Binding of gonadotropin-releasing hormone (Gn-RH) to pituitary membranes in male rats

The binding of gonadotropin-releasing hormone (Gn-RH) to membranes of male rats pituitary cells was studied. It appeared that Gn-RH was bound to 3 classes of sites and that the concentration in high affinity binding sites was very low. Moreover there was negatively cooperative interaction among these high affinity binding sites leading to a rapid decrease in binding affinity and consequently unmeasurable displacement of labeled hormone with large amounts of radio-inert ligand. These facts are inconsistent with the possible estimation of Gn-RH by radioreceptor assay.     

Relationship between pituitary GnRH-binding sites and pituitary release of gonadotrophins in post-partum beef cows

Thirty primiparous suckling beef cows were slaughtered on Day 7, 14, 28, 42 or 56 after parturition. Some had resumed oestrous cyclicity by the time they were slaughtered on Days 42 and 56. Amongst acyclic cows between Days 7 and 42, pituitary LH concentrations and basal and GnRH-induced release of LH from pituitary explants doubled. Pituitary FSH concentration and basal release in FSH increased only by 15-20%, while GnRH-induced release of FSH in vitro was unchanged. During postpartum anoestrus, overall mean concentrations of serum FSH did not change, whereas overall mean concentrations and pulse amplitudes of serum LH increased. Numbers and affinity constants of GnRH-binding sites in pituitary glands remained constant during the post-partum period studied. We conclude that, under these experimental conditions, numbers and affinity constants of GnRH-binding sites in the pituitary gland of post-partum beef cows do not limit the ability of the anterior pituitary gland to release gonadotrophins.     

Oestrus, time of ovulation, ovulation rate and conception rate in progestagen-treated ewes given Gn-RH, Gn-TH analogues and gonadotrophins.

The influence of Gn-RH, hCG and a PMSG-hCG mixture (PG600) on the time of ovulation, ovulation rate and on the occurrence of oestrus in ewes treated with progestagen-impregnated sponges for 12 days examined. The effects of Gn-RH analogues on plasma LH, oestrus, ovulation and conception rate were also investigated. Six separate experiments were carried out. When 50 micrograms Gn-RH were given 24 h after sponge removal ovulation occurred in 44--46% of ewes within 24 h and in all ewes by 34 h. Gn-RH was a more potent ovulation synchronizer than hCG. Both hCG and PG600 reduced the incidence of overt oestrus. Gn-RH also had this effect in ewes treated during February and May but not in August and September. Gn-RH analogues given 2 days before sponge removal significantly increased ovulation rate. The display of oestrus was not affected in ewes treated 2 days before sponge removal but was suppressed in 43-69% of ewes treated with an analogue at the time of sponge removal. Ovulation occurred in 50-62% of ewes within 30-35 h of injection of Gn-RH analogues, regardless of the time of their administration. The release of LH in response to one analogue was not influenced by the presence of the progestagen-impregnated sponge in the vagina. When given a Gn-RH analogue 2 days before sponge removal or at the time of sponge removal 63 and 62% of mated ewes became pregnant compared with 70% of control ewes.     

Effects of a systematic application of human chorionic gonadotropin, gonadotropin-releasing hormone analog and bovine anterior pituitary gonadotropin in cows with cystic ovarian disease

Fourty-one cows with ovarian follicular cysts (cysts) diagnosed by rectal palpation and observation of estrus behavior were injected intramuscularly (i.m.) with 20,000 IU of human chorionic gonadotropin (HCG). Ten days after this treatment all the cows were examined per rectum for changes in the ovaries with special regard to luteinization of cysts. Cows not responding to HCG were administered 500(10)mug of an analog of gonadotropin-releasing hormone (Gn-RH), Des-Gly-LH-RH-ethylamide, i.m.. If the cysts remained unchanged 10 days subsequent to the second treatment, a third treatment, 400 KE of bovine anterior pituitary gonadotropin (APG), was given i.m. HCG was clinically effective in only 8 cows (20 %). All of the 8 cows conceived. Of 33 cows not responding to HCG, 18 cows (55 %) responded to Gn-RH analog and 12 cows (36 %) conceived. Eleven (73 %) of 15 cows failing to respond to Gn-RH analog were successfully treated with APG and 6 cows (40 %) conceived. In 37 cases, treatment effects were also evaluated by determining serum levels of progesterone prior to and 10 days subsequent to each treatment. When effects of treatment were judged by 1.0 ng/ml or more increase of serum progesterone levels 10 days after treatment, HCG was effective in 13 of 37 cows (35 %), retreatment with Gn-RH analog was successful in 8 of 16 cows (50 %) and APG was ineffective in 3 cows not responding to both HCG and Gn-RH analog. It may be concluded that the therapeutic effect of HCG is disappointing and about half of the cases not responding to HCG were successfully treated with Gn-RH analog. If the cows did not respond to both HCG and Gn-RH analog, they may not respond to APG either.     

Plasma levels of LH, FSH, prolactin, oestradiol-17beta and progesterone during natural and induced oestrus in the dairy goat

The patterns of LH, FSH, prolactin and oestradiol-17beta, before and during natural oestrus, and of progesterone during the following cycle were studied in four French Alpine dairy goats and compared with those obtained after synchronization of oestrus in the same animals. The highest concentration of oestradiol-17beta was measured at the beginning of oestrus and was followed 3 hours later by simultaneous rises of LH, FSH and prolactin. A second FSH peak was observed 48h after the first one. On D(3) (D(0) = day of oestrus) progesterone concentration was over 1 ng/ml. The luteal phase lasted 15 days. Peak concentrations of oestradiol-17beta and progesterone were higher in animals when oestrus was induced. This was attributed to their higher ovulation rate. The second FSH peak was lower, and the interval between oestradiol-17beta peak and gonadotrophin surge longer, than at natural oestrus.     

Effects of norgestomet-implants and fluorogestone acetate-impregnated sponges on oestrous cycle length and luteal function of ewes

Plasma progesterone profiles were used to assess luteal function and length and synchronization of oestrous cycles in ewes after insertion of subcutaneous ear implants containing Norgestomet or intravaginal sponges impregnated with fluorogestone acetate (FGA) for 12 or 14 days. Insertions were made 2, 9 or 16 days after synchronization of the oestrous cycle with FGA-sponges. An i.m. injection of 500 IU pregnant mares' serum gonadotrophin was given at the time of sponge or implant removal. Norgestomet- implants inserted 9 or 16 days after FGA-sponge treatment had no effect on luteal function but delayed the onset of a new oestrous cycle for the duration of treatment. Following withdrawal of implants, oestrus was effectively synchronized. When Norgestomet-implants were inserted 2 days after FGA-sponge treatment, luteal function was normal. At the time of implant removal, plasma progesterone levels were elevated suggesting the presence of functional corpora lutea. In contrast, insertion of FGA-sponges early in the oestrous cycle shortened the luteal phase and a new oestrous cycle was initiated within 48 h after sponge removal. These results indicate that Norgestomet- implants can artificially prolong the length of the oestrous cycle and do not affect the functional lifespan of corpora lutea in cycling ewes. However, when Norgestomet-implants are inserted early in the oestrous cycle, they are unable to cause premature regression of corpora lutea.     

Studies of pituitary function in lactating ewes.

The release of LH from the pituitary of lactating ewes was studied. In Exp. 1, ewes were injected with 50 microng oestradiol benzoate (OB), 2-0 mg testosterone propionate (TP) or oil only (control) on days 5, 10, or 20 after lambing. LH was measured in peripheral plasma samples obtained 20-38 h after treatment, and the ovulations were recorded. The number of ewes in which an LH release was detected, and the amount released, declined between Day 5 and 20 after OB treatment but increased after TP treatment. The releases of LH were not always accompanied by ovulation and the incidence of ovulation was higher in ewes treated with TP. In Exp. 2, lactating ewes were injected with 1 or 5 (at 2-h intervals) doses of 50 microng Gn-RH, on Days 12 or 25 after lambing. LH was measured in peripheral plasma samples collected every 2 h for 10 h and every 3 h for a further 70 h. Release of LH occurred in all ewes, the amount being greater in ewes receiving multiple injections and in ewes treated on Day 25. The incidence of ovulation was higher after treatment on Day 25. Multiple injections of Gn-RH appeared to reduce the incidence of abnormal corpora lutea.     

Effects of chronic treatment with a GnRH agonist on oestrous behaviour and on the secretion of LH and progesterone in the ewe

A study was carried out to investigate a novel approach to oestrus synchronization in the ewe by treatment with a gonadotrophin releasing hormone (GnRH) agonist. Groups of ewes were initially treated on Day 2, 10 or 14 of the oestrous cycle with 10 mug GnRH analogue (D-Ser(Bu(t)) 6 des Gly GnRH ethylamide) per ewe per day for 14 days. Behavioural oestrus was inhibited during GnRH agonist treatment and recurred from 8 to 38 days after the treatment in an unsynchronized manner. Luteal activity during treatment was not impaired but reduced progesterone concentrations occurred in cycles after the treatment. The rhythm of ovarian function, generally characterized by prolonged follicular development, was impaired. During the treatment and subsequent recovery period, integrity of pituitary function was examined by measuring luteinizing hormone (LH) after GnRH agonist was injected, and after stimulation test doses of 150 ng natural GnRH were administered. During treatment there was, with time, a decline in pituitary response to the agonist which suggested that pituitary release of LH was exhausted. After the 14-day treatment the stimulation test with GnRH revealed a gradual return to normal responsiveness although this was not complete three weeks after the treatment when compared to control ewes. This lowered pituitary activity could cause the impaired ovarian function.     

Oestrous cycle dynamics in peri-pubertal and mature Javanese thin-tail sheep

Duration of oestrus, time of ovulation and hormone profiles for progesterone and LH in prepubertal, pubertal and mature Javanese thin-tail sheep were studied at synchronized oestrus following progestagen-PMSG treatment and at the first natural oestrus after synchronization.The ewe lambs responded to progestagen-PMSG treatment by showing earlier onset of oestrus and an earlier and higher peak of LH concentration than mature ewes. For pre-pubertal, pubertal and mature ewes the mean LH peaks were 49.9, 43.9 and 37.9 ng/ml (P>0.05) at mean intervals of 7.5, 8.4 and 16.5 h (P < 0.05), respectively, after onset of oestrus. Duration of oestrus was 41.2 h in pubertal lambs and averaged 37.5 h in the other two groups (P>0.05). Except in one mature ewe, ovulation occurred between 24 and 36 h after onset of oestrus and the majority ovulated at around the end of oestrus. The corpora lutea developed normally, as indicated by plasma-progesterone changes. The patterns of plasma-progesterone changes were similar in all three groups, though the concentrations were lower in the ewe lambs.At the first natural oestrus after synchronization, mature ewes showed longer (P>0.05) oestrus (31.5 vs. 24.3 h), longer time interval from onset of oestrus to the LH peak (16.0 vs. 12.0 h) and from the LH peak to ovulation (21.0 vs. 19.6 h) than peri-pubertal lambs. Six of eight pre-pubertal lambs did not ovulate at their first natural oestrus, resulting in a conception rate of 11% for that group, while in pubertal lambs and mature ewes conception rates were 70% and 100%, respectively.     

Plasma hormone levels and reproductive behaviour in anoestrous ewes after treatment with progesterone and PMSG

Plasma progesterone and gonadotrophin levels were studied in anoestrous ewes treated during June or July with a subcutaneous progesterone implant and/or an injection of oestradiol or PMSG. Of 32 ewes treated with progesterone during July, 9 showed a gonadotrophin surge after removal of the implant, and 10 ewes showed oestrous behaviour during the following 4 days. Six ewes conceived at this induced oestrous. Progesterone treatment during June was much less effective, with only 2 of 19 treated ewes showing a gonadotrophin surg and oestrous behaviour. Administration of PMSG at the time of implant removal in the June experiment was followed by a gonadotrophin surge and oestrous behaviour in 18 of 19 ewes, and 15 ewes conceived at the induced oestrus. An injection of PMSG, without progesterone pretreatment, stimulated a gonadotrophin surge and ovulation, but did not result in oestrous behaviour. The treatments employed appeared to initiate cyclic ovarian activity in the July experiment, but not in the June experiment.     

Oestradiol-17beta responsiveness, plasma LH profiles, pituitary LH and FSH concentrations in long-term ovariectomised Holstein cows at 24 h, 48 h and 21 days following treatment with an absorbable GnRH agonist implant

Non-lactating OVX Holstein cows (N = 34) were used to investigate the effect of s.c. placement of an absorbable GnRH agonist implant (Ovuplant; deslorelin 2.1mg, Peptech Animal Health, Australia) on the relationship of plasma LH, oestradiol responsiveness and pituitary LH content. On the day of implant insertion (Day 0), one group (OVU-48h; N = 5) received Ovuplant and had blood samples collected at hourly intervals to characterize the LH response, while a second group (CON-48 h; N = 5) remained untreated and acted as controls. Blood samples were collected every 10 min over 6 h from CON-48 h and OVU-48 h, at 24 h post-implant insertion. These cows were then slaughtered at 48 h post-implant insertion and their pituitaries recovered. Another group received Ovuplant (OVU-21d+E2; N = 10) or were left untreated (CON-21d+E2) and 21 days later were injected i.m. with 0.5 mg 17beta-E2. Blood samples were collected every 10 min for 4 h on the day before E2 injection to characterize LH pulse frequency and amplitude. Beginning 14 h later, blood samples were collected hourly for 12 h to characterize the expected LH surge. These cows were slaughtered and their pituitary glands recovered and assayed for LH and FSH content. Peak plasma LH concentrations (59 +/- 19 ng/ml) were measured after 30 min of Ovuplant insertion. They had returned to pre-treatment levels by 7 h. By 24 h post-implant insertion, OVU-48 h plasma LH profiles were characterized by reduced LH pulse frequency (0.23 +/- 0.09 pulses/h versus 0.75 +/- 0.26 pulses/h; OVU-48 h versus CON-48 h; P < 0.05). The cows that received Ovuplant had lower LH pulse amplitude, LH pulse frequency and mean LH concentrations after 20 days. Injection of 0.5 mg 17beta-E2 induced an LH surge in every one of the control cows with their peak concentrations measured 18 h post injection. No increase in LH was detected in any Ovuplant treated cows. Pituitary FSH content was reduced in Ovuplant treated cows after 48 h, but not that of LH. In conclusion, absorbable deslorelin implants induced a substantial but temporary release of LH, but even 21 days later their LH profiles were characterized by marked suppression of pulsatile LH and an absence of response to E2. These results suggest the implant has prolonged biological activity.     

Pituitary and gonadal function in hypogonadotrophic hypogonadal (hpg) mice bearing hypothalamic implants

GnRH receptor values are 30-50% of normal in pituitaries of hpg male mice, and testicular LH receptors only 8% of normal (160.4 +/- 17.6 and 2013 +/- 208.1 fmol/testis respectively). In male hpg mice bearing fetal preoptic area (POA) hypothalamic implants for 10 days there was no change in pituitary GnRH receptors, pituitary gonadotrophin content, or seminal vesicle weight. However, testicular weights and LH receptors were doubled in 4/10 mice and 2 had increased serum FSH levels. Between 26 and 40 days after implantation pituitary GnRH receptors and pituitary LH increased to normal male levels, although at 40 days serum and pituitary FSH concentrations had reached only 50% of normal values. Testicular and seminal vesicle weights increased more than 10-fold by 40 days after implantation and LH receptors to 70% of normal. In hpg female mice bearing hypothalamic implants for 30-256 days pituitary gonadotrophin concentrations were normal, even though GnRH receptors reached only 60% of normal values (6.18 +/- 0.4 and 9.8 +/- 0.4 fmol/pituitary respectively). Serum FSH was substantially increased from values of less than 30 ng/ml in hpg mice to within the normal female range in hypothalamic implant recipients. Ovarian and uterine weights increased after hypothalamic grafting from only 4-5% to over 74% of normal values. LH receptors increased from 6.5 +/- 1.3 fmol/ovary for hpg mice to 566.9 +/- 39.2 fmol/ovary for implant recipients. Vaginal opening occurred about 23 days after implantation and these animals displayed prolonged periods of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cytoplasmic binding of oestradiol-17beta in several brain regions, pituitary and uterus of ferrets ovariectomized while in or out of oestrus

The reproductive status at the time of ovariectomy had no effect on oestradiol binding. In both groups of ferrets the oestradiol binding capacity of the uterus was approximately 10 times greater than that of the other tissues studied; of these other tissues the highest oestradiol binding capacity was present in the pituitary, followed by hypothalamus, midbrain, amygdala and cerebral cortex. The binding affinity of hypothalamic receptors for oestradiol was the same in ferrets which had been previously ovariectomized while either in oestrus or anoestrus. It is suggested that the occurrence of seasonal oestrus does not depend on changes in the binding capacity or affinity of hypothalamic receptors for oestradiol.     

Alteration of somatostatin but not growth hormone-releasing factor pituitary binding sites in obese Zucker rats.

The present study was designed to determine whether the diminution of growth hormone (GH) secretion that occurs in obese Zucker rats is related to alterations of GH-releasing factor (GRF) or somatostatin (SRIF) pituitary binding sites. Cold saturation studies were performed in pituitary homogenates of 4-month-old lean and obese rats, using [125I-Tyr10]hGRF(1-44)NH2 as radioligand and [127I-Tyr10]hGRF-(1-44)NH2 as competitor, and in pituitary membrane preparations, using [125I-Tyr0, D-Trp8]SRIF14 as radioligand and [127I-Tyr0, D-Trp8]SRIF14 as competitor. In lean rats, analysis of the curves by the Ligand program revealed the presence of two distinct classes of GRF binding sites, the first being of high affinity (0.74 +/- 0.11 nM) and low capacity (118 +/- 31 fmol/mg protein), the second being of lower affinity (880 +/- 240 nM) and higher capacity (140 +/- 35 pmol/mg protein), and of a single class of SRIF binding sites (affinity: 0.40 +/- 0.12 nM; capacity: 24 +/- 6 fmol/mg protein). In obese rats, no difference was observed in GRF binding parameters for both classes of sites, but the concentration of somatostatin binding sites was reduced by 67% when compared to their lean littermates. These findings suggest that the SRIF pituitary receptors are down-regulated in obese Zucker rats and indicate that no alteration of GRF pituitary binding sites contribute to the blunted GH secretion observed in this model of obesity.     

Reproductive responses of early postpartum dairy cattle to continuous treatment with a GnRH agonist (deslorelin) for 28 days to delay the resumption of ovulation

An experiment was conducted to investigate the potential of chronic delivery of a potent GnRH agonist (deslorelin) via subcutaneous implants to delay the resumption of ovulatory cycles in postpartum dairy cattle. Cows received either a single deslorelin implant (n=40; DES) within 7 days of calving or were untreated (n=24; CON). Blood samples were collected thrice weekly during the period the implants were in place. Plasma concentrations of progesterone (P4) and 17beta-oestradiol (E2) were measured along with selected serum metabolites. Implants were removed after 28 days and cattle monitored daily for behavioral oestrus. Serial weekly blood samples were collected to detect the occurrence of ovulation. Cows were artificially inseminated as they were detected in oestrus from 30 days after implant removal. Pregnancy status was subsequently determined by manual palpation of uterine contents at strategic intervals.Insertion of implants induced ovulation in 3/40 cows as determined by a rise in progesterone 7 days later. Deslorelin implants delayed the onset of ovulatory cycles compared with untreated herdmates (mean 43.4+/-4.2 versus 57.3+/-1.6 days postpartum; P<0.001). A noticeable delay of at least 12 days was observed between implant removal and the first animals ovulating. Mean plasma E2 concentrations during the period the implants were in place were similar for DES and CON cows that experienced a prolonged spontaneous postpartum anoestrus (low P4 >60 days), although both groups had concentrations only 20% of CON cows that had ovulated prior to 30 days postpartum.The patterns of recovery following implant removal were highly variable. A number of DES cows showed a low and transient rise in plasma progesterone around 21 days after implant removal. Some cows displayed oestrus but did not appear to form a fully functional corpus luteum with this phenomenon being more prevalent among DES cows (7 of 37 versus 1 of 21; P<0.05). Overall, significantly more DES cows were detected in oestrus without ovulating compared to CON cows. Final pregnancy rates did not differ between DES and CON groups. The mean time to conception for DES cows was longer (21.2+/-5.6 versus 41.1+/-7.4 days, CON versus DES; P<0.01). This difference was not present if the time from first ovulation to conception was compared (50.5+/-5.3 versus 43.5+/-9.3 days, CON versus DES; P>0.05). Deslorelin implants provided a reliable method of inducing anoestrus when treatment was initiated prior to 3 days postpartum. A variable pattern of recovery was observed which delayed conception but did not ultimately reduce the final proportion pregnant at the completion of mating. The study demonstrates the potential of GnRH agonists to control postpartum reproductive function to manipulate the fertility of dairy cows.     

The effect of “Estrumate” followed by progesterone coils on oestrus synchronization and conception of post-partum beef and dairy cows

To synchronize oestrus, a single dose of 0.5 mg of the prostaglandin F_2α analogue cloprosterol (Estrumate) was administered to 200 post-partum beef and dairy cows. Six, 9 or 13 days following the injection of Estrumate a progesterone-releasing intravaginal device (PRID) was inserted for 12 to 16 days. Among the synchronization treatments investigated, the most promising one was to administer 0.5 mg of Estrumate followed 13 days later by a PRID inserted for 12 days. Following the use of this treatment 87% of post-partum suckling beef and dairy cows manifested oestrus within the first 3 days after PRID removal; 80% appeared in heat between approximately 36 and 72 h following PRID removal. Conception rates for untreated controls, cows inseminated once at 55 h, and cows inseminated twice at 48 and 72 h following PRID removal, were 52, 50 and 48% respectively.     

Localization of putative gonadotrophin releasing hormone receptor protein in the anterior pituitary

Specific binding of a fully biologically active 125I-gonadotrophin releasing hormone (GnRH) to isolated anterior pituitary cells is time dependent, saturable and the concentration dependent binding curves exhibit positive cooperativity. Binding to intact or solubilized plasma membranes and an affinity purified GnRH receptor protein reveals in all instances multiple high affinity binding sites. Thus, GnRH receptor protein appears to be an intrinsic constituent of the cell membrane, and perhaps, other membranous organelles. To investigate the latter, the binding of 125I-GnRH to various subcellular fractions was studied and its affinity and time requirements determined. GnRH binding to plasma membranes and secretory granules was to multiple high affinity sites, while that to nuclei and microsomes was to a single high affinity site. Binding was 1.83 +/- 0.07, 0.78 +/- 0.04, 0.31 +/- 0.03 and 0.27 +/- 0.03 fmol micrograms-1 protein for isolated plasma membranes, secretory granules, microsomes and nuclei, respectively, after 30 min incubation with 10(-9) M GnRH. The magnitude of binding to microsomes did not change during the incubation period. It did not show any decrease (p greater than 0.05) in isolated nuclei and plasma membranes, except for the 24 h time period, when a significant drop (p less than 0.001) was seen. Binding to the secretory granule fraction culminated at 15 min and then decreased (p less than 0.001) steadily to a non-detectable level at 24 h. Thus GnRH receptor protein or its portion may be an integral part of some membranous particles in the anterior pituitary cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of oestrous synchronization regimens for lactating dairy cows.

The objective of this experiment was to evaluate various programmes for synchronization of oestrus. The focus of the study was to evaluate rates of detection of oestrus, synchrony of oestrus, pregnancy rate, and effect of ovarian status at initiation of the programmes on rates of detection of oestrus and pregnancy rate. Spring-calving, lactating dairy cows (n = 2009) were allocated at random to one of six treatments: (1) A (n = 335), progestogen (controlled intravaginal drug release; CIDR) inserted per vaginum 10 d before breeding season for 8 d, 10 microg of buserelin at CIDR insertion, PGF2alpha treatment on the day prior to CIDR removal, and AI of cows detected in oestrus within 6 d after CIDR withdrawal; (2) B (n = 330), as in A, plus 1 mg of oestradiol benzoate i.m. 10 h post CIDR withdrawal; (3) C (n = 347), as in A, except buserelin was replaced by 10 mg of oestradiol benzoate; (4) D (n = 335), as in A, plus PGF2alpha and oestradiol benzoate at CIDR insertion; (5) E (n = 332), CIDR containing a 10 mg oestradiol benzoate capsule inserted per vaginum for 12 d; or (6) F (n = 330), as in E, plus PGF2alpha on the day prior to CIDR withdrawal. The oestrous detection rate (number of cows detected in oestrus within 6 days of CIDR withdrawal as a proportion of the number of cows submitted for synchronization of oestrus) and oestrous synchrony (oestrous detection rate within 2 d of CIDR withdrawal), respectively, were greater (P<0.05) following B (95.7% of 330, 98.7% of 316) compared with any of the other programmes for synchronization of oestrus (A: 87.5 of 335, 79.4% of 293; C: 86.7% of 347, 80.0% of 301; D: 90.1% of 335, 89.8% of 302; E: 74.4% of 332, 70.4% of 247; F: 76.4% of 330, 78.5% of 252). The oestrous detection rate was reduced (P<0.05) among cows in metoestrus administered E (64.0% of 50) relative to similar cows administered F (82.8% of 64). Pregnancy rate was greater (P<0.05) following B (57.9% of 330) than A (48.9% of 335, P = 0.06), C (43.2% of 347), E (40.0% of 332), and F (35.1% of 330) but not D (59.3% of 302), when based on those cows presented for oestrous synchronization programmes. In conclusion, 1 mg of oestradiol benzoate administered 10 h post CIDR withdrawal (B) resulted in the best overall oestrous detection, oestrous synchrony, and pregnancy rates, which would be beneficial to a fixed-time AI program.     

Studies on factors affecting release of gonadotrophins during the superfusion of isolated rat pituitaries.

Isolated pituitary glands from adult male rats were maintained in a continuous flow system. Gn-RH (1000 pmol/ml) caused a characteristic release of cyclic AMP, LH, and FSH. Cyclic AMP (1000 nmol/ml) liberated a similar amount of both gonadotrophins. Theophylline (1 mmol/ml) enhanced the effect of cyclic AMP by 21% for LH and 41% for FSH. The infusion of oestradiol (184 pmol/ml) alone or before Gn-RH infusion did not produce a significant effect on the secretion of either gonadotrophin or cyclic AMP. In contrast, there was a significant reduction in the amount of LH (P less than 0-025) and FSH (P less than 0-05) released by pituitaries infused with oestradiol and cyclic AMP.     

Induction of fertile oestrus in seasonally anoestrous ewes with low doses of Gn-RH

Oestrus, conception and lambing performance were assessed in progesterone-primed seasonally anoestrous ewes induced to ovulate with gonadotrophin-releasing hormone (Gn-RH), which was administered intravenously for 48 h as either injections of 250 ng at 2-h intervals (n = 15) or as a continuous infusion at the rate of 125 ng/h (n = 12) or 250 ng/h (n = 12).In $\text{14}\text{15}$ of the ewes injected with Gn-RH, a preovulatory LH peak was recorded at a mean time interval of 33.9 ± 1.8 h after the start of treatment. All ewes displayed oestrus and all ovulated, with a mean ovulation rate of 1.67 ± 0.13. Eleven ewes were diagnosed as pregnant and subsequently lambed. Following infusion of Gn-RH, preovulatory LH peaks were recorded in $\text{21}\text{24}$ ewes at a mean time of 36.1 ± 2.9 h (125 ng/h) and 34.7 ± 2.0 h (250 ng/h). All but two of the ewes displayed oestrus and $\text{23}\text{24}$ ovulated. The group mean ovulation rates of 1.27 ± 0.14 (125 ng/h) and 1.75 ± 0.22 (250 ng/h) were not significantly different. Eleven of the 22 ewes mated were diagnosed as pregnant and produced live lambs.These results suggest that fertility of Gn-RH-induced ovulations in seasonally anoestrous ewes is comparable to that apparent in ewes ovulating spontaneously during the breeding season.