Effectiveness of cyclic intragastric feeding as a circadian zeitgeber in the squirrel monkey

To investigate the mechanism of circadian synchronization by oral food consumption, we tested the effectiveness of cyclic intragastric feeding in synchronizing squirrel monkeys to a 24-hour schedule. Five monkeys were prepared with intragastric catheters and were studied free-ranging within a cage in an environmentally controlled chamber with continuous monitoring of body temperature (every 15 min) and locomotor activity (every 30 min). A balanced mixture of dextrose, amino acids, fat emulsion, vitamins and minerals was infused from 0800-2000 every day via the gastrostomy, first during a 12-hour light and 12-hour dark cycle (lights on 0800-2000 hours), and then subsequently in constant illumination. Serial phase analysis was performed by computing cycle-by-cycle acrophases of best-fitting sinusoids. One animal was synchronized by the feeding regimen while four others exhibited relative coordination. We conclude from these results that the timing of intragastic feeding may have only a weak synchronizing effect on primate circadian rhythms and that pre-gastric cues associated with oral food ingestion contribute to entrainment by feeding schedules.     

Circadian pacemaker control of feeding in the rat,at dawn

Previous studies suggest that feeding at dusk is probably dependent on the rat's immediate energy requirements, while feeding at dawn may have an ‘anticipatory’ function. However, little is known regarding the relative contribution of habit, energy deficits and circadian pacemakers in the expression of feeding behavior. The aim of the present study was to investigate the involvement of, and interplay between, habit, pacemaker synchronization and energy deficits in the occurrence of dawn feeding in rats. Light onset exerts a strong control over the timing of dawn feeding. The motivation to feed at dawn persists even when access to food is prevented during this period, and shows accompanying shifts with changes in light onset. Rats compensate their caloric deficit imposed by food restriction at dawn by eating earlier in the light phase. This feeding probably occurs in response to feedback mechanisms signalling an energy deficit. The rapid shift in dawn feeding with changes in light onset and food restriction, and its quick reappearance after discontinuing food restriction, argue against habit formation as solely responsible for the occurrence and maintenance of dawn feeding. Habit formation does play a secondary role in the maintenance, but occurs within the ‘boundaries’ set by the pacemaker. These experiments show that the timing and maintenance of dawn feeding are under the control of a circadian pacemaker which can be shifted by light onset only.     

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.     

Food and light as entrainers of circadian running activity in the rat

The purpose of this experiment was to determine the extent to which circadian running activity could be controlled by two different environmental cues: periodic access to food and ambient light cycles. Seven male rats were successively exposed to the following four conditions: (1) With light cycles (LD 12:12) and continuous access to food, the animals displayed a basically nocturnal pattern of running. (2) With access to food for two hours during the illuminated portion of the LD cycle, all of the animals showed a burst of running preceding food presentation. Three of these seven animals concurrently showed diminished running associated with the light cycle. (3) The animals continued to run before the periodic presentation of food when constant light replaced the LD cycle. (4) When allowed continuous access to food, and still exposed to constant light, the animals continued to show bursts of running around the time that food had been delivered. Rats' circadian activity cycles were therefore influenced by periodic access to food. Moreover, when periodic food was offered during the illuminated portion of the LD cycle (when rats are normally inactive) food presentation was a more potent entrainer than photic cues in controlling running activity.     

Overeating,dietary selection patterns and sucrose intake in growing rats

Weanling rats were allowed access to food for either 5 hours or 24 hours per day. Within each food availability condition one group had access to a complete diet and one group had access to the complete diet and a 32% solution of sucrose. Caloric intake and rates of growth were considerably higher in the 24 hour access groups. The availability of sucrose led to a small (10–15%) but consistent elevation in caloric intake in the ad lib condition but did not influence growth in either food availability condition. Absolute levels of sucrose intake and the proportion of calories taken from the sucrose solution were consistently higher in the ad lib group and increased with increases in body size for both groups. Dilution of the chow component or the sucrose component of the diet did not alter dietary selection patterns in either food availability condition. It appears that access to a palatable carbohydrate solution can lead to overeating in the rat but these solutions do not induce the rat to select imbalanced diets that compromise growth.     

Circadian variation of food intake and digestive tract contents in the rat

The variation of food intake and digestive tract contents over a period of 24 hours was measured in the free feeding rat kept under a 12:12 hr light:dark cycle (lights on at 0700 hr). After a cessation between 0700 and 1000 hr, the rate of food intake increased progressively during daytime followed by a marked increase and a plateau after lights went off. In the dark, dry matter in the stomach was almost ten times the levels of the light phase. In the major part of the high feeding period, the rates of food intake and gastric emptying equilibrated. Dry matter content of the intestine also followed circadian variations that were specific for each segment. The time-to-time content of the mid-gut segment was closely related to gastric emptying. The extent of dry matter absorption in the proximal intestine decreased continuously from the time lights went on and reached a minimum at 22 hr. Absorption in the distal segments was much less subjected to variation over 24 hours. In the free feeding situation, the changes induced by the light cycle on the rhythm of food intake were accompanied by marked modifications of digestive tract contents including those of the intestine. Data indicate a close relationship between gastric function and the middle portion of the intestine.     

Restricted access to food, but not sucrose, saccharine, or salt, synchronizes the expression of Period2 protein in the limbic forebrain

Restricted feeding schedules (RF) in which daily access to food is limited to a few hours each day can entrain the rhythms of expression of circadian clock genes in the brain and periphery in rodents. The critical factors mediating the effect of RF on rhythms of clock gene expression are unknown. Previously, we demonstrated that daytime RF shifts the phase of expression of the clock protein, Period2 (PER2) in the oval nucleus of the bed nucleus of the stria terminalis in rats kept on a 12-h light/dark cycle, and restored the rhythm of PER2 expression in rats housed in constant light. We now report that RF also modifies the rhythms of PER2 expression in the central and basolateral nuclei of the amygdala and in the dentate gyrus, such that all three areas become synchronized, peaking 12 h after the time of food presentation. Daily limited access to sucrose or saccharine in freely fed rats or scheduled access to saline in sodium-deprived rats had no effect on these PER2 rhythms. Thus, it would appear that the rhythms of PER2 in limbic forebrain structures are sensitive to signals that arise from the alleviation of a negative metabolic state associated with scheduled feeding and that access to rewarding substances in the absence of food deprivation or metabolic challenges, per se, is not sufficient to alter the rhythms of PER2 expression in these regions.     

Circadian variation in cell division of the mouse alimentary tract,bone marrow and corneal epithelium

Circadian rhythms in DNA synthesis are described for the tongue epithelium, five different regions of the alimentary canal (gut) –esophagus, stomach, duodenum, jejunum and rectum– and bone marrow in a group of BDF¹ male mice. A circadian rhythm is also described for the mitotic index in the corneal epithelium in the same mice. The data document for the first time in the same animals the dramatic variation in cell division encountered from one region of the gut to another. This variation is seen in the amplitudes of the rhythms as well as in the over-all 24-hour means. On the contrary, the phasings of the rhythms in the different regions of the gut are remarkably similar. In this study, where the mice were standardized to 12 hours of light (0600-1800) alternating with 12 hours of darkness, the peak of the DNA-synthesis rhythm occurred around the time of transition from dark to light, and the trough around the time of transition from light to dark. The implications of these findings, and those of others, to the study of cell kinetics and to cancer chemotherapy are discussed.     

Changes in certain diurnal hormonal rhythms in developing male rats exposed to adiurnal environmental cues

It has traditionally been thought that diurnal rhythmicities cannot be entrained to follow adiurnal environmental cues that deviate greatly from a 24-hour periodicity. In this study, exposure of male rats pre- and postnatally to an adiurnal lighting schedule of 9 hours light: 9 hours dark resulted in the entrainment of certain hormonal rhythms (such as plasma corticosterone and plasma TSH) but not of others (such as pituitary TSH and plasma T₃). Even though thyroid rhythms were influences only partially by the shorter, 18-hour day, absolute levels of pituitary TSH and plasma T₃ were significantly higher in experimental rats (9 hours light:9 hours dark) than controls (12 hours light:12 hours dark). Whole-body and kidney weights and tail length were also significantly increased in the 9 hours light:9 hours dark animals, suggesting a possible relationship of shorter day cycles to faster growth and higher thyroid hormone levels.     

Partial isolation of the suprachiasmatic nuclei: Effects on circadian rhythms of rat drinking behavior

The rat's daily pattern of contact with a drinking tube was observed in a light-dark cycle (LD 12:12) and in constant light (LL) after partial isolation of the suprachiasmatic nuclei (SCN) with a wire microknife that transected their lateral, dorsal, and caudal connections or the passage of a wire microknife through the SCN. Animals whose SCN had been partially isolated remained entrained to an LD cycle and phase shifted drinking behavior when the LD cycle was delayed 4 hr. Disintegration of circadian rhytmicity was observed when these rats were exposed to LL. Passage of the knife through the SCN resulted in immediate and persistent disruption of circadian rhythmicity as severe as that seen after electrolytic lesions of the SCN. These data confirm the necessity of the SCN in regulating circadian behavior and suggest that the SCN may mediate entrainment and freerunning rhythms via different neural systems.     

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 access, the aged rats, like young rats, rapidly reverted 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.     

Daily rhythms of feeding in the genetically obese and lean Zucker rats

Feeding patterns were examined in obese (fa/fa) and lean (Fa/-) adult Zucker rats over the light-dark cycle during 14 days. Obese rats eat more than lean rats especially during the dark phase. Light and dark feeding expressed as percentage of 24 hr intake showed no significant differences between the lean and obese groups. The higher food intake in obese rats is mainly caused by larger meals since obese rats ate fewer meals than lean rats. Only for the obese group differences were observed between mean meal size in light and dark phase. There is some indication that the circadian controlled temporal distribution of meals is different in obese rats compared to lean rats since obese rats eat fewer but larger meals during the first half of the dark phase. During this phase meal size increases gradually in the obese rats, suggesting that the circadian influence on feeding motivation is increased.     

Effect of an inverted light regime on the biological rhythms of the mitotic index of mouse esophageal epithelium in different periods

Changes of the mitotic index in the esophageal epithelium were studied every 20 min for 24 hours in control mice and in mice kept under conditions of an inverted light regime for 10 days. A monophasic circadian rhythm was revealed, which was virtually completely resynchronized to the new light regime after 10 days of photoinversion. Circahoralian fluctuations of the mitotic index with 1–2-hour periods were noted. In controls the period of these fluctuations was shorter in the active phase of the circadian rhythm than during the passive phase. After photoinversion this regularity was no longer observed. Hence, in contrast to the circadian rhythm, the hierarchical structure of the temporal organization of the mitotic index rhythm of the esophageal epithelium in different periods was not restructured during the 10 days of the new light/dark regime. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 121, N ^o 1, pp. 94–97, January, 1996     

Circadian variation in the effects of nitric oxide synthase inhibitors on body temperature, feeding and activity in rats

We have investigated whether there is circadian variation in the effects of nitric oxide synthase inhibitors on body temperature, physical activity and feeding. We used nocturnally active Sprague-Dawley rats, housed at approximately 24 degrees C with a 12:12 h light:dark cycle (lights on 07:00 hours) and provided with food and water ad libitum. Nitric oxide synthesis was inhibited by intraperitoneal injection of the unspecific nitric oxide synthase inhibitor N-nitro- L-arginine methyl ester ( L-NAME, 100, 50, 25, 10 mg/kg), or the relatively selective inducible nitric oxide synthase inhibitor aminoguanidine (100, 50 mg/kg), during the day ( approximately 09:00 hours) or night ( approximately 21:00 hours). Body temperature and physical activity were measured using radiotelemetry, while food intake was calculated by weighing each animal's food before as well as 12 and 24 h after each injection. We found that daytime injection of L-NAME and aminoguanidine had no effect on daytime body temperature. However, daytime injection of both drugs did decrease nocturnal food intake ( P<0.05) and activity ( P<0.05). When injected at night, L-NAME reduced night-time body temperature ( P<0.01), activity ( P<0.05) and food intake ( P<0.05) in a dose-dependent manner, but night-time injection of aminoguanidine inhibited only night-time activity ( P<0.05). The effects of nitric oxide synthase inhibition on body temperature, feeding and activity therefore are primarily a consequence of inhibiting constitutively expressed nitric oxide synthase, and are subject to circadian variation.     

Hypothalamic hyperphagia despite imposed diurnal or nocturnal feeding and drinking rhythms.

Rats normally do most of their eating at night. When ad lib fed rats are made hyperphagic with lesions or parasagittal hypothalamic knife cuts the increases in eating occur primarily during the day. This suggests that a disruption of circadian rhythms may mediate the overeating. However, when knife cut rats were food and water deprived all day excessive eating occurred at night. Similarly, when they were deprived all night overeating occurred during the day. Under both conditions od deprivation the food intakes and rapid weight gains of the ad lib fed knife cut group were defended. It was concluded that: (1) in hypothalamic hyperphagia either the excessive food intake or the excessive weight gain is defended when food and water are available only half of each day, and (2) disruption of nocturnal feeding and drinking rhythms is not the cause of hypothalamic hyperhagia.     

Circadian organization of thirteen liver and six brain enzymes of the mouse

The activities of 13 liver and 6 brain enzymes were studied in 7–12 week old CD₂F₁ male mice that had been fed ad libitum and standardized either to 12 hours of light (0600–1800) alternating with 12 hours of darkness (1800–0600) (LD 12:12), or to a reversed light-dark cycle (darkness 0600–1800; light 1800–0600) (DL 12:12). Three separate studies were performed on two different days; in each experiment, subgroups of 14 animals were sacrificed at 3-hour intervals. Livers were assayed for: isocitrate dehydrogenase, glutamate dehydrogenase, lactate dehydrogenase, alcohol dehydrogenase, glutathione reductase, glyoxylate reductase, L-alanine aminotransferase, glutamate oxalacetate transaminase, pyruvate decarboxylase, fructose-1-phosphate aldolase, fructose diphosphate aldolase, fructose 1,6-diphosphatase, and fatty acid synthetase. Brains were assayed for phosphoglucose isomerase, adenosine triphosphatase, creatine phosphokinase, pyruvate kinase, adenylate kinase, and malate dehydrogenase. All 19 enzymes demonstrated a prominent circadian rhythm in at least one experiment. Moreover, each rhythmic variable showed a statistically significant fit to a 24-hour cosine(sine) curve by the method of least squares. In general, peak activities of the liver enzymes analyzed were associated with the beginning of the dark cycle and initiation of the animal's activity, while the group of brain enzymes had peak activities which occurred at the beginning of the animals' rest span and were near the beginning of the light cycle. The phasing of each of the rhythms could be reversed within a two-week span after reversing the environmental light-dark cycle 180°.     

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.     

Effect of skeleton photoperiod and food availability on the circadian pattern of feeding and drinking in rats

Feeding and drinking behavior were measured in rats maintained under a 12:12 light-dark (LD) cycle or skeleton photoperiod (SPP). Feeding and drinking were closely associated during the normal LD cycle but under SPP conditions an increased feeding activity during the subjective day was not accompanied by an equivalent increment of water intake. This indicates a stronger coupling of drinking to the subjective night. A restriction of food availability to the subjective light phase did not cause an accompanying complete shift in drinking behavior. These results suggest that drinking is largely dependent on the influence of a circadian oscillator and this association is not disrupted by changes in feeding schedule. A change in food access to the subjective light phase caused partial but not permanent desynchronization between feeding and drinking behavior. Synchrony was reestablished within one day once food was available ad lib. Complete return to the original feeding and drinking patterning took 3 days. It is suggested that separate slave oscillators controlling feeding and drinking are governed by a hypothesized "master" circadian oscillator which remains definitely entrained to the original rhythm by the light pulses of the SPP condition.     

Rapid reentrainment of the circadian clock itself, but not the measurable activity rhythms to a new light-dark cycle in the rat.

Experiments were performed to determine if the circadian clock reentrains more quickly to an 8-hour phase shift in light-dark (LD) cycles than does the overt rhythm of activity. To investigate the reentrainment of the clock itself to an 8-hour advance or delay in the LD cycle, the rats were released into constant darkness only two or three days after a shift in LD cycle, and the amount of the phase shift of the clock itself was estimated from where free-running rhythm started by backward extrapolation. If the circadian clock could rapidly reset itself to the new LD cycle, it was predicted that the free-running rhythm of activity would start from near the dark period of the new LD cycle rather than the preceding one. When rats were released into constant darkness three days after the LD cycle was advanced by 8 hours, the activity of the free-running rhythm started near time of dark period of the new LD cycle in all rats (n = 16). When rats (n = 24) were released into constant darkness two days after the LD cycle was advanced by 8 hours, 12 rats started the activity near time of dark period of the new LD cycle, while 9 rats started the activity near time of dark period of the preceding LD cycle. The remaining 3 rats showed the activity of the free-running rhythm near intermediate phase (transient phase). On the other hand, when the rats were not released into constant darkness after LD cycle was advanced by 8 hours, it took 6.4 days for activity rhythm to reentrain to the advanced LD cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of restricted food access on circadian fluctuation of serotonin N-acetyltransferase activities in hereditary microphthalmic rats

The characteristics in diurnal fluctuation of serotonin N-acetyltransferase activity were examined in normal and microphthalmic mutant rats of the Donryu strain under ad lib or restricted feeding conditions. Under a 12:12-h light:dark (12-h LD) cycle with free access to food, normal-sighted rats exhibited typical nocturnal increases in the activity of pineal serotonin N-acetyltransferase, being more than 50-fold higher in the dark period than that in the light period, but hereditary blind rats showed nonperiodic change in the pineal enzyme activity in the average, suggesting that the rhythms in individuals have become free-running, asynchronous. When the subjective night or subjective day of the mutants was discerned by active or inactive in the locomotor activity, the pineal enzyme activities in the mutants increased at the subjective night but depressed at the subjective daytime. When food access was restricted only for 6 h in the light period of the LD cycle, normal rats still showed the nocturnal increases in the pineal enzyme activity, but hereditary blind rats manifested a blunt peak in the activity of the pineal enzyme at eating time in the light period. The results suggest that microphthalmic mutant rats maintain the ability to shift and to synchronize their circadian phases induced by restricted access to food, even if they completely lack their optic nerve and visual input to the circadian clock.