Blockade of pulsatile LH, FSH and testosterone secretion in rams by constant infusion of an LHRH agonist

Adult Soay rams were infused for 21 days with 50 micrograms buserelin/day, using s.c. implanted osmotic mini-pumps. The continuous treatment with this LHRH agonist induced a supraphysiological increase in the blood concentrations of LH (15-fold) and testosterone (5-fold) followed by a decrease below pre-treatment values after 10 days. The blood concentrations of FSH showed only a minimal initial increase but the subsequent decrease was dramatic, occurring within 1 day. By Day 10 of treatment, the blood concentrations of all 3 hormones were low or declining, LH pulses were absent in the serial profiles based on 20-min blood samples and the administration of LHRH antiserum failed to affect the secretion of LH or testosterone. By Day 21, the secretion of FSH, LH and testosterone was maximally suppressed. The i.v. injection of 400 ng LHRH was totally ineffective at stimulating an increase in the blood concentrations of LH while the i.v. injection of 50 micrograms ovine LH induced a normal increase in the concentrations of testosterone; this confirmed that the chronic treatment with the LHRH agonist had desensitized the pituitary gonadotrophs without markedly affecting the responsiveness of the testicular Leydig cells. The ratio of bioactive: radioimmunoactive LH did not change during the treatment. The long-term effect of the infusion was fully reversible as shown by the increase in the blood concentrations of FSH, LH and testosterone and the return of normal pulsatile fluctuations in LH and testosterone within 7 days of the end of 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.     

Evidence that testosterone and follicular fluid do not interact in the control of FSH secretion in rams

The hypothesis that testosterone and inhibin interact in the control of FSH secretion in rams was tested. Adult rams were castrated and were simultaneously given testosterone implants and 3-times daily sc injections of 0, 0.4, 0.8 or 1.6 ml charcoal-treated bovine follicular fluid (bFF). After 1 wk, the implants were removed, and the bFF injections continued as before. Blood samples were taken daily for mean LH, FSH and testosterone concentrations, and every 10 min for 12 h in the presence and in the absence of testosterone for assessment of pulsatile LH release. The bFF specifically inhibited FSH secretion from rat pituitary cells in culture. In the presence of testosterone, there were no main effects of bFF on mean plasma FSH or LH concentrations, nor were these values different from their pre-treatment means (P>0.05). Treatment with bFF did not affect LH pulse frequency or amplitude, but the number of rams showing LH pulses was reduced in the 0.8 and 1.6-ml dose groups (P<0.05). Removal of testosterone increased (P<0.05) both gonadotropins. In the absence of testosterone, no main effect of bFF on mean LH or FSH concentrations was observed, although the 1.6-ml dose suppressed the postcastration rise of both LH and FSH. These data suggest that inhibin does not interact with testosterone and that a physiological level of testosterone is sufficient for the regulation of FSH secretion in adult rams.     

Stress induced changes in testis function

The mechanism through which chronic stress inhibits the hypothalamic-pituitary-testicular axis has been investigated. Chronic restraint stress decreases testosterone secretion, an effect that is associated with a decrease in plasma gonadotropin levels. In chronically stressed rats there was a decrease in hypothalamic luteinizing hormone-releasing hormone (LHRH) content and the response on plasma gonadotropins to LHRH administration was enhanced. Thus the inhibitory effect of chronic stress on plasma LH and FSH levels seems not to be due to a reduction in pituitary responsiveness to LHRH, but rather to a modification in LHRH secretion. It has been suggested that beta-endorphin might interfere with hypothalamic LHRH secretion during stress. Chronic immobilization did not modify hypothalamic beta-endorphin, while an increase in pituitary beta-endorphin secretion was observed. Since we cannot exclude that changes in beta-endorphin secreted by the pituitary or other opioids may play some role in the stress-induced decrease in LHRH secretion, the effect of naltrexone administration on plasma gonadotropin was studied in chronically stressed rats. Naltrexone treatment did not modify the decrease in plasma concentrations of LH or FSH. These findings suggest that the inhibitory effect of restraint on the testicular axis is exerted at hypothalamic level by some mechanism other than opioids.     

Plasma LH, FSH and testosterone concentrations in adult rams which were homozygous carriers or non-carriers of the Booroola fecundity gene

No gene-specific differences were found with respect to LH or testosterone pulsatile secretion (over 12 h), or in 12 hourly mean FSH concentrations in adult Booroola FF and ++ rams. Also, no differences between genotypes in the LH response to an injection of testosterone propionate, the FSH response to an infusion of bovine follicular fluid, or the testosterone response to injections of PMSG were noted. However, during the phase of seasonal testicular development, mean testosterone pulse amplitude (over 12 h) and the FSH response to 25 micrograms GnRH were higher in FF than in ++ rams (P less than 0.05); there were also significant effects of sire (P less than 0.05 in FF genotype only) and litter size (P less than 0.05) on testosterone pulse amplitude and GnRH-stimulated FSH release, respectively. During the breeding season, mean LH, but not FSH, concentrations were higher in FF than in ++ rams, after an injection of 0.5 micrograms GnRH; LH release was not affected by sire or litter size (P greater than 0.05). Long-term studies revealed that the FF rams were born in significantly larger litters, they weighed significantly less than ++ rams (P less than 0.05), and that bodyweight was significantly correlated (P less than 0.05) with litter size. There were no differences in testis size, and testis size was not significantly correlated with bodyweight. There was a strong tendency (P = 0.056) for overall mean FSH concentrations, measured weekly for 9 months, to be highest more often in FF than in ++ rams.(ABSTRACT TRUNCATED AT 250 WORDS)     

Some effects of epidermal growth factor on reproductive function in Merino sheep

During the i.v. infusion of a depilatory dose (100 micrograms/kg bodyweight) of mouse epidermal growth factor (EGF) into ovariectomized Merino ewes the frequency of pulsatile LH release was significantly reduced. However, the amplitude of pulses of LH secretion, either those naturally occurring or those induced by LHRH injection, was unchanged or only slightly reduced. Similar infusions of mouse EGF were made in progestagen-treated anoestrous Merino ewes in which LH secretion was maintained by injections of LHRH. These ewes did not experience oestrus or ovulate in response to PMSG injected 1 day after mouse EGF treatment (2 days before progestagen withdrawal); both responses occurred in controls. The EGF-treated ewes experienced oestrus and ovulated following progestagen-PMSG treatment 6 weeks later. These results suggest that mouse EGF inhibits the hypothalamic pulse generator responsible for LH release in the ewe but has little if any effect on pituitary sensitivity to LHRH; and mouse EGF apparently has a direct effect on the ovaries, temporarily impairing their ability to ovulate in response to exogenous gonadotrophin.     

Influence of the degree of stimulation of the pituitary by gonadotropin-releasing hormone on the action of inhibin and testosterone to suppress the secretion of the gonadotropins in rams

This experiment determined if the degree of stimulation of the pituitary gland by GnRH affects the suppressive actions of inhibin and testosterone on gonadotropin secretion in rams. Two groups (n = 5) of castrated adult rams underwent hypothalamopituitary disconnection and were given two i.v. injections of vehicle or 0.64 microg/kg of recombinant human inhibin A (rh-inhibin) 6 h apart when treated with i.m. injections of oil and testosterone propionate every 12 h for at least 7 days. Each treatment was administered when the rams were infused i.v. with 125 ng of GnRH every 4 h (i.e., slow-pulse frequency) and 125 ng of GnRH every hour (i.e., fast-pulse frequency). The FSH concentrations and LH pulse amplitude were lower and the LH concentrations higher during the fast GnRH pulse frequency. The GnRH pulse frequency did not influence the ability of rh-inhibin and testosterone to suppress FSH secretion. Testosterone did not affect LH secretion. Following rh-inhibin treatment, LH pulse amplitude decreased at the slow, but not at the fast, GnRH pulse frequency, and LH concentrations decreased at both GnRH pulse frequencies. We conclude that the degree of stimulation of the pituitary by GnRH does not influence the ability of inhibin or testosterone to suppress FSH secretion in rams. Inhibin may be capable of suppressing LH secretion under conditions of low GnRH.     

Short-term pituitary-testicular responses of prepubertal ram lambs to hemicastration

To examine the short-term effects of hemicastration on pituitary-gonadal responses, 12 ram lambs were anesthetized and hemicastrated at 4 mo of age and killed (n = 4) at 2 (HC2), 7 (HC7), or 14 (HC14) days following surgery. Four intact (INT) rams were killed 14 days following anesthesia. Testis and pituitary weights were similar between HC and INT rams. Serum follicle-stimulating hormone (FSH) in HC rams increased within 6 h, peaked at 12 h, and remained elevated above INT levels throughout the study. Overall mean serum testosterone levels in HC rams were lower than in INT rams for the first 48 h, but were similar by 3 days post-surgery. Pulsatile luteinizing hormone (LH) and testosterone secretion was suppressed for the first 9.5 h following anesthesia and/or surgery in both HC and INT animals. A single LH pulse and succeeding testosterone pulse occurred in 10/12 HC and 4/4 INT rams between 10 and 14 h post-surgery, both of which were lower in amplitude in HC than INT animals. However, on Day 7, pulsatile secretory patterns of LH and testosterone were similar, suggesting compensatory androgen secretion had occurred in HC rams. Pituitary LH content was unaffected by hemicastration. In contrast, pituitary FSH content was greater in HC7 and HC14 compared to HC2 and INT animals. Pituitary gonadotropin hormone-releasing hormone (GnRH) receptor concentrations were similar in INT, HC7, and HC14 rams, but were slightly reduced in HC2 rams. Neither testicular LH nor FSH receptor concentrations were altered by hemicastration at any time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of intermittent pulsatile infusion of luteinizing hormone-releasing hormone on dihydrotestosterone-suppressed gonadotropin secretion in castrate rams

The feedback effects of dihydrotestosterone (DHT) on gonadotropin secretion in rams were investigated using DHT-implanted castrate rams (wethers) infused with intermittent pulsatile luteinizing hormone-releasing hormone (LHRH) for 14 days. Castration, as anticipated, reduced both serum testosterone and DHT but elevated serum LH and follicle-stimulating hormone (FSH). Dihydrotestosterone implants raised serum DHT in wethers to intact ram levels and blocked the LH and FSH response to castration. The secretory profile of these individuals failed to show an endogenous LH pulse during any of the scheduled blood sampling periods, but a small LH pulse was observed following a 5-ng/kg LHRH challenge injection. Dihydrotestosterone-implanted wethers given repeated LHRH injections beginning at the time of castration increased serum FSH and yielded LH pulses that were temporally coupled to exogenous LHRH administration. While the frequency of these secretory episodes was comparable to that observed for castrates, amplitudes of the induced LH pulses were blunted relative to those observed for similarly infused, testosterone-implanted castrates. Dihydrotestosterone was also shown to inhibit LH and FSH secretion and serum testosterone concentrations in intact rams. In summary, it appears that DHT may normally participate in feedback regulation of LH and FSH secretion in rams. These data suggest androgen feedback is regulated by deceleration of the hypothalamic LHRH pulse generator and direct actions at the level of the adenohypophysis.     

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Comparative response of rams and bulls to long-term treatment with gonadotropin-releasing hormone analogs

The objective was to compare the relative response between rams and bulls in characteristics of LH, FSH and testosterone (T) secretion, during and after long-term treatment with GnRH analogs. Animals were treated with GnRH agonist, GnRH antagonist, or vehicle (Control) for 28 days. Serial blood samples were collected on day 21 of treatment, and at several intervals after treatment. Injections of natural sequence GnRH were used to evaluate the capacity of the pituitary to release gonadotropins during and after treatment. Treatment with GnRH agonist increased basal LH and T concentrations in both rams and bulls, with a greater relative increase in bulls. Endogenous LH pulses and LH release after administration of GnRH were suppressed during treatment with GnRH agonist. Treatment with GnRH antagonist decreased mean hormone concentrations, LH and T pulse frequency, and the release of LH and T after exogenous GnRH, with greater relative effects in bulls. Rams previously treated with antagonist had a greater release of LH after administration of GnRH compared with control rams, while rams previously treated with agonist showed a reduced LH response. Bulls previously treated with agonist had reduced FSH concentrations and LH pulse amplitudes compared with control bulls while bulls previously treated with antagonist had greater T concentrations and pulse frequency. The present study was the first direct comparison between domestic species of the response in males to treatment with GnRH analogs. The findings demonstrated that differences do occur between rams and bulls in LH, FSH and testosterone secretion during and after treatment. Also, the consequences of treatment with either GnRH analog can persist for a considerable time after discontinuation of treatment.     

Effect of testosterone and bovine follicular fluid on concentrations of luteinizing hormone and follicle-stimulating hormone in plasma of castrated rams that are homozygous carriers or non-carriers of the Booroola fecundity gene.

Castrated adult FecBFecB and Fec+Fec+ Booroola rams were injected with charcoal-treated bovine follicular fluid (bFF) (a source of inhibin-like activity) or given testosterone implants to examine whether the fecundity gene (FecB) influences sensitivity to negative feedback hormones in males. Mean concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) did not differ between genotypes before treatment. In Expt 1, injections of 5 ml bFF, but not of 1 ml (each given four times at intervals of 8 h), significantly (P < 0.05) depressed concentrations of LH and FSH, but there was no effect of genotype. After treatment, gonadotrophin concentrations returned to pretreatment values and for 2-2.5 days scaled (divided by pretreatment mean) LH values (235 +/- 49 for FecBFecB and 96 +/- 26% for Fec+Fec+ rams; P < 0.05) and scaled FSH values (106 +/- 5 for FecBFecB and 85 +/- 5% for Fec+Fec+ rams; P < 0.05) were significantly higher in FecBFecB than in Fec+Fec+ rams in the group that received 5 ml bFF. Irrespective of genotype, treatment with 5 ml bFF did not reduce mean FSH to concentrations observed in testis-intact rams. In Expt 2, Silastic envelopes were implanted subdermally to give physiological or supraphysiological circulating concentrations of testosterone. Both doses significantly reduced scaled LH values in a biphasic manner, such that there was an initial suppression followed by a short-lived increase. During the initial period of suppression in the lower dose group, mean scaled LH values were significantly higher in FecBFecB than in Fec+Fec+ rams (48.3 +/- 7.5 versus 23.1 +/- 5.5%; P < 0.05). Low doses of testosterone decreased LH pulse frequency in both genotypes but decreased (P < 0.05) pulse amplitude and mean concentrations in the Fec+Fec+ animals only. In nonimplanted control rams, mean LH concentrations (in samples taken every 10 min for 12 h) were significantly lower in FecBFecB than in Fec+Fec+ rams (0.6 +/- 0.2 versus 1.3 +/- 0.1 ng ml-1; P < 0.05). The mean FSH response to testosterone was not related to genotype. These data suggest that expression of the FecB gene results in an altered sensitivity of the pituitary gland to changes in negative feedback from testicular hormones and that, irrespective of genotype, neither testosterone nor inhibin-like activity alone can fully control FSH secretion in castrated rams.     

The effects of gastric inhibitory polypeptide (GIP) on the release of anterior pituitary hormones

Several members of the secretin family of hormones have been demonstrated to alter anterior pituitary hormone secretion. Here we report the action of gastric inhibitory polypeptide (GIP) on gonadotropin and somatotropin release. Intraventricular injection of 1 microgram (0.2 nmole) GIP (2.5 microliters) produced a significant decrease in plasma FSH at 30 (p less than 0.02) and 60 min after its injection (p less than 0.01). The FSH-lowering effect of a higher dose of 5 micrograms (1 nmole) of GIP was already developed at 15 min (p less than 0.01) and was prolonged until the end of the experiment (60 min, p less than 0.05). No change in plasma LH was detected at any time during the experimental period. If 5 micrograms of estradiol-benzoate were given SC 48 hr prior to experiment, the initial values of FSH and LH were markedly decreased. In these animals GIP failed to influence plasma FSH and LH. When dispersed anterior pituitary cells from OVX rats were cultured overnight and incubated in vitro with GIP, the peptide was found to induce both FSH and LH release. Highly significant release occurred with the lowest dose tested of 10(-7) M and there was a dose-response effect for both hormones. The slope of the dose-response curve was similar for both FSH and LH release. GIP was less potent than LHRH which produced a greater stimulation of both FSH and LH release at a dose of 10(-9) M than did 10(-7) M GIP. The two peptides had an additive effect on the release of both FSH and LH.(ABSTRACT TRUNCATED AT 250 WORDS)     

A change in basal FSH release accompanies the onset of the second or selective phase of increased serum FSH in the cyclic rat

Experiments were undertaken to investigate if changes occur at the level of the anterior pituitary gland to result in selective follicle-stimulating hormone (FSH) release during late proestrus in the cyclic rat. At 1200 h proestrus, prior to the preovulatory luteinizing hormone (LH) surge in serum and the accompanying first phase of FSH release, serum LH and FSH concentrations were low. At 2400 h proestrus, after the LH surge and shortly after the onset of the second or selective phase of FSH release, serum LH was low, serum FSH was elevated about 4-fold, pituitary LH concentration was decreased about one-half and pituitary FSH concentration was not significantly decreased. During a two hour $\text{in}$$\text{vitro}$ incubation, pituitaries collected at 2400 h released nearly two-thirds less LH and 2.5 times more FSH than did pituitaries collected at 1200 h. Addition of luteinizing hormone releasing hormone (LHRH) to the incubations caused increased pituitary LH and FSH release. However, the LH and FSH increments due to LHRH in the 2400 h pituitaries were not different from those in the 1200 h pituitaries. The results indicate that a change occurs in the rat anterior pituitary gland during the period of the LH surge and first phase of FSH release which results in a selective increase in the basal FSH secretory rate. It is suggested that this change is primarily responsible for the selective increase in serum FSH which occurs during the second phase of FSH release.     

Ontogeny of pituitary gonadotrophin secretion in the fetal and post-natal pig in response to LHRH in vitro

Monolayer cultures of anterior pituitary cells from male or female pigs of 60, 80, 105 days of fetal life or of 60, 160 and 250 days of post-natal life were prepared and treated with LHRH (1 pM to 10 nM). Dose-related increases of LH were first seen at 80 days of gestation in both sexes, while only female fetuses responded to maximal LHRH at 60 days. Basal and stimulated LH release doubled in cultures from 105-day-old fetuses when compared with those at 80 days. Compared to late fetal stages LH release was 20- to 30-fold higher in cell cultures from 60-day-old (post-natal) donors without further change during the post-natal period. In all pre- and post-natal age groups basal and maximal LH release of pituitary cells from males was lower than that of females. FSH stimulation started in male and female cells at 80 days of gestation only at LHRH concentrations exceeding or equal to 0.1 nM. By 105 days FSH secretion was dose-related and pituitary cells of females responded with higher FSH values than did those of males. In general, post-natal cells released much higher amounts of FSH than did prenatal cells. Basal and maximal release of FSH decreased during post-natal development in both sexes. Basal as well as maximal FSH release of cultures from female donors was higher than that found in cultures from male donors. Determination of total LH and FSH content in fetal pituitary cell cultures indicated that the developmental increase in gonadotrophin release potential is a function of the total gonadotrophin content in vitro. We conclude that (1) the in-vitro release of gonadotrophins to LHRH is dose-, age- and sex-dependent; (2) in the female fetal pig LH responsiveness develops earlier than FSH responsiveness; (3) apparently, these maturational changes mainly reflect alterations in pituitary gonadotrophin content; and (4) there is no simple relationship between in-vitro release and circulating gonadotrophins.     

Cysteamine reduces serum gonadotropin concentrations in adult male rats

We have examined the effects of cysteamine on the hypothalamic-pituitary-gonadal axis of the adult male rat. A single subcutaneous injection of cysteamine (300 mg/kg) reduces significantly (p less than or equal to 0.05 serum concentrations of LH, FSH and T. Cysteamine blocked LH secretion induced by castration and administration of naloxone and LHRH. Neither acute nor chronic treatment (7 days) altered the hypothalamic LHRH content. These results suggest that cysteamine acts to reduce pituitary responsiveness to LHRH, resulting in lower mean serum gonadotropin and testosterone concentrations. It is possible, however, that cysteamine acts also at the hypothalamus to reduce LHRH secretion and/or at the testes to reduce testosterone release.     

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.     

Inhibition of pituitary-gonadal function in male rats by a potent GnRH antagonist

The inhibitory effects of the potent GnRH antagonist, [Ac-D-pCl-Phe1,2,D-Trp3,D-Arg6,DAla10]GnRH (GnRHant) upon pituitary-gonadal function were investigated in normal and castrated male rats. The antagonist was given a single subcutaneous (s.c.) injections of 1-500 micrograms to 40-60 day old rats which were killed from 1 to 7 days later for assay of pituitary GnRH receptors, gonadal receptors for LH, FSH, and PRL, and plasma gonadotropins, PRL, and testosterone (T). In intact rats treated with low doses of the antagonist (1, 5 or 10 micrograms), available pituitary GnRH receptors were reduced to 40, 30 and 15% of the control values, respectively, with no change in serum gonadotropin, PRL, and T levels. Higher antagonist doses (50, 100 or 500 micrograms) caused more marked decreases in free GnRH receptors, to 8, 4 and 1% of the control values, which were accompanied by dose-related reductions in serum LH and T concentrations. After the highest dose of GnRHant (500 micrograms), serum LH and T levels were completely suppressed at 24 h, and serum levels of the GnRH antagonist were detectable for up to 3 days by radioimmunoassay. The 500 micrograms dose of GnRHant also reduced testicular LH and PRL receptors by 30 and 50% respectively, at 24 h; by 72 h, PRL receptors and LH receptors were still slightly below control values. In castrate rats, treatment with GnRHant reduced pituitary GnRH receptors by 90% and suppressed serum LH and FSH to hypophysectomized levels. Such responses in castrate animals were observed following injection of relatively low doses of GnRHant (100 micrograms), after which the antagonist was detectable in serum for up to 24 h. These data suggest that extensive or complete occupancy of the pituitary receptor population by a GnRH antagonist is necessary to reduce plasma gonadotropin and testosterone levels in intact rats. In castrate animals, partial occupancy of the available GnRH receptor sites appears to be sufficient to inhibit the elevated rate of gonadotropin secretion.     

Inhibitory effects of a luteinizing-hormone-releasing hormone agonist implant on ovine fetal gonadotrophin secretion and pituitary sensitivity to luteinizing-hormone-releasing hormone.

Sheep fetuses at day 70 of gestation (term = 145 days) were implanted subcutaneously with a biodegradable implant containing a luteinizing-hormone-releasing hormone (LHRH) agonist (buserelin) to investigate whether treatment with LHRH agonist would induce a state of desensitization of the fetal gonadotrophs and thus influence fetal gonadal development. Treatment with the LHRH agonist for 35-40 days caused a significant reduction in mean fetal plasma concentrations of LH and follicle-stimulating hormone (FSH) compared with control fetuses. LH pulses were evident in control fetuses but were completely abolished by buserelin treatment. Furthermore, the pituitary content of LH and FSH was significantly depleted in fetuses implanted with LHRH agonist. A bolus intravenous injection of 500 ng LHRH given to control fetuses caused a rapid and significant increase in plasma LH and FSH concentrations which was sustained for at least 60 min after injection. Pretreatment with buserelin completely abolished the LH and FSH responses to a bolus injection of LHRH. There were no differences between the sexes in fetal gonadotrophin concentrations or pituitary sensitivity to LHRH in control or agonist-treated fetuses. Furthermore, buserelin treatment for 35-40 days had no effect on the morphological appearance of the fetal gonads when compared with control fetuses, at least to day 110 of pregnancy. These results provide evidence for the induction of a state of desensitization of the LHRH receptors of the fetal pituitary gonadotrophs following long-term treatment with an LHRH agonist, but provide no evidence for a role for gonadotrophin secretion in gonadal development at this stage in fetal life.     

The rapid effects of luteinizing hormone releasing hormone agonists and antagonists on the mouse pituitary in vitro

The effect of luteinizing hormone releasing hormone (LHRH) and its analogs on the release of FSH and LH by 20 day old whole mouse pituitary incubated in vitro for 3-4 hrs was investigated. Three agonistic analogs (AY 25650, 25205 and Buserelin) all of which are reported to be superactive in vivo showed approximately the same potency in this in vitro test system. Preincubation of the pituitaries for 1 h with the antagonistic analogs [Ac Dp Cl Phe1,2, D Trp3, D Phe6, D Ala10] LHRH and [Ac Dp Cl Phe1,2, D Trp3, D Arg6, D Ala10] LHRH inhibited the secretion of LH and FSH induced by 2.5 x 10(-9)M LHRH. The inhibitory response was dose dependent. The continued presence of the antagonists was not required for effective suppression of the LHRH effect. Experiments designed to find out the minimum time required for eliciting suppression of LHRH revealed that preincubation of the pituitary with the second antagonist for 5 mins followed by removal was adequate to produce effective inhibition of gonadotropin release. At lower doses of the antagonist, LH release was more effectively inhibited than FSH release. The results suggest that antagonistic analogs can effectively bind to LHRH receptors in the whole pituitary incubation preventing the subsequent action of LHRH. With the present incubation system assessment of bioactive LH and FSH release is possible within 24 hrs.