Light-induced Phase Advances Research Articles

Decreased sensitivity to light of the photic entrainment pathway during chronic clorgyline and lithium treatments.

Certain antidepressant drugs (ADs) cause disturbances in sleep that could result from their capacity to alter the timing of circadian rhythms. Effects on the timing of rhythms could be due to the drugs' known capacity to alter the frequency of the intrinsic rhythm of the circadian pacemaker, or to a capacity to modify the pacemaker's response to external stimuli that serve as time cues (Zeitgebers) that regulate the timing (phase) of its rhythm. To examine the possibility that ADs alter the sensitivity of the system that mediates the phase-shifting effects of light, hamsters were treated chronically with the MAOI, clorgyline, and lithium. Each hamster was then exposed to a single 5-min light pulse (intensity range = 0.00137 to 137 microW/cm2) at circadian phases known to elicit phase advances (CT18) and phase delays (CT13.5) in the daily onset of wheel running. The half-saturation constant (sigma), photic sensitivity (1/sigma), and maximum phase-shifting response to light were estimated from the best-fit stimulus response curves. In addition, threshold sensitivity, the light intensity required to produce a threshold phase-shifting response, was determined. Clorgyline decreased the magnitude of light-induced phase advances at each of the light intensities tested. Clorgyline also decreased the magnitude of light-induced phase delays at low light intensities, but increased the magnitude of phase delays at higher light intensities. Clorgyline decreased the sensitivity of the photic phase-shifting system, as indicated both by the threshold sensitivities at CT13.5 and CT18, and by 1/sigma at CT13.5. Lithium decreased the threshold sensitivity at CT18, and 1/sigma at CT13.5. Lithium decreased the magnitude of phase delays, but not phase advances. Clorgyline's effects on the photic entrainment pathway may be mediated by its effects on serotonin, which has been shown to modulate the pacemaker's response to morning and evening light, and by tolerance to this effect of serotonin. The fact that both clorgyline and lithium decrease the photic sensitivity of the entrainment pathway suggests that other psychoactive drugs might also share this property. It is possible that the decreased sensitivity to light of the entrainment pathway affects the clinical response to these and other psychoactive medications.

Local administration of serotonin agonists blocks light-induced phase advances of the circadian activity rhythm in the hamster.

Circadian rhythms in mammals are synchronized to environmental light-dark cycles through a direct retinal projection to the suprachiasmatic nucleus (SCN), a circadian clock. This process is thought to be modulated by other afferents to the SCN, including a dense serotonergic projection from the midbrain raphe. Previous work from this laboratory demonstrated that a systemically administered 5-hydroxytryptamine1A/7 (5-HT1A/7) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) dose dependently attenuates light-induced phase shifts of the circadian activity rhythm of the Syrian hamster. In this study, we demonstrate that local injections (1-100 microM) of the 5-HT1A/7 agonists 8-OH-DPAT or 5-carboxamidotryptamine into the region of the SCN inhibit light-induced phase advances of the circadian wheel-running rhythm. In addition, the inhibitory effects of systemically administered 8-OH-DPAT were unaffected by either radiofrequency-induced lesions of the intergeniculate leaflet or 5,7-dihydroxytryptamine-induced lesions of serotonergic projections to the SCN. These findings support a modulatory role of serotonin in photic regulation of circadian phase through an action at the level of the SCN.

Neuropeptide Y blocks light-induced phase advances but not delays of the circadian activity rhythm in hamsters

In mammals, the suprachiasmatic nuclei (SCN) are the anatomical site of localization of the light-entrainable circadian clock responsible for the generation of daily rhythms in physiology and behavior. In addition to direct retinohypothalamic innervation, the SCN receive a prominent projection of fibers from the intergeniculate leaflet (IGL) of the thalamus, the geniculohypothalamic tract (GHT), some of which contain the neurotransmitter, neuropeptide Y (NPY). Since the GHT has been suggested to play a role in the modulation of photic entrainment of the SCN circadian clock in rodents, we investigated the effects of local administration of NPY into the region of the SCN on light-induced phase shifts of the free-running activity rhythm in hamsters. Injection of 60 nmol of NPY into the SCN region 10 min prior to light exposure at circadian time 19 completely blocked light-induced phase advances. Similar treatment at circadian time 14 had no significant effect on the magnitude of light-induced phase delays. Injection of NPY at either time point without light exposure did not alter circadian phase. The findings support a modulatory role for NPY in the photic entrainment of the SCN circadian clock.

Aging alters the serotonergic modulation of light-induced phase advances in golden hamsters.

Recent findings have raised the possibility that some of the age-related changes in the circadian system and the response of the circadian pacemaker to environmental stimuli may involve central serotonergic mechanisms. The present study compared the effects ofpretreatment with the serotonin agonist 8-hydroxy-2(di-n-propylamino)tetralin (5 mg/kg ip) on the magnitude of light-induced phase advances in young (2-4 mo) and old (18-20 mo) golden hamsters. The ability of this serotonin agonist to attenuate the photic phase resetting of circadian locomotor rhythmicity in young animals was decreased by 46% in old hamsters (P < 0.05). These results suggest that deficits in the mechanisms for serotonergic control of circadian function may interfere with the optimal adaptation of the senescent organism to its temporal environment.

5HT1BReceptor Agonists Inhibit Light-Induced Phase Shifts of Behavioral Circadian Rhythms and Expression of the Immediate–Early Genec-fosin the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is a circadian oscillator and a critical component of the mammalian circadian system. It receives afferents from the retina and the mesencephalic raphe. Retinal afferents mediate photic entrainment of the SCN, whereas the serotonergic afferents originating from the midbrain modulate photic responses in the SCN; however, the serotonin (5HT) receptor subtypes in the SCN responsible for these modulatory effects are not well characterized. In this study, we tested the hypothesis that 5HT1B receptors are located presynaptically on retinal axon terminals in the SCN and that activation of these receptors inhibits retinal input. The 5HT1B receptor agonists TFMPP and CGS 12066A, administered systemically, inhibited light-induced phase shifts of the circadian activity rhythm in a dose-dependent manner at phase delay and phase advance time points. This inhibition was not affected by previous systemic application of either the selective 5HT1A receptor antagonist (+)WAY 100135 or by the 5HT2 receptor antagonist mesulergine, whereas pretreatment with the nonselective 5HT1 antagonist methiothepin significantly attenuated the effect of TFMPP. TFMPP also produced a dose-dependent reduction in light-stimulated Fos expression in the SCN, although a small subset of cells in the dorsolateral aspect of the caudal SCN were TFMPP-insensitive. TFMPP (1 mM) infused into the SCN produced complete inhibition of light-induced phase advances. Finally, bilateral orbital enucleation reduced the density of SCN 5HT1B receptors as determined using [125I]-iodocyanopindolol to define 5HT1B binding sites. These results are consistent with the interpretation that 5HT1B receptors are localized presynaptically on retinal terminals in the SCN and that activation of these receptors by 5HT1B agonists inhibits retinohypothalamic input.

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 I 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.

Differences in colocalization between Fos and PHI, GRP, VIP and VP in neurons of the rat suprachiasmatic nucleus after a light stimulus during the phase delay versus the phase advance period of the night

Two groups of four rats each received a 15-minute light stimulus during the first part of the night (ZT14) and the second part (ZT19), respectively. After 45–60 minutes, the animals were killed by perfusion fixation. Adjacent Vibratome sections through the suprachiasmatic nucleus (SCN) were double-immunostained for the presence of peptide histidine isoleucine (PHI), gastrin releasing peptide (GRP) or vasoactive intestinal peptide (VIP) with Fos by using fluorophore-conjugated secondary antibodies. A few sections were triple-immunostained for PHI, GRP or VIP with vasopressin (VP) and Fos. Sections were analyzed with a confocal laser scanning microscope. It turned out that the ZT19 light stimulus induced 4.2 times more nuclear profiles in the SCN immunoreactive for Fos than the light stimulus given at ZT14. The SCN of control animals did not show any Fos immunoreactivity. After the ZT14 light stimulus, approximately 33% of the Fos profiles showed colocalization with a perikaryal profile immunoreactive for PHI, GRP or VIP, whereas at ZT19, this percentage had doubled to approximately 65%. After the light stimulus at ZT14, the relatively low Fos induction was numerically and proportionally most prominent in the PHI-immunoreactive perikarya. As compared with ZT14, the increase of Fos after the ZT19 light stimulus was most pronounced in the GRP-immunoreactive perikarya (21×) followed by VIP (15×) and PHI (5×) This outcome suggests that at least three different cell groups characterized by, respectively, PHI alone, GRP, and VIP fully or partly colocalized with PHI, play a prominent role during light-induced phase shifts: the PHI neurons during light-induced phase delays, the GRP and VIP/(PHI) neurons during light-induced phase advances. © 1996 Wiley-Liss, Inc.

Methamphetamine modifies the photic entraining responses in the rodent suprachiasmatic nucleus via serotonin release.

We examined whether methamphetamine modifies the photic entraining responses in the rat suprachiasmatic nucleus. Optic nerve stimulation increased vasoactive intestinal polypeptide release from rat suprachiasmatic nucleus slices, and methamphetamine inhibited this increase in a concentration-dependent manner. Optic nerve stimulation has been reported to evoke field potentials in rat suprachiasmatic nucleus slices. Methamphetamine attenuated this field potential, and maximal inhibition (75.5%) was achieved at a concentration of 100 microM. Systemic administration of methamphetamine (1-5 mg/kg) inhibited light (300 lux, 1h)-induced Fos expression in the suprachiasmatic nucleus; methamphetamine at a dose of 5 mg/kg, i.p. caused 40% inhibition of light-induced Fos expression. We examined whether the inhibitory effect of methamphetamine on photic entraining responses mediates serotonin release from the suprachiasmatic nucleus. High-performance liquid chromatographic analysis revealed that methamphetamine application increased serotonin release from rat suprachiasmatic nucleus slices in a concentration-dependent manner, but did not affect noradrenaline release. In addition, reduction of serotonin content attenuated the effect of methamphetamine on field potential induced by optic nerve stimulation in vitro and also light-induced phase advances of wheel running activity rhythm in vivo. The present results support the idea that methamphetamine produces an inhibitory effect on photic entrainment in the suprachiasmatic nucleus via serotonin release.

Diazepam modulates the period of locomotor rhythm in mice ( Mus booduga) and attenuates light-induced phase advances

Experiments were carried out on the continuous action of diazepam (benzodiazepine) offered through drinking water in 2% ethanol on ψ and τ of the activity rhythms under LD (12 : 12) and DD conditions. Under entraining conditions diazepam failed to evoke striking changes in ψ. On the other hand, under free-running conditions period-lengthening and period-shortening effects were observed. Further experiments conducted on the continuous effect of diazepam on light pulse evoked phase shifts revealed that phase advances were attenuated significantly in diazepam-treated animals at CT 20 and 24. These results were discussed with regard to the action of diazepam on the light sensitivity of the circadian pacemaker.

CGMP-dependent protein kinase inhibitors block light-induced phase advances of circadian rhythms in vivo.

Synchronization of circadian rhythms is thought to be accomplished primarily through daily phase delays and advances of the endogenous circadian clock that, in mammals, is located in the hypothalamic suprachiasmatic nucleus (SCN). In the SCN, numerous second messenger pathways may participate in photic signal transduction. In these studies, the involvement of cyclic nucleotide-dependent kinases was examined in vivo using inhibitors of adenosine 3',5'-cyclic monophosphate (cAMP)- and guanosine 3',5'-cyclic monophosphate (cGMP)-dependent kinase (PKA and PKG, respectively). In constant dark, selective and nonselective inhibitors of PKG injected near the SCN of hamsters had no effect on phase delays produced by light pulses given in the early subjective night (early in the animals' active period) but significantly attenuated phase advances induced late in the subjective night. PKA inhibition had no effect at either time point. In addition, cGMP agonists had no effect on rhythmicity in the absence of light. The results suggest that PKG activity is necessary, but not sufficient, for normal photic responsiveness and that PKA activity is not required. The phase dependence of the effect of PKG inhibition supports the notion that photic entrainment is influenced by biochemical pathways that differentially regulate sensitivity in a phase-dependent manner.

CGMP-dependent protein kinase inhibitor blocks light-induced phase advances of circadian rhythms in vivo

The suprachiasmatic nucleus (SCN) contains the primary mammalian circadian clock. Light synchronizes these circadian rhythms through a mechanism involving the release of excitatory amino acids (EAA) and synthesis of nitric oxide (NO) in the SCN. In the current study, we investigated whether cGMP-mediated activation of cGMP-dependent protein kinase (PKG) is associated with light-induced phase shifts of the circadian oscillator. Local administration of the specific PKG inhibitor, KT-5823, significantly attenuated light-induced advances in the phase of activity rhythms when administered during late subjective night (CT 19). Similar treatment at CT 14 had no significant effect on light-induced phase delays. These results are the first to implicate PKG in the biochemical pathway(s) responsible for photic phase advances, and suggest a divergence in biochemical pathways involved in photic phase shifts.

Wavelength dependence of light-induced phase shifts and period changes in hamsters

The wavelength dependence of three different circadian responses to light was compared in Syrian hamsters: phase advances and phase delays induced by 15-min light pulses at CTl8 (circadian time 18) and CTl3, respectively, and changes in free-running period under different levels of constant illumination (LL) intensity. Fluorescent lamps with transmission maxima at 436, 514, and 658 run were used for delivering blue, green, and red light, respectively. White fluorescent light was also used in the LL portion of the study. The magnitude of each circadian response was plotted as a function of light intensity expressed in photons cm −2 s −1, and intensity-response relationships for the different wavelengths were quantified by fitting the data with a 4-parameter sigmoidal function using a least-squares curve-fitting routine. For each response type, the curves for the different wavelengths were identical but displaced along the horizontal axis (i.e., the functions differed only in the intensity (σ) required for producing a half-maximal response]. The values of a for green and blue light were similar, while that for red light was several times greater. These results indicate that a single class of photoreceptors with peak sensitivity in the blue-green region mediates light-induced phase advances, phase delays, and changes in the period of the hamster circadian pacemaker.

Tryptophan loading modulates light-induced responses in the mammalian circadian system.

Enhanced endogenous serotonergic activity, stimulated by L-tryptophan (TRYPT) loading, was found to have a substantial impact on neurochemical and behavioral aspects of the circadian response to light in the male Syrian hamster. An intraperitoneal (i.p.) injection of 150 mg/kg TRYPT significantly stimulated serotonin (5-HT) release in the suprachiasmatic nuclear (SCN) region, as reflected by a 205 +/- 30% maximal increase in the extracellular concentration of 5-HT assessed using microdialysis. Administration of TRYPT 1 h before exposure to a light pulse (30 min, 40 lux) delivered during late subjective night dose-dependently suppressed the number of SCN cells expressing light-induced Fos-like immunoreactivity (Fos-LI; maximal suppression @200 mg/kg was 77 +/- 4%, p < 0.001). This action of TRYPT was attenuated by pretreatment with the 5-HT1a antagonist, NAN-190, and was abolished by the 5-HT2/5-HT7 antagonist, ritanserin, or the nonselective 5-HT antagonist, metergoline (all 10 mg/kg). These antagonists alone had no effect on light-induced Fos. In a second experiment, pretreatment of free-running hamsters housed under constant darkness with 150 mg/kg TRYPT 45-60 min prior to light exposure (10 min, 20 lux) during late subjective night (CT 19) significantly attenuated the light-induced phase advances of the circadian activity rhythm (66 +/- 7 min vs. 100 +/- 6 min for vehicle controls; p < 0.001). The same dose of TRYPT given 1 h before lights-on for 5 consecutive days in hamsters maintained under 14L:10D altered the phase angle of entrainment such that activity onsets were delayed by 36 +/- 8 min relative to controls (p < 0.05). The same dose of TRYPT administered during late subjective night also suppressed the extracellular concentration of glutamate in the SCN region assessed using microdialysis (55 +/- 8% suppression; p < 0.05 vs. baseline). These results support the hypothesis that the ascending serotonergic projection to the SCN modulates photic entrainment processes within the circadian oscillator.

Altered circadian pacemaker functions and cyclic AMP rhythms in the drosophila learning mutant dunce

Neural circadian pacemakers can be reset by light, and the resetting mechanism may involve cyclic nucleotide second messengers. We have examined pacemaker resetting and free-running activity rhythms in Drosophila dunce (dnc) and DC0 mutants, which identify a cAMP-specific phosphodiesterase and the catalytic subunit of cAMP-dependent protein kinase, respectively. dnc mutants exhibit augmented light-induced phase delays and shortened circadian periods, which indicate altered pacemaker function. Interestingly, however, light-induced phase advances are normal in dnc, suggesting a selective effect on one component of the pacemaker resetting response. Furthermore, we demonstrate the presence of circadian rhythms in cAMP content in head tissues and show that dnc mutations increase the amplitude of daily cAMP peaks. These results show that cAMP levels are not chronically elevated in the dnc mutant. A role for cAMP signaling in circadian processes is also suggested by an analysis of DC0 mutants, which have severe kinase deficits and display arrhythmic locomotor activity.

Serotonin modulates photic responses in the hamster suprachiasmatic nuclei

The aim of the present study was to examine the effects of serotonin agonists on three elements of the photic response in the hamster suprachiasmatic nuclei (SCN). Both serotonin and the selective 5-HT1A agonist 8-OH-DPAT inhibited field potentials recorded in the SCN in response to optic nerve stimulation in the hypothalamic slice preparation. The effects of both drugs were dose related over a concentration range of 1-50 microM, and, in both cases, a maximal inhibition of approximately 60% was achieved at a concentration of 25-50 microM. Systemic administration of 8-OH-DPAT inhibited light-stimulated Fos expression in the SCN. A regionally selective pattern of inhibition was observed, with decreases restricted predominately to the ventral and dorsal borders of the SCN. Finally, systemic administration of 8-OH-DPAT was found to dose-dependently attenuate light-induced phase shifts of the free-running activity rhythm. The effects of 8-OH-DPAT on light-induced phase advances were dose dependent. Injection of 8-OH-DPAT at a dose of 0.5 mg/kg caused 57% inhibition of light-induced phase advances, while a dose of 5 mg/kg inhibited the phase advance by 82%. Injection of 0.05 mg/kg 8-OH-DPAT did not significantly inhibit light-induced phase advances. Injection of 5 mg/kg 8-OH-DPAT alone did not significantly alter the phase of the activity rhythm. Similarly, injection of 5 mg/kg 8-OH-DPAT 30 min prior to light stimulation at CT14 completely inhibited light-induced phase delays, while this dose of the drug did not alter the phase of the activity rhythm when administered alone.(ABSTRACT TRUNCATED AT 250 WORDS)

An inhibitor of protein phosphorylation stops the circadian oscillator and blocks light-induced phase shifting in Gonyaulax polyedra.

The expression of circadian rhythmicity in Gonyaulax polyedra is strikingly altered by an inhibitor of protein phosphorylation. The effects of 6-dimethylaminopurine (6-DMAP), known to reversibly block cell division in many systems through inhibition of protein kinase activity, are described here for Gonyaulax. Its action appears to be exclusively tonic in nature; in cells continuously exposed, the period is lengthened in a concentration-dependent fashion. Shorter treatments at a higher concentration of 6-DMAP (5 mM) apparently stop the circadian oscillator, but reversibly so, since the rhythm resumes after drug removal with a phase delay approximately equal to the duration of the treatment. Pulses of the inhibitor are effective in causing phase delays at all times of the circadian cycle. In addition, 6-DMAP completely blocks light-induced phase advances and is effective in inhibiting many Gonyaulax protein kinases in vitro.

Local administration of EAA antagonists blocks light-induced phase shifts and c-fos expression in hamster SCN.

Syrian hamsters were fitted with guide cannulas stereotaxically aimed at the suprachiasmatic nuclei (SCN) and housed in constant darkness in cages equipped with running wheels. Animals received 300-nl injections of either vehicle, CNQX, or MK-801 into the region of the SCN 5 min before a brief (10 min at 20 lx) light exposure. Local administration of either 1 mM CNQX or 1 mM MK-801 at circadian time (CT) 18 significantly inhibited light-induced phase advances (vehicle = 52 +/- 9 min; CNQX = 12 +/- 7 min; MK-801 = 12 +/- 5 min; P < 0.05 relative to vehicle+light group). The effects of both drugs were reversible and dose related. Injection of 1 mM MK-801 at CT13.5 resulted in a 71% inhibition of light-induced phase delays (vehicle = -51 +/- 6 min; MK-801 = -15 +/- 5 min; P < 0.05), while CNQX failed to significantly inhibit light-induced phase delays (-39 +/- 10 min). Local administration of either 1 mM CNQX or MK-801 into the SCN region reduced the number of Fos-immunoreactive cells relative to vehicle-injected controls by approximately 32 and 44%, respectively (vehicle = 951 +/- 79; CNQX = 643 +/- 135, P < 0.05; MK-801 = 533 +/- 143, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral inhibition of circadian responses to light.

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.

NMDA as well as non-NMDA receptor antagonists can prevent the phase-shifting effects of light on the circadian system of the golden hamster.

The present experiments were designed to evaluate whether the intraventricular administration of excitatory amino acid (EAA) receptor antagonists would prevent light-induced phase shifts of the circadian rhythm of wheel-running activity in the hamster. Administration of the non-N-methyl-D-aspartate (non-NMDA) antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) blocked light-induced phase advances and delays. Similarly, administration of the competitive NMDA receptor antagonist, 3(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP), prevented light-induced phase advances and delays. Neither drug by itself caused any consistent effect on the phase of the rhythm. These data provide further evidence that EAA receptors mediate the effects of light on the circadian system, and suggest that both NMDA and non-NMDA receptor types may be involved.

Different populations of cells in the suprachiasmatic nuclei express c- fos in association with light-induced phase delays and advances of the free-running activity rhythm in hamsters

Circadian rhythmicity is controlled by a light-entrainable pacemaker located in the suprachiasmatic nuclei (SCN) of the mammalian hypothalamus. Brief light exposure during the subjective night causes phase shifts of the free-running activity rhythm and expression of c- fos-related proteins (Fos) among a population of cells in the hamster SCN. Light exposure (30 lux for 15 min) during the early subjective night (CT13) causes phase delays (−60 ± 12 min), while exposure at mid-subective night (CT18) causes phase advances (114 ± 48 min) of t free-running activity rhythm. Light exposure at mid-subjective day (CT6) does not cause phase alterations of the rhythm. Similarity, only light exposure at CT13 or CT18 induces Fos expression in the SCN. The distributi on of Fos-immunoreactive cells in the SCN is more widespread in animals stimulated with light at CT18. In addition, a group of cells located dorsal and anterior to the SCN express Fos only after stimulation at CT18. The data are consistent with the hypothesis that Fos expression represents an event in the signal transduction pathway leading to light-induced alterations in circadian pacemaker function. Furthermore, the data raise the possibility that different populations of cells in the suprachiasmatic hypothalamus may participate in light-induced phase advances and delays of the circadian pacemaker.