Neurosecretory Axons Research Articles

Nitric oxide synthase in the caudal neurosecretory system of the teleost Oreochromis niloticus

This study provides evidence that, within the caudal neurosecretory system of the teleost Oreochromis niloticus, neurons express nitric oxide synthase (NOS)-like molecules. The presence of NOS-like molecules was demonstrated by means of NADPH-diaphorase (NADPHd) staining and NOS immunohistochemistry. In the caudal spinal cord, NOS-positive neurosecretory cell bodies and neurosecretory fibers were observed. In addition, NOS-positive structures were found in the urophysis which correspond to neurosecretory axon terminals. Cellular co-localization of NOS and ovine corticotropin-releasing factor (oCRF) immunoreactivities confirmed that the NOS-positive structures belong to the caudal neurosecretory system. The present results suggest that NO may participate in the caudal neuroendocrine function.

An electron microscopic study of the endocrine dorsal bodies in reproductively active and inactive Siphonaria pectinata ( Pulmonata: Mollusca)

The fine structure of the dorsal bodies of the pulmonate limpet Siphonaria pectinata is described in the context of female reproduction involving egg production. In reproductively-active (egg-laying) animals, the ciliated dorsal body cells are filled with lipid droplets and mitochondria. Gap junctions are commonly seen between the cells. The Golgi complexes and the smooth endoplasmic reticulum constitute the other prominent cell organelles. In reproductively-inactive (non-egg-laying) animals, there is a significant reduction in the number of lipid droplets and evidence of reduced synthetic activity in the dorsal bodies. About 12 dorsal body cells are present immediately underneath the perineurium of each cerebral ganglion of the central nervous system. These internal cells are structurally similar to those outside the central nervous system. Cell processes of some of these cells exit the central nervous system at a minimum of three locations on each side and they come in close proximity to the dorsal body cells outside the cerebral ganglia. Like the external cells, the internal cells also communicate via gap junctions and exhibit structural differences according to whether or not the animals are reproductively active. The dorsal body cells, inside and outside the central nervous system, appear to be innervated by neurosecretory axons suggesting neuronal control of dorsal body activity.

Taurine in rat posterior pituitary: Localization in astrocytes and selective release by hypoosmotic stimulation

Taurine, a gamma-aminobutyric acid (GABA)-like acidic amino acid, has previously been shown to be prominently localized to astrocytes in the supraoptic nucleus, the neurons of which contain only small amounts, and to have inhibitory actions on supraoptic neuronal activity. In the present study, taurine distribution in the neurohypophysis was determined by using a well-characterized monoclonal antibody against taurine itself. Preembedding immunohistochemistry was performed at light and electron microscopic levels by using diaminobenzidine and gold-substituted silver-intensified peroxidase (GSSP) methods. At the light microscopic level, the distribution pattern and cellular localization of taurine immunoreactivity corresponded to that of glial fibrillary acidic protein. Pituicyte cell bodies and processes displayed dense taurine immunoreactivity. Electron microscopic observations revealed strong taurine GSSP reactions in these neural lobe astrocytes, but weak taurine reactivity was seen within only some neurosecretory axons. High-performance liquid chromatography analyses demonstrated that in vitro hypoosmotic stimulation (reduction of 40 mOsm/kg) of isolated posterior pituitaries resulted in preferential increases in taurine release into the bathing medium without increased release of other amino acids. Conversely, tissue concentrations of taurine significantly decreased with hypoosmotic perfusion, while glutamate, glutamine, and GABA concentrations were not reduced. These results indicate that taurine is mainly concentrated in neurohypophysial astrocytes, which are known to engulf the neurosecretory axonal processes and terminals. Taurine released from pituicytes under basal and hypoosmotic conditions may act to suppress axon terminal depolarization and thereby depress release of neurohypophysial peptides.

Structural characterization of peripheral nerve cells and nerve-muscle junctions of the oviduct of stable fly (Diptera:Muscidae).

Fine structure of both peripheral nerve cells and neuromuscular junctions associated with the oviduct of stable fly. Stomoxys calcitrans (L.), was described. Twelve or more multipolar peripheral neurons were found along major branch nerves that enter the ovipositor. Several were suspended in the haemacoel and others were in close proximity to the surface of the oviduct. Some peripheral neurons contained an abundance of neurosecretory granules that ranged in size from 32 to 180 nm in diameter. No glial elements enveloped the perikarya of such cells. Neurosecretory axons were usually found in the boundary region of large nerves just beneath the stroma. Peripheral nerve cells in close apposition to oviduct muscles were generally non-neurosecretory and were ensheathed in a glial perineurium. Peripheral neurons were surrounded occasionally by an extensive network of extracellular spaces in the glial perineurium. Other smaller neurons were found within large nerve trunks. Nerve-muscle junctions contained large clusters of synaptic vesicles and occasionally included small groups of neuro-secretory granules. Many nerve terminals on the surface of the oviduct contained a complex postsynaptic folding of sarcolemma. Active transmission sites were indicated by increased densities along the neurolemma. Some neurohemal release sites were also evident.

Development and secretory function of neurosecretory A cell in brain ofBombyx mori

Histological and immunohistochemical observations showed that four pairs of neurosecretory A cells coincide with the bombyxin neuron which produces the 4K-prothoracicotropic hormone (4K-PTTH). The A cells are characterized by a large cytoplasmic vacuole containing homogeneous granular materials, but the vacuole does not contain the bombyxin. Bombyxin axons from the A cells are distributed on the surface of the corpus allatum (CA), and neurosecretory granules (NSGs) are released by exocytosis at earlier stages than those in other neurosecretory axons (NSAs). Embryonic bombyxin neurons show characteristics of secretory function after the bristle formation stage (156 h after egg laying) and become the neurosecretory A cells in the larval stage. © 1996 Wiley-Liss, Inc.

Specific Associations of Neurosecretory or Neuromodulatory Axons with Insect Skeletal Muscles

The general process of neuromodulation in the skeletal muscles of various insects is accomplished via several different structural forms. In addition to motor axons, which may contain a modulatory substance as a co-transmitter, a second class of axons is found in close association with insect skeletal muscles. These axons typically contain dense cored vesicles. Some come directly into contact with the sarcolemma, but do not form a typical neuromuscular junction. Others have finely distributed branches within the muscle but only directly contact glial cells of the motor nerve branches. Immunocytochemistry has shown that these nerve types contain one or more of several potential transmitters: glutamate, octopamine, serotonin, leucokinin, proctolin, or insulin. While some of these substances are known to modulate muscle fiber contractile abilities directly, or to affect the mechanisms of neuromuscular transmission, others can be hypothesized to be involved in development, respiration, and other undiscovered functions.

Pentoxifylline inhibits the proliferation and the fibrogenic properties of human hepatic myofibroblastic cells

In the region of the distal optic chiasma of each optic lobe of Periplaneta americana, there is a group of about 120 monopolar neurosecretory cells. These cells do not stain with paraldehyde fuchsin but remain acidophilic after oxidation. They stain red or sometimes indigo with the azan technique. Histochemically, the neurosecretory material is positive for protein and the amino acids tryptophan and arginine but negative for 1, 2-glycols and strongly acidic groups. At the ultrastructural level, the cytoplasm of the cells contain many elementary neurosecretory granules 100 to 170 nm in dia. The cells also contain well-developed Golgi bodies and endoplasmic retieulum. The axons from these cells run toward the interior of the optic lobe. In this region, axons containing dense granules (mean diameter 70 nm) and synaptic vesicles synapse onto the axons from the neurosecretory cells. The neurosecretory axons then cross over to the anterior side of the optic lobe and run towards the brain. The function of these neurosecretory cells is unknown, but they may be involved with photoperiodically controlled activity rhythms.

Innervation of the rat anterior and neurointermediate pituitary visualized by immunocytochemistry for the growth-associated protein GAP-43.

Immunocytochemical localization of the neuronal growth associated protein GAP-43 revealed a dense axonal plexus throughout the neurointermediate lobe of the rat pituitary. These axons were fine, presumably monoaminergic fibers, whereas magnocellular neurosecretory axons did not appear to contain detectable GAP-43. These experiments also revealed the presence of an extensive nerve plexus within the anterior lobe. Fine beaded fibers were present throughout the parenchyma of the anterior lobe, and punctate staining suggestive of nerve terminals was seen surrounding numerous endocrine cells. Nerve fibers did not appear to cross directly between the intermediate and anterior lobes, but rather entered the anterior lobe directly from its margins or in association with blood vessels. Preabsorption of antisera with GAP-43 purified from neonatal rat brain completely eliminated immunoreactivity. These findings confirm the existence of a direct innervation of the anterior pituitary of the rat; moreover, the presence of GAP-43 in these fibers suggests that they may be capable of growth and terminal reorganization in the adult animal.

Nerve-dependent expression of high polysialic acid neural cell adhesion molecule in neurohypophysial astrocytes of adult rats

The adult hypothalamo-neurohypophysial system of the rat retains the capacity to express highly polysialylated isoforms of neural cell adhesion molecule normally expressed in developing tissues. Here we report that the expression of these isoforms in neurohypophysial astrocytes (pituicytes) may be regulated by neurosecretory cells. In the intact neurohypophysis a strong and homogeneously distributed immunostaining for the “embryonic”, highly sialylated form of neural cell adhesion molecules was detected by light-microscopic immunocytochemistry. By electron-microscopy, both neurosecretory axons and pituicytes were immunoreactive for this isoform. However, in contrast to the rather uniform staining on nerve fibres, polysialic acid immunolabelling on glial surfaces was uneven: immunostaining could be observed on glial surfaces facing neuronal elements, but not at contact sites between pituicytes. In addition, most glial and neuronal elements were heavily and evenly labelled with the polyclonal antibody recognizing “total” neural cell adhesion molecule. Surgical transection of the hypophysial stalk, a procedure that eliminates descending neurosecretory axons from the neurohypophysis, resulted in the complete disappearance of polysialic immunoreactivity from the neurohypophysis. The electron-microscopic analysis confirmed that cell surfaces of pituicytes lacked this immunoreactivity after the lesion. When residual neurosecretory axons were observed following an incomplete lesion, immunoreactivity on axons and glial processes was maintained. Transection did not affect the distribution of “total” neural cell adhesion molecule. We postulate that the presence of neurosecretory axons in the neurohypophysis is necessary to maintain the capacity of pituicytes to express immunoreactivity for the polysialylated isoforms of neural cell adhesion molecule but not the neural cell adhesion molecule itself since immunoreactivity for “total” neural cell adhesion molecule was unaltered after hypophysial stalk transection. This constitutes an example of how neuron-glia interactions could influence properties of the glial cell surface, which, in turn, could be a critical determinant of plasticity and regenerative capacities of the hypothalamo-neurohypophysial system.

Intrahypothalamically transected neurosecretory axons do not regenerate in the absence of glial cells.

Fifteen days after transection of the hypothalamo-neurohypophysial tract at the lateral retrochiasmatic hypothalamic area, neurosecretory axons had vigorously regenerated into transplants of explanted hypophysial neural lobe, to a lesser extent into sciatic nerve transplants, and least into optic nerve transplants. Regenerating axons were always closely associated with the specific glial cells of these grafts. When these glial cells were killed by cryotreatment prior to transplantation, neurosecretory axons did not regenerate into the abundant extracellular matrix of the transplants, including persisting basal lamina tubes in neural lobe and sciatic nerve grafts. The presence of viable glial cells is a prerequisite for neurosecretory axon regeneration.

Fine structure of the rectal pads in Grylloblatta compodeiformis (walker) (Orthoptera : Grylloblattidae) with reference to fluid transport

The rectal pads of the primitive insect Grylloblatta compodeiformis (Orthoptera : Grylloblattidae) were studied using light and electron microscopy. In this species, the rectal epithelium is thickened to form 6 prominent rectal pads, each of which is composed of tall columnar epithelial cells and laterally placed slender junctional cells, but is devoid of secondary cells. The rectal pads are interconnected by simple rectal epithelium, and are lined by a thin cuticular intima. They are surrounded by an extensive connective tissue space, which contains bundles of delicate connective tissue fibers, neurosecretory axons, and tracheae and tracheoles, which do not penetrate into the pads. The epithelial cells exhibit extensive infoldings of the apical plasma membranes that are closely associated with mitochondria. The lateral membranes are also highly folded around large mitochondria that possess longitudinally oriented cristae. These membrane folds form mitochondrial-scalariform junctional complexes and enclose intercellular channels and spaces. The apical cytoplasm of the epithelial cells contains numerous coated vesicles, dense tubular elements, multivesicular bodies and lysosomes, which suggests receptor-mediated endocytosis of macromolecules. The presence of large whorls of rough endoplasmic reticulum and abundant free ribosomes in the cytoplasm and nuclei with multiple, well-developed nucleoli indicate that the epithelial cells are actively engaged in protein synthesis. The ultrastructural features were examined in relation to their role in fluid transport in a cold habitat.

Regeneration of neural lobe-like neurovascular contact regions in explanted neural lobes placed in the hypothalamo-neurohypophysial tract in the lateral retrochiasmatic area

Neural lobes that had been explanted 30 days earlier were transplanted into retrochiasmatic lesions of the hypothalamo-neurohypophysial tract 15 days before being observed by electron microscopy and neurophysin immunohistochemistry. Neurovascular contact regions consisting of microvascular networks surrounded by neurophysin-immunoreactive terminals developed in 86% of the grafted explants. The fine structure of such regions resembled that of the neural lobe, with palisades of neurosecretory axon terminals abutting the basal laminae associated with the microvessels and plexuses of neurosecretory axons occupying the spaces within the vascular network. Both continuous and fenestrated blood vessels were present. Lamellopodia from glial cells partially ensheathed both axons and terminals, and sometimes separated the terminals from the perivascular basal lamina. Profiles in which neurosecretory granulated vesicles were depleted and many microvesicles were present were interpreted as terminals from which hormones had been released. No regeneration occurred into explants that had been cryotreated to kill their pituicytes and other cells before transplantation. These observations demonstrate that neurosecretory axons severed in the hypothalamus can regenerate to form a new neural lobe-like structure when an appropriate microenvironment is available, and that neural lobes explanted 21 days earlier retain the elements required to supply that microenvironment. They also provide evidence that viable pituicytes are essential for regeneration of neurosecretory axons and terminals into transplanted explants.

Fine structural characteristics of the zone of contact between the lower infundibular stem and the pituitary pars distalis in the little brown bat, Myotis lucifugus.

The purpose of this study was to examine the morphological characteristics of the pituitary gland in the little brown bat that might influence mechanisms of hypothalamic releasing hormone transport. Paraffin sections were prepared from whole crania to examine in situ the orientations of the three parts of the adenohypophysis (pars distalis, pars intermedia, and pars tuberalis) relative to the components of the neurohypophysis (pars nervosa and infundibular stem) and the basal hypothalamus. Of particular interest was the observation that the axis of the infundibular stem is directed posteriorly from the median eminence and occupies a depression in the dorsal surface of the pars distalis as it approaches the pars nervosa. Previous studies have revealed that neuronal projections containing releasing hormones extend into the infundibular stem in this species. Therefore, we conducted a fine structural study to determine whether the zone of contact between the infundibular stem and the pars distalis could represent a site of specialized interaction between hypophysiotropic hormones and their target cells. The results show that the sparse connective tissue along this boundary contains abundant fenestrated capillaries that are exposed on one side to neurosecretory axons and on the other to cells of the pars distalis. Furthermore, secretory cells nearest these capillaries exhibit ultrastructural evidence of heightened secretory activity. We conclude that the fine structural characteristics of this zone are consistent with localized mechanisms of releasing hormone transport.

Arginine vasopressin mobilises intracellular calcium via V 1-receptor activation in astrocytes (pituicytes) cultured from adult rat neural lobes

An extremely close association exists between the membranes of the neurosecretory endings and the resident astrocytes (pituicytes) of the neurohypophysis. Indeed, synaptoid contacts involving neurosecretory vesicle-containing axons contacting pituicytes have been observed. suggesting pituicytes as targets of the products released from neurosecretory axons. We have investigated the effects of various neural lobe peptides on pituicytes in primary culture from adult neurohypophyses. Using Fura-2 loaded cells and dynamic ratio imaging, we have determined that arginine vasopressin (AVP) or V 1- but not V 2-receptor agonists, mobilise potuicyte intracellular Ca 2+([Ca 2+] i) in the absence of extracellular Ca 2+. AVP was consistently effective at concentrations of 10 nM or higher in elevating [Ca 2+] i by 200–1000 nM. These responses could be blocked by V 1-antagonists and were shown to be associated with accumulation of phosphoinositides. Oxytocin was also found to mobilise [ [Ca 2+] i but was effective only at higher concentrations than for AVP. Oxytocin-evoked [Ca 2+] i elevations were also blocked by V 1-antagonists. Raising [K +] 0 was ineffective in changing [Ca 2+] i suggesting that these cells lack voltage-gated Ca 2+ channels. We conclude that pituicytes possess V 1-receptors, activation of which mobilises [Ca 2+] i, possibly functioning to initiate a Ca 2+-activated K + conductance which could contribute to further depolarisation of secretory terminals and facilitate exocytosis.

Fine structure of the neurohemal sinus gland of the shore crab, Carcinus maenas, and immuno-electron-microscopic identification of neurosecretory endings according to their neuropeptide contents.

The sinus gland of the shore crab, Carcinus maenas, is a compact assembly of interdigitating neurosecretory axon endings abutting upon the thin basal lamina of a central hemolymph lacuna. Four types of axon endings are distinguishable by the size distribution, shape, electron density and core structure of their neurosecretory granules. One additional type of axon ending is characterized by electron-lucent vacuoles and vesicles. The axon profiles are surrounded by astrocyte-like glial cells. Various fixations followed by epoxy- or Lowicryl-embedding were compared in order to optimize the preservation of the fine structure of the granule types and the antigenicity of their peptide hormone contents. By use of specific rabbit antisera, the crustacean hyperglycemic, molt-inhibiting, pigment-dispersing, and red-pigment-concentrating hormones were assigned to the four distinct granule types which showed no overlap of immunostaining. Epi-polarization microscopy and ultrathin section analysis of immunogold-stained Lowicryl-embedded specimens revealed that immunoreactivity to Leu-enkephalin and proctolin is co-localized with molt-inhibiting hormone immunoreactivity in the same type of granule. The size and core structure of the immunocytochemically identified granule types vary little with the different pretreatments but, in some cases, to a statistically significant extent. The present results are compared with those from earlier studies of sinus glands in different crustaceans. The methods of granule identification used in this study supplement the classical approach in granule typing; they are easier to perform and more reliable for the analysis of release phenomena in identified secretory neurons supplying the neurohemal sinus gland.

Development of neural lobe-like neurovascular contact regions after intrahypothalamic transection of the hypothalamo-neurohypophysial tract

Fifteen days after bilateral transection of the hypothalamo-neurohypophysial tract at the level of the lateral retrochiasmatic area, neurovascular contact regions had developed proximal to 66% of the lesions. Contact regions developed in every case when neural lobe explants were placed into the lesions, and near approximately half of the lesions into which small pieces of sciatic or optic nerve were transplanted. Neurovascular contact regions were characterized by microvascular networks surrounded by dense neurophysin-immunoreactive plexuses. At the fine structural level, the organization of such regions resembled that of the neural lobe, with the single exception that capillaries were not fenestrated. Nemerous neurosecretory axons were present, and palisades of neurosecretory axon terminals abutted perivascular basal laminae. Lamellopodia from glial cells partially ensheathed regenerating neurosecretory axons and often lay between terminals and the perivascular basal lamina. Terminals with many microvesicles and few neurosecretory granulated vesicles provided morphological evidence of hormone release.

Effects of interleukin-1 on the stress-responsive and -nonresponsive subtypes of corticotropin-releasing hormone neurosecretory axons.

Administration of interleukin-1 (IL-1) induces increases in plasma ACTH and glucocorticoids. Numerous experiments have implicated the hypothalamic CRH neurosecretory system in these responses, but have failed to provide evidence for involvement of the ACTH secretagogue vasopressin (VP). The rat CRH neurosecretory system contains two types of cells: VP expressing and VP deficient. Hence, the above findings suggested that IL-1 may selectively activate the VP-deficient subtype of CRH neurosecretory cells. In this study we employed postembedding electron microscopic immunocytochemistry to directly assay IL-1-induced depletion of secretory vesicles from identified VP-expressing and VP-deficient CRH neurosecretory axons. IL-1-induced depletion of secretory vesicles from these axons was correlated with increases in plasma ACTH and decreases in plasma PRL. No dose of IL-1 was found that could selectively activate one subtype of CRH neurosecretory axons; at doses of 0.67 microgram/100 g and above for both IL-1 alpha and IL-1 beta, equal depletion of vesicles from the two subtypes was observed. Similar results were previously found after the injection of bacterial lipopolysaccharide, which induces the release of IL-1 from macrophages. The findings unequivocally establish for the first time that IL-1 activates hypothalamic CRH neurosecretory cells in the absence of surgical stress, anesthesia, disruption of the infundibular area, or administration of toxic drugs. In addition, these data clearly demonstrate that IL-1 induces the release of VP from neurosecretory axons in the portal capillary zone of the external zone of the median eminence. Previous studies have shown that the VP-deficient subtype of CRH neurosecretory axons is not strongly activated by several types of stress; therefore, activation of the system by inflammatory mediators involves mechanisms different from those mediating the stress response.

Effects of interleukin-1 on the stress-responsive and -nonresponsive subtypes of corticotropin-releasing hormone neurosecretory axons

Effects of interleukin-1 on the stress-responsive and -nonresponsive subtypes of corticotropin-releasing hormone neurosecretory axons

Degenerative changes in the structure of neuromuscular junctions of Manduca sexta during metamorphosis.

During the degenerative processes that precede and accompany metamorphosis of the larval mesothoracic dorsal longitudinal muscles of Manduca sexta, the motor nerves and neuromuscular junctions undergo a variety of structural changes that are largely secondary to the changing morphologies of their respective glia. In the central region of the main motor nerve, the multiple layers of glial processes surrounding each of the large axons withdraw, leaving them apposed. In the peripheral region of the main motor nerve and in the secondary and tertiary nerve branches supplying the muscle, the outer glial processes of the nerve sheath and those that loosely wrap accompanying small neurosecretory axons all swell. Phagocytic cells and cells of unknown function invade the outer region of the nerve. In the neuromuscular junctions, the glial cells withdraw their processes from a complicated interdigitation with processes from the muscle fiber and from their relationship with the nerve terminal. As degeneration proceeds, this allows a greater area of contact between each nerve terminal and the muscle fiber. Within each junction there is a mixture of both functional and non-functional regions and active zones, as determined by both thin-section and freeze-fracture observations. No correlation was found between the degree of degeneration of a neuromuscular junction and its association with a particular muscle fiber or its position on the fiber relative to the origin or insertion.

Chapter 46: Development of the median eminence during ontogenesis (morpho-functional aspects)

This chapter discusses the observations on the development of the median eminence (ME) during ontogenesis that has been obtained mainly in recent years. Information of this kind is of particular importance for the evaluation of the establishment of the direct and the feedback regulations of the neuroendocrine system. At the level of the ME hypothalamic neurohormones are released from the neurosecretory axons to the hypophyseal portal circulation and transferred to the adenohypophysis, thus directly regulating this gland. In turn, adenohypophyseal hormones ascend via the same portal circulation to the ME, and finally to the hypothalamic neuroendocrine centers contributing to their short feedback regulation. At the level of the ME, neurohormones are discharged from the supraependymal axons to the cerebrospinal fluid (CSF). A part of the neurohormones circulating in the CSF penetrates through the CSF-brain barrier and controls the neuroendocrine centers in an ultrashort feedback manner. Another part is transferred via tanycytes to the hypophyseal portal circulation reinforcing the direct regulation of the adenohypophyseal functions. The axonal pathways for the transfer of adenohypophyseotropic neurohormones to the median eminence in rats are established by the end of fetal life.