Neurohaemal organs in the medial nervous system of insects

Glial cells in peripheral nerves of the cockroach, Periplaneta americana

The ultrastructural organization and the junctional complexes of peripheral nerves have been investigated in the cockroach Periplaneta americana. Nerve 5 is surrounded by a layer of connective tissue, the neural lamella, beneath which is a layer of perineurial glial cells wrapping the axons. Adjacent perineurial cells are joined to one another by septate, gap and tight junctions. Septate and gap junctions were observed in freeze-fracture replicas of main trunk nerve 5. Septate junctions were found as rows of PF particles mainly in perineurial cell membranes. Gap junctions exhibited EF macular aggregates in perineurial and subperineurial glial cells. During incubations in vivo with extracellularly applied ionic lanthanum, the lanthanum did not penetrate beyond the perineurium. Where nerve 5 branches and contacts the muscle, lanthanum penetrated freely between the muscle fibres and the nerve branches. In small peripheral branches where the axons are surrounded by single a glial layer, lanthanum is unable to penetrate to the axolemma.

Morphology and metamorphosis of the peptidergic Va neurons and the median nerve system of the fruit fly, Drosophila melanogaster

Metamorphosis is a fundamental developmental process and has been intensively studied for various neuron types of Drosophila melanogaster. However, detailed accounts of the fate of identified peptidergic neurons are rare. We have performed a detailed study of the larval morphology and pupal remodelling of identified peptidergic neurons, the CAPA-expressing Va neurons of D. melanogaster. In the larva, Va neurons innervate abdominal median and transverse nerves that are typically associated with perisympathetic organs (PSOs), major neurohaemal release sites in insects. Since median and transverse nerves are lacking in the adult, Va neurites have to undergo substantial remodelling during metamorphosis. We have examined the hitherto uncharacterised gross morphology of the thoracic PSOs and the abdominal median and transverse nerves by scanning electron microscopy and found that the complete reduction of these structures during metamorphosis starts around pupal stage P7 and is completed at P9. Concomitantly, neurite pruning of the Va neurons begins at P6 and is preceded by the high expression of the ecdysone receptor (EcR) subtype B1 in late L3 larvae and the first pupal stages. New neuritic outgrowth mainly occurs from P7-P9 and coincides with the expression of EcR-A, indicating that the remodelling of the Va neurons is under ecdysteroid control. Immunogold-labelling has located the CAPA peptides to large translucent vesicles, which are released from the transverse nerves, as suggested by fusion profiles. Hence, the transverse nerves may serve a neurohaemal function in D. melanogaster.

Identical cellular distribution of all abundant neuropeptides in the major abdominal neurohemal system of an insect (Periplaneta americana)

The median neurosecretory cells in abdominal ganglia of insects synthesize a number of putative hormones, which are abundant in the abdominal perisympathetic organs (PSOs). The peptide inventory of these prominent neurohemal release sites is best investigated in the American cockroach and strongly differs from that of head/thoracic neurohemal organs. In this study, we found a complete colocalization of all abundant neuropeptides in this hormonal system, including periviscerokinin-1 and -2, pyrokinin-5, YLSamide, VEAacid, and SKNacid. The first immunoreactive cells were detected on day 18 of embryonic development and already contained the complete set of peptides. By using antisera against the above-mentioned peptides, the development of this neurohormonal system could be studied and is described in detail. Subsequent electron microscopic immunogold stainings in PSO preparations revealed the costorage of PSO peptides in a single vesicle species. Surprisingly, all these peptides were found in axons containing clear vesicles, whereas all axons with dense core vesicles were totally devoid of immunoreactivity. Unlike the axons with dense core vesicles, immunostained axons ramify in the center of the PSO but exhibit only rare morphological signs of exocytosis. Instead, putative release sites of the clear vesicle-containing axons were detected peripherally to the PSOs, namely, on the hyperneural muscle.

Identification of the cellular target for eclosion hormone in the abdominal transverse nerves of the tobacco hornworm, Manduca sexta

The isolated abdominal central nervous system of Manduca sexta undergoes an increase in cyclic GMP (cGMP) when exposed to the insect peptide eclosion hormone (EH) before pupal ecdysis. Previously, cGMP immunocytochemistry revealed that the EH-stimulated increase in cGMP was contained in numerous filamentous processes within the transverse nerve associated with each abdominal ganglion. These processes seemed to be the axons of neurosecretory cells projecting to this neurohemal organ. In the present paper, we now show that the EH-stimulated cGMP is not present in neurosecretory terminals. There is no colocalization of the EH-stimulated cGMP with immunoreactivity of two peptides, known to be present in axons in the transverse nerves. Furthermore, there is no colocalization of EH-stimulated cGMP with the synaptic vesicle protein, synaptotagmin. The neurosecretory axons are localized to a narrow band at the anterior margin of the transverse nerve, whereas the cellular elements showing an EH-stimulated cGMP increase are primarily present in the posterior region. There are two cell types in this region: a granular and a nongranular type. The cGMP immunoreactivity seems to be contained within the nongranular type. During adult development, the cells of the posterior compartment spread in a thin layer between the transverse and dorsal nerves, become positive for myosin immunoreactivity between pupal stages 5 and 8, and seem to form the adult ventral diaphragm muscles. We conclude that the EH-sensitive filaments in the transverse nerves of Manduca are most likely to be intrinsic cells that subsequently develop into the ventral diaphragm muscles of the adult.

Neuropeptide-stimulated cyclic guanosine monophosphate immunoreactivity in the neurosecretory terminals of a neurohemal organ

The neuropeptide eclosion hormone acts on the nervous system of the tobacco hornworm, Manduca sexta, to increase cyclic guanosine monophosphate (cGMP) levels. In this study I describe the localization of some of the sites where these increases occur. Prior to pupal ecdysis, eclosion hormone stimulates an increase in cGMP in a network of fibers in the transverse nerve of each abdominal ganglion. Double-label experiments with propidium iodide suggest that the cGMP immunoreactivity is primarily localized in neurosecretory nerve endings. The time course of the increase in cGMP immunoreactivity and its requirement for lipid metabolism is similar to that of the cGMP increase measured by radioimmunoassay. The cGMP response in the transverse nerve is stage-specific, occurring prior to pupal ecdysis and not prior to larval or adult ecdysis.

Characterization and partial purification of different factors with contraction-potentiating activities from neurohaemal organs of the locust

Phe-Met-Arg-Phe (FMRF-NH2) and structurally related peptides enhance neuromuscular transmission and contraction of the M. extensor tibiae preparations of the locusts Locusta migratoria and Schistocerca gregaria (Walther et al.: Neurosci. Lett. 45:99-104, '84). Similar effects could also be obtained with extracts of locust ganglia (Walther and Schiebe: Neurosci. Lett. 77:209-214, '87). By using two HPLC systems, we have partially purified extracts of the unpaired median nerves (including their neurohaemal organs) of different locust ganglia. The biological activity of the extracts served as an estimate for the degree of purification. Six different bioactive fractions were identified migrating at and close to retention times of known -RFamide peptides with similar bioactivity. No fraction coeluted with authentic FMRF-NH2 or FLRF-NH2. We demonstrate that extensor tibiae muscle contractions were potentiated by HPLC fractions from raw material with -RF-NH2 immunoreactivity, but also by HPLC fractions from raw material without such immunoreactivity.

Formation of the transverse nerve in moth embryos. II. Stereotyped growth by the axons of identified neuroendocrine neurons

We are interested in the cellular mechanisms that guide neuroendocrine axons to their neurohaemal target regions and that regulate the extent and positioning of their terminal arbor. The neurohaemal organ we have studied is the segmentally repeated transverse nerve of the moth Manduca. In the mature animal, two motor neurons and a heterogeneous set of identified neuroendocrine neurons project to this nerve; the latter release hormonal peptides from along its length. In the preceding report, we demonstrated that during embryogenesis, the position, trajectory and extent of the transverse nerve are anticipated by two sets of nonneuronal cells, the strap and the bridge. In this paper we show that four identified neuroendocrine neurons (L1 and B1-3), like the identified motor neurons before them, elaborate growth cones that use this preexisting scaffolding as a substrate for axonal elongation. Moreover, growth cone navigation by these neuroendocrine neurons is as precise and invariant as that displayed by the motor neurons. One feature that differentiates the behavior of the developing neuroendocrine cells from that of the motor neurons is a stereotyped interaction that the L1 and B1-3 axons undergo with an identified syncytial cell that lies in close proximity to the strap. Each neuroendocrine neuron specifically adheres to the syncytium by extending numerous filopodia, and an occasional large lamellopodium, over its surface. These contacts are maintained by the neuroendocrine axons after their growth cones have left the vicinity of the syncytium and proceeded into the strap/bridge complex. Adhesion to the syncytium is transient and specific to the neuroendocrine neurons: although motor neuron axons are present at this same time and place, they display no affinity for the syncytium. This distinction correlates with the fact that the neuroendocrine neurons go on to elaborate arbor within the confines of the transverse nerve, while the motor neurons do not. We suggest that the syncytium may act as a "fictive target" for these neurons to aid in the differentiation of features that are specific to their cellular phenotype.

Neurosecretory cells with 'synaptoid perikarya' in Helix. A definitive description of secretory release from the somata of endocrine neurones

Neurosecretory cells in the mollusc Helix have perikarya that show clear signs of adaptation for both the synthesis and the discharge of secretory material. They are characterized morphologically by juxtaposition with the neural lamella. Passage of hormone into the haemocoel is apparently facilitated in many cases by the extreme attenuation of areas of the lamella and perineurium adjacent to the perikarya and by other forms of histological differentiation. Presumptive sites of release are characterized by prominent aggregations of synaptoid vesicles and the discharge of the contents of secretory granules by exocytosis.

FMRF-NH2-like factor from neurohaemal organ modulates neuromuscular transmission in the locust

YGGFMRFamide, FMRFamide and related peptides potentiate transmission at locust slow motor synapses. Since immunohistochemical evidence points to neurohaemal organs (NHO) as potential sources for endogenous RFamide-like peptides we have applied extracts from the metathoracic NHO, equivalent to 1/3 NHO, to the metathoracic extensor tibiae muscle. NHO-extract depolarizes the muscle fibre and increases its membrane resistance; enhances transmitter release; and increases the amplitude of contraction and the rate of relaxation. These effects are in good qualitative and quantitative agreement with those of YGGFMRFamide (5 X 10(-8) to 10(-7) M). Octopamine which partly acts like YGGFMRFamide cannot account for the NHO-effects.

Diversity of neurohormonal release sites in insects: coexistence of two different (alpha and beta) types of neurohemal structure in the perisympathetic organs

A comparative ultrastructural study of insects was made whose primitive perisympathetic organs consisted either of a single median neurohemal formation per segment (Periplaneta americana) or of two transverse formations (Carausius morosus), or again, of three--one median and two transverse--as in Locusta migratoria. The results showed that the perisympathetic organs always comprised two types of structure (alpha and beta), even when a single formation exists. In Periplaneta and Carausius, both types were seen to coexist in the same neurohemal formation. In Locusta, however, they were separated, the alpha structure constituting the median organ and the beta structure, the two transverse organs. Each of the two structures have special features: the beta structure displays the usual characteristics of a loose neurohemal organ, i.e., it is penetrated by sinuses and has neurosecretory endings devoid of a glial coat; the alpha structure, on the contrary, forms dense compact organs whose endings are covered with a continuous glial layer. During the evolution, this last structure gradually separated from the other neurohemal formations. It was observed to correspond to a single type of neurosecretory cell, distinct from all other cell types by the particular mode of its release. These results for primitive perisympathetic organs confirm earlier findings for advanced organs. They indicate that both the alpha and beta structures are generally present in insects, thus showing their importance in the physiology of these animals.

Central and peripheral neurosecretory pathways to an insect flight motor nerve

Ultrastructural examination of the IIN1b nerve to the dorsal longitudinal flight muscle of Manduca sexta L. verified the presence of neurosecretory processes. Subspherical and irregular vesicles were found where the nerve enters the muscle, while spherical vesicles were found in the proximal region only. A dorsal unpaired median (DUM) cell, the median nervous system, and two or more peripheral cells are the sources of these neurosecretory inclusions. Light the electron microscopy CoCl2 backfills of the transverse nerve produced intensification of a peripheral neuron (#1) and processes in nerves IIN1a and IIN1b. Similar backfills of nerve IIN1b produced intensification of a DUM cell, a second peripheral neuron (#2), and processes in the transverse nerve and nerve IIN1a. Neuron #1 contained large spherical electron-dense vesicles while neuron #2 contained smaller subspherical vesicles. These cells were situated upon the link and/or transverse nerves. Based on these results, we suspect central and peripheral neurosecretory processes reach nerve IIN1b as follows: the link nerve projects prothoracic median nervous system and neuron #2 processes, nerve IIN1a projects neuron #1 processes, and nerve IIN1 projects mesothoracic DUM cell processes, although this latter pathway was less clear.

Ultrastructural comparison of the perisympathetic organs in three Coleoptera: Chrysocarabus auronitens F., Oryctes rhinoceros L. and Tenebrio molitor L

Ultrastructural comparison of different types of perisympathetic organs (POs) in three species of Coleoptera (Chrysocarabus auronitens, Oryctes rhinoceros, and Tenebrio molitor) showed that the structure of these organs was not related to their morphological types but to their topography. Two kinds of PO structure may be distinguished: compact median and diffuse lateral. They were similar in that both were surrounded by thin neural lamellae and exhibited numerous glial cells originating in the perineurium (type I perineurial cells) as well as abundant neurosecretory endings. They were different in as much as in median POs, the neurosecretory endings were generally surrounded by perineurial processes but in transverse POs, these endings were sheathless. Only one type of neurosecretory axon was distinguished in the median organs but three or four in the transverse. The nature of the processes by which neurosecretory granules are released may depend on the type of neurosecretory axon. For instance, exocytosis always occurred for dense spherical granules, and granule fragmentation was visualized for granules of smaller size.

Ultrastructural identification of neurohaemal sites in a tick: evidence that the dorsal complex may be a true endocrine gland

A fine structural study has been made of the 'paraganglionic plates' associated with the perineurium and of the 'retrocerebral organ' associated with the periganglionic sheath of the tick Boophilus microplus; these structures have been postulated, from descriptions by light microscopy, to be tick neurohaemal organs. Neurosecretory terminals are observed frequently in the neural lamella/perineurial sheath, particularly in a dorso-lateral area which may correspond to the 'paraganglionic plates'. No evidence was found of a discrete peripheral neurohaemal organ such as the corpus cardiacum of insects. The 'retrocerebral organ' is comprised of periganglionic sheath cells, which appear to be glandular rather than neurohaemal, and peripheral ganglionic cells.

The neurosecretory system of the adult Melanogryllus desertus Pall. (Orthoptera, Gryllidae). I. Ultrastructural study of the median neurosecretory cells in the brain

Based on the nature of their granules, eight prinicipal types of neurons, six of which are thought to be neurosecretory, are recognized in the median neurosecretory cell group of the brain of Melanogryllus desertus. Most of the neurosecretory cells contain granules with diameters of 200-300 nm. In a few the granules are smaller with diameters varying from 60-100 nm. Most of the cells have well developed Golgi areas and dense bodies of different sizes. Dense bodies are closely associated the neurosecretory granules. Accumulations of electron-dense granular material occur in expanded cisterns of endoplasmic reticulum, particularly in type-I cells.

Autolysis in axon terminals of a new neurohaemal organ in the cockroach Periplaneta americana

An ultrastructural investigation showed that there was a neurohaemal organ in the wall of the ampulla of the antennal pulsatile organ. The neurosecretory axon terminals occurred singly or in small groups, rather than closely packed together as in other neurohaemal organs. All axons contained the same type of neurosecretory granule. The granules had varying electron density and a diameter in the range 1000-2500 A. Some terminals contained small, elliptical electron-transparent vesicles and the axolemma was apposed to the stroma. Other terminals were large and enveloped by glial tissue and the contents of the terminals exhibited varying degrees of autolytic degeneration. Autolysis was characterized by the occurrence of dense bodies and multilaminate bodies which enclosed mitochondria and neurosecretory granules. It was suggested that the neurosecretory material affects antennal function.

Neuroanatomy of the mesothoracic ganglion of the cockroach Periplaneta americana (L.). I. The roots of the peripheral nerves

Bodian silver-stained sections and Procion Yellow impregnation w ere used to examine the general neuroanatomy and details o f the nerve roots in the mesothoracic ganglion of adult males ofPeriplaneta americana(L .). General structure of the ganglion is summarized and the basic plan of its tracheation outlined. Fresh details are given of the seven paired longitudinal fibre tracts and ten transverse commissures, and three new oblique tracts of characteristic shape are described : the ring tract, G -tract and I-tract. Regions of the ventral association centre are briefly described. Groups of neuron cell bodies, which lie peripherally in the ganglion, are designated according to position. The fibre bundles that form the roots of each of the six paired peripheral nerves (nerves 2 -7 ) and the single median nerve (nerve 8) are numbered from dorsal to ventral and their courses and fibre com position described. In all, 30 roots are characterized on each side o0f the ganglion, containing over 150 efferent, presumed motor, fibres and over 2000 afferent, presumed sensory, fibres. Function and fibre diameter are not consistently related and almost all sizes of both motor and sensory fibres occur. Nerves 2 and 6 divide into distinct dorsal and ventral roots; the roots of nerves 4 and 8 are dorsal and of nerve 7 solely ventral; roots of nerves 3 and 5 form fairly continuous series from dorsal to ventral. The more dorsal nerve roots tend to be motor in function and the ventral roots sensory. Nearly all motor cell bodies are located ventrally or ventrolaterally and their processes run m ore or less dorsally to give off dendritic branches into dorsal or lateral neuropile before sending axons peripherally in the nerve trunks. Cell bodies of nerves 3 -6 , except for one in the midline, are ipsilateral, those of nerves 3 and 4 lying almost wholly anteriorly and of nerves 5 and 6 both anteriorly and posteriorly. Cell bodies of nerve 2 are contralateral and anterior except for one in the dorsal midline. Nerve 8 receives axons from posterior cell bodies of both sides. An apparently common inhibitorym otoneuron branches to nerves 3 -6 . The topology of some other motoneurons, chiefly of nerves 4 and 5, is outlined and the probable correspondence of some of them with previously identified metathoracic neurons is discussed. Sensory roots, predominantly of very small fibres, of nerves 2, 3 and 5 -7 enter the ventral association centre. Mainly coarser sensory fibres branch into mid-level neuropile or below , though some of those of nerves 2, 3 and 7 ascend more dorsally.