Effects of testosterone, dihydrotestosterone and estradiol on gonadotropin release after gonadotropin releasing hormone administration in cyclic mares

Sixteen intact cyclic mares were treated on the fourth day of estrus and then every other day for a total of six injections with 1) testosterone propionate, 2) dihydrotestosterone (DHT) benzoate, 3) estradiol (E2) benzoate or 4) safflower oil. Mares were given gonadotropin releasing hormone (GnRH) on Day 3 of estrus (pretreatment) and again 24 h after the last steroid or oil injection. Treatment with testosterone propionate resulted in a greater (P less than 0.05) follicle-stimulating hormone (FSH) response to the second injection of GnRH compared with all other treatments. Treatment with DHT benzoate also resulted in greater (P less than 0.05) FSH response to GnRH compared with control and E2 benzoate-treated mares. Testosterone propionate and E2 benzoate administration suppressed (P less than 0.05) the normal diestrous rise in FSH concentrations exhibited by the control and DHT benzoate-treated mares. Steroid treatment did not affect the luteinizing hormone (LH) response to GnRH, although testosterone propionate treatment did suppress concentrations of LH in daily blood samples during Days 3 to 6 of treatment. It is concluded that testosterone's effect on FSH after GnRH treatment observed in this and previous experiments can be attributed to two different properties of the hormone or its metabolites acting simultaneously. That is, testosterone increased the secretion of FSH in response to GnRH as did DHT (an androgenic effect). At the same time, testosterone suppressed FSH concentrations in daily blood samples in a manner identical to that of E2 benzoate (an estrogenic effect).     

Androgen and progesterone effects on follicle-stimulating hormone and luteinizing hormone secretion in anestrous mares

Anestrous lighthorse mares were treated in December with dihydrotestosterone (DHT; 150 micrograms/kg of body weight), progesterone (P; 164 micrograms/kg), both DHT and P (DHT+P), testosterone (T; 150 micrograms/kg), or vehicle (n = 4/group). Daily blood sampling was started on Day 1, and on Day 4 all mares were administered a pretreatment injection of gonadotropin-releasing hormone (GnRH) and were bled frequently to characterize the responses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations. Treatment injections were given on Day 4 and then daily through Day 17. On Day 18, all mares were again administered GnRH and were bled frequently. Treatment of mares with DHT, P, or T increased (p less than 0.01) plasma concentrations of these steroids to approximately 1.5 ng/ml during the last 10 days of treatment. There was no effect (p greater than 0.10) of treatment on LH or FSH concentrations in daily blood samples. Relative to the pretreatment GnRH injection, mares treated with T or DHT+P secreted approximately 65% more (p less than 0.01) FSH in response to the post-treatment GnRH injection; FSH response to the second GnRH injection was not altered (p greater than 0.10) in control mares or in DHT- or P-treated mares. There was no effect of any steroid treatment on LH secretion after administration of GnRH (p greater than 0.10). Averaged over all mares, approximately 94 times more FSH than LH was secreted in response to injection of GnRH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Active immunization of gilts against gonadotropin-releasing hormone: effects on secretion of gonadotropins, reproductive function, and responses to agonists of gonadotropin-releasing hormone

Sexually mature gilts were actively immunized against gonadotropin-releasing hormone (GnRH) by conjugating GnRH to bovine serum albumin, emulsifying the conjugate in Freund's adjuvant, and giving the emulsion as a primary immunization at Week 0 and as booster immunizations at Weeks 10 and 14. Antibody titers were evident by 2 wk after primary immunization and increased markedly in response to booster immunizations. Active immunization against GnRH caused gonadotropins to decline to nondetectable levels, gonadal steroids to decline to basal levels, and the gilts to become acyclic. Prolactin concentrations in peripheral circulation were unaffected by immunization against GnRH. The endocrine status of the hypothalamic-pituitary-ovarian axis was examined by giving GnRH and two agonists to GnRH and by ovariectomy. An i.v. injection of 100 micrograms GnRH caused release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in control animals, but not in gilts immunized against GnRH. In contrast, administration of 5 micrograms D-(Ala6, des-Gly-NH2(10] ethylamide or 5 micrograms D-(Ser-t-But6, des-Gly-NH2(10] ethylamide resulted in immediate release of LH and FSH in both control and GnRH-immunized gilts. Circulating concentrations of LH and FSH increased after ovariectomy in the controls, but remained at nondetectable levels in gilts immunized against GnRH. Prolactin concentrations did not change in response to ovariectomy. We conclude that cyclic gilts can be actively immunized against GnRH and that this causes cessation of estrous cycles and inhibits secretion of LH, FSH, and gonadal steroids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gonadotrope function in ovariectomized ewes actively immunized against gonadotropin-releasing hormone (GnRH)

The gonadotrope cells of the ovine anterior pituitary were insulated from hypothalamic inputs by imposing an immunologic barrier generated by active immunization of ovariectomized ewes against gonadotropin-releasing hormone (GnRH) conjugated to keyhole limpet hemocyanin (KLH) through a p-aminophenylacetic acid bridge. All GnRH-KLH animals immunized developed titers of anti-GnRH that exceeded 1:5000. The antisera were specific for GnRH and cross-reacted with GnRH agonists modified in position 10 to an extent that was less than 0.01%. Ewes actively immunized against GnRH-KLH displayed levels of basal and GnRH agonist-induced gonadotropin secretion that were markedly lower (p less than 0.05) than comparable parameters in ewes actively immunized against KLH. In contrast, basal and thyrotropin-releasing hormone (TRH)-induced prolactin (PRL) secretion were not compromised by active immunization. Immunization against the GnRH-KLH conjugate, but not KLH alone, prevented expression of the positive feedback response to exogenous estradiol (E2). Pituitary stores of immunoactive luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were significantly (p less than 0.001) reduced in ewes immunized against GnRH-KLH but stores of PRL were not affected by such immunization. Further, the biopotency of the residual LH stores in tissue of animals from the anti-GnRH group was significantly (p less than 0.05) lower than LH biopotency in anti-KLH animals. Serum levels of LH in anti-GnRH ewes were restored by circhoral administration of a GnRH agonist that did not cross-react with the antisera generated. Pulsatile delivery of GnRH agonist in anti-GnRH ewes significantly (p less than 0.05) elevated serum LH within 48 h and reestablished LH levels comparable to anti-KLH ewes within 6 days of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of in vivo administration of testosterone propionate on in vitro production of follicle-stimulating hormone and luteinizing hormone by pituitaries of pony mares

The in vitro incorporation of [3H]leucine into immunoprecipitable follicle-stimulating hormone (FSH) and luteinizing hormone (LH) was assessed for pituitaries from pony mares treated with testosterone propionate (TP) or oil (controls). Mares were treated every other day with TP (n = 4) at 350 micrograms/kg of body weight or with an equivalent volume of oil (n = 4). One day following the sixth injection of TP, each mare received an intravenous injection of gonadotropin releasing hormone (GnRH) at 1.0 micrograms/kg body weight and was bled frequently for 4 h. Treatment of mares with TP reduced FSH (P less than 0.05) and LH (P less than 0.01) concentrations in daily blood samples and increased (P less than 0.01) the amount of FSH secreted in response to GnRH compared with control mares. Incorporation of [3H]leucine into immunoprecipitable FSH was also greater (P less than 0.01) in pituitaries from TP-treated mares compared with control mares on both a per mg tissue and per anterior pituitary basis. The amount of LH secreted after GnRH, the amount left in the pituitary and the incorporation of [3H]leucine into LH were not affected by treatment. These results confirm earlier conclusions drawn from indirect evidence that androgens increase the production of FSH in the mare.     

Effect of gonadotropin-releasing hormone antagonists on serum follicle-stimulating hormone and luteinizing hormone under conditions of singular follicle-stimulating hormone secretion

Previous work has indicated that in long-term ovariectomized rats a potent antagonist to gonadotropin-releasing hormone (GnRH) suppressed serum luteinizing hormone (LH) more successfully than follicle-stimulating hormone (FSH). The present studies examined whether the rise in serum FSH which occurs acutely after ovariectomy, or during the proestrous secondary surge, depends on GnRH. In Experiment A, rats were ovariectomized at 0800 h of metestrus and injected with (Ac-dehydro-Pro1, pCl-D-Phe2, D-Trp3,6, NaMeLeu7)-GnRH (Antag-I) at 1200 h of the same day, or 2 or 5 days later. Antag-I blocked the LH response completely, but only partially suppressed serum FSH levels. Experiment B tested a higher dose of a more potent antagonist [( Ac-3-Pro1, pF-D-Phe2, D-Trp3,6]-GnRH; Antag-II) injected at the time of ovariectomy. The analog suppressed serum LH by 79% and FSH by 30%. Experiment C examined the effect of Antag-II on the day of proestrus on the spontaneous secondary surge of FSH, as well as on a secondary FSH surge which can be induced by exogenous LH. Antag-II, given at 1200 h proestrus, blocked ovulation and the LH surge expected at 1830 h, as well as increases in serum FSH which occur at 1830 h and at 0400 h. Exogenous LH triggered a rise in FSH in rats suppressed by Antag-II. In Experiment D proestrous rats were injected with Antag-II at 1200 h and ovariectomized at 1530 h. By 0400 h the antag had suppressed FSH in controls, but in the ovariectomized rats, a vigorous FSH response occurred.(ABSTRACT TRUNCATED AT 250 WORDS)     

Response of plasma LH and FSH to gonadotropin-releasing hormone in pony foals and ovariectomized pony mares

Plasma FSH and LH response to a synthetic GnRH analog was measured in adult ovariectomized pony mares (OVX) and in pony foals (<70 days of age) during late spring (May-June). FSH and LH responded in a similar fashion (200% increase) in the OVX mare, which is different from other reports for intact mares. There was a greater mean response to a comparable dose of GnRH in the prepubertal foal for both FSH (500%) and LH (900%) than in the OVX mare. There was a positive correlation between age and the maximum FSH response to GnRH in male and female foals. The LH response was positively correlated with age in male foals, but not in females. The response to GnRH in the prepubertal foals was consistent with the previously observed patterns of gonadotropin secretion during this age period.     

Passive immunization of the pig against gonadotropin releasing hormone during the follicular phase of the estrous cycle

Three experiments were conducted to determine the effects of passively immunizing pigs against gonadotropin releasing hormone (GnRH) during the follicular phase of the estrous cycle. In Experiment 1, sows were given GnRH antibodies at weaning and they lacked estrogen secretion during the five days immediately after weaning and had delayed returns to estrus. In Experiment 2, gilts passively immunized against GnRH on Day 16 or 17 of the estrous cycle (Day 0 = first day of estrus) had lower (P<0.03) concentrations of estradiol-17beta than control gilts, and they did not exhibited estrus at the expected time (Days 18 to 22). When observed three weeks after passive immunization, control gilts had corpora lutea present on their ovaries, whereas GnRH-immunized gilts had follicles and no corpora lutea. The amount of GnRH antiserum given did not alter (P<0.05) serum concentrations of LH or pulsatile release of LH in sows and gilts. In Experiment 3, prepuberal gilts were given 1,000 IU PMSG at 0 h and GnRH antiserum at 72 and 120 h. This treatment lowered the preovulatory surge of LH and FSH, but it did not alter serum estradiol-17beta concentrations, the proportion of pigs exhibiting estrus, or the ovulation rate. These results indicate that passive immunization of pigs against GnRH before initiation of or during the early part of the follicular phase of the estrous cycle retards follicular development, whereas administration of GnRH antibodies during the latter stages of follicular development does not have an affect. Since the concentration of antibodies was not high enough to alter basal or pulsatile LH secretion, the mechanism of action of the GnRH antiserum may involve a direct ovarian action.     

Changes in the hypothalamic-hypophyseal axis of mares associated with seasonal reproductive recrudescence

Four groups of mares, representing anestrus (AN; n = 8), early transition (ET; n = 7), late transition (LT; n = 8) and estrus (EST; n = 12) were used to examine changes in the hypothalamus and anterior pituitary during the period of transition from winter anestrus into the breeding season. Mares were of mixed breeding, between the ages of 3 and 20 years, and had shown normal patterns of estrous behavior and ovulation during the breeding season previous to this experiment. Hypothalamic content of gonadotropin-releasing hormone (GnRH) and anterior pituitary content of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of receptors for GnRH in anterior pituitary tissue was also determined. There was no effect of stage of transition into the breeding season on receptors for GnRH or content of FSH (p greater than 0.05). Likewise, content of GnRH in the hypothalamus did not differ between the four groups (p greater than 0.05). However, pituitary content of LH increased progressively from anestrus to the breeding season (p less than 0.05). Means for the AN, ET, LT and EST groups were 1.1 +/- 0.2, 2.2 +/- 0.3, 6.3 +/- 1.4 and 15.2 +/- 1.8 micrograms LH/mg pituitary, respectively. In addition, serum concentrations of LH associated with the first ovulation of the year for 5 of the EST mares were significantly lower (p less than 0.01) than those associated with the second ovulation of the year.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary gonadotropin-releasing hormone receptor content in rats with polycystic ovaries

A single injection of estradiol valerate (EV) induces, after a lag period of 4-6 wk, a chronic anovulatory polycystic ovarian (PCO) condition in adult rats. This condition is associated with a selective compromise of luteinizing hormone (LH) release and/or synthesis reflected in low basal serum LH concentrations, decreased pituitary content of LH, and decreased gonadotropin-releasing hormone (GnRH)-stimulated LH secretion. The present study was undertaken to determine to what extent the aberrant LH release in rats with PCO could be related to alterations in pituitary content of GnRH receptors. Pituitary GnRH-receptor content was assessed by the evaluation of saturation binding of a GnRH analog, [125I]-D-Ala6-des-Gly10-GnRH, to pituitary membrane preparations. The receptor content of pituitaries from rats with PCO was compared to that obtained from intact animals at estrus and diestrus. Receptor levels in ovariectomized normal rats and rats with PCO were also assessed. The pituitary GnRH receptor content in PCO rats was similar to that observed in normal controls at estrus and was significantly lower than that for rats at diestrus. Although a twofold increase in pituitary GnRH receptor content was observed at 28 days following the castration of control rats, GnRH receptor content in the pituitaries of PCO rats, at 28 days following ovariectomy, remained unchanged. Although, castration-induced elevations in mean serum LH and follicle-stimulating hormone (FSH) concentrations were observed in both the PCO and control animals, the rise in both gonadotropins was significantly attenuated in the PCO-castrates when compared to the ovariectomized controls. Since GnRH is a major factor in the regulation of pituitary GnRH receptor content, these findings suggest that hypothalamic GnRH release is impaired in rats with PCO and that this impairment is independent of any influences from the polycystic ovaries.     

Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content

These experiments tested the hypothesis that administration of steroid hormones to ovariectomized (OVX) mares during the vernal transition to the breeding season would influence LH and FSH secretion. Circulating gonadotropin concentrations, response to exogenous GnRH, and pituitary gonadotropin content were monitored. Experiments 1 and 2 were conducted, beginning 10 March, and 3 February, respectively, utilizing a total of 30 long-term OVX pony mares. In experiment 1, mares were administered vehicle (n = 5) or estradiol-17 beta (E2, n = 5, 5 mg/3 ml sesame oil), twice daily for 16 days. Blood samples were collected daily for assessment of circulating LH and FSH concentrations. On Day 10 of treatment, 400 micrograms GnRH were administered to all mares. LH increased significantly over days of treatment in the estradiol-treated group, but pituitary response to GnRH tended to be less than in control mares. Circulating FSH tended to decline over days of treatment in estradiol-treated mares, and the pituitary response to GnRH was significantly reduced. Pituitary LH, but not FSH, was increased on Day 16 of treatment with estradiol. In experiment 2, 20 OVX mares received, twice daily, vehicle (n = 5), E2, n = 5; 5 mg), progesterone (P4, n = 5; 100 mg), or progesterone plus estradiol (P4/E2, n = 5; 100 + 5 mg). Treatment continued for 14 days. GnRH (100 micrograms) challenges were administered on Days 6 and 13 of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of follicular fluid treatment on follicle-stimulating hormone, luteinizing hormone and compensatory ovarian hypertrophy in prepuberal gilts

Prepuberal 130-day-old gilts were treated with 10 ml of charcoal-stripped porcine serum (PS), whole porcine follicular fluid (WpFF) or charcoal-stripped pFF (CpFF) twice daily beginning the day before and continuing 8 days after unilateral ovariectomy (ULO). Follicle-stimulating hormone (FSH) declined for the first 14 h after ULO in WpFF and CpFF gilts and then by 24 h returned to values observed at or before ULO, whereas FSH was increased nearly twofold at 14 h in PS gilts. At 8 days after ULO the remaining ovaries from PS-treated gilts were heavier than ovaries from follicular fluid-treated gilts. In a second experiment, ovariectomized 130-day-old gilts were assigned to either a group infused with PS, a group infused with 5 ml CpFF, or a group infused with 10 ml Cpff at 18 and 2 h before a gonadotropin-releasing hormone (GnRH) challenge. Porcine follicular fluid had no effect on luteinizing hormone (LH) response to GnRH, depressed the FSH response to a 10-micrograms challenge of GnRH, but had no effect on FSH response to a 50-micrograms challenge of GnRH. In a third study, gilts were subjected to sham ovariectomy (Sham) or ULO at 130 days of age. GnRH (10 micrograms) was given on Days 1, 2 or 8 after surgery. The response to GnRH in ULO versus Sham gilts did not differ for FSH or LH on any day.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gonadotropin-releasing hormone at estrus: luteinizing hormone, estradiol, and progesterone during the periestrual and postinsemination periods in dairy cattle

Administering gonadotropin-releasing hormone (GnRH) improved conception rates in our previous studies. Our objective was to determine if the effect of GnRH was mediated through serum luteinizing hormone (LH) and/or by altered secretion of serum progesterone (P) and estradiol-17 beta (E) during the periestrual and post-insemination periods. Cattle were given either GnRH (n = 54) or saline (n = 55) at 72 h and inseminated artificially (AI) 80 h after the second of two injections of either prostaglandin F2 alpha or its analog, cloprostenol. Progesterone and E were measured in blood serum collected during 3 wk after AI (estrus) from 60 females. Blood was collected for LH determinations via indwelling jugular cannulae from 14 cows and 11 heifers. Collections were taken every 4 h from 32 to 108 h after the second PGF injection (PGF-2) (periestrual period) and at more frequent intervals during 240 min after administration of GnRH (n = 18) or saline (n = 7). Ten females had a spontaneous preovulatory LH surge before GnRH treatment (GnRH-spontaneous), whereas GnRH induced the preovulatory LH surge in six females. A spontaneous LH surge appeared to be initiated in two heifers at or near the time of GnRH treatment (spontaneous and/or induced). The remaining seven cows had spontaneous LH surges with no subsequent change in LH after saline treatment. Serum P during the 21 days after estrus was lower (p less than 0.05) in both pregnant and nonpregnant (open) cattle treated previously with GnRH compared with saline. Serum P during the first week after estrus was greater (p less than 0.01) and increased (p less than 0.05) more rapidly in saline controls and in GnRH-spontaneous cattle than in those exhibiting GnRH-induced or GnRH-spontaneous and/or-induced surges of LH. Conception rate of cattle receiving GnRH was higher (p = 0.06) than that of saline-treated controls. These data suggest that GnRH treatment at insemination initiated the preovulatory LH surge in some cattle, but serum P in both pregnant and open cows was compromised during the luteal phase after GnRH treatment. Improved fertility may be associated with delayed or slowly rising concentrations of serum progesterone after ovulation.     

Luteinizing hormone release in intact and castrate rams is altered with immunoneutralization of endogenous estradiol

Changes in the dynamics of luteinizing hormone (LH) release in the adult ram following immunoneutralization of endogenous estradiol were investigated. Castrate rams were actively immunized against estradiol-6-bovine serum albumin for 7 months and then their patterns of episodic LH release and LH response to multiple injections of gonadotropin-releasing hormone (GnRH, two 5-micrograms doses given iv 2 h apart) were assessed (April). In comparison with control rams immunized against rabbit gamma globulin, estradiol-immunized rams (antibody titre approximately 1:5000) exhibited more frequent LH releases (11.7 +/- 0.3 vs. 9.3 +/- 0.8 pulses/8 h, P less than 0.05) and a greater LH response to the first GnRH injection (peak delta value 190 +/- 8 vs. 130 +/- 25 ng/mL, P less than 0.01). Estradiol antiserum collected from the castrate rams was used in the passive immunization of intact rams (antibody titre approximately 1:200) for 1 month (beginning mid-July). Although episodic LH release was always similar for control and immunized rams, testosterone levels in the latter group increased approximately 150%. In contrast with the castrate ram response, GnRH treatment (two 5-micrograms doses given iv 80 min apart) produced a "self-priming" effect on LH release in the intact rams, an effect that was dampened with estradiol immunoneutralization. Consequently, peak 2:peak 1 ratios for delta value and 80-min mean incremental increase were much smaller (P less than 0.01) for the immunized rams (approximately 2:1 vs. 4:1 for the control rams).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effectiveness of an antagonist to gonadotrophin releasing hormone on the FSH and LH response to GnRH in perifused equine pituitary cells,and in seasonally acyclic mares

We wish to use a gonadotrophin-releasing hormone (GnRH) antagonist in the mare as a tool for investigating the control of the oestrous cycle. The aim of this study was to test the effectiveness of the antagonist cetrorelix by testing both in vitro, using perifused equine anterior pituitary cells, and in vivo in seasonally acyclic mares. Pituitary cells were prepared and after 3-4 days incubation, loaded onto columns and given four pulses of GnRH (at 0, 30, 60 and 90 min; dose-response study). After the second GnRH pulse, infusion of cetrorelix began (0, 100, 1000 and 2000 pmol/l) and continued until the end of the experiment. To mimic luteal phase conditions, cells were pre-incubated and perifused with progesterone (25 nmol/l) and GnRH pulses given at 0, 90, 180 and 270 min. Cetrorelix (0 or 1000 pmol/l) began after the second GnRH pulse. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) concentrations were measured in 5 min fractions. Both FSH and LH response areas (above baseline) after GnRH were inhibited by 1000 pmol/l cetrorelix (P < 0.01, P < 0.01, respectively) but not by 100 pmol/l cetrorelix. Similarly, in the presence of progesterone, cetrorelix inhibited the FSH (P < 0.001) and LH (P = 0.0002) response area. Seasonally acyclic mares, pre-treated for 3 days with progesterone (150 mg i.m. per day) were given cetrorelix as (i) a loading dose of 1 microg/kg then infusion at 2.2 ng/(kg min) for 90 min, (ii) a s.c. injection at 20 microg/kg, (iii) infusion at 2.2 ng/(kg min) for 48 h, and (iv) no cetrorelix (control mares). At 90 min, 6, 24 and 48 h after cetrorelix was first administered, mares were given a bolus injection of GnRH (22.2 ng/kg i.v.) and the FSH and LH responses measured. All doses of cetrorelix inhibited the FSH response at 90 min. The response was no longer suppressed at 6 h in the 90 min infusion group, showing a rapid recovery from inhibition. At 24 h, the FSH responses in the injected and 48 h infusion group were suppressed. The LH concentrations were low and showed no significant changes. This study has defined the time course and dose of cetrorelix with respect to its effect on FSH in the horse. It is concluded that cetrorelix could be used to elucidate the role of FSH in follicular development in cyclic mares.     

Pituitary gonadotropins, hypothalamic gonadotropin-releasing hormone, and testicular traits of boars exposed to natural or supplemental lighting during pubertal development

Seventy crossbred boars were reared under natural (30 lux) or supplemental lighting (1000 lux) beginning at 4 wk of age. Boars received supplemental lighting from six 40-watt fluorescent bulbs between 0530 and 2030 h. Five boars from each treatment were killed at 67, 91, 119, 155, 182, 210, or 246 days of age. No differences (p greater than 0.05) in pituitary concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) were found between treatment groups at any age. Total pituitary content of LH, FSH and PRL increased as boars became older, but when expressed as hormone concentration, only PRL increased with age. Content of gonadotropin-releasing hormone (GnRH) in the pituitary stalk-median eminence, preoptic area, and hypothalamus proper was similar (p greater than 0.05) between treatments. When GnRH contents were totaled and combined for the treatment groups, it was found that GnRH content increased (p less than 0.05) as boars became older. No differences (p greater than 0.05) were observed in testicular volume percentage of seminiferous tubules and tubular diameter between lighting treatments. These data demonstrate that the supplemental lighting does not influence puberty in boars by altering hypothalamic content of GnRH or pituitary stores of LH, FSH, and PRL.     

Gonadotropin secretion in ovariectomized ewes: effect of passive immunization against gonadotropin-releasing hormone (GnRH) and infusion of a GnRH agonist and estradiol

Gonadotropin secretion was examined in ovariectomized sheep after passive immunization against gonadotropin-releasing hormone (GnRH). Infusion of ovine anti-GnRH serum, but not control antiserum, rapidly depressed serum concentrations of luteinizing hormone (LH). The anti-GnRH-induced reduction in serum LH was reversed by circhoral (hourly) administration of a GnRH agonist that did not cross-react with the anti-GnRH serum. In contrast, passive immunization against GnRH led to only a modest reduction in serum concentrations of follicle-stimulating hormone (FSH). Pulsatile delivery of the GnRH agonist did not influence serum concentrations of FSH. Continuous infusion of estradiol inhibited and then stimulated gonadotropin secretion in animals passively immunized against GnRH, with gonadotrope function driven by GnRH agonist. However, the magnitude of the positive feedback response was only 10% of the response noted in controls. These data indicate that the estradiol-induced surge of LH secretion in ovariectomized sheep is the product of estrogenic action at both hypothalamic and pituitary loci. Replacement of the endogenous GnRH pulse generator with an exogenous generator of GnRH-like pulses that were invariant in frequency and amplitude could not fully reestablish the preovulatory-like surge of LH induced by estradiol.     

Effects of active immunization against gonadotropin releasing hormone on serum luteinizing hormone,progesterone levels and estrous cycles in the guinea pig

Mature female guinea pigs that had been observed to undergo three consecutive periods of estrus at approximately 16-day intervals were immunized with either 100 μg gonadotropin releasing hormone (GnRH) conjugated to 100 μg bovine serum albumin (BSA) or 100 μg BSA alone during diestrus (day 5–10) of the fourth cycle. Booster immunizations were administered 32 days after the first injection. Animals were bled by cardiac puncture at the time of first injection and at 16, 32, 48 and 64 days. Animals were necropsied at 64 days after first treatment.Daily observation indicated that vaginal manifestation of estrus was not apparent after a period equal to one estrous cycle in seven of ten GnRH immunized guinea pigs and after two cycles in the remaining three GnRH immunized guinea pigs. Estrous cycles persisted in BSA treated females throughout the experiment.Serum luteinizing hormone (LH) declined significantly by 32 days after the first immunization against GnRH and remained lower than both pretreatment values and levels in control animals at the same bleeding times throughout the experiment. Serum progesterone levels were significantly lower in the GnRH immunized group than in the control group at 48 and 64 days.At necropsy the weight of the ovaries of GnRH immunized guinea pigs was significantly lower than that of controls. Corpora lutea and antral follicles were present in both GnRH treated and control females. The presence of serum progesterone levels and of antral follicles in the GnRH immunized females suggests that a low level of gonadotropic support may have persisted to 64 days after initiation of treatment.Results indicate that immunization against GnRH can reduce LH and progesterone levels and induce cessation of estrous cycles in the guinea pig.     

Feedback effects of ovarian steroids on the hypothalamic-hypophyseal axis in the rabbit

The feedback effects of two ovarian steroids, estradiol-17 beta (E2) and 20 alpha-hydroxypregn-4-en-3-one (20 alpha OH), were examined in both intact (INT) and ovariectomized (OVEX) does. We measured steroid-induced alterations in endogenous gonadotropin-releasing hormone (GnRH) from sequential 10-min samples of hypothalamic perfusates, simultaneous changes in peripheral plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the modification of pituitary responsiveness, i.e., increments in plasma LH (delta LH) and plasma FSH (delta FSH), after 50 ng, 250 ng, and 1 microgram of exogenous GnRH in individual does of 6 treatment groups. The groups were: INT does, OVEX does, OVEX does receiving either one (1 E2) or two (2 E2) E2-filled Silastic capsules, OVEX does receiving a 20 alpha OH-filled capsule (20 alpha OH), and OVEX does receiving both capsules of E2 and 20 alpha OH (1 E2 + 20 alpha OH). Ovariectomy enhanced the pulsatile release of hypothalamic GnRH and pituitary LH and FSH, and increased the LH response (delta LH) to exogenous GnRH (OVEX vs. INT, p less than 0.05). Replacement of E2 at the time of ovariectomy prevented the increased GnRH and gonadotropin secretion as well as the enhanced delta LH that were observed in untreated OVEX does. The release of hypothalamic GnRH in the 20 alpha OH group was lower (p less than 0.05) than that in the OVEX group and not different from that in the INT group. The release of pituitary LH and FSH and the delta LH in the 20 alpha OH group was not different from that in the OVEX group, but these parameters were greater (p less than 0.05) than those in the INT group. The hypothalamic GnRH pulse frequency in the 1 E2 + 20 alpha OH group was lower (p less than 0.05) than that in either the 1 E2 or the 20 alpha OH group, but the delta LH in the 1 E2 + 20 alpha OH group was not different from that in either the 1 E2 or the 20 alpha OH group. The highest dose (1 microgram) of exogenous GnRH stimulated a modest increase in FSH in the OVEX, 20 alpha OH, 1 E2 + 20 alpha OH, and 1 E2 groups; but a steroid effect on delta FSH among these 4 groups was not apparent.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal variation in hypothalamic content of gonadotropin-releasing hormone (GnRH), pituitary receptors for GnRH, and pituitary content of luteinizing hormone and follicle-stimulating hormone in the mare

Seasonal changes in the hypothalamic-hypophyseal axis were investigated using tissue from 49 light-horse mares, of mixed breeding. Hypothalamic and pituitary tissues were collected at 5 intervals throughout the years 1981 and 1982, representing midbreeding season (July, n = 10), transition out of the breeding season (October, n = 11), midanestrus (December, n = 8), transition into the breeding season (March, n = 10), and again in the following midbreeding season (July, n = 10). The hypothalamic region was dissected into preoptic area, body and median eminence. Gonadotropin-releasing hormone (GnRH) was extracted from hypothalamic samples with methanol-formic acid and quantified by radioimmunoassay. The anterior pituitary was homogenized and receptors for GnRH were quantified in a crude membrane fraction. Concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in the resulting supernatant. Content of GnRH in each of the 3 hypothalamic areas varied with season (P less than 0.01) and was lowest during midanestrus (P less than 0.05). There was no effect of season (P greater than 0.01) on either concentration or total number of receptors for GnRH, or concentration of FSH in the anterior pituitary. Concentrations of LH in the anterior pituitary varied with season (P less than 0.001). Means (+/- SEM) for the 5 collection times were 15.5 +/- 2.7, 9.7 +/- 2.4, 2.3 +/- 0.5, 2.7 +/- 0.4 and 11.7 +/- 1.5 microgram LH/mg anterior pituitary, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)