Neurosecretory Elements Research Articles

Morphology and monoaminergic modulation of Crustacean Hyperglycemic Hormone-like immunoreactive neurons in the lobster nervous system.

Neuronal somata located near branch points in the second thoracic nerve roots of the lobster are immunoreactive for Crustacean Hyperglycemic Hormone (CHH)-like peptides, a family of putative stress hormones. We have employed intracellular dye injection, immunostaining, and confocal imaging to observe the anatomy of these root neurons, which are morphologically diverse and dye coupled. Some root neurons contribute to neurosecretory structures at the points of exit of the root from the nerve cord. Other CNS-projecting root neurons send projections into the T5-A1 interganglionic connectives. Neurosecretory elements of the serotonin (5HT) and octopamine (OCT) systems, implicated in postural control and aggression, terminate densely in the vicinity of the second thoracic root neurons. We have confirmed by double immunostaining for 5HT and CHH-like peptides that the endings of the 5HT neurons are in close apposition to root neurons in the superficial regions of the root. We have also extended previous studies documenting electrophysiological responses of the root neurons to 5HT or OCT. Bath-applied 5HT and OCT inhibit the spontaneous bursting activity of root neurons at concentrations higher than 100 nM. The root neurons desensitize to the persistent presence of high concentrations of 5HT, but not OCT, in the bath. Nanomolar concentrations of OCT, but not 5HT have an excitatory effect on the spontaneous bursting activity of root neurons. This region of the lobster nervous system is of continuing interest, as identified neurons of three neuromodulatory systems implicated in stress and aggression converge and interact at the level of identified neurons.

Electron Microscopy Study of Anterior Neuropituitary of the Sterlet Acipenser ruthenus

Structural organization of sterlet anterior neuropituitary (ANP), a homologue of the median tubercle of Tetrapoda, was studied by method of electron microscopy. The ANP was conventionally divided in three zones in dorsoventral direction. Neurosecretory fibers (NsF) were concentrated mainly in the central and in the external zones. Terminals of NsF are directly adjacent to the basal membrane (BM) of the brain vascular sheath to form a morphological substrate for release of neurohormones into the pericapillary space. The presence of invaginations of the brain connective tissue sheath, which are in a close contact with NsF, indicates an increase of the area of neurosecretory contacts. Neurosecretory granules (NsG) of various shape and size are revealed in NsF and their terminals. Comparison of light-microscopy immunohistochemical data with localization of NsF terminals containing NsG of different diameters allows reconstructing distribution of neurosecretory elements of various ergicity in the ANP. Patterns of co-localization of various biologically active substances were revealed in the same varicose dilatations or terminals. Detection of typical synaptic contacts in the sterlet ANP indicates possibility of the cholinergic pituitary regulation, alongside with the peptide- and monoaminergic regulation.

A central negative feedback action of thyroid hormones on thyrotropin- releasing hormone secretion

Two experiments were conducted to test the hypothesis that thyroid hormones exert central negative feedback effects on the secretion of TRH from the hypothalamus in the ewe. In the first experiment, we examined the effects of thyroidectomy on the secretion of TRH and TSH. Thyroidectomy was followed by an unambiguous increase in TRH in pituitary portal plasma and TSH in the peripheral circulation. In the second experiment, we tested the effects of T4 replacement to thyroidectomized ewes. T4 replacement reversed the effects of thyroidectomy on TRH and TSH release. Of interest, TRH secretion in thyroidectomized ewes was continuously elevated during the collection, raising the possibility that TRH is secreted continuously, rather than exclusively in a strictly pulsatile manner indicative of phasic discharges synchronized among TRH neurosecretory elements. Collectively, these results suggest that thyroid hormones can act centrally to inhibit TRH (and thus TSH) release in the ewe, and they support the concept that at least part of the negative feedback action of thyroid hormones is exerted at the hypothalamic level.

A central negative feedback action of thyroid hormones on thyrotropin-releasing hormone secretion.

Two experiments were conducted to test the hypothesis that thyroid hormones exert central negative feedback effects on the secretion of TRH from the hypothalamus in the ewe. In the first experiment, we examined the effects of thyroidectomy on the secretion of TRH and TSH. Thyroidectomy was followed by an unambiguous increase in TRH in pituitary portal plasma and TSH in the peripheral circulation. In the second experiment, we tested the effects of T4 replacement to thyroidectomized ewes. T4 replacement reversed the effects of thyroidectomy on TRH and TSH release. Of interest, TRH secretion in thyroidectomized ewes was continuously elevated during the collection, raising the possibility that TRH is secreted continuously, rather than exclusively in a strictly pulsatile manner indicative of phasic discharges synchronized among TRH neurosecretory elements. Collectively, these results suggest that thyroid hormones can act centrally to inhibit TRH (and thus TSH) release in the ewe, and they support the concept that at least part of the negative feedback action of thyroid hormones is exerted at the hypothalamic level.

Neuroendocrine regulation of ovarian maturation in the Indian white prawn Penaeus indicus H. Milne Edwards

The control and regulation of ovarian maturation by neurosecretory elements in the eyestalk, brain and thoracic ganglia of Penaeus indicus have been examined. In the X- organ complex of the eyestalk, secretions from type A and B cells are apparently involved in inhibiting ovarian maturation, while in the brain and thoracic ganglia, secretions from type GN, A and B cells had a stimulatory effect on the ovary. The pyriform C cells were not involved with ovarian maturation. Both unilateral and bilateral eyestalk ablation resulted in precocious maturation of the ovary. The histology of the remaining eyestalk in unilaterally ablated animals revealed that all the NSCs in the X-organs were in a suppressed state and the sinus gland was devoid of granular aggregations. Abnormal behaviour was noticed in bilaterally ablated prawns and strangely few animals moulted with developing ovaries. Due to the synchronous occurrence of moulting and reproductive activities the probability of both gonad and moult inhibiting hormones being the same is discussed. In addition the relationship between gonadal maturation and the moult cycle has also been studied.

Neuronal and glial gap junctions in the goldfish preoptic area, a thin section and freeze-fracture study

In freeze-fracture, both large macular gap junctions and long thin gap junctions surrounded by a strand of tight junction were found on neurosecretory cells. Preoptic neurons show large areas of soma-to-soma apposition, but thin section showed no evidence for gap junctions between neuronal somata. Neurosecretory cell neurites formed parallel bundles in neuropil lateral to the nucleus, and gap junctions were found between the neurites. These junctions apparently correspond to macular junctions seen on neurosecretory elements in freeze-fracture. Some large macular gap junctions found in freeze-fracture presumably correspond to junctions seen between glial cells in thin section. However, glial membranes lacked characteristics distinguishing them from neuronal membranes. In one instance, a large apparent glial sheet process formed both macular and long thin gap junctions on different surfaces. The long thin gap junctions that were surrounded by a strand of tight junction were formed with a large neurosecretory cell soma. Extensive pinocytosis was observed at some membranes forming gap junctions.

Optimizing release from peptide hormone secretory nerve terminals.

Secretion of the peptide neurohormones oxytocin and vasopressin from terminals of magnocellular neurones in the mammalian neurohypophysis is elicited by conduction of depolarizing action potentials into terminal membranes, inducing opening of voltage-sensitive Ca2+ channels, entry of Ca2+ from the extracellular space and a rise in cytoplasmic Ca2+ concentration. The amount of peptide released per action potential is not immutable. In particular, the patterns in which action potentials are generated at the cell somata of the two types of neurone each appear exquisitely suited to optimize the release process at the terminal by utilizing a frequency-facilitation mechanism and by minimizing a mechanism of fatigue in the release process. The different properties of oxytocin and vasopressin neurones are of important physiological significance. The secretory terminals are also a site of receptor-mediated influences of neuromodulators which can greatly alter release efficiency. The mechanisms underlying facilitation and fatigue are not clearly understood. The evidence suggests that processes both prior to depolarization of the terminals (propagation and form of the action potentials) and directly at the terminals (frequency/pattern-dependent Ca2+ entry and channel openings) are involved. Transient activity-related increases in extracellular K+ concentration may be involved at both sites. Two types of neuromodulation have been partly characterized. Kappa-Opioid receptors in secretory terminal membranes directly modulate depolarization-evoked peptide release probably via interactions with Ca2+ channels. beta-Adrenergic receptors localized on neurohypophyseal astroglial cells mediate more subtle effects of noradrenaline. In the more chronic situation the neurohypophyseal astroglia alter their morphological relationships with neurosecretory elements and the basal lamina at release sites, changes which may also serve to optimize the secretory process.

Neuronal components of the circadian clock in the scorpion, Androctonus australis: Central origin of the efferent neurosecretory elements projecting to the median eyes

The central origin of efferent neurosecretory fibers (ENSF) supplying the median eyes of the scorpion, Androctonus australis, with circadian information was investigated at the light- and electron-microscopic levels by the use of several tracer techniques. The results obtained convincingly show that the central neurons stained via the optic nerve are the source of the ENSF in the retina. Almost all of these neurons give rise to two different axon collaterals, one of which projects into the ipsilateral, the other into the contralateral median-eye nerve. Three protocerebral contact zones of the ENSF system are described, and their functional role within the circadian clock system is discussed.

The hypothalamo-hypophysial system of the frog Rana temporaria

The median eminence (ME) of the adult frog, Rana temporaria, was studied by means of electron microscopy including quantitative electron-microscopic autoradiography. In frogs captured in May and June numerous peptidergic neurosecretory fibres extending via the internal zone to the pars nervosa display large swellings containing few granules, mitochondria, neurotubules and cisternae of the smooth endoplasmic reticulum. In addition, few secretory globules up to 1.5 μm in diameter occur in these varicosities. In animals collected during the autumn period many of these neurosecretory swellings filled with neurosecretory granules and polymorphic inclusions resemble Herring bodies. Three types of granule-containing neurosecretory fibres were observed in the external zone (EZ) of the ME of adult R. temporaria. Peptidergic A1- and A2-type fibres are characterized by granules 150–220 nm and 100–160 nm in diameter, respectively. Monoaminergic fibres of type B with granules approximately 100 nm in diameter represent ∼ 50% of all neurosecretory elements in the EZ of the frog ME; ∼12% of the total number of granule-bearing axons in the EZ actively taking up radiolabelled 5-hydroxytryptophan are thought to be serotoninergic terminals. Neurosecretory terminals of all types and glial vascular endfeet establish direct contacts with the perivascular space of the primary portal capillaries. Some neurosecretory terminals are separated from the lumen of the third ventricle by a thin cytoplasmic lamella of tanycytes. The possible physiological significance of this structural pattern is discussed.

Evidence for structural plasticity in the supraoptic nucleus of the rat hypothalamus in relation to gestation and lactation

Supraoptic nuclei of lactating rats present a particular anatomical organization that could serve to facilitate the synchronization of neuronal firing observed during suckling-induced reflex milk ejections. Although magnocellular neurones are usually separated by neuropil elements, particularly glial fibers, in lactating rats, numerous neurosecretory soma and dendritic profiles are in direct apposition, without glial interposition. Concomittantly, there is also a higher incidence of presynaptic terminals contacting two neurosecretory elements in the same plane of section (“double” synapses). In the present study, a quantitative ultrastructural analysis was used to trace the evolution of the structural reorganization of the nucleus at different stages of the reproductive cycle. The percentage of neurosecretory soma and dendritic profiles in direct apposition was low two weeks after the beginning of pregnancy, but the day prior to parturition, as during lactation, over 40% of all neurosecretory profiles were directly in contact and involved about 10% of the total neuronal surface membrane measured (a 5-fold increase over the corresponding frequencies recorded in virgin rats at oestrus). The contiguous neuronal membranes and associated intercellular space appeared unmodified, except for the presence of attachment plaques, that also increased in frequency at late gestation and lactation. The incidence of “double” synapses also increased by late gestation, so that at lactation, they bridged 8% of all the recorded neurosecretory somata and dendrites, (as compared to 1% in the virgin rats). Similar changes were observed during a first and second gestation and lactation. The incidence of direct appositions and “double” synapses then diminished gradually after weaning: 2 months after the end of lactation, the ultrastructure of the nucleus resembled that of virgin animals. These observations demonstrate a plasticity in the structural organization of the supraoptic nucleus that appears closely related to changing physiological states of the animal and that involves both neurone-glial relationships and the neurones' synaptic configuration.

Somatostatin-like immunoreactive material in the rabbit retina

Somatostatin-like immunoreactivity was detected by a specific radioimmunoassay in extracts of retinas of Dutch-belted rabbits at a mean concentration of1.1±0.1 ng/retina, or 170±30 pg/mg protein. On both gel permeation chromatography and reversed phase high performance liquid chromatography, the majority of this material was indistinguishable from somatostatin-14; a smaller amount of immunoreactive material co-chromatographed with somatostatin-28. The somatostatin-like immunoreactivity was concentrated in a P 2 (synaptosomal) fraction prepared by differential centrifugation. The concentration of somatostatin-like immunoreactivity was unaffected by the prior intravitreal injection of the selective neurotoxins 6-hydroxydopamine and 5, 7-dihydroxytryptamine. The rabbit retina, therefore, contains readily detectable amounts of material chemically similar to somatostatin-14 and somatostatin-28. This material is concentrated in neurosecretory elements of the retina, but is not contained in cells which accumulate dopamine or indoleamines. The rabbit retina is suitable for in vitro studies of the central nervous system function of the somatostatin family of peptides.

Evidence that TRH, somatostatin and substance P are present in neurosecretory elements of the vertebrate retina

Evidence that TRH, somatostatin and substance P are present in neurosecretory elements of the vertebrate retina

The microanatomy and fine structure of the rostellum of Hymenolepis diminuta.

The microanatomic arrangement of the muscular, nervous, and excretory systems as well as the organization of the tegument are described for the rostellum of Hymenolepis diminuta. An inner circular and an outer longitudinal layer of muscle comprise the rostellar capsule, delimiting the rostellum from the scolex proper. A similar muscular arrangement surrounds an apical invagination of the rostellar tegument, the anterior canal. Elements of the excretory and nervous systems enter the rostellum basally through a discontinuous region of the capsule. Although excretory canals extend into the rostellum, flame cells and their associated collecting ducts are absent. The rostellar nervous system is comprised of a single bilateral pair of ganglia, which provide motor innervation of the anterior canal, and the circular muscles of the rostellar capsule; it also receives dendrites from apical uniciliate sensory receptors. The tegument lining the anterior canal and covering the apical rostellum is syncytial and continuous with the tegument of the scolex proper. Twelve to 15 cytons are radially arranged around the anterior canal. They stain selectively with paraldehyde-fuchsin between days 3 and 35 postinfection, but are clearly tegumental, not neurosecretory elements. The appearance of ovoid granules in the rostellar tegumentary cytons coincides with the onset of fuchsinophilia, but the granules persist despite the subsequent loss of this staining characteristic. Intact granules are secreted into the lumen of the anterior canal, although their function has not been ascertained.

Modulation of juvenile hormone synthesis by an analogue in the cockroach

The juvenile hormones (JH) are novel insect sesquiterpenoid products synthesised by paired endocrine glands known as the corpora allata (CA). These glands are innervated by neural and neurosecretory elements originating in the central nervous system (CNS)1–3 and are known to undergo predictable changes in synthesis and release of JH during the reproductive cycle4–8. This implies that the CA are regulated by intrinsic physiological signals. For example, in the viviparous cockroach, Diploptera punctata, if the haemolymph titres of JH are experimentally altered by removal of one member of the pair or by implantation of additional CA, the original CA undergo appropriate corrective responses in JH synthesis9,10. This is partly due to signals from the CNS because denervation of the CA attenuates the compensatory change10. The depression in JH synthesis after addition of exogenous JH to the system has also been observed11,12. However, there have been no direct measurements of JH synthesis following treatment with exogenous JH analogues in vivo. We show here that in Diploptera punctata, topical application of a JH analogue (JHA) depresses the synthesis of JH at high doses but stimulates synthesis at low doses and that the analogue can support oocyte growth and maturation in the absence of the CA and that application to intact animals stimulates oocyte growth rates. These results indicate that the analogue can replace authentic JH.

THE ANATOMY OF THE DECAPOD CRUSTACEAN AUXILIARY HEART

The anatomy of an auxiliary heart found in many decapod crustaceans is described. This heart is found at the anterior end of the dorsal median artery before the artery divides to supply the cerebral nervous system. The heart is essentially two strips of modified somatic muscle located inside a sinus in the dorsal median artery. These muscles are innervated by four motoneurons located in the supraesophageal ganglion. Sensory innervation and possible neurosecretory elements are also described.

Immunocytochemical study of the hypothalamo-neurohypophysial system. I. Localization of neurosecretory neurons containing neurophysin-I and neurophysin-II in the domestic pig.

Antibodies raised against porcine neurophysin-I and porcine neurophysin-II using an injection regime in rabbits over a short time period, were used to localize neurophysin-I and neurophysin-II in hypothalamic neurosecretory elements of the domestic pig. In transverse section, neurophysin-II containing cells were more abundant in the dorsal medial region of the rostral supraoptic nucleus (SON) as compared with the distribution of neurophysin-I neurons. The main bulk of the cells of the SON were heavily stained for neurophysin-I with neurophysin-II containing cells positioned dorsal from the edge of the optic chiasma. Neurosecretory cells of the SON as seen in sagittal section also showed a differential staining for neurophysins-I and -II. Rostral regions of the pig paraventricular nucleus (PVN) contained magnocellular elements near the third ventricle which were stained predominantly for neurophysin-II. In regions corresponding to the caudal PVN there appeared two populations of neurosecretory neurons: (a) an area of cells adjacent to the third ventricle which contained neurophysin-II antigen and (b) a group of densely populated cells in the dorsal-lateral region which was stained for neurophysin-I. The results support the existence in the pig of at least two distinct populations of neurosecretory neurons corresponding to the neurophysin-I and neurophysin-II neurosecretory system.

Neurosecretory system of the brain-corpus cardiacum complex in diapausing Tyria jacobaeae L. (Lepidoptera : Arctiidae)

The neurosecretory elements of the brain-corpora cardiaca complex were examined in early- (ED) and late-diapausing (LD) pupae of Tyria jacobaeae (Lepidoptera), an insect with obligatory diapause. Neurosecretory granules were present in pars intercerebralis (PI) and lateral (LA) cells of the brain, as well as in the axon component of corpora cardiaca (cc). Variations were observed in the shape, density and distribution of the granules. Differences exist between ED and LD brain cells. Axonal pathways linking PI-LA cells and cc were devoid of granules, but exhibited changes at the microtubular level. This is the first time such observations have been made in an obligatory diapause pupa.

On degeneration of peptidergic neurosecretory fibres in the albino rat

Three types of degenerating peptidergic neurosecretory fibres have been found in the posterior pituitary of chronically dehydrated albino rats. "Dark" neurosecretory fibres and their swellings contain neurosecretory granules, neurotubules, shrunken mitochondria and diffusely distributed fine dense material. Some swellings are filled with synaptic vesicles and/or conglomerations of dense membranes. The transitional forms exist between these fibres and extracellular accumulations of electron dense material. Synaptic vesicles, single neurosecretory granules, lipid-like droplets and lamellar bodies occur in the latter. Some neurosecretory fibres and swellings have numerous polymorphous inclusions arising due to degradation of secretory inclusions and organelles, mitochondria and neurotubules in particular. "Dark" neurosecretory elements and those with numerous polymorphous inclusions are enveloped by pituicyte cytoplasm. Sometimes the plasma membranes both of the pituicytes and neurosecretory fibres are destroyed or transformed into a multi-membrane complex. It is assumed that pituicytes may phagocytize degenerating neurosecretory elements. N urosecretory fibres with a locally dissolved neuroplasm and/or large lucent vacuoles seem to be due to axonal degeneration by the "light" type. These neurosecretory elements, the largest of them in particular, may transform into large cavities bordered by a membrane and containing flake-like material and single-membrane vacuoles. Degeneration of neurosecretory elements seems to occur mainly due to hyperfunction of the hypothalamo-hypophysial neurosecretory system.

The infracerebral gland and cerebral neurosecretory system—a probable neuroendocrine complex in phyllodocid polychaetes

Three components of probable neuroendocrine significance have been detected within or associated with the cerebral ganglion of Eulalia and Phyllodoce. In Eulalia, the neurosecretory system consists of three types of neuron, whose axons terminate on the inner surface of the brain capsule. Neurosecretory elements (interpreted as axon collaterals) also form synapses within the brain. The secretory end-foot system is a neuronal complex of probable glandular significance which forms terminals packed with mitochondria that intermingle with the typical neurosecretory endings. The infracerebral gland is made up of C 1 and C 2 cells. C 2 cells are neurons of possible glandular function. They give rise to “transcapsular fibres” that extend into the brain, and to processes that probably terminate adjacent to a blood vessel or coelomic sinus.

Localization of neurophysin in the neurosecretory elements of the hypothalamus and neurohypophysis of the normal and osmotically stimulated guinea-pig as demonstrated by immunofluorescence histochemical techniques.

Antisera to porcine neurophysin-II and ovine neurophysin-III were used to localize neurophysin-like material in the hypothalamus and neurohypophysis of guinea-pigs with an immunofluorescence technique. Although the guinea-pig appears to have only one major neurophysin it was found to be localized in both of the bilateral magnocellular nuclei. Neurophysin-like material was also present in extreme rostral portions of the hypothalamus, in cells lying between the third ventricle and the supraoptic nucleus and in a cluster of cells dorsomedial to the fornix. Immunofluorescence was observed in neurosecretory fibres that followed pathways previously characterized with classical histological stains for “neurosecretory material”.