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The Hypothalamo-Pituitary–Gonadal Axis

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The Hypothalamo-Pituitary–Gonadal Axis T M PLANT The hypothalamo-pituitary– 226:2 T41–T54 Thematic Review gonadal axis 60 YEARS OF NEUROENDOCRINOLOGY The hypothalamo-pituitary–gonadal axis Correspondence Tony M Plant should be addressed to T M Plant Department of Obstetrics, Gynecology and Reproductive Sciences, Magee-Womens Research Institute, University of Email Pittsburgh School of Medicine, 204 Craft Avenue, Room B311, Pittsburgh, Pennsylvania 15213, USA [email protected] Abstract This review provides an outline of how our understanding of the neuroendocrine control Key Words of the hypothalamo-pituitary–gonadal axis has evolved since the publication of Geoffrey " gonadotrophin releasing Harris’ renowned monograph in 1955. Particular attention is directed to the neurobiology hormone underlying pulsatile GnRH release from the hypothalamus, the neuroendocrine control of " reproduction ovarian cycles, puberty and seasonality of gonadal function, and to ideas that have " hypothalamus emerged as a result of examining the relationship between growth and the reproductive " neuroendocrinology axis. The review closes with i) a brief discussion of how knowledge gained as a result " ovulation of pursing the early hypotheses of Harris has led to major clinical and therapeutic applications, and ii) a personal glimpse into the future of research in this fascinating area of biology. Journal of Endocrinology Journal of Endocrinology (2015) 226, T41–T54 Introduction In the fourth chapter of Geoffrey Harris’ widely acclaimed in this regard. After elegantly interpreting and sum- monograph (Harris 1955), which now 60 years later marizing the extant data, Harris proposed that of the provides the corner stone of this special issue of the hypotheses that were being debated at the time, ‘the most journal, he succinctly and systematically presented his likely seems to be that nerve fibres from the hypothalmus views, and the evidence upon which they were based, on liberate some humoral substance(s) into the capillaries of the neural mechanisms controlling the pituitary–gonadal the primary plexus in the median eminence and that this axis. That gonadal function was under control by the CNS substance is carried by the portal vessels to excite or was well established at the time of Harris’ monograph, as inhibit the cells of the pars distalis’. It is to be recalled, that was the recognition of the gonad-stimulating properties this idea had been proposed by Harris and Green on of pituitary gonadotropin, the relative insignificance of several occasions prior to publication of the monograph gonadal nerves to gonadal function and the concept of (Harris 1955). It is also worth noting that the evidence neurosecretion. The problem for Harris and his fellow upon which Harris’ hypothesis was based had been neuroendocrinologists was how did the hypothalamus obtained primarily from studies of the female, most likely regulate the secretion of the anterior pituitary hormones, because ovulation was a discrete and readily detected specifically gonadotropin in the context of this review, event and, at the time, the only reliable surrogate marker and what the role was of the hypopophysial portal system of acute hypothalamic activation. http://joe.endocrinology-journals.org Ñ 2015 Society for Endocrinology This paper is part of a thematic review section on 60 years of neuroendocrinology. DOI: 10.1530/JOE-15-0113 Printed in Great Britain The Guest Editors for this section were Ashley Grossman and Clive Coen. Published by Bioscientifica Ltd. Downloaded from Bioscientifica.com at 09/25/2021 03:43:35AM via free access Thematic Review T M PLANT The hypothalamo-pituitary– 226:2 T42 gonadal axis The main purpose of this Thematic Review is to pituitary, introduction of transgenic models, advances describe the essential refinements and additional com- in molecular genetics, and the arrival of the ‘omics’ era. plexities that have been added to the fundamental model After discussing refinements to the Harris model resulting Harris put forward in 1955 for the neuroendocrine control from the application of the foregoing technologies, this of gonadal function (Fig. 1). The major additions to the review will close with a brief outline of ways in which neuroendocrinologist’s armamentarium that have knowledge of the operation of this neuroendocrine axis facilitated the development of the ideas of Harris are, has been applied to clinical practice, and a personal according to an historical timeline, the introduction of glimpse into the future of this field. RIA to measure concentrations of pituitary and gonadal hormones in the peripheral circulation and gonadotropin- Harris’ ‘humoral’ substance (GnRH) and its releasing hormone (GnRH) in portal blood, development mode of release of immunohistochemical techniques to localize neuro- peptides and neurotransmitters in the hypothalamus, After many years of herculean effort, a struggle between application of techniques in molecular endocrinology to two rival laboratories and many setbacks (recounted by the study of gene expression in the hypothalamus and Nicholas Wade in The Nobel Duel (Anchor Press/Double- Day, 1981) and in a series of three articles published in External Science in April/May, 1978), the isolation of the humoral environment substance of hypothalamic origin that is secreted into the hypophysial portal circulation to regulate the synthesis and secretion of the gonadotopins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), was inde- pendently achieved from bovine and ovine brain by the groups of Schally and Guilleman respectively. Ironically Hypothalamus this work was published in 1971, the year of Harris’ death (Amoss et al. 1971, Matsuo et al. 1971). It was a decapeptide Hypophysial portal vessels that was initially termed LH-releasing hormone or LH-releasing factor (LRF), but the molecule is now generally ? Anterior referred to as GnRH1 (in this review GnRH1 (mammalian Journal of Endocrinology pituitary Gonadotropic GnRH) will be referred to as GnRH). For their labors, Schally “Feed- hormone and Guilleman were awarded the Nobel Prize in Physiology back” or Medicine in 1977. Had it not been for Harris’ untimely death at an age of 58, he surely would have shared this most Gonad prestigious award for laying the conceptual underpinnings of Schally and Guilleman’s work. In 1969, 2 years before Schally and Guilleman Gonadal steroid reported the isolation of GnRH, Knobil’s laboratory hormone studying the ovariectomized rhesus monkey had found that LH was secreted in a pulsatile or episodic manner with a frequency in the agonadal condition of approximately Accessory one pulse per hour. They proposed that this pulsatile reproductive mode of LH secretion may be due to intermittent signals organs from the CNS that are relayed to the pituitary by an LRF (Dierschke et al. 1970, Knobil 1992). Knobil’s laboratory went on to demonstrate in 1978, that intermittent GnRH stimulation of the pituitary was essential for sustained secretion of both LH and FSH (Belchetz et al. 1978, Fig. 2). Figure 1 Interestingly, it was not until 1982, after Clarke & Model proposed by Harris in his 1955 monograph to illustrate the relationship Cummins (1982) had developed a technique to sample between the external environment and the reproductive organs. Reproduced from Harris GW (1955) Neural Control of the Pituitary Gland.London,UK: portal blood in the unsedated ewe, that the episodic Edward Arnold. nature of hypothalamic GnRH release was empirically http://joe.endocrinology-journals.org Ñ 2015 Society for Endocrinology Published by Bioscientifica Ltd DOI: 10.1530/JOE-15-0113 Printed in Great Britain Downloaded from Bioscientifica.com at 09/25/2021 03:43:35AM via free access Thematic Review T M PLANT The hypothalamo-pituitary– 226:2 T43 gonadal axis 999 the day of proestrus, was responsible for eliciting the 20 Pulsatile Continuous Pulsatile 200 pre-ovulatory gonadotropin surge from the pituitary. The prediction that this neural signal was relayed to the 15 150 pituitary by Harris’ humoral substance was confirmed when Fink et al. demonstrated a large surge in the /ml) 10 100 g concentration of GnRH in portal blood during the critical LH (ng/ml) period of the proestrus rat (Sarkar et al. 1976). The neural FSH (n 5 50 mechanism responsible for the preovulatory surge of GnRH that occurs over a period of several hours is now 0 0 conceptualized as the GnRH surge generator. The precise 10 5 0 5 10 15 20 25 30 35 Days relationship between the GnRH neurons mediating surge and pulsatile release of the decapeptide, however, remains to be determined. Figure 2 Demonstration of the need of intermittent GnRH stimulation of the pituitary for sustained secretion of LH (closed data points) and FSH (open data points). A monkey rendered hypogonadotropic by a hypothalamic Other ‘humoral’ substances regulating lesion was treated initially with an intermittent i.v. infusion of GnRH (1 mg/min for 6 min every h). On day 0 the pulsatile regimen was terminated gonadotroph function and replaced with a continuous GnRH infusion (1 mg/min). On day 20 the pulsatile mode of GnRH stimulation was re-instituted. Reproduced, with The notion that regulation of anterior pituitary hormone permission, from Belchetz PE, Plant TM, Nakai Y, Keogh EJ & Knobil E (1978) secretion by humoral substances from the hypothalamus Hypophysial responses to continuous and intermittent delivery of might be achieved by inhibiting the cells of the pars hypopthalamic gonadotropin-releasing hormone. Science 202 631–633. Copyright 1978, American
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