Melatonin receptor signaling in pregnant and nonpregnant rat uterine myocytes as probed by large conductance Ca2+-activated K+ channel activity

The moon cycle influences reproductive and productive traits in guinea pigs (Cavia porcellus) from a tropical Andean area

There is currently innumerable evidence showing that the lunar cycle affects various reproductive aspects in farm animals. However, there is very little information on the effect of the lunar cycle on productive traits in these species. A retrospective study was conducted (2015-2018) to evaluate the influence of the lunar cycle on some reproductive and productive traits in a guinea pig (Cavia porcellus) production system in the southern Andean region of Ecuador. A total of 7352 productive and reproductive records of guinea pig females housed in 3 m2 cages with a breeding male were analyzed. The following variables were considered: offspring sex ratio, litter size per cage, number of weaned guinea pigs, mortality, individual and litter weaning weight of guinea pigs, and calving frequency. The lunar cycle was split into eight periods of ~3.7-d length each. Data were analyzed by logistic regression and general linear model, and means were compared by the least mean squares method of the SAS. The offspring sex ratio was not correlated to the lunar cycle. The lunar cycle at mating and calving influenced the litter size per cage, number of weaned, and mortality of guinea pigs. The individual and litter-weaning weights were influenced by the lunar cycle at calving. The frequency of calving was greater around the new and full moon than in the remaining periods of the lunar cycle. In conclusion, the lunar cycle influenced several productive and reproductive traits in guinea pigs, such as litter size, mortality, number of pups weaned, and individual and litter weaning weights. This valuable information may have practical applications in management of guinea pig production systems.

Hypotensive effects of melatonin in rats: Focus on the model, measurement, application, and main mechanisms

The hypotensive effects of melatonin are based on a negative correlation between melatonin levels and blood pressure in humans. However, there is a positive correlation in nocturnal animals that are often used as experimental models in cardiovascular research, and the hypotensive effects and mechanism of melatonin action are often investigated in rats and mice. In rats, the hypotensive effects of melatonin have been studied in normotensive and spontaneously or experimentally induced hypertensive strains. In experimental animals, blood pressure is often measured indirectly during the light (passive) phase of the day by tail-cuff plethysmography, which has limitations regarding data quality and animal well-being compared to telemetry. Melatonin is administered to rats in drinking water, subcutaneously, intraperitoneally, or microinjected into specific brain areas at different times. Experimental data show that the hypotensive effects of melatonin depend on the experimental animal model, blood pressure measurement technique, and the route, time and duration of melatonin administration. The hypotensive effects of melatonin may be mediated through specific membrane G-coupled receptors located in the heart and arteries. Due to melatonin's lipophilic nature, its potential hypotensive effects can interfere with various regulatory mechanisms, such as nitric oxide and reactive oxygen species production and activation of the autonomic nervous and circadian systems. Based on the research conducted on rats, the cardiovascular effects of melatonin are modulatory, delayed, and indirect.

Melatonin Receptors: A Key Mediator in Animal Reproduction

Melatonin, a hormone produced by the mammalian pineal gland, influences various physiological activities, many of which are related to animal reproduction, including neuroendocrine function, rhythm regulation, seasonal behavior, gonadogenesis, gamete development and maturation, sexual maturation, and thermoregulation. Melatonin exerts beneficial actions mainly via binding with G-protein-coupled receptors (GPCR), termed MT1 and MT2. Melatonin receptors are crucial for mediating animal reproduction. This paper reviews the characteristics of melatonin receptors including MT1 and MT2, as well as their roles in mediating signal transduction and biological effects, with a focus on their function in animal reproduction. In addition, we briefly summarize the developments in pharmacological research regarding melatonin receptors as drug targets. It is expected that this review will provide a reference for further exploration and unveiling of melatonin receptor function in reproductive regulation.

Melatonin activates BKCa channels in cerebral artery myocytes via both direct and MT receptor/PKC-mediated pathway

The pineal hormone melatonin is a neuroendocrine hormone with high membrane permeability that is involved in regulation of circadian rhythm of several biological functions. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels are abundantly expressed in vascular smooth muscle cells and play an important role in vascular tone regulation. We investigated the mechanisms through which myocyte BK_Ca channels mediate effects of melatonin on cerebral arteries (CAs). Arterial contractility measurements showed that melatonin alone did not change vascular tone in CAs; however, it induced concentration-dependent vasodilation of phenylephrine-induced contraction in CAs. In the presence of the potent endothelial oxide synthase inhibitor, N^ω-nitro-L-arginine methyl ester, melatonin-elicited relaxation was significantly inhibited by iberiotoxin (BK_Ca channel blocker). Melatonin significantly increased BK_Ca currents but not voltage-gated K⁺ (K_V) currents in whole-cell recordings. Melatonin decreased the amplitude of Ca²⁺ sparks and spontaneous transient outward currents (STOCs), however, a significant increase in open probability of BK_Ca channels was observed in both inside-out and cell-attached patch-clamp recordings. This melatonin-induced enhancement of BK_Ca channel activity was significantly suppressed by luzindole (melatonin MT₁/MT₂ receptor inhibitor), U73122 (phospholipase C (PLC) inhibitor), and Ro31-8220 (protein kinase C (PKC) inhibitor). Melatonin had no significant effects on sarcoplasmic reticulum release of Ca²⁺. These findings indicate that melatonin-induced vasorelaxation of CAs is partially attributable to direct (passing through the cell membrane) and indirect (via melatonin MT₁/MT₂ receptors-PLC-PKC pathway) activation of BK_Ca channels on CA myocytes.

Melatonin receptors: molecular pharmacology and signalling in the context of system bias

Melatonin, N-acetyl-5-methoxytryptamine, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Melatonin mediates vasodilation through both direct and indirect activation of BKCa channels

Melatonin, synthesized primarily by the pineal gland, is a neuroendocrine hormone with high membrane permeability. The vascular effects of melatonin, including vasoconstriction and vasodilation, have been demonstrated in numerous studies. However, the mechanisms underlying these effects are not fully understood. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels are expressed broadly on smooth muscle cells and play an important role in vascular tone regulation. This study explored the mechanisms of myocyte BK_Ca channels and endothelial factors underlying the action of melatonin on the mesenteric arteries (MAs). Vascular contractility and patch-clamp studies were performed on myocytes of MAs from Wistar rats. Melatonin induced significant vasodilation on MAs. In the presence of N^ω-nitro-l-arginine methyl ester (l-NAME), a potent endothelial oxide synthase (eNOS) inhibitor, melatonin elicited concentration-dependent relaxation, with lowered pIC₅₀ The effect of melatonin was significantly attenuated in the presence of BK_Ca channel blocker iberiotoxin or MT1/MT2 receptor antagonist luzindole in both (+) l-NAME and (-) l-NAME groups. In the (+) l-NAME group, iberiotoxin caused a parallel rightward shift of the melatonin concentration-relaxation curve, with pIC₅₀ lower than that of luzindole. Both inside-out and cell-attached patch-clamp recordings showed that melatonin significantly increased the open probability, mean open time and voltage sensitivity of BK_Ca channels. In a cell-attached patch-clamp configuration, the melatonin-induced enhancement of BK_Ca channel activity was significantly suppressed by luzindole. These findings indicate that in addition to the activation of eNOS, melatonin-induced vasorelaxation of MAs is partially attributable to its direct (passing through the cell membrane) and indirect (via MT1/MT2 receptors) activation of the BK_Ca channels on mesenteric arterial myocytes.

Melatonin: an inhibitor of breast cancer

The present review discusses recent work on melatonin-mediated circadian regulation, the metabolic and molecular signaling mechanisms that are involved in human breast cancer growth, and the associated consequences of circadian disruption by exposure to light at night (LEN). The anti-cancer actions of the circadian melatonin signal in human breast cancer cell lines and xenografts heavily involve MT1 receptor-mediated mechanisms. In estrogen receptor alpha (ERα)-positive human breast cancer, melatonin suppresses ERα mRNA expression and ERα transcriptional activity via the MT1 receptor. Melatonin also regulates the transactivation of other members of the nuclear receptor superfamily, estrogen-metabolizing enzymes, and the expression of core clock and clock-related genes. Furthermore, melatonin also suppresses tumor aerobic metabolism (the Warburg effect) and, subsequently, cell-signaling pathways critical to cell proliferation, cell survival, metastasis, and drug resistance. Melatonin demonstrates both cytostatic and cytotoxic activity in breast cancer cells that appears to be cell type-specific. Melatonin also possesses anti-invasive/anti-metastatic actions that involve multiple pathways, including inhibition of p38 MAPK and repression of epithelial-mesenchymal transition (EMT). Studies have demonstrated that melatonin promotes genomic stability by inhibiting the expression of LINE-1 retrotransposons. Finally, research in animal and human models has indicated that LEN-induced disruption of the circadian nocturnal melatonin signal promotes the growth, metabolism, and signaling of human breast cancer and drives breast tumors to endocrine and chemotherapeutic resistance. These data provide the strongest understanding and support of the mechanisms that underpin the epidemiologic demonstration of elevated breast cancer risk in night-shift workers and other individuals who are increasingly exposed to LEN.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.

Dual effects of melatonin on uterine myoelectrical activity of non-pregnant rats

OBJECTIVE

In this experimental study, we aimed to investigate the role of melatonin on uterine myoelectrical activity of non-pregnant rats.

MATERIAL AND METHODS

Forty-six female rats were assigned to six groups: (1) control; (0.2 mL 0.9% NaCl was injected intravenously (IV), n=6); (2) melatonin applied as 0.4 mg/kg/IV (n=8); (3) melatonin applied as 4 mg/kg/IV (n=8); (4) single dose of oxytocin (100 mU/kg) injected IV (n=8); (5) melatonin (0.4 mg/kg) plus oxytocin (100 mU/kg) (n=8); and (6) melatonin (4 mg/kg) plus oxytocin (100 mU/kg) injected IV (n=8). Each rat underwent a laparotomy, and uterine myoelectrical signals were recorded. The mean spectrum, averaged over the spectral content of signals in each group, was compared.

RESULTS

Melatonin induced uterine myoelectrical activity in a dose-dependent manner. Treatment of melatonin after oxytocin suppressed the mean power of the signals. Serum melatonin concentrations were significantly higher in melatonin-treated rats.

CONCLUSION

Melatonin itself at two different dose levels was found to be equally effective in stimulating the uterine electrical signals, although oxytocin-induced uterine electrical activity was suppressed by melatonin. These findings merit further investigations on the possible beneficial role of melatonin in the treatment of conditions associated with abnormal uterine activity.

Melatonin prevents experimental preterm labor and increases offspring survival

Preterm delivery is the leading cause of neonatal mortality and contributes to delayed physical and cognitive development in children. At present, there is no efficient therapy to prevent preterm labor. A large body of evidence suggests that intra-amniotic infections may be a significant and potentially preventable cause of preterm birth. This work assessed the effect of melatonin in a murine model of inflammation-associated preterm delivery which mimics central features of preterm infection in humans. For this purpose, preterm labor was induced in BALB/c mice by intraperitoneal injections of bacterial lipopolysaccharide (LPS) at 10.00 hr (10 μg LPS) and 13.00 hr (20 μg LPS) on day 15 of pregnancy. On day 14 of pregnancy, a pellet of melatonin (25 mg) had been subcutaneously implanted into a group of animals. In the absence of melatonin, a 100% incidence of preterm birth was observed in LPS-treated animals, and the fetuses showed widespread damage. By comparison, treatment with melatonin prevented preterm birth in 50% of the cases, and all pups from melatonin-treated females were born alive and their body weight did not differ from control animals. Melatonin significantly prevented the LPS-induced rises in uterine prostaglandin (PG) E2 , PGF2α, and cyclooxygenase-2 protein levels. In addition, melatonin prevented the LPS-induced increase in uterine nitric oxide (NO) production, inducible NO synthase protein, and tumor necrosis factor-alpha (TNFα) levels. Collectively, our results suggest that melatonin could be a new therapeutic tool to prevent preterm labor and to increase offspring survival.

Circadian regulation of reproduction: from gamete to offspring

Few challenges are more critical to the survival of a species than reproduction. To ensure reproductive success, myriad aspects of physiology and behaviour need to be tightly orchestrated within the animal, as well as timed appropriately with the external environment. This is accomplished through an endogenous circadian timing system generated at the cellular level through a series of interlocked transcription/translation feedback loops, leading to the overt expression of circadian rhythms. These expression patterns are found throughout the body, and are intimately interwoven with both the timing and function of the reproductive process. In this review we highlight the many aspects of reproductive physiology in which circadian rhythms are known to play a role, including regulation of the estrus cycle, the LH surge and ovulation, the production and maturation of sperm and the timing of insemination and fertilisation. We will also describe roles for circadian rhythms in support of the preimplantation embryo in the oviduct, implantation/placentation, as well as the control of parturition and early postnatal life. There are several key differences in physiology between humans and the model systems used for the study of circadian disruption, and these challenges to interpretation will be discussed as part of this review.

Melatonin anticancer effects: review

Melatonin (N-acetyl-5-methoxytryptamine, MLT), the main hormone produced by the pineal gland, not only regulates circadian rhythm, but also has antioxidant, anti-ageing and immunomodulatory properties. MLT plays an important role in blood composition, medullary dynamics, platelet genesis, vessel endothelia, and in platelet aggregation, leukocyte formula regulation and hemoglobin synthesis. Its significant atoxic, apoptotic, oncostatic, angiogenetic, differentiating and antiproliferative properties against all solid and liquid tumors have also been documented. Thanks, in fact, to its considerable functional versatility, MLT can exert both direct and indirect anticancer effects in factorial synergy with other differentiating, antiproliferative, immunomodulating and trophic molecules that form part of the anticancer treatment formulated by Luigi Di Bella (Di Bella Method, DBM: somatostatin, retinoids, ascorbic acid, vitamin D3, prolactin inhibitors, chondroitin-sulfate). The interaction between MLT and the DBM molecules counters the multiple processes that characterize the neoplastic phenotype (induction, promotion, progression and/or dissemination, tumoral mutation). All these particular characteristics suggest the use of MLT in oncological diseases.

Melatonin and the circadian timing of human parturition

Although the onset of spontaneous human parturition has long been known to occur preferentially during the nighttime and early morning hours, no convincing physiological explanation for this pattern has yet been proposed. This review focuses on the circadian timing of mammalian parturition, particularly in the human. It is proposed that differences in the phasing of parturition among different species are likely a function of opposite uterine responses to humoral cues, in particular those coding for time of day. The brain hormone melatonin fulfills many of the prerequisites to serve as a circadian signal for initiating uterine contractions that lead to human parturition. These encompass direct actions of melatonin on myometrial smooth muscle cells that are synergistic with oxytocin in facilitating greater uterine contractions at night. This may not only help to explain the nocturnal phasing of human parturition but also open new avenues for the management of term and preterm labor.

Melatonin influences insulin secretion primarily via MT(1) receptors in rat insulinoma cells (INS-1) and mouse pancreatic islets

Several studies have revealed that melatonin affects the insulin secretion via MT(1) and MT(2) receptor isoforms. Owing to the lack of selective MT(1) receptor antagonists, we used RNA interference technology to generate an MT(1) knockdown in a clonal β-cell line to evaluate whether melatonin modulates insulin secretion specifically via the MT(1) receptor. Incubation experiments were carried out, and the insulin concentration in supernatants was measured using a radioimmunoassay. Furthermore, the intracellular cAMP was determined using an enzyme-linked immunosorbent assay. Real-time RT-PCR indicated that MT(1) knockdown resulted in a significant increase in the rIns1 mRNA and a significantly elevated basal insulin secretion of INS-1 cells. Incubation with melatonin decreased the amount of glucagon-like peptide 1 or inhibited the glucagon-stimulated insulin release of INS-1 cells, while, in MT(1) -knockdown cells, no melatonin-induced reduction in insulin secretion could be found. No decrease in 3-isobutyl-1-methylxanthine-stimulated intracellular cAMP in rMT(1) -knockdown cells was detectable after treatment with melatonin either, and immunocytochemistry proved that MT(1) knockdown abolished phosphorylation of cAMP-response-element-binding protein. In contrast to the INS-1 cells, preincubation with melatonin did not sensitize the insulin secretion of rMT(1) -knockdown cells. We also monitored insulin secretion from isolated islets of wild-type and melatonin-receptor knockout mice ex vivo. In islets of wild-type mice, melatonin treatment resulted in a decrease in insulin release, whereas melatonin treatment of islets from MT(1) knockout and MT(1/2) double-knockout mice did not show a significant effect. The data indicate that melatonin inhibits insulin secretion, primarily via the MT(1) receptor in rat INS-1 cells and isolated mouse islets.

Circadian aspects of mammalian parturition: a review

The identification of circadian clocks in endocrine tissues has added considerable depth and complexity to our understanding of their physiology. A growing body of research reveals circadian clock gene expression in the uterus of non-pregnant and pregnant rodents. This review will focus on the mammalian uterus and its rhythmicity, particularly as it pertains to the circadian timing of parturition. This key event in the reproductive axis shows dramatic species-specific differences in its circadian phase. It is proposed here that these differences in the phasing of mammalian parturition are likely a function of opposite uterine cell responses to humoral cues. The argument will be made that melatonin fulfills many of the criteria to serve as a circadian signal in the initiation of human parturition, including specific actions on uterine smooth muscle cells that are consistent with a role for this hormone in the circadian timing of parturition.

Melatonin and IP3-induced Ca2+ release from intracellular stores in the malaria parasite Plasmodium falciparum within infected red blood cells

IP(3)-dependent Ca(2+) signaling controls a myriad of cellular processes in higher eukaryotes and similar signaling pathways are evolutionarily conserved in Plasmodium, the intracellular parasite that causes malaria. We have reported that isolated, permeabilized Plasmodium chabaudi, releases Ca(2+) upon addition of exogenous IP(3). In the present study, we investigated whether the IP(3) signaling pathway operates in intact Plasmodium falciparum, the major disease-causing human malaria parasite. P. falciparum-infected red blood cells (RBCs) in the trophozoite stage were simultaneously loaded with the Ca(2+) indicator Fluo-4/AM and caged-IP(3). Photolytic release of IP(3) elicited a transient Ca(2+) increase in the cytosol of the intact parasite within the RBC. The intracellular Ca(2+) pools of the parasite were selectively discharged, using thapsigargin to deplete endoplasmic reticulum (ER) Ca(2+) and the antimalarial chloroquine to deplete Ca(2+) from acidocalcisomes. These data show that the ER is the major IP(3)-sensitive Ca(2+) store. Previous work has shown that the human host hormone melatonin regulates P. falciparum cell cycle via a Ca(2+)-dependent pathway. In the present study, we demonstrate that melatonin increases inositol-polyphosphate production in intact intraerythrocytic parasite. Moreover, the Ca(2+) responses to melatonin and uncaging of IP(3) were mutually exclusive in infected RBCs. Taken together these data provide evidence that melatonin activates PLC to generate IP(3) and open ER-localized IP(3)-sensitive Ca(2+) channels in P. falciparum. This receptor signaling pathway is likely to be involved in the regulation and synchronization of parasite cell cycle progression.

Melatonin inhibits tetraethylammonium-sensitive potassium channels of rod ON type bipolar cells via MT2 receptors in rat retina

By challenging specific receptors, melatonin synthesized and released by photoreceptors regulates various physiological functions in the vertebrate retina. Here, we studied modulatory effects of melatonin on K+ currents of rod-dominant ON type bipolar cells (Rod-ON-BCs) in rat retinal slices by patch-clamp techniques. Double immunofluorescence experiments conducted in isolated cell and retinal section preparations showed that the melatonin MT₂ receptor was expressed in somata, dendrites and axon terminals of rat Rod-ON-BCs. Electrophysiologically, application of melatonin selectively inhibited the tetraethylammonium (TEA)-sensitive K+ current component, but did not show any effect on the 4-aminopyridine (4-AP)-sensitive component. Consistent with the immunocytochemical result, the melatonin effect was blocked by co-application of 4-phenyl-2-propionamidotetralin (4-P-PDOT), a specific MT₂ receptor antagonist. Neither protein kinase A (PKA) nor protein kinase G (PKG) seemed to be involved because both the PKA inhibitor Rp-cAMP and the PKG inhibitor KT5823 did not block the melatonin-induced suppression of the K+ currents. In contrast, application of the phospholipase C (PLC) inhibitor U73122 or the protein kinase C (PKC) inhibitor bisindolylmaleimide IV (Bis IV) eliminated the melatonin effect, and when the Ca²+ chelator BAPTA-containing pipette was used, melatonin failed to inhibit the K+ currents. These results suggest that suppression of the TEA-sensitive K+ current component via activation of MT₂ receptors expressed on rat Rod-ON-BCs may be mediated by a Ca²+-dependent PLC/inositol 1,4,5-trisphosphate (IP₃/PKC signaling pathway.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

p63RhoGEF--a key mediator of angiotensin II-dependent signaling and processes in vascular smooth muscle cells

The purpose of our study was to investigate the role of endogenous p63RhoGEF in G(q/11)-dependent RhoA activation and signaling in rat aortic smooth muscle cells (RASMCs). Therefore, we studied the expression and subcellular localization in freshly isolated RASMCs and performed loss of function experiments to analyze its contribution to RhoGTPase activation and functional responses such as proliferation and contraction. By this, we could show that p63RhoGEF is endogenously expressed in RASMCs and acts there as the dominant mediator of the fast angiotensin II (ANG II)-dependent but not of the sphingosine-1-phosphate (S(1)P)-dependent RhoA activation. p63RhoGEF is not an activator of the concomitant Rac1 activation and functions independently of caveolae. The knockdown of endogenous p63RhoGEF significantly reduced the mitogenic response of ANG II, abolished ANG II-induced stress fiber formation and cell elongation in 2-D culture, and impaired the ANG II-driven contraction in a collagen-based 3-D model. In conclusion, our data provide for the first time evidence that p63RhoGEF is an important mediator of ANG II-dependent RhoA activation in RASMCs and therewith a leading actor in the subsequently triggered cellular processes, such as proliferation and contraction.

Melatonin and reproduction revisited

This brief review summarizes new findings related to the reported beneficial effects of melatonin on reproductive physiology beyond its now well-known role in determining the sexual status in both long-day and short-day seasonally breeding mammals. Of particular note are those reproductive processes that have been shown to benefit from the ability of melatonin to function in the reduction of oxidative stress. In the few species that have been tested, brightly colored secondary sexual characteristics that serve as a sexual attractant reportedly are enhanced by melatonin administration. This is of potential importance inasmuch as the brightness of ornamental pigmentation is also associated with animals that are of the highest genetic quality. Free radical damage is commonplace during pregnancy and has negative effects on the mother, placenta, and fetus. Because of its ability to readily pass through the placenta, melatonin easily protects the fetus from oxidative damage, as well as the maternal tissues and placenta. Examples of conditions in which oxidative and nitrosative stress can be extensive during pregnancy include preeclampsia and damage resulting from anoxia or hypoxia that is followed by reflow of oxygenated blood into the tissue. Given the uncommonly low toxicity of melatonin, clinical trials are warranted to document the protection by melatonin against pathophysiological states of the reproductive system in which free radical damage is known to occur. Finally, the beneficial effects of melatonin in improving the outcomes of in vitro fertilization and embryo transfer should be further tested and exploited. The information in this article has applicability to human and veterinary medicine.

Melatonin: signaling mechanisms of a pleiotropic agent

Melatonin acts both as a hormone of the pineal gland and as a local regulator molecule in various tissues. Quantities of total tissue melatonin exceed those released from the pineal. With regard to this dual role, to the orchestrating, systemic action on various target tissues, melatonin is highly pleiotropic. Numerous secondary effects result from the control of the circadian pacemaker and, in seasonal breeders, of the hypothalamic/pituitary hormonal axes. In mammals, various binding sites for melatonin have been identified, the membrane receptors MT(1) and MT(2), which are of utmost chronobiological importance, ROR and RZR isoforms as nuclear receptors from the retinoic acid receptor superfamily, quinone reductase 2, calmodulin, calreticulin, and mitochondrial binding sites. The G protein-coupled receptors (GPCRs) MT(1) and MT(2) are capable of parallel or alternate signaling via different Galpha subforms, in particular, Galpha(i) (2/) (3) and Galpha(q), and via Gbetagamma, as well. Multiple signaling can lead to the activation of different cascades and/or ion channels. Melatonin frequently decreases cAMP, but also activates phospholipase C and protein kinase C, acts via the MAP kinase and PI3 kinase/Akt pathways, modulates large conductance Ca(2+)-activated K(+) and voltage-gated Ca(2+) channels. MT(1) and MT(2) can form homo and heterodimers, and MT(1) interacts with other proteins in the plasma membrane, such as an orphan GPCR, GPR50, and the PDZ domain scaffolding protein MUPP1, effects which negatively or positively influence signaling capacity. Cross-talks between different signaling pathways, including influences of the membrane receptors on nuclear binding sites, are discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

M2 muscarinic receptors induce airway smooth muscle activation via a dual, Gbetagamma-mediated inhibition of large conductance Ca2+-activated K+ channel activity

Airway smooth muscle is richly endowed with muscarinic receptors of the M(2) and M(3) subtype. Stimulation of these receptors inhibits large conductance calcium-activated K(+) (BK) channels, a negative feed back regulator, in a pertussis toxin-sensitive manner and thus facilitates contraction. The underlying mechanism, however, is unknown. We therefore studied the activity of bovine trachea BK channels in HEK293 cells expressing the M(2) or M(3) receptor (M(2)R or M(3)R). In M(2)R- but not M(3)R-expressing cells, maximal effective concentrations of carbamoylcholine (CCh) inhibited whole cell BK currents by 53%. This M(2)R-induced inhibition was abolished by pertussis toxin treatment or overexpression of the Gbetagamma scavenger transducin-alpha. In inside-out patches, direct application of 300 nm purified Gbetagamma decreased channel open probability by 55%. The physical interaction of Gbetagamma with BK channels was confirmed by co-immunoprecipitation. Interestingly, inhibition of phospholipase C as well as protein kinase C activities also reversed the CCh effect but to a smaller (approximately 20%) extent. Mouse tracheal cells responded similarly to CCh, purified Gbetagamma and phospholipase C/protein kinase C inhibition as M(2)R-expressing HEK293 cells. Our results demonstrate that airway M(2)Rs inhibit BK channels by a dual, Gbetagamma-mediated mechanism, a direct membrane-delimited interaction, and the activation of the phospholipase C/protein kinase C pathway.

Transcriptional inhibition of oxytocin receptor expression in human myometrial cells by melatonin involves protein kinase C signaling

CONTEXT

Our laboratory recently characterized the expression of the melatonin receptors in the human myometrium and showed that the expression of these receptors is suppressed during late pregnancy.

OBJECTIVE

In an effort to understand better the significance of melatonin in the human myometrium, we explored the mechanisms through which this hormone influences the expression of the oxytocin receptor in vitro.

DESIGN

The stable melatonin analog iodomelatonin was presented to cultured telomerase-immortalized myometrial cells of the human telomerase reverse transcriptase line under physiological doses and durations. Pharmacological inhibitors of melatonin binding, gene transcription, phospholipase C, and protein kinase C signaling were used to define the mechanism of melatonin action.

RESULTS

Our results reveal that melatonin significantly inhibits oxytocin receptor mRNA expression primarily via the melatonin 2 receptor. The melatonin-dependent decrease in oxytocin receptor transcripts involves suppression of gene transcription rather than enhanced rates of transcript degradation. Melatonin effects were abolished by pretreating the cells with the phospholipase C inhibitor U73122 or the protein kinase C inhibitor C1.

CONCLUSIONS

Melatonin, like oxytocin, can negatively regulate oxytocin receptor transcription in human myometrial cells via modulation of protein kinase C signaling. This is consistent with the hypothesis that the reduced melatonin receptor expression during late pregnancy, which occurs at a time when oxytocin receptors are up-regulated, may be physiologically important for the subsequent timing of labor.

Oxytocin Receptors Differentially Signal via Gq and Gi Proteins in Pregnant and Nonpregnant Rat Uterine Myocytes: Implications for Myometrial Contractility

Oxytocin (OT) receptors are important regulators of myometrial contractility. By using the activity of large conductance Ca2+-activated K+ (BKCa) channels as readout, we analyzed OT signaling in cells from nonpregnant (NPM) and pregnant (PM) rat myometrium in detail. In nystatin-perforated whole-cell patches from NPM cells, which leave the intracellular integrity intact, OT transiently increased BKCa-mediated outward currents (Iout). This OT-evoked Iout was caused by the Ca2+ transients in response to the Gq/11-mediated activation of phospholipase C and was inhibited by activation of protein kinase A (PKA). In an open-access whole-cell patch (OAP), the OT-induced transient rise in Iout was disrupted whereas the regulation of BKCa by the cAMP/PKA cascade remained intact. OT counteracted the isoprenaline, i.e. the β-adrenoceptor/Gs-mediated effect in NPM cells measured in OAP. In contrast, OT further enhanced the β-adrenoceptor/Gs-mediated effect on BKCa activity in PM cells. All OT effects in the OAP were mediated by pertussis toxin-sensitive Gi proteins and PKA. By quantitative real-time PCR and overexpression of the recombinant protein, we demonstrate that an up-regulation of the Gβγ-stimulated adenylyl cyclase II during pregnancy is most likely responsible for this switch. By studying the OT-evoked Iout in nystatin-perforated whole-cell patches of PM cells, we further detected that the OT receptor/Giβγ-mediated coactivation of adenylyl cyclase II enhanced the β-adrenoceptor/Gs-induced suppression of the OT-evoked Ca2+ transients and thus diminishes and self-limits OT-induced contractility. The differential regulation of the PKA-mediated suppression of OT-evoked Ca2+ transients and BKCa activity likely supports uterine quiescence during pregnancy.

Melatonin effects on the hypothalamo-pituitary axis in fish

Melatonin, a hormonal output signal of vertebrate circadian clocks, contributes to synchronizing behaviors and neuroendocrine regulations with the daily and annual variations of the photoperiod. Conservation and diversity characterize the melatonin system: conservation because its pattern of production and synchronizing properties are a constant among vertebrates; and diversity because regulation of both its synthesis and modes of action have been profoundly modified during vertebrate evolution. Studies of the targets and modes of action of melatonin in fish, and their parallels in mammals, are of interest to our understanding of time-related neuroendocrine regulation and its evolution from fish to mammals, as well as for aquacultural purposes.

Pregnancy and estradiol modulate myometrial G-protein pathways in the guinea pig

OBJECTIVE

Coupled to hundreds of receptors, G-proteins modulate signal transduction pathways and are important hormonal targets. The first objective was to determine the effect of pregnancy and estradiol on myometrial guanosine triphosphatase activity. The second objective was to begin dissecting the molecular mechanism(s) underlying alterations in guanosine triphosphatase activity.

STUDY DESIGN

Myometrial tissue was obtained from pregnant, nonpregnant, and ovariectomized untreated and estradiol-treated guinea pigs. Myometrial membranes were prepared by homogenization and differential centrifugation. Basal high-affinity specific guanosine triphosphatase activity was quantitated by enzymatic assay and expressed in rhomol 32Pi per milligram protein per minute. Guanosine triphosphatase activity was stimulated using oxytocin, isoproterenol, and prostaglandin F2alpha. Specific G-protein subunits were quantitated using Western blots. G-protein associated gene expression was semiquantitated using HGU133A gene array chips from Affymetrix.

RESULTS

Basal myometrial guanosine triphosphatase activity was increased in pregnant compared with nonpregnant animals. Estradiol increased basal myometrial guanosine triphosphatase activity, compared with untreated controls. The effect of estradiol on stimulated activity was agonist dependent. Both Galphas and Galphai isoform 1 protein levels were increased in myometrium from late pregnant compared with nonpregnant animals. By late gestation, the messenger ribonucleic acid levels of those genes were unaltered, compared with the nonpregnant animal. In general, the impact of pregnancy on G-protein family member gene messenger ribonucleic acid expression was modest. Only the small guanosine triphosphatase Rap1b demonstrated altered expression more than 2-fold during either myometrial quiescence (midpregnancy) or activation (term pregnancy) (up 3-fold during quiescence). Genomic network analyses revealed that the expression of another small guanosine triphosphatase, Rab7, was exclusively up-regulated (80%) during quiescence. During late pregnancy, network analysis showed that only G-protein beta was exclusively altered (up-regulated). Estradiol mimicked the pregnancy effect on both transcription and translation of G-protein family members for some but not all potentially relevant genes.

CONCLUSION

The increase in functional myometrial guanosine triphosphatase activity during pregnancy may reflect increased synthesis of 1 or more small guanosine triphosphatase.

Functional coupling of beta3-adrenoceptors and large conductance calcium-activated potassium channels in human uterine myocytes

CONTEXT

Beta3-adrenoreceptor modulation in human myometrium during pregnancy is linked functionally to myometrial inhibition. Maxi-K+ channels (BK(Ca)) play a significant role in modulating cell membrane potential and excitability.

OBJECTIVE

This study was designed to investigate the potential involvement of BK(Ca) channel function in the response of human myometrium to beta3-adrenoceptor activation.

DESIGN

Single and whole-cell electrophysiological BK(Ca) channel recordings from freshly dispersed myocytes were obtained in the presence and absence of BRL37344, a specific beta3-adrenoreceptor agonist. The in vitro effects of BRL37344 on isolated myometrial contractions, in the presence and absence of the specific BK(Ca) channel blocker, iberiotoxin (IbTX), were investigated.

SETTING

The study was carried out at the Clinical Science Institute.

PATIENTS OR OTHER PARTICIPANTS

Myometrial biopsies were obtained at elective cesarean delivery.

INTERVENTION

No intervention was applied.

MAIN OUTCOME MEASURES

Open state probability of single channel recordings, whole cell currents, and myometrial contractile activity were measured.

RESULTS

Single-channel recordings identified the BK(Ca) channel as a target of BRL37344. BRL37344 significantly increased the open state probability of this channel in a concentration-dependent manner (control 0.031 +/- 0.004; 50 microM BRL37344 0.073 +/- 0.005 (P < 0.001); and 100 microM BRL37344 0.101 +/- 0.005 (P < 0.001). This effect was completely blocked after preincubation of the cells with 1 microM bupranolol, a nonspecific beta-adrenoreceptor blocker, or 100 nM SR59230a, a specific beta3-adrenoreceptor antagonist. In addition, BRL37344 increased whole-cell currents over a range of membrane potentials, and this effect was reversed by 100 nM IbTX. In vitro isometric tension studies demonstrated that BRL37344 exerted a significant concentration-dependent relaxant effect on human myometrial tissue (P < 0.05), and preincubation of these strips with IbTX attenuated this effect on both spontaneous and oxytocin-induced contractions (44.44 and 57.84% at 10(-5) M, respectively).

CONCLUSIONS

These findings outline that activation of the BK(Ca) channel may explain the potent uterorelaxant effect of beta3-adrenoreceptor agonists.

Deletion of the Ca2+-activated potassium (BK) alpha-subunit but not the BKbeta1-subunit leads to progressive hearing loss

The large conductance voltage- and Ca2+-activated potassium (BK) channel has been suggested to play an important role in the signal transduction process of cochlear inner hair cells. BK channels have been shown to be composed of the pore-forming alpha-subunit coexpressed with the auxiliary beta1-subunit. Analyzing the hearing function and cochlear phenotype of BK channel alpha-(BKalpha-/-) and beta1-subunit (BKbeta1-/-) knockout mice, we demonstrate normal hearing function and cochlear structure of BKbeta1-/- mice. During the first 4 postnatal weeks also, BKalpha-/- mice most surprisingly did not show any obvious hearing deficits. High-frequency hearing loss developed in BKalpha-/- mice only from approximately 8 weeks postnatally onward and was accompanied by a lack of distortion product otoacoustic emissions, suggesting outer hair cell (OHC) dysfunction. Hearing loss was linked to a loss of the KCNQ4 potassium channel in membranes of OHCs in the basal and midbasal cochlear turn, preceding hair cell degeneration and leading to a similar phenotype as elicited by pharmacologic blockade of KCNQ4 channels. Although the actual link between BK gene deletion, loss of KCNQ4 in OHCs, and OHC degeneration requires further investigation, data already suggest human BK-coding slo1 gene mutation as a susceptibility factor for progressive deafness, similar to KCNQ4 potassium channel mutations.