The importance of signal pattern in the transmission of endocrine information: pituitary gonadotropin responses to continuous and pulsatile gonadotropin-releasing hormone

We tested the hypothesis that pulsatile GnRH stimulation of the pituitary is required for normal gonadotropin secretion in humans. We administered GnRH in pulsatile and continuous regimens in varying order to each of five women with hypothalamic amenorrhea and presumed endogenous GnRH deficiency. Mean serum levels of GnRH were similar during the pulsatile and continuous regimens. All women ovulated during the pulsatile regimen (progesterone, greater than 31.8 nmol/L (10 ng/mL); none ovulated during the continuous regimen. Compared to pretreatment levels, FSH and estradiol, as measured by RIA, and LH, as measured by bioassay, increased significantly during the pulsatile GnRH regimen, but not during the continuous regimen. However, LH and alpha-subunit, as measured by RIA, increased significantly during both continuous and pulsatile GnRH administration. We conclude that a pulsatile pattern of GnRH is essential to normal functioning of the human female reproductive axis. Continuous administration of GnRH, producing mean serum levels of the peptide indistinguishable from those found during pulsatile administration, stimulates some rise in a nonbioactive form of radioimmunoassayable LH-like material and alpha-subunit, but does not stimulate bioactive LH, FSH, estradiol, or progesterone and does not lead to ovulation.     

Clinical review 15: Management of ovulatory disorders with pulsatile gonadotropin-releasing hormone

Pulsatile GnRH therapy has yet to achieve widespread acceptance as an alternative to exogenous gonadotropin therapy in women with hypothalamic amenorrhea and complete GnRH deficiency. However, when a physiologically based replacement regimen of pulsatile GnRH is used, a high rate of ovulation and conception can be anticipated in patients with complete GnRH deficiency and hypothalamic amenorrhea. Women with polycystic ovarian syndrome may also benefit from pulsatile GnRH, although rates of ovulation are lower. Pretreatment with a GnRH agonist may improve these rates considerably, but experience is limited. Whether an iv or sc route of administration is chosen, a simplified clinical monitoring protocol can be created which requires a minimum of patient monitoring while assuring maximum safety. Seventy five nanograms per kg appears to be a reasonable initiating dose, with subsequent increases in those who do not respond. The frequency of GnRH administration is best based on the GnRH pulse frequency in normal women. However, further information is needed to determine whether such a variable frequency is clearly superior to a fixed frequency regimen. When used appropriately, pulsatile GnRH is safe, effective, and offers an excellent alternative to conventional gonadotropin therapy for women with disordered endogenous GnRH secretion. Most importantly, and as opposed to exogenous gonadotropin therapy, pulsatile GnRH can be administered by most physicians in the office setting without the necessity of on-line E2 monitoring. This feature will enable more patients to receive treatment by their local physicians, whereas exogenous gonadotropin therapy should be administered by appropriately equipped referral centers. In the future, further studies will be required to determine which other categories of patients might benefit from pulsatile GnRH.     

Absence of regular pulsatile LH secretion during pre- and post-implantation periods in sheep

Two experiments were conducted to determine the patterns of LH secretion and to evaluate the LH responses to pulsatile administration of GnRH during early pregnancy in ewes. In experiment 1, pregnant ewes (n=16) were used to determine the concentration of LH in plasma of jugular blood samples collected every 15 min for 6h before (day 10 post-mating) and after (days 20 and 30 post-mating) implantation. In experiment 2, the pituitary LH responses to exogenous pulsatile administration of GnRH were examined on day 10 post-mating in 4 pregnant ewes. A small dose of GnRH (200 ng/ml saline) was injected (i.v.) every 3h and jugular blood samples were collected every 15 min for 12h beginning at the onset of GnRH administration and continuing through the 4th GnRH pulse. During the frequent-sample bleeding at any of the stages of pregnancy examined, LH concentrations oscillated in a pulsatile manner. However, pulsatile LH release occurred irregularly and infrequently. Overall mean LH concentrations, frequency and amplitude of LH pulses were not significantly different between any of the stages of pregnancy examined. Pulsatile administration of GnRH on day 10 post-mating induced regular pulses of LH. In conclusion, these data demonstrate that: (i) pulsatile LH secretion occurs irregularly during early pregnancy, and (ii) the absence of regular pulsatile LH release during early pregnancy is not attributed to a lack of pituitary responsiveness to GnRH.     

Hormonal responses in pubertal males to pulsatile gonadotropin releasing hormone (GnRH) administration

Twenty-two boys (9 with delayed puberty and 13 with short stature) ages 12.3 - 17.8 yr, and 10 adult males with idiopathic hypogonadotropic hypogonadism (ages 17.3 - 41.1 yr) have been studied following pulsatile, sc GnRH therapy (240 ng/kg/pulse) over 6 days. Mean pre- and post-therapy LH and FSH concentrations were estimated by 15 min blood sampling over 3-h periods immediately before and at the end of the treatment period. There were significant correlations between the mean pre- and posttreatment LH and FSH concentrations (r = 0.82, p less than 0.001 and r = 0.51, p less than 0.02, respectively) for the 2 groups of peripubertal boys when assessed together. Nine of the 10 adults with hypogonadism showed proportionately greater gonadotropin increments following pulsatile therapy when compared with the peripubertal boys. Standard bolus GnRH tests (100 micrograms iv) did not differentiate between the three groups of patients before pulsatile GnRH therapy. Bolus GnRH tests could predict the subsequent response to pulsatile therapy in the peripubertal boys only. There was no significant change in LH increments following the GnRH bolus tests in either group, after pulsatile GnRH administration (p greater than 0.1). Early response to pulsatile GnRH administration is dependent upon the maturity of the hypothalamic-pituitary-testicular axis in males with delayed puberty or short stature. Patients with hypogonadotropic hypogonadism do not show this relationship.     

Increase of serum leptin after short-term pulsatile GnRH administration in children with delayed puberty

OBJECTIVE: Leptin is known to play an important role in pubertal development in humans, probably acting as one permissive factor for the onset of puberty. Leptin serum concentrations change during pubertal development and an initial increase before the onset of puberty has been reported. The underlying mechanism for this increase in leptin levels is unknown. We hypothesized that the pulsatile release of GnRH stimulates leptin metabolism. In this study, the effect of short-term pulsatile GnRH administration on leptin levels in children with delayed onset of puberty was investigated. METHODS: Nineteen children (15 males and four females, mean age 15.5 years, range 13.1-20.5 years), who underwent evaluation for delayed sexual maturation, were included in the study. Sixteen subjects received 36 h of pulsatile intravenous GnRH, using an infusion pump that released 5 microg GnRH every 90 min. Serum concentrations of LH, FSH, testosterone, estradiol and leptin were analysed before and up to 36 h after GnRH administration. Eight patients received a single dose GnRH-agonist stimulation test (buserelin acetate test, 10 microg/kg body weight) with a 24-h follow-up (five patients underwent both tests). RESULTS: Mean (+/-s.e.m.) serum leptin increased significantly (P<0.01) after 36 h of pulsatile GnRH administration (7.26+/-1.35 vs 9.75+/-1.76 ng/ml). In contrast, no increase in leptin concentrations was observed after administration of a single dose of buserelin. CONCLUSIONS: These findings suggested that the increase in serum leptin at the onset of puberty is triggered by the pulsatile release of GnRH.     

Control of gonadotropin secretion in the ovine fetus: the effects of a specific gonadotropin-releasing hormone antagonist on pulsatile luteinizing hormone secretion

To demonstrate the dependence of fetal pituitary LH secretion endogenous GnRH, we studied the effects of bolus iv administration of a specific GnRH antagonist analog [GnRH-Ant; (N-acetyl-D-p-chloro-Phe1,2,D-Trp3,D-Arg6,D-Ala10)GnRH] on pulsatile LH release in 10 chronically cannulated ovine fetuses of 104-129 days gestation (term, 147 days). Vehicle alone was given to 13 control fetuses of 107-125 days gestation. Blood samples for LH determination by RIA (NIH LH S16 standard) were taken after injection of either GnRH-Ant (175-300 micrograms dissolved in 1 ml 5% dextrose in water) or vehicle alone for 1.75-5 h. The efficacy of GnRH receptor blockade was then assessed by a bolus iv challenge with 50 micrograms synthetic GnRH. The mean (+/- SEM) observation period per animal was similar for the two groups (3.8 +/- 0.2 h for GnRH-Ant; 3.6 +/- 0.2 h for controls). The frequency of spontaneous pulsatile LH secretion was significantly decreased in the fetuses given GhRH-Ant (2 pulses over 38 h total observation vs. 13 pulses over 47.3 h in control fetuses; P = 0.006). The average interpulse interval was 19.0 h in the GnRH-Ant group compared to 3.6 h in controls. Although the mean pulse amplitude was lower in the GnRH-Ant group (2.8 +/- 1.2 vs. 7.6 +/- 1.1 ng/ml for controls), this difference was not statistically significant (P = 0.065, by one-tailed t test). The mean peak serum LH concentration in response to the GnRH challenge was significantly blunted in the GnRH-Ant group (4.6 +/- 0.8 vs. 20.6 +/- 1.8 ng/ml for controls; P less than 0.001). These results indicate that GnRH-Ant administration causes a virtual cessation of pulsatile LH discharge. As this GnRH-Ant blocks GnRH action at the receptor level, these data demonstrate that pulsatile LH secretion in the ovine fetus is dependent on endogenous GnRH release as early as 104 days gestation.     

Failure to induce puberty in a man with X-linked congenital adrenal hypoplasia and hypogonadotropic hypogonadism by pulsatile administration of low-dose gonadotropin-releasing hormone

To elucidate the mechanism of hypogonadotropic hypogonadism in a patient with X-linked congenital adrenal hypoplasia, we studied the effects on serum LH and FSH of repeated iv administration of GnRH (400 micrograms, over 2 h, once a day, for 14 consecutive days), pulsatile sc administration of GnRH (5 micrograms every 90 min during days 1 approximately 56, 10 micrograms every 90 min during days 57 approximately 91) and an iv bolus injection of 10 mg of naloxone. The repeated administration of GnRH restored the hyporesponsiveness of serum FSH and increased serum testosterone level from less than 1.0 to 1.7 nmol/l, but the impaired LH response to the standard GnRH test was not improved. The pulsatile administration of GnRH for 91 consecutive days did not induce a clinical or a biochemical change of puberty. Serum testosterone remained undetectable less than 1.0 nmol/l, the hyporesponsiveness of serum LH was not improved, but basal FSH level was significantly increased and the impaired FSH response to the standard GnRH test was slightly improved. Naloxone had no effect on serum LH or FSH before or during the pulsatile administration. We conclude that hypogonadotropic hypogonadism in our patient is due to the pituitary dysfunction and that the endogenous opioid peptides may not play a role in the mechanism of inhibited gonadotropin secretions.     

Dose response effects of pulsatile GnRH administration on restoration of pituitary GnRH receptors and pulsatile LH secretion during lactation

Ovariectomized (OVX) rats suckling 8 pups have a complete suppression of pulsatile LH secretion and a decrease in pituitary GnRH receptor (GnRH-R) content. Removing the suckling stimulus for 24 h results in a sharp increase in GnRH-R and a restoration of pulsatile LH secretion. These findings suggest that the suckling stimulus induces a suppression of GnRH secretion, and removal of the suckling stimulus permits the restoration of GnRH secretion. Indeed, if GnRH antiserum is injected at the time of pup removal, the restoration of pituitary GnRH-R and LH secretion is prevented. The present studies were designed to test our hypothesis that the deficits in pituitary gonadotroph function observed during lactation are due to suckling-induced suppression of GnRH. Exogenous GnRH was administered in a pulsatile regimen to OVX lactating rats on days 10 and 11 postpartum, and the effects on pituitary GnRH-R levels, pituitary sensitivity to GnRH, and pulsatile LH secretion were assessed. GnRH doses of 0, 0.5, 2.0 or 5.0 ng/pulse were administered every 50 min for 24 h beginning on day 10. Administration of 0.5 ng GnRH/pulse for 24 h increased GnRH-R from 35 +/- 3 to 63 +/- 8 fmol/pituitary. There was a clear GnRH dose-related upregulation of GnRH-R to approach nonsuckling levels (140-160 fmol/pituitary) with the 5 ng GnRH dose. At the beginning of GnRH administration, the pituitary was very unresponsive to GnRH. Consistent LH pulses were only observed with 5 ng GnRH/pulse.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lack of gonadotropic response to pulsatile gonadotropin-releasing hormone in isolated hypogonadotropic hypogonadism associated to congenital adrenal hypoplasia

Congenital adrenal hypoplasia (AH) is a rare condition, known to be associated with isolated hypogonadotropic hypogonadism (IHH). Three studies have reported attempts to stimulate gonadotropin secretion with pulsatile gonadotropin-releasing hormone (GnRH) in a total of 4 patients presenting such a syndrome, with conflicting results. In the present study, one patient with idiopathic IHH and AH was treated with pulsatile sc GnRH--doses ranging from 2.5 to 10.0 micrograms/pulse, every 90 min--during 8 weeks in an attempt to induce puberty. The prepubertal basal plasma levels of LH, FSH and testosterone, and saliva testosterone levels remained unaltered throughout treatment, at all doses of GnRH tested. The gonadotropin response to an acute iv GnRH administration (0.1 mg) also remained at the prepubertal level after pulsatile GnRH treatment. No circulating anti-GnRH antibodies were detected. The absence of gonadotropic response to exogenous pulsatile GnRH suggests that the IHH of patients with AH is due to an abnormal pituitary function rather than to a lack of endogenous GnRH.     

Alternate-day GnRH therapy for ovarian hypofunction induced by weight loss: treatment of six patients who remained amenorrhoeic after weight gain

objective Body weight loss has an adverse effect on the pituitary-ovarian function, and no restoration of the function occurs in 20–30% of patients even after the recovery of body weight. In the present study, the efficacy of alternate-day GnRH therapy was investigated in six patients whose pituitary-ovarian function had not recovered at 15 months to 6 years after the recovery of body weight. DESIGN Synthetic GnRH (100 μg) was injected intramuscularly on alternate days for a total of 12 doses. On the completion of GnRH administration the efficacy of clomi-phene-human chorionic gonadotrophin (hCG) therapy was evaluated. A GnRH loading test and evaluation of LH secretion patterns were also performed to investigate the changes in hypothalamic-pituitary-ovarian function secondary to GnRH treatment. RESULTS All six patients ovulated in response to clomiphene-hCG therapy after 1–3 courses of GnRH treatment. The ovulatory response to clomiphene-hCG continued during follow-up periods of 5 months to 5 years. The initial hormonal change induced by GnRH treatment was a rise in the basal serum FSH level and a return of the FSH response to GnRH loading. This was followed by an increase in serum LH and the return of LH response to GnRH loading. In all six patients, pulsatile LH secretion was absent before starting GnRH treatment. GnRH treatment produced LH pulses in five of the six patients, although the pulse frequency was less than that in normally cycling women. A further increase in the pulse frequency was observed during clomiphene therapy. CONCLUSIONS Alternate-day GnRH administration is effective in inducing responsiveness to clomiphene in patients with anovulation secondary to weight loss.     

Resistance of hypogonadic patients with mutated GnRH receptor genes to pulsatile GnRH administration

We have studied a kindred with three siblings with isolated hypogonadotropic hypogonadism caused by compound heterozygote mutations in the GnRH receptor gene. The disorder was transmitted as an autosomal recessive trait. The R262Q mutation in intracellular loop 3 of the receptor was associated with a mutation in the third transmembrane domain of the receptor, A129D, that has never been described before. This A129D mutation results in a complete loss of function, indicated by the lack of inositol triphosphate (TP3) 3 production by transfected Chinese hamster ovary (CHO) cells after GnRH stimulation. The two brothers had microphallus and bilateral cryptorchidism and were referred for lack of puberty, whereas their sister had primary amenorrhea and a complete lack of puberty. Their basal gonadotropin concentrations were below the reference range, and their endogenous LH secretory patterns were abnormal, with a low-normal frequency of small pulses or no apparent LH pulse. Pulsatile GnRH administration (10 microg/pulse every 90 min for 40 h) resulted in increased mean LH without any significant changes in testosterone levels in the two brothers, whereas the LH secretory profile of their sister remained apulsatile. Larger pulses of exogenous GnRH (20 microg every 90 min for 24 h) caused the sister to produce recognizable low amplitude LH pulses. The concentrations of free alpha-subunit significantly increased in all patients during the pulsatile GnRH administration. Thus, these hypogonadal patients are partially resistant to pulsatile GnRH administration, suggesting that they should be treated with gonadotropins to induce spermatogenesis or ovulation rather than with pulsatile GnRH.     

Subcutaneous administration of gonadotropin-releasing hormone: absorption kinetics and gonadotropin responses

To evaluate the suitability of the sc route for the pulsatile delivery of GnRH, plasma GnRH, LH, and FSH levels were measured by RIA in five women with hypothalamic amenorrhea after sc injection of single doses of 2.5, 5, and 10 micrograms GnRH. The results were compared with those obtained after bolus iv injection of 10 micrograms GnRH. After sc injection, plasma GnRH levels rose to a dose-related maximum after 5-10 min and fell to less than 10% of the peak value by 90 min. The mean plasma disappearance half-time was 24 min (range, 18-30 min). After bolus iv injection, an initial rapid phase of disappearance (t1/2, 2.8 min) was followed by a slower phase (t1/2, 33 min), falling within the 95% confidence intervals for the disappearance half-time after sc administration (12-36 min). The patterns of LH response to sc and iv GnRH were similar, with maximum levels reached between 20 and 30 min after injection, then declining to 50-69% of the peak value by 90 min after sc injection and 61% of the peak value 90 min after iv injection. There was no significant difference between peak LH responses to 10 micrograms iv and sc doses of GnRH [15.2 +/- 2.5 (+/- SEM) vs. 13.2 +/- 2.2 IU/L]. Subcutaneous administration of three consecutive GnRH pulses at 90-min intervals to four women resulted in gonadotropin responses to each GnRH pulse. We conclude that sc GnRH administration results in pulsatile plasma GnRH and gonadotropin responses, the latter resembling those seen after iv GnRH. These results confirm the suitability of the sc route for pulsatile GnRH delivery.     

Ovulation induction with pulsatile gonadotropin-releasing hormone and continuous human menopausal gonadotropin in polycystic ovarian disease

Various treatments have been applied to polycystic ovarian (PCO) type of anovulation. However, none of them was definitive in terms of the efficacy and side effects. Six anovulatory women of PCO type were treated with pulsatile gonadotropin-releasing hormone (GnRH) of various pulse intervals and continuous human menopausal gonadotropin (hMG). The efficacy and rationale of the treatments were discussed. The subjects were diagnosed PCO by GnRH test and/or laparoscopy. They did not ovulate with clomiphene, clomiphene-hCG and hMG-hCG therapies. Their pretreatment serum FSH and LH levels and FSH/LH ratios were 6.9 +/- 1.2 mIU/ml, 15.7 +/- 5.1 mIU/ml, and 0.54 +/- 0.19 (Mean +/- SD), respectively. The treatment consisted of 3 protocols: 1) pulsatile GnRH (5-10 micrograms/pulse) of 90 min interval, 2) pulsatile GnRH (5-10 micrograms/pulse) of 120 min interval and 3) continuous hMG (150 IU/day) through subcutaneous route. Follicular growth was monitored sonographically and an intramuscular bolus of 10,000 IU hCG was given when the dominant follicle reached 20 mm in diameter. During both GnRH treatments serum FSH levels and FSH/LH ratios did not elevate substantially. Serum LH, E2 and PRL levels elevated acutely and transiently during the initial phase of GnRH treatments. Follicular growth was observed in a small fraction of the cases, but none of them ovulated. In contrast, continuous hMG treatment induced significant elevation in serum FSH levels (8.2 +/- 1.7 mIU/ml; p less than 0.01) and FSH/LH ratios (1.73 +/- 0.57; p less than 0.001). Transient hyperprolactinemia was accompanied with the preovulatory E2 rise. All the cases ovulated and 3 singleton pregnancies followed. These findings draw conclusions as follows. Pulsatile GnRH administration may desensitize the pituitary presumably due to increased GnRH pulse frequency as a consequence of two independent pulse generators, intrinsic and exogeneous. It may induce transient hyperprolactinemia through a paracrine system between gonadotrophs and lactotrophs. As a due course pulsatile GnRH therapy is questionable for ovulation induction in cases with functioning hypothalamic-pituitary axis. The fact that continuous hMG effectively induced follicle maturation with elevated FSH/LH ratios suggested that FSH dominance might be a prerequisite for folliculogenesis. The fluctuating nature of gonadotropins might not be mandatory for folliculogenesis.     

The abnormal response of polycystic ovarian disease patients to exogenous pulsatile gonadotropin-releasing hormone: characterization and management

Pulsatile GnRH administration for induction of ovulation is often ineffective in polycystic ovarian disease (PCOD) patients. To clarify and correct the endocrine mechanisms underlying this deranged response we gave pulsatile GnRH (5 micrograms, iv, every 60 min) to idiopathic hypogonadotropic hypogonadism (IHH) patients with primary amenorrhea for 19 cycles and to PCOD patients for 24 cycles before (pre-A) and for 25 cycles after (post-A) GnRH analog suppression. Compared to IHH, pre-A cycles were characterized by elevated LH, estradiol, and testosterone; reduced luteal phase progesterone; and low ovulatory (38%) and pregnancy rates (8%). Conversely, LH, estradiol, and follicular phase testosterone levels were lower in post-A than in pre-A cycles, while luteal phase progesterone was higher; the endocrine pattern of post-A cycles closely resembled the one of IHH cycles. The ovulatory and pregnancy rates of PCOD patients improved remarkably in post-A cycles (90% and 38%, respectively). Excessive body weight was associated with a lower incidence of ovulation in both pre-A (15%) and post-A cycles (75%). A worse endocrine pattern and a lower ovulatory rate (50%) were obtained when a second consecutive post-A cycle occurred without repeating GnRH analog suppression. No signs of even mild ovarian hyperstimulation and no multiple pregnancies were recorded in the post-A cycles. We conclude that in PCOD 1) deranged pituitary sensitivity, excessive ovarian androgen secretion, and obesity critically affect folliculogenesis and ovulation; 2) pituitary-gonadal suppression with a GnRH analog markedly improves the endocrine and clinical responses to pulsatile GnRH ovulation induction; 3) optimal results can be achieved only when each pulsatile GnRH cycle is preceded by GnRH analog suppression; and 4) pulsatile GnRH is highly effective and safe for ovulation induction, provided that PCOD subjects are pretreated with a GnRH analog.     

Utility of free alpha-subunit as an alternative neuroendocrine marker of gonadotropin-releasing hormone (GnRH) stimulation of the gonadotroph in the human: evidence from normal and GnRH-deficient men.

To examine the hypothesis that the secretion of free alpha-subunit (FAS) can serve as an alternative to LH as a neuroendocrine marker of gonadotroph stimulation by GnRH in euthyroid humans, we have investigated the relationship of pulsatile FAS secretion in euthyroid GnRH-deficient men (n = 10) before and after exogenous GnRH stimulation and in normal men under the influence of endogenous GnRH secretion (n = 18). Before GnRH exposure, the GnRH-deficient men showed a complete absence of both LH and FAS pulses. During the initial 7 days of GnRH exposure, all GnRH-deficient men exhibited pulsatile release of FAS by the third day, whereas the appearance of pulsatile release of LH and FSH was more variable. Long term administration of GnRH led to pulses of LH and FAS that were 100% concordant with a demonstrable dose-response relationship between GnRH and FAS, which was quantitatively similar to but more exuberant than that for LH. All doses of GnRH that produced LH pulses within the normal adult range yielded supraphysiological FAS pulses. Analysis of distribution histograms of interpulse intervals and pulse amplitudes of LH and FAS in both normal and GnRH-deficient subjects demonstrated no significant difference between these glycoproteins in interpulse intervals in either the normal or GnRH-deficient groups or in the pulse amplitudes in the GnRH-deficient subjects. There was, however, a significant difference (P less than 0.01) between the distribution histogram of LH and FAS pulse amplitudes in normal men. We conclude that the pulsatile secretion of FAS in euthyroid men 1) is determined by GnRH secretion, 2) is the initial glycoprotein to be secreted in a pulsatile fashion from the gonadotroph during early GnRH exposure in GnRH-deficient men, 3) demonstrates a dose-response relationship to exogenous GnRH which is more robust than that of LH in GnRH-deficient men receiving GnRH, and 4) can, therefore, serve as a complementary and powerful tool with LH for the study of GnRH neurosecretory dynamics.     

The effect of long-term pulsatile GnRH administration on the 24-hour integrated concentration of GH in hypogonadotropic hypogonadic patients

Measurement of integrated concentration of GH by means of continuous withdrawal sampling is a method of evaluating physiological hormonal secretion. Integrated concentration of GH was evaluated in 5 subjects with idiopathic hypogonadal hypogonadism (range 19-27 years) and in a 17-year-old male with idiopathic delay of puberty (5 males, 1 female) before and 30-240 days after the start of pulsatile GnRH administration. Gonadotropins and testosterone or 17 beta-estradiol were restored, whereas 24-h integrated concentration of GH (before therapy 5.4 +/- 1.3 IU/1; during GnRH 8.1 +/- 2.0 IU/1; P less than 0.05) was increased by GnRH therapy. However, no correlation was found between GH levels and sex steroid concentrations during GnRH pulsatile administration. These data further confirm that a physiological increase in gonadotropins and sex steroids can modulate GH synthesis and/or release.     

Suppression of mediobasal hypothalamic gonadotropin-releasing hormone and plasma luteinizing hormone pulsatile patterns by phentolamine in ovariectomized rhesus macaques

In gonadectomized animals, pulses of LH are secreted concurrently with pulsatile hypothalamic GnRH and it is hypothesized that pulses of GnRH are either driven or modulated by episodes of catecholamine release. The objective of this study was to determine if the alpha-adrenergic antagonist phentolamine (PHEN) can simultaneously block the release of GnRH and LH in ovariectomized (OVX) rhesus macaques. In Exp 1, simultaneous peripheral blood and mediobasal hypothalamic push-pull perfusion (PPP) samples were collected remotely at 10-min intervals for 24 h via a swivel/tether device in eight conscious, freely moving OVX rhesus monkeys. Phentolamine was continuously infused iv for 6 h at the rate of 4 mg/kg BW.h in five animals and 20 mg/kg BW.h in three animals. Infusion started at 6 h after the commencement of PPP. Sampling of PPP and blood continued for 12 h after the cessation of PHEN infusion. Exp 2 was carried out to determine if PHEN affects pituitary responsiveness to exogenous GnRH under conditions similar to those in Exp 1. Exogenous GnRH (5 micrograms, iv) was injected as a single bolus at 10-h intervals before, during, and after either a saline (4 ml/h for 6 h) infusion or, 3 weeks later, a PHEN infusion (4 mg/kgBW.h for 6 h) in three OVX females. The results of Exp 1 show that pulsatile patterns of hypothalamic GnRH and LH were either dampened or abolished by PHEN infusion. During the recovery period after PHEN infusion, pulse amplitudes of LH were enhanced, but pulse amplitudes of endogenous GnRH did not differ, as compared to those of corresponding LH and GnRH before infusion of PHEN. Data from Exp 2 suggested that the alpha-adrenergic blocking agent had no effect on the pituitary LH response to exogenous GnRH administration. These results directly support the hypothesis that adrenergic neuronal activities are critical for the pulsatile release of hypothalamic GnRH which governs the pulsatile release of LH in gonadectomized animals.     

Does cortisol inhibit pulsatile luteinizing hormone secretion at the hypothalamic or pituitary level?

Elevations in glucocorticoids suppress pulsatile LH secretion in sheep, but the neuroendocrine sites and mechanisms of this disruption remain unclear. Here, we conducted two experiments in ovariectomized ewes to determine whether an acute increase in plasma cortisol inhibits pulsatile LH secretion by suppressing GnRH release into pituitary portal blood or by inhibiting pituitary responsiveness to GnRH. First, we sampled pituitary portal and peripheral blood after administration of cortisol to mimic the elevation stimulated by an immune/inflammatory stress. Within 1 h, cortisol inhibited LH pulse amplitude. LH pulse frequency, however, was unaffected. In contrast, cortisol did not suppress either parameter of GnRH secretion. Next, we assessed the effect of cortisol on pituitary responsiveness to exogenous GnRH pulses of fixed amplitude, duration, and frequency. Hourly pulses of GnRH were delivered to ewes in which endogenous GnRH secretion was blocked by estradiol. Cortisol, again, rapidly and robustly suppressed the amplitude of GnRH-induced LH pulses. We conclude that, in the ovariectomized ewe, cortisol suppresses pulsatile LH secretion by inhibiting pituitary responsiveness to GnRH rather than by suppressing hypothalamic GnRH release.     

Bioactive follicle-stimulating hormone responses to intravenous gonadotropin-releasing hormone in boys with idiopathic hypogonadotropic hypogonadism

To test the hypothesis that exogenous pulsatile administration of GnRH will increase serum bioactive FSH (bFSH) levels, we studied four boys with suspected idiopathic hypogonadotropic hypogonadism (IHH). These boys presumably secreted relatively little GnRH. By virtue of their low baseline serum gonadotropin levels yet responsive pituitary gonadotrophs, these boys with IHH proved to be an excellent clinical model to test this hypothesis. Administration of GnRH (0.025 microgram/kg.dose) iv at 1- or 2-h intervals for 3-5 days resulted in an increase in serum bFSH after 91% of the GnRH doses. Serum immunoreactive FSH (iFSH) and LH (iLH) levels increased after 42% and 64% of the GnRH doses, respectively. Ninety percent of the iLH responses were concordant with bFSH responses, but only 33% of the iLH responses were concordant with iFSH responses. The serum bFSH responses occurred consistently within 20 min after GnRH administration and resulted in an increased serum bioactive to immunoreactive FSH ratio. By 60 min, serum bFSH levels had returned to preinjection levels. Serum testosterone and estradiol levels did not change during the period of GnRH administration in three of the four boys. We conclude that pulsatile, low dose iv GnRH administration in boys with IHH elicits significant serum bFSH increases by 20 min; the newly secreted FSH is preferentially enriched with increased in vitro FSH bioactivity, and it is rapidly cleared from serum (60 min). Therefore, serum bFSH measurements may provide a sensitive index of GnRH effects on the gonadotrophs.