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Function of gonadotropin releasing hormone and inhibin (2022)
Marja Brolinson, Ariel Dunn, Joshua Morris, and Micah Hill
Gonadotropin-releasing hormone
Neuroanatomy
The hypothalamus is located at the base of the brain and forms the floor of the third ventricle. Neuroendocrine cells, which share characteristics of both neurons and endocrine glands, are located in the hypothalamus and respond to signals including neurotransmitters and subsequently produce and release hormones into the bloodstream or neural synapse
Gonadotropin-releasing hormone (GnRH) producing neuroendocrine cells arise from the olfactory placode of the developing brain. These cells migrate during embryogenesis through the olfactory bulb and forebrain to their final location in the arcuate nucleus of the hypothalamus, projecting toward the median eminence [1,2]. The number of GnRH-releasing neurons in humans is between 1000 and 1500 [3]. The median eminence is surrounded by a robust capillary network known as the hypophyseal portal system which links the hypothalamus to the anterior pituitary. Impaired migration of the GnRH neurons is characterized by a physiological impact both on the olfactory area, represented by anosmia, and impaired GnRH secretion seen in Kallmann syndrome (Fig. 8.1)
GnRH structure
GnRH is a neuroendocrine-releasing hormone responsible for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. GnRH is initially synthesized and secreted by the GnRH neurons within the hypothalamus into the superior hypophyseal capillary network, which downstream mediates the control of the gonadotropic production cells in the anterior pituitary. This is the initial step in the hypothalamic-pituitary-gonadal axis. The neurons exist in a clustered network of other cells allowing for close interaction with secreted neurotransmitters and hormones. GnRH is a small tropic peptide hormone composed of 10 amino acids (Fig. 8.2).
GnRH secretion
GnRH is secreted into the vast capillary network of the hypophyseal portal system. It is quickly degraded once secreted and has a half-life of approximately 2–4 min. GnRH activates the GnRH receptor (GnRH-R), a G protein-coupled receptor with a seven-transmembrane domain. The gene for the receptor is located on Chromosome 4 and codes for the 328 amino acid protein [12]. After binding, phosphatidylinositol4-5-bisphosphate (PIP2) is cleaved into second messengers diacylglycerol (DAG) and inositol triphosphate (IP3), and IP3 stimulates intracellular calcium release. DAG, IP3, and calcium stimulate Protein Kinase C, and the mitogen-activated protein kinase (MAPK) cascades leading to increased transcription factors for gonadotropin production [13].
Gonadotrope responsiveness to GnRH secretion is modulated based on the pulsatility of GnRH release. Neuronal activity is characterized by intrinsic short pulsatile patterns that change based on releasing and inhibiting factors including stress, energy availability, sex steroids, and neurotransmitters [14]. The pulsatility of GnRH is posited to emanate from the neurons in the arcuate nucleus of the hypothalamus [15]. Two modes of GnRH secretion have been identified: pulsatile and surge. Pulsatile secretion is mainly responsible for the daily intermittent secretion of GnRH. Surge secretion is responsible for the pre-ovulatory LH surge that immediately precedes ovulation (Fig. 8.3). The pulsatile intermittent release maintains receptor reactivity to the presence of GnRH; however, increasing frequency of GnRH release may reduce responsiveness and continuous GnRH secretion may lead to near-complete desensitization of the receptors to GnRH. This desensitization to GnRH can be exploited for therapeutic treatment with GnRH agonists to turn off the HPG axis and induce a pseudo-menopause state. These medications have broad clinical applications and can be considered for the treatment of precocious puberty and endometriosis.
Additionally, different patterns of GnRH pulsatility differentially affect gonadotropes to induce transcription and production of LH or FSH. High-frequency pulses stimulate LH production, whereas low-frequency GnRH pulses favor FSH secretion [16]. In males, GnRH pulse frequency remains constant, whereas, in females, pulse frequency varies throughout the menstrual cycle. LH pulse frequency, a surrogate for GnRH pulsatility, is slow in the luteal phase, speeding up during the follicular phase, and culminates in a pre-ovulatory surge in conjunction with similarly timed GnRH surge secretion. Estrogen and progesterone released from the corpus luteum provide a negative feedback mechanism for GnRH post-ovulation.
Control/regulation of GnRH
Regulation of GnRH by neuronal inputs
KNDy neurons
Neurons in the arcuate nucleus also express several neurotransmitters including kisspeptin, neurokinin, and dynorphin which have been shown to regulate GnRH via a “pulse generator” and are essential for puberty and reproduction regulation [17]. These three neurotransmitters have lent their names to the KNDy neurons which produce these neurotransmitters [18].
Kisspeptin and its G-protein receptor GPR54 are responsible for normal pubertal development and loss of function mutations of the receptor causes hypogonadotropic hypogonadism [19,20]. This pubertal development is contingent upon the careful synthesis and secretion of GnRH. Additionally, kisspeptin neurons express estrogen and progesterone receptors and are therefore sex-steroid-dependent modulators of GnRH secretion. Neurokinin B stimulates kisspeptin and loss of function mutations in neurokinin B and its receptor TACR3 have been shown in humans to result in absent or delayed puberty, similar to kisspeptin [21]. Dynorphin, however, inhibits the release of GnRH [22].
Other neuropeptides
Several other neuropeptides impact GnRH either directly or through other intermediaries. Neuropeptide Y stimulates the appetite by way of insulin and leptin and has a potentiating effect on gonadotropins and an indirect effect on GnRH. In the absence of estrogen, neuropeptide Y has been shown to inhibit gonadotropin secretion and is likely reflective of the body’s nutritional status, and acts as an indicator of the body’s energy state in order to regulate reproductive function [23]. Glutamate stimulates GnRH secretion by activating nitric oxide as an intermediary which acts on cyclic GMP [24]. γ-aminobutyric acid (GABA) and noradrenaline appear to have dual roles on GnRH, both inhibiting and stimulating GnRH neurons. Serotonin, dopamine, and prolactin have been shown to suppress GnRH pulsatility, although the pathways are not clearly understood.
Regulation of GnRH by gonadal feedback
In response to GnRH, gonadotropes produce both FSH and LH. In both males and females, FSH stimulates the production of gametes, while LH stimulates the steroid hormone production by the gonads. The androgens and estrogens produced suppress GnRH production via negative feedback at both the levels of the pituitary gonadotrophs and the arcuate nucleus. This is likely via the KNDy neuronal network, as GnRH-releasing neurons do not have androgen or estrogen receptors themselves.
Regulation of GnRH by other regulating factors
Nutrition and stress also impact GnRH regulation and play a significant role in puberty and reproductive function. GnRH pulsatility is suppressed in energy-deprived periods such as stress and starvation. Insulin and leptin/ghrelin suppress the GnRH neurons via neuropeptide Y and increased cortisol from stress suppresses the KNDy neurons [25,26].
Clinical application of GnRH
Approximately 10%–30% of GnRH neurons are required to achieve normal physiologic function in the hypothalamus. However, the synchronized release of GnRH in a pulsatile fashion is necessary to achieve appropriate secretion of LH and FSH [27]. At the pituitary level, the expression of GnRH-Rs is dynamic and increases or decreases in response to feedback loops of FSH and LH, GnRH via an ultra-short feedback loop, and estradiol and progesterone [12].
GnRH has a half-life of 2–4 min, limiting its use as a therapeutic agent; pulsatile GnRH administration has been used to induce the development of follicles and induce ovulation; however, GnRH analogs have been developed with longer plasma half-lives in order to exert a desired clinical effect [28]. GnRH agonists bind to GnRH-Rs, mimicking the activity of native GnRH. Continuous administration of GnRH or a GnRH agonist (GnRH-a) leads to an initial stimulation of pituitary GnRH-R, known as the flare, followed by desensitization and downregulation of pituitary GnRH-R, resulting in suppression of gonadotrope secretion [28,29]. Pulsatile stimulation is necessary to avoid downregulation of the GnRH-R.
GnRH-a is utilized to both stimulate and suppress reproductive function according to the timing of the administration regimen. Pulsatile treatment mimics GnRH activity and maintains pituitary function, while continuous administration results in the suppression of pituitary gonadotropins following the initial flare effect [30]. GnRH-a can also be used as an alternative to hCG to trigger ovulation via induction of an LH surge in cycles utilizing GnRH-antagonists (GnRH-ant) for controlled ovarian stimulation, as the pituitary remains responsive to GnRH-a activity [28]. GnRH-antagonists are designed to shut down the pituitary-gonadal axis without the flare effect noted with GnRH or GnRH-a via competitive binding of GnRH to GnRH-Rs [31].
It has been demonstrated that GnRH/GnRH-R are located not only in the hypothalamus and pituitary but also in the periphery, including in the ovary and endometrium [30]. Peripheral GnRH-R is associated with anti-proliferative activity and has been suggested as a target for GnRH-analog-based therapies to treat ovarian tumors and other steroid-dependent diseases [28].
*GnRH in the ovary
*GnRH in the endometrium
Inhibin Structure
Inhibins are part of the transforming growth factor (TGF)-β superfamily [38]. They function to suppress FSH secretion without affecting LH levels. Inhibins are glycoproteins produced by the granulosa and corpus luteal cells of the ovaries as well as the Sertoli cells of the testis [39]. The two main isoforms are inhibin A and inhibin B, which are disulfide-linked heterodimers [40,41]. They each share an identical α subunit and two different β subunits: β-A for inhibin A and β-B for inhibin B [38]. They are unique to the TGF-β family in that they act as antagonists and function primarily in an endocrine fashion [42]
Inhibins are synthesized as prohormones and intracellular cleavage of sulfhydryl-linked dimers releases mature C-terminal portions of the hormones to allow their biological activity [43]. Mature inhibin ab dimers have molecular weights of 31–34 kDa [44].
Receptors
GnRH promotes both FSH and LH and does not differentiate between the two. Inhibins resolve this issue by specifically blocking pituitary FSH production. As such, they permit the separate modulation of LH and FSH. Inhibins have not been found to have signaling activity and function more as antagonists to activin-mediated signaling in the pituitary [42,43]. Activins are structurally related to inhibins and are potent stimulators of FSH production from gonadotropes [44]. Both, inhibin A and inhibin B can form complexes with activin receptors to prevent activin’s intracellular phosphorylation cascades, ultimately inhibiting FSH production.
Betaglycan, a membrane-bound proteoglycan, is an obligate and high-affinity co-receptor for the α subunit of inhibin A. This complex of inhibin A and betaglycan then binds activin type II receptors (ActRII) A/B via inhibin’s β subunit in pituitary gonadotrope cells to suppress FSH secretion [39,42,44]. Inhibin B is functional even in the absence of betaglycan and may have an unidentified co-receptor [42].
*Production and inhibin expression in females
*Production and expression in males
Sertoli cells produce inhibin A and B in male fetuses; however, in adult males, inhibin B is preferentially expressed and serum inhibin A levels should be undetectable. Prior to puberty, Sertoli cells produce both the α and the β-B subunits. After puberty, only the α subunit is expressed by Sertoli cells, and production of the β-B subunit is taken over by maturating germ cells [39,51]. Inhibin B serum levels markedly rise during the pubertal transition from Tanner stage G1 to G2, which is concurrent with rising LH and testosterone.
Completion of spermatogenesis requires low FSH signaling and high LH signaling. This is accomplished by Sertoli cell inhibins suppressing FSH release from the pituitary. This is supported by the finding of infertile men with elevated serum FSH have an inverse correlation with inhibin B levels (Fig. 8.4) [39,52,53].
Summary
GnRH produced and released by the hypothalamus is the key regulator in the HPG axis. Variations in pulsatility of GnRH production drive multiple physiologic functions including the production of gonadotropes FSH and LH and therefore play a pivotal role in reproduction. GnRH is regulated by many factors including an ultrashort feedback loop, KNDy neurons, and many other neurotransmitters in addition to diet, sleep, and stress patterns. Research has further demonstrated that GnRH also has effects in extra-pituitary tissues as well through the action of peripheral GnRH receptors. New possibilities for novel treatment strategies, particularly in the locally expressed GnRH/GnRH-R system targeting the ovary and the endometrium
Inhibin A and B are glycoproteins produced by the granulosa and corpus luteal cells of the ovaries as well as the Sertoli cells of the testis.
