Hypothalamic Pituitary Hormones

Question 1

What are hormones? What are the ways in which they can act?

Answer 1

chemical messengers produced by cells, carried by blood and detected by non-adjacent target cells that manufacture a specific high affinity receptor to which the hormone binds and initiates a response

paracrine - act locally, fast response
autocrine - act on the cell that produced it
endocrine - act on distant targets, slow response
direct signalling across gap junctions - target cell is connect by a gap junction

Question 2

What are the two main classes in which hormones fall into?

Answer 2

steroids
- chemically derived from cholesterol by adrenal cortex, testis and ovary
- enter the cell in combination with receptor and interacts with DNA
= cortisol, estradiol

peptides/proteins
- modified amino acids, short peptide chains or proteins
- interact with membrane bound receptors
= amines (NA), peptides (oxytocin), proteins (growth hormone) and glycoproteins (FSH)

Question 3

Where are steroids synthesised?

Answer 3

adrenal cortices
- glucocorticoids (cortisol), mineralocorticoids (aldosterone), androgens (testosterone), estrogens and progestin (progesterone)

gonads = ovaries and testes
- oestrogens, progesterone and testosterone

Question 4

How do steroids work in the body?

Answer 4

steroid diffuses through the cell membrane into the cytosol
= allowed due to its cholesterol backbone

steroid binds to cytosolic receptor
- receptor is associated with a heat shock protein

binding causes the heat shock protein to dissociate from the hormone-receptor complex

hormone-receptor complex enters the nucleus and binds with high affinity DNA sequences
- hormone response elements (HRE)/ steroid response elements (SRE)

binding initiates
- transcription of genes, formation of mRNA and protein synthesis

OR it can diffuse directly into the nucleus and bind with a nuclear receptor then bind to high affinity DNA sequences
- slower effects than ion channels or GCPR

Question 5

What are positive and negative feedback loops?

Answer 5

feedback loops control release of hormone

negative feedback loop
- the response counteracts the stimulus shutting off the response loop

positive feedback loop
- the response reinforces the stimulus sending the parameter further from the set point
- outside factor is required to shut off the feedback loop

Question 6

What is the hypothalamic pituitary axis?

Answer 6

hypothalamus
- connects the nervous and endocrine systems
- produce peptide hormones that control secretions of the pituitary gland (pituitary hormones)

pituitary gland (hypophysis)
- anterior lobe = adenohypophysis
- posterior lobe = neurohypophysis

Question 7

What types of cells does the anterior lobe (adenohypophysis) contain and what do they secrete?

Answer 7

somatotrophs
- secrete growth hormone

lactotroph
- secrete prolactin

thyrotrophs
- secrete thyrotropin (TSH)

corticotroph
- secrete adrenocorticotroph (ACTH)

gonadotroph
- secrete gonadotrophins (FSH and LH)

Question 8

How is the thyroid gland axis stimulated?

Answer 8

hypothalamus releases thyroid releasing hormone (TRH) into the hypophyseal portal system
- TRH is a tripeptide (glu-his-pro)

TRH stimulates the thyrotrophs of the anterior pituitary to synthesise and secrete thyroid stimulating hormone (TSH)

TSH stimulates the thyroid gland by binding to and activating specific TSH receptors which increases cAMP production

increases iodine uptake by the thyroid gland
- thyroid hormone synthesis and release = thyroxin (T4) and triiodothyronine (T3)

Question 9

How is the adrenal gland axis stimulated?

Answer 9

hypothalamus releases corticotrophin releasing factor/hormone (CRF/CRH)

CRH stimulates adrenocorticotrophin (ACTH) production by the anterior pituitary

ACTH released enters systemic circulation and stimulates receptors found in the adrenal cortex that generate cAMP

ACTH regulates the production and secretion of
- cortisol, aldosterone and dehydropiandrosterone (DHEA)