PROJECT SUMMARY Puberty is an intricate developmental process that results in reproductive maturation. In female rodents, reproductive circuits are considered mature when estrogen positive feedback is able to elicit a luteinizing hormone (LH) surge. The LH surge triggers ovulation and the luteinization of the ruptured ovarian follicle - critical events in reproduction. It was previously shown in the Micevych laboratory that peripheral estradiol (E2) increases progesterone (P4) synthesis from hypothalamic astrocytes, which initiates the LH surge. Interestingly, this E2-facilitated P4 synthesis by hypothalamic astrocytes was only observed in post-pubertal females and never in males. How the pubertal process affects hypothalamic astrocytes to begin to allow E2- facilitated P4 release is yet to be elucidated. The canonical view of puberty and reproductive maturation has been that the hypothalamic-pituitary-gonadal (HPG) axis is awakened at puberty through some combination of removal of a brake and application of the gas pedal, suggesting that the cellular machinery involved in the regulation of the HPG axis is in place long beforehand. My previous studies have shed light on the possibility that the cells responsible for reproductive maturation are not in place before puberty, given that during puberty a new population of astrocytes is added to the anteroventral periventricular nucleus (AVPV) of the hypothalamus, the brain region that controls estrogen positive feedback and the LH surge. About half of these pubertally born AVPV cells are active during the LH surge and inhibition of cell proliferation during puberty or early adulthood blunts the LH surge. These studies have led to the hypothesis that pubertally born hypothalamic astrocytes are responsible for E2-facilitated P4 release that leads to the LH surge that underlies female reproduction. The Micevych laboratory is the ideal environment for me, given that my hypothesis could explain why male hypothalamic astrocytes don't have E2-facilitated P4 synthesis. If there are ovarian hormone-induced sex differences in cell addition to AVPV, males might simply not have this population of P4-synthesizing astrocytes. The Micevych lab has the wisdom and infrastructure to aptly test this hypothesis and I propose to run parallel in vivo and in vitro studies. In vivo, I will determine where in the hypothalamus E2- facilitated P4 release occurs and then selectively ablate proliferating astrocytes before, during, or after puberty to determine if E2-signaling is disrupted. In vitro, I will examine the E2-responsiveness of pure populations of hypothalamic astrocytes born before, during, or after puberty. Together, the results from these proposed studies have the potential to change the classical view of the events that lead to reproductive maturation.