Induction of the self-priming effect of luteinizing hormone releasing hormone during the sexual maturation of the male rat

Pubertal and young adult male rats release more luteinizing hormone (LH) in response to luteinizing hormone releasing hormone (LHRH) if pretreated with LHRH than if pretreated with saline. Immature male rats do not show this self-priming effect. In order to examine the role of acute changes in testicular steroids in this process, immature (29-30 days old) or pubertal (50-51 days old) male rats were castrated or sham operated under ketamine HCl anesthesia. Beginning immediately after completion of the surgery, they were given three priming injections of 10 ng LHRH/100 g body wt or saline at 30-min intervals. Thirty minutes after the third priming injection, a blood sample was obtained by cardiac puncture followed immediately by a challenge injection of 50 ng LHRH/100 g body wt given to both saline and LHRH primed groups. Ten minutes after the challenge injection a final blood sample was obtained by heart puncture. Serum was assayed for LH concentration by radioimmunoassay. Sham-operated pubertal rats showed a typical self-priming effect. Animals pretreated with LHRH released significantly (P less than 0.01) more LH in response to the challenge injection than did rats pretreated with saline. Acute castration also resulted in a significant (P less than 0.001) self-priming effect in pubertal rats. As anticipated, sham castrated immature males did not show a self-priming effect. Acutely castrated immature rats however, showed a significant (P less than 0.05) self-priming effect. These data provide support for the hypothesis that, prior to puberty, increases in testosterone during the priming process inhibit the expression of the self-priming effect.     

Changes in circulating levels of immunoreactive follicle stimulating hormone, luteinizing hormone, and testosterone during sexual development in the rhesus monkey, Macaca mulatta

Basal serum levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) and the responsiveness of these hormones to a challenge dose of luteinizing hormone releasing hormone (LHRH), were determined in juvenile, pubertal, and adult rhesus monkeys. The monkey gonadotrophins were analyzed using RIA reagents supplied by the World Health Organization (WHO) Special Programme of Human Reproduction. The FSH levels which were near the assay sensitivity in immature monkeys (2.4 +/- 0.8 ng/ml) showed a discernible increase in pubertal animals (6.4 +/- 1.8 ng/ml). Compared to other two age groups, the serum FSH concentration was markedly higher (16.1 +/- 1.8 ng/ml) in adults. Serum LH levels were below the detectable limits of the assay in juvenile monkeys but rose to 16.2 +/- 3.1 ng/ml in pubertal animals. When compared to pubertal animals, a two-fold increase in LH levels paralleled changes in serum LH during the three developmental stages. Response of serum gonadotrophins and T levels to a challenge dose of LHRH (2.5 micrograms; i.v.) was variable in the different age groups. The present data suggest: an asynchronous rise of FSH and LH during the pubertal period and a temporal correlation between the testicular size and FSH concentrations; the challenge dose of LHRH, which induces a significant rise in serum LH and T levels, fails to elicit an FSH response in all the three age groups; and the pubertal as compared to adult monkeys release significantly larger quantities of LH in response to exogenous LHRH.     

Nutritional influences on sexual maturation in the rat

The effect of altered nutrition on sexual maturation may depend in part on the nature and timing of the dietary change. The data are conflicting as to whether rats undernourished before weaning but normally fed after weaning have delayed puberty, but such undernourished rats clearly weigh less at vaginal opening than do normally fed animals. Altered nutrition after weaning can change the timing of puberty, and in such cases the body weight at puberty of the animals given the modified diet is frequently abnormal. The factors regulating the age and weight at puberty of rats fed altered diets seem to include the degree of underfeeding, as reflected in the growth rate, and the composition of the diet. Undernourished immature male rats have low serum testosterone secondary to gonadotropin deficiency. Basal luteinizing hormone (LH) in these animals is either low or "inappropriately normal" relative to their hypoandrogenic state (low serum testosterone and sexual accessory gland weights), and serum LH increases after luteinizing hormone-releasing hormone (LHRH) or castration are normal or minimally reduced. Serum follicle-stimulating hormone (FSH) in undernourished rats is subnormal basally and after administration of LHRH, but not after castration, which suggests that the low basal serum FSH is due to inhibition of FSH output by a testicular factor. Spermatogenesis may be unaltered by dietary changes severe enough to cause hypoandrogenism, although very severe under-nutrition will impair sperm production.     

Alteration in the normal pattern of serum testosterone and 5α-androstane-3α,17β-diol in the immature male rat following chronic treatment with luteinizing hormone releasing hormone or luteinizing hormone

A major component of sexual maturation in the male rat is a progressive decline in serum concentrations of 5α-androstane-3α,17β-diol (3α-diol) and a concomitant increase in testicular testosterone biosynthesis and secretion. Chronic administration of synthetic luteinizing hormone releasing hormone (LHRH) or luteinizing hormone (LH)/human chorionic gonadotropin (hCG) to immature male rats has been shown to result in a delay in sexual maturation as evidenced by decreased sex accessory gland weights and altered testicular testosterone production. We have examined the postulate that such treatments may either reverse or retard the normal developmental pattern of serum testosterone and 3α-diol concentrations. Chronic $\text{in vivo}$ treatment of 28 day old immature male rats for 2 weeks with daily injections of either 0.5 μg of LHRH, 1.0 μg of LHRH, or 30 μg of LH was found to result in significant reductions in weights of the seminal vesicles and ventral prostate glands and diminutions in serum testosterone concentrations. Serum content of 3α-diol was either unchanged or slightly elevated in the LHRH treated animals and increased significantly in the LH treated animals. These data suggest that either a reversal of or retardation in the normal developmental pattern of serum testosterone and 3α-diol content has been achieved in the immature male rat by chronic LHRH or LH treatment.     

A new male hypogonadism mutant rat (hgn/hgn): concentrations of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in the serum and the responsiveness of accessory sex organs to exogenous T, FSH, human chorionic gonadotropin, and luteinizing hormone-releasing hormone

To determine the etiology of male hypogonadism in a newly found mutant rat (hgn/hgn, with a single autosomal recessive trait), concentrations of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured, and the responsiveness of the urogenital organs, hypothalamus, and pituitary gland to testosterone (1 mg/kg s.c. for 7 days), FSH (0.3 AU/kg s.c. for 7 days), human chorionic gonadotropin (hCG) (40 IU/kg s.c. for 7 days), and luteinizing hormone-releasing hormone (LHRH) (0.5 or 5.0 micrograms/kg s.c. for 7 days) were tested. Treatment with testosterone only increased the weights of all of the accessory sex organs, whereas treatment with FSH, hCG, or LHRH did not. Levels of serum FSH and LH were extremely higher and testosterone was lower in hgn/hgn males than in normal males. Serum FSH and LH decreased to levels found in intact animals after treatment with testosterone, suggesting that hypothalamic responsiveness to exogenous testosterone is present in the hgn/hgn males. Thus, the status of the hgn/hgn males was indicated to be due to primary Leydig cell dysfunction.     

Testicular weight, tubular diameter and number of Sertoli cells in rats are decreased after early prepubertal administration of an LHRH-antagonist; the quality of spermatozoa is not impaired

To suppress gonadotropin secretion during the sensitive period in development of the testes, immature male rats were treated with an antagonist of luteinizing hormone-releasing hormone (LHRH; ORG. 30276) from postnatal days 6-15. Previously, it has been demonstrated that this treatment results in delayed pubertal development, decreased testicular weight, impaired fertility and adult sexual behavior. In the present experiments it was investigated whether the decreased testicular weight was correlated with morphological changes in the testis. Also, by using an artificial insemination technique, the biological activity of spermatozoa of adult male rats, treated during early prepuberty with the LHRH antagonist (LHRH-A), was tested. The present results demonstrated a decrease in the diameter of the testicular tubuli of LHRH-A-treated rats. The number of Sertoli cells per tubular cross-section was also smaller. But qualitatively no differences could be observed in the testis. All stages of maturation of the seminiferous epithelium were equally frequently represented in LHRH-A-treated males compared with controls. Artificial insemination using spermatozoa obtained from the epididymis of LHRH-A-treated rats, resulted in a pregnancy rate of 100%, similar to the control rate. From the present data, we conclude that the infertility in adult male rats, treated with an antagonist to LHRH during prepubertal life, does not result from malfunction in the maturational processes in the germinal cells and the testes as a whole, despite the observation of changes in the testicular morphology. The infertility of LHRH-A-treated male rats can be explained by the observed impairment of sexual behavior. We suggest, that a central action of the antagonist of LHRH when administered to immature male rats may lead to permanent changes in the development of sexual behavior.     

Serum concentrations of reproductive hormones after administration of various anesthetics to immature and young adult male rats

Immature and young adult male rats were either castrated or unoperated. One of seven anesthetic agents (Rompun, Bio-Tal, Thiopental, pentobarbital, ketamine, halothane, or ether) was administered. When the animals were clearly anesthetized, they were decapitated. Control rats were decapitated without anesthesia. Serum concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin, testosterone, and androstenedione were determined by radioimmunoassay. None of the anesthetics was clearly suitable for study of all these hormones. Most would be suitable for acute LH studies. Ketamine and halothane appeared inappropriate for FSH studies in immature rats. Pentobarbital, Rompun, and ether caused increases in serum prolactin. Most of the agents appeared to cause a reduction in serum testosterone in intact rats but an increase in castrated animals, suggesting an inhibition of testicular androgen secretion and a stimulation of adrenal androgen secretion.     

Inhibin B,follicle stimulating hormone,luteinizing hormone and testosterone during childhood and puberty in males: changes in serum concentrations in relation to age and stage of puberty

Inhibin B is a gonadal dimeric polypeptide hormone that regulates synthesis and secretion of follicle stimulating hormone (FSH) in a negative feedback loop. The aim of the present study was to determine changes in serum inhibin B, gonadotropins and testosterone concentrations during childhood and puberty in males. We studied the relationship between circulating inhibin B, gonadotropins and testosterone in serum of healthy boys during the first two years of life and then in pubertal development. Using a recently developed two-side enzyme-linked immunosorbent assay (ELISA), inhibin B levels were measured in the serum of 78 healthy boys divided into eleven age groups from birth to the end of pubertal development. In addition, serum levels of gonadotropins and testosterone were measured. Serum inhibin B, gonadotropins and testosterone increased during the first months of postnatal life. A peak in serum inhibin B and gonadotropins concentrations was observed around 3-4 months of age. There was a significant positive correlation between serum inhibin B and gonadotropins and testosterone levels during the first 2 years of life. After this early increase, serum inhibin B, gonadotropins and testosterone levels decreased significantly and remained low until puberty followed by an increase beginning with the onset of puberty. Serum levels of inhibin B reached a peak at stage G3 of puberty. Around midpuberty, inhibin B lost its positive correlation with luteinizing hormone (LH) and testosterone from early puberty, and developed a strong negative correlation with FSH, which persisted into adulthood. We conclude that inhibin B plays a key role in the regulation of the hypothalamic-pituitary-gonadal hormonal axis during male childhood and pubertal development. Inhibin B is a direct marker of the presence and function of Sertoli cells and appears to reflect testicular function in boys.     

Alterations in hypothalamic content of luteinizing hormone-releasing hormone associated with pineal-mediated testicular regression in the golden hamster

The adult male golden hamster will undergo testicular regression when exposed to a short photoperiod, blinding, or late afternoon injections of melatonin. The present study was conducted to compare the effects of all three treatments on serum gonadotropin levels and testicular weights, and to evaluate the effects of these treatments on hypothalamic content of both immunoreactive and bioactive luteinizing hormone-releasing hormone (LHRH) levels. Hamsters were blinded (BL), exposed to a short photoperiod (SP), or received daily injections of melatonin (MEL) for 15 wk. Each treatment (BL, SP, MEL) induced a temporally similar decline in serum luteinizing hormone (LH), serum follicle-stimulating hormone (FSH), and testicular weight. Spontaneous recrudescence occurred earliest in the MEL group, with serum gonadotropins and testicular weight returning to normal by 15 wk. The SP group exhibited recovery of serum gonadotropins but not testicular weight by 15 wk. The BL group demonstrated partial recovery of serum FSH levels by 15 wk, with no recovery in either serum LH or testicular weight. Each treatment group demonstrated increased hypothalamic content of immunoreactive LHRH which was temporally correlated with the decreases of serum gonadotropins. Additionally, the MEL and SP groups demonstrated decreased immunoreactive LHRH levels during spontaneous recrudescence. Extracts of hypothalami from all treatment groups were bioactive on control hamster pituitary cells. These results indicate that there are temporal differences among the three common treatments and that these differences are manifested in serum gonadotropins, testicular weight and hypothalamic LHRH. Hypothalamic LHRH levels determined by radioimmunoassay and bioassay show periods of increase and decrease which coincide with periods of altered serum gonadotropin levels in all groups.     

Flutamide blocks the self-priming effect of luteinizing hormone-releasing hormone in pubertal male rats

Castration of pubertal or young adult male rats eliminates the self-priming effect of luteinizing hormone-releasing hormone on luteinizing hormone secretion. Testosterone, dihydrotestosterone, or estradiol will maintain this effect in castrated animals. In order to explore the mechanism by which both dihydrotestosterone and estradiol are capable of maintaining the effect, intact rats as well as castrated animals implanted with testosterone capsules were treated with the antiandrogen Flutamide. In both intact animals and castrated rats bearing testosterone-filled Silastic capsules, Flutamide blocked the self-priming effect. These data suggest that the androgen receptor is of primary importance in the maintenance of the self-priming effect.     

The luteinizing hormone-releasing hormone (LHRH) agonist, [D-Trp6, des-Gly-NH2(10)]-LHRH ethylamide, has no direct effect on spermatogenesis in the rat

Treatment of intact rats with luteinizing hormone-releasing hormone (LHRH) agonists has been shown to produce atrophy of a variable number of testicular seminiferous tubules. These findings raised the question of a possible direct versus indirect action of LHRH agonists on spermatogenesis. To answer this question, we treated hypophysectomized rats with the LHRH agonist [D-Trp6, des-Gly-NH2(10)]-LHRH ethylamide, dihydrotestosterone (DHT), or a combination of these two compounds for a period of 1 mo. Treatment of hypophysectomized animals with the LHRH agonist alone had no significant effect on the atrophy of seminiferous tubules found after hypophysectomy. DHT, however, maintained spermatogenesis at 80% of the level seen in intact animals. When DHT and the LHRH agonist were administered in combination, the stimulatory effects of DHT were observed with no significant interference caused by the LHRH agonist. This study shows that an LHRH agonist has no direct effect on the morphology of the seminiferous tubules in the absence of the pituitary gland and strongly suggests that the atrophy observed in the testis after LHRH agonist treatment in intact animals is mediated by the LHRH agonist-induced changes in luteinizing hormone secretion and/or direct action of the peptide on Leydig cells.     

Modulation of luteinizing hormone pulse amplitude by the frequency of luteinizing hormone-releasing hormone stimulation and by testosterone in castrated, hypothalamic-lesioned male rats

The frequency of spontaneous luteinizing hormone (LH) pulses is thought to be a direct result of the frequency of luteinizing hormone-releasing hormone (LHRH) pulses from the hypothalamus. By contrast, the amplitude of spontaneous LH pulses may be controlled by several factors other than the amplitude of LHRH pulses. We tested two hypotheses: 1) that LH pulse amplitude is determined in part by the frequency of LHRH pulses of constant magnitude, and 2) that testosterone (T) exerts a direct feedback effect on the pituitary gland to regulate LH pulse amplitude. Gonadal feedback was eliminated by castrating adult male rats (n = 20). Endogenous LHRH secretion was eliminated by lesioning the medial basal hypothalamus. Serum LH levels (0.19 +/- 0.04 ng/ml RP-2, mean +/- SEM) and T levels (0.15 +/- 0.02 ng/ml), measured several weeks after hypothalamic lesioning, confirmed the hypogonadotropic hypogonadal state of the animals. During a 8-h period, unanesthetized, unrestrained animals were injected with 40-ng pulses of LHRH via catheters into the jugular vein, and blood samples for LH measurement were drawn at 10-min intervals. The LHRH pulse interval was 20 min during the first 4 h in all animals. The pulse interval was doubled to 40 min in half of the animals (n = 10) during the next 4 hours; in the other 10 animals, the pulse interval was maintained constant at 20 min throughout the study. Within both of these groups, one-half of the animals (n = 5) were infused with T to achieve a physiological level of T in serum (2.46 +/- 0.36 ng/ml at 4 h), while the other half received vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of galanin-like peptide on luteinizing hormone secretion in the rat: sexually dimorphic responses and enhanced sensitivity at male puberty

Reproductive function is exquisitely sensitive to adequacy of nutrition and fuel reserves, through mechanisms that are yet to be completely elucidated. Galanin-like peptide (GALP) has recently emerged as another neuropeptide link that couples reproduction and metabolism. However, although the effects of GALP on luteinizing hormone (LH) secretion have been studied, no systematic investigation on how these responses might differ along sexual maturation and between sexes has been reported. Moreover, the influence of metabolic status and potential interplay with other relevant neurotransmitters controlling LH secretion remain ill defined. These facets of GALP physiology were addressed herein. Intracerebral injection of GALP to male rats induced a dose-dependent increase in serum LH levels, the magnitude of which was significantly greater in pubertal than in adult males. In contrast, negligible LH responses to GALP were detected in pubertal or adult female rats at diestrus. Neonatal androgen treatment to females failed to "masculinize" the pattern of LH response to GALP. In addition, metabolic stress by short-term fasting did not prevent but rather amplified LH responses to GALP in pubertal males, whereas these responses were abrogated by pharmacological inhibition of nitric oxide synthesis. We conclude that the ability of GALP to evoke LH secretion is sexually differentiated, with maximal responses at male puberty, a phenomenon which was not reverted by manipulation of sex steroid milieu during the critical neonatal period and was sensitive to metabolic stress. This state of LH hyperresponsiveness may prove relevant for the mechanisms relaying metabolic status to the reproductive axis in male puberty.     

Changes in the isoelectric focusing profile of pituitary follicle-stimulating hormone in the developing male rat

Pituitary glands, hypothalami, and trunk blood were obtained from male rats at 5, 15, 18, 21, and 29 days of age, on the day of balanopreputial separation (Days 42-45), and during adulthood. The forms of follicle-stimulating hormone (FSH) present within each pituitary were separated by polyacrylamide gel isoelectric focusing. Serum and pituitary gonadotropins, hypothalamic luteinizing hormone-releasing hormone (LHRH), and the profile of FSH forms across the isoelectric focusing gel were determined by radioimmunoassay. No change in the relative proportions of FSH forms were observed between 5 and 21 days of age. Likewise, only slight changes in serum and pituitary gonadotropin levels and hypothalamic LHRH content were observed at these times. After 21 days of age, dramatic increases in serum and pituitary gonadotropin levels were observed. Similarly, a shift in FSH forms within the pituitary to more basic and bioactive forms was observed at this time. These results demonstrate that, during the transition through puberty in the male rat, not only the absolute amount, but also the isoelectric focusing profile, of FSH change.     

Effects of luteinizing hormone releasing hormone on gonadotrophins in the hamster

The changes in serum gonadotrophins in male hamsters following one injection of 15 μg luteinizing hormone releasing hormone (LHRH) (Group A) were compared with those following the last injection of LHRH in animals receiving an injection approximately every 12 hr for 4 days (Group B) or 12 days (Group C). Peak follicle stimulating hormone (FSH) levels (ng/ml) were 1776±218 (Group A), 2904±346 (Group B), and 4336±449 (Group C). Peak luteinizing hormone (LH) values (ng/ml) were 1352±80 (Group A), 410±12 (Group B), and 498±53 (Group C). Serum FSH:LH ratios, calculated from the concentrations measured 16 hr after the last LHRH injections, were higher in Groups B and C than in Group A. Similar injections of LHRH (100 ng or 15 μg/injection) for 6 days elevated the serum FSH:LH ratio in intact males. Five such LHRH injections (100 ng/injection) blunted the rise in serum LH in orchidectomized hamsters. Direct effects of LHRH on gonadotrophin secretory dynamics or altered brain-pituitary-testicular interactions may alter the ratio of FSH to LH in the hamster.     

Effects of aging on pituitary and testicular luteinizing hormone-releasing hormone receptors in the rat

Aging exerts profound influences on the function of the hypothalamic-pituitary-testicular-axis. This work has been performed in order to verify whether, in male rats, the decreased secretion of LH and testosterone (T) occurring in old animals is reflected by modifications of luteinizing hormone-releasing hormone (LHRH) receptors at the level of the anterior pituitary and of the testes. To this purpose, the affinity constant (Ka) and the maximal binding capacity (Bmax) for the LHRH analog [D-Ser(tBu)6]des-Gly10-LHRH-N-ethylamide were evaluated, by means of a receptor binding assay, in membrane preparations derived from the anterior pituitary and testicular Leydig cells of male rats of 3 and 19 months of age. Serum levels of LH and T were measured by specific RIAs. The results obtained show that, in aged male rats, the concentration of pituitary LHRH receptors is significantly lower than that found in young animals. On the other hand, the concentration of LHRH binding sites is significantly increased on the membranes of Leydig cells of old rats. In no instance the Ka for the LHRH analog is significantly affected. Serum levels of LH and T are significantly lower in old than in young male rats. In conclusion, these results suggest that the reduced secretion of LH in old male rats may be linked, at least partially, to a decrease of the number of pituitary LHRH receptors. The impaired production of testosterone occurring in aged rats is accompanied by a significant increase of the number of testicular LHRH receptors, indicating that also the intratesticular mechanisms controlling testosterone release undergo significant alterations with aging.     

Effect of postnatal treatment with a gonadotropin-releasing hormone antagonist on sexual maturation of male rats

The role of postnatal pituitary-testicular activity in sexual maturation at puberty was studied in male rats. Rats were injected twice daily with a potent gonadotropin-releasing hormone antagonist (N-Ac-4-Cl-D-Phe1, 4-Cl-D-Phe2, D-Trp3, D-Phe6, D-Ala10-NH2-GnRH) (GnRH-Ant.), 2 mg/kg, on Days 1-15 of life, and killed on Day 48, 56 or 90 of life. The treatment delayed the onset of puberty (monitored by balano-preputial separation) by 8 days (from the age of 48 to 56 days). The weights of testes, seminal vesicles and ventral prostates were reduced by 50-60% on days 48 and 56 of life, but only the testis weights remained suppressed by Day 90. Levels of serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH), but not those of prolactin (Prl), were elevated 2-to-4-fold in the treated animals at the three ages studied. Serum and testicular testosterone (T) and the receptors for LH and Prl were suppressed in the peripubertal animals (48 and 56 days), but serum T was elevated and the receptor levels were normal in the 90-day group. The testicular FSH receptors were 50% suppressed at all ages studied. Only minor changes were observed in testicular histology when studied at 48 and 56 days. The 85-day-old animals treated with GnRH-Ant. were infertile when mated with females.(ABSTRACT TRUNCATED AT 250 WORDS)     

The response of the anterior pituitary and testes to synthetic luteinizing hormone-releasing hormone (LHRH) and the effect of castration on pituitary responsiveness in the maturing chicken fed aflatoxin

The responsiveness of the anterior pituitary to exogenous luteinizing hormone-releasing hormone (LHRH; 20 micrograms/kg body weight) and the subsequent stimulation of testosterone secretion by the testes was studied after administration of dietary aflatoxin (10 ppm) to 9-wk-old male chickens. In both control and aflatoxin-treated males, there were significant (p less than 0.05) increases in plasma luteinizing hormone (LH) concentrations following LHRH administration, which peaked at 5 min post injection and declined thereafter. Plasma testosterone levels increased soon after the LHRH injection in control males, secondary to elevated LH levels in the peripheral circulation, and continued to increase throughout the experimental period. In contrast, this LH-induced elevation in plasma testosterone was delayed in aflatoxin-treated males, with no substantial increase until 20 min post-LHRH injection. In a subsequent experiment, castration of aflatoxin-fed males resulted in an altered response to exogenous LHRH, as compared to their intact counterparts. Based on these data, it appeared that while the LH-secretory capacity of the anterior pituitary was not diminished in birds receiving aflatoxin, the testicular response to exogenous LHRH was altered during aflatoxicosis. Additionally, the effect of castration on plasma LH profiles after LHRH administration provides preliminary evidence for extra-testicular effects of dietary aflatoxin on reproduction in the avian male.     

Ovulation inhibition in the pregnant mare's serum gonadotropin-treated immature rat: a bioassay for luteinizing hormone-releasing hormone antagonists

A convenient method for evaluating the biological activity of luteinizing hormone-releasing hormone (LHRH) antagonists was devised. Pregnant mare's serum gonadotropin (PMSG) treatment of immature rats is known to stimulate follicular growth and estrogen production, that in turn stimulates the release of LHRH which triggers an ovulatory discharge of luteinizing hormone (LH) from the pituitary. The present bioassay of the antagonists is based on the inhibition of ovulation in the PMSG-treated rats. Twenty-eight-day-old Sprague Dawley rats maintained under a light period of 12 h/day (lights on at 0630 h) were given 10 IU of PMSG s.c. at 0930 h. On Day 30 of age the antagonist was given s.c. at 1430 h. The rats were killed on the following morning and the oviducts examined for the presence of ova. In addition, the antagonists were compared in their ability to inhibit serum testosterone levels in adult male rats. In the PMSG-treated rats the order of ovulation-inhibiting potency of the following antagonists was: [Ac-D-NAL(2)1,4FD-Phe2,D-Trp3,D-Arg6]-LHRH (LHRH-1) greater than [Ac-delta 3 Pro1,4FD-Phe2,D-NAL(2)3.6]-LHRH (LHRH-2) greater than [Ac-delta 3 Pro1,4FD-Phe2,D-Trp3,6]-LHRH (LHRH-3). The order of potency was confirmed by their antitesticular effects in adult male rats.     

Effects of chronic D-Leu6, des-Gly10-gonadotropin releasing hormone ethylamide on male sex tissues

The chronic administration of superactive agonists of gonadotropin releasing hormone (GnRH-A) have been reported to have a direct inhibitory effect on the sex tissues of the male rat. In an attempt to confirm or refute this statement, adult male rats were either left intact or were castrated and then treated daily for 14 days with either testosterone (T), dihydrotestosterone (DHT) or sesame oil (vehicle). Half of the intact and castrate animals also received daily injections of 200 ng of the GnRH agonist, D-Leu6, des-Gly10-GnRH ethylamide for 14 days. Twenty-four hours after completing treatment, blood levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH) and T were measured by radioimmunoassay and the ventral prostate gland (VP), seminal vesicle (SV) and penis were weighed. After 2 weeks of GnRH-A treatment, the plasma T level was reduced from 2506 +/- 170 (pg/ml +/- SEM) in the intact, nontreated animals to 907 +/- 69 in the intact, GnRH-A-treated group, indicating that the dosage of GnRH-A used in this study had an inhibiting effect on T secretion. No differences were observed in the VP, SV and penile weights between the castrate, GnRH-A and the castrate, nontreated groups. When exogenous T or DHT was given for 14 days to these castrated animals, the concomitant administration of GnRH-A did not appear to have any effect on the plasma T levels or the sex accessory tissue weights. These data suggest that GnRH-A itself does not appear to have a direct inhibitory or stimulatory effect on the sex tissues of the adult male rat.