Neurosecretory cells

As its name implies, the neurohypophysis is derived embryonically from nervous tissue. It is essentially an outgrowth of the hypothalamus and is composed of bundles of axons, or neural tracts, of neurosecretory cells originating in two hypothalamic nuclei. These neurons are referred to as neurosecretory cells because they generate action potentials as well as synthesize hormones. The cell bodies of the neurosecretory cells in the supraoptic nuclei produce primarily antidiuretic hormone (ADH) and the cell bodies of the paraventricular nuclei produce primarily oxytocin. These hormones are then transported down the axons to the neurohypophysis and stored in membrane-bound vesicles in the neuron terminals. Much like neurotransmitters, the hormones are released in response to the arrival of action potentials at the neuron terminal. 

As discussed previously, the neurohypophysis has a direct anatomical connection to the hypothalamus. Therefore, the hypothalamus regulates the release of hormones from the neurohypophysis by way of neuronal signals. Action potentials generated by the neurosecretory cells originating in the hypothalamus are transmitted down the neuronal axons to the nerve terminals in the neurohypophysis and stimulate the release of the hormones into the blood. The tracts formed by these axons are referred to as hypothalamic-hypophyseal tracts (see Figure 10.2). The action potentials are initiated by various forms of sensory input to the hypothalamus. Specific forms of sensory input that regulate the release of ADH and oxytocin are described in subsequent sections in this chapter. 

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary leaves the blood by way of the secondary capillary plexus and exerts its effect on the appropriate cells of the adenohypophysis. The hypophyseal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue. 

The primary factor that influences ADH secretion is a change in plasma osmolarity. Osmoreceptors in the hypothalamus are located in close proximity to the ADH-producing neurosecretory cells. Stimulation of these osmoreceptors by an increase in plasma osmolarity results in stimulation of the neurosecretory cells an increase in the frequency of action potentials in these cells and the release of ADH from their axon terminals in the neurohypo-... 

Greenberg DA, Carpenter CL, Messing RO (1987) Calcium channel antagonist properties of the antineoplastic antiestrogen tamoxifen in the PC12 neurosecretory cells. Cancer Res 47 70-74... 

From these studies, some patterns of effect potentially mediated by the endocrine system have been observed. These include the accumulation of vitellogenin in ticks [50-52] and accelerated ovarian development in beetles and ticks [51, 53]. Different researchers have attributed these effects to a range of different mechanisms, including induced excitation of neurosecretory cells releasing juvenile hormone [50], and ecdysteroid disruption either by blocking the neuropeptide itself or at the epidermal site of synthesis [51]. Other researchers have not corroborated these results and have even found contradictory effects, e.g., suppressed ovarian development [52], Therefore at present for SPs, there is no clear evidence for adverse population-relevant effects with an underlying endocrine mode of action. 

The highly vascularized respiratory epithelium is composed of five major cell types ciliated cells, nonciliated cells, columnar cells, goblet cells, and basal cells. Low numbers of neurosecretory cells are present in the basement membrane [17]. Approximately 20% of the total number of cells in the lower turbinate area is ciliated with fine projections ( 100 per cell) on the apical cell surface. Cilia are used to transport the mucus toward the nasopharynx. These long (4-6 fxm) and thin projections are mobile and beat with a frequency of 1,000 strokes per min. Ciliated and nonciliated columnar cells are populated with about 300 microvilli per cell, which help in enlarging the surface area. 

The neurosecretory cells of the supraoptic nucleus receive both the GABAergic inhibitory input mentioned above (Section 2.7) and an excitatory glutamate input Whereas the D4 heteroreceptor-mediated depression of GABA release disinhibits the neurosecretory cells (Section 2.7), the D4 heteroreceptor-mediated depression of glutamate release (Table 1) inhibits them and, hence, the pituitary secretion of oxytocin and vasopressin. 

Ichikawa T. (1998) Activity patterns of neurosecretory cells releasing pheromonotropic neuropeptides in the moth Bombyx mori. Proc. Natl. Acad. Sci. USA 95, 4055 4-060. 

Ichikawa T., Hasegawa K., Shimizu I., Katsuno K., Kataoka H. and Suzuki A. (1995) Structure of neurosecretory cells with immunoreactive diapause hormone and pheromone biosynthesis activating neuropeptide in the silkworm, Bombyx mori. Zool. Sci. 12, 703-712. 

Sato Y., Ikeda M., Yamashita O. (1994) Neurosecretory cells expressing the gene for common precursor for diapause hormone and pheromone biosynthesis activating neuropeptide in the subesophageal ganglion of the silkworm, Bombyx mori. Gen. Comp. Endocrin. 96, 27-36. 

Malosio, M. L., Giordano, T., Laslop, A., and Meldolesi, J. (2004). Dense-core granules A specific hallmark of the neuronal/neurosecretory cell phenotype.. Cell Sci. 117, 743—749. 

Bern, H. A. (1962). The properties of neurosecretory cells. In General and comparative endocrinology Supplement, 1 117-32. 

Evidence for neurosecretory cells in the cestode Hymenolepis microstoma. Canadian Journal of Zoology, 55 1726-33. 

Alexander MJ (1993) Estrogen-regulated synthesis of neurotensin in neurosecretory cells of the hypothalamic arcuate nucleus in the female rat. Endocrinology 733 1809-1816. 

Q3 Vasopressin is a small peptide hormone consisting of nine amino acids, most of which is synthesized in neurosecretory cells of the supraoptic nucleus of the hypothalamus. Small quantities are also produced in the neighbouring paraventricular nucleus. The hormone is transported down the axons of the neurosecretory cells via the infundibulum to the posterior pituitary, where it is stored until release into the blood is triggered by nerve impulses from the hypothalamus. Vasopressin is better known as antidiuretic hormone (ADH). The name vasopressin relates to its vasoconstrictor action, which increases pressure in the vascular system. This action was discovered before its effects on water retention were known. 

Prothoracicotropic hormone (Bombyx PTTH) of the silkworm, Bombyx mori, was extracted from adult heads. Bombyx PTTH is suggested to be a glyco-peptide and consists of two essentially identical subunits. Amino acid sequencing and cDNA analysis revealed the whole amino acid sequence of the subunit, composed of 104-109 residues. Bombyx PTTH stimulated adult development in brainless Bombyx pupae at a dose of ca. 0.1 ng and also enhanced the release of ecdysone in vitro at a concentration of 10 M. Immunohistochemistry and in situ hybridization showed that Bombyx PTTH was transcribed and translated in two pairs of dorso-lateral neurosecretory cells in the brain. 

By use of a synthetic pentadecapeptide corresponding to the amino-terminal region of Bombyx PITH, a monoclonal antibody that specifically recognized Bombyx PITH was prepared. Immunohistochemistry of Bombyx brains using this antibody revealed that two pairs of dorso-lateral neurosecretory cells were immunoreactive (Mizoguchi, et aK in preparation). 

Further, in situ hybridization using S-labeled complementary RNA probe showed that the Bombyx PITH gene was transcribed in the same cells, indicating that Bombyx PITH is synthesized in these cells (12). In Manduca sexta two pairs of brain neurosecretory cells in a similar location have been immunohistochemically identified as the PTTH-producing cells (14). 

Although these rhythmic contractions of the extensor>tibiae are initiated by spontaneous depolarizations in certain fibers within the muscle, they are regulated by a specific cell on the dorsal surface of the metathoracic ganglion. This cell is found in a cluster of neurosecretory cells called the dorsal unpaired median (DUM) neurons. Neuromuscular transmission and muscular contraction in the extensor-tibiae muscle is modulated by the biogenic amine octopamine and by the peptides, proctolin, and FMRFamide. 

These neurons were of different t3rpes interneurons, motor neurons, and neurosecretory cells. A small, stereotyped population of PLI neurons was found in the Drosophila larval central nervous system (CNS). In the periphery, proctolin-immunoreactive neuromuscular endings were identified on both visceral and skeletal muscle fibers. On the hindgut, the neuropeptide is associated with endings on intrinsic circular muscle fibers. In this study, the presence of proctolin was verified in the CNS, hindgut, and segmental body wall by tissue extraction followed by reverse-phase HPLC and quantitative bioassay. Evidence for a proctolin-like substance has also been found in the adult Colorado potato beetle... 

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