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PubMed1976

[Results of vaginal cytology studies and examination of the arborization phenomenon of the vaginal mucus for the determination of estrus cycle activity in sheep].

Vaginal smears were taken by means of a metal spatula from the dorsal part of the vaginal vestibule of sexually mature sheep with intach genitals for the purpose of following up changes in the vaginal cell structure. The smears were taken daily over the whole testing period of 35 days. The cells were defferentiated by means of the dyeing method according to Papanicolaou as well as by using Shorr's staining which had been modified according to Pundel. Differentiation was made between occurrences ob basal, parabasal, intermediary, and superficial cells, with the view to figuring out the cycle phases. That vaginal smear method, when used in two or three folloow-up examinations on the basis of random checks, has proved to enable determination of cyclic activity of a complete herd. This is of great importance to heart synchronization or heart induction, since all results so far have shown that all success depended strongly on the given degree of cyclic activity of the sheep concerned.

Animals↗
PubMed1985

Relationships among dietary lipid intake, serum cholesterol and ovarian function in Holstein heifers.

Experiments were performed to determine whether serum concentrations of total cholesterol (TC) undergo cyclic changes during the estrous cycle of dairy heifers and to assess the relationship between serum concentrations of TC and ovarian steroid hormones. The effects of a hypercholesterolemic diet upon luteal progesterone secretion also were determined. Experiment 1 involved five dairy heifers exhibiting normal estrous cycles. Serum concentrations of TC, progesterone, testosterone and estradiol-17 beta were determined in blood samples collected throughout a complete estrous cycle. A transient decline in TC was observed during the luteal phase of all heifers beginning on d 2 and reached a nadir 6 d after estrus. Highest mean concentrations of TC occurred between d -2 and +2 (96.3 +/- 8.2 mg/dl), which were markedly higher (P less than .05) than the lowest mean concentrations (76.3 +/- 10.3 mg/dl) observed on d 6. Concentrations of serum TC were negatively correlated (r = -.40; P less than .01) with progesterone between d 2 and 9. Serum TC was not correlated with testosterone or estradiol-17 beta. In Exp. 2, seven cycling Holstein heifers were fed a control diet for 70 d (Stage I), a diet containing 15% whole sunflower seed as a source of supplemental dietary lipid for 70 d (Stage II) and then the control diet for 70 d (Stage III). Diets were isocaloric and isonitrogenous. All heifers were synchronized with prostaglandin F2 alpha after wk 5 in each of the three feeding stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

Long-term suppression of reproductive function by a single dose of gonadotropin-releasing hormone antagonists in a sheep model.

OBJECTIVE: To determine the effect of single long-acting doses of GnRH antagonists on reproductive function in a sheep model. DESIGN: Observational, model study. SETTING: University-affiliated research unit. ANIMAL(S): Nine intact mature Merino sheep in experiment 1 and 12 mature Merino-crossed ewes with the ovary autotransplanted to the neck in experiment 2. INTERVENTION(S): Synchronization of estrous cycle either with intravaginal progestins or prostaglandin F2alpha analogues and treatment with a single dose of GnRH antagonist; evaluation of reproductive activity, plasma sampling, and ovarian ultrasonography. MAIN OUTCOME MEASURE(S): Determination of estrus behavior; plasma concentrations of P, FSH, LH, and inhibin A; and number and size of ovarian follicles. RESULT(S): In both experiments, the concentrations of FSH and LH were suppressed when compared with those in control ewes. In experiment 1, the ovulatory cycles were suppressed for > or = 55 days in treated sheep. In experiment 2, there were no follicles sized > or = 5 mm in treated ewes for 50 days. CONCLUSION(S): The suppression of the development of large follicles for > or = 30 days after a single injection of a long-acting GnRH antagonist provides a novel convenient method of pretreatment before COS.

Animals↗
PubMed2007

Prevalence and risk factors for postpartum anovulatory condition in dairy cows.

The objectives of this research were to determine the prevalence of the anovulatory condition within a temperate region of North America and identify cow-level and herd-level risk factors for this condition. A total of 1,341 cows from 18 herds were classified as cycling or anovular based on skim milk progesterone concentration determined at 46 and 60 +/- 7 d in milk. Calving history, periparturient disease incidence, body condition score, milk ketone concentration in the first 2 wk of lactation, and first 305-d mature-equivalent milk projections were recorded. Reproductive and culling information was retrieved monthly from the Dairy Herd Improvement Association. The cow-level prevalence of anovulation was 19.5%, with a herd-specific range from 5 to 45%. Accounting for the effect of clustering at the herd level, cows experiencing a difficult calving, cows with twin calvings, displaced abomasum, and cows with subclinical ketosis in the first week after calving were at greater risk for diagnosis of anovulation. Anovular cows within herds using ovulation synchronization programs were inseminated at the same time postpartum with a 6-percentage point reduction in the probability of pregnancy relative to cycling herdmates (29.7 vs. 35.9%, respectively), whereas anovular cows in herds breeding based on observed estrus were inseminated 8 d later and suffered a 10-percentage point reduction in the probability of pregnancy at first insemination (20.3 vs. 30.5). Time to pregnancy was delayed in anovular cows by 30 d (156 vs. 126 d). Using survival analysis, the impact of anovulation decreased with time. The daily probability of pregnancy (hazard ratio) was similar to cycling cows by 165 d in milk. The results underline the important associations of peripartum health with reproductive function and performance.

Animals↗
PubMed1998

Scanning electron microscopic and morphometric analysis of the guinea pig uterine luminal surface: cyclic and ovarian steroid-induced modifications.

A morphometric and scanning electron microscopic analysis of endometrial, luminal topographical changes of cyclic and ovariectomized, ovarian steroid-treated guinea pigs was undertaken to determine the effects of cyclic endocrine influences on the uterine surface features recognized to be involved in the process of interstitial blastocyst implantation. Estrogenic stimulation of the uterine epithelial layer induced microvillus growth and closed the endometrial gland openings on the luminal surface. Progestins antagonized these parameters, as indicated by short, sparse microvillus formation over a flat luminal uterine surface on which gland openings were both numerous and prominent. The combined effects of estrogen and progesterone therapies induced luminal epithelial changes resembling those characterizing the cyclic uterine surface at the time of blastocyst implantation. These data indicate that the cyclic alterations in the uterine luminal surface are directly regulated by ovarian steroid hormones, and that these changes in surface structure are synchronized and directly involved in the preparation for blastocyst-uterine interaction during the nidatory process in this species.

Animals↗
PubMed1989

Induction of fertile oestrus in anoestrous dogs by constant infusion of GnRH agonist.

The continuous, subcutaneous administration of the GnRH superagonist lutrelin for 14 days was evaluated as a means to induce oestrus in 24 anoestrous beagle bitches, beginning 90-146 days after the previous oestrus. The recent occurrence of pregnancy and lactation (N = 12) vs a non-pregnant cycle (N = 12) did not appear to affect the incidence of pro-oestrus (10/12 vs 11/12 bitches), or oestrus (8/12 vs 11/12 bitches), the timing of pro-oestrus (5 +/- 0.4 days) or oestrus (13 +/- 0.3 days) or the incidence of ovulation (8/12 vs 10/12 bitches), but did reduce the number of ovulating bitches that gave birth to litters after mating the induced oestrus (3/8 vs 6/10). The mean magnitude of the endogenous preovulatory or preovulatory-like LH surges at Day 8-15 varied (P less than 0.5) among the 9 bitches that became pregnant (4.7 +/- 0.9 ng/ml), the 9 bitches that ovulated but did not become pregnant (2.7 +/- 0.6 ng/ml), and the 6 bitches that failed to ovulate in response to treatment (1.9 +/- 0.3 ng/ml). Overall, mean LH surges (3.3 +/- 0.5 ng/ml) were lower than those in 12 contemporary spontaneous control cycles (16 +/- 3 ng/ml) and may have been compromised by pituitary down-regulation caused by continued agonist administration. The latter was suggested in ovariectomized bitches treated similarly with agonist by the suppression, within 10 days of start of treatment, of LH from 35 +/- 6 to 3 +/- 0.5 ng/ml. The 50% success rate obtained in early anoestrous bitches after non-pregnant cycles suggests that constant infusion of GnRH agonist might be useful for the induction or synchronization of fertile oestrus in dogs, particularly if initiated later in anoestrus.

Animals↗
PubMed1996

Effect of interval from melengestrol acetate to prostaglandin F2 alpha on timed and synchronized pregnancy rates of beef heifers and cows.

The objective of this experiment was to determine the optimal interval from the last day of melengestrol acetate (MGA) feeding to prostaglandin F2 alpha (PGF2 alpha) treatment on pregnancy rates of beef heifers and cows. All females (149 heifers and 399 postpartum cows) were fed MGA (.5 mg) daily for 14 d and then administered PGF2 alpha (25 mg Lutalyse) 13, 15, and 17 d (Groups 1, 2, and 3, respectively) after the last day of MGA feeding. Females not in estrus the first 52 h after PGF2 alpha treatment were artificially inseminated 72 h after PGF2 alpha treatment. Females in estrus 0 to 52 h and 78 h to 6 d after PGF2 alpha treatment were inseminated at estrus. Blood sera (collected immediately before and 3 d after PGF2 alpha treatment) were assayed for progesterone concentrations. Pregnancy was determined 44 to 47 d after the 72-h AI by rectal examination. The intervals from MGA feeding to PGF2 alpha that had the highest 72-h AI pregnancy rates were 17 d for heifers (43%) and 15 d for cows (43%). Heifers with a 17-d interval had a higher (P < .05) 72-h AI pregnancy rate than heifers with 13-d and 15-d intervals, and cows with a 15-d interval had a higher (P < .05) 72-h AI pregnancy rate than cows with a 17-d interval. The 4-d synchronized pregnancy rates (the 72-h AI and the succeeding 3 d inseminations) for both heifers (44%) and cows (53%) were not different (P > .05) among groups. Fewer (P < .05) cows with a 17-d interval from MGA to PGF2 alpha had corpora lutea regression (by 72 h) after PGF2 alpha treatment than cows with 13-d and 15-d intervals. We interpret the results to indicate that the interval from MGA to PGF2 alpha treatment may influence 72-h AI pregnancy rates, that optimal intervals are 17 d for heifers and 15 d for cows, and that pregnancy rates are improved by insemination for 3 d after the 72-h AI.

Animals↗
PubMed1988

Pineal and ovarian response to 22- and 24-h days in the ewe.

Melatonin secretion in ewes was entrained by 22-h light-dark cycles whether of long (16L:6D) or short (6L:16D) photoperiod. In photoperiods of 6L:16D, a phase-delay of melatonin secretion was evident, leading to a dark-phase duration shorter than that found in 8L:16D. Early onset of estrus was induced in anestrous ewes kept in 8L:16D, but not 6L:16D, from 22 July compared to controls in natural light. In photoperiods of 16L:6D, the melatonin profile corresponded precisely to the dark phase. Early offset of estrus was induced in estrous ewes kept in both 18L:6D and 16L:6D from 18 December compared to controls in natural light. Thus, when the duration of melatonin secretion was appropriate to the long photoperiod (16L:6D), but with a constantly changing phase position, a long-day reproductive response was found. Activity-rest cycles were not entrained by 16L:6D; thus the synchronization of melatonin and activity-rest cycles does not appear to be essential for the induction of a long-day reproductive response. These results support the hypothesis that the duration, not the circadian-phase position, of melatonin is critical to the induction of photoperiodic effects.

Animals↗
PubMed1996

Changes in morphological appearance and functional capacity of recruited follicles in cows treated with FSH in the presence or absence of a dominant follicle.

This study was designed to determine the effect of the presence of a dominant follicle at the beginning of FSH stimulation on the morphological appearance and functional capacity of recruited follicles during FSH stimulation in cattle. Synchronized nonlactating dairy cows were assigned to 1 of 2 groups and treated with FSH in the presence (n = 5) or absence (n = 6) of a dominant follicle between Days 7 and 12 of the estrous cycle (Day 0 = estrus) to stimulate follicular growth. Dominant follicles were identified by daily ultrasonographic observations, beginning on Day 3 of the estrous cycle. Dominant follicle had an ultrasonographic diameter > or = 10 mm and were in a growing phase, or maintaining a constant diameter (> or = 10 mm) for less than 4 d. Ovaries were collected at slaughter on the morning of the third day following initiation of the FSH stimulation. All follicles > 2 mm were dissected, classified according to diameter (Class 1: 2 to 4.4 mm; Class 2: 4.5 to 7.9 mm; Class 3: > 8 mm), and incubated individually for 90 min in medium M-199 (37 degrees C, 5% CO2). Following incubation, integrity of each follicle was evaluated histologically to assess the level of atresia and biochemically to determine the in vitro release of estradiol (E2) and androstenedione in culture media. On Day 3 of the FSH treatment, mean number of follicles in each class was similar (P > 0.1) between the 2 groups. The percentage of atretic follicles in Classes 1 and 3 on Day 3 of the FSH stimulation did not differ (P > 0.1) between the 2 groups. However, the percentage of atretic follicles in Class 2 was higher (P < 0.005) in cows treated with FSH in presence than in absence of a dominant follicle (60.8 vs 38.2%). The release of E2 in culture media by small Class 1 atretic or healthy follicles, by Class 2 atretic and by Class 3 healthy follicles was not affected (P > 0.1) by the ovarian status. However (P < 0.001), the release of E2 in culture media of Class 2 healthy and Class 3 atretic follicles was less for follicles harvested from cows bearing than from those not bearing a dominant follicle. Within each follicular class, concentrations of androstenedione in the culture media did not differ between the 2 groups (P > 0.1). These results suggest that the presence of a dominant follicle at the beginning of FSH stimulation alters the population of follicles recruited FSH stimulation. This may be associated with the reported decrease of the superovulatory response in cows superovulated in presence of a dominant follicle.

Journal Article↗
PubMed2002

Progesterone profiles of fat-tailed tuj ewes following tactile and visual separation of rams at the beginning of the breeding season.

The aim of this study was to determine the level and duration of progesterone secretion during the sequential oestrous cycles in fat-tailed Tuj ewes following tactile and visual separation of rams during the breeding season. For these purposes, rams were separated from the main flock for 50 days starting from the beginning of the breeding season and 21 ewes were randomly selected for the current experiment. In order to assess luteal activity and the length of oestrous cycles, the ewes were blood-sampled thrice or twice weekly for the measurement of progesterone in plasma. The data showed that 3 +/- 0.2 oestrous cycles were observed in this time window, and the first oestrous cycle observed was shorter (P < 0.05) than the following cycles (mean +/- SEM, 14.6 +/- 0.82, 16.5 +/- 0.48 and 17.0 +/- 0.54 days, respectively, for cycles 1, 2 and 3). Progesterone production was significantly lower in the first oestrous cycle compared with the second and third cycles on days 9, 12 and 14 of the cycles. The follicular periods of ewes showed four accumulations (maximum values using a 3-day moving average technique) throughout the study and the percentages of ewes at each accumulation (peak day +/- 1 day) were 50, 35, 65 and 80% for the first, second, third and fourth accumulations, respectively (P < 0.05). It was concluded that progesterone production was lower and the oestrous cycles were shorter during the first oestrous cycle and that tactile and visual separation of rams at the beginning of the breeding season might enhance the synchronizing effect towards the end of the breeding season in the fat-tailed Tuj ewes.

Animals↗
PubMed1988

Timing the breeding season of the ewe: what is the role of daylength?

Photoperiod is the proximate cue which times the reproductive period of the sheep and the role of daylength as a "zeitgeber" is evaluated in this report. Although sheep are regarded as a short-day breeding species, the reproductive period of the Suffolk ewe begins on daylengths which are on average 2.5 hours longer than those on which it ends. The reasons for this apparent anomaly are considered. Experimental evidence is presented which demonstrates that the transitions between the breeding season and anoestrus are not actively generated by changing ambient photoperiod. Rather the ewe expresses an endogenous circannual rhythm of reproduction and the role of daylength is to entrain this reproductive rhythm to a period of 365 days. It appears that only portions of the annual cycle of photoperiod are necessary to synchronize the breeding season, however, additional photic cues may be required to produce a reproductive period of normal duration.

Animals↗
PubMed2002

Use of estradiol cypionate in a presynchronized timed artificial insemination program for lactating dairy cattle.

Experiment 1 evaluated pregnancy rates when estradiol cypionate (ECP) was used to induce ovulation as part of a timed artificial insemination (TAI) protocol in comparison to Ovsynch for lactating dairy cows in Florida (n = 371) and Texas (n = 321). Cows were presynchronized with two injections of PGF2, (25 mg, im) given 14 d apart with TAI protocols beginning 14 d after the second injection of PGF20. The TAI protocols consisted of an injection of GnRH (100 microg, im) followed by PGF2alpha 7 d later. Then, cows either received an injection of GnRH (Treatment I, Ovsynch) at 48 h after PGF2alpha and inseminated 16 to 24 h later or received an injection of ECP (1 mg, i.m.) at 24 h after PGF2alpha, (Treatment II; Heatsynch) and inseminated 48 h later. In Florida, pregnancy rates after TAI were 37.1 +/- 5.8% for Ovsynch compared with 35.1 +/- 5.0% for Heatsynch. In Texas, pregnancy rates were 28.2 +/- 3.6% for Ovsynch and 29.0 +/- 3.5% for Heatsynch. Overall pregnancy rates did not differ between Ovsynch and Heatsynch treatments. In Experiment 2, estrus and ovulation times were determined in lactating dairy cows submitted to the Heatsynch protocol. Frequencies of detected estrus and ovulation after ECP were 75.7% (28/37) and 86.5% (32/37), respectively. Mean intervals to ovulation were 55.4 +/- 2.7 h (n = 32) after ECP and 27.5 +/- 1.1 h (n = 27) after onset of estrus. Estrus occurred at 29.0 +/- 1.8 h (n = 28) after ECP. It is recommended that any cow detected in estrus by 24 h after ECP injection be inseminated at 24 h and all remaining cows be inseminated at 48 h because 75% (n = 24/32) of the ovulations occurred between > or = 48 h to < or = 72 h after ECP. Synchronization of ovulation and subsequent fertility indicated that estradiol cypionate could be used to induce ovulation for successful timed insemination.

Animals↗
PubMed1992

Effects of parity and synchronization of donors by an oral progestogen on embryo transfer in swine.

The aim of this study was to establish whether the quantity or quality of embryos collected is affected by 1) the reproductive status of donors (nulliparous gilts vs parous sows); 2) pretreatment of donors with Oxolven. Embryos were collected from gilts (n=38) and from weaned sows (n=35). Approximately half of each group (gilts and sows) had been subjected to oral treatment with the progestogenic 19-nortestosterone derivative Oxolven for 14 to 21 days. After induction of estrus with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) and following artificial insemination, the embryos were recovered surgically. The ovulation rate of donors averaged 21.6, with no significant difference among groups. Most of the ova and embryos collected were at the 4-cell stage (53%). There was a high incidence of uncleaved ova (28%) in both groups of animals. This was particularly so in gilts, both the Oxolven-treated and controls (35 and 32%). The incidence of uncleaved ova was lower, however, in Oxolven-treated (29%) and control sows (14%). Embryos at the 4- to 8-cell stage were transferred to synchronous recipient gilts. Transfer results, expressed by the survival of transferred embryos, were not significantly affected by the progestogen treatment (30% for Oxolven treatment vs 34% for the controls) or by the reproductive status of the donors (33% for gilts vs 28% for sows).

Journal Article↗
PubMed1987

Economic evaluation of fourteen methods of estrous detection.

Costs of 14 methods to detect estrus were compared under simulated commercial dairy situations using artificial insemination. Alternative methods were twice or thrice daily visual observation and factorial combinations with various estrous detection aids [teaser bull, heat mount detector, and tail paint (also in combination with visual observation only during routine chores), estrous synchronization by prostaglandin F2 alpha, estrous synchronization plus heat mount detector]. A sensitivity analysis was used to compare tail paint and heat mount detector methods. Total costs for labor and other inputs were estimated for alternative 120-cow dairy herds with cumulative conception rate greater than or equal to 90% by considering five labor wage rates, four first-service conception rates, and three detection rates for each estrous detection method. Heat mount detector and tail paint methods with visual observation only during routine dairy chores always had lowest cost when average wage rate exceeded @2.25/h. To make heat mount detectors competitive, corresponding detection rate would have to be at least 10% higher than for the tail paint method. Unaided visual observation with two checks/d was less expensive than heat mount detector methods when labor had few alternative demands and cost less than $2.25/h. These methods were generally less than half as costly as teaser bull and prostaglandin alternatives.

Animal Husbandry↗
PubMed1990

Physiological mechanisms controlling anestrus and infertility in postpartum beef cattle.

Postpartum infertility is caused by four factors: general infertility, lack of uterine involution, short estrous cycles and anestrus. The general infertility component is common to any estrous cycle and reduces potential fertility by 20 to 30%. Incomplete uterine involution prevents fertilization during the first 20 d after calving but is not related to anestrus. Short estrous cycles prevent fertility during the first 40 d after calving by causing the cow to return to estrus before pregnancy recognition occurs. Anestrus is the major component of postpartum infertility and is affected by several minor factors: season, breed, parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy as well as two major factors: suckling and nutrition. These major factors have direct effects on anestrus but also interact with one or more other factors to control postpartum anestrus. Physiological mechanisms associated with anestrus involve blockage of the GnRH "pulse generator" in the hypothalamus, but other pathways also must be involved because bypassing the pulse generator is not an effective treatment for all cows. The primary cause of anestrus probably is different for different stages of anestrus. The mediating mechanisms for anestrus are not involved with prolactin, oxytocin, the adrenal or direct neural input from the mammary gland but are at least partially involved with blood glucose and the endogenous opioid peptide system. Management options to decrease the impact of anestrus and infertility include: 1) restrict breeding season to less than or equal to 45 d; 2) manage nutrition so body condition score is 5 to 7 before calving; 3) minimize effects of dystocia and stimulate estrous activity with a sterile bull and estrous synchronization; and 4) judicious use of complete, partial or short-term weaning.

Anestrus↗
PubMed1983

Bovine embryo morphology and evaluation.

The following paper briefly reviews the morphology of the bovine embryo and presents a retrospective analysis of bovine embryo transfer results accumulated from April to December of 1982 at a commercial embryo transfer center. Of particular interests were bovine embryo morphology, assessment of embryo quality, and recipient-donor, recipient-embryo synchrony requirements. Embryos were recovered from superovulated donors five to nine days after estrus (estrus = day O). All embryos were individually examined at 200X for cell stage of development and embryo quality. Embryos were nonsurgically transferred to recipients that were within two days of estrous cycle synchrony with the donor. Attempts were made to synchronize estimated developmental age of embryos to the day of the recipient cycle. A high degree of variability was observed in morphological development and embryo quality within and among donors. Embryo recovery in individual donors resulted in a wide range of embryonic cell stages, often differing in estimated developmental ages from 24 to 48 hours. A total of 783 embryos were transferred, resulting in 308 pregnancies. Stage of embryonic development (16-cell through hatched blastocyst) had little effect on pregnancy rates. Embryo quality was a more accurate predictor of success. Embryos of excellent, good, fair and poor categories resulted in 45%, 44%, 27% and 20% pregnancy rates, respectively. Recipient-donor estrous cycle synchrony of two days in either direction did not significantly alter pregnancy rates. However, 88% of 258 pregnancies (584 total transfers) occurred with a +/-1 day recipient-embryo synchrony compared to 74% based on +/-1 day recipient-donor cycle synchrony (P<0.001). Results suggest that transfer of bovine embryos based on synchrony between day of recipient cycle and state of embryonic development provides higher pregnancy rates than transfers based on recipient-donor cycle synchrony.

Journal Article↗
PubMed1993

Secretory patterns and rates of gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone revealed by intensive sampling of pituitary venous blood in the luteal phase mare.

We used our unique nonsurgical technique for collecting pituitary venous (pit) blood to study GnRH, FSH, and LH secretion patterns in midluteal phase mares. This method does not perturb endocrine function and allows continuous monitoring of GnRH and gonadotropin (Gn) secretion, determination of the amount of GnRH perfusing gonadotropes, and direct measurements of the amounts of Gn secreted. In a total of 80 h of 5-min sampling in four mares, eight Gn peaks occurred; however, more frequent sampling was needed to define secretory events precisely. Therefore, pit blood was collected continuously and split into 30-sec segments in six mares. To ensure a peak during sampling, the opioid antagonist naloxone was given after 4-6 h of sampling to try to replicate a physiological signal for GnRH release. Naloxone induced Gn peaks in jugular blood that were indistinguishable in amplitude from spontaneous peaks. Intensive sampling of pit blood showed that jugular peaks reflected major episodes of GnRH and Gn secretion lasting 30-55 min, which were similar in profile whether naloxone induced or spontaneous and consisted of a train of three to six peaks of diminishing amplitude. Peaks of GnRH and, less often, Gn also occurred outside major episodes. Despite markedly variable size, GnRH peak maxima were correlated with the amount of LH and FSH secreted in concurrent peaks. Likewise, cross-correlation analyses (n = 960 samples/mare) showed close correspondence between patterns of GnRH and secreted FSH and LH. The delay (+/- SEM) between GnRH and Gn maxima was 0.62 +/- 0.18 min for LH and 0.18 +/- 0.22 min for FSH. The majority of GnRH and Gn peaks were concurrent; however, 34.7% of GnRH peaks occurred without Gn peaks. These peaks had a lower amplitude than those with Gn peaks (P < 0.001). For Gn, secretion (i.e. ratio between pit and jugular concentrations, > 1.5) continued at a low level for 40 +/- 9% (LH) or 64 +/- 14% (FSH) of the time between Cluster-defined peaks during the basal period. We conclude that in the luteal phase 1) the predominant mode of GnRH and Gn secretion is as concurrent, large amplitude, prolonged episodes that appeared to be the summation of a train of peaks; and 2) a GnRH dose-Gn response relationship operates endogenously. This along with the synchronicity of secretion patterns of the three hormones suggest that GnRH is the major secretagogue for both LH and FSH.

Animals↗