Journal of Neuroendocrinology
From Molecular to Translational Neurobiology
Journal of Neuroendocrinology 20, 719–726
ª 2008 The Author. Journal Compilation ª 2008 Blackwell Publishing Ltd
REVIEW ARTICLE
Hypothalamic Control of the Pituitary-Gonadal Axis in Higher Primates:
Key Advances over the Last Two Decades
T. M. Plant
Department of Cell Biology and Physiology, University of Pittsburgh, Magee-Womens Research Institute, Pittsburgh, PA, USA.
Journal of
Neuroendocrinology
Correspondence to:
Tony M. Plant, Department of Cell
Biology and Physiology, University of
Pittsburgh, Magee-Womens Research
Institute, 204 Craft Avenue; Rm.
B311, Pittsburgh, PA 15213, USA
(e-mail: plant1@pitt.edu).
This review provides a brief historical background to the foundation of primate reproductive
neuroendocrinology that was laid by Ernst Knobil during the late 1960s and early 1970s. This
is followed by a discussion of studies conducted over the last two decades that I view as
having contributed to the current understanding of the field of primate reproductive neuroendocrinology. The review concludes with a short summary of key questions that remain to
be addressed.
Key words: puberty, ovulation, primate, GnRH, kisspeptin.
The purpose of this review is to identify some of the studies that
have captured my interest over the last two decades, and which I
view as having contributed to our current understanding of the
neuroendocrine mechanisms that control postnatal development
and function of the gonad in higher primates1. Before embarking
on this journey, it is first necessary to briefly outline where this
rather circumscribed and relatively scantly studied area of neuroendocrinology stood in 1988: a recent starting point when it is
recalled that the neuroendocrine control of the gonad was first
appreciated, albeit dimly, at the turn of the 20th Century as a
result of the work of W. Heape (1) with the rabbit, a reflex ovulator,
and of F. H. A. Marshall (2), who stressed the correlation between
the external environment and reproduction.
Early milestones
The hypothalamus became the focus of the ‘neuro’ component of
neuroendocrine control systems governing gonadal function as a
result of lesion and stimulation studies conducted by Smith, by
Marshall and Verney, and by Fevold, Hisaw and Greep in the late
1920s and 1930s (3–5). This was followed during the 1940s by the
development of the tenet championed by Geoffrey Harris that the
hypothalamic control of the anterior pituitary gland, including those
cells regulating gonadal function, was achieved by neurohormones
released into the primary plexus of the hypophysial portal circulation (6). Final proof of Harris’ neurovascular hypothesis was
obtained in 1970 when the laboratories of Guillemin and of Schally
1
Higher primates are comprised of three families: Old World monkeys, apes
and humans.
doi: 10.1111/j.1365-2826.2008.01708.x
published the isolation of the hypothalamic-releasing factor, thyroid-stimulating hormone-releasing factor or thyrotrophin-releasing
hormone. One year later, the isolation and structure of luteinising
hormone releasing factor (LRF) or follicle-stimulating hormone
(FSH)- and luteinising hormone-releasing hormone (LHRH) was
announced by the same groups (7, 8)2. Although Harris died in
1971, he has come to be recognised as the father of neuroendocrinology, while Guilleman and Schally, who outlived Harris (both are
alive at the time of writing), were awarded the 1977 Nobel Prize in
Physiology or Medicine for their work.
Primate reproductive neuroendocrinology in the late
1980s
The foundation for this area of neuroendocrinology was based largely on an elegant and systematic series of experiments conducted
by Ernst Knobil at the University of Pittsburgh during the 1960s
and early 1970s that provided a rational model for the control of
the menstrual cycle. Borrowing from control theory, he proposed
that the pattern of gonadotrophin secretion throughout the ovarian
cycle of the rhesus monkey could be accounted for by the negative- and positive-feedback actions of ovarian oestradiol secretion
on LH and FSH release (9). In a characteristic linear approach, he
then addressed the sites of the negative and positive feedback
actions of oestradiol on gonadotrophin secretion in the female
monkey. Using animals in which the spontaneous hypophysiotrophic
drive to the gonadotrophs was first abolished with hypothalamic
2
The decapeptide, gonadotropin-releasing hormone (GnRH), which is synonymous with LRF and LHRH, is used throughout this review to describe mammalian GnRH (GnRH-1).
T. M. Plant
3
Strictly speaking, ‘on-diminished-on’.
240
LH (ng/ml)
lesions and subsequently restored with a pulsatile infusion of synthetic GnRH, he demonstrated that invariant intermittent GnRH
stimulation of the pituitary was sufficient to drive ovulatory menstrual cycles with normal follicular and luteal phases (10). Knobil
concluded that a pituitary site of action could account for both the
negative- and positive-feedback actions of oestradiol on gonadotrophin secretion (10). The same experimental model led to the discovery that an intermittent hypothalamic GnRH signal to the pituitary
was an obligatory component of the neuroendocrine control system
that governs normal gonadotrophin secretion (11). Although the
concept of a hypothalamic GnRH pulse generator remains a corner
stone of reproductive neuroendocrinology, Knobil’s conclusions
regarding the pituitary as a major site of oestradiol action in the
control of gonadotrophin secretion throughout the menstrual cycle,
including the periovulatory period, are now often ignored. I believe
this stems from the fact that the molecular and cellular bases of
oestradiol action to regulate gonadotrophin secretion are being
studied primarily in rats and mice; species in which hypothalamic
sites of oestradiol action are critical for the unfolding of the
4 ⁄ 5-day oestrous cycle. It is ironic to note that Knobil’s interest in
the control of the menstrual cycle of the rhesus monkey was born
in the lecture hall where he found that, using data obtained
primarily from studies of the laboratory rat, it was impossible to
explain the regulation of ovulation in women to medical students.
By the late 1980s, it was also well established that in primates, as
in nonprimate species, the onset of gametogenesis at the time of
puberty resulted from an increase in gonadotrophin secretion triggered, in turn, by an increase in GnRH pulse generator activity at this
critical stage of development (12, 13). It was also known at this time
that the postnatal ontogeny of GnRH pulsatility in primates is fundamentally different from that in rodents. This is because the increase
in GnRH pulsatility that triggers puberty at the end of the protracted
juvenile phase of primate development reflects a resurgence in hypothalamic GnRH pulse generator activity that has been held in check
since late infancy. This is in striking contrast to rodents where the
postnatal development of the pubertal GnRH signal unfolds progressively and without interruption (14). The characteristic ‘on–off–on’
pattern3 of GnRH pulse generator activity during postnatal development in higher primates was convincingly revealed in 1975 by Grumbach and his colleagues (15), who described, in agonadal human
subjects, a characteristic ‘diphasic’ pattern of gonadotrophin secretion from birth to adulthood with elevated levels during infancy and
puberty separated by a hiatus during the childhood and juvenile
years. This feature of primate development is exemplified by the
agonadal rhesus monkey in which gonadotrophin secretion is amplified because of the absence of testicular or ovarian feedback signals
(Fig. 1). The qualitative preservation of the diphasic pattern of gonadotrophin secretion in agonadal monkeys and human subjects also led
to the rejection of the ‘gonadostat’ hypothesis to account for the
onset of puberty in higher primates (16). The latter idea was replaced
over time by the notion of a gonad independent central inhibition or
restraint to account for the prepubertal reduction in pulsatile GnRH
release (17–20), and such a neurobiological brake now represents a
160
80
0
320
FSH (ng/ml)
720
240
160
80
0
0
20
40
60
80
100
Age (weeks)
120
140
160
Fig. 1. The on–off–on pattern of gonadotrophin-releasing hormone pulse
generator activity during postnatal development in agonadal male (stippled
area) and female (closed data points error bars) rhesus monkeys as
reflected by circulating mean luteinising hormone (LH) (top panel) and follicle-stimulating hormone (FSH) (bottom panel) concentrations from birth to
142–166 weeks of age. Note, in females, the intensity and duration of the
prepubertal hiatus in the secretion of FSH, and LH to a lesser extent, is truncated in comparison to males. Adapted with permission (16) from TM. Plant.
Puberty in primates. In: E. Knobil, JB. Neill, eds. The Physiology of Reproduction, 2nd edn. New York, NY: Raven Press Ltd., 1994; Vol. 2, Chapter
42,453–485. Copyright Elsevier, 1994.
major component of contemporary models of puberty onset in primates. It was also recognised prior to the 1980s that the pubertal
increase in GnRH could result from either the removal of an inhibitory input or the application of a stimulatory signal to the GnRH
neuronal network of the juvenile hypothalamus, or a combination of
the two (21) (i.e. the notion of a brake is conceptual). In 1988, however, the neural components of the brake that is imposed on GnRH
pulsatility during juvenile development (and childhood in man) were
unknown. Similarly, the control system that dictates when the brake
is applied and when it is released was unknown, as it remains today.
In this regard, the notion of a brain clock to time the initiation of
puberty, or a hypothalamic somatometer to track growth and thereby
coordinate the onset of puberty with a sufficient level of somatic
development, has been in the literature for several decades (22, 23).
Primate reproductive neuroendocrinology: the last two
decades
Puberty
GnRH neurones are not limiting to the onset of puberty
As demonstrated by my laboratory in the late 1980s (24), the network of GnRH neurones in the hypothalamus of the juvenile male
ª 2008 The Author. Journal Compilation ª 2008 Blackwell Publishing Ltd, Journal of Neuroendocrinology, 20, 719–726
Hypothalamic control of the primate gonad
are important components of the neurobiological brake that holds
or releases the check on GnRH pulsatility from infancy until puberty. A major role for GABA in inducing a state of relative dormancy in the network of GnRH neurones of the prepubertal female
was firmly established during the 1990s by an elegant series of
studies by Terasawa and her colleagues at the Wisconsin National
Primate Research Centre employing the rhesus monkey. Using the
demanding technique of hypothalamic perfusion, the Wisconsin
group found that release of GABA into the median eminence
declined as pulsatile GnRH release was increasing at the onset of
puberty (25). On the other hand, acute administration to the median eminence of the GABAA receptor antagonist, bicuculline, or the
antisense oligodeoxynucleotide for the mRNA coding the GABA
synthesising enzyme, glutamate acid decarboxylase 67 (GAD67), elicited a discharge of GnRH in the juvenile animal (25, 26). Moreover,
initiation of chronic repetitive administration of bicuculline into the
base of the third cerebroventricle at approximately 15 months
of age in premenarcheal monkeys led to precocious puberty,
as indicated by a dramatically premature menarche and first
ovulation (27).
Because of the sex difference in the intensity of the prepubertal
brake on GnRH pulsatility, analogous studies of the role of GABA in
the male primate would be of considerable interest, but these have
not been conducted; a state of affairs probably related to the high
cost of experimentation with nonhuman primates. In the male, on
the other hand, exploration of inhibitory inputs to the GnRH neuronal network during juvenile development has focused on NPY and
compelling indirect evidence is available to support the view that
NPY is an important inhibitory component of this neuroendocrine
control system (28).
monkey may be reawakened at any time and, with surprising
ease, by intermittent chemical stimulation with N-methyl-D-aspartate, a glutamate receptor agonist (Fig. 2). This finding indicates
that, in primates, the network of GnRH neurones, which in adulthood provides the drive to the gonadotrophs, must be viewed
together with the pituitary and gonads as a nonlimiting component of the control system that governs the onset of puberty in
these species. The subsequent finding that levels of GnRH and of
GnRH mRNA in the hypothalamus of the juvenile monkey are
similar to those in the pubertal ⁄ adult brain (20) is consistent with
the notion of a dormant GnRH pulse generator, and suggests that
the molecular substrate for the biosynthesis and secretion of
GnRH is maintained in a state of suspended animation throughout the juvenile phase of development when release of the decapeptide into the hypophysial portal circulation is restrained. It
should be noted here that the intensity of the prepubertal brake
that is imposed on GnRH release between infancy and puberty
appears to be sexually differentiated. In the female, the brake is
less intense and is imposed for a shorter duration; a sex difference that is reflected in the postnatal time course of gonadotrophin secretion in the agonadal situation (Fig. 1). Whether this
difference, which is presumably the result of testicular programming of the fetal hypothalamus, is solely quantitative is unknown.
This, however, would seem to be the most parsimonious situation.
Components of the neurobiologic brake on prepubertal
GnRH pulsatility
During the last 20 years, we have learnt that glutamate (see above),
c–amino butyric acid (GABA), neuropeptide Y (NPY) and kisspeptin
Weeks of NMDA treatment
0
1
2
3
721
5
Post-NMDA
7
11
15
10
6
4
2
0
50
30
10
LH (ng/ml)
Testosterone (ng/ml)
8
09 13 09 13 09 13 09 13 09 13 09 13 09 13 09 13 09 13
Time of day (h)
Fig. 2. Chronic intermittent stimulation of the gonadotrophin-releasing hormone (GnRH) neuronal network in the hypothalamus of juvenile male monkeys
(15–16 months of age) with brief i.v. infusions of N-methyl-D-aspartate (NMDA) once every 3 h for 15 weeks results in the immediate initiation of pubertal
activity of the hypothalamic-pituitary-testicular axis. After 11 weeks of stimulation, the pulsatile pattern of hormone activity in the pituitary-Leydig cell axis
was indistinguishable from that observed spontaneously in adults, and motile sperm were recovered in the epididymides of two animals at 16 and 22 weeks
after initiation of NMDA treatment. The latency to full activation of the pituitary-testicular axis is due to a progressive upregulation of pituitary gonadotrophin
synthesis, and the action of NMDA on the axis was abolished by concomitant treatment with a GnRH receptor antagonist (not shown). LH, luteinising hormone. Reprinted with permission (24) from TM. Plant et al. Puberty in monkeys is triggered by chemical stimulation of the hypothalamus. Proc Nalt Acad Sci
USA 1989;86:2506–2510. Copyright PNAS, 1989.
ª 2008 The Author. Journal Compilation ª 2008 Blackwell Publishing Ltd, Journal of Neuroendocrinology, 20, 719–726
T. M. Plant
LH (IU/l)
(A) 20
10
0
0
2
4
25 ng/kg
(B )
6
8
10 12 14 16 18 20 22 24
Time (h)
75 ng/kg
7.5 ng/kg
250 ng/kg
60
LH (IU/l)
During the last 5 years, kisspeptin has emerged as a totally
unexpected and novel component of the neurobiological machinery
underlying the on–off–on pattern of GnRH release throughout
postnatal development. Kisspeptins are encoded by the KiSS-1 gene
and signal at the G-protein coupled receptor, GPR54 (29). Significantly, because of the primate focus of this review, the initial evidence for the importance of this signalling pathway in regulating
the hypothalamic-pituitary-gonadal axis in any species was
obtained by contemporary genetic studies of human pathophysiology and the findings that were reported were incontrovertible. Two
groups, one led by de Roux in Paris (30) and the other by Crowley
in Boston (31), each almost simultaneously reported that several
members of a large consanguineous family presenting with
hypogonadotrophic hypogonadism and absent puberty carried
homozygous mutations for GPR54. In one of these studies (31), it
was further reported that, in an additional subject bearing a compound heterozygote mutation of GPR54, pituitary responsiveness to
pulsatile GnRH administration was not compromised (interestingly,
it was actually exaggerated), indicating a hypothalamic locus for
the deficit associated with this genetic disorder (Fig. 3). The impact
of these studies on the field of reproductive neuroendocrinology in
general has been, and continues to be, enormous, and this is
reflected by the extent to which the initial human findings have,
and are being, reversely translated to nonprimate species serving as
paradigms for investigating a variety of modulators of the hypothalamic-pituitary-gonadal axis, such as development, season, nutrition, metabolism and stress. In the field of reproductive
neuroendocrinology, it is difficult to identify a precedent for such a
scenario.
In collaboration with the Boston group and the Ojeda laboratory
at the Oregon National Primate Research Centre, my laboratory
pursued the notion that kisspeptin signalling at GPR54 is a major
component of the trigger for the pubertal resurgence of pulsatile
GnRH release in primates. It was reasoned that, if this is indeed
the case, then the following premises should apply. First, KiSS-1
and the gene coding for GPR54 should be expressed in the primate
hypothalamus. Second, an increase in kisspeptin signalling should
occur in association with the pubertal resurgence of pulsatile GnRH
release and, third, an increase in hypothalamic kisspeptin levels in
the juvenile monkey should lead to a precocious activation of a
pubertal mode of pulsatile GnRH release. Using in situ hybridisation, KiSS-1 expressing neurones were found to be restricted to the
region of the arcuate nucleus in the medial basal hypothalamus
(MBH), whereas GPR54 mRNA was more diffusely distributed in the
MBH (32). More recently, we have shown that the location of kisspeptin perikarya, as revealed by immunohistochemistry, is restricted
to the arcuate nucleus, and therefore consistent with the expression of KiSS-1. Parenthetically, the absence of kisspeptin perikarya
in the more rostral regions of the monkey hypothalamus, such as
the anteroventral periventricular nucleus, has also been reported in
the human (33), and contrasts with the situation in rodents: a
comparative difference that may be related to differences in gonadal feedback control systems governing the preovulatory gonadotrophin surge in primates and rodents (see above). Reverse
transcription-polymerase chain reaction revealed that KiSS-1
40
20
0
0
(C)
2
4
Time (h)
6
8
IHH patient with GPR54 mutations: R331X, X399R
Regression line
Regression line, mean LH Amp for 6 IHH men,
no GPR54 coding sequence abnormalities
95% confidence limits
60
LH amplitude
722
40
20
0
0
1
2
3
LOG (10) GnRH
Fig. 3. Spontaneous and induced luteinising hormone (LH) secretion in a
male patient carrying a GPR54 mutation. (A) Solid circles show spontaneous
moment to moment changes in circulating LH concentrations at baseline
over a 24-h period. The range for normal men is shown by the crosshatched area. Arrow heads show LH pulses. (B) LH responses to brief i.v.
boluses of synthetic gonadotrophin-releasing hormone (GnRH) (7.5–
250 ng ⁄ ml) administered after the patient had received intermittent s.c.
GnRH treatment for several months to heighten pituitary responsiveness to
the decapeptide. (C) Dose–response curve describing LH pulse amplitude
versus GnRH dose (log10 ng ⁄ kg) for the patient compared to the
same regression for subjects without GPR54 mutations. Reprinted with
permission (31) from SB. Seminara et al. The GPR54 gene as a regulator of
puberty. N Engl J Med 2003;349:1614–1627. Copyright Massachusetts Medical Society, 2003.
expression was increased in association with the pubertal resurgence in pulsatile GnRH release in agonadal males and intact
females, whereas an increase in GPR54 mRNA levels was only
observed in the females. A precocious sustained release of pulsatile
GnRH from juvenile monkeys, however, could only be elicited by
intermittent stimulation with kisspeptin (34) (Fig. 4). Continuous i.v.
infusions of the same peptide failed to initiate a sustained release
of LH, and downregulation of GPR54 became apparent at high
doses of uninterrupted administration (35).
Taking the foregoing considerations together with the finding
that GnRH neurones express GPR54 (29), this leads to the conclu-
ª 2008 The Author. Journal Compilation ª 2008 Blackwell Publishing Ltd, Journal of Neuroendocrinology, 20, 719–726
Hypothalamic control of the primate gonad
releasing the check on GnRH pulsatility during juvenile development. Thus, the role of kisspeptin in triggering the pubertal resurgence in pulsatile GnRH release must be integrated with that of
other neural signals, and probably additional signals of glial origin
in order to provide a rational model for the neurobiological bases
of puberty onset: this is a major challenge for the future and
requires much more than just pasting neurones ⁄ glia of different
phenotypes onto a wiring diagram of positive and negative inputs
to GnRH neurones.
8
LH (ng/ml)
6
4
Leptin: an obligatory, albeit permissive, factor for the
pubertal resurgence of pulsatile GnRH release
2
0
723
0900
1200
Day 1
1200
Day 2
Time (h)
0900
1200
Day 3
Fig. 4. Luteinising hormone (LH) discharges induced precociously in agonadal juvenile male monkeys (20–24 months of age) by an intermittent i.v.
infusion of gonadotrophin-releasing hormone (GnRH) that mimics a castrate
adult hypophysiotrophic drive to the gonadotrophs (open arrows on day 1)
and by brief infusions of kisspeptin every hour (closed arrows) initiated on
day 1 immediately following termination of GnRH priming and maintained
for 48 h in a crossover design (kisspeptin-closed data points; vehicle-open
data points). LH responses are shown for the first three kisspeptin or vehicle
pulses on day 1, three such pulses on day 2 and the last two pulses on day
3. GnRH priming (open arrows) was reinitiated after completion of kisspeptin or vehicle administration on day 3. The LH response to pulsatile kisspeptin administration was abolished by concomitant treatment with a GnRH
receptor antagonist (not shown) indicating a hypothalamic site of action of
kisspeptin. Reprinted with permission (34) from TM. Plant et al. Repetitive
activation of hypothalamic G protein - coupled receptor 54 with intravenous
pulses of kisspeptin in the juvenile monkey (Macaca mulatta) elicits a sustained train of gonadotropin - releasing hormone discharges. Endocrinology
2006:147:1007–1013. Copyright 2006, The Endocrine Society.
sion that kisspeptin signalling at GPR54 is a critically important
component of the neurobiological substrate that triggers the pubertal resurgence of GnRH release. The question remains, however, as
to the precise role of this neuropeptide. Do kisspeptin neurones in
the arcuate nucleus constitute a pubertal clock, or do they mediate
a signal from a pubertal clock located elsewhere in the brain? Do
kisspeptin neurones constitute a hypothalamic somatometer that
senses a circulating signal reflecting somatic development? At the
cellular level, the finding that robust GnRH pulsatility was induced
precociously only when the kisspeptin stimulus is applied intermittently (Fig. 4) must be reconciled with the intriguing clinical observations of pulsatile LH release with low amplitude and
approximately normal frequencies in patients with inactivating
mutations of GPR54 (Fig. 3) (36). The latter findings suggest that
kisspeptin may not be responsible for GnRH pulse generation, but
rather functions to amplify the activity of the GnRH pulse generator. Moreover, as discussed above, there is compelling evidence for
the involvement of other neurotransmitters (GABA and glutamate)
and neuropeptides (NPY) in the neurobiological brake holding or
Based on unchanging plasma leptin levels throughout peripubertal
development in the male monkey, my laboratory proposed, in
1997, that the role of this adipocyte hormone in primate puberty
was permissive (37). The evidence that engrained this notion into
our thinking, however, was provided by studies of a classical
endocrine paradigm in man. In 1999, Farooqi and colleagues (38)
reported that leptin replacement for 12 months to a 10-year-old
prepubertal child with advanced bone age and severe obesity and
congenital leptin deficiency due to homozygosity for a frameshift
mutation in the LEP gene was associated with the onset of puberty. Over the last decade, this finding has been confirmed in an
additional two children of pubertal age whereas, significantly, in
younger children (n = 4), there has been no evidence of premature puberty following leptin replacement (38, 39; I. S. Farooqi,
unpublished observations). These key clinical observations emphasising the obligatory, albeit permissive, action of leptin in the onset
of puberty also provide a precedent for the view that a circulating
hormone reflecting somatic status (fat mass in this case) can
serve as a profound modulator of the hypothalamic GnRH pulse
generator.
Structural remodelling of the GnRH neuronal network
and its afferent inputs
That neuroendocrine systems exhibit morphological plasticity was
first established for vasopressin and oxytocin neurones, where glial
ensheathment and synaptic input may be modified when magnocellular nuclei are activated by either an osmotic challenge or by
application of a suckling stimulus (40). Building on these fascinating findings, my laboratory demonstrated that the postnatal mediobasal hypothalamus of the monkey continues to express the
embryonic form of neural cell adhesion molecule (NCAM), polysialic acid-NCAM (41), a previously established marker of neuronal
plasticity, and subsequently demonstrated at the ultrastructural
level that synaptic innervation of GnRH perikarya is reduced in
animals in which the resurgence in pulsatile GnRH release has
occurred compared with juvenile monkeys in which pulsatile GnRH
release has not yet escaped the prepubertal check (42). Recent
application of contemporary imaging techniques to the rodent
hypothalamus indicate that synaptic plasticity of the GnRH neurone across development may be most extensive at the dendritic
level (43).
ª 2008 The Author. Journal Compilation ª 2008 Blackwell Publishing Ltd, Journal of Neuroendocrinology, 20, 719–726
724
T. M. Plant
Adulthood
The preovulatory GnRH surge
As discussed above, studies by Knobil three decades ago had
demonstrated that oestradiol induced pre-ovulatory gonadotrophin
surges could be elicited in female monkeys in the absence of a
discharge of GnRH, leading to the proposal that the role of the
hypothalamus in the control of the menstrual cycle was permissive
(10). Knobil’s model was not accepted with universal enthusiasm
because it was fundamentally different from those proposed to
account for the ovarian cycle of the rodent, which included an
obligatory discharge of hypothalamic GnRH to trigger the preovulatory LH surge. The polarising nature of the Knobil model led
many to overlook the experimental data, which, of course, do not
exclude the possibility of an additional action of oestradiol at the
level of the hypothalamus. Thanks to the perseverance of Spies
and his colleagues (44), who demonstrated, in the rhesus monkey,
an unambiguous increase in GnRH release in the median eminence
concomitant with the preovulatory LH surge, such an additional
hypothalamic action of oestradiol in the monkey is now established. Interestingly, the situation in the human female remains a
topic of debate. Although ovulatory menstrual cycles may be
induced in women with deficiencies in hypothalamic GnRH with
invariant pulsatile GnRH treatment (45, 46), studies by Hall using a
GnRH antagonist to indirectly quantitate endogenous GnRH secretion in normal women suggest that GnRH release during the preovulatory gonadotrophin surge may actually be decreased (47). If
this is indeed the case, it would have to be argued, nevertheless,
that it is the result of a hypothalamic action of oestradiol or a
related ovarian signal. We are therefore left to determine the relative importance of hypothalamic versus pituitary sites of the positive-feedback action of oestradiol between species, and the extent,
if any, to which the neurobiology of the circadian coupled hypothalamic signal for ovulation in the rodent may be translated to
the human female.
Interplay of nutrition and other stressors in modulating
hypothalamic GnRH pulsatility
That the hypothalamic network of GnRH neurones is the primary
locus of the negative impact of undernutrition on the reproductive
axis was convincingly demonstrated in 1986 by the finding that a
chronic intermittent GnRH infusion could restore a normal pattern of
gonadotrophin secretion in castrate male monkeys maintained on a
calorie deficient diet (48). This study provided the impetus for Cameron, a co-author of the 1986 paper, to subsequently initiate a systematic series of studies aimed at elucidating the nutritional control of
gonadotrophin secretion in the monkey (49). During the course of
these studies, Berga was beginning to explore the notion that, in
many cases, stress induced amenorrhea in women was the result of
exposure to a combination of psychogenic and metabolic stresses,
the latter arising from both nutritional factors and exercise. At this
time, these investigators were both at the Center for Research in
Reproductive Physiology at the University of Pittsburgh, and a successful collaboration was initiated to systematically explore the
impact of combinations of stressors on GnRH pulsatility and menstrual cyclicity using the nonhuman primate as an experimental
model. Recently, these investigators have provided compelling evidence that, in the monkey, the addition of a mild psychosocial stress
(moving from one cage room to another) to an exercise schedule
coupled with caloric restriction results in an interruption of normal
menstrual cyclicity (50). A better understanding of interactions
between stressors to impair GnRH pulsatility may have direct relevance to the treatment of life style amenorrhea in women.
Conclusions
In writing this review, I was struck by the fact that, despite Herculean efforts by several laboratories over the last 20 years, what
might be called fundamental advances or breakthroughs in our
understanding of the neuroendocrine physiology of the primate
hypothalamic-pituitary-gonadal axis are few and far between. Certainly, the discovery that kisspeptin-GPR54 signalling is a dominant
pathway regulating GnRH release across all mammalian species is
such a breakthrough. At the same time, the control of the timing
of human puberty remains a mystery, the precise neuroendocrine
mechanisms underlying the preovulatory gonadotrophin surge in
women is far from being clarified, and the cell biology responsible
for GnRH pulse generation continues to be debated.
Acknowledgements
I would like to thank my colleagues, Drs Melvin Grumbach, Ei Terasawa,
Harold Spies, William Crowley and Stephanie Seminara for their thoughts
and help while I was writing this review. I am also grateful to Dr Sadaf
Farooqi for sharing with me unpublished data on her leptin deficient
patients. Work from my laboratory during the last 30 years has been generously supported by the National Institutes of Health (Grants HD 13254 and
08160).
Received: 29 February 2008,
accepted 10 March 2008
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