Progress towards the development of non-peptide orally-active gonadotropin-releasing hormone (GnRH) antagonists: therapeutic implications

Gonadotropin-releasing hormone (GnRH) is a decapeptide (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly.NH2) which is produced from a precursor polypeptide in hypothalamic neurons and secreted in a pulsatile fashion to stimulate the secretion of LH and FSH via its interaction with a cognate receptor on gonadotropes. Low doses of the native peptide delivered in a pulsatile manner to mimic that found in the hypothalamic portal vessels restore fertility in hypogonadal patients, and are also effective in treating cryptorchidism and delayed puberty. Administration of high doses of GnRH, or agonist analogues, causes desensitization of the gonadotrope with consequent decline in gonadal gametogenesis and steroid and peptide hormone synthesis. This phenomenon finds extensive therapeutic application in clinical medicine in a wide spectrum of disease (Table 1). In addition, GnRH analogues have promise as new generation male and female contraceptives in conjunction with steroid hormone replacement. GnRH antagonists inhibit the reproductive system through competition with endogenous GnRH for the receptor and, in view of their rapid effects, are being increasingly used for the above mentioned applications. The peptide agonists and antagonists currently available require parenteral administration, typically in the form of long-acting depots. A new generation of non-peptide GnRH antagonists are beginning to emerge which should allow oral administration and, therefore, may provide greater flexibility of dosing, lower costs and increased patient acceptance.

A new photoreactive antagonist cross-links to the N-terminal domain of the gonadotropin-releasing hormone receptor

A new photoreactive gonadotropin-releasing hormone (GnRH) antagonist [Ac-(4-azidobenzoyl)-D-Lys1, D-4-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10]GnRH (PAnt-1) was synthesized and shown to bind covalently to mouse and human GnRH receptors after ultraviolet irradiation. PAnt-1 exhibited high binding affinity (Ki = 3.1 +/- 0.8 nM), and high crosslinking efficiency as shown by loss of 78% of binding sites following crosslinking at saturating concentration. Crosslinking resulted in irreversible receptor blockade as shown by inhibition of GnRH-stimulated inositol phosphate production. PAnt-1 has a photoreactive group at residue 1 of the peptide, a region believed to be critical in determining antagonist versus agonist properties of GnRH analogues. The attachment site of PAnt- to the receptor was localized between residues 11 and 19 of the extracellular N-terminal domain of the receptor by peptide mapping studies using natural sequence differences between human, mouse and sheep GnRH receptors, as well as a panel of GnRH receptor constructs with a series of engineered protease cleavage sites. A disulphide bridge between Cys14 and Cys200 was cleaved during crosslinking, suggesting that Cys14 is the crosslinked residue. These results suggest that peptide GnRH antagonists bind to the receptor with the N-terminal end of the peptide positioned in a site comprising the constrained regions of the N-terminal domain and second extracellular loop in the vicinity of the Cys14-Cys200 disulphide bridge.

The functional microdomain in transmembrane helices 2 and 7 regulates expression, activation, and coupling pathways of the gonadotropin-releasing hormone receptor

Structural microdomains of G protein-coupled receptors (GPCRs) consist of spatially related side chains that mediate discrete functions. The conserved helix 2/helix 7 microdomain was identified because the gonadotropin-releasing hormone (GnRH) receptor appears to have interchanged the Asp(2.50) and Asn(7.49) residues which are conserved in transmembrane helices 2 and 7 of rhodopsin-like GPCRs. We now demonstrate that different side chains of this microdomain contribute specifically to receptor expression, heterotrimeric G protein-, and small G protein-mediated signaling. An Asn residue is required in position 2.50(87) for expression of the GnRH receptor at the cell surface, most likely through an interaction with the conserved Asn(1.50(53)) residue, which we also find is required for receptor expression. Most GPCRs require an Asp side chain at either the helix 2 or helix 7 locus of the microdomain for coupling to heterotrimeric G proteins, but the GnRH receptor has transferred the requirement for an acidic residue from helix 2 to 7. However, the presence of Asp at the helix 7 locus precludes small G protein-dependent coupling to phospholipase D. These results implicate specific components of the helix 2/helix 7 microdomain in receptor expression and in determining the ability of the receptor to adopt distinct activated conformations that are optimal for interaction with heterotrimeric and small G proteins.

Two gonadotropin-releasing hormone receptor subtypes with distinct ligand selectivity and differential distribution in brain and pituitary in the goldfish (Carassius auratus)

In the goldfish (Carassius auratus) the two endogenous forms of gonadotropin-releasing hormone (GnRH), namely chicken GnRH II ([His5, Trp7,Tyr8]GnRH) and salmon GnRH ([Trp7,Leu8]GnRH), stimulate the release of both gonadotropins and growth hormone from the pituitary. This control is thought to occur by means of the stimulation of distinct GnRH receptors. These receptors can be distinguished on the basis of differential gonadotropin and growth hormone releasing activities of naturally occurring GnRHs and GnRHs with variant amino acids in position 8. We have cloned the cDNAs of two GnRH receptors, GfA and GfB, from goldfish brain and pituitary. Although the receptors share 71% identity, there are marked differences in their ligand selectivity. Both receptors are expressed in the pituitary but are differentially expressed in the brain, ovary, and liver. Thus we have found and cloned two full-length cDNAs that appear to correspond to different forms of GnRH receptor, with distinct pharmacological characteristics and tissue distribution, in a single species.

Evidence for differential regulation of multiple transcripts of the gonadotropin releasing hormone receptor in the ovine pituitary gland; effect of estrogen

The number of pituitary gonadotropin-releasing hormone receptors (GnRH-R) varies across the estrous cycle. We report that there is variable expression of the differently-sized GnRH-R transcripts in cyclic ewes and in an experimental model. During the follicular phase of the cycle, and compared to the luteal phase, there was increased expression of the 1.5, 2.3 and 3.7 kilobase (kb) transcripts with no change in the levels of the 5.6 or the 1.2 kb transcripts. Steady state levels of mRNA for luteinising hormone beta and common alpha subunit were also increased in the follicular phase of the cycle. In hypothalamo-pituitary disconnected ovariectomised ewes given pulsatile GnRH replacement, injection of estrogen increased the 1.5, 2.3 and 3.7 kb, while the levels of the 5.6 and 1.2 kb transcripts were not altered. We conclude that the differential regulation of GnRH-R mRNA occurs through a direct effect of E on the pituitary.

A single amino acid substitution in transmembrane helix VI results in overexpression of the human GnRH receptor

OBJECTIVE

Construction of constitutively active mutants of the GnRH receptor, a member of the G-protein coupled receptor superfamily, would facilitate investigation of the mechanism of receptor activation.

DESIGN

Point mutations were introduced in the human GnRH receptor in positions corresponding to those which caused constitutive activity in other G-protein coupled receptors. The effects of these mutations on ligand binding, receptor intracellular signaling and receptor expression were determined.

METHODS

Wild type and mutated receptor cDNAs were expressed in COS-1 cells. Basal and agonist-stimulated inositol phosphate production and ligand binding were determined. In addition, receptor mRNA levels, cell surface receptor stability and rate of internalization were measured.

RESULTS AND CONCLUSIONS

Although none of the mutant receptors exhibited constitutive activity, mutation of Phe-2 72 in transmembrane helix VI to Leu increased cell surface receptor numbers, with unchanged affinities for radiolabeled agonist, superagonist and antagonist peptides compared with wild type receptor. The cell surface receptor stability and rate of internalization were similar for wild type and F272L GnRH receptors. Thus a single amino acid mutation in transmembrane helix VI causes an increase in cell surface receptor numbers, which appears to result from an increased rate of receptor protein translation, processing or insertion into membranes.

A high affinity gonadotropin-releasing hormone (GnRH) tracer, radioiodinated at position 6, facilitates analysis of mutant GnRH receptors

Cloning of GnRH receptors from several animal species has made it possible to investigate receptor function using site-directed mutagenesis. However, many mutant GnRH receptors exhibit decreased ligand binding, which makes analysis of their ligand binding characteristics technically difficult. To increase the affinity of binding to the GnRH receptor, a novel tracer ligand, 125I-[His5,D-Tyr6]GnRH, was designed and synthesized to allow radioiodination at position 6 rather than the usual position 5. In competition binding assays, total binding of 125I-[His5,D-Tyr6]GnRH was higher than binding of a conventional tracer ligand, 125I-[D-Ala6,N-MeLeu7,Pro9NHEt]GnRH. The bindable fractions and specific activities of both peptides were similar, and the receptor binding affinities of the unlabeled peptides were indistinguishable. However, comparison of the radiolabeled peptides in saturation binding assays showed that the affinity of the peptide, 125I-[His5,D-Tyr6]GnRH, (Kd, 0.19 nM), was approximately 2-fold higher than that of the conventional tracer. The increased binding of 125I-[His5,D-Tyr6] GnRH has allowed the development of a sensitive GnRH receptor binding assay for analysis of mutant GnRH receptors that exhibit decreased ligand binding.

Cloning and characterization of the chicken thyrotropin-releasing hormone receptor

We report on the cloning of the full-length complementary DNA for the chicken TRH receptor. Although the TRH receptor has been cloned from several mammalian species, this is the first report from another vertebrate class. The ligand binding pocket, which is situated in the transmembrane helices of the mouse and rat TRH receptors, is completely conserved in the chicken receptor. Pharmacological studies (receptor binding and signaling) employing several TRH analogs revealed that there are no significant differences between the chicken and mouse receptors. These findings show that there have been considerable evolutionary constraints on TRH receptor structure and function. Several truncated forms of the chicken TRH receptor that appear to retain a part of an intron and are truncated in the putative third intracellular loop were also cloned, but were nonfunctional. This study provides a useful tool for further studies on the roles of TRH in avian growth and TSH regulation.

Alanine-261 in intracellular loop III of the human gonadotropin-releasing hormone receptor is crucial for G-protein coupling and receptor internalization

Gonadotropin-releasing hormone (GnRH) is a decapeptide that regulates reproductive function via binding to the GnRH receptor, which is a G-protein-coupled receptor (GPCR). For several members of this family, the C-terminal domain of intracellular loop III is important in ligand-mediated coupling to G-proteins; mutations in that region can lead to constitutive activity. A specific alanine residue is involved in certain GPCRs, the equivalent of which is Ala-261 in the GnRH receptor. Mutation of this residue to Leu, Ile, Lys, Glu or Phe in the human GnRH receptor did not result in constitutive activity and instead led to complete uncoupling of the receptor (failure to support GnRH-stimulated inositol phosphate production). When this residue was mutated to Gly, Pro, Ser or Val, inositol phosphate production was still supported. All the mutants retained the ability to bind ligand, and the affinity for ligand, where measured, was unchanged. These results show that Ala-261 cannot be involved in ligand binding but is critical for coupling of the receptor to its cognate G-protein. Coupling is also dependent on the size of the residue in position 261. When the amino acid side chain has a molecular mass of less than 40 Da efficient coupling is still possible, but when its molecular mass exceeds 50 Da the receptor is uncoupled. Internalization studies on the Ala261-->Lys mutant showed a marked decrease in receptor internalization compared with the wild type, indicating that coupling is necessary for effective receptor internalization in the GnRH receptor system. Activation of protein kinase C (with PMA), but not protein kinase A (with forskolin) markedly increased the internalization of the mutant receptor while having a small effect on the wild-type receptor.

Functional microdomains in G-protein-coupled receptors. The conserved arginine-cage motif in the gonadotropin-releasing hormone receptor

An Arg present in the third transmembrane domain of all rhodopsin-like G-protein-coupled receptors is required for efficient signal transduction. Mutation of this Arg in the gonadotropin-releasing hormone receptor to Gln, His, or Lys abolished or severely impaired agonist-stimulated inositol phosphate generation, consistent with Arg having a role in receptor activation. To investigate the contribution of the surrounding structural domain in the actions of the conserved Arg, an integrated microdomain modeling and mutagenesis approach has been utilized. Two conserved residues that constrain the Arg side chain to a limited number of conformations have been identified. In the inactive wild-type receptor, the Arg side chain is proposed to form an ionic interaction with Asp3.49(138). Experimental results for the Asp3. 49(138) --> Asn mutant receptor show a modestly enhanced receptor efficiency, consistent with the hypothesis that weakening the Asp3. 49(138)-Arg3.50(139) interaction by protonation of the Asp or by the mutation to Asn favors activation. With activation, the Asp3. 49(138)-Arg3.50(139) ionic bond would break, and the unrestrained Arg would be prevented from orienting itself toward the water phase by a steric clash with Ile3.54(143). The mutation Ile3.54(143) --> Ala, which eliminates this clash in simulations, causes a marked reduction in measured receptor signaling efficiency, implying that solvation of Arg3.50(139) prevents it from functioning in the activation of the receptor. These data are consistent with residues Asp3.49(138) and Ile3.54(143) forming a structural motif, which helps position Arg in its appropriate inactive and active receptor conformations.

Neonatal immunization against gonadotropin-releasing hormone (GnRH) results in diminished GnRH secretion in adulthood

The effects of neonatal immunization against GnRH were studied in sheep after they had reached adulthood (3-4 yr) and the antibody titers had fallen to undetectable levels. The immunized animals had small gonads, and the females did not have large follicles (>3 mm) or corpora lutea in their ovaries. Compared with controls, the immunized animals had low or nondetectable levels of LH and FSH in peripheral plasma, and the immunized animals generally failed to respond to a single i.v. GnRH challenge. After ovariectomy, the control ewes, but not the immunized ewes, showed an elevation in plasma LH and FSH levels. The sampling of hypophysial portal blood, with a newly described method, showed that the secretion of GnRH was reduced in the immunized animals, but the amount of GnRH in the median eminence was similar in the control and immunized ewes. The pituitary content of LH and FSH was reduced in the immunized ewes as was messenger RNA for the gonadotropin subunits and the GnRH receptor. These data indicate that neonatal immunization does not affect the synthesis of GnRH in adulthood but reduces the secretion of GnRH, causing long-term sterility in these animals.

Contrasting internalization kinetics of human and chicken gonadotropin-releasing hormone receptors mediated by C-terminal tail

The chicken gonadotropin-releasing hormone receptor (GnRH-R) is notable for having a cytoplasmic C-terminal tail, which is not present in the mammalian GnRH-Rs. We report here that the cytoplasmic tail mediates rapid agonist-promoted receptor internalization. The chicken GnRH-R mediated internalization of gonadotropin-releasing hormone (GnRH) agonist (125I[His5-D-Tyr6]GnRH) at a rate of 11.3%.min-1, compared with only 0.71 %.min-1 for the human GnRH-R. To determine whether the presence of the cytoplasmic tail was responsible for the more rapid internalization kinetics of the chicken GnRH-R we truncated the tail after the Ile336 residue (S337stop). Receptor-mediated internalization of GnRH agonist by the S337stop-chicken GnRH-R was much slower than in the wild-type chicken receptor, and was similar to the wild-type human GnRH-R (0.55 %.min-1). These data indicate that rapid agonist-promoted internalization of the chicken GnRH-R is mediated through elements in the cytoplasmic C-terminal tail, distal to or including Ser337 and suggests that elimination of the C-terminal tail during evolution of mammalian GnRH-Rs may be related to its effects on internalization.

A second form of gonadotropin-releasing hormone (GnRH) with characteristics of chicken GnRH-II is present in the primate brain

The primate brain was thought to contain only the GnRH known as mammalian GnRH (mGnRH). This study investigates whether a second form of GnRH exists within the primate brain. We found that brain extracts from adult stumptail and rhesus monkeys contained two forms of GnRH that were similar to mGnRH and chicken GnRH-II (cGnRH-II) based on the elution position of the peptides from HPLC and on cross-reactivity with antisera that are specific to mammalian or chicken GnRH-II in RIAs. The fetal brain of rhesus monkeys also contained mGnRH and a cGnRH-II-like peptide by the same criteria. Immunocytochemistry with a cGnRH-II-specific antiserum in adult and fetal rhesus monkeys showed immunopositive neurons generally scattered in the periaqueductal region of the midbrain, with a few positive cells in the posterior basal hypothalamus. Neurons immunopositive for cGnRH-II were fewer in number and smaller in size, with less defined nuclei and thinner neurites compared with those for mGnRH. Administration of synthetic cGnRH-II to adult rhesus monkeys resulted in a significant increase in the plasma LH concentration during the luteal phase of the menstrual cycle, but not during the midfollicular phase. We conclude that the primate brain contains mGnRH and a cGnRH-II-like molecule, although the function of the latter is unknown.

Behavioral regulation of gonadotropin-releasing hormone production

In vertebrates reproductive readiness requires coordination between the sexes. Behavioral interactions with potential mates can initiate the neuroendocrine events that are required for successful copulation, ovulation, and fertilization. Regardless of the efferent pathway used, their targets are the neurons that produce and secrete gonadotropin-releasing hormone (GnRH). Several excellent animal models are currently under use to study the relationship between behavior and GnRH. In the musk shrew (Suncus murinus) starting 15 h after mating, prior to ovulation, GnRH-ir cell numbers are elevated along with GnRH content in brain and estradiol in plasma. Immunoreactive GnRH cell numbers also change in brains of female musk shrews sacrificed during, and directly after, brief interactions with males. These rapid changes in GnRH-ir cells are not correlated with measurable increases in GnRH content or elevations in plasma concentrations of estradiol. To determine which aspect(s) of the behavioral interaction is salient for the change in GnRH-ir, studies have been conducted in which interactions with males and their sensory cues were restricted during a 1-h interaction. In this study, behavioral interactions with an awake male behind a screen barrier resulted in a decrease in the numbers of GnRH-ir cells in the forebrain. Further studies with this animal model will help determine how behavioral inputs stimulate processing and release of GnRH.

Absence of mutations in exon 3 of the GnRH receptor in human gonadotroph adenomas

BACKGROUND AND OBJECTIVE

The mechanisms of tumourogenesis for the majority of pituitary tumours are unknown. Mutations of G-protein coupled receptors (GPCRs) have recently been described as important in diverse human diseases, including thyroid adenomas. To test this hypothesis in pituitary gonadotroph adenomas, we amplified and sequenced the GnRH receptor gene in 12 human tumours. We restricted our analysis to the third exon, since this represents the hotspot for activating mutations in other GPCRs.

PATIENTS

Pituitary adenoma tissue was identified from patients who had tumours resected and where a diagnosis of gonadotroph adenoma had been made on the basis of immunohistochemical demonstration of LH and/or FSH.

METHODS

Genomic DNA was extracted from paraffin-embedded tissue of 18 gonadotroph adenomas. The third exon was successfully amplified by PCR in 12 cases and directly sequenced.

RESULTS

We found no missense point mutations or even silent polymorphisms in any tumour studied.

CONCLUSION

We conclude that activating mutations of the GnRH receptor gene do not represent an important mechanism of pituitary gonadotroph tumourogenesis.

Testosterone acts directly at the pituitary to regulate gonadotropin-releasing hormone-induced calcium signals in male rat gonadotropes

We have recently shown that castration alters GnRH-induced calcium (Ca2+) signaling in the gonadotropes of male rats. Instead of generating spike-plateau Ca2+ responses to high concentrations of GnRH (100 nM), the majority of gonadotropes from castrated rats have oscillatory Ca2+ responses, which are generally only seen with low concentrations of GnRH in the gonadotropes of intact rats. This change in the nature of GnRH-induced Ca2+ responses is prevented by in vivo testosterone treatment. The aims of the present study were, therefore, to determine if testosterone acts directly at the pituitary or via the regulation of hypothalamic GnRH secretion. Accordingly, castrated male rats were treated with a GnRH antagonist to ablate the effects of increased GnRH secretion at the pituitary gland. GnRH antagonist treatment (10 microg/100 g BW, twice daily for 7 days from the time of castration) decreased the concentration of LH in the serum of castrated rats (0.4 +/- 0.1 ng/ml vs. 11.2 +/- 0.4 ng/ml in untreated castrated rats, mean +/- SEM) but had no effect on the proportion of gonadotropes having oscillatory Ca2+ responses to 100 nM GnRH when compared with untreated castrated rats (63% in antagonist-treated castrated rats vs. 70% in untreated castrated rats). The GnRH antagonist treatment did not, however, interfere with the ability of in vivo testosterone treatment (100 microg/100 g body weight/day) to decrease the proportion of gonadotropes having oscillatory Ca2+ responses to 100 nM GnRH (26% in testosterone-treated rats vs. 25% in testosterone and antagonist-treated rats). These findings indicate that testosterone acts directly at the pituitary, and not by altered GnRH secretion, to modulate GnRH-induced Ca2+ signals. To confirm this suggestion, cultured gonadotropes of castrated male rats were treated in vitro with 10 nM testosterone. Testosterone treatment for twelve, but not 4 h, restored the proportion of gonadotropes having oscillatory Ca2+ responses to that seen in gonadotropes from intact rats. The in vitro effects of testosterone over 12 h were prevented by concomitant treatment with the protein synthesis inhibitor cycloheximide (10 microM), which, when given alone, had no effect on GnRH-induced Ca2+ signals in cells from castrate male rats. Taken together, these findings suggest that testosterone has a direct genomic action at the pituitary to regulate GnRH-induced Ca2+ signals, via a process that involves new protein synthesis.

Worldwide frequency of a common genetic variant of luteinizing hormone: an international collaborative research. International Collaborative Research Group

OBJECTIVE

To determine the worldwide frequency of a common immunological LH variant because of two point mutations in the LH beta-subunit gene (Trp8Arg and Ile15Thr).

DESIGN

Cross-sectional study on LH status (variant and wild-type) in serum (or DNA) samples from Finland (Finns and Lapps), Estonia, Poland, Sweden, The Netherlands, United Kingdom, Italy, South Africa (blacks), Thailand, China, Japan, and the United States (Hispanics and blacks).

SETTING

Academic research environment.

PATIENT(S)

Ambulatory adult men and women (n = 2,936) with minor illnesses and no known endocrinological disorders.

INTERVENTION

A single blood sample was collected from each subject.

MAIN OUTCOME MEASURE(S)

The LH status was determined by two immunofluorometric assays using monoclonal antibodies. One (assay 1) only recognizes the wild-type LH, the other (assay 2) recognizes equally variant and wild-type LH. The ratio of assay 1 to assay 2 indicates the LH status: wild-type, > 0.9; heterozygote, 0.2 to 0.9; and homozygote, < 0.15. One population (Lapps) was studied by DNA analysis using polymerase chain reaction and allele-specific oligonucleotide hybridization.

RESULT(S)

The carrier frequency of the variant LH beta allele varied from 7.1% in U.S. Hispanics to 41.9% in Lapps of northern Finland. The variant LH beta allele tended to be more common in populations from Northern Europe as compared with those from Asia.

CONCLUSION(S)

The high frequency of the LH beta variant worldwide makes it an important confounding factor when obtaining disproportionately low LH levels with some immunometric assays. The LH variant may contribute to some pathologies of the pituitary-gonadal function.

Chicken GnRH II-like peptides and a GnRH receptor selective for chicken GnRH II in amphibian sympathetic ganglia

Amphibia, like most vertebrate species, have two forms of GnRH, namely [Arg8]GnRH (mammalian GnRH) and [His5,Trp7,Tyr8] GnRH (chicken GnRH II). The differential distribution of the two peptides in the amphibian brain suggests that they may play different roles. Mammalian GnRH, which is found predominantly in the hypothalamus, is most likely the prime regulator of gonadotropin release, while chicken GnRH II, which occurs predominantly in the midbrain and hindbrain, may play a neuromodulatory role. In amphibian sympathetic ganglia, GnRH has been demonstrated to be a neurotransmitter where its release from the presynaptic nerve terminals reversibly inhibits M current, a time- and voltage-dependent potassium current. The occurrence of GnRH in sympathetic ganglia extracts from two amphibian species was investigated. Chicken GnRH II-like immunoreactivity was detected in extracts of bullfrog (Rana catesbeiana) and platanna (Xenopus laevis) sympathetic ganglia after high performance liquid chromatography. Under the chromatographic conditions used, a second unknown peptide co-eluted with synthetic mammalian GnRH, but showed no cross-reactivity with specific mammalian GnRH antisera. To test the possibility of the presence of a chicken GnRH II receptor in sympathetic ganglion neurones, competition binding of membranes extracted from the sympathetic ganglia of the two amphibian species was investigated with 125I-labelled GnRH agonists. The binding of 125-I-[His5,D-Arg6,Trp7,Tyr8]GnRH (a chicken GnRH II agonist) to membranes from the sympathetic ganglia of both amphibian species was specific and had a higher affinity than chicken GnRH II, mammalian GnRH and a mammalian GnRH agonist [D-Ala6,NMe-Leu7,Pro9-NHEt]GnRH. These findings suggest that endogenous chicken GnRH II may play a role in synaptic transmission in the sympathetic ganglia via a receptor specific for chicken GnRH II.

Advances in understanding gonadotrophin-releasing hormone receptor structure and ligand interactions

Gonadotrophin-releasing hormone (GnRH) is the central regulator of the reproductive system and its analogues are used widely in the treatment of diverse diseases. The GnRH receptor is a member of the large family of G-protein-coupled receptors (GPCRs) which have seven transmembrane domains. Knowledge of these receptors has assisted the development of molecular models of the GnRH receptor that allow prediction of its three-dimensional configuration and the way GnRH binds and activates its receptor. Comparison with other GPCRs led to the discovery that Lys121, in the third transmembrane domain, has a role in agonist binding. The history of GnRH structure-activity studies has allowed the identification of an acidic residue in the third extracellular loop of the receptor that is required for binding of mammalian GnRH, while synthetic GnRH analogues have showed that Asn102, in the second extracellular loop, may interact with the carboxy-terminus of GnRH. These residues can now be incorporated into the receptor models that are being used to design orally active non-peptide GnRH analogues for contraception and treatment of a variety of reproductive disorders.

Two populations of luteinizing hormone-releasing hormone neurons in the forebrain of the rhesus macaque during embryonic development

To investigate the possibility that a second luteinizing hormone-releasing hormone (LHRH) population appears during development in primates, embryos and fetal brains of rhesus monkeys were immunostained with antisera specific to different LHRH forms. Two LHRH cell populations were discernible by immunoreactivity to antisera LR-1 and GF-6. Because one LHRH cell type migrated out from the olfactory placode several days earlier than the other, they were referred to as "early" and "late" LHRH cells, respectively. Although late LHRH neurons were immunoreactive to all anti-mammalian LHRH antisera tested, early LHRH neurons were only detected by antiserum GF-6. Early LHRH neurons (approximately 10 x 7 microm) were smaller than late LHRH neurons (approximately 18 x 7 microm). Early LHRH neurons were first found around the olfactory placode, in the nasal mesenchyme, and in the rostroventral forebrain on embryonic day 30 (E30), whereas late LHRH neurons were first seen in the olfactory pit on E32. Early LHRH cells were located throughout the basal forebrain on E32-E42, whereas late LHRH cells were found in the olfactory pit and along the terminal nerve on E34-E36 and were not seen in the forebrain until E38. By E51-E62, late LHRH neurons reached into the basal hypothalamus in a distribution resembling that in the older brain, while early LHRH neurons were found in the septum, preoptic region, stria terminalis, medial amygdala, claustrum, internal capsule, and globus pallidus. Based on the distribution pattern of immunopositive cells with antiserum LR-1, late LHRH cells are bona fide LHRH neurons that regulate the pituitary-gonadal axis. In contrast, the molecular form of early LHRH cells is unclear, although it is plausible that early LHRH cells may contain the molecule in which the C-terminal epitope of LHRH is modified or absent. It is concluded that in primates there is a second population of LHRH neurons that originates from the embryonic olfactory placode before the origin of mammalian LHRH-like neurons, and that these two populations of LHRH-immunopositive neurons have different morphologic features and different final distributions in the brain.

Immunocytochemical localization of GnRH precursor in the hypothalamus of European starlings during sexual maturation and photorefractoriness

Immunocytochemistry with quantitative image analysis, for both GnRH and its precursor proGnRH-GAP, was used in male European starlings (Sturnus vulgaris) to investigate four stages of a photoperiodically-induced reproductive cycle. Four different groups of birds were examined: photosensitive buy sexually immature, sexually mature, undergoing gonadal regression, and after the completion of regression and fully photorefractory. The size of cells staining for GnRH and proGnRH-GAP increased during gonadal maturation. A reduction in the number of cells staining for GnRH and the size of cells staining for both GnRH and proGnRH-GAP occurred during gonadal regression, though staining for GnRH and proGnRH-GAP in the median eminence remained high at this stage. Birds examined after completion of regression showed significantly reduced staining for both GnRH and its precursor. These observations suggest that photorefractoriness is promoted by a reduction in proGnRH-GAP production and in GnRH synthesis, rather than requiring inhibition of release of GnRH at the median eminence.

Asn102 of the gonadotropin-releasing hormone receptor is a critical determinant of potency for agonists containing C-terminal glycinamide

We demonstrate a critical role for Asn102 of the human gonadotropin-releasing hormone (GnRH) receptor in the binding of GnRH. Mutation of Asn102, located at the top of the second transmembrane helix, to Ala resulted in a 225-fold loss of potency for GnRH. Eight GnRH analogs, all containing glycinamide C termini like GnRH, showed similar losses of potency between 95- and 750-fold for the [Ala102]GnRHR, compared with wild-type receptor. In contrast, four GnRH analogs that had ethylamide in place of the C-terminal glycinamide residue, showed much smaller decreases in potency between 2.4- and 11-fold. In comparisons of three agonist pairs, differing only at the C terminus, glycinamide derivatives showed an 11-20-fold greater loss of potency for the mutant receptor than their respective ethylamide derivatives. Thus Asn102 is a critical determinant of potency specifically for ligands with C-terminal glycinamide, while ligands with C-terminal ethylamide are less dependent on Asn102. These findings indicate a role for Asn102 in the docking of the glycinamide C terminus and are consistent with hydrogen bonding of the Asn102 side chain with the C-terminal amide moiety. Taken with previous data, they suggest a region of the GnRH receptor formed by the top of helices 2 and 7 as a binding pocket for the C-terminal part of the ligand.

Incorporation of an additional glycosylation site enhances expression of functional human gonadotropin-releasing hormone receptor

Mutation ofN-glycosylation sites in the mouse gonadotropin-releasing hormone receptor was previously shown to impair its expression in COS-1 cells. We therefore investigated the effects of adding an extra glycosylation site to the human gonadotropin-releasing hormone receptor, as a means for increasing its expression. Covalent labeling of the mutant receptor expressed in COS-1 cells with a gonadotropin-releasing hormone (GnRH) photoreactive analog demonstrated a shift in apparent molecular weight, indicating that the new site was in fact glycosylated. The receptor with extra glycosylation site displayed normal binding affinities for agonists buserelin and [D: -Ala(6)-Pro(9)-NHEt]-GnRH, and the antagonist antide, and a slightly increased affinity for GnRH. Receptor number was increased by 1.7-fold in membrane preparations from cells expressing the mutant receptor, compared with wild-type. Photoaffinity labeling of cell-surface receptors in intact cells demonstrated a 1.8-fold increase in binding sites on the cell surface. The GnRH receptor (GnRHR) with extra glycosylation site conferred a markedly enhanced signaling response to agonist. Dose-response curves for GnRH-stimulated inositol phosphate production were left-shifted by an average of 4.4-fold, and maximal inositol phosphate responses were increased by 1.2 fold, in cells transfected with mutant compared with wild-type receptor, indicating that the increase in binding sites represented functional receptors. These results demonstrate that addition of an extra glycosylation site enhances expression of the human GnRHR, a strategy that may be applicable to other cell-surface receptors.

The rate of CpG mutation in Alu repetitive elements within the p53 tumor suppressor gene in the primate germline

Cytosine to thymine transition mutations at the CpG dinucleotide are the most common point mutations in cancer and genetic disease. We calculated the in vivo rate of CpG mutation in the primate germline by deriving a primordial consensus sequence for an Alu repetitive element which inserted into intron 6 of the primate p53 gene 35 to 55 million years ago. Comparison of this primordial sequence to the Alu sequence in intron 6 of present-day primates was used to determine the nature and rate of mutations which occurred during evolution. We estimate the half-life of a CpG nucleotide to be 24 to 60 million years, and the rate constant for mutation at this dinucleotide to be 1.2 x 1O(-8) to 2.9 x 1O(-8) years(-1). These results were confirmed by the analysis of a second Alu sequence in intron 10 of the p53 gene. The in vivo mutation rate is at least 1250-fold slower than the in vitro chemical rate of 5-methylcytosine deamination in double-stranded DNA, showing that current estimates of CpG mutation repair have been significantly underestimated. Furthermore, the mutability of the CpG dinucleotide has led to the depletion of this dinucleotide from the vertebrate genome, and calculations in this study suggest that current levels of the CpG dinucleotide in the primate genome are very close to a steady state equilibrium in which the rate of CpG mutation is equal to the rate of CpG formation by random mutation.

Development of methods for purification of membrane associated gonadotropin-releasing hormone binding proteins

Gonadotropin-releasing hormone (GnRH) is the primary regulator of mammalian reproduction. It stimulates the release of luteinizing hormone and follicle stimulating hormone via receptors on the cell membranes of pituitary gonadotrope cells. This paper describes the development of a protocol for purification of GnRH binding proteins from sheep pituitary membranes. Membranes were best solubilized using a zwitterionic detergent. Solubilized membranes were applied to an affinity column prepared with a GnRH analogue. The most effective analogue was the agonist [D-Lys6,Pro9-NHEt]-GnRH. The column was washed with a gradient of sodium chloride up to 0.4 M and GnRH binding activity was eluted from the column using the acidic buffer. Eluted fractions bound labelled GnRH agonist after neutralization of the buffer. Sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis revealed a major protein band with a relative molecular weight of 67 kD. Amino acid sequence analysis showed that the protein is different from the cloned GnRH receptor, but homologous with a similar protein recently purified from bovine pituitary. This protein may have a function which is modulated by binding of GnRH, GnRH fragments or GnRH-related peptides.

A locus of the gonadotropin-releasing hormone receptor that differentiates agonist and antagonist binding sites

The decapeptide gonadotropin-releasing hormone controls reproductive function via interaction with a heptahelical G protein-coupled receptor. Because of molecular model of the receptor predicts that Lys121 in the third transmembrane helix contributes to the binding pocket, the function of this side chain was studied by site-directed mutagenesis. Substitution of Arg at this position preserved high affinity agonist binding, whereas Gln at this position reduced binding below the limits of detection. Leu and Asp at this locus abolished both binding and detectable signal transduction. The EC50 of concentration-response curves for coupling to phosphatidyl inositol hydrolysis obtained with the Gln121 receptor was more than 3 orders of magnitude higher than that obtained for the wild-type receptor. In order to determine whether the increased EC50 obtained with this mutant reflects an altered receptor affinity, the effect of decreases in wild-type receptor density on concentration-response curves was determined by irreversible antagonism. Progressively decreasing the concentration of the wild-type receptor increased the EC50 values obtained to a maximal level of 2.4 +/- 0.2 nM. Comparison of this value with the EC50 of 282 +/- 52 nM observed with the Gln121 receptor mutant indicates that the agonist affinity for this mutant is reduced more than 100-fold. In contrast, antagonist had comparable high affinities for the wild-type, Arg121, and Gln121 mutants. The results indicate that a charge-strengthened hydrogen bond donor is required at this locus for high affinity agonist binding but not for high affinity antagonist binding.

Localization and characterization of gonadotropin-releasing hormones in the brain, gonads, and plasma of a dipnoi (lungfish, Protopterus annectens)

Two molecular forms of GnRH (chicken GnRH II and a second variant) are present in the brains of species from all the major vertebrate groups. Their differential distribution in the brain and temporal expression during development suggests that have different functional roles. We investigated the nature of GnRH molecular forms in the brain, plasma, testis, and ovary of adult and juvenile lungfish (Protopterus annectens), using high performance liquid chromatography and radioimmunoassay with specific GnRH antisera. In the brain of adult and juvenile lungfish, two peptides with identical chromatographic and immunologic properties to mammalian GnRH and chicken GnRH II were detected. Chicken GnRH II predominated in both the adult and juvenile brain, and the percentage of chicken GnRH II relative to mammalian GnRH was greater in the juvenile brain. In the plasma, only mammalian GnRH was present. Immunoreactive GnRH was not detected in the testis and ovary. Chicken GnRH II and mammalian GnRH were found in the cells of the preoptic nucleus and in the ganglion of the nervus terminalis. Fibers were seen in the ventral hypothalamus, and chicken GnRH II immunoreactivity was detected within the neural lobe of the pituitary. The finding of chicken GnRH II in a sarcopterygian fish adds further support to our hypothesis that this ubiquitous structural variant is highly conserved and likely to have an important functional role. Mammalian GnRH, previously described in several early-evolved actinopterygian fish, also has a fairly widespread distribution and early evolutionary origin. The immunocytochemical distribution of mammalian GnRH and chicken GnRH II fibers in the lungfish brain suggests that both forms are hypophysiotropic. In addition, the presence of mammalian GnRH in the plasma of the lungfish suggests that this molecular form of GnRH has a hypophysiotropic function reaching target organs (pituitary and gonads) via the general circulation.

Differential regulation of the two forms of gonadotropin-releasing hormone (mGnRH and cGnRH-II) by sex steroids in the European female silver eel (Anguilla anguilla)

The effect of steroids on the two gonadotropin-releasing hormone (GnRH) forms present in the eel (mammalian GnRH, mGnRH and chicken GnRH-II, cGnRH-II), as well as on gonadotropin (GTH), was studied using specific radioimmunoassays. Female silver eels received chronic treatments with various steroids (estradiol, testosterone, androstenedione, 5 alpha-androstane-3 beta, 17 beta-diol). Estradiol or the combination of estradiol and androgens induced increases in brain and pituitary mGnRH levels and pituitary GTH level, whereas androgens given alone had no significant effect. In contrast, androgens or their combination with estradiol reduced brain cGnRH-II levels (this form remaining undetectable in the pituitary), estradiol given alone having no significant effect. This work demonstrates that the two forms of GnRH undergo a differential regulation by steroids, with a positive estrogen-dependent feedback on mGnRH (as well as on GTH) and a negative androgen-dependent feedback on cGnRH-II. These data are in agreement with previous results obtained in experimentally matured female eels (induced by a gonadotropic treatment which stimulates the production of both estrogens and androgens) showing increases in mGnRH and GTH levels, as well as a decrease in cGnRH-II [1]. The positive feedback of steroids on the mGnRH-GTH axis adds credence to the hypothesis according to which mGnRH would be the main form involved in the control of the gonadotropic function. This positive feedback would play an important role, amplifying pubertal stimulation of the gonadotropic axis, in this fish species.

Agonist activity of mammalian gonadotropin-releasing antagonists in chicken gonadotropes reflects marked differences in vertebrate gonadotropin-releasing receptors

The pharmacology of mammalian and avian gonadotropin-releasing (GnRH) receptors differs for agonist analogues. We have therefore compared the activities of mammalian-based GnRH antagonists in sheep and chicken gonadotropes to further elucidate the different structural requirements of the receptors. The antagonist activities of ten GnRH analogues were compared in cultured sheep and chicken pituitary cells by determining the dose required to cause a 50% inhibition of luteinizing hormone secretion (IC50) induced by GnRH at its half-maximal concentration (EC50). Nine analogues showed high antagonist activity in the sheep bioassay. Analogue IC50s varied between half and twice ((1.22-6.06) x 10(-10) M) the GnRH EC50 (3 x 10(-10) M). One of these peptides exhibited partial agonist activity. In contrast, eight of the analogues showed low antagonist activity in chicken pituitary cells, with IC50s varying from 46 to 1490 times ((1.4-44.7) x 10(-7) M) the GnRH EC50 (3 x 10(-9) M) and had a different order of potencies compared with that in the sheep. Furthermore, two analogues did not display antagonist activity at all in the chicken bioassay, but acted as pure agonists, stimulating LH secretion. These findings demonstrate marked differences in pharmacology between the avian and mammalian pituitary GnRH receptors and emphasize that GnRH antagonists, selected for their efficacy in mammals, cannot necessarily be used for physiological studies in non-mammalian vertebrates. The distinctly different pharmacology of the receptors and structural requirements of analogues for agonist/antagonist activity establish a basis for identifying receptor features involved in ligand-induced signal propagation using chimaeras of cloned sheep and chicken receptors.

Evolutionary aspects of gonadotropin-releasing hormone and its receptor

1. Gonadotropin-releasing hormone (GnRH) was originally isolated as a hypothalamic peptide hormone that regulates the reproductive system by stimulating the release of gonadotropins from the anterior pituitary. However, during evolution the peptide was subject to gene duplication and structural changes, and multiple molecular forms have evolved. 2. Eight variants of GnRH are known, and at least two different forms are expressed in species from all vertebrate classes: chicken GnRH II and a second, unique, GnRH isoform. 3. The peptide has been recruited during evolution for diverse regulatory functions: as a neurotransmitter in the central and sympathetic nervous systems, as a paracrine regulator in the gonads and placenta, and as an autocrine regulator in tumor cells. 4. Evidence suggests that in most species the early-evolved and highly conserved chicken GnRH II has a neurotransmitter function, while the second form, which varies across classes, has a physiologic role in regulating gonadotropin release. 5. We review here evolutionary aspects of the family of GnRH peptides and their receptors.

Functional domains of the gonadotropin-releasing hormone receptor

1. The cloning of the mammalian gonadotropin-releasing hormone receptor sets the stage for rapid progress in understanding the structure of the receptor, its interaction with ligand, and its mechanisms of activation. 2. The receptor is a 327 to 328-amino acid seven-transmembrane domain G protein-coupled receptor. 3. Recent site-direct mutagenesis studies have provided considerable insight into glycosylation of the receptor, the arrangement of the helices, and the ligand binding domains.

Identification of N-glycosylation sites in the gonadotropin-releasing hormone receptor: role in receptor expression but not ligand binding

The asparagine residues of the three N-glycosylation consensus sequences in the mouse gonadotropin-releasing hormone receptor were mutated to determine which residues were glycosylated and the function of glycosylation. Photoaffinity labelled Gln4 and Gln18 receptor mutants exhibited lower apparent molecular weight on SDS polyacrylamide gel electrophoresis, while the Gln102 receptor showed wildtype mobility. This indicates that the receptor is glycosylated at Asn4 and Asn18 but not at Asn102. Binding affinities of all the mutant receptors were normal, indicating that carbohydrate moieties are not involved in ligand binding interactions. However, expression of the Gln4 and Gln18 receptors were substantially decreased, indicating a role for glycosylation in receptor expression or stability. All the glycosylation site mutants were capable of normal signal transduction, as indicated by their ability to stimulate inositol phosphate production.

Identification of chicken GnRH II in brains of metatherian and early-evolved eutherian species of mammals

Two molecular forms of GnRH (chicken GnRH II and a second variant) are present in the brains of species from all the major vertebrate groups. In mammals, two forms are present in metatherian species and early-evolved eutherian species, but chicken GnRH II has not been identified in more advanced eutherian species. We investigated the nature of GnRH molecular forms in several early-evolved mammalian species, using high performance liquid chromatography and radioimmunoassay with specific GnRH antisera. These chromatographic and immunological data indicate that in the brains of a metatherian species (possum, Trichosurus vulpecula) and in two early-evolved eutherian species (order Insectivora: musk shrew, Suncus murinus and mole, Chrysochloris asiatica), both mammalian and chicken II GnRHs are present, while in another relatively early-evolved eutherian species (order Chiroptera: bat, Miniopterus schreibersii) only mammalian GnRH is present. In the adult possum and mole brains the proportion of chicken GnRH II was lower than that of mammalian GnRH, while in the musk shrew brain chicken GnRH II predominated. A peptide likely to be mammalian proGnRH was detected in the brains of the three eutherian species (musk shrew, mole, and bat). These findings suggest that metatherian and primitive eutherian species of mammals continue to express chicken GnRH II as in the vast majority of nonmammalian vertebrates, while the peptide is apparently not expressed in modern placental mammalian species. The functional significance of chicken GnRH II is not yet clear, but there are indications that it has a neurotransmitter or neuromodulator role in addition to that of regulating pituitary hormone release in certain vertebrate species.

Gonadotropin-releasing hormones in microdissected brain regions of an amphibian: concentration and anatomical distribution of immunoreactive mammalian GnRH and chicken GnRH II

Mammalian and chicken II gonadotropin-releasing hormones (mGnRH, cGnRH II) were extracted from 350 microns diameter punches from brains of a urodele amphibian, Taricha granulosa, and measured by means of radioimmunoassay (RIA) with specific antisera. Measurable quantities of both peptides were found in the lateral pallium, the subpallium (along the course of the nervus terminalis), the preoptic area, habenula, optic tectum, infundibulum, paraventricular organ/posterior tubercle of the caudal diencephalon, medulla, and cerebrospinal fluid. Highest concentrations of both peptides were in the preoptic area and infundibulum, suggesting a role in gonadotropin release. In most extrahypothalamic regions, cGnRH II concentrations exceeded those of mGnRH, suggesting that cGnRH II may function as a neurotransmitter in many sites, perhaps to control reproductive behaviors. Results are largely consistent with immunocytochemical (ICC) analyses, except that RIA revealed small amounts of both peptides not found by ICC in some areas of the brain. Results from this microdissection/RIA study and prior ICC studies in amphibians support the conclusions that GnRH cell bodies in the terminal nerve and preoptic area, which project mainly to the median eminence and habenula, express mGnRH, and that GnRH cell bodies in the caudal diencephalon, which project widely throughout the CNS, express cGnRH II. Comparative data support the view that cGnRH II, and the neural systems in which it is expressed, evolved early in vertebrate phylogeny and have been highly conserved.

Identification of the molecular forms of and steroid hormone response to gonadotropin-releasing hormone in the Australian lungfish, Neoceratodus forsteri

Gonadotropin-releasing hormone (GnRH) peptides in the hypothalamus of the lungfish, Neoceratodus forsteri, were investigated by reverse-phase HPLC and RIA with region-specific antisera. Both chicken GnRH II and mammalian GnRH were identified, the latter being present in greater concentration. The steroidogenic response to a single intracardiac injection of synthetic mammalian GnRH was investigated in early and late spring (beginning and end of spawning season) and in early autumn. In early spring, both sexes responded with rapid and transient elevation of circulating steroid hormones. Testosterone in males showed the greatest response by elevating from 120 to 240 nmol/liter within 2 min of the injection and returning to approximately 100 nmol/liter within 15 min. Female lungfish showed a similar but slightly less dramatic response in circulating estradiol and testosterone. The responses of both males and females were reduced in late spring and abolished in early autumn, which is indicative of a period of seasonal refractoriness.

Incorporation of D-Ala2 in growth hormone-releasing hormone-(1-29)-NH2 increases the half-life and decreases metabolic clearance in normal men

D-Ala2-GHRH-(1-29) has increased binding affinity and exhibits enhanced biological activity in man. It is not known whether changes in the metabolic clearance of this and other GHRH analogs contribute to their increased biological activity. GHRH-(1-29)-NH2 and D-Ala2-GHRH-(1-29)-NH2 were administered by constant iv infusion at a rate of 25 ng/kg.min to 10 normal men. Blood was sampled during the 90-min infusion and for 20 min afterward and assayed for the infused analog. The MCR of the D-Ala2 analog (mean +/- SE) was significantly less (21 +/- 1.2 mL/kg.min) than that of GHRH-(1-29)-NH2 (39.7 +/- 3.9 mL/kg.min; P < 0.001). The disappearance half-time of the D-Ala2 analog was 6.7 +/- 0.5, whereas that of GHRH-(1-29)-NH2 was 4.3 +/- 1.4 min (P < 0.05). These findings demonstrate that the D-Ala2 substitution contributes to the enhancement of biological activity by reducing metabolic clearance.

Glutamate 301 of the mouse gonadotropin-releasing hormone receptor confers specificity for arginine 8 of mammalian gonadotropin-releasing hormone

The Arg residue at position 8 of mammalian GnRH is necessary for high affinity binding to mammalian GnRH receptors. This requirement has been postulated to derive from an electrostatic interaction of Arg8 with a negatively charged receptor residue. In order to identify such a residue, 8 conserved acidic residues of the mouse GnRH receptor were mutated to isosteric Asn or Gln. Mutant receptors were tested for decreased preference for Arg8-containing ligands by ligand binding and inositol phosphate production. One of the mutants, in which the Glu301 residue was mutated to Gln, exhibited a 56-fold decrease in apparent affinity for mammalian GnRH. The mutant receptor also exhibited decreased affinity for [Lys8]GnRH, but its affinity for [Gln8]GnRH was unchanged compared with the wild type receptor. The apparent affinity of the mutant receptor for the acidic analogue, [Glu8]GnRH, was increased more than 10-fold. The mutant receptor did not, therefore, distinguish mammalian GnRH from analogues with amino acid substitutions at position 8 as effectively as the wild type receptor. This loss of discrimination was specific for the residue at position 8, because the mutant receptor did distinguish mammalian GnRH from analogues with favorable substitutions at positions 5, 6, and 7. These findings show that Glu301 of the GnRH receptor plays a role in receptor recognition of Arg8 in the ligand and are consistent with an electrostatic interaction between these 2 residues.

Absence of rapid desensitization of the mouse gonadotropin-releasing hormone receptor

Desensitization of gonadotropin release by the pituitary gland in response to gonadotropin-releasing hormone (GnRH) agonists has clinical applications in the treatment of gonadal-hormone-dependent disorders. We therefore investigated possible desensitization of inositol phosphate (IP) responses of GNRH receptors. No short-term homologous desensitization of the IP response to GnRH was observed in either alpha T3 gonadotrope cells line or GH3 cells transfected with GnRH receptor cDNA. The absence of homologous desensitization is unusual among G-protein-coupled receptors, and may be due to the absence of a C-terminal cytoplasmic tail, a unique feature of the GnRH receptor. Several potential protein kinase C phosphorylation sites which might mediate heterologous desensitization are present on the GnRH receptor. In both alpha T3 cells and GnRH-receptor-transfected Cos-1 cells, activation of protein kinase C by pretreatment with phorbol ester caused a 35-53% decrease in the IP response to GnRH. However, phorbol ester also inhibited guanosine 5'-[gamma-thio]triphosphate-stimulated IP production in permeabilized Cos-1 cells, suggesting that this inhibition is mediated at a post-receptor site.

Mast cells with gonadotropin-releasing hormone-like immunoreactivity in the brain of doves

Using an antiserum (LR-1) raised against mammalian gonadotropin-releasing hormone (GnRH), we previously identified a nonneuronal cell that was more numerous in the medial habenula (MH) of courting ring doves than in individuals housed in visual isolation. The current studies suggest that they are mast cells. Both acidic toluidine blue and toluidine blue dissolved in water/butanediol revealed metachromatic cells with a distribution and morphology similar to that obtained by immunostaining with the GnRH antiserum in the MH. Some cells had granules reactive to safranin in the presence of alcian blue, indicative of a highly sulfated proteoglycan of the heparin family. Immunocytochemical studies demonstrated that all MH cells containing GnRH-like immunoreactivity contained histamine, another mast cell marker. The GnRH-immunoreactive cells had a unilobular, ovoid nucleus. Secretory granules within the cells were electron dense and displayed a variety of internal structures. Fine filamentous processes appeared evenly distributed on the cell surface whether cells were located on the pial surface or within the brain parenchyma. All of these features are characteristic of mast cells. To test whether the epitope recognized by the GnRH antiserum was produced by the mast cells or endocytosed from the cerebrospinal fluid, an iodinated GnRH analog was injected intracerebroventricularly at the initiation of courtship. Radioautography revealed no radioactive cells in the brain, indicating that the GnRH antibody recognized a molecule synthesized by the nonneuronal cells rather than internalized by a receptor-mediated mechanism. These observations suggest an interaction between a component of the immune network and specific regions of the central nervous system.

Differential regional distribution of gonadotropin-releasing hormones in amphibian (clawed toad, Xenopus laevis) brain

In most vertebrate species two forms of gonadotropin-releasing hormone (GnRH) are present in the brain, and their differential distribution suggests they have different functional roles. The regional distribution and relative concentrations of GnRH molecular forms in the brain of adult clawed toad (Xenopus laevis) were determined using high performance liquid chromatography and radioimmunoassay with a library of region-specific GnRH antisera. Four immunoreactive forms of GnRH were detected: mammalian, hydroxyproline mammalian, chicken II, and an unidentified form of GnRH. Mammalian GnRH was distributed throughout the brain, and hydroxyproline mammalian was present in the forebrain, midbrain (excluding hypothalamus), and hypothalamus. Chicken GnRH II also occurred throughout the brain, but was present in greater amounts in the hindbrain and midbrain (excluding hypothalamus). An unidentified form of GnRH with properties of salmon GnRH was detected in the forebrain. Considering the relative proportions of mammalian GnRH and chicken GnRH II in the major brain areas, the concentration of mammalian GnRH was high in the forebrain, midbrain (excluding hypothalamus), and in particular in the hypothalamus, and very little chicken GnRH II was present in these areas. In the hindbrain, chicken GnRH II predominated and the concentration of chicken GnRH II was highest in the medulla. These findings suggest: (1) mammalian GnRH is the prime regulator of gonadotropin release from the pituitary, and (2) chicken GnRH II has an extrapituitary role.

A reciprocal mutation supports helix 2 and helix 7 proximity in the gonadotropin-releasing hormone receptor

Activation of the pituitary gonadotropin-releasing hormone receptor, a member of the seven-transmembrane G protein-coupled receptor (GPCR) family, triggers a cascade of events leading to gonadotropin release and stimulation of the reproductive system. An unusual feature of this receptor, observed in mice, rats, and humans, is the presence of Asn87 in the second putative transmembrane helix at the location of a highly conserved aspartate in the GPCR family and of Asp318 in the putative seventh transmembrane helix where nearly all other GPCRs have asparagine. The possibility that these residues interact was suggested by this reciprocal pattern and by a three-dimensional model of the gonadotropin-releasing hormone receptor and was investigated by site-directed mutagenesis. Replacing Asn87 in the second transmembrane domain by aspartate eliminated detectable ligand binding. A second mutation, generating the double-mutant receptor Asp87Asn318, recreated the arrangement found in other GPCRs and re-established high affinity agonist and antagonist binding. The restoration of binding by a reciprocal mutation indicates that these two specific residues in helices 2 and 7 are adjacent in space and provides an empirical basis to refine the model of the transmembrane helix bundle of the receptor.

Comparative sequence analysis and functional characterization of the cloned sheep gonadotropin-releasing hormone receptor reveal differences in primary structure and ligand specificity among mammalian receptors

The cloned sheep gonadotropin-releasing hormone (GnRH) receptor was analysed for sequence homology/differences among mammalian receptors and its pharmacology characterized in COS-1 cells. Transmembrane domains TM2, TM3, TM5, TM6 and TM7, and extracellular loop 1 are most highly conserved (> 90%) in this G-protein coupled receptor. The Kd of the sheep receptor in binding assays (4.9 nM) was similar to the human and rat receptors, but lower than the mouse receptor. The rank order of potency of a series of GnRH analogues for binding and inositol phosphate stimulation in transfected COS-1 cells was identical to that of the receptor characterized in sheep pituitary gonadotropes. Northern blot analysis identified four transcripts sized 5.4 kb, 3.6 kb, 2.3 kb and 1.3 kb in sheep pituitaries which were upregulated by castration.

LH responses to single doses of exogenous GnRH by freshly captured Damaraland mole-rats, Cryptomys damarensis

Pituitary function in reproductive and nonreproductive colony members of Damaraland mole-rats, Cryptomys damarensis, was investigated by measuring the LH responses to single doses of 2 micrograms exogenous GnRH and physiological saline in 29 females and 37 males (31 of these animals from two entire colonies). In females, basal LH concentrations were significantly greater in reproductive (n = 9) than in nonreproductive animals (n = 11): 7.6 +/- 1.0 versus 4.3 +/- 0.6 miu ml-1, respectively (P < 0.001). Reproductive females had a significantly greater LH response to 2.0 micrograms GnRH (7.6 +/- 1.0 to 37.7 +/- 6.2 miu ml-1; n = 9) than did nonreproductive females (4.3 +/- 0.6 to 11.8 +/- 1.0 miu ml-1; n = 11, P < 0.001). In contrast, there was no significant difference in basal LH concentrations between reproductive (n = 8) and nonreproductive males (n = 20): 5.3 +/- 4.3 versus 3.2 +/- 1.2 miu ml-1, respectively. There was also no difference in LH response to the administration of 2.0 micrograms GnRH between reproductive and nonreproductive males (5.3 +/- 4.3 to 21.8 +/- 8.6 miu ml-1; n = 8; versus 3.2 +/- 1.2 to 21.1 +/- 8.5 miu ml-1; n = 21; P = 0.5). When the results from the two entire colonies were analysed separately, LH responses to GnRH in the 11 nonreproductive females were less than in the two reproductive females. In contrast, the response of two reproductive males in the colonies did not differ from that of 16 nonreproductive males, although these latter comparisons could not be validated statistically.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of commercial oral amino acid supplements to increase serum growth hormone concentrations in male body-builders

Amino acids are commonly ingested as ergogenic acids in the belief that they enhance protein synthesis and stimulate growth hormone release. The aim of this study was to determine the acute effect that amino acid supplements have on serum growth hormone (GH) concentration. Seven male body-builders reported to the laboratory on four occasions after an 8-hr fast and ingested, in random order, either a placebo, a 2.4-g arginine/lysine supplement, a 1.85-g ornithine/tyrosine supplement, or a 20-g BovrilR drink. Blood was collected before each treatment and again every 30 minutes for 3 hours for the measurement of serum GH concentration. On a separate occasion, subjects had an intravenous infusion of 0.5 microgram GH-releasing hormone.kg-1 body weight to confirm that GH secretory response was normal. The main finding was that serum GH concentrations were not altered consistently in healthy young males following the ingestion of the amino acid supplements in the quantities recommended by the manufacturers.

Presence and differential distribution of distinct forms of immunoreactive gonadotropin-releasing hormone in the musk shrew brain

Gonadotropin-releasing hormone (GnRH) immunoreactive cells and fibers were revealed in olfactory regions, the ventral forebrain, and in the midbrain of the musk shrew (Suncus murinus). Immunoreactive neurons in olfactory and telencephalic areas were specific for the mammalian form of GnRH. Cell bodies in the midbrain, however, cross-reacted with an antibody specific for chicken-II GnRH. High-performance liquid chromatography and radioimmunoassay analyses confirmed these results; high levels of chicken II GnRH were present in the midbrain, and mammalian GnRH was detected in both forebrain and midbrain. In addition, a third, late-eluting form of GnRH was revealed using high-performance liquid chromatography in both forebrain and midbrain of the musk shrew. Midbrain neurons containing GnRH have not been reported previously in a mammal, although mesencephalic GnRH immunoreactivity within cell bodies is common among nonmammalian vertebrates. Likewise, while multiple forms of GnRH have been reported in nonmammalian vertebrates and several metatherian species of mammals, this is the first report on multiple forms of GnRH in the brain of a placental mammal. Taken together, the findings suggest that this primitive eutherian mammal has retained the ability to produce GnRH protein in the midbrain. This feature of the GnRH system has been conserved among nonmammalian vertebrates, but appears to have been lost in modern placental mammal species. The functional significance of this group of neurons has yet to be determined.

Inhibition of pituitary hormone exocytosis by a synthetic peptide related to the rab effector domain

GTP-binding proteins of the rab family are believed to function at several steps in intracellular vesicular transport. We examined the effects of a rab-related peptide in permeabilized pituitary cells, in which exocytosis can be triggered by distinct Ca(2+)-dependent or Ca(2+)-independent pathways. We report that a synthetic peptide of 18 amino acids related to the rab effector domain, rab3AL (30-47) inhibited luteinizing hormone (LH) and growth hormone (GH) exocytosis triggered by either pathway. Ca(2+)-stimulated LH and GH release were inhibited by more than 80% and 50%, respectively, by 100 microM peptide. The peptide (100 microM) also inhibited LH and GH exocytosis stimulated by phorbol myristate acetate plus cAMP by more than 45% and 80%, respectively. The effect was sequence-specific since a second peptide, lacking the first 3 amino acids but otherwise identical failed to inhibit exocytosis. These results suggest that a protein of the rab family is involved in regulated pituitary hormone exocytosis, and they identify 3 amino acids of the putative rab effector domain which may be functionally important in exocytosis.