Naloxone enhances LH but not FSH release during various phases of the estrous cycle in the ewe

Suffolk x whiteface ewes were infused with 0.5 mg/kg/hr naloxone hydrochloride (NAL) for 6 hrs during the early, mid and late luteal and early follicular phases of the estrous cycle. Basal serum luteinizing hormone (LH) concentration was increased by NAL during each trial in the luteal phase and LH pulse amplitude was proportionately increased by 158%, 164% and 350% during the early luteal, mid luteal and early follicular phases, respectively. The apparent NAL induced increase (92%) in LH pulse amplitude during the late luteal phase was not significant. NAL only affected LH pulse frequency during the early follicular phase, when it was decreased. Mean serum follicle stimulating hormone (FSH) concentration was not affected by NAL. The results of this study indicate that endogenous opioid peptides (EOPs) may partially mediate the suppressive influence of estradiol-17 beta (E2) on LH pulse amplitude and also the stimulatory effect of E2 on LH pulse frequency in the early follicular phase. The data may suggest that NAL enhances the amplitude of pulses of gonadotropin releasing hormone (GnRH) by counteracting E2 inhibitory effects on LH release at the level of the pituitary. Alternately, some component of E2 feedback may be an EOP mediated component at the level of the hypothalamus.     

Changing frequency of pulsatile luteinizing hormone and progesterone secretion during the luteal phase of the menstrual cycle of rhesus monkeys

Experiments were conducted to examine the pulsatile nature of biologically active luteinizing hormone (LH) and progesterone secretion during the luteal phase of the menstrual cycle in rhesus monkeys. As the luteal phase progressed, the pulse frequency of LH release decreased dramatically from a high of one pulse every 90 min during the early luteal phase to a low of one pulse every 7-8 h during the late luteal phase. As the pulse frequency decreased, there was a corresponding increase in pulse amplitude. During the early luteal phase, progesterone secretion was not episodic and there were increments in LH that were not associated with elevations in progesterone. However, during the mid-late luteal phase, progesterone was secreted in a pulsatile fashion. During the midluteal phase (Days 6-7 post-LH surge), 67% of the LH pulses were associated with progesterone pulses, and by the late luteal phase (Days 10-11 post-LH surge), every LH pulse was accompanied by a dramatic and sustained release of progesterone. During the late luteal phase, when the LH profile was characterized by low-frequency, high-amplitude pulses, progesterone levels often rose from less than 1 ng/ml to greater than 9 ng/ml and returned to baseline within a 3-h period. Thus, a single daily progesterone determination is unlikely to be an accurate indicator of luteal function. These results suggest that the changing pattern of mean LH concentrations during the luteal phase occurs as a result of changes in frequency and amplitude of LH release. These changes in the pulsatile pattern of LH secretion appear to have profound effects on secretion of progesterone by the corpus luteum, especially during the mid-late luteal phase when the patterns of LH concentrations are correlated with those of progesterone.     

Effects of an opioid antagonist on pulsatile luteinizing hormone secretion in the ewe vary with changes in steroid negative feedback

In ewes during the breeding season, estradiol (E) and progesterone (P) synergistically regulate pulsatile luteinizing hormone (LH) secretion. E primarily inhibits LH pulse amplitude and P inhibits LH pulse frequency. To determine if endogenous opioid peptides (EOP) mediate these negative feedback effects, we administered the long-acting opioid antagonist WIN 44,441-3 (WIN) to intact ewes during the luteal and follicular phases of the estrous cycle and to ovariectomized ewes treated with no steroids, E, P, or E plus P. Steroid levels were maintained at levels seen during the estrous cycle by Silastic implants placed shortly after surgery. WIN increased LH pulse frequency, but not amplitude, in luteal phase ewes. In contrast, during the follicular phase, LH pulse amplitude was increased by WIN and pulse frequency was unchanged. Neither LH pulse frequency nor pulse amplitude was affected by WIN in long-term ovariectomized ewes untreated with steroids. In contrast, WIN slightly increased LH pulse frequency in short-term ovariectomized ewes. WIN also increased LH pulse frequency in ovariectomized ewes treated with P or E plus P. WIN did not affect pulse frequency but did increase LH pulse amplitude in E-treated ewes. These results support the hypothesis that EOP participate in the negative feedback effects of E and P on pulsatile LH secretion during the breeding season and that the inhibitory effects of EOP may persist for some time after ovariectomy.     

Effects of melatonin on luteinizing hormone secretion in anestrous ewes following dopamine and opiate receptor blockade

In the present investigation we have examined the ability of melatonin to modify the pulsatile LH secretion induced by treatment with a DA antagonist (sulpiride, SULP) or opioid antagonist (naloxone, NAL) in intact mid-anestrous ewes. The experimental design comprised the following treatments-in experiment 1: (1) intracerebroventricular (i.c.v.) infusion of vehicle (control I); (2) pretreatment with SULP (0.6 mg/kg subcutaneously) and then i.c.v. infusion of vehicle (SULP + veh); (3) pretreatment with SULP and then i.c.v. infusion of melatonin (SULP + MLT, 100 microg per 100 microl/h, total 400 microg). In experiment 2: (4) i.c.v. infusion of vehicle (control II); (5) i.c.v. infusion of NAL (NAL-alone, 100 microg per 100 microl/h, total 300 microg); (6) i.c.v. infusion of NAL in combination with MLT (NAL + MLT, 100 microg + 100 microg per 100 microl/h). All infusions were performed during the afternoon hours. Pretreatment with SULP induced a significant (P < 0.01) increase in LH pulse frequency, but not in mean LH concentration, compared with control I. In SULP + MLT-treated animals, the LH concentration was significantly (P < 0.01) higher during MLT infusion, but due to highly increased LH secretion in only one ewe. The significant changes in the SULP + MLT group occurred in LH pulse frequency. A few LH pulses were noted after melatonin administration compared with the number during the infusion (P < 0.05) and after vehicle infusion in the SULP + MLT group (P < 0.05). The i.c.v. infusion of NAL evoked a significant increase in the mean LH concentration (P < 0.001) and amplitude of LH pulses (P < 0.01) compared with these before the infusion. The enhanced secretion of LH was also maintained after i.c.v. infusion of NAL (P < 0.01) with a concomitant decrease in LH pulse frequency (P < 0.05). In NAL + MLT-treated ewes, mean plasma LH concentrations increased significantly during and after the infusion compared with that noted before ( P < 0.001). No difference in the amplitude of LH pulses was found in the NAL + MLT group, but this parameter was significantly higher in ewes during infusion of both drugs than during infusion of the vehicle (P < 0.01). The LH pulse frequency differed significantly (p < 0.05), increasing slightly during NAL + MLT administration and decreasing after the infusion. In conclusion, these results demonstrate that: (1) in mid-anestrous ewes EOPs, besides DA, are involved in the inhibition of the GnRH/LH axis; (2) brief administration of melatonin in long-photoperiod-inhibited ewes suppresses LH pulse frequency after the elimination of the inhibitory DA input, but seems to not affect LH release following opiate receptor blockade.     

Changes in patterns of luteinizing hormone secretion before and after the first ovulation in the postpartum mare

To determine whether luteinizing hormone (LH) secretion during the first estrous cycle postpartum is characterized by pulsatile release, circulating LH concentrations were measured in 8 postpartum mares, 4 of which had been treated with 150 mg progesterone and 10 mg estradiol daily for 20 days after foaling to delay ovulation. Blood samples were collected every 15 min for 8 h on 4 occasions: 3 times during the follicular phase (Days 2-4, 5-7, and 8-11 after either foaling or end of steroid treatment), and once during the luteal phase (Days 5-8 after ovulation). Ovulation occurred in 4 mares 13.2 +/- 0.6 days postpartum and in 3 of 4 mares 12.0 +/- 1.1 days post-treatment. Before ovulation, low-amplitude LH pulses (approximately 1 ng/ml) were observed in 3 mares; such LH pulses occurred irregularly (1-2/8 h) and were unrelated to mean circulating LH levels, which gradually increased from less than 1 ng/ml at foaling or end of steroid treatment to maximum levels (12.3 ng/ml) within 48 h after ovulation. In contrast, 1-3 high-amplitude LH pulses (3.7 +/- 0.7 ng/ml) were observed in 6 of 7 mares during an 8-h period of the luteal phase. The results suggest that in postpartum mares LH release is pulsatile during the luteal phase of the estrous cycle, whereas before ovulation LH pulses cannot be readily identified.     

Influence of naloxone and yohimbine administration on pulsatile LH secretion in luteal phase beef cows

This study examined the effects of two specific neurotransmitter receptor antagonists, naloxone (NAL; mu-opioid) and yohimbine (YOH; alpha(2)-adrenergic), on pulsatile luteinizing hormone (LH) release during the luteal phase (Day 10; Day 0 = estrus) of beef cows. Treatments were saline i.m. (C; n = 4); 1mg/kg NAL i.m. followed 3 h later by two 0.5 mg/kg injections spaced 2.5 h apart (N; n = 4); 0.2 mg/kg YOH i.v. (Y; n = 3); or combined N and Y regimens, with Y preceding N by 30 min (NY; n = 4). Blood samples were collected for 8 h before (Period I) and after (Period II) initiation of treatment. Respiration rates of Y cows were similar to C cows during Period II. However, respiration rates of N and NY animals increased 70% within 30 min of the first NAL injection. Acute LH release was not observed in response to either NAL or YOH. Pulsatile LH secretion was unchanged in N, Y and NY cows during Period II when compared with Period I. In contrast, basal and pulsatile LH secretion was inhibited in C cows during Period II. The inhibition of LH secretion in C animals following NAL indicate that the cows were under stress during Period II. Thus, these data suggest that the inhibition of LH release in stressed animals can be overcome by pharmacologic attenuation of inhibitory (N) or accentuation of stimulatory (Y) signals to LHRH-containing neurons.     

Hypothalamic site of progesterone action on gonadotropin release

Tonic gonadotropin secretion was monitored at 20 min intervals for a total of 9 hours in 3 female volunteers during the mid-luteal phase of an ovulatory cycle. This control period was followed by repeated LH-RH stimulation (12 micrograms LH-RH as i.v. bolus once every hour for another 5 hours). During the control period spontaneous albeit low-frequent pulsatile secretion was observed for LH (a pulse occurring once every 3-8 hours) but not for FSH. While intermittent exogenous LH-RH stimulation was being performed at circhoral LH-RH pulse frequency pulsatile gonadotropin release was established at synchronous episodicity and systemic gonadotropin levels consecutively increased. These data provide indirect evidence that the pituitary gland is not rendered refractory to LH-RH by luteal progesterone secretion but readily responds to LH-RH stimuli even when these simulate a follicular phase LH-RH pulse frequency. Thus, it is concluded that spontaneous pulsatile LH release at low frequency during the luteal phase of the cycle reflects low frequent LH-RH discharges from the hypothalamus. Underlying mechanisms are discussed.     

Different effects of subnormal levels of progesterone on the pulsatile and surge mode secretion of luteinizing hormone in ovariectomized goats

This study tested the hypothesis that endocrinological threshold levels of progesterone that induce negative feedback effects on the pulsatile and surge modes of LH secretion are different. Our approach was to examine the effects of subnormal progesterone concentrations on LH secretion. Long-term ovariectomized Shiba goats that had received implants of silastic capsules containing estradiol were divided into three groups. The high progesterone (high P) group received a subcutaneous implant of a silastic packet (50 x 70 mm) containing progesterone, and the low progesterone (low P) group received a similar implant of a small packet (25 x 40 mm) containing progesterone. The control (non-P) group received no treatment with exogenous progesterone. Blood samples were collected daily throughout the experiment for the analysis of gonadal steroid hormone levels and at 10-min intervals for 8 h on Days 0, 3, and 7 (Day 0: just before progesterone treatment) for analysis of the pulsatile frequency of LH secretion. Then estradiol was infused into the jugular vein of all animals at a rate of 3 microg/h for 16 h on Day 8 to determine whether an LH surge was induced. Blood samples were collected every 2 h from 4 h before the start of the estradiol infusion until 48 h after the start of the infusion. In each group, the mean +/- SEM concentration after progesterone implant treatment was 3.3 +/- 0.1 ng/ml for the high P group, 1.1 +/- 0.1 ng/ml for the low P group, and <0.1 ng/ml for the non-P group, concentrations similar to the luteal levels, subluteal levels, and follicular phase levels of the normal estrous cycle, respectively. The estradiol concentration ranged from 4 to 8 pg/ml after estradiol capsule implants in all groups. The LH pulse frequency was significantly (P < 0.05) suppressed on Day 3 (6.2 +/- 0.5 pulses/8 h) and on Day 7 (2.6 +/- 0.9 pulses/8 h) relative to Day 0 (9.0 +/- 0.5 pulses/8 h) in the high P group. In both the low P and non-P groups, however, the changes of pulsatile frequency of LH were not significantly different, and high pulses (7-9 pulses/8 h) were maintained on each of the 3 days they were tested. An LH surge (peak concentration, 100.3 +/- 11.0 ng/ml) occurred in all goats in the non-P group, whereas there was no surge mode secretion of LH in either the high P or the low P group. The results of this study support our hypothesis that the threshold levels of progesterone that regulate negative feedback action on the LH pulse and the LH surge are different. Low levels of progesterone, around 1 ng/ml, completely suppressed the LH surge but did not affect the pulsatile frequency of LH secretion.     

The effect of the presence and pattern of luteinizing hormone stimulation on ovulatory follicle development in sheep

The aim of this study was to examine the role of LH on the growth of the large preovulatory follicle and its secretion of hormones in sheep. Ewes with ovarian autotransplants were treated with GnRH-antagonist at the time of luteal regression and different LH regimes applied for 60-66 h before administration of an ovulatory stimulus (hCG). In Experiment 1 (N = 24; n = 8), ewes received either no LH or constant or pulsatile infusion of LH at the same dose (1.25 microg/h). In Experiment 2 (N = 12, n = 6), LH was constantly infused at a rate of 1.25 microg or 2.5 microg oLH/h. In Experiment 1, animals receiving either pulsatile or constant LH exhibited increases in estradiol and inhibin A secretion (P < 0.001) and a depression in FSH (P < 0.001) that resembled the normal follicular phase. Similarly in Experiment 2, doubling the dose of LH resulted in a two-fold increase in ovarian estradiol secretion (P < 0.05) but no other changes. All animals receiving LH, regardless of the pattern of stimulation, ovulated and established a normal luteal phase. In contrast, no LH treatment resulted in constant immuno-active LH without pulses, unchanged FSH and inhibin A concentrations (P < 0.05), and basal estradiol secretion (P < 0.001). Morphologically normal large antral follicles were observed in this group and although corpora lutea formed in response to hCG, progesterone profiles were abnormal. In conclusion, these results suggest that LH is an essential requirement for normal ovulatory follicle development and subsequent luteal function and show that a pulsatile mode of LH stimulation is not required by ovulatory follicles.     

Pulsatile secretion of LH during the periovulatory and luteal phases of the oestrous cycle in the Père David's deer hind (Elaphurus davidianus)

Changes in the secretion of LH during the oestrous cycle were studied in 5 tame Père David's deer in which ovulation was synchronized with progesterone implants and prostaglandin injections. Plasma LH concentrations were measured in samples collected at 15-min intervals for a 36-h period, starting 16 h after the removal of the progesterone implants (follicular phase), and for a further 10-h period 10 days after the removal of the progesterone implants (luteal phase). In all animals, there was a preovulatory surge of LH and behavioural oestrus which occurred at a mean time of 59.6 h (+/- 3.25) and 69 h respectively following implant removal. LH pulse frequency was significantly higher during the follicular phase (0.59 +/- 0.03 pulses/h) than the luteal phase (0.24 +/- 0.2 pulses/h), thus confirming in deer findings from research on domesticated ruminants. There were no significant differences between the follicular and luteal phases in mean plasma LH concentrations (0.57 +/- 0.09 and 0.74 +/- 0.13 ng/ml) or mean pulse amplitude (0.99 +/- 0.14 and 1.05 +/- 0.21 ng/ml) for the follicular and luteal phase respectively. The long interval from the removal of progesterone to the onset of the LH surge and the absence of a significant difference in mean LH concentration or pulse amplitude in the follicular and luteal phases resemble published data for cattle but differ from sheep in which there is a short interval from luteal regression to the onset of the surge and a marked increase in LH pulse amplitude during the luteal phase.     

Opioid regulation of luteinizing hormone secretion in the male rat

Current evidence suggests that endogenous opioid peptides (EOPs) tonically inhibit secretion of luteinizing hormone (LH) by modulating the release of gonadotropin-releasing hormone (GnRH). Because of their apparent inhibitory actions, EOPs have been assumed to alter both pulse frequency and amplitude of LH in the rat; and it has been hypothesized that EOP pathways mediate the negative feedback actions of steroids on secretion of GnRH. In order to better delineate the role of EOPs in regulating secretion of LH in the male rat, we assessed the effects of a sustained blockade of opiate receptors by naloxone on pulsatile LH release in four groups: intact male rats, acutely castrated male rats implanted for 20 h with a 30-mm capsule made from Silastic and filled with testosterone, acutely castrated male rats implanted for 20 h with an osmotic minipump dispensing 10 mg morphine/24 h, and male rats castrated approximately 20 h before treatment with naloxone. We hypothesized that if EOPs tonically inhibited pulsatile LH secretion, a sustained blockade of opiate receptors should result in a sustained increase in LH release. We found that treatment with naloxone resulted in an immediate but transient increase in LH levels in intact males compared to controls treated with saline. Even though mean levels of LH increased from 0.15 +/- 0.04 to a high of 0.57 +/- 0.14 ng/ml, no significant difference was observed between the groups in either frequency or amplitude of LH pulses across the 4-h treatment period. The transient increase in LH did result in a 3- to 4-fold elevation in levels of plasma testosterone over baseline. This increase in testosterone appeared to correspond with the waning of the LH response to naloxone. The LH response to naloxone was eliminated in acutely castrated rats implanted with testosterone. Likewise, acutely castrated rats treated with morphine also failed to respond to naloxone with an increase in LH. These observations suggest that chronic morphine and chronic testosterone may act through the same mechanism to modulate secretion of LH, or once shut down, the GnRH pulse-generating system becomes refractory to stimulation by naloxone. In acutely castrated male rats, levels of LH were significantly increased above baseline throughout the period of naloxone treatment; this finding supports the hypothesis that the acute elevation in testosterone acting through mechanism independent of opioid is responsible for the transient response of LH to naloxone in the intact rat.     

Effect of alterations in pulsatile luteinizing hormone release on ovarian follicular atresia and steroid secretion on diestrus 1 in the rat estrous cycle

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.     

Suppression of pulsatile luteinizing hormone secretion by gonadotrophin-releasing hormone antagonist does not affect episodic progesterone secretion or corpus luteum function in ewes.

Progesterone secretion has been observed to be episodic in the late luteal phase of the oestrous cycle of ewes and is apparently independent of luteinizing hormone (LH). This study investigated the effects of suppressing the pulsatile release of LH in the early or late luteal phase on the episodic secretion of progesterone. Six Scottish Blackface ewes were treated i.m. with 1 mg kg-1 body weight of a potent gonadotrophin-releasing hormone (GnRH) antagonist on either day 4 or day 11 of the luteal phase. Six ewes received saline at each time and acted as controls. Serial blood samples were collected at 10 or 15 min intervals between 0 and 8 h, 24 and 32 h, and 48 and 56 h after GnRH antagonist treatment and daily from oestrus (day 0) of the treatment cycle for 22 days. Oestrous behaviour was determined using a vasectomized ram present throughout the experiment. Progesterone secretion was episodic in both the early and late luteal phase with a frequency of between 1.6 and 3.2 pulses in 8 h. The GnRH antagonist abolished the pulsatile secretion and suppressed the basal concentrations of LH for at least 3 days after treatment. This suppression of LH, in either the early or late luteal phase, did not affect the episodic release of progesterone. Daily concentrations of progesterone in plasma showed a minimal reduction on days 11 to 14 after GnRH antagonist treatment on day 4, although this was significant (P < 0.05) only on days 11 and 13. There was no effect of treatment on day 11 on daily progesterone concentration, and the timing of luteolysis and the duration of corpus luteum function was unaffected by GnRH antagonist treatment on either day 4 or day 11. These results indicate that the episodic secretion of progesterone during the luteal phase of the oestrous cycle in ewes is independent of LH pulses and normal progesterone secretion by the corpus luteum can be maintained with minimal basal concentrations of LH.     

Pulsatile patterns of gonadotropins and ovarian steroids during estrus and pseudopregnancy in the rabbit

Consecutive daily plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol-17 beta (E2), progesterone (P4) and 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP) were monitored in estrous rabbits and in these same doses during pseudopregnancy (PSP); these daily hormone levels, as well as the immediate post-coital changes in gonadotropin secretion, were similar to those in previous reports. To examine the pulsatile patterns of the gonadotropins and ovarian steroids, sequential, 10-min plasma samples were collected for 6 h from estrous does and on Days 3, 10, and 17 of PSP. All five hormones were measured in the serial samples from estrous and PSP Day 10 does; LH and FSH only were assayed in the remaining sequential samples. The amplitude and frequency of FSH pulses did not differ between any of these stages. In marked contrast, LH pulse amplitudes, and even pulse frequencies in Day 17 does, were profoundly increased during PSP above those in estrous does. Progestin secretions, both P4 and 20 alpha-OHP, also were sharply elevated in PSP Day 10 does as compared with those in estrous rabbits; the pulse amplitudes of both progestins were severalfold higher during PSP. P4 pulse frequencies were also increased at this time. Conversely, the parameters of E2 secretion did not differ between estrous and PSP Day 10 animals. In PSP Day 10 does, high amplitude pulses of both P4 and 20 alpha-OHP occurred simultaneously with high amplitude LH pulses. Simultaneous E2 and P4 pulses were evident in these same sequential plasma samples, and this E2-P4 pulse association was greater than that of 20 alpha-OHP pulses with E2 pulses. Our findings failed to identify conclusively the trophic stimulus for the progestin pulse patterns, but the mechanism may involve the coordinated action of LH and E2. The results do demonstrate that each gonadotropin and ovarian steroid is secreted in a pulsatile manner in both estrous and pseudopregnant rabbits. There are altered profiles in LH and progestin pulses, without major changes in FSH and E2 patterns, between the stages of estrus and PSP. The causes and consequences of these divergent endocrine shifts cannot be deduced from these data.     

Follicle populations,ovulation rates and plasma profiles of LH,FSH and prolactin in Scottish Blackface ewes in high and low levels of body condition

Scottish Blackface ewes in high body condition (mean score = 2.86) had a higher mean ovulation rate (1.8 v. 0.9; P < 0.05) and more large (⪖ 4 mm diameter) follicles (4.6 v 2.2; P < 0.05) than ewes in low condition (mean score = 1.84) but similar numbers of small (1–4 mm diameter) follicles (6.3 v 6.0; NS). There was little difference in LH profiles with body condition but FSH and prolactin concentrations were significantly greater, during both luteal and follicular phases of the cycle, in ewes in high condition.Despite the relationships between body condition and ovulation rate and between condition and hormone concentrations, within the high condition groups, there was no significant difference in FSH levels with ovulation rate. Prolactin levels were higher in ewes with a single ovulation than in ewes with two or three ovulations. There was a trend towards a higher mean LH pulse frequency in the luteal phase and a higher mean LH pulse amplitude in the follicular phase in ewes with multiple ovulations compared with ewes with a single ovulation. During oestrus, only circulating prolactin concentrations differed with body condition, being significantly higher in ewes in high condition, but mean LH concentrations were higher and FSH concentrations lower in ewes with multiple ovulations. Subsequent luteal function, as measured by circulating progesterone concentrations, was normal in all ewes. It is concluded that body condition affected the size of the large follicle (⪖ 4 mm diameter) population through changes in FSH and possibly pulsatile LH secretion and prolactin secretion during the luteal and follicular phases of the cycle and that the number of follicles that were potentially ovulatory was probably determined during the luteal phase of the cycle. However, their ability to undergo the final stages of development and to ovulate may be related to the amount of LH secreted during the follicular phase.     

Perioestrous patterns of circulating LH,FSH, prolactin and oestradiol-17β in the gilt

Concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) were measured in jugular blood and those of oestradiol-17β (E₂17β) in utero-ovarian blood. Samples were taken from five intact gilts every 15 min for 108 h starting between day 15 and day 18 of the oestrous cycle. In the late luteal/early follicular phase, high pulsatile LH secretion, close to one pulse per hour, was observed. This could be the stimulus necessary for the final maturation of the ovarian follicles.Thereafter, frequency and amplitude of pulses, and the baseline value, decreased and were low at least between 36 and 12 h before the preovulatory LH surge. PRL and FSH concentrations also declined. This was probably due to the increase of oestrogen secretion. As E₂17β concentrations were still high, the surge of LH which was accompanied by increase in FSH and PRL, occurred for approximately 13 to 20 h. While LH and PRL mean levels decreased, FSH concentrations continued to increase. Peaks of PRL were observed during the late luteal/early follicular phase and during the LH discharge. During the period of estrus, each exposure to the boar was immediately followed by one of these peaks, which could play a role in the sexual behavior of the gilt.     

Gonadotropin-releasing hormone in third ventricular cerebrospinal fluid of the heifer during the estrous cycle

The release profile of GnRH in cerebrospinal fluid (CSF) and its correlation with LH in peripheral blood of ovary-intact heifers during the estrous cycle were investigated. A silicon catheter was placed into the third ventricle of six heifers using ultrasonography. During the mid-luteal phase, the heifers were injected with prostaglandin F(2alpha) to induce luteolysis. Surges of CSF GnRH (66.7 h after prostaglandin F(2alpha) administration) and peripheral LH (66.3 h) occurred simultaneously and were coincident with the onset of estrus (67.0 h). Duration of elevated GnRH concentration considerably overlapped with the estrous phase in each of the heifers. Mean pulse frequencies of both GnRH and LH were significantly higher during the proestrous and early luteal phases than during the mid-luteal phase, while mean concentration and pulse amplitude of both GnRH and LH were not different between these three phases. Of all the GnRH pulses identified, more than 80% were accompanied by an LH pulse during the proestrous and early luteal phases. However, the proportion of GnRH pulses that were coincident with an LH pulse during the mid-luteal phase decreased to 60%. The results clearly demonstrate that a dynamic (pulse) and longer-term (surge) changes of GnRH release into CSF are physiologically expressed during the estrous cycle in heifers, and the pattern of pulsatile GnRH secretion in heifers depends upon their estrous cycle.     

Seasonal changes of gonadotropin-releasing hormone secretion in the ewe.

A sustained volley of high-frequency pulses of GnRH secretion is a fundamental step in the sequence of neuroendocrine events leading to ovulation during the breeding season of sheep. In the present study, the pattern of GnRH secretion into pituitary portal blood was examined in ewes during both the breeding and anestrous seasons, with a focus on determining whether the absence of ovulation during the nonbreeding season is associated with the lack of a sustained increase in pulsatile GnRH release. During the breeding season, separate groups (n = 5) of ovary-intact ewes were sampled during the midluteal phase of the estrous cycle and following the withdrawal of progesterone (removal of progesterone implants) to synchronize onset of the follicular phase. During the nonbreeding season, another two groups (n = 5) were sampled either in the absence of hormonal treatments or following withdrawal of progesterone. Pituitary portal and jugular blood for measurement of GnRH and LH, respectively, were sampled every 10 min for 6 h during the breeding season or for 12 h in anestrus. During the breeding season, mean frequency of episodic GnRH release was 1.4 pulses/6 h in luteal-phase ewes; frequency increased to 7.8 pulses/6 h during the follicular phase (following progesterone withdrawal). In marked contrast, GnRH pulse frequency was low (mean less than 1 pulse/6 h) in both groups of anestrous ewes (untreated and following progesterone withdrawal), but GnRH pulse amplitude exceeded that in both luteal and follicular phases of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Perturbation of estradiol-feedback control of luteinizing hormone secretion by immunoneutralization induces development of follicular cysts in cattle

We used immunoneutralization of endogenous estradiol to investigate deficiencies in the estradiol-feedback regulation of LH secretion as a primary cause of follicular cysts in cattle. Twenty-one cows in the prostaglandin (PG) F(2alpha)-induced follicular phase were assigned to receive either 100 ml of estradiol antiserum produced in a castrated male goat (n = 11, immunized group) or the same amount of castrated male goat serum (n = 10, control group). The time of injection of the sera was designated as 0 h and Day 0. Five cows in each group were assigned to subgroups in which we determined the effects of estradiol immunization on LH secretion and follicular growth during the periovulatory period. The remaining six estradiol-immunized cows were subjected to long-term analyses of follicular growth and hormonal profiles, including evaluation of pulsatile secretion of LH. The remaining five control cows were used to determine pulsatile secretion of LH on Day 0 (follicular phase) and Day 14 (midluteal phase). The control cows exhibited a preovulatory LH surge within 48 h after injection of the control serum, followed by ovulation of the dominant follicle that had developed during the PGF(2alpha)-induced follicular phase. In contrast, the LH surge was not detected after treatment with estradiol antiserum. None of the 11 estradiol-immunized cows had ovulation of the dominant follicle, which had emerged before estradiol immunization and enlarged to more than 20 mm in diameter by Day 10. Long-term observation of the six immunized cows revealed that five had multiple follicular waves, with maximum follicular sizes of 20-45 mm at 10- to 30-day intervals for more than 50 days. The sixth cow experienced twin ovulations of the initial persistent follicles on Day 18. The LH pulse frequency in the five immunized cows that showed the long-term turnover of cystic follicles ranged from 0.81 +/- 0.13 to 0.97 +/- 0.09 pulses/h during the experiment, significantly (P < 0.05) higher than that in the midluteal phase of the control cows (0.23 +/- 0.07). The mean LH concentration in the immunized cows was also generally higher than that in the luteal phase of the control cows. However, the LH pulse and mean concentration of LH after immunization were similar to those in the follicular phase of the control cows. Plasma concentrations of total inhibin increased (P < 0.01) concomitant with the emergence of cystic follicles and remained high during the growth of cystic follicles, whereas FSH concentrations were inversely correlated with total inhibin concentrations. In conclusion, neutralization of endogenous estradiol resulted in suppression of the preovulatory LH surge but a normal range of basal LH secretion, and this circumstance led to an anovulatory situation similar to that observed with naturally occurring follicular cysts. These findings provide evidence that lack of LH surge because of dysfunction in the positive-feedback regulation of LH secretion by estradiol can be the initial factor inducing formation of follicular cysts.     

An ultrasound-aided study of temporal relationships between the patterns of LH/FSH secretion, development of ovulatory-sized antral follicles and formation of corpora lutea in ewes

To characterize the pulsatile secretion of LH and FSH and their relationships with various stages of follicular wave development (follicles growing from 3 to > or =5 mm) and formation of corpora lutea (CL), 6 Western white-faced ewes underwent ovarian ultrasonography and intensive blood sampling (every 12 min for 6 h) each day, for 10 and 8 consecutive days, commencing 1 and 2 d after estrus, respectively. Basal serum concentrations of LH and LH pulse frequency declined, whereas LH pulse duration and FSH pulse frequency increased by Day 7 after ovulation (P<0.05). LH pulse amplitude increased (P<0.05) at the end of the growth phase of the largest ovarian follicles in the first follicular wave of the cycle. The amplitude and duration of LH pulses rose (P<0.05) 1 d after CL detection. Mean and basal serum FSH concentrations increased (P<0.05) on the day of emergence of the second follicular wave, and also at the beginning of the static phase of the largest ovarian follicles in the first follicular wave of the cycle. FSH pulse frequency increased (P<0.05) during the growth phase of emergent follicles in the second follicle wave. The detection of CL was associated with a transient decrease in mean and basal serum concentrations of FSH (P<0.05), and it was followed by a transient decline in FSH pulse frequency (P<0.05). These results indicate that LH secretion during the luteal phase of the sheep estrous cycle reflects primarily the stage of development of the CL, and only a rise in LH pulse amplitude may be linked to the end of the growth phase of the largest follicles of waves. Increases in mean and basal serum concentrations of FSH are tightly coupled with the days of follicular wave emergence, and they also coincide with the end of the growth phase of the largest follicles in a previous wave, but FSH pulse frequency increases during the follicle growth phase, especially at mid-cycle.