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Most living organisms maintain cell autonomous circadian clocks that synchronize critical biological functions with daily environmental cycles. In mammals, the circadian clock is regulated by inputs from signaling pathways including glycogen synthase kinase 3 (GSK3). The drug lithium has actions on GSK3, and also on inositol metabolism. While it is suspected that lithium's inhibition of GSK3 causes rhythm changes, it is not known if inositol polyphosphates can also affect the circadian clock. We examined whether the signaling molecule inositol hexaphosphate (IP 6 ) has effects on circadian rhythms. Using a bioluminescent reporter (Per2::luc) to measure circadian rhythms, we determined that IP 6 increased rhythm amplitude and shortened period in NIH3T3 cells. The IP 6 effect on amplitude was attenuated by selective siRNA knockdown of GSK3B and pharmacological blockade of AKT kinase. However, unlike lithium, IP 6 did not induce serine-9 phosphorylation of GSK3B. The synthesis of IP 6 involves the enzymes inositol polyphosphate multikinase (IPMK) and inositol pentakisphosphate 2-kinase (IPPK). Knockdown of Ippk had effects opposite to those of IP 6 , decreasing rhythm amplitude and lengthening period. Ipmk knockdown had few effects on rhythm alone, but attenuated the effects of lithium on rhythms. However, lithium did not change the intracellular content of IP 6 in NIH3T3 cells or neurons. Pharmacological inhibition of the IP 6 kinases (IP6K) increased rhythm amplitude and shortened period, suggesting secondary effects of inositol pyrophosphates may underlie the period shortening effect, but not the amplitude increasing effect of IP 6 . Overall, we conclude that inositol phosphates, in particular IP 6 have effects on circadian rhythms. Manipulations affecting IP 6 and related inositol phosphates may offer a novel means through which circadian rhythms can be regulated. [ABSTRACT FROM AUTHOR]

Food intake and sleep are two mutually exclusive behaviors and both are normally confined to opposing phases of the diurnal cycle. The temporal coordination of behavior and physiology along the 24-h day–night cycle is organized by a network of circadian clocks that orchestrate transcriptional programs controlling cellular physiology. Many of the peptide hormones of the gastrointestinal tract are not only secreted in a circadian fashion, they can also affect circadian clock function in peripheral metabolic tissues and the brain, thus providing metabolic feedback to metabolic and neurobehavioral circuits. In this review, we summarize the current knowledge on this gastrointestinal peptide crosstalk and its potential role in the coordination of nutrition and the maintenance of metabolic homeostasis. [ABSTRACT FROM AUTHOR]

In mammals, the master circadian clock resides in the suprachiasmatic nucleus (SCN). The SCN is characterized by robust circadian oscillations of clock gene expression and neuronal firing. The synchronization of circadian oscillations among individual cells in the SCN is attributed to intercellular coupling. Previous studies have shown that gap junctions, specifically those composed of connexin-36 (Cx36) subunits, are required for coupling of electrical firing among SCN neurons at a time scale of milliseconds. However, it remains unknown whether Cx36 gap junctions also contribute to coupling of circadian (∼24 h) rhythms of clock gene expression. Here, we investigated circadian expression patterns of the clock gene Period 2 ( Per2 ) in the SCN of Cx36-deficient mice using luminometry and single-cell bioluminescence imaging. Surprisingly, we found that synchronization of circadian PER2 expression rhythms is maintained in SCN explants from Cx36-deficient mice. Since Cx36 expression levels change with age, we also tested circadian running-wheel behavior of juvenile (3–4 weeks old) and adult (9–30 weeks old) Cx36-deficient mice. We found that impact of connexin-36 expression on circadian behavior changes greatly during postnatal development. However, consistent with the intact synchrony among SCN cells in cultured explants, Cx36-deficient mice had intact locomotor circadian rhythms, although adults displayed a lengthened period in constant darkness. Our data indicate that even though Cx36 may be required for electrical coupling of SCN cells, it does not affect coupling of molecular clock gene rhythms. Thus, electrical coupling of neurons and coupling of circadian clock gene oscillations can be regulated independently in the SCN. [ABSTRACT FROM AUTHOR]

Background Major depressive disorder is associated with disturbed circadian rhythms. To investigate the causal relationship between mood disorders and circadian clock disruption, previous studies in animal models have employed light/dark manipulations, global mutations of clock genes, or brain area lesions. However, light can impact mood by noncircadian mechanisms; clock genes have pleiotropic, clock-independent functions; and brain lesions not only disrupt cellular circadian rhythms but also destroy cells and eliminate important neuronal connections, including light reception pathways. Thus, a definitive causal role for functioning circadian clocks in mood regulation has not been established. Methods We stereotactically injected viral vectors encoding short hairpin RNA to knock down expression of the essential clock gene Bmal1 into the brain’s master circadian pacemaker, the suprachiasmatic nucleus (SCN). Results In these SCN-specific Bmal1 -knockdown (SCN- Bmal1 -KD) mice, circadian rhythms were greatly attenuated in the SCN, while the mice were maintained in a standard light/dark cycle, SCN neurons remained intact, and neuronal connections were undisturbed, including photic inputs. In the learned helplessness paradigm, the SCN- Bmal1 -KD mice were slower to escape, even before exposure to inescapable stress. They also spent more time immobile in the tail suspension test and less time in the lighted section of a light/dark box. The SCN- Bmal1 -KD mice also showed greater weight gain, an abnormal circadian pattern of corticosterone, and an attenuated increase of corticosterone in response to stress. Conclusions Disrupting SCN circadian rhythms is sufficient to cause helplessness, behavioral despair, and anxiety-like behavior in mice, establishing SCN- Bmal1 -KD mice as a new animal model of depression. [ABSTRACT FROM AUTHOR]

Endogenous circadian (∼24 h) clocks regulate key physiological and cognitive processes via rhythmic expression of clock genes. The main circadian pacemaker is the hypothalamic suprachiasmatic nucleus (SCN). Mood disorders, including bipolar disorder (BD), are commonly associated with disturbed circadian rhythms. Dopamine (DA) contributes to mania in BD and has direct impact on clock gene expression. Therefore, we hypothesized that high levels of DA during episodes of mania contribute to disturbed circadian rhythms in BD. The mood stabilizer valproic acid (VPA) also affects circadian rhythms. Thus, we further hypothesized that VPA normalizes circadian disturbances caused by elevated levels of DA. To test these hypotheses, we examined locomotor rhythms and circadian gene cycling in mice with reduced expression of the dopamine transporter (DAT-KD mice), which results in elevated DA levels and mania-like behavior. We found that elevated DA signaling lengthened the circadian period of behavioral rhythms in DAT-KD mice and clock gene expression rhythms in SCN explants. In contrast, we found that VPA shortened circadian period of behavioral rhythms in DAT-KD mice and clock gene expression rhythms in SCN explants, hippocampal cell lines, and human fibroblasts from BD patients. Thus, DA and VPA have opposing effects on circadian period. To test whether the impact of VPA on circadian rhythms contributes to its behavioral effects, we fed VPA to DAT -deficient Drosophila with and without functioning circadian clocks. Consistent with our hypothesis, we found that VPA had potent activity-suppressing effects in hyperactive DAT -deficient flies with intact circadian clocks. However, these effects were attenuated in DAT -deficient flies in which circadian clocks were disrupted, suggesting that VPA functions partly through the circadian clock to suppress activity. Here, we provide in vivo and in vitro evidence across species that elevated DA signaling lengthens the circadian period, an effect remediated by VPA treatment. Hence, VPA may exert beneficial effects on mood by normalizing lengthened circadian rhythm period in subjects with elevated DA resulting from reduced DAT. [ABSTRACT FROM AUTHOR]

Abstract: Acetylcholine (ACh) has always been regarded as a classical neurotransmitter that binds to nicotinic or muscarinic receptors and mediates signal transmission. The traditional view, that ACh acts solely as a neurotransmitter, has to be revised based on numerous findings demonstrating the existence of a non-neuronal cholinergic system. It is noteworthy that murine and human embryonic stem cells also synthesize ACh and express the enzyme acetylcholinesterase and muscarinic ACh receptors. Here, we investigated the possible role of ACh and AChRs in the regulation of embryonic stem cells. First, the expression of α3, α4, α7 and β2 nicotinic receptor subunits in embryonic stem cells was investigated by RT-PCR. Second, in vitro studies have been conducted to assess the effects of ACh and its agonists on calcium dynamics, cell survival and proliferation. ACh and nicotine, but not muscarine could induce the mobilization of the intracellular Ca2+. Interestingly, ACh increased the viability, but decreased the proliferation of embryonic stem cells. Our data provide evidence that ACh might exert its effect on stem cells by binding to specific receptors and modulating cell death and proliferation. [ABSTRACT FROM AUTHOR]