W2 L2 Thu Hypothalamo-Pituitary-Gonadal Axis

Question 1

Pituitary Gland role and structure

Answer 1

• Pituitary gland: regulates reproduction, metabolism, growth + stress response
  § Hormones: LH/FSH (anterior pituitary), oxytocin (posterior pituitary), prolactin, GH (growth/development), TSH (regulates thyroid gland), ACTH (regulates adrenal gland), MSH
• Develops in utero + has dual embryological origin
  § Roof of mouth/pharynx (Rathke’s pouch) forms the anterior pituitary (adenohypophysis)
  § Neural outgrowth forms the posterior pituitary (neurohypophysis)
• Pituitary joined to hypothalamus by median eminence (contains portal veins)
• AP cells: gonadotrophs secrete LH +/or FSH, lactotrophs secrete prolactin

Question 2

Hypothalamus

Answer 2

bilateral structure split by 3rd ventricle containing cerebrospinal fluid
§ Nuclei (11): clusters of neurons with the same function
* Coronal section through optic chiasm: paraventricular nucleus (PVN) on either side of 3rd ventricle, supraoptic, suprachiasmatic nuclei
* Coronal section through infundibulum + median eminence: ventromedial nucleus (VMN), arcuate nucleus
* Median eminence: no blood brain barrier, where neurons of hypothalamus terminate, capillary network that allows neuropeptides to drain from brain + be passed down into anterior/posterior pituitary
-Consist of 2 type of cell, Hormones are passed to posterior pituitary along axons via hypothalamo-hypophyseal tract

Question 3

2 types of cells in hypothalamic nuclei Pavocellular

Answer 3

Ø Small cell bodies in preoptic area, PVN, ventromedial nucleus, arcuate nucleus
Ø Axons terminate at capillary beds in median eminence where factors are released
* Capillaries coalesce to form hypothalamo-pituitary portal vessels to anterior pituitary
* In anterior pituitary, GnRH stimulates gonadotrophs to release LH + FSH
Ø Produces various neuropeptide hormones incl. gonadotrophin (GnRH)

Question 4

2 types of cells in hypothalamic nuclei Magnocellular neurons Magnocellular neurons::

Answer 4

Ø Large cell bodies in paraventricular nucleus + supraoptic nuclei
Ø Axons run down pituitary stalk (infundibulum) + terminate in posterior pituitary
Ø Produce oxytocin + vasopressin (related nonapeptides)
* Oxytocin: stimulates luteolysis, uterine contractions (myometrium), milk ejection in lactation, initiates nursing behaviour, stimulates contractions of seminiferous tubules + epididymis
* Vasopressin (AVP): water reabsorption, similar effect to oxytocin in high concentrations

Question 5

HPG Axis Endocrinology Sequence

Answer 5

§ Hypothalamus releases GnRH to anterior pituitary which produces LH + FSH → systemic circulation to gonads

§ Sertoli cells produce inhibin which inhibits FSH secretion by pituitary gonadotrophs
§ Testosterone produced by gonads provide negative feedback to hypothalamus or pituitary (inhibits GnRH + LH/FSH)
Ø Testosterone injections suppress LH secretion (↓pulse frequency + amplitude of steroid release)

Question 6

HPG Axis Endocrinology Experimental Models Rhesus Monkey

Answer 6

ovaries removed, hypothalamus lesioned + GnRH infused
Ø Pulsating GnRH: LH + FSH levels are high
Ø Continuous GnRH: LH + FSH levels drop (pituitary GnRH receptors downregulated by itself)

Question 7

Portal blood system of Pituitary gland

Answer 7

• neural and oral ectoderm origin
• ectodermal  anterior pituitary (endocrine cells)
• neural  posterior pituitary (nerve terminals)

Question 8

GnRH secretion is pulsatile

Answer 8

GnRH is a decapeptide
― 10 amino acids
― Rapidly broken down in blood
GnRH secreted in pulses
― § GnRH: secreted in pulses into portal system in median eminence = pulsatile release of FSH/LH
Ø Pulse generator in hypothalamus sets frequency of pulses
Ø Feedback from body regulates amplitude + frequency of pulses
Ø Pituitary is larger than hypothalamic nuclei so produces more LH which takes longer to break down
Ø Pulse frequency varies with species + reproductive state (ewe 1 pulse/2hr; rhesus monkey 1 pulse/hr)

Question 9

Expiriment: gonadectomy

Answer 9

testes/ovaries removed = LH/FSH no effect = ↑GnRH = ↑LH/FSH

Question 10

Hypophysectomy

Answer 10

remove pituitary gland = GnRH cannot bind + LH/FSH not produced to stimulate steroid production
Ø Spermatogenesis halted + accessory sex glands (ASG) regress
Ø Administer high dose testosterone = spermatogenesis resumed at ↓level + ASG re-develop

Question 11

Antibodies experiment

Answer 11

bind hormones to remove its functionality (GnRH, inhibin, steroids)
Ø Anti-GnRH antiserum: binds GnRH = not effective in anterior pituitary + low levels of LH/FSH produced
§ Administrate exogenous FSH = partial spermatogenesis resumed (up to A3-4) + ASG regress
Ø Anti-LH antiserum: bind LH to prevent stimulation of Leydig cells + stops testosterone production

Question 12

Pituatary stalk lesion exp

Answer 12

prevent GnRH from entering pituitary, investigate role of portal vessels + effects on LH/FSH

Question 13

Neuroendocrine Control of Testis (Ram)

Answer 13

§ Non-breeding period: testis small, no testosterone produced due to low LH/FSH levels = no sperm produced
§ Onset of breeding: ↑frequent pulses of FSH + LH, some testosterone produced = sperm production + testes grow
§ Breeding period: small pulsatile FSH + LH levels, high testosterone = full sperm production + testes large
§ GnRH from hypothalamus → anterior pituitary release FSH/LH → gonads
Ø Testosterone/oestrogen: negative feedack for GnRH + therefore FSH/LH
Ø Inhibin (sertoli/granulosa cells): negative feedback on FSH
§ Castration: testosterone source removed = no negative feedback on GnRH = ↑FSH/LH levels
Ø Higher frequency + amplitude of pulses as system thinks T levels are too low + need more GnRH

Question 14

HPG Axis (Ram)

Answer 14

§ Castrated rams (wethers): given T/DHT/E2 = ↓LH, ↓FSH, ↓GnRH pulse frequency
Ø Hormone treatment provides negative feedback to regulate GnRH + therefore LH/FSH
§ Hypothalamo-pituitary disconnected (HPD) wethers with GnRH pulses infused every 2 hours
Ø HPD disconnection = no GnRH reaches pituitary to stimulate LH/FSH release
Ø Adding T/DHT/E2 has little to no effect on LH/FSH levels as negative feedback cannot reach pituitary
§ Wether infused with testosterone results in ↓GnRH concentration, pulse frequency + pulse amplitude

Question 15

Regulation of Ovarian Function

Answer 15

§ GnRH stimulates LH + FSH; oestrogen negative feedback on LH/FSH
§ Prior to ovulation, high levels of oestrogen build until LH surge
§ LH pulsatility varies duing menstrual cycle:
Ø Early follicular to mid-follicular phase: small LH ↑frequency
Ø Late follicular phase: LH pulse ↑frequency, ↑levels
Ø Ovulation: eventually pulses become so rapid + build up to 1 LH surge
Ø Luteal phase: ↓pulsatility, ↑amplitude (change in signal = produce progesterone)
§ For ovulation, ↑oestradiol + positive feedback occurs leading to LH/FSH surge
§ Post-menopausal: low oestradiol = no feedback + inhibition of LH/FSH release (pulse frequency change + high LH/FSH levels)

Question 16

Pituitary Steroids regulation for ovulation

Answer 16

§ GnRH (hypothalamus) stimulates LH + FSH production from anterior pituitary
Ø FSH ↑follicle growth + ↑oestrogen amount
Ø Oestrogen rise triggers LH surge (positive feedback) by increase response to GnRH
Ø Oestrogen induces + maintains GnRH receptors in pituitary
Ø GnRH pulses can change expression of GnRH receptors (can downregulate)
§ After ovulation, oestrogen has a negative feedback effect on FSH/LH

Question 17

Sexual Dimorphism

Answer 17

Hormones in utero determine patterning + number of nuclei; how individual responds after puberty
§ A male foetus exposed to androgens in brain which is converted to oestrogen which masculinises the brain
Ø Neonatal androgen exposure prevents response to E2 in adults; does not masculinise hypothalamus

Question 18

Prolactin in reproduction

Answer 18

hormone secreted by lactotrophs of anterior pituitary gland that promotes lactation
Ø Luteotrophin (maintains corpus luteum) in some species (rat, mouse)
§ Surge of prolactin induced by mating causes rescue of CL + pseudopregnancy/1st stages of pregnancy
Ø Circadian pattern of release – elevated during sleep hours
* Very high PRL decreases oestrogen and testosterone
In rats prolactin surges with GnRH before ovulation
In human no clear cycle

Question 19

Control of Prolactin secretion

Answer 19

Pituitary disconnection increases prolactin, so ‘suppression’ from hypothalamus
Prolactin inhibitory factors (PIF)
• Dopamine (DA)
• GABA
• GnRH-associated peptide (GAP)
Prolactin releasing factors
• vasoactive intestinal polypeptide (VIP)
• thyrotrophin releasing hormone (TRH)
• oestrogen
Prolactin short loop feedback
prolactin receptors on tuberoinfundibular dopamine associated neurons (TIDA) prolactin inhibits its own release
Oestrogen
stimulates prolactin synthesis and release by lactotrophs

Question 20

Hyperprolactinaemia

Answer 20

↓dopamine levels or excess prolactin production form a pituitary gland adenoma tumour
Ø Women: amenorrhoea + ↓libido, no pulsatile release of LH, reduced response to GnRH, no positive feedback
Ø Men: no pulsatile release of LH, ↓testosterone + libido, erectile dysfunction, infertility, breasts simulated
Ø Treatment: drugs like bromocriptine (activate D2), surgery (tumours in pituitary gland)

Question 21

Regulation of ovarian function - rat

Answer 21

GnRH- Stimulates LH and FSH
LH- needed for ovulation- release of ovum
FSH- Increases follicle growth & oestrogen
Oestrogen - increase LH

Question 22

LH secretion pattern in menstrual cycle

Answer 22

• LH pulsatility varies during the menstrual cycle- increases during follicular phase (day 4 vs day 8)
• Pre-ovulation LH pulse frequency steady but LH levels rise (day 8 vs day 13)
• Luteal phase decreased pulsatility, increased amplitude
  OV = ovulation
  E2 = oestrogen Johnson & Everitt Fig 6.14

Question 23

Steroids can directly affect pituitary experiment

Answer 23

Rhesus monkey with lesions in medial basal hypothalamus (MBH) that blocks GnRH secretion given GnRH pulses
Ø Large dose of oestradiol benzoate (EB) causes initial suppression of FSH + LH
Ø If E2 remains high enough for long enough = LH surge

Question 24

Regulation of steroid production - pituitary

Answer 24

GnRH – hypothalamus
LH & FSH - anterior pituitary
FSH - ovary (growing follicle secretes increasing amounts of oestrogen, E2)
Oestrogen rise triggers LH surge (positive feedback) by increasing response to GnRH
Oestrogen induces and maintains GnRH receptors in pituitary
GnRH pulses also change expression of GnRH receptors (can downregulate itself) AFTER ovulation, oestrogen has a negative feedback effect on FSH/LH