SUN-238 Estrogen Modulates Expression Levels of Gonadotropin-Releasing Hormone Receptor (GNRHR) in Immortalized Kisspeptin Neurons in Vitro

In female animals, ovarian estradiol (E2) can act as both a negative feedback inhibitor of GnRH secretion, as well as a positive feedback stimulator at the time of ovulation. Both of these E2-regulated mechanisms work via stimulation or repression of two distinct neuronal populations of Kisspeptin (KP)-synthesizing neurons. While it is clear that AVPV KP neurons increase kiss1 expression during the preovulatory surge on proestrus, subsequent secretory mechanisms required for potentiation of GnRH surge release remain unclear. Two KP-secreting cell lines, KTaV-3, which demonstrate increased kiss1 expression under high E2 exposure, and KTaR-1, which exhibit kiss1 suppression under low E2 exposure, were used to probe the presence of GnRH receptor (GnRHR) expression under different E2 exposure conditions. KTaV-3 and KtaR-1 cells were treated with a range of doses of E2 (5-100pM) and/or progesterone (20nM) for varying durations (4-96h), exposed to steroid hormones either constitutively or via modulating levels over time, approximating concentration changes found during the murine estrous cycle. Following RNA isolation, cDNAs were probed with primers for gnrhr. Preliminary results in KTaV-3 cells reveal the expression of gnrhr is induced only following elevated (50-100pM) E2 treatment for 18-24h. These same E2 exposure conditions were also found to increase expression of the homeobox protein dlx3, a transcription factor required for GnRHR expression in pituitary gonadotropes. In Arc-derived KTaR-1 cells, gnrhr expression was observed only following decreases in E2 concentration, while dlx3 remained constitutively elevated in this cell line. While reciprocal GnRH-Kisspeptin connections have not yet been observed in vivo, these observations suggest the potential for Kisspeptin neurons to respond to GnRH secretory changes under particular E2 exposure conditions, by modulating receptivity to GnRH at the level of the AVPV and/or Arcuate nuclei. We are continuing to explore the temporal parameters of this induction of GnRHR in KP cells, and if exposure of immortalized KP neurons to GnRH in vitro elicits expression and signaling changes in a time- and E2-dependent manner. Results will provide a more complete understanding of positive and negative feedback mechanisms required for normal neuroendocrine regulation of reproduction.