Compositional changes of AP-1 DNA-binding proteins are regulated by light in a mammalian circadian clock

Remodeling of the cycling transcriptome of the oyster Crassostrea gigas by the harmful algae Alexandrium minutum

As a marine organism, the oyster Crassostrea gigas inhabits a complex biotope governed by interactions between the moon and the sun cycles. We used next-generation sequencing to investigate temporal regulation of oysters under light/dark entrainment and the impact of harmful algal exposure. We found that ≈6% of the gills' transcriptome exhibits circadian expression, characterized by a nocturnal and bimodal pattern. Surprisingly, a higher number of ultradian transcripts were also detected under solely circadian entrainment. The results showed that a bloom of Alexandrium minutum generated a remodeling of the bivalve's temporal structure, characterized by a loss of oscillations, a genesis of de novo oscillating transcripts, and a switch in the period of oscillations. These findings provide unprecedented insights into the diurnal landscape of the oyster's transcriptome and pleiotropic remodeling due to toxic algae exposure, revealing the intrinsic plasticity of the cycling transcriptome in oysters.

REVIEW ■ : Jun, Fos, and CREB/ATF Transcription Factors in the Brain: Control of Gene Expression under Normal and Pathophysiological Conditions

The expression and activation of transcription factors and the control of gene transcription in the nervous system is a recent and rapidly expanding field in neurosciences. This research area may provide insights concerning the information transfer that arises from postsynaptic potentials or ligand-coupling of membrane receptors and terminates in gene expression. Visualization of both de novo synthesis of inducible transcription factors (ITFs) and phosphorylation of preexisting transcription factors have been used to mark neurons, pathways, and networks excited by various stimuli. This article summarizes basics of the transcription process and the complex functions of Jun, Fos, and CREB/ATF proteins, as well as the use of ITFs as experimental instruments in neurophysiology and neurobiology. The major focus is on the alterations in ITF expression following acute or chronic pathophysiological stimuli as mirrors of alterations in neuronal programs underlying adaptation, dysfunctions, or the development of diseases affecting the nervous system. NEUROSCIENTIST 2:153-161, 1996

Neural plasticity and memory: molecular mechanism

Deciphering the cellular and molecular mechanisms of memory has been an important topic encompassing the learning and memory domain besides the neurodegenerative disorders. Synapses accumulate cognitive information from life-lasting alterations of their molecular and structural composition. Current memory storage models identify posttranslational modification imperative for short-term information storage and mRNA translation for long-term information storage. However, the precise account of these modifications has not been summarized at the individual synapse level. Therefore, herein we describe the spatiotemporal reorganization of synaptic plasticity at the dendritic spine level to elucidate the mechanism through which synaptic substructures are remodeled; though at the molecular level, such mechanisms are still quite unclear. It has thus been concluded that the existing mechanisms do not entirely elaborate memory storage processes. Further efforts are therefore encouraged to delineate the mechanism of neuronal connectivity at the chemical level as well, including inter- or intramolecular bonding patterns at the synaptic level, which may be a permissive and vital step of memory storage.

KAYAK-α modulates circadian transcriptional feedback loops in Drosophila pacemaker neurons

Circadian rhythms are generated by well-conserved interlocked transcriptional feedback loops in animals. In Drosophila, the dimeric transcription factor CLOCK/CYCLE (CLK/CYC) promotes period (per), timeless (tim), vrille (vri), and PAR-domain protein 1 (Pdp1) transcription. PER and TIM negatively feed back on CLK/CYC transcriptional activity, whereas VRI and PDP1 negatively and positively regulate Clk transcription, respectively. Here, we show that the α isoform of the Drosophila FOS homolog KAYAK (KAY) is required for normal circadian behavior. KAY-α downregulation in circadian pacemaker neurons increases period length by 1.5 h. This behavioral phenotype is correlated with decreased expression of several circadian proteins. The strongest effects are on CLK and the neuropeptide PIGMENT DISPERSING FACTOR, which are both under VRI and PDP1 control. Consistently, KAY-α can bind to VRI and inhibit its interaction with the Clk promoter. Interestingly, KAY-α can also repress CLK activity. Hence, in flies with low KAY-α levels, CLK derepression would partially compensate for increased VRI repression, thus attenuating the consequences of KAY-α downregulation on CLK targets. We propose that the double role of KAY-α in the two transcriptional loops controlling Drosophila circadian behavior brings precision and stability to their oscillations.

The circadian clock in cancer development and therapy

Most aspects of mammalian function display circadian rhythms driven by an endogenous clock. The circadian clock is operated by genes and comprises a central clock in the brain that responds to environmental cues and controls subordinate clocks in peripheral tissues via circadian output pathways. The central and peripheral clocks coordinately generate rhythmic gene expression in a tissue-specific manner in vivo to couple diverse physiological and behavioral processes to periodic changes in the environment. However, with the industrialization of the world, activities that disrupt endogenous homeostasis with external circadian cues have increased. This change in lifestyle has been linked to an increased risk of diseases in all aspects of human health, including cancer. Studies in humans and animal models have revealed that cancer development in vivo is closely associated with the loss of circadian homeostasis in energy balance, immune function, and aging, which are supported by cellular functions important for tumor suppression including cell proliferation, senescence, metabolism, and DNA damage response. The clock controls these cellular functions both locally in cells of peripheral tissues and at the organismal level via extracellular signaling. Thus, the hierarchical mammalian circadian clock provides a unique system to study carcinogenesis as a deregulated physiological process in vivo. The asynchrony between host and malignant tissues in cell proliferation and metabolism also provides new and exciting options for novel anticancer therapies.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Role of signal transducer and activator of transcription-3 in estradiol-mediated neuroprotection

Estradiol is protective in experimental cerebral ischemia, but the precise mechanisms remain unknown. Signal transducer and activator of transcription-3 (STAT3) is a transcription factor that is activated by estrogen, translocates to the nucleus, and induces the transcription of neuroprotective genes, such as bcl-2. We determined whether estradiol increases STAT3 activation in female rat brain after focal cerebral ischemia and whether STAT3 activation contributes to estradiol-mediated neuroprotection against ischemic brain injury. Ovariectomized (OVX) female rats with and without estradiol replacement were subjected to 2 h of middle cerebral artery occlusion (MCAO), and phosphorylated STAT3 (P-STAT3) and total STAT3 (T-STAT3) were quantified by Western blot analysis at 3 and 22 h of reperfusion. STAT3 activation was colocalized with neuronal and survival markers microtubule-associated protein 2 (MAP2) and Bcl-2 using immunohistochemistry. Infarct size was measured at 22 h after MCAO in estradiol-treated OVX animals in the presence and absence of STAT3 inhibitor cucurbitacin I (JSI-124) using 2,3,5-triphenyltetrazolium chloride staining. Estradiol increased P-STAT3 in the ischemic cortex cytosolic fraction at 3 h after MCAO without affecting T-STAT3. This was associated with increased P-STAT3 in the nuclear fraction, which remained elevated at 22 h after MCAO. The nuclear P-STAT3 colocalized with MAP2 and Bcl-2 within the peri-infarct zone. The P-STAT3 inhibitor JSI-124 abolished the protective effect of estradiol without affecting infarct size in untreated OVX rats. We conclude that estradiol increases STAT3 phosphorylation in neurons after MCAO and that STAT3 activation plays an important role in estradiol-mediated neuroprotection.

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.

The circadian clock: pacemaker and tumour suppressor

The circadian rhythms are daily oscillations in various biological processes that are regulated by an endogenous clock. Disruption of these rhythms has been associated with cancer in humans. One of the cellular processes that is regulated by circadian rhythm is cell proliferation, which often shows asynchrony between normal and malignant tissues. This asynchrony highlights the importance of the circadian clock in tumour suppression in vivo and is one of the theoretical foundations for cancer chronotherapy. Investigation of the mechanisms by which the circadian clock controls cell proliferation and other cellular functions might lead to new therapeutic targets.

Sleep deprivation-induced c-fos and junB expression in the rat brain: effects of duration and timing

Expression of the immediate-early genes (IEGs) c-fos and junB in the rat brain was studied in response to sleep deprivation (SD) starting at four time points during the light phase of a 12:12 light:dark cycle. Animals were confined to slowly rotating wheels for 3 or 6 h in order to prevent sleep. The numbers of c-Fos- and JunB-immunoreactive cells were assessed in seven brain regions previously reported to respond to SD with increased c-fos expression (medial preoptic area (MPA), cortex, anterior and posterior paraventricular thalamic nuclei, amygdala, caudate-putamen, and laterodorsal tegmental nucleus). While c-Fos was induced by SD in all regions studied, there were differences in levels of induction depending on the duration of deprivation and on the timing of the deprivation period during the light phase. The most robust induction occurred in most regions in response to 3-h deprivation periods beginning 3 h into the light phase. A similarly timed peak of induction was observed in the MPA and cortex after 6 h of SD. In two regions, the posterior paraventricular thalamic nucleus and amygdala, 6 h of deprivation induced greater c-Fos immunoreactivity than did 3 h of deprivation, collapsed across all phases tested. Increased JunB immunoreactivity in response to either duration of deprivation was more limited and was significant only in the MPA, cortex, caudate-putamen and amygdala. c-Fos and JunB immunoreactivity in the paraventricular hypothalamic nucleus was low and similar in control and deprived animals. These results indicate that both duration of prior wakefulness and time of day influence the extent of IEG expression differentially in brain regions responsive to SD. The results also suggest that the posterior paraventricular thalamic nucleus and amygdala might be primarily responsive to length of wakefulness (sleep drive), while the MPA and anterior paraventricular thalamic nucleus might integrate input related to both homeostatic sleep drive and circadian clock influences on sleep regulation.

Is light-regulated AP-1 binding in the rat suprachiasmatic nucleus gated by the circadian clock?

In mammals, photic entrainment of circadian rhythms likely involves light- and clock-dependent expression of immediate early genes, including fos-like and jun-like genes, in the rat suprachiasmatic nucleus. Using an electrophoretic mobility shift assay, we evaluated whether the photic regulation of DNA-binding activity and composition of activating protein-1 (AP-1) complexes in the suprachiasmatic nucleus is also dependent on circadian phase. Phase-dependent light inducibility in the expression of fra-2 and c-fos genes and in immunoreactive Fra-2 and c-Fos protein expression was also evaluated, by in situ hybridization and immunocytochemistry. Light's effects on AP-1 DNA-binding differed both qualitatively and quantitatively according to the circadian phase at which light was applied. This phase dependence accounted for by both compartmentalization of proteins involved in constitutive AP-1 complexes within the nucleus or cytoplasm and control of the extent to which the expression of specific complexes was induced. It was then shown that the mechanisms by which the circadian clock gates the photic induction of AP-1 components differed according to the nature of the protein.

The Mouse Clock Locus: Sequence and Comparative Analysis of 204 Kb from Mouse Chromosome 5

The Clock gene encodes a basic helix-loop-helix (bHLH)–PAS transcription factor that regulates circadian rhythms in mice. We previously cloned Clock in mouse and human using a battery of behavioral and molecular techniques, including shotgun sequencing of two bacterial artificial chromosome (BAC) clones. Here we report the finished sequence of a 204-kb region from mouse chromosome 5. This region contains the complete loci for the Clock andTpardl (pFT27) genes, as well as the 3′ partial locus of the Neuromedin U gene; sequence analysis also suggests the presence of two previously unidentified genes. In addition, we provide a comparative genomic sequence analysis with the syntenic region from human chromosome 4. Finally, a new BAC transgenic line indicates that the genomic region that is sufficient for rescue of the Clock mutant phenotype is no greater than 120 kb and tightly flanks the 3′ end of the Clockgene.

[The sequence data reported in this paper have been submitted to the GenBank data library under accession no. AF146793.]

The mouse Clock locus: sequence and comparative analysis of 204 kb from mouse chromosome 5

The Clock gene encodes a basic helix-loop-helix (bHLH)-PAS transcription factor that regulates circadian rhythms in mice. We previously cloned Clock in mouse and human using a battery of behavioral and molecular techniques, including shotgun sequencing of two bacterial artificial chromosome (BAC) clones. Here we report the finished sequence of a 204-kb region from mouse chromosome 5. This region contains the complete loci for the Clock and Tpardl (pFT27) genes, as well as the 3' partial locus of the Neuromedin U gene; sequence analysis also suggests the presence of two previously unidentified genes. In addition, we provide a comparative genomic sequence analysis with the syntenic region from human chromosome 4. Finally, a new BAC transgenic line indicates that the genomic region that is sufficient for rescue of the Clock mutant phenotype is no greater than 120 kb and tightly flanks the 3' end of the Clock gene.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.

Differential regulation of fos family genes in the ventrolateral and dorsomedial subdivisions of the rat suprachiasmatic nucleus

Extensive studies have established that light regulates c-fos gene expression in the suprachiasmatic nucleus, the site of an endogenous circadian clock, but relatively little is known about the expression of genes structurally related to c-fos, including fra-1, fra-2 and fosB. We analysed the photic and temporal regulation of these genes at the messenger RNA and immunoreactive protein levels in rat suprachiasmatic nucleus, and we found different expression patterns after photic stimulation and depending on location in the ventrolateral or dorsomedial subdivisions. In the ventrolateral suprachiasmatic nucleus, c-fos, fra-2 and fosB expression was stimulated after a subjective-night (but not subjective-day) light pulse. Expression of the fra-2 gene was prolonged following photic stimulation, with elevated messenger RNA and protein levels that appeared unchanged for at least a few hours beyond the c-fos peak. Unlike c-fos and fra-2, the fosB gene appeared to be expressed constitutively in the ventrolateral suprachiasmatic nucleus throughout the circadian cycle; immunohistochemical analysis suggested that delta FosB was the protein product accounting for this constitutive expression, while FosB was induced by the subjective-night light pulse. In the dorsomedial suprachiasmatic nucleus, c-fos and fra-2 expression exhibited an endogenous circadian rhythm, with higher levels during the early subjective day, although the relative abundance was much lower than that measured after light pulses in the ventrolateral suprachiasmatic nucleus. Double-label immunohistochemistry suggested that some of the dorsomedial cells responsible for the circadian expression of c-Fos also synthesized arginine vasopressin. No evidence of suprachiasmatic nucleus fra-1 expression was found. In summary, fos family genes exhibit differences in their specific expression patterns in the suprachiasmatic nucleus, including their photic and circadian regulation in separate cell populations in the ventrolateral and dorsomedial subdivisions. The data, in combination with our previous results [Takeuchi J. et al. (1993) Neuron 11, 825-836], suggest that activator protein-1 binding sites on ventrolateral suprachiasmatic nucleus target genes are constitutively occupied by DeltaFosB/JunD complexes, and that c-Fos, Fra-2, FosB and JunB compete for binding after photic stimulation. The differential regulation of fos family genes in the ventrolateral and dorsomedial suprachiasmatic nucleus suggests that their circadian function(s) and downstream target(s) are likely to be cell specific.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Expression profiles of JunB and c-Fos proteins in the rat circadian system

The immediate-early genes c-Fos and JunB are implicated in light signaling within the suprachiasmatic nucleus (SCN), the mammalian circadian clock. Light induces phase-dependent expression of c-Fos and JunB within the retinorecipient SCN. In the absence of light, rhythmic expression of these genes has been observed in the dorsal region of the SCN with peak expression observed near dawn. The present study examined the pattern of induction of c-Fos and JunB in the SCN and intergeniculate leaflet (IGL) of rats housed in constant conditions, under light-dark cycles, or in dark-adapted light-stimulated animals. In contrast with previous studies, no evidence of expression of c-Fos and JunB was observed within the dorsal SCN, regardless of circadian time. Strain differences could account for the absence of rhythmic JunB expression, whereas the use of a monoclonal anti-c-Fos antibody in the present study may account for the absence of dorsal SCN c-Fos staining. The profile of light-induced c-Fos and JunB expression was consistent with previous observations. In the SCN, light-induced expression of c-Fos and JunB was phase dependent, whereas in the IGL light-induced both c-Fos and JunB regardless of circadian time.

Immediate early gene expression within the visual system: light and circadian regulation in the retina and the suprachiasmatic nucleus

Immediate early genes are a family of genes that share the characteristic of having their expression rapidly and transiently induced upon stimulation of neuronal and non-neuronal cells. In this review, first a short description of the IEGs is given, then it is discussed the stimulus-induced and circadian-induced variations in the expression of IEGs in the visual system, mainly in the retina and the suprachiasmatic nucleus. The possible physiological consequences of these variations in IEG expression are also considered. Finally, we refer to two aspects of our recent studies and those of other laboratories involving light-driven IEG expression. The first is the finding that in the chick retina, the expression of c-fos is differentially modulated in the different cell types and that c-fos regulates the synthesis of the quantitatively most important lipids of all cells, the phospholipids, by a non-genomic mechanism. The second is the occurrence of differential waves of IEG expression in the mammalian suprachiasmatic nucleus regarding light induction or spontaneous oscillations.

Daily rhythm of spontaneous immediate-early gene expression in the rat suprachiasmatic nucleus

Nocturnal light induces the expression of various immediate-early genes (IEGs) in the suprachiasmatic nucleus (SCN), the primary pacemaker of the circadian system of mammals, and causes phase shifts of behavioral rhythms. In the hamster SCN, some IEGs show both sensitivity to light induction at night and a daily peak of spontaneous expression near dawn in different regions of the nucleus. To investigate whether both patterns of IEG expression are observed in the rat SCN, the authors studied the expression of NGFI-A, junB, c-fos, and fosB near the time of subjective dawn in rats entrained to a light-dark 12:12 cycle and then maintained in constant total darkness for approximately 48 h. They found that there were two independent rhythms of expression for junB and c-fos mRNAs in the SCN: (1) a rhythm of photic sensitivity expressed throughout the night and (2) a spontaneous rhythm of expression triggered around dawn and persisting for at least 2 h into the day. By contrast, fosB and NGFI-A transcripts were expressed only after light exposure at night and did not exhibit significant levels of spontaneous expression in the absence of photic input. These observations demonstrate that the circadian clock gates expression of two independent rhythms related to IEG expression in the rat SCN. The rhythm of sensitivity to nocturnal light exposure is expressed more strongly in the ventral SCN and may be related to photic entrainment. The second rhythm is triggered spontaneously in darkness around subjective dawn and is expressed in more dorsal parts of the SCN.

The expression of the melatonin synthesis enzyme: arylalkylamine N-acetyltransferase in the suprachiasmatic nucleus of rat brain

The hormone melatonin, secreted primarily from the pineal gland, plays an important physiological role in synchronizing biological rhythms and neuroendocrine. Presently, we find the expression of the serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT) mRNA, the rate-limiting enzyme in the conversion of serotonin to melatonin, in the rat suprachiasmatic nucleus (SCN) which contains the biological circadian clock in mammals. AA-NAT mRNA content in rat SCN did not show a significant circadian rhythm. However, AA-NAT enzyme activity was lowest at midday and highest at early night, and the rhythm persisted under constant dark conditions. These results indicate that the rat SCN is capable of synthesizing melatonin and suggest that melatonin synthesis in the SCN may be regulated by the circadian clock at the post transcriptional level.

Circadian and photic regulation of immediate-early gene expression in the hamster suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus is the site of an endogenous circadian clock synchronized by daily light-dark cycles. At some daily phases, light exposure both shifts the clock and alters the expression of several immediate-early genes in cells of the suprachiasmatic nucleus. We have studied both spontaneous circadian and light-induced expression of several immediate-early gene messenger RNAs and proteins in hamsters in constant darkness or in response to brief light exposure. There was no detectable spontaneous expression of NGFI-A messenger RNA in suprachiasmatic nucleus cells at any circadian phase, but light pulses induced its expression selectively during the subjective night, with highest levels of expression 6 h into the night. We also found that there are two independent rhythms of expression of junB messenger RNA and JunB protein, as well as c-fos messenger RNA and c-Fos protein, in the suprachiasmatic nucleus of hamsters: a rhythm of photic sensitivity expressed throughout the night and a spontaneous rhythm of expression triggered around dawn. Induction of NGFI-A messenger RNA and c-fos messenger RNA and c-Fos protein in response to a light pulse were found throughout the suprachiasmatic nucleus, with the highest levels of expression in the ventrolateral subdivision; however, the spontaneous expression of JunB and c-Fos proteins was confined mainly to the dorsomedial suprachiasmatic nucleus. The temporal and anatomical differences in the expression of these immediate-early genes in the mammalian suprachiasmatic nucleus suggest that their protein products may be involved in different signaling mechanisms mediating either photic entrainment or endogenous oscillations within distinct subpopulations of suprachiasmatic nucleus cells.

Defensive conditioning-related increase in AP-1 transcription factor in the rat cortex

In the studies reported herein, electrophoretic mobility shift assay (EMSA) and immunocytochemistry have been applied to document increased levels of AP-1 transcription factor, and its major component, c-Fos in the rat brain following behavioral training of two-way active avoidance. A single training session (50 trials) provoked elevation of AP-1 in the visual, sensory and limbic cortex but not in the hippocampus. A session following long term training (10 sessions, up to asymptotic level of performance) had much smaller effect on AP-1 levels in the visual cortex than single training session. The long term training was used to ensure that observed effects were related to acquisition of the reaction rather than simply to behavioral performance. Supershift EMSA analysis with antibodies directed at individual AP-1 components revealed that AP-1 extracted from the brains of trained as well as naive animals is composed of the same proteins, i.e., in order of relative level within the protein family: c-Fos, Fos B, Fra-2, and Jun D, Jun B, c-Jun. These studies reinforce the notion that transcription factors as regulators of gene expression-and AP-1 in particular-may respond to behavioral stimulation and furthermore may play a role in acquisition of behavioral reactions.

Developmental changes in nicotinic receptor mRNAs and responses to nicotine in the suprachiasmatic nucleus and other brain regions

Our previous studies demonstrated that nicotine induces c-fos expression in the suprachiasmatic nucleus (SCN) of the rat during a narrow developmental window occurring in the perinatal period. We have extended these observations by showing that c-fos cannot be induced in the adult SCN by nicotine even during the subjective night, when phase shifts do occur. In contrast to the SCN, significant induction of c-fos and NGFI-A was observed in the medial habenula and paraventricular nucleus at all circadian times. In the fetal rat SCN we show that NGFI-A and junB are also induced by nicotine, but not c-jun. To investigate whether changes in nicotinic acetylcholine receptor (nAChR) expression in the SCN may underlie this change in sensitivity during the perinatal period, we examined nAChR mRNAs across this developmental period. By Northern analyses, alpha2, alpha3 and alpha4 subunit mRNAs are relatively abundant in the fetal SCN but decline substantially in the adult. alpha7 mRNA increases substantially while beta2 mRNA is relatively abundant throughout development. We also examine expression in the whole mouse brain beginning at embryonic day 11. Many mRNA sizes for nAChR subunits in both the rat and mouse are characterized here for the first time by Northern analyses and some show very large changes in expression across development. In particular, a small 1.4 kb alpha2-related mRNA is highly expressed during early development, perhaps indicating an important novel function for this subunit.

Photic entrainment and induction of immediate-early genes within the rat circadian system

Immediate-early genes (IEGs) are transiently expressed within the rodent circadian system in response to nocturnal light. The two most studied light-induced IEGs within this system are Fos and Jun-B. Molecular expression of these two genes within the hypothalamic suprachiasmatic nucleus (SCN) correlates with light-induced behavioral phase shifts. Previous studies of the role of Fos and Jun-B in circadian clock resetting have used light stimuli that induce strong phase shifts. However, the relationship of Fos and Jun-B expression in the SCN and light-induced phase shifts in an entrainment context is undocumented in rats. In this study, male rats for which the free running period was determined were entrained to a 0.5 h:23.5 h LD cycle. On the fifteenth day of stable entrainment, the entraining light pulse was reduced to 10 min. Animals were killed 50 min later and brains were processed for IEG immunocytochemistry. Strong Fos induction was observed in the SCN and the intergeniculate leaflet (IGL). Strong Jun-B immunoreactivity was observed only in the SCN whereas Jun-B labeling in the IGL was weak. Significant correlations were obtained between the magnitude of light-induced IEGs in the SCN and the magnitude of the daily phase shift required for stable entrainment to the 0.5 h:23.5 h LD cycle. Further, a significant correlation was observed between the number of Fos and Jun-B immunoreactive cells in the SCN and IGL. These data suggest that the magnitude of Fos and Jun-B induction within the SCN is related to the magnitude of the daily phase shift required for stable entrainment.

Diurnal regulation of a DNA binding protein to the period repeat sequence in the SCN nuclear extract of rat brain

We previously reported the mammalian period repeat mRNA fluctuates during circadian time in the rat suprachiasmatic nucleus (SCN) which is considered to be a clock pacemaker in mammalian brain. Presently we discovered a period repeat sequence (PR) DNA-binding protein in the rat SCN nuclear extract. In the SCN, the binding activity of PR DNA-binding protein to (ACAGGC)3 was most highest during the late day and most lowest during the late night by electro-mobility shift assay (EMSA). In the cortex nuclear extract, the binding of PR DNA-binding protein did not show a significant variation during a day. This is the first report to show the existence of diurnal regulated PR DNA-binding protein in the SCN.

Inhibition of light- or glutamate-induced mPer1 expression represses the phase shifts into the mouse circadian locomotor and suprachiasmatic firing rhythms

mPer1, a mouse gene, is a homolog of the Drosophila clock gene period and has been shown to be closely associated with the light-induced resetting of a mammalian circadian clock. To investigate whether the rapid induction of mPer1 after light exposure is necessary for light-induced phase shifting, we injected an antisense phosphotioate oligonucleotide (ODN) to mPer1 mRNA into the cerebral ventricle. Light-induced phase delay of locomotor activity at CT16 was significantly inhibited when the mice were pretreated with mPer1 antisense ODN 1 hr before light exposure. mPer1 sense ODN or random ODN treatment had little effect on phase delay induced by light pulses. In addition, glutamate-induced phase delay of suprachiasmatic nucleus (SCN) firing rhythm was attenuated by pretreatment with mPer1 antisense ODN, but not by random ODN. The present results demonstrate that induction of mPer1 mRNA is required for light- or glutamate-induced phase shifting, suggesting that the acute induction of mPer1 mRNA in the SCN after light exposure is involved in light-induced phase shifting of the overt rhythm.

Rapid resetting of the mammalian circadian clock

The suprachiasmatic nuclei (SCN) contain the principal circadian clock governing overt daily rhythms of physiology and behavior. The endogenous circadian cycle is entrained to the light/dark via direct glutamatergic retinal afferents to the SCN. To understand the molecular basis of entrainment, it is first necessary to define how rapidly the clock is reset by a light pulse. We used a two-pulse paradigm, in combination with cellular and behavioral analyses of SCN function, to explore the speed of resetting of the circadian oscillator in Syrian hamster and mouse. Analysis of c-fos induction and cAMP response element-binding protein phosphorylation in the retinorecipient SCN demonstrated that the SCN are able to resolve and respond to light pulses presented 1 or 2 hr apart. Analysis of the phase shifts of the circadian wheel-running activity rhythm of hamsters presented with single or double pulses demonstrated that resetting of the oscillator occurred within 2 hr. This was the case for both delaying and advancing phase shifts. Examination of delaying shifts in the mouse showed resetting within 2 hr and in addition showed that resetting is not completed within 1 hr of a light pulse. These results establish the temporal window within which to define the primary molecular mechanisms of circadian resetting in the mammal.

Light-induced variations in AP-1 binding activity and composition in the rat suprachiasmatic nucleus

Expression of immediate early genes, including fos-like and jun-like genes, in the suprachiasmatic nucleus is believed to be part of the mechanism for photic entrainment of circadian rhythms to the environmental light/dark cycle. However, the effects of a light stimulus on activating protein-1 (AP-1) complexes in the suprachiasmatic nucleus remain unclear. The photic regulation of AP-1 DNA-binding activity and composition in the rat suprachiasmatic nucleus was evaluated by using an electrophoretic mobility shift assay. A light pulse given during subjective night induced an increase in AP-1 binding activity when either nuclear or whole-cell extracts from suprachiasmatic nuclei were used. Under constant dark conditions, proteins that are predominant components of AP-1 complexes are Fra-2 and Jun-D. Under light stimulation, c-Fos and Jun-B consistently increased, as expected, but this was also the case for Fra-2, Jun-D, and c-Jun, although to a lesser extent. An immunocytochemical study of the Fra-2 expression pattern demonstrated the presence of the protein in the ventrolateral as well as in the dorsomedial subdivisions of the suprachiasmatic nucleus. Light regulation of Fra-2 immunoreactivity, however, appeared to be restricted to the ventrolateral subdivision. It is concluded that light may be acting both by increasing constitutive AP-1 complexes and by inducing the expression of specific complexes.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Expression of Fos in the circadian system following nonphotic stimulation

Syrian hamsters, Mesocricetus auratus, were confined to novel running wheels for a 3-h period, starting at approximately circadian time (CT) 4.5 (i.e., approaching the middle of their subjective day). It can be reliably predicted from the amount of running in this situation whether or not there will be a subsequent phase-shift. Expression of the immediate early genes c-fos and fosB was examined by immunocytochemistry in the suprachiasmatic nucleus (SCN), the intergeniculate leaflet (IGL) of the thalamus, and the medial pretectal area of hamsters that ran vigorously in the novel wheel and would have phase-shifted. c-Fos was increased, compared to levels in a control group left in their home cages, in the IGL, and the pretectum (PT), but decreased in the SCN. No significant changes in FosB were detected in any region examined. An additional experiment argued against the possibility that the changes in c-Fos could be attributed to a rapid advance of the pacemaker to a different phase in the circadian cycle. Counts of c-Fos-positive cells in the IGL were similar in animals given pulses of running starting at CT 4.5 and starting at CT 12.5-16 (i.e., in the subjective night when they would have been active anyway). Altogether the results support the view that activation of the IGL is important in nonphotic clock resetting, and raise the possibility that the PT may also be involved in nonphotic resetting. However, the results also indicate that novelty-induced running does not alter c-Fos induction in a phase-specific manner in the IGL. The inhibition of c-Fos in the SCN by nonphotic phase-shifting events contrasts with the well-known inducing effects of light pulses. These different effects might underlie some of the interactions between nonphotic and photic zeitgebers when both act together on the circadian system.

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.

Circadian behavior and plasticity of light-induced c-fos expression in SCN of tau mutant hamsters

In hamsters homozygous for the circadian clock mutation tau, the photic history dramatically affects the magnitude of light-induced circadian phase shifts. The maximum amplitude of phase shifts produced by 1-h light pulses presented at CT 14 was less than 2 h in animals that had been in DD for 2 days, whereas animals that had been kept in DD for 49 days could be shifted by more than 8 h. In this study, the authors compared the effect of previous light history on the amplitude of circadian phase shifts and on c-fos expression in the SCN of tau mutant hamsters. Although the maximum amplitude of behavioral phase shifts was drastically different between animals that had been held for either 2 or 49 days in DD, maximal fos induction was not significantly different in these two groups. However, photic thresholds for light-induced behavioral phase shifts, c-fos mRNA, and Fos immunoreactivity were closely correlated within both groups, and these thresholds were lower (more sensitive to light) after 49 than after 2 days in DD. The correlation between phase shifting and Fos induction thresholds, under conditions where both responses are dramatically altered by the previous light history, demonstrates an association between changes in circadian behavioral phase-shifting responses of tau mutant hamsters and plasticity of light-induced c-fos expression in SCN. However, because the maximum amplitudes of Fos induction and phase shifting were not correlated in animals that had been in DD for 2 days, we speculate that the level of c-fos expression does not directly determine phase shift amplitude.

Characterization of the circadian system of NGFI-A and NGFI-A/NGFI-B deficient mice

The genes NGFI-A (also known as EGR-1, zif/268, and Krox-24) and NGFI-B (nur/77) have previously been shown to be induced in the SCN of rats and hamsters by photic stimulation during the subjective night. The purpose of this study is to determine whether these genes are also induced in the SCN of mice and, if so, to characterize the circadian system of animals in which either NGFI-A or both NGFI-A and NGFI-B were eliminated by homologous recombination. In wildtype mice, NGFI-A mRNA was found to be induced in the SCN as in other rodent species. Therefore, wheel-running activity was recorded from null mutants and wildtype controls under LD 12:12 and DD conditions. Mice of all three strains appeared to entrain normally to LD 12:12 and could re-entrain to both phase advances and phase delays of the light cycle. The response of the circadian pacemaker of all three genotypes to acute light pulses appeared to be normal. The retinal innervation of the SCN in NGFI-A-/- mice and the photic induction of Fos in the SCN of both NGFI-A-/- and NGFI-A-/-/B-/- mice were indistinguishable from wildtype mice. These results indicate that induction of NGFI-A and NGFI-B is not required for photic entrainment or phase shifting of the mouse circadian system.

Examination of DNA-binding activity of neuronal transcription factors by electrophoretical mobility shift assay

Electrophoretical mobility shift assay (EMSA) is a simple, rapid, and highly sensitive technique for detection of single- or double-stranded DNA-binding proteins such as transcription factors in crude nuclear extracts (F.M. Ausubel, R. Brent, R.E. Kingston, D. D. Moore, J.G. Seidman, J.A. Smith, K. Struhl (Eds.), Current Protocols in Molecular Biology, Greene Publishing Associates and Wiley-Interscience, 1989, pp. 12.0.1-12.2.10 [1]; J. Carey, Gel Retardation. Methods Enzymol., 208 (1991) 103-117 [2]). By using this technique, it is possible to quantify the abundance, relative affinity and binding specificity of DNA-binding proteins. Since proteins which bind specifically to radiolabeled DNA probes retard the mobility of the probe during electrophoresis (it also called gel retardation assay), discrete bands correspond to the individual DNA-protein complexes. Furthermore, EMSA allows one to determine which member(s) of a certain protein family are included in the DNA-protein complex by means of specific antibodies raised against the DNA-binding protein (supershift assay).

Light regulates Homer mRNA expression in the rat suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) of the mammal is the circadian pacemaker responsible for generation of circadian rhythms. Several immediate-early genes are expressed in the SCN by light stimuli which induce phase shifts of animal activity rhythms. In the present study, we investigated whether Homer, a PDZ-like protein which is rapidly induced following synaptic activation, mRNA expression is regulated by light in rat SCN. Homer mRNA expression in the SCN of rat killed at 4 h after onset of the light and dark phases was very low. One hour light stimuli during the subjective night dramatically induced Homer mRNA expression in the ventrolateral portion of the SCN, whereas light stimuli during the subjective light phase did not. This finding implies that Homer may be involved in the photic entrainment of the circadian clock.

Resetting the biological clock: mediation of nocturnal CREB phosphorylation via light, glutamate, and nitric oxide

Synchronization between the environmental lighting cycle and the biological clock in the suprachiasmatic nucleus (SCN) is correlated with phosphorylation of the Ca2+/cAMP response element binding protein (CREB) at the transcriptional activating site Ser133. Mechanisms mediating the formation of phospho-CREB (P-CREB) and their relation to clock resetting are unknown. To address these issues, we probed the signaling pathway between light and P-CREB. Nocturnal light rapidly and transiently induced P-CREB-like immunoreactivity (P-CREB-lir) in the rat SCN. Glutamate (Glu) or nitric oxide (NO) donor administration in vitro also induced P-CREB-lir in SCN neurons only during subjective night. Clock-controlled sensitivity to phase resetting by light. Glu, and NO is similarly restricted to subjective night. The effects of NMDA and nitric oxide synthase (NOS) antagonists on Glu-mediated induction of P-CREB-lir paralleled their inhibition of phase shifting. Significantly, among neurons in which P-CREB-lir was induced by light were NADPH-diaphorase-positive neurons of the SCN's retinorecipient area. Glu treatment increased the intensity of a 43 kDa band recognized by anti-P-CREB antibodies in subjective night but not day, whereas anti-alpha CREB-lir of this band remained constant between night and day. Inhibition of NOS during Glu stimulation diminished the anti-P-CREB-lir of this 43 kDa band. Together, these data couple nocturnal light, Glu, NMDA receptor activation and NO signaling to CREB phosphorylation in the transduction of brief environmental light stimulation of the retina into molecular changes in the SCN resulting in phase resetting of the biological clock.

Daily variation of CNS gene expression in nocturnal vs. diurnal rodents and in the developing rat brain

Expression of c-fos has been shown to vary throughout the brain over the course of the 24-h day. The magnitude of these changes appear to be similar in a light:dark (LD) cycle or in constant dark (DD). To further examine whether the diurnal and circadian changes in c-fos and other immediate-early gene (IEG) expression in brain are related to waking behaviors such as locomotor activity, we conducted three experiments using Northern analysis. First, we compared IEG expression in nocturnal vs. diurnally active species. Second, we investigated IEG expression in a hibernating species during its active and inactive phases. Third, we examined the development of IEG expression in the young post-natal rat. As a comparison to results obtained in extra-SCN brain regions, we also examined IEG and vasopressin expression in the SCN itself across the circadian cycle. Animals maintained under a 12:12-h LD cycle were sacrificed in the morning (10:00-11:00 h, ZT2-ZT3) or night (22:00-23:00 h, ZT14-ZT15) or at the corresponding circadian times (CT) when kept in DD. Rats sacrificed in the morning always showed lower c-fos expression than at night in all brain areas examined while the reverse pattern was seen in squirrels under both LD and DD conditions, suggesting a direct correlation between c-fos message and activity. The cerebellum displayed the greatest magnitude change between morning and night (often reaching 10-fold). Among other IEGs examined, the expression of NGFI-A and junB are similar to c-fos, but of lesser magnitude, whereas c-jun appears to be invariant in the rat but is increased during the active phase in squirrels. During the hibernation season, squirrels have lower levels of c-fos consistent with their low levels of activity even during their euthermic interbout periods. c-fos expression in the cerebellum and rest of brain of 1-week-old rats sacrificed at ZT3 and ZT15 showed low levels at both timepoints whereas 2- and 3-week-old animals had higher levels at night as do adults. Among other IEGs, junB and NGFI-A again were similar to c-fos while c-jun and junD were more constant. Our observations support the idea of a diurnal rhythm of IEG expression in the CNS that is related to waking behaviors. Among IEGs, c-fos exhibits the greatest daily variation in expression.

Visual sensitivities of nur77 (NGFI-B) and zif268 (NGFI-A) induction in the suprachiasmatic nucleus are dissociated from c-fos induction and behavioral phase-shifting responses

Mammalian circadian rhythms are regulated by a pacemaker in the suprachiasmatic nucleus of the hypothalamus. Recent work from several laboratories has shown that light induces the IEGs, c-fos and jun-B, in the rodent suprachiasmatic nucleus. In hamsters, there is a strong correlation between circadian entrainment and the induction of c-fos and jun-B in the suprachiasmatic nucleus by light. Previous work has shown that the IEGs, nur77 and zif268, both of which encode transcription factors, are also light-inducible in the rat suprachiasmatic nucleus [Rusak, B., McNaughton, L., Robertson, H.A. and Hunt, S.P., Circadian variation in photic regulation of IEG mRNAs in rat suprachiasmatic nucleus cells, Mol. Brain Res., 14 (1992) 124-130.; Sutin, E.L. and Kilduff, T.S., Circadian and light-induced expression of IEG mRNAs in the rat suprachiasmatic nucleus, Mol. Brain Res., 15 (1992) 281-290.]. To characterize the photic-regulation of these genes in the suprachiasmatic nucleus of golden hamsters, we used in situ hybridization to measure nur77 and zif268 mRNA levels with 33P-labeled complementary RNA probes. 5-min monochromatic light pulses at CT19 induced a dramatic increase in both nur77 and zif268 mRNA levels. Peak mRNA levels occurred 45-60 min after light onset for both nur77 and zif268. In addition, the induction of both nur77 and zif268 mRNA levels was gated by the circadian pacemaker. Light pulses during subjective day (CT3 and CT9), which do not cause behavioral phase-shifts, did not significantly alter mRNA levels of either nur77 or zif268; whereas light pulses during the subjective night (CT14 and CT19), which induce phase-shifts, dramatically increased both nur77 and zif268 mRNA levels. In contrast to c-fos induction, which has a photic threshold indistinguishable from that of the behavioral phase-shifting response, nur77 and zif268 mRNA induction were found to have visual sensitivities greater than the phase-shifting response by 1-2 log units (10-100-fold). Although light and circadian phase regulate nur77 and zif268 expression in the SCN, these results demonstrate that their induction is not rate-limiting for photic entrainment of the hamster circadian system.

Sensory regulation of immediate-early gene expression in mammalian visual cortex: implications for functional mapping and neural plasticity

The expression of immediate-early genes that code for transcription factors has been extensively studied in the brain with regard to imaging functional activity. The components of the AP-1 transcription factor--in particular, c-Fos--and Zif268 have been widely used for this purpose. However, the precise details by which they are induced after synaptic stimulation remain unknown. Furthermore, the roles of these two proteins in neurons remains speculative and include such varied functions as short-term maintenance of cellular homeostasis to long-term changes that guide cortical plasticity. Current efforts at elucidating the physiological roles of AP-1 and Zif268 rely on assessing their expression in response to different conditions of sensory and pharmacological stimulation. In this review, we have examined the expression patterns of these transcription factors in the mammalian visual cortex under different conditions, with particular emphasis on the constitutive levels and how they change after visual deprivation and stimulation. A synthesis of this information offers further insight into their likely functions and the extent to which transcription factors may represent patterns of neural activity as a possible prelude to plastic events.

Regulation of cAMP response element binding protein (CREB) binding in the mammalian clock pacemaker by light but not a circadian clock

Mammalian circadian rhythms are considered to be regulated by a clock pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The molecular mechanism of entrainment and oscillation of circadian rhythm are not well understood but photic induction of immediate-early gene (IEG) expression in the SCN is thought to play a role. Here we show that under 12 h light:12 h dark (LD) condition, the cAMP response element binding protein (CREB) binding to cAMP responsive promoter element (CRE) of NMDAR1/zeta1 promoter region in the SCN is higher during the light than the dark by electro-mobility shift assay (EMSA). When animals are placed in constant dark, CREB DNA binding activity in the SCN is low and does not vary with circadian time when compared with cortex nuclear extract as a control. Most significantly, photic induction of CREB binding activity in the SCN occurs at all circadian times tested, indicating that CREB DNA binding in the SCN is not gated by the endogenous clock. These results implicate the role of CREB in photic neuronal signaling in the SCN and suggest that CREB DNA binding activities may not be regulated by a circadian clock.

Light-induced c-fos mRNA expression in the suprachiasmatic nucleus and the retina of C3H/HeN mice

Light-induced expression of c-fos mRNA was studied over a circadian period (approximately 24 h) in C3H/HeN mice maintained in constant dark. This mouse strain expresses an rd mutation (retinal degeneration) which does not affect light-induced phase shifts of circadian rhythms. c-fos mRNA expression in the retina and the suprachiasmatic nucleus (SCN) after a light pulse (300 lux) was determined by in-situ hybridization autoradiography using a 35S-labeled c-fos riboprobe. Light induced the expression of c-fos mRNA in retino-recipient areas of the SCN. This response was dependent on the circadian time (CT) and was observed only during the subjective night (CT14-CT22) and early subjective day (CT2). However, the period of photosensitivity for c-fos induction extended 1 h over the period of photosensitivity for phase shifts in circadian behavior. In the retina of C3H/HeN mice, light-induced c-fos mRNA expression was observed in a small number of cells in the ganglion cell layer (approximately 0.2%) which may represent ganglion cells projecting to the SCN. A dependence of c-fos expression with the circadian time was observed in retinal ganglion cells, suggesting that retinal photosensitivity may also be controlled by a circadian oscillator. In conclusion, we demonstrated light-induced expression of the immediate early gene c-fos mRNA in both the retina and SCN of C3H/HeN mice expressing the rd mutation.

Age-related changes in light-induced Jun-B and Jun-D expression: effects of transplantation of fetal tissue containing the suprachiasmatic nucleus

Fos and Jun mRNA and peptide exhibit a daily light-induced rhythm in the suprachiasmatic nucleus (SCN). The authors previously have reported that Fos expression in the SCN is elevated prematurely during the dark, light-induced Fos expression is attenuated in middle-aged rats, and transplantation of fetal SCN tissue into the third ventricle of rats of this age restores the daily pattern of Fos expression to that of the young. Using immunocytochemistry, the authors performed the present study to determine whether Jun-B and Jun-D expression in the SCN is altered at the same stage during aging and, if so, whether transplantation of fetal tissue containing the SCN can restore the light-induced rhythms of these two immediate early genes. All groups of rats were transcardially perfused 90 min prior to and after light onset. In young rats, light induced a robust increase in the number of Jun-B positive cells in the SCN, whereas very few cells were labeled before light onset. In middle-aged rats, the light-induced increase in the number of Jun-B positive cells was significantly attenuated. Transplantation of fetal SCN tissue into the middle-aged rats successfully restored light-induced Jun-B expression to the levels of young rats. By contrast, Jun-D exhibited a constitutively high level of expression in the SCN both before and after light onset, and light induced only a slight but significant increase. No age-related changes were detected in the expression of Jun-D either before or after light onset. Transplantation of fetal SCN tissue did not alter the daily pattern of Jun-D expression in the middle-aged rats. These data suggest that (1) light-induced activation of SCN neural activity is blunted during aging, (2) fetal SCN tissue can provide the critical support to allow the host to respond properly to light cues, and (3) the age-related change in Jun-B expression in the middle-aged host SCN can be rescued by fetal SCN transplants.

Critical period for cycloheximide blockade of light-induced phase advances of the circadian locomotor activity rhythm in golden hamsters

Recent studies indicate that the protein products of genes involved in transcriptional regulation play an important role in light-induced phase-shifting of the circadian clock, and suggest that protein synthesis may be necessary for some critical steps in the process by which light can induce a phase shift. In order to determine if this protein-dependent step occurs before, during or at some time after the light pulse is presented, golden hamsters were treated with the protein synthesis inhibitor, cycloheximide (65 mg/kg, injected subcutaneously), at various times relative to the presentation of a light pulse that normally induces an advance in the circadian locomotor activity rhythm. When hamsters were treated with cycloheximide 1 h or 30 min before, as well as 0, 1, 2, 3 or 4 h after the onset of the light pulse; the phase advancing effects of light were completely blocked and delay phase shifts were observed. The observed phase shifts in these groups of animals were not significantly different from those observed in animals treated with cycloheximide without exposure to light. Treatment with cycloheximide 6 or 3 h before the light pulse partially blocked light-induced phase advances. The phase advancing effects of light were not altered in those animals injected with cycloheximide 6 or 9 h after the light pulse. These results support the hypothesis that protein synthesis is necessary for light-induced phase advances in the mammalian circadian clock, and indicate that such synthesis is necessary for at least 4 h after the light pulse for phase shifts to occur. Thus, light may not induce an instantaneous (< 1 h) phase shift in the circadian clock of hamsters, but instead depends on the synthesis of proteins for about 4 h after light stimulation.

Expression of fosB mRNA in the hamster suprachiasmatic nucleus is induced at only selected circadian phases

We have studied the expression of fosB mRNA in the suprachiasmatic nucleus (SCN) of hamsters by in situ hybridization using oligonucleotides with sequences complementary to the C-terminal of the fosB mRNA sequence. In animals exposed for 48 h to darkness, there was little or no background expression in SCN cells of fosB mRNA at any circadian phase. Light pulses (30 min) were able to induce fosB expression only during the subjective night. Transcripts of fosB increased rapidly to peak by the end of a 30-min light pulse. Light-induced increases gradually declined in darkness, but levels were still elevated for up to 150 min after the light pulse. Induction in response to a light pulse was largely restricted to the ventrolateral portion of the nucleus which receives the heaviest retinal projection. The temporal and anatomical pattern of fosB mRNA expression in the hamster SCN therefore resembles that reported previously for other immediate-early genes, such as c-fos.

Spontaneous and light-evoked expression of JunB-like protein in the hamster suprachiasmatic nucleus near subjective dawn

Some cells in the hamster suprachiasmatic nucleus (SCN) show a circadian rhythm of expression of junB mRNA in constant darkness, while others show junB mRNA only in response to light at night. We found that both the light-induced and spontaneous expressions of junB mRNA are translated into protein in SCN cells. In constant darkness, JunB-like immunoreactivity (lir) appears spontaneously in cells in the dorsal SCN around subjective dawn and persists for at least 4 h into the subjective day. During the subjective night, there is no spontaneous expression, but a light pulse can induce JunB-lir in cells throughout the SCN, and especially in the ventrolateral portion. As a component of AP-1 proteins, JunB may play a role both in mediating circadian responses to photic stimuli and in spontaneous oscillation of elements of the SCN circadian pacemaker.

c-fos CRE-binding activity of CREB/ATF family in the SCN is regulated by light but not a circadian clock

The DNA-binding activity of cAMP-responsive element binding protein (CREB) in the suprachiasmatic nucleus (SCN) was examined with Ca or cAMP-responsive element (Ca/CRE) in upstream sequence of c-fos gene using electro-mobility shift assay (EMSA). By using supershift assay, Ca/CRE-binding activity in the SCN was shown to contain not only CREB but also activating transcription factor-1 (ATF-1). Furthermore, photic-induction of CREB binding activity to Ca/CRE in the SCN occurred at all circadian times tested, indicating that CREB DNA-binding in the SCN was not gated by the endogenous clock. These results implicated the role of CREB/ATF family in photic neuronal signaling in the SCN.