Endocrinology, lecture on Hypothalamo-Hypophysial Communication
X. Hypothalamo-Hypophysial Communication
A. Hypothalamus produces hypophysiotropic hormones: releasing hormones
and release-inhibiting hormones
1. hormone release stimulated by depolarization ® slow travelling
action potential in neurosecretory neurons
2. located below the thalamus,
just below and beside the 3rd ventricleof the brain
a. nuclei with endocrine function:
POA, SCN, SON, PVa, AH, PVN, DMN, VMN, ARC
3. POA + AH produce and release GnRH and TRH
a. coordinate GnRH and PRL surges
4. SCN coordinates rhythmic hormone release (e.g. CRH, melatonin)
5. SON + PVN make OT and AVP (magnocellular)
a. parivcellular division makes CRH and TRH (TRH from PVN)
6. PVa + DMN synthesize CRH, TRH and somatostatin(GHRIH)
7. VMN also makes CRH, TRH & somatostatin, but also GHRH
a. regulates insulin and glucagon (glucoreceptors)
b. GnRH to MCG (midbrain central gray, aka PAG)
8. ARC synthesizes CRH, TRH, GHRH
a. GnRH is made in mediobasal portion
b. DA (PRIH) from tubero-infundibular region
9. Hypothalamus also has normal neurons which send signals
by action potential to other parts of the brain
a. e.g. posterior hypothalamus regulates sympathetic NS, VMN
regulates female sex behavior
b. neurohormones (such as RHs and neurohypophysial nonapeptides)
may be the neurotransmitters for these normal neurons
(e.g. AVP, CRH, DA, GnRH, OT, TRH, somatostatin)
B. Hypothalamo-Hypophysial Portal System
1. Neurohemal organ in the Median Eminence
a. superior hypophysial artery delivers blood to capillary bed
in the median eminence
b. RHs released from neurosecretory neurons into capillary bed
i. neurohemal organ
c. RHs carried by Hypothalamo-Hypophysial Portal Vein(s)
to the adenohypophysis (pars distalis)
i. also to the intermediate lobe (pars intermedia)
C. Adenohypophysis = Anterior Pituitary
1. Derived from Rathke's pouch
a. fusion of a ventral growth of the diencephalon, the
infundibulum, with an ectodermal sac (Rathke's)
b. APUD - common origin from neuroectoderm
i. hypothesis - all peptide hormone-producing cells are of
neural origin - amine precursor uptake and decarboxylation series
(1) supported by hormone production as
neurotransmitters in the brain
2. corticotropic cells - produce/release ACTH
a. adrenocorticotropic hormone or corticotropin
b. comes from a preprohormone called POMC or pro-opiomelanocortin
i. also cleaved from POMC:
a-MSH, b-endorphin, g-lipotropin...
3. somatotropic cells - GH
4. gonadotropic cells - LH and FSH
5. lactotropic cells - PRL
6. thyrotropic cells - TSH
7. PACAP (pituitary AC activating peptide)
i. pituitary paracrine enhances tropic hormone release
8. Pars tuberalis - thin layer of cells connecting the
adenohpyophysis/median eminence with the 3rd ventricle
a. tanycytes - capable of absorbing and moving material (including
hormones) from median eminence to cerebrospinal fluid (CSF)
or vice versa
9. Pars Intermedia - a-MSH producing area
between the pars distalis and pars nervosa
D. Neurohypophysis = Pars Nervosa (posterior pituitary)
1. axons from hypothalamic neurosecretory neurons of the supraoptic
and paraventricular nuclei project into the pars nervosa
2. Supraoptic Nucleus (SON)
a. produces mostly AVP
3. Paraventricular nucleus (PVN)
a. produces mostly OT
4. secreted into neurohemal organ of the general circulation
E. Regulation of Hypophysiotropic Secretion
1. hypothalamus is innervated by other areas of the brain
a. hippocampus, amygdala, BNST, septum, tegmentum...
b. hypothalamus also innervates other areas of the brain
2. a variety of neurotransmitters effect hypophysiotropic secretion
a. the same factor may stimulate the release of one
and inhibit the release of another
i. Serotonin (5-HT) stimulates CRH, GHRH, inhibits TRH
ii. NE stimulates GnRH and TRH, inhibits or stimulates CRH
iii. ACh stimulates GHRH and CRH, inhibits somatostatin
iv. Glu stimulates GnRH
v. GABA inhibits CRH
F. Regulation of Tropic Secretion
A. RHs are synergistically enhanced by a adenohypophysial paracrine
1. PACAP stimulates tropic hormone release by potentiating cAMP activity
in pituitary corticotropes, gonadotropes, lactotropes, melanotropes,
somatotropes, thryrotropes
a. PACAP in the secretin/glucagon hormone family
