Resetting the biological clock: mediation of nocturnal circadian shifts by glutamate and NO

Ethanol modulates mammalian circadian clock phase resetting through extrasynaptic GABA receptor activation

Ethanol modulates the actions of multiple neurotransmitter systems, including GABA. However, its enhancing effects on GABA signaling typically are seen only at high concentrations. In contrast, although GABA is a prominent neurotransmitter in the circadian clock of the suprachiasmatic nucleus (SCN), we see ethanol modulation of clock phase resetting at low concentrations (<50 mM). A possible explanation is that ethanol enhances GABAergic signaling in the SCN through activating GABA(A) receptors that contain the delta subunit (GABA(Adelta) receptors), which are sensitive to low ethanol concentrations. Therefore, we investigated whether ethanol acts on GABA(Adelta) receptors in the SCN. Here we show that acute application of the GABA(Adelta) receptor antagonist, RO15-4513, to mouse hypothalamic slices containing the SCN prevents ethanol inhibition of nighttime glutamate-induced (photic-like) phase delays of the circadian clock. Diazepam, which enhances activity of GABA(A) receptors containing the gamma subunit (GABA(Agamma) receptors), does not modulate these phase shifts. Moreover, we find that RO15-4513 prevents ethanol enhancement of daytime serotonergic (non-photic) phase advances of the circadian clock. Furthermore, diazepam phase-advances the SCN circadian clock when applied alone in the daytime, while ethanol has no effect by itself at that time. These data support the hypothesis that ethanol acts on GABA(Adelta) receptors in the SCN to modulate photic and non-photic circadian clock phase resetting. They also reveal distinct modulatory roles of different GABA(A) receptor subtypes in circadian clock phase regulation.

Nuclear receptors homo sapiens Rev-erbbeta and Drosophila melanogaster E75 are thiolate-ligated heme proteins which undergo redox-mediated ligand switching and bind CO and NO

Nuclear receptors E75, which regulates development in Drosophila melanogaster, and Rev-erbbeta, which regulates circadian rhythm in humans, bind heme within their ligand binding domains (LBD). The heme-bound ligand binding domains of E75 and Rev-erbbeta were studied using electronic absorption, MCD, resonance Raman, and EPR spectroscopies. Both proteins undergo redox-dependent ligand switching and CO- and NO-induced ligand displacement. In the Fe(III) oxidation state, the nuclear receptor hemes are low spin and 6-coordinate with cysteine(thiolate) as one of the two axial heme ligands. The sixth ligand is a neutral donor, presumably histidine. When the heme is reduced to the Fe(II) oxidation state, the cysteine(thiolate) is replaced by a different neutral donor ligand, whose identity is not known. CO binds to the Fe(II) heme in both E75(LBD) and Rev-erbbeta(LBD) opposite a sixth neutral ligand, plausibly the same histidine that served as the sixth ligand in the Fe(III) state. NO binds to the heme of both proteins; however, the NO-heme is 5-coordinate in E75 and 6-coordinate in Rev-erbbeta. These nuclear receptors exhibit coordination characteristics that are similar to other known redox and gas sensors, suggesting that E75 and Rev-erbbeta may function in heme-, redox-, or gas-regulated control of cellular function.

Noise-induced coherence in multicellular circadian clocks

In higher organisms, circadian rhythms are generated by a multicellular genetic clock that is entrained very efficiently to the 24-h light-dark cycle. Most studies done so far of these circadian oscillators have considered a perfectly periodic driving by light, in the form of either a square wave or a sinusoidal modulation. However, in natural conditions, organisms are subject to nonnegligible fluctuations in the light level all through the daily cycle. In this article, we investigate how the interplay between light fluctuations and intercellular coupling affects the dynamics of the collective rhythm in a large ensemble of nonidentical, globally coupled cellular clocks modeled as Goodwin oscillators. On the basis of experimental considerations, we assume an inverse dependence of the cell-cell coupling strength on the light intensity, in such a way that the larger the light intensity, the weaker the coupling. Our results show a noise-induced rhythm generation for constant light intensities at which the clock is arrhythmic in the noise-free case. Importantly, the rhythm shows a resonancelike phenomenon as a function of the noise intensity. Such improved coherence can be only observed at the level of the overt rhythm and not at the level of the individual oscillators, thus suggesting a cooperative effect of noise, coupling, and the emerging synchronization between the oscillators.

Light Pulse‐Induced Heme and Iron‐Associated Transcripts in Mouse Brain: A Microarray Analysis

Synchronization of circadian oscillators with the outside world is achieved by the acute effects of light on the levels of one or more clock components. In mammals the PAS transcription factors Clock, NPAS2, and BMAL1 regulate gene expression as a function of the day-night cycle. Both PAS domains of NPAS2 were found to bind heme as a prosthetic group, form a gas-regulated sensor, and exert heme-status control of DNA binding in vitro. In a microarray analysis comparing overall changes in brain transcript levels between mice subjected to light pulses during the dark phase with animals maintained in darkness, we traced consistent changes in more than 200 different transcripts. Of these, 20 are associated with heme and iron biosynthesis and catabolism. A model for the pathway of induction of heme and iron homeostasis-related transcripts resulting from light pulses suggests that light signals (as stressors) induce transcription of heme oxygenase 2 (Hmox2) and cytochrome P450 oxidoreductase (Por), which may serve as a primary line of cellular defense. HMOX2 degrades heme from proteins such as hemoglobin. This degradation generates CO, a signal molecule, and may also change the redox state of the cell by reducing the NADPH/NADP ratio. This could lead to up-regulation of globin gene transcription, thereby releasing iron that in turn controls production of ferritins, and further up-regulating aminolevulinate synthase 2 (Alas2).

Constitutive Activation of Ras in Neurons: Implications for the Regulation of the Mammalian Circadian Clock

Almost all organisms living on earth exhibit rhythms under the control of autonomous timekeeping mechanisms referred to as circadian clocks. In mammals, peripheral clocks are synchronized (entrained) with high precision in a 24 h periodicity by the master circadian clock located in the suprachiasmatic nucleus (SCN) of the ventral hypothalamus. Light is the strongest phase-adjusting stimulus of the circadian clock. Circadian oscillations are driven by transcription/translation-based feedback/feedforward loops, comprising a set of clock genes and their protein products. The signalling pathways that couple light input to transcriptional, translational, and post-translational changes to ensure precise entrainment of the clock are not yet well characterized. A candidate pathway for transmission of photic information in the SCN is represented by the extracellular signal-regulated kinases ERK1/ERK2. In neurons, the ERK pathway is activated by a large array of stimuli, including trophic factors, neurotransmitters, and modulatory peptides. An upstream element of the ERK signalling route is the small intracellular membrane-anchored G-protein, Ras. In order to study its possible role in the entrainment of the circadian clock we are using transgenic gain-of-function mice expressing constitutively activated Val-12 Ha-Ras selectively in neurons (synRas mice). The Ha-Ras transgene protein is expressed in the SCN of synRas mice neurons serving as a model for interfering with the normal rhythmic changes in Ras activities in the SCN. This will allow us to investigate whether the associated modulation of the downstream targets such as ERK activities will interfere with the mechanisms of entrainment.

Temporally restricted role of retinal PACAP: integration of the phase-advancing light signal to the SCN

Circadian rhythms in physiology and behavior are temporally synchronized to the day/night cycle through the action of light on the circadian clock. In mammals, transduction of the photic signal reaching the circadian oscillator in the suprachiasmatic nucleus (SCN) occurs through the release of glutamate and pituitary adenylate cyclase-activating peptide (PACAP). The authors' study aimed at clarifying the role played by PACAP in photic resetting and entrainment. They investigated the circadian response to light of PACAPnullmice lacking the 5th exon of the PACAP coding sequence. Specifically, they examined free-running rhythms, entrainment to 12-h light:12-h dark (LD)cycles, the phase-response curve (PRC) to single light pulses, entrainment to a23-h T-cycle, re-entrainment to 6-h phase shifts in LD cycles, and light-induced c-Fos expression. PACAP-null and wild-type mice show similar free-running periods and similar entrainment to 12:12 LD cycles. However, the PRC of PACAP-null mice lacks a phase-advance portion. Surprisingly, despite the absence of phase advance to single light pulses, PACAP-null mice are able to entrain to a 23-h T-cycle, but with a significantly longer phase angle of entrainment than wild types. In addition, PACAP-null mice re-entrain more slowly to a 6-h phase advance of the LD cycle. Nevertheless, induction of c-Fos by light in late night is normal. In all experiments, PACAP-null mice show specific behavioral impairments in response to phase-advancing photic stimuli. These results suggest that PACAP is required for the normal integration of the phase advancing light signal by the SCN.

Phase shifting capacity of the circadian pacemaker determined by the SCN neuronal network organization

Background

In mammals, a major circadian pacemaker that drives daily rhythms is located in the suprachiasmatic nuclei (SCN), at the base of the hypothalamus. The SCN receive direct light input via the retino-hypothalamic tract. Light during the early night induces phase delays of circadian rhythms while during the late night it leads to phase advances. The effects of light on the circadian system are strongly dependent on the photoperiod to which animals are exposed. An explanation for this phenomenon is currently lacking.

Methodology and Principal Findings

We recorded running wheel activity in C57 mice and observed large amplitude phase shifts in short photoperiods and small shifts in long photoperiods. We investigated whether these different light responses under short and long days are expressed within the SCN by electrophysiological recordings of electrical impulse frequency in SCN slices. Application of N-methyl-D-aspartate (NMDA) induced sustained increments in electrical activity that were not significantly different in the slices from long and short photoperiods. These responses led to large phase shifts in slices from short days and small phase shifts in slices from long days. An analysis of neuronal subpopulation activity revealed that in short days the amplitude of the rhythm was larger than in long days.

Conclusions

The data indicate that the photoperiodic dependent phase responses are intrinsic to the SCN. In contrast to earlier predictions from limit cycle theory, we observed large phase shifting responses in high amplitude rhythms in slices from short days, and small shifts in low amplitude rhythms in slices from long days. We conclude that the photoperiodic dependent phase responses are determined by the SCN and propose that synchronization among SCN neurons enhances the phase shifting capacity of the circadian system.

The structural basis of gas-responsive transcription by the human nuclear hormone receptor REV-ERBbeta

Heme is a ligand for the human nuclear receptors (NR) REV-ERBalpha and REV-ERBbeta, which are transcriptional repressors that play important roles in circadian rhythm, lipid and glucose metabolism, and diseases such as diabetes, atherosclerosis, inflammation, and cancer. Here we show that transcription repression mediated by heme-bound REV-ERBs is reversed by the addition of nitric oxide (NO), and that the heme and NO effects are mediated by the C-terminal ligand-binding domain (LBD). A 1.9 A crystal structure of the REV-ERBbeta LBD, in complex with the oxidized Fe(III) form of heme, shows that heme binds in a prototypical NR ligand-binding pocket, where the heme iron is coordinately bound by histidine 568 and cysteine 384. Under reducing conditions, spectroscopic studies of the heme-REV-ERBbeta complex reveal that the Fe(II) form of the LBD transitions between penta-coordinated and hexa-coordinated structural states, neither of which possess the Cys384 bond observed in the oxidized state. In addition, the Fe(II) LBD is also able to bind either NO or CO, revealing a total of at least six structural states of the protein. The binding of known co-repressors is shown to be highly dependent upon these various liganded states. REV-ERBs are thus highly dynamic receptors that are responsive not only to heme, but also to redox and gas. Taken together, these findings suggest new mechanisms for the systemic coordination of molecular clocks and metabolism. They also raise the possibility for gas-based therapies for the many disorders associated with REV-ERB biological functions.

Acute ethanol impairs photic and nonphotic circadian phase resetting in the Syrian hamster

Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal, we used microdialysis to characterize the pharmacokinetics of intraperitoneal injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase resetting of the circadian locomotor activity rhythm. Peak EtOH levels (approximately 50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of approximately 3 h. EtOH treatment dose-dependently attenuated photic phase advances but had no effect on phase delays and, contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase advances. In a complementary experiment using reverse microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase advances were blocked, similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such actions could underlie the disruption of circadian rhythmicity associated with alcohol abuse.

Remodeling the clock: coactivators and signal transduction in the circadian clockworks

Most organisms on earth such as cyanobacteria, fungi, plants, insects, animals, and humans synchronize their physiological and behavioral activities with the environmental cycles of day and night. Significant progress has been made in unraveling the genetic components that constitute a molecular circadian clock, which facilitates the temporal control of physiology and behavior. Clock genes assemble interlocked transcriptional/translational feedback loops that underlie the circadian oscillations. Recent investigations revealed that posttranslational regulation of clock proteins is crucial for functioning of the molecular oscillator and for precise temporal control of circadian transcription. This review provides an overview of the homologous clockworks in Drosophila and mammals, with a special focus on recent insights in the posttranslational regulation of clock proteins as well as the role of coactivators, repressors, and signal transduction for circadian controlled genome-wide transcription. The emerging mechanisms of clock gene regulation provide an understanding of the temporal control of transcription in general and the circadian orchestration of physiology and behavior in particular.

Mice lacking the PACAP type I receptor have impaired photic entrainment and negative masking

The retinohypothalamic tract (RHT) is a retinofugal neuronal pathway which, in mammals, mediates nonimage-forming vision to various areas in the brain involved in circadian timing, masking behavior, and regulation of the pupillary light reflex. The RHT costores the two neurotransmitters glutamate and pituitary adenylate cyclase activating peptide (PACAP), which in a rather complex interplay are mediators of photic adjustment of the circadian system. To further characterize the role of PACAP/PACAP receptor type 1 (PAC1) receptor signaling in light entrainment of the clock and in negative masking behavior, we extended previous studies in mice lacking the PAC1 receptor (PAC1 KO) by examining their phase response to single light pulses using Aschoff type II regime, their ability to entrain to non-24-h light-dark (LD) cycles and large phase shifts of the LD cycle (jet lag), as well as their negative masking response during different light intensities. A prominent finding in PAC1 KO mice was a significantly decreased phase delay of the endogenous rhythm at early night. In accordance, PAC1 KO mice had a reduced ability to entrain to T cycles longer than 26 h and needed more time to reentrain to large phase delays, which was prominent at low light intensities. The data obtained at late night indicated that PACAP/PAC1 receptor signaling is less important during the phase-advancing part of the phase-response curve. Finally, the PAC1 KO mice showed impaired negative masking behavior at low light intensities. Our findings substantiate a role for PACAP/PAC1 receptor signaling in nonimage-forming vision and indicate that the system is particularly important at lower light intensities.

Effects of chronic expression of the HIV-induced protein, transactivator of transcription, on circadian activity rhythms in mice, with or without morphine

Patients with human immunodeficiency virus (HIV) infection exhibit changes in sleep patterns, motor disorders, and cognitive dysfunction; these symptoms may be secondary to circadian rhythm abnormalities. Studies in mice have shown that intracerebral injection of an HIV protein, transactivator of transcription (Tat), alters the timing of circadian rhythms in a manner similar to light. Therefore, we tested the hypothesis that chronic Tat expression alters circadian rhythms, especially their entrainment to a light-dark (LD) cycle, by using transgenic mice in which Tat expression in the brain was induced via a doxycycline (DOX)-sensitive, glial fibrillary-associated, protein-restricted promoter. Because opiate substance abuse, which shares comorbidity with HIV infection, also disrupts sleep, a final experiment assessed the effects of morphine exposure on circadian rhythms in wild-type and Tat transgenic mice. Mice housed in cages equipped with running wheels were fed chow with or without DOX. Experiment 1 revealed a small but significant (P < 0.05) difference between groups in the phase angle of entrainment and a 15% decrease in the wheel running in the DOX group (P < 0.005). During exposure to constant darkness, DOX did not alter the endogenous period length of the circadian rhythm. Experiment 2 investigated the effect of DOX on circadian rhythms in wild-type and Tat(+) mice during exposure to a normal or phase-shifted LD cycle, or morphine treatment without any change in the LD cycle. Tat induction significantly decreased wheel running but did not affect entrainment to the normal or shifted LD cycle. Morphine decreased wheel running without altering the phase angle of entrainment, and the drug's effects were independent of Tat induction. In conclusion, these findings suggest that chronic brain expression of Tat decreases locomotor activity and the amplitude of circadian rhythms, but does not affect photic entrainment or reentrainment of the murine circadian pacemaker.

Oxidative Stress - Clinical Diagnostic Significance

Elevated free radical production and/or insufficient antioxidative defense results in cellular oxidant stress responses. Sustained and/or intense oxidative insults can overcome cell defenses resulting in accumulated damage to macromolecules, leading to loss of cell function, membrane damage, and ultimately to cell death. Oxidative stress (OS) can result from conditions including excessive physical stress, exposure to environmental pollution and xenobiotics, and smoking. Oxidative stress, as a pathophysiological mechanism, has been linked to numerous pathologies, poisonings, and the ageing process. Reactive oxygen species and reactive nitrogen species, endogenously or exogenously produced, can readily attack all classes of macromolecules (proteins, DNA, unsaturated fatty acid). The disrupted oxidative-reductive milieu proceeds via lipid peroxidation, altered antioxidative enzyme activities and depletion of non-enzymatic endogenous antioxidants, several of which can de detected in the pre-symptomatic phase of many diseases. Therefore, they could represent markers of altered metabolic and physiological homeostasis. Accordingly, from the point of view of routine clinical-diagnostic practice, it would be valuable to routinely analyze OS status parameters to earlier recognize potential disease states and provide the basis for preventative advance treatment with appropriate medicines.

Mass spectrometry-based discovery of circadian peptides

A significant challenge to understanding dynamic and heterogeneous brain systems lies in the chemical complexity of secreted intercellular messengers that change rapidly with space and time. Two solid-phase extraction collection strategies are presented that relate time and location of peptide release with mass spectrometric characterization. Here, complex suites of peptide-based cell-to-cell signaling molecules are characterized from the mammalian suprachiasmatic nucleus (SCN), site of the master circadian clock. Observed SCN releasates are peptide rich and demonstrate the co-release of established circadian neuropeptides and peptides with unknown roles in circadian rhythms. Additionally, the content of SCN releasate is stimulation specific. Stimulation paradigms reported to alter clock timing, including electrical stimulation of the retinohypothalamic tract, produce releasate mass spectra that are notably different from the spectra of compounds secreted endogenously over the course of the 24-h cycle. In addition to established SCN peptides, we report the presence of proSAAS peptides in releasates. One of these peptides, little SAAS, exhibits robust retinohypothalamic tract-stimulated release from the SCN, and exogenous application of little SAAS induces a phase delay consistent with light-mediated cues regulating circadian timing. These mass spectrometry-based analyses provide a new perspective on peptidergic signaling within the SCN and demonstrate that the integration of secreted compounds with information relating time and location of release generates new insights into intercellular signaling in the brain.

Role for the NR2B subunit of the N-methyl-D-aspartate receptor in mediating light input to the circadian system

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that utilize glutamate as a neurotransmitter. A variety of evidence suggests that the release of glutamate then activates N-methyl-D-aspartate (NMDA) receptors within the SCN and triggers a signaling cascade that ultimately leads to phase shifts in the circadian system. In this study, we first sought to explore the role of the NR2B subunit in mediating the effects of light on the circadian system of hamsters and mice. We found that localized microinjection of the NR2B subunit antagonist ifenprodil into the SCN region reduces the magnitude of light-induced phase shifts of the circadian rhythm in wheel-running activity. Next, we found that the NR2B message and levels of phospho-NR2B vary with time of day in SCN tissue using semiquantitative real-time polymerase chain reaction and western blot analysis, respectively. Functionally, we found that blocking the NR2B subunit with ifenprodil significantly reduced the magnitude of NMDA currents recorded in SCN neurons. Ifenprodil also significantly reduced the magnitude of NMDA-induced Ca2+ changes in SCN cells. Together, these results demonstrate that the NR2B subunit is an important component of NMDA receptor-mediated responses within SCN neurons and that this subunit contributes to light-induced phase shifts of the mammalian circadian system.

Acute ethanol modulates glutamatergic and serotonergic phase shifts of the mouse circadian clock in vitro

Alcohol abuse is associated with sleep problems, which are often linked to circadian rhythm disturbances. However, there is no information on the direct effects of ethanol on the mammalian circadian clock. Acute ethanol inhibits glutamate signaling, which is the primary mechanism through which light resets the mammalian clock in the suprachiasmatic nucleus (SCN). Glutamate and light also inhibit circadian clock resetting induced by nonphotic signals, including 5-HT. Thus, we investigated the effects of acute ethanol on both glutamatergic and serotoninergic resetting of the mouse SCN clock in vitro. We show that ethanol dose-dependently inhibits glutamate-induced phase shifts and enhances serotonergic phase shifts. The inhibition of glutamate-induced phase shifts is not affected by excess glutamate, glycine or d-serine, but is prevented by excess brain-derived neurotrophic factor (BDNF). BDNF is known to augment glutamate signaling in the SCN and to be necessary for glutamate/light-induced phase shifts. Thus, ethanol may inhibit glutamate-induced clock resetting at least in part by blocking BDNF enhancement of glutamate signaling. Ethanol enhancement of serotonergic phase shifts is mimicked by treatments that suppress glutamate signaling in the SCN, including antagonists of glutamate receptors, BDNF signaling and nitric oxide synthase. The combined effect of ethanol with these treatments is not additive, suggesting they act through a common pathway. Our data indicate further that the interaction between 5-HT and glutamate in the SCN may occur downstream from nitric oxide synthase activation. Thus, acute ethanol disrupts normal circadian clock phase regulation, which could contribute to the physiological and psychological problems associated with alcohol abuse.

Cholecystokinin-A receptors regulate photic input pathways to the circadian clock

Daily behaviors are strongly dominated by internally generated circadian rhythms, but the underlying mechanisms remain unclear. In mammals, photoentrainment of behaviors to light-dark cycles involves signaling from both intrinsically photosensitive retinal ganglion cells and classic photoreceptor pathways to the suprachiasmatic nucleus (SCN). How classic photoreceptor pathways work with the photosensitive ganglion cells, however, is not fully understood. Although cholecystokinin (CCK) peptide has been shown to be present in a variety of vertebrate retinas, its function at a systems level is also unknown. In the present study we examined a possible role of CCK-A receptors in photoentrainment using CCK-A receptor knockout mice. The lacZ reporter gene within a gene-knockout cassette revealed precise localization of CCK-A receptors in the circadian clock system. We demonstrated that CCK-A receptors were located predominately on glycinergic amacrine cells but were rarely found on SCN neurons. Moreover, Ca(2+) imaging analysis demonstrated that the CCK-A agonist, CCK-8 sulfate (CCK-8s), mobilized intracellular Ca(2+) in amacrine cells but not glutamate-receptive SCN neurons. Furthermore, light pulse-induced mPer1/mPer2 gene expression in SCN, behavioral phase shifts, and the pupillary reflex were significantly reduced in CCK-A receptor knockout mice. These data indicate a novel function of CCK-A receptors in the nonimage-forming photoreception presumably via amacrine cell-mediated signal transduction pathways.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+]i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i, suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

PACAP and PDF signaling in the regulation of mammalian and insect circadian rhythms

Endogenous circadian clocks are inherent to all living organisms. They are needed to guarantee successful life since they regulate very important biological processes such as behavior and reproduction. Secretin-like G-protein coupled receptors are very important factors in the signal transduction pathways of circadian clocks. In this review, we will focus on the role of two secretin-like signaling pathways that play an important role in the regulation of the mammalian and the insect clock, respectively: the pituitary adenylate cyclase-activating polypeptide (PACAP) and pigment dispersing factor (PDF) signaling pathways. Both pathways are most likely related although their function in the biological clock differs.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Circadian profiling of the transcriptome in NIH/3T3 fibroblasts: comparison with rhythmic gene expression in SCN2.2 cells and the rat SCN

To screen for output signals that may distinguish the pacemaker in the mammalian suprachiasmatic nucleus (SCN) from peripheral-type oscillators in which the canonical clockworks are similarly regulated in a circadian manner, the rhythmic behavior of the transcriptome in forskolin-stimulated NIH/3T3 fibroblasts was analyzed and compared relative to SCN2.2 cells in vitro and the rat SCN. Similar to the circadian profiling of the SCN2.2 and rat SCN transcriptomes, NIH/3T3 fibroblasts exhibited circadian fluctuations in the expression of the core clock genes, Per2, Cry1, and Bmal1, and 323 functionally diverse transcripts, many of which regulate cellular communication. Overlap in rhythmic transcripts among NIH/3T3 fibroblasts, SCN2.2 cells, and the rat SCN was limited to these clock genes and four other genes that mediate fatty acid and lipid metabolism or function as nuclear factors. Compared with NIH/3T3 cells, circadian gene expression in SCN oscillators was more prevalent among genes mediating glucose metabolism and neurotransmission. Coupled with evidence for the rhythmic regulation of the inducible isoform of nitric oxide synthase (iNos) in SCN2.2 cells and the rat SCN but not in fibroblasts, studies examining the effects of a NOS inhibitor on metabolic rhythms in cocultures containing SCN2.2 cells and untreated NIH/3T3 cells suggest that the gaseous neurotransmitter nitric oxide may play a key role in SCN pacemaker function. This comparative analysis of circadian gene expression in SCN and non-SCN cells may have important implications in the selective analysis of circadian signals involved in the coupling of SCN oscillators and regulation of rhythmicity in downstream cells.

Glutamatergic activity modulates the phase-shifting effects of gastrin-releasing peptide and light

Previous studies have established that microinjection of gastrin-releasing peptide (GRP) into the suprachiasmatic nucleus (SCN) region or third ventricle causes circadian phase shifts similar to those produced by light pulses. Activation of N-methyl-d-aspartate (NMDA) receptors in the SCN region also produces light-like phase shifts. This study was designed to test the effects of (+/-)-2-amino-5-phosphonopentanoic acid (AP5), an NMDA antagonist, and l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a glutamate reuptake inhibitor, on GRP-induced phase shifts. Adult male Syrian hamsters equipped with a surgically implanted guide cannula aimed at the third ventricle were housed in constant darkness until stable free-running rhythms of wheel-running activity were apparent. Microinjection of GRP into the third ventricle at circadian time (CT)13 induced large phase delays. These GRP-induced phase delays were completely blocked by co-administration of AP5, suggesting that GRP-induced phase delays require concurrent activation of NMDA receptors. Microinjection of AP5 alone did not induce significant phase shifts. A second set of experiments was designed to test whether GRP-induced phase shifts would be enhanced by PDC. Co-administration of PDC and GRP elicited significantly larger phase delays at CT13 than GRP alone. However, administration of PDC alone did not induce a significant phase shift. Finally, when administered just prior to a light pulse, PDC elicited significantly larger phase delays than light pulse plus vehicle controls. These data suggest that the effects of GRP on the circadian clock phase are highly dependent on the level of excitation provided by activated NMDA receptors.

The molecular gatekeeper Dexras1 sculpts the photic responsiveness of the mammalian circadian clock

The mammalian master clock, located in the suprachiasmatic nucleus (SCN), is exquisitely sensitive to photic timing cues, but the key molecular events that sculpt both the phasing and magnitude of responsiveness are not understood. Here, we show that the Ras-like G-protein Dexras1 is a critical factor in these processes. Dexras1-deficient mice (dexras1-/-) exhibit a restructured nighttime phase response curve and a loss of gating to photic resetting during the day. Dexras1 affects the photic sensitivity by repressing or activating time-of-day-specific signaling pathways that regulate extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK). During the late night, Dexras1 limits the capacity of pituitary adenylate cyclase (PAC) activating peptide (PACAP)/PAC1 to affect ERK/MAPK, and in the early night, light-induced phase delays, which are mediated predominantly by NMDA receptors, are reduced as reported previously. Daytime photic phase advances are mediated by a novel signaling pathway that does not affect the SCN core but rather stimulates ERK/MAPK in the SCN shell and triggers downregulation of clock protein expression.

Exploring spatiotemporal organization of SCN circuits

Suprachiasmatic nucleus (SCN) neuroanatomy has been a subject of intense interest since the discovery of the SCN's function as a brain clock and subsequent studies revealing substantial heterogeneity of its component neurons. Understanding the network organization of the SCN has become increasingly relevant in the context of studies showing that its functional circuitry, evident in the spatial and temporal expression of clock genes, can be reorganized by inputs from the internal and external environment. Although multiple mechanisms have been proposed for coupling among SCN neurons, relatively little is known of the precise pattern of SCN circuitry. To explore SCN networks, we examine responses of the SCN to various photic conditions, using in vivo and in vitro studies with associated mathematical modeling to study spatiotemporal changes in SCN activity. We find an orderly and reproducible spatiotemporal pattern of oscillatory gene expression in the SCN, which requires the presence of the ventrolateral core region. Without the SCN core region, behavioral rhythmicity is abolished in vivo, whereas low-amplitude rhythmicity can be detected in SCN slices in vitro, but with loss of normal topographic organization. These studies reveal SCN circuit properties required to signal daily time.

Circadian Clock Gene Expression in the Ovary: Effects of Luteinizing Hormone1

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.

Extracellular long-term recordings of the isolated accessory medulla, the circadian pacemaker center of the cockroach Leucophaea maderae, reveal ultradian and hint circadian rhythms

In the cockroach Leucophaea maderae transplantation studies located the circadian pacemaker center, which controls locomotor activity rhythms, to the accessory medulla (AMe), ventromedially to the medulla of the brain's optic lobes. The AMe is densely innervated via GABA- and manyfold peptide-immunoreactive neurons. They express ultradian action potential oscillations in the gamma frequency range and form phase-locked assemblies of synchronously spiking cells. Peptide application resulted in transient rises of extracellularly recorded activity. It remained unknown whether transient rises in spontaneous electrical activity as a possible indication of peptide release occur in the isolated circadian clock in a rhythmic manner. In extracellular glass electrode recordings of the isolated AMe in constant darkness, which lasted at least 12 h, the distribution of daytime-dependent changes in activity independently of the absolute action potential frequency was examined. Rapid, transient changes in activity preferentially occurred at the mid-subjective night, with a minimum at the middle of the subjective day, hinting the presence of circadian rhythms in the isolated circadian clock. Additionally, ultradian rhythms in activity change that are multiples of a fundamental 2 h period were observed. We hypothesize that circadian rhythms might originate from coupled ultradian oscillations, possibly already at the single cell level.

The brain's master circadian clock: implications and opportunities for therapy of sleep disorders

The suprachiasmatic nuclei (SCN) residing in the anterior hypothalamus maintains a near-24-h rhythm of electrical activity, even in the absence of environmental cues. This circadian rhythm is generated by intrinsic molecular mechanisms in the neurons of the SCN; however, the circadian clock is modulated by a wide variety of influences, including glutamate and pituitary adenylate cyclase-activating peptide (PACAP) from the retinohypothalamic tract, melatonin from the pineal gland, and neuropeptide Y from the intergeniculate leaflet. By virtue of these and other inputs, the SCN responds to environmental cues such as light, social and physical activities. In turn, the SCN controls or influences a wide variety of physiologic and behavioral functions, including attention, endocrine cycles, body temperature, melatonin secretion, and the sleep-wake cycle. Regulation of the sleep-wake cycle by the SCN has important implications for development of therapies for sleep disorders, including those involving desynchronization of circadian rhythms and insomnia.

Second messenger and Ras/MAPK signalling pathways regulate CLOCK/CYCLE-dependent transcription

The heterodimeric complex of the transcription factors CLOCK (CLK) and CYCLE (CYC) constitutes the positive element of the circadian clock in Drosophila and mammals. Phosphorylation of clock proteins represents an essential mechanism for promotion and control of the molecular oscillator. However, the kinases and signalling pathways that regulate CLK/CYC function remain largely elusive. In the present study we performed a chemical screen of kinase inhibitors in a cell culture reporter assay to identify functional regulators of CLK/CYC-dependent gene expression. These studies and analysis of constitutively active forms of kinases revealed that cyclic nucleotide/protein kinase A (PKA), calcium/calmodulin-dependent kinase (CaMK) II and Ras/mitogen-activated protein kinase (MAPK) regulate CLK/CYC activity. In vitro phosphorylation analysis showed a direct phosphorylation of CLK by CaMK II and p42 MAPK [extracellular signal-regulated kinase (ERK) 2], suggesting that these kinases regulate CLK/CYC-dependent transcription by direct phosphorylation of CLK.

Brain-derived neurotrophic factor and neurotrophin receptors modulate glutamate-induced phase shifts of the suprachiasmatic nucleus

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells. Previous work raised the possibility that brain-derived neurotrophic factor (BDNF) and its high-affinity tropomyosin-related receptor kinase may be important as modulators of this excitatory input into the SCN. In order to test this possibility, we used whole-cell patch-clamp methods to measure spontaneous excitatory currents in mouse SCN neurons. We found that the amplitude and frequency of these currents were increased by BDNF and decreased by the neurotrophin receptor inhibitor K252a. The neurotrophin also increased the magnitude of currents evoked by application of N-methyl-d-aspartate and amino-methyl proprionic acid. Next, we measured the rhythms in action potential discharge from the SCN brain slice preparation. We found that application of K252a dramatically reduced the magnitude of phase shifts of the electrical activity rhythm generated by the application of glutamate. By itself, BDNF caused phase shifts that resembled those produced by glutamate and were blocked by K252a. The results demonstrate that BDNF and neurotrophin receptors can enhance glutamatergic synaptic transmission within a subset of SCN neurons and potentiate glutamate-induced phase shifts of the circadian rhythm of neural activity in the SCN.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

A novel suction electrode recording technique for monitoring circadian rhythms in single and multiunit discharge from brain slices

The study of spontaneous neuronal discharge for several days in vitro poses substantial technical difficulties not readily addressed by existing methodologies. Here we describe a novel method for recording bioelectrical signals from brain slices, using media-filled, glass micropipettes stabilized at the recording interface by negative pressure. Such electrodes are durable, economical and easily constructed using standard laboratory equipment. Through these suction electrodes, we monitored approximately 24 h oscillations in spontaneous single and multiunit discharge from acutely prepared adult mouse suprachiasmatic nuclei (SCN) slices for up to 4 days in vitro, with very high success rates. Neuronal oscillations exhibited all the characteristics previously determined in SCN recordings using other techniques. Importantly, our electrode assembly enables SCN activity in acutely prepared brain slices to be monitored for substantially longer than with other methodologies. In summary, our data suggest that suction electrodes are likely to provide a favorable alternative to existing methods for recording long-term neuronal activity from brain slices.

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Regulation of glutamatergic signalling by PACAP in the mammalian suprachiasmatic nucleus

Background

Previous studies indicate that light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that contain both glutamate and pituitary adenylyl cyclase activating peptide (PACAP). While the role of glutamate in this pathway has been well studied, the involvement of PACAP and its receptors are only beginning to be understood. Speculating that PACAP may function to modulate how neurons in the suprachiasmatic nucleus respond to glutamate, we used electrophysiological and calcium imaging tools to examine possible cellular interactions between these co-transmitters.

Results

Exogenous application of PACAP increased both the amplitude and frequency of spontaneous excitatory postsynaptic currents recorded from SCN neurons in a mouse brain slice preparation. PACAP also increased the magnitude of AMPA-evoked currents through a mechanism mediated by PAC1 receptors and the adenylyl cyclase-signalling cascade. This enhancement of excitatory currents was not limited to those evoked by AMPA as the magnitude of NMDA currents were also enhanced by application of PACAP. Furthermore, PACAP enhanced AMPA and NMDA evoked calcium transients while PACAP alone produced very little change in resting calcium in most mouse SCN neurons. Finally, in rat SCN neurons, exogenous PACAP enhanced AMPA evoked currents and calcium transients as well evoked robust calcium transients on its own.

Conclusion

The results reported here show that PACAP is a potent modulator of glutamatergic signalling within the SCN in the early night.

Interleukin-6 and Nitric Oxide Synthase Expression in the Vasopressin and Corticotrophin-releasing Factor Systems of the Rat Hypothalamus

Nitric oxide synthase (NOS) and interleukin-6 (IL-6) are constitutively expressed in hypothalamic cells. However, phenotypic and functional aspects of these cells remain unknown. We have studied the expression pattern of these two molecules in hypothalamic cells expressing corticotropin-releasing factor (CRF) and arginin-vasopressin (AVP), two major regulatory peptides in the hypothalamus-pituitary system, using immunofluorescence, intracerebroventricular injection of colchicine, and the study in parallel of the labeling pattern of axons in the median eminence. Within AVP cells, we distinguished two different populations: large, intensely stained AVP cells coexpressing IL-6; and large, intensely stained AVP cells coexpressing IL-6 and NOS. Within the CRF cells, we distinguished three different populations: large, intensely stained CRF cells immunonegative for AVP, NOS, and IL-6; large cells weakly stained for CRF and AVP, immunopositive for NOS and immunonegative for IL-6; and small cells intensely stained for CRF and AVP and immunonegative for IL-6 and NOS. In addition, we also found AVP cells containing IL-6 in the suprachiasmatic nucleus. These results suggest that neuronal NOS and IL-6 may be involved in different modulatory processes in hypophysiotropic and non-hypophysiotropic cells.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function

It is becoming increasingly clear that the intrinsic properties of both the heart and vasculature exhibit dramatic oscillations over the course of the day. Diurnal variations in the responsiveness of the cardiovascular system to environmental stimuli are mediated by a complex interplay between extracellular (i.e., neurohumoral factors) and intracellular (i.e., circadian clock) influences. The intracellular circadian clock is composed of a series of transcriptional modulators that together allow the cell to perceive the time of day, thereby enabling preparation for an anticipated stimulus. These molecular timepieces have been characterized recently within both vascular smooth muscle cells and cardiomyocytes, giving rise to a multitude of hypotheses relating to the potential role(s) of the circadian clock as a modulator of physiological and pathophysiological cardiovascular events. For example, evidence strongly supports the hypothesis that the circadian clock within the heart modulates myocardial metabolism, which in turn facilitates anticipation of diurnal variations in workload, substrate availability, and/or the energy supply-to-demand ratio. The purpose of this review is therefore to summarize our current understanding of the molecular events governing diurnal variations in the intrinsic properties of the heart, with special emphasis on the intramyocardial circadian clock. Whether impairment of this molecular mechanism contributes toward cardiovascular disease associated with hypertension, diabetes mellitus, shift work, sleep apnea, and/or obesity will be discussed.

A novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus

The suprachiasmatic nucleus of the anterior hypothalamus is the center of an internal biological clock in mammals. Glutamate is the neurotransmitter of retino-hypothalamic tract responsible for mediating the circadian actions of light in rodents. N-methyl-d-aspartate receptors, particularly NR2B subunit are reported to be principally involved in photic resetting of the biological clock in vivo and in slice culture. But, the precise cellular mechanisms of the resetting are not elucidated, because no adequate neuronal cell lines derived from the suprachiasmatic nucleus have been established. We established a neuronal cell line, N14.5, derived from the suprachiasmatic nucleus of a transgenic rat harboring the temperature-sensitive simian virus 40 large T-antigen gene. When the cells were cultured at 39 degrees C, the morphological features were turned fibroblastic into neuronal round cell body with neurite extensions. These cells showed immunoreactivities for neuronal markers (betaIII-tubulin, microtubule-associated protein 2 and TAU2) and as well as for vasoactive intestinal peptide which is expressed in the ventrolateral region of the suprachiasmatic nucleus. The cells expressed N-methyl-d-aspartate receptors, particularly NR1 and NR2B subunits as revealed by quantitative PCR. N-methyl-d-aspartate activated phosphorylation of p44/42 mitogen-activated protein kinase and increased expression level of Per1 and Per2 mRNA. These results suggest that the N14.5 is a novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus, and that N-methyl-d-aspartate receptors expressed in the cells are a functional receptor. The N14.5 cells may be a useful tool to elucidate numerous chronobiological processes, especially resetting mechanism induced by an external light signal.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Neonatal alcohol exposure permanently disrupts the circadian properties and photic entrainment of the activity rhythm in adult rats

BACKGROUND

Alcohol exposure during the period of rapid brain development produces structural damage in different brain regions, including the suprachiasmatic nucleus (SCN), that may have permanent neurobehavioral consequences. Thus, this study examined the long-term effects of neonatal alcohol exposure on circadian behavioral activity in adult rats.

METHODS

Artificially reared Sprague-Dawley rat pups were exposed to alcohol (EtOH; 4.5 g/kg/day) or isocaloric milk formula (gastrostomy control; GC) on postnatal days 4-9. At 2 months of age, rats from the EtOH, GC, and suckle control (SC) groups were housed individually, and properties of the circadian rhythm in wheel-running behavior were continuously analyzed during exposure to a 12-hr light:12-hr dark photoperiod (LD 12:12) or constant darkness (DD).

RESULTS

Neonatal alcohol exposure had distinctive effects on the rhythmic properties and quantitative parameters of adult wheel-running behavior. EtOH-treated animals were distinguished by unstable and altered entrainment to LD 12:12 such that their daily onsets of activity were highly variable and occurred at earlier times relative to control animals. In DD, circadian regulation of wheel-running behavior was altered by neonatal alcohol exposure such that the free-running period of the activity rhythm was shorter in EtOH-exposed rats than in control animals. Total amount of daily wheel-running activity in EtOH-treated rats was greater than that observed in the SC group. In addition, the circadian activity patterns of EtOH-exposed rats were fragmented such that the duration of the active phase and the number of activity bouts per day were increased.

CONCLUSIONS

These data indicate that neonatal alcohol exposure produces permanent changes in the circadian regulation of the rat activity rhythm and its entrainment to LD cycles. These long-term alterations in circadian behavior, along with the developmental alcohol-induced changes in SCN endogenous rhythmicity, may have important implications in clinical sleep-wake disturbances observed in neonates, children, and adults exposed to alcohol in utero.

Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

Circadian rhythms in Syrian hamsters can be phase shifted by behavioral arousal during the usual rest phase of the circadian rest-activity cycle. Phase shifts can be greatly potentiated by exposing the animals to constant light for 1 or 2 cycles. This could reflect a change in a specific nonphotic input pathway to the suprachiasmatic nucleus (SCN) circadian pacemaker, or it could be caused by a change in the amplitude of the pacemaker. If the latter, then phase shifts to any stimulus, including those activating the photic input pathway, should be potentiated. This hypothesis was tested by measuring phase shifts induced by microinjections of NMDA (500 nl, 10 mM) into the SCN area of hamsters exposed to constant light or dark for 2 days. NMDA induced significant phase delay shifts that mimicked those induced by light exposure early in the night. The magnitude of these shifts did not differ by prior lighting condition. Shifts induced by NMDA (200 nl, 10 mM) microinjections on day 3 and 13 of LL also did not differ. Phase shifts induced by a nonphotic stimulus (3 h of running stimulated by confinement to a novel wheel) were significantly potentiated by 2 days of exposure to constant light. These results indicate that exposure to constant light for 2 circadian cycles differentially affects phase resetting responses to photic and nonphotic inputs to the circadian pacemaker, suggesting that potentiation of shifts to nonphotic stimuli reflect changes in a nonphotic input pathway rather than in an amplitude dimension of the circadian pacemaker.

A pilot study of nitric oxide blood levels in patients with chronic orofacial pain

BACKGROUND

Control of pain is the major goal in the management of chronic orofacial pain (COP) patients. The pathogenesis of COP is currently not well understood. Consequently, the treatment of COP may be suboptimal or even harmful. Based on independent observations, we propose that local elevated levels of nitric oxide (NO) may have a central role in the pathogenesis of COP.

HYPOTHESIS

NO level in the orofacial region of COP patients is elevated. A regional increased level of NO causes excessive vasodilatation. This hyperperfusion is manifested by hyperthermia of the overlying skin, while NO enhances nociception, aggravating orofacial pain. An alternative mechanism involving NO as a neurotransmitter at the CNS level may contribute to orofacial pain, but seems not to account for all the known clinical observations.

METHODS

Two groups of subjects were studied: 5 patients with COP and 59 control subjects. For each subject we collected blood samples for analysis of nitrite\nitrate (or NOx).

RESULTS

(1) NOx blood levels for 5 patients diagnosed with COP was 65.9 microM (SD of 10.4) verses 42.7 microM (SD of 24.2) for 59 control subjects, the difference being statistically significant, t-statistic = -2.12 (P > .05). (2) No statistical difference was found for NOx blood levels for 59 control subjects divided by gender (male vs female), with 23 female controls having NOx blood levels of 42.6 microM (SD of 25.2) and male controls having NOx blood levels of 42.8 microM (SD of 24.0), t-statistic = -0.03, P = .98.

CONCLUSION

This pilot study suggests that NO blood levels may have an association with COP. A better understanding of the mechanism of chronic orofacial pain is expected to lead to more precise diagnostic staging and management of this disorder.

The Drosophila nuclear receptor e75 contains heme and is gas responsive

Nuclear receptors are a family of transcription factors with structurally conserved ligand binding domains that regulate their activity. Despite intensive efforts to identify ligands, most nuclear receptors are still "orphans." Here, we demonstrate that the ligand binding pocket of the Drosophila nuclear receptor E75 contains a heme prosthetic group. E75 absorption spectra, resistance to denaturants, and effects of site-directed mutagenesis indicate a single, coordinately bound heme molecule. A correlation between the levels of E75 expression and the levels of available heme suggest a possible role as a heme sensor. The oxidation state of the heme iron also determines whether E75 can interact with its heterodimer partner DHR3, suggesting an additional role as a redox sensor. Further, the E75-DHR3 interaction is also regulated by the binding of NO or CO to the heme center, suggesting that E75 may also function as a diatomic gas sensor. Possible mechanisms and roles for these interactions are discussed.

HIV protein, transactivator of transcription, alters circadian rhythms through the light entrainment pathway

Patients infected with the human immunodeficiency virus (HIV), and other mammals infected with related lentiviruses, exhibit fatigue, altered sleep patterns, and abnormal circadian rhythms. A circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) temporally regulates these functions in mammals. We found that a secretary HIV transcription factor, transactivator of transcription (Tat), resets the murine circadian clock, in vitro and in vivo, at clinically relevant concentrations (EC50= 0.31 nM). This effect of Tat occurs only during the subjective night, when N-methyl-d-aspartate (NMDA) receptor [d-2-amino-5-phosphonovaleric acid (0.1 mM)] and nitric oxide synthase ( NG-nitro-l-arginine methyl ester, 0.1 mM) inhibitors block Tat-induced phase shifts. Whole cell recordings of SCN neurons within the brain slice revealed that Tat did not activate NMDA receptors directly but potentiated NMDA receptor currents through the enhancement of glutamate release. Consistent with this presynaptic mechanism, inhibitors of neurotransmission block Tat-induced phase shifts, such as tetrodotoxin (1 μM), tetanus toxin (1 μM), P/Q/N type-calcium channel blockers (1 μM ω-agatoxin IVA and 1 μM ω-conotoxin GIVA) and bafilomycin A1(1 μM). Thus the effect of Tat on the SCN may underlie lentiviral circadian rhythm dysfunction by operating as a disease-dependent modulator of light entrainment through the enhancement of excitatory neurotransmission.