EEG delta activity during undisturbed sleep in the squirrel monkey.

The squirrel monkey (Saimiri sciureus) exhibits a robust daily rhythm of sleep-wakefulness that is under circadian control, but the nature of homeostatic sleep regulation in this diurnal primate is poorly understood. Since delta frequency (0.5-2.0 Hz) activity in the electroencephalogram (EEG) during non-Rapid Eye Movement (NREM) sleep is thought to reflect homeostatic factors contributing to sleep tendency, we measured EEG delta power density and slow wave incidence and amplitude during NREM sleep during spontaneous sleep, occurring when monkeys were housed undisturbed in a 24-hour light-dark (LD) cycle and in constant light (LL). In LD and LL conditions, monkeys exhibited circadian rhythms in delta power density, wave incidence and wave amplitude that peaked in the middle of the subjective night, several hours after consolidated sleep onset. These results differ from predictions of a purely homeostatic model of sleep that would include maximal levels of delta activity at sleep onset.

Circadian and homeostatic influences on sleep in the squirrel monkey: sleep after sleep deprivation.

A series of sleep deprivation (SD) experiments were performed to examine the relative influence of circadian and homeostatic factors on the timing of sleep in squirrel monkeys free-running in constant illumination. All SDs started at the beginning of subjective night and lasted 0, 1/4, 1/2, 1, 1 1/4, or 1 1/2 circadian cycles. These six lengths represented three pairs: (0.1), (1/4, 1 1/4), (1/2, 1 1/2). Within each pair, SD ended at the same circadian phase but differed by one circadian cycle in duration. Both before and after SD, consolidated sleep (CS) episodes occurred predominantly during subjective night, even after long SDs ending at the beginning of subjective day. CS duration was strongly influenced by circadian phase but had no overall correlation with prior wake duration. Sleep loss incurred during SDs longer than 1/4 cycle was only partially recovered over the next two circadian cycles, though total sleep duration was closer to baseline levels after the second circadian cycle after SD. There was a trend toward a positive correlation between prior wake duration and the amount of NREM and delta activity measures during subjective day. Delta activity was not increased in the first 2 hours of CS after the SD. Relatively high levels of delta activity occurred immediately after the SD ended and again at the time of baseline CS onset. These data indicate that the amount of sleep and delta activity after SD in squirrel monkeys is weakly dependent on prior wake duration. Circadian factors appear to dominate homeostatic processes in determining the timing, duration and content of sleep in these diurnal primates.

MidnightSun: software for determining light exposure and phase-shifting schedules during global travel.

The application of circadian principles has the potential to alleviate jet-lag in global travelers, but their application is hampered by the difficulty of determining light exposure along international flight routes. Computerized tools can solve this problem algorithmically. We have developed a program for Macintosh computers, called MidnightSun, which allows researchers to display ambient lighting conditions at any geographical location at any time of the year. The program contains a data base with the latitudes and longitudes of over 3000 airports. It calculates flight paths and durations, and prints a graphical itinerary indicating times of daylight during flights and layovers. Given a travel itinerary and a user-defined phase response curve (PRC) for light, it recommends light exposure times that may accelerate the reentrainment of circadian rhythms to new time zones and reduce the deleterious effects of jet-lag (depending on the efficacy of the PRC and the compliance of the traveler). Other potential applications include determining lighting protocols for photoperiodism experiments and providing data sets for mathematical circadian simulations under naturalistic lighting conditions.

Thermoregulatory responses of rhesus monkeys during spaceflight.

This study examines the activity, axillary temperature (T(ax)), and ankle skin temperature (Tsk) of two male Rhesus monkeys exposed to microgravity in space. The animals were flown on a Soviet biosatellite mission (COSMOS 1514). Measurements on the flight animals, as well as synchronous flight controls, were performed in the Soviet Union. Additional control studies were performed in the United States to examine the possible role of metabolic heat production in the T(ax) response observed during the spaceflight. All monkeys were exposed to a 24-h light-dark cycle (LD 16:8) throughout these studies. During weightlessness, T(ax) in both flight animals was lower than on earth. The largest difference (0.75 degree C) occurred during the night. There was a reduction in mean heart rate and Tsk during flight. This suggests a reduction in both heat loss and metabolic rate during spaceflight. Although the circadian rhythms in all variables were present during flight, some differences were noted. For example, the amplitude of the rhythms in Tsk and activity were attenuated. Furthermore, the T(ax) and activity rhythms did not have precise 24.0 hour periods and may have been externally desynchronized from the 24-h LD cycle. These data suggest a weakening of the coupling between the internal circadian pacemaker and the external LD synchronizer.

Renal response to 7 days of lower body positive pressure in squirrel monkeys.

The purpose of this study was to characterize the renal response to central volume expansion using lower body positive pressure (LBPP) in the adult male squirrel monkey (Saimiri sciureus). Urinary excretion of sodium, potassium, and water and plasma aldosterone concentrations were measured during a control day, 7 days of LBPP, and a recovery day. Time control experiments in the same animals included chair sitting without exposure to LBPP. Seven monkeys (600-1,000 g) were trained to sit in a specially designed metabolism chair before chronic implantation of arterial and venous catheters to facilitate maintenance infusion of saline, monitoring of vascular pressures, and blood sampling; urine was collected via a condomlike tube. A cephalad shift in body fluid was induced by applying 20 mmHg of air pressure from the waist down. Urine volume and sodium excretion were increased significantly (28 to 52 ml/day and 0.4 to 3.5 meq/day, respectively) during the initial 24 h of LBPP with most of this response occurring in the first 6-12 h. From the 2nd to the 7th day of LBPP, urinary excretion rates for sodium and water were not different from chair-sitting controls. Water and sodium balance significantly decreased from +15 to -12 ml/day and from +1.1 to -2.2 meq/day, respectively, from the control to the first day of LBPP. This change in balance was not observed in the time-control group. Removal of the stimulus resulted in a modest conservation of sodium and water. The renal responses were not associated with any changes in plasma aldosterone levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The benzodiazepine triazolam phase-shifts circadian activity rhythms in a diurnal primate, the squirrel monkey (Saimiri sciureus).

Triazolam can shift the phase of circadian rhythms in hamsters recorded in constant light or dark. This effect is apparently mediated by physical activity stimulated by the drug. We examined whether triazolam can shift the phase of circadian rhythms in a diurnal primate, the squirrel monkey, that is sedated by triazolam. Single injections of triazolam at 0.15-0.20 mg/kg induced phase advances or delays of monkey activity rhythms recorded in constant light. The phase-response curve is similar to that obtained for hamsters. Behavioral activation is evidently not necessary for a phase-shifting action of triazolam in this primate. A companion study in which triazolam was administered to squirrel monkeys in the dark in a light-dark cycle reentrainment paradigm failed to find evidence for a phase shifting action of triazolam. Shifts induced by triazolam in monkeys recorded in constant light may thus reflect changes in light exposure as a consequence of sedation or altered retinal processing of light.

Effects of aging on food-entrained circadian rhythms in the rat.

Twenty-four hour schedules of restricted food availability entrain a component of the circadian activity rhythm in rats via a food-entrainable pacemaker separate from the light-entrainable pacemaker. The effect of aging on food-entrained circadian rhythms was examined in 6 rats maintained on a restricted diurnal feeding schedule from age 3-21 months and again from 24-25 months. Food-entrainment, measured as behavioral anticipation of a 1-hr daily mealtime during the middle of the light period and persistence of this anticipation rhythm during food deprivation, was apparent in the aged rats when recorded in wheel-running cages from 20-21 months of age. Despite the long duration of restricted diurnal food diurnal food access, the aged rats, like young rats, rapidly to nocturnal activity when transferred to ad lib feeding. When restricted diurnal feeding was reinstated at 24 months age, these rats, now recorded in food-bin monitoring cages, required more time for a food anticipation pattern to emerge and showed a lower amplitude food anticipation rhythm compared to a group of young adult rats. These age-related changes are similar to those that characterize photically entrained circadian rhythms and suggest that both components of the rat's multioscillatory circadian timekeeping system deteriorate in parallel over the life span.

Lithium lengthens circadian period in a diurnal primate, Saimiri sciureus.

Lithium lengthens the period of free-running circadian rhythms in a variety of species, but this effect has not been demonstrated unequivocally in primates. Because of the possible link between lithium's action on the circadian clock and its therapeutic action in human mood disorders, we tested the ability of lithium to lengthen circadian period in a diurnal primate with circadian properties similar to those of humans. Lithium carbonate was administered in food pellets to 8 adult male squirrel monkeys (Saimiri sciureus) for at least 27 consecutive days. Serum lithium levels on the last day of lithium administration ranged from 0.76 to 2.02 mEq/liter, comparable to the therapeutic range for treatment of bipolar disorder in humans (0.6-1.2 mEq/liter). Circadian periods of perch-hopping activity were longer during lithium treatment than during baseline in 7 of the 8 monkeys (changes of -0.08 to +1.41 hr, mean +0.55 hr, p = 0.01), and returned toward baseline values when lithium was discontinued. In most cases, the period change was evident within a few days after beginning full lithium dose, and was not accompanied by changes in level or pattern of activity, nor in amplitude of the circadian rhythm. Food consumption and body weight were reduced during lithium treatment, and rebounded on return to lithium-free diet. Period change was related to lithium dose (p less than 0.05), but did not correlate with food consumption, body weight, or baseline circadian period. These results, by establishing that lithium lengthens circadian period in primates, suggest that studying the cellular mechanisms of this circadian effect may be relevant to understanding lithium's therapeutic effect on mood in humans.

Characteristics of food-entrained circadian rhythms in rats during long-term exposure to constant light.

Rats possess a system of circadian oscillators that permit entrainment of circadian activity rhythms independently to 24 hr cycles of light-dark and food access. The nature of interactions between food- and light-entrainable oscillators was examined by observing the generation and persistence of food-entrained circadian rhythms in rats whose light-entrainable rhythms were eliminated by long-term exposure to constant light. Most of these rats showed a delayed generation of food-entrained rhythms and only one of eight animals showed persistence of food associated rhythms during a 4-day food deprivation test. Rats whose light-entrainable rhythms are eliminated by suprachiasmatic nuclei ablation show, in contrast, normal generation and persistence of food-entrained rhythms. The results suggested a disruptive influence of constant light on non-photic entrainment, possibly due to coupling forces between damped light-entrainable oscillators and the food-entrainable oscillators.

Food-anticipatory rhythms under 24-hour schedules of limited access to single macronutrients.

Food-restricted rats anticipate a fixed daily mealtime by entrainment of a circadian timekeeping mechanism separate from that which generates daily light-entrainable activity rhythms. The entrainment pathways and rhythm-generating substrates for food-anticipatory rhythms are unknown. In this study, we attempted to define minimal food-related stimuli necessary or sufficient for food anticipation by employing schedules of restricted macronutrient availability, with or without free access to a complementary diet. Rats did not anticipate a daily meal of protein, carbohydrate, or fat, as measured by tilt-cage, running-wheel, or food-bin activity, when they had free access to other nutrients. However, rats did anticipate single-macronutrient meals when they were limited to only two, larger, complementary meals each day (protein-fat, protein-carbohydrate) providing a reduced total number of calories. Previous work has shown that caloric restriction per se is not a prerequisite for food anticipation. In combination with that study, the present results indicate that the size of a nutrient meal, in absolute terms or relative to total daily nutrient intake, is of pre-eminent importance in determining its value as a synchronizer of anticipatory rhythms. The results further suggest that physiological responses unique to the ingestion and absorption of any particular macronutrient are not necessary components of the entrainment pathway.

Effects of restricted feeding schedules on circadian organization in squirrel monkeys.

Free running circadian rhythms of motor activity, food-motivated lever-pressing, and either drinking (N = 7) or body temperature (N = 3) were recorded from 10 squirrel monkeys maintained in constant illumination with unlimited access to food. Food availability was then restricted to a single unsignaled 3-hour interval each day. The feeding schedule failed to entrain the activity rhythms of 8 monkeys, which continued to free-run. Drinking was almost completely synchronized by the schedule, while body temperature showed a feeding-induced rise superimposed on a free-running rhythm. Nonreinforced lever-pressing showed both a free-running component and a 24-hour component that anticipated the time of feeding. At the termination of the schedule, all recorded variables showed free-running rhythms, but in 3 animals the initial phase of the postschedule rhythms was advanced by several hours, suggesting relative coordination. Of the remaining 2 animals, one exhibited stable entrainment of all 3 recorded rhythms, while the other appeared to entrain temporarily to the feeding schedule. These results indicate that restricted feeding schedules are only a weak zeitgeber for the circadian pacemaker generating free-running rhythms in the squirrel monkey. Such schedules, however, may entrain a separate circadian system responsible for the timing of food-anticipatory changes in behavior and physiology.

In vivo metabolic activity of the suprachiasmatic nuclei: non-uniform intranuclear distribution of 14C-labeled deoxyglucose uptake.

Anatomical techniques have shown that the suprachiasmatic nuclei (SCN) are organized into distinct dorsomedial and ventrolateral subdivisions. As a functional correlate to this morphological organization, the intranuclear distribution of SCN glucose utilization was mapped using the autoradiographic 14C-labeled deoxyglucose method. In nocturnal rats and diurnal squirrel monkeys injected with the tracer during the light portion of the light-dark cycle, the middle of the SCN was metabolically more active than its rostral or caudal ends. No obvious dorsomedial/ventrolateral parcellation of SCN functional activity was disclosed. The rostrocaudal metabolic contour persisted unchanged in the absence of external light and resembled the 3-dimensional shape of the SCN (the highest metabolic activity was generally found at the largest cross-sectional area). This result is discussed with respect to its implications for the generation of circadian rhythmicity by the endogenous pacemaker in the SCN.

Resynchronization of circadian sleep-wake and temperature cycles in the squirrel monkey following phase shifts of the environmental light-dark cycle.

Circadian rhythms in physiological and behavioral functions gradually resynchronize after phase shifts in environmental time cues. In order to characterize the rate of circadian resynchronization in a diurnal primate model, the temperature, locomotor activity, and polygraphically determined sleep-wake states were monitored in squirrel monkeys before and after 8-h phase shifts of an environmental light-dark cycle of 12 h light and 12 h dark (LD 12:12). For the temperature rhythm, resynchronization took 4 d after phase delay shift and 5 d after phase advance shift; for the rest-activity cycle, resynchronization times were 3 d and 6 d, respectively. The activity acrophase shifted more rapidly than the temperature acrophase early in the post-delay shift interval, but this internal desynchronization between rhythms disappeared during the course of resynchronization. Further study of the early resynchronization process requires emphasis on identifying evoked effects and measuring circadian pacemaker function.

Estrogen modifies the circadian timing and amplitude of the luteinizing hormone surge in female hamsters exposed to short photoperiods.

LH surges occur 3 h later in intact anovulatory hamsters exposed to nonstimulatory photoperiods (6L:18D) for 8 wk than the proestrous LH surges from the same hamsters housed in 6L:18D for 3 weeks. In ovariectomized hamsters housed in 6L:18D for 3 wk, the LH surge was observed at the same time of day as in intact anovulatory hamsters at 8 wk. Implanting Silastic capsules containing estradiol benzoate (EB) advanced the timing of the daily surge of LH in ovariectomized hamsters housed in 6L:18D for 8 wk. EB also affected the magnitude of the LH surge in hamsters housed in 6L:18D for 8 wk. Two days after receiving EB implants, daily LH surges in anovulatory hamsters were suppressed by 75% and in ovariectomized "regressed" hamsters by 37%. This difference between groups was probably due to ovarian progesterone in intact animals. Estrogen is not required for LH surges in anovulatory hamsters but suppresses LH release when administered exogenously. The delay in the timing of the LH surge in anovulatory hamsters may result from the decline in estrogen resulting from short photoperiod exposure.

Physiology of the circadian timing system: predictive versus reactive homeostasis.

Since Cannon first formulated the concept of homeostasis 60 years ago, attention has been focused on the corrective responses initiated after the steady state of the organism is perturbed. In this lecture it is argued that the concept of homeostasis should be extended to include the precisely timed mechanisms of the circadian (and circannual) timing system which enables organisms to predict when environmental challenges are most likely to occur. A mature understanding of homeostasis should encompass both "reactive" responses to changes in physiological variables which have already occurred and the "predictive" responses initiated in anticipation of predictably timed challenges. Such predictive responses have particular value because they enable physiological mechanisms to be utilized immediately, even if they involve a delay of several hours, by activating them at a suitable time in advance of a probable challenge. However, conflicts may often occur between predictions and reality. Examples from sleep-wake behavior, thermoregulation, blood volume homeostasis, and the regulation of potassium balance show that predictive responses often compromise the effectiveness of reactive homeostatic mechanisms even to the point of risking the survival of the organism. It must be concluded that the day-night cycle of the natural environment has played a fundamental role in shaping the evolutionary development of homeostatic mechanisms because of the dominating predictability of diurnal changes in illumination, temperature, food availability, and predator activity.

GABA and circadian timekeeping: implications for manic-depression and sleep disorders.

Circadian rhythms, evident in a wide variety of physiological and behavioural parameters, are under the control of central neural pacemakers, the best characterized of which is the suprachiasmatic nucleus of the hypothalamus. The neurophysiological mechanisms involved in central pacemaker function are unknown. Recent biochemical, pharmacological and behavioural evidence suggests that the inhibitory transmitter gamma-aminobutyric acid (GABA), present in the small interneurones of the suprachiasmatic nucleus, plays an important role in circadian timekeeping. This has enabled the formulation of strategies for treatment of patients with manic depressive illness and certain sleep disorders in which disorders of circadian timekeeping may be fundamental.

Sleep-wake stages during the subjective night of the squirrel monkey.

The study of the circadian sleep-wake cycle is beset by unique technical challenges. Continuous polygraphic recordings are necessary to characterize circadian phenomena; however, the traditional method of recording sleep at high (15 mm/sec) chart speed is impractical for continuous animal studies that may last several weeks at a stretch. A system to determine four sleep-wake stages (awake, transitional, non-REM, REM) from low chart speed (1.5 mm/sec) recordings was developed and validated by direct behavioral observation using four adult male squirrel monkeys prepared for chronic recording of EEG, EOG and EMG. The polygraphic stages "transitional," non-REM and REM were highly correlated with behavioral observations of sleep, although individual sleep stages could not be resolved by behavioral parameters alone.

Daily rhythms of benzodiazepine receptor numbers in frontal lobe and cerebellum of the rat.

Behavioural, biochemical and neurophysiological evidence suggests that gamma-aminobutyric acid (GABA) may play an important role in the neural control of circadian rhythms. Central receptors for benzodiazepines are functionally coupled to GABA receptors and appear to mediate behavioural effects of exogenous benzodiazepines. The binding of 3H-flunitrazepam to synaptic plasma membranes prepared from various regions of rat brain was examined at 6-hour intervals over a 36-hour period. Prominent daily rhythms in receptor number (Bmax) were observed in the frontal lobe and the cerebellum but not in the temporoparietal regions, hypothalamus or medulla/pons. Binding was highest during periods of sleep/low activity with a significant decrease occurring just prior to waking. These results suggest that daily fluctuations in benzodiazepine receptor numbers may be related to the temporal control of sleep/wake and muscle activity cycles.

Forced internal desynchronization between circadian temperature and activity rhythms in squirrel monkeys.

In an attempt to force internal desynchronization between the rest-activity rhythm and the body temperature rhythm of the squirrel monkey (Saimiri sciureus), five animals were studied in a 14:14 light-dark cycle. In four animals a 28-h spectral component was found to predominate in the rest-activity rhythm, whereas an unentrained circadian component (tau = 25.9 +/- 0.4 h) predominated in the body temperature rhythm. Plots of the cycle-by-cycle acrophases of the two rhythms confirm that they desynchronize, due to the failure of the temperature rhythm to entrain to the light-dark cycle. These data from intact animals provide further support for the hypothesis that the squirrel monkey circadian timing system has at least two pacemakers. A rhythm for which the supra-chiasmatic nuclei (SCN) have previously been shown to be essential (rest-activity) simultaneously exhibited a different period from a rhythm (body temperature) that has been shown to persist after destruction of the SCN.

Role of heat loss and heat production in generation of the circadian temperature rhythm of the squirrel monkey.

To study heat production and heat loss in determination of the daily body temperature rhythm, we examined colonic temperature, skin (tail, foot and abdomen) temperatures and oxygen consumption in chair-restrained squirrel monkeys maintained in isolation in an environmental chamber with a 24-hr light-dark cycle (LD 12:12), maintained at a constant thermoneutral temperature (26 degrees C). In all experiments repeated high amplitude (2 degrees C) diurnal rhythms in colonic temperature were observed. Heat loss, estimated from changes in skin temperature, also displayed a circadian rhythm, although there was considerable variation in waveform. On average, a rhythm in heat production, indicated by changes in the rate of oxygen consumption, was also present. However, a large degree of variability was seen in oxygen consumption, and in several cycles from various animals there were no observable 24-hr rhythms. The circadian body temperature rhythm is thus not simply a consequence of daily changes in metabolism, but rather a regulated response that involves both heat production and heat loss.