3) HPG Axis Flashcards
What are some examples of hypothalamic releasing hormones?
CRH, TRH, GHRH, somatostatin, gonadotropin releasing hormone, prolactin RH, dopamine
What are the features of hypothalamic releasing hormones?
Secretion in pulses tied to biological clock
Act on specific receptors and through secondary messengers
Stimulate release and synthesis of pituitary hormones
Hyperplasia and hypertrophy of target cells
Where is the pituitary gland located?
Beneath hypothalamus in sella turcica of sphenoid bone
What is the anterior pituitary also known as?
Pars distalis or adenohypophysis
What is the posterior pituitary also known as?
Pars nervosa or neurohypophysis
How are the hypothalamus and anterior pituitary connected?
Superior hypophyseal artery
What does the HPG axis regulate?
Development
Reproduction
Ageing
Describe the release of GnRH:
Pulsatile, every 1-3 hours
Travels to pituitary in hypophyseal portal system
What affects the intensity of the GnRH stimulus?
Frequency and intensity of release
What are the main actions of FSH and LH?
Stimulate sex hormone synthesis and control gamete production
Where does LH act in men and what does this do?
Leydig cells to cause production of testosterone
What effect does testosterone have on the HPG axis?
Negative feedback on hypothalamus and AP, reducing GnRH and LH + FSH release
Where does FSH act in men and what does this do?
Receptors on Sertoli cells on seminiferous tubules
Causes Sertoli cells to grow and secrete spermatogenic substances
What is ABP and what is its function?
Androgen binding protein, need testosterone for Sertoli cell function so ABP binds testosterone to keep it in tubules
What is the effect of inhibin on HPG axis?
Reduces FSH selectively
Describe the release of testosterone:
Circadian rhythm, highest in morning
Affected by environmental stimuli
Describe the changes in release of hormones and gonadal steroids in ageing men:
Testosterone falls with LH and FSH rising
Where do LH and FSH bind in females?
Theca interna and granulosa cells respectively
What is the effect of moderate titres of oestrogen on HPG axis?
Negative feedback (reduced GnRH)
What is the effect of high titres of oestrogen on HPG axis?
Positive feedback if no progesterone
Causes LH surge
What effect does progesterone have on the HPG axis?
Increases inhibitory effects of moderate oestrogen and prevents positive feedback at high oestrogen
How do oestrogen and progesterone affect GnRH release?
Oestrogen reduces GnRH per pulse, progesterone frequency of pulses
Where is inhibin released from in females?
Granulosa cells
What are the effects of testosterone on the body?
Maintenance of male internal genitalia Metabolic effects (anabolic) Behavioural effects (aggression and sexual activity)
