Behavioral inhibition of circadian responses to light

Light exposure during daytime modulates expression of Per1 and Per2 clock genes in the suprachiasmatic nuclei of mice

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the master circadian clock in mammals. Nocturnal light pulses that reset the circadian clock also lead to rapid increases in levels of Per1 and Per2 mRNA in the SCN, suggesting that these genes are involved in the synchronization to light. During the day, when light has no phase-shifting effects in nocturnal rodents, the consequences of light exposure for Per expression have been less thoroughly studied. Therefore, the effects of light exposure during the day were assessed on Per1 and Per2 mRNA in the SCN of mice. Expression of Per1 and Per2 was generally increased by 30-min light pulses during the subjective day, with more pronounced effects in the morning. One exception was noted for a transient decrease in Per2 expression after a short light pulse applied at midday. Prolonged light exposure (up to 3 hr) starting at midday markedly increased Per2 expression but not that of Per1. Moreover, the amplitude of the daily variations of both Per and the duration of Per1 peak was increased in mice exposed to a light-dark cycle compared with those transferred to constant darkness. Finally, the amplitude of the daily variations of both Per and the basal level of Per1 were increased in mice under a light-dark cycle compared with animals synchronized to a skeleton photoperiod (i.e., with daily dawn and dusk 1-hr exposures to light). Taken together, the results indicate that prolonged light exposure during daytime positively modulates daily levels of Per1 and Per2 mRNA in the SCN of mice.

Response of the mouse circadian system to serotonin 1A/2/7 agonists in vivo: surprisingly little

Serotonin (5-HT) is thought to play a role in regulating nonphotic phase shifts and modulating photic phase shifts of the mammalian circadian system, but results with different species (rats vs. hamsters) and techniques (in vivo vs. in vitro; systemic vs. intracerebral drug delivery) have been discordant. Here we examined the effects of the 5-HT1A/7 agonist 8-OH-DPAT and the 5-HT1/2 agonist quipazine on the circadian system in mice, with some parallel experiments conducted with hamsters for comparative purposes. In mice, neither drug, delivered systemically at a range of circadian phases and doses, induced phase shifts significantly different from vehicle injections. In hamsters, quipazine intraperitoneally (i.p.) did not induce phase shifts, whereas 8-OH-DPAT induced phase shifts after i.p. but not intra-SCN injections. In mice, quipazine modestly increased c-Fos expression in the SCN (site of the circadian pacemaker) during the subjective day, whereas 8-OH-DPAT did not affect SCN c-Fos. In hamsters, both drugs suppressed SCN c-Fos in the subjective day. In both species, both drugs strongly induced c-Fos in the paraventricular nucleus (within-subject positive control). 8-OH-DPAT did not significantly attenuate light-induced phase shifts in mice but did in hamsters (between-species positive control). These results indicate that in the intact mouse in vivo, acute activation of 5-HT1A/2/7 receptors in the circadian system is not sufficient to reset the SCN pacemaker or to oppose phase-shifting effects of light. There appear to be significant species differences in the susceptibility of the circadian system to modulation by systemically delivered serotonergics.

Contribution of classic photoreceptors to entrainment

The ability to phase shift and entrain in response to light is spared in retinally degenerate mice (rd/rd). In the present work, fewer retinally degenerate C57BL/6 mice than wildtypes entrained in dim lights, suggesting that rods and/or cones contribute toward entrainment even though they are not necessary. Thresholds for entrainment appear to be a more sensitive test of deficits in entrainment than phase shifts in response to light pulses.

Attenuation of phase shifts to light by activity or neuropeptide Y: a time course study

Circadian rhythms in mammals can be synchronised to photic and non-photic stimuli. Interactions between photic and behavioural stimuli were investigated during the late subjective night, 6 h after activity onset in Syrian hamsters (CT18). Light pulses of 130 lx for 15 min at this time resulted in phase advance shifts. Novel wheel exposure, for a period of 3 h, following photic stimulation was able to attenuate the phase advancing effects of light. A time delay of up to 60 min between photic and behavioural stimuli also resulted in significant attenuation of light-induced phase shifts (P<0.05). A 90-min interval between stimuli resulted in no significant attenuation. Novel wheel exposure mediates its effects via the intergeniculate leaflet, which conveys information to the SCN and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Phase shifts to light pulses given at CT18 were attenuated by NPY administration. Neuropeptide Y injections up to 60 min post-light exposure significantly attenuated phase shifts by 50% on average. However a 90-min interval between light and NPY microinjection did not significantly affect light-induced phase shifts. These results confirm previous work indicating that novel wheel exposure and NPY administration can modulate light-induced phase shifts during the late night. Further, they show for the first time that the time course for this interaction is similar between wheel running and NPY. Most significantly, our work indicates that the time course in vivo in the late night is similar to that shown previously in vitro during the early night.

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 24 h 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.

Neuropeptide Y differentially suppresses per1 and per2 mRNA induced by light in the suprachiasmatic nuclei of the golden hamster

Neuropeptide Y (NPY), present in an input pathway to the suprachiasmatic nuclei (SCN), can block the effects of light on circadian rhythms. The authors have studied this interaction using an in vitro brain slice technique. Effects of NPY on light-induced period1 and period2 mRNA in the SCN were examined in vitro following a light pulse during early subjective night. Golden hamsters (n = 91) were housed under a 14:10 LD cycle and then moved to constant dim red light for 3 days. Hamsters were exposed to a 5-min light pulse previously shown to induce phase shifts and prepared for in vitro application of NPY. Hypothalamic slices containing the SCN were maintained in vitro for 40 min to 4 h after the light pulse, then quick-frozen. Sections were evaluated by in situ hybridization with [35S]-labeled cRNA probes for per mRNA. Rapid light induction of both per1 and per2 by 40 min and 1 h after the light pulse, respectively, was apparent, with NPY inhibition of this response significant by at least these same time points. However, although striking suppression of per2 mRNA by the NPY continued through the peak for per2 at 2 h, per1 mRNA levels rebounded quickly to equal the per1 induction peak at 1 h and mirrored the control light induction pattern for per1 thereafter. Delaying NPY to 30 min after slice preparation demonstrated that NPY is capable of suppressing peak per1 levels. These results confirm the feasibility of measuring light-induced gene expression in the SCN in vitro. A differential regulation of per1 and per2 transcription might be of critical importance for the modulation of circadian responses to light.

Circadian phase-delaying effects of bright light alone and combined with exercise in humans

In a within-subjects (n = 18), counterbalanced design, the circadian phase-shifting effects of 3 h of 1) bright light (3,000 lx) alone 2) and bright light combined with vigorous exercise were compared. For each treatment, volunteers spent 3 nights and 2 days in the laboratory, typically receiving the treatment from approximately 2300 to 0200 on night 2. Bedtimes and waketimes were fixed to the volunteers' habits. Illumination was 50 lx during other wake hours and 0 lx during sleep. Bright Light Alone elicited a significant phase delay in rectal temperature minimum (70 min), but not in urinary 6-sulphatoxymelatonin (6-SMT) acrophase (20 min). Bright Light + Exercise elicited a significant phase delay in 6-SMT (68 min), but did not result in a significant difference in shift compared with Bright Light Alone. The study had adequate statistical power (80%) to detect phase-shift differences between treatments of approximately 2-2.5 h. Thus any antagonism of light shifts with exercise could not have been revealed. Within the limited exercise and light parameters of this study, the results suggest that exercise does not reliably modulate phase-shifting effects of late night bright light in humans.

Glutamate blocks serotonergic phase advances of the mammalian circadian pacemaker through AMPA and NMDA receptors

The phase of the mammalian circadian pacemaker, located in the suprachiasmatic nucleus (SCN), is modulated by a variety of stimuli, most notably the environmental light cycle. Light information is perceived by the circadian pacemaker through glutamate that is released from retinal ganglion cell terminals in the SCN. Other prominent modulatory inputs to the SCN include a serotonergic projection from the raphe nuclei and a neuropeptide Y (NPY) input from the intergeniculate leaflet. Light and glutamate phase-shift the SCN pacemaker at night, whereas serotonin (5-HT) and NPY primarily phase-shift the pacemaker during the day. In addition to directly phase-shifting the circadian pacemaker, SCN inputs have been shown to modulate the actions of one another. For example, 5-HT can inhibit the phase-shifting effects of light or glutamate applied to the SCN at night, and NPY and glutamate inhibit phase shifts of one another. In this study, we explored the possibility that glutamate can modulate serotonergic phase shifts during the day. For these experiments, we applied various combinations of 5-HT agonists, glutamate agonists, and electrical stimulation of the optic chiasm to SCN brain slices to determine the effect of these treatments on the rhythm of spontaneous neuronal activity generated by the SCN circadian pacemaker. We found that glutamate agonists and optic chiasm stimulation inhibit serotonergic phase advances and that this inhibition involves both AMPA and NMDA receptors. This inhibition by glutamate may be indirect, because it is blocked by both tetrodotoxin and the GABA(A) antagonist, bicuculline.

Sleep deprivation decreases phase-shift responses of circadian rhythms to light in the mouse: role of serotonergic and metabolic signals

The circadian pacemaker in the suprachiasmatic nuclei is primarily synchronized to the daily light-dark cycle. The phase-shifting and synchronizing effects of light can be modulated by non-photic factors, such as behavioral, metabolic or serotonergic cues. The present experiments examine the effects of sleep deprivation on the response of the circadian pacemaker to light and test the possible involvement of serotonergic and/or metabolic cues in mediating the effects of sleep deprivation. Photic phase-shifting of the locomotor activity rhythm was analyzed in mice transferred from a light-dark cycle to constant darkness, and sleep-deprived for 8 h from Zeitgeber Time 6 to Zeitgeber Time 14. Phase-delays in response to a 10-min light pulse at Zeitgeber Time 14 were reduced by 30% in sleep-deprived mice compared to control mice, while sleep deprivation without light exposure induced no significant phase-shifts. Stimulation of serotonin neurotransmission by fluoxetine (10 mg/kg), a serotonin reuptake inhibitor that decreases light-induced phase-delays in non-deprived mice, did not further reduce light-induced phase-delays in sleep-deprived mice. Impairment of serotonin neurotransmission with p-chloroamphetamine (three injections of 10 mg/kg), which did not increase light-induced phase-delays in non-deprived mice significantly, partially normalized light-induced phase-delays in sleep-deprived mice. Injections of glucose increased light-induced phase-delays in control and sleep-deprived mice. Chemical damage of the ventromedial hypothalamus by gold-thioglucose (600 mg/kg) prevented the reduction of light-induced phase-delays in sleep-deprived mice, without altering phase-delays in control mice. Taken together, the present results indicate that sleep deprivation can reduce the light-induced phase-shifts of the mouse suprachiasmatic pacemaker, due to serotonergic and metabolic changes associated with the loss of sleep.

The neuropeptide Y Y5 receptor mediates the blockade of "photic-like" NMDA-induced phase shifts in the golden hamster

Circadian or daily rhythms generated from the mammalian suprachiasmatic nuclei (SCN) of the hypothalamus can be synchronized by light and nonphotic stimuli. Whereas glutamate mediates photic information, nonphotic information can in some cases be mediated by neuropeptide Y (NPY) or serotonin. NPY or serotonin can reduce the phase-resetting effect of light or glutamate; however, the mechanisms and level of interaction of these two kinds of stimuli are unknown. Here we investigate the effect of NPY on the NMDA-induced phase shift of the hamster SCN circadian neural activity rhythm by means of single-unit recording techniques. NMDA (10-100 microm) applied in the early subjective night induced phase delays in the time of peak firing, whereas doses in the millimolar range disrupted firing patterns. The NMDA-induced phase delay was blocked by coapplication of NPY (0.02-200 microm). NPY Y1/Y5 and Y5 receptor agonists, but not the Y2 receptor agonist, blocked the NMDA-induced phase delay in a similar manner as NPY. The coapplication of a Y5 but not Y1 receptor antagonist eliminated NPY blockade of NMDA-induced phase delays, suggesting that the Y5 receptor is capable of mediating the inhibitory effect of NPY on photic responses. These results indicate that nonphotic and photic stimuli may interact at a level at or beyond NMDA receptor response and indicate that the Y5 receptor is involved in this interaction. Alteration of Y5 receptor function may therefore be expected to alter synchronization of circadian rhythms to light.

Opposing effects of behavioural activity and light on neurons of the suprachiasmatic nucleus

The mammalian circadian pacemaker is located in the suprachiasmatic nuclei. It can be shifted in phase by photic cues and by the behavioural activity of the animal. When presented together, light and behavioural activity attenuate each others' phase-shifting effect. Still unclear is how behavioural activity affects the suprachiasmatic nuclei and how it interacts with photic information. Previously, we reported the occurrence of behaviourally induced suppressions of neuronal activity. The present study investigates the characteristics of these suppressions as a function of circadian time and, additionally, in the presence of photic cues. We performed long-term multiunit activity recordings of neurons in freely moving rats and found that these suppressions of neuronal firing in the suprachiasmatic nucleus occurred at every phase of the circadian cycle. The magnitude of the suppressions showed a circadian variation, with larger suppressions during subjective day. When a light pulse was applied during a suppression, light and activity appeared to oppose each others' effects within the recorded population of neurons. The resulting discharge level appeared to be the sum of both responses. The opposing effects of light and activity were also found in single unit recordings, indicating that photic and behavioural stimuli interact at the level of a single neuron.

Neuropeptide Y in the mammalian circadian system: effects on light-induced circadian responses

The present review examine the role of neuropeptide (NPY) in the circadian system, focusing on the interactions between light and NPY, especially during the subjective night. NPY has two different effects on the circadian system of mammals. On one hand, NPY, similar to behavioral stimulation, can change the phase of the clock by itself during the subjective day. On the other hand, NPY, again similar to behavioral stimulation, can inhibit the phase-shifting effect of light during the night. These effects of NPY may occur through different receptor subtypes, the Y2 receptor mediating day-time effects and the Y5 receptor mediating night-time effects of NPY. Our results also indicate that there are differences between in vivo and in vitro studies: NPY inhibition of in vivo light-induced phase shifts was observed only late in the subjective night; however, NPY applied in vitro could block light-induced phase shifts early in the subjective night as well. Contrasting these in vivo and in vitro results led us to suggest that the time of day of maximal effect of NPY in the intact animal may be a time when exogenous administration of NPY has little effect, due to saturation of the system. This situation could be an example of how the measurable output of the clock can be affected by the behavioral state in a different way at different time points, depending not only on the clock itself but also on behavior. If verified in human beings, the ability of NPY to modulate the circadian-clock responses to light may be of clinical importance.

Circadian binding activity of AP-1, a regulator of the arylalkylamine N-acetyltransferase gene in the rat pineal gland, depends on circadian Fra-2, c-Jun, and Jun-D expression and is regulated by the clock's zeitgebers

The daily rhythm in circulating melatonin is driven by a circadian rhythm in the expression of the arylalkylamine N:-acetyltransferase gene in the rat pineal gland. Turning off expression of this gene at the end of night is believed to involve inhibitory transcription factors, among which Fos-related antigen 2 (Fra-2) appears as a good candidate. Circadian rhythms in the expression of three proteins of activating protein-1 (AP-1) complexes, namely, Fra-2, c-Jun, and Jun-D, are shown here to account for circadian variations in AP-1 binding activity. Quantitative variations in the Fra-2 component over the circadian cycle were associated with qualitative variations in protein isoforms. Destruction of the suprachiasmatic nucleus resulted in decreased nocturnal AP-1 activity, showing that AP-1 circadian rhythm is driven by this nucleus. Exposure to light during subjective night and administration of a serotonin 5-HT(1A)/5-HT(7) receptor agonist during subjective day, respectively, induced a 50% decrease and a 50% increase in both AP-1 and Fra-2 expression. These effects were impaired by suprachiasmatic nucleus lesions. These data show that pineal AP-1 binding activity, which results from Fra-2 expression, can be modulated by light and serotonin through the suprachiasmatic nucleus according to a "phase dependence" that is characteristic of the rhythm of clock sensitivity to both zeitgebers.

Behavioral feedback regulation of circadian rhythm phase angle in light-dark entrained mice

Induced and spontaneous wheel running can alter the phase and period (tau) of circadian rhythms in rodents. The relationship between spontaneous running and the phase angle (psi) of entrainment to 24-h light-dark (LD) cycles was evaluated in C57BL/6j mice. With a wheel freely available, psi was significantly correlated with the absolute (r = 0.32) and relative (r = 0.44) amount of activity during the first 2 h of the activity period. When wheels were locked during the first half of the night in LD and then unlocked in constant dark (DD), mice exhibited a delayed psi and lengthened tau compared with mice that had wheels locked during the second half of the night. In DD, tau correlated negatively with total daily activity. To evaluate if wheel running modulates the phase-resetting actions of LD, phase shifts to light pulses were measured at two time points in DD, when daily activity levels differed by 40%. Phase delays to light were 56% greater when activity levels were lower. However, in a counterbalanced follow-up experiment, phase advances and delays to light pulses were not affected by the availability of wheels, although an effect of time in DD was replicated. Spontaneous activity can regulate psi and tau without altering the response of the pacemaker to light.

Photic phase response curve in Octodon degus: assessment as a function of activity phase preference

Light exposure during the early and late subjective night generally phase delays and advances circadian rhythms, respectively. However, this generality was recently questioned in a photic entrainment study in Octodon degus. Because degus can invert their activity phase preference from diurnal to nocturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activity phase preference in a 24-h light-dark cycle (LD 12:12), degus were released in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many circadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-24: n = 7). The circadian phase of body temperature (Tb) onset was defined as CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when delivered during the early (CT 13-16) and late (CT 20-23) subjective night, respectively. No significant phase shifts were observed during the middle of the subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar to most other diurnal and nocturnal species, suggesting that entrainment mechanisms do not determine overt diurnal and nocturnal behavior.

Age-dependent changes of the circadian system

This review summarizes the current knowledge on changes of the circadian system in advanced age, mainly for rodents. The first part is dedicated to changes of the overt rhythms. Possible causes are discussed, as are methods to treat the disturbances. In aging animals and humans, all rhythm characters change. The most prominent changes are the decrease of the amplitude and the diminished ability to synchronize with a periodic environment. The susceptibility to photic and nonphotic cues is decreased. As a consequence, both internal and external temporal order are disturbed under steady-state conditions and, even more, following changes in the periodic environment. Due to the high complexity of the circadian system, which includes oscillator(s), mechanisms of external synchronization and of internal coupling, the changes may arise for several reasons. Many of the changes seem to occur within the SCN itself. The number of functioning neurons decreases with advancing age and, probably, so does the coupling between them. As a result, the SCN is unable, or at least less able, to produce stable rhythms and to transmit timing information to target sites. Initially, only the ability to synchronize with the periodic environment is diminished, whereas the rhythms themselves continue to be well pronounced. Therefore, the possibility exists to treat age-dependent disturbances. This can be done pharmacologically or by increasing the zeitgeber strength. So, some of the rhythm disturbances can be reversed, increasing the magnitude of the light-dark (LD) zeitgeber. Another possibility is to strengthen feedback effects, for example, by increasing the daily amount of activity. By this means, the stability and synchronization of the circadian activity rhythm of old mice and men were improved.

Intermittent bright light and exercise to entrain human circadian rhythms to night work

Bright light can phase shift human circadian rhythms, and recent studies have suggested that exercise can also produce phase shifts in humans. However, few studies have examined the phase-shifting effects of intermittent bright light, exercise, or the combination. This simulated night work field study included eight consecutive night shifts followed by daytime sleep/dark periods (delayed 9 h from baseline). There were 33 subjects in a 2 x 2 design that compared 1) intermittent bright light (6 pulses, 40-min long each, at 5,000 lx) versus dim light and 2) intermittent exercise (6 bouts, 15-min long each, at 50-60% of maximum heart rate) versus no exercise. Bright light and exercise occurred during the first 6 h of the first three night shifts. The circadian phase marker was the demasked rectal temperature minimum. Intermittent bright-light groups had significantly larger phase delays than dim-light groups, and 94% of subjects who received bright light had phase shifts large enough for the temperature minimum to reach daytime sleep. Exercise did not affect phase shifts; neither facilitating nor inhibiting phase shifts produced by bright light.

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.

5-HT1B receptor-mediated presynaptic inhibition of retinal input to the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain, and serotonin (5-HT) can modify the response of the SCN circadian oscillator to light. 5-HT1B receptor-mediated presynaptic inhibition has been proposed as one mechanism by which 5-HT modifies retinal input to the SCN (Pickard et al., 1996). This hypothesis was tested by examining the subcellular localization of 5-HT1B receptors in the mouse SCN using electron microscopic immunocytochemical analysis with 5-HT1B receptor antibodies and whole-cell patch-clamp recordings from SCN neurons in hamster hypothalamic slices. 5-HT1B receptor immunostaining was observed associated with the plasma membrane of retinal terminals in the SCN. 1-[3-(Trifluoromethyl)phenyl]-piperazine HCl (TFMPP), a 5-HT1B receptor agonist, reduced in a dose-related manner the amplitude of glutamatergic EPSCs evoked by stimulating selectively the optic nerve. Selective 5-HT1A or 5-HT7 receptor antagonists did not block this effect. Moreover, in cells demonstrating an evoked EPSC in response to optic nerve stimulation, TFMPP had no effect on the amplitude of inward currents generated by local application of glutamate. The effect of TFMPP on light-induced phase shifts was also examined using 5-HT1B receptor knock-out mice. TFMPP inhibited behavioral responses to light in wild-type mice but was ineffective in inhibiting light-induced phase shifts in 5-HT1B receptor knock-out mice. The results indicate that 5-HT can reduce retinal input to the circadian system by acting at presynaptic 5-HT1B receptors located on retinal axons in the SCN.

Morphine-induced activity attenuates phase shifts to light in C57BL/6J mice

Circadian rhythms can be phase shifted and entrained by daily schedules of light and by non-photic stimuli such as locomotor activity. Relatively little is known of how photic and non-photic stimuli interact to regulate circadian phase. Morphine injections were used to examine the effects of locomotor activity on phase shifts to light pulses in mice free-running in constant dark. Morphine injections scheduled early or late in the active period (subjective night) induced hyperactivity, but did not induce phase shifts. Light pulses late in the subjective night induced phase advance shifts that were significantly attenuated (63% smaller, p<0. 01) by pretreatment with morphine. This inhibitory effect of morphine on light-induced phase advances was blocked by preventing mice from running for 6 h after the injections. Light pulses early in the subjective night induced phase delay shifts that were only weakly attenuated (15% smaller, p=0.06) by morphine. These results demonstrate behavioral inhibition of light-induced phase resetting of circadian rhythms in mice, and suggest that the strength of this effect may be phase dependent, although other interpretations are possible.

Gold-thioglucose-induced hypothalamic lesions inhibit metabolic modulation of light-induced circadian phase shifts in mice

The circadian clock located in the suprachiasmatic nuclei is entrained by the 24-h variation in light intensity. The clock's responses to light can, however, be reduced when glucose availability is decreased. We tested the hypothesis that the ventromedial hypothalamus, a key area in the integration of metabolic and hormonal signals, mediates the metabolic modulation of circadian responses to light by injecting C57BL/6J mice with gold-thioglucose (0.6 g/kg) which damages glucose-receptive neurons, primarily located in the ventromedial hypothalamus. Light pulses applied during the mid-subjective night induce phase delays in the circadian rhythm of locomotor activity in mice kept in constant darkness. As previously observed, light-induced phase delays were significantly attenuated in fed mice pre-treated with 500 mg/kg i.p. 2-deoxy-D-glucose and in hypoglycemic mice fasted for 30 h, pre-treated with 5 IU/kg s.c. insulin or saline, compared to control mice fed ad libitum. In contrast, similar metabolic challenges in mice with gold-thioglucose-induced hypothalamic lesions did not significantly affect light-induced phase delays compared to mice treated with gold-thioglucose and fed ad libitum. These results indicate that destruction of gold-thioglucose-sensitive neurons in the ventromedial hypothalamus prevent metabolic regulation of circadian responses to light during shortage of glucose availability. Therefore, the ventromedial hypothalamus may be a central site coordinating the metabolic modulation of light-induced phase shifts of the circadian clock.

A nonphotic stimulus inverts the diurnal-nocturnal phase preference in Octodon degus

Mechanisms differentiating diurnal from nocturnal species are thought to be innate components of the circadian timekeeping system and may be located downstream from the circadian pacemaker within the suprachiasmatic nucleus (SCN) of the hypothalamus. In the present study, we found that the dominant phase of behavioral activity and body temperature (Tb) is susceptible to modification by a specific modality of behavioral activity (wheel-running activity) in Octodon degus, a mammal that exhibits multiple chronotypes. Seven Octodon degus exhibited diurnal Tb and locomotor activity (LMA) circadian rhythms while entrained to a 24 h light/dark cycle (LD 12:12). When the diurnal animals were provided unrestricted access to a running wheel, the overt daily rhythms in these animals inverted to nocturnal. This nocturnal pattern was sustained in constant darkness and returned to diurnal after removal of the running wheel. Six additional animals exhibited nocturnal chronotypes in LD 12:12 regardless of access to running wheels. Wheel-running activity inverted the phase preference in the diurnal animals without changing the 24 hr mean LMA or Tb levels. Because wheel running did not increase the amplitude of the pre-existing diurnal pattern, simple masking effects on LMA and Tb cannot explain the rhythm inversion. The diurnal-nocturnal inversion occurred without reversing crepuscular-timed episodes of activity, suggesting that diurnal or nocturnal phase preference is controlled separately from the intrinsic timing mechanisms within the SCN and can be dependent on behavioral or environmental factors.

Destruction of serotonergic neurons in the median raphe nucleus blocks circadian rhythm phase shifts to triazolam but not to novel wheel access

Systematic treatment of hamsters with triazolam (TRZ) or novel wheel (NW) access will yield PRCs similar to those for neuropeptide Y. Both TRZ and NW access require an intact intergeniculate leaflet (IGL) to modulate circadian rhythm phase. It is commonly suggested that both stimulus types influence rhythm phase response via a mechanism associated with drug-induced or wheel access-associated locomotion. Furthermore, there have been suggestions that one or both of these stimulus conditions require an intact serotonergic system for modulation of rhythm phase. The present study investigated these issues by making serotonin neuron-specific neurotoxic lesions of the median or dorsal raphe nuclei and evaluating phase response of the hamster circadian locomotor rhythm to TRZ treatment or NW access. The expected effect of TRZ injected at CT 6 h on the average phase advance was virtually eliminated by destruction of serotonin neurons in the median, but not the dorsal, raphe nucleus. No control or lesioned animal engaged in substantial wheel running in response to TRZ. By contrast, all median raphe-lesioned hamsters that engaged in substantial amounts of running when given access to a NW had phase shifts comparable to control or dorsal raphe-lesioned animals. The results demonstrate that serotonergic neurons in the median raphe nucleus contribute to the regulation of rhythm phase response to TRZ and that it is unlikely that these neurons are necessary for phase response to NW access. The data further suggest the presence of separate pathways mediating phase response to the two stimulus conditions. These pathways converge on the IGL, a nucleus afferent to the circadian clock, that is necessary for the expression of phase response to each stimulus type.

Expression of Fos in the circadian system following nonphotic stimulation

Syrian hamsters, Mesocricetus auratus, were confined to novel running wheels for a 3-h period, starting at approximately circadian time (CT) 4.5 (i.e., approaching the middle of their subjective day). It can be reliably predicted from the amount of running in this situation whether or not there will be a subsequent phase-shift. Expression of the immediate early genes c-fos and fosB was examined by immunocytochemistry in the suprachiasmatic nucleus (SCN), the intergeniculate leaflet (IGL) of the thalamus, and the medial pretectal area of hamsters that ran vigorously in the novel wheel and would have phase-shifted. c-Fos was increased, compared to levels in a control group left in their home cages, in the IGL, and the pretectum (PT), but decreased in the SCN. No significant changes in FosB were detected in any region examined. An additional experiment argued against the possibility that the changes in c-Fos could be attributed to a rapid advance of the pacemaker to a different phase in the circadian cycle. Counts of c-Fos-positive cells in the IGL were similar in animals given pulses of running starting at CT 4.5 and starting at CT 12.5-16 (i.e., in the subjective night when they would have been active anyway). Altogether the results support the view that activation of the IGL is important in nonphotic clock resetting, and raise the possibility that the PT may also be involved in nonphotic resetting. However, the results also indicate that novelty-induced running does not alter c-Fos induction in a phase-specific manner in the IGL. The inhibition of c-Fos in the SCN by nonphotic phase-shifting events contrasts with the well-known inducing effects of light pulses. These different effects might underlie some of the interactions between nonphotic and photic zeitgebers when both act together on the circadian system.

Neuropeptide Y blocks serotonergic phase shifts of the suprachiasmatic circadian clock in vitro

The mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) can be reset in vitro by various neurochemical stimuli. This study investigated the phase-shifting properties of neuropeptide Y (NPY) and serotonin (5-HT) agonists when applied alone, as well as their combined effects on clock resetting. These neurotransmitters have both been shown to advance the SCN clock in vitro when applied during the daytime. By monitoring the SCN neuronal activity rhythm in vitro, I first confirm that the 5HT1A/5HT7 agonist (+)DPAT maximally advances the SCN clock when applied at zeitgeber time 6 (ZT6). Conversely, NPY only phase advances the neuronal activity rhythm when applied at ZT 10. This effect occurs through stimulation of Y2 receptors. NPY, again acting through Y2 receptors, blocks (+)DPAT-induced phase shifts at ZT 6, while neither (+)DPAT nor 5-HT affect NPY-induced phase shifts at ZT 10. NPY appears to block (+)DPAT-induced phase shifts by preventing increases in cyclic AMP. These data are the first to demonstrate in vitro interactions between daytime resetting stimuli in the rat, and provide critical insights into mechanisms controlling circadian clock phase.

The suprachiasmatic nucleus: a 25-year retrospective

The suprachiasmatic nuclei (SCN) of the anterior hypothalamus contain the master circadian pacemaker in mammals. On the occasion of the 25th anniversary of the discovery of the SCN as the circadian clock, Charles A. Czeisler and Steven M. Reppert organized a meeting to review milestones and recent developments in the study of the SCN. The discovery that the SCN contain tissue necessary for generation of circadian rhythmicity was established by lesion studies published in 1972. The second phase of study demonstrated unequivocally that the SCN contain an autonomous circadian pacemaker. The principal studies in this period showed the presence of metabolic and electrical activity rhythms in the SCN in vivo and progressed to studies showing that the SCN maintain rhythmicity in vitro, demonstrating that the transplanted SCN can restore circadian function following destruction of the host SCN and ultimately showing that single SCN "clock cells" exhibit independent rhythms in firing rate. The third phase of study, aimed at identifying the biochemical and molecular mechanisms responsible for rhythmicity within the SCN, has begun with the identification of circadian mutants (tau mutant hamsters and Clock mutant mice) and the isolation of the Clock gene. This report traces the important steps forward in our understanding of the suprachiasmatic circadian clock by recounting the information presented at the SCN Silver Anniversary Celebration.

Behavioral inhibition of light-induced circadian phase resetting is phase and serotonin dependent

Circadian rhythms in Syrian hamsters can be phase shifted by light exposure during the subjective night and by a bout of wheel running induced during the subjective day. Interactions between photic and behavioral stimuli were examined by comparing phase shifts to 15 min, 50 lux light pulses with and without a bout of running induced by confinement to a novel wheel 30 min prior to and extending through light exposure. Light pulses 6 h after dark onset on the first night of constant dark induced phase advance shifts averaging 80 min. Wheel running attenuated these shifts by 45% on average (p<0.01). Light pulses 1 h or 2.25 h after dark onset induced phase delay shifts averaging 50 min and 20 min, respectively, that were not affected by stimulated running. A significant running response to the novel wheel was evident at all 3 time points, but was greater to wheel confinement at both times early in the night. Stimulated running alone early or late in the night did not produce significant phase shifts. Behavioral attenuation of phase advances to light late in the night was prevented by pretreatment with the general 5HT1 antagonist metergoline (2 mg/kg i.p.). Metergoline did not significantly attenuate running in novel wheels. These results indicate that modulation of light-induced phase shifts by behavior is phase dependent and may involve direct or indirect actions of serotonin within the circadian system.

Both neuropeptide Y and serotonin are necessary for entrainment of circadian rhythms in mice by daily treadmill running schedules

This study investigated the role of the suprachiasmatic nucleus (SCN) circadian pacemaker and its neuropeptide Y (NPY) and serotonin (5-HT) afferents in entrainment (synchronization) of mouse circadian rhythms by treadmill running. Blind C57BL/6j mice were run in treadmills for 3 hr/d for 3-10 weeks after receiving radio-frequency lesions of the SCN or the intergeniculate leaflet (IGL, the source of SCN NPY) or infusions of the 5-HT neurotoxin 5,7-DHT into the SCN area. Of 25 intact mice, 22 entrained and three showed period (tau, the mean duration of the circadian cycle) modulations to scheduled running. Arrhythmic SCN-ablated mice did not synchronize to scheduled running in a way suggestive of circadian pacemaker mediation. Of 15 mice with IGL lesions, only two with partial lesions entrained. Mice with complete IGL lesions (five), confirmed by immunocytochemistry, showed no entrainment or tau changes. Of 19 mice with 5-HT lesions, only two with partial lesions entrained. All but two mice with complete (10) or nearly complete (4) 5-HT denervation, confirmed by immunocytochemistry, showed tau modulations during the treadmill schedule. Failure to entrain was not explained by group differences in tau before the treadmill schedules. The results indicate that the SCN and both NPY and 5-HT are necessary for entrainment to 24 hr schedules of forced running but that complete loss of 5-HT does not prevent modulations of pacemaker motion by behavioral stimuli. Treadmill entrainment in mice may involve synergistic interactions between 5-HT and NPY afferents at some site within the circadian system.

Serotonin-containing fibers in the suprachiasmatic hypothalamus attenuate light-induced phase delays in mice

Photic and non-photic stimuli phase shift and entrain circadian rhythms through distinct but interacting mechanisms which impinge on the suprachiasmatic nucleus (SCN), the circadian pacemaker. Our understanding of this mechanism is incomplete. Serotonin (5-HT) injected locally at the SCN reduces light-induced glutamate release and decreases the expression of c-fos, a marker of photic transduction. Furthermore, in SCN slices, 5-HT application reduces field potentials after optic nerve stimulation. We therefore predicted that 5-HT-terminal destruction restricted to the SCN would augment phase shifts of circadian rhythms induced by light exposure. To investigate this possibility, we compared photic phase delays and Fos-like immunoreactivity in mice which had previously received bilateral infusions directed at the SCN containing either the selective 5-HT neurotoxin 5,7-dihydroxytryptamine (DHT, n = 16) or vehicle (VEH, n = 12). Phase delays after a light pulse given during the mid-subjective night (30 lux, 30 min starting at circadian time (CT) 12-20) in DHT-mice were 50% greater than in VEH-mice (P = 0.017). DHT mice (n = 5) had 76% larger Fos responses to a mid-subjective night light pulse than VEH-mice (n = 5) (P = 0.029). We conclude that 5-HT at or near the SCN in mice reduces photic phase shifts and modulates the magnitude of the photic phase response in the mouse.

Scheduled activity reorganizes circadian phase of Syrian hamsters under full and skeleton photoperiods

Circadian rhythms can be shifted or entrained by light and by arousing nonphotic stimuli. Interactions between photic and nonphotic stimuli were examined by subjecting hamsters to a daily 3 h bout of induced running under full (FPP) or skeleton (two daily light pulses; SPP) photoperiods. Activity scheduled in mid-day of a FPP induced large phase delays (260 +/- 63 min) in hamsters that ran more than 4000 rev/3 h. Split rhythms were not evident in constant dark (DD) tests. Activity scheduled in mid-subjective day of a SPP induced 180 degrees inversions of circadian phase, apparently achieved by oscillator splitting in some cases. Activity scheduled late-day and early-night induced a mix of phase delays, advances and no responses. Activity scheduled at two phase ranges late in the night had no effect, but scheduled 1 h later (beginning the last hour of darkness) induced large phase delays (238 +/- 30 min). There was no evidence of oscillator splitting during DD tests, but free-running period was significantly longer in groups that showed large phase delays. Induced running schedules have powerful effects on the phase of photic entrainment and can alter intrinsic pacemaker properties, including internal oscillator coupling and period.

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.

Microinjection of NMDA into the SCN region mimics the phase shifting effect of light in hamsters

Although there is considerable data that glutamate is the primary transducer of photic information to the circadian clock in the suprachiasmatic nucleus (SCN), the ability of glutamate to mimic the phase-shifting effects of light has yet to be demonstrated in vivo. In the present study, microinjections of the glutamate agonist NMDA directly into the SCN of Syrian hamsters induced significant phase delays at circadian time (CT) 13.5 and phase advances at CT 19. These results support the hypothesis that glutamate is the primary neurotransmitter responsible for the transduction of photic information to the SCN.

Multiunit activity recordings in the suprachiasmatic nuclei: in vivo versus in vitro models

The suprachiasmatic nuclei (SCN) of the hypothalamus continue to oscillate when they are isolated in a brain slice preparation. We recorded multiunit activity in the SCN of the rat both in vivo and in vitro to determine the circadian discharge pattern. The variability of the discharge pattern is larger and the amplitude of the rhythm is smaller in vivo than in vitro. Moreover we found evidence for a direct effect of the animal's behavioural activity on electrical activity of the SCN in vivo. These findings may provide an electrophysiological basis for the known effects of behavioural stimuli on the circadian pacemaker. This study underscores the importance of recordings in intact preparations in addition to in vitro work when generalisations to physiological conditions are to be made.

Neuropeptide Y and glutamate block each other's phase shifts in the suprachiasmatic nucleus in vitro

The suprachiasmatic nuclei contain a circadian clock whose activity can be recorded in vitro for several days. Photic information is conveyed to the nuclei primarily via a direct projection from the retina, the retinohypothalamic tract, utilizing an excitatory amino acid neurotransmitter. Photic phase shifts may be mimicked by application of glutamate in vitro. A second, indirect pathway to the suprachiasmatic nuclei via the geniculohypothalamic tract utilizes neuropeptide Y as a transmitter. Phase shifts to neuropeptide Y in vitro are similar to those seen to non-photic stimuli in vivo. We have used the hypothalamic slice preparation to examine the interactions of photic and non-photic stimuli in the suprachiasmatic nuclei. Coronal hypothalamic slices containing the suprachiasmatic nuclei were prepared from Syrian hamsters and 3 min recordings of the firing rate of individual cells were performed throughout a 12 h period. Control slices receiving either no application or application of artificial cerebrospinal fluid to the suprachiasmatic nucleus showed a consistent daily peak in their rhythms. Glutamate produces phase shifts of the circadian clock in the hamster hypothalamic slice preparation during the subjective night but not during the subjective day. These phase shifts were similar in timing and direction to the photic phase response curve in vivo confirming previous work with the rat slice preparation. Neuropeptide Y produces phase shifts of the circadian clock during the subjective day but not during the subjective night. The phase shifts are similar in timing and direction to the non-photic phase response curve in vivo, confirming previous in vitro work. We then examined the interaction of these neurochemicals with each other at various times during the circadian cycle. We found that both advances and delays to glutamate in the slice are blocked by application of neuropeptide Y. We also found that phase shifts to neuropeptide Y in the slice are blocked by application of glutamate. These results indicate that photic and non-photic associated neurochemicals can block each others phase shifting effects within the suprachiasmatic nucleus in vitro. These experiments demonstrate the ability of photic and non-photic associated neurochemicals to interact at the level of the suprachiasmatic nucleus. It is clear that neuropeptide Y antagonizes the effect of glutamate during the subjective night, and that glutamate antagonizes the effect of neuropeptide Y during the subjective day. Great care must be taken when devising treatments where photic and non-photic signals may interact.

Exercise and human circadian rhythms: what we know and what we need to know

A synopsis of the effects of exercise on the circadian system in nocturnal rodents is followed by a review of the few studies investigating the influence of exercise on the human circadian system. It is premature to make specific recommendations about using exercise to promote synchronization in people because of the lack of information on the best times of exercise, the amounts required, and interactions between nonphotic and photic zeitgebers.

Circadian phase-response curves for simulated dawn and dusk twilights in hamsters

Phase shifts of the circadian rhythm of wheel-running activity were compared in Syrian hamsters maintained in constant darkness and exposed to 1-h naturalistic dawn or dusk twilight ramps (0.003-10 lx), or to 1-h rectangular light pulses (1 lx) providing equal photon exposure. The phase-response curves (PRCs) for dusk and rectangular pulses were virtually identical and resembled the PRC for dawn pulses, except that the mean phase advance caused by dawn pulses at circadian time 19 (CT 19) was approximately 1 h smaller. This difference could not be accounted for by differences in the amount of wheel-running observed during light pulse exposure, because the animals ran more during dusk pulses than during either of the other two pulse types. In a second experiment, 15-min rectangular light pulses (1 lx) immediately preceded by a 47-min dawn ramp caused smaller phase delays at CT 13 than rectangular pulses alone, despite a 40% increase in total photon exposure, but phase advances at CT 19 did not differ between the two light treatments. These results indicate that phase shifts of the circadian pacemaker in hamsters are determined primarily, though not entirely, by total photon exposure. They also indicate that dawn pulses may be less effective than dusk or rectangular pulses at certain circadian phases, possibly due to light adaptation during the early portion of the dawn twilight.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.

Locomotor activity and non-photic influences on circadian clocks

Some of the main themes in this review are as follows. 1. The notion that non-photic zeitgebers are weak needs re-examining. Phase-shifts to some non-photic manipulations can be as large as those to light pulses. 2. As well as being able to phase-shift and entrain free-running rhythms, non-photic events have a number of other effects: these include after-effects of entrainment, period changes, and promotion of splitting. 3. The critical variable for non-photic shifting is unknown. Locomotor activity is more likely to be an index of some other necessary state rather than being causal itself. This index may be better when tendencies to move are channelled into easily measured behaviours like wheel-running. 4. Given ignorance about the critical variable, quantification of activity may be the best presently available measure of zeitgeber intensity. Therefore, the behaviour during non-photic manipulations must be examined as carefully as the shifts themselves. When no phase-shifting follows manipulations such as IGL lesions or serotonin depletion, if the animals are inactive, then little can be inferred. 5. Lack of information on the critical variable(s) for non-photic shifting makes it problematic to compare data from studies using different non-photic manipulations. However, the presence of locomotor activity (or its correlate) does appear to be necessary for triazolam to produce shifts. 6. Novelty-induced wheel-running in hamsters depends on the NPY projection from the IGL to SCN. It remains to be determined how important NPY is in other species or in clock-resetting by other manipulations, but methods are now available to study this. 7. Interactions between photic and non-photic zeitgebers remain virtually unexplored, but it is evident that photic and non-photic stimuli can attenuate the phase-shifting effects of each other. Interactions are not purely additive or predictable from PRCs. 8. The circadian system does more than synchronize free-running rhythms to the solar day. Its non-photic functions and their interactions with photic inputs probably account for some of the anatomical complexity of circadian circuitry.

Circadian rhythms and the pharmacology of affective illness

The chronic effects of antidepressant drugs (ADs) on circadian rhythms of behavior, physiology and endocrinology are reviewed. The timekeeping properties of several classes of ADs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, serotonin agonists and antagonists, benzodiazepines, and melatonin are reviewed. Pharmacological effects on the circadian amplitude and phase, as well as effects on day-night measurements of motor activity, sleep-wake, body temperature (Tb), 3-methoxy-4-hydroxyphenylglycol, cortisol, thyroid hormone, prolactin, growth hormone and melatonin are examined. ADs often lower nocturnal Tb and affect the homeostatic regulation of sleep. ADs often advance the timing and decrease the amount of slow wave sleep, reduce rapid eye movement sleep and increase or decrease arousal. Together, AD effects on nocturnal Tb and sleep may be related to their therapeutic properties. ADs sometimes delay nocturnal cortisol timing and increase nocturnal melatonin, thyroid hormone and prolactin levels; these effects often vary with diagnosis, and clinical state. The effects of ADs on the coupling of the central circadian pacemaker to photic and nonphotic zeitgebers are discussed.

Cellular colocalization of Fos and neuropeptide Y in the intergeniculate leaflet after nonphotic phase-shifting events

Nonphotic and photic stimuli that phase shift circadian rhythms were presented to hamsters, Mesocricetus auratus. The nonphotic stimulus was a 3-h pulse of novelty-induced wheel running starting at circadian time 4-5. The photic stimulus used was a 0.5 h light pulse starting at circadian time 18. Double immunocytochemistry was used to determine the neurochemical phenotype of cells in the intergeniculate leaflet that were activated by these stimuli. Both the nonphotic and the photic phase-shifting stimuli induced the expression of c-fos in the intergeniculate leaflet compared to unstimulated controls. However, after nonphotic stimulation, Fos-like immunoreactivity was common in neurons that also were NPY positive. Such colocalization of Fos and NPY after photic stimuli was rare. These findings suggest that the NPY pathway from the intergeniculate leaflet to the suprachiasmatic nucleus carries information about nonphotic events.

Efferent projections of the paraventricular thalamic nucleus in the rat

The paraventricular nucleus of the thalamus (PVT) receives input from all major components of the circadian timing system, including the suprachiasmatic nucleus (SCN), the intergeniculate leaflet and the retina. For a better understanding of the role of this nucleus in circadian timing, we examined the distribution of its efferent projections using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). The efferent projections of the PVT are loosely organized along its dorsal-ventral and anterior-posterior axes. The anterior PVT sends projections to the SCN; the dorsomedial and ventromedial hypothalamic nuclei; the lateral septum; the bed nucleus of the stria terminalis; the central and basomedial amygdaloid nuclei; the anterior olfactory nucleus; the olfactory tubercle; the nucleus accumbens; the infralimbic, piriform, and perirhinal cortices; the ventral subiculum; and the endopiriform nucleus. A small PHA-L injection, restricted to the ventral portion of the anterior PVT, produces a similar pattern of labeling, except for a marked decrease in the number of labeled fibers in the hypothalamus, cortex, and lateral septum and an increase in labeling in the endopiriform nucleus and basolateral amygdaloid nucleus. The posterior PVT has a more limited efferent distribution than the anterior PVT, terminating in the anterior olfactory nucleus; the olfactory tubercle; the nucleus accumbens; and the central, basolateral, and basomedial nuclei of the amygdala. Our results show that the anterior PVT is ideally situated to relay circadian timing information from the SCN to brain areas involved in visceral and motivational aspects of behavior and to provide feedback regulation of the SCN.

Circadian responses to light: the calmodulin connection

KN-62, an inhibitor of CaM kinase II, attenuated phase shifts induced by low intensity light pulses and reduced light-induced phosphorylation of the transcription factor, CREB, in the suprachiasmatic nucleus. The calmodulin inhibitor, W-7, had similar effects: neither drug produced a complete block of photic responses. The results support the hypothesis that circadian responses to light are mediated in part by CaM kinase activity and CREB, and suggest that other signal transduction pathways also take part.

Nonphotic phase shifting in the old and the cold

When confined to novel running wheels or when given injections of triazolam in their home cages, old hamsters do not become as active as young hamsters. Therefore, lack of nonphotic phase shifting following such manipulations may stem from insufficient activity or arousal. Phase advances can be obtained in some 10-month-old animals when wheel running during the pulse is increased by the presence of females in estrous condition and in most 18-month-old hamsters by combining confinement to a novel wheel with triazolam injections. These data suggest that there is relatively little if anything wrong in aging hamsters with the nonphotic phase-shifting mechanism itself. The reason why in certain situations old hamsters do not shift appears to be because the nonphotic inputs to these shifting mechanisms are not strong enough. However, when running in novel wheels is increased by carrying out the tests at cold temperatures, most old animals did not show subsequent phase shifts. Evidently it is not running per se that is critical for phase shifts, but probably the motivational context for such running.

Intergeniculate leaflet lesions and behaviorally-induced shifts of circadian rhythms

The aim of this work was to assess the effect of lesions of the intergeniculate leaflet on nonphotic phase shifts produced by confining hamsters to novel running wheels for 3 h in the middle of the subjective day. In intact hamsters this procedure produces large phase advances provided that the hamsters maintain high levels of wheel running during the confinement. Intergeniculate leaflet lesions blocked or reduced phase shifts after confinement to a novel wheel. However, for most animals these lesions also reduced both the amount of activity during the 3 h pulse in the novel wheel and the amount of daily wheel running in the home cage. To boost activity of lesioned hamsters to levels associated with large phase shifts, the animals were confined to novel wheels at low ambient temperature. The lesioned hamsters still failed to show large phase shifts. The benzodiazepine triazolam also failed to induce phase shifts in lesioned animals, but it induced less activity in lesioned animals as compared to sham-operated controls. The data support the hypothesis that the intergeniculate leaflet conveys information about nonphotic phase-shifting to the circadian pacemaker in the suprachiasmatic nucleus. They also raise the possibility that some effects of intergeniculate leaflet lesions previously interpreted as having a photic basis, might be due to the activity-lowering effect of the lesions.