The hypothalamo neurohypophysial axis (3)

Question 1

What are the two types of hypothalamic nuclei

Answer 1

paraventricular nuclei (axons pass through median eminence and terminate in the
neurohypophysis) or the supraoptic nuclei (axons pass through the median emninence and terminate just above the
optic chiasma  in the neurohypophysis)

Question 2

what are the two types of hypothalamic neurones within the paraventricular nuclei

Answer 2

Magnocellular neurones (larger) and they terminate in the neurohypophysis. Parvocellular neurones (smaller)which terminate either in primary capillary plexus in the median eminence or in the other parts of the brain

Question 3

what two molecules is the neurohypophysis associated with

Answer 3

vasopressin and oxytocin

Question 4

What neurones secrete vasopressin

Answer 4

Suprachiasmatic Nucleus and Vasopressinergic neurones

Question 5

Are suparoptic neurones magnocellular or parvocellular

Answer 5

magnocellular

Question 6

Features of Supraoptic neurones

Answer 6

They leave the hypothalamic supraoptic nuclei. Pass through the median eminence. They have herring bodies
which are specific to magnocellular neurones from the supraoptic and paraventricular nuclei. They release neurosecretions (hormones) in the neurohypophysis where they terminate. Molecules can be released from the herring bodies. They are either vasopressinergic or oxytocinergic

Question 7

What are herring bodies

Answer 7

granules that accumulate the newly synthesised hormones within the
axon/dendrites, forming swellings which then release the hormones into the general circulation

Question 8

Features of paraventricular neurones

Answer 8

Originate in paraventricular nuclei. Paraventricular neurones have a magnocellular AND a parvocellular component. Some parvocellular vasopressinergic neurones terminante in median eminence. Some parvocellular neurones pass to other parts of the brain. The majority of them are magnocellular and these pass down to the neurohypophysis. They are either vasopressinergic or oxytocinergic.

Question 9

which neurones have herring bodies

Answer 9

magnocellular neurones (supraoptic)

Question 10

What neurones secrete oxytocin

Answer 10

Oxytocinergic neurones

Question 11

What are the differences between oxytocin and arginine vasopressin (AVP)

Answer 11

AVP has Phenylalanine instead of Isoleucine. AVP has Arginine instead of Leucine
So they differ by two amino acids. AVP prohormone is cleaved to produce three molecules (AVP + Neurophysin + Glycopeptide). Oxytocin prohormone is cleaved to produce two molecules (Oxytocin + Neurophysin)

Question 12

What are the similarities betwee oxytocin and arginine vasopressin (AVP)

Answer 12

They are both nonapeptides. Both have a ring structure with 6 amino acids and a small attached chain. They are both initially synthesised as prohormones. Their prohormones are cleaved to form hormones and their neurophysin protein

Question 13

What is the synthesis of vasopressin

Answer 13

Pre-prohormone is pre-provasopressin. This is synthesised and then processed in granules (like the herring bodies) to form pro vasopressin
(pro-hormone). Prohormone breaks down into: Arginine Vasopressin (AVP), Neurophysin (NP)
and Glycopeptide. These products are formed in equimolar amounts. This break down takes place in the nerve axon.

Question 14

What is the synthesis of oxytocin

Answer 14

Pre-prooxytocin splits into pro-oxytocin. The pro-oxytocin is further processed to form oxytocin and a slightly different neurophysin. It does not have glycopeptide.

Question 15

What are the different vasopressin receptors

Answer 15

V1a and V1b. V2

Question 16

how do hormones affect the brain

Answer 16

by entering the cerebrospinal fluid

Question 17

Where is V1a receptor found

Answer 17

Vascular smooth muscle (vasoconstriction). Hepatocytes (glycogenolysis). CNS parvocellular neurones (behavioural effects)

Question 18

Where is V1b receptor found

Answer 18

Adenohypophysial corticotrophs (ACTH production)

Question 19

Where is V2 receptor found

Answer 19

Collecting duct cells (water reabsorption). Other unidentified sites (e.g. endothelial cells, blood clotting factors VIII and Von Willbrandt factor

Question 20

What does the V1 receptor do

Answer 20

A Gq protein linked receptor. Linked via G proteins to   
phospholipase C (PLC). Once activated, PLC converts PIP3 to IP3 and DAG which increase cytoplasmic [Ca2+] and activate other mediators such as Protein Kinase C (PKC) which produce cellular response

Question 21

What does the V2 receptor do

Answer 21

A Gs protein linked receptor. Linked via G protein to Adenylate Cyclase. Once activated, adenylate cyclase converts ATP to cAMP. cAMP activates Protein Kinase A (PKA). PKA activates other intracellular mediators which produce cellular responses (recruitment of aquaporins which causes the antidiuretic effect)

Question 22

What is the physiological action of vasopressin

Answer 22

Its principle physiological action is in the renal collecting duct (principle cells) where is stimulates water reabsorption. This results in its antidiuretic effect. Controls final concentration of urine

Question 23

What are the other actions of vasopressin

Answer 23

vasoconstriction, CNS effects, acting on neurotransmitters, corticotrophin release, synthesis of blood clotting factors, hepatic glycogenolysis

Question 24

how does vasopressin control renal water reabsorption

Answer 24

V2 receptors are found in the basolateral membrane. Vasopressin binds to the V2 receptor and activates adenylate cyclase. Adenylate cyclase converts ATP to cAMP. cAMP activates protein kinase A which stimulates the synthesis of AQUAPORIN 2 molecules

Question 25

Why do cells need aquaporins

Answer 25

All cells need aquaporins to allow the movement of water across the cell membrane

Question 26

What kind of aquaporin is aquaporin 2 (AQP2)

Answer 26

a vasopressin dependent aquaporin

Question 27

What happens when Aquaporin 2 is synthesised, under the influence of vasopressin

Answer 27

Aquaporins assemble into accumulations called Aggraphores.
These migrate to the apical membrane (facing the inside of the collecting duct). AQP2 is inserted into the membrane. AQP2 then acts as protein channel for water absorption from the tubule lumen into the cell. Water is transported out of the cells and into the blood via AQP3 and AQP4
An osmotic gradient is needed for the movement of water from the collecting duct, through the principal cells and into the blood

Question 28

Why is an osmotic gradient needed for the vasopressin physiological action

Answer 28

An osmotic gradient is needed for the movement of water from the collecting duct, through the principal cells and into the blood

Question 29

describe the control of vasopressin in water reabsorption

Answer 29

Increased plasma osmolality stimulates osmoreceptors leading to stimulation of vasopressinergic neurones. More vasopressin is released. More water is reabsorbed. Plasma osmolarity returns to normal. Stimulus is removed. Vasopressin is switched off.

Question 30

what is dehydration

Answer 30

increase in plasma osmolality

Question 31

Osmolality

Answer 31

measure of the concentration of osmotically active particles in the blood

Question 32

Why are osmoreceptors circumventricular organs

Answer 32

they must be in contact with vasopressinergic neurones but also with the blood so they lie outside the blood-brain barrier

Question 33

describe the control of vasopressin in vasoconstriction

Answer 33

fall in blood pressure is a massive stimulus for VP. When the blood pressure falls there is decreased baroreceptor activity. Therefore there is decreased inhibition of VP. More VP is released. So sympathetic activity increases because the baroreceptors are linked to the sympathetic nervous system which is the neural way in which vasoconstriction is controlled.

Question 34

What are baroreceptors

Answer 34

stretch receptors that are stimulated by an increase in pressure

Question 35

What are the major physiological actions of oxytocin

Answer 35

act on the uterus during parturition by causing the myometrial cells to contract to allow the delivery of baby. Also act on the breast during lactation by causing the myoepithelial cells to contract. This causes milk ejection.

Question 36

what stimulates milk production

Answer 36

prolactin

Question 37

what stimulates milk ejection

Answer 37

oxytocin

Question 38

what are the other actions of oxytocin

Answer 38

cardiovascular system- it causes transient vasodilation and tachycardia and constriction of umbilical arteries and veins. kidney - anti diuresis and secondary hyponatraemia. CNS - maternal behaviour and social recognition

Question 39

What does oxytocin do in the uterus

Answer 39

it promotes rhymic contraction from the fundus to the cervix. it increases local prostanoid production. it dilates the cervix

Question 40

what effects the uterine actions of oxytocin

Answer 40

progesterone suppresses it and oestrogen enhances is. most marked in late stages of pregnancy

Question 41

What is the endocrine efferent limb of the neuroendocrine reflex arc

Answer 41

It goes to the neurohypophysis where it stimulates the production of oxytocin causing contraction of the myoepithelial cells in the breast resulting in milk ejection

Question 42

what are the major clinical uses of oxytocin

Answer 42

induction of labour at term. prevention treatment of post partum haemorrhage. facilitation of milk let-down. autism- social responsiveness

Question 43

what happens if there is a lack of oxytocin

Answer 43

doesn’t cause major issues. parturition and milk ejection effects can be induced or replaced by other means

Question 44

what happens if there is a lack of vasopressin

Answer 44

Diabetes Insipidus - Characterised by polydipsia and polyuria. Central Diabetes Insipidus is when - no vasopressin is produced. Nephrogenic Diabetes Insipidus is when the kidneys are resistant to vasopressin.

Question 45

what happens when there is too much vasopressin

Answer 45

SIADH (Syndrome of Inappropriate ADH)

Too much ADH produced. Decreases plasma osmolality. Urine is concentrated