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.     

Orchidectomy unleashes pulsatile luteinizing hormone secretion in the rat

We charted the development of pulsatile luteinizing hormone (LH) secretion as a function of the time elapsed after removal of the testes. On seven occasions between the moment of castration and 80 days afterwards, we obtained consecutive blood samples at frequent (2.5- to 5-min) intervals from cannulated male rats. Orchidectomy increased both the amplitude and frequency of LH release within 1 day after surgery. Amplitude: From 19 h through 80 days postcastration, peak LH levels rose steadily, and LH pulses grew progressively more pronounced in nadir-to-peak amplitude. Frequency: Our findings offer new evidence establishing an increase in LH pulse frequency from less than 1 per h to 2-3 per h within 1 day after orchidectomy. Once deprived of testicular influences, the frequency of pulsatile LH discharges remained static through 80 days. The sudden onset (less than 1 day after castration) and temporal uniformity of high-frequency LH pulses demonstrate that LH release is governed by an intrinsic, 20- to 30-min neural periodicity in castrate rats. Most important, these findings imply that the testes mask or modulate the expression of an intrinsic, 20- to 30-min neural generator directing the periodic discharge of LH in the intact male rat.     

Characteristics of pulsatile luteinizing hormone secretion in middle-aged ovariectomized rats

Experiments were performed to characterize the pulsatile patterns of circulating luteinizing hormone (LH) in the middle-aged ovariectomized (OVX) rat. Frequent blood samples were taken from OVX rats at 6, 7-8, and 9-10 mo of age, and LH was measured by radioimmunoassay. Rats had been OVX either 2 wk (STO) or 10-20 wk (LTO) previously. Mean LH levels were significantly lower with increasing age, reflecting effects on both pulse amplitude and pulse frequency. Mean LH levels were higher in LTO than STO groups, reflecting primarily an increase in pulse amplitude, but there was also a small, significant decrease in pulse frequency with increased time following OVX. In a second experiment, a random selection of the rats in the STO groups was tested again 10 wk after OVX. A significantly higher number of 9- to 10-mo-old rats had pulsatile patterns at 10 wk than at 2 wk following OVX. Furthermore, mean plasma LH concentrations were higher in all three groups. We conclude that decreases in several parameters of LH secretion are seen in middle-aged OVX rats, at the time when irregularities are first seen in the estrous cycle in the intact rat.     

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.     

Orexin-A suppresses the pulsatile secretion of luteinizing hormone via beta-endorphin

Orexins, the novel hypothalamic neuropeptides that stimulate feeding behavior, have been shown to suppress the pulsatile secretion of LH in ovariectomized rats. However, the mechanism of this action is still not clear. We examined the effect of naloxone, a specific opioid antagonist, on the suppression of the pulsatile secretion of LH by orexins to determine whether beta-endorphin is involved in this suppressive effect. We administered orexins intracerebroventricularly and injected naloxone intravenously in ovariectomized rats, and we measured the serum LH concentration to analyze the pulsatile secretion. Administration of orexin-A significantly reduced the mean LH concentration and the pulse frequency, but coadministration of naloxone significantly restored the mean LH concentration and the pulse frequency. Administration of orexin-B also significantly reduced the mean LH concentration and the pulse frequency, and coadministration of naloxone did not restore them. These results indicate that orexin-A, but not orexin-B, suppresses GnRH secretion via beta-endorphin.     

Effects of season and lactation on luteinizing hormone secretion in postpartum ewes

Effects of season, postpartum interval and short-term weaning were investigated on luteinizing hormone (LH) secretion in ewes. Blood samples were collected at 10-min intervals for 4 h (basal period). Then gonadotropin-releasing hormone (GnRH) was administered and 10 more blood samples were collected over an additional 4 h period. The effects of day post partum (5, 20 or 40) and short-term weaning (weaned Day 37, tested Day 40 post partum) on basal and GnRH-induced LH secretion were tested. Mean basal concentrations of LH for ewes on Day 5, 20 or 40 post partum ranged from 1.6 to 4.6 ng/ml and did not differ. Mean concentrations of LH during the post-GnRH sampling interval were greater (P<0.01) for ewes bled on Day 20 or 40 post partum (12.3 and 11.8 ng/ml, respectively) than for ewes bled on Day 5 or for unbred control ewes (6.7 and 5.8 ng/ml, respectively). Weaning on Day 37 depressed GnRH-induced LH secretion on Day 40 post partum (8.18 ng/ml; P<0.05). Seasonal changes in LH secretion on Day 20 or 40 post partum in January, March or June lambing ewes were also tested. There was no difference in basal or GnRH-induced LH secretion between Day 20 or 40 post partum among groups in January or March.. In June, ewes had lower (P<0.01) basal and GnRH-induced LH secretion on Day 20 post partum than ewes did on Day 40 post partum. Across month of the year, on Day 20 post partum, ewes lambing in March released more LH in response to GnRH than ewes lambing in January (P=0.07) or June (P<0.05). Response to GnRH on Day 20 post partum was similar for ewes lambing in January or June (P>0.1). Ewes lambing in January released less (P<0.01) LH on Day 40 post partum than ewes lambing in March or June; however, no difference was detected between the latter two groups (P>0.1). Thus, seasonal modifications of the releasable pool of LH may mask or modify the effect of the postpartum interval upon this endocrine response.     

Effects of weaning and naloxone on luteinizing hormone secretion in postpartum ewes

The effects of weaning and naloxone on concentrations of luteinizing hormone (LH) at 20 days postpartum were examined. March-lambing Finnish Landrace x Southdown ewes (n = 20) were bled via jugular venipuncture at 10-min intervals for 4 h. Naloxone (1 mg/kg bodyweight) was administered i.v. at 60, 120, and 180 min. Treatment groups were suckled (S), weaned on Day 17 (W), suckled plus naloxone (SN), and weaned plus naloxone (WN). Mean concentrations of LH were calculated for 0-60, 70-120, 130-180, and 190-240 min time intervals. Analysis of variance indicated a group effect (p = 0.03) and a group x time interaction (p = 0.02). Concentrations of LH followed a cubic pattern in SN (p = 0.03) and WN (p = 0.08) ewes, whereas LH levels decreased (p less than 0.05) in a pattern consisting of linear and quadratic trends in S and W ewes. Concentrations of LH in S and W ewes were similar at 0-60 and 190-240 min. W ewes had lower (p less than 0.05) concentrations of LH than S ewes at 70-120 and 130-190 min. Further analysis revealed that LH was elevated in SN ewes (p = 0.01) and WN ewes (p = 0.07) at 70-120 min, but was not significantly elevated at 130-180 min. At 190-240 min LH was increased in SN ewes (p = 0.03), but LH levels in WN ewes were similar to those of SN ewes as well as to those of S control ewes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intracerebroventricular administration of ghrelin rapidly suppresses pulsatile luteinizing hormone secretion in ovariectomized rats.

Ghrelin, an endogenous growth hormone (GH) secretagogue, is shown to increase food intake, which action is similar to that of orexin, also a hypothalamic peptide. Since orexin suppresses pulsatile LH secretion in ovariectomized (OVX) rats, the present study was undertaken to investigate whether ghrelin also suppresses LH secretion. Effects of intracerebroventricularly injected ghrelin (0.1 nmol/0.3 microl) were examined in OVX rats treated with a small dose of 17beta-estradiol (E(2)). After ghrelin injection, pulsatile LH secretions which were ongoing in these E(2)-treated OVX rats were significantly suppressed for about 1 h, whereas GH secretion increased, peaking at 30 min. The main parameter suppressed by ghrelin was the pulse frequency, not the pulse amplitude, suggesting the hypothalamus as the site of ghrelin action. This study provides evidence that ghrelin acts not only in the control of food intake but also in the control of LH secretion.     

Reduced pulsatile luteinizing hormone and testosterone secretion with aging in the male rat

To identify possible age-dependent changes in the feedback relationship between the brain-pituitary and testes, we examined the minute-to-minute patterns of plasma luteinizing hormone (LH) and testosterone (T) in intact, young male rats and compared these profiles to those of old animals. Young (3 mo; n = 11) and old (22 mo; n = 12) Sprague-Dawley rats were fitted with indwelling venous catheters and between 24 and 48 h later, were bled without anesthesia, by remote sampling, at 10-min intervals for 8 h. Blood samples of 400 microliter were withdrawn, and an equivalent volume of a blood replacement mixture was infused after each sample. Plasma LH and T levels in each sample were measured by radioimmunoassay (RIA). Plasma T levels in old animals failed to show the transient oscillations observed in young animals. Mean plasma T levels were 50% lower in old compared to young animals (P less than 0.001). Plasma patterns of LH in old animals, like their younger counterparts, showed statistically significant episodic increases, whose apparent pulse frequency was inappropriately low for their circulating T level (although not statistically different from the young group). Pulse amplitude in the old animals was 66% lower in the old compared to the young group (P less than 0.015). We conclude that age-associated alterations in brain mechanisms governing LH secretion underline these endocrine changes.     

Photoperiodic regulation of gonadal growth and pulsatile luteinizing hormone secretion in male ferrets

Testicular growth was monitored in male ferrets subjected to one of the following photoperiodic treatments begun at weaning (8 weeks of age): 8 hr light/day (short days), 18 hr light/day (long days), or short days followed by transition to long days at either 10, 12, or 14 weeks of age. Mean ages to achieve adult testis width of greater than or equal to 12 mm were 27.5 +/- 1.3, 25.0 +/- 1.5, 23.6 +/- 2.9, 20.0 +/- 0.8, and 21.2 +/- 1.0 weeks in ferrets raised from weaning in long days, raised from weaning in short days, and transferred from short to long days at 10, 12, or 14 weeks, respectively. This criterion was met significantly earlier by ferrets experiencing the photoperiod transition at 12 or 14 weeks of age than by ferrets housed in long days from weaning. At the end of the experiment (30 weeks of age), mean testis width was significantly smaller in ferrets raised in long days from weaning or transferred to long days at 10 weeks of age, compared to that of the other three groups (p less than 0.05). In a second experiment, photoperiod experience with long or short days was begun at birth, and testicular size was monitored for a longer period of time. The time courses for testicular maturation were similar to that obtained when these treatments began at weaning. By 40 weeks of age, mean testis width of ferrets raised in long days was comparable to that of ferrets raised in short days. A third study determined that the retarded testicular growth observed in ferrets exposed to long days from weaning was correlated with diminished pulsatile luteinizing hormone (LH) secretion. At 28 weeks of age, mean LH pulse frequency was 0.86 +/- 0.09 pulses/hr in ferrets undergoing spontaneous puberty in short days or photoinduced puberty after a short-to-long-day transition; pulse frequency was significantly lower (0.46 +/- 0.26 pulses/hr; p less than 0.05) in ferrets raised in long days. These results indicate that gonadal growth can be precociously induced in male ferrets by exposure to a sequence of short days followed by long days, and that the absence of sufficient prepubertal exposure to short days compromises pulsatile LH secretion and rate of gonadal growth. Experience with short days during development may be necessary for manifestation of stimulatory responses to long days.     

Effects of pulsatile infusion of luteinizing hormone-releasing hormone on luteinizing hormone secretion and ovarian function in hypophysial stalk-transected beef heifers

Hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were investigated in beef heifers by infusing LH-releasing hormone (LHRH) in a pulsatile manner (1 microgram/ml; 1 ml during 1 min every h) into the external jugular vein of 10 hypophysial stalk-transected (HST) animals. The heifers were HST approximately 30 mo earlier. All heifers had increased ovarian size during the LHRH infusion. The maximum ovarian size (16 +/- 2.7 cm3) was greater (P less than 0.01) than the initial ovarian size (8 +/- 1.4 cm3). Ovarian follicular growth occurred in 4 of 10 HST heifers in response to pulsatile LHRH infusion. In 2 heifers, an ovarian follicle developed to preovulatory size, but ovulation occurred in only 1 animal after the frequency of LHRH was increased (1 microgram every 20 min during 8 h). In blood samples obtained at 20-min intervals every 5th day, LH concentrations in peripheral serum remained consistently low (0.9 ng/ml) and nonepisodic in the 10 HST heifers during infusion of vehicle on the day before beginning LHRH. In 7 of 10 HST animals, episodic LH secretion occurred in response to pulsatile infusion of LHRH. In 3 of these long-term HST heifers, however, serum LH remained at basal levels and the isolated pituitary seemingly was unresponsive to pulsatile infusion of LHRH as indicated by sequential patterns of gonadotropin secretion obtained at 5-day intervals. These results indicate that pulsatile infusion of LHRH induces LH release in HST beef heifers.     

Photoperiodic regulation of pulsatile luteinizing hormone secretion and adenohypophyseal gene expression in female golden hamsters.

LH concentrations were measured in serum collected at 10-min intervals from chronically ovariectomized female Syrian hamsters that had been maintained for 9 wk in stimulatory (long) or inhibitory (short) photoperiods. Short days reduced the number of detectable LH pulses during both the morning and the afternoon. Most short-day hamsters experienced a gradual afternoon rise in serum LH concentrations; this rise was not composed of multiple pulses. In separate groups of similarly treated hamsters, pituitary LH-beta mRNA abundance was significantly reduced by short-day exposure at both times of day even though serum LH concentrations rose in the afternoon. Estradiol treatment induced an afternoon surge of serum LH in both photoperiods, and eliminated the effect of photoperiod on LH-beta mRNA abundance in the afternoon. Serum prolactin (PRL) concentrations were not consistently influenced by day length in castrated hamsters with or without estrogen treatment, but PRL mRNA abundance was significantly suppressed by short-day exposure in all groups. The results indicate that day length exerts profound steroid-independent effects upon hypophyseal gene expression, and that the regulation of LH-beta mRNA abundance may be due to photoperiodic control of the neural GnRH pulse generator.     

Regulation of pulsatile luteinizing hormone secretion by insulin in the diabetic male lamb

This study tested the hypothesis that LH secretion is modulated by insulin and that the responsiveness to hypoinsulinemia is enhanced by sex steroids. The model was the developing male lamb (12-26 wk of age) rendered diabetic by chemically induced necrosis of insulin-secreting tissue (streptozotocin). Our approach was to monitor LH secretion under diabetic conditions, with or without insulin supplementation, either in the presence or in the absence of gonadal steroids. The first experiment determined if chronic insulin supplementation could sustain LH secretion in diabetic lambs. After documentation of the induced diabetic condition, twice-daily treatment with a long-acting insulin preparation (Lente) minimized diabetes-induced hyperglycemia, sustained growth, and maintained LH pulse frequency at levels comparable to pre-diabetic conditions. A second experiment evaluated the acute regulation of LH secretion by insulin. Twenty-four hours of insulin withdrawal decreased LH pulse frequency, increased circulating glucose levels, increased the concentration of plasma non-esterified fatty acids (NEFAs), and increased urinary output of ketones. LH pulse frequency continued to decline after 96 h of insulin withdrawal. By contrast, 24 h of insulin re-supplementation increased LH pulse frequency, reduced circulating glucose and NEFA concentrations, decreased plasma cortisol, and reduced urinary output of ketones. After 96 h of insulin re-supplementation, LH pulse frequency increased further, to levels comparable with those before insulin withdrawal. A third experiment determined if the effects of insulin withdrawal on LH secretion are influenced by the presence of gonadal steroids. The same individuals were treated with a physiologic dose of estradiol (Silastic capsule, s.c.) and subsequently monitored for changes in LH secretion in the presence and in the absence of exogenous insulin. Prior to insulin withdrawal, estradiol decreased both LH pulse frequency and pulse amplitude. Moreover, after 96 h of insulin withdrawal, estradiol potentiated the decline in LH pulse frequency (47% reduction in LH pulse frequency in the presence of estradiol versus 26% reduction in LH pulse frequency in the absence of estradiol). These findings support the contention that insulin and/or insulin-dependent changes in glucose availability modulate LH(GnRH) pulse frequency, and that such effects are potentiated by, but not dependent upon, gonadal steroids.     

Serum glucose and free fatty acids modulate growth hormone and luteinizing hormone secretion in the pig

Two experiments were conducted to determine the effect of free fatty acids (FFA) and glucose treatment on growth hormone (GH) and luteinizing hormone secretion in the pig. In Experiment (Exp) 1, 15 prepuberal gilts received an intravenous infusion of FFA (n = 5; 3 ml of 10% Liposyn II/kg), glucose (n = 5; 1 g/kg), or saline (n = 5; 3 ml of 0.9%/kg). Jugular blood samples were collected every 15 min for 2 hr before and 3 hr after intravenous infusion of saline, FFA, and glucose. Synthetic [Ala15]-h growth hormone-releasing factor-(1-29)NH2 (1 microgram/kg) and gonadotropin-releasing hormone (0.2 micrograms/kg) were administered 30 min after infusion (Time 0 = infusion). In Exp 2, eight prepuberal gilts received either FFA (n = 4) or saline (n = 4) as described in Exp 1, except that treatments were given every hour ove a 10-hr period. Blood samples were collected every 15 min from 1 hr before to 10 hr after the start of FFA or saline infusion. In Exp 1, the peak GH response to growth hormone-releasing factor was delayed by 45 min (P less than 0.01) by glucose treatment and suppressed (P less than 0.01) by FFA treatment. The luteinizing hormone response to gonadotroph-releasing hormone was suppressed (P less than 0.03) by glucose and enhanced (P less than 0.03) by FFA. In Exp 2, the number of GH pulses was increased (P less than 0.05) by FFA infusion and GH concentrations were positively correlated (r = 0.58, P less than 0.0003) with FFA concentrations, while luteinizing hormone pulse amplitude was greater (P less than 0.01) in FFA gilts than in saline gilts. These results indicate that FFA are more effective modulators of GH secretion than acute hyperglycemia, while metabolic status can alter pituitary responsiveness to gonadotropin-releasing hormone.     

Effects of time after ovariectomy, season and oestradiol on luteinizing hormone and follicle-stimulating hormone secretion in ovariectomized ewes.

During the breeding season, five groups of three ewes were implanted at ovariectomy with 0.36, 0.5, 1.0 and 6.0 cm oestradiol implants or implants containing no steroid. Eleven days after receiving implants, blood samples were taken every 10 min for 6 h; implants were then removed. Treatments were repeated three times during each of two consecutive breeding seasons and four times during the intervening anoestrus. In ovariectomized ewes without steroid treatment, luteinizing hormone (LH) pulse frequency increased from early to mid-breeding season, decreased to a minimum at mid-anoestrus and increased to reach a maximum at the mid-point of the second breeding season, subsequently declining. LH pulse amplitude was inversely related to frequency. Basal serum LH concentrations decreased gradually from the first breeding season to reach a minimum at mid-anoestrus and gradually increased to reach a maximum at the end of the second breeding season. Mean serum LH and follicle-stimulating hormone (FSH) concentrations were higher at the end of the second breeding season compared with the beginning of the first breeding season. All parameters of gonadotrophin secretion were decreased much more by oestradiol during the anoestrus than during the breeding season. LH pulse frequency was decreased during anoestrus and at high oestradiol concentrations during the first breeding season. Apart from LH pulse amplitude, the decreases in all parameters of gonadotrophin secretion were less during the second compared with the first breeding season. The minimum effective dose of oestradiol required to decrease mean and basal serum concentrations of LH during anoestrus was lower than in the breeding season. The minimum effective dose of oestradiol required to decrease mean serum concentrations of FSH was lower in the first compared with the second breeding season. Oestradiol depression of LH pulse amplitude and mean serum concentrations of LH and FSH showed a dose dependency during the breeding season. During anoestrus dose dependency was seen for basal concentrations of LH and mean serum concentrations of LH and FSH. We conclude that significant chronic changes in gonadotrophin secretion occur in the ewe with time after ovariectomy. Sensitivity to oestradiol also changes, and the effects of oestradiol are not always dose dependent. We suggest that the circannual pattern of LH pulse frequency and basal LH secretion are directly linked to the circannual cycle of photoperiod.(ABSTRACT TRUNCATED AT 400 WORDS)     

Central action of insulin regulates pulsatile luteinizing hormone secretion in the diabetic sheep model

This study tested the hypothesis that central mechanisms regulating luteinizing hormone (LH) secretion are responsive to insulin. Our approach was to infuse insulin into the lateral ventricle of six streptozotocin-induced diabetic sheep in an amount that is normally present in the CSF when LH secretion is maintained by peripheral insulin administration. In the first experiment, we monitored cerebrospinal fluid (CSF) insulin concentrations every 3-5 h in four diabetic sheep given insulin by peripheral injection (30 IU). The insulin concentration in the CSF was increased after insulin injection, and there was a positive relationship between CSF and plasma concentrations of insulin (r = 0.80, P < 0.01). In the second experiment, peripheral insulin administration was discontinued, and the sheep received either an intracerebroventricular (i.c.v.) infusion of insulin (12 mU/day in 2.4 ml saline) or saline (2.4 ml/day) for 5 days (n = 6) in a crossover design. The dose of insulin (i.c.v.) was calculated to approximate the increase in CSF insulin concentration found after peripheral insulin treatment. To monitor LH secretory patterns, blood samples were collected by jugular venipuncture at 10-min intervals for 4 h on the day before and 5 days after the start of i.c.v. insulin infusion. To monitor the increase in CSF insulin concentrations, a single CSF sample was collected one and four days after the start of the central infusion. The i.c.v. insulin infusion increased CSF insulin concentrations above those in saline-treated animals (P < 0.05) and maintained them at or above the peak levels achieved after peripheral insulin treatment. Central insulin infusion did not affect peripheral (plasma) insulin or glucose concentrations. LH pulse frequency in insulin-treated animals was greater than that in saline-treated animals (3.5 +/- 0.2 vs. 2.3 +/- 0.3 pulses/4 h, P < 0.01), but it was less than that during peripheral insulin treatment (4.8 +/- 0.2 pulses/4 h, P < 0.01). Our findings suggest that physiologic levels of central insulin supplementation are able to increase pulsatile LH secretion in diabetic sheep with low peripheral insulin. These results are consistent with the notion that central insulin plays a role in regulating pulsatile GnRH secretion.     

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.     

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.     

Absence of an increase in gonad-independent drive to pulsatile luteinizing hormone secretion during photoperiod-induced puberty

This study was conducted to determine if photoperiod can influence the pattern of luteinizing hormone (LH) secretion in the absence of the ovaries in the developing female sheep. Lambs were raised in a photoperiod sequence (short, long, short days) known to induce puberty between 30 and 35 wk of age, or in a photoperiod (only short days) that prevents puberty during the first year. Their ovaries were removed at 10 wk of age, and the detailed pattern of LH was assessed (samples at 12-min intervals for 4 h) each 3- to 5-wk period between 9 and 45 wk of age. Rapid LH pulses (40- to 50-min interpulse interval) were evident within a few weeks after ovariectomy in both groups of females. Those exposed to the artificial photoperiod sequence that induces normal sexual maturity did not increase their pulse frequency further during the pubertal period. Moreover, their LH pulse frequencies were not greater than those in agonadal females exposed to the photoperiod that delays puberty. These findings indicate that photoperiodic induction of puberty in the sheep does not require steroid-independent modulation of pulsatile LH secretion.