Hypothalamic Pulse Generator Research Articles

In vivo imaging of the GnRH pulse generator reveals a temporal order of neuronal activation and synchronization during each pulse

A hypothalamic pulse generator located in the arcuate nucleus controls episodic release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and is essential for reproduction. Recent evidence suggests this generator is composed of arcuate "KNDy" cells, the abbreviation based on coexpression of kisspeptin, neurokinin B, and dynorphin. However, direct visual evidence of KNDy neuron activity at a single-cell level during a pulse is lacking. Here, we use invivo calcium imaging in freely moving female mice to show that individual KNDy neurons are synchronously activated in an episodic manner, and these synchronized episodes always precede LH pulses. Furthermore, synchronization among KNDy cells occurs in a temporal order, with some subsets of KNDy cells serving as "leaders" and others as "followers" during each synchronized episode. These results reveal an unsuspected temporal organization of activation and synchronization within the GnRH pulse generator, suggesting that different subsets of KNDy neurons are activated at pulse onset than afterward during maintenance and eventual termination of each pulse. Further studies to distinguish KNDy "leader" from "follower" cells is likely to have important clinical significance, since regulation of pulsatile GnRH secretion is essential for normal reproduction and disrupted in pathological conditions such as polycystic ovary syndrome and hypothalamic amenorrhea.

Encoding and Decoding Mechanisms of Pulsatile Hormone Secretion

Ultradian pulsatile hormone secretion underlies the activity of most neuroendocrine systems, including the hypothalamic-pituitary adrenal (HPA) and gonadal (HPG) axes, and this pulsatile mode of signalling permits the encoding of information through both amplitude and frequency modulation. In the HPA axis, glucocorticoid pulse amplitude increases in anticipation of waking, and, in the HPG axis, changing gonadotrophin-releasing hormone pulse frequency is the primary means by which the body alters its reproductive status during development (i.e. puberty). The prevalence of hormone pulsatility raises two crucial questions: how are ultradian pulses encoded (or generated) by these systems, and how are these pulses decoded (or interpreted) at their target sites? We have looked at mechanisms within the HPA axis responsible for encoding the pulsatile mode of glucocorticoid signalling that we observe in vivo. We review evidence regarding the 'hypothalamic pulse generator' hypothesis, and describe an alternative model for pulse generation, which involves steroid feedback-dependent endogenous rhythmic activity throughout the HPA axis. We consider the decoding of hormone pulsatility by taking the HPG axis as a model system and focussing on molecular mechanisms of frequency decoding by pituitary gonadotrophs.

Melatonin effects on luteinizing hormone in postmenopausal women: a pilot clinical trial NCT00288262

BackgroundIn many mammals, the duration of the nocturnal melatonin elevation regulates seasonal changes in reproductive hormones such as luteinizing hormone (LH). Melatonin's effects on human reproductive endocrinology are uncertain. It is thought that the same hypothalamic pulse generator may both trigger the pulsatile release of GnRH and LH and also cause hot flashes. Thus, if melatonin suppressed this pulse generator in postmenopausal women, it might moderate hot flashes. This clinical trial tested the hypothesis that melatonin could suppress LH and relieve hot flashes.MethodsTwenty postmenopausal women troubled by hot flashes underwent one week of baseline observation followed by 4 weeks of a randomized controlled trial of melatonin or matched placebo. The three randomized treatments were melatonin 0.5 mg 2.5–3 hours before bedtime, melatonin 0.5 mg upon morning awakening, or placebo capsules. Twelve of the women were admitted to the GCRC at baseline and at the end of randomized treatment for 24-hour sampling of blood for LH. Morning urine samples were collected twice weekly to measure LH excretion. Subjective responses measured throughout baseline and treatment included sleep and hot flash logs, the CESD and QIDS depression self-ratings, and the SAFTEE physical symptom inventory.ResultsUrinary LH tended to increase from baseline to the end of treatment. Contrasts among the 3 randomized groups were statistically marginal, but there was relative suppression combining the groups given melatonin as contrasted to the placebo group (p < 0.01 one-tailed, Mann-Whitney U = 14.) Similar but not significant results were seen in blood LH. There were no significant contrasts among groups in hot flashes, sleep, depression, or side-effect measures and no significant adverse effects of any sort.ConclusionThe data are consistent with the hypothesis that melatonin suppresses LH in postmenopausal women. An effect related to the duration of nocturnal melatonin elevation is suggested. Effects of melatonin on reproductive endocrinology should be studied further in younger women and in men. Larger studies of melatonin effects on postmenopausal symptoms would be worthwhile.

Aging of the male reproductive system.

Reproductive and sexual physiology, changes in body composition and mental performance in the aging male cannot simply be reduced to presumptive hypogonadism defined by low androgen serum levels or by decreasing levels of growth hormone (GH) and melatonin. Morphological changes in organs at different regulatory levels of hormonal networks governing, for example reproduction, such as diminished hypothalamic pulse generator mass, focal degeneration and loss of Leydig cells in testicular tissue, lead to diminished reserve capacities in production and to loss of coordinated pulsatile release of hypothalamic neuropeptides (e.g. gonadotropin releasing hormone, GnRH) and consequently diminished release of pituitary protein and glycoprotein hormones and testicular steroid hormones. Owing to presumptive alterations in feedback sensitivity, decreased testosterone levels do not necessarily upregulate pituitary LH secretion. Alternatively, increased serum levels of LH and FSH can be observed in old men either because of primary hypogonadism or to decreased hypothalamic opioid tone. In general, endocrine functions are sufficient to maintain fertility in elderly men because, except for sperm motility, quantitative and qualitative functional semen parameters are apparently not affected by age. Nevertheless, reduced endocrine and organic functions might become critical at different levels, with high inter-individual variability, of the hypothalamo/pituitary/gonadal-axis. One of the most intriguing organic manifestations of male aging is benign prostatic hyperplasia (BPH), the pathologic prevalence of which closely matches age. Age-associated changes in the endocrine system and in local networks of epithelial, stromal and luminal factors may play important roles in BPH development.

The Wisdom of the Body Revisited.

Being selected a Cannon lecturer by the American Physiological Society is a singular honor under any circumstances, but a particularly poignant one for me because of my origins in Cannon’s old Department at the Harvard Medical School. In fact, when I began my career there as an instructor 45 years

Gonadotropin-releasing hormone secretion into third-ventricle cerebrospinal fluid of cattle: correspondence with the tonic and surge release of luteinizing hormone and its tonic inhibition by suckling and neuropeptide Y.

Objectives of the current studies were to characterize the pattern of GnRH secretion in the cerebrospinal fluid of the bovine third ventricle, determine its correspondence with the tonic and surge release of LH in ovariectomized cows, and examine the dynamics of GnRH pulse generator activity in response to known modulators of LH release (suckling; neuropeptide Y [NPY]). In ovariectomized cows, both tonic release patterns and estradiol-induced surges of GnRH and LH were highly correlated (0.95; p < 0.01). Collectively, LH pulses at the baseline began coincident with (84%) or within one sampling point after (100%) the onset of a GnRH pulse, and all estradiol-induced LH surges were accompanied by corresponding surges of GnRH. A 500- microg dose of NPY caused immediate cessation of LH pulses and lowered (p < 0.001) plasma concentrations of LH for at least 4 h. This corresponded with declines (p < 0.05) in both GnRH pulse amplitude and frequency, but GnRH pulses were completely inhibited for only 1.5-3 h. In intact, anestrous cows, GnRH pulse frequency did not differ before and 48-54 h after weaning on Day 18 postpartum, but concentrations of GnRH (p < 0.05) and amplitudes of GnRH pulses (4 of 7 cows) increased in association with weaning and heightened secretion of LH. We conclude that the study of GnRH secretory dynamics in third-ventricle CSF provides a reasonable approach for examining the activity and regulation of the hypothalamic pulse generator in adult cattle. However, data generated using this approach must be interpreted in their broadest context. Although strong neurally mediated inhibitors of LH pulsatility (suckling; NPY) had robust effects on one or more GnRH secretory characteristics in CSF, only high doses of NPY briefly abolished GnRH pulses. This implies that the GnRH signal received at the hypophyseal portal vessels under these conditions may differ quantitatively or qualitatively from those in CSF, and theoretically would be undetectable or below a biologically effective threshold when LH pulses are absent.

Mid-Follicular Phase Pulses of Inhibin B Are Absent in Polycystic Ovarian Syndrome and Are Initiated by Successful Laparoscopic Ovarian Diathermy: A Possible Mechanism Regulating Emergence of the Dominant Follicle

The hypothalamic pulse generator of GnRH is recognized to be central to ovulatory function as evidenced by the anovulation of women with hypogonadotrophic hypogonadism due to Kallmann’s syndrome or severe anorexia nervosa. LH is released from the anterior pituitary in pulses, the frequency of which is closely entrained with those of GnRH. In contrast, secretion of FSH is influenced by a number of regulatory molecules, including GnRH, estradiol, inhibin, and activin. The close temporal relationship between changes in levels of inhibin B and FSH in the mid-follicular phase suggests that the release of inhibin B by the preovulatory follicle critically regulates pituitary FSH secretion. Polycystic ovarian syndrome (PCOS) is one of the most common endocrine disorders affecting ovulation, and abnormal ovarian morphology as detected by ultrasonography remains the most sensitive diagnostic marker for this disorder. The etiology of PCOS is unclear, but its effective treatment by both anti-estrogens and by exogenous FSH suggests that a primary disorder of FSH regulation may be central. To investigate the possible role of inhibin B in the pathology of PCOS, serum inhibin B levels were measured in 10 women with PCOS on cycle day 5 of a spontaneous or progestrogen-provoked bleed and compared with levels on cycle day 5 of 10 women with regular ovulatory cycles. The mean serum inhibin B levels in the PCOS patients were significantly higher at 248 (±43.4) pg/mL compared with normal controls, 126 (±18.6) pg/mL (P < 0.01). Ten women with clomiphene resistant PCOS and 5 normal controls consented to undergo serial blood sampling on cycle day 5. Time Series Analysis using a Fourier Transformation to analyze the power spectrum of the data revealed that in normal women there is a distinct periodicity in inhibin B levels with a clear peak detectable every 60–70 min (P < 0.05), whereas in women with ovulatory dysfunction due to PCOS, no such pattern of regular pulsatility was seen. Four women with PCOS whose anovulation was successfully treated with laparoscopic ovarian diathermy (LOD) underwent repeat venous sampling following LOD. Their serum inhibin B levels fell to the upper limit of the normal range (160± 38.5) pg/mL, and pulsatility was initiated It is possible that inhibin B pulses are being generated directly by the ovary in response to pulses of GnRH in the peripheral circulation, or indirectly in response to FSH pulses arising in the pituitary. The function of inhibin B pulses in the mid-follicular phase of the normal cycle remains to be elucidated, but the absence of the normal pulsatile pattern in women with PCOS, in conjunction with high basal levels of inhibin B arising from the multiple small follicles characteristic of the PCOS ovary, appears to reinforce the development of a large cohort of small, developmentally arrested, and ultimately atretic follicles in these patients. Initiation of normal inhibin B pulsatility by LOD in patients with polycystic ovaries appears to correlate with the post-operative onset of ovular cycles.

Mid-follicular phase pulses of inhibin B are absent in polycystic ovarian syndrome and are initiated by successful laparoscopic ovarian diathermy: a possible mechanism regulating emergence of the dominant follicle.

The hypothalamic pulse generator of GnRH is recognized to be central to ovulatory function as evidenced by the anovulation of women with hypogonadotrophic hypogonadism due to Kallmann's syndrome or severe anorexia nervosa. LH is released from the anterior pituitary in pulses, the frequency of which is closely entrained with those of GnRH. In contrast, secretion of FSH is influenced by a number of regulatory molecules, including GnRH, estradiol, inhibin, and activin. The close temporal relationship between changes in levels of inhibin B and FSH in the mid-follicular phase suggests that the release of inhibin B by the preovulatory follicle critically regulates pituitary FSH secretion. Polycystic ovarian syndrome (PCOS) is one of the most common endocrine disorders affecting ovulation, and abnormal ovarian morphology as detected by ultrasonography remains the most sensitive diagnostic marker for this disorder. The etiology of PCOS is unclear, but its effective treatment by both anti-estrogens and by exogenous FSH suggests that a primary disorder of FSH regulation may be central. To investigate the possible role of inhibin B in the pathology of PCOS, serum inhibin B levels were measured in 10 women with PCOS on cycle day 5 of a spontaneous or progestrogen-provoked bleed and compared with levels on cycle day 5 of 10 women with regular ovulatory cycles. The mean serum inhibin B levels in the PCOS patients were significantly higher at 248 (+/- 43.4) pg/mL compared with normal controls, 126 (+/- 18.6) pg/mL (P < 0.01). Ten women with clomiphene resistant PCOS and 5 normal controls consented to undergo serial blood sampling on cycle day 5. Time Series Analysis using a Fourier Transformation to analyze the power spectrum of the data revealed that in normal women there is a distinct periodicity in inhibin B levels with a clear peak detectable every 60-70 min (P < 0.05), whereas in women with ovulatory dysfunction due to PCOS, no such pattern of regular pulsatility was seen. Four women with PCOS whose anovulation was successfully treated with laparoscopic ovarian diathermy (LOD) underwent repeat venous sampling following LOD. Their serum inhibin B levels fell to the upper limit of the normal range (160 +/- 38.5) pg/mL, and pulsatility was initiated. It is possible that inhibin B pulses are being generated directly by the ovary in response to pulses of GnRH in the peripheral circulation, or indirectly in response to FSH pulses arising in the pituitary. The function of inhibin B pulses in the mid-follicular phase of the normal cycle remains to be elucidated, but the absence of the normal pulsatile pattern in women with PCOS, in conjunction with high basal levels of inhibin B arising from the multiple small follicles characteristic of the PCOS ovary, appears to reinforce the development of a large cohort of small, developmentally arrested, and ultimately atretic follicles in these patients. Initiation of normal inhibin B pulsatility by LOD in patients with polycystic ovaries appears to correlate with the post-operative onset of ovular cycles.

Melatonin inhibits naloxone-induced luteinizing hormone release in ovariectomized estrogen-primed rats.

The present study aimed to examine the effect of melatonin on naloxone-induced luteinizing hormone (LH) secretion in ovariectomized estrogen-primed rats. A single intracerebroventricular (i.c.v.) injection of naloxone (mu opioid receptor blocker, 15 micrograms) or an intravenous (i.v.) injection of LH-releasing hormone (LHRH, 50 ng/kg) elicited a transient and significant increase in the serum LH concentration within 10 min. While an i.c.v. injection of 100 ng melatonin by itself did not change the basal LH release, it almost completely inhibited the naloxone-induced LH release. Melatonin (10 ng) also significantly reduced the effect of naloxone. However, an i.c.v. injection of 100 ng melatonin did not affect the LHRH-induced LH release. In separate experiments, the effect of melatonin on naloxone-induced pulsatile LH secretion was studied in estrogen-treated rats. A continuous i.v. infusion of naloxone (20 mg/kg/h) induced LH pulses in rats treated i.c.v. with saline. An i.c.v. administration of 100 ng melatonin, which by itself did not affect basal LH secretion, significantly reduced the frequency, but not the amplitude, of LH pulses induced by the naloxone infusion. These results show that melatonin has a suprapituitary site of action to inhibit naloxone-induced LH release, and suggest that melatonin has an effect in inhibiting the activity of the hypothalamic LHRH pulse generator, either directly or indirectly, in female rats.

SEMI-PARAMETRIC AND NON-PARAMETRIC MODELING OF THE TEMPORAL ACTIVITY OF THE HYPOTHALAMIC PULSE GENERATOR IN CASTRATE MONKEYS

The luteinizing hormone (LH) is released by the pituitary in discrete pulses. Electro-physiological studies in monkeys have demonstrated that sharp intermittent increases in the electrical activity of a hypothalamic pulse generator (HPG) are associated in a one-to-one manner with the occurrence of LH pulses in the plasma and exhibits a circadian modulation. In order to investigate the temporal structure of the HPG, we develop a semi-parametric stochastic point process model generalizing the Cox's periodic regression model. We apply this approach to the study of memory range of the process underlying HPG activity, using experimental data from one ovariectomized rhesus monkey. A non-parametric approach is also described.

Hypothalamic multiunit activity and pulsatile luteinizing hormone release in the castrated male rat

Using chronically implanted microelectrodes, multiunit electrical activity (MUA) was recorded from the arcuate nucleus of freely moving gonadectomized male rats. Intermittent increases in MUA activity (MUA volleys) closely associated with luteinizing hormone pulses measured in the peripheral circulation were observed, which confirms that this experimental approach can be used for monitoring the activity of the gonadotropin-releasing hormone-associated hypothalamic pulse generator in the male rat. The mean MUA volley frequency was 22.2 min (range 13-38 min), whereas the mean MUA volley duration was 2.7 +/- 0.8 min (standard deviation). In addition to a large inter-individual variability. MUA volley intervals also showed an important intra-individual variability. This observation suggests that, beside the mean frequency of pulse generator activation, the degree of variability in gonadotropin-releasing hormone-associated pulse generator activity might be an additional relevant parameter in the characterization of the reproductive function in the male rat.

Stochastic modeling of the hypothalamic pulse generator activity

Luteinizing hormone (LH) is released by the pituitary in discrete pulses. In the monkey, the appearance of LH pulses in the plasma is invariably associated with sharp increases (i.e, volleys) in the frequency of the hypothalamic pulse generator electrical activity, so that continuous monitoring of this activity by telemetry provides a unique means to study the temporal structure of the mechanism generating the pulses. To assess whether the times of occurrence and durations of previous volleys exert significant influence on the timing of the next volley, we used a class of periodic counting process models that specify the stochastic intensity of the process as the product of two factors: 1) a periodic baseline intensity and 2) a stochastic regression function with covariates representing the influence of the past. This approach allows the characterization of circadian modulation and memory range of the process underlying hypothalamic pulse generator activity, as illustrated by fitting the model to experimental data from two ovariectomized rhesus monkeys.

Effect of human prolactin administration on gonadotropin and thyrotropin secretion in normal men.

To test the hypothesis that PRL is able to feedback negatively on its own secretion (short-loop feedback) in humans via augmentation of the turnover of tuberoinfundibular dopamine (TIDA), the effects of the administration of purified hPRL on endogenous LH, FSH and TSH were assessed. Purified hPRL, given in an i.v. loading dose of 90 micrograms followed by a continuous infusion of 1.39 micrograms/min to 4 normal male volunteers resulted in a tripling of PRL levels (10.5 +/- 1.9 micrograms/L increasing to 30.9 +/- 3.6 micrograms/L) at the end of 90 min. There were no changes in LH, FSH or TSH levels, however, during or following the infusion. Purified hPRL was also given in 1 and 8 micrograms/kg doses IM to 5 normal male volunteers. Although PRL levels did not rise significantly with the 1 microgram/kg dose, levels almost doubled with the 8 micrograms/kg dose (9.5 +/- 2.2 micrograms/L increasing to 17.4 +/- 1.5 micrograms/L). Again, LH, FSH and TSH levels did not change significantly over the three hour period of sampling with either dose. In conclusion, in this study we found that a 2-3 fold increase of circulating PRL levels maintained for 1.5-3 h exerted no apparent effects on the secretion of endogenous LH, FSH and TSH. This study provides direct evidence against the existence of a short-loop feedback occurring via TIDA activation in humans over this time interval but does not rule out the possibility that such feedback may occur with more prolonged states of hyperprolactinemia or via other mechanisms or the possibility of an effect on the hypothalamic pulse generator.

Role of neuropeptide-Y in episodic luteinizing hormone release in ovariectomized rats: an excitatory component and opioid involvement.

We tested the hypothesis that hypothalamic neuropeptide-Y (NPY) is an excitatory signal in the episodic secretion of LH in ovariectomized (ovx) rats and that the suppression of LH secretion that consistently follows intracerebroventricular administration of NPY is due to concurrent release of opioids or CRH, both previously shown to readily inhibit LH release. In the first experiment, ovx rats received continuous intraventricular infusion of either serum containing NPY antibodies (NPY-Ab) or normal rabbit serum (control) at dilutions of 1:5 or 1:1. NPY-Ab infusion at a 1:5 dilution significantly decreased mean plasma LH levels and LH pulse amplitude without affecting LH pulse frequency over a 3-h period of observation. However, infusion of relatively more concentrated NPY-Ab (1:1) markedly decreased not only mean plasma LH levels and LH pulse amplitude, but also the frequency of LH episodes. In the next experiment, we observed that intraventricular administration of NPY (0.2 nmol) suppressed LH release for 60 min. However, blockade of opiate receptors with iv infusion of naloxone (2 mg/h) before and after NPY injection completely counteracted the NPY-induced inhibition of LH release. On the other hand, prior blockade of the CRH receptors with alpha-helical CRH-(9-41) (25 or 100 micrograms/rat) was ineffective in reversing the inhibitory LH response of NPY (0.125 nmol). These results together with our previous demonstration of morphological communication between NPY and beta-endorphin neurons, show that suppression of LH by exogenous NPY in ovx rats may result from concurrent stimulation of opioids, primarily beta-endorphin. However, diminution of all parameters of episodic LH secretion by NPY-Ab affirms the notion that the NPY network is a physiologically important excitatory component of the hypothalamic pulse generator circuitry that regulates episodic LH secretion in rats.

Role of neuropeptide-Y in episodic luteinizing hormone release in ovariectomized rats: an excitatory component and opioid involvement

We tested the hypothesis that hypothalamic neuropeptide-Y (NPY) is an excitatory signal in the episodic secretion of LH in ovariectomized (ovx) rats and that the suppression of LH secretion that consistently follows intracerebroventricular administration of NPY is due to concurrent release of opioids or CRH, both previously shown to readily inhibit LH release. In the first experiment, ovx rats received continuous intraventricular infusion of either serum containing NPY antibodies (NPY-Ab) or normal rabbit serum (control) at dilutions of 1:5 or 1:1. NPY-Ab infusion at a 1:5 dilution significantly decreased mean plasma LH levels and LH pulse amplitude without affecting LH pulse frequency over a 3-h period of observation. However, infusion of relatively more concentrated NPY-Ab (1:1) markedly decreased not only mean plasma LH levels and LH pulse amplitude, but also the frequency of LH episodes. In the next experiment, we observed that intraventricular administration of NPY (0.2 nmol) suppressed LH release for 60 min. However, blockade of opiate receptors with iv infusion of naloxone (2 mg/h) before and after NPY injection completely counteracted the NPY-induced inhibition of LH release. On the other hand, prior blockade of the CRH receptors with alpha-helical CRH-(9-41) (25 or 100 micrograms/rat) was ineffective in reversing the inhibitory LH response of NPY (0.125 nmol). These results together with our previous demonstration of morphological communication between NPY and beta-endorphin neurons, show that suppression of LH by exogenous NPY in ovx rats may result from concurrent stimulation of opioids, primarily beta-endorphin. However, diminution of all parameters of episodic LH secretion by NPY-Ab affirms the notion that the NPY network is a physiologically important excitatory component of the hypothalamic pulse generator circuitry that regulates episodic LH secretion in rats.

The luteinizing hormone-releasing hormone-secreting hypothalamic hamartoma is a congenital malformation: natural history.

The LHRH-secreting hypothalamic hamartoma (HH), a congenital malformation consisting of a heterotopic mass of nervous tissue that contains LHRH neurosecretory neurons attached to the tuber cinereum or the floor of the third ventricle, can cause true or central precocious puberty (TPP). We have suggested that it functions as an ectopic LHRH pulse generator independent of the central nervous system inhibitory mechanism that normally restrains the hypothalamic LHRH pulse generator. TPP associated with a hamartoma has all of the hormonal hallmarks of puberty, including a pubertal pattern of pulsatile LH and a pubertal plasma LH response to LHRH administration. Little is known about the natural history of HH. We present long term data on 10 children (5 females and 5 males) with TPP due to HH. Physical signs of puberty were observed at a mean age of 2.2 +/- 1.6 yr (range, 0.5-5.1). Two of 10 had a pedunculated mass, and 8 of 10 had a sessile mass. The hamartoma varied in diameter from 4-25 mm and did not change with time (3.5-8.7 yr). Four patients have a seizure disorder, 3 with gelastic seizures (1 with mental retardation) and 1 with tonic-clonic seizures. The shape of the hamartoma, sessile or pedunculated, did not correlate with the occurrence of seizures. At presentation with sexual precocity, the mean height SD for chronological age was +3.5 +/- 0.4, the mean height SD for bone age was -1.9 +/- 0.4, and the mean bone age SD for chronological age was +6.8 +/- 0.7. Baseline data were comparable to those of 10 females with idiopathic TPP. Nine of 10 HH patients and all idiopathic TPP patients were treated with a LHRH agonist. The response to therapy was excellent in both groups and indistinguishable. Nine of 10 HH children attend school regularly and, aside from those with seizures, have no neurological handicap. While surgical resection of the hamartoma has been recommended, it carries an increased risk of morbidity and mortality and, if removal is incomplete, does not arrest the sexual precocity. In our experience, LHRH agonist therapy for TPP due to HH is the preferable approach.

Treatment with naltrexone in hypothalamic ovarian failure: induction of ovulation and pregnancy

Sixty-six women suffering from various grades of hypothalamic ovarian failure were treated with the opiate antagonist naltrexone at doses ranging from 25 to 150 mg per day. This treatment resulted in complete normalization of the menstrual cycle in 49 of 66 patients, as indicated by the pattern of circulating levels of gonadotrophins and ovarian steroids. Five patients failed to respond, three of whom were suffering from primary hypothalamic amenorrhoea. In patients who responded to the administration of naltrexone, there was a dramatic increase in the amplitude and frequency of gonadotrophin pulses, reflecting disinhibition of the hypothalamic gonadotrophin-releasing hormone (GnRH) pulse generator. Eighteen pregnancies were achieved in 16 women who were also treated for infertility, resulting in a cumulative pregnancy rate closely resembling that of a normal population. There were only minor side-effects that could be attributed to the drug. These data demonstrate that chronic administration of an opiate antagonist will normalize ovarian function in women suffering from different grades of hypothalamic ovarian failure. The data therefore support the view that suppression of the activity of the hypothalamic pulse generator, that directs GnRH release, is mediated by endogenous opioids. Also, that hypothalamic ovarian failure is the consequence of an inappropriate increase in opioid tone impinging on neurons that release GnRH in a pulsatile manner into the pituitary portal circulation.

Effect of melatonin implants on secretion of luteinizing hormone in intact and castrated rams

Rams were treated with melatonin implants in 2 experiments designed to examine the control of reproductive seasonality. In Exp. 1, rams (n = 12) were allocated to 3 treatment groups: 2 groups were treated with 2 melatonin implants per ram for 4 months from 11 November (N) and 9 December (D) and the remaining group was untreated (C). The seasonal increase in luteinizing hormone (LH) pulse frequency and testes size was advanced in Groups N and D. A second seasonal cycle in LH secretion and testes size occurred in Groups N and D after melatonin implants became exhausted. In Exp. 2, rams (n = 20) were allocated to 4 treatment groups: 10 rams were castrated on 6 October and 1 group of entire rams (EM) and one group of castrated rams (CM) were treated with 2 melatonin implants per ram each month from 3 November until 8 January. The other group of entire rams (EC) and castrated rams (CC) was untreated. An increase in LH pulse frequency occurred after castration. Melatonin treatment increased LH pulse frequency in entire rams and reduced LH pulse frequency in castrated rams. The results demonstrated that the advanced reproductive development as a result of treatment with melatonin implants was due to an effect of melatonin on the hypothalamic pulse generator to increase LH pulse frequency. The ability of melatonin to influence LH pulse frequency in entire and castrated rams indicated that an effect of melatonin on the hypothalamic pulse generator is independent of testicular steroids.

Maturation of the Hypothalamic Control of Pulsatile Gonadotropin-Releasing Hormone Secretion at Onset of Puberty: II. Reduced Potency of an Inhibitory Autofeedback*

At the time of onset of puberty in the male rat, between 15 and 25 days of age, we have reported that pulsatile GnRH secretion by hypothalamic explants showed an increased frequency as indicated by the reduction of the mean interpulse interval from 66 to 40 min (P less than 0.05). This study aimed to evaluate whether these changes in GnRH secretion involved a self-regulatory mechanism. A 7.5-min exposure of explants obtained at 50 days to 0.1 microM superagonist D-TRP6-PRO9-N-Et-GnRH (GnRH-A) resulted in a delay of the next GnRH secretory pulse so that the mean interpulse interval increased from 35 to 67 min (P less than 0.001). In addition, after a 7.5-min exposure to GnRH-A, there was a 15-min period with absent or reduced release of GnRH in response to 50 microM veratridine, a depolarizing agent. A similar refractory period of 15 min was observed using explants obtained at 25 and 50 days whereas, at 15 days, the period of refractoriness lasted for 52.5 min. The inhibitory effect of GnRH-A on the subsequent response to veratridine occurred at similar concentrations of GnRH-A at the three studied ages and the inhibition was prevented using an antagonist of GnRH together with GnRH-A. The involvement of GnRH itself in an autofeedback mechanism was evaluated by studying the period of refractoriness separating two GnRH pulses elicited by 7.5-min exposures to veratridine. The initial responsiveness to veratridine was recovered after a refractory period of 52.5, 22.5, and 15 min when studied at 15, 25, and 50 days, respectively. While refractoriness occurred during repeated depolarization with K+ or veratridine, such an effect was not observed using N-methyl-D-aspartate (NMDA). During exposure to GnRH-A, the NMDA-induced release of GnRH was only reduced by 38% whereas veratridine-induced secretion showed a 94% reduction. Thus, exogenous activation of NMDA-sensitive receptors could bypass the inhibitory autofeedback. We conclude that: 1) pulsatile GnRH secretion is controlled by an inhibitory autofeedback involving NMDA sensitive receptors, 2) the increased frequency of pulsatile GnRH secretion at onset of puberty may be related to a reduced sensitivity of the hypothalamic pulse generator to an inhibitory autofeedback.

Disorders of the Hypothalamic Pulse Generator: Insufficient or Inappropriate Gonadotropin-Releasing Hormone Release

Disorders of the Hypothalamic Pulse Generator: Insufficient or Inappropriate Gonadotropin-Releasing Hormone Release