Luteinizing Hormone Pulse Frequency Research Articles

Characteristics of temporal patterns of cortisol and luteinizing hormone in primiparous, postpartum, anovular, suckled, beef cows exposed acutely to bulls

BackgroundThe physiological mechanism by which bulls stimulate resumption of ovarian cycling activity in postpartum, anovular, suckled cows after calving may involve the concurrent activation of the hypothalamic-hypophyseal-ovarian (HPO) axis and hypothalamic-hypophyseal-adrenal (HPA) axis. Thus, the objectives of this experiment were to determine if characteristics of temporal patterns of cortisol and luteinizing hormone (LH) in postpartum, anovular, beef cows are influenced by acute exposure to bulls. The null hypotheses were that daily, temporal characteristics of cortisol and LH concentration patterns do not differ between cows exposed acutely to bulls or steers.MethodsSixteen cows were assigned randomly 67 +/- 4 (+/- SE) after calving to be exposed to bulls (EB, n = 8) or steers (ES, n = 8) 5 h daily for 9 d (D 0 to 8). Blood samples were collected daily from each cow via jugular catheters at 15-min intervals for 6 h from 1000 to 1600 h each day. The 5-h exposure period began 1 h after the start of the intensive bleeding period. Characteristics of cortisol and LH concentration patterns (mean, baseline, pulse frequency, pulse amplitude, and pulse duration) were identified by PULSAR analyses.ResultsMean cortisol concentrations decreased (P < 0.05) in cows in both treatments from D 0 to D 2. Thereafter, mean cortisol concentrations stabilized and did not differ (P > 0.10) between EB and ES cows. The decrease in mean cortisol concentrations in EB and ES cows from D 0 to D 2 was attributed to cows acclimatizing to intensive blood sampling and handling procedures. Consequently, analyses for characteristics of cortisol and LH concentration patterns included D 2 through 8 only. Cortisol mean and baseline concentrations, and pulse amplitude did not differ (P > 0.10) between EB and ES cows. However, cortisol pulse duration tended to be longer (P = 0.09) and pulse frequency was lower (P = 0.05) in EB than ES cows. LH pulse frequency was greater (P = 0.06) in EB than ES cows. All other characteristics of LH concentration patterns did not differ (P > 0.10) between EB and ES cows. Characteristics of cortisol concentration patterns were not related to characteristics of LH concentration patterns for ES cows (P > 0.10). However, as cortisol pulse amplitude increased, LH pulse amplitude decreased (b1 = -0.04; P < 0.05) for EB cows.ConclusionsIn conclusion, exposing primiparous, postpartum, anovular, suckled cows to bulls for 5-h daily over a 9-d period did not alter mean concentrations of cortisol or LH compared to mean concentrations of cortisol and LH in cows exposed to steers. However, exposing cows to bull in this manner altered characteristics of temporal patterns of both LH and cortisol by increasing LH pulse frequency and decreasing cortisol pulse frequency. Interestingly, in cows exposed to bulls, as amplitude and frequency of cortisol pulses decreased, amplitudes of LH pulses increased and frequency of LH pulses tended to increase. Thus, the physiological mechanism of the biostimulatory effect of bulls may initially involve modification of the HPA axis and these changes may facilitate activation of the HPO axis and resumption of ovulatory cycles in postpartum, anovular, suckled cows.

Influence of ovarian hormones on endocrine activity of gonadotroph cells in the adenohypophysis of lambs during the postnatal transition to prepuberty

There is juvenile hiatus during maturation of larger mammals with relatively long life spans. Using histomorphological and functional criteria we describe the feedback mechanisms which could play a role in the regulation of the gonadotrophic axis during the postnatal transition to the quiescent prepubertal period in sheep. The aim of this study was to determine the influence of ovarian factors on the endocrine activity of gonadotroph cells, the site of synthesis, storage and release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), in adenohypophyses of weanling and weaned prepubertal lambs. The examination was made in (i) 9-week-old infantiles, suckling lambs undergoing weaning, ovary-intact (OVI) and ovariectomised (OVX) at the 6th week of age, and (ii) 16-week-old juveniles OVI and OVX at the 12th week of age ( n = 5 per group). Changes in gonadotrophs were assayed with hybridohistochemistry, immunohistochemistry and radioimmunoassay. The percentage of the adenohypophyseal area (PA) occupied by gonadotrophs containing LHβ-mRNA and immunoreactive for LHβ was lower ( P < 0.05), whereas the PA occupied by cells containing FSHβ-mRNA and immunoreactive for FSHβ was higher ( P < 0.05) in the 16-week-old OVI lambs in comparison with the 9-week-old ones. The mean concentration and basal level of LH in the peripheral blood plasma were greater ( P < 0.05) in the 16-week-old OVI lambs in comparison with the 9-week-old group, whereas the circulating FSH was not different. In the OVX 9-week-old lambs, the PA occupied by gonadotrophs containing LHβ-mRNA and the plasma LH concentration, basal level, pulse frequency and amplitude were greater ( P < 0.05), whereas the PA occupied by cells immunoreactive for LHβ was lower ( P < 0.05) in comparison with the OVI group. In the OVX 16-week-old lambs, the PA occupied by gonadotrophs containing LHβ-mRNA and immunoreactive for LHβ, the LH plasma concentration, basal level and pulse frequency were ( P < 0.05) greater in comparison with the OVI group. The PA occupied by gonadotrophs containing FSHβ-mRNA and the plasma FSH concentration were greater ( P < 0.05) in the OVX 9- and 16-week-old lambs in comparison with the OVI ones. The ovariectomy had no effect on the PA occupied by cells immunoreactive for FSHβ in both age stages. In conclusion, ovarian factors may play inhibitory role in regulating both LH and FSH synthesis rate and release and stimulatory role in regulating LH storage in adenohypophyseal gonadotrophs in infantile lambs. In lambs at the beginning of the juvenile period, ovarian factors may play only inhibitory role in regulating both LH and FSH synthesis and release and LH storage. The effects of ovarian hormones on the gonadotrophin storage, i.e. releasable pools in adenohypophyseal cells, are specific for LH, no such effects are apparent on FSH in lambs during the postnatal transition to prepuberty. Thus, the initiation of the juvenile period in female sheep is a function of change of the stimulatory role of ovarian hormones in regulating LH storage to the inhibitory one.

Kisspeptin signalling in the hypothalamic arcuate nucleus regulates GnRH pulse generator frequency in the rat.

BackgroundKisspeptin and its G protein-coupled receptor (GPR) 54 are essential for activation of the hypothalamo-pituitary-gonadal axis. In the rat, the kisspeptin neurons critical for gonadotropin secretion are located in the hypothalamic arcuate (ARC) and anteroventral periventricular (AVPV) nuclei. As the ARC is known to be the site of the gonadotropin-releasing hormone (GnRH) pulse generator we explored whether kisspeptin-GPR54 signalling in the ARC regulates GnRH pulses.Methodology/Principal FindingsWe examined the effects of kisspeptin-10 or a selective kisspeptin antagonist administration intra-ARC or intra-medial preoptic area (mPOA), (which includes the AVPV), on pulsatile luteinizing hormone (LH) secretion in the rat. Ovariectomized rats with subcutaneous 17β-estradiol capsules were chronically implanted with bilateral intra-ARC or intra-mPOA cannulae, or intra-cerebroventricular (icv) cannulae and intravenous catheters. Blood samples were collected every 5 min for 5–8 h for LH measurement. After 2 h of control blood sampling, kisspeptin-10 or kisspeptin antagonist was administered via pre-implanted cannulae. Intranuclear administration of kisspeptin-10 resulted in a dose-dependent increase in circulating levels of LH lasting approximately 1 h, before recovering to a normal pulsatile pattern of circulating LH. Both icv and intra-ARC administration of kisspeptin antagonist suppressed LH pulse frequency profoundly. However, intra-mPOA administration of kisspeptin antagonist did not affect pulsatile LH secretion.Conclusions/SignificanceThese data are the first to identify the arcuate nucleus as a key site for kisspeptin modulation of LH pulse frequency, supporting the notion that kisspeptin-GPR54 signalling in this region of the mediobasal hypothalamus is a critical neural component of the hypothalamic GnRH pulse generator.

18 EFFECT OF GnRH TREATMENT IN LH SECRETION IN INTACT AND OVARIECTOMIZED NELLORE HEIFERS

The gonadotropin-releasing hormone (GnRH) and its analogues become control reproductive function in farm animals. The GnRH agonist (GnRHa), which is used in veterinary practice, has a longer half-life and higher affinity to GnRH receptors than the native decapeptide. GnRH or GnRHa effects depend on the species, dose, route, and the moment of administration. As Nellore is the main bovine breed in Brazil, corresponding to 70% of Brazilian herds, more studies on its reproductive physiology are needed. The objective was to study luteinizing hormone (LH) secretion in intact (I, n = 4) and ovariectomized (Ov, n = 4) prepubertal Nellore heifers (18 months) after gonadorelin acetate administration (25 μg, iv; Lecirelina, Gestran Plus, ARSA S.R.L., Buenos Aires, Argentina) from samples collected every 15 min for 10 h. LH was quantified by RIA, with sensitivity of 0.03 ng mL-1 and CV of 11%. Maximum LH amplitude was considered as the peak for each period. Peaks were identified as an increase 2 times higher than intraassay CV and involving at least 3 samples. The time for the highest peak occurrence was determined by GraphPad Prism program (Prism 3.00 for Windows, GraphPad Software, San Diego CA, USA). Results were compared by the ANOVA followed by t-test either paired or not using GraphPad InStat (3.06 version). After GnRH treatment, maximum LH peak amplitude was higher in Ov (12.48 ± 1.41 ng mL-1) (P ≤ 0.05) than in intact heifers (6.42 ± 1.30 ng mL-1), but there was no significant difference (P ≥ 0.05) on LH pulse frequency, with only one peak in both groups during the 10-h period. Although there was a small delay in peak occurrence in I (63.75 ± 21.48 min) compared with Ov heifers (45 ± 16.43 min), there was no difference (P ≥ 0.05) between groups. The results suggested a higher hypothalamus sensitivity to GnRH in absence of gonadal steroids or a higher LH storage during the hypergonatropism period. Support by FAPESP (São Paulo, Brazil); fellowship from CAPES.

Hypothalamic Ghrelin Suppresses Pulsatile Secretion of Luteinizing Hormone via β-Endorphin in Ovariectomized Rats

Objectives: Ghrelin, an endogenous growth hormone secretagogue, is a known accelerator of feeding behavior and suppresses pulsatile secretion of luteinizing hormone (LH) in ovariectomized rats. However, the mechanisms underlying this action remain unclear. We examined the effects of naloxone (NAL), a specific opioid antagonist, on the suppression of pulsatile LH secretion by ghrelin to determine whether β-endorphin (β-END) is involved in this suppressive effect. Methods: Ghrelin was administered intracerebroventricularly, and NAL was injected intravenously in ovariectomized rats; then, serum LH concentrations were measured by radioimmunoassay in blood samples drawn every 6 min for 2 h to analyze pulsatile secretion. Results: Administration of ghrelin significantly reduced mean LH concentration and pulse frequency. Coadministration of NAL with ghrelin significantly restored mean LH concentration and pulse frequency. Conclusion: Suppressive effect of intracerebroventricular injection of ghrelin on pulsatile LH secretion was mediated by β-END, suggesting that hypothalamic ghrelin suppressed pulsatile gonadotropin-releasing hormone secretion via β-END in female rats.

Altered “set-point” of the hypothalamus determines effects of cortisol on food intake, adiposity, and metabolic substrates in sheep

Chronic elevation of glucocorticoid concentrations is detrimental to health. We investigated effects of chronic increase in plasma cortisol concentrations on energy balance and endocrine function in sheep. Because food intake and reproduction are regulated by photoperiod, we performed experiments in January (JAN) and August (AUG), when appetite drive is either high or low, respectively. Ovariectomized ewes were treated (intramuscularly) daily with 0.5 mg Synacthen Depot® (synthetic adrenocorticotropin: ACTH) or saline for 4 wk. Blood samples were taken to measure plasma concentrations of cortisol, luteinising hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), leptin, insulin, and glucose. Adrenocorticotropin treatment increased concentrations of cortisol. During JAN, treatment reduced food intake transiently, but increased food intake in AUG. Leptin concentrations were reduced and glucose concentrations were greater in AUG, and insulin concentrations were similar throughout the year. Treatment with ACTH increased leptin concentrations in AUG only, whereas insulin concentrations increased in JAN only. Synacthen treatment increased glucose concentrations, with a greater effect in JAN. Changes in truncal adiposity and ACTH-induced cortisol secretion were positively correlated in JAN and negatively correlated in AUG. Treatment reduced the plasma LH pulse frequency in JAN and AUG, with an effect on pulse amplitude in JAN only. Treatment did not affect plasma GH or FSH concentrations. We conclude that chronically elevated cortisol concentrations can affect food intake, adiposity, and reproductive function. In sheep, effects of chronically elevated cortisol concentrations on energy balance and metabolism depend upon metabolic setpoint, determined by circannual rhythms.

Arrest of pulsatile luteinizing hormone (LH) secretion during insulin-induced hypoglycemia (IIH): improvement by intrahypothalamic perfusion with glucose.

Insulin-induced hypoglycemia (IIH), as with many other acute stressors, restrains the activity of the reproductive axis, reducing luteinizing hormone (LH) release. In adult ovariectomized, steroid-primed rats, we investigated the effect of IIH and of mediobasal intrahypothalamic perfusion with glucose (200 mg/dl) on pulsatile LH secretion. IIH led to a significant decrease in all pulsatility parameters studied using PC-Pulsar analysis, e.g. pulse amplitude and frequency, maximum and baseline LH levels (p < 0.05 versus control), and LH overall mean release (p < 0.01 versus control). Intrahypothalamic perfusion with glucose normalized LH pulse frequency, improved maximum and baseline levels, and partially ameliorated LH pulse amplitude and overall mean release. Thus, our results show that the glucoprivic cessation of LH release is restored, at least partially, by an adequate glucose supply to the hypothalamus; it is proposed, in view of these and previous results, that different mechanisms in the CNS may be involved in LH suppression observed during IIH.

Effects of ghrelin on Kisspeptin mRNA expression in the hypothalamic medial preoptic area and pulsatile luteinising hormone secretion in the female rat

The orexigenic gut peptide ghrelin negatively modulates the hypothalamic–pituitary–gonadal (HPG) axis. Hyperghrelinaemia results during negative energy balance, a state often associated with delayed puberty and disrupted fertility, whilst exogenous ghrelin suppresses pulsatile luteinising hormone (LH) secretion. The recent identification of kisspeptin (Kiss1) and its G protein-coupled receptor (GPR)54 (Kiss1r) as an essential component of the HPG axis controlling gonadotrophin secretion raises the possibility that kisspeptin-Kiss1r signalling may play a critical role in the transduction of ghrelin-induced suppression of LH. Ovariectomised oestrogen-replaced rats were implanted with intravenous catheters and blood samples collected for detection of LH pulses prior to and after intravenous administration of ghrelin (3 nM/250 μl) or saline (250 μl) during ad libitum feeding or after overnight fasting. Quantitative RT-PCR was used to determine Kiss1 and Kiss1r mRNA levels in brain punches of the key hypothalamic sites regulating gonadotrophin secretion, the medial preoptic area (mPOA) and arcuate nucleus (ARC), collected 6 h following administration of ghrelin. Ghrelin significantly lowered LH pulse frequency in fed rats, an effect significantly enhanced by food deprivation. Fasting, ghrelin or their combination down-regulated Kiss1, without affecting Kiss1r, expression in the mPOA, and affected the expression of neither in the ARC. Considering the pivotal role for kisspeptin signalling in the activation of the HPG axis, the ability of ghrelin to down-regulate Kiss1 expression in mPOA may be a contributing factor in ghrelin-related suppression of pulsatile LH secretion.

Bovine C-terminal octapeptide of RFamide-related peptide-3 suppresses luteinizing hormone (LH) secretion from the pituitary as well as pulsatile LH secretion in bovines

Gonadotropin-inhibiting hormone (GnIH), observed in quail as a member of the RFamide neuropeptide family, suppresses luteinizing hormone (LH) secretion from the avian pituitary. Rats and cattle have an active gene of another member of the RFamide neuropeptide family, termed RFamide-related peptide-3 (RFRP-3), although bovine RFRP-3 is different from that of rats in both length and amino-acid sequence. A single injection of GnIH or RFRP-3 inhibited LH secretion in rodents, which continued for various periods. This study was conducted to evaluate the effects of bovine C-terminal octapeptide of RFRP-3 (RFRP-3-8) on LH secretion from cultured anterior pituitary (AP) cells of cattle, and the effects of RFRP-3-8 injections on pulsatile LH secretion in castrated male calves. The suppressive effect of RFRP-3-8 on LH secretion from AP cells was observed in the presence of gonadotropin-releasing hormone (GnRH), but not in the absence of GnRH in culture media. In another experiment collecting blood samples serially from castrated male calves with repeated intravenous injections of RFRP-3-8 ( n = 6) or saline ( n = 6), the RFRP-3-8 group showed suppressed LH pulse frequency during the injection period ( P < 0.05); however, the RFRP-3-8 group showed no difference from the saline group in all measures of LH secretion in the postinjection period. In conclusion, our results suggested that RFRP-3-8 suppresses LH secretion from cultured AP cells, as well as LH pulse frequency in cattle.

Atrazine Inhibits Pulsatile Luteinizing Hormone Release Without Altering Pituitary Sensitivity to a Gonadotropin-Releasing Hormone Receptor Agonist in Female Wistar Rats1

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-tri-azine] is one of the most commonly used herbicides in the United States. Atrazine has been shown to suppress luteinizing hormone (LH) release and can lead to a prolongation of the estrous cycle in the rat. The objectives of this study were to examine the effects of atrazine on normal tonic release of LH and to elucidate the site of action of atrazine in the hypothalamic-pituitary-gonadal axis. Episodic release of gonadotropin-releasing hormone (GnRH) and the corresponding release of LH from the anterior pituitary gland are required for normal reproductive function. To determine if atrazine affects pulsatile LH release, ovariectomized adult female Wistar rats were administered atrazine (50, 100, or 200 mg/kg of body weight daily by gavage) or vehicle control for 4 days. On the final day of atrazine treatment, blood samples were obtained using an indwelling right atrial cannula. In the group receiving 200 mg/kg, there was a significant reduction in LH pulse frequency and a concomitant increase in pulse amplitude. To determine if the effects of atrazine on LH release were due to changes at the level of the pituitary, animals were passively immunized against endogenous GnRH, treated with atrazine, and challenged with a GnRH receptor agonist. Atrazine failed to alter pituitary sensitivity to the GnRH receptor agonist at any dose used. Taken together, these findings demonstrate that high doses of atrazine affect the GnRH pulse generator in the brain and not at the level of gonadotrophs in the pituitary.

Responses of gonadotropin secretion to short-term dietary supplementation in ovariectomized goats with different body weights

The responses of luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion to acute dietary supplementation were studied in goats with different body weights. Ovariectomized Shiba goats ( n = 11) were used and were maintained with a feed of 100% of their energy requirement. They were implanted subcutaneously with an oestradiol capsule and were divided into light (LBW; ≤24 kg, n = 6, mean ± S.D., 21.8 ± 2.7 kg) and heavy body weight (HBW; >24 kg, n = 5, mean ± S.D., 32.0 ± 6.3 kg) groups on the basis of their body weights at 8 days before the start of treatment. At the start of treatment (Day 1), the level of a feed changed to 250% of their energy requirement and this level was maintained for 7 days in both groups. Blood samples were collected daily from Day −7 to Day 7 for the analysis of FSH, glucose, and insulin profiles in plasma. Frequent blood samples were also collected at 10 min intervals for 6 h on Day 0, Day 3, and Day 7 for analysis of LH pulses. LH pulse frequency increased significantly on Day 3 as compared with that on Day 0 in both the HBW (7.4 ± 0.5 pulses/6 h vs. 6.2 ± 0.8 pulses/6 h, p < 0.05) and LBW (6.5 ± 0.8 pulses/6 h vs. 5.5 ± 0.5 pulses/6 h, p < 0.05) groups, whereas it decreased on Day 7 (HBW, 6.4 ± 0.9 pulses/6 h; LBW, 6.3 ± 1.6 pulses/6 h, p > 0.05 vs. Day 0). Plasma glucose and insulin concentrations increased temporarily from Day 2 to Day 4 and then decreased to the level before the start of dietary supplementation in both groups. There was no significant difference in the LH pulse frequency or daily concentrations of FSH, glucose, or insulin between the HBW and LBW groups throughout the experimental period. The present study indicated that acute dietary supplementation stimulates pulsatile LH secretion in parallel with a rise of blood glucose and insulin levels. However, the influence of body weight on these responses between light and heavy animals was not observed.

Role of Estradiol in Cortisol-Induced Reduction of Luteinizing Hormone Pulse Frequency

Precise control of pulsatile GnRH and LH release is imperative to ovarian cyclicity but is vulnerable to environmental perturbations, like stress. In sheep, a sustained (29 h) increase in plasma cortisol to a level observed during stress profoundly reduces GnRH pulse frequency in ovariectomized ewes treated with ovarian steroids, whereas shorter infusion (6 h) is ineffective in the absence of ovarian hormones. This study first determined whether the ovarian steroid milieu or duration of exposure is the relevant factor in determining whether cortisol reduces LH pulse frequency. Prolonged (29 h) cortisol infusion did not lower LH pulse frequency in ovariectomized ewes deprived of ovarian hormones, but it did so in ovariectomized ewes treated with estradiol and progesterone to create an artificial estrous cycle, implicating ovarian steroids as the critical factor. Importantly, this effect of cortisol was more pronounced after the simulated preovulatory estradiol rise of the artificial follicular phase. The second experiment examined which component of the ovarian steroid milieu enables cortisol to reduce LH pulse frequency in the artificial follicular phase: prior exposure to progesterone in the luteal phase, low early follicular phase estradiol levels, or the preovulatory estradiol rise. Basal estradiol enabled cortisol to decrease LH pulse frequency, but the response was potentiated by the estradiol rise. These findings lead to the conclusion that ovarian steroids, particularly estradiol, enable cortisol to inhibit LH pulse frequency. Moreover, the results provide new insight into the means by which gonadal steroids, and possibly reproductive status, modulate neuroendocrine responses to stress.

Rapid Induction of Cell Proliferation in the Adult Female Ungulate Brain (Ovis aries) Associated with Activation of the Reproductive Axis by Exposure to Unfamiliar Males1

In many species, the reproductive centers of the brain are profoundly affected by sociosexual stimuli. This is particularly evident in female ungulates such as sheep, in which exposure to males switches them from reproductively quiescent to fertile. In two experiments with female sheep, we tested whether the brain centers that control gonadotropin-releasing hormone (GnRH) neuronal activity respond differentially to "novel" vs. familiar males and whether the neuroendocrine response is associated with increased cell proliferation in the hippocampus, a site associated with memory formation. In experiment 1, groups of 10 female sheep that had previously been habituated to males for 3 mo were re-exposed to familiar males or were exposed to novel males. Only the novel males increased luteinizing hormone (LH) pulse frequency, indicating stimulation of GnRH neuronal activity. In experiment 2, groups of six female sheep were injected with bromodeoxyuridine (BrdU) and then maintained in isolation from males or exposed to novel males. Two days later, the hippocampus and hypothalamus were removed and processed for fluorescence immunohistochemistry. Again, exposure to males increased LH pulse frequency. Most important, male exposure also doubled the number of BrdU-positive cells in the dentate gyrus of the hippocampus. No BrdU-positive cells were detected in the hypothalamus. We conclude that the stimulus from novel males switches on the reproductive centers of the brain of female sheep and rapidly doubles the rate of cell proliferation in the hippocampus. The rapidity of this response contrasts with rodents, in which several days of exposure to male pheromones seem necessary for an effect on neurogenesis.

Impact level of dihydrotestosterone on the hypothalamic‐pituitary‐leydig cell axis in men

Dihydrotestosterone (DHT) the physiologically most potent androgen cannot be aromatised into oestrogen. DHT is used as a treatment for idiopathic gynaecomastia. In order to investigate the different sites of action of DHT on the hypothalamic-pituitary-testicular axis, two groups of adult men were studied. Group I included 10 gonadotropin-releasing hormone (GnRH)-deficient men who were evaluated before and during a pulsatile infusion of GnRH alone for 2 weeks and then in association with DHT given transdermally at doses used in the treatment of gynaecomastia for further two weeks. Luteinizing hormone (LH) pulsatility was assessed at the end of each step of the study. Plasma LH levels were measured every 15 min. Plasma testosterone (T), DHT, oestradiol (E2), free alpha-subunit (FAS) of glycoproteic hormones and LH bioactivity were measured on pooled plasma samples. Group II included 12 healthy men in whom plasma T, DHT and E2 were measured before and then 24, 48 and 72 h after the injection of 5000 IU hCG alone or in combination with either DHT or the pure anti-androgen nilutamide. Two weeks separated each of the 3 hCG testing. In group I, except for bioactive/immunoreactive (B/I) LH ratio which was unchanged, GnRH treatment induced significant rises (p < 0.01) in all plasma hormone levels, LH pulse amplitude and frequency. During treatment with GnRH+DHT, plasma DHT levels increased up to 16.8 +/- 2.5 nm, while plasma hormone levels, B/I LH ratio, LH pulse amplitude and frequency were similar to those obtained with GnRH alone. In group II, the peak of hCG-induced T rise was not modified by either DHT or nilutamide. In contrast, DHT reduced by 50% (p < 0.01) the E2 peak in response to hCG. These data show that DHT exerts no direct action on the pituitary to retroregulate LH secretion and to modify either B/I LH ratio or FAS secretion. Its reducing effect on LH secretion is likely mediated at the hypothalamic level. DHT does not appear to have a physiological influence on Leydig cells steroidogenesis. Administered at therapeutic doses, DHT directly reduces testicular aromatase activity that combined with its antigonadotropic effect leads to the gain in the symptomatic treatment of gynaecomastia.

Food Deprivation Induces Monocarboxylate Transporter 2 Expression in the Brainstem of Female Rat

Ketone bodies are considered to act as a signal to suppress gonadotropin release during negative energy balance. The present study examined the effect of 48-h fasting on the mRNA expressions of monocarboxylate transporter 1 (MCT1) and MCT2, which are involved in ketone body transport, in several brain regions. Quantitative real-time RT-PCR analysis showed that the MCT2 mRNA levels were significantly increased by 48-h fasting in the area postrema-solitary tract nucleus (AP-NTS) region but not the arcuate nucleus-ventromedial hypothalamic nucleus (ARC-VMH) and central gray-supragenual nucleus around the 4th ventricle (CG-SGe) regions. Fasting did not significantly affect MCT1 mRNA expression in any of the brain areas examined. Luteinizing hormone (LH) pulse frequency significantly decreased and plasma concentrations of beta-hydroxybutyric acid, a ketone body, significantly increased after 48-h fasting. The present results suggest that increased uptake of ketone bodies via MCT2 in the AP-NTS region is likely involved in the mechanism of fasting-induced suppression of LH secretion in rats.

A role for the medial preoptic area in CGRP-induced suppression of pulsatile LH secretion in the female rat

Calcitonin gene-related peptide (CGRP) is involved in a variety of stress responses and plays a pivotal role in stress-induced suppression of the GnRH pulse generator in the rat. Intracerebroventricular administration of CGRP suppresses luteinizing hormone (LH) pulses and increases Fos expression within the medial preoptic area (mPOA) and paraventricular nucleus (PVN). The aims of the present study were to investigate whether the mPOA or PVN are sites of action for CGRP-induced suppression of LH pulses and whether lipopolysaccharide (LPS), restraint or insulin-induced hypoglycaemia, stressors known to suppress LH pulses, affect mRNA expression for CGRP and its receptor subunits (calcitonin receptor-like receptor (CL) and RAMP-1) in the mPOA and PVN. Micro-infusion of CGRP (50, 250 or 500 pmol) into the mPOA, but not the PVN, dose-dependently suppressed LH pulse frequency. LPS, restraint and hypoglycaemia suppressed RAMP-1 mRNA, but not CL or CGRP mRNA expression in the mPOA. In the PVN, all three stressors suppressed CL mRNA expression, but only LPS or restraint suppressed RAMP-1 mRNA, and CGRP mRNA was unaffected. These results provide evidence that, unlike the PVN, the mPOA might play an important role in the inhibitory effect of CGRP on pulsatile LH secretion. Additionally, CGRP receptor function may be involved in this brain region in stress-induced suppression of the GnRH pulse generator.

Can audio–visual or visual stimuli from a prospective mate stimulate a reproductive neuroendocrine response in sheep?

Stimuli from a prospective mate increase the secretion of luteinising hormone (LH) in sheep. This ‘male effect’ in ewes and ‘female effect’ effect in rams is predominantly mediated by olfactory signals, though it is thought that non-olfactory signals play synergistic or substitutive roles. In this study, we tested whether exposure to visual or audio–visual stimuli from a prospective mate would stimulate an increase in LH secretion in ewes (Experiment 1) and rams (Experiment 2). In Experiment 1, groups of eight Merino ewes were exposed to one of three stimuli midway through a frequent blood-sampling regimen: full ram contact, still images of rams, a video of ewes and rams mating. Control ewes (n = 8) were completely isolated from rams. Exposure to still images of rams appeared to stimulate an increase in mean LH concentrations (P < 0.05) and tended to increase LH pulse frequency (P < 0.1), but the response was significantly smaller than that observed in ewes exposed to rams (P < 0.01). Audio–visual stimuli had no effect on any parameters of LH secretion (P > 0.1). In Experiment 2, Merino rams were allocated to either an Exposure (n = 7) or a Control (n = 7) group. Exposure rams underwent two exposure periods midway through a frequent blood-sampling regimen; exposure to still images of ewes and audio recorded during mating of ewes and rams (audio–visual exposure); exposure to oestrous ewes (ewe exposure). Control rams were sampled at the same frequency but remained isolated from ewe stimuli. Exposure of rams to the audio–visual stimuli did not affect any parameters of LH secretion (P > 0.1). In contrast, exposure to oestrous ewes increased LH pulse frequency (P < 0.05) and advanced the onset of the next LH pulse (P < 0.05). In conclusion, visual signals appear to be involved in eliciting the neuroendocrine response of ewes to rams and are of greater importance to this phenomenon in ewes (male effect) than rams (female effect). However, overall the visual and audio–visual signals used in this study were far less effective than stimulus animals, suggesting that these stimuli are less important than olfactory signals, or a combination of olfactory and audio–visual signals.

Insulin Modulation of Luteinizing Hormone Secretion in Normal Female Volunteers and Lean Polycystic Ovary Syndrome Patients

Background/Aims: Endocrine features of polycystic ovary syndrome (PCOS) include altered ovarian steroidogenesis, hyperinsulinemia and abnormal luteinizing hormone (LH) secretion. This study was undertaken to further evaluate the role of insulin to modulate LH secretion in lean PCOS patients with normal insulin sensitivity and normal volunteers. Methods: The study was performed in five nonobese patients diagnosed with PCOS on the basis of amenorrhea and a polycystic morphology at ovarian ultrasound, and 5 normal controls in early to mid-follicular phase and matched for weight and age. All subjects were phenotyped, and then admitted for 12 h of frequent (q 10’) blood sampling on two separate occasions, once for a baseline study and the other time for a hyperinsulinemic and euglycemic clamp study. LH was measured in samples obtained throughout each admission in order to perform LH pulse analysis. Results: Baseline LH secretion in PCOS subjects was significantly different from controls: they had higher LH levels, higher LH/FSH ratios as well as a faster LH pulse frequency than normal women. Insulin administration did not affect the pattern of LH secretion of PCOS patients, whereas it significantly increased the LH pulse frequency while decreasing the LH interpulse intervals in the controls. Conclusions: These data confirm that an abnormal pattern of LH secretion characteristic of PCOS can be observed in lean patients, and appears independent of peripheral insulin levels. Furthermore, our results in lean controls provide the first direct evidence that peripheral insulin can modulate the activity of hypothalamic gonadotropin-releasing hormone (GnRH) neurons in the human.

Effect of transient hypothyroidism during infancy on the postnatal ontogeny of luteinising hormone release in the agonadal male rhesus monkey (Macaca mulatta): implications for the timing of puberty in higher primates.

The present study examined whether a transient thyroid hormone (T(4)) deficit during infancy in male monkeys would compromise the arrest of luteinising hormone (LH) secretion during the infant-juvenile transition, and/or interfere with the pubertal resurgence of LH. Animals were orchidectomized and thyroidectomized (n = 3; Tx) or sham Tx (n = 3) within 5 days of birth. T(4) replacement was initiated in two Tx monkeys at age 19 weeks to reestablish a euthyroid condition. Blood samples were drawn weekly for hormone assay. Body weight, crown-rump length, and bone age were assessed throughout the study. Within a week of Tx, plasma T(4) declined to undetectable levels and, by 6-8 weeks of age, signs of hypothyroidism were evident. Transient hypothyroidism during infancy failed to prevent either arrest of LH secretion during the infant-juvenile transition or the pubertal resurgence of LH secretion, both of which occurred at similar ages to sham Tx animals. Although body weight exhibited complete catch-up with T(4) replacement, crown-rump length and bone age did not. Thus, bone age at the time of the pubertal LH resurgence in Tx animals was less advanced than that in shams. Although Tx did not influence qualitatively the pattern of gonadotrophin secretion, LH levels during infancy and after pubertal LH resurgence were elevated in Tx monkeys. This was not associated with changes in LH pulse frequency and amplitude, but half-life (53 versus 65 min) of the slow second phase of LH clearance was greater in Tx animals. These results indicate that hypothalamic mechanisms dictating the pattern of gonadotrophin-releasing hormone release from birth to puberty are not dependent on T(4) action during infancy, and fail to support the notion that onset of puberty is causally coupled to skeletal maturation. They also indicate that LH renal clearance mechanisms may be programmed in a T(4) dependent manner during infancy.

Black cohosh has central opioid activity in postmenopausal women

To test whether black cohosh (BC) exhibits an action on the central endogenous opioid system in postmenopausal women. This was a mechanistic study conducted in the same individuals of luteinizing hormone pulsatility with a saline/naloxone challenge (n = 6) and positron emission tomography with [C]carfentanil, a selective micro-opioid receptor radioligand (n = 5), before and after 12 weeks of unblinded treatment with a popular BC daily supplement. BC treatment for 12 weeks at a standard dose (Remifemin, 40 mg/day) had no effect on spontaneous luteinizing hormone pulsatility or estrogen concentrations. With naloxone blockade, there was an unexpected suppression of mean luteinizing hormone pulse frequency (saline vs naloxone = 9.0 +/- 0.6 vs 6.0 +/- 0.7 pulses/16 h; P = 0.056), especially during sleep when the mean interpulse interval was prolonged by approximately 90 minutes (saline night interpulse interval = 103 +/- 9 min vs naloxone night interpulse interval = 191 +/- 31 min, P = 0.03). There were significant increases in mu-opioid receptor binding potential in the posterior and subgenual cingulate, temporal and orbitofrontal cortex, thalamus, and nucleus accumbens ranging from 10% to 61% across brain regions involved in emotional and cognitive function. In contrast, binding potential reductions of lesser magnitude were observed in regions known to be involved in the placebo response (anterior cingulate and anterior insular cortex). Using two different challenge paradigms for the examination of central opioid function, a neuropharmacologic action of BC treatment was demonstrated in postmenopausal women.