hpg axis

What is the HPG axis?

The hypothalamic–pituitary–gonadal axis. The brain senses hormone levels and signals the anterior pituitary to maintain hormonal homeostasis.

What are the basic components of the HPG axis?

Hypothalamus → GnRH → pituitary → LH/FSH → gonads → sex steroids → feedback to brain.

what is GnRH produced

by GnRH neurons in the hypothalamus → motor neurons of the reproductive system

funx of GnRH 

acts on anterior pituitary gnadotropes → induces synthesis and release of LH and FSG 

does continuous release of GnRH cause continuous release of LH and FSH

NO- must be pulsatile release, if it is not, GnRH receptors will be down regulate and this will stop release of LH and FSH

what is the hypothalamic-pituitary portal system

a communication passage that transports hypthalamic products to the anterior piuitary

GnRH analogues (antagonists/agonists) are used therapeutically to

• TURN OFF gonadotropin secretion, if necessary, to stop precocious puberty OR

• slow down the progression of prostate cancer

one cause of hypogonadism is 

the absence of GnRH neurons (and interestingly, if the person has this form of hypogonadism, they often also have anosmia - the inability to smell

what is a glycoprotein

a hormone w 2 subunits:

• common alpha unit 

• unique beta unit 

what hormones are glycoproteins (peptide)

• TSH

• LH 

• FSH 

• hCG 

which subunit of glycoproteins confers biological activity

beta

LH and hCG have a similar beta unit excpet…

hCG has unique moieties so it has a longer ½ life in the blood

what hormones are considered gonadotropins

• LH

• FSH

• hCG

LH target in males

leydig cells → testosterone

LH target in females

thecal cells → testosterone production

FSH target in males

sertoli cells → inhibin and spermatogenesis

funx of inhibin in males

inhibits FSH at pituitary 

funx of inhibin in females

inhibits FSH production and release

FSH target in females 

granulosa cells → estradiol and later progesterone synthesis 

hCG target in males (fetus)

fetal testes → inc testosterone

hCG target in females

corpus luteum → inc estrogen and progesterone

what neuron type in the hypothalamus responds to testosterone to regulate GnRH

KINDy neuron

funx of KNDy neurons

• drive pulses of GnRH → pulses of LH 

• primary target for sex steroids

• central processors of HPG axis 

• source of GnRH pulses 

• form the pulse generator