Effects of feeding cycles on circadian rhythms in squirrel monkeys

Squirrel monkeys (Saimiri sciureus) were housed singly in cages equipped with a tree for climbing to measure locomotor activity, and with a movable food cup that could be arrested automatically. The animals were kept in continuous dim illumination (LL), twice interrupted by several weeks of entrainment by a light-dark (LD) 12:12 cycle. Apart from three control sections in which the food cups were unlocked continuously (ad libitum feeding), food was accessible for 3 hr per day only, with interfeeding intervals varying from 23 to 26 hr (periodic restricted feeding, or RF). During LD entrainment, the imposition of an RF schedule resulted in anticipatory behaviors, represented by increased tugs at the food cup and a pause in locomotor activity preceding the feeding time. In LL, the animals showed free-running circadian rhythms of locomotor and "feeding" activity that nearly always persisted when ad libitum feeding was replaced by RF. The period (tau) of the free-running rhythm was slightly modulated in relation to the varying interfeeding intervals (T), but entrainment was never achieved except in one test with an animal whose tau was very close to T. It is concluded that periodic availability of food represents an extremely weak zeitgeber, if any, for the circadian pacemaker of squirrel monkeys.     

The internal synchronization of five circadian functions of the rabbit

Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL).

(1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66±8 (S.D.) %), 'water intake' (64±15 (S.D.) %) and 'urine excretion' (58±10 (S.D.) %). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 5S±13 (S.D.) % of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours.

(2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with-: = 24.8±0.5 (S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber.

(3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters.

(4) During LL the level of locomotor activity was reduced for 13±16 (S.D.) %, the events of food intake were increased for 17±12 (S.D.) %.

(5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days.

(6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.     

Circadian organization of running-wheel activity, food intake and drinking of old male rats

Circadian rhythms of running-wheel activity, food intake and drinking were monitored in old male rats of Long-Evans strain over 22 months of age in both entrained (light:dark 12:12, LD) and free running condition (continuous illumination, LL) and were compared with those of young adult male rats of 3.5 to 6.5 months of age. Twenty-four hour distribution of running activity, feeding events and licking events of young rats as well as old rats showed bi- or tri-modal patterns during the 12 hr dark period of the LD schedule. In the light period, 2 out of 8 old rats, 6 out of 10 old rats and 1 out of 6 old rats had 1 or 2 medium or high peaks in running activity, feeding events and licking events, respectively, leading to equal distribution between the dark and light period. In the LD schedule, old rats showed a decrease in running-wheel activity, its patterns and power spectra, a decrease in feeding events and its power spectra in 6 rats which lost circadian rhythms and increase in feeding events and its power spectra in 4 rats which still showed circadian rhythms and increase in licking events. LL suppressed running-wheel activity, its patterns and power spectra, licking events and its power spectra and feeding events in young rats. However, LL could suppress only feeding events of 4 rats which still showed circadian rhythms and licking events and its spectral level in old rats. The possible causes of decreased response to LL in old rats and its implication are discussed.     

Feeding entrainment of locomotor activity rhythms in the goldfish is mediated by a feeding-entrainable circadian oscillator

Periodic food availability can act as a potent zeitgeber capable of synchronizing many biological rhythms in fishes, including locomotor activity rhythms. In the present paper we investigated entrainment of locomotor rhythms to scheduled feeding under different light and feeding regimes. In experiment 1, fish were exposed to a 12:12?h light/dark cycle and fed one single daily meal in the middle of the light phase. In experiment 2, we tested the effect of random versus scheduled feeding on the daily distribution of activity. During random feeding, meals were randomly scheduled with intervals ranging from 12 to 36?h, while scheduled feeding consisted of one single daily meal set in the middle of the light or dark phase. Finally, in experiment 3, we studied the synchronization of activity rhythms to feeding under constant darkness (DD) and after shifting the feeding cycle by either advancing or delaying the feeding cycle by 9?h. The results revealed that goldfish synchronized to feeding, overcame light entrainment and significantly changed their daily distribution of activity according to their feeding schedule. In addition, the daily activity pattern modulated by feeding differed between layers: a peak of activity being noticeable directly after feeding at the bottom, while an anticipatory behaviour was obvious at the surface of the tank. Under DD and no food, free-running rhythms averaging 25.5?± 1.9?h (mean?±?SD) were detected. In conclusion, some properties of feeding entrainment (e.g. anticipation of the feeding time, free-running rhythms following termination of periodic feeding, and the stability of ø after shifting the feeding cycle) suggested that goldfish have (a) separate but tightly coupled light- and food-entrainable oscillators, or (b) a single oscillator that is entrainable by both light and food (one synchronizer being eventually stronger than the other).     

Circadian entrainment by feeding cycles in house sparrows,Passer domesticus

Summary We studied the potential zeitgeber qualities of periodic food availability on the circadian rhythms of locomotor and feeding activity of house sparrows. The birds were initially held in a LD-cycle of 12:12 h, with food restricted to the light phase. After transfer to constant dim light, the birds remained entrained by the restricted feeding schedule. Following an exposure to food ad libitum conditions, the rhythms could be re-synchronized by the feeding cycle. Shortening of the zeitgeber period to 23.5 h resulted in the loss of entrainment in most birds, whereas a longer zeitgeber period of 25 h re-entrained the rhythms of most birds. Although these results prove that periodic food availability can act as a zeitgeber for the circadian rhythms of house sparrows, several features of our data indicate that restricted feeding is only a weak zeitgeber. The pattern of feeding activity prior to the daily time of food access shown under some experimental conditions suggests that anticipation is due to a positive phase-angle difference of the birds' normal circadian system rather than being caused by a separate pacemaker.     

Circadian components of semilunar and lunar timing mechanisms

The timing of semilunar as well as lunar reproductive rhythms has been analyzed in different geographic populations of the intertidal chironomid Clunio. In stocks of three populations differing in period and phase relationship with the lunar month, these long-term rhythms were synchronized in the laboratory by using artificial moonlight cycles of 30 days in otherwise 24-hr light-dark (LD) cycles (0.4 lux during 4 successive nights every 30 days in LD 12:12). In LD cycles of various periods, a strong synchronization was only possible in LD 12:12 and LD 11:11, whereas in LD 10:10 and LD 15:15 the synchronization by the 30-"day" moonlight cycle was weak or even absent. The study demonstrates a limited range of circadian periods for entrainment of the long-term rhythms. It is concluded that an LD cycle with a period near 24 hr is an essential zeitgeber condition for semilunar and lunar timing in this marine insect. Further, it is suggested that the underlying physiological timing mechanism of Clunio consists of a circadian function for the perception of the monthly moonlight zeitgeber cycles that entrain the endogenous, temperature-compensated oscillator of the circasemilunar (or circalunar) period. The long-term oscillator triggers the metamorphosis of the insect, and thereby determines the time of its eclosion and reproduction on the shorelines, in correlation with days of spring tides recurring about every 14-15 days.     

Meal feeding schedule and ventromedial hypothalamic lesions do not affect rhythm of pineal serotonin N-acetyltransferase activity in rats

Effects of meal feeding schedule and bilateral lesions of the ventromedial hypothalamus (VMH) on the circadian rhythm of pineal serotonin N-acetyltransferase (SNAT) activity were examined in rats, under LD (12:12) condition. Neither meal feeding nor VMH lesions affected the phase of the circadian rhythm of pineal SNAT activity, but the VMH lesions reduced the level. Meal feeding caused a shift of the phases of the daily rhythms of phosphoenolpyruvate carboxykinase and tyrosine aminotransferase activities in the liver. These findings suggest that the circadian rhythm of pineal SNAT activity is not entrained by the food intake, and that the VMH does not function as a master oscillator of the rhythm.     

Recording and Analysis of Circadian Rhythms in Running-wheel Activity in Rodents

When rodents have free access to a running wheel in their home cage, voluntary use of this wheel will depend on the time of day^1-5. Nocturnal rodents, including rats, hamsters, and mice, are active during the night and relatively inactive during the day. Many other behavioral and physiological measures also exhibit daily rhythms, but in rodents, running-wheel activity serves as a particularly reliable and convenient measure of the output of the master circadian clock, the suprachiasmatic nucleus (SCN) of the hypothalamus. In general, through a process called entrainment, the daily pattern of running-wheel activity will naturally align with the environmental light-dark cycle (LD cycle; e.g. 12 hr-light:12 hr-dark). However circadian rhythms are endogenously generated patterns in behavior that exhibit a ~24 hr period, and persist in constant darkness. Thus, in the absence of an LD cycle, the recording and analysis of running-wheel activity can be used to determine the subjective time-of-day. Because these rhythms are directed by the circadian clock the subjective time-of-day is referred to as the circadian time (CT). In contrast, when an LD cycle is present, the time-of-day that is determined by the environmental LD cycle is called the zeitgeber time (ZT).Although circadian rhythms in running-wheel activity are typically linked to the SCN clock^6-8, circadian oscillators in many other regions of the brain and body^9-14 could also be involved in the regulation of daily activity rhythms. For instance, daily rhythms in food-anticipatory activity do not require the SCN^15,16 and instead, are correlated with changes in the activity of extra-SCN oscillators^17-20. Thus, running-wheel activity recordings can provide important behavioral information not only about the output of the master SCN clock, but also on the activity of extra-SCN oscillators. Below we describe the equipment and methods used to record, analyze and display circadian locomotor activity rhythms in laboratory rodents.     

Entrainment of Circadian Wheel-Running Rhythms of the Northern Brown Bandicoot, Isoodon macrourus, by Daily Restricted Feeding Schedules

Northern brown bandicoots (Isoodon macrourus) were subjected to restricted feeding for 3 h in the middle of the light period of a 14: 10 light/dark cycle and immediately following this in constant dark. When feeding was restricted to the middle of the light period of the light/dark cycle, all bandicoots maintained a nocturnal activity rhythm. In addition to the nocturnal rhythm, a few bandicoots showed meal-anticipatory activity during the light period. In bandicoots that did not show meal-anticipatory activity, diurnal activity was sometimes evident either during or shortly after the daily meal time. The observation of meal-anticipatory activity in some bandicoots suggests that this species may have a mechanism separate from the light-entrainable mechanism that allows the daily anticipation of periodically available food sources. In the next stage of the experiment, which was in constant dark, the meal was presented at the same time of day as it had been in the previous stage. In all bandicoots, the previously light-entrained component of activity free-ran and was eventually affected by the restricted feeding schedule to some degree. Bandicoots showed weak entrainment and relative coordination, suggesting that restricted feeding is a weak zeitgeber in this species. Evidence also suggesting that two separate but coupled pacemakers control the activity rhythms of the bandicoot was that (a) bandicoots simultaneously showed free-running light-entrainable rhythms and meal-entrained anticipatory rhythms; (b) in several bandicoots, the light-entrainable rhythm was phase advanced when it free-ran through the meal time; and (c) in one bandicoot, meal-entrained anticipatory activity was forced away from the meal time when the previously light-entrained component of activity free-ran through it.     

Chronopharmacological study of furosemide; (VIII) influence of feeding restriction

We have previously reported that a time-dependent variation is observed in the diuretic effect of furosemide and the light-dark cycle is a potent zeitgeber for this chronopharmacological phenomenon of the agent in rats. The present study was undertaken to examine whether a time of food intake is another zeitgeber for this event. In study I, rats were maintained with free access to food for 3 weeks. Furosemide (30 mg/kg) was given orally at 12 am or 12 pm. Urine was collected for 8 hours after the agent and urinary excretion of sodium and furosemide were determined. Thereafter, these rats were maintained under a daytime-restricted feeding schedule (9 am-11 am) for 3 weeks (study II) and a night-time-restricted feeding schedule (9 pm-11 pm) for 3 weeks (study III). The identical protocol of study I was repeated at the end of study II and III. Diuretic effect of furosemide and its urinary excretion were significantly greater at 12 am than at 12 pm in study I and III. However such an administration time-dependent change in the effect of furosemide and its urinary amount disappeared in study II. These data indicate that a time of food intake is another potent zeitgeber for the time-dependent variation in the diuretic effect of furosemide.     

Circadian clocks for all meal-times: anticipation of 2 daily meals in rats

Anticipation of a daily meal in rats has been conceptualized as a rest-activity rhythm driven by a food-entrained circadian oscillator separate from the pacemaker generating light-dark (LD) entrained rhythms. Rats can also anticipate two daily mealtimes, but whether this involves independently entrained oscillators, one 'continuously consulted' clock, cue-dependent non-circadian interval timing or a combination of processes, is unclear. Rats received two daily meals, beginning 3-h (meal 1) and 13-h (meal 2) after lights-on (LD 14:10). Anticipatory wheel running began 68±8 min prior to meal 1 and 101±9 min prior to meal 2 but neither the duration nor the variability of anticipation bout lengths exhibited the scalar property, a hallmark of interval timing. Meal omission tests in LD and constant dark (DD) did not alter the timing of either bout of anticipation, and anticipation of meal 2 was not altered by a 3-h advance of meal 1. Food anticipatory running in this 2-meal protocol thus does not exhibit properties of interval timing despite the availability of external time cues in LD. Across all days, the two bouts of anticipation were uncorrelated, a result more consistent with two independently entrained oscillators than a single consulted clock. Similar results were obtained for meals scheduled 3-h and 10-h after lights-on, and for a food-bin measure of anticipation. Most rats that showed weak or no anticipation to one or both meals exhibited elevated activity at mealtime during 1 or 2 day food deprivation tests in DD, suggesting covert operation of circadian timing in the absence of anticipatory behavior. A control experiment confirmed that daytime feeding did not shift LD-entrained rhythms, ruling out displaced nocturnal activity as an explanation for daytime activity. The results favor a multiple oscillator basis for 2-meal anticipatory rhythms and provide no evidence for involvement of cue-dependent interval timing.     

Vasopressin deficiency provides evidence for separate circadian oscillators of activity and temperature

Vasopressin-containing Long-Evans and vasopressin-deficient Brattleboro rats were maintained in individual cages while telemetered activity (AC) and body temperature (BT) data were collected. Rats were initially exposed to a 12 h/12-h light/dark cycle (photic zeitgeber) and were allowed ad-libitum access to food and water. Daily feeding, care, and handling (nonphotic zeitgebers) occurred at the beginning of the second hour of the dark cycle. After a 14-day habituation period, rats were subjected to continuous light (LL) or dark (DD) and nonphotic cues were presented irregularly. During the habituation period, both strains exhibited clear 24-h circadian rhythms of AC and BT. In LL or DD, photic cues were removed and nonphotic cues were presented irregularly. There was a shift in the rhythm for Long-Evans animals to 26 h for both AC and BT in LL and 24.6 h in DD. Feeding, care, and handling were seen as minor artifact. In Brattleboro rats, although there were robust 26-h and 24.6-h circadian rhythms of AC in the LL and DD, respectively, BT data were inconsistent and showed sporadic fluctuations. In the BT rhythm of Brattleboro animals, strong peaks were associated with feeding, care, and handling times and trough periods were characterized by a dramatic drop in temperature. This experiment demonstrates that AC and BT are controlled by separate oscillators. In addition, the importance of vasopressinergic fibers in the control of circadian rhythms of BT is evidenced by the loss of circadian rhythms in animals lacking these functional fibers when exposed to free-running paradigms where there is no entrainment of photic or nonphotic oscillators.     

Characteristics of Food-Entrained Orcadian 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.     

Characteristics of Food-Entrained Orcadian 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 ingestion and circadian rhythmicity

Feeding is organised within the 24-h of the light - dark (LD) cycle. Food is ingested in a circadian manner in nature and in laboratory animals kept under constant conditions. The circadian rhythmicity in food ingestion is driven by a biological clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The circadian organisation of food ingestion not only allows animals to live in harmony with their environment but food intake could also act as a zeitgeber for other rhythmic functions. Lesions in the area of the SCN result in the loss of most rhythmic functions as well as to a disrupted circadian rhythmicity of food and water intake. These findings, together with observations from daytime feeding experiments conducted in nocturnal animals, suggest that food intake may serve as a temporal signal for some peripheral organs to oscillate in phase with the SCN. This paper overviews and discusses how food intake interacts with the circadian system.     

Circadian rhythms of locomotor activity and temperature selection in sleepy lizards, <Emphasis Type="Italic">Tiliqua rugosa</Emphasis>

This study examined whether the daily rhythms of locomotor activity and behavioural thermoregulation that have previously been observed in Australian sleepy lizards (Tiliqua rugosa) under field conditions are true circadian rhythms that persist in constant darkness (DD) and whether these rhythms show similar characteristics. Lizards held on laboratory thermal gradients in the Australian spring under the prevailing 12-hour light : dark (LD) cycle for 14 days displayed robust daily rhythms of behavioural thermoregulation and locomotor activity. In the 13-day period of DD that followed LD, most lizards exhibited free-running circadian rhythms of locomotor activity and behavioural thermoregulation. The predominant activity pattern displayed in LD was unimodal and this was retained in DD. While mean levels of skin temperature and locomotor activity were found to decrease from LD to DD, activity duration remained unchanged. The present results demonstrate for the first time that this species’ daily rhythm of locomotor activity is an endogenous circadian rhythm. Our results also demonstrate a close correlation between the circadian activity and thermoregulatory rhythms in this species indicating that the two rhythms are controlled by the same master oscillator(s). Future examination of seasonal aspects of these rhythms, may, however, cause this hypothesis to be modified.     

The effect of various photoperiods on daily oscillations of serum corticosterone and insulin in rats.

The effect of various photoperiods on circadian rhythms of chosen parameters was investigated in laboratory rats. SPF male Wistar rats were adapted for six weeks to artificial light-dark cycles (LD 8:16, 12:12, 16:8). The light was switched on at 07.00 h in all regimens. The rats were killed at 3-hour intervals within 24 h, the serum concentration of corticosterone, insulin, glucose, food and water intake was determined. The external and computative acrophases of corticosterone varied in every photoperiod being dependent on the duration of light, the mesor values decreased in LD 16:8 in comparison with other photoperiods. The external acrophase of insulin was located 4 h after light onset in LD 8:16 and 12:12, in LD 16:8 one hour before light onset. The mesor values were approximately equal in all photoperiods. The circadian rhythms of glucose were similar in all regimens. Circadian variation of food and water consumption culminated at the same time in all regimens, the amount of food consumed in light increased with the light duration. Various photoperiods remarkably influenced circadian oscillations of corticosterone and in part food and water intake which could be considered as photoperiodic traits.     

Circadian rhythms of locomotor activity in cockroach nymphs: free running and entrainment

The temporal organization of locomotor activity was investigated in nymphs of the cockroach Leucophaea maderae. Approximately 40% of the animals examined between 1 and 50 days of age exhibited a circadian activity rhythm in constant darkness (n = 172) with an average free-running period of 23.7 +/- 0.68 hr. Twelve of 17 animals in which activity was recorded for most or all of the final instar also exhibited periods of rhythmic activity. The rhythms of the nymphs could be entrained by light-dark (LD) cycles with periods of 22, 24, or 26 hr. In contrast, neither maternal influences during embryogenesis nor hatching from the egg was effective in synchronizing the rhythms. Although adult cockroaches can be readily entrained by temperature cycles, in nymphs temperature appeared at best to be a weak zeitgeber. Embryonic exposure to an LD cycle until 6 days prior to egg hatch was effective in synchronizing the activity rhythms of the nymphs, indicating that differentiation of an entrainable pacemaking system occurs prior to hatching.     

Effects of cholecystokinin octapeptide (CCK-8) on food intake in adult and aged rats under different feeding conditions

The effects of CCK on food intake were investigated under fixed feeding conditions in comparison to a test meal taken after 16 h of food deprivation. The experiments were performed on young adult rats (8 weeks old) as well on aged rats (23 months old). Intraperitoneal CCK-8 (8 and 40 μg/kg) significantly reduced the size of a test meal following 16-h food deprivation. This effect was independent of the age of the rats. However, under fixed feeding conditions neither of the doses used in this study reduced food intake in the young adult rats, whereas the highest dose of 40 μg/kg did so in the aged rats. These results suggest that the hypophagic effect of exogenous CCK-8 depends on experimental conditions, food intake being reduced after a period of food deprivation but not under a fixed feeding regimen in adult animals. Furthermore, the data suggest that age is a factor contributing to the complex behavioral actions of CCK, because only old animals were more susceptible to an anorectic action of CCK under the fixed feeding schedule. An explanation may lie in an interaction of other known behavioral effects of CCK (e.g., anxiogenic, mnemonic action) with its effects under the different feeding schedules.     

Time-dependent effects of leptin on food intake and locomotor activity in goldfish

The present study investigates the possible circadian dependence of leptin effects on food intake, locomotor activity, glycemia and plasma cortisol levels in goldfish (Carassius auratus). Fish were maintained under 12L:12D photoperiod and subjected to two different feeding schedules, one group fed during photophase (10:00) and the other one during scotophase (22:00). Leptin or saline were intraperitoneally injected at two different times (10:00 or 22:00), coincident or not with the meal time. To eliminate the entraining effect of the light/dark cycle, goldfish maintained under 24 h light (LL) were fed and leptin-injected at 10:00. A reduction in food intake and locomotor activity and an increase in glycemia were found in goldfish fed and leptin-injected at 10:00. No significant changes in circulating cortisol were observed. Those effects were not observed when leptin was administered during the scotophase, regardless the feeding schedule; neither in fish maintained under LL, suggesting that a day/night cycle would be necessary to observe the actions of leptin administered during the photophase. Changes in locomotor activity and glycemia were only observed in goldfish when leptin was injected at daytime, coincident with the feeding schedule, suggesting that these leptin actions could be dependent on the feeding time as zeitgeber. In view of these results it appears that the circadian dependence of leptin actions in goldfish can be determined by the combination of both zeitgebers, light/dark cycle and food. Our results point out the relevance of the administration time when investigating regulatory functions of hormones.