Schematic of neuroendocrine feedback systems involved in the control of GnRH and LH secretion. Feedback 1: GnRH and LH release is under the control of negative feedback action of estradiol, which is predominant during the prepubertal and anestrus period. Feedback 2: positive feedback actions of estradiol responsible for generation of the preovulatory LH surge and onset of cyclicity. After puberty (adulthood, right panel), a third feedback mechanism comes into play (Feedback 3), namely negative feedback action of progesterone, operational during the luteal phase (modified from Foster et al., 2007).