The hypothalamo neurohypophysial axis (3) Flashcards
What are the two types of hypothalamic nuclei
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)
what are the two types of hypothalamic neurones within the paraventricular nuclei
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
what two molecules is the neurohypophysis associated with
vasopressin and oxytocin
What neurones secrete vasopressin
Suprachiasmatic Nucleus and Vasopressinergic neurones
Are suparoptic neurones magnocellular or parvocellular
magnocellular
Features of Supraoptic neurones
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
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
Features of paraventricular neurones
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.
which neurones have herring bodies
magnocellular neurones (supraoptic)
What neurones secrete oxytocin
Oxytocinergic neurones
What are the differences between oxytocin and arginine vasopressin (AVP)
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)
What are the similarities betwee oxytocin and arginine vasopressin (AVP)
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
What is the synthesis of vasopressin
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.
What is the synthesis of oxytocin
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.
What are the different vasopressin receptors
V1a and V1b. V2
how do hormones affect the brain
by entering the cerebrospinal fluid
Where is V1a receptor found
Vascular smooth muscle (vasoconstriction). Hepatocytes (glycogenolysis). CNS parvocellular neurones (behavioural effects)
Where is V1b receptor found
Adenohypophysial corticotrophs (ACTH production)
Where is V2 receptor found
Collecting duct cells (water reabsorption). Other unidentified sites (e.g. endothelial cells, blood clotting factors VIII and Von Willbrandt factor
What does the V1 receptor do
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
What does the V2 receptor do
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)
What is the physiological action of vasopressin
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
What are the other actions of vasopressin
vasoconstriction, CNS effects, acting on neurotransmitters, corticotrophin release, synthesis of blood clotting factors, hepatic glycogenolysis
how does vasopressin control renal water reabsorption
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
