Negative feedback as an obligatory antecedent to the estradiol-induced luteinizing hormone surge in ovariectomized pigs

In ovariectomized pigs, estradiol treatment induces a preovulatory-like luteinizing hormone (LH) surge, but only after serum LH concentrations are suppressed for 48 h. This inhibition of LH release is attributable in large part to inhibition of gonadotropin-releasing hormone (GnRH) release. The present report examines the dependency of the estradiol-induced LH surge on this preceding phase of negative feedback. Ten ovariectomized gilts were given an i.m. injection of estradiol benzoate (10 micrograms/kg BW). Beginning at the time of estradiol treatment, 5 of these gilts received 1-microgram GnRH pulses i.v. every 45 min for 48 h, i.e. during the period of negative feedback. The remaining 5 control gilts received comparable infusions of vehicle. Estradiol induced the characteristic biphasic LH response in control gilts. On the other hand, the inhibitory LH response to estradiol was prevented and the ensuing LH surge was blocked in 4 of the 5 gilts given GnRH pulses during the negative feedback phase. These results indicate that suppressing release of GnRH and/or LH is an important antecedent to full expression of the LH surge in ovariectomized pigs. Assimilation of this observation with the existing literature provides novel insights into the neuroendocrine control of LH secretion in castrated and ovary-intact gilts.     

Luteinizing hormone secretion as affected by hypophyseal stalk transection and estradiol-17beta in ovariectomized gilts

The objectives were to determine hypothalamic regulation of pulsatile luteinizing hormone (LH) secretion in female pigs and the biphasic feedback actions of estradiol-17beta (E(2)-17beta). In the first study, the minimum effective dosage of E(2)-17beta that would induce estrus in ovariectomized gilts was determined to be 20microg/kg body weight. In the second study, ovariectomized gilts were assigned randomly on day 0 to treatments: (a) hypophyseal stalk transection (HST), (b) cranial sham-operated control (SOC), and (c) unoperated control (UOC). On day 3, gilts from each group received a single i.m. injection of either E(2)-17beta (20microg/kg body weight) or sesame oil. Blood was collected from an indwelling jugular cannula at 15min intervals for 3h before (day -2) and after treatment (day 2) from HST, SOC and UOC gilts. On day 3, blood was collected at 2h intervals for 12h after E(2)-17beta or sesame oil injection and at 4h intervals thereafter for 108h. Pulsatile LH secretion in all gilts 2 days after ovariectomy exhibited a frequency of 0.9+/-0.06peaks/h, amplitude of 1.3+/-0.13ng/ml, baseline of 0.8+/-0.07. Serum LH concentrations from SOC and UOC gilts were similar on day 2 and profiles did not differ from those on day -2. In HST gilts pulsatile LH release was abolished and mean LH concentration decreased compared with controls (0 versus 0.9+/-0. 06peaks/h and 0.77+/-0.03 versus 1.07+/-0.07ng/ml, respectively; P<0. 05). E(2)-17beta or sesame oil did not affect serum LH concentration in HST gilts, and LH remained constant throughout 120h (0.7+/-0. 07ng/ml). In SOC and UOC control gilts, E(2)-17beta induced a 60% decrease (P<0.05) in LH concentration within 12h, and LH remained low until 48h, then increased to peak values (P<0.05) by 72h, followed by a gradual decline to 120h. Although pituitary weight decreased 31% in HST gilts compared with controls (228 versus 332mg, P<0.05), an abundance of normal basophils was evident in coronal sections of the adenohypophysis of HST comparable to that seen in control gilts. The third and fourth studies determined that hourly i. v. infusions of LHRH (2microg) and a second injection of E(2)-17beta 48h after the first had no effect on the positive feedback action of estrogen in UOC. However, in HST gilts that received LHRH hourly, the first injection of E(2)-17beta decreased (P<0.05) plasma LH concentrations while the second injection of E(2)-17beta failed to induce a positive response to estrogen. These results indicate that both pulsatile LH secretion and the biphasic feedback action of E(2)-17beta on LH secretion depend on hypothalamic regulatory mechanisms in the gilts. The isolated pituitary of HST gilts is capable of autonomous secretion of LH; E(2)-17beta will elicit direct negative feedback action on the isolated pituitary gland if the gonadotropes are supported by exogenous LHRH, but E(2)-17beta at high concentrations will not induce positive feedback in isolated pituitaries. Thus, the direct effect of E(2)-17beta on the pituitary of monkeys cannot be mimicked in pigs.     

Pre- and Postpubertal LH and Estradiol Pattern in Gilts Subjected to Intermittent Inescapable Electroshock

The effect of intermittent electroshock on LH and es-tradiol secretory pattern and on reaching puberty was studied in 24 prepubertal gilts. Twelve gilts 115-168 days of age received unpredictable and inescapable electroshocks 0-5 times daily between 8 am and 4 pm and 12 gilts served as controls. At an age of 168 ± 0.7 days all gilts were moved, regrouped and exposed to a boar for 30 min. Observa-tions for signs of oestrus were carried out twice daily. Indwelling jugular catheters were inserted into 8 gilts on each treatment after the initial boar contact. Blood samples were collected to determine LH profiles for 4 h every 15 min on day 2 and day 4 after the in-itial boar contact. The remaining 4 gilts on each treatment were catheterized one day prior to the initial boar contact and blood was collected to determine LH profiles the day before initial boar contact and day 1 and day 2 after initial boar contact for 6 h every 15 min. In addition, blood samples were collected and analyzed for LH and estradiol from all gilts daily at 8 am, 12 am and 4 pm for the first 3 days following the initial boar con-tact and thereafter every 4 h until the end of oestrus (diurnal samples). Samples taken daily at noon the first 5 days following initial boar contact were analyzed for Cortisol. The electroshock treatment significantly increased the age at puberty (p=0.04) and tended to decrease the mean LH concentration prior to the preovulatory LH surge (p=0.08) and the maximal concentration of LH during the preovulatory LH surge (p=0.07). The apparent down regulation of the plasma concentration of LH was not as-sociated with increased activity in the hypothalamus-pituitary-adrenal axis in that the basal concentration of Cortisol was not affected by treatment. This indicates that other physiological mechanisms are involved in stress-induced suppression of LH.     

Passive immunization of the pig against gonadotropin releasing hormone during the follicular phase of the estrous cycle

Three experiments were conducted to determine the effects of passively immunizing pigs against gonadotropin releasing hormone (GnRH) during the follicular phase of the estrous cycle. In Experiment 1, sows were given GnRH antibodies at weaning and they lacked estrogen secretion during the five days immediately after weaning and had delayed returns to estrus. In Experiment 2, gilts passively immunized against GnRH on Day 16 or 17 of the estrous cycle (Day 0 = first day of estrus) had lower (P<0.03) concentrations of estradiol-17beta than control gilts, and they did not exhibited estrus at the expected time (Days 18 to 22). When observed three weeks after passive immunization, control gilts had corpora lutea present on their ovaries, whereas GnRH-immunized gilts had follicles and no corpora lutea. The amount of GnRH antiserum given did not alter (P<0.05) serum concentrations of LH or pulsatile release of LH in sows and gilts. In Experiment 3, prepuberal gilts were given 1,000 IU PMSG at 0 h and GnRH antiserum at 72 and 120 h. This treatment lowered the preovulatory surge of LH and FSH, but it did not alter serum estradiol-17beta concentrations, the proportion of pigs exhibiting estrus, or the ovulation rate. These results indicate that passive immunization of pigs against GnRH before initiation of or during the early part of the follicular phase of the estrous cycle retards follicular development, whereas administration of GnRH antibodies during the latter stages of follicular development does not have an affect. Since the concentration of antibodies was not high enough to alter basal or pulsatile LH secretion, the mechanism of action of the GnRH antiserum may involve a direct ovarian action.     

Control of ovulation in farm animals

Examination of hormonal changes occurring in farm species at the onset of puberty, during the follicular phase of the oestrous cycle, and at those times when ovarian activity is re-established after periods of seasonal or lactational anoestrus, provides circumstantial evidence that the final phases of follicular development are dependent on a pattern of tonic (episodic) LH secretion. A suppression of episodic LH secretion is associated with periods of anovulation. Stimulation of tonic LH secretion by repeated injections of small doses of synthetic Gn-RH or purified LH restores normal reproductive function in all but deeply anoestrous animals. Continuous infusion of Gn-RH is as effective as repeated injections. It is suggested that an additional inadequacy, possibly endocrine, contributes to the anovulatory state in deep anoestrus.     

Compensation by pulsatile GnRH infusions for incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and castrated male pigs.

The aim of this study was to investigate incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and male pigs. Gilts (250 days old; n = 36), which had been ovariectomized 30 (OVX 30) or 100 days (OVX 100) before the start of treatment, were challenged i.m. with oestradiol benzoate and were either given no further treatment, fed methallibure to inhibit endogenous GnRH release or fed methallibure and given i.v. pulses of 100 or 200 ng GnRH agonist at 1 h intervals during the LH surge (48-96 h after oestradiol benzoate). The same treatments were applied to long-term orchidectomized male pigs (ORC, n = 23). In addition, one ORC group was not injected with oestradiol benzoate but was fed methallibure and given pulses of 200 ng GnRH agonist. Oestradiol benzoate alone induced an LH surge in the OVX 30 group only (5/6 gilts), methallibure suppressed (P < 0.05) oestradiol benzoate-induced LH secretion, while pulses of 100 ng GnRH agonist in animals fed methallibure produced LH surges in four of six OVX 30 and four of six OVX 100 gilts. The induced LH surges were similar to those produced by oestradiol benzoate alone in OVX 30 gilts. Pulses of 200 ng GnRH agonist produced LH surges in OVX 30 (6/6) and OVX 100 (6/6) gilts and increased the magnitude of the induced LH surge in OVX 100 gilts (P < 0.05 compared with 100 ng GnRH agonist or OVX 30 control). Pulses of 200 ng GnRH agonist also induced LH surge release in ORC male pigs (5/6), but were unable to increase LH concentrations in a surge-like manner in ORC animals that had not been given oestradiol benzoate, indicating that oestradiol increases pituitary responsiveness to GnRH. These results support the hypothesis that oestradiol must inhibit secretion of LH before an LH surge can occur. It is concluded that incompetence for oestradiol-induced LH surges in long-term ovarian secretion-deprived gilts and in male pigs is due to the failure of oestradiol to promote a sufficient increase in the release of GnRH.     

Short-term relationships between plasma LH, FSH and progesterone concentrations in post-partum dairy cows and the effect of Gn-RH injection

Jugular venous blood samples were obtained from 7 dairy cows every 10 min for 10-19 h during the early- or mid-luteal phase of the oestrous cycle, and each cow was given 1 or 2 i.v. injections of 100 micrograms synthetic Gn-RH. Four of these cows were also sampled in a different cycle with no treatment being administered. Peaks of plasma LH, FHS and progesterone were detected in each animal in the absence of treatment; those of LH and progesterone often occurred in parallel. Injection of Gn-RH was always followed by a significant increase in plasma LH and progesterone concentrations and in most cases by a significant FSH increase. There was a significant temporal relationship between the peaks of all 3 hormones. A further 8 cows were sampled during the first 10 days post partum when the mean plasma progesterone concentration was low. An i.v. injection of 200 micrograms synthetic Gn-RH was given to each animal and this resulted in a significant increase in plasma LH and FSH concentrations, but in only one cow was the Gn-RH injection followed by a significant increase in plasma progesterone concentration. The LH response to Gn-RH injection was significantly less in cows injected on or before Day 5 post partum than in cows injected on Days 7-10 post partum.     

Attainment of puberty in female pigs: Influence of boar stimulation

The object of the present study was to investigate whether the presence of a boar is of importance for the age at puberty and the expression of external heat symptoms in female pigs. Altogether 60 crossbred gilts were purchased at an age of 12 weeks and grouped, three or four per pen, in three separate barns (treatment groups A, B and C). Treatment group A: Boars were kept in adjacent pens to the gilts during the whole period. One vasectomized boar was penned with the gilts daily for 20 min during the experimental period, which started at a mean age of 142 days. Treatment group B: Boars were kept in adjacent pens to the gilts during the whole period. Treatment group C: No boars were kept in the barn at any time.The study was performed six times. Daily heat control was carried out during the experimental period, which was terminated when the gilts had passed at least two oestrous cycles. Blood samples for progesterone analysis were taken once a week. Laparoscopy was performed in all gilts after their first observed heat. The gilts were slaughtered after their third or fourth heat. The effects of treatment group were estimated by the method of least squares analysis.All gilts except one showed external heat symptoms at regular intervals during the experimental period. A few gilts did not stand for the boar at each oestrous period, regardless of oestrous number. Gilts belonging to group A showed their first oestrus on average 20 days earlier than gilts in group B and 35 days earlier than gilts in group C. These differences were highly significant (P < 0.001). The stimulatory effect of the boar on the age at puberty varied between the six experimental groups and the interaction between treatment and experimental groups was significant (P < 0.01).     

Gonadotropin and prolactin secretion following intraventricular administration of morphine in gilts

The effects of central nervous system administration of morphine on secretion of luteinizing hormone (LH), follicle-stimulating hormone, and prolactin were investigated in ovariectomized gilts stereotaxically implanted with lateral ventricular cannulas. In Experiment 1, mean serum LH and follicle-stimulating hormone concentrations and serum LH pulse frequency were unaffected by artificial cerebrospinal fluid administration (P greater than 0.1), but decreased (P less than 0.01) in 8 of 11 gilts when 500 micrograms of morphine were given 3 hr later. Serum LH pulse amplitude was unaffected (P greater than 0.1) by cerebrospinal fluid or morphine injection. In Experiment 2, luteinizing hormone concentrations decreased (P less than 0.0001) and prolactin concentrations increased (P less than 0.0001), but follicle-stimulating hormone concentrations did not change (P greater than 0.1) after 500 micrograms of morphine. Gonadotropin responses to 10 micrograms of gonadotropin-releasing hormone, given 2 hr after intraventricular injection, were similar (P greater than 0.1) for morphine- and cerebrospinal fluid-treated gilts. These results indicate that morphine inhibits LH secretion at the level of the central nervous system, and are consistent with the concept that endogenous opioid peptides participate in the regulation of gonadotropin and prolactin release in pigs.     

Evidence that progesterone may influence subsequent luteal function in the ewe by modulating preovulatory follicle development

Ovulation was induced in seasonally anoestrous ewes by repeated 2-h injections of 250 ng Gn-RH, after 12 days (Group 1, N = 7; Group 2, N = 8), 2 days (Group 3, N = 8) or no (Group 4, N = 7) progesterone pretreatment. A preovulatory LH peak occurred spontaneously at a mean (+/- s.e.m.) time of 43.1 +/- 2.0 h, 38.5 +/- 3.1 h and 26.8 +/- 1.7 h after the start of Gn-RH treatment in Groups 1, 3 and 4 respectively, and was artificially induced in ewes in Group 2, after 24 h of treatment, by a single i.v. injection of 150 micrograms Gn-RH. Normal luteal function occurred in all progesterone-pretreated ewes, but in only 1/7 animals not treated with progesterone. These results demonstrate that, although normal luteal function in progesterone-primed ewes induced to ovulate with repeated injections of low doses of Gn-RH is associated with a delayed preovulatory LH peak, it is not this extended period of follicle development which is responsible for functional competence of the resultant corpus luteum. Since as little as 2 days of exposure to elevated plasma progesterone concentrations is effective, it is suggested that progesterone may act directly on the preovulatory follice.     

Serum concentrations of prolactin, oestrogen and LH during the perioestrous period in prepubertal gilts induced to ovulate and mature gilts

The temporal relationships of serum prolactin, oestrogen and LH concentrations during the perioestrous period were compared in prepubertal gilts induced to ovulate by PMSG and hCG and in mature gilts. In Exp. 1, 2 sustained prolactin surges, beginning 4 days and 1 day before the preovulatory LH surge, occurred in all mature gilts. A single preovulatory prolactin surge occurred in 3 prepubertal gilts, starting just before the preovulatory LH surge, but 4 prepubertal gilts had neither a prolactin nor an LH surge. A status (prepubertal or mature) versus time interaction (P less than 0.01) was detected for serum prolactin concentrations. A preovulatory oestrogen surge occurred in all gilts but was of lesser magnitude (P less than 0.01) and duration (P less than 0.05) in the prepubertal gilts without prolactin and LH surges compared to mature gilts and of lesser magnitude (P less than 0.01) compared to prepubertal gilts with prolactin and LH surges. The relative timing of the oestrogen surge in prepubertal gilts corresponded with that of mature gilts when adjusted to the LH surge (if present) but was delayed (P less than 0.01) in all prepubertal gilts if standardized to the hCG injection. In Exp. 2, mature gilts were examined to determine whether 2 perioestrous prolactin surges were characteristic of all cycling gilts. Of 9 gilts, 8 exhibited an initial prolactin surge 4-5 days before oestrus and 5/9 gilts exhibited a periovulatory prolactin surge. The presence of 2 perioestrous serum prolactin surges was not a requirement for subsequent pregnancy maintenance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Absence of brain opioid peptide modulation of luteinizing hormone secretion in the prepubertal gilt

Three experiments were conducted to evaluate the role of endogenous opioid peptides (EOP) in modulating luteinizing hormone (LH) secretion in the prepubertal gilt. In Experiment I, 8 prepubertal (P) gilts, 160-170 days of age (puberty = 197 +/- 10 days), received either 1 (n = 2), 3 (n = 3), or 6 (n = 3) mg/kg BW of naloxone (NAL), an opiate antagonist, in saline i.v. Blood was collected by jugular vein cannula every 15 min for 2 h before and 2 h after NAL. All doses of NAL failed to alter serum LH concentrations. In Experiment II, 21 P gilts 160-170 days of age and 21 mature (M) gilts were ovariectomized (OVX). At the time of OVX, gilts were classified as prepubertal if their ovaries were devoid of corpora albicantia and corpora lutea. Three weeks after OVX, P and M gilts were injected twice daily for 10 days with either 0.85 mg/kg BW of progesterone (P4) or oil vehicle (V), resulting in the following groups: PP4 (n = 11), PV (n = 10), MP4 (n = 11), and MV (n = 10). All gilts received 1 mg/kg BW of NAL on the last day of treatment. Blood samples were collected via a jugular cannula every 15 min for 4 h before and 2 h after NAL treatment. NAL treatment resulted in an increase (p less than 0.05) in serum LH concentrations only in the MP4 gilts. In Experiment III, 15 OVX gilts 280 days of age were used. Ten of the 15 gilts were OVX prior to puberty at 160 days of age and were classified as chronologically mature (CM) at the time of treatment. The remaining 5 gilts were OVX after puberty, and were classified as sexually mature (SM) at the time of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Alterations in gonadotropin secretion and ovarian function in prepubertal gilts by elevated environmental temperature

The effect of chronic exposure to elevated environmental temperature on gonadotropin secretion and ovarian function was studied in prepubertal gilts. Gilts were maintained under control (15.6 degrees C) or elevated temperature (33.3 degrees C) conditions from 150 to 180 days of age. Endocrine and ovarian responses to bilateral (BLO), unilateral (ULO), and sham ovariectomy were evaluated between 175 and 180 days of age. During the 96-h sampling period after BLO, plasma concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were suppressed in heat-stressed females. Similarly, elevated temperatures abolished the transient rise in FSH and subsequent follicular growth normally associated with ULO. In contrast, environmental treatment had no effect on the secretion of FSH and LH after sham ovariectomy, yet the number of small follicles was lower in gilts exposed to elevated temperatures than in females maintained under control conditions. These results indicate that a chronic exposure to elevated environmental temperature during pubertal development diminished the ability of the hypothalamo-hypophyseal axis to secrete FSH and LH, which had physiological consequences on follicular growth. When provided an appropriate stimulus (ULO), an acute period of FSH secretion and subsequent development of follicles failed to occur in females exposed to elevated temperatures. Consequently, we propose that delayed puberty in gilts during periods of elevated environmental temperatures is due, in part, to a diminished capacity for gonadotropin secretion.     

Assessment of the sexual and antler potential of the male white-tailed deer (Odocoileus virginianus) by Gn-RH stimulation test

Twelve mature white-tailed bucks were injected with gonadotropin regulating hormone (Gn-RH, 100 micrograms/deer) during the rut (November) and during the spring (April). In the rut, superior bucks (with actual or potential large body weight, trophy antlers and a high social rank) responded to Gn-RH with a small increase of LH (below 20 micrograms/ml) and a profound rise in testosterone (T) (30-50 ng/ml). The inferior animals exhibited high increase of LH (30-40 ng/ml) but a low rise in T (below 10 ng/ml). FSH levels increased only slightly after Gn-RH and the concentrations were not related to reproductive performance. During the spring, increase in LH levels after Gn-RH administration greatly exceeded the rise of T, but no relationship was found between hormonal levels and the reproductive potential. FSH levels increased remarkably after Gn-RH administration. Gn-RH (administered during the rut) might be used for assessment of the potential for reproductive and antler performance.     

Time of the preovulatory LH surge in the gilt and sow relative to the onset of behavioral estrus

Two experiments were conducted to study the time of occurrence of the preovulatory LH surge in pigs. Sampling every ten minutes in six cycling gilts before and after onset of standing estrus revealed the preovulatory surge began from 8 hr before to 12 hr after the lordosis reflex was elicited. Three of six gilts initiated the preovulatory LH release coincident with the onset of estrus. Data from 28 postpartum sows, with samples drawn every six hours commencing with the onset of estrus, indicated maximum LH levels were present at the first observance of estrus. Six of the 28 sows had an LH peak 18-24 hr after the onset of estrus.     

Induction of ovulation in seasonally anoestrous ewes by continuous infusion of low doses of Gn-RH

Two groups of 12 seasonally anoestrous ewes were infused with Gn-RH at the rate of 125 or 250 ng/h for 48 h. Four control ewes were infused with the saline vehicle alone. Mean LH concentrations increased significantly in response to Gn-RH infusion and were significantly higher (P less than 0.05) in ewes receiving 250 ng Gn-RH/h. LH concentrations remained unchanged in the control ewes. Oestrus was detected in 22/24 Gn-RH-treated ewes and occurred at a mean time of 37.0 +/- 1.2 h after the start of infusion. Ovulation occurred in all but one of the 24 Gn-RH-treated ewes with mean ovulation rates of 1.27 +/- 0.14 (125 ng-Gn-RH/h) and 1.75 +/- 0.22 (250 ng Gn-RH/h). These results demonstrate that a sustained elevation in mean circulating concentrations of LH induced by continuous administration of Gn-RH is sufficient to invoke the final phases of follicular development, and thereby ovulation, in the seasonally anoestrous ewe.     

Attainment of puberty in female pigs: Clinical appearance and patterns of progesterone,oestradiol-17β and LH

The object of the study was to investigate the clinical and endocrine patterns of progesterone, oestradiol-17β and LH during the peripubertal period in female pigs. Crossbred gilts were penned in groups at an age of 10–12 weeks and boars were kept in adjacent pens during the entire experimental period. Daily oestrous checks started at 4.5 months of age and the gilts were slaughtered after their third heat. At the age of 4.5–5 months a permanent catheter was inserted in the cephalic vein and blood samples were collected from the gilts once daily until either the first or second oestrus. In three gilts hourly blood samples were taken during their first and second oestrus, beginning at early pro-oestrus.The gilts showed their first oestrus at the average age of 183 days. No corpora lutea from earlier ovulations were observed in gilts laparoscoped after their first detected oestrus. During the 30-day period before first oestrus the mean daily progesterone levels varied between 32 and 329 pmol/l. The average levels of oestradiol-17β varied between 15.6 and 30.8 pmol/l. There was no tendency for the oestradiol-17β level to rise before onset of first pro-oestrus. The average levels of LH varied between 0.15 and 0.94 μg/l. The statistical analyses revealed no significant relationship between the level of the hormones studied and onset of first oestrus. The mean progesterone levels during the first and second oestrous cycles were almost identical, however. Oestradiol-17β increased gradually during pro-oestrus, reaching maximum levels before onset of oestrus and thereafter decreasing sharply to values around 30 pmol/l. The oestradiol-17β levels were higher at the second than at the first pro-oestrous period. The concentrations of plasma LH rose sharply with declining plasma levels of oestradiol-17β. The duration of elevated plasma LH levels (> 1 μg/l) was, on average, 26 h and the LH levels were higher during the first oestrus than during the second oestrus. The first rise in progesterone was observed 11–29 h after the LH levels had decreased to concentrations below 1 μg/l.     

Identification of the gut microbiota biomarkers associated with heat cycle and failure to enter oestrus in gilts

Failed puberty is one of the main reasons for eliminating gilts from production herds. This is often caused by disorders of sex hormones. An increasing number of studies have suggested that the gut microbiota may regulate sex hormones and vice versa. Whether the gut microbiota is involved in the failure of oestrus in gilts remains unknown. We used 16S rRNA gene sequencing, network-based microbiota analysis and prediction of functional capacity from 16S rRNA gene sequences to explore the shifts in the gut microbiota throughout a heat cycle in 22 eight-month-old gilts. We found that a module of co-occurrence networks composed of Sphaerochaeta and Treponema, co-occurred with oestrus during a heat cycle. The mcode score of this module reflecting the stability and importance in the network achieved the highest value at the oestrus stage. We then identified bacterial biosignatures associated with the failure to show puberty in 163 gilts. Prevotella, Treponema, Faecalibacterium, Oribacterium, Succinivibrio and Anaerovibrio were enriched in gilts showing normal heat cycles, while Lachnospiraceae, Ruminococcus, Coprococcus and Oscillospira had higher abundance in gilts failing to show puberty. Prediction of functional capacity of the gut microbiome identified a lesser abundance of the pathway ‘retinol metabolism’ in gilts that failed to undergo puberty. This pathway was also significantly associated with those bacterial taxa involved in failed puberty identified in this study (P < 0.05). This result suggests that the changed gut bacteria might result in a disorder of retinol metabolism, and this may be an explanation for the failure to enter oestrus.