Interactions of photoperiod, testosterone, and naloxone on GnRH and LH pulse parameters in the male sheep

This study tested the hypothesis that the effects of the opiate antagonist naloxone on GnRH (and LH) secretion is affected by photoperiod length and testosterone (T) concentrations. The effect of infusing naloxone on GnRH and LH pulse patterns was determined in four groups of orchidectomized sheep: long day (LD) photoperiod treated with T, LD without T (LDC), short day photoperiod (SD) with T, SDC (n = 5-7/group). Hypophyseal-portal and jugular blood samples were collected at 10 min intervals for 4 h before and 4 h during naloxone infusion (1 mg/kg/h). Neither photoperiod nor T affected either mean GnRH or LH whereas naloxone (P < 0.01) increased both. LD photoperiod (P < 0.01), T (P < 0.01) and naloxone (P < 0.01) all increased LH pulse amplitude whereas only naloxone increased GnRH pulse amplitude (P < 0.01). There was an interaction (P < 0.01) between steroid and naloxone on LH, but not GnRH, pulse amplitude. Both LD photoperiod and T increased both LH and GnRH (P < 0.01) interpulse-interval (IPI). Naloxone decreased GnRH IPI (P < 0.01). The LH/GnRH pulse amplitude ratio was (P < 0.02) increased by T--likely a secondary response to the T-induced increase in IPI. These results are interpreted as showing that in the ram the endogenous opiate peptides regulate both GnRH pulse frequency and amplitude, but that their specific role is modulated by photoperiod and T. These results do not support the concept that the opiate peptides are the primary mediators of the negative feedback effects of T.     

Adenohypophyseal receptors for LHRH, pituitary content of gonadotropins and plasma concentrations of LH in cyclic, pregnant and postpartum beef cows

Adenohypophyseal concentrations of LHRH receptors, pituitary content of LH and FSH, and plasma concentrations of LH were determined in thirty Hereford, Angus or Hereford-Angus heifers that were randomly assigned by breed and weight to five periods including day 3 of the estrous cycle (CY), pregnant day 120 (P120), 200 (P200), 275 (P275), or day 2 postpartum (PP). Jugular blood samples were collected at 10-min intervals for 8 hr from all cows. Within 2 hr after completion of blood sampling, animals were slaughtered and the pituitary gland frozen at −196 C. LH pulse frequency/8 hr was reduced (P<.05) during gestation (.5, .2, and 1.5 ± .5/8 hr, for P120, P200, and P275, respectively) and PP (.5 ± .5/8 hr) compared to CY (7.8 ± .5/8 hr). Frequency of LH pulses/8 hr was not different (P>.1) among P120, P200 or PP periods but was different (P<.05) between P200 and P275. There were no differences in LH pulse height (P>.1) among periods; however, pulse amplitude was greatest (P<.05) at P120 (1.3 ± .2 ng/ml) and lowest between P200 and PP (.6 to .8 ± .2 ng/ml). Baseline concentrations of plasma LH did not differ (P>.1) among P and PP periods (.3 ± .1 ng/ml), but were lower (P<.05) than in CY animals (.7 ± .1 ng/ml). Concentration of adenohypophyseal LHRH receptors was approximately two-fold greater (P<.05) at P120 (25.85 ± 2.2 fmol/mg) than at all other periods (9.5 to 14.9 ± 2.2 fmol/mg). Pituitary content of LH was greatest at P120 (1.56 ± .11 ug/mg) and lowest (P<.05) at P275 and PP (0.46 to 0.52 ± .11 ug/mg). Pituitary content of FSH was greatest (P<.05) in P (12.7 to 17.0 ± 1.4 ug/mg) and PP (18.3 ± 1.4 ug/mg) vs CY (5.0 ± 1.4 ug/mg) cows and increased from P120 to PP (P<.05). Results indicate that physiological changes occurring during gestation may have an effect on subsequent function of the adenohypophysis in beef cows.     

Progesterone and Luteinizing hormone secretion patterns in early pregnant gilts

We studied luteinizing hormone (LH) pulsatility and episodic progesterone release of the corpus luteum (CL) on Day 11 and Day 21 in inseminated gilts and aimed to establish a relationship between these two hormones. Blood was collected at 15-min intervals for 12 hr on Days 11, 16 and 21 from a vena cava caudalis catheter. At euthanasia, eight gilts were pregnant and six gilts were not pregnant. Progesterone parameters (basal, mean, pulse frequency and pulse amplitude) did not differ between pregnant and non-pregnant gilts on Day 11, LH pulse frequency and amplitude tended to differ (p = .07 and p = .079). In pregnant gilts, basal and mean progesterone, progesterone pulse amplitude and frequency declined significantly from Day 11 to Day 21 (p < .05). A significant decline was also seen in the LH pulse amplitude from Day 11 to Day 21 (p < .05). None of the LH pulses was followed by a progesterone pulse within 1 hr on Day 21. On Day 11 and Day 21 appeared a synchronicity in the LH pulse pattern, as there were two or three LH pulses in 12 hr and these LH pulses appeared in the same time window. We conclude that on Day 11 and Day 21 of pregnancy in gilts, progesterone pulses do not follow an LH pulse within one hour. Further we demonstrated that the successful or not successful formation of a CL of pregnancy is independent of progesterone release on Day 11 after insemination. We confirmed the decline of progesterone from Day 11 to Day 21 in the vena cava caudalis and could demonstrate that this decline is partly due to lower progesterone pulse amplitude and frequency and that the decline occurs simultaneously with a decline in LH pulse amplitude.     

Endotoxin induces delayed ovulation following endocrine aberration during the proestrous phase in Holstein heifers

The effect of endotoxin on follicular growth and on secretion of LH, estradiol-17β, progesterone and cortisol during the proestrous phase in cattle was investigated. Holstein heifers were treated with PGF₂ at 11–13 d after ovulation to induce luteolysis. At 42 hr after PGF₂ treatment, heifers were administered either lipopolysaccharide (LPS; Escherichia coli, O111:B4, 5 μg/kg, n = 6) or saline (control; n = 6) by i.v. bolus injection. Ovarian structures were monitored daily by transrectal ultrasonography, and blood samples were collected at various times for hormonal analysis. The duration from PGF₂ treatment to ovulation was significantly longer in the LPS group (8.0 ± 1.3 d) than in the control group (4.2 ± 0.2 d). LPS significantly reduced the pulse frequency of LH for 6 hr after the administration, and increased the mean concentration and pulse amplitude of LH from 3 to 6 hr after the administration. The plasma concentrations of progesterone and cortisol were transiently increased after LPS administration. The plasma concentration of estradiol-17β was significantly decreased at 24 hr after LPS administration compared to that in the controls. Five of six LPS-treated heifers exhibited no preovulatory LH surge until 120 hr after PGF₂ treatment and the remaining heifer exhibited the surge at 108 hr after PGF₂ treatment, while the LH surge was observed at 54–78 hr after PGF₂ treatment in control heifers. These results suggest that endotoxin disrupts progression of the proestrous phase of cattle, interrupting the preovulatory estradiol rise and thus delaying the LH surge and the subsequent ovulation.     

Effect of exogenous estradiol on plasma concentrations of somatotropin, insulin-like growth factor-I, insulin-like growth factor binding protein activity, and metabolites in ovariectomized angus and braham cows

To determine the effect of breed and estradiol-17β on selected hormones and metabolites, ovariectomized (3 mo) Angus (n = 14) and Brahman (n = 12) cows were paired by age and body weight and randomly assigned as either nonimplanted controls (CON) or implanted with estradiol (E2) for 45 d. After Day 7 and through Day 42, plasma concentration of somatotropin was greater for E2 than CON cows (treatment X day, P < 0.05). During an intensive blood sampling on Day 36, E2 cows tended (P < 0.10) to have greater somatotropin pulse amplitudes than CON cows, but other parameters of somatotropin release were not affected (P > 0.10) by E2 treatment. The effect of breed was apparent on Day 36 as Brahman cows had greater (P < 0.05) somatotropin pulse amplitude, basal secretion, and mean concentration than Angus cows. Overall, plasma concentration of IGF-I was greater (P < 0.01) for E2 than CON cows (158.3 vs. 104.2 ng/ml) and was greater for Brahman than Angus cows (164.1 vs. 98.4 ng/ml). However, there was a trend (P < 0.10) for a treatment X breed X day interaction for IGF-I (i.e., the magnitude of increase in IGF-I concentration was greater in E2-Angus than E2-Brahman cows). After Day 7 and through Day 42, total plasma IGF binding protein (IGFBP) activity was greater (P < 0.01) for E2 than CON cows. Ligand blotting revealed at least five forms of IGFBP activity, and E2 cows had greater (P < 0.05) binding activity of IGFBP-3 and the 30- and 32-kDa IGFBP than CON cows. Brahman cows had greater (P < 0.05) IGFBP-3 and the 32-kDa IGFBP than Angus cows. After Day 14 and through Day 42, concentration of urea nitrogen (PUN) was greater (P < 0.001) for CON than E2 cows (treatment X day, P < 0.001). Brahman had greater (P < 0.01) PUN than Angus cows (16.6 vs. 14.2 mg/dl). Plasma concentration of glucose was greater (P < 0.01) for E2 than CON cows (78.9 vs. 76.4 mg/dl) but was not affected (P > 0.10) by breed. In summary, these data suggest that some, but not all, of the positive effects of estradiol on peripheral concentration of IGF-I and IGFBP activity can be attributed to increased somatotropin. Moreover, breed influenced basal and E2-induced secretion of somatotropin and IGF-I such that differences between Brahman and Angus cows in plasma IGF-I concentrations were abated within 3 wk of estradiol implantation. Thus, breed influences the metabolite and hormonal response of cattle to estrogenic implants.     

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.     

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.     

Effects of progesterone and weaning on LH and FSH responses to naloxone in postpartum beef cows

Two experiments were conducted to evaluate the effects of naloxone, an endogenous opioid receptor antagonist, on LH and FSH secretion in postpartum beef cows. In Experiment 1, 24 cows were divided into three equal groups. On day 15 postpartum, all cows were bled for 8 hr at 10 min intervals to evaluate LH secretory parameters. On day 18 postpartum, three treatments were administered: (a) saline at 0730 and 1130 hr; (b) 275 mg naloxone at 0730 and 1130 hr; (c) naloxone as in (b) above, plus this group was also treated with 50 mg progesterone (P4) twice daily from day 16 to day 19. In each treatment, jugular vein samples were collected at 10 min intervals from 0800 to 1600 hr. On day 19 the same treatments were administered at the same times, however, all cows were given 25 micrograms GnRH at 1200 hr to evaluate the LH secretory response. Naloxone increased mean LH concentration (P less than .05) and tended to increase pulse amplitude and frequency compared to controls. However, the most dramatic difference was due to P4 treatment which suppressed mean LH, pulse amplitude and frequency. Treatments had no effect on LH secretion in response to a 25 micrograms dose of GnRH. In Experiment 2, the effects of suckling on the naloxone response were examined in 16 postpartum cows. On day 21 postpartum, blood was collected at 10 min intervals for 8 hr and then calves were removed from half the cows. After 3 days of calf removal, all cows were sampled at 10 min intervals for 4 hr; then naloxone was injected after each 10 min sample at a dose rate of 200 mg/hr (33 mg per injection). Naloxone treatment and sampling continued for an additional 8 hr. Calf removal alone had very little effect on LH pulsatility. However, naloxone resulted in increased pulse frequency and mean LH compared to the control period. We conclude that LH release in the early postpartum cow is partially regulated by endogenous opioid peptides. We were unable to detect any effects on FSH secretion nor on pituitary sensitivity to exogenous GnRH.     

N-methyl-D, L-aspartate induces a transient increase in LH secretion in the seasonally anestrous ewe

The primary objective of this study was to determine the LH response to an excitatory amino acid agonist, N-methyl-D, L-aspartate (NMA) in the seasonally anestrous ewe. In experiment 1, 3 i.v. injections of NMA were given; doses of 0.5, 1.5 and 4.5 mg/kg BW were tested. LH response to NMA depended on the dose. There was little response to the lowest dose. All animals responded to the first injection of the intermediate and the highest doses (mean pulse amplitude: 9.2 +/- 0.4 and 6.8 +/- 1.2 ng ml, respectively). The responses to the second or third injections of both doses were variable and were either absent or reduced compared to that of the first. In experiments 2 and 3, ewes were given 3 injections of normal saline (NS) followed by 3 injections of NMA (1.25 and 4.5 mg/kg BW, respectively) at 2 hr intervals. The last injection of NMA was followed 2 hr later by an injection of GnRH (3.0 ng/kg BW). In experiment 2, the first NMA injection induced an immediate LH pulse (mean pulse amplitude: 8.0 +/- 1.6 ng/ml) in all ewes, however, the second and third injections induced LH pulses in only 25% and 75% (mean pulse amplitude: 2.2 and 2.4 +/- 0.6 ng/ml) of the ewes, respectively. In experiment 3, NMA increased mean LH release (P less than 0.05) after all injections, but responsiveness to the third injection was reduced in some ewes. GnRH injections induced LH release in all ewes in experiments 2 and 3 (mean pulse amplitude: 6.9 +/- 1.8 and 6.4 +/- 2.2 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of dose, frequency, and duration of infused gonadotropin-releasing hormone on secretion of luteinizing hormone and follicle-stimulating hormone in nutritionally anestrous beef cows.

Nutritionally induced anovulatory cows were ovariectomized and used to determine the relationships between dose, frequency, and duration of exogenous gonadotropin-releasing hormone (GnRH) pulses and amplitude, frequency, and concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in serum. In Experiment 1, cows were given pulses of saline (control) or 2 micrograms of GnRH infused i.v. during a 0.1-, 1.25-, 5-, 10-, or 20-min period. Concentrations of LH and FSH during 35 min after GnRH infusion were greater than in control cows (P < 0.01), and FSH concentrations were greater when GnRH infusions were for 10 min or less compared with 20 min. In Experiment 2, the effect of GnRH pulse frequency and dose on LH and FSH concentrations, pulse frequency, and pulse amplitude were determined. Exogenous GnRH (0, 2, or 4 micrograms) was infused in 5 min at frequencies of once every hour or once every 4th hr for 3 d. There was a dose of GnRH x frequency x day effect on LH and FSH concentrations (P < 0.01), indicating that gonadotropes are sensitive to changes in pulse frequency, dose, and time of exposure to GnRH. There were more LH pulses when GnRH was infused every hour, compared with an infusion every 4th hr (P < 0.04). Amplitudes of LH pulses were greater with increased GnRH dose (P < 0.05), and there was a frequency x dose x day effect on FSH pulse amplitude (P < 0.0006). We conclude that LH and FSH secretion in the bovine is differentially regulated by frequency and dose of GnRH infusions.     

Pituitary-testicular responses of estradiol-17 beta-implanted bull calves to continuous versus pulsatile infusion of luteinizing hormone releasing hormone

The luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone response of bull calves implanted with estradiol-17 beta to continuous and pulsatile infusion of luteinizing hormone releasing hormone (LHRH) has been examined. Estradiol-17 beta reduced serum LH and FSH concentrations and suppressed testosterone secretion and testicular growth when compared with sham-implanted bulls. Pulsatile iv infusion of LHRH [500 ng every 2 h (6 micrograms/d)] for a 4-wk period to estradiol-17 beta-implanted bulls resulted in elevated mean serum LH and testosterone concentrations that were characterized by discrete secretory episodes. Mean serum FSH was also increased by LHRH pulse infusion, but LHRH-coupled secretory episodes were not apparent. Continuous infusion of LHRH (6 micrograms/d) did not increase the low serum gonadotropin levels observed in estradiol-17 beta-implanted calves. Testicular growth was normal in LHRH pulse-infused calves, but was markedly curtailed in continuously infused calves. These results suggest that estradiol-17 beta inhibits testicular development by blocking gonadotropin release at the level of the hypothalamus because pulsatile administration of LHRH can override the inhibitory effect by increasing LH and FSH secretion.     

Circulating concentrations of 17-estradiol influence pattern of LH in circulation of cows

The objective of the research was to determine the relationship between circulating 17β-estradiol (E2) and secretion of luteinizing hormone (LH) in cows. A second objective was to determine if response to E₂ was influenced by interval between ovariectomy and the start of E₂ treatment. Thirty-one nulliparous cows 3 yr of age were randomly assigned to a 2 × 4 factorial arrangement of treatments. Sixteen cows were ovariectomized at 18 mo of age (long term), and the other 15 cows were ovariectomized at 36 mo of age (short term). At the time of ovariectomy of cows in the short term group, 11 cows in the short term group and 12 cows in the long term group were implanted subcutaneously with 1, 2 or 4 polydimethylsiloxane capsules containing E₂. The other eight cows served as non-implanted controls (n=4-short term, n=4-long term). All cows were fitted with jugular vein catheters on day 29 of treatment, and on day 30 blood samples were collected at 12-min intervals for 6 hr. At the end of 6 hr, luteinizing hormone-releasing hormone (LHRH) was administered and blood sampling continued at 12-min intervals for an additional hour. Serum was analyzed for LH and E₂. Variables of LH secretion analyzed were mean concentration, frequency of pulses, amplitude of pulses and maximum concentration after LHRH. There were no significant interactions for any of the variables of LH among cows ovariectomized for the long and short term. There was a significant linear increase in mean concentration of LH with increased circulating concentration of E₂. Frequency of LH pulses was not affected by circulating concentration of E₂. As circulating concentration of E₂ increased, amplitude of LH pulses increased and response to LHRH increased - resulting in an increase in mean LH. Interval from time of ovariectomy to the start of E₂ treatment only had a minor influence on mean concentration of LH and profile of LH concentrations in circulation.     

Response to luteinizing hormone releasing hormone in postpubertal boars with large testes

The objective of the present study was to determine if postpubertal boars (12-13 months of age; 156 +/- 8 kg) with large testes had altered hypothalamic control of secretion of luteinizing hormone (LH). Seven boars with the highest estimated 150 d, paired testis weights from a line selected for large testes (769 +/- 60 g = mean weight of excised testes) and 8 boars from a control group (control, 544 +/- 20 g) were tethered in stalls and fitted with indwelling jugular catheters. Males were bled when they were intact, 14 days after castration and during administration of sodium pentobarbital anesthetic (subsequent to castration) to block secretion of endogenous LH-releasing hormone (LHRH). Blood samples were collected at 12-min intervals for 6 hr before and 1 hr after intravenous injection of LHRH in intact and castrated males. During anesthesia, LHRH was administered 4 times at 1-hr intervals and blood samples were collected every 6 min. All samples were analyzed for concentrations of LH and pooled samples were analyzed for concentrations of 17-beta estradiol (E2) and testosterone (T). In intact and castrated males, mean concentrations of LH, frequency and amplitude of pulses of LH, and concentrations of E2 and T were not different between boars of the two groups (P greater than .10). Response to exogenous LHRH was less (P less than .05) in intact males with large testes than in corresponding males from the control group (P less than .05). Fourteen days after castration, males that had larger testes before castration had less of a response to LHRH than males from the control group (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Serum leptin concentrations, luteinizing hormone and growth hormone secretion during feed and metabolic fuel restriction in the prepuberal gilt

Two experiments were conducted to determine 1) the effect of acute feed deprivation on leptin secretion and 2) if the effect of metabolic fuel restriction on LH and GH secretion is associated with changes in serum leptin concentrations. Experiment (EXP) I, seven crossbred prepuberal gilts, 66 +/- 1 kg body weight (BW) and 130 d of age were used. All pigs were fed ad libitum. On the day of the EXP, feed was removed from four of the pigs at 0800 (time = 0) and pigs remained without feed for 28 hr. Blood samples were collected every 10 min from zero to 4 hr = Period (P) 1, 12 to 16 hr = P 2, and 24 to 28 hr = P 3 after feed removal. At hr 28 fasted animals were presented with feed and blood samples collected for an additional 2 hr = P 4. EXP II, gilts, averaging 140 d of age (n = 15) and which had been ovariectomized, were individually penned in an environmentally controlled building and exposed to a constant ambient temperature of 22 C and 12:12 hr light: dark photoperiod. Pigs were fed daily at 0700 hr. Gilts were randomly assigned to the following treatments: saline (S, n = 7), 100 (n = 4), or 300 (n = 4) mg/kg BW of 2-deoxy-D-glucose (2DG), a competitive inhibitor of glycolysis, in saline iv. Blood samples were collected every 15 min for 2 hr before and 5 hr after treatment. Blood samples from EXP I and II were assayed for LH, GH and leptin by RIA. Selected samples were quantified for glucose, insulin and free fatty acids (FFA). In EXP I, fasting reduced (P < 0.04) leptin pulse frequency by P 3. Plasma glucose concentrations were reduced (P < 0.02) throughout the fast compared to fed animals, where as serum insulin concentrations did not decrease (P < 0.02) until P 3. Serum FFA concentrations increased (P < 0.02) by P 2 and remained elevated. Subcutaneous back fat thickness was similar among pigs. Serum IGF-I concentration decreased (P < 0.01) by P 2 in fasted animals compared to fed animals and remained lower through periods 3 and 4. Serum LH and GH concentrations were not effected by fast. Realimentation resulted in a marked increase in serum glucose (P < 0.02), insulin (P < 0.02), serum GH (P < 0.01) concentrations and leptin pulse frequency (P < 0.01). EXP II treatment did not alter serum insulin levels but increased (P < 0.01) plasma glucose concentrations in the 300 mg 2DG group. Serum leptin concentrations were 4.0 +/- 0.1, 2.8 +/- 0.2, and 4.9 +/- 0.2 ng/ml for S, 100 and 300 mg 2DG pigs respectively, prior to treatment and remained unchanged following treatment. Serum IGF-I concentrations were not effected by treatment. The 300 mg dose of 2DG increased (P < 0.0001) mean GH concentrations (2.0 +/- 0.2 ng/ml) compared to S (0.8 +/- 0.2 ng/ml) and 100 mg 2DG (0.7 +/- 0.2 ng/ml). Frequency and amplitude of GH pulses were unaffected. However, number of LH pulses/5 hr were decreased (P < 0.01) by the 300 mg dose of 2DG (1.8 +/- 0.5) compared to S (4.0 +/- 0.4) and the 100 mg dose of 2DG (4.5 +/- 0.5). Mean serum LH concentrations and amplitude of LH pulses were unaffected. These results suggest that acute effects of energy deprivation on LH and GH secretion are independent of changes in serum leptin concentrations.     

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.     

Lamprey gonadotropin-releasing hormone-III selectively releases follicle stimulating hormone in the bovine

Recent studies have shown that lamprey gonadotropin-releasing hormone (l-GnRH) is localized in the mammalian brain, and that l-GnRH-III, can selectively induce FSH secretion in the rat both in vivo and in vitro. Consequently, the purpose of this study was to determine if l-GnRH-III could elicit selective FSH release in cattle and compare this response with that to mammalian luteinizing hormone releasing hormone (m-LHRH). Cattle were chosen as the animal model because previous studies have demonstrated that FSH and LH are secreted by separate gonadotropes in that species. For these studies, crossbred cycling heifers were implanted with jugular cannulae and l-GnRH-III was infused either between Days 9–14 or on Day 20 of the estrous cycle. Blood samples were collected both before and following peptide infusion. Our results demonstrate that during Days 9–14 of the estrous cycle (luteal phase), when progesterone levels averaged between 4 and 5 ng/ml, a dose of 0.25 mg of l-GnRH-III induced the release of FSH (P < 0.05), but not LH. A 0.5 mg dose of l-GnRH-III caused a greater release of FSH (P < 0.01), but still did not induce LH release. Higher doses of the peptide were capable of significantly releasing both gonadotropins. Importantly, during the luteal phase, doses of 0.5 and 2 mg of m-LHRH were ineffective in stimulating FSH, but did elicit marked increases (P < 0.001) in LH. Again, progesterone levels averaged 4–5 pg/ml. In order to assess gonadotropin releasing ability of l-GnRH-III at a different phase of the estrous cycle, some animals were administered the peptide on Day 20, when progesterone levels were below 1.0 pg/ml. At this time, the l-GnRH-III induced the release of LH (P < 0.01), but not FSH. Overall, our results demonstrate that l-GnRH-III can selectively induce FSH in cattle during the luteal phase, whereas m-LHRH was ineffective in that regard. Furthermore, the fact that l-GnRH-III can selectively stimulate FSH when serum progesterone is high, and LH when serum progesterone is low, suggests its actions are under strong control of this steroid. We suggest the FSH releasing capacity of l-GnRH-III in cattle could render this peptide useful for enhancement of reproductive efficiency in this species.     

Influence of age and sex on basal secretion of growth hormone (GH) and on GH-induced release by porcine GH-releasing factor pGRF(1–29NH2) in growing pigs

The aim of this study was to determine the effect of age and sex on basal secretory patterns of growth hormone (GH) and growth hormone-releasing factor (GRF) induced GH release. Eighteen pigs (9 castrated males and 9 females) were stimulated with pGRF(1–29)NH₂ at 7,11,15,19 and 23 weeks of age. Blood samples were taken from each animal via jugular vein cannulate every 20 min, from 6 hr before to 5 hr after iv GRF administration at a dose of 4 μg/kg. GH baseline levels, amplitude of the GH peaks, area under the GH peaks and the overall mean of GH serum levels decreased (P<.001) with age in both sexes. Age also had a marked effect on GRF-induced GH release: the amplitude of GH peaks and area under the GH peaks decreased (P<.001) with age. The GH response to pGRF(1–29)NH₂ varied considerably, depending on the timing of the episodic endogenous secretion of GH. An immediate response (<30 min) was observed when GRF was injected at the end of a trough period or at the beginning of a peak, but there was no immediate response when GRF was injected at the end of a peak or at the beginning of a trough period. Our results show that both endogenous GH secretion and pGRF(1–29)NH₂-induced GH release declines with age, suggesting a decreased sensitivity of the somatotroph cells to GRF with age; and that the high variability of the GH response to pGRF(1–29)NH₂ stimulation depends greatly on the timing of the episodic endogenous GH release, thus implying a possible episodic endogenous somatostatin secretion by the hypothalamus.     

Prolactin and Gonadotropin Responses in Geldings to Injections of Estradiol Benzoate in Oil, Estradiol Benzoate in Biodegradable Microspheres, and Estradiol Cypionate

We previously reported success in inducing early ovulation in seasonally anovulatory mares with a combination of estradiol pretreatment followed by daily administration of a dopamine antagonist (sulpiride). Although every-other-day injections of estradiol benzoate (EB) were effective in that experiment, practical application of this technology would require simplification of the treatment regimen. The current experiment was designed to compare, in a gelding model, the biologic responses of two alternative, one-injection regimens for estradiol delivery to the established EB treatment used previously. Fifteen long-term geldings were sampled via jugular venipuncture from November 5 to 7, 2006, and were then administered intramuscular injections of vegetable oil (n = 4); EB, 11 mg in oil (n = 4; controls); EB in biodegradable microspheres (300 mg; n = 3); or estradiol cypionate, 100 mg in oil (n = 4). Injections of EB in oil were repeated every other day for a total of 10 injections, as was done in our previous experiment. Jugular blood samples were drawn from all geldings at 3, 6, 12, 24, 36, and 48 hours relative to injections, and then on the mornings of days 3, 4, 6, 8, 10 to 18, 22, 26, and 30. On days 10 through 13, all geldings received subcutaneous injections of 125 mg sulpiride, a dopamine receptor antagonist, to stimulate prolactin secretion. On day 12, each gelding received an intravenous injection of 30 μg gonadotropin-releasing hormone (GnRH) analog and 3 mg thyrotropin-releasing hormone (TRH); frequent blood samples were drawn to characterize the luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin responses. Relative to geldings receiving oil, all geldings receiving estradiol injections had a rise (P < .05) in estradiol concentrations lasting at least 12 days. Daily LH concentrations increased (P < 0.01) in all treated groups, but the response was delayed approximately 14 days in the geldings receiving EB in microspheres. Daily FSH concentrations decreased (P < .01) in all treated groups, with the greatest response in the geldings receiving EB in microspheres. Prolactin in daily samples increased (P < .01) similarly in all estradiol-treated groups after injection of sulpiride. The LH response to GnRH analog was greatest (P < .05) in geldings receiving EB in oil and estradiol cypionate; the FSH response was not altered by treatment. The prolactin response to TRH was greater (P < .01) in estradiol-treated geldings relative to controls, but did not differ among groups. Compared with the responses to every-other-day EB injections in oil, as we used previously, a single injection of 100 mg estradiol cypionate gave the most similar and consistent responses. Because of these similar responses in this gelding model, it is likely that a single injection of 100 mg estradiol cypionate can be used in lieu of every-other-day injections of EB in oil in the treatment regimen we reported previously for stimulating ovarian activity in seasonally anovulatory mares.     

Feed intake and serum GH, LH and cortisol in gilts after intracerebroventricular or intravenous injection of urocortin

In three experiments (Exp), ovariectomized gilts received intracerebroventricular (ICV; Exp 1 - with restraint, Exp 2 - without restraint) or intravenous (i.v.; Exp 3) injections of urocortin or saline to assess effects on feed intake and serum GH, LH, and cortisol. Following a 20-hr fast, feed was presented at 1 hr (Exp 1) or 30 min (Exp 2 and 3) after injection (time = 0 hr) of saline or 5 (U5) or 50 (U50) μg/pig (Exp 1 and 2) or 5 μg/kg BW (Exp 3) of urocortin. Blood samples were collected every 15 min from –2 to 6 hr relative to injection and hormone data pooled 2 hr before and hourly after treatment. Treatment with U50 decreased feed intake, relative to saline (treatment x time interaction; P < 0.05), when delivered ICV but not i.v. A treatment by time interaction was detected for GH (Exp 1, 2, 3) and LH (Exp 1 and 2; P < 0.01). Serum GH increased over time (relative to −2 hr; P < 0.05) following treatment with urocortin but not saline regardless of route of administration. Conversely, in Exp 1 (U5 and U50) and Exp 2 (U50), LH decreased relative to −2 hr with a delayed decrease during Exp 1. Serum cortisol was not affected by treatment in Exp 1, but increased following urocortin in Exp 2 and 3 (treatment by time interaction, P < 0.01). These data provide evidence that urocortin modulates GH and LH concentrations and suppresses feed intake in gilts via mechanisms which may be independent of cortisol and may depend upon dose and route of administration.     

Endogenous opiates and the hypothalamic-pituitary-gonadal axis in male sheep

The effects of morphine and the opiate receptor antagonist, naloxone, on the secretory pattern of luteinizing hormone (LH) were assessed in male sheep. Morphine infusion (250 mg/hr) abruptly stopped LH pulsatile secretion in castrates (wethers) and decreased mean serum LH concentrations by nearly 70 percent. Response of the pituitary to exogenous LH releasing hormone was not affected by morphine suggesting that the effects of morphine on LH secretion were mediated through the hypothalamus. Estradiol-implanted wethers, characterized by a nonpulsatile LH secretory pattern, responded to intravenous injection of naloxone (20, 50 and 200 mg Lv.) with an immediate release (pulse) of L.H. Similarly, LH release was significantly increased following naloxone infusion (200 mg/hr for four hours) in intact rams and wethers implanted with testosterone or estradiol. In contrast, naloxone infusion altered the pattern of LH secretion in wethers but without affecting mean serum LH concentrations. These results support the notion that LH secretion in male-sheep is tonically regulated by endogenous opiates and further suggests that opioid modulation of the hypothalamic-pituitary-LH axis in sheep involves an interaction with the steroid negative feedback system.