Neurosecretory Axons Research Articles

Insect peripheral nerves: accessibility of neurohaemal regions to lanthanum.

During incubation in vivo, exogenously applied ionic lanthanum comes to surround the numerous neurosecretory terminals which are found lying within or immediately beneath the acellular neural lamella ensheathing the nerves from fifth instar and adult specimens of Rhodnius prolixus. The lanthanum does not penetrate beyond the cellular perineurium, which completely surrounds the non-neurosecretory axons in these nerves and constitutes a form of 'blood-brain barrier'. In some cases, however, lanthanum is found in the vicinity of a neurosecretory axon lying beneath the perineurium, where it can be assumed to have leaked in from the neurosecretory terminal lying free in the neural lamella. When nerves are incubated in calcium-free media, regions with an attenuated perineurium become 'leaky', in that lanthanum is found lying in those extracellular spaces between axons and glia which lie immediately below the thin part of the perineurial layer. Bathing solutions made slightly hyperosmotic to the haemolymph with sucrose have no apparent disruptive effects on the barrier. When the tissues are incubated in more hypertonic solutions, the perineurial barrier becomes 'leaky' throughout, and tracer pervades beyond its cells into all the intercellular spaced between glia and axons. The possible role of the zonulae occludentes in both the maintenance of the perineurial barrier and in the formation of interglial occlusions to local penetration of exogenous substances is considered.

Ultrastructural study of the prothoracic glands of Galleria mellonella L. in the penultimate, last larval, and pupal stages

The prothoracic gland (PGL) of Galleria mellonella is a Y-shaped, paired organ, consisting of 45-50 polyploid giant cells. The PGL cells are supplied by neurosecretory axons; release of neurosecretory granules (1000-1300 A in diameter) directly on the surface of PGL cells was frequently observed. Based on ultrastructure, the last two larval instars can be divided into three phases: 1) restitutive phase immediately after moulting; 2) gradual activation in mid-intermoult as indicated by the logarithmic cell growth, decrease of nucleo-cytoplasmic ratio, increase in the number of cell organelles participating in protein synthesis, and the structural changes of these organelles; 3) "release' period preceding moulting, characterized mainly by the extreme dilatation of peripheral invaginations. From the prepupal stage onward cellular activity is asynchronous. Part of the cells already show the signs of involution, while others histolyse only after the activation phase subsequent to moulting. PGL in G. mellonella is one of the larval tissues. In the course of activation its ultrastructure changes as a function of juvenile hormone (JH) cocentration, in the absence of which it histolyses. Accordingly, it has seemed to us to be a suitable model for the cytological study of JH activity.

Retrograde transport of horseradish peroxidase in the magnocellular neurosecretory system of the rat

Horseradish peroxidase (HRP) was injected into the pituitary in adult rats. After 2-3 days, the neurones of the supraoptic nuclei, the magnocellular parts of the paraventricular nuclei, and the various accessory neurosecretory hypothalamic nuclei showed accumulation of HRP. The HRP reaction product consisted of fine, discrete cytoplasmic granules, and in electron micrographys it was seen to be located in the lysosome-like dense bodies of 0.4-0.6 mum diameter which are normally present in the cytoplasm of the neurosecretory neuron.es. Very little reaction product was found in the neurosecretory axons. Reaction product was also found in the hypothalamic arcuate nuclei. This was the result of an endogenous peroxidase-like activity, since it occurred in control animals which had not received HRP. This endogenous reaction product is non-neuronal. Morphologically, it takes the form of distinctive clusters of coarse granules which are seen in electron micrographs to be characteristic angular bodies of 0.7-1.0 mum diameter located in the cytoplasm of astrocytes or their processes.

The pituitary of Mugil cephalus during adaptation to biotopes of different salinities

The epithelial and nervous components of the pituitary of Mugil cephalus were analysed by histological and electron microscope techniques in specimens caught in freshwater ponds, the sea and a hypersaline lagoon. The most pronounced differences were found in the prolactin secreting cells and the neurosecretory axons which innervate them. Changes in cell structure in the different biotopes were also found in the somatotrophs, the gonadotrophs, the corticotrophs and the lead-cells of the intermediate lobe. During the spawning season, the gonadotroph cell region of seawater specimens is characterized by a large number of both A- and B-type neurosecretory axons. It is suggested that Mugil breeders find a method of stimulating synthetic and secretory activity of the gonadotrophs by chlomiphene-like factors as a means towards achieving gonadal activity in freshwater specimens.

The neurosecretory cells of the optic lobe in Carcinus maenas (L).

The six types of neurosecretory cell in the optic lobe of Carcinus maenas described by light microscopy are recognised by electron microscopy. They are categorised according to size, distribution of organelles and type of neurosecretory product. The neuro-secretory material,produced as granules by the Golgi bodies, migrates to the cell periphery eventually reaching the sinus gland via the neurosecretory cell axon extension. No change in size occurs in the granules but the density does alter. Each cell type has its own characteristic type of neurosecretory granule based on size and electron density. Multivesicular and lytic bodies in cell types 1,2,3,4 and 6 suggest a cycle for degrading neurosecretory material. Such a cycle is not so evident in cell type 5. Peripheral release of neurosecretory material is suggested for cell type 6 although the fate of the material is unknown.

Movement of neurosecretory product through the anatomical compartments of the neural lobe of the pituitary gland. An electron microscopic autoradiographic study.

Electron-microscope autoradiographs have been prepared from the neural lobes of the pituitary glands of rats which had received intracisternal injections of [35S] cysteine at various times before gland removal. The rate of appearance and disappearance of autoradiographically demonstrable radioactivity in the neural lobe closely paralleled that previously determined, biochemically, for radioactive hormones and neurophysins. Radioactivity was appreciably associated with the undilated parts of neurosecretory axons only during the first few hours after injection of the label. The axonal dilations were subdivided into those in which small vesicles could be seen ("endings") and those in which no small vesicles could be seen ("swellings"). Radioactivity appeared first in "endings" and then in progressively larger and larger profiles of "swellings". It appeared that newly arrived granules were found close to the limiting membrane of the nerve swelling and that as time progressed they moved deeper and deeper into the swelling. On the basis of the results, suggestions were made for an anatomical explanation of the readily-releasable pool of hormone which has been demonstrated pharmacologically.

Neurosecretion and the control of visceral organs in insects.

Since Van der Kloot's review on neurosecretion in insects in 1960 (116), the struc­ tures of juvenile hormone and ecdysone, the molting hormone, have been published. As predicted by Van der Kloot these events have vastly increased the number of experimental approaches at hand and have expanded knowledge accordingly. In the interim, the subject of neurosecretion in insects and related animals has been re­ viewed on numerous occasions (5, 7, 26, 31, 38, 41, 61, 66, 67, 95, 105, 107, 119, 120). As of this writing there are no less than four reviews on various aspects of neurosecretion in insects which are completed or in preparation (34, 44, 68, 90). Much of the work on neurosecretion has centered upon the release of neurohor­ mones from neurohemal organs and their effect upon bodily processes. Hormonal control of visceral muscles and scattered reports of innervation of glands and organs by neurosecretory axons have not received as much research attention as other areas of neuroendocrinology, and only very recently have electrical properties of insect neurosecretory neurons been investigated in detail. These latter subjects will be reviewed below along with a few other selected topics in neurosecretion.

Hypothalamohypophysial vascular and neurosecretory link in the teleost Bagarius bagarius (Ham.).

A three-dimensional picture of the hypothalamohypophysial neurosecretory system of B. bargarius could be obtained by adopting in situ staining techniques. The paired NPO give rise to left and right neurosecretory tracts which, although they approximate as a common tract, maintain their individuality till entering the pars intermedia. The ventral hypothalamic and the hypophysial arteries take their origin from the internal carotid artery. The former contributes to the formation of the primary plexus of the median eminence and the latter enters the pituitary directly, giving rise to the neuroadreno-interface vasculature. The vasculature of the median eminence lies in close contact with the long common neurosecretory tract. Morphological evidence suggests that in B. bagarius there are three pathways of hypothalamic control of the hypophysis: (1) A direct neuroglandular pathway, where the neurosecretory axons come directly in contact with the adrenohypophysial cells. (2) An indirect neurovasculoglandular pathway, where the blood exposed to the NSM at the median eminence comes in contact with gland cells. (3) Another indirect neurovasculoglandular pathway, where the blood is exposed to NSM at the neurohypophysis prior coming in contact with the gland cells. Thus, B. bagarius has the 'median eminence equivalent' neuroadeno-interface vasculature typical of the teleosts and the median eminence comparable to the Elasmobranchs, Holocephali and primitive Actinopterygians. This shows that the pituitary portal system of teleosts is in general agreement with that of other fish groups, except that in some it is restricted to the neuroadreno-interface, whereas in others like B. bagarius it extends to the hypothalamus also. These may be termed anterior and posterior median eminence.

Vasopressin as a Neurotransmitter in the Central Nervous System: Some Evidence from the Supraoptic Neurosecretory System

This chapter proposes the supraoptic neurosecretory system as a model and provides some evidence from the supraoptic neurosecretory system. There is some evidence to suggest that the neurohormones might be considered as central synaptic transmitters. These polypeptides are synthesized by neurons whose location in the brain is well established (the magnocellular system for antidiuretic hormone and oxytocin) or still uncertain (the parvocellular system for pituitary releasing factors). A major difficulty in validating these hypotheses arises from the lack of a morphologically and functionally defined set of neurons in which the capacity of a neurohormone to act as a neurotransmitter may be tested. It is suggested that the excitation-inhibition sequence produced in the supraoptic neurosecretory neurons of the anaesthetized monkeys after osmotic stimulation may result from recurrent inhibition (RI) mediated by neurosecretory axon collaterals. Electrophysiological evidence for RI was given by the observation of a period of reduced firing following antidromic invasion of the cell. The only observation inconsistent with a neurotransmitter role for vasopressin in mediating RI is that RI persists in Brattleboro rats which apparently lack vasopressin.

The autoradiographical assoc-iation of fast transported material with dense core vesicles in the central nervous system of Anodonta cygnea (l).

An autoradiographical study of the distribution of fast transported material in the cerebro-visceral connective of Anodonta cygnea (L.) following the injection of 3H-5-hydroxytryptophan into the cerebral ganglion has been carried out at both the light- and electronmicroscope levels. Statistical analysis of the results at the electron microscope level revealed that the distribution of grains within the tissues of the cerebro-visceral connective was highly non-random. Both the “circle analysis” and the “hypothetical grain density analysis” methods showed that most of the 3H-activity was associated with dense core vesicles (1000 A diameter). These dense core vesicles were most frequently seen in the neurosecretory axons found near the periphery of the connective, but a number of labelled vesicles were also present in the small axons. The significance of these findings in relation to the distribution and the fast axonal transport of 5-hydroxytryptamine in the cerebro-visceral connective is discussed.

Ultrastructure of the cell types and of the neurosecretory innervation in the pituitary of Mugil cephalus L. from freshwater, the sea, and a hypersaline lagoon II. The proximal pars distalis

The ultrastructure of the proximal pars distalis was studied in the pituitary of grey mullet taken from freshwater, the sea, and a hypersaline lagoon. Variations were found in the mean size of the presumptive somatotrop cell secretory granules in the three biotopes. The presumptive gonadotrops displayed a wide array of cytological differences but it is uncertain whether these are due to the influence of the biotope or to the functional state of the cell in correlation with the seasonal secretory cycle. Numerous neurosecretory axons of type B and a few of type A were in direct contact with the somatotrops. During the spawning season, the gonadotrop cell region of seawater specimens was characterized by a large number of A- and B-type axons, pointing to the possibility that both the synthesizing and releasing neurosecretory influence on the gonadotropin-secreting cells may be stimulatory.

Potassium-induced release of the diuretic hormones of Rhodnius prolixus and Glossina austeni: Ca dependence, time course and localization of neurohaemal areas.

ABSTRACT Exposure of neurohaemal areas to solutions of elevated K concentration (above 40 mM) causes a maximal release of diuretic hormone in Rhodnius prolixus and Glossina austeni. An involvement of Ca in hormone release is indicated by the reduction caused by low concentrations of this cation (below 2 mM) or by the presence of Mn. During prolonged treatment with K-rich solutions the rate of hormone release is initially high, but then declines. This response parallels that for Ca entry into squid giant axons during maintained potassium depolarization and suggests that the rate of Ca entry controls the rate of hormone release. Tetrodotoxin did not reduce the potassium-induced release of the hormone, suggesting that K acts directly on the neurosecretory axon endings in the neurohaemal areas.

Electron microscopic study of rat pituitary primordium in organ culture.

To see whether adenohypophysial tissue has the capacity of self-differentiation, Rathke's pouch together with part of the neurohypophysial primordium was isolated from 12-(Group A), 14- (Group B) and 15-day-old (Group C) rat fetuses and cultivated for 9, 6 and 5 days, respectively. Electron microscopic examination of these explants showed many adenohypophysial cells containing electron dense granules, indicative of cytodifferentiation in all groups. Three cell types were distinguishable on the basis of the size of their granules. Variable amounts of neurohypophysial tissue were seen in some explants of Groups B and C, but not A. This “pars nervosa” contained pituicytes with lipid-like inclusions in their cytoplasm but no neurosecretory axons. The adenohypophysial cells abutting on the pars nervosa were arranged rather regularly like cells of the pars intermedia which contained a few granules of about 200 mμ diameter.

Histology of the neurosecretory system and neurohaemal organs of the spider, Argiope aurantia (Lucas).

AbstractHistological studies of the neurosecretory system during the postembryonic development of a spider, Argiope aurantia, were made at the light‐microscopic level.Neurosecretory cells which are found in all stages are classified into type I and type II cells. The type I cells are present in the aboral region of the brain and in pedipalpal, ambulatory and abdominal ganglia of the subesophageal mass. The type II cells which appear from the seventh stage are confined to the cheliceral ganglia. Three stages of secretory activity (poor, medium and full) based on stainability are described in these cells.In both types clear axonal transportation of neurosecretory material is observed. The discrete tracts and commissures formed by these neurosecretory axons are described in the brain and subesophageal ganglion. The complexity of some of these pathways is comparable to that of the ordinary neurons.One pair of nerves from the brain and four pairs of nerves from the subesophageal mass enter a neurohaemal organ, the Tropfenkomplex. This is a paired structure, situated dorsally, on either side of the subesophageal mass. The neurosecretory axons branch extensively within the organ and on their course they from sacs or pools filled with secretory material.The Tropfenkomplex is enveloped by a thin neural sheath which runs deep into the organ dividing it into a series of lobes. Glial cells are distributed within the organ. As in the neurosecretory cells, changes in stainability of secretory material were also observed in the Tropfenkomplex.During intermolt periods two peaks of stainability have been noticed. The first peak lasts for 24 hours after the molt, and this is followed by a low activity period between second and fifth day. From the sixth to the tenth day after the molt the second peak commences. It is suggested that the second peak may be responsible for bringing about molting.The cheliceral group appears (seventh stage) at a time when external indication of reproductive characters are visible. In the ninth stage, by the tenth day after the last molt, several of the type I and type II cells contain much secretion. This is followed by maturation of gonads and oviposition. Thus both type I and type II cells are believed to be involved in the reproduction of the animal.

Electron microscopic studies of the development of new neurohaemal contacts in the median eminence of the rat after hypophysectomy

In the normal rat the axons of the parvicellular neurosecretory system form terminals which are characterised by dense core vesicles of 80–160 nm diameter and which lie against the fenestrated portal capillaries in the upper neurohypophysis (median eminence). The magnocellular neurosecretory axons traverse the interanal zone of the median eminence and form terminals against the fenestrated capillaries of the distal neurohypophysis (neural lobe). In the neural lobe it is assumed that the more electron dense vesicles (of around 200 nm diameter) characterise the vasopressin containing terminals of neurones in the supraoptic nucleus and that the less numerous terminals with the more electron lucent vesicles (also of around 200 nm diameter) belong to the axons of the paraventricular neurones and contain oxytocin. For the first few days after hypophysectomy, the proximal stumps of the cut neurosecretory axons in the internal zone of the median eminence are distended with neurosecretory material while axonal sprouts which are rich in neurotubules and completely ensheathed in satellite cell cytoplasm gradually penetrate the external zone. Subsequently there is a massive hypertrophy of fenestrated capillaries and the newly formed axonal sprouts progressively lose their satellite cell cover and thus acquire neurohaemal contacts with the outer basement membrane of the perivascular space. The development of these contacts is associated with the appearance of increasing numbers of synaptic vesicles and prejunctional dense projections. This reconstitution of the morphological features of the normal neurohaemal contacts is associated with recovery of antidiuretic function. In the hypertrophic median eminence the neurosecretory axon terminals with the electron lucent type of vesicles far outnumber those with the electron dense vesicles. It is suggested that the greater degree of cell loss from the supraoptic as opposed to the paraventricular nuclei puts the paraventricular axons at an advantage in competing for space on the basement membrane around the perivascular space of the newly formed capillaries. It is suggested that an important factor in promoting the regeneration of the neurosecretory axons is the remarkable hypertrophy of the fenestrated capillaries against which they come to terminate.

Neuroendocrine Phenomena in Aphroditid and Related Polychaetes

AbstractA description is given of the neurosecretory system and a possible neuroendocrine system in polychaetes of the families Aphroditidae, Polynoidae and Sigalionidae. Two paired tracts and one unpaired tract of neurosecretory axons, and the ganglionic nuclei from which they arise, have been detected. The neurosecretory tracts are directed to the brain floor where they form a storage‐and‐release complex or neurosecretory neuropile, containing secretion‐laden fibre terminals which are diverse in character and can be distinguished on the basis of staining properties. Bundles of axons (transcapsular fibres) pass through the neurallamella and come into close association with the epithelioid cells of the infracerebral gland, a possible endocrine structure. The “gland” cells resemble monopolar neuron cell bodies in morphology and are of two types: a cells are devoid of secretory material, whereas b cells contain accumulations of PAF‐positive inclusions. Possible simple photoreceptors are associated with part of the cerebral neurosecretory system.

Excitatory Transmission in Insect Neuromuscular Systems

Many, but not all, visceral muscles in insects are innervated by neurosecretory axons. The neurosecretory junctions with the heart muscle of the American cockroach, Periplaneta americana , show ultrastructural and electrophysiological evidence of chemically transmitting synapses, and cytochemical evidence for the presence of monoamines. Electron microscopy of nerve terminals shows that synaptic vesicles may be formed directly from electron-dense “neurosecretory” granules Neurotomy of motor axons to skeletal muscles in insects leads to aggregation and clumping of synaptic vesicles after 48 hours. Treatment of in vitro nerve-muscle preparations with various respiratory poisons caused aggregation similar to that developed in neurotomized animals. This suggested that vesicle aggregation in both cases may have resulted from a decrease in available adenosine triphosphate in the nerve terminal with subsequent alteration in the normal charge density which supports a repulsive force between the vesicles.

Axon pathways of the intermediate neurosecretory cells in Culex tarsalis Coquillett (Diptera: Culicidae)

The axons of the intermediate neurosecretory cells in the brain of Culex tarsalis Coquillett larvae cross over in the pars intercerebralis, pass ventrolaterally through the protocerebrum, and then pass through the circumoesophageal connectives and the ventral nerve chain as far as the eighth abdominal ganglion. These axons possess branches in the region of juncture of the proto- and deuto-cerebrum, and in the suboesophageal, thoracic, and eighth abdominal ganglia; no branches were visible in the tritocerebrum or abdominal ganglia 1 to 7. The branches appear to terminate within the ganglia, usually near the neuropile boundary. The large ventral neurosecretory axons described in larvae of Culiseta inornata (Williston) by L. Burgess in 1971 are probably the axons of the intermediate neurosecretory cells of that mosquito.

Ultrastructural studies on the ontogenesis of the caudal neurosecretory system in the roach, Leuciscus rutilus

The ontogenesis of the urophysial system (caudal neurosecretory system) in the roach (Leuciscus rutilus) was ultrastructurally analyzed. In newly hatched spawns the urophysial components are differentiated in the form of neurosecretory perikarya, axons and terminals and display the characteristics of a functional system although the neurohemal area is not developed. It is assumed that the system is active simultaneously with the preopticohypophysial system during ontogenesis. On the contrary, organogenesis of the urophysis is late, and only at the 14 mm stage do the neurosecretory axons first penetrate the meninx to participate in the formation of the organ. Assumed aminergic cell types associated with the urophysial system are differentiated at the time of hatching, thus indicating a functional relationship. A secretory ependyma releasing its granules into the central canal is described.

The protocerebral neurosecretory system and associated cerebral neurohemal area of Acheta domesticus.

Six principal types of neurons are recognized in the median neurosecretory-cell group of the pars intercerebralis in the house cricket, Acheta domesticus. Four of the six cell types are neurosecretory neurons. The majority of the 900 cells contain granules 1500–3000 A in diameter and are designated as type I. These neurons, which are associated with the nervus corporis cardiaci I (NCCI), undergo asynchronous secretory cycles. Two cell types (II and IV), which contain 700–1600 A granules, terminate in a neurohemal area on the ventral median surface of the brain, a site not previously recognized as neurohemal in insects. Neurosecretory axons that lie within and below the perineurial sheath in the cerebral neurohemal area contain typical synaptoid figures, but no evidence of exocytosis was found. A single pair of neurosecretory cells (type III) has been recognized only from light microscope sections. Two distinct types of vesicle-bearing axons occur in protocerebral neuropile. The majority contain electron-dense and dense-cored vesicles 600–1000 A in diameter. Some axons contain granules of type I cells and may be collaterals or loops of NCCI axons.