The KiNG of reproduction: Kisspeptin/ nNOS interactions shaping hypothalamic GnRH release
Introduction
ὁρμῶν (hormôn), a Greek word meaning “to set in motion, to excite, to stimulate”. Since its introduction by Ernest Starling in 1905 (Starling, 1905), this word has been used to describe the chemical messengers used as a means of communication between different organs in an animal. When the term was introduced, practically nothing was known about the nature or the action of these messengers, which were believed to be produced by only a few specialized organs of the endocrine system (i.e. the glands). We have come a long way since then, with many conceptual changes occurring over the years, the most important of which is possibly the acknowledgement that the nervous and endocrine systems work together to transmit physiological information. The discipline of “Neuroendocrinology” was launched by Geoffrey Harris with his publication in 1955, which not only provided the first proof that the endocrine system could be controlled by the central nervous system (CNS), but also laid the foundations for the notion of the hypothalamic-pituitary-gonadal axis (HPG) (Harris, 1955). In the past two decades, we have come to acknowledge that the field of neuroendocrinology extends far beyond the traditional neuron-endocrine pathways to encompass the production of hormones by non-traditional cells and tissues, with new and often non-catalytic roles in the regulation of an organism's development, physiological homeostasis, reproductive capacity and behavior.
The three components of the HPG axis – the hypothalamus, pituitary gland and gonads (i.e. the testes and ovaries) – closely interact and depend on each other to allow the complex dialogue between the CNS and the periphery that is indispensable for reproductive function. The hypothalamus is undeniably the single most important brain region integrating vegetative and endocrine signals, and controls diverse processes including cardiovascular function, sleep, metabolism, stress, thermoregulation, water and electrolyte balance, growth and reproduction. Within the hypothalamus, specialized neuronal populations sense moment-to-moment changes in circulating levels of hormones and nutrients, to regulate neuroendocrine function (Elmquist et al., 2005). Among these hypothalamic neuronal populations are the neurons producing gonadotropin-releasing hormone (GnRH), the main orchestrators of reproductive function, which act as integrators of various signals coming from both the central and the peripheral nervous system.
In spite of their crucial role, GnRH neurons are an extremely small population of cells across mammalian species, counting only 1000–3000 neurons in the rodent brain. In rodents, the GnRH neuronal soma are primarily distributed in the preoptic hypothalamic area (POA) extending their nerve terminals to the pericapillary space of the median eminence (ME), located in the more mediobasal area of the hypothalamus (MBH) (Barry J et al., 1973), releasing the GnRH decapeptide in an episodic manner in both sexes (Sarkar et al., 1976; Moenter et al., 1991; Sarkar and Minami, 1995; Terasawa et al., 1999). Indeed, both immortalized GnRH-secreting GT1 cells and primary GnRH neurons release GnRH in a pulsatile manner, at species-specific intervals (for review see Terasawa, 2019). GnRH is then carried through the pituitary portal circulation for delivery to the anterior pituitary, where it stimulates gonadotropes to synthesize and secrete the gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). These gonadotropins then act on the gonads (i.e. the testes and ovaries) to promote gonadal development and the secretion of sex steroids, which in turn provide positive or negative feedback back to the brain to regulate GnRH release (Prevot, 2015). The pulsatile pattern of GnRH secretion is reflected in the pulsatile secretion of LH during the negative feedback action of sex steroids, while GnRH surge, taking place during the positive feedback action of sex steroids, results in a crucial peak of LH release, triggering ovulation in females (Nett et al., 1974).
Thus, correct release of LH and FSH depends on the proper regulation of the frequency and timing of GnRH secretion by sex steroids as well as other neuronal and non-neuronal factors. In turn, proper development of GnRH neurons, GnRH expression and GnRH signaling are all essential for normal functioning of the mammalian HPG axis (Cattanach et al., 1977; Mason et al., 1986; Schwanzel-Fukuda et al., 1989). Justified by the authority of the GnRH system over the regulation of key physiological events, the network surrounding GnRH neurons, ensuring the controlled and timely regulation of their response is complex and multidimensional. Among these cells, kisspeptin neurons have so far been considered the master excitatory driving force behind GnRH/LH release during both positive and negative feedback phases (Navarro et al., 2009; Pielecka-Fortuna et al., 2010; Clarkson et al., 2017).
In this review, based primarily on research carried out in rodents, we challenge the view of kisspeptin neurons as the sole regulators or supreme “monarchs” of the GnRH network, controlling both GnRH pulse and surge generation. Instead, we explore the implication of the much overlooked population of neuronal nitric oxide synthase (nNOS) neurons, producing the diffusible messenger nitric oxide (NO), in the control of the GnRH system. Finally, we will discuss how the tripartite Kisspeptin, nNOS, GnRH (KiNG) network, through a mechanism consisting of the alternation of neuronal activation and the release of tonic inhibition, regulates LH pulsatility and LH surge generation and thus reproductive function.
Section snippets
Milestones during the developmental maturation of the GnRH network in the mouse
GnRH neurons originate from stem cells of the olfactory placode. Around embryonic (E) day 11.5 in mice, these neurons embark on their migratory path from the nose, entering the forebrain following a series of guidance cues (for review see Wray et al., 2010), and in close association with the vomeronasal nerve fibers. By E16, GnRH neurons enter the forebrain (Schwanzel-Fukuda et al., 1989). The HPG axis is believed to be already somewhat functional during embryonic development (Aubert et al.,
The involvement of ovarian steroid hormones in the GnRH pulse and the GnRH surge generators
Even though the secretory profiles of estrogen and progesterone are poorly characterized in mice, both gonadal steroids exert critical inhibitory and stimulatory actions upon the brain to control GnRH release, shaping the estrous cycle in all female mammals.
It is widely accepted that estrogens play a key role, exerting a negative feedback action on the GnRH system, thus maintaining the constant frequency of the episodic release of GnRH/LH throughout the follicular phase. According to the
The mechanism underlying the ability of nNOS cells to promote the synchronized activity of GnRH neurons
Since mammalian GnRH neurons are widely scattered in the POA, one could hypothesize that GnRH pulsatility and GnRH surge are dependent upon a form of oscillatory network activity rather than individual synaptic connectivity. Although this concept remains to be uncovered in vivo, this oscillatory activity may ultimately reflect the wave-like shape of both pulsatile and surge GnRH secretion. Considering that nNOS neurons can communicate with each other via NO volume transmission, NO signaling is
Concluding remarks and perspectives
Overall, the kisspeptin/nNOS neuronal network is ideally poised to generate and regulate episodic GnRH release in response to developmental and physiological cues, thus ensuring the precise sequence of events that constitutes pubertal activation and adult fertility (Choe et al., 2013; Messina et al., 2016). However, upstream of its release, GnRH must also be produced. Recent findings shed some light on the transcriptional mechanisms controlling the postnatal maturation of the GnRH system,
Acknowledgements
The authors acknowledge S. Rasika for editorial assistance with the manuscript before submission. The author's work is supported by a doctoral fellowship from the University of Lille School of Medicine, Lille, France (to V.D.), the Fondation pour la Recherche Médicale (Equipe FRM, DEQ20130326524 to V.P) and the Agence Nationale de la Recherche (ANR-17-CE16-0015 to V.P) and the European Union Horizon 2020 research and innovation program (No 847941 to K.C. and V.P.).
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