Neurohypophysial Disorders

Question 1

Name the two main nuclei within which neurones of the neurohypophysis have their cell bodies.

Answer 1

Paraventricular Nucleus - Contains magnocellular and projections to other parts of the CNS not just neurohypophysis
Supraoptic Nucleus - Only contains magnocellular oxytocin and vasopressin neurons

Question 2

What two hormones are produced by the neurohypophysis?

Answer 2

Vasopressin

Oxytocin

Question 3

What is the principal action of vasopressin and how does it carry out this action?

Answer 3

Vasopressin’s main action is on the V2 receptors in the renal cortical and medullary collecting ducts
It stimulates the synthesis and assembly of aquaporin 2, which then increases water reabsorption and has an antidiuretic effect (less urine production)

Question 4

State some other actions of vasopressin.

Answer 4

Vasoconstriction
Corticotrophin release
Factor VIII and von Willebrand factor
Central effects

Question 5

What are the main actions of oxytocin?

Answer 5

It is a contractile molecule that binds to oxytocin receptors
It causes contraction of the myometrium during parturition and is involved in milk ejection
It also has central effects

Question 6

What are the consequences of a lack of the neurohypophysial hormones?

Answer 6

Lack of Oxytocin – not clinically significant

Lack of Vasopressin – Diabetes Insipidus

Question 7

What are the two forms of diabetes insipidus?

Answer 7

Central (cranial) and Nephrogenic Diabetes Insipidus

Question 8

What can cause central diabetes insipidus?

Answer 8

Damage to neurohypophysial system (injury, surgery, cerebral thrombosis, tumours, granulomatous infiltration, pituitary surgery, craniopharyngioma)
Idiopathic
Familial (rare)

Question 9

What can cause nephrogenic diabetes insipidus?

Answer 9

Familial (rare)

Drugs e.g. lithium, dimethyl chlortetracycline (DMCT)

Question 10

State some signs and symptoms of diabetes insipidus.

Answer 10

Polyuria 
Polydipsia 
Hypo-osmolar urine  
Dehydration 
Possible disruption of sleep  
Possible electrolyte imbalance

Question 11

State another cause of polydipsia that isn’t diabetes.

Answer 11

Psychogenic polydipsia
This is a central disturbance that increases the drive to drink
Osmotic stimuli still preserved

Question 12

What test can be used to distinguish between normal, psychogenic polydipsia, central DI and nephrogenic DI? Describe the results you would expect.

Answer 12

Fluid deprivation test
 Normals and psychogenic polydipsia will show a rise in urine osmolality
 Central and nephrogenic diabetes insipidus will show little or no change in urine osmolality
Fluid deprivation with administration of DDAVP (Desmopressin)
 Central diabetes insipidus will show a rise in urine osmolality
 Nephrogenic DI will still have a low urine osmolality (because of end-organ resistance)

Question 13

Why is the urine osmolality of someone with psychogenic polydipsia lower (in the fluid deprivation test) than a normal subject?

Answer 13

Over time, the constant passage of large volumes of water through the kidneys will wash out the osmotic gradient that is necessary for AVP to exert its diuretic effect

Question 14

Describe the normal change in urine osmolality as plasma osmolality increases.

Answer 14

Normally, urine osmolality will increase as plasma osmolality increases (in a graph of urine osmolality against plasma osmolality it will show a sigmoid shape)
In DI, there is little change in urine osmolality as plasma osmolality increases

Question 15

Describe changes in plasma vasopressin following administration of hypertonic saline in a normal subject, psychogenic polydipsia, central DI and nephrogenic DI.

Answer 15

Hypertonic saline will increase the plasma osmolality and hence will increase the vasopressin secretion in patients that have the capacity to produce vasopressin (normal, psychogenic polydipsia and nephrogenic DI)
Patients with central DI can’t produce vasopressin at all so the hypertonic saline will show no change in plasma vasopressin

Question 16

What is SIADH?

Answer 16

Syndrome of Inappropriate ADH = when the plasma vasopressin concentration is inappropriate for the existing plasma osmolality

Question 17

State some signs of SIADH.

Answer 17

Decreased urine volume Increased urine osmolality

Question 18

What is the main consequence of SIADH?

Answer 18

HYPONATRAEMIA

Question 19

State some symptoms of SIADH that are caused by the hyponatraemia.

Answer 19

Can be symptomless
At relatively mild hyponatraemia = generalized weakness, poor mental function, nausea <120mM
Severe hyponatraemia = confusion, coma, death <110mM

Question 20

State some causes of SIADH.

Answer 20

Tumours (ectopic secretion)
Neurohypophysial malfunction (e.g. meningitis, cerebrovascular disease)
Thoracic disease (e.g. pneumonia)
Endocrine disease (e.g. Addison’s)
Physiological – it can happen under normal circumstances where AVP is release is stimulated by non-osmotic stimuli (e.g. hypovolaemia, pain, surgery)
Drugs
Idiopathic

Question 21

How is SIADH treated?

Answer 21

Fluid Restriction
Provide appropriate treatment when the cause is identified (e.g. surgery for a tumour)
NOTE: if someone is hyponatraemic you need to deal with that as soon as possible – e.g. use drugs that prevent vasopressin action in the kidneys

Question 22

What is the name given to exogenous vasopressin?

Answer 22

Argipressin

Question 23

Where are V1 and V2 receptors found?

Answer 23

V1
 Vascular smooth muscle 
 Non-vascular smooth muscle 
 Anterior pituitary
 Liver 
 Platelets 
CNS
V2
 Kidney
 Endothelial cells

Question 24

State the pharmacological actions of argipressin.

Answer 24

NATRIURESIS – this is one of the unexplained side-effects of having a large amount of vasopressin – it is V2 mediated and only happens when given at high doses
PRESSOR ACTION – V1 mediated vasoconstriction – the coronary vessels are particularly sensitive to vasopressin (this can cause cardiac ischaemia and angina attacks)
Contraction of vascular smooth muscle
Increased ACTH secretion
Increased factor VIII and vWF production

Question 25

State two selective peptidergic vasopressin selective agonists.

Answer 25

V1 – Terlipressin

V2 - Desmopressin

Question 26

Compare the effects of argipressin and desmopressin.

Answer 26

Argipressin acts on V1 and V2
Argipressin is more effective in causing vasoconstriction via V1 receptors Desmopressin is more effective in the kidneys in causing water reabsorption via V2 receptors

Question 27

State some clinical uses of Desmopressin.

Answer 27

Treatment of diabetes insipidus
Treatment of nocturnal eneuresis
Haemophilia (need V2 stimulation)
NOTE: it is taken orally or nasally

Question 28

State some unwanted effects of Desmopressin.

Answer 28

Nausea
Headaches
Abdominal pain
Fluid retention and hyponatraemia

Question 29

State two peptidergic V1 receptor agonists and their uses.

Answer 29

Terlipressin – used to treat oesophageal varices (it causes vasoconstriction)
Felypressin – injected by dentists along with local anaesthetic (the vasoconstriction keeps the local anaesthetic at the site of injection for longer thus prolonging the action of the local anaesthetic)

Question 30

State one treatment used for nephrogenic diabetes insipidus and its possible mechanism of action.

Answer 30

Thiazide Diuretics (e.g. bendroflumethiazide)
This inhibits the Na+/Cl- pump in the DCT leading to a diuretic effect
This leads to volume depletion resulting in a compensatory increase inNa+ reabsorption from the PCT
This increases proximal water reabsorption so less water reaches the collecting duct
This ultimately leads to reduced urine volume

Question 31

What are vaptans?

Answer 31

Non-peptide vasopressin receptor antagonists
Tolvaptan = V2 receptor antagonist
It is used to treat hyponatraemia associated with SIADH and may be useful in treating congestive heart failure

Question 32

State some drugs that increase or decrease vasopressin secretion.

Answer 32

Increase vasopressin secretion = nicotine

Decrease vasopressin secretion = alcohol + glucocorticoids

Question 33

Where are osmoreceptors located?

Answer 33

Neurons located in organum vasculosum which project to hypothalamic PVN and SON.

Question 34

When does osmorecptor firing increase?

Answer 34

Increase in extracellular sodium leads to h20 leaving and causes shrinkage of osmoreceptors. This increases osmoreceptor firing leading to VP release from hypothalamic and PVN&SON neurons.

Question 35

Summarise the biochemical features of diabetes insipidus

Answer 35

Hypernatraemia
Raised urea
Increased plasma osmolality
Dilute urine - low urine osmolality.

Question 36

Summarise the biochemical features of psychogenic polydipsia

Answer 36

Mild hyponatraemia - excess water intake
Low plasma osmolality
Dilute urine - low osmolality