Daily novel wheel running reorganizes and splits hamster circadian activity rhythms.

The phenomenon of splitting of locomotor activity rhythms in constant light has implied that the mammalian circadian pacemaker is composed of multiple interacting circadian oscillators. Exposure of male Syrian hamsters to novel running wheels also induces splitting in some reports, although novel wheel running (NWR) is better known for its effects on altering circadian phase and the length of the free-running period. In three experiments, the authors confirm and extend earlier reports of split rhythms induced by NWR. Male Syrian hamsters, entrained to LD 14:10, were transferred for 6 to 11 consecutive days to darkened novel Wahmann wheels at ZT 4 and were returned to their home cages at ZT 9. All hamsters ran robustly in the novel wheels. NWR caused a marked reorganization of home cage wheel-running behavior: Activity onsets delayed progressively with each additional day of NWR. After 11 days, activity onset in the nighttime scotophase was delayed by 7 h and disappeared completely in 2 hamsters (Experiment 1). After 6 to 7 days of NWR (Experiment 2), activity onset delayed by 5 h. Transfer of hamsters to constant darkness (DD) after 7 days of NWR revealed clearly split activity rhythms: The delayed nighttime activity bout was clearly identifiable and characterized by a short duration. A second bout associated with the former time of NWR was equally distinct and exhibited a similarly short duration. These components rejoined after 3 to 5 days in DD accomplished via delays and advances of the nighttime and afternoon components, respectively. The final experiment established that rejoining of activity components could be prevented by perpetuating the light-dark:light-dark cycle used to induce split rhythms. The data suggest that NWR causes selective phase shifting of some circadian oscillators and that component oscillators interact strongly in constant darkness.     

Phase response curve and light-induced fos expression in the suprachiasmatic nucleus and adjacent hypothalamus of Arvicanthis niloticus

This article describes the phase response curve (PRC), the effect of light on Fos immunoreactivity (Fos-IR) in the suprachiasmatic nucleus (SCN), and the effect of SCN lesions on circadian rhythms in the murid rodent, Arvicanthis niloticus. In this species, all individuals are diurnal when housed without a running wheel, but running in a wheel induces a nocturnal pattern in some individuals. First, the authors characterized the PRC in animals with either the nocturnal or diurnal pattern. Both groups of animals were less affected by light during the middle of the subjective day than during the night and were phase delayed and phase advanced by pulses in the early and late subjective night, respectively. Second, the authors characterized the Fos response to light at circadian times 5, 14, or 22. Light induced an increase in Fos-IR within the SCN during the subjective night but not subjective day; this effect was especially pronounced in the ventral SCN, where retinal inputs are most concentrated, but was also evident in other regions. Both light and time influenced Fos-IR within the lower subparaventricular area. Third, SCN lesions caused animals to become arrhythmic when housed in a light-dark cycle as well as constant darkness. In summary, Arvicanthis appear to be very similar to nocturnal rodents with respect to their PRC, temporal patterns of light-induced Fos expression in the SCN, and the effects of SCN lesions on activity rhythms.     

Signaling in the mammalian circadian clock: the NO/cGMP pathway

Mammalian circadian rhythms are generated by a hypothalamic suprachiasmatic nuclei (SCN) clock. Light pulses synchronize body rhythms by inducing phase delays during the early night and phase advances during the late night. Phosphorylation events are known to be involved in circadian phase shifting, both for delays and advances. Pharmacological inhibition of the cGMP-dependent kinase (cGK) or Ca2+/calmodulin-dependent kinase (CaMK), or of neuronal nitric oxide synthase (nNOS) blocks the circadian responses to light in vivo. Light pulses administered during the subjective night, but not during the day, induce rapid phosphorylation of both p-CAMKII and p-nNOS (specifically phosphorylated by CaMKII). CaMKII inhibitors block light-induced nNOS activity and phosphorylation, suggesting a direct pathway between both enzymes. Furthermore, SCN cGMP exhibits diurnal and circadian rhythms with maximal values during the day or subjective day. This variation of cGMP levels appears to be related to temporal changes in phosphodiesterase (PDE) activity and not to guanylyl cyclase (GC) activity. Light pulses increase SCN cGMP levels at circadian time (CT) 18 (when light causes phase advances of rhythms) but not at CT 14 (the time for light-induced phase delays). cGK II is expressed in the hamster SCN and also exhibits circadian changes in its levels, peaking during the day. Light pulses increase cGK activity at CT 18 but not at CT 14. In addition, cGK and GC inhibition by KT-5823 and ODQ significantly attenuated light-induced phase shifts at CT 18. This inhibition did not change c-Fos expression SCN but affected the expression of the clock gene per in the SCN. These results suggest a signal transduction pathway responsible for light-induced phase advances of the circadian clock which could be summarized as follows: Glu-Ca2+-CaMKII-nNOS-GC-cGMP-cGK-->-->clock genes. This pathway offers a signaling window that allows peering into the circadian clock machinery in order to decipher its temporal cogs and wheels.     

Exotic photoperiods induce and entrain split circadian activity rhythms in hamsters

The split circadian activity rhythm that emerges in hamsters after prolonged exposure to constant light has been a theoretical cornerstone of a multioscillator view of the mammalian circadian pacemaker. The present study demonstrates a novel method for splitting hamster circadian rhythms and entraining them to exotic light:dark cycles. Male Syrian hamsters previously maintained on a 14-h day and 10-h night were exposed to a second 5-h dark phase in the afternoon. The 10-h night was progressively shortened until animals experienced two 5-h dark phases beginning 10 h apart. Most hamsters responded by splitting their activity rhythms into two components associated with the afternoon and nighttime dark phases, respectively. Each activity component was entrained to this light:dark:light:dark cycle. Transfer of split hamsters to constant darkness resulted in rapid joining of the two activity components with the afternoon component associated with onset of the fused rhythm. In constant light, the nighttime component corresponded to activity onset of the fused rhythm, but splitting emerged again at an interval characteristic for this species. The results place constraints on multi-oscillator models of circadian rhythms and offer opportunities to characterize the properties of constituent circadian oscillators and their interactions.     

Light pulses do not induce c-fos or per1 in the SCN of hamsters that fail to reentrain to the photocycle

Circadian activity rhythms of most Siberian hamsters (Phodopus sungorus sungorus) fail to reentrain to a 5-h phase shift of the light-dark (LD) cycle. Instead, their rhythms free-run at periods close to 25 h despite the continued presence of the LD cycle. This lack of behavioral reentrainment necessarily means that molecular oscillators in the master circadian pacemaker, the SCN, were unable to reentrain as well. The authors tested the hypothesis that a phase shift of the LD cycle rendered the SCN incapable of responding to photic input. Animals were exposed to a 5-h phase delay of the photocycle, and activity rhythms were monitored until a lack of reentrainment was confirmed. Hamsters were then housed in constant darkness for 24 h and administered a 30-min light pulse 2 circadian hours after activity onset. Brains were then removed, and tissue sections containing the SCN were processed for in situ hybridization. Sections were probed with Siberian hamster c-fos and per1 mRNA probes because light rapidly induces these 2 genes in the SCN during subjective night but not at other circadian phases. Light pulses induced robust expression of both genes in all animals that reentrained to the LD cycle, but no expression was observed in any animal that failed to reentrain. None of the animals exhibited an intermediate response. This finding is the first report of acute shift in a photocycle eliminating photosensitivity in the SCN and suggests that a specific pattern of light exposure may desensitize the SCN to subsequent photic input.     

The effect of light on melatonin secretion in the cultured pineal glands of Anolis lizards

Melatonin, a hormone produced by the pineal gland, is important for regulating circadian rhythms in many animals. Light at night causes an acute suppression of melatonin in nearly all vertebrate species. A previous study found that light failed to suppress melatonin in the lizard Anolis carolinensis. This is a surprising result given that the Anolis pineal gland is intrinsically photosensitive, is a key pacemaker controlling locomotor activity, and can be directly entrained to a light-dark cycle. To find out if the lack of photic suppression is widespread in the Anolis genus, we investigated the acute effects of light on melatonin secretion in five different species of Anolis using flow-through tissue culture. We administered a two-hour pulse of bright light to isolated pineal glands during the night. The results show photic suppression of melatonin in all five Anolis species, but the suppression is weak relative to that seen in other vertebrates. Moreover, Anolis species differ in the magnitude of the effect. These findings are discussed in the context of vertebrate pineal evolution and the ecology of Anolis lizards. Given their extensive phylogenetic and ecological divergence, Anolis lizards provide a promising system for investigating the ecology and evolution of circadian organization.     

Djungarian hamsters: a species with a labile circadian pacemaker? Arrhythmicity under a light-dark cycle induced by short light pulses

In most cases, phase-shifting effects of light pulses are studied in animals kept in constant darkness (DD) or in animals released into DD following the stimulus. In this study, the authors exposed Djungarian hamsters (Phodopus sungorus) to short light pulses during the dark phase of a 16:8 light-dark (LD) cycle and thus obtained a type VI phase response curve. Light pulses early in the night caused phase delays of the activity onset as well as phase advances of the activity offset, whereas light pulses later in the night resulted in phase advances of the activity offset only. A combination of two 15-min light pulses-the first one given late in the scotophase and the second given early in the dark phase of the following night-led to a strong compression of the activity phase alpha. In 75% of all animals, daily rhythms were no longer visible after complete alpha compression, and long-term arrhythmicity (up to 145 days) persisted despite continued exposure to an LD cycle. Because three independent output rhythms of the clock (i.e., activity, body temperature, and melatonin rhythms) were equally affected, the authors conclude that overt arrhythmicity was due not merely to disrupted output pathways but to an altered state of the central pacemaker. The authors suggest a qualitative two-oscillator model to explain this phenomenon. Their hypothesis assumes that, due to loose coupling, the pacemaker of Djungarian hamsters can be driven to a state of zero phase difference between the two oscillators, with zero amplitude of their outputs.     

The two-oscillator circadian system of tree shrews (Tupaia belangeri) and its response to light and dark pulses

The wheel-running activity rhythm of tree shrews (tupaias; Tupaia belangeri) housed in constant darkness (DD) phase-advanced following a 3-hr light pulse at circadian time (CT) 21. Dark pulses of 3 hr presented to tupaias in bright constant light (LL) did not induce significant phase shifts of the free-running activity rhythm, irrespective of the CT. In dim LL, tupaias showed simultaneous splitting of their circadian rhythm of wheel-running activity, nest-box activity, and feeding behavior. Light pulses of 6 hr and 2300 lux were presented to 13 tupaias with split wheel-running activity rhythms. These light pulses induced immediate phase shifts in the two components of the split rhythm in opposite directions. No differences were observed between the light-pulse phase response curves of the two components. Equally large immediate phase advances were induced in both components by light pulses of 230 lux, but not by 23 lux. The final phase shifts were small at all CTs. In two tupaias, activity rhythms transiently split and re-fused. Analysis of the relative position of the components in one of these indicates asymmetry in the coupling between the components.     

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.     

Melatonin Shifts Human Orcadian Rhythms According to a Phase-Response Curve

A physiological dose of orally administered melatonin shifts circadian rhythms in humans according to a phase-response curve (PRC) that is nearly opposite in phase with the PRCs for light exposure: melatonin delays circadian rhythms when administered in the morning and advances them when administered in the afternoon or early evening. The human melatonin PRC provides critical information for using melatonin to treat circadian phase sleep and mood disorders, as well as maladaptation to shift work and transmeridional air travel. The human melatonin PRC also provides the strongest evidence to date for a function of endogenous melatonin and its suppression by light in augmenting entrainment of circadian rhythms by the light-dark cycle.     

Melatonin Shifts Human Orcadian Rhythms According to a Phase-Response Curve

A physiological dose of orally administered melatonin shifts circadian rhythms in humans according to a phase-response curve (PRC) that is nearly opposite in phase with the PRCs for light exposure: melatonin delays circadian rhythms when administered in the morning and advances them when administered in the afternoon or early evening. The human melatonin PRC provides critical information for using melatonin to treat circadian phase sleep and mood disorders, as well as maladaptation to shift work and transmeridional air travel. The human melatonin PRC also provides the strongest evidence to date for a function of endogenous melatonin and its suppression by light in augmenting entrainment of circadian rhythms by the light-dark cycle.     

cGMP-phosphodiesterase inhibition enhances photic responses and synchronization of the biological circadian clock in rodents

The master circadian clock in mammals is located in the hypothalamic suprachiasmatic nuclei (SCN) and is synchronized by several environmental stimuli, mainly the light-dark (LD) cycle. Light pulses in the late subjective night induce phase advances in locomotor circadian rhythms and the expression of clock genes (such as Per1-2). The mechanism responsible for light-induced phase advances involves the activation of guanylyl cyclase (GC), cGMP and its related protein kinase (PKG). Pharmacological manipulation of cGMP by phosphodiesterase (PDE) inhibition (e.g., sildenafil) increases low-intensity light-induced circadian responses, which could reflect the ability of the cGMP-dependent pathway to directly affect the photic sensitivity of the master circadian clock within the SCN. Indeed, sildenafil is also able to increase the phase-shifting effect of saturating (1200 lux) light pulses leading to phase advances of about 9 hours, as well as in C57 a mouse strain that shows reduced phase advances. In addition, sildenafil was effective in both male and female hamsters, as well as after oral administration. Other PDE inhibitors (such as vardenafil and tadalafil) also increased light-induced phase advances of locomotor activity rhythms and accelerated reentrainment after a phase advance in the LD cycle. Pharmacological inhibition of the main downstream target of cGMP, PKG, blocked light-induced expression of Per1. Our results indicate that the cGMP-dependent pathway can directly modulate the light-induced expression of clock-genes within the SCN and the magnitude of light-induced phase advances of overt rhythms, and provide promising tools to design treatments for human circadian disruptions.     

Diurnal, circadian and photic regulation of calcium/calmodulin-dependent kinase II and neuronal nitric oxide synthase in the hamster suprachiasmatic nuclei

Mammalian circadian rhythms are entrained by light pulses that induce phosphorylation events in the suprachiasmatic nuclei (SCN). Ca²⁺-dependent enzymes are known to be involved in circadian phase shifting. In this paper, we show that calcium/calmodulin-dependent kinase II (CaMKII) is rhythmically phosphorylated in the SCN both under entrained and free-running (constant dark) conditions while neuronal nitric oxide synthase (nNOS) is rhythmically phosphorylated in the SCN only under entrained conditions. Both p-CaMKII and p-NOS (specifically phosphorylated by CaMKII) levels peak during the day or subjective day. Light pulses administered during the subjective night, but not during the day, induced rapid phosphorylation of both enzymes. Moreover, we found an inhibitory effect of KN-62 and KN-93, both CaMKII inhibitors, on light-induced nNOS activity and nNOS phosphorylation respectively, suggesting a direct pathway between both enzymes which is at least partially responsible of photic circadian entrainment.     

Entrainment of 2 subjective nights by daily light:dark:light:dark cycles in 3 rodent species

Recent work with exotic 24-h light:dark:light:dark (LDLD) cycles indicates surprising flexibility in the entrainment patterns of Syrian hamsters. Following exposure to an LDLD cycle, hamsters may adopt a form of rhythm splitting in which markers of subjective night (e.g., activity, melatonin) are expressed in each of the twice daily scotophases. This pattern contrasts markedly with that of conventionally entrained hamsters in which markers of subjective night are expressed once daily in only 1 of the 2 dark periods. The "split" entrainment pattern was examined further here in Syrian and Siberian hamsters and in mice exposed to LDLD 7:5:7:5, a condition that reliably induces split activity rhythms in all 3 species. The phase angle of entrainment and activity duration were generally similar comparing the 2 daily activity bouts in each species. The stability of this split entrainment state was assessed by deletions of photophases on individual days, by exposure to skeleton photoperiods, and by transfer to constant darkness. As in Syrian hamsters, the one-time substitution of darkness for one 7-h photophase did not grossly alter activity patterns of Siberian hamsters but acutely disrupted the split rhythms of mice. Skeleton light pulses of progressively shorter duration did not significantly alter split entrainment patterns of either Syrian or Siberian hamsters. Both species continued to exhibit stable entrainment with activity expressed in alternate scotophases of an LD 1:5 cycle presented 4 times daily. In contrast, the split activity rhythms of mice were not maintained under skeleton pulses. In constant darkness, rhythms of Siberian hamsters remained distinctly split for a minimum of 2 cycles. Split entrainment to these novel LDLD and 4-pulse skeleton lighting regimes demonstrates a marked degree of plasticity common to the circadian systems of several rodent species and identifies novel entrainment patterns that may be reliably elicited with simple environmental manipulations. Inter- and intraspecific differences in the stability of split activity rhythms likely reflect differences in coupling interactions between the component circadian oscillators, which, adopting separate phase relations to these novel LD cycles, yield a split entrainment pattern.     

c-Fos expression in the brains of behaviorally "split" hamsters in constant light: calling attention to a dorsolateral region of the suprachiasmatic nucleus and the medial division of the lateral habenula

"Splitting" of circadian activity rhythms in Syrian hamsters maintained in constant light appears to be the consequence of a reorganized SCN, with left and right halves oscillating in antiphase; in split hamsters, high mRNA levels characteristic of day and night are simultaneously expressed on opposite sides of the paired SCN. To visualize the splitting phenomenon at a cellular level, immunohistochemical c-Fos protein expression in the SCN and brains of split hamsters was analyzed. One side of the split SCN exhibited relatively high c-Fos levels, in a pattern resembling that seen in normal, unsplit hamsters during subjective day in constant darkness; the opposite side was labeled only within a central-dorsolateral area of the caudal SCN, in a region that likely coincides with a photo-responsive, glutamate receptor antagonist-insensitive, pERK-expressing cluster of cells previously identified by other laboratories. Outside the SCN, visual inspection revealed an obvious left-right asymmetry of c-Fos expression in the medial preoptic nucleus and subparaventricular zone of split hamsters killed during the inactive phase and in the medial division of the lateral habenula during the active phase (when the hamsters were running in their wheels). Roles for the dorsolateral SCN and the mediolateral habenula in circadian timekeeping are not yet understood.     

Melatonin and its generating system in vertebrate retina: circadian rhythm, effect of environmental lighting and interaction with dopamine

Retinas of rats, rabbits, chicks and carp possess enzymes, i.e. serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), which convert serotonin (5-HT) to melatonin, NAT activity and melatonin levels, but not HIOMT activity, show distinct circadian rhythms, with peak values occurring during the dark (night) phase of the 12 h light-dark cycle. Exposure of the animals to light at night inhibited the night-stimulated NAT activity. Treatment of rats and rabbits with the dopaminergic agonist, apomorphine, inhibited the retinal NAT activity. Dopamine levels in the rabbit retina showed diurnal variations, with higher contents seen during the light phase of both the 12 h light-dark cycle with lights on between 06:00–18:00, and that with reversed periods of illumination (lights on between 18:00–06:00). Melatonin potently inhibited the electrically-evoked calcium-dependent release of [³H]dopamine from pieces of retina from both albino and pigmented rabbits. Our results indicate that the light-regulated melatonin-generating system does operate in the vertebrate retina. The present data, together with other findings, suggest that in the retina there is an antagonistic interplay between melatonin and dopamine. Thus, melatonin inhibits dopamine synthesis in, and release from, the retinal dopaminergic cells, whilst dopamine inhibits the night (dark)-stimulated melatonin formation by decreasing NAT activity. Since light increases metabolic activity of the retinal dopaminergic cells (it enhances the amine synthesis, levels and release), it seems likely that the retinal dopamine plays a role of a “light” messenger in the inhibition of melatonin synthesis. It is suggested that an interplay between melatonin and dopamine in the retina is responsible for regulation of those retinal events which follow circadian rhythmicity, and/or are dependent on light-dark conditions.     

Influence of light intensity on plasma melatonin and locomotor activity rhythms in tench

Melatonin production by the pineal organ is influenced by light intensity, as has been described in most vertebrate species, in which melatonin is considered a synchronizer of circadian rhythms. In tench, strict nocturnal activity rhythms have been described, although the role of melatonin has not been clarified. In this study we investigated daily activity and melatonin rhythms under 12:12 light-dark (LD) conditions with two different light intensities (58.6 and 1091 microW/cm2), and the effect of I h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1091.4 microW/cm2) applied at middarkness (MD) on nocturnal circulating melatonin. The results showed that plasma melatonin in tench under LD 12:12 and high light conditions displayed rhythmic variation, where values at MD (255.8 +/- 65.9 pg/ml) were higher than at midlight (ML) (70.7 +/- 31.9 pg/ml). Such a difference between MD and ML values was reduced in animals exposed to LD 12: 12 and low light intensity. The application of 1 h light pulses at MD lowered plasma melatonin to 111.6 +/- 3.2 pg/ml (in the 3.3-10.5 microW/cm2 range) and to 61.8 +/- 18.3 pg/ml (with the 1091.4 microW/cm2 light pulse) and totally suppressed nocturnal locomotor activity. These results show that melatonin rhythms persisted in tench exposed to low light intensity although the amplitude of the rhythm is affected. In addition, it was observed that light pulses applied at MD affected plasma melatonin content and locomotor activity. Such a low threshold suggests that the melatonin system is capable of transducing light even under dim conditions, which may be used by this nocturnal fish to synchronize to weak night light signals (e.g., moonlight cycles).     

Interactions between light and melatonin on the circadian clock of mice.

Melatonin and light synchronize the biological clock and are used to treat sleep/wake disturbances in humans. However, the two treatments affect circadian rhythms differently when they are combined than when they are administered individually. To elucidate the nature of the interaction between melatonin and light, the present study assessed the effect of melatonin on circadian timing and immediate-early gene expression in the suprachiasmatic nucleus (SCN) when administered in the presence of light. Male C3H/HeN mice, housed in constant dark in cages equipped with running wheels, were treated with either melatonin (90 microg, s.c.) or vehicle (3% ethanol-saline) 5 min prior to exposure to light (15 min, 300 lux) at various times in the circadian cycle. Combined treatment resulted in lower magnitude phase delays of circadian activity rhythms than those obtained with light alone during the early subjective night and advances in phase when melatonin and light were administered during the subjective day (p < .001). The reduction in phase delays with combined treatment at Circadian Time (CT) 14 was significant when light exposure measured 300 lux but not at lower light levels (p < .05). When light preceded melatonin administration, the inhibition of phase delays attained significance only when the light exposure reached 1000 lux (p < .05). Neither basal nor light-induced expression of c-fos mRNA in the SCN was modified by melatonin administration at CT 14 or CT 22. Together, these results suggest that combined administration of melatonin and light affect circadian timing in a manner not predicted by summing the two treatments given individually. Furthermore, the interaction is not likely to be due to inhibition of photic input to the clock by melatonin but might arise from a photically induced enhancement of melatonin's actions on circadian timing.     

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.