Tissue neurosecretory

In humans, the hypothalamic-derived protein and the hormone noncovalent complexes are packaged in neurosecretory granules, then migrate along axons at a rate of 1 4 mm/h until they reach the posterior pituitary where they are stored prior to release into the bloodstream by exocytosis (67). Considerable evidence suggests that posterior pituitary hormones function as neurotransmitters (68) vasopressin acts on the anterior pituitary to release adrenocorticotropic hormone [9002-60-2] (ACTH) (69) as well as on traditional target tissues such as kidneys. Both hormones promote other important central nervous system (CNS) effects (9,70). 

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. 

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 hypothalamic control of the posterior pituitary is quite different than that of the anterior and intermediate lobes. Specific neurons have their cell bodies in certain hypothalamic nuclei. Cell bodies in the paraventricular nuclei manufacture oxytocin, whereas the supraoptic nuclei contain cell bodies that synthesize ADH. The axons from these cells extend downward through the infundibulum to terminate in the posterior pituitary. Hormones synthesized in the hypothalamic cell bodies are transported down the axon to be stored in neurosecretory granules in their respective nerve terminals (located in the posterior pituitary). When an appropriate stimulus is present, these neurons fire an action potential, which causes the hormones to release from their pituitary nerve terminals. The hormones are ultimately picked up by the systemic circulation and transported to their target tissues. 

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... 

Elucidation of the primary structure of PBAN opened up a new field of research with immense possibilities. The exact site and the specific neurosecretory cells that produce PBAN need to be identified. We also need to determine where, when, and how PBAN is released. Development of antibodies for their use in immunohistochemistry should prove useful. A radioimmunoassay for PBAN will be useful to more precisely detect the hormone in various tissues such as hemolymph, and follow the time-course of release and determine PBAN titer at various times of the day. Identification of the receptors for PBAN is crucial to the study of its mechanism of action (how does PBAN induce pheromone production ). 

Proteohormones, steroid hormones and peptide hormones may be differentiated on the basis of their chemical structure. Hormones may be classified as neurosecretory (e. g. hypothalamus), glandular (endocrine glands) or aglandular according to their source tissue. Aglandular hormones are also known as tissue hormones they are synthesized in special cells located in specific organs. Hormones may be differentiated according to their transport characteristics into unbound hormones and hormones which are coupled to a carrier protein. A further classification is based on their principal functions, (s. tab. 3.9)... 

Research increasingly shows that the distinction between the nervous and endocrine systems is not as clear as was once thought. For example, certain nerve cells, referred to as neurosecretory cells, synthesize and release hormones into the blood. Oxytocin and vasopressin (see p. 125) are two prominent examples. In addition, several neurotransmitters act through second messengers. Epinephrine, which can function as a neurotransmitter and a hormone, induces Tissue-specific effects dependent upon the nature of the receptor to which it binds. 

Many hormones are rich in cysteine and the tissues in which they accumulate can be easily recognized by a marked uptake of S-L-cysteine. For instance, mature virgin mice, mature mice of both sexes and castrated males display a S-labelled juxtamedullary X-zone in the brain, whereas normal adult male mice do not . The neurosecretory system of the earthworm markedly accumulates cysteine- S The neurosecretory cells of rapidly developing female locusts and females in the second gonotropic cycle take up cysteine- S at a greater rate than either newly emer d or slowly developing females . 

Typical for arthropod (such as insects, spiders, crustaceans) neurosecretory tissue is that the secretory part of the glands is the outside surface see Fig. 25.1b 1), which, therefore, is readily accessible for MALDI matrix application. Furthermore, the tiny size of the tissue allows for whole mount tissue analysis (i.e., without the need of a cryostat) and for full gland analysis within a relatively short time frame. An additional advantage is that live... 

All these features have made the cockroach neurosecretory system our favorite model for direct tissue (neuro)secretory peptide investigations, for many years (2). 

As already discussed neurophysin-M consisted of two protein components separable by electrophoresis. One of the two constituents occurred in neurosecretory granules and the other appeared to be a degradation product. When attempts were made to separate them by fractional precipitation in the presence of vasopressin by the addition of increasing amounts of salt the proportion of the two remained constant. However, after a few days the amorphous precipitate kept at crystallized. This was interesting because it was the first time that vasopressin had been obtained in crystalline form 26, The need to separate the two proteins in neurophysin-M was removed when it was found that only one of them was present in extracts of an acetone powder of pituitary tissue. 

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