Endocrinology 3 - The Hypothalamo-neurohypophysial axis Flashcards
What are the two hypothalamic nuclei associated with the posterior pituitary gland?
- Paraventricular nuclei
- Supra-optic nuclei
What is a hypothalamic nucleus?
A collection of cell bodies.
What types of neurones are present the paraventricular neurones?
- The majority are magnocellular, which pass down to the neurohypophysis via the medial eminence.
- Some are parvocellular, These are small and pass to other parts of the brain or the median eminence.
What types of parvocellular neurones terminate in the median eminence?
Some vasopressinergic neurones.
What types of neurones are supraoptic neurones?
These neurones are magnocellular.
Describe the position and structure of magnocellular neurones.
- They pass through the median eminence and terminate in the neurohypophysis.
- They have herring bodies along the axon.
What are herring bodies?
Granules that accumulate the newly synthesised hormones within the
axon/dendrites, forming swellings which then release the hormones into the general circulation.
What hormones are produced by the supraoptic neurones and the paraventricular neurones?
They produce vasopressin and oxytocin. Therefore, they are vasopressinergic or oxytocinergic.
Compare the chemical structures of vasopressin and oxytocin.
- Both have 9 amino acids, with a connecting disulfide bridge.
- There are 2 amino acids different - Vasopressin contains Phe and Arg, while oxytocin contains Ile and Leu.
Describe the process of synthesis of vasopressin.
- Pre-provasopressin has a signal peptide so it is converted to provasopressin.
- Provasopressin contains arginine vasopressin, neurophysin and glycopeptide.
What is different between oxytocin synthesis and vasopressin synthesis?
Oxytocin synthesis has different neurophysin and the glycopeptide is absent.
What happens in the cell upon vasopressin binding to V1 receptors on cells?
- Linked by Gg proteins to phospholipase C.
- IP3 is produced alongside DAG, which causes Ca2+ release and PKC production.
What are the two types of V1 receptors, and what are their affects on tissues?
- V1a causes vasoconstriction in arteriel/arteriolar smooth muscle, as well as glycogenolysis.
- V1b causes production of adrenocorticotrophic hormone
What is the effect when vasopressin binds to V2 receptors?
- Water reabsorption in the collecting duct (ADH)
Describe the process that occurs when vasopressin binds to V2 receptors in the collecting duct.
- Vasopressin binds to Gs receptors on the basolateral membrane
- Adenyl cyclase converts ATP to cAMP, which activates protein kinase A
- This stimulates synthesis of aquaporin 2 in aggraphores, which insert aquaporin into the apical membrane so water can be reabsorbed.
- Aquaporin 3 and 4 in the basolateral membrane transport the water into the blood.
List the physiological actions of oxytocin.
- Major effects are in the uterus and mammary gland.
- Minor effects in the cardiovascular system and kidney
Describe the physiological action of oxytocin in the uterus.
- Here oxytocin is acting on myometrial cells, causing a rhythmic contraction form fundus to cervix.
- This increases prostanoid production, and dilation of the cervix
Which hormones suppress and enhance oxytocin?
- Progesterone suppresses oxytocin
- Oestrogen enhances oxytocin
Describe the physiological action of oxytocin on the mammary gland.
Here oxytocin causes contraction in the myoepithelial cells, resulting in milk ejection.
How is oxytocin controlled by neuro-endocrine reflex?
- Sucking causes a neural response to the hypothalamus, causing the production of oxytocin to increase and the cells contract to release milk
How can oxytosin be used in the kidney and cardiovascular systems?
- It is similar to vasopressin, so may be used to increase uptake of water from urine (anti-diuretic).
- It can treat tachycardia and increase vasodilation, so blood flow to the organs increases.
Describe the action of oxytocin in the CNS
Causes tend and befriend, meaning it increases affiliative behaviour. This is like fight or flight but in women.
What are the major clinical uses of oxytocin?
- Induction of labour at term by IV infusion
- Prevention treatment of post-partum haemorrhage (supresses bleeding)
- Encourage release of milk in breast feeding mothers
Describe the homeostatic control of vasopressin in the kidney.
- Increased plasma osmolarity stimulates release of vasopressin, this results in a decrease in plasma osmolarity so vasopressin is no longer released.
- Decreased arterial blood pressure detected by baroreceptors stimulates release of vasopressin, which increases vasoconstriction and therefore increases the blood pressure (stimulus is removed - vasopressin is no longer produced)
What does a lack of oxytocin cause?
Partuition (giving birth) and milk ejection effects must be replaced by other means.
What happens if there is a lack of circulating vasopressin?
- Diabetes insipidus (large water consumption, constant thirst and large amounts of dilute urine passed)
- This is central (/cranial), meaning too little is produced.
What is the second type of diabetes insipidus, other than cranial?
- Nephrogenic diabetes insipidus, where the kidney has a resistance to vasopressin
- In this case, the levels of vasopressin will be high.
