Circadian regulation of Fos B is different from c-Fos in the rat suprachiasmatic nucleus

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

Functional Divergence of Mammalian TFAP2a and TFAP2b Transcription Factors for Bidirectional Sleep Control

Sleep is a conserved behavioral state. Invertebrates typically show quiet sleep, whereas in mammals, sleep consists of periods of nonrapid-eye-movement sleep (NREMS) and REM sleep (REMS). We previously found that the transcription factor AP-2 promotes sleep in Caenorhabditiselegans and Drosophila In mammals, several paralogous AP-2 transcription factors exist. Sleep-controlling genes are often conserved. However, little is known about how sleep genes evolved from controlling simpler types of sleep to govern complex mammalian sleep. Here, we studied the roles of Tfap2a and Tfap2b in sleep control in mice. Consistent with our results from C. elegans and Drosophila, the AP-2 transcription factors Tfap2a and Tfap2b also control sleep in mice. Surprisingly, however, the two AP-2 paralogs play contrary roles in sleep control. Tfap2a reduction of function causes stronger delta and theta power in both baseline and homeostasis analysis, thus indicating increased sleep quality, but did not affect sleep quantity. By contrast, Tfap2b reduction of function decreased NREM sleep time specifically during the dark phase, reduced NREMS and REMS power, and caused a weaker response to sleep deprivation. Consistent with the observed signatures of decreased sleep quality, stress resistance and memory were impaired in Tfap2b mutant animals. Also, the circadian period was slightly shortened. Taken together, AP-2 transcription factors control sleep behavior also in mice, but the role of the AP-2 genes functionally diversified to allow for a bidirectional control of sleep quality. Divergence of AP-2 transcription factors might perhaps have supported the evolution of more complex types of sleep.

Fos immunoreactivity in rat subcortical visual shell in response to illuminance changes

Immediate early gene expression has been used frequently as a marker of activity in the circadian visual system. Recent evidence suggests that the pretectum participates in orchestrating sleep and circadian responses to light. Lesions of the pretectum eliminate dark shift-induced rapid eye movement sleep triggering in albino rats, and compromise circadian phase shifts in hamsters. We hypothesized that regions of the pretectum respond to light with robust and region-specific Fos activation, similar to the suprachiasmatic nucleus and intergeniculate leaflet. We used Fos expression, the protein product of the immediate early gene c-fos, as a functional marker to measure the responses of neurons following acute lighting changes. Rats maintained on a 12:12 light-dark cycle were subjected to a shift from light-to-dark or from dark-to-light at midday (Zeitgeber time 6) or midnight (Zeitgeber time 18). Fos expression was visualized with immunocytochemistry and quantified with an automated scoring system. We found three regions in the pretectum (the olivary pretectal nucleus, posterior limitans, and a region homologous to the hamster commissural pretectal nucleus), and two regions in the lateral geniculate complex (the intergeniculate leaflet and ventral lateral geniculate nucleus) that demonstrated significant Fos activation in response to light. Furthermore, the olivary pretectal nucleus, the posterior limitans, and the ventral lateral geniculate nucleus showed preferential Fos activation after acute light onset rather than following chronic exposure to light at midday, whereas at midnight these nuclei showed Fos activation following both chronic light exposure and acute light onset. Given the extensive anatomical connections between pretectal nuclei and other nuclei in the subcortical visual shell, as well as with centers for sleep and arousal, it is highly plausible that these pretectal nuclei integrate information about changes in illuminance, and aid in the coordination of acute behavioral responses to light.

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.

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.

Sleep and wakefulness in c-fos and fos B gene knockout mice

G-protein coupled receptor (GPCR) stimulation has been implicated in the regulation of sleep. Upon stimulation of a GPCR an intracellular cascade involving second and third messengers is initiated. The latter include the fos-family of immediate early genes (IEGs). Although there is considerable evidence indicating that IEGs are expressed in response to sleep, the effects of their deletion on sleep is not known. The present study examined sleep-wakefulness in mice lacking the c-fos or fos B genes. Null c-fos mice compared to their wildtype (WT) and heterozygote (het) siblings had more wakefulness and less slow wave sleep (SWS); REM sleep was not affected. The null c-fos mice also had increased delta activity (0.3-4 Hz). In contrast, the null and heterozygote fos B mice had less REM sleep, but the time spent in SWS or wakefulness was not different from their wild-type (WT) siblings. In the null c-fos mice, the increased wakefulness and the reduction in SWS could not be due to a systemic alteration in temperature since the core temperature was similar in all mice. By demonstrating that these IEGs are involved in sleep, we suggest that the deletion of specific genes, even within a family of genes, can have a specific effect on sleep.

Neural connections of the anterior hypothalamus and agonistic behavior in golden hamsters

In male golden hamsters, offensive aggression is regulated by an interaction between arginine-vasopressin and serotonin at the level of the anterior hypothalamus. The present studies were conducted to study a neural network underlying this interaction. The connections of the anterior hypothalamus were examined by retrograde and anterograde tracing in adult male hamsters. Several limbic areas were found to contain both types of tracing suggesting reciprocal connections with the anterior hypothalamus. Their functional significance relating to the consummation of aggression was tested by comparing neuronal activity (examined through quantification of c-Fos-immunolabeling) in two groups of animals. Experimental animals were sacrificed after attacking an intruder. Control animals were sacrificed after exposure to a woodblock carrying the odor of an intruder that elicited behaviors related to offensive aggression without its consummation. An increased density of Fos-immunoreactivity was found in experimental animals within the medial amygdaloid nucleus, ventrolateral hypothalamus, bed nucleus of the stria terminalis and dorsolateral part of the midbrain central gray. These data suggest that these areas are integrated in a neural network centered on the anterior hypothalamus and involved in the consummation of offensive aggression. Finally, c-Fos-immunoreactivity was combined with labeling of serotonin and vasopressin neurons to identify sub-populations particularly associated with offensive aggression. Vasopressin neurons in the nucleus circularis and medial division of the supraoptic nucleus showed increased neuronal activity in the fighters, supporting their role in the control of offensive aggression.

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.

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.

Differential effects of glutamatergic blockade on circadian and photic regulation of gene expression in the hamster suprachiasmatic nucleus

Nocturnal light exposure induces immediate-early gene (IEG) expression in the hypothalamic suprachiasmatic nucleus (SCN) and causes phase shifts of activity rhythms in mammals. Some IEGs also show a circadian rhythm of expression in the SCN. While excitatory amino acids (EAAs) are known to be involved in mediating photic regulation of entrainment and gene expression, their involvement in spontaneous rhythms of gene expression has not been studied. We assessed the role of NMDA receptors in the expression of NGFI-A, junB and fosB mRNAs induced by light pulses of different intensities late in the night (Zeitgeber Time [ZT] 18). We also examined the spontaneous expression of junB mRNA near subjective dawn (ZT 0). Both dim (5 lx) and bright (100 lx) light pulses induced similar levels of expression of NGFI-A and junB in the SCN late in the night. fosB mRNA was strongly induced by bright light but was less sensitive to dim light. At ZT 18, dizocilpine (MK-801) (3 mg/kg, i.p. ), a non-competitive NMDA receptor antagonist, almost completely blocked light-evoked expression of IEG mRNAs in the ventral SCN but not in the dorsolateral region at a mid-caudal level using either light intensity. At ZT 0, MK-801 strongly reduced light-evoked expression of junB mRNA in both SCN subdivisions, but inhibited spontaneous expression significantly only in the dorsal region. NMDA receptors appear to play an important role in mediating photic input regulating IEG expression only in the ventral SCN at night. At dawn, however, NMDA receptors are involved in mediating photic effects in both parts of the SCN, as well as being involved in spontaneous activation of junB expression selectively in the dorsal SCN. These findings support the idea that the effects in the dorsolateral SCN of nocturnal light exposure are mediated by different mechanisms than those in other portions of the nucleus.

Comparison of the expression of two immediate early gene proteins, FosB and Fos in the rat preoptic area, hypothalamus and brainstem during pregnancy, parturition and lactation

Medial preoptic area (MPA), supraoptic nucleus (SON), magnocellular (MaPVN) and parvocellular (PaPVN) paraventricular hypothalamic nuclei, and mesencephalic lateral tegmentum (MLT) are involved in maternal behavior, parturition and lactation. This study investigated the FosB and Fos immunoreactivity in these regions of virgin, pregnant, parturient, lactating, and lactating-arrested rats. The patterns of FosB and Fos expression were compared between the sections taken from the same animals. Quantitative immunohistochemistry revealed a significant increase in the numbers of FosB-positive neurons in the MPA, SON, MaPVN, and MLT of parturient and lactating females as compared with pregnant or virgin animals. In lactating rats, the numbers of FosB-positive neurons in the MPA, PaPVN, and MLT were increased, but the numbers in the SON and MaPVN were decreased as compared with parturient females. Many Fos-positive neurons were also seen in parturient and lactating rats, and the patterns of Fos expression in each region were quite similar to those of FosB. Moreover, double-labeling immunohistochemistry revealed that: (1) many FosB-positive nuclei were observed in oxytocin and vasopressin neurons of the SON and PVN in parturient rats; (2) within FosB-positive neurons, 89.5% in the MPA, 86.8% in the MLT of parturient rats, and 92% in the MPA and 90.8% in the MLT of lactating animals were also Fos-positive. Only a small number of FosB and Fos-positive neurons were seen in females that were killed in the early stage of parturition. Removal of the litters immediately after parturition completely eliminated FosB and Fos expression in each region in the dams. Taken together, the present results suggest that FosB expression is co-involved with Fos in the neural activation during parturition and lactation in rats.

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.

The induction of Fos-like proteins in the suprachiasmatic nuclei and intergeniculate leaflet by light pulses in degus (Octodon degus) and rats

In nocturnal rodents, exposure to light results in an increase in Fos expression in two regions that receive direct retinal input: the suprachiasmatic nuclei (SCN) of the hypothalamus and the intergeniculate leaflet (IGL) of the thalamus. The induction of Fos within the SCN of nocturnal rodents is phase dependent, with light presented during the subjective night increasing Fos expression and light presented during the subjective day having little effect. By contrast, Fos expression increases in the IGL when light is presented during the subjective day or night. It is unclear whether Fos is part of the pathway mediating light-induced phase shifts in diurnal rodents. In the present study, the ability of light to induce immunostaining for Fos in the SCN and IGL was compared in diurnal rodents, Octodon degus (degus), and nocturnal rats. Degus and rats were either maintained in constant darkness or exposed to a 1-h light pulse at circadian time (CT) 4 or 16. Degus exhibit robust phase shifts at each of those circadian hours, whereas rats demonstrate phase shifts only at CT 16. In degus, exposure to a 1-h light pulse at CT 16 resulted in an increase in the number of Fos-immunopositive (Fos+) cells in the ventrolateral SCN. By contrast, a 1-h light pulse at CT 4 resulted in a decrease in the number of Fos+ cells in the dorsomedial portion of the SCN. In rats, a light pulse presented at CT 16 resulted in an increase in Fos+ cells throughout the SCN, and a pulse at CT 4 had no effect on Fos staining. Both degus and rats showed increases in Fos expression in the IGL after light exposure at CTs 4 and 16. The authors conclude that light pulses presented at times that produce phase shifts in activity rhythms also alter Fos expression in the SCN and IGL of degus. Although these effects of light exposure on Fos expression are not identical in diurnal and nocturnal rodents, it is likely that Fos and other immediate early genes are part of the pathway mediating the effects of light in both diurnal and nocturnal rodents.

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 circadian and light-induced expression of junB mRNA in the hamster suprachiasmatic nucleus

We examined both spontaneous and light-evoked expression of junB mRNA in the hamster suprachiasmatic nucleus (SCN), an endogenous circadian pacemaker. junB was expressed in the SCN in response to a light pulse during the subjective night and early subjective day as well as spontaneously during the transition from subjective night to subjective day. Light-evoked expression was strongest in the ventral SCN, while spontaneous expression was stronger in the dorsal SCN. Spontaneous expression began around subjective dawn and persisted for at least 4 h into the subjective day. The expression of junB mRNA may play a role in both phase-shifting responses to light and in spontaneous oscillation of the SCN pacemaker.

c-Fos expression in the rat intergeniculate leaflet: photic regulation, co-localization with Fos-B, and cellular identification

Ambient light alters the level of the transcriptional regulatory protein c-Fos in the suprachiasmatic nucleus, the site of an endogenous circadian clock in mammals, and in one other retino-recipient area, the intergeniculate leaflet of the lateral geniculate complex. Complementing previous work by ourselves and others on the photic and temporal regulation of c-Fos expression in the suprachiasmatic nucleus, the present studies investigated c-Fos regulation in the rat intergeniculate leaflet, revealing some important differences between the two brain regions. In the intergeniculate leaflet, the levels of c-fos mRNA (by in situ hybridization) and immunoreactive c-Fos protein (by immunohistochemistry) were elevated by light pulses administered either during the subjective day or subjective night. The regulation of immunoreactive Fos-B protein was similar to c-Fos, and 98% of Fos-B-expressing cells were also c-Fos-positive (by double-label immunofluorescence). By combining c-Fos immunofluorescence with stereotaxic injections of the retrograde tract tracer FluoroGold, we found photically-induced c-Fos in 15% of intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus and in 34% of those projecting to the contralateral intergeniculate leaflet. Intergeniculate leaflet cells that express c-Fos after photic stimulation appear to represent a functionally-defined population that does not correspond to anatomically-defined categories based on connectivity or peptidergic phenotype.

FosB in the suprachiasmatic nucleus of the Syrian and Siberian hamster

The suprachiasmatic nucleus (SCN) generates circadian rhythms of behavior and hormone secretion in mammals, and integrates responses to light and nonphotic stimuli to synchronize such rhythms with the external environment. Previous studies have demonstrated a close association between the induction of the immediate early gene (IEG) c-fos in the SCN by light and phase shifts of circadian rhythms induced by light, but nonphotic stimuli (e.g., arousal), which also cause phase shifts, do not increase c-fos expression in the SCN. Because c-fos is now known to be a member of a large family of IEGs which can regulate transcription and thus cellular function, the aim of the current study was to determine whether induction of another member of this immediate early gene family, fosB, is associated with photic and nonphotic phase shifts. An antiserum that recognizes a unique peptide sequence derived from FosB was produced so that the expression of fosB could be investigated in cells within the SCN by immunocytochemical detection of its protein product. The regional distribution of FosB-immunoreactive (ir) cells in the SCN of Syrian and Siberian hamsters was broadly similar to that for c-Fos-ir cells. However, whereas c-fos expression in the SCN was constitutively low, but could be massively induced by light at particular circadian phases, FosB-ir cells were present at all circadian phases studied, irrespective of photic stimulation, and light only produced marginal increases in the number of FosB-ir cells compared with nonstimulated controls. Moreover, blockade of glutamatergic neurotransmission by pretreatment of hamsters with the NMDA receptor antagonist MK801 significantly reduced photic induction of c-Fos-ir cells, but did not influence the number of FosB-ir cells in the SCN. Finally, an arousing nonphotic stimulus known to cause phase advances in wheel-running behavior in Syrian hamsters did not alter significantly the number of FosB-ir cells in the SCN. These observations indicate that light and nonphotic stimuli are not potent regulators of fosB expression in the SCN. However, because fosB and c-fos can be present in the SCN at the same time after a light pulse, these studies indicate the potential for interactions with each other and with members of the Jun family in the regulation of the circadian timing system.

Changes in circadian rhythms and sleep quality with aging: mechanisms and interventions

Literature is reviewed indicating that aging is characterized by changes in circadian rhythms and sleep quality. The most marked change is an attenuation of amplitude. An advance of phase, a shortening of period, and a desynchronization of rhythms are also evident. The mechanisms underlying these changes are unknown. However, age-related changes in the retina, suprachiasmatic nucleus, and pineal gland seem relevant along with behavioral changes such as a reduction in physical activity and exposure to photic stimulation. Changes in circadian rhythms are frequently associated with a reduction in nighttime sleep quality, a decrease in daytime alertness, and an attenuation in cognitive performance; reversing such changes could enhance the quality of life for a large and rapidly increasing percentage of the population. Reversal appears possible by increasing melatonin levels with either appropriately timed exposure to photic stimulation and/or appropriately timed administration of exogenous melatonin. These interventions may increase aspects of genetic expression that have changed with aging. A hypothesis concerning the potential benefits of enhanced circadian amplitude is also offered.