No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes1

Abstract

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.

LIM Homeodomain Transcription Factor Isl-1 Enhances Follicle Stimulating Hormone-β and Luteinizing Hormone-β Gene Expression and Mediates the Activation of Leptin on Gonadotropin Synthesis

“…Furthermore, LH and FSH also exhibit different secretion patterns. Previous studies in sheep have confirmed that LH is stored in electron-dense granules within the gonadotrope (55) associated with SgII (56 -58), and FSH appears in electron light granules, possibly associated with chromogranin A (57)(58)(59). The release and subsequent plasma concentrations of FSH appear more closely related to the amount of FSH being produced within the gonadotrope, and also to the levels of transcription of FSH␤ mRNA (4).…”

In utero exposure to low doses of environmental pollutants disrupts fetal ovarian development in sheep

“…Fetal serum levels of follicle-stimulating hormone (FSH), estradiol and prolactin (PRL) were measured by radioimmunoassays that have been described and validated previously for sheep ( McNeilly et al ., 1986 ; Mann and Lamming, 1995 ; Lincoln et al ., 2003 ; Crawford et al ., 2004 ). The assay standards used, assay sensitivities and intra-assay coefficients of variation were: FSH; NIDDK-FSH-RP2 and NIH-LS18, 0.1 ng/ml, <10%, estradiol; MAIA estradiol kit (Serono Diagnostics, Fleet, Hants, UK), 0.2 pg/ml, <12%, PRL; NIDDK-PRL—RP3, 0.5 ng/ml, 10%).…”

KISS1R: Hallmarks of an Effective Regulator of the Neuroendocrine Axis

“…GnRH priming is not only believed to be important for the normal onset of puberty. In reproductively mature female animals, including human females, it has also been proposed that GnRH secreted during progression of the follicular phase increases the number of GnRH receptors on gonadotropes and this serves to potentiate the LH preovulatory surge [72,73,74]. Thus, the constitutive activity of KISS1R may act to prime the gonadotrope.…”