Endocrine mechanisms of puberty in heifers: estradiol negative feedback regulation of luteinizing hormone secretion

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the first experiment, seven heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day 0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and subcutaneously implanted with estradiol-17 beta (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A second experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the second experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. These results indicate that: a) LH secretion in prepubertal heifers is responsive to estradiol negative feedback; and b) estradiol negative feedback decreases during the prepubertal period in beef heifers.     

Effects of estradiol on the prolactin surges and the feedback mechanisms of gonadotropins in pseudopregnant rats

The influences of estradiol on the prolactin (PRL) surges and on the secretion of gonadotropins (LH and FSH) were investigated in the pseudopregnancy (PSP) of acutely ovariectomized rats. The four following experimental groups were prepared: 1) intact PSP as a control, 2) ovariectomy was performed on day 0 of PSP (OVX), 3) a Silastic tube containing estradiol was implanted for day 1-4 into the OVX rats (OVX-E 1-4), and 4) the Silastic tube was implanted for day 5-8 by the same manner into the OVX rats (OVX-E 5-8). In the OVX group nocturnal (N) PRL surges were observed at 0500 h on days 4, 8 and 12 examined, and increased secretions of LH and FSH were noted. In the OVX-E 1-4 group, the N PRL surge was suppressed on day 4, and the suppressed N PRL surge did not occur on day 8, after the removal of the implanted tubes. Diurnal (D) PRL surges with LH surges were observed at 1700 h on day 4 in these rats. Similarly, more remarkable results were obtained on days 8 and 12 in the OVX-E 5-8 group than in the OVX-E 1-4.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of suckling and ovariectomy on the control of luteinizing hormone secretion during the postpartum period in beef cows

Twenty-two mature pluriparous beef cows were randomly assigned to one of six treatments in a 2 X 3 factorial experiment in order to study the role of suckling and ovarian factors on control of the tonic and episodic release of luteinizing hormone (LH). Twelve cows remained intact (INT) and 10 were ovariectomized (OVX) within 4 days following the day of parturition (Day 0). The suckling intensities were nonsuckled (0), suckled once daily for 30 min (1) and suckled ad libitum by two calves (2). Blood samples were collected at 15-min intervals for 6 h weekly, from Days 6 to 76 postpartum. The postpartum intervals to initiation of ovarian luteal function were 31 +/- 3, 41 +/- 4 and 67 +/- 1 days (means +/- SEM) for INT cows with 0, 1 and 2 suckling intensities, respectively. Mean LH concentrations and frequency of LH pulses increased as time of ovulation approached in INT cows. In OVX animals, both mean LH concentrations and frequency of LH pulses increased as time postovariectomy progressed. No differences were detected in mean LH concentrations or frequency of LH pulses between the two suckled OVX groups. Mean LH in the OVX-0 cows was greater on Days 13, 20 and 27 postpartum when compared to the respective days in suckled OVX cows. Frequency of LH pulses tended to be lower (P less than 0.10) in both suckled OVX groups when compared with OVX-0 cows from Day 6 to Day 55 postpartum. It is postulated that suckling and ovarian factors act together during the postpartum period to suppress LH levels and frequency of LH pulses in beef cows.     

Seasonal LH profile in ovariectomized cattle

Two experiments were conducted to examine seasonal changes in circulating LH concentrations in ovariectomized heifers. In experiment 1, four Holstein heifers were ovariectomized in April 1977 during middiestrus. Blood samples were collected daily for 30 days surrounding each equinox and solstice for one year to examine changes in plasma LH levels at the time of seasonal photoperiod changes. The LH concentrations were highest during the winter solstice period and lowest during the summer solstice period. In addition, samples taken at two-week intervals indicated a distinct LH profile with maximal LH concentrations during November-April and minimal concentrations during May-October. In experiment 2, eight Holstein heifers were ovariectomized in June-July, 1979 and given an estradiol or a control implant in October. A distinct LH profile for the interval extending from January, 1980 to February, 1981 was found in the heifers that were not treated with estradiol. Concentrations were maximal during December-April and minimal during May-November. The LH profile followed a similar pattern in the estradiol-treated heifers; however, the overall profile was at a higher level. These data indicate that underlying seasonal reproductive mechanisms are present in cattle even though the species ovulates and breeds throughout the year.     

Nutritional anestrus in beef cows: effects of body condition and ovariectomy on serum luteinizing hormone and insulin-like growth factor-I

The objectives of this experiment were to determine if a postcastration increase in concentrations of LH occurs in nutritionally anestrous beef cows and to examine the relationship between body energy reserves and secretion of LH and insulin-like growth factor-I (IGF-I). Nonpregnant, nonlactating, Hereford cows were fed to maintain (M) body weight (BW), body condition score (BCS), and normal estrous cycles (n = 5) or were fed a restricted (R) diet for 26 wk to lose BW and BCS and to become anestrus (n = 10). At 5-7 wk after the initiation of anestrus, R cows were randomly allotted to be ovariectomized (OVX) via flank incision (n = 5) or to remain intact (INT, n = 5). OVX was performed when R cows became anestrous. All M cows were OVX. Serum was collected frequently the day before and during the first 10 days after OVX, and concentrations of progesterone, estradiol, LH and IGF-I were quantified. On Day 10 after OVX, 1 mg of estradiol was injected into 3 cows from each group and serum was collected for 30 h. After OVX, there was a treatment-by-day effect for mean serum LH and IGF-I concentrations. Concentrations of LH increased (p less than 0.01) and concentrations of IGF-I decreased (p less than 0.05) in M-OVX cows when compared with R-OVX and R-INT cows. Concentrations of LH and IGF-I were similar for R-OVX and R-INT cows. The number of LH pulses was similar for M and R cows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progesterone directly inhibits pituitary luteinizing hormone secretion in an estradiol-dependent manner.

We recently demonstrated that progesterone and estradiol inhibit pituitary LH secretion in a synergistic fashion. This study examines the direct feedback of progesterone on the estradiol-primed pituitary. Nine ovariectomized (OVX) ewes underwent hypothalamic-pituitary disconnection (HPD) and were infused with 400 ng GnRH every 2 h throughout the experiment. After 7 days of infusion, estradiol was implanted s.c. Four days later, estradiol implants were exchanged for blank implants in 4 ewes and for progesterone implants in 5 ewes. These implants remained in place for another 4 days. Blood samples were collected around exogenous GnRH pulses before and 0.5 to 96 h after implant insertion and exchange. Serum LH and progesterone concentrations were determined through RIA. One month later, 4 of the HPD-OVX ewes previously implanted with steroids were reinfused with GnRH and the implantation protocol was repeated using blank implants only. In estradiol-primed ewes, progesterone significantly lowered LH secretion after 12 h of implantation and LH secretion remained inhibited while progesterone implants were in place (p less than 0.05). Removing estradiol transiently lowered LH secretion, and this effect was significant only 24 h after estradiol withdrawal (p less than 0.05). These data suggest that progesterone has a direct, estradiol-dependent inhibitory effect on pituitary LH release and that estradiol may sustain pituitary gonadotrope response to GnRH.     

Effect of photoperiod on LH, FSH, prolactin and melatonin patterns in ovariectomized prepubertal heifers

Angus and Angus crossbred prepubertal heifers were ovariectomized and randomly assigned to either increasing light simulating the photoperiod of the vernal equinox to the summer solstice (I) or decreasing light simulating the photoperiod of the autumnal equinox to the winter solstice (D) for 43 degrees N latitude. Three blood samples were taken each week for 14 weeks, the first at 11:00 h and two others 2 days later, 1 h before lights on (dark), 1 h before lights off (light). At the end of 14 weeks 4 heifers from each treatment group were cannulated and samples were taken for 12 h at 15-min intervals, 6 h in the light and 6 h in the dark. All sera were assayed for LH, FSH and prolactin. In addition, the samples taken at 15-min intervals were assayed for melatonin. In samples taken weekly at 11:00 h circulating concentrations of LH and prolactin were higher among animals in Group I, while FSH concentrations were not different between Groups D and I. In samples collected weekly in the light or the dark, LH and prolactin concentrations were higher in Group I animals. However, prolactin concentrations were higher and LH concentrations tended to be higher in samples taken in the dark. FSH concentrations were not different between either D or I or dark and light. In samples taken at 15-min intervals the prolactin baseline was higher and pulse amplitude tended to be higher for Group I animals. Neither LH nor FSH pulse characteristics differed between I and D; however, LH baseline and LH pulse amplitude were higher in the dark. Melatonin pulse amplitude was higher among animals in Group D and higher in serum collected in the dark. These results suggest that photoperiod alters circulating concentrations of LH and prolactin and alters pulsatile release of LH, prolactin and melatonin in the prepubertal heifer.     

Influence of season on estrous and luteinizing hormone responses to estradiol benzoate in ovariectomized sows

The objective of this experiment was to determine whether seasonal differences existed in estrous and LH responses to estradiol benzoate (EB) in ovariectomized sows. Sows were ovariectomized after weaning their first litter, and treatment was begun 120 d after ovariectomy. Sows were given 400 mug EB intramuscularly (i.m.) on July 24, 1982 (summer), October 24, 1984 (fall), January 29, 1985 (winter), and March 27, 1985 (spring). Beginning 24 h after EB, sows were checked for estrus four times daily. Proportion in estrus was affected by season, with all sows exhibiting estrus within 5 d after EB in summer, winter, and spring. Only three of five sows exhibited estrus within 5 d after EB in fall. Interval (h) to estrus was delayed in fall (80 h) compared to other seasons (62.6 h; SEM = 4.5). Concentrations of LH were suppressed within 6 h after EB in all seasons but rebounded to pre-injection levels more slowly in fall and spring than in winter and summer. Frequency of LH peaks (3.2 +/- .4 4 h ) was not affected by season, but amplitude (1.9 vs 0.9 ng/ml) and baseline (2.7 vs 1.6 ng/ml) were greater (P < 0.05) for summer than for the other seasons combined. At 6 h after injection, concentrations of estradiol-17beta (pg/ml) were greater in summer (58.3) than in fall (19.0), winter (32.4), or spring (16.6; SEM = 10.4). We conclude that environmental factors associated with season alter responsiveness of the brain to estradiol, thereby controlling sexual behavior and LH secretion.     

Evidence for a direct negative effect of estradiol at the level of the pituitary gland in sheep

The purpose of this experiment was to determine if pituitary stores of LH could be replenished by administration of GnRH when circulating concentrations of both progesterone and estradiol-17 beta (estradiol) were present at levels observed during late gestation. Ten ovariectomized (OVX) ewes were administered estradiol and progesterone via Silastic implants for 69 days. One group of 5 steroid-treated OVX ewes was given GnRH for an additional 42 days (250 ng once every 4 h). Steroid treatment alone reduced (p less than 0.01) the amount of LH in the anterior pituitary gland by 77%. Pulsatile administration of GnRH to steroid-treated ewes resulted in a further decrease (p less than 0.01) in pituitary content of LH. Compared to the OVX ewes, concentrations of mRNAs for alpha- and LH beta-subunits were depressed (p less than 0.01) in all steroid-treated ewes, whether or not they received GnRH. The ability of the dosage of GnRH used to induce release of LH was examined by collecting blood samples for analysis of LH at 15 days and 42 days after GnRH treatment was initiated. Two of 5 and 3 of 5 steroid-treated ewes that received pulses of GnRH responded with increased serum concentrations of LH after GnRH administration during the first and second bleedings, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary receptors for gonadotropin-releasing hormone in relation to changes in pituitary and plasma luteinizing hormone in ovariectomized-hypothalamo pituitary disconnected ewes. I. Effect of changing frequency of gonadotropin-releasing hormone pulses

Studies were undertaken to determine if changes in the amplitude of luteinizing hormone (LH) pulses that occur in response to changes in the frequency of gonadotropin-releasing hormone (GnRH) pulses are due to an alteration in the number of GnRH receptors. Ewes were ovariectomized (OVX) and the hypothalamus was disconnected from the pituitary (HPD). Ewes were then given pulses of GnRH at a frequency of 1/h or 1/3 h. Two control groups were included: OVX ewes not subjected to HPD, and HPD ewes that were not OVX. At the end of one week of treatment, blood samples were collected to determine the amplitude of LH pulses. The treated ewes were killed just before the next scheduled pulse of GnRH, and the content of LH and number of GnRH receptors were measured in each pituitary. The amplitude of LH pulses was highly correlated with the amount of LH in the pituitary gland (r = 0.71, p less than 0.01), and both LH content and pulse amplitude (mean + SEM) were higher in ewes receiving GnRH once per 3 h (189.7 +/- 39.3 microgram/pituitary, 10.3 +/- 1.1 ng/ml, respectively) than in ewes receiving GnRH once per h (77.8 +/- 11.4 microgram/pituitary, 5.2 +/- 1.3 ng/ml). The pituitary content of LH was highest in the OVX ewes (260.2 +/- 57.4 micrograms/pituitary) and lowest in the nonpulsed HPD ewes (61.7 +/- 51.2 micrograms/pituitary). The number of GnRH receptors was similar in all groups, and was not correlated with any other variable.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in the ewe: relationship of frequency of LH pulses to day length and response to estradiol negative feedback

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in ovariectomized ewes were examined over the course of 2 yr in relation to annual changes in environmental photoperiod, shifts in response to estradiol negative feedback control of LH secretion, and timing of the breeding season. Under natural environmental conditions, the frequency of LH pulses in individual ovariectomized ewes changed gradually and in close association with the annual cycle of day length. As days became shorter in late summer and autumn, LH pulse frequency increased; conversely, as day length increased in late winter and spring, frequency declined. Under artificial conditions in which ovariectomized ewes were exposed to different photoperiods, a similar inverse relationship was observed between day length and LH pulse frequency. The seasonal changes in frequency of LH pulses in ovariectomized ewes, although symmetric with the annual photoperiodic cycle, were not temporally coupled to the dramatic shifts in response to estradiol feedback inhibition of LH secretion at the transitions between breeding season and anestrus. The feedback shifts occurred abruptly and at times when LH pulse frequency in ovariectomized ewes was at, or near, the annual maximum or minimum. The tight coupling between LH pulse frequency and photoperiod leads to the conclusion that there is a photoperiodic drive to the LH pulse-generating system of the ewe. The temporal dissociation between changes in this photoperiodic drive and the seasonal shifts in response to estradiol negative feedback support the hypothesis that the neuroendocrine basis for these two phenomena is not one and the same.     

Nocturnal variation of prolactin secretion in the Mouflon (Ovis gmelini musimon) and domestic sheep (Ovis aries): seasonal changes

Seasonal changes in nocturnal prolactin secretion and their relationship with melatonin secretion were monitored in wild (Mouflon, Ovis gmelini musimon) and domesticated sheep (breed Manchega, Ovis aries). Two groups of eleven adult females each, were maintained outdoors under natural photoperiod. Plasma concentrations of prolactin and melatonin were determined during the summer and winter solstices and the autumn and spring equinoxes. Blood samples were collected every 3h during the night hours, and 1h before and after the onset of darkness and sunrise. Maximum mean plasma concentrations of prolactin during the dark-phase in Mouflons were observed in the summer solstice, (P<0.001) and in the summer solstice and spring equinox in Manchega ewes (P<0.001). Mean plasma concentrations of prolactin were higher in the wild species (P<0.001) during the summer solstice. In contrast, during the spring equinox, mean levels of prolactin were higher in Manchega ewes than in Mouflons (P<0.05). Plasma prolactin concentrations showed a nocturnal rhythm in both breeds, with seasonal variations (P<0.001). The increase in plasma melatonin levels during the first hour after sunset was accompanied to increasing concentrations of PRL 1h after the onset of darkness, only in the autumn and spring equinox for the Mouflon, and in the summer solstice and spring equinox for the Manchega ewes. In Mouflons, the fall of plasma PRL concentrations about the middle dark-phase in all the periods studied, coincided with high levels of melatonin. A similar relation was observed in Manchega ewes only in the winter solstice and spring equinox. The current study shows that the nocturnal rhythm of prolactin secretion exhibits seasonal variation; differences in the patterns of prolactin secretion between Mouflon and Manchega sheep are taken to represent the effects of genotype.     

Effects of neuromedin U on the pulsatile LH secretion in ovariectomized rats in association with feeding conditions

We examined the effects of intracerebroventricular injection of neuromedin U (NMU), at a dose that is reported to induce satiety in rats, on the pulsatile luteinizing hormone (LH) secretion in adult ovariectomized (OVX) rats under a normal feeding or a 48-h fasted condition. In OVX rats under the normal feeding condition, injection of NMU (1 nmol/3 microl) significantly decreased the mean LH concentration without affecting the frequency or amplitude of LH pulses, but under the 48-h fasted condition, it significantly decreased the mean LH concentration and the frequency of LH pulses without affecting the amplitude. The interpulse interval was significantly lengthened by NMU injection under the normal and the 48-h fasted condition, but the effect under the 48-h fasted condition was greater than under the normal feeding condition. We also confirmed that the 48-h fasted condition per se did not affect the pulsatile LH secretion in OVX rats. We suggest that NMU and fasting synergistically inhibit the pulsatile LH secretion, even though NMU has been said to act as a satiety factor.     

Estrogen decreases the sensitivity of anterior pituitary to the inhibitory effect of nitric oxide on prolactin release

OBJECTIVE: We investigated the effect of chronic estrogen treatment on the inhibitory action of nitric oxide (NO) on prolactin release. METHODS: The effect of NO on prolactin release was studied in anterior pituitaries of female Wistar rats, intact at random stages, ovariectomized (OVX), and OVX treated for 15 days with 17beta-estradiol (OVX-E(2)). RESULTS: Sodium nitroprusside (NP, 0.5 mM), a NO donor, inhibited prolactin release from anterior pituitaries and was able to stimulate cGMP synthesis in intact and OVX rats. Only a high, supraphysiological concentration of NP (2 mM) inhibited prolactin release from anterior pituitaries of OVX-E(2) rats and increased cGMP synthesis in OVX-E(2) rats. 8-Br-cGMP, a cGMP analogue, decreased prolactin release from anterior pituitaries of OVX rats but did not affect it in OVX-E(2) rats. CONCLUSION: Our results suggest that estrogen may modify the sensitivity of the anterior pituitary to the inhibitory effect of NO on prolactin release by affecting guanylyl cyclase activity and the cGMP pathway.     

Effect of plane of nutrition on seasonality of reproduction in Spanish Payoya goats

The aim of this study was to determine if there is a seasonal pattern of sexual activity in female Payoya goats and if this seasonality could be modulated by nutrition. During the experimental period of 20 months, 43 non-pregnant adults goats were penned under natural photoperiod at latitude 37 degrees 15'N. At the onset of the experiment, the animals were allocated to three experimental groups differing in the level of nutrition and whether the animals were entire or ovariectomized does. The high nutrition group (H, n = 16 entire does) receiving 1.5 times maintenance requirements. The low nutrition group (L, n = 16 entire does) and an ovariectomized and oestradiol treated group (OVX, n = 11 ovariectomized does) received a diet supporting their maintenance requirements. The groups were balanced for live weight (LW) and body condition score (BCS) at the beginning of the study. In entire goats, oestrus was tested daily using aproned males, ovulation rate was assessed by laparoscopy 7 days after identification of oestrus and plasma samples were obtained twice per week for progesterone assay. OVX goats were isolated from the other groups and bucks, plasma samples were assayed twice per week for LH and there were four intensive sampling periods during the year to determine LH pulsatility. LW and BCS were recorded for all animals once a week. A clear circannual cycle in live weight change was observed in all experimental groups, being relatively stable or slightly decreasing in summer and autumn and increasing during winter and spring. The effect of exposure to high (H) rather than low (L) nutrition was to cause earlier onset of ovarian activity (5 versus 17 August; P < 0.05), and expression of oestrous (16 August versus 2 September; P < 0.01) and later cessation of reproductive activity (ovulation 11 February versus 17 January; P < 0.01). Consequently, seasonal anoestrus was 32 days shorter in does on the higher plane of nutrition (P < 0.01). The seasonality of reproductive activity was confirmed in the OVX does, with reduced LH concentrations during spring and summer, and increased LH concentrations in autumn and winter. There was no effect of nutrition on ovulation rate. These results demonstrate that the female Payoya goat exhibits marked reproductive seasonality which is modulated by nutrition but possibly not ovulation rate.     

Effect of melatonin on daily LH secretion in intact and ovariectomized ewes during the breeding season.

This study was conducted to find out whether daily LH secretion in ewes may be modulated by melatonin during the breeding season, when the secretion of both hormones is raised. Patterns of plasma LH were determined in luteal-phase ewes infused intracerebroventricularly (icv.) with Ringer-Locke solution (control) and with melatonin (100 microg/100 microl/h). Response in LH secretion to melatonin was also defined in ovariectomized (OVX) ewes without and after estradiol treatment (OVX+E2). Basal LH concentrations by themselves did not differ significantly before, during and after both control and melatonin infusions in intact, luteal-phase ewes. However, single significant (P<0.05) increases in LH concentration were noted during the early dark phase in the control and 1h after start of infusion in melatonin treated ewes. In both OVX and OVX+E2 ewes, melatonin decreased significantly (P<0.01, P<0.05, respectively) mean plasma LH concentrations as compared to the levels noted before the infusions. In OVX+E2 ewes, a single significant (P<0.05) increase in LH occurred 1h after start of melatonin treatment, similarly as in luteal-phase ewes. No significant differences in the frequencies of LH pulses before, during and after melatonin infusion were found in all treatments groups. In conclusion, melatonin may exert a modulatory effect on daily LH secretion in ewes during the breeding season, stimulating the release of this gonadotropin in the presence of estradiol feedback and inhibiting it during steroid deprivation. Thus, estradiol seems to be positively linked with the action of melatonin on reproductive activity in ewes.     

Restoration of LH output and 17beta-oestradiol responsiveness in acutely ovariectomised holstein dairy cows pre-treated with a GnRH agonist (deslorelin) for 10 days

The objectives of the study were firstly to identify the role of the ovary in maintaining plasma luteinising hormone (LH) concentrations in cows treated with an implant of a potent GnRH agonist (deslorelin), and secondly to characterise the changes in LH following ovariectomy (OVX) in the same animals. Oestrus was synchronised in mature Holstein dairy cows and deslorelin implants were inserted 17 days later into two-third of the cows. A further 10 days later (day 0) all cows had bilateral OVX performed. A control group (CON; n=4) received no treatment and had blood samples collected at 15-min intervals for 8h on the day prior to OVX (day -1) and similarly on days 4 and 10. One group (DES_IN; n=4) had implants in place for the duration of the study while another group had implants removed (DES_OUT; n=4) at the time of OVX. DES_IN cows were sampled hourly at each sampling session (days -1, +4 and +10), whereas DES_OUT cows were sampled similarly to CON except on day -1 when hourly samples were collected.Predictable post-operative increases in mean LH (0.61 ng/ml versus 1.79 ng/ml; P<0.01) and LH pulse amplitude (0.66 ng/ml versus 1.56 ng/ml; day -1 versus day +10; P<0.01) occurred after CON cows were ovariectomised. Smoothed LH means showed a delayed effect of time compared to arithmetic means. Pulse frequency was unchanged following OVX in CON cows. A comparison of all cows that had been treated with deslorelin from day -1 showed a significant elevation of smoothed mean LH compared to untreated cows (0.80 ng/ml versus 0.34 ng/ml; DES_IN and DES_OUT versus CON; P<0.05). DES_IN cows had a 54% reduction in mean LH from day -1 to +4 following OVX (1.05 ng/ml versus 0.48 ng/ml; P<0.01) indicating the probable involvement of the ovary in the maintenance of elevated basal LH. No further reduction was detected by day +10. The LH response to an intramuscular (IM) injection of 500 microg 17beta-oestradiol (E2) on day +11 varied significantly between treatment groups (P<0.01). CON cows showed a typical LH surge, reaching maximum concentrations (10.3 ng/ml) at 17.3h post-injection. Even though low amplitude LH pulsatility had been restored in DES_OUT cows by day +4, there was an inconsistent response to E2 on day +12; one cow had an apparently normal surge yet, others showed only attenuated responses. Pulse amplitude in DES_OUT cows was lower at days +4 and +10 compared to CON (P<0.05). DES_IN cows did not produce any surge after E2. Mean LH prior to OVX (day -1) remained unchanged following the 500 microg oestradiol injection (0.38 ng/ml versus 0.45 ng/ml pre-E2 versus post-E2 compared to 1.05 ng/ml pre-OVX).The results of this experiment implicated ovarian involvement in maintaining elevated basal LH output in cows that were chronically treated with a GnRH agonist. Individual cows varied in their LH surge response to exogenous E2 given 12 days after implant removal, even though LH pulse amplitude and frequency had been restored.     

Central actions of neuropeptide-Y may provide a neuromodulatory link between nutrition and reproduction.

Central injection of neuropeptide-Y (NPY) has been shown to attenuate secretion of LH in ovariectomized rats, rabbits, and monkeys. Several investigators have reported elevated concentrations of NPY in the central nervous system of undernourished animals. The relationship between nutrition and reproduction positions NPY as a potential neuromodulator involved in nutritionally induced changes in secretion of LH. Three experiments were conducted with the following objectives: 1) to examine the effects of NPY on secretion of LH in ovariectomized (OVX) ewes and the influence of estradiol-17 beta (E) on these effects; 2) to determine whether NPY may act through direct effects on the pituitary to influence secretion of LH; and 3) to determine changes in concentrations of NPY in laterocerebro-spinal fluid (CSF) of food-restricted ewes compared to well-fed ewes. In Experiment 1, OVX ewes with s.c. implants of E (OVX + E, n = 4) or no steroid treatment (OVX, n = 4) were fitted with intracerebroventricular (i.c.v.) and jugular cannulae. One of 4 doses of porcine NPY (pNPY; 0, 0.5, 5, or 50 micrograms) was injected i.c.v. and blood samples were collected every 10 min for 4 h prior to and following i.c.v. injection. Blood serum was assayed for LH. The experiment was replicated four times such that each ewe received each dose of pNPY. Mean concentrations of LH as well as frequency and amplitude of pulses of LH were attenuated in response to i.c.v. injection of pNPY in a dose-related manner in both OVX and OVX + E ewes.(ABSTRACT TRUNCATED AT 250 WORDS)     

NO mediates effects of estrogen on central regulation of blood pressure in restrained,ovariectomized rats

We tested the hypotheses that estrogen replacement in ovariectomized (OVX) rats attenuates cardiovascular responses to psychological stress and that nitric oxide (NO) in the brain mediates these effects. Female rats were OVX; one group received 17beta-estradiol (OVX-E) for 11-12 days and the other received vehicle (OVX-V). Seven days after OVX, OVX-E and OVX-V rats were chronically instrumented for arterial pressure measurements and intracerebroventricular injections. Later (4-5 days), OVX-E and OVX-V rats received intracerebroventricular injections of NG-nitro-l-arginine (88 microg/kg), an inhibitor of constitutive NO production, or vehicle. Mean arterial pressure (MAP) and heart rate responses were then measured in conscious rats exposed to two cycles of 1-h restraint/1-h rest. We show that MAP responses in restrained OVX-E rats were attenuated both during restraint and during rest. Although inhibition of NO production in the brain had no effect on MAP responses to restraint in OVX-V rats, it augmented responses in restrained OVX-E rats, especially during periods of rest, so that MAPs in restrained OVX-E and OVX-V rats were indistinguishable. Finally, NO levels in hypothalami and brain stems were elevated in restrained OVX-E, but not OVX-V, rats compared with their respective unrestrained controls. These results show that estrogen replacement in OVX rats reduces arterial pressure responses to psychological stress and that these effects are mediated, at least in part, by NO.     

Ovarian estradiol modulates the stimulatory effect of ovulation-inducing factor (OIF) on pituitary LH secretion in llamas

This study was designed to: 1) characterize the effect of ovulation-inducing factor (OIF) on pituitary LH secretion in ovariectomized (OVX) llamas; and 2) determine the effect of OIF on LH secretion in OVX llamas pretreated with estradiol-17β (E-17β) or estradiol benzoate (EB). In Experiment 1, intact and OVX llamas (n = 5 or 6 per group) were assigned to a two by two factorial design: 1) Intact llamas treated with 1 mL of phosphate buffered saline (PBS); 2) Intact llamas treated with 1 mg of purified OIF; 3) OVX llamas treated with 1 mL of PBS; or 4) OVX llamas treated with 1 mg of purified OIF. In Experiment 2, intact and OVX llamas (n = 5 or 6 per group) were randomly assigned to the following groups: 1) Intact llamas treated with 1 mg of purified OIF; 2) OVX llamas treated with 1.0 mL of PBS; 3) OVX llamas treated with 1.0 mg of purified OIF; 4) OVX llamas primed with E-17β, followed by 1.0 mg of purified OIF. Experiment 3 was similar as described for Experiment 2, except that priming was done with EB. In Experiment 1, animal category by treatment and animal category by treatment by time interactions tended (P = 0.08) to affect LH concentration. The effect of OIF on LH released was partly restored (P < 0.05), to the values observed for the intact OIF-treated females, when OVX llamas were primed with E-17β or BE (Experiments 2 and 3). We concluded that peripheral estradiol concentrations in llamas partially modulates the effect of OIF on pituitary LH secretion; however, other ovarian factor(s) could also participate in this modulatory action.