Hypothalamic Control of the Pituitary Gland Flashcards
Mechanism of hypothalamic-pituitary communication
- The hypothalamo-hypophysial portal system connects the hypothalamus to the anterior pituitary
- Hormones are carried in this tiny portal system.
- local release restricts the amount of blood exposed to the released hormone
- thus the hypothalamic hormones don’t get diluted by the time they reach the anterior pituitary.
- This allows for very tiny amounts to be released to reach the anterior pituitary in high concentrations.
Hypothalamic hormones that regulate anterior pituitary fxn
- Thyroid Releasing Hormone (TRH)
- Corticotropin Releasing Hormone (CRH)
- Growth Hormone Releasing Hormone (GHRH)
- Somatostatin (SST)
- Prolactin Inhibiting Hormone (PIH)*
- PIH = Dopamine (DA)
- Gonadotroph Releasing Hormone (GnRH)
Thyroid releasing hormone (TRH): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- Receptor/cell type
- Gq; Thyrotrophs
- AP action
- ↑ TSH
- (↑ PRL)
- Downstream
- ↑ Thyroid Hormone from thyroid
Corticotropin releasing hormone (CRH): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- Receptor/cell type
- Gs; Corticotrophs
- AP action
- ACTH
- Downstream
- ↑ Cortisol from adrenal cortex
Growth Hormone releasing hormone (GHRH): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- receptor/cell type
- Gs; Somatotrophs
- AP action
- ↑ GH
- Downstream
- Acts alone
Somatostatin (SST): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- receptor/cell type
- Gi; Somatotrophs.
- AP action
- ↓ GH
- Downstream
- Acts alone
Prolactin Inhibiting Hormone (PIH): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- receptor/cell type
- Gi; Lactotrophs
- AP action
- ↓ PRL
- Downstream
- Binds cytokine receptor, causes dimerization → activation of JAK/STAT → gene expression.
- ~300 effects
- Mammary gland → mammogenesis, lactogenesis, and galactoporesis.
- Under normal circumstances, prolactin is inhibited.
Gonadotropin Releasing Hormone (GnRH): Ant. Pituitary Receptor/Cell type + AP action + downstream action
- receptor/cell type
- Gq; Gonadotrophs.
- AP action
- ↑ LH + ↑ FSH
- Downstream
- ↑ Estrogen and Progesterone
One sentence summary of mechanism hormone communication from hypothalamus ==> anterior pituitary
Stimulus-dependent (AP arrival) secretion of hypothalamic hormones is analagous to neurotransmitter release (just switch hormone for NT), and is thus calcium-dependent secretory vesicle release.
Principles of hormone release from hypothalamic neurons
- Calcium dependent exocytosis (same mechanism as neurotransmitter release)
- Pulsatile release (plasma shows spikes)
- Endogenous pacemakers regulate pulsatile secretion
- Pulsatile secretion of hypothalamic hormone signals pituitary secretion, e.g. pulse of GnRH precedes LH pulse
- Circadian rhythms of secretion (GH high after bedtime, ACTH high in early a.m.)
Main receptors/signal transduction mechanisms for hypothalamic hormones
- All receptors for hypothalamus hormone are G coupled → Gs, Gi, Gq.
- There is a lot of interaction with various receptor types and a given hormone.
- Multiple pathways can be activated by a single hormone
Hypothalamus origins/connections to brain
- Hypothalamus is derived from the third ventricle
- receives inputs from the thalamus, limbic system (olfactory bulb, hippocampus, habenula (stalk of pineal gland), & amygdala), retina, reticular activating substance & neocortex.
- Basically, this allows things like emotions, light, pain, etc. to affect hormone release.
Hypothalamic nerve terminal location & mechanism of neurohormone release
- Nerve terminals from hypothalamic neurons terminate @ median eminence
- secrete hormones into a capillary plexus formed by blood coming from the superior hypophyseal arteries.
- These neurohormones ==> portal venous system into the anterior pituitary into a second capillary plexus.
- This is all outside of the blood brain barrier.
Posterior pituitary hormones synthesis location + cell types
- Posterior pituitary hormones (ADH & oxytocin) are synthesized in the supraoptic nucleus and paraventricular nucleus which have two cell types:
- Magnocellular neurons terminate in pars nervosa; make both ADH & oxytocin
- Parvocellular neurons terminate at median eminence, thus hormones can be taken up into the portal system and affect the anterior pituitary (e.g., CRH can ↑ ACTH).
Characteristics of type 1 feedback loop control of hypothalamic hormone release
- Type 1 feedback loop → Hormone is regulated variable
- Negative feedback loop: most common, regulated variable is plasma conc. of hormone. Often times excess hormone inhibits further production of itself by acting upstream. (e.g. T4 inhibits Thyrotrophs)
- Positive feedback loop: rare, ends with expulsion of something (like a baby, w/oxytocin)
