Lecture 3 - Hypothalamo-hypophysial axis: Neurohypophysis Flashcards
What are the key features of hypothalamo-neurohypophysial system (HNHS)?
Suprachiasmatic nucleus - makes vasopressin
Paraventricular nucleus - axons go to NH and to other parts of CNS
Supraoptic nucleus - above optic chiasm
Median eminence
Hypothalamus
Where are the cell bodies present (of HNHS)?
In supraoptic and paraventricular nuclei
Where do the magnocellular neurones terminate and what main feature do they have?
In neurohypophysis and have a long axon
Where do parvoventricular neurones originate?
Paraventricular nuclei
Where do parvoventricular neurones terminate?
In median eminence or other parts of brain
What do supraoptic neurones synthesise?
EITHER vasopressin OR oxytocin - so they’re either VPergic or oxytocinergic
Describe supraoptic nuclei’s way of reaching the neurohypophysis:
They leave hypothalamic supraoptic nuceli Pass through the median eminence Terminate in neurohypophysis Has Herring bodies along axon - store hormones and can release it into surrounding medium
What are the types of paraventricular neurones?
Parvocellular Magnocellular
Where do parvocellular neurones terminate?
Some to other parts of the brain Some are VP neurones and terminate in median eminence
How is vasopressin synthesised?
Initially synthesised as prohormones: Pre-provasopressin > Pro-vasopressin > Vasopressin + Neurophysin + glycopeptide
How is oxytocin synthesised?
Pre-prooxytocin > pro-oxytocin > oxytocin + Neurophysin
Difference between oxytocin and vasopressin synthesis
The neurophysin differs by two amino acid: AVP has an arginine in 2nd position and Phe in square bit OT has a Leucine in 2nd position and Ile in square bit
Give examples and definition of a nonapeptide
A nonapeptide is an oligopeptide containing 9 a.a. residues Examples: Oxytocin and VP
Name the principal action of VP
Stimulates water reabsorption in the renal collecting ducts - results in ANTIDIURETIC effect Na+ reabsorption in other parts of kidneys
Describe other actions of VP
VASOCONSTRICTION
Corticotrophin release
CNS effects
Acts as neurotransmitter (aspects of behaviour - finding a partner/looking after young)
Synthesis of blood clotting factors (VIII and von Willbrandt factor)
Hepatic glycogenolysis (caused by every stressor)
List the different Vasopressin receptors
V1a V2 V1b (V3) - adenohypophysial corticotrophs (corticotrophin production)
Describe the function of V1
Involved in vasoconstriction - Arterial smooth muscle Glycogenolysis - hepatocytes Behavioural and other effects - CNS neurones
Describe the function of V2 - MOST IMPORTANT RECEPTOR
Water reabsorption - collecting duct cells Factor VIII and von Willbrandt factor synthesis
Describe how V1 is activated
Linked via G proteins to phospholipase C PLC acts on PL to produce inositol triphosphate (IP3) (and DAG) IP3 increases cytoplasmic [Ca2+] and PKC Producing cellular response
Describe how V2 is activated
Linked via G proteins to Adenyl Cyclase Actos on ATP to form cAMP cAMP activates PKA which activates other celular mediators Which produce cellular response (AQ2)
Describe the process of AQ2 insertion into cell membrane
VP in the blood attaches to V2 receptors on cell membrane Activating G-protein, ATP > cAMP cAMP activates PKA which activates other intracellular mediators This synthesises AQ2 which migrate the aggraphores to the apical membrane AQ2 are inserted into apical membrane, allowing water to enter AQ3 and 4 are inserted into the basolateral membrane allowing water to enter into plasma
How is VP controlled when there is increased plasma osmolality?
Osmoreceptors detect the dehydration - which have a stimulatory effect on hypothalamic nuclei, increasing VP production Increasing water reabsorption DECREASING plasma osmolality
How is VP controlled when there is decreased arterial blood pressure?
Detected by baroreceptors/volume receptors
Reduces inhibition of VP, Stimulates hypothalamic nuclei to produce more VP
VP released in to blood Increases vasoconstriction INCREASES blood pressure (also can be influenced by higher centres - stress, so more VP secreted)
List the actions of oxytocin:
Central effects - behavioural Affects breast during lactation > myoepithelial cells contract > causing milk ejection Affects uterus at parturition > myometrial cells > contraction > delivery of baby
What is the neuroendocrine reflex arc for oxytocin?
List the clinical conditions caused by VP
Diabetes insipidus
Syndrome of Inappropriate ADH
Describe the 2 types of DI:
Central DI - no VP present
Nephrogenic DI - tissue insensitivity DI causes Polyurea and Polydipsia
What are the causes of DI?
Central: problem in brain - trauma to head, tumour, autoimmune, inflammation Nephrogenic: problem in kidneys (psychogenic polydipsia - caused by psychological issues)
