Hormones of Male Reproduction
Endocrine Regulation of Testicular Function
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH), protein hormones (see Chapter 12) from the adenohypophysis (anterior pituitary), are the primary endocrine regulators of testicular function.
Their overall effect is to stimulate testicular function, so both are considered to be gonadotrophins. FSH promotes spermatogenesis by its actions on the germ cells in the seminiferous tubules and the sustentacular cells that support the development of the spermatozoa. LH acts on testicular interstitial cells to promote the secretion of androgens, primarily testosterone. The testosterone produced by the interstitial cells is necessary for the completion of spermatogenesis, so both FSH and LH are required for normal spermatogenesis.A single hormone from the hypothalamus, gonadotrophin-releasing hormone (GnRH), stimulates the release of both FSH and LH from the adenohypophysis. Negative feedback to the hypothalamus to regulate GnRH is provided by serum testosterone, which is produced by testicular interstitial cells when stimulated by LH. Testosterone also has direct effects on the adenohypophysis to suppress LH release directly. When stimulated by FSH, sustentacular cells produce a protein hormone, inhibin. inhibin has a negative feedback effect on the adenohypophysis to suppress further releases of FSH.
in farm animals, the feedback regulation of GnRH, FSH, and LH secretion is such that spermatogenesis is maintained at rates adequate for breeding throughout the year. However, plasma levels of FSH, LH, and testosterone do vary with season in some species, and these have been associated with differences in sexual activity. For example, FSH, LH, and testosterone levels are highest in rams while the days are getting shorter, and this is associated with increased sexual activity.
Testosterone and Its Effects
Testosterone is a steroid hormone that enters its target cells to exert its effects. Within target cells, testosterone is converted to dihydrotest- erone, which binds to intracellular receptors. In addition to supporting the maturation of spermatozoa within the testis, testosterone promotes the development and function of male accessory sex organs, causes development of secondary sex characteristics, and promotes male sexual behavior.
Lack of libido (sex drive) and inability to produce offspring are two of the most obvious effects of castration and the resultant lack of testosterone. However, animals castrated after attaining sexual maturity may continue to mate for some time if they had sexual experience before castration. If an animal is castrated before puberty, many of the masculine secondary sex characteristics fail to develop, and the castrated animal tends to resemble the female of the species. In addition, the accessory sex glands fail to develop normally if castration occurs early in life, and they regress and become nonfunctional if castration occurs after sexual maturity. Even though testosterone production by the testis is necessary for a normal libido, it is not testosterone that directly affects neurons within the brain to produce a normal libido. Within neurons, testosterone is converted to estradiol, an estrogen, and it is this estradiol that actually stimulates the appropriate neurons. Anabolic steroids (discussed next) used to promote growth cannot be converted to estradiol and thus these do not increase the libido.
Anabolic steroids are synthetic compounds used to increase net protein synthesis and skeletal muscle mass. In this manner, these compounds are similar to endogenous androgens such as testosterone. Because of their similarity to testosterone and endogenous androgens, anabolic steroids also promote the development of secondary sexual characteristics and exert a negative feedback effect on the hypothalamic-pituitary axis. As a result of this negative feedback, endogenous testosterone production and spermatogenesis are suppressed. It is not clear whether these can return to normal levels if animals receive anabolic steroids for prolonged periods.