Circadian rhythm dissociation in an environment with conflicting temporal information.

The relative contributions of light-dark (LD) cycles and feeding (EF) cycles in providing temporal information to the circadian time-keeping system were examined in chair-acclimatized squirrel monkeys (Saimiri sciureus). The circadian rhythms of drinking, colonic temperature, urine volume, and urinary potassium excretion were measured with the LD and EF cycles providing either conflicting phases or periods. In conflicting phase experiments, animals were exposed to 24-h LD cycles consisting of 12 h of 600 lx followed by 12 h of less than 1 ls and concurrent 24-h EF cycles in which the animals ate for 3 h and then fasted for 21 h. One group had food available at the beginning and a second group at the end of the light period. In conflicting period experiments, monkeys were exposed to 23-h LD cycles (LD 11.5:11.5) and 24-h EF cycles (EF 3:21). Analysis of the rhythms showed that both phase and period information were conveyed to the drinking and urinary rhythms by the EF cycle, and to the temperature rhythm by the LD cycle.     

Comparison of synchronization of primate circadian rhythms by light and food.

Several circadian rhythms in squirrel monkeys (Saimiri sciureus) entrained by two different agents were studied to compare their mode of coupling with the environmental zeitgebers. Synchronization was accomplished either by light-dark cycles consisting of 12 h of 600 lx followed by 12 h of less than 1 lx (LD 12:12), or by eat-fast cycles in which the animals could eat for 3 h and then had to fast for the remaining 21 h each day (EF 3:21). The rhythms of drinking, colonic temperature, and urinary potassium and water excretion were measured in chair-acclimatized monkeys. The drinking and urinary rhythms were more reproducible (smaller mean variance) and more stable (smaller standard deviation of the timing of a phase reference point) in EF than in LD cycles, whereas the temperature rhythm was more tightly controlled by LD cycles than by EF cycles. In constant light an 8-h phase delay in the EF cycle caused the drinking and urinary rhythms to resynchronize to the EF cycle within one day, while the temperature rhythm required about 6 days to resynchronize. In contrast, previously published data for a similar phase delay in the LD cycle with food available ad libitum show that the drinking and temperature rhythms resynchronized more rapidly than the urinary rhythms. These results indicate that separate mechanisms are involved in transducing temporal cues from LD and EF cycles in the circadian timekeeping system of these nonhuman primates.     

Renal electrolyte circadian rhythms: independence from feeding and activity patterns.

The interrelationships between urinary electrolyte circadian rhythms and rhythms of feeding, drinking and activity were studied in six conscious chair-acclimatized squirrel monkeys (Saimiri sciureus) kept in temperature-controlled isolation chambers on a light-dark (LD) 12:12 h cycle. With lights on (600 lx) from 0800 to 2000 h and off (less than 1 lx) from 2000 to 0800 h, renal potassium excretion in monkeys fed ad libitum fell to a daily minimum of 64 +/- 6 mueq/h at 0500 h and rose to a maximum of 274 +/- 13 mueq/h at 1700 h. Sodium excretion fell to a minimum of 13 +/- 2 mueq/h at 1000 h and rose to a maximum of 43 +/- 6 mueq/h at 2100 h, while water excretion fell to a minimum of 869 +/- 63 mul/h at 0500 h and rose to a maximum of 2,307 +/- 222 mul/h at 1700 h. Feeding, drinking, and activity occurred only during the lights-on period. Independence of the urinary rhythms from diurnal variations in feeding, drinking, and activity was established a) by depriving monkeys of food, b) by depriving monkeys of water, and c) by training monkeys to perform a 2-hourly schedule of feeding, drinking, and activity throughout day and night. None of these three regimens resulted in major reductions of the amplitude, or changes in the phase of the circadian rhythms of urinary electrolyte or water excretion. These findings indicate that the circadian rhythms of urinary potassium, sodium, and water excretion are controlled by mechanisms that are not passively dependent on the behavioral patterns of feeding, drinking, and activity.     

Circadian control of thermoregulation in the squirrel monkey, Saimiri sciureus.

The characteristics and control of the circadian rhythms of core body temperature (colonic) and skin temperature (tail) were studied in chair-acclimatized squirrel monkeys (Saimiri sciureus). When animals were entrained to a light-dark cycle (12 h 600 lx; 12 h less than 1 lx) these two temperatures displayed prominent, reproducible, tightly coupled circadian rhythms. In contsant light of 600 lx, where no other effective circadian time cues were present, both temperature rhythms persisted with free-running periods. Within each animal, however, these rhythms were not as tightly coupled to one another as in LD. On occasion colonic and tail temperature rhythms free-ran with different circadian periods and some animals demonstrated "splitting" of the colonic temperature rhythm, with the colonic temperature rhythm displaying a bimodal pattern. These results suggest that the circadian rhythm of body temperature in primates is under the control of more than one potentially independent circadian oscillator.     

Cortisol-mediated synchrinization of circadian rhythm in urinary potassium excretion.

Conscious chair-acclimatized squirrel monkeys (Saimiri sciureus) studied with lights on (600 lx) from 0800 to 2000 h daily (LD 12:12) display a prominent circadian rhythm in renal potassium excretion. The characteristics of this rhythm were reproduced in adrenalectomized monkeys by infusing 5 mg cortisol and 0.001 mg aldosterone, or 5 mg cortisol alone, between 0800 and 0900 h daily. When the timing of cortisol adminisration (with or without aldosterone) was phase-delayed by 8 h, the urinary potassium rhythm resynchronized by 80% of the cortisol phase shift, but only after a transient response lasting 3-4 days. With the same daily dose of adrenal steroids given as a continuous infusion throughout each 24 h, urinary potassium excretion showed free-running oscillations no longer synchronized to the light-dark cycle. These results indicate that the cirdacian rhythm of plasma cortisol concentration acts as an internal mediator in the circadian timing system, synchronizing a potentially autonomous oscillation in renal potassium excretion to environmental time cues and to other circadian rhythms within the animal.     

Effects of feeding and light cycles on activity rhythms of maternally isolated rat pups

Motor activity of infant rat pups was measured continuously between days 3 and 18 of postnatal age. Mother-reared rats on a 12:12 LD cycle exhibited significant rhythmic activity in the circadian range as early as day 5 of postnatal age. Some of the pups reared in isolation from maternal and sibling contact and kept on 12:12 LD cycles, feeding cycles, or combinations of feeding, temperature, and LD cycles also showed rhythmic activity but it was less persistent and of a lower amplitude than the rhythms of the mother-reared group. In the isolated rat pups nocturnal partitioning of activity was strengthened in the presence of both a light-dark cycle and a feeding cycle but only when the feeding resembled more natural nursing rhythms. In animals kept on constant light and a feeding cycle, activity occurred slightly more often during the 12-hr interval of decreased food intake. The addition of a temperature cycle--cooler nocturnal temperature--decreased the proportions of nocturnal motor activity. These results indicate that feeding and light-dark cycles may contribute to the synchronization of activity rhythms during the early postnatal period.     

Rhythms in behaviors, body temperature and plasma corticosterone in SCN lesioned rats given methamphetamine

In aperiodic rats with lesions in the suprachiasmatic nuclei (SCN), rhythms with a circadian period in spontaneous locomotion, wheel-running, feeding, drinking, body temperature and plasma corticosterone were restored by chronic administration of methamphetamine. These rhythms were not entrained by a light-dark cycle. Wheel-running, feeding and drinking rhythms in individual rats were in phase in terms of ultradian bout as well as circadian fluctuation. Rhythms of the intraperitoneal temperature appeared accompanying the spontaneous locomotor rhythm. The phase relation between the two rhythms was similar to that of SCN dependent rhythms. Plasma corticosterone also fluctuated in a circadian fashion. The corticosterone peak preceded the activity onset of locomotor rhythm by a few hours, which was similar to the phase relation observed in the SCN intact animals. It is concluded that the oscillatory mechanism underlying the spontaneous locomotor rhythm in SCN lesioned and methamphetamine treated rats drives also other physiological rhythms. The phase-relations among them were similar to those of rhythms driven by the circadian pacemaker in the SCN.     

Effect of a short photoperiod on circadian rhythms of body temperature and motor activity in old rats

Circadian rhythms of body temperature and motor activity were documented in young and old rats (four 8-week-old and five 22-month-old male Wistars, implanted with telemetric probes and housed in a chronobiological facility) under two different photoperiod conditions. The animals were maintained in a light:dark (LD) cycle of 12 h each (LD 12:12) for 4 weeks and then exposed to a LD 6:18 cycle for 7 weeks to assess the effect of age on the desynchronization of the temporal structure of the rhythms. In old rats under LD 12:12, the power of the 24-h component and the circadian amplitude of body temperature and motor activity were markedly lower than in the young and both rhythms were phase-advanced. After the shift to LD 6:18, the circadian rhythmicity was maintained for both variables and the same phase delay (+5+/-1 h) was observed in both age groups, as was a gradual expansion of the patterns of both functions with the longer night. The photoperiod reduction (6 weeks under LD 6:18) did not modify the power of the 24-h component of body temperature and motor activity in old rats. In young rats, however, the power and amplitude of the 24-h component of motor activity rhythm fell to the levels of those in old rats, while the power of the 24-h component of body temperature rhythm and the amplitude did not change. Our data show that the circadian rhythm of motor activity, but not of body temperature, responds age dependently to a photoperiod reduction.     

Feeding time synchronizes primate circadian rhythms.

Circadian rhythms of squirrel monkeys maintained in constant light and temperature can be entrained by 24 hr cycles of food availability with eating for 3 hr and fasting for 21 hr (EF 3:21). Rhythms of drinking, body temperature and urinary potassium and water excretion exhibited periods which matched the 24 hr period of the EF 3:21 cycle. These results suggest that temporal patterns of food intake are capable of synchronizing the circadian timekeeping system which underlies the observed rhythms.     

Adaptation to daily meal-timing and its effect on circadian temperature rhythms in two inbred strains of mice

Inbred strains of mice differ in their adjustment to a mealtime out of phase with the light-dark cycle. When food access was restricted to the first 4 h of the light span, C3H/2Ibg mice steadily lost weight and died, while C57BL/6J mice recovered baseline levels of food intake within a few days. C57BL meal-timed mice also showed delayed circadian body temperature rhythms so that peak temperatures coincided with the time of food availability. Both strains were able to adapt when the period of food availability was gradually shifted to the first 12 h of the 16-h light period. C57BL mice again phase-delayed their body temperature rhythms, while C3H mice exhibited highly variable individual responses, and over half continued to increase temperature in anticipation of lights-off. These results suggest that the timing of body temperature rhythms of C57BL mice may be more easily altered relative to the light-dark cycle than that of C3H mice.This work was supported in part by NIH Grant GM 21993 to C.B.L.     

Behavior and body temperature in rats following chronic foot shock or psychological stress exposure

In an attempt to examine stress-induced behavioral disorders, including circadian rhythm disturbances, we measured motor activity, feeding, drinking, and body temperature over a 14-day period following a long-term stress exposure in rats. Male Wistar rats were exposed to foot shock (physical) or non-foot shock stress (psychological) induced by the communication box for 1 h daily over 12 weeks. Two to three months after the termination of the stress sessions, motor activity, food intake, water intake, and body temperature were measured by means of an automatic behavioral measurement system under a 12:12-h light:dark cycle. Motor activity, feeding, and drinking patterns were not influenced by either of the previous stress exposures. Daily rhythm of body temperature was also unchanged in either stress group, however, a significant elevation in body temperature (by 0.20 degrees C, p<0.05) was observed only in non-foot-shocked rats. The present study suggests that only psychological stress induces an elevation of body temperature following the stress exposures; however, long-term stress exposures in the present experiment do not disturb behavioral activities and daily rhythms of behaviors.     

Temperature rhythm reentrains faster than locomotor rhythm after a light phase shift

Mammalian endogenous circadian rhythms are entrained to the environmental light-dark (LD) cycle. Although the circadian rhythms of core body temperature (Tb) and spontaneous locomotor activity (LA) are well synchronized under stable LD conditions, it is thought that these two parameters are regulated by distinct mechanisms. The purpose of the present study was to examine the adaptability of these two rhythms to an abrupt change in the environmental light phase. Tb and LA were simultaneously recorded in individual mice kept under 12:12-h LD cycle conditions before and after an 8-h photic phase advance. The onset of LA required 8 days to reentrain to the new LD cycle, whereas 6 days were required for reentrainment of the acrophase of Tb. Resting Tb, i.e., the Tb level independent of LA, was extracted from the same data source. The resting Tb level exhibited a robust daily rhythm with a difference of 1.0 degrees C between LD phases. After the photic phase advance, the resting Tb rapidly reached a stable level within 4 days, whereas the uncorrected Tb required 6 days for reentrainment. Based on these findings, we revealed that, independent of LA, the adaptability of the Tb rhythm to a new light cycle is half as rapid as that of LA. These results therefore suggest that the circadian rhythms of Tb and LA are intrinsically regulated by different pacemaker or effector mechanisms.     

Sleep-onset insomniacs have delayed temperature rhythms

It was predicted from free running and ultradian cycle studies that sleep-onset insomniacs would have endogenous circadian rhythms that were phase delayed compared to good sleepers. Thirteen sleep-onset insomniacs and nine good sleepers were selected to differ only in their sleep-onset latencies as confirmed by polysomnography. their rectal temperatures were measured over a 26-h constant routine and analyzed with best-fit Fourier curves including 24-h fundamental and 12-h harmonic components. The temperature rhythm markers of the insomniacs' rhythms were approximately 2.5 h later than the respective phases of the good sleepers. The usual bedtimes of the insomniacs fell within the "wake maintenance zone" of their delayed temperature rhythm. The good sleepers had typical bedtimes several hours after their "wake maintenance zone" and closer to their body temperature minimum. It was suggested that manipulations to phase advance the insomniacs' rhythms would reduce their sleep-onset latencies. It was also predicted that early morning insomnia results from phase advanced circadian rhythms and that sleep maintenance insomnia results from an abnormal phase relationship between the 24-h temperature rhythm and 12-h sleep-alert rhythm.     

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.     

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.     

Effects of two-hour light-dark cycles on feeding, drinking and motor activity of the rat

The effect of two-hour light-dark cycles on feeding, drinking and motor activity in the rat was compared with behavior under the usual $\text{12}\text{12}$ hour cycle. The two-hour cycles consisted of $\text{60}\text{60}$ min, $\text{80}\text{40}$ min and $\text{40}\text{80}$ min light-dark schedules which were maintained each for 7 days. Water intake, frequency of feeding, and motor activity were still significantly higher during dark than during light, although their occurrence during dark was reduced as compared to the $\text{12}\text{12}$ hour control schedule. A free-running circadian rhythm of consummatory behavior with a period length exceeding 24 hours was present throughout the experimental period. The amplitude of the circadian feeding rhythm gradually decreased over time, whereas the percentage of feeding during dark increased. During the circadian phase of minimal food intake, illumination changes affected feeding behavior more strongly than during the phase of maximal food intake. After restoration of the orginal $\text{12}\text{12}$ hour cycle, the amplitude of the nocturnal feeding rhythm increased gradually over several days, whereas the amplitude of the drinking rhythm showed a more rapid recovery. The experiments show that even short cycles of illumination may exert control over the rat's consummatory and motor activity. Short light-dark schedules provide a way for studying separately effects of illumination and of circadian rhythms.     

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.     

Continuous exposure to dim illumination uncouples temporal patterns of sleep, body temperature, locomotion and drinking behavior in the rat

Dissociable circadian rhythms of sleep and body temperature in primates are thought to be regulated by independent oscillators whereas the uncoupling of circadian rhythms has not been well described in other mammals. Therefore, we made simultaneous recordings of non-rapid-eye-movement-sleep (NREMS), rapid-eye-movement-sleep (REMS), brain temperature, intraperitoneal temperature, locomotion and drinking activity under light-dark (LD) and continuous dim illumination (dim LL) and analyzed their interrelations. The rhythmic patterns of body temperature, locomotion and drinking were modified on the 12th circadian day of dim LL, while the mean body temperature as well as mean occurrence of drinking and locomotor activities did not change significantly. In contrast, dim LL exposure significantly increased the total time spent in NREMS during the resting phase of dim LL and increased REMS episodes during the active phase of dim LL. The diverse effects of dim LL exposure on the recorded phenomena suggest that temporal patterns of sleep were the most sensitive to perturbations of lighting and that differential oscillatory mechanisms may regulate sleep and other circadian rhythms in the rat.     

Food anticipatory activity and photic entrainment in food-restricted BALB/c mice

The BALB/c mouse was evaluated as a model for the study of entrainment of circadian rhythms by feeding schedules. Mice were housed in a 12:12-h light-dark (LD) environment with food available for 3-5 h/day (5 h before dark onset). Food anticipatory activity (FAA) rhythms were evident in all mice, ranging from robust in some to weak and variable in others. Advancing transients of the end of nocturnal activity were evident in many cases, culminating in a significant shortening of the main bout of nocturnal activity. Transients and contraction of nocturnal activity were not dependent on the expression of FAA. Following restricted feeding, nocturnal activity expanded by a series of delaying transients. On the first day of constant dark (DD) with ad libitum food access following restricted feeding in LD, the phase from which activity free-ran was advanced by comparison with control tests. Transients, compressed nocturnal activity, and advanced phase of free-run suggest that feeding schedules cause phase advancement of light-entrained rhythms in BALB/c mice. When restricted feeding was imposed in DD, several mice expressed robust FAA concurrent with a free-running activity component. In some cases, free-running rhythms entrained to feeding time, and in other cases, the period of the free run lengthened toward 24 h. These data show that restricted feeding in BALB/c mice can engage a circadian mechanism driving FAA rhythms and can also modulate the phase of photic entrainment, possibly by a direct entraining effect on the light-entrained rhythm. The BALB/c mouse strain, in several respects, appears to be a useful model for the study of scheduled feeding and circadian rhythms.     

Rat circadian rhythms entrain to a descending saw-tooth light intensity cycle.

We reexamined whether rat circadian rhythms entrained to the light intensity cycle of a descending saw-tooth (ST-d) form, in which illuminance decreased rectilinearly from 300 lx to 0 lx in 24 h, and abruptly returned to 300 lx (lights-on time). Ambulation, drinking and subcutaneous body temperature were simultaneously monitored in 5 intact, 5 pinealectomized and 5 orchiectomized rats. Additionally, sleep was monitored in the intact rats. In all the rats, entrainment was confirmed during 65 days' exposure to the ST-d cycle. The waveforms of the entrained rhythms were much modified compared with those during LD 12:12. The estimated activity periods of the entrained rhythms straddled the lights-on time of the ST-d cycle. In all the groups of rats, administration of a single ST-d cycle in constant dim red light produced only delay shifts irrespective of its circadian phase, and there was no significant circadian variation in the magnitude of phase shifts. The results indicate that rat circadian rhythms entrain to the ST-d cycle with an unexpected phase position, which cannot be explained by the phase-response curve.