Early Subjective Day Research Articles

Differential responses of circadian activity onset and offset following GABA-ergic and opioid receptor activation.

The circadian pacemaker in the mammalian suprachiasmatic nuclei is responsive to photic and nonphotic stimuli. In the present study, the authors have investigated the response of activity onset and offset to application of nonphotic stimuli: the benzodiazepine midazolam and the opioid receptor agonist fentanyl. In correspondence with previous studies, both stimuli induced phase advances of the activity onset when given in the mid- to late subjective day. In contrast, activity offset did not phase advance following these injections. Injections during the early subjective day induced small phase delays of the activity onset, while large phase delays occurred in activity offset. Phase shifts, induced at both circadian time zones, were paralleled by an increase in the length of daily activity (alpha). The increase in a remained present during several days after the injection. The different kinetics in phase shifting of the activity onset and offset indicate complexity in phase-shifting behavior of the circadian pacemaker in response to nonphotic stimuli. Moreover, the data show responsiveness of the circadian system to GABA-ergic and opioid receptor activation, not only during the mid- to late subjective day but also during the early subjective day. The data implicate that the early subjective day is an interesting phase for analysis of molecular and biochemical processes involved in nonphotic phase shifting.

NOVEL DINOFLAGELLATE CLOCK‐RELATED GENES IDENTIFIED THROUGH MICROARRAY ANALYSIS1

In a genome‐wide study of circadian gene expression, we constructed microarrays containing 3500 cDNAs from the dinoflagellate Pyrocystis lunula Schütt and compared the abundance of transcripts at circadian times separated by 12 h. About 3% of the unique genes screened were identified as circadian controlled, more than 50% of which could be identified with diverse known genes. Most were preferentially expressed in either early subjective day or late subjective night. Light exposures at times expected to induce phase shifts in the rhythm revealed 30 differentially expressed genes, including some potentially participating in photic entrainment and others in pathways connecting a central oscillator to output rhythms. Those that appeared in both screens were considered as possible core clock genes, but there were no similarities with such genes from other organisms. This is the first report of circadian regulation of transcript levels in a dinoflagellate.

Spatial and temporal regulation of mitogen-activated protein kinase phosphorylation in the mouse suprachiasmatic nucleus

Circadian and photic regulation of mitogen-activated protein kinase (MAPK) has been shown to associate closely with the function of the circadian clock in vertebrate clock tissues such as the mouse suprachiasmatic nucleus (SCN). Here we show that, in the central region of the mouse SCN, MAPK exhibited circadian and daily rhythms in phosphorylation with a peak at (subjective) night, and this activation was sustained for at least 8 h. In contrast, in the dorsomedial region of the SCN, MAPK showed an overt rhythm in phosphorylation with a transient peak at early subjective day, which was antiphase to that in the central region. Noticeably, the phospho-MAPK-immunoreactive cells observed in the dorsomedial region were distributed from the rostral to the caudal end of the SCN, whereas those observed in the central region were localized within the middle SCN along the rostral–caudal axis. Furthermore, a 15-min light pulse given at subjective night transiently evoked MAPK phosphorylation throughout the ventrolateral region of the SCN peaking within 15 min after the light onset, whereas nighttime-phosphorylated MAPK signals in the central–middle SCN become undetectable within 60 min after the light onset. Thus, the mode of circadian and photic regulation of MAPK phosphorylation varies remarkably among the three subregions within the SCN, suggesting divergent and cell type-specific roles of MAPK in the clock system of the mouse SCN.

Effects of 5,7-dihydroxytriptamine (5,7-DHT) on circadian locomotor activity of the blow fly, Calliphora vicina

The biogenic amine serotonin (5-HT) is a neuromodulator in both vertebrates and invertebrates. It has been shown that serotonin, apart from its distinct effects on behavior, also plays a morphoregulatory role during the ontogeny of the insect's nervous system. The role of serotonin in modulating circadian locomotor activity of the blow fly, Calliphora vicina was explored. Injection of a specific neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the hemolymph appeared to significantly reduced the level of locomotor activity and lengthened the period (τ) of circadian rhythmicity. After drug injection in constant darkness flies continued with their free-running rhythm of a locomotor activity, depending on the time of 5,7-DHT injection. This compound causes phase delay when administered in the early subjective day, and phase advance in the late subjective day. This effect is the opposite of the phase response curve obtained for 5-HT injections. This suggests that 5-HT might act as an entraining agent via the output pathway by feedback to clock neurons in the brain. Some of the injected insects regained their normal level of activity after a few days. These findings suggest a potential role for serotonin as modulator of circadian rhythms in insect including regulation of the level of locomotor activity.

Intrinsic Role of Polysialylated Neural Cell Adhesion Molecule in Photic Phase Resetting of the Mammalian Circadian Clock

The suprachiasmatic nuclei (SCN), the location of the mammalian circadian clock, are one of the few adult brain regions that express the highly polysialylated form of neural cell adhesion molecule (PSA-NCAM). A role for the polysialic acid (PSA) component of PSA-NCAM, which is known to promote tissue plasticity, has been reported for photic entrainment of circadian rhythmicity in vivo. The in vivo results, however, do not discriminate between PSA acting upstream or downstream of the glutamatergic synapses that convey photic information to the SCN. To address this key issue, we exploited an in vitro rat brain slice preparation that retains robust circadian function. As in the intact SCN, PSA levels in the isolated SCN are rhythmic, with higher levels during the early subjective day and lower levels during subjective night. Importantly, bath application of glutamate to SCN slices rapidly and transiently increases PSA levels during both the subjective day and night. Pretreating the slices with endoneuraminidase, which selectively removes PSA from NCAM and thereby prevents this increase, abolishes glutamate- and optic chiasm stimulation-induced phase delays of the SCN circadian neuronal activity rhythm. These results support the hypothesis that PSA expression in the SCN is controlled by both the circadian clock and photic input to the clock and that expression of PSA in the SCN is critical for photic-like phase shifts of the clock. Together, these results establish that such actions of PSA are manifested downstream from presynaptic retinohypothalamic terminals and therefore are intrinsic to the SCN itself.

Effects of 5,7-dihydroxytriptamine (5,7-DHT) on circadian locomotor activity of the blow fly, Calliphora vicina

The biogenic amine serotonin (5-HT) is a neuromodulator in both vertebrates and invertebrates. It has been shown that serotonin, apart from its distinct effects on behavior, also plays a morphoregulatory role during the ontogeny of the insect's nervous system. The role of serotonin in modulating circadian locomotor activity of the blow fly, Calliphora vicina was explored. Injection of a specific neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the hemolymph appeared to significantly reduced the level of locomotor activity and lengthened the period (tau) of circadian rhythmicity. After drug injection in constant darkness flies continued with their free-running rhythm of a locomotor activity, depending on the time of 5,7-DHT injection. This compound causes phase delay when administered in the early subjective day, and phase advance in the late subjective day. This effect is the opposite of the phase response curve obtained for 5-HT injections. This suggests that 5-HT might act as an entraining agent via the output pathway by feedback to clock neurons in the brain. Some of the injected insects regained their normal level of activity after a few days. These findings suggest a potential role for serotonin as modulator of circadian rhythms in insect including regulation of the level of locomotor activity.

Expression profiles of PER2 immunoreactivity within the shell and core regions of the rat suprachiasmatic nucleus: lack of effect of photic entrainment and disruption by constant light.

The circadian clock cells of the mammalian suprachiasmatic nucleus (SCN) generate oscillations in physiology and behavior that are synchronized (entrained) by the external light/dark (LD) cycle. The mechanisms that mediate the effect of light on the core molecular mechanism of the clock are not well understood, but evidence suggests that the Period2 gene, which encodes a key clock regulator (PER2), might be involved. We assessed the expression of PER2 immunoreactivity in the retinorecipient core and shell compartments of the SCN of rats entrained to cycles of discrete light pulses presented at the early subjective day (dawn) or night (dusk), or housed in constant light. We found that in animals entrained to a 0.5 h:23.5-h LD cycle (light falls near dawn), PER2 expression is rhythmic both in the shell and in the core regions of the SCN and indistinguishable from that seen in the SCN of control rats housed in complete darkness. Similarly, the pattern of PER2 expression in the SCN of rats entrained to a 0.5-h:25.5-h LD cycle (light falls near dusk) resembled that in dark-housed controls. We also found that presentation of a discrete light pulse in the early subjective night did not induce PER2 protein expression in the SCN, even 6 h after photic stimulation. Finally, in constant light-housed, behaviorally arrhythmic rats, PER2 expression in the SCN was low and nonrhythmic. These results show that rhythmic PER2 expression occurs both in the shell and core regions of the rat SCN. Furthermore, they show that the expression of PER2 in the SCN is not regulated by entraining light. Finally, constant light-induced behavioral arrhythmicity is associated with a disruption of rhythmic PER2 expression in the whole SCN. Together, the results are consistent with a proposed role for PER2 in the core mechanism of the circadian clock but argue against an important role for PER2 in the mechanism mediating photic entrainment.

Light suppresses Fos expression in the shell region of the suprachiasmatic nucleus at dusk and dawn: implications for photic entrainment of circadian rhythms

The transcription factor Fos is implicated in neuronal signaling in the suprachiasmatic nucleus, the mammalian circadian clock ( Ikonomov and Stoynev, 1994; Klein et al., 1991; Kornhauser et al., 1996). Fos is expressed in two different regions within the suprachiasmatic nucleus. In the ventrolateral, retinorecipient, core region ( Leak et al., 1999; Moga and Moore, 1997), Fos is induced by light and expression is closely linked, both temporally and functionally, to clock resetting and entrainment of circadian rhythms ( Aronin et al., 1990; Beaulé and Amir, 1999; Hastings et al., 1995; Kornhauser et al., 1996; Kornhauser et al., 1990; Rea, 1989, 1998; Rusak et al., 1990; Wollnik et al., 1995). In the dorsomedial shell region ( Leak et al., 1999), Fos expression is rhythmic ( Guido et al., 1999a,b; Rusak et al., 1992; Sumova and Illnerova, 1998; Sumova et al., 1998). Expression is high during the subjective day when photic sensitivity of the core is minimal, and low in the subjective night, when photic sensitivity of the core is maximal. Although it has been shown that the pattern of Fos expression in the shell tracks the photoperiod ( Sumova et al., 2000), nothing is known about whether light influences the expression of Fos in the shell region or about the role of Fos expression in the shell in clock resetting and entrainment. In the present study we found that, in rats maintained in constant darkness, brief exposure to light in the early subjective day or night induced Fos in the core, as expected, and acutely suppressed the levels of Fos immunoreactivity in the shell region. Similar changes in Fos expression in the core and shell regions were seen after exposure to a brief entraining light. Light exposure in the mid-subjective day or night differentially affected Fos expression in the core, as previously shown, but had no effect on Fos expression in the shell region. The finding that Fos expression in the shell region of the suprachiasmatic nucleus is suppressed by light at dawn and dusk suggests a critical role for the shell in photic entrainment of circadian rhythms in nocturnal rodents.

Nonphotic Entrainment by 5-HT1A/7Receptor Agonists Accompanied by ReducedPer1andPer2mRNA Levels in the Suprachiasmatic Nuclei

In mammals, the environmental light/dark cycle strongly synchronizes the circadian clock within the suprachiasmatic nuclei (SCN) to 24 hr. It is well known that not only photic but also nonphotic stimuli can entrain the SCN clock. Actually, many studies have shown that a daytime injection of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH DPAT), a serotonin 1A/7 receptor agonist, as a nonphotic stimulus induces phase advances in hamster behavioral circadian rhythms in vivo, as well as the neuron activity rhythm of the SCN in vitro. Recent reports suggest that mammalian homologs of the Drosophila clock gene, Period (Per), are involved in photic entrainment. Therefore, we examined whether phase advances elicited by 8-OH DPAT were associated with a change of Period mRNA levels in the SCN. In this experiment, we cloned partial cDNAs encoding hamster Per1, Per2, and Per3 and observed both circadian oscillation and the light responsiveness of Period. Furthermore, we found that the inhibitory effect of 8-OH DPAT on hamster Per1 and Per2 mRNA levels in the SCN occurred only during the hamster's mid-subjective day, but not during the early subjective day or subjective night. The present findings demonstrate that the acute and circadian time-dependent reduction of Per1 and/or Per2 mRNA in the hamster SCN by 8-OH DPAT is strongly correlated with the phase resetting in response to 8-OH DPAT.

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 ΔFosB 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 ΔFosB/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.

Locomotor Activity Rhythm in the Field Mouse Mus booduga Phase-shifts to Melatonin Injections in a Dose-dependent Manner

Melatonin is known to shift the phase of the locomotor activity rhythm in the field mouse Mus booduga in accordance with a type-I phase response curve (PRC), with phase delays during the subjective day and phase advances during late subjective night and the early subjective day. At CT4 (circadian time 4; i.e. 16 hr. after activity onset) and CT22 of the circadian cycle, a single dose of melatonin (1 mg/kg) is known to evoke maximum delay and maximum advance phase-shifts, respectively. We investigated the dose-dependent responses of the circadian pacemaker of these mice to a single dose of melatonin at the times for maximum delay and maximum advance. The circadian pacemaker responsible for the locomotor activity rhythm in these mice responded to various doses of melatonin in a dose-dependent manner with the magnitude of phase shifts increasing with dose.

In the field mouse Mus booduga melatonin phase response curves (PRCs) have a different time course and wave form relative to light PRC

The phase shifting effects of the pineal hormone melatonin on the circadian locomotor activity rhythm of the field mouse. Mus booduga was examined at various phases of the circadian cycle using single melatonin injections of two concentrations (10 mg/kg, high dose; and 1 mg/kg, low dose) and two phase response curves (PRCs) were constructed. A single dose of melatonin administered during the early subjective day evoked maximum phase delays, and during the late subjective night evoked phase advances in the locomotor activity rhythm. Other phases of the circadian cycle also responded to melatonin. The interval between circadian time 19 (CT19) and CT2 of the high dose melatonin PRC is marked by significant phase advances, whereas the interval between CT2 and 19 is marked by significant phase delays. A single dose of melatonin of strength 10 mg/kg was found to evoke phase shifts that were of comparable magnitude to those of the phase shifts evoked by natural daylight pulses. Control animals, treated with 50% dimethyl sulfoxide (DMSO), did not respond with phase shifts significantly greater than zero. Significant differences between the shapes of the two melatonin PRCs exist. Further melatonin PRCs appear to have a different time course and wave form relative to light-induced PRC.

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.

Circadian Period Modulation and Masking Effects Induced by Repetitive Light Pulses in Locomotor Rhythms of the Cricket, Gryllus bimaculatus

Effects of 15 min light pulses given at various intervals (every 1, 2, 4, 6, 8 and 12 hr) under constant darkness on the locomotor rhythm were investigated in the adult male cricket, Gryllus bimaculatus. A single pulse per 24 hr induced period modulation in a circadian phase dependent manner, yielding a period modulation curve (PMC): the 15 min light pulse lengthened the period in the early subjective night (CT11-16) and shortened it during the late subjective night to the early subjective day (CT20-5). Frequent light pulses modulated the freerunning period of the rhythm dependent on the interval of the pulses: when compared with the freerunning period in DD (23.74 +/- 0.03 hr) the period was significantly shorter in intervals of 2 and 4 hr, but lengthened when the interval was 1 and 12 hr. Frequent light pulses also resulted in entrainment of the rhythm to run with the period of 24 hr and the ratio of the entrained animals varied from 12% to 72% depending on the interval of the light pulses. The period modulation and the entrainment by the repetitive light pulses could be interpreted according to the PMC. In about 15% of animals, the light pulses induced a rhythm dissociation, suggesting that the bilaterally paired circadian pacemakers have their own sensitivity to the entraining photic information. The light pulse caused a masking effect, i.e., an intense burst of activity. The magnitude of the light induced responses was dependent on the circadian phase. The strongest masking effect was observed in the subjective night. The phase of the prominent period modulation and of the marked masking effects well coincides with the previously reported sensitive phase of the photoreceptive system.

Cyclic AMP resets the circadian clock in cultured Xenopus retinal photoreceptor layers.

The Xenopus retinal photoreceptor layer contains a circadian oscillator that regulates melatonin synthesis in vitro. The phase of this oscillator can be reset by light or dopamine. The phase-response curves for light and dopamine are similar, with transitions from phase delays to phase advances in the mid-subjective night. Light and dopamine each can inhibit adenylate cyclase in retinal photoreceptors, suggesting cyclic AMP as a candidate second messenger for entrainment of the circadian oscillator. We report here that treatments that increase intracellular cyclic AMP reset the phase of the photoreceptor circadian oscillator, and that the phase-response curves for these treatments are 180 degrees out of phase with the phase-response curves for light and dopamine. Activation of adenylate cyclase by forskolin during the late subjective day or early subjective night caused phase advances. The same treatment during the late subjective night or early subjective day caused phase delays. Similar phase shifts were induced by 3-isobutyl-1-methylxanthine (a phosphodiesterase inhibitor) or 8-(4-chlorophenylthio)cyclic AMP. All of these treatments also acutely increased melatonin release. Forskolin and 3-isobutyl-1-methylxanthine increased the accumulation of intracellular cyclic AMP, but not cyclic GMP, in photoreceptor layers. The results indicate that cyclic AMP-dependent pathways regulate the photoreceptor circadian oscillator and suggest that a decrease in cyclic AMP may be involved in circadian entrainment by light and/or dopamine.

Nuclear localization is required for function of the essential clock protein FRQ.

The frequency (frq) gene in Neurospora encodes central components of a circadian oscillator, a negative feedback loop involving frq mRNA and two forms of FRQ protein. Here we report that FRQ is a nuclear protein and nuclear localization is essential for its function. Deletion of the nuclear localization signal (NLS) renders FRQ unable to enter into the nucleus and abolishes overt circadian rhythmicity, while reinsertion of the NLS at a novel site near the N-terminus of FRQ restores its function. Each form of FRQ enters the nucleus soon after its synthesis in the early subjective day; there is no evidence for regulated sequestration in the cytoplasm prior to nuclear entry. The kinetics of the nuclear entry are consistent with previous data showing rapid depression of frq transcript levels following the synthesis of FRQ, and suggest that early in each circadian cycle, when FRQ is synthesized, it enters the nucleus and depresses the level of its own transcript.

Circadian rhythm of brain-derived neurotrophic factor in the rat suprachiasmatic nucleus

This study was conducted to determine whether the rat suprachiasmatic nucleus (SCN) is characterized by circadian expression of brain-derived neurotrophic factor (BDNF). In constant darkness, SCN content of both BDNF mRNA and protein oscillated in a circadian fashion. BDNF mRNA and protein levels in the SCN reached peak values during the early subjective day and during the subjective night, respectively. In contrast, the hippocampus showed no sign of circadian rhythmicity in its expression of BDNF mRNA and protein. Since BDNF enhances synaptic transmission in other brain regions, the coincidence between peak expression of BDNF protein in the SCN and the known interval of circadian pacemaker sensitivity to the phase-shifting effects of light may have some implications for the role of BDNF in the circadian regulation of the SCN pacemaker by photic signals from the retinohypothalamic tract.

Variation in the expression of the mRNA for protein kinase C isoforms in the rat suprachiasmatic nuclei, caudate putamen and cerebral cortex

Using in situ hybridization, we have examined mRNA expression for five isoforms of protein kinase C (PKC α, β1, β2, γ and ε) in the rat suprachiasmatic nuclei (SCN) and other central sites during the 24 h cycle. The signal for each of these isoforms shows a marked local density within the SCN. In the absence of photic cues, there are changes in the expression of the mRNAs for the four isoforms that are Ca 2+-dependent ( α, β1, β2 and γ), but not for one of the Ca 2+-independent PKCs ( ε). PKC α mRNA exhibits a monophasic rhythm of expression in the SCN with a peak at early subjective night, circadian time (CT) 14. In contrast, the mRNAs for PKC β1, β2 and γ show a biphasic rhythm in the SCN with peaks at early subjective day, CT 0, and early subjective night, CT 14. The four Ca 2+-dependent isoforms may therefore subserve phase-related functions within the SCN at the onset of subjective night and, in the case of β1, β2 and γ, also at the onset of subjective day. Variation in the mRNAs for PKC β1 and γ (but not for α, β2 or ε) is also found in the caudate putamen and in the cingulate and parietal cortex; the biphasic pattern of expression for these mRNAs is precisely in phase with that observed in the SCN. The β1 and γ isoforms may therefore contribute to temporally regulated functions at sites outside the SCN. The present observations raise the possibility that receptor-mediated regulation of circadian functions is modulated or even gated by circadian changes in intracellular components that participate in distinct signal cascades.

Cell Cultures of Rat Suprachiasmatic Nucleus: Circadian Oscillation of Vasopressin Release

A dissociated cell culture system has been developed for the suprachiasmatic nucleus (SCN), in which release of vasopressin showed a clear circadian oscillation. The oscillation peaked at early subjective day and appeared within one day in culture. This system may provide a valuable model for the study of cellular and molecular mechanisms of the mammalian circadian pacemaker.

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 (∼ 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 (∼ 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.