Endocrine Physiology - Hypothalamus and Pituitary

Question 1

Where is the hypothalamus and what does it do?

Answer 1

Floor of the third ventricle, closely associated to the pituitary gland, posterior to the optic chiasm, primarily maintaining homeostasis.

Widespread connections to brainstem, limbic system, higher centres & pituitary.
Involved in monitoring & managing the autonomic nervous system, thirst, appetite, thermoregulation, sexual activity.

Hormones released by the hypothalamus affect the pituitary - it is connected both functionally and anatomically to the anterior pituitary via the hypophyseal portal system.

Question 2

What hormones does the hypothalamus secrete?

Answer 2

Releasing hormones
CRH - Corticotrophin releasing hormone
Stimulates release of ACTH (Adrenocorticotrophic hormone) and melanocyte stimulating hormone
TRH (Thyrotropin releasing hormone)
GnRH (Gonadotropic hormone releasing hormone)
GHRH (Growth hormone releasing hormone)
PRLH (Prolactin releasing hormone)

Inhibitory factors
Somatostatin
Inhibits pituitary release of Growth hormone, thyroid stimulating hormone, prolactin
Inhibits pancreatic release of insulin and glucagon, and inhibits GI release of gastrin & secretin.
Dopamine inhibits prolactin release

Question 3

What is a tropic hormone?

Answer 3

A hormone that affects its target organ indirectly by first stimulating another endocrine organ.

This allows massive amplification of a signal.

Question 4

What does the pituitary gland do?

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Answer 4

Central component of the endocrine system, with effects on
Water balance and osmoregulation
Growth
Metabolism and cell turnover
Smooth muscle tissue (especially uterine and breast)

Divided into anterior and posterior parts

Anterior pituitary connected to hypothalamus via hypothalamo-hypophyseal portal circulation, with three subsections

Pars tuberalis

Pars intermedius
Melanocyte stimulating hormone (stimulation of melanin in the skin)

Pars distalis
Produces the 6 main hormones
Prolactin (stimulates lactation)
Growth Hormone (Protein anabolism, lipolysis, tissue repair, cell growth via insulin-like growth factors)
Thyroid stimulating hormone (stimulates thyroxine T4 and triiodothyronine T3)
Adrenocorticotropic hormone (release of gluco- and mineralo-corticoids from adrenal cortex) - G protein linked, adenyl cyclase -> cAMP -> Protein kinase A
Luteinising hormone (Testosterone vs ovulation & ovarian secretion of oestrogen, stimulating corpus luteum formation and secretion of progesterone)
Follicle Stimulating Hormone (Spermatogenesis or follicle growth)

Posterior pituitary
Secretes two hormones (produced in hypothalamus, released by pituitary via a direct neural connection)
Oxytocin (lactation, renal water retention, uterine contraction)
Vasopressin (ADH - vasoconstriction particularly arteriolar, renal water reabsorption, factor VIII synthesis)

Question 5

Explain the hypothalamic-pituitary-adrenal axis and the control of cortisol release

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Answer 5

Endocrine pathway via which hypothalamus, pituitary and adrenal glands coordinate secretion of stress hormones.

Hypothalamus releases CRH (Corticotropin releasing hormone)

Stimulates ACTH (Adrenocorticotropic hormone) release from pituitary

Stimulates release of cortisone and aldosterone from adrenals

Both demonstrate negative feedback inhibition of both CRH and ACTH release

Question 6

Explain the mechanism of action of ADH

Answer 6

ADH = Vasopressin

9 amino acid peptide hormone

Synthesised in the supraoptic nucleus of hypothalamus, released from posterior pituitary gland

Acts on V1 and V2 receptors

V1: At high doses, induces vascular smooth muscle contraction of arterioles via IP3 and DAG as downstream messengers

V2: Found in basolateral membrane of cortical/medullary collecting ducts int he kidney. Active at lower doeses, and via G-protein stimulation of cAMP production, increases permeability of ducts to water via recruitment of aquaporins (fluid retention, expanding plasma volume)

Lower doses,