Avian reproductive endocrinology.

Hypothalamic-releasing factors regulate the secretion of anterior pituitary hormones. The anterior pituitary gland secretes the same six hormones as found in mammals: FSH, LH, prolactin, GH (somatotropic hormone), ACTH, and TSH, plus the melanotropic hormone. The endocrine hormones of the avian posterior pituitary gland concerned with reproduction are mesotocin and AVT. The pineal gland, through the secretion of the hormone melatonin, modulates the periodic autonomic functions of the central nervous system. The ovary produces estrogens, progestogens, and androgenic compounds. The testes produce testosterones and progesterone. The thyroid glands produce two hormones, T4 and T3. The avian adrenal glands produce corticosterone and aldosterone. The bursa of Fabricius is considered an endocrine organ since it is involved in the production of humoral factors. The male reproductive system undergoes hormonal changes associated with puberty, the breeding season, and molt. Some avian species undergo a type of disintegration and seasonal reconstruction of the testis and epididymis. The relationship of the ovarian follicular hormones and the plasma hormones varies depending on the stage of the reproductive cycle and the seasonal photostimulation. Female birds may conceive in the absence of a mate as a result of the fertile period phenomena. The blood chemistry of laying birds is different from that seen in nonlaying hens. Domestication has had a definite influence on the hormone cycles of some avian species. This may lead to certain reproductive problems.     

Alterations in the ability of the bovine pituitary gland to secrete gonadotropins in vitro during the first follicle-stimulating hormone increase of the estrous cycle and in response to exogenous steroids

The objective was to determine if the endocrine status of the animal dictates the responsiveness of gonadotrophs to estradiol, activin, inhibin and follistatin; hormones implicated in the differential release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Bovine pituitaries were obtained at 13 (n=8), 30 (n=24) and 66 (n=8) h after the onset of estrus, corresponding to before, during and the end of the first FSH increase of the estrous cycle which follows the pre-ovulatory gonadotropin surge in heifers. Heifers slaughtered at 30 h received no treatment, or were treated with progesterone with or without estradiol before slaughter to suppress the first transient FSH increase. Secretion of FSH from cultured pituitary cells, reflecting the prior in vivo status, was greater (P<0.01) at 30 h than 13 or 66 h, whereas, LH secretion was less (P<0.01) at 13 h compared with 30 h. Treatment with exogenous steroids decreased (P<0.05) the pituitary gland's ability to subsequently secrete FSH and LH. Inhibin and, to a greater extent, estradiol decreased (P<0.01) mean FSH secretion but increased (P<0.05) mean LH secretion. These findings suggest that estradiol and inhibin both have the ability to differentially modulate basal gonadotropin secretion during the first FSH increase of the bovine estrous cycle. Differential regulation of LH and FSH is mediated via an alteration in gonadotropin biosynthesis and basal secretion. Furthermore, the secretory capability of cultured pituitary cells and basal gonadotropin secretion reflect the prior endocrine status of the animal from which pituitaries were obtained.     

日粮能量对育成柴鸡外周血浆促性腺激素与垂体分泌促性腺激素的影响

采用单因素方差随机试验,选用14周龄育成柴鸡36只,随机的分成3个处理,每个处理3个重复,每个重复4只鸡,分别饲喂低能、对照、高能的日粮,试验期为28d。结果显示,育成柴鸡外周血浆FSH浓度,低能、对照与高能差异极限著（P〈0.01）,并且随着日龄增加FSH浓度逐渐增加。育成柴鸡外周血浆LH浓度第1周时低能组与对照、高能组差异极显著（P〈0.01）,对照与高能组差异不显著;第2、3周处理低能组、对照、高能组差异极显著（P〈0.01）,并且随着日龄增加浓度增加。育成柴鸡体外培养垂体细胞分泌FSH、LH高能组与低能、对照组差异极显著（P〈0.01）,而低能组与对照组差异不显著。结果表明,日粮能量通过下丘脑-垂体-性腺轴影响育成柴鸡生殖机能。     

日粮能量对乌鸡外周血清促性腺激素与垂体分泌促性腺激素的影响

采用体外细胞培养和放射免疫测定法（RIA）测定日粮能量对鸟鸡外周血清促性腺激素与垂体分泌促性腺激素的影响。日粮能量处理：低能Ⅱ组、低能I组、对照组、高能I组、高能Ⅱ组;体外培养垂体细胞组分：高能组、对照纽、低能组。结果显示,血清中FSH含量,对照组与高能Ⅱ组相比差异显著（P〈0．05）,与低能I、Ⅱ组相比差异极显著（P〈0．01）;血清中LH含量,对照组与高能I组、高能Ⅱ组组间差异显著（P〈0．05）;低能Ⅱ组与高能I组、低能Ⅱ组与高能Ⅱ组、低能I组与高能I组、低能I组与高能Ⅱ组组间差异极显著（P〈0．01）;单层垂体细胞中FSH含量,高能组和对照组与低能组比差异显著（P〈0．05）;单层垂体细胞中LH含量,高能组与低能组比较差异极显著（P〈0．01）,对照组与低能组比较差异不显著。结果表明,日粮能量对处理体外培养乌鸡垂体细胞分泌FSH、LH有促进作用。     

Pituitary and testicular responses of beef bulls to active immunization against inhibin alpha

Prepubertal crossbred beef bulls served as controls or were actively immunized against the N-terminal, 30-amino acid synthetic fragment of porcine inhibin alpha, pI alpha (1-30). Antibody titers were detected in sera (greater than 40% B/BO in sera diluted 1,000-fold) but not in rete testis fluid of 390-d-old bulls. Serum FSH and inhibin remained static during a 5-h intensive bleed; inhibin was not acutely affected by a 15-fold LH rise and a threefold FSH rise induced by exogenous GnRH. Serum FSH, but not LH or testosterone, was consistently elevated (P less than .05) in immunized bulls compared with control bulls. Neither pituitary weight, pituitary gonadotropin content nor pituitary FSH/LH ratios were affected (P greater than .10) by pI alpha(1-30) active immunization. Testicular sperm density was greater (60 x 10(6) vs 45 x 10(6) sperm/g testis; P less than .10) in immunized bulls, but testes weight, epididymides weight and total daily sperm production remained unchanged. These results suggest that inhibin is important for regulation of FSH secretion and testicular function. Immunization with suitable inhibin vaccines may improve bull fertility.     

The Effects of Isoflavones on Androgens and Glucocorticoids During Puberty on Male Wistar Rats

Isoflavones are the most common phytoestrogens found in human diets. However, it is still not clear whether isoflavones have effects on the reproductive and the endocrine systems under normal dietary intake and overdose. The aim of this study was to determine how the most important isoflavones, genistein and daidzein, affect androgen and glucorticoid levels on male prepuberal rats. A hundred and seventy‐five 30‐day‐old male Wistar rats were dosed orally by stomach tube every day for 35 days, with saline solution, low and high doses of genistein, daidzein and a mixture of both. Serum samples were analysed by an enzyme immunoassay for hormone determinations. In control group, there was a peak of testosterone (T) and dihydrotestosterone levels associated to the onset of puberty, at the third week. However, in low‐dose groups, the same peak was found at the fourth week (p < 0.05), indicating a delay in the onset of puberty in these groups. Moreover, high doses groups serum androgen levels were significantly lower (p < 0.05) than the control group from the first week until fifth week. This fact was supported by a epididymal histological analysis that indicate in low doses there were several content of spermatozoa at fourth week and in high doses there were few content of spermatozoa. Besides, corticosterone levels followed the same pattern of androgens in all groups. We can conclude that oral administration of isoflavones in male rats decreased the secretion of androgens and glucocorticoids causing a delay in the onset of puberty and may cause physiological and developmental problems.     

Pituitary effects of steroid hormones on secretion of follicle-stimulating hormone and luteinizing hormone

Steroid hormones have a profound influence on the secretion of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These effects can occur as a result of steroid hormones modifying the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, or a direct effect of steroid hormones on gonadotropin secreting cells in the anterior pituitary gland. With respect to the latter, we have shown that estradiol increases pituitary sensitivity to GnRH by stimulating an increase in expression of the gene encoding the GnRH receptor. Since an estrogen response element (ERE) has not been identified in the GnRH receptor gene, this effect appears to be mediated by estradiol stimulating production of a yet to be identified factor that in turn enhances expression of the GnRH receptor gene. However, the importance of estradiol for enhancing pituitary sensitivity to GnRH during the periovulatory period is questioned because an increase in mRNA for the GnRH receptor precedes the pre-ovulatory rise in circulating concentrations of estradiol. In fact, it appears that the enhanced pituitary sensitivity during the periovulatory period may occur as a result of a decrease in concentrations of progesterone rather than due to an increase in concentrations of estradiol. Estradiol also is capable of altering secretion of FSH and LH in the absence of GnRH. In a recent study utilizing cultured pituitary cells from anestrous ewes, we demonstrated that estradiol induced a dose-dependent increase in secretion of LH, but resulted in a dose-dependent decrease in the secretion of FSH. We hypothesized that the discordant effects on secretion of LH and FSH might arise from estradiol altering the production of some of the intrapituitary factors involved in synthesis and secretion of FSH. To examine this hypothesis, we measured amounts of mRNA for activin B (a factor known to stimulate synthesis of FSH) and follistatin (an activin-binding protein). We found no change in the mRNA for follistatin after treatment of pituitary cells with estradiol, but noted a decrease in the amount of mRNA for activin B. Thus, the inhibitory effect of estradiol on secretion of FSH appears to be mediated by its ability to suppress the expression of the gene encoding activin.     

Pituitary chromophobe carcinoma with a low level of serum gonadotropin and an aspermatogenesis in a dog

A 5-year-old male Shiba dog with progressive neurologic signs was examined by computed tomography (CT). A CT image of the brain disclosed a large, spherical high-density lesion in the thalamus and diencephalon. Serum LH, FSH and testosterone levels were all low. Macroscopically the large mass was connected with the sella turcia, and it was histopathologically diagnosed as a pituitary chromophobe carcinoma. An aspermatogenesis was observed in the testes. Therefore, it was suggested that the low levels of gonadotropin secretion from the pituitary gland due to the pituitary tumor resulted in the failure of maturation of spermatozoa and spermatids.     

Patterns of secretion of luteinizing hormone, follicle stimulating hormone and testosterone in stallions during the summer and winter

Samples of jugular blood were drawn from each of five stallions every 15 min for 12 h during the summer and winter to determine the short-term fluctuations in plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone. Concentrations of LH and FSH were generally not pulsatile, although one stallion exhibited three distinct pulses in these hormones during the winter. In general, patterns of secretion of all three hormones were similar in both seasons and the number of significant rises in hormonal concentrations did not differ between seasons. Concentrations of LH and FSH were positively correlated (P less than .05) for eight of the ten sampling periods, indicating a close relationship between the secretion rates of these two gonadotropins. Testosterone concentrations varied in an episodic manner during the 12-h period, and all stallions exhibited at least one episode of high testosterone secretion regardless of the pattern of LH concentrations. The response in testosterone concentrations to the three LH pulses exhibited by the one stallion in winter was not the same for each pulse. The correlations between a single random sample and mean concentrations over the 12-h period were high (r between .88 and .99) for all three hormones, indicating that a single sample of blood would be representative of overall concentrations. It appears that the stallion differs from males of other domestic species in that concentrations of gonadotropins and testosterone vary in a much less pulsatile manner.     

Changes in the pituitary-gonadal axis associated with puberty in Holstein bulls

The temporal pattern of the endocrine changes associated with puberty were studied using 52 bulls born in October or April. Blood samples were taken weekly and at 30-min intervals for 5 h every 4-wk. Bulls were castrated at one of six 4-wk intervals between 12 and 32 wk and blood samples were taken. Season of birth affected concentrations of testosterone (greater for spring-born) in intact bulls, but not luteinizing hormone (LH) or follicle stimulating hormone (FSH). The concentration of FSH increased about 30% between 4 and 32 wk, without evidence of pulsatile discharge. Basal concentration of LH was low and pulsatile discharges were infrequent at 4 or 8 wk. At 12, 16 and 20 wk, however, basal LH concentration was elevated and LH discharges were at less than 2-h intervals. Testosterone concentration did not rise until 18 to 20 wk, but then continued to rise; LH discharge was suppressed concomitantly. Bulls castrated at 16 or 20 wk had higher concentrations of LH in their blood both before and shortly after castration values for bulls, but by 21 d after castration values for bulls of all ages were similar. It was concluded that elimination of an unidentified suppressive factor allows frequent discharges of LH between 12 an 16 wk, but the testes do not respond by secreting more testosterone until 18 to 20 wk. By 24 wk, the testes are secreting more testosterone and pituitary production of LH is restored to a lower level; LH discharges decline in frequency and basal LH level declines. The high frequency discharges of LH between 12 and 20 wk are postulated to induce responsiveness of Leydig cells to LH and, thus, enable elevation of intratesticular testosterone to levels necessary for Sertoli cell differentiation and initiation of spermatogenesis.     

Changes in concentrations of plasma immunoreactive follicle-stimulating hormone, luteinizing hormone, estradiol-17beta, testosterone, progesterone, and inhibin in heifers from birth to puberty

This study was designed to clarify the characteristics of changes in plasma concentrations of reproductive hormones in heifers from birth to puberty. Weekly or daily hormonal changes were observed in 39 heifers. Daily changes in the concentration of follicle-stimulating hormone (FSH) demonstrated a consistent cycle of hormone changes over a 7- to 8-day period in heifers from approximately 10 days to 9 months old. Weekly changes in reproductive hormones showed that there were three brief periods in heifers between birth and puberty in which dramatic changes occur. The first period was the first week after birth, during which a reciprocal relationship between steroid hormones and gonadotropins was observed. At birth, the concentrations of steroid hormones were higher than those at any other age. These hormone levels rapidly decreased within the first week after birth. Gonadotropin levels, however, increased from birth to 1 week of age. The second period of major change was at approximately 4 weeks of age when there was an increase in the concentrations of luteinizing hormone (LH), estradiol-17beta, testosterone, and immunoreactive inhibin. The third period was the last 5 weeks before the first ovulation, when there was an increase in the concentrations of estradiol-17beta followed by an increase in (LH). These results suggest that regular hormone changes start from 10 days after birth and that the periods from birth to 4 weeks of age and the last 5 weeks before the first ovulation in heifers are important to the development of reproductive functions before puberty.     

Inhibin secretion in the golden hamster (Mesocricetus auratus) testis during active and inactive states of spermatogenesis induced by the restriction of photoperiod

Gonadal function in the male golden hamster (Mesocricetus auratus) was investigated during exposure to a short photoperiod condition. Within 3 weeks of exposure to the short photoperiod condition, FSH and testosterone in the plasma significantly decreased, and subsequently immunoreactive (ir)-inhibin significantly decreased. Testicular contents of ir-inhibin and testosterone, and pituitary contents of LH and FSH also significantly decreased by 3 weeks with regression of weight of testes, epididymis and seminal vesicles and sperm head count. Circulating LH varied but not significantly. Thereafter, all reproductive parameters and secretion of LH, FSH, ir-inhibin and testosterone gradually recovered after 17 weeks of exposure even though animals continued to be subjected to the short photoperiod condition. Plasma concentrations of inhibin B and inhibin pro-alphaC were detectable and were significantly decreased after 15 weeks of exposure to the short photoperiod, but their levels were still detectable. Immunopositive reaction of inhibin alpha and betaB subunits was found in Sertoli cells and Leydig cells in the regressed testes of animals subjected to short photoperiod as was also seen in animals before exposure to the short photoperiod. Although the spermatogenic cycle was suppressed like prepubertal animals, the present study showed that the testicular recovery, so-called refractoriness, is functionally different from the developing stage of immature animals, especially with regard to inhibin secretion. The present results showed that changes in FSH preceded changes in inhibin during the regression and recovery phases, indicating that FSH is a major regulatory factor of inhibin secretion in male golden hamsters. The present study also demonstrated that regressed testes still secrete a small amount of bioactive inhibin during exposure to a short-photoperiod condition.     

Superovulation beim Rind: Hormonprofile bei Stimulation mit Serum-gonadotropin (PMSG) bzw. hypophysärem FSH*)

Contents: Superovulation in cattle: Hormonal profdes during superovulation with PMSG or pituitary FSH.: Hormonal profiles for LH, FSH, PMSG, progesterone in peripheral blood and oestrogens in urine during superovulation with PMSG (1500–3100 IU i. m.) or pituitary porcine FSH (5 mg i.m. 2 times daily for 5 days) were evaluated radioimmunologicaly. Two days after the begin of treatment luteolysis was induced by means of 0.5 mg Estrumat® i. m. The experiments were performed with 14 heifers and 2 cows. Blood samples were taken in 6 or 12 h intervals. Preovulatory LH and FSH peaks occurred coinciding with the onset o f oestrus in good responding animals instead of a few hours later, as in weakly responding animals or during normal oestrous cycle. PMSG could be still measured in peripheral blood 10 days after application. Preovulatory gonadotropin peaks are preceded in good responding animals by clear oestrogen peaks contrary t o weak responding ones. Progesterone concentrations of good responding animals increase faster after the preouulatory LH peak and reach higher absolute values compared t o bad responding ones or during normal oestrous cycle. Furthermore there was a clear difference in progesterone values between both stimulation methods. Progesterone concentrations after induction of superovulation with PMSG are significantly higher f o r about the same number of corpora lutea as after stimulation with pituitary FSH. For judgement of the success of stimulation the determination of progesterone can be considered as parameter. From the hormonal profiles no conclusion can be drawn why some animals don't respond to the stimulation.     

抑制素、活化素和卵泡抑素研究进展

抑制素、卵泡抑素和活化素是3种参与垂体促卵泡素调控过程的糖蛋白激素,随着对促卵泡素调控过程的深入了解,发现这3种蛋白在动物生殖周期中发挥着重要的作用。文章主要就抑制素、卵泡抑素和活化素的结构特征、生理功能以及抑制素和卵泡抑素对活化素生物学活性的抑制机理进行了综述。     

Hypothalamic control of gonadotropin secretion by LHRH,FSHRF, NO,cytokines, and leptin

Gonadotropin secretion by the pituitary gland is under the control of luteinizing hormone-releasing hormone (LHRH) and the putative follicle stimulating hormone-releasing factor (FSHRF). Lamprey III LHRH is a potent FSHRF in the rat and seems to be resident in the FSH controlling area of the rat hypothalamus. It is an analog of mammalian LHRH and may be the long sought FSHRF. Gonadal steroids feedback at hypothalamic and pituitary levels to either inhibit or stimulate the release of LH and FSH, which is also affected by inhibin and activin secreted by the gonads. Important control is exercised by acetylcholine, norepinephrine (NE), dopamine, serotonin, melatonin, and glutamic acid (GA). Furthermore, LH and FSH also act at the hypothalamic level to alter secretion of gonadotropins. More recently, growth factors have been shown to have an important role. Many peptides act to inhibit or increase release of LH and the sign of their action is often reversed by estrogen. A number of cytokines act at the hypothalamic level to suppress acutely the release of LH but not FSH. NE, GA, and oxytocin stimulate LHRH release by activation of neural nitric oxide synthase (nNOS). The pathway is as follows: oxytocin and/ or GA activate NE neurons in the medial basal hypothalamus (MBH) that activate NOergic neurons by alpha₁ (α₁) receptors. The NO released diffuses into LHRH terminals and induces LHRH release by activation of guanylate cyclase (GC) and cyclooxygenase. NO not only controls release of LHRH bound for the pituitary, but also that which induces mating by actions in the brain stem. An exciting recent development has been the discovery of the adipocyte hormone, leptin, a cytokine related to tumor necrosis factor (TNF) α. In the male rat, leptin exhibits a high potency to stimulate FSH and LH release from hemipituitaries incubated in vitro, and increases the release of LHRH from MBH explants. LHRH and leptin release LH by activation of NOS in the gonadotropes. The NO released activates GC that releases cyclic GMP, which induces LH release. Leptin induces LH release in conscious, ovariectomized estrogen-primed female rats, presumably by stimulating LHRH release. At the effective dose of estrogen to activate LH release, FSH release is inhibited. Leptin may play an important role in induction of puberty and control of LHRH release in the adult as well.     

Effects of sodium pentobarbital on release of gonadotropins in ewes immediately after ovariectomy and after treatment with gonadotropin-releas ing hormone

Two experiments were conducted with ewes 9 to 11 days after estrus to determine whether the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are controlled differentially. In experiment 1, gonadotropin-releasing hormone (GnRH) was injected (100 (μg/ewe) at time = 0 min into ewes in four treatment groups. The treatment groups (9 ewes/group) were: 1) periodic iv sodium pentobarbital (NaPen) vehicle from 0 min; 2) periodic iv NaPen from 0 min; 3) vehicle iv for 120 min then iv NaPen from 120 min; 4) vehicle iv for 150 min then iv NaPen from 150 min. A surgical plane of anesthesia was maintained from the initiation of NaPen injection until the experiment ended. Jugular blood was sampled at 30-min intervals from ?30 to + 210 min for LH and FSH assays, and profiles of hormone concentrations were compared by time-trend analyses. GnRH released LH (P<.001) and FSH (P<.001), but NaPen did not affect the profiles of hormone concentrations; this indicated that NaPen did not reduce the ability of the pituitary to secrete gonadotropins in response to GnRH. Experiment 2 was a 2x2 factorial with ovariectomy (time = 0 hr) and NaPen as the main effects. One group of ovariectomized (n = 6) and one group of sham ovariectomized (n = 6) ewes were anesthetized only during surgery, while a group of ovariectomized (n = 7) and a group of sham ovariectomized (n = 6) ewes were kept at a surgical plane of anesthesia until 10 hr after surgery. Patterns of LH and FSH were compared in jugular blood collected hourly from 0 hr until 10 hr after surgery and in samples collected at 24 hr intervals from -24 to +72 hr of surgery. After ovariectomy, LH increased (P<.001) hourly and daily, but anesthesia suppressed (hourly, <.001 and daily, P<.005) these increases, which resulted in an interaction (hourly, P<.001 and daily, P<.01) of ovariectomy and anesthesia. FSH after ovariectomy increased hourly and daily (hourly, P<.02 and daily, P<.001), but the effect of anesthesia and interaction of ovariectomy and anesthesia were not significant. Because NaPen did not alter secretion of LH or FSH after exogenous GnRH in experiment 1 while it blocked the postovariectomy increase in LH but not FSH in experiment 2, we concluded that the postovariectomy increase in LH resulted from increased hypothalamic secretion of GnRH. The mechanisms responsible for the postovariectomy increase in FSH secretion are not identical to those for LH. The mechanisms that control the postovariectomy secretion of FSH might involve factors that are not suppressible by NaPen or, alternatively, the differences in LH and FSH release after ovariectomy might reflect the removal of ovarian factors that suppress FSH but not LH secretion in intact ewes.     

山羊GnRH和促性腺激素的释放特点

通过外科手术分别连续收集活体山羊中黄体期及早卵泡期的垂体门脉血样和外周血样，经放射免疫测定，山羊中黄体期和早卵泡期的促性腺激素释放激素（ＧｎＲＨ）、促黄体生成素（ＬＨ）和促卵泡素（ＦＳＨ）均呈波动式释放。在早卵泡期，ＦＳＨ单位时间内波动次数和血浆平均水平显著高于中黄体期；ＧｎＲＨ与ＬＨ的波动型基本一致，ＦＳＨ的变化不太规则。表明山羊垂体促性腺激素的释放受丘脑下部ＧｎＲＨ的调节，但ＦＳＨ似乎还存在其他调节机理。     

Luteinizing hormone, follicle stimulating hormone and prolactin secretion in ewes and wethers after zeranol or estradiol injection

Plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) were determined over a 24-h period using radioimmunoassay in sheep injected with corn oil (control) or various doses of zeranol or estradiol-17 beta. Injection of .333, 1 or 10 mg of zeranol caused dose-related increases (P less than .01) in plasma PRL (peak levels at 12 to 18 h) and LH (peak levels at 12 to 20 h) in ovariectomized ewes. Similarly, PRL and LH increased following doses of 33 or 100 microgram of estradiol. Before the LH surge, plasma LH levels were significantly depressed (4 to 8 h). Plasma FSH levels were significantly decreased 4 to 8 h after zeranol and estradiol injection. Slight surges of FSH were observed at times similar to those of LH, but the peak level was never greater than control levels. Injection of 1 mg of zeranol or 100 microgram of estradiol into wethers resulted in a 24-h pattern of PRL secretion not significantly different of LH concentration and significantly prolonged inhibition of FSH secretion. These results indicate similarities in the effects of zeranol and estradiol on anterior pituitary hormone secretion within groups of animals of the same sex or reproductive state. Differences in secretion and plasma concentrations of LH, FSH and PRL due to underlying sexual dimorphism are maintained and expressed even when animals are challenged with structurally different compounds of varying estrogenic potencies.     

Effect of LPS on Reproductive System at the Level of the Pituitary of Anestrous Ewes

In our research we focused our attention on the effect of the immune stress induced by bacterial endotoxin–lipopolysaccharide (LPS) on the hypothalamic–pituitary–gonadal axis (HPG) at the pituitary level. We examined the effect of intravenous (i.v.) LPS injection on luteinizing hormone (LH) and follicle‐stimulating hormone (FSH) release from the anterior pituitary gland (AP) in anestrous ewes. The effect of endotoxin on prolactin and cortisol circulating levels was also determined. We also researched the effect of immune challenge on the previously mentioned pituitary hormones and their receptors genes expression in the AP. Our results demonstrate that i.v. LPS injection decreased the plasma concentration of LH (23%; p < 0.05) and stimulates cortisol (245%; p < 0.05) and prolactin (60%; p < 0.05) release but has no significant effect on the FSH release assayed during 6 h after LPS treatment in comparison with the control levels. The LPS administration affected the genes expression of gonadotropins’β‐subunits, prolactin and their receptors in the AP. Endotoxin injection significantly decreased the LHβ and LH receptor (LHR) gene expression (60%, 64%; p < 0.01 respectively), increased the amount of mRNA encoding FSHβ, FSH receptor (FSHR) (124%, 0.05; 166%, p < 0.01; respectively), prolactin and prolactin receptor (PRLR) (50%, 47%, p < 0.01; respectively). The presented, results suggest that immune stress is a powerful modulator of the HPG axis at the pituitary level. The changes in LH secretion could be an effect of the processes occurring in the hypothalamus. However, the direct effect of immune mediators, prolactin, cortisol and other components of the hypothalamic pituitary–adrenal (HPA) axis on the activity of gonadotropes has to be considered as well. Those molecules could affect LH synthesis directly through a modulation at all stages of LHβ secretion as well as indirectly influencing the GnRHR expression and leading to reduced pituitary responsiveness to GnRH stimulation.     

Clinical use of GnRH agonists in canine and feline species

GnRH (gonadotrophin releasing hormone) is a key hormone of reproductive function in mammals; agonist forms have been largely developed, and data concerning their use in small animal reproduction are now abundant. GnRH agonists act by a two-step mechanism. First, their agonist properties on the pituitary will cause marked LH (luteinizing hormone) and FSH (follicle-stimulating hormone) secretion into the bloodstream, accompanied by an increase in the concentrations of sex steroid hormones. Then, in case of constant administration, GnRH agonists will lead to pituitary desensitization, and FSH and LH levels will collapse. These two effects have been widely documented, and these compounds have many potential benefits in a clinical context, capitalizing both on their stimulating and sterilizing effects.