Motor activity correlates negatively with free-running period, while positively with serotonin contents in SCN in free-running rats

Effects of prenatal exposure to fluoxetine on circadian rhythmicity in the locomotor activity and neuropeptide Y and 5-HT expression in male and female adult Wistar rats

Serotonin (5-HT) reuptake inhibitors, such as fluoxetine, are the most prescribed antidepressant for maternal depression. In this sense, it exposes mothers and the brains of infants to increased modulatory and trophic effects of serotonergic neurotransmission. 5-HT promotes essential brain changes throughout its development, which include neuron migration, differentiation, and organization of neural circuitries related to emotional, cognitive, and circadian behavior. Early exposure to the SSRIs induces long-term effects on behavioral and neural serotonergic signalization. We have aimed to evaluate the circadian rhythm of locomotor activity and the neurochemical content, neuropeptide Y (NPY) and 5-HT in three brain areas: intergeniculate leaflet (IGL), suprachiasmatic nuclei (SCN) and raphe nuclei (RN), at two zeitgebers (ZT6 and ZT18), in male and female rat's offspring early exposed (developmental period GD13-GD21) to fluoxetine (20mg/kg). First, we have conducted daily records of the locomotor activity rhythm using activity sensors coupled to individual cages over four weeks. We have lastly evaluated the immunoreactivity of NPY in both SCN and IGL, and as well the 5-HT expression in the dorsal and medial RN. In summary, our results showed that (1) prenatal fluoxetine affects phase entrainment of the rest/activity rhythm at ZT6 and ZT18, more in male than female specimens, and (2) modulates the NPY and 5-HT expression. Here, we show male rats are more susceptible to phase entrainment and the NPY and 5-HT misexpression compared to female ones. The sex differences induced by early exposure to fluoxetine in both the circadian rhythm of locomotor activity and the neurochemical expression into SCN, IGL, and midbrain raphe are an important highlight in the present work. Thus, our results may help to improve the knowledge on neurobiological mechanisms of circadian rhythms and are relevant to understanding the "broken brains" and behavioral abnormalities of offspring early exposed to antidepressants.

Wheel-Running Facilitates Phase Advances in Locomotor and Peripheral Circadian Rhythm in Social Jet Lag Model Mice

The circadian clock maintains our health by controlling physiological functions. Social jet lag is one factor that can disrupt the body clock. This is caused by the difference in sleeping hours between weekdays when we live according to social time and holidays when we live according to our body clock. The body clock can be altered by exercise, nutrition, and stress, and several studies have reported that these factors can be used to improve a disturbed body clock. Here we focused on exercise and examined whether continuous wheel-running could improve the disordered body clock in a mouse model that mimics social jet lag. The results showed that the wheel-running exercise group showed faster synchronization of the onset of activities on weekdays which had been delayed by social jet lag and the results were even more pronounced in the high-fat diet feeding condition. Also, when the expression rhythms of the clock genes were examined, they experienced a sudden time shift in the advance light condition or social jet lag condition, it was found that the wheel-running group had a higher ability to adapt to the advance direction. Thus, it is possible that the effective inclusion of exercise in human, especially those who eat high-fat foods, life can improve the disordered body clock in terms of social jet lag.

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

GABA- and serotonin-expressing neurons take part in inhibitory as well as excitatory input pathways to the circadian clock of the Madeira cockroach Rhyparobia maderae

In the Madeira cockroach, pigment-dispersing factor-immunoreactive (PDF-ir) neurons innervating the circadian clock, the accessory medulla (AME) in the brain's optic lobes, control circadian behaviour. Circadian activity rhythms are entrained to daily light-dark cycles only by compound eye photoreceptors terminating in the lamina and medulla. Still, it is unknown which neurons connect the photoreceptors to the clock to allow for light entrainment. Here, we characterized by multiple-label immunocytochemistry the serotonin (5-HT)-ir anterior fibre fan and GABA-ir pathways connecting the AME- and optic lobe neuropils. Colocalization of 5-HT with PDF was confirmed in PDF-ir lamina neurons (PDFLAs). Double-labelled fibres were traced to the AME originating from colabelled PDFLAs branching in accessory laminae and proximal lamina. The newly discovered GABA-ir medial layer fibre tract connected the AME to the medulla's medial layer fibre system, and the distal tract fibres connected the AME to the medulla. With Ca²⁺ imaging on primary cell cultures of the AME and with loose-patch-clamp recordings in vivo, we showed that both neurotransmitters either excite or inhibit AME clock neurons. Because we found no colocalization of GABA and 5-HT in any optic lobe neuron, GABA- and 5-HT neurons form separate clock input circuits. Among others, both pathways converged also on AME neurons that coexpressed mostly inhibitory GABA- and excitatory 5-HT receptors. Our physiological and immunocytochemical studies demonstrate that GABA- and 5-HT-immunoreactive neurons constitute parallel excitatory or inhibitory pathways connecting the circadian clock either to the lamina or medulla where photic information from the compound eye is processed.

Suprachiasmatic Nucleus Interaction with the Arcuate Nucleus; Essential for Organizing Physiological Rhythms

The suprachiasmatic nucleus (SCN) is generally considered the master clock, independently driving all circadian rhythms. We recently demonstrated the SCN receives metabolic and cardiovascular feedback adeptly altering its neuronal activity. In the present study, we show that microcuts effectively removing SCN-arcuate nucleus (ARC) interconnectivity in Wistar rats result in a loss of rhythmicity in locomotor activity, corticosterone levels, and body temperature in constant dark (DD) conditions. Elimination of these reciprocal connections did not affect SCN clock gene rhythmicity but did cause the ARC to desynchronize. Moreover, unilateral SCN lesions with contralateral retrochiasmatic microcuts resulted in identical arrhythmicity, proving that for the expression of physiological rhythms this reciprocal SCN-ARC interaction is essential. The unaltered SCN c-Fos expression following glucose administration in disconnected animals as compared to a significant decrease in controls demonstrates the importance of the ARC as metabolic modulator of SCN neuronal activity. Together, these results indicate that the SCN is more than an autonomous clock, and forms an essential component of a larger network controlling homeostasis. The present novel findings illustrate how an imbalance between SCN and ARC communication through circadian disruption could be involved in the etiology of metabolic disorders.

Exercise Distributed across Day and Night Does Not Alter Circadian Period in Humans

In rodents, increased activity due to running-wheel access is associated with a change in observed circadian period. In humans, exposure to exercise has failed to demonstrate similar effects on period. Methodological issues with prior studies such as light exposure during exercise, length of study, and method of measuring period confounded those evaluations of the effect of exercise on period in humans. In the present experiment, the authors examined the effect of exercise on period in 8 subjects using a 44-day within-subjects inpatient study. They used a 20-h forced desynchrony protocol, in which subjects were exposed to exercise across circadian phases under dim light conditions. Exercise consisted of three 45-min sessions per wake period on an ergometer. Target exercise intensity was ~65% of maximal heart rate. Intrinsic circadian period was measured using both core body temperature and hourly plasma melatonin samples. Consistent with previous reports, the authors find no effect of exercise on endogenous circadian period as measured by either core body temperature or melatonin. Exercise distributed across biological day and night does not appear to affect circadian period.

In vivo monitoring of multi-unit neural activity in the suprachiasmatic nucleus reveals robust circadian rhythms in Period1⁻/⁻ mice

The master pacemaker in the suprachiasmatic nucleus (SCN) controls daily rhythms of behavior in mammals. C57BL/6J mice lacking Period1 (Per1^−/−) are an anomaly because their SCN molecular rhythm is weak or absent in vitro even though their locomotor activity rhythm is robust. To resolve the contradiction between the in vitro and in vivo circadian phenotypes of Per1^−/− mice, we measured the multi-unit activity (MUA) rhythm of the SCN neuronal population in freely-behaving mice. We found that in vivo Per1^−/− SCN have high-amplitude MUA rhythms, demonstrating that the ensemble of neurons is driving robust locomotor activity in Per1^−/− mice. Since the Per1^−/− SCN electrical activity rhythm is indistinguishable from wild-types, in vivo physiological factors or coupling of the SCN to a known or unidentified circadian clock(s) may compensate for weak endogenous molecular rhythms in Per1^−/− SCN. Consistent with the behavioral light responsiveness of Per1^−/− mice, in vivo MUA rhythms in Per1^−/− SCN exhibited large phase shifts in response to light. Since the acute response of the MUA rhythm to light in Per1^−/− SCN is equivalent to wild-types, an unknown mechanism mediates enhanced light responsiveness of Per1^−/− mice. Thus, Per1^−/− mice are a unique model for investigating the component(s) of the in vivo environment that confers robust rhythmicity to the SCN as well as a novel mechanism of enhanced light responsiveness.

Amplitude of the SCN clock enhanced by the behavioral activity rhythm

Circadian rhythms are regulated by the suprachiasmatic nucleus (SCN), a small structure at the base of the hypothalamus. While light effects on the SCN are well established, little is known of behavioral effects. This study elucidates direct modulating action of behavioral activity on the SCN by use of in vivo electrophysiology recordings, assessments of general locomotor behavior, and video-tracking of mice. The results show suppression of SCN neuronal activity by spontaneous behavior, the magnitude being dependent on the intensity, duration and type of behavioral activity. The suppression was moderate (32% of circadian amplitude) for low-intensity behavior and considerable (59%) for locomotor activity. Mild manipulation of the animals had reversed effects on the SCN indicating that different mechanisms are involved in the regulatory effect of spontaneous versus induced activity. The results indicate that exercise at the proper time of the cycle can boost the amplitude of the rhythm of the SCN clock itself. This has potentially beneficial effects for other rhythmic functions that are under the control of the SCN.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Neonatal alcohol exposure permanently disrupts the circadian properties and photic entrainment of the activity rhythm in adult rats

BACKGROUND

Alcohol exposure during the period of rapid brain development produces structural damage in different brain regions, including the suprachiasmatic nucleus (SCN), that may have permanent neurobehavioral consequences. Thus, this study examined the long-term effects of neonatal alcohol exposure on circadian behavioral activity in adult rats.

METHODS

Artificially reared Sprague-Dawley rat pups were exposed to alcohol (EtOH; 4.5 g/kg/day) or isocaloric milk formula (gastrostomy control; GC) on postnatal days 4-9. At 2 months of age, rats from the EtOH, GC, and suckle control (SC) groups were housed individually, and properties of the circadian rhythm in wheel-running behavior were continuously analyzed during exposure to a 12-hr light:12-hr dark photoperiod (LD 12:12) or constant darkness (DD).

RESULTS

Neonatal alcohol exposure had distinctive effects on the rhythmic properties and quantitative parameters of adult wheel-running behavior. EtOH-treated animals were distinguished by unstable and altered entrainment to LD 12:12 such that their daily onsets of activity were highly variable and occurred at earlier times relative to control animals. In DD, circadian regulation of wheel-running behavior was altered by neonatal alcohol exposure such that the free-running period of the activity rhythm was shorter in EtOH-exposed rats than in control animals. Total amount of daily wheel-running activity in EtOH-treated rats was greater than that observed in the SC group. In addition, the circadian activity patterns of EtOH-exposed rats were fragmented such that the duration of the active phase and the number of activity bouts per day were increased.

CONCLUSIONS

These data indicate that neonatal alcohol exposure produces permanent changes in the circadian regulation of the rat activity rhythm and its entrainment to LD cycles. These long-term alterations in circadian behavior, along with the developmental alcohol-induced changes in SCN endogenous rhythmicity, may have important implications in clinical sleep-wake disturbances observed in neonates, children, and adults exposed to alcohol in utero.

MDMA alters the response of the mammalian circadian clock in hamsters: effects on re-entrainment and triazolam-induced phase shifts

Serotonin (5-hydroxytryptamine or 5-HT) is a neurotransmitter that is involved in a wide range of behavioural and physiological processes. Previous work has indicated that serotonin is important in the regulation of the circadian clock, which is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy'), which is widely used as a recreational drug of abuse, is a serotonin neurotoxin in animals and non-human primates. Previous work has shown that MDMA exposure can alter circadian clock function both in vitro and in vivo. Evidence shows that 5-HT may have a modulatory role in the regulation of the circadian clock by non-photic stimuli, such as the benzodiazepine triazolam (TRZ). Triazolam is a short-acting benzodiazepine that results in phase advances of the wheel running activity in hamsters when administered during the mid-subjective day. In the present study, male Syrian hamsters treated with TRZ (5 mg/kg) at ZT6 significantly phase advanced their clock. Treatment with MDMA significantly diminished the TRZ induced phase shift in hamsters. Previous evidence shows the involvement of 5-HT in the re-synchronisation of the endogenous clock to a new shifted light-dark cycle. Untreated animals were successfully entrained to a new, 6 h advanced light-dark cycle within an average of 4.5 +/- 0.1 days. Following treatment with MDMA, these animals took an average of 8.3 +/- 0.1 days to re-entrain to a shifted environmental cycle. Immunohistochemical analysis revealed that animals treated with MDMA showed reduced serotonin staining, as evidenced by a decrease in innervation density in the SCN. No significant differences were found in cell counts within the raphe nuclei. These results demonstrate the importance of the serotonergic system in the modulation of photic and non-photic responses of the circadian pacemaker.

Let there be “more” light: enhancement of light actions on the circadian system through non-photic pathways

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.

Different effects of intensity and duration of locomotor activity on circadian period

An outstanding unresolved issue in chronobiology is how the level of locomotor activity influences length of the free-running, endogenous circadian period (tau). To address this issue, the authors studied a novel model, 4 replicate lines of laboratory house mice (Mus domesticus) that had been selectively bred for high wheel-running activity (S) and their 4 unselected control (C) lines. Previous work indicates that S mice run approximately twice as many revolutions/day and exhibit an altered dopaminergic function as compared with C mice. The authors report that S mice have a tau shorter by about 0.5 h as compared with C mice. The difference in tau was significant both under constant light (control lines: tau = 25.5 h; selected: tau = 24.9 h) and under constant dark (control lines: 23.7 h; selected: 23.4 h). Moreover, the difference remained statistically significant even when the effects of running speed and time spent running were controlled in ANCOVA. Thus, something more fundamental than just intensity or duration of wheel-running activity per se must underlie the difference in tau between the S and C lines. However, despite significant difference in total wheel-running activity between females and males, tau did not differ between the sexes. Similarly, among individuals within lines, tau was not correlated with wheel-running activity measured as total revolutions per day. Instead, tau tended to decrease with average running speed but increase with time spent running. Finally, within individuals, an increase in time spent running resulted in decreased tau in the next few days, but changes in running speed had no statistically significant effect. The distinctions between effects of duration versus intensity of an activity, as well as between the among- versus within-individual correlations, are critical to understanding the relation between locomotor activity and pace of the circadian clock.

Age-Related Changes in the Spontaneous Motor Rhythms of theSenescence-Accelerated Mouse (SAMP8)

The present study examined the effect of age on the spontaneous motor rhythms of mice during wheel running. The spontaneous motor tempo (SMT) of wheel running was measured for the P8 strain of the senescence-accelerated mouse (SAMP8) by recording the sequence of time intervals (measured in milliseconds) for successive revolutions ofa run-wheel over the course of 16 days. Analyses of the distribution of interrevolution intervals of 2-, 7-, and 12-month-old SAMP8 revealed an age-related slowing of wheel running and a corresponding increase in variability consistent with Weber's law. All three age groups also demonstrated a practice effect over the course of testing best described by a power law. These findings provide evidence of age-related changes in the spontaneous motor rhythms of the SAMP8 that occur as early as 7 months of age. The results are consistent with age-related changes in human subjects and suggest that spontaneous wheel-running behavior in rodents may be a good model for studying SMT.

Inhibition of hibernation by exercise is not affected by intergeniculate leaflets lesion in hamsters

The circadian clock of mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, has been demonstrated to integrate day length change from long (LP) to short photoperiod (SP). This photoperiodic change induces in Syrian hamsters a testicular regression through melatonin action, a phenomenon that is inhibited when hamsters have free access to a wheel. The intergeniculate leaflets (IGL), which modulate the integration of photoperiod by the SCN, are a key structure in the circadian system, conveying nonphotic information such as those induced by novelty-induced wheel running activity. We tested in hamsters transferred from LP to a cold SP the effects of wheel running activity on a photoperiod-dependent behavior, hibernation. Lesions of the IGL were done to test the role of this structure in the inhibition induced by exercise of photoperiod integration by the clock. We show that wheel running activity actually inhibits hibernation not only in sham-operated animals, but also in hamsters with a bilateral IGL lesion (IGLX). In contrast, IGL-X hamsters without a wheel integrate slower to the SP but hibernate earlier compared with sham-operated animals. Moreover, some hibernation characteristics are affected by IGL lesion. Throughout the experiment at 7 degrees C, IGL-X hamsters were in hypothermia during 18% of the experiment vs. 32% for sham-operated hamsters. Taken together, these data show that the IGL play a modulatory role in the integration of photoperiodic cues and modulate hibernation, but they are not implicated in the inhibition of hibernation induced by wheel running activity.

Serotonin phase-shifts the mouse suprachiasmatic circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) receives multiple afferent signals that could potentially modulate its phase. One input, the serotonin (5-HT) projection from the raphe nuclei, has been extensively investigated in rats and hamsters, yet its role(s) in modulating circadian clock phase remains controversial. To expand our investigation of 5-HT modulation of the SCN clock, we investigated the phase-shifting effects of 5-HT and its agonist, (+)8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), when applied to mouse SCN brain slices. 5-HT induced 2-3 h phase advances when applied during subjective day, while non-significant phase shifts were seen after 5-HT application at other times. These phase shifts were completely blocked by the 5-HT antagonist, metergoline. DPAT also induced phase shifts when applied during mid-subjective day, and this effect appeared dose-dependent. Together, these results demonstrate that the mouse SCN, like that of the rat, is directly sensitive to in vitro phase-resetting by 5-HT.

Effect of cage enrichment on the daily use of running wheels by Syrian hamsters

Institutional animal care committees may one day require for the welfare of captive hamsters more floor space and the introduction of tunnels and toys. As hamsters are popular animal subjects in chronobiological research, and as clock phase is usually measured through running wheel activity, it is important to determine what effect cage enrichment might have on daily wheel use. Here the daily number of wheel revolutions, the daily duration of the running activity phase, the phase relationship between lights-off and onset of running activity, and the free-running period of circadian activity rhythms were measured in Syrian hamsters, Mesocricetus auratus, housed in single cages or in multiple cages linked by tunnels and supplied with commercial wooden toys. Free-running periodicity was not affected by cage enrichment. In multiple-cage systems, there were fewer daily revolutions, shorter wheel-running activity phases, and delayed running activity onsets. These effects, however, were small as compared to interindividual and week-to-week variation. They were statistically significant only under a light:dark cycle, not in constant darkness, and only when interindividual variation was eliminated through a paired design or when the number of cages was increased to five (the maximum tested). Daily wheel use is thus affected by cage enrichment, but only slightly.

Circadian rhythm of drinking and running-wheel activity in rats with 6-hydroxydopamine lesions of the ventral tegmental area

Circadian rhythms in drinking and running-wheel (locomotor) activity of rats with 6-hydroxydopamine (6-OHDA, 4 microg/2 microl per rat)-induced lesions in the ventral tegmental area (VTA) were examined under a light-dark (LD) cycle and constant dim light (5 lux). Under the LD cycle, the length of the locomotor activity period was decreased during the dark, and increased during the light period in the lesioned rats. Under the constant dim light conditions, the free-running circadian period (tau) of drinking and activity rhythm was longer in lesioned rats than in sham-operated controls. The elongation of the circadian period was accompanied by decrements in activity. These observations suggest that the mesolimbic dopaminergic system modulates rhythms in circadian drinking and locomotor activity.

N-methyl-D-aspartate receptor subtype 2C is not involved in circadian oscillation or photoic entrainment of the biological clock in mice

Ishida et al. [1994: Neurosci Lett 166: 211-215] reported the circadian change of N-methyl-D-aspartate (NMDA) receptor subtype 2C mRNA and photic induction of this receptor's mRNA in the suprachiasmatic nucleus (SCN). Therefore, we investigated the role of NMDA receptor subtypes in the biological clock using NMDA receptor 2A (NR2A)- or 2C (NR2C)-deficient mice. However, NR2C-/- mice showed normal light-dark (LD)-entrained locomotor activity rhythms and free-running rhythms under constant darkness and also exhibited normal reentrainment to 6-hr LD shifts and phase delays with single light pulses. Thus, present results demonstrated no significant NR2C contribution to circadian oscillation and photic entrainment, even though expression of NR2C mRNA was highly observed in the SCN. On the other hand, the period of the free-running activity rhythm in NR2A-/- mice but not NR2C-/- mice was slightly longer than that in wild-type mice in spite of low expression of NR2A in the SCN. Furthermore, reentrainment to an LD advance in NR2A-/- mice was slower under low-intensity light conditions. Thus, we suggest that NR2A plays a role in determining the behavioral state that affects the circadian rhythm. In order to elucidate the role of NR2A and NR2C in the SCN, we examined NMDA-induced Ca(2+) elevations in the SCN of mutant mice using a Ca(2+) imaging method. A partial reduction in Ca(2+) elevation was observed in both NR2A-/- and NR2C-/- mice when high concentrations (100 or 300 microM) of NMDA were applied. The present results suggest that NR2A plays a weak role in oscillation or entrainment of the biological clock, and that NR2C does not participate in the functions of circadian oscillation and light entrainment.

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.

Serotonin agonist quipazine induces photic-like phase shifts of the circadian activity rhythm and c-Fos expression in the rat suprachiasmatic nucleus

Nonphotic stimuli can reset and entrain circadian activity rhythms in hamsters and mice, and serotonin is thought to be involved in the phase-resetting effects of these stimuli. In the present study, the authors examined the effect of the serotonin agonist quipazine on circadian activity rhythms in three inbred strains of rats (ACI, BH, and LEW). Furthermore, they investigated the effect of quipazine on the expression of c-Fos in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN). Quipazine reduced the amount of running wheel activity for 3 h after treatment, however, no long-term changes in tau and in the activity level were observed. More important, quipazine induced significant phase advances of the activity rhythm and c-Fos production in the SCN at the end of the subjective night (Circadian Time [CT] 22), whereas neither phase shifts nor c-Fos induction were observed during the subjective day. Quipazine injections also resulted in moderate phase delays at the beginning of the subjective night (CT 14). A similar phase-response characteristic typically can be observed for photic stimuli. By contrast, nonphotic stimuli normally produce phase advances during the subjective day. The present results suggest species differences between the hamster and the rat with respect to the serotonergic action on circadian timekeeping and indicate that serotonergic pathways play a role in the transmission of photic information to the SCN of rats.

Rhythmic properties of the hamster suprachiasmatic nucleus in vivo

We recorded multiple unit neural activity [multiunit activity (MUA)] from inside and outside of the suprachiasmatic nucleus (SCN) in freely moving male golden hamsters housed in running-wheel cages under both light/dark cycles and constant darkness. The circadian period of MUA in the SCN matched the period of locomotor activity; it was approximately 24 hr in wild-type and 20 hr in homozygous tau mutant hamsters. The peak of MUA in the SCN always occurred in the middle of the day or, in constant darkness, the subjective day. There were circadian rhythms of MUA outside of the SCN in the ventrolateral thalamic nucleus, the caudate putamen, the accumbens nucleus, the medial septum, the lateral septum, the ventromedial hypothalamic nucleus, the medial preoptic region, and the stria medullaris. These rhythms were out-of-phase with the electrical rhythm in the SCN but in-phase with the rhythm of locomotor activity, peaking during the night or subjective night. In addition to circadian rhythms, there were significant ultradian rhythms present; one, with a period of approximately 80 min, was in antiphase between the SCN and other brain areas, and another, with a period of approximately 14 min, was in-phase between the SCN and other brain areas. The periods of these ultradian rhythms were not significantly different in wild-type and tau mutant hamsters. Of particular interest was the unique phase relationship between the MUA of the bed nucleus of the stria terminalis (BNST) and the SCN; in these two areas both circadian and ultradian components were always in-phase. This suggests that the BNST is strongly coupled to the SCN and may be one of its major output pathways. In addition to circadian and ultradian rhythms of MUA, neural activity both within and outside the SCN was acutely affected by locomotor activity. Whenever a hamster ran on its wheel, MUA in the SCN and the BNST was suppressed, and MUA in other areas was enhanced.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

Serotonin and feedback effects of behavioral activity on circadian rhythms in mice

Wheel running activity can shorten the period (tau) of circadian rhythms in rats and mice. The role of serotonin (5HT), in this effect of behavior on circadian pacemaker function, was assessed by measuring tau during wheel-open and wheel-locked conditions in mice sustaining neurotoxic 5HT lesions directed at the suprachiasmatic nucleus (SCN). Intact mice exhibited a significant lengthening of tau (approximately 10 min) within 3 weeks when running wheels were locked. Mice with immunocytochemically confirmed 5HT depletion showed significantly longer tau than intact mice during wheel access, and did not show a significant change in tau up to 6 weeks after wheels were locked. In these mice, variability of tau across wheel access conditions was similar in magnitude to tau variability in intact mice at two time points without wheel access (+/- 3 min). 5HT-depleted mice also exhibited significantly longer activity periods (alpha), and a significantly delayed peak of activity within alpha. Previous studies show that a delayed peak of activity within alpha is associated with longer tau. Group differences in tau, and apparent failure of wheel-locking to lengthen tau in mice with 5HT lesions, may thus be due to loss of a serotonergic behavioral input pathway to the SCN, or to a lesion-induced change in the waveform of the activity rhythm.

Coupling effect of locomotor activity on the rat's circadian system

Exercise is recognized to affect circadian rhythmicity in a variety of ways. It masks the expression of other behavioral and physiological rhythms, entrains the master pacemaker, and influences the free-running period of other rhythms. In this paper we study the influence of exercise on the organization of the timing system by analyzing the effect of voluntary locomotor activity on the circadian feeding behavior of rats subjected to different lighting conditions. The availability of wheel running prevented loss of feeding circadian rhythmicity under constant bright light (LL) but did not elicit any circadian pattern in rats showing a previous arrhythmic pattern. Under dim red light (DR), the rhythm was more pronounced in exercising than in sedentary rats, while wheel-running availability accelerated the emergence of circadian rhythmicity in arrhythmic animals that were moved from LL to DR. These results can be explained by the existence of a positive feedback loop between physical exercise and its pacemaker and also suggest that exercise changes the functioning of the circadian system to facilitate the emergence of circadian rhythms in previously arrhythmic animals.

Locking and unlocking of running wheel affects circadian period stability differently in three inbred strains of rats

Running-wheel access has been shown to shorten the circadian period length (tau) of various mammalian species. Due to the close correlation between tau and the level of activity, running wheel-induced changes of the activity level are thought to be responsible for the observed changes in tau. In the present study, the influence of the running wheel on tau and the activity level was examined in three inbred strains of rats (ACI, BH, LEW). Four animals of each strain had free access to their running wheels, while the wheels of the other 4 animals of each strain were mechanically locked. These conditions were changed twice, so that each animal encountered both kinds of changes, that is, from a locked to an unlocked running wheel and vice versa. During the whole study, overall activity was measured by infrared detectors. Running-wheel access resulted in a significant increase of overall activity in strains LEW and ACI. However, significant changes of tau were observed only in LEW rats. These rats showed a significant shortening of tau after the second change of the housing conditions regardless of whether the wheel was locked or unlocked. Consequently, no causal relationship was found between changes of tau and running wheel-induced changes of overall activity. Instead, the results suggest that subtle environmental influences like locking or unlocking the running wheel affect tau in a strain-dependent manner, whereas changes in the activity level are neither necessary nor sufficient to induce changes of tau.

Repeated short-fasting modifies the macronutrient self-selection pattern in rats

The daily caloric intake and circadian pattern of macronutrient self-selection were examined in rats subjected to 3 h of food and water deprivation at the beginning or at the end of darkness. When one sole 3-h period of deprivation was applied, rats showed a compensatory response characterized by an unscheduled diurnal and nocturnal increase in the intake of the three macronutrients. However, repeated short restrictions during 15 days promoted a scheduled time-dependent feeding response, characterized by an exclusive increase in carbohydrate and fat intake and a decrease in protein intake. Repeated deprivation at the onset of dark produced a feeding response confined to the dark phase, while late dark deprivation produced both a diurnal and nocturnal increase in feeding. After 15 days of repeated restriction, rats showed no body weight variations with respect to control rats fed ad libitum. These results show that short fasting elicits a time- and macronutrient-dependent feeding response in rats, which involves reorganization of the macronutrient self-selection pattern to promote a total daily caloric compensation. These results suggest that animals principally respond to the energy deficit produced by restriction.

Aggressive and sexual social stimuli do not phase shift the circadian temperature rhythm in rats

The objective of the present study was to determine whether the rat circadian system is sensitive to social stimuli. Male rats were subjected to a sociosexual interaction with an estrous female or to an aggressive interaction with a dominant male conspecific. The interactions lasted for 1 h and took place in the middle of the circadian resting phase. Control animals were picked up and handled for a few minutes, but were otherwise left undisturbed. Animals were housed under constant dim red light during the whole period of the experiment. To assess the effects of the interactions on free-running circadian rhythmicity, body temperature was measured by means of radio telemetry. neither the sociosexual interaction with a female nor the aggressive interaction with another male induced phase shifts or changes in the free-running period. The rat circadian system does not seem to be sensitive to social stimuli directly. Moreover, the finding that aggressive interactions do not phase shift circadian rhythms indicates that the endogenous pacemaker in rats is not sensitive to stressors.

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.

Restricted daytime feeding modifies suprachiasmatic nucleus vasopressin release in rats

The authors have shown previously that vasopressin (VP) release from suprachiasmatic nucleus (SCN) efferents in rats is important for the timing of the circadian activity of the hypothalamo-pituitary-adrenal (HPA) axis, resulting in a circadian rise in corticosterone at dusk. When meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosterone peak at dusk. This feeding-associated rise in corticosterone is regarded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors investigated a putative involvement of SCN-derived VP in the control of the prefeeding corticosterone peak by measuring the intranuclear release of VP in the SCN and plasma corticosterone levels in rats in ad libitum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted daytime feeding caused clear changes in the daily release pattern of VP from SCN terminals. Both a delayed onset of the diurnal rise and a premature decline of the elevated daytime levels were observed, but the acrophase of the VP rhythm was not phase shifted. Concerning the circadian corticosterone peak, no phase shift of its acrophase was observed either. It is concluded that (1) restricted daytime feeding does affect SCN activity, (2) intranuclear release of VP within the SCN is an important mechanism to amplify and synchronize the circadian rhythms as dictated by the light/dark-entrained circadian pacemaker, and (3) VP release observed in animals on restricted feeding is completely compatible with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.

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.

Long-term fitness training improves the circadian rest-activity rhythm in healthy elderly males

In old age, the circadian timing system loses optimal functioning. This process is even accelerated in Alzheimer's disease. Because pharmacological treatment of day-night rhythm disturbances usually is not very effective and may have considerable side effects, nonpharmacological treatments deserve attention. Bright light therapy has been shown to be effective. It is known from animal studies that increased activity, or an associated process, also strongly affects the circadian timing system, and the present study addresses the question of whether an increased level of physical activity may improve circadian rhythms in elderly. In the study, 10 healthy elderly males were admitted to a fitness training program for 3 months. The circadian rest-activity rhythm was assessed by means of actigraphy before and after the training period and again 1 year after discontinuation. As a control for possible seasonal effects, repeated actigraphic recordings were performed during the same times of the year as were the pre and post measurements in a control group of 8 healthy elderly males. Fitness training induced a significant reduction in the fragmentation of the rest-activity rhythm. Moreover, the fragmentation of the rhythm was negatively correlated with the level of fitness achieved after the training. No seasonal effect was found. Previous findings in human and animal studies are reviewed, and several possible mechanisms involved in the effect of fitness training on circadian rhythms are discussed. The results suggest that fitness training may be helpful in elderly people suffering from sleep problems related to circadian rhythm disturbances.

Phase shifts to refeeding in the Syrian hamster mediated by running activity

Circadian rhythms in hamsters can be entrained by restricted daily feeding schedules. Phase control may be exerted by feeding per se, or by wheel running in anticipation of food access. Phase modulation by feeding was examined here by depriving hamsters of food for 9-24 h and refeeding at 1 of 7 different zeitgeber times on the first day of constant dim light. Significant group mean phase-advance shifts were observed only following 24 h and 17 h deprivations ending in the mid-subjective day, 7 h before the usual time of lights off (mean shifts 28 min and 66 min, respectively). The largest phase shifts were associated with wheel running during the first 6 h of refeeding. When running wheels were locked during this time in an additional group, no phase shifts were observed. A trend for small phase delays was evident for 14 h deprivations ending at the beginning of the subjective night, but no significant group mean or individual shifts were observed at other refeeding times. Refeeding after food deprivation, thus, appears to have minimal effects on circadian phase in hamsters; wheel running associated with refeeding may account for occasional shifts observed.

Persistence of nonphotic phase shifts in hamsters after serotonin depletion in the suprachiasmatic nucleus

Serotonin-containing fibres (5-HT) project from the raphe complex to the suprachiasmatic nucleus (SCN). Previous studies have suggested that this pathway may be involved in nonphotic resetting of the circadian clock. For example, 5-HT agonists are capable of phase shifting the biological clock both in vivo and in vitro, producing phase response curves (PRCs) similar in shape to those of other nonphotic stimuli. Therefore we studied the role of the serotonergic projection to the SCN in nonphotic phase shifts by bilateral injection of the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) onto the SCN of hamsters. About 50 days after the administration of the neurotoxin, the 5-HT and 5-HIAA (5-hydroxyindole acetic acid) levels were severely depleted in the SCN, as revealed by high performance liquid chromatography (HPLC), and immunocytochemistry (ICC). The average level of 5-HT depletion was 88% in Experiment 1 and 95% in Experiment 2. This treatment had no effect on the magnitude of phase shifts produced by 3 h of novelty-induced wheel-running starting at circadian time (CT) 4, the peak of the advance region of the PRC to this stimulus. The effect of 5-HT depletion on shifts produced by running at CT 22 were inconclusive because of changes in the behavior of control animals. No changes in the phase angle of entrainment of animals in a 14:10 light:dark (LD) cycle were detected in depleted animals. The results suggest that the 5-HT projection from the raphe to the SCN is not essential for activity-induced phase shifts in hamsters.

Effects of fluoxetine and olfactory bulbectomy on mouse circadian activity rhythms

Olfactory bulbectomy (OBX) in SWR outbred male mice lengthened the free-running period and delayed the phase of a circadian rhythm for wheel-running activity. OBX also increased mean levels of activity. Two weeks of daily intraperitoneal injections of Fluoxetine (8 mg/kg), a serotonin re-uptake inhibitor, reversed the effects of bulbectomy on the mean level of activity and significantly shortened the free-running period of the activity rhythm. The phase of the activity rhythm was not significantly affected by the Fluoxetine treatment. These results are consistent with a hyposerotonergic mediation of the effects of OBX on circadian period and activity level.

Serotonergic stimulation and nonphotic phase-shifting in hamsters

Stimuli that make hamsters active, such as dark pulses or triazolam administration, also phase shift their circadian clocks, producing phase advances during the subjective day and phase delays during the subjective night. Activity or its correlate appears to be important in producing the shifts because preventing locomotion blocks the phase shifts associated with these stimuli. The physiological basis of clock resetting induced by activity is not fully understood. The serotonergic (5-HT) projection from the raphe to the suprachiasmatic nucleus (SCN) is a possible route by which nonphotic information could reach the pacemaker. Administration of 8-HYDROXY-2-(DI-N-PROPYLAMINO) TETRALIN HYDROBROMIDE (8-OH-DPAT), a 5-HT1A and 5-HT7 receptor agonist, at circadian time (CT) 8 produces phase advances in the circadian rhythms of hamsters. Before concluding that 5-HT mediates the effect of activity on the pacemaker, it must be shown that 5-HT agonist do not produce shifts simply because they make animals more active. Therefore, we investigated the contribution of activity to 8-OH-DPAT-produced shifts. Preventing hamsters from moving around after administering 8-OH-DPAT did not abolish phase shifts. Moreover, higher doses of 8-OH-DPAT diminished activity on the day of injection but did not affect the amplitude of phase shifts. Suprisingly, quipazine (a non specific 5-HT agonist), when injected in the middle of subjective day did not phase shift the activity rhythm of hamsters, as it has been reported to do in rats.

Effect of lithium carbonate on activity level and circadian period in different strains of rats

Lithium, an important pharmacological agent for the treatment of manic-depressive illness in humans, is known to lengthen the circadian period in a number of different species. Recent experiments, on the other hand, suggest that pharmacological agents may affect the circadian system indirectly through an increase or decrease of activity. To explore the interaction between pharmacological and activity effects on the circadian system, lithium was administered chronically to three different strains of rats (ACI, BH, and LEW) while wheel-running activity was studied quantitatively. Two of these inbred strains (BH and LEW) show profound abnormalities in their circadian activity rhythms, namely, a reduced overall level of activity and bimodal or multimodal activity patterns. Wheel-running activity was monitored for 4 weeks under baseline conditions, followed by 3 weeks with lithium treatment (0.3% Li2CO3 administered with food) and 4 weeks with normal food. Treatment with lithium (average intake per day = 3.6 +/- 0.2 mg) consistently decreased both the overall level and the circadian amplitude of the activity rhythm. The free-running period tau was slightly lengthened during lithium treatment, while the most dramatic effect on period was observed after lithium withdrawal. Correlation analysis, however, revealed only a small negative correlation between activity level and period length, which proved significantly only for animals of the ACI strain. Our data support the traditional interpretation that lithium lengthens circadian period by a direct pharmacological effect on the circadian pacemaker rather than through indirect effects of activity feedback.

Effect of chronic tryptophan depletion on the circadian rhythm of wheel-running activity in rats

The effect of chronic treatment with a tryptophan (TRP)-free diet on the free-running circadian wheel-running rhythm and the central serotonergic system was investigated in blinded male rats. The long-term TRP-free diet did not change periods of activity, but disordered their patterns. This seemed to be due to masking, entrainment, enhancement of the morning activity, and obscuring of the activity onset as well as appearance of some periodic activities within the subjective night. A long-term TRP-fre diet decreased the concentration of TRP, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in all brain regions tested: frontal cortex, hippocampus, thalamus, hypothalamus, midbrain, and pons. Density of 5-HT1A receptor binding was significantly decreased in the frontal cortex and hypothalamus, whereas no significant change was observed in the density of 5-HT2 receptor binding in all regions. These results suggest that the period of primary circadian pacemaker is not affected, but its oscillation, as well as the coupling strength between the primary and secondary pacemakers, is weakened by the dysfunction of the serotonergic system caused by chronic TRP depletion.

Neonatal desipramine treatment alters free-running circadian drinking rhythms in rats

Neonatal treatment with monoamine reuptake inhibitors results in a constellation of neurobehavioral alterations in adult rats that may model human depression. Since alterations in circadian rhythmicity have been reported in both depressed patients and in animal depression models, the present study examined the effects of neonatal desipramine treatment (5.0 mg/kg SC from postnatal day 7 through 22) on free-running circadian drinking rhythms. Rhythmicity was examined in constant darkness (DD), constant light (LL), and during adult desipramine treatment (0.25 mg/ml via the drinking water). Compared with saline-treated controls, neonatal desipramine lengthened free-running period in DD, blunted the period-altering effect of LL, and potentiated the period-altering effect of adult desipramine treatment. Neonatal desipramine treatment also increased circadian amplitude and spectral magnitude, but did not modify the effects of light or adult desipramine on these parameters. These results provide further evidence that behavioral depression is associated with alterations in circadian rhythmicity, and are consistent with the hypothesis that such relationships are mediated by brain monoaminergic systems.

Aftereffects of scheduled daily exercise on free-running circadian period in Syrian hamsters

This study examined whether a nonphotic factor, scheduled daily exercise, could cause aftereffects on the free-running circadian period of Syrian hamsters. Groups of hamsters were kept under a cycle of 14-h light:10-h dark with access to their running wheel for only 3 h a day. Depending on the group, this 3-h period coincided with early day, midday, late day, early night, or late night. Controls did not have access to wheels. After 12 days, all hamsters were released into constant darkness (DD) and given free access to their wheel. Late-day runners showed a significantly shorter free-running period in DD compared to night runners and to controls, indicating that free-running periods can be shortened by nonphotic factors in this species. On the first day of DD, the activity onset of hamsters preceded (midday and late-day runners), coincided with (night runners and controls), or followed (early day runners) the previous time of D onset. Advanced activity onsets in late-day runners were consistent with both their short free-running periods and probable phase-advancing effects of late-day exercise; in contrast, delayed activity onsets in early day runners could only be explained by phase-delaying effects of the scheduled exercise.

Specific destruction of the serotonergic afferents to the suprachiasmatic nuclei prevents triazolam-induced phase advances of hamster activity rhythms

Administration of Triazolam (Tz)--a short acting benzodiazepine (BZ)--induces permanent phase-shifts in locomotor activity of golden hamsters (Mesocricetus auratus). However, the target area(s) as well as the mechanism involved in the Tz-induced changes are not known. Previous results indicated that raphe nuclei (RN) would appear to be a likely site for Tz-induced phase shifts. Therefore, we specifically destroyed the 5-HT fibers connecting the RN with the SCN--the site of the endogenous mammalian clock--by microinjections of the selective neurotoxin 5,7 dihydroxytryptamine (5,7-DHT) at the level of SCN. Infusion of 5,7-DHT resulted in long lasting damage of the ascending serotonergic projection from RN to the hypothalamus. Subsequently, the phase-shifting effect of Tz was investigated. Only complete or almost complete depletion of the 5-HT input to the SCN was accompanied with a pronounced reduction of the phase shift together with a significant reduction of wheel-running activity during the 6 h following Tz injection. Our present results support the view that the 5-HT innervation of the SCN represents an essential link in the phase-shifting action following peripheral Tz injections.

The interaction of thyroid state, MAOI drug treatment, and light on the level and circadian pattern of wheel-running in rats

In order to examine the relationship between thyroid status, the circadian system, and antidepressant drug response, the antidepressant drug clorgyline, a monoamine oxidase inhibitor (MAOI), was administered chronically to sham-operated or thyroparathyroidectomized rats. Wheel-running was monitored continuously in a light-dark (LD) cycle, and then in constant dim light. In LD, MAOI treatment increased levels of running. This effect was delayed in hypothyroid rats relative to euthyroid rats. In constant light, the MAOI-induced increase in running was diminished in euthyroid but not hypothyroid animals. Hypothyroid animals were less responsive to the change in lighting than were euthyroid animals, and this was more apparent in hypothyroid rats given MAOI. The daily pattern of running differed with lighting condition as well as with treatment group. MAOI-treatment of hypothyroid animals phase-advanced the pattern of wheel-running. MAOI-treatment of control animals increased the amplitude of wheel-running particularly in the LD cycle. These results indicate that thyroid status, lighting, and MAOI treatment interact to alter the behavioral response to chronic drug treatment.

Circadian drinking rhythms in SHR and WKY rats: effects of increasing light intensity

This study sought to define the generality of a previous finding that the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rat strains differ in free-running circadian period when maintained in running-wheel cages under constant light. Circadian drinking rhythms were monitored in SHRs and WKYs housed without access to running wheels under an increasing series of light intensities beginning with constant darkness. Strain differences in circadian period were seen only at relatively high light intensities, indicating that SHRs and WKYs differ in circadian light sensitivity. Since SHRs and WKYs differ in circadian period with or without access to running wheels, this strain difference is not likely to depend on differential locomotor activity levels. SHRs and WKYs also differed in spectral profile and circadian waveform, but only under low light intensities. At higher intensities, dissociation of rhythmicity was seen in both strains.

Monoamine depletion alters the entrainment and the response to light of the circadian activity rhythm in hamsters

Reduced amplitude, shorter free-running periods and desynchronization among a number of circadian rhythms are associated with advanced age in rodents. The response of the hamster circadian system to photic stimuli is also altered during senescence. Decreased monoamine levels, receptor sites and neuronal populations are commonly observed in the aging brain. The objective of the present study was to determine if monoamine depletion with reserpine in young hamsters induces changes in the circadian rhythm of locomotor activity similar to those that occur spontaneously with aging. Wheel-running activity of 12 young hamsters under a 14 h-light/10 h-dark cycle was continuously monitored. The total activity level, the times of activity onset, peak and offset and the duration of activity were determined during a 1-week period after vehicle treatment and for three 1-week periods after reserpine treatment (4 mg/kg). A second group of eight reserpine-treated and six vehicle-treated animals was kept in constant darkness (DD). The period of the circadian activity rhythm in DD and the phase-shifts after short light pulses at circadian time 19 (CT19) were determined in the control and reserpine-treated groups. Brain monoamines in the hypothalamus, striatum and pons/medulla after reserpine and vehicle treatment were determined by high-pressure liquid chromatography. The data were analyzed with x2 periodogram and one-way ANOVA followed by Duncan's post hoc test. Reserpine treatment significantly reduced total wheel-running activity and the monoamine levels in the hypothalamus, striatum and pons/medulla.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin and the mammalian circadian system: II. Phase-shifting rat behavioral rhythms with serotonergic agonists

The suprachiasmatic nuclei (SCN) receive primary afferents from the median and dorsal raphe, but the role of these projections in circadian timekeeping is poorly understood. Studies of the SCN in vitro suggest that quipazine, a general serotonin (5-HT) receptor agonist, can produce circadian time-dependent phase advances and phase delays in circadian rhythms of neuronal activity. The present study addresses whether quipazine and the selective 5-HT1A receptor agonist 8-OH-DPAT are similarly effective in vivo. Drinking and wheel-running patterns of male Wistar rats individually housed in constant darkness were monitored before and after subcutaneous administration of quipazine (5-10 mg/kg) at either circadian time (CT) 6 or CT 18, with and without running wheels available. Dose-dependent phase advances (20-180 min) were produced at CT 6. Significant phase shifts were not observed at CT 18. CT 6 quipazine-treated animals also showed a sustained and significant shortening of rhythm period (tau) following treatment (-0.28 hr; p < 0.002). tau shortening was inconsistently observed in CT 18 quipazine-treated rats. Neither quipazine-induced phase shifts nor tau effects were dependent on wheel-running activity per se. 8-OH-DPAT delivered via intracerebral ventricular treatment into the third ventricle (5 microliters at 100 microM in saline) produced slightly smaller phase advances (20-90 min) at CT 6, but did not produce phase delays at CT 18 or changes in tau. These findings support in vitro evidence that 5-HT-ergic agonists can phase-shift the circadian pacemaker.

No triazolam-induced expression of Fos protein in raphe nuclei of the male Syrian hamster

While the visual projections to the suprachiasmatic nuclei (SCN) play a role in mediating the effects of light on circadian rhythms, the functional significance of the serotonergic projection from the raphe nuclei (RN) to the SCN is uncertain. Because previous results indicated that RN would appear to be a likely site for triazolam (Tz)-induced phase shifts, we used the expression of Fos-protein as a marker of Tz-induced neuronal activation. Immunocytochemistry was used to visualize the presence of Fos-like protein. Tz-induced Fos-labeled nuclei were found in superior colliculi, Edinger-Westphal nuclei (EW) and dorsal tegmental nuclei (DTg), but not in the RN. The SCN showed only occasionally labeled nuclei in all experimental groups, whereas there was no Tz-induced Fos-immunoreactivity in the intergeniculate leaflet (IGL). The present data not necessarily exclude the implication of the RN in the phase shifting effect of Tz. The phase shift could still be accomplished using a different set of immediate early genes (IEG), or without an IEG response. Alternatively, as will be discussed, other pathways could mediate the phase shifting effect of Tz.

Circadian activity rhythms in SHR and WKY rats: strain differences and effects of clonidine

The spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) inbred rat strains have been subjected to extensive behavioral and neurochemical characterization. The present study examined free-running circadian activity rhythms in these two strains. Because previous studies indicated that free-running rhythms are altered during chronic clonidine administration, and that SHRs and WKYs may respond differentially to clonidine, the effects of this agent on rhythmicity were compared in the two strains. SHRs were hyperactive and showed shorter free-running periods than did WKYs. Clonidine administration altered free-running rhythms similarly in the two strains, but reduced activity levels only in the relatively hyperactive SHRs. These results are consistent with the hypothesis that central noradrenergic systems influence circadian locomotor activity rhythms.