Serotonin-immunoreactive neurons in the brain of Manduca sexta during larval development and larval-pupal metamorphosis

Serotonergic Neurons in the Brain and Gnathal Ganglion of Larval Spodoptera frugiperda

The fall armyworm Spodoptera frugiperda (S. frugiperda) (Lepidoptera: Noctuidae) is a worldwide, disruptive, agricultural pest species. The larvae of S. frugiperda feed on seedling, leave, and kernel of crops with chewing mouthparts, resulting in reduced crop yields. Serotonin is an important biogenic amine acting as a neural circuit modulator known to mediate lots of behaviors including feeding in insects. In order to explore the serotonergic neural network in the nervous system of larval S. frugiperda, we performed immunohistochemical experiments to examine the neuropil structure of the brain and the gnathal ganglion with antisynapsin and to examine their serotonergic neurons with antiserotonin serum. Our data show that the brain of larval S. frugiperda contains three neuromeres: the tritocerebrum, the deutocerebrum, and the protocerebrum. The gnathal ganglion also contains three neuromeres: the mandibular neuromere, the maxillary neuromere, and the labial neuromere. There are about 40 serotonergic neurons in the brain and about 24 serotonergic neurons in the gnathal ganglion. Most of these neurons are wide-field neurons giving off processes in several neuropils of the brain and the gnathal ganglion. Serotonergic neuron processes are mainly present in the protocerebrum. A pair of serotonergic neurons associated with the deutocerebrum has arborizations in the contralateral antennal lobe and bilateral superior lateral protocerebra. In the gnathal ganglion, the serotonergic neuron processes are also widespread throughout the neuropil and some process projections extend to the tritocerebrum. These findings on the serotonergic neuron network in larval S. frugiperda allow us to explore the important roles of serotonin in feeding and find a potential approach to modulate the feeding behavior of the gluttonous pest and reduce its damage.

Distribution of Serotonin-Immunoreactive Neurons in the Brain and Gnathal Ganglion of Caterpillar Helicoverpa armigera

Serotonin (5-hydroxytryptamine, 5-HT) is an important biogenic amine that acts as a neural circuit modulator. It is widespread in the central nervous system of insects. However, little is known about the distribution of serotonin in the nervous system of the cotton bollworm Helicoverpa armigera. In the present study, we performed immunohistochemical experiments with anti-serotonin serum to examine the distribution of serotonin in the central nervous system of H. armigera larvae. We found about 40 serotonin-immunoreactive neurons in the brain and about 20 in the gnathal ganglion. Most of these neurons are wide-field neurons giving rise to processes throughout the neuropils of the brain and the gnathal ganglion. In the central brain, serotonin-immunoreactive processes are present bilaterally in the tritocerebrum, the deutocerebrum, and major regions of the protocerebrum, including the central body (CB), lateral accessory lobes (LALs), clamps, crepine, superior protocerebrum, and lateral protocerebrum. The CB, anterior ventrolateral protocerebrum (AVLP), and posterior optic tubercle (POTU) contain extensive serotonin-immunoreactive process terminals. However, the regions of mushroom bodies, the lateral horn, and protocerebral bridges (PBs) are devoid of serotonin-immunoreactivity. In the gnathal ganglion, the serotonin-immunoreactive processes are also widespread throughout the neuropil, and some process projections extend to the tritocerebrum. Our results provide the first comprehensive description of the serotonergic neuronal network in H. armigera larvae, and they reveal the neural architecture and the distribution of neural substances, allowing us to explore the neural mechanisms of behaviors by using electrophysiological and pharmacological approaches on the target regions.

Behavioral and genomic characterization of molt-sleep in the tobacco hornworm, Manduca sexta

During the transition from feeding to molting, larval insects undergo profound changes in behavior and patterns of gene expression regulated by the neuroendocrine system. For some species, a distinctive characteristic of molting larvae is presence of a quiescent state sometimes referred to as "molt-sleep". Here, observations of 4th instar Manduca sexta larvae indicate the molting period involves a predominantly quiescent state that shares behavioral properties of adult insect sleep in that it is rapidly reversible and accompanied by a reduced responsiveness to both mildly arousing and noxious stimuli. When subjected to noxious stimuli, molting larvae exhibit locomotory and avoidance behaviors similar to those of inter-molt larvae. Although less consolidated, inter-molt quiescence shares many of the same behavioral traits with molting quiescence. However, when subjected to deprivation of quiescence, inter-molt larvae display a compensatory rebound behavior that is not detected in molting larvae. This suggests that molting quiescence is a specialized form of inactivity that affords survival advantages to molting larvae. RNA-seq analysis of molting larvae shows general reduction in expression of genes encoding GPCRs and down regulation of genes connected with cyclic nucleotide signaling. On the other hand, certain ion channel genes are up-regulated, including transient receptor potential (TRP) channels, chloride channels and a voltage-dependent calcium channel. These findings suggest patterns of gene expression consistent with elevation of quiescent state characteristic of the molt in a model holometabolous insect.

Organization and functional roles of the central complex in the insect brain

The central complex is a group of modular neuropils across the midline of the insect brain. Hallmarks of its anatomical organization are discrete layers, an organization into arrays of 16 slices along the right-left axis, and precise inter-hemispheric connections via chiasmata. The central complex is connected most prominently with the adjacent lateral complex and the superior protocerebrum. Its developmental appearance corresponds with the appearance of compound eyes and walking legs. Distinct dopaminergic neurons control various forms of arousal. Electrophysiological studies provide evidence for roles in polarized light vision, sky compass orientation, and integration of spatial information for locomotor control. Behavioral studies on mutant and transgenic flies indicate roles in spatial representation of visual cues, spatial visual memory, directional control of walking and flight, and place learning. The data suggest that spatial azimuthal directions (i.e., where) are represented in the slices, and cue information (i.e., what) are represented in different layers of the central complex.

A developmental study of serotonin-immunoreactive neurons in the embryonic brain of the marbled crayfish and the migratory locust: evidence for a homologous protocerebral group of neurons

It is well established that the brains of adult malacostracan crustaceans and winged insects display distinct homologies down to the level of single neuropils such as the central complex and the optic neuropils. We wanted to know if developing insect and crustacean brains also share similarities and therefore have explored how neurotransmitter systems arise during arthropod embryogenesis. Previously, Sintoni et al. (2007) had already reported a homology of an individually identified cluster of neurons in the embryonic crayfish and insect brain, the secondary head spot cells that express the Engrailed protein. In the present study, we have documented the ontogeny of the serotonergic system in embryonic brains of the Marbled Crayfish in comparison to Migratory Locust embryos using immunohistochemical methods combined with confocal laser-scan microscopy. In both species, we found a cluster of early emerging serotonin-immunoreactive neurons in the protocerebrum with neurites that cross to the contralateral brain hemisphere in a characteristic commissure suggesting a homology of this cell cluster. Our study is a first step towards a phylogenetic analysis of neurotransmitter system development and shows that, as for the ventral nerve cord, traits related to neurogenesis in the brain can provide valuable hints for resolving the much debated question of arthropod phylogeny.

3D-Reconstructions and Virtual 4D-Visualization to Study Metamorphic Brain Development in the Sphinx Moth Manduca Sexta

DURING METAMORPHOSIS, THE TRANSITION FROM THE LARVA TO THE ADULT, THE INSECT BRAIN UNDERGOES CONSIDERABLE REMODELING: new neurons are integrated while larval neurons are remodeled or eliminated. One well acknowledged model to study metamorphic brain development is the sphinx moth Manduca sexta. To further understand mechanisms involved in the metamorphic transition of the brain we generated a 3D standard brain based on selected brain areas of adult females and 3D reconstructed the same areas during defined stages of pupal development. Selected brain areas include for example mushroom bodies, central complex, antennal- and optic lobes. With this approach we eventually want to quantify developmental changes in neuropilar architecture, but also quantify changes in the neuronal complement and monitor the development of selected neuronal populations. Furthermore, we used a modeling software (Cinema 4D) to create a virtual 4D brain, morphing through its developmental stages. Thus the didactical advantages of 3D visualization are expanded to better comprehend complex processes of neuropil formation and remodeling during development. To obtain datasets of the M. sexta brain areas, we stained whole brains with an antiserum against the synaptic vesicle protein synapsin. Such labeled brains were then scanned with a confocal laser scanning microscope and selected neuropils were reconstructed with the 3D software AMIRA 4.1.

In vitro expression and pharmacology of the 5-HT7-like receptor present in the mosquito Aedes aegypti tracheolar cells and hindgut-associated nerves

We have previously reported the cloning of a 5-hydroxytryptamine receptor (Aedes 5-HT7-like receptor) from adult Aedes aegypti. For functional expression of the Aedes 5-HT7-like receptor, CHO-K1 cells were stably transfected with a receptor expression construct, pC5-HT7. The Aedes 5-HT7-like receptor positively coupled to Gs protein, increasing intracellular cAMP in response to 5-HT; adenylyl cyclase activity was induced in a concentration-dependent, saturable manner. Only 5-HT, and not octopamine, dopamine or tyramine, caused the induction of cAMP. At 10 nM 5-HT a weak synergism was observed between octopamine and 5-HT. Other known agonists of the mammalian 5-HT7 receptor were tested. Their order of potency was: 5-HT >> 5-CT = 8-OH-DPAT >> pimozide. This is the first report on the functional expression of a mosquito neurohormone receptor.

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.

Metamorphic control of cyclic guanosine monophosphate expression in the nervous system of the tobacco hornworm, Manduca sexta

During metamorphosis of Manduca sexta, defined sets of neurons show a dramatic accumulation of cyclic guanosine monophosphate (cGMP). Although many of these cells show low but detectable levels of cGMP during specific developmental windows, these levels are enhanced dramatically during dissection of the central nervous system (CNS). The ability of these neurons to show this induced cGMP expression depends on the developmental stage. Larvae do not show this capacity but it appears during the transition from the larval to the pupal stage. There are two different classes of response: the early expressing neurons start to show a cGMP response at the beginning of the prepupal stage while the late expressing cGMP neurons start at different times during the pupal-adult transition. The former set includes larval neurons that will likely be remodeled during metamorphosis, and a number of them are serotonergic. The late-expressing group also includes some larval cells, but most are adult-specific neurons. At least for one adult-specific cluster, the antennal lobe neurons, the cGMP expression parallels the maturation phase of these cells.

A developmental study of serotonin-immunoreactive neurons in the larval central nervous system of the spider crab Hyas araneus (Decapoda, Brachyura)

Larval development in crabs is characterized by a striking double metamorphosis in the course of which the animals change from a pelagic to a benthic life style. The larval central nervous system has to provide an adequate behavioural repertoire during this transition. Thus, processes of neuronal reorganization and refinement of the early larval nervous system could be expected to occur in the metamorphosing animal. In order to follow identified sets of neurons throughout metamorphosis, whole mount preparations of the brain and ventral nerve cord of laboratory reared spider crab larvae (Hyas araneus) were labelled with an antibody against the neurotransmitter serotonin. The system of serotonin-immunoreactive cell bodies, fibres and neuropils is well-developed in newly hatched larvae. Most immunoreactive structures are located in the protocerebrum, with fewer in the suboesophaegeal ganglia, while the thoracic and abdominal ganglia initially comprise only a small number of serotonergic neurons and fibres. However, there are significant alterations in the staining pattern through larval development, some of which are correlated to metamorphic events. Accordingly, new serotonin-immunoreactive cells are added to the early larval set and the system of immunoreactive fibres is refined. These results are compared to the serotonergic innervation in other decapod crustaceans.

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.

Postembryonic development of gamma-aminobutyric acid-like immunoreactivity in the brain of the sphinx moth Manduca sexta

We have investigated the distribution of immunocytochemical staining for the neurotransmitter gamma-aminobutyric acid (GABA) in the brain of the sphinx moth Manduca sexta during larval, pupal, and adult development. In the larval brain, about 300 neurons are GABA-immunoreactive. All neuropil areas except the mushroom bodies and central complex show intense immunostaining. Only minor changes in the pattern of immunoreactivity occur during larval development. During metamorphosis, changes in immunostaining occur in two phases. Beginning in wandering fifth-instar larvae (stage W2), immunoreactivity appears in numerous neurons of the central body and optic lobe and becomes more intense during early pupal stages. At the same time, GABA-like immunoreactivity disappears in most neuropil areas of the brain and becomes faint in many immunoreactive somata. Neurons with arborizations in the ventrolateral protocerebrum, however, continue to exhibit intense immunostaining during this period, and strongly immunolabeled fibers connect these areas with the ventral nerve cord. The second phase of transformation begins around pupal stage P5/P6, when faint immunostaining appears in many previously nonimmunoreactive somata and most neuropil areas of the brain. In subsequent stages (P8-P10), this immunoreactivity disappears again in most somata, but in certain cell groups, it becomes more intense and gradually develops to the adult pattern. Most larval GABA-immunoreactive neurons appear to survive through metamorphosis into the adult. Neurons in the midbrain that acquire GABA-like immunoreactivity during metamorphosis usually lie adjacent to larval immunostained neurons, suggesting common lineages. The onsets of the two developmental phases of GABA-like immunoreactivity correlate with sharp rises in hemolymph titers of ecdysteroid hormones, suggesting a role for ecdysteroids in the regulation of GABA synthesis. We hypothesize that the disappearance of GABA in many areas of the brain starting 2 days prior to pupation dramatically alters its functional circuitry and thus may account for profound changes in the behavior of the animal.

Improvements for the anatomical characterization of insect neurons in whole mount: the use of cyanine-derived fluorophores and laser scanning confocal microscopy

The optical sectioning capability of the laser scanning confocal microscope was utilized to image dye-filled neurons within whole-mounted insect ganglia. Specific pterothoracic interneurons, in the moth Manduca sexta, were retrogradely filled with Neurobiotin and subsequently visualized with a monoclonal anti-biotin conjugated with one of the following fluorophores: fluorescein, and the newly developed cyanines, Cy3.18 (Cy3) and Cy5.18 (Cy5). Overall, the Cy5 fluorophore was best suited for imaging insect neurons within ganglia. This new methodology allowed us to identify and characterize morphologically a collection of descending multisegmental interneurons with large or small diameter somata. A variety of larger molecular weight (10,000 daltons) tracers was also used to examine the possibility of nonselective filling of neurons with Neurobiotin, possibly through gap junctions. We also investigated the usefulness of Cy3 and Cy5 as fluorophores for transmitter immunostaining of neurons in whole mount. Neurons immunoreactive for serotonin and the neuropeptides, FMRFamide and SCPB, were imaged in the brain and the pterothoracic ganglion. The central projections of some of these immunoreactive neurons were imaged in their entirety.

FMRFamide-like immunoreactivity in the ventral ganglion of the fly Sarcophaga bullata: metamorphic changes

1. Localization of FMRFamide-like immunoreactivity was examined in the ventral ganglion of the fly Sarcophaga bullata using the indirect immunofluorescent method. 2. There are six large cells in the thoracic ganglion which are highly immunoreactive at all stages of development. 3. During metamorphosis the thoracic FLI neurons shift their position from ventrolateral to mid-ventral position and their axons terminate and elaborate a highly immunoreactive dorsal neural sheath. 4. It is suggested that the dorsal neural sheath may function as a neurohaemal organ from which FMRFamide-like substances may be released into the haemolymph to act as neurohormones.

Serotonin immunoreactivity in the optic lobes of the sphinx moth Manduca sexta and colocalization with FMRFamide and SCPB immunoreactivity

In the optic lobes (OLs) of the sphinx moth Manduca sexta, 300-350 neurons per hemisphere are immunoreactive with an antiserotonin antiserum. Two groups of weakly serotonin-immunoreactive cells (OL1) appear to be amacrine cells of the medulla, whereas more intensely immunoreactive cells (OL2) are probably centrifugal neurons that innervate the lobula, medulla, and lamina, as well as the superior protocerebrum. At least one other OL2 cell is a local optic-lobe interneuron with arborizations in the dorsal medulla and lobula. The serotonin-immunoreactive cells are also immunoreactive with an antiserum against Drosophila melanogaster DOPA decarboxylase. All OL2 cells, but not the OL1 cells, are furthermore immunoreactive with an anti-FMRFamide antiserum and an anti-SCPB antiserum. This suggests that neuropeptides related or identical to FMRFamide and SCPB are localized and may serve as cotransmitters with serotonin in OL2 optic-lobe interneurons.

Development and distribution of serotonin in the central nervous system of Manduca sexta during embryogenesis. I. The brain and frontal ganglion

Development of the serotonergic system in the brain and frontal ganglion of the Manduca embryo between 35 and 100% of development was studied immunocytochemically with an antiserum to serotonin (5-HT). Serotonin immunoreactivity was initially detectable at 40-45% development in short fibers in the head region, prior to differentiation of the brain. Immunoreactive cell bodies were first seen in the brain at 60% development, located in the protocerebrum and tritocerebrum. Thick fiber tracts crossing the midline (commissures) could also be observed at this early stage. As development of the embryo progressed, eight immunoreactive cell groups, containing a total of about 38-40 cells, and four commissures with terminal arborizations appeared successively in the brain. From 75 to 100% development, no obvious changes occurred in the number or distribution of cells, and the brain exhibited the same pattern of 5-HT immunoreactive cells, fiber tracts and arborizations as in last instar larvae of Manduca. However, an increase in the size of the cells in both the brain and frontal ganglion was noted between 75 and 80% development, followed by a decrease by 100% development. The frontal ganglion was found to contain three 5-HT immunoreactive cells, which appeared to send bilateral projections into the frontal connectives and the recurrent nerve. During embryonic development, the dendritic arborizations of these frontal ganglion cells increased, while the amount of 5-HT immunoreactivity in the cell bodies decreased. Thus, the serotonergic system first appears in the Manduca embryo at an early stage of development, similar to the situation in other insects as well as vertebrates. By the end of the embryonic period, the same number of serotonergic neurons are present in the brain as in larval and adult Manduca, suggesting that once formed, these cells persist through postembryonic development and metamorphosis.

Development and distribution of serotonin in the central nervous system of Manduca sexta during embryogenesis. II. The ventral ganglia

The distribution of serotonin (5-HT) immunoreactive cells and their projections was mapped in the ganglia of the ventral nerve cord of the tobacco hornworm, Manduca sexta, during embryonic development, using an antiserum to 5-HT. Immunoreactive cells were first seen at 60% development. By 75% of embryonic development, a total of 94 immunoreactive cells were found in the ventral ganglia, including the suboesophageal ganglion. This number decreased to 80 neurons by 100% of development. About 50% of these cells were arranged in the abdominal ganglia and the rest were located in both the suboesophageal and thoracic ganglia. The suboesophageal ganglion exhibited immunoreactive segmental interneurons in areas corresponding to the mandibular, maxillary and labial neuromeres. Two pairs of immunoreactive interneurons were also observed to occur bilaterally in each of the thoracic and abdominal ganglia, with the exception of the prothoracic ganglion. This ganglion contained three pairs of bilaterally arranged immunoreactive neurons as early as 60% of embryonic development. Serotonin immunoreactivity was also found in a number of efferent neurons in the mandibular and labial neuromeres of the suboesophageal ganglion and in the prothoracic, mesothoracic and posterior abdominal ganglia. The occurrence of 5-HT in these efferent neurons suggests an involvement of serotonin in fore- and hindgut function via its effect on the visceral muscles. Immunoreactive lateral longitudinal fibers extended along the entire length of the ventral nerve cord together with dense segmental arborizations. The latter had regressed by the time the embryo was fully developed. This regression of the arborizations in the ganglia at the end of embryonic development indicates that a reorganization of 5-HT innervation occurs to support new larval functions. The time of appearance of 5-HT immunoreactive cells and fibers suggests that serotonin may play a role in the development of the ventral nerve cord.