Seasonal changes in the concentrations of plasma gonadotropins and prolactin in wild mallard drakes

Seasonal changes in the concentrations of plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin were measured in serial samples taken from seven captive wild mallard drakes exposed to natural lighting and temperature in Kiel, West Germany (54 degrees N), for 20 months. The seasonal pattern of plasma LH levels was characterized by high titers during the reproductive phase in the spring, a steep decrease toward the end of this phase (May/June), low levels during the summer, and a second annual peak in the fall. Plasma FSH levels increased during February and March, the period of rapid testicular growth, and reached the highest values at the end of March/beginning of April. Later in the spring FSH levels decreased and remained low for the rest of the year. The concentrations of plasma prolactin increased progressively during April and May, reaching their highest values at the end of the breeding season, coinciding with the steep fall in the levels of plasma gonadotropins. Prolactin concentrations fell during July and August and were at their lowest level in the autumn. It is concluded that the development of photorefractoriness is associated with an increase in the concentrations of plasma prolactin.     

Plasma prolactin, thyroxine, triiodothyronine, testosterone, and luteinizing hormone during a photoinduced reproductive cycle in mallard drakes

The temporal relationships between plasma concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) were determined in a group of six wild mallard drakes during the development and maintenance of long-day refractoriness after transfer from 6 h light: 18 h darkness (6L:18D) to 20L:4D for 24 weeks. As shown by changes in the plasma concentrations of luteinizing hormone (LH) and testosterone, the birds came into breeding condition and then became long-day refractory within 5 weeks of photostimulation. Long-day refractoriness was maintained for the remainder of the study. Plasma prolactin began to increase immediately after photostimulation, although not as fast as the increases in plasma LH and testosterone. The concentration of plasma T4 also increased after photostimulation but, as shown by decreased plasma LH and testosterone levels, only after the birds had become long-day refractory. The development of long-day refractoriness was thus directly correlated with an increased plasma prolactin and not with a change in plasma concentration of T4. Plasma T3 decreased after photostimulation but returned to prestimulation values as the birds became long-day refractory and remained stable for the remainder of the study. Concentrations of plasma T4 and prolactin returned to baseline values after about 15 weeks photostimulation showing that the long-term maintenance of long-day refractoriness is not directly related to continuously high plasma concentrations of either hormone.     

Effect of heat stress on plasma concentrations of prolactin and luteinizing hormone in ewes.

Basal concentrations of prolactin but not luteinizing hormone were elevated in ewes by 8--10 h of heat stress given daily during the first 11 days of their oestrous cycle. However, the prolactin and luteinizing hormone responses to thyrotrophin releasing hormone and gonadotrophin releasing hormone were unaffected.     

Change in concentrations of luteinizing hormone subunit messenger ribonucleic acids in the estrous cycle of beef cattle.

Changes in the concentrations of LH subunit messenger ribonucleic acids (mRNAs) and in the LH content of the anterior pituitary of beef cattle were studied during the estrous cycle. Japanese beef cows were classified according to the expected day of the estrous cycle: stage I (early-luteal phase, days 1-4; day 1=day of ovulation), stage II (early-mid-luteal phase, days 5-10), stage III (late-mid-luteal phase, days 11-17) and stage IV (follicular phase, days 18-20), according to the morphology of the ovaries. The anterior pituitaries of the cows were collected and the levels of alpha and LHbeta subunit mRNAs were determined by slot-blot analyses. The LH content of the anterior pituitary was measured by radioimmunoassay. The level of alpha subunit mRNA in the pituitary of cows was highest in stage I and decreased significantly by stage II (P<0.05); thereafter it tended to increase. The level of LHbeta subunit mRNA did not change significantly during the estrous cycle. The LH content of the pituitary of cows was low in stage I and tended to increase by stage II, then to decrease from stage II to III, and to increase significantly from stage III to IV (P<0.05). These results suggest that the highest levels of gene expressions of alpha subunit in the anterior pituitary occur in the early-luteal phase of beef cows, while the LH content is increased most in the follicular phase. The enhanced gene expressions of common alpha subunit in the early-luteal phase could be important in replenishing the bovine anterior pituitary with LH, which is depleted of hormone by the LH surge or the enhanced pulsatile release.     

Lithium: Short-term and chronic effects on plasma testosterone and luteinizing hormone concentrations in mice

The effects of short-term and chronic lithium administration on the concentrations of plasma testosterone (T) and luteinizing hormone (LH) were evaluated in C57BL/6 mice, maintained on a fixed photo-period of LD 14:10 (white lights on at 06:00 h, CST). Lithium chloride was injected intraperitoneally twice daily (at 09:00 and 16:00 h) in groups of adult male mice at a dosage of 2.5 meq/kg for 7 days, and 1.25 meq/kg for 21 days. Circulating levels of T and LH were measured by standard radioimmunoassay (RIA) methods. Plasma T levels showed a significant increase in mice treated with lithium for 7 days as compared to those in saline-injected control animals. However, there was no significant difference in the concentrations of plasma T between chronic (21 days) lithium-treated mice and the matched control. Plasma LH levels remained unchanged following both short-term and chronic lithium treatment.     

Effects of acutely increased plasma testosterone concentrations on pulsatile luteinizing hormone secretion in the male rat

To assess the role of testosterone (T) in regulating the minute-to-minute release of pulsatile luteinizing hormone (LH) secretion in the adult male rat, we investigated the negative feedback of acute increases in plasma T concentrations on pulsatile LH secretion in acutely castrated male rats. At the time of castration, we implanted T-filled Silastic capsules, s.c., which maintained plasma T concentrations at approximately 1.8 ng/ml and suppressed LH pulses. On the next day, the capsules were removed; blood sampling (every 6 min) was started 8 h after implant removal, thereby allowing LH pulses to be reinitiated. Immediately following a control bleeding interval of 2 h, either T or vehicle alone was infused s.c., and blood sampling continued for another 4 h. In animals receiving vehicle alone, LH pulse frequency and mean LH levels increased over the 6 h bleeding period. The administration of 200 ng T/min caused a rapid rise in plasma T concentrations of about 4 ng/ml ("physiological") and prevented the increase in pulse frequency that occurred in the control group; it did not, however, reduce pulse frequency over the 4 h infusion period. When T was infused at the rate of 400 ng/ml, plasma T concentrations rose to approximately 18 ng/ml ("supraphysiological") and LH pulse frequency was significantly reduced, but not completely inhibited, during the last 2 h of the infusion. The pulse amplitude of luteinizing hormone did not change significantly in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoperiod and luteinizing hormone secretion in domestic and wild pigs

During seasonal anoestrus (long-days), oestradiol can effectively inhibit the pulsatile secretion of luteinizing hormone (LH) in sheep. The aim of our trial was to determine whether the same regulatory mechanism exists in the pig. Altogether, 20 ovariectomized and oestradiol-implanted gilts (16 domestic pigs, 4 European wild boars) were randomly allocated to two treatment groups. The first group was kept under a short-day light-dark cycle of 8L:16D, and the second group under a long-day light regime of 16L:8D. After a 6-week treatment period, blood samples were taken at 20-min intervals for 12h. After sampling, the light regimens were switched. Sampling was then repeated following another 6 weeks of treatment. In both treatment groups, 2.3 LH pulses occurred every 12h. The basal LH level was 0.7+/-0.4 ng/ml for the short-day group and 1.0+/-0.5 ng/ml for the long-day group. The mean LH level was 0.9+/-0.4 and 1.3+/-0.6 ng/ml and the LH pulse amplitude 0.5+/-0.4 and 0.6+/-0.5 ng/ml, respectively. The basal and mean LH levels were therefore lower in short-day gilts (P<0.05), while LH pulse amplitude and frequency remained unaffected by treatment. In conclusion, the 6-week period under two different light regimes resulted in higher basal LH concentration in long-day gilts but was not able to produce changes in LH frequency in prepubertal gilts.     

Effect of castration on plasma luteinizing hormone in postpubertal boars

Two experiments were conducted to test the working hypothesis that mean plasma concentrations of luteinizing hormone (LH) increase as a result of an increase in the frequency and amplitude of the pulsatile releases of LH in postpubertal boars after removal of gonadal steroid hormones by castration. It was further hypothesized that these changes in secretion of LH would be the result of changes in sensitivity of the pituitary to gonadotropin releasing hormone (GnRH). In Experiment 1, plasma LH was monitored in 10 postpubertal crossbred boars (13 to 14 mo old and weighing 159 +/- 6.0 kg) at 12-min intervals for 6 h before and 1 h after GnRH (375 ng/kg of body weight) on Days -1, 7, 14, 21 and 29 relative to castration. In Experiment 2, plasma LH was monitored in four castrated and five intact postpubertal boars (11 to 12 mo old and weighing 150 +/- 5.1 kg) after each of three doses of GnRH (94, 188 and 375 ng/kg) were administered to each animal. Sample collection occurred 5 wk after castration. Mean LH and frequency of pulsatile releases of LH increased as a result of castration (P<0.0001), with changes evident by Day 7 after castration. However, the amplitude of the LH pulses increased minimally after castration (P<0.10). The response to exogenous GnRH increased throughout Experiment 1 (P<0.0001), even though the amplitude of the pulsatile releases of LH (response to endogenous GnRH) did not change. Castrated animals in Experiment 2 had a greater response of LH to GnRH stimulation than intact boars (P<0.05). The dose-response curve of castrated animals was not parallel (P<0.001) to that of intact boars, and indicated that sensitivity of the pituitary to GnRH had increased in the absence of gonadal steroids. Thus, the hypotheses stated above can be accepted with the exception that castration may have a minimal effect on LH pulse amplitude. Based on the results of these experiments, we suggest that gonadal steroid hormones modulate both the size of releasable stores of LH and pituitary sensitivity to GnRH in boars.     

Ovulatory responses and plasma luteinizing hormone concentrations in dairy heifers after treatment with exogenous progesterone and estradiol benzoate

The objectives of this experiment were to determine the effects of 0.5 mg estradiol benzoate, administered intramuscularly 24 h after removal of CIDR-B progesterone containing intravaginal devices, on the time to estrus, ovulation and peak LH concentration in dairy heifers. Ovulatory responses and plasma LH concentrations were examined using 14 Friesian dairy heifers in 2 separate treatment periods. All heifers received a CIDR-B progesterone-containing intravaginal device with an attached 10-mg estradiol benzoate capsule for 12 d. Within each period, 24 h after CIDR-B removal, 7 heifers received an intramuscular injection of 0.5 mg estradiol benzoate while the remaining 7 heifers received an intramuscular injection of a placebo. Blood samples for LH assay were collected at 0, 6 and 12 h, and then every 4 h for 60 h after estradiol injection. Detection of estrus was conducted at 4-h intervals, and ultrasonographical examination to detect ovulation was conducted every 8 h for 88 h after removal of the CIDR-B device. Treatment with estradiol benzoate tended to reduce the time from device removal to the LH peak in Period 1 (median time to LH peak 40.1 vs 63.9 h; P = 6.07). In Period 2, treatment with estradiol had no significant effect on the time to the LH peak, standing estrus or ovulation. We hypothesize that the period effect was due to the stage of cycle at the time of treatment. For heifers treated in Period 1, the stage of cycle was random. However, because of the prior synchronization of estrus, which was implicit in the experimental design, heifers in Period 2 tended to be in late diestrus. The administration of estradiol benzoate after treatment with exogenous progesterone appears to overcome the variability in timing of LH peaks typically occurring in a herd of synchronized heifers due to different stages of follicular development.     

Effects of ovariectomy or force feeding on the plasma concentrations of prolactin and luteinizing hormone in incubating turkey hens

The effect of ovariectomy (OVX) on plasma concentrations of prolactin (PRL) and luteinizing hormone (LH) in incubating turkey hens was studied. Neither the sham-operated nor the OVX hens exhibited any change in the pattern of incubation behavior as a result of the surgery. Plasma concentrations of estradiol decreased to less than approximately 3 pg/ml by 2 days after surgery in the OVX hens. There were no significant differences in plasma levels of PRL between the sham-operated and OVX hens throughout the study. The concentration of PRL did not change in either the sham-operated or OVX hens and was maintained at high levels after surgery and during incubation of the eggs. By 2 days after hens were placed into cages, plasma levels of PRL significantly decreased and were maintained at low levels in both groups. The concentration of LH did not change in either group during the two wk after surgery when the hens were incubating eggs. After the hens were placed into cages, the concentration of LH increased in the OVX hens and was maintained at significantly higher levels than in the sham-operated hens. By contrast, the concentration of LH increased within 4 days after OVX of out-of-lay but nonincubating hens. The delay in the postcastration increase in plasma level of LH in the OVX hens was not associated with anorexia of incubating hens, since plasma levels of LH were not affected by force-feeding unless plasma levels of PRI were suppressed by nest deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein and energy self-selection of turkey hens. Serum prolactin and luteinizing hormone concentrations.

1. The incidence of broodiness was four times as high among turkey hens fed a complete control diet than among hens allowed to self-select their diet from two different feed sources, one being relatively high in protein and the other relatively high in energy (i.e. split-diet). 2. Among non-broody birds, hens fed the split-diet had a significantly lower serum prolactin concentration in the third month of production as compared to control hens. 3. Hens in their second season of egg production had significantly lower serum luteinizing hormone concentrations during the latter stages of egg production than did first season hens.     

The effect of ketamine/xylazine and carbon dioxide on plasma luteinizing hormone releasing hormone and testosterone concentrations in the male Norway rat

The effect of a commonly used anaesthetic, ketamine/xylazine and/or carbon dioxide (CO(2)) on plasma luteinizing hormone releasing hormone (LHRH) and testosterone concentrations was determined in male Sprague-Dawley rats. These values were compared with values obtained from pre-anaesthetic control samples. Ketamine/xylazine treatment did not significantly affect testosterone concentrations. In contrast, LHRH started to decrease one hour after ketamine/xylazine administration and continued to significantly decrease after 24 h. In addition, in the CO(2) euthanasia-only group, LHRH concentrations were also significantly decreased. These results suggest that ketamine/xylazine anaesthesia followed by CO(2) euthanasia 24 h later is exerting a significant effect on LHRH concentrations 24 h after anaesthetizing, while only having a slight effect on testosterone, and that CO(2) is exerting an immediate significant effect on LHRH. In conclusion, LHRH analysis should be avoided after ketamine/xylazine anaesthesia and CO(2) euthanasia.     

Bioactive and immunoreactive concentrations of circulating luteinizing hormone during sexual maturation in the bovine.

The characteristics of circulating LH during sexual maturation in cattle were assessed by examining bioactive and immunoreactive LH concentrations, as well as their ratio (B/I ratio). Male and female intact control (CONT), gonadectomized (GNX; at 241 +/- 3 days of age, Day 0 of the study), and gonadectomized animals administered 17 beta-estradiol (GNXE) were evaluated. Serum samples were collected at 15-min intervals for 24 h at 1, 7, 13, 17, 21, 25, 29, 33, 37, and 43 wk subsequent to Day 0. Bioactive LH was assessed with an in vitro bioassay using mouse testicular interstitial cells. In initial experiments, immunoreactive LH was quantified in RIAs using three different antibodies. The two RIAs employing polyclonal antibodies overestimated low LH concentrations, but the absolute values obtained in each of the three assays were highly correlated. Hence, immunoreactive LH was measured in an RIA using monoclonal anti-bovine LH (bLH) (JR-518B7). No significant changes in the B/I ratios were observed during individual pulses of LH secretion. Accordingly, pools consisting of equal volumes of the serial blood samples collected during the 24-h period for each animal at each stage of maturation (pools) were compared. LH B/I ratios for GNX females increased significantly with time (p less than 0.01) and the B/I ratios for GNX males were significantly higher than for GNX females (p less than 0.05). Concentrations of LH in most of the pools for GNXE and CONT animals were extremely low or nondetectable until the later bleeding periods.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenal luteinizing hormone releasing hormone receptors.

Paraffin sections of mouse adrenals processed with antiserum to luteinizing hormone-releasing hormone (LHRH) in the unlabeled antibody enzyme method reveal moderate staining in the cytoplasm of cells of zona fasciculata and reticularis. The stain is intensified upon pretreatment of sections with LHRH. Pretreated sections processed with solid phase immunoabsorbed LHRH are unstained. Analogues of LHRH deficient in the C-terminal glycine amide inhibit staining, while analogues deficient in the N-terminal pyroglutamic acid have no effect. It is concluded that the adrenal contains receptors for a ligand resembling LHRH in receptor and immunoreactivity. The possibility is considered that the ligand may be an inhibitor of pineal origin.     

Effects of removal of chicks from hens on concentrations of prolactin, luteinizing hormone and oestradiol in plasma of brooding Gifujidori hens.

Gifujidori hens were allowed to repeat a breeding cycle in one season. In the first breeding cycle the duration of the brooding (raising chicks) stage was limited to 3 weeks, whereas in the second breeding cycle it was limited to 1 week by removing all chicks from mother hens. In the first breeding cycle, plasma prolactin (PRL) was high during the incubation period, but rapidly decreased on the day of hatching and reached minimum values about 1 week after hatching. In contrast, plasma luteinizing hormone (LH) concentrations were low during the incubation period, but after hatching they gradually increased and reached peak values immediately after removal of chicks. Concentrations of oestradiol in plasma were low in the incubation and brooding stages but increased significantly immediately after removal of chicks. In the second breeding cycle, changes in PRL and LH concentrations were similar to those observed in the first breeding cycle except that even greater increases in plasma LH and oestradiol concentrations were observed one week after hatching when the chicks were removed. These results suggest that coexistence of newly hatched chicks may suppress LH secretion from the pituitary of the hen in the natural breeding cycle.