Effect of metoclopramide on luteinizing hormone secretion in postpartum anestrous cows.

The effect of metoclopramide (MC), a dopamine antagonist on luteinizing hormone (LH), was examined in anestrous primaparous cows. Metoclopramide has been found to be beneficial in overcoming fescue toxicosis; increasing LH secretion stimulates return to ovulatory function after parturition. Consequently, if MC had negative effect on LH secretion, it would indicate that administration of MC to reproducing animals might be limited. Of 14 postpartum (47 to 66 days) cows, 7 were given MC (4 mg/kg of body weight, IV), and 7 served as controls. Blood was obtained via jugular cannulas at 15-minute intervals for 8 hours; MC was given at the end of the first hour, and gonadotropin-releasing hormone (GnRH, 7 mg/kg), was given IV at the end of hour 7 as a challenge stimulus for LH secretion. Prior to GnRH administration, MC did not have significant effect on LH secretion, as judged by mean serum LH concentration, LH pulse frequency, and LH pulse amplitude. Administration of MC resulted in greater (P less than 0.05) LH response to GnRH, indicating enhanced secretory ability when the pituitary gland was challenged. Serum prolactin concentration was increased (P less than 0.01) by MC administration. Therefore, MC did not have adverse effect on LH secretion in postpartum cows.     

Effect of naloxone on serum luteinizing hormone, cortisol and prolactin concentrations in anestrous beef cows

Two experiments were conducted with the opioid antagonist naloxone to determine the effect of opioid receptor blockade on hormone secretion in postpartum beef cows. In Exp. 1, nine anestrous postpartum beef cows were used to measure the effect of naloxone on serum luteinizing hormone (LH), cortisol and prolactin concentrations. Cows received either saline (n = 4) or 200 mg naloxone in saline (n = 5) iv. Blood samples were collected at 15-min intervals for 2 h before and after naloxone administration. Serum LH concentrations increased (P less than .01) in naloxone-treated cows from 1.8 +/- .04 ng/ml before treatment to 3.9 +/- .7 ng/ml and 4.2 +/- .5 ng/ml at 15 and 30 min, respectively, after naloxone administration. In contrast, LH remained unchanged in saline-treated cows (1.6 +/- .3 ng/ml). Serum cortisol and prolactin concentrations were not different between groups. In Exp. 2, 12 anestrous postpartum beef cows were used to examine the influence of days postpartum on the serum LH response to naloxone. Four cows each at 14 +/- 1.2, 28 +/- .3 and 42 +/- 1.5 d postpartum received 200 mg of naloxone in saline iv. Blood samples were taken as in the previous experiment. A second dose of naloxone was administered 2 h after the first, and blood samples were collected for a further 2 h. Serum LH concentrations increased (P less than .01) only in cows at 42 d postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of exogenous gonadotropin-releasing hormone on ovarian function in beef cows after short- and long-term nutritionally induced anovulation

The effect of pulsatile infusion of gonadotropin-releasing hormone (GnRH) on follicular function was evaluated in nutritionally induced anovulatory beef cows. After 4 (short; n = 12) or 18 wk (long; n = 12) of anovulation, cows were randomly assigned within anovulatory group to either 2 microg of GnRH treatment or saline (control; i.v.) every hour for 5 d. Ovarian structures were monitored by daily ultrasonography. Growth rate of the largest follicle (P < 0.01) and maximal size of the largest follicle during treatment were greater (P < 0.01) for GnRH vs control cows. At exsanguination after 5 d of GnRH treatment, the size of the second-largest follicle was greater (P < 0.05) in short (i.e., 4 wk) anovulatory cows than in long (i.e., 18 wk) anovulatory cows and the largest follicle tended (P < 0.10) to be larger in long vs short anovulatory cows. Short anovulatory GnRH-treated cows had more small follicles than short anovulatory control cows or long anovulatory GnRH-treated or control cows (anovulation x GnRH; P < 0.10). Follicular fluid (FFL) concentrations of estradiol (P < 0.01) and androstenedione (P < 0.05) were greater in GnRH vs control cows. Concentrations of insulin-like growth factor-I were greater (P < 0.10) in large vs small follicles in cows that were anovulatory for 4 wk, but not in cows that were anovulatory for 18 wk. The amount of insulin-like growth factor-binding protein (IGFBP)-3 in FFL was greater (P < 0.05) in 4- vs 18-wk anovulatory cows. Amounts of IGFBP-2, -4, and -5 were greater (P < 0.001) in FFL of small (< 5 mm) vs large (> or = 5 mm) follicles regardless of treatment. We conclude that pulsatile treatment with GnRH for 5 d stimulates similar growth of the largest follicles in short- and long-term anovulatory beef cows, and that the duration of anovulation is not a major factor that limits follicular growth w hen anovulatory cowsare treated with GnRH. The primary intrafollicular factors associated with increased follicular size were increased concentrations of estradiol, progesterone, and insulin-like growth factor-I,and decreased concentrations of IGFBP-2, -4, and -5. Increased duration of anovulation was associated with decreased concentrations of IGF-I and IGFBP-3 in FFL.     

Effects of zeranol on reproduction in beef bulls: luteinizing hormone, follicle-stimulating hormone, and testosterone secretion in response to gonadotropin-releasing hormone and human chorionic gonadotropin

Effects of zeranol on the maturation of the adenohypophyseal-gonadal axis were studied in beef bulls. Calves were implanted with 36 mg of zeranol at 3-month intervals from birth through 6 months of age (group 2, n = 10) or were not treated (control group 1, n = 10). After 9 months, group-2 calves were given implants of 36 mg of zeranol at 3-month intervals through 18 months of age (group 2B, n = 5) or were not reimplanted (group 2A, n = 5). Areas under the curves outlined by concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone for 6 hours after the administration of 100 micrograms of gonadotropin-releasing hormone (GnRH) were calculated. Gonadotropin-releasing hormone was administered at 3-month intervals from 1.5 through 19.5 months of age. Areas under the curves for concentrations of testosterone for 4 hours after the administration of 10,000 IU of human chorionic gonadotropin (HCG) at 4.5, 7.5, and 10.5 months or 1,000 IU at 13.5 and 16.5 months of age also were calculated. The amount of FSH released was greater (P less than 0.05) for group-2 than for group-1 calves at 4.5 and 7.5 months of age. The amount of FSH released in groups 2A and 2B tended (P less than 0.10) to be greater than that for group 1. Significant differences between groups 2A and 2B were not observed. The amount of LH released at 7.5 months of age was less for groups 1 and 2 than that at earlier ages, and the decrease was greater (P less than 0.05) for group 2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progesterone concentration,estradiol pretreatment,and dose of gonadotropin-releasing hormone affect gonadotropin-releasing hormone-mediated luteinizing hormone release in beef heifers

We examined whether progesterone (P₄)-induced suppression of LH release in cattle can be overcome by an increased dose of exogenous gonadotropin-releasing hormone (GnRH) or pretreatment with estradiol (E₂). In Experiment 1, postpubertal Angus-cross heifers (N = 32) had their 2 largest ovarian follicles ablated 5 d after ovulation. Concurrently, these heifers were all given a once-used, intravaginal P₄-releasing insert (CIDR), and they were randomly assigned to be given either prostaglandin F_2α (Low-P₄) or no treatment (High-P₄) at follicle ablation, and 12 h later. Six days after emergence of a new follicular wave, half of the heifers in each group (n = 8) were given either 100 or 200 μg of GnRH i.m. Plasma luteinizing hormone (LH) concentrations were higher in the Low- vs High-P₄ groups, and in heifers given 200 vs 100 μg of GnRH (mean ± SEM 15.4 ± 2.2 vs 9.1 ± 1.2, and 14.8 ± 2.1 vs 9.8 ± 1.4 ng/mL, respectively; P ≤ 0.01). Ovulation rate was higher (P = 0.002) in the Low-P₄ group (15/16) than in the High-P₄ group (6/16), but it was not affected by GnRH dose (P = 0.4). In Experiment 2, heifers (n = 22) were treated similarly, except that 5.5 d after wave emergence, half of the heifers in each group were further allocated to be given either 0.25 mg estradiol benzoate i.m. or no treatment, and 8 h later, all heifers were given 100 μg GnRH i.m. Both groups treated with E₂ (Low- and High-P₄) and the Low-P₄ group without E₂ had higher peak plasma LH concentrations compared to the group with high P₄ without E₂ (12.6 ± 1.8, 10.4 ± 1.8, 8.7 ± 1.3, and 3.9 ± 1.2 ng/mL, respectively; (P < 0.04)). However, E₂ pretreatment did not increase ovulation rates in response to GnRH (P = 0.6). In summary, the hypotheses that higher doses of GnRH will be more efficacious in inducing LH release and that exogenous E₂ will increase LH release following treatment with GnRH were supported, but neither significantly increased ovulation rate.     

Patterns of tonic luteinizing hormone release and ovulation frequency in suckled anestrous beef cows following varying intervals of temporary weaning

Thirty postpartum Brahman crossbred cows were utilized to determine the effects of varying intervals of temporary weaning on tonic LH secretion and ovulation. Cows were assigned randomly on day 17–21 postpartum to one of five groups: 1) Suckled Ad libitum, 2) 48-hr weaned, 3) 72-hr weaned, 4) 96-hr weaned, or 5) 144-hr weaned. The mean maximal rise in LH pulse frequency due to weaning occurred within 2 days and averaged 221 percent of time 0 values. The frequency of LH pulses was greater (P<.06) in weaned than in suckled controls. This temporal increase was self-limiting, displaying an acute rise followed by a variable rate of decline in all groups. However, pulse frequency remained elevated relative to suckled controls for the longest period of time for weaning durations of 96 and 144 hr (P<.10). In 48-hr and 72-hr weaned cows, a rapid reversal of the initial increase in LH pulse frequency was observed following calf return. A significant linear regression (y = 1.9 ± .64x; P<.03) described the increase in LH pulse frequency that occurred in cows which ovulated following weaning. Nonovulators were sensitive to calf return and responded by exhibiting a linear decline (y = 2.87 − .43x; P<.04) in LH pulse frequency following this event. The amplitude of LH pulses increased (P<.02) during the period after calf return in ovulators, but did not change in nonovulators. Percentage ovulating by day 10 increased (P<.05) with increased weaning duration past 72 hr. We conclude that calf return before 96 hr markedly attenuates weaning-induced increases in LH secretion and ovulation.     

Endogenous opioid suppression of release of luteinizing hormone during suckling in postpartum anestrous beef cows

Beef cows were used to determine if suckling influences release of LH via endogenous opioids at 28 +/- 4 d after parturition. Cows of similar weight and body condition (6.8 +/- .1, 1 = emaciated, 9 = obese) were assigned randomly to five groups (n = 6 to 7): 1) control-suckled/saline (suckled 15 min every 6 hr for 48 hr); 2) control-suckled/naloxone; 3) calf-removal/saline (calf removal for 52 hr); 4) calf-removal/naloxone; and 5) control-suckled/GnRH (Gonadotropin-Releasing Hormone). At 0 hr, saline was administered to all cows. This treatment was continued at 6 hr intervals for 24 hr. Either naloxone (0.5 mg/kg), GnRH (40 ng/kg) or saline was administered to cows in their respective groups every 6 hr during the ensuing 24-hr period in calf-removal groups, or immediately preceding each suckling episode in the control-suckled groups. Blood samples for analysis of luteinizing hormone (LH) were collected at 15-min intervals for 1 hr prior to and 3 hr after treatment at 0, 24, 36 and 48 hr. Cows were observed for estrus twice daily. All cows in the control-suckled/GnRH group released LH (P less than .05) in response to exogenous GnRH, indicating the presence of releasable quantities of the gonadotropin. Mean concentrations of LH were not effected (P greater than .05) by the control-suckled regime. However, calf-removal alone, or in combination with naloxone, increased (P less than .05) mean concentrations of LH by 48 hr.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of body condition and estradiol on luteinizing hormone secretion in post-partum beef cows

An experiment was conducted to test the hypothesis that the effect of body fatness on LH pulsatility in post-partum cows is entirely independent of the negative feedback effects of ovarian steroids. Forty beef cows were fed in the last 100 d of gestation so that they achieved either a thin (mean score 1.97) or fat (mean score 2.79) body condition (0 to 5 scale) at calving and were fed after calving to maintain live weight and body condition. At 15 (sd 3.7) d post partum all cows were ovariectomised and half from each body condition score treatment group received a subcutaneous estradiol implant (+EST) while the remainder received no implant (−EST). At weeks 5 and 9 post-partum blood samples were collected via jugular catheter every 20 minutes for 10 hr on two consecutive d and on the third d cows were injected via the jugular vein with 2.5 μg GnRH. Blood samples were collected every 15 minutes for 1 hr before and 2 hr after GnRH injection. At 5 and 9 weeks the fatter cows had significantly higher mean LH concentrations, baseline LH concentrations, LH pulse amplitudes and pulse frequencies (P<0.01). Implantation with estradiol in both fat and thin cows reduced mean LH concentrations, baseline LH concentrations, LH pulse amplitudes and pulse frequencies (P<0.001). The lack of interaction between body condition and the presence or absence of estradiol implies that the effect of body condition on LH release is independent of ovarian steroid feedback mechanisms. Fat cows showed a greater release of LH in response to exogenous GnRH (P<0.01) than thin cows while implantation with estradiol in both fat and thin cows decreased (P<0.01) LH release. The pituitary responsiveness to GnRH with the −EST cows was greater at 9 compared to 5 weeks, but there was no difference with time in the +EST cows. However, there was no such interaction in endogenous LH pulse amplitude suggesting that in the absence of estradiol the magnitude of GnRH pulses declined with time post-partum.     

Relationship between weaning and secretion of luteinizing hormone, cortisol and transcortin in beef cows

The effects of suckling on secretion of luteinizing hormone, cortisol and transcortin were investigated in anovulatory postpartum cows. On d 35 postpartum, calves were separated from 12 cows to prevent suckling and eight calves continued to suckle their dams ad libitum. Between 35 and 41 d postpartum, samples of jugular blood were collected every 15 min for two periods of 6 h/d. In non-suckled cows, frequency of pulses and basal luteinizing hormone increased but amplitude of pulses did not change. Concentrations of total cortisol in serum of cows were not altered during 3 d after weaning calves and did not differ among intervals before, during and after a suckling event. Affinity of transcortin for cortisol was not affected by postpartum interval or treatment. Capacity of transcortin to bind cortisol tended to increase after weaning. We found no evidence to support the hypothesis that suckling reduces binding capacity of transcortin or increases unbound cortisol. Differences in preovulatory secretion of luteinizing hormone between suckled and non-suckled cows could not be accounted for by differences in secretion of cortisol. In beef cows that are fed to satisfy requirements for energy and have average body condition, we conclude that negative modulation of luteinizing hormone by suckling is not mediated by cortisol.     

Hypothalamic deafferentation in prepuberal beef heifers: Effects of gonadotropin-releasing hormone and estradiol benzoate on luteinizing hormone secretion

Hypothalamic control of luteinizing hormone (LH) secretion was investigated in crossbred beef heifer calves by comparing anterior (AHD), posterior (PHD), and complete (CHD) hypothalamic deafferentation with sham operated controls (SOC). Heifers (n = 16) were fitted with an indwelling jugular catheter for 6 days before cranial surgery, and assigned randomly to treatments. Blood for radioimmunoassay of LH was collected sequentially at 15-min intervals during an 8-h period on days ? 1 before and day 6 after hypothalamic deafferentation or sham operation. On the day of surgery, blood samples were collected sequentially at 15-min intervals 2 h before induction of anesthesia and throughout surgery and early recovery. Seven months after hypothalamic deafferentation, all experimental and sham operated heifers were ovariectomized and treated with vegetable oil (i.m.) plus saline (i.v.), vegetable oil plus gonadotropin releasing hormone (GnRH), estradiol benzoate (EB, 1 mg) in vegetable oil. After ovariectomy basal plasma concentrations of LH increased (P < 0.01) compared with the low circulating hormone levels before ovariectomy. The amplitude of LH response to GnRH was greater (P < 0.01) in CHD and PHD when compared with SOC and AHD heifers. Injection of EB failed to induce a LH surge in CHD and PHD 900–1100 min later when compared with the robust response seen in SOC and AHD heifers. Injection of EB plus GnRH elicited LH release in all deafferentated and sham operated heifers. These results indicate a transient change in LH secretion after AHD or CHD in prepuberal heifers with intact ovaries. After OVX, the integrity of the neural connection of the posterior hypothalamus is required for EB-induced LH release in beef heifers.     

Opioid inhibition of luteinizing hormone secretion during the postpartum period in suckled beef cows

Recent studies have shown that naloxone (N), an opioid antagonist, increases concentrations of luteinizing hormone (LH) in the postpartum anestrous beef cow. However, the LH response to N was influenced by the postpartum interval. For example, a significant LH response to 200 mg of N occurred on d 42 but not on d 14 or 28 postpartum. The present study was conducted to determine the effect of different doses of N on LH secretion during the postpartum period of beef cows. Twelve cows were given 200, 400 or 800 mg of N on d 14, 28 and 42 postpartum in a Latin square design with repeat measures within cells. On d 14, serum concentrations of LH increased (P less than .01) from .5 +/- .1 ng/ml (mean +/- SE) before N to a peak of 2.0 +/- .5 and 1.4 +/- .5 ng/ml for cows given 400 and 800 mg of N, respectively. In contrast, 200 mg of N had no effect on serum concentrations of LH. On d 28 and 42 all three doses of N elevated (P less than .01) serum concentrations of LH. Therefore, a larger dose of N was required to increase serum concentrations of LH on d 14 postpartum compared with d 28 and 42. Based on these data we suggest that endogenous opioids participate in the regulation of LH secretion in the early postpartum period. The differential response to naloxone may be due to changes in endogenous opioid inhibition of LH secretion during the postpartum period.     

Effects of feeding diets containing endophyte-infected fescue seed on luteinizing hormone secretion in postpartum beef cows and in cyclic heifers and cows.

Two experiments were conducted to investigate the effect of feeding endophyte (Acremonium coenophialum)-infected fescue (Festuca arundinacea Shreb.) seed on LH secretion in postpartum beef cows and in cycling heifers and cows. In Exp. 1, spring-calving primiparous Angus cows (n = 16) were pair-fed for 75 d diets that contained endophyte-free or endophyte-infected (95%) fescue seed that contained 1.3 micrograms/g of ergovaline and 5.2 mg/g of saturated pyrrolizidines. Serial blood samples for basal and GnRH-stimulated serum LH analysis were obtained on d 7, 28, 42, and 56 of the study. The endophyte had no effect on LH secretion (basal, pulse frequency, and amplitude) or milk production. Average daily gain was decreased (P < .05) in cows that consumed infected fescue seed compared with controls (-.20 vs -.01 kg, respectively). Basal serum prolactin concentrations were reduced (P < .01) in treated compared with control cows (8.9 vs 25.4 ng/mL, respectively) on d 70. In Exp. 2, cycling Angus heifers (n = 8; age = 2 yr) and cows (n = 8; age = 4 yr) stratified by age were pair-fed for 40 d diets that contained the noninfected or the highly infected fescue seed. Estrus was synchronized by prostaglandin F2 alpha (d 18 and 28). Serial blood samples for serum LH analysis were obtained on d 28 (luteal phase) and d 30 (follicular phase). The endophyte did not affect LH (P > .28) or prolactin (P > .16) secretion, whereas ADG was decreased (P < .05) in treated compared with control animals (.32 vs .70 kg/d, respectively).     

Pituitary effects of steroid hormones on secretion of follicle-stimulating hormone and luteinizing hormone

Steroid hormones have a profound influence on the secretion of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These effects can occur as a result of steroid hormones modifying the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, or a direct effect of steroid hormones on gonadotropin secreting cells in the anterior pituitary gland. With respect to the latter, we have shown that estradiol increases pituitary sensitivity to GnRH by stimulating an increase in expression of the gene encoding the GnRH receptor. Since an estrogen response element (ERE) has not been identified in the GnRH receptor gene, this effect appears to be mediated by estradiol stimulating production of a yet to be identified factor that in turn enhances expression of the GnRH receptor gene. However, the importance of estradiol for enhancing pituitary sensitivity to GnRH during the periovulatory period is questioned because an increase in mRNA for the GnRH receptor precedes the pre-ovulatory rise in circulating concentrations of estradiol. In fact, it appears that the enhanced pituitary sensitivity during the periovulatory period may occur as a result of a decrease in concentrations of progesterone rather than due to an increase in concentrations of estradiol. Estradiol also is capable of altering secretion of FSH and LH in the absence of GnRH. In a recent study utilizing cultured pituitary cells from anestrous ewes, we demonstrated that estradiol induced a dose-dependent increase in secretion of LH, but resulted in a dose-dependent decrease in the secretion of FSH. We hypothesized that the discordant effects on secretion of LH and FSH might arise from estradiol altering the production of some of the intrapituitary factors involved in synthesis and secretion of FSH. To examine this hypothesis, we measured amounts of mRNA for activin B (a factor known to stimulate synthesis of FSH) and follistatin (an activin-binding protein). We found no change in the mRNA for follistatin after treatment of pituitary cells with estradiol, but noted a decrease in the amount of mRNA for activin B. Thus, the inhibitory effect of estradiol on secretion of FSH appears to be mediated by its ability to suppress the expression of the gene encoding activin.     

Influence of an agonist of gonadotropin-releasing hormone (buserelin) on estrus synchronization and fertility in beef cows.

The objective of this experiment was to determine the effect of sequential treatment with buserelin (a GnRH agonist) and cloprostenol (a prostaglandin F2 alpha analog) on estrous response and fertility in beef cattle with different ovarian conditions. On d 0 (1st d of treatment), the control group (n = 52, 10 heifers and 42 cows) and the GnRH group (n = 48, 10 heifers and 38 cows) received 2 mL of saline or 2 mL of Receptal (8 micrograms of buserelin), respectively. On d 6, all cows that had not exhibited spontaneous estrus were given i.m. 500 micrograms of cloprostenol (PGF). Ultrasonography on d 0 and assays of progesterone in blood on d -11, 0, and 6 were used to identify follicular and luteal status of animals. Cattle were observed for estrus from d 0 to 10. Cows showing estrus were bred artificially 12 h after onset of estrus. Over the 10-d period, the number of cows detected in estrus and pregnancy and conception rates were identical for the two groups. However, between d 0 and 6, the proportion of cows exhibiting estrus was lower (P less than .01) in the GnRH group than in the control group. Between d 6 and 10, the synchronization rate and precision of estrus were greater (P less than .01) in the buserelin-treated group than in the control group. Conception rate and interval from PGF injection to onset of estrus were not different between the two treatment groups. Presence of a large (greater than 10 mm) follicle on d 0 enhanced synchronization rate and precision of estrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of the ovary and suckling on luteinizing hormone response to naloxone in postpartum beef cows

The influence of the suckling stimulus and ovarian secretions on LH response to naloxone was studied in 16 postpartum anestrous beef cows that were assigned randomly to one of four groups (n = 4/group): intact suckled (IS), intact nonsuckled (IN), ovariectomized suckled (OS) or ovariectomized nonsuckled (ON). Ovariectomy (OS + ON) and calf removal (IN + ON) were performed on d 2, 3 or 4 after parturition. Jugular venous blood was collected at 15-min intervals for 4 h before and 4 h after administration of naloxone (1 mg/kg BW, i.v.) on d 14 and d 28 after parturition. Gonadotropin-releasing hormone (5 micrograms, i.v.) was given 3 h after naloxone. Both IN and OS increased (P less than .05) mean pretreatment LH above IS values (mean +/- SE, ng/ml; IS 1.6 +/- .1 vs IN 2.5 +/- .3 and OS 2.7 +/- .4; P less than .01), whereas ON increased (P less than .01) LH (3.7 +/- .3 ng/ml) even further. Mean LH increased (P less than .05) after naloxone administration in all treatment groups. However, magnitude of this response was variable and dependent on ovarian status. Amplitude of the naloxone-induced LH response was greater (P less than .05) for ovariectomized (5.9 +/- 1.1 ng/ml) than for intact groups (2.7 +/- .5 ng/ml). Gonadotropin-releasing hormone increased mean LH concentrations in all groups. We suggest that ovarian secretions and the suckling stimulus contribute to endogenous opioid inhibition of LH during the postpartum interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Steroid hormone and luteinizing hormone concentrations in the anestrous sow.

This investigation characterized serum concentrations of luteinizing hormone (LH), estradiol-17 beta (E2), progesterone (P4) and cortisol (C) in anestrous sows. Twenty-two sows that had not returned to estrus within 45 days after weaning (anestrous sows), and ten sows that had returned to estrus within seven days following weaning (cyclic sows) were nonsurgically fitted with indwelling jugular vein cannulae. Blood samples were collected at 6 h intervals for seven days and at 15 min intervals for 8 h on the fifth day after cannulation. Serum LH concentrations were determined in all samples, while C, E2 and P4 levels were quantitated in serum collected at 6 h intervals. Serum P4 concentrations in anestrous sows were consistently less than 0.5 ng/mL, and E2 levels ranged from 10 to 19 pg/mL. Concentrations of LH remained less than 1.0 ng/mL in anestrous sows, whereas a preovulatory LH surge was observed in five of ten cyclic sows. There was a circadian rhythm in mean C levels with C peaks occurring at 0600 or 2400 h and nadir levels observed at 1200 and 1800 h. Few differences in C levels were detected between anestrous and cyclic sows. It was evident that anestrous sows did not exhibit cyclic or predictable variations in steroid hormone concentrations. Unfortunately, the results of this study failed to elucidate the endocrine pathogenesis of the anestrous sow.     

Sociosexual stimuli and gonadotropin-releasing hormone/luteinizing hormone secretion in sheep and goats

Sociosexual stimuli have a profound effect on the physiology of all species. Sheep and goats provide an ideal model to study the impact of sociosexual stimuli on the hypothalamic-pituitary-gonadal axis because we can use the robust changes in the pulsatile secretion of luteinizing hormone as a bioassay of gonadotropin-releasing hormone secretion. We can also correlate these changes with neural activity using the immediate early gene c-fos and in real time using changes in electrical activity in the mediobasal hypothalamus of female goats. In this review, we will update our current understanding of the proven and potential mechanisms and mode of action of the male effect in sheep and goats and then briefly compare our understanding of sociosexual stimuli in ungulate species with the "traditional" definition of a pheromone.     

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.     

Detection of urinary luteinizing hormone in Japanese black cows after administration of gonadotropin-releasing hormone

The blood luteinizing hormone (LH) surge in cows is well studied. However, little is known about urinary LH in cows. This study examined urinary LH concentrations after administration of gonadotropin-releasing hormone (GnRH) in six Japanese black cows to induce LH secretion from the pituitary gland into the bloodstream. Abrupt rises in plasma and urinary LH were observed after GnRH administration. Plasma and urinary LH peaked at 2 and 5 hr, respectively. A positive correlation was observed between plasma LH concentrations and urinary LH amounts. Ovulation was confirmed in the cows after 48 hr of GnRH administration. These data strongly suggest that urinary LH is derived from plasma LH, which triggers ovulation in cows.