Hypothalamic Control of the Pituitary Gland Flashcards
hypothalamic hormones are typically what type
peptides, except for dopamine, which is a catecholamine
how do hypothalamic hormones get to anterior pituitary
hypothalamo-hypophyseal portal system
- blood enters median eminence through superior hypophyseal arteries and form capillary plexus
- nerve terminals of hypothalamic neurons terminate here where neurohormones are released into capillary bed and transported via portal system to second cap plexus in anterior lobe
hypothalamic hormones
Thyrotropin releasing hormone, gonadotropin releasing hormone, growth hormone releasing hormone, somatostatin, prolactin inhibiting factor (DA)
action of thyrotropin releasing hormone (TRH)
increase TSH/PRL release from AP
action of GnRH
increased LH/FSH release from AP
Corticotropin releasing hormone action
increase POMC and ACTH release from AP
growth hormone releasing factor action
increased GH release from AP
action of somatostatin (GH inhibiting hormone)
decrease GH/TSH release from AP
Prolactin inhibiting factor action
decrease PRL from AP
how are hypothalamic hormones secreted from posterior pituitary
appropriate stimulation of neuron results in action potential generation –> Ca enters nerve terminal through VDCC causing liberation of hormone from secretory vesicles
what kinds of receptors do the hypothalamic hormones act on
G proteins
- CRH and GHRH are coupled to Gs
- Somatostatin with Gi
- PIH/DA– Gi
- GnRH - Gq
- Gq also with TRH lactotrophs/thyrotrophs
what contributes to release of hormones from anterior pituitary
release of hormones dependent on calcium, but several intracellular cascades implicated in various pituitary cells. Membrane conductances also appear to play a role. The intracellular cascades also probably involved in regulating synthesis of various pituitary hormones.
Pituitary gland
endocrine control center responding to neural signals and catering to needs of various target tissues (e.g. thyroid, gonads, adrenal gland, bone), coordinating and regulating their functions
- connected to hypothalamus via pituitary stalk
major divisions of pituitary
anterior (adenohypophysis) and posterior (neurohypophysis)
from what is anterior pituitary derived
Rathke’s pouch (pharyngeal epithelium)
from what is posterior pituitary derived
neural tissue arising from an embryological evagination of diencephalon
what comprises the anterior pituitary
pars tuberalis, pars intermedia (intermediate lobe), and pars distalis (anterior lobe)
from which part of adenohypophysis are hormones secreted
pars distalis/anterior lobe
TRH
acts on cells called thyrotrophs to stimulate secretion of TSH and lactotrophs to release PRL via Gq
GnSH
acts on gonadotrophs to stimulate secretion of luteinizing hormone (LG) and follicle stimulating hormone (FSH)
GHRH
act on somatotrophs to stimulate secretion of growth hormone
somatostatin
act on somatotrophs to inhibit GH secretion
CRH
acts on corticotrophs to stimulate secretion of corticotropin (ACTH) and proopiomelanocortin (POMC)
PIH
(prolactin inhibiting hormone) acts on lactotrophs to inhibit prolactin secretion
Prolactin releasing factor
possibly also TRH; act on mammotrophs to stimulate secretion of prolactin
polypeptide hormones from AP
PRL and GH
glycoproteins from adenohypophysis
TSH, FSH, LH
- each comprised of identical alpha-subunit and unique beta subunit
from what is ACTH derived
prohormone POMC, which is cleaved in both anterior and intermediate lobes of pituitary to give rise to ACTH, an N-terminal fragment and beta lipotropin
what does cleavage of beta lipotropin produce
gamma-lipotropin adn beta-endorphin.
- beta-endorphin contains sequence of metenkephalin
- gamma-lipotropin contains sequence of beta-MSH
- different in intermediate lobe, where alpha-MSH mostly generated after ACTH further cleaved
parts of posterior pituitary
median eminence, infundibular stem, infundibular process (pars nervosa)
where are hormones from posterior pituitary synthesized
in 2 nuclei in hypothalamus - supraoptic and paraventricular
which peptides secreted by posterior pituitary
ADH and oxytocin
neurons of hypothalammic nuclei extending into post pituitary
magnocellular neurons (processes extend into posterior pituitary and end in pars nervosa) - parvocellualr neurons -- end in median eminence close to endings of hypothalamic neurons producing anterior pituitary hormones -- aka post pituitary hormones can reach anterior lobe and have some function
what neurons secrete ADH/oxytocin
magnocellular neurons from larger prohormone packaged into secretory vesicles and cleaved into the hormone + neurophysin protein as vesicles travel down axon
ADH
antidiuretic hormone; secreted in response to increased plasma osmolarity or decreased blood pressure; acts on renal tubular cells and collecting ducts to alter water permeability adn conserve water.
ADH receptors
V1–coupled to Gq and the phospholipase C pathway mediates vasopressive action of ADH
V2– coupled to Gs and cAMP pathway regulates effects of ADH on glomerular filtration rates in kidney
when is oxytocin secreted
during passage of infant through cervix at childbirth, during sex, in response to suckling of infant during breast-feeding
- acts on uterus around birth for myometrium contraction and contraction of myoepithelial cells in lactating women for milk ejection
how often are pituitary hormones secreted
not constant but varied and pulsatile, with spikes in plasma levels. Can also have circadian rhythm (i.e. GH)
most common way hormones are controlled
negative feedback by hormone itself or a metabolite
2 places you can regulate hormone production
- hormone production (can regulate timing and amplitude of pulsatile release over shorter term; also have circadian regulation over 24 hrs) itself
- at the target: i.e. too much hormone, target cell can internalize receptors to prevent overstimulation (downregulation)
- can also have full effect of some hormones even if some receptors taken away (spare receptors)
function of hypothalamo-hypophyseal portal system
allows tiny amounts of hormone to be carried with minimal dilution to anterior pituitary
