Brain-testicular interactions. Tonic secretion of gonadotropins is controlled by testicular negative feedback inputs to the hypothalamic-pituitary axis. Both FSH and LH are regulated by steroidal feedback at the level of the MBH. Inhibin acts selectively on the gonadotrope to modulate secretory patterns of FSH.
Secretions of FSH and LH are induced by GnRH. An increase in pituitary release of LH stimulates testicular production of testosterone, which in turn inhibits hypothalamic output of GnRH, and therefore secretion of gonadotropins. Production of testosterone and inhibin declines in the face of low levels of gonadotropins. There is a reciprocal elevation in secretion of GnRH and gonadotropins with the decrease in circulatory testosterone. The complete sequence of pulsatile endocrine events in males is repeated every few hours.
A hierarchy of reproductive control as applied specifically to the male is presented in Figure 3-20.
Testicular steroidogenesis. Testosterone is the secretory product of the testis of utmost physiological importance. Luteinizing hormone acts at various enzymatic sites within Leydig cells to prompt synthesis of testosterone. A latent hydrolase is activated (becomes phosphorylated), which mobilizes cholesterol from cholesterol esters. Transport of cholesterol into the mitochondria and synthesis of the mitochondrial C-27 and microsomal C-21 side-chain cleavage enzymes are also induced. The delta-5 pathway of steroidogenesis prevails in the male.
Estradiol is synthesized by the testis via a two-cell (Leydig/Sertoli), two-gonadotropin (LH/FSH) process. Testosterone secreted from Leydig cells in response to LH can be aromatized to estradiol by Sertoli cells under the directive of FSH. Cyclic AMP is a mediator of gonadotropic actions (Figure 3-21). The testes of most mammals have a low capacity to aromatize androgens (in contrast to the ovarian follicle); therefore, very little estradiol of testicular origin reaches the bloodstream (boars and stallions produce large amounts of gonadal estrogens).
End-organ steroid metabolism. Under some circumstances testosterone is enzymatically modified by its target cell. Testosterone is aromatized by nervous tissue, and as indicated above, by Sertoli cells. Seminiferous, ductal, accessory, and secondary tissues convert testosterone to DHT (Figure 3-22). Hence, behavior, functional aspects of the seminiferous tubule, feedback regulation of GnRH, and developmental responses can result from the intracellular action of an agent of testosterone - rather than due to testosterone itself.