Medical Physiology, 3rd Edition

Hypothalamic-Pituitary-Gonadal Axis

The hypothalamic-pituitary-gonadal axis (Fig. 54-2) controls two primary functions: (1) production of gametes (spermatogenesis in males and oogenesis in females), and (2) gonadal sex steroid biosynthesis (testosterone in males and estradiol and progesterone in females). In both sexes, the hypothalamus produces gonadotropin-releasing hormone (GnRH), which stimulates the gonadotrophs in the anterior pituitary to secrete the two gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Although the names of these hormones reflect their function in the female reproductive system (see pp. 1111–1112) they play similar roles in controlling gonadal function in both sexes. The hypothalamic-pituitary axis is therefore the central regulator of male and female reproductive systems. In the male, LH and FSH control, respectively, the Leydig and Sertoli cells of the testes.

image

FIGURE 54-2 Hypothalamic-pituitary-gonadal axis. Small-bodied neurons in the arcuate nucleus and preoptic area of the hypothalamus secrete GnRH, a decapeptide that reaches the gonadotrophs in the anterior pituitary via the long portal veins (see Fig. 47-3). Stimulation by GnRH causes the gonadotrophs to synthesize and release LH, which stimulates Leydig cells, and FSH, which stimulates Sertoli cells. Negative feedback on the hypothalamic-pituitary-gonadal axis occurs by two routes. CNS, central nervous system.

The hypothalamus secretes GnRH, which acts on gonadotrophs in the anterior pituitary

Gonadotropin-releasing hormone (GnRH), which is synthesized by small-bodied peptidergic neurons in the hypothalamus, stimulates the synthesis, storage, and secretion of gonadotropins by gonadotroph cells in the anterior pituitary. The hypothalamic-pituitary-portal system (see p. 978) describes the route by which GnRH and other releasing hormones emanating from the hypothalamus reach the anterior pituitary gland. The neurons that synthesize, store, and release GnRH are dispersed throughout the hypothalamus but are principally located in the arcuate nucleus and preoptic area. During embryonic development, GnRH neurons originate in the olfactory placode and migrate to the hypothalamus. Studies involving both rats and primates show that sites of GnRH production other than the hypothalamus (e.g., the limbic system) can also participate in the control of sex behavior. Neuronal systems originating from other areas of the brain impinge on the hypothalamic GnRH-releasing neurons and thus form a functional neuronal network that integrates multiple environmental signals (e.g., diurnal light-dark cycles) and physiological signals (e.g., extent of body fat stores, stress) to control GnRH release and, ultimately, the function of the reproductive system.

GnRH is a decapeptide hormone encoded by a single gene on chromosome 8. Like many other peptide hormones, GnRH is synthesized as a prohormone—69 amino acids long in this case. Cleavage of the prohormone yields the decapeptide GnRH (residues 1 to 10), a 56–amino-acid peptide (residues 14 to 69) referred to as GnRH-associated peptide (GAP), and three amino acids that link the two (Fig. 54-3). Via the secretory pathway (see p. 34), the neuron transports both GnRH and GAP down the axon for secretion into the extracellular space. The role of GAP is unknown.

image

FIGURE 54-3 Map of the GnRH gene. The mature mRNA encodes a preprohormone with 92 amino acids. Removal of the 23–amino-acid signal sequence yields the 69–amino-acid prohormone. Cleavage of this prohormone yields GnRH. AA, amino acid.

GnRH neurons project axons directly to a small swelling on the inferior boundary of the hypothalamus, known as the median eminence, which lies above the pituitary stalk. The axons terminate near portal vessels that carry blood to the anterior pituitary (see p. 978). Consequently, GnRH secreted at the axon terminals in response to neuron activation enters the portal vasculature and is transported directly to gonadotrophs in the anterior pituitary.

GnRH stimulates the release of both FSH and LH from the gonadotroph cells in the anterior pituitary by interacting with high-affinity membrane receptors on the gonadotroph cell surface (see Fig. 55-5). The GnRH receptor (GnRHR) is a G protein–coupled receptor (GPCR; see pp. 51–52) linked to Gαq, which activates phospholipase C (PLC; see p. 58). PLC acts on membrane phosphatidylinositol 4,5-bisphosphates (PIP2) to liberate inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates the release of Ca2+ from internal stores, which triggers exocytosis LH and FSH. DAG stimulates protein kinase C, which indirectly increases expression of genes encoding LH and FSH. The net effect is an increase in the synthesis and release of LH and FSH from the gonadotrophs. Because secretion of GnRH into the portal system is pulsatile, secretion of LH and FSH by the gonadotrophs is also episodic. The frequency of pulsatile LH discharge in men is ~8 to 14 pulses over a 24-hour period. FSH pulses are not as prominent as LH pulses, both because of their lower amplitude and because of the longer half-life of FSH in the circulation.

Upon binding GnRH, the GnRH receptor is internalized and partially degraded in the lysosomes. However, some GnRH receptors are shuttled back to the cell surface, and de novo receptor synthesis continues from GnRH receptor gene expression. Return of the GnRH receptor to the cell membrane is referred to as receptor replenishment. A consequence of receptor internalization, however, is that the responsiveness of gonadotrophs to GnRH can be decreased by prolonged exposure to GnRH. Thus, although pulsatile GnRH discharge elicits a corresponding pulsatile release of LH and FSH, continuous administration of GnRH—or intermittent administration of high doses of GnRH analogs—suppresses the release of gonadotropins. This effect occurs because continuous (rather than pulsatile) exposure to GnRH causes a decrease in the number of GnRH receptors on the surface of the gonadotroph (i.e., receptor internalization exceeds replenishment). The induced desensitization to GnRH can be used therapeutically to control the reproductive function. A clinical application of this principle is chemical castration in prostatic cancer. Here, the administration of long-acting GnRH analogs desensitize the gonadotrophs to GnRH, which leads to low LH and FSH levels and thereby reduces testosterone production (see Box 55-2).

Under the control of GnRH, gonadotrophs in the anterior pituitary secrete LH and FSH

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are members of the same family of hormones as human chorionic gonadotropin (hCG; see p. 1139) and thyroid-stimulating hormone (TSH; see p. 1010). These glycoprotein hormones are composed of two polypeptide chains designated α and β, both of which are required for full biological activity. The α subunits of LH and FSH, as well as the α subunits of hCG and TSH, are identical. In humans, the common α subunit has 92 amino acids and a molecular weight of ~20 kDa. The β subunits differ among these four hormones and thus confer specific functional and immunological characteristics to the intact molecules.

Each of the unique β subunits of FSH and LH is 115 amino acids in length. The β subunits of LH and hCG are identical except that the β subunit of hCG has an additional 24 amino acids and additional glycosylation sites at the C terminus. imageN54-1 The biological activities of LH and hCG are very similar. Indeed, in most clinical uses (e.g., in an attempt to initiate spermatogenesis in oligospermic men), hCG is substituted for LH because hCG is much more readily available. imageN54-2

N54-1

Human Chorionic Gonadotropin

Contributed by Ervin Jones, Walter Boron, Emile Boulpaep

hCG is secreted by the placenta, and some reports have described that small amounts of this substance are made in the testes, pituitary gland, and other nonplacental tissues.

hCG appears in the urine of pregnant women about 12 to 14 days after conception—the basis for pregnancy tests. In former times, hCG was extracted from the urine of pregnant women.

N54-2

Plasma Lifetime of Luteinizing Hormone, Human Chorionic Gonadotropin, and Follicle-Stimulating Hormone

Contributed by Ervin Jones

The disappearance of exogenous LH from the circulation is independent of gonadal function and follows a dual exponential time course. The half-life of the fast component is 40 minutes and that of the slow component is 120 minutes. Because of its increased glycosylation, hCG has an even longer half-life. FSH has a slower turnover rate; its disappearance from the blood is described by two exponentials with half-lives of about 4 hours and 3 days, respectively.

Differential secretion of FSH and LH is affected by several other hormonal mediators, including sex steroids, inhibins, and activins (see pp. 1113–1115). Thus, depending on the specific hormonal milieu produced by different physiological circumstances, the gonadotroph produces the α and β subunits of FSH and LH at different rates.

The specific gonadotropin and the relative proportions of each gonadotropin released from the anterior pituitary depend on the developmental age. The pituitary gland of the male fetus contains functional gonadotrophs by the end of the first trimester of gestation. Thereafter, gonadotropin secretion rises rapidly and then plateaus. Gonadotropin secretion begins to decline in utero during late fetal life and increases again during the early postnatal period.

Male primates release LH in response to GnRH administration at 1 to 3 months of age, a finding indicative of functional competence of the anterior pituitary gland. Also during this time, a short-lived postnatal surge of LH and testosterone secretion occurs in males. Although the cause of this short-lived surge of gonadotropins remains to be understood, it is clearly independent of sex steroids. The sensitivity of the gonadotrophs to stimulation subsequently diminishes, and the system remains quiescent until just before puberty.

Release of FSH is greater than that of LH during the prepubertal period, a pattern that is reversed after puberty. GnRH preferentially triggers LH release in men. This preferential release of LH may reflect maturation of the testes, which secrete inhibins, a specific inhibitor of FSH secretion at the level of the anterior pituitary gland. Increased sensitivity of the pituitary to increasing gonadal steroid production may also be responsible for the diminished secretion of FSH.

LH stimulates the Leydig cells of the testis to produce testosterone

LH derives its name from effects observed in the female, that is, from the ability to stimulate ovulation and the formation and maintenance of the corpus luteum (see p. 1116). The comparable substance in the male was originally referred to as interstitial cell–stimulating hormone (ICSH). Subsequently, investigators realized that LH and ICSH are the same substance, and the common name became LH.

LH stimulates the synthesis of testosterone by the testes. Testosterone production decreases in males after hypophysectomy. Conversely, LH (or hCG) treatment of men increases testosterone levels, but only if the testes are intact and functional. The interstitial Leydig cells are the principal targets for LH and the primary source of testosterone production in the male. The plasma membranes of Leydig cells have a high-affinity LH receptor, a GPCR coupled to Gαs (Fig. 54-4). Binding of LH to this receptor activates membrane-bound adenylyl cyclase (see p. 53), which catalyzes the formation of cAMP, which in turn activates protein kinase A (PKA). Activated PKA modulates gene transcription (see Fig. 4-13) and increases the synthesis of enzymes and other proteins necessary for the biosynthesis of testosterone (see pp. 1097–1100).

image

FIGURE 54-4 Leydig and Sertoli cell physiology. The Leydig cell (left) has receptors for LH. The binding of LH increases testosterone synthesis. The Sertoli cell (right) has receptors for FSH. (Useful mnemonics: L for LH and Leydig, S for FSH and Sertoli.) FSH promotes the synthesis of androgen-binding protein (ABP), aromatase, growth factors, and inhibin. There is crosstalk between Leydig cells and Sertoli cells. The Leydig cells make testosterone, which acts on Sertoli cells. Conversely, the Sertoli cells convert some of this testosterone to estradiol (because of the presence of aromatase), which can act on the Leydig cells. Sertoli cells also generate growth factors that act on the Leydig cells.

FSH stimulates Sertoli cells to synthesize hormones that influence Leydig cells and spermatogenesis

The Sertoli cells are the primary testicular site of FSH action (see Fig. 54-4). FSH also regulates Leydig cell physiology via effects on Sertoli cells. The signaling events after FSH binding are similar to those described above for LH on the Leydig cell. Thus, binding of FSH to a GPCR activates Gαs, causing stimulation of adenylyl cyclase, an increase in [cAMP]i, stimulation of PKA, transcription of specific genes, and increased protein synthesis. These proteins are important for synthesis and action of steroid hormones, including the following:

1. Androgen-binding protein (ABP), which is secreted into the luminal space of the seminiferous tubule, near the developing sperm cells. ABP helps to keep local testosterone levels high.

2. P-450 aromatase (P-450arom; see p. 1117 and Table 50-2), a key steroidogenic enzyme that converts testosterone, which diffuses from the Leydig cells to the Sertoli cells, into estradiol.

3. Growth factors and other products that support sperm cells and spermatogenesis. These substances significantly increase the number of spermatogonia, spermatocytes, and spermatids in the testis. The stimulatory effect of FSH on spermatogenesis is not a direct action of FSH on the spermatogonia; instead, stimulation of spermatogenesis occurs via the action of FSH on Sertoli cells. FSH may also increase the fertility potential of sperm; it appears that this effect of FSH results from stimulation of motility, rather than from an increase in the absolute number of sperm.

4. Inhibins, which exert negative feedback on the hypothalamic-pituitary-testicular axis to inhibit FSH secretion (see below). Inhibins are members of the transforming growth factor-β (TGF-β) superfamily, which also includes the activins and antimüllerian hormone (see p. 1080). Inhibins are glycoprotein heterodimers consisting of one α and one β subunit that are covalently linked. The granulosa cells in the ovary and the Sertoli cells in the testis are the primary sources of inhibins. We discuss the biology of inhibins and activins in more detail on pages 1113–1115. Inhibins are secreted into the seminiferous tubule fluid and into the interstitial fluid of the testicle. In addition to exerting an endocrine effect on the axis, inhibins also have paracrine effects, acting as growth factors on Leydig cells.

Leydig cells and Sertoli cells engage in crosstalk (see Fig. 54-4). For example, the Leydig cells make testosterone, which acts on Sertoli cells. In the rat, β endorphin produced by fetal Leydig cells binds to opiate receptors in Sertoli cells and inhibits their proliferation. Synthesis of β endorphins could represent a local feedback mechanism by which Leydig cells constrain the number of Sertoli cells. Conversely, Sertoli cells affect Leydig cells. For example, Sertoli cells convert testosterone—manufactured by Leydig cells—to estradiol, which decreases the capacity of Leydig cells to produce testosterone in response to LH. In addition, FSH acting on Sertoli cells produces growth factors that may increase the number of LH receptors on Leydig cells during development and thus result in an increase in steroidogenesis (i.e., an increase in testosterone production).

What, then, is required for optimal spermatogenesis to occur? It appears that two testicular cell types (Leydig cells and Sertoli cells) are required, as well as two gonadotropins (LH and FSH) and one androgen (testosterone). First, LH and Leydig cells are required to produce testosterone. Thus, LH, or rather its substitute hCG, is used therapeutically to initiate spermatogenesis in azoospermic or oligospermic men. Second, FSH and Sertoli cells are important for the nursing of developing sperm cells and for the production of inhibin and growth factors, which affect the Leydig cells. Thus, FSH plays a primary role in regulating development of the appropriate number of the Leydig cells so that adequate testosterone levels are available for spermatogenesis and the development of secondary sex characteristics.

During early puberty in boys, both FSH and LH levels increase while, simultaneously, the Leydig cells proliferate and plasma levels of testosterone increase (Fig. 54-5). imageN54-3

image

FIGURE 54-5 Plasma testosterone level as a function of age in human males. (Data from Griffin JE, et al: The testis. In Bondy PK, Rosenberg LE [eds]: Metabolic Control and Disease. Philadelphia, WB Saunders, 1980; and Winter JS, Hughes IA, Reyes FI, Faiman C: Pituitary-gonadal relations in infancy: 2. Patterns of serum gonadal steroid concentrations in man from birth to two years of age. J Clin Endocrinol Metab 42:679–686, 1976.)

N54-3

Effects of Follicle-Stimulating Hormone on Leydig and Sertoli Cells during Puberty

Contributed by Ervin Jones

As noted in the upper panel of eFigure 54-1, both FSH and LH levels increase during early puberty in boys, while simultaneously the Leydig cells proliferate. As a result, the Leydig cells increase their production of testosterone, and plasma levels of this steroid hormone increase, as shown in the lower panel of eFigure 54-1.

image

EFIGURE 54-1 Plasma levels of FSH, LH, and testosterone from puberty to adulthood. The upper panel shows how plasma levels of biologically active LH and FSH increase during puberty, expressed in terms of both the stages of puberty and bone age. The lower panel shows the concomitant rise in plasma levels of testosterone. LH stimulates Leydig cells to synthesize testosterone. FSH indirectly promotes testosterone synthesis by stimulating the Sertoli cells to produce factors that act on Leydig cells. (Upper panel,modified from Reiter EO, Beitins IZ, Ostrea TR, et al: Bioassayable luteinizing hormone during childhood and adolescence and in patients with delayed pubertal development. J Clin Endocrinol Metab 54:155–161, 1982; and Beitins IZ, Padmanabhan V, Kasa-Vubu J, et al: Serum bioactive follicle-stimulating hormone concentration from prepuberty to adulthood: A cross-sectional study. J Clin Endocrinol Metab 71:1022–1027, 1990; lower panel, modified from Grumbach MM, Styne DM: Puberty: Ontogeny, neuroendocrinology, physiology, and disorders. In Wilson JD, Foster DW, Kronenberg HM, Larsen PR [eds]: Williams Textbook of Endocrinology, 9th ed. Philadelphia, WB Saunders, 1998, pp 1509–1625.)

The primary target of FSH in the testis is the Sertoli cell (see Fig. 54-4). Via this action on Sertoli cells, FSH indirectly increases the number of Leydig cells, which is a key part of pubertal development. In hypogonadotropic-hypogonadal men (i.e., individuals who have decreased levels of both LH and FSH), treating with exogenous FSH stimulates the Sertoli cells to release factors that induce differentiation and maturation of Leydig cells. Subsequent treatment with hCG (i.e., which acts like LH) acts on these Leydig cells to synthesize testosterone and thereby support spermatogenesis.

During puberty, a related change is that Sertoli cells become relatively less sensitive to FSH, but at the same time they become more dependent on the testosterone that the Leydig cells produce. Thus, there is a continuum in the development of the Sertoli cells: as proliferation and maturation of the Sertoli cells proceeds during puberty, the responsiveness of the Sertoli cell to FSH declines while its responsiveness to testosterone increases. The mechanism for this switch appears to be that FSH stimulates the synthesis of androgen receptors on Sertoli cells.

The hypothalamic-pituitary-testicular axis is under feedback inhibition by testicular steroids and inhibins

The hypothalamic-pituitary-testicular axis in postpubertal males not only induces production of testosterone and inhibin by the testes but also receives negative feedback from these substances (see Fig. 54-2).

Normal circulating levels of testosterone inhibit the pulsatile release of GnRH by the hypothalamus and thereby reduce the frequency and amplitude of the LH- and FSH-secretory pulses. Testosterone also has negative-feedback action on LH secretion at the level of the pituitary gonadotrophs.

The inhibins also feed back on FSH secretion. Evidence for such negative feedback is that plasma FSH concentrations increase in proportion to the loss of germinal elements in the testis. Thus, FSH specifically stimulates the Sertoli cells to produce inhibin, and inhibin “inhibits” FSH secretion. Inhibin appears to diminish FSH secretion by acting at the level of the anterior pituitary rather than the hypothalamus.

The secretion of LH and FSH is under the additional control of neuropeptides, amino acids such as aspartate, corticotropin-releasing hormone (CRH), and endogenous opioids.