Effect of prolonged infusion of small doses of LH-RH on the release of LH and ovulation in ewes during mid-anoestrus

Abstract

Doma\l=n'\ski& Kochman (1968) showed that sheep were more responsive to infusion of purified hypothalamic extract into the adenohypophysis in late anoestrus than in mid-anoestrus. Natural (Schally, Redding, Matsuo & Arimura, 1972) and synthetic (Matsuo, Arimura, Nair & Schally, 1971) LH-RH have been used to induce ovulation in many species, including sheep (Reeves et al., 1972). Administration of a single intravenous injection of 150 \g=m\gsynthetic LH-RH to seasonally anoestrous ewes induces LH release in all injected animals, but the LH peak is much smaller than that observed at oestrus and luteal function, assessed by peripheral progesterone levels, is absent in most animals (Crighton, Foster, Haresign & Scott, 1975). When the LH-RH was given as five injections the LH peak resembled that at natural ovulation but plasma progesterone levels were still low. Shareha, Ward & Birchall (1976) gave much smaller doses of LH-RH (25 \g=m\g/day in continuous infusions for 2 days) to anoestrous ewes but the results were again unsatisfactory.Jeffcoate, Greenwood & Holland (1974) showed that the rate of disappearance of LH-RH from the circulation in normal human subjects is represented as a double exponential with half-lives of the two components being 5-3 and 27-4 min. Kochman, Kerdelhué, Zor & Jutisz (1975) reported that degradation of the hormone, caused by polypeptidases in the hypothalamic and pituitary tissues, occurs within 30 min. We therefore investigated the effects of prolonged infusions of small doses of

Release of Luteinizing Hormone-Releasing Hormone, β-Endorphin and Noradrenaline by the Nucleus Infundibularis/Median Eminence during Periovulatory Period in the Sheep

“…However, the LH peak in the peripheral blood plasma during the natural process ranged from 100 to 240 ng/ml in 4 cases and 50 ng/ml in the other, were much higher, than the analogous values of this hormone during the cloprosterol-induced process, ranging from 8.0 to 45.0 ng/ml. The peak values of LH surge (34.72 ± 4.35 ng/ml) observed during ovulation induced in mid-anestrous ewes by an infusion of 3.0 pg synthetic LHRH into the internal maxillary artery for one 6-hour period a day for 6 con secutive days were also much lower than those observed as preovulatory surges during natural ovulation [9]. Our observa- tions show that preovulatory LH surges in the ewe are accompa nied by changes in the intensity and pattern of hypothalamic LHRH release to the hypophysial portal blood.…”

Effects of modifying gonadotrophin-releasing hormone input before and after the oestrogen-induced LH surge in ovariectomized ewes with hypothalamo-pituitary disconnection

“…First, it may be that a small but constant background of GnRH is secreted during the follicular phase of the oestrous cycle and that this, in addition to pulsatile GnRH secretion, primes the pituitary gland for the ensuing positive-feedback event. Secondly, these data may explain the conundrum whereby anoestrous ewes may be induced to ovulate by a continuous low-dose infusion of GnRH (Domanski, Przekop, Skubiszewski et al 1977;McLeod, Haresign & Lamming, 1983;McLeod & Haresign, 1984); by superimposing a con¬ stant background of GnRH upon the endogenous pulsatile secretion, it may be possible to initiate the events that culminate in a preovulatory LH surge. The effects on the oestrogen-induced LH surge of a basal mode of GnRH input followed by pulses of GnRH has important implications, and should be the subject of further study.…”