Regulation Of Reproductive Hormone Secretion
The hypothalamus of the brain, the anterior pituitary gland, and the testes (figure 19.7) produce hormones that influence the male reproductive system. Gonadotropin-releasing (gō ′ nad-ō -trō ′ pin) hormone (GnRH) is released from neurons in the hypothalamusand passes to the anterior pituitary gland (table 19.1). GnRH causes cells in the anterior pituitary gland to secrete two hor-mones, luteinizing (loo′ tē -̆ı -nı̄ z-ing) hormone (LH) and follicle-stimulating hormone (FSH), into the blood. LH and FSH arenamed for their functions in females, but they are also essential reproductive hormones in males.
LH binds to the interstitial cells in the testes and causes them to secrete testosterone. FSH binds primarily to sustentacular cells in the seminiferous tubules and promotes sperm cell development. It also increases the secretion of a hormone called inhibin (in-hib′ in; to inhibit).
The blood levels of reproductive hormones are under negative-feedback control. Testosterone has a negative-feedback effect on the secretion of GnRH from the hypothalamus and the secretion of LH and FSH from the anterior pituitary gland. Inhibin has a negative-feedback effect on the secretion of FSH from the anterior pituitary gland.
For GnRH to stimulate LH and FSH release, the pituitary gland must be exposed to a series of brief increases and decreases in GnRH. If GnRH is maintained at a high level in the blood for days or weeks, the anterior pituitary cells become insensitive to it. GnRH can be produced synthetically and is useful in treating some people who are infertile. Synthetic GnRH must be administered in small amounts in frequent pulses or surges. GnRH can also inhibit reproduction because long-term administration of GnRH can sufficiently reduce LH and FSH levels to prevent sperm cell production in males or ovulation in females.