Pituitary Gonadotropin-Releasing Hormone Receptorsduring Gonadotropin Surges in Ovariectomized-Estradiol-Treated Rats*

Abstract

In intact cycling rats, the number of pituitary GnRH receptors varies markedly during the estrous cycle. Concentrations are maximal on diestrus and early proestrus, before falling rapidly for a brief period immediately before the preovulatory gonadotropin surge. In this study we investigated whether dynamic changes in ovarian steroids, pituitary hormones, and GnRH itself, all of which are changing at the time of the surge, play a role in the acute transient down-regulation of the pituitary GnRH receptors. We used the ovariectomized-estradiol-treated female rat as a model, as these animals exhibit daily gonadotropin surges at a predictable time of the day and also allow studies in a situation where concentrations of ovarian steroids are stable. The pituitary GnRH binding capacity (GnRH-BC) was measured using the analog D-Ala6des Gly10-GnRH ethylamide as ligand. GnRH-BC was stable between 0900-1530 h [range, 288 +/- 29 to 262 +/- 33 fmol protein (mean +/- SE)] and fell abruptly to 123 +/- 17 fmol/mg at 1630 h, before returning to the initial level by 1730 h. This abrupt fall in GnRH-BC preceded the afternoon gonadotropin surge and was similar in timing, magnitude, and duration to that observed in intact cycling rats. Serum PRL decreased from peak levels at 1630 h, coincident with the fall in GnRH-BC, before rebounding at 1730 h. Pentobarbital given at 1400 h abolished both the gonadotropin surge and the acute fall in GnRH-BC, but did not change serum PRL levels, suggesting that PRL is not causally related to the fall in GnRH-BC. The stable morning levels of GnRH-BC were not reduced after iv injections of LH, FSH, or both hormones despite elevations in serum gonadotropins to concentrations greater than those seen during the afternoon surge. Additionally, multiple iv injections of GnRH at 30- or 10-min intervals did not decrease the stable morning levels of GnRH-BC, although serum LH and FSH were markedly elevated. The data suggest that dynamic fluctuations in ovarian steroids, gonadotropins, PRL, and GnRH are not causally related to the acute transient reduction of pituitary GnRH receptors before the afternoon gonadotropin surge. These results also suggest that another hypothalamic or pituitary factor(s) is involved in the acute regulation of GnRH receptors, and the ovariectomized-estradiol-treated rat appears to be a good model for the elucidation of the factor(s) involved.

Diurnal and estradiol-dependent changes in gonadotropin-releasing hormone neuron firing activity

“…In rodents, the relationship of pituitary GnRH receptor expression to LH surge generation is unclear. Some studies in OVX rats indicated that pituitary GnRH receptor expression decreases before an estradiolinduced surge (48), suggesting that estradiol exerts a negative feedback effect at the pituitary before the LH surge, making a GnRH surge critical for LH surge generation. Other studies, however, have indicated an increase in GnRH receptor mRNA before both proestrus and estradiol-induced surges that persists throughout the LH surge in rats (49,50).…”

Estradiol-Induced LH Release in Juvenile Female Rats: Nembutal Blocks both the Decline and the Exposure of Nonavailable Pituitary LHRH Receptor Sites

“…This decline may develop before [3,30] or dur ing [8,27] the LH surge, and it appears to be of a transient nature [3,27]. A similar reduction in receptor content can be induced in ovariectomized rats by the administration of es tradiol, at a dose capable of eliciting a proestrous-type of LH discharge [3],…”

“…Earlier findings have demonstrated that Nembutal blockade of either the proestrous LH surge [1,3,27] or of an Ej-induced LH release [3] prevents the associated decline in pituitary LHRH receptors. Our results confirm these obser vations in their entirety.…”

“…[27] found that LHRH decreased LHRH receptors only after a peak LH response had been achieved. More re cently, Barkan et al [3] have examined this problem in de tail and have failed to decrease LHRH receptor content af ter several modes of LHRH administration. The possibility exists, however, that none of these regimes reproduced the proestrous pattern of endogenous LHRH release.…”

“…The LH surge induced by this procedure is ac companied by a corresponding drop in pituitary LHRH re ceptors [3]. Since Nembutal blockade of the proestrous LHRH and LH discharges prevents the decline in LHRH receptors [1,3,27], administration of Nembutal was used to test the hypothesis that increased release of LHRH and/or LHRH-related substances on proestrus renders pituitary LHRH receptors unavailable for binding of exogenous lig ands. We have reasoned that, if this is so, then pituitary membranes from Nembutal-blocked animals should not exhibit an increased number of available LHRH receptors after the in vitro dissociation procedure.…”