Previous studies suggest two roles for estradiol in inducing the LH surge in ewes: a neural action to evoke a sudden release of GnRH and a pituitary action to maximize response to GnRH. We tested two hypotheses: a follicular phase estradiol rise induces a GnRH surge; and the surge-inducing action of estradiol does not vary with season. In the breeding season, ewes in the midluteal phase of the estrous cycle were ovariectomized and treated with implants producing luteal phase levels of estradiol and progesterone, and an apparatus was surgically installed for later sampling of pituitary portal blood. At the normal time of luteolysis (1 week later), progesterone implants were removed, simulating luteal regression. Ewes were divided into two groups: estradiol implants also removed (n = 6) and estradiol implants added 16 h after progesterone removal to produce a rise in estradiol to levels that mimic those that circulate in the late follicular phase (n = 6). In anestrus, the estradiol rise treatment was replicated in ewes (n = 5) after an artificial luteal phase produced by sequential insertion and subsequent removal of progesterone implants. Regardless of season, the LH surge induced by estradiol was invariably accompanied by a massive GnRH surge, ranging from 73- to 394-fold over presurge values. The GnRH and LH surges began together, but the GnRH surge continued well beyond the surge of LH. There was no seasonal difference in time course or amplitude of the GnRH surge. Control ewes not treated with estradiol exhibited regular pulses of LH and GnRH every 1-2 h, but no surge of either hormone. We conclude that, regardless of season, a rise in estradiol to late follicular phase levels initiates a large and abrupt GnRH surge coincident with the onset of the LH surge. The LH surge ends despite continued elevation of GnRH.

The estradiol-induced surge of gonadotropin-releasing hormone in the ewe.

Postpartum acyclicity in suckled beef cows: a review.

An injection of 500 micrograms prostaglandin (PG) analogue was given on Day 12 (mid-luteal phase) of the oestrous cycle to 8 cows. An LH surge occurred 59 +/- 2 h later. LH, FSH, prolactin, oestradiol-17 beta, progesterone and oxytocin concentrations were determined in blood samples collected from the caudal vena cava and/or the jugular vein at 20-min (5 cows) or 5-min (3 cows; only LH and FSH concentrations were determined) intervals for 24 h, beginning 48 h after the PG injection. Oxytocin concentrations were low and similar in the vena cava and the jugular vein. In blood samples collected every 5 min the interpulse interval for LH and FSH during the period before the LH surge was 38-40 min. In the 20-min samples the interpulse interval for oestradiol was similar to that for LH and FSH, but pulse amplitude and basal concentrations steadily increased to reach maximum concentrations 6-8 h before and again during the LH surge. A decrease in oestradiol concentrations, lasting at least 60 min, occurred just before the start of the LH surge. Progesterone concentrations also increased at the same time as the LH surge. The magnitude of the LH surges varied from 7 to 32 ng/ml, but all cows ovulated and had oestrous cycles of normal length. Distinct pulses of LH and FSH were observed throughout the LH and FSH surges. Pulsatile secretion of LH was not detected for a period of up to 6-12 h following the LH surge, but then low-amplitude pulses occurred. In contrast, the pulsatile secretion of FSH remained at a frequency similar to that observed during the descending phase of the FSH surge. Furthermore, a second increase in FSH concentrations occurred, beginning 4-12 h after the LH-surge. It is concluded that: (1) the frequent, high-amplitude pulses of oestradiol that occur before and during the LH surge are probably due to stimulation by pulses of LH; (2) the LH surge is the result of an increase in frequency and amplitude of the LH pulses; (3) the second increase in FSH that follows the LH and FSH surges appears to be the result of an increase in the amplitude (not frequency) of the FSH pulses; and (4) very little, if any, oxytocin is secreted from the ovary during the periovulatory phase of the oestrous cycle.

/pdf/pulsatile-secretion-of-gonadotrophins-ovarian-steroids-and-27qir0zd5z.pdf

Pulsatile secretion of gonadotrophins, ovarian steroids and ovarian oxytocin during the periovulatory phase of the oestrous cycle in the cow.

Summary

(1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre-pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH.



(2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion.



(3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone.



(4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day-length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day-length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses.



(5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol.



(6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses.



(7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not known.

Factors affecting the secretion of luteinizing hormone in the ewe.

When doses of progestin used commercially are administered to synchronize the stage of the estrous cycle among bovine females, fertility is reduced compared with that of untreated controls. The reduced fertility that results from the use of progestin based estrus synchrony regimens is associated with the development of persistent ovarian follicles. Persistent ovarian follicles develop as a result of the greater frequency of LH pulses that occurs during the period of treatment with progestins. The greater release of LH pulses results in enhanced secretion of 17 beta-estradiol from persistent ovarian follicles. The greater frequency of LH release or the greater secretion of 17 beta-estradiol associated with development of persistent ovarian follicles probably contributes to the reduced fertility that occurs when progestins are used to synchronize stage of the estrous cycle.

/pdf/progestin-and-estrogen-regulation-of-pulsatile-lh-release-3cx3maej22.pdf

Progestin and estrogen regulation of pulsatile LH release and development of persistent ovarian follicles in cattle

The objectives of these studies were to determine 1) the influence of estradiol on the ability of the pituitary gland to release LH in response to LHRH in vivo, and 2) whether LHRH is required to cause the preovulatory LH surge in cattle. To minimize the confounding effect of endogenous LHRH or LHRH-induced LH release, we used the estradiol-treated steer as a model for the cow. The rationale for this choice is that estradiol does not normally cause an LH surge in steers, but it does exert a negative feedback effect inhibiting LHRH secretion. In the first experiment, an LH surge was induced in steers by injecting 1 μg LHRH at 20-min intervals between 12 and 20 h after estradiol. This surge resembled the preovulatory LH surge in cows with respect to both duration (10 h) and amplitude (49 ng/ml). After the peak, LH concentrations returned to baseline despite continued LHRH injections. In control steers given oil, repetitive injections of LHRH increased serum LH to only 13 ng/ml. In a second experiment, steer...

Evidence that Estradiol Induces the Preovulatory LH Surge in Cattle by Increasing Pituitary Sensitivity to LHRH and then Increasing LHRH Release

The objective was to determine whether the suckling-induced delay in return to estrus postpartum could be explained by changes in hypothalamic LHRH content or ability of the pituitary to release LH and FSH in response to LHRH or 59 mM K+ in vitro. In addition, serum concentrations of several other hormones were measured. Nine Holstein cows were suckled ad libitum by two calves and milked by machine twice daily and eight were milked by machine only from calving until slaughter on day 14 postpartum. On day 13 postpartum, blood was collected at 15-min intervals from 0815 to 1200 hr and from 2015 to 2400 hours. Suckled cows had lower (P less than .05) mean serum LH concentrations on day 13 postpartum than did nonsuckled controls. This decrease resulted from a 60% reduction in frequency and a 40% reduction in amplitude of episodic LH peaks. Suckling did not affect body weight change postpartum or serum concentrations of progesterone, estradiol-17 beta, total glucocorticoids, prolactin or FSH during the first 14 days postpartum. The suckling-induced decrease in serum LH was not reflected by a reduction in hypothalamic LHRH or pituitary LH on day 14 postpartum. However, pituitary explants from suckled cows on day 14 postpartum secreted 50% less (P less than .01) LH in response to LHRH (25 ng/ml for 30 min) or K+ (59 mM for 30 min) in vitro than did those from nonsuckled cows. Secretion of FSH was increased 20-fold by LHRH and K+ in vitro, but differences due to suckling treatment were not significant. Decreased frequency and amplitude of episodic LH secretion in vivo and reduced capacity of pituitaries to respond to LHRH may be the cause of suckling-induced inhibition of postpartum ovulation in cattle.

The hypothalamo-pituitary gonadotrophic axis of suckled and nonsuckled dairy cows postpartum.

The effect of estradiol-17a(E2) in regulation of basal and luteinizing hormone releasing hormone (LHRH) mediated release of LH was studied. Bovine anterior pituitaries were dispersed and "o5 X iocells in 1 ml medium were dispensed to each well of multiwell culture plates. LHRH at concentrations of 0.01, 0.1, 1 and 10 ng/ml increased LH release 3, 23, 94 and 183%, relative to controls without LHRH. In addition, experiments were conducted as 4 X 5 factorials where concentrations of E2 (0, 5, 50 or 500 ng/ml) and concentration of LHRH (0, 0.1, 1, 10 and 100 ng/ml) were main effects. Estradiol was present in media beginning 2 h (acute) or 26 h (chronic) before LHRH was added. Estradiol exposure for 2 h did not affect basal LH release. In contrast, basal LH release by pituitary cells incubated for 26 h with 5, 50 and 500 ng E2 /ml media averaged 26, 37 and 33 ng/ml, greater (P<0.005) than the 6 ng/ml average for controls. LH release increased linearly (P<0.1) with log dose of LHRH when 0, 5, or 50.ng/ml E2 was added to the media beginning 2 or 26 h before LHRH. Dose response slopes were 0.75, 1.40 and 2.12 (acute) and 1.5, 13.4 and 10.5 (chronic), respectively. Acute, but not chronic exposure of pituitary cells to 500 ng/ml E2 inhibited (P<0.01) LHRH-induced LH release. LU content in cells not exposed to E2 was 854 ng. LH content was increased 201, 360 and 542% relative to untreated controls by chronic exposure of cells to 5, 50 or 500 ng E2 /ml media. We conclude that chronic exposure of bovine pituitary cells to E2 increased basal LH release and synthesis. In addition, acute exposure of bovine pituitary cells to 5 or 50 ng/ml E2 or chronic exposure to 5, 50 or 500 ng/ml E2 increased sensi- tivity of bovine pituitary cells to LU RH.

/pdf/effects-of-estradiol-on-basal-and-luteinizing-hormone-5fb9oojrqd.pdf

Effects of Estradiol on Basal and Luteinizing Hormone Releasing Hormone (LHRH)-Induced Release of Luteinizing Hormone (LH) from Bovine Pituitary Cells in Culture

Objectives were to determine: 1) whether estradiol, given via implants in amounts to stimulate a proestrus increase, induces preovulatory-like luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges; and 2) whether progesterone, given via infusion in amounts to simulate concentrations found in blood during the luteal phase of the estrous cycle, inhibits gonadotropin surges. All heifers were in the luteal phase of an estrous cycle when ovariectomized. Replacement therapy with estradiol and progesterone was started immediately after ovariectomy to mimic luteal phase concentrations of these steroids. Average estradiol (pg/ml) and progesterone (ng/ml) resulting from this replacement were 2.5 and 6.2 respectively; these values were similar (P greater than 0.05) to those on the day before ovariectomy (2.3 and 7.2, respectively). Nevertheless, basal concentrations of LH and FSH increased from 0.7 and 43 ng/ml before ovariectomy to 2.6 and 96 ng/ml, respectively, 24 h after ovariectomy. This may indicate that other ovarian factors are required to maintain low baselines of LH and FSH. Beginning 24 h after ovariectomy, replacement of steroids were adjusted as follows: 1) progesterone infusion was terminated and 2 additional estradiol implants were given every 12 h for 36 h (n = 5); 2) progesterone infusion was maintained and 2 additional estradiol implants were given every 12 h for 36 h (n = 3); or 3) progesterone infusion was terminated and 2 additional empty implants were given every 12 h for 36 h (n = 6). When estradiol implants were given every 12 h for 36 h, estradiol levels increased in plasma to 5 to 7 pg/ml, which resembles the increase in estradiol that occurs at proestrus. After ending progesterone infusion, levels of progesterone in plasma decreased to less than 1 ng/ml by 8 h. Preovulatory-like LH and FSH surges were induced only when progesterone infusion was stopped and additional estradiol implants were given. These surges were synchronous, occurring 61.8 +/- 0.4 h (mean +/- SE) after ending infusion of progesterone. We conclude that estradiol, at concentrations which simulate those found during proestrus, induces preovulatory-like LH and FSH surges in heifers and that progesterone, at concentrations found during the luteal phase of the estrous cycle, inhibits estradiol-induced gonadotropin surges. Furthermore, ovarian factors other than estradiol and progesterone may be required to maintain basal concentrations of LH and FSH in heifers.

Estradiol induces and progesterone inhibits the preovulatory surges of luteinizing hormone and follicle-stimulating hormone in heifers.

Luteinizing hormone releasing hormone-induced release of luteinizing hormone from pituitary explants of cows killed before or after oestradiol treatment.

J. S. Kesner

Papers

Evidence that Estradiol Induces the Preovulatory LH Surge in Cattle by Increasing Pituitary Sensitivity to LHRH and then Increasing LHRH Release

The hypothalamo-pituitary gonadotrophic axis of suckled and nonsuckled dairy cows postpartum.

Effects of Estradiol on Basal and Luteinizing Hormone Releasing Hormone (LHRH)-Induced Release of Luteinizing Hormone (LH) from Bovine Pituitary Cells in Culture

Estradiol induces and progesterone inhibits the preovulatory surges of luteinizing hormone and follicle-stimulating hormone in heifers.

Luteinizing hormone releasing hormone-induced release of luteinizing hormone from pituitary explants of cows killed before or after oestradiol treatment.