56598

Cortex

Modę of secretlon Gonadotropin releasing hormone

y-aminobutyrate Dopaminel Endorphins -

Noradrenalinę +

Hypo-

thalamus

Luteinizing

hormone

Follicle

stimulating

hormone

Hypo-

physis-

Gonadotropin releasing hormone

Testosterone

Follide

stimulating

hormone

Luteinizing

hormone

Inhibln

Testosterone

Estrogens

Adipose tissue

n-1-1-r

6    12    18    24

Hours ofthe day

Testis

(+) Stimulation Inhibition

Figurę 1.2: Requlation of testicular function and secretion pattern of the hormones involved. Gonadotropin re leasing hormone (GnRH) is secreted from the hypothalamus under the influence of neurotransmitters (y-aminobutyrate, catecholamines, endorphins) and from the cortcx in a pulsatilefashion. and stimulates produc-*.ion ofthe gonadotropins luteinizing hormone (LH) and follicle stimulating hormone {FSH) in the hypophysis. LH inown in red) follows the secretion profile of GnRH very closely, whereas FSH (blue) does not display such elear oulsatility owing to res longer half-life. LH stimulates testosterone biosynthesis in the Leydig cells. Testosterone exerts a direct, and after metabolism to estrogens an indirect, negative feedback effect on the secretion of GnRH and gonadotropins. Together with testosterone, FSH stimulates the production of peptides such as inhibm, which in turn exerts a negative feedback effect on FSH.