Regeneration of the neurosecretory tract in hypophysectomized catfish Ompok bimaculatus (Bloch)--an in situ study

Regeneration and reorganization of the proximal cut end of the pituitary stalk is demonstrated in Ompok bimaculatus with the aid of in situ staining technique. The lesioned end exhibited an increase of axonal density and some axons became prominent by their uncontrolled growth. In few instances the cut ends were reorganized into a neurohypophysis-like organ.     

The caudal neurosecretory system

Summary Recent information on the caudal neurosecretory system (urophysis) is collated, with special reference to cellular biology including neural relations, activity and chemistry of the biological principles associated with the urophysis, pharmacological analysis of the receptors for these principles, and their possible functions in a physiological sense. The existence of at least two principles, urotensins I and II, is well established. They differ pharmacologically and chemically and may arise from different cell types. At present, osmoregulation, cardiovascular regulation and reproduction are the most likely aspects of organismal physiology wherein these principles may be involved.Dedicated to Professor Doctor Berta Scharrer on the occasion of her seventieth birthday, with profound admiration and affection     

Pseudopodia formation by neurosecretory granules

Summary Ultrastructural studies of the mouse neurohypophysis, under various experimental conditions, revealed a number of neurosecretory granules (NSG) bearing single pseudopodia-like protrusions. Some NSG adhered to the axolemma via pseudopodia; other NSG, distant from the axolemma, budded electron lucent microvesicles from the tip of the pseudopod.Pseudopodia counts were made on electron micrographs, and calculated as a percentage of the NSG population. In neural lobes from intact mice, small numbers of pseudopodia were observed (0.3%); the count increased significantly after injections of large doses of horseradish peroxidase (HRP) (9.4–14.5%); hypertonic saline augmented the count, as did histamine.In vitro incubation experiments with isolated neural lobes in Krebs Ringer revealed concomitant pseudopodia formation and elevated vasopressin release (measured by antidiuretic bioassay) in the presence of HRP and di-butyryl cyclic AMP respectively. Histamine and excess potassium also increased hormone secretion, but did not induce pseudopodia formation in vitro; pseudopodia were observed neither in controls, nor in the presence of ineffective secretagogues.It is suggested that the pseudopod may represent the active site on the granule membrane. Different ultrastructural images of granule release suggest that several modes of hormone release may be operative in the neurohypophysis. The role of HRP in pseudopodia formation and vasopressin release is enigmatic.This study is dedicated to Prof. Berta Scharrer, esteemed mentor and beloved friendProf. Zwi Selinger, Department of Biological Chemistry of the Hebrew University, kindly collaborated in the in vitro experiments. Thanks are due to Mrs. Ilana Sabnai and Mrs. Sara Eimerl for excellent technical assistance. Research supported by the Binational Israel-United States Science Foundation (BNSF), grant 200     

Morphofunctional basis of neurosecretory cell plasticity

Mass accumulation and storage of neurosecretory products are typical only for nonapeptidergic elements, as it has been shown by our study of the structure and function in neurosecretory cells of different nature. All liberinergic, statinergic and monoaminergic neurosecretory cells keep constancy in the state of high functional activity of extrusive processes at normal conditions. Morpho-functional features of these elements principally differ from those of nonapeptidergic neurosecretory cells, which are characterized by remarkable secretory cycles. The extremely large size of elementary secretory granules, maximum development of the Herring bodies, various modes of secretion, secretory and extrusive cycles in neurosecretory function, and massive accumulation of neurosecretory granules occurring in neurosecretory terminals finally, all these characters are considered to be the primary features of a high plasticity of the nonapeptidergic neurosecretory cell. A high reactivity of nonapeptidergic neurosecretory cells has been demonstrated here by the quantitative ultrastructural research of the dynamics of functional activity of neurosecretory terminals at both experimental and physiological stressful states. The highest plasticity of nonapeptidergic neurosecretory cells compared to all other neurosecretory cell types may be provided by their ability to restore the initial law level of functional activity, referred to as "functional reversion".     

Retrograde axoplasmic transport of neurosecretory material

The axonal transport of neurosecretory material was studied in neurosecretory axons of the supraoptico-posthypophyseal system after in-situ transection of the median eminence. Two hours, 8 h, and 18 h after the lesion, both vasopressin and oxytocin antibodies revealed progressive accumulations of immunoreactive material not only in the proximal but also in the distal stumps of the transected axons. The electron-microscopic examination of these axonal portions revealed that such intense immunopositive labelings could be correlated, in both stumps, to a conspicuous accumulation of neurosecretory granules. It is concluded that, under normal physiological conditions, a significant amount of axoplasmic neurosecretory material is transported in retrograde direction and that such a retrograde transport mainly involves neurosecretory granules.     

Plasticity in the hypothalamic magnocellular neurosecretory system

Over the past decade or so, plasticity has emerged as an important, quantifiable property of the mammalian hypothalamic magnocellular neurosecretory system. This plasticity has turned out to be genuinely related to normal function in the sense that it is a set of responses to physiological stimulation rather than only the sequelae of insult or injury, and it is generally completely reversible. This latter property, of course, distinguishes it further from the plasticity observed after injury. Four features of this magnocellular system that have been shown to display predictable and reversible intercellular plasticity are reviewed: the relationships between neurons and their associated astrocytic glia at various levels (dendritic somatic and terminal) of the magnocellular elements; the extent of terminal and glial contact with the basement lamina in the neurohypophysis; the type and possible efficacy of synaptic input, and the extent of electrotonic coupling among the magnocellular neurons.     

PIKfyve negatively regulates exocytosis in neurosecretory cells

Regulated secretion depends upon a highly coordinated series of protein-protein and protein-lipid interactions. Two phosphoinositides, phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3-phosphate, are important for the ATP-dependent priming of the secretory apparatus prior to Ca(2+)-dependent exocytosis. Mechanisms that control phosphoinositide levels are likely to play an important role in priming fine tuning. Here we have investigated the involvement of PIKfyve, a phosphoinositide 5-kinase that can phosphorylate phosphatidylinositol 3-phosphate to produce phosphatidylinositol 3,5-bisphosphate on large dense core vesicle exocytosis from neuroendocrine cells. PIKfyve localizes to a subpopulation of secretory granules in chromaffin and PC12 cells. Nicotine stimulation promoted recruitment of PIKfyve-EGFP onto secretory vesicles in PC12 cells. YM-201636, a selective inhibitor of PIKfyve activity, and PIKfyve knockdown by small interfering RNA potentiated secretory granule exocytosis. Overexpression of PIKfyve or its yeast orthologue Fab1p inhibited regulated secretion in PC12 cells, whereas a catalytically inactive PIKfyve mutant had no effect. These results demonstrate a novel inhibitory role for PIKfyve catalytic activity in regulated secretion and provide further evidence for a fine tuning of exocytosis by 3-phosphorylated phosphoinositides.