hypothalamic control of pituitary Flashcards
what connects the hypothalamus with the anterior pituitary
hypothalamo-hypophyseal portal system
describe the hypothalamo-hypophyseal portal system
Blood enters the median eminence through the superior hypophyseal arteries, which forms a capillary plexus. Nerve terminals of hypothalamic neurons terminate here, and their neurohormones are released into this capillary bed and then are transported via the portal system vasculature to a second capillary plexus in the anterior lobe
What classes of hormones does the hypothalamus secrete
peptides and dopamine (catecholamine)
List the hypothalamic hormones
Thyrotropin releasing hormone (TRH), Gonadotropin releasing hormone (GnRH), Corticotropin releasing hormone (CRH), Growth hormone releasing factor (GHRH), Somatostatin (GH inhibiting hormone; GIH), Prolactin inhibiting factor (PIH, aka dopamine)
What effect does TRH have on anterior pituitary
increases release of TSH and PRL
What effect does GnRH have on anterior pituitary
increases release of LH and FSH
What effect does CRH have on anterior pituitary
Increases POMC and ACTH release
What effect does GHRH have on anterior pituitary
release of GH
What effect does somatostatin have on anterior pituitary
decreases release of GH and TSH
What effect does PIH have on anterior pituitary
decreases PRL
describe release of hypothalamic hormones
Stimulation of hypothalamic neurons > action potential > Ca entery at nerve terminal through voltage gated channels > hormone released via secretory vesicles
describe cellular mechanisms of hypothalamic hormones on the anterior pituitary
hypothalamic hormone binds to G-protein receptor on target cells in anterior pituitary > cell signaling mechanisms > hormone release from the receptor is mediated by extracellular Ca
Which hypothalamic hormones bind to Gs and what is the second messenger pathway involved
CRH and GHRH- Bind Gs > stimulate adenylate cyclase to produce cAMP in corticotrophs and somatotrophs respectively.
Which hypothalamic hormones bind to Gi and what is the second messenger pathway involved
somatostatin and DA- Binds Gi > decreased levels of cAMP
GnRH signaling
In gonadotrophs, GnRH receptor activation leads to the hydrolysis of membrane phosphatidyl inositol
TRH signaling
In thyrotrophs and lactotrophs, hydrolysis of phosphatidyl inositol plays role
role of arachidonic acid signaling pathway
may be involved in release mechanism used by pituitary cells
Mechanisms of hormone release at pituitary
- Second messengers cause Ca flux from outside into cell to mediate release. 2. Production of IP3 (mediated by Gq and PLC) increases cytosolic Ca from intracellular stores mediating release
what is the adenohypophysis and the neurohypophysis
adenohypophysis: anterior pituitary. Neurohypophysis: posterior pituitary
Where are hormones secreted from the anterior pituitary
the pars distalis aka anterior lobe
List the hormones of the anterior pituitary
TSH, LH, FSH, growth hormone, corticotropin (ACTH) and proopiomelanocortin (POMC), prolactin
For each of the anterior pituitary hormones, list the hypothalamic hormone affecting its release and the cell that secretes it
TSH: TRH stimulates release via thyrotrophs. LH and FSH: GnRH stimulates release via gonadrotrophs. GH: GHRH stimulates and somatostatin inhibits release from somatotrophs. ACTH and POMC: CRH stimulates release from corticotrophs. Prolactin: PIH (DA) inhibits release and prolactin releasing factor (TRH) stimulates release from mammotrophs.
Which pituitary hormones are glycoproteins
TSH, LH and FSH
Which pituitary hormones are polypeptides
growth hormone and prolactin
which pituitary hormones are derived from POMC
ACTH- POMC is cleaved in anterior and intermediate lobes of pituitary giving rise to ACTH, and N terminal fragment and beta-lipotropin.
Describe the pattern of hormone release from adenohypophysis
pulsatile- plasma levels show spikes. Directed by pulsatile secretion of hypothalamic hormones. Strong evidence for LH, FSH, and ACTH. Also, circadian rhythms affect the pattern of release
What controls the pulsatile release of hypothalamic hormones
an endogenous pacemaker
Hyperprolactinemia
Excess prolactin inhibits pulsatile secretion of GnRH causing gonadal dysfunction
Which hormone is elevated shortly after sleep and which is elevated in the morning
GH secretion is elevated shortly after sleep onset, whereas ACTH is highest during the early morning hours.
Where are neurohypophyseal hormones synthesized
In the supraoptic and paraventricular nucleus of the hypothalamus.
describe the cell types of the supraoptic and paraventricular nuclei
1) the magnocellular neurons, whose processes extend into the posterior pituitary and end in the pars nervosa and 2) the parvocellular neurons, which end at the median eminence and produce the anterior pituitary regulating hormones
Posterior pituitary control of anterior pituitary
some of the posterior pituitary hormones can reach the anterior lobe where they can have some functions e.g. ADH can act on corticotrophs to increase ACTH production. Cortisol, regulated by ACTH, can in turn, inhibit both ADH function in the kidneys and release at the hypothalamus.
List hormones released by neurohypophysis
antidiuretic hormone (ADH) and oxytocin
Synthesis of neurohypophyseal hormones
Synthesized as prohormone > packaged into secretory vesicles and cleaved into hormone and protein neurophysin as vesicles travel down axon of neuron into posterior pituitary > AP activates Ca channels > release of neurohormone
what causes release of ADH
Increased plasma osmolarity or decreased BP
ADH actions
It acts on the cells of the renal tubule and collecting ducts to alter water permeability and conserve water. Also acts as pressor agent, increasing BP
ADH receptors
There are two kinds of ADH receptors: 1. V1 coupled to Gq and the Phospholipase C pathway, mediates the vasopressive action of ADH. 2. V2 coupled to Gs and the cAMP pathway regulates the effects of ADH on glomerular filtration rates in the kidney
When is oxytocin secreted
- During the passage of the infant through the cervix at childbirth. 2. During sexual intercourse. 3. In response to suckling by the infant during breast-feeding. This neuroendocrine reflex can be conditioned
