gated Ca2+ channels, activated by nAChR-mediated membrane depolarization. To test whether single action potentials can induce Ca2+ transients detectable by Fura Red, individual leech Retzius neurons were injected iontophoretically with the Ca2+ indicator, and the membrane potential was recorded during Ca2+ imaging. Single action potentials induced transient increases in the Fura Red ratio measured in the axon, while trains of action potentials elicited Ca2+ transients that could also be recorded in the cell body and the nucleus. The results show that Fura Red can be used as a ratiometric Ca2+ indicator for confocal imaging.

Dock and Pak regulate olfactory axon pathfinding in Drosophila

The convergence of olfactory axons expressing particular odorant receptor (Or) genes on spatially invariant glomeruli in the brain is one of the most dramatic examples of precise axon targeting in developmental neurobiology. The cellular and molecular mechanisms by which olfactory axons pathfind to their targets are poorly understood. We report here that the SH2/SH3 adapter Dock and the serine/threonine kinase Pak are necessary for the precise guidance of olfactory axons. Using antibody localization, mosaic analyses and cell-type specific rescue, we observed that Dock and Pak are expressed in olfactory axons and function autonomously in olfactory neurons to regulate the precise wiring of the olfactory map. Detailed analyses of the mutant phenotypes in whole mutants and in small multicellular clones indicate that Dock and Pak do not control olfactory neuron (ON) differentiation, but specifically regulate multiple aspects of axon trajectories to guide them to their cognate glomeruli. Structure/function studies show that Dock and Pak form a signaling pathway that mediates the response of olfactory axons to guidance cues in the developing antennal lobe (AL). Our findings therefore identify a central signaling module that is used by ONs to project to their cognate glomeruli.

Developmental changes in the electrophysiological properties and response characteristics of Manduca antennal-lobe neurons

Using whole cell patch-clamp recordings, we have examined changes in the electrophysiological properties and response characteristics of antennal lobe (AL) neurons associated with the metamorphic adult development of the sphinx moth, Manduca sexta. Whole cell current profiles and electrical excitability were examined in dispersed AL neurons in vitro, and in medial-group AL neurons in situ in semi-intact brain preparations. Around stages 2-4 of the 18 stages of metamorphic adult development, whole cell current profiles were dominated by large outward (K+) currents. Calcium-dependent action potentials could be elicited at this stage, but only a small percentage of cells exhibited sodium spikes. From stages 3 to 10, there was a rapid increase in the proportion of AL neurons exhibiting rapidly activating, transient sodium currents, and many cells in vitro exhibited spontaneous bursts of spike activity at this time. As development progressed, action-potential waveforms became shorter in duration and larger in amplitude. Cell-type-specific differences in the prevalence of spontaneous activity, and in the electrophysiological properties and response characteristics of AL neurons, were most apparent late in metamorphosis. While removal of antennal sensory input to the ALs early (stage 1-2) in metamorphosis had no detectable effect on the development of cell excitability, a significantly higher percentage of neurons in vitro from stage 4 pupae exhibited sodium-based action potentials following the addition of serotonin to the culture medium. Characteristic forms of electrical excitability in developing Manduca AL neurons, and their modulation by serotonin, seem likely to play a central role in the functional development of the ALs.

Radial glia development in the mouse olfactory bulb

Radial glia are critical for cell migration and lamination of the cortex. In most developing cortical structures, radial glia, as their name suggests, extend processes from the ventricle to the pia in regular parallel arrangements. However, immunohistochemical labeling from several laboratories suggests that radial glia have a more branched morphology in the olfactory bulb. To investigate the morphology of radial glia in the mouse olfactory bulb we (1) labeled radial glia and olfactory receptor neuron axons at 24-hour intervals by immunohistochemistry; and (2) developed a novel method of generating and applying "nanocrystals" of 1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate (DiI) to the ventricle surface such that the processes of single olfactory bulb radial glia are labeled in the embryonic olfactory bulb. We examined the structure and interactions of radial glia with ingrowing olfactory receptor neuron (ORN) axons in late embryonic olfactory bulb development. These results showed that olfactory bulb radial glia do not form straight parallel structures as do radial glia in the neocortex but rather have a convoluted trajectory from the ventricle to the bulb surface. Moreover, olfactory bulb radial glia consistently extend tangential branches at the level of the internal plexiform layer. Beginning at embryonic day 17.5, two types of radial glia can be distinguished: type I radial glia have a process that extends from the ventricle into the glomerular layer. These apical processes form highly restricted tufts, or "glial glomeruli" at the same time that ORN axons are forming "axonal glomeruli." In type II radial glia the apical process does not enter the glomerular layer but instead ramifies within the external plexiform layer. The tight spatiotemporal relationship between the glomerulization of radial glia processes and ORN axons during development suggest that radial glia processes could play a role in the formation and/or stabilization of mammalian glomeruli.

Regulation of cyclic GMP elevation in the developing antennal lobe of the Sphinx moth, Manduca sexta

In the moth, Manduca sexta, 3',5'-guanosine monophosphate (cGMP) is transiently elevated during adult development in about 100 neurons of the antennal lobe. We demonstrate that nearly all of these neurons are local interneurons of the lateral cluster I, that their capacity to show a strong cGMP response during development is regulated by the steroid hormone 20-hydroxyecdysone, and that in a subpopulation of these neurons cGMP elevation seems to be controlled directly by the gaseous messenger molecule nitric oxide (NO). Treatment with the acetylcholine esterase inhibitor eserine, antennal nerve transection, and electrical stimulation of the antennae suggest that NO/cGMP signaling during development is an activity-dependent process. Besides input from the antennae, input from the central brain and the ventral ganglia is involved in upregulating cGMP in the antennal-lobe neurons. Possible sources are centrifugal aminergic neurons, since application of serotonin and histamine enhances the GMP signal in local interneurons. Comparing the time course of cGMP elevation with events occurring during development leads us to the hypothesis that the NO/cGMP signaling pathway might be involved in synapse formation of a subset of antennal-lobe neurons.

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.

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

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