168 Part III / Insects / Plants / Comparative until Wisconsin researchers demonstrated that melato- nin implanted into another mustelid, the short-tailed weasel, forced molting of their brown summer fur and the growth of white winter fur. Subsequently, similar studies were conducted in which mink were treated with this indoleamine. The results of these studies confirmed that melatonin, when administered as an implant to mink during the summer, induced molting of the sum- mer fur and early growth of winter pelage. This sea- sonal effectiveness of melatonin became obvious once it was demonstrated that in all vertebrate species exam- ined thus far, the concentrations of melatonin in the pineal gland and plasma are increased during the dark portion of the daily light/dark cycle. It is now well estab- lished that the nocturnal rise in melatonin production occurs because norepinephrine released from innervat- ing sympathetic neurons binds to pinealocyte β-adren- ergic receptors, resulting in cAMP-mediated induction of N-acetyltransferase, the rate-limiting enzyme in the biochemical pathway leading to melatonin synthesis. Thus, as summer turns to fall, the average daily endo- genous levels of melatonin to which mink are exposed increase and are sufficient to promote changes in pel- age growth as just described. It was generally assumed that melatonin acted direct- ly on the hair follicle to evoke molting and regrowth. However, the discovery that seasonal changes in daily systemic levels of PRL occurred that were inversely related to those of melatonin suggested the possibility that this protein hormone might actually mediate the apparent effect of melatonin on the pelage cycle. Indeed, in mink, the spring and autumn molts were found to be correlated with increasing and decreasing daily plasma concentrations of PRL, respectively. Proof that photoperiod-related changes in prolactin secretion in mink are regulated, at least in part, by melatonin was provided by results of research demon- strating that the administration of melatonin to mink prior to the spring molt reduced systemic PRL levels and delayed the molt. Further evidence that PRL played an important role in controlling the pelage growth cycle was provided by data of studies in which mink were treated with bromocryptine. This ergot alkaloid sup- presses PRL secretion and when given to mink during the summer induces molting of the summer pelage and rapid out-of-season growth of winter fur, just as in response to exogenous melatonin. Collectively, the available data suggest that PRL secretion as regulated by the seasonal changes in melatonin production stimu- lates fur growth of mink during the spring molt and may inhibit the autumn molt until mean daily levels become markedly suppressed owing to increased pro- duction of melatonin. Although it is apparent that melatonin and PRL are primary regulators of the seasonal changes in hair growth, it should be noted that hormones such as MSH, adrenocorticotropic hormone, and even gonadal steroids have also been shown to be involved in this process, but perhaps more so in species other than mustelids. 9.2. Delayed Implantation Delayed implantation is a form of diapause during which development of the embryo is retarded at the blastocyst stage. There are two types of delayed im- plantation: facultive (lactational) delay, as occurs in mice and rats, and obligate delay, as occurs in bats, roe deer, and various carnivores. The endocrinology of delayed implantation has been extensively studied in mink and the Western spotted skunk. Mink generally begin mating during late February or early March in the northern hemisphere. Ova fertilized at these early matings undergo development to the blastocyst stage and enter a diapause state. Interestingly, although diapaused embryos resulting from an early mating may be in residence in the uterus, the female may mate again. Fertilized ova from this second mating may also only develop to the blastocyst stage, with further develop- ment being arrested. Mating of the female to different males at the first and subsequent matings, which might occur as much as 1 wk later, can result in superfetation in this species. The duration of delayed implantation in mink is vari- able, depending on the time of mating. After ovulation, corpora lutea are formed, but these structures appear to be almost translucent and devoid of complete vascular- ization during diapause. In both mink and spotted skunks, the corpora lutea apparently produce low quan- tities of progestins, but neither administration of proges- terone nor of estrogens will induce implantation in intact or ovariectomized mink and skunks. Yet, the small amount of progestin produced by corpora lutea or per- haps some unknown ovarian protein hormone is essen- tial to maintain embryo viability. Bilateral ovariectomy of mink during the delayed implantation period prevents implantation and results in death of the blastocysts. As with the endocrine regulation of pelage growth, research has established that seasonal changes in the photoperiod act as the “zeitgeber” that times implanta- tion in mustelids. Implantation of embryos occurs shortly after the vernal equinox in the northern hemi- sphere and coincides with the daily increased quanti- ties of PRL being secreted. The uterus and ovaries of the mink contain relatively high concentrations of PRL receptors. In fact, the ovarian concentration of PRL receptors during diapause is about 30 times greater than the concentration of unoccupied receptors mea- Chapter 11 / Comparative Endocrinology 169 sured after the vernal equinox. The high concentration of PRL receptors in the ovary prior to the increase in PRL secretion reflects the fact that in mink PRL has been shown to be luteotropic and essential for func- tional activation of the corpora lutea to synthesize and secrete progesterone. As might be expected, treat- ment of mink with bromocryptine (a dopaminergic agonist) or melatonin suppresses PRL and pro- gesterone secretion and prolongs the period of delayed implantation. It is to be noted that exogenous mela- tonin also decreases uterine concentrations of PRL receptors. Whether this is owing to inhibition of PRL secretion or some other indirect or direct effect of melatonin is not known. Collectively, these data might be interpreted to suggest that implantation is initiated by activation of corpora lutea to produce progesterone. However, as indicated, progesterone by itself cannot initiate implan- tation of diapaused mink embryos. Similarly, there is no evidence that increased estrogen secretion is required for renewed blastocyst development or induction of implantation in carnivores as it is in rodents. Although evidence suggests that PRL and progesterone are involved in initiating implantation and maintaining pregnancy, the key biochemical(s) essential for termi- nating embryonic diapause in mustelids remains an enigma. Expression of LIF (a cytokine) in the endo- metrium of the mink uterus during embryo expres- sion suggests the possibility that this compound may at least be another component of the implantation phe- nomenon. SELECTED READING Adkins-Regan E. Hormonal mechanisms of mate choice. Am Zool 1998;38:166–178. Davis JS, Rueda BR. The corpus luteum: an ovarian structure with maternal instincts and suicidal tendencies. Front Biosci 2002;7: 1949–1978. Foster DL. Puberty in the sheep. In: Knobil E, Neill JD, eds. The Physiology of Reproduction, 2nd Ed., vol 2. New York, NY: Raven, 1994:411–451. Geist V. Mountain Sheep. A Study in Behavior and Evolution. Chi- cago, IL: University of Chicago Press, 1971. Ginther OJ, Berg MA, Bergfelt DR, Donadeu FX, Kot K. Follicle selection in monovular species. Biol Reprod 2001;65:638–647. Keverne EB, Kendrick KM. Oxytocin facilitation of maternal behav- ior in sheep. Ann NY Acad Sci 1992;652:83–101. Ojeda SR, Urbanski HE. Puberty in the rat. In: Knobil E, Neill JD, eds., The Physiology of Reproduction, 2nd Ed., vol. 2. New York, NY: Raven, 1994:363–409. Resko JA, Perkins A, Roselli CE, Stellflug JN, Stormshak F. Sexual behavior of rams: male orientation and its endocrine correlates. J Reprod Fertil 1999;Suppl 54:259–269. Straus DS. Nutritional regulation of hormones and growth factors that control mammalian growth. FASEB J 1994;8:6–12. Williams GL, Amstalden M, Garcia MR, Stanko RL, Nizielski SE, Morrison CD, Keisler DH. Leptin and its role in the central regu- lation of reproduction in cattle. Dom Anim Endocrinol 2002;23: 339–349. Chapter 12 / Hypothalamic Hormones 171 HYPOTHALAMIC–PITUITARY PART IV 172 Part IV / Hypothalamic–Pituitary Chapter 12 / Hypothalamic Hormones 173 173 From: Endocrinology: Basic and Clinical Principles, Second Edition (S. Melmed and P. M. Conn, eds.) © Humana Press Inc., Totowa, NJ 12 Hypothalamic Hormones GnRH, TRH, GHRH, SRIF, CRH, and Dopamine Constantine A. Stratakis, MD, DSc and George P. Chrousos, MD CONTENTS INTRODUCTION GNRH TRH GHRH SRIF CRH DOPAMINE hormones, including gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), growth hormone–releasing hormone (GHRH), soma- tostatin (SRIF), corticotropin-releasing hormone (CRH), and the neurotransmitter dopamine. 2. GnRH 2.1. GnRH Protein and Its Structure The existence of GnRH as a hypothalamic factor was demonstrated in 1960. Systemic injection of acid hypo- thalamic extracts released LH from rat anterior pituitar- ies. The structure of GnRH was elucidated in 1971. The decapeptide pyroGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg- Pro-Gly-amide was named luteinizing hormone-releas- ing hormone (LHRH). The term has been supplanted by GnRH, since this peptide not only releases LH from the gonadotropes, but also follicle-stimulating hormone (FSH). An FSH-specific hypothalamic-releas- ing hormone, however, may also exist and be similar to the LHRH/GnRH protein, explaining the difficulty researchers have met with its purification. 1. INTRODUCTION Alcmaeon, a sixth-century BC physiologist philoso- pher, introduced the brain as the center of human think- ing, organizer of the senses, and coordinator for survival. However, the need for a visible connection between the brain and the rest of the body to explain a rapid and effective way of communication that would maintain homeostasis led Aristotle to the erroneous conclusion that the heart was the central coordinating organ and blood the means of information trans- mission. In contemporary medicine, the two ancient concepts are integrated in the exciting field of neuroen- docrinology. The traditional distinctions between neu- ral (brain) and hormonal (blood) control have become blurred. Endocrine secretions are influenced directly or indirectly by the central nervous system (CNS), and many hormones influence brain function. The hypothalamic-pituitary unit is the mainstay of this nonstop, interactive, and highly efficient connection between the two systems. Its function is mediated by hypothalamic-releasing or hypothalamic-inhibiting 174 Part IV / Hypothalamic–Pituitary GnRH plays a pivotal role in reproduction. Phylo- genetically, this protein has been a releasing factor for pituitary gonadotropins, since the appearance of verte- brates. The structures of its gene and encoded protein have been highly preserved. Only one form of GnRH has been identified in most placental mammals, but six additional highly homologous GnRH forms have been found in other more primitive vertebrates. Only three amino acids vary in these six molecules, which together with the mammalian protein (mGnRH) form a family of molecules with diversity of function, including stimulation of gonadotropin release; regulation of sexual behavior and placental secretion; immuno- stimulation; and, possibly, mediation of olfactory stimuli. In the human brain, placenta, and other tis- sues, where the gene is expressed, GnRH protein is the same. In other species, however, several GnRH forms are expressed in the various tissues and have different functions. In amphibians, mGnRH releases gonado- tropins from the pituitary, but another, nonmammalian GnRH is responsible for slow neurotransmission in sympathetic ganglia. Marked diversification of function exists within the relatively small GnRH peptide. The residues at the amino (N)- and corboxy (C)-termini appear to be prima- rily responsible for binding to the GnRH receptor, whereas release of LH and FSH depends on the presence of residues 1–4. These critical residues are conserved in evolution. In addition, residues 5, 7, and 8 form a struc- tural unit, which is important for the biologic activity of GnRH receptors. Thus, the functional unit formed by the side chains of His 2 , Tyr 5 , and Arg 8 is necessary for full biologic activity of mGnRH. Substitution of the Arg residue reduces potency in releasing both LH and FSH, whereas replacement of the Leu 7 increases the potency for LH release, but does not alter that for FSH. Similar structure-function specificity is present in the remain- ing GnRH family members. The secondary structure of all GnRH peptides is highly conserved, too. A β-turn, formed by residues 5–8, creates a hairpin loop, which aligns the N- and C-termini of the GnRH molecule and provides the active domain of the hormone. 2.2. GnRH Gene and Its Expression GnRH is synthesized as part of a larger peptide, the prepro-GnRH precursor. The latter contains a signal sequence, immediately followed by the GnRH decapep- tide; a processing sequence (Gly-Lys-Arg) necessary for amidation; and a 56-amino-acid-long fragment, called GnRH-associated peptide, or GAP. Thus, the structure of prepro-GnRH is similar to that of many secreted proteins, in which the active sequence is coded along with a signal and processing sequences, and an “associated” peptide that is cleaved prior to secretion. GAP appears to coexist with GnRH in hypothalamic neurons, but its function remains elusive. Its sequence is considerably less preserved among species, and it does not appear to bind to specific receptors. GAP was ini- tially thought to inhibit the secretion of prolactin (PRL), but this was not confirmed in vivo. The human GnRH gene is located on the short arm of chromosome 8 (Table 1) and in all mammals consists of four exons. The first exon encodes the 5´-untranslated region (UTR). The second exon encodes prepro-GnRH up to the first 11 amino acids of GAP. The third and fourth exons encode the remaining sequence of the GAP and the 3´-UTR. Interestingly, the opposite strand of DNA is also transcribed in the hypothalamus and the heart. The function of this transcript, named SH, is unknown and may be involved in GnRH gene regula- tion. Despite the presence of many sequence changes among the GnRH genes of different species, the intro/ exon boundaries have been preserved through evolu- tion. The presence of highly homologous other GnRH forms in nonmammalian vertebrates suggests a com- mon evolutionary process, that of the duplication of one common ancestor gene. Expression of the GnRH gene is subject to significant species- and tissue-specific regulation. One example is the alternative splicing of the first GnRH gene exon in the mammalian brain and placenta. The promoter region of the rat GnRH gene has been sequenced and studied extensively. Sequences that can bind transcription fac- tors, such as Pit-1, Oct-1, and Tst-1, as well as estrogen and other steroid hormone response elements exist in the 5´-flanking region of the rat GnRH gene, suggesting a quite complex and extensive hormonal regulation of its expression. 2.3. GnRH Receptor The first step in GnRH action is recognition of the hormone by a specific cell membrane receptor (GnRH- R). The latter was recently cloned from several species, including human. It is a member of the seven-transmem- brane segment class, characteristic of G protein–linked receptors. Several differences exist, however, between the GnRH-R and the other members of this superfamily of membrane proteins. The highly conserved Asp-Glu, which is essential for function and is found in the second seven-transmembrane segment of many receptors, is replaced in the GnRH-R with Asp. In addition, the GnRH-R lacks a polar cytoplasmic C-terminal region and has a novel phosphorylation site adjacent to the third seven-transmembrane segment. The concentration of GnRH-Rs in the pituitary gland is tightly regulated and changes with the physiologic Chapter 12 / Hypothalamic Hormones 175 state of the organism. During the estrous cycle of rats, hamsters, ewes, and cows, the maximum number of receptors is observed just prior to the preovulatory surge of LH; thereafter, the number decreases and may require several days to achieve proestrous levels. Ovariectomy increases the number decreases significantly after expo- sure to androgens and during pregnancy and lactation. Several in vitro models employing pituitary cell cul- tures have indicated a biphasic response of GnRH-R to physiologic concentrations of GnRH. An initial desen- sitization of gonadotropes to GnRH is associated with downregulation of the receptor. This phase followed by an upregulation of the receptor number, which, how- ever, is not associated with increased sensitivity to GnRH, since gonadotropes respond with near-maximal LH release, when only 20% of available GnRH-Rs are occupied. The regulation of GnRH-R gene expression and protein function by GnRH provides the basis for the effects of constant GnRH infusion of GnRh super- agonists on LH and FSH secretion. Whereas low or physiologic concentrations of GnRH stimulate the syn- thesis of GnRH-R, constantly high concentrations of this hormone downregulate the receptor in a process that involves physical internalization of agonist-occu- pied receptors. This is accompanied by loss of a func- tional calcium channel and other mechanisms. Indeed, GnRH regulates pituitary LH and FSH synthesis and release by a Ca 2+ -dependent mechanism involving GnRH-R-mediated phosphoinositide hydrolysis and protein kinase C (PKC) activation. A G protein or multiple G proteins coupled to GnRH-R also play(s) and intermediatory role. This protein appears to be dif- ferent from G s or G i , and similar to that hypothesized to be involved in TRH mediation of action. Following GnRH stimulation, an increase in phospholipid meta- bolism and intracellular Ca 2+ and accumulation of inositol phosphates occur in pituitary gonadotropes. Calmodulin and its dependent protein system are impor- tant intracellular mediators of the Ca 2+ signal in the gonadotropes. In addition to its action on the gonadotropes, GnRH exerts a variety of effects in the CNS. Lordosis and mounting behaviors are facilitated by intraventricular and subarachnoid administration of GnRH, or local infusion of this peptide in the rat hypothalamic ventro- medial nucleus (VMN) and central gray. GnRh can change the firing patterns of many neurons and is present in presynaptic nerve terminals. These actions are mediated through GnRH-R. The latter has been found to be widely distributed in the rat brain, in areas such as the hypothalamic VMN and arcuate nucleus (but not the preoptic region), the olfactory bulb and the nucleus olfactorius, the septum, and the amygdala and hippocampus. With few exceptions, CNS GnRH-R binds to GnRH analogs with the same affinity as the pituitary GnRH-R does. However, the former may not share the same second-messenger system(s) with the latter, since it is unclear whether Ca 2+ is needed for hippocampal GnRh action. Aside from the CNS, GnRH-R is present in the gonads (rat and human ovary, rat testis) and rat immune system. GnRH has also been demonstrated to stimulate the production of ovarian steroidogenesis from isolated rat ovaries. The physi- ologic significance of these actions, however, remains unclear. Table 1 Genes, Pathophysiology, and Clinical Use of Hypothalamic Hormones Hormone Chromosome Receptor Associated disorders Clinical Use GnRH 8p GnRH-R Kallmann syndrome, precocious puberty, GnRH test, GnRH superagonists hpg mouse. and antagonists TRH 3 TRH-R “Hypothalamic” TRH test hypothyroidism GHRH 20p GHRH-R lit–, dw–, and dwj– mice, GHRH test, GHRH analogs “hypothalamic” GH deficiency and antagonists SRIF 3q SSTR-1–5 SRIF analogs CRH 8q CRH-R 1α, 1β “Hypothalamic” adrenal insufficiency, CRH test, CRH analogs CRH-R2 chronic fatigue, fibromyalgia, and antagonists atypical and melancholic depression, stress, autoimmune states Dopamine D-1R–D-5R Nonadenomatous D-2R agonists (pituitary: D2-R) hyperprolactinemia 176 Part IV / Hypothalamic–Pituitary 2.4. GnRH-Secreting Neurons: Embryology and Expression Almost all the GnRH in mammalian brains is present in the hypothalamus and regions of the limbic system, hippocampus, cingulate cortex, and olfactory bulb. GnRH-expressing neurons migrate during develop- ment from their original place on the medial side of the olfactory placode into the forebrain. The GnRH neu- rons, which are generated by cells of the medial olfac- tory pit, do not have a GnRH secretory function before they attain their target sites in the basal forebrain. They do, however, express the GnRH gene, a feature that allowed their detection by in situ hybridization. In mice, these cells are first noted in the olfactory epithelium by d 11 of embryonic life. By d 12 and 13, they are seen migrating across the nasal septum toward the forebrain, arriving at the preoptic area (POA) of the developing hypothalamus by d 16–20. GnRh neuron migration is dependent on a neural cell adhesion molecule, a cell- surface protein that mediates sell-to-cell adhesion, is expressed by cells surrounding the GnRH neurons, and appears to be a “guide” for their migration. By immunocytochemistry, GnRH cell bodies are found scattered in their final destination, the POA, among the fibers of the diagonal band of Broca and in the septum, with fibers projecting not only to the median eminence, but also through the hypothalamus and mid- brain. In primates, more anteriorly placed cell bodies in the POA and septum are connected with dorsally pro- jecting fibers that enter extrahypothalamic pathways presumably involved in reproductive behavior, whereas more posteriorly placed cell bodies in the medial hypo- thalamus itself give rise to axons that terminate in the median eminence. The two types of GnRH neurons are also morphologically different; the former have a smooth cytoplasmic contour, whereas the latter have “spiny” protrusions. Similar anatomic and functional plasticity has been documented at the level of the GnRH neuronal terminal. GnRH may be present in other areas of the nervous system. In frogs, a GnRH-like peptide in sympathetic ganglia is thought to be an important neurotransmitter. GnRH can enhance or suppress the electrical activity of certain neurons in vitro. GnRH is also present in the placenta, where its mRNA was first isolated. Interest- ingly, GnRH, like TRH, is secreted into milk. 2.5. GnRH Secretion and Action Secretion of hypothalamic GnRH is required for reproductive function in all species of mammals stud- ied. Its secretion is subject to regulation by many hor- mones and neurotransmitters that act on the endogenous GnRH secretory rhythm, the “GnRH pulse generator.” The latter provides a GnRH pulse into the hypophyseal- portal vessels at approx 90 intervals, which can be slowed down or accelerated by gonadal hormones. Tes- tosterone and progesterone in physiologic concentra- tions and hyperprolactinemia slow the discharge rate of the generator, whereas estrogens have no effect on the frequency of the GnRH pulses. Females of all species respond to estrogens with an acute increase in LH and, to a lesser degree, FSH, a phenomenon that explains the “ovulatory LH surge” via positive estrogen feedback on the pituitary. The mechanism of the estrogen-induced LH release has yet to be elucidated. The presence of testicular tissue prevents the estrogen-stimulatory effect on GnRH and LH secretion, but testosterone, although it slows down the GnRH pacemaker, does not completely abolish the estrogen effect. Since estrogen releases LH in castrated male monkeys, a nontestosterone testicular hormone other than inhibin may be responsible for this blocking effect in males. GnRH secretion responds to emotional stress, changes in light-dark cycle, and sexual stimuli through the inputs that GnRH neurons receive from the rest of the CNS. Norepinephrine stimulates LH release through the activation of α-adrenergic receptors, and administration of α-antagonists blocks ovulation. A population of β-adrenergic neurons, which are inhibi- tory of GnRH secretion, has also been identified. Dopa- mine has inhibitory effects, but the role of epinephrine, G-aminobutyric acid (GABA), and serotonin is less clear. Acetylcholine may increase GnRH secretion, because it can induce estrus in the rat that is blocked by atropine. Glutamate stimulates GnRH secretion via the N-methyl- D-aspartate (NMDA) receptor. Naloxone can stimulate LH secretion in humans, but this effect is modulated by the hormonal milieu. Thus, administra- tion of naloxone increases LH levels in the late follicu- lar and luteal phases, but not in the early follicular phase or in postmenopausal women. It has been postulated that endogenous opioids may mediate the effects of gonadal steroids on GnRH secretion, since β-endor- phin levels are markedly increased by administration of estrogen and progesterone. Disruption of reproductive function in mammals is a well-known consequence of stress. This effect is thought to be mediated through activation of both the central and peripheral stress system. CRH directly inhibits hypo- thalamic GnRH secretion via synaptic contacts between CRH axon terminals and dendrites of GnRH neurons in the medial POA. The role of CRH regulation of GnRH secretion may be species specific with important differ- ences noted between rodents and primates. Endogen- ous opioids mediate some of these effects of CRH, but Chapter 12 / Hypothalamic Hormones 177 their importance varies with species, as well as with the period of the cycle and the gender of the animals. CNS cytokines also regulate GnRH secretion and function. Central injection of interleukin-1 (IL-1) inhibits GnRH neuronal activity and reduces GnRH synthesis and release. These effects are in part mediated through endo- genous opioids and CNS prostaglandins (PGs). IL-1 and possibly other central cytokines may act as endogenous mediators of the inflammatory stress-induced inhibi- tion of reproductive function. 2.6. Gonadotropin Deficiency: Kallmann Syndrome In 1943, Kallmann and associates described a clini- cal syndrome of hypogonadism and anosmia affecting both men and women. The pathologic documentation of the characteristic neuroanatomic defects of the syn- drome led to the term olfactory-genital dysplasia for what is now known as Kallmann syndrome. With the discovery of GnRH in 1971, the defect was determined to be hypothalamic in all patients with the syndrome, who subsequently were shown to resume normal gona- dotropin secretion after repeated and/or pulsatile ad- ministration of GnRH. The genetic basis of Kallmann syndrome, which has in most cases an X-linked inheritance, was recently elu- cidated at the molecular level. The earlier evidence that GnRH-secreting neurons migrate to the hypothalamus from the olfactory placode during development, com- bined with the observation that many patients with the X-linked form of ichthyosis caused by steroid sulfatase deficiency also had deafness and hypogonadotropic hypogonadism, led to identification of the KAL gene. The latter maps at chromosomes Xp22.3, is contiguous to the steroid sulfatase gene, and codes for a protein that is homologous to the fibronectins, with an important role in neural chemotaxis and cell adhesion. Since the identification of the KAL gene, several defects have been described in patients with Kallman syndrome. Contiguous gene deletions have been found in patients with other genetic defects, such as ichthyo- sis, blindness, and/or deafness, whereas smaller dele- tions of the KAL gene are found in patients with anosmia and GnRH deficiency. These patients also demonstrate cerebellar dysfunction, oculomotor abnor- malities, and mirror movements. Mutations of the gene that cause only anosmia in some affected patients have been described, and recently, KAL gene defects were reported in few patients with isolated gonadotro- pin deficiency. Selective, idiopathic GnRH deficiency (IGD) is thought to be caused by various genetic defects that may include the GnRH gene itself. Patients with IGD and hereditary spherocytosis were recently described and are believed to have contiguous gene deletions involv- ing the 8p11-p21.1 locus. In a murine model of hypo- gonadotropic hypogonadism (the mouse), the defect was found to be caused by a deletion of the GnRH gene and was recently repaired by gene replacement therapy. 2.7. Clinical Uses of GnRH GnRH and its long-acting agonist analogs are, respec- tively, used in the treatment of GnRH deficiency, includ- ing menstrual and fertility disorders in women and hypothalamic hypogonadism in both sexes, and the treatment of central precocious puberty (CPP) in both boys and girls. Soon after the pulsatile nature of gona- dotropin secretion was characterized, the requirement for intermittent stimulation by GnRH to elicit physi- ologic pituitary responses was determined. This led to the development of long-acting GnRH analogs, which provide the means of medical castration not only in CPP, but in a variety of disorders, ranging from endometrio- sis to uterine leiomyomas and prostate cancer. GnRH antagonists are currently being developed for the treat- ment of hormone-dependent cancers, such as prostate cancer, and for potential use of a male contraceptive in combination with testosterone. GnRH is also used in clinical testing for the identifi- cation of CPP in children and the diagnosis of GnRH deficiency in all age groups. The gonadotropin response to 100 µg GnRH (intravenously [iv]) changes from an FSH-predominant response during the prepubertal years to an LH-predominant response during puberty. Signifi- cant gender differences exist in the peak hormonal val- ues attained following GnRH stimulation, and the test is used in combination with other criteria for establish- ment of the diagnosis of precocious puberty. The same test is used in adults with suspected central hypogo- nadism. The lack of LH and FSH response to 100 µg GnRH iv is compatible with GnRH deficiency or pitu- itary hypogonadism, and repeated stimulation with GnRH may be needed to distinguish patients with Kallmann syndrome or selective IGD. The GnRH stimulation test is particularly useful in testing the effi- cacy of medical castration by GnRH agonists. 3. TRH 3.1. Prepro-TRH and Its Structure TRH was the first hypothalamic-releasing factor to be isolated in 1969. Its discovery was followed by the description of GnRH, somatostatin, CRH, and GHRH, all in the early 1070s. TRH is a tripeptideamide (pGlu- His-Pro-NH 2 ), synthesized as part of a large prohor- mone termed prepro-TRH. The latter contains repeating sequences (Gln-His-Pro-Gly), the number of which [...]... 4pl6, and 4p16 for the D-1R, D-2R, D-3R, D-4R, and D-5R, respectively) and are intronless for the activating D-1R and D-5R but contain 6, 5, and 4 introns for the inhibitory D-2R, D-3R, and D-4R, respectively Posttranslational processing is extensive for the latter three receptors, resulting in a greater number of receptor isoforms The action of dopamine on pituitary PRL release is mediated through D-2R,... receptors (D2-R) Five DRs exist (D-1R–5R) and all their genes were cloned before 1991 They belong to the seven-transmembrane segment domain GPCR family and have common structural organization and some homology with serotoninergic and adrenergic receptors D-IR and D-5R activate, whereas D-2R, D-3R, and D-4R inhibit adenylate cyclase The third cytoplasmic loop is short in the former and long in the latter It... are oligopeptides with GH-releasing effects that bind to receptors different from the GHRH-R in the hypothalamus and elsewhere in the CNS The original GHRP was a synthetic, met-enkephalin-derived hexapeptide (His-D-Trp-Ala- 183 Trp-D-Phe-Lys-NH2), which was a much more potent GH secretagogue than GHRH both in vivo and in vitro When administered in large doses, GHRPs enhance ACTH and PRL release from the... systemic secretion of IL-6, which by inhibiting the other two inflammatory cytokines, tumor necrosis factor-α (TNF-α) and IL-1, and by activating the HPA axis, participates in the stress- 192 Part IV / Hypothalamic–Pituitary induced suppression of the immune inflammatory reactions Stress-associated CRH hypersecretion, and the resultant glucocorticoid- , catecholamine-, and IL-6mediated immunosuppression... that mediate both pituitary- and hormone-specific signaling Pit-1, a 33-kDa tissuespecific transcription factor, binds to specific sites on the promoter This factor is expressed in lactotropes, somatotropes, and thyrotropes and is critical for GH, PRL, and TSH-β gene transcription GH-releasing hormone (GHRH) stimulates GH transcription, and insulin-like growth factor-1 (IGF-1) inhibits GH mRNA expression... Amino acid no Normal range ACTH: 4 .5 266 (ACTH-39) ACTH, 4–22 pmol/L 22 191 . receptors differ- ent from the GHRH-R in the hypothalamus and else- where in the CNS. The original GHRP was a synthetic, met-enkephalin-derived hexapeptide (His-D-Trp-Ala- Trp-D-Phe-Lys-NH 2 ), which. of acid hypo- thalamic extracts released LH from rat anterior pituitar- ies. The structure of GnRH was elucidated in 1971. The decapeptide pyroGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg- Pro-Gly-amide was. somatostatin, and do not require iv administration. The best-studied among these analogs is octreotide (D-Phe-Cys-Phe-D-Trp- Lys-Thr-Cys-Thr[ol]), which is currently used exten- sively in neuroendocrine