Light is a prominent stimulus that synchronizes endogenous circadian rhythmicity to environmental light/ dark cycles. Nocturnal light elevates mRNA of the Period1 (Per1) gene and induces long term state changes, expressed as phase shifts of circadian rhythms. The cellular mechanism for Per1 elevation and light-induced phase advance in the suprachiasmatic nucleus (SCN), a process initiated primarily by glutamatergic neurotransmission from the retinohypothalamic tract, was examined. Glutamate (GLU)-induced phase advances in the rat SCN were blocked by antisense oligodeoxynucleotide (ODN) against Per1 and Ca 2؉ /cAMP response element (CRE)-decoy ODN. CRE-decoy ODN also blocked light-induced phase advances in vivo. Furthermore, the CRE-decoy blocked GLU-induced accumulation of Per1 mRNA. Thus, Ca 2؉ /cAMP response element-binding protein (CREB) and Per1 are integral components of the pathway transducing light-stimulated GLU neurotransmission into phase advance of the circadian clock.Mammalian circadian rhythmicity is generated by endogenous alternations in transcription/translation of putative clock genes within the suprachiasmatic nucleus (SCN) 1 of the basal hypothalamus. As a projection site of the retinohypothalamic tract, the SCN is poised to respond to retinal light information, mediated primarily by glutamatergic (GLU) neurotransmission, to assure time-of-day congruence between the endogenous pacemaker and the external environment. The mechanisms by which the SCN decodes and processes light information are complex and change as the biochemical clock states progress through their 24-h cycle (1). Light resets the clock throughout the night via glutamatergic-N-methyl-D-aspartate receptor-mediated Ca 2ϩ influx, which activates nitric-oxide synthase to liberate nitric oxide (NO) (2). At this point, the light signaling pathway diverges. In the early night, the light-induced state change, which delays subsequent rhythms, proceeds through NO-dependent activation of a neuronal ryanodine receptor. Light-induced state changes in the late night are independent of ryanodine receptor activation, but require activation of protein kinase G (PKG) (3-5). The discovery of several specific genes associated with circadian rhythmicity, including Period (Per) and Timeless (Tim) (for review, see Ref. 6), raises questions regarding the mechanisms that interface nocturnal light signals with the molecular clockwork. Throughout the night, light stimuli sufficient to cause long term state changes, or phase shifts, of circadian rhythms of rodent wheel running correlate with increased phosphorylation of the transcription factor, Ca 2ϩ /cAMP response element-binding protein (CREB) (7, 8), activation of Ca 2ϩ /cAMP response element (CRE)-mediated transcription (9), and a rise in Per1 mRNA (10 -15). This investigation was undertaken to determine whether CRE-mediated activation of Per1 is required for light/GLU-induced phase resetting of the SCN clock. We hypothesized that the GLU-induced phase advance requires activation of CRE and elevation ...

Circadian clocks are complex biochemical systems that cycle with a period of approximately 24 hours. They integrate temporal information regarding phasing of the solar cycle, and adjust their phase so as to synchronize an organism's internal state to the local environmental day and night. Nocturnal light is the dominant regulator of this entrainment. In mammals, information about nocturnal light is transmitted by glutamate released from retinal projections to the circadian clock in the suprachiasmatic nucleus of the hypothalamus. Clock resetting requires the activation of ionotropic glutamate receptors, which mediate Ca2+ influx. The response induced by such activation depends on the clock's temporal state: during early night it delays the clock phase, whereas in late night the clock phase is advanced. To investigate this differential response, we sought signalling elements that contribute solely to phase delay. We analysed intracellular calcium-channel ryanodine receptors, which mediate coupled Ca2+ signalling. Depletion of intracellular Ca2+ stores during early night blocked the effects of glutamate. Activators of ryanodine receptors induced phase resetting only in early night; inhibitors selectively blocked delays induced by light and glutamate. These findings implicate the release of intracellular Ca2+ through ryanodine receptors in the light-induced phase delay of the circadian clock restricted to the early night.

Despite a central circadian role in Drosophila for the transcriptional regulator Timeless (dTim), the relevance of mammalian Timeless (mTim) remains equivocal. Conditional knockdown of mTim protein expression in the rat suprachiasmatic nucleus (SCN) disrupted SCN neuronal activity rhythms, and altered levels of known core clock elements. Full-length mTim protein (mTIM-fl) exhibited a 24-hour oscillation, where as a truncated isoform (mTIM-s) was constitutively expressed. mTIM-fl associated with the mammalian clock Period proteins (mPERs) in oscillating SCN cells. These data suggest that mTim is required for rhythmicity and is a functional homolog of dTim on the negative-feedback arm of the mammalian molecular clockwork.

BACKGROUND/OBJECTIVES Epidemics of obesity and diabetes are escalating. High-calorie/high-fat food is a major cause for these global health issues, but molecular mechanisms underlying high-fat, diet-induced obesity are still not well understood. The aryl hydrocarbon receptor (AhR), a transcription factor that acts as a xenobiotic sensor, mediates environmental toxicant-induced obesity, insulin resistance and development of diabetes. AhR also influences lipid metabolism and diet-induced obesity. The effects of AhR deficiency on diet-induced obesity, hepatic steatosis and insulin resistance were examined. METHODS : Male wild type (WT), AhR null (AhR−/−) and AhR heterozygote (AhR+/−) mice were fed a normal chow diet (NCD, 10% kcal from fat) or a high-fat diet (HFD, 60% kcal from fat) for up to 14 weeks. Adiposity, adipose and liver morphology, insulin signaling, metabolic parameters and gene profiles were assessed. RESULTS AhR deficiency protected against HFD-induced obesity, hepatic steatosis, insulin resistance and inflammation. Moreover, AhR deficiency preserved insulin signaling in major metabolic tissues. These protective effects result from a higher energy expenditure in AhR-deficient mice compared to WT. Levels of transcript for both the thermogenic gene, uncoupling protein 1 (Ucp1), in brown adipose tissue and mitochondrial β-oxidation genes in muscle were significantly higher in AhR−/− and AhR+/− mice compared to WT. CONCLUSIONS This work documents a physiologically relevant function for AhR in regulation of body weight, hepatic fat deposition, insulin sensitivity and energy expenditure under HFD exposure, suggesting that AhR signaling may be developed as a potential therapeutic target for treatment of obesity and metabolic disorders.

A molecular device that measures time on a daily, or circadian, scale is a nearly ubiquitous feature of eukaryotic organisms. A core group of clock genes, whose coordinated function is required for this timekeeping, is expressed both in the central clock and within numerous peripheral organs. We examined expression of clock genes in the rat ovary. Transcripts for core oscillator elements (Arntl, Clock, Per1, Per2, and Cry1) were present in the ovary as indicated by quantitative real-time RT-PCR. Rhythmic expression patterns of Arntl and Per2 transcripts and protein products were out of phase with respect to the central oscillator and in complete antiphase to each other. Expression of Arntl was significantly elevated after the LH surge on the day of proestrus. Finally, hCG treatment induced cyclic expression of both Arntl and Per2 gene products in hypophysectomized, immature rats primed with eCG. Collectively, these data suggest that the core underpinnings of the transcriptional/translational feedback loop that drives circadian rhythmicity is present in the rat ovary. Furthermore, the study identifies LH as a potential regulator of circadian clock gene rhythms in the ovary.