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Regulation of puberty
Best Practice & Research Clinical Endocrinology and Metabolism Vol. 16, No. 1, pp. 1±12, 2002
doi:10.1053/beem.2002.0176, available online at http://www.idealibrary.com on
1 Regulation of puberty Henriette A. Delemarre-van de Waal
MD, PhD
Professor of Paediatric Endocrinology Department of Pediatrics, VU University Medical Center, PO Box 7057, 1007 MB Amsterdam, The Netherlands
Pubertal development is the last phase of a continuum of changing gonadotrophin releasing hormone (GnRH) activities. Whether or not puberty tends to start at a younger age, as has been recently described in a population of black Americans, remains under debate. Such early onset has not been con®rmed in dierent European countries. Ideas about the underlying mechanisms responsible for the reawakening of GnRH release at the onset of puberty have changed signi®cantly during the last decades. At this moment, the common opinion is that neuronal outgrowth of both GnRH and other regulatory neurons results in changing interactions and activities. Sex steroids, as well as various central neurotransmitters, play a role in modulating GnRH release. Active release after birth is followed by the restraint of childhood. A re-onset of GnRH excitatory activities heralds the onset of puberty. This chapter gives an overview of the many factors involved. Key words: puberty; pubertal development; secular trend; precocious puberty; gonadostat; GnRH; GABA; glutamate; NPY; leptin; growth factors.
INTRODUCTION Puberty can be de®ned as a maturational process of the hypothalamus±pituitary± gonadal axis resulting in growth and development of the genital organs and, concomitantly, in physical and psychological changes towards adulthood leading to the capacity to reproduce. Increasing levels of gonadotrophins stimulate gonadal growth. Subsequent production of sex steroids leads to the development of secondary sex characteristics. In girls breast development starts at a mean age of 10.7 years.1 About 2.4 years after the onset of breast development menarche occurs. Thereafter menstrual cycles are generally irregular and anovulatory. A regular pattern of ovulatory cycles is established within 5 years of menarche in most adolescents.2 In boys, spermaturia occurs at about 13.5 years. This hallmark of gonadal maturation can be expected when mean testicular size is 11.5 ml, which occurs at genital stage 3.3 The timing of pubertal events is similar in the various European countries. Trends in the timing of puberty are the subject of debate at present. As a result of the secular trend, ®nal height has been increasing over the last century, while pubertal 1521±690X/02/01000112 $35.00/00
c 2002 Elsevier Science Ltd. *
2 H. A. Delemarre-van de Waal
onset has become earlier. However, the trend towards earlier puberty seemed to come to a halt during the last decades: Dutch studies have shown that earlier timing of puberty did not occur during the period 1980±1997.1 In contrast to the Dutch data, investigations in the USA have shown an ongoing trend of earlier onset of breast development in girls and genital growth in boys.4,5 There, 5% of white American girls had breast development at the age of 7. In African-American girls this percentage was as high as 15%. Since menarche remained the same at 12.7 years of age, it appears that puberty in American girls is characterized by earlier onset of breast development but with a slower progression of pubertal development. The earlier onset of puberty in girls is associated with an increased body mass index, which is clearer in white than in black girls.6 It has been suggested that this increased body mass index plays a role in the earlier onset. Oestrogens stored in body fat would cause increased bioactivity. However, during prepuberty oestrogen levels are low and, therefore, cannot account for the increased oestrogen availability. Another option is that fat tissue exerts a central eect via the neurotransmitter neuropeptide Y (NPY), which is involved in the regulation of gonadotrophin releasing hormone (GnRH). Several studies have shown that increasing body fat may trigger the onset of puberty.6 In American boys, recent data have also shown evidence of earlier genital growth.5 These studies elicited much criticism since the children were not investigated appropriately, while very important conclusions were drawn from the data. Therefore, future studies are needed to con®rm such earlier onset in male and female American adolescents. Several other studies in dierent European countries also do not support the American experience of strikingly earlier onset of puberty, since, in Dutch studies, a halt in the trend towards earlier puberty has been found during the last decades.7 With respect to growth and development the most recent survey in the Netherlands showed a striking increment of body weight in healthy children.8 The eect of this shift to a higher prevalence of obese children and adolescents may have its eect on the regulation of pubertal development in a later phase. Another group of children with earlier pubertal onset are children adopted from developing countries. The timing of pubertal events is comparable with that of their Asian and South American peers.9 These adopted children often present with malnourishment and infectious problems on arrival in their new home-country. In general, these children grow remarkably well during infancy and childhood, which is very promising for ®nal height. However, since most of them have a relatively early or precocious puberty ®nal height is often much lower than that expected, although it is not dierent from the ®nal height achieved by their peers living in the country of origin.9 It could be hypothesized that the abnormal intensive growth after adoption is corrected by an earlier onset of puberty in order to achieve a ®nal height that accords with the `programmed' original growth pattern. ENDOCRINE REGULATION OF PUBERTY The hypothalamus±pituitary±gonadal axis is already mature in the fetal period. The decapeptide gonadotrophin releasing hormone (GnRH) is produced by hypothalamic neurons that originate from the medial olfactory placode of the nose. These cells migrate across the nasal septum, arching into the septal±preoptic area and the hypothalamus.10 The GnRH producing cells are then located in the arcuate nucleus, in the preoptic area and in the medial basal hypothalamus. GnRH is present in the fetal
Regulation of puberty 3
hypothalamus from 9 weeks of gestation. During gestation there is an increase of GnRH content with maximum levels at 22±25 weeks of gestation in the female fetus and at 34±38 weeks in the male fetus. Higher levels are reported in the female.11 The continuity of the primary and secondary plexus of the portal capillary network is completed by weeks 19±21 and may explain the striking rise of gonadotrophin levels in circulation at mid-gestation in both male and female fetuses. After mid-gestation, plasma gonadotrophin levels decrease from their castrate-like (i.e. high) levels to very low levels at birth. In the female, the pituitary content of luteinizing hormone (LH) and follicle stimulating hormone (FSH), as well as the FSH plasma levels, are remarkably higher than in the male fetus at mid-gestation. The decrease of gonadotrophin levels in the second half of gestation seems to be the result of a developing negative feedback to sex steroids as well as a developing system of inhibiting in¯uences on the GnRH neurons. Since primates have a similar control of gonadotrophin secretion to humans, research has focused on this model to gain insight into the human mechanisms of puberty. In primate studies, g-aminobutyric acid (GABA) and other substances have been related to a decreasing GnRH release, although stimulating eects of GABA have also been found. The low gonadotrophin levels during childhood may result from tonic inhibition of GnRH by GABA neurons. In primates, release of GnRH neurons from GABA control is critical for the onset of puberty.12 GnRH stimulates the production and release of both LH and FSH. GnRH levels are dicult to measure, since it is secreted into the portal circulation and directly transported to the pituitary. Its short half-life of 4±7.8 minutes may contribute to this phenomenon.13 GnRH is secreted in a pulsatile pattern. Simultaneous episodic ¯uctuations of GnRH in portal blood and LH in peripheral blood have been demonstrated in sheep.14 In humans, pulsatile patterns of LH have been found and it can be assumed that this re¯ects pulsatile GnRH release. FSH oscillations are not as marked as those of LH and are not always synchronized with the LH pulses. The discrepancy between the pulses of plasma LH and FSH levels may be caused by the relatively long half-life of FSH (4±6 h) compared with the relatively short half-life of LH (20±30 mins).15,16 A transient increase of both LH and FSH levels can be seen during the ®rst months after birth (Figure 1). LH shows a pulsatile secretion pattern.17 This increase in gonadotrophin may be due to the rapid withdrawal of placental sex steroids and subsequent disturbance of the negative feedback equilibrium between sex steroids and GnRH release. This spontaneous, transient activity of the gonadal axis provides a golden window for establishing the diagnosis in hypogonadism. Central and gonadal problems can be dierentiated with relative ease. After the central restraint is switched on diagnostic work-up has to wait until puberty. During prepuberty the GnRH pulse generator is `asleep'. Gonadotrophin levels are suppressed. Whether or not gonadotrophin levels are measurable depends on the type of assay. Using highly sensitive immuno¯uorometric assays, very low levels of pulsatile secretion of LH have been measured in prepubertal boys and girls, while neither detectable levels nor pulses were found using immunoradiometric assays.18,19 The question arises whether the pulses are the result of more sensitive assays, the method of pulse detection or due to bias. It remains doubtful whether or not this highly sensitive assay can measure real LH pulses in Kallmann's syndrome patients. These patients have an organic GnRH de®ciency caused by an absent migration of the GnRH neurons from the olfactory placode.20
4 H. A. Delemarre-van de Waal
80
LH/FSH level
40 20 15 10 5 0 Fetus
Infancy
Childhood
Puberty
Adult
Figure 1. Pattern of plasma gonadotrophin levels during prenatal and postnatal development. LH, luteinizing hormone; FSH, follicle stimulating hormone.
With the onset of puberty, LH is secreted, ®rstly, only during the night. In boys this nocturnal LH increase is associated with a testosterone rise; in girls the rise in oestradiol occurs the following morning. With the progression of puberty the LH secretion gradually increases during both day and night. This increment is caused by the enhancement of both the LH pulse frequency and the pulse amplitude. During puberty the day±night rhythm is maintained and disappears in adulthood, presumably as a result of increasing sex steroid levels.19,21 With the progression of puberty the response to a challenge of exogenous GnRH also increases. During prepuberty, when endogenous GnRH stimulation is low, there is no, or hardly any, increase in gonadotrophins following such a challenge. From Tanner stage 2 to 5 a GnRH challenge increases LH and FSH in boys and LH in girls.22 For FSH in girls there is an exception at stage 2. This stage is characterized by high GnRH-stimulated levels of FSH only. The response is much lower in stage 3. Such a strong FSH response is also seen during infancy at the time that the central restraint is not completely developed and GnRH release is still decreasing. Incomplete suppression of this high FSH response is helpful in diagnosing premature thelarche in female infants. With the further development of the central restraint this gonadal activity and, in turn, the thelarche will disappear spontaneously. Although generally, basal gonadotrophin levels supply appropriate information about puberty, it is good to remember that a blunted response to GnRH only indicates that the pituitary is not stimulated by endogenous GnRH. This may be the result of a `normal' prepubertal state or it may be due to a state of hypogonadotrophic hypogonadism. Great eort has been put into the development of a test that can discriminate between these states, for instance by using a GnRH analog test or the multiple GnRH challenge test. However, a reliable test for discriminating between the normal prepubertal state and a GnRH de®ciency state is not available. In the case of a hypogonadotrophic hypogonadism it is possible to dierentiate between a pituitary or
Regulation of puberty 5
Feedback
High
Decreasing sensitivity
Adult
Prepuberty
Onset of puberty
Adult
GnRH
LH/FSH
Figure 2. The `gonadostat' hypothesis is based on the occurrence of a change in the sensitivity of a gonadotrophin regulating system (gonadostat) to the negative feedback of gonadal steroids. During prepuberty the sensitivity is highest, resulting in a low release of GnRH and gonadotrophins. Initiation of puberty is secondary to a decreased sensitivity inducing increasing GnRH and gonadotrophin release until a new equilibrium has been obtained in adulthood. GnRH, gonadotrophin-releasing hormone; LH, luteinizing hormone; FSH, follicle stimulating hormone. (Adapted from Grumbach et al).28.
hypothalamic defect by long-term pulsatile GnRH administration. This treatment schedule may be applied to induce pubertal development as well.23±26
HYPOTHESES ON THE MECHANISM OF THE ONSET OF PUBERTY The so-called `gonadostat' theory was ®rst suggested by Hohlweg in 1931 and was later developed further by Grumbach and coworkers.27,28 This theory is based on the concept of a changing sensitivity of the gonadotrophin regulating system (gonadostat) to the negative feedback of gonadal steroids (Figure 2). Already in 1932 Hohlweg & Junkmann29 observed that in the castrated immature rat, small doses of steroid hormones were able to stop pituitary hyperfunction, while in adult rats larger amounts were needed. In humans, the development of this negative feedback continues from fetal life throughout childhood when small doses of oestrogens are able to suppress gonadotrophin levels. According to this concept, the onset of puberty is the result of a decreasing sensitivity: thus, gonadotrophin secretion increases and, in turn, there is an increment in gonadal steroid levels. At the end of puberty the feedback of the gonadal steroids on gonadotrophin secretion reaches a new equilibrium at a higher setpoint. Findings that contradicted the `gonadostat' hypothesis came from studies in agonadal patients, who showed an identically changing pattern throughout infancy and prepuberty, although their gonads were not capable of producing gonadal steroids. Therefore, in the 1980s, an alternative theory was proposed, the `intrinsic restraint concept' (Figure 3). This concept assumes that, in addition to a sex steroid negative feedback, a central inhibitory system restrains the GnRH release and induces the prepubertal phase. At the onset of puberty the intrinsic restraint itself is inhibited or
6 H. A. Delemarre-van de Waal
Late neuronal growth
Early neuronal growth
Intrinsic restraint GABA
Stimulation Glutamate NPY Leptin TGFs
Opioids Sex steroids Negative feedback GnRH
LH FSH Sex steroids Figure 3. A conceptual model of the control of the onset of puberty. The early prepubertal GnRH quiescence is due to early neuronal growth resulting in a restraint of GnRH release. Stimulating factors overcome this restraint and initiate GnRH release again at the onset of puberty. GABA, g-aminobutyric acid; GnRH, gonadotrophin-releasing hormone; LH, luteinizing hormone; FSH, follicle stimulating hormone; NPY, neuropeptide Y; TGF, transforming growth factor.
becomes overruled by a stimulating system.30 This concept is supported by data that show that hypothalamic lesions in children may result in precocious pubertal development. During the last decades increasing evidence has been found supporting this intrinsic restraint hypothesis (see Factors involved in the onset of puberty, below). The `desynchrony' theory, a third hypothesis, is based on the ®nding that concentrations of GnRH in the hypothalamus of primates during prepuberty are similar to those seen in adulthood. According to this hypothesis the lack of GnRH stimulation of the pituitary is due to desynchronization of the `®ring' of GnRH neurons.31 In vitro studies in immortalized GnRH neurons revealed that these neurons might be coupled electrically through gap junctions, resulting in the co-ordination of the secretion between separate cells. The organization of the GnRH neuronal network synchronizes the GnRH secretion in individual cells.32
FACTORS INVOLVED IN THE ONSET OF PUBERTY The hypothalamic GnRH secretion is under inhibitory as well as stimulatory control. Central inhibition of GnRH release results in the prepubertal `sleep'. The onset of puberty is due to the removal or diminution of this central restraint. As described
Regulation of puberty 7
above, GABA is the dominant inhibitory neurotransmitter in the hypothalamus. However, in primates direct innervation of GnRH neurons by GABA neurons has not been found. Since a reciprocal innervation between GABAergic and glutamatergic neurons has been found33 it is possible that inhibition of GnRH neurons by GABA is mediated via glutamergic neurons. Several developmental changes in GABA levels and metabolism have been observed. The glutamic acid decarboxylases (GAD) GAD 65 and GAD 67 are two dierent forms of the GABA-synthetic enzyme. During development in the rat spinal cord, the GABA concentration and the number of GABAergic neurons show an increase from embryonic day 13 to the second postnatal week, which is followed by a decline in the third postnatal week.34 During the same period, GAD 65 and GAD 67 mRNAs increase exponentially from embryonic day 11 until the ®rst postnatal week and then decline into the adult range during the second postnatal week.35 Prior to the onset of puberty in female rats, GABA release in the pre-optic area decreases, while in the female rhesus monkey GABA release in the median eminence decreases concomitantly with the pubertal increase of GnRH release.36,37 Supportive evidence has been gained from patient observation. Bourguignon et al38 observed regression of pubertal signs in an 11-month old boy with severe epileptic seizures and precocious puberty under GABA agonist treatment. These ®ndings support the idea that GABA contributes to the suppression of GnRH release before the onset of puberty. In humans the opioid-peptides decrease gonadotrophin levels, which suggests that the opioidergic system plays a role in the intrinsic restraint.39 Naloxone, an opiate receptor antagonist, induces an increase of, in particular, LH secretion in normal adult men and women. During the prepubertal years, naloxone is not able to initiate any gonadotrophin release. However, in early puberty, when gonadotrophins and sex steroids are in the pubertal range, naloxone results in a gonadotrophin increase. The involvement of the opioidergic system in the negative feedback of sex steroids on gonadotrophin secretion at the hypothalamic level becomes operative during pubertal development when sex steroids increase.40 The major excitatory amino acid neurotransmitter, glutamate, is also involved in the regulation of GnRH release. The GnRH neuronal network receives axo-dendritic synaptic input from glutamatergic neurons, which are located in the hypothalamus.41 Glutamate regulates the GnRH pulsatile release via two major excitatory amino acid receptors: (1) the ionotropic receptors, which are coupled to ion channels and to which the N-methyl-D-aspartate (NMDA) receptor belongs and (2) the metabotropic receptors, which are coupled to G proteins.42 Glutamate stimulates GnRH release in sexually immature monkeys and rats. The pubertal increase of GnRH secretion has been suggested to be the result of increased glutaminase activity.43 Glutaminase is the enzyme responsible for the conversion of glutamine into glutamate, a prerequisite for the pulsatile GnRH release. In addition, in male rats, an increase in NMDA receptor activity is seen in the period preceding the onset of puberty, probably partly induced by the increased glutamate activity.44 The peripubertal changes support the concept that the neurotransmitter glutamate plays a major role in stimulating GnRH release at the onset of puberty. Neuropeptide Y (NPY) is widely distributed in the mammalian central nervous system and is involved in the regulation of several brain functions including food intake and reproductive function. It also plays a role in the central eects of leptin. A NPY infusion into the median eminence elicits GnRH release in pubertal female monkeys but not in prepubertal monkeys. In prepubertal monkeys, NPY release is low, but
8 H. A. Delemarre-van de Waal
along with the increase of GnRH release, an increase in NPY occurs. Since NPY in prepuberty does not in¯uence GnRH release, it seems that it may contribute to the pubertal process rather than playing a major role in triggering the onset of puberty.12 Growth factors may in¯uence the regulation of GnRH release as well. Astroglia cells synthesize and release growth factors such as transforming growth factor-a and -b (TGFa and TGFb). Other glial-derived substances, which may alter GnRH release are basic ®broblast growth factor, epidermal growth factor, insulin-like growth factor-1 (IGF-1), neuronal cell adhesion molecules and the cytokines interleukin-1 and -6. In the rat and in the rhesus monkey TGFa mRNA levels in astrocytes increase with normal puberty. Lesions in the hypothalamic area initiate precocious puberty, whereby the underlying mechanism may be the induced astrogliosis. Astrogliosis may be responsible for the increase in TGFa in the hypothalamus. TGFa stimulates GnRH release as well as stimulating the glia cells to produce bio-active substances, such as prostaglandin E2, which, in turn, also stimulates the release of GnRH.45 It is clear that the glia tissue is involved in the regulation of GnRH release. However it is uncertain whether it plays a critical role in the triggering of puberty. GnRH neurons contain IGF-1 receptors. IGF-1 plasma levels increase during puberty and, therefore, may contribute to the GnRH release. Whether or not this plays a role in the initiation of puberty remains unclear. IGF-1 may exert its eect on the amount of GnRH release: there are some reports describing a faster tempo of puberty in non-growth hormone de®cient children during growth hormone treatment.46 It is well known that nutritional factors in¯uence pubertal development. In chronic malnutrition a pubertal delay will occur. In the adult, the reproductive axis may return to the prepubertal state. In 1971, Frisch & Revelle47 proposed the hypothesis of a direct relationship between a critical weight and menarche. In their study, they observed that the mean weight of 48 kg at menarche did not alter as menarcheal age increased, whereas mean height had increased signi®cantly. In later studies more indications have been found that in both the female rat and the human female a particular ratio of fat to lean body mass is necessary for the start of puberty and maintenance of reproductive capacity. In 1973, Ruf wondered `how the brain can be informed of the nutritional state of the organism and how it ``knows'' when to initiate the process of puberty'.48 The peptide leptin plays a role in this by informing the brain about the peripheral energy stores. Leptin is produced and secreted by white adipose tissue and has potent eects on feeding behaviour, thermogenesis and neuroendocrine processes. In cases of leptin absence, obesity occurs as well as infertility.49 Leptin treatment will decrease food intake and restore reproductive function. Leptin exerts its eect through the NPY neurons. It decreases NPYergic signalling by acting directly at the level of NPY perikarya and possibly at the level of the NPY nerve terminals.50 In the fasting state, leptin levels decline and gonadotrophin secretion is suppressed. Such ®ndings underline the fact that nutrition, particularly when mediated by fat tissue and leptin, contributes to pubertal development. However, it is uncertain whether leptin is essential for the triggering of pubertal onset. Data in the rat have shown that leptin levels remain constant in the prepubertal and postpubertal stages as well as the fact that leptin gene expression in the hypothalamus does not show any developmental changes. These ®ndings do suggest that leptin is not an important metabolic trigger for the onset of puberty, but that leptin acts as a permissive signal for the onset of puberty.51,52 This is also in agreement with the ideas of the Frisch hypothesis that a critical weight triggers the onset of puberty.
Regulation of puberty 9
SUMMARY Puberty is the ®nal phase in a continuum of changing GnRH activities from the fetal period onwards. Before the last decade there was a trend towards earlier onset of puberty and increased ®nal height. Recently a halt in the trend of earlier pubertal onset has been described in various European countries. However, recently a very young onset of puberty was observed in American girls, especially in Black Americans. These alarming data should be con®rmed in a non-selected group of children. GnRH is already active in the fetus during early gestation. At a gestational age of 7 weeks GnRH, LH and FSH are already detectable. At midgestation LH and FSH blood levels are very high. These levels decrease, presumably as the result of a developing negative feedback to sex steroids as well as due to a developing GnRH suppression system. At birth, gonadotrophin levels are low, but show a transient increase during infancy. Incomplete suppression after birth may lead to the occurrence of the benign premature thelarche syndrome in female infants. The underlying mechanisms
Practice points . although not con®rmed in European studies, a trend towards earlier onset of puberty, as has been recently described in children in the USA, may be found during the coming years. Guidelines for work-up and treatment of disorders in the timing of puberty would then have to be reconsidered . preventative programmes for overweight and obese children may, in addition to general health care, have an eect on pubertal development . adopted children may have an earlier onset of puberty, which will have a negative impact on their ®nal height. However, the ®nal height achieved may be normal for their original population and, therefore, intervention should be reconsidered . puberty is the last developmental phase of a continuum that began during the fetal period . premature thelarche during infancy is the result of a partial suppression of the physiological postnatal transient increase of gonadotrophins . the postnatal spontaneous activity of the hypothalamic±pituitary±gonadal axis oers the clinician a golden window to evaluate this axis. After the development of central restraint all diagnostic procedures have to be delayed until the age that puberty is normally expected. Therefore work-up of sexual disorders has to be commenced during the ®rst weeks of life . the mechanisms that result in the changing pattern of gonadotrophin levels and the onset of puberty re¯ect complex anatomical interactions among GnRH neurons and other regulatory neurons. Factors modulating these neurons may play inhibitory, stimulatory, facilitatory or permissive roles . the neurotransmitters GABA and glutamate play a crucial role in the inhibition and stimulation, respectively, of GnRH release during development . modulation of GnRH release by sex steroid negative feedback is partly mediated by the opioidergic system . growth factors synthesized by glia cells may aect GnRH release. Their physiological role remains to be elucidated . leptin appears to serve as a metabolic signal to the gonadotrophin axis, more as a permissive factor than as an indispensable factor for the initiation of puberty
10 H. A. Delemarre-van de Waal
Research agenda . studies are needed to investigate claims that the age of onset of puberty in American adolescents is becoming earlier
responsible for the changing pattern of GnRH release are based on interactions between the GnRH neurons and other neurons. These neurons are subject to a continuing development in growth and function. Various neurotransimitters such as GABA, glutamate, opioids, NPY, leptin and growth factors play inhibitory, stimulatory, facilitatory or permissive roles in this development.
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Regulation of puberty 11 *18. Dunkel L, Alfthan H & Stenman Uhm Perheentupa J. Gonadal control of pulsatile secretion of luteinizing hormone and follicle stimulating hormone in prepubertal boys evaluated by ultrasensitive time-resolved immuno¯uorometric assays. Journal of Clinical Endocrinology and Metabolism 1990; 70: 107±114. *19. Wennink JMB, Delemarre-van de Waal HA, Schoemaker R et al. Luteinizing hormone and follicle stimulating hormone secretion patterns in boys throughout puberty measured using highly sensitive immunoradiometric assays. Clinical Endocrinology 1989; 31: 551±564. 20. Wu FCW, Butler GE, Kelnar CJH et al. Patterns of pulsatile luteinizing hormone and follicle stimulating hormone secretion in prepubertal (midchildhood) boys and girls and patients with idiopathic hypogonadotrophic hypogonadism (Kallmann's syndrome): a study using an ultrasensitive timeresolved immuno¯uorometric assay. Journal of Clinical Endocrinology and Metabolism 1991; 72: 1229±1237. 21. Wennink JMB, Delemarre-van de Waal HA, Schoemaker R et al. Luteinizing hormone and follicle stimulating hormone secretion patterns in girls throughout puberty using highly sensitive immunoradiometric assays. Clinical Endocrinology 1990; 33: 333±344. 22. Dickerman Z, Prager-Lewin R & Laron Z. Response of plasma LH and FSH to synthetic LH-RH in children at various pubertal stages. American Journal of Diseases in Childhood 1976; 130: 634±638. 23. Kletter GB, Rolfes-Curl A, Goodpasture JC et al. Gonadotropin-releasing hormone agonist analog (nafarelin): a useful diagnostic agent for the distinction of constitutional growth delay from hypogonadotropic hypogonadism. Journal of Pediatric Endocrinology and Metabolism 1996; 9: 9±19. 24. Snijder PJ, Rudenstein RS, Gardner DF & Rothman JG. Repetitive infusion of gonadotropin releasing hormone distinguishes hypothalamic from pituitary hypogonadism. Journal of Clinical Endocrinology and Metabolism 1979; 48: 864±870. 25. Delemarre-van de Waal HA. Induction of testicular growth and spermatogenesis by pulsatile, intravenous administration of gonadotrophin-releasing hormone in patients with hypogonadotropic hypogonadism. Clinical Endocrinology 1993; 38: 473±480. *26. Delemarre-van de Waal HA, van den Brande JL & Schoemaker J. Prolonged pulsatile administration of LRH in (pre)pubertal children. Journal of Clinical Endocrinology and Metabolism 1985; 61: 859±867. 27. Hohlweg W & Dohrn M. Beziehungen zwischen Hypophysenvorderlappen und KeimdruÈsen. Wiener Archivs Innere Medizin 1931; 21: 337±339. 28. Grumbach MM, Roth JC, Kaplan SL & Kelch RO. Hypothalamic±pituitary regulation of puberty in man: evidence and concepts derived from clinical research. In Grumbach MM, Grave GD & Mayer FE (eds) Control of the Onset of Puberty, pp 115±166. New York: WLiley & Sons, 1974. 29. Hohlweg W & Junkmann K. Die Hormonal±NervoÈse Regulierung der Funktion des Hypophysenvorderlappens. Klinische Wochenschrift 1932; 8: 321±324. 30. Conte FA, Grumbach MM, Kaplan SL & Reiter EO. Correlation of luteinizing-hormone releasing factor induced luteinizing hormone and follicle stimulating hormone release from infancy to 19 years with the changing pattern of gonadotropin secretion in agonadal patients: relation to the restraint of puberty. Journal of Clinical Endocrinology and Metabolism 1980; 50: 163±168. 31. Plant TM, Fraser MO, Medamurthy R & Gay VL. Somatogenic control of GnRH neuronal synchronization during development in primates: a speculation. In Delemarre-van de Waal HA, Plant TM, van Rees GP & Schoemaker J (eds) Control of the Onset of Puberty III, pp 111±122. Amsterdam: Elsevier, 1989. 32. Wetsel WC, Valenca MM, Merchenthaler I et al. Intrinsic pulsatile secretory activity of immortalized luteinizing hormone-releasing hormone secreting neurons. Proceedings of the National Academy of Science of the USA 1992; 89: 4149±4153. 33. Goldsmith PC & Thind KK. Morphological basis for neuronal control of GnRH secretion in the monkey. Journal of Endocrinology 1995; 73±85. 34. Schaner AE, Beher T, Nadi S & Barker JL. Quantitative analysis of transient GABA expression in embryonic and early postnatal rat spinal cord neurons. Brain Research. Developmental Brain Research 1993; 72: 265±276. 35. Somogyi R, Wen X, Ma W & Barker JL. Developmental kinetics of GAD family mRNAs parallel neurogenesis in the rat spinal cord. Journal of Neuroscience 1995; 15: 2575±2591. 36. Goroll D, Arias P & Wuttke W. Preoptic release of amino acid neurotransmitters evaluated in peripubertal and young adult female rats by push-pull perfusion. Neuroendocrinology 1993; 58: 11±15. 37. Terasawa E, Luchansky LL, Kasua E & Nyberg CL. An increase in glutamate release follows a decrease in g-aminobutyric acid and the pubertal increase in luteinizing hormone releasing hormone release in female rhesus monkeys. Journal of Neuroendocrinology 1999; 11: 275±282. *38. Bourguignon JP, Jaeken J, Gerard A & de Zegher F. Amino acid neurotransmission and initiation of puberty: evidence from non-ketotic hyperglycaemia in a female infant and gonadotropin-releasing hormone secretion by rat hypothalamic explants. Journal of Clinical Endocrinology and Metabolism 1997; 82: 1899±1903.
12 H. A. Delemarre-van de Waal 39. Delitala G, Guisti M, Mazzocchi G et al. Participation of endogenous opiates in regulation of the hypothalamic±pituitary±testicular axis in normal men. Journal of Clinical Endocrinology and Metabolism 1983; 57: 1277±1281. 40. Delemarre-van de Waal HA. Central Regulation of Puberty. PhD Thesis. Nieuwkoop: De Boer, 1984. 41. Goldsmith PC, Thind KK, Perera AD & Plant TM. Glutamate-immunoreactive neurons and their gonadotropin-releasing hormone-neuronal interactions in the monkey hypothalamus. Endocrinology 1994; 134: 858±868. 42. Brann DW & Mahesh VB. Exitatory amino acids: evidence for a role in the control of reproduction and anterior pituitary hormone secretion. Endocrine Reviews 1994; 15: 3±49. 43. Brann DD & Mahesh VB. Excitatory amino acid neurotransmission. Evidence for a role in neuroendocrine regulation. Trends in Endocrinology and Metabolism 1992; 3: 122±126. *44. Bourguignon JP, Gerard A, Alvarez-Gonzalez ML et al. Gonadal-independent developmental changes in activation of N-methyl-D-aspartate receptors involved in gonadotropin-releasing hormone secretion. Neuroendocrinology 1992; 55: 634±641. 45. Ma YJ, Berg-van der Emde K, Rage F et al. Hypothalamic astrocytes respond to transforming growth factor-alpha with the secretion of neuroactive substances that stimulate the release of luteinizing hormone-releasing hormone. Endocrinology 1997; 138: 19±25. 46. Hopwood NJ, Hintz RL, Gertner JM et al. Growth response of children with non-growth-hormone de®ciency and marked short stature during three years of growth hormone therapy. Journal of Pediatrics 1993; 123: 215±222. *47. Frisch RE & Revelle R. Height and weight at menarche and a hypothesis of menarche. Archives of Disease in Childhood 1971; 46: 695±701. 48. Ruf KB. How does the brain control the process of puberty? Zeitschrift for Neurologie 1973; 204: 95±98. 49. Kalra SP, Dube MG, Pu S et al. Interacting appetite-regulating pathways in the hypothalamic regulation of body weight. Endocrine Reviews 1999; 20: 68±100. 50. Zhang Y, Proenca R, Maei M et al. Positional cloning of the mouse obese gene and its human homologue. Nature 1994; 372: 425±432. 51. Flier JS. What's in a name? In search of leptin's physiologic role. Journal of Clinical Endocrinology and Metabolism 1998; 83: 1407±1413. *52. Cheung CC, Thornton JE, Nurani SD et al. A reassessment of leptin's role in triggering the onset of puberty in the rat and mouse. Neuroendocrinology 2001; 74: 12±21.
doi:10.1053/beem.2002.0176, available online at http://www.idealibrary.com on
1 Regulation of puberty Henriette A. Delemarre-van de Waal
MD, PhD
Professor of Paediatric Endocrinology Department of Pediatrics, VU University Medical Center, PO Box 7057, 1007 MB Amsterdam, The Netherlands
Pubertal development is the last phase of a continuum of changing gonadotrophin releasing hormone (GnRH) activities. Whether or not puberty tends to start at a younger age, as has been recently described in a population of black Americans, remains under debate. Such early onset has not been con®rmed in dierent European countries. Ideas about the underlying mechanisms responsible for the reawakening of GnRH release at the onset of puberty have changed signi®cantly during the last decades. At this moment, the common opinion is that neuronal outgrowth of both GnRH and other regulatory neurons results in changing interactions and activities. Sex steroids, as well as various central neurotransmitters, play a role in modulating GnRH release. Active release after birth is followed by the restraint of childhood. A re-onset of GnRH excitatory activities heralds the onset of puberty. This chapter gives an overview of the many factors involved. Key words: puberty; pubertal development; secular trend; precocious puberty; gonadostat; GnRH; GABA; glutamate; NPY; leptin; growth factors.
INTRODUCTION Puberty can be de®ned as a maturational process of the hypothalamus±pituitary± gonadal axis resulting in growth and development of the genital organs and, concomitantly, in physical and psychological changes towards adulthood leading to the capacity to reproduce. Increasing levels of gonadotrophins stimulate gonadal growth. Subsequent production of sex steroids leads to the development of secondary sex characteristics. In girls breast development starts at a mean age of 10.7 years.1 About 2.4 years after the onset of breast development menarche occurs. Thereafter menstrual cycles are generally irregular and anovulatory. A regular pattern of ovulatory cycles is established within 5 years of menarche in most adolescents.2 In boys, spermaturia occurs at about 13.5 years. This hallmark of gonadal maturation can be expected when mean testicular size is 11.5 ml, which occurs at genital stage 3.3 The timing of pubertal events is similar in the various European countries. Trends in the timing of puberty are the subject of debate at present. As a result of the secular trend, ®nal height has been increasing over the last century, while pubertal 1521±690X/02/01000112 $35.00/00
c 2002 Elsevier Science Ltd. *
2 H. A. Delemarre-van de Waal
onset has become earlier. However, the trend towards earlier puberty seemed to come to a halt during the last decades: Dutch studies have shown that earlier timing of puberty did not occur during the period 1980±1997.1 In contrast to the Dutch data, investigations in the USA have shown an ongoing trend of earlier onset of breast development in girls and genital growth in boys.4,5 There, 5% of white American girls had breast development at the age of 7. In African-American girls this percentage was as high as 15%. Since menarche remained the same at 12.7 years of age, it appears that puberty in American girls is characterized by earlier onset of breast development but with a slower progression of pubertal development. The earlier onset of puberty in girls is associated with an increased body mass index, which is clearer in white than in black girls.6 It has been suggested that this increased body mass index plays a role in the earlier onset. Oestrogens stored in body fat would cause increased bioactivity. However, during prepuberty oestrogen levels are low and, therefore, cannot account for the increased oestrogen availability. Another option is that fat tissue exerts a central eect via the neurotransmitter neuropeptide Y (NPY), which is involved in the regulation of gonadotrophin releasing hormone (GnRH). Several studies have shown that increasing body fat may trigger the onset of puberty.6 In American boys, recent data have also shown evidence of earlier genital growth.5 These studies elicited much criticism since the children were not investigated appropriately, while very important conclusions were drawn from the data. Therefore, future studies are needed to con®rm such earlier onset in male and female American adolescents. Several other studies in dierent European countries also do not support the American experience of strikingly earlier onset of puberty, since, in Dutch studies, a halt in the trend towards earlier puberty has been found during the last decades.7 With respect to growth and development the most recent survey in the Netherlands showed a striking increment of body weight in healthy children.8 The eect of this shift to a higher prevalence of obese children and adolescents may have its eect on the regulation of pubertal development in a later phase. Another group of children with earlier pubertal onset are children adopted from developing countries. The timing of pubertal events is comparable with that of their Asian and South American peers.9 These adopted children often present with malnourishment and infectious problems on arrival in their new home-country. In general, these children grow remarkably well during infancy and childhood, which is very promising for ®nal height. However, since most of them have a relatively early or precocious puberty ®nal height is often much lower than that expected, although it is not dierent from the ®nal height achieved by their peers living in the country of origin.9 It could be hypothesized that the abnormal intensive growth after adoption is corrected by an earlier onset of puberty in order to achieve a ®nal height that accords with the `programmed' original growth pattern. ENDOCRINE REGULATION OF PUBERTY The hypothalamus±pituitary±gonadal axis is already mature in the fetal period. The decapeptide gonadotrophin releasing hormone (GnRH) is produced by hypothalamic neurons that originate from the medial olfactory placode of the nose. These cells migrate across the nasal septum, arching into the septal±preoptic area and the hypothalamus.10 The GnRH producing cells are then located in the arcuate nucleus, in the preoptic area and in the medial basal hypothalamus. GnRH is present in the fetal
Regulation of puberty 3
hypothalamus from 9 weeks of gestation. During gestation there is an increase of GnRH content with maximum levels at 22±25 weeks of gestation in the female fetus and at 34±38 weeks in the male fetus. Higher levels are reported in the female.11 The continuity of the primary and secondary plexus of the portal capillary network is completed by weeks 19±21 and may explain the striking rise of gonadotrophin levels in circulation at mid-gestation in both male and female fetuses. After mid-gestation, plasma gonadotrophin levels decrease from their castrate-like (i.e. high) levels to very low levels at birth. In the female, the pituitary content of luteinizing hormone (LH) and follicle stimulating hormone (FSH), as well as the FSH plasma levels, are remarkably higher than in the male fetus at mid-gestation. The decrease of gonadotrophin levels in the second half of gestation seems to be the result of a developing negative feedback to sex steroids as well as a developing system of inhibiting in¯uences on the GnRH neurons. Since primates have a similar control of gonadotrophin secretion to humans, research has focused on this model to gain insight into the human mechanisms of puberty. In primate studies, g-aminobutyric acid (GABA) and other substances have been related to a decreasing GnRH release, although stimulating eects of GABA have also been found. The low gonadotrophin levels during childhood may result from tonic inhibition of GnRH by GABA neurons. In primates, release of GnRH neurons from GABA control is critical for the onset of puberty.12 GnRH stimulates the production and release of both LH and FSH. GnRH levels are dicult to measure, since it is secreted into the portal circulation and directly transported to the pituitary. Its short half-life of 4±7.8 minutes may contribute to this phenomenon.13 GnRH is secreted in a pulsatile pattern. Simultaneous episodic ¯uctuations of GnRH in portal blood and LH in peripheral blood have been demonstrated in sheep.14 In humans, pulsatile patterns of LH have been found and it can be assumed that this re¯ects pulsatile GnRH release. FSH oscillations are not as marked as those of LH and are not always synchronized with the LH pulses. The discrepancy between the pulses of plasma LH and FSH levels may be caused by the relatively long half-life of FSH (4±6 h) compared with the relatively short half-life of LH (20±30 mins).15,16 A transient increase of both LH and FSH levels can be seen during the ®rst months after birth (Figure 1). LH shows a pulsatile secretion pattern.17 This increase in gonadotrophin may be due to the rapid withdrawal of placental sex steroids and subsequent disturbance of the negative feedback equilibrium between sex steroids and GnRH release. This spontaneous, transient activity of the gonadal axis provides a golden window for establishing the diagnosis in hypogonadism. Central and gonadal problems can be dierentiated with relative ease. After the central restraint is switched on diagnostic work-up has to wait until puberty. During prepuberty the GnRH pulse generator is `asleep'. Gonadotrophin levels are suppressed. Whether or not gonadotrophin levels are measurable depends on the type of assay. Using highly sensitive immuno¯uorometric assays, very low levels of pulsatile secretion of LH have been measured in prepubertal boys and girls, while neither detectable levels nor pulses were found using immunoradiometric assays.18,19 The question arises whether the pulses are the result of more sensitive assays, the method of pulse detection or due to bias. It remains doubtful whether or not this highly sensitive assay can measure real LH pulses in Kallmann's syndrome patients. These patients have an organic GnRH de®ciency caused by an absent migration of the GnRH neurons from the olfactory placode.20
4 H. A. Delemarre-van de Waal
80
LH/FSH level
40 20 15 10 5 0 Fetus
Infancy
Childhood
Puberty
Adult
Figure 1. Pattern of plasma gonadotrophin levels during prenatal and postnatal development. LH, luteinizing hormone; FSH, follicle stimulating hormone.
With the onset of puberty, LH is secreted, ®rstly, only during the night. In boys this nocturnal LH increase is associated with a testosterone rise; in girls the rise in oestradiol occurs the following morning. With the progression of puberty the LH secretion gradually increases during both day and night. This increment is caused by the enhancement of both the LH pulse frequency and the pulse amplitude. During puberty the day±night rhythm is maintained and disappears in adulthood, presumably as a result of increasing sex steroid levels.19,21 With the progression of puberty the response to a challenge of exogenous GnRH also increases. During prepuberty, when endogenous GnRH stimulation is low, there is no, or hardly any, increase in gonadotrophins following such a challenge. From Tanner stage 2 to 5 a GnRH challenge increases LH and FSH in boys and LH in girls.22 For FSH in girls there is an exception at stage 2. This stage is characterized by high GnRH-stimulated levels of FSH only. The response is much lower in stage 3. Such a strong FSH response is also seen during infancy at the time that the central restraint is not completely developed and GnRH release is still decreasing. Incomplete suppression of this high FSH response is helpful in diagnosing premature thelarche in female infants. With the further development of the central restraint this gonadal activity and, in turn, the thelarche will disappear spontaneously. Although generally, basal gonadotrophin levels supply appropriate information about puberty, it is good to remember that a blunted response to GnRH only indicates that the pituitary is not stimulated by endogenous GnRH. This may be the result of a `normal' prepubertal state or it may be due to a state of hypogonadotrophic hypogonadism. Great eort has been put into the development of a test that can discriminate between these states, for instance by using a GnRH analog test or the multiple GnRH challenge test. However, a reliable test for discriminating between the normal prepubertal state and a GnRH de®ciency state is not available. In the case of a hypogonadotrophic hypogonadism it is possible to dierentiate between a pituitary or
Regulation of puberty 5
Feedback
High
Decreasing sensitivity
Adult
Prepuberty
Onset of puberty
Adult
GnRH
LH/FSH
Figure 2. The `gonadostat' hypothesis is based on the occurrence of a change in the sensitivity of a gonadotrophin regulating system (gonadostat) to the negative feedback of gonadal steroids. During prepuberty the sensitivity is highest, resulting in a low release of GnRH and gonadotrophins. Initiation of puberty is secondary to a decreased sensitivity inducing increasing GnRH and gonadotrophin release until a new equilibrium has been obtained in adulthood. GnRH, gonadotrophin-releasing hormone; LH, luteinizing hormone; FSH, follicle stimulating hormone. (Adapted from Grumbach et al).28.
hypothalamic defect by long-term pulsatile GnRH administration. This treatment schedule may be applied to induce pubertal development as well.23±26
HYPOTHESES ON THE MECHANISM OF THE ONSET OF PUBERTY The so-called `gonadostat' theory was ®rst suggested by Hohlweg in 1931 and was later developed further by Grumbach and coworkers.27,28 This theory is based on the concept of a changing sensitivity of the gonadotrophin regulating system (gonadostat) to the negative feedback of gonadal steroids (Figure 2). Already in 1932 Hohlweg & Junkmann29 observed that in the castrated immature rat, small doses of steroid hormones were able to stop pituitary hyperfunction, while in adult rats larger amounts were needed. In humans, the development of this negative feedback continues from fetal life throughout childhood when small doses of oestrogens are able to suppress gonadotrophin levels. According to this concept, the onset of puberty is the result of a decreasing sensitivity: thus, gonadotrophin secretion increases and, in turn, there is an increment in gonadal steroid levels. At the end of puberty the feedback of the gonadal steroids on gonadotrophin secretion reaches a new equilibrium at a higher setpoint. Findings that contradicted the `gonadostat' hypothesis came from studies in agonadal patients, who showed an identically changing pattern throughout infancy and prepuberty, although their gonads were not capable of producing gonadal steroids. Therefore, in the 1980s, an alternative theory was proposed, the `intrinsic restraint concept' (Figure 3). This concept assumes that, in addition to a sex steroid negative feedback, a central inhibitory system restrains the GnRH release and induces the prepubertal phase. At the onset of puberty the intrinsic restraint itself is inhibited or
6 H. A. Delemarre-van de Waal
Late neuronal growth
Early neuronal growth
Intrinsic restraint GABA
Stimulation Glutamate NPY Leptin TGFs
Opioids Sex steroids Negative feedback GnRH
LH FSH Sex steroids Figure 3. A conceptual model of the control of the onset of puberty. The early prepubertal GnRH quiescence is due to early neuronal growth resulting in a restraint of GnRH release. Stimulating factors overcome this restraint and initiate GnRH release again at the onset of puberty. GABA, g-aminobutyric acid; GnRH, gonadotrophin-releasing hormone; LH, luteinizing hormone; FSH, follicle stimulating hormone; NPY, neuropeptide Y; TGF, transforming growth factor.
becomes overruled by a stimulating system.30 This concept is supported by data that show that hypothalamic lesions in children may result in precocious pubertal development. During the last decades increasing evidence has been found supporting this intrinsic restraint hypothesis (see Factors involved in the onset of puberty, below). The `desynchrony' theory, a third hypothesis, is based on the ®nding that concentrations of GnRH in the hypothalamus of primates during prepuberty are similar to those seen in adulthood. According to this hypothesis the lack of GnRH stimulation of the pituitary is due to desynchronization of the `®ring' of GnRH neurons.31 In vitro studies in immortalized GnRH neurons revealed that these neurons might be coupled electrically through gap junctions, resulting in the co-ordination of the secretion between separate cells. The organization of the GnRH neuronal network synchronizes the GnRH secretion in individual cells.32
FACTORS INVOLVED IN THE ONSET OF PUBERTY The hypothalamic GnRH secretion is under inhibitory as well as stimulatory control. Central inhibition of GnRH release results in the prepubertal `sleep'. The onset of puberty is due to the removal or diminution of this central restraint. As described
Regulation of puberty 7
above, GABA is the dominant inhibitory neurotransmitter in the hypothalamus. However, in primates direct innervation of GnRH neurons by GABA neurons has not been found. Since a reciprocal innervation between GABAergic and glutamatergic neurons has been found33 it is possible that inhibition of GnRH neurons by GABA is mediated via glutamergic neurons. Several developmental changes in GABA levels and metabolism have been observed. The glutamic acid decarboxylases (GAD) GAD 65 and GAD 67 are two dierent forms of the GABA-synthetic enzyme. During development in the rat spinal cord, the GABA concentration and the number of GABAergic neurons show an increase from embryonic day 13 to the second postnatal week, which is followed by a decline in the third postnatal week.34 During the same period, GAD 65 and GAD 67 mRNAs increase exponentially from embryonic day 11 until the ®rst postnatal week and then decline into the adult range during the second postnatal week.35 Prior to the onset of puberty in female rats, GABA release in the pre-optic area decreases, while in the female rhesus monkey GABA release in the median eminence decreases concomitantly with the pubertal increase of GnRH release.36,37 Supportive evidence has been gained from patient observation. Bourguignon et al38 observed regression of pubertal signs in an 11-month old boy with severe epileptic seizures and precocious puberty under GABA agonist treatment. These ®ndings support the idea that GABA contributes to the suppression of GnRH release before the onset of puberty. In humans the opioid-peptides decrease gonadotrophin levels, which suggests that the opioidergic system plays a role in the intrinsic restraint.39 Naloxone, an opiate receptor antagonist, induces an increase of, in particular, LH secretion in normal adult men and women. During the prepubertal years, naloxone is not able to initiate any gonadotrophin release. However, in early puberty, when gonadotrophins and sex steroids are in the pubertal range, naloxone results in a gonadotrophin increase. The involvement of the opioidergic system in the negative feedback of sex steroids on gonadotrophin secretion at the hypothalamic level becomes operative during pubertal development when sex steroids increase.40 The major excitatory amino acid neurotransmitter, glutamate, is also involved in the regulation of GnRH release. The GnRH neuronal network receives axo-dendritic synaptic input from glutamatergic neurons, which are located in the hypothalamus.41 Glutamate regulates the GnRH pulsatile release via two major excitatory amino acid receptors: (1) the ionotropic receptors, which are coupled to ion channels and to which the N-methyl-D-aspartate (NMDA) receptor belongs and (2) the metabotropic receptors, which are coupled to G proteins.42 Glutamate stimulates GnRH release in sexually immature monkeys and rats. The pubertal increase of GnRH secretion has been suggested to be the result of increased glutaminase activity.43 Glutaminase is the enzyme responsible for the conversion of glutamine into glutamate, a prerequisite for the pulsatile GnRH release. In addition, in male rats, an increase in NMDA receptor activity is seen in the period preceding the onset of puberty, probably partly induced by the increased glutamate activity.44 The peripubertal changes support the concept that the neurotransmitter glutamate plays a major role in stimulating GnRH release at the onset of puberty. Neuropeptide Y (NPY) is widely distributed in the mammalian central nervous system and is involved in the regulation of several brain functions including food intake and reproductive function. It also plays a role in the central eects of leptin. A NPY infusion into the median eminence elicits GnRH release in pubertal female monkeys but not in prepubertal monkeys. In prepubertal monkeys, NPY release is low, but
8 H. A. Delemarre-van de Waal
along with the increase of GnRH release, an increase in NPY occurs. Since NPY in prepuberty does not in¯uence GnRH release, it seems that it may contribute to the pubertal process rather than playing a major role in triggering the onset of puberty.12 Growth factors may in¯uence the regulation of GnRH release as well. Astroglia cells synthesize and release growth factors such as transforming growth factor-a and -b (TGFa and TGFb). Other glial-derived substances, which may alter GnRH release are basic ®broblast growth factor, epidermal growth factor, insulin-like growth factor-1 (IGF-1), neuronal cell adhesion molecules and the cytokines interleukin-1 and -6. In the rat and in the rhesus monkey TGFa mRNA levels in astrocytes increase with normal puberty. Lesions in the hypothalamic area initiate precocious puberty, whereby the underlying mechanism may be the induced astrogliosis. Astrogliosis may be responsible for the increase in TGFa in the hypothalamus. TGFa stimulates GnRH release as well as stimulating the glia cells to produce bio-active substances, such as prostaglandin E2, which, in turn, also stimulates the release of GnRH.45 It is clear that the glia tissue is involved in the regulation of GnRH release. However it is uncertain whether it plays a critical role in the triggering of puberty. GnRH neurons contain IGF-1 receptors. IGF-1 plasma levels increase during puberty and, therefore, may contribute to the GnRH release. Whether or not this plays a role in the initiation of puberty remains unclear. IGF-1 may exert its eect on the amount of GnRH release: there are some reports describing a faster tempo of puberty in non-growth hormone de®cient children during growth hormone treatment.46 It is well known that nutritional factors in¯uence pubertal development. In chronic malnutrition a pubertal delay will occur. In the adult, the reproductive axis may return to the prepubertal state. In 1971, Frisch & Revelle47 proposed the hypothesis of a direct relationship between a critical weight and menarche. In their study, they observed that the mean weight of 48 kg at menarche did not alter as menarcheal age increased, whereas mean height had increased signi®cantly. In later studies more indications have been found that in both the female rat and the human female a particular ratio of fat to lean body mass is necessary for the start of puberty and maintenance of reproductive capacity. In 1973, Ruf wondered `how the brain can be informed of the nutritional state of the organism and how it ``knows'' when to initiate the process of puberty'.48 The peptide leptin plays a role in this by informing the brain about the peripheral energy stores. Leptin is produced and secreted by white adipose tissue and has potent eects on feeding behaviour, thermogenesis and neuroendocrine processes. In cases of leptin absence, obesity occurs as well as infertility.49 Leptin treatment will decrease food intake and restore reproductive function. Leptin exerts its eect through the NPY neurons. It decreases NPYergic signalling by acting directly at the level of NPY perikarya and possibly at the level of the NPY nerve terminals.50 In the fasting state, leptin levels decline and gonadotrophin secretion is suppressed. Such ®ndings underline the fact that nutrition, particularly when mediated by fat tissue and leptin, contributes to pubertal development. However, it is uncertain whether leptin is essential for the triggering of pubertal onset. Data in the rat have shown that leptin levels remain constant in the prepubertal and postpubertal stages as well as the fact that leptin gene expression in the hypothalamus does not show any developmental changes. These ®ndings do suggest that leptin is not an important metabolic trigger for the onset of puberty, but that leptin acts as a permissive signal for the onset of puberty.51,52 This is also in agreement with the ideas of the Frisch hypothesis that a critical weight triggers the onset of puberty.
Regulation of puberty 9
SUMMARY Puberty is the ®nal phase in a continuum of changing GnRH activities from the fetal period onwards. Before the last decade there was a trend towards earlier onset of puberty and increased ®nal height. Recently a halt in the trend of earlier pubertal onset has been described in various European countries. However, recently a very young onset of puberty was observed in American girls, especially in Black Americans. These alarming data should be con®rmed in a non-selected group of children. GnRH is already active in the fetus during early gestation. At a gestational age of 7 weeks GnRH, LH and FSH are already detectable. At midgestation LH and FSH blood levels are very high. These levels decrease, presumably as the result of a developing negative feedback to sex steroids as well as due to a developing GnRH suppression system. At birth, gonadotrophin levels are low, but show a transient increase during infancy. Incomplete suppression after birth may lead to the occurrence of the benign premature thelarche syndrome in female infants. The underlying mechanisms
Practice points . although not con®rmed in European studies, a trend towards earlier onset of puberty, as has been recently described in children in the USA, may be found during the coming years. Guidelines for work-up and treatment of disorders in the timing of puberty would then have to be reconsidered . preventative programmes for overweight and obese children may, in addition to general health care, have an eect on pubertal development . adopted children may have an earlier onset of puberty, which will have a negative impact on their ®nal height. However, the ®nal height achieved may be normal for their original population and, therefore, intervention should be reconsidered . puberty is the last developmental phase of a continuum that began during the fetal period . premature thelarche during infancy is the result of a partial suppression of the physiological postnatal transient increase of gonadotrophins . the postnatal spontaneous activity of the hypothalamic±pituitary±gonadal axis oers the clinician a golden window to evaluate this axis. After the development of central restraint all diagnostic procedures have to be delayed until the age that puberty is normally expected. Therefore work-up of sexual disorders has to be commenced during the ®rst weeks of life . the mechanisms that result in the changing pattern of gonadotrophin levels and the onset of puberty re¯ect complex anatomical interactions among GnRH neurons and other regulatory neurons. Factors modulating these neurons may play inhibitory, stimulatory, facilitatory or permissive roles . the neurotransmitters GABA and glutamate play a crucial role in the inhibition and stimulation, respectively, of GnRH release during development . modulation of GnRH release by sex steroid negative feedback is partly mediated by the opioidergic system . growth factors synthesized by glia cells may aect GnRH release. Their physiological role remains to be elucidated . leptin appears to serve as a metabolic signal to the gonadotrophin axis, more as a permissive factor than as an indispensable factor for the initiation of puberty
10 H. A. Delemarre-van de Waal
Research agenda . studies are needed to investigate claims that the age of onset of puberty in American adolescents is becoming earlier
responsible for the changing pattern of GnRH release are based on interactions between the GnRH neurons and other neurons. These neurons are subject to a continuing development in growth and function. Various neurotransimitters such as GABA, glutamate, opioids, NPY, leptin and growth factors play inhibitory, stimulatory, facilitatory or permissive roles in this development.
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