Endocrine Mechanisms of Puberty in Heifers: Estradiol Negative Feedback Regulation of Luteinizing Hormone Secretion

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the first experiment, seven heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day 0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and subcutaneously implanted with estradiol-17β (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A second experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the second experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. These results indicate that: a) LH secretion in prepubertal heifers is responsive to estradiol negative feedback; and b) estradiol negative feedback decreases during the prepubertal period in beef heifers

Increasing Concentrations of 17β-Estradiol Has Differential Effects on Secretion of Luteinizing Hormone and Follicle-Stimulating Hormone and Amounts of mRNA for Gonadotropin Subunits during the Follicular Phase of the Bovine Estrous Cycle

The hypothesis tested was that 17β-estradiol (E2) would increase amounts of mRNA for α, LHβ, and FSHβ subunits during the follicular phase of the estrous cycle prior to the preovulatory surge of gonadotropins in cows. On Day 16 (Day 0 = estrus) of the estrous cycle, all cows were treated with prostaglandin F2α (PGF2α). Cows served as intact controls (CONT, n = 4) were ovariectomized (OVX, n = 5), or were ovariectomized and administered E2 (OVXE, n = 6) in increasing doses starting at the time of treatment with PGF2α. Cows were bled for 6 h before and for 40 h after PGF2α treatment to characterize pulsatile secretion of LH and FSH. Forty hours after PGF2α treatment, pituitaries were collected for evaluation of amounts of mRNA for α, LHβ, and FSHβ subunits. Amplitude of LH pulses was greater (p \u3c 0.05) in cows from the OVXE than from the CONT group. Concentrations of FSH were greater in cows from both the OVXE and OVX (p \u3c 0.01) groups than from the CONT group. Amounts of mRNA for α and FSHβ subunits were greater (p \u3c 0.01) in pituitaries of cows from the OVX than from the CONT or OVXE groups. Amounts of mRNA for LHP subunit in pituitaries of cows from the OVX group tended to be greater (p \u3c 0.08) than from the CONT group. Cows in the OVXE group tended (p \u3c 0.08) to have greater amounts of mRNA for FSHβ subunit than did CONT cows. Amounts of mRNA for α and LHβ subunits in cows from the OVXE and CONT groups did not differ (p \u3e 0.10). Pituitary weight and content of LH and FSH were not different (p \u3e 0.10) among cows of the different groups. Ovariectomy resulted in enhanced secretion of gonadotropins and increased amounts of mRNA for gonadotropin subunits above values detected in CONT cows. Ovariectomized cows administered E2, in follicular-phase patterns had amounts of mRNA for a and LHβ similar to those in CONT cows even though secretion of LH was enhanced in the OVXE group. We reject our hypothesis and conclude that E2 has a divergent role in regulation of gonadotropins. Release of LH is enhanced by E2, but E2 reduced mRNA for gonadotropin subunits (tended to reduce mRNA for FSHβ subunit) in ovariectomized cows to amounts detected in intact cows during the follicular phase of the bovine estrous cycle

Melengestrol Acetate at Greater Doses Than Typically Used for Estrous Synchrony in Bovine Females Does Not Mimic Endogenous Progesterone in Regulation of Secretion of Luteinizing Hormone and 17β-Estradiol

Our working hypothesis was that doses of melengestrol acetate (MGA) greater than those typically administered in estrous synchrony regimens would regulate secretion of LH and 17β-estradiol (E2) as endogenous progesterone (P4) does during the midluteal phase of the estrous cycle. We also hypothesized that endogenous P4 from the CL would interact with MGA to further decrease the frequency of LH pulses and E2. Cows on Day 5 of their estrous cycle (Day 0 = estrus) were randomly assigned to an untreated control group (CONT, n = 5) or to one of six MGA treatment groups (n = 5 per group): 1) MGA administered orally each day via a gelatin capsule at a dose of 0.5 mg MGA/cow with the CL present (0.5CL); 2) 0.5 mg MGA/cow daily in the absence of CL (0.5NO); 3) 1.0 mg MGA with CL present (1.OCL); 4) 1.0 mg MGA without CL (1.ONO); 5) 1.5 mg MGA with CL present (1.5CL); 6) 1.5 mg without CL (1.5NO). MGA was administered for 10 days (Day 5 = initiation of treatment). To regress CL, cows assigned to groups without CL received injections of prostaglandin F2α (PGF, 0; 25 mg) on Days 6 and 7 of their estrous cycle. All cows were administered PGF2α. at the end of the 10-day treatment period. During the treatment period, daily blood samples were collected to determine concentrations of E2. Serial blood samples were collected at 15-min intervals for 24 h on Days 8, 11, and 14 to determine pattern of LH secretion. Frequency of LH pulses on Days 8, 11, and 14 was greater (p \u3c 0.05) in cows without CL (0.5NO, 1.ONO, and 1.5NO) than in cows with CL (0.5CL, 1.OCL, 1.5CL, and CONT). Mean concentrations of LH were greater (p \u3c 0.05) in cows from the 0.5NO group on Days 8 and 11 and were greater (p \u3c 0.05) in cows from the 0.5NO, 1.ONO, and 1.5NO groups on Day 14 as compared to cows with CL. Overall mean concentrations of LH across Days 8, 11, and 14 were greatest (p \u3c 0.05) in cows from the 0.5NO group and were also greater (p \u3c 0.05) in cows from the 0.5NO, 1.ONO, and 1.5NO groups as compared to cows in the 0.5CL, 1.OCL, 1.5CL, and CONT groups. Mean concentrations of E2 during the treatment period were greater (p \u3c 0.05) in cows from the 0.5NO group than in cows from either the 1.ONO or the 1.5NO group; these values were also greater (p \u3c 0.05) in cows of the 0.5NO, 1.ONO, and 1.5NO groups as compared to cows of the 1.OCL and CONT groups. Therefore, we reject our working hypothesis because doses of MGA greater than those typically used in estrous synchrony protocols did not suppress LH and E2 to the same extent that endogenous P4 does. In addition, MGA treatment when CL were present did not result in a further suppression of LH pulse frequency or of E2 as compared to the values in control cows with functional CL

Effects of 17β-Estradiol on Distribution of Pituitary Isoforms of Luteinizing Hormone and Follicle-Stimulating Hormone during the Follicular Phase of the Bovine Estrous Cycle

The objective of this study was to examine the influence of 17β-estradiol (E2) on distribution of LH and FSH isoforms during the follicular phase of the bovine estrous cycle prior to the preovulatory surges of LH and FSH. On Day 16 of the estrous cycle (Day 0 = estrus), intact controls (CONT; n = 4) were treated with prostaglandin F2α (PGF2α) to induce luteal regression and initiation of the follicular phase. Other cows were also treated with PGF2α and either ovariectomized (OVX; n = 5) or ovariectomized and given E2 implants (OVXE; n = 6) to mimic the pattern of increasing E2 concentrations during the follicular phase of the estrous cycle. Pituitaries were collected 40 h after treatment with PGF2α, or ovariectomy (0 h). Aliquots of pituitary extracts were chromatofocused on pH 10.5-4.0 gradients. The LH resolved into thirteen isoforms (designated A-L and S, beginning with the most basic form) while FSH resolved into nine isoforms (designated I-IX, beginning with the most basic form). The percentage of LH as isoform F (elution pH = 9.32 + 0.01) was greater (p \u3c 0.05) in the OVX group (48.5%) than in the OVXE group (45.0% ). LH isoforms I (elution pH = 6.98 ± 0.01) and J (elution pH = 6.48 ± 0.01) were more abundant (p \u3c 0.05) in cows from the OVXE (2.3 and 5.8%, respectively) than the OVX group (1.4 and 3.7%, respectively). Distribution of LH isoforms in cows from the three groups did not differ (p \u3e 0.10). Distribution of FSH isoforms were similar (p \u3e 0.05) among all groups. In summary, removal of the ovary (OVX) resulted in a slight increase in percentage of the basic LH isoform F, while removal of the ovary and administration of E2 (OVXE) in a pattern that mimicked increasing concentrations of E2 during the follicular phase of the estrous cycle resulted in a slight increase in the percentage of acidic LH isoforms (I and J). There was no influence of ovariectomy or treatment with E2 on distribution of FSH isoforms in the pituitary. Thus, gonadotropin heterogeneity does not appear to change significantly during the follicular phase of the bovine estrous cycle

Comparison of Circulating Concentrations of Reproductive Hormones in Boars of Lines Selected for Size of Testes or Number of Ovulations and Embryonal Survival to Concentrations in Respective Control Lines

The objectives of this study were to determine whether circulating concentrations of gonadotropins and gonadal hormones of boars were altered as a result of selection of pigs for size of testes or for embryonal survival and(or) number of ovulations. Included in Exp. 1 and 2 were boars with the greatest estimated paired weight of testes (TS) and boars from a control (C) line. Concentrations of FSH were similar ( P \u3e .10) in boars from the TS and C lines. In Exp. 3, 4, and 5, circulating concentrations of FSH and 17β-estradiol (E2) were evaluated in neonates, during pubertal development, and in mature boars of lines selected for an index of number of ovulations and embryonal survival ( I ) , and data were compared to those for boars from a respective C line. Concentrations of E2 were not different in boars from the I line and those from the C line during the early neonatal period but were greater ( P \u3c .05) in boars of the C line than in those from the I line during pubertal development. Concentrations of FSH were greater ( P \u3c .05) in mature boars from the I line than in those from the C line. In summary, selection for size of testes did not influence circulating concentrations of FSH in mature boars. The secretory pattern of E2 in boars before puberty changed as a result of selection for embryonal survival and number of ovulations in females of the I line, and the different patterns of circulating E2 early in life may result in enhanced circulating concentrations of FSH in adult boars of the I line compared with boars of the C line

EC85-219 1985 Nebraska Swine Report

This 1985 Nebraska Swine Report was prepared by the staff in Animal Science and cooperating departments for use in the Extension and Teaching programs at the University of Nebraska-Lincoln. Authors from the following areas contributed to this publication: Swine Nutrition, swine diseases, pathology, economics, engineering, swine breeding, meats, agronomy, and diagnostic laboratory. It covers the following areas: breeding, disease control, feeding, nutrition, economics, housing and meats

Modulation of luteinizing hormone and follicle-stimulating hormone in circulation by interactions between endogenous opioids and oestradiol during the peripubertal period of heifers

The aim of this study was to determine whether the decline in oestradiol inhibition of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the peripubertal period of heifers is associated with a change in opioid modulation of LH and FSH secretion. Opioid inhibition of LH secretion was determined by response to administration of the opioid antagonist naloxone. Prepubertal heifers (403 days old) were left as intact controls, ovariectomized or ovariectomized and chronically administered oestradiol. Control heifers were used to determine time of puberty. Three weeks after ovariectomy, four doses of naloxone (0·13-0·75 mg kg-1 body weight) or saline were administered to heifers in the treatment groups in a latin square design (one dose per day). Blood samples were collected at intervals of 10 min for 2 h before and 2 h after administration of naloxone. This procedure was repeated four times at intervals of 3 weeks during the time intact control heifers were attaining puberty. All doses of naloxone induced a similar increase in concentration of serum LH within a bleeding period. During the initial bleeding period (before puberty in control heifers), administration of naloxone induced an increase in LH concentration, but the response was greater for heifers in the ovariectomized and oestradiol treated than in the ovariectomized group. At the end of the study when control heifers had attained puberty (high concentrations of progesterone indicated corpus luteum function), only heifers in the ovariectomized and oestradiol treated group responded to naloxone. Opioid inhibition of LH appeared to decline in heifers during the time control heifers were attaining puberty. Heifers in the ovariectomized group responded to naloxone at the time of administration with an increase in FSH, but FSH did not respond to naloxone at any other time. Administration of naloxone did not alter secretion of FSH in ovariectomized heifers. These results suggest that opioid neuropeptides and oestradiol are involved in regulating circulating concentrations of LH and possibly FSH during the peripubertal period. Opioid inhibition of gonadotrophin secretion appeared to decline during the peripubertal period but was still present in ovariectomized heifers treated with oestradiol after the time when age-matched control heifers had attained puberty. We conclude that opioid inhibition is important in regulating LH and FSH in circulation in heifers during the peripubertal period. However, opioids continue to be involved in regulation of circulating concentrations of LH after puberty

Endocrine Mechanisms of Puberty in Heifers: Estradiol Negative Feedback Regulation of Luteinizing Hormone Secretion

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the first experiment, seven heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day 0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and subcutaneously implanted with estradiol-17β (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A second experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the second experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. These results indicate that: a) LH secretion in prepubertal heifers is responsive to estradiol negative feedback; and b) estradiol negative feedback decreases during the prepubertal period in beef heifers

Effect of Suckling and Ovariectomy on the Control of Luteinizing Hormone Secretion During the Postpartum Period in Beef Cows

Twenty-two mature pluriparous beef cows were randomly assigned to one of six treatments in a 2 x 3 factorial experiment in order to study the role of suckling and ovarian factors on control of the tonic and episodic release of luteinizing hormone (LH). Twelve cows remained intact (INT) and 10 were ovariectomized (OVX) within 4 days following the day of parturition (Day 0). The suckling intensities were nonsuckled (0), suckled once daily for 30 mm (1) and suckled ad libitum by two calves (2). Blood samples were collected at 1 5-mm intervals for 6 h weekly, from Days 6 to 76 postpartum. The postpartum intervals to initiation of ovarian luteal function were 31 ± 3, 41 ± 4 and 67 ± 1 days (X ± SEM) for INT cows with 0, 1 and 2 suckling intensities, respectively. Mean LH concentrations and frequency of LH pulses increased as time of ovulation approached in INT cows. In OVX animals, both mean LH concentrations and frequency of LH pulses increased as time postovariectomy progressed. No differences were detected in mean LH concentrations or frequency of LH pulses between the two suckled OVX groups. Mean LH in the OVX-O cows was greater on Days 13, 20 and 27 postpartum when compared to the respective days in suckled OVX cows. Frequency of LH pulses tended to be lower (

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: 1) that ovarian estradiol would inhibit luteinizing hormone (LH) secretion in heifers that were anestrus as a result of restricted dietary energy intake and 2) 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-17β-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