The Hypothalamo-Neurohypophysial System Flashcards
What are the two hypothalamic nuclei associated with posterior pituitary gland and where do their axons terminate?
Paraventricular nucleus - axons pass through median eminence and terminate in neurohypophysis
Supraoptic nucleus - axons pass through the median eminence and terminate just above the optic chiasma in neurohypophysis
Two types of neurones in paraventricular nucleus?
Magnocellular - larger, terminate on neurohypophysis (supraoptic plexus neurones are also magnocellular)
Parvocellular - smaller, terminate on primary capillary plexus
What do the paraventricular and supraoptic neurones produce?
Both can be either vasopressinergic or oxytocinergic and release either vasopressin or oxytocin
Process of vasopressin production?
Pre-provasopressin is synthesised and processed in granules to form pro-vasopressin
Provasopressin then forms vasopressin, neurophysin proteins, and glycopeptide in EQUIMOLAR amounts
These are released as neurosecretions of hormone and neurophysin proteins
What is similar and different about oxytocin synthesis?
The initial events (preprohormone to prohormone to hormone) are the same
The neurophysin produced is different
NO GLYCOPEPTIDE PRODUCED
Similarities in vasopressin and oxytocin structures?
Both have a ring structure with 6 amino acids
Both have a small attached chain of pre-amino acids
Differences in vasopressin and oxytocin structures?
Differ by 2 amino acids in different locations
There is a difference in the ring structure, Vasopressin has Phenylalanine replaced by Isoleucine in Oxytocin
There is a difference in the chain structure, Vasopressin has Arginine replaced by Leucine in Oxytocin
Principal physiological action of vasopressin?
Increased water reabsorption in the principal cells of the collecting duct of the kidney
Some other actions of vasopressin throughout the body?
Vasoconstriction
Corticotrophin (ACTH) release
Synthesis of blood clotting factors (VIII and Von Willebrand factor)
Hepatic glycogenolysis
What are the different vasopressin receptors and their locations?
V1a receptors - vascular smooth muscle, hepatocytes, CNS parvocellular neurones
V1b receptors - Adenohypophysial corticotroph cells
V2 receptors - Collecting duct principal cells
Process of V1 receptor activation?
Vasopressin binds
G-protein-linked receptor which activates phospholipidase C enzyme
IP3, DAG produced which increase cytoplasmic Ca2+ and other mediators
Produces a cellular response
Process of V2 receptor activation?
Vasopressin binds
G-protein linked receptor which activates Adenylate cyclase
Increases ATP to cAMP conversion
cAMP activates protein kinase A which activates cellular response
Cellular response is increased synthesis of aquaporins, particularly AQP2
Which membranes do V2 receptors lie on and AQP2 molecules insert into?
Basolateral membrane of principal cell for receptor (vasopressin in blood)
Apical membrane for vasopressin-dependent AQP2 (water in lumen moves down conc gradient through AQP2)
Where do AQP3/4 channels lie and why?
In BASOLATERAL MEMBRANE for water movement out of cell into blood
Process of synthesis of AQP2 to insertion into apical membrane?
AQP2 synthesised
Packaged into aggraphores which migrate to apical membrane
Fuse with membrane leading to insertion of AQP2 into membrane
How is vasopressin stimulated to release to change water reabsorption and process of doing so?
Increased plasma osmolarity detected by osmoreceptors in the brain Increased activation of neurones Increased VP secretion into circulation Increased water reabsorption in kidneys --> DECREASE in plasma osmolarity
How is vasopressin stimulated to release to alter vasoconstriction and process of doing so?
Decreased arterial blood pressure detected by baroreceptors (decreased stimulus of sympathetic NS)
Increased VP secretion
Increased vasoconstriction
–> INCREASE in arterial blood pressure
Difference in prolactin and oxytocin effects on breast milf production/
Prolactin - SYNTHESIS
Oxytocin - EJECTION
Oxytocin actions on uterus and breast?
Uterus - during parturition, myometrial cells contract, baby is delivered
Breast - during lactation, myoepithelial cells contract, milk ejected
Neuroendocrine reflex arc of oxytocin? (same arc as prolactin with different end response)
Suckling stimulates tactile receptors Activation of neural afferent limb Increased oxytocin release from neurohypophysis Activation of endocrine efferent limb Milk ejection response
Comparative clinical importance of oxytocin and vasopressin and any conditions arising from problems?
Oxytocin - not hugely important, can be replaced by artificial milk and delivery of baby can be induced
Vasopressin - very important, insufficient = diabetes insipidus, too much = SIADH (syndrome of inappropriate ADH)
What is diabetes insipidus and characteristic symptoms?
Condition where you lose too much water in urine due to reduced water reabsorption
Polyuria - excessive urination
Polydipsia - excessive drinking
What are the two types of DI?
Central - caused by having/producing no vasopressin
Nephrogenic - caused by tissue insensitivity to vasopressin
