Masking of locomotor activity in hamsters

The inhibition of locomotion by light (masking) was investigated in Syrian hamsters. When 1-h pulses of light were presented in the early night, activity was strongly suppressed by irradiances of about 1 lx or greater. Ultradian light-dark cycles were used as another way to study masking. Hamsters were unable to entrain to 3.5:3.5-h light-dark cycles, thus permitting the masking and the entraining effects of light to be distinguished. Light had greater suppressive effects on activity in home cages than on activity in novel running wheels. Moreover, in home cages activity remained very low for about 30 min after lights were turned off. Post-pulse suppression of activity was not simply a consequence of reduced running, as shown by experiments in which running was temporarily prevented by locking the wheels. A phase response curve for masking was obtained by placing hamsters in novel wheels for 3-h periods at various times throughout their circadian cycles, and then superimposing a 30-min light pulse. The suppressive effect of light was maximal around the onset of activity, which normally coincides with dusk in hamsters. This may have adaptive value in limiting foraging to the hours of darkness. Accepted: 8 February 1999     

c-fos expression in the putative avian suprachiasmatic nucleus

c-fos induction was investigated as a potential component in the avian photic entrainment pathway and as a possible means of locating the central pacemaker in birds. In both quail (Coturnix coturnix japonica) and starlings (Sturnus vulgaris) exposure to 1 h of light induced Fos-lir in the visual suprachiasmatic nucleus but not in the medial suprachiasmatic nucleus. However, the degree of c-fos induction in the visual suprachiasmatic nucleus was similar at different circadian times despite the fact that the light pulses caused differential phase shifts in the locomotor rhythm. For golden hamsters the same experiment resulted in significantly different levels of Fos-lir in the suprachiasmatic nucleus, as well as different phase shifts. Starlings and hamsters were also entrained to T-cycles that caused a large daily phase shift (T = 21.5 h in starlings, T = 22.67 hours in hamsters), or no daily phase shift (T = free running period). No difference in the induced levels of Fos-lir in the visual suprachiasmatic nucleus region was observed between the two groups of starlings, but in hamsters there were significantly different levels of Fos-lir in the suprachiasmatic nucleus between the two groups. Accepted: 15 November 1996     

Disruption of masking by hypothalamic lesions in Syrian hamsters

Negative masking of locomotor activity by light in nocturnal rodents is mediated by a non-image-forming irradiance-detection system in the retina. Structures receiving input from this system potentially contribute to the masking response. The suprachiasmatic nucleus (SCN) regulates locomotor activity and receives dense innervation from the irradiance-detection system via the retinohypothalamic tract, but its role in masking is unclear. We studied masking in adult Syrian hamsters (Mesocricetus auratus) with electrolytic lesions directed at the SCN. Hamsters were exposed to a 3.5:3.5 ultradian light/dark cycle and their wheel-running activity was monitored. Intact hamsters showed robust masking, expressing less than 20% of their activity in the light even though light and dark occurred equally during their active times. In contrast, hamsters with lesions showed, on average, as much activity in the light as in the dark. Tracing of retinal projections using cholera toxin subunit showed that the lesions damaged retinal projections to the SCN and to the adjacent subparaventricular zone. Retinal innervation outside the hypothalamus was not obviously affected by the lesions. Our results indicate that retinohypothalamic projections, and the targets of these projections, to the SCN and/or adjacent hypothalamic areas play an important role in masking.     

The circadian <Emphasis Type="Italic">Clock</Emphasis> mutant mouse: impaired masking response to light

Synchronization of an internal clock (entrainment) and a direct response to light (masking) are complementary ways of restricting activity of an animal to day or night. The protein CLOCK has an important role in the oscillatory mechanism of mammalian pacemakers. Our data show that it is also involved in masking responses. Mice with the Clock/Clock mutation reduced their wheel running less than wildtypes when given 1-h light pulses of light (2–1,600 lx) in the night. With dimmer lights (<2 lx), there were no significant differences between mutant and wildtype mice. Impaired masking responses to light in Clock/Clock mice were confirmed in tests with ultradian light–dark cycles (3.5:3.5 h and 1:1 h). Tests with pulses of light longer than 1 h revealed that, although the mutants responded more slowly to light, they sustained the suppression of activity over the course of the 3-h tests better than wildtypes.     

A molecular explanation of interactions between photic and non-photic circadian clock-resetting stimuli

Non-photic clock-resetting events (arousal and locomotor activity) in the subjective day reduced expression of Period genes in the suprachiasmatic nucleus of hamsters. This decrease was attenuated by a 30-min light pulse occurring during the last 0.5 h of 3.5 h of confinement to a novel running wheel. This provides an example at the molecular level of an interaction between different modalities of synchronizing agents.     

Enhanced masking response to light in hamsters with IGL lesions

Syrian hamsters with intergeniculate leaflet or sham lesions were given tests with a series of light pulses of gradually decreasing intensities. The light pulses were given early in the night, at zeitgeber time 14–15. The amount of wheel running during the pulses was compared to that in the same hour on a night with no light pulses. Hamsters with intergeniculate leaflet lesions showed a significantly greater suppression of their wheel running in response to light than the sham-lesioned animals. The lesioned animals also had larger negative phase angles of entrainment to the 14:10-h light-dark cycle than sham-operated controls. However, phase shifting in response to light pulses at either zeitgeber time 14 or 18 was not significantly altered by the lesions. Preferences for spending more time in a dark than a light area were not abolished by the lesions. It is concluded that the intergeniculate leaflet in the Syrian hamster cannot be of paramount importance for masking of locomotor activity by light but may play a modulating role. Accepted: 30 January 1999     

Intensive voluntary wheel running may restore circadian activity rhythms and improves the impaired cognitive performance of arrhythmic Djungarian hamsters

Circadian rhythms are highly important not only for the synchronization of animals and humans with their periodic environment but also for their fitness. Accordingly, the disruption of the circadian system may have adverse consequences. A certain number of animals in our breeding stock of Djungarian hamsters are episodically active throughout the day. Also body temperature and melatonin lack 24-h rhythms. Obviously in these animals, the suprachiasmatic nuclei (SCN) as the central pacemaker do not generate a circadian signal. Moreover, these so-called arrhythmic (AR) hamsters have cognitive deficits. Since motor activity is believed to stabilize circadian rhythms, we investigated the effect of voluntary wheel running. Hamsters were bred and kept under standardized housing conditions with food and water ad libitum and a 14 L/10 D lighting regimen. AR animals were selected according to their activity pattern obtained by means of passive infrared motion detectors. In a first step, the daily activity behavior was investigated for 3 weeks each without and with running wheels. To estimate putative photic masking effects, hamsters were exposed to light (LPs) and DPs and also released into constant darkness for a minimum of 3 weeks. A novel object recognition (NOR) test was performed to evaluate cognitive abilities both before and after 3 weeks of wheel availability. The activity patterns of hamsters with low wheel activity were still AR. With more intense running, daily patterns with higher values in the dark time were obtained. Obviously, this was due to masking as LPs did suppress and DPs induced motor activity. When transferred to constant darkness, in some animals the daily rhythm disappeared. In other hamsters, namely those which used the wheels most actively, the rhythm was preserved and free-ran, what can be taken as indication of a reconstitution of circadian rhythmicity. Also, animals showing a 24-h activity pattern after 3 weeks of extensive wheel running were able to recognize the novel object in the NOR test but not so before. The results show that voluntary exercise may reestablish circadian rhythmicity and improve cognitive performance.     

Plasticity of Hamster Circadian Entrainment Patterns Depends on Light Intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (～350 lux) were alternated with either dim (<0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (～45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

The Annual Activity Pattern of Djungarian Hamsters (Phodopus sungorus) Is Affected by Wheel‐Running Activity

Djungarian hamsters (Phodopus sungorus) exhibit pronounced winter acclimatization with changes in body mass, gonads, fur, and thermogenic capacity induced by decreasing daylength. To determine whether the annual activity pattern reflects the crucial role of the photoperiod for the hamsters' seasonality, animals with and without access to a running wheel (RW) were exposed to natural lighting conditions (～52°N) and ambient temperatures. Registration of locomotion in hamsters with a RW revealed a clear activity pattern closely related to dusk and dawn throughout the year. In contrast, animals without RW access showed a less stable phase relationship between the activity and the day‐night cycle in autumn and winter. During these seasons, the activity phase either exceeded the dark phase or even became indistinguishable from the rest phase. This correlated not only with increased locomotion during the light phase but also over the whole 24 h period, especially in autumn. In RW hamsters, a similar but attenuated trend was found that possibly reflects foraging due to increased food hoarding before winter. The more stable correlation between activity time and night length in RW hamsters might be explained by a suppressing effect of light on wheel‐running behavior (negative masking) and/or a stabilizing effect of running exercise on rhythmicity. In a further experiment, the phase‐reference points lights‐off and lights‐on within artificial light‐dark (LD) cycles were compared to sunset and sunrise in an intermediate ratio of light and dark and in long days. With respect to the defined phase‐reference points of the zeitgeber, the phase relation between activity and the LD cycle was similar in natural and corresponding artificial lighting conditions, and dependent on the LD ratio.     

Neuropeptide Y microinjected into the suprachiasmatic region phase shifts circadian rhythms in constant darkness

The geniculohypothalamic tract (GHT) is a projection from the intergeniculate leaflet to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity and appears to communicate photic information to the SCN. Microinjection of NPY into the SCN has been found to phase shift circadian rhythms of hamsters housed in constant light in a manner similar to the phase shifts produced by pulses of darkness or triazolam injections. In the present study, NPY was injected into the SCN of Syrian hamsters housed in constant darkness and was found to produce phase shifts similar to those seen in hamsters housed in constant light. Microinjections were not followed by wheel running during the subjective day (the time when NPY microinjections are followed by significant phase advances). These data suggest that NPY produces phase shifts by some mechanism other than by inducing wheel running or by inhibiting the response of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian clock.     

Circadian clock resetting by sleep deprivation without exercise in Syrian hamsters: dark pulses revisited

Circadian rhythms in Syrian hamsters can be phase shifted by procedures that stimulate wheel running ("exercise") in the mid-subjective day (the hamster's usual sleep period). The authors recently demonstrated that keeping hamsters awake by gentle handling, without continuous running, is sufficient to mimic this effect. Here, the authors assessed whether wakefulness, independent of wheel running, also mediates phase shifts to dark pulses during the midsubjective day in hamsters free-running in constant light (LL). With running wheels locked during a 3 h dark pulse on day 3 of LL, hamsters (N = 16) averaged only 43+/-15 min of spontaneous wake time and phase shifted only 24+/-43 min. When wheels were open during a dark pulse, two hamsters remained awake, ran continuously, and showed phase advance shifts of 7.3 h and 8.7 h, respectively, whereas the other hamsters were awake <60 min and shifted only 45+/-38 min. No animals stayed awake for 3 h without running. Additional time in LL (10 and 20 days) did not potentiate the waking or phase shift response to dark pulses. When all hamsters were sleep deprived with wheels locked during a dark pulse, phase advance shifts averaged 261+/-110 min and ranged up to 7.3 h. These shifts are large compared to those previously observed in response to the 3 h sleep deprivation procedure. Additional tests revealed that this potentiated shift response is dependent on LL prior to sleep deprivation but not LL after sleep deprivation. A final sleep deprivation test showed that a small part of the potentiation may be due to suppression of spontaneous wheel running by LL. These results indicate that some correlate of waking, other than continuous running, mediates the phase-shifting effect of dark pulses in the mid-subjective day. The mechanism by which LL potentiates shifting remains to be determined. The lack of effect of subsequent LL on the magnitude of shifts to sleep deprivation in the dark suggests that LL reduces responsivity to light by processes that take >3 h of dark to reverse.     

Running activity mediates the phase-advancing effects of dark pulses on hamster circadian rhythms

Summary Pulses of darkness can phase-shift the circadian activity rhythms of hamsters,Mesocricetus auratus, kept in constant light. Dark pulses under these conditions alter photic input to the circadian system, but they also commonly trigger wheel-running activity. This paper investigates the contribution of running activity to the phase-shifting effects of dark pulses. A first experiment showed that running activity by itself can phaseshift rhythms in constant light. Hamsters were induced to run by being confined to a novel wheel for 3–5 h. When this was done at circadian times (CT) 0, 6, and 9, the mean steady-state phase-shifts were 0.6 h, 3.5 h, and 2.3 h, respectively. The latter two values are at least as large as those previously obtained with dark pulses of similar durations and circadian phases. A second experiment showed that restricting the activity of hamsters during 3-h dark pulses at CT 9 reduces the amplitude of the phase-shifts. Unrestrained animals phase-advanced by 1.1 h, but this shift was halved in animals whose wheel was locked, and completely abolished in animals confined to nest boxes during the dark pulse. Activity restriction in itself (without dark pulses) had only minimal phase-delaying effects on free-running rhythms when given between ca. CT 10 and CT 13. These results support the idea that, in hamsters at least, dark pulses affect the circadian system mostly by altering behavioural states rather than by altering photic input to the internal clock.Abbreviations CT circadian time - DD constant darkness - LD light-dark - LL constant light - PRC phase response curve - period of rhythm     

Plasticity of hamster circadian entrainment patterns depends on light intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (approximately 350 lux) were alternated with either dim (< 0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (approximately 45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

The effect of taurine depletion on the contractile properties and fatigue in fast-twitch skeletal muscle of the mouse

Summary. Taurine as well as tauret (retinyliden taurine) levels were measured in locust Locusta migratoria compound eyes. HPLC measurements revealed relatively low taurine levels (1.9 ± 0.16 mM) in dark-adapted eyes. Glutamate, aspartate and glycine levels were 2.0 ± 0.2, 2.7 ± 0.4 and 3.0 ± 0.37 mM, respectively, while GABA was present only in trace amounts. After about 4 h of light adaptation at 1500–2000 lx, amino acid levels in the compound eye were as follows: taurine, 1.8 ± 0.17 mM; glutamate, no change at 2.1 ± 0.2 mM; aspartate sharply increased to 4.7 ± 0.7 mM; glycine slightly decreased to 2.8 ± 0.3 mM; and GABA trace levels. In the compound eye of locust Locusta migratoria, the existence of endogenous tauret in micro-molar range was established. In the dark, levels were several times higher compared with compound eye after light adaptation 1500 lx for 3 h, as estimated by TLC in combination with spectral measurements. Existence of tauret in compound eye is of special interest because in the compound eye, rhodopsin regeneration is based on photoregeneration.     

Age-related changes in circadian responses to dark pulses

Aging involves many alterations in circadian rhythms, including a loss of sensitivity to both photic and nonphotic time signals. This study investigated the sensitivity of young and old hamsters to the phase advancing effect of a 6-h dark pulse on the locomotor activity rhythm. Each hamster was tested four times during a period of approximately 9 mo; periods of exposure to a 14-h photoperiod were alternated with the periods of exposure to constant light (20-80 lx), during which the dark pulses were administered. There was no significant difference in the phase shifts exhibited by the young (4-10 mo) and old hamsters (19-25 mo) or in the amount of wheel running activity displayed during each dark pulse. However, young hamsters had a significantly greater propensity to exhibit split rhythms immediately after the dark pulses. These results suggest that, although aging does not reduce the sensitivity of the circadian pacemaker to this nonphotic signal, it alters one property of the pacemaker, i.e., the flexibility of the coupling of its component oscillators.     

Lesions of the Intergeniculate Leaflet Lead to a Reorganization in Circadian Regulation and a Reversal in Masking Responses to Photic Stimuli in the Nile Grass Rat

Light influences the daily patterning of behavior by entraining circadian rhythms and through its acute effects on activity levels (masking). Mechanisms of entrainment are quite similar across species, but masking can be very different. Specifically, in diurnal species, light generally increases locomotor activity (positive masking), and in nocturnal ones, it generally suppresses it (negative masking). The intergeniculate leaflet (IGL), a subdivision of the lateral geniculate complex, receives direct retinal input and is reciprocally connected with the primary circadian clock, the suprachiasmatic nucleus (SCN). Here, we evaluated the influence of the IGL on masking and the circadian system in a diurnal rodent, the Nile grass rat (Arvicanthis niloticus), by determining the effects of bilateral IGL lesions on general activity under different lighting conditions. To examine masking responses, light or dark pulses were delivered in the dark or light phase, respectively. Light pulses at Zeitgeber time (ZT) 14 increased activity in control animals but decreased it in animals with IGL lesions. Dark pulses had no effect on controls, but significantly increased activity in lesioned animals at ZT0. Lesions also significantly increased activity, primarily during the dark phase of a 12:12 light/dark cycle, and during the subjective night when animals were kept in constant conditions. Taken together, our results suggest that the IGL plays a vital role in the maintenance of both the species-typical masking responses to light, and the circadian contribution to diurnality in grass rats.     

Phase shifting by novelty-induced running: activity dose-response curves at different circadian times

This study compared phase shifting after novelty-induced running at different circadian times (CTs). In Experiment 1, hamsters were confined to novel wheels for 3 h, starting at CTs 2, 4, 6, 8, 10 or 22. The largest shifts were found at CTs 2, 4 and 6. At each CT there was a relationship between the number of revolutions during the pulse and the size of phase shift. Maximum shifts were usually observed at each CT when animals ran 5000–9000 revolutions during the pulse. In Experiment 2, hamsters were confined to novel wheels for 1 h, also starting at CTs 2, 4, 6, 8, 10 or 22. Unlike with 3-h pulses, the largest shifts with 1-h pulses occurred at CT 8. In Experiment 3, hamsters were shut into a small nest box after a 1-h pulse at CT 8; phase shifting was unaffected, showing that movement about the home cage after a 1-h pulse had ended was not required for shifting. At CTs 2, 4 and 22, 3-h pulses produced shifts but 1-h pulses did not. Possibly, there are two different mechanisms of nonphotic phase shifting that can be activated by being placed in a novel wheel, but the results can also be explained in terms of a single mechanism. Accepted: 8 August 1997     

Dim nocturnal illumination alters coupling of circadian pacemakers in Siberian hamsters,<Emphasis Type="Italic"> Phodopus sungorus</Emphasis>

The circadian pacemaker of mammals comprises multiple oscillators that may adopt different phase relationships to determine properties of the coupled system. The effect of nocturnal illumination comparable to dim moonlight was assessed in male Siberian hamsters exposed to two re-entrainment paradigms believed to require changes in the phase relationship of underlying component oscillators. In experiment 1, hamsters were exposed to a 24-h light-dark-light-dark cycle previously shown to split circadian rhythms into two components such that activity is divided between the two daily dark periods. Hamsters exposed to dim illumination (<0.020 lx) during each scotophase were more likely to exhibit split rhythms compared to hamsters exposed to completely dark scotophases. In experiment 2, hamsters were transferred to winter photoperiods (10 h light, 14 h dark) from two different longer daylengths (14 h or 18 h light daily) in the presence or absence of dim nighttime lighting. Dim nocturnal illumination markedly accelerated adoption of the winter phenotype as reflected in the expansion of activity duration, gonadal regression and weight loss. The two experiments demonstrate substantial efficacy of light intensities generally viewed as below the threshold of circadian systems. Light may act on oscillator coupling through rod-dependent mechanisms.Abbreviations activity duration - DD constant dark or dim - E evening oscillator - ETV estimated testis volume - LDLD light-dark-light-dark cycle - LED light emitting diode - M morning oscillator - SCN suprachiasmatic nuclei - free-running period     

Vesicular glutamate transporter 2 (VGLUT2) is co-stored with PACAP in projections from the rat melanopsin-containing retinal ganglion cells

The retinal ganglion cell layer of the eye comprises a subtype of cells characterized by their intrinsic photosensitivity and expression of melanopsin (ipRGCs). These cells regulate a variety of non-image-forming (NIF) functions such as light entrainment of circadian rhythms, acute suppression of locomotor activity (masking), and pupillary light reflex. Two neurotransmitters have been identified in ipRGCs, glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP). To date, little is known about their release and interplay. Here, we describe the presence and co-localization of vesicular glutamate transporter 2 (VGLUT2; a marker of glutamate signaling) and PACAP in ipRGCs and their projections in the brain. Nine adult male Wistar rats were assigned to one of three groups; anterograde tracing (n = 3), eye enucleation (n = 3), and untreated (n = 3). Under anaesthesia, rats were transcardially perfusion-fixated, after which the brains and eyes were removed for double immunohistochemical staining using a polyclonal anti-VGLUT2 antibody and a mouse monoclonal anti-PACAP antibody. Results revealed that VGLUT2- and PACAP-immunoreactivity (-ir) were present in ipRGCs and co-localized in their projections in the suprachiasmatic nucleus, the intergeniculate leaflet, and the olivary pretectal nucleus. We conclude that there is evidence to support the use of glutamate and PACAP as neurotransmitters in NIF photoperception by rat ipRGCs, and that these neurotransmitters are co-stored and probably released from the same nerve terminals. Furthermore, we conclude that VGLUT2 is the preferred subtype of vesicular transporter used by these cells.     

THREE DAYS OF NOVEL WHEEL ACCESS DIMINISHES LIGHT-INDUCED PHASE DELAYS IN VIVO WITH NO EFFECT ON PER1 INDUCTION BY LIGHT

The mammalian circadian clock, located in the hypothalamic suprachiasmatic nuclei, synchronizes endogenous behavioral and physiological rhythms to a 24h period through responses to two types of stimuli: photic (light) and nonphotic (behaviorally induced arousal and/or increases in activity). Photic stimuli can block nonphotic effects and vice versa, although the mechanisms and levels of interactions between these two stimuli types are unknown. Here, we investigated whether 3 d of access to a novel running wheel alters the phase shift to light in vivo, and whether this effect could be seen on induction by light of the circadian gene per1. Through measurement of running wheel activity of golden hamsters, access to a new wheel for 3 d was shown to diminish photic phase delays with no effect on phase advances. As seen using in situ hybridization, however, there was no effect on levels of light-induced per1 mRNA. This study indicates a possible role for this paradigm as a model of interactions between photic and nonphotic stimuli.