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The Hypothalamic-Pituitary-Gonadal Axis Page!

Hypothalamic Gonadotropin-Releasing Hormone (GnRH)

GnRH is a deca-peptide (a small, simple, straight chain protein composed of 10 amino acids) with a MW of ~ 1200, and a circulating t_1/2 (half-life) of 2-10 minutes. Because GnRH is so small, and its half-life is so short, in addition to the fact that it is released in very small quantities, GnRH is not measured clinically. GnRH is stimulatory to the reproductive system. GnRH is a very primitive molecule, and is recognized as the mating factor released by Saccharomyces cerevisiae (a-mating yeast, also known as brewer's yeast or budding yeast). GnRH is not only synthesized in the arcuate nucleus of the hypothalamus, but also in very small quantities by the ovary (ovarian GnRH is known as gonadocrinin) and by the placenta. In addition, GnRH has been recognized as a transmitter in sympathetic ganglia, which simply tells us that GnRH is not strictly limited to the reproductive system. Our interest is primarily in GnRH produced by the hypothalamus.

GnRH is produced in the arcuate nucleus of the hypothalamus, secreted from the median eminence of the hypothalamus and is carried down to the anterior pituitary via a connecting vascular network called the "hypothalamic-pituitary portal system." (see pics 1 2 3 of this vascular portal system). It is important that you recognize that the connections between the hypothalamus and the anterior pituitary are via this hypothalamic-pituitary portal system (a vascular connection). Later, we will see that the connection between the hypothalamus and the posterior pituitary is a direct nervous connection.

GnRH, released from the hypothalamus, stimulates the synthesis and release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary. GnRH is released from neurons in the hypothalamus in distinct pulses. This, in turn, causes LH to be released from the anterior pituitary in distinct pulses, and these LH pulses are easily measured in blood plasma. (The types of GnRH pulses that can be experimentally measured in the hypothalamic pituitary portal vessels and the types of LH and androgen pulses measureable in the bloodstream of males are depicted here. The pulses occur about every 1-2 hours in males. The frequency of GnRH secretion varies in females, being faster during the follicular phase and slower during the luteal phase - more on these phases later).

In humans, the hypothalamic "GnRH pulse generator" is in the arcuate nucleus, which is a tiny nucleus at the bottom of the hypothalamus, located centrally beneath the floor of the middle ventricle (IIIrd ventricle), immediately above the median eminence. The median eminence lies above the pituitary stalk (the stalk is also known as the infundibulum). The GnRH pulse generator is regulated by stimulatory and inhibitory inputs. The stimulatory inputs are primarily catecholaminergic (eg. norepinephrine and epinephrine). The inhibitory inputs may be primarily opioidergic (eg. beta-endorphin and other peptides with morphine-like activity). Catecholaminergic neurons have gonadal steroid and opioid (see endogenous opioid peptide) receptors and these substances generally reduce the firing rate of the catecholaminergic neurons in the hypothalamus. This, in turn, reduces activity of the GnRH neurons.

The GnRH pulse generator is responsible for day-to-day (tonic) pulsatile LH secretion. An additional phasic (surge) center may exist outside of the GnRH pulse generating system. The phasic center may be activated during the preovulatory gonadotropin surge (the massive surge of LH and FSH shown in the top panel of the figure), stimulating GnRH pulse frequency to maximal levels. This high frequency of GnRH secretion causes the anterior pituitary to release a huge "mid-cycle" surge of LH and FSH. The LH component of the preovulatory gonadotropin surge causes ovulation. The phasic center is postulated to lie in the anterior hypothalamus and exists only in females. (did you see this info on the surge?)

Preovulatory gonadotropin surge

Midcycle Surge

Surge

GnRH is synthesized in the arcuate nucleus and is axonally transported to nerve terminals in the median eminence. GnRH released from the median eminence diffuses into the pituitary-portal vasculature and is carried to the anterior pituitary where GnRH binds to specific GnRH receptors and stimulates LH and FSH synthesis and secretion. Other hypophyseotropic hormones released from the median eminence include TRH, CRH, GHRH and SRIF. These, in turn, cause the release of a number of pituitary hormones. The pituitary is also known as the hypophsyis, so we also call pituitary hormones, "hypophyseal hormones."

Pulsatile hormone release helps maintain responsiveness of target tissues (there is a small lesson on this below). Molecules of hormone released in a pulse bind to receptors on target cells. The receptors then aggregate and are internalized. During the interval preceding the next hormone pulse, receptors are recycled back to the surface of the target cells. In fact, this is the case with GnRH. If hormones were constantly present, the receptors would be internalized so quickly that the cell would not be able to recycle enough receptors to the cell surface to respond. This means that long acting GnRH analogs, which continuously down-regulate GnRH receptors on the pituitary, can be used to shut down LH and FSH secretion from the anterior pituitary. You might be interested in how this relates to treatment of precocious puberty!

The pituitary is only able to respond to GnRH when it is administered in "pulses," or short bursts. If we treat people with GnRH superagonists with continuous GnRH activity, this continuous GnRH activity will actually cause the gonadotropes of the anterior pituitary to "shut down!" In cases of precocious puberty, this will actualy cause inactivation of the reproductive axis and prevent the young child from entering puberty too early in life. Precocious puberty has been known to occur as early as 2 years.

Do you understand how GnRH suppresses gonadotropin secretion in precocious puberty; if so, you should be able to come up with a plausible explanation for how super GnRH analogs (agonists) can help in endometriosis patients!

Anterior Pituitary Luteinizing Hormone & Follicle Stimulating Hormone

LH and FSH are referred to as gonadotropins because they have tropic, or stimulatory, effects on the gonads. LH and FSH are produced by "gonadotrope" cells of the anterior pituitary. Prolactin and growth hormone may have small amounts of gonadotropin activity, because the promote development of gonadotropin hormone receptors on gonadal cells... but remember, in general, when people talk about gonadotropins, they are talking about LH and FSH... hormones produced by the gonadotropes of the anterior pituitary. Prolactin and growth hormone are also produced by the anterior pituitary, but prolactin is produced by "lactotrope cells (lactotropes)," and growth hormone is produced by "somatotrope cells (somatotropes)."

LH and FSH are glycoproteins with a and b chains. Glycoproteins are proteins which are somewhat protected from enzymes in circulation by a coat of carbohydrate material... thus the name, glycoprotein. They have molecular weights of 30,000 and 33,000, respectively. The a chain is common to several hormones, including LH, FSH, hCG and TSH. The b-subunit varies between hormones and so imparts hormone specificity. LH and FSH are secreted by gonadotrope cells in the anterior pituitary. GnRH is released in a pulsatile fashion. LH is clearly released in a pulsatile fashion. Circulating FSH profiles appear less distinctly pulsatile than circulating LH profiles. It has often been suggested that the reason that FSH appears less pulsatile is because of a longer t_1/2 than LH (FSH has a t_1/2 of 2-2.5 hours while that of LH is 20-25 minutes). Thus, it was suggested that FSH had less time to be cleared from circulation between GnRH pulses. However, even in circumstances when GnRH pulses are several hours apart, we see that FSH levels are relatively stable (ie. FSH does not fall between peaks of GnRH secretion and the t_1/2 explanation for the apparent lack of pulsatility of FSH does not hold water). It has also been suggested that there may be alternate FSH releasing factors (ie. a specific "follicle-stimulating- hormone-releasing-hormone" other than GnRH... although one has never been found). Generally, pulses of LH occur about once every 1-2 hours, although, as we will see when discussing the menstrual cycle, the frequency and amplitude of LH pulses are subject to regulation by gonadal steroids. Variation in gonadotropin secretion is greatest in females, varying with ovarian steroid levels during different phases of the ovarian/menstrual cycle. Pulses of LH are caused by pulses of GnRH.

Receptors for Protein Hormones

As proteins are not lipid soluble, they cannot pass through cell membranes. This means that receptors for protein hormones such as GnRH, LH and FSH must be located on the surface of the cell membrane. Generally, protein hormone receptors span the cell membrane. When the receptors bind their specific hormone, the portion of the receptor extending through the cell membrane activates intracellular signalling mechanisms that tell the cell how to respond. These intracellular signals, sometimes called "second messengers," are generally enzymes, but sometimes involve opening of chemically-gated calcium channels. It appears that many protein hormones actually activate more than a single "second messenger" system.

Gonadal Steroid & Glycoprotein Feedback on the Hypothalamic-Pituitary Axis

Much of what we know about gonadal feedback on the hypothalamic-pituitary axis has been learned and extrapolated from rodents and domestic species, particularly the gonadectomized, steroid treated animal models.

steroid receptors

Steroid receptors occur both in the cytoplasm and nucleus. Receptors in the cytoplasm may need to bind steroid, undergo a period of transformation during which they form dimers (large molecules with 2 subunits), and then undergo an active process of translocation into the nucleus prior to binding with nuclear chromatin "acceptor sites." Steroid receptors have i) protein binding subunits which link the steroids in proximity to DNA regulatory sites, and ii) DNA binding sites which can bind and affect DNA regulatory sites. Multiple estrogen receptors have been identified and different estrogens cause the different estrogen receptors to form dimers, which take on multiple conformations to exert different levels of activity. The principal actions of steroids involve genomic regulation. Thus, the effects of steroids are suspected to occur several hours after hormone-receptor binding. There is growing evidence that steroid hormones may induce specific biological effects at cell membranes. Here is a somewhat simpler construct of steroid hormone action.

You need to know that...

-steroid receptors are found in the cytoplasm and the nucleus
-steroid receptors interact directly with DNA to alter gene expression
-the effects of steroid receptor activation are not seen for at least 3-12 hours, because the effects are exerted through changes in genetic expression of cell proteins (enzymes, hormones), and this takes time!

In females, estrogens and progestagens (progestins) are the most important reproductive steroids. When interpreting effects of estrogen and progesterone on the hypothalamic-pituitary axis, it appears that, within the physiological range, the "functional ratio" or "dominance ratio" of progesterone:estrogen bioactivity may be more important than the specific concentration of either class of hormone by itself.

estrogen feedback

Estrogens are known to exert both negative and positive feedback effects on gonadotropin secretion. Pharmacologically, estrogen feedback has been shown to have 3 components; initial negative feedback, a secondary positive feedback effect (the kind that causes the mid-cycle preovulatory gonadotropin surge") and, finally, continued suppression of gonadotropin secretion if estrogen is maintained. So where does estrogen act to have these negative and positive effects? Physiologically, estrogen treatment, in the absence of progesterone, tends to decrease LH pulse amplitude and increase LH pulse frequency. Estrogens reduce LH pulse amplitude by reducing pituitary responsiveness to GnRH and indirectly by increasing GnRH pulse frequency such that the amount of GnRH and LH stored between hormone pulses is reduced. This classical inverse relationship between frequency and amplitude of hormone secretion is a recurring theme in endocrinology. Estrogens synergize with progestins to further suppress LH pulse frequency and amplitude. What all this means is that, during the follicular phase (more on this below), when estrogens are dominant, there is a high frequency of low amplitude LH pulses (this pattern drives follicular development in the follicular phase - see the follicular phase below).

When estrogens are high enough for long enough, they paradoxically increase pituitary responsiveness to GnRH. This "estrogen positive feedback" causes a high speed burst of GnRH pulses. This is the type of GnRH secretory pattern which likely drives the preovulatory gonadotropin surge. High frequency, high amplitude LH pulses occur throughout the preovulatory k surge, with the pulses decreasing in frequency during the declining phase of the surge. As we will see, these estrogens that eventually trigger the preovulatory gonadotropin surge come from the developing ovarian follicles!

progesterone feedback

Progesterone suppresses gonadotropin secretory frequency. Suppression of LH pulse frequency suggests that progesterone acts at the hypothalamus to reduce the frequency of GnRH secretion. Progesterone does not exert significant negative feedback effects at the pituitary. Progesterone synergizes with estrogen to further suppress LH pulse frequency and amplitude during the luteal phase. The luteal phase is the phase following ovulation, during which progesterone acts to prevent uterine contractions in the event of fertilization and implantation (more on this below).

androgen feedback

Androgen feedback is of primary concern with respect to gonadotropin secretion in males. When contemplating the nature of negative feedback of androgens on gonadotropin secretion, two important observations should be considered. First, androgens are aromatized to estrogens when diffusing through the blood-brain barrier. Therefore, "behavioral effects" of androgens in the CNS of males are actually exerted by estrogens. Secondly, it should be noted that males are not susceptible to any type of "positive steroid feedback." This is because of differences in "hardwiring" of the GnRH regulatory centers in the male and female hypothalamus (ie. neither estrogens nor androgens can be used to experimentally exert positive feedback in intact or gonadectomized males). Thus, the appearance of the "estrogen positive feedback phenomenon" associated with onset of puberty is a characteristic of puberty which is unique to females (more on this below).

Like gonadotropins, androgens (secreted by interstitial/Leydig cells of the testes) are secreted in a somewhat pulsatile fashion, with pulses of androgen closely following LH pulses. Again, it is presumed that exposure of the testis to pulses of LH helps maintain Leydig cell responsiveness to LH.

gonadal inhibin/activin (more glycoproteins)

Inhibin and activin are glycoproteins secreted by ovarian granulosa cells and Sertoli cells of the testis. Activin is a bb homodimer (meaning that the subunits are the same) with a MW of approximately 24,000 and inhibin is an a,b heterodimer (meaning that the subunits are not the same) with a MW of approximately 32,000. Inhibin specifically suppresses synthesis and secretion of FSH by the pituitary. Activin blocks inhibitory effects of inhibin on FSH secretion. Specific and distinct receptors for inhibin and activin have been identified in the pituitary. Inhibin and activin receptors have also been identified in the gonads, suggesting paracrine effects of these glycoproteins in the gonads, although these paracrine effects have not been well established.