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

The Suprachiasmatic Nucleus: A Clock of Multiple Components

Although impressive progress has been made in understanding the molecular basis of pacemaker function in the suprachiasmatic nucleus (SCN), fundamental questions about cellular and regional heterogeneity within the SCN, andhowthis heterogeneity might contribute toSCNpacemaker function at a tissue level, have remained unresolved. To reexamine cellular and regional heterogeneity within the SCN, the authors have focused on two key questions: which SCN cells are endogenously rhythmic and/or directly light responsive? Observations of endogenous rhythms of electrical activity, gene/protein expression, and protein phosphorylation suggest that the SCN in mammals examined to dateis composed of anatomically distinct rhythmic and nonrhythmic components. Endogenously rhythmic neurons are primarily found in rostral, dorsomedial, and ventromedial portions of the nucleus; at mid and caudal levels, the distribution of endogenously rhythmic cells in the SCN has the appearance of a “shell.” The majority of nonrhythmic cells, by contrast, are located in a central “core” region of the SCN, which is complementary to the shell. The location of light-responsive cells, defined by direct retinohypothalamic input and light-induced gene expression, largely overlaps the location of nonrhythmic cells in the SCN core, although, in hamsters and mice light-responsive cells are also present in the ventral portion of the rhythmic shell. While the relative positions of rhythmic and light-responsive components of the SCN are similar between species, the precise boundaries of these components, and neurochemical phenotype of cells within them, are variable. Intercellular communication between these components may bea key featurer esponsiblefor theuniquepace maker properties of the SCN observed at a tissue and whole animal level.

Circadian firing-rate rhythms and light responses of rat habenular nucleus neurons in vivo and in vitro

The suprachiasmatic nuclei of the anterior hypothalamus serve as the principal pacemaker of the mammalian circadian system. Among its efferent targets are the habenular nucleus (Hb), especially the lateral Hb (LHb), which plays an important role in conveying input from the limbic forebrain to midbrain structures. We recorded extracellularly from single neurons in the LHb and medial Hb (MHb), both in vivo and using an in vitro slice preparation, to assess their responses to retinal illumination and the rhythmicity of their firing rates. Of cells recorded in the LHb, 42% were tonically activated or suppressed by retinal illumination, while significantly fewer cells recorded in the MHb responded to retinal illumination (19%). Of photically responsive cells, 68% in the LHb were activated and the remainder suppressed, while only 25% of those recorded in the MHb were activated. Cells in both the LHb and MHb showed higher baseline firing rates during the day than during the night in vivo, while photic responses were of significantly larger amplitude among LHb cells during the projected night than during the projected day. LHb cells recorded in vitro maintained their rhythmicity for two circadian cycles, but MHb cells did not show a rhythm in vitro. The habenula may play a role in linking circadian and motivational systems and may contribute to photic regulation of these systems, as well as to the rhythmicity of their function.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Anatomical and temporal differences in the regulation of ZIF268 (NGFI-A) protein in the hamster and mouse suprachiasmatic nucleus

Several immediate-early genes have been shown to be induced in the rodent circadian pacemaker, the suprachiasmatic nucleus, by retinal illumination at night. We compared spontaneous and light-evoked levels of the immediate-early gene protein ZIF268 (NGFI-A) in the Syrian hamster and C57BL/6J mouse suprachiasmatic nucleus. Exposure of both hamsters and mice to light pulses early and late in the subjective night caused increased ZIF268 immunoreactivity in the region of the suprachiasmatic nucleus that receives retinal innervation. In contrast to hamsters, mice also showed spontaneous increases in ZIF268 at both subjective night phases at the lateral edges of the suprachiasmatic nucleus. Light also evoked a significant increase in ZIF268 levels during the subjective day in the lateral suprachiasmatic nucleus, with few labeled cells in the ventral and dorsal suprachiasmatic nucleus. These results demonstrate a novel circadian pattern and regional differentiation of ZIF268 immunoreactivity in the suprachiasmatic nucleus of mice that differ from those in other rodents. There are pronounced species differences in both spontaneous and light-evoked expression of ZIF268 immunoreactivity.

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.

Spontaneous c-Fos rhythm in the rat suprachiasmatic nucleus: location and effect of photoperiod

A recently reported circadian rhythm in the spontaneous c-Fos immunoreactivity in the rat suprachiasmatic nucleus (SCN) is expressed mostly in the dorsomedial (dm) SCN, where vasopressinergic cells are located. The aim of the present study is to find out whether day length, i.e., photoperiod, affects the dm-SCN rhythm and, if so, how the rhythm adjusts to a change from a long to a short photoperiod. In addition, a question of whether the spontaneous c-Fos production is localized in vasopressin- producing cells or in other cells is also studied to characterize further the dm-SCN rhythmicity. Combined immunostaining for c-Fos and arginine vasopressin (AVP) revealed that most of c-Fos immunopositive cells were devoid of AVP; the results suggest that c-Fos-producing cells in the dm-SCN are mostly not identical with those producing AVP. In rats maintained under a long photoperiod with 16:8-h light-dark cycle (LD 16:8) daily and then released into darkness, the time of the afternoon and evening decline of the spontaneous c-Fos immunoreactivity in the dm-SCN differed just slightly from the time in rats maintained originally under a short LD 8:16 photoperiod; however, the morning c-Fos rise occurred about 4 h earlier under the long than under the short photoperiod. After a change from a long to a short photoperiod, a rough but not yet a fine adjustment of the morning c-Fos rise to the change was accomplished within 3-6 days. The results show that similar to the recently reported ventrolateral SCN rhythmicity, the intrinsic dm-SCN rhythmicity is also affected by the photoperiod and suggest that the whole SCN state is photoperiod dependent.

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.

Acute effects of light on body temperature and activity in Syrian hamsters: influence of circadian phase

Light exposure at night causes an acute increase in human body temperature, which normally falls during the night. This change is largely attributable to the suppression by light of the nocturnal rise in melatonin levels. Little is known, however, about the effects of light on body temperature in nocturnally active mammals in which the nightly peak in melatonin secretion coincides with the circadian phase of elevated, rather than decreased, body temperature. We investigated the effects of a 1-h exposure to light on body temperature and activity of Syrian hamsters, Mesocricetus auratus, at two phases during the night and at two phases during the projected day. Brain or abdominal temperature was recorded continuously using implanted radio transmitters while locomotor activity was monitored simultaneously using a passive infrared movement detector. Responses to light exposure were strongly circadian phase dependent; light during the night caused elevations in both brain and core body temperature, whereas light during the projected day did not. Temperature increases at night could not be attributed solely to activity increases at the onset of light pulses, indicating a contribution from nonbehavioral mechanisms of thermogenesis. These results provide the first evidence for circadian modulation of acute temperature responses to light in a nocturnal mammal.

Selective regional blockade of junB gene expression in the hamster suprachiasmatic nucleus by a tyrosine kinase inhibitor

The hypothalamic suprachiasmatic nucleus (SCN) functions as a circadian pacemaker regulating a variety of physiological and behavioral rhythms in mammals. Retinal illumination evokes expression of several immediate-early genes, including junB, in the ventral SCN early in the subjective night and throughout the SCN later in the subjective night. junB mRNA and protein are also expressed spontaneously around subjective dawn in nocturnal rodents, but only in the dorsal SCN. We examined the biochemical signaling mechanisms underlying both spontaneous and light-evoked expression of junB mRNA in the SCN of Syrian hamsters. Hamsters were injected (i.p.) before subjective dawn with vehicle or with either tyrphostin or genistein, inhibitors of protein tyrosine kinase, and maintained in the dark for 30 min. They were then exposed to a light pulse or kept in darkness for another 30 min. In situ hybridization studies demonstrated that tyrphostin pretreatment (12 or 24 mg/kg) reduced both spontaneous and light-evoked expression of junB mRNA only in the dorsal, and not the ventral, portion of the SCN. Conversely, genistein had little effect on either spontaneous or light-evoked expression of junB mRNA in any part of the SCN. These results indicate that a protein tyrosine kinase sensitive to tyrphostin but not to genistein is involved in the transduction pathways leading to expression of junB mRNA selectively in the dorsal SCN, independently of circadian phase and independently of the involvement of light.

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.

Spontaneous rhythm in c-Fos immunoreactivity in the dorsomedial part of the rat suprachiasmatic nucleus

In rats maintained for 2 days in constant darkness, the suprachiasmatic nucleus exhibited a circadian rhythm in c-Fos immunoreactivity, with the maximum in the morning and trough during the subjective night. In contrast to the night-time photic c-Fos induction occurring in the ventrolateral part of the nucleus, the spontaneous rhythmic c-Fos induction in darkness occurred in the dorsomedial part and might indicate an elevated dorsomedial neuronal activity in the early subjective day.

Molecular circadian oscillators: an alternative hypothesis

Results from experiments in different organisms have shown that elements of input pathways can themselves be under circadian control and that outputs might feed back into the oscillator. In addition, it has become clear that there might be redundancies in the generation of circadian rhythmicity, even within single cells. In view of these results, it is worth reevaluating our current working hypotheses about the pacemaker's molecular mechanisms and the involvement of single autoregulatory genes. On one hand, redundancies in the generation of circadian rhythmicity might make the approach of defining a discrete circadian oscillator with the help of single gene mutations extremely difficult. On the other hand, many examples show that components of signal transduction pathways can indeed be encoded by single genes. The authors have constructed a model placing an autoregulatory gene and its products on an input pathway feeding into a separate oscillator. The behavior of this model can explain the majority of results of molecular circadian biology published to date. In addition, it shows that different qualities of the circadian system might be associated with different cellular functions that can exist independently and, only if put together, will lead to the known circadian phenotype.

Melatonin does not influence the expression of c-fos in the suprachiasmatic nucleus of rats and hamsters

We have assessed whether melatonin can induce c-fos expression at various circadian phases, and whether melatonin can inhibit photically induced c-fos expression in the suprachiasmatic nucleus (SCN) in both rats and Syrian hamsters. Subcutaneous administration of melatonin at a dose of 100 microg/kg neither induced expression of Fos, the protein product of the c-fos proto-oncogene, nor inhibited the expression of Fos-like immunoreactivity (Fos-lir) induced by a light pulse in the SCN of rats and hamsters. In situ hybridization studies also demonstrated the absence of induction by acute melatonin treatments of c-fos mRNA in the SCN. Taken together, these results demonstrate that melatonin effects on SCN cells involve signal transduction pathways that do not include regulation of c-fos gene expression.

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.