Changes in inhibin activity, and progesterone, oestrogen and androstenedione concentrations, in rat follicular fluid throughout the oestrous cycle

Follicular fluid was aspirated from all visible surface follicles of rats at selected times of the oestrous cycle. Fluids from a pair of rat ovaries were pooled and assayed for inhibin activity by the rat anterior pituitary cell culture assay. Serum LH, FSH and progesterone as well as follicular fluid progesterone, total oestrogens and androstenedione were also measured. Follicular fluid inhibin activity was relatively constant throughout the oestrous cycle (30.7 +/- 3.4% inhibition of FSH per 0.1 microliter follicular fluid) except for a well defined surge at pro-oestrus (09:00-16:00 h, peak at 14:00 h = 84.0 +/- 7.2% inhibition of FSH per 0.1 microliter follicular fluid). The follicular fluid was not treated with charcoal before assay because a pilot experiment showed that such treatment did not alter the inhibin activity of follicular fluid. Steroids in follicular fluid were generally lowest on the afternoon of oestrus and the morning of dioestrus I and generally elevated during pro-oestrus.     

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Effects of ageing and exogenous melatonin on pituitary responsiveness to GnRH in rats

The effect of age and melatonin on the activity of the neuroendocrine reproductive system was studied in young cyclic (3-5 months-old), and old acyclic (23-25 month-old) female rats. Pituitary responsiveness to a bolus of GnRH (50 ng per 100 g body weight) was assessed at both reproductive stages in control and melatonin-treated (150 micrograms melatonin per 100 g body weight each day for 1 month) groups. After this experiment, female rats were treated for another month to study the influence of ageing and melatonin on the reproductive axis. Plasma LH, FSH, prolactin, oestradiol and progesterone were measured. A positive LH response to GnRH was observed in both control groups (cyclic and acyclic). However, a response of greater magnitude was observed in old acyclic rats. Melatonin treatment reduced this increased response in acyclic rats and produced a pituitary responsiveness similar to that of young cyclic rats. FSH secretion was independent of GnRH administration in all groups, indicating desynchronization between LH and FSH secretion in response to GnRH in young animals and during senescence. No effect on prolactin was observed. Significantly higher LH (3009.11 +/- 1275.08 pg ml(-1); P < 0.05) and FSH concentrations (5879.28 +/- 1631.68 pg ml(-1); P < 0.01) were seen in acyclic control rats. After melatonin treatment, LH (811.11 +/- 89.71 pg ml(-1)) and FSH concentrations (2070 +/- 301.62 pg ml(-1)) decreased to amounts similar to those observed in young cyclic rats. However, plasma concentrations of oestradiol and progesterone were not reduced. In conclusion, the results of the present study indicate that, during ageing, the effect of melatonin is exerted primarily at the hypothalamo-pituitary axis rather than on the ovary. Melatonin restored the basal concentrations of pituitary hormones and pituitary responsiveness to similar values to those observed in young rats.     

Changes in the feedback control of gonadotrophin secretion in ewes from lines selected for testis size in the ram lamb

The dynamics of FSH and LH secretion were studied in sheep genetically selected for High (H) and Low (L) rates of testis growth. Gonadotrophin secretion had previously been shown to be affected in the ram lamb with H-line lambs more sensitive to steroid feedback than L. While there were significant differences in mean LH concentrations during the luteal and follicular phases of the oestrous cycle, mean LH values were essentially similar in the two lines in response to ovariectomy, the effect of oestradiol implants on the response to ovariectomy and the response to LHRH. However, the frequency of LH pulses in the H line was similar during both phases of the oestrous cycle, showing a surprising insensitivity to steroid feedback. By contrast, LH pulse frequency was markedly lower in the L-line ewes in the luteal than the follicular phase (0.6 vs 1.1 pulses/h) as expected from the literature. Mean FSH concentrations were significantly higher in the L-line ewes during the follicular phase of the oestrous cycle and after ovariectomy but no significant differences were detected at the other sampling periods. There were no differences in ovulation rate between the lines. It was concluded that selection for testis size had affected the feedback control of gonadotrophin release in the ewe, as in the ram, and hence the expression of the genes controlling this is not sex limited.     

Effect of mouse epidermal growth factor on plasma concentrations of LH, FSH and testosterone in rams

Two experiments were conducted to examine the effects of mouse epidermal growth factor (EGF) on the concentrations of testosterone, LH and FSH in jugular blood plasma and on the pituitary responsiveness to LHRH. In 20 rams treated with subcutaneous doses of EGF at rates of 85, 98 or 113 micrograms/kg fleece-free body weight, mean plasma LH and testosterone concentrations were significantly reduced (P less than 0.05) at 6 h after treatment but not at 24 h. EGF treatment at 130 micrograms/kg fleece-free body weight suppressed the plasma content of these hormones for up to 48 h. Mean plasma FSH concentrations decreased significantly (P less than 0.05) for up to 48 h after EGF treatment, the effect being most pronounced in rams with mean pretreatment FSH values greater than or equal to 0.5 ng/ml. Intravenous injections of 1.0 micrograms LHRH given to each of 5 rams before and at 6 h, 24 h and 72 h after EGF treatment produced LH and testosterone release patterns which paralleled those obtained in 5 control rams similarly treated with LHRH. These results suggest that, in rams, depilatory doses of mouse EGF temporarily impair gonadotrophin and androgen secretion by inhibiting LHRH release from the hypothalamus. Such treatment appears to have no effect on the responsiveness of the pituitary to LHRH.     

Superfused pituitary cell cultures: comparative responsiveness of cells derived from various stages of the estrous cycle to LHRH stimulation administered as short duration pulses

We have reinvestigated the question of maintenance of differential LHRH sensitivity in culture and further investigated the role of pulsatile LHRH in the in vitro release of pulsatile LH and FSH at different stages of the estrous cycle. Pituitaries were collected on each day of the 4 day cycle at 0800. In addition, pituitaries were also collected at 1500 and 1900 on proestrous. The cells were dispersed and exposed 48 hrs later to short duration 4 ng LHRH pulses; this dose was optimized for LH release and was applied at a frequency of 1 pulse/60 min. In terms of absolute magnitude of LH response, observed responsiveness was ranked in the following order: proestrous 1900 greater than estrous 0800 greater than diestrous 1 0800 greater than proestrous 1500 greater than diestrous 2 0800. Responsiveness was significantly greater at proestrous 1900 (p greater than 0.01), estrous 0800 (p greater than 0.05) and diestrous 1 0800 (p greater than 0.05) when compared to either of the other stages tested. The heightened LHRH sensitivity of proestrous was therefore maintained in cell culture indicating that the system should be valid for conducting studies on the control of gonadotropin secretion during this period. FSH did not respond in pulsatile manner to the LHRH levels employed further substantiating recent evidence that LHRH seems to function somehow less directly in FSH as compared to LH secretion.     

The induction of divergent gonadotropin secretion in vitro by variations in luteinizing hormone releasing hormone pulse regimen

The effects of luteinizing hormone releasing hormone (LHRH) pulse amplitude, duration, and frequency on divergent gonadotropin secretion were examined using superfused anterior pituitary cells from selected stages of the rat estrous cycle. Cells were stimulated with one of five LHRH regimens. With low-amplitude LHRH pulses (regimen 1) in the presence of potentially estrogenic phenol red, LH response in pituitary cells from proestrus 1900, estrus 0800, and diestrus 1,0800 were all significantly larger (P less than 0.05) than the other stages tested. In the absence of phenol red, responsiveness at proestrus 1900 was significantly larger than proestrus 0800, proestrus 1500, and estrus 0800 (P less than 0.01, 0.05, and 0.05, respectively); other cycle stages tested were smaller. No significant differences were observed between cycle stages for follicle-stimulating hormone (FSH) secretion in the presence or absence of phenol red. Because pituitary cells at proestrus 1900 were the most responsive to low-amplitude 4 ng LHRH pulses, they were also used to study the effects of LHRH pulses of increased amplitude or duration and decreased frequency. Increasing the amplitude (regimen 2) or the duration (regimens 3 to 5) increased FSH secretion; this effect was greatest with regimens 3 and 5. When regimens 3 and 5 were studied in pituitary cells obtained at proestrus 1500, FSH was significantly increased by both regimes, but most by regimen 5; furthermore, LH release was significantly reduced. When regimens 3 and 5 were studied in pituitary cells obtained at estrus 0800, FSH release was elevated most significantly by regimen 5. Thus, variations in LHRH pulse regimen were found to be capable of inducing significant divergence in FSH release from superfused anterior pituitary cells derived from specific stages of the estrous cycle.     

Pituitary response to exogenous LHRH in superovulated women

The response of the pituitary to exogenous LHRH was investigated in 9 normally ovulating women during the late follicular phase of a spontaneous (control) cycle, a cycle during treatment with clomiphene and a cycle during treatment with 'pure' FSH. During clomiphene treatment, basal FSH concentrations increased significantly up to Day 6 of the cycle and then decreased progressively while basal LH values showed a continuous rise. During treatment with FSH, basal LH concentrations decreased significantly. The response of both FSH and LH to LHRH showed a significant and quantitatively similar decrease during clomiphene or FSH administration as compared to the spontaneous cycles. It is suggested that basal secretion of FSH and LH is regulated by two separate mechanisms, and that an ovarian inhibitory factor(s) attenuates the response of both FSH and LH to exogenous LHRH and possibly the endogenous LH surge in superovulated cycles.     

Effects of abdominal vagotomy on serum LH concentrations in female rats

Vagotomy on the morning of pro-oestrus did not prevent the pro-oestrous LH surge and rats became oestrous on the following day. However, vagotomized rats then exhibited a period of acyclicity which lasted for 20.4 +/- 1.3 (s.e.m.) days. Food intake and body weight also declined after vagotomy. During the first week after vagotomy, afternoon LH surges generally did not occur, a pattern which was similar in animals pair fed with vagotomized rats. However, pair-fed rats showed oestrous cycles while vagotomized rats were acyclic. At 7 days after vagotomy, LH surges were induced by oestradiol benzoate and progesterone treatment of ovariectomized rats. Vagotomy suppressed the post-ovariectomy increase in serum LH at 7 and 21 days after surgery. These results, combined with those of other studies, suggest impairment of LH release in vagotomized rats.     

Dissociation of LH and FSH Responses to LHRH during estrogen therapy of patients with ovarian failure

This study examines the effect of oral estrogen treatment on gonadotropin secretion in three young women with gonadal failure. Each subject was treated with 0.1 mg BID of ethinyl estradiol for four weeks, and the LH and FSH responses to 200 microgram of intravenously administered LHRH were measured basally and weekly during therapy. Significant reduction of basal levels of FSH occurred within one week of treatment, with obliteration of LHRH-mediated FSH responsiveness within two weeks. By contrast, basal levels of LH were significantly reduced by the end of the second week of treatment, and LHRH-mediated LH levels were sustained for three weeks. In one subject an LHRH test was performed every other day for two weeks after cessation of therapy. Return of FSH responsiveness was delayed one week beyond that of LH, which occurred within three days of discontinuation of estrogen. These results indicate that during the early phase of oral estrogen replacement therapy, FSH secretion may be selectively blunted; after discontinuation of treatment, recovery of FSH secretion lags behind recovery of LH.     

Steroid regulation of gonadotropins in genetically hypoprolactinemic females (IPL nude rats)

IPL nude females present an absence of lactation with hypoprolactinemia. While males present a slight but significant decrease in serum testosterone and gonadotropins, females show normal values of estradiol, progesterone, LH and FSH during all estrus cycle stages. In this work, we observed that the postovariectomy rise of LH and FSH was significantly lower in the IPL nude females. We studied also the effect of acute (1 injection of 25 micrograms/rat E2Bz) or long-term (E2Bz capsule for 8 days) estradiol benzoate (E2Bz) treatment, with or without progesterone injection (5 mg/rat) in ovariectomized (OVX) IPL and normal females. The sensitivity of gonadotropins to E2 negative feedback is decreased in the IPL nude rats, result in agreement with previous reports and which could be linked to both hypoprolactinemia and decreased beta-endorphin observed in the IPL nude rat. The responsiveness of LH to LHRH was also tested in OVX and OVX + E2Bz or OVX + E2B + P treated. In OVX females responsiveness of LH to LHRH was similar in IPL nude to that of normal females. However, LH responsiveness in acute and long-term steroid-treated OVX IPL nude was significantly depressed. Since the mechanism whereby PRL interacts with steroids to modify gonadotropin secretion is still unexplained, IPL nude rat could be a good model to study it.     

Alteration of gonadotrophin and steroid hormone release, and of ovarian function by a GnRH antagonist in gilts

This study examined the impact of the gonadotrophin-releasing hormone (GnRH) antagonist Antarelix on LH, FSH, ovarian steroid hormone secretion, follicular development and pituitary response to LHRH in cycling gilts. Oestrous cycle of 24 Landrace gilts was synchronised with Regumate (for 15 days) followed by 800 IU PMSG 24h later. In experiment 1, Antarelix (n=6 gilts) was injected i.v. (0.5mg per injection) twice daily on four consecutive days from day 3 to 6 (day 0=last day of Regumate feeding). Control gilts (n=6) received saline. Blood was sampled daily, and every 20 min for 6h on days 2, 4, 6, 8 and 10. In experiment 2, gilts (n=12) were assigned to the following treatments: Antarelix; Antarelix + 50 microg LHRH on day 4; Antarelix + 150 microg LHRH on day 4 or control, 50 microg LHRH only on day 4. Blood samples were collected daily and every 20 min for 6h on days 2, 4 and 6 to assess LH pulsatility. Ovarian follicular development was evaluated at slaughter.Antarelix suppressed (P<0.05) serum LH concentrations. The amount of LH released on days 4-9 (experiment 1) was 8.80 versus 36.54 ngml(-1) (S.E.M.=6.54). The pattern of FSH, and the preovulatory oestradiol rise was not affected by GnRH antagonist. Suppression of LH resulted in a failure (P<0.05) of postovulatory progesterone secretion. Exogenous LHRH (experiment 2) induced a preovulatory-like LH peak, however in Antarelix treated gilts the LH surge started earlier and its duration was less compared to controls (P<0.01). Furthermore, the amount of LH released from day 4 to 5 was lower (P<0.01) in Antarelix, Antarelix + 50 and Antarelix + 150 treated animals compared to controls. No differences were estimated in the number of LH pulses between days and treatment. Pulsatile FSH was not affected by treatment. Mean basal LH levels were lower (P<0.05) after antagonist treatment compared to controls. Antarelix blocked the preovulatory LH surge and ovulation, but the effects of Antarelix were reduced by exogenous LHRH treatment. The development of follicles larger than 4mm was suppressed (P<0.05) by antagonist treatment.In conclusion, Antarelix treatment during the follicular phase blocked preovulatory LH surge, while FSH and oestradiol secretion were not affected. Antarelix failed to alter pulsatile LH and FSH secretor or pituitary responsiveness to LHRH during the preovulatory period.     

Changes in pituitary and ovarian LHRH receptors after active immunization of female rats against LH or LHRH

The hypothalamic-pituitary-ovarian axis of female rats was disrupted at the site of LHRH stimulation by active immunization against LHRH or at the site of LH action by active immunization against LH. Active immunization against LH was associated with an increase in pituitary LHRH receptors to levels comparable to control values at pro-oestrus whereas immunization against LHRH led to a marked reduction in receptor numbers. Ovarian LHRH receptor concentrations were increased by both treatments. It is concluded, therefore, that (1) LHRH receptors in the pituitary and ovary are not concomitantly controlled, and (2) pituitary receptor numbers are primarily under positive autoregulatory control by LHRH and that ovarian LHRH receptor concentrations may be under long-term influence of LH.     

Sex differences in pituitary LH storage and release in LHRH-stimulated pubertal rats

Summary This study investigates the relationship between pituitary LHRH responsiveness and the depletion of LH in pubertal rats. The anterior pituitaries of 7-week-old rats of both sexes were stimulated for a maximum of 24 h with either a continuous, or pulsatile exposure to LHRH in vitro. Immunohistochemical examination revealed that most LH-cells in females became depleted of immunoreactive material, regardless of the mode of LHRH administration. In contrast, the majority of LH-cells in the male gland retained a strong immunostaining intensity. Radioimmunoassay showed that the initial pituitary LH content was significantly lower in the female rats (P< 0.001), but, even so, they released a higher percentage of stored LH in response to LHRH stimulation in vitro. A similar result was also obtained after a single injection of LHRH in vivo. Thus, the lower LH content and higher LHRH responsiveness of the female pituitary explain why LHRH treatment induced a pronounced LH depletion in this sex. These results are discussed in relation to available data on heightened LH secretion in maturing female rats.     

Effect of LHRH immunoneutralization on follicular development, the LH surge and luteal function in the stumptailed macaque monkey (Macaca arctoides)

A dose of 100 microliter of a potent ovine LHRH gamma globulin inhibited ovulation in the cyclic rat when administered at 12:00 h on the day of pro-oestrus. A dose of 10 ml of the preparation was administered i.v. to female stumptailed macaques to achieve circulating antibody titres 3-4-fold higher than in the rat. In an ovariectomized macaque, this caused a marked fall in serum concentrations of LH to less than 10% of pretreatment values and also a significant, though less pronounced, fall in FSH. Six monkeys were treated with the LHRH gamma globulin during the mid-late follicular phase of the cycle. In 2 monkeys in which serum oestradiol concentrations were less than 100 pg/ml at the time of antibody administration, the rising oestradiol levels were abruptly suppressed and the normal mid-cycle LH surge failed to occur. Serum concentrations of LH and FSH declined to low levels for 8-10 days after which time normal follicular development occurred. In the remaining 4 monkeys in which follicular development was more advanced as indicated by serum oestradiol concentrations of greater than 100 pg/ml, the antibodies induced either a transient decline or had no effect on the rising serum concentration of oestradiol. An LH/FSH surge followed by a rise in serum progesterone occurred in these macaques. When the antibodies were administered to a further 6 macaques, which had also been treated with oestradiol benzoate during the early follicular phase to induce an LH surge, the neutralization of LHRH again failed to block the surge even when the dose of antibody was increased to 20 ml. The results show that LHRH antibodies were unable to block the LH surge in the macaque. They contrast with results obtained with LHRH immunoneutralization in the sheep, rat, hamster, mouse and bird and suggest that the ability of oestrogen to induce an LH surge by acting directly on the LHRH-primed anterior pituitary gland is more dominant in the primate.     

Polycystic ovarian condition in estradiol valerate-treated rats: spontaneous changes in characteristic endocrine features

A chronic anovulatory polycystic ovarian (PCO) condition can be induced in rats with estradiol valerate (EV). We have previously shown that the early stages (8-10 wk after EV treatment) of the condition are characterized by low basal plasma luteinizing hormone (LH) and estradiol concentrations, as well as poor LH responsiveness to LH-releasing hormone (LHRH). These observations suggested that alterations in pituitary LH secretory activity may be involved in induction and maintenance of the PCO condition. In order to examine this possibility we have measured basal plasma LH and follicle-stimulating hormone (FSH) concentrations at various times (6, 15, 20 and 22 wk) after treatment with EV. AT 22 wk animals were subjected to a double LHRH pulse or equivalent treatment with saline. Basal plasma LH concentrations in EV-treated animals doubled between 6 and 22 wk. Despite this sharp increase, basal plasma LH concentrations at 22 wk were still significantly lower in EV-treated animals compared to proestrous controls. Basal FSH in EV-treated animals, remained in the proestrous range throughout the 22-wk period. Pituitary FSH and LH secretions in response to the LHRH challenge were significantly greater in EV-treated animals compared to proestrous controls. Plasma estradiol was significantly greater at 22 wk post-EV treatment than at 9 wk and this difference was reflected in the histology of the endometrium. These results indicate that a PCO condition is compatible with radical alterations in basal LH, and responsiveness to LHRH. Thus, aberrations in the ability to secrete LH do not appear to be causal in maintaining the condition.     

Modulation of pituitary responsiveness to LHRH by androgens in ovariectomized female rats.

The effect of androgens on pituitary response to luteinizing-hormore-releasing hormone (LHRH) and their ability to modify effects of 17beta-estradiol (E2) on pituitary responsiveness to LHRH were tested in ovariectomized rats maintained on a daily dose of 0.25 microgram estradiol benzoate per rat for 6 d before androgen administration. Testosterone propionate (TP) (4, 40, 400, or 4000 microgram per rat), administered 24 h before LHRH (500 ng per rat), had no significant effect on luteinizing hormone (LH) or follicle-stimulating hormone (FSH) response. Similar doses of dihydrotestosterone (DHT) did not significantly alter the LH response but significantly suppressed the FSH response. Even the lowest dose completely blocked the FSH response to LHRH. TP in combination with 4 or 400 microgram of E2 suppressed the stimulatory effect of E2 on both LH and FSH response to LHRH in a dose-related manner. DHT and E2 in combination affected LH response inconsistently, whereas their ratio determined FSH response; there was pronounced inhibition of FSH response in rats given high doses of DHT combined with low doses of E2; DHT inhibition of FSH response in animals receiving 4 microgram of DHT with 400 microgram E2 was partially overcome by the stimulatory effect of E2. Our results indicate that TP and DHT affect LH and FSH response to LHRH differently. The ratio of androgen to estrogen is important in determining the response to LHRH.     

Hypothalamic luteinizing hormone-releasing hormone (LHRH) and LH secretion in aging female rats

Age-related changes in hypothalamic luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) secretion were studied in young (6 months), middle-aged (12 months) and old (18 months) female rats. The LHRH levels in the mid-hypothalamic area were higher in intact middle-aged and old females than in young ones. Additionally, there was no age difference in the hypothalamic LHRH levels in male rats. In order to clarify the significance of this age-related increase in female rats, we examined the effects of progesterone treatment in estrogen-primed ovariectomized young and old rats on the LHRH levels in the median eminence (ME) and on plasma LH levels. We found phasic changes in ME-LHRH and plasma LH levels in estrogen-primed rats following progesterone treatment in rats of both ages, but the progesterone-induced change in ME-LHRH levels tended to be delayed in old rats compared with young females. This delay may correspond to the delayed onset, slow and low magnitude of plasma LH increase in old females. The ME-LHRH levels were generally higher in old rats than in young rats. Nevertheless, we found that the increase in plasma LH in response to progesterone treatment in estrogen-primed ovariectomized females was smaller in old rats than young rats. These results suggest that the LHRH secretory mechanism changes with age in female rats. Such alterations may result in the accumulation of LHRH in the mid-hypothalamic area and an increase in ME-LHRH.     

Ontogeny of pituitary gonadotrophin secretion in the fetal and post-natal pig in response to LHRH in vitro

Monolayer cultures of anterior pituitary cells from male or female pigs of 60, 80, 105 days of fetal life or of 60, 160 and 250 days of post-natal life were prepared and treated with LHRH (1 pM to 10 nM). Dose-related increases of LH were first seen at 80 days of gestation in both sexes, while only female fetuses responded to maximal LHRH at 60 days. Basal and stimulated LH release doubled in cultures from 105-day-old fetuses when compared with those at 80 days. Compared to late fetal stages LH release was 20- to 30-fold higher in cell cultures from 60-day-old (post-natal) donors without further change during the post-natal period. In all pre- and post-natal age groups basal and maximal LH release of pituitary cells from males was lower than that of females. FSH stimulation started in male and female cells at 80 days of gestation only at LHRH concentrations exceeding or equal to 0.1 nM. By 105 days FSH secretion was dose-related and pituitary cells of females responded with higher FSH values than did those of males. In general, post-natal cells released much higher amounts of FSH than did prenatal cells. Basal and maximal release of FSH decreased during post-natal development in both sexes. Basal as well as maximal FSH release of cultures from female donors was higher than that found in cultures from male donors. Determination of total LH and FSH content in fetal pituitary cell cultures indicated that the developmental increase in gonadotrophin release potential is a function of the total gonadotrophin content in vitro. We conclude that (1) the in-vitro release of gonadotrophins to LHRH is dose-, age- and sex-dependent; (2) in the female fetal pig LH responsiveness develops earlier than FSH responsiveness; (3) apparently, these maturational changes mainly reflect alterations in pituitary gonadotrophin content; and (4) there is no simple relationship between in-vitro release and circulating gonadotrophins.     

Effects of ovarian hyperstimulation and isolated preovulatory follicles on LH responses to GnRH in rats

Adult rats were pretreated with a 3-day regimen of human menopausal gonadotrophin (hMG), PMSG, human FSH or hCG and experiments were carried out on the day of pro-oestrus. Treatment with hMG and hFSH induced a significant increase in the number of preovulatory follicles on the day of pro-oestrus and this was correlated with increased circulating concentrations of oestradiol. There was a parallel increase in the self-priming effect of GnRH, as observed from the biphasic LH response to a continuous GnRH challenge. PMSG treatment did not stimulate increased numbers of maturing follicles and was less effective in raising circulating oestrogen concentrations compared with hMG and hFSH. However, pituitary responsiveness was much higher after PMSG treatment and the biphasic response to continuous perfusion with GnRH was absent; LH release was high from the initiation of the stimulus. hCG alone failed to stimulate follicular maturation but enhanced pituitary LH responses. Hemi-pituitary glands perfused in the presence of isolated preovulatory follicles also showed augmented biphasic LH responses to GnRH compared with control hemi-pituitary glands. The apparent dissociation which can occur between follicular maturation, circulating oestrogen concentrations and pituitary responsiveness to GnRH supports the idea of non-steroidal ovarian factors modulating LH release.