Pattern of Gonadotropin-Releasing Hormone (GnRH) Secretion Leading up to Ovulation in the Ewe: Existence of a Preovulatory GnRH Surge^*

Abstract

We have previously shown LH surges induced by physiological estradiol levels are invariably accompanied by robust and sustained GnRH surges in the ewe. Such an increase, however, has not been observed consistently during the preovulatory LH surge. In the present study, we examined GnRH secretion in Suffolk and Ile de France ewes during the preovulatory period using a method for pituitary portal blood collection which allows simultaneous portal and jugular blood samples to be taken at frequent intervals for up to 48 h. Ewes were sampled either during the mid-late luteal phase (n = 8) or follicular phase (n = 20). During the follicular phase, a robust increase in GnRH secretion occurred at the onset of the LH surge in 11 of 12 ewes sampled during the LH surge. The GnRH increase in most ewes was a massive surge, reaching values averaging 40-fold greater than baseline and extending well beyond the end of the preovulatory LH surge. In the single ewe not exhibiting a GnRH surge during the LH surge, postmortem inspection indicated blood was probably not sampled from the pituitary portal vessels. In the early follicular phase, GnRH-pulse frequency was greater than that observed in the luteal phase and, within the follicular phase, GnRH-pulse frequency increased further and amplitude decreased as the surge approached. These data demonstrate GnRH secretion leading up to ovulation in the ewe is dynamic, beginning with slow pulses during the luteal phase, progressing to higher frequency pulses during the follicular phase and invariably culminating in a robust surge of GnRH. The LH surge, however, ends despite continued elevation of GnRH.

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

“…Studies of rats and sheep have indicated that a GnRH surge accompanies and is required for the LH surge (7)(8)(9)42), and more recent studies in rhesus monkeys have indicated that a GnRH surge is present and required in this species as well (10,43). In rodents, the amplitude of the GnRH surge has been reported to be too small to trigger a LH surge without an accompanying increase in pituitary responsiveness to GnRH (44,45).…”

“…This switch, in turn, leads to a large, continuous increase in GnRH levels in pituitary portal blood, or a GnRH surge, which serves as the final, prerequisite signal for the pituitary luteinizing hormone (LH) surge, subsequently initiating ovulation. Preovulatory GnRH surges have been demonstrated in rats (7), sheep (8,9), and monkeys (10).…”

Lack of a C-terminal Tail in the Mammalian Gonadotropin-releasing Hormone Receptor Confers Resistance to Agonist-dependent Phosphorylation and Rapid Desensitization

“…This may not only allow the mammalian GnRH-R to maintain signaling during sustained agonist exposure but may also prevent functional desensitization in circumstances where the receptors are repetitively challenged with agonist. These features may serve to maintain cellular sensitivity and responsiveness during events such as the GnRH surge that precedes the preovulatory luteinizing hormone surge (42) and allow the frequency encoded pattern of the hypothalamic pulsatile GnRH release (43) to be faithfully maintained at the level of the pituitary gonadotropes.…”

Parturition to resumption of ovarian cyclicity: comparative aspects of beef and dairy cows

“…It is well established from studies using sheep as a model (and in cattle to a lesser extent) that LH pulsatile secretion reflects the GnRH pulsatile secretion pattern (Moenter et al, 1991;Skinner et al, 1995;Skinner et al, 1997;Gazal et al, 1998). This has been tested and validated by the LH pulsatile infusion studies of Duffy et al (2000) in early postpartum anoestrous beef cows.…”