Tonic Luteinizing Hormone Secretion Research Articles

Ad libitum feeding triggers puberty onset associated with increases in arcuate Kiss1 and Pdyn expression in growth-retarded rats.

Increasing evidence shows that puberty onset is largely dependent on body weight rather than chronological age. To investigate the mechanism involved in the energetic control of puberty onset, the present study examined effects of chronic food restriction during the prepubertal period and the resumption of ad libitum feeding for 24 and 48 h on estrous cyclicity, Kiss1 (kisspeptin gene), Tac3 (neurokinin B gene) and Pdyn (dynorphin A gene) expression in the hypothalamus, luteinizing hormone (LH) secretion and follicular development in female rats. When animals weighed 75 g, they were subjected to a restricted feeding to retard growth to 70–80 g by 49 days of age. Then, animals were subjected to ad libitum feeding or remained food-restricted. The growth-retarded rats did not show puberty onset associated with suppression of both Kiss1 and Pdyn expression in the arcuate nucleus (ARC). 24-h ad libitum feeding increased tonic LH secretion and the number of Graafian and non-Graafian tertiary follicles with an increase in the numbers of ARC Kiss1- and Pdyn-expressing cells. 48-h ad libitum feeding induced the vaginal proestrus and a surge-like LH increase with an increase in Kiss1-expressing cells in the anteroventral periventricular nucleus (AVPV). These results suggest that the negative energy balance causes pubertal failure with suppression of ARC Kiss1 and Pdyn expression and then subsequent gonadotropin secretion and ovarian function, while the positive energetic cues trigger puberty onset via an increase in ARC Kiss1 and Pdyn expression and thus gonadotropin secretion and follicular development in female rats.

Sex bias of the birth litter affects surge but not tonic LH secretion in gilts.

The physiology and behavior of gilts that develop in a male-biased litter can differ from gilts that develop in a female-biased litter. We hypothesized that gilts from male-biased litters will have a delayed and attenuated luteinizing hormone (LH) surge, and reduced LH pulse frequency and amplitude compared to gilts from female-biased litters. Gilts were selected at birth from male-biased (>60% males n = 10) or female-biased (>60% females n = 9) litters. From 18 wk of age, detection of puberty using daily boar contact began and their subsequent estrous periods were synchronized with oral progestogen (altrenogest). On day 3 after altrenogest withdrawal, blood samples were obtained from 6 gilts per sex bias group at 10 min intervals from 0900 to 2100 h to determine LH pulse amplitude and frequency. From 0900 on day 4, all 19 gilts were sampled every 4 h until the end of estrus to characterize LH surge dynamics. There were no differences between groups in LH pulse characteristics. Compared to gilts from female-biased litters, the LH surge in gilts from male-biased litters was delayed [56.00 ± 3.32 h vs. 43.11 ± 3.76 h (mean ± standard error of the mean (SEM)), P < 0.05], the duration was decreased [29.78 ± 2.12 h vs. 37.71 ± 1.19 h (mean ± SEM), P < 0.05] and the total secretion as measured by area under the curve was decreased (91.42 ± 9.52 ng/mL vs. 120.28 ± 9.48 ng/mL, P < 0.05). Our results indicate that a male-biased uterine environment has different effects on the tonic secretion of LH than the LH surge, with only some elements of the LH surge being affected.

Leptin Signaling in GABA Neurons, But Not Glutamate Neurons, Is Required for Reproductive Function

The adipocyte-derived hormone leptin acts in the brain to modulate the central driver of fertility: the gonadotropin releasing hormone (GnRH) neuronal system. This effect is indirect, as GnRH neurons do not express leptin receptors (LEPRs). Here we test whether GABAergic or glutamatergic neurons provide the intermediate pathway between the site of leptin action and the GnRH neurons. Leptin receptors were deleted from GABA and glutamate neurons using Cre-Lox transgenics, and the downstream effects on puberty onset and reproduction were examined. Both mouse lines displayed the expected increase in body weight and region-specific loss of leptin signaling in the hypothalamus. The GABA neuron-specific LEPR knock-out females and males showed significantly delayed puberty onset. Adult fertility observations revealed that these knock-out animals have decreased fecundity. In contrast, glutamate neuron-specific LEPR knock-out mice displayed normal fertility. Assessment of the estrogenic hypothalamic-pituitary-gonadal axis regulation in females showed that leptin action on GABA neurons is not necessary for estradiol-mediated suppression of tonic luteinizing hormone secretion (an indirect measure of GnRH neuron activity) but is required for regulation of a full preovulatory-like luteinizing hormone surge. In conclusion, leptin signaling in GABAergic (but not glutamatergic neurons) plays a critical role in the timing of puberty onset and is involved in fertility regulation throughout adulthood in both sexes. These results form an important step in explaining the role of central leptin signaling in the reproductive system. Limiting the leptin-to-GnRH mediators to GABAergic cells will enable future research to focus on a few specific types of neurons.

The effects of prolonged, intracerebroventricular prolactin treatment on luteinizing hormone secretion, catecholaminergic activity and estrous behavior in ewes.

Searching for the role of prolactin (PRL) in controlling gonadotropic axis activity in sheep, we studied the effects of prolonged, intracerebroventricular (i.c.v.) PRL infusion on luteinizing hormone (LH) secretion and catecholaminergic activity in the hypothalamic infundibular nuclei/median eminence (IN/ME) in sexually active ewes during the periovulatory period. Three groups of animals received the following treatments: 1). i.c.v. infusion of PRL at a dose of 200 microg/day (Lower dose, n = 5); 2). i.c.v. infusion of PRL at a dose of 400 microg/day (Higher dose, n = 6), and 3). i.c.v. infusion of the vehicle (control, n = 5). Each dose of PRL was infused in a pulsatile manner, 4 x 50 microg/h and 4 x 100 microg/h, in 30-min intervals, respectively, during four consecutive days before oncoming ovulation. The estrous behavior of ewes following treatments was also monitored as a determinant of the GnRH/LH surge. Two series of blood collections were made in every ewe, the first on the day preceding the infusion (day 0 of the experiment), the second on the day after the infusion (day 5 of the experiment). In addition, on day 5 of the experiment, perfusions of the IN/ME were made by the push-pull method, either in control or lower dose-treated animals. It was shown that a significant (p < 0.01, p < 0.001) increase in tonic LH secretion during the periovulatory period remained in ewes irrespective of the kind of infusion. No statistical differences were found in LH pulse frequency, amplitude, or in the length of the pulse when compared with values from day 0 and 5 of the experiment within each group. A significant (p < 0.001) increase in IN/ME perfusate concentrations of dopamine and noradrenaline metabolites was noted in PRL-treated ewes in comparison with those in the control. The estrous behavior in PRL-treated animals was delayed for a few days, 3.80 +/- 0.80 days at the lower dose (p < 0.01), and 2.83 +/- 0.98 days at the higher dose (p < 0.05) in comparison with the control, 0.20 +/- 0.20 days. These data indicate that maintenance of an increased PRL concentration within the central nervous system (CNS) for a few days before oncoming ovulation has no inhibitory effect on tonic LH secretion. A few-day shift of the preovulatory GnRH/LH surge, as determined by estrous behavior, might, however, be a consequence of the PRL-induced increase in catecholamine turnover in the IN/ME.

Lack of effect of intracerebroventricular infusion of H1 or H2 histamine receptor antagonists on luteinizing hormone secretion in fed or fasted rats

The effects of intracerebroventricular infusion of histamine receptor antagonists on pulsatile secretion of luteinizing hormone (LH) were examined in ovariectomized, estradiol-primed rats under conditions of normal feeding or fasting for 48 h. Blood samples for determination of the plasma LH concentration by radioimmunoassay were collected every 6 min for 3 h through an atrial indwelling catheter. Ten microliters of either the H1 receptor antagonist chlorpheniramine (3 mM), the H2 receptor antagonist ranitidine (3 mM), or artificial cerebrospinal fluid (vehicle) were infused into the third ventricle during the second 1-h period of blood sampling. The infusion of either receptor antagonist failed to restore the pattern of pulsatile LH release in fasted animals to that observed in fed rats. The plasma LH concentration as well as the pulse frequency and pulse amplitude of LH secretion in fed rats were also not affected by either receptor antagonist. These results indicate that H1 or H2 receptor antagonists did not affect on tonic LH secretion in fed or fasted rats.

Suppression of Tonic Luteinizing Hormone Secretion and Norepinephrine Release near the GnRH Neurons by Estradiol in Ovariectomized Rats

One of the major neurotransmitters that controls pulsatile luteinizing hormone (LH) secretion is norepinephrine (NE). NE pulses detected in the median eminence of ovariectomized rhesus monkeys are highly correlated with both GnRH and LH pulses. In contrast, previous reports suggest that this is not the case in rats, thus it remains to be determined whether NE stimulates LH release on a pulse-by-pulse basis in that species. Further, a variety of indirect evidence supports the hypothesis that in rats, estradiol exerts its negative feedback action on LH secretion in part by inhibiting noradrenergic neurotransmission that is stimulatory to LH release, but there is no direct evidence to support this hypothesis. Therefore the following study was designed to test the hypothesis that estradiol suppresses NE release in the vicinity of the GnRH neurons after ovariectomy. In addition, we examined whether episodes of NE release are correlated with LH pulses in ovariectomized rats. Blood samples and microdialysates of the diagonal band of Broca/medial preoptic area (DBB/MPOA) were collected every 5 min from 09:00 to 14:00 h from untreated or estradiol-treated (4–5 days), long-term ovariectomized (1–4 months) rats for determination of plasma LH by RIA and NE release by HPLC. The results indicate that in both untreated and estradiol-treated ovariectomized rats, LH pulses are not correlated with episodes of NE. Thus, NE may play a permissive role in the control of pulsatile LH secretion in rats. Further, estradiol treatment leads to a suppression of both plasma LH levels and NE release in the DBB/MPOA, supporting the hypothesis that a decrease in NE neurotransmission that is stimulatory to LH release mediates the negative feedback action of estradiol on tonic LH secretion.

Glucose availability modulates the timing of the luteinizing hormone surge in the ewe.

To determine if glucose availability modulates the timing of the positive feedback action of oestrogen on gonadotropin secretion, we monitored the estradiol-induced luteinizing hormone (LH) surge in sheep (n = 5/group) made transiently hypoglycemic by insulin. Experiment 1 determined an effective insulin treatment, one which would depress tonic LH secretion. Two injections of insulin (5 IU/kg iv) 4 h apart were found to induce extended hypoglycemia (10-13 h) and to decrease the LH pulse frequency for 8 h (5.0 +/-0.32 pulses/4 h before versus 2.5+/-0.34 pulses/4 h after insulin; P<0.05; mean +/- SEM). Using this same paradigm, experiment 2 determined the influence of the transient hypoglycemia on the LH surge mechanism. In control sheep, estradiol (subcutaneous implants at hour 0) evoked an LH surge with a latency period of 12.4+/-0.5 h. When insulin was administered either before (hours -4 and 0) or after the estradiol stimulus (hours 4 and 8, or 12 and 16), the onset of the LH surge was delayed to 29.0+/-2.4 h (average of all three time groups, P <0.05). Infusion of glucose from hours 12-30, along with insulin, prevented hypoglycemia and restored the normal timing of the oestrogen-induced LH surge to that of controls (15.4+/-0.93 h, P>0.05). These findings suggest that not only is the tonic mode of LH secretion sensitive to metabolic fuel availability, but the surge mode of LH secretion is as well.

Effect of adrenocorticotrophic hormone on gonadotrophin releasing hormone-induced luteinizing hormone secretion in vitro.

An in vitro perifusion study investigated the effect of different forms of adrenocorticotrophic hormone (ACTH) on gonadotrophin releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion, particularly GnRH self-priming, and oestradiol sensitisation of the ovine pituitary. Fragments of pituitaries were obtained from mixed-breed adult nonpregnant female sheep (without corpora lutea, unless otherwise stated). The amount of LH released by different doses of GnRH (2.5 x 10(-10) M (n = 9 chambers), 1 x 10(-10) M (n = 9), or 5 x 10(-11) M (n = 6)) was evaluated by giving two GnRH pulses (5 min each) 2 h apart. In a duplicate set of chambers, ACTH1-24 (5 x 10(-7) M) was included in the perifusate 0.5 h before the first GnRH challenge. Potassium chloride (KCl; 100 mM) was administered 2 h after the second GnRH challenge to assess the viability of the tissue and the size of the releasable LH pool. Results were expressed as percentage of LH secretion. The influence of ACTH1-24 on oestradiol sensitisation was also examined using pituitaries obtained during the luteal phase. Pituitary tissues were perifused throughout with 1 x 10(-9) M or 6 x 10(-11) M oestradiol in the medium. The LH response to the second GnRH challenge (GnRH 2) was significantly greater (p < 0.01) than after the first (GnRH 1) at the highest dose of GnRH (2.5 x 10(-10) M; 2547 +/- 804 vs. 4547 +/- 1013%), but at the lower doses (1 x 10(-10) M or 5 x 10(-11) M), the self-priming effect of GnRH was not evident (3016 +/- 550 vs. 2932 +/- 490% and 841 +/- 205 vs. 711 +/- 87%). Treatment with ACTH1-24 (5 x 10(-7) M) did not affect tonic LH secretion nor the LH response to the first or second GnRH challenge at any of the GnRH doses tested. The LH released in response to KCl was also similar from control and ACTH1-24-treated tissue at all GnRH doses. Both lower doses of GnRH (1 x 10(-10) M or 5 x 10(-11) M) produced the self-priming effect when the pituitary tissue was sensitised with the higher dose of oestradiol (1 x 10(-9) M; 1711 +/- 239 vs. 5085 +/- 1307%, and 1502 +/- 376 vs. 2619 +/- 629%). In the presence of lower concentrations of oestradiol (6 x 10(-11) M), self-priming was observed only after the higher dose of GnRH (1 x 10(-10) M; 1293 +/- 214 vs. 2865 +/- 436%), not the lower dose (5 x 10(-11) M; 985 +/- 203 vs. 1271 +/- 436%). In spite of these differences, ACTH1-24 treatment did not affect LH secretion (neither basal nor potassium-induced). The effect of ACTH1-39 (1 x 10(-8) M or 5 x 10(-7) M; n = 6 chambers per combination) on GnRH-induced LH secretion was examined using the higher (2.5 x 10(-10) M) or lower dose of GnRH (1 x 10(-10) M), with or without oestradiol sensitisation (1 x 10(-9) M). At the lower dose (1 x 10(-8) M), ACTH1-39 influenced neither tonic nor GnRH-induced LH secretion. The LH released by KCl was also similar to the control and ACTH-treated tissue. In contrast, the higher dose of ACTH1-39 (5 x 10(-7) M) increased tonic LH secretion immediately after inclusion in the medium (104 +/- 3 vs. 161 +/- 20%), but suppressed the GnRH self-priming effect after 2.5 x 10(-10) M, i.e., the LH responses to GnRH 1 and 2 were similar (1786 +/- 294 vs. 1553 +/- 373%). However, the LH response to KCl was not significantly different (p > 0.05) between the control and ACTH-treated tissues (2333 +/- 286 vs. 2638 +/- 431%). When the effect of this higher dose of ACTH1-39 on oestradiol-priming was investigated, ACTH increased tonic LH secretion but suppressed the self-priming effect of GnRH (1 x 10(-10) M GnRH; 945 +/- 274 vs. 922 +/- 323%; p > 0.05), and decreased (p < 0.05) the LH released in response to KCl compared to the controls (1803 +/- 409 vs. 4302 +/- 1017%). In summary, in vitro, ACTH1-24 did not affect either tonic LH secretion, the GnRH self-priming effect, or oestradiol sensitisation. The entire ACTH1-39 increased tonic LH secretion, but reduced GnRH self-priming and oestradiol sensitisation. (ABSTRACT TRUNCATED)

Prenatal testosterone differentially masculinizes tonic and surge modes of luteinizing hormone secretion in the developing sheep.

In sheep, prenatal exposure to androgens during a critical period for sexual differentiation can masculinize tonic luteinizing hormone (LH) secretion and defeminize the LH surge. The present study investigated the possible independent control of these two modes of LH secretion, as revealed by their developmental history. Specifically, we tested the hypothesis that separate critical periods exist for androgenization of tonic and surge LH secretion. Pregnant ewes were treated weekly with testosterone cypionate (200 mg in oil). As a control and to induce robust masculinization of reproductive neuroendocrine function, one group of females received testosterone from day 30 to 86 of gestation (LONG group). To determine if masculinization of tonic LH secretion develops separately from that of the LH surge, two additional groups were treated from day 30 to 51 (EARLY group) or 65-86 (LATE group). At birth, the external genitalia of the LONG- and EARLY-treated females were masculinized; those of the LATE-treated group were normal. At 2 weeks of age, all androgenized females, together with normal males and females (n = 8 each), were gonadectomized and steroids replaced using an estradiol-filled Silastic capsule. First, to determine the timing of the pubertal decrease in steroid sensitivity, circulating LH was monitored twice weekly. Second, to test the function of the LH surge system, LH was measured every 1-2 h for 60 h after an acute increase in estradiol at 9 months of age. With regard to tonic LH secretion, in control males and LONG-treated females, a sustained increase in tonic LH in the presence of constant steroid feedback occurred at 7.1 +/- 0.3 and 10.9 +/- 1.7 weeks of age, respectively (mean +/- SE). In control females, tonic LH increased at 27.1 +/- 0.8 weeks. Despite the differences in their genitalia, EARLY and LATE testosterone treatment produced intermediate effects: LH secretion increased at 19.3 +/- 1.2 and 20.4 +/- 0.8 weeks, respectively. In response to acute estradiol stimulation, all control females produced a surge of LH that peaked 18.4 +/- 0.6 h after steroid treatment. For the control males and LONG-treated females, LH concentrations were not sustained above unsuppressed pretreatment levels throughout the 60-hour sampling period. All but 4 of the 18 EARLY- and LATE-treated females responded to estradiol stimulation with a surge of LH that peaked at 29.8 +/- 1.6 and 31.8 +/- 1.3 h, significantly later than that of control females.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenocorticotrophin (ACTH) 1–39 and corticotrophin releasing hormone (CRH) induced changes in luteinizing hormone secretion from perifused ovine pituitaries

Stress is known to inhibit luteinizing hormone (LH) secretion, possibly via the actions of adrenocorticotrophin (ACTH) or corticotrophin releasing hormone (CRH). The aim of the present study was to investigate whether ACTH 1–39 or CRH could influence tonic and/or gonadotrophin releasing hormone (GnRH) induced LH secretion from the ovine pituitary in vitro. Perifused pituitaries ( n = 44 chambers) from mature ewes were given two GnRH challenges (5 × 10 −10 M): the first 45 min after sampling began and the second 2.5 h later. Some were perifused from 15 min prior to the first GnRH challenge with medium containing either 10 −7 M ( n = 8) or 3.75 × 10 −8 M ( n = 7) ACTH 1–39, or 2.5 × 10 −8 M ( n = 12) CRH. Results were expressed as a percentage of baseline values within a chamber. Neither of the two doses of ACTH 1–39 nor CRH had an effect on tonic LH secretion (control, 99.5 ± 5%; ACTH, 96 ± 8% and 97.5 ± 11%, respectively; CRH, 103 ± 11%) or the initial GnRH-induced LH secretion (control, 1049.1 ± 18.6%; ACTH, 1110.6 ± 352.2% and 1237.1 ± 610%, respectively; CRH, 1040.9 ± 240.7%). However, the LH response to the second GnRH challenge was significantly ( P < 0.001) inhibited (control, 1574.6 ± 224.9%; ACTH, 915.3 ± 426% and 1055.7 ± 326.4%, respectively; CRH, 857.1 ± 192%). Both doses of ACTH 1–39 significantly ( P < 0.001) inhibited the degree of GnRH self-priming, as did CRH (control, 152.4 ± 22.8%; ACTH, 79.3 ± 17.2% and 91.0 ± 18.5%; CRH, 82.8 ± 7.6%) when the second LH response was expressed as a percentage of the first LH response. In conclusion, both ACTH 1–39 and CRH had no effect on tonic LH secretion, but inhibited GnRH-induced LH secretion by preventing GnRH self-priming. The actions of CRH could be direct or be mediated by ACTH.

Studies on Regulatory Mechanisms for Gonadotropins Secretion in Lactating Rats

The present study was designed to clarify mechanisms responsible for regulation of FSH and LH secretion and ovarian follicular maturation during lactation in rats. Throughout lactation in rats, plasma levels of follicle-stimulating hormone (FSH) remain within the range normally found during the diestrous phase of the estrous cycle, regardless of litter size, although levels of luteinizing hormone (LH) in plasma are consistently lower in dams nursing 8 pups than in dams nursing 2 pups. All antral follicles larger than 401μm in diameter degenerate during the first half of lactation and do not reappear until the second half of lactation in rats nursing 8 pups. Healthy follicles of this size, however, are always present in mothers nursing 2 pups. These changes in follicular degeneration corresponded with a decrease in concentrations of plasma LH in the first half of lactation, and follicular development was noted with an increase in basal levels of plasma LH in the second half of lactation. Corpora lutea formed after postpartum ovulation secrete a large amount of progesterone under the influence of prolactin released as a result of suckling. Despite continued suckling, plasma concentrations of prolactin and progesterone decline gradually and a slight increase in plasma concentrations of estradiol-17β occurs during the second half of lactation. Concentrations of bioactive and immunoreactive inhibin in plasma vary with the number of healthy antral follicles and no positive correlation has been observed with luteal function throughout lactation in the rat.On the basis of the present results it is indicated that 1) tonic secretion of LH is an important factor in normal follicular maturation and maintenance in lactating rats, 2) the suckling stimulus, rather than ovarian factors, is mainly responsible for the suppression of FSH as well as LH secretion during the first half of lactation in rats nursing 8 pups. On the other hand, during the second half of lactation in rats nursing 8 pups, ovarian inhibin plays a primary role in the suppression of FSH secretion, whereas ovarian steroids act to suppress LH secretion. The present findings also indicate that 3) the suckling stimulus alone suppressed the secretion of both LH and FSH and this effect may be mediated by the inhibition of LH releasing hormone (LH-RH) secretion from the hypothalamus, 4) the suckling-induced inhibition of LH-RH may be primarily mediated by endogenous corticotropin-releasing hormone and opioid peptides and 5) the pituitary-adrenal system is capable of influencing the maintenance of a normal secretion of gonadotropins and prolactin as well as the maintenance of ovarian function during the lactation in the rat.

LH secretion in ovariectomized ewes: effects of morphine and ovarian steroid interactions with naloxone during the breeding season and anestrum

Morphine (MOR) infusion (i.v.) was used to mimic effects of endogenous opioid peptides (EOPs) on LH secretion in ovariectomized (OVX) ewes during the breeding season and anestrum. MOR decreased luteinizing hormone (LH) pulse frequency and/or amplitude at higher doses (0.5 and 1.0 mg kg−1 h−1), and increased basal serum LH concentrations at a lower dose (0.01 mg kg−1 h−1). Simultaneous MOR and naloxone (NAL) infusion (0.5 mg kg−1 h−1, each) did not affect LH secretion, suggesting that effects of MOR were EOP receptor-specific. EOP involvement in ovarian steroid feedback was examined by NAL infusion and ovarian steroid replacement in OVX ewes. NAL infusion increased LH pulse frequency in OVX ewes during the breeding season, suggesting ovarian steroid-independent, but EOP-dependent suppression of LH secretion. NAL did not alleviate seasonal suppression of LH pulse frequency during the anestrum, suggesting ovarian steroid-independent, EOP-independent suppression of LH secretion. NAL eliminated progesterone (P4) suppression of LH pulse frequency during the breeding season, suggesting that a major portion of P4 feedback on tonic LH secretion is EOP-dependent. P4 replacement in the anestrum did not suppress LH pulse frequency. NAL infusion, following P4 replacement in the anestrum, increased LH pulse frequency to levels observed in untreated OVX ewes during the breeding season. It was concluded that EOP-dependent P4 feedback experimentally over-rode EOP-independent suppression of LH pulse frequency during the anestrum. EOP activity was not required for estradiol (E2) to suppress LH secretion from the pituitary during the breeding season. Key words: LH, morphine, naloxone, progesterone, estradiol

Inhibitory feedback action of oestradiol on tonic secretion of luteinizing hormone in pre-and postpubertal gilts

Experiments were performed in gilts to test whether the inhibitory feedback action of oestradiol-17β on tonic luteinizing hormone (LH) secretion decreases around puberty and whether a stimulus to tonic LH secretion occurs which is independent of ovarian steroid control. Fifty-eight German Landrace gilts were either ovariectomized (OVX), OVX and treated with oestradiol-17β via subcutaneous silastic implants to maintain chronic low levels of oestradiol-17β, or served as controls. Treatments began at either 130, 180 or 210 days of age. All 130- and 180-day-old gilts providing data used for analysis were prepubertal, while 210-day-old gilts were OVX between Days 9–15 of their first oestrous cycle. Blood samples for analysis of LH were taken over a 4 h period at 10 min intervals 1 day before OVX and on Days 2, 7, 14, 21 and 28 after OVX. Intact controls were sampled on corresponding days. Mean LH, nadir LH levels, LH pulse frequency and amplitude were calculated for each 4 h sampling period. Mean plasma LH levels were not affected by age in intact controls. Within 2 days of ovariectomy plasma LH concentrations increased rapidly and then further increased only marginally without any difference between the three age groups. In the youngest age group which received a 3 cm oestradiol-17β implant resulting in oestradiol levels and uterine weights below those of respective controls ( P < 0.05), plasma LH was suppressed until 145 days of age when circulating LH increased ( P < 0.01). Between 180 and 210 days of age oestradiol-17β implants of 6 cm length (producing oestradiol levels and uterine weights similar to those of respective controls) suppressed LH to levels of controls. In postpubertal gilts oestradiol-17β implants of the same length and with similar effects on plasma oestradiol levels were unable to suppress LH until 21 days after ovariectomy when LH returned to pretreatment levels. Changes in mean LH in all age and reproductive status groups reflected changes in LH pulse frequency and nadir LH levels, whereas LH pulse amplitude was not affected. Since the postovariectomy levels of LH did not differ between pre- and postpubertal animals, it appears that in the gilt a stimulus to tonic LH secretion which is independent of ovarian steroid control does not occur during puberty. A major part of the reduction in the sensitivity to oestradiol negative feedback became apparent after first ovulation, which does not support the view that a resetting of the “gonadostat” initiates ovarian cyclicity. The transition from high to low oestradiol feedback sensitivity may be temporary or not complete even after first ovulation and a return to the hypersensitive state may occur.

Acute and subacute effects of naturally occurring estrogens on luteinizing hormone secretion in the ovariectomized rat: Part 2

Acute intravenous administration of the phytoestrogen genistein (G) blocks the gonadotropin-releasing hormone-(GnRH)-induced rise of luteinizing hormone (LH) in ovariectomized rats. The present experiments were performed to determine whether subacute administration of G or the mycoestrogens zearalenone and zearalenol would affect GnRH-induced or progesterone-induced LH secretion in ovariectomized rats. Charles River CD rats were ovariectomized and used 2 to 5 weeks later. Blood samples were obtained either via decapitation or via intraatrial cannulae three days after compounds were injected subcutaneously in sesame oil or corn oil vehicle. LH was measured by RIA. Pretreatment with estradiol benzoate suppressed LH levels at 1200 h, while G had no effect. Challenge with progesterone (8 mg/kg BW, sc) evoked LH release at 1600 h in rats pretreated with estradiol benzoate, but LH levels did not change in rats pretreated with G, zearalenone, or zearalenol. While GnRH-induced LH secretion was preserved in rats pretreated with estradiol, no LH response was detected in rats pretreated with the higher dose of G (8 mg/kg BW) or either dose of zearalenol (0.8 mg/kg BW or 8 mg/kg BW). We conclude that in the ovariectomized rat 1) subacute administration of G, zearalenone, or zearalenol do not inhibit tonic LH secretion, 2) G, zearalenone, and zearalenol do not provide “estrogenic priming” for progesterone-induced LH secretion; however, 3) G and zearalenol do block GnRH-induced LH secretion. The seemingly selective neuroendocrine effects of these naturally-occurring dietary estrogens emphasize that actions of each putative estrogen must be characterized for each “estrogenic” endpoint.

Specific Physiological Regulation of Luteinizing Hormone Secretory Events Throughout the Human Menstrual Cycle: New Insights into the Pulsatile Mode of Gonadotropin Release

Abstract To investigate the physiological regulation of luteinizing hormone (LH) secretory events and the endogenous clearance of this hormone, we applied multiple-parameter deconvolution analysis to serum LH concentration-time series obtained from normal women during three phases of the menstrual cycle. The number of significant LH secretory bursts (/24 h) was maximal in the late follicular (LF) phase (27 +/- 1.6; mean +/- SEM), minimal in the mid-luteal (ML) phase (10 +/-1.0) and intermediate in the early follicular (EF) phase (18 +/- 1.4). Similarly, the half-duration of the secretory impulse (min) was different at each phase of the cycle with values of 6.5+/-1.0, 3.5+/-0.9 and 11 +/- 1.1 during the EF, LF and ML phases, respectively. In contrast, there were no cycle-dependent differences in the LH half-life or in the total daily secretion of LH. When maximal secretory impulse amplitudes were examined, a putative bimodal distribution was found in the ML but not the EF or LF phases. The amplitudes for the large ML impulses, the LF and EF impulses and the small ML impulses were 0.95 +/- 0.05, 070 +/- 0.03, 0.43 +/- 0.02 and 0.26 +/- 0.02, respectively. The mass (mlU/ml) of hormone secreted within bursts was minimal in the LF phase (2.1 +/- 0.1), maximal in the large ML impulses (10.2 +/- 0.5) and intermediate in the EF (2.8 +/- 0.1) and small ML (3.1 +/-0.3) secretory impulses. There was no evidence of tonic (i.e. inter-secretory burst) LH secretion during any phase of the menstrual cycle. The early morning hours of the EF phase were characterized by fewer secretory bursts of greater amplitude. During the ML phase, autocorrelation analysis of inter-secretory burst intervals revealed a negative association indicating that high frequency events both precede and follow secretory pauses. In addition, secretory burst amplitude and both the preceding and following inter-secretory burst interval was correlated to secretory burst amplitude. These new data on the nature of regulated LH secretion indicate that specific facets of spontaneous LH secretory events are controlled throughout the menstrual cycle. Such observations offer a basis for defining altered secretory dynamics in a variety of pathophysiologic situations.

Absence of an effect of naloxone, an opioid antagonist, on luteinizing hormone release in vivo and luteinizing hormone-releasing hormone I release in vitro in intact, castrated, and food restricted cockerels

The possibility that the tonic secretion of luteinizing hormone (LH) and chicken luteinizing hormone-releasing hormone I (LHRH-I) is regulated by an inhibitory action of endogenous opioid peptides was investigated in cockerels using the opiate receptor antagonist, naloxone. Baseline concentrations of plasma LH in the experimental cockerels were increased by surgical castration or reduced by limiting food intake. Baseline and K +-induced releases of LHRH-I from perifused mediobasal-preoptic hypothalami from castrated cockerels were higher than those from hypothalami from intact cockerels. Similarly, baseline and K +-induced releases of LHRH-I from perifused mediobasal hypothalami from fully fed cockerels were higher than those from the hypothalami from fasting cockerels. Intravenous injections of 0.1, 1, or 10 mg naloxone/kg body weight failed to increase the concentration of plasma LH in castrated, intact, fully fed, or fasted cockerels. Perifusion of mediobasal-preoptic hypothalami from castrated or intact cockerels with 200 μ M naloxone or mediobasal hypothalami from fully fed or fasted cockerels with 10 μ M naloxone failed to stimulate the release of LHRH-I. These observations suggest in the cockerel that endogenous opioid peptides may not play an obligatory role in the inhibitory control of the tonic secretion of luteinizing hormone.

Sexual Differentiation of the Mechanism Controlling Pulsatile Secretion of Luteinizing Hormone Contributes to Sexual Differences in the Timing of Puberty in Sheep*

Sexual differences in the regulation of tonic luteinizing hormone (LH) secretion were examined in immature female and male sheep (eight each, including six pairs of female/male twins). After gonadectomy of lambs at 2 weeks of age, Silastic capsules filled with estradiol, a primary central feedback steroid in both females and males, were implanted every 3 weeks for 3 days, and then removed, so that the pattern of LH secretion could be repeatedly determined in the same individuals both with and without steroid feedback. Implanted capsules yielded circulating steroid levels of 2-5 pg/ml. Circulating LH concentrations were determined by radioimmunoassay in blood samples collected at 12-min intervals for 4 h immediately before estradiol was implanted, and again, immediately before it was removed 3 days later. In male lambs, a decrease in responsiveness of the hypothalamic-pituitary axis to inhibition by estradiol began at 8-11 weeks, as evidenced by the progressive increase in mean LH concentrations and frequency of LH pulses. This correlated temporally with the onset of spermatogenesis in intact male controls (n = 8). In females, a similar decrease in responsiveness did not occur until 26-29 weeks of age, corresponding to the onset of ovulatory cycles in intact female controls (n = 6). In the absence of estradiol implants, LH pulse frequencies were higher in male lambs than in female lambs between 5 and 35 weeks of age. There was no further increase in LH pulse frequency in the absence of the gonads in either sex during the pubertal period. These findings suggest that the mechanism regulating tonic LH secretion in developing lambs is sexually differentiated in its responsiveness to inhibition by estradiol. This differentiation also occurs at a more fundamental steroid-independent level, but any causal relationship between the higher steroid-independent LH pulse frequency and the lower responsiveness to estradiol negative feedback in males is not evident. We hypothesize that these sexual differences in the regulation of tonic LH underlie the difference in the timing of puberty in male and female lambs.

A Role for Catecholaminergic Neurons in the Suppression of Pulsatile Luteinizing Hormone Secretion in the Prepubertal Ewe Lamb

These experiments tested the hypothesis that inhibitory catecholaminergic neural systems suppress tonic luteinizing hormone (LH) secretion in the prepubertal ewe lamb, as they do in the adult anestrous ewe. This was done by determining the effects of the alpha-adrenergic antagonist, phenoxybenzamine, and the dopaminergic antagonist, pimozide. In the first experiment, pimozide at 0.24 mg/kg increased LH plus frequency in 28-week-old lambs, but not in 17-week-old animals. Phenoxybenzamine (0.8 mg/kg) had no effect at either age. In experiment 2, a dose-response curve for pimozide (0.008, 0.08, 0.8 mg/kg) and a high dose of phenoxybenzamine were tested at two ages. The highest dose of pimozide and phenoxybenzamine stimulated LH release at both 22 and 28 weeks, although the magnitude of the response was blunted in older lambs. In the third experiment, we tested whether age or time of year determined the response to pimozide by using two groups of lambs born 5 weeks apart. Two doses of pimozide (0.24 and 0.8 mg/kg) were tested at the same time of year in these lambs. The highest dose of pimozide again stimulated LH secretion in 22-week-old lambs, but not in 17-week-old animals; the lower dose produced a partial (not statistically significant) response only in older lambs. These data suggest that inhibitory catecholaminergic (probably dopaminergic) neurons contribute to the suppression of tonic LH secretion prior to puberty in the ewe lamb, but that this neural system only becomes functional between 17 and 22 weeks of age.

The postweaning rise of tonic luteinizing hormone secretion in anestrous cows is not prevented by chronic milking or the physical presence of the calf.

The objective of the following study was to examine the ability of frequent milking, the physical presence of the calf, and their combination to prevent a postweaning rise in tonic luteinizing hormone (LH) secretion, estrus, and ovulation. Thirty Hereford cows were allowed to suckle their calves ad libitum until 17-21 days post partum and confirmed as anestrus. They were then assigned alternately by order of calving to 1 of 5 treatment groups: (1) Suckled (S) ad libitum; (2) Nonsuckled (NS)--calf removed for 102 h; (3) Nonsuckled--calf present (NSC)--calf remained with cow, but muzzled to prevent suckling for 102 h; (4) Nonsuckled--milked 8 times a day (NSM)--calf removed for 102 h and cow hand-milked for 10 min every 2 h from 0700 to 2100 h; (5) Nonsuckled--calf present--milked 8 times a day (NSMC)--combination of 3 and 4. Luteinizing hormone secretion patterns, estrous activity, and ovulation were monitored throughout the experiment. Prior to treatment (Day 0), mean pulse frequency (pulses/6 h), mean concentrations (ng/ml), and median concentrations (ng/ml) of LH did not differ (p greater than 0.45) between groups, and were 0.7 +/- 0.15, 2.8 +/- 0.14, and 2.6 +/- 0.11, respectively. Marked rises (p less than 0.01--p less than 0.03) in LH pulse frequency were observed in all groups except S between 48 and 54 h after onset of treatment. Mean and median concentrations of LH were lower (p less than 0.02) in S cows than in all other groups at 48-54 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Separate neural systems mediate the steroid-dependent and steroid-independent suppression of tonic luteinizing hormone secretion in the anestrous ewe.

In the ewe, two types of seasonal fluctuations in secretion of tonic luteinizing hormone (LH) have been described: a steroid-dependent change whereby estradiol gains the capacity to suppress LH pulse frequency in anestrus, and a steroid-independent decrease in pulse frequency in ovariectomized animals during anestrus. We have proposed that the former reflects activation, in anestrus, of estradiol-sensitive catecholaminergic neurons that inhibit gonadotropin-releasing hormone (GnRH). Three results reported here support this hypothesis: dopaminergic (pimozide) and alpha-adrenergic (phenoxybenzamine) antagonists increased LH in intact anestrous ewes without altering pituitary responses to GnRH; other dopaminergic (fluphenazine) and alpha-adrenergic (dibenamine) antagonists also increased LH in anestrus; agonists for dopaminergic (apomorphine) and alpha-adrenergic (clonidine) receptors suppressed LH secretion in both seasons, suggesting that the appropriate receptors are present in breeding-season ewes. In contrast, catecholamines do not appear to mediate the steroid-independent suppression of pulse frequency; neither pimozide nor phenoxybenzamine increased LH pulse frequency in ovariectomized ewes during anestrus. When antagonists for 6 other neurotransmitter receptors (muscarinic and nicotinic cholinergic, GABAnergic, serotonergic, opioid, and beta-adrenergic) were tested in anestrus, only cyproheptadine, the serotonergic antagonist, increased pulse frequency in ovariectomized ewes. Cyproheptadine had no effect on frequency during the breeding season. On the basis of these results, we propose that the steroid-dependent and -independent actions of anestrous photoperiod occur via catecholaminergic and serotonergic neurons, respectively.