Mechanism of the postcoital contraceptive effect of luteinizing hormone-releasing hormone: ovarian luteinizing hormone receptor interactions

Properties of a potent LHRH antagonist (Org 30850) in female and male rats

Org 30850 (Ac-D-pClPhe1,2,D-Bal3,D-Lys6,D-Ala10-LHRH) is a novel LHRH antagonist, which is being developed for the treatment of hormone-dependent disorders. The activities of this compound with respect to its endocrinological properties and side-effects were tested in rats and the results were compared with one of the first LHRH antagonists: Ac-D-pClPhe1,2,D-Trp3,D-Arg6,D-Ala10-LHRH (Org 30276). A single subcutaneous (s.c.) dose of 0.3 micrograms/kg Org 30850 administered to rats in pro-estrus gave inhibition of ovulation in approx. 50% of the rats, whereas Org 30276 was approx. 4 times less potent. The effect of a single s.c. injection of Org 30850 on testosterone levels in young adult male rats was also studied. The administration of 250 micrograms/kg or higher of Org 30850 induced a significant decrease in testosterone levels after 3 h, this effect lasted for at least 48 h. Treatment of female rats for 14 days with a daily dose of 12 micrograms/kg Org 30850 decreased statistically significantly uterine and ovarian weights. At a daily dose of 50 micrograms/kg Org 30850 completely suppressed estrous cycles and significantly decreased estradiol and FSH serum levels. The LH levels were below the detection level in both control and treated animals on the (expected) second day of di-estrus. Treatment of male rats for 14 days (25-200 micrograms/kg) resulted in a dose-dependent reduction of the gonads, accessory sex organs, testosterone levels and gonadotrophins. The decrease in gonadal function in both sexes was reversible since the females proved to be as fertile as the controls 6 weeks after the last treatment and an almost complete recovery of the weight of testes, seminal vesicles and ventral prostate was observed in the males 4 weeks after cessation of treatment. In contrast to Org 30276, Org 30850 exerted very slight irritation at the site of injection and no edematous reactions in the extremities at a daily dose of up to 8 mg/kg in male rats. It is concluded that Org 30850 is a very potent LHRH antagonist without edematous reactions and with a more favourable therapeutic index than Org 30276.

Ovarian steroid production in rats treated with gonadotropin-releasing hormone during early pregnancy

In the pregnant rat, luteinizing hormone (LH) stimulates the ovarian production of testosterone (T) which is aromatized to estradiol (E2). E2 promotes progesterone (P) synthesis by the ovary. To determine if the administration of gonadotropin-releasing hormone (GnRH) disrupts pregnancy by suppressing ovarian steroid production, rats were treated on days 7-12 of pregnancy with 25, 50 or 100 micrograms/day of GnRH or 0.2, 1 or 5 micrograms/day of a GnRH agonist (GnRH-Ag). The higher two doses of GnRH or GnRH-Ag within 24 h suppressed peripheral levels of plasma P and terminated pregnancy within 48 h. By day 12, P levels in the ovarian vein in rats treated with GnRH or GnRH-Ag in respective doses were 2098 +/- 261, 732 +/- 437, 110 +/- 15, and 2575 +/- 463, 49 +/- 9, 43 +/- 8 compared to 1833 +/- 433 ng/ml in controls. Daily treatment of P (4 mg) and E2 (0.5 microgram) simultaneously with GnRH-Ag at its maximum dose reversed the abortifacient effect of GnRH-Ag and maintained pregnancy. Peripheral levels of Plasma LH in all groups were higher than controls on days 10 and 12. Ovarian vein levels of T on days 10 or 12 of pregnancy were either not significantly different from controls (at 2703 +/- 607 or 3249 +/- 690 pg/ml, respectively) or increased dramatically to 9547 +/- 1769 on day 10 and to 5985 +/- 1426 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. Similarly, ovarian vein levels of E2 on days 10 or 12 were either not significantly different from controls (at 2022 +/- 227 or 2793 +/- 184 pg/ml, respectively) or increased dramatically to 2980 +/- 58 pg/ml on day 10 in rats treated with 25 micrograms of GnRH or to 3296 +/- 241 on day 10 and to 3420 +/- 325 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. These results indicate that the abortifacient effect of GnRH administration in rats is not due to its effect on the uterus, but to its suppressive effects on ovarian P secretion. There was no evidence to show that a GnRH-induced fall in ovarian secretion of either T or E2 were involved in this process.

Antigonadotropic actions of GnRH agonist on ovarian cells in vivo and in vitro

GnRH and its agonists have recently been shown to inhibit a variety of reproductive functions, in addition to its well-known gonadotropin releasing action in the pituitary. In order to determine the inhibitory mechanism and the site of action of these peptides in ovary, the direct actions of an agonistic analogue [D-Leu6, des-Gly-NH2] GnRH ethylamide on hypophysectomized immature rat ovaries in vivo and on rat luteal cells as well as porcine granulosa cells incubated in vitro were investigated. 125I-Labeled GnRH agonist, when injected to immature female rats, bound specifically not only to pituitary but also to ovaries. GnRH agonist inhibited hCG stimulation of progesterone production and ovarian weight augmentation in hypophysectomized immature female rats in vivo. FSH-stimulated induction of ovarian LH/hCG receptors and ovarian weight gain in diethylstilbestrol (DES)-treated hypophysectomized immature female rats were also suppressed by GnRH agonist. Moreover, treatment with GnRH agonist inhibited hCG-stimulated progesterone production by rat luteal cells incubated in vitro. In short time incubation of porcine granulosa cells obtained from medium follicles, the capacity and affinity of LH/hCG receptors were not affected by GnRH agonist. However, in long term culture of porcine granulosa cells obtained from small follicles, concomitant treatment with GnRH agonist markedly inhibited the induction of LH/hCG receptors stimulated by FSH and insulin. It may be possible that GnRH agonist prevents the differentiation of granulosa cells and subsequent acquisition of LH/hCG receptors, but has no effect on the LH/hCG receptors already induced. On the other hand, GnRH agonist delayed hCG-stimulated accumulation of cyclic AMP in porcine granulosa cells obtained from medium follicles. This delay of cyclic AMP accumulation may be responsible for the inhibition of progesterone production by ovarian cells. These findings suggest that GnRH agonist acts directly on ovarian cells and inhibits the action of FSH and LH/hCG, independently.

LRFD6a has a dose-related stimulatory or inhibitory effect on the ovary in normal luteal phase women

A dose response curve for LRFD6a-a potent LRF agonist-was established in normal midluteal phase women. At low doses (3-10 micrograms) an acute stimulatory effect on gonadotropins and ovarian steroidogenesis of estradiol and progesterone was observed. Despite large increases in circulating gonadotropins following the highest dose (30 micrograms LRFD6a), no acute changes in plasma steroids were noted. A sharp drop in progesterone was present after 3 days, with significant shortening of the cycle in 3 of 4 subjects in the group. While the stimulatory effect may be due to gonadotropin release and local action, the luteolytic effect observed is suggested to be a consequence of the direct inhibitory effect of the analog on ovarian function.

Resistance of the mouse to the antifertility effects of LHRH agonists

In the mouse, the LH-releasing activity of the LHRH agonist, D-Trp6-N alpha-MeLeu7-DesGly10-Pro9-NHEt-LHRH (Wy-40,972), was established by its ability both to induce ovulation when administered at 1600 hours on the the second day of diestrus and to elevate serum LH in adult males. While Wy-40,972 was only slightly less active in terms of these and points than it was in the rat, the predictive and possibly causal association between LH-releasing and antifertility activity established for this LHRH analog in the rat could not be clearly identified in the mouse. A total daily dose of 1000 microgram Wy-40,972/mouse was required to completely inhibit pregnancy during days 1-7 of pregnancy and produced only partial inhibition during days 7-12. This dose represents, on a body weigh basis, 8250 times the 100 percent effective pregnancy-terminating dose for the rat during equivalent intervals. The resistance of the mouse to the antifertility activity of Wy-40,972 was found not to be restricted to this particular LHRH analog or to the reproductive state. Administration of another potent LHRH analog, D-Ala6-DesGly10-Pro9-NHEt-LHRH (Wy-18,481), to adult male mice at a dose of 100 microgram/mouse/day for up to 14 days had no inhibitory effect on the weights of the testes or sex accesory organs. This dose of Wy-18,481 is 7500 times that necessary for significant reduction of these reproductive organ weights in rats within 7 days of treatment. Investigation as to the nature of the mouse's apparently divergent response to the LHRH agonists may further elucidate the antifertility mechanism of such compounds in susceptible species.

In vivo studies on prolactin function in the female rat: divergent effects of treatment with bromergocryptine and antisera to rat prolactin

The effect of suppression of prolactin (PRL) serum levels on early pregnancy and lactation in the rat by means of either bromergocryptine (BEC) or rabbit antiserum to rat PRL has been examined. BEC terminated early pregnancy when given on any day from 1 to 6 and significantly impaired lactation performance when injected during the lactation period. Antibodies against rat PRI, however, showed no effect on either early pregnancy or lactation, although the sera of the treated rats contained free PRL antibodies as evidence for sufficient dosage of antiserum. This finding can be explained in two ways: either the antibodies are not able to neutralize the biological activity of the bound PRL, or the effect of BEC is not mediated by PRL-suppression alone but requires a second component not provided by the specific antiserum.

Luteinising hormone-releasing and anti-fertility properties of a glucagon-selective somatostatin analogue

The hypothalamic tetradecapeptide, somatostatin (SRIF), inhibits the secretion of growth hormone (GH) and numerous other hormones, including insulin and glucagon. Attempts to use SRIF as an adjunct in the treatment of diabetes mellitus met with limited success due to its short biological half-life and the undesirable diabetogenic activity of its insulin-lowering properties. Efforts at synthesis have yielded SRIF derivatives with prolonged GH-lowering activity which did not suppress glucagon or had equivalent insulin-inhibiting activity as well as several short-acting compounds with the appropriate glucagon specificity. A dodecapeptide analogue [des-Ala, Gly] His-D-Trp-SRIF (Wy-41, 747) has been identified that combines selective inhibition of GH and glucagon release with prolonged activity. However, in routine pharmacological tests chronic treatment of mature rats with Wy-41, 747 produced anti-reproductive effects resembling those described for luteinising hormone (LH)-releasing hormone (RH) and its agonists. We report here that Wy-41, 747, unlike SRIF and other of its analogues tested, releases LH, induces ovulation and inhibits pregnancy when administered before or after implantation; these properties are traditionally associated with the separate LH-releasing class of peptides.

Local antifertility effect of luteinizing hormone-releasing hormone (LRH)

Injection of luteinizing hormone-releasing hormone (LRH) into the right uterine horn of the rat on D5 produced a dose related inhibition of pregnancy in the treated horn. The number of viable fetuses as well as the uterine weight was reduced while the number of placental sites was increased. The contralateral uninjected horn was unaffected. These data demonstrate that LRH can inhibit pregnancy in the rat through an extrapituitary effect.