Salinity Alters Toxicity of Commonly Used Pesticides in a Model Euryhaline Fish Species (Menidia beryllina)

Changing salinity in estuaries due to sea level rise and altered rainfall patterns, as a result of climate change, has the potential to influence the interactions of aquatic pollutants as well as to alter their toxicity. From a chemical property point of view, ionic concentration can increase the octanol–water partition coefficient and thus decrease the water solubility of a compound. Biologically, organism physiology and enzyme metabolism are also altered at different salinities with implications for drug metabolism and toxic effects. This highlights the need to understand the influence of salinity on pesticide toxicity when assessing risk to estuarine and marine fishes, particularly considering that climate change is predicted to alter salinity regimes globally and many risk assessments and regulatory decisions are made using freshwater studies. Therefore, we exposed the Inland Silverside (Menidia beryllina) at an early life stage to seven commonly used pesticides at two salinities relevant to estuarine waters (5 PSU and 15 PSU). Triadimefon was the only compound to show a statistically significant increase in toxicity at the 15 PSU LC₅₀. However, all compounds showed a decrease in LC₅₀ values at the higher salinity, and all but one showed a decrease in the LC₁₀ value. Many organisms rely on estuaries as nurseries and increased toxicity at higher salinities may mean that organisms in critical life stages of development are at risk of experiencing adverse, toxic effects. The differences in toxicity demonstrated here have important implications for organisms living within estuarine and marine ecosystems in the Anthropocene as climate change alters estuarine salinity regimes globally.

An Inter-Institutional Collaboration to "Make Teaching Matter": The Teaching Academy of the Consortium of West Region Colleges of Veterinary Medicine

Veterinary medical education is a relatively small community with limited numbers of institutions, people, and resources widely dispersed geographically. The problems faced, however, are large-and not very different from the problems faced by (human) medical education. As part of an effort to share resources and build a community of practice around common issues, five colleges in the westernmost region of the United States came together to form a regional inter-institutional consortium. This article describes the processes by which the consortium was formed and the initiation of its first collaborative endeavor, an inter-institutional medical/biomedical teaching academy (the Regional Teaching Academy, or RTA). We report outcomes, including the successful launch of three RTA initiatives, and the strategies that have been considered key to the academy's success. These include strong support from the consortium deans, including an ongoing financial commitment, a dedicated part-time Executive Coordinator, regular face-to-face meetings that supplement virtual meetings, an organization-wide biennial conference, an effective organizational structure, and a core group of dedicated leaders and RTA Fellows. The western consortium and RTA share these processes, insights, and outcomes to provide a model upon which other colleges of veterinary medicine can build to further leverage inter-institutional collaboration.

Autocrine production of reproductive axis neuropeptides affects proliferation of canine osteosarcoma in vitro

Background

Osteosarcoma strikes hundreds of people each year, of both advanced and younger ages, and is often terminal. Like many tumor types, these bone tumors will frequently undergo a neuroendocrine transition, utilizing autocrine and/or paracrine hormones as growth factors and/or promoters of angiogenesis to facilitate progression and metastasis. While many of these factors and their actions on tumor growth are characterized, some tumor-derived neuropeptides remain unexplored.

Methods

Using validated canine osteosarcoma cell lines in vitro, as well as cells derived from spontaneous tumors in dogs, we explored the autocrine production of two neuropeptides typically found in the hypothalamus, and most closely associated with reproduction: gonadotropin-releasing hormone (GnRH) and kisspeptin (Kiss-1). We evaluated gene expression and protein secretion of these hormones using quantitative RT-PCR and a sensitive radioimmunoassay, and explored changes in cell proliferation determined by MTS cell viability assays.

Results

Our current studies reveal that several canine osteosarcoma cell lines (COS, POS, HMPOS, D17, C4) synthesize and secrete GnRH and express the GnRH receptor, while COS and POS also express kiss1 and its cognate receptor. We have further found that GnRH and kisspeptin, exogenously applied to these tumor cells, exert significant effects on both gene expression and proliferation. Of particular interest, kisspeptin exposure stimulated GnRH secretion from COS, similarly to the functional relationship observed within the neuroendocrine reproductive axis. Additionally, GnRH and kisspeptin treatment both increased COS proliferation, which additionally manifested in increased expression of the bone remodeling ligand rankl within these cells. These effects were blocked by treatment with a specific GnRH receptor inhibitor. Both neuropeptides were found to increase expression of the specific serotonin (5HT) receptor htr2a, the activation of which has previously been associated with cellular proliferation, suggesting that production of these factors by osteosarcoma cells may act to sensitize tumors to circulating 5HT of local and/or enteric origin.

Conclusions

Here we report that kisspeptin and GnRH act as autocrine growth factors in canine osteosarcoma cells in vitro, modulating RANKL and serotonin receptor expression in a manner consistent with pro-proliferative effects. Pharmacological targeting of these hormones may represent new avenues of osteosarcoma treatment.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5363-4) contains supplementary material, which is available to authorized users.

Dietary resveratrol increases mid-life fecundity of female Nothobranchius guentheri

The decline of female reproductive function is an early phenotype of aging in humans, occurring only midway through the lifespan. Yet the number of women delaying pregnancy continues to rise in industrialized societies due to personal or socioeconomic circumstances, often resulting in subfertility or difficulty conceiving. There are few defined mechanisms associated with this etiology, and equally few effective therapies. To combat this problem, we used a novel emerging model, Nothobranchius guentheri, that recapitulates the age-associated spectrum of changes that adversely affect human fertility. We hypothesized that resveratrol (RSV), which activates SirT1 as an oxidative stress sensor and longevity assurance enzyme, would improve female fecundity in mid-life. RSV, a polyphenol found in grapes and red wine, has been touted as an anti-aging dietary supplement due to its ability to prolong both lifespan and health span. SirT1 is an NAD+ dependent histone deacetylase, whose activity is regulated by the nicotinamide to NAD+ salvage pathway, especially the rate-limiting enzyme NAMPT. We found that female N. guentheri fed 600μgRSV/g food into mid-life (~20weeks), beginning at sexual maturity, showed increased embryo production compared to those on Control diet. Furthermore, the RSV-fed fish had significantly increased NAMPT. This suggests that dietary RSV has a positive effect on female fertility, and that it may become an effective therapy to regulate sirtuin activity and combat reproductive senescence.

Presynaptic inhibition decreases when estrogen level rises

The objective was to determine estrogen's influence on control of a skeletal muscle through measurements of motorneuron excitability (H:M ratio) and presynaptic inhibition (PI). Estrogen serum concentrations were measured at menses and ovulation of female subjects and compared to male controls. Data were analyzed from 12 women and 13 men reporting no history of knee ligament injury. Women reported regular menstrual cycles and no hormone-based contraceptive use for the previous year. Women were tested at menses (Time1) and ovulation (Time2). Men were tested twice, approximately 14 days apart. Analysis indicated no difference in the H:M ratio between the sexes at either time point. A significant difference for the sexes was detected in the magnitude of estrogen change (∆EST) between observations. At Time1, the male and female estrogen concentrations were not different; however, they were different at Time2, primarily due to the large rise observed in the women. A significant difference between the sexes was also seen in the magnitude of change for PI (∆PI) between observations. As with EST, the levels of PI between the sexes at Time1 were not different; however, a difference existed at Time 2. Estrogen interacts with GABA at several nervous system locations affecting inhibition of synaptic transmission. This is the first study to investigate changes in PI of a skeletal muscle between times of low and high estrogen. Improving the understanding of estrogen's influence on skeletal muscles may provide answers to why noncontact anterior cruciate ligament injuries of the knee occur more frequently in women.

Paternally Inherited DLK1 Deletion Associated With Familial Central Precocious Puberty

CONTEXT

Central precocious puberty (CPP) results from premature activation of the hypothalamic-pituitary-gonadal axis. Few genetic causes of CPP have been identified, with the most common being mutations in the paternally expressed imprinted gene MKRN3.

OBJECTIVE

To identify the genetic etiology of CPP in a large multigenerational family.

DESIGN

Linkage analysis followed by whole-genome sequencing was performed in a family with five female members with nonsyndromic CPP. Detailed phenotyping was performed at the time of initial diagnosis and long-term follow-up, and circulating levels of Delta-like 1 homolog (DLK1) were measured in affected individuals. Expression of DLK1 was measured in mouse hypothalamus and in kisspeptin-secreting neuronal cell lines in vitro.

SETTING

Endocrine clinic of an academic medical center.

PATIENTS

Patients with familial CPP were studied.

RESULTS

A complex defect of DLK1 (∼14-kb deletion and 269-bp duplication) was identified in this family. This deletion included the 5' untranslated region and the first exon of DLK1, including the translational start site. Only family members who inherited the defect from their father have precocious puberty, consistent with the known imprinting of DLK1. The patients did not demonstrate additional features of the imprinted disorder Temple syndrome except for increased fat mass. Serum DLK1 levels were undetectable in all affected individuals. Dlk1 was expressed in mouse hypothalamus and in kisspeptin neuron-derived cell lines.

CONCLUSION

We identified a genomic defect in DLK1 associated with isolated familial CPP. MKRN3 and DLK1 are both paternally expressed imprinted genes. These findings suggest a role of genomic imprinting in regulating the timing of human puberty.

Evaluation of Immortalized AVPV- and Arcuate-Specific Neuronal Kisspeptin Cell Lines to Elucidate Potential Mechanisms of Estrogen Responsiveness and Temporal Gene Expression in Females

In females, ovarian estradiol modulates kisspeptin (Kiss-1) synthesis to act as an obligatory regulator of downstream gonadotropin release in vivo, via stimulation of GnRH neurons. Changes in the ovarian condition are relayed to the neuroendocrine hypothalamus via two sexually dimorphic Kiss-1 populations, located in the anteroventral periventricular (AVPV) and arcuate nuclei, conveying estradiol-positive and -negative feedback, respectively. To elucidate how differential responsiveness to estradiol is mediated in these populations, we generated two kisspeptin-secreting cell lines from an adult kiss1-green fluorescent protein (GFP) female mouse. These lines recapitulate in vivo responsiveness to estradiol, with KTaV-3 (AVPV) cells demonstrating significantly increased kiss1 expression under high physiological estradiol exposure, whereas KTaR-1 (arcuate) cells exhibit kiss1 suppression after lower estradiol exposure. Baseline expression of estrogen receptor-α (esr1) differs significantly between KTaV-3 and KTaR-1 cells, with KTaR-1 cells demonstrating higher basal expression of esr1. Estradiol stimulation of kiss1 expression in KTaV-3 cells is modulated in a dose-dependent manner up to 25.0 pM, with less responsiveness observed at higher doses (>50.0 pM). In contrast, KTaR-1 kiss1 attenuates at lower estradiol doses (2.0-5.0 pM), returning to baseline levels at 25.0 pM and greater. Furthermore, the expression of the core clock genes bmal1 and per2 show normal rhythms in KTaV-3 cells, regardless of estradiol treatment. Conversely, KTaR-1 antiphasic transcription of bmal1 and per2 is phase delayed by low estradiol treatment. Strikingly, estradiol induces circadian rhythms of kiss1 expression only in KTaV-3 cells. Further exploration into estradiol responsiveness will reveal mechanisms responsible for the differential expression pattern demonstrated in vivo between these cell types.

Antagonism of serotonin receptor 1B decreases viability and promotes apoptosis in the COS canine osteosarcoma cell line

Serotonin receptor 1B (5HTR1B) traditionally exhibits anti-proliferative activity in osteoblasts. We examined the expression and function of 5HTR1B in the COS canine osteosarcoma cell line and normal canine osteoblasts. Equal levels of 5HTR1B gene and protein expression were found between normal and malignant osteoblasts. Treatment with serotonin enhanced viability of osteosarcoma cells but not normal osteoblasts. Challenge with the 5HTR1B agonist anpirtoline caused no change in cell viability. Rather incubation with the specific receptor antagonist SB224289 caused reduction in osteoblast viability, with this effect more substantial in osteosarcoma cells. Investigation of this inhibitory activity showed 5HTR1B antagonism induces apoptosis in malignant cells. Evaluation of phosphorylated levels of CREB and ERK, transcriptional regulators associated with serotonin receptor signalling in osteoblasts, revealed aberrant 5HTR1B signalling in COS. Our results confirm the presence of 5HTR1B in a canine osteosarcoma cell line and highlight this receptor as a possible novel therapeutic target.

The expression and role of serotonin receptor 5HTR2A in canine osteoblasts and an osteosarcoma cell line

Background

The significance of the serotonergic system in bone physiology and, more specifically, the importance of the five hydroxytryptamine receptor 2A (5HTR2A) in normal osteoblast proliferation have been previously described; however the role of serotonin in osteosarcoma remains unclear. Particularly, the expression and function of 5HTR2A in canine osteosarcoma has not yet been studied, thus we sought to determine if this indoleamine modulates cellular proliferation in vitro. Using real time quantitative reverse transcription PCR and immunoblot analyses, we explored receptor expression and signaling differences between non-neoplastic canine osteoblasts (CnOb) and an osteosarcoma cell line (COS). To elucidate specific serotonergic signaling pathways triggered by 5HTR2A, we performed immunoblots for ERK and CREB. Finally, we compared cell viability and the induction of apoptosis in the presence 5HTR2A agonists and antagonists.

Results

5HTR2A was overexpressed in the malignant cell line in comparison to normal cells. In CnOb cells, ERK phosphorylation (ERK-P) decreased in response to both serotonin and a specific 5HTR2A antagonist, ritanserin. In contrast, ERK-P abundance increased in COS cells following either treatment. While endogenous CREB was undetectable in CnOb, CREB was observed constitutively in COS, with expression and exhibited increased CREB phosphorylation following escalating concentrations of ritanserin. To determine the influence of 5HTR2A signaling on cell viability we challenged cells with ritanserin and serotonin. Our findings confirmed that serotonin treatment promoted cell viability in malignant cells but not in normal osteoblasts. Conversely, ritanserin reduced cell viability in both the normal and osteosarcoma cells. Further, ritanserin induced apoptosis in COS at the same concentrations associated with decreased cell viability.

Conclusions

These findings confirm the existence of a functional 5HTR2A in a canine osteosarcoma cell line. Results indicate that intracellular second messenger signal coupling of 5HTR2A is different between normal and malignant cells, warranting further research to investigate its potential as a novel therapeutic target for canine osteosarcoma.

Clocks on top: the role of the circadian clock in the hypothalamic and pituitary regulation of endocrine physiology

Recent strides in circadian biology over the last several decades have allowed researchers new insight into how molecular circadian clocks influence the broader physiology of mammals. Elucidation of transcriptional feedback loops at the heart of endogenous circadian clocks has allowed for a deeper analysis of how timed cellular programs exert effects on multiple endocrine axes. While the full understanding of endogenous clocks is currently incomplete, recent work has re-evaluated prior findings with a new understanding of the involvement of these cellular oscillators, and how they may play a role in constructing rhythmic hormone synthesis, secretion, reception, and metabolism. This review addresses current research into how multiple circadian clocks in the hypothalamus and pituitary receive photic information from oscillators within the hypothalamic suprachiasmatic nucleus (SCN), and how resultant hypophysiotropic and pituitary hormone release is then temporally gated to produce an optimal result at the cognate target tissue. Special emphasis is placed not only on neural communication among the SCN and other hypothalamic nuclei, but also how endogenous clocks within the endocrine hypothalamus and pituitary may modulate local hormone synthesis and secretion in response to SCN cues. Through evaluation of a larger body of research into the impact of circadian biology on endocrinology, we can develop a greater appreciation into the importance of timing in endocrine systems, and how understanding of these endogenous rhythms can aid in constructing appropriate therapeutic treatments for a variety of endocrinopathies.

The Changes They are A-Timed: Metabolism, Endogenous Clocks, and the Timing of Puberty

Childhood obesity has increased dramatically over the last several decades, particularly in industrialized countries, often accompanied by acceleration of pubertal progression and associated reproductive abnormalities (Biro et al., 2006; Rosenfield et al., 2009). The timing of pubertal initiation and progression in mammals is likely influenced by nutritional and metabolic state, leading to the hypothesis that deviations from normal metabolic rate, such as those seen in obesity, may contribute to observed alterations in the rate of pubertal progression. While several recent reviews have addressed the effects of metabolic disorders on reproductive function in general, this review will explore previous and current models of pubertal timing, outlining a potential role of endogenous timing mechanisms such as cellular circadian clocks in the initiation of puberty, and how these clocks might be altered by metabolic factors. Additionally, we will examine recently elucidated neuroendocrine regulators of pubertal progression such as kisspeptin, explore models detailing how the mammalian reproductive axis is silenced during the juvenile period and reactivated at appropriate developmental times, and emphasize how metabolic dysfunction such as childhood obesity may alter timing cues that advance or delay pubertal progression, resulting in diminished reproductive capacity.

Oestrogen induces rhythmic expression of the Kisspeptin-1 receptor GPR54 in hypothalamic gonadotrophin-releasing hormone-secreting GT1-7 cells

Oestrogen-stimulated preovulatory gonadotrophin surges are temporally regulated in a way that remains not fully understood. Mammalian ovulation requires surges of gonadotrophin-releasing hormone (GnRH), released from specialised neurones in the hypothalamus. Surge regulation is mediated by ovarian oestrogen (17 β-oestradiol; E(2) ) feedback-acting as a negative signal until the early afternoon of the pro-oestrous phase, at which point it stimulates robust increases in GnRH release. Multiple lines of evidence suggest a role for the circadian clock in surge generation, although the presence of endogenous oscillators in several neuronal populations throughout the mediobasal hypothalamus complicates an elucidation of the underlying mechanisms of circadian regulation. In the present study, we propose that endogenous oscillators within GnRH neurones are modulated by oestrogen to elicit GnRH surge secretion. One mechanism by which this may occur is through the up-regulation of receptors of known stimulators of GnRH, such as kisspeptin's cognate receptor, GPR54. Through analysis of mRNA and protein abundance patterns, we found that high levels of E(2) elicit circadian expression profiles of GPR54 in vitro, and that disruption of endogenous GnRH oscillators of the clock dampens this effect. Additionally, although kisspeptin administration to GT1-7 cells does not result in surge-level secretion, we observed increased GnRH secretion from GT1-7 cells treated with positive feedback levels of E(2) . These results in this in vitro neuronal model system suggest a possible mechanism whereby receptor expression levels, and thus GnRH sensitivity to kisspeptin, may change dramatically over the pro-oestrous day. In this way, elevated ovarian E(2) may increase kisspeptidergic tone at the same time as increasing GnRH neuronal sensitivity to this neuropeptide for maximal surge release.

Modulation of gonadotrophin-releasing hormone secretion by an endogenous circadian clock

The mechanisms mediating positive feedback effects of oestradiol on pre-ovulatory gonadotrophin releasing-hormone (GnRH) surge generation in female mammals, although well-explored, are still incompletely understood. In addition to binding to and signalling through classical nuclear receptor-mediated pathways in afferent hypothalamic neurones, recent evidence suggests that ovarian steroids may use membrane-bound receptors or nonclassical signalling pathways to directly influence cell function leading to the generation of GnRH surge secretion. We review recent investigations into the role of the endogenous molecular circadian clock on modulation of GnRH gene expression and neuropeptide secretion, and will explore potential molecular mechanisms by which ovarian steroids may directly induce secretory changes at the level of the GnRH neurone, examining closely whether circadian clock gene oscillations may be involved.

Circadian clock and output genes are rhythmically expressed in extratesticular ducts and accessory organs of mice

Circadian clocks regulate multiple rhythms in mammalian tissues. In most organs core clock gene expression is oscillatory, with negative components Per and Cry peaking in antiphase to Bmal1. A notable exception is the testis, where clock genes seem nonrhythmic. Earlier mammalian studies, however, did not examine clock expression patterns in accessory ductal tissue required for sperm maturation and transport. Previous studies in insects demonstrated control of sperm maturation in vas deferens by a local circadian system. Sperm ducts express clock genes and display circadian pH changes controlled by vacuolar-type H(+)-ATPase and carbonic anhydrase (CA-II). It is unknown whether sperm-processing rhythms are conserved beyond insects. To address this question in mice housed in a light-dark environment, we examined temporal patterns of mPer1 and Bmal1 gene expression and protein abundance in epididymis, vas deferens, seminal vesicles, and prostate. Results demonstrate variable tissue-specific patterns of expression of the two genes, with variations in levels of clock proteins and their nucleo-cytoplasmic cycling observed among examined tissues. Strikingly, mPer1 and Bmal1 mRNA and proteins oscillate in antiphase in the prostate, with similar peak-trough patterns as observed in the suprachiasmatic nuclei, the brain's central clock. Genes encoding CA and a V-ATPase subunit, which are rhythmically expressed in sperm ducts of moths, are also rhythmic in some segments of murine sperm ducts. Our data suggest that some sperm duct segments may contain peripheral circadian systems whereas others may express clock genes in a pleiotropic manner.

The circadian clock protein BMAL1 is necessary for fertility and proper testosterone production in mice

Although it is well established that the circadian clock regulates mammalian reproductive physiology, the molecular mechanisms by which this regulation occurs are not clear. The authors investigated the reproductive capacity of mice lacking Bmal1 (Arntl, Mop3), one of the central circadian clock genes. They found that both male and female Bmal1 knockout (KO) mice are infertile. Gross and microscopic inspection of the reproductive anatomy of both sexes suggested deficiencies in steroidogenesis. Male Bmal1 KO mice had low testosterone and high luteinizing hormone serum concentrations, suggesting a defect in testicular Leydig cells. Importantly, Leydig cells rhythmically express BMAL1 protein, suggesting peripheral control of testosterone production by this clock protein. Expression of steroidogenic genes was reduced in testes and other steroidogenic tissues of Bmal1 KO mice. In particular, expression of the steroidogenic acute regulatory protein (StAR) gene and protein, which regulates the rate-limiting step of steroidogenesis, was decreased in testes from Bmal1 KO mice. A direct effect of BMAL1 on StAR expression in Leydig cells was indicated by in vitro experiments showing enhancement of StAR transcription by BMAL1. Other hormonal defects in male Bmal1 KO mice suggest that BMAL1 also has functions in reproductive physiology outside of the testis. These results enhance understanding of how the circadian clock regulates reproduction.

Orexin A induces GnRH gene expression and secretion from GT1-7 hypothalamic GnRH neurons

Orexin A, a recently discovered hypothalamic peptide, has been shown to have a stimulatory effect on release of gonadotropin-releasing hormone (GnRH) from rat hypothalamic explants in vitro. However, it is presently unclear whether in vivo this effect is mediated directly at the level of the GnRH neuron, or via multiple afferent neuronal connections. Therefore, in the present study, we investigated the direct action of orexin A on GnRH neurons using the immortalized GnRH-secreting GT1-7 hypothalamic cells. Orexin-1 receptor (OX1R) expression was detected in GT1-7 cells by RT-PCR and Western blot. Results showed that 0.1-1 nM orexin A, when administered in culture media for 4 h, can significantly stimulate GnRH mRNA expression in GT1-7 cells (p < 0.05). Administration of 1 microM OX1R antagonist, SB-334867, completely blocked the observed orexin A responses in these cells, indicating that orexin A stimulation of GnRH neurons is specifically through OX1R. Moreover, 0.1 nM orexin A stimulated GnRH release after 30-45 min. To examine possible signal transduction pathways involved in mediating these effects, a MEK inhibitor (UO-126), PKC inhibitor (calphostin C), and PKA inhibitor (H-89), were used, with each blocking orexin A-induced GnRH transcription and release from immortalized cells. Collectively, our results show that orexin A is capable of directly stimulating GnRH transcription and neuropeptide release from these immortalized hypothalamic neurons, and that the effects of orexin A appear to be mediated via the OX1R, coupled with activation of the PKC-, MAPK- and PKA-signaling pathways. It is suggested that the stimulatory effect of orexin A on GnRH transcription and release may also occur directly at the level of GnRH neurons in vivo.

Clocks and the black box: circadian influences on gonadotropin-releasing hormone secretion

Although the mechanisms underlying hypothalamic surge secretion of gonadotropin-releasing hormone (GnRH) in rodent models have remained enduring mysteries in the field of neuroendocrinology, the identities of two fundamental constituents are clear. Elevated ovarian oestrogen, in conjunction with circadian signals, combine to elicit GnRH surges that are confined to the afternoon of the proestrus phase. The phenomenon of oestrogen positive feedback, although extensively investigated, is not completely understood, and may involve the actions of this steroid directly on GnRH perikarya, as well as on the activity of neuronal afferents. Additionally, whereas many studies have focused upon regulation of GnRH surge secretion by the neuroanatomical biological clock, the suprachiasmatic nucleus, it remains unclear why this daily signal is capable of stimulating surges only in the presence of oestrogen. This review re-examines multiple models of circadian control of reproductive neurosecretion, armed with the recent characterisation of the intracellular transcriptional feedback loops that comprise the circadian clock, and attempts to evaluate previous studies on this topic within the context of these new discoveries. Recent advances reveal the presence of oscillating circadian clocks throughout the central nervous system and periphery, including the anterior pituitary and hypothalamus, raising the possibility that synchrony between multiple cellular clocks may be involved in GnRH surge generation. Current studies are reviewed that demonstrate the necessity of functional clock oscillations in generating GnRH pulsatile secretion in vitro, suggesting that a GnRH-specific intracellular circadian clock may underlie GnRH surges as well. Multiple possible steroidal and neuronal contributions to GnRH surge generation are discussed, in addition to how these signals of disparate origin may be integrated at the cellular level to initiate this crucial reproductive event.

Circadian gene expression regulates pulsatile gonadotropin-releasing hormone (GnRH) secretory patterns in the hypothalamic GnRH-secreting GT1-7 cell line

Although it has long been established that episodic secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus is required for normal gonadotropin release, the molecular and cellular mechanisms underlying the synchronous release of GnRH are primarily unknown. We used the GT1-7 mouse hypothalamic cell line as a model for GnRH secretion, because these cells release GnRH in a pulsatile pattern similar to that observed in vivo. To explore possible molecular mechanisms governing secretory timing, we investigated the role of the molecular circadian clock in regulation of GnRH secretion. GT1-7 cells express many known core circadian clock genes, and we demonstrate that oscillations of these components can be induced by stimuli such as serum and the adenylyl cyclase activator forskolin, similar to effects observed in fibroblasts. Strikingly, perturbation of circadian clock function in GT1-7 cells by transient expression of the dominant-negative Clock-Delta19 gene disrupts normal ultradian patterns of GnRH secretion, significantly decreasing mean pulse frequency. Additionally, overexpression of the negative limb clock gene mCry1 in GT1-7 cells substantially increases GnRH pulse amplitude without a commensurate change in pulse frequency, demonstrating that an endogenous biological clock is coupled to the mechanism of neurosecretion in these cells and can regulate multiple secretory parameters. Finally, mice harboring a somatic mutation in the Clock gene are subfertile and exhibit a substantial increase in estrous cycle duration as revealed by examination of vaginal cytology. This effect persists in normal light/dark (LD) cycles, suggesting that a suprachiasmatic nucleus-independent endogenous clock in GnRH neurons is required for eliciting normal pulsatile patterns of GnRH secretion.

In vivo gonadotropin-releasing hormone secretion in female rats during peripubertal development and on proestrus

Pubertal development in female rats is characterized by increased LH levels and the appearance of estrogen-dependent afternoon LH mini-surges. In these studies we performed the first analysis of GnRH patterns in peripubertal rats to determine whether there are similar changes in pulsatile GnRH release. Microdialysis samples were collected at 5-min intervals throughout a 5-h afternoon period from 22 rats sampled on a single day between 30-47 days of age. Adult female rats were sampled on proestrus for comparison. In 30- to 33-day-old rats, GnRH release was infrequent (2.7 pulses/5 h; n = 3), whereas intermediate pulse frequencies were observed in 34- to 37-day-old rats (6.4 pulses/5 h; n = 9) and 38- to 42-day-old (5.0 pulses/5 h; n = 5) rats. The highest GnRH pulse frequencies were observed in 43- to 47-day-old rats (9.4 pulses/5 h; n = 5). Mean GnRH pulse amplitude did not vary significantly with age. Animals sampled before vaginal opening (VO) exhibited significantly slower GnRH pulse frequencies than those sampled after vaginal opening (1.3 pulses/5 h pre-VO vs. 7.6 pulses/5 h post-VO; P = 0.01). An afternoon increase in GnRH secretion, defined operationally as a greater than 25% increase in mean GnRH levels in the last half of the sampling period and tentatively termed a mini-surge, was observed in 0%, 33%, 40%, and 60% of 30- to 33-, 34- to 37-, 38- to 42-, and 43- to 47-day-old rats, respectively. An overall increase in GnRH pulse frequency was observed in females displaying a mini-surge (9.0 pulses/5 h with mini-surge compared with 4.7 pulses/5 h with no mini-surge). The mini-surge itself, however, was associated with a late afternoon increase in GnRH pulse amplitude and not in pulse frequency. In adult proestrous rats, peak levels during the GnRH surge were an order of magnitude greater than those reached in pubertal animals. Our findings demonstrate that pubertal maturation in the female rat is associated with an acceleration of GnRH pulse generator activity and that later stages of pubertal maturation are characterized by the appearance of afternoon increases in GnRH release that may underlie previously reported mini-surges in LH.

Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcription factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicated in the physiological regulation of several important neuroendocrine processes, including estrous behavior and periovulatory hormone secretions. We review evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrate neural and endocrine signals which direct several critically important neuroendocrine processes. An integrative function for PRs is reviewed in several physiological contexts, including the display of lordosis behavior in female rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulatory follicle stimulating hormone surges. The weight of evidence indicates that cross talk at the intracellular PR is an essential component of the integrative mechanisms that direct each of these neuroendocrine events. The recurrence of PR's integrative actions in several different physiological contexts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine processes.

Stimulation of gonadotropin-releasing hormone surges by estrogen. I. Role of hypothalamic progesterone receptors

Estrogen (E2) stimulates GnRH surges by coupling a daily neural signal to neuronal circuitries governing GnRH release. We have hypothesized that E2 promotes this coupling process by inducing expression of neuronal transcription factors, which are subsequently activated by neurotransmitter-mediated mechanisms representing the daily neural signal. These experiments tested the specific hypothesis that the progesterone receptor (PR) functions in this manner, viz. as an E2-induced factor whose activation is necessary for the stimulation of GnRH surges. Two complimentary experiments were performed to determine whether activation of hypothalamic PRs is obligatory for the stimulation of GnRH surges by E2. In the first, the effects of a PR antagonist on GnRH and LH surges were assessed in ovariectomized (OVX), E2-primed rats. Rats were OVX on diestrous day 2, treated with 30 microg estradiol benzoate or oil vehicle, sc, and then administered either oil vehicle or the type I antiprogestin, ZK98299 at 0900 h on proestrus. GnRH release rates and plasma LH levels were determined in each animal by microdialysis of median eminence and atrial blood sampling, respectively. Estrogen, but not oil vehicle, treatment evoked robust and contemporaneous GnRH and LH surges in animals that received no PR antagonist on proestrus. Additional treatment with ZK98299, however, completely blocked both GnRH and LH surges. In a second experiment, specific involvement of anteroventral periventricular (AVPV) PRs in E2-induced GnRH surges was assessed. Additional groups of OVX, E2-primed rats were fitted with intracerebroventricular cannulas, and PR antisense oligonucleotides were infused into the third ventricle adjacent to the AVPV to prevent expression of PR in this periventricular region. Animals infused with PR antisense oligos did not exhibit any LH surges, whereas surges were observed in saline-, missense-, and sense oligo-treated controls. Immunohistochemistry confirmed the effectiveness of PR antisense oligonucleotides in blocking PR expression. These findings provide direct support for the hypothesis that activation of PRs, specifically those in hypothalamic regions including the AVPV, is an obligatory event in the stimulation of GnRH surges by E2.

Stimulation of gonadotropin-releasing hormone surges by estrogen. II. Role of cyclic adenosine 3'5'-monophosphate

Release of GnRH surges in female rats is directed by a daily neural signal and occurs only after exposure of the hypothalamus to sustained, elevated estrogen (E2) levels in serum. We have proposed that preovulatory E2 couples the daily neural signal to the circuitry governing GnRH release by a two-step process, which includes stimulation of neuronal progesterone receptors (PRs) by E2 and subsequent activation of PRs by the daily neural signal. In the preceding report we documented that PR activation is obligatory for the stimulation of GnRH surges by E2. In these studies we assess the validity of a second essential feature of this model, that neural signals can activate PRs and thereby prompt the release of GnRH and LH surges. Our efforts specifically focused on the role of cAMP in mediating neural PR trans-activation leading to GnRH surges. To assess whether cAMP may function as a daily neural signal, cAMP levels were examined via a competitive binding assay in anteroventral periventricular nucleus (AVPV) homogenates obtained at 0900, 1200, 1500, 1800, and 2100 h on all days of the estrous cycle. A significant rise in cAMP concentrations was observed at 1500 h on all estrous cycle days. A similar rise at the same time was observed in AVPV tissues of ovariectomized (OVX) rats regardless of steroid treatment. No significant increase in cAMP levels was observed at any time point in homogenates of ventromedial nucleus or cerebral cortex. In a second experiment, female rats were OVX on the afternoon of diestrous day 2 and simultaneously administered 30 microg estradiol benzoate or oil vehicle. On the following day of presumptive proestrus, rats received intracerebroventricular infusions of the cAMP analog, 8-bromo-cAMP, or saline vehicle at 0900 h. Rats treated with 8-bromo-cAMP exhibited LH surges that were advanced by 3 h compared with those in saline-treated controls. This advance did not occur in 8-bromo-cAMP-treated rats not primed with E2, or in E2-treated rats given the antiprogestin RU486. In a third experiment, OVX, estradiol benzoate-primed rats received intracerebroventricular infusions of saline vehicle or the adenylyl cyclase inhibitor SQ22536; although saline-treated rats exhibited normal LH surges, no surges were observed in the rats receiving SQ22536. In additional SQ22536-treated animals, however, LH surge release was rescued and greatly augmented by a pharmacological dose of progesterone. These results demonstrate that 1) cAMP levels in the AVPV are significantly elevated at 1500 h on a daily basis; 2) cAMP elevations in the AVPV can prematurely evoke LH surges by a mechanism that requires PR activation; 3) inhibition of adenylyl cyclase activity in the AVPV blocks LH surges, an action that can be reversed by progesterone; and 4) cAMP generation leads to PR transactivation in the AVPV. Our observations thus provide support for the hypothesis that an increase in intracellular cAMP in the AVPV acts as a component of the daily neural signal required to initiate GnRH and subsequent LH surges, and that transmission of this signal is mediated by cAMP-induced PR trans-activation in the AVPV.

Absence of gonadotropin surges and gonadotropin-releasing hormone self-priming in ovariectomized (OVX), estrogen (E2)-treated, progesterone receptor knockout (PRKO) mice

It is well known that estrogen (E2) stimulates expression of progesterone receptors (PRs), thereby inducing responsiveness of several tissues to the actions of progesterone (P). Recent studies have also suggested, however, that biological actions previously ascribed to E2 alone may also be mediated by activation of E2-induced PRs, even independently of signal changes in P concentrations. In the present experiments, the progesterone receptor knockout (PRKO) mice were used to assess the role of PR activation in the positive feedback actions of E2 on gonadotropin release. Ovariectomized (OVX) PRKO mice were tested for their capacity to mount primary gonadotropin surges in response to exogenous E2, and to exhibit a GnRH self-priming effect in response to sequential injections of the decapeptide. Wild-type (WT) and PRKO mice were OVX, treated with both 17beta-estradiol and estradiol benzoate (EB), and then killed at 1900 h on day 7 postOVX. Plasma LH RIA revealed that WT mice exhibited surges in response to the E2 treatment; the PRKO mice, however, showed no elevation in plasma LH above untreated controls. Instead, plasma LH levels in E2-treated, OVX PRKO mice decreased significantly in comparison to untreated OVX PRKO mice, suggesting that E2 can exert a negative feedback influence on LH release in PRKO mice, despite the absence of positive feedback effects. A slight but significant rise in plasma FSH was observed in E2-treated OVX WT mice in comparison to untreated controls: an effect not seen in E2-treated OVX PRKO mice, reinforcing the observation that estrogen's positive feedback effects are compromised in PRKO mice. In a second experiment, E2-treated OVX WT and PRKO mice were given either one or two pulses of GnRH 60 min apart, and killed 10 min later. The WT mice were found to exhibit a robust GnRH self-priming effect, as WT mice receiving two GnRH pulses displayed LH responses approximately 2-fold greater than those receiving only one pulse. By contrast, PRKO mice receiving two GnRH pulses exhibited no additional increase in plasma LH levels. We conclude that PR activation is obligatory for expression of the GnRH self-priming effect as well as for generation of E2-induced LH and FSH surges. The extent to which failure of LH surge secretion in PRKO mice is due to the absence of GnRH self-priming, lack of hypothalamic GnRH surges, and/or defects in other processes remains to be determined. These observations clearly demonstrate, however, that the presence of PR is an absolute requirement for the transmission of E2-induced signals leading to gonadotropin surges.

Endocrine defects in mice carrying a null mutation for the progesterone receptor gene

Mice carrying a null mutation of the progesterone receptor gene exhibit several reproductive abnormalities, including anovulation, attenuated lordotic behavior, uterine hyperplasia, and lack of mammary gland development. The hormonal correlates of these abnormalities are unknown, however, and were the focus of these studies. Serum samples from female wild-type (WT) and progesterone receptor knockout (PRKO) mice were obtained and analyzed by RIA for LH, FSH, PRL, estrogen (E2), and progesterone. Hypothalamic tissues were also processed for measurement of LHRH by RIA. Serum LH levels in PRKO mice were found to be elevated by approximately 2-fold over basal (metestrus) values in WT mice. By contrast, basal FSH levels were not different in PRKO and WT mice. Basal levels of E2 and progesterone in serum were likewise similar in the two groups, as were hypothalamic LHRH concentrations. Basal PRL levels were slightly higher in PRKO vs. WT mice. Ovariectomy of both groups of mice was accompanied by significant increases in both LH and FSH. At 5 days following ovariectomy, LH levels were elevated in both groups by 2-fold over PRKO basal and 4-fold over WT basal levels; however, by 10 days postovariectomy LH levels had continued to rise to a greater extent in PRKO mice than in WT animals. The FSH response to ovariectomy was greater for the PRKO mice at 5 days, but was no different from WT at 10 days. Of seven PRKO mice that were exposed to male odor, none exhibited preovulatory surges 3 days later, on the day of presumptive proestrus; this was in marked contrast with WT females, in which 100% exhibited robust LH surges. These results confirm the essential role of progesterone receptors in the regulation of hypothalamic and/or pituitary processes that govern gonadotropin secretion. The finding that basal LH levels are elevated in PRKO mice confirms that circulating progesterone normally conveys a significant portion of the total ovarian negative feedback control of the gonadotropin. That gonadotropin responses to ovariectomy are slightly enhanced in PRKO mice suggests that adrenal progesterone may contribute to the imposition of negative feedback control. The apparent inability of PRKO mice to respond to male odor suggests that anovulation in these mice may not be solely due to reproductive abnormalities within the ovary itself; rather, PRKO mice additionally harbor neuroendocrine defects that render them incapable of mounting normal preovulatory gonadotropin surges. It remains to be determined how the absence of PR in brain and pituitary of PRKO mice may produce this hormonal acyclicity and, conversely, how the presence of PR in brain and pituitary of WT mice may be obligatory in the generation of gonadotropin surges.

An alternative gonadotropin-releasing hormone (GnRH) RNA splicing product found in cultured GnRH neurons and mouse hypothalamus

Gonadotropin-releasing hormone (GnRH) is encoded by the proGnRH gene which contains four exons and three introns. In this study, two immortalized GnRH-expressing cell lines (Gn11 and NLT) were characterized. The NLT and Gn11 cells, derived from a same brain tumor in a transgenic mouse, display neuronal morphology and neuron-specific markers. However, NLT cells secrete much higher levels of GnRH than Gn11 cells. To delineate the mechanism underlying this difference, reverse transcriptase-polymerase chain reaction and RNase protection assays were performed to examine proGnRH gene expression. While the mature proGnRH mRNA was predominately expressed in NLT cells, Gn11 cells express an abundant short transcript. Sequence analysis revealed that this short transcript contains exons 1, 3, and 4, but not exon 2, which encodes the GnRH decapeptide. RNase protection assays demonstrated that NLT cells express much higher levels of mature proGnRH mRNA than Gn11 cells. The lower level of GnRH secreting capacity in Gn11 cells is due, in part, to decreased expression of mature proGnRH mRNA. When proGnRH gene expression in the mouse brain was examined, the same short splicing variant was observed in the olfactory area and preoptic area-anterior hypothalamus. But the prevalent transcript in these regions was the mature proGnRH mRNA. In contrast, only the mature proGnRH mRNA was found in the caudal hypothalamus. These results suggest that alternative splicing may be one of the mechanisms regulating proGnRH gene expression in the animal brain.