NU BIOL 5541 - The Pituitary Gland (2): The Posterior Lobe (Neurohypophysis)

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The Pituitary Gland (2): The Posterior Lobe (Neurohypophysis)Embryological derivation and general structure• The posterior lobe of the pituitary gland is derived from a downward growth of neural tissue from the developing brain• The posterior pituitary is comprised of the nerve axons descending through the pituitary stalk from their cell bodies in the hypothalamus, and numerous glialcells called pituicytes which make up the bulk of the neural lobe• Also called the neurohypophysis• Separated from the adenohypohysis by an intermediate lobe (fairly non-existent in humans)• Magnocellular unmyelinated neurons, with swellings along the axons called Herring bodies• Fenestrated capillaries allow for passage of molecules released from the nerve endings into the general circulation• Neurons are described as being either vasopressinergic (approximately 80%) or oxytocinergic• The paraventricular nuclei, unlike the supraoptic nuclei, are composed of both magnocellular and parvocellular neurons which project either to the medianeminence or to other parts of the brain• Act as hormones on their target cells in the adenohypophysis, the main one being vasopressin which acts as a corticotrophin releasing factor• The parvocellular axons going to other parts of the CNS release their neurosecretions across synapses to other neurons (act as neurotransmitters)Synthesis, storage, release and transport of neurohypophysial hormones• Vasopressin and oxytocin are both nonapeptides synthesized in the neuronal cell bodies of the hypothalamic SON and PVN• Pro-vasopressin is processed into vasopressin, a 93-amino acid neurophysin II and and a 39-amino acid glycopeptide called copeptin• Pro-oxytocin is similarly processed within the vesicles to just oxytocin and a slightly different neurophysin I• Vesicles are transported down the nerve axons by axoplasmic flow, involving the intracellular microtubular network and protein “motor” molecules kinesin anddynein• Calcium ions, in conjunction with the microfilament network in the cytoplasm, bring about the movement of vesicles towards the cell membrane to which theyfuse• Release of vesicle contents by exocytosis is accompanied by the separation of hormone from neurophysin in the presence of the more alkaline pH of theextracellular fluid• It is believed that the Herring bodies and even undiluted parts of the nerve axon can also release the molecules by exocytosis• Potential release of the hormones into the cerebrospinal fluid of the third ventricle could explain some of their central effects, particularly in those parts of thebrain devoid of vasopressinergic and oxytocinergic neurons• Overall firing rates increase or decrease with changing stimuli• Some degree of local coordination of firing activity in adjacent neurons• Oxytocin and vasopressin are transported unbound to their target tissues• Relatively short half-lives (approximately 5 minutes)Vasopressin (VP)• Receptors and mechanisms of action• 3 forms, GPCRs• Vasopressin also has some small affinity for the oxytocin receptor• V1a receptor is found in a variety of vasopressin’s target tissues, including vascular smooth muscle, hepatocytes, cardiomyocytes, platelets, the adrenalcortex, the kidneys, and the CNS• V1b receptors are found on the corticotrophs of the adenohypophysis, the heart, lungs, thymus, mammary glands, CNS• V2 receptor is found in the principal cells of the renal cortical and medullary collecting ducts and the loops of Henle• The V1 receptor and its signaling pathways• GPCR activates the membrane-bound enzyme phospholipase C which catalyzes the hydrolysis of a membrane phospholipid called PIP2, producing DACand IP3• IP3 opens calcium channels in intracellular calcium stores such as the ER, allowing calcium ions to move into the cytoplasm (CREB can be induced bycalcium/calmodulin-dependent kinase)• DAG phosphorylates intracellular proteins (in the presence of calcium, activates PKC)• The V2 receptor and its signaling pathways• The enzyme activated by the GPCR is an adenyl cyclase - activates PKA• The principal cells of the renal collecting ducts synthesize a vasopressin-dependent aquaporin which is essential for the movement of water from tubularfluid into the cells• Physiological actions of vasopressin• The magnocellular neurons which terminate within the neurohypophysis release vasopressin into the general circulation which transports it to its targetcells in tissues throughout the body• The parvocellular neurons terminating on the walls of the primary capillary plexus in the median eminence release vasopressin into thehypothalamo-adenohypophysial portal system which transports it to its specific target cells (corticotrophs) in the anterior pituitary• Peripheral actions• Antidiuretic effect: stimulation of the water reabsorption process in the collecting duct (dysfunction leads to DI)• Migration of aggraphores (vesicles) containing AQP2 proteins towards the apical membranes of cells, where they are inserted• Longer term synthesis of new AQP2 molecules via the activation of CREB which enters the cell nucleus and promotes gene expression• Degradation or recycling of V2 receptor via beta-arrestin activation• Additional synthesis of another aquaporin (AQP3) which enables water to leave the cell via the basolateral membrane• Other renal effects: stimulation of sodium and chloride reabsorption from the ascending limb of the loop of Henle, and the stimulation of reabsorption inthe thin descending limb of the loop of Henle and the collecting duct• Vasoconstrictor activity: in situations such as dehydration, hemorrhage, or other conditions of volume depletion, the pressor effect of vasopressin is ofphysiological relevance in maintaining, or raising, blood pressure to normal levels• Vasodilator activity: in the kidneys• Blood coagulation: stimulation of molecules which participate in coagulation, namely Factor VIII and the von Willebrand factor (vasopressin used intreatment of hemophilia)• Hepatic glycogenolysis: contributes to the increase in the blood glucose concentration which is an essential element of the normal stress response• Adenohypophysial action: stimulate corticotrophin release• Central actions• Vasopressin is released from dendrites abutting the walls of the third ventricle, and the concentration of vasopressin within the CSF is higher than in theplasma• Main functions are

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NU BIOL 5541 - The Pituitary Gland (2): The Posterior Lobe (Neurohypophysis)

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