The anterior pituitary gland: lessons from livestock

There has been extensive research of the anterior pituitary gland of livestock and poultry due to the economic (agricultural) importance of physiological processes controlled by it including reproduction, growth, lactation and stress. Moreover, farm animals can be biomedical models or useful in evolutionary/ecological research. There are for multiple sites of control of the secretion of anterior pituitary hormones. These include the potential for independent control of proliferation, differentiation, de-differentiation and/or inter-conversion cell death, expression and translation, post-translational modification (potentially generating multiple isoforms with potentially different biological activities), release with or without a specific binding protein and intra-cellular catabolism (proteolysis) of pituitary hormones. Multiple hypothalamic hypophysiotropic peptides (which may also be produced peripherally, e.g. ghrelin) influence the secretion of the anterior pituitary hormones. There is also feedback for hormones from the target endocrine glands. These control mechanisms show broadly a consistency across species and life stages; however, there are some marked differences. Examples from growth hormone, prolactin, follicle stimulating hormone and luteinizing hormone will be considered. In addition, attention will be focused on areas that have been neglected including the role of stellate cells, multiple sub-types of the major adenohypophyseal cells, functional zonation within the anterior pituitary and the role of multiple secretagogues for single hormones.     

Development, validation, and utilization of a novel antibody specific to the type III chicken gonadotropin-releasing hormone receptor

Two gonadotropin-releasing hormone receptors (GnRH-Rs) have been characterized in chickens to date: cGnRH-R-I and cGnRH-R-III, with cGnRH-R-III being the predominant pituitary form. The purpose of the present study was to first validate a novel antibody for the specific detection of cGnRH-R-III and second, using this antibody, detect changes in cGnRH-R-III protein levels in the pituitary gland of male and female chickens during a reproductive cycle. The localization of cGnRH-R-III within the anterior pituitary gland was also determined. Western blotting of pituitary extracts and transiently transfected COS-7 cell lysates revealed that our antibody is highly specific to cGnRH-R-III protein. Similarly, when used in immunocytochemistry, this antibody specifically detects cells expressing cGnRH-R-III and not cGnRH-R-I. Western blot analyses of chicken pituitary gland homogenates show that cGnRH-R-III protein levels are significantly greater in sexually mature birds than in immature birds or birds at the end of a reproductive cycle (P < 0.0001). A similar pattern was observed for both males and females. Additionally, the antibody was able to detect cGnRH-R-III in cells along the periphery of the cephalic and caudal lobes of the anterior pituitary where the cells containing the gonadotropins are located. In summary, we successfully validated a novel antibody to cGnRH-R-III and showed levels of cGnRH-R-III protein in the pituitary fluctuate with respect to the reproductive status in both male and female chickens.     

Effects of season and sex on in vitro aromatase and 17β-oxidoreductase activities in the brain and anterior pituitary gland of gonadectomized sheep

To improve understanding of seasonal control of reproduction, we studied effects of season and sex on in vitro conversion of androstenedione to estrone and testosterone within the brain and anterior pituitary gland of recently gonadectomized sheep. For both sexes, aromatase activity was relatively high in the amygdala and hypothalamus plus preoptic area (HPOA), but aromatase was not detected in the parietal cortex or the anterior pituitary gland. Aromatase activity of the amygdala was not affected by sex or season. For castrated rams, aromatase activity in the HPOA in May was <25% of that in December (P<.05). For ovariectomized ewes, however, aromatase activity in the HPOA was similar in May and December (P>.05). All tissues contained 17β-oxidoreductase. For the amygdala and HPOA, conversion of androstenedione to testosterone was greater (P<.05) in the male than the female, regardless of season. In rams, 17β-oxidoreductase activity was lower in the HPOA and higher in the anterior pituitary gland in May than in December. In the anterior pituitary of ewes, however, 17β-oxidoreductase activity was higher in December. Thus, conversion of androstenedione to testosterone and estrogens within the brain and anterior pituitary gland could help to control the secretion of luteinizing hormone. We postulate that in rams a seasonal reduction of aromatase activity in the HPOA and increase in 17β-oxidoreductase in the anterior pituitary gland during May may be involved with onset of the seasonal increase in LH secretion.     

Regulation of the Estrous Cycle in Domestic Animals — A Review

Neuroendocrine and endocrine factors involved in the regulation of reproductive cycles in domestic animals are discussed. Although research data from many species are considered, emphasis is placed on their relevance for the cow, sow, ewe and, to a lesser extent, the mare. Literature cited is not designed to be complete, but rather to be representative of the large volume of material which has been written on the subject.

Gonadotropin-releasing hormone is synthesized and secreted in response to various exteroceptive stimuli, but both its release and its effects on the anterior pituitary are modified by feedback of target gland hormones. A modulating role for nonsteroidal regulators such as inhibin has yet to be proven. Prostaglandins are important for corpus luteum regression and ovulation.

The relative roles of the gonadotropins and the above substances in hypothalamic, adenohypophyseal and ovarian function are considered. General mention is made of mechanisms of hormonal action.

     

Differences in reproductive pattern between wild and domestic rams are not associated with inter-specific annual variations in plasma prolactin and melatonin concentrations

This study compares morphological changes, performed with real-time transrectal ultrasonography, of testis and accessory glands and seasonal endocrine changes in a wild (Mouflon) and a domesticated (Spanish Merino) breed of sheep. In Mouflons, the maximum plasma testosterone concentrations, testicular diameter and vesicular gland size occurred synchronously during autumn (P < 0.001). In Merino rams the highest circulating levels of testosterone (P < 0.05) and maximum testicular diameter (P < 0.001) occurred during summer, with no seasonal variations in vesicular and bulbourethral glands. The seasonal changes in plasma concentrations of prolactin were not correlated with annual variations in testicular and glandular size in neither wild nor domestic species. No differences were observed between both species for the seasonal pattern of prolactin secretion and mean amplitude of melatonin. Wide differences in reproductive patterns between wild and domestic types of rams do not appear to be attributed to seasonal changes in prolactin and melatonin secretion.     

Neuroendocrine mechanism of seasonal reproduction in birds and mammals

In temperate zones, animals use changes in day length as a calendar to time their breeding season. However, the photoreceptive and neuroendocrine mechanisms of seasonal reproduction are considered to differ markedly between birds and mammals. This can be understood from the fact that the eye is the only photoreceptive organ, and melatonin mediates the photoperiodic information in mammals, whereas in birds, photoperiodic information is directly received by the deep brain photoreceptors and melatonin is not involved in seasonal reproduction. Recent molecular and functional genomics analysis uncovered the gene cascade regulating seasonal reproduction in birds and mammals. Long day‐induced thyroid stimulating hormone in the pars tuberalis of the pituitary gland regulates thyroid hormone catabolism within the mediobasal hypothalamus. Further, this local thyroid hormone catabolism appears to regulate seasonal gonadotropin‐releasing hormone secretion. These findings suggest that although the light input pathway is different between birds and mammals (i.e. light or melatonin), the core mechanisms are conserved in these vertebrates.     

Hormonal responses to exercise and training

Current knowledge and understanding of the hormonal response to exercise are limited, whether in relation to horses, humans, or other species. The changes in plasma concentration of some hormones occur early in exercise, apparently owing to a neuronal stimulation, whereas others, being pituitary dependent, require hormonal stimulation. Also, although it is possible to observe changes in plasma concentrations of hormones, the mechanism by which this is achieved is not always understood, and unless the nonprotein-bound, or active, form of the hormone is also determined, changes in plasma concentration are less informative. Both the intensity and duration of exercise may be of importance in initiating or maintaining the hormonal response. Impulses from either the working muscles or motor centers, via the central nervous system, modify the response of the glands of the endocrine system directly via pituitary hormones or indirectly via the sympathoadrenal system. The initial response to the onset of exercise is enhancement of sympathoadrenal activity and secretion of pituitary hormones, which result in a reduction in the plasma concentration of insulin and a rise in that of virtually all other hormones. Because of this shift in hormone balance, a modification of the metabolism of intra- and extra-muscular triglycerides and glycogen as fuels for muscular exercise occurs. The variation in mobilization of one fuel source may well influence its combustion, together with both the mobilization and combustion of the other. When exercise is prolonged, the hormonal response is influenced by additional factors such as temperature, glucose availability, oxygen tension, and changes in plasma volume. The effect of training on hormonal responses is generally an ameliorating one that reflects an increased efficiency of muscular energy metabolism as a result of the training process.     

Disorders of the parathyroid glands

The three calcitropic hormones, parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol and calcitonin are together responsible for calcium homeostasis in the mammal. Feline PTH is an 84 amino acid, single chain polypeptide with a molecular weight of 9449, which is secreted by the parathyroid glands. The principle secretagogue for PTH is a low plasma ionised calcium concentration, although both 1,25-dihydroxycholecalciferol and phosphate have significant roles in regulating PTH secretion. The ability to accurately measure circulating PTH in the cat has simplified the evaluation of disorders of calcium metabolism in this species. In primary parathyroid disorders the lesion is located within the parathyroid gland, with parathyroid secretion being inappropriate to the prevailing mineral balance. By contrast, in secondary conditions a pathological state out with the parathyroid gland alters mineral homeostasis and the parathyroid gland responds in an appropriate manner. The measurement of circulating PTH may then be used to determine if PTH secretion is appropriate to the prevailing calcium concentrations to differentiate primary from secondary disorders. Although primary hyper and hypoparathyroidism are generally considered rare endocrine conditions of the cat, the ability to measure PTH has led to their increasing recognition.     

The Role of Uterine and Ovarian Hormones in Luteolysis: A Comparison among Species

Luteolytic mechanisms have evolved in mammals to improve reproductive efficiency. The hormonal interactions that control the onset and progress of luteolysis are complex. They involve endocrine and paracrine signals that link the corpus luteum, uterus and posterior pituitary gland. Current concepts concerning these interactions will be examined in the five major domestic ungulate species commonly raised in Europe and North America (cattle, sheep, goats, pigs and horses). Some of these interactions are similar across species. All five depend on prostaglandin F_2α secreted from the uterus, to induce luteolysis. Three hormones, progesterone, estradiol and oxytocin interact to regulate uterine secretion of PGF_2α. Oxytocin is an acute stimulus for uterine PGF_2α secretion. Progesterone and estradiol interact to regulate uterine secretary responsiveness to oxytocin. Precisely how these hormones interact varies across species.     

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.     

The control of milk ejection in the mammal

Extract

Occasionally the need arises for a review of a subject aimed at putting the available evidence into perspective. Over the past decade a considerable amount of work has been done on the milk-ejection mechanism of the mammal. Too frequently this reflex has been regarded as isolated from other aspects of mammalian reproduction. In this review, an attempt is made to outline our basic knowledge of the nature of the milk-ejection process and the mechanisms involved in it against the background of the mammalian reproductive system. For along time it has been known that the suckling stimulus has an effect on the ability of the mammal to secrete milk. At the same time it is known that there are no neural pathways to the anterior pituitary which releases the hormones controlling mammary gland activity. Thus the mechanism by which the stimuli of suckling could affect the release of anterior lobe hormones has not been at all clear. Evidence is rapidly accumulating for the idea that the posterior pituitary hormones, which must pass through the anterior pituitary gland on their way to the systemic circulation, may have an effect on the release of anterior lobe substances. This effect is made possible by the peculiar system of circulation which exists between the neurohypophyseal tract and the anterior pituitary. Evidence for this view is summarized in what follows, and its significance as a factor in the maintenance of milk secretion is considered. It has been thought worth while to present a generalized picture of the milk-ejection reflex and of certain other reproductive reflexes as recent developments within the mammals superimposed upon the “water balance” regulating mechanisms which are to be found throughout the entire vertebrate phylum. In doing this, a certain amount of over-simplification has been inevitable, but it is hoped that the review will place some of the problems of lactation in perspective and, in so doing, show the many gaps in our knowledge which must be filled before we have an adequate understanding of the process controlling the secretion and ejection of milk in the mammal.     

激素调控促进动物生长发育的研究

激素对机体的代谢、生长、发育、繁殖、性别、性欲和性活动等起重要的调节作用。动物机体通过各种内分泌腺分泌的激素,间接调节动物机体的活动。内分泌腺分泌的激素直接进入血液、随着血液循环到达身体各个部分,在一定的器官或组织中发生作用,从而协调动物机体新陈代谢、生长、发育、生殖及其它生理机能,使这些机能得到兴奋或抑制,使它们的活动加快或减慢。生长激素可以促进蛋白质的合成与骨的生长,甲状腺激素促进新陈代谢和生长发育,可以提高神经系统的兴奋。胰岛分泌的激素可以控制糖类代谢,调节血糖含量。性激素促进生殖器官的生长发育,维持第二性征。     

Neuroendocrine interactions and seasonality

Sheep in temperate latitudes are seasonal breeders. Of the different seasonal cues, photoperiod is the most reliable parameter and is used by animals as an indication of the time of the year to synchronize endogenous annual rhythms of reproduction and physiology. The photoperiodic information is transduced into neuroendocrine changes through variations in melatonin secretion from the pineal gland. Melatonin triggers variations in the secretion of luteinizing hormone-releasing hormone, luteinizing hormone and follicle stimulating hormone (LHRH/LH/FSH) responsible for seasonal changes in reproductive activity. In female sheep, the seasonal changes in the hormonal LH pattern mainly reflect an increase in the negative feedback exerted by estradiol under long days on the frequency of pulsatile LH secretion. The resulting seasonal inhibition of LH secretion involves the activation of monoaminergic and especially dopaminergic systems by estradiol. Other types of physiological regulation subject to seasonal changes such as voluntary food intake (VFI), fat metabolism, body mass and pelage growth also occur in sheep, goats or related wild species. Several neuroendocrine intermediates seem to be shared by these different systems and may participate in their synchronization, providing the advantage that this helps mammalian species to adapt to their environment.     

Cellular localization of IL-18 and IL-18 receptor in pig anterior pituitary gland

Pro-inflammatory cytokine interleukin 18 (IL-18) has been proposed to have a role in modulating immuno-endocrine functions. Our previous study showed that IL-18 and IL-18 receptor (IL-18R) colocalized in somatotrophs of the bovine anterior pituitary gland, and the possibility that IL-18 acts on somatotrophs as an autocrine factor. In the present study, we investigated the localization of IL-18 and IL-18R in the pig anterior pituitary gland. RT-PCR analysis showed the expression of IL-18 and IL-18R mRNAin the pig anterior pituitary gland. Immunohistochemistry of IL-18 and specific hormones revealed the presence of IL-18 in somatotrophs, mammotrophs, thyrotrophs and gonadotrophs. IL-18R was localized in somatotrophs and thyrotrophs. Furthermore, the somatotrophs immunoreactive for IL-18 did not contain IL-18R. Thus, IL-18R and IL-18 were not colocalized in an identical somatotroph. These findings suggest that the localization of IL-18 in pig somatotrophs is different from that in bovine somatotrophs, although IL-18 closely associates with somatotrophs in the anterior pituitary glands in both species.     

苦马豆素抑制N-聚糖加工对生殖激素分泌功能的影响

苦马豆素（SW）中毒可引起生殖激素分泌紊乱,其中,促性腺激素是糖蛋白,由N-聚糖糖基化修饰调控其活性和功能。SW是N-聚糖加工过程中ɑ-甘露糖苷酶的抑制剂,而SW如何通过改变N-聚糖加工过程进一步影响生殖激素的分泌功能尚不明确。因此,本试验建立SW染毒妊娠期小鼠模型,利用MALDI-TOF-MS质谱法检测糖蛋白N-聚糖糖链结构的变化,分析N-聚糖加工过程中糖基酶的活性和生殖激素水平及其受体蛋白表达量。随着SW染毒时间的延长,染毒组小鼠垂体前叶糖蛋白的5种复合型N-聚糖糖链结构消失,而新增加3种杂合型N-聚糖糖链结构;N-聚糖糖基转移酶和糖苷酶的活性均显著下降;进一步发现染毒组小鼠卵巢促性腺激素受体、雌二醇和孕酮受体蛋白表达量显著低于对照组,并且生殖激素分泌水平也出现显著下降。SW可显著抑制N-聚糖糖基酶的活性使糖蛋白上N-聚糖糖链结构发生改变,影响促性腺激素及其受体的活性,使其对下游类固醇激素分泌的调控作用失衡,最终导致小鼠妊娠期生殖激素分泌紊乱。     

Swainsonine Affects the Secretion of Reproductive Hormones by Inhibiting N-glycan Processing

Swainsonine (SW) can cause disorders of reproductive hormones. Gonadotropins are glycoprotein hormones, so they are regulated by N-glycosylation modifications. ɑ-mannosidase, a key enzyme that accelerates the processing of N-glycosylation modifications, can be inhibited by SW. So how does SW affect the structure of N-glycan and the secretion performance of reproductive hormone is unclear. Thus, this test was conducted by intraperitoneal injection of SW exposed mice to establish models of poisoning. The changes of N-glycan structure in their pituitary tissues were detected by MALDI-TOF-MS mass spectrometry; the activity of glycosylase, the level of reproductive hormone, the quantity of reproductive hormone receptors were analyzed. With the extension of injection time, the five composite glycosides of the pituitary glycoprotein in the poisoned group disappeared, and three hybrid glycosides were added. The activities of glycosyltransferase and glycosidase in the poisoned group were significantly decreased. There were further found that the expression levels of gonadotropin receptor, estradiol and progesterone receptor proteins in the poisoned group were significantly lower than those in the control group, and the secretion levels of reproductive hormones were also significantly decreased. SW can significantly inhibit the activity of N-glycan glycosylase and cause changes of normal N-glycan structure; it has a negative influence on the activities of gonadotropins and their receptors, causing the regulation of downstream steroid hormone secretion out of balance, and eventually, reproductive hormone regulation can be disrupted.     

Hypothalamic control of the thyroidal axis in the chicken: over the boundaries of the classical hormonal axes

The pituitary gland, occupying a central position in the hypothalamo-pituitary thyroidal axis, produces thyrotropin (TSH), which is known to stimulate the thyroid gland to synthetize and release its products, thyroid hormones. TSH is produced by a specific cell population in the pituitary, the so-called thyrotropes. Their secretory activity is controlled by the hypothalamus, releasing both stimulatory and inhibitory factors that reach the pituitary through a portal system of blood vessels. Based on early experiments in mammals, thyrotropin-releasing hormone (TRH) is generally mentioned as the main stimulator of the thyrotropes. During the past few decades, it has become clear that the hypophysiotropic function of the hypothalamus is more complex, with different hormonal axes interacting with each other. In the chicken, it was found that not only TRH, but also corticotropin-releasing hormone (CRH), the main stimulator of corticotropin release, is a potent stimulator of TSH secretion. Somatostatin (SRIH), a hypothalamic factor known for its inhibitory effect on growth hormone secretion, was demonstrated to blunt the TSH response to TRH and CRH. In this review we summarize the latest studies concerning the "interaxial" hypothalamic control of TSH release in the chicken, with a special emphasis on the molecular components of these control mechanisms. It remains to be demonstrated if these findings could also be extrapolated to other species or classes of vertebrates.     

Differential expression of myostatin and its receptor in the porcine anterior pituitary gland

Myostatin (MSTN), known as growth and differentiation factor 8 (GDF-8), is a member of the transforming growth factor β (TGF-β) superfamily that negatively regulates skeletal muscle mass. Myostatin binds with high affinity to the receptor serine threonine kinase activin receptor type IIB (ActRIIB). Activins that also belong to the TGF-β superfamily, stimulate follicle-stimulating hormone production in gonadotrophs and suppress growth hormone and adrenocorticotropic hormone production in somatotrophs and corticotrophs, respectively. The aim of the present paper was therefore to clarify the endocrine action of MSTN in adenohypophysis. The present study details the expression and cellular localization of MSTN and ActRIIB in porcine anterior pituitary gland. The mRNA of MSTN and ActRIIB was consistently expressed in RT-PCR. Immunohistochemistry of MSTN and specific hormones showed that MSTN localized in thyrotrophs and gonadotrophs, in which most of the MSTN immunoreactive cells were identified as thyrotrophs. The immunostaining of ActRIIB was restricted to corticotrophs. These results indicate that MSTN was mainly produced in thyrotrophs and its receptor, ActRIIB, was restrictively contained in corticotrophs. Interestingly, thyrotrophs immunoreactive for MSTN were frequently close to corticotrophs immunoreactive for ActRIIB. The present study suggests that MSTN from thyrotrophs may regulate corticotroph function as a paracrine mediator among the porcine anterior pituitary cells.