Demonstration of a 60K molecular weight luteinizing hormone-releasing hormone receptor in solubilized adrenal membranes by a ligand-immunoblotting technique

A new technique for the identification of LHRH receptors has been developed and applied to demonstrate an adrenal LHRH binding protein. Solubilized membrane proteins were separated electrophoretically and transferred to nitrocellulose paper. This was followed by sequential incubations with LHRH, anti-LHRH antiserum, peroxidase-conjugated second antibody, and 4-chloro-1-naphthol. Using an antiserum directed towards the middle region of LHRH, a 60K mol wt band was visualized in rat adrenal and pituitary membranes. A band of slightly higher molecular weight was present in membranes of bovine adrenal cortex but was absent in the medulla. The 60K band was not visualized when nonimmune rabbit serum was used. The 60K band was also not visualized when an antiserum requiring the NH2 and COOH termini of LHRH was used, suggesting that these regions of LHRH are not accessible to the antiserum after binding to the receptor. These studies have demonstrated the existence of LHRH binding protein in adrenal cortical tissue with a molecular size similar to that of the pituitary receptor. Adrenal membrane binding sites were less clearly demonstrated by conventional 125I-ligand binding techniques as nonspecific binding was high. The ligand-immunoblotting technique is a sensitive, specific and rapid procedure with potential application in screening normal and tumor tissues for LHRH receptors and studying LHRH interactions with its receptor.

Growth hormone responses to growth hormone-releasing hormone (1-29)-NH2 and a D-Ala2 analog in normal men

Synthetic analogs of growth hormone-releasing hormone, GHRH(1-29)-NH2 and D-Ala2 GHRH(1-29)-NH2 were administered as a bolus intravenous injection to five normal men in a dose range of 0.015 to 0.5 micrograms/kg body weight. Vehicle only was administered in a control study. Peak responses to GHRH analogs occurred at 15 or 30 min. An increase in the integrated plasma growth hormone (GH) response was observed at each dose. The dose-response curve of GHRH(1-29)-NH2 indicated that it has a similar molar potency to GHRH(1-40) and GHRH(1-44). The potency of D-Ala2 GHRH(1-29)-NH2 was approximately twice that of GHRH(1-29)-NH2. Neither analog affected blood levels of PRL, TSH, LH, FSH, ACTH, insulin, glucagon, glucose, cortisol, free thyroxine, and free triiodothyronine. No side effects were noted other than transient flushing with the highest dose administered. The findings demonstrate GHRH(1-29)-NH2 and its D-Ala2 analog are potent stimulators of GH release and have potential application in clinical medicine.

Structure-activity relationships of mammalian, chicken, and salmon gonadotropin releasing hormones in vivo in goldfish

Mammalian, chicken, and salmon gonadotropin releasing hormones (GnRHs), and anlogs of each peptide, were injected either alone or in combination with pimozide into goldfish, and the changes in serum gonadotropin (GtH) levels determined. The native peptides had similar potencies in terms of magnitude and duration of the GtH response. Analogs of LHRH that are superactive in mammals are also superactive in goldfish; although [(imBzl)-D-His6, Pro9-NEt]-LHRH is very highly superactive in mammals it has activity similar to [D-Ala6, Pro9-NEt]-LHRH in goldfish. D-Ala6 or (imBzl)-D-His6 substitutions of [Trp7, Leu8, Pro9-NEt]-LHRH are not superactive in goldfish, whereas the D-Arg6 substitution is highly superactive, indicating that there are differences in the factors that make salmon and mammalian GnRH superactive. These results also indicate that the structural modifications that determine superactivity of GnRHs in goldfish differ from what is known for mammals.

Immunoreactive and biologically active somatostatin in human and sheep milk

The presence of immunoreactive and biologically active somatostatin in sheep and human milk has been demonstrated. Milk somatostatin exhibits similar chromatographic behavior to that of synthetic somatostatin-14 on both reversed-phase C18 and cation-exchange high-performance liquid chromatography columns. Milk, in contrast to plasma, contains only somatostatin-14-like material. Milk somatostatin was capable of inhibiting the basal and the prostaglandin-induced release of growth hormone from anterior pituitary cell cultures in a pattern similar to synthetic somatostatin-14. The concentrations of the peptide, as determined by radioimmunoassay, were found to be 113 pg/ml in human milk and 150 +/- 4.8 pg/ml (mean +/- range) in sheep milk. These values are severalfold higher than the corresponding concentration of the peptide in the plasma of these species. These findings are analogous to our previous observations concerning two other hypothalamic hormones, luliberin and thyroliberin [Baram, T., Koch, Y., Hazum, E. and Fridkin, M. (1977) Science (Wash. DC) 198, 300-302]. The high concentration of somatostatin and other neuropeptides in milk implies either an active concentrating mechanism in the mammary gland or an additional extrahypothalamic source for the synthesis and release of these peptides.

Hypothalamic dysfunction in overtrained athletes

Some athletes who undertake strenuous training programs for a prolonged period of time develop the overtraining syndrome. The pathophysiology of the condition is unknown. Hypothalamic-pituitary function was studied by determining the hormonal responses to insulin-induced hypoglycemia in five asymptomatic male marathon runners during a 4-month period in which they ran 42-, 56-, and 92-km races and in four overtrained male athletes. The response of the asymptomatic runners was not different when tested 1 month before and within 48 h after the 42- and 92-km races. All four overtrained athletes presented with impaired training and racing times, apathy, and a heavy-legged feeling and were tested when overtrained and again after 4 weeks of rest. The plasma cortisol, ACTH, GH, and PRL responses to insulin-induced hypoglycemia in the four overtrained athletes were lower than their responses after the rest and lower than the responses of the asymptomatic runners. In both groups, the LH, TSH, and PRL responses to LHRH and TRH were normal. The impaired hormonal responses to insulin-induced hypoglycemia, with recovery after 4 weeks of rest, indicate hypothalamic dysfunction and may be a diagnostic marker of the overtraining syndrome.

[Trp7,Leu8]LH-RH in reptilian brain

Luteinizing hormone-releasing hormone (LH-RH) immunoreactive peptides in acetic acid extracts of lizard (Cordylis nigra) brain were studied by high performance liquid chromatography (HPLC) and radioimmunoassay with region-specific antisera. Four different LH-RH immunoreactive peptides were detected. The major form co-eluted with salmon brain LH-RH, [Trp7,Leu8]LH-RH, in a cation exchange and three reverse phase HPLC systems which were specifically designed to separate a range of LH-RH analogues. The interaction of this major LH-RH immunoreactive peptide with a number of antisera directed against different regions of mammalian, chicken and salmon LH-RH was similar to the relative interaction of [Trp7,Leu8]LH-RH with these antisera. These data strongly indicate that the major form of lizard brain LH-RH is identical to salmon brain LH-RH [( Trp7,Leu8]LH-RH). The three additional molecular forms of immunoreactive LH-RH in lizard brain appear to differ from mammalian LH-RH in the middle to C-terminal region of the molecule.

Binding studies of substance P anterior pituitary binding sites: changes in substance P binding sites during the rat estrous cycle

Previous studies have shown that substance P (SP), an undecapeptide widely distributed in the gastrointestinal tract and in the peripheral and central nervous system, is a putative regulatory peptide involved in the control of reproductive function. Specifically, SP inhibited, at the anterior pituitary (AP) level, the stimulatory action of a physiological concentration (10(-8) M) of Gonadotropin Releasing Hormone (GnRH) on the release of the luteinizing hormone (LH). In the present work, we have demonstrated the presence of specific SP binding sites in the AP and related changes in the number of these sites to GnRH receptor number, hypothalamic SP and GnRH content and LH secretion during the rat estrous cycle. High affinity saturable SP binding sites (Kd, 1.5 approximately equal to 10 nM) were demonstrated in AP membranes using [3H]-SP or a novel analog, [125I]-(D-Tyr0, NorLeu11)SP. The binding affinity of SP fragments decreased with progressive removal of amino acid residues from N or C termini of the molecule. Other neuropeptides had low affinity for the SP binding sites. During the rat estrous cycle, SP and GnRH binding capacity of the anterior pituitary were inversely related. At the time of the proestrous LH surge, the AP binding capacity was low for GnRH but high for SP. The highest content of SP in the hypothalamus were recorded during the afternoon of proestrus when hypothalamic GnRH levels were lowest and the preovulatory surge occurred. These studies have established the presence of high affinity specific binding sites for SP in the AP which alter during the estrous cycle in a manner appropriate for mediating the direct inhibitory effects of SP on LH release in vitro.

Structure-activity relations of LHRH in birds

Structural requirements in LHRH for gonadotropin-releasing activity were investigated by comparing the activity of the natural vertebrate LHRH structural variants and synthetic analogues. Substitution of Arg8 results in a loss of activity of LHRH in mammals, whilst a number of amino acid substitutions for Arg8 retain high gonadotropin-releasing activity in the chicken. Thus Arg8 of LHRH comprises an integral part of the binding site of LHRH and/or contributes towards the conformation of the binding site for the mammalian receptor while this does not pertain in the bird. The possibility that relative conformational stabilization of LHRH is important for biological activity in mammals but not birds, was supported by the demonstration that a gamma-lactam conformationally constrained analogue of LHRH was more active than LHRH in the mammalian system but equipotent in the bird. The chicken LHRH receptor is also relatively undiscriminating with regard to amino acid substitutions in positions 5 and 7. A series of LHRH analogues with pure antagonist activity in rats exhibited a spectrum of activities, from pure agonist to mixed activity and pure antagonist, in the chicken. These differences in LHRH structural requirements of the mammalian and avian receptor are reflected by a difference in molecular size of the chicken receptor (67,000) and mammalian receptor (60,000). Nevertheless, like the mammalian pituitary, the chicken pituitary does exhibit "desensitization" on prolonged exposure to LHRH.

Isolation and structural characterization of chicken hypothalamic luteinizing hormone releasing hormone

Studies on partially purified chicken hypothalamic luteinizing hormone releasing hormone (LHRH) utilizing chromatography, radioimmunoassay with region-specific antisera, enzymic inactivation, and chemical modification established that the peptide is structurally different from mammalian hypothalamic LHRH. These studies demonstrated that arginine in position 8 is substituted by a neutral amino acid. On the basis of conformational criteria and evolutionary probability of amino acid interchange for arginine, the most likely substitution was glutamine. We therefore synthesized [Gln8]-LHRH and established that it had identical chromatographic, immunologic, and biological properties to the natural chicken peptide. In concurrent studies, purification of 17 micrograms of an LHRH from 249,000 chicken hypothalami was achieved using acetic acid extraction, immuno-affinity chromatography, and cation exchange and reverse phase high performance liquid chromatography. Amino acid composition and sequence analyses confirmed the structure of this form of chicken LHRH as pGlu-His-Trp-Ser-Tyr-Gly-Leu-Gln-Pro-Gly-NH2.

Ligandin concentrations in the steroidogenic tissues of the rat during development

Ligandin, a ubiquitous multifunctional cytoplasmic protein which exhibits glutathione S-transferase, glutathione peroxidase and delta 5-3-ketosteroid isomerase activities and binds to cortisol metabolites, is present in relatively high concentrations in gonadal and adrenal tissue. In contrast to hepatic ligandin, little is known about the ontogeny of ligandin in steroid-synthesising tissues. We report here the intracellular concentrations of ligandin as well as the serum concentrations of testosterone and progesterone measured by radioimmunoassay at different stages of development in the rat. Ligandin levels in testis, ovary and adrenal tissue were relatively high soon after birth, decreased by day 9 and increased rapidly during puberty to reach adult levels. These changes appeared to be paralleled by changes in the circulating levels of testosterone and progesterone. In contrast, ligandin levels in non-steroidogenically active tissues, such as liver and kidney, were low at birth and rose progressively to reach adult levels. Whereas hepatic ligandin concentration could be increased at all stages of development by phenobarbital induction, no induction occurred in the endocrine tissues.

Arginine hydrochloride stimulation of serum potassium and aldosterone is enhanced by somatostatin-28

Potassium, aldosterone and insulin responses to arginine infusion were compared in 5 normal men during infusions of somatostatin-14 (SS-14), somatostatin-28 (SS-28) and a control infusion. SS-14 and SS-28 were infused continuously at a rate of 8.6 pmol/min/kg and arginine (0.5 g/kg) was infused from 30-60 min. Following the control infusion with arginine hydrochloride, serum aldosterone and potassium levels increased slightly. The rise in potassium appears to be due to an exchange of cellular potassium for the proton from the arginine hydrochloride. SS-14 had no significant effect on serum potassium or aldosterone but an equimolar dose of SS-28 significantly enhanced the rise in potassium and aldosterone. SS-28 completely inhibited the arginine-stimulated insulin increase while SS-14 only partially inhibited the insulin increase. Since insulin opposes the increase in serum potassium by stimulating cellular uptake of this cation, the enhanced rise in serum potassium in response to arginine hydrochloride during the SS-28 infusion is likely due to the potent insulin suppressing effect of SS-28. The rise in serum aldosterone is directly related to the degree of elevation of serum potassium. These findings caution against the infusion of SS-28 during an arginine stimulation test.

A radioimmunoassay specific for [Gln8]LH-RH: application in the confirmation of the structure of chicken hypothalamic luteinizing hormone-releasing hormone

In the first report on the chemical structure of a nonmammalian LH-RH, chicken hypothalamic LH-RH was demonstrated to be [Gln8]LH-RH [2-4]. However, these studies and subsequent reports [7,8] did not totally exclude the possibility of a reverse sequence of the two amino acids Leu-Gln. In view of the recently described structure of salmon brain LH-RH as [Trp7,Leu8]LH-RH [9], we undertook to confirm our earlier conclusion that chicken LH-RH is [Gln8]LH-RH and not [Gln7, Leu8]LH-RH. The immunologic, chromatographic and biological properties of natural chicken hypothalamic LH-RH were compared with those of the two synthetic peptides, [Gln8]LH-RH and [Gln7,Leu8]LH-RH. A radioimmunoassay highly specific for [Gln8]LH-RH was developed. Natural chicken LH-RH cross-reacted fully with the antiserum which requires the COOH-terminal Gln8 to Gly10-NH2 for binding, while [Gln7,Leu8]LH-RH showed less than 0.1% cross-reaction. On a high resolution reverse phase high performance liquid chromatography system, natural chicken LH-RH co-eluted with [Gln8]LH-RH and was well separated from [Gln7,Leu8]LH-RH. In a chicken anterior pituitary cell bioassay, natural chicken LH-RH and [Gln8]LH-RH were equipotent in stimulating luteinizing hormone release, while the relative potency of [Gln7,Leu8]LH-RH was 4.4%. These data, in particular the use of a specific [Gln8]LH-RH antiserum, provide conclusive evidence that chicken LH-RH is [Gln8]LH-RH.

Synthesis, luteinizing hormone-releasing activity, and receptor binding of chicken hypothalamic luteinizing hormone-releasing hormone

Chicken hypothalamic LHRH, which was recently structurally characterized, was synthesized by solid-phase methodology, characterized, and tested for LH-releasing activity using anterior pituitary cells from chickens and sheep, as well as for binding to chicken and rat anterior pituitary membrane receptors. Synthetic chicken LHRH exhibited identical properties to the isolated natural chicken LHRH in several chromatographic systems. Synthetic chicken LHRH displayed identical activity to natural chicken LHRH in stimulating LH release from dispersed chicken anterior pituitary cells, and synthetic mammalian LHRH was equipotent in this system. Both the natural and synthetic chicken peptides had similar low potency in stimulating LH release from cultured ovine anterior pituitary cells (approximately 1% of the potency of synthetic mammalian LHRH). The ED50 of binding to rat anterior pituitary membrane receptors was 2.5 X 10(-6) M for synthetic chicken LHRH, compared to 6.3 X 10(-8) M for synthetic mammalian LHRH.

Inhibition of pancreatic hormone secretion by somatostatin-28 and somatostatin-14 in man

The effects of a 210 min infusion of 1.8 nmol/kg somatostatin-14 (SS-14), somatostatin-28 (SS-28), and vehicle (Haemaccel) alone, on arginine- and insulin-stimulated release of pancreatic hormones were tested in 5 normal male subjects. Arginine administered at 30-60 min induced an increase in plasma glucose concentrations which was enhanced by SS-14 and further increased by SS-28. SS-28 was more effective than SS-14 in suppressing the arginine-induced secretion of insulin. Arginine-stimulated and insulin-stimulated (at 120 min) glucagon release was equally suppressed by SS-14 and SS-28, as was insulin-stimulated pancreatic polypeptide secretion. At the end of the SS-14 infusion the mean plasma somatostatin level was approximately 28% of that which occurred during the SS-28 infusion. The results are discussed in relation to similar studies in vitro and in vivo in laboratory animals and to a possible role of the two forms of SS in carbohydrate homeostasis.

Synthesis and biological activity of [D-Trp6] chicken luteinizing hormone-releasing hormone

An agonist of chicken hypothalamic luteinizing hormone-releasing hormone (cLH-RH), [D-Trp6] cLH-RH, was synthesized and tested for luteinizing hormone (LH)-releasing activity using dispersed chicken anterior pituitary cells, as well as for binding to rat anterior pituitary membrane receptors. cLH-RH and mammalian LH-RH (mLH-RH) gave identical dose-response curves in stimulating chicken LH release (ED50 = 1.6 and 1.8 X 10(-9) M respectively) and similar estimates of potency. The [D-Trp6] analogs of cLH-RH and mLH-RH stimulated LH release at lower doses (ED50 = 7.0 and approximately 7.0 X 10(-11) M respectively) and were approximately 20-fold more potent. In contrast to the activity in the chicken bioassay, cLH-RH bound to rat anterior pituitary membrane receptors with a much lower affinity than did mLH-RH and had a relative potency of 2%. [D-Trp6] cLH-RH was approximately 100-fold more potent than cLH-RH in the rat receptor assay while [D-Trp6] mLH-RH was 28-fold more active than mLH-RH. These data demonstrate that substitution of Gly6 of LH-RH with D-Trp enhances the LH release from chicken pituitary cells to a similar extent to that observed in mammals, and indicate that the approaches used to produce active LH-RH analogs in mammals are likely to be applicable to birds.

Evaluation of LH-RH stimulation of testosterone as an index of reproductive status in rams and its application in wild antelope

In rams a positive correlation (P less than 0.001) existed between average testosterone levels from 30-min blood sampling for 18 h and average testosterone levels of samples taken 0, 1 and 2 h after injection of LH-RH administered 90 min after anaesthesia. Attempts were therefore made to assess testosterone status by LH-RH challenge and limited blood sampling in animals immobilized in their natural habitat. In impala (Aepyceros melampus) territorial males had higher plasma testosterone values than did bachelors after LH-RH challenge (8.1 compared with 2.6 ng/ml, P less than 0.05). In blesbok (Damaliscus dorcas), the relationship was less clear, but testicular volume was correlated with plasma testosterone concentration and with testicular responsiveness measured by testosterone produced per unit of LH (P less than 0.001 and P less than 0.05, respectively). The LH-RH challenge technique therefore has value as a measure of testicular function and permits study of ungulates in their natural environment.

Comparative structure-activity studies on mammalian [Arg8] LH-RH and chicken [Gln8] LH-RH by fluorimetric titration

Fluorimetric titrations of mammalian [Arg8] LH-RH, chicken [Gln8] LH-RH and an analogue [Lys8] LH-RH revealed pK values of 5.80, 6.22 and 6.01 for His2, and 9.65, 9.88 and 9.88 for Tyr5. The titration ranges for His2 were 1.72, 2.03 and 1.71 while the range for Tyr5 was rather similar (approximately 1.7) for all three peptides. Biological activity and receptor binding in the mammalian system for chicken LH-RH was 1% relative to mammalian LH-RH while [Lys8] LH-RH had a relative activity of approximately 10%. In contrast, mammalian and chicken LH-RH were equipotent in stimulating LH release from chicken pituitary cells. The results indicate differences in the receptors related to the conformations of LH-RH and position 8-substituted analogues.

Somatostatin-28 and somatostatin-14 suppression of arginine-, insulin-, and TRH-stimulated GH and PRL secretion in man

GH and PRL responses to arginine infusion and to the combined pituitary stimulation test (0.1 u/kg of insulin, 200 micrograms of TRH, 100 micrograms of LHRH) were compared in five normal men during infusions of somatostatin-14 (SS-14) and somatostatin-28 (SS-28), and during control infusions of vehicle alone. SS-14 and SS-28 were infused at a rate of 1.8 nmol/kg over 210 min. Arginine (0.5 g/kg) was infused from 30 to 60 min and the combined pituitary stimulation test commenced at 120 min. Arginine (0.5 g/kg) infused from 30 to 60 min induced an increase in GH secretion in all subjects and this increase was completely abolished in these same subjects when infused with SS-14 and SS-28. Arginine-induced hyperglycaemia was significantly greater during infusion of SS-14 and further enhanced by infusion of SS-28. A small increase in PRL secretion occurred after arginine infusion and this was not inhibited by SS-14 or SS-28. Insulin and TRH administration induced marked increases in both GH and PRL secretion. The mean GH increase was significantly inhibited by SS-14 and SS-28 up to 165 min but not thereafter. The PRL increase was significantly inhibited by SS-28 but not by SS-14 and this greater efficacy was also indicated by administering different doses of SS-28 to one subject. Taken together with the demonstration that SS-28 is released from the median eminence, these findings indicate that SS-28 has a hormonal role in the regulation of GH and PRL secretion.

Somatostatin-28 is an hormonally active peptide secreted into hypophysial portal vessel blood

Somatostatin 14 and 28 (SS-14 and SS-28) have been measured in hypophysial portal blood of male Wistar rats. The concentration and total amount of SS released into portal blood were increased 6- to 7-fold by electrical stimulation of the median eminence. On HPLC, only two peaks of immunoreactivity were detected in extracts of portal plasma and these peaks corresponded with SS-14 and SS-28, respectively. The amount of SS-28 in portal plasma was, on a molar basis, similar or greater than that of SS-14 and this together with other evidence suggests that SS-28 is an hormonally active peptide.

Somatostatin-28 inhibits thyroid-stimulating hormone release in man

The effects of an equimolar (1,8 nmol/kg body weight) infusion of somatostatin-28 (SS-28) and somatostatin-14 (SS-14) on the thyroid-stimulating hormone (TSH) response to a combined pituitary function test consisting of 200 micrograms thyrotrophin-releasing hormone (TRH) as well as 100 micrograms luteinizing hormone-releasing hormone (LH-RH) and insulin 0.1 U/kg were compared in 5 normal men. SS-28 significantly inhibited the TSH response to TRH compared with the control infusion. SS-14 was ineffective in 1 subject but overall did not differ significantly from SS-28. In 1 of the subjects the inhibitory effects of SS-28 on the TSH response was determined at three different doses of SS-28. In this subject SS-28 was found to be approximately 16 times more potent than SS-14 in the inhibition of TSH release. Suppression of TSH by SS-28 adds further support for a hormonal role for the peptide.

Structure of chicken hypothalamic luteinizing hormone-releasing hormone. II. Isolation and characterization

Avian luteinizing hormone-releasing hormone (LH-RH) has been isolated from 249,000 chicken hypothalami and shown to differ structurally from mammalian hypothalamic LH-RH. Purification was achieved by acetic acid extraction, anti-LH-RH affinity chromatography, and cation exchange and reverse phase high performance liquid chromatography. The isolated peptide eluted as a single peak on reverse phase high performance liquid chromatography. Acid hydrolysis of the peptide yielded integral molar ratios of amino acids and a composition identical with that of mammalian decapeptide LH-RH, except for the presence of an additional glutamic acid residue and the absence of arginine. The isoelectric point of chicken LH-RH (7.3) is consistent with the glutamic acid representing a glutamine residue. We therefore synthesized [Gln8]LH-RH and established that it has chromatographic properties identical with natural chicken LH-RH. These studies indicate that the structure of chicken hypothalamic LH-RH is: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Gln-Pro-Gly-NH2.

Characterization of Leydig cell gonadotropin-releasing hormone binding sites utilizing radiolabeled agonist and antagonist

Gonadotropin-releasing hormone (GnRH) binding sites in intact Leydig cells and in membrane preparations were investigated using 125I-labeled GnRH agonist and antagonist. Binding was saturable and involved a single class of high affinity sites. Intact Leydig cells and a membrane preparation had a higher affinity for GnRH agonist (Kd 3.0 +/- 1.7 X 10(-10) M) than for GnRH antagonist (Kd 10.0 +/- 1.8 X 10(-10) M). With anterior pituitary membranes the Kd was 2.8 +/- 0.7 X 10(-10) M for the agonist and 2.4 +/- 1.4 X 10(-10) M for the antagonist. The Kd for GnRH was similar for Leydig cells and the anterior pituitary. Chymotrypsin and trypsin digestion decreased receptor binding, but neuraminidase increased Leydig cell binding in contrast to the decrease in binding observed with pituitary receptors. The results suggest that the Leydig cell GnRH binding sites may differ from the pituitary receptor which may be related to structural differences in GnRH-like peptides recently described in extracts of rat testis.