Circadian variation of nitric oxide synthase activity in mouse tissue

Circadian and Diurnal Regulation of Cerebral Blood Flow

Circadian and diurnal variation in cerebral blood flow directly contributes to the diurnal variation in the risk of stroke, either through factors that trigger stroke or due to impaired compensatory mechanisms. Cerebral blood flow results from the integration of systemic hemodynamics, including heart rate, cardiac output, and blood pressure, with cerebrovascular regulatory mechanisms, including cerebrovascular reactivity, autoregulation, and neurovascular coupling. We review the evidence for the circadian and diurnal variation in each of these mechanisms and their integration, from the detailed evidence for mechanisms underlying the nocturnal nadir and morning surge in blood pressure to identifying limited available evidence for circadian and diurnal variation in cerebrovascular compensatory mechanisms. We, thus, identify key systemic hemodynamic factors related to the diurnal variation in the risk of stroke but particularly identify the need for further research focused on cerebrovascular regulatory mechanisms.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

The Circadian Clocks, Oscillations of Pain-Related Mediators, and Pain

The circadian clock is a biochemical oscillator that is synchronized with solar time. Normal circadian rhythms are necessary for many physiological functions. Circadian rhythms have also been linked with many physiological functions, several clinical symptoms, and diseases. Accumulating evidence suggests that the circadian clock appears to modulate the processing of nociceptive information. Many pain conditions display a circadian fluctuation pattern clinically. Thus, the aim of this review is to summarize the existing knowledge about the circadian clocks involved in diurnal rhythms of pain. Possible cellular and molecular mechanisms regarding the connection between the circadian clocks and pain are discussed.

Time of day as a critical variable in biology

BACKGROUND

Circadian rhythms are important for all aspects of biology; virtually every aspect of biological function varies according to time of day. Although this is well known, variation across the day is also often ignored in the design and reporting of research. For this review, we analyzed the top 50 cited papers across 10 major domains of the biological sciences in the calendar year 2015. We repeated this analysis for the year 2019, hypothesizing that the awarding of a Nobel Prize in 2017 for achievements in the field of circadian biology would highlight the importance of circadian rhythms for scientists across many disciplines, and improve time-of-day reporting.

RESULTS

Our analyses of these 1000 empirical papers, however, revealed that most failed to include sufficient temporal details when describing experimental methods and that few systematic differences in time-of-day reporting existed between 2015 and 2019. Overall, only 6.1% of reports included time-of-day information about experimental measures and manipulations sufficient to permit replication.

CONCLUSIONS

Circadian rhythms are a defining feature of biological systems, and knowing when in the circadian day these systems are evaluated is fundamentally important information. Failing to account for time of day hampers reproducibility across laboratories, complicates interpretation of results, and reduces the value of data based predominantly on nocturnal animals when extrapolating to diurnal humans.

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

Circadian Control of Sodium and Blood Pressure Regulation

The attention for the control of dietary risk factors involved in the development of hypertension, includes a large effort on dietary salt restrictions. Ample studies show the beneficial role of limiting dietary sodium as a lifestyle modification in the prevention and management of essential hypertension. Not until the past decade or so have studies more specifically investigated diurnal variations in renal electrolyte excretion, which led us to the hypothesis that timing of salt intake may impact cardiovascular health and blood pressure regulation. Cell autonomous molecular clocks as the name implies, function independently to maintain optimum functional rhythmicity in the face of environmental stressors such that cellular homeostasis is maintained at all times. Our understanding of mechanisms influencing diurnal patterns of sodium excretion and blood pressure has expanded with the discovery of the circadian clock genes. In this review, we discuss what is known about circadian regulation of renal sodium handling machinery and its influence on blood pressure regulation, with timing of sodium intake as a potential modulator of the kidney clock.

Circadian Rhythm in Adipose Tissue: Novel Antioxidant Target for Metabolic and Cardiovascular Diseases

Obesity is a major risk factor for most metabolic and cardiovascular disorders. Adipose tissue is an important endocrine organ that modulates metabolic and cardiovascular health by secreting signaling molecules. Oxidative stress is a common mechanism associated with metabolic and cardiovascular complications including obesity, type 2 diabetes, and hypertension. Oxidative stress can cause adipose tissue dysfunction. Accumulating data from both humans and experimental animal models suggest that adipose tissue function and oxidative stress have an innate connection with the intrinsic biological clock. Circadian clock orchestrates biological processes in adjusting to daily environmental changes according to internal or external cues. Recent studies have identified the genes and molecular pathways exhibiting circadian expression patterns in adipose tissue. Disruption of the circadian rhythmicity has been suggested to augment oxidative stress and aberrate adipose tissue function and metabolism. Therefore, circadian machinery in the adipose tissue may be a novel therapeutic target for the prevention and treatment of metabolic and cardiovascular diseases. In this review, we summarize recent findings on circadian rhythm and oxidative stress in adipose tissue, dissect the key components that play a role in regulating the clock rhythm, oxidative stress and adipose tissue function, and discuss the potential use of antioxidant treatment on metabolic and cardiovascular diseases by targeting the adipose clock.

Nitric oxide: To be or not to be an endocrine hormone?

Nitric oxide (NO), a highly reactive gasotransmitter, is critical for a number of cellular processes and has multiple biological functions. Due to its limited lifetime and diffusion distance, NO has been mainly believed to act in autocrine/paracrine fashion. The increasingly recognized effects of pharmacologically delivered and endogenous NO at a distant site have changed the conventional wisdom and introduced NO as an endocrine signalling molecule. The notion is greatly supported by the detection of a number of NO adducts and their circulatory cycles, which in turn contribute to the transport and delivery of NO bioactivity, remote from the sites of its synthesis. The existence of endocrine sites of synthesis, negative feedback regulation of biosynthesis, integrated storage and transport systems, having an exclusive receptor, that is, soluble guanylyl cyclase (sGC), and organized circadian rhythmicity make NO something beyond a simple autocrine/paracrine signalling molecule that could qualify for being an endocrine signalling molecule. Here, we discuss hormonal features of NO from the classical endocrine point of view and review available knowledge supporting NO as a true endocrine hormone. This new insight can provide a new framework within which to reinterpret NO biology and its clinical applications.

Effect of Light-Dark Cycle Misalignment on the Hypothalamic-Pituitary-Gonadal Axis, Testicular Oxidative Stress, and Expression of Clock Genes in Adult Male Rats

This study investigated the influence of circadian misalignment on the male reproductive system. Adult Sprague-Dawley male rats were exposed to prolonged light (20 h light : 4 h dark) or prolonged darkness (4 h light : 20 h dark) for 12 consecutive weeks. The somatic index of seminal vesicles and prostates increased due to prolonged light exposure. Sperm count and motility were enhanced solely by prolonged light exposure, whereas the percentage of sperm abnormalities was reduced by both prolonged light and darkness. The serum levels of reproductive hormones (follicle-stimulating hormone, luteinizing hormone, testosterone, and prolactin) were elevated, and the estradiol level was reduced by long-term light and dark exposure. Testicular total antioxidant capacity and antioxidant enzyme activities were improved, and lipid peroxidation was inhibited following chronic exposure to light or dark. Chronic light exposure increased, but chronic darkness decreased, testicular nitric oxide production. The mRNA expression of the hypothalamic and testicular clock genes including PER1-2, CRY1-2, BMAL1, CLOCK, and Rev-Erbα was altered by circadian disruption. Prolonged light exposure decreased the levels of thyroid hormones and suppressed the mRNA expression of adiponectin receptors 1 and 2. The immunohistochemical expression of proliferating cell nuclear antigen was decreased only by chronic darkness. The present study thus provides new insights into the physiological changes associated with long-term exposure to light or darkness, in which the expression levels of various clock gene mRNAs are modulated, reproductive hormones are increased, and the antioxidant enzyme system is ameliorated as mechanisms of adaptation to chronic circadian disruption.

Diurnal Regulation of Renal Electrolyte Excretion: The Role of Paracrine Factors

Many physiological processes, including most kidney-related functions, follow specific rhythms tied to a 24-h cycle. This is largely because circadian genes operate in virtually every cell type in the body. In addition, many noncanonical genes have intrinsic circadian rhythms, especially within the liver and kidney. This new level of complexity applies to the control of renal electrolyte excretion. Furthermore, there is growing evidence that paracrine and autocrine factors, especially the endothelin system, are regulated by clock genes. We have known for decades that excretion of electrolytes is dependent on time of day, which could play an important role in fluid volume balance and blood pressure control. Here, we review what is known about the interplay between paracrine and circadian control of electrolyte excretion. The hope is that recognition of paracrine and circadian factors can be considered more deeply in the future when integrating with well-established neuroendocrine control of excretion.

Sub-acute restraint stress progressively increases oxidative/nitrosative stress and inflammatory markers while transiently upregulating antioxidant gene expression in the rat hippocampus

We have previously demonstrated that acute stress decreases neuronal nitric oxide synthase (NOS) expression in the hippocampus despite increased concentrations of nitric oxide which may indicate feedback inhibition of neuronal NOS expression via inducible NOS-derived nitric oxide. Moreover, the hippocampus undergoes an initial oxidative/nitrosative insult that is rapidly followed by upregulation of protective antioxidants, including the zinc-binding metallothioneins, in order to counter this and restore redox balance following acute stress exposure. In the present study, we have utilized indicators of oxidative/nitrosative stress, members of the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) pathway, antioxidant metallothioneins, and neuroinflammatory markers to observe the changes occurring in the hippocampus following short term repeated stress exposure. Male Wistar rats were subjected to control conditions or 6 h of restraint stress applied for 1, 2, or 3 days (n = 8 per group) after which the hippocampus was isolated for redox assays and relative gene expression. The hippocampus showed increased oxidative stress, transient dys-homeostasis of total zinc, and increased expression of the Nrf2 pathway members. Moreover, repeated stress increased nitrosative status, nitric oxide metabolites, and 3-nitrotyrosine, indicative of nitrosative stress in the hippocampus. However, levels of neuronal NOS decreased over all stress treatment groups, while increases were observed in inducible NOS and xanthine dehydrogenase. In addition to inducible NOS, mRNA expression of other inflammatory markers including interleukin-6 and interleukin-1β also increased even in the presence of increased anti-inflammatory glucocorticoids. Together, these results demonstrate that despite increases in antioxidant expression, sub-acute stress causes an inflammatory phenotype in the hippocampus by inducing oxidative/nitrosative stress, zinc dys-homeostasis, and the accumulation of nitrotyrosinated proteins which is likely driven by increased inducible NOS signaling.

An overview of the emerging interface between cardiac metabolism, redox biology and the circadian clock

At various biological levels, mammals must integrate with 24-hr rhythms in their environment. Daily fluctuations in stimuli/stressors of cardiac metabolism and oxidation-reduction (redox) status have been reported over the course of the day. It is therefore not surprising that the heart exhibits dramatic oscillations in various cellular processes over the course of the day, including transcription, translation, ion homeostasis, metabolism, and redox signaling. This temporal partitioning of cardiac processes is governed by a complex interplay between intracellular (e.g., circadian clocks) and extracellular (e.g., neurohumoral factors) influences, thus ensuring appropriate responses to daily stimuli/stresses. The purpose of the current article is to review knowledge regarding control of metabolism and redox biology in the heart over the course of the day, and to highlight whether disruption of these daily rhythms contribute towards cardiac dysfunction observed in various disease states.

Generation and intracellular trafficking of a polysialic acid-carrying fragment of the neural cell adhesion molecule NCAM to the cell nucleus

Polysialic acid (PSA) and its major protein carrier, the neural cell adhesion molecule NCAM, play important roles in many nervous system functions during development and in adulthood. Here, we show that a PSA-carrying NCAM fragment is generated at the plasma membrane by matrix metalloproteases and transferred to the cell nucleus via endosomes and the cytoplasm. Generation and nuclear import of this fragment in cultured cerebellar neurons is induced by a function-triggering NCAM antibody and a peptide comprising the effector domain (ED) of myristoylated alanine-rich C kinase substrate (MARCKS) which interacts with PSA within the plane of the plasma membrane. These treatments lead to activation of the fibroblast growth factor (FGF) receptor, phospholipase C (PLC), protein kinase C (PKC) and phosphoinositide-3-kinase (PI3K), and subsequently to phosphorylation of MARCKS. Moreover, the NCAM antibody triggers calmodulin-dependent activation of nitric oxide synthase, nitric oxide (NO) production, NO-dependent S-nitrosylation of matrix metalloprotease 9 (MMP9) as well as activation of matrix metalloprotease 2 (MMP2) and MMP9, whereas the ED peptide activates phospholipase D (PLD) and MMP2, but not MMP9. These results indicate that the nuclear PSA-carrying NCAM fragment is generated by distinct and functionally defined signal transducing mechanisms.

Acute intake of quercetin from onion skin extract does not influence postprandial blood pressure and endothelial function in overweight-to-obese adults with hypertension: a randomized, double-blind, placebo-controlled, crossover trial

PURPOSE

To determine whether postprandial metabolic and vascular responses induced by a high-fat and high-carbohydrate meal are attenuated by ingestion of the flavonol quercetin.

METHODS

Twenty-two overweight-to-obese hypertensive patients participated in a randomized, double-blind, controlled, crossover meal study. They consumed a test meal (challenge) rich in energy (4754 kJ), fat (61.6 g), saturated fatty acids (53 % of total fatty acids), and carbohydrates (113.3 g) with either placebo or 54 mg quercetin. Blood pressure, reactive hyperemia index (RHI), high-sensitive C-reactive protein (hs-CRP), soluble endothelial-derived adhesion molecules, parameters of lipid and glucose metabolism, and markers of antioxidant status were measured before the meal and at 2 and 4 h postprandially.

RESULTS

Systolic and diastolic blood pressure increased significantly over time, but were not affected by treatment (placebo or quercetin). During both treatments, serum endothelin-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and plasma asymmetric dimethylarginine slightly decreased over time, whereas RHI increased. Serum triglycerides, total cholesterol, and insulin significantly increased, whereas HDL cholesterol and glucose significantly decreased over time, again with no effect of treatment. Plasma α-tocopherol significantly increased, and plasma Trolox equivalent antioxidative capacity decreased over time. Serum hs-CRP, plasma retinol, and β-carotene did not significantly change during the trial.

CONCLUSION

In hypertensive patients, a high-energy meal did not lead to postprandial impairment of vascular endothelial function. Postprandial metabolic responses induced by the challenge, such as lipemia and insulinemia, were not attenuated by the concomitant ingestion of quercetin.

CLINICAL TRIAL

This trial was registered at www.germanctr.de/ and http://apps.who.int/trialsearch/ as DRKS00000555.

Redox regulation and pro-oxidant reactions in the physiology of circadian systems

Rhythms of approximately 24 h are pervasive in most organisms and are known as circadian. There is a molecular circadian clock in each cell sustained by a feedback system of interconnected "clock" genes and transcription factors. In mammals, the timing system is formed by a central pacemaker, the suprachiasmatic nucleus, in coordination with a collection of peripheral oscillators. Recently, an extensive interconnection has been recognized between the molecular circadian clock and the set of biochemical pathways that underlie the bioenergetics of the cell. A principle regulator of metabolic networks is the flow of electrons between electron donors and acceptors. The concomitant reduction and oxidation (redox) reactions directly influence the balance between anabolic and catabolic processes. This review summarizes and discusses recent findings concerning the mutual and dynamic interactions between the molecular circadian clock, redox reactions, and redox signaling. The scope includes the regulatory role played by redox coenzymes (NAD(P)+/NAD(P)H, GSH/GSSG), reactive oxygen species (superoxide anion, hydrogen peroxide), antioxidants (melatonin), and physiological events that modulate the redox state (feeding condition, circadian rhythms) in determining the timing capacity of the molecular circadian clock. In addition, we discuss a purely metabolic circadian clock, which is based on the redox enzymes known as peroxiredoxins and is present in mammalian red blood cells and in other biological systems. Both the timing system and the metabolic network are key to a better understanding of widespread pathological conditions such as the metabolic syndrome, obesity, and diabetes.

Daily cycling of nitric oxide synthase (NOS) in the hippocampus of pigeons (C. livia)

Background

Nitric oxide synthase (NOS) is essential for the synthesis of nitric oxide (NO), a non-conventional neurotransmitter with an important role in synaptic plasticity underlying processes of hippocampus-dependent memory and in the regulation of biological clocks and circadian rhythms. Many studies have shown that both the NOS cytosolic protein content and its enzymatic activity present a circadian variation in different regions of the rodent brain, including the hippocampus. The present study investigated the daily variation of NOS enzymatic activity and the cytosolic content of nNOS in the hippocampus of pigeons.

Results

Adult pigeons kept under a skeleton photoperiod were assigned to six different groups. Homogenates of the hippocampus obtained at six different times-of-day were used for NOS analyses. Both iNOS activity and nNOS cytosolic protein concentrations were highest during the subjective light phase and lowest in the subjective dark phase of the circadian period. ANOVA showed significant time differences for iNOS enzymatic activity (p < 0.05) and for nNOS protein content (p < 0.05) in the hippocampus. A significant daily rhythm for both iNOS and nNOS was confirmed by analysis with the Cosinor method (p < 0.05). The present findings indicate that the enzymatic activity of iNOS and content of nNOS protein in the hippocampus of pigeons exhibit a daily rhythm, with acrophase values occurring during the behavioral activity phase.

Conclusions

The data corroborate the reports on circadian variation of NOS in the mammalian hippocampus and can be considered indicative of a dynamic interaction between hippocampus-dependent processes and circadian clock mechanisms.

Diurnal variation in vascular and metabolic function in diet-induced obesity: divergence of insulin resistance and loss of clock rhythm

Circadian rhythms are integral to the normal functioning of numerous physiological processes. Evidence from human and mouse studies suggests that loss of rhythm occurs in obesity and cardiovascular disease and may be a neglected contributor to pathophysiology. Obesity has been shown to impair the circadian clock mechanism in liver and adipose tissue but its effect on cardiovascular tissues is unknown. We investigated the effect of diet-induced obesity in C57BL6J mice upon rhythmic transcription of clock genes and diurnal variation in vascular and metabolic systems. In obesity, clock gene function and physiological rhythms were preserved in the vasculature but clock gene transcription in metabolic tissues and rhythms of glucose tolerance and insulin sensitivity were blunted. The most pronounced attenuation of clock rhythm occurred in adipose tissue, where there was also impairment of clock-controlled master metabolic genes and both AMPK mRNA and protein. Across tissues, clock gene disruption was associated with local inflammation but diverged from impairment of insulin signaling. We conclude that vascular tissues are less sensitive to pathological disruption of diurnal rhythms during obesity than metabolic tissues and suggest that cellular disruption of clock gene rhythmicity may occur by mechanisms shared with inflammation but distinct from those leading to insulin resistance.

Diurnal variation in vascular function: role of sleep

Although vascular function is lower in the morning than afternoon, previous studies have not assessed the influence of prior sleep on this diurnal variation. The authors employed a semiconstant routine protocol to study the contribution of prior nocturnal sleep to the previously observed impairment in vascular function in the morning. Brachial artery vascular function was assessed using the flow-mediated dilation technique (FMD) in 9 healthy, physically active males (mean ± SD: 27 ± 9 yrs of age), at 08:00 and 16:00 h following, respectively, 3.29 ± .37 and 3.24 ± .57 h prior sleep estimated using actimetry. Heart rate and systolic and diastolic blood pressures were also measured. The data of the experimental sleep condition were compared with the data of the "normal" diurnal sleep condition, in which FMD measurements were obtained from 21 healthy individuals who slept only during the night, as usual, before the morning test session. The morning-afternoon difference in FMD was 1 ± 4% in the experimental sleep condition compared with 3 ± 4% in the normal sleep condition (p = .04). This difference was explained by FMD being 3 ± 3% lower in afternoon following the prior experimental sleep (p = .01). These data suggest that FMD is more dependent on the influence of supine sleep than the endogenous circadian timekeeper, in agreement with our previous finding that diurnal variation in FMD is influenced by exercise. These findings also raise the possibility of a lower homeostatic "set point" for vascular function following a period of sleep and in the absence of perturbing hemodynamic fluctuation.

α1-Adrenoreceptor activity does not explain lower morning endothelial-dependent, flow-mediated dilation in humans

Early morning reduction in endothelium-dependent, flow-mediated dilation (FMD) may contribute to the high incidence of sudden cardiac death at this time of day. The mechanisms underpinning diurnal variation in FMD are unclear, but potentially relate to a circadian rhythm in sympathetic nerve activity. We hypothesized that blockade of α1-mediated sympathetic nerve activity would act to attenuate the diurnal variation in FMD. In a randomized and placebo-controlled design, we measured brachial artery FMD in 12 participants (mean age = 26 yr, SD = 3) at 0600 and 1600 after ingestion of an α1-blocker (prazosin, 1 mg/20 kg body mass) or placebo. Arterial diameter and shear rate were assessed using edge-detection software. Heart rate and blood pressure were also measured. Data were analyzed using linear mixed modeling. Following placebo, FMD was 8 ± 2% in the morning compared with 10 ± 3% in the afternoon ( P = 0.04). Blockade with prazosin led to a slight but nonsignificant increase in morning FMD ( P = 0.24) and a significant ( P = 0.04) decrease in afternoon FMD, resulting in no diurnal variation ( P = 0.20). Shear rate did not differ in the morning or afternoon under either condition ( P > 0.23). Blood pressure was lower following prazosin compared with placebo ( P < 0.02), an effect that was similar at both times of day ( P > 0.34). Heart rate and norepinephrine levels were higher in the afternoon following prazosin. These data indicate that α1-adrenoreceptor activity does not explain lower morning endothelium-dependent FMD.

Cadmium-Induced Disruption in 24-h Expression of Clock and Redox Enzyme Genes in Rat Medial Basal Hypothalamus: Prevention by Melatonin

In a previous study we reported that a low daily p.o. dose of cadmium (Cd) disrupted the circadian expression of clock and redox enzyme genes in rat medial basal hypothalamus (MBH). To assess whether melatonin could counteract Cd activity, male Wistar rats (45 days of age) received CdCl₂ (5 ppm) and melatonin (3 μg/mL) or vehicle (0.015% ethanol) in drinking water. Groups of animals receiving melatonin or vehicle alone were also included. After 1 month, MBH mRNA levels were measured by real-time PCR analysis at six time intervals in a 24-h cycle. In control MBH Bmal1 expression peaked at early scotophase, Per1 expression at late afternoon, and Per2 and Cry2 expression at mid-scotophase, whereas neither Clock nor Cry1 expression showed significant 24-h variations. This pattern was significantly disrupted (Clock, Bmal1) or changed in phase (Per1, Per2, Cry2) by CdCl₂ while melatonin counteracted the changes brought about by Cd on Per1 expression only. In animals receiving melatonin alone the 24-h pattern of MBH Per2 and Cry2 expression was disrupted. CdCl₂ disrupted the 24-h rhythmicity of Cu/Zn- and Mn-superoxide dismutase (SOD), nitric oxide synthase (NOS)-1, NOS-2, heme oxygenase (HO)-1, and HO-2 gene expression, most of the effects being counteracted by melatonin. In particular, the co-administration of melatonin and CdCl₂ increased Cu/Zn-SOD gene expression and decreased that of glutathione peroxidase (GPx), glutathione reductase (GSR), and HO-2. In animals receiving melatonin alone, significant increases in mean Cu/Zn and Mn-SOD gene expression, and decreases in that of GPx, GSR, NOS-1, NOS-2, HO-1, and HO-2, were found. The results indicate that the interfering effect of melatonin on the activity of a low dose of CdCl₂ on MBH clock and redox enzyme genes is mainly exerted at the level of redox enzyme gene expression.

Circadian clocks and vascular function

The circadian clock regulates many aspects of physiology, including cardiovascular function. Internal oscillators exist in endothelial, smooth muscle cells, and fibroblasts of the vasculature. Vascular tone and thrombus formation, 2 key elements of vascular function with regard to adverse cardiovascular events, exhibit diurnal rhythmicity. In this review, we describe changes in vascular function that result from genetic disruption of discrete elements of the circadian clock.

Pressed for time: the circadian clock and hypertension

Hypertension is a major risk factor for cardiovascular disease and death. The "silent" rise of blood pressure that occurs over time is largely asymptomatic. However, its impact is deafening-causing and exacerbating cardiovascular disease, end-organ damage, and death. The present article addresses recent observations from human and animal studies that provide new insights into how the circadian clock regulates blood pressure, contributes to hypertension, and ultimately evolves vascular disease. Further, the molecular components of the circadian clock and their relationship with locomotor activity, metabolic control, fluid balance, and vascular resistance are discussed with an emphasis on how these novel, circadian clock-controlled mechanisms contribute to hypertension.

Normal responses to restraint stress in mice lacking the gene for neuronal nitric oxide synthase

The hormonal changes associated with immobilization stress (IMO) include a swift increase in corticosterone (CORT) concentration and a decrease in circulating testosterone (T) levels. There is evidence that the production of the short-lived neuromodulator nitric oxide (NO) is increased during stress in various tissues, including the brain. NO also suppresses the biosynthesis of T. Both the inducible and the neuronal isoforms of NO synthase (iNOS and nNOS, respectively) have been implicated in this suppression, but the evidence has not been conclusive. We used adult wild-type (WT) and nNOS knockout male mice (nNOS-/-) to assess the respective roles of CORT and nNOS-derived NO in stress mediated inhibition of T production. Animals were assigned to either basal control or 3-hour IMO groups. No difference in basal plasma and testicular T levels were observed between WT and nNOS-/-, although testicular weights of mutant mice were slightly lower compared to WT animals. The plasma contents of luteinizing hormone (LH) and CORT in unstressed mice of both genotypes were similar. Exposure to 3 hours of IMO increased plasma CORT and decreased T concentrations in mice of both genotypes. However, comparable levels of plasma LH and testicular nitrite and nitrate (NOx), NO stable metabolites, were detected in control and stressed WT and nNOS-/- mice. Adrenal concentrations of NOx declined after IMO, but the reduction was not statistically significant. These findings implicate CORT rather than NO generated by nNOS in the rapid stress-induced suppression of circulating T.

Twenty-four-hour variation of L-arginine/nitric oxide/cyclic guanosine monophosphate pathway demonstrated by the mouse visceral pain model

The L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway is known to be involved in central and peripheral nociceptive processes. This study evaluated the rhythmic pattern of the L-arginine/NO/cGMP pathway using the mouse visceral pain model. Experiments were performed at six different times (1, 5, 9, 13, 17, and 21 h after light on) per day in male mice synchronized to a 12 h:12 h light-dark cycle. Animals were injected s.c. with saline, 2 mg/kg L-arginine (a NO precursor), 75 mg/kg L-N(G)-nitroarginine methyl ester (L-NAME, a NOS inhibitor), 40 mg/kg methylene blue (a soluble guanylyl cyclase and/or NOS inhibitor), or 0.1 mg/kg sodium nitroprusside (a nonenzymatic NO donor) 15 min before counting 2.5 mg/kg (i.p.) p-benzoquinone (PBQ)-induced abdominal constrictions for 15 min. Blood samples were collected after the test, and the nitrite concentration was determined in serum samples. L-arginine or L-NAME caused both antinociception and nociception, depending on the circadian time of their injection. The analgesic effect of methylene blue or sodium nitroprusside exhibited significant biological time-dependent differences in PBQ-induced abdominal constrictions. Serum nitrite levels also displayed a significant 24 h variation in mice injected with PBQ, L-NAME, methylene blue, or sodium nitroprusside, but not saline or L-arginine. These results suggest that components of L-arginine/NO/cGMP pathway exhibit biological time-dependent effects on visceral nociceptive process.

Mutation of the circadian clock gene Per2 alters vascular endothelial function

BACKGROUND

The circadian clock regulates biological processes including cardiovascular function and metabolism. In the present study, we investigated the role of the circadian clock gene Period2 (Per2) in endothelial function in a mouse model.

METHODS AND RESULTS

Compared with the wild-type littermates, mice with Per2 mutation exhibited impaired endothelium-dependent relaxations to acetylcholine in aortic rings suspended in organ chambers. During transition from the inactive to active phase, this response was further increased in the wild-type mice but further decreased in the Per2 mutants. The endothelial dysfunction in the Per2 mutants was also observed with ionomycin, which was improved by the cyclooxygenase inhibitor indomethacin. No changes in the expression of endothelial acetylcholine-M3 receptor or endothelial nitric oxide synthase protein but increased cyclooxygenase-1 (not cyclooxygenase-2) protein levels were observed in the aortas of the Per2 mutants. Compared with Per2 mutants, a greater endothelium-dependent relaxation to ATP was observed in the wild-type mice, which was reduced by indomethacin. In quiescent aortic rings, ATP caused greater endothelium-dependent contractions in the Per2 mutants than in the wild-type mice, contractions that were abolished by indomethacin. The endothelial dysfunction in the Per2 mutant mice is not associated with hypertension or dyslipidemia.

CONCLUSIONS

Mutation in the Per2 gene in mice is associated with aortic endothelial dysfunction involving decreased production of NO and vasodilatory prostaglandin(s) and increased release of cyclooxygenase-1-derived vasoconstrictor(s). The results suggest an important role of the Per2 gene in maintenance of normal cardiovascular functions.

Dosing time-dependent variation of bone resorption by cyclosporin A in rats' femurs

The dosing time-dependent difference of bone resorption by cyclosporin A was determined in normal rats. Rats were kept in rooms with a 12-h light/dark cycle. Cyclosporin A (3 mg/kg, once a day) or vehicle was given at either 2 h after light on (2 HALO) or 8 HALO, 14 HALO, 20 HALO for 24 weeks. Serum and 4-h urine samples were obtained before and at 12 and 24 weeks after the treatment. Body weight, creatinine clearance, serum parathyroid hormone, the trough level of cyclosporin A in whole blood and urinary excretion of Ca and P were not changed by the drug at every any dosing time. Serum Ca and P concentrations by the vehicle treatment differed with the dosing time. Furthermore, increases of these two parameters by the drug varied with dosing time; most prominently at the 2 HALO dosing, and were not seen at the 8 and 14 HALO dosings. Degree of bone resorption of the femur determined by dual-energy X-ray absorption, also varied with dosing time, most prominently at 2 HALO and less prominently at 14 HALO. Increase of urine deoxypyridinoline excretion, a marker of osteoclast activity, by the drug was highest at 2 HALO and lowest at 14 HALO, however parathyroid hormone and osteocalcin concentrations after cyclosporin A treatment did not vary with dosing time. Reduction of urinary nitric oxide (NO) was most prominent at 2 HALO and negligible at 14 HALO. We concluded that cyclosporin A-induced bone resorption and serum Ca and P increases were varied with dosing time. Sensitivity of osteoclasts by the drug was the major mechanisms of the phenomenon, while differences in pharmacokinetics, the parathyroid gland, osteoblasts and renal handling of Ca and P did not contribute to the phenomenon.

Testosterone production in mice lacking inducible nitric oxide synthase expression is sensitive to restraint stress

Immobilization stress (IMO) induces a rapid increase in glucocorticoid secretion [in rodents, corticosterone CORT)] and this is associated with decreased circulating testosterone (T) levels. Nitric oxide (NO), a reactive free radical and neurotransmitter, has been reported to be produced at higher rates in tissues such as brain during stress. The biosynthesis of T is also known to be dramatically suppressed by NO. Specifically, the inducible isoform of nitric oxide synthase (iNOS) was directly implicated in this suppression. To assess the respective roles of CORT and NO in stress-mediated inhibition of T production, adult wild-type (WT) and inducible nitric oxide synthase knockout (iNOS(-/-)) male mice were evaluated. Animals of each genotype were assigned to either basal control or 3-h IMO groups. Basal plasma and testicular T levels were equivalent in both genotypes, whereas testicular weights of mutant mice were significantly higher compared with WT animals. Exposure to 3-h IMO increased plasma CORT and decreased T concentrations in mice of both genotypes. Testicular T levels were also affected by stress in WT and mutant males, being sharply reduced in both genotypes. However, the concentrations of nitrite and nitrate, the stable metabolites of NO measured in testicular extracts, did not differ between control and stressed WT and iNOS(-/-) mice. These results support the hypothesis that CORT, but not NO, is a plausible candidate to mediate rapid stress-induced suppression of Leydig cell steroidogenesis.

Evidence supporting a circadian control of natural killer cell function

Natural killer (NK) cells participate in the immune response against infection and cancer. An emerging body of epidemiological data supports that circadian homeostasis may constitute a factor risk for cancer development. Physiological rhythms under circadian control persist in the absence of light entrainment and ultimately rely on a molecular clock. We have previously shown that NK cell cytolytic activity follows a daily rhythm and that NK cells enriched from light-entrained rats present 24-h oscillations of clock genes, cytolytic factors, and cytokines. To investigate whether these oscillations are under a genuine circadian control, we assessed the daily expression of clock genes (Per1, Per2, Clock, and Bmal1), a clock-controlled gene (Dbp), cytolytic factors (granzyme B and perforin), and cytokines (IFN-gamma and TNF-alpha) in NK cells enriched from rats maintained in constant darkness (DD). In addition, we investigated whether the disruption of the NK cell clock by RNA interference (RNAi) affects the expression of cytolytic factors and cytokines. Persistent 24-h oscillations were found in the expression levels of clock genes, cytolytic factors, and cytokines in NK cells enriched from DD rats. In addition, RNAi-mediated Per2 knockdown caused a significant decrease of granzyme B and perforin levels in the rat derived NK cell line RNK16. Taken together, these results provide evidence supporting that NK cell function is under circadian regulation.

Role of cyclooxygenase isoforms and nitric-oxide synthase in the modulation of tracheal motor responsiveness in normal and antigen-sensitized Guinea pigs

The effects of selective cyclooxygenase (COX) isoform (COX-1, COX-2) inhibition, alone or in combination with nitric-oxide synthase (NOS) blockade, on in vitro tracheal muscle responsiveness to histamine were investigated in healthy and ovalbumin (OVA)-sensitized guinea pigs. Immunohistochemistry showed that COX-1 and COX-2 are constitutively present in normal guinea pig trachea, particularly in the epithelial layer, and that COX-2 expression is enhanced in OVA-sensitized animals both in epithelial and subepithelial tissues. In normal guinea pigs, SC-560 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole] (COX-1 inhibitor) or DFU [5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl)phenyl-2(5H)-furanone] (COX-2 inhibitor) significantly increased the contractile response to histamine, these effects being not additive. NOS inhibition by l-N(G)-nitro-arginine methyl ester (l-NAME) did not affect histamine-induced contraction but reversed the increase caused by COX-1 blockade while not modifying the enhancement associated with COX-2 inhibition. In guinea pigs subjected to OVA sensitization and challenge, COX-2, but not COX-1, inhibition enhanced the motor responses to histamine without any influence by l-NAME. In normal, but not in sensitized animals, the removal of epithelial layer from tracheal preparations abolished the enhancing action of DFU on histamine-mediated contraction. A COX-2-dependent release of prostacyclin (PGI(2)), but not prostaglandin E(2), was observed in tracheal tissues from normal and OVA-sensitized guinea pigs. In conclusion, both COX-1 and COX-2 are constitutive in guinea pig trachea, and COX-2 expression is enhanced by OVA sensitization; in normal animals, epithelial COX-2 exerts a PGI(2)-dependent inhibitory control on tracheal contractility, and this isoform is subjected to upstream regulation by epithelial COX-1 and NOS through a complex interplay; and following antigen sensitization, the inhibitory control on tracheal contractility is maintained by COX-2 induced at subepithelial cell sites.

Antioxidative protection by melatonin: multiplicity of mechanisms from radical detoxification to radical avoidance

Melatonin has been shown to protect against oxidative stress in various, highly divergent experimental systems. There are many reasons for its remarkable protective potential. Signaling effects comprise the upregulation of antioxidant enzymes, such as superoxide dismutases, peroxidases, and enzymes of glutathione supply, down-regulation of prooxidant enzymes, such as nitric oxide synthases and lipoxygenases, and presumably also the control of quinone reductase 2. Other mechanisms are based on direct interactions with several reactive oxygen and nitrogen species. Among these reactions, the capacity of easily undergoing single-electron transfer reactions is of particular importance. Electron donation by melatonin is not only an aspect of direct radical scavenging, but additionally represents the basis for formation of the protective metabolites AFMK (N1-ace-tyl-N2-formyl-5-methoxykynuramine) and AMK (N1-acetyl-5-methoxykynuramine). Recent investigations on mitochondrial metabolism indicate that melatonin as well as AMK are capable of supporting the electron flux through the respiratory chain, of preventing the breakdown of the mitochondrial membrane potential, and of decreasing electron leakage, thereby reducing the formation of superoxide anions. Radical avoidance is a new line of investigation, which exceeds mitochondrial actions and also comprises antiexcitatory effects and contributions to the maintenance of internal circadian phase relationships.

Reversal of the circadian expression of tyrosine-hydroxylase but not nitric oxide synthase levels in the spinal cord of dopamine D3 receptor knockout mice

Circadian rhythms have been described for numerous transmitter synthesizing enzymes in the brain but rarely in spinal cord. We measured spinal tyrosine-hydroxylase (TH) and nitric oxide synthase (NOS) levels in the thoracic intermediolateral nucleus, the location of sympathetic preganglionic neurons, in male wild type (WT) and dopamine D(3) receptor knockout mice (D(3)KO). TH and NOS levels both displayed circadian patterns in WT and D(3)KO animals with overall reduced TH and increased NOS expression in the D(3)KO mice. The circadian pattern of NOS expression was similar in WT and D(3)KO mice. In contrast, TH expression was inverted in D(3)KO mice, with TH levels consistently lower than in WT throughout the day, but strongly increased temporarily 1 h prior to daylight. TH is the rate-limiting enzyme for the production of dopamine. Spinal dopamine dysfunction is implicated in a sleep disorder called restless legs syndrome (RLS). RLS follows a circadian rhythm and is relieved clinically by dopamine D(3) receptor agonists. Our observations of an altered circadian pattern in spinal dopamine synthesis in D(3)KO animals may provide insight into putative dopaminergic mechanisms contributing to RLS.

Reciprocal regulation of haem biosynthesis and the circadian clock in mammals

The circadian clock is the central timing system that controls numerous physiological processes. In mammals, one such process is haem biosynthesis, which the clock controls through regulation of the rate-limiting enzyme aminolevulinate synthase 1 (Alas1). Several members of the core clock mechanism are PAS domain proteins, one of which, neuronal PAS 2 (NPAS2), has a haem-binding motif. Indeed, haem controls activity of the BMAL1-NPAS2 transcription complex in vitro by inhibiting DNA binding in response to carbon monoxide. Here we show that haem differentially modulates expression of the mammalian Period genes mPer1 and mPer2 in vivo by a mechanism involving NPAS2 and mPER2. Further experiments show that mPER2 positively stimulates activity of the BMAL1-NPAS2 transcription complex and, in turn, NPAS2 transcriptionally regulates Alas1. Vitamin B12 and haem compete for binding to NPAS2 and mPER2, but they have opposite effects on mPer2 and mPer1 expression in vivo. Our data show that the circadian clock and haem biosynthesis are reciprocally regulated and suggest that porphyrin-containing molecules are potential targets for therapy of circadian disorders.

Circadian rhythms, oxidative stress, and antioxidative defense mechanisms

Endogenous circadian and exogenously driven daily rhythms of antioxidative enzyme activities and of low molecular weight antioxidants (LMWAs) are described in various phylogenetically distant organisms. Substantial amplitudes are detected in several cases, suggesting the significance of rhythmicity in avoiding excessive oxidative stress. Mammalian and/or avian glutathione peroxidase and, as a consequence, glutathione reductase activities follow the rhythm of melatonin. Another hint for an involvement of melatonin in the control of redox processes is seen in its high-affinity binding to cytosolic quinone reductase 2, previously believed to be a melatonin receptor. Although antioxidative protection by pharmacological doses of melatonin is repeatedly reported, explanations of these findings are still insufficient and their physiological and chronobiological relevance is not yet settled. Recent data indicate a role of melatonin in the avoidance of mitochondrial radical formation, a function which may prevail over direct scavenging. Rhythmic changes in oxidative damage of protein and lipid molecules are also reported. Enhanced oxidative protein modification accompanied by a marked increase in the circadian amplitude of this parameter is detected in the Drosophila mutant rosy, which is deficient in the LMWA urate. Preliminary evidence for the significance of circadian rhythmicity in diminishing oxidative stress comes from clock mutants. In Drosophila, moderately enhanced protein damage is described for the arrhythmic and melatonin null mutant per0, but even more elevated, periodic damage is found in the short-period mutant per(s), synchronized to LD 12:12. Remarkably large increases in oxidative protein damage, along with impairment of tissue integrity and--obviously insufficient--compensatory elevations in protective enzymes are observed in a particularly vulnerable organ, the Harderian gland, of another short-period mutant tau, in the Syrian hamster. Mice deficient in the per2 gene homolog are reported to be cancer-prone, a finding which might also relate to oxidative stress. In the dinoflagellate Lingulodinium polyedrum [Gonyaulax polyedra], various treatments that cause oxidative stress result in strong suppressions of melatonin and its metabolite 5-methoxytryptamine (5-MT) and to secondary effects on overt rhythmicity. The glow maximum, depending on the presence of elevated 5-MT at the end of subjective night, decreases in a dose-dependent manner already under moderate, non-lethal oxidative stress, but is restored by replenishing melatonin. Therefore, a general effect of oxidative stress may consist in declines of easily oxidizable signaling molecules such as melatonin, and this can have consequences on the circadian intraorganismal organization and expression of overt rhythms. Recent findings on a redox-sensitive input into the core oscillator via modulation of NPAS2/BMAL1 or CLK/BMAL1 heterodimer binding to DNA indicate a direct influence of cellular redox balance, including oxidative stress, on the circadian clock.

Pharmacokinetics and pharmacodynamics of L-arginine in rats: a model of stimulated neuronal nitric oxide synthesis

Nitric oxide (NO) is believed to be involved in a variety of central nervous system (CNS) functions, including opioid responsivity. Elucidation of the role of NO in the CNS requires the ability to elevate systematically neuronal NO concentrations in vivo. This study was conducted to assess the pharmacokinetics of L-arginine, a NO precursor, and to relate the disposition of this amino acid to the pharmacodynamic endpoint of neuronal NO production. L-Arginine (250-, 500-, or 1000-mg/kg/h) or saline was infused intravenously for 6 h to rats. L-Arginine was quantified in brain and blood (after in vivo microdialysis) with high-performance liquid chromatography. NO was quantified simultaneously with a sensitive and specific amperometric sensor placed in the hippocampus. The data were fit with a comprehensive pharmacokinetic-pharmacodynamic (PK/PD) model to obtain parameters governing the systemic disposition of L-arginine, the uptake of L-arginine into the brain, and subsequent NO production. Exogenous administration of L-arginine resulted in incremental elevations in hippocampal NO, with a approximately 33, 48, and approximately 50% increase from control for the 250-, 500-, and 1000-mg/kg/h L-arginine treated rats, respectively. The PK/PD model, which incorporated known characteristics of the system (saturable uptake of L-arginine into brain; NO production governed by circadian changes in enzyme activity) was capable of describing accurately the observed data. The model developed herein will be invaluable in characterizing the numerous roles of NO in the CNS.