Vasopressin- and proctolin-like immunoreactive efferent neurons in blowfly abdominal ganglia: development and ultrastructure

Neuroarchitecture of peptidergic systems in the larval ventral ganglion of Drosophila melanogaster

Recent studies on Drosophila melanogaster and other insects have revealed important insights into the functions and evolution of neuropeptide signaling. In contrast, in- and output connections of insect peptidergic circuits are largely unexplored. Existing morphological descriptions typically do not determine the exact spatial location of peptidergic axonal pathways and arborizations within the neuropil, and do not identify peptidergic in- and output compartments. Such information is however fundamental to screen for possible peptidergic network connections, a prerequisite to understand how the CNS controls the activity of peptidergic neurons at the synaptic level. We provide a precise 3D morphological description of peptidergic neurons in the thoracic and abdominal neuromeres of the Drosophila larva based on fasciclin-2 (Fas2) immunopositive tracts as landmarks. Comparing the Fas2 "coordinates" of projections of sensory or other neurons with those of peptidergic neurons, it is possible to identify candidate in- and output connections of specific peptidergic systems. These connections can subsequently be more rigorously tested. By immunolabeling and GAL4-directed expression of marker proteins, we analyzed the projections and compartmentalization of neurons expressing 12 different peptide genes, encoding approximately 75% of the neuropeptides chemically identified within the Drosophila CNS. Results are assembled into standardized plates which provide a guide to identify candidate afferent or target neurons with overlapping projections. In general, we found that putative dendritic compartments of peptidergic neurons are concentrated around the median Fas2 tracts and the terminal plexus. Putative peptide release sites in the ventral nerve cord were also more laterally situated. Our results suggest that i) peptidergic neurons in the Drosophila ventral nerve cord have separated in- and output compartments in specific areas, and ii) volume transmission is a prevailing way of peptidergic communication within the CNS. The data can further be useful to identify colocalized transmitters and receptors, and develop peptidergic neurons as new landmarks.

Proctolin in the post-genomic era: new insights and challenges

Complete understanding of how neuropeptides operate as neuromodulators and neurohormones requires integration of knowledge obtained at different levels of biology, including molecular, biochemical, physiological and whole organism studies. Major advances have recently been made in the understanding of the molecular basis of neuropeptide action in invertebrates by analysis of data generated from sequencing the genomes of several insect species, especially that of Drosophila melanogaster. This approach has quickly led to the identification of genes encoding: (1) novel neuropeptide sequences, (2) neuropeptide receptors and (3) peptidases that might be responsible for the processing and inactivation of neuropeptides. In this article, we review our current knowledge of the biosynthesis, receptor interaction and metabolic inactivation of the arthropod neuropeptide, proctolin, and how the analysis and exploitation of genome sequencing projects has provided new insights.

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

Cellular colocalization of diuretic peptides in locusts: a potent control mechanism

Locust abdominal ganglia are shown to colocalize Locusta-diuretic peptide-, leucokinin I-, and lysine vasopressin-like immunoreactivity in posterior lateral neurosecretory cells. Extracts of abdominal ganglia were partially purified by RP-HPLC then dot immunoassay screened with the same antisera used for immunocytochemistry. Locusta-diuretic peptide-like immunoreactive material coeluted with synthetic Locusta-diuretic peptide, and leucokinin-like immunoreactive material coeluted with locustakinin. Lysine vasopressin-like material eluted in fractions that also showed Locusta-diuretic peptide and leucokinin I immunoreactivity. The diuretic activity of synthetic Locusta-diuretic peptide and locustakinin is demonstrated, and they are shown to act at least additively to promote Malpighian tubule fluid secretion. The immunoreactive neurosecretory cells are assumed to express at least these two peptides, and a model for promoting fluid secretion is proposed.

Morphology of the vasopressin-like immunoreactive (VPLI) neurons in many species of grasshopper

It has previously been shown that the pair of vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria, have cell bodies on the ventral midline of the suboesophageal ganglion and extensive arborisations in all ganglia of the central nervous system. In the present study, we have stained vasopressin-like immunoreactive neurons in 16 additional species of grasshopper, and consistently find this pair of extensive neurons: we assume these to be interspecies homologues. However, the anatomy of these neurons falls into two morphological types: the first, typified by Schistocerca gregaria, has most of its processes distributed in dorsal and lateral neuropil of all ganglia; the second, typified by Locusta migratoria, is equally extensive in its arborisation, but the distribution of branches is shifted peripherally into the optic lobes and the proximal portions of peripheral nerves. It has been suggested that the peripheral fibres in Locusta migratoria are neurohaemal organs for the release of a vasopressin-like diuretic peptide. Our sample of 17 Acridoid species has deliberately selected animals from very different habitats, but our extensive survey of VPLI anatomy shows that peripheral fibres are only present in species from the subfamily Oedipodinae (of which Locusta migratoria is a member) and that no peripheral fibres are present in any of the species from the 4 other subfamilies of the Acridoidea that we have examined. The presence of peripheral fibres is therefore determined by phylogeny and not by habitat. The absence of peripheral VPLI fibres in most grasshopper species examined in this study probably means that the release of putative diuretic hormone from VPLI to control water homeostasis cannot be a conserved function of this ubiquitous neuron. In contrast, the extensive central arborisations and rare antigenicity, which are highly conserved features of the VPLI neuron in all those grasshoppers we have examined, suggests that any conserved role is more likely to be central. A central role for the VPLI neuron has yet to be determined.

Peptidergic regulation of the Limulus midgut

1. The morphology and innervation of the midgut (intestine) in the horseshoe crab, Limulus polyphemus was investigated. The organization of this tissue was examined with routine histology. Radioimmunoassay, immunohistochemistry and high performance liquid chromatography were employed to detect, localize and identify peptidergic innervation of the midgut. The actions of synthetic and native proctolin-like and FMRFamide-like peptides were compared on the isolated midgut preparation. 2. Levels of proctolin and FMRFamide were determined in extracts of Limulus midgut tissue using radioimmunoassay. High levels of proctolin-like immunoreactivity (69.5 +/- 11.3 ng/g) were detected, while levels of FMRFamide-like immunoreactivity (0.8 +/- 0.2 ng/g) were less. Proctolin levels were equally distributed, while the levels of FMRFamide-like immunoreactivity exhibited an anterior bias. 3. Proctolin- and FMRFamide-like immunoreactivities in the Limulus midgut were localized with immunohistochemistry. Proctolin- and FMRFamide-immunoreactive elements were detected in intestinal nerve branches and individual fibers running along the surface of the midgut in whole-mount preparations. In sectioned tissue, staining for these peptides was observed throughout the midgut, typically associated with muscle bands and fibers. Only a few immunoreactive cell bodies were observed. 4. Proctolin, and several FMRFamide-like peptides produced distinct and opposing actions on the isolated Limulus midgut preparation. Proctolin elicited contracture and rhythmic contractions of this tissue, while FMRFamide and N-terminally extended analogs of FLRFamide relaxed gut tension. FMRFamide-like peptides partially reversed the excitatory actions of proctolin. 5. Proctolin- and FMRFamide-like peptides in Limulus midgut extracts were partially characterized with high performance liquid chromatography. One peak of proctolin-like activity was detected on a linear gradient of 18 to 31.5% acetonitrile. The native proctolin-like peptide produced excitatory actions on the isolated midgut preparation which were indistinguishable from those produced by synthetic proctolin. Several peaks of FMRFamide-like bioactivity (Busycon radula protractor muscle assay) were detected with a linear gradient of 5 to 30% acetonitrile. Fractions from two distinct peaks produced FMRFamide-like inhibitory effects on the isolated Limulus midgut preparation. These findings suggest a role for proctolin-like and FMRFamide-like peptides as regulators of intestinal motility in Limulus.

Segmental peptidergic innervation of abdominal targets in larval and adult dipteran insects revealed with an antiserum against leucokinin I

An antiserum against the cockroach neuropeptide leucokinin I (LKI) was used to study peptidergic neurons and their innervation patterns in larvae and adults of three species of higher dipteran insects, the flies Drosophila melanogaster, Calliphora vomitoria, and Phormia terraenovae, as well as larvae of a primitive dipteran insect, the crane fly Phalacrocera replicata. In the larvae of the higher dipteran flies, the antiserum revealed three pairs of cells in the brain, three pairs of ventro-medial cells in the subesophageal ganglion, and seven pairs of ventro-lateral cells in the abdominal ganglia. Each of these 14 abdominal leucokinin-immunoreactive (LKIR) neurons innervates a single muscle of the abdominal body wall (muscle 8), which is known to degenerate shortly after adult emergence. Conventional electron microscopy demonstrates that this muscle is innervated by at least one axon containing clear vesicles and two axons containing dense-cored vesicles. Electron-microscopical immunocytochemistry shows that the LKIR axon is one of these two axons with dense-cored vesicles and that it forms terminals on the sarcolemma of its target muscle. The abdominal LKIR neurons appear to survive metamorphosis. In the adult fly, the efferent abdominal LKIR neurons innervate the spiracles, the heart, and neurohemal regions of the abdominal wall. In the crane fly larva, dorso-medial and ventrolateral LKIR cell bodies are located in both thoracic and abdominal ganglia of the ventral nerve cord. As in the larvae of the other flies, the abdominal ventrolateral LKIR neurons form efferent axons. However, in the crane fly larva there are two pairs of efferent LKIR neurons in each of the abdominal ganglia and their peripheral targets include neurohemal regions of the dorsal transverse nerves. An additional difference is that in the crane fly, a caudal pair of LKIR axons originating from the penultimate pair of dorso-median LKIR cells in the terminal ganglion innervate the hind-gut.

Localization of vasopressin-like immunoreactivity in the CNS of Aplysia californica

Chromatographic and immunological evidence indicates that a vasopressin-like peptide might be present in the CNS of Aplysia californica, and that this peptide may be involved in modulating the behaviour of the gill. Immunocytochemical techniques using antisera raised against various vasopressin-like peptides were used to localize the sites containing these peptides in the CNS of Aplysia. Vasopressin-like immunoreactivity was found to be restricted to one single neuron in the abdominal ganglion and two small neurons located bilaterally in each pedal ganglion. Immunoreactive fibres were present in the neuropile of the abdominal, pedal, pleural and cerebral ganglia, but not in the buccal ganglion. The identification of these neurons provides a morphological localization for vasopressin-like substances detected previously in CNS extracts of Aplysia californica. In addition, the possibility of electrophysiological studies involving the immunoreactive neurons identified in the present paper will allow a more direct approach to study the physiological role of vasopressin-like peptides in Aplysia.

Neurons in the cockroach nervous system reacting with antisera to the neuropeptide leucokinin I

Antisera were raised against the myotropic neuropeptide leucokinin I, originally isolated from head extracts of the cockroach Leucophaea maderae. Processes of leucokinin I immunoreactive (LKIR) neurons were distributed throughout the nervous system, but immunoreactive cell bodies were not found in all neuromeres. In the brain, about 160 LKIR cell bodies were distributed in the protocerebrum and optic lobes (no LKIR cell bodies were found in the deuto- and tritocerebrum). In the ventral ganglia, LKIR cell bodies were seen distributed as follows: eight (weakly immunoreactive) in the subesophageal ganglion; about six larger and bilateral clusters of 5 smaller in each of the three thoracic ganglia, and in each of the abdominal ganglia, two pairs of strongly immunoreactive cell bodies were resolved. Many of the LKIR neurons could be described in detail. In the optic lobes, immunoreactive neurons innervate the medulla and accessory medulla. In the brain, three pairs of bilateral LKIR neurons supply branches to distinct sets of nonglomerular neuropil, and two pairs of descending neurons connect the posterior protocerebrum to the antennal lobes and all the ventral ganglia. Other brain neurons innervate the central body, tritocerebrum, and nonglomerular neuropil in protocerebrum. LKIR neurons of the median and lateral neurosecretory cell groups send axons to the corpora cardiaca, frontal ganglion, and tritocerebrum. In the muscle layer of the foregut (crop), bi- and multipolar LKIR neurons with axons running to the retrocerebral complex were resolved. The LKIR neurons in the abdominal ganglia form efferent axons supplying the lateral cardiac nerves, spiracles, and the segmental perivisceral organs. The distribution of immunoreactivity indicates roles for leucokinins as neuromodulators or neurotransmitters in central interneurons arborizing in different portions of the brain, visual system, and ventral ganglia. Also, a function in circuits regulating feeding can be presumed. Furthermore, a role in regulation of heart and possibly respiration can be suggested, and probably leucokinins are released from corpora cardiaca as neurohormones. Leucokinins were isolated by their myotropic action on the Leucophaea hindgut, but no innervation of this portion of the gut could be demonstrated. The distribution of leucokinin immunoreactivity was compared to immunolabeling with antisera against vertebrate tachykinins and lysine vasopressin.

Vasopressin-immunoreactive neurons and neurohemal systems in cockroaches and mantids

Vasopressin-like neuropeptides of insects are of special interest because of their possible function as hormones and neuromodulators. Therefore, this study was undertaken by using whole-mount immunofluorescent staining by two antisera that recognize different types of vasopressin-like immunoreactive groups of neurons in the cockroaches Periplaneta americana, Leucophaea maderae, Nauphoeta cinerea, Diploptera punctata, and Blaberus discoidalis and in the mantids Litaneuria minor and Tenodera aridifolia sinensis. Using an antiserum to Arg/vasopressin, only two cells, the paired ventral paramedian (PVP) neurons, were immunostained in the central nervous system (CNS) of the cockroaches. These cells are located in the subesophageal ganglion, project throughout the CNS, and appear to be neurosecretory. Their varicose collaterals extend into the dorsal (motor) neuropil of the segmental ganglia, and this neuropil may be the principal site of the release of their neurosecretion. The PVP neurons were also stained by an antiserum to Lys/vasopressin; in addition, this antiserum stained several other groups of neurons, most of which appeared to be neurosecretory. Two pairs of Lys/vasopressin-immunoreactive cells are similar to the PVP neurons in that they are located in the subesophageal ganglion, extend through the ventral nerve cord, have collaterals in the dorsal neuropil of the segmental ganglia, and appear to be neurosecretory within the CNS. In addition, midventral and anteroventral clusters of Lys/vasopressin-immunoreactive neurosecretory neurons in the subesophageal ganglion project neurohemal release sites on the corpora allata. Other types of Lys/vasopressin-immunoreactive neurons include median and lateral neurosecretory cells of the protocerebrum and neurosecretory cells in the tritocerebrum, all of which project to the corpora cardiaca. In the abdominal ganglia there are posterolateral clusters of Lys/vasopressin neurosecretory neurons, and these cells extend to neurohemal release sites on the transverse and lateral cardiac nerves. In mantids the anti-Arg/vasopressin and anti-Lys/vasopressin antisera stained most of the same groups of neurons that these antisera recognized in cockroaches. The results of this study suggest that there are two or more vasopressin-like peptides in cockroaches and mantids and that these peptides may be released either as hormones in the blood or as neurosecretions within the CNS. Their function(s) in these insects remains to be determined.

Galanin immunoreactivity in the blowfly nervous system: localization and chromatographic analysis

In this study chromatographic, immunochemical, and immunocytochemical methods provide evidence of a galanin-like peptide(s) in an invertebrate, the blowfly Phormia terraenovae. The major portion of the galanin-like immunoreactivity (GAL-LI) in fly heads was extractable in acetic acid but not in boiling water, which suggests that the peptide(s) may be highly basic in nature. GAL-LI was present both in the head and body portion of the blowfly in roughly the same amounts. Initial gel filtration data, using a G-50 Sephadex column and a weak phosphate-buffer (pH 6.5) as eluent, suggested that a fly GAL-LI peptide(s) from fly heads, eluting as an apparent single peak, was smaller than porcine GAL(1-29) and GAL(1-15). However, concomitant analysis using a G-25 Sephadex column and acetic acid (0.2 M) as eluent, spread the immunoreactive material over a great portion of the chromatogram, although the main portion of the material eluted in the same size range as porcine GAL(1-29). Taken together, the gel filtration data thus suggest that fly GAL-LI peptide(s) may be highly basic but presumably similar in size to vertebrate GAL(1-29). However, the hydrophobic properties of the fly GAL-LI peptide(s) differ from that of porcine GAL as demonstrated by the presence of several immunoreactive components eluting both early as well as late in the chromatogram when using reverse-phase high performance liquid chromatography (HPLC); early peaks may represent highly basic and/or possibly smaller GAL-immunoreactive peptide(s), whereas later peaks may represent less basic and possibly elongated forms. Immunocytochemistry indicated that GAL-LI was present in the nervous system of the blowfly. About 160 GAL-immunoreactive neurons were found in the brain and subesophageal ganglion, 26 in the fused thoracic ganglion and 30 in the fused abdominal ganglion. In the brain, GAL-immunoreactive fibers supply specific subdivisions of the central body, optic lobe, superior protocerebrum, and tritocerebrum as well as neuropil in the subesophageal ganglia. In the thoracico-abdominal ganglia, GAL-immunoreactive neuron processes are found inside synaptic neuropil as well as in the neural sheath of the ganglia and several of the dorsal nerve roots. Many of the GAL-immunoreactive neurons react also with an antiserum against porcine galanin message associated peptide, a peptide present in the preprogalanin protein. Immunocytochemical double-labeling indicated that some GAL-immunoreactive neurons also reacted with antisera against the molluscan peptides FMRFamide and SCPB, whereas no evidence could be found for colabeling with antisera against tyrosine hydroxylase, substance P and physalaemin.(ABSTRACT TRUNCATED AT 400 WORDS)

The vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria. I. Anatomy

Antiserum to arginine-vasopressin has been used to characterise the pair of vasopressin-like immunoreactive (VPLI) neurons in the locust. These neurons have cell bodies in the suboesophageal ganglion, each with a bifurcating dorsal lateral axon which gives rise to predominantly dorsal neuropilar branching in every ganglion of the ventral nerve cord. There are extensive beaded fibre plexuses in most peripheral nerves of thoracic and abdominal ganglia, but in the brain, the peripheral plexuses are reduced while neuropilar branching is more extensive, although it generally remains superficial. An array of fibres runs centripetally through the lamina-medulla chiasma in the optic lobes. Lucifer Yellow or cobalt intracellular staining of single VPLI cells in the adult suboesophageal ganglion shows that all immunoreactive processes emanate from these two neurons, but an additional midline arborisation (that was only partially revealed by immunostaining) was also observed. Intracellularly staining VPLI cells in smaller larval instars, which permits dye to reach the thoracic ganglia, confirms that there is no similar region of poorly-immunoreactive midline arborisation in these ganglia. It has been previously suggested that the immunoreactive superficial fibres and peripheral plexuses in ventral cord ganglia serve a neurohaemal function, releasing the locust vasopressin-like diuretic hormone, F2. We suggest that the other major region of VPLI arborisation, the poorly immunoreactive midline fibres in the suboesophageal ganglion, could be a region where VPLI cells receive synaptic input. The function of the centripetal array of fibres within the optic lobe is still unclear.

Dual peptidergic innervation of the blowfly hindgut: a light- and electron microscopic study of FMRFamide and proctolin immunoreactive fibers

1. The innervation of the hindgut, rectal valve, rectum and rectal papillae of the adult blowfly, Calliphora erythrocephala, was studied by means of light and electron microscopic immunocytochemistry, using antibodies against the neuropeptides proctolin and FMRFamide. 2. Branches from the abdominal nerves reaching the posterior portion of the gut were found to contain mostly neurosecretory type axons and to innervate the muscle coat of all hindgut structures studied. 3. Some of the axons found in these nerve branches innervating the gut display proctolin- others FMRFamide-like immunoreactivity. Both types of peptidergic axons were found to have abundant terminals in the muscle coat of the hindgut, rectum and rectal valve and in the medulla of the rectal papillae. 4. It is clear that two separate peptidergic systems derived from the abdominal ganglion are supplying the hindgut structures, and, possibly, they use proctolin- and FMRFamide-like peptides respectively as their transmitters or modulators.

Histaminelike immunoreactive neurons innervating putative neurohaemal areas and central neuropil in the thoraco-abdominal ganglia of the flies Drosophila and Calliphora

The fused thoraco-abdominal ganglia of the flies Calliphora vomitoria and Drosophila melanogaster were investigated immunocytochemically with antisera against histamine. In both insect species, 18 histaminelike immunoreactive (HA-IR) neurons were resolved in these ganglia. Six of these neurons have cell bodies in the thoracic neuromeres and 12 in the fused abdominal neuromeres. All cell bodies are situated ventrally. In Calliphora all cell bodies are arranged in a segmental pattern. In Drosophila only the thoracic cell bodies have a segmental arrangement, whereas the abdominal ones are clustered anteriorly close to the last thoracic neuromere. In both species the six thoracic neurons supply processes to the synaptic neuropil in the thoracic neuromeres and to the dorsal neural sheath. The processes in the neural sheath run anteriorly in the lateral portions of the ganglion into the cervical connective. In a few regions laterally arborizing terminals are found in putative neurohaemal areas. These areas were investigated by electron microscopic immunocytochemistry in Calliphora. The HA-IR terminals (containing small granular vesicles) were found outside the "blood-brain barrier" below the acellular basal lamina of the neural sheath. Release of histamine into the circulation is therefore theoretically possible. The central processes of the six thoracic HA-IR neurons may interact synaptically with large numbers of other neurons in the neuropil, and the peripheral varicose fibers from the same HA-IR neurons possibly are neurohaemal release sites. The abdominal HA-IR neurons, in contrast, form extensive arborizations within the synaptic neuropil only. Both thoracic and abdominal neurons have ipsilateral and contralateral branches as well as processes that invade more than one neuromere. A single HA-IR neuron thus invades large volumes of synaptic neuropil. Histamine may be used by neurons of the ventral ganglia both as neurotransmitter (or neuromodulator) and as a circulating neurohormone released from the neural sheath.

Substance P-, FMRFamide-, and gastrin/cholecystokinin-like immunoreactive neurons in the thoraco-abdominal ganglia of the flies Drosophila and Calliphora

Immunocytochemical analysis of the thoraco-abdominal ganglia of the flies Drosophila melanogaster and Calliphora vomitoria revealed neurons displaying substance P- (SPLI), FMRFamide-(FLI), and cholecystokinin-like (CCKLI) immunoreactivity. It could be demonstrated that a number of neurons contain peptides reacting with antisera against all the three types of substances, others were either FLI or CCKLI alone. No neurons displayed only SPLI. Instead, the total number (about 30) of SPLI neurons constitute a subpopulation of the FLI/CCKLI neurons. Many of the identifiable immunoreactive neurons seem to be homologous in the two fly species. One set of six large neurons, termed ventral thoracic neurosecretory neurons (VTNCs), are among those that are SPLI, FLI, and CCKLI in both Drosophila and Calliphora. With the present immunocytochemical technique, the detailed morphology of the VTNCs could be resolved. These neurosecretory neurons supply the entire dorsal neural sheath of the thoraco-abdominal ganglia with terminals, thus forming an extensive neurohaemal area. The VTNCs also have processes connecting the thoracic neuromeres to the cephalic suboesophageal ganglion, as well as extensive arborizations in the thoracic ganglia, suggesting an important role in integrating and/or regulating large portions of the central nervous system, in addition to their neurosecretory function. Most of the other SPLI, FLI, and CCKLI neurons in the thoraco-abdominal ganglia seem to be interneurons. However, there are four FLI neurons that appear to be efferents innervating the hindgut and a few abdominal FLI and CCKLI neurons may be additional neurosecretory cells. From the present study it appears as if neuropeptides related to substance P, FMRFamide and CCK have roles as neurotransmitters/neuromodulators and circulating neurohormones in Drosophila and Calliphora.

Substance P-like immunoreactive neurons in the nervous system of Drosophila

With an antiserum against substance P a small number of neurons could be resolved in great detail in the nervous system of the fruitfly Drosophila melanogaster. In the brain, 10 substance P-like immunoreactive (SPLI) neurons were individually identified. Two of these form extensive bilateral connections with dorsal and ventral protocerebral neuropil. Another two neurons have cell bodies located ventrally in the subesophageal ganglion and processes throughout subesophageal neuropil. In the thoracico-abdominal ganglia 10 SPLI neurons could be identified. Eight of these have large cell bodies located ventrally in thoracic ganglia and two have small cell bodies located posteriorly in the abdominal ganglia. Six of the 8 thoracic SPLI neurons could be resolved in detail and were found to form: (1) processes in dorsal thoracic and abdominal neuropil as well as processes running through the cervical connective into the subesophageal ganglia; and (2) processes running into the dorsal neural sheath of the thoracic ganglia. The latter processes form an extensive network of varicose terminals over the thoracic ganglia. Our results indicate that a substance P-like neuropeptide can act as a neurohormone released into the circulation from terminals in the neural sheath as well as a neurotransmitter/neuromodulator released by interneurons in the brain.