Daily variations in the sensitivity of proestrous LH surge in the inhibitory effect of intraventricular injection of 5-HT or GABA in rats.

Intraventricular injection of 5-hydroxytryptamine (5-HT) into female rats at 11:00 h on the day of proestrus inhibited the preovulatory surge of luteinizing hormone (LH) and ovulation. A similar response was observed after the activation of the serotonergic system by stimulation of the median raphe nucleus. A diurnal rhythm of these responses was observed. In rats acclimated to a 14-h:10-h light:dark cycle the potency of 5-HT to inhibit the LH surge and ovulation was 2.06 and 2.3 times greater, respectively, when injected at 11:00 h than at 13:00 h. Also stimulation of the median raphe nucleus at 11:00 h was significantly more effective in inhibiting these parameters than stimulation at 13:00 h. Similarly, the ability of gamma-amino-butyric acid (GABA) to inhibit the preovulatory LH surge and ovulation was greater in rats injected in the morning than in the afternoon. The results of this study indicate that during proestrus the sensitivity of 5-HT and GABA to induce inhibition of preovulatory LH release and ovulation shows daily variations with maximal effect before the critical period.     

Pituitary and ovarian hormone secretion and ovulation in gilts injected with gonadotropins during and after oral administration of progesterone agonist (Altrenogest)

We determined changes in plasma hormone concentrations in gilts after treatment with a progesterone agonist, Altrenogest (AT), and determined the effect of exogenous gonadotropins on ovulation and plasma hormone concentrations during AT treatment. Twenty-nine cyclic gilts were fed 20 mg of AT/(day X gilt) once daily for 15 days starting on Days 10 to 14 of their estrous cycle. The 16th day after starting AT was designated Day 1. In Experiment 1, the preovulatory luteinizing hormone (LH) surge occurred 5.6 days after cessation of AT feeding. Plasma follicle-stimulating hormone (FSH) increased simultaneously with the LH surge and then increased further to a maximum 2 to 3 days later. In Experiment 2, each of 23 gilts was assigned to one of the following treatment groups: 1) no additional AT or injections, n = 4; 2) no additional AT, 1200 IU of pregnant mare's serum gonadotropin (PMSG) on Day 1, n = 4); 3) AT continued through Day 10 and PMSG on Day 1, n = 5, 4) AT continued through Day 10, PMSG on Day 1, and 500 IU of human chorionic gonadotropin (hCG) on Day 5, n = 5; or 5) AT continued through Day 10 and no injections, n = 5. Gilts were bled once daily on Days 1-3 and 9-11, bled twice daily on Days 4-8, and killed on Day 11 to recover ovaries. Termination of AT feeding or injection of PMSG increased plasma estrogen and decreased plasma FSH between Day 1 and Day 4; plasma estrogen profiles did not differ significantly among groups after injection of PMSG (Groups 2-4). Feeding AT blocked estrus, the LH surge, and ovulation after injection of PMSG (Group 3); hCG on Day 5 following PMSG on Day 1 caused ovulation (Group 4). Although AT did not block the action of PMSG and hCG at the ovary, AT did block the mechanisms by which estrogen triggers the preovulatory LH surge and estrus.     

Interaction between ovarian and adrenal steroids in the regulation of gonadotropin secretion

Recent work from our laboratory suggests that a complex interaction exists between ovarian and adrenal steroids in the regulation of preovulatory gonadotropin secretion. Ovarian estradiol serves to set the neutral trigger for the preovulatory gonadotropin surge, while progesterone from both the adrenal and the ovary serves to (1) initiate, (2) synchronize, (3) potentiate and (4) limit the preovulatory LH surge to a single day. Administration of RU486 or the progesterone synthesis inhibitor, trilostane, on proestrous morning attenuated the preovulatory LH surge. Adrenal progesterone appears to play a role in potentiating the LH surge since RU486 still effectively decreased the LH surge even in animals ovariectomized at 0800 h on proestrus. The administration of ACTH to estrogen-primed ovariectomized (ovx) immature rats caused a LH and FSH surge 6 h later, demonstrating that upon proper stimulation, the adrenal can induce gonadotropin surges. The effect was specific for ACTH, required estrogen priming, and was blocked by adrenalectomy or RU486, but not by ovariectomy. Certain corticosteroids, most notably deoxycorticosterone and triamcinolone acetonide, were found to possess "progestin-like" activity in the induction of LH and FSH surges in estrogen-primed ovx rats. In contrast, corticosterone and dexamethasone caused a preferential release of FSH, but not LH. Progesterone-induced surges of LH and FSH appear to require an intact N-methyl-D-aspartate (NMDA) neurotransmission line, since administration of the NMDA receptor antagonist, MK801, blocked the ability of progesterone to induce LH and FSH surges. Similarly, NMDA neurotransmission appears to be a critical component in the expression of the preovulatory gonadotropin surge since administration of MK801 during the critical period significantly diminished the LH and PRL surge in the cycling adult rat. FSH levels were lowered by MK801 treatment, but the effect was not statistically significant. The progesterone-induced gonadotropin surge appears to also involve mediation through NPY and catecholamine systems. Immediately preceding the onset of the LH and FSH surge in progesterone-treated estrogen-primed ovx. rats, there was a significant elevation of MBH and POA GnRH and NPY levels, which was followed by a significant fall at the onset of the LH surge. The effect of progesterone on inducing LH and FSH surges also appears to involve alpha 1 and alpha 2 adrenergic neuron activation since prazosin and yohimbine (alpha 1 and 2 blockers, respectively) but not propranolol (a beta-blocker) abolished the ability of progesterone to induce LH and FSH surges. Progesterone also caused a dose-dependent decrease in occupied nuclear estradiol receptors in the pituitary.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ability of progesterone to reverse anti-androgen (hydroxyflutamide)-induced interference with the preovulatory LH surge and ovulation in PMSG-primed immature rats

In Exp. 1, PMSG was injected to 26-day-old prepubertal rats to induce ovulations. On Day 2 (2 days later, the equivalent of the day of pro-oestrus) they received at 08:00 h 5 mg hydroxyflutamide or vehicle and at 12:00 h 2 mg progesterone or testosterone or vehicle. Animals were killed at 18:00 h on Day 2 or at 09:00 h on Day 3. Progesterone but not testosterone restored the preovulatory LH surge and ovulation in hydroxyflutamide-treated rats. In Exp. 2, 2 mg progesterone or testosterone were injected between 10:30 and 11:00 h on Day 2, to advance the pro-oestrous LH surge and ovulation in PMSG-primed prepubertal rats. Injection of hydroxyflutamide abolished the ability of progesterone to advance the LH surge or ovulation. Testosterone did not induce the advancement of LH surge or ovulation. In Exp. 3, ovariectomized prepubertal rats implanted with oestradiol-17 beta showed significantly (P less than 0.01) elevated serum LH concentrations at 18:00 h over those observed at 10:00 h. Progesterone injection to these animals further elevated the serum LH concentrations at 18:00 h, in a dose-dependent manner, with maximal values resulting from 1 mg progesterone. Hydroxyflutamide treatment significantly (P less than 0.003) reduced the serum LH values in rats receiving 0-1 mg progesterone but 2 mg progesterone were able to overcome this inhibition. It is concluded that progesterone but not testosterone can reverse the effects of hydroxyflutamide on the preovulatory LH surge and ovulation. It appears that hydroxyflutamide may interfere with progesterone action in induction of the LH surge, suggesting a hitherto undescribed anti-progestagenic action of hydroxyflutamide.     

Neuroendocrine control of luteinizing hormone secretion and reproductive function in spontaneously persistent-estrous aging rats

Middle-aged female rats cease to display estrous cycles and exhibit a state of persistent estrus (PE). Under PE and chronic anovulatory conditions, there is a lack of spontaneous luteinizing hormone (LH) surges, but ovulations often occur after the females are caged with males. This study examined the effects of caging and mating with male rats on LH release in PE females, and assessed their reproductive capacity. Young cyclic rats received intra-atrial cannulae, and subsequently were sampled every 90 min during 1400-2130 h on proestrus for plasma LH measurement. PE females were similarly cannulated and sampled. Two days later, these PE rats received an s.c. injection of estradiol benzoate (EB) and were sampled on the following day. While young females exhibited the proestrous LH surge, PE rats maintained low plasma LH levels persistently and were unable to increase LH release after EB administration. On the other hand, when cannulated PE females were caged with fertile males, 92% displayed lordotic responses, and 75% of those sexually receptive PE females exhibited LH surges followed by ovulation. The initiations of the lordotic response and the LH surge both were more rapid in PE females caged with males beginning at 1500 h than at 1400 h. In contrast, when individual PE rats were placed in clean boxes without males, only one of 13 females showed an increase in LH release followed by ovulation. Separate groups of PE rats were mated with fertile males, and subsequently used for counting the number of blastocysts in the uteri on Day 5 of pregnancy and the number of pups delivered at term.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progesterone blocks the estradiol-stimulated luteinizing hormone surge by disrupting activation in response to a stimulatory estradiol signal in the ewe

The preovulatory surges of GnRH and LH are activated by increased concentrations of circulating estradiol, but ovulation is blocked when progesterone concentrations are elevated. Although it is has been shown that this action of progesterone is due to a central inhibition of the GnRH surge, the mechanisms that underlie the blockade of the GnRH surge are poorly understood. In this study we investigated whether progesterone can block the estradiol-dependent activation stage of the GnRH surge induction process, and thus prevent expression of the LH surge. The results demonstrated that exposure to progesterone for half or the full duration of the activation stage can prevent the stimulation of LH surges by estradiol (experiment 1), whereas exposure to progesterone midway though a period of estradiol exposure, which in itself is sufficient to activate the surge, did not block the LH surge (experiment 2). These results suggest that progesterone 1) disrupts activation of the surge induction system in response to a stimulatory estradiol signal and 2) does not compromise the ability of animals to respond to a stimulatory estradiol signal applied immediately after progesterone exposure. Because the disruptive effects of activated progesterone in response to estradiol are rapid but transient, it may be that progesterone directly interferes with the activation of estradiol-responsive neural systems to block the GnRH/LH surge.     

LH release and luteal function in post-partum acyclic ewes after the pulsatile administration of LH-RH

The administration of LH-RH in a pulsatile regimen (100 ng i.v./h for 48 h) to acyclic ewes 26-30 days post partum increased plasma LH concentrations, and both the frequency and amplitude of plasma LH pulses. In 12/14 ewes these increases were followed by plasma LH surges similar to the preovulatory surges observed in 10 control cyclic ewes. Subsequent luteal function in the post-partum ewes was deficient. Plasma progesterone was detected in 7/12 post-partum ewes showing plasma LH surges. The concentrations were lower (1.3 +/- 0.2 ng/ml) and detected for shorter periods (3-10 days) than in cyclic ewes (2.4 +/- 0.2 ng/ml, 12/15 days). In the post-partum ewes the increases in plasma LH concentrations before the LH surge were higher but of shorter duration than in the cyclic ewes. The inadequate luteal function in the post-partum ewes could therefore have been due to inappropriate LH stimulation of the ovary before the LH surge.     

Increase in ovulation rate after treatment of ewes with bovine follicular fluid in the luteal phase of the oestrous cycle

Administration of charcoal-treated bovine follicular fluid to Damline ewes twice daily (i.v.) from Days 1 to 11 of the luteal phase (Day 0 = oestrus) resulted in a delay in the onset of oestrous behaviour and a significant increase in ovulation rate following cloprostenol-induced luteolysis on Day 12. During follicular fluid treatment plasma levels of FSH in samples withdrawn just before injection of follicular fluid at 09:00 h (i.e. 16 h after previous injection of follicular fluid) were initially suppressed, but by Day 8 of treatment had returned to those of controls. However, the injection of follicular fluid at 09:00 h on Day 8 still caused a significant suppression of FSH as measured during a 6-h sampling period. Basal LH levels were higher throughout treatment due to a significant increase in amplitude and frequency of pulsatile secretion. After cloprostenol-induced luteal regression at the end of treatment on Day 12, plasma levels of FSH increased 4-fold over those of controls and remained higher until the preovulatory LH surge. While LH concentrations were initially higher relative to those of controls, there was no significant difference in the amount of LH released immediately before or during the preovulatory surge. These results suggest that the increase in ovulation rate observed during treatment with bovine follicular fluid is associated with the change in the pattern of gonadotrophin secretion in the luteal and follicular phases of the cycle.     

Serum concentrations of prolactin, oestrogen and LH during the perioestrous period in prepubertal gilts induced to ovulate and mature gilts

The temporal relationships of serum prolactin, oestrogen and LH concentrations during the perioestrous period were compared in prepubertal gilts induced to ovulate by PMSG and hCG and in mature gilts. In Exp. 1, 2 sustained prolactin surges, beginning 4 days and 1 day before the preovulatory LH surge, occurred in all mature gilts. A single preovulatory prolactin surge occurred in 3 prepubertal gilts, starting just before the preovulatory LH surge, but 4 prepubertal gilts had neither a prolactin nor an LH surge. A status (prepubertal or mature) versus time interaction (P less than 0.01) was detected for serum prolactin concentrations. A preovulatory oestrogen surge occurred in all gilts but was of lesser magnitude (P less than 0.01) and duration (P less than 0.05) in the prepubertal gilts without prolactin and LH surges compared to mature gilts and of lesser magnitude (P less than 0.01) compared to prepubertal gilts with prolactin and LH surges. The relative timing of the oestrogen surge in prepubertal gilts corresponded with that of mature gilts when adjusted to the LH surge (if present) but was delayed (P less than 0.01) in all prepubertal gilts if standardized to the hCG injection. In Exp. 2, mature gilts were examined to determine whether 2 perioestrous prolactin surges were characteristic of all cycling gilts. Of 9 gilts, 8 exhibited an initial prolactin surge 4-5 days before oestrus and 5/9 gilts exhibited a periovulatory prolactin surge. The presence of 2 perioestrous serum prolactin surges was not a requirement for subsequent pregnancy maintenance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dose-dependent inhibition of the preovulatory surges of gonadotropins and prolactin by the antiestrogen CI-628: possible sites of action

The present series of experiments was conducted in an attempt to correlate previously reported dose-dependent and site-selective inhibitory effects of an antiestrogen, CI-628, on 17 beta-estradiol (E2)-receptor interactions in the anterior pituitary gland (AP) and hypothalamus with its effects on the preovulatory surges of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin. The effects of CI-628 on the response of the AP to luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH) also were examined. In the first study, rats exhibiting 4-day estrous cycles were injected with various doses (0.02, 0.20, 2.0, and 20 mg/kg) of CI-628 or vehicle at 0900 h on diestrus-2 and proestrus. The preovulatory LH surge and both preovulatory and secondary FSH surges were marginally affected by 0.02 mg/kg CI-628, but were completely abolished by higher doses. In contrast, a dose of 0.20 mg/kg only delayed the prolactin surge; however, higher doses were effective in extinguishing cyclic prolactin release. In a second experiment, CI-628 in rats treated on diestrus-2 and proestrus exerted a dose-dependent suppression of the AP LH response to an initial injection of LHRH on proestrous afternoon in rats whose endogenous LH surges were blocked by phenobarbital. However, AP LH responses to a second LHRH injection to assess the self-priming capacity of LHRH were attenuated only in rats given 0.20, 2.0, and 20 mg/kg CI-628. Contrastingly, the AP prolactin response to TRH was suppressed only in rats given 0.20 mg/kg CI-628.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endocrine alterations that underlie endotoxin-induced disruption of the follicular phase in ewes

Two experiments were conducted to investigate endocrine mechanisms by which the immune/inflammatory stimulus endotoxin disrupts the follicular phase of the estrous cycle of the ewe. In both studies, endotoxin was infused i.v. (300 ng/kg per hour) for 26 h beginning 12 h after withdrawal of progesterone to initiate the follicular phase. Experiment 1 sought to pinpoint which endocrine step or steps in the preovulatory sequence are compromised by endotoxin. In sham-infused controls, estradiol rose progressively from the time of progesterone withdrawal until the LH/FSH surges and estrous behavior, which began approximately 48 h after progesterone withdrawal. Endotoxin interrupted the preovulatory estradiol rise and delayed or blocked the LH/FSH surges and estrus. Experiment 2 tested the hypothesis that endotoxin suppresses the high-frequency LH pulses necessary to stimulate the preovulatory estradiol rise. All 6 controls exhibited high-frequency LH pulses typically associated with the preovulatory estradiol rise. As in the first experiment, endotoxin interrupted the estradiol rise and delayed or blocked the LH/FSH surges and estrus. LH pulse patterns, however, differed among the six endotoxin-treated ewes. Three showed markedly disrupted LH pulses compared to those of controls. The three remaining experimental ewes expressed LH pulses similar to those of controls; yet the estradiol rise and preovulatory LH surge were still disrupted. Our results demonstrate that endotoxin invariably interrupts the preovulatory estradiol rise and delays or blocks the subsequent LH and FSH surges in the ewe. Mechanistically, endotoxin can interfere with the preovulatory sequence of endocrine events via suppression of LH pulsatility, although other processes such as ovarian responsiveness to gonadotropin stimulation appear to be disrupted as well.     

Temporal relationships among LH, estradiol, and follicle vascularization preceding the first compared with later ovulations during the year in mares

Diameter of the preovulatory follicle, plasma concentrations of LH and estradiol, and vascularization of the follicle wall, based on color-Doppler signals, were characterized in 40 pony mares for 6 days preceding ovulation (Days -6 to -1; preovulatory period). Comparisons between the preovulatory periods preceding the first compared with a later ovulation during the year were used to study the relationships between LH and estradiol and between vascularization and estradiol. Diameter of the preovulatory follicle was greater (P<0.02) and concentration of LH was less (P<0.02) during the first preovulatory period, whereas concentration of estradiol was not different between the first and second preovulatory periods. Vascularized area (cm(2)) of the follicle wall increased at a reduced rate during the first preovulatory period, as indicated by an interaction (P<0.03) between day and group. Vascularized area was similar between the preovulatory groups on Day -6, and a reduced rate of increase resulted in a lesser (P<0.001) area on Day -1 before the first ovulation (1.4+/-0.1cm(2)) than before a later ovulation (2.2+/-0.2 cm(2)). Results demonstrated that follicle vascularization and the LH surge were attenuated preceding the first ovulation of the year with no indication that estradiol was involved in the differences between the first and later ovulations.     

Elevated peripheral concentrations of LH block ovulation and increase thecal and serum progesterone in the hamster

Insertion of osmotic minipumps containing 1 mg ovine LH on Day 1 (oestrus) elevated circulating serum concentrations of LH, progesterone and androstenedione when compared with values at pro-oestrus. Ovulation was blocked for at least 2 days at which time there were twice the normal numbers of preovulatory follicles. Follicular and thecal progesterone production in vitro was elevated when compared with that in pro-oestrous controls. Follicular and thecal androstenedione production in vitro was lower than in controls even though serum concentrations of androstenedione were elevated; the higher androstenedione values may be due to the increase in number of preovulatory follicles when compared with pro-oestrous controls. Follicles from LH-treated hamsters aromatized androstenedione to oestradiol and follicular production of oestradiol was similar to that in pro-oestrous follicles despite low follicular androstenedione production in the LH-treated group. Treatment with 20 i.u. hCG on Days 4 or 6 after insertion of an LH osmotic minipump on Day 1 induced ovulation of approximately 30 ova, indicating that the blockade of ovulation was not due to atresia of the preovulatory follicles. Serum progesterone concentrations on Days 2, 4 and 6 in LH-treated hamsters were greater than 17 nmol/l, suggesting that the blockade of ovulation might have been due to prevention of the LH surge by high serum progesterone concentrations.     

Synchronization of estrus and ovulation and associated endocrine changes in Bos indicus cows

The effects of 4 estrus synchronization treatments on intervals to and synchrony of estrus and ovulation, on timing of the preovulatory LH surge and associated changes in plasma progesterone, LH, FSH, and 17beta-estradiol (E(2)) were investigated in 48 Bos indicus cows. Treatment 1 consisted of 2 injections of PGF(2alpha) 14 d apart (n = 12); Treatment 2 of a subcutaneous 3-mg norgestomet implant and an intramuscular injection of 3 mg of norgestomet and 5 mg estradiol valerate, with the implant removed 10 d later (n = 12; norgestomet-estradiol); Treatment 3 of norgestomet-estradiol, with a subcutaneous injection of PMSG given at time of implant removal (Day 10; n = 12); and Treatment 4 of norgestomet implant (as for Treatments 2 and 3) inserted for 10 d, with an intramuscular injection of PGF(2alpha) given at the time of implant removal (n = 12). The experiment was conducted in 2 replicates (24 cows/replicate, 6 cows/group). Estrus, ovulation and timing of the preovulatory surge of LH varied less in cows treated with norgestomet-estradiol and PMSG than in cows in Treatments 1 and 4 (P < 0.008). Treatment with PMSG reduced variation in ovulation times and timing of the LH surge in cows treated with norgestomet-estradiol (P < 0.02). Concentrations of E(2) were higher in cows in Treatments 2 and 3 on the final day of treatment and at about 6 h post ovulation compared with cows in Treatments 1 and 4 (P < 0.05). Different methods for synchronizing estrus did not alter sequential endocrine and behavioral changes in relation to the timing of the LH peak, and the results were consistent with current recommendations for insemination times in Bos taurus cattle.     

Ovarian capillary blood flow in seasonally anoestrous ewes induced to ovulate by treatment with GnRH

The microsphere technique was used to obtain estimates of ovarian capillary blood flow near ovulation, in 8 seasonally anoestrous ewes, which were induced to ovulate by GnRH therapy. Plasma progesterone concentrations were monitored in jugular blood sampled between Days 4 and 7 after the onset of the preovulatory LH surge. The ewes were then slaughtered. Three of the ewes were treated with a single injection of 20 mg progesterone before GnRH therapy. In these ewes and 1 other, plasma progesterone values increased after ovulation and reached 1.0 ng/ml on Day 7 following the preovulatory LH surge (normal, functional CL), whilst in the other 4 ewes progesterone concentrations increased initially then declined to 0.5 ng/ml by Day 7 (abnormal CL). In the ewes exhibiting normal luteal function, the mean ovarian capillary blood flow was significantly greater (P less than 0.01) than that for ewes having abnormal luteal function. Irrespective of the type of CL produced, capillary blood flow was significantly greater (P less than 0.05) in ovulatory ovaries than in non-ovulatory ovaries. These findings indicate that the rate of capillary blood flow in ovaries near ovulation may be a critical factor in normal development and maturation of preovulatory follicles and function of subsequently formed CL.     

Effect of prolonged administration of small doses of LH-RH and LH-RH analogue [D-Ser (But)6] Des Gly-NH210 ethylamide on the release of LH and induction of ovulation in mid-anoestrous ewes

A natural process of LH release and induction of ovulation in anoestrous ewes was simulated by prolonged administration of small doses of LH-RH and its analogue [D-Ser(But)⁶] Des Gly-NH₂¹⁰ ethylamide. In the first series of experiments on 40 Merino ewes infusions of LH-RH were made into the maxillaris interna artery for 6 consecutive days for 6 h each day. Total doses of 24.0, 26.0, 28.0 and 32.0 μg per animal of varying and progressively increasing daily quantities of the hormone were administered in groups I, II, III and IV, respectively. In group V the animals were infused with a total dose of 28.0 μg LH-RH and injected additionally i.m. with 3.0 μg 17β-oestradiol on days 4 and 5 of the infusion of LH-RH. Ovulation did not occur earlier than days 4, 5 and 6 after the beginning of infusions. The highest number of positive reactions occurred in group IV ($\text{8}\text{10}$) and in group V ($\text{7}\text{8}$ animals). The pattern of LH peaks in general was correlated with the time of ovulations. The LH concentrations of the preovulatory peaks in experimental ewes were mostly lower than those in naturally ovulating animals. The corpora lutea were functional during the first 7 days after ovulation.In the second series of experiments on 26 Merino ewes the LH-RH analogue [D-Ser -(But)⁶] Des Gly-NH₂¹⁰ ethylamide was injected i.m. or i.a. for 6 consecutive days. Total doses of 15.5, 9.5 and 7.5 μg of the analogue per animal, administered at varying and progressively increasing daily doses in respective groups, induced several surges of LH in the same individuals for 2 or even 3 consecutive days. Corpora lutea and degenerating follicles in the form of cysts were found in the ovaries of animals of these groups. Very small daily doses ranging from 0.1 μg administered during the first 3 days, to 1.5 μg on day 5 of the treatment, released one surge of LH on day 5 of the treatment in all individuals with peaks ranging from 30.0 to 58.0 ng/ml and induction of ovulation with almost normal luteal function. On the basis of these experiments it is suggested that the evaluation of the effect of active substance (LH-RH or its analogue), its suitability and application of rightly chosen doses to induce the full physiological process of ovulation should be based not only on the release of LH and luteal function but also on tests of the ability of the released ovum to undergo fertilization and its further development.     

Evidence against a role of prolactin in the timing of ovulation in the turkey

Two separate experiments in which blood was sampled at 2-h intervals from turkeys hens failed to show a significant change in plasma prolactin (Prl) concentrations in relation to the preovulatory surge of luteinizing hormone (LH) for the first (C1) ovulation of a sequence. Intravenous injection of 125 IU of ovine Prl (NIH-P-S10) or of 1 or 2 ml of antiserum to turkey Prl at varying intervals before C1 ovulation had no effect on the timing or incidence of C1 ovulation. However, injection of Prl before C1 ovulation tended to inhibit ovulation of the second (C2) egg of the sequence, while injection of antiserum to Prl before C1 ovulation tended to either advance or inhibit C2 ovulation. Possibly, the effects of Prl and Prl antiserum on C2 ovulation reflect interference with maturation of the C2 ovarian follicle rather than interference with neuroendocrine processes that regulate the timing of the preovulatory surge of LH. The data for C1 ovulation argue against a change in circulating levels of Prl as a factor in the timing of the preovulatory surge of LH.     

Participation of the lower brain stem in induction of preovulatory gonadotropin surges in female rats

The role of the lower brain stem in controlling preovulatory gonadotropin surges was investigated in female rats under acute experimental conditions. Electrolytic lesions or diethyldithiocarbamate implantations in the ventrolateral part of the medulla oblongata (VLMO), which were carried out at 1100-1330 h on the day of proestrus, resulted in a blockade of the preovulatory surges of LH, FSH and PRL as well as subsequent ovulation. Such treatments in the dorsomedial part of the medulla oblongata did not affect gonadotropin surges or ovulation. By means of electrolytic lesions in the VLMO, norepinephrine concentrations were significantly reduced in the preoptic-anterior hypothalamic area at 1700-1800 h on proestrus, though they did not change in the mid-posterior hypothalamus. Electrochemical stimulations of the suprachiasmatic part of the preoptic area or norepinephrine injections into the third ventricle at 1400-1500 h on proestrus in animals with VLMO lesions succeeded in induce gonadotropin surges and ovulation. These results suggest that the lower brain stem is involved in the induction of preovulatory gonadotropin surges and that the process may be mediated by the ascending noradrenergic system which originates in the VLMO.     

Induction of ovulation in the acyclic postpartum ewe following continuous, low-dose subcutaneous infusion of GnRH

Pituitary and ovarian responses to subcutaneous infusion of GnRH were investigated in acyclic, lactating Mule ewes during the breeding season. Thirty postpartum ewes were split into 3 equal groups; Group G received GnRH (250 ng/h) for 96 h; Group P + G was primed with progestagen for 10 d then received GnRH (250 ng/h) for 96 h; and Group P received progestagen priming and saline vehicle only. The infusions were delivered via osmotic minipumps inserted 26.6 +/- 0.45 d post partum (Day 0 of the study). Blood samples were collected for LH analysis every 15 min from 12 h before until 8 h after minipump insertion, then every 2 h for a further 112 h. Daily blood samples were collected for progesterone analysis on Days 1 to 10 following minipump insertion, then every third day for a further 25 d. In addition, the reproductive tract was examined by laparoscopy on Day -5 and Day +7 and estrous behavior was monitored between Day -4 and Day +7. Progestagen priming suppressed (P < 0.05) plasma LH levels (0.27 +/- 0.03 vs 0.46 +/- 0.06 ng/ml) during the preinfusion period, but the GnRH-induced LH release was similar for Group G and Group P + G. The LH surge began significantly (P < 0.05) earlier (32.0 +/- 3.0 vs 56.3 +/- 4.1 h) and was of greater magnitude (32.15 +/- 3.56 vs 18.84 +/- 4.13 ng/ml) in the unprimed than the primed ewes. None of the ewes infused with saline produced a preovulatory LH surge. The GnRH infusion induced ovulation in 10/10 unprimed and 7/9 progestagen-primed ewes, with no significant difference in ovulation rate (1.78 +/- 0.15 and 1.33 +/- 0.21, respectively). Ovulation was followed by normal luteal function in 4/10 Group-G ewes, while the remaining 6 ewes had short luteal phases. In contrast, each of the 7 Group-P + G ewes that ovulated secreted progesterone for at least 10 d, although elevated plasma progesterone levels were maintained in 3/7 unmated ewes for >35 d. Throughout the study only 2 ewes (both from Group P + G) displayed estrus. These data demonstrate that although a low dose, continuous infusion of GnRH can increase tonic LH concentrations sufficient to promote a preovulatory LH surge and induce ovulation, behavioral estrus and normal luteal function do not consistently follow ovulation in the progestagen-primed, postpartum ewe.     

Effects of reducing the remating interval after parturition on the fertility and plasma concentrations of luteinizing hormone, prolactin, oestradiol-17 beta and progesterone in lactating domestic rabbits

Primiparous crossbred does were remated on Day 1 (n = 15) or 14 (n = 25) post partum and killed on Day 10 post coitum to assess their fertility. Blood samples were taken during the pre- (0-12 h post coitum) and post- (1-10 days post coitum) ovulatory periods and plasma was assayed for luteinizing hormone (LH), prolactin, oestradiol-17 beta and progesterone. Ovulation response was significantly greater (P less than 0.01) and ovulation rate significantly lower (P less than 0.001) in does mated on Day 1 than in those mated on Day 14 post partum. Does failing to ovulate on Day 14 post partum exhibited no preovulatory LH surge and had significantly lower (P less than 0.05) premating concentrations of oestradiol-17 beta and prolactin than those ovulating at this time. No significant differences in hormone concentrations were observed during the preovulatory period between does ovulating on Days 1 and 14 post partum, with the exception of oestradiol-17 beta. Concentrations of this hormone were significantly lower (P less than 0.01) in does mated on Day 1, at 1 h post coitum. We conclude that (i) fertility was affected by the remating interval after parturition, (ii) ovulation failure was associated with an absence of the preovulatory LH surge and a reduction in premating concentrations of oestradiol-17 beta and prolactin and (iii) the lower ovulation rate in early lactation was apparently caused by a reduction in ovarian competence to respond to the gonadotrophic stimulus.