It is well established that the time of ovulation in the rat is related to the environmental light/dark cycle (Everett, 1948). Rats also show a diurnal rhythm of spontaneous running, and it seemed a theoretical possibility that the effect of light on the luteinizing hormone (LH)-release mechanism might be mediated via general bodily activity. As a first step in testing this hypothesis, the relationship of ovulation time to the activity cycle was investigated under conditions of constant darkness, both activity patterns and oestrous cycles having first been disrupted by prolonged exposure to constant light.

Twelve regularly cyclic control rats were exposed to 14 h light and 10 h darkness daily, and were kept for at least 8 days in individual cages, each having an activity wheel attached. The number of revolutions/h was recorded by means of a Miniscript recorder. Daily vaginal smears were taken, and on a day of oestrus

Previous data suggest that developmental increases in peripheral concentrations of insulin-like growth factor-I (IGF-I) may be one of several neuroendocrine signals that regulate the timing of puberty. In order to test this hypothesis further, normal juvenile female rhesus monkeys (Con; n = 6) were compared with age-matched animals (Igf; n = 4) which received a constant subcutaneous infusion of recombinant human IGF-I (110 micrograms/kg/day) from 18 through 36 months of age. Menstrual bleeding was monitored and ovulation was inferred from a sustained rise in serum progesterone. In order to assess the sensitivity of luteinizing hormone-releasing hormone (LHRH) neurons to excitation, the response of serum LH to the acute administration of the glutamate receptor agonist N-methyl-D, L-aspartic acid (NMDA) was assessed prior to menarche, 2 months following menarche, and during the follicular phase of a female's third ovulation or 50 days after a female's first ovulation. In addition, the pituitary response of LH secretion to an LHRH agonist was assessed during the follicular phase of a female's fourth ovulation or 75 days following her first ovulation. IGF-I treatment effectively elevated serum concentrations by more than 86% of the values observed in Con animals. Although the treatment also enhanced the developmental increase in IGF binding protein-3 (IGFBP-3), IGF-I was increased proportionately more, resulting in a significantly higher molar ratio of IGF-I:IGFBP-3 in treated females throughout the course of the study. Treatment with IGF-I did not affect age at menarche but did significantly advance the age of first ovulation. Consequently, the interval between menarche and first ovulation was significantly shorter in Igf compared with Con females. Although the total number of ovulations exhibited by Igf (3.8 +/- 0.3) and Con females (3.0 +/- 0.5) in the 12 months following menarche was similar, significantly more of these were characterized by normal luteal phase progesterone secretion in Igf (100% +/- 0) compared with Con females (64% +/- 17). An analysis of cycles with a normal luteal phase revealed that serum estradiol during the luteal phase was significantly higher in Igf compared with Con females. Finally, IGF-treated females responded to NMDA treatment with a significantly greater increase in serum LH following menarche but not prior to menarche. In contrast, the response of serum LH to an LHRH agonist did not differ between Igf and Con females. These data suggest that the premature elevation in IGF-I levels, and consequently the ratio of IGF-I:IGFBP-3, accelerates the tempo of the final stages of puberty in rhesus monkeys. This action of IGF-I is probably the result of an increase in LHRH neuronal activity and is not due to a change in pituitary sensitivity to LHRH stimulation. In addition, ovarian sensitivity to LH stimulation during the luteal phase is also increased by IGF-I. Taken together, these data suggest that developmental increases in peripheral IGF-I secretion participate in the neuroendocrine regulation of puberty in female primates.

When 1·0 μg luteinizing hormone releasing hormone (LH-RH) was given i.v. three times at 1 h intervals from 17.00 to 19.00 h on the day of dioestrus (day 0) to regular 4 day cyclic rats, premature ovulation was induced the next morning (day 1) with the number of ova present comparable to normal spontaneous ovulation. The next spontaneous ovulation occurred on the morning of day 5, 4 days after premature ovulation induced by LH-RH.

Plasma concentrations of FSH and LH showed transient rises and falls within 1 h of administration of LH-RH; concentrations of FSH in the plasma decreased from 20.00 h on day 0 but markedly increased again from 23.00 h on day 0 to 02.00 h on day 1 and these high levels persisted until 14.00 h on day 1, with only a small increase of plasma LH during this period. The duration of increased FSH release during premature ovulation induced by LH-RH treatment was 6 h longer than the FSH surge occurring after administration of HCG on day 0. Surges of gonadotrophin were absent on the afternoon of day 1 (the expected day of pro-oestrus) and the surges characteristic of pro-oestrus occurred on the afternoon of day 4 and ovulation followed the next morning. The pituitary content of FSH did not decrease despite persisting high plasma levels of FSH during premature ovulation induced by either LH-RH or HCG on day 0.

The changes in uterine weight indicated that the pattern of oestrogen secretion from the day of premature ovulation induced by LH-RH to the day of the next spontaneous ovulation was similar to that of the normal 4 day oestrous cycle. When 10 i.u. HCG were given on day 0, an increase in oestrogen secretion occurred on day 2, 1 day earlier than in the group given LH-RH on day 0. This advancement of oestrogen secretion was assumed to be responsible for the gonadotrophin surges on day 3.

Similar numbers of fully developed follicles were found by 17.00 h on day 2 after premature ovulation induced by either LH-RH or HCG, suggesting that the shorter surge of FSH during premature ovulation induced by HCG had no serious consequences on the initiation of follicular maturation for the succeeding oestrous cycle in these rats.

Administration of LH-RH on day 0 had no direct effect on the FSH surge during premature ovulation. Secretory changes in the ovary during ovulation may be responsible for this prolonged selective release of FSH.

ABSTRACT

The suppression of ovulation in subordinate female marmosets was associated with suppressed pituitary LH secretion and reduced pituitary LH response to gonadotrophin-releasing hormone (GnRH). In subordinate females, basal plasma LH concentrations were commonly below 2 IU/1 (n = 5) (maximum 10·7 IU/1). Plasma oestrogen concentrations were similarly low (maximum 0·62 nmol/l) and plasma progesterone concentrations of below 30 nmol/l confirmed the anovulatory condition. This infertility condition was rapidly reversed when subordinate females (n = 5) were removed from their social groups and housed singly, when plasma LH (maximum 140·0 IU/1) and oestrogen (maximum 7·84 nmol/l) concentrations increased preceding ovulation. Infertility was rapidly reimposed when these singly housed females were re-introduced to subordinate status in new social groups, when plasma LH concentrations fell to their previous low values within 4 days; no ovulation occurred thereafter. Plasma oestrogen levels also fell, but less dramatically. The luteal phases of three of the subordinate females were shortened following the re-instatement of subordinate status. The maximum LH response of subordinate females to the highest dose of GnRH (200 ng) was only 19·1±6·7 IU/1 (mean ± s.e.m.; n=8): this contrasted with that in dominant females in either the follicular phase (40·0±13·3 IU/1; n=6) or the luteal phase (126·7±24·9 IU/1; n=10) of the ovarian cycle.

These results suggest that the social suppression of fertility in subordinate female marmosets is mediated by impaired hypothalamic GnRH secretion. Such an immediate and precise behavioural control of LH secretion and ovulation is without equal in anthropoid primates.

J. Endocr. (1988) 117, 329–339

SUMMARY

Plasma levels of luteinizing hormone (LH) were measured by radioimmunoassay in serial samples obtained from unanaesthetized, unrestrained rats by means of intracardiac cannulae (i) in relation to the critical period of neural facilitation (CP) during the afternoon of pro-oestrus and (ii) before and after an injection of sodium pentobarbitone given at specific times in relation to the critical period. Samples were also obtained by cardiac puncture before and after hypophysectomy carried out at specific times in relation to the CP. The effect of pentobarbitone or hypophysectomy on ovulation was also determined in the same animals. The CP as defined by pentobarbitone injection and hypophysectomy lasted from 15.00 to 16.00 h in the majority of rats. During this period the LH levels rose gradually above the basal value, and rose sharply thereafter to reach peak values at 17.00–18.00 h. Pentobarbitone given before the CP delayed the surge by 24 h, while its administration at the end of the CP caused an abrupt fall in plasma LH, indicating a reliance on the neurogenic stimulus both to initiate and to maintain the LH surge. LH levels reached immediately after pentobarbitone administration or hypophysectomy were correlated with the subsequent pattern of ovulation. Only a small increase in LH, amounting to about 14% of the peak values attained during the afternoon of prooestrus, was required for ovulation per se. This amount is similar to the small rise in circulating LH observed during the CP. The bulk of LH secreted during the 'pre-ovulatory surge' does not appear to be required for ovulation.

SUMMARY

A bioassay of gonadotrophin depending on the induction of ovulation in immature mice is described.

Dose-response data to various preparations of gonadotrophin are examined.

Results by this method in some instances contrasted more sharply with those of the ovarian augmentation assay for follicle-stimulating hormone than did the results of the assay for total gonadotrophin.

The addition of prolactin, adrenocorticotrophic hormone and stilboestrol did not interfere with the assay.

Mice were selected on the basis of ovulatory responses to injections of pregnant mare serum gonadotrophin (PMSG) and human chorionic gonadotrophin (hCG). Various parameters of pituitary and gonadal function as well as responsiveness to exogenous gonadotrophins were examined in males from high induced-ovulation rate (HIOV) and low induced-ovulation rate (LIOV) lines. Testicular weight, seminal vesicle weight and plasma LH levels were lower in HIOV than in LIOV males, while plasma concentrations of FSH and testosterone did not differ. Binding of FSH, but not of LH, in the testes was significantly higher in HIOV mice. Twenty-four hours after administration of hCG, plasma testosterone levels were higher and testicular LH binding sites appeared more depleted in HIOV than in LIOV males. Production of testosterone by decapsulated testes in vitro was significantly higher in HIOV than in LIOV mice under basal conditions, as well as in the presence of LH, FSH, hCG or PMSG. It was concluded that selection for differences in gonadotrophin-induced ovulation rate produced correlated differences in the steroidogenic response of the testes to gonadotrophins and that these differences may be due to divergence in the number of gonadal FSH binding sites.

ABSTRACT

An increased daily excretion of pregnanediol, relative to that early in the menstrual cycle, is often taken to be evidence that a woman has ovulated. This paper assesses the value of alternative procedures for this purpose. Urine, plasma and saliva samples were collected during a 24-h period from 20 women during the follicular phase and from 20 women during the luteal phase. The 24-h excretion of pregnanediol was compared with (1) the concentration of progesterone in plasma, (2) the concentration of progesterone in saliva, (3) the concentration of pregnanediol in small urine samples, (4) the rate of excretion of pregnanediol and (5) the ratio of pregnanediol to creatinine in small urine samples. Each analyte increased substantially during the luteal phase. The median increases (ratio of luteal to follicular phase values) were 14·8, 3·2, 10·6, 11·9 and 11·1 respectively. By comparison, the median increase in 24-h pregnanediol output was 9·2. When the other analytes were used instead of the 24-h excretion of pregnanediol to assess the possibility of ovulation, the incidence of misclassifications (follicular samples classed as luteal and luteal samples classed as follicular) was 0, 12·8, 5·9, 2·0 and 1·0% respectively. It was concluded that the most satisfactory alternative to the measurement of 24-h pregnanediol output for the biochemical assessment of ovulation based on progesterone production was the measurement of the concentration of progesterone in plasma; the least satisfactory alternative was determination of the concentration of progesterone in saliva. If blood was not available, measurement of the ratio of pregnanediol to creatinine in a small urine sample was the preferred method.

J. Endocr. (1984) 100, 75–80

The effects of chronic adrenalectomy on the oestrous cycle, periovulatory gonadotrophin surges in sera and ovulation in the rat were assessed. Rats which were exposed to a 14 h light: 10 h darkness lighting schedule for at least 3 weeks and those which showed at least two consecutive 4 day oestrous cycles were divided into three groups. One group (controls) was left untreated. The rest were adrenalectomized (ADX) or sham-adrenalectomized (Sham) during oestrus and then placed in a different animal room with the same lighting schedule but with the onset and end of the photoperiod delayed by 4 h. These operations did not interrupt the length of the oestrous cycles. Rats were decapitated for collection of trunk blood on the fifth pro-oestrous or oestrous day after the operation. Both Sham and ADX rats synchronized to the new lighting schedule in terms of the temporal patterns of their periovulatory surges in serum LH and FSH and the timing of ovulation. Neither adrenalectomy nor the sham-operation altered the magnitude of the pro-oestrous or oestrous increases in serum LH or FSH when compared with those in control rats. Body, uterine and ovarian weights as well as the number of ova shed were similar in the ADX and Sham groups. The results indicate that adrenal secretions are not necessary for rats to synchronize the timing of the periovulatory surges in serum LH and FSH or ovulation to a 14 h light: 10 h darkness schedule and the periovulatory surges of LH and FSH are unaltered in chronically adrenalectomized rats.

In order to elucidate the mechanism of the resumption of follicular activity and ovulation in rats, levels of FSH, LH and prolactin in plasma and pituitary gland and ovarian follicular development were quantified after removal of the litter on day 3 of lactation (day of parturition = day 0 of lactation). Such removal resulted in ovulation of 13 oocytes 4 days later, a number comparable with that found in normal cyclic rats. Plasma levels of prolactin were high during lactation but markedly decreased after removal of the litter. Although plasma concentrations of FSH and LH did not change during days 3–7 of lactation, there was an FSH surge between 24 and 30 h after removal of the litter. Plasma concentrations of LH also increased slightly but significantly by 24 h after removal of the litter and this value persisted during the following 2 days. Surges of FSH, LH and prolactin occurred at 17.00 h 3 days after pups were removed. Removal of the litter did not increase pituitary contents of FSH, LH and prolactin and a marked reduction in pituitary levels of FSH and LH, but not of prolactin, occurred at 17 00 h 3 days after removal of the litter.

A quantitative study of follicular development indicated that follicles larger than 401 μm in diameter were absent during days 3–7 of lactation. However, the number and size of antral follicles increased by 30 h after removal of the litter, probably due to the increases in plasma levels of FSH and LH, and follicles larger than 601 μm in diameter appeared 3 days after the young were removed. Although ovulation could not be induced by human chorionic gonadotrophin from days 3 to 5 of lactation, its administration 30 h after removal of the litter produced ovulation in all rats by the following morning.

These results indicated that a moderate increase in FSH, although below the amounts released at the preovulatory surge, together with basal levels of LH which were within the range observed on the day of dioestrus during the normal cycle were responsible for the initiation of follicular maturation after removal of the litter.