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— Modę of secretion Gonadotropin releasing hormone

iluM

Cortex

y-aminobutyrate Dopaminei Endorphins -

Noradrenalinę +

Hypo-

thalamus

Luteinizing

hormone

Follicle

stimulating

hormone

Hypo-

physis-

Gonadotropin releasing hormone

Testosterone

Follicle

stimulating

hormone

Luteinizing

hormone

Inhibin

Testosterone

Estrogens

Adiposetissue

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Hours of the day

© Stimulation Inhibition

Figurę 1.2: Regulation of testicular function and secretion pattern of the hormones involved. Gonadotropin re-ieasing hormone (GnRH) is secreted from the hypothalamus under the influence of neurotransmitters (y-aminobutyrate, catecholamines, endorphins) and from the cortex in a pulsatile fashion, and stimulates produc-: on of the gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH) in the hypophysis. LH i shown in red) follows the secretion profile of GnRH very closely, whereas FSH (blue) does not display such elear pulsatility owing to its 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 inhibin, which in turn exerts a negative feedback effect on FSH.