Peripheral Neurosecretory Cells Research Articles

Recent topics on the regulatory mechanism of ecdysteroidogenesis by the prothoracic glands in insects.

Molting and metamorphosis are strictly regulated by steroid hormones known as ecdysteroids. It is now widely recognized that ecdysteroid biosynthesis (ecdysteroidogenesis) in the prothoracic gland (PG) is regulated by the tropic factor prothoracicotropic hormone (PTTH). However, the importance of PTTH in the induction of molting and metamorphosis remains unclear, and other mechanisms are thought to be involved in the regulation of ecdysteroidogenesis by the PG. Recently, new regulatory mechanisms, prothoracicostatic factors, and neural regulation have been explored using the silkworm, Bombyx mori, and two circulating prothoracicostatic factors, prothoracicostatic peptide (PTSP) and Bommo-myosuppressin (BMS), have been identified. Whereas PTTH and BMS are secreted from the brain, PTSP is secreted from the peripheral neurosecretory system – the epiproctodeal gland – during the molting stage. The molecular basis of neural regulation of ecdysteroidogenesis has been revealed for the first time in B. mori. The innervating neurons supply both Bommo-FMRF related peptide (BRFa) and orcokinin to maintain low levels of ecdysteroids during the feeding stage. These complex regulatory mechanisms – involving tropic and static factors, peripheral neurosecretory cells as well as the central neuroendocrine system, and neural regulation in addition to circulating factors collaborate to regulate ecdysteroidogenesis. Thus, together they create the finely tuned fluctuations in ecdysteroid titers needed in the hemolymph during insect development.

Bombyx prothoracicostatic peptides activate the sex peptide receptor to regulate ecdysteroid biosynthesis

Insect molting and metamorphosis are induced by steroid hormones named ecdysteroids, whose production is regulated by various neuropeptides. We cloned the gene and analyzed the expression of the prothoracicostatic peptide, a unique neuropeptide shown to suppress the production of ecdysteroids in the prothoracic gland of the silkworm, Bombyx mori. We also characterized a Bombyx G protein-coupled receptor, which has previously been identified as an ortholog of the Drosophila sex peptide receptor, as a functional prothoracicostatic peptide receptor. This receptor responded specifically to the prothoracicostatic peptides when examined using a heterologous expression system. The receptor was highly expressed in the prothoracic gland on the day before each larval and pupal ecdysis, when prothoracicostatic peptides are synthesized at a high level in the epiproctodeal glands. These results suggest that the sex peptide receptor functions as a prothoracicostatic peptide receptor in Bombyx and that the peripheral neurosecretory cells as well as the central neuroendocrine system play stage-specific roles in regulating ecdysteroidogenesis.

Insect ion transport peptides are derived from alternatively spliced genes and differentially expressed in the central and peripheral nervous system

Insect ionic and fluid homeostasis relies upon the Malpighian tubules (MT) and different hindgut compartments. Primary urine formed in MTs is finally modified by ion, solute and water reabsorptive processes primarily in the hindgut under the control of several large peptide hormones. One of these, the ion transport peptide (ITP), is a chloride transport-stimulating and acid secretion-inhibiting hormone similar to crustacean hyperglycaemic hormones (CHHs). In locusts, moths and fruit flies, ITP together with the slightly longer ITPL isoforms, inactive in hindgut bioassays, arise by alternative splicing from very similar itp genes. ITP and ITPL are differentially distributed in (1) pars lateralis/retrocerebral complex neurosecretory cells (NSCs) containing both splice forms, (2) interneurons with either one of the splice forms, (3) hindgut-innervating abdominal ITP neurons (in Drosophila only), and (4) intrinsic, putative sensory NSCs in peripheral neurohaemal perisympathetic/perivisceral organs or transverse nerves (usually containing ITPL). Both splice forms occur as hormones released into the haemolymph in response to feeding or stress stimuli. ITPL mainly released from the peripheral NSCs is discussed as a competitive inhibitor (as established in vitro) of ITP action on yet to be identified hindgut ITP receptors. Furthermore, some evidence has been provided for possible ecdysis-related functions of ITP and/or ITPL in moths. The comparative data on the highly similar gene, precursor and primary structures and similar differential distributions in insect and crustacean NSCs suggest that CHH/ITP and ITPL neuropeptide-producing cells and their gene products share common phylogenetic ancestry.

Localization of RFamide-like Immunoreactivity in the Visceral Organs and Peripheral Neurosecretory Cells Related to the Terminal Abdominal Ganglion in the Cricket, Gryllus bimaculatus

The distribution of RFamide-like immunoreactivity was examined in the terminal abdominal ganglion (TAG)-related peripheral nervous system, and in the visceral tissues of the female cricket, Gryllus bimaculatus. In the TAG, most of the RFamide-like immunoreactive (RFaLI) neurons were bilaterally paired, and were found in the neuromeres of the 7th to the 10/11th segment. The remaining neurons were found in the midline region. These included anterior unpaired neurons, a cluster of neurons in the posterior end, and ventral paired neurons in the 9th segment. Immunocytochemistry combined with Lucifer Yellow back-filling indicated that the immunoreactive paired medial neurons innervated the rectum. In the visceral tissues innervated by the TAG, RFaLI fibers were seen in the oviducts and rectum, while the spermathecal duct and spermatheca did not exhibit any immunoreactivity. In the peripheral nervous system, RFaLI peripheral neurosecretory cells (PNCs) were found on the fifth and seventh segmental nerve roots of the TAG in both sexes. The perikaryon of the PNCs contained several immunoreactive large electron-dense granular vesicles. Intracellular dye injection showed that the PNCs probably form neurohaemal-release sites for RFamide-like peptide. The PNCs did not project any processes into the TAG.

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.

Peripheral Neurosecretory Cells of Insects Contain a Neuropeptide with Bursicon-Like Activity

ABSTRACT In insects, neurosecretory cells have been found not only within ganglia of the central and stomatogastric nervous system, but also in the peripheral nerves (Fifield & Finlayson, 1978; Wasserman, 1985; Baudry-Partiaoglou, 1987). These peripheral neurones and their processes along peripheral nerves contain electron-dense granules and stain with various dyes for neurosecretory cells, suggesting that they produce, store and probably release neurosecretory material. The nature of this material, however, is unknown. Only the results of Raabe (1986), showing that some abdominal peripheral neurosecretory cells of three insects are glucagonimmunoreactive, have suggested that such cells are peptidergic. We demonstrate in this study that a group of peripheral neurosecretory cells in crickets contains such a peptide. Moreover, we are able to suggest a function for this peptide and thus for an identified set of peripheral neurosecretory neurones.

Peripheral neurosecretory cells on the thoracic median nerves of the locust, Schistocerca gregaria.

The distribution of neutral red staining peripheral cell bodies along the nerve trunks of the thoracic median nervous system of the locust, Schistocerca gregaria, is described. Backfilling of the cells with cobalt chloride solution reveals that they are neurones with characteristic axonal processes that terminate in the neurohaemal areas of the median nerve. The neurones react with the dye acridine orange, indicating their neurosecretory nature. This is confirmed by their ultrastructural appearance at the electron microscope level. The distribution and staining properties of the cells are compared with those of peripheral neurones from other insects and the nature of their neurosecretory product is discussed.

Diversity of neurohormonal release sites in insects: Neurohemal areas associated with peripheral neurosecretory cells in Periplaneta americana L. (Dictyoptera: Blattidae) and Locusta migratoria R. & F. (Orthoptera : Locustidae)

Neurohemal areas are located on the distal region of the transverse nerve and on the link nerve of the cockroach, Periplaneta americana (Dictyoptera : Blattidae) and the locust, Locusta migratoria (Orthoptera : Locustidae). They constitute the sites at which the peripheral neurosecretory cells on these nerves discharge their products. Histological and ultrastructural studies suggest that, at least in Periplaneta, these areas also serve to release neurosecretory products different from those found in the perisympathetic organs (POs). These products come from the ganglion cells, some of them via the transverse nerve, and others, via the somatic nerves. The existence of these neurohemal areas broadens the problem of the release zones and shows that the POs are not the only neurohemal structures associated with ventral ganglion cells. An attempt is made to explain the reason for the existence of these different release sites, as well as the role of the peripheral cells.

Comparative immunocytochemical study of release sites of insulin, glucagon and AKH-like products in Locusta migratoria, Periplaneta americana, and Carausius morosus

Insulin, glucagon and adipokinetic hormone antisera were applied to the corpora cardiaca, perisympathetic organs, neurohemal areas and peripheral neurosecretory cells of three insect species, the locust Locusta migratoria, the cockroach Periplaneta americana, and the stick insect Carausius morosus. The neurohemal part of the corpora cardiaca was shown to be immunoreactive to both insulin and glucagon antisera while the glandular cells reacted to adipokinetic hormone antisera. The perisympathetic organs seem to be devoid of these three substances, but certain peripheral neurohemal areas contained AKH and glucagon immunoreactive products. The latter were found to originate in the peripheral neurosecretory cells.

Metamorphosis of the cerebral neuroendocrine system in the moth Manduca sexta.

We have examined the morphology and neuronal elements of the cerebral neuroendocrine system in the larval, pupal, and adult stages of the moth Manduca sexta with a variety of neuroanatomical techniques. The larval brain contains several discrete groups of neurosecretory and non-neurosecretory cells that project to the associated neurohemal organs (the corpora cardiaca-allata complex, or CC-CA) and to a variety of more peripheral structures. A previously undescribed set of cells in the subesophageal ganglion have also been found that project out the neurosecretory nerves. During metamorphosis, the cerebral neuroendocrine system undergoes a dramatic structural reorganization, including the reduction or loss of many larval nerves and a repositioning of the cell groups and their dendritic fields. Despite these changes, most of its central elements are retained. In addition, by the completion of adult development a new cluster of cells can be found on either side of the dorsal midline of the brain. We have also determined the relative contributions of the different cell groups to the moth neuroendocrine system by intracellular iontophoresis of dye into individual cells. Within the dorsal protocerebrum, five separate morphological types of cells can be recognized, each with a distinctive pattern of dendritic arborization in the brain and terminal neurohemal processes that project to the CC, the CA, the aorta, or to a combination of these regions. The large intrinsic cells of the CC have also been filled, revealing an unusual set of morphological features in these peripheral neurosecretory cells.

Peripheral neurosecretory cells in some Lepidoptera

A segmental series of peripheral neurosecretory cells was found in larvae of some Lepidoptera. They occur in the thoracic segments and in the first eight abdominal segments, towards the distal end of the transverse nerves. In the thorax, and in abdominal segments 1, 2, and 8 there is usually a single cell, but in abdominal segments 3–7 there are 2–14 cells. The latter have the appearance of small ganglia and lie on the aliform muscles where the transverse nerve divides to innervate the individual muscle fibres. All of the peripheral neurosecretory cells appear to produce the same secretion, some of which is discharged through the transverse nerves and released at the proximal neurohaemal dilations. Other fibers extend distad from the cell bodies and may supply the aliform muscles.

Electrical activity of neurohaemal tissue in the stick insect, Carausius morosus

Preparations of the isolated transverse nerve of the median (unpaired) nervous system of the stick insect show spontaneous electrical activity in the form of action potentials. There are no cell bodies of neurones in these preparations so the action potentials must arise in isolated processes of the cells. These may be axons belonging to neurosecretory cells in the central nervous system or processes of peripheral neurosecretory cells. Most of the action potentials resemble those described for other neurosecretory cells in having a spike duration that is longer than that of an ordinary neurone, in this case the sensory neurone of the dorsal stretch receptor. A few have a similar form and duration to the sensory spike. It is postulated that these action potentials are generated either by the passage of neurosecretion through the cell membrane or by excitation of processes by the newly released material. If they have adaptive significance it may be to monitor the release of secretion.