Opiatergic inhibition of pulsatile luteinizing hormone release during the menstrual cycle of rhesus macaques

The endogenous opioid peptides (EOPs) may inhibit the rate of hypothalamic gonadotropin-releasing hormone (GnRH) release and hence the frequency of pulsatile luteinizing hormone (LH) release, particularly in the luteal phase of the menstrual cycle. Our objectives were to compare the effects of an opiate antagonist, naloxone (NAL), on the patterns of LH, estradiol-17 beta (E2), and progesterone (P4) secretion during the follicular and luteal phases of the macaque menstrual cycle. Plasma levels of E2, P4, and bioactive LH were measured in serial, 15-min blood samples during 8-hr infusions of NAL (2 mg/hr) or saline, either on Days 5 or 6 of the follicular phase (FN and FS, n = 5 and 4, respectively) or on Days 8, 9, or 10 of the luteal phase (LN and LS, n = 5 each) of a menstrual cycle. The pulsatile parameters of each hormone were determined by PULSAR analysis and the correspondence of steroid pulses with those of LH were analyzed for each cycle stage in each animal. As expected, LH mean levels and pulse frequencies in LS monkeys were only about one-third of those values in FS animals. NAL had no effects on pulsatile LH, E2, or P4 release during the follicular phase. In contrast, luteal phase NAL infusions increased both LH mean levels and pulse frequencies to values which were indistinguishable from those in FS animals. LH pulse amplitudes did not differ among the four groups. Mean levels and pulse frequencies of P4 secretion in LS monkeys were about 4- and 14-fold greater than those values in FS animals. Mean levels and pulse amplitudes of P4 release in LN animals were greater than those values in all other groups. LH and E2 pulses were not closely correlated in follicular phase animals, and this pulse association was not altered by NAL. In FS monkeys, LH and P4 pulses were not correlated; however, NAL increased this LH-p4 pulse correspondence. LH and P4 pulses were closely correlated in luteal phase animals and this association was not affected by NAL. Our data suggest that the EOPs inhibit the frequency of pulsatile LH secretion in the presence of luteal phase levels of P4. During the midfollicular phase when LH pulses occur every 60 to 90 min, the opioid antagonist NAL alters neither the pulsatile pattern of LH release nor E2 secretion, but NAL may directly affect P4-secreting cells.     

Prolactin suppresses GnRH but not TSH secretion

BACKGROUND/AIMS: In animal models, prolactin increases tuberoinfundibular dopamine turnover, which has been demonstrated to suppress both hypothalamic GnRH and pituitary TSH secretion. To test the hypothesis that prolactin suppresses GnRH and TSH secretion in women, as preliminary evidence that a short-feedback dopamine loop also operates in the human, the effect of hyperprolactinemia on GnRH and TSH secretion was examined. METHODS: Subjects (n=6) underwent blood sampling every 10 min in the follicular phase of a control cycle and during a 12-hour recombinant human prolactin (r-hPRL) infusion preceded by 7 days of twice-daily subcutaneous r-hPRL injections. LH and TSH pulse patterns and menstrual cycle parameters were measured. RESULTS: During the 7 days of r-hPRL administration, baseline prolactin increased from 16.0+/-3.0 to 101.6+/-11.6 microg/l, with a further increase to 253.7+/-27.7 microg/l during the 12-hour infusion. LH pulse frequency decreased (8.7+/-1.0 to 6.0+/-1.0 pulses/12 h; p<0.05) with r-hPRL administration, but there were no changes in LH pulse amplitude or mean LH levels. There were also no changes in TSH pulse frequency, mean or peak TSH. The decreased LH pulse frequency did not affect estradiol, inhibin A or B concentrations, or menstrual cycle length. CONCLUSION: These studies demonstrate that hyperprolactinemia suppresses pulsatile LH secretion but not TSH secretion and suggest that GnRH secretion is sensitive to hyperprolactinemia, but that TSH secretion is not. These data further suggest that the degree of GnRH disruption after 7 days of hyperprolactinemia is insufficient to disrupt menstrual cyclicity.     

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.     

The dopaminergic inhibition of LH secretion during the menstrual cycle

To assess the potential inhibitory role of hypothalamic dopaminergic input on the LRF-LH system, the gonadotropin response to a dopamine receptor antagonist, metoclopramid (MCP, 10 mg iv bolus) was examined during different phases of the menstrual cycle in 12 women. In addition, the role of dopamine infusion on naloxone (opiate receptor antagonist) induced LH increments was examined. MCP induced an abrupt increase in circulating LH levels in the mid-luteal phases but not in the early and late follicular phase subjects. No significant changes in serum FSH levels were observed. Dopamine, when infused concomitantly with naloxone, completely suppressed the naloxone induced pulsatile increments of LH in mid-luteal subjects. These findings support the contention that an increased dopaminergic inhibition of LRF-LH system occurs during the high estrogen-progesterone phase of the menstrual cycle, and provide preliminary evidence that the inhibitory role of endogenous opioids on LRF release may involve the dopaminergic system.     

Naloxone evokes large-amplitude GnRH pulses in luteal-phase ewes

Ewes were sampled during the mid-late luteal phase of the oestrous cycle. Hypophysial portal and jugular venous blood samples were collected at 5-10 min intervals for a minimum of 3 h, before i.v. infusions of saline (12 ml/h; N = 6) or naloxone (40 mg/h; N = 6) for 2 h. During the 2-h saline infusion 2/6 sheep exhibited a GnRH/LH pulse; 3/6 saline infused ewes did not show a pulse during the 6-8-h portal blood sampling period. In contrast, large amplitude GnRH/LH pulses were observed during naloxone treatment in 5/6 ewes. The mean (+/- s.e.m.) amplitude of the LH secretory episodes during the naloxone infusion (1.07 +/- 0.11 ng/ml) was significantly (P less than 0.05) greater than that before the infusion in the same sheep (0.54 +/- 0.15 ng/ml). Naloxone significantly (P less than 0.005) increased the mean GnRH pulse amplitude in the 5/6 responding ewes from a pre-infusion value of 0.99 +/- 0.22 pg/min to 4.39 +/- 1.10 pg/min during infusion. This episodic GnRH secretory rate during naloxone treatment was also significantly (P less than 0.05) greater than in the saline-infused sheep (1.53 +/- 0.28 pg/min). Plasma FSH and prolactin concentrations did not change in response to the opiate antagonist. Perturbation of the endogenous opioid peptide system in the ewe by naloxone therefore increases the secretion of hypothalamic GnRH into the hypophysial portal vasculature. The response is characterized by a large-amplitude GnRH pulse which, in turn, causes a large-amplitude pulse of LH to be released by the pituitary gland.     

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.     

Augmentation, by naloxone, of the frequency and amplitude of LH-RH pulses in hypothalamo-hypophyseal portal blood in the castrated ram

The effect of naloxone administration on the LH-RH secretion in hypophyseal portal blood and LH secretion in peripheral blood was studied in four short term castrated rams (between 2 to 4 days after castration). For two animals (A and B) given a single naloxone injection, an increase of LH-RH pulse amplitude was observed (A, 22.3 to 80.5 pg/ml and B, 22.5 to 34.5 pg/ml) with only a small (nonsignificant) increase in LH-RH pulse frequency. For animals C and D given four injections of naloxone, both LH-RH pulse amplitudes and LH-RH pulse frequency were increased. Means of LH-RH pulse amplitude increase from 29.3 to 65.1 pg/ml and from 34.6 to 50.8 pg/ml for animals C and D respectively and the number of LH-RH pulses detected during the 3 hrs. before and after the first injection of naloxone were respectively 3 vs. 5 and 3 vs. 7. Whereas all LH pulses were preceded with a LH-RH pulse in animals A and B, after the multiple naloxone injections in animals C and D, a rapid LH-RH pulse frequency was associated with a sustained increment of LH secretion in peripheral blood in such a way that individual LH pulses were not clearly defined. The present report is the first documentation on naloxone increasing the release of LH-RH secretion in hypophyseal portal blood of conscious, unrestrained, short-term castrated rams. The results indicate: (1) that the opiate antagonist naloxone is able to increase both the amplitude and the frequency of LH-RH discharge by the hypothalamus and (2), when the LH-RH pulse frequency exceeds one pulse every 30 min., discrete LH secretory episodes are not observed in peripheral blood.     

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.     

Neuroendocrine regulation of pulsatile luteinizing hormone secretion in elderly men

Leydig cell function is driven by LH, secreted in a pulsatile manner by the anterior pituitary in response to episodic discharge of hypothalamic LHRH into the pituitary portal circulation, under control of a yet to be defined neural mechanism, the "hypothalamic LHRH pulse generator". The normal aging process in elderly men is accompanied by a decline in Leydig cell function. Whereas primary testicular factors undoubtedly play an important role in the decrease of circulating (free) testosterone levels with age, recent studies demonstrated that aging also affects the central compartment of the neuroendocrine cascade. Hypothalamic alterations comprise changes in the regulation of the frequency of the LHRH pulse generator with an inappropriately low frequency relative to the prevailing androgen impregnation and opioid tone, and with an increased sensitivity to retardation of the LHRH pulse generator by androgens. As observed by some authors in basal conditions and by others after endocrine manipulations. LH pulse amplitude seems also to be reduced in elderly men as compared to young subjects. This is most probably the consequence of a reduction in the amount of LHRH released by the hypothalamus. Indeed, challenge of the gonadotropes with low, close to physiological doses of LHRH in young and elderly men reveals no alterations in pituitary responsiveness when looking at either the response for immunoreactive LH or bioactive LH. Deconvolution analysis on data obtained after low-dose LHRH suggests a markedly prolonged plasma half-life of LH in elderly men, a finding which may explain the paradoxical increase of mean LH levels in face of the reduced or unchanged frequency and amplitude of LH pulses.     

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.     

Modulation of luteinizing hormone pulse amplitude by the frequency of luteinizing hormone-releasing hormone stimulation and by testosterone in castrated, hypothalamic-lesioned male rats

The frequency of spontaneous luteinizing hormone (LH) pulses is thought to be a direct result of the frequency of luteinizing hormone-releasing hormone (LHRH) pulses from the hypothalamus. By contrast, the amplitude of spontaneous LH pulses may be controlled by several factors other than the amplitude of LHRH pulses. We tested two hypotheses: 1) that LH pulse amplitude is determined in part by the frequency of LHRH pulses of constant magnitude, and 2) that testosterone (T) exerts a direct feedback effect on the pituitary gland to regulate LH pulse amplitude. Gonadal feedback was eliminated by castrating adult male rats (n = 20). Endogenous LHRH secretion was eliminated by lesioning the medial basal hypothalamus. Serum LH levels (0.19 +/- 0.04 ng/ml RP-2, mean +/- SEM) and T levels (0.15 +/- 0.02 ng/ml), measured several weeks after hypothalamic lesioning, confirmed the hypogonadotropic hypogonadal state of the animals. During a 8-h period, unanesthetized, unrestrained animals were injected with 40-ng pulses of LHRH via catheters into the jugular vein, and blood samples for LH measurement were drawn at 10-min intervals. The LHRH pulse interval was 20 min during the first 4 h in all animals. The pulse interval was doubled to 40 min in half of the animals (n = 10) during the next 4 hours; in the other 10 animals, the pulse interval was maintained constant at 20 min throughout the study. Within both of these groups, one-half of the animals (n = 5) were infused with T to achieve a physiological level of T in serum (2.46 +/- 0.36 ng/ml at 4 h), while the other half received vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of increasing and decreasing daylength to the photoperiodic control of LH secretion in the Ile-de-France ram

Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P less than 0.01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P less than 0.05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

LH secretion around the time of the preovulatory gonadotrophin surge in the ewe

Blood samples were taken once an hour from 17 ewes starting on Day 15 of a natural oestrous cycle and continuing for 4 days or until 36 h after the onset of oestrus. On Days 12, 16, 17 and 18 of the cycle, blood samples were also taken every 5 min for 6 h, between 09:00 and 15:00 h. LH pulse frequency rose and amplitude fell between the luteal and follicular phase of the oestrous cycle ( ). In the period from 48 h before to 40 h past the peak of the preovulatory LH surge, LH pulse frequency did not change. LH pulse amplitude was similar prior to and following the LH surge. During the preovulatory LH surge, LH pulse amplitude rose markedly ( ), with the visible, discrete components of pulses ranging from twice to 20 times those seen prior to or following the surge. The amplitude of LH pulses on the downslope of the LH surge was greater than that on the upslope of the surge (P < 0.05). We conclude that the preovulatory LH surge may consist of an amalgamation of high frequency, high amplitude pulses of LH secretion.     

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.     

Does the seasonal increase in estradiol negative feedback prevent luteinizing hormone surges in anestrous ewes by suppressing hypothalamic gonadotropin-releasing hormone pulse frequency?

To test the hypothesis that the anestrous increase in estradiol negative feedback prevents estrous cycles by suppressing hypothalamic gonadotropin-releasing hormone (GnRH) pulse frequency, a variety of regimens of increasing GnRH pulse frequency were administered to anestrous ewes for 3 days. A luteinizing hormone (LH) surge was induced in 45 of 46 ewes regardless of amplitude or frequency of GnRH pulses, but only 19 had luteal phases. Estradiol administration induced LH surges in 6 of 6 ewes, only 3 having luteal phases. Anestrous luteal phase progesterone profiles were similar in incidence, time course, and amplitude to those of the first luteal phases of the breeding season, which in turn had lower progesterone maxima than late breeding season luteal phases. In the remaining ewes, progesterone increased briefly or not at all, the increases being similar to the transient rises in progesterone occurring in most ewes at the onset of the breeding season. These results demonstrate that increasing GnRH pulse frequency induces LH surges in anestrus and that the subsequent events are similar to those at the beginning of the breeding season. Finally, they support the hypothesis that the negative feedback action of estradiol prevents cycles in anestrus by suppressing the frequency of the hypothalamic pulse generator.     

Changes in episodic luteinizing hormone secretion leading to puberty in the lamb

In this study, we monitored episodic luteinizing hormone (LH) secretion throughout development in eight April-born ewe lambs to determine if a change in LH pulse patterns preceded first ovulation at puberty. LH pulses were measured in samples collected every 12 min for 6 h once in July, twice a month from 22 August to 2 October, and then weekly until puberty. Progesterone concentrations, measured in samples taken 3/wk, were used as an index of first ovulation, which occurred at 29.3 +/- 0.7 wk of age. LH pulse frequencies throughout most of this period ranged from 0 to 2 pulses/6 h, with no change over time. However, during the week prior to the first progesterone rise, there was a significant increase in pulse frequency to a level seen during the follicular phase in post-pubertal lambs. This increase in pulse frequency was evident in 7 of 8 lambs; pulses were not analyzed in the last lamb because samples were taken during the LH surge. In contrast, LH pulse amplitude did not increase prior to puberty. In fact, pulse amplitude declined linearly during the 3 wk before first ovulation and then increased during the follicular phase in post-pubertal animals. These results support the hypothesis that an increase in the frequency of episodic LH secretion is a key event leading to the onset of ovarian cycles in the lamb. Whether an increase in pulse amplitude is also necessary remains unclear. If so, it must occur just before the LH surge, since it was not detected in any samples taken before puberty in this study.     

Effect of the introduction of rams during the anoestrous season on the pulsatile secretion of LH in ovariectomized ewes

Introduction of rams to ovariectomized ewes treated with oestradiol implants (N = 10) increased the frequency of LH pulses from 4 X 8 to 10 X 6 pulses per 12 h. This effect was reflected by increases in mean levels of LH and the basal levels upon which the pulses were superimposed. In ewes that had not been treated with oestradiol (N = 5), there was no significant increase in pulse frequency but mean and basal levels of LH increased slightly after the introduction of rams. In a second experiment, similar effects of the introduction of rams were seen in ovariectomized ewes treated with oestradiol or oestradiol + androstenedione (N = 16), but no significant effects of the rams were observed in untreated ewes (N = 8) or ewes treated only with androstenedione (N = 7). No preovulatory surges of LH were observed in the 30-h period after the introduction of rams. It was concluded that the ram stimulus probably evokes the increase in pulse frequency by inhibiting the negative feedback action of oestradiol, and that the surge normally observed in entire ewes is dependent on the ovarian response to these pulses. However, the observation of responses in some ewes not treated with oestradiol also raises the possibility that the ram stimulus can act directly on the hypothalamic neurones that control the secretion of LH, and that this effect is enhanced in the presence of oestrogen.     

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.     

FACTORS AFFECTING THE SECRETION OF LUTEINIZING HORMONE IN THE EWE

(1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre-pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH. (2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion. (3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone. (4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day-length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day-length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses. (5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol. (6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses. (7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not known.     

Pattern of ovarian progesterone secretion during the luteal phase of the ovine estrous cycle

Pulsatile secretion of progesterone has been observed during the late luteal phase of the menstrual cycle in the rhesus monkey and human. As the luteal phase progresses in each of these species, there is a pattern of decreased frequency and increased amplitude of progesterone pulses. The present study was designed to determine the pattern of progesterone secretion during the late luteal phase (Days 10-16) of the normal ovine estrous cycle. Five unanesthetized ewes, each bearing an indwelling cannula in the utero-ovarian vein, were bled every 15 min from 0800 h on Day 10 through 0800 h on Day 16 of the estrous cycle. With the computer program PULSAR, it was determined that progesterone secretion was episodic, with pulsations observed on all days. Analysis of variance was used to determine differences in frequency, amplitude, and interpeak interval (IPI) of progesterone pulses among ewes and days. The ewes averaged 8.0 +/- 0.63 pulses of progesterone per 24 h. Mean frequency of pulses was not different between days but showed differences between ewes. Mean amplitude of progesterone pulses was 7.0 +/- 0.27 ng/ml, with no differences observed either between days or between ewes. Mean IPI was 197 +/- 7.1 min, and, like frequency, the IPI was not different between days, but varied between ewes. No consistent temporal relationship was found between progesterone pulses and luteinizing hormone (LH), as determined by bioassay and radioimmunoassay, on Day 14 of the cycle in one ewe. The results indicate that progesterone secretion is episodic during the luteal phase of the ovine estrous cycle and is independent of LH pulses.(ABSTRACT TRUNCATED AT 250 WORDS)