The final stages of follicular and oocyte maturation, ovulation, and fertilization take place during a narrow window of time, typically less than 24 h in rodents. These events, which are a prelude to normal embryonic development, can potentially be targeted by environmental contaminants or pharmaceuticals in a variety of ways, resulting in infertile reproductive cycles or developmental disorders in offspring. This chapter synthesizes experimental evidence, where available, for specific compounds targeting preovulatory follicles, the ovulatory process, or oocytes. We also point out biologically plausible sites and modes of potential toxicant-induced insult based upon studies with experimental chemicals. We hope this approach will stimulate further research focused on how acute toxicant exposure during brief but critical periods of oocyte development might derail these essential processes and place the female at risk for reproductive failure.

Concentrations of LH in female rats do not increase to concentrations seen in the male at 24 h after castration until up to one week after ovariectomy, while FSH concentrations increase rapidly in both sexes. We hypothesized that the lag in LH rise is due to the imposition of a prolonged suppression by the high concentrations of oestradiol seen during each cycle at pro-oestrus, which prevents a rapid response to the removal of negative feedback. This hypothesis was tested by studying the effect of exposure to pro-oestrus oestradiol concentrations, administered in Silastic capsules, for 24 h on the pulsatile release of LH and FSH. In the first experiment, treatment with oestradiol for 24 h began on day 3 after ovariectomy and resulted in a significant suppression of LH pulse frequency, which appeared to persist for up to 4 days after removal of the implant. In the second experiment, exposure for 24 h to high, pro-oestrous oestradiol concentrations beginning on day 3 or 7 after ovariectomy significantly suppressed mean LH concentrations and LH pulse amplitude compared with vehicle-treated controls, while FSH secretion did not differ between the two treatments. These results suggest that the acute lag in the rate of LH rise after ovariectomy depends on a prolonged suppressive effect of oestradiol and that the rise in oestradiol during the cycle, which exerts a positive feedback to trigger the preovulatory gonadotrophin surges, also exerts a negative feedback for 3-4 days, and this contributes to the relative stability of cycle duration in female rats.

Peripheral administration of N-methyl-D,L-aspartate (NMA), an analogue of the excitatory amino acid aspartate, elicits LH and prolactin (PRL) release in rats, most likely by increasing endogenous releasing-hormone secretion. These experiments were carried out to assess the degree to which NMA stimulates FSH and to analyze the relationship between endocrine status and responsiveness to NMA in female rats, in contrast to male rats, as described in the companion paper [Biol Reprod 48:000-000]. In experiment 1, estrous rats (n = 10) and diestrous rats (n = 10) and in experiment 2, estrous rats (n = 11) and rats ovariectomized (OVX) 8 days previously (n = 10) were fitted with atrial catheters and injected s.c. with 100 micrograms of an LHRH antagonist or vehicle at 2100 h. Starting at 0900 h the next day (metestrus, proestrus, or Day 9 post-OVX), blood was withdrawn every 10 min for 3 h. Each animal received i.v. 5 mg NMA after the first hour and i.v. 500 ng LHRH after the second hour. NMA significantly increased LH in metestrous and proestrous females, and LHRH antagonist blunted the increases. In OVX females, LH decreased after NMA. FSH was not affected by NMA in any group. PRL increased after NMA in proestrous and metestrous animals. LHRH caused surge-like LH and small FSH increases in vehicle groups; these increases did not differ in amplitude between intact and OVX animals and were blunted by pretreatment with LHRH antagonist. In experiment 3, 10 diestrous rats were fitted with atrial catheters and were serially bled at 2-h intervals from 1200 h on the following day (proestrus) until 0600 h on estrus morning. After the first sample the animals were injected s.c. with 0.2 mg/kg MK801, a noncompetitive NMA receptor antagonist, or with saline. Four of the 5 saline-treated animals exhibited surges of LH and FSH as well as elevated progesterone levels, with LH and progesterone peaking at 2000 h. Five of 5 MK801-treated animals failed to have elevated LH, FSH, or progesterone levels at any time point. These data demonstrate that LHRH mediates the LH response to NMA in rats and that endogenous NMA receptor binding may be necessary for the preovulatory gonadotropin surges. The lack of FSH responses to NMA during periods of low-level gonadotropin secretion suggests that physiological increments in endogenous LHRH secretion sufficient to induce a pulse of LH are insufficient to stimulate pulse-like FSH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral administration of N-methyl-D,L-aspartate (NMA), a neuroexcitatory amino acid agonist, probably stimulates 1,H release through an increase in endogenous LHRH secretion. In the present study, NMA and a potent LHRH antagonist were used to determine the degree to which release of FSH is similarly dependent upon the acute secretion of LHRH. A second aim was to compare responsiveness of LHRH neurons to NMA in castrated and intact male rats. Adult male rats were castrated (n = 10) or sham castrated (n = 11) on the morning of Day 0. After 8 days, rats were fitted with atrial catheters between 0900 and 1200 h; at 2 100 h they received s.c. either oil vehicle or 100 mug of an LHRH antagonist. Starting at 0900 h on Day 9, 0.5-ml blood samples were collected every 10 min for 3 h. After 1 h of sampling each animal received i.v. 5 mg of NMA in 0.5 ml 0.9% saline. An hour later each rat received i.v. 500 ng of LHRH in 0.5 ml saline. Plasma LH, FSH, and prolactin (PRL) levels were determined by RIA. In the oil-treated sham castrates, mean plasma LH levels were increased by 110% (p < 0.0 1) within 10 min and remained elevated for 30 min after the injection of NMA. The profile of this LH secretory response was similar to or slightly more robust than endogenous LH pulses observed previously. The NMA-induced I.H release was completely blocked by pretreatment with LHRH antagonist. In both oil- and antagonist-treated sham-castrated rats, NMA administration failed to elicit a concomitant increase in plasma FSH levels. In both castrated groups, neither LH nor FSH release was elevated after administration of NMA. Treatment with NMA produced similar PRL increments in sham-castrated and castrated groups. In all animals, the injection of LHRH stimulated robust increases in LH secretion and much smaller increases in FSH release. Our data indicate that although a large dose of exogenous LHRH can stimulate FSH release, the amount of endogenously secreted LHRH required to induce a pulse of LH is inadequate to stimulate a coincident pulse of FSH secretion. Thus, it is hypothesized that FSH secretion is regulated by two or more mechanisms; LHRH maintains, in part, basal secretion of FSH, while pulse-like increments in FSH secretion arc either constitutive endocrine events or are driven by a separate hypothalamic FSH-releasing factor. On the basis of our second major finding-that NMA-stimulated LH secretion is attenuated in castrated rats-it is also hypothesized that the readily releasable pool of LHRH is diminished following castration.

In male rats, LH pulse frequency and amplitude increase dramatically by 24 h after gonadectomy; in females they increase only slightly by this time. Mean FSH levels increase significantly in both sexes by 24 h after gonadectomy. The objectives of the present studies were to compare pulsatile LH, FSH, and prolactin (PRL) secretion in intact versus gonadectomized and in male versus female rats, and to determine whether the acute postovariectomy lag in LH rise is due to a lingering effect of the higher PRL and/or progesterone (P) levels seen in intact females. LH pulse amplitude, frequency, and mean levels increased significantly by 24 h after gonadectomy in both sexes, but the increases were greater in the males. FSH mean levels, but not pulse amplitude or frequency, increased similarly in both sexes by 24 h after gonadectomy. PRL did not change with gonadectomy. Treatment with CB-154 (a dopamine agonist), with or without RU486 (a P antagonist), 1 h before gonadectomy significantly suppressed pulsatile PRL secretion 1 day later in both sexes. There was no effect of either treatment on LH secretion. We have demonstrated that there is a sex difference in LH, but not FSH or PRL, pulsatility at 24 h after gonadectomy, and that female rats' higher PRL and P levels do not account for their slow rate of LH rise after ovariectomy.