17β-estradiol exposure accelerates skeletal development in Xenopus laevis tadpoles.

Although it is well established that estrogen regulates skeletal growth and ossification in mammals, the effects of estrogen on skeletal development in amphibians are relatively uncharacterized. This study was conducted to characterize the impact of 17β-estradiol exposure on skeletal development in Xenopus laevis tadpoles. On day 48 postfertilization, tadpoles were placed in tanks containing 50% Holtfreter's Solution ±17β-estradiol at one of four concentrations (10(-11), 10(-10), 10(-9), and 10(-8) M).

Using a classic paper by I. E. Lawton and N. B. Schwartz to consider the array of factors that control luteinizing hormone production.

Two significant benefits derived from reading and discussing classic scientific papers in undergraduate biology courses are 1) providing students with the realistic perspective that science is an ongoing process (rather than a set of inarguable facts) and 2) deepening the students' understanding of physiological processes. A classic paper that is useful in both of these regards is by I. E.

Teaching aldosterone regulation and basic scientific principles using a classic paper by Dr. James O. Davis and colleagues.

Classroom discussion of scientific articles can be an effective means of teaching scientific principles and methodology to both undergraduate and graduate science students. The availability of classic papers from the American Physiological Society Legacy Project has made it possible to access articles dating back to the early portions of the 20th century. In this article, we discuss a classic paper from the laboratory of Dr.

Galanin enhancement of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion in female rats is estrogen dependent.

The hypothalamic peptide GnRH is the primary neuroendocrine signal regulating pituitary LH in females. The neuropeptide galanin is cosecreted with GnRH from hypothalamic neurons, and in vitro studies have demonstrated that galanin can act at the level of the pituitary to directly stimulate LH secretion and also augment GnRH-stimulated LH secretion. Several lines of evidence have suggested that the hypophysiotropic effects of galanin are important for the generation of preovulatory LH surges.

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats.

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion.

Pituitary follistatin regulates activin-mediated production of follicle-stimulating hormone during the rat estrous cycle.

Follistatin, an activin-binding protein, plays a key role in the modulation of activin-dependent functions. In the anterior pituitary, activin stimulates the synthesis and secretion of FSH. In the current study, we assessed the roles of locally produced activin and follistatin in the control of FSH gene expression and secretion.

Expression of ryanodine receptors in the pituitary gland: evidence for a role in gonadotropin-releasing hormone signaling.

GnRH elicits secretion of LH and FSH from gonadotropes by activating an array of intracellular signals including the generation of inositol triphosphate and the release of intracellular calcium. Given the important role of calcium in the secretory responses to GnRH, we examined the expression and function of the ryanodine receptors, which are known to modulate calcium release from intracellular stores. Using RT-PCR analysis, we found that ryanodine receptor (RyR) types 2 and 3, but not type 1, are expressed in rat pituitaries.

Gonadotropin-releasing hormone regulates follicle-stimulating hormone-beta gene expression through an activin/follistatin autocrine or paracrine loop.

The FSH beta gene is stimulated by low frequency pulses of GnRH, but is unaffected or suppressed when GnRH is applied at higher frequencies or continuously. The current studies explored the hypothesis that GnRH frequency-dependent regulation of FSH beta may be mediated by pituitary expression of activin, which stimulates FSH beta messenger RNA (mRNA), and follistatin, which blocks activin. Using a system of perifused male rat pituitary cells, a reciprocal relationship was observed between FSH beta and follistatin mRNAs in response to different patterns of GnRH treatment.

Sexually dimorphic transcriptional responses to gonadotropin-releasing hormone require chronic in vivo exposure to estradiol.

GnRH regulates secretion of the gonadotropins, LH and FSH, in a sexually dimorphic manner. In the present study, we examined GnRH regulation of the gonadotropin alpha-subunit promoter to assess whether sex-dependent hormonal effects are manifest at the transcriptional level. Primary cultures of male or female rat pituitary cells were transfected with a reporter gene containing the alpha-promoter linked to luciferase (-420 alpha-LUC) and then subjected to treatment with GnRH for 24 h.

Gonadotropin-releasing hormone regulation of pituitary follistatin gene expression during the primary follicle-stimulating hormone surge.

Follistatin is produced in the gonadotrope and folliculostellate cells of the pituitary gland and is thought to indirectly regulate FSH biosynthesis and secretion through its ability to bind activin. Recent measurements of follistatin gene expression during the rat estrous cycle revealed a marked increase in pituitary follistatin messenger RNA (mRNA) levels at the time of the preovulatory FSH surge. This finding suggests a role for follistatin in the regulation of FSH at this dynamic time of the cycle.

Gonadotropin-releasing hormone receptor messenger ribonucleic acid expression in the ovary during the rat estrous cycle.

Recent evidence indicates that the GnRH receptor (GnRH-R) gene is expressed in a number of tissues besides the anterior pituitary gland, suggesting that GnRH may serve other functions in addition to its role as a hypothalamic releasing factor. In particular, high levels of GnRH-R transcripts have been detected in rat and human ovarian granulosa cells. To better understand the role of the GnRH-R in the ovary under physiological conditions and to determine which follicles are potentially responsive to the actions of GnRH, we used in situ hybridization histochemistry and quantitative reverse transcriptase-polymerase chain reaction for measurement of ovarian GnRH-R messenger RNA (mRNA) expression during the rat ovulatory cycle.

Roles of estrogen, progesterone, and gonadotropin-releasing hormone (GnRH) in the control of pituitary GnRH receptor gene expression at the time of the preovulatory gonadotropin surges.

Pituitary GnRH receptor (GnRH-R) messenger RNA (mRNA) levels are regulated dynamically during the rat estrous cycle. GnRH-R mRNA levels increase 3-fold on the morning of proestrus and remain elevated throughout the gonadotropin surges, after which they decline rapidly. Because the day of proestrus is characterized by complex changes in the steroidal milieu and increased release of hypothalamic peptides such as GnRH, a series of in vivo steroid replacement and in vitro perifusion studies was used to assess the relative contributions of estrogen (E), progesterone (P), and GnRH to the induction and decline of GnRH-R gene expression during the gonadotropin surges.

Amplitude and frequency modulation of pulsatile luteinizing hormone-releasing hormone release.

1. A variety of neuroendocrine approaches has been used to characterize cellular mechanisms governing luteinizing hormone-releasing hormone (LHRH) pulse generation. We review recent in vivo microdialysis, in vitro superfusion, and in situ hybridization experiments in which we tested the hypothesis that the amplitude and frequency of LHRH pulses are subject to independent regulation via distinct and identifiable cellular pathways.

Unmasking of neuropeptide-Y inhibitory effects on in vitro gonadotropin secretion from pituitaries of metestrous, but not proestrous, rats.

We have recently demonstrated that GnRH-stimulated gonadotropin secretion is augmented by coadministration with neuropeptide-Y (NPY) in anterior pituitaries removed in the afternoon from proestrous, but not metestrous, rats. To test the hypothesis that these effects of NPY are due to an interaction with progesterone (P4), we conducted another cycle stage experiment using NPY, adding an in vitro treatment with P4. Pituitaries were taken from rats at 0900 h (before the rise of P4 on proestrus) on proestrus or metestrus and were perifused for 8 h, with and without GnRH pulses.

Dynamic regulation of pituitary follistatin messenger ribonucleic acids during the rat estrous cycle.

FSH synthesis and secretion are regulated by a complex interplay of hypothalamic, gonadal, and pituitary factors. Recent evidence suggests that inhibin, activin, and follistatin, although originally identified as gonadal peptides, are also expressed in the pituitary, where they may be secreted and play an autocrine/paracrine role in the control of FSH beta gene expression. Attempts to study pituitary regulation of the genes encoding these proteins have been hampered by low levels of mRNA expression.

RU486 administration blocks neuropeptide Y potentiation of luteinizing hormone (LH)-releasing hormone-induced LH surges in proestrous rats.

We previously demonstrated that NPY potentiates LHRH-induced LH secretion specifically under endocrine conditions in which preovulatory LH surges are generated. The present study was designed to test the hypothesis that NPY's facilitatory actions are dependent upon preovulatory progesterone secretion. In Exp 1, female rats were fitted with atrial catheters on diestrus.

Estrous cycle stage-dependent effects of neuropeptide-Y on luteinizing hormone (LH)-releasing hormone-stimulated LH and follicle-stimulating hormone secretion from anterior pituitary fragments in vitro.

We recently demonstrated that neuropeptide Y (NPY) potentiates LH-releasing hormone (LHRH)-stimulated LH secretion in vivo, and that these actions of NPY are exerted only under the endocrine conditions leading to preovulatory LH surges, viz. the proestrous or estrogen- and progesterone-primed ovariectomized animal. The present experiments tested the hypothesis that NPY's facilitatory actions are exerted directly at the level of the anterior pituitary gland and depend on in vivo exposure of gonadotropes to the preovulatory endocrine milieu.

Dynamic regulation of gonadotropin-releasing hormone receptor mRNA levels in the anterior pituitary gland during the rat estrous cycle.

The number of gonadotropin-releasing hormone (GnRH) receptors on pituitary gonadotropes varies substantially during the rat estrous cycle and may modulate pituitary responsiveness to GnRH. The present studies were undertaken to determine to what extent these changes in GnRH receptor number reflect a change in GnRH receptor mRNA expression in the anterior pituitary gland. Using quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), pituitary GnRH receptor mRNA levels were measured at various timepoints throughout the rat estrous cycle.

Neuropeptide Y gene expression in the arcuate nucleus: sexual dimorphism and modulation by testosterone.

Neuropeptide Y (NPY) peptide concentrations in the arcuate nucleus have recently been shown to be modulated by gonadal steroids in the male rat. The present study was designed to determine whether NPY messenger RNA (mRNA)-synthesizing cells in the arcuate nucleus (Arc) of the male rat are regulated by testosterone (T) and whether there is a sexual dimorphism in the expression of the NPY gene in this region. In situ hybridization and quantitative autoradiography were used to assess the level of NPY gene expression in the Arc.

Neuropeptide Y gene expression in the arcuate nucleus is increased during preovulatory luteinizing hormone surges.

Recent studies have suggested that neuropeptide Y (NPY) plays an important role in the induction of the preovulatory LH surge. The present study was performed in order to determine if a change in NPY gene expression within arcuate nucleus NPY neurons is associated with the generation of the preovulatory LH surge. In Exp 1, in situ hybridization was used to measure NPY messenger RNA (mRNA) levels in the arcuate nucleus of female rats at 0900 h and every 2 h from 1400-2200 h on the day of proestrus (PRO).

Neuropeptide Y potentiates luteinizing hormone (LH)-releasing hormone-induced LH secretion only under conditions leading to preovulatory LH surges.

We recently demonstrated that neuropeptide Y (NPY) potentiates the ability of pulsatile LHRH infusions to restore LH surges in pentobarbital (PB)-blocked, proestrous rats. In the present study we determined if specific endocrine conditions are necessary for the expression of these direct pituitary effects of NPY. Facilitatory actions of NPY were examined in the absence of gonadal feedback [ovariectomy (OVX)], in the presence of negative gonadal feedback (metestrus), after estrogen priming of the pituitary gland [OVX plus 30 micrograms estradiol benzoate (EB) 2 days before experiments], and after treatments which evoke preovulatory-like LH surges (OVX plus EB and 5 mg progesterone or P the morning of experiments).

Neuropeptide Y potentiates luteinizing hormone (LH)-releasing hormone-stimulated LH surges in pentobarbital-blocked proestrous rats.

Recent evidence suggests that hypothalamic neurosecretion of neuropeptide Y (NPY) may be required for the preovulatory LH surge in female rats. Results of immunoneutralization and portal blood collection studies have suggested that NPY may serve to enhance the response of gonadotropes to the stimulatory action of LHRH. To directly test this hypothesis, the effects of NPY on LHRH-stimulated LH secretion were assessed in proestrous rats that were anesthetized with pentobarbital (PB) to block endogenous LHRH neurosecretion.

Neuroendocrine regulation of the luteinizing hormone-releasing hormone pulse generator in the rat.

We have analyzed the mechanisms by which several known regulators of the LHRH release process may exert their effects. For each, we have attempted to determine how and where the regulatory input is manifest and, according to our working premise, we have attempted to identify factors which specifically regulate the LHRH pulse generator. Of the five regulatory factors examined, we have identified two inputs whose primary locus of action is on the pulse-generating mechanism--one endocrine (gonadal negative feedback), and one synaptic (alpha 1-adrenergic inputs) (see Fig.