<P> In addition, leptin and insulin have stimulatory effects and ghrelin has inhibitory effects on gonadotropin - releasing hormone (GnRH) secretion from the hypothalamus . Kisspeptin also influences GnRH secretion . </P> <P> One of the most important functions of the HPG axis is to regulate reproduction by controlling the uterine and ovarian cycles . In females, the positive feedback loop between estrogen and luteinizing hormone help to prepare the follicle in the ovary and the uterus for ovulation and implantation . When the egg is released, the empty follicle sac begins to produce progesterone to inhibit the hypothalamus and the anterior pituitary thus stopping the estrogen - LH positive feedback loop . If conception occurs, the placenta will take over the secretion of progesterone; therefore the mother cannot ovulate again . If conception does not occur, decreasing excretion of progesterone will allow the hypothalamus to restart secretion of GnRH . These hormone levels also control the uterine (menstrual) cycle causing the proliferation phase in preparation for ovulation, the secretory phase after ovulation, and menstruation when conception does not occur . The activation of the HPG axis in both males and females during puberty also causes individuals to acquire secondary sex characteristics . </P> <P> In males, the production of GnRH, LH, and FSH are similar, but the effects of these hormones are different . FSH stimulates sustentacular cells to release androgen - binding protein, which promotes testosterone binding . LH binds to the interstitial cells, causing them to secrete testosterone . Testosterone is required for normal spermatogenesis and inhibits the hypothalamus . Inhibin is produced by the spermatogenic cells, which, also through inactivating activin, inhibits the hypothalamus . After puberty these hormones levels remain relatively constant . </P> <P> The activation and deactivation of the HPG axis also helps to regulate life cycles . At birth FSH and LH levels are elevated, and females also have a lifetime supply of primary oocytes . These levels decrease and remain low through childhood . During puberty the HPG axis is activated by the secretions of estrogen from the ovaries or testosterone from the testes . This activation of estrogen and testosterone causes physiological and psychological changes . Once activated, the HPG axis continues to function in men for the rest of their life but becomes deregulated in women, leading to menopause . This deregulation is caused mainly by the lack of oocytes that normally produce estrogen to create the positive feedback loop . Over several years, the activity the HPG axis decreases and women are no longer fertile . </P>

How is the spermatogenesis in the testis inhibited by the brain testicular axis