Simultaneous measurement of luteinizing hormone-releasing hormone and luteinizing hormone during estradiol-induced luteinizing hormone surges in the ovariectomized ewe

Sequential bleeding and push-pull perfusion of the hypothalamus were used to characterize luteinizing hormone (LH) and LH-releasing hormone (LHRH) release in ovariectomized (OVX) ewes after injection of corn oil or estradiol benzoate (EB). Push-pull cannulae were surgically implanted into the stalk median eminences of 24 OVX ewes. Seven to 14 days later each of 20 animals was given an i.m. injection of 50 micrograms EB. Blood samples and push-pull perfusate were collected at 10-min intervals for 6-12 h beginning 12-15 h after EB injection. Four OVX ewes were given i.m. injections of corn oil 7 days after implantation of push-pull cannulae. Blood samples and push-pull perfusate were collected at 10-min intervals for 4 h between 18 and 22 h after injection of corn oil. Luteinizing hormone remained below 2 ng/ml throughout most of the sampling periods in 9 of 20 EB-treated ewes. In 5 of these 9 LHRH also was undetectable, whereas in 4 LHRH was detectable (1.84 +/- 0.29 pg/10 min), but did not increase with time. Preovulatory-like surges of LH occurred in 11 EB-treated ewes, but LHRH was undetectable in 5. In 4 of 6 ewes showing LH surges and detectable LHRH, sampling occurred during the onset of the LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Postnatal ovary is necessary for the maturation of estradiol-induced luteinizing hormone and prolactin surges in female rats

The role of postnatal ovary in the maturation of estradiol (E2)-induced luteinizing hormone (LH) and prolactin (PRL) surges was examined in female rats of Wistar-Imamichi strain. Animals were bilaterally ovariectomized at 24 h after birth, 1 week (w), 2 w, 3 w, 4 w or 6 w of age. At about 10 w of age, every group was primed with estradiol benzoate (E2B) for two days, and on the third day was decapitated at either 0900 h or 1900 h. Anterior pituitary (AP) LH and PRL content was determined in every group of no E2B treatment. Surge-like secretions of LH and PRL were observed at 1900 h, only in rats ovariectomized on or after 4 w of age. AP LH and PRL content was the higher, as ovariectomy was delayed. These results indicate that postnatal ovary is necessary for the maturation of E2-induced LH and PRL surges. Such an effect of ovary is mediated at least by its stimulation of AP LH and PRL content.     

Ovulation inhibition in the pregnant mare's serum gonadotropin-treated immature rat: a bioassay for luteinizing hormone-releasing hormone antagonists

A convenient method for evaluating the biological activity of luteinizing hormone-releasing hormone (LHRH) antagonists was devised. Pregnant mare's serum gonadotropin (PMSG) treatment of immature rats is known to stimulate follicular growth and estrogen production, that in turn stimulates the release of LHRH which triggers an ovulatory discharge of luteinizing hormone (LH) from the pituitary. The present bioassay of the antagonists is based on the inhibition of ovulation in the PMSG-treated rats. Twenty-eight-day-old Sprague Dawley rats maintained under a light period of 12 h/day (lights on at 0630 h) were given 10 IU of PMSG s.c. at 0930 h. On Day 30 of age the antagonist was given s.c. at 1430 h. The rats were killed on the following morning and the oviducts examined for the presence of ova. In addition, the antagonists were compared in their ability to inhibit serum testosterone levels in adult male rats. In the PMSG-treated rats the order of ovulation-inhibiting potency of the following antagonists was: [Ac-D-NAL(2)1,4FD-Phe2,D-Trp3,D-Arg6]-LHRH (LHRH-1) greater than [Ac-delta 3 Pro1,4FD-Phe2,D-NAL(2)3.6]-LHRH (LHRH-2) greater than [Ac-delta 3 Pro1,4FD-Phe2,D-Trp3,6]-LHRH (LHRH-3). The order of potency was confirmed by their antitesticular effects in adult male rats.     

Stimulation of ornithine decarboxylase activity by luteinizing hormone releasing hormone in the testis of immature rat

The activity of ornithine decarboxylase (ODC) was found to increase in the testis of immature rats following intratesticular injection with luteinizing hormone releasing hormone (LHRH). Maximal stimulation of ODC activity occurred with 1 μg of the hormone at 2 h. The enzyme activity returned to control levels at 4 h. The minimal effective dose was found to be 0.1 μg per testis. The stimulating effect of LHRH was confined to Leydig cells alone. The seminiferous tubules did not show any change in ODC activity following LHRH treatment. These results show that LHRH acts directly on the testis and influences the levels of ODC in the Leydig cells of rat.     

2-hydroxyestrone induced rise in serum luteinizing hormone in the immature male rat.

Subcutaneous injection of 50 or 100 μg of 2-hydroxyestrone in 35 day old male rats resulted in a dramatic (4–7 fold) rise in serum luteinizing hormone at 6 hours post treatment. Estrone in similar doses (1–200 μg) caused a small decrease in serum luteinizing hormone which was not statistically significant. The discrepancy between the actions of naturally occurring estrogens may be useful in explaining the time and dose related biphasic effects of estrogens on gonadotropin levels.     

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

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.     

Frequency of luteinizing hormone surges and egg production rate in turkey hens

Whether the interval between preovulatory surges of LH was different between lines of turkey hens with either poor (RBC3 line, peak at 55%) or excellent rate of egg production (Egg line, peak at 85%) was examined. Laying hens were cannulated and bled hourly for 10 days at peak of production. A constant light photoschedule was used to avoid diurnal masking of innate circadian rhythms. The mean interval between LH surges in the RBC3 line was longer than in the Egg line and had a higher coefficient of variation. A few longer LH surge intervals (72 h) were found in some RBC3 line hens (2 of 7 hens), but none were found in Egg line hens (0 of 11 hens). All progesterone (P4) surges were coupled with LH surges, but not all LH-P4 surges were coupled with ovipositions (blind LH-P(4) surges). The percentage of blind LH-P4 surges was not different between lines. The baseline concentration of LH was higher in Egg line than RBC3 line hens, but LH surge amplitude, and surge duration were not different. The baseline and surge amplitude concentrations of P4 were not different between lines, nor was the concentration of estradiol-17beta. The longer interval between LH surges was the major factor tested that was associated with the poorer egg production rate in RBC3 line hens in comparison to Egg line hens. A higher incidence of blind LH surges further contributed to lower egg production in RBC3 line turkey hens.     

Attenuation of the estradiol-induced surge release of luteinizing hormone by antihistamine in ovariectomized ewes

Ovariectomized ewes received intramuscular (i.m.) injections of an H₁-histamine receptor antagonist, diphenhydramine, or saline during the anestrous and breeding seasons to determine if histamine may regulate the estradiol-induced surge release of LH in ewes. In addition, concentrations of histamine and GnRH in hypothalamic regions and histamine and LH in the pituitary gland were determined during the estradiol-induced surge of LH. Pretreatment mean, basal, and estradiol-induced secretion of LH did not differ (P > 0.05) among seasons. However, the quantity of LH (ng) measured during the estradiol-induced surge of LH was less (P < 0.05) in ewes treated with diphenhydramine (411 ± 104) than saline (747 ± 133). Treatment with diphenhydramine did not (P > 0.05) influence steady-state concentrations of histamine in hypothalamic or pituitary gland tissues, hypothalamic concentrations of GnRH, or anterior pituitary concentrations of LH during the estradiol-induced surge of LH. It is concluded that histamine may modulate the estradiol-induced surge release of LH in ewes by affecting the secretion of GnRH.     

Restoration of the periovulatory follicle-stimulating hormone surges in sera by luteinizing hormone releasing hormone in phenobarbital-blocked rats.

The periovulatory increases of follicle-stimulating hormone (FSH) in rat sera can be divided into two phases. The first phase consists of a rise and fall during proestrus and the second phase consists of a rise and fall during estrus. The second phase was not blocked by phenobarbital (100 mg/kg BW) injected i.p. between the first and second phases. In contrast, phenobarbital administered prior to the onset of the first phase blocked both phases of increased serum FSH. In phenobarbital-blocked rats, administration of luteinizing hormone releasing hormone (LHRH) during proestrus, either by s.c. injection (10 μg) or by a 3 hr constant-rate i.v. infusion (50 ng/hr), simulated both the proestrous and estrous phases of increased serum FSH. These results indicate that 1) the second phase of the serum FSH rise is itself not susceptible to phenobarbital blockade, 2) a proestrous mechanism susceptible to phenobarbital alteration is necessary for both phases of increased serum FSH to occur, and 3) administration of LHRH to phenobarbital-blocked rats during proestrus restores both phases of FSH release.     

Changes in luteinizing hormone-releasing hormone content of discrete hypothalamic areas associated with spontaneous and induced preovulatory luteinizing hormone surges in the domestic hen

It is widely assumed that luteinizing hormone-releasing hormone (LHRH) neuronal activation is involved in the preovulatory surge of LH in the hen. In addition, this LH surge may be initiated by ovarian progesterone (P4) release. Thus, spontaneous and P4-induced LH surges should be associated with acute changes in LHRH content of discrete hypothalamic areas associated with LHRH cell bodies and/or LHRH axon terminals. Medial preoptic area (mPOA) and infundibulum (INF) LHRH content was measured by radioimmunoassay at intervals before, at, and following peak LH levels of a spontaneous preovulatory surge of LH, as well as when this surge was advanced by P4 administration in laying hens. Nonlaying birds served as additional controls. Levels of serum LH, P4, 17 beta-estradiol and pituitary LH were also measured. Increased (P less than 0.05) LHRH content in mPOA without changes in the INF are associated with peak serum LH levels of the spontaneous LH surge. By contrast, decreased (P less than 0.05) LHRH content in both mPOA and INF is associated with peak serum LH levels when the spontaneous surge was advanced 8 h by P4 administration to laying hens. Medial preoptic area and INF LHRH contents were significantly lower (P less than 0.05) in nonlaying than in laying hens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Duration and amplitude of the luteal phase progesterone increment times the estradiol-induced luteinizing hormone surge in ewes

Progesterone (P) powerfully inhibits the neuroendocrine reproductive axis, but the mechanisms and site or sites of action of this steroid remain poorly understood. Progesterone exposure during the luteal phase also alters the responsiveness of the hypothalamus to increased concentrations of estrogen (E) during the follicular phase. Using an ovariectomized ovine follicular phase model, we investigated whether the amplitude and duration of the luteal phase increase in circulating P affects the E-induced surge in LH. Treatment of ewes for 10 days with two, one, or half an intravaginal P-releasing implant or with an empty implant demonstrated that P concentrations significantly (P: < 0.0001) delayed the time to surge onset upon exposure to an equal concentration of E. This delay was not due to a time-related difference in responsiveness to E after P clearance because the time of surge onset was not different when E treatment began 6, 12, or 24 h after the withdrawal of two P implants that had been present for 10 days. The final study demonstrated that the duration of P before treatment (5, 10, or 30 days) significantly (P: < 0.0001) delayed the responsiveness of the estradiol-dependent surge-generating system. There was no effect on surge amplitude or duration in any experiment. Thus, the amplitude and duration of exposure to luteal phase P significantly affect the neural elements targeted by E to induce the preovulatory LH surge.     

Negative feedback as an obligatory antecedent to the estradiol-induced luteinizing hormone surge in ovariectomized pigs

In ovariectomized pigs, estradiol treatment induces a preovulatory-like luteinizing hormone (LH) surge, but only after serum LH concentrations are suppressed for 48 h. This inhibition of LH release is attributable in large part to inhibition of gonadotropin-releasing hormone (GnRH) release. The present report examines the dependency of the estradiol-induced LH surge on this preceding phase of negative feedback. Ten ovariectomized gilts were given an i.m. injection of estradiol benzoate (10 micrograms/kg BW). Beginning at the time of estradiol treatment, 5 of these gilts received 1-microgram GnRH pulses i.v. every 45 min for 48 h, i.e. during the period of negative feedback. The remaining 5 control gilts received comparable infusions of vehicle. Estradiol induced the characteristic biphasic LH response in control gilts. On the other hand, the inhibitory LH response to estradiol was prevented and the ensuing LH surge was blocked in 4 of the 5 gilts given GnRH pulses during the negative feedback phase. These results indicate that suppressing release of GnRH and/or LH is an important antecedent to full expression of the LH surge in ovariectomized pigs. Assimilation of this observation with the existing literature provides novel insights into the neuroendocrine control of LH secretion in castrated and ovary-intact gilts.     

Site of action for endotoxin-induced cortisol release in the suppression of preovulatory luteinizing hormone surges

The study was conducted to identify the mechanisms of endotoxin/cortisol action in the suppression of preovulatory LH surges in heifers infused with Escherichia coli (E. coli ) endotoxin. The hypotheses tested were that 1) endotoxin stimulates the release of progesterone, possibly from the adrenal leading to the LH blockade; 2) cortisol released in response to endotoxin infusion blocks the synthesis of estradiol at the ovarian level, culminating in a failure of the LH surge. Eight Holstein heifers were given two injections of prostaglandin F(2alpha) (PG), 11 d apart, to synchronize estrus. Starting from 25 h after the second injection of PG (PG-2), the uterus of each heifer was infused either with 5 ml of pyrogen-free water (control, n = 3) or with E. coli endotoxin (5 mug/kg of body weight) in 5 ml of pyrogen-free water (treated, n = 5), once every 6 h for 10 treatments. Blood samples were obtained every 15 min for 1 h before infusion and again 2 h after each infusion, then hourly until 1 h before the next infusion. After the tenth infusion, blood was collected daily until estrus. Serum progesterone concentrations remained at baseline values (< 1 ng/ml) in control and treated heifers. The total amount of progesterone measured starting 24 to 84 h after PG-2 injection was not different between control and treated heifers (P 0.05). In the control heifers, serum estradiol concentrations remained basal (< 10 pg/ml) until 4 h before the LH surge. Serum estradiol concentrations increased to 20 +/- 5.6 pg/ml, 4 h before the LH surge in control heifers (LH surge occurred 60 to 66 h after the PG-2 injection). There were no changes in serum estradiol concentrations in treated heifers during the sampling period, and the concentrations remained < 10 pg/ml. The total amount of estradiol measured in control heifers was higher (P < 0.05) than in treated heifers. The results if this study suggest that increases in cortisol concentrations after the infusion of endotoxin might block the synthesis of estradiol at the ovarian level, resulting in the failure of a preovulatory LH surge to occur.     

Daily rhythms of serum luteinizing hormone in the immature hamster are estradiol-dependent

In the female Syrian hamster (Mesocricetus auratus), daily rhythms of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) begin several weeks before regular vaginal estrous cycles are initiated. These rhythms, which appear rather abruptly at about 16 days of age, are dependent on the presence of the ovaries. The experiments described here were intended to determine the nature of the ovarian information required for the initiation and maintenance of the daily LH surge. This surge characterizes the daily cycle of LH and occurs each afternoon at about 1700 h in the intact animal between 2 and 5 weeks of age. Females were ovariectomized at 14 or 15 days of age and implanted with constant-release Silastic capsules of estradiol (E) or progesterone (P). Blood samples were collected at 21 days of age at 1400 or 1700 b, and the serum was assayed for LH, P, and E. While ovariectomy abolished the afternoon surge of serum LH that was observed in sham-operated controls, implantation of E effectively replaced the ovaries. Implantation of P was without effect on LH levels; when P plus E was implanted, the effect was similar to that of E alone. These results suggest that ovaries of the 2-week-old hamster secrete estrogen necessary for the initiation of cyclical LH release.     

Interval between preovulatory surges of luteinizing hormone increases late in the reproductive period in turkey hens

In turkey hens, the egg production rate is relatively high early during a reproductive period, but declines as the period progresses. Among lines with different egg production potential, the interval between preovulatory surges of LH is the primary determinant of the egg production rate. The main objective of this study was to determine whether the decline in egg production rate late during an egg production period is also associated with a difference in the interval between LH preovulatory surges. A group of photosensitive turkey hens (Early) were photostimulated with continuous light (24L:0D) at 40 wk of age to induce egg laying, and serial blood samples were collected after about 3 wk of egg production. A second group of hens (Late) were housed in floor pens and photostimulated with 14L:10D at 40 wk of age for a normal 36-wk reproduction period and were then switched to 24L:0D lighting for 2 wk before collection of serial blood samples. Continuous light photostimulation was used for at least 2 wk before and during serial blood sampling to avoid potential masking effects of diurnal lighting on the interval between LH surges. The Early (n = 12) and Late (n = 16) hens were cannulated 3 days before being serially bled hourly for 10 days. The mean interval between preovulatory surges of LH was shorter in the Early hens than in the Late hens (26.1 +/- 2.5 h and 34.7 +/- 3.9 h, respectively). The intra-hen LH surge interval coefficient of variation was lower in the Early hens than in the Late hens (7.2% and 18.6%, respectively). The inter-hen LH surge interval coefficient of variation was similar in the Early and Late hens (9.5% and 11.2%, respectively). The incidence of blind surges of LH (those not retrospectively associated with ovipositions) was not different between Early and Late laying hens (8.4% +/- 15.2% and 7.3% +/- 14.6%, respectively). In conclusion, in turkey hens, longer intervals and greater intra-hen variation between LH surges were associated with a poorer rate of egg production late in the reproductive period relative to early in the reproductive period.