Central Projections of Gustatory Receptor Neurons in the Medial and the Lateral Sensilla Styloconica of Helicoverpa armigera Larvae

Food selection behavior of lepidopteran larvae is predominantly governed by the activation of taste neurons present in two sensilla styloconica located on the galea of the maxilla. In this study, we present the ultrastructure of the sensilla styloconica and the central projection pattern of their associated receptor neurons in larvae of the heliothine moth, Helicoverpa armigera. By means of light microscopy and scanning electron microscopy, the previous findings of two morphologically fairly similar sensilla comprising a socketed conic tip inserted into a large peg were confirmed. However, the peg size of the medial sensillum was found to be significantly bigger than that of the lateral sensillum. The sensory neurons derived from each sensillum styloconicum were mapped separately using anterograde staining experiments combined with confocal laser-scanning microscopy. For determining the afferents’ target regions relative to each other, we reconstructed the labeled axons and placed them into a common reference framework. The sensory axons from both sensilla projected via the ipsilateral maxillary nerve to the suboesophageal ganglion and further through the ipsilateral circumoesophageal connective to the brain. In the suboesophageal ganglion, the sensory projections targeted two areas of the ipsilateral maxillary neuropil, one located in the ventrolateral neuromere and the other adjacent to the neuromere midline. In the brain, the axon terminals targeted the dorso-anterior area of the ipsilateral tritocerebrum. As confirmed by the three-dimensional reconstructions, the target regions of the neural projections originating from each of the two sensilla styloconica were identical.     

Central projections from contact chemoreceptors of the locust ovipositor and adjacent cuticle

Contact chemoreceptors (basiconic sensilla) located on the ovipositor and genital segments of the locust serve to control the chemical features of the substrate before and during oviposition. They occur dispersed and also crowded in fields between mechanosensory exteroceptors sensitive to touch or wind (trichoid and filiform sensilla). The central nervous projections of the four chemosensory and one mechanosensory neurons from single basiconic sensilla were stained selectively, focusing on receptors on the ovipositor valves, which usually contact the substrate during the pre-oviposition probing movements. All axons and neurites from one contact chemoreceptor usually stay close together in most of their projections. Segregation occurs mainly when single axons terminate in one neuromere while the others proceed to a different neuromere or ganglion. For projections from one chemoreceptor, there is evidence neither for functional segregation of mechanosensory from chemosensory afferent terminals nor for specific segregation between different chemosensory afferents. The projections from sensilla of dorsal cuticle tend to project rather uniformly along the midline of the terminal ganglion. Comparative staining of touch- and wind-sensitive hair receptor neurons shows mostly central projections, similar to those of neighbouring contact chemoreceptors. From the typical intersegmental projections of most primary afferents and from the lack of segregation into glomerular structures, we conclude that integration of chemosensory information from the genital segments is distributed in the terminal and the 7th abdominal ganglion.     

Primary sensory projections from the labella to the brain of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

Golgi silver impregnation of sensory neurons arising from labellar taste sensilla of Drosophila melanogaster Meigen (Diptera : Drosophilidae) revealed 7 distinct types I-VII of primary (sensory) fibres projecting to the suboesophageal ganglion (SOG) of the brain. Each fibre was classified on the bases of the neuropil volume occupied by its terminal arborisation, the shape of neuropil region receiving the arborisations and the detailed morphology of the arborisations. The primary sensory fibre projections from the labella are confined to the SOG where they project mainly in the anterior and central neuropils. No labellar sensory fibres project to posterior SOG. Of these 7 types of sensory fibres, three (III, IV and VII) show ipsilateral projections, while others have both ipsi-, and contralateral branches.Four types of interneurons are suggested to be associated with taste perception. Type A interneurons are local interneurons with arborisations confined only to the taste sensory neuropil of the SOG. The types B - D interneurons are interganglionic/output neurons with axons projecting to various brain regions-SOG, calyces of the mushroom bodies, tritocerebrum and thoracic ganglia. These projections suggest that more than one centre (SOG, tritocerebrum, calyces of the mushroom bodies and thoracic ganglia) are involved in processing gustatory information.     

Cephalic sensory pathways in the central nervous system of larval Manduca sexta (Lepidoptera : Sphingidae)

Central projections of neurons innervating sensory structures on the head of larval Manduca sexta were traced by using methods of anterograde cobalt-diffusion. Regions of the deutocerebrum and tritocerebrum in the brain receive input from the antenna, labrum, maxilla, labial palps, hypopharynx and other unidentified regions of the buccal cavity. Antennal, maxillary and labial inputs project to the larval antennal centre (LAC) of the deutocerebrum. Stemmatal neurons and a few antennal neurons project into the protocerebrum. The suboesophageal ganglion (SEG) receives input from mechanosensory neurons in all parts of the head and its sensory appendages. Some mechanosensory neurons project further to the first thoracic ganglion. In addition to receiving input from chemosensory neurons of the maxilla, the SEG may also receive chemosensory input from epipharyngeal sensilla of the labrum.     

Function and central projections of gustatory receptor neurons on the antenna of the noctuid moth Spodoptera littoralis

Chemosensory information is crucial for most insects to feed and reproduce. Olfactory signals are mainly used at a distance, whereas gustatory stimuli play an important role when insects directly contact chemical substrates. In noctuid moths, although the antennae are the main olfactory organ, they also bear taste sensilla. These taste sensilla detect sugars and hence are involved in appetitive learning but could also play an important role in food evaluation by detecting salts and bitter substances. To investigate this, we measured the responses of individual taste sensilla on the antennae of Spodoptera littoralis to sugars and salts using tip recordings. We also traced the projections of their neuronal axons into the brain. In each sensillum, we found one or two neurons responding to sugars: one NaCl-responsive and one water-sensitive neuron. Responses of these neurons were dose-dependent and similar across different locations on the antenna. Responses were dependent on the sex for sucrose and on both sex and location for glucose and fructose. We did not observe a spatial map for the projections from specific regions of the antennae to the deutocerebrum or the tritocerebrum/suboesophageal ganglion complex. In accordance with physiological recordings, back-fills from individual sensilla revealed up to four axons, in most cases targeting different projection zones.     

Sensilla on the palps and legs of the adult soft tick argas persicus oken (IXODOIDEA: ARGASIDAE) and their projections to the central nervous system

The sensory structures present on the palps and legs of adult Argas persicus Oken (Ixodoidea: Argasidae) were studied by light, scanning and transmission electron microscopy. The number, distribution, surface morphology and the fine structure of the prominent sensilla present on these appendages were determined. The palps have 2 morphologically prominent types of sensilla: one with a grooved surface of the hair and the other having a non-grooved hair. The TEM distinguishes at least 4 prominent subtypes in grooved sensilla with single or double lumina and dendrites occupying the periphery of the central lumen or distributed all over the central lumen. Amongst the sensilla with non-grooved hair-shaft, a rare type of Olfactory Mechanoreceptive (OM) sensillum was found on the palps and the first legs of A. persicus. At the base of the hair-shaft, the OM sensillum has 2 mechanosensory dendrites. The hair-shaft of the sensillum has a porous cuticle, characteristic of an olfactory sensillum. The lumen of the hair-shaft is invested with branching dendrites from 3–8 neurons, which are surrounded by 4 sheath cells. The sensilla on the legs, including those present in the Hallers organ, are of at least 3 prominent categories. (i) Single wall with un-innervated hair-shaft. (ii) Single wall, multiporous sensillum with dendrites present in the hair shaft. (iii) Double walls with spoke channels and dendrites present in the central lumen. Sensory projections from the crown of sensilla located on the distal end of the palp extend to the palpal and suboesophageal (SOG) ganglia. Projections in the SOG extend further to the contralateral side. Sensilla in the Hallers organ project to the first pedal ganglion and to the anterodorsal region of supraoesophageal ganglion. As expected, the primary sensory projections from the sensilla of the other 3 legs extend to the respective pedal ganglia.     

Sensory projections of identified coxal hair sensilla of the scorpionHeterometrus fulvipes (Scorpionidae)

The topography of long hair sensilla on the coxae of walking legs and pedipalps of the scorpionHeterometrus fulvipes is described. Identified long hair sensilla are cobalt filled, and central projections of sensory fibres are reported for the first time in the suboesophageal ganglion of this scorpion. The afferent fibres arising from each long hair sensilla segregate into ventral, dorsomedial and dorsal tracts upon their entry into the suboesophageal ganglion. These transverse tracts bifurcate towards the middle of the leg neuromere and form three ipsilateral, plurisegrnental, longitudinal sensory pathways. Filling a pair of bilaterally distributed long hair sensilla shows bilaterally arranged longitudinal afferent tracts interconnected by distinct transverse commissures. Similar patterns of sensory projections are observed when filling homologous hairs on other legs and pedipalps. Numerous fine collaterals arise from the longitudinal sensory trancts that subdivide and end in small blebs presumed to be presynaptic endings. The dorsal and dorsomedial longitudinal tracts and their respective commissures are in close association with the dendritic arborisations of pedipalpal and leg motor neurons, suggesting direct contact between them. The probable functions of these multisegmental hair afferent pathways are discussed.     

Leu-callatostatin gene expression in the blowflies Calliphora vomitoria and Lucilia cuprina studied by in situ hybridisation: comparison with Leu-callatostatin confocal laser scanning immunocytochemistry

In situ hybridisation studies using a digoxigenin-labelled DNA probe encoding the Leu-callatostatin prohormone of the blowflies Calliphora vomitoria and Lucilia cuprina have revealed a variety of neurones in the brain and thoracico-abdominal ganglion, peripheral neurosecretory neurones, and endocrine cells of the midgut. With two exceptions, the hybridising cells are the same as those previously identified in immunocytochemical studies of sections and whole-mounts using Leu-callatostatin COOH-terminal-specific antisera. Within the brain and suboesophageal ganglion, there is a variety of neurones ranging from a single pair of large cells situated in the dorsal protocerebrum, to the several pairs of neurones in the tritocerebrum, some of which, in immunocytochemical preparations, can be seen to project via axons in the cervical connective to the thoracico-abdominal ganglion. In the medulla of the optic lobes, numerous small interneurones hybridise with the probe, as do clusters of similar-sized neurones close to the roots of the ocellar nerves. These results indicate that the Leu-callatostatin neuropeptides of the brain play a variety of roles in neurotransmission and neuromodulation. There are only three pairs of Leu-callatostatin-immunoreactive neurones in the thoracico-abdominal ganglion, at least two pairs of which project axons along the median abdominal nerve to provide extensive innervation of the hindgut. The Leu-callatostatin peripheral neurosecretory cells are located in close association with both nerve and muscle fibres in the thorax. In addition to neuronal Leu-callatostatin, the presence of the peptide and its mRNA has been demonstrated in endocrine cells in the posterior part of the midgut. These observations provide an example of a named brain/gut peptide in an insect.     

Serotonin-immunoreactive neurons in the antennal lobes and suboesophageal ganglion of the honeybee

Summary We have used immunohistochemical methods to investigate the morphology of identified, presumptive serotonergic neurons in the antennal lobes and suboesophageal ganglion of the worker honeybee. A large interneuron (deutocerebral giant, DCG) is described that interconnects the deutocerebral antennal and dorsal lobes with the suboesophageal ganglion and descends into the ventral nerve chord. This neuron is accompanied by a second serotonin-immunoreactive interneuron with projections into the protocerebrum. Two pairs of bilateral immunoreactive serial homologues were identified in each of the three suboesophageal neuromeres and were also found in the thoracic ganglia. With the exception of the frontal commissure, no immunoreactive processes could be found in the peripheral nerves of the brain and the suboesophageal ganglion. The morphological studies on the serial homologues were extended by intracellular injections of Lucifer Yellow combined with immunofluorescence.     

An identified dorsal unpaired median neurone and bilaterally projecting neurones exhibiting bovine pancreactic polypeptide-like/FMRFamide-like immunoreactivity in abdominal ganglia of the migratory locust

Summary Three antisera were used to study the distribution and anatomy of bovine pancreatic polypeptide (BPP)-like/FMRFamide-like immunoreactive neurones within the unfused abdominal ganglia of the migratory locust, Locusta migratoria. All the antisera used stained two or more clusters of perikarya, localized anteriorly and posteriorly near the midline within each unfused abdominal ganglion. Double labelling experiments with intracellular dye injection, or differential backfilling, combined with subsequent immunostaining were carried out to identify these neurones. Two of the antisera (antisera 1 and 2, both raised against FMRFamide) stained three groups of midline neurones, located anterior dorsal, anterior ventral and posterior dorsal within the ganglion. Neurones of the former of these two clusters projected via the anterior median nerve to a neurohaemal organ. The posterior cluster of midline cells comprised immunopositive perikarya all but one of which also projected via the anterior median nerve to innervate the neurohaemal organ. Double labelling with Lucifer yellow and antisera 1 and 2 showed that the remaining neurone was the previously identified doral unpaired median (DUM)heart1 neurone. The third antiserum (AK141), also raised against FMRFamide, stained neurones within an anterior dorsal cluster, and in a posterior cluster. Double labelling with differential Co²⁺/Ni²⁺-backfilling and the antiserum 3 (AK141) demonstrated that the large neurones of both clusters belonged to the population of bilaterally projecting neurones (BPNs), including the DUMheart1 neurone. Since the antisera cross-react with BPP and fail to label neurones when preadsorped with BPP or FMRFamide, we conclude that the labelled neurones contain polypeptides of the FMRFamide/BPP-family.     

The central projection of cephalic mechanosensory axons in the haematophagous bug Triatoma infestans

The projections of mechanosensory hairs located on the dorsal and lateral head of the adult haematophagous bug Triatoma infestans were analyzed by means of cobalt filling. Axons run into the anterior and posterior tegumentary nerve and project through the brain to the ventral nerve cord. The fibres are small in diameter and run as a fascicle. Some branches run into suboesophageal and prothoracic centres; others run as far as to the mesothoracic ganglion. These sensory projections resemble that of wind-sensitive head hairs of the locust. The functional role of this sensory system in this species is discussed.     

Control of diapause-factor secretion from the suboesophageal ganglion in the silkworm,Bombyx mori: The roles of the protocerebrum and tritocerebrum

To localize any functional areas in the brain which control the secretion of the diapause factor from the suboesophageal ganglion in the Bombyx silkworm, various parts of the brain were excised or transected surgically. It was demonstrated that cortices slightly lateral to the median line on the dorsal side of the protocerebrum (Lmdp) are closely related to the inhibitory mechanism in the pupa producing non-diapause eggs. On the other hand, cortices of anterolateral areas on the ventral side of the protocerebrum (Alvp) participate in the stimulatory mechanism in the pupa producing diapause eggs. Moreover, it is suggested that the tritocerebrum is competent to stimulate secretion of the diapause factor from the suboesophageal ganglion not only in the pupae producing diapause eggs but also in the pupae producing non-diapause eggs. From these results, we propose a possible relationship between the Alvp, Lmdp and tritocerebrum concerning regulation of diapause-factor secretion from the suboesophageal ganglion.     

Projection pattern of poreplate sensory neurones in honey bee worker,Apis mellifera L. (Hymenoptera : Apidae)

The projections of olfactory receptor cells of the poreplate sensilla were studied in the worker honey bee, Apis mellifera L. (Hymenoptera : Apidae) by filling single sensilla iontophoretically with cobalt chloride. Successful fillings of individual sensilla lead to staining of one to 22 sensory neurones. All stained receptor cell axons are uniglomerular. Seven fillings of poreplates from the 5th flagellar segment in different animals were compared to analyse the distribution patterns of the receptor cell axons in the antennal lobe. The sensory neurones of individual poreplates project to widely distributed glomeruli in the antennal lobe. The projection patterns of different poreplates are not the same, but may be overlapping.     

Regeneration of the tibia and somatotopy of regenerated hair sensilla in Schistocerca gregaria (Forskål)

After injury many arthropods are able to regenerate lost body parts and their innervation. Here, regeneration was studied in the desert locust Schistocerca gregaria after amputation of the midleg tibia and tarsus in the first larval instar. A regenerate was formed first in the third larval instar and it increased in size with each larval moult. The regenerate was always unsegmented and remained much shorter than the intact leg parts. The growth rate was initially rather high and decreased thereafter to that of intact parts. The amputation also influenced the growth rate of proximal leg parts (femur and trochanter) resulting in shortened leg segments. The regenerate carried many sense organs like trichoid sensilla and canal sensilla. The primary mechanosensory neurons of the trichoid sensilla projected somatotopically into the mesothoracic ganglion. A comparison of these projections from intact leg segments and regenerates showed a regrow into the target neuropil areas and a restoration of the somatotopy. Intact sensilla on the injured leg and regenerated sensilla expanded their central projections lateral-medially.     

The innervation of the wind-sensitive head hairs of the locust, Schistocerca gregaria

ABSTRACT. The innervation of the locust head hairs was investigated by filling the sub-cuticular neurones with cobalt and by recording antidromic spikes at individual hairs when the circumoesophageal and cervical connectives were stimulated. The central projections from most head hairs ( c. 80%) terminate in the suboesophageal ganglion, whereas about 20% extend into the thoracic ganglia. Sensilla with projections to the thoracic ganglia are structurally no different from those whose fibres terminate in the suboesophageal ganglion and they are scattered throughout the hair fields with no consistent positions in different individuals. We have re-investigated the 'accessory response' (Camhi, 1969) and conclude that it is not indicative of a separate afferent or efferent system but is simply cross-talk from adjacent axons. We relate our findings to the fibre composition of the dorsal tegumentary nerve.     

Localization of octopaminergic neurones in insects

This paper reviews data on the localization of octopaminergic neurones revealed by immunocytochemistry in insects, primarily the locusts Schistocerca gregaria and Locusta migratoria, cricket Gryllus bimaculatus, and cockroach Periplaneta americana. Supporting evidence for their octopaminergic nature is mentioned where available. In orthopteran ventral ganglia, the major classes of octopamine-like immunoreactive (-LI) neurones include: (1) efferent dorsal and ventral unpaired median (DUM, VUM) neurones; (2) several intersegmentally projecting DUM interneurones in the suboesophageal ganglion; other DUM interneurones are probably GABAergic; (3) a pair of anterior median cells in the prothoracic ganglion; (4) a single pair of ventral cells in most thoracic and some other ganglia; these appear to be plurisegmentally projecting interneurones. Eight categories of octopamine-LI neurones occur in the orthopteran brain. The basic projections of three types are described here: one class project to the optic lobes to form wide field projections. Another type descends to cross into the tritocerebral commissure and may invade the contralateral brain hemisphere. A further class is the median neurosecretory cells with axons in the nervi corpori cardiaci I. Available data for the honey bee Apis mellifera and moth Manduca sexta indicate that the octopamine-LI cell types found in orthopterans also occur in holometabolous insects. Immunocytochemical evidence suggests that some octopaminergic DUM cells contain an FMRFamide-related peptide and the amino acid taurine as putative cotransmitters.     

棉铃虫幼虫脑和咽下神经节的三维结构构建

【目的】解剖棉铃虫Helicoverpa armigera (Hübner) 5龄幼虫脑和咽下神经节及其内部神经髓形态结构,并分析和构建幼虫脑和咽下神经节以及各神经髓的三维结构模型。【方法】采用免疫组织化学方法解剖脑和咽下神经节的内部神经髓结构,利用激光共聚焦显微镜获取脑和咽下神经节扫描图像,然后利用AMIRA 三维图像分析软件进行图像分析,从而构建脑和咽下神经节的三维结构模型,并测量脑和咽下神经节以及内部各神经髓的体积,并分析了相对比例。【结果】 棉铃虫5龄幼虫脑和咽下神经节由围咽神经索连接在一起。脑主要由前脑、中脑和后脑3部分组成。前脑内包括视叶、蕈形体和中央体等形态结构较明显的神经髓。此外,前脑还包括其他位于脑的左右两侧以及背侧和腹侧大量神经髓区域,约占脑总神经髓的59.65%。这些神经髓区域边界不明显。中脑主要包括1对触角叶;后脑位于脑的腹侧和触角叶的下方,体积较小。咽下神经节由3个神经节融合构成,从前到后分别为上颚神经节、下颚神经节和下唇神经节,由于融合的紧密程度高,3个神经节间的边界不明显。【结论】阐明了棉铃虫5龄幼虫脑和咽下神经节的神经髓形态结构,构建了脑和咽下神经节以及内部神经髓的三维结构模型。三维模型可以任意旋转,能从任何角度观察脑、咽下神经节和内部不同神经髓的结构及其它们之间的空间关系。本研究结果对研究棉铃虫脑和咽下神经节信息接收、处理及调控行为的机制奠定了解剖学基础。     

Fine structure and primary sensory projections of sensilla on the maxillary palp of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

Three types of hairs were identified on the maxillary palp of Drosophila melanogaster Meigen (Diptera : Drosophilidae): (i) single-walled, multiporous sensilla basiconica, which constitute 75% of the innervated hairs; (ii) thick walled non-porous sensilla trichodea, which make up the remaining 25% of the innervated hairs; and (iii) numerous spinules, which are un-innervated. These sensilla basiconica uniformly contain 2 bipolar sense cells, whereas sensilla trichodea have a single dendrite with a tubular body at the base of each hair. A majority of the sensilla basiconica is located on the distal half of the dorsal surface, whereas sensilla trichodea are positioned on the tip and entire ventrolateral ridge of the palp. Approximately 125 axons of the sense cells join to form a single nerve. The structure of sensilla basiconica and sensilla trichodea suggests that they are olfactory and mechanosensory respectively. The contact chemoreceptors (gustatory sensilla) are conspicuously absent on the maxillary palp.Golgi silver impregnations and cobalt fills show that the primary sensory fibres from sensilla trichodea and sensilla basiconica on the maxillary palp project in the posterior suboesophageal ganglion (SOG) and the antennal lobe respectively. A single fibre projects separately either in the SOG or in the antennal lobe. In the antennal lobe, the input received from sensilla basiconica is usually bilateral and at least 5 glomeruli are innervated symmetrically on either side from both the palps.This study suggests that the sensory neurons are capable of making selective projections in the specific regions of the brain. Accordingly, the fibres from a sensillum project to the brain with respect to their functions and the individual glomeruli represent functional units of the brain, receiving inputs in a characteristic combination.     

A map of taste neuron projections in the Drosophila CNS

We provide a map of the projections of taste neurons in the CNS of Drosophila. Using a collection of 67 GAL4 drivers representing the entire repertoire of Gr taste receptors, we systematically map the projections of neurons expressing these drivers in the thoracico-abdominal ganglion and the suboesophageal ganglion (SOG). We define 9 categories of projections in the thoracico-abdominal ganglia and 10 categories in the SOG. The projection patterns are modular, and can be interpreted as combinations of discrete pattern elements. The elements can be interpreted in terms of the taste organ from which the projections originate, the structures from which they originate, and the quality of taste information that they represent. The extensive diversity in projection patterns provides an anatomical basis for functional diversity in responses elicited by different taste stimuli.     

Organizational principles of outputs from Dipteran brains

Two major classes of Descending Neurones (DNs) originate in fly cerebral ganglia: (1) uniquely identifiable DNs, most of which arise preorally in duetocerebral neuropil of the supraoesophageal ganglion, the brain proper (2) parallel projecting DNs (PDNs) most originating in the suboesophageal ganglion. Brain DNs receive inputs directly from sensory systems and indirectly via higher center and peptidergic interconnections of the protocerebrum. Direct inputs include primary mechanosensory afferents, first order relay neurones from the olfactory lobes and ocellar receptor cups, and higher order visual neurones that interact with retinotopic inputs from compound eyes. Uniquely identifiable DNs arising in the brain are arranged in uniquely identifiable clusters. Each cluster receives a unique combination of inputs which are shared wholly or in part by the dendritic trees of its constituent DNs. Axons arising from a cluster diverge to different targets in the thoracic ganglia. PDNs form groups of as many as 40 neurones, as determined from outgoing axon bundles. Dendrites of PDNs are thin and diffuse, and arborize amongst collaterals from through-going axons of descending neurones arising in the brain. Axon bundles of PDNs are typically organized in rather simple ladder-like patterns in thoracic ganglion. A third type of uniquely identifiable DN also arises in the suboesophageal ganglion but does not seem to be arranged in clusters.