Regulation of anterior pituitary gonadotropin subunit mRNA levels during the preovulatory gonadotropin surge: a physiological role of progesterone in regulating LH-beta and FSH-beta mRNA levels

A novel reproductive peptide, phoenixin

Normal anterior pituitary function is essential for fertility. Release from the gland of the reproductive hormones luteinising hormone and follicle-stimulating hormone is regulated primarily by hypothalamically-derived gonadotrophin-releasing hormone (GnRH), although other releasing factors (RF) have been postulated to exist. Using a bioinformatic approach, we have identified a novel peptide, phoenixin, that regulates pituitary gonadotrophin secretion by modulating the expression of the GnRH receptor, an action with physiologically relevant consequences. Compromise of phoenixin in vivo using small interfering RNA resulted in the delayed appearance of oestrus and a reduction in GnRH receptor expression in the pituitary. Phoenixin may represent a new class of hypothalamically-derived pituitary priming factors that sensitise the pituitary to the action of other RFs, rather than directly stimulating the fusion of secretary vesicles to pituitary membranes.

Hirsutism, virilism, polycystic ovarian disease, and the steroid-gonadotropin-feedback system: a career retrospective

This career retrospective describes how the initial work on the mechanism of hormone action provided the tools for the study of hirsutism, virilism, and polycystic ovarian disease. After excessive ovarian and or adrenal androgen secretion in polycystic ovarian disease had been established, the question whether the disease was genetic or acquired, methods to manage hirsutism and methods for the induction of ovulation were addressed. Recognizing that steroid gonadotropin feedback was an important regulatory factor, initial studies were done on the secretion of LH and FSH in the ovulatory cycle. This was followed by the study of basic mechanisms of steroid-gonadotropin feedback system, using castration and steroid replacement and the events surrounding the natural onset of puberty. Studies in ovariectomized rats showed that progesterone was a pivotal enhancer of estrogen-induced gonadotropin release, thus accounting for the preovulatory gonadotropin surge. The effects of progesterone were manifested by depletion of the occupied estrogen receptors of the anterior pituitary, release of hypothalamic LHRH, and inhibition of enzymes that degrade LHRH. Progesterone also promoted the synthesis of FSH in the pituitary. The 3α,5α-reduced metabolite of progesterone brought about selective LH release and acted using the GABA(A) receptor system. The 5α-reduced metabolite of progesterone brought about selective FSH release; the ability of progesterone to bring about FSH release was dependent on its 5α-reduction. The GnRH neuron does not have steroid receptors; the steroid effect was shown to be mediated through the excitatory amino acid glutamate, which in turn stimulated nitric oxide. These observations led to the replacement of the long-accepted belief that ovarian steroids acted directly on the GnRH neuron by the novel concept that the steroid feedback effect was exerted at the glutamatergic neuron, which in turn regulated the GnRH neuron. The neuroprotective effects of estrogens on brain neurons are of considerable interest.

Morphometry and immunohistochemistry of follicles growth and steroidogenesis in saharian wild sand rat, Psammomys obesus, ovary

The sand rat (Psammomys obesus) constitutes a model to study seasonal changes and several metabolic disorders. In order to perform breeding laboratory conditions, the reproductive function of this species living in North Occidental Algerian Sahara was studied. The aim of this work was to investigate the follicular growth changes and the steroidogenic associated aspects. The study was performed using morphometrical and immunohistochemical methods. From primordial to preantral states, the follicle diameter increased progressively from 17-20 mum to 192-225 mum. The preovulatory follicles reached about 500 mum in diameter. Immunoreactivity to progesterone, androstenediol and estradiol, varied in the different parts of the ovary and follicular cells. The progesterone antibody appeared clearly labelled in the theca interna of the growing follicle and increased in the granulosa; the androgen antibody was continuously weak and diffuses in all follicles; the estradiol labelling appeared weak and diffuse in preantral follicles then increased in antral follicles in both theca and granulosa or only in granulosa. In antral follicles, estradiol label was clearly localized in granulosa cells and totally devoid in theca cells. In Psammomys ovary, labels of hormone were diffuse or localized, weak or intense in the theca and or in the granulosa according to the follicle size.

Mechanisms controlling the function and life span of the corpus luteum

The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follicular granulosal and thecal cells after ovulation. Luteinizing hormone (LH) from the anterior pituitary is important for normal development and function of the corpus luteum in most mammals, although growth hormone, prolactin, and estradiol also play a role in several species. The mature corpus luteum is composed of at least two steroidogenic cell types based on morphological and biochemical criteria and on the follicular source of origin. Small luteal cells appear to be of thecal cell origin and respond to LH with increased secretion of progesterone. LH directly stimulates the secretion of progesterone from small luteal cells via activation of the protein kinase A second messenger pathway. Large luteal cells are of granulosal cell origin and contain receptors for PGF(2alpha) and appear to mediate the luteolytic actions of this hormone. If pregnancy does not occur, the corpus luteum must regress to allow follicular growth and ovulation and the reproductive cycle begins again. Luteal regression is initiated by PGF(2alpha) of uterine origin in most subprimate species. The role played by PGF(2alpha) in primates remains controversial. In primates, if PGF(2alpha) plays a role in luteolysis, it appears to be of ovarian origin. The antisteroidogenic effects of PGF(2alpha) appear to be mediated by the protein kinase C second messenger pathway, whereas loss of luteal cells appears to follow an influx of calcium, activation of endonucleases, and an apoptotic form of cell death. If the female becomes pregnant, continued secretion of progesterone from the corpus luteum is required to provide an appropriate uterine environment for maintenance of pregnancy. The mechanisms whereby the pregnant uterus signals the corpus luteum that a conceptus is present varies from secretion of a chorionic gonadotropin (primates and equids), to secretion of an antiluteolytic factor (domestic ruminants), and to a neuroendocrine reflex arc that modifies the secretory patterns of hormones from the anterior pituitary (most rodents).

Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.

Neuroendocrine mechanisms underlying the control of gonadotropin secretion by steroids

There is considerable evidence that although estradiol may trigger the preovulatory surge of gonadotropins, progesterone is required for its full magnitude and duration and that glucocorticoids bring about selective follicle-stimulating hormone release. The luteinizing hormone-releasing hormone (LHRH) neuron does not have steroid receptors and is regulated by excitatory amino acid neurotransmission. Steroids do not appear to modulate excitatory amino acid receptors directly but increase release of glutamate in the preoptic area. This may be due to the suppression by steroids of the enzyme glutamatic acid decarboxylase67 that converts glutamate into GABA. NMDA receptors colocalize with nitric oxide synthase-containing neurons that surround the LHRH neurons in the preoptic area and intersect the LHRH fibers in the median eminence. Other potential novel pathways of LHRH release that are currently being explored include carbon monoxide generated by the action of heme oxygenase-2 on heme molecules and bradykinin acting via bradykinin B2 receptors.

Pituitary Sex Steroid Receptors: Localization and Function

The pituitary contains estrogen receptor (ER), progesterone receptor (PR), and androgen receptor (AR). In accordance with immunocytochemistry, it is agreed that sex hormone receptors reside into the nucleus. All three receptors are found predominantly in gonadotrophs and lactotrophs, and less frequently in other cell types. ER plays a major role in prolactin (PRL) production and lactotroph proliferation, and protracted estrogen administration induces lactotroph hyperplasia and adenoma in rodents. Most research on PR and AR is focused on their role in the fine-tuning of gonadotropin secretion during estrous cycle. Contrary to the effect in nontumorous pituitary, estrogens can inhibit the proliferation of transplantable rat pituitary tumors and of cell lines derived from them. In humans, despite the presence of ER in all types of adenohypophysial tumors, the role of estrogen in tumor cell proliferation is still unclear. Few results indicate that tumor growth is stimulated by estrogen, and inhibited by progesterone and androgen. Novel data reveal that steroid hormones can act directly on plasma membrane or via other receptors, and interact with growth factors, oncogenes, and other transcription factors. The mechanisms by which steroid hormones control cell proliferation remain a major challenge for future research.

Diverse modes of action of progesterone and its metabolites

Progesterone and its metabolites have a variety of diverse effects in the brain, uterus, smooth muscle, sperm and the oocyte. The effects include changes in electrophysiological excitability, induction of anesthesia, regulation of gonadotropin secretion, regulation of estrogen receptors, modulation of uterine contractility and induction of acrosome reaction and oocyte maturation. The latency of the effects vary from several seconds to several hours. Thus, it is not surprising that multiple mechanisms of action are involved. The classical mechanism of steroid hormone action of intracellular receptor binding has been supplemented by the possibility of the steroid acting as a transcription factor after the binding of the receptor protein to DNA. Other mechanisms include influence of the steroids on membrane fluidity and acting through other cell signalling systems, membrane receptors and GABA(A) receptors. Of particular interest are multiple mechanisms for the same types of action. For example the effect of progesterone on gonadotropin release is largely exerted via the classical intracellular receptor as well as membrane receptors, whereas 3(alpha),5(alpha)-tetrahydroprogesterone-induced LH release occurs via the GABA(A) receptor system. The inhibition of uterine contractility by progesterone is regulated by progesterone receptors while the action of 3(alpha),5(alpha)-tetrahydroprogesterone on uterine contractility is regulated by GABA(A) receptors. The regulation of the differences in the pattern of progesterone effects on estrogen receptor dynamics in the anterior pituitary and the uterus in the same animal are also of considerable interest.

Glutamate: a major neuroendocrine excitatory signal mediating steroid effects on gonadotropin secretion

The preovulatory gonadotropin surge is induced by progesterone in the cycling female rat or in the ovariectomized estrogen-treated female rat after adequate estrogen-priming activity is present. The source of progesterone under physiological conditions could be the ovary and/or the adrenal. Since the GnRH neuron does not possess estrogen and progesterone receptors, its function is modulated by other CNS neurotransmitters and neurosecretory products. Among these, excitatory amino acids (EAAs) have now been shown to play an important role in the regulation of pulsatile gonadotropin release, induction of puberty and preovulatory and steroid-induced gonadotropin surges. Glutamate, the major endogenous EAA exerts its action through ionotropic and metabotropic receptors. The ionotropic receptors consist of two major classes, the NMDA (N-methyl-D-aspartate) and non-NMDA: kainate and AMPA (DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. EAA receptors are found in hypothalamic areas involved with reproduction. While both NMDA and non-NMDA receptors are involved in the regulation of LH secretion, the NMDA receptors appear to be involved with the regulation of puberty and FSH secretion as well. Steroids increase the release rates of glutamate and aspartate in the preoptic area during the gonadotropin surge. Steroids may also regulate the hypothalamic AMPA receptors.

Evidence that progesterone modulates anterior pituitary neuropeptide Y levels during the progesterone-induced gonadotropin surge in the estrogen-primed intact immature female rat

In a previous study we reported that in vivo estrogen-priming alone, without subsequent progesterone-treatment, was sufficient to maximize NPY potentiation of gonadotropin hormone-releasing hormone responsiveness exhibited in vitro by the rat anterior pituitary. This observation suggests that the necessity, as reported by others, for both estrogen-priming and progesterone-treatment to maximize NPY potentiation of GnRH responsiveness in vivo may be due to progesterone acting primarily at the hypothalamus. Consequently, the current study was performed to determine whether progesterone facilitates gonadotropin secretion in vivo by acting to stimulate hypothalamic synthesis of NPY and the subsequent elevation of anterior pituitary tissue levels of NPY. Intact immature female rats were injected with estradiol at 1700 h on days 27 and 28. On day 29 at 0900 h, the animals received an injection of progesterone (2 mg/kg) or vehicle and were subsequently sacrificed at 1200, 1330 and 1500 h. Rats which received only estradiol injections were used as controls. Surge levels of serum LH and FSH were observed at 1330 and 1500 h. Hypothalamic levels of NPY mRNA at 1200 h on day 29 were higher (P < 0.01) in estradiol-primed rats which received progesterone; there was no accompanying statistically significant change in hypothalamic NPY content. NPY content in the anterior pituitary was significantly increased (P < 0.01) at 1200 h on day 29 in estradiol-primed rats which received progesterone; there was no accompanying significant change in anterior pituitary NPY mRNA levels. Hypothalamic GnRH mRNA content was significantly increased (P < 0.01) at 1330 h on day 29 concomitant with the peak of the gonadotropin surge in the estradiol-primed, progesterone-treated rat. The data indicate that progesterone modulates hypothalamic NPY mRNA and anterior pituitary NPY levels as well as GnRH mRNA levels and that modulation of NPY levels in the hypothalamic-pituitary axis occurs prior to modulation of GnRH gene expression. These studies support the hypothesis that in the estrogen-primed rat, progesterone facilitates the induction of the gonadotropin surge by maintaining hypothalamic synthesis of NPY as well as by modulating anterior pituitary NPY tissue levels.