Effects of pituitary stalk-transection and type of barrier on pituitary and luteal function during the estrous cycle of the ewe

Abstract

Effects of pituitary stalk-transection on plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) prolactin (PRL) and progesterone were investigated during the estrous cycle of ewes. Pituitary stalk (SS) or sham (SH) transection was performed on day 1 (estrus = day 0) of the estrous cycle. A Teflon or Silastic barrier was placed between the cut ends of the stalk to prevent reorganization of the portal vasculature. Immediately following surgery, pulsatile administrations of gonadotropin releasing hormone (GnRH, 200ng/hr) or 0.9% NaCl was initiated and continued for the duration of the experiment. Estradiol benzoate (EB, 50 ug im) was administered to all ewes on day 3. Mean concentrations of LH were greater in SS ewes than in SH ewes (P<.O5). There was a trend (P=.06) for the concentration of LH to be higher in ewes with Teflon compared with Silastic barriers between the cut ends of the stalk. Infusion of GnRH elevated concentrations of LH in both SS and SH ewes (P<.05). Concentrations of progesterone were reduced (P<.O1) in saline infused SS ewes while infusion of GnRH in SS ewes maintained concentrations of progesterone similar to saline infused SH ewes. The concentrations of FSH or PRL were unaffected by SS, type of barrier or treatment with GnRH. Administration of EB failed to induce a surge of LH except in a SH ewe infused with GnRH. Ewes were more responsive to infusion of GnRH following SS than after SH as reflected by increased plasma concentrations of LH and progesterone.

Platelet-activating factor may act as an endogenous pulse generator for sheep of luteolytic PGF2αrelease

“…The synergistic interaction between PAF and OT in the generation of PGF pulses supports substantial evidence for a role of OT in luteolysis in sheep (19,37). However, the persistence of PGF pulses in both ovariectomized ( 54) and hypothalamic-pituitary stalk-sec- tioned (16,35) ewes has raised doubts about an essential role for OT in the luteolytic process. The observation that PGF pulsatility occurred in steroid-replaced ovariectomized ewes is tempered by the observation that the amplitude of the PGF pulses was markedly reduced compared with intact ewes (54).…”

“…Although constant infusion of OT in ruminants prevents pulsatile release of PGF 2␣ and luteolysis (21), exhaustion of 75% of ovarian OT by use of norepinephrine (29) and inhibition of OT receptors (OTR) with an antagonist (30) did not disrupt luteolysis. Furthermore, pulsatile release of PGF 2␣ from the uterus occurs spontaneously even in ovariectomized (54) and hypothalamic-pituitary stalksectioned ewes (16,35). Although the pulses of PGF are of smaller amplitude, their persistence in these ewes questions the role of OT.…”

Components of a platelet-activating factor-signaling loop are assembled in the ovine endometrium late in the estrous cycle

“…These observations explain many aspects of luteolytic regulation in sheep; however, uterine PGF 2␣ pulses persist in the absence of the ovary (albeit with smaller amplitude), and luteolysis occurs without a functional pituitary (8,19,29). The effect of the combined removal of both the ovaries and pituitary on luteolysis is yet to be assessed; thus a redundant action of these two sources of oxytocin cannot be excluded.…”

Aspects of luteal function in pigs and sheep

“…In the ewe both pituitary stalk-transection and complete hypophysectomy have been used to investigate the components of the luteotrophic complex. Stalk-transection appears not to dramatically affect luteal function in ewes (Denamur et al, 1966;Mallory et al, 1986) and therefore the interplay between the hypothalamus and the pituitary is unclear in this species. Hypophysectomy, on the other hand, does influence luteal function in sheep but authors differ in their interpretation of the data depending on the measurements used to assess it and the actual time of assessment relative to the stage of the estrous cycle.…”