Paraventricular alpha1- and alpha2-adrenergic receptors mediate hindbrain lipoprivation-induced suppression of luteinizing hormone pulses in female rats

Acute central lipoprivation suppresses pulsatile luteinizing hormone (LH) release and increases blood glucose levels through noradrenergic input to the hypothalamic paraventricular nucleus (PVN) in female rats. The present study was conducted to identify adrenergic receptor subtypes involved in central lipoprivation-induced suppression of pulsatile LH secretion and increases in plasma glucose levels in female rats. Acute hindbrain lipoprivation was produced by injection into the fourth cerebroventricle (4V) of 2-mercaptoacetate (MA), an inhibitor of fatty acid oxidation, in estradiol-implanted ovariectomized rats. Two min before MA injection, alpha1-, alpha2- or beta-adrenergic receptor antagonist was injected into the PVN. Injection of MA into the 4V suppresses pulsatile LH release in PVN vehicle-treated rats, whereas pretreatment of animals with injection of alpha1- or alpha2-adrenergic antagonist into the PVN blocked the effect of the 4V MA injection on LH pulses. beta-Adrenergic antagonist did not affect MA-induced suppression of LH pulses. The counter-regulatory increase in plasma glucose levels after 4V MA injection was also partially blocked by pretreatment with alpha1- and alpha2-adrenergic receptor antagonists. These results suggest that alpha1- and alpha2-adrenergic receptors in the PVN mediate hindbrain lipoprivation-induced suppression of LH release and counter-regulatory increases in plasma glucose levels in female rats.     

Role of noradrenergic receptors in the bed nucleus of the stria terminalis in regulating pulsatile luteinizing hormone secretion in female rats

The bed nucleus of the stria terminalis (BNST) is one of the brain areas densely innervated by noradrenergic neurons originating in the brain stem. The present study aims to determine the role of noradrenergic receptors in the BNST in regulating pulsatile luteinizing hormone (LH) secretion in female rats. Ovariectomized (OVX) or estrogen-primed OVX (OVX+E2) rats received three 1-h-interval injections of 0.05 micromol of noradrenaline (NA), phenylephrine (alpha1-adrenergic receptor agonist), clonidine (alpha2-agonist), or isoproterenol (beta-agonist) into the BNST. Injection of NA or alpha1-adrenergic agonist into the BNST strongly suppressed pulsatile LH secretion in OVX+E2 rats with a significant (P < 0.05) decrease in the mean LH level for 3 h and LH pulse frequency, but alpha2-and beta-agonists did not affect any of the LH pulse parameters. In OVX animals, alpha1- and alpha2-adrenergic agonists caused a significant change in LH pulse frequency and amplitude, respectively, though the effect was not as apparent as the NA- or alpha1-agonist-induced changes in OVX+E2 animals. These results indicate that NA inputs to the BNST suppress pulsatile LH secretion via alpha1-adrenergic receptors and that estrogen enhances this suppression.     

Central estrogen action sites involved in prepubertal restraint of pulsatile luteinizing hormone release in female rats

The present study aimed to determine estrogen feedback action sites to mediate prepubertal restraint of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release in female rats. Wistar-Imamichi strain rats were ovariectomized (OVX) and received a local estradiol-17β (estradiol) or cholesterol microimplant in several brain areas, such as the medial preoptic area (mPOA), paraventricular nucleus, ventromedial nucleus and arcuate nucleus (ARC), at 20 or 35 days of age. Six days after receiving the estradiol microimplant, animals were bled to detect LH pulses at 26 or 41 days of age, representing the pre- or postpubertal period, respectively. Estradiol microimplants in the mPOA or ARC, but not in other brain regions, suppressed LH pulses in prepubertal OVX rats. Apparent LH pulses were found in the postpubertal period in all animals bearing estradiol or cholesterol implants. It is unlikely that pubertal changes in responsiveness to estrogen are due to a change in estrogen receptor (ER) expression, because the number of ERα-immunoreactive cells and mRNA levels of Esr1, Esr2 and Gpr30 in the mPOA and ARC were comparable between the pre- and postpubertal periods. In addition, kisspeptin or GnRH injection overrode estradiol-dependent prepubertal LH suppression, suggesting that estrogen inhibits the kisspeptin-GnRH cascade during the prepubertal period. Thus, estrogen-responsive neurons located in the mPOA and ARC may play key roles in estrogen-dependent prepubertal restraint of GnRH/LH secretion in female rats.     

Central injection of ketone body suppresses luteinizing hormone release via the catecholaminergic pathway in female rats

Ketosis is found in various pathophysiological conditions, including diabetes and starvation, that are accompanied by suppression of gonadal activity. The aim of the present study was to determine the role of ketone body in the brain in regulating pulsatile luteinizing hormone (LH) secretion in female rats. Injection of 3-hydroxybutyrate (3HB), a ketone body, into the fourth cerebroventricle (4V) induced suppression of pulsatile LH secretion in a dose-dependent manner in ovariectomized (OVX) rats with an estradiol (E2) implant producing diestrus plasma E2 levels. Plasma glucose and corticosterone levels increased immediately after the 4V 3HB injection, suggesting that the treatment caused a hunger response. The 3HB-induced suppression of LH pulses might be mediated by noradrenergic inputs to the hypothalamic paraventricular nucleus (PVN) because a local injection of α-methyl- p-tyrosine, a catecholamine synthesis inhibitor, into the PVN blocked 3HB-induced suppression of LH pulses and PVN noradrenaline release was increased by 4V 3HB injection in E2-primed OVX rats. These results suggest that ketone body sensed by a central energy sensor in the hindbrain may suppress gonadotropin release via noradrenergic inputs to the PVN under ketosis.     

Glucoprivation-induced Fos expression in the hypothalamus and medulla oblongata in female rats

Glucoprivation induced by 2-deoxy-D-glucose (2DG) suppresses pulsatile luteinizing hormone (LH) secretion in female rats. The suppression is enhanced in the presence of estrogen. In the present study, 2DG-induced Fos expression was examined in the solitary tract nucleus (NTS), hypothalamic paraventricular nucleus (PVN), raphe obscurus nucleus (ROb) and raphe pallidus nucleus (RPa), which have been previously suggested to be involved in glucoprivation-induced suppression of LH secretion in female rats. Ovariectomized (OVX) or estrogen-primed ovariectomized (OVX+E(2)) rats were injected intravenously with 2DG (400 mg/kg BW). The brain was removed 1 h after the injection. The number of Fos-like-immunoreactive (Fos-li) cells in the PVN and NTS was significantly increased in OVX+E(2) rats compared with control groups, but did not show a significant increase in the OVX group. Few Fos-li cells were observed in the ROb and RPa in all groups. All of the Fos-li cells in the PVN and NTS were neurons because they had immunoreactivities to microtubule-associated protein 2. Some Fos-li cells (8.3%) had tyrosine hydroxylase-like immunoreactivities in the NTS in 2DG-treated OVX+E(2) rats. These results suggest that neurons in the PVN and NTS are involved in the estrogen-dependent neural cascade mediating glucoprivic suppression of LH secretion in female rats.     

Dynorphin-Kappa Opioid Receptor Signaling Partly Mediates Estrogen Negative Feedback Effect on LH Pulses in Female Rats

Accumulating evidence suggests that the arcuate nucleus (ARC) kisspeptin/neurokinin B (NKB)/dynorphin (KNDy) neurons play a role in estrogen negative feedback action on pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release. The present study aimed to determine if dynorphin (Dyn) is involved in estrogen negative feedback on pulsatile GnRH/LH release. The effect of the injection of nor-binaltorphimine (nor-BNI), a kappa-opioid receptor (KOR) antagonist, into the third cerebroventricle (3V) on LH pulses was determined in ovariectomized (OVX) adult female rats with/without replacement of negative feedback levels of estradiol (low E₂). The mean LH concentrations and baseline levels of LH secretion in nor-BNI-injected, low E₂-treated rats were significantly higher compared with vehicle-treated controls. On the other hand, the nor-BNI treatment failed to affect any LH pulse parameters in OVX rats without low E₂ treatment. These results suggest that Dyn is involved in the estrogen negative feedback regulation of pulsatile GnRH/LH release. The low E₂ treatment had no significant effect on the numbers of ARC Pdyn (Dyn gene)-,Kiss1- and Tac2 (NKB gene)-expressing cells. The treatment also did not affect mRNA levels of Pdyn and Oprk1 (KOR gene) in the ARC-median eminence region, but significantly increased the ARC kisspeptin immunoreactivity. These findings suggest that the negative feedback level of estrogen suppresses kisspeptin release from the ARC KNDy neurons through an unknown mechanism without affecting the Dyn and KOR expressions in the ARC. Taken together, the present result suggests that Dyn-KOR signaling is a part of estrogen negative feedback action on GnRH/LH pulses by reducing the kisspeptin release in female rats.     

Circulating estradiol suppresses luteinizing hormone pulse frequency during dietary restriction

The influence of dietary restriction on the negative feedback potency of 17-beta-estradiol (E2) was evaluated in both castrated male (wethers) and female sheep (OVX ewes) during the breeding season. In study 1, OVX ewes received maintenance or restricted dietary energy for 7 weeks or maintenance energy for 6 weeks prior to a 5 day fast (n=12ewes/feeding group). Estradiol (0.31microg E2/50kg/h) or vehicle (10% EtOH-saline) was continuously infused into half the animals in each dietary treatment for the final 54h of the study. The dynamic pattern of LH secretion was assessed during the final 6h of infusion. Estradiol inhibited luteinizing hormone (LH) pulse amplitude independent of nutrition (P=0.02); fasting increased mean LH, LH peak height, and LH nadir in the absence of E2 (P=0.004, P=0.02, and P=0.02, respectively); while E2 inhibited pulse frequency (P=0.02) and increased peak width (P=0.04) in restricted ewes. Interestingly, despite uniform E2 delivery, serum concentrations of E2 differed with feeding status. Therefore, 12 wethers were infused with 0.31microg E2/50kg/h (6 fed, 6 fasted) and six wethers received 0.19microg E2/50kg/h (fasted) to establish similar serum concentrations of E2 in fed (0.31microg/50kg/h) and fasted (0.19microg/50kg/h) wethers. When fed and fasted wethers had uniform serum concentrations of E2 LH pulse frequency was suppressed (P<0.05) in fasted relative to fed animals, supporting the postulate that energy restriction enhances the E2 negative feedback potency. Collectively, these studies demonstrate that nutrition affects E2 feedback potency and clearance.     

Negative feedback control of luteinizing hormone secretion in prepubertal beef heifers at 60 and 200 days of age

Prepubertal beef heifers at 60 and 200 d of age, born in the fall or spring, were assigned randomly to one of three treatment groups: (1) intact = 1; (2) bilateral ovariectomy (OVX); or (3) OVX plus estradiol-17 beta(E2) administered in silastic implants (OVX + E2). Luteinizing hormone (LH) was measured in serum samples collected at 20-min intervals for 4 h from heifers on -1, +7, +21, +35 and +49 d after OVX. Luteinizing hormone concentrations increased in the serum by 7 d after OVX in heifers at both 60 and 200 d of age (P less than .001; time X treatment). Prior to OVX, the LH patterns were characterized by low levels and infrequent episodic pulses. By 49 d after OVX, the mean LH concentrations increased and the pattern changed to one of rhythmic LH pulses with a periodicity of 1 h (P less than .001; time X treatment). Estradiol-treated OVX heifers did not exhibit a postovariectomy rise in serum LH concentrations. Serum E2 concentration 49 d after OVX in OVX heifers was threefold greater than in 1 or OVX heifers, thus demonstrating that E2 exerted negative feedback on pituitary LH secretion in prepubertal heifers. There was no measurable difference in serum E2 concentrations between I and OVX heifers; however, the contrast in the concentration and pattern of serum LH between the two groups was dramatic and suggested gonadal factors in addition to E2 are involved in controlling LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of zinc deficiency on glucose tolerance]

18 male rats (Sprague Dawley) were divided into 3 groups each receiving the following amounts of dietary zinc: 2 mg/kg in the depletion group and 100 mg/kg in both the ad libitum-fed control group and in the pair-fed control group. After 34 days of experiment the rats were fasted for 12 hrs and then received 50 mg of glucose per 60 g of body weight injected into the femoral muscle. With equal intial glucose concentrations Zn-depleted rats exhibited a significantly reduce glucose tolerance compared with that of the ad libitum-fed control animals. The reduced glucose tolerance in the Zn-deficient animals does, in this case, not arise as a result of the reduced food intake and the resulting weakened condition of the animals. This was shown by results obtained with the pair-fed animals exhibiting a significantly more stable glucose tolerance than the Zn-depleted rats, which was even more rigid than that of the ad libitum-fed controls.     

Simultaneous observation of the GnRH pulse generator activity and plasma concentrations of metabolites and insulin during fasting and subsequent refeeding periods in Shiba goats

The time course of GnRH pulse generator activity and plasma concentrations of energy substrates and insulin were simultaneously observed in female goats during 4-day fasting and subsequent refeeding in the presence or absence of estrogen for a better understanding of the mechanism of energetic control of gonadotropin secretion in ruminants. The GnRH pulse generator activity was electrophysiologically assessed with the intervals of characteristic increases in multiple-unit activity (MUA volleys) in the mediobasal hypothalamus. In estradiol-treated ovariectomized (OVX+E2) goats, the MUA volley intervals increased as fasting progressed. Plasma concentrations of non-esterified fatty acid and ketone body increased, while those of acetic acid and insulin decreased during fasting. The MUA volley intervals and plasma concentrations of those metabolites and insulin were restored to pre-fasting levels after subsequent refeeding. In ovariectomized (OVX) goats, changes in plasma metabolites and insulin concentrations were similar to those in OVX+E2 goats, but the MUA volley intervals were not altered. The present results demonstrated that fasting suppressed GnRH pulse generator activity in an estrogen-dependent manner. Changes in plasma concentrations of energy substrates and insulin during fasting were associated with the GnRH pulse generator activity in the presence of estrogen, but not in the absence of the steroid in female goats.     

Nursing enhances the negative effect of estrogen on LH release in the cow

Twenty-three crossbred beef cows between 4 and 5 yr of age were assigned at random to one of six treatments: (1) ovariectomized 4 d postpartum (OVX) with early weaning of calves 21 d postpartum (OVX-EW; n = 4), (2) OVX-EW and 17 beta-estradiol implants (OVX-E2-EW; n = 4), (3) OVX and normal nursing by calves throughout the experiment (OVX-NN; n = 3), (4) OVX-NN and 17 beta-estradiol implants (OVX-E2-NN; n = 4), (5) intact cows and early weaning of calves 21 d postpartum (EW), (6) intact cows and normal nursed (NN). Blood was collected at 15-min intervals over a 4-h period once weekly during the 12-wk postpartum period in the OVX cows. Early weaned intact cows exhibited estrus 23 d sooner (P less than .05) than normally nursed cows. A hormone level for each cow at each week was determined from the mean of the 17 samples collected over the 4 h period each week. There were no significant changes due to E2 treatment, for concentrations of LH, FSH or number of pulses during wk 1 through 3. However, during wk 4 through 12 the linear and quadratic contrasts of wk X estrogen X nursing were significant for serum LH, indicating there was no difference between the treatments for EW and NN without E2 but there was a large difference in the presence of E2. During nursing E2 suppressed serum LH below that of nonestrogen-treated cows while after weaning E2 stimulated LH release above that of nonestrogen-treated cows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Feedback of 17 beta-estradiol on secretion of luteinizing hormone during different seasons of the year

The working hypothesis was that the amount of increase in secretion of luteinizing hormone (LH) that results from positive feedback of 17 beta-estradiol (E2) is dependent on season of the year in mature bovine females. Seven beef cows, ovariectomized approximately 2 mo before the initiation of the experiment, were used in the initial year (1983) of the study. Three of the ovariectomized cows (OVX-E2) received an sc E2 implant, which provided low circulating levels of E2. The remaining four cows (OVX) were not implanted. Blood samples were collected serially (at 10-min intervals for 6 h) at each spring and fall equinox and at each summer and winter solstice. This protocol was replicated with a different group of cows in 1985 (OVX-E2, n = 4; OVX, n = 6). Concentration of LH in blood serum was quantified in all samples. Concentration of E2 in blood serum was measured in pools of samples from each serial blood collection. Concentrations of E2 were higher (P less than .05) in the implanted cows. Mean concentration of LH and amplitude of pulses of LH were higher (P less than .05) at each season of the year in cows that were ovariectomized and implanted with E2 than in cows that were ovariectomized and did not receive E2. An effect of season of the year on mean concentration of LH was detected (P less than .01). No influence of season or E2 was detected for frequency of pulses of LH. There was no significant treatment X season interaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulatory roles of leptin in reproduction and metabolism: a comparative review

Leptin plays an important role in signaling nutritional status to the central reproductive axis of mammals and appears to be at least a permissive factor in the initiation of puberty. The expression and secretion of leptin are correlated with body fat mass and are acutely affected by changes in feed intake. Moreover, circulating leptin increases during pubertal development in rodents, human females and heifers. Effects of leptin are mediated mainly via receptor activation of the JAK-STAT pathway; however, activation of alternative pathways, such as MAP kinase, has also been reported. Although the leptin receptor (LR) has not been found on GnRH neurons, leptin stimulates the release of GnRH from rat and porcine hypothalamic explants. Moreover, leptin increases the release of LH in rats and from adenohypophyseal explants and/or cells from full-fed rats and pigs. In contrast, stimulation of the hypothalamic-gonadotropic axis by leptin in cattle and sheep is observed predominantly in animals and tissues pre-exposed to profound negative energy balance. For example, leptin prevents fasting-mediated reductions in the frequency of LH pulses in peripubertal heifers, augments the magnitude of LH and GnRH pulses in fasted cows, and enhances basal secretion of LH in vivo and from adenohypophyseal explants of fasted cows. However, leptin is incapable of accelerating the frequency of LH pulses in prepubertal heifers, regardless of nutrient status, and has no effect on the secretion of GnRH and LH in full-fed cattle or hypothalamic/hypophyseal explants derived thereof. Similar to results obtained with LH, basal secretion of GH from anterior pituitary explants of fasted, but not normal-fed cows, was potentiated acutely by low, but not high, doses of leptin. Mechanisms through which undernutrition hypersensitize the hypothalamic-gonadotropic axis to leptin may involve up-regulation of the LR. However, an increase in LR mRNA expression is not a requisite feature of heightened adenohypophyseal responses in fasted cattle. To date, leptin has not been successful for inducing puberty in ruminants. Future therapeutic uses for recombinant leptin that exploit states of nutritional hypersensitization, and identification of genetic markers for genotypic variation in leptin resistance, are currently under investigation.     

Effects of suckling and low doses of estradiol on luteinizing hormone secretion during the postpartum period in beef cows

An experiment was conducted to test if suckling acutely suppressed circulating levels of LH during the postpartum period in beef cows. In addition, the influence of exogenous administration of low concentrations of estradiol on LH secretion during the postpartum period was evaluated. Twelve mature cows were randomly assigned before parturition to one of three treatments. Four intact cows were used as controls (INT). Eight cows were ovariectomized within the first 7 days following parturition. Four of these cows received a silastic 17β-estradiol implant subcutaneously at the time of ovariectomy (OVX-E); the remaining four cows received no further treatment (OVX). All cows were allowed to nurse one calf for 30 min daily between 1200 and 1230 hours for the duration of the experiment. Blood samples were collected at 12 min intervals for 6 hr before and 6 hr after suckling on days 9, 30, 44 and 58 postpartum. Mean interval (mean ± SE) to the first increase in peripheral progesterone concentrations indicative of the onset of ovarian luteal activity was detected in INT cows 37 ± 4.9 days postpartum. An acute effect of suckling on LH secretion did not occur in INT and OVX cows but mean LH concentrations were reduced in OVX-E cows following suckling on days 44 and 58. Mean LH concentrations remained low in INT cows; whereas, in OVX and OVX-E cows LH concentrations increased linearly (P<0.05) as the interval from time of ovariectomy increased. Cows in the OVX-E group had a higher mean concentration of LH than cows in the OVX group at 30, 44 and 58 days postpartum (P<0.05). Frequency of LH pulses did not differ between cows in the OVX and OVX-E groups at any period. Data from this experiment support the concept that suckling is acting in a chronic fashion to inhibit LH secretion during the postpartum period. In the absence of ovaries, chronic administration of exogenous estradiol in low concentrations has a positive effect on secretion of LH in the postpartum cow.     

Effects of Estradiol and Progesterone on the Numbers and Distribution of Mastocytes in the Uterus of Ovariectomized Rats

The purpose of this study was to investigate the effects of estradiol(E)and progesterone(P)on mastocyte distribution in the uterus of ovariectomized rats.Thirty-five adult female rats were divided randomly into seven groups:one sham operated control group(SHAM);one ovariectomized group(OVX);three ovariectomized plus E treatment groups(OVX+E 20,100,or 500 μg/kg body weight·d);and two ovariectomized plus P groups(OVX+P 2 or 10 mg/kg body weight·d).Seven days after treatment,the contents of estradiol and progesterone in serum were detected by radioimmunoassay,and mastocytes in the uterus were stained by toluidine blue staining.Results were as following:① Compared to ovariectomized rat,the concent ration of estradiol in serum increased by 97.13 % in OVX+E 20(P0.05),204.84 % in OVX+E 100(P0.05),and 936.45 % in OVX + E 500 group(P0.05);the progesterone concent ration increased by 77.25 % in OVX+P 2(P0.05)and 235.25 %in OVX+P 10 group(P0.05).② Compared to ovariectomized rat,the number of mast cells in uteri decreased by 32.65% in OVX+E 20,64.50 % in OVX+E 100(P0.05),74.49 % in OVX+E 500(P0.05)and 70.67 % in OVX+P 10 groups(P0.05).However,the number of mast cells increased by 66.73% in OVX+P 2 group(P0.05)compared with OVX.The trend of mast cells number in the rat uterus was decreased gradually with the increase of estrogen or progesterone concent ration.The number of mast cells in ovariectomized rat uterus was affected by estrogen or progesterone.These results demonstrated that estrogen or progesterone directly affected the number of mast cells in the uterus of rat.     

Circulating concentrations and pattern of luteinizing hormone and follicle-stimulating hormone in circulation are changed by the circulating concentration of 17 beta-estradiol in the bovine male and female.

The objectives of the present study were 1) to determine whether 17 beta-estradiol (E2) regulation of tonic secretion of LH and FSH is sexually differentiated in the bovine and 2) to evaluate the effects of various physiological concentrations of E2 on the profiles and concentrations of gonadotropins in circulation. This was accomplished by administering different numbers of implants containing E2 to gonadectomized bovine males and females. Mean age at initiation of the study was 18.5 mo. Animals received 1, 2, 4, 8, or 16 implants of E2 or a sham implantation. Mean concentrations of LH in circulation and amplitude of LH pulses were similar between males and females after administration of E2. There was a cubic response for mean concentrations of LH and amplitudes of LH pulses across the dosages of E2 administered; lower concentrations of E2 had little effect, whereas higher concentrations of E2 suppressed both mean LH and amplitude of LH pulses. A linear decline in frequency of LH pulses occurred as concentrations of E2 in circulation increased. A treatment x sex interaction resulted for mean concentrations of FSH in circulation. Low doses of E2 resulted in a greater enhancement of circulating concentrations of FSH in males than in females. Tonic secretion of LH in bovine males and females responded in a similar manner to administration of various physiological concentrations of E2; however, a differential response between sexes was observed for FSH.     

Endocrine changes during restoration of estrous cycles following induction of anestrus by restricted nutrient intake in beef heifers

The working hypotheses in this experiment were: that ovarian estradiol would inhibit luteinizing hormone (LH) secretion in heifers that were anestrus as a result of restricted dietary energy intake and the responsiveness of LH secretion to estradiol negative feedback would decrease during the period when restoration of estrous cycles occurred following feeding of diets adequate in energy. Fifteen heifers weighing 341 +/- 12 (mean +/- SE) kg were fed a diet containing 50% of the energy required for maintenance until 40 to 50 d following cessation of estrous cycles. Heifers were assigned to intact control (C, n = 5), ovariectomized (OVX, n = 5) or ovariectomized-estradiol-17 beta-implanted (OVX + E2, n = 5) treatments. Heifers were subsequently provided a high-energy (HE) diet until termination of the study. Progesterone concentrations indicating cessation of corpus luteum function were detected after heifers had lost 71 +/- 8 kg body weight over 186 +/- 28 d. Control heifers re-initiated estrous cycles as indicated by increased progesterone concentrations in serum at 49 +/- 9 d after initiation of feeding the HE diet (360 +/- 18 kg body weight). Initiation of pulsatile LH secretion was observed in heifers by d 12 following OVX. Estradiol suppressed LH secretion in OVX + E2 heifers during the period of nutritional anestrus in C heifers. Suppressive effects of E2 on LH secretion continued in OVX heifers after C heifers had initiated corpus luteum function. Therefore, the working hypothesis that LH secretion is inhibited by E2 in the nutritionally anestrous heifer is accepted but responsiveness to estradiol does not subside with re-initiation of estrous cycles, thus this working hypothesis is rejected.     

Measurement of the amount of mRNA for gonadotropins during an estradiol-induced preovulatory-like surge of LH and FSH in ovariectomized ewes

The amount of messenger RNA (mRNA) for luteinizing hormone beta-subunit (LH beta), follicle-stimulating hormone beta-subunit (FSH beta) and alpha-subunit was measured during estradiol-17 beta (E) positive feedback in ovariectomized (OVX) ewes. During the anestrous season, OVX ewes were given an i.m. injection of E (25 micrograms: n = 5) or oil (control; n = 4) and hourly blood samples were collected for 16 hr. After blood collection, ewes were killed and anterior pituitary glands were removed for analysis of hormone and mRNA content. Preovulatory-like increases in serum concentrations of LH and FSH were measured in E-treated OVX ewes. In two E-treated OVX ewes the serum concentrations of LH and FSH were still increasing, whereas in the remaining three E-treated OVX ewes, serum concentrations of LH were on the decreasing portion of the E-induced preovulatory-like surge. Pituitary content of LH was lower (P less than .10) in E-treated OVX ewes when serum concentrations of LH were decreasing than that measured in control ewes or E-treated OVX ewes in which serum concentrations were still increasing. Pituitary content of FSH and prolactin were similar (P greater than .05) among all groups. The amount of mRNA for LH beta-subunit was similar (P greater than .05) in ewes in which serum concentrations of LH were increasing and in control ewes, but was lower (P less than .05) in ewes with decreasing levels of LH. The amount of mRNA for FSH beta-subunit was lower (P less than .05) in all E-treated OVX ewes (independent of whether serum concentrations of FSH were increasing or decreasing) than that measured in control ewes. There was no difference (P greater than .05) in the amount of mRNA for alpha-subunit among any groups. Thus, amounts of mRNA for the beta-subunits of gonadotropins are reduced, while amounts of mRNA for alpha-subunit are unchanged during estradiol positive feedback in OVX ewes.     

Effects of ovariectomy on bone metabolism and bone mineral density in spontaneously diabetic Torii-Lepr(fa) rats

The Spontaneously Diabetic Torii-Lepr(fa) (SDT- fa/fa) rat is a new model of obese type 2 diabetes. The female SDT-fa/fa rat shows obesity, hyperglycemia and hyperlipidemia from a young age. However, it is not known whether diabetes and estrogen deficiency can lead to bone abnormalities in the female SDT-fa/fa rat. The objective of the present study was to investigate the effects of ovariectomy (OVX) on bone metabolism and bone mineral density (BMD) in the female SDT-fa/fa rat. Female Sprague-Dawley rats were used as control animals. The BMDs of the whole tibia and fifth lumbar (L5) vertebral body were analyzed at 30 weeks after OVX. Serum osteocalcin, a bone formation marker, and urine deoxypyridinoline (DPD), a bone resorption marker, were sequentially analyzed before and at 5, 15 and 30 weeks after OVX. Serum osteocalcin and urine DPD levels were lower in SDT-fa/fa rats than in control rats before OVX. Both serum osteocalcin and urine DPD levels were elevated in control rats 5-30 weeks after OVX, but only the urine DPD levels were elevated in SDT-fa/fa rats 5-30 weeks after OVX. SDT-fa/fa rats showed a decrease in the BMDs of the whole tibia and L5 vertebral body compared with control rats. OVX decreased the BMDs of the whole tibia and L5 vertebral body in control rats, but not in SDT-fa/fa rats. These data suggest that estrogen deficiency is not a risk factor for bone loss in type 2 diabetes mellitus.     

Cortisol disrupts the ability of estradiol-17β to induce the LH surge in ovariectomized ewes

Stress disrupts the preovulatory luteinizing hormone (LH) surge in females, but the mechanisms are unknown. We tested the hypothesis that cortisol compromises the ability of estrogen to induce a preovulatory-like LH surge in ovariectomized ewes in both the breeding and nonbreeding season. Luteinizing hormone surges were induced in ovariectomized ewes by treatment with progesterone followed by a surge-inducing estradiol-17β (E2) stimulus using a crossover design. The experiment was replicated in the breeding and nonbreeding seasons. Cortisol reduced the incidence of LH surges irrespective of season. Cortisol increased the latency from E2 stimulus to the onset of the surge in the breeding season only and suppressed the LH surge amplitude during both seasons (P < 0.01). We conclude that cortisol can interfere with the LH surge in several ways: delay, blunt, and in extreme cases prevent the E2-induced LH surge. Furthermore, the effect of cortisol to delay the E2-induced LH surge is more pronounced in the breeding season. These results show that cortisol disrupts the positive feedback effect of E2 to trigger an LH surge and suggest the involvement of multiple mechanisms.