Effects of the steroid hormone 20-hydroxyecdysone and prior sensory input on the survival and growth of moth central olfactory neurons in vitro

Nitric oxide has differential effects on currents in different subsets of Manduca sexta antennal lobe neurons

Nitric oxide has been shown to regulate many biological systems including olfaction. In the moth olfactory system nitric oxide is produced in the antennal lobe in response to odor stimulation and has complex effects on the activity of both projection neurons and local interneurons. To examine the cell autonomous effects of nitric oxide on these cells, we used patch-clamp recording in conjunction with pharmacological manipulation of nitric oxide to test the hypothesis that nitric oxide differentially regulates the channel properties of these different antennal lobe neuron subsets. We found that nitric oxide caused increasing inward currents in a subset of projection neurons while the effects on local neurons were variable but consistent within identifiable morphological subtypes.

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.

Sensory cell proliferation within the olfactory epithelium of developing adult Manduca sexta (Lepidoptera)

BACKGROUND

Insects detect a multitude of odors using a broad array of phenotypically distinct olfactory organs referred to as olfactory sensilla. Each sensillum contains one to several sensory neurons and at least three support cells; these cells arise from mitotic activities from one or a small group of defined precursor cells. Sensilla phenotypes are defined by distinct morphologies, and specificities to specific odors; these are the consequence of developmental programs expressed by associated neurons and support cells, and by selection and expression of subpopulations of olfactory genes encoding such proteins as odor receptors, odorant binding proteins, and odor degrading enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We are investigating development of the olfactory epithelium of adult M. sexta, identifying events which might establish sensilla phenotypes. In the present study, antennal tissue was examined during the first three days of an 18 day development, a period when sensory mitotic activity was previously reported to occur. Each antenna develops as a cylinder with an outward facing sensory epithelium divided into approximately 80 repeat units or annuli. Mitotic proliferation of sensory cells initiated about 20-24 hrs after pupation (a.p.), in pre-existing zones of high density cells lining the proximal and distal borders of each annulus. These high density zones were observed as early as two hr. a.p., and expanded with mitotic activity to fill the mid-annular regions by about 72 hrs a.p. Mitotic activity initiated at a low rate, increasing dramatically after 40-48 hrs a.p.; this activity was enhanced by ecdysteroids, but did not occur in animals entering pupal diapause (which is also ecdysteroid sensitive).

CONCLUSIONS/SIGNIFICANCE

Sensory proliferation initiates in narrow zones along the proximal and distal borders of each annulus; these zones rapidly expand to fill the mid-annular regions. These zones exist prior to any mitotic activity as regions of high density cells which form either at or prior to pupation. Mitotic sensitivity to ecdysteroids may be a regulatory mechanism coordinating olfactory development with the developmental choice of diapause entry.

Insect neuronal cultures: an experimental vehicle for studies of physiology, pharmacology and cell interactions

The current status of insect neuronal cultures is discussed and their contribution to our understanding of the insect nervous system is explored. Neuronal cultures have been developed from a wide range of insect species and from all developmental stages. These have been used to study the morphological development of insect neurones and some of the extrinsic factors that affect this process. In addition, they have been used to investigate the physiology of sodium, potassium and calcium channels and the pharmacology of acetylcholine and GABA receptors. Insect neurones have also been grown in culture with muscle and glial cells to study cell interactions.

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.

Brain-Derived Neurotrophic Factor Promotes Neurite Growth and Survival of Antennal Lobe Neurons in Brain from the Silk Moth, Bombyx moriin vitro

This study was conducted to investigate effects of brain-derived neurotrophic factor on the neurite growth and the survival rate of antennal lobe neurons in vitro, and secretion of brain-derived neurotrophic factor-like neuropeptide from brain into hemolymph in the silk moth, Bombyx mori. In primary culture of antennal lobe neurons with brain-derived neurotrophic factor, it promoted both a neurite extension of putative antennal lobe projection neurons and an outgrowth of branches from principal neurites of putative antennal interneurons with significance (p<0.05). Brain-derived neurotrophic factor also increased significantly a survival rate of antennal lobe neurons (p<0.05). Results from immunolabeling of brain and retrocerebral complex, and ELISA assay of hemolymph showed that brain-derived neurotrophic factor-like neuropeptide was synthesized by both median and lateral neurosecretory cells of brain, then transported to corpora allata for storage, and finally secreted into hemolymph for action. These results will provide valuable information for differentiation of invertebrate brain neurons with brain-derived neurotrophic factor.

Effects of 20-hydroxyecdysone and serotonin on neurite growth and survival rate of antennal lobe neurons in pupal stage of the silk moth Bombyx mori in vitro

Effects of 20-hydroxyecdysone and serotonin on the morphological development and the survival of antennal lobe neurons from day-2 pupal brains of the silk moth Bombyx mori were investigated in vitro. Four morphologically distinct neuronal types could be identified in the cultured antennal lobe neurons: unipolar, bipolar, multi-polar and projection neurons. Antennal lobe neurons in culture with 20-hydroxyecdysone and serotonin showed different patterns of the morphological development from those described in Manduca sexta. Projection neurons extend their neurites remarkably by 20-hydroxyecdysone in B. mori, but there is no extension from antennal lobe neurons in M. sexta. Multi-polar neurons conspicuously increase only formation of new branches from their primary neurites by serotonin in B. mori, but there are both extension and branching of the neurites in M. sexta. On day-5, antennal lobe neurons in lower titers of 20-hydroxyecdysone had significantly higher survival rates than those in higher titers. Neurons cultured for 7 days at different levels of 20-hydroxyecdysone generally showed significantly lower survival rates than neurons cultured for 5 days under the same conditions.

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.

Development of depolarization-induced calcium transients in insect glial cells is dependent on the presence of afferent axons

Changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) induced by depolarization have been measured in glial cells acutely isolated from antennal lobes of the moth Manduca sexta at different postembryonic developmental stages. Depolarization of the glial cell membrane was elicited by increasing the external K(+) concentration from 4 to 25 mM. At midstage 5 and earlier stages, less than 20% of the cells responded to 25 mM K(+) (1 min) with a transient increase in [Ca(2+)](i) of approximately 40 nM. One day later, at late stage 5, 68% of the cells responded to 25 mM K(+), the amplitude of the [Ca(2+)](i) transients averaging 592 nM. At later stages, all cells responded to 25 mM K(+) with [Ca(2+)](i) transients with amplitudes not significantly different from those at late stage 5. In stage 6 glial cells isolated from deafferented antennal lobes, i.e., from antennal lobes chronically deprived of olfactory receptor axons, only 30% of the cells responded with [Ca(2+)](i) transients. The amplitudes of these [Ca(2+)](i) transients averaged 93 nM and were significantly smaller than those in normal stage 6 glial cells. [Ca(2+)](i) transients were greatly reduced in Ca(2+)-free, EGTA-buffered saline, and in the presence of the Ca(2+) channel blockers cadmium and verapamil. The results suggest that depolarization of the cell membrane induces Ca(2+) influx through voltage-activated Ca(2+) channels into antennal lobe glial cells. The development of the depolarization-induced Ca(2+) transients is rapid between midstage 5 and stage 6, and depends on the presence of afferent axons from the olfactory receptor cells in the antenna.

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.

Developmental changes in the density of ionic currents in antennal-lobe neurons of the sphinx moth, Manduca sexta

Early in metamorphic adult development, action potentials elicited from Manduca sexta antennal lobe neurons are small in amplitude, long in duration, and calcium dependent. As development proceeds, the action potential waveform becomes larger in amplitude, shorter in duration, and increasingly sodium dependent. Whole cell voltage-clamp analysis of Manduca antennal-lobe neurons in vitro has been used to identify voltage-activated currents that contribute to developmental changes in the electrical excitability of these cells. Proximal Branching neurons [putative projection (output) neurons] and Rick Rack neurons (putative local antennal-lobe interneurons) are examined in detail early (pupal stage 5) and late (pupal stage 14) in adult metamorphosis. In both cell types, four voltage-gated and two calcium-dependent ionic currents have been identified. Cell-type-specific changes in the density of sodium, calcium, and potassium currents correlate temporally with changes in cell excitability and spike waveform. Developmental changes in ionic current profiles are accompanied also by the emergence of cell-type-specific response characteristics in the cells. Together with the accompanying paper, this study provides an important foundation for examining the impact of developmental changes in electrical excitability on the growth, electrical properties and connectivity of neurons in central olfactory pathways of the moth.

Dual effect of ecdysone on adult cricket mushroom bodies

Mushroom bodies, which are the main integrative centre for insect sensorial information, play a critical role in associative olfactory learning and memory. This paired brain structure contains interneurons grouped in a cortex, sending their axons into organized neuropiles. In the house cricket (Acheta domesticus) brain, persistent neuroblasts proliferate throughout adult life. Juvenile hormone (JH) has been shown to stimulate this proliferation [Cayre, M., Strambi, C. & Strambi, A. (1994) Nature, 368, 57-59]. In the present study, the effect of morphogenetic hormones on mushroom body cells maintained in primary culture was examined. Whereas JH did not significantly affect neurite growth, ecdysone significantly stimulated neurite elongation. Moreover, ecdysone also acted on neuroblast proliferation, as demonstrated by the reduced number of cells labelled with 5-bromodeoxyuridine following ecdysone application. Heterospecific antibodies raised against ecdysone receptor protein and ultraspiracle protein, the two heterodimers of ecdysteroid receptors, showed positive immunoreactivity in nervous tissue extracts and in nuclei of mushroom body cells, indicating the occurrence of putative ecdysteroid receptors in cricket mushroom body cells. These data indicate a dual role for ecdysone in adult cricket mushroom bodies: this hormone inhibits neuroblast proliferation and stimulates interneuron differentiation. These results suggest that a constant remodelling of mushroom body structure could result from physiological changes in hormone titres during adult life.

Distribution of dopamine receptors and dopamine receptor homologs in the brain of the honey bee, Apis mellifera L

In the brain of the honey bee, Apis mellifera, the radioligands [3H]-SCH23390 and [3H]-spiperone recognise D1- and D2-like receptors, respectively. In addition to being pharmacologically distinct and exhibiting significantly different expression profiles during the lifetime of the bee, [3H]-SCH23390- and [3H]-spiperone-binding sites differ markedly in their distribution within the brain. Estimates of [3H]-SCH23390-binding site density are highest in the somatal rind, whereas [3H]-spiperone-binding sites are most concentrated in the beta lobe neuropil of the mushroom bodies. Molecular cloning techniques have been used to identify two honey bee genes encoding dopamine receptor homologs. The first is the honey bee counterpart of a Drosophila D1-like dopamine receptor and is expressed in the mushroom bodies of both workers and drones. The second is related to D2-like dopamine receptors from vertebrates and is expressed in the brain of the bee, but the precise distribution of expression is not yet known.

Antennal lobe neurons of the honey bee, Apis mellifera, express a D2-like dopamine receptor in vitro

We have used the D2-specific dopamine receptor ligand spiperone [N-(p-aminophenethyl) spiperone; NAPS] coupled to the fluorophore 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD) to visualize dopamine receptors expressed in vitro by neurons of the primary antennosensory centers (antennal lobes) of the brain of the honey bee, Apis mellifera. Changes in the percentage of antennal lobe neurons exhibiting spiperone binding sites over time in culture and at different stages of metamorphic adult development have been investigated. Neurons obtained from animals at all stages of development exhibited spiperone binding sites, but only after 2 days or more in vitro. The percentage of antennal lobe neurons in vitro expressing spiperone binding sites increased significantly with the development of the antennal lobe neuropil. Fluorescently labelled spiperone (120 nM) could be displaced effectively by 1 mM dopamine but not by the same concentration of tyramine, octopamine, or serotonin. In addition, the D2 antagonist spiperone and the D2/D1 antagonist fluphenazine were more effective at displacing the fluorescent ligand than the D1-specific antagonist SCH23390. Our results indicate that Apis antennal lobe neurons in culture express a dopamine receptor and that this receptor is more likely to be D2-like than D1-like in nature. The receptor is expressed early in the metamorphic adult development of the antennal lobe neuropil of the brain.

Preparation of primary cultures and acute slices of the nervous system of the moth Manduca sexta

Interactions among receptor neurons, glial cells and neurons intrinsic to the antennal lobe of the moth underlie the formation of olfactory glomeruli. To isolate these interactions, as well as to understand the effect of a variety of humoral agents on differentiation of the neurons and glia, we generate primary cultures of neurons or glia. These methods are described. In addition. we describe a protocol for producing slice preparations of the developing moth brain that we are using to study the biophysical and morphological development of glial cells. This technique allows us to examine a class of glial cells associated with the glomeruli that otherwise are nearly inaccessible using standard intracellular recording techniques. It also preserves the 3-dimensional arrangement of glia that may strongly influence the development of glomeruli.

Transmitter-induced calcium signalling in cultured neurons of the insect brain

The insect brain offers a unique opportunity for cell biological investigation of calcium signalling in relatively homogeneous neuronal populations, such as the mushroom body neurons. This review summarizes fluorescence imaging experiments with the indicator dye fluo-3 to investigate the calcium responses to transmitter stimulation of honeybee mushroom body neurons in primary culture. Application of acetylcholine (ACh) or nicotine promoted a calcium influx into the cell body and neurites. The increase in intracellular calcium after ACh stimulation was blocked by alpha-bungarotoxin. These results support previous histochemical studies that suggested the expression of nicotinic cholinergic receptors on Kenyon cells. An increase in cytoplasmic calcium levels leads in specific neurons to the generation of nitric oxide (NO) by a Ca2+/calmodulin activated NO synthase. NO is thought to diffuse as a short-lived messenger molecule through the plasma membrane. Using a sensitive photometric assay it could be shown that dissociated cells from the locust brain release NO after stimulation by agents elevating cytoplasmic Ca2+ levels and by the excitatory neurotransmitter ACh. The experiments in dissociated culture therefore allow the demonstration of a Ca(2+)-dependent release during nerve cell depolarization, which is a basic requirement for identifying NO as a messenger molecule.

Long-term culture of fully differentiated adult insect neurons

An identified group of cells, dorsal unpaired median (DUM) neurons, isolated from the central nervous system of adult cockroaches can be grown in vitro for extended periods. These cells often develop morphological characteristics that differ from their in situ appearance but physiological experiments demonstrate that they retain their distinctive in vivo membrane properties. A method for culturing insect CNS neurons, which includes the use of a haemolymph-derived growth-enhancing factor is described, and this technique is compared with other attempts to develop an efficient system for producing in vitro preparations of well-defined, identifiable neurons that would provide a model system for aspects of neuron developmental growth, repair and function.

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.

Enhancement by serotonin of the growth in vitro of antennal lobe neurons of the sphinx moth Manduca sexta

Cell culture experiments have been used to examine the effects of serotonin [5-hydroxytryptamine (5-HT)] on the morphological development of antennal lobe (AL) neurons in the brain of the sphinx moth, Manduca sexta. The majority of cells used in this study were from animals at stage 5 of the 18 stages of metamorphic adult development. 5-HT did not affect the survival of M. sexta AL neurons in culture, but did increase the numbers of cells displaying features characteristic of certain cell types. Three morphologically distinct cell types were examined in detail. The principal effect of 5-HT on these neurons was enhancement of cell growth. The magnitude of responses to this amine was cell-type specific. Site-specific responses to 5-HT were apparent also in one cell type. Our results suggest that the effects of 5-HT can change during the course of metamorphic development. These changes coincide temporally with the development of fast, sodium-based action potentials.

Serotonin-induced changes in the excitability of cultured antennal-lobe neurons of the sphinx moth Manduca sexta

The modulatory actions of 5-hydroxy-tryptamine (5HT or serotonin) on a morphologically identifiable class of neurons dissociated from antennal lobes of Manduca sexta at stages 9-15 of the 18 stages of metamorphic adult development were examined in vitro with whole-cell patch-clamp recording techniques. Action potentials could be elicited from approximately 20% of the cells. These cells were used to examine effects of 5HT (5 x 10(-6) to 5 x 10(-4) M) on cell excitability and action-potential waveform. 5HT increased the number of spikes elicited by a constant depolarizing current pulse and reduced the latency of responses. 5HT also led to broadening of action potentials in these neurons and increased cell input resistance. Modulation of potassium channels by 5HT is likely to contribute to these responses. 5HT causes reversible reduction of at least 3 distinct potassium currents, one of which is described for the first time in this study. Because effects of 5HT on antennal-lobe neurons in culture mimic those observed in situ in the brain of the adult moth, in vitro analysis should contribute to elucidation of the cellular mechanisms that underlie the modulatory effects of 5HT on central olfactory neurons in the moth.

20-Hydroxyecdysone stimulates proliferation of glial cells in the developing brain of the moth Manduca sexta

The steroid hormone 20-hydroxyecdysone (20-HE) controls diverse aspects of neuronal differentiation during metamorphosis in the hawkmoth Manduca sexta. In the present study we have examined the effect of 20-HE on glial cells of the brain during the metamorphic period. The antennal (olfactory) lobe of Manduca provides an ideal system in which to study effects of hormones on glial cells, since three known classes of glial cells participate in its development, and at least one type is critically important for establishment of normal neuronal morphology. These glial cells, associated with the neuropil, form boundaries for developing olfactory glomeruli as a result of proliferation and migration. We determined whether glial cells proliferate in response to 20-HE by injecting a pulse of 20-HE into the hemolymph at different stages of development and monitoring proliferation of all three types of glial cells. Hormone injections at the beginning and end of metamorphic development, when hormone titers are normally low, did not stimulate proliferation of neuropil-associated glial cells. Injections during the period when hormone titers are normally rising produced significant increases in their proliferation. Injections when hormone titers are normally high were ineffective at enhancing their proliferation. One other class of glial cells, the perineurial cells, also proliferate in response to 20-HE. Thus, glial proliferation in the brain is under the control of steroid hormones during metamorphic development.

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.

Factors that influence the development of cultured neurons from the brain of the moth Manduca sexta

During metamorphic adult development, neurons and glial cells in the developing olfactory (antennal) lobes of the moth undergo characteristic and extensive changes in shape. These changes depend on an interplay among these two cell types and ingrowing sensory axons. All of the direct cellular interactions occur against a background of changing steroid hormone titers. Antennal-lobe (AL) neurons dissociated from stage-5 (of 18 stages) metamorphosing animals survive at least 3 wk in primary cell culture. We describe here the morphological influences on AL neurons of (1) exposure to the steroid hormone 20-hydroxyecdysone, (2) exposure to sensory axons, and (3) interactions among the AL neurons. Cultured AL neurons respond only weakly, if at all, to 20-hydroxyecdysone. They do, however, show greater total outgrowth and branching when they had been exposed in vivo to sensory axons. Because there is no direct contact between some of the neuronal types and the sensory axons at the time of dissociation, the increase in outgrowth must have been mediated via a diffusible factor(s). When AL cells (neurons and glia) are plated at high density in low volumes of medium, or when the cells are plated at low density but in the presence of medium conditioned by high-density cultures, neurite outgrowth and cell survival are increased. Nerve growth factor (NGF), epidermal growth factor (EGF), fibroblast growth factor-basic (bFGF), transforming growth factor-beta (TGF beta) and insulin-like growth factor (ILGF) had no obvious effect on neuronal morphology and thus are unlikely to underlie these effects. Our results suggest that the mature shape of AL neurons depends on developmental interactions among a number of diffusible factors.

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.