Estradiol and gonadotropin-releasing hormone (GnRH) interact to increase GnRH receptor expression in ovariectomized ewes after hypothalamic-pituitary disconnection.

Gonadotropin-releasing hormone (GnRH) receptor expression is regulated by estradiol and GnRH itself. The objective of this experiment was to determine the extent to which low levels of estradiol, similar to those observed during the transition from the luteal to the follicular phase of the estrous cycle, and GnRH interact to regulate expression of GnRH receptors and GnRH receptor mRNA. Ewes were ovariectomized (OVX) at least 2 wk prior to initiation of the experiment, and the pituitary gland was surgically disconnected from the hypothalamus to remove ovarian and hypothalamic inputs to the pituitary. Within 24 h after hypothalamic-pituitary disconnection, ewes received pulses of GnRH (250 ng/pulse) every 2 h for 6 d. At the end of 6 d, ewes were randomly assigned to treatments in a 2 x 2 factorial arrangement as follows: half of the animals received a single estradiol implant and half received an empty implant (placebo). At the same time, animals also received one of the following treatments: (1) saline or (2) GnRH (100 ng/pulse/2 h). Additionally, one group of ewes was ovariectomized, but not subjected to hypothalamic-pituitary disconnection (OVX controls). Blood samples were collected 15 min prior to each pulse of GnRH or saline and at 15-min intervals for 1 h after each pulse until tissues were collected and concentrations of luteinizing hormone (LH) were determined. Anterior pituitaries were collected 24 h after implant insertion to quantitate steady-state amounts of GnRH receptor mRNA and numbers of GnRH receptors. Mean LH was greatest in ovariectomized control ewes compared to all other treatments (p < 0.05). Mean LH and LH pulse amplitude in the placebo and GnRH-treated group most closely mimicked LH secretion in ovariectomized control animals. Mean LH and LH pulse amplitude were similar between both GnRH-treated groups (p < 0.05). Mean LH and LH pulse amplitude were significantly lower in all animals treated with saline compared to OVX controls (p < 0.05). Treatment with an estradiol implant and pulsatile GnRH increased (p < 0.05) relative amounts of GnRH receptor mRNA and the number of GnRH receptors compared to all other treatments. There were no differences in GnRH receptor expression between the remaining treatment groups (p > 0.05). Therefore, in OVX ewes after hypothalamic-pituitary disconnection, low levels of estradiol and GnRH are required to increase GnRH receptor mRNA and GnRH receptor numbers. Since we only observed an increase in GnRH receptor expression in the presence of both estradiol and GnRH, we conclude that there is a synergistic interaction between these two hormones in the regulation of GnRH receptor expression.