Activity feedback to the mammalian circadian pacemaker: influence on observed measures of rhythm period length

In the mouse, activity is precisely timed by the circadian clock and is normally most intense in the early subjective night. Since vigorous activity (e.g., wheel running) is thought to induce phase shifts in rodents, the temporal placement of daily exercise/activity could be a determinant of observed circadian rhythm period. The relationship between spontaneous running-wheel activity and the circadian period of free-running rhythms was studied to assess this possibility. With ad libitum access to a running wheel, mice exhibited a free-running period (tau) of 23.43 +/- 0.08 hr (mean +/- SEM). When running wheels were locked, tau increased (23.88 +/- 0.04 hr, p less than 0.03), and restoration of ad libitum wheel running again produced a shorter period (tau = 23.56 +/- 0.06 hr, p less than 0.05). A survey of free-running activity patterns in a population of 100 mice revealed a significant correlation between the observed circadian period and the time of day in which spontaneous wheel running occurred (r = 0.7314, p less than 0.0001). Significantly shorter periods were observed when running was concentrated at the beginning of the subjective night (tau = 23.23 +/- 0.04), and longer periods were observed if mice ran late in the subjective night (tau = 23.89 +/- 0.04), F (1, 99) = 34.96, p less than 0.0001. It was previously believed that the period of the circadian clock was primarily responsive to externally imposed tonic or phasic events. Systematic influences of spontaneous exercise on tau demonstrate that physiological and/or behavioral determinants of circadian timekeeping exist as well.

Influence of running wheel activity on free-running sleep/wake and drinking circadian rhythms in mice

Previous studies have indicated that manipulation of activity levels can modify characteristics of sleep/wake and activity rhythms. The generality of these observations was evaluated by simultaneously measuring drinking and sleep/wake rhythms while mice had free or no access to a running wheel in constant conditions (DD). Robust circadian rhythms in all parameters were observed in the "wheel free" (unrestricted) condition. When wheels were locked, the peak amplitude of the sleep/wake circadian rhythm decreased by approximately 50% without affecting the amplitude of the drinking rhythm. Total wake time decreased 11% per circadian day when wheels were locked with increases in both NREM and REM sleep. Whereas the amplitude of the drinking waveform was unaffected, wheel restriction caused an equivalent increase in period length (tau) for both rhythms. These results indicate that, unlike the generalized effects of activity on tau, activity restriction influences on rhythm amplitude do not generalize to all behavioral and/or physiological variables. This work also supports the notion that activity influences on sleep/wake rhythm amplitude reflect behavioral "masking" rather than a fundamental change in the direct coupling mechanisms of the biological clock.

Are ground squirrels sleep deprived during hibernation?

Hibernation is an adaptation for energy conservation, which probably evolved as an extension of non-rapid-eye-movement sleep mechanisms. Yet, during periodic arousals from bouts of deep hibernation, ground squirrels (Spermophilus lateralis) spend most of their time asleep. Spectral analysis of the electroencephalogram revealed that cortical slow-wave intensity during sleep is high at the beginning of a euthermic period and declines thereafter. Sleep slow-wave intensity is greater after longer bouts of hibernation than after shorter bouts. We hypothesize that low body temperatures during hibernation are incompatible with the restorative function of sleep as reflected in cortical slow-wave activity. Animals must incur the energetic costs of periodic arousals from hibernation to receive the restorative benefits of euthermic slow-wave sleep. The timing of arousals from hibernation may be a function of accumulated sleep debt.

Triazolam fails to induce sleep in suprachiasmatic nucleus-lesioned rats

Rats with suprachiasmatic nuclei (SCN) lesions did not show increased sleep after triazolam (TRZ) injections at any dose from 0.2 to 1.6 mg/kg, whereas 0.4 mg/kg TRZ given intact rats in the middle of their activity phase significantly increased sleep. Across SCN-lesioned and intact rats, the amount of sleep before and after TRZ 0.4 mg/kg was negatively correlated. SCN-lesioned rats did not have a circadian activity-dominant period and so did not accumulate a biological sleep debt. Their lack of response to TRZ may have resulted from the absence of a sleep debt compared to intact rats injected in the middle of their activity phase. These data support our hypothesis that the homeostatic process controlling sleep gates benzodiazepine hypnotic efficacy.

Reticular formation activity during wakefulness and sleep in a hibernator (Spermophilus lateralis)

This study examined ground squirrel reticular formation (RF) cell activity as a function of: (1) arousal state, and, (2) phasic alterations in neck muscle activity. A total of 37 neurons were recorded from 11 behaving squirrels (Spermophilus lateralis). Five cells were located in the midbrain RF, 15 were in the lateral pontine site implicated in inhibition of motor output, i.e. the subcoeruleus area, 11 were in the medial pontine RF, and 6 were in the medullary RF. Regardless of location, the majority of the cells (81%; 30 of 37) exhibited greatest activity when the animals were awake and/or in rapid-eye-movement (REM) sleep. However, the relationship between neuronal activity and phasic alterations in neck muscle activity differed as a function of location. The activity of 53% of cells (8 of 15) located in the subcoeruleus area increased with phasic decreases in electromyographic (EMG) activity, whereas the activity of cells (22 of 22) in other RF sites decreased or did not change with phasic decreases in EMG activity. The increased activity of subcoeruleus area cells during phasic decreases in neck muscle activity is further evidence suggesting that the lateral pontine RF is involved in promotion of muscle atonia during REM sleep and possibly wakefulness.

Real-time automated sleep scoring: validation of a microcomputer-based system for mice

Long-term circadian studies of sleep and wakefulness in rodents have been hindered by the labor required to analyze long polygraph records. To expedite such studies, we have designed and implemented SCORE, a microcomputer-based real-time sleep scoring system for rodents. The electroencephalograph is digitized in 10-s epochs at 100 Hz. Frequency and amplitude information from the waveform are extracted into a 48-dimension vector that is then compared to previously taught vectors representing the canonical features of four arousal states: wakefulness, theta-dominated wakefulness, rapid eye movement (REM) sleep, and nonREM (NREM) sleep. Match values are assigned for each state to each epoch; after excluding states based on wheel-running or drinking activity data, the nonexcluded state with the best match value for the epoch is scored. Analysis of over 23,000 epochs for four mice yielded an overall agreement of 94.0% between two human scorers and the program, compared with a 94.5% agreement between the two human scorers. The SCORE algorithm matched the human concensus best for wakefulness (97.8%) and NREM sleep (94.7%), but was lower for REM sleep (75.2%) and theta-dominated wakefulness (83.3%). Most errors in scoring of REM sleep were in close temporal proximity to human-scored REM epochs. SCORE is capable of scoring arousal states for eight animals simultaneously in real time on a standard IBM PC equipped with a commercially available analog-to-digital conversion board, and should considerably facilitate the performance of long-term studies of sleep and wakefulness in the rodent.

Triazolam-induced sleep in the rat: influence of prior sleep, circadian time, and light/dark cycles

Rats entrained to 12-h on /12-h off light schedule and injected with triazolam 0.4 mg/kg at the mid-point of their activity phase (6 h after lights out: circadian time = CT-18) had a stronger hypnotic response than animals free-running in constant dark injected at the equivalent circadian time. In contrast, entrained rats injected 5 h after lights on (CT-5) showed increased wake after injection relative to baseline, largely due to REM sleep inhibition. Hypnotic efficacy was found to be inversely related to prior accumulated sleep. During the 6 h before injection, entrained rats injected at CT-18 slept significantly less than the free-running rats, which in turn slept significantly less than entrained rats injected at CT-5. Taken together, the results suggest that the amount of prior sleep was a more important influence on the response to triazolam than either light/dark per se or circadian phase. Methodologically, automated sleep scoring was found to be an efficient method for examining drug effects, particularly when corroborated by concurrent independent physiological variables and spectral analysis.

Systematic 24-hr behavioral observations of sleep and wakefulness in a skilled-care nursing facility

Sleep patterns of aged, infirm, demented, chronically institutionalized residents of a skilled-care nursing facility were studied. The purpose of this naturalistic study was to describe sleep and wakefulness (S/W) within the limits afforded by brief behavioral observations and to examine homeostasis and diurnal rhythmicity of S/W as a function of psychoactive drug intake. Observers noted S/W every 15 min, 24 hr a day for 10 days in 24 Ss. Results indicated substantial individual variation in daytime hours. Daily and weekly variation within Ss was minimal. Sleep was least likely near sunset. Ss on psychoactive drugs showed dampened diurnal variation in S/W rhythms. In Ss not on such drugs, there was a suggestion of homeostasis of S/W between sleep during the morning and evening hours. Results are discussed methodologically (viability of approach), theoretically (age-related change in sleep), and practically (potential treatments).

Systematic 24-hr behavioral observations of sleep and wakefulness in a skilled-care nursing facility

Sleep patterns of aged, infirm, demented, chronically institutionalized residents of a skilled-care nursing facility were studied. The purpose of this naturalistic study was to describe sleep and wakefulness (S/W) within the limits afforded by brief behavioral observations and to examine homeostasis and diurnal rhythmicity of S/W as a function of psychoactive drug intake. Observers noted S/W every 15 min, 24 hr a day for 10 days in 24 Ss. Results indicated substantial individual variation in daytime hours. Daily and weekly variation within Ss was minimal. Sleep was least likely near sunset. Ss on psychoactive drugs showed dampened diurnal variation in S/W rhythms. In Ss not on such drugs, there was a suggestion of homeostasis of S/W between sleep during the morning and evening hours. Results are discussed methodologically (viability of approach), theoretically (age-related change in sleep), and practically (potential treatments).

Action potential duration increases as body temperature decreases during hibernation

The effect of temperature on the duration of both population spikes and action potentials of single neurons has been investigated in a variety of in vitro preparations. A few studies have examined the influence of temperature on spike potentials from spinal motoneurons of intact, anesthetized mammals. In all cases, the duration of the action potential or population spike increased as temperature decreased. A similar increase in the duration of action potentials accompanied hibernation in the ground squirrel (Spermophilus lateralis). Oscilloscope traces of 2 brainstem reticular formations, and 8 posterior thalamic single units were photographed at a body temperature (Tb) of 34-36 degrees C during euthermia prior to entrance into hibernation and at Tb's ranging from 10 to 27 degrees C during hibernation. There was a significant increase in the duration of the second component of the diphasic action potential at the lower Tb (P less than 0.01). This temperature effect was reversible, i.e. action potential durations returned to preentrance euthermic values following arousal from hibernation (Tb = 34-36 degrees C). This study is the first to use behaving animals to demonstrate that changes in biophysical characteristics of central nervous system neurons occur at low Tb. These changes in membrane characteristics probably result in alterations in neuronal functioning and information processing during hibernation.

Effects of light intensity on the circadian temperature and feeding rhythms in the squirrel monkey

The circadian rhythms of body temperature and feeding appear to be timed by separate pacemakers. Tonic administration of light has been used to investigate the response of the pacemaker timing behavioral rhythms; however, the response of the body temperature rhythm has not been similarly examined. This study investigates the circadian timing of the body temperature rhythm under conditions of different light intensity. We simultaneously recorded the patterns of both feeding and body temperature in squirrel monkeys free-running in an environment free of external time cues. In each lighting condition, the periods of the body temperature and feeding rhythms were identical. In constant bright light the rhythm periods were longer than when the animals were exposed to constant dim light. In addition, the variability of the periods was dependent on light intensity. The feeding rhythm period variance of animals in constant bright light was smaller than when in dim light. Conversely, the period of the free-running body temperature rhythm exhibited more variability in bright light than in dim light. Further, in each condition, there were changes in phase angle relationship between feeding and body temperature which were qualitatively similar to those observed in humans, although quantitatively smaller in magnitude. Thus, in the squirrel monkey, tonic light studies reveal that the mean circadian period of the body temperature and feeding rhythms are similar. However, changes in phase relationship, and differential rhythm period stabilities suggest differences in the period of the underlying, tightly coupled pacemakers.

Primate body temperature and sleep responses to lower body positive pressure

Cephalic fluid shifts, induced by lower body positive pressure (LBPP) are known to influence various physiological systems (i.e. cardiovascular and renal). In earlier experiments, we observed an apparent change in the arousal state of primates in such LBPP conditions. This study was designed to examine the effects of LBPP on arousal state and body temperature level which is normally correlated with sleep. Chair restrained male squirrel monkeys were exposed to 40 mmHg LBPP for 90-100 minutes between the daytime hours of 13:00-15:00. Each monkey was placed in a specially modified restraint chair to which they were highly restrained [correction of trained]. Deep body temperature (DBT) was collected from 10 animals. Sleep parameters were obtained from 6 animals chronically implanted for sleep recording. A video camera was used to observe each animal's apparent state of arousal. LBPP resulted in an approximate 0.9 degree C decrease in DBT. During video observation some animals appeared drowsy during LBPP, however, sleep recording revealed no significant changes in the state of arousal. Thus, LBPP is capable of inducing a mild hypothermia [correction of hyperthermia]. Further, the mechanisms underlying the observed lowering of body temperature appear to be independent of arousal state.